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LECTURES

ON THE

PHYSIOLOGY OF PLANTS

SACHS

[3]

HENRY FROWDE

Oxford University Press Warehouse
Amen Corner, E.C.

LECTURES

ON THE

PHYSIOLOGY OF PLANTS

BY

JULIUS VON SACHS

TRANSLATED BY

H. MARSHALL WARD, M.A., F.L.S.

FELLOW OF Christ's college, Cambridge, and professor of botany in

THE forestry SCHOOL, R. I. E. COLLEGE, COOPER's HILL

WITH 455 WOODCUTS

£DjcforD

AT THE CLARENDON PRESS

MDCCCLXXXVII

\^All rig] Us reserved^

PREFACE.

After the fourth edition of my 'Text-Book of Botany' (1874) had nearly
passed out of print, I received from the publishers, as well as from botanical friends,
repeated invitations to prepare a fifth edition. It is, however, an old experience
that while one works up with pleasure a second and even a third edition of a
comprehensive work, frequent repetition eventually becomes inconvenient or even
painful to the author. Having experienced this sufficiently with the fourth edition,
I was unable to make up my mind to a fifth. Apart from other circumstances,
I was driven to this to an important extent by the progressive development of
my scientific convictions. My mode of comprehending important questions of the
Physiology of Plants had undergone changes in various directions, particularly in
consequence of my compilation of the 'History of Botany'; like others, more or
less subject to the prevailing opinions of the present, I had held as important
matters which I was gradually impelled to recognise as insignificant and tran-
sitory; higher stand-points and freer prospects opened out to me in the course of
time, and the form of my text-book would no longer adapt itself to the advanced
view. The artist may touch up his composition here and there with a few
strokes of the pencil, or even make greater alterations ; but that is not sufficient
when the composition itself has ceased to be the expression of his idea. This is
the position in which I find myself with respect to my text-book, since the chief
thing in it to me is the composition, the form of the exposition as a whole.

Moreover for several years past the wish had been taking a more and more
definite form in my mind, to set forth the most important results of the physiology of
plants in such a manner that not only students, but also wider circles, should be
interested in them. That object, however, is only to be attained by a freer form
of exposition, and I believe I have found it in the choice of lectures. It is not
only the right but also the duty of any one who lectures, however, to place in
the foreground his own mode of viewing the matter ; the audience wish to know
and should know how the science as a whole shapes itself in the mind of the
lecturer, and it is comparatively unimportant whether others think the same or
otherwise.

I would have the present book criticised from this point of view. It is
intended to introduce students and cultivated readers generally to the study of

VI PREFACE.

the Physiology of Plants, free from the trammels of learned descriptions of appa-
ratus which of course could not be dispensed with in a text-book or hand-book
for specialists.

Perhaps no other branch of Natural Science is so unknown to the educated
public as the Physiology of Plants ; because, in spite of the important progress
which has been made in it during the last twenty years, and in spite of the uses to
which it may be applied, no one has undertaken to publish its established results
in a convenient and intelligible form. This really serious want in our literature
I wish to supply by means of my ' Lectures,' and this is of course only possible by
the contents being strictly scientific ; only the form of the exposition is to differ from
that hitherto customary, in running in phrases which are universally intelligible.
The object indicated, ho\\ever, requires also that much which is apparently self-
evident to the specialist must here be expressly brought forward and explained, so
that a certain prolixity of description is often unavoidable, while, on the other
hand, some questions of the day important to the Botanist are entirely passed
over or can only be briefly touched upon. Again, having regard to the super-
abundance of material, a suitable selection must be made, for, as is well known,
the secret of being tedious lies in trying to say all one knows.

The notes on the literature attached to the separate lectures are only intended
for those readers who may be by any chance stimulated by my book to wish for
further direction along the untrodden grounds of our literature.

The publishers were of opinion that a new edition of the systematic part of
my ' Text-book ' might conveniently be attached to my ' Lectures.' Since I have
myself neither the time nor the inclination to undertake such a new working-up
of this domain of Botany, I have made arrangements with Professor Goebel, and
he is to compile the systematic portion of my Text-book independently, and
according to his own judgment, and publish it as a separate Book, which the
reader may employ as a supplement to my ' Lectures.'

Dr. J. v. SACHS.

Würzburg, /?/w^ 27///, 18S2.

PREFACE TO THE ENGLISH EDITION.

The work of translating Professor Sachs' 'Vorlesungen über Pflanzen-physio-
logie ' has been carried on during intervals between scientific duties of other kinds,
and some delay has resulted from the pressure of certain of these duties of late.

To put the graphic language of the original into English, which should be
as widely read in this country as the German lectures have been on the Continent,
has been a distinct wish and aim on my part ; but it is well known that difficulties
often arise in the attempt to render a German scientific sentence into simple English
intelligible to the general reader, and that the force of many expressions may be
readily injured or destroyed by displaying too much fear of their foreign form. That
it has not always been possible to reproduce the living ideas of the author with their
full force in the new language, I am only too well aware, but it is hoped that
the faults are venial : they would have been more numerous but for the kindness
of Dr. S. H. Vines, F.R.S., of Christ's College, Cambridge, who has been so good as
to look over the proofs before I finally revised them for the press, I am also
indebted to the courtesy of Mr. W. T. Thiselton Dyer, F.R.S., C.IVI.G., the Director
of the Royal Gardens, Kew, and to Dr. Bayley Balfour, F.R.S., Professor of Botany
in the University of Oxford, for several suggestions.

The index is the only real departure from the original: this I have greatly
extended, in the hope that it will be correspondingly useful to students and teachers.
With regard to the bibliography, I have not added to the notes selected by Professor
Sachs ; but it may be pointed out that further references to the literature connected
with special points are available in the English editions of De Bary's ' Comparative
Anatomy of the Ferns and Phanerogams,' by Professor Bower and Dr. Scott ;
Goebel's ' Outlines of the Classification and Special Morphology of Plants,' by
Professor Bayley Balfour and the Rev. H. E. F. Garnsey ; De Bary's ' Comparative
Morphology and Biology of the Fungi, INIycetozoa and Bacteria,' by the same ; and in
the ' Lectures on the Physiology of Plants,' by Dr. S. H. Vines.

PI. MARSHALL WARD.
Forestry School,

Cooper's Hill.

CONTENTS.

PART I. — Organography.

PAGE

Lecture I. — Introductory Remarks on the Physiological Organography of the

Vegetative Organs i

Shoot and Root. — Typical, Rudimentary, and Reduced Forms. — Substance and
Form of Organs.

Lecture II.— The Typical Roots of Vascular Plants tl

Branching of Roots of Seedlings. — Place of Origin of Roots. — Development of
Roots. — Entrance of Roots into Soil. — Importance of Root-hairs. — Shortening of
Roots. — Conversion of Roots into Shoots.

Lecture III. — Roots, continued. Metamorphosed and Reduced Roots of Vascular

Cryptogams ; Rudimentary Roots of Mosses and Thallophytes ... 23
Lignified and Napiform Roots. — Roots of Parasites. — Roots of Muscinese. — Roots
of Liverworts. — Roots of Algse. — Roots of Fungi.

Lecture IV. — Typical Forms of Shoot of the Vascular Plants .... 36
Typical Shoots. — Continuity of Shoot-axis and Leaf. — Crowding of Leaves at the
Growing-point. — Buds. — Vascular bundles of Shoots. — Segmentation of Leaves. —
Venation of Leaves.

Lecture V. — Metamorphosed and Reduced Shoots of Vascular Plants. Shoots of

Mosses, Algae, and Fungi ........... 54

Succulent Shoots. — Cladodes. — Twining Shoots. — Tendrils.— Thorns. — Rimners,
Tubers, Bulbs. — Subterranean Shoots. — Root-like Shoots. — Shoots of Parasites. —
Relation of Chlorophyll to the Forms of Shoots. — Shoots of Mosses. — Shoots
of Liverworts. — Shoots of Algas. — Concluding remarks.

Lecture VI. — The Cellular Structure of Plants. Protoplasm, Nucleus, Cell-
Wall 73

Original meaning of the word ' Cell.' — Isolation of Cells. — Cell-wall and Cell-
contents. — Chemical nature of Protoplasm. — Protoplasm. — Movements of Proto-
plasm.— Chlorophyll-corpuscles. — Nucleus. — Cell-wall. — Deliquescence of cell- walls.
— Stratification, Striation, Pits. — Structure of Cell-wall.

Lecture VII. — Development of Cells 94

Growth and Cell-division. — Formation of Chambers in Mother-cells.— Development
of Reproductive Cells. — Uniformity of Cell-formation. — Behaviour of Nucleus
during division. — Origin of the New Septum. — Abnormal Nuclear division. — Non-
cellular Plants.

Lecture VIII. — Forms and Systems of Tissue : Epidermal Tissue and Vascular

Bimdles no

Common Wall of Tissue-cells. — Systems of Tissue. — Epidermis. — Cuticle. — Waxy
Coverings. — Stomata. — Epidermis of Moss. — Hairs. — Physiological significance of
Hairs. — Structure of Hairs. — Vascular Bundles (Strands). — Structure of Vascular
Bundles. — Xylem and Phloem of the Strand. — Arrangement of Xylem and Phloem.
— Vessels. — Sieve-tubes.

CONTENTS.

PAGE
141

Lecture IX. — Systems of Tissue, continued. Fundamental Tissue. Rudimentary
Differentiations of Tissue ...........

Fundamental Tissue. — Hypoderm. — Sheaths. — Sclerenchyma. — Assimilatory Paren-
chyma.— Tissue-systems of Mosses. — Differentiations of Tissues in Algse. — Differ-
entiations of Tissues in Fungi.

Lecture X. — Secondary Growth in Thickness of Shoot- Axes and Roots . . 155

Correlation between Growth in Thickness and extent of Foliage. — Cambium-ring. —
Products of Cambium. — Wood. — Heart-wood and Alburnum. — Cork, Periderm. —
Bark. — Lenticels. — Secondary Growth in Thickness of Monocotyledons.

Lecture XL — Laticiferous Vessels and Receptacles for Secretions . . . 171

Latex-tubes. — Variety of Secretions. — Calcium Oxalate.— Calcium Carbonate. —
Secretion-vesicles. — Resin- and Gum-canals. — Internal Glands. — Epidermal Glands.

PART II. — The External Conditions of Vegetable-life, and the
Properties of Plants.

Lecture XI L— The General External Conditions of Plant-Life .... 189
Organic Structure and External Influences. — Cardinal Points of Temperature
suitable for Vegetation.- — Representation of the Dependence by Curves. — General
Law of Dependence. — Dependence on Light. — Daily Periodicity. — Influence of
Gravitation, Light, Electricity. — Dependence on Habitat. — Dependence on Animals.

Lecture XI 1 1. — The Molecular Structure of Plants and its Physiological Im-
portance 205

Molecules, Molecular Complexes. — Swelling. — Diosmosis. — Turgescence. — Artificial
Cells. — Tissue-tensions. — Rigidity due to Tissue- tensions. — Elasticity, &c. due to
Lignified Sclerenchyma.

PART III.— Nutrition.

Lecture XIV. — The Ascent of Water in Transpiring Land-Plants . . .225

Importance of the Water-current in Nutrition. — Transpiration from Leaves. —
Ascending Current in the Wood. — Sclerenchyma conducts water. — Distribution of
the Water-current in the Leaves. — Rapidity of the Upward Flow. — Filtration
through Wood. — Condemnation of the Capillarity Theory. — Calculation of the
Cubic Contents of the Wood-cavities. — Specific Properties of Wood. — Suction in
Cut Branches.

Lecture XV. — Conditions of Transpiration — Absorption of Water and Nutritive

Matters by the Roots of Land-Plants 246

Regulation of Transpiration. — Mechanics of Stomata. — Stomata as the Regulators of
Transpiration. — Transport of Nutritive Salts in Wood. — Absorption of Water and
Salts by Leaves. — Water contained in the Soil. — Absorption of Water by Root-
hairs. — Absorption of Substances held in the Soil. — Attachment of Root-hairs to
Particles of Soil. — Corrosion of Minerals by Roots.

Lecture XVI.— Excretion of Water in the Liquid State 266

Mobility under Pressure of the Water in Wood. — Water-currents in the Wood due
to Changes of Temperature. — Weeping of Root-stock. — Periodic variations in the
Excretion of Water. — Mechanics of the Weeping of Root-stocks. — Relation of Root-
pressure to Transpiration. — Excretion of Drops from Leaves. — Excretion of Drops
by Non-cellular Plants.

CONTENTS. xi

PAGE

Lecture XVII.— The Nutritive Materials of Plants 282

Artificial Nutrition of Plants. — Influence of Iron. — Chlorosis. — Quantitative Selection.
— Importance of Silica. — Quantity of Ash in Plants. — Source of Nitrogen. — Source
of Carbon. — Action of Light in Assimilation.

Lecture XVIII. — The Production of the Organic Substance of Plants — Assimila-
tion 296

Evolution of Oxygen. — Chlorophyll the Instrument of Assimilation. — Influence of
Light on the Development of Chlorophyll. — Assimilation in the various parts of the
Solar Spectrum. — Assimilation not effected by the so-called Chemical Rays. —
Dependence on the Length of Waves of Light. — The first visible product of Assimi-
lation.

Lecture XIX. — Origin of Starch in the Chlorophyll, and in the Starch-forming

Corpuscles, Further Behaviour and Fate of the Chlorophyll .... 309
Starch in Chlorophyll. — Energy of Assimilation. — Visible Processes in the Chloro-
phyll.— Starch-forming Corpuscles. — Cheinical Processes in Chlorophyll. — Emptying
of Leaves in Autumn. — Colouring Matter of Chlorophyll.

Lecture XX. — Chemical Metamorphoses of the Products of Assimilation. Phy-
siological Classification of the Products of Metabolism 323

Origin of Proteids. — Employment of Reserve-materials. — Biological Significance of
products of Metabolism. — Reservoirs of Reserve-materials. — Forms of Reserv^e-
materials. — Aleurone-grains and Crystalloids. — Inulin. — Starch-grains. — Granulöse
and Cellulose of Starch-grains.

Lecture XXL— Renewal of Activity of Reserve-Materials. Ferments. Dormant

Periods 341

Ferments. — Growing Organs produce Ferments. — Peptones. — Asparagin. — Fats. —
Naegeli's distinction between Ferment-action and Fermentation. — Resting Periods
of Vegetation.

Lecture XXII. — Passage of the Plastic-Materials through the Tissues . . . 353
Transport of Constructive Materials over large Distances. — Employment of the
Constructive Materials in Growing Organs. — Movement and Consumption of the
Constructive Materials. — Mechanics of the Movements.— Movement in Latex-tubes.
— Movements induced by Growth.

Lecture XXIII. — The Absorption of Organic Nutritive Materials. Parasites.

Saprophytes. Insectivorous Plants 366

Nutrition of Parasites. — Action of Parasites on their Hosts. — Connection of Parasites
with their Hosts. — Sources of the Plastic Substances of Parasites. — Comparison with
Seedlings. — Diotiaa vmscipida. — Drosera. — Nepenthes and other Insectivorous
Plants.

Lecture XXIV. — Lecture xxiii, continued. — Nutrition of Fungi. Lichens . . 380
Various effects of Fungi on their Substratum. — Nutritive Materials of Fungi. —
Nutritive and Non-nutritive Organic Substances. — Formation of Fat. — Fennentation
of Yeast. — Bacteria. — Putrefaction. — Tree-killing Fungi. — Ferment-action and Fer-
mentation of Fungi.— Parasitism of Lichen-fungi. — Configuration of Lichens con-
ditioned by Chlorophyll.

Lecture XXV. — The Respiration of Plants. Spontaneous Evolution of Heat.

Phosphorescence 395

Absorption of Oxygen and Evolution of Carbon Dioxide. — Quantity of Carbon
Dioxide respired. — Destruction of Organic Substance by Respiration. — Intra-
molecular Respiration. — Purpose of Respiration. — Measurement of Spontaneous
Heat. — Production of Light by Respiration.

CONTENTS.

PART IV.— Growth.

Lecture XXVI. — The Distribution of the Phases of Growth in Space and Time . 41 1
The Growth of Plants compared with that of Crystals. — The Three Phases of
Gro\vth. — Embryonic Conditions of Organs. — Elongation of Organs. — The Com-
pletion of Internal Differention in Organs. — Distribution of Growth in Non-Cellular
Plants. — Growth of Fungi. — Growth of Algse and Myxomycetes.

Lecture XXVIL — Relations between Growth and Cell-division in the Embryonic

Tissues 431

Dependence of Divisions on the Form of the Mother-cell. — The Arrangement of
Cells Independent of the Morphological and Physiological Nature of the Organ. —
Net-works of Cell-walls. — Periclines, Anticlines, Trajectories. — Arrangement of Cells
in an Elliptical Disc. — Examples agreeing with the Diagram. — Sequence of Anti-
clines and Periclines immaterial. — Displacement of Division-walls by Growth. —
Anticlines and Periclines in an Ellipsoidal body. — Anticlines and Periclines in
Growing-points. — The Apex of the Growing-point grows most slowly. — Confocal
and Co-axial Arrangement of Cells. — Apical Cells. — Transverse Segments of Apical
Cells. — Tetrahedral Apical Cells. — Significance of the Apical Cell.
Lecture XXVIIL — Formation of Organs at the Growing-point. Branching . 460
Forms of Growing-points. — Depressed Growing-points. — Metamorphosis of Growing-
point into an Organ. — Progressive Development of Organs at the Growing-point. —
Development of Leaves. — Development of Intemodes. — Shoots with and without
Branches. — Dichotomy or Bifurcation. — Apparently Endogenous Inception of Shoots.
Adventitious Growing-points.

Lecture XXIX. — Axis of Growth. Polarity. Laterality. Relations of Position 481
Axis of Growth. — Polarity in the Growing Axis. — Radial, Bilateral, Dorsi-ventral. —
Extension of the idea of Radial Structure. — Dorsi-ventral, Bilateral. — Dorsi-
ventrality. — Dorsi-ventral Shoots. — Laterality and Phyllotaxis. — Rejection of the
Spiral-theory. — Parastichies. — Divergences.

Lecture XXX. — Causal Relations of Growth of the different Organs of the Plant

one to another (Correlations) 502

Conception of the Form of Plants. — Influence of Growing Shoots on other Shoots. —
Bud-scales are suppressed Foliage-leaves. — Organs of like kind as Competitors
for the same Formative Substances. — Assimilatory Tissue forms thin Lamellae. —
Assimilating Surfaces influence the Formation of Wood and Roots. — The Activity
of the Roots affects the Functions of Leaves.

Lecture XXXI.— Influences of the Environment on the Processes of Configura-
tion in the Plant 515

Inherited Disposition and External Influences. — Influence of Gravitation on the
Origin of Growing-points. — Influence of Gravitation on the Orientation of Growing-
points. — Movement of Shoot-forming and Root-forming Substances. — Development
of Roots on the side turned away from Light. — Dorsiventral Structure induced by
Illumination.. — Influence of Light and Gravitation on Post-embryonic Growth. —
Influence of Gravitation on Post-embryonic Growth. — Influence of Light on Post-
embryonic Growth. — Etiolation. — Flower-forming Reserve Substance in Bulbs,
Tubers, &c. — Influence of Light on Post-embryonic Growth. — Gall-formations. —
Fertilisation.

Lecture XXXII. — The Course of Growth during Elongation. Periodic Varia-
tions 539

The Grand Period of Growth. — Distribution of Growth in the Growing Region. —
Length of the Growing Region. — Passive Motion of the Apices of Buds and Roots.
—Nutations due to Unequal Growth. Torsions due to Growth. — Growth by Day
and by Night. — Auxanometer. — Daily periods of Growth. — Daily periods in con-
tinued darkness. — Apparatus for Auxanometrical observation.

CONTENTS.

Lecture XXXIII. — Mechanical Causes and Effects of the Growth of Cells and

Organs 563

Turgescence as a Cause of Growth. — Cause of Turgescence. — Drooping.— Plas-
molysis retards Growth. — Tissue-tensions during Growth. — Transverse Tension in
Tissues. — Behaviour of an Isolated Prism of Pith. — Cortical Pressure. — Tissue-
tension in roots. — Effect of Pressures in Tissues on the form and size of Cells. —
Changes in Wood during alterations of Cortical Pressure. — Growth in Thickness of
the Branches of Trees. — Tyloses ; Callus. — Internal and External Work by means
of Growth.

PART v.— IrritabiUty.

Lecture XXXIV. — General Considerations on Irritability 587

Disproportionality between Stimulus and Effect. — Stimuli, Stimulation, Irritable
Structure. — Stimulation in Crystals. — Rigor of Irritable Organs. — Propagation and
after-effect of a Stimulus. — Propagation of a Stimulus compared with Physical
Processes. — Spontaneous Periodic Movements, — Specific Energy of Irritable Organs.

Lecture XXXV. — Irritability and Mobility of Protoplasmic Structures . . . 603
Swarm-spores and Antherozoids. — Velocity of Swarm-spores. — Rotation. — Influence
of Temperature on the Movement of Swarm-spores. — Emulsion-figures. — Action of
Light on Swarm-spores. — Influence of the Colour and Intensity of the Light. —
Amoebje, Plasmodia.— Circulation of Protoplasm in the Cell. — Circulation and
Rotation of Protoplasm. — Movements of Chlorophyll.— Causes of the Move-
ments.

Lecture XXXVI. — Periodic Movements of Foliage-Leaves and Flowers (Sleep-
Movements) 623

Variations in Illumination induce Movements. — Combination of different causes of
Movement. — Phototonus and Paratonic Light-stimuli. — Structure of Motile Organs.
— Tissue-tension in Motile Organs. — Histology of Motile Organs. — Changes in the
Water-contents due to alterations in the Light. — Description of the Daily
Periodicity. — Colours of Light. — Mechanics of the Movement. — Lever Action. —
Leaves without special Motile Organs. — Opening and Closing of Flowers. — Use
of Sleep-movements.

Lecture XXXVI I.— The Irritability of Mimosa and other Plants . . . -644
Description of the Phenomena of Irritability. — Escape of Water on Stimulation.—
The Irritable Movement is the consequence of the Expulsion of Water. — Irritable
Stamens of the Cynarese. — Cause of the Expulsion of Water from Stimulated Organs.
The Movement results from Changes in the Protoplasm and Cell-wall. — The
Irritability of Mimosa as means of protection.

Lecture XXXVI 1 1.— The Revolving of Tendrils and Twining Plants . . -657
Organography of Tendrils. — The Coiling of Tendrils. — Mechanics of the Coiling. —
Irritability. — Thick and thin Tendrils. — Advantageous properties of Tendrils. —
Nutation previous to Coiling. — Behaviour of Revolving Shoots on Supports. —
Twining without Supports. — Klinostat-movement of Free-shoots. — Feeble Shoots.
— Comparison of Twining-shoots and Tendrils. — Geotropism.

Lecture XXXIX. — Geotropism and Heliotropism 677

Geotropism. — Positive and Negative Geotropism. — Nature of the Geotropic Sti-
mulus.— Centrifugal Force. — Klinostat. — Form of Geotropic Curvature depends on
the Distribution of Growth. — Changes in Form of Shoot-axes during Erection. —
Geotropic Curvature of the Nodes of Grasses. — Motile Organs of Leaves. — Down-
ward curvatures. — Changes during Geotropic Downward curvatures. — Heliotropism.
Demonstration of Heliotropism. — Theory of Heliotropism. — Action of Colours of
the Spectram.— Non-cellular Plants.

CONTENTS.

Lecture XL. — The Anisotropy of the Organs of Plants 698

Definition of Anistropy. — Anistropy a cause of the Configuration of Plants. — Radial
Organs are Orthotropic, Dorsi-ventral ones Plagiotropic. — Radial Organs react
alike on all sides, Dorsiventral ones differently. — Rolled-up Plagiotropic Organs are
Orthotropic— Plagiotropic Lateral Roots. — Shoots of Marchantia. — Plagiotropic
Organs consist of Orthotropic parts. — Plagiotropism of the Ivy. — TropcTobini,
Gourd, etc. — Torsion in Plagiotropic Lateral Shoots.— Hydrotropism. — Positive
and Negative Hydrotropism.

PART VI.— Reproduction.

Lecture XLL — The Organs of Reproduction — General Considerations — Algae ;

Fungi; Archegoniata 721

Vegetative Multiplication. — Alternation of Generations. — Ascomycetes. — Con-
jugatEe. — Gametes. — Zygotes. — Oospheres and Antherozoids (Spermatozoa, Zoo-
sperms). — Reproduction of Vauchcria. — Alternation of Generations in the Moss. —
Archegonia and Antheridia of the Moss. — Embryo of the Moss. — Reproduction of
the Vascular Cryptogams. — Sporangia and Spores of Eqiiisetuni. — Prothallium
oi Equisetiii/i. — Embryology of the Fern.

Lecture XLIL — The Organs of Reproduction, continued. — Heterosporous Vas-
cular Cryptogams ; Gymnosperms ; Angiosperms 745

Reproductive apparatus of Marsilia. — Embryology of Marsilia. — Formation of
Embryo inside the Macrospore. — Pollen-grains, Pollen-tube, Embryo-sac. — Naked-
seeded Plants. — Nature of the Ovule and of the Seed.— Fertilisation by means of a
Pollen-tube. — Development of the Seed in Gymnosperms. — Angiosperms or tnie
Flowering Plants. — Flowers of Angiosperms. — Fertilisation of Angiosperms. — Stigma
and PoUen-tube.^Seeds with and without Endosperm. — Embrj'ology of Angiosperms.

Lecture XLIIL — The Action of Sexual Cells upon one another (Continuity of

Embryonic Substance) ........... 767

The Essentials of Fertilisation. — Continuity of Embryonic Substance. — Mutual
Action at a distance of Sexual Cells. — Strasburger on the Fertilisation of Ferns. —
Juranyi on the Fertilisation of CEdogojimm. — Behaviour of Conjugating Gametes. —
Action at a distance in PeronosporeK. — Pollen-tubes.

Lecture XLIV. — Transmission and Blending of the Paternal and Maternal Pro-
perties by Fertilisation (Hybrids) 779

Only nearly allied Forms hybridise.— Mingling of Parental Characters. — Sexuality
of Hybrids. — Millardet on the Hybrids of the Vine. — Resistance of Hybrid Vines
to the attacks of Fungi and Phylloxera.

Lecture XLV. — Influence of the Origin of the Sexual Cells of the same species on

the Result of Fertilisation 787

Dichogamy, Heterostylism, Herkogamy. — Pollination by the aid of Insects. — Aris-
tolochia Clematitis. — Pollination in Salvia. — Pollination in Viola and Epipactis.

Lecture XLVL — The Object of Fertilisation. Apogamy 798

Importance of Sexual Organs and Sexual Propagation. — Apogamy. — Vegetative
multiplication of Apogamous Plants.

INDEX 803

PART I.

ORGANOGRAPHY.

LECTURE I.

INTRODUCTORY REMARKS ON THE
PHYSIOLOGICAL ORGANOGRAPHY OF THE VEGETATIVE ORGANS.

In the vegetable, as in the animal kingdom, we find extremely simply organised
forms, in which all the processes necessary for the maintenance and reproduction of
the individual are carried on in the limited space of microscopically small cells, in a
scarcely ponderable mass of vegetable substance. In such cases as the Yeast-fungi
and many very small Algae, the body of the plant appears therefore, at least from the
exterior, of very simple construction, in the form of a cell, globular or ellipsoidal,
discoid, tubular, or of some other shape. In such cases nothing is to be seen of a
segmentation into different organs distinct from one another externally. With
increasing perfection of organisation, however, parts of various form, organs with
different functions, make their appearance as segments of the body of the plant, the
life-functions of which supplement one another; and this end is attained the more
completely the more each individual organ discharges but one function. With this
division of physiological labour, the perfection of organisation of a living being
increases.

The most important division of physiological labour consists in this, that in
addition to the organs which serve for the maintenance of a plant already existing,
reproductive organs, i.e. such as have the sole object of producing new plants
of like kind, also arise. We may class together the former as vegetative organs in
contradistinction to these reproductive organs.

Apart from quite isolated phenomena in plants of simple organisation, it is the
vegetative organs alone which constitute the whole body of a plant that strikes the
eye : the reproductive organs in the narrow sense of the word, the spores, oospheres,
antherozoids, pollen-grains, are always of microscopic minuteness, although larger
parts, which belong, strictly speaking, to the vegetative body, may assume special
forms and functions, by means of which they are enabled to act as accessory organs
in reproduction. To this category belong, for example, the parts of the flowers of
phanerogamous plants.

Only later, when we are concerned more in detail with the physiology of the
reproductive phenomena, shall we examine closely also the forms of the reproductive
organs. As a preparation for the theory of nutrition, of growth, and of the phenomena
of irritability, however, it suffices for the present to make ourselves acquainted with

2 LECTURE T.

the most important characteristics of the vegetative organs. In this I do not aim in
any way at setting forth, in the sense of descripti\'e or morphological Botany, all
possible varieties of form of homonymous organs in the whole vegetable kingdom ;
the point in question here is rather to show the beginner, proceeding from physio-
logical points of view, the most important modifications of form, since the purely
formal morphological contemplation of the organs of the plant customary hitherto
has left their physiological relations entirely out of account. Any one who has
been exclusively concerned with the formal morphology of the vegetable kingdom
prevalent during the last thirty or forty years, can scarcely concei\e the importance
which the vegetative organs possess physiologically.

Considering the grand variety of forms in the vegetable kingdom, which com-
mences with simple spheroidal organisms only visible with the microscope, and
ascends to such forms as we meet with in phanerogamous trees and shrubs, it
is not easy to state in a few words the correspondences in nature of similar organs,
and their essential differences. We know, it is true, that from the lowest stages of
vegetable organisation up to the most highly developed plants, the same plan of
organisation is always adhered to in all essential points. It is found, however, that
very often organs which are in their nature of the same kind, may be endowed with
not only very different forms, but also with different physiological functions. Since
we assume from the standpoint of the theory of descent that the more highly
organised forms have been developed from the simpler, the most varied shapes
from the same primary form, it becomes clear that very different organs may
nevertheless be of like original nature ; but that, in the course of changes which the
whole organisation of the vegetable kingdom has undergone in time, they have been
able to obtain new qualities, and to lose older ones. Thence follows, however, that
we are not in a position to say in a few words which properties of an organ are
essential, and correspond originally with the general plan of construction of the
vegetable kingdom; in other words, it is not possible to express organographical
ideas clearly and exhaustively by means of simple definitions.

We adopt, therefore, a totally different mode of consideration. Without concerning
ourselves in any way with definitions, we regard first the various organs where they
present themselves in the highest perfection in their typical character, and then seek
to establish which organs, in other regions of the vegetable kingdom, present also the
same essential peculiarities more or less modified. In doing this, however, we place
in the foreground the physiological properties, which very often correspond but litde
with the relations of outward form which constitute the subject-matter of morphology.
I believe, however, that this comparative physiological method of consideration of the
organs apprehends their true nature in a more fundamental manner than morphology
has done hitherto.

Above all, physiological organography has to do, not merely with the incidental
visible forms of organs, but more especially with their functional importance for the
life of the plant; these functional activities, however, are nothing but reactions to
external influences, even where the case appears to be otherwise. Each reaction
of an organ to external influence is, however, what is usually termed irritability, as
will be shown more fully later on ; all the organs of the plant, without exception, are
in some one sense irritable, and the essential differences and agreements of the

ROOT AND SHOOT.

various organs depend especially upon their irritability, i.e. upon the manner in which
they react to external influences. In what follows, the organs of vegetation will be
characterii-ed from this point of view.

Departing from the customary mode of view, we divide the body of the more highly
developed plants into two groups of organs, — Root and Shoot. In accordance with its
original signification, the root is that part of a
plant which becomes fixed on or in a substratum
as an organ of attachment, and in the latter case
serves for the absorption of nutritive matters con-
tained in the substratum. The shoot, or, to ex-
press it more generally, the system of shoots of
a plant, is on the other hand originally that part
which, becoming developed outside the substra-
tum, produces and increases the substance of the
plant, and brings forth, in addition, the reproduc-
tive organs, which never appear on a root.

That both fulfil their characteristic functions
depends upon their different reactions to the uni-
versal forces of nature, gravitation, light, contact,
&c. That, as we shall see later, there are roots
which grow forth above the substratum like shoots,
that there are also numerous shoots which pene-
trate into the substratum like roots, does not alter
their original nature ; and only proves, as has been
mentioned already, that w^ith progressive develop-
ment of the vegetable kingdom, with progressive
adaptation of various organs to special require-
ments of life, some peculiarities may be lost,
whilst new ones on the other hand may arise.
This, however, will become clear only in the
further course of our investigations.

In the first place, it will be advisable to illus-
trate more fully, by a few examples, the difference
between root and shoot.

If we allow an Almond, for example, to ger-
minate in moist earth, we find after a few weeks
that the young plant already contained in the seed
becomes further developed (Fig. i): the primary
root {iv) has grown down into the earth, and
numerous thin filiform lateral rootlets {iv) spring
forth from it, growing obliquely or horizontally.

Language has not waited for Botany to recognise this portion of our plant extended
in the substratum, as a special and peculiar organ, and to distinguish it from the
remaining parts by the name of Root.

On the other hand, there rises from the germinating Almond, above the sub-
stratum, that part of the plant which will later on develope in the air and in the

B 2

Fig. I.— Seedling of the Almond (Atni%\ial:<s
comtnunis. iv primary root; m' secondary roots;
he hypocotyledonary (first) segment of the seedling
shoot; <- cotyledons (first leaves); st their stalks;
i first internode of the shoot-axis of the seedling ;
h its young leaves (nat. size).

LECTURE I.

presence of light as the Ahiiond-tree : it consists, in the first place, of a cylindrical
part (/) tending upwards, which we recognise immediately as the future main stem of
the young Almond-tree, and at the upper portion of which the young leaves {b) are
already visible. The two bodies; densely filled with nutritive matters marked {c) in
our figure (the Cotyledons), are also to be regarded as leaves which spring from the
seedling stem. We comprehend the whole of this structure under the name of Shoot,
as opposed to Root, and distinguish on it two categories of parts, viz. the leaves
(b and c), and the Shoot-axis (/). However sharply the leaves are here and in other
cases separated off from the shoot-axis, an extended comparison of different forms of
plants leaves no doubt that they are nothing but portions or outgrowths of the shoot-
axis, and must be taken together with this as
one whole. Our plant is thus in the first
instance segmented into root and shoot, as
the two chief forms of vegetative organs.

If, in contradistinction to this highly organ-
ised seedling, which developes into a tree, we
now contemplate a small plant of extremely
simple nature, growing even without cell divi-
sions, as Botrydium (Fig. 2), we here again
find the two parts, Root and Shoot. The
first, consisting of thin, branched tubes, has
penetrated into the substratum (wet clay), it
fixes the entire plant, and at the same time
absorbs mineral nutritive materials from the
soil, and it does both by virtue of a series of
properties which it shares with the highly organ-
ised root of the Almond plant ; it is only by the
external form and visible internal structure
that it is distinguished from it; the function,
performed in virtue of special irritability, is in
all essential points the same in both. The
other portion of our Botrydium plant is a
globular swelling of the vesicle of which the
whole plant consists : this part, however,
comes forth above the substratum, and meet-
ing the light, the usual green colouring matter of plants is produced — viz., chlo-
rophyll ; by means of this, it is enabled to decompose carbon dioxide, and, with the
aid of the mineral substances absorbed by the roots, to produce organised plant-
substance. In this pointv the globular shoot, in spite of its extremely simple
organisation, corresponds "^to the highly developed germinal shoot of the Almond.
It does so also in a second point ; the reproductive organs arise in it sooner or later,
though in a much simpler form than is the case with the Almond-tree, on which the
reproductive bodies (oospheres and pollen-grains), also microscopically small, only
arise after some years — when the germinal shoot is strongly developed and much
branched — in the interior of the flowers, which are simply altered shoots. A detailed
exposition of these matters would present a thousand examples, which, even on

Fig. 2.—Botrydi
nified about 30 tinif
Rostafinski).

Hulaliim. An AI.5a, mag-
ot ; s green shoot (after

TYPICAL, RUDIMENTARY, AND REDUCED FORMS.

a purely external examination, would exhibit all possible transitional forms between the
organisation of the germinating Almond and the BoUydiwn\ but for a physiologically
correct comprehension of the actual state of affairs we scarcely require this aid, since
we can prove that that part of either plant which penetrates into the substratum, and
equally that rising above the substratum, undertakes in both cases essentially the
same functions for the whole plant, in spite of all difference of form, and is to this
end endowed with the same kinds of irritability.

In order to obtain at once more general points of view for physiological
organography, we will take into consideration yet a third example, a plant of structure
equally simple to that of Bol/ydium, which also consists simply of a single, branched,
tubular cell, a mould-fungus of the
Mucor group, which is represented
in Fig. 3. The part (/;/), with its
thousands of branches, has arisen
from the germinating spore, and
has extended in the substratum
(e.g. moist bread, the flesh of an
apple, &c.). After some time, there
grow forth from the main arms of
this branch-system thicker simple
tubes, which rise above the sub-
stratum, swell up in a globular
manner at their ends, and develope
reproductive organs in these spo-
rangia. Our whole Mucor plant
is devoid of chlorophyll, and is
therefore unable to produce or-
ganic vegetable substance by de-
composition of carbon dioxide; on
the contrary it absorbs it for its de-
velopment out of the substratum,
that is, by means of the portion
m (contained in the substratum)
which, in spite of its different or-
ganisation, behaves itself, physio-
logically, exactly as the root of the

Bolrydhim and of the Almond, since it penetrates into the substratum, urged by the
same kind of irritability, and absorbs water and nutritive matters from it. We are
therefore completely justified in regarding this portion of our fungus, distinguished
by botanists as the mycelium, as its root, and accordingly the fruit-bearing portions,
protruding above the substratum, appear as shoots ; it is true they show, as already
said, an essential difference from the forms of shoot hitherto considered, inasmuch as
they are devoid of the capacity of producing vegetable substance from carbon dioxide,
since they contain no chlorophyll ; instead of this, however, the other property of the
shoot — to bear reproductive organs — has been preserved to them.

We have started in our organographical consideration, as also appears in the

FIG. ^.—ritycomyces ftiteits. m mycelium (root) grown in gela
jg conidiophores, rising above the substratum.

6 LECTURE I.

history of science, from a highly developed plant, from a plant the roots of which
every one knows as such, the essential root-properties of which are developed in
the highest perfection. The germinal shoot of the Almond is likewise that which
language has all along distinguished as a shoot. I name such organic forms which
present the essential peculiarities in great perfection, and from which therefore a clear
scientific consideration best proceeds, typical forms, and believe myself in this use of
the word to be completely in accordance with the original sense of the word Type.
When we now, proceeding from typical forms of organs, compare abnormal forms
with them, we find two different categories of these. For, on the one hand, we meet
in the lower regions of the vegetable kingdom with such forms of organs, especially
of roots and shoots, in which the organic differentiation generally is not yet so far
advanced as in the typical ones ; w^e have to do with feeble beginnings, which have not
yet attained to the typical standard. In this sense, we may term the roots of our
Boirydium and Mucor rudimentary roots, in so far as the word rudiment signifies a
state of beginning which has not yet attained to completeness. In strong contrast
to these rudimentary forms, we have to distinguish further the degenerate or reduced
forms. As it is to be assumed, from the standpoint of the theory of descent, that
the more perfect have gradually developed from the simple organisms, we are also
driven to the further assumption, that, in consequence of special modes of life, more
simply organised forms have again arisen from those more highly organised, and
this process we term degeneration, or reduction. This process is strikingly exhibited
when plants, which we must regard as originally always containing chlorophyll,
become parasitic. Since through parasitism they come into the condition of
absorbing organic material from wiihout, they cease to form it by decomposition
of carbon dioxide ; under these circumstances the chlorophyll is superfluous as an
instrumeht of assimilation, and in consequence of this, the whole remaining
organisation becomes simplified, so far as it is connected with the activity of the
chlorophyll; and since it is especially the shoots which are the bearers of the
assimilating organs, these lose, in parasites, together with the loss of chlorophyll,
all those peculiarities which arise from the activity of the chlorophyll, viz., the larger
development of surface, and the external segmentation connected therewith, and so
on. There then remains to the shoot only the second primary peculiarity, viz., to be
a bearer of fructification, and such we find to be the case in the third plant considered
above, the Mucor, as with most of the other fungi, and the phanerogams devoid of
chlorophyll. To recapitulate what has been said, we have, as the typical forms, the
highly organised ones which first offer themselves for consideration. Comparison with
these brings into notice, on the one hand, the rudimentary organs which have not
yet come up to the typical standard ; and on the other hand, we regard those organs
as degenerated, or reduced, which have been developed from typical forms by
simplification of their functions.

It is, however, always very difficult so to form our general conceptions, that they
shall comprehend all the cases occurring in nature. So it is also here. For very often
organic forms are met with, with regard to which one knows not in which of these
categories they should be placed. For example, it is certain that the flowers of phanero-
gamous plants belong to the category of leafy shoots ; from a purely aesthetic point of
view, one might be led to consider them as the most highly developed form of shoot.

DERIVED AND METAMORPlIüSED FÜRMS. 7

whereas the systematic or phylogenetic mode of consideration leads us to the
conclusion, that the flower shoots are to be considered as altered leaf shoots, of
which the capacity for assimilation has ceased, because they take part in the function
of fertilisation. They are in this sense, therefore, reduced forms of shoot. In the same
^\-ay, we may assume that the tendrils of the Vine, in spite of their containing
chloroph\ll, yet scarcely come into consideration in the assimilation of the whole plant ;
whilst they serve it as climbing organs, and in so far contribute to the green
leaves being able vigorously to carry on their function of assimilation. Again, we
may regard the numerous subterranean forms of shoot as derived from green leafy
shoots, which, however, on their part, assist in the performance of the processes of
nutrition by the latter in this or that manner. It is against common sense to regard
those kinds of organic forms, which have it is true only been derived later from the
typical ones but which contribute to the greater perfection of the entire organism,
simply as degenerations. In order to express this objection adequately in language,
I shall designate these kinds of organs, derived or metamorphosed forms ; a flower,
a vine-tendril, a subterranean runner, &c. are thus, for us, derived or metamorphosed
shoots ; and in the same way, roots which serve the function of nutrition either not
exclusively or not at all, but rather do duty as climbing organs, reservoirs of reserve
material, and so forth, are not necessarily to be termed reduced, but more generally
as derived or metamorphosed forms of root.

In this setting forth of the ideas of rudimentary, typical, derived and reduced
organs, we also find at once the reason why it is in general quite impossible to
characterise correctly by means of short, precise definitions, a comprehensive categoiy
of organs : thus, in comparison with the typical forms of a group, various characters
may yet be wanting to the rudimentary forms of the same group ; on the other hand,
certain characters may become lost by degeneration, and it is obvious how very
diflicult it is, under such circumstances, to have regard to all these various cases in
a definition.

It is therefore, in my opinion, the best, as well for research as for my exposition,
to find out first by careful comparison of very many cases, what organic forms present
themselves as typical within a category; these are precisely those organs where the
forms concerned are developed in the highest degree : one has then only vet to
establish, whether and how far other forms are to be looked upon as rudimentary
or derived. The setting up of a typical form supplies in a certain manner the ideal
towards which the rudimentary forms are striving, and from which the derived forms
have fallen away again\

1 The usual division of all organs of the plant into root, stem, and leaf, no longer comes
up to the present state of science — at best it might hold good exclusively of the vegetative organs.
Nevertheless, it will, as I believe, clearly result from what has already been said in the text, as well
as from the following lectures, that there is no meaning in considering stem and leaves separately
from one another, as two groups co-ordinated with the roots ; that, rather, both together are to
be co-ordinated with the root, as shoot. Since what is necessary has been said in the text, it is
superfluous to attempt any further demonstration here.

Yet a few words in anticipation on the reproductive organs, to which I ^hall, however, return
in the last part of this book. That the reproductive organs in the first place of the phanerogams
were regarded as leaves, or as appendages of such, in the sense of the doctrine of metamorphosis,
was oidy justified so long as the stamens and carpels of the phanerogams were considered as the
true sexual organs. No attempt was made to arrange the archegonia and aulheridia under one of

« LECTURE I.

It is however necessary yet to refer to some other points of general significance.
We seek the characteristic properties of organs, from our point of view, not in
their outward form, not even especially in their visible anatomical structure, but, as
already stated, in their manner of reacting to external influences, or of being irritable :
and this depends, as we shall yet see in what follows, less upon the external form and
anatomical structure, than upon those invisible relations of structure which are usually
termed molecular, and in the sense of chemistry, atomic. This implies, however,
that the characteristic similarities and differences of organs depend upon their
material constitution. From general scientific principles, we must assume that to
each visible external difference of the organ, there corresponds also a difference of
its material substance, exactly as we regard the form of a crystal as an expression
of the material properties of the crystallising substance. In our time of advanced
scientific knowledge, it might appear, strictly speaking, scarcely necessary to lay
special stress upon this principle; but the thoroughly scholastic way of thinking,
which, coming from former centuries, has been kept alive up to the most modern times
in the region of botanical morphology, makes it seem still advisable to devote a few
words to the matter. As late as the year 1880 one of the most renowned German
botanists, from the stand-point of these antiquated views, used the following words in
an academical address: "The figure of the entire organism, which will only be realised
in a material form in the future, already operates virtually in the present, as a motive
cause, before and during the development of the parts, just as does the plan accord-
ing to which the builder places his blocks." Such a mode of looking at organic forms

the general, so-called morphological conceptions, and yet nobody ventured to form of them a special
category of organs, in contradiction to the obsolete doctrine of metamorphosis.

In the present state of our knowledge, however, we have without doubt to take as the typical
reproductive organs of the whole vegetable kingdom, on the one hand sporangia, on the other the
archcgonia and antheridia of the Mosses and Vascidar Cryptogams. It would be ridiculous to wish
to arrange these under the morphological conception, leaf or stem ; we have here to do, rather, with
two groups of organs, well characterised physiologically and morphologically, viz. sporangia and sexual
organs. In the comparison of the Algae and Fungi with the Muscinens and Vascular Cr)'ptogams, it is
shown at once, not only that the proper spores of both groups correspond to one another, but also
that the so-called oogonia of the Thallophytes, are rudimentary simpler archegonia, and the same
holds good with respect to the antheridia. On the j)ther hand, following up the sporangia and
sexual organs of the Vascular Cryptogams into the clashes of the Gymnosperms and Angiosperms,
progressive degenerations of them, and a remarkable combination of the sexual organs with the
spores are found. That the pollen-grains correspond to the male microspores, the embryo sacs to the
female macrospores of the higher Cryptogams, is now generally accepted, after this relation had
long been established by Hofmeister and myself. Numerous more recent works bearing on the
history of development, especially those of Goebel, leave no doubt that the anthers, as well as the
ovules, are true though reduced sporangia, &c.

Since I shall return to these matters later on, what has been said is only to serve to emphasize,
in connection with the division of the vegetative organs into root and shoot, that two peculiar
groups of organs stand opposed to them, viz. the sporangia and sexual organs, in their typical form
as archegonia and antheridia. We have thus to distinguish hve categories of organs : (I) the vege-
tative organs — (i) root and (2) shoot; (II) the reproductive organs — a asexual sporangia and
spores (3), b the sexual archegonia (4) and antheridia (5). All other forms of organs are rudi-
mentary or reduced organs of these groups.

I have good reason for entirely discarding the term ihalhis. In the first place, the Greek word
6a\\os means simply shoot ; and, secondly, the thallus of the Algte and Fungi, taken purely as
a matter of fact, according to the customary view, is simply characterised by the want of leaves,
although the morphologists are by no means able to say what a leaf really is.

SUBSTANCE AND FORM OF ORGANS. g

and their existence, to wliicii I replied at the time^ is only possible if, as has happened
hitherto, the organic form is regarded as something existing per se ; as if the organs of
the plant themselves by no means consisted of real matter, with its forces and reactions
to external influences, but, like platonic ideas, existed only in the abstract, but were
nevertheless able to operate upon the actual matter of plants.

If now the outward form and internal structure, and the functional capacity of
an organ resulting therefrom, constitute the necessary expression of its material sub-
stance, then a very simple consideration leads us to the perception, that this material
substance of an organ is itself again the result of the physiological activity of preceding
organs of the same plant. The first organs of the seedling arise from materials
or chemical compounds, which the mother-plant has produced and apportioned
to thenij the later organs arising after germination, the shoot and its parts, roots and
so forth, however, are constructed from the materials which the organs of the seedling
have taken up from without, and then further altered according to the specific nature
of the plant; each subsequent organ is the result of the constructive activity of
preceding organs. Thence it follows that the construction of organs, their deve-
lopmental history and growth, is also a subject-matter of vegetable physiology,
and that, as one may say, not only the most important but also the most
difficult part of it. Vegetable physiology has thus to do not merely with the
functions of organs already existing, but also with the origination of the organs
themselves, which is itself a function of preceding organs. In reproduction, we must
assume that certain particles of matter pass over into the reproductive cells, which
have been previously produced and their nature determined by the organs of the
mother-plant, and now possess the capability, whilst appropriating to themselves new
matter from without, of imprinting upon this the same series of material differences
as had already declared themselves in the mother-plant ^. This repetition of the
formative processes which depend on chemical processes is, as a rule, a very exact
one in each organic form, so that the descendants resemble the progenitors in all
points. It is this process which is termed Heredity. It is at once perceived that
Heredity is simply a fact of experience, the cause of which we do not know, and
nothing more. It can therefore only lead to more complete confusion in science, if,
as often happens in these days, Heredity is treated in a very thoughtless manner as a
force of nature, by which it is imagined to be possible to explain all manner of things.

But this constant repetition of the peculiarities of the ancestors in the de-
scendants is not without exception ; sometimes, as cultivated plants especially show in
a thousand ways, more or less extensive deviations from the peculiarities of the parents
appear in the descendants, which, under certain circumstances, may become more and
more intensified in the following generations, by further propagation. This fact is
expressed by the term 'formation of varieties.' Its cause is in general unknown; in
any case, however, we are correct in assuming that the changes, perceptible externally
in variation, consist in material alterations in the parents, during procreation or

^ Compare my treatise ' Stoff und Form der rßiDizcnorgane' in the Arbeiten des botan. Instituts
in Würzburg, 1880, Bd. II, p. 453.

^ I have explained in my two treatises on ' S/off und Form' in the Arb.'iten des botan. Instituts
in Wiirzburg-, Bd. II, that the 'gemmules' in Darwin's Pangenesis are not meant here, but matter in
the sense of chemistry and physics.

10 LECTURE I.

immediately after it, most probably in a change of the proper generative substances.
This is not the place to go into these matters more in detail ; we ought rather to
emphasise only that the theory of descent has been developed from the consideration
of variation in reproduction, from the fact of the accumulation of new peculiarities
in the varieties. These facts, in combination with the observation that from the
simplest forms of plants up to the most highly organised there is an unbroken series
of transitional or intermediate forms, have led to the hypothesis, as daring as
fruitful, that the most highly developed organisms have been gradually developed from
the simplest, by means of the progressive formation of varieties. This is essentially
the meaning of the theory of descent, which has during the last twenty years pro-
duced so great a revolution in the views of naturalists. We also cannot dispense with it
in our organographical considerations, simply for the reason that the understanding of
any organic form whatever is only possible if one looks upon it as the result of the
ever progressing organising tendency of the substance of the plant, i. e. of the
variation of other preceding organic forms ; or in other words, every organic form
is the outcome of a history, which is as old as the organic world itself. This
principle asserts itself far less in the consideration of the specific functions of given
organs, than in the comparison of their external form.s, and the natural system is the
attempt to state clearly this historical relationship of all plants to one another. Like
all new and comprehensive hypotheses, the theory of descent has also caused con-
fusion and devastation in weak minds, and perhaps done as much harm as good.
We shall be on our guard against laying any value whatever upon the excrescences
of the theory of descent, without casting aside the good and helpful which is
contained in its scientific nucleus. Even the above statements concerning typical,
rudimentary, reduced and metamorphosed forms, have a clear, actual meaning only
on the basis of the theory of descent, and so I shall, in our further considerations,
have recourse to the theory where the facts require it.

Concerning one point I should wish to anticipate : viz. the use of the word
Purpose, a word which many fanatics of the theory of descent would, if possible,
banish entirely from the language. But the fact that, formerly, Purpose in the
mechanism of organisms was referred to causes other than now, is no reason for
robbing our language of a pregnant term. By the expression. This or that mechanism
has a Purpose in an organism, one understands really nothing more than that this
contributes to the ability of the organism to exist. It is now obvious, however, without
further discussion, that all properties of any organism must necessarily be so arranged,
that they at least do not call in question its existence under the circumstances of
life natural to it. ' To the purpose' means, therefore, in general, the same as ' capable
of existence,' and it would be foolishness to waste even a word as to whether one
may use the term in this sense or not. This implies, however, that there is
absolutely no scientific merit in maintaining of any organic mechanism whatever,
that it is in general to the purpose, or contributes to the capability of existence ;
since that is self-evident. On the other hand, it is in certain circumstances very
important and profitable to demonstrate how far, and under what conditions, a given
mechanism in the organism is of purpose ; in what way this contributes, in combination
with other mechanisms, to the capability of existence of a given organism ; and strictly
speaking, the whole of physiology is essentially occupied with such demonstrations.

LECTURE IL

THE TYPICAL ROOTS OF VASCULAR PLANTS.

Lr has been already remarked in the first lecture, while considering the
differences between roots and shoots, that the primary and essential character of
the root consists in that it becomes, first of all, developed as an organ of attachment
on or in a substratum, and, when the latter is the case, it is at the same lime the
medium of absorption of nutritive matters contained in the substratum. Since,
moreover, assimilation — i. e. the production of organic vegetable substance — is foreign
to its physiological purpose, the instrument of assimilation, chlorophyll, is also
wanting to it : though it is not thereby precluded that in certain rare cases, when
the roots are developed in air or water and thus exposed to light, chlorophyll may
also arise in them. However I shall return to the physiological properties of roots
afterwards. INIeanwhile, it will be to the purpose to consider the relations of
organisation ; and we will concern ourselves, according to the scheme of comparative
organography given above, first with the typical, and then with the rudimentary
and reduced forms of roots.

We find typical, and at the same time very highly organised roots, in the
large majority of vascular })lants, the phanerogams and higher cryptogams :
botanists for about the last forty years have accustomed themselves to term these
organs exclusively roots, which is from a purely formal point of \iew but half
justifiable, and from the standpoint of physiological organography must be entirely
rejected.

In vascular plants with upright main stems, as the Sunflower, Tobacco, Hemp,
and others, we meet with a root-system developed comjjletely in the earth, and
composed of long, cylindrical fibres of varying thickness, which is in ordinary
language simply termed the root; in scientific language it is however more to the
purpose to call each one of these fibres a root. In many cases the first root,
already present in the embryo of the plant, becomes vigorously developed, pene-
trating perpendicularly into the earth [w, Fig. 4) : from it spring, radiating
laterally in two, three, or more directions, numerous new roots {n) which grow
horizontally or obliquely downwards, and in their turn again produce new root-fibres,
and in such a way that the new, young roots always arise behind the growing
end of what is the mother-root for the time being. In the case here considered,
which is realised especially in trees and annual erect plants, the whole root-system

LECTURE II.

of a plant thus arises from the embryonic root (radicle) : this is the oldest, grows most
vigorously, and penetrates deepest into the earth. It is, however, more frequently the
case, that the first root of the seedling, with its ramifications, soon ceases to grow, and
new, more vigorous roots are then formed from the lower parts of the shoot-axis,
which likewise, although they arise above the ground, penetrate into the earth
^_^ and become branched there.

^\ In such a case, more vigor-

-ous roots may go on con-
tinually appearing higher
up the stem, so that the
root - system ramifying in
the ground, arises, not as
before from a tap-root but
from the main stem of
the plant. The Palms and
the Maize furnish excel-
lent examples of this. In
creeping plants — e.g. the
Gourd — more especially
when the stem creeps for-
ward beneath the earth,
roots usually spring from
the under side of the shoot-
axis along its whole length,
in many cases even close
behind the bud, and these
may branch also in their
turn. In subterranean creep-
ing stems, the root- system
originally formed on the
seedling may then die off,
and rot away, together with
the hinder part of the stem
itself, new roots being con-
tinually produced behind
the apex of the growing bud.
But even when the primary
root-system remains pre-
served, but the stem climbs,
it frequently happens that
new roots are -coTftinually
produced below the bud ; these then cling to solid bodies as climbing organs, as
in the Ivy and many tropical Aroids. Roots of this kind are commonly termed
aerial roots ; they may also grow down from a considerable height in the form of thick,
cylindrical cords several metres long, and finally penetrate into the ground, there to
become branched (tropical Aroids). But even in stems growing upright, it happens

Fig. 4

-Seedling of Goui

rd (CucHi-bila

rcpd).

w

primary

root; n

: secondary

roots: /:

the hypocotyledon

ous segment

of tlie

si

loot-axis ;

c the

cotyledons

(nat. size).

SPACE OCCUPIED BY ROOTS.

13

frequently enough that roots arise immediately beneath the growing point, and even
that the whole root-system of the plant consists of them, as occurs in many Ferns,
particularly the Marattiacese and many tree ferns. In several species of Cactus,
moreover, the tendency also exists to put forth roots close beneath the growing
point of the shoot; finally, roots may even spring out of leaves, as occasionally
happens in a fortuitous manner, but occurs
quite normally in our common large Fern,
Aspidium Filix-mas, where the shoot-axis is
so densely beset with leaves, that it generally
shows no proper surface at all ; all the
numerous roots of this plant spring from
the basal parts of the leaves.

Since the roots developed in the earth
especially absorb water and nutritive matters
dissolved in it, their number and ramifica-
tions, and thus the whole formation of the
root-system, are the richer, the more vigor-
ously the green, transpiring and assimilat-
ing leaf-surface of the aerial shoots is de-
veloped. In time, in proportion as the
assimilating foliage increases in surface, the
subterranean root- system becomes larger,
and fresh portions of the soil are pene-
trated by it. The number of root-fibres,
and the space which they traverse in all
directions within the soil, is much larger
than one usually supposes. It is cer-
tainly not put too high if we assume this
space to be more than a cubic metre in
the case of a well-developed Sunflower,
Hemp, or Gourd plant; and in the case
of large trees we may estimate it even
at hundreds of cubic metres. This space
then is so traversed in all directions by
thousands of fine roots, that scarcely a
cubic centimetre of it remains exempt. By
this means the plant," abundantly provided
with foliage, succeeds in extracting from
the soil the very large quantities of water
which are transpired from the leaves, to-
gether with the nutritious matters contained
in it. S. Clark has taken the trouble to

measure the length of all the roots of a large Gourd ])lant, and found that
it amounted to twenty-five kilometres. This relation between root-formation
and leaf-surface makes itself evident again in that, in floating aquatic plants
{Hydrochari's, Siratiotes, Lemnd). the number and lcn,L;lh of the roots are

Fig. 5.— Diagrammatic longitudinal section through a yoims
Maize plant (Zea Mays), lu primary root ; ^ <j)' ((,'", the
roots springing from the shoot axis (.r) ; * leaves ; i buds.
The growing points are represented black, the elongating parts
grey ; tlie portions left white arc fully grown.

14

LECTURE II.

generally inconsiderable, since they find the nutritive fluid in abundance, while
the leaves, floating on the water or surrounded by the damp air, use up but
little water ; plants living in humus and root-parasites which have no green
foliage leaves, as Neottia, Lafhrcea, Orohatiche, have also but an insignificant
root-system in proportion to their body-weight, because in the absence of green
leaves the most important transpiratory surface is wanting. In general the root
fibres are the thinner, the more numerous they are in the root-system ; since, in the
formation of roots, it is essentially a matter of a large surface, with least possible

mass of the organ. Hence
the less numerous, short
roots of plants devoid of
chlorophyll, as well as of
water-plants, are remark-
ably thick and fleshy; while
the young absorbing roots
of large trees and shrubs
^■ery often scarcely attain
the thickness of a horse-
hair. If we now examine
a single root fibre more
closely, we have at its free
end the growing point, out
of which the whole cellular
tissue of the fibre becomes
developed. This growing
point is moreover encased
with a cap of firmer per-
manent tissue. The fore
end of the root is, in fact,
destined to be pushed for-
wards in the earth, much
as the point of a nail which
is driven into a board. It
is obvious that the solid
root-cap, with its smooth
slippery surface and its coni-
cal form, facilitates the pro-
gress forwards, and at the
same time protects the delicate tissue of the growing point. In aerial and aquatic
roots, which we may regard as derived or even reduced forms, the root-cap
is feebler, and is only in general an organ of protection against the drying up
of the growing point, or against its immediate contact with w^ater. The anatomical
relations need only be briefly referred to: each root-fibre consists of a central
or axial vascular bundle, or fibro-vascular cord, and a soft cortex of ex-
ceedingly thin-walled parenchyma cells surrounding it. The axial cord, the
histological constitution of which diff'ers considerably from the vascular bundles

FIG. 6.-

iigitudii

of the :

of a youngf A)tgiopteris
eziecta. Above, the youngest leaves (*) are still completely enveloped in the
stipules (ii b) ; st stalk of an unfolded leaf, with its stipules lib; n everywhere
the leaf-scars on the basal portions yy from which the leaf-stalks have been
separated ; cc the commissures of the stipules in longitudinal section ; iinu roots
(nat. size).

DEVELOPMENT OF ROOTS.

of the shoot, is at its posterior end connected with the vascular bundles of the
organ bearing it; it runs through the whole length of the root, and terminates in
the growing point, from which it, as well as the cortex, obtains the elements for
its further growth in length. Through this cord, and particularly through the non-
lignified elements of it, the formative matters, especially such as are of proteid
nature, are conducted to the growing point from the mother-organ. Since the
roots have to absorb water and the nutritive matters dissolved in it, and do not
need protection against evaporation in the soil, they are devoid of the resistant
epidermis of the shoot-axis, the outer wall of the cells is not at all or very slightly
cuticularised, and obviously the stomata are wanting, since in roots there is no
question of a rapid interchange of gases with the
environment.

When a new root-fibre is about to arise, there
is formed in the interior of the tissue of the
mother-organ, for instance inside a root, or a shoot-
axis, or even a leaf, in the first place a new grow-
ing point, which usually consists of small-celled
embryonic tissue, which becomes at once clothed on
the outside with the root-cap. These new growing
points generally arise on the outer side of the vas-
cular bundles of the mothei'-organ, and when the
young roots begin to grow, they must first break
through its cortical tissue, as may easily be observed
in fresh roots : the lateral roots of these protrude
from slits, the edges of which are often raised up
in a lip-like manner. As a rule, a root, and likewise
a shoot-axis, gradually produces a large number of
root-fibres, which on their part generally act in the
same manner. In this the rule holds good which
prevails generally in the formation of organs in the
vegetable kingdom, that the roots appear in acro-
petal succession — i, e. the youngest roots are always
nearest the growing point of the mother-root, or
of the root-forming shoot, and the further they
are removed from the growing point of the mother-
organ, the older and longer they are. Since the

young roots arise on the outer side of the vascular bundles, or at an)- rate in
definite relation to the vascular bundles of the mother-organ, and these run more
or less parallel to one another, it follows obviously that the roots M-hich spring from
an organ are arranged in longitudinal rows. This arrangement appears much clearer
in the branching of the root itself, than in root-forming stems ; we find here the
lateral roots arranged in 2, 3, 4, ^^ or more longitudinal rows, or orthostichies. In
general, however, the arrangement of the roots on their mother-organ is not so
strictly regular as, for example, the arrangement of leaves on their shoot-axes. It
is also to be insisted upon that the young roots always appear first at some distance
from the advancing growing point of the mother-root or mother-shoot, even though

Fig. 7.— Longitudinal section through the
root-apex of Marsilia sat7iatv'x. -ws the apical
cell ; xy its last segments ; o the epidermis ;
gf the axial vascular bundle of the root ; wh
layers of the root-cap; h the parts of the root-
cap extending further back on the root (highly
magnified).

i5

LECTURE II.

they have been previously developed from the embrj'onal tissue of the same ; a root
beset with numerous lateral roots, therefore, usually possesses a naked end some
or several centimetres long.

I now pass on, finally, to a short description of the proper physiological peculiar-
ities of the root. I confine myself again in so doing to the typical forms, more espe-
cially as we meet with them as subterranean roots in the land plants vvith green leaves.
The main purpose of these is, as already insisted upon, in addition to the fastening of
the plant into the ground, the absorption of fluid nourishment out of it ; and this must
be kept in view in judging of all the physiological properties of the root. Above all,
it of course depends upon this that the roots really penetrate into the substratum : to
this end the tap-roots, of seedlings especially, as well as those roots springing from
stems and destined to become further developed in the ground, are endowed with

geotropism ; that is, the
growing portion, some milli-
metres long, lying behind
the growing point, reacts to
the influence of the gravi-
tation of the earth in such
a manner, that the grow-
ing point becomes di-
rected downwards exactly
vertical. This, if the root
had previously another direc-
tion, is effected by a corre-
sponding curvature behind
the growing point. If such
a root is placed in the
light first, it may be also
heliotropic, i. e. it becomes
curved at the same place
in such a way that the
growing point is directed
away from the source of
light; and both kinds of
reaction work therefore, in general, to the same end— that the apex of the root
penetrates into the ground. The same result is yet further secured, by the growing
end of the root being sensitive to slight pressure and to moist surfaces ; the
growing end becomes concave on the pressed and on the moister side, and it is
obvious that, under ordinary conditions of life, these two irritabilities also will
act like the geotropism and heliotropism of the root, and contribute to gi\e the root-
apex that direction by which it penetrates into the substratum.

Concerning the origin of roots from shoot-axes, it is frequently to be
observed that the place of origin of the growing points is already determined by
external influences alone, or together with others : it is either the side lying
towards the centre of the earth which alone produces roots, in consequence
of an effect of gravity ; or, in climbing stems, it is the shaded side turned away

Fig. 8.— Root o( Acorus calamus. Transverse section of the axial cylinder, with
surrounding cortical tissue; s endodermis; p pericainbiuin ; fh phloem; g groups of
vessels (strongly magnified).

GROWTH OF ROOTS IN THE SOIL.

17

and o;ro\vn forwartls

.izr wj^

v*S^*-

Y

/^^

w

It

Mi

%j7,

/I

^'T

'M

from the light, wliich is also in general at the same time moister, from \vhich
alone new roots arise.

A root having in this way penetrated into the eartl
perpendicularly within it, lateral roots arise progres-
sively from above downwards, in the order stated.
These also are geotropic, and sensitive to contact and
to moisture, but in a different degree from the tap-root :
their geotropism especially is different ; the lateral
roots are caused by the influence of gravitation
to grow, not vertically downwards, but obliquely, or
even horizontally. However difficult it may be to
conceive this varying sensitiveness of the different
roots of a system, thus much is obvious, that ^^
the entire root-system is compelled to grow into the
earth in all directions by it. At a later period, when
we are particularly concerned with the irritability of
plants, I can speak in detail concerning the pro-
perties of roots here mentioned : the point now is
simply to establish the existence of these properties,
and to insist upon their purpose for the life of the
plant.

In this connection, however, there are 3'et a few
points of special interest : in the first place stress is
to be laid upon the fact, that in roots growing into the
earth, the portion which is becoming elongated behind
the growing point is very short ; even in vigorous
tap-roots only 8-10 mm. long, and in thin lateral
roots often only 2-3. This appears remarkable, when
one considers that in aerial leaf-shoots and flower-
scapes the elongating portion is commonly many centi-
metres long. In the shortness of the root, however, an
arrangement is afforded which renders the progress of
the apex of the root within the earth considerably
easier, since the whole force of the push by which
the growing point is driven forward is concen-
trated immediately behind it. In the long aerial roots
of tropical Aroids, and others, where this mechanical
arrangement is unnecessary, I found, accordingly, a
much more considerable length of the elongating-
region.

Fundamentally, however, the roots, as we have
hitherto learned to understand them, are only the bearers
of the proper organs for the absorption of food, namely,
the root-hairs. These are exceedingly delicate-walled,
narrow tubes, some millimetres in length and a few tÄtt ths of a millimetre in diameter,

/-

7^

and 1

Fig. 9.— Diagram of the primary root
er part of the shoot-axis of a seeciling
of Pisurn sativtttn (Pea). Cot cotj'ledons ;
rt, *, c, d, e transverse sections at the vari-
ous heights; ß (everywhere) ihe pliloem of
the vascular bundles; vih a secondary root
(behind) ; vis second iry roots at the sides
of the seedhng ; Hit tlie first leaf (behind) ;
?■// tertiary roots.

which grow forth from the surface of the roots in \erv large numbers.

[3]

If roots

8

LECTURE IT.

become developed in moist air, they appear to be covered with a brilliant white
velvety pile, which consists of the densely crowded root-hairs. These tubes are
simple protuberances of the outermost cortical cells of the root; they arise on the
recently developed part, and therefore behind the elongating region. In a root
about I2-20 centimetres long, they cover, however, by no means the whole surface,
but only a piece of a few centimetres in length; the root-hairs in fact die off
again after a few days, and completely disappear. It is therefore always only a
young, but completely elongated portion of the root, which is covered with vigorous
hairs. If we thus picture to ourselves a root as it grows longer, the posterior

Fig. to.— a seedling of Vjaa Faba, fastened by means
of a needle («) into a cork (not figured) to show how the
apex of the root (o) grows beyond the point of the needle.
The marks 5 and 10 are to be observed. Comparing B
with ^, point 10 is not displaced at all, and 5 but little.
The growth has taken place chiefly between 0 and 5.

;. II.— Seedling of Si>ia/>is Alba
nustard). .-/ with the particles of
iging to the root-hairs ; B after
nioval by washing in water.

hairs die off, in proportion as new ones arise behind the growing apex ;
thus the part of the root provided with hairs moves forward. If this takes
place in the earth, it is obvious that the portion beset with vigorous root-hairs
continually comes in contact with such particles of soil as have before remained
untouched. Now, it is the root-hairs by which the roots become capable of actually
taking up the water and the nutritive matters of the soil. As these fine tubes grow in
between the particles of soil, they apply themselves here and there to them so
intimately and firmly, that they cannot be separated again without injury. How
necessary this is, I shall be able to make quite clear in the theory of nutrition;
so much, however, is understood without further remark, that by means of this
arrangement, particles of earth which we may imagine to ourselves in the form of

ROOT-HAIRS.

^9

a cylinder of 6-10 mm. thick around the root (about | mm. thick), are called into
requisition in nutrition. We can demonstrate this at once in a neat and instructive
manner, if fresh, vigorously growing roots are carefully lifted out of the loose earth :
the particles. of earth cling so fast to the tract which is beset with root-hairs, that
they cannot be shaken off, whereas the growing end of the root is quite smooth and
dean, and the particles of earth also fall off from that part of the root which lies
behind the hairs, because the hairs have there died off and disappeared. A clear
representation of the activity of the roots in the earth is only to be obtained when
we picture to ourselves the richly branched root-system of a large, vigorous land-
plant, and reflect how the thin root-fibres travel through the soil in all directions,
and that each thin fibre exhausts a cylinder of earth several millimetres in
diameter, and how the absorbing
part of each root- fibre penetrates
continually from day to day into
new, fresh portions of soil, while
the hairs disappear from the older
parts of the fibres, because there
is nothing more there for them to
seek. It may be added, that only
by the intimate groM'ing together
of the root-hairs with the earth
particles is it possible for transpir-
ing land-plants still to take up
large quantities of water from a
soil apparently almost dry, in
order to transmit it to the green
leaves.

To the most remarkable pro-
perties of roots belongs the short-
ening of the tract of a root-fibre,
already grown to its full length ^
It begins immediately behind the
place where the elongation has
ceased, and may then continue
for a long time in the older

parts. This shortening, which may amount to 10, or even 25 per cent, of the
original length, is brought about by the parenchymatous cortex, and the axial
fibro-vascular cord becomes passively drawn together, so that its vessels assume
a serpentine course, while the outer layers of the cortex, also passive, develope
transverse folds, which can be easily seen with the unaided e}e, especially in

KIG. 12— Root-hairs of a Se/nf^ineUa—some \
to tliem. The outlines show tlie impressions of the particles of
the tubes had closely applied themselves (strongly magnified).

cles of soil clii

' The shortening of roots at the termination of tlie growth in length was discovered by Fitt-
manti (Flora, 1819, Bd. II. p. 651), again found later by myself (Arbeiten des botan. Inst. Würz-
burg, I. p. 419), then brought forward anew in its biological bearing by Irmisch (Beitr. zur vergl.
Morph.5. Abtheilung, Aroide», Halle XI IT. 2. 1S74. p. 11). Hugo de Vries has furnished a detailed
investigation of the mechanics of this shortening, from the point of view of more recent knowledge
(Bot. Zeitung, 1879, p. 650).

C 2

LECTURE II.

the roots of bulbous plants, but also on other thick root-fibres. It is clear that,
from the place where a root-fibre has closely applied itself to the soil by means of
its root-hairs, this shortening of the older tract of fibre must tend forcibly to
draw its point of attachment back to the mother root, or to the part of the stem

Fu:

-IG. 14

A wheat plant during g-emiination. Fig. 14 four weeks older than Fig. 13. .S the seed (or, properly, the fruit) ; /> first
leaf; W^W^ apices of roots not yet covered with root-hairs; ee in Fig. 13 parts of root the hairs of which are attached to
particles of soil— in Fig. 14 these portions (f) are older and the root-hairs have perished— younger parts [e' €') have own
become attached to the soil.

whence it rises. A vertical tap-root will thus be drawn tight in various
directions b}^ its numerous side rootlets, very much as the mast of a ship
is strained and made fast in different directions by the cordage. This ten-
sion, ari.sing from subsequent shortening, shows itself especially in the aerial

SHORTENING OF THE ROOTS. 2 1

roots of many fig-trees, which grow vertically downwards ; these hang down slack
until they penetrate into the earth; as soon, however, as the lower end becomes
fixed and branched within the earth, the part of the aerial root which is in the
air appears tightly stretched, like the string of a piano, in consequence of the
after-shortening. It is easily understood that the horizontal branches of the
Indian fig-trees, from which such aerial roots grow down into the earth, must
obtain by this mechanism a very firm hold, which becomes enhanced still
more by the roots referred to growing thicker later on, and becoming woody.
Much more general, however, is the effect which is produced through the
shortening of the tap-roots in herbaceous plants. If, for example, the seeds of
Umbelliferous plants or Composites, or others, are allowed to germinate, thinly
covered with earth, the seedling shoot at first rises i to 2 cm. above the earth ;
after several days, however, we remark that the part of the seedling shoot beneath the
first leaves is quite sunk into the earth, so that the primary leaves lie on the surface of
the earth. This change can only have been effected by the tap-root, the under
part of which is fast anchored in the earth by the root-hairs, shortening in the
ground, and drawing the stem of the seedUng into the earth. In many cases, indeed,
this shortening of the tap-root appears to proceed steadily even for years. For only
by means of this assumption can it be explained how the rosettes of radical leaves,
in plants with so-called polycephalous main-roots, remain close to the surface of the
earth, instead of becoming gradually lifted up. We have a very instructive example
of this kind in the common Dandelion {Taraxacum officinale). From the upper
part of the fleshy and strong tap-root spring several shoots, which continue to
grow for years, and annually bear a rosette of leaves close to the surface of
the earth. These shoots grow upwards, though slowly, and yet the leaf-rosette,
renewed annually, always remains close to the surface of the earth. The phenomenon
can certainly not be explained otherwise than by the assumption that the tap-root
always creeps in the soil, and draws down the shoots just as deep as they have
become elongated abo\e. The case appears to be the same in the species of
Verbascum, in Geniiana lutea, and others ; accordingly, the tap-roots of older plants
of this kind also present deep transverse folds.

The creeping of a root in the earth may thus, in some degree, be compared
with the mode in which an earthworm bores into it: the anterior part becomes pushed
in forcibly between the particles of earth, while the posterior becomes drawn after it
by shortening.

Finally, we may mention the fact that the roots of vascular plants frequently
appear as organs of regeneration, since they produce shoots which, growing up
in their turn into the air, constitute new plant individuals. To the best known
examples in this connection belong the common Acacia {Robinia pseudacacia) as
well as Ailauthus glandulosa, from the horizontally spreading roots of which
young trees grow forth out of the earth. In a perennial species of cucumber,
Thladianiha dubia, the annual regeneration is, in fact, confined to such root-born
shoots. On the very long thin root-fibres of this plant roundish tubers become
developed, from the ui^per side of which numerous shoot-buds arise, which grow
forth the following year above the earth, independent of the mother plant, and
represent new plants. I regard it as simply a modification of this process w'hen

22 LECTURE II.

leaf-forming shoots spring not laterally from roots, but direct from their growing
points, of which, however, but few examples are known as yet.

This fact has been longest known in Neotiia Nidus Avis, an orchid of our
woods, the leaves of which are not green ; of the numerous and short, stout roots
of its subterranean stem, some produce leaf-buds from their growing points, the
root-cap becomes torn and pushed aside, and the root apex forthwith transformed
into a shoot. Goebel has described the same fact in the root of an Aroid
{Anthuriiun longi/oIium)\n our gardens ^ and apparently the fact is just the same in a
fern cultivated here {Platycerium Willitigkii). In this category w-e may also place
the transformation of the roots of Selaginella into leaf-shoots ^ In these plants,
belonging to the class Lycopodiaceae among the Vascular Cryptogams, the leaf-
shoots become branched in a forked manner, and at the points of branching
appear fihform structures, also dichotomously branched, which in many species
are at once possessed of the properties of roots, in other species, however, are
somewhat different from common roots, and are then called Rhizophores, because
they assume the form of ordinary roots only on penetrating the soil. In some
species of Selaginella {S. Martensii, inocgicalifolia, hrviga/a), these rhizophores may
acquire the form of leafy shoots.

' Compare Goebel, Bot. Zeit. 1878, p. 645.

- Compare Pfeffer, in Hanstein's botanischen Abhandlungen, Bonn, I. p. 67, and Sachs, Lehr-
buch, IV. Aufl. pp. 171, 470.

LECTURE III.

ROOTS {continiuir^f.

METAIMORPHOSED AND REDUCED ROOTS OF THE VASCULAR
CRYPTOGAMS; RUDIMENTARY ROOTS OF THE MOSSES AND
THALLOPHYTES.

In the last lecture, I kept in view only the typical forms of roots of the
vascular plants, and their most important peculiarities. According to the mode of
life of the plants concerned, however, more or less extensive deviations from the
typical form of root may appear even in vascular plants. These deviations are to be
distributed chiefly in three categories ; thus, we have in the first place roots which,
originally quite typical, only modify their form .and properties at a later period
of life, in order to serve special objects for the plant ; then are to be noticed
the aerial and aquatic roots which, from the first, undertake a function more or
less departing from the normal, and possess accordingly an abnormal organi-
zation ; thirdly, we find in parasites among vascular plants a continuous series
of degenerations, through which their roots, in consequence of more and more
pronounced parasitism, finally lose all the anatomical peculiarities of typical roots,
though the most important physiological properties remain preserved to them.
And finally we shall consider, in addition to these metamorphoses, the roots of
Mosses, Algae, and Fungi, especially pointing out that the so-called rhizoids of
these plants are in fact nothing more than simply organised roots, different, it
is true, in their anatomical structure from those of vascular plants, but agreeing
with them completely in physiological relations.

The first category named above comprehends the lignified older roots of
Conifers and other trees, the soft, perennial, turnip-shaped roots of many vegetables,
true subterranean root-tubers, as well as some aerial roots. They have all
the common pecuHarity of being, fundamentally, typical roots, which eventually
acquire other properties. This is perceived most plainly in perennial woody
roots. The tap-root of the seedling of a tree with its first ramifications is, in
the beginning, thoroughly typical, even with reference to those peculiarities which
stamp it as a nutritive organ. When, however, the young tree becomes larger,
it requires a firmer hold in the ground, and the roots become thicker and more

24 LECTURE III.

solid, by the formation of wood ; as they become surrounded with a cork layer, they
cease entirely to take up nutritive matters and water from the earth : they are now
simply organs of attachment, and the function of nutrition is restricted to the younger
fibres of the root system. In tap-roots thickened to a turnip shape, which, espe-
cially by cultivation, swell up to huge, soft masses of tissue, a tissue, anatomically
similar to the wood but not lignified, also becomes formed in the ordinary thin
root-fil)res by the activity of a cambial layer : this causes the upper part of the
tap-root, with occasionally the lower part of the stem, to swell up in a napiform
manner. Common examples are Beet-root, Radish, Carrot, Chicory root, &c.
In this case the object is not only to attain greater solidity; these kinds of
roots are, rather, reservoirs of reserve materials, in which sugar, starch, or
inulin become stored together with proteid substances, which are employed in
the next period of vegetation for the production of new shoots. Here, also, the
function of absorbing nutritive matters remains to the lateral roots, which are
attached as fine threads to the large tap-root. Lateral roots may also swell up
to tuberous reservoirs of reserve material, as happens in the Dahlia, Hop, and
conspicuously in Ipomcea purga and Thladianiha dubia. In many other root-tubers,
the facts are otherwise ; the stout, round or lobed tubers of the Ophrydece, which
furnish salep, and the napiform tubers of the Monks' hood {Aconitum napellus) and
others, as well as those of Ranunculus Ficaria, are formed at the base of a subter-
ranean bud of the stem, and appear forthwith as thick, fleshy, short swellings, the
root nature of which is indeed not doubtful, which, however, together with the
other filiform roots of these plants, have essentially the purpose of serving as
reservoirs of reserve material for the shoot-buds connected with them. In the
species named, in fact, the whole plant disappears after the ripening of the fruit,
with the exception of these tubers, out of each of which a new plant arises in
the next period of vegetation.

Of aerial roots, many may be regarded as quite ordinary typical roots, the full
development of which, however, becomes prevented by accidental outward circum-
stances. Thus, the aerial roots of the Ivy, which arise densely crowded and in
rows on the shaded side of the shoot axis, are capable, when supplied with earth, of
growing out into long, typical, branched roots, because darkness and moisture favour
their development ; usually, on the other hand, where they become too much dried
up in the air and hindered in growth by the light, they remain simple, short threads.
However, being sensitive to touch and at the same time negatively heliotropic,
they cling fast to the trunks of trees, rocks, and walls, on which the shoots climb
up, and so serve as clasping organs. Very instructive, with reference to the
capacity of the roots to adapt themselves to outward conditions of life, are also the
long aerial roots of some tropical Aroids (e.g. Monstera). These roots, arising in
the bud of the climbing stem, are vigorous, often 6-8 mm. and more thick, and
will develope into ordinary, branched, absorbing roots, if a shoot is cut off and stuck
into the earth. If, on the other hand, they are compelled to grow down through
the air, which sometimes occurs for several meters, they remain quite simple and
unbranched, until the apex finally penetrates into the earth, and there produces
a much-branched root system. If these aerial roots ha\e the opportunity of clinging
to a wall, a thick stem, &c., the apex soon becomes closely applied to it, and root-hairs

PARASITISM OF THE MISTLETOE. 25

and lateral roots arise behind it, which also cling to the support. The rhizophores
of some Selaginellas, already mentioned, behave in a manner lundamentall}' quite
similar : in them also, the true root-nature only appears when they penetrate into the
earth. On the other hand, the aerial roots of many tropical orchids, which live on
the branches of high trees, are specially organised for life in the air. The aerial
roots are here in the first place clinging organs : stimulated by contact \\itli the
cortex of the tree, they wind round and become applied fost to it ; at the same time,
however, they possess the function of conveying water to the plant, and, where
possible, of absorbing soluble matters. The latter is attained by their putting forth
root-hairs where they come into contact with a solid body, to which they cling fast.
The absorption of water is favoured in these aerial roots by the so-called velamen
arising from the outer layer of tissue behind the growing green end : it is
several cell-layers thick, and appears as a white spongy covering, since its cells
contain air. The cell-walls are capable of imbibing, and are able to absorb not
only rain and dew, but e\en the vapour of the atmosphere. The aerial roots of
some tropical Aroids also behave similarly, e.g. species of Aiithuriiim, Philodendron,
Rhaphidophora, Monsiera, and others.

As parasitism acts generally in a degrading manner on organisms, and causes
degeneration of organs, so also in roots; the more decidedly parasitism makes
itself evident in phanerogamous parasites, the more do their roots lose their ordinary
typical structure, and at last only amorphous masses of tissue or even isolated
cells remain, which have still in common Mith true roots only the property of
penetrating into the substratum, and absorbing food. By l)otanical authors such
roots are termed hausioria. A few examples ma}' further illustrate what has been
said ^

In our INIistletoe {Viscum album), which, as is known, lives on the stems and
branches of apple-trees, horse-chestnuts, pines, poplars, and other plants, the roots
penetrate, it is true, into the cortex and wood, but chiefly, as one may assume, only
to absorb water and minerals dissolved in it, i. e. the so-called crude sap, which
is being conveyed in the wood of the tree to the leaves ; whether the roots of the
mistletoe, losing themselves in the cortex, possibly extract organic matters also from
the tree, is uncertain. So much is at any rate established, that the shoots of the
mistletoe, unusually well su[)plietl with chlorophyll, produce organic materials inde-
pendently; even the roots flourishing in the wood and cortex are rich in chlorophyll,
and green ; the parasitism of the mistletoe, as regards its nutrition, is in any case
oiily partial, and, accordingly, also the degrading influence of the parasitism but
insignificant.

If a viscid mistletoe berry sticks to a young branch of a tree, covered with a
thin cork layer, the somewhat large embryo, very rich in chloroplull, germinates.

' The most important treatises concerning phanerogamous parasites are: Franz Unger, äv'-
ti-äge zur Komiiiiss der parasitischen Pflanzen ; Eichler, Die BalanopJiorcen, in Flora Brasiliensis,
Heft. 47, 1869 ; Graf zu Sohiis-Laubach, Ucbcr Bau und Entiuicklung der Ernährungsorgane
parasitischer Phancrogamcn, in Jahrb. f. Wiss. Bot. VI. p. 509 ; the same, Ueber den Thalliis 7>on
Pilostyles, Bot. Zeit. 1874, Nos. 4 and 5 ; the same, Die Entwicklung der Blüthc hei Bruginansia,
Bot Zeit. 1S76, p. 449; Ludwig Koch, Die A'/re und P/t/chsscid; Heidelberg, 1880; Robert
Harlig, L'/icr I'lsr/////. Zcitsclir. f. Korst- und Jagdwesen, Bd. VIII.

2.6

LECTURE III.

Its root-apex becomes turned away from the light to the branch, the radicle bores
through the cortex, and makes its way through the cambium to the wood ; if its
apex is situated deep in the wood, later on, this is the result of the new woody
layers of the branch growing round the base of the root, while this becomes elon-
gated a little to a correspondmg extent. Later on, as it seems, roots arising from
the base of the stem grow within the soft living cortex, and from these again lateral
roots arise on the side turned towards the wood of the host : they penetrate through
the cambium layer to the wood, are then surrounded by the younger, growing, woody
layers, becoming meanwhile elongated to a corresponding extent at the base, and
now constitute the so-called suckers of the mistletoe. Of these three forms of root,
only those which grow in a sinuous manner within the cortex, apparently, have a root-
cap, which, however, even here is not well developed. The anatomical structure of the
Viscum root allows little to be recognised of the structure so distinctive of true roots ;

Fig. 15.— Lower portion of the stem (<r) of the Mistletoe (P'iscuni albtim). h the wood of the shoot-
axis; i primary root; .#" roots growing in the cortex of the host-branch (<rl ; g two buds arising from
these ; ee so-called Haiistoria. roots which penetrate through the cambium into the young wood, and
become surrounded by it later ; bb wood of host-branch (half cut across at dd) showing the annual
rings (nat. size).

even the characteristic axial vascular bundle is considerably reduced, and the
adaptive peculiarity of the sucker is especially striking— its growing point is
converted into permanent tissue, while the growth in length, inconsiderable it is
true, is carried on at the part which lies in the cambium layer of the branch of
the tree. In spite of all this, no unbiassed person will hesitate to regard these green
organs of the mistletoe, even though devoid of a cap and anatomically abnormal, as
reduced roots, and this is so far of some importance, since here we have one
of the first stages of reduction produced on roots by parasitism. Finall)-, it may
be mentioned that the Viscum roots losing themselves in the cortex of the host, repre-
sent very active organs of multiplication : from them spring shoots, which, breaking
through the cortex of the tree, come forth into the light, and from these, new roots
then again run into the cortex, so that as occasionally occurs on old apple-trees
the whole tree, from crown to root, is infested with the mistletoe.

PARASITISM OF CUSCUTA.

27

-Like the mistletoe, so also are the species of Cuscula parasitic on the aerial
green shoots of woody plants : their parasitism is, however, complete, since they not
only possess no roots fastening them into the soil, but they also completely lack
chlorophyll, and are necessitated to take the whole of their nourishment from the
host. This they do by means of hausioria, which arise within the twining stem of
the Cuscuta only where this closely surrounds the host-plant, and it is apparently
the pressure hereby exercised which induces the origin of the haustoria. That these
latter are to be regarded as reduced roots can hardly be doubtful from all the
researches before us, as well as from our figure. Not only the place and the occa-
sion of their origin, but also the primary young stages of these organs, correspond
with those of typical roots. In the further development, however, a striking deviation
from these appears. The tissue of the haustorium, corresponding to the body of the

FIG. 1^.— Haustorium of Cuscula epiUnu)n on the axial vascti'ar bundle {s). betieall
{rr) of tile shoot-axis of the Cuscuta. ee epidermis of latter; E epidermis of Lh
R its cortex ; // its wood. The axis of the Cuscuta and its hauslorimii in longitudit
stem oi Ltnuin in transverse section (niagn.)

root, becomes arranged into a bundle of rows of elongated cells, which, growing
forward at the end, break through first the cortex of die Cuscula, and then the cortex
of the host-shoot, to penetrate to the woody body of the latter, or e\en to break
through into its pith. Hereupon, indeed, the cell-rows of the body of the root
may become isolated as separate threads, which grow into the tissues of the host.
In the axis of the hauslon'um, the axial cord of vascular bundles is also still to be
recognised, and the vessels of this become fused with the vessels in the wood
of the host-plant. This intimate union of the haustorium of the Cuscula with the
stem of the host-plant becomes aided yet more by an outgrowth of the tissue of
the Cuscuta, which surrounds the haustorium in the form of an annular wall, anel
becomes closely applied to the shoot of the host.

Similar haustoria, departing still further from the type of true roots, become
developed on the otherwise typical roots, branching in the earth, of Thesium and

•8

LECTURE III.

Rhinanthits, green-leaved plants which are parasitic only by-the-way, and fasten
themselves to the roots of neighbouring plants by some of their root-fibres. We
find the last stage in the reduction of the root-formation in phanerogamous para-
sites, finally, in the Balanophorece and Raßesmcece, in which, as has been mentioned
before, the vegetative body, seen apart from the flower-shoots, no longer allows
the differentiation into shoot and root to be recognised. In the RafflesiacecB the root
presents amorphous masses of cells, which become extended in the tissue of the host,
and in the Balanophorece their parasitism brings about out-growths of tissue on the

Fig. 17.—.-

L.

v\n

al icLtiun

of the flower

of Brugmnnsi

1 Z

//t

li,

the vegetative body of

which is parasit

c in

the

rout

(7«)ofaC

sstis. fmaxy.

represented in

tra

ISVL

rse

section at

B (nat. size.

after Sohns-La

ibac

!)■

FIG. i\i.—B,ilanofho!-nfiniocisa. iv root of host plant,
out of wliich tlie tuberims part of the parasite grows; the
woody bundles of the host-root (represented as dark streaks)
penetrate into the latter; a, h, c young inflorescences (nat.

affected roots of the host-plant, which make the union of the parasite with this
yet closer, and call to mind in many respects the gall-formations produced by
insects (cf. Figs. 17 and 18).

From what has been said concerning parasites, it follows that the typical roots
of vascular plants may lose their external form and anatomical structure completely,
under certain circumstances, so that their chief physiological properties alone are
maintained.

If we now turn to the more simply organised plants, to the Mosses and Algse,

ROOTS OF MOSSES.

29

we there find, in correspondence with the simpler organisation of the whole,
roots also of simpler structure— rudimentary forms of root in which, however, the
essential physiological properties of true roots appear quite as evident as in them ;
it is therefore, though unimportant in itself, a logical error to distinguish these
organs of the Mosses and Algae with a special term as Rhizoids, since it must be
the aim of comparative organography to name alike organs which are alike in
their nature.

Meanwhile, with regard to this matter also a few examples must suffice.
Let us turn first to the true Mosses \ which come so near to the vascular
plants in their shoot formation. From
the asexually produced reproductive
cell (the spore) of a moss, there arises
not immediately the proper moss-plant,
but a much simpler plantlet, con-
sisting of jointed and much-branched
cell filaments, the so-called Protonema.
Already in the germination, and yet
more in the further growth of this, the
contrast of shoot and root comes out
distinctly. While at the one end of the
germinating spore a filament containing
chlorophyll becomes developed, ascend-
ing above the substratum or creeping on
its surface, with branches which in many
respects remind us of leaves, there ap-
pears from the other end of the spore a
thread devoid of chlorophyll, which bores
forthwith into the substratum, in certain
respects comparable to a tap-root, and
producing, like this, a branch-system of
roots.

As, however, e\en in the vascular
plants the primary root usually remains
feeble, and becomes replaced by later
ones arising from the stem, so also on
the protonema of the moss, roots arise
here and there from the creeping shoot-
axes, as from the creeping shoots of vascular plants. At the same time there
arise from the protonema-shoots the proper moss-stems furnished with leaves ;
a process which corresponds to the formation of flowering leaf-shoots from the
rhizomes and stolons of vascular plants. Out of the shoot- axes of this moss-stem,
fresh root-fibres now arise, either from the basal parts only, just as in the INIaize

Fir -it)— A joung phntlet (
{/) the f rwiid growing ends of
of soil It / 1 superficial root i
chloioph>Il (I e protoiieina) at
subtenanein root, h the latter more
B X ^00)

ot a Moss {Farbiila) witli roots
lich (w) .are attached to granules
putting out branches containing

a tuberous bud is situated on a
ougly magnified (.-? X =o ;

' Compare Schimper, Rccherchcs anatomiqiies et physiologiqucs sur Ics Mousses, .S'^rasslnirg,
1848 ; Sachs, Lehrb. d. Bot. Cap. Laubmoose; Hermann Müller, in Arbeiten des Txit. Inst. \Vuiz-
l)iii-g, I. p. 475.

3°

LECTURE III.

and other ]Monocot3-ledons, or close beneath the growing point of the stem, so
that this is finally covered by a dense feltwork of roots, like the stems of many
tree-ferns. If the foliage shoot of the moss under consideration is dorsiventral, and
expanded in an oblique or horizontal position, with the upper and under side differently
organised, root-fibres arise only in acropetal succession from the shaded side turned
towards the substratum. In all these points, the rhizoids of the moss completely
resemble the typical roots of vascular plants ; the abnormality consists only in that,
in keeping with the simple cellular structure of the moss, they consist not of

Fig 20. — A, ß, C germination of Fititaria hygro>n€ty
the soil. From the creeping protonema, which branches at
the soil. (After Müller ; highly magnified.)

' (a Moss) ; D older seedling creeping on the surface (a. />) of
and forms leafy shoots at /v/, arise roots which penetrate into

masses of tissue, but of jointed cell filaments. However, so far as this anatomical
structure admits, the relations of growth agree also with those of true roots :
a long cell occupying the end of the root-fibre of the moss corresponds to the
growing and cell-forming end of a phanerogamous root, and the growth in
length takes place only in this one cell ; its free end corresponds to the growing
point, and further backwards it represents the portion of a root which is becoming
elongated. New segments are added to the root-fibre by obliqueh' placed walls, and

ROOTS OF HEPA T ICE.

3'

lateral root-fibres arise from them with properties like those possessed by the lateral
roots of vascular plants. As the younger part only of a true root attaches itself
to the soil by its root-hairs, so in the roots of mosses a similar connection appears
at the young end only; further backwards from this, the cell- wall becomes
thicker, and takes on more or less the solidity, resistance, and dark colour found
on the older root-fibres, e. g. of Ferns and Horse-tails. Although the geotropism,
heliotropism and the sensitiveness to pressure and moisture in the roots of mosses
have not been hitherto directly investigated, it may nevertheless be concluded with
certainty, judging from their whole biological behaviour, that they completely resemble
true roots in all these respects.

Still more simply organised are the roots of the Liverworts, especially those with
flat, extended, ribbon-like shoots, e.g. of
Marcha7itm. From the shaded underside
of their dorsiventral shoots, thin-walled,
narrow, but very long tubes appear, simply
as protuberances of certain epidermis cells,
which penetrate deep into the earth with-
out becoming branched. In the simplicity
of their anatomical structure, they resemble
apparently only the root-hairs of the vascu-
lar plants, with which, in fact, they agree in
so far that they, like those, are immediately
concerned Avith the function of nutrition.
These simple tubes, however, possess in
addition the essential physiological peculi-
arities of typical roots, as is to be concluded
with certainty from their whole behaviour ;
their sensitiveness to light and moisture,
and their origin under the influence of
gravitation and pressure, are beyond
doubt. We have before us, in the root-
hairs of the Marchanh'cB and other flat-
shooted Liverworts, organs of the simplest
structure, in which are united all the phy-
siological peculiarities of the root-hairs and
root-bodies of the more highly organised
vascular plants, as further proof that these
physiological properties are quite indepen-
dent of the cellular structure of the higher plants. The prothallia of Ferns agree
with the Liverworts, as regards the formation of roots, so completely, that it is
sufficient to refer to Fig. 21 for the elucidation of both.

If we now turn to the Algce, we meet with great variety both in the
root formation, and with respect to all other relations of organisation. But the
Algae, with rare exceptions, live entirely in water, and can al)sorb this and
the food-matters dissolved in it with their whole surface ; and their roots
must thus be of very subordinate importance in the absorption of food, whereas

FIG. 21.— Pro-embryo {Prothal/tu) of a Fern {Os»tiiH</a
alls) seen from tlie under side, a antlieridia ; iv root-
rs ; V the growini; point (magnified).

32

LECTURE III.

the second main function of roots, to be organs of attachment, asserts itself promi-
nently.

We may first consider the genera
Fucus and Laininaria, including
large plants which are also highly
organised anatomically; here we
find the base of the whole plant an-
chored fast to stones, rocks, &c., by
means of a much-branched root.
The attempt to penetrate into a nu-
tritive substratum is hardly made,
since it is superfluous, as has been
said ; it suffices here that the roots
cling as organs of attachment to
any solid body, since the whole
surface of the plant takes up food,
but is also exposed to the brunt
of the waves. If the student has
made himself sufficiently acquaint-
ed with the peculiarities of the typi-
cal roots described, there cannot
be the slightest doubt that these
organs of attachment of the large
sea-wracks are roots, in which the
function of nutrition is a second-
ary matter. Their anatomical and
cellular structure corresponds with
the whole character of the plant ;
these roots consist of masses of
tissue as do those of the vascular
plants, and they are branched
dichotomously like those of the
LycopodiacecT.

If we then proceed to Algae of
simpler structure, we meet among
others with the group Characeas,
the roots of which agree in most
essential peculiarities with those of
the Mosses — simple cell-filaments
with oblique cross-walls, which
penetrate into the substratum and
become branched there. Finally
are to be mentioned here the roots
of the non-cellular Algae, with
which we have become acquainted already in the genus Botrydmm (cf. p. 4). It is
obvious that in these essentially non- cellular plants, the much-branched roots are

Fig. 22.— Entire plant of Laminaria CL
bb o'd leaf divided up; i'*' newly arising le;
from the growing point e ; y boundary betwi
At f. the new leaf is beginning to split ; at a- ;
(J nat. size).

ROOTS OF ALGJE AND FUNGI.

?>3

not divided into cell-chambers; but it has already been pointed out that this simple
non-cellular structure nevertheless does not prevent
them from exhibiting all the essential physiological
properties of a typical root.

In those Algae the shoots of which are simple
segmented filaments, or rows of cells, as in the genera
(Edogoiiiu?7i, Cladophora, and others, the root is usually
very small in proportion to the green shoot, since this
small size, with very simple organisation, suffices to
supply an organ of attachment, the mechanical
action of which, considering the mode of life of
these plants, is very little called into play. In the
genus Spirogyra we meet even with the extreme case
in this connection, that in the germination of the
spore the shoot portion becomes vigorously developed,
while the root end is only indicated, as it were, and
no longer used even as an organ of attachment ; the
simple unbranched filaments of this alga float, in fact,
quite free in the water. Nevertheless the ability to
form roots is not wanting in Spirogyra; if its long
filaments are cut up, and the pieces laid on wet peat,
some of their cells put out branched colourless tubes,
which behave like roots. In this connection we find
similar cases also in highly developed plants ; on the
one hand, the rooting of severed portions of shoots in
many vascular plants ; on the other hand, the eff'ect
which continued contact with a solid body exerts on
the development of new roots, as e.g. in Cusaila, and
the gemmae of Marchanlia.

Passing over finally to the Fungi, it has already
been mentioned in the first lecture (p. fi), that the mycelium of these resembles

Fig 23.— Development of zoospores of
aidogoniuin (after Pringsheiin X 350). AB
zoospores arising from an old filament ;
C free zoospore (motile) ; D the same be-
ginning to germinate ; E a zoospore formed
from the entire contents of a germinated
swarm-spore ; root and shoot are to be dis-
tinguished in the latter.

e

Fig. 24.— Germination of Sftrogyra ju£nlis (after Pringslieim in • Flnra,' no. 30. 1852). / resting zygospore ; // the
same beginning to germinate; /// further developed (the zygospore was enclosed in the cell C of a filament on
which the conjugating apparatus is still visible) ; e outer cellulose wall of spore ; / yellowish brown membrane ;

g the third innermost membrane of the spore, which forms the germii
the posterior end (d) of wliich grows out as a narrow root process.

tube

' w' first septa of a germinal tube.

typical roots from the physiological point of view ; it has, however, to take up not
[3]

34

LECTURE IIT.

Peltigera hori
size).

^.~>^^^

-A portion of the foliaceous tliallu

merely water and dissolved salts, but also organic food-matters. Apart from certain
special cases of complex formation of tissue, the m}-celium consists of much-branched
tubes, which are unsegmented in the Phycomycetes, but in all other cases divided by
transverse walls. From the manner in which these cell-filaments penetrate into the
substratum, become branched there, and, under certain circumstances, again come
forth from the substratum, and so on, it follows that the mycelium of the Fungus
agrees entirelv with typical roots in respect to geotropism and heliotropism, and in

its sensitiveness to moisture and contact.
The mycelium, it is true, possesses the
property of producing fruit-bearing hyphse,
^«/V^-'s-. Ji^" which we have already di^tinguished as

T^^^te^ '^*^^^"-* J ^^ formation of the shoot in Fungi.

V-i^'^^^ÄJ^^^^^^^^. This is not an essential deviation from

the typical roots, since in many phane-
rogams and vascular cryptogams the
tendency exists in a very prominent de-
gree to produce shoots out of roots ; and
we meet further with the formation of
shoots out of roots in the ^Mosses. It
cannot therefore be urged against the
root-nature of the mycelium of the Fungi,
that the fruit-bearing portions ordinarily
spring out of it as shoots. This happens
in fact even in Alonotropa — a phanerogam
devoid of chlorophyll — in a quite similar
manner.

Among the Fungi, the group of Lichens
is distinguished by a very remarkable
form of parasitism. The fungus tissue, in
fact, surrounds the Algae containing chloro-
phyll which nourish it, so that the latter
behave like a histological constituent of
the Fungus, which has now become in a
certain sense a plant containing chloro-
phyll. In accordance with this, the shoot-
formation of the Lichens is also often far
more complete than in other Fungi, and
thence follows, again, that in them the con-
trast between shoot and root is more sharply
expressed than in other Fungi.
Here too, again, the root may act not merely as an organ of attachment
only, but also at the same time as an organ of nutrition. The first is the case in
many so-called fruticose lichens, which are attached by a narrow base to the
dry bark of trees, e.g. the genus Us7iea. The roots of the so-called foliaceous
lichens, the shoots of which are extended as flat dorsi-ventral plates on the earth, or
on tree trunks, as in the large genus Peltigera, appear not only as organs of attach-

FIG. ■^—Uiuea f.
lification ; f organ of .
Dn tlie bark of a tree.

vhicli becomes firmly fixed

ROOTS OF LICHENS,

35

tnent, but also as organs of nutrition. From these highly organised forms of Lichens
down to the so-called crustaceous Lichens, the vegetative body of which grows in
the interior of the dry bark of trees, on dry earth, or even on hard stones, we find
a series of transitional forms, ending in cases in which a proper formation of shoot
and root are hardly to be spoken oP.

' The reason why I term all these organs roots, will follow sufficiently from the connection of the
text. If the prejudice against descriptive botany, still frequently existing even in scientific
circles, is ever to cease, it will be well entirely to get rid of the superfluous nomenclature
exemplified by the words Ji/iizoids, Rhizines, &:c. Where would Zoology be if the feet of insects,
as well as their eyes and wings, were distinguished by such names ? Yet the mania for inventing
names has gone so far of late years, that the root of vascular plants has been termed a thallus,
simply because it has no leaves.

LECTURE IV.

THE TYPICAL FORINIS OF SHOOT OF THE VASCULAR PLANTS.

We distinguish, generally, as the shoot, the vegetative organ standing in op-
position to the root. There are, it is true, derived (metamorphosed) forms of shoot
which live hidden in the substratum, as there are indeed derived forms of root also,
which, abandoning their primitive characters, become developed outside the nutri-
tive substratum. The primary and most prominent peculiarity of the shoots, however,
is that they raise themselves above the substratum, in order to fulfil their most important
vital function in the air (occasionally in water) under the influence of light, that
is, to decompose carbon dioxide by means of their chlorophyll, and to pro-
duce organic substance, from which new shoots and new roots may be formed.
A second function of the shoots, perhaps as essential, consists in that earlier or later
they bear the proper reproductive organs, viz. the sporangia and sexual organs,
wWch are never formed on roots.

The mode of life, outward form, and internal structure of the shoot depend
upon the way in which, according to circumstances, these two main problems, as-
similation and the development of reproductive organs, are solved. In many cases,
it suffices that the shoot containing chlorophyll simply rises above the substratum
to the light ; it then assimilates, and subsequently developes the reproductive organs.
Generally, however, a further division of labour appears within .a shoot or .shoot-
system ; some branches, as subterranean shoots, store up the products of the as-
similation of those above ground, and become transformed into reservoirs of reserve
material, while, very often, certain branches are specially entrusted with the formation
of reproductive organs. That roots also may arise on the shoots, according to their
mode of life, has already been mentioned several times ; they represent, therefore,
the proper body of the plant, on which all other organs appear, as the limbs on the
trunk of an animal.

These deviations from the primitive nature of the shoot, and its metamorphoses
and reductions, go much further, and are far more various than in roots, and
it is not so easy as with the latter to express the fundamental physiological
character clearly and exhaustively. It will be the object of later lectures to give clear
ideas of this enormous variety of the development of shoots, by a series of examples.
To-day, however, we shall confine ourselves exclusively to the peculiarities of typically
developed shoots, as they occur in the great majority of vascular plants. What tl.e

THE SHOOT.

?>7

non-botanical student sees and knows of the vegetable world, apart from the
larger flowers of phanerogams, are the shoot-formations which we have here in
view. They are the so-called stalks or stems, with the leaves situated upon them,
and, in order to avoid any misunderstanding, I may add that I shall consider first
exclusively those shoots the leaves of which contain chlorophyll. Such leaves are
called foliage-leaves, and accordingly the shoots are called foliage-shoots. We have
at first to do with these only. A Palm-stem with its huge crown of leaves is thus

FIG. 27.— W Ofhioglofsitm vitlgatum; B Botryckium Limarui. (Both natural size.) 71» roots;
St stem ; bs leaf-stalk ; x point of branching of the leaf, where the sterile lamina (*) separates froiii
the fertile one {/). The shoot a.vis remains in the earth ; only its leaves (one each year) come forth to
the light

a foliage-shoot, as is also the upright stem, furnished with large leaves, of a tobacco
plant. The foliage-shoot generally becomes branched, i.e. a foliage-shoot produces
new shoots at definite points, which in their turn do the same, and so there arises from
one shoot step by step a shoot-system, on which every single member is to be dis-
tinguished as mother-shoot or daughter-shoot, according as its relation to a preceding
or following^ shoot is to be indicated. Mother- and daughter-shoots may be like or
unlike between themselves. The Conifera3, e.g. Firs and Pines, as well as other forest
trees, and a great number of annual plants, e. g. Thorn-apple, Hemp, &c., are in

3«

LECTURE IV.

their aerial parts branch-systems which have become developed from the original

germinal shoot (Plumule) of the seed.

As already indicated above, a typical shoot consists of the leaves and the shoot-
axis, which are, however, in the
first place not properly to be
■i\ "" . considered as different organs,

but essentially only as por-
tions of one organ, although,
by peculiar development and
further formations later, the
leaves as well as the shoot-
axes may assume distinctive
characters. Essentially, and as
the history of development
shows, the leaves are how-
ever, strictly speaking, nothing
more than protuberances or
out-growths of the shoot-axis,
which, by means of their large
development of surface, are qua-
lified to present the chlorophyll
contained in them to the light,
and to the air containing car-
bon dioxide, in the most ap-
propriate manner, so that
the process of assimilation, the
production of organic substance
for the whole plant, may take
place with the greatest possible
energy. In this connection, the
shoot-axis appears first only in
the simple character of a sup-
port, on which these organs
of assimilation are suitably
arranged in large numbers.
The products of assimilation
are also conducted upwards
as well as downwards in
it ; while at the same time
water and nutritive matters
are conveyed from the roots
to the leaves through it. The
whole structure of a higher

F.G. .8 -Gemunation of Indian corn (Z.. «,.»): successive ages,/,//, plant Is Ouly tO bc UUdcr-

///. A and B the seedling removed from /, witli its scutellum sc. Every- (.(-nnH if tVlP'JP mnffprn n rp kpnt

where, w primar>' root; ifs root sheath; -W. a" secondary roots; * first StOOQ H tnebC maUCrS are KCpL

leaves; k first segment of slioot-axis ; r edge of scutellum; e endosperm, • vMp,y

surrounded by the pericarp (nat. size. Cf. Fig. 5). "' View.

RELATION OF LEAF TO SIIOOT-AXIÜ.

39

The interdependence of the leaves and their shoot-axes is made especially
clear at the growing point of the shoot, where it is easy to see that the leaves
are, fundamentally, only protuberances of the substance of the shoot-axis itself;
outer and inner layers of the tissue of this form protuberances, the tissues of which

Fig 29.— Diagram of a dicotyledonous plant. / and // embryonic stages; /// after germination. <-. f cotyle-
dons ; w, w' roots ; h hypocotyledonous segment of shoot axis ; b—b'" leaves ; i, k' buds. The growing points
are shaded black, the parts becoming elongated grey.

are from the first, and during the whole duration of life of the loaf, in complete
continuity with the tissue-systems of the shoot-axis. A root is connected with its
mother-root, or with the stem from which it arises, as something extraneous, like a
parasite which has first to establish its connection with the mother-organ. The leaves

40

LECTURE IV.

sland in quite a diflerent relation towards the shoot-axis : the epidermis of the latter
passes over to the leaf without visible interruption, and the cortical tissue of both
is also in complete continuity, and, apart from a few cases, the vascular bundles
of the stem or shoot-axis are really nothing more than the lower ends of the
vascular bundles which curve out above into the leaves, and produce in these the
so-called venation. In its original condition, indeed, the shoot-axis has generally
no surface of its own at all, since the leaves come forth so closely one upon an-
other at the growing point, that no free surface of the shoot-axis remains. If the
growth in length of the latter is very slight, even the completely formed shoot-
axis has no free surface ; it is quite covered with leaves, as in our wood-fern,
Aspidiiun Filix-vias,. and in such stems of phanerogams as bear their leaves in
so-called radical rosettes, and in many other cases. But even where a more vigorous
growth in length of the shoot occurs, the bases of the leaves may grow with it, in
such a manner that the whole surface of the shoot-axis is nevertheless clothed with

FIG. 30.— Upper end of a shoot-axis of
Hippuris vulgaris, after removal of the
surrounding older leaves. Above is the
naked groviiing point, from vvliich arise the
young leaves visible lower down (mag-
nified).

Fig. 31.— Longitudinal section through the apex
of an erect shoot of hippuris vulgaris, s apex of
shoot; b, b, b leaves (in whorls); k^ k, their axillary
buds, which become flowers; g, g primary vessels.
The dark parts indicate the cortex, with its intercel-

leaf-substance, as occurs especially with very small leaves, e. g. in the Selaginellcz
and many Ctipressinece, such as Thuja. In the typical development of a shoot,
however, the growth of the shoot-axis proceeds in such a manner that the bases
of the leaves, at first closely crowded, become separated, since between each two
superimposed leaves of the growing point, a piece of the shoot-axis is as it were
intercalated, and then attains a more or less considerable length. Such a portion
of the axis is then called an internode, or inter-foliar part of the shoot-axis.
Only among the vascular cryptogams are single cases known {SalviniacecT) where,
immediately on the development of the leaves at the growing point, such inter-
foliar parts of the axis with free surfaces are present. One of the most striking
phenomena in shoots, viz. the position of the leaves on their axis, can be in part
explained causally from the fact that the youngest leaves arise at the growing point,
as said, so close above and by the side of one another, that the whole surface of
the shoot-axis is covered by them, so that with the growth of the leaves mutual
pressure must necessarily occur.

BUDS.

41

Yet another, and much more striking phenomenon, however, is produced by
the crowded position of the youngest leaves at the end of the shoot-axis, in
connection with a further iact, viz. the much quicker growth of the leaves, as
contrasted with the elongation of the shoot-axis. This is the formation of Buds.
By the word bud, we distinguish generally the young condition of a shoot :
either the whole young shoot, or the young portion at the free end of a shoot
already further developed, is a bud. Shortly put, the bud is the growing point of
a shoot, surrounded by its leaves We can thus only speak of buds in connection

Mc;. 32. — Setaj^inella iiia-i/ieali/olüt : longitudinal section tlirough the rii;li
base of leaf; h ligule ; sp sporangium; ii poir.t of connection of the ca
air cavities ; x series of cells traversing the cavities (X 12c).

with leaf-forming shoots : in contrast to them, stand the naked growing points of the
leafless shoots of Algas and Fungi, to which I shall refer later.

The development of every new plant individual begins with the production of a
young shoot ; in leaf-forming plants, therefore, with the formation of buds. Thus
there arises, even in the embryo of the vascular plants as soon as any organs become
recognisable on them, a bud (the plumule), and in the same way the development of
any new shoot, after a growing point has been formed, consists in the development
of a bud, which then grows out further, at once or later. While the growing point
itself, just as the youngest parts of the shoot-axis on which leaves are already

4a

LECTURE IV.

situated, only grows very slowly in length, the growth of the young leaves is on the
other hand much more vigorous ; as each older leaf lying further back from the

PIQ. j3._ Apical regions of two primary shoots of 2V-rt /1/a!.r. Apex
of the very small-celled growing point, from which the leaves *, *', *",
*'" arise as multicellular protuberances, which soon surround the stem,
and envelope it and the younger leaves like sheaths. In the axil of
the third youngest leaf i'. the very young rudiment of a branch is
visible as a roundish protuberance (cf. Fig. 5).

FIG. 3i.— Longitudinal section through the
apical region of the primary stem of Helianlhus
annuus, immediately before the development of
the flowers, j apex of the broad growing
point ; *, b youngest leaves ; r cortex ; m pith.

growing point has a start of all the younger ones, and as, at the same time, each
older leaf is arched concave inwards and envelopes the younger more or less,

leaf thus lying on leaf, a bulb-like body,
the young shoot, is formed. If this
now becomes further unfolded, the leaves
expand according to their succession
in age, attain their complete develop-
ment, and become thrown outwards,
while at the same time the corresponding
segments of the shoot-axis are further
developed. Sometimes the bud consists
only of a few very young leaves, as in
the climbing-buds of the Ivy; in other
cases, however, dozens of leaves and
internodes are found in the bud state. If
the period of vegetation is interrupted
by the winter, the bud may maintain its
form, simply because the further de-
velopment of the parts is discontinued,
until, with favourable weather, the older
parts of the bud again begin to grow,
and new ones are formed at the growing
point ; e. g. in our so-called Acacia (RobiJiia pseudacacia) and the Arbor vitae
{Thuja). In the majoril}- of trees and shrubs, however, peculiarly organised winter-

FIG. 35.— I-ongitudinal section of the fruit of Zea Mais (X 6).
c pericarp ; n remains of stigma \/s base of the fruit ; eg dense
yellowish portion of endosperm ; e-w whiter, less dense por-
tion of endosperm ; sc scutellum of embryo ; ss its apex ; e its
epithelium ; k plumule ; w (below) primary root ; -ws its root-
sheath ; -ui (above) secondary roots arising from the first inter-
node (J<) of the embryonal stem (cf. Fig. 27).

WINTER-BUDS.

43

buds are formed, which are situated at the end or at the sides of the branch as special
organs. When a winter-bud is about to be formed at the end of a leaf-shoot,
e, g. of a Fir, Oak, Horse-chestnut, &c., the development of foliage-leaves suddenly
ceases, and a number of corresponding leaf-rudiments assume the form of scales,
which tightly surround the }-ounger parts, and are often glued together with resin or
balsam. Very commonly, it is the lateral shoots of woody plants which on their origin
assume at once the form of winter-buds, and commence with the formation of scale-
leaves. In some special cases, as in the Fir and Pine, the apex of the shoot which
becomes developed in the next period of vegetation into the leaf-shoot, is already
present in a quite embryonic state, enveloped by the bud-scales. In other cases,
again, as in the Horse-chestnut and our fruit trees, we find already in the autumn,
within the winter-bud, not only the end of the young shoot, but a branch-system,
with flower buds and more or less developed foliage leaves, and all these parts
already so far formed, that the first warm days of spring suffice to bring them to
complete development, after the bud-scales
have opened.

In herbaceous plants it often occurs
that all the organs produced in one
period of vegetation — root, shoots, and
flowers — completely disappear, and that only
a few buds, with or without enveloping
scales, remain over the winter ; and, de-
veloping in the next spring and forming
new roots, represent a new transitory
plant individual. So it is in many water
plants, as Aldrovanda vesiculosa and Ulri-
cularia, and many land plants, where
the bud, remaining behind in connection
with a reservoir of reserve material, repre-
sents a tuber, capable of germinating, as in
our species of Op/irys, in the Arrow-head
{Sagittaria), Crocus vermis, and in Ficaria
ranunculoides, &c. Strictly speaking, bulbs
also are only peculiarly developed persistent
buds, the outer leaves of which are thick,

swollen, and filled with reserve materials, the substances which are used in the
beginning of the period of vegetation for the development of the young shoot-
rudiments in the interior of the bulb. One need only cut through a common
kitchen onion {Allium Cepd), in the direction of its length, or a Hyacinth or Tulip
bulb, in autumn or winter, in order to recognise the state of affairs at once.

Entering now more closely upon the study of the organisation of the shoot,
it is better to consider the shoot-axis and leaves separately. No one will w-ish
to compare the organisation of the leaves with that of the root ; but the typical
form of the shoot-axis, with its cylindrical or prismatic figure, suggests a comparison
between it and the root. In spite of this external similarity, however, the organisation
of the leaf shoot-axes is very strikingly different from that of the roots. If the fully

Fig. so.— Longitudinal section tlirougli tlie apical region o
a bud of Equisetiim ai-uense. ss apical cell; *— *; the sheath-
like leaves (magnifiedj.

44

LECTURE IV.

developed shoot-axis possesses inter-foliar parts, as is usually the ca-:e, these are
usually clothed with a well-developed epidermis, which is in its turn covered by
a cuticle, and perforated by slomata. Hairs also of the most varied form, prickly,
woolly, or glandular, are very common appendages. If the typical leaf-shoots are
destined to last several or many years, their epidermis becomes sooner or later
replaced by a uniform cork layer, a so-called periderm, which carries on the main
function of the epidermis (viz. the prevention of the loss of water by evaporation),
more completely than the epidermis itself.

Fig. 37.— Median longitudinal section throuifh a small
■winter bud oi Abies fectinata. ss scale-like envelopintj
leaves, which arise from a cup-like outgrowth of the
cortex (w) of the preceding year; .ipith of the new
rudimentary slioot ; v its growing point.

KlG. 38.— Median longitudinal section through a vigorous
.vinter bud of the Horse-chestnut {Aesculus hippocaslaiiuin).
'/t, /:, r pith, wood, and cortex of shoot-axis ivr) of the pre-
;e<ling year; ss bud scales; 1'/ young foliage-leaves; in the
niddle is the young inflorescence— the dotted space is filled
vith woolly hairs.

In the interior, the young shoot-axes consist of a basis of succulent par-
enchymatous tissue, in which run a few or very many vascular bundles, often also
sclerenchymatous cords. While in the roots the vascular bundles are united into a
single axial strand, surrounded by the parenchymatous cortex of the root, the vascular
bundles of the shoot-axes on the other hand run, primarily at least, as isolated
filaments, the upper ends of which curve out into the leaves, while their lower
ends attach themselves to the middle parts of the preceding bundles. Only in
rarer cases, e. g. in some water plants {Hippuris) and a few Cryptogams {Marsilea,
Pilularia, &c.), is the shoot-axis traversed by an axial strand of vascular
bundles, a so-called cauline bundle, to which the bundles of the leaves become
attached. Thus, in contrast to the roots, each single vascular bundle of the shoot-
axis is surrounded by parenchymatous ground tissue. If the shoot-axes, however,
form true wood later on, which in fact only occurs in the Coniferse and Dicoty-
ledons, then a so-called Cambium-layer arises, which is developed in part in the

STRUCTURE OF SHOOT-AXES.

45

vascular bundles, in part in the parenchyma between these, and from which true
wood-tissue is produced towards the inside, and so-called secondary cortex towards
the outside. In this way, the originally tender shoot-axes of the plants named become
transformed into solid, persistent, woody stems and branches, from which the leaves
fall later on, and which serve henceforward only as supports for the shoots existing
for the time being In the stems of Palms and Ferns, such a secondary formation

/?■ ■' A-

lis: A corn seen from above, ^ from below, C from tlie side, and
1 of the cataphyllary leaves are seen at /,/./, and at k. k the aNill.-
d leafstem, by its side hk (in C), the bud which will replace it, fr..ni

stem will arise D longfitudinal section through this bud ; ;;, ;/ its cataphyllary leaves ;

/ perianth ; a anthers ; k a bud in the axil of a foliage-leaf

Pig y).—Crri
ring-like lines of i
base of decayed fl(

in longitudinal section. The
,ry buf^ls belonging to them ; b

which a new corm and flower-
l, I foliage-leaves ; h bract ;

of wood and cortex by means of a cambium does not take place. In them, the
shoot-axis is at once from the beginning so thick, and so penetrated by bundles
and layers of elastic fibres, that it is able, with progressive elongation of the
stems, to bear the burden of the leaves arising
at the apex. It would carry us much too far
to go more closely into the anatomical and
exceedingly various relations of organisation
of shoot-axes. The main point for physiologi-
cal consideration is, that, as supporters of the
leaves, and, later on, of entire and often huge
branch-systems, they must possess not only
the necessary solidity, which is afforded
by true wood or by numerous bundles of
elastic fibres; but provision must also be made,
that the water taken up by the roots, ascend-
ing in the woody parts, is not evaporated on
its way through the stem and the shoot-
axes. This is sufficiently provided against
in younger shoots by the epidermis, in older

ones by the cork-periderm, and in the very old ones by the solid bark. Another
function of the shoot-axes is, finally, to convey the substance assimilated by the foliage-
leaves, on the one hand to the root, and on the other to the buds and shoots ; this is
accomplished in part by means of the so-called phloem (soft-bast) of the vascular
bundles, or the secondar\- cortex, in part b}- the parcnchvmatous fundamental tissue.
If the variety of form of the shoot-axis makes itself evident especially in

Fig. 40.— Bud in
removal of the seal
which the scales are
short sheath of foliage leaves.

\ Cefa.

ofbulbof.-;//n,

roa<l, short part of stem on

ed ; / (in A) lannna ; sh the still

lie outer leaves have

46

LECTURE IV.

its anatomical structure and in its formation of wood and cortex, in the leaves
of the shoot, on the other hand, it is especially the outward form which
exhibits in astonishing variety the unbounded formative tendency of vegetable
substance. I may well expect from my readers that the ordinary external forms

^7/e

fed

r\G. \\.~Clematis viticeUa (after Nägeli). End of the
shoot rendered transparent, to show the course of the
vascular-bundles (leaf-traces), which curve out above into
the (removed) leaves.

Fig. 42-— Course of the vascular-bundles in a
monocotyledonous shoot of the Palm type, r
growing point ; j-j shoot-axis ; ** bases of leaves.
(After Kalkenberg.)

of foliage leaves are to some extent familiar to them; that they know the chief
division of the same into lamina, petiole and sheath ; that the term stipule, and,
further, the terms entire, divided, lobed, pitmaJifid, compound leaves, &c., are not
quite strange to them. On the other hand, it accords with the main object of this

FOLIAGE LEAVES.

47

lecture to turn our attention to some other points in the organisation of the foliage-
leaves.

The physiologically important part of any foliage leaf is the blade {lamina),
a lamella, consisting of several layers of tissue, in the typical case however always
very thin, a few tenths of a millimeter thick, in which the cells containing chlorophyll,
the so-called mesophyll, play the chief part. Upon the activity of this layer con-
taining chlorophyll the structure of the foliage-leaves depends.

All other arrangements in a foliage leaf have as their object the presenting of
this very thin la}'er of tissue, extended flat, to the light, of moderating and regu-
lating according to requirement the too rapid evaporation of the water out of it,
of bringing about the flow of new assimilable matters to the cells containing
chlorophyll, and making possible the return flow of the products of assimilation

to the shoot-axis, as well as, finally, of protecting these thin lamellae of tissue against
being torn under the influence of the wind. All the relations of organisation of i^c
foliage leaves are intelligible from this point of view.

The epidermis of foliage leaves is the most completel}- organised which is
found on plants. On their upper and lower sides, it envelopes the thin lamella
of tissue containing chlorophyll : it protects it by means of its elasticity and
solidit}' : its strongly developed cuticle prevents a too rapid evaporation of the
water flowing to the mesophyll : its innumerable stomata, the openings of which
widen or close according to need, allow to the watery vapour arising in the inter-
cellulai- spaces of the mesophyll a passage .out into the atmosphere, regulated
according to need, and at the same time facilitate the entrance of the carbon
dioxide, and, after its decomposition, the exit of the oxygen. Hairy coverings of

48

LECTURE IF.

the most varied kind— prickles, stinging hairs, woolly hairs, and glandular hairs-
protect the leaves against too intense sun-light, against too strong cooling, and
against numerous attacks of insects, &c., according to the biological relations of
the plants concerned. Of great importance to the function of the foliage
leaves, and consequently to the existence of the whole plant, is the so-called
ve7iation. The essential parts of this are the vascular bundles, which, bending out from

Fig. 44. — Several meshes from the leaf of A}ithyllts vitlneraria. 7H midrib ; l\ b secondary cross-veins
radiating from it; a, a a closed mesh; r, c endings of the finest veins. The figure shows only the spiral cells
of the veins ; but the phloem of the vascular bundles runs with these, and the meshes are filled with the parenchy-
matous niesophjll. (Strongly magnified.)

the shoot-axis into the base of the leaf, and running through the petiole when it is
present, become branched in the leaf-blade or lamina. It depends solely and
simply upon the nature of the leaf what the form and importance of its venation
shall be. In the first place, the vascular bundles of the leaf venation have the duty
of conveying the w^ater, laden with nutritive matters, to the assimilating mesophyll,
and of conducting back a p.irt of the products of assimilation into the shoot-a.xis.

THE VENATION OF LEAVES.

49

If the leaves are small, and already rendered sufficiently stiff by means of their
epidermis, the vascular bundles lose themselves within the mesophyll to fulfil the
functions named, and little or nothing of the venation is to be remarked externally.
The venation is the more prominent, however, the larger the leaf surface and the
thinner the leaf tissue. For in this case it is no longer merely a matter of the con-
duction of water and the carrying away of the products of assimilation, as in
small, thick and stiff leaves; it is now, rather, an important ^function, of the
venation to keep the thin lamina of the leaf expandad flat, > much as • t^he ribs
of an umbrella extend its thin covering. This purely mechanical office falls
essentially to the mid-rib and to its stronger ramifications in the leaf: the vas-
cular bundles of these are surrounded by more or less thick layers of succulent
and tightly stretched parenchyma, and their epidermis is further supported by
elastic tissue. These so-called primary veins of the leaves project on the under-
side as strong ribs ; their rigidity (upon which
almost everything depends), is due to strong
tensions between the succulent parenchyma
and the epidermis. By their growth in
length, the veins strive to obtain greater linear
dimensions than the thin lamina of mesophyll
stretched between them ; hence they hold
the latter tightly extended, exactly as the
ribs of an umbrella hold the silk. From these
mechanically effective primary veins which
project on the underside of the foliage leaf,
arise further, as lateral branchings, thinner
vascular bundles, by means of which the spaces
between the primary veins become so con-
nected, that the lamina of the leaf is divided
up into a large number of small and still
smaller fields or areolae. Out of these anas-
tomoses, finally, in the highly organized thin
leaves of the Dicotyledons, ramifications of

vascular bundles arise which become branched within the smallest areolce of the
venation, and at length terminate blindly (Fig. 44).

The venation of thin and large foliage leaves has thus two chief functions.
First, a mechanical one, with the object of keeping the thin lamina sl^f and
extended flat; and secondly, it has the object of promoting the processes of
nutrition in the leaf, since the finer ramifications of the venation permeate the
leaf much in the same way as the smaller furrows of a well-designed irrigation
system run through a meadow in all directions, in order to bring and carry off
water. Even in large foliage leaves, the space is so closely permeated by the ends
of veins, that every square millimeter of mesophyll possesses its canals for bringing
and carrying off: these all run into the primary veins of the leaf, and thence
into the petiole, or into the shoot-axis.

In ordinary very thin leaves, the venation has, however, yet a second purely
mechanical end to fulfil, in addition to the stretching of the green lamina, namely,

[3]

FIG. 4^
hypO!_^lossu

of the leaf-like shoot of Riiscus
ityledon (after Httingshausen).

50

LECTURE IV.

to protect the leaf from rupture. This problem is not satisfactorily solved in all
leaves: the large, sometimes gigantic leaves of the Banana {Musd), Strelitzia,
Ravenala, &c., are provided with so inadequate a venation, that not only in our
inclement climate, but even in their milder native home, they become torn by
the currents of air; so that the broad leaf surface (3-6 m. long and 0*5 to
I m, wide) is regularly slit into a number of lobes and shreds, still held together
only by the powerful mid-rib. From this, innumerable lateral veins proceed,
close to one another, and parallel, and directed perpendicularly towards the
margin of the large leaf, to end there without adequate connection. When
the wind whips the large leaf surface, it tears from the margin inwards, like
an unhemmed flag, and the slits go parallel to the lateral veins up to the mid-
rib. The large majority of leaves, on the other hand, and especially very thin

ones, are protected by the mode of distribu-
tion of their veins, in a remarkably effi-
cient manner, against the so-called shearing
action of the wind, i.e. against the tearing
from the margin inwards. First, it is to
be remarked that the epidermis at the leaf
margins generally undergoes considerable
strengthening ; and that the cuticle especially,
and perhaps elastic fibres as well, usually form
a thick, and often very solid border at the
margin of the leaf. The resistance to in-
cision is again strengthened by the course of
the leaf-ribs in the neighbourhood of the mar-
gin of the leaf. Without entering into too much
detail, or into an exhaustive survey of all the
cases occurring, the subject is still sufficiently
interesting to bring forward some at least of
the most important examples. In this I com-
mence with the most complete arrangements
known to me, such as we find in the large thin
and entire leaves of dicotyledonous plants.
The commonest case appears toi be this. Starting from the strong mid-rib
of the leaf, each of the primary lateral ribs, which alternate right and left, runs
forwards and outwards at an acute angle towards the apex of the leaf, to become
joined, finally, in a curve convex towards the leaf margin, on to the rib next in
front. There thus arises a series of arches, which run immediately within the
leaf margin {Ficus acuminata, religiosa). If the leaf margin is compared say with
a bridge, the lateral ribs, with their arch-like marginal connections, represent
the piers with their arches. In very large and delicate leaves, this arrangement
is strengthened by the formation of a second system of smaller arches, convex
outwards, the piers of which rest on the primary lateral ribs ; or there is
even a third system of still smaller pier-arches, with their convexities lying
towards the margin. The leaf margin is then comparable to a railway viaduct
constructed of two or three storeys of arches ; a comparison which is by no means

VARIETIES OF VENATION. 5 1

a merely superficial or formal one, but completely describes the fact itself,
since this mechanical arrangement of the venation has a similar mechanical
significance to that of the arch-piers of a bridge. We find this double or triple
arch system perfectly evident in the leaves of Tobacco, Catalpa, and the
Tulip-tree; less evident in those of the Rhubarb, and in Asarum (Fig. 43),
and Salix grandi/oh'a (Fig. 46). This arrangement is also well seen in the
individual leaflets of the Walnut-tree, as well as in the large leaves of NymphcBa
alba, floating on the surface of water : and it is expressly to be remarked here,
that leaves floating on or in water must be insured against being torn by the
shock of its waves, in the same manner as aerial leaves are protected against
the wind.

In the large thin leaves of the Dicotyledons we find, however, besides the
arrangement named of a single, double, or triple arch system, quite another form of
distribution of the ribs, in that the side ribs spreading from the primary rib run
out straight to the margin, and terminate there. Out of these spring secondary and
tertiary lateral ribs, which behave in exactly the same way; so that the leaf
margin is met by a very large number of ribs coming from the interior of the
leaf and terminating in it, which are supported beneath the margin by feeble cross-
connections. It is obvious that this arrangement ofl'ers less guarantee for the
solidity of the leaf margin than that described above. Nevertheless, it is often
enough very distinctly developed, e.g. in the foliage leaves of the common Bottle
Gourd, in those of Corylus colurna and maxima, and in the floating aquatic leaves
of Euryale ferox. This form of venation asserts itself much more distinctly in many
Ferns with dichotomously (forked) branched ribs {Scolopendriu7n^ Aspidium spinu-
lostim, Osmunda regalis, Sec). Mixed systems, according to both the principles
named, are also frequently to be observed.

A much rarer form of distribution of the ribs for protection against incision
is found in the magnificent leaves of Cyanophyllum fonnosum, and also in a
monocotyledonous plant — Smilax sarsaparilla. In the lanceolate lamina, close
to the right and left margin, and proceeding from the base of the mid-rib, a
thin rib runs on each side, which is again united with the mid-rib at the apex of
the leaf. Further removed from the right and left leaf margin, a strong rib
then rises on each side, also springing from the base to the apex of the mid-rib.
Starting from the mid-rib, numerous ribs pass across right and left through the leaf-
substance to the two last named, and in like manner from these over to the marginal
ribs; and the fields of the leaf-surface so formed are reticulately penetrated bj
numberless water-veins. This venation, also occasionally occurring in Dicotyledons,
leads us to the majority of the broad monocotyledonous leaves, e.g. of Potamogeton
natans, Ah'smaplanlago, Majatithemutn bifolium, Convallaria latifolia, and others, where,
proceeding from the base of the mid-rib, two, three, four, or more lateral ribs arise,
which run in an arch-like manner to the apex of the leaf, approximately parallel to
the leaf margin. Feebler cross veins then often divide the longitudinal bands of the
lamina thus arising into smaller fields. In the commonest form of monocoty-
ledonous leaves, however, where the lamina forms a long narrow band, as in
the Grasses, Lilies, and Dracaenas, a number of weak ribs run parallel with the
leaf margin and the mid-rib to the apex of the leaf— an arrangement which

52

LECTURE IV.

thoroughly satisfies the requirements in. the face of the clanger of transverse and
longitudinal tearing.

If we now return to the broad dicotyledonous leaves, we find, first, those the
margin of which is coarsely toothed, or variously incised apd lobed. In such
cases as the leaf of the Vine, Petasites albus, and many others, the course of the
ribs resembles the second case described above, e. g. that of Cucurbita. The lateral
ribs spreading from the mid-rib, as well as their secondary ramifications, run
direct to the leaf margin, and generally end in a prominent tooth of the same :
this occurs also in small toothed leaves, e. g. of Viburnum latitana. Inside the
depression between the teeth, in this case, a smaller vein generally ends ; or two

small lateral veins meet here at an obtuse angle.
With this arrangement, also, the arch-like mar-
ginal connection of the lateral veins may again
be mixed.

If we now pass to the divided and com-
pound leaves, the important question is whether
the diff'erent parts of the lamina have each a con-
siderable extent of surface, as in the Walnut, or
whether they are small and narrow. In the
first case, the venation has the same mechanical
function as in large, entire leaves ; in the second
case, on the other hand, as in the finely divided,
doubly and triply pinnatisected leaves of many
Umbelliferae {Coniuvi, Anthriscus\ in the Milfoil
and others, a connected leaf-surface, and a
tearing of it from the edge inwards, are
hardly to be spoken of, and the corresponding
arrangements of the venation dt) not exist. The
same is the case also in the great majority of the
finely divided leaves of Ferns ; where, moreover,
the leaf tissue already possesses considerable
soHdity on its own account.

If the mechanical principles of leaf-venation
here made evident are kept in view, it is always
easy to understand the distribution of the coarser
leaf-ribs, even in cases not here considered. It is further obvious that in
very small leaves, as those of the Lycopodiaceae, most Conifers, and many
Dicotyledons, the mechanical arrangements described would be quite superfluous ;
and consequently they are not present. In the same way it is clear that in very
stiff leathery leaves, as those of the Oleander, the mechanical aspect of the venation
becomes wholly insignificant in contrast to its importance in connexion with the
carrying to and fro of nutritive substances ; and that finally, in the so-called suc-
culent plants, the foliage leaves of which are very thick, and sufficiently solid besides,
nothing at all is to be noticed of the mechanical venation. These considerations,
however, teach us at the same time how fruitful is every conception of organic
forms based on the principle of causality, as contrasted with the merely formal com-

FlG. 47.— Venation of the leaf of C
latifolia (after Ettingshausen).

FUNCTIONS OF THE PETIOLE.

53

pajison of the same. Ettingshausen has in the latter sense described thousands of
leaf-venations, without arriving at any important result whatever; while our simple
principle, according to which the leaf-venation fulfils, on the one hand, the carrying to
and fro of nutritive matter, and, on the other, the mechanical office of keeping the
lamina tighdy extended and protecting it from rupture, affords a very clear insight
into the variety of forms here prevalent.

As regards the mechanical arrangements, however, we have yet to refer to one
point hitherto not touched upon. It is quite a common phenomenon that small
narrow leaves are situated immediately upon the shoot-axis ;
while the majority of large, thin leaves of aerial plants, as well
as of aquatics, are fastened on more or less long and thin
stalks. I see in this disposition a further expression of the
mechanical principle in the organization of the leaves : for
it is at once obvious, that the described arrangements of the
venation against rupture by the wind, or the wave-shock of
water, are necessarily aided when the large thin leaf-surfaces
swing on elastic movable stalks. Storm and wave-shock
hardly affect the lamina when it is situated on an elastic
stalk which yields readily to the pressure of the wind
or water current, and turns so that the leaf resembles a
weather-cock rather than a common flag. If, on the other
hand, the leaf is situated on the axis without a stalk, all
depends upon whether the axis itself is flexible and elastic.
If we now reflect, that an extremely important feature in the
general appearance of plants consists in whether their leaves
are small or large, whether they are entire or divided, whether

they are stalked or sessile, &c., it is obvious that we obtain by means of the
mechanical principle here expounded, a right understanding of one of the most
striking and widely extended phenomena of the vegetable world. And if we ask,
finally, why it is just in the case of the foliage-leaves that this mechanical principle
is so clearly manifested, the answer is already given above : it is simply a matter of
presenting the chlorophyll cell-tissue of the plant to the light in the form of a very
thin lamina, and at the same time of providing that by means of a regulated
evaporation of water out of this lamina the food-materiak taken up by the root
also flow to the leaf ^

' In accordance with the purpose of the present book, I must refrain from the treatment of the
venation of leaves more in detail, although there is much temptation to attempt to refer these
relations of organization, so obvious but hitherto not understood, to a principle so simple as
that indicated in the text. Schwendener (D(7s mechanische Princip ini anat. Bau der Monocoiylen.
Leipzig, 1879) has also concerned himself, it is true, with the rigidity of leaves, so far as it is based
on the anatomical structure of the transverse section, but has scarcely noticed the relations pointed
out by me in the text. Compare also Haberlandt in Jahrb. fur wiss. Bot. Bd. XIII. pp. i6o, &c.
I hope later to establish further the short notices given in the text, in a detailed exposition.

LECTURE V.

METAMORPHOSED AND REDUCED FORMS OF SHOOT OF THE
VASCULAR PLANTS. THE SHOOTS OF MOSSES, ALGJE, AND FUNGL

The further, with progressive division of labour in the body of a plant, the
particular biological r^Ie assigned to a shoot becomes removed from that of the
typical leaf-shoot, the more its external form, internal structure, and irritability also
are modified. A deeper study of these metamorphoses and degenerations of the
shoot might easily lead to a very long series of monographs of the most various
plants; we must content ourselves, however, with a condensed survey of the more
common cases.

Let us first consider some modified forms of sub-aerial shoots, among which
those of the so-called Succulents are distinguished in that, while they possess
massive shoot-axes or leaves, a surprisingly small extension of the surface is
produced : in contrast with the slender, graceful structure of the typical leaf-
shoot, we have thus to do here with stout, massive forms. By means of this
constitution, the succulent plants are enabled to live, even in hot dry air, with a
small supply of water from the earth; of course at the expense of their increase
in body-weight, since the small development of surface allows such plants only a
relatively inconsiderable superficial extension of the chlorophyll, and accordingly
a less plentiful production of organic substance.

Confining ourselves first to the very decided cases of succulence, this may
appear either in the shoot-axis or in the leaves.

In the succulent shoot-axes, the leaves, although formed at the growing point,
usually become partly or wholly arrested later; the developed shoot then consists
only of the leafless axis. This is the case in the true Cactus forms, Echinocacius,
Mamillaria, Cereus, Opwiiia, &c., the shoot-axes of which resemble either roundish
tubers or thick prismatic bodies, on the surface of which the places whence the
leaves have disappeared are marked by tufts of hairs and prickles, situated on pro-
jecting cushions or ridges. Occasionally, the shoot-axes of Cadi also assume flat,
leaf-like forms {Phyllocachis, &c.). In other families, succulent leafless shoot-axes are
found in individual species, the habitats of which are favourable to such an arrange-
ment. Thus we have Cactus-like forms in the family of the Asclepiadacese in the
African genus Stapdia ; and in the genus Euphorbia in a series of species likewise

SUCCULENT PLANTS, AND CLADODES.

55

M

African, as Euphorbia canariensis, globosa, Caput medusoB, &c. The assimilating
tissue containing chlorophyll is, in these leafless succulents, extended in the form of
a very thin layer beneath the epidermis of the shoot-axis.

We find weak, thin shoot-axes, with very succulent leaves, in the family
of the Crassulaceae, and in the genus Mesembryanthemum ; among the Mono-
cotyledons especially in the genus Aloe, and here and there also in other plants.
The leaves in these cases entirely give up their usual expansion as thin laminae;
they become thick, tuber-like, or prismatic, and so forth. The parenchyma con-
taining chlorophyll forms also in them only a thin laj'er extending beneath the
epidermis, while the main mass of such leaves consists of colourless, succulent, slimy
tissue.

In contrast to the Succulents, there are also plants the shoot-axes of which
remain thin, and possess abundant chlorophyll beneath the epidermis, while the
leaves are all arrested as small scales; so that here also, as in the Cactus-like
succulent plants, the shoot-axes themselves undertake the function of assimilation.
Such plants are found in various families of Phanero-
gams ; among the Papilionaceae, e. g. Spariium junceum,
and among the Scrophularinae, Russelia juncea, &C\ The
whole Cryptogamic class Equisetaceae (Horsetails) also
belongs here, as well as the genus Psilolum included
in the Lycopodiacese.

Especially instrucdve, M'ith reference to the relations
between axis and leaf, are the plants which produce so-
called Cladodes. These are shoots, the leaves of which
are arrested as very small scales, while the shoot-axes
themselves assume the flat form of ordinary leaves, to
such an extent that an unpractised observer would un-
doubtedly take shoot-axes of this kind for ordinary
foliage leaves. Among the Monocotyledons the genus
Ruscus from the family of the Asparagineae, among
the Coniferae the genus Phyllocladus from the family
of the Taxineae, and among the Dicotyledons the genus
PhyUanihus from the family Euphorbiaceae, require special
mention.

In such cases, it is generally only lateral shoots of the main axis which assume
the form of thin foliage leaves; it may also occur, however, that all the shopts, eien
if not exactly leaf-shaped, are flattened, broad, and abound in chlorophyll, tnd so
replace the arrested foliage leaves, as in Muhlenbeckia platyclada (Polygonaoeae), and
in Carmichaelia (Papilionaceae).

In the cases of metamorphosed shoots mentioned hitherto, it is essentially
a question of the distribution of the assimilating tissue containing chlorophyll,
either in the leaves or in the shoot-axis itself. The climbing shoots, on the other
hand, belong to quite another category of metamorphoses, in which the relation
of the foliage leaves to the shoot remains essentially undisturbed ; where, how-
ever, arrangements exist by means of which the feeble, thin, shoot-axes, which
arc not able to bear the weight of the leaves, flowers, and fruit, are enabled to

I-IG. if).— Ruscus acultatus (after
Duchartre). a main shoot of the usual
form ; eld the leaf-like lateral shoots
(cladodia) on which the flower shoots
(7?) are situated.

5Ö

LECTURE V.

climb up foreign bodies, usually other plants. In the Ivy, for example, and Ficiis
scandens, this takes place simply by the shoot-axes attaching themselves firmly to
tree trunks or walls, and fastening themselves by means of clinging roots. Much
more perfect is the arrangement for climbing, however, in the foliage shoots of
twining plants, where, in the long drawn shoot-axes, a tendency to spiral curvature is
combined with the irritability for the geotropic influence of gravitation S by which
shoot-axes of this kind are caused to wind themselves spirally close around upright
poles, stems, or branches, and so push their leaf-forming buds continually higher.

Well-known examples of such twining
plants are the Hop, Bindweed {Convol-
vulus), Bean {Phaseolus), and Dioscorea.

We find the most perfect mechanism of
climbing shoots, however, in the tendril-
plants; the leafy shoot-axes of which are
likewise so thin and flexible, that they are
not able to support the weight of leaves,
flowers, and fruits. Such plants, therefore,
just as true twining plants, must climb
upon shrubs and similar supports, since,
if they find no opportunity of doing this,
they become arrested. The climbing
organs, now, are simply the tendrils.
These are long, thin, filiform organs,
which, by the continual contact of a thin
solid body, are induced to wind fast around
it, so that at the same time the whole
leaf shoot is caused to elevate "its apex
higher and higher. Such sensitive tendrils
may be developed in very different ways :
sometimes it is the long thin stalks of
the proper foliage leaves themselves, which
wind around their supports, as in the
Spanish cress {Tropccoluni), Clettialts, Mau-
randia, Fumaria, Solaman jasminoides, &c. In other cases, it is the elongated mid-
rib of the foliage leaf, with or without ramifications, as in Nepenthes, Cobea scandens,
Püum sativum (Pea), and species of Vetch {yicici). Finally, also, metamorphosed
shoot-axes, with entirely arrested leaflets, may occur as irritable tendril filaments ;
thus, the tendrils of the Vine and the Virginian Creeper {A?}ipelopsts) are meta-
morphosed lateral shoots on the nqrmal leaf shoots of these plants. They may in
fact be considered as flower shoots degenerated to climbing organs. To the same
category belong, probably, the extraordinarily sensitive tendrils of the Gourd-like
plants and the Passion-flowers {Passiflorecs) ; while in the genus S})ulax (Monocoty-
ledons) the tendrils are outgrowths of the leaf stalks. I shall return later to the highly

Fig. so.— Apex of a shoot of Akcbia qui>iata, growing
out beyond the supporting rod ; four free spiral turns have
been produced.

* That the theory of the twining of climbing plants proposed by Darwin is wrong, has been
shown by H. de Vries. Compare Arb. des Bot. Instit. I. p. 317, and II. p. 718.

TENDRILS AND THORNS.

57

remarkable phenomena of irritability of the tendrils, and shall only mention here
that these climbing organs may arise by metamorphosis of very different parts of
the shoot ; and this either at the expense of the development of leaves, or without
the latter being essentially affected.

In a certain sense the so-called thorns stand in contrast to the tendrils ; they
agree with them, however, in that they arise from various parts of the shoot,
generally with suppression of the leaf-formation. As thorns we distinguish certain
hard pointed, simple or branched bodies, which serve the plants concerned either
as a protection against the attacks
of larger animals, or as climbing-
organs.

Frequently, thorns arise from
the metamorphosis of foliage
leaves, as in the Barberry [Ber-
beris); or they are remnants of
fallen foliage leaves, as in Isoeies
hystrix, Astragalus tragacantha ;
or the end of a foliage shoot be-
comes transformed into a thorn, as
in Ramnus cathartica; or an entire
branched shoot, furnished with
small leaflets, at first delicate and
soft, eventually hardens to a system

Fig. <,i.—Trofccolum minus. The long
petiole (a, a, a) of the leaf (/) is sensitive to
continued contact, and has so wound itself
round a support and its own stem (rt), that the
latter becomes firmly fixed to the support ; z
ti>e axillary shoot of the leaf.

1-lG. ^z.—IiryoHta dioica. B a portion c.f the stem
from which the tendril arises, together with the leaf-stalk
(b) and the bud (k) ; the lower part of the tendril («) is stiff
(not tendril-like) ; the upper portion (x) has coiled round
a branch. The long intermediate part of the tendril,
between the rigid basal portion (») and the point of attach-
ment (jf), has become spirally coiled, and thus raised the
stem B; luiu' the points whore the direction of the coil is
reversed.

of thorns, as in GlediiscJiiaferox, Iriacajilhus, Sec. Thorns of this kind are shoot-axes,
on which the development of leaves ceases, while the tissue, and even that of the
growing point, is changed into hard, woody, permanent tissue. The older nomen-
clature has hitherto carefully avoided classing together with thorns the prickles,
which agree with them physiologically. These are hard, pointed, short or long bodies,

58

LECTURE V.

developed from various places on the shoot-axes, or even from leaves, and which
cannot be looked upon as metamorphosed shoots or leaves. In the Roses and
Brambles they are known to every one ; among the American speeies of Solanum,
many are remarkable for conspicuous brightly coloured prickles, e. g. Solanum pyra-
catitha and alro-sanguineuin ; many Palms, on the other hand, have very long and
hard ones resembling those of a porcupine.

Among sub-aerial shoots the filiform, often very long runners, which usually
spring from the base of an upright leafy shoot and bear a few inconspicuous
leaf-scales on their long internodes, deserve even if only passing mention here.

Fig. 53-— a potato plant grown from seed. ;- primary root; c/ cotyledons ; //foliage leaves; ** lateral shoots,
with leaf scales c' c' ; tb the tubers at the ends of these shoots (after Duchatre).

These produce, at some distance from the mother-plant, a rosette of foliage leaves,
from which spring, downwards, a tuft of roots, and upwards, flower shoots.
The Strawberry affords a very good example. Runners of this kind are funda-
mentally organs of multiplication, since from each rooted tuft of foliage leaves at
the end of a runner, the filiform part of which perishes later, a new independent
plant arises. Very many other plants behave in a similar manner; their runners
or stolons grow forth as horizontal fibres beneath the surface of the earth, and
produce from their terminal bud a new rooted plant, often far removed from the
mother-plant. This is the case for example in the Umbellifer JIjgopodiu?n poda-
graria, in the common Valerian ( Valeriana oßcinalis), and in many labiate plants

STOLONS AND BULBS.

59

{Siachys, Mentha). With these forms are connected those in which the end of a
subterranean runner does not develope into a sub-aerial leaf shoot at once and
in the same period of vegetation, but swells up as a thick tuber, on which, in
the axils of very small arrested leaflets, buds (eyes) are situated, from which rooted
sub-aerial leafy shoots arise in the next period of vegetation : from these again
arise tuber-producing subterranean runners. We find an example of this, as
well known as instructive, in the Potato,
and the Jerusalem artichoke {Helianlhus
tuberosus).

Many bulbs also arise at the end of
subterranean runners, e.g. those in young
tulip plants. Bulbs are distinguished from
tubers, however, in that, in them, not
the shoot-axis, but the leaves envelop-
ing the leaf-bud become thick, succulent,
and fleshy, and filled with reserve mate-
rials like the tubers. In the majority of
bulbs, however, the buds arising in the
axils of the bulb-scales are transformed
at once into new bulbs, which are
not removed by far - reaching runners
from the parent bulb. This is the case,
for example, in our common kitchen
Onion, the Garlic, Crown Imperial, and
many others. It is evidently an advanta-
geous arrangement when tubers and bulbs,
from which new plants arise in the next
year, are formed at the end of more
or less long runners ; since the newly
arising individuals are thus developed far
from the mother-plant, in a soil which has
not yet been exhausted by the latter.

It is clear that all plants with run-
ners (above ground or below, forming
tubers and bulbs or simply bearing leaf-
shoots at the end) are constantly travelling, ^'""f'^^i p<'"'<'".°f ='«•" bearing the MwRe leaves ;/'/')

■' / O' which ends above in the terminal flower ; ^ ovary ; o anthers ;

and consequently they appear each year >» pe"^'«h ; ^ a lateral bud (young buib) in the axii of the

^ J -J i i^ J youngest bulb-scale; A the apex of the first leaf of this lateral

in places further distant. If such plants ^^^- The lateral bud developes as the bulb of next year, .a

t^ ^ the roots which arise on the fibro-vascular bundles of the base

are particularly well adapted for the soil oft'>'=buib.
and prevailing climate, they may at length

completely occupy wide stretches of ground, since they dispossess all other plants.
Such is the case with the Couch Grass {Triticum repens) and with many Sedges
{Carex), and particularly with the Horsetails (Equisetum) : Equiselum h'mosum,
and paluslre, for example, are able, by means of their subterranean runners creeping
in the mud, to occupy large stretches in bogs and the margins of ponds and lakes,
to the exclusion of other plants. Plants provided with subterranean runners play an

FIG. S4-— Longitudinal section through a germinating bulb
of Ttilipa precox, h brown membrane covering the bulb; *
the part of the stem bearing the bulb-scales (sh. sk); si the

6o

LECTURE V.

important part in nature ; and the dense grass of meadows, as well as the ineradi-
cable weeds, owe their existence in great measure to the mode of life described.

In plants provided with runners, the main shoot for the time being, with its foliage
leaves and flower-stalks, is developed chiefly above ground. It occurs, however, not
rarely that the whole shoot-system of a plant consists of subterranean creeping shoot-
axes, from which only the foliage leaves appear above the earth, in order to form
products of assimilation in the light; these are carried down through the leaf-
stalks, and deposited as reserve material in the subterranean thick shoot-axes. A
very perfect example of this occurs in our Bracken fern (Pteris aqtalina), from
the subterranean shoot-axes of which only a single large foliage leaf from each
bud comes forth each year above ground; similarly also in the Fern Lygodhm,
where likewise only the foliage leaves live above ground, and their mid-ribs climb
like the stem of a twining plant. It is indilferent, from a physiological point of view,
whether the system of subterranean perennial shoot-axes forms a Monopodium
or a Sympodium. Thus, a Monopodium arises when the growing points of the

Fig. 55.— Rhizome of Pteris aquilina. I, II, III the subterranean creeping shoot axes; ss the apex of
one of them ; i, 2 to 6 the basal portions of the leaf-stalks ; 7 a young leaf ; * a rotted leaf stalk, the still living
basal portion of which bears a bud a III. The fibres covered with hairs are roots, which develope behind the
forward-growing apex of the stem.

subterranean system continue to grow, while the annually renewed sub-aerial
shoots bearing leaves and flowers spring from lateral growing points of the
subterranean system, e.g. Herb Paris {Pan's quadrifolia). A subterranean Sym-
podium occurs, on the other hand, when from the growing point of a sub-
terranean creeping shoot there finally arises a sub-aerial foliage shoot or flower
stem, which then again disappears; only the subterranean creeping piece of
the shoot-axis still remains, and a subterranean lateral bud, growing further
in the direction of the previous subterranean bud, again developes at its end
into a transitory leaf-shoot. The subterranean system of shoot-axes then consists
of numerous basal pieces of diff'erent shoot-generations, the apical portions
of which have come forth year by year above the earth, and then disappeared.
We find an excellent example of this in the so-called Solomon's Seal {Poly-
gonahon).

To these aphoristic remarks concerning subterranean shoots, I have to add
finally that these shoots are distinguished as Rhizomes, when, creeping horizontally or

RHIZOMES.

6i

obliquely from the shoot-axes, as is generally the case, they produce numerous roots.
I could scarcely mention a more illustrative example of Rhizomes or Root-stocks
than those of the genus In's, the Flag {Acorus calamus), and especially the common
Asparagus {Asparagus officinalis). The examples named, moreover, offer only
a small selection from the great variety of subterranean forms of shoot. It is for
our purpose, however, quite superfluous to enter further into particulars, since any
one who has rightly understood what has been said will, on careful observation
of any subterranean shoot, easily comprehend the true state of the matter. Con-
cerning this, I may simply remark that one has to guard occasionally against
confounding subterranean shoots with roots; since the former very often assume
externally the aspect of roots. Above all, they contain no chlorophyll : the sub-
terranean shoot-axes are white, brown, or yellow, instead of green. The decisive
point always lies, however, in that subterranean shoot-axes possess leaves, and bear

Fig. cß.—Polygonatii>ii miiltißnruiii. The anterior piece of a mucli longer rhizome, consisting of
four annual growths, A seen in profile, ß from above ; all the adventitious roots have been cut off, their
position being indicated by the roundish scars. The numbers 1864, 1865, 1866, indicate the years in
which the respective pieces of the sympodiuni have been produced.

a leaf-forming bud at the end. Of course the leaves of runners and rhizomes are
usually small, inconspicuous, membranous, or scale-like, and arrested generally,
since the true leaf-nature can only appear when the leaf rudiments develope under
the influence of light as green foliage leaves. That these inconspicuou? sub-
terranean leaves, like the scales of sub-aerial winter buds, are only arrested
foliage leaves, may be in part experimentally demonstrated ' ; since it is possible,
by proper interference with the normal conditions of vegetation, to produce true
foliage leaves from such scales. With respect to Bulbs, it may finally be mentioned
that their stout, soft scales are also leaves, according to their developmental history
— generally entire metamorphosed leaves, as in the Crown Imperial and Tulip.

' That the leaf-scales are only foliage leaves prevented from their normal development, but do
not represent a so-called leaf-formation in the sense of the Braunian Morpholoijy. has been
experimentally demonstrated by (locbcl, 15ot. Zeitg. iSSo, pp. 75.^, &c.

62

LECTURE V.

Sometimes, however, the body of the bulb is composed of the subterranean basal
portions of leaves, the sub-aerial parts of which possess the properties of green foliage
leaves : our common kitchen Onion presents the most familiar example. As may
be readily supposed, subterranean shoots may resemble true roots not only in their
form but also in their functions; since -they, like the latter, take up food from
the earth, and are endowed with irritabilities by means of which they grow through
the substratum in all directions like true roots. We find subterranean shoots of this
kind sometimes covered with absorbing root-hairs, e. g. in the Horsetails. Besides
the horizontal creeping runners, there are others which penetrate obliquely or vertically
downwards into the earth, and behave also in other respects like primary roots. 'This

occurs to a very marked extent in the subter-
ranean shoots of the genus Draccetia; and from
the subterranean shoot-system of Equiseium
spring single branches, which grow down
2-3 m. perpendicularly into the earth. Since,
consequently, shoot-axes can undertake the
functions of roots, the development of true
roots may be entirely wanting in such cases.
There is a whole series of examples known
in this connection.

The genus Psiloium, belonging to the
Lycopodiacece, developes a subterranean sys-
tem of shoots, the leaf rudiments of which
are only to be traced by careful microscopic
research ; while they behave in oth.er re-
spects exactly like roots. Completely rootless
are, further, some Orchideae devoid of chlo-
rophyll (jCorallorhiza, Epipoguvi), the sub-
terranean shoots of which may in truth be
recognised as such at once, but nevertheless
completely undertake the function of roots.
I take this opportunity of remarking that
the genus Salvinia is also completely root-
less; in a manner, however, quite different
from the plants hitherto named. Salvinia is,
in fact, a plant floating horizontally on the
surface of water, ana provided with broad aerial leaves, from the shoot axis of which
long root-like fibres hang down into the water : the latter are, however, neither true
roots nor metamorphosed shoots, but, according to the history of development,
peculiarly modified leaves, which have thus here undertaken the function of roots.

In all the cases considered hitherto, we have been concerned with derived or
metamorphosed forms of shoot of those vascular plants in which at least the main
shoots either produce green assimilating leaves, or are abundantly provided with
chlorophyll in their stems. The departures from the type are in these cases brought
about by division of the physiological labour. The deviations from the typical
forms of shoot proceed much further in plants devoid of chlorophyll, which either

3f Allium cfpa divided lengthwise.
:h remains behind
as a bulb scale after the upper part of the leaf has died
down ; s the membranous part of the sheath ; .; the hollow
-Janiina ; h cavity, and ;' inner side of the lamina ; *■ ligule.

HUMUS-PLANTS AND PARASITES.

'3

take up organic food from the remains of other plants that are becoming
decomposed, or obtain it directly from other living plants, to which they are
parasitically attached. To the first category, which are usually distinguished as Co-
prophytes, but which I, however, prefer to name Humus-plants (saprophytes), l^elong
as well-known examples some of our native Orchids — Neoitia, Epipogum, Corallorhiza,
and also Laihraa and Monotropa. In these the consequence of the want of chloro-
phyll is that the leaves, which are otherwise numerously developed, remain small
and scale-like ; since large leaves have only a meaning and object when they abound
in chlorophyll, and serve as organs of nutrition. Since, thus, the function of assi-
milation is absent, so also is the conveyance of mineral nutritive matters, dissolved out
of the earth in corresponding quantities
of water, wholly or in part superfluous.
Accordingly, such plants have but few
roots ; and, likewise, a vigorously de-
veloped woody body in the shoot-axes is
wanting. The same is also the case with
many Parasites, particularly in the genus
Orobanche, very rich in species, which
though parasitic on the roots of other ■
plants, nevertheless developes some roots
in the earth; for the flower stem shoot-
ing up above the ground must needs
have a certain supply of water, since it
loses at least small quantities of vapour
for months. The species of Cuscu/a
possess after their germination no roots
whatever in the earth : by means of the
numerous haustoria, which penetrate into
the tissue of the host-plant from the long
filiform shoot-axes winding around it,
they extract not only all food matters,
but also the necessary water, the quantity
of which is of course extremely small,
since the leaves are reduced to minute
membrane-like scales. Much more com-
plete than in these cases is the degradation of the shoot-formation in those parasites
the whole vegetative body of which developes in the tissue of the host-plant
until the production of flowers, or, after taking its origin in these, grows forward
below ground. This is the case in the BalanophorccB, the HydnorecB, and
RafflesiacccE, of which I have already mentioned that the organs penetrating into
the nutritive substratum entirely lose the root-form common in the vascular
plants. Up to the time when the flower buds arise, one can scarcely distinguish
the vegetative body of such plants as a shoot. That we have to do with plants
from the group of Phanerogams, is only to be recognised when the flowers' or in-
florescences arise ; and these, in their turn, are to be regardeil even in the typical
jjlants containing clilorophyll as highly metamorphosed shoots. l>ut even the parts of

Fig. s^'—f^alanophora fitiig^osa. 7u the root of the Ivost
plant on which it is parasitic ; the darker veins are woody
bundles wliich run from the root into the tissue of the parasite ;
<7, tyC young flower shoots (natural size).

64

LECTURE V.

parasites of this kind which protrude, from the substratum have, in comparison with
the typical shoots of the vascular plants, something uncommonly strange about them.
They sometimes resemble in their whole aspect the fructifications of large Fungi ;
in a very striking manner, for instance, in the genus Scyhalium. That in all the
saprophytes and parasites hitherto mentioned, it is in fact only the want of chloro-
phyll by which the degradation of all the vegetative organs has been brought about,
is clear at once, if we compare with them the Misletoe and the whole family of the
LoranihacecB. These plants also are parasitic by their roots, as has already been
shown, on the tissues of their host-plants. They are, however, abundantly provided
with chlorophyll, and accordingly their shoot-formation leaves nothing to be desired.
Their broad, dark green foliage leaves, on green, sharply segmented axes, possessing the
anatomical structure of highly organized vascular plants, show that the strange aspect
of the saprophytes and parasites devoid of chlorophyll is simply to be ascribed to the
want of chlorophyll; for the Loranthacese, with their little altered roots, absorb in
the main only water and mineral food-matters from the wood of the trees inhabited

by them, and they are able, by means of their
shoots containing chlorophyll, to produce
organic substances by the decomposition of
the carbon dioxide of the atmosphere. In
accordance with this, the woody body of the
shoot-axis is strongly developed, the water
provided with mineral matters being con-
veyed through this to the assimilating leaves.
From the comparison of the Saprophytes
and Parasites with normal plants possessing
green leaves, we gain the conviction that the
whole vegetative segmentation, the distinction
of root and shoot, the differentiation of the
shoot into leaf and axis, and the production
of wood and other anatomical elements, have
essentially been called forth by the activity of the chlorophyll in nutrition. In pro-
portion as plants strive to present to the light and the air flat plates of tissue rich in
chlorophyll (i.e. leaves) from their shoot-axes, the shoot-axes must also develope
those forms of tissue which serve for the conveyance of nutritive materials
into the leaves and back from them; and, in proportion as this happens, the roots
contained in the substratum must also be capable of absorbing the water lost at the
evaporating leaves, together with the mineral matters necessary to assimilation. The
richer in chlorophyll and the larger the leaves (or the shoot-axes themselves, as in
the Cacti and cladodia), the more perfect also must the roots thus be. It is,
therefore, easily intelligible, that with increasing parasitism, — with the decrease,
and, finally, the entire disappearance of the chlorophyll-contents of the shoots, the
formation of roots also becomes simplified, and finally ceases altogether ; and that when
parasitism is carried to the highest degree, even the difierentiation of root, shoot, and
leaves in the vegetative body finally ceases, till, at last, the relation of plants of this
kind to highl}- organised phanerogams is recognised only in the development of the
flowers or inflorescences. Similar degradations are also met with in animal parasites.

FIG. ^g.—Scybalütm fungifo
Kibes) the root of the host plant ;
niiinerous small flowers densely ci
face ; l< young shoot ; yy"fibro-vas

(after Eichler).

EFFECTS OF PARASITISM.

65

Firmly fixed on or in the animal host, and relatively incapable of movement, the
animal parasite requires only feebly developed feeding and motile organs, or even
none at all, and accordingly no organs of sense. In animal parasites also, with
increasing parasitism, the external segmentation and the anatomical structure proper
to the parasite as a member of a highly organised group disappear more and
more; and here in general, just as in plants, the reproductive organs succumb
less than the vegetative organs to the destructive action of laziness. It is, in fact,
in both cases inactivity — laziness, which distinguishes parasitism, and effects the
degeneration of organs. As an animal which clings by means of suckers during its

Fig. do.—Hydnora Africana. tt a small piece of the subterranean vegetative body of the host-
plant ; out of this spring a mature flower bl and flower buds bi' bl" (| natural size).

whole life on or in another animal can dispense with the various activities which
other animals require for the seizing of their food, and the corresponding movements
and use of the sense-organs; so a plant loses, with the loss of chlorophyll, the
necessity for raising itself above the substratum to the light and of developing all
those adaptations which subserve this purpose.

If we now turn to the more simply organised plants, the ]\Iosses and Algae,
we find, the lower we descend, continually simpler forms of shoot : we have to regard
these, however, not as reduced, but rather as rudimentary — as not yet typically
developed. Numerous transitional forms, however, indicate the passage from the

[3]

66 LECTURE V.

typical leaf-shoots of vascular plants down to the simplest beginnings of shoot-
formation; where, finally, there still remains of the shoot-nature only the rising
above the substratum, the production of the generative organs, and the capacity
for assimilation with the irritabilities necessary thereto.

We shall here refer only to some important points and give a few examples,
since these matters have been already spoken of.

In the true Mosses, and, in the subdivision of the Liverworts, in the foliose
Jungermanniae, we find quite typical shoots, differentiated into leaves and axes,
but anatomically much simpler than those of the vascular plants. We have here

Fig. 6i.— Portion of transverse section of root of dssus. c cambium ; h
The tissue bb belongs to the vegetative body oi Brugmansia Zippelii, parasitii
thin fibres creep in the cortex of the Cissiis root, and swell up here and there into thick cushions (as in
the figure), from which flowers are developed, which then appear externally as in Fig. 17 A (strongly
magnified ; after Solms-Laubach).

always to do with small plantlets, usually growing together in dense swards,
which only vegetate in moist air, and make use of a dried-up condition as a
period of rest. Accordingly, the necessity for the formation of wood and the con-
duction of water, and the arrangement of those anatomical points which bring about
the solidity of large land plants — all those arrangements generally which are only
necessary to large land plants — are wanting. In particular, the foliage leaves of
the Mosses, considering their small size and the life-conditions mentioned, do
not need to form a network of ribs and veins, as we found to be the case
with the large leaves of land plants : or, to express the causal relations more

SHOOTS OF MOSSES.

67

correctly, since the formative forces in the class of Mosses tend neither to the
formation of wood, vascular bundles, and elastic fibres, nor to a great ex-
tension of the assimilating surface, the plants remain relatively small— their mass is
insignificant in comparison with the majority of the more highly organised plants.
Moreover, the most various metamorphoses may occur in the shoots of Mosses, as
in tliose of the vascular plants. They produce runners with small leaves, which

Fig. f>^.—Catharinca undulata, a moss. From the creeping rooted rhizomes are developed
upright leafy shoots, the older of wliicli bear tlie long-stalked sporogonia (natural size ; after
Schimper).

sooner or later become rooted again and form upright leaf shoots, or they produce
tubers, rhizomes, &c. ; these all very small, and of simple cell -structure. A
special peculiarity of the true Mosses, however, lies in the production of a system
of shoots consisting merely of cell -filaments, the so-called protonema ; which
possesses in all essential points the nature of a shoot, without developing the
typical perfection of the moss-shoot. From this protonema are developed, as

68

LECTURE V.

lateral outgrowths, not only roots but also the proper shoots of the Moss, which in
their turn are the bearers of the sexual organs ; the protonema may thus be looked
upon as a simple preliminary stage of the whole vegetative body of the Moss. The
leaf-like plates of tissue, abounding in chlorophyll, which sometimes occur on the

Fig. 6%.—Funarüi liygrometrica. A germinating spores ; w vacuole ; -w root ; s exosporium ; H part of a
developed protonema, about three weeks after germination ; h a prostrate main shoot, with brown walls and oblique '

septa, from which erect branches proceed ; K rudiment of a leaf shoot, with root iv (..4 x 55° i ^ ^ about go).

protonema, but which cannot be regarded as leaves in the narrower sense of the
word {Teiraphis), also deserve special mention. Like numerous similar structures
in the Algae, Liverworts, and even the prothallia of the Ferns, we have here to do
with shoot-formations which, like the cladodes of vascular plants, serve essentially

Fig. 64.— Protoneraatous outgrowth from the root of Mnittnt koritnm, with leaf-forming buds fC,
roots of an inverted sod, from which the protonema filaments (« «) shoot forth (X90) ■

only the purpose of presenting a larger quantity of chlorophyll to the light, and so
aid nutrition.

In some subdivisions of the Liverworts we meet with flat broad shoot-axes,
which, provided with rhizoids on their under sides, either possess leaf-like mem-
branes or entirely dispense with them : this, however, from our point of view, does

SHOOTS OF ALGM.

69

not affect the shoot-nature of these kinds of structure. We have here to deal
simply with flat shoots; which by means of their chlorophyll contents, their rising
above the substratum, and their ability to form reproductive organs, make their
shoot-nature sufficiently evident. Nothing can be more superfluous than the mor-
phological hair-splitting, which sees in organs of this kind anything essentially
different from the leaf-shoots of other plants, simply because the external segmen-
tation into axis and leaf is wanting, a want which is compensated by the flat
extension and the abundance of chlorophyll.

In the endless variety of forms of the Algae, any one who is accustomed to the
great constancy of the typical form of shoot of the vascular plants and higher Mosses,
is struck by the fact that here, even in closely allied subdivisions, the development
of shoots containing chlorophyll, so far as their segmentation into leaf and axis
is concerned, is extremely various. Nature, one might almost say, has here
given free play to the formative forces of vegetable substance, under the simpler
conditions of life which water presents ; while for the higher development under more
difficult relations, such as the life of the
land-plant brings with itself, the typical form
of leaf-shoot has evidently proved itself to
be most to the purpose, and with progres-
sive development of the vegetable kingdom
has been retained almost without exception.
Where in the Algae a sharper segmentation
appears, there the difference between parts
usually extended flat and rich in chloro-
phyll, and thin stem-like parts — that is, the
main difference of leaves and axes — makes
itself evident. This occurs in a very pe-
culiar, and, in comparison with the vascular
plants and Mosses, very strange form, in
the Laminariae. The whole of the peren-
nial plant (Fig. 66) consists of a shoot

rooted below, the lower part of which {s), like a leaf-stalk, bears at its upper
end {b) a lamina, which may be compared to a large foliage leaf. The growing
point of this remarkable shoot is situated at the boundary of the stalk and lamina
(at e) ; it produces annually, to a certain extent by intercalation, a new lamina (b'),
upon which the older one {b) decays away.

Thus, although a great difference exists formally between a shoot of this
kind and a leaf-shoot of the higher plants, it is nevertheless clear that, by means
of the segmentation indicated, the same physiological advantages are attained on
the whole as are provided by the ordinary segmentation into leaf and axis. The
physiological similarity is exhibited still more in the throwing off of the flattened
leaf-like portion, and in the replacement of it by a new organ of a similar
kind. In another also highly organised genus of Algae, Sargassum, we find
forms of shoot which deviate but little from those of vascular plants. From
the growing point of a thin axis arise flat leaves (b), separated by internodes;
and, in addition, portions of the shoot serve as swim bladders (/), and sexual

Fig. 6$.—Afarc/ia»(!a fotymoypha (slightly magnified).
A B young shoots ; C the two shoots, with cupules and
gemmae, which arise from a gemma ; v v the depressed apical
region ; D a portion of epidermis seen from above ; sp stomata
on the rhomboid areola? (more strongly magnified).

70

LECTURE V.

organs (/) are exclusively formed on others — a division of labour which is
elsewhere met with only in highly organised plants. In the species of Fucus, on

the other hand, there is generally
no segmentation into leaf and axis,
and the branched shoots are them-
selves fiat, or even band-shaped,
like the cladodes of many Phane-
rogams. In many other genera
of Algae are found richly branched
shoots without any flat tissue-
plates whatever ; the assimilating
chlorophyll lies beneath the sur-
face of the cylindrical body of tis-
sue (Fig. 68), the physiological
behaviour of which may very well
be compared with the shoot sys-
tem of Spartiuni junceum or of
Psilotum. Again, in other cases
the whole shoot consists of a short
rooted basal portion, on which are
situated leaf-like, thin plates of
tissue (Fig. 69), from the margin
of which similar organs often
grow forth, like the segments
of an Opimlia. I must unfor-
tunately deny myself in this su-
perficial survey, from going more
deeply into the variety of forms
prevailing here ; since just those
organographical relations which
are most important for us could
only be rendered clear by means
of numerous drawings and de-
tailed descriptions. On account
of their importance, however,
mention must finally be made of
those Algae the growth of which
is not accompanied by cell-divi-
sion, and which thus, as one is
accustomed to say, only consist
of one cell : this, however, does
rir,. 66.-La>m„^yia cu„simu. Hot prcvcnt thcsc plants occa-

sionally attaining considerable
magnitude, and, in addition to the formation of roots, the most varied segmentation
of their shoots also. In this sense the genera Caukrpa, Bolrydhm, Vaucheria, and
others (cp. p. 4) have already been referred to.

SHOOTS OF ALGJE.

71

I have previously insisted upon the fact that in the simplest Algae, as in the
simplest Fungi, the segmentation of the vegetative body into root and shoot does
not occur. It has already been mentioned, also, that the accepted description of
the segmentation of more highly developed Fungi into mycelium and fructification,
signifies essentially, from our point of view, nothing further than the segmentation
into root and shoot ; only, we have here to do with plants devoid of chloro-
phyll, in which, as in all other plants devoid of chlorophyll, the shoot is no

FIG. 67.—Srtr£-asstim vuls-at-e (natura! size). Cp. the

Fig. eS.—Ge/iriim

Fig. fx).—Delesseria sangi<iiitn ;
leaf-like shoot, with anchoring root
below.

longer an organ of assimilation, but only the bearer of reproductive cells, as
which, with few exceptions, it also protrudes above the nourishing substratum.
In the large common Mushrooms and Toadstools of the woods and fields, the
stalked pileus, on the under side of which the spores arise, is thus the fructi-
fication, or, according to our view, the shoot reduced to a mere fructification;
the proper root-system being extended in the earth as the mycelium. That
we are here again concerned with innumerable varieties of form, is evident

7 2 LECTURE V.

in the richness in species of the Fungi, and in the great variety of their modes
of life.

In this condensed treatment of comparative organography, I have aimed
essentially at bringing forward only those points which may serve for the under-
standing of subsequent general physiological expositions. To this end we have
regarded the different parts of plants simply as organs, i. e. as instruments for definite
physiological functions; and in particular, only those which, in contrast to the Re-
productive organs, have been distinguished
as the Vegetative organs. This physio-
logical treatment of organography, hitherto
unduly repressed in Botany under the sway
of the morphological school, is moreover
by no means intended to exclude the purely
formal comparison, as it has hitherto been
conducted under the name of morphology;
its effect on the latter is only to be that
of explaining and enlightening. That the
comparison of organic forms from purely
physiological points of view, which are ex-
clusively concerned with the functions of
the organs, can exhaust one side only of
the subject) and that, besides the functional
signification of the organs, phylogenetic re-
lations also exist, which, quite independently
of the mode of life of organisms, concern
their relationship and their descent, requires
in our time no proof The whole of syste-
matic botany and morphology, so far as
they have any meaning at all, have to do
exclusively with this ; and it is just because
this morphological and phylogenetic mode
of consideration of plants has always pre-
vailed in Botany, and now threatens to
overgrow every other interest in plants,
that I have considered it to the purpose to
obtain, by means of the preceding lectures, a more fitting basis for our physio-
logical considerations.

Only this one point I would yet emphasise here. According to my view,
progress in the department of Botany lies in the physiological direction; and
sooner or later, starting from physiological points of view, we shall also come
to understand the proper morphological hereditary properties of the organs, and
the causes of their variations. The purely formal relations of organisms among
themselves also, chiefly disclosed by the history of development, must sooner or
later be expressed as the effects of physiological causes which can be definitely stated.

(m) beset with fructification ; /—T development of the fructi
fication (natural size).

LECTURE VI

ON THE CELLULAR STRUCTURE OF PLANTS. PROTOPLASM.
NUCLEUS. CELL-WALL.

That plants consist of cells is now known to every well-informed man;
yet the true meaning of the word Cell may be quite clear to but few, the less so
since biologists themselves, even now, hold and discuss the most different opinions
upon it. To many, the cell is always an independent living being, which sometimes
exists for itself alone, and sometimes 'becomes joined with' others — millions of its like,
in order to form a cell-colony, or, as Hackel has named it for the plant particularly,
a cell-republic. To others again, to whom the author of this book also belongs,
cell-formation is a phenomenon very general, it is true, in organic life, but still only
of secondary significance ; at all events, it is merely one of the numerous expressions
of the formative forces which reside in all matter, in the highest degree, however, in
organic substance.

Such being the case, it is certainly best to leave every theory aside for the
present, and confine ourselves to the most immediate experience, and to the consider-
ation of a few objects which, without any controversy, consist of cells.

Let us first turn for this purpose to Fig. 71, which represents the transverse
section of the flower-stalk of, a plant closely allied to our common kitchen Onion.
We see at once that its area is occupied by a mesh-work, or is divided into numerous
closed chambers. The walls of the latter consist, as is perceived at once, of solid,
and, as further experience will show, even of extraordinarily solid substance : the
spaces bounded by these contain fluid, and, as we shall see later, other constituents
still more important. The size of the chambers is very various, and so is their
form. From our figure it is at once observed, however, that the chambers re-
sembling one another in size and form are aggregated in layers or groups, arranged
on the transverse section of the organ of the plant.

The individual mesh-like spaces or chambers are the cells : the groups of
similar cells are the various forms of tissue.

An equally thin longitudinal section through the same flower stalk would present
a less evident, and apparently less orderly picture. Careful consideration would,
however, even on the longitudinal section, again enable us to recognise chambers
closed on all sides, and bounded by a net-work of solid Malls ; only in this case the

74

LECTURE VI.

chambers appear more extended in the longitudinal direction (especially those
marked {g) in the figure) in the form of very long narrow tubes.

A careful comparison of the two figures — i. e. the transverse and longitudinal
sections — leads to the conclusion that the various linear networks of the longi-
tudinal and transverse sections are due to the presence of chambers closed on all
sides, the cavities of which are separated from one another by solid walls, just
like the rooms in a large building. The transverse section of a portion of a plant

thus corresponds to the ground-
plan of a building; the longitu-
dinal secdon to the elevation of
the same.

Apart from those Algs and
Fungi which we have already
learned to recognise as non-cellular
plants, we find in all plants and
parts of plants this chamber-like
structure; and we still designate
the single chambers by the same
term which their discoverer, Robert
Hooke (1667), employed for them,
because he was struck by the
similarity of the chambers with
the cell-structure of a honeycomb.

Fig. 71. — Transverse section of the flower scape of Allium Schoaio- vy^ chall r>f rmir<;P tSPP fhnt tVlic
/mx«;«(X30). ^ epidermis ; M cells containing chlorophyll ;»- colourless ^^ ^ ''"''-'^ "^ COUFbC bCC UldL LUli,

oHcTeSyrnr"'' "' p"" P^^^"'^"^""^ = ^ ^' ™='^"'^-- ^""'"« • ^'^ ""«: similarity is an extremely super-
ficial one, although in the cells
of plants, as in those of a honeycomb, living contents become formed, and formative
nutritive materials are stored up.

If we regard the longitudinal or transverse section of a very young portion
of a plant, such as a growing point (Fig. 72) or the transverse section through
ordinary wood or cork, we remark that all the cells fit closely into one another on
all sides ; since between any two neighbouring cells there is always only one solid
wall, like a simple wall between two chambers of a house. This kind of
cellular structure originally exists in every cellular plant, and accords with the
origin of the cells during growth. But very often it happens that in the course of the
further development, the chambers separate partly or entirely from one another, the
simple primary partidon walls becoming more or less split ; and each single chamber
thus becomes bounded by a wall of its own, at certain places, or over its whole
circumference. This case is observed very generally in the tissue of green leaves,
especially in the lower half of the thin lamina. The cells, at first fitting closely on all
sides, have separated from one another during the growth of the leaf, and are now
connected only at isolated points. The case is similar also in the seed-coat of the
Gourd, of which Fig. 73 represents a transverse section. The inner layer of this
seed-coat consists of cells which have eventually become separated from one
another, since through growth they have assumed various irregular forms. In such
cases empty spaces exist between the now independent chambers or cells, which are

ISOLATION OF CELLS. 75

termed intercellular spaces, or lacunae. In the ordinary succulent tissues of Stems,
Roots, Fruits, etc., these intercellular spaces are relatively very small, since the cells
of these portions of the plant, at first fitting closely on all sides, separate a little from
one another only here and there. It often happens, however, especially in aquatics
and bog-plants, that the large cells of the succulent tissue (parenchyma) become
separated from one another in layers, so that large chamber-like cavities arise, which
are separated from one another by simple layers of cells. An excellent example of
this, easily perceptible even with the unaided eye, is presented by the leaf-sheaths
of the species of Musa.

This mutual isolation of cells, resulting spontaneously during growth, may
under certain circumstances be produced artificially, especially when the partition-
walls are thick ; either simply by means of continued boiling in water, or by long
maceration in a mixture of chlorate of potash and nitric acid, or even by slow
rotting. In the excrement of plant-eating animals, also, we find the undigested cells
often completely isolated from one another under the influence of the gastric and

...(-'— rtj

Fig. 72. — Longitudinal section tlirougli the growing point of a winter bud of Abies pectniata (x about joo).
5 the apex of the growing point ; b h youngest leaves ; r cortex ; m pith.

intestinal juices. In such cases, a splitting of the originally simple partition wall
of neighbouring cells is effected by external influences; or a thin layer, the so-
called middle lamella, lying in the thick partition wall, is dissolved. However,
even in the normal course of vegetation, and especially in the formation of repro-
ductive organs, a complete isolation of the cells occurs in such a manner that,
finally, they become emptied out of the reproductive organs in the form of a more
or less fine dust. The pollen from the opened anthers of the stamens of flowering
plants, for instance, consists of such isolated cells ; and occasionally, as in the
Pines and many Nettle-like plants, these are carried away in great quantities by
the wind. The spores of most Cryptogams are further examples. The fine heavy
dust, bought in the apothecaries' shops under the name of Lycopodium, consists
entirely and simply of such spores ; and similarly we can shake out of any moss
capsule, or blow away from a tuft of the commonest mould {Penicillium glaticum),
a still finer powder. If the unfolded pileus of a ripe Mushroom or Toad-stool is
laid with the lower side on paper, it is found, after some hours, that a picture of
its lamellated or tubular underside has been formed, — a figure in dust, which has

76

LECTURE VI.

arisen by millions of small cells (i.e. spores) having become loosened from the

Fungus, and fallen on to the paper beneath.

In the first place, then, and espe-
cially when we investigate anatomi-
cally the more highly developed
plants, the cells appear as chambers,
or as a framework in the substance of
the plant. We see, however, that these
chambers may also become separated
in part or wholly from one another, so
that they finally appear as independent
bodies. In many Algae and Fungi,
(e.g. the Yeast fungus) indeed, all
the cells arising during growth be-
come separated from one another.
Finally I may again refer to the
non-cellular plants or Cceloblastce.
We have found examples of these
already in the genera Catilerpa,
Boirydtum, and Vauchen'a, and have
seen that their substance is not
divided up into cell-chambers. But
every such plant arises primarily by
the growth of an isolated cell, which
may assume the most various forms
later on. A fully grown plant of this
kind may thus, in a certain sense, be
considered as a single cell; on the
other hand, however, it appears in
comparison with ordinary plants as
a non-cellular plant, because in it
the formation of chambers during
growth, on which the cellular struc-
ture depends, is suppressed.

It follows from what has already
been said, and will become still more
evident subsequently, that we must
not imagine the cellular structure of
a plant to be due to cells previously
independent becoming united with
one another to form an aggregate ;
but we must regard the cells as
small portions of the growing plant-
substance, which either remain united
to one another, as is usually the case,

or eventually become separated from one another. According as we keep the

Fig. 74.— Ripe pollen grain of Cichorium Intybus. The almost
globular cell-meinbrane is beset with ridges connected in a iiet-like
manner, each of which bears pectinate series of spines.

CONTENTS OF CELLS. 77

one or the other case in view, the cells appear as mere chambers and parts of
the growing plant-body, or as independent living organisms from which new plants
arise by growth. It depends, therefore, entirely upon our mode of consideration,
and upon the point of departure of our consideration, whether we regard the cells
as independent so-called elementary organisms, or merely as parts of a multi-
cellular plant.

Our consideration of cells has hitherto been particularly concerned with their
external boundary, which is afforded by the solid cell-wall or cell-membrane ; and in
a larger cellular plant these walls or membranes form the solid framework, within
which, in the cell-chambers and partly in the framework itself, the fluid sap moves
and all other substances of the plant are contained. It now concerns us to
make ourselves more closely acquainted with the matters which are contained in
the chambers — in the cells. In very many cases, e. g. ordinary wood, cork, bark,
or even old dried-up pith from shoot-axes (Elder pith), or the tissue of fallen
leaves, &c., we find the cell-chambers empty ; i. e. they contain either merely air, or
at most clear water or a few granules. The same is the case also with the integuments
of seeds and dry pericarps. We have become accustomed to consider siy:h parts
of plants as dead or perished; since experience teaches, without exception, that
when the cell-chambers are empty in the manner stated, the parts grow no more,
that no more metabolism occurs in them, and that no new cells are formed in
them. They are in these respects inactive, physiologically dead, but they may be
nevertheless of great use in the general economy of the plant, as we shall learn
subsequently with respect to wood, elastic fibres, and cork. It is, moreover, a
peculiarity of the more highly developed plants, especially of the so-called vascular
plants, that masses of such dead cells (contributing to the life of the plant however)
become accumulated. In the Mosses, Algse, and Fungi this occurs only incidentally.
In every living plant, however, are found layers, strands, and other aggregates of
living cells; i.e. such in which further growth, cell-multiplication, and chemical
processes of life take place. In highly organised plants, it is the soft cortex of
older parts, the succulent shoot-axes and leaves, the growing points of the shoots
and roots, the flowers, unripe fruits and seeds, which consist of such living cells.
In Mosses, Algse, and Fungi the whole living body of the plant generally is
composed of them.

The contents of the cell-chambers in such living parts of the plant, in
the narrower sense of the word, may be exceedingly various. Very generally
there are found together with a watery fluid, the cell-sap, more or less numerous
starch-grains, drops of fat, small crystals, and in ripe seeds so-called aleurone-
grains. In green leaves, and in other green parts of the plant, green roundish
or polygonal granules of soft substance are particularly conspicuous : these are
the chlorophyll granules, which in many Algae are replaced by green bands,
plates, and the like. All these contents of the living cells when sufficiently
magnified attract the attention at once; and up to about the year 1840 they were
almost the only contents which were noticed in the cell. But on more careful
observation of the interior of the cell-chambers, especially when the granular
structures named are not present in too great quantity, there is perceived in every
living vegetable cell another substance of a very peculiar kind, which generally

78

LECTURE VI.

presents itself as a slimy, or occasionally gelatinous, or sometimes more or less
solid mass; and either completely fills the cavity of the cell, or only clothes the ,
wall as a thin layer, or traverses the "sap cavity of the cell in the form of a
network.

This is the universally known and yet essentially unknown protoplasm, that
substance which, according to the researches of the last forty years, represents
the proper living body of every cell ; and which, as has been gradually proved,

is the actual basis of life of
all organisms, both of animals
and of plants, and the investiga-
tion of which therefore, in our
time especially, is the object
of the Zoologist and Botanist.
In the protoplasm itself, again,
is found the nucleus of the
cell, which generally pre-
sents itself as a definitely
formed part of the protoplasm.
Only one nucleus is usually
present ; but in large, and espe-
cially in elongated cells, several
or many nuclei may be distri-
buted in the protoplasm. More-
over, particularly in the non-
cellular plants among the Algae
and Fungi, hundreds, or even
thousands of nuclei may be
found in the protoplasm, where
formerly, up to the year 1878,
their presence had been alto-
gether overlooked.

From what has been stated,
the protoplasm, with its nu-
cleus, and the cell-membrane
or cell-wall, appear as the
essentials of every vegetable
cell. The other visible sub-
stances are temporary struc-
tures, nutritive materials, pro-
ducts of metabolism, or refuse matters and the like, to which we shall return in
detail when we are considering the theory of nutrition.

For the present, however, we will keep to the . essential ■ constituents of the
vegetable cell— the protoplasm, nucleus, and membrane ; and these we will now
study more in detail.

The Protoplasm, the fundamental significance of which for the whole life of
the plant has been already indicated, consists of albuminous substances, i.e. of

Fig. 75.— Parenchyma cells from the middle layer of the root-cortex of
Fritillayia itnperialis (longitudinal section X 550). A very young cells still de-
void of cell-sap, lying close to the apex of the root ; B cells of the same descrip-
tion about 2 mm. from the apex of the root ; the cell-sap (j) forms isolated drops
in the protoplasm (p) ; plates of protoplasm separate these drops. C cells of the
same description about 7—8 m. m. from the apex of the root ; the two cells to the
right below are seen from the front, the large cell to the left below is in optical
section. The cell to the right above is opened by the section ; the cell-nucleus
shows a peculiar appearance of swelling under the influence of the penetrating
water \x y).

COMPOSITION OF PROTOPLASM.

n

chemical compounds which behave essentially like common egg-albumen, or the
casein of milk, or like the coagulable substance of blood, &c. These chemical
compounds are composed of the elements carbon, hydrogen, oxygen, nitrogen, and
sulphur : they are the most complex of all organic chemical combinations, and
as the result of their decomposition a large number of simpler organic carbon com-
pounds are formed. The statement that protoplasm consists of albuminous sub-
stances is, however, not to be understood to mean that proteid and proto-
plasm are identical. For by the former is meant only the chemical compound
of the elements mentioned; whereas the protoplasm composed of this pos-
sesses a definite organisation, which is foreign to the chemical compound as
such. The case is similar to that of the chemical compound of chlorine and
sodium (common salt) on the one hand, and, on the other hand, of the
crystals of common salt. It is, moreover, onl}' an abbreviation to say that
protoplasm consists of proteids.
This is only its essentially distinc-
tive character ; but, as a matter
of fact, living protoplasm always
contains a larger or smaller quan-
tity of water, and if this is with-
drawn up to a certain minimum
it loses its vital activity, and on
the withdrawal of more water even
its ability to live. The water be-
longs to the molecular structure of
the living protoplasm in the same
sense as the water of crystallization
is necessary to the structure of very
many crystals, which lose their crystal-
line form on the withdrawal of the
water of crystallization.

On examination we find, more-
over, that in the protoplasm mineral
constituents are further contained,
which, after its combustion, remain over
as incombustible ash; a series of salts

in which alkalies, hme, magnesia, phosphoric acid, and sulphuric acid predominate.
Even so far, the protoplasm appears to be a mixture of numerous chemical
compounds; and we have cause to believe, in addition, that continual chemical
changes are connected with the vital processes in it, the products of which remain,
at least for a time, between the molecules of the protoplasm. The substance of the
protoplasm here described appears quite homogeneous under the microscope, even
with very strong magnifying powers. In actively vegetating cells, however, this
substance, in itself homogeneous and transparent, is thickly set with very numerous
small granules (microsomes); these appear even with the strongest magnifying
powers as dots, the true nature of which, from a chemical point of view, and
the importance of which for the life of the protoplasm, are still doubtful, although

Fig. 76. — Forms of the protoplasm enclosed m cells A and B of
Zea Mays. A cells from the first leaf-sheath of a seedling ; B from the
first internode of the same ; C from the tuber of Heliaiilhiis tuberosus,
after the action of Iodine and dilute Sulphuric acid; h cell-wall;
k nucleus ; / protoplasm.

8o

LECTURE VI.

we may assume as probable that they are very finely divided nutritive matters which
are employed in the vital processes.

In the tissue of growing points and embryos, the protoplasm usually fills
up the entire space enclosed by the thin cell-walls, and shares it as yet only
with the relatively large nucleus (compare Fig. 72). As Fig. 75 shows, how-
ever, the protoplasm assumes different forms with the growth of the cells.
As the cells become larger, the protoplasm does not increase in an equal
degree, but there are formed in it cavities, filled with fluid, i.e. with cell-
sap. The protoplasm itself then assumes the form of plates and threads, united

in a net-like manner, which
radiate from a central clump
investing the nucleus, to the
periphery of the cell, there
to pass over into a more
or less thin layer of proto-
plasm, which here clothes the
cell-wall like a hollow sac.
The more the cells enlarge
with progressive growth, the
larger the sap-vacuoles in the
protoplasm become ; until, very
often, there remains of the
latter only a thin lamella
lying on the cell-wall, some-
what as a wall-paper on a
chamber wall. The elonga-
tion of the cells, as we see,
is essentially connected with
the absorption of water, with-
out a corresponding increase
of the protoplasm. In very
succulent parts of plants it
is therefore by no means easy
to see the protoplasm with

FIG. 77.— Stellate hair on the calyx of the young flower bud of Althaa rosea.

the microscope : in the large
parenchyma cells of the root-cortex, the pith and the cortex of vegetating slioots,
in leaves, fruits, and so forth, it often requires special methods of investiga-
tion to bring into view the exceedingly thin protoplasmic utricle which encloses
the large sap-cavity of the cell, and lies close upon the cell-wall. This is
accomplished by means of various contracting media, e.g. dilute solutions of
iodine, alcohol, glycerine, &c., by which the protoplasm is killed, and compelled
to separate from the cell-wall in the form of a pellicle, sometimes exceedingly
thin.

It often happens, however, that, with vigorous growth of the cells, and therefore
with considerable enlargement of their cavities, the protoplasm also becomes
highly nourished and increased in mass. In such cases there are found, even in

MOVEMENTS OF PROTOPLASM.

8i

large cells, considerable quantities of protoplasm, usually so distributed that
a thicker or thinner layer of it clothes the inner side of the cell-wall, while a
more or less massive clump envelopes the nucleus, from which threads or bands
of protoplasm pass out to the wall. This arrangement of the protoplasm is very
easily seen under the microscope, without disturbance to its life, in the hairs on
the epidermis of many plants. If we observe such a living cell attentively
and for a long time, the substance of the protoplasm is seen to be in con-
tinual movement — one of the most remarkable of phenomena, which allows of the

FIG. iS.—B to G protoplasm from a ruptured filament of faiicherza terrestris, slowly emerging in water, and m various
successive conditions, at intervals of about five minutes, h cell-membrane of the ruptured filament ; ;' the portion of protoplasm
still in the filament ; a (in A", C, D, F) a sphere of protoplasm becoming detached, forming vacuoles, and then becoming
deliquescent (in F) ; If a branch of the protoplasm from which the mass d' separates otf. This is isolated in D, and then
becomes deliquescent in F; c and c' behave similarly. G shows the further changes of the portion c" in F ; j4 a recently
escaped clump' of protoplasm, rounded off into a sphere, the chlorophyll granules lying all together inside, and hyaline
protoplasm enveloping the whole as a skin.

immediate recognition of the internal disturbances of equilibrium taking place in the
protoplasm. These movements are rendered visible by means of the finer or
coarser granules distributed in the protoplasm. These are seen to travel from
the cell-wall in the strands of protoplasm towards the nucleus, returning thence by
the same or other strands to the peripheral layer; and here gliding onwards
again, sooner or later to be carried once more through other threads of proto-
plasm to the nucleus, and thence to turn back as before. In many water-plants
the movement of the protoplasm is simpler : it forms a relati\-el\- thick layer on the
[3]

82

LECTURE VI.

inner side of the cell-wall, and is very watery, therefore resembling a fluid, and
moves as a flowing stream along the cell-wall, while with it float the nucleus and
the chlorophyll corpuscles, and incidentally small crystals of calcium oxalate. This
form of movement has been distinguished as rotation of the protoplasm from that
first described, which is termed circulation.

Circulation and Rotation, however, are only special forms of the movements
of protoplasm within closed cells. If cells rich in protoplasm, especially those
of the Coeloblastic Algae, are ruptured under the microscope, the protoplasm
gushes forth out of the opened cell-membrane as a viscid mass, which may
assume the most various forms : these are by no means simply those of a viscous
fluid body, but are produced by peculiar movements within the protoplasm. An

example of this is illustrated in Fig. 78. Here
may be noted the remarkable fact that sepa-
rated portions of the exuded protoplasm, provided
they contain one or more nuclei, may become
surrounded after some time with a cell-mem-
brane, grow, and form a new plant. Such is the
case not only with the Vaucheria represented in
the figure, but, according to Schmitz, also with
some other non-cellular Algae {Siphonodadium
and Vallonid). What happens in such cases by
means of external accidental influences, is con-
stant in the reproduction of very many Algae.
Either the whole mass of protoplasm of their
cells becomes loosened from the surrounding
wall, and emerges through an aperture in the
latter ; or the protoplasmic body is first broken
up into a large number of individual nucleated
portions, which are then expelled from the
has parent cell-membrane. Such free protoplasmic

: fixed and 1

Fig. 79- — C swarm-spore of (Edogc
motile ; D the same after it has becom
formed the organ of attachment; F. exit of all the , ,. , r i a i

protoplasm of a germinating plant in the form of a bodlCS thC SWarm-SpOrCS OI thC AlgBB arC

swarm-spore. (X 350. After Pringsheim. y«/»*. /. r • 1 i • 1 1 n -i- 1 .1

■zviss.Bot.^.T^t.-L.) furnished with more or less fine ciha, by the

aid of which they swim about in water, just
like hifusoria, until they become fixed in some place, surround themselves with a
cell-membrane, and begin to grow. The movement of such protoplasmic bodies,
which are generally ovoid in shape, is two-fold ; they revolve round the axis,
and at the same time travel forward in the water, a motion similar to that of a
planet in space, or of a shot fired from a rifled cannon.

To the most remarkable phenomena in the life of protoplasm belong the
processes observed in a subdivision of the Fungi, the Myxomyceies. During
their vegetative condition, before they develope their spores, these organisms
consist entirely of naked protoplasm (i.e. not surrounded with a cell-membrane)
which occasionally creeps forth in large quantities out of the nourishing sub-
stratum of foliage or tan on to the surface, and is found for hours in
active movement from place to place as a so-called plasmodium. This
phenomenon is easily observed in the so-called 'flowers of tan.' On moist.

PLASMODIA

«3

sultry summer mornings, there issues from old tan in the pits, or in green-
houses and hot-beds, a bright yellow, apparently liquid substance, which collects
in the form of large flat cakes, occasionally pounds in weight and some
centimetres thick, and the surface of which is sometimes smooth, sometimes
covered with numerous branched excrescences. In greenhouses it may happen
that this substance {jEthaliiim sepiicum) creeps up the stems of plants a
metre high and more, in the form of thin threads, and becomes collected
above on large leaves as thick cakes the size of the hand. If large quantities
of tan, in which the yellow clumps of ^thaliwn are already recognisable, are
placed on a dish early in the morning, and covered with a bell-glass and

Fig. 80— a Plasmodium of Didyminm leucopus (after Cienkowsky, X 350). B fructification ai Arcyria incarnata, stiU
closed ; C the same after rupture of the wall / and extension of the capillitium cp (after De Bary, X 20).

allowed to stand in a room, the movements of the JEthalium may be observed
for days together. Occasionally it creeps over the edge of the dish on to the table,
and spreads on this, forming threads and broad cakes with undulating or moss-like
surfaces, until it finally becomes rigid and breaks up into innumerable small
cells. If wet glass slips are placed vertically in the tan already containing the
JEthalium, it creeps up upon these ; and if brought under the microscope the form
and movement of the substance may be more exactly observed (Fig. 80). There
remains no doubt whatever that we have here to do with a structure which resembles
in every detail the circulating protoplasm in living plant-cells, only its mass is re-
latively extraordinarily large.

Besides the internal mobility of these free masses of protoplasm, by means of

84

LECTURE VI.

which they are enabled to creep upwards, they present the striking phenomenon
that sooner or later, when the active external movement ceases, they assume
definite forms (often in the highest degree characteristic, and resembling those
of mushrooms, etc.) and then become rigid; solid substances are secreted on
the surface, and partly also in the interior, while the remainder breaks up into
innumerable round cells. The plasmodium of the Myxomyceles may be con-
sidered as the simplest type of a growing plant; a plant which during its growth
produces no cell-walls at all, even at its outer surface, but is at the same
time able to assume certain simple and characteristic forms. From these relations
of form of the plasmodia up to the processes of growth of the Coeloblastse, or
non-cellular Algse and Fungi, there is practically only one step ; for if we
imagine a plasmodium externally surrounded by a cell-membrane, the latter not

essentially hindering the configuration
of the protoplasm, we have somewhat
the same process as in the growth of a
Plasmodium ; only with the difference, that
by means of the solid outer membrane,
less dependence upon the external world
and a greater individuality of the con-
figuration are attained. If we imagine
further that within a Coeloblast, such
as a Caulerpa, Vaiicheria, Bryopsis, etc.,
with progressive growth, a cleaving of
the protoplasmic mass takes place in the
interior by partition walls placed trans-
versely and longitudinally, we obtain an
ordinary plant, consisting of cell-cham-
bers. The continuation of our descrip-
tion will show that the ordinary structure
of the plant can in fact be understood
F«;. si.-üiaymücm farinaaum. a Myxomycete. The accordlug to this schcmc. Fundamentally,

whole structure at first consists of protoplasm, from which the 1,1 i • i_ 1 • j •

solid fabric here figured is separated out later, while the CVCry plant, hOWCVer highly OrgaUlSCd, IS

remainder (inside the capsule) breaks up into spores (after • 1 j 1 ^ • V If

Rostafinski). The stellate bodies on the capsule are crj'stals. a prOtOplaSmiC body COhcrent m itSell,

which, clothed without by a cell-wall and
traversed internally by innumerable transverse and longitudinal walls, grows ; and it
appears that the more vigorously this formation of chambers and walls proceeds with
the nutrition of the protoplasm, the higher also is the development attained by the
total organisation. It is perhaps impossible to make the significance of the pro-
toplasm for the life of the plant clear in any other manner than this ; although to
those not yet famiHar with the facts it certainly cannot be easy to apprehend the
true meaning of the matters here stated. Nevertheless it was an ingenious thought
of Hofmeister's to put down the apparent creeping motion of the plasmodia of the
Myxoniyceies, and their later transformation into fructification, as the simplest type of
growth, even for more highly organised plants.

In spite of its internal mobility and apparently viscid or slimy consistence,
the protoplasm exhibits occasionally, besides the coarser structure already described,

CHL OR OPHFLL-B ODIES.

a still finer organisation, which is only to be detected by means of strong magnifying
powers. This consists in reticulated fibrillse, or in a system of very small chambers,
the walls of which appear to be relatively more solid and the contents more fluid : as
yet, however, but little is certainly known concerning this very fine structure.

We may regard the chlorophyll-bodies as a constituent of the protoplasm.
These generally appear in the form of roundish or polygonal granular structures of
soft consistence; in many lower Algae they occur also in the form of bands or
plates with similar properties. The chlorophyll-bodies, whatever form they possess,
always lie embedded in the substance of the protoplasm, and usually in a middle layer
of the peripheral protoplasm ; so that they
are separated from the cell-wall itself as well
as from the sap-cavity of the cell, by a layei
of colourless protoplasm. If parts of plants
containing chlorophyll are placed in strong
alcohol, the green colouring matter becomes
dissolved in the latter, and imparts to it a
magnificent colouration, which in transmitted
light appears green, and in reflected sunlight
blood-red. This green colouring matter, dis
tinguished by the fluorescence just indicated
and by other optically remarkable properties, is
however, only a very small part of the chlo-
rophyll-granule relatively to its mass ; for in
cells extracted with alcohol the latter are
found still of their original size and form, but
now colourless. It is easily recognised that
the proper substance of the chlorophyll-gra-
nules is a colourless mass, which was homo-
geneously permeated, wholly or in part, by
the colouring matter. The colourless matrix
remaining behind after the extraction, ex-
hibits towards chemical reagents the essential
properties of proteid substances; and the
origin of the chlorophyll-granules in young
cells, as well as the fact that in many lower
plants the whole protoplasmic body of a cell,
or the greater part of it, behaves like a
chlorophyll-body, leaves no doubt that a chlo-
rophyll-granule is practically a portion of the protoplasm tinged with chloroph}!!
colouring-matter. This, however, does not interfere with the fact that the chlo-
rophyll-granules conduct themselves in many respects like individual independent
organisms inside the protoplasm : for they can not only grow and under certain
circumstances change their form; but they divide also, becoming constricted in
the middle until the two halves separate entirely from one another. B}- means of
such divisions into two — which are easy to discover and to follow in the leaves
of the Mosses, in the cells of the Characece, and in the prolhallia of Ferns, and which

hygy

(a moss). A cell

Fig. 82.— j
of a leaf with protoplasm and chlorophyll-grains em-
bedded in it; B chlorophyll-grains with their enclosed
starch ; b,b', h» a chlorophyll-grain dividing \ f, s ^ chloro-
phyll-grain disorganised hy water.

86

LECTURE VI.

appear to occur very commonly also elsewhere in the vegetable kingdom — the
number of the chlorophyll-granules increases somewhat proportionally to the increase
in surface of the growing cell-membrane.

It may be reserved for other opportunities to treat of the properties of the

chlorophyll colouring-matter, and of the grand
part which the chlorophyll-bodies play in the
vegetable kingdom as instruments of assimila-
tion ; and, finally, of the small starch-granules
commonly arising in them in consequence of
assimilation. Here it was only proposed to
state preliminarily the most necessary facts
concerning the external appearance of the
chlorophyll-bodies.

The cell-nucleus occurs, as already stated,
in the ordinary cells of the higher plants,
only singly in each cell as a rule. In very
long vesicular cells, and in the latex-tubes and
bast-cells, on the other hand, nuclei are present in larger numbers ; and in Algee and
Fungi it happens, when the cells are spacious or very long, that two, several, or
even hundreds and thousands of nuclei (which are then usually very small and
difficult to observe) exist in a cell ^ In accordance with its general behaviour,
the cell-nucleus may always be considered as a peculiarly individualised and sub-
stantially somewhat distinct part of the protoplasm, from which it is commonly
sharply separated in the form of a globular or ovoid mass, often lens-shaped
later, or rarely in the form of a more band-like or vermiform body. In older, still
living, vigorous cells, it commonly appears in the form of a vesicle; or a sphere
filled with granular substance, in which is almost always to be detected a so-
called nucleolus, or two, or even several. So far as the discussion (vigorously

Fig. 83.— Embryos in the embryo sac
they contain very large cell-nuclei, eac
cleoli.

Fig. 84. — Cell-nuclei of Nothoscordptt /i-a^rans (after Flemming). i. resting nucleus ; 2. the nuclein
of- the nucleus arranged in a filamentous coil ; 3. optical section of the coil, where apparently only
granules are visible.

revived of late) concerning the nature of the cell-nucleus allows the forming ot
an opinion, the nucleus consists of two kinds of substances : the main mass,

' More details concerning what has been stated in the text are found in my 'Text Book
of Botany.' With regard to what specially concerns the multi-nuclear cells, at that time (i.e. up
to 1 874) still unknown, cf. Schmitz, Beobachtungen über die vielkernigen Zellen der Siphonocladicueen.
Festschrift der Naturforsch. Gesellsch., Halle, 1879, The same in Sitzungsber. der niederrhein,
Gesellsch. für Natur- und Heilk., Bonn, 1880, June 7, and 1879, August 4. Treub, Sur les cellules
vegetales ä phisieiirs noyaiix, Archives neerlandaises t. xv, Johow, Untosuchtingen über die
Zellkerne in den Secretbehältern und Parenchymzellen der Monocotylen, Bonn, 1880.

THE CELL-NUCLEUS.

87

a watery protcinaccous substance, differs apparently not very essentially from
the surrounding protoplasm. Within this is present, however, a second sub-
stance in the form of granules or filaments, which, according to certain state-
ments, is distinguished by containing phosphorus ^ and appears in a more
definite form especially when, at the commencement of cell-division, the cell-
nucleus itself prepares to divide. These two substances may perhaps be
distinguished most advantageously by the terms nucleoplasm and nuclein. The
latter is especially remarkable in that with colouring media, and particularly
with Haematoxylin, it stains more quickly and deeply than the nucleoplasm;
whence this reagent is also employed to make the nuclei evident, and indeed
generally in very large numbers, in cells where no nuclei at all were detected
formerly. In the larger fully grown cells
of the higher plants, the nucleus appears
generally as an inactive mass ; in cells poor
in protoplasm usually lying on the wall.
Its prominent significance is clear, on
the other hand, in two cases. First, in
the growing point (Fig. 72), wehere the
nuclei almost entirely fill up the space
of the otherwise small cells, so that the
mass of a growing point consists to a
very great extent of nuclear substance.
Yet more striking, however, appears the
nucleus as an essential element of the cell
during cell-formation itself: there results
at this time a more definite separation
of its two constituents, the nucleoplasm
and the nuclein, to which I shall return
later.

The third essential constituent of a
vegetable cell, the cell-membrane or cell- fig. 8s.-/'to-,> «7;,,/»,«: structure of ti,e brown scieren-

11 r 11 ,• 1 .1 chyma in the stem (X ^iTO). ^ a fresh thin transverse section;

wall, forms, as already mentioned, the ex- ^ the lonRltudinal wall between two cells (fresh), a twisted pit-

, 1 i-Ji J C t.\, 11 TV '^^■nsX at the lower end; C transverse section in concentrated

ternal solid boundary of the cell. In its sulphuric acid ;/? longitudinal secti,™ of tl,e wan in sulphunc

, . ., • i. r r ^c\A; « the middle lamella of the wall ; * second shell ; irthird,

primitive state it consists Ot a peculiar innermost shell of the wan ;/ pore canals ; nmnen of the cell.

chemical compound, cellulose, which is

composed of Carbon, Hydrogen, and Oxygen. This substance is remarkable for
its great resistance to the most various chemical solvent reagents ; and its extra-
ordinary solidity and elasticity are of especial importance for the plant. The
sharp outHne and impressive form of the parts of plants, and their great solidity,
though containing enormous quantities of water, depend essentially upon this
property of cellulose. Nevertheless, only the very thin cell-membranes in the
young parts of plants, and perhaps in older parenchyma, consist of true cellulose,
which is moreover always mixed with water and incombustible mineral substances.
With increasing age, and according to the physiological work which the

Zacharias, Über die ehcmisehe Beschaffenheit des Zellkerns, Bot. Zeitg. 18S1, p. 170.

88

LECTURE VI.

cells concerned have to fulfil in the life of the plant, both the chemical and the
physical properties of the cell-wall become changed. In the great variety here
met with, there may be distinguished three chief cases of metamorphoses of the
substance of the cell-wall as of especially frequent occurrence : these are lignifica-
tion, suberisation, and the conversion into mucilage. Lignification, which we find in
the typical form in the empty cells of ordinary wood, but also in many other cases,
is usually associated with a considerable thickening of the otherwise very thin cell-
walls. It is due to the formation of a peculiar chemical compound, which is
soluble in potash solution and also in a mixture of potassium chlorate and nitric
acid, and which causes the lignified cell-wall to become highly coloured when
treated with iodine solution and various other substances (e.g. with aniline
sulphate a bright yellow). It is to this woody substance, the so-called xylogen,
apparently, that the peculiar properties of the lignified cell-walls are to be as-
cribed. It is distinguished above all by its great hardness and elasticity, so
that it absorbs relatively but little water, and consequently increases but litde in

volume when thoroughly wetted, and on
drying up loses but little in volume —
properties upon which the endless varieties
of uses of wood in the arts especially
depend, and which in like manner come
into consideration for the life of the plant
itself Among the most prominent peculiar-
ities of lignified cell-walls (to which, more-
over, I shall return in detail at a later
period) is its capacity of allowing im-
bibed water to move with facility between
the molecules of the substance; upon which,
as we shall see, depends the physiological
significance of wood as an organ for con-
ducting water in transpiring land plants.
If the xyhgen is extracted by means of
proper solvents from the cell-wall, a skeleton of the wall of similar form remains
behind, which then exhibits the ordinary reactions of cellulose.

In a certain sense the suberisation of the cell-wall forms the contrast to its
lignification ; it consists in that in the basis of cellulose another substance,
suberin or cork substance, is deposited. Such suberised cell-walls, of which
common bottle-cork, the skin of potatoes, the outer cortex or periderm of most
young branches of trees, &c. consists, may be somewhat thick ; they are generally,
however, relatively thin, upon which depends in part the compressibility of common
cork. The suberin, together with which are often found considerable deposits of
silica and other mineral substances in addition, confers upon the cell-wall the property
of being able to absorb water only in very small quantities ; and of hindering with
great energy the passage of watery vapour and other gases through the cell-wall. In
a word, the suberised cells have essentially the properties to which common bottle-
cork owes its varied uses, because it allows the vapours of fluids to pass through it
so exceedingly slowly. On this account also every part of a plant the fluids of which

Fig. 86.— Transverse
siliqita, a granular cell
them ; c the so-called inl
swollen portion of the '
strongly magnified).

:tion of the endosperm of C
ntents ; b solid walls sur
cellular substance — i. e. tti
II between each two cell

SUßERISATlON OF THE CELL-WALL.

89

require to be protected against evaporation, is surrounded by suberised cell-tissue ;
as the stems and older branches of woody plants, in the wood of which the current
of water moves.

In like manner the outside of leaves and of young shoot-axes is clothed with
a pellicle, generally exceedingly thin, the so-called cuticle, the substance of which
possesses all the essential properties of suberised cell-walls. While lignificalion

87.— Cells from a leaf of Hoj.r mriiosa
(X 800), showing the striation ; though far less
strongly marked in nature, the stricc are quite
as evident, a optical longitudinal section of the
crossed annular striation ; b external view of the
side where the annular stri.-? cross ; c. ä external
view of the side where they do not cross ; e a
portion of cell-wall where only isolated annular

Fir,. 88.— Cells with brown walls in the stem
of Fleri's aquilina. .1 h.ilf a cell, isolated and
decolorised hy Schulze's maceration ; B a piece
more highly magnified {.< 550)— the fissure-like
pits are crossed, i. e. the fissure becomes twisted
with mcrease of thickening ; / lateral view of a
fissure, which here appears as a simple canal,
since it exhibits the narrow section.

makes the cell-walls predominantly hard and rigid, the}- gain by means of suberisation
in extensibility and elasticity.

The third common form of metamorphosis of the cell-wall, the conversion
into mucilage, consists in a chemical alteration of the cellulose, in consequence of
which it obtains the property of absorbing large quantities of water, and of swelling up
in a corresponding degree, in many cases so strongly that the volume increases a
hundred-fold or more. According as this property is more or less developed, the
mucilaginous change of the cell-wall makes itself evident in a more or less gelatinous
consistence ; and it may even be converted into a liquid slime with water. As a further

90

LECTURE VI.

metamorphosis, we may consider the formation of Bassorin, and finally that of soluble
gum-arabic. This mucilaginous change of the cell-walls may, under certain cir-
cumstances, occur as a diseased condition, as in the gum-formation of Plums and
Cherries ; in other cases, on the other hand, especially when the mucilaginous change
is associated with only a slight alteration of the original cellulose, it appears as a
normal change serving definite purposes of life. Numerous seeds and dry indehiscent-
fruits possess within their epidermis mucilaginous layers of cell-membrane, which, on
being moistened with water, break through the cuticle and surround the seed or the
fruit as a gelatinous envelope. This is the case for example with the seeds of the
Quince, the Flax, Plantago psyllium, &c. ; if a large quantity of the small grains
is moistened with water, a viscid paste is formed, the swollen-up mucilaginous layers
forming a coherent mass.

In the economy of the Algse and many Fungi (especially certain Gastero-
mycetes), and some Lichens (e. g. the Collemacese), the mucilaginous change of the

iW^o

I

Fig. 89.— a parenchyma cell from the
cotyledon of Phaseolus multiflorus isolated
by maceration, i, i the rounded corners
bounding intercellular spaces ; t, t the walls
in contact with neighbouring cells, and pro-
vided with pits.

Fig. go.— Transverse section of a scle-
renchymacell from the root-tuber of Z)«A/iVr
variabilis (x 800). / the lumen of the Cell ;
K pit-canals which penetrate the stratifica-
tion ; sp a fissure by which an inner system
of layers has become separated.

cell-walls plays a very prominent part ; so much so, indeed, that the form and mode
of life of such plants are to a certain extent determined by it.

These chemical alterations of the cell-walls, here briefly described, need by
no means always invade the entire thickness of a cell-wall. It is often only
definite layers or shells of it which are affected by the changes named. The
suberisation or cuticularisation frequently takes place only at the exterior, espe-
cially in the case of isolated cells ; whereas lignification tends to affect the
middle layers of strongly-thickened cell-walls ; while the formation of mucilage may
affect either the middle lamella or any other layer of the cell-wall.

The extremely thin walls of young vigorously growing cells usually appear quite
homogeneous, even with strong magnifying powers. In thicker, and especially in very
thick cell-walls, on the other hand, a concentric stratification is recognised on the trans-
verse section, to which a corresponding marking is again found also in the longitudinal
section. With particularly clear objects it is perceived that this concentric strati-
fication of thickened cell-walls depends upon the alternation of lamellae, some poor in
water and hard, others richer in water and soft, which together make up the cell-wall.

STRIATION OF THE CELL-WALL.

91

Besides these concentric layers there is to be recognised, however, in very many cases
in thick cell-walls, occasionally also in thinner ones, the so-called striation, which is
perceived best with very strong magnifying powers when the cell-walls are seen in
surface-view. The striation appears then in the form of parallel lines, which
generally run obliquely across the object, or travel round the cell-wall in the form
of a spiral, or even in closed circles. In the first case are recognised occasionally,

Fig. 91.— Forms of cells oi Maychantia polymoi-pha (a
Liverwort) with thickenings projecting inwards. A half an
elater from the sporogonium, with two spiral bands; A' 71.
portion more highly magnified ; B a parenchyma cell from
the middle portion of the thallus, with reticulate thickenings
projecting inwards J C a fine root-hair with thickenings pro-
jecting inwards— these are arranged on a spiral constric-
tion of the cell-wall; i? a thicker root-hair— the projections
are thicker and branched, and the spiral arrangement still
more evident.

Fig. 92.— Portion of an annular vessel from
thefibro-vascular bundle of Zed Mays (X 550).
h, li the thin cell-wall of the vessel, on which
are clearly to be seen the boundary lines of the
neighbouring cells ; r, r the annular thickenings
of the wall of the vessel ; y the inner substance
of one of the rings cut through transversely ;
! the denser layer which extends over the ring
on its inner side projecting into the lumen of

the

cell.

with sufficient magnifying power, two systems of striation crossing one another.
Nageli has also referred the striations of the cell-wall to the existence of layers,
alternately richer and poorer in water, which traverse the cell-wall from without
inwards. The whole cell-wall, according to this view, resembles a crystal
cleavable in three directions, the intersecting lamellae of which are alternately
richer and poorer in water.

In addition to this very fine structure of the cell-wall, we have to consider,

92

LECTURE VI.

however, a far more conspicuous coarser structure in the same cells : this is visible
with w^eaker magnifying powers, and makes itself chiefly evident by individual parts
of a cell-wall being much thinner or much thicker than accords with the common
thickness. We are here concerned with what may be termed the sculpture of the
surface. The commonest form of this sculpture consists in that, on cell-walls not
excessively thick, single or grouped roundish areas remain thin during the growth
in thickness of the whole wall, or do not take part in the growth in thickness
generally. We term such areas pits, and the cell-wall itself pitted. If the parts
of the wall lying between the pits grow very much in thickness, the pits appear
no longer merely as thin areas, but as canals running through the thickness
of the wall from the cavity of the cell, and closed exteriorly by a very thin
membrane. The pits lie in the partition wall between two neighbouring cells of
a tissue, and the pit-canals run on both sides so as to meet one another.
The thin membrane which closes the pit of one side forms also at the same time
the closure for the pit of the other side. If, therefore, we imagine the cells not
isolated, but in connection as usual, like the chambers of a house, and separated by

their walls, the pits and pit-canals appear
like holes, through which the neighbouring
chambers communicate ; in such a manner,
however, that a very fine membrane is
always still present in the middle, by means
of which the neighbouring cell-cavities are
in fact separated from one another. It is
easy to see, and is to be insisted upon still
more in detail later on, that the exchange
of sap between neighbouring cells must be
facilitated by means of the pitting; especi-
ally when the partition-walls are of con-
siderable thickness, whereby of course the diffusion movements from cell to cell
would be rendered very difficult without the presence of numerous pits.

Another form of sculpture of the cell-wall, also very frequent, shows itself in
that not thin areas, as in the pitting, but strongly thickened areas stand out
as peculiarly formed parts of the wall, which otherwise remains very thin.
Sometimes the thickened areas appear as cones projecting inwards, or they form
massive rings, which often become loose and fall out on cutting succulent stems
and leaves; yet more frequently, the thickened part of the cell-wall forms a
spiral band, running on the inner side of the thin wall, somewhat like a spirally-
wound wire fitted in a glass tube of suitable width. Very often two or three such
spiral bands with parallel windings are present, and, like the rings, these also often
become loosened on cutting or tearing off leaves and stalks, and may then be
perceived even with the unaided eye as threads of extreme fineness but considerable
length : these are very distinct for example in the leaves of Agapanihus when torn
across. Finally, a common form is the so-called reticulate thickening, which we
may suppose to be derived from the spiral by connections or anastomoses being
established between the parallel spiral bands, so that a mesh-work arises. If we
suppose the meshes of this net-work very narrow, and the thick bands relativel}-

Fig. 93.— Ripe pollen grain of Ciclwi-imn Intybus. The
almost splierical cell-wall is beset witii reticulately connected
thickening ridges, each of which supports comb-like series of
spinose thickenings, projecting still more prominently.

■SCULPTURE OF CELL-WALLS.

93

broad, then the meshes appear as pits, as is clearly seen in the so-called pitted
vessels.

In these cases of sculpture the thicker portions of the cell-wall necessarily
project inwards, when the cells are tightly closed in on all sides by other
cells. In isolated cells, on the other hand, as the pollen-grains of the Phane-
rogams and the spores of the Cryptogams, the thicker parts of the cell-wall project
towards the exterior, and present the most various forms of knobs, cones, prickles,
combs, ledges united into net-works, and so forth. Generally the whole outer surface
of such cell-walls is cuticularised, and the sculptured parts mentioned consist also
of cuticular substance.

LECTURE VII.

DEVELOPMENT OF CELLS.

We have hitherto considered the cells in the condition in which they immediately
present themselves in the living parts of plants under the microscope. The organs,
however, at first themselves microscopically small, grow up later to a considerable
size. This growth must necessarily also affect the cells of which the organs consist ;
and thus, with increasing growth of the organs, the cells also would have to attain
a considerable size if, during the growth itself, a diminution and at the same time
multiplication of them did not take place by division. In general, the growth of
the organs of the plant, especially in its first stages, is associated with multiplication
of the existing cells ; and only in the final processes of growth, where the organs
attain their definitive form and size, does the multiplication of the cells in them cease,
and the cells themselves attain their ultimate form and size.

Thus, to observe the formation of new cells we must not make use of organs
already fully grown; we must rather examine the growing points and the parts
which are becoming elongated. However, although there is no doubt that cell-divisions
are constantly taking place in the growing points, it is still very difficult to observe
this process there. On the other hand, it is relatively easy to see the multiplication of
the cells further distant from the growing points in the parts becoming elongated ;
and moreover, as experience has shown, it is particularly in the development of the
organs of reproduction that the processes of cell-formation may be easily seen.

Above all, it is to be premised that the multiplication of cells is always brought
about by the division of cells already existing ; and that new cells at no time shoot
forth like crystals out of a fluid. Each newly arising cell originates by division
of one already existing ; and very generally this division is a bipartition, i. e. two
daughter-cells arise from a mother-cell from the substance of the former becoming
divided into two equal halves.

Before I enter more in detail into the processes which take place in the bipar-
tition of a cell, it will be advisable to consider the more obvious facts.

Since the cells generally appear as chambers in the substance of the plant, their
new formation or division may also be represented, especially during growth, as a
progressive formation of chambers or compartments. Within the cells already

INSERTION OF PARTITION WALLS. n^

existing and dividing, new partition walls arise, somewhat as if the chambers
of a house were each divided into two smaller rooms, by putting in partition
walls. The formation of cells in growing organs where the existing cells in-
crease in size, gives therefore the impression of each cell being only able to
attain a definite size; and, when it passes beyond this, a division wall appears
by means of which the cavity becomes divided generally into two equal halves.
In most cases these new partition walls can be recognised in the framework of the
older cell-walls on transverse and longitudinal sections as thin lines, which are
usually set at right angles on the older walls, as the accompanying figure clearly
shows. The cells already present grow; and as they grow and the chambers
formed by them become larger, partition walls arise, by means of which the
increasing size of the cells is to a certain extent again equalised, so that in a
given plant, in spite of the progressive growth, the cell cavities never exceed
a certain dimension. The man-
ner in W'hich the new partition
walls are inserted in the growing
cells, gives in a certain sense the
impression of the latter being, so
to speak, cut through, and so
divided each into two portions.
The cell-division appears there-
fore (and this should be especially
insisted upon) as a diminution in
size of the chambers into which the
living plant substance is divided,
following upon growth and con-
ditioned by growth. This mutual
relation of growth and cell-division
is particularly clear if one examines
from this point of view plants of
simple construction, such as the
CharacecB. Fig. 95 represents a
longitudinal section through the

apex or terminal bud of such a plant, which is preeminendy suited, on careful
consideration, to render clear the relation of cell-division to growth and the whole
external and internal configuradon of a plant. From the so-called apical cell (?')
found at the end of the stem, each time this attains a certain height by growth,
a lower transverse disk is cut off; and thus a new cell, a so-called segment,
is formed. This is again divided by means of a transverse wall into two cells
lying one above the other; a lower lenticular one, and an upper one which
resembles a biconcave lens. The former is no more divided on further growth ; and
gradually assumes the forms i" , i", i"" , by which, as seen, it becomes transformed
into a cylinder, continually increasing in length, and constitutes a joint or interfoliar
part of the axis of the Chara. The upper biconcave daughter-cell of the segment
developes, on the other hand, a number of outgrowths around the axis of the stem,
from which the leaves originate ; while the middle portion becomes the node between

FIG. 94.— Epulermis and adjacent cortical parencliyma of the hypocotyl
of Hclianihus annuus, wliicli rapidly increases in thickness at the conclusion
of germination : the darker thicker cell walls are the original ones, the thinner
radial ones those recently developed. The vigorous tangential growth of the
epidermis cells and the cuticle is especially interesting.

96

LECTURE VIT.

two interfoliar parts. These leaves and nodes now continually undergo repeated
divisions during their growth, which follow a quite definite and exactly known law,
both as regards their direction in space and also their sequence one upon the other
in time.

It is not necessary here to enter more particularly into these matters.
We shall be able to apprehend in the main the relation between growth and the
repeated division of the cells into two, if we observe that in our figure the parts

Fig. 95.— Longitudinal section througli the apex of Chara. Each portion of tissue bounded by a thick
contour has originated from a segment, 1. e. from a cell immediately segmented off from the apical cell.
(Partly diagrammatic.)

distinguished by /, b, respectively, always proceed from a segment of the apical
cell ; and that with the increasing growth of the same the number of division walls
contained in them increase also. If we compare, for example, the portion i" h"
included by the thick contours, with the younger portion i' b', we easily perceive
how with the growth of the former, which previously possessed exactly the form and
size of the latter, the cell-divisions have proceeded; and if, further, we take into
consideration the portion z"'' b"\ in like manner enclosed by thick contours, we again
recognise how, with progressive growth, the cell-divisions have proceeded in definite
order also.

The development of the stomata in the epidermis of a leaf (Fig. 96) may serve as

CELL-DIVISION.

97

a second example showing how the division of cells already existing proceeds
with their growth. In Fig. A are represented a number of epidermis cells of a
very young leaf, seen from the outer surface. The chambers {s) are those from
which the two guard-cells of a stoma are formed later. These, as well as the
surrounding epidermis cells, grow, become enlarged, and change their form ;
thereupon new cell-divisions appear, as in figure B, where the older cell-walls are
indicated by thick lines, those most recently formed with thin ones. It is observed
that around each mother-cell of a stoma {s) a number of new cell-walls have
appeared, by which pieces have become, as it were, cut out of the neighbouring
epidermis cells ; so that each mother-cell of a stoma {s) is surrounded by a group
of new cells. Fig. C shows a mother-cell now divided into two guard-cells {s s),
with the group of adjoining cells and neighbouring epidermis cells surrounding

I on the leaf of Commelyna ccxkstis. A and B very young; C nearly mature ; ss (in A and B)
ss (in C) guard cells. In A and B is shown the formation of the neicfhbourmg cells.

them : all the cells already indicated in B have grown further, i. e. they have en-
larged and changed their form, and the cell-walls have become thicker. By
means of the further growth, which now proceeds without cell-divisions, they
finally become fully developed.

It is evident from what has been hitherto said concerning the division of
cells, that each newly produced cell is not only a portion of the mother-cell,
but that even its original form depends upon what form the mother-cell possessed
at the moment of division, and in what direction the division wall was formed.
Very generally the latter happens in such a manner that the division wall is
set at right angles on the walls already existing, and, when necessary to this
end, it presents a corresponding curvature. The newly arising division walls
are therefore commonly not flat plates, but in most cases more or less, though
it may be almost imperceptibly, curved, as seen in the figures. The figures also

[31 ' H-

98

LECTURE VII.

show at once that a certain relation exists between the form of the organ in
which growth and cell-division are taking place, and the direction, curvature,
and general arrangement of the newly arising cell-walls — a relation of which
we shall speak more in detail later on, when considering growth in its relation to
cell-formation.

The relations just mentioned are partly or entirely wanting in the develop-

FlG. 97 —Peziza convexuta. A vertical section tlirougli the
whole Fungus (X 20); h Hymenium, i. e. the layer in which the
spore-producing asci are situated ; j the tissue body which
surrounds the hymenium at the margin (?) like a cup. Fine
filaments proceed from the base and penetrate into the soil.
B Small portion of hymenium (X 550) ; sh sub-hymenial layer of
densely interwoven hyphae ; n—f asci, with spores in various
stages of development ; between are thinner filaments— para-
physes— containing red granules.

Fig. 98.— Zoosporangia of Aclüya (X S5o)-
A still unopened; B the zoospores are escap-
ing; a zoospores just emitted; b membranes
left behind, from which the swarming zoo-
spores (f) have escaped ; c a lateral bud.

ment of reproductive cells, such as oospheres, spores, and pollen-grains : for, in the
majority of such cases, the formation of new cells appears as if it did not depend
upon a mere cutting-through by means of a new partition wall. The cell-
formation gives here the impression of being not a mere division into chambers

FREE CELL-FORMATION.

99

or compartments of the chambers ah-eady present; the new cells appear rather
from the very beginning as more or less rounded, independent bodies, isolated from
the sister-cells.

This difference is especially conspicuous where a large number of rounded
daughter-cells are formed inside a mother-cell, in such a manner that a portion

Figs. 99 and 100.— Fniiki'a cordata. A transverse
section of a young pollen sac before tlie isolation of the
mother-cells (j;«); cp the epithelium (Tapetum) clothing
the loculus ; 7u wall of the pollen sac. ß loculus
after isolation of the mother-cells (sm); ep remains
of Tapetum (X 550. For further development cf. the
following figs.)

Fig. loi.—Fiiiiiia ovnta. Development of pollen
(X 550) VII. The wall of the daughter-cell has absorbed
water and burst : the protoplasmic body forces itself out
through the fissure, and lies in front of it rounded off as a
sphere.

Fig. 102.— S a young pollen cell of Fuiikia ovala.
The ctternally projecting thickenings are still small (in
C they are larger) and arranged as lines connected into a

of the existing protoplasm of the mother-cell remains unemployed, as in Fig. 97.
This mode of cell-formation has long been distinguished from the cell-division
previously described as an essentially different process, as free cell-formation : more
recent researches have shown, however, that between these apparently very
different modes of cell-formation the most various intermediate stages occur, which

lOO

LECTURE VII.

make a fundamental distinction impossible. It is more a matter of an external
difference concerning the form of the daughter-cell, than an essential difference
in the process of formation itself. The chief difference between the so-called
free cell-formation and ordinary cell-division lies especially in that, in the former,
the developing daughter-cells appear not as mere segments of the mother-cell,
but as rounded off individuals; and whether in this a part of the protoplasm
of the mother-cell remains unemployed or not, may be considered as unessential
for the cell-formation itself, although specifically important for its purpose at the
time. We have, for example in Fig. 98, a case of cell-division where numerous
daughter-cells, at first polyhedral but subsequently rounded off, are developed from

V\C. 103. — Althiza rosea. — Division of the pollen mother-cells into four, A — I- ^ /'and G a tetrad. The membrane
of the 'special mother-cell" has burst under the influence of water, and allowed the protoplasmic body of the youn;;
pollen cells to escape. H a fully developed pollen grain seen from without, and magnified to the same extent.

the protoplasm of a relatively large mother-cell ; here the first processes come under
the same category as ordinary cell-division, while the later ones resemble free cell-
formation. On the other hand, we find in the origin of the pollen grains in Fig. loi,
in the ordinary repeated bipartition of a mother-cell, with the simultaneous secretion
of a cell-membrane and rounding ofi' of the mother-cell itself, a case in which the cell-
division presents itself in the form previously described as a development of chambers
inside chambers already existing; only that the chamber-walls, as they rapidly
grow in thickness, round off the cavities enclosed by them. To be sure, there
appears later a very conspicuous difference from the ordinary bipartition of
cells in vegetati\'e growing organs, in so far, that in this case the walls formed in

VARIETIES OF CELL-DIVISION. 1 01

the division are destroyed, and each protoplasmic body becomes enveloped in
a new specially formed cell-wall ; so that finally only these newly formed cell-
walls remain after the destruction of the older ones, while the whole complex of
cells, produced by repeated bipartition, now consists of individual pollen grains
entirely separated from one another; each of which, from its origin and organisa-
tion, however, must nevertheless be regarded as a cell. We may here perceive
clearly how cells, appearing inside a growing organ as mere chambers, arise in con-
nection with reproduction as individual bodies entirely separated from one another.
This process is still more conspicuous in the formation of the pollen grains of most
dicotyledonous plants, of which an example is represented in Fig. 103. Here the
four daughter-cells of a pollen mother-cell arise (at least apparently and so far as
concerns the development of the wall) simultaneously, and not by means of a
repeated bipartition of the existing cell-cavity. After the preparatory stages to be
described later, in which also we can still recognise the principle of bipartition,
the protoplasmic body of the pollen mother-cell becomes so constricted around
the four nuclei produced by repeated bipartitions, that it assumes in the first
place a four-lobed form ; and the relatively very thick wall of the mother-cell
growing towards the centre in the form of ledges so projects, that four chambers
closed off from one another result {E), in which the nucleated masses of proto-
plasm lie. Here then the rounding off of the developing daughter-cells is particularly
clear. It becomes still more so in that now, inside each of the chambers of the
mother-cell (the so-called special mother-cell), a new cell-wall is secreted by each
protoplasmic body, which it retains during further growth, while the chamber-
walls arising before and during the division dissolve and disappear V

The more recent researches in the province of the cell theory have shown
that at the foundation of all these different forms of cell-formation there lies a
common principle ; that especially the first preparatory stages, which are particularly
clear in the behaviour of the cell-nucleus, are everywhere essentially the same ; that
it always depends originally upon a bipartition of the substance of the cell-nucleus,
and a corresponding grouping of the protoplasm around the centres so arising ;
the process being completed by the formation of a new cell-wall.

What has been hitherto stated especially concerns the external appearances, which
are to be recognised clearly enough even with relatively low powers of the microscope.
Only recently, with very strong magnifying powers and new micro-chemical reagents
and methods, a series of processes have been shown to take place in the interior
of the cell-nucleus and the protoplasm, which afford a deeper insight into the
behaviour of the living cell contents during the formation of new cells. We owe
especially to the researches of Strasburger, Flemming, Schmitz, and others, a very
thorough knowledge of the most minute details in the changes of the protoplasm
and nucleus during division^. From the statements of the former, as well as from
those of a number of zoologists, moreover, the very important fact results that the

* More details concerning what has been said in the text up to this point are found in my ' Text
Book,' the fourth edition of which appeared before the new researches on cell-division.

^ Of Strasburger's many works on cell-division, mention need be made only of his last
book, ' Zcllbildung tiud Zclltheibmg,' Jena, 18S0. Further important are Flemming, 'Beiträge zur

102 LECTURE VII.

processes during the cell-formation of plants and animals are in all essential
points alike.

The following description is taken chiefly from the statements of Strasburger,
with which my own observations agree, at least in the points which appear important
to me : the questions still under discussion between the specialists in this province
may here be avoided as much as possible.

The first preparations for the division of a vegetable cell are noticeable in the
changes of the cell-nucleus; and it is in fact in the visible elements of the nuclein
that the first indications and further course of the division of the nucleus are to be
traced.

'In general,' says Strasburger, 'the contents of the nucleus (the nuclein) become
coarsely granular : hereupon the granules fuse with one another into shorter or longer

FIG. 104.— Changes in the ccll-nncleu
of Iris pumila (i). In 9 the division
produced. The dark portions are nuclei

during the division of the mother cell of a stoma
I completed and the two guard cells have been
(X 880— after Strasburger).

filaments, curved in all directions. The nucleoli often remain preserved in vegetable
cells for a long time, much longer than in animal cells : finally they too enter into the
formation of filaments ; and the wall of the nucleus also becomes concerned in the
process. In the rare cases where this does not happen, they disappear, at least at
the poles of the nucleus. When the wall of the nucleus has become, as is usual,
completely absorbed, the filaments lie immediately in the surrounding protoplasm of
the cell, and may in many cases be dispersed somewhat widely in it. The nuclear
filaments (filiform arrangement of the nuclein) are, particularly in round cell-nuclei,
nearly equally distributed through the entire space of the nucleus ; in elongated cell-
nuclei they follow more or less the longitudinal axis. They begin later in all cases to
place themselves parallel to one another. If an elongation of the cell-nucleus in a
definite direction has taken place meanwhile, the filaments become elongated in the
same direction. Two poles become plainly distinguishable in the cell-nucleus. The

Kenntniss der Zelle tind ihrer Lebensersc/ieintingen,' I. and II ; in Archiv für mikroskopische Ana-
tomie, Bonn, Bd. XVIII. and XX. 1880-81 ; and Schmitz, 'Struktur des Protoplasmas und der
Zellkerne,' in Sitzungsber. der niederrhein. Ges. für Nat. und Heilk. zu Bonn, 13 July, 1880.

BEHAVIOUR OF THE NUCLEUS IN DIVISION. 103

more or less linear extended filaments are usually connected at their ends ; now and
again they become connected also at other points by cross bridges. Later, in cell-
nuclei poor in contents (i. e. containing little nuclein), the individual filaments become
drawn together in such a manner, that they form a simple layer of rodlets or granules
in the equatorial plane. If the filaments were fused at their polar ends, this con-
nection is first loosened. In cell-nuclei rich in contents (i.e. rich in nuclein) the
filaments also maintain a considerable length in the stage now treated of, and they
may even reach from the one pole of the nucleus to the other. In nuclear figures
with equatorial bridges, the loops open, not only in the filaments turned towards
the poles but also in those lying equatorially. The equatorial connecting filaments
hereupon resolve themselves chiefly into V-shaped figures placed radially with the
free legs directed outwttrds. The filaments directed towards the poles become
more or less drawn towards the equator and at the same time shortened, or they
maintain their original lengths. They converge somewhat towards the poles,
or run almost parallel to one another, or curve, even spreading' strongly out-
wards from one another. In the formation of the nuclear figure described, the
whole stainable substance (the nuclein) of the cell -nucleus is demonstrably
employed^.'

Strasburger distinguishes this everywhere as thenuclear disc. This consists
therefore in the simplest case of a simple layer of granules or of straight parallel
rodlets. These elements are visibly separated from one another; and, looked at
from pne of the nuclear poles, a radial arrangement is sometimes to be discovered,
sometimes not. The rodlets of the nuclear disc (the nuclein rodlets) may, how-
ever, also be of more considerable length, and at the same time be curved in an
irregular manner, spreading very much towards the exterior. On both sides
of the nuclear disc, in the great majority of cases, delicate fibrillse are visible
which (with colouring media such as hsematoxylin) are generally stained only
faintly or not at all. These are the so-called spindle strias or fibrillas of
Strasburger.

' These form together with the nuclear disc (the described arrangement of
the nuclein) the nuclear spindle. The spindle fibrillse show themselves most beau-
tifully and plainly when the nuclear disc is limited to only one equatorial layer of
granules or rodlets. They are so much the less visible the greater the extension
of the nuclear disc (i. e. again the nuclein rods) towards the poles.' The so-called
spindle fibrillse of Strasburger are, according to his view, formed from the substance
of the surrounding protoplasm, or they belong, as we may w-ell assume, to the
ground substance of the cell-nucleus, the niicleo-plasma. ' Since now the formation
of the spindle fibrillae,' says Strasburger, ' starts from the two poles of the cell-
nucleus and proceeds towards the equator, the continuous spindle fibrillae, which
can be traced between the elements of the nuclear disc (the rodlets of nuclein) from
one pole to another, can only have originated by fusion of the ends meeting one
another. Other spindle fibrillae join the elements of the nuclear disc (the nuclein
rodlets) on both sides.

■ 1 The staining of the nuclein is brought about especially by means of haematoxylin, with the
aid of picric-acid, alum, and other reagents. Compare the works mentioned in note 2, p. loi.

104

LECTURE VII.

' The division of the nuclear disc (which consists of nuclein rodlets) is ac-
complished in the equator, and both halves separate from one another. Elements
which lie in the equatorial plane, or run through it, undergo division. With granules,
rods, and rodlets, this takes place simply by constriction. If the nuclear disc
consists of accumulated granules or rodlets, one part goes over on the one side, the
other part on the other. The process is more complicated in nuclear discs which
present filaments placed equatorially. These form mostly two or more legged
figures, with the ends of the legs turned outwards. The figures become doubled

Fig. 105.— Cbanpes of the cell
yyagrans ; 4 — 8 from Allittin odon
magnified).

The dark portions ;

-3 from the endospern
the Nuclein (after Fie

ling : !

into two similar ones, or now pass over to the corresponding daughter-nuclei in
such a manner that their fused ends are directed towards the pole, their free ends
towards the equator.'

' Diuing the development of the daughter-nuclei,' proceeds Strasburger, ' they
are usually nourished at the same time from the surrounding protoplasm, that
they may grow up to the size of the mother-nucleus. This may be followed very
well in Spirogyra, on account of the free suspension of the nucleus in the cell. All
the protoplasm collected at the polar side of the daughter-nucleus, and here forming

FORMATION OF THE CELL-PLATE AND SEPTUM. \0$

a layer of considerable thickness, is finally consumed by the young daughter-nuclei,
which increase in size accordingly. An absorption of the protoplasm of the cell,
collected together at the poles of the nucleus, can also be clearly made out in
the staminal hairs of Tradescantia!

After these changes have occurred in the nucleus and protoplasm, the forma-
tion of the new partition walls begins, and I will describe this also according to
Strasburger's statements. ' That may be regarded as the commonest form of cell-
division in the vegetable kingdom,' he says, 'which is accomplished by means of
a partition wall originating in the connecting fibrillae between the nuclei. The
few filaments which finally remain behind between the separating halves of the
nuclear disc, and which are to be referred to spindle fibrillae, become elongated
and increased in number by the deposition of new cell protoplasm, which becomes
like them differentiated into filaments. The newly added filaments are not to be
distinguished from those originally present : they react like these, and consequently
again support the view that the spindle fibrillae are cell protoplasm.' Strasburger
understands by the expression ' cell-plate,' a plate-like arrangement of small granules
from which the new partition-wall arises. He says it is difficult to decide as to
the chemical nature of these granules.

' So much is certain,' says Strasburger, ' they enter direcdy into the formation
of the cell-wall. There is thus probably not a layer formed of protoplasm, which
subsequently becomes split up and secretes cellulose at the separating surfaces;
but the cellulose wall probably originates direcdy from material conveyed to the
spot. I have been able to establish in the living Spirogyra that the granules destined
for the formation of the partition-wall wander as such to the place where they
are employed. In other (the most numerous) cases, on the contrary, the granules
appear to be formed at the spot, there and then.

' It is especially striking that they are at first small, gradually becoming larger.
The connecting fibrillae usually become laterally extended so far, that they spread
over the entire transverse section of the cell. Where this has happened, the cell-
plate also extends across the whole cell. The cellulose wall is formed simul-
taneously from this, and fits close on the wall of the mother-cell at the periphery.
Where the complex of connecting fibrillae is not able to extend completely across
the cell, it attaches itself first to one side wall of the cell, and, fitting close
to this, the formation of the partition-wall out of the cell-plate begins. From the
developed wall, however, the complex of fibrillae becomes slowly withdrawn,
growing at the same time at its free edges by the continual formation of new
connecting fibrillae ; and within this the cell-plate becomes completed, until it
extends completely across the cell. These diflferences are conditioned by the
size of the lumen of the cell, in relation to the mass of the protoplasm. In the
cases where the cell-plates at once traverse the lumen of the cell, or at least but
few movements are executed to this end, the cell-nucleus also lies approximately
in the middle of the cell : where, however, the cell-plate has to extend across the
lumen progressively, the cell-nucleus lies on one side wall of the celi, and becomes
divided in this parietal position. Only after the foundation of the cell-wall is a
connected layer of protoplasm produced on both sides of it. In this numerous
granules are often found still embedded, which have not been used up in the

io6

LECTURE VII.

formation of the cellulose wall. The connecting fibrillse hereupon become indis-
tinct; they sink together to a structureless mass of plasma, which is withdrawn
to the wall of the cell, or they fuse to several coarser fibres, or they simply dis-
appear in the surrounding protoplasm.'

The processes of cell-division hitherto described may be considered as typical :
but, under certain circumstances, more or less deviating forms occur.

Passing over isolated cases, we may turn at once to the processes which have
hitherto been distinguished as essentially different from those above described,
under the name of 'free cell-formation.' Apart from the cases already mentioned
in cryptogamic plants, it is especially in the formation of the endosperm in the
embryo-sac within the ovule of the Phanerogams that the so-called free cell-
formation occurs. The opinion was for a long time held, that in the protoplasm
of the embryo-sac a great number of cell-nuclei arise simultaneously and inde-
pendently of one another, as it were
like crystals out of a mother-liquor,
and that around each of these a por-
tion of protoplasm collects which then
becomes enveloped with a cell-mem-
brane. According to Strasburger's pub-
lications, however, the numerous cell-
nuclei visible in the protoplasm of the
embryo-sac, also arise by the division
of one originally existing nucleus; and
these nuclei then multiply rapidly by
bipartition. ' Between the freely mul-
tipl}ing nuclei,' says the observer men-
tioned, ' connecting fibrillse are visible
in the usual manner. In some cases
these disappear very quickly, without
increasing; in others they grow but
imperfectly; in yet others, however, the
connecting fibrillae are seen to become considerably increased as in ordinary cell-
division, and a cell-plate appears in them.

'Between the formation of free endosperm and the formation of numerous
coherent cells, a distinction no longer exists. If, for instance, numerous spores are
to be developed in a sporangium (cf Fig. 98), or numerous oospheres in an
oogonium, or, finally, numerous spermatozoids in an antheridium, the nuclei in
most cases are seen in the first place multiplying freely by bipartition, and arranging
themselves at approximately regular distances; and then separating layers appear,
by means of which the mass becomes cut up into as many divisions as there are
cell-nuclei. Each nucleus then occupies the middle of a cell. Connecting threads
are here not to be observed.'

The departures from the above described typical form of cell-division, and
especially of nuclear division, go still further in many Algae and Fungi. Thus,
Schmitz observed in the older cells of the Characece, in which the most various
kinds of nuclear division are to be found, that the nucleus breaks up into two

Fig. io6.-C(
the nuclei produced by
AgrimoHia eupatoriun
irfied.)

ng development of partition-walls between
divisions in the embryo sac of
(After Strasburger— very highly mag-

THE NUCLEUS AND CELL-DIVISION. 107

nuclei by an annular constriction; or there is formed in the interior of the nucleus
a fissure, which extends towards the exterior, and so leads to the division
of the old nucleus. It is not necessary here to enter further upon other cases of
division of the cell-nucleus described by Strasburger and Schmitz; however, the
fact discovered by the former, that a fusion into one of previously distinct nuclei
occasionally occurs, may be mentioned here. On the other hand, according to the
statements of Schmitz, it may also happen in very simply constructed filamentous
Algae {C/iroococcacecE, Oscillaricc, Noslocacect,) that a properly defined nucleus is
not to be demonstrated at all in the protoplasmic bodies of the small cells. Granules,
which are characterised as nuclein by their reactions towards haematoxylin, are
found distributed in the whole mass of the protoplasm of the cell.

When the cell-division runs its course in the typical manner above described, the
behaviour of the cell-nucleus easily gives the observer the impression that the impulse
to cell-formation proceeds from it ; and that the division of the protoplasmic body and
the later origin of the cell-wall are caused by the activity of the nucleus. There
are, however, facts which show that, on the one hand, cell-nuclei may be repeatedly
divided without a corresponding division of the cell ensuing, as is the case in the
large-celled Algae with very many nuclei : on the other hand, again, cell-divisions
(i.e. divisions of the protoplasmic body with subsequent formation of partition walls)
may also appear in the same plants quite independent of the division of one or
several nuclei. The above described tj'pical process of cell-division is thus only
the ordinary case, and occurs in higher plants almost without exception. It must
not be concluded from this, however, that the described changes of the cell-nucleus
are the cause of the division of the cell itself: we have here rather a coincidence
of two processes, which in other cases may appear separately — the division of the
nucleus, and the cell-division itself.

There are yet a few other remarks of general importance to be added here.
Above all, is to be mentioned the fact that the whole protoplasmic body, with the
nuclear substance contained in it, may, under certain circumstances, begin a new
life. This, again, happens more frequently in the Algge. The protoplasm &c.,
becoming loosed from the already existing cell-wall, contracts by driving out
water, rounds off, and leaves the cavity of what had hitherto been the cell to
swim about for a longer or shorter period as a swarm-spore, in the form of
a naked protoplasmic body: finally it becomes fixed, and, after the formation
of a new cell-wall, developes into a new plant. Also the case must be mentioned
where, by the union of two cells hitherto foreign to one another (or at least com-
pletely separate from one another), a single new cell is formed; two protoplasmic
bodies containing nuclei fuse together, and represent a single body, which now
becomes surrounded with a cell-wall, and sooner or later grows forth into a new
plant. If the two fusing cells are of equal size, and generally similar, the process
is termed ' conjugation ' : this is the normal form of reproduction in many Algae
and sorhe Fungi. Conjugation is, however, only the simplest form of sexual repro-
duction, the essence of which consists in that two nucleated masses of protoplasm,
which are not in themselves capable of any further development, furnish by their
fusion a product which is capable of development. In its typical form, however,
this sexual act consists in a relatively large cell, formed only of protoplasm

io8

LECTURE VIT.

and nuclein but without a cell-wall — the oosphere, taking up into itself very small
movable corpuscles, the spermatozoids, and being thereby impelled, by the secretion
of a cell-wall, to constitute itself into a true cell surrounded with a membrane, from
which a new plant now proceeds. This is the case in all Mosses, many Algae, and
all Vascular Cryptogams. In the Phanerogams, however, the oosphere is incited' to
further development by a substance passing over into it from the fertilising pollen-
tube. According to Strasburger's statement it appears that this fertilising substance
is essentially the nuclein of the pollen tube. Zacharias and Schmitz are of opinion
that in the Cryptogams the fertilising spermatozoid originates from the nucleus of
its mother-cell ; so that fertilisation appears to be essentially a matter of the trans-
ference of the substance of the cell-nucleus, especially of the nuclein, out of the

Fig. lo-j. — yaiic/icyi'tr xessilis (X 30). A exil of a swarm-spore {sp) ; B, C its germination ;
D germinating plant devoid of a root; ir', Folder plants with roots {7t'} ; s/> the original spore
still visible as a swelling ; o£ and /t (in /") sexual organs.

male organ into the oosphere \ A further pursuit of this topic, however, belongs
to the theory of sexual reproduction, to be treated of later.

Finally, with reference to the facts presented hitherto, we have still to cast a
glance at the non-cellular plants, the Coeloblasta. These plants are often of
considerable size, and develope roots and shoots, even leaf-forming shoots, fructi-
fication, and sexual organs, without their internal substance being converted by cell-
division into a system of chambers. A plant of this nature, as we have already learnt
from several examples {Caulerpa, Bryopsis, Vaucheria, Mucor), is a much branched
vesicle, the various protuberances of which represent the difTerent organs; roots,
shoots, leaves, sexual organs, sporangia, and so forth. The wall of the vesicle is
a cell-membrane, and its contents protoplasm and cell-sap ; and, with reference solely

' On the nature of the spermatozoa, see Zacharias. Bot. Zeitg. 1881, Nos. 50, 51,

CCELOBLAST^ OR NON-CELLULAR PLANTS. 109

to this condition of affairs, the whole plant may be considered as a single cell, or
as a uni-cellular plant. It affects the fact little, that under certain circumstances, at
particular places of such a plant, transverse septa arise in the vesicle. The main
point is, that growth, i.e. the increase in volume and external configuration, is
not here accompanied by corresponding cell-divisions as in other plants. So much
the more remarkable appears now the fact, established by Schmitz since 1878,
that in the protoplasm of these CceloblastcE numerous, even hundreds and thousands
of cell-nuclei are contained, which, with the advancing growth of the plant, are
multiplied by division, and obtain a definite arrangement within the protoplasm ;
thus behaving in a certain measure as if it were simply that the corresponding
partition-walls are not formed. This impression obtains in importance still more
in that, according to Schmitz, the cell-nuclei are most numerous at the growing
point of the vesicle, thus behaving as in the growing point of a cellular plant.
Nevertheless, it is not at all probable that the CaloblastcB are degraded or reduced
forms — i.e. we cannot well assume that they have arisen from proper cellular
plants by the cell-division, assumed as formerly existing, having ceased to develope
partition-walls. Be that, however, as it may, the fact remains for physiological
consideration that in the protoplasm of these plants a certain quantity of nuclear
substance (especially the nuclein characteristic of it) is distributed in formed
portions and at small intervals, and is especially aggregated in the growing
points. From this fact, we obtain once more, as we have already obtained from
other sides, a certain insight into the true significance of the cellular structure
of plants. We need only imagine in a not too complex cellular plant
(a higher Alga, a Moss, or even a vascular plant) that in the substance enveloped
by the outer walls of the epidermis, the cell-walls are simply wanting; whereas
the protoplasm, with the cell-nuclei distributed in it, behaves essentially just
as if these cell-walls were present. Thus we have, on the whole, the structure
of a Cceloblasi. On the contrary, we need only imagine the inner cavity of
such a Cceloblast to be divided up by numerous transverse and longitudinal
partition-walls into very numerous small chambers, each of which encloses one
or several of the cell-nuclei present, and we should thus have an ordinary cellular
plant. It is however very easily intelligible that not only the solidity, but also the
shutting off of various products of metabolism, the conduction of the sap from place
to place, and so forth, must attain greater perfection if the whole substance of
a plant is divided up by numerous transverse and longitudinal walls into cell-
chambers sharpl}' separated off from one another.

LECTURE VIII.

FORMS AND SYSTEMS OF TISSUE : EPIDERMAL TISSUE AND
VASCULAR BUNDLES.

By the term ' cell-tissue ' is designated in general the cellular structure of
plants. In particular, however, we understand by a form of tissue, a layered, fibriform,

or other mass of cells, which, in their
growth and other physiological rela-
tions, present a certain agreement,
and are distinguished from the other
neighbouring masses of tissue. It
commonly happens that several forms
of tissues are again connected with
one another ; so that they constitute a
whole of definite physiological cha-
racter. Such a union of tissues is
termed a ' system of tissues.' Before
entering more closely into the descrip-
tion of tissues and systems of tissues,
however, it is perhaps desirable to
learn how the expression ' tissue ' in
general has been introduced into
vegetable anatomy, and later also
into the histology of animals. In
this expression we have in fact to
do with a historical curiosity; since
it is clear from what has been already
stated, that the cellular structure of
plants possesses very little resem- '
blance to what are usually called
tissues, such as linen, cloth, or
Brussels lace, &c. Nevertheless, the
expression cell-tissue depends upon an erroneously assumed resemblance of cellular
structure to the things mentioned. One of the first founders of vegetable anatomy,

Fig. io8.— Transverse section of the shoot-axis of Setaginella ina-
q)(ali/olia. The epidermis and several layers of external tissue pos-
sess dark-coloured thick cell-walls; the fundamental tissue, with thinner
walls, envelopes three fibro-vascular bundles, which are separated from
it by large intercellular spaces (/).

THE COMMON WALL OF ADJACENT CELLS.

Nehemiah Grew (1682), believed that he had discovered that the cell-wall frame-
work of plants consists of exceedingly fine fibres ; and that the cellular structure
itself is to be compared to a great number of pieces of Brussels lace arranged
upon one another in layers. In this sense he spoke of a cell-tissue {contextus
cellulosus) of plants. Although the erroneous character of this view has long been
known, the name has still been preserved, and even transferred from vegetable
anatomy to animal histology. This need not be wondered at, since a very great
number of other scientific terms date likewise from early erroneous conceptions.

I shall in this lecture only attempt to expound the most general principles
of tissue formation ; and in doing this, I shall again first deal with the typical
forms of tissue, leaving many abnormal tissue structures to be mentioned
later.

As will be remembered from what was said in the last lecture, the cells of
a cellular plant arise by the intercalation of new partition-walls in the cell-cavities
already existing; and as the cel-
lular bodies grow, new partition-
walls are repeatedly intercalated in
the cell-cavities, in various direc-
tions, and in definite order as
regards time. It follows im-
mediately from this, however,
that it is utterly incorrect to re-
gard the cells of a growing plant
as structures originally free, and
only fusing with one another later.
This erroneous view, which phy-
totomists entertained in earlier
decades, must necessarily lead to
the further, but also unfounded

assumption, that the partition-wall between each two neighbouring cells consists from
the beginning of two separate lamellae ; and accordingly a cement was further believed
to be necessary, by means of which these wall-lamellas of neighbouring cells were
stuck fast together. This cement was called the intercellular substance. The organ-
isation of the partition-walls, especially in cells with thick walls, appeared thoroughly
to confirm this view. This has long been given up, however, since we now know
that each new partition-wall between two neighbouring cells of a tissue appears
as an immeasurably thin solid membrane; and that, from the mode of origin of
the latter, as described previously, not the remotest reason exists for considering this
membrane — the new partition-wall — as consisting of two lamellae. If later on, with
progressive growth of the tissue-cells, the thickened partition-wall nevertheless
presents itself as a double lamella, or as already mentioned becomes split into two
lamellae so that spaces arise between the cell-chambers, this is simply due to
a subsequent splitting of the originally simple partition-wall into two lamellae, and
is not a proof that the partition-wall was composed, in the first instance, of two
such lamellae.

If the partition-walls between neighbouring cells attain a considerable thickness

Fig. 109.— Transverse section through the soft parenchyma of the stem
of Zea Mays, o-iv partition wall common to each of the two cells ; •: inter-
cellular spaces produced by the splitting of this wall.

12

LECTURE VIII.

by means of subsequent growth, we observe between each two chambers (as Fig. no
shows) not a double wall, but three layers. The cavity of each one of the
neighbouring cell-chambers is in fact enclosed by a more or less thick layer;
while between the two lies a middle layer, which usually behaves differently
towards chemical reagents. This is either capable of swelling to a great extent, or
is very strongly lignified or cuticularised. I distinguished this layer fifteen years
ago, in order to exclude every theoretical expla-
nation as to its origin, as the middle lamella of
the thickened partition-wall of adjoining cells.

As one of the most general results of the his-
tological researches of the last four or five decades,
it is remarkable that the whole internal structure
even of the most highly organised plants arises
from cells in the manner described. The older
phytotomists were by no means of opinion that
all the histological elements of highly organised plants
could be considered as cells : they distinguished,
rather, three chief forms, viz. cell-tissue, fibres,
and vessels. Fibres are the long-drawn elements
of the bast and wood running to a point above
and below ; vessels, again, are long prismatic or cylin-
drical tubes which run particularly in the wood.
It was one of Mohl's greatest services to demon-
strate that the fibres, as well as the vessels and
all other elements of the plant, arise by the pro-
gressive growth and changes of form of primitive
ordinary cells. For as, according to what has
been already said, the leaves, roots, and such
organs are constructed in the form of outgrowths
from the growing-points of the higher plants; so
also all the various forms of tissues arise from
the embryonal-tissue of the growing-point. In this
we find, in fact, very small thin-walled cells, all
alike, and entirely filled with protoplasm and cell-
nucleus : these cells are constantly multiplied during
their growth by the intercalation of new partition-
walls. The youngest organs which grow forth
from the growing-points themselves consist of such
embryonal tissue; and in proportion as the young
organs increase externally in size and complete
their configuration, the embryonal tissue in the interior also undergoes further
changes. The tissue, quite homogeneous at first, becomes differentiated into layers
and strings of various nature; and with the growth of the organ itself, this
internal differentiation into various forms and systems of tissues proceeds also,
until in the fully grown organs the typical tissue forms have attained com-
plete de\'elopment. It is not my object to describe in detail this progressi\'e

FIG.

of thickened

cells, showing the middle lamella (m); i the
whole mass of cellulose lying upon this middle
lamella; / lumen of the cell from which the
contents have been removed. A from the cor-
tical tissue of the stem of Lycopodium cliama-
cypariisHs ; B wood cells from the inner wood of
a young fibro-vascular bundle of Helianthus
ajtnuus; C wood of Piiius sylvestris, st a
medullary ray.

S y STEMS OF TISSUES. II3

differentiation of the forms of tissue out of the homogeneous embryonal tissue
of growing-points and embryos; it suffices for our purpose to understand the
forms and systems of tissues in their developed condition. Even in this, however,
I confine myself to the most necessary points. It only concerns , us at first to
describe the tissues and systems of tissues generally distributed in the vegetable
kingdom : the description of all those histological matters which subserve special
physiological functions may be deferred until I have to speak of the latter themselves.
We might otherwise at this period easily deteriorate into a dry formalism and a
wearisome systematic arrangement, which will best be avoided by treating of the
various points when discussing their physiological significance.

If we imagine an}- plant whatever consisting of cell-tissue, and li\ing in water,

FIG. III.— Fructification o( HfMiis ßavüiiis in lon-
gitudinal section, and slif;htly magnified, st stipes ; >tii
pileus ; /ty hymenium; f velum ; h tlie cavity beneath
hymenium; y^ continuation of the liymenial layi

Fig. 112.— Transverse section of the stem o( Br_
eitm {X 90). 7u root hairs, produced by the
nvths of single cells of the external layer.

the

s; ht the ren.ovable yelk

pi!

earth, or air, the first thing demanded of its organisation is an efficient shutting
off" of the tissue-masses from the surrounding world : an external layer of cells, or
according to circumstances several such layers, obtains greater solidity and other
peculiarities, by means of which this shutting off is effected. Thus there arises
in the first instance the differentiation of an epidermal tissue as distinguished from
the inner mass of tissue; and it need hardly be added that the contrast between
the two is the more marked the more important is the actual shutting off from
without. In the roots of land-plants and in the shoots of submerged water-plants,
where the whole surface sub.serves the absorption of nutritive matters, the formation
of epidermis will obviously be less conspicuous than in shoot-axes and leaves
which live in tlie air, and have to be protected above all against the loss of
[3] • I

114

LECTURE VIII :

water by evaporation; and if true roots here and there come into the same
position, as is the case with the aerial roots of epiphytal Orchids and many
Aroids, in these also the development of epidermis is more pronounced than in
ordinary roots.

If a plant consisting of cell-tissue attains a certain size, and a sharper dif-
ferentiation of root and shoot, and of assimilating leaf-like portions in contrast
to the shoot-axes results, the demand arises in the plant for channels through
which, on the one hand, the matters absorbed by the roots may be conveyed to
the organs of assimilation, and, on the other hand, the organic substance there
produced may be conducted to the remaining parts of the body of the plant.

Fig. 113, — Apex of the shoot of Tradescatttia
albiflora rendered transparent in order to show
the course of the vascular bundles, i, 2, 3, the
cut-off leaves. (After De Gary.)

FlC. 114. — Transverse section throuj::h a young internode
of the shont axis of Tyadesca}itia albißoya. b, b vascular
bundles. The large cells are parenchyma of the fundamental
tissue. (After De Bary.)

This is usually accomplished by means of fibriform arrangements of elongated
cells, running from the roots through the shoot-axes to the organs of assimi-
lation. In lower stages of organisation, as in the Algae (e.g. many Flon'decp)
and Mosses, this stops short with the differentiation of elongated cells, which differ
otherwise but little from the surrounding tissues. In higher stages of organ-
isation, on the contrary, i. e. in all vascular plants (the higher Cryptogams and
Phanerogams), however, these undergo a very peculiar and complicated develop-
ment, as strands of tissue serving as channels for the sap, and constitute the vascular
bundles or fibro-vascular strand.«.

THE THREE SYSTEMS OF TISSUES. TI5

Thus, with a little consideration, and bearing in mind the most general vital
conditions of the plant, we find tliat there is a triad oi" tissue-systems which
compose the bodies of highly organised plants; or, put better, three systems of
tissues arise in these plants by differentiation. These are the epidermal tissue, the
fibro-vascular strands or vascular bundles, and the remaining tissue, which I have
already distinguished from the other two as the fundamental tissue. It has
recently been attempted, it is true, to resolve these three systems of tissues
into a greater number of different forms of tissues; in this, however, either quite
subordinate physiological points of view, or even purely formal morphological con-
siderations have been started from. I find, after all that has been said in this
direction in the literature of the Jast six or eight years, no ground whatever for
giving up the classification into the three systems of tissues named, which is, moreover,
almost universally accepted ; and so much the less, since all other histological con-
siderations can be harmonised with these three systems in a perfectly unconstrained
manner. This is possible on the one hand even with very highly organised vascular
plants, where, at least in the young states of the organs, epidermal tissue, vascular
bundles, and fundamental tissue are at once prominent as the three essential systems ;
on the other hand, I do not know how this triple division is to be evaded in
the Mosses, especially in the more highly organised true Mosses, as well as in the
more highly developed Algoe. The last named subdivisions of plants present, rather,
very fine examples of rudimentary forms of the three systems of tissues. If we
take into consideration, finally, the forms reduced or degraded from the highly
developed types, such as phanerogamic water-plants and parasites, it again results
that all other differentiations of tissues fall completely into the background as
secondary matters in contrast to those here brought forward.

As in the consideration of the external segmentation of plants, I shall here
also introduce in the first place the typical highly developed forms, in their most
important relations of configuration, and then come back to some rudimentary
and derived or reduced forms.

We find the typical forms of the three systems of tissues, however, developed
chiefly in the axes and leaves of aerial shoots, and the following statements are
in the first place intended to refer to these.

The Epidermis is the superficial tissue of the }'ounger shoot-axes and leaves, not
yet altered by the requirements of an advanced age ; since in true roots a specially
organised epidermis can but rarely be spoken of, and even in submerged
water-plants the outer layer of tissue is often scarcely to be distinguished from
those lying deeper. In its typical form, the epidermis is thus found on the
leaves and shoots exposed to air and light, and is but little modified even on
those underground. Physiologically understood, the epidermis is, as already
mentioned, the la}er of tissue sharply marking off the organ concerned from
the exterior, and its essential properties chiefly amount to this, — to hinder the
evaporation of the fluids contained in the cell-tissue, and to prevent injurious
influences entering from without; while, on the other hand, through more or
less numerous stomata, a means is afforded of discharging the aqueous vapour
formed in the interior, according to need, and at the same time of facilitating
the entrance and exit of carbon dioxide and oxygen gas. We may regard

I 2

ii6

LECTURE VIII.

b' A

this as the proper function of the epidermis ; though it is b}- no means gain-
said that the most various other functions are required of it according to circum-
stances.

Let us first consider the arrangements of the epidermis serving for exchision^
It consists usually of a single layer of tabular, or more rarely of columnar cells, the
lateral walls of which are in close contact on all sides; so that the layer usually
constitutes a very thin but, in relation to its thickness, extremely firm membrane
formed of cells, which on leaves and young shoot-axes can in many cases be
stripped off over wide areas. The external wall of the epidermis cells is usually
of considerable thickness, and is occasionally so thickened that the cavity is
relatively insignificant. In rarer cases it is the wall of the inner side which
presents the strong thickening. The contents of the epidermis cells are usually
devoid of chlorophyll grains, even on leaves and shoot-axes abounding in chloro-
phyll ; yet not unfrequently, especially in Ferns
and some Phanerogams, and particularly those
growing in the shade, the protoplasmic lining
of the wall also contains chlorophyll grains;
and in submerged water-plants the chloro-
phyll contents of the external cell-layer (repre-
senting the epidermis) may be even particu-
larly abundant. The shutting off, already more
or less completely ensured by the thickening
of the outer walls, is however considerably
promoted by means of a pellicle, usually
very thin but sometimes thicker, which runs
continuously over the outer surface of all the
epidermis cells, and is distinguished as the
Cuticle. The substance of this thin pellicle,
the properties of which agree essentiall}'
with those of cork, differs from cellulose :
it is dissolved by alkalies, but withstands
concentrated sulphuric acid (which liquifies
cellulose), and its most important property consists in that it is permeated only
with difficulty by water, thus hindering the entrance and exit of that liquid.
Hence the cuticle is especially thick in leaves and shoot-axes which have need
of considerable protection against the evaporation of the water of the cells, as on
the leaves of ever-green plants and of succulents (e.g. Agave, Aloe, Firs, and
species of Cachis). Very frequently the protection afforded by the cuticle is
enhanced by the cutin or cuticular substance being also deposited in the outer
walls of the epidermis cells themselves, and forming in these the so-called
cuticularised layers of the walls of the epidermis cells. As a further step towards
the shutting off from the outer world, and especially against the entrance of
rain-water and dew, we may regard the formation of wax, partly in the sub-

Fig. 1 15.— Epidermis of th.
aqiti/oliiim. A transverse sec
without ; a, b the very thick cu

niidril>of the leaf of//<-.v
ion ; H surface view from
icle, with its processes /'' ;

■ wall of the cells of the epidermis.

' The most thorough description of the epidermis is found in Da Bary's classical Work, '/'
yleichende Anatomic der J'cgi'tatioiis-or^^a/ie der r/iaiicrogaiiicii und Ferne,' Leipzig, 1S77.

DEPOSITS OF WAX.

117

Stance of the outer wall of the epidermis and the cuticle itself, partly on the
outside of the latter. These waxy deposits, first exactly investigated by De Bary,
appear in the form of exceedingly small granules on the surface of the cuticle,
or as fine rodlets, or lastly as continuous, occasionally very thick, crusts of wax,
which, in virtue of their fatty nature, do not allow water to adhere to the
epidermis; so that rain and dew hanging on the shoots usually form round
drops. If such leaves clothed with wax are dipped in water, they exhibit in it
a silvery-glancing layer of air, and after withdrawal, it is found that they have
not been wetted by the water, but are dry.

By means of this arrangement, the internal tissue is protected against the
loss of those substances which woukl
difi'use out through surfaces coming di-
rectly in contact w^ith water — a point
to which I shall return at another
opportunity. It is obvious, by the way,
that the arrangements just described are
wanting in subterranean and submerged
shoots which absorb water and nutritive
materials from without.

Although the leaf-surfaces are pro-
tected in the manner stated against the
entrance of water from without, as w-ell
as against the unlimited evaporation
of the water of the cell-sap; there yet
remains on the other hand the necessity
of letting aqueous vapour escape according
to circumstances from the assimilatory
surfaces into the atmosphere, in order that
fresh water containing nutritive materials
absorbed by the roots may follow on into
the organs of assimilation; and it is like-
wise necessary to provide passages for
the entrance of carbon dioxide as well as

the exit of the oxygen gas produced by its decomposition. We find these in the
so-called stomata of the epidermis. The more vigorous the transpiration, and,
consequently, the more copious the absorption and decomposition of carbon
dioxide by means of the chlorophyll-corpuscles, the more numerous in general
are the stomata. Hence it is pre-eminently on the surfaces of the green organs of
assimilation, the foliage-leaves, that they occur in enormous numbers ; while they are
always wanting at the roots, and are only found on subterranean and submerged
shoots incidentally and in small numbers. On the sub-aerial shoot-axes and other
organs they are more or less numerous according to circumstances. The extra-
ordinary minuteness of the stomata, in connection with the properties of the cuticle
which clothes them, prevents water under ordinary circumstances from entering
by capillarity, and likewise prevents dust, fungus-spores, &c., from penetrating
their openings; while an adequate exchange of gases and discharge of aqueous

Fig. 116.— Epiderni'
subjacent, of the hauin
cane). A internode ; B
surface. (De Bary.)

and layers of tissue immediately
if Sacchariim qfficiitarum (Sugar-
de— showing wax formations on the

ii8

LECTURE VIII.

FIG. 117.— Transverse section of the leaf of Püuis
Pinaster (X 800). s guard-cells of a stoma (/), under
which is the air-cavity /; 7' entrance, surrounded by the
epidermis cells, the cuticle (r) of which is very thick ;
a the middle lamella, and t the thickening layers of the
sclerenchyma cells ; g cells of the leaf parenchym.i, with
the contents contracted.

vapour can always be provided for by a corresponding increase in the number
of these fine openings, and by the plant at the same time possessing the power
of enlarging or diminishing the width of the stomata according to requirement.

The extreme narrowness of these openings,
even at the maximum, is shown by the
statements of Mohl, according to which the
very large stomata of various species of Lilium
may be widened up to -^l-^ mm. at most, and
those of Indian corn {Zea Mays) up to yig mm.
at most ; while Unger gives the size of the open
fissures of Agapanihus umbellaliis as 0-000047
sq. mm. This extraordinary minuteness is
compensated by the large number of the
stomata. On the foHage leaves, they amount
per sq. mm. in the most frequent cases to
40-100, very often 100-300, occasionally even
600-700 and more; so that the epidermis
of a foliage leaf of any considerable size is
penetrated by many millions of these fine
openings. — In accordance with the object of
the stomata above indicated, they are funda-
mentally nothing but the mouths of the inter-
cellular spaces of the internal tissue which
are developed pre-eminently in the organs
rich in chlorophyll ; and the stomata are
also again only intercellular spaces between
peculiarly formed cells of the epidermis, the
so-called guard-cells. Every stoma, where
it penetrates the epidermis, is surrounded by
two peculiarly formed 'cells, the guard-cells,
the partition-wall of w'hich, originally simple,
has become split, and so formed the inter-
cellular space or pore of the stoma. The
figures here annexed show the condition of
things sufficiently for our purpose. If we
strip off the epidermis from a foliage-leaf,
and spread it out beneath the microscope,
the peculiarly formed pairs of cells which have
proceeded from one mother-cell, and have
produced the proper stoma by the splittmg of
their partition-wall, and thus surround it, are
easily recognised. Very often, especially in the
highly organised Phanerogams, another group
of peculiarly placed and formed epidermis cells are found around -the pair of guard-
cells ; these have arisen, before or after the development of the proper guard-cells,
from neighbouring epidermis cells by the corresponding formation of partition-walls,

Fig. 118, young, and (FIG. 119) fully developed sto
mata of the leaf of the Hyacinth, s stoma in transverse
section ; t air-cavity; e, e neighbouring cells of the epi
dermis ; /, / parenchyma of the leaf.

WATER-STOMATA.

IJ9

and further growth. Our figures show several cases of such accessory cells of
the stomata. As a peculiarity of the guard-cells, it is moreover to be added, that

1-"1G. 120.— Development of stomata on the leaf of Sedicm furfurascens. A very
young; B nearly mature; e,e epidermis cells. The numbers denote the order of
succession of the divisions.

they always contain chlorophyll-grains in somewhat large quantity, even when the
rest of the epidermis is devoid of them.

There is no doubt that the stomata are originally destined for the exchange
of gases of the assimilating organs, and for the regulated discharge of aqueous

ff\S^^

^^ o

:lopment of stomata of Pterisflabetlata (a Fern). A very young
• cells of the epidermis ; v preparatory divisions ; s Cin -■:;) mothe
le two guard-cells (ss) in B.

vapour. Nevertheless, these organs, as occurs so often elsewhere, may also be
made serviceable for totally different purposes; and thus we find in many cases,
especially in the highly organised Phanerogams, that stomata, but little modified
in other respects, are employed for the purpose of conducting to the surface water
excreted from the internal tissues. Such stomata, which are generally situated on
the margins of the foliage leaves or on the teeth of the leaf-margin, or more

120

LECTURE VIII.

rarely at definite points on the surface of the leaf, are distinguished from the ordinary
air-stomata as water-stomata : beneath them are the terminations of fibro-vascular
bundles which convey the water. Later on, when the movements of water in the
plant is being treated of, we shall return to these modified stomata. If, proceeding
from the vascular plants down towards the more simply organised groups, we trace the
formation of epidermis, we often find, especially in the Muscinece, that where it is a matter
not of simple tissue-layers but of solid masses of tissue containing chlorophyll, a com-
pletely typical epidermis, even with stomata, is present. In the true Mosses, the spore-
forming capsule, also histologically complex in other respects, possesses stomata in

u;. 123.— Transverse section of tlie liorizontal flat slioot of Manlicintia
•orfha. A middle portion, witli leafy appendages (b) and roots (//) on tlie
lower surface (X 30); B part of margin of the shoot, more highly magnified;
/ colourless parenchyma with reticulate thickenings ; 0 epidermis of the upper
surface; chl chlorophyll cells; sp stomata; j partitions between the broad
intercellular spaces; u lower epidermis, with dark-coloured cell-walls.

<-ia liygyometricn. Portion of a longitudinal section of the
I'hich develope later the inner and outer peristome.

;; / parenchyma ;

a highly organised epidermis ; and among the flat-shooted Liverworts, the shoot, com-
posed of several layers of cells, exhibits on the surface conspicuously large stomata,
deviating from the typical structure, and also differing in their development. In all
Algae and Fungi, on the other hand, these organs are completely wanting ; and in
most cases, the superficial tissue of these plants cannot well be termed epidermis, in
the narrower sense of the word, although wherever the tissue of a plant consists of
several layers, the outer layer, formed of one or more strata of cells, is so organised
that it sharply shuts off the internal tissue from the external world. The cells of
the outer layer especially fit together without inter-spaces, and are, like the
epidermis cells of the higher plants, generally somewhat smaller than those of the

EPIDERMIS OF THALLOPHFTES. 121

internal tissue, and cuticularised on the outer side, or clothed by a cuticle : and even

FIG 124.— Examples of the forms of hairs. AofPlectiaiithus/rttticosus; B Cajophoya lalei-itia; C Hüraceiim
piti/crimi ; D Cheiranthus Cheiri. (After De Bary.)

in organs where a single layer of cells constitutes the lissue-as in the prothallia
of Fejns, the leaves of Mosses, and in many Alg«,— and where, .the vegetative-body

122

LECTURE VIII.

consists only of septate filaments, or even only of isolated cells, as in many Algae,
we still find at the surface, at least cuticularised outer layers of the cell-wall, or
even an actual cuticle ; and this is not wanting even on the isolated cells of vascular
plants, e.g. spores, and pollen-grains, — and indeed is often strongly developed on them.
To the characteristic peculiarities of tjie epidermis, especially of vascular plants,
belong the structures known as hairs. In the narrower sense of the word, hairs
are outgrowths of individual epidermis-cells, which however, by means of their
growth and physiological adaptations, may assume the most various forms. According
to circumstances, these outgrowths of the epidermis cell are wart-like .projections,
vesicles, elongated tubes, shield-like outspread scales, prickly outgrowths, or soft

Fig. 125.— Development of the hairs on the calyx of a flower-bud of Althaa rosea (x 300) ; ivh (in A'\
woolly hairs on the inner surface ; b and c glandular hairs in various stages of development ; a (to the right)
rudiment of a glandular hair; ep everywhere, the (stil! young) epidermis. «The figures a in ^, ^ (to the
left) and y (to the right, lower figure) first stages of development of the stellate (or rather tufted) hairs, of
which later stages may be compared in Fig. 126. In A, a shows the hair in longhudinal section; /3 and Y
seen from above. The cells are rich in protoplasm; in 7 the development of vacuoles (•?') is beginning.

woolly coverings, and so forth. IVIoreover, these outgrowths may remain simple
tubes, or, as they grow further, become transformed by means of more or less
numerous transverse and longitudinal divisions into multicellular organs. Among the
vascular plants, there are only few, as the aerial shoots of Equisetums, Conifers, and
Duckweeds {Lettmacece), where the formation of hairs generally is wanting. Usually,
indeed, one and the same plant is furnished with various kinds of hairs, often so
densely that hardly anything else is to be seen on the surface of the organ.

There is scarcely any other organ in the vegetable kingdom where the two
different principles — on the one hand the mere formative force of organic substance,
and on the other hand the adaptation to definite life-purposes— come so clearly into

THE USES OF HAIRS.

23

view as in the hairs. For on the one hand we find them peculiar and constant
in form in nearly whole families — e.g. the large bristle hairs of the Boragtnecc,
the stellate hairs of the Cruci/ero', the tufted hairs of the Malvacece and so
forth — without our being able to assign any use for them to the plant concerned.
On the other hand, we find a great variety of forms of hairs, the biological
significance of which can scarcely be doubtful, and the presence of which varies
within closely allied groups. Among the simplest, and yet most important forms
of hairs, are the root-hairs, already mentioned; these are mere protuberances

FIG. 126.—-^ stellate hair on the calyx of a young flower-bud of Althcra rosea: thicker projections
of protoplasm he on the wall of each cell, and are in 'streaming' motion (indicated by the arrows);
B epidermis (ep) with the basal portion of a fully developed stellate hair, showing the structure of
the wall (X 55°)-

of the superficial cells of the roots which grow out into long tubes, and which, as has
been already shown, play such a very important part (in the land-plants especially) in
the taking up of water and mineral nutritive matters. Much more various are the
physiological uses of hairs on the aerial, shoots, on leaves as well as shoot-axes.
There we find in the first place hairs of the simplest structure, as long, twisted,
thin-walled vesicles very soon losing their fluid contents, — the woolly hairs, which
form woolly coverings on the young parts of shoots before their complete develop-
ment, and are thus to be looked upon as means of protection for the parts of the
bud against injurious external influences ; such hairy coverings are found, for ex-
ample, on the unfolding buds of the Horse-chestnut, Alpine-rose, various Aralias, etc.

124

LECTURE VIII.

These become loosened from the epidermis on the complete unfolding of the bud-parts,
so that later on no trace remains of their former existence. In other cases, on the
contrary, these coolly hairs, which contain air and which are occasionally branched,
multicellular, etc., and devoid of sap, remain even on the completely formed parts of
the shoot : the latter then appear in the fuller developed condition covered as with a
white glistening wool — e.g. the leaves of the Mullein ( Verbasawi), oi Stachys germanica,
and so forth. A very common form of hairs is the glandular hair; these are
generally formed of a short or long stalk, with a little head situated upon it. Ethereal
oils, resin, gum, strong-smelling matters, &c. are secreted in the latter, which we there-
fore relegate to the category of organs of secretion, to be considered more closely later.
To this category of organs belong also the so-called Collciers. These are usuall\-
multicellular, very variously formed hairs on the young bud-parts of widely different
plants (e.g. Rumex, Ri'bes, AInus, Syritiga), out of which, on contact with water, a bal-

4Ai:

Ö3?r0)

I IG. \i-].—Chenopodium quinoa. I—IV development of the flower (longitudinal
sections) ; / the calyx, beset with glandular hairs ; a anthers ; k, k carpel ; sk ovule ;
X apex of floral axis. K transverse section of an anther showing four pollen sacs and the
: (on) highly magnified.

samic mixture of gums and resins exudes, which clothes the surfaces of the unfolding
bud-parts, and subsequently disappears. Commonly intermingled with such secreting
hairs in very many Phanerogams are stinging hairs, of the most various form : long,
conically projecting, pointed out-growths, the wall of which is usually strongly silicified,
and among which the stinging hairs of the nettle-like plants {Uriicacece), the LoasacecE
and others, are especially to be mentioned. The tissue of the shoot-axes and leaves
usually forms cushion-like outgrowths at the base of such hairs, as is also the case with
the climbing hairs of twining stems (e.g. of the Hop); these develope, from a short
base sunk into the epidermis, on one, or two, or more sides, hook-shaped or needle-
like, thick-walled, very hard outgrowths, which ^ipparently contribute to increase the
friction of the climbing stems on their supports. In many insectivorous plants,
the glandular hairs secrete peptonising digestive juices, and by this means indeed
contribute to the nourishing of the plant {Pinguicula, Drosera^. On the other
hand, again, the peltate or scale-like hairs (as in ElcBagnus) springing from a narrow

FORMS OF HAIRS AND PRICKLES.

base in the form of a ratlially construcletl shield, closely appressed to the surface of the

leaf, as well as the stellate or tufted hairs proceeding from a single epidermis cell—

e.g. the Rose-mallow {Alihcsa) — are organs of quite unknown function. On the

seeds and small fruits of very many Phanerogams are found hairs containing air,

which appear in part as soft, twisted, woolly hairs (e.g. on the seeds of Cotton), or

as stiff bristles (seeds of Asclepias Syriaca). They serve, as organs of flight, for the

distribution of the seeds and small fruits ; and they are especially well developed as the

so-called Pappus of many Compositce and Valerianecß. Proceeding from the stinging

hairs, we find, further, a series of stronger outgrowths

of the epidermis, consisting of hard, lignified tissue,

and, under the name of prickles, clothing the shoot-axes

and occasionally the leaf-ribs of many phanerogamous

plants. Some of these, which serve either for protection

against the attacks of larger animals or as climbing

organs, are, like the hairs, mere outgrowths of the

epidermis — e.g. the prickles of the Bramble; more

frequently, however, the deeper tissue also takes

part with the epidermis in the formation of prickles,

and even vascular bundles may terminate in them.

Finally may be mentioned the fiat, outspread, often

relatively large hairs of many Ferns, empty of sap in

the complete state, which clothe the leaf-stalk ami

occasionally the leaf-ribs of these plants with a dense

' ramentaceous ' covering.

The hair-like structures certain!}- owe their existence
and great variety to the circumstance that the\- are
able to grow forth unhindered from the free surface
of the plant. Hence cells and tissue-bodies which
possess exactly the aspect of ordinary hairs are found
generally in such places where free space for their
formation exists; thus, single cells are developed in
the form of hairs in internal tissues which are
traversed by large, roomy, intercellular spaces. To
the most conspicuous of such objects belong the
stellate hairs in the leaf-stalks of the Aymphcracetv,
traversed by large intercellular spaces; and those in
many tropical AroidecE, the tissue of which indeed
obtains, by means of such internal hair-like structures,
a tenacious stringy texture. According to the principle

stated (which regards the epidermis as the place of origin of the hairs less than
the space free for the development of long outgrowths), hairs, often of consider-
able length, occur even on those \ery simple Algae antl Fungi where a true
epidermis, or even indeed a distinct superficial layer, cannot be said to exist :
the hairs are in such cases simple outgrowths of the free surflices of flat single-
layered masses of tissue, as in the Alga CoJeocJuvle scutala, or they are formed
at the apex of a filament consisting of cells.

Fig. 128.— Part of a longitudinal section
of the leaf-stalk of Monstera ddiciosa (an
Aioid). p, p parenchyma; j, j a scleren-
chynia cell or internal hair in the form uf
the letter H

126 LECTURE VI IT.

From the exceedingly various structure and biological significance of the hairs
indicated above, it is self-evident that they present the greatest possible diflferences
even in their material constitution. It is therefore hardly possible to say anything
general concerning them, except that they are all, in their young states, simple cells,
or portions of cells, or aggregates of cells, and that their membrane consists originally
of cellulose, and their contents of protoplasm and cell-sap. With the further functional
development the most various alterations may take place : the protoplasm and cell-
sap may either completely disappear, and the hairs thus represent empty vesicles, as
in cotton, and many other woolly hairs; or, on the other hand, the cell-contents
are not only preserved, but the protoplasm is even very strongly nourished, and
is then distinguished by. circulatory movements and other vital peculiarities, as in
the stinging hairs of the Nettle, the stellate hairs of Althcea, the jointed hairs on the
stamens of Tradescantia, and in many other cases. According to circumstances, and
corresponding to its biological importance, the cell-wall remains thin and pliant,
or it becomes thickened, silicified, or stony — a process which may be extended to
the tissue at the base of the hair (especially well seen, for instance, in the Gourd plant) ;
or in contrast to this, certain layers of the cell-wall of the hair become mucilaginous,
and swell up in contact with water, or a balsamic substance (ethereal oil, resin, etc.)
is deposited between its fine cuticle and the inner layer of the cell-wall; or, finally,
the capitula of the hairs become covered with waxy threads, as on the underside
of the leaves of the Fern, Gynmogramme calotnelanos, and others.

In contrast to the numerous forms of hairs which can be distinguished as organs
with definite functions, the frequent occurrence of hairs of which a biological
significance is neither perceptible nor probable is of special interest. We find,
for example, in some water-plants an uncommonly thick covering of hairs, especially
on the young parts of the shoot, e.g. in the Water-lily {Nuphar); while in other water-
plants again, with a very similar mode of life, this hairiness is wanting.

The second of the above systems of tissue are the Vascular bundles, or Fibro-
vascular strands. I have already pointed out that the simplest rudimentary forms of
these are observable already in some Algae consisting of masses of tissue, as well as in
the more strongly developed shoots of Mosses, where they are constituted in general
as filiform bundles of thin-walled cells, distinguished from the rest of the tissue
by their length. The typical form of vascular bundle, however, is so essential a
characteristic of the higher Cryptogams and Phanerogams, that these two subdivisions
have been placed from of old, under the name of vascular plants, in contrast to the
MuschiecE and Thallophytes as the so-called cellular plants. The typical vascular
bundles run in the form of thin filaments, often of considerable length, through the
soft fundamental tissue of the roots, shoot-axes, and leaves. In the roots, as
already mentioned, there always runs but one strand, which lies in the axis, and
with which the strands of the lateral roots are joined. It is however question-
able whether these root-strands are not rather to be regarded as composed of two,
three, four, or more proper vascular bundles.

Much more various are the relations in the shoot-axes. Only in some water-
plants does a single strand of vascular bundles traverse the interfoliar part up to the
growing-point; and with reference to its morphological nature the same probably
holds good of it as of the axial root-strands. The vascular bundles of the leaves are

VASCULAR BUNDLES OF THE AXIS. I27

connected with this so-called cauline strand. Usually, however, a larger number of
six, eight, ten, or even of hundreds of single thin vascular bundles run in a shoot-
axis ; and each single vascular bundle generally belongs in its lower course to the
shoot-axis, but curves outwards into a leaf with its upper limb, in such a manner

Fig. 12g.— CUmaiis -.■ittcella (after Nägeli). Apex of
a shoot rendered transparent in order to show the course
Df the vascular bundles, the upper ends of which curve out-
wards into the leaves. The youngest leaves (a-5) possess
as yet no vascular bundles.

Fig. 130.— Diagram showing the course of the vascular
bundles in a Monocotyledon of the Palm type (after Fal-
kenberg). *, b bases of the leaves; v growing points of
the shoot.

that each leaf takes up into itself one, two, three, or many such terminations of
the vascular bundles of the shoot-axis. In the interior of the latter, however,
the single bundles are joined together so tha't the lower end of each becomes
fitted somewhere on the course of an older strand curving into a leaf situated

128

LECTURE VIII.

lower down. As lor the rest, great variety prevails in the other relations
of the strands in the shoot-axes as well as in the leaf-stalks; and the cross
connections of the strands at those parts of the shoot-axes which, as so-called
nodes, represent swollen places close to the leaf-insertions, or occur as diaphragms
at the corresponding places inside hollow stems (particularly evident in the
Grasses), are especially to be alluded to. Besides these common (to the axes
and leaves) bundles, however, cauline bundles may also be found, the upper ends
of which, growing further in the growing-point of the shoot-axis, do not curve
out into leaves. In the leaf itself, especially in the lamina of green foliage
leaves, the vascular bundles run in the leaf-ribs described previously, where
they are surrounded by a special parenchymatous envelope. Their generally very

Convallaria Inti/olia

FIC. 131.— Course of the vascular bundles (v

fine terminations however lose themselves, ^vithout projecting exteriorly, in the
green leaf parenchyma itself, and there form the fine network of veins by which
the broad dicotyledonous leaves especially are distinguished; or they branch
there in a dichotomous manner, as in many Ferns, or extend in open curves, or
almost straight lines in the leaves of the IMonocotyledons, — relations which we ha%'e
considered previously from another point of view, and shall yet treat of later on.

The vascular bundles are generally very thin. In succulent shoot-axes, thinner
roots, and in the venation of the leaves, they are often scarcely so thick as a human
or horsehair; but occasionally they attain the thickness of a common thread,
or thicker. Only in the stems of the Tree-ferns, where they are broad and
band-like, and form a coarse net-work, 'do they reach more considerable dimensions

VASCULAR BUNDLE SKELETONS.

129

in transverse section. In general, they are just to be distinctly perceived with the
unaided eye on transverse and longitudinal sections of the organs, especially by
transmitted light. When they contain strongly lignified elements, which of course

FIG. zy^.—Satiibiicus Efiit/ns : the leaf-traces in two internodes. They
are arranged in a cylinder, which is here flattened out in one plane. Each
internode bears two opposite leaves, and each leaf receives from the stem
a median bundle {/;), and two strong lateral bundles {s's'). The descending
bundles split below, and the limbs so formed are inserted into the spaces
between the lower bundles. In addition to these, finer bundles {s" s")
exist, which are connected by horizontal branches from which bundles {nn)
ascend into the stipules. (After Hanstein.)

FIG. iTß.—Asf'ülium fllix-ma.
vascular bundles (j.>) in the stem,
with the basal portions of finer bu
into the leaves.

. h network of
l^ a single mesh,
idles which pass

only occurs in land-plants, and when, in consequence of this, they are tough and
hard, they may often be drawn out from the tissues to considerable distances, in the
form of extensible elastic threads (leaf-stalks of Plantago major, Pri?mila sinensis) ; or
they may be laid bare in
large pieces by scraping off
the soft fundamental tissue,
as in numerous Ferns, e.g.
the stem of the Bracken,
Pkri's aqm'h'na, and in our
common male fern {Aspi-
diwn filix mas). After
taking away all the leaf-
stalks, it is possible by
means of careful pressure,
and bruising and washing, to
obtain even the whole vas-
cular bundle system of the
stem, as a hollow cylindrical
net-work. Particularly fine
objects, very instructive for
physiological purposes, are
obtained as so-called vas-
cular bundle skeletons, when

suitable organs (parts of stems, foliage-leaves, fruits, &c.) are exposed to slow
rotting under water, by which all the softer tissue is destroyed. Fine vascular
bundle skeletons are often j)roduced spontaneously in the open by frequent
[3]

FIG. \2A—Pteris aquilina. A transverse section of the subterranean stem (natural
size) : r hard, brown sub-epidermal tissue ; p soft, slimy parenchyma, rich in starch ;
pr dark walled sclerenchyma forming two broad bands traversing the stem ; ag fibre-
vascular bundles which run externally to these bands of sclerenchyma ; is bundles
running internally to the same bands— 5 the fibro-vascular bundle ag (in A) isolated by
scraping off the parenchyma ; it divides and branches. The dotted lines (k) show the
outline of the stem (st) and its branches (si' and sf) and a leaf-stalk [}>).

130

LECTURE VITT.

freezing and tha^ving, and washing out in the rain. Of course such skeletons
can only be obtained if the vascular bundles are lignified, or surrounded with
lignified sheaths; since these, however, are found in the most various subdivisions
of vascular plants, it is easy to make a collection of vascular bundle skeletons,
certainly among the most instructive objects that can be preserved in a botanical
museum. The careful and repeated study of successful vascular bundle skele-
tons of the most various leaves and shoot-axes, as (Enanthe phellandrnnn, Zea
Mais, or old withered stems of Palms and Draccrnas, as well as the fruits of the
Thorn-apple [Bahira Siramonmm), old Banana fruits, and the like, is a most
attractive occupation, provided that the observer is to a certain extent familiar with
the physiological significance and origin of these structures. Only to one possessing

a sufficient knowledge of these mi-
croscopic relations can the micro-
scopic structure of the vascular
bundles be intelligible ; and in order
to arrive at clear conceptions con-
cerning the conduction of materials
which devolves upon the vascular
bundles, and their co-operation in
ensuring the solidity of the plant, the
study of vascular bundle skeletons is
indispensable. Their significance in
teaching is even in our time much
too little appreciated.

It is not very easy, in the short
space at our disposal, to give a clear
insight into the microscopical struc-
ture of vascular bundles : this, as
need hardly be mentioned, undergoes
great variations in the different sub-
divisions of plants. Only the most
necessary points will be mentioned
here, since I must repeatedly refer
in detail to more special matters
subsequently, when we are concerned
with physiological functions. In the fu'st place, it is to be observed that a bundle
by no means maintains the same structure through its entire course ; the lower
and upper ends are generally thinner than the middle parts, and therefore more
simple in structure. As the thickness — i.e. the surface of the transverse section —
grows, not only the number but also the variety of cell-forms increases. Thicker
bundles may exhibit hundreds of cells on the transverse section, while the thinnest
(e. g. in the venation of the leaves) consist of a few cells only.

It may also be pointed out that the microscopic picture of the vascular bundle
in transverse section is generally extremely characteristic, and easily impresses itself
on the imagination ; whereas, in consequence of the great length, frequent thinning
out, and oblique course of the individual elements, the picture of the longitudinal

Fig. -i^S'—Cyaf^ea Imrayana (a Fern) Portion of a stem with the
bases of four leaves, after the removal of the cortex, and showing the
course of the vascular bundles. The dark bodies are roots. (After De

THE FIBRO-VASCULAR BUNDLES.

f3f

section of the vascular bundle often presents a confusing chaos of lines (contours of
the cell-walls). In order to obtain a clear insight into the microscopic structure of
the bundles, it is necessary carefully to compare transverse and longitudinal sections ;
since the characteristic forms of the cells are generally visible only in the lateral
view (and therefore in the longitudinal section), while the arrangement and grouping
of the elements are more evident in the transverse section. In addition, however,
it happens that it is just in the study of the vascular bundles that the relations
of symmetry prevailing in the anatomical structure of the plant, which we shall
bring under general consideration later on, are particularly conspicuous ; so that a
longitudinal section through a bundle, carried in the radial direction from the
outside inwards, affords a view quite different from that obtained when the knife
meets the bundles parallel to the surface of the organ, or in any other direction. As

FIG. 136.— Transverse section of a feebly developed •
Fern), s phloem ; sp narrow spiral tracheides— the v
cndodermis. (After De Eary.)

ciliar bundle from the rhizome of Polypodium vulgare
er lumina belong to the broad scalariform tracheides.

a guide to the structure of a vascular bundle, always somewhat complicated, the dis-
tinction (first given by De Bary^) of the total mass of its cells into two subdivisions,
the Bast-portion {Phloem of Nägeli) and the Wood-portion {Xylan of Nageli), is
especially serviceable. Both groups consist in general of elongated, often very
long, vesicular or tubular cells, narrow, or very narrow in transverse section ; so
that on a transverse section through an organ the vascular bundles usually come
into view at once as groups of cells with peculiarly narrow lumina, situated in the
large-celled parenchyma of the fundamental tissue. Individual elements, especially

* De liary, I.e. p. 330. I here bring forward De Bary's subdivision of the tissue of the vascular
bundles into Gcfiissthcil and Sicbthcil — the earlier subdivision into Wood and Bast {Xylon and
Phloem), — because by Schwendener's misusage of the word 'Bast,' by which he distinguishes all
possible sclerenchymatous tissues not belonging to the vascular bundles, a confusion pernicious
to the beginner has been introduced into the good old nomenclature.

132 LECTURE VIII.

certain forms of vessels, may occasionally be very large, however, in transverse
section. The phloem and xylem likewise agree in that, apart from rare excep-
tions and later transformations, they possess no intercellular spaces of any kind :
the cells of a bundle all fit together closely on every side ; and in this they differ
conspicuously from the parenchyma of the fundamental tissue, which is interrupted
by intercellular spaces.

The xylem, as well as the phloem, consists of various forms of cells ; and we may
say, for the purposes of a rapid survey, that in each of these two groups, vascular,

FIG. 137.— Transverse section of a closed fibro-vascular bundle of the stem of Zca A/ays (x SSo). It consists of
xylem {o-, £■, S, r, i) and phloem {v, v). The surrounding thick-walled tissue is the bundle sheath, belonging to the
fundamental tissue, /./thin-walled parenchyma of the fundamental tissue; a outer side; i inner side (facing the
axis of the stem) ; g, g two large pitted vessels ; J- spiral vessel ; r isolated ring of an annular vessel ; / air cavity
produced by rupture during growth.

fibrous, and parenchymatous elements occur. The homologous, and especially the
vascular, elements of the one group, however, are different from those of the other
group. In the more highly developed fibro-vascular bundles of terrestrial plants, the
cell-walls in the xylem are either totally, or at any rate to a great extent, lignified ;
especially the walls of the vessels and woody-fibres. When the lignification in the
xylem is reduced to a minimum, as occurs in very succulent tubers, tuberous roots,
and aquatic plants, the lignification is not unfrequently confined to the walls of the
vessels. In the phloem, either no lignification of the cell-walls occurs at all, or it

GENERAL CHARACTERS OF XYLEM AND PHLOEM.

33

follows only late in the thick-walled fibres — the proper bast-cells: these, however,
often also remain entirely unlignified. The pliability of true bast, e. g. of Flax- and
Hemp-fibres, simply depends upon their being not lignified at all, or only to a very
slight extent. The other elements of the phloem are remarkable for their soft, pure
cellulose walls. Diflicult though it is, however, to separate phloem and xylem
sharply from one another by words, the difference of their aspect under the
microscope is nevertheless usually conspicuous. It is especially characteristic for

Fig. 138.— Part of a transverse section of the completely extended hypocotyledonous portion of the stem of
Ricinus communis, r parenchyma of the primary cortex, and m pith, both belonging to the fundamental tissue.
Between r and b is the simple vascular-bundle sheath (containing starch), which also belongs to the fundamental
tissue. The bundle consists of phloem {b,y), xylem (g, t), and cambium [c. c), and is therefore an open bundle. The
cambium (c, <-) extends into the fundamental tissue situated between the bundles, as interfascicular cambium (f, *),
which arises by subsequent division of the large parenchyma cells, b, b bast fibres ; y, y soft bast (partly paren-
chyma, partly sieve-tubes) ; t, t narrow, and g, g wide, dotted vessels, with wood prosenchyma between.

the xylem, that at least some air-conducting vessels are present; while the
vascular elements of the phloem, the sieve-tubes, are filled with slime containing
albuminous matters, or clear sap, and never contain air. So far as we have
yet succeeded in obtaining an insight into the physiological significance of the
vascular bundles, the xylem appears to be the tissue which provides chiefly for the
conveyance of the water taken up by the roots, and the mineral nutritive matters
contained in it, to the organs of assimilation: however the true function of the
cavities of the vessels, which, as we shall see later, contain very rarified air, and

134

LECTURE VIII.

are only filled with water in special cases, still remains doubtful. On the other
hand, we may regard the phloem, in the main, as the form of tissue in which
albuminous substances containing nitrogen are produced, and by which they are
distributed for long distances in the organs : the elements of the phloem may also
serve for the transport of products of assimilation. The vascular bundles of simpler
structure, especially in water-plants, are probably confined to these functions ; while
in cases where, with the increase of the assimilating foliage, a more rapid bringing
of water and carrying away of products of assimilation are necessary, greater com-
plexities in structure occur. In land-plants especially the tendency prevails to

produce elastic fibres within the vas-
cular bundle itself; that is, wood-
cells in the xylem, and true bast-
fibres in the phloem. In other cases
the vascular bundle is enveloped, on
one side or all round, with such
elastic fibres : this contributes to the
solidity of the organs, especially of
the shoot-axes and leaves, and as
I hope to show later, the conduction
of water is facilitated at the same
time.

The arrangement of the xylem
and phloem in a vascular bundle
presents many noteworthy differences,
according to the organ and the class
of plant : it is not possible however,
apart from details, to establish a re-
lation between them and definite
physiological principles. It may
therefore suffice, with the help of the
accompanying figures, simply to point
out here a few of the most important
differences in this connection. Thus,
we find in the Ferns, as Fig. 139
shows, the xylem enveloped all
round by a more or less thick layer
of phloem. In the Monocotyledons,
on the other hand, and particularly in the Grasses, the latter forms a strand, which
is accompanied on its inner side (i. e. the side turned towards the axis of growth of
the organ) by the xylem, which also envelopes it more or less laterally (Fig 137).
Fig. 138 gives an approximate representation of the ordinary structure of the vas-
cular bundle of the Dicotyledons. With respect to details I may refer to the
explanations of the figures, with the remark that in the monocotyledonous bundles
(Fig. 137) the thick-walled cells, forming a sheath surrounding the whole proper
vascular bundle, do not belong to the bundle itself, but represent one of the
frequently occurring arrangements to ensure solidity appertaining to the fundamental

Fig. 139.— Portion of the transverse section of a large vascular bundle
from the stem of Pleris aquilina, with some of the surrounding paren-
chyma {P): the latter is filled (in winter) with starch, s spiral vessel in
one focus of the elliptical transverse section : this is surrounded by thin-
walled wood-cells, containing starch. £, g scalariform vessels and sp
sieve tubes. Between these and the xylem is a layer of cells bearing
starch in winter, b bast cells with thick soft walls; sg vascular bundle
sheath. Between sg and i is a layer of cells containing starch.

THE FIBRO-VASCULAR BUNDLES OF ROOTS.

tissue, to which I shall come back later. It would carry us here much too far to
enter more in detail into the relations indicated : moreover this would be superfluous,
since their physiological significance, as already explained, are either not known, or,
where it is possible, will be brought forward subsequently as opportunity serves.

Strikingly different from the vascular bundles of the shoot-axes and leaves are
the fibro-vascular strands of all true roots; and they are usually so characteristic
that it is recognised at once on a transverse section whether a root-strand is under
observation or not. The most striking feature is that the xylem and phloem-bundles
are so placed in groups at the periphery of the axial cylinder, that they alternate
with one another laterally; the phloem-bundles being situated chiefly on the surface
of the strands, and the xylem, radially placed, forming plates proceeding from the
periphery towards the inside, between the phloem - bundles, as shown in the
accompanying figures. One
of the most striking pecu-
liarities of the root-strand is
that the narrowest, thinnest
vessels lie on the outer cir-
cumference, and are those
first formed: further towards
the interior of the strand,
vessels with continually
wider lumina become de-
veloped. Only when the
root-strand possesses amore
considerable thickness, is
its axial space occupied by
a kind of pith — i. e. paren-
chymatous tissue : in very
thin root-strands this is en-
tirely wanting, and, indeed,
a vessel, or a group of
vessels may run in the axis
of the strand. IMoreover,
between the vascular plates
and phloem -bundles are

formed small quantities of parenchymatous tissue, filling the interspaces to a certain
extent; as in the bundles of the shoot, parenchymatous elements are present in
addition to the vascular and fibrous elements.

If we now, finally, take into consideration the cell-forms of the vascular bundles
themselves, already mentioned, the vascular elements are most prominent in both
parts. Those in the xylem are generally termed, shortly, vessels ; while those of the
phloem, corresponding to them, are named sieve-tubes.

As vessels in the wider sense, and thus comprehending both the forms men-
tioned, are to be distinguished longitudinal rows of cells, the cavities of which
communicate directly with one another by the transverse septa being either com-
pletely absorbed, or perforated by pores.

Fig. 140.— Root of Acorus Calamus. Transverse section of the axial cylmde
vith surrounding cortical tissue, j- Endodermis ; /, / narrow, peripheral (oldest) ves
els ; g wide, inner (younger) vessels ; ph phloem.

136

LECTURE VIII.

The vessels (wood-vessels) appear in relatively thick, vigorous bundles in various
forms, as annular vessels, as spiral vessels with one or more spiral bands, or as

Fig. 141— Transverse section of a primary root of a seedling of Phtiscolits vntltifloriis ; taken from the upper swoUe
portion, at the time when the first leaves and lateral roots are already developed, b bast and phloem ; s endoderniis— vascula
bundle sheath ; /c pericambium inside the endodermis ; f primary and ^ secondary vessels ; m pith ; c cambium.

Fig. 142.— Longitudinal section of the vascular bundle oi Ricinus (cf. transverse section in Fig. 138). r cortical paren-
chyma ; gs vascular bundle sheath ; ni parenchyma of the pith ; b bast fibres ; / phloem parenchyma ; c cambium. The
cells between c and / eventually form a sieve-tube. In the xyleni the elements are gradually developed from j- to t'.
s primary, narrow, and very long spiral vessel ; s' wide spiral vessel— both with unreliable spirals ; / scalariform— in part
reticular— vessel ; h and h' wood-cells ; t pitted vessel, with a resorbed septum at q ; h" , h'" wood-cells ; f young pitted
vessel— the borders of the pits develope first, and then the pore arises inside At /, t, t', the boundary lines of the adjoining
cells — now removed— are observed on the walls of the vessels.

reticulately thickened vessels, and finally as so-called dotted vessels or vessels provided
with bordered pits. In the development of a vascular bundle from the embryonal

VESSELS WITH BORDERED PITS. 1 37

tissue of the youngest organs, these vessels are gradually constructed in the order
here mentioned, and in general their breadth increases in the same sequence ; while
the length of the individual segments, or cells, of which the vessels are composed,
decreases at the same time and in the same order. Hence the first formed, narrow,
annular and spiral vessels, which have to take part in the whole growth in length of
the organ, are in the completed state very long tubes ; whereas in the last formed^
pitted vessels, the individual segments, especially when they only attain completion
after the termination of the growth in length of the organs, appear short and barrel-
shaped. These also may occasionally however, as in roots, form very long tubes.

The nature of the sculpture on the walls of the vessels has already been
explained (in lecture VI). Here is still to be added, that the vessels with
bordered pits owe their striking aspect on the surface view to the circumstance
that the pits are here not only very closely crowded, and are thus separated
fundamentally only by thickening ridges;
but the thickening ridges become arched,
growing forth from the primary thin cell-
wall into the interior of the cell, laterally
and on all sides over the spaces between
the meshes, so that each pit is connected
with the inner cavity of the cell only by
means of a very narrow pore or slit. We
may also say, a bordered pit is a short
canal, leading out from the interior of the
cell, and suddenly becoming widened at the
primary cell-wall. In the surface view of fig. us.-Pa« of the longitudinal «aii of a pitted vessel.

,,,.,,, ^ ,., a the primary thickening ridges, which become arched over

a bordered pit, therefore, a roundish pore the areoloe (/.), and only leave the sllt W exposed.

or slit is perceived in the middle, which is

surrounded by a circular or polygonal border, corresponding to the outer cir-
cumference of the pit. In the Ferns (as Fig. 144 shows), where the segments of
the pitted vessels are situated upon one another with very oblique septa, the bordered
pits are so extended in breadth, that the thickening ridges between them often
appear like the rounds of a ladder ; hence the old name, ladder-like (scalariform)
vessels. In addition to these proper vessels, there are found in the xylem of
thicker strands other more fibrous elements, with the upper and lower ends
obliquely cut off, or drawn out to long points, the finer structure of the walls and
pits of which is similar to that of the true vessels. These, therefore, have been
termed tracheides, in contradistinction to the true vessels, which are named
tracheae. All these tracheal elements lose their protoplasm, together with the
cell nucleus, completely, as soon as their wall-structure is fully developed; so that
later not the smallest remnants of them are to be perceived. Even the cell-sap
disappears completely, and the vascular tubes only contain air; and indeed it is not
improbable that they occasionally become even empty of air.

The resemblance of the vascular constituents of the phloem, the sieve-tubes,
to the tracheal elements of the xylem is very slight. The latter, with their ligni-
fied cell-walls devoid of contents, are conspicuous on account of the sculpture of
their wall; while the sieve-tubes, on the contrary, are provided with soft, supple,

,38

LECTURE VIII.

mostly thin walls, and, at least in the younger organs, are filled with slimy proteid
matter. The transverse septa, which lie closer to one another the older the part of
the plant had already become before the formation of the sieve-tubes, are also
of a soft, or even gelatinous consistence ; and they are never entirely, or to any
great extent absorbed; but by absorption taking place at isolated spots, they
become transformed into a network of ridges, in the meshes of which lie the canals

Fig. 144.— y*. A third portion of a vessel from the rhizome of Pteris aqiiitina. The oblique scalariform end (/)
and a portion of the lateral wall in surface view: the areas not pitted correspond to the angles of neighbouring
vessels. B is the part x (in A) more highly magnified ( x 375). C and D very thin longitudinal sections perpendicular
to the lateral wall of two neighbouring vessels; showing the thin primary wall, on which are situated the sections of
the thick projections between the pits. (After De Bary.)

by means of which the segments of a sieve-tube, situated one over the other, com-
municate, and through which the slimy contents can be pressed. Where sieve-tubes
border immediately on one another laterally, so-called sieve-plates may also be
formed on their side-walls, the structure of which resembles that of the sieve-like
transverse septa.

Amongst the tracheal structures of the xylem, and still more between the
sieve-tubes, is found more or less abundant, parenchymatous, mostly thin-walled
tissue : this consists of more or less elongated, soft cells, which contain fluids
of various kinds, and frequently starch.

THE PHLOEM.

'39

Finally, as to the elastic fibres of the vascular bundle already mentioned.
These occur in the phloem as true bast-fibres, in the xylem as wood-fibres :
the latter have been distinguished from the fibre-Hke trachei'des more particularly
as the libriform fibres of the wood. At another opportunity we shall see how,
in cases where subsequent growth in thickness of the shoot-axes occurs (i. e. in
the proper development of wood) the libriform fibres contribute to the for-
mation of the true wood; and, so far as the true bast-fibres are concerned^ it

Fig. 145.— Transverse section of the phloem
of a fibro-vascular bundle in the stem of Cticiir-
/>ila Pepo (X 550). j;' septa of young sieve-tubes
showing areolae — the pores not yet developed j
/, / phloem parencliyma ; c,c cambium. There
are no bast fibres here, the whole phloem con-
sisting of soft bast.

Fig. 146.— Longitudinal section of the phloem of C/<-
curbita Pepo; showing three sieve-tubes, the transverse
septa (^, q) of which are not yet perforated. The slimy
mass \sl and ps) contained in them is contracted, si a
young sieve-plate on the lateral wall : pores will also be
found later at x and /. z narrow parenchymatous cells
between the sieve-tubes.

may here be pointed out simply that they are wanting in many vascular bundles,
and in other cases appear more isolated ; in others, again, they exist in the form
of layers or thick strands on the outside of the phloem.

The peculiar characteristics of the vascular bundle lie, according to my view,
which agrees with that of De Bary, in the presence of tracheal elements in the
xylem, and of sieve-tubes, or of tissue-elements similar to them, in the phloem.

» It will be clear from the text and from note 2, that by 'bast-fibres' are not to be understood
here the Stereom elements distinguished as ' bast ' by Sch wendener.

140

LECTURE VIII.

By the presence of these two groups of tissue, even when each of them
only asserts itself by a few characteristic elements in the transverse section,
the vascular bundles may always be distinguished from the strand-like structures,

occurring frequently enough elsewhere, which
run in the fundamental tissue of many shoot-
axes and leaf-stalks in a similar manner to the
vascular bundles ; but which are essentially dif-
ferent from these, being merely special forms
of fundamental-tissue. As the true nature of
leaves and roots through the whole series of the
vascular plants, though the structural relations
are the most different possible, is always to
be recognised; so also the vascular bundles
of all vascular plants, even when they depart
far from the typical structure, as in many
water-plants and parasites, yet always appear
as tissues of essentially the same nature. I
interpolate this remark, because it has recently
been attempted to confound these character-
istic and constant constituents of the ana-
tomical structure with other strand-like ar-
rangements of tissue, which may be present
or be wanting according to purely biological
requirements ; and this to the great injury of Phytotomy, which only loses thereby
in clearness and scientific depth. One might just as well name any given filiform
organ a root, or any flat structure a leaf, as place the vascular bundles on a
level with other haphazard strand-like masses of tissue \

Fig. 147.— Parts of sieve-tubes where the segments
unite, showing the perforation of the septa after solution
of the cell-wall by sulphuric acid. ^ and B from the
petiole oi Cucurbita; C from the stem of Dahlia. In A
the cell-wall h, h' is not yet dissolved, s slimy contents ;
o and ti accumulation at the upper and under side of the
septum; / the threads of slimy substance which connect
these accumulations, and pass through the pores of the
sieve-plate.

> The attempts repeatedly made of late to place the vascular bundles, as a subordinate form of
tissue, approximately on an equal footing with mere sclerenchyma strands, only shows how little the
younger botanists have succeeded in comprehending the province of vegetable anatomy, cultivated
with such great results by Mohl, Nägeli, and De Bary.

LECTURE IX.

THE SYSTEMS OF TISSUES {co?tthiued).

FUNDAMENTAL TISSUE; RUDIMENTARY DIFFERENTIATIONS OF

TISSUE.

I CLASS together all the masses of tissue which are enclosed by the epidermal
tissue, and traversed by the vascular bundles, under the itxm ßmdamenial tissue^.
In the younger and still succulent organs, covered only by the epidermis,
and the vascular bundles of which have not yet been altered by subsequent
growth in thickness, and in organs generally in which the formation of true
wood and secondary cortex has not yet commenced, the main mass of the
entire substance consists of funda-
mental tissue. This is perhaps
best seen in an Apple, the whole
edible substance of which con-
sists of it. The succulent mass
of tissue in the leaf of an Aloe,
again, apart from the very thin
vascular bundles and the epider-
mis, is formed entirely of funda-
mental tissue.

The most widely spread, and,
as may well be assumed, the primi-
tive and typical form of funda-
mental tissue, is the ordinary thin-
walled parenchyma; in which we

may at the same time perceive the typical form of all true cell-tissue. The
parenchyma cells are usually the largest in the body of the plant; and are
either roundish, polyhedral, or elongated and prismatic, or more rarely pointed
above and below. Very commonly, although not quite generally, small, or often
very capacious intercellular spaces run between them. The contents consist of
living protoplasm with a nucleus; the former, usually small in quantity, forming

Fig. 148 —Transverse section through the soft parenchyma of the stem
of Zea Mays. g-Jv common wall between each two cells ; z inter-cellular
space produced by splitting of the wall.

1 I first characterised the fundamental tissue as a system of tissue co-ordinated with the
epidermis and vascular bundles in the first edition of my ' Text Book' (1868).

143 LECTURE IX.

a thin clothing to the cell-walh The remaining space is filled with watery sap;
and the parenchyma cells may contain in addition the most various products of
assimilation and metabolism. It is chiefly in these cells that such substances are
stored temporarily, or even up to the next period of vegetation and longer,
to be dissolved and used up later for the purposes of growth. In this form we
find the parenchymatous tissue as a succulent envelope surrounding the axial
strand of the young root, as pith, and primary cortex ; and to a certain extent as
the mass surrounded by the epidermis, and filling up the parts between the vascular
bundles, in the shoot-axes and leaf-stalks, and in the leaves themselves, as well
as in the coverings of the fruit. Even the endosperm of seeds and the coty-
ledons of embryos may be classed under this head.

However, this parenchymatous cell-tissue is by no means the only form
in the system of the fundamental tissue; for the parenchyma itself undergoes

FIG. 149.— A portion of the transverse section of the flower stem of Fa-nicuhim officutnle. slightly magnified,
f epidermis ; A/ wood ; A resin passages. All the rest is fundamental tissue—»« parenchyma of the pith; r paren-
chyma of the cortex ; ch parenchyma containing chlorophyll; c coUenchyma.

more or less extensive differentiations, without losing its essential peculiarities, and,
very frequently, cells and forms of tissue of the most various kind appear in it
besides.

A differentiation within the parenchymatous fundamental tissue is very gene-
rally exhibited, firstly, in so far that the intercellular spaces towards the epidermis,
as well as at the borders of the vascular bundles, disappear, and the sizes of the
cells, and especially their diameters, diminish ; and, generally, alterations in the
thickness, substance, and pitting of the walls occur also. These changes not rarely
proceed so far that the layers of tissue concerned entirely lose their parenchymatous
character : they then have different names assigned to them.

Thus, it is a very common phenomenon that beneath the epidermis^ of the
shoot-axes, petioles and thicker leaf-ribs of dicotyledonous plants, the cells of the
fundamental tissue are developed as CoUenchyma (Fig. r^o); the longitudinal

HYPODERM: ENDODERMTS.

H3

walls exhibit pad-like projecting thickenings deposited in the angles of the cells,
which swell up strongly in water, and still more in dilute potash solution, and
give to the transverse section an uncommonly characteristic appearance. In the
leaves of Conifers, Cycads, and in many other cases, the collenchyma is replaced by
very thick-walled, prismatic, and even fibrous
cells, in part lignified; these, as in Figs. 149
and 151, run close beneath the epidermis, and
are grouped together in strands or layers.
These, and similar so-called Hypodermal
structures, serve to render the organs con-
cerned elastic and solid ; since, by means of
their own stiffness, they increase, or, in
the case of collenchyma, aid the elastic re-
sistance which the epidermis opposes to the
expansion of the succulent, turgid paren-
chyma.

Where the fundamental tissue bounds
the vascular bundles, there are always formed
layers, commonly sharply marked, which may
be distinguished generally as vascular bundle-
sheaths; these however, like the hypodermal
layers, are, according to circumstances, of very
different constitution. The commonest form

of these vascular bundle-sheaths is usefully distinguished by De Bary as Endo-
dermis. It is particularly well marked in the roots of all vascular plants, at the
circumference of the axial strand, in the form of a simple layer of small cells,
the walls of which are more or less suberized, cf. Fig. 8 s. In the shoot-axis

Fig. 150.— Epidermis (f) and Collenchyma \cl) from the
petiole of Hegoiiia. The outer walls of the epidermis cells
are equally thickened ; where they adjoin the collenchyma
the thickening only occurs at the angles where three cells
meet. These thickenings have a great capacity for swell-
ing, clil chlorophyll grains; / parenchyma.cell.

Fig. 151.— Transverse section of the leaf of
Pinus Pinaster ( X 3o)- ' epidermis ; es hypo-
dermal prosenchyma ; sp stomata ; h resin-pas-
sages ; / parenchyma containing chlorophyll; g, *
colourless internal tissue e"'''"''""" * — *^' — '
vascular bundles.

two fibro-

FIG. 152.— The lefl-hand comer of the previous
figure (X 500). c cuticularlsed layer of the epidermal
cells ; i inner layer of the same, not cuticularlsed ;
f' very strongly-thickened outer wall of the epidermis
cell lying in the comer ; g, i' hypoderm cells : g-
middle lamella; i' stratified thickening mass; p
parenchyma containing chlorophyll ; fr contracted

also a very similar endodermis is frequently found, enveloping all the leaf-traces
as a hollow cylinder, and therefore appearing on the transverse section as a
continuous ring, by which the whole tissue is separated into a cortex, lying outside
the endodermis, and an internal core of tissue (pleromc), c*^. Fig. 138. Such

144

LECTURE IX.

plerome-sheaths are very distinct in the rhizomes of many Monocotyledons, as
Iris, Acorus, and so forth ; and also in the sub-aerial shoot-axes of many Dicoty-
ledons. More frequently, a layer of tissue similar to the endodermis is present in
sub-aerial shoot-axes and leaves, either only on the outside, or also on the inside
of each single vascular bundle; or each individual bundle may be entirely sur-
rounded by such a layer of tissue, as in Fig. 153. In many other Ferns, this
vascular bundle-sheath is distinguished by a strong thickening of the longitudinal
walls turned towards the bundle, and often also by a dark brown colour (cf.
Fig. 136). In other cases, particularly in the upright shoot-axes and leaves of
many Monocotyledons, especially of the Grasses, Palms, Dracanas, etc., which require

greater elasticity and rigidity,

the vascular bundle-sheath
consists of a more or less
thick, often (especially in
Palms) extraordinarily thick
layer of very thick-walled,
lignified, long, spindle-
shaped fibres, which fit to-
gether without intercellular
spaces. While the function
of the ordinary endodermis
is apparently only to render
slow the exchange of sap be-
tween the parenchymatous
fundamental tissue and the
vascular bundles, these large
lignified fibrous strands
(Sclerenchyma) not only
serve this purpose, but upon
them depends, in plants not
properly forming wood

FIG. 153.— Portion of the transverse section of one of the large vascular bundles of OtherWlSC, thC rigidity of

the stem of /"/^^/i- «?!«/!««, with surrounding parenchyma />: the latter is filled (in ,1, u <■ " iV,

winter) with starch, j spiral vessel in the focus of the elliptical transverse section ; this InS SDOOt-aXeS. SinCC tlie

is surrounded by thin-walled wood-cells containing starch, i'^ the scalariform vessels ; i U ^1 C U

j;> sieve-tubes. 3 bast cells with thick soft walls; Ji>- bundle sheath. Between Ä and j^^ VaSCUlar DUnOleS Ol SUCtl

is a layer of cells containing starch. -^^ ^ i j i

Monocotyledons, and some
similarly constructed Dicotyledons, in themselves very thin and feeble, are not at all
calculated to give the necessary rigidity to the stems and shoot-axes. These
sclerenchyma-sheaths, moreover, do not always surround the entire vascular bundle,
as in Fig. 154; frequently they accompany it only on its outer or inner side, or on
both. Moreover, such layers of sclerenchyma, which belong to the fundamental
tissue, do not always immediately accompany the vascular bundles; but very
frequently are quite independent from these, as layers and strands running in
the parenchyma of the fundamental tissue. They are particularly well seen
as stout, dark brown, hard bands in the stem of the Bracken-fern (Fig. 134,
A, pr.) as well as in the stems of Tree-ferns, where they not rarely form
a hard protective coat, both under the epidermis and around each of the large

SCLERENCHYMATOUS FUNDAMENTAL TISSUE.

145

vascular bundles. In the long, thin, and yet very elastic and firm flowering scapes
of Rushes {JuncHs, Scirpus, Cladinm, etc.), lignified sclerenchymous strands either
run close beneath the epidermis, or a closed ring ofthat tissue lies in the neighbour-
hood of the periphery, and gives the slender column the necessary rigidity.
Schwendener, who first called attention to the significance of these sclerenchyma
layers and strands with reference to the elasticity and rigidity of the organs concerned,
distinguished their cells as bast-cells. This nomenclature, however, has not been
accepted by botanists : the name bast was given long ago to the elastic fibres in the

Fig. 154.— Transverse section of a fibro-vascular bundle from the stem ot Zea Mays (Indian Corn) suiTOunded by its
sheath of sclerenchyma; a is towards the exterior, and ;' towards the centre of the stem, //large-celled parenchyma of the
fundamental tissue. Only the parts v (sieve-tubes), j^ (pitted vessels), J (spiral vessels), r (annular vessels), and the elements
lying between these constitute the vascular bundle— the thick-walled shaded tissue is the sclerenchymatous sheath.

phloem of the vascular bundle and the secondary cortex. Generally speaking, this
true bast is not lignified, its long fibres being rather distinguished in fact by their
flexibility. Besides, plenty of examples occur where the sclerenchyma layers described
consist of cells which cannot be in any way compared externally w'ith bast-fibres : the
sclerenchyma cylinder in the flowering scape of species oi AUium (Fig. 155 Jr.), for
instance, consists of cells transversely or obliquely truncated above and below, the
relation of which admits of no doubt whatever that the entire sclerenchyma ring is
only a layer of narrower cells of the parenchymatous fundamental tissue, which is
[3]

14^

LECTURE IX.

distinguished from the rest of the fundamental parenchyma by possessing much
smaller intercellular passages, or none at all, and by the walls being strongly
lignified. The same is the case with the brown strands of the Bracken-fern, which
are prosenchymatous, it is true, but in part contain starch. The stem oi Lycopodiiim
chamcBcyparisms (Fig. 157) shows that under certain circumstances the main mass of the
fundamental tissue may assume the sclerenchymatous condition : the very thick-walled

cells, figured in transverse section,
are pointed above and below, and
are arranged in prosenchymatous
layers. Not rarely, however, scle-
renchyma cells occur isolated,
or arranged together in small
roundish groups, or even forming
loose layers in the fundamental
tissue. Thus, the hard concre-
tions, about the size of a grain of
sand, in the soft flesh of the Pear,
consist of groups of very thick
sclerenchymatous cells, which are
apparently nothing further than
peculiarly developed parenchyma
cells. The same holds good of
the layers or groups of so-called
stone cells, beneath the layer of
cork in the cortex of dicotyle-
donous woody plants (e.g. in the
Poplar), In other cases, these
isolated sclerenchyma cells ex-
hibit more characteristic forms, as
in Fig. 1 5 7 3, which represents a
many armed, large sclerenchyma
cell from the leaf of a Camelia.
Similar forms are found in the
cortex of some Conifers ; very fine
and numerous, for example, in
the Fir {Abies pectinafd). In
the hard stiff leaves of the Pro-
ieacecB and some other evergreen
plants, sclerenchymatous cells of
the most various form are to be
met with. Also in the pith of
ouped, lignified sclerenchyma cells occur, mostly

FIG. 155-

Transver

se section

of the

flower scape

of Al/ii

m Schoeno-

fyasum (X 30

e epid

erm

s ; c!

chloi

ophyll ceHs;

r colourless cortical

parenchyma ;

tn pith

par

sncliyr

la ; i>-

g' vascular

bundles ;

sr ring of

sclerenchyma ,

h centra

1 ca

ity.

Fig.

a. h

-Transverse section from the rhizome of Pteris aquili
root-hairs ; c strongly thickened brown walled cells beneath the epidermis ;
q a more deeply situated cell, less thickened— part of the wall seen en face,
se cells of deeper layers containing starch, passing over to the internal colour-
less parenchyma of the fundamental tissue.

some wood plants, isolated or
however of simple form.

If possible, yet more various than in the shoot-axes and leaves, are the forms of
fundamental tissue developed in the fruits and seeds of the Phanerogams. To select
only a few examples from the almost endless variety, it may be mentioned that the

SCLERENCHVMATOUS FUNDAMENTAL TISSUE.

47

of the stem of Lycfljia,

Plums and Cherries owe their name — stone fruit — to the circumstance that the
fundamental tissue of the pericarp, at first homogeneous and parenchymatous,
becomes separated into two layers, of which the outer, enclosed by a solid
epidermis, forms the ecHble flesh of the fruit, and the inner one the so-called
stone. Each consists at first of thin-walled large parenchyma cells filled with sap
containing sugar, while the cells of the tissue of the stone become exceedingly
thick-walled, and finally, on ripening, are very strongly lignified. Especially
various are the cell-formations
in dry fruits, capsules, and in-
dehiscent fruits, as well as in
the testa of many seeds. It
is, however, difficult to give a
survey of these forms at all
comprehensive, since a com-
parative study of them is
still wanting ; and we have
here to do everywhere with
specific adaptations to definite
biological relations of indivi-
dual species of plants. In the
tissue-formations of the coats
of seeds and fruits it is some-
times simply a matter of solid-
ity and mechanical protection ;
sometimes of arrangements
which, on the ripening of the
capsule, bring about its dehi-
scence and the scattering of
the seeds. Berries and stone-
fruits by means of nourishing
masses of tissue attract animals
to eat them, and the latter then
scatter the hard-shelled seeds
at other places. In other cases
again, wings or parachute-like
outgrowths (Pappus of the Com-
positce) arise on seeds or dry
fruits ; and many other such ar-
rangements exist, which are na-
turally connected with peculiar
developments of the portions of tissue concerned.

Among the various forms of fundamental tissue is also to be included one which
is the most widely spread and physiologically by far the most important, viz. the chlo-
rophyll parenchyma of the green parts of plants, and especial!}' of the foliage leaves.
With reference to the forms of the cells and the formation of intercellular spaces, the
absence of suberisation and lignification, and the usually thin walls and soft contents,

L 2

Fig. 15
P parenchy

—Part of the tn

cells with chlorophyll grai

idle ; If V a. large branched sclerenchy

ween the parenchyma cells.

section of a leaf o( Camellia Ja fonica .
and oil drops ; F a very thin vascular
. cell, the arms of which are inserted

T48

LECTURE IX.

the assimilating tissue does not differ essentially from ordinary parenchymatous
fundamental tissue, which may also be termed nutritive tissue in the wider
sense. The distinctive point — not only because the green colour of the plant is
caused by it, but much more on account of its fundamental significance for
the whole nutrition of the plant — lies in the presence of the chlorophyll-grains,

which we may shortly distinguish
as green-coloured portions of
the protoplasm of these cells.
Since, as I shall show later, the
chlorophyll cells of the assimilat-
ing parenchyma, the typical forms
of which are found in the green
foliage leaves, must maintain a
vigorous exchange of gases for
the purpose of assimilation ; and
since, at the same time, the nu-
tritive water of the soil is con-
ducted to them, which they exhale
into the air in the form of aque-
ous vapour ; it is- intelligible why
the assimilating parenchyma in
general possesses a spongy cha-
racter. Its cells separate from
one another until nearly isolated,
often supporting one another only
at single, narrow, circumscribed
places, and forming numerous,
large intercellular spaces, which
usually communicate in all direc-
tions, and in which carbon dioxide,
oxygen, and aqueous vapour can
move from place to place. Since the
assimilating tissue can perform its
function only under the influence
of light, we find it always in the
form of thin layers immediately
accessible to the light. Thick
layers of chlorophyll would have
no purpose : since thin layers of
even o-i to 0-5 mm. thick absorb
the useful light rays, layers lying deeper would thus obtain no more useful rays.
Hence the assimilating tissue generally occurs in the form of plates, which are
very thin but extensive in surface, and which in ordinary thin foliage leaves are
covered only by an epidermis, abundantly supplied with stomata. For the reasons
named, moreover, even in the very thick leaves of succulent plants and the Crassu-
laceae, species of Aloe, Agave, &c., only a thin lamella of green assimilating tissue is

through the testa of the

PARENCHYMA: ASSIMILATING TISSUE.

[49

present close beneath the epidermis ; and the same is the case in the green shoot-axes
of Equisehim, and still more evidently in the species of Cactus, the huge shoot-axes of
which possess a thin green lamella only at the circumference. However, this is not
the place to enter more closely into detail respecting the important physiological pro-
perties of the assimilating parenchyma, since this can be done in a proper manner
only in connection with the theory of the nutrition of plants to be expounded later.

In what has been said concerning the epidermis, the vascular bundles, and the
fundamental tissue, we have a description, superficial though it be, of the typical
histological structure of vascular plants, i.e. the Phanerogams and Cryptogams.
According to the requirements of the mode of life, the most various deviations from
the forms of tissue mentioned
may occur; of these we do
not propose to treat further,
since what has been stated suf-
fices as a basis for further phy-
siological considerations. As
in the treatment of the external
segmentation of plants, I shall,
in connection with the three
typical systems of tissue, again
allude to the corresponding
differentiations of tissue in the
simply organised plants; where
we find, according to the nomen-
clature introduced earlier, the
rudimentary beginnings of the
three systems of tissues. If it
were here proposed to exhaust
all the various histological re-
lations of the lower plants, the
material for several volumes
would present itself; it suffices

however for my purpose to confine ourselves to a few short remarks, simply to
serve for the guidance of the reader.

It is already known from the first lectures, that in the Mosses, Algse, and Fungi,
many organs which in the vascular plants consist of multicellular masses of tissue,
are constructed only of single cells, jointed filaments, or simple cell-layers. In such
cases, it is obvious that a differentiation into epidermis, fundamental tissue,
and vascular bundles cannot be spoken of. Where, however, the organs of
the lower plants consist of several or numerous layers of tissue, we always
meet with a more or less evident differentiation, which then presents itself as
a rudimentary form of the three systems of tissue adopted by us. This is
to be seen particularly clearly in the true Mosses, which are nevertheless highly
organised. The spore-forming capsule (the sporogonium or moss-fruit) shows,
especially in the highly developed typical Mosses, a sharply diff"eienliated epider-
mis, which may even be provided with stomata, marked off" from an inner mass

(550). A in the
cellular spaces (

ansverse section through the leaf of Sclaginella ittaeqiiati/olia
iddle ; B at the margin ; ch chlorophyll grains— the small dots in
rains ; eu epidermis of lower side ; eo that of upper side ; / inter-
ntaining air ; sp <

I50

LECTURE IX,

of tissue, chiefly parenchymatous fundamental tissue, and which is generally
differentiated into a compact colourless portion, and chlorophyll-tissue traversed
with numerous intercellular spaces. Of special interest in this respect is the
segmenting off of a portion of the epidermis as the deciduous cover of the
moss-fruit, and, in the more highly organised forms, the formation of the so-called
peristome. This consists of four, eight, sixteen or more so-called teeth, which on
their part arise from peculiarly differentiated rows of cells beneath the cover mentioned,
by the strong thickening and lignification of the cell-walls : this is illustrated in
part by Fig. 122, and the accompanying Fig. 160. It has been already pointed
out that in the fiUform thin shoot-axis of the true Mosses, a strand, consisting of
narrow, elongated cells, and which is undoubtedly to be regarded as a rudimentary
vascular bundle, runs within a well-marked fundamental tissue, which is surrounded

Fig. :59.— Tran
single cells of tlie

:rse section of tlie stem of Bryitm
:-hairs produced by the outgrowth
itermost layer.

Fig. is^b.—Fiinaria hyoroiiietyfca. .1 a small leafy
shoot {g) with the calyptra (r) ; B a plant (g) with the
almost ripe sporogonium ; s the seta ; y the capsule ; c the
calyptra; Ca longitudinal section through the middle of
the capsule; (/operculum; a annulus ;/ peristome ; c c'
columella; h air cavity; s mother-cells of spores. At t
the loose tissue of the columella presents the appearance
of confervoid filaments.

by a more or less sharply defined epidermal layer; and when the leaves of the
Moss, elsewhere consisting of a simple cell-layer, possess a mid-rib, in this also
a rudimentary vascular bundle runs, joining that of the shoot-axis. Since the
roots of the INIoss only consist of jointed cell-filaments, such tissue differentiations
obviously cannot exist in them.

Much simpler are the forms assumed by the tissues in the majority of the
Liverworts ; which, however, in their most highly organised forms, the Marchantiae,
nevertheless attain a very considerable degree of organisation. The ribband-Hke
foliar shoots of these plants, lying flat on the substratum, produce root-hairs on the
lower side, as well as leaf-like outgrowths ; while the upper side of the shoot developes
into an organ of assimilation. A sharply marked epidermis invests an inner
mass of tissue consisting of several layers, which is to be distinguished as funda-

TISSUES OF MUSCINE.E, ETC. 15I

mental tissue. Correspoiuling to the dorsi-vcntral structure spoken of, the under side
dispenses with stomata : these however are so much tlie more numerous on the
upper side, and are developed in a form deviating from that usual elsewhere, as
is shown in Fig. 161 .7^. The predominating parenchymatous fundamental tissue of
the Marchantia shoot is also differentiated in accordance with its ventral and dorsal
side. It consists of a large-celled parenchyma, in which, according to Goebel,
there are forms of cells which may be looked upon at any rate as feeble indications
of vascular bundles ; beneath the epidermis of the upper side of the shoot lies
a spongy assimilating tissue, which is divided up into sharply bounded areolce, each
opening on to the exterior through a stoma.

From these highly organised true Mosses and Liverworts, where the most
essential relations of the typical systems of tissue are still to be recognised, a series
of forms lead by imperceptible stages down to the simplest representatives of these
two classes of plants, in the histological structure of which scarcely any traces of
differentiation are to be found.

FIG. 160.— The mouth of the cap-
sule (*) oi Fontinalis antipyretica (X50).
ap outer, and i inner peristome (after
Schiniper).

FIG. 161.— Transverse section of the flattened shoot oi Marchantia poly-
morpha. u epidermis of lower surface ; 0 that of the upper surface ; sp
stoma ; clU chlorophyll tissue within an areola (bounded by ss\ ; p colourless
parenchyma.

I have already, in the introduction to the systems of tissue, mentioned by the
way that in the Algai and Fungi, when they consist of solid masses of tissue
(which is, of course, a necessary condition), a differentiation into epidermal and
fundamental tissue takes place, in which not rarely there runs a rudimentary
vascular bundle consisting of long cells; and here the additional remark suffices,
that stomata are always wanting to the epidermal tissue, even when it is otherwise
sharply marked off.

In the consideration of the external segmentation of the Algx and Fungi, it
was necessary to allude to the fact that instances of organisation occur, which often
depart so completely from the typical relations of the INIosses and vascular plants, that
we must look upon them no longer as rudimentary forms of the latter, but as quite
peculiar. The same holds also with reference to the differentiation of tissue of many
Alg£e and .Fungi. Here, again, it must however suffice to illustrate by a few
examples what has been said ; and indeed it is chiefly a matter of establishing
the fact that even when the histological relations are entirely abnormal, the

[52

LECTURE IX.

differentiation into a firmer, more resistent, epidermal layer, contrasted with an
inner mass of tissue corresponding to the fundamental tissue, still recurs ; and that
where the relations of form otherwise admit of it, a bundle formed of parallel,
elongated elements runs in the fundamental tissue, which again is to be regarded
as the rudiment of a vascular bundle.

Apart from the Mucorini and a few other non-cellular Fungi, the hypha, or
jointed fungus filament, is maintained with striking consistency as the elementary
form of histological structure in the Fungi. The hypha is a filament segmented

by transverse septa, mostly very thin,
Y_,^^ _-,x growing forward at its end, and

often much branched. In the so-
called Mould-fungi, the mycelium, as
well as the fructification, consists of
single hyphffi ; where, however, the
vegetative body is more massive, as
in the fungi commonly known as
Truffles, Mushrooms and Toad-stools,
Gasteromycetes, etc., the mass of
tissue, often variously differentiated,
is likewise composed entirely of
such hyphse. The tissue of these
plants is thus constructed, not, as in
the majority of Algae and all Mosses
and vascular plants, by means of bi-
partition and a corresponding forma-
tion of chambers, commencing ori-
ginally in the unicellular embryo
and proceeding with the growth ; but
the internal structure of the Fungus
gives the impression of very numerous
hyphae, each one of which strictly
speaking leads an independent exist-
ence, having become united into a
colony, the single individuals of which
— that is, the hyphae — are subordi-
nate, however, to a common plan of
configuration. Instead of detailed
explanations, the consideration of
the half diagrammatic Fig. 162 may give an idea of the facts indicated. In
spite of this structure, entirely deviating from the usual histological type, here again
a sharply marked differentiation of tissue nevertheless results: in Fig. 162 this is
expressed chiefly by the closely packed (and, as it seems, lignified or other-
wise altered) hyphae running on the surface of the tissue-body, representing an
epidermal tissue, to which even hair-like outgrowths are not wanting. ' Moreover,
we perceive how in the internal mass of tissue the hyphae bound certain spaces ;
of which the darker roundish portions enclose hollow chambers, in which the

|,.;»,«#f' ,.

Fig. 162.— Half of a longitudinal section through the fructification of
Cruciliiilnm ■vulgare (a Gasteromycete). ap the so-called outer, ip the
inner peridium ; these together constitute the skin ; j^^liairs. The figure
is diagrammatic in so far that the hyphae are represented far too thick.

TISSUES OF LICHENS.

^53

reproductive cells (spores) are produced,
ditferentiated, however, as in the fruits
of Phanerogams, at the ripening of fruc-
tification. The previously slimy internal
hyphal tissue then dries up completely,
and the dense portions containing the
spores then appear as peculiar organs,
which remain free in the cavity of the
entire Fungus : this now, enclosed by
the firm epidermal layer, has some-
what the shape of an ordinary flower-
pot.

The fruticose Lichen, represented
in Fig. 163, may serve as a second ex-
ample of hyphal tissue. The Lichens
are, as we now know, true Fungi,
which have the peculiar habit of en-
closing in their tissue their host-plants
which contain chlorophyll (that is small
Algae) without interfering with their re-
production : these appear in our draw- t
ing as dark granules. These elements, ;
serviceable, if is true, to the proper
bo(^y of the Lichen, but foreign in
other respects, behave within the hy-
phal tissue just as if they only con-
stituted a special layer of tissue, cor-
responding to the assimilating paren-
chyma of a green plant ; and this to
such an extent, that up to sixteen years
ago, when De Bary first perceived the
true nature of Lichens, these en-
closed Algaj were considered as a
special form of tissue of the body of the
Lichen itself. Apart, however, irom
these remarkable facts, Fig. 163 shows
with all requisite clearness the differen-
tiation of the hyphal tissue into three
systems. The outermost of these may
be at once designated epidermal tissue ;
and in the interior is a strand separated
from the epidermal tissue. Thus again
we meet with the three forms, which we
have to regard as the most rudimentary
indication of the three typical systems of ^
tissue of the higher plants.

The tissues in this case are only fully

Wf|.

zk' 2ta, a fruticose Lichen. ^ optical longi-
1 branch, in potash; B transverse section of a
1/1,0- (so-called Gonidia) ; r epidermal layer;

Fig. \6i.—Ha/y)iie<ia «puyttia. A a shoot— natur
idinal section of a segment (slightly magnified) perpendii
ane of the paper. The segments of.-/ are flat.

154

LECTURE IX.

A similar internal differentiation results, however, even in the case where
a plant from the subdivision of the Cceloblastge only consists of a single
vesicle, the growth and branching of which is not accompanied by cell divisions at
all. Among the Coeloblastag in this connection are especially to be mentioned the
marine Algce Codium and Halymeda. These are plants of considerable size, variously
segmented, and apparently composed of masses of tissue ; a transverse or longitudinal
section, slightly magnified, presents apparently an ordinary plant tissue, resembling
that of many other Algae, until closer inspection shows that the apparent cell tissue
consists of the ramifications of a tubular cell in itself continuous, the thousand-fold
repeated outgrowths of which are more or less segmented by constrictions. Here
also the comparison of Fig. 164 will show the true state of the case. One has only
to observe that Fig. B represents the longitudinal section of A^ perpendicular to the
plane of the paper, and indeed only through a small portion of the length of one
of the Opuntia-like segments of the shoot. It is perceived at once, that the vesicles
form at the periphery of the shoot a dense layer, representing to a certain extent an
epidermis; and that in the axis the vesicles run longitudinally and are long-jointed,
representing a looser vesicular system traversed by interstices, between the epidermal
tissue and the axial strand, which however, corresponding to the form of the shoot,
is to be imagined broad and flat.

LECTURE X.

THE SECONDARY GROWTH IN THICKNESS OF SHOOT-AXES
AND ROOTS.

Having started with the typical three systems of tissues, and traced the
anatomical differentiation to its most rudimentary beginnings in the INIosses, Algae,
and Fungi, we may now pass to the consideration of the highest stage of development
which the formation of tissue undergoes in the vegetable kingdom. We find this
chiefly in true woody plants, the trees and shrubs of the natural classes Gymnosperms
(especially the Conifers) and Dicotyledons. But in many species of both these
sub-divisions, not generally considered as woody plants, exactly the same processes
of tissue-formation take place as in perennial woody plants ; in fact much of what
is to be said concerning the formation of wood and the processes accompanying
it is often especially evident in annual and biennial plants. In anticipation, however,
it may be mentioned at once that the phenomena of growth to be treated of here
do not, as a rule, occur in water-plants ; since the biological relations connected with
subsequent growth in thickness have in general a meaning and purpose only in
transpiring land plants. We may comprehend the phenomena here concerned under
the idea of the secondary giowth in thickness of shoot-axes and roots'.

As the whole process of growth in length and the origin of new organs is
referred to the growing point of the roots and shoots ; so the entire growth in
thickness springs from the functional activity of a thin layer of tissue of similar
character — i.e. from the cambium. Where this remains active for a long time,
as it does occasionally even for centuries, the originally thin root-fibres and shoot-
axes gradually develop into those huge thick bodies met with as old tree-trunks
and their branches, and napiform roots and tubers. Although these organs
originally (i.e. immediately after the conclusion of their growth in length) are com-
posed, as already described, of epidermis, fundamental tissue, and vascular bundles ;
later, when the growth in thickness by means of the cambium has continued
long enough, but little remains of that original structure, the whole of the thick
stem, branch, or root, consisting, apart from insignificant remnants of the primary
pith and vascular bundles, entirely of the products of the cambium. With the

' The extensive subject of the secondary growth in thickness, and especially the numerous
abnormal cases not touched upon in the text, are thoroughly and accurately treated in De Har\'s
' Vcrgl. Aiiat. der Vegcfafioiisorgaiic,^ i^77-

1^6 LECTURE X.

increasing age of such plants, the assimilating leaf-surface enlarges ; and accord-
ingly there gradually arises the necessity for a larger and more extended root-
system, the parts of which, as well as the shoot-axes, gradually grow in thickness.
The thickened roots, stems, and branches present not only the necessary solidity of
structure, but they are also essentially adapted to meet the increased requirements
of the large leaf-surface. In the first place, it is necessary to convey from the
earth to the leaves large quantities of water and dissolved salts : this is attained by
the woody body, which is the organ for conducting water, continually increasing in
thickness. In addition, large quantities of products of assimilation are elaborated in the
crown of foliage ; these must, at least in part, be conveyed to the subterranean roots
and lower portions of the stem. Accordingly a so-called secondary cortex (secondary
phloem), gradually increasing in thickness, becomes developed from the cambium ; and
in this secondary cortex, sieve-tubes and parenchyma form the chief conducting organs.
At the same time masses of parenchymatous tissue in the cortex, as well as in the
wood, are employed for the storing up of reserve materials. If, however, we consider
the distances which the ascending as well as the descending nutritive sap traverse
in the roots, stems, and branches, it is evident that the ordinary thin epidermis
could not afford sufficient protection against injurious evaporation during this
movement. In many cases, therefore, in woody plants, the epidermis becomes further
developed. Generally, however, it is replaced by a stronger layer — on the parts still
young by typical cork-tissue, which possesses in an enhanced degree the properties of
the epidermis ; on older parts by the formation of bark, which in its turn affords
protection not merely against prejudicial drying up of the sap-passages in the
cortex and wood, but also against mechanical injuries which might occur in the
course of time. These remarks may serve to indicate simply the meaning of all
the structural arrangements which are the result of the activity of the cambium.
In true water plants, which transpire but little, or not at all, the necessity of a
vigorous supply of water from the roots is of course entirely wanting, and accord-
ingly no subsequent formation of true wood takes place ; consequently the increase
in circumference of stem and root is unnecessary; and all those adaptations accom-
panying the formation of the secondary cortex, and of the cork and bark of the
woody land-plants, are also wanting. Of course nature employs yet other means
for the attainment in land plants of a considerable size of the body, and for ensuring
a long continuity of life to the roots and shoot-axes. Among the Palms and
palm-like Liliaceae, for instance, as well as among the Ferns, plants of large
dimensions are found, in which great solidity of the older roots and shoot-axes is
necessary ; and where practically the same remarks hold good as to the conduction
of the sap. The circumstances here, however, are entirely different, in so far that
in such plants the stem supporting the huge crown of leaves soon obtains its
final thickness beneath the growing point. The tissues serving as passages for
the sap, and as elastic masses, are developed equally from the beginning, without
any subsequent enlargement in diameter; and this again is connected with the
circumstance that the assimilating crown of foliage, although continually renewed,
f3oes not annually increase in size as in true dicotyledonous tr^es.

I will now, in the first place, attempt to characterise as shortly as possible the
tissue formations proceeding from the cambium and caused by the growth in thick-

THE CAMBIUM-RING. l57

ness. In this, however, I shall again keep in view only the commonest or typical
cases. That in the great variety of the woody plants, innmnerable more or less
extensive deviations from this type occur, hardly needs mention. What is to follow
concerns, in the first place, the growth in thickness of the shoot-axes ; deviations in
the roots (always insignificant) may be mentioned with them as occasion arises.

The whole growth in thickness is connected, as stated, with the functional activity
of the cambium. The origin of the latter, however, is itself again dependent upon the
original nature of the vascular bundles. Here are concerned only those parts of the
bundles which run as so-called leaf-traces in the interfoliar parts, the upper ends
being cast off in the leaves at their death. In the plants in question, these leaf-
traces are seen, on the transverse section of the shoot-axis, to be arranged in a
circle; and their longitudinal course is in general parallel to the surface, as
represented in Fig. 129. The phloem-portions of these usually not very numerous
bundles are all turned towards the surface of the shoot-axis ; the xylem-portions
being directed towards the centre of the transverse section. From the very first,
even before the origin of the cambium ring, the elements of the vascular bundles
are arranged in radial rows.

The first indication of the commencing growth in thickness consists in that a
layer of cells, lying in the vascular bundle between the phloem and xylem, grows in the.
radial direction, and accordingly becomes divided by tangentially placed partition-
walls. Thus originates the fascicular cambium. The cells of this, arising by con-
tinually repeated divisions, when they lie towards the phloem side develope into elements
of the secondary cortex (secondary phloem), and when they arise on the inner side of
the cambium constitute new elements of the wood. In many cases, the growth in
thickness progresses in such a manner that this cambium layer, lying in each
individual vascular bundle, causes the phloem as well as the xylem of the bundle to
increase in the radial direction, so that the primitively rounded transverse section of
the bundle becomes gradually elongated radially, and widened outwards into a wedge
shape. The fundamental tissue lying between these vascular bundles which are
growing in thickness, developes at the same time, by repeated cell-divisions. In this
manner, however, a closed mass of wood is not formed ; nor is a continuous layer of
secondary cortex formed around the wood (e.g. stem of Gourd).

In the typical case of growth in thickness, there is formed, after the production
of a cambium layer in each leaf-trace, a similar layer also in the fundamental tissue
bet\^een each two neighbouring bundles (see Fig. 165,.^); and this always in such
a manner that the parenchyma cells of the fundamental tissue are elongated in the
radial direction, and divide by means of tangentially placed longitudinal walls. Thus
arises the interfascicular cambium, which becomes joined on to the fascicular cambium,
and together with this constitutes a continuous cambium layer. This appears on
the transverse section as a ring; but of course it is really a hollow cylinder,
running in the tissue of the shoot-axis. On the inside of this cambium-ring lie
the xylem-portions, and on its outside the phloem-portions of the leaf-traces ; and
since the cambium-ring produces wood progressively on the whole of its inner
side, a woody ring or hollow cylinder of wood is produced, enclosing the pith
or inner portion of fundamental tissue. On the outside of the cambium-ring arises
in like manner a hollow cylinder of secondary cortex. It may be mentioned

158

LECTURE X.

here that with this mode of growih, certain differences between the arrangement and
form of the secondary cortex and that of the secondary wood necessarily arise.
The cambium-ring increases progressively in diameter, and the wood-elements arising
from it, and which grow but little in transverse section, need only be deposited
so to speak by apposition on the wood-elements already present; hence the
transverse section of the wood comes to present a very evident arrangement of

its elements in radial rows, and
in layers concentric with the
periphery. On the outside of
the cambium-ring, on the other
hand, it by no means suffices
that the layers of cortex already
existing receive new deposits
on the inside ; for the conti-
nually progressing increase in
circumference of the cambium-
ring necessitates a further
growth in the peripheral direc-
tion of at least .a part of the
existing elements of.the cortex.
In consequence of this, subse-
quent alterations of the tissue-
elements in the cortex take
place, by means of which, under
certain circumstances, their ra-
dial arrangement is much dis-
turbed : the arrangement in
concentric layers, on the other
hand, is usually more marked.
Finally, however, the peripheral
growth frequently ceases in the
most external (i. e. the oldest)
cortical layers ; and then, in
th the development of conscquencc of the formation

■n at different stages ni '■

and grow^th of internal lawyers,
longitudinal cracks arise on the
exterior, or other distortions of
the outermost, oldest layers of
tissue take place.

When the terms wood and
cortex are employed in what follows, the secondary wood and secondary cortex
developed from the cambium-ring are always to be understood. We will now
consider somewhat more closely the anatomical constitution of these tissues.

The wood, as well as the cortex, consists of two systems of tissue, the origin of
which is already to be seen in the cambium. First, of elements which are elongated
longitudinall}-, and generally deposited together in the form of bundles or groups of

FIG. 165.— Diagram of ordinary growth in thickni
a compact woody mass. ^, B, C the same transverse
order of age. A before the origin of the interfascicul
duction; C .ifter the c.imbiimi has been active for
primary cnrt. \ : 1/ lili : - |]liloeni ; x xylein of tlie \
groups of t ' : phloem. X.B these ar.

fascicular c,i . ;.- , ,;/ >rl,iscicular cambium ; fh t

fascicular cdiui.uim ; i/'i ilie wood developed from

ir cambium ; B after its pro-
ome time. Everywhere, R
iscular bundles \ b b b three
widely separated in C\ fc
le wood developed from the
lie interfascicular cambium ;

i/p the seconder;
diagram is based

tissue deve'oped from the interfascicular cambium— the
1 of the hypocotyl of Ricinus cotnj/nniis.

MEDULLARY RA VS. 159

fibres with a sinuous course. The meshes of the network which thus arise, forming
longitudinally elongated slits between the bundles named, are filled up by horizon-
tall}' elongated parallel rows of cells, running in the radial direction from the interior
to the outside ; these are the so-called medullary rays, which, according as they run
in the wood or in the secondary cortex, are to be distinguished as xylem-rays (the
"silver-grain'' of the carpenter) or cortex-rays (phloem-rays). It is to be noted,
however, that each cortex-ray is simply the outer continuation of a xylem-ray,
formed by the cambium. This coarser structure of the product of the cambium
is to be seen very plainly in the decomposed stems of Brassica (Cabbage), Carica
papaya, and other plants, and still more conspicuously in the woody skeletons of
various species of Cactus — e. g. Cereus, Opuniia, and others, where the meshes between
the wood- and bast-bundles are unusually distinct. In like manner, the broad bands of
the somewhat strongly ligniried(an exceptional occurrence) commercial bast of the Lime
illustrate what is here said. In these cases the radial tissue of the rays, which fills
up the meshes between the wood- and bast-bundles as they undulate tangentially,
becomes destroyed by rotting and the action of the weather in general, because it
consists of soft non-lignified cells ; in the true woody plants, on the other hand, as
may frequently be observed in the decomposed stems of the Red Beech in woods, the
substance of the xylem-rays is occasionally more resistent than that of the wood
bundles, and while the latter become destroyed by rotting, the former remain behind,
constituting to a certain extent a skeleton of radially disposed plates. On a tan-
gential section, moreover, as well as on radial split surfaces of ordinary wood, the
larger rays are perceived as bands running horizontally, which traverse the woody
mass from within outwards ; the smaller and very small ones are only to be seen
with the microscope. Moreover only the first few rays, already existing at the
beginning of the growth in thickness, run from the pith through the whole thickness
of the wood into the cortex : they break up the mass of wood (seen on the transverse
section) into a small number of wedge-shaped portions, with the broad side outwards.
The rays subsequently developed with the progressive growth in thickness are much
more numerous, and the later they arise, the further they are removed from the pith
in the wood. They break this up, as seen on the transverse section, into continually
finer, radially disposed portions, arranged in a fan-like manner. It is always to be
understood, however, that this breaking-up apparent on the transverse section, is only
the expression of the longitudinally elongated meshes in the undulating course of
the wood ; and that the rays themselves only represent the filling up of these meshes.
I may take this opportunity of remarking, as to the principle lying at the base of all
the tissue formation of the higher plants, that, apart from certain organs of secretion
and isolated idioblasts lying in the tissue, similar tissue-elements of a plant stand
everywhere in contact with one another. As the epidermis, the vascular bundles, and
the fundamental tissue, as well as the individual cells of the two latter are all in con-
tinuous connection throughout the entire plant ; so with the elements of the wood
and cortex, now to be described more in detail, so far as concerns tissue-elements
of the same kind. This principle comes out particularly clearly in the case of the
medullary rays, inasmuch as these run as parenchymatous nourishing tissue con-
tinuously Irom the w^ood into the cortex, and in their turn are connected v/ith the
parenchymatous elements of the wood-bundles as well as with those of the cortex.

i6o

LECTURE X.

As in the primary vascular bundles, on the existence of which the formation of
the cambium-ring and the entire growth in thickness depends, we can also distinguish
two main groups of tissue-elements in the products of the cambium. The whole of the
secondary cortex arising on the outer side of the cambium-ring is in the main a continued
development of the phloem of the vascular bundle ; and the wood-mass arising on the
inner side of the cambium-ring is in like manner a further development of the xylem of
the original vascular bundle. We here neglect the medullary rays for the moment. In

the cortex, as well as in the wood, we
have again, in addition to vascular ele-
ments (sieve-tubes and wood vessels re-
spectively), also parenchymatous tissue
(xylem- and phloem-parenchyma) and
elastic fibres (wood and bast-fibres).

We will, in the first place, consider
the composition of the wood arising
from the cambium-ring. In the Coni-
fers, this consists entirely of elongated
tracheVdes, or with a very small admix-
ture of parenchyma. These are long
fibres, pointed above and below, on the
radial side walls of which are remark-
ably large, isolated, bordered pits, which
produce a very characteristic appear-
ance (Fig. 1 66). In the Dicotyledons, on
the other hand, such trachei'des usually
enter, it is true, into the composi-
tion of the wood; together with these,
however, three other elements occur.
First the wood - vessels, which tra-
verse the wood -bundle as continuous
tubes, isolated or in groups, and usually
with bordered pits. More rarely they
are reticulately thickened (annular and
ordinary spiral vessels do not occur
in the secondary wood, though the
Cactacese possess trachei'des with the
walls thus sculptured), and then follow
parenchymatous cells — the wood-parenchyma. These cells occur either in the
form of spindle-shaped fibres pointed above and below, but with living contents ;
or they resemble these but are chambered by several transverse septa. Sometimes
forming the main mass of the wood, sometimes less conspicuous in the mass,
are the proper wood-fibres, or libriform fibres. These are narrow, thick-walled,
elongated cells, obliquely pointed above and below ; the wall-structure of which
may exhibit all transitions from that of trachei'des with bordered pits to that of
bast fibres. Of these histological elements of the wood, the one or the other may
predominate, or exist in small quantity, or be wholly wanting. Generally they

Fig. z66.—Pi!iits sylvestris. Radial longitudinal section
the wood of a vigorous branch; cb cambial wood-cell; a-
wood cells; ^/'/"bordered pits of the wood-cells in order
St large pits where cells of the medullary rays are in contact
wood cells (X 550)-

THE WOOD.

i6i

are all present, and their relative quantity and mode of grou])ing determine the
character of the wood ; which appears accordingly as a loose porous mass abounding
in vessels and tracheides, or even chiefly parenchymatous, or, on the contrary,
as a firm, dense mass of tissue chiefly composed of libriform fibres. Libriform
fibres, trachei'des, and wood-parenchyma form in ordinary cases the dense matrix
of wood in which the vessels run ; these usually possess far wider lumina,
and are often recognisable on the transverse section even by the unaided eye as

#-s.^/.ä\i !

FIG. 167.— Tangential longitudinal section through the ho
St medullary rays cut across ; / wood parenchyma ; t tracheidc

punctate openings, and on the longitudinal section as more or less evident narrow
canals.

The vessels, trachei'des, and libriform fibres of the wood, as soon as they are fully
developed, contain air, and their walls are usually strongly lignified ; only the paren-
chymatous fibres {^Ersatz-fasern and proper wood parenchyma) contain protoplasm
and products of assimilation, and possess thin walls, which are pitted in the usual
way and are often feebly or not at all lignified.

Apart from a few still doubtful exceptions, the woody mass arising from the
cambium in the course of years is composed of concentric layers, which appear

[3]

[62

LECTURE X.

on the transverse section as rings ; and, since one of these is formed each
year, they are distinguished as annual rings. These are, of course, in reahty hollow
cylinders disposed one around the other. Each annual ring is the product of the
wood-forming activity of the cambium-ring during one period of vegetation. That
these appear, even to the unaided eye, as layers of the wood-body sharply separated
from one another, depends upon the fact that within any one period of vegetation
(i.e. in the time during which an annual ring is formed), the formation of wood itself
varies periodically. It is evident that the inner portion of any one annual ring is the
first to be developed in the period of vegetation — that is, it is formed in the spring ;
while the outer portion of the same annual ring has arisen towards the conclusion of
the wood-forming activity in the same period of vegetation. At the boundary of suc-
cessive annual rings, therefore,
the spring-wood of the follow-
ing ring is always deposited
on the autumnal woo i of the
preceding next inner ring ;
so that the usually different
wood formations of two pe-
riods of vegetation lie imme-
diately adjoining one another.
Owing to this, the separation
of two neighbouring annual
rings is more clearly marked.
Put quite generally, the spring-
wood is composed of ele-
ments with wide lumina and
thin walls ; the autumn wood
last formed in a period of ve-
getation consists, on the con-
trary, of elements with narrow
lumina, and often also thick
walls. This difference comes
out very clearly in the simple
wood of the Conifers, which
consists, as stated, entirely of tracheVdes. The transverse sections of these are nearly
square in the spring wood ; in the autumn wood, on the contrary, they are narrow
and rectangular, so that the cells appear compressed in the radial direction to
one-third or one-fourth the diameter of those of the spring wood.

Apart from the stronger thickening of the walls, the autumn wood is al^o
richer in wall-substance and poorer in cavities than the spring wood. In the
dictyledonous woods there is associated with these differences affecting the paren-
chyma, tracheides, and libriform fibres, a corresponding difference in the vessels,
so that in general the vessels l}-ing in the spring-wood are far larger in trans^•erse
section than those formed in the summer and autumn. Frequently, the relative
number of vessels in the rings of wood also diminishes with the advancing season.
Thus, taken all together, the spring wood is more porous, looser, and poorer in

FIG. i68.— Transverse section of the wood of Rhainnns fj
autumn-wood of an older annual ring ; w vessels in the spring-woe
ring (after Rossmann).

ALBURNUM AND DURAMEN. 163

substance than the autumn wood. The transition from the one to the other in
the same annual ring may be gradual or sudden ; and it is to be added that the
middle part of an annual ring, between the spring and autumn wood, is often
especially rich in libriform fibres.

On the transverse and longitudinal section of the older stems and branches of
trees, the difference between the so-called splint (alburnum) and the heart-wood
(duramen) is usually very marked. The alburnum is a pale, more or less thick
zone of wood lying under the cortex, and in which two, three, or many annual rings
may be recognised; the wood which lies within this zone is generally dark-coloured
(red, yellow, brown, or black), and formed of a much harder mass. This is the
duramen, alone employed in the arts. Since the thickness of the alburnum remains
practically the same, while the duramen increases in thickness year by year, it is
obvious that an inner layer of alburnum becomes transformed annually into a new
outer layer of duramen. This difference between the alburnum and the duramen is
not always evident in the colouring and hardness — in the Fir, for example; and in-
deed it is stated of the Box-tree and some kinds of Maple, that this difference does
not exist at all, but that the entire wood of an older stem consists of alburnum. The
alburnum is the wood in the condition in which it has arisen in a period of vegetation,
by the development of the various wood elements from the cambium : it is the normal
wood, which comes immediately and alone into consideration for the vital activity of
the plant. The water absorbed by the roots ascends in it, and reserve materials are
deposited in its medullary rays and other parenchymatous cells. The wood which
has passed over into the condition of duramen, on the contrary, takes no more direct
part in the vital activity of the plant; it is to be regarded as a mass of tissue passing
over into decomposition, and perhaps also destined for the deposition of excretions.
The duramen owes its darker hue, as well as its increased hardness and re-
sistance to destruction, to the infiltration of dark-coloured substances, often soluble
in water as colouring matters (Red-wood, Blue-wood, Yellow-wood) ; or of resinous
bodies {Guaiacum, Conifers) ; or occasionally even of silica (Tectona, Iron-wood), and
other substances which, like these, in extreme cases not merely impregnate the walls,
but partly, or even entirely, fill up the cavities of the fibres and vessels.

In contrast to the resistent, elastic woody mass, the secondary cortex (secondary
phloem) appears on the outside of the cambium-ring, generally as a softer mass of
tissue composed chiefly of active living cells ; in this tissue, however, bast-fibres,
singly or in groups, produce a conspicuous fibrous structure of great toughness. As
already mentioned, the sie\e-tubes are to be regarded as the characteristic main
element of the secondary cortex ; in addition to which a larger or smaller quantity of
thin-walled, elongated, soft parenchyma is never wanting. It is usual to distinguish
both these together as the soft bast, in contradistinction to the proper bast-fibres.
The latter are generally very thick-walled, often very long (but occasionally short)
tough fibres ; which in some cases are completely wanting in the secondary cortex,
and in others are scattered as isolated elements in the soft bast. Frequently,"
however, these predominate throughout the structure of the secondary cortex, in the
form of more or less thick bundles, or even layers. Like the various elements
of the wood, those of the secondary cortex may also be present in very different-
quantities and variously arranged.

M 2

164

LECTURE X.

The whole structure of the secondary cortex is influenced by the horizontal
tissue of the medullary rays to an extent even greater than is the case in the
wood. To mention two extreme cases only; the medullary rays, when they traverse
the cortex in great numbers, may cause the whole of the remaining tissue to appear
arranged chiefly in radial rows on the transverse section {Ctnchoiia); and when bast
fibres are present, these also appear on the transverse section to be arranged chiefly
in radial rows. In the other extreme case, on the contrary, individual medullary rays
become much wider as they proceed from the cambium to the exterior ; since their
parenchyma cells grow very strongly in the tangential direction, in correspondence
with the increase in circumference of the stem, and become chambered by radial
longitudinal walls, thus constitudng a parenchymatous mass of concentrically
arranged layers of cells, in which lie thicker bundles of soft bast, traversed under
certain circumstances by bast-fibres. In the Conifers the arrangement in radial rows

Fig. 169. — Transverse section of the stem of ^U7izperi{s conimitJtzs» jrjf Cambium ; hh
autumn wood of the youngest annual ring-; bb bast-fibres of the secondary cortex, in peri-
pheral rows and with sieve-tubes between them ; »z medullary rays (De Bary).

as well as that in concentric layers is usually evident ; and Fig. 169 shows at once
how the thick-walled bast fibres are here disposed in layers within the soft bast.

Observed on a tangential section, the secondary cortex, like the wood,
is composed of sinuous or undulating bundles of elongated elements connected
in a net-like manner ; the meshes of which, likewise elongated longitudinally,
are filled up by the parenchyma of the medullary rays. This structure is to be
seen even by the unaided eye when the bundles abound in bast-fibres.

The activity of the cambium is generally less energetic towards the exterior
than on the inside; i.e. the formation of cortex proceeds in the radial direction
much more slowly than the formation of wood. This is obvious at once, on
comparing the cortex in the transverse section of a large Beech stem, where it is
only a few millimeters thick, with the huge mass of wood ; both of them being of the
same age.

In shoot-axes which eventually proceed to perennial growth in thickness, the

PERIDERM.

165

formation of periderm occurs before the commencement of the growth in thickness,
or simultaneously with it, or later. By periderm is understood a layer of tissue
consisting of cork-cells, which in normal cases surrounds the whole of the shoot-
axis or root as a continuous envelope. In most cases it is of inconsiderable
thickness ; occasionally, however, as in the Cork-Oak, it forms a layer (bottle-cork)
several centimeters thick. We have in the peel of the potato a very instructive
example of the formation of simple periderm. In some cases, as in the Elm, the cork-
periderm grows out in the form of isolated longitudinal ridges. Thicker periderm
may consist of alternate layers of thick-walled and thin-walled cork-cells; and the
individual thin layers of cork not rarely become stripped off from one another, as on

V~.h

Fig. 170.— Commencing development
of periderm in the shoot-axis of ^ttojia
cheirimolia (transverse section), e epi-
dermis ; cc cells dividing to form cork ; r
cortical parenchyma containing chloro-
phyll.

^ftÖ;V

Fig. 171.— Cork formation in a first year's branch oK Rihes ntgrmn
part of transverse section, c epidermis ; h hair; b bast-cells ; pr cor
tical parenchyma, distorted by the growth in thickness of the branch
A' the total product of the phellogcn (f) ; k the radial rows of cork-cells,
developed from c in centrifugal order ; /rfphelloderm— parenchyma con^
taining chlorophyll, and arising centripetally from c (X 550).

the older branches and younger stems of the Birch, and especially in many species
of Melaleuca. The periderm is a strengthened substitute for the epidermis. The
suberised cell-walls fit together on all sides without intercellular spaces, and possess
the properties of the cuticle and the cuticularised outer cell-walls of the epidermis.
The protection which they afford to the inner tissues against the evaporation of their
fluids is increased by the cork-cells dying and losing their sap immediately after be-
coming fully developed ; while granular or amorphous matters not rarely fill up the
cavities in part or entirely (Birch). The origin of the periderm is due to a formation
of tissue similar to the cambium. This cork-cambium or Phellogen either (and more
rarely) originates in the epidermis itself, or in the layer of primary cortical tissue
immediately beneath it; or in a layer lying somewhat deeper. The cells referred to

l66 LECTURE X.

grow first in the radial direction, and then become divided by tangential longitudinal
walls. Of the two daughter-cells so produced, the outer developes into a cork-cell ;
while the inner one, continuously increasing in width radially, again divides. Of
the two daughter-cells now existing, the outer one is again transformed into a
cork-cell ; while the inner one remains active, and repeats the same process.
This layer of cells which continually become regenerated is the phellogen; the
similarity of which to the cambium is increased yet more by the fact that layers
of tissue often originate from the phellogen on the inner side also. These, as
the so-called PheUoder7n, strengthen the parenchymatous living cortex (cp. Fig. 171).
The arrangement in radial rows and at the same time in concentric layers (which
immediately results in part from the origin of the cork-cells mentioned above, and
in part from their quickly terminated growth) are almost universally characteristic of
cork-tissue.

As in many cases {Viscian, Acer sln'afwn, species of Coniiis, SiC.) the epidermis
itself follows the growth in thickness of the organ for many years; so also the
periderm (and this is oftener the case) may for years follow the increase in
circumference of a branch or stem by passive stretching and growth.

Sooner or later^ however, Avhere the growth in thickness persists for a long
time, a repeated changing of the peripheral tissues usually results — Bark is formed.
In rarer cases this occurs as so-called ring-bark. This arises from the development
of new layers of periderm in the deeper cortex, some cell-layers distant from the
preceding periderm ; and the older layers thus become exfoliated across the stem
( Vi'/i's, Clemalis, Cupressinese, according to De Bary). Scale-bark, of which we find
the simplest and most instructive example on the stems of Plalanus, is however
commoner. As is well known, there peel off from these stems in the summer
larger or smaller plates of tissue, often as large as the hand and 1-2 mm. thick;
and a somewhat rough surface, beset with corky warts, is left behind on the^tem
after their fall. If we now suppose that, at the same places in the cortex, such scales
gradually die, and do not fall off, but remain suspended and clinging to one another
on the stem, the latter would, after some years, become surrounded with a scaly
layer of dead masses of tissue, deposited one over the other. These together constitute
scale-bark; and at the same time, in consequence of the progressive increase in
circumference of the stem, these dead packets of scales become more and more
separated from one another by means of longitudinal clefts running between them.
This process is to be recognised very clearly on close observation of the bark of old
Pine stems. The origin of these scales of bark is brought about by the repeated
formation of thin lamellcC of cork within the living cortex ; these do not however run
all round the organ, l)ut their edges come to the surface at circumscribed places, so
that a portion is cut out of the cortex, so to speak, as if with a gouge. Not only the
phellogen-layer concerned, but also the whole of the cortical tissue lying on its
exterior dies off and dries up; whereupon the same process is again repeated towards
the interior. Since now these cork-lamellas cut out indiscriminately the most
various forms of tissue from connection with the living parts, the bark-scales con-
sist of all the different cell formations which occur in the cortex generally ; and it
is clear that, since by the progressive development of bark a thicker and thicker coat
grows around the stem, the living cortex itself, whilst being regenerated in its turn

liARk'. 167

by the cambium, always represents only a relatively thin layer of the tissues produced
from the cambium. After the primary cortex has been removed by the processes
mentioned, the older layers of the secondary cortex gradually take part in the
formation of bark ; and as the inner older wood is transformed into heart wood, and
the original pith and the primary vascular bundles surrounding it have likewise long
ceased to take part in the vital processes, the organ concerned (an older shoot-axis
or root) now consists only of such living masses of tissue (secondary cortex and
alburnum) as owe their existence entirely to the activity of the cambium. The
varying hardness and other material peculiarities of the bark (and especially the
way in which it gradually becomes split up by means of continually deeper longitu-
dinal cracks, often connected in a net-like manner) depend on the properties of
those masses of cortical tissue which provide the material for the development of
bark. If large quantities of elastic fibres are formed in it, the bark also exhibits a
fibrous fracture ; if, as very often happens, numerous stone-cells {scleroblasls) are
present in the parenchymatous cortical tissue, these are found again in the bark-
scales. The latter will also be rich in calcium oxalate when this has been
accumulated previously in large masses in the secondary cortical tissue, as is
commonly the case.

As in the epidermis the stomata exist as passages of communication between the
atmosphere and the air contained in the intercellular spaces ; so also in the ])eriderm,
and later in the bark, peculiar organs occur by means of which, it is assumed,
a certain communication between the atmosphere and the interior of the cortical
tissue is established. These organs are the Lenticels. They arise even in the
first period of vegetation of lignifying shoot-axes, and project from the smooth
periderm of annual and perennial shoots usually as pale coloured roundish warts.
With the increase in circumference of the organ they become widened, and at length
appear as transversely elongated masses of tissue, which swell up in moist weather
like cushions, their number increasing with the advancing age of the branch or
stem. The lenticels may be considered as peculiar localised growths of the peri-
derm. Where the periderm is developed in the epidermis or close beneath it, the
lenticels arise before it or simultaneously with it beneath the not very numerous
stomata of the shoot-axis. In the tissue lying under a stoma, a phellogen, convex
inwards, is formed; and from this, serially arranged cork-tissue is developed
externally, and phelloderm internally. This phellogen is immediately continuous
with the rest of the periderm-forming tissue of the shoot-axis; only its activity in
developing cells is more vigorous, especially towards the outside. Moreover the cells
originating from the phellogen externally are distinguished from those of the ordinary
periderm, in that they possess intercellular spaces, and that the external mass of the
lenticel forms a very loose powdery tissue, broken up into its individual cells. This
may be termed lenticel-tissue, and is plainly shown in Fig. 172. The cells of this
lenticel-tissue remain thin-walled and for a long time living, and, on contact with water,
capable of swelling and even of growth. At the conclusion of the period of vegetation
a dense layer of cork, not traversed by intercellular spaces, is formed in the lenticel;

' What is here said concerning the Lenticels is taken from Dc Uary's 'I'crgl. Anat. der
Vc^ctations-organc,'' § 179.

l68 LECTURE X.

by means of which the communication of the cortical intercellular spaces with the
atmosphere during the period of rest is interrupted. At the beginning of the new
period of vegetation, however, new looser lenticel-tissue arises from the phellogen of
the lenticel, which splits the closing layer of cork and opens the lenticel during the
period of vegetation. The origin of the lenticels is not always connected with the

Fig. 172.— Transverse section through a lenticel oi Belula alba. <• epidermis ; Ja stoma. Beneath
this is tlie loose tissue of the lenticel, and further inwards the phellogen ; at the margin of the lenticel the
development of periderm is beginning (after Ue Bary). ■

existence of stomata, however. In the formation of internal layers of periderm,
lenticels are developed quite independently, but to a certain extent as local growths
of the new layer of periderm ; and since internal layers of periderm are necessarily
connected with the formation of bark, lenticels thus arise within the bark. This is
particularly clear after the fall of the bark scales in Plaiaims, on the fresh exposed
surface of the cortex, which is now beset with corky warts.

What has hitherto been said concerning growth
P V J. f^ in thickness, is true especially of the shoot-axes.

f^ ^ As already mentioned, however, the roots of woody

plants also undergo a subsequent growth in thick-
ness, which only differs in a few points from that
of the shoot-axes. Here, as there, the first origin
of the cambium is connected with the vascular
bundles. These, as we know already, form an
axial cylinder in the roots ; in which the phloem-
,A portions are placed at the periphery, alternating
"^ with the xylem. The cambium arises therefore as

r- "" an annular layer with sinuosities, so running that

here again the phloem comes to lie on the outside,
and the xylem on the inside of the cambium. The
formation of secondary wood and cortex then
follows exactly as in the shoot-axis; except that,
in general, the peculiarity prevails in the root that
the secondary wood springing from the cambium-ring (seen in transverse section)
does not fit on to the xylem-portions of the axial string, but is developed between
them, — i.e. on the inside of the primary phloem-portions. In the growth in thick-
ness of the root, also, the production of parenchymatous tissue very commonly

FIG.

I73--

-Tra

nsverse section

through

1 the

upper s«

•oil en po

rt.on of the pri:

UKiry roc

rt of

Phaseolu

s m

ult.ß^

j; v^j— less magnified than

Fig.

141. On

COD

iparir

ig the two figures, the c

po!

iition

of the primary

bast (bb) is

observed.

GROWTH IN THICKNESS OF ROOTS, ETC.

69

predominates. In roots which grow in thickness, the formation of periderm usually
occurs at an early period; and this always takes its origin deep in the internal
tissue. Inside the endodermis before mentioned, which envelopes the entire axial
cord, lies a layer of parenchyma, the so-called pericambium ; and it is in this
(according to De Bary) that the periderm of the root arises. Thus the whole of the
cortical tissue of the root dies oft", and a new cortical layer — i, e. a phelloderm —
is produced by the activity of the phellogen, on the outer side of which is formed
at the same time a periderm, consisting of cork.

Mention may here be made, finally, of the peculiar growth in thickness of many
napiform roots, e. g. Radish, Turnip, &c., as well as of the tuberous swellings of
some shoot-axes, such as the Potato, Turnip, &c. These parts of the plant, edible on
account of their soft, thin-walled, non-lignified masses of tissue, owe (since they too
are thin and filiform at first) their later thickness and massiveness to the subsequent
growth in thickness brought about
by means of a cambium - layer.
Essentially, and considered purely
formally, the processes are the same
here as in the ordinary cases
where the cambium produces true
wood ; only here, in place of the
development of tracheides and libri-
form fibres, the formation of paren-
chyma predominates, and the ligni-
fication of the cells produced on the
inner side of the cambium-ring is
entirely suppressed, or affects in a
slight degree only the not numerous
vessels which traverse the secondary
wood consisting entirely of non-lig-
nified parenchyma. Occasionally, as
is known. Turnips and Potato-tubers
become woody : they are then tra-
versed by tough inedible fibres — i. e.
by actually lignified strands.

Passing over the very numerous cases of abnormal growth in thickness*
in dicotyledonous woody plants (since these, in spite of considerable deviations
from the type, nevertheless differ in no way essentially from it), I may still say
a few words as to the growth in thickness of a small group of monocotyledonous
plants — Monocotyledons not elsewhere exhibiting this process. Here are concerned
a sub-division of the Liliaceae, to which the well-known genera Draccena, Yucca, and
a few others belong. These plants, palm-like in advanced age, possess when young a
thin stem, scarcely as thick as a finger, which may subsequently attain a considerable
thickness. This occurs of course by growth in thickness ; and this extends also into

FIG. 174.— Transverse section of a thin root on the rhizome of the
Nettle (Urtica dioica). irthe series of primary vessels arranged right
and left. The secondary wood forms two groups— lying above and below
in the fijfure. c the cambium. The primary cortex lias been thrown off;
the periderm is developing at the circumference (after De Bary).

1 The essential facts on abnormal growth in thickness are found in De Bary's 'Anat. da
Vcgetations-orgaiie^ Cap. XVI.

170

LECTURE X.

the roots. Now the young, thin stem is traversed, as in ^lonocotyledons generally,
by isolated closed vascular bundles, which, originating near the surface, ascend
M'ithin the stem in a radially oblique direction, reach about to the centre, and then
curve out somewhat suddenly into the leaves (cp. Fig. 130). The paths of the
numerous bundles arising at various heights and curving in and out, thus cross :
and it is scarcely conceivable how, under these circumstances, a cambium

layer, connected as in the true wood
plants, and at the same lime running
through the vascular bundles, could
come into existence. As a matter of
fact, the growth in thickness of these
plants is initiated and carried forward
in a somewhat different manner. A
zone of the fundamental tissue, annular
in transverse section, outside of which
there lies a thin cortical tissue, becomes
transformed by radial growth and the
appearance of tangential partition-walls
into a tissue capable of division — a
meristem; the activity of which cer-
tainly possesses great similarity to that of
the cambium. Some considerable differ-
ences, however, are apparent. Apart
from the fact that the development of
secondary cortex from this meristem
is generally very inconsiderable, no mass
of wood, homogeneous and compact
as in the Coniferae and Dicotyledons, is
deposited on the inner side of the
continually widening meristem circle ;
but the products of the layer of meri-
stem are secondary vascular bundles,
consisting of sieve-tubes and tra-
chei'des. These secondary vascular
bundles exhibit a sinuous undulating
course, anastomose radially and tangentially, and thus form a dense network,
the meshes of which are filled with radially elongated parenchyma which originates
from the zone of meristem, and, as is easily observed, corresponds to the medullary
rays of true wood. With this growth in thickness is also connected a develop-
ment of periderm at the surface of the organ; this, however, is usually confined
to the production of a tliin cork membrane, which, like the epidermis, completely
envelopes tlie whole organ as a smooth layer.

Fig. 175.— Part of trnnsverse section of a stem of Dracann {>■
ßexa^) about i^mm. thick and i ni. liigli, taken about 20 cm. beneat
the apex, f epidermis; /■ cork (periderm); ?■ cortical fundament-
tissue; * transverse section of a fibro-vascular bnndl»' which I'.-nr
out into a leaf; m the primary fundamental ti^-'i (j ul:i ; ; il
primary bundles: x the zone of meristem, in \\' ; h it. vmhi
vascular bundles— older ones 4" have already. | .iii.\ . r i\li II
emerged from the zone, its inner portion being transformed in
radially arranged fundamental tissue {st).

LECTURE XL

LATICIFEROUS VESSELS AND RECEPTACLES FOR SECRETIONS.

The epidermal tissue, the vascular bundles, and the fundamental tissue
may be traced from their most rudimentary beginnings in the Thallophytes,
through the Muscine» and Vascular Cryptogams up to the most highly developed
Phanerogams as structures phylogenetically identical. In them is expressed an
essentially similar plan of organisation of the whole vegetable kingdom ; and even in
the product of the cambium of the woody plants we discover nothing essentially
new, but only a further development of these tissue-differentiations.

The fact is quite otherwise with the laticiferous vessels and organs of secretion,
which allow no such phylogenetic continuity to be recognised. At the most
various stages of development of the vegetable kingdom, we meet with organs
of this kind in isolated small subdivisions; while they are wanting in other groups,
often very extensive, or appear only here and there in them. In general, we
have always to recognise in their existence, however, a sign of further advanced
physiological division of laliour ; only the matter is especially one of chemical
problems, which are to be solved by means of these relations of organisation.
Since the division of physiological labour is most distincdy expressed in the
Phanerogams generally; so, too, the forms of tissue here to be considered occur
in them more frequently and in greater variety than in the Cryptogams, though
they are by no means wanting to the latter.

It is especially characteristic of the laticiferous vessels and organs of secretion
that, while similar in nature in other respects, they are not exclusively peculiar to
any one of the three systems of tissue, but are curiously independent in their
occurrence. We find laticiferous vessels and organs of secretion sometimes, though
more rarely, in the epidermal tissue ; at others in the fundamental tissue, or in the
vascular bundles. In this connection the most that can be said is that they })articu-
larly affect the fundamental tissue.

We may first consider the Laticiferous Vessels^.

In a large number of families, and genera within certain families, or even
species within certain genera (e.g. in the Euphorbiaceaj, Urticacese, Asclepiadeas,
Papaveraceae, Campanulaceae, Lobeliacese, Cichoriacese, etc.), it is remarkable that from
any wound, however small, there exudes at once a thick drop, or c\cn a sti'eam

' In so far as the description here given differs from that in my ' Tcxt-bool<,' it is founded on
De Bary's ' Vergleichende Anat. der Vegetations-organe,'' Cap. VI.

172 LECTURE XI.

(the larger Euphorbiacese) of milk-like fluid. This is usually white, like animal
milk, more rarely yellow {C/ieh'donmm) or orange-coloured {Bocconid) ; and the
outflow ceases very soon after the injury. This milk-like fluid (the latex), as will
be shown more clearly later on, is contained in narrow tubes, generally much-
branched or anastomosing. These are the laticiferous vessels, which permeate
all the organs of the plants concerned (in some cases perhaps with the exception
of the roots) in such a manner that any puncture or cut, however insignificant,
opens some of them, and occasions the outflow of the latex. The latex itself
consists of two chief constituents ; a watery fluid, and granules or drops, generally
exceedingly small, suspended in it, which, as in animal milk, produce the opaque,
milky appearance. In the watery fluid are usually dissolved, in addition to the
mineral salts which occur in all the fluids of the plant, small quantities of sugar,
gum, starch, and proteid-like substances; and, according to circumstances, peculiar
alkaloids or vegetable acids and their salts. The substances which cause the
latex when drawn off from the plant to form, on mere contact with air, water,
alcohol, ether, or acids, flocky coagula which become more or less clumped
together and separated from the watery fluid, are still unknown. The small
corpuscles suspended in the fluid, which cause the opacity and cloudiness, appear
when strongly magnified as round bodies, often almost immeasureably small,
in other cases larger. In the latter case, especially in the Ficus-like plants,
the corpuscles exhibit a concentric stratification, but are at the same time soft
and sticky. These emulsified substances generally show a tendency to cohere
among themselves in the latex drawn from the plant ; and so form coherent
masses, which after the evaporation of the watery constituent present themselves
as dense unctuous mixtures like Opium, or as brittle resins like Euphorbium, or
finally as elastic caoutchouc or India-rubber. In dried latex are also found small
quantities of wax and fat. The most important product of the laticiferous vessels
besides the Opium from Papaver somtii/erimi (which contains Morphia) is perhaps
Caoutchouc : this is obtained chiefly from a subdivision of arborescent Brazilian
Euphorbiacese {Hevea), Indian species of Fiais, as well as from Apocynaceae and
some Asclepiadese.

The medicinal employment of a variety of kinds of dried latex shows that
besides the predominant substances already mentioned, smaller quantities of nar-
cotically active alkaloids or other peculiar matters are contained in them. The
occurrence of bodies resembling ferments is of especial interest for vegetable
physiology. Among these, besides the peptonising ferment in Ficus carica, the
Papayotin contained in the latex of Carica papaya is particularly well known ;
and, according to the most recent researches, it is not improbable that ferments
are distributed in many, or perhaps in most kinds of latex.

It appears that the laticiferous vessels play a part similar to that of the blood-
vessels, and especially the veins of animals. They contain on the one hand matters
which find immediate employment in growth; and, on the other hand, secretions
and excretions which accumulate in them, and are of no further use. When
they contain ferments, their physiological significance is still further enhanced.
Of particular interest in this relation also is the existence of starch grains in the
latex of many Euphorbiacese ; since, with respect to these, we know that, after being^

LATICIFEROUS VESSELS.

173

dissolved and undergoing further chemical changes, they play a most important part
in the metabolism of the plant. As however the occurrence of laticiferous vessels
themselves is very variable, even within narrow circles of alliance ; so also the
chemical composition appears to be variable in the highest degree, from species
to species and from genus to genus even within the families concerned. The
substances mentioned as contained in the latex may be present or wanting ; or one
or another of them may predominate
or be reduced to a minimum.

The laticiferous vessels themselves
are always so narrow that they can never
be seen on a transverse section of the
organ with the unaided eye. The micro-
scope, however, shows that they may be
of very different diameter in the same
plant. In the roots, shoot-axes, and
nerves of the leaves, run thicker tubes,
from which thinner and yet thinner ones
arise. The substance of the walls of the
tubes always consists of soft cellulose,
sometimes capable of swelling : they are
never lignified, suberised, or otherwise
essentially altered by infiltration. One
of the most prominent characteristics of
the laticiferous vessels is their continuity
throughout the whole plant, or at any
rate over wide areas. This may obvi-
ously, even if not in every point, be
closely compared with the vascular sys-
tem of an animal. This continuity is
also the reason why a relatively con-
siderable quantity of sap flows from a
small injury at any part of the plant, in
spite of the narrow diameter of the
laticiferous vessels ; for their walls are
under high pressure on all sides, which
is brought about by the turgescence
of the surrounding tissues. On the
injury of any tube, therefore, the sap
is pressed forward even from distant
parts towards the opening.

According to their origin and form, two kinds of laticiferous vessels are to
be distinguished — the segmented and the unsegmented.

The segmented laticiferous vessels, which occur in the Cichoriacea; (par-
ticularly fine in the cortex of Scorzonera), in the Papaveracea?, Campanulaco:v,
etc., originate in the embryo, and in the embryonal tissue of the growing
point, in the following manner. In certain scries of cells which arc previously

176.-

4 Tangential longitudinal section through the root
of Scoyzo)tcra hisfanica. In the parencliymatous tissues run
numerous laticiferous vessels, anastomosing laterally. K small por-
tion of a laticiferous vessel with adjacent parenchyma, more highly

magnified.

174

LECTURE XT.

distinguishable by their peculiar contents, the transverse septa become entirely, or in
rarer cases {Chelidoniuni) partially absorbed, so that an actual vessel is formed ; at the
same time, in many cases, lateral anastomoses are
developed, the segments of these vessels sending
forth protuberances between the cells of neighbour-
ing tissues, which end blindly, or communicate
directly with other protuberances of the same kind^
Thus the laticiferous vessels form a reticular system
which extends up to the youngest leaves and grow-
ing points. If in such plants a subsequent growth
in thickness takes place, laticiferous vessels may
gradually arise, singly or grouped in layers, as ele-
ments of the secondary, cortex between the previ-
ously mentioned constituents of the latter (these are
particularly abundant in the roots of Taraxacum
oßcinale).

The unsegmented laticiferous vessels, which are
at present only known in the Euphorbiaceoe, Ascle-
piadese, and Ficus-like plants (Moraceae), arise in
quite another way. Schmalhausen showed some
years ago that in the young embryo of these plants,
at the place where the cotyledons spring from the
axis of the seedling, some few (4 — 6) cells, recog-
nisable by their contents, are present, which in the
further development of the embryo put forth pro-
tuberances into the root as well as into the plumule
(its axis and leaves). These much branched tubes
now penetrate as far as the growing point of the
root and shoot, grow continually with these, and
become branched with the organs arising from the
growing points, just as in the shoot-axes them-
selves ; so that thus, in a vigorously developed plant
of this kind, the whole system of unsegmented lati-
ciferous vessels consists of a few multifariously
branched and very long tubes, which were originally
simple cells. If it were feasible by any means to
destroy all the other tissues of such a plant as a
large Euphorbia or Asclepias, the entire form of the
plant would still be preserved as a mass of very
fine threads of various thickness, representing the
ramifications of the original latex-cells; just as the
injected vascular system of a vertebrate animal after
the removal of all other tissue allows the whole

Fig. 177.— Latex-cells (unsegmented latici-
ferous vessels) oi Euphorbia spUtideiis, prepared
free from the apex of a shoot by rotting. A
ramifications ending blindly; B a piece more
strongly magnified, with 'bone-shaped' starch
grains.

* With respect to these anastomoses of the segmented laticiferous vessels, and the origin of the
latter, see W. H. Scott's paper ' Zur Enttmckchingsgcschichtc der gegliederten Milehrohreii' in
'Arbeiten des bot. Inst, in Würzburg,' II. 648.

LATICIFEROUS VESSELS, ETC. 1 75

organisation of the body to be recognised. Like the segmented laticiferous vessels,
these unsegmented tubes may also send forth branches from the cortex across
through the vascular bundles and secondary wood into the pith, in which they
ramify all over or chielly in the outer part. By maceration it is possible to free
the ends of their branches in the growing points and leaves, and to convince
oneself of the fact that we have here to do with continuously growing free ends of
individual tubes, and not with the fusion of originally separated cells, as in the
segmented laticiferous vessels. The walls of the thicker, older latex tubes, especially
of the Euphorbiae, may attain a very considerable thickness.

While the finer ramifications of both kinds of laticiferous vessels are inserted
between the most various tissues within the organs, the tendency prevails for the
main stem of the tubies to accompany the vascular bundles, and especially to run
in the neighbourhood of the sieve-tubes, or even to replace them. This is espe-
cially striking where the laticiferous vessels in the secondary cortex arising from
the cambium are the more numerous the fewer the sieve-tubes, or vice versa {Papaver
B-hocas, Argemone viexicana, Chelidoniian rnajus, and Glauciuin luleum, according to
De Bary).

The laticiferous vessels contain, as stated, two essentially different groups of
substances; those which are again utilised in metaboHsm (proteids, carbo-hydrates,
fats, ferments), and those which must be regarded as excreta useless in metabolism
(resins, gums, caoutchouc, alkaloids, etc.).

The Receptacles of Secretions, now to be considered, and which should more
properly bear the name of excretory organs, contain on the other hand exclusively
auch matters as find no further use in the metabolism of tlie living plant. This
may be concluded with certainty from the fact that when these matters have once
arisen in a' given receptacle, they remain there, and are not again dissolved and
used for the purposes of growth. It is not implied that these matters produced
as bye-products in metabolism are completely useless to the plant, like the refuse
in chemical works; they may, rather, according to circumstances, be of use
for the well-being of the plants concerned in this or that sense. This is particularly
well seen in many epidermal glands, to be described later. It is only to be insisted
on here that the matters referred to are not further concerned in the nutritive
processes and metabolism accompanying growth. Since such substances, as already
mentioned, also accumulate in the laticiferous vessels and become withdrawn from
the processes^ of interchange of fluids, it is intelligible that, with few exceptions, the
plants provided with laticiferous vessels possess none of the receptacles for secretion
here to be described ; and also that the presence of the latter generallv precludes
that of laticiferous vessels (De Bary).

The refuse matters collected together in the receptacles for secretions are, from
a chemical point of view, of very various nature. Calcium oxalate, in the form
of beautiluUy constructed crystals or crystalline granules, occurs very commonU-,
especially in Phanerogams. More confined to single orders and families are found
resins and ethereal oils, both usually combined into a so-called lialsam : mucus,
swelling in water, and various kinds of gums also occur, and these, when they
are accompanied in the receptacles for secretions In- resins and ethereal oils
and form emuUions in water, resemble latex. Fmm the presence of these

176

LECTURE XI.

substances in laticiferous vessels, in the narrower sense, it cannot 3-et be concluded
whether a sharp distinction exists between segmented laticiferous vessels and
serially arranged secretion-vesicles. Very widely distributed are, further, certain
tannins, often mixed with a red colouring matter. These are found in special
individuaHsed cells or in rows of cells, and are not again employed in metabolism,
and are therefore to be considered as excreta; whereas in other cases {e.g. the
plumule of the Oak) tannins of another kind are to be recognised, from their
origin and disappearance, and by their behaviour in growth, as special forms of
reserve-materials, which find further employment in metabolism, and are therefore
to be excluded from our present considerations.

It would, however, be in vain to attempt a thorough classification of the
organs of secretion according to the chemical nature of their excreta; and
a purely histological classification would lead just as little to a satisfactory
result. We will confine ourselves in what follows, therefore, chiefly to the
general characters of the organs concerned, without attempting a rigid sifting
according to this or that point of view. It may be mentioned in anticipation,
however, that the receptacles for secretion^ present anatomically the greatest
possible variety. Very often they are individual cells, lying in the tissues, which
contain calcium oxalate, mucus, resin, tannin, etc. : or such cells are arranged
in long rows, which then usually follow the vascular bundles, or lie in the soft
bast. Or the receptacles for secretion are intercellular spaces, which become filled
with the secretion from the cells bounding them; and these intercellular spaces
may either be long, more or less narrow canals, or form roundish or elongated
sacs. Or serially arranged or roundish cell groups become disorganised, and
the cavity thereby arising (according to De Bary a lysigenous intercellular space)
remains filled with the products of decomposition of the cells. Finally, secretions
may arise in the wall separating neighbouring cells; or even between the cuticle
and the proper cell wall in the epidermis and hair-like structures, as is often the
case with the numerous epidermal glands in the Phanerogams.

The fact that the receptacles of secretions, as well as the laticiferous vessels,
with rare exceptions, make their appearance in the very youngest state of the
organs, at the beginning of the differentiation of their embryonal tissues, throws
light on their physiological signification. They are already sketched out or deve-
loped when the tissues of the vascular bundles and the fundamental tissue first
commence to obtain their characteristic structure. Apparently the developing tissue
rids itself during nutrition of certain products of decomposition even thus early; and
these remain lying in the receptacles of secretion as such without further use.

Here also the higher degree of organisation of the shoot, in comparison with the
root, makes itself evident, in that receptacles of secretions, which appear in the shoot-
axes and leaves, are not rarely wanting to the roots, or are more feebly developed in
them ; whereas they usually appear very abundantly in the flowering shoots of the
Phanerogams. Families which possess no receptacles of secretions at all are
relatively rare among the Phanerogams — e.g. the Gravmiece and Cyperacece,

^ De Bary's description (' Vcrgl. Anat' Cap. Ill) of the receptacles for secretions is the only
satisfactory one hitherto.

RECEPTACLES FOR SECRETIONS. 177

CrucifercE, Ranunculacecr ; and Taxus, among the Conifers otherwise rich in
secretions.

I now pass to a somewhat more detailed description of the most important forms of
receptacles for secretions, and shall confine myself here also to the more common cases.

The commonest of all the receptacles for secretions are the vesicles containing
crystals (Lithocysts) in which calcium oxalate is accumulated as refuse matter.
More rarely this occurs — as in many SolanecB, some species of Amaranlhus, in
the pith of Samhucus nigra, etc. — in the form of exceedingly small crystalline
particles, which are deposited in countless multitudes in individual cells, and fill
these entirely. More frequently the oxalate of lime appears in the form of very
thin needles, pointed at both ends, which lie parallel to one another and form
bundles of so-called raphides, often filling up the entire cavity of elongated vesicles.
This is to be observed in many Monocotyledons (Aroidea?, Liliacese) ; as well as
in some Dicotyledons (the Vine and its allies, Cinnamon, Impa/iais, etc.). In many
Monocotyledons (species oi Allium, Iridese, Amaryllidese) and in the great majority
of Dicotyledons (particularly abundant and well developed in the bark of Guiacum
oßcinale), the calcium oxalate appears in the form of single crystals, well formed on
all sides, which are contained in the cells as simple individuals, or as twin crystals,
or as clusters of crystals. The great variety of forms in which this salt appears
is partly explained from the fact that, according to the rapidity of its separation, it
crystallises either with two molecules of water of crystallisation in the klino-rhombic
system, or with six molecules of water of crystallisation in the quadratic system. The
much more frequent klino-rhombic forms in the plant are referred to the ground
form of the hendyohedron, and develope as prisms, tables and twins with truncated
angles of the most various kinds. The raphides also apparently belong here. The
fundamental form of the calcium oxalate crystallising in the quadratic system is the
quadrate-octahedron, the main axis of which is sometimes extremely short, so that
the crystal assumes the form of a letter cover ; in other cases combinations of the
quadratic prism with the corresponding pyramid occur, and so forth \ As a rule
the crystal-vesicles contain a slimy substance : if the crystals are very large, the
slime is less abundant.

In some cases, on the other hand (tubers of Orchids), the cell is chiefly filled
with slime, and contains only a small group of crystals. Where the crystals of
oxalate appear as well-formed individuals or clusters, they often stand in relation with
the cell-wall, and are embedded in cellulose projections, or in the cell-wall itself
(pith oi Kerria, Ricinus, in the vascular bundles of the petiole of various AroidecE,
leaf parenchyma of Hoya carnosa, leaves of Citrus, cortex of Salix, Populus, Celtis,
Fagus and others). In other cases also a relation of this kind is at least not im-
probable. Very small crystals of calcium oxalate — their form no longer distinctly
recognisable, but rendered evident by means of polarised light — are also found
occasionally deposited in thickened cell-walls. These are particularly common in the
bast of Coniferse, and in a few Dicotyledons (leaves of Sedum, Mesemhryanihemuni).

^ As regards the crystals of calcium oxalate, the chief work is that of Holzner, ' Flora,' 1 S64,
p. 273, and 1867, p. 499. Further, Rosanoff, 'Bot. Zeitg.' 1865 and 1867. Graf Solms-Laubach,
'Bot. Zeitg.' 1871 ; and Pfitzer, 'Flora.' 1872, p. 97, are also important.

[3]

78

LECTURE XI.

Among the Gymnosperms, Wehvilschia mirabilis — so very remarkable in other
respects also — has thousands of well-developed oxalate crystals contained in the
substance of the very thick walls of the schlerenchymatous cells (Fig. 178).

The quantity 01 oxalate of lime may occasionally be enormous. According
to Schleiden, the dry substance oiCereiis senilis contains more than 85 per cent, of its
weight of it. This substance is also generally abundant in the parenchyma of foliage
leaves, as well as in the secondary cortex and pilh of dicotyledonous woody plants, and
occasionally in the medullar}- rays also {Camellia, Vilis). Sometimes, however, the

crystals are entirely wanting, as in the Equi-
setacese, and most Ferns and Grasses; and
even in families of Phanerogams which are
otherwise rich in calcium oxalate it is absent
in single species, e. g. in Petimia nyctaginiflora
(Solanacese), in Tulipa silveslris, Lilium mar-
fagon, and Lilium candidium (De Bary).

With regard to its distribution in the tissues,
we may notice the frequency of crystals of
oxalate in rows of cells which accompany
the»vascular bundles, or run in the secondary
cortex, or which, on the other hand, occur in
groups or rows beneath the epidermis. Many
Monocotyledons are especially remarkable for
the serially arranged large vesicles, containing
slime and raphides, in the petioles, leaves,
and bulb-scales — e. g. the Commelynaceae,
Amaryllideae, and Palms.

In many crustaceous Lichens, and to a
less extent in some other Fungi, calcium
oxalate is excreted in large quantities on the
exterior of the cells. Although oxalate of
lime is occasionally met with in the form of
small crystals suspended in the protoplasm ;
in living cells rich in protoplasm, and even
in those containing chlorophyll, this does not
affect its being a useless secretion, since even
in living cells — e. g. in hairs rich in pro-
toplasm— such an excretion may obviously result without the life of the cell itself
being endangered.

In comparison with the exceedingly common occurrence of calcium oxalate, the
excretion of calcium carbonate in solid masses is rare. Among the Phanerogams
there are two families especially — the Urticaceae in the wider sense, and the Acan-
thaceae — which are remarkable for the presence of so-called Cystoliths; in the first
family in connection with the epidermal system, in the others in the interior of
the fundamental tissue. Cystoliths are massive bodies, often resembling a bunch of
grapes, attached by a thin short stalk to the wall of the cells in which they are
contained. The substance of this body consists of cellulose, which is permeated by

FIG. 178.— Half of a sclerencliyina cell from U'ehv
schia Htirabitis, with numerous crystals of oxalate of lir

embedded in the outer layer of the very thick wall.

SECRETIONS.

179

a large quantity of exceedingly fine granules of carbonate of lime, so that the whole
represents a mass of stony hardness, from which the calcium salt may be dissolved
out by weak acids, with the formation of carbon dioxide gas. Among the lower
Cryptogams also cases of calcification are found — i.e. deposits of calcium
carbonate in the substance of the cell w-all — so that the whole plant is of stony
hardness, as in the Corallines and Mclobesiacese among the Florideai, and even in
single species of Halyvicda, as well as in Acclahiilaria among the Si[)hone3e
(Coeloblastce).

If we now turn to those receptacles for secretions which contain fluids, we find
the contents in the form of gummy mucus, or of latex- like emulsions, which may flow
out in abundance on wounding, or of
solutions of tannin, balsams and the like.
We here meet with tissue-formations
which, at the one extreme, pass almost
imperceptibly into segmented laticiferous
vessels, as in the so-called tannin-bear-
ing laticiferous vessels of many Aroidece
and species of Musa ; while the other
extreme is exhibited in the occurrence
of roundish cells, scarcely differing from
the surrounding parenchyma, and iso-
lated or grouped in all kinds of ways,
in which the substances mentioned are
contained, usually as mixtures. We may
term these structures collectively secre-
tion-vesicles, and speak in more special
cases of gum-vesicles, resin-vesicles,
latex-vesicles, tannin-vesicles, and so
forth. By this means the characteris-
tic constituents of the contents are in-
dicated. Apart from the laticiferous
vessels of the AroidecB and species of
Musa, from their nature still somewhat
doubtful, there are found in the most
various subdivisions of the INIonocoty-
ledons, Dicotyledons, and some Ferns,

secretion-vesicles which are formed of thin-walled more or less long, often very
long tubular cells, standing one over the other in rows, and accompanying the
vascular bundles on the outer side, or at the circumference of the pith: or, as is
the case with the rows of vesicles in Phaseohis, running in the soft bast of the
vascular bundle itself Among the most remarkable forms belonging here are
the vesicles abounding in tannin which run in the internodes of Samhucus 7iigra
both in the cortex and in the circumference of the pith ; these, according to
De Bary, probably extend through the entire length of an internode (20 c. m.),
and are at the same time remarkable for their breadth (o-i mm. and more). The
so-called utricular vessels also, occurring in the Leeks, and especially in the bulb

N 2

cell of the hypoderni (A) of llie
ciastica, c epidermis ; ch cliloropliyll-

i8o

LECTURE XT.

scales o{ Allitwi Cepa, A. fishilosim, etc., deserve mention ; they run somewhat close
beneath the epidermis, and consist of serially arranged, rather broad vesicles, the
ends of which communicate with one another by means of broad, pitted, transverse
septa, and they contain a granular coagulable latex, which flows out in some quantity
when a bulb is cut through at the base. The erroneously so-called laticiferous
vessels of the Maples (well developed in Acer plafanoides) are similarly constituted;
they run at the boundaiy between the phloem of the vascular bundle, and the
sclerenchyma strands accompanying it. The resin of the Co7ivolvulacece, partly
used in medicine (e. g. Scammony from the roots of Convolvulus Scammonium,
Jalap resin from the root of Ipomea purga), and others, are obtained, like the
dried latexes of the druggists' shops, by mere inspissation of the contents which flow
out in the form of latex from the serially arranged more or less elongated secretion-
vesicles of these plants. Like the true la-
ticiferous vessels, the rows of utricles men-
tioned may also be repeated in continually
increasing numbers in the secondary tissue,
originating from the activity of a cambium
ring. The same holds good of the short resin
and gum-resin vesicles found singly or in
small groups in the cortex ; and which are
distinguished by their refractive contents,
and occasionally by their considerable size,
in the Ginger-like plants, and in Calamus
{Acorns), Piperacese, Laurineae and INIagno-
liacese, and also in some Euphorbiaceae (Cas-
carilla bark) and Aristolochise. To these
forms are to be added the mucus-vesicles in
the parenchyma of the Malvaceae, Tiliacege,
Laurineae, Ulmeae, and species of Cactus, and
in the tubers of Orchis and the cortex of
the Pines. They are generally distinguished
from the ceils of the surrounding parenchyma
by their larger size ; and are filled with a gum-
like mucus, which dissolves in water. This
latter is in the majority of cases {according to
De Bary) nothing more than strongly swollen
cellulose, which fills up the lumen of the cell, and arises from alteration of the
cell-walls, still showing in part the layering and pitting of the latter. The mucus
in the tubers of Orchis (Salep) arises however, according to Frank, in the interior
of the protoplasm in the form of a vacuole, which grows up together with a small
bundle of raphides and presses the remaining contents aside. When, as in
Althea rosea, peculiarly grouped mucus-cells of this kind become completely
disorganised, there arise lacunae in the parenchyma of various shapes and sizes,
filled with this translucent mucus. Such gum-reservoirs are distinguished from the
cases of proper Gummosis, where large groups of tissue in the older organs are
changed into basorin and other kinds of gum (Cherry gum, Tragacanth, etc.),

Fig. i8o.— Longitudinal section through a scale of the
bulb of Allium Cepa. e epidermis; c cuticle; / paren-
chyma ; sg^ the latex of the utricular vessel coagulated in
potash solution ; g q septum of the vessel. The longi-
tudinal wall is pitted, and separates the utricular vessel
from one lying behind.

RESIN-PASSAGES, ETC. l8l

in that they assume their characteristic condition at the beginning of the differen-
tiation of the tissues in the very young organs. Further are to be mentioned the
tannin-vesicles of many plants. These are isolated parenchyma cells, often hardly
distinguished in any other way, which are filled with a concentrated solution of tannin,
frequently accompanied by a red colouring-matter. Exquisite examples of them are
found in the cortex of the stem of the seedling of Ricinus and many other germinat-
ing woody plants, where they attract the attention of the microscopist either directly
by the red colour, or by forming ink on the addition of solutions of iron salts.

A group of receptacles for secretions, well characterised anatomically, are the
resin-passages and gum-passages ; these arise by the separation of the walls of
neighbouring rows of cells, and thus constitute cellular passages filled with secre-
tions. In the Cycadese, some Lycopodioe, IMarattiaceae, and species of Canna,

Fig. i8i.— Pai t of the transverse section of the stem of Fceniculuiit officinale (1-ennel) slightly magni-
fied, e epidermis ; t/i green, and »-colourless parenchyma of the cortex ; cc bundles of coUenchyma; hk
resin canals ; /:/ wood ; g vessels ; ni pith

OpunLiiiä, and Araliacece, these passages contain mucous substances ; in the Coniferae
(with the exception of Taxus) the contents are mixtures of ethereal oil and resin
(balsam), and the same also occurs in the Terebinthaceae, Umbelliferce and those
Compositas which possess no laticiferous vessels. These intercellular canals are
generally elongated, and often penetrate the whole of the organs of the plant ;
or they may be wanting in the roots, but are then so much the more abundant
in the primary and secondary cortex of the shoot-axes, and occur also in the
foliage leaves. The length of these canals, and apparently also tlieir occasional
lateral communications, explain why trees which produce resin gradually exude
such large quantities of balsam from local wounds, which then generally stiffens
to pasty resinous masses in the air (resins of Conifera^, e.g. Sandarach, Mastic
derived from Pistacea terebinthus, etc.). In other cases, again, as in the Alismaceae,
Butomacese and the Aroidese among the Monocotyledons, and in the Clusiacese, species
of MamiUaria, and Umbelliferae, among the Dicotyledons, such intercellular passages

l82

LECTURE XT.

contain latex-like emulsions of resinous and gummy substances (De Bary). Oc-
casionally these secretion-bearing intercellular passages only form short gaps in
the tissue, as in the small leaves of the Cupressineas and in the seed-coats of some
plants of the same group.

Fig. 182.— Resin canals in the young stem of the Ivy (transverse
boundarj' of tlie cambium (<:) and soft bast (-wb) ; /; wood. D and i
boundary between the bast (W and the cortical parenchyma (rf).

.1, B. C young canals U^) .
nd older canals (^) lying 1

The intercellular secretion-passages arise early in the tissue-differentiation in
young organs ; and may also be repeatedly formed in the secondary tissue-layers
produced from the cambium, especially in woody plants. It is generally by the
separation of the angles of four neighbouring longitudinal series of cells that the
passage is produced: this becomes filled at once with the secretion. If the passage
remains narrow in diameter, the cells surrounding it either remain undivided, or,
growing slightly, they undergo a single division only, so that the secretion-passage
is only bounded by a few cells on the transverse section. If, on the other hand, the
organ referred to increases in circumference generally, the secretion-passages running
in it may also be considerably extended in width ; the cells surrounding them grow-
to a corresponding degree with it,- and become divided by radial as well as by
tangential walls (taken with regard to the passages), so that the passage is now
surrounded by two, or even by several layers of a special tissue, the epithelium of
the passage, the granular contents and delicate wall of which distinguish it from
the surrounding tissue. In the leaves of species of Pinus, the bordering of the
epithelium of the passages by a closed sheath of schlerenchyma cells is very
characteristic. As regards the source of the secretion, there can be no doubt
that it originates from the epithelium; although it is not quite certain that the

GLANDS. 183

characteristic contents of the passage itself are ah'eady recognisable as such in the
epithelium cells. Possibly the substance of the wall of the epithelium itself becomes
chemically altered and concerned in the formation of the secretion.

All receptacles for secretions not consisting of long vesicles, series of vesicles,
or long intercellular passages, were formerly collected together under the name of
Glands. We shall employ this expression quite arbitrarily for a special kind of re-
ceptacles for secretion, which chiefly contain ethereal oils and resins dissolved in them ;
and we shall at the same time distinguish two subdivisions of these structures — the

Fig. 183.— Gland on tlie upper side of the leaf
oi Dictamiuis Fraxinella (after Rauter). -■/, B
early stages of development ; C mature gland ; d
the covering layer forming a continuation of the
epidermis; c and/ mother-cells of the gland-
tissue ; c a large drop of ethereal oil.

Fig. 184.— Gland with hair, from the inflorescence
ct{ Dictainiiiis Fraxinrlla (after Rauter). ./I, A' ejirly
stages of development ; r mature gland with the hair
{fi) at its apex.

internal glands, i.e. those situated under the epidermis, on the one hand; and the so-
called epidermal glands, which belong to the epidermis and its hairs, on the other.

The internal glands are often perceptible to the unaided eye as bright, trans-
lucent dots in the tissue of foliage leaves or stems, e. g. in the leaves of the Citrons
and Oranges, many species of Hypericum, Lysimachia, etc. ; or they are contained in
protuberances of the outer surface, as in Dictamnus Fraxinella. The internal glands
in the skin of the fruit of the Orange are very conspicuous and large ; they abound
in ethereal oils, and appear in transverse and longitudinal sections as roundish
cavities, from which the inflammable ethereal oil spurts on the application of pres-
sure. Such glands originate, so far as investigation extends, from a single mother-

i84

LECTURE XI.

cell, which, as it slowly developes, undergoes many divisions in all directions, so that
a multicellular mass of tissue of roundish form arises, the cells of which subsequently
become remarkable as containing very granular, apparently dead protoplasm. Later
on, the thin cell-walls dissolve ; the process commencing in the middle of the
spheroidal group and proceeding outwards. There thus arises a roundish cavity,
filled partly with watery sap, partly with drops of ethereal oil or balsam — the
products of solution of the mass of cells. The layers of tissue surrounding this
cavity fit closely on all sides, without intercellular spaces, and thus form a kind of
wall to the receptacle for the secretion (well seen in the leaves of Citrus). These
processes will be sufficiently understood on comparing figures 183 and 184; the
relations there illustrated appear with few deviations to be exactly the same in the
internal glands of the Myrtaceae, other Rutaceae, and in Hypericum, Gossypitim (Cotton),
Lysimachia and species of Oxalis.

Very many plants\ particularly among the Dicotyledons, owe the more or less

Fig. 1S5. — A portion of the epidermis with ijlandular
hairs, from the petiole of Primula sinensis, a com-
mencement of tlie development of secretion ; b a large
bubble of secretion; rf after the bursting of the bubble
(De Bary).

Fig. 186.— Lupulin gland of
the Hop. A young, before the
secretion separates ; B older,
the cuticle raised up by the
secretion (De Bary).

Sticky condition of the surfaces of the leaves and young shoot-axes to epidermal
glands, as is easily perceived by passing the fingers over the surface of such plants.
As a rule, the secretion at the same time possesses a strong specific odour, as in
the glands before mentioned. The glandular condition of the epidermis proceeds in
many cases from secretions developed by the epidermis cells themselves, as is very
conspicuous under the nodes of the stem of Lychnis viscaria, on the young
shoot-axes of the White Birch {Betula alba), on the teeth of the leaf of Prunus and
species of Salix, and on small areas on the underside of the leaf of Prunus lauro-
cerasus and others. The viscid secretion appears here on the epidermis cells
(which are otherwise intact, but sometimes distinguished by being particularly small
and peculiar in shape) between the proper cell-wall and the cuticle situated upon

* Cp. Johannes Hanstein, ' Ueber die Organe der Harz- Jtnd Schleiviabsondentug^ ' Bot. Zeitg.'
3, p. 697 ; and also the much better description in De Bary's ' Vergl. Anat' § 19.

EPIDERMAL GLANDS.

18'

it. As the secretion increases, the cuticle becomes more and more raised up
from the underlying cellulose wall (De Bary), The balsamic secretion between the
cellulose wall and cuticle in the hair-glands or glandular hairs appears in exactly
the same way. It is usually the so-called capitate hairs, i. e. hairs consisting of
a pedicle-like support and a roundish or broad shield-shaped capitulum, which
show this peculiarity ; and it is practically indifferent whether the whole hair
consists of a single cell, or a cell series, or a mass of dssue. The secredon
appears, as a rule, first at the apex of the capitulum, or, where none is present, at the
apex of the cell series, between the wall
and the cuticle; and, as the secretion
extends from that point, the thin cuticle
becomes gradually raised up in the
form of a vesicle, the cavity of which is
filled with the secretion. An unusually
good example of this process is pre-
sented in the glandular hairs of Primula
sinensis (Fig. 185). The so-called Lu-
pulin also, a pulverulent strongly smell-
ing substance which may be shaken
from the ripe, cone-like, female inflo-
rescences of the Hop, consists of glan-
dular hairs, which, as Fig. 186 shows,
are short, stalked, cup-shaped plates
of tissue, the cuticle of which is
raised up by the bulky secretion as a
hemispherical vesicle, while the cells
themselves die. The so-called Hashish
arises similarly, in the long-stalked, many-
celled capitate hairs of the female plant
of the Indian Hemp. In very many
other cases, e. g. in the Patschouli plant
{Pogosiefuon patschouli), in the Thyme
( Thyinus vulgaris), in Cishis, Pelargonium
etc., and also in the Fern Aspidium, the
processes are essentially the same. In
other cases again the balsamic secretion
appears in multicellular glandular hairs,
within the generally radially arranged

septa of the peltate short-stalked capitulum of the hair itself: the septa sjilit into
two lamellae, and the intercellular space thus produced becomes filled with the
secretion. Such ' schizogenous glands ' are found on the under surface of the leaves
of Rhododendron ferugineum and elsewhere. But schizogenous glands may also arise
in the epidermis itself, as in the Papilionaceous ^^\■AXi\.Psoralea hirta (Fig. 187), where,
by the division of one epidermis cell, a roundish group of cells is developed which
extends deep into the dssue beneath and presents an almost globular aggregate,
surrounded by the leaf-parenchyma. The septa of this aggregate stand per^

l-IG. 187.— Vertical section of the surface of the leaf \^i PsoraUa.

ta (Papilionacea:). a an almost mature gland after tlie removal of

I from between the walls by alcohol ; c commencing form -

tweeii the cell walls ; * very young gland, before

secretion. It is seen that the glandular cells are only elongated

epidermal cells (fJe Hary).

l86 LECTURE XL

pendiculaily to the surface of the leaf, and first exhibit the splitting and separation
of the secretion mentioned in the middle: this proceeds so far later on, that. the
individual cells of the complex appear to lie quite separate in the mass of secreted
substances (De Bary).

Johannes Hanstein distinguished as Colleters certain massive multicellular
hairs, the secretion of which consists of resin and gum, or gum alone, and
effects the sticking together of the parts of the bud in many plants. This secre-
tion is very conspicuous on the just opening buds of the Horse-chestnut, Poplar,
and Sjringa ; and is evident in a less degree in very many other woody plants
{Ribes, Corylus, Carptnus, Lomcera, Sarnbucus), and also in many herbaceous plants
{Heliajithus, Datura, Salvia, Viola, &c.). In the Polygoneae, especially the Docks
and Rhubarbs, it is chiefly mucilage, which becomes diffluent in water and fills
up the spaces between the folded parts of the bud and covers the developing
leaves and buds. These secretions, which make their appearance in the very young
parts of the bud, are lost on the complete unfolding of the leaves and internodes, and
appear to be a means of protection for the young organs against drying up, and
other injurious influences. These substances are excreted from the hairs or colleters
mentioned, which are developed especially at the edges of the young stipules,
and elsewhere on the parts of the bud. They may present the most various
forms, but the secretion always takes place between the cuticle and the outer cell-wall
of the body of the hair : the cuticle, as in other cases, is raised up in a vesicular
manner by the secretion, until it finally bursts (especially on the admission of
water) by the swelling of the slime, and the mass spreads over the surface of
the organ. Examples of schizogenous glands occur here also; and occasionally
the epidermis itself is concerned in the formation of the secretion between its
cuticle and cell-v/all.

In connection with the glandular hairs may be briefly mentioned here the
mealy, dust-like, capitate hairs, described by De Bary, to which the under surface
of the leaf of the so-called Gold and Silver Ferns {Gymnogramnie, Noihochlaena,
Cheilatithes) owe their white or golden yellow {Pieris aurala) meal-like covering.
The same is true of the mealy-dusted foliage leaves of many Primroses, e.g. Primula
marginata, farinosa, auricula. The mealy covering is exclusively produced by
the round capitula of shortly-stalked hairs ; from all points of the surface of these
capitula are developed very thin, but relatively long rodlets or threads of an
apparently resinous substance, which dissolves in alcohol, ether, acetic acid, and
alkalies, and (it is said) again crystallises out from the solutions. This mode of excre-
tion appears to be only another form of the excretion of balsam occurring elsewhere
between the cuticle and cell-wall of the hair; since, according to De Bary, the
vesicular glandular hairs previously described may occur in place of these dusty
hairs on the leaves of the same or allied plants.

Physiologically the most remarkable of all external glands are the digestive
glands of the insectivorous plants. I contemplate treating of these later, however,
when dealing with the theory of nutrition.

PART II.

THE GENERAL CONDITIONS OF VEGETABLE LIFE, AND
THE PROPERTIES OF PLANTS.

LECTURE XII.

THE GENERAL EXTERNAL CONDITIONS OF PLANT-LIFE.

The vital activity of plants is, like that of animals, the result of the co-operation
of two factors; the internal structure, or inherited disposition, and the influences
operating from without, or stimuli.

It is of the greatest importance to be perfectly clear on this point ; therefore it
may be permitted to make the nature of the two factors mentioned more intelligible,
where possible, by the comparison of an organism with a machine.

The work done by a steam-engine, for example, depends in like manner
upon two factors : namely its structure, and the energy which is supplied to it.
Upon the form and combination of its individual parts depends what kind of
work it can perform — whether, as a locomotive, it will be in a position to draw
a train; or, as an engine, to plough an arable field; or, as a spinning machine,
to spin yarn out of thread ; or whether it puis a loom in motion, or planes the
iron parts for another machine, and so forth. But be the machine constructed
as you will, it performs the work intended by the constructor only if it be set in
motion; and this, as is well known, is accomplished by the tension of the steam
which moves the piston to and fro in the cylinder.

Moreover, it is not enough merely that the steam is evolved in the machine ;
since, so long as it possesses too slight a tension, not sufficient to overcome the
friction of the parts of the machine, these latter do not move. The performance
of work will only be attained if, by the supply of sufficient quantities of heat,
the tension of the steam reaches a magnitude which gives to the parts of the machine
the quickening energy by means of which it can perform the work in view.

The case is similar with the work performed by an organised body, particularly
of a plant, or part of a plant. The work which it is able to perform depends es-
sentially upon its structure — upon the combination and chemical nature of its smallest
parts. Whether these parts will be set in motion, however, and whether in ac-
cordance with their nature and combination they will actually accomplish that which
they are able to perform, depends entirely upon whether they obtain from without
the energy in virtue of which they are put into those forms of movement, the
co-operation of which we distinguish as the vital activity of the organism.

After these consideradons," it is clear that the investigation of the phenomena
of life — i. e. Physiology— has to do with two problems : on the one hand, with

igo LECTURE XII.

the investigation of the internal structure of the organs, and, on the other hand, with
the exposition of those external influences by means of which this internal struc-
ture is impelled to the movements and performance of work specifically peculiar
to it.

As an engineer who wishes to criticise a steam-engine must know exactly, on
the one hand its structure, and, on the other, the properties of heat and of steam ;
so it is the aim of the physiologist to explain the phenomena of life from the internal
structure of the organs, and the nature of the external influences acting upon them.
Only he meets, of course, with far greater difficulties than the engineer ; since the
organism is an incomparably more complicated machine than any, however perfect,
constructed by man. In the first place, the structure of the latter is relatively easy
to comprehend ; and it is set in motion by the well-understood activity of heat and
steam. On the other hand, our knowledge of the actual internal structure of the
organs of a plant is still in the highest degree incomplete; since in these matters
it concerns the coarser visible portions far less than the molecular structure and
atomic composition. This, since it depends upon invisible parts, can only be known
by indirect methods, and by means of conclusions slowly obtained. Moreover,
it is shown that the structure of the organism is not simply sensitive to one kind
of external influence, as is that of a steam-engine ; but that all known natural forces
may affect the living machine. Plants do not re-act only to the motion which is
communicated to them as heat, but also to the movements of the aether which
our eye perceives as light. They are, at the same time, sensitive to electrical changes,
and they re-act in a manner not yet understood to the influence of gravitation, and
even slight contact may produce very great effects. But above all — a matter which
does not come into consideration at all in a machine constructed by man — the
organism nourishes itself — i. e. it absorbs material into itself from without, and itself
joins this to the substance of the organs already present. Here, also, it again de-
pends upon the substance already existing, and its structure, that the nutrition in
a given organism takes place in a certain manner, and not otherwise. When
a seed or a bud becomes detached from a plant, in order to begin an in-
dependent life, the course of the latter is already traced out beforehand. The nature
and combination of the smallest particles in the seed are specifically determined by
the nature of the mother-plant — they are established beforehand. The question is
only whether the germ will develope ; whether it will, as a matter of fact, perform all
that which it can perform according to the laws prescribed for it by the mother-plant;
and this 'whether' depends upon the external influences of heat, light, gravitation,
electricity, nutrition and respiration, the contact of solid and fluid bodies, and so
forth.

The difficult part of physiological investigation always lies in the explanation
of the structural relations of the organs, by which the latter, in consequence of the
influence of external forces, are put in motion in the way specifically determined ;
and, unfortunately, experience shows that the coarser visible structural relations only
play a more subordinate part in the matter, and that physiological investigation is
compelled to go back to the invisible, smallest particles of matter, the mutual
relations of which, again, can only be inferred from the entire working of all the
visible organs. We may therefore reserve until we enter upon the consideration of

EXTERNAL CONDITIONS OF LIFE. 191

the individual phenomena ol' vegetation, a detailed study of the corresponding
structural relations of the organs.

Much, however, may be stated generally with reference to the influence of ex-
ternal causes on the phenomena of life ; since here it is only a matter of proving, by
means of observation and experiment, what kinds of vital phenomena appear when
certain alterations are produced in the environment of the living plant. In this way
it has been shown, even in the still very imperfect condition of our knowledge in
this respect, that a far greater portion of the phenomena of life are called forth by
external influences than one formerly ventured to assume ; and that what the living
organism performs of itself, by reason of its inherited structure, always concerns only
the specific nature of the performance. Whether and with what intensity this takes
place depends, however, entirely upon the play of external influences.

What I would here emphatically lay down, is the fact that every phenomenon
of life arises from two factors : on the one hand, from the structure transmitted
from the mother organism, and, on the other, from external forces working on this
structure. Every phenomenon of life is the product of these two factors, neither of
which is at all effective by itself. The one factor, the external influences, which we
may also distinguish as the external conditions of life, we will now take into view
somewhat more closely in its most general outlines.

The life of the plant is a coherent chain of the most various motions of the
smallest particles, the atoms and molecules of its substance, from which arise in the
plant mass-motions of the entire organs, which, however, are slow and mostly difficult
to perceive. The most general and important source of vital force through which
the Hfe-motions in the bodies of plants are called forth is, however, Heat^ As
is well known, heat is to be imagined as a motion of the smallest particles of matter,
which is transmitted from one substance to another. The intensity of this motion, or
the force with which the smallest particles vibrate, is called the temperature, which
in its turn is measured by means of the thermometer. Experience shows now
that the vital motions in the interior of the plant do not occur until the tem-
perature of the environment, which by degrees becomes communicated to the plant,
reaches a certain height — a definite degree of the thermometer; i.e. the heat-motion
imparted to the plant must effect a certain intensity of the vibrations of its smallest
particles, in order that those further motions of the atoms and molecules may take
place by which the various chemical processes of nutrition, and all the various
molecular motions of which the Hfe of the plant consists, are called forth.

Numerous, but by no means final researches^ have shown that temperatures

' I have given a comprehensive description of the effects of heat on vegetation, with detailed
references to the literature up to the year 1865, in my 'Handbuch der Experimcntal-physiologie der
Pßanzen'' (Leipzig, 1865), pp. 47-68 ; and supplied more details concerning freezing in my
' Lehrbuch der Botanik,' 1868, p. 562. Cp. also the succeeding editions of the latter book.

- Concerning the upper and lower limits of temperature of vegetation, cp. my treatises in the
' Regensburger Flora,' 1863 — 'Die von'ibergchettdett Star7-e-zustande periodisch beweglicher umi
reizbarer Pßajizenorgane,'' pp. 449, &c. ; and 'Flora,' 1864, 'Über die Temperaturgrenzen der
Vegetation' p. 8 ; and further, p. 497, ' Über den Einfluss der Temperatur auf das Ergrünen der
Blätter: More details in this connection are to be found in my treatise, 'Physiologische Unter-
suchungen über die Abhängigheit der Keimung von der Temperatur,' Jahrb. für wiss. Bot B. II,
1S60, p. 338.

192 LECTURE KIT.

between the freezing-point of water, on the one hand, and about 50° C. on the
other, indicate those intensities of heat-motion at which plant life generally is still
possible. It is quite conceivable, and even occurs occasionally, that certain phe-
nomena of vegetation may still occur even below the freezing-point of water, because
from various causes the water contained in the cells only begins to crystallise at
a few degrees below zero. However, these are isolated cases : in the great majority
the vital movements in general only begin at a few or several degrees above the
freezing-point. On the other hand, the temperature of 50° C. also denotes the
upper limit only quite generally, since the majority of the vegetative processes are
brought to a standstill below this temperature ; and temperatures of 45° to 50°
persisting for a long time are simply fatal to most plants. It depends here essentially
upon the abundance of water or the succulence of the parts of the plant concerned
whether they freeze^ at lower temperatures, or perish at too high a temperature
below or above 50° C. Parts containing little water, as the majority of ripe, dry seeds
and winter buds, are exceedingly resistent even to low degrees of cold, and the
former even resist temperatures of 60° to 80° for a long time ; whereas very succulent
organs often freeze even at temperatures in the neighbourhood of the freezing-
point (this, however, is a specific peculiarity of certain plants), and are killed at 50°
or even under.

Within the specified range of temperatures there lies for each individual vegeta-
tive function a certain degree of temperature at which it developes its greatest
activity. We call this the optimum temperature ; thus distinguishing that point of the
thermometer at which any one vegetative phenomenon exhibits its maximum
activity. At every degree of temperature above or below this optimum, the vegetative
phenomenon in question will be less active. Exact investigation shows also
that, proceeding from the lower limit of temperature, at each higher constant degree
of temperature already mentioned, the activity of the phenomena of vegetation is
greater, until at the specific optimum temperature itself the maximum activity is
attained ; and if the temperatures rise still higher, the activity decreases step by step,
until, on attaining the upper limit of temperature, it ceases entirely. If this high
temperature acts for a short time only, the vegetative phenomenon may be renewed
when a lower temperature recurs ; if, however, the high temperature mentioned
has continued too long, death ensues.

In this case especially, where it is a matter of the dependence of the life of the
plant upon the temperature, we may again take the example of the steam-engine
already employed. We may compare the lower zero-point with that condition
where the tension of the steam just suffices to overcome the friction of the machine,
and to bring about a slow movement, which with increasing tension of the steam

' Concerning the freezing of plants, cp. Sachs, ' KrysiaUbildiingen bei dan Gefrieren und
Veränderung der Zellhaüte bei dem Anfthatien saftiger rflanzentheile,' in the 'Berechten der kgl.
Sachs. Gesellsch. der Wiss.,' Febr. i860, where I first described the separation of crystalline water
from cells. In the 'Untersuchimgen über das Erfriei-etz der Pßanze/i'' in H. 5 of the periodical,
'Die landwirthschaftlichen Versuchsstationen' (Dresden, i860), I gave a theory of freezing on
which all more recent researches in this direction have been based. This was improved in my
' Lehrbuch der Botanik" {\%(i%-'j/^. Müller (Thurgau) made valuable further researches, ' Land-
wirthschaftliche Jahrbücher,' pub. Thiel (Berlin, 1880).

THREE CARDINAL POINTS OF TEMPERATURE. 193

becomes accelerated, until the machine works not only with a certain velocity, but
also in such a manner that all the individual movements work together in the most
advantageous manner ; this would correspond to the behaviour of the plant at the
optimum temperature. If the tension of the steam is increased by supplying more
heat to the machine, dangers arise with the increasing velocity of its motion :
individual parts are heated and expanded too much, and others are strained until
they rupture ; increased centrifugal force may cause the fly-wheel to break, and the
machine shordy becomes destroyed by its own motion. We may compare these
processes in a certain sense with the death of the plant from heat.

By the establishment of the three cardinal points of temperature (v.'hich
expression includes the lower and higher temperatures as well as the optimum),
it is seen that each single vegetative phenomenon in any one species of plant
in general possesses its particular cardinal points. In other words, the lowest
temperature at which perceptible growth takes place, does not necessarily suffice
for the development of chlorophyll, or for assimilation, or for the irritability of
motile organs, and so forth ; and when this is determined for one species of plant,
the lower zero-points of these functions in another species are by no means necessarily
the same. This is likewise the case with reference to the optimum temperatures
and the upper limits of temperature. In general it is shown, however, that most
plants flourish best at certain medium temperatures, somewhere between 15° and
30° C, because within these limits the various phenomena of vegetation take their
course with sufficient energy and work harmoniously together. The diversity of the
lower zero-points of the various functions may, however, bring it about that at certain
lower temperatures, somewhere between 5° and 10° C, the various functions no
longer work harmoniously together, so that pathological conditions are induced.
It is observed, for instance, that in the spring time the young leaves of cereal
plants grow, it is true, but in spite of bright illumination they remain yellow,
because the lower limit of temperature for growth is not so high as that for the
development of chlorophyll. Similarly, it is occasionally observed during the
regularly recurrent cooling of the air about the middle of June, that plants which
require a relatively high temperature, e.g. Beans {Phaseolus), Maize, Gourd,
Buckwheat, etc., unfold new leaves, it is true, during such periods of cool weather,
but for the reasons given above they remain yellow, until at a higher temperature
the development of the chlorophyll is rendered possible.

No object would be served by bringing forward here all the numbers which
various observers have to a certain extent established as to the limits of temperature
and the optima of the various vegetative phenomena. Better opportunities for this
will occur later on, when considering the individual phenomena of vegetation them-
selves. Yet it may in some measure contribute to the explanation of what has been
said hitherto if a few numbers at least are mentioned as examples. The growth of
the seedling in its dependence upon temperature has been most frequently observed',

^ In my ueatise, 'Physiologische Untersuchtingen über die Abhängigkeit der Keimung von der
Temperatur; in the Jahrb. für Wiss. Bot. (i860), I first showed that the growth of roots and shoots
above an optimum temperature is again retarded, and may thus be represented by a curve which
first ascends from the abscissa and then returns to it ; ^vliile up to that time, even by Boussingault,
simple proportionality between temperature and growth had bi.cn asbumcd. That the form of a curve

[3]

194

LECTURE XII.

and it has been established that the seeds of cereals, for example, can still germinate
— i. e. develope their roots and seedling shoots — at a temperature very near to
the freezing-point of water ; whereas the ]\Iaize and the Kidney-bean [Phaseolus
vmltifloriis) require at least a temperature of about 9° C. ; and Date-stones, as it
appears, only germinate at 15° C. The optimum temperatures at which the quickest
growth of the organs of the seedling results, appears in our cereals to be 28° or 29''
C. ; in the other plants mentioned, however, it seems to lie above 30° C. At a
continued temperature above 40° C. the germination of all these plants is abnormal,
or they perish.

Of the other most general vital phenomena of the plant, may be men-
tioned, in the first place, the streaming of the protoplasm. In the hairs of
the Gourd-plant this movement appears only to begin when the temperature
reaches 10-11° C; the optimum temperature of the streaming of the protoplasm
may lie between 30° and 40° C. ; and towards 50° it decreases more and more, to
cease entirely if the exposure continues sufficiently long.

The relations which we have so far explained between the vegetative processes
and the temperature may also be graphically represented. If we suppose a straight
line drawn horizontally, and its length divided into a number of equal parts, which,
proceeding from left to right, are marked like a thermometer scale, 0°, 1°, 2", 3°, and
so forth to about 50°; and if we then further suppose the growth in length of roots
or shoots attained at the temperatures named, and in equal times, to be denoted by
lines of corresponding lengths, erected perpendicularly on our horizontal line (the
so-called abscissa) at the places where the temperatures belonging to it are recorded,
these vertical lines represent ordinates of a curve obtained by connecting their upper
ends together by a continuous line. It is clear that this line, usually curved, as al-
ready said, attains its highest point above the optimum temperature of the abscissa
line, and from thence again sinks down to the latter. This curved line we term
simply the temperature curve of the growth in length; and to any one at all
familiar with these matters, such a curve presents the easiest guide to the relations
of law between temperature and growth established by investigation. In like
manner the velocity of the protoplasmic movements may of course also be repre-
sented by vertical ordinates on a temperature abscissa, and thus in the form of a
curve ; and, in general, each function of the plant dependent upon the temperature
may be so represented.

So far as we at present understand the dependence of growth, of the move-
ments of protoplasm, the formation of chlorophyll, assimilation, and of the various
other irritabilities upon temperature, light, electricity, and external influences in
general, all these relationships may be likewise expressed in the form of curves ;
in each case the external influences being registered according to their intensity
on an abscissa line, and the corresponding efi^ects on the plant being expressed by
ordinates. Everywhere, so far as sufficiently strict investigations are to hand,
the curves of function so constructed show the main property of temperature

ascending from the abscissa and returning to it may also be employed for other relations of
dependence of the plant upon external influences, was further established subsequently by myself
and other observers. This law is, however, expressed generally for the first time in the present
lecture.

GRAPHIC REPRESENTATION OF EFFECTS OF LIGHT, HEAT ETC.

'95

curves : /. e., they begin at a certain intensity of tlie external influence, and thus
at a definite place on the abscissa line, then ascend more and more until
a maximum of the effect appears above a given point of the abscissa line, which
is always to be distinguished as the optimum point, from whence onwards the
curves again sink down to the abscissa. In order to illustrate this generalisation
of our law of curves more clearly by an example, I may cite the dependence
of the evolution of oxygen from the organs containing chlorophyll, under the
influence of light of various colours ; for the evolution of oxygen effected by
the chloroph}-!! is a function of tlie wave-lengths of light, in so far that only light
the wave-length of which amounts to at least 0-0003968 mm., and does not
exceed 0-0006866 mm., effects the separation of the oxygen. Starting from both
extremes, this effect of the light increases, and reaches the maximum at an optimum

Hed. Or at Ige Yclhnv.

Indigo.

Fig. 188.— The band A—H denotes the absorption spectrum of a sohition of chlot
Fraunhofer's lines, TIte spectrum is employed as the abscissa-lme for the curve
Pfeffer).

liyll. The lines A. B-H are
explained in the fi^nire (after

wave-length of 0-0005889 mm. As is well known, the various wave-lengths of
light produce in our eyes the sensation of the various colours of the spectrum ; and
since the number first mentioned represents the wave-length of the blue, and the
second that of the red, while the third — the optimum — denotes the wave-length
of yellow light, we may also say that the evolution of oxygen begins in the blue
light, ascends thence through the green of the spectrum up to the middle of the
yellow, and there reaching its highest point again descends in the orange-coloured
rays, to cease within the red portion of the spectrum. Thus if we have a sufficiently
large solar spectrum, and a green leaf is placed during equal times in the various
coloured regions of the spectrum, the quantity of oxygen evolved each time is
expressed by the curve of which the cardinal points were specified above^

* With reference to the effect of various coloured lights on assimilation, cp. my treatise in 'Kot.
Zeit.' 1864, pp. 353, &c., where the older literature, up to that lime scarcely noticed at all, is

196 LECTURE XII.

Indeed, it is not going too far to say that every dependence of a physio-
logical function upon any one external influence assumes the form of a curve,
first ascending and then" again descending; and that we have here one of the
fundamental laws of physiology. Since now we may term each dependence of
a vegetadve phenomenon upon external influences irritability, the form of curve
mentioned represents the fundamental law of irritability. This, expressed in words,
would run thus ; — If the intensity of an external influence {i. e. of a stimulus) increases
more and more, the effect of the stimulus, or the corresponding function of the
plant also rises, but only up to a certain degree, the optimum of the former; if
it exceeds this optimum, the effect on the plant is diminished, until at a certain
most intense influence the functional capability of the plant ceases. And further :
as the influence of the temperature only stimulates the plant when it attains
a certain height, so also every other external force or stimulus only then appears
to exert a perceptible effect when it has attained a certain intensity, sufficient to
overcome the resistances existing in the plant.

The law of the dependence of the phenomena of vegetation upon external
influences, which I have here attempted to make clear, gives, as, is ever the case
with natural laws, the relations between cause and effect in the most general
and therefore abstract form possible. Thus, even the simply formulated law of
gravitation is only a quite general abstract formula for an endless variety of
events. The path described by a stone thrown into the air, as weU as the
arrangement of the planetary system ; the apparent irregularity of the movement
of comets, as well as the flowing of water from the continents towards the ocean,
and the ebb and flow of the latter, are ruled by the abstract proposition that
bodies attract one another in proportion to their mass, and in inverse proportion
to the square of their distances : or, to select yet another example, the endless
variety of natural phenomena produced by the reflection of rays of light ; the
ordinary reflected image of our own person seen in the mirror, the focus of
a concave mirror, the Fata IMorgana, and the signals of the Heliograph, all
come under the abstract natural law that rays of light are reflected from the
surface of a body at the same angle as that at which they meet it. Just as
little as we regard the fact that this geometrical expression underlies this endless
variety of figures and phenomena, so litUe do we regard the above-described
curves as representing in endless variety the relations between plants and the
external world. Since it is the object of this lecture to give an account as clear
and general as possible, and not merely abstract but also concrete, we will now
pause again at a sketch, though slight, of those phenomena of vegetation which
illustrate the dependence of the life of the plant upon external influences in a
particularly impressive and intelligible form.

collected. Pfeffer gave further investigations, made in my laboratory, in Arbeiten des bot. Inst,
in Wzbg., B. I. p. i. I referred to the errors of other observers on this question in the same (B. I.
p. 276), in the treatise 'Die Pflanze und das Auge als verschiedene Reagentien für das Lichte Here
I have still to remark that the expression for the dependence of the evolution of oxygen upon
coloured ligiit — namely, a dependence upon the wave-length — correctly employed in the text, was
first established by me in the third edition of my 'Text Book' (1873) and in the fourth edition
(1874), p. 718, which Pfeffer has not quoted in his 'Pflanzenphysiologie,'' p. 21 j.

DEPENDENCE OF PLANTS ON HEAT, LIGHT, ETC. 197

There is, in the first place, the striking contrast of the winter rest of vegetation,
as opposed to tlie vilaUty wliicli the unfolding of the shoots and flowers in
spring and summer presents. In the main it is the lower temperature of winter
which renders the vital phenomena of the plant impossible ; only when the
temperature of the air and of the earth rises several degrees above the freezing
point of water, with a higher position of the sun, do the buds of trees, subterranean
rhizomes, bulbs, and tubers begin to be active and grow. At first, this is hardly per-
ceptible, and very slow ; but with the appearance of the first warm spring days, the
shoots come forth into the light with striking rapidity, and in a few days the
whole aspect of nature is changed. The surface of the earth is brilliant with
vivid green, flowers make their appearance, and in a few weeks one hardly
remembers how bare and lifeless the winter landscape was. The contrast appears
no less conspicuous when, coming from the hot air of the plains, in the middle of
summer, we ascend into the cool climate of Alpine heights, or journey towards
the far north ; we then leave a fully developed vegetation, to go towards a delayed
spring. When our trees and fields are preparing for the autumn, vegetable life
is just beginning on the mountains, and in high northern latitudes. If closely allied
forms of plants are compared, of which the one is native with us and the others are
at home between the Tropics, we very often find the former small, succulent
herbs and shrubs, while the latter develope into woody bushes or huge trees — a
difference which depends chiefly upon the fact that our summer is cool and short,
while that of the Tropics is hot and long. Any one, finally, who has to cultivate
plants of diflTerent climates on the same spot, e. g. in a botanical garden, knows
with what great cost and trouble this is done. The construction of greenhouses
and the costly labour are chiefly, if not exclusively, caused by the different require-
ments of warmth of plants coming from different climates.

Still more various, if possible, are the phenomena due to the dependence of »
the life of the plant upon Lights

If, for example, we lead the terminal bud of a vigorous leafy shoot into the dark
cavity of a box with opaque walls, a system of shoots, leaves, flowers, roots, tendrils,
and even ripe fruits with seeds capable of germinating, develope on it. The whole,
however, presents an exceedingly strange appearance ; the shoot-axes and leaf-stalks
are quite white ; the laminae of the leaves are small, and coloured yellow instead of
green, and generally are not extended flat ; and the entire substance of these so-called
etiolated parts is richer in water, more delicate, and more sensitive to injurious
influences than normal ones developed in the light. The flowers developed in the
dark, however, attain their- full beauty of colour and size. If now a bud of this
shoot-system, grown in the dark, is conducted through another narrow opening of
the box out into the light again, normal, green, flat-extended and large leaves
are produced once more ; and the shoot, growing forward in the light, again obtains
its normal peculiarities in every respect. This experiment, as simple as it is in-
structive, shows that the growth of all the organs can take place, even in deep

1 I published the first detailed investigation on the so-called etiolation of plants in my treatise
' rc/>er den Einfluss des Tageslichts auf Neubildung und Enfallung verschiedener PJI an zen organ c*
in Bot. Zeit. 186.5, and further in the treatise, ' Wirkung des Lichts auf die Blüthenbildung unter
]^ermittlung der Laubblätter ^ Bot. Zeit., p. 117.

198 LECTURE XII.

darkness, though with some inconsiderable abnormalities ; that, however, the organs
which are coloured green in the normal condition remain yellow or even colourless in
the dark, whilst flowers, fruits and seeds are developed in the normal manner. The
most important point here, however, is that such vigorous growth in the dark is only
possible when the etiolated shoot is nourished by normal green leaves exposed to
the sunlight. If these latter are shut off from the light, or are even much shaded,
by which means their assimilation is destroyed or injured, the growth of the parts
in the dark box also ceases, or becomes in a high degree abnormal. Stated
generally, the phenomenon described follows from the fact that the nutrition of the
plant is a function of the chlorophyll in the green leaves ; and that this chlorophyll
is only formed under the influence of light, and is enabled by means of the latter
to decompose the carbon dioxide of the air, and thereby to produce organic
vegetable substance, which becomes transferred from the assimilating leaves into
the buds, there to produce new organs even in the dark cavity. From the same
fact is to be explained why the culture of plants in dwelling-rooms in general yields
such unsatisfactory results. Even when provided with the best soil they remain small
and inconspicuous, the older leaves fall off prematurely, and flowers, or it may
be fruits, appear only in small numbers or not at all, because the influence of the
light through the window on the green nutritive organs is too feeble to bring about
a vigorous nutrition. These evils of chamber-culture only come forward the more
actively the warmer the plants are maintained ; since the higher temperature forces
them to vigorous growth, for which, however, in the feeble light, the corresponding
production of constructive materials is wanting. The plants here grow themselves to
death, so to speak. It may be allowed here also to interpolate the rule, based on
physiology, for the practical culture of plants in green-houses, that so far as plants
are concerned warmth chiefly signifies growth, while light, on the other hand,
brings about nutrition. Much light with a low temperature usually produces a
superfluity of nutritive matters, which the plant bears without injury ; a high tempera-
ture with feeble illumination brings about growth without the corresponding nutrition,
and is highly injurious to the plant, and in extreme cases even fatal. The
too short continuance of daylight also accounts for the fact that in hot, tropical
countries many of our native cultivated plants do not flourish ; because, although
the high temperature stimulates them, it is true, to vigorous growth, the short
tropical days do not allow of a sufficiently energetic nutrition. The larger the green
surfaces, especially of the leaves, the greater is the number of rays of light by which
•the plant is stimulated to active nutrition. Careful observations have shown that a
square metre of the green leaf-surface of a vigorously growing plant produces,
in ten sunny hours, 3-8 grams of dry plant-substance by the decomposition of carbon
dioxide^. If one reflects how many square metres of leaf-surface a large tree
possesses, and that the nutritive activity in one period of vegetation continues with
us for about 150 days, it is clear that a tree forms many kilograms of organic sub-
stance in the course of a single summer, affording the material for the development

^ The investigation 'Über specifische Assitnilationscnergie'' ('Arbeiten des bot. Inst, in Wzbg.,
B. II. p. 346) made by Karl Weber in my laboratory, may be consulted on this subject. See also
Sachs, 'Ein Beitrag zur Kenntniss der Ernährungsthätigkeit der Blätter'' in ' Arb. des Bot. Inst.'
1884, B. III. p. I, where it is shown that very much more than this may be produccd.

PERIODICITY OF DAY AND NIGIIT. 199

of buds and flowers which lakes place usually in the ncxl spring. This vegetable
substance produced in the green organs is constructed from carbon dioxide and
water, with the separation of a ver}- considerable quantity of ox}-gcn ; it is therefore
a substance poor in oxygen. Plants, as is well known, are combustible, i. e. their sub-
stance, poor in oxygen, is again converted, as it burns in the air, into the compounds
rich in oxygen (carbon dioxide and water) from which it was originally produced
in the cells containing chlorophyll. Just as much heat as is set free in the burning
of a tree, must have been fixed during the production of its organic substance.
This heat of combustion of a plant, however, represents a definite amount of energy
which can be made useful — in a steam-engine, for instance : an amount of mechanical
work exactly as great, only in another form, was performed in the cells of the plant
containing chlorophyll during the production of the organic combustible substance
in them, from water and carbon dioxide with the separation of oxygen. Or, in other
words, the energy which is produced by the combustion of vegetable substance existed
originally in the form of luminous vibrations of the ether, the energy of which has
been employed in the cells containing chlorophyll for the separation of the oxygen.

This work of the organs containing chlorophyll also changes with the alternation
of day and night ; but the resulting daily periodicity in vegetable life is made evident
in many other phenomena. In the first place, in a periodical growth of the young
organs, which may be accelerated by the darkness of night, unless the temperature
sinks too low, and brings about, on the contrary, a retarding of the growth: thus,
the rapidity of growth during the night may be greater or less than during the day\
according to circumstances. To the daily periodic changes most easily recognisable
belong the so-called sleep-movements of leaves ; these are particularly conspicuous
in the compound leaves of the Leguminosese, e. g. Rohinia and the Oxalide» ; they
take place in such a manner that the parts of a leaf fold themselves together in the
evening, either upwards or downwards, and open out again on the following morning.
But even simple foliage leaves move. In Jul}- and August one need only look around
a garden after sundown to perceive the altered position of the leaves of almost
all the plants ; the large foliage leaves of the Sun-flower [Helianthus aiinmis), for
instance, all bend downwards, so that the upper ones in part cover the lower,
while the foliage-leaves of the large Balsam {Ijupatiens glandidiferci) all stand upright
at night. These are only examples, however, since these movements are quite
general. Still better known are the so-called sleep-movements of flowers, many
of which close before sundown to open again next morning — phenomena which
we shall consider in detail later on, and which depend upon the diurnal changes of
intensity of the light and temperature. I shall also have to speak more in detail
later on of the heliotropic curvatures of the growing parts of plants, and of the
influence of light on the hitherto unexplained swimming movements of the so-called
swarm-spores of the Algse. The manner in which light operates as a stimulus on
plants is exceedingly various, and by its periodic daily alternations it leads conse-
(juently to daily periodical changes in plants.

■ I criticised, and corrected by my own observations, the older views (in great part quite
wrong) on the daily periodicity of growth in length, in my treatise, ' Über den Einßiiss der Lufttem-
peratur des Tageslichtes auf die stündlichen und täglichen Aenderungcn des Ldngenii.'achsthums'' in
Sachs' ' iVrbeiten des bot. Inst, in Wzbg.,' B, I, p. 99.

200 LECTURE XII.

Of the cosmic forces universally operating on the plant, I have still to mention
Gravitation. Plants possess an irritability — one might almost say a perception— with
respect to the angle which their organs make with the perpendicular where they
grow \ They are irritable as to the direction in which gravitation acts on each of
their organs, and this independently of their weight, or of any pressure. They
possess a sensitiveness to gravity, as we do to light or heat; while a direct
perception of gravitation is completely wanting to us, since we perceive it only
through the effect of weight and pressure. If a plant in full growth, the roots
of which have developed in a flower-pot, is brought from the ordinary erect position
into some other — e.g. laid horizontally — it is noticed after several hours, or, according
to circumstances, after some days, that all the growing organs, and some apparently
already fully grown, have been caused to move by means of this change of position :
the apices of the roots, the growing shoot-axes and leaves, flower stalks, etc., describe
the most various curvatures, until the free moveable parts of the organs have again
assumed those directions with respect to the horizon which they possessed before
the change of position of the whole plant. Those previously directed vertically
upwards or downwards become curved until their apices are again directed upwards
or downwards ; and parts previously growing obliquely or horizontally become
curved, after the change of position, until they can again grow forward obliquely or
horizontally in the same manner. We shall see later on that these movements
are called forth by an effect of gravitation on those organs of a plant which are
capable of growing. Stimulations are produced so that the organs respond to
every change of position with respect to the direction of gravitation, and these
only cease when the organs have again attained their original direction. This
phenomenon is termed Geotropism. The influence of gravitation makes itself felt,
however, in another and quite difi"erent manner, in that growing points arise at definite
places, the position of which is determined by the direction of gravitation. To
this point also we shall return subsequently. It may here be mentioned, in anticipation,
that the influence of gravitation on the direction of the position of equilibrium in
which the various organs of the plant grow onwards undisturbed can also be
established with certainty without special experiments ; it suffices to observe ^ that
the perpendicular stem of a Fir-tree or Palm stands vertical at every place on the globe
where it grows — i. e. has exactly that direcdon which, conUnued downwards, leads to

^ My views on Geotropism, presented in this book, are based on the following treatises by me —
' Längcnwachstlmni der Ober- und Unterseite horizontal gelegter sich aufwärtskrüninicnder Sprosse'
(Arbeiten des bot. Inst, in Wzbg., B. I. p. 193), and ' Über das Wachsthum der Haupt- und
Nebeniüurzeln'' (ibid. pp. 385 and 584), where I also first showed that organs growing obliquely
to the horizon are geotropic like lateral roots : this was afterwards also demonstrated by Elfving
for horizontal subterranean stolons. I wrote on the Geotropism of shoot-axes in 'Flora' (1873)
the essay, ' Über IVaehsthicm nnd Geotropism aufrechter Stengel,' a short notice on a very long
investigation. In addition are to be compared on this subject my treatises, ' Über Anschliessting der
geotropischen und helioti-opischeti I^riiinmnngen,' and further, ' Über orthotrope tind plagiotrope
Pflanzenthcilc,' in ' Arbeiten des TDot. Inst, in Wzbg ,' B. II. I siiecially refer to the concluding
remarks in the essay last mentioned.

^ I first showed in my 'Handbuch der Experimentalphysiologie'' (iSö^:;"), p. 100, that the
demonstration of the influence of gravitation on plants by means of rapid rotation — undoubtedly
first given by Knight in 1S06 — was not needed, since the same conclusion is to be drawn with the
same certainty fiom every day observations, as I have pointed out in the text.

EFFECTS OF GRAVITY, ELECTRICITY, ETC. 201

the centre of gravity of the earth. The terminal biul of such a stem grows away
from the centre of the earth liere just as it does at our antipodes, while the apex of
the primary root tends towards it ; and no other force than the gravitation of the
earth is conceival)le wliich could bring about this behaviour of the growing parts of
plants. Further consideration also shows that this is true for all parts of the plant,
no matter under what angle they grow with respect to the horizon or the radius of
the earth. Each organ of a plant has its specific irritability as to the direction in
■which it is met by gravity ; and this in such a manner that it only obtains a definite
position of equilibrium or rest when it is intersected by the vertical at a certain de-
finite angle. It would be simply impossible for any one acquainted with this fact to
imagine how the vegetable world could be formed, or exist in general without this
effect of gravitation on the processes of growth, if we were not in a position entirely
to neutralise the effect of gravitation on a living plant, by means of a simple
instrument — the Klinostat. It suffices to fasten the plant in any position whatever on
an exactly horizontal axis, and to keep it slowly rotating, so that the growing
organs continually change their direction with respect to the horizon, and come into
reversed positions during equal periods. In this case the stimuli act in opposite
directions on the same organ, and the angles under which the various organs grow
forth from their mother-organs are only determined by internal forces. In a similar
manner, moreover, the so-called heliotropic curvatures effected by light can also be
set aside by continual rotation opposite the source of light.

Relatively little is as yet known concerning the influence of Electricity on the
fife of the plant ^ Experiments have been made chiefly on the action of induction
shocks on protoplasm, and on the irritable and motile foliar structures of many plants ;
but the results obtained have not afforded a deeper insight into the nature of plants.
In general, all that can be said is that very feeble constant currents or induction
shocks during short periods produce no visible effects on protoplasm, but that, on the
other hand, with a certain strength of the currents, disturbances appear in the
protoplasm which resemble those brought about by a high temperature, and that
with still further increase of strength of the current the protoplasm is killed. Feebler
induction shocks act on the irritable organs of the leaves of Mimosa, the stamens of
Berberis, Centaiirea, etc., like shaking or contact — i. e. the organs perform the
corresponding movements. I found that constant currents proceeding outwards
from the pistil in the flowers of Berberis stimulate the anthers to irritable mo\ements,
whereas similar currents in the opposite direcdon are without effect.

That electro-motive mechanisms are present also in the normal life of the plant
itself may be in part directly demonstrated, in part presumed on general grounds. It
has been established, for instance, that every movement of water in a tissue, even in
the woody mass, is connected with slight electric disturbances; and that these even
appear when displacements of water are caused by the mere passive bending of a por-
tion of a plant, or by movements of irritability on its part — processes of which we shall
treat still more in detail in the proper place. In addition, however, we may assume

' I collected what was known up to the year 1S65 on electrical mechanisms and efTects in
plants in the ' Experimcntal-physiologie' (1S65), p. 74. Kunkel made more recent reseai-ches in my
laboratory, and at my suggestion, on the electrical action and conductivity of the living parts of
plants — 'Arbeiten des bot. Inst, in Wzbg.,' B. II, p. i and p. 333.

202 LECTURE XII.

that the chemical processes in nutrition, continually going on in the plant, and the
molecular movements during growth and the passage of fluids from place to place,
are all connected with electrical disturbances of various kinds, although it has not
been possible hitherto to demonstrate this experimentally. We may also suppose
that in the ordinary life of land-plants especially, during the continually altering
differences of electrical tension between the atmosphere and the soil, equahsations
take place through the bodies of the plants themselves. The land-plant rooted
in the soil offers a large surface to the air by means of its branches, and the
roots are still more closely in contact with the m.oist earth, while the whole plant is
filled with fluids which conduct electricity and are decom.posed by currents. Such
being the case, it can scarcely be otherwise than that the electrical tensions between
the atmosphere and the earth become equalised through the plant itself. Whedier
this acts favourably on the processes of vegetation, however, has not yet been
scientifically investigated, since what has been done here and there in the way of
experiments in this sense can scarcely lay claim to serious notice.

We possess, on the other hand, more exact and deeper knowledge, reaching into
the very essence of the matter, as to the action of Chemical forces in the plant. Among
the very numerous chemical elements of the earth, there are but few which enter
from without into the body of the plant (either in the condition of an element,
as the oxygen of the air, or in the form of very simple compounds, as carbon dioxide
and water, or, finally, in the form of salts) and there undergo decompositions
producing new chemical combinations, from which the organisable substance of
the plant itself proceeds. It is the object of the theory of nutrition to study these
processes in detail. Mention is made of them here only in so far as they con-
stitute external conditions of the life of the plant. If only a single element of those
necessary to nutrition is by any chance absent, or is present in too small quantity,
a plant cannot be nourished at that place, and thus cannot even live for long. The
well-being of any plant, therefore, depends upon all the elements contributing to life
being present in suitable chemical combinations and available to the plant. That
nearly the whole surface of the earth is covered with vegetation, and that even the
waters and the sea abound in plants, results simply from the fact that the few materials
subserving life are almost universally present in the necessary combinations. Or, we
may say, plants are constructed from those chemical compounds which are present
in quantity almost everywhere on the surface of the globe. Apart from oxygen,
carbon dioxide and water, there are a small number of salts — potassium, nitrate, potas-
sium chloride, calcium and magnesium sulphates and phosphates, and compounds
of iron — which, as we know, suffice for the nutrition of every plant, and which are,
moreover, absolutely indispensable. These chemical compounds, however, are found
together nearly everywhere, though mingled in the most various relative quantities :
this influences the thriving of the plant for good or ill, and therefore co-operates in
determining its distribution. To mention only a few examples in this connection :
there are certain species of plants which flourish in fresh water only, and others which
do so only in sea water. A certain large number of land-plants grow only in the
neighbourhood of the sea, or around salt springs, or on the saline soil of dried-up
seas in places abounding in common salt; which places, however, other plants
avoid as unsuited to them. The presence or absence of water at a given spot is

IMPORTANCE OF WATER. 203

quite decisive as to the possibility of vegetation. The deserts of Asia and Africa owe
their paucity of plants essentially to the droup^ht prevailing there ; since around
every spring by chance occurring even in the desert, a luxuriant oasis of vegetation
becomes developed, simply because all other nutritive materials are present in the
sand of the desert and the water of the spring. Water also affects the whole
organisation of the plant in a definite manner. That submerged and floating aquatic
plants in general present a different aspect from land-plants, and are of a more delicate
and simpler structure, strikes every observer at once. To lay stress on one point
only ; it is obvious that land-plants, the large green leaf-surfaces of which are
extended in diy air for the purpose of producing vegetable substance under the
influence of sunlight by assimilation, are necessitated not only to absorb the salts of
the soil which co-operate in this process by means of roots, but also to transport
them into the assimilating leaves. This takes place, however, by means of a current
of water ascending from the roots through the stem and branches into the leaves ; and
this is maintained by means of the continuous evaporation from the leaves. This
ascending current of water now requires a special organ in which to move, and that is
the woody body; it also requires a richly-developed system of roots to collect the small
quantities of moisture of the soil, which contains but little water, and so forth. These
arrangements, as is at once obvious, are superfluous in a submerged water-plant ; and,
therefore, proper wood is wanting to it, and its roots are insignificant in comparison
with those of a land-plant. It scarcely needs special mention that the most various
intermediate forms exist in the connection referred to ; and these are still more
various because nature generally can attain her object by very diflerent means.
Under conditions, for example, where the transpiration of leaves is too copious, forms
appear without leaves, as the majority of the species of Cactus, and similarl)- formed
Euphorbiacese and Stapelias ; or forms with thick succulent leaves like the Crassu-
laceae, in which transpiration is likewise only very feeble, or, finally, woody shrubs
with few and small leaves. But just the most interesting and instructive arrange-
ments w^hich would here present themselves must be passed over for the time being,
since they require too lengthy a description.

Just as plants react towards cosmical and inorganic influences generally, so
also they exist in a relation of dependence towards various other plants and animals.
In this, again, they react in such a manner that the external shape of their bodies
and their internal organisation become modified in the most various ways. This is
evident in a very conspicuous manner in the life of parasitic plants. When they
absorb the whole of their food from other plants, or occasionally even from animals,
they themselves do not require leaves containing chlorophyll : they are, therefore,
devoid of leaves, and accordingly, agreeing willi what has already been said, the
development of wood also is suppressed. The majority of parasites are massive
bodies of tissue, abounding in parenchyma, and with little development of surface,
the strange aspect of which usually strikes e\en the non-botanical observer, not to
speak of the further abnormalities, particularly of the sexual organs, which arise from
parasitism. On the other hand, however, even plants which contain chlorophyll and
are self-supporting may not only be enfeebled in vigour by parasitic plants, but also
altered in form. Of this, the Euphorbia infested with Fungi, as well as the so-called
Witches'-brooms (branches of a Fir altered by Fungi) offer well-known examples.

204 LECTURE XII.

Gall-formations are also to be added here. By means of the stimulus exerted by
insects during their development in the interior of the plant the growing parts of the
plant may develop in monstrous forms, or bodies of very peculiar sharply defined
form grow forth from them. In this connection the fact is of special interest
that the quality of these galls on the same plant depends chiefly upon the specific
peculiarity of the animal which produces the gall by its irritation. On our Oaks
alone, more than a dozen different forms of galls are produced by different insects.
Nevertheless, although very common, these phenomena are more isolated and
incidental. The most remarkable and beautiful dependence of the highly-organised
flowering plants upon insects makes itself evident in the mechanisms of flowers,
discovered by Conrad Sprengel in 1794. Sprengel showed even then that all
beautifully formed and coloured and odorous flowers, in all their relations of con-
figuration, are adapted for being visited by insects of a certain form and size for
the sake of their nectar; at the same time these animals transport the fertilising
pollen from the anthers on to the stigmas of other flowers of the same kind. Since
the development of seeds is only complete in these cases, the reproduction of these
plants depends upon the visits of insects; as, again, on the other hand, the entire
existence of the insects referred to is conditioned by the flowers of these plants.

Even this small selection of examples wiU suffice to show bow the whole life of a
plant — not only with respect to its origin, but also to its maintenance in the widest
sense of the word — depends upon external influences of the most various kind. It would
be incorrect to suppose, however, that the vegetable world as such, or any individual
plant-form, can be called into life at any time whatever by these external causes. All
that we have here considered are simply and solely reactions of vegetable substance
already existing, towards external influences on the same. The mode in which these
reactions are manifested depends, however, upon the nature of the given plant ;
and a further problem of physiology — and, indeed, the most difficult one of all — lies
in the investigation of this innermost nature of the plant, by means of which it is
rendered capable of these reactions.

LECTURE XIII.

THE MOLECULAR STRUCTURE OF PLANTS, AND ITS
PHYSIOLOGICAL IMPORTANCE.

The coarser structure of plants as perceived with the unaided eye in their
external form, as well as the structure visible with the microscope, have been
treated of in previous lectures ; and, as occasion offered, reference has also been
made to the fact that the explanation of the phenomena of hfe (that is physiological
investigation) cannot be satisfied with the knowledge of this visible structure,
but that we are obliged to form more definite conceptions of that structure which
is no longer visible, and no longer perceivable even with the strongest powers
of the microscope. The attempt may now be made, preparatory to what is to be
said later, to bring forward here some of the most important and most general
results of investigation in the latter direction. The problem is to draw conclusions
from the phenomena evident to the senses, which give us definite information as to
the structural relations no longer perceivable by the senses. In such cases it is
always advisable not to build conclusions on conclusions, and hypotheses on
hypotheses ; but to draw, from safely established facts only, the conclusions which
immediately follow. At the same time it will be well, for the better guidance of
those who are not quite at home in scientific matters, to go back somewhat further
than may appear necessary.

Not only physiological, but physical and chemical investigation also, were
long ago driven to form certain conceptions with respect to the minute invisible
structure of bodies, in order to obtain a more definite insight into natural processes.
In chemistry this is done by the assumption of the existence of atoms — exceedingly
small, indivisible masses of matter with which the chemical forces are associated.
It is imagined that the chemical properties of an elementary substance such as
Hydrogen, Oxygen, Potassium, Phosphorus, etc., are still existent even in atoms
of these substances. Certain chemical phenomena, however, necessitate the as-
sumption that two or more atoms of an element may come together into a closer
combination, which is distinguished as a molecule. Chemical combinations of
different elements must always necessarily be composed of two or more atoms,
and therefore always form molecules. A molecule is, therefore, according to the
view of the chemist, the smallest conceivable mass of a chemical compound ;
since, if the molecule were split up still further, the chemical luuure of the object

2o6 LECTURE XIII.

would necessarily be altered, because the combination of the atoms of different
kinds would be loosened. Thus carbon dioxide consists of molecules, each of
which is composed of one atom of Carbon and two atoms of Oxygen. In like
manner water consists of molecules, each of which consists of two atoms of
Hydrogen and one atom of Oxj'gen. The molecules of most other inorganic
compounds are more complicated : the molecule of potassium nitrate, for example,
consists of one atom of Potassium, one atom of Nitrogen, and three atoms of
Oxygen. The composition of the organic chemical compounds produced by plants,
however, are to be conceived as being much more complex. They all contain Carbon
and Hydrogen, and generally Oxygen also, and the most important of all organic
combinations — the proteid substances — contain Nitrogen and Sulphur in addition ;
and this in such a manner that in one molecule dozens, or even hundreds of atoms
of the elements named are combined with one another. The body of the plant,
then, consists chiefly of such polyatomic chemical combinations. The cellulose of
the solid frame-work of the plant, and the protoplasm and nuclear substance
consist of molecules, each of which contains very numerous atoms of three, four,
or five elements. Even in the province of pure chemistry, however, it is necessary
for the explanation of certain phenomena to assume that polyatomic molecules
may come together among themselves into closer molecular unions; and that in
this manner new chemical properties arise which do not belong to the individual
molecules.

With chemical processes in the narrower sense of the word are connected other
natural phenomena, in which it is no longer merely a matter of chemical changes,
but of movements in space of quite another kind. Here belong, on the one hand,
those movements of molecules which a dissolved or melted body exhibits on its
solidification as a crystal of definite form, as well as those movements by which
a crystal becomes again dissolved into its individual molecules, by a soluble medium
or by melting ; and, on the other hand, the entrance of water into organised bodies,
and the changes in volume effected thereby, as well as numerous other phenomena.
We are here particularly concerned with these latter processes, belonging to the
domain of molecular physics. For it appears that for the explanation of most
processes of life, the assumption of atoms and chemical molecules no longer
suffices, but that we are rather obliged to assume combinations of molecules which
form very, large numbers of small particles, or Micellae (Noegeli), which are never
visible with the microscope however, and the arrangement of which gives rise
to certain very peculiar properties of organised bodies^

1 It is not necessary to go more closely here into the theory of the internal structure of
organised bodies founded by Naegeli. Only the following need be noticed. According to Nsegeli
a series of the most highly characteristic properties of organised bodies — i.e. of starch grains, cell-
walls, and crystalloids— may be explained by the assumption that the molecules in the sense of the
chemist are united into larger unions, up to many thousands, and so constitute molecules of a higher
order, or, as Ncegeli more recently terms them, Micellae (particles). ' Organised substances,' says
Ntegeli ('Bot. Mittheilungen' in den Sitzungsberichten der kgl. bayr. Akademie der Wissenschaften,
1862, März, p. 203), 'consist of crystalline, doubly-refracting molecules (Micellje), which lie near
one another loosely, but arranged in a certain regular manner. In the moist state each of these
(Micellae) is surrounded with an envelope of water in consequence of its powerful attraction: in the
dry state they are in mutual contact.' In my ' Experimental-Physiologie,' 1 865, p. 443, I fust

SWELLING OF ORGANISED BODIES. 207

Crystals are either solul)le in water or not. In the latter case the water which
is in contact with the crystal, in spite of the force of attraction which exists between
both, is unable to tear off any molecules; and the construction of the insoluble
crystal also prevents the entrance of water molecules into its interior. If the crystal
is soluble in water, however (as a cube of common salt for instance), the attraction
between the two substances results in molecules being torn off from the surfaces
of the crystal, and intercalated between the molecules of the water.

By this means molecules of the salt l\ing deeper come into contact willi
the water, and suffer the same fate in it, until the whole crystal is dissolved into
its molecules, which now move about within the mass of water; and this movement
continues until a completely uniform distribution of the salt molecules in the mass
of water results. With the attainment of this condition of equilibrium, where every
salt molecule is surrounded by exactly as many molecules of water as ever}- other
in the same solution, relative rest now occurs. If, instead of the common salt, a
small portion of Iodine had been placed at the bottom of a large vessel filled with
Avater, a watery solution of Iodine would have l^een formed in the same way;
and the uniform distribution of the Iodine molecules in the water would be
recognisable at once in its uniform colouradon. This condition of equilibrium,
however, we could at once bring into a condition of movement if we suspended
in the upper part of the solution a bag filled with starch. The Iodine molecules
coming immediately in contact with the starch, would then penetrate into the
starch grains, affording opportunity to the more distant Iodine molecules also to
move towards the starch ; and this process would continue until all the Jodine
molecules, even the most distant ones, had travelled from the bottom of the vessel
up to the bag of starch, and therefore in opposition to gravitation.

Organised bodies — to which, so far as the plant is concerned, we may consider
the cell-wall, the protoplasm and nucleus, starch-grains, and the so-called crystalloids
to belong — behave towards water quite differently from insoluble and soluble crystals.
If a dry body of this kind is laid in water, its volume is increased more or less
according to circumstances, and by this the consistence of the body is altered.
Previously hard and brittle, it now becomes soft and flexible. A closer examination
at once shows that the increase in volume which the body has undergone by
swelHng up in water, is almost equal to the volume of the water which it has
absorbed. If this so-called imbibed water is again withdrawn from the swollen bod}-,
by evaporation or by means of some medium which abstracts water, c. g. absolute
alcohol, its volume again shrinks until it has reached the original size. Between
the organised body capable of swelling and the water, there exists a mutual at-
tracdon, just as between water and a soluble crystal. The great difference between
the two, however, lies in that the molecules of the latter become separated from
one another and distributed between the molecules of the w-ater; in the swelling
of an organised body, on the other hand, the water penetrates between its micellse
without these completely losing their connecdon. They only separate further from

suggested that protoplasm also is an organised sut)stance in Nsegeli's sense : it had previously been
regarded as a structureless slime or even as a lluid. This view established by me, that protoplasm

is an organised body, is now generally accc[4ed.

2o8 LECTURE XIII.

one another ; or rather they become forcibly driven apart by the penetrating water.
This process of swelling is thus something quite other than the entrance of water
into a porous unorganised body, e.g. dry gypsum ^ or into a heap of sand, etc. In

^ Although the view of the structure of organised bodies and those capable of swelling cited
in the previous note had been established and accepted by Naegeli, the view was nevertheless
held tenaciously for a long time, that the imbibition of water by such bodies could be referred
to the laws of capillarity in narrow, hollow tubules. That the capillary theory is in no way
capable of explaining particularly the movements of fluids in wood follows from Nsegeli and
Schwendener's considerations {'Das Alikroskop^ II Aufl. § 371), and indeed the more forcibly since
these investigators proceeded from the view that it was self-evident that capillarity was concerned in
the matter. I have in my work, ' Ucbcr die Porosität des Holzes^ (' Arbeiten des bot. Inst, in Würz-
burg,' II. Bd., p. 305, 1879), and jireviously in a preliminary notice, expressed myself on the matter
in question as follows: 'This view, that imbibition is only a special case of capillarity, was first
expressed by De Luc, and in fact because hygroscopic bodies after being completely saturated with
water on being brought into alcohol apparently maintain their condition of imbibition. The fact is
however incorrectly apprehended. If bodies capable of swelling and free from water, such as animal
glue, coagulated dry proteid, dry stems of Laiiiiuaria, &c., are placed in alcohol almost free from
■water (98 "/o) they never swell up in it at all, and never increase in weight, or only very slightly. If
they are jDlaced dry in water they absorb very much of it, as is shown by weighing, and their volume
increases to nearly the volume of the absorbed water. This increase in volume proves that the
water does not penetrate into pre-formed cavities (capillaries), but that it drives asunder the mole-
cules of the substance, and that to the extent of its own volume. If such a saturated body is again
allowed to dry up it again assumes the previous volume, and the cavities which the water had
produced and filled disappear, the molecules again applying themselves to one another. Alcohol
and thick glycerine are not able to diive asu«der the molecules of dry bodies capable of swelling,
and therefore do not penetrate into them. Since, therefore, cavities in which water or glycerine or
alcohof could penetrate forthwith do not exist in dry bodies of this kind, a comparison of these
processes with the capillary entrance of fluids in large bodies can of course scarcely be spoken of.

' When water, alcohol, or other fluids penetrate into bodies such as cast gypsum, chalk, or burnt
clay, which in the dry state possess actual capillary cavities, they drive before them the air
contained in the cavities, and this can be exhausted and measured ; when water, on the other hand,
penetrates into a dry body capable of swelling, no air is driven out, simply because it penetrates
into spaces which it first opens itself.'

If dry bodies capable of swelling, which do not absorb alcohol or glycerine, are first placed in
water until they are completely swollen, and then brought into very strong alcohol or glycerine, the
effect may be very different according to the nature of the body. Glue contracts energetically, since
the imbibed water is withdrawn from it without an equal volume of alcohol or glycerine passing in.
Laminaria behaves quite otherwise : it contracts but little in 98 % alcohol, and, as is shown by
weighing and determining the volume, alcohol passes into the spaces left by the water. Here,
however, the internal condition of the Laminaria is altered : in the watery state it was flexible and
soft, in the alcohol it becomes hard and brittle. Even if the alcohol which has replaced the
water is driven out by heat, the Laminaria no longer contracts to its previous volume in the dry
state: it now contains evident capillary cavities filled with air, since it swims on water, while the
dry Laminaria sinks at once. The alcohol is thus unable to drive asunder the molecules
of the cell- walls when dry ; if the water however has driven these apart, the alcohol penetrates
into the cavities occupied by the water, because as it advances it makes the molecules of the cell-
walls immovable and prevents contraction. These experiments explain also why alcohol is so
extremely serviceable as a medium for preserving and maintaining the form of plants : it takes the
place of the water of the cell-walls, while it prevents the contraction of the molecules of the latter.
If plants are laid in alcohol quite fresh they preserve their fresh appearance, if drooping portions
are placed in it they maintain their drooping aspect. The protoplasm lying within the cell-walls
so stiffened contracts on the other hand, since it becomes rigid in alcohol.

The imbibition of the cell-wall may be better compared with the process of solution of a salt,
than with the capillarity of porous bodies. Just as the water seizes upon the molecules of a crystal,
and takes them between its own molecules in solution; so the dry body capable of imbibition seizes
upon the water molecules, and forces them between its own. Both processes require time. \\ hen

IMBIBITION. 209

these cases the water penetrates into cavities — into small visible and invisible pores,
previously filled with air which now becomes forcibly expelled by the water
passing in : no pushing asunder of the solid parts occurs here, as is obvious from
the fact that the volume of the porous body is not perceptibly enlarged by the
penetrating water.

The chief point in connection with the swelling of an organised body, and upon
which stress is to be especially laid, is, as follows from the preceding remarks, that
the water of imbibition by no means passes in through pre-formed cavities or pores ;
but that it penetrates by forcing asunder the minute particles {niicellcB) of the
swelling body, as is evident at once by the increase in volume. Organised bodies
are therefore not porous in the ordinary sense of the word; and the penetration
of water into them does not occur by capillarity, so-called. A second point of
the greatest importance in the phenomena of the swelling of organised bodies,
is with respect to the extraordinary force with which the water penetrates
and drives asunder the solid particles.- This may be recognised, for instance, in
that dry wedges of wood driven into blocks of granite, and then moistened, expand
with a force so great that the stone may be split. The impossibility of completely
squeezing the water out of swollen vegetable cell-walls by pressure, however great,
also shows with what immense force the molecules of water are held between those
of the cell-wall, and gives a measure of the force with which they have penetrated into
the dry membrane. We shall make use of these facts in the theory of the move-
ment of water, and they will remove for us all those difficulties which have previously
been felt. The further fact that bodies capable of imbibing, such as dry cell-walls and
starch-grains, condense the aqueous vapour from the surrounding air to such an
extent that they become entirely or almost entirely saturated with water, or swollen,
may also be taken as evidence of the magnitude of the attractive forces between
molecules of water and these organised bodies. These however are only incidental

however the water molecules are at length distributed equally between those of the swelling body,
they are there held as fast as the salt molecules distributed in the water of the solution.

The water-molecules contained in a cell-wall in a state of imbibition evidently exert mutual
pressure just as little as the salt-molecuIe5 in a solution : the imbibed water-molecules form a
coherent mass of fluid just as little as the dissolved salt-molecules of a crystal do. Of course the
case is otherwise in a porous capillary body. In such a body provided with pre-formed capillaries
the height of the capillary column depends upon the weight of the continuous column of water,
and this exerts a pressure on the walls according to its height. In an imbibing body the weight
of the water does not come into consideration. It is therefore immaterial whether the imbibed
water in the cell-walls of a tree is 20 or 100 metres high.

The comparison of the imbibed water in a cell-wall or any other body capable of swelling and
imbibing with the condition of the water of crystallisation is perhaps still more apparent : no one
would assume that this is contained in capillary cavities of the crystal. The water of crj-stallisation
is also present between the molecules of the salt in a form in which it can no longer be designated
a fluid : it exists in a form which prevents the water molecules from exerting pressure on one
another and obeying the laws of hydrostatics applicable to a capillary water column, however fine.
Like the water of imbibition, the water of crystallisation also may be driven off by heat, at least in
some cases; but of course the crystalline form is then destroyed. P'undamentally, however, some-
thing of the same kind seems to take place in the conducting cell-walls of the wood ; for their
desiccation even at lower temperatures produces essential alterations in the imbibing properties, since
air-dry wood loses the specific property of rapidly conducting the water of imbibition ; it may
therefore be assumed that a permanent alteration in the molecular structure of the cell-walls is
effected by a certain degree of desiccation.

[3j.

21 O LECTURE XIII.

phenomena. We obtain an idea of the enormous magnitude of the forces here
coming into consideration only through knowledge of the fact that during the
penetration of water into dry starch-grains ^ a rise of temperature of several degrees
centigrade takes place; for this fact can scarcely be otherwise explained than by
assuming that the penetrating water is condensed, and a corresponding heating
effect follows. Since, however, a condensation of water by which a rise of
temperature of only 07ie degree is brought about requires a pressure of several
hundred atmospheres, we arrive at the conclusion that the condensation of the
water as it forces asunder the particles of the imbibing body must be equivalent
to a pressure of many hundred atmospheres.

But organic bodies capable of swelling are able to absorb not merely pure
water, but also aqueous solutions : here however peculiar phenomena make them-
selves apparent. It depends entirely upon the nature of the atoms distributed in
the water of the solution, on the one hand, and that of the swelling body on
the other hand, how much of the former enters simultaneously with the
water between the particles of the latter. In many cases a swelling cell-wall
absorbs a larger quantity of water, but a smaller quantity of dissolved matter,
from a somewhat concentrated salt solution, than corresponds with the concen-
tration of the solution. It follows from certain phenomena which we will examine
more closely later on, that living protoplasm absorbs only pure water from
certain solutions, leaving the dissolved substances behind. In other cases, again, the
swelling body absorbs a far larger quantity of the dissolved substances than accords
with the percentage composition of the solution. This is particularly conspicuous
with many colouring-matters, which may be so strongly absorbed, especially by
bodies consisting of proteids, such as dead protoplasm and crystalloids, that these
bodies become intensely and darkly coloured, even when the solution itself contains
but little colouring matter and is very light-coloured.

According to circumstances, the various phenomena of swelling shortly
indicated here play a part in the vital phenomena of plants. The changes in
volume which cell-membranes especially undergo by imbibition and desiccation,
may cause various movements of dead masses of tissue or of individual
cells. The dehiscence of dry capsular fruits, for the purpose of distributing
their seeds, is in general brought about by the fact that, on the drying up
of the pericarps, either their outer or inner sides lose relatively more of the
water of imbibition, by which curvatures and even ruptures of particular parts
of the pericarp are produced. In some cases these curvatures produced by
unequal contraction and dilatation are, according to the stmcture of the organ,
combined with spiral windings or with the rolling up and extension of band-like

1 That heat is set free when water enters into organised, and to a smaller extent also when
it passes into unorganised bodies, was, according to Pfeffer, first established by Pouillet. Jimgk and
I observed the increase of temperature during imbibition by starch in 1865 (cp. 'Lehrbuch der
Botanik," 1868, p. 500). Naegeli (' Theorie der Gährung; 1879, p. 133) found that the increase
of temperature when perfectly dry starch absorbed water, amounted to 11.6° C, both bodies before
the combination being at the temperature of 22° C. Since now, according to Joule, water is heated
0.03° C. by a pressure of 34.3 atmospheres, it follows that a rise of temperature of 11° must corre-
spond to a pressure of enormous magnitude. The water penetrating first into the starch grains
undergoes the most considerable rise of temperature, and therefore the greatest compression.

OSMOSE. 2 I I

parts. The pentamerous fruii of Erodium gruinuvi, for example, separates into
five single parts, each enclosing a seed, and each of which is provided with a
long awn which, on the drying up of the fruit, becomes twisted into a spiral at
its lower end ; if this organ is moistened the awn is extended quite straight,
and the very energetic movements produced by alternate drying and wetting,
together with various additional adaptations, lead finally to the result that the
fruit bores into the earth, to germinate there in the following spring. The
awns of many grasses behave similarly. The so-called warping of wood on drying
is also caused by the unequal changes of volume during the drying — i. e. the loss
of the water of imbibition of the wood ; and in the same way many movements
of the branches of trees during intense cold are to be referred to the same
principle, since the solidification to ice of the imbibed water acts in the same
way as desiccation, and, if it occurs unequally on different sides of a branch,
must produce curvatures.

The great force with which water penetrates into cells capable of swelling
will, as already mentioned, subsequently enable us to understand how the highest
trees are enabled to raise the large quantities of water transpired from their foliage,
from the roots through the stem into the leaves. A third fact also comes into
consideration here : the fluid moving in the substance of the cell-walls as water
of imbibition is not pure water, but a very dilute solution of those materials which
the roots absorb from the soil, and on account of which (since they are necessary
for assimilation) the entire water-current towards the leaves is set in motion. The
conveyance of substances produced in any cell whatever towards neighbouring cells
can also only take place by the cell-walls, as well as the protoplasmic linings of
the cells being able to absorb not merely pure water but also aqueous solutions.
This process of the movement of substances from cell to cell requires, however, the
consideration of a far more complicated phenomenon, depending upon the processes
of solution and swelling hitherto described, and termed Diosmosis (Osmose).

If a wide glass tube is closed at the lower end with an organic mem-
brane, and a quantity of salt solution is poured into the tube, which is then im-
mersed in pure water, the latter penetrates the closing membrane as water of
imbibition, is taken up by the molecules of salt of the solution in the tube, and
serves for the dilution of this solution. If this process continues sufficiently
long, a remarkable increase in the volume of the liquid in the interior of the glass
tube takes place, and the fluid ascends in it, under certain circumstances, to
a very considerable height : the cause of this movement lies in the attraction
of the particles of salt for the water which permeates the closing membrane
from below. This process, here described so simply, is designated Endosmosts
(Endosmose). Under certain circumstances Exosmosis (Exosmose) also may occur.
If the membrane closing the tube is capable of absorbing or imbibing the salt
solution in the tube, the attraction of the external water causes a portion of the
dissolved salt molecules which have penetrated into the membrane to diffuse out
into the water, while at the same time a larger quantity of water molecules ascend
through the membrane into the salt solution. It depends however entirely upon the
nature of the membrane and the dissolved salt, whether the latter can exude at all
by exosmose. In the process of osmose therefore, there are two points of prominent

P 2

31 a LECTURE XIII.

interest— in the first place, the question whether a substance dissolved in water is
able to pass through a given membrane; and, secondly, the force with which the
water on the one side and the dissolved substance on the other side of the
membrane attract one another. It depends upon this, whether a given substance
can pass from one cell into another in the living plant, and with what force cells
are enabled to take up water into themselves. If, for example, there arises by
chemical decompositions, in any cell whatever, a combination of atoms which
cannot diffuse out through the protoplasm and the cell-wall into the neighbouring
cells, this solution must accumulate in the cell in question, and even attain a
high degree of concentration, without passing into the neighbouring cells ; on the
other hand, chemical compounds which are contained in a cell-tissue may become
accumulated to a large extent in any given cell, if they suffer a change of their
condition of aggregation in the latter. If, for example, the sugar penetrating into a
cell is used for the formation of starch-grains, fresh sugar is enabled to penetrate
continually into this cell so long as this change takes place ; and, as we shall
see later, the plant makes the most abundant use of this and similar processes.
The distribution of the various chemical compounds in the tissues of plants, their
travelling over wide distances, and their accumulation in definite organs, depend
upon such processes ; a point to which we shall return more in detail subsequently
in the theory of nutrition.

The use made of the force with which water penetrates by endosmose
into the cell is no less varied and general. The first phenomenon produced by
this is the so-called turgescence of the celP; which however can take place
only in living cells still provided with protoplasm. We have to picture such a
cell as a double-walled vesicle, closed on all sides : the external wall consists of
cellulose, the internal or second wall, closely applied to it, consists of protoplasm,
and the enclosed cavity is filled with solutions of salts (the cell-sap). Let us suppose
the simplest case, and that such a cell lies in water. The water of imbibition
contained in the walls is taken up by the salt-molecules contained in the cell-sap,
and an equal quantity penetrates into the two peripheral layers from the exterior.
If this process continues for a long time, a large quantity of water gradually
penetrates into the interior of the cell ; and this is rendered possible only by the
double wall becoming distended to a corresponding extent. When this distension
finally ceases, the wall opposing it, no further flow of water inwards can take
place. The cell is now in the state of turgescence — i. e. the walls are distended
by the water which has forcibly penetrated into the cell, and since they strive
to contract elastically, they exert a pressure on the internal fluid. The main
point to be noticed here, is that the force by which the wall is pressed outwards
arises from the attracting force of the salts dissolved in the cell-sap towards the
water surrounding the cell ; and the opposite pressure, which prevents the further
penetration of water, is afforded by the elasticity and cohesion of the cell-wall.
In this, moreover, another fact is to be especially observed. The cellulose
wall, as is known from numerous observations, is, it is true, but slightly extensible

' Further particulars as to the turgescence of cells and its significance in growth as well as in
irritable movements will be given later.

TüRGESCENCE. 213

and very elastic, and is thus In so far suited to withstand the endosmotic pressure
acting from within; but this property of the cellulose wall would not by itself
allow turgescence of the cell to be set up, since the cellulose permits filtration to a
very great extent— i. e. the cell-sap pressing from within would be again forced
out through the cellulose wall, even with very feeble pressure, and consequently
no observable turgescence could arise in this way. As a matter of fact, also, it
is shown that all cells merely enveloped with a cellulose wall (e.g. wood cells)
are incapable of becoming turgescent. Only those cells in which a protoplasmic
utricle is deposited all around the inner surface of the cellulose wall are capable
of becoming turgescent. The protoplasmic membrane in fact permits the entrance
of the water absorbed by endosmose into the sap-cavity, but it is in a high degree
resistent against the pressure of filtration which arises from the increase of the
volume of the sap\ To this pressure the protoplasmic membrane of the cell
is impermeable, and thus completes the necessary properties of the cell-wall,
in such a manner that the water which has penetrated by endosmose cannot be
again pressed out. Or, in other words, the cellulose wall as well as the proto-
plasmic membrane permit the entrance of the water absorbed by endosmose into
the sap-cavity of the cell, which in consequence of this tends to enlarge; the
protoplasmic membrane prevents the water filtering out again in consequence of
this pressure, while the external cellulose membrane forms a solid, elastic support,
to which the protoplasmic membrane becomes appressed by the endosmotic pressure.
It depends upon the extensibility and elasticity of the cellulose membrane how
far the volume of the cell-sap can be increased; for the protoplasmic membrane
itself is in a very high degree extensible and but slightly elastic, and if the sap-cavity
were surrounded only by it the vesicle would extend without resistance, in con-
sequence of the increase in volume by endosmose. The properties of the
protoplasmic membrane and the cellulose membrane thus supplement one another
in offering resistance to the endosmotic pressure of the cell-sap. That this is
actually the case is proved by the behaviour of a turgescent cell when, by
evaporation or by exosmose, a portion of the water of its cell-sap is wdthdrawn^
If, for instance, turgescent cells are placed in a highly concentrated but otherwise
uninjurious salt solution — e. g. potassium nitrate — a considerable quantity of the
water of its cell-sap is withdrawn from the cell by the attraction of the salt ; in
consequence of this the protoplasmic membrane becomes very strongly contracted,
in proportion to the decrease in volume of the cell-sap. The cellulose membrane
on the other hand is only slightly contracted, because it was only slightly extended.
Both membranes thus become separated from one another; and the protoplasmic
membrane lies as a closed utricle free in the cavity of the slightly contracted
cellulose membrane. If such a cell is again placed in pure water, the latter is
attracted by the salts of the cell-sap and penetrates both membranes, the proto-

* With respect to the co-operation of the protoplasm in the turgescence of cells, cp. my ' Lehr-
buch der Botanik; IV Aufl. 1874, p. 866.

- The relations between turgescence, the protoplasm, and the cell-wall, established by Naegeli,
Pfeffer, and myself, were first clearly explained by Hugo de Vries in his ' Untersuchtingen über die
mechanischen Ursachen der Zeilstreckimg'' (Leipsic, 1877), a publication which may be recommended
in the highest terms to those commencing the study of vegetable physiology.

214

LECTURE XIII.

plasmic utricle becomes extended, applies itself in the first place close to the
cellulose wall, and as the endosmose proceeds further the latter also becomes
distended again to a certain extent, until its elasticity opposes further extension,
and the cell is now again turgescent. The cellulose wall may in this sense be
compared with a very firm but large meshed wire net, the protoplasmic membrane
on the contrary with a very extensible but exceedingly fine, and therefore scarcely
permeable, net.

The turgescence of a vegetable cell may be imitated in its most general
features by an artificial apparatus. If a pig's bladder is fastened on to one end
of a short wide glass tube, and the tube is then completely filled with a solution
of salt or sugar, and the second opening also closed with bladder, a sort of

Fig. i8g.— I. Youngf, half-grown cell from the cortical parenchyma of the peduncle of
Ctphalaria leucantha. 2. The same cell in a 40/0 solution of potassium nitrate. 3. The
same cell in a e^/o solution. 4. The same cell in a 10 o/^ solution, i and 4 after nature;
2 and 3 diagrammatic. AH in optical longitudinal section, h cell-wall ; / protoplasmic
lining of the wall ; k cell nucleus ; c chlorophyll grains ; s cell-sap ; e penetrated salt solution
(De Vries).

artificial cell results. If this is placed in a quantity of pure water, the latter
penetrates by endosmose into the cell, and the increase in volume causes the two
distended membranes to project externally like hemispheres, and at the same time
strongly resists pressure with the finger, like a solid body. If a prick is made with a
fine needle in one of the two membranes, the liquid spurts up to a considerable height,
the membranes at the same time collapsing elastically. Here also it is evidently
the attraction of the dissolved substance for the water of imbibition of the membrane,
which supplies the force by which the two membranes are so forcibly distended,
and the turgescent condition produced. The diff'erence consists only in that here,
there are not two difi'erent membranes which oppose the pressure of the penetrated
water, as in the vegetable cell, but that one and the same membrane — i. e. the
pig's bladder — allows the entrance of the endosmotic current on the one hand,
and, on the other, is at the same time elastic and resistent to filtration ; while in
the living vegetable cell the two latter properties are distributed between the cellulose
membrane and the protoplasmic sac. The so-called precipitation membranes also
behave similarly, though their resemblance to living vegetable cells has been to

ARTIFICIAL CELLS. 215

a great extent exaggerated \ If, for instance, a drop of concentrated solution of
chloride of copper is placed in a vessel filled with yellow prussiate of potash, a closed
precipitation membrane of ferrocyanide of copper is instantaneously produced on
the contact of the two fluids : there thus arises a cell-like structure, and, since the
precipitation membrane is permeable to the water of the surrounding solution, this
penetrates into the cell, attracted by the chloride of copper in the interior. The
increase in volume effects a corresponding pressure on the very thin precipitation
membrane, which, since it is not extensible, bursts after a time ; it is, however, at
once completed again to a closed membrane, since the two salts come in contact
for a moment at the gaping fissure, and at once produce a new precipitation
membrane. In this way such a cell may gradually grow considerably. Somewhat
greater is the similarity of the growth of a precipitation membrane of gelatine
tannate, formed when a drop of non-gelatinising solution of glue is placed in a
solution of tannin. Such a cell grows more uniformly and without the violent
eruptions referred to above. How far this growth may be compared with that
of a living vegetable cell, even with reference to the molecular processes in the
membrane, depends, however, upon our knowledge of the growth of true vegetable
cells. Here we have only to do with turgescence, and it is obvious from what
has been said that in these artificial cells the turgescence depends upon properties of
the membrane other than in the natural ones.

The capability of becoming turgescent is one of the most important properties
of the vegetable cell, since a long series of vital phenomena depend entirely or in
part upon it. In the first place, the fact is to be insisted upon, that growth and the
increase in circumference generally of living vegetable cells only take place when
they are turgescent, a point to which I shall return later in the theory of growth.
The opposite of the turgescent condition of a vegetable organ is that of drooping.
Every one knows that cut-off leaves or branches, if not placed with the cut surface
in water, soon become flaccid ; the shoot-axes, previously stiff" and brittle, become
highly flexible, and are no longer able to support the weight of the leaves, which
have likewise become flaccid — the parts sink down : they droop. If the whole
was previously weighed in the fresh state, it may be easily demonstrated that the
drooping shoot has become lighter : it has given off" water by evaporation, and it is
simply this loss of water, by which the turgescence of the cells has become
diminished, which causes the drooping, since if the shoot is allowed to take up
water (which of course does not always succeed to a sufficient extent), the drooping
condition disappears, and the young shoot-axes and leaves again become tense and
rigid, because the cells again become turgescent. With the consideration of this
phenomenon we come to the important question, upon what does the stiff'ness and
elastic rigidity of the succulent parts of plants depend.? a question which here
requires still closer consideration.

' Traube, the discoverer of the so-called precipitation membranes and the artificial cells con-
sisting of them, studied the properties of the latter, and thereby added to our knowledge of the
processes of diffusion, upon which I wrote critically in detail and on the basis of my own researches
in my ' Lehrlmch der Botanik; III Aufl., 1873. Traube's unfounded claims to priority as the
founder of the theory of growth by intussusception, and the astonishing confusion of his artificial
precipitation cells with actual vegetable cells, I replied to in 1878 in the 'Bot. Zeitung,' p. 308
(^' Zur Geschichte der viechanischcit Theorie des Wachstltnins der organischen Zellen ).

2l6 LECTURE XIII.

If a large leaf-stalk from a Rhubarb plant, for instance, or a Hcracleum, &c.,
or even a portion of the growing flowering stem of these plants, is cut off, an
excellent object is to hand for making clear the question here brought forward. Let
us suppose the object cut off square above and below, and its length to be about
50 cm. If now a strip of the epidermal tissue together with the collenchyma layers
which strengthen it are removed completely, and the attempt is then carefully made
to lay this collection of tissues again in its place, it is observed that the epidermal
strip is too short : it has become elastically contracted during the separation, and
thus, in the natural condition of the object, had been passively extended. If the whole
epidermis is now removed all round, and the length of the very succulent cylinder
(which consists chiefly of parenchyma and very extensible vascular bundles which
scarcely come into consideration here) is measured, it is found to have increased
very considerably in length during the manipulation \ Not rarely, such a cylinder
becomes extended from 50 cm. to 53 or 55 cm., or even more. In the natural
condition, where the epidermis enveloped the succulent cylinder of tissue, the latter
was thus passively contracted, and had a tendency to become extended ; it was
prevented from so doing, however, by the elasticity of the epidermis and collen-
chyma. There existed, therefore, in the natural condition of the whole, a mutual
tension between the epidermal tissue and the succulent fundamental tissue; the
latter behaved to a certain extent like the contents of a turgescent cell, which
distend the membrane. But of course it is not to be supposed that in the passive
compression of the tissue, it depended upon the compression of the water con-
tained in it, since this is, as regards forces coming into consideration here, simply to
be regarded as not compressible. The lengthening of the peeled cylinder of tissue
depends rather, as we shall see later, on a sudden alteration in the form of its cells —
they become longer and narrower. Nevertheless, the comparison is apt in other
respects, since it can be shown that in the natural objects also a transverse tension
exists, of such a kind that the inner tissue exerts a pressure on the surrounding
epidermal tissue in the transverse direction also. INIoreover, this condition of
so-called tissue-tensions is only found when the objects named are abundantly
supplied with water : if they had previously been allowed to droop through loss of
water, the separation of the masses of tissue would only give inconsiderable dif-
ferences in length between the epidermis and the internal body of tissue, or even
none at all.

We must now, however, regard yet another point in our simple experiment.
The leaf-stalk or portion of stem in the fresh state, or at any rate after having
been previously submerged for some hours in water, was tense and stiff: it
possessed considerable elasticity and rigidity. The removed strips of epidermis,

' The alterations in dimension taking place on the separation of tissues from one another were
first scientifically established by Brücke in 1848 in his ' Untersuchtmg über die Bewegung der
Mimosen ' on the motile organs of the latter. In my investigation, ' Über das Bewegungsorgan und
die periodischen Bnvegungen der Blätter von Phaseolus und Oxa/is' (Bot. Zeitung, 1S57), I treated
of the same phenomenon. The views on the tensions of tissues were lat^r, however, led in a wrong
direction by various publications of Hofmeister, since he chiefly observed the cell-walls only, and
did not pay sufficient attention to the pressure between the cell-sap and the wall. I then (in my
'Lehrbuch der Botanik,' especially in the III and IV editions) again brought the theory of the
tensions of tissues, as a consequence of the turgescence of cells, into the direction previously pursued.

TENSIONS OF TlSSr/ES. 21 7

however, are limp as damp paper : the uncovered inner succulent body of tissue
is now likewise highly flexible; it is quite impossible, for example, to hold it
suspended horizontally, since it at once bends limp downwards. We have here
then the case of an elastic, solid, stiff body consisting of two parts, each in a high
degree flexible and by no means stiff: only in their natural connection do the
epidermal and internal tissues together form an elastic and rigid body ; and in fact it
is the mutual tension, and the circumstance that the inner tissue is strictly too large
for the extensible epidermis (or, conversely, the epidermis too small for the former),
which brings about the rigidity of the whole. The same is also the case, however,
with a turgescent cell. Its membrane taken by itself is flaccid, and of course we
cannot speak of solidity in connection with the fluid contents ; nevertheless a tur-
gescent cell is as elastic as a billiard ball. We have the same condition of affairs also
in a thin-walled caoutchouc balloon, which, when empty, is a limp wrinkled sac, but
which may be converted into a firm elastic sphere by being strongly inflated with air :
the solidity of this again depends simply upon the mutual pressure of contents and
skin. If we suppose some hundreds of thousands of small caoutchouc balloons
thus inflated with air, and all contained together in an extensible caoutchouc vesicle,
the latter, together with its contents, would also form a rigid bar like the stem of a
plant. If we suppose the small caoutchouc balloons not inflated with air, but filled
tense with water, the same effect results ; and it is somewhat in this manner that we
have to imagine the rigidity of a petiole or stem produced by the turgescence of
the cells. It is at once clear that if the small balloons in the supposed system lose
a part of their turgescence by the withdrawal of water, and each thence becomes
somewhat smaller, that the tension of these cellular contents towards the enveloping
caoutchouc vesicle also diminishes ; the latter would then become shorter, and the
rigid system must at the same time relax. We must picture somewhat in this way
the drooping of a cut-off shoot when it loses water by evaporation.

The rigidity of succulent shoot-axes and leaves, especially those which are
still growing in length, depends essentially upon this condition of the layers
of tissue, brought about by turgescence and tissue-tensions; the same is true
of the peculiar firmness of succulent fruits, tubers, bulbs, and roots, which all
become limp and soft, or as one generally expresses it, shrivel, by the loss of water.
The upright position of the young flowering stalks which numerous plants put forth
in the spring, and the rigidity of young shoot-axes and the leaves of trees in the
spring, are due simply and entirely to the state of things described.

If we further suppose that in a stem, petiole or root so conditioned, a loss
of water takes place by evaporation from the turgescent cells only on one side of
the longitudinal axis, the object must become shortened a little on this side, and the
necessary consequence is that it becomes bent or curved, since the shortened side
becomes concave. In like manner the increase of turgescence and extension of the
cells on one side of the longitudinal axis would cause this side to become convex.
The latter fact may be elegantly demonstrated on the roots, 10-15 cm. long, of the
seedHng of the Bean, INIaize, Gourd, &c. If these are allowed to dry up for a few
minutes in the air, they become shortened a little through slight drooping, and if the
roots are then dexterously laid on the surface of water, so that only the under side
of the root is moistened, the cells of this side at once absorb water, and become

21 8 LECTURE XIII.

larger and longer; the consequence is that since a long stretch of the root
becomes curved and convex on the under side\ the free apex of the root rises
high above the level of the water. This movement takes place so rapidly that it can
be conveniently followed with the eye.

A long series of induced movements, with which we shall become familiar more
in detail subsequently and among which those of the leaves of Mimosa are best
known, are similarly effected by alterations in the turgescence of one side; only these
alteradons are not caused by evaporation of the water of the cell-sap. These sensitive
phenomena depend upon the remarkable fact that, by simple contact or shaking,
the protoplasm of the irritable cells suddenly loses the resistance to filtration w^iich
is necessary for turgescence, so that a portion of the cell-sap is driven through the
cell-walls of the sensitive organ into neighbouring parts, while the walls, which
were previously distended elastically, contract, and thus bring about a shortening of
the one side of the sensitive organ, in consequence of which the latter becomes
curved concave on this side.

We have already seen that the rigidity of succulent stems and petioles during
growth in length, and often also for some time after its conclusion, is caused by
ihe tension of the tissues, depending on the turgescence of the parenchymatous
tissue and the opposite pressure of the epidermis often strengthened by collenchyma.
The rigidity of the older portions of plants which are traversed by woody
sclerenchyma strands, and which no longer grow in length, are produced in
another M-ay, however. That a tree stem, or a woody branch, or even an older
lignified flowering stalk of a shrub, or the haulm of a grass, is rigid and elastic,
depends on quite other causes. In these cases, where lignified masses of tissue
are always present in the organ, it is these alone, or with the co-operation of tissue
tensions also, which determine the rigidity of the organ. As is well known, a thin,
woody Willow twig stripped of its cortex is firm and elastic, and thin rods cut out of
the wood of the stem are likewise extremely rigid; and even very thin slips
of wood possess this property in a high degree. Here the rigidity by no means
depends upon the mutual tensions of layers which are themselves limp ; but upon
the fact that the woody tissue is itself rigid, hard, and elastic, much as a metallic
rod or a crystal. In addition to the remarkable power of rapidly conducting the
water absorbed by imbibition in the substance of the cell-wall, the lignified cells
have the office in the vegetable world of enhancing the rigidity of the organs,
without the intervention of tissue tensions; and the enormous density of the shell
of a Cocoa-nut, or of a Cherry-stone, shows how great may be the solidity of lignified
cells under certain circumstances. In these examples, however, it is rather the bulky
accumuladon of the solid materials which brings about the solidity of the body in
quesdon. In the construction of their flower stems and leaf stalks, on the other
hand, only a relatively small quantity of lignified masses of tissue comes into use in
the form of very thin strands or layers ; these, however, are so distributed accord-
ing to mechanical principles in the organs, that they nevertheless produce a high

^ The upward curvature of partially drooping radicles laid horizontally on a surface of water
here referred to was first observed by Ciesielski, but was regarded as a phenomenon of growth. 1
gave the correct explanation in the ' Arbeiten des bot. Inst, in Würzburg,' B. I, pp. 395-4CI.

ELASTICITY OF LIGNIFIED TISSUES.

219

degree of rigidity. We find particularly exquisite examples of this in the
haulms of Grasses, and in the extraordinarily long scapes of many Rushes and
Sedges, &c. {/uncus, Sdrpus, Cyperus). These organs form columns, which, though
one, two, or even three metres high, only possess a diameter of a few millimetres.
In spite of their exceedingly slender form, they are very rigid : under the
pressure of the wind they may be bent down into a semicircle, and in spite of
being weighted at the apex with fruits and leaves, nevertheless spring up again like
elastic steel wire. Examination shows, however, that these slender columns are
either hollow, as the haulms of the Grasses, where the wall of the hollow cylinder is
often only of the thickness of ordinary paper ; or the interior of the columnar organ
consists principally of very loose pith. At the circumference of the cylinder, however,
either quite close beneath the epidermis, or on the outer and inner sides of the slender
vascular bundles, which are arranged in a circle on the transverse section, run thin
cords of strongly lignified elastic
fibres, upon which alone the
rigidity of such organs depends.
These lignified strands of fibre are
scarcely at all extensible, and their
elasticity may be not inaptly com-
pared to that of wrought iron. As
in the artificial construction of a
thin hollow column, so Nature distri-
butes these firm layers and strands
chiefly at the circumference of the
organs, because there their essen-
tial properties are at the greatest
advantage. If the haulm of a
Grass or the scape of a Rush,
or any other similarly constructed
cylindrical organ is bent, the elastic
layers and strands on the convex side
must be thereby slightly elongated,
and those of the concave side slightly

compressed ; since they possess a high degree of elasticity, however, they forcibly
assume their natural length again when the external pressure ceases, and straighten
the cylindrical organ. Schwendener\ who has closely investigated the relations

* The significance of the strings and layers of lignified sclerenchymatous cells which accompany
the vascular bundles or which run without these beneath the epidermis or in the parenchymatous
tissue, as conducing to the rigidity of stems and leaves, especially in the Monocotyledons,
was first correctly recognised and established by Schwendener in his book 'Das tncclianischc
Princip in anatomischen Bau der Monocoty/en' (Leipsic, 1874). That Schwendener sought to
introduce these forms of tissue into science as ' bast ' was however a mistake. The supporting ring in
the flowering scape of the species of Allium for instance consists of parenchymatous fundamental
tissue, the cells of which are only narrower, thicker-walled, and strongly lignified. I also find that
Schwendener laid too little stress upon the fact that the rigidity of stems actively growing in
length must depend on quite another principle, since in these the strings and layers in question are
not yet lignified and consequently are very extensible. Schwendener's researches essentially account
only for the rigidity of fully developed intemodes and leaves.

Fig. 190.— Transverse section of the flowering scape of Allium
loenoprasum. sr the cyUnder of supporting tissue, consisting of
lified fundamental tissue.

2 20

LECTURE XIII.

of construction of shoot-axes and leaves here referred to, showed how the distri-
bution of the elastic strands and layers strictly accords with the principles of
theoretical mechanics. That the mechanical elements effective in the construction
of thin stems also find their use in the construction of leaves, is easily intelligible ;
in this, however, it is a matter not merely of the elasticity and rigidity which
determine their free suspension in the air, but also of that kind of solidity which
prevents the tearing from the margin inwards. It has already been explained in a
previous lecture (IV), with a series of examples, how foliage leaves are protected
against such injury by the mode of distribution of the ribs and veins.

After what has been said, it scarcely requires special mention that the kind of
rigidity last described, since it depends on the subsequent lignification of the elastic
fibres, only comes into play in the older stems and in leaves which are already fully
grown ; whereas the kind of rigidity previously described, and effected by the turges-
cence and tensions of the tissue, serves the organs which are still elongating, or which
at any rate are not provided with lignified parts. For the sake of completeness I may

add at the same time that young shoot-
axes and leaf-stalks in an earlier stage of
their growth in length, exhibit yet a third
kind of rigidity : this depends also, it is
true, on turgescence, but is characteristic
in that the walls of the turgescent cells are
more extensible and less elastic than in
later stages of growth. The portions of the
shoot-axes, some 10-20 cm. long, under
the terminal buds of Clematis and Aristo-
lochia, and under the young flowering spike
o{ Plantago major, &c., are in this condition
highly extensible. A longitudinal pull acts
on these organs as on a thin thread of caout-
chouc, and they are so flexible, like such
a thread, that they can literally be wound round the finger. Stretching, and strong
bending at the same time, however, make them limp, and effect a diminution of
the rigidity: this is brought about by the fact that the limits of elasticity of the
still very thin cell walls are easily exceeded by the stretching. On this peculiarity, by
which the young filiform structures obtain the consistence of a leaden wire, or perhaps
of a wax thread, depends an easily observed phenomenon, which Hofmeister errone-
ously explained as a phenomenon of sensitiveness \ If shoots with long, slender, still
growing apical portions are shaken, it is remarked that after the shaking they hang
down limp, to become upright again, in consequence of further growth, only after some
time. I showed several years ago that the apparently irritable curvature of young shoot-
axes is nothing more than an effect of their great extensibility, in combination with
very small elasticity, and that one cannot here speak of sensitive effects. That such

Fig. 191. — Transverse section of a petiole of Mitsa Eiisete.
The lamella: of tissue at the periphery together form an elastic
system ; between these are stretched soft parenchymatous
plates, in which run thin vascular bundles.

' The bending down of the growing apices of shoots, caused by shakings or blows, was first
closely investigated by Hofmeister, but erroneously regarded as a phenomenon of irritability.
I gave the true explanation of this phenomenon from the great flexibility and feeble elasticity of
growing shoot-axes in the third edition of my 'Text-book,' 1873.

ELASTICITV OF LIGNIFIED TISSUES. 221

young shool-axes, and likewise younger portions of root-fibres do, as a matter of
fact, so behave, is most easily observed thus : it is possible to give to them with the
finger almost any chosen curvature, which they then maintain for a long time, until
further changes take place through growth. The shaking of the objects, employed
by Hofmeister, may be better replaced by several blows given with a rod, applied
on one side of the older lower portions of the shoot. The free apex is thus made
to vibrate violently, and the apex in part maintains the strong curvature, so
that on coming to rest it hangs down. It is obvious that the powerful vibrations
given to slender flower stems by the wind in spring must call forth similar pheno-
mena; and upon this depends, at least in part, the unpleasant aspect of a garden
in a strong wind.

PART III.

NUTRITION.

LECTURE XIV.

THE ASCENT OF WATER IN TRANSPIRING LAND-PLANTS.

Plants require water for the growth of the younger organs, the cells of
which must be rendered turgid by endosmose. At the same time the water
taken up serves for the production of chemical compounds in nutrition ; and
if the surfaces of the shoot are in contact with the atmosphere, a part of the
absorbed water is exhaled in the form of vapour— a loss which must be again
made good by a fresh supply of water, if complete drying up is to be avoided.
It depends essentially upon the nature of a plant and its mode of life how the taking
up and giving off of the water are accomplished. It is in the first place obvious that
these matters will be otherwise in plants submerged in water, or in subterranean
plants, than in those which expose large evaporating surfaces in the air ; and in
these again it is important whether they are small and can endure an occasional
desiccation, as most Mosses and Lichens, or whether we have to do with larger
and more highly organised plants, the root-system of which extends in the m.oist
earth, and the assimilating foliage of which unfolds in the open air, and cannot
endure severe drying up. In the latter circumstances are found especially the erect
or climbing Ferns, Conifers, and Flowering plants. On account of the great variety
of facts which here present themselves, it is advantageous to commence our con-
siderations on a typical case which allows us to recognise the important points
clearly on all sides. Let us picture to ourselves for this purpose a tree, or even an
annual plant, the stem of which stands erect and unfolds its crown of foliage to
the air, i.e. plants of the common form, as met with in the Tobacco, Sun-flower,
ordinary Palms, Firs and Pines, Poplars, Oaks, or other forest trees.

When in such plants the buds at the apex are to put forth shoots and unfold,
they require for the purpose a quantity of water which is nearly as large as the
organs themselves which are to be developed ; since the latter, in the fresh, fully
grown condition, consist of water to the extent of about nine-tenths or more of
their volume. This water, however, is absorbed far below, by the roots in the
earth, and must thus be conveyed to the growing shoots through the stem. As
soon as the leaves of the latter are unfolded, moreover, they constantly give off
aqueous vapour to the atmosphere; and, as may easily be observed on cut-oflf
shoots, they would completely dry up in a few days if water were not constantly
conveyed to them through the stem from the roots. Experience shows now that
in land-plants with large and thin leaves, the volume of water made use of for

[3] Q

226 LECTURE XIV.

the purpose last named, in one period of vegetation, may be many times larger
than the volume of the whole plant itself. We have thus to do here with an
exceedingly energetic function; and, disregarding for the time being all other
movements of water in the plant, we will concern ourselves in the first place
exclusively with the phenomenon last indicated, which is called into play by
the evaporation of water at the surfaces of the leaves, and which we may
best distinguish as the ascent of water in transpiring land-plants. When the
phenomena which here take place and co-operate have been made clear, it
will be easy to understand deductively the movements of water in other plants
also.

In the first place it will be advantageous to point out the purpose for which the
flow of water is brought about by the transpiration of the leaves. The green leaves
containing chlorophyll are, as has been already mentioned several times, the organs
of assimilation of the plant ; and it is in their cells that the carbon dioxide of the air
is decomposed, and employed for the production of carbonaceous vegetable substance.
Water also is necessary for the formation of this substance ; and it is known, and will
be shown in detail later on, that a series of salts which the roots take up from the
earth are absolutely necessary in addition, if the process of assimilation in the cells
containing chlorophyll is to take place. These salts (especially sulphates and
phosphates of calcium and magnesium, potassium nitrate, and salts of iron) must
therefore be conveyed to the green leaves for the purpose of producing organic sub-
stance. This is accomplished as follows : an exceedingly dilute solution of these salts,
which may be compared at once to ordinary drinking water, flow? from the roots
through the stem into the leaves. Considering the small amount of salts contained in
the ascending water, however, only an extremely small quantity of the nutritive sub-
stances would be conveyed into the assimilating cells if the matter ended with the
mere flowing in. The assimilating leaves, however, are induced by the warmth
of the surrounding air, and especially by the rays of light, to let the water which
has streamed into them escape in the form of vapour ; this enables a fresh
quantity of water charged with nutritive matter to flow up to them from the
roots. Thus, by means of evaporation or transpiration, a continuous flow into the
organs of assimilation is rendered possible : as the water evaporates from these
organs, the salts brought by it from the soil remain behind in the assimilating leaf-cells,
and take part in the chemical processes of nutrition. This obvious explanation of the
ascent of the water, so far as it is caused by transpiration, has hitherto been almost
entirely misunderstood in a remarkable manner ; and short-sighted people have even
maintained that the transpiration of land-plants is a faulty arrangement, which has to
be compensated by means of the currents of water. As a matter of fact, however,
the whole organisation of a land-plant is only intelligible if one keeps in view the
purpose of the water current indicated. The envelopment of the stem with an
epidermis which possesses few stomata and hinders evaporation, or, in woody
plants, even with a thick periderm, and in large trees with bark, has essentially the
object of protecting the nutritive water ascending in the stem against evaporation.
The small thickness and large area of the leaves, combined with the existence of
the stomata which pierce the epidermis in millions, and which may be closed or
open according to circumstances, are likewise only intelligible when it is known

CONDITIONS OF TRANSPIRATION. 227

that they laciHtate and regulate the formation of vapour, in order to render
possible the inflow of fresh nutritive water to the organs of assimilation according
to requirement. Where this is not necessary, as in submerged water-plants, the
arrangements mentioned are also wanting; and in land-plants which possess a large
transpiring surface, and are therefore devoid of a large assimilating surface, as
in species of Cacfiis and Crassulacea?, nutrition and accordingly growth also
are relatively feeble. When it has been sought, very thoughtlessly, to lay stress
upon the fact that land-plants assimilate and grow even in an atmosphere
saturated with vapour, where transpiration from the assimilating surfaces is not
possible, it has been overlooked that in such cases it is a matter of an extremely
feeble assimilation only, and accordingly also of a very small supply of nutritive
water to the leaves, — a supply so small that, as can be demonstrated, it is main-
tained without transpiration-currents. INIoreover, there is the fundamental error
of assuming it as at all possible to maintain a land-plant even but 50 cm. high
in a space constantly saturated with aqueous vapour : the fact being that even in this
case (since saturation with vapour is effected with extreme difficulty) opportunity
occurs for transpiring and maintaining a feeble flow of water. Let it but be attempted
to bring any tree, or even only a Sunflower or a plant o^ Ricinus, to normal vigorous
development with actual prevention of all transpiration, and it will then be seen what
comes of it : every horticulturist knows that land-plants grown in very moist air, and
therefore with feeble ascending currents of water, are much too poor in substance
and too watery to pass as healthy plants.

Considering the great importance of the movement of water called forth by
transpiration, for the whole well-being of land plants, the chief relations of organi-
sation of which are subjected to this function, it is worth while to consider the
phenomena in question more closely, and above all to examine those organic
arrangements which specially serve to produce the movements of water, as well as
to become acquainted with the mechanics of the remarkable movement itself.

In the first place, it is necessary to convince ourselves that the leaves con-
tinually exhale considerable quantities of aqueous vapour. It is only necessary to lay
a leaf on the scale of a balance, to notice that it becomes continually lighter,
and finally dries up entirely ; this simply takes place by the escape of aqueous vapour.
In the same way a plant eventually withers and dries up when rooted in a flower-
pot and not watered. If a plant is allowed to grow in a glass or metal vessel
filled with nutritious soil, so that a large number of green leaves become developed,
and the upper surface of the vessel is then closed by a lid consisting of two halves,
which only allows the stem to pass through the centre; and if the whole is then put on a
scale, first placed in equilibrium, it is noticed that the scale-pan loaded with the plant
l*ecomes lighter, and rises, although no water can evaporate from the vessel enclosing
the roots. Evaporation takes place, however, through the leaves ; and if a Tobacco
plant, for instance, has been used for this investigation, the total leaf-surface of which
amounts even to only a few square millimetres, and the experiment is arranged in
ordinary daylight, or in sunshine, the balance shows a loss by transpiration of one or
several cubic centimetres of water per hour, and it is easy to calculate that in the
course of a few days the same plant evaporates from its leaves several hundred cubic
centimetres of water, amounting to more than its own volume. This result is obtained

Q 2

228 LECTURE XIV.

much more readily if Maize, Tobacco, Beans, Cabbage, or other plants are allowed to
develop their roots, not in earth, but in aqueous solutions of nutritive matters. If the
glass vessel containing the nutritive solution and the roots of the plant is closed by
a halved cork, which allows the stem to pass through and fit tight, the level of the
nutritive solution is seen to sink from day to day ; and in the course of several days
the whole of the fluid within reach of the roots disappears from the vessel. The
whole of this water has ascended through the stem into the green leaves, there to
be evaporated. If the student wishes to convince himself of this, which of course is
scarcely necessary on a Htde reflection, it suflices to cover a plant arranged as
described with a bell-glass previously cooled : the aqueous vapour escaping from the
leaves soon becomes condensed on the glass wall, and runs down in the form of drops.
Of course no measure of the evaporation in the open air would be obtained by this
means, because the space under the bell-glass becomes nearly saturated with aqueous
vapour, and then almost entirely prevents further transpiration from the leaves.
A substance which absorbs water (e. g. sulphuric acid or calcium chloride) could be
brought under the bell glass, however, and the vapour exhaled by the plant would
then be absorbed by this substance, and the saturation with vapour of the air sur-
rounding the leaves prevented : the increase in weight of the substance named would
then indicate the quantity of water exhaled in a given time. If plants with large
leaves stand in spring-time or autumn behind a closed window, it is noticed in the
morning that at those parts of the glass near the upper surfaces of the leaves, there
is a deposit of water, which has evaporated from the leaves and been condensed on
the cold window panes.

It would be quite in vain, however, to attempt to give from researches of
the kind above described, an exact account of how much water a plant exhales in
the form of aqueous vapour, and takes up through the roots, in a day or in a
week. Transpiration depends upon the environment, quite as much as upon the
organisation of the plant; experience shows that the formation of vapour from
the leaves is increased as the warmth and dryness of the air, and especially the
intensity of the light, increase. In moist air, or when fog, dew or rain cover
the surfaces of the leaves with water, it is obvious that but little aqueous
vapour, or even none at all, can escape from these organs : the transpiration, and
accordingly the amount of the water moving in the plant, thus depend upon the
alteration of external circumstances ; and so far as experience teaches, the well-being
of the plant is not interfered with within a wide range of play of these circumstances.
However, the extremes must be avoided, since just as the long continued prevention
of transpiration is injurious to a land-plant, so a too energetic formation of vapour
in the leaves during hot sunshine may lead to the result that the roots do not at the
same time absorb as much water as is given off at the leaves; hence the latter
droop or wither — a phenomenon observed often enough during dry weather on
very hot July days, but which passes away without injury when the temperature sinks
at night, and the relative moisture of the atmosphere rises, and the transpiration
diminishes so far that the supply of water from the roots suffices to make the leaves
again turgid and fresh — i. e., to fill them with water.

Although it is not possible to give an exact idea of the quantity of water
sent upwards in a given plant during a period of vegetation, it is nevertheless of some

AMOUNT OF WATER TRANSPIRED.

229

interest to know how high the possible maximum of the water carried up by the
current may rise. That a tolerably vigorous Tobacco plant at the time of flowering,
or a Sunflower of the height of a man, or a Gourd-plant with fifteen or twenty large
leaves, takes up and transpires 800-1000 cubic centimetres of water during a warm
July day, is certainly no rarity ; and so far as may be judged from the quantities
made use of by branches placed with the cut surface in the water, it may be
believed that large fruit-trees, or Oaks, Poplars, &c., absorb, transport through the
stem, and transpire from the leaves 50-100 and more litres of water daily ^ Now,
these large masses of water are carried up, in tall trees, to a height of 50-100 metres
in a direction opposed to that of gravity ; and it is clear that in these cases the plant
performs a work, the magnitude of which is evident most simply by supposing the
same mass of water to be drawn
up to the height stated by means
of a windlass, for instance,
worked by a man. The plant
of course accomplishes this
work in quite another way, and
we will now see how it does it.
In the first place it is
important to establish in what
part, i. e. in which tissue of
the root, stem, and leaves the
water moves. The answer, as
already known for nearly 200
years, may shortly be given
thus : in the proper woody
plants (i.e. the Conifers and
Dicotyledons) the ascending
stream of water passes through
the wood. In the case of the
plants mentioned the proof
of this assertion is easily fur-
nished ; it is simply necessary
to separate and remove a ring
of cortex, by means of a double

annular cut on a stem, to completely interrupt' all the layers of tissue lying outside
the mass of wood. It is advisable to wrap tinfoil, or some other body which
prevents drying up, closely round the exposed wood. If the ascending current of

Fig. 192.— Transverse section of the wood of Rhamnus Fraiigiila. g autumn-
wood of the older, w vessels of the spring-wood of the younger, annual ring
(afier Rossinann).

' Although it is not possible to state for a particular plant any definite quantity of water which
it must transpire in order to flourish vigorously (and it is certain that every land-plant has wide
limits in this respect), it is nevertheless of value for general guidance to know approximately, on the
base of experimental determinations, how large the quantity of water transpired by a plant during its
whole vegetative period may be imder certain circumstances. According to Haberlandt (cited by
Pfeffer, ' Pflanzen-physiologic,^ B. I. p. 153) the water transpired by the Maize in the course of a
vegetative period of 173 days = 14 litres ; by the Hemp = 27 litres in 140 days; by a Sunflower =
66 litres in 140 days. According to Höhnel, a hectare of beech forest, 115 years old, evaporates
2'3 to 3'5 millions of litres of water between June i and December i.

230 LECTURE XIV.

water were conducted entirely, or even only to a small extent through the various
tissues of the cortex, the fact would soon be recognised, since the leaves of the tree
would droop, and finally dry up. This, however, does not happen : on the contrary,
they remain quite fresh, thus proving that in spite of the interruption of the cortex
exactly as much water flows to them as they require for their transpiration. The
pith in the middle of the stem does not come into consideration at all, since it is dry
or already destroyed ; and in the cases of thicker stems its mass is much too small 10
be taken into account here. But even the usually darker coloured and harder heart-
wood of the stem and older branches is not concerned with the conduction of water ;
since if a ring of younger wood or alburnum is removed at the same time with the
ring of cortex, so that only the ' heart ' remains uninjured, the withering of the crown
of leaves betrays the fact that the water-supply has ceased. It is thus, put shortly,
in the alburnum that the water of trees ascends. But even within the alburnum a
difference prevails, in so far that the dense autumnal wood of any one annual ring
is less capable of conveying water than the large-celled spring wood of the same
ring ; and it appears, indeed, that in this way the spring wood of any individual
annual ring represents an isolated conducting layer, not in immediate communication
with the homonymous layer of an older or younger annual ring \

It is demonstrated by means of the above experiments, chiefly for the
compact woody body of the Conifers and Dicotyledons, that the ascending current
of water moves in the wood only ; but no such simple proof is possible, on the other
hand, in the case of Monocotyledons and Tree ferns. These, as is known, form
no proper woody body. Lignified cells are found, it is true, in the xylem of the
individual isolated vascular bundles ; but the quantity of this wood of the vascular
bundles is so small, that it seems scarcely conceivable how the water necessary for
evaporation could be conveyed through the thin lignified strands to the huge leaf-
crown of a Date palm, for instance. In addition to this, the connection of the
vascular bundles in the palm stem does not favour the assumption that only the
xylem of the individual bundle can undertake the conveyance of water. The
vascular bundles of the stem of a palm commence below as strands of hair-
like fineness, which apply themselves where their diameter is extremely small
to the older bundles bending out into the leaves. The difficulty disappears,
however, if the sclerenchymatous, thick, lignified vascular bundle-sheaths are claimed
at the same time as the principal water-conducting organs of the Palms, Dracaenas,
and other Monocotyledons. In their anatomical and histological structure, moreover,
as well as in their lignification, these sclerenchymatous strands resemble the more
solid parts of true wood ; and in view of their considerable diameter, it seems far
more probable that the large quantity of water evaporating in the leaf crown ascends
in them. If this assumption, which I hold as more than probable, becomes estab-
lished, the sclerenchymatous vascular bundle-sheaths in the stem and leaf-stalks of
large Ferns must also be regarded in the same manner.

^ The proof that the autumnal wood of each annual ring offers great resistance to filtration, as
well as the heart-wood, and that the former has probably as little to do with the transpiration-current
as the heart-wood, is found in my treatise, ' Üöcr die Porosität des Holzes' (Arb. des bot. Inst, in
Wzbg. 1879, B. II, § 3).

ASCENT OF WATER IN MüNOCOTVLEDONS.

^31

The necessity of the assumption that in the vascular bundles of the Monoco-
tyledons it is not, or is only in a very subordinate degree, the lignified xylem itself
which conveys the ascending current of water, but that rather the lignified scle-
renchymatous sheaths play the main part in the process, is very evident on
regarding the transverse section of the vascular bundle of a graminaceous plant
(as Fig. 195). Here the area of the lignified xylem is only a fraction of that
of the sclerenchymatous sheath. Besides, the fibrous elements of the latter in
such cases agree in all essential points with the proper wood fibres of the
secondary dicotyledonous wood. In other cases, as in the scapes of species

Fig. 193.— Part of the transverse section of the stem of
a Dracdna, about 13 mm. diameter. The very slender and
thin walled proper vascular bundles are surrounded by thick,
lignified sheaths of sclerenchyma, which, according to my view,
convey the ;

Fig. 194.— Transverse section of the stem of Cyathta Im-
rayaiia (nat. size). All the black streaks (s s') indicate lignified
sclerenchyma; the grey streaks and dots vascular bundles (a).
(De Bary.)

of Allium, the hollow cylinder of sclerenchyma is practically only lignified
parenchyma; but here, as likewise in the Piperacea;, it is not clear in which
forms of tissue the current of water ascends, if not in the layers of scleren-
chyma. Even in Dicotyledons it often happens that lignified strands or layers of
sclerenchyma run through the shoot-axes, in addition to feeble vascular bundles
deficient in wood.

Hitherto all these sclerenchymatous structures have been considered only
as means for promoting elasticity and rigidity, as they and the true wood
in fact are. I conclude, on the other hand, that all layers of lignified tissue which
may be followed continuously from the root through the stem into the transpiring

232

LECTURE XIV,

organs serve as conducting organs for the transpiration-current, though they are at
the same time of use as elastic and rigid masses.

The wood and lignified sclerenchyma are certainly not the only tissues which
can conduct water within the plant ; for simple reflection shows that large fruits,
as those of the Gourd, absorb enormous masses of water into their parenchymatous
tissues, though no corresponding lignified strands pass through them from the
stem. The absorption of water in all young buds, as well as in soft non-lignified
napiform roots and tubers, shows that a movement of water is likewise possible in

Fig. igs.— Transverse section of a vascular bundle in the stem oi Zea .Ifnys. It consists of the parts
'V,£; s r, I. f p thin-walled parenchyma of the fundamental tissue Apart from the lignified cells betweer
g and^, only the dark shaded sclerenchymatous sheath which sunounds the vascular bundle constitutes a
woody layer worth mentioning and apparently suitable for the conveyance of the ascending current of water,

parenchymatous tissue, even without the presence of lignified cell walls. The dis-
tinguishing feature of the wood, however, is the great rapidity with which the
particles of water can move in it; and just this rapidity is necessary where the
transpiration -current of land-plants here considered is concerned. Hence we see that
even very highly organised plants, when they do not transpire, or do so but little,
possess no woody bundles, or only very thin ones — as, for instance, submerged and
floating phanerogamous water plants, and those root parasites which develop chiefly
under ground, as the Balanophorea;, &c. The lignified layers and strands in the

SLOWER CURRENTS OF WATER. 233

roots, Stems, and leaf-stalks of the land-plants thus serve for the rapid movements of
water, caused by transpiration ' ; this does not preclude, however, that slow move-
ments of water of the most various kind may also take place in all the other
forms of tissue of these plants. In the first place, it is quite certain that the water
in the younger absorbing roots must pass through the external parenchymatous
layers of tissue, in order to reach the axial woody bundles ; and it is easily
intelligible, again, that the thin fibro-vascular bundles of the petiole, which spread
out and branch within the lamina as the venation, transfer their water to the
parenchymatous chlorophyll-cells of the leaf, to which end, in fact, the whole
mechanism of the water currents is set in motion. In the same way the cortical
parenchyma of the shoot-axes, and even their epidermis, require certain quantities of
water in order to make good the feeble evaporation at their surfaces ; and there is
no doubt that they draw this laterally to themselves from the stream ascending in
the wood. Finally, the drooping and renewed turgescence of shoot-axes and thick
leaf-stalks abounding in parenchyma, teach us that the water ascending in the
woody bundles also passes over into the parenchymatous layers, because otherwise
the restoration of the turgescent condition would not be possible. The disdnguishing
feature about all these movements of water in the non-lignified tissues, however, is
the slowness and difficulty with which they occur ; and this is clearly indicated in
the whole organisation of the land-plants. The course of the woody bundles conducting
the water in the leaves is so regulated, and the ramifications of the venation so con-
sdtuted, that the water conveyed in the latter need only traverse an exceedingly
short distance in order to enter into all the parenchyma cells of the leaf. We see also,
at the same time, why the venation, especially of the strongly transpiring thin leaves
of Dicotyledons, divides up the entire area of the leaf into so many exceedingly small
areolae ; because by this means the paths of the water particles in the badly con-
ducting parenchyma are rendered the shortest possible. In like manner the path
taken by the water from the woody portions of the stem across into the cortex is
never more than a few millimetres long, although a rapid supply is not at all necessary
here. In the cases where large gourds, apples and pears, tuberous roots and tubers
gradually absorb large masses of water into their parenchymatous tissue, which is not
traversed by strong woody bundles, it simply depends not upon a rapid transpira-
tion-current, but, on the contrary, upon an exceedingly slow movement of water, just
sufficiendy copious to bring about growth — the increase in volume of the cells. In
contrast to this we find the woody body, or the strands of tissue equivalent to it, always
the more strongly developed the more it is a matter of conveying large quantities of
water in a short dme to widely distant and large transpiring surfaces; and, remarkably
enough, these considerations also confirm the fact that the same distribution of the
lignified layers and strands, by which the elasticity and rigidity are obtained in the

' That the wood cell-walls differ essentially and entirely from other cell-walls as regards their
conductibility for water, and the minute quantities of substances dissolved in it, and that, while
imbibing less, they hold the water in a very mobile condition, I have also made evident in the
treatise mentioned in note i, p. 230. This is opposed to the assumption previously current, and
especially contrasts with the views put forth by Nägeli and Schwendcner. In note i, p. 20S, all
that is necessary has been mentioned concerning the fact that imbibition is essentially different
from capillaiily, although the two have hitherto been always coufoumlcd.

234

LECTURE XIV.

Stem and in the venation of the leaves, serves at the same time to deliver the water
in a proper manner to the badly conducting masses of parenchyma. The lignified
supporting tissues in slender upright stems are therefore brought close to the
periphery, not only because they thus best promote elasticity, &c., but also because the
loss of water at the surface of the epidermis can thus be most easily made good.

-Lignified, water-conducting spiral cells and fibres in the venation of the leaf of Aiitkyllü
The enclosed interspaces or areolae are filled up with transpiratory assimilating tissue

The same consideration shows that the venation of the leaves serves at the same
time their elasticity and rigidity, and the appropriate supply of water.

The wood is thus endowed with the specific property of allowing rapid movements
of water to take place in it : it is therefore of interest to know how rapidly a particle
of water moves in the wood of the stem under particularly favourable circumstances
— that is, during very active transpiradon from the leaves. Numerous researches
which I have made in this direction with various kinds of plants, show that

RAPIDITF OF TRANSPIRATION CURRENTS.

235

a distance of 50 to 80 or 100, occasionally even of 200 centimetres, is frequently
traversed within an hour by a particle of water in the wood. It is obvious
that when the transpiration at the leaves is feeble, and the quantity of water
consumed small, the rapidity of the flow in the w^ood also diminishes; and that
with the complete stand-still of transpiration, the movement in the wood likewise
ceases, or at any rate sinks to a minimum in the event of a small supply of water
being necessary for the purposes of growth of parts situated higher. Formerly it was
sought in various ways to obtain an idea of the velocity of the ascending current of
water, but all the different methods employed for this purpose have been shown to be
faulty in the highest degree. I have shown that researches with cut-off branches are
for our purpose to be rejected once for all; and that likewise the experiments
made for centuries, in which
coloured solutions were allowed
to ascend in the wood, must ne-
cessarily give velocities which are
too small. This is easily seen on
reflecting that the colouring of the
wood cell-walls simply consists in
their seizing upon and holding fast
the colouring matter, while the
water of solution thus freed from
the colouring matter speeds on
in advance. This may be very
clearly illustrated by means of the
apparatus here represented. A
strip of filter paper hangs with its
lower end in a solution of colour-
ing matter {a), e.g. of aniline violet
or indigo. Even a few minutes
after the immersion a disassocia-
tion of the fluid is noticed : the
upper limit of the coloured part
{(/) only ascends slowly in the pk; j^^,

paper strip, while the medium of

solution {Ö c), freed from the colouring matter, ascends much more rapidly. The
like must also happen when a cut-off branch is placed with the cut surface in a solu-
tion of colouring matter; only in this case it is not observed how quickly the water
speeds on in advance of the colouring matter. Moreover in this mode of experimenting
still further errors are introduced through the rarefaction of the air in the wood,
to which I shall refer later. Really useful observations on the velocity of the ascend-
ing water current are obtained, however, when weak solutions of lithium nitrate are
allowed to be absorbed by the uninjured roots of a transpiring plant. For this purpose
the plants must have been previously grown for some time in earth in a flower-pot, or
have developed all their roots in a nutritive solution, according to the method to be
described later ; since it is impossible to obtain uninjured roots by digging them up
and washing them. The lithium solution possesses, as I convinced myself with the

236

LECTURE XIV.

aid of the paper strips previously mentioned, the advantageous property of ascending
■without becoming decomposed ; and the lithium is taken up by the cell walls not
more energetically than the water. It thus becomes a matter of ascertaining how high

the lithium has ascended in one hour in
the stem, and thence into the leaves, when
a weak lithium solution of say 1-20/0 has
previously been offered to the roots for
absorption. The lithium has, moreover,
the advantageous property of being re-
cognisable in the interior of the plant,
even in the minutest quantities, in an
exceedingly simple manner, by means
of the well-known intense red line per-
ceived in the spectrum of incandescent
lithium vapour by means of a spec-
troscope. It thus suffices to burn in
a Bunsen's flame small chips of wood
from the stem of the plant investigated,
after this has been cut into small por-
tions, to recognise by means of the
spectroscope the presence of lithium ;
or the same observation may be made
with portions of the leaves of the plant
investigated. We owe the excellent
idea of allowing lithium salts, on ac-
count of their easy detection, to be
absorbed by plants, to Professor Mac-
Nab ; his researches however were so
uncritically conducted that it required
a protracted investigation on my part
to put to actual use for our pur-
pose the excellent properties of this
substance \

With all that has been said hitherto,
however, we have still obtained no idea
as to the form in which, and as to the forces by means of which, the water ascends in

Fig. igS.—Piiius sylvestris: radial longitudinal section through
the wood of a vigorously growing branch, c* cambium wood-cells ;
a—e older wood-cells ; /, t', l" bordered pits of wood-cells, in order
of age ; st large pits where the cells of the medullary rays lie in
contact with the wood-cells (X 55°)-

1 In my treatise, ' Ei7i Beitrag zur Kenntniss des aufsteigenden Wasserstromes in transpiriren-
den Pflanzen'' (Arb.des bot. Inst, in Wzbg., 1878, II. p. 148), I subjected to criticism the investigations
made by MacNab and Pfitzer with solutions of lithium, and showed that only plants with uninjured
roots can give satisfactory results. To quote some numbers here, for example ; in a branch of Salix
fragilis, with abundance of roots which absorbed the lithium solution direct, and not from the earth,
the lithium ascended 85 cm. high in i hour: in two plants of Zca Mays under similar circumstances
it ascended 30 and 42 cm. respectively. With plants rooted in the soil and watered with the
lithium solution, and placed favourably as regards transpiration, the lithium ascended, in i hour,
118 cm. in Nicotiana tabacum, 206 cm in Albizzia lophantha, 107 cm. in Musa sapientuni. 70 cm. in
Helianthus annuus, and 98 cm. in Vitis vinifera. That experiments with coloured solutions give
no certain results, I have insisted upon in my ' Experimental-physiologie,'' 1865, p. 217.

STRUCTURE OF WOOD.

237

meantime to the wood of Conifers,
these woods possess no vessels, but

the wood. To tliis end we must enter somewhat more in detail into the structure
and the physical properties of wood. In order to avoid unnecessary complications
we will confine our consideration in the
such as the Pine, Yew, and Fir, because
consist only of tracheides, and are there-
fore relatively simple and homogeneous
in structure. We must state, in the first
place, the very important fact (shown by
Theodore Hartig 20-30 years ago, but
denied later by all phytotomists) that
the histological elements of the wood are
not in open communication one with
another, and that the bordered pits of the
wood-cells (and vessels of foliage-trees)
are not actually perforated, but closed by
fine membranes. Thence follows that the
woody body by no means represents a sys-
tem of continuous capillary tubes, but is
formed of chambers distinctly shut off on all
sides from one another. The purely micro-
scopical and anatomical confirmation of
this fact would, for our purpose, sdll leave
room for much doubt, if the closure of
the bordered pits in the wood were not
established with absolute certainty in
other ways. This is done most simply
by means of the apparatus here figured.
A piece of the stem of a Fir, or other
Conifer, several centimetres long, and some
2-3 centimetres thick, and still provided
with the cortex, is cut smooth at both sec-
tions, and the one end fastened by means
of caoutchouc to a long glass tube which
is connected above with a wide vessel : the
tube and vessel are now filled with an
emulsion of cinnabar which has previously
passed through several layers of filter-
paper and therefore contains only cinnabar
particles of the most extreme minuteness,
which even with the strongest magnifica-
tion appear merely as dots, and exhibit the
so-called Brownian motion. By means of

the hydrostatic pressure in the apparatus figured the fluid now filters through the
wood : the water running off below appears quite clear and free from cinnabar, for
the latter remains in the uppermost pordon of the wood, not penetrating deeper
than some 1-5 to 2 millimetres. The microscopic examination of this piece after

/

FIG. 199.— Apparatus for the filtration of water or emulsion
of zinnabar through pine wood. The fresh piece of wood /i is
fastened by means of india-rubber tubing d to the wide glass
tube c: c is connected with the narrow glass tube t> (about
60 cm. long) by means of a cork. This tube is fastened above
to the vessel a, which contains w,ater or emulsion of cinnabar ;i.
The water filtered through the wood passes through the
funnel e, and is collected in the graduated vessel/.

23«S LECTURE XIV.

several days' filtration now shows that the long wood-cells, which were opened by the
section, are completely filled with the fine cinnabar particles ; and that these have
penetrated even into the cavities of the bordered pits and completely filled them.
It may here be definitely observed that the very minute cinnabar particles have pene-
trated even through the narrow pore of the pit into its cavity, but are there held fast,
since a thin membrane prevents their passage over into the neighbouring cells of the
wood. Only the cut cells of the upper end of the piece of wood have taken up
particles of cinnabar, not the uninjured ones. A similar result is obtained if, instead
of cinnabar emulsion, mercury is poured into the apparatus described, and left in it
for some time. The mercury does not pass through the wood of Conifers ; but it
fills the cells opened at the upper transverse section and the cavities of their bor-
dered pits, without passing over into the neighbouring cells.

After thus establishing that the wood-cells of the Coniferse are not in open
comrhunication (which is likewise true of the wood- cells and vessels of ordinary
foliage trees), the theory which has existed for two hundred years, according to which
the water ascending in the wood is considered to move as in capillary tubes, falls to
the ground of its own accord. The hypothesis established by Quincke^, that the
ascending water may be drawn up as an extremely thin molecular layer on the
surfaces of the walls of these capillaries, is likewise definitely refuted by means
of our discovery. Both theories, however, would be wrecked without that, on
the fact that even if continuous capillaries were present in the wood, they are
closed in the roots below and in the leaves above ; and that, further, the transverse
section of these capillaries — i. e. the wood-cells and vessels respectively — is far too
large to explain, according to the known laws of capillary tubes, an ascent of the
water in the wood to a height of more than a few metres.

Now, however, there comes in another quite decisive fact, by which the capillary
theory of the ascent of the sap is likewise put aside : the fact, namely, that at the
time when transpiration is going on in the leaves — that is, at the time when a rapid
current of water is ascending in the wood — the cavities of the w^ood-cells and vessels
of foliage-trees are not filled with water, as should be the case according to the
capillary theory ; rather, the wood-cells and vessels are only to a small extent filled
with water, the vessels, however, being quite empty. This highly important fact
is already to be concluded from the simple observation that fresh wood, cut out
of the stem or branch of a transpiring tree in summer, and then thrown into
water, floats ; and since the cell-walls of the wood are without doubt specifically
heavier than water, if their cavities v/ere at the same time filled with water,
such a piece of wood must sink like a stone. Instead of this, however, it
floats, i. e. it is lighter than water ; and this fact is not otherwise explicable than
by the assumption that cavities exist in the wood which are not filled with water.

' The possibility that the ascending water may pass up to the leaves as an extremely thin layer
on the inner side of the wood cell-walls, was not suggested by me, but by the physicist Quincke ;
though it is true it was made public by me in my text-book. This view was of course only tenable
so long as the bordered pits of the wood were supposed to be open, according to the older statements
of Schacht. Since the proof that the pits of the wood are closed was afforded, however, by Sanio's
and De Bary's anatomical investigations, and by my repetition of the elder Hartig's filtration experi-
ments, Quincke's theory can, of course, no longer be spoken of.

WATER TN THE CELL-CAVITIES.

239

This supposition, moreover, is proved to be correct by two further facts. If a piece
of fresh wood is placed in the fire, bubbles of gas are seen to be emitted- with
violence, evidently because the air contained in the wood is expanded by the
heat and forced out at the transverse section, at the same time driving out
with it a portion of the water contained in the wood. Again, we find that a
piece of wood cut from a living tree, when laid upon water, gradually becomes
heavier and sinks deeper and deeper, because it absorbs water: this would be

-Tang-cntial long

St medullary rays

of Aümithits i^iandiilosa. £ ^ vesr.els ;
tracheVdes; ^ libriform cells.

quite impossible if the cavities of the wood were actually already completely filled
with water.

These observations prove then most decisively that just at the time of the most
rapid ascent of water in the wood, the cavities in the wood-cells cannot be filled with
water ; that, rather, cavities containing air and vapour must exist in them. By means
of more exact considerations and experimental researches, I have succeeded in finding
a method by which we are able to determine in any given piece of wood, by a
simple calculation, how large the empty space in the wood-cells is. For this purpose
the specific gravit)- of the wood cell-walls must first bo exactly determined. Among

240 LECTURE XIV.

Other methods, I succeeded in doing this in the following manner: — Since it is
impossible to fill all the cavities in a large piece of wood with water, cross slices
0-I-0-2 millimetre thick were taken of the wood of Firs and ordinary foliage trees :
since now the wood cells are several millimetres long, all the elements in these cross
slices must be opened and cut through transversely. They were then boiled for a long
time in salt solutions, to remove any bubbles of air. These solutions were of calcic
nitrate and zinc nitrate. The result was that the cross slices mentioned sank in such
solutions, though extremely slowly, when the areometer indicated a specific gravity of
1-56. This number thus expresses almost exactly the specific gravity of the wood cell-
walls ; or, in other words, if one had a cubic centimetre consisting entirely of the substance
of the wood cell-walls, it would weigh 1-56 gramme, whence results immediately that
one gramme of wood cell-wall occupies a space of o'64i cubic centimetre. Furnished
with this number, it is easy now to calculate in any piece of wood cut from a living
tree how large the spaces occupied by the lignified walls, by the water, and by the
cavities filled with air must be. First is determined the volume and the weight of
the fresh wood; it is then dried at 100°, and the weight of the dry wood determined.
The difference in weight obviously gives the weight of the evaporated water, from
which its volume follows immediately, since one cubic centimetre of water is exactly
one gramme. The specific gravity of the wood found above now allows us to
calculate from its absolute dry weight the volume of the cell-walls, since we divide
that weight by the specific gravity, and all else follows. To illustrate this by an
example — a cylindrical piece of wood, consisting of five annual rings, was taken from
the stem of a living Fir on the 2nd of January: the piece was 105 millimetres long
and 33 millimetres thick. From these dimensions the calculated volume is 89*8
cubic centimetres, and it was found, by immersion in mercury, to be 90 cubic centi-
metres. That the wood, though containing much water, still contained air, was clear
at once from the fact that it floated in water.

Weight of the fresh wood 87-60 grammes.

Weight of the dry wood 34'83 grammes.

Water in the fresh wood 52"77 grammes.

From the dry weight of the wood we get ~ = 2 2'33 cubic centimetres as

the cubic contents of the cell-walls. ^

From these data it is calculated that 100 cubic centimetres of fresh wood
consist of —

24-81 cubic centimetres = mass of wall (calculated dry).
58-63 ,, = water (in the cavities, and imbibed).

16-56 ,, = air cavities.

Since intercellular spaces and vessels do not exist in the wood of the Fir, the
16*560/0 air cavities were thus contained in the wood-cells themselves ; and since the
wood-walls, as we shall see, absorb by imbibition only about half their volume of water,
they thus contained only 12*4 cubic centimetres of water, the remaining water (viz.
46-23 cubic centimetres) must have been contained in the cell-cavities.

DISTRIBUTION OF WATER IN THE WOOD. 24I

Consequently the volume of the cell-cavities is calculated as —
i6'56 cubic centimetres containing air.
+ 46*23 ,, containing water.

= 6279 cubic centimetres of cavity altogether.
The volume of the saturated cell-walls as —

24-81 cubic centimetres dry mass of wall.
+ 12-4 „ water of imbibition.

= 37 "2 1 cubic centimetres saturated wood walls.

The space occupied by the saturated walls, therefore, in this case stands in the
proportion to the space occupied by the water and air, as i : i-68; or, the space oc-
cupied by the saturated walls is greater than a third of the total volume of the wood.

It is to be noticed that in this pardcular case the piece of wood examined
contained much water. Had less water been present the calculated space containing
air would have turned out to be greater, since we may assume that so long as water
is still present in the cavity of the cells, the cell-walls themselves are completely
saturated with it ^

The calculation given depends in part on the assumption that cell-walls, taken
as dry, only imbibe water to the extent of the half of their volume. I was led to this
assumption by the following considerations and results : if a thin cross slice of fresh
wood is suspended in dry air, there is generally formed during the drying of the wood
a radial fissure, which forces its way from the exterior to the centre. Hereupon the
cross slice is dried at 100° C, and the weight of the wood cell-walls determined; from
which its volume is reckoned by means of the specific gravity. The dry slice of wood
is now again suspended in a space saturated with aqueous vapour, where it gradually
condenses so much water, and at the same time becomes so distended by swelling,
that the fissure produced during the drying again closes up, and this so completely that
it is finally no longer to be recognised at all. In this condition the cell-walls must
necessarily be completely saturated with water ; and there is no fear of water beino- con-
tained in the cell-cavities also. Weighing once more, gives the result that the water
thus absorbed until the cell-walls are saturated constitutes about half their volume.

This last discovered fact is now, however, of particular interest, since it shows
that the wood cell-walls have a strikingly small power of swelling, compared with
other cell membranes which are capable of swelling, and especially with those which
become mucilaginous in water, and take up enormous quantities of that liquid.

Here, then, we are face to face with the proper and specific physiological signi-
ficance of the wood cell-walls. This consists in that they absorb relatively but little
water, but that this small quantity of water of imbibition is strikingly mobile in them.

According to all that has been said hitherto, the ascending current of water in
transpiration (in the wood generally) thus moves in the substance of the ccll-tvaUs

* The knowledge of the specific gravity of the wood cell-walls gives us also the power of
calculating the extent of surface of all the cell-walls in a given piece of wood. For a piece of fresh
Fir-wood in winter I found, for example, that 100 c.cm. contained 25 c.cm. of solid wall. Since now
the thickness of a saturated wall may be assumed to average about 0.0025 mm., by dividing the
volume of wall named by this thickness it results that the superficial extent of the walls in a
piece of Fir wood i m. long and i sq. cm. in diameter = 10 sq. m.

[3]

242 LECTURE XIV.

lhe?)iselres. We have thus to imagine the water of imbibition of the latter as being in
motion; and this is the main result of our considerations hitherto ^ Any one may
easily supply a striking proof of this, moreover. If the delicate stem of the Hop,
Flax, etc. be sharply bent beneath a few transpiring leaves, the cells at the sharply
bent place must all be so compressed together that cavities containing water no
longer exist. Thence follows, however, that the water ascending into the transpiring
leaves can only go through the walls of the wood-cells ; and that this actually
occurs, follows from the fact that the shoots remain fresh above the sharply bent
place — they remain thus fresh for weeks and months, even in the sunshine in
the open.

The great mobility of the imbibed water of the wood cell-walls already estab-
lished above, and on which the whole phenomenon of the water-flow thus depends, is
a specific property of the lignified cell-walls ; a fact which appears the more striking
when it is known that other cell-walls capable of swelling to a great extent, though
it is true they absorb water in large quantities, hold it immovably fast. This is very
conspicuous, for example, in the case of the stalks of Laminaria, the main mass of which
consist of cell-walls capable of swelHng; if a fresh stalk of this Alga is so placed in water
that some centimetres project above, the immersed part remains swollen, but the part
projecting above dries and shrivels up — a proof that the water ascends in it not at all
or only exceedingly slowly. Ordinary parenchyma cells appear to behave similarly.

As further characteristic of the wood cell-walls, in so far as they tire the con-
ducting organs for the ascending water, it is a remarkable but easily confirmed fact
that they lose for ever their main property of conveying water when they have once
become air dry. It would be utterly in vain to look for an ascent of water at all
rapid in an old dried up rod of wood placed in water. The wood once dried has,
it is true, the capacity of still becoming saturated with water of imbibition ; but the
mobility of the latter no longer exists.

It is evident that by the drying of the wood cell-walls, an essential alteration of
their molecular structure has taken place, which, however, is not to be detected
with the microscope. Nevertheless this fact is established, and it is of the greatest
importance for the life of plants. The so-called freezing of woody plants in the
long- continued cold of winter may serve as proof of this; for this phenomenon is
something quite other than the freezing of succulent shoots and living leaves in the
late autumn or in the spring time. The dying of trees in long-continued winter cold
depends to a great extent on the drying up of their wood, as we may convince

* For the guidance of readers not sufficiently acquainted with phytotomy, it may not be super-
fluous to quote the following from my treatise, ' Über die Porosität des Holzes^ p. 292. ' The wood
consists of a framework of lignified lamella;, which enclose cavities (cell-cavities). According to
circumstances the cavities may contain water, or air (with aqueous vapour), or both. The walls
themselves may be dry, or contain water (imbibed) ; and their volume or condition of swelling alters
with the amount of water contained. The cell cavities of the wood are capillary spaces ; the cell-walls,
on the contrary, contain no capillaries into which liquid or air could directly penetrate. In order to be
able to judge of the movement of water in the wood produced by transpiration and other causes, it is
necessary to sharply distinguish between the capillarity of the cavities and the imbibition of the cell-
walls.' That the ascending transpiration-current moves in the substance of the wood cell-walls
I assumed (though with a certain reserve) in my ' Pßanzen-pkysiologie' p. 216: it was first expressed
definitely, however, in my 'Text-book.'

CONDUCTiniLITF OF WOOD FOR WATER

243

ourselves directly by cutting off single pieces. This wood, when once dried up, is
no longer in a condition to convey water from the roots to the buds ; and so the
plants perish.

We thus see that the wood owes its significance as the organ for conducting
water to a series of most remarkable properties, which are found in no other natural
body ; and after what has been said, it would be simply childish still to attempt, as
has been done previously, to derive the properties of the wood and the mechanics
of the ascending water current from observations on capillary and porous bodies,
such as gypsum, or from the endosmotic processes in an apparatus made with
animal bladder, or even from the properties of parenchymatous tissues. The wood
is a body sui generis, and especially adapted by nature for the purpose of conveying
water (and even water laden with nutritive substances) from the roots up into the
transpiring organs of assimilating plants. The utterly incorrect views of the
mechanics of the ascending water-current which were held previously to my re-
searches 'On the porosity of wood,' 1879, were especially characterised by the
difficulties arising from the fact that water ascends in the wood up to heights of more
than TOO m. into the leaves of tall trees, because it was always assumed here
that it was a matter of capillary tubes. It is true water can also ascend in capillary
tubes, but to a considerable height only when they are extremely narrow.

Here, however, appears the difficulty perceived, but not overcome, by Nageli
and Schwendener, that the capillary movement in very narrow cavities is so
exceedingly slow that it no longer suffices for the requirements of transpiration.
It was simply an entirely false principle from which observers formerly proceeded ;
since it depends not upon a phenomenon of capillarity, but upon imbibition and
swelling, where, as I have shown in the preceding lecture, quite other molecular
relations and forces are put in requisition. The force wiih which water is held
fast in bodies which imbibe it is so enormously great, that it is quite immaterial
whether the mass of wood saturated with water extends 10 m. or 100 m. into the
air above the absorbing roots ; just as in the case of the salt solution of sea-water it
is immaterial whether the dissolved salt molecules are suspended 100 m. or 1000 m.
above the sea-bottom. The one point of special importance to be considered here
is the facile mobility of the water thus held fast by the cell-walls. This, however,
is to be understood on observing that the wood cell-walls are saturated with
water from their origin, and that every displacement of a molecule of water in
them causes a movement of other molecules of water ; whence it will be immaterial
to the forces of imbibition whether the water molecule A or B enters into the
sphere of attraction of a given molecule of wood.

The main result of all these considerations is this, that the ascending cur-
rent of water depends upon the motion of the relatively small number of water
molecules which are contained between the molecules, or ' micellae,' of the wood cell-
walls. This much is established, that this movement can only occur when the
wood cell-walls at the upper end of this system lose a portion of their water molecules.
By this loss its state of saturation with water becomes disturbed, and the equilibrium
altered : the parts of the wood cell-walls which have become poorer in water will
tend to restore the equilibrium by attracting water from the nearest wood-cells,
which, in their turn and for the same reason, take it up again from parts of the

R 2

244

LECTURE XIV.

wood situated lower, until finally this movement, extending backwards, proceeds
from the foliage of a land-plant down through the stem, into the young roots which
absorb the water out of the earth.

That this motion of the water molecules depends upon the activity of their
evaporation from the leaves is obvious at once, and has already been stated as
a fact. On the other hand, however, it is also evident that the rapidity of the
flow of the water must depend upon certain properties of the wood walls,
which we designate by the word conductivity; if this becomes lessened by any
means whatever, the evaporation at the leaves need not on that account be
diminished to a like extent. If the latter now continue to transpire vigorously,
while the supply is smaller than the loss of water, a deficiency of water must finally
result in the leaves, and then even in the younger shoot-axes, and they will

droop ; and since further arrangements (to
be considered later) are met with, by
means of which, with a want of water in
the organs of transpiration, the transpira-
tion itself becomes lessened, it follows that
under certain circumstances a diminished
conductivity can be inferred from di-
minished transpiration. The behaviour of
cut-off shoots placed with the lower cut
surface in water, gives occasion for these
reflections. It might evidently be sup-
posed that the cut surface of such a
shoot w^ould absorb the water in this
form, more easily than when the cut surface
is still in connection with the wood of
the lower part containing water. This is
not the case, however, since various experi-
ments show that cut-ofT shoots placed in
water gradually transpire less and less,
and droop, which is evidently caused
only by the imperfect conduction of water.
If the shoot has been cut through far
from the apex, at a part already strongly lignified, or if the lignification reaches
right up into the apex when the winter buds are already formed and all the
upper leaves are completely developed, then the phenomenon mentioned appears
in a small degree only, and the drooping takes place only after some days ; it
occurs after a few hours, however, when a young shoot-axis is cut through, which
is not yet, or is but little lignified, and in which the lignification does not yet
extend into the apical part— e. g. in the case of the apex of a shoot of the Sun-flower
20-30 centimetres long, or one of Arislolochia si'pho 40-50 centimetres long. On
fixing such drooping shoots into 'a U-tube, as in the accompanying figure, and
pressing the water (w) by means of a column of mercury {q) into the cut end,
I found that the shoot after some time again became turgescent and tense : the
diminished absorption of water was again increased by the pressure, and the power

FIG. 201— The U-tube is first filled with water ; thei
bored india-rubber stopper i, in which the stem of the pi.
fitted, is inserted. The shoot droops, as in a. On po
mercury into the other limb, so that g' stands some 8-i
above q, the shoot becomes turgid, as in * ; and remair
even when the level g subsequently stands higher than g'.

EXPERIMENTS WITH CUT-OFF SHOOTS. 245

of conduction, even when transpiration was energetic, restored to such an extent
that after the pressure of the column of mercury {q q') had become cquaHsed,
the shoot nevertheless continued to absorb water, and indeed with such force
that the level q of the mercury was raised many centimetres high above q. De
Vries, who investigated this phenomenon further, found now that the withering
of cut-off shoots only occurs very late, or does not occur at all, if the shoot-axis,
before being cut, is bent down into a bowl of water, and the portion under water
then cut through. This proves that the contact of the cut surface with air is one
of the essential causes of the diminished conductivity at the transverse section ; and
that the alteration concerns not exclusively the section, but also parts of the con-
ducting tissue higher up, may be concluded from the fact that the conductivity is again
increased when a shoot, which has been cut off in the air and then placed in water,
is cut through under the level of the water, but several centimetres above the first
section. It is still questionable on what this injurious influence of the air, even
on only momentary contact with the section, depends, and perhaps the sudden
entrance of the air into the cavities of the wood filled with very rarefied gases plays
a part in the matter^. That the absorption of water at a cut surface of wood
gradually diminishes, even apart from these circumstances, is shown by the fact that,
by cutting off daily a small piece from the lower end, shoots standing in water may
be maintained fresh for a much longer time. Evidently the absorbing section
becomes altered and diseased by slimy substances and colonisations of Bacteria ;
and even when this is not the case, since the cut surfaces of the wood cell-walls
gradually absorb large masses of water, the fine dust particles always contained in
the latter must collect and cover the section with an impenetrable layer. This
circumstance also brings it about that, when apparently pure water is filtered through
fresh wood, it goes through at first with great ease, after which the filtration becomes
slower and slower; if an extremely thin cross lamella is then cut off from the wood,
the filtration again proceeds more rapidly for a short time.

* With respect to the drooping of the apices of cut-off shoots, cp. Hugo de Vries (Arb. des bot.
Inst, in Wzbg. I, p. 287).

LECTURE XV.

CONDITIONS OF TRANSPIRATION— ABSORPTION OF WATER AND
NUTRITIVE MATTERS BY THE ROOTS OF LAND-PLANTS.

The main purpose attained by means of transpiration is, as already men-
tioned, that large masses of water, containing very small quantities of dissolved
nutritive substances, are gradually carried to the organs of assimilation ; these
nutritive substances in their turn take part in the process of assimilation, while the
greater portion of the water in which they were dissolved evaporates. The flow of
water described in the preceding lecture thus subserves nutrition in this sense. It
now concerns us to become acquainted more "in detail with the conditions under
which transpiration itself takes place, and which accelerate or diminish the rapidity
of the flow of water, and therefore that of the supply of nutritive matters also.
Upon this depends, again, the rapidity with which the water enters into the ab-
sorbing roots ; since in a land-plant growing under normal conditions, the quantity
of water exhaled as vapour at the leaves is always nearly equal to that taken up by
the roots, except in so far as possible additional circumstances, which I shall treat
of in the following lecture, cause diff"erences in this relation.

It is very easy to establish the fact that leaves or succulent shoot-axes, from w hich
the epidermis has been taken off", as well as tubers and turnips after the peeling off
of their periderm, dry up very quickly. That this happens much more slowly in the
natural condition, is evidently due to the epidermis or cork-tissue of the periderm
respectively. These are permeable with difficulty, not only by water itself, but also
by aqueous vapour ; and this is certainly true in a still higher degree of the thick
bark of old tree stems. Plants produce well-developed periderm and bark, moreover,
only at those parts where transpiration is not to be permitted to take place.

We are thus concerned properly with the epidermis only; this, on the one
hand, affords protection against the excessive evaporation of the water from the leaves
and young shoot-axes, and, on the other hand, is specially organised for the purpose
of rendering transpiration possible, and yet more, of limiting or accelerating it
according to circumstances. The epidermis accomplishes the first object by means
of the cuticle, and the waxy coatings, which, it is true, do not absolutely prevent the
evaporation of water from the epidermis cells, but still render it exceedingly slow.
The second object — the regulation of the evaporation of water — is accomplished by

REGULATION OF TRANSPIRATION,

247

means of the stomata, in so for that they can be opened and closed. The stomata
are, as was mentioned before, the external openings of the intercellular spaces of the
parenchyma ; and, according as these openings are closed or open, the exit of the
aqueous vapour exhaled from the surfaces of the moist cell-walls in the intercellular
spaces is permitted to a greater or less extent. The stomata and their movements

which depend on irritability, have, as we see, a meaning only in so far as the epidermis
lying between them, and provided with cuticle and wax, prevents transpiration
in the narrower sense. The aqueous vapour is evidently to be given off not at any
haphazard spot of the epidermis, but by the walls of the inlercellular spaces;
and these are jjarticularly large and numerous in the assimilating parenchyma.

24^ LECTURE XV.

As the moist walls of the assimilating cells transpire their water into these
spaces, they are enabled to attract a fresh supply of water from the wood
bundles, the nutritive materials of which are absorbed in the interior of the as-
similating cells. It is easy to see that in this way — that is, by the formation of
vapour in the intercellular spaces — a more uniform supply of food to each cell is
rendered possible, than if the water evaporated at the surfaces of the leaves them-
selves. Since, now, this aqueous vapour, arising at the proper places, can only escape
through the stomata, the possibility is afforded of concentrating the supply of
nutritive matters brought by means of the transpiration at those times when, under the
influence of intense light, assimilation is actually taking place in the cells containing
chlorophyll : for at this time the stomata are open, while they become narrowed
in the shade, and closed in darkness. The mechanism of the opening and closing
of the stomata is thus adapted to ensure a flow of nutritive materials from the
soil to the assimilating cells, at those times when, through the open stomata, the
entrance and exit respectively of the carbon dioxide of the atmosphere and the
oxygen set free by its decomposition are much facilitated.

It would be extremely rash to suppose that, because submerged water-plants
possess no stomata and do not transpire, the considerations here put forward are
unimportant. The mode of life and organisation of water-plants are both essentially
different from those of the land-plants. The transpiration and corresponding flow
of material which they lack, are simply replaced by their being able, by means
of their feebly cuticularised epidermis, to take up water and substances dissolved in
it from outside at all parts; accordingly all those arrangements are absent which
we are here studying simply as conditions of the life of land-plants. The im-
portance of our considerations is not altered by the fact that small quantides of
aqueous vapour can escape from the epidermis even without the aid of stomata.
The question as to what extent this happens and whether it is of any use at all for
the plant may, in fact, be altogether neglected. It may indeed be said that in no
region of vegetable physiology has the concern for insignificant minutiae led people
so far astray from the insight into the great significance of the really important
matters of organisation, as in connection with this matter of transpiration and all
the concomitant phenomena. All possible minutiae have been studied ; but the
principal fact, that it is a matter of the supply of nutritive matters to the organs of
assimilation has been scarcely noticed. Since, however, as is clear from the pre-
ceding considerations, it is just the opening and closing of the stomata which
regulate the transpiration, and with it the ascending current of water, and finally
the absorption of nutritive matters from the soil, we will briefly enter a little more
in detail upon the mechanics of these movements.

The opening and closing of the stomata are brought about by alterations in the
form of the guard-cells, the nature of which is to be perceived most clearly from the
figure here given ^ (Fig. 203). The thick lines mark the contours of the guard-cells in
transverse section, at right angles to the surface of the leaf when the aperture is open.

* The statements in onr text on the structuie of the stomata, so far as the mechanics are con-
cerned, are taken freely from Schwendener's treatise, ' Ueber Ban und Mcclianik der Spaltliff-
>!i(figen,' Monatsbericht d, Kgl. Akad. der Wiss. zu Berlin, July, 18S1.

MECHANICS OF THE STOMA TA.

249

The thin lines give the contours of the same cells when the aperture is closed.
The figure shows at once that it is not simply a matter of the projection of the inner
thin lamella, indicated by d; but that the whole guard-cell changes its form in
the process of opening and closing. On account of their firm connection with the
neighbouring epidermis cells, this has for its consequence at the same time a dis-
placement of both the guard-cells in space also. The other figure (203 d) shows the
form of the two guard-cells as seen from the surface ; the guard-cell A is drawn in
the condition in which it is when the aperture is closed, while the cell £ repre-
sents the form when the aperture is open. Here, however, it is to be noted that the
dark part a a, by no means corresponds to the parts marked d in the above figure,
but to the ledge there marked a. It is to be noticed in this figure that with the
opening of the aperture, the curvature of the outline of the guard-cell B increases ;
and that this behaves like a bent tube, which on swelling presses with its two ends
against the corresponding parts of the other guard-cell, while the back portion
recedes, and the aperture opens. Stomata are found in various plants, it is
true, in which the form and position of the guard-cells deviate from those here

Fig. 203.— Transverse section of a stoma at right angles to the surface of the leaf The thick i
of the guard-cells in the open condition, the thin ones the form when close

show the form

described : in principle, however, the mechanical relations to be considered are
always the same. In the first place, the guard-cells are always thick and
strongly cuticularised at their outer and inner walls (Fig. 203 a and d), and the cuticle
generally forms more or less strongly projecting cushions or ridges at the places
indicated, which surround the outer and inner entrances to the aperture. On the
other hand, two thin walls exist in every guard-cell : one, usually the larger, where
the guard-cell joins the next neighbouring epidermis cell (^ in Fig. 203); while a thin
lamella, generally lower down, bounds the proper aperture of the stoma at d. In
the relaxed condition, with slight turgescence, the thick places a and 3 tend to
become straight, and parallel with the slit-like aperture; hence they compress
the relaxed cell in such a manner that the thin part at d becomes pushed
outwards. If, however, the guard-cell absorbs more water, which is facilitated by
the thin wall at e, the thin portion d tends to expand in the vertical direction:
now, however, it necessarily becomes itself drawn back, since it assumes a more
vertical position, as shown by the thick contours in Fig. 203. It is easily seen that

250

LECTURE XV.

the thick ridges a and b, which are fastened at the two ends of the guard-cells (cf.
Fig. 203, b), must become curved by this ; and by means of their elasticity they bring
it about, that, as soon as the turgescence is diminished, the guard-cell becomes com-
pressed and straight, and that the walls are driven outwards as in Fig. 203. These
are only the most important and obvious points in the mechanics of the opening and
closing of the stomata. So far as concerns the transverse section of a guard-cell, the
matter may be briefly put thus : the guard-cell, as turgescence increases, tends to
assume a more symmetrical form, and, as the turgescence diminishes and the
aperture closes, a less symmetrical form, as is shown at once by" Fig. 203 \

Although the desired certainty does not yet exist with respect to the external
conditions under which the stomata open and close, so much is at any rate certain,
that the apertures open in sunshine, and under strong illumination generally, when the
guard-cells increase in turgescence; and that they close in shade and darkness, when the
turgescence of the guard-cells diminishes. In the first place, it is clear that the altera-
tions in turgescence must result from the entrance and exit of water ; and that in this
process it is the epidermal cells bordering upon the guard-cells which give up or again

absorb the water. In this connection the thin
lamella e, e (Fig. 203) is important, in so far
that it facilitates the passage of the water.
The question is now, how does the light bring
about an increase of the turgescence of the
guard-cells ; and why does this diminish with
declining illumination ? With regard to these
points, Schwendener, from whom the preced-
ing description is in the main borrowed, has
expressed no opinion. It appears to me,
however, that the fact which I have repeatedly
brought forward during the last twenty years
— that the guard-cells of all foliage leaves
contain chlorophyll-grains and the products
of their assimilation, which are usually wanting in the surrounding epidermal cells —
can now be explained. The chlorophyll-grains in the guard-cells will, as the
illumination increases, produce carbo-hydrates by assimilation ; these are at least
partly soluble, and act endosmotically, and thus bring about an inflow of water
from the neighbouring epidermal cells through the thin boundary wall. Or at
least so much may be assumed with certainty, that by the presence of the chloro-
phyll in the guard-cells generally, a supply of organic substance is ensured in
them, which according to circumstances may cause a diffusion current. It will
require further researches to decide whether this is really sufficient. Besides this,
however, a direct influence of light upon the molecular condition of the protoplasm
may come into consideration ; so, indeed, that we may here have to do with a phe-
nomenon of irritability in the narrower sense of the word. We may imagine that,
as the intensity of the light increases, the protoplasmic utricle clothing the cell-wall
becomes more resistant, opposes the hydrostatic pressure of the cell-sap more strongly.

Fig. 203 *.— Stoma seen from above (externally). The
upper half of the figure shows the form of a guard-cell
when the stoma is closed ; the lower half that when
open.

See note i, p. 248.

IMPORTANCE OF STOMATA. 25 1

and prevents the filtration of the water from the guard-cells into the epidermis,
without, however, interfering with the filling of the cells by endosmose ; in the dark,
on the contrary, or even as the intensity of the light diminishes, the protoplasmic
utricle would, according to my assumption, become more permeable and less
resistant to filtration, and in consequence of this a portion of its water might be
pressed out from the guard-cells, previously forcibly distended, through the wall e.
That such an irritability is not opposed to the phenomena of irritability elsewhere
existing in the vegetable world will be sufficiently demonstrated by subsequent
lectures on the irritability of plants.

To any one ignorant of the facts, the theme here treated of may appear somewhat
small and insignificant, on considering how extremely minute the stomata are, and
that the diameter even in the open condition amounts to only a few hundredths
of a millimetre. Through such extremely fine openings (of which we may ob-
tain an idea by drawing out glass tubes to hair-like fineness) only extremely little
water can evaporate or other gas pass even in a long time. But the fact
is altered when we reflect how extremely numerous the stomata may be on
the assimilating green foliage-leaves — one hundred to two hundred on a square
millimetre of the epidermis are common, and they are not rarely much more
numerous; so that a leaf say the size of the hand possesses many millions of such
fine openings, which simultaneously open when the sun shines on the leaf, and
simultaneously close when it becomes shaded. The alteration in the discharge of
the aqueous vapour developed in the intercellular spaces, effected by the opening '
and closing under varying conditions of illumination, is enhanced by the warming
of the tissues of the leaf, which usually accompanies intense illumination, so that
an increased formation of vapour then takes place. Consequently the tension of
the vapour is raised, and it is forcibly expelled from the open apertures. In the
shade and in darkness, on the other hand, the formation of vapour in the interior
diminishes as the temperature falls, and the stomata being closed, the vapour at
low tension is retained ; whence complete saturation of the intercellular spaces with
vapour must soon occur, preventing the further formation of vapour, especially during
the sinking of the temperature at night. It scarcely needs mention that all these con-
siderations can only refer to the stomata on the organs of transpiration ; and that,
self-evidently, those occurring on subterranean shoots and on parasites containing
chlorophyll, only come into consideration in so far as they present, since they exist,
open paths of communication for an extremely slow exchange of gases, and that we
have here to do with organs of modified or even entirely suppressed function.

Besides the varying illumination, and the consequent opening and closing of
the stomata, account has to be taken of many other factors, by means of which
transpiration may be accelerated or diminished. In the first place, so much is
clear, that even when the stomata are open, the escape of aqueous vapour from
them must be less rapid in proportion as the surrounding air is already laden
with vapour. In other words, transpiration must be diminished by the moisture of
the surrounding air; it is entirely prevented, however, only when the air is com-
pletely saturated, and the temperature of the leaves is not higher than that of the
surrounding air. Were the latter the case, the acjueous vapour, expanded even more
by the higher temperature, could still escape. Since, further, the formation of vapour

252

LECTURE XV.

from moist surfaces, and thus from the cell-walls which bound the intercellular
spaces of a leaf, is accelerated by a rise of temperature, transpiration must
increase with a rising temperature, always provided the stomata be open. According
to some observations made several years ago in my laboratory, it is not improbable
that the mere shaking of shoots facilitates the exit of water from the leaves ; this
is in so far worthy of closer investigation that the significance of the wind, and
the vigorous shaking of foliage-shoots effected by it, might be explained more

in detail. That motion of the air
acts favourably on the transpi-
ration of leaves, as it does on eva-
poration from any moist body, is
obvious on a little reflection ; but the
shaking of shoots may, in addition,
favourably affect the mere expulsion
of the vapour from the intercellular
spaces.

More than twenty years ago I fur-
ther confirmed the remarkable fact,
already in part noticed by Senebier,
that the transpiration from leaves
may also be altered by the presence
of materials dissolved in the water
which the roots take up^. Weak
solutions of those salts which plants
employ as nutritive materials, — e.g.
potassium nitrate, gypsum, &c., —
poured on the soil in which the roots
are, bring about a noticeable retard-
ing of the transpiration. The same
takes place when uninjured roots,
developed in solutions of nutritive
materials, take up solutions of nutri-
tive salts instead of pure water. Cul-
tivated plants in a strongly manured
soil will therefore in general transpire
more feebly, and make use of less
water, than those in a soil poor in
food-matters. It is at present hardly
possible to give an entirely satis-
factory explanation of this fact ; since, although it is known that aqueous vapour
is given off less easily from a salt solution than from pure water, still the pheno-

FIG. 204.— Apparatus for demonstrating the suction and transpiration
of a shoot under various external conditions. The shoot a is fastened
water-tight into the middle glass tube : at * a thermometer is inserted
in the same way, which, by being pushed in, serves at the same time to
bring the level (c) of the water in the narrow tube at any time to its
original height again, when it has sunk by the suction of the shoot during
1—3 minutes. If the shoot is large enough, and the apparatus not too
heavy, it may be placed on a scale and the loss in weight by transpiration
be determined at the same time.

On the changes in transpiration by the influence of salt solutions at the roots, cp. my treatise
n the paper, 'Die Lafuhvirthschafil. Versiuhsstaiiotien,' 1858, 1, p. 203, and 'Bot. Zeitung,' i860,
NT- - • More recent facts are quoted in Pfeffer's ' Pßanzen-physiologie,' Bd. i, p. 151.

No

SALTS IN THE ASCENDING WATER. 253

menon in question is far from being explained by that. However, liere again we
recognise the significance of transpiration for the nutrition of the plant; since it
is clear that water strongly charged with nutritive salts requires to be supplied to
the green organs of assimilation in less quantity than if it contained but little of
these substances.

In concluding the preceding remarks, and as an introduction to what follows,
I may now briefly advert to the question whether the water ascending in the wood
cell-walls, which evaporates in the leaves, does really carry with it the nutritive
matters — the sulphates and phosphates of the alkahs and alkaline earths — dissolved
in it, and transport them into the assimilating cells of the leaves. This question
is justified, because for a long time the transport of these materials was supposed to
take place in another manner. It was assumed that the living cells of the root-
cortex, abounding in sap and containing protoplasm, take up these substances from the
water of the soil surrounding the roots, according to the laws of endosmosis ; and
that likewise, proceeding from cell to cell, from the roots right up into the leaves,
endosmotic processes transport the salts into the organs of assimilation, without the
co-operation of a continuous current of water. What is here considered is a
movement of the molecules of salt dissolved in the water, independent of a current
of the water itself. If we suppose an isolated cell, lying in water which contains
saltpetre, for example, while none is present in the cell, then a certain quantity
of the molecules of this salt will, in spite of the cell-membrane and protoplasm,
pass into the cell; and if this stands in connection with a series of other cells,
these also will gradually take up the molecules of salt. This would take place to an
extent so much the greater if the salt-molecules were decomposed in the interior
of the distant cells, and by this means an endosmotic equilibrium prevented from
re-establishing itself. This form of the movement of material certainly occurs in
submerged water-plants, in root-parasites growing underground, and here and there
also in the tissues of the land-plants ; but considerations of the most various kinds
led me years ago to the view that in transpiring land-plants, and especially in shrubs
and trees, the vital conditions of the plants could not be correlated in this manner.
The endosmotic movements of the salt-molecules referred to, are so slow that they
could not possibly supply the foliage of a tree with the large quantities of nutritive
salts which there co-operate in assimilation, and are actually shown to be present
in the leaves by analysis. When I had fully substantiated the ideas, already pointed
out incidentally by Unger, that the transpiration current of land-plants moves not
in- the cavities but in the substance of the cell-walls of the wood, the question
arose whether the water ascending in the wood cell-walls is not perhaps pure
water, or whether it carries with it the soluble salts contained in the soil, and
conveys them into the leaves. The probability of the latter assumption was in-
creased by the consideration that we have to deal with only extremely dilute
solutions — water in which the dissolved molecules of the salts concerned are
contained only in extremely small quantity; perhaps i : 2000 or even less.
Further, it was to be observed that even in the ordinary endosmotic movement
salt molecules go through the substance of cell-walls and protoplasm. I was thus
able to conclude, further, that also the few molecules contained in the ascending
current are carried forward in the substance of the wood walls, simultaneously with

254 LECTURE XV.

the water. These considerations receive full experimental confirmation by the
fact that even a salt (lithium nitrate) not belonging to the ordinary nutritive
materials answers the condition required by me, and ascends in the wood walls with
the transpiration current. If, as already mentioned, a weak solution of lithium
is poured on the roots of a land-plant, the latter continues to transpire, and after
one or two hours the lithium is already found in the leaves situated fifty to two hun-
dred centimetres above the roots. If the leaves be cut off and immersed, with the
exclusion of the stalk, in pure water, lithium may be recognised in the water after a
few hours, having come out of the epidermis by diffusion. It is by no means to be
supposed that this rapid movement of the lithium in the plant has taken place
by endosmose from cell to cell : it has evidently ascended with the transpiration
current in the wood cell-walls, and in the same manner the nutritive salts men-
tioned are likewise sent into the leaves. What has been said, however, does
not exclude the fact that endosmotic movements of salt-molecules take place where
necessary even in vroody plants : when the cells of the living cortex, or the
assimilating cells of the transpiring leaves decompose phosphate of lime or sulphate
of magnesia and the like, in their interior, molecules of these salts, ascending with
the water in the wood walls, will be able to enter by endosmose into the interior
of the cells named. We shall obtain a correct idea on the whole if we suppose
that the water ascending in the wood extends in the first place into all the cell-walls,
even of the parenchymatous tissues of the cortex and leaves, and takes with it the salt
particles dissolved in it; thus the cell-contents become surrounded on all sides by
a layer of water, which is between the molecules (micellae) of the cell-membrane.
The cell-wall framework of the entire plant contains at the same time the store of
water and salts for all the cells. If one of the latter requires water, it finds it in the
first place in its wall ; if, on the other hand, it requires a number of salt molecules,
these also are contained in the water of the cell-wall, and can enter at once into the
interior. Everything is of course far more simple in submerged water-plants, which
can take up water and the salts dissolved in it at every point of their surface ; with
the slender structure and the numerous large intercellular spaces of the water-plants,
the path which the materials in the cell-walls have to describe is rarely greater than
I mm., and even the slowest movement suffices to satisfy the requirement.

The question has for a long time been ventilated whether the leaves are not able,
at least under certain circumstances — e. g. during continuous rain, or when they
are covered with dew — to absorb water and substances dissolved in it. Numerous
researches directed to this end have yielded no satisfactory result w^hatever;
but it is by no means difficult to apprehend the essential points without that. If
the whole plant is filled and turgid with water, and particularly the leaves, it is
not clear how the latter are to take up more water from without; if, on the other
hand, they are drooping, and not quite filled with water, it depends upon the consti-
tution of the cuticle whether and how rapidly they are able to absorb water.
The fact is, that drooping shoots, especially of wood-plants, when inverted and
plunged in water, so that the cut end of the stem projects, often remain drooping
for days in spite of the surrounding water, and thus absorb none ; and it is
obvious that this is the case in general so long as a layer of air clings to the
surface and prevents the wetting of the leaves. If the leaves become actually wetted.

ABSORPTION OF LIQUIDS, ETC. BY LEAVES. 255

however, small quantities of water can also penetrate into the tissue, according to
circumstances, and even any particles of salt contained in the water may enter.
This must be concluded with certainty from the fact that leaves permeated by
lithium nitrate give up the lithium to the surrounding water. The same follows
also from the fact that drops of water which are allowed to cling to the surfaces of
leaves show an alkaline reaction after some time, because small quantities of alkaline
salts have diffused out from the tissue ^ ; and that substances of the most various kind
penetrate from outside into the leaves, is shown by the extraordinary sensitiveness of
the latter to vapours of ammonia, chlorine, nitric and sulphuric acids, for example, which
injure fields and gardens in the proximity of factories to a large extent. But all this
does not prove that any considerable quantities of water, and salts dissolved in it, are
conveyed by means of the leaves to the land-plants, and that the activity of the roots
and of transpiration is supplemented by this means. When leaves and shoot-axes
droop after a hot day, and again become fresh as the sun goes down, this happens
in no case by the absorption of aqueous vapour from the air which has again become
moist; but simply depends upon the fact that with diminution of the transpiration
the turgescence of the organs becomes restored by the continued supply of water
from below.

It appears from what has just been said, that the normal supplying of land-
plants with water and nutritive matters dissolved in it, is the function of the roots
distributed in the earth. The peculiarities and difficulties, to be spoken of afterwards,
with which the absorption of these matters from the soil is connected, as well as the
very large quantities of water which must be absorbed, evidently constitute one of
the causes which bring it about that transpiring land-plants containing chlorophyll,
in contrast to the water-plants and parasites, produce such copiously branched root-
systems, consisting mostly of hundreds and thousands of individual root-fibres. I have
expressed myself more in detail concerning this in a previous lecture. The copiousness
of the root-system corresponds simply to the extent in surface and function of the
organs of transpiration. Where, as in submerged water-plants, transpiration is
completely wanting, only very few roots or none at all exist; and in the former
case these serve in the lower plants (Algae), as has been shown previously, only as
anchoring organs. The floating plants provided with an evaporating surface (e. g.
Stratiotes, Hydrochan's, Lemnd) have, it is true, more or less developed roots, the
function of which, however, is but little in demand, because the leaves only transpire
feebly in the moist atmosphere; and, on the other hand, the roots surrounded by
liquid water can absorb it without hindrance. Bog-plants are also in a similar
position with regard to the activity of their roots : the crown of leaves, it is true,
transpires considerable quantities of water, but their roots are in a soil completely
saturated with water, and are able to absorb it unhindered. They are in fact much
more abundantly rooted than the floating and submerged water-plants ; but still are

' With regard to the alkaline reaction of drops of water placed on living leaves, and the fact,
already discovered by Theodore de Saussure, 1804, that alkalies can be washed out of living leaves,
more is to be found in my treatise, ' Ueber alkalische und satire Riaktioncn der Säfte lebender
Pßanzenzellen^ Bot. Zeitung, 1862, p. 257.

256 • LECTURE XV.

provided with root-fibres, not so numerous by far or so densely filling up the soil as
in the case of the proper land-plants. It is characteristic of the mode of life of the
latter that they only flourish, as a rule, when their roots are distributed in a soil which
is relatively dry, and only incidentally saturated with water. Thus, just those plants
which exhale the largest quantities of aqueous vapour by means of their assimilating
leaves are adapted by their roots to a soil in which relatively little water is contained
ordinarily, and especially at the time when most water is made use of. In order to
understand this completely, it is to be noted that it is in the months of May, June,
July, August, and September that the transpiration and nutrition of our cultivated
plants, field plants, and forest trees are particularly active, that is, at a time when the
earth is completely saturated with water by the rain only now and again, while weeks
and months often pass, during which these plants are necessitated to take up by vigor-
ous transpiration large quantities of water from a soil which, as the most superficial
inspection shows, contains relatively only small quantities of that liquid. And this
apparently unfavourable circumstance is really quite necessary for the well-being of
the truly terrestrial plants ; since, as is well known, fields of which the soil is too
damp are made in a high degree favourable to vegetation by drainage, and the like
is true of gardens and forests. The cultivation of plants in greenhouses and rooms
also teaches that land-plants which are rooted in flower-pots very easily perish from
rotting of the roots if they are watered too often; and it is one of the most
elementary rules of the cultivation of plants in pots, to let the earth in the latter
become very dry each time before fresh water is poured on. The roots of land-plants
thus properly carry on their functions continuously only when the soil surrounding
them is as a rule relatively poor in water ; although a complete saturation of the soil
for a short time does not at once act injuriously. The condition normally favourable
for the roots of transpiring land-plants is therefore this; that they are distributed
in a soil which, in addition to small quantities of water, contains at the same time
spaces filled with air, by means of which the respiration of the roots is maintained.
Very often, however, during long continued drought, the quantity of water contained
in the soil sinks so low that the latter appears almost air-dry ; and it seems scarcely
intelligible how the roots are able to extract from it the large quantities of water
transpired through the leaves. It thus concerns us to obtain an accurate notion
as to how the roots of land-plants accomplish the absorption of such large quantities
of water, from a soil relatively poor in water. I made this problem the subject of deep
reflections and experiments so long ago as 1859 \ and presented the results in a con-
nected form in my Handbook of the Experimental Physiology of Plants, 1865. The
figure already employed there to illustrate the behaviour of the roots in the soil, may
here again serve for further explanation. For the absorption of water, it is only the
younger portions of the individual root-fibres, distant some centimetres from the
root-cap, which essentially come into consideration — parts which, not yet covered
with periderm, are provided with thousands of root-hairs. It is these root-hairs
which bring about the immediate connection of land-plants with the soil which
nourishes them. We may therefore confine our considerations to the behaviour of

' For the behaviour of capillary water in the soil, I refer to my treatise in the paper,
■ Laiuhvirthsiliaftl. Versuchsstationen,' 1S59. Heft. IV, p. i.

ATfACHMENT OF RüüT-HAIRS.

^57

a single root-hair, which has been protruded from a young root into the surrounding
soih

In Fig. 205, e is tlie epidermis of a root which has grown perpendicularly down-
wards ; the root-hair h h has been developed as a protuberance of an epidermal cell,
and at 2 and s it is closely applied to single particles of soil, as seen below. The bodies
T, shaded dark, are microscopically small particles of earth, between which are cavities
containing air — left completely white. Each particle of soil is enveloped by a thin layer
of water, which is held fast by surface-attraction : where the attraction of neighbouring
particles of earth co-operates at the re-entering angles, these otherwise thin layers of
water form thicker masses. These aqueous spheres are indicated in the drawing by waved
lines (as at ß and y). The surface of the root-hair is also (as at a) clothed with a thin layer
of water, and its walls are saturated with that fluid ; the spaces left white are filled

with air. Let us now regard the root-hair for a moment as inactive, and suppose no
disturbance at all to be taking place in the soil. Then all the aqueous spheres of the
particles of earth are not only in contact with one another, but also in equilibrium.
If we were to take away the layer of water at y, for example, the equilibrium would
be disturbed throughout the entire system, and water would flow from Ö and ß
and other places, towards y, until the forces were in equilibrium. If we now assume
the root-hair h h to absorb the water a or t, this penetrates through the membrane
into the interior of the hair, or it moves along t, a, S, in the substance of the wall
itself: the surface of this wall at a or r thus possesses less w^ater than corresponds to its
power of attraction. It attracts it, therefore, at the spot r : this then absorbs water from
ß, and the movement proceeds towards y and 8, Ac, until the molecular equilibrium of
all the aqueous spheres is again established. By this means they all become thinner
and thinner, and the soil as a whole drier. This desiccation, however, mav make itself
[3]

258 LECTURE XV.

evident not merely in the immediate neighbourhood of the root-hair, but at the same
time affect the more distant parts, since by the absorption through the root-hair at
n or r, a continuous flow of the adherent water of S takes place towards y, ß, and a.
This assumption is supported by the observation that the earth of large glass vessels
in which plants are grown, dries up not merely in the immediate neighbourhood of the
absorbing roots, but, so far as the colour of the earth allows us to recognise it,
the desiccation increases equally in all parts even far from the roots. This move-
ment of water on the surfaces of the particles of soil, easily deduced from molecular
forces, is confined therefore not simply to microscopic distances. Every root-hair is
itself the centre of a current directed from all sides towards it ; and at the surface
of a small piece of root covered with thousands of root-hairs there results a similar
movement, which carries the aqueous particles in the soil from all sides, but especially
radially, towards the axis of the root.

If we suppose the aqueous envelope of a particle of soil to consist of several
very thin layers, a, d, c . . . . n, according to its thickness, so that n is the outermost,
and a the one immediately in contact with the particle of soil ; then the water
molecules of the elementary layer a will be attracted with a maximum force, and
in the layers 5 c, &c., further removed, this attraction diminishes progressively ;
and if n represents the outermost layer, when the soil is just saturated with water,
the molecular attraction in it is only just great enough to prevent the water
from dropping off. In the case last assumed, when an absorption of water takes
place at a or r, the outermost layer of the aqueous spheres moves first, in order to
restore the disturbed equilibrium of the whole system, and flows towards a and t;
because this outermost elementary layer is most loosely held, and is therefore most
easily put in motion. The more water the root-hair has already taken up, the thinner
are the aqueous spheres of the entire system, and the greater the force with which the
elementary layer now outermost {c, for instance) is held fast ; and the greater must the
forces be which draw the water into the wall of the root-hair, and the more difficult
and slow the transmission of a disturbance from a to ß, y, and 8. A condition of
the aqueous envelopes may finally ensue, where the elementary layers still remaining
are held so fast by the particles of soil that no more water enters the wall of the
root-hair. In this case, their surfaces may possibly be still clothed with a very thin
layer of fluid, which indeed is wanting to no saturated body. If now the root is
in connection with a leafy stem above ground, transpiration from this organ will go
on removing water from the plant; this loss, however, under the circumstances
given, can no longer be compensated by absorption on the part of the root, and the
interior of the plant becomes deficient in water, the cells, no longer sufficiently turgid,
become flaccid, and the leaves droop. Conversely, it is also possible under
certain circumstances to conclude from the drooping of the leaves, and from
the quantity of water known to be contained in the soil, as to the condition which
denotes equilibrium between the suction of the root and the forces of adhesion
in the soil.

I have in a few cases sought to determine the percentage of water contained in the
soil when Tobacco plants rooted in it were no longer in a condition to withdraw
the minimum of water from it. This takes place when the leaves droop in a moist
atmosphere, even at night, and therefore when the loss by transpiration is very

ABSORPTION OF WATER FROM THE SOIL. 259

small and the compensation afforded by means of the roots is at a minimum ;
when this is no longer afforded by the roots, equilibrium has nearly set in between
the absorbing forces at the surface of the root and the absorptive power of the soil
for water. The carrying out of such determinations cannot from their very nature
be quite exact, and thus the following numbers are only intended to afford a definite
idea of the matters described. Under certain circumstances a young Tobacco
plant began to droop when the soil, a mixture of sand and black beech humus
(in a room), still contained 12-30/0 of its dry weight of water," determined at ioo°C.
This soil, dried at loo^C, was able, however, to retain 460/0 of its weight of
water by adhesion. Consequently, of the possible amount of water in this soil,
only 46 — 12-3 (i.e. SS-V^/o) was at the disposal of the Tobacco plant; the
12-30/0 of the imbibed water still present was held so fast, that the roots were
no longer able to take it up.

Another almost similar Tobacco plant, standing near this, drooped during
a rainy night, when the loamy soil surrounding its roots still contained 80/0 of water ;
100 grammes of this loam, however, were able to retain 52-1 grammes of water by
adhesion or absorption ; accordingly this soil, saturated with water, gives up to the
plant only 52-1 — 8 (i.e. 44-10/0) of its water.

Under like conditions a third Tobacco plant drooped when its roots were in
coarse-grained quartz sand, which, in 100 parts of its weight, still contained
1-5% water. This sand, dried at 100° C, was able, however, to retain 20-80/0
water; consequently 20-8-1-5 (i.e. 19-30/0) of water altogether were at the disposal
of the plant, after saturation of the soil had taken place.

The more the last remnant of water adheres to the particles of soil, the greater
also will be the amount of water contained in the soil at the time when the
root is no longer able to extract water from it; the humus soil still contained
at this time 12-30/0, the loam 80/0, the sand only 1-50/0. Since the drooping
only took place when the soil showed this poverty in water, these examples
demonstrate that the Tobacco roots still withdraw at least small quantities of
water from the soil up to the time when the soil is air-dry, since the propor-
tions of water named correspond approximately to the air-dry condition of such
soils ; and these researches show at the same time that plants still withdraw
water from the soil even when it is no longer possible to squeeze any out of it
by pressure.

From these facts, now, it is to be seen that it is by the intimate contact or attach-
ment of the root-hairs with the particles of soil that they succeed in sucking up into
the plant the extremely thin layers of water of the latter. This union of root-hairs
and particles of soil has, however, still another very important significance. Only
by this means are they enabled to take from the soil the nutritive materials
necessary in addition to the water. In fact, certain materials indispensable for
the plant, such as sulphates of lime and magnesia, as well as extremely small
traces of other nutritive salts, are without doubt dissolved in the thin layers of
water which surround the particles of soil: this follows from the fact that the
water running off from the drain pipes of tillage soil contains these substances.
But a number of the most valuable nutritive materials are held so fast in vegetable
soil, that it is impossible to w^ash them out with such quantities of water as are

26o

LECTURE XV

conveyed to them by the rain. These materials (chiefly potash, ammonia, phos-
phoric acid, and the less important silica) are found in the soil in a peculiar
combined condition : they are, as we are accustomed to say, absorbed. An

Fig. 206. Seedling of Wheat plant. Fig. 207. Tlie same four weeks older. 5 the seed coats, &c ; 71» the apices
of roots not yet furnished «itli hairs; e parts attached to the soil (in Fig. 206). In Fig. 207 the hairs are already
perished on these parts, e' younger parts of roots attached to soil.

illustration of this fact^ is obtained most easily by filling an ordinary funnel

» On the absorption and taking up of absorbed matters, a very thorough account is found in
my 'Handbuch der Experimental-physiologie,' 1865, p. 178. The corresponding section in Pfeffer's
' Pflanzeji'physiologie'' (1881) shows that essentially nothing new is to be added concerning the
subjects mentioned in the previous and following nolts.

ABSORPTIVE PROPERTIES OF SOIL.

26]

with about a kilogramme of damp soil from a field or garden, and then pouring
over it a weak solution of potash salts, phosphates, or ammonia compounds.
If the fluid running off below be then examined, only slight traces of the materials
named are found in it, or even none at all ; though lime and magnesium sulphate
are generally present in large quantities. It
is therefore necessary to pour on very con-
siderable quantities of pure water, in order
to gradually wash out again part of the
materials previously held fast in the earth ;
the soil of vegetation thus acts on these
nutritive materials of plants, as animal
charcoal on colouring matters and other
chemical compounds.

FIG. 2o3. — Seedling of Wliite Mustard.. ^
taken out of the soil and shows the particles of i
clinging to the root hairs : in £ these have been
moved by washing in water.

Fig. 209. — Root-hair of a seedling of Wheat
closely attached to particles of soil (highly magnified,
cf. Fig. 206).

Meanwhile it is of little interest for us to know in how far purely chemical
or even molecular forces come into consideration in the absorption of the materials
in the soil. So much is at any rate established, that chemical compounds of
potash, ammonia, and phosphoric acid, are retained with great force on the

202 LECTURE XV.

surfaces of the small particles of soil, forming extremely fine coats on them.
But it is these substances which must be taken up by the root-hairs, and it is obvious
that this is only possible by the root-hairs coming into the closest and most
extensive contact with the particles of soil. Since the nutritive materials clinging
to the particles of soil are not soluble — or but very slightly so — in the layers of
water, the root-hairs, as they apply themselves fast to the surfaces of the particles,
must themselves effect the solution of the absorbed materials. This they ac-
complish by means of the extremely thin membrane of the root-hair being
permeated with an acid fluid, which, coming in contact with the surfaces of the
particles of soil, renders soluble the molecules of absorbed materials adhering there :
it thus becomes possible for these substances to penetrate into the. root-hairs accord-
ing to the laws of diffusion, and thence to pass over into the stream of sap, to be
carried finally to the organs of assimilation. That the root-hairs are, as a matter
of fact, able to bring the whole of the potash salts and phosphates necessary
to vigorous vegetation from the absorbed condition into the plant, may be easily
demonstrated by repeating an experiment first proposed by Naegeli in 1861.
I am accustomed to make the experiment in the following way: bits of peat
which, according to chemical analysis, contain practically no potash salts and
phosphates, are laid for some days in a 1-2 per cent, solution of any potassium
salt (e. g. potassium phosphate), or even in a solution of a complete nutritive
mixture such as we shall learn more about later on, until they are completely
soaked. The bits of peat are now lixiviated for some days in pure water, fre-
quently renewed, until the latter no longer contains any traces of potassium and
phosphoric acid : they are then broken up into small pieces, and a large flower
pot filled with them. Seeds of Maize, Wheat, Tobacco, Hemp, Gourd, Beans, or
other plants are now sown in this medium. It suffices to water in the usual
manner with ordinary spring water, which is always rich in sulphates of lime and
magnesia, or with a very dilute solution of gypsum, magnesium sulphate, and calcium
nitrate in distilled water. On the completion of germination the plants then go
on growing as in good garden soil, and attain to complete development and
vigorous production of seeds; while plants of the same species cultivated in a
similar manner, except that the peat soil had not previously absorbed any phosphoric
acid and potassium salts, remain exceedingly feeble, and languish. Such experi-
ments prove that the quantities of phosphoric acid and potash necessary* to
vigorous development are taken up by the root-hairs from the surfaces of the
pieces of peat.

By means of the acid fluid which permeates the walls of the root-hairs, the
latter are able to dissolve even solid and crystallised minerals. In the year 1859
I showed^ that roots which become closely applied to the polished surfaces of
marble plates, corrode them, so that after some time a corrosion figure of the roots
is obtained on the marble surface. Of course it is not to be supposed that these
corrosions penetrate to any considerable depth ; they are evident rather only
as extremely fine rugosities on the polished surface. I subsequently showed that

^ I first drew attention to the corrosion of polished stone plates, 'Bot, Zeitung^ i860, p. 118,
&c. More details in my ' Experitnental-physiologie,'' p. 188.

CORROSION FIGURES. 26^

similar corrosion figures arc to be obtained also on polished surfaces of Dolo-
mile (a mixture of calcium carbonate and magnesium carbonate), of Magnesite
(crystalline magnesium carbonate), and of Osteolite (earthy apatite, chiefly tribasic
calcium phosphate) when the roots of Beans, Maize, Wheat, Gourd, Tropceolum,
Peas, and others are caused to apply themselves closely, and grow over the
polished surfaces of these stones. The experiments were made by laying the
plate of stone, with the polished surface turned upwards, at the bottom of
a vessel 10-15 cm. deep ; this was then filled with clean washed sand, and
several seeds placed in the latter. With sufficient moisture the radicles grow
vertically downwards until they abut upon the polished surface ; there they
bend to one side, grow on horizontally, chnging to the polished surface, and
form lateral roots which likewise become closely attached. When after 8-10
days, in summer, the vessel is upturned, and the stone plate carefully washed
and dried, the corrosion figure of the roots is seen on the polished surface.
The experiment succeeds with such certainty that it may be used for demonstration
in lectures.

It follows from these facts that the root-hairs, when they meet in the
soil with small pieces of stone which contain carbonate of lime, compounds
of phosphoric acid, magnesite and dolomite, dissolve small quantities of these
salts at their surfaces, and carry them into the plant. Since now the soil of
vegetation is a very various mixture of small particles of stone with organic
remains, so-called humus, different hairs of the same root, and likewise the hairs
of different roots of a plant, find opportunity here and there of taking up sometimes
chiefly phosphates, at others potassium compounds, &c., and conveying them into
the sap of the plant. In this, each individual root-hair of course accomplishes
but very little ; the quantity which it is able to take up will perhaps only amount
to the millionth part of a milligram. As we know already from previous study,
each root-hair remains capable of action but a few days ; however, on a plant
even of inconsiderable size there are millions of active root-hairs, and as the
older of these die, millions of new ones appear in their place, to use up portions
of the soil not yet laid under contribution. We must here remember what
was said in the organography of the roots, that behind each root-apex as it
grows forward in the soil, new root-hairs are continually arising, while the older
ones further behind on the root perish (Fig. 207). Since now, at the same time,
new root-fibres spring in various directions from those already existing, and
extend in the soil, new portions of the latter are continually being placed in
requisition by the root-hairs ; and thus the whole of the soil, permeated by the
thousands and hundreds of thousands of fine root-fibres of a plant, is gradually
used up.

The roots thus evidently contribute to the chemical decomposition of the solid
particles of stone in the soil of vegetation, — a process which proceeds at the
same time slowly by means of the carbonic acid contained in the soil, and of
the nitric acid contained in the rain water; and if the rotting remains of
vegetation are again incorporated in the soil, the vegetable mould must in this
way continually become richer in soluble and absorbed nutritive matters. This
influence of plants on their mineral substratum is clearly evident where Lichens

204 LECTURE XV.

and INIosses attach themselves to the free surfaces of rocks, e. g. on high moun-
tains. The solid crystalline surface of the stone becomes gradually converted, by
the activity of the roots of these plants, into a friable crumbling loose substance,
which continually penetrates deeper into the stone, and so presents a substratum
in which even the stronger roots of larger plants can then obtain a hold.
In a certain sense, the roots thus behave towards the mineral constituents of
the soil much as the roots of parasites behave towards the tissues of their host
plants; we shall return to this subject later, however.

If we now throw another glance on the matters treated of in the present
lecture, the fact is clearly demonstrated that the plant undertakes the absorp-
tion of its nutritive matters by means of its own activity, and is by no means
simply passive. In the first place the root-hairs, after they have become closely
applied to the particles of soil, have to take up the water clinging fast to
the latter, as well as the equally closely attached nutritive substances. The
next step is to convey these substances to the assimilating organs, the
green leaves. The wonderful properties of the wood cell-walls, which are
comparable with nothing else, permit the ascent of the water and the small
quantities of salt dissolved in it through the stem up to the leaves, but only
in so far as these latter themselves regulate this movement according to their
needs : when strong light promotes turgescence in the sensitive guard-cells of
the stomata, causing them to open, the aqueous vapour escapes from the inter-
cellular spaces of the leaf simultaneously with the oxygen of the decomposed
carbon dioxide; and as assimilation thus commences, the ascending water cur-
rent is set in motion by this continued open condition of the stomata, and,
starting from the apex of the root-hairs, conveys the constituents of the soil
which are indispensable in assimilation into the tissues of the leaves containing
chlorophyll.

That the roots are not merely passively concerned in the absorption of water
from the soil, but are independently and actively engaged in it in consequence
of certain irritabilities, I convinced myself first in the autumn of 1859, on ob-
serving^ that the leaves of plants of Tobacco and Gourd rooted in flower-pots
drooped when the earth was cooled down to a few degrees above zero, and
thus absorbed less water than was requisite to cover the feeble transpiration.
The warming of the flower-pot sufficed to increase the absorption of water so
far that the drooping leaves again became turgid. This phenomenon can also
be brought about in summer, if the pots in which sensitive plants are rooted
are cooled by surrounding them with pieces of ice. It is not all plants however
which are provided with roots so sensitive to temperature, but only those, as it
appears, which come from warm climates. Cabbages, for instance, and other native
plants did not droop when I cooled their roots. On the other hand, again,
it is certain that in all plants the absorbing activity of the roots is invigorated
by an increase of temperature up to 25° — 30° C ; as is apparent, for instance,

' i have described the drooping of leaves when the roots are in earth^too cold for them in the
paper, ' Landwirthschaftliche Versuchsstationen^ 1859. I> P- 238.

ACTIVITY OF ROOTS. 265

from my success in causing a great number of very different plants to excrete
water at the edges of the leaves in the form of drops, by warming the roots —
the consequence being a vigorous absorption and forcing up of the water into
the leaves.

The excretion of drops here mentioned I shall consider more in detail
in the next lecture.

LECTURE XVI.

EXCRETION OF WATER IN THE LIQUID STATE.

Although the phenomena to be treated of here are far less important for the
life and maintenance of the plant than transpiration — i. e. the excretion of aqueous
vapour by land-plants — it is nevertheless worth while to examine more closely
the excretion of liquid water which takes place in plants under very various
circumstances; especially since it affords us a deeper insight into relations of
structure and of mechanical arrangements in the plant. From this point of view
the excretions of water termed ' bleeding ' and ' weeping ' particularly excited the
interest of the older vegetable physiologists at the end of the seventeenth and
beginning of the eighteenth centuries.

By the terms Bleeding and Weeping are designated those excretions of water
which occur under certain circumstances in consequence of injuries, generally such
that water is exuded in greater or less quantity from fresh transverse sections
of the wood. Two essentially different cases have to be distinguished, however,
viz., first, the case in which water is exuded from the wood-body in consequence
of a rise of temperature, and in which, above all, the activity of the roots does
not take any part; and secondly, the case where, from wounds in the wood,
relatively large quantities of water are exuded during long periods, which did
not previously exist in the wood, but must first have been absorbed by the roots.
The first phenomenon, which I shall name the bleeding of wood in winter, can
be produced just as well with isolated cut-off pieces of wood as with rooted
woody plants ; while the other phenomenon, which I distinguish as the weeping of
the root-stock, only occurs in vigorous, rooted, living plants, although similar exuda-
tions of water, but very much less in quantity, may also occur under certain
circumstances with separate portions.

The bleeding of wood in winter^ is observed in its most typical form if, on
a cold but frostless winter day, a portion of a branch of a tree {Rhamnus,
Hazel, Pine, Walnut, Birch, &c.), about 25-50 centimetres long and 2-5
centimetres thick, is cut off, and the two cut ends smoothed with a sharp knife.
Outside in the cold air the smooth sections appear relatively dry, and no
liquid water is to be seen, even at the lower section, when the piece of branch

^ Cp. my treatise, ' Qiulhtngserscheijntngen an Hölzern' (Bot. Zeit, i860, p. 253), where the
older literature is collected also. The explanation of the phenomena there described by me is found
in my treatise, ' Über die Porosität des Holzes'' (Arb. des bot. Inst, in Wzbg. B. II, p. 291).

BLEEDING OF WOOD. 267

is held vertically. When the object is brought into a warm room, however, or,
still better, wrapped in a warm cloth, clear water gradually oozes out from the
wood at the lower cut end ; and if the latter is then held for some few minutes
through the opened window out in the cold air, the exuded water is plainly seen
to be slowly sucked in again, until the surface of the section appears distinctly
dry. If the cold piece of branch is placed in a cylinder of warm water at
25 — 30°C., so that only the upper section protrudes, water again oozes forth: this
occurs first at the most external layer of the wood and then progressively oozes
from the inner and older rings of the alburnum, in accordance with the progressive
warming from without inwards. Small bubbles of air come out of the vessels
with the water, and are forcibly ejected. If the object is lifted out and at once
brought in the same position into a cylinder of cold water, the layer of water on the
cut end then sinks again into the wood; again also progressively from without
inwards as the cooling effect penetrates, until the section appears dry.

From this simple experiment several things are to be learnt. In the first
place, it is by the warming of the wood that water is expelled at the cut end;
and in fact the water is driven from the warm to the cold place. In accord-
ance with this, cooling brings about the re-absorbtion into the wood of the
expelled water. Secondly, detailed studies in the winter of 1859 showed me that
the quantity of water expelled always represents only a relatively small proportion
of the total quantity of water contained in the wood. It is obvious, and is
demonstrated by observation, that the phenomenon only appears at all when
the wood contains relatively large quantities of water. The poorer in water it is,
the more it must be warmed to bring about the emission of water. As Hofmeister
first recognised, basing his opinion on my observations, the whole phenomenon
depends chiefly upon the expansion of the air contained in the wood. We saw
in the lecture before last, that the wood-cells, even of wood very rich in water,
are never entirely filled with water; but that a portion of their cavity is occupied
by air-bubbles. It is the expansion of this air, saturated with aqueous vapour,
which forces out the water from the wood-cells ; and the cooling of these air-
bubbles, on the other hand, effects the suction by which the water is again drawn in.
That the change of volume of the water itself is far from sufficient to explain the
quantity driven out, I had already established by my investigation. Since, however,
the wood-cells are closed on all sides, and the vessels may here be neglected for
the time being, and since the phenomenon appears also in the wood of Conifers,
which has no vessels, the water driven out by the expansion of the air-bubbles
in the wood must obviously be forced through the cell-walls themselves; this,
of course, will take place most easily and rapidly through the thin portions of wall
which close the pits. With a slight rise of temperature, now, the force with
which the air-bubbles drive out the water is relatively feeble; and from the
fact that the water filters with facility through the wood cell-walls, we draw the con-
clusion that wood, even when it contains no vessels, must be in a very high degree
permeable. In how high a degree this is the case I convinced myself^ some

» With respect to the extraordinary facility with which the filtration of pure water takes place
through fresh wood, further facts are also to be found in the treatise last cited.

268 LECTURE XVI.

years ago, with fresh, vigorous pieces of living Pine. When a film of water was
placed with a brush on the upper transverse section of a piece of the stem even
I — 3 m. long, it sank in a few seconds into the wood, while an equal quantity of water
appeared at the lower section, — a proof that the smallest pressure can be equalised
by means of filtration through the wood. By this and other experiments I con-
vinced myself, also, that the spring wood of the annual rings is more permeable by
far than the autumnal and heart wood.

I may take this opportunity also of mentioning the very remarkable fact
that the air in the wood-cells, as well as in the vessels of living plants, is in a
high degree rarefied — a fact which I had already stated hypothetically in 1865,
but which is now better known, and can be easily demonstrated. The simplest
proof that the air in the wood-cells exerts a feebler pressure than the atmosphere,
lies in the fact that a freshly cut piece of wood, when laid in water of like
temperature, absorbs it eagerly, as may be easily proved from the increasing
weight of the wood. This absorption, however, is simply nothing further than
the forcing in of the water by the pressure of the external air, which, in its turn,
can only exert this influence when and so long as the tension of the air contained
in the wood, together with its aqueous vapour, is feebler than the pressure of the
atmosphere; or in other words, if the air-bubbles saturated with vapour in the
wood had the same tension as the atmospheric air, it is by no means obvious
how fresh wood, the cell-walls of which are saturated with water, and the cavities,
at least in part, filled with it, could still take up water; and this is, of course, still
much less the case if the cavities of the wood were already quite full of water.
In 1874 Höhnel made known the remarkable fact that when a branch of a living
plant is bent down into a bowl of mercury, and there cut through, the mer-
cury penetrates far into the vessels. In a few seconds the mercury pene-
trates several centimetres into the vessels of both parts of the shoot, and so
much the further the wider the vessels are. If the capillary repulsion between
the vascular walls and the mercury is also regarded, one is led to the conclusion
that the latter must be forced into the vessels by a considerable pressure, or, if
one will, suction. The pressing force, however, is no other than the pressure
of the atmosphere on the mercury; and this is only in so far effective as the
pressure of the air in the vessels is smaller, or, in other words, because the air
in these is rarefied. The rarefaction may be very considerable, and Höhnel
calculated from his experiments that the pressure of the air in the vessels may
sink to ^ or even J of the atmospheric pressure; and I have sought to make
it probable that, under certain circumstances, younger vessels and wood-cells may
even be completely empty of air and only contain aqueous vapour \ In fact,

1 I have already propounded my view that wood-cells and vessels may be devoid of air under
certain circumstances in 'Arbeiten des bot. lust, in Wzbg.' (II, 1879, p. 324). I may simply add
one remark here. When, on the death and disappearance of the protoplasm from wood-cells and
vessels, the watery sap contained in their cavities comes into immediate contact with the lignified
walls, it is imbibed under the influence of the neighbouring wood cell-walls. The water contained
in the recently developed wood-cells and vessels enters into the ascending transpiration current and
is carried forward by this. The imbibing force of the wood cell- walls here effective, however, is,
as we know already, enormous, and may be able to withdraw the water from the fresh wood-cells

AIR TN THE WOOD-CELLS, ETC. 269

air from without can only penetrate into the cavities of young vessels and wood-
cells with difficulty, when the latter lose the protoplasm and sap with which they
were previously filled. Meanwhile, without entering more into detail respecting this
point, it is obvious that the air in the wood-cells, which is always rarefied, must become
still further expanded and rarefied if a great portion of the water contained in them
is quickly conveyed through the wood cell-walls to the transpiring leaves. Höhnel's
experiment, moreover, as I have shown, may be made more evident so far as
the vessels are concerned, with dark solutions of colouring matter; and again,
I have, by cutting off the branches of living Conifers under a solution of
lithium, also been able to convince myself that this solution quickly penetrates to
a considerable distance into the wood-tissue. This is to be recognised by the
spectroscope, in the manner previously indicated ; and demonstrates anew that
the air in the wood-cells as well as in the vessels is rarefied.

It is clear that these facts may be employed to explain several still obscure
phenomena concerning the water contained in the wood of living trees. The
obscure facts to which I here refer consist, not in that the water ascends in the
wood cell-walls up into the crown of the highest trees, since this, as w^e saw
before, is quite intelligible from the imbibing force of the wood cell-walls : it has
been hitherto unintelligible, however, how the liquid water comes into the cavities of
the wood-cells. If only woody plants of a few metres high were concerned, or the
lower portions of a tall tree stem, one might believe that the rarefaction of the
air in the wood-cells acts as a sucking apparatus. But every such suction is simply
nothing more than the difference of pressure between the atmosphere and the
rarefied internal air. Thus, if the cavity of the wood-cells were entirely empty
of air, and devoid of aqueous vapour, the suction, or, what is the same thing in this
case, the whole external pressure of the air could only force the water about
ten metres high in the wood of the stem, even if the cell-walls opposed no
resistance.

It still remains unexplained, moreover, how the action of the atmospheric
pressure on the surfaces of the root is to be imagined. Moreover, the wood-
cells situated much higher in a tree contain liquid water, in addition to air, and
this at any rate cannot be explained so simply by differences of atmospheric
pressure.

Meanwhile, I only bring these doubts forward here because they render
clear the question with which we are concerned. Sooner or later it will yet be
shown that the rarefaction of the air in the wood, in combination with peculiar
arrangements not understood hitherto, brings it about that liquid water gets into
the wood-cells, even at considerable heights. Concerning this, also, I may mention
that the rarefaction of the air is easily observed even in the vessels of small
herbaceous plants; and that a fact so general can certainly not be accidental
and without significance for the life of the plant. Whether it may be regarded

and vessels with such force that the latter become completely empty. Vapour under a pressure
corresponding to the temperature alone remains in their cavities. When observations show that
air (though very much rarified) is contained in the cavities of wood, this must have penetrated by
slow diffusion from the exterior.

2/0 LECTURE XVI.

as promoting the movement of the imbibed Avater in the Hgnified cell-walls may
well be conjectured, but cannot be proved.

If we bear in mind the phenomena observed on warming and cooling a piece of
wood cut off in winter, it is clear that within the stem of a Hving tree, movements of
water will also occur in the interior of the wood, through the unequal warming at
different heights w^hich must necessarily occur with sudden changes of temperature.
When, for example, the sun shines on a tree after a cold night, the thinner branches
are more rapidly warmed than the thick stem, and the water is driven from them
into the latter; and conversely, with the increasing cold of night the branches become
more rapidly cooled than the thick stem, and take up water from the latter. With
a rise of temperature in winter, tension in the wood generally must occur, since the
air-bubbles in the wood-cells on becoming warmed tend to force the water through
the walls, which is prevented by the cortex. If, however, a hole penetrating as far as
the alburnum is made with an auger, and an exit-tube, bent downwards and opening
at the bottom of a flask, is placed in it, these effects of changes of temperature may
be made visible. As the temperature rises, water flows out of the hole into the
flask ; this has simply been expelled by the expanding air-bubbles of the wood.
On the recurrence of cold weather, however, the air-bubbles contract; and the
consequence of this is that the water which had flowed out into the flask is
again sucked up through the tube into the wood. Although, as was shown above,
the water expelled by the rise of temperature is always only a small portion of that
present in the wood, nevertheless several litres of water may be driven out from
a large tree in an experiment of this kind, if the increase of temperature is suf-
ficient, since the water contained in the wood of a large tree may certainly amount
to hundreds of litres. Moreover, the phenomenon need not be confined to the
winter ; it may continue also in the spring, and even in summer (e. g. with the
Birch), since it only depends upon how much water is contained in the wood, and
upon the corresponding changes of temperature. The characteristic fact of this
kind of bleeding, however, simply lies in that it takes place even in winter, and
independent of the period of vegetation.

The second kind of bleeding, or the weeping of the root-stock, is distinguished
from that hitherto considered, in that it only takes place during the period of proper
vegetative activity, when the roots have already commenced to absorb water from
the warm soil. This phenomenon has been observed in the Vine from of old.
In spring, when the soil is to a certain extent warmed and the roots incited to
activity, if a Vine is cut through close to the earth, or even higher, a quantity of
clear water instantly exudes from the wood ; and this outflow of water may continue
for many days, and, as I know by personal experience, may yield several litres
of water. The same happens with many trees in the spring time. Birch-stems, for
example, sawn across a little above the soil, send forth streams of water, coming from
the alburnum, which exude from the root-stock for weeks. This bleeding is,
moreover, by no means confined to woody plants in the narrower sense of the word,
as is proved by the old custom of the Mexicans of obtaining enormous quantities of
their national drink, pulque, from the huge plants of Agave americana (the so-called
'hundred-years Aloe'), by cutting out the so-called 'heart' or leaf-bud, within
the rosette of leaves, upon which water, driven up from the roots, is excreted

WEEPING OF THE ROOT-STOCK.

371

into the hollow of the stem. This contains sugar and albuminous substances, and
therefore passes over into alcoholic fermentation ; and, according to the statement of
Alexander von Humboldt, hundreds of litres of sap may be gradually obtained in
this manner from a single plant. It was formerly believed to be a special peculiarity
of some few plants thus to expel the water absorbed by the roots at the transverse section
of the stem, until Hofmeister showed (about 1850) that the same phenomenon may
be observed in any given plant, even small annuals which form little wood; it is
only necessary to cut off the stems of Ricinus, Tobacco, Digitalis, Nettle, Sunflower,
Maize, and such like well-known cultivated plants abo\'e the roots, when they are
already actively vegetating, and to connect
a glass tube at the section of the stem, to
observe most of the phenomena referred to;
and this may be done the more conveniently
since even plants cultivated in flower-pots
are suitable for the purpose. Such an
experiment proceeds best and simplest if
the plant is allowed to grov/, not in soil,
but in an aqueous solution of nutritive sub-
stances (Fig. 210), until it has developed a
vigorous root-system ; the stem is then cut
off, and the root-stock connected with an
exit-tube, and as the roots absorb water
from the nutritive fluid, an equal quantity
exudes above at the transverse section of
the stock.

The weeping of the root-stock has,
since the time of Hales (1721), been in-
vestigated a thousand times by the most
various observers, and it would require
many hours to refer to all the results in
detail. Instead of this, however, we will
commence our further considerations on
the phenomenon in question with a definite
example. In August, 1881, a very vigor-
ous specimen of the Sun-flower {Heliantlms
annuus), about 3 metres high and with a
stem 4-5 centimetres in diameter, which

was growing in the open land in my experimental garden, and was thus quite
normal, was cut off transversely about twenty-five centimetres above the sur-
face of the earth, and an appropriate exit-tube at once placed on the stump of
the stem; the descending curved limb of the latter allowed the water expelled
from the root-stock to pass into a graduated cylinder. Since the experiment
was commenced on a hot day, the following phenomenon, to which I shall
return below, presented itself: the root-stock, instead of expelling water at once,
on the contrary absorbed a not inconsiderable quantity of water through the trans-
verse section of the stem during the first few hours. The outflow from the section

FIG. 210.— A Maize plant developed in a nutritive solution
in the vessel a, is cut off above the cork (A) at <-, and here con-
nected with the glass tube e/^ by means of caoutchouc (rf)-
The narrow glass tube h passes through the cork g: egh is
filled with water, and mercury is then poured in h. Tliis
mercury is driven up by the water expelled from c, as shown at

272

LECTURE XVI.

of the Stem only began afterwards, and then continued for fourteen days. During
the first few days the expulsion of water was copious : later on it gradually
diminished. On the whole, however, 1061 cubic centimetres of water were expelled
in thirteen days — a quantity which was at least three times as large as the volume of
the whole root-stock ; whence follows without further remark that the escaped w-ater
could not possibly have been previously contained in the root-stock, but must have
been taken up only during the outflow — one of the most fundamental facts connected
with this subject.

The circumstances under which the outflow of water takes place, will best be
made evident if I here put together in the form of a table the observations made,
at least for the first six days. It is only to be noticed that the letters n, e, m, a signify
noon, evening, morning, and afternoon respectively ; the numbers in the fifth column
are obtained by division of the quantity of w^ater discharged in the given period of
time by the corresponding number of hours.

HELIANTHUS ANNUUS. AUGUST, 1881.

Date.

Hours,
from — to.

Weather.

Temperature-
degrees R. in
the earth.

Water
discharged
per hour-
cubic cm.

i
Remarks.

25 Aug.

I2in. — 7e,

Sunny

8-5

7 e. —8 m.

—

H

2.46

Night.

26 Aug.

8 m. — 92 m.

—

13

14

Maximum.

—

9I m. — 12 n.

Clear

16.5

12.8

—

12 n. — 4 6.

19

2-5

Transverse section of

—

46. — 6 e.

17

2-5

wood washed, at once
set in progress again.

27 Aug.

6 e. — 54 m.

Rain

1.6

Night.

51 m. — 8 m.

■ —

J3

7-2

1

8 m. — 9 m.

—

13-2

14

Maximum.

—

9 m. — II m.

—

13.2

13

—

11 m. — 3 a.

—

135

12

3 a. — 7e.

—

135

8

28 Aug.

7e. -5|ni.

—

12

3-6

Night. j

5| m. — 8 m.

—

12

8

8 m. — io| m.

Cloudy

12.3

10-4

io| m. — 12 n.

—

12.8

10.6

Maximum.

12 n. — 4 e.

—

1 3- 1

8-5

■

4 e. — 5 e.

—

13

6

5 e. — 7 e.

Rain

13

7

29 Aug.

7 e. — 6^ m.

Clear

9-5

2.6

Night.

—

5^ m. — 8 m.
8 m. — 9 m.

— ■

10

.V6

—

II

8

_

9 m. — II m.

—

13

9

Maximum.

_

II m. — 3 a.

—

15

8.2

—

3 a. —5 6.

—

14-5

5

5 6. -7 6.

—

13

4-5

30 Aug.

7 6. - 5l ni.

51 m. — 8 m.

Cloudy

10

2-5

Night.

—

10-5

5-2

—

8 m. — 10 m.

—

12.5

13.5

Maximum.

10 m. — 12 n.

—

15

5-.=;

_

12 n. — 3 a.

Rain

16

5-7

—

3 a. -5e.

~

15

3-5

What is conspicuous at once in our table, and has been known for a long time,
are the continual fluctuations in the quantity of water hourly discharged. These take

FLUCTUATIONS IN THE DISCHARGE OF WATER 273

place in such a manner tliat at a certain hour daily — in the present case between
8 and 1 1 o'clock — a maximum of the discharge occurs ; from thonce onwards
the quantities hourly discharged decrease into the night, to increase again towards
morning. It appears, however, that even with the same species of plant the hour of
the maximum may be considerably retarded, since another observer, who likewise
investigated Helianthiis animus, but grown in a flower-pot, gives the time of maximum
discharge as between 12 and 2 o'clock, and in general the statements of various
observers differ considerably on this point. Even in the same specimen, the hour
of the maximum discharge may alter on successive days, as our table in part shows ;
still for myself I always hold it to be important that a series of observations made
in i860, under peculiarly favourable conditions, on a plant of the common Potato S
likewise rooted in the open land, gave results which agree with our table in all
essential points, especially in that the maximum discharge always occurred in the
forenoon. The most remarkable fact in this periodicity lies, however, in that
it is independent of small fluctuations of temperature, and even of considerable
fluctuations of the moisture of the soil, to such an extent that in spite of these
it is still perceptible. Thus our table shows, on August 26, a maximum dis-
charge of fourteen centimetres between 8 and 9 o'clock in the morning, the tempera-
ture being only 13° R.; while in the afternoon, between 12 and 4 o'clock, at 19° R.,
only 2-5 cubic centimetres were excreted, and similar relations occur repeatedly in
the table. In like manner the quantity of water contained in the earth is not
strictly proportional, since even when the soil contains less water a larger outflow
may take place ; however, numerous experiments in this direction teach that a drying
up of the soil to any great extent, as well as cooling of the roots, influences the
quantity of outflow. It has been attempted to represent this periodicity in the
activity of a root-stock as a consequence of the preceding daily periodicity of
illumination, so long as the plant was still intact — an assumption which is hardly
supported by the existing series of observations, and from general grounds possesses
little probability in itself. Meanwhile the cause of the daily periods is simply
unknown.

The inspection of our table yields the further result that the quantity of
discharge at the time of the maxima is greater during tlie first two days than in
those which follow ; although on the 30th of August, that is on the fifth day, it
rises again nearly up to the original height. On the other hand, the minimum hourly
discharges in the later days are rather greater than at first. In other words, as
the experiment continues, the diff"erence between the daily maximum and minimum
diminishes; as is also to be gathered from my old observations of i860, on the

^ The older literature on the weeping of the root-stock, as well as the facts from which our
more recent view as to the mechanism of this process has been gradually developed, are found in the
following works. Hofmeister—' Über Spannung, Ansfliissmcngc und Ausflussgcschwindighcit von
Säften lebender Pflanzen ' (Flora, 1S62, pp. 97, &c.). Sachs' ' Experimental Physiologie,' 1865, VII.
I established the foundation for an understanding of the transpiration-current as well as of the
excretion of water in the liquid state in my oft-cited treatise, ' Über die Porosität des Holzes' :
the obscurity prevailing in Pfeffer's ' Pflanzen-physiologie' (18S1), in the section on the move-
ments of water in the plant, might have been avoided if the author had suniciently regarded
this treatise. With respect to my observations on the Potato plant mentioned above, they are
tabulated in mv ' Experimental Physiologie' (p. 210).
[3]

2 74

LECTURE XVT.

Potato plant mentioned above. All these relations come out clearly, moreover, if
the observations are graphically represented on a system of co-ordinates, in such
a manner that the days and hours of the observation are expressed on an abscissa
line, while the quantities discharged per hour appear as ordinates. The curve so
constructed facilitates a survey of the periodic fluctuations more than is the case with
a tabl .

The liquid flowing out from the transverse section of the wood is by no means
pure water, but always contains small quantities of those salts which the roots
absorb from the soil. With appropriate reagents, it is easy to detect potassium,
phosphoric acid, sulphuric acid, and Hme in the discharged water; and small

quantities of organic matters, such as sugar
and traces of proteids, may also be found
— substances which have evidently been
washed to a certain extent out of the wood
of the root-stock.

The water flowing out at the transverse
section of the root-stock moves from below
upwards, and must therefore be set in mo-
tion by pressure acting in opposition to
gravitation. This pressure, or, as I have
previously termed it, root-pressure, may,
however, be far more considerable than
is necessary for forcing the water out
of the short stump of a stem. Even
with smaller woody summer plants, as
Sunflower, Potato, Tobacco, Nettle, &c.,
we may convince ourselves, with the aid
of the simple apparatus represented in
the accompanying figure, that the water
exudes from the transverse section of the
wood with a force capable of overcoming
the pressure of a column of mercury of
20-30 centimetres in height. The root-
pressure in old well-rooted stocks of the
Vine is much greater; Hales found, for
instance, that the water is discharged from the transverse section of the wood with
a force which holds in equilibrium a column of mercury more than 100 centimetres
high, and some more recent observations show still greater energy on the part of the
root-pressure.

If we now attempt to obtain an idea of the nature of this pressure, it is to be
noticed above all that the water at the transverse section of the wood wells forth
from the vessels and wood-cells, as may be directly observed. This water, how-
ever, is evidently taken up from the soil by the root-hairs, or by the outer cells of
the absorbing roots generally. Hence the problem to be answered may be stated
in this form : how, by means of absorption by the external cells of the root, can the
water obtain admittance to the cavities of the wood, and be pressed upwards in these

Fig. 211.— Apparatus for observing tJie force with which
■water is extruded from tlie root-stocli at the cut surface of tlie
stem r. The glass tube R is first tightly connected with the
root-stock, and the manometer tube r then fixed firmly, by
means of the cork k. R is then completely filled with water,
and closed by the upper cork k. Finally, mercury is poured
into r, so that g' stands higher than g from the first. The level
g' ascends above g according to the magnitude of the root-
pressure in each case. This apparatus is much more conve-
nient than those formerly employed.

ROOT-PRESSURE AND ENDOSMOSE.

275

with great force? I attempted, in my 'Handbook of Experimental Physiology,' 1865
(p. 204), to make this problem clear, and partly to solve it, with the aid of the
diagram here reproduced.

Fig. 212 is intended to represent, diagrammatically and simply, a piece of
a young absorbing root. A, A are so many cortical cells of the root, on which we
may suppose protuberances to arise as root-hairs ; these immediately surround
a vessel, B. By means of the dissolved substances contained in the cells A, the
water of the surrounding soil is absorbed by endosmose ; this causes the cells, since
they possess a protoplasmic lining to the walls on the inside, to become highly
turgescent. Now, as I have already shown, turgescence in general depends upon
the absorption of water by endosmose taking place through cell-walls which resist
filtration to so great an extent that a strong hydrostatic pressure becomes estab-
lished in the cells. Now we may imagine hypothetically that the outer walls a, while
favourable to the inflow of the water by endosmose, offer much resistance to its filtra-
tion, whereas the portions of wall d, which bound the vessel, may allow filtration
to a greater extent. It is then obvious that
when a cell A is highly turgescent, the
water taken up through a may be filtered
out with force through the wall d ; in con-
sequence of this, the vessel B must be-
come filled with water, and this will
escape above at the transverse section of
the root-stock, since the vessel is closed at
the root-tip below. Since we know now
that the turgescence of a cell may be equal
to a pressure of several atmospheres, it is
intelligible that the water enters from the
cells A into the vessel B with a pressure
which is able to drive it up to a height
of 10-15 metres. To a certain extent,
this argument may be demonstrated by an
artificial apparatus. In the accompanying fig. 212.

figure (Fig. 213), 0 indicates an artificial cell

composed of a wide glass tube g, g, which is closed at a with a double bladder, and
at b with a single one. After filling z with a solution which acts endosmotically, the
glass tube r is fastened over b by means of a caoutchouc cap K, K. If this apparatus
is now laid in a vessel full of water, the cell z absorbs the water by endosmose
through the double membrane a, and when the turgescence has reached a suflfiiciently
high degree the fluid absorbed through a again filters out through the single mem-
brane b, which offers a smaller resistance to filtration ; it then collects and ascends
in r. Incomplete though this apparatus may be, it at any rate illustrates the funda-
mental ideas on which our conception of the essence of root-pressure depends,
and in the sixteen years which have elapsed since my publication quoted above,
nothing better has been put in its place. I then neglected to mention the various
functions which devolve on the one hand upon the cellulose membrane, and on
the other hand upon the protoplasmic utricle of the cell, in turgescence and

T 2

27^

LECTURE XVI.

filtration. After I had demonstrated the extraordinary capacity for filtration of the
cell-wall, however, the resistance to filtration had to be chiefly ascribed to the
protoplasmic utricle. This, supported by Naegeli's old observations on the
properties of the latter, has subsequently been done by Pfeffer.

Although as a rule the forcing up of water into the stem only takes place to any
considerable extent in copiously branched root-stocks, it is conceivable according
to my explanation that here and there similar effects may also occur in parts of
shoots without roots. All that is necessary is that vigorously absorbing and
turgescent parenchyma cells force out their water into neighbouring vessels, whence
it can be extruded. As a matter of fact I found that cut-off portions of the young
haulms of various Grasses, when stuck with the basal ends in damp sand and
protected from evaporation, allowed drops of water to well forth at the upper

cut ends. According to Pitra's very ex-
tensive publications, leafy branches of wood
plants wiih the leaves immersed in water,
so that the cut end of the shoot-axis
projects, allow water under pressure to
exude from the latter; this, according to
the principle given, appears quite credible,
although I will not deny that my own
researches in this direction have remained
without result.

The forcing up of the water out of the
roots into the stem was formerly considered
to be probably one of the causes by which,
in the transpiration of plants, the water
absorbed from the soil is despatched into
the leaves. Since we know, however, that
the root-pressure is able to lift the water
30-50 feet at most, the ascent of water
in trees 200-300 feet high cannot be ex-
plained in this manner ; recourse can only
^^^- "^- be had to the root-pressure for the explan-

ation of the transpiration-current in plants
of a few metres high. Since I have proved, however, that the transpiration- current
ascends in the walls of the wood-cells as imbibed water, and that any pressure what-
ever from beneath is superfluous, and taking into full consideration the fact that the
water ascending by means of the root-pressure moves in the cavities of the wood, the
view mentioned above must be regarded as untenable ; and the fact that the suction
brought about by transpiration requires far more water than is supplied by the root-
pressure, shows that it is completely unfounded, as indeed was already to be deduced
from the older observations of Hofmeister \ If the leafy top of a plant of the Gourd,
Tobacco, or Sunflower, is cut off above the earth and placed in water, while the

-' ^ That far more water is disposed of in transpiration than the root pressure is able to supply
in an equal time, is shown more in detail by my statements, as well as the older observations of
others, published by Hugo de Vries in 'Ari'. des bot. Inst' (B. I. p. 288).

TRANSPIRATION AND ROOT-PRESSURE.

277

root-Stock is connected with an exit tube, the water disposed of by the leafy top,
as well as the outflow from the root-stock, may be directly observed. This
proves now that the quantity of water required in suction or transpiration at the
top is far larger than that driven into the stem by the root-pressure ; the transpiring
leafy top of a Tobacco-plant, for instance, took up 200 cubic centimetres of water
in the same time that the root-stock discharged only fifteen cubic centimetres.
It follows even from this fact that the transpiration-current cannot be explained
by the root-pressure. Yet more definitely, however, is this conclusion to be drawn
from the fact that, as a rule, no root-pressure at all exists in a plant in which
vigorous transpiration is going on. If the leafy top of a Tobacco-plant, Sunflower,
Potato, or Gourd-plant, &c., is cut off in sunshine, that is while vigorous transpiration

Fig. 214.—-^ a stein of Helianthus annuus (a) r^
tube b,c,d\ the water forced out at c by the ro
same plant (/) placed in a graduated cylinder (£).

:-pressure is measur

er-pot and furnished with an
ed in e. B the top of

and ascent of water are proceeding, and a glass tube is at once placed on the
stump projecting out of the earth, and if a quantity of water is then poured into
this, it is observed that the latter is sucked into the wood of the root-stock.
This proves that at the time of transpiration the cavities of the root-stock are
not filled with water, but that they must contain rarefied air, which enables water
to penetrate into the cavities. This fact, again, completely excludes the assumption
that the forcing up of the water by the root can in any way co-operate in the transpi-
ration-current ; it shows, on the contrary, that the phenomenon of the weeping of
the root-stock only comes into existence when the transpiration-current has been

278

LECTURE XVI.

made ineffectual by cutting oft" the top, or when, as in the case of the Vine in spring,
the transpiring leaf-surfaces are not yet present.

However, even in the case of plants in full leaf, when the transpiration of the
leaves is diminished at a low temperature and in moist air— the roots remaining
in a moist Avarm soil — water may be taken up with great energy, and be pressed
forcibly into the leaves, to well out in the form of drops, if the necessary mechanisms
exist. Thus we have the exudation of liquid water from the living and uninjured
plant. The phenomenon is very easily observed when young plants of Cabbage,
Maize or other Grasses, species of Alchemilla and many other plants have been
cultivated in flower-pots, the pots warmed up to 20°-25°, and the leaves protected

from too vigorous transpiration by being
covered with a large bell-glass^. After
a few hours, occasionally even after 15-20
minutes, drops of water appear at certain
places at the apices and margins of the
leaves, especially on their teeth; these
gradually increase in size, and finally fall
off", whereupon new drops become formed.
Such drops of water occur particularly co-
piously at the apices of the leaves of many
Aroids, e.g. Colocasia aniiquorum and
Calla JEthiopica. In the natural course
of events in the open air, the excretion of
drops on the leaves is easy to observe
when, after a hot day, the air becomes
cooler and moister as the sun goes down,
while the earth still retains its diurnal
warmth, and incites the roots to absorb
vigorously. It may then be directly seen
that drops of water are extruded at the
margins of the leaves of Potatoes, Grasses,
species of Alchemilla, and many other
plants. Much of the water which we find
early in the morning on the margins of
the leaves of many field and garden plants
in the form of large drops, and which are generally taken for drops of dew, is
really water excreted by the plants themselves.

That as a matter of fact it is the root-pressure which drives the water forcibly
into the leaves, and expels it in the form of drops at appropriate places, may be
proved by fastening cut-off" branches on the shorter limb of a wide glass tube, as in
Fig. 216, so that the cut surface dips into water, which is then forced into the shoot
by means of mercury poured into the other limb of the tube. If the leaves are

Fig. 215.— The double walled vessel ?', a stands on the tripod
d, d. which is furnished with the lamp /; water is contained
between the two zinc walls of the vessel. The flower-pot ty
with the plant p, stands on a support, i, i. and is covered by
the bell-glass ^, ^, which rests upon the hooks h, k.

' All essential points respecting the e.xtrus'on of drops from leaves known up to that time were
collected in my 'pflanzen- Physiologic' (p. 237), and the excretion of nectar was discussed at the
same time.

EXUDATION OF DROPS OF WATER FROM LEAVES.

279

protected from active evaporation, a pressure of 10-20 centimetres of mercury
generally suffices to render drops of water visible on them after ten minutes, or an
hour, or longer, at any time of the day ; and with branches of Alchemilla alpina, the
common Vine, and others, the excretion of drops in this manner continues for eight
or ten days, so that several hundred cubic centimetres of water drop away. That
the water forced in reaches the leaves through the wood can be made evident by
removing a ring of cortex.

It is generally at specially organised places on the leaves that the drops
appear. These are often the so-called water-stomata (cf. Lecture VIII. p. 119);
in other cases, however, they are ordinary stomata, and occasionally, in Grasses for
instance, mere fissures in the epidermis; or, finally, as Moll has shown ^, the drops
occur even at places on the margins or on the surfaces of the leaves where no
special openings are to be perceived. In Aroideae, Fuchsia, Tropaoluni, Helleborus
m'ger, Primula sinensis, and others, the drops exude through water-stomata; and
here the water-stomata are usually at the margins of the leaf, and especially at
the upper side of the apex of the teeth,
several or many existing together. But
in some plants, such as Platanus, Ulnius,
and Vi/is, according to Moll, the excre-
tion of drops may take place on the
under side of the teeth, although they
possess water-stomata on the upper side.
In Cestrum rosewn, Datura, &c., indeed,
he found drops excreted at places where
no stomata at all exist. In general,
according to Moll, younger leaves are
more apt to exude drops than older
ones, where the organs of exit not rarely
become impassable. If in such cases
water is forced into the shoot-axes, it
penetrates into and fills the inter-cellular
spaces of the mesophyll, and the leaf
becomes injected ; this does not directly
injure the leaves, however, and they re-
main living when the injected water has

disappeared from the intercellular spaces by evaporation. A peculiar adaptation
to these processes is found in the leaves of the Aroideoe mentioned. Following
the margins, in company with the vascular bundles, are certain wide canals,
which conduct the water up to the extraordinarily large water-stomata at the
apex of the leaf. From these as mafly as eighty-five drops per minute are
said to be excreted at night; and one observer even collected 22-6 grammes

FIG. 216.— This apparatus, already described above (p. 244).
may be employed to demonstrate the extrusion of drops from
leaves by forcing in water at the cut surface of the shoot.

' I have demonstrated the phenomenon of the excretion of drops on the leaves of cut shoots by
pressing water into them from below, in my lectures since 1S69, and occasionally given instruction
in my laboratory on the same subject. Moll has published more detailed researches on this subject
in 'Bot. Zeitg' (1880, p. 49) and in ' Mitt/ici/ungcn der Kgl. Akad. der ]Viss. in Holland''
(Amsterdam, R. II. Th. 15).

28o

'.ECTURE XVI.

of water dropped in one night from the leaves of Colocasia antiquorum. According
to a statement by Müsset, the drops of water forced out may even be projected
to a distance of several centimetres. Whether the excretion of drops has any
favourable significance for the plant, and what, is still unknown ; though, con-
sidering the generality of the phenomenon, it seems scarcely possible to deny
that it must be of advantage in the economy of the plant. Moreover this water
is, of course, not pure water : on the contrary, it contains traces of the salts taken
up by the plant, and occasionally also organic substances as well. On rapid
evaporation of the excreted drops, these matters may remain behind as solid
masses; and in this way are explained the calcareous scales on the margins of

the leaves of many Saxifrages.
This shows, however, that spe-
cial relations of organisation
co-operate here, since they do
not occur in the majority of
other plants \

I have already repeatedly
mentioned in preceding lec-
tures that the relations of or-
ganisation and vital pheno-
mena of highly organised plants
consisting of differentiated
masses of tissue, are again
usually to be met with also in
such very simply organised
plants as in the CccloblasfcB,
which never undergo cell-di-
vision in the course of their
growth. So also the excretion
of drops is observed among the
Fungi, on the sporangia of
species of Mucor. These plants
consist, as was shown at p. 5, of
copiously branched tubes, their
root-portion or mycelium (Fig.
2 1 7 ni) being extended in the nourishing substratum, while thicker tubes grow per-
pendicularly from these into the air, and form the sporangia at their ends. On these
sporangia, when they have attained a height of several centimetres, numerous small
drops of water now appear, which are evidently being forced out through the closed
membrane; the pressure necessary for this, however, is evidently afforded by the
endosmotic absorption of the roots in the substratum, and we have here at once
•the simplest conceivable case of root-pressure, since the entire plant consists of a
single tube, never divided off by transverse walls. Thus it is the parts of the tube

-Phycomyces nitens.

1 With respect to the calcareous scales on the leaves referred to, cf. De Bary's ' Vcrgl. Anat.
der Vegetations-organe' (1877, p, 57).

EXCRETION OF NECTAR. 28l

extended in the substratum which absorb the water, and this is forced out again in
the form of drops on those parts which protrude above the substratum. Hence, how-
ever, it follows that the absorbing root-hyphse must offer more resistance to filtration
than the walls of the sporangium, and we have here at once a case which supports
my theory, adduced above, of the causes producing root-pressure. IMoreover other
Fungi, which consist of ordinary segmented hyphse, can also excrete water on
their fructification above the substratum ; this happens, for example, with the com-
monest of all mould-fungi, PenicilUum glauciwi. When it fruits, after forming
a dense membrane on the surface of a nutritive fluid, numerous drops of fluid
collect on the upper side of the fungus membrane. The Fungus Merulius lachry-
mans, which is found in cellars and is so exceedingly destructive, has evidently
obtained its name from a similar excretion of water.

The drops of so-called nectar are also to be referred to the excretion of liquid
water. At the bottom of many flowers, at the base of the petals or stamens, or
more particularly on the cushion at the base of the flower, juices containing sugar
are excreted at the time of pollination, which are collected by bees and serve
for the formation of honey. These juices are distinguished in Botany as nectar.
Similar organs — i. e. nectaries — also occur however on other parts of plants, as on
the under side near the base of the lamina of Prunus lauro-cerasus, and right
and left on the petiole of Prunus avium, &c., also on the stipules of the common
field bean ( Vicia /aba). These nectaries excrete juices containing sugar when the
parts of the plant concerned are highly turgescent. The arrangements referred
to are especially conspicuous in the common Crown imperial {Fritillaria wiperialis),
where large, clear drops of nectar are excreted from rounded somewhat depressed
spots at the base of the petals; when these drops are shaken off", they become
replaced by others, and this even when the flower-shoot is cut off and placed in
water, whence it follows that in this case root-pressure is not needed, but rather
that masses of tissue exist in the neighbourhood of the nectary, from which the
water and its saccharine contents are excreted — a fact which is also to be observed
in other flowers. According to Wilson's researches, the renewal of the drops is
suppressed when the external surfaces of the nectary are washed with water;
they re-appear, however, if a very small particle of sugar is laid on the surface of
the washed nectary. The attraction of the sugar evidently then acts by exosmose
on the water contained in the tissue of the nectary; and we may well assume
with Wilson that even in the normal condition of the nectary, the solution of
the sugar already excreted affects the cell-sap of the nectary osmotically, especially
when the sap already excreted becomes by evaporation more concentrated than
the solution in the tissue of the nectary. The excretion of the nectar in the first
instance, however, must depend upon other causes: either, as has been supposed^,
the outer wall of the nectar-cells becomes transformed into a soluble product, as
in the coUeters of leaf-buds, or, in so far as this is not the case, a pressure similar
to the root-pressure forces out the sap.

' Respecting the mechanics of the secretion of nectar, cf. 'The Cause of the Excretion of Water
on the Surface of Nectaries,'' by Dr. W. P. Wilson (Cambr. Mass., U. S. A.). The matter, however,
needs further investigation.

LECTURE XVII.

THE NUTRITIVE MATERIALS OF PLANTS.

In my 'History of Botany' I have shown' how, resuming the teaching of
Aristotle, Caesalpinus, so early as the sixteenth century and before any clear idea of
chemical science was possessed, attempted to obtain an insight into the mechanism
of the nutrition of plants ; and how in the seventeenth century van Helmont
undertook the first scientific botanical experiment calculated to give information as
to the sources of the food of plants. Malpighi, the founder of vegetable anatomy,
as early as 1671 made the statement that the proper nutritive organs of the plant
are the green leaves, and that these take up food from the air with the co-operation
of light. Hales, the founder of the mechanics of the movements of the sap (1727),
was much occupied with the view that a large portion of the vegetable substance
is obtained from the gaseous condition by condensation; and much useful know-
ledge of various kinds was collected later, especially by Du Hamel (1758). With
the foundation of modern scientific chemistry by Lavoisier, at the end of the last
century, the theory of the nutrition of plants also at once assumed a more definite
form. Ingenhouss proved (1779) that the most abundant constituent of every
vegetable body, carbon, originates from the carbon dioxide of the atmosphere, and
a short time afterwards Theodore de Saussure not only established this fact for
all time, but it was he also who first clearly perceived the true significance for
the production of vegetable substance of the mineral matters taken up from the
earth by the roots. From the appearance of his classical work in 1804, up to the
beginning of 1840 or so, when Justus von Liebig and Boussingault made renewed
and zealous studies of the nutrition of plants, there lies a barren period; but,
stimulated by these men, the study of the nutrition of plants has been pursued
during the last forty years with great zeal and excellent results. We may in fact
say that few departments of science have been cultivated more patiently and with
better results than this. The complete revolution which rational agriculture and
forestry have experienced through the establishing of the theory of the nutrition of
plants, proves how much has been accomplished in this department. It would
extend far beyond the scope of this lecture to reproduce even briefly the substance of
the literature of the subject. The most significant result of the development pf

' Cf. my ' Geschichte der Botanik^ München, 1S75.

ARTIFICIAL CULTURES. 283

.the nutrition theory mentioned is met with, however, in the fact that we are
now able to rear plants artificially — that we are in a position, with chemically
pure water to which we add some few chemically pure salts, to rear artificially
highly developed plants as well as the simplest Algae (and. mutatis mutandis,
also Fungi) — that from inconspicuous and often scarcely ponderable quantities of
vegetable substance, quantities of it as large as we choose may be produced in
this way.

Such being the favourable position of affairs, I regard it as the simplest and
most instructive method to connect the main points of the theory of the nutrition
of plants, so far as they concern the food materials, wäth the description of an
experiment in artificial nutrition made with a highly organised plant. I think that
in this manner the essential and important points come into view more clearly than
with any other mode of exposition. In the year i860, I published the results of
experiments which demonstrated that land-plants are capable of absorbing their
nutritive matters out of watery solutions, without the aid of soil, and that it is
possible in this way not only to maintain plants alive and growing for a long
time, as had long been known, but also to bring about a vigorous increase of
their organic substance, and even the production of seeds capable of germination*.
Much disputed at first, this method of artificial plant-culture has been more and
more developed and perfected by various experimenters during the last twenty
years, so that it now affords one of the most important aids to the study of the
various questions of nutrition. The description of the ways and means by which
such an experiment is arranged and runs its course, in itself gives a clear illustration
of the most important processes of nutrition in a highly organised plant.

We will assume that we have to do with the Garden Bean {Phaseolus) or the
Field Bean {Vicia Faba),oxM-a.\ze {Zea Mays), or Buckwhea-t {Polygonum /agopyrum),
plants with which the greatest number of experiments of this kind have hitherto
been conducted, although others, e.g. the Cabbage {Brassica) and Horse-chestnut,
&c., are also suitable for the purpose.

It is best to allow the seeds of these plants to germinate in a box filled with
well-washed damp saw-dust, until the radicle is several centimetres long. After
the seedling has been carefully taken out and washed, it is fastened into a perforated
cork, Ä', as in Fig. 218, so that only the root dips into the water, N, of the vessel in
which the culture is being made. It is then only necessary to be careful that
the portion of the seed filled with reserve materials, ^ — the endosperm or the
thick cotyledons according to circumstances — remains sufficiently moist, without
however being submerged in the water. The apparatus, thus arranged, is placed
in a sunny window or in a suitable green-house ; care being taken however that
the root immersed in the water is sufficiently darkened. This is best accomplished
by placing the glass vessel in a hollow cylinder of cardboard. This precaution
is necessary, not so much because the roots might be injured by the light, as to
prevent green Algse from growing in the water and settling on the surfaces of the root.

' Cf. 'Die iand7uir(Asi:haßt. Versuchsstationen,' Dresden, H. VI. i860, p. 219; and Bot. Zeitg.
i860, p. 113; and, further, Pfeffer's ' Pßamenphysiologie,' i88i, I. p. 253.

3^4

LECTURE XVII.

I assume the vessel in which the culture is made to have been first filled
with pure distilled water. In this case the seedling would nevertheless at first
grow vigorous and healthy, and, if w-e are dealing with large seeds of Beans or
Maize, 'produce three or four normal large leaves in the air, and several dozens
of lateral rootlets in the water. These organs arise and grow at the expense of
the materials — the so-called reserve materials — which were already present in the
seed. Soon, however, the moment arrives when these materials have disappeared
from their reservoirs, and are completely used up : there is then found in the
endosperm of the Maize seed, or in the thick cotyledons of the Bean, nothing
more of the starch and proteid matters previously abounding, and which in
these cases chiefly represent the nutritive materials reserved for the first stages
of germination. Microscopic examination would show
also that, in the roots, shoot-axes and leaves, the
stores of organic plastic materials are exhausted.
So far, therefore, the mere taking up of water by the
roots has sufficed for the construction of a number of
new plant organs, at the cost of the organic substance
stored up in the seed. The seed also contains a quan-
tity of mineral matters— compounds of potassium, mag-
nesium, calcium, iron, phosphorus, and sulphur. If
now the plants remain with their roots immersed in
distilled water, they hold out, it is true, for a long time,
but without growing at all considerably, and at last
they perish.

If now several seedling plants of the same kind, e. g.
Maize or Bean, have been allowed to grow up to this
point, we may employ one or several of these indivi-
duals for the decisive experiment. We replace (which
of course is better done previously) the distilled water
by a solution of various salts, known to us from a long
series of earlier experiments. The simplest plan will
be to give here at once in a tabular form the salt
'" solution offered to the plants. I mention the quanti-
ties I am accustomed to use generally in water cultures,
with the remark, however, that a somewhat wide margin may be permitted
with respect to the quantities of the individual salts and the concentration of
the whole solution; it does not matter if a little more or less of the one or
the other salt is taken, if only the nutritive mixture is kept within certain limits
as to quality and quantity, which are established by experience. Thus the following
would be a useful solution : —

Water . . ■ •

Potassium nitrate ....

Sodic chloride (common salt)

Calcium sulphate (Gypsum)

Magnesium sulphate

Common calcic phosphate (finely pulverised)

,000 cc.m.
I gramme.
0-5 »

0-5 »
o-.f-, „

CHLOROSIS.

285

The calcium phosphate, last mentioned, is but sparingly soluble in the solution
and forms a sediment during the course of the experiment.

The result of this proceeding becomes evident after a few days, in that the
plants appear more vigorous as contrasted with those the roots of which have
remained in pure water, and in that new lateral roots grow from their roots, and
new leaves from their buds. After some time, however, when the third or fourth
leaf of our experimental plant is unfolding, a diseased condition appears. The
new leaves which develope henceforth remain completely white, that is, they
produce no chlorophyll ; and microscopical investigation shows, as Gris first
observed in similar cases, that no chlorophyll grains are present in the protoplasm
of such colourless leaves. This then is a proof that our mixture of nutritive
materials still lacks something, and from the older observations of Gris, we
know that the disease of our plants — so-called Chlorosis — is due to a want of
iron'. Our experiment has thus taught us at the same time how this disease
(which occurs not rarely also under ordinary conditions of growth, e. g. in Robinias,
Horse-chestnuts, and other plants) may be produced artificially. However, the
curing of this disease may also be efi"ected at once : it suffices to add an
extremely small quantity of a soluble salt of iron, e. g. a few drops of solution of
iron chloride or ferrous sulphate, to the water which the roots take up, in order
even after forty-eight hours (or, according to circumstances, after three or four
days) to see the white leaves of the Bean or Maize becoming green : after
several days they are quite normally green. The action of the iron on the
formation of chlorophyll may be demonstrated even more simply; if a very
dilute solution of a salt of iron is painted with a camel-hair pencil on the
surface of a chlorotic leaf, the formation of chlorophyll in the leaf begins at
this place after one or two days, and gradually extends thence. It may here
be mentioned that I made auger-holes in the stem of an Acacia, the foliage
of which was completely chlorotic, and placed funnels, filled with solutions of
iron, in the holes ; the solution of iron carried up from the auger-hole through
the wood into the leaves of the next branch, in a few days caused the leaves
of this branch to become completely green, while the others remained chlorotic.
These experiments evidently prove that iron is necessary for the formation of
chlorophyll, but they do not show whether the iron forms a constituent of the
green colouring matter itself: it is possible, according to recent statements, that
the latter is actually the case. It must be mentioned further that only an ex-
tremely small quantity of iron is necessary : several milligrammes of a soluble
iron salt per litre of nutritive fluid are quite sufficient to remove the chlorosis
of the plant under experiment, and larger quantities of iron act as a poison
on the roots as well as on the leaves. At the same time, however, the course
of our experiment shows how important these small quantities of iron are for
the total life of the plant: since if they are withheld the white chlorotic leaves
perish after a time. The feeble nutrition by means of the first two or three
green leaves of the plant does not suffice to bring about any considerable growth ;

* The literature of these and other facts appertaining here is quoted in my ' Expcrimeiiial-
physiologie^ 1S65, p. 141, &c.

286 LECTURE XVIT.

and the plant which is supplied with a nutritive solution devoid of iron perishes
at length, simply because it is not able to form the proper organs of assimilation,
the chlorophyll-grains in the cells of the leaf, and when these are wanting the plant
is also unable to produce organic substances.

It is quite unnecessary, and in no way advantageous, to leave the roots of the
experimental plants continually in the nutritive solution to which the iron has been
added ; experience shows rather that the plants flourish excellently if their roots are
placed now and again for a few days in pure water, or still better in an almost saturated
solution of gypsum. The roots, usually not perfectly healthy in the nutritive solution,
then grow again vigorously, and numerous new lateral roots are formed ; and when
this has happened the plant may again be placed in a new nutritive solution. We
may also proceed so that not all the salts given in the above table are simultaneously
presented to the roots; the salts may be distributed in two or three fractionated
solutions, and the plant allowed to absorb alternately the one or the other of them.
In this case also vigorous vegetation is maintained, and indeed I made the first
successful experiments on nutrition in this manner.

The plants, then, being treated in the way described, new leaves and roots are
continually developed, flowers appear after six or eight weeks, and it is only necessary
to provide for the pollination of the female reproductive organ to produce fruit.
As a rule it suffices to let the plants stand at the open window or outside during
the time of flowering ; insects then provide for the transference of the pollen from
the anthers to the stigmas in the usual manner. The ovary then swells, and
after a few weeks a greater or less number of fruits are formed containing seeds
capable of germination. With the Maize, I have frequently obtained in this
manner heads of fruit with more than loo grains capable of germinating, and
with Beans 6-10 pods with 12-20 ripe seeds. If to the weight of these seeds
that of the organic substance of the shoots, leaves, and roots produced in our water-
culture is reckoned, it is shown that the organic substance, in comparison with that
of the seed from which the experimental plant was developed, may amount to three-
hundredfold and more.

We have thus by artificial nutrition raised a plant which has gone through
the whole course of its development, and we may employ its seeds for a second
experiment of the same kind.

From this experiment it follows that the materials which we have placed at the
disposal of the plant completely suffice for the whole process of nutrition, and any
other addition of material is entirely superfluous. It should be mentioned here,
indeed, that the common salt specified in our table, or any other chlorine compound,
may be omitted, since numerous culture experiments have shown that sodium
and chlorine are, strictly speaking, superfluous in nutrition. On the other hand,
the presence of common salt or some other chloride is of use, in so far that the
nutritive solution can thereby be prevented from becoming alkaline ^ ; since by
the absorption at the roots the chemical composition of the nutritive solution
becomes altered more and more in time. Of course this can also be avoided by
completely renewing the entire nutritive solution every three or four days.

Cf. Pfeffer, ' Pßauzcnpliysiologie^ I. p. 254.

SELECTIVE POWER OF ROOTS. 287

This opportunity may be taken of remarking that the roots possess a sort
of selective power, as it has been called, as to quantity at any rate, even if not
as to quality. That is to say, the roots absorb dissolved matters of the most various
kind, it is true, even those which are injurious, and thus are not able to reject any ;
but on the other hand it was shown by De Saussure, and has been confirmed by more
recent investigations, that the roots absorb the nutritive salts at their disposal by no
means in the same quantitative proportions as those in which they are met with in the
mixed solution. If, for instance, the solution contains more calcium than potassium,
the subsequent analysis of the plant-ash may yield more potassium than calcium, and
so forth. This fact, depending upon a power of quantitative selection on the part of
the plant, is evident in the thousands of ash analyses which have been made of the
most various plants ' developed in the open. Plants of different species which have
grown close together on the same soil, or in the same water, exhibit totally different
combinations in their ash, — the one having much, the other little, of calcium com-
pounds ; in the one magnesium and calcium predominate, in the other potassium,
compared with the quantity in which these substances are contained in the soil or (in
the case of water-plants) in the water. The latter is particularly conspicuous in marine
plants. While sea-water contains 3°/o of common salt, and only very small quantities
of salts of potassium, magnesium, and calcium, the salts last named nevertheless pre-
dominate in the ash of these plants, and it contains relatively but little of the common
salt of the sea-water. Plants thus take what they require from a mixture of salts,
without particular reference to the composition of the nutritive mixture. But, again,
even this does not exactly occur in the sense that they only and exclusively take up
just so much of each material as is necessary for nutrition ; since in the ash of plants
of the same species are found more or less potassium, calcium, magnesium, phos-
phoric or sulphuric acids according as they have grown on this or that soil. The
composition of the ash of a species of plant is thus not specifically constant, but is
dependent, at least in part, upon the composition of the soil or the water. Least
variable in this respect are, of course, the fruits and seeds : in these are collected
only the materials specifically necessary for the seedling, while in the shoots and
roots superfluous matters also may be lodged.

The question still remains, however, whether all the salts which we have added to
our nutritive solution are actually necessary for the plant. These salts were, potassium
nitrate, calcium sulphate, and magnesium and calcium phosphates; we have already
learnt that the iron salt is indispensable, and recognised the incidental significance
of the sodium chloride. The question raised may be in the first place simplified by
regarding as the essentials, not the salts named as such, but the elements contained
in them. As numerous investigations in this direction show, it is simply necessary
that the nutritive mixture should contain the elements potassium, calcium, magnesium,
iron, phosphorus, and sulphur, in suitable neutral combinations containing oxygen.
For reasons the explanation of which would here carry us too far, however, it is best
to choose the salts given in our table ; and it is at the same time certain that land-
plants, as well as water-plants, as a rule, find and take up just these very salts in their
environment, since potassium nitrate, gypsum, magnesium sulphate, calcium phosphate,

' Emil Wolff, ' Ascheiuvialyscn von landwirthschaftlichcn rrodiic/cu," Berlin, 1S71.

288 LECTURE XVII.

and some compound of iron, are found in every soil in which plants grow, and in
the water of rivers, wells, and seas, and are undoubtedly used by the plants.

Mere chemical analysis, by which the presence of the materials in plants
is proved, would by no means alone suflfice to show whether all the substances
detected by the analysis are really indispensable for the nourishment of the plant.
As a matter of fact, numerous other elements in addition to those mentioned
have been discovered now and then in plants, occasionally indeed in considerable
quantity. I need only mention iodine in marine j)lants, manganese in many land-
plants, and the very considerable quantities of zinc in plants which grow on
calamine soils, but which in other places flourish just as well without zinc. One
of the commonest phenomena of this kind, however, is the accumulation of silica in
the tissues of very many plants, although we may regard it as certain that this
substance is superfluous for the chemical processes of nutrition, as well as for the mole-
cular movements connected with growth. Among the plants in the epidermis of which
particularly large quantities of silica are deposited, we find, besides the Equisetacese
and some others, the Grasses, and among these the Maize. Although in such plants
half of the total ash often consists of silica, I (and subsequently other observers
also) nevertheless succeeded in bringing Maize plants to vigorous and complete
development with the help of nutritive solutions to which not even a trace of silica was
added ^ ; the small traces of silica detected with difiiculty in these experimental plants
could only have been derived from the glass walls of the vessels, or from the dust.
Although it is thus shown that this substance, widely distributed in plants, is
superfluous for the purposes of nutrition and growth, it would be going too far
to regard it as superfluous also in every other connection. The silica is generally
deposited in the outer walls of the cells of the epidermis of the leaves and stems,
i. e. of the transpiring organs : it is conveyed there with the water of nutrition, and, on
the evaporation of this, remains behind in the outer cell-wall. By burning the latter
on platinum foil, especially with the addition of sulphuric acid, the silica is obtained
in the form of so-called skeletons, that is, thin plates which still possess with great
exactness all the finer structure of the epidermis cell-walls, and especially of the
hairs. The form of these skeletons shows that the molecules of silica have been
regularly deposited in the cell-walls, between the molecules or micellae of cellulose ;
the walls of vessels, wood-cells, and parenchyma cells in old leaves (e.g. of Oaks
and Beeches in the autumn), moreover, aff"ord exquisite siliceous skeletons. Among
the most remarkable phenomena of this kind is the deposition of silica in the cell-
walls of the unicellular Algae of the subdivision of the Diatomaceae, of the siliceous
skeletons of which the well-known silica deposit of Bilin in Bohemia, for example,
consists. It is scarcely probable that these Algae could flourish in a medium devoid of
silica, and whether the Equisetums and Grasses which abound in silica would maintain
themselves permanently if the supply of silica were cut off is more than questionable,
although it is established that this substance is superfluous for the processes of
nutrition and growth, and quite certain that the exceedingly fine masses of silica
in the epidermis of these plants scarcely at all afiects the elasticity and rigidity

' Cf. my treatise in ' Anitalcii der Landwirthschaft in den kgl. preuss. Staaten,' Wochenblatt,
t862, p. 1S4.

THE LAYING OF WHEAT. 289

of the haulms and leaves. The somewhat thoughtless assumption that the rigidity
of the haulms of cereals is essentially promoted by the silica which they contain,
impelled agriculturists, thirty years ago, to manure their wheat fields with costly
preparations of silica, hoping thereby to prevent the laying of the wheat — by
laying is understood the giving way of the lower joints of the haulms, especially in
continuous rainy weather, so that whole wheat fields have their haulms laid flat on
the earth before the ripening of the grain. In my Experimental Physiologie (1865),
and still earlier in my lectures, I demonstrated the incorrectness of this view, and
showed that the laying of the wheat has nothing whatever to do with the silica,
but is due to deficient lignification of the supporting tissues in the haulms,
which stand too closely together, shade one another, and so assume the diseased
condition of etiolation, to be described later ; and just those wheat-fields best sup-
plied with food, and the plants of which shade one another to the greatest extent,
are most exposed to laying. It would not now occur to any agriculturist to dig
his gold into his wheat-fields in the form of siHca\ The physiological significance
of the silicification of cell-walls is best illustrated by the observation that in numerous
plants the deposition of silica begins in the hairs, and especially in the prickles,
and extends centrifugally from the base of these to the surrounding epidermis cells.
This is particularly well seen in the Hop, Hemp, Gourd, etc.; and takes place
on stems and leaves already fully grown, or nearly so.

We now know, therefore, that the silica so common in plants, although it
may occasionally be of some advantage, is nevertheless not a nutritive material in
the narrower sense of the word — i.e. it takes no part in the chemical processes
of assimilation and metabolism. We know further, that the small traces of iron
salts are of the greatest importance in the process of nutrition of ordinary green
plants, since without iron the proper instrument of nutrition, chlorophyll, is not
formed. With this, however, our definite knowledge of the physiological significance
of the nutritive substances mentioned is practically exhausted : we know, to put
it shortly, nothing certain as to the parts played by potassium, calcium, magnesium,
and phosphorus in assimilation and metabolism. With respect to sulphur, we
know at least that it forms an indispensable constituent in the chemical composition
of Proteids, and is therefore necessary for the building up of protoplasm. With
respect to the others, however, only so much is established, that they are absolutely
indispensable for assimilation — i.e. for the production of organic plant-substance;
for researches on vegetation show that when even only one of these elements
(potassium, calcium, magnesium, and phosphorus) is excluded from the nutritive
mixture, assimilation soon ceases, and the production of organic plant-substance
proceeds no further. The salts of these elements, therefore, certainly take part in
the chemical processes which occur during the formation of organic plant-substance
from inorganic material, though we do not know what part they play in it.

Moreover, it is also not to be forgotten that the elements mentioned, or
their compounds, take part not only in assimilation itself, but also in the meta-

' With respect to silica in plants, and the fact that the so-called laying of wheat is caused by
shading and etiolation, and not by want of silica, as hitherto supposed, what is necessary is stated
in my ' Experimental-physiologie^ 1865, P- 150-
[3]

290 LECTURE XVII.

holism which occurs during growth, etc. In this connection, for example, lime often
plays a similar part to silica : just as numerous plants have their cell-walls silicified,
others may be calcified by the deposition of large quantities of carbonate of lime,
and thus become stone-hard, as the Melobesiacese and Corallineae among the Algae ;
and there is scarcely room for doubt that all, or at any rate very many older cell-
walls leave behind on combustion an ash containing lime, and occasionally, as in the
vessels of the Gourd, it is even possible to obtain chalk skeletons by combustion.

Without wishing to enter more in detail into the thousand-fold investigated
ashes of plants, it will still be well to bring forward two facts concerning them.
In the first place, every part of a plant, every cell-wall, even the youngest, and
likewise the protoplasm, and even starch-grains, leave an ash behind after com-
bustion; and from their general occurrence it may be concluded that certain
constituents of the ash at least are indispensable for the chemical composition
or for the molecular structure of the cell-walls and protoplasm. A second fact
worth noting is the abundance of ash-constituents in the green assimilating leaves :
this is at once explained when we know that these constituents are continually
being carried by the transpiration-current to the leaves, and are retained in them
on the evaporation of the water, and when we observe that no assimilation occurs
in their absence, as experiments on vegetation demonstrate.

The quantity of ash, however, as a rule, amounts only to a small proportion
of the plant-substance, reckoned without water. It usually varies between lo/o and
iqo/o; the wood and seeds belonging to the parts poorest in ash — the former evidently
because the ash constituents do not need to be accumulated in it for the purposes
of metabolism, but are conveyed to the organs of assimilation. The poverty
of seeds in ash-constituents, on the other hand, is explained on reflecting
that the seeds take up water laden with mineral matters when they germinate.
Finally, ash analyses show that the quantitative composition — i. e. the relative
richness in potassium, calcium, magnesium, and phosphorus — changes according
to the nature and the age of the organ of a plant. Ripe seeds, for example,
usually contain relatively much potassium, magnesium, and phosphorus. Since,
however, it is not possible to bring the facts so far known as to the composition
of the ash into immediate connection with definite physiological functions, we will
now leave this subject for the present. ^

Turning once more to our water- cultures, we have now to take into con-
sideration another most important question. Our plants, as we have already seen,
consist only to a very small extent of the substances which were dissolved in the
water, and which have been absorbed by them. In the main, the substance of
our experimental plant consists, like that of any other plant, of cellulose, proteid
substances, small quantities of fat, and a few other organic matters. Besides
nitrogen and sulphur, which are contained in the proteid substances of the
protoplasm, the latter, as well as the cellulose and all organic substances employed
for the normal construction of the plant, contain hydrogen, oxygen, and carbon.
The source of these elements of the organic substance, so far as the hydrogen
and oxygen are concerned, presents no difficulties whatever : these are in fact the
elements of water, which the plant absorbs in such enormous quantities, and which
permeates all parts of it, however small. The sulphur in the proteid substances,

ORTGIN OF THE NITROGEN. 29 1

where of course it only occurs in very small quantity, also presents no difficulty
■with respect to its origin : we added it in fact to the nutritive solution in the form of
calcium and magnesium sulphates, and if we now find it also in the proteid sub-
stances of the protoplasm of our plants, no longer in the form of sulphuric acid
but as an elementary constituent in the formula of proteid substance, there is little
to be surprised at, since we know how easily sulphur compounds may be
decomposed. We also find in the micro-chemical investigation of our plants, as
a residuum of this decomposition, crystals of oxalate of lime, the calcium of which
passed into the plant in the form of calcium sulphate.

Of the elements of which protoplasm, cellulose, and the other organic com-
pounds produced by the plant consist, the nitrogen and carbon still remain to be
considered. Our table (p. 284) shows that in the water of the nutritive mixture, a
salt containing nitrogen, viz. potassium nitrate, is contained, and indeed predominates.
The nitrogen in the proteid substances of the plant is, as we may thus assume,
the same nitrogen which we presented to the roots in the form of saltpetre. On
the decomposition of this salt, its nitrogen must have become a constituent of the
proteid substances newly produced in the plant. At any rate the active vegetation
of our experimental plants, and the very significant increase of substances of a proteid
nature which had been especially stored up in the gathered seeds, show that plants
are able to take up from nitrates as much nitrogen as is necessary for the formation
of the proteid substances of a vigorous plant.

Theodore de Saussure had already proposed the question whether the nitrogen
of the atmosphere (which, as is well known, consists of about ^ nitrogen and \ oxygen)
is not used by plants to produce their nitrogenous compounds. Thirty or forty
years ago Boussingault endeavoured to decide this question by a long series of
exceedingly careful experiments on vegetation, and came to the conclusion that
the atmospheric nitrogen is not employed in the process of the assimilation of
plants. His experimental plants in each case grew vigorously and produced proteid
compounds when he presented to their roots, in addition to the other nutritive sub-
stances, various kinds of nitrogenous compounds. They grew, on the other hand,
very badly, and their proteid substances did not increase, when such a supply
of nitrogen compounds was wanting ; although the nitrogen of the atmosphere
was at the disposal of the plants so treated. By means of the now much further
developed art of nourishing plants artificially, and especially by means of water-
cultures, we are in a position at any time to afford ocular demonstration of
the important result obtained by Boussingault, by means of simple experiments.
Whenever a sufficient quantity of saltpetre is added to the nutritive solution, the
experimental plants grow vigorously, produce numerous ripe seeds capable of
germination, and give on analysis a corresponding increase of the nitrogenous
substance. If on the other hand the nitrate is withheld from the nutritive mixture,
the experimental plant continues to grow for a time, it is true, making use of the
proteid substances already contained in the seed for the formation of the protoplasm
of its organs ; and this stunted growth may even continue for some time, since the
protoplasm of the first leaves is again dissolved and absorbed from them, to be
employed for the formation of a few new leaves ; but even in this case analysis
gives no increase of the proteid substances.

U 2

292

LECTURE XVII.

The view was once held that it was ammonia salts particularly which,
being absorbed by the roots of plants, or perhaps indeed by their leaves in the dew
and rain water, yielded the nitrogen for the proteid compounds of the plant. But
the result of our water-culture proves, in the first place, that plants are able at
any rate to absorb the whole of their nitrogen in the form of compounds of nitric
acid ; but if, on the other hand, the attempt is made to replace the latter by ammonia
salts, experimental difficulties make their appearance which we will not here discuss
in detail. If, further, it is remembered that the ammonia produced in nature by
the rotting and decomposition of organic remains, especially in vegetable soil, is
easily transformed in the presence of potassium salts into compounds of nitric acid,
which may be detected everywhere in vegetable soils and waters, and that nitric
acid is contained in rain water, although in very small quantities, one comes to
the conclusion that (apart perhaps from certain special cases, and particularly apart
from parasites and Fungi) ordinary green plants obtain the nitrogen for the formation
of their proteid substances, and therefore of their protoplasm, from salts of nitric
acid. It may here be remarked in addition that the soil of gardens, wheat-fields,
vineyards, orchards, and so forth, is usually somewhat deficient in compounds of
nitric acid, and that on this account the production of vegetable substance, although
all other nutritive matters are present in the soil, proves to be relatively small; a
suitable addition of saltpetre is in such cases always calculated to increase the
vegetation to its utmost luxuriance, and if other nitrogenous manures act similarly,
we may assume that they give rise in the soil, sooner or later, to salts of nitric acid,
which are absorbed by the roots.

Finally, the question remains as to the source of the carbon in our experimental
plants. If we suppose the plant, after driving off the water, to be dried at
ioo-i20°C., about half of the total dry substance consists of carbon. This
element in fact is contained in every organic compound, as well as in the cellu-
lose, protoplasm, and fatty bodies which play the most prominent part in plants.
Our plants, nourished with aqueous solutions of salts, could not absorb any
carbon compound, however, from this nutritive solution. Investigation shows,
on the contrary, that they continually excrete small quantities of a carbon com-
pound, viz. carbon dioxide ; and I shall subsequently show that it is entirely
erroneous to suppose that the carbon dioxide which usually abounds in the soil
is absorbed by the roots and conveyed to the leaves. The question for us now,
however, is whence comes this great mass of carbon which is gradually accu-
mulated in the organic compounds of our experimental plants? It is, at any
rate, not derived from the nutritive solution. There remains, therefore, only
one source, the atmosphere — half of the dry weight of the organic substance of
plants must be derived from the atmosphere. This fact now, after two hundred
years of scientific progress, follows with absolute certainty from every experi-
ment on nutrition with green-leaved plants. But the founders of the theory of
the nutrition of plants, Ingenhouss and De Saussure, discovered this fact — one
might almost say the most important fact of biological science — by very different
methods ; and in my ' History of Botany ' I have attempted to show how this
important natural process was gradually discovered by these men, and to what
misconceptions and foolish objections by incompetent minds, even this now in-

ORIGIN OF THE CARBON. 293

disputable fact, like every other great discovery, has been exposed, to such an
extent, indeed, that some forty years ago the weighty word of Justus von Liebig
was needed, before the fact that the carbon of plants is derived solely and simply
from the atmosphere was again completely established. We shall not be concerned
more in detail with how plants obtain the carbon from the carbon dioxide of the
atmosphere until the following lecture : meanwhile the source only of this substance
interests us here, quite apart from the physiological work of the plant. Many
so-called investigators were prevented, for more than forty years after the discoveries
of Ingenhouss and De Saussure, from believing in the atmosphere as the exclusive
source of the carbon, by the fact, known even then, that the relative quantity of
carbon dioxide contained in the air is extremely small. Innumerable analyses of air
have shown, in fact, that in 10,000 litres (that is in 10 cubic metres) there are
contained on an average but 4-6 litres of carbon dioxide ; and since i litre of
carbon dioxide weighs about 2 grams, 10 cubic metres of air contain 8-12
grams of carbon dioxide. This quantity certainly appears very small, the more
so since only j\ of it consist of carbon, and, as we shall see later, the whole
of the oxygen of the carbon dioxide (that is y\) are given off by the plant.
Nevertheless so much is clear, that 10 cubic metres of air contain somewhat
more than 2 grams of carbon, from which, by combination with hydrogen,
oxygen, nitrogen, and sulphur, according to circumstances in each case, 4-5
grams of vegetable substance may be formed. A plant, however, which in the
living state contains 4-5 grams of organic substance, weighs, together with
the water contained in it, about 20-25 grams on the average, and this would
certainly be a rather small specimen. However, not only a plant growing in the
open, but even one in a room, has at its disposal much more than 10 cubic metres
of air, for, on account of the continual movement of the atmosphere, new air con-
taining carbon dioxide is always being conveyed to the leaves ; and on closer
consideration it is seen to depend essentially only upon whether the green leaves
are able to snatch these relatively small quantities of carbon dioxide from the
atmosphere with sufficient rapidity. That they actually do this is shown by the
result— the fact that very considerable quantities of carbon are accumulated in a
plant in a relatively short time. I found, for instance, that a very vigorous
Tobacco plant in the course of 100 days during its development in the garden
absorbed more than 400 grams of carbon from the air, and a Sunflower
fixed more than 800 grams in the same time; quantities which were contained
in several thousand cubic metres of air. Moreover, the further fact that a Gourd
plant, with a leaf-surface measuring one square metre, is able in the course of ten
hours of daylight to produce as much as 8 grams of starch, in which neai'ly
4 grams of carbon are contained, shows that the carbon dioxide of 20 cubic
metres of air has been made use of by this area of leaf-surface in one day ^.

These reflections on the source of the carbon enable us at the same time to
perceive that it would be exceedingly unreasonable to make such experiments on
nutrition as have been described, in a completely closed space to which the air has

1 [See further Sachs, in Arbeiten des hot. Institut zu Würzburg, B. Ill, 18S4, where the
quantities of starch produced are shown to be considerably greater.]

294

LECTURE XVII.

not free access. In this way the necessary supply of carbon would be cut off from
the experimental plant; and without this no formation of vegetable substance
whatever can take place, since all organic compounds contain considerable quantities
of carbon. After these considerations, some readers might be in doubt as to
whether the great masses of carbon accumulated year by year over the whole
surface of the globe in the form of plant-substance are really contained in the
atmosphere. Nevertheless, one may be satisfied of this: the carbon dioxide in
the whole terrestrial atmosphere is to be reckoned, not by litres, but by thousands of
cubic miles; and, as has long been calculated, would provide the total vegetation
of the earth with carbon for many years, even if new supplies of carbon dioxide were
not continually being formed. It is, it is true, an optimistic fable to suppose that
the world is so perfectly arranged that all the carbon dioxide produced by the
respiration of animals and men exactly suffices to equalise the consumption of carbon
dioxide of the whole vegetation of the earth. Nevertheless, an enormous quantity of
carbon dioxide is continually being supplied to the atmosphere by this very respira-
tion of animals, as well as by the processes of decomposition, especially of dead
plants themselves, which are taking place everywhere, and by means of innumerable
springs charged with carbon dioxide, and by hundreds of smoking volcanoes, and in
our time also by hundreds and thousands of smoking chimneys, through which the
carbon of coal escapes as carbon dioxide, so that a lack of it need scarcely be
expected for thousands of years. An approximate idea of how enormously large
the condensation of the carbon of the atmosphere by the assimilation of plants
must have been hitherto, is obtained on reflecting that the deposits of coal, lignite,
and turf, spread over the whole earth, and the bituminous substances as great or even
greater in quantity which permeate mountain formations, besides asphalt, petroleum,
&c., are products of the decomposition of earlier vegetations, which in the course of
millions of years have taken from the atmosphere the carbon contained in these
substances and transformed it into organic substance.

We have not yet done, however, with the consideration of our experimental
plant. Any one conducting such an experiment, and not prepared with a knowledge
of vegetable physiology, might possibly for convenience, or for some other reason,
place the experimental plants with their nutritive solution at the back of a chamber,
or in the middle of an ordinary dwelling-room or laboratory. He would then find,
after a few weeks, that the plants had grown it is true to some extent, and even
that a few fresh leaves had been formed after germination. These plants would,
however, look unhealthy, shrivel up, and die ; and after drying them and weighing
their organic substance, it would be found that the latter, according to circumstances,
is scarcely so large as the weight of the seeds employed, and may be considerably
smaller. That plants cannot flourish under such circumstances is self-evident, how-
ever, on learning from the researches of Ingenhouss and De Saussure, as well as
from our own experiments, that the decomposition of the atmospheric carbon
dioxide in the green parts, and the consequent production of new plant-substance,
only occurs when the organs containing chlorophyll meet with light of sufficient
intensity. By this is to be understood in general, so far as vigorous vegetation
is concerned, all the daylight reflected from, the sky, or, in the case of plants
growing entirely in the open, direct sunhght. The whole nature of the plant is

CONDITIONS AFFECTING THE EXPERIMENTS. 295

adapted to just this intensity of light, and but few species can flourish in the
deep shade of woods. In a word, the experimental plants, to which we have
hitherto confined our attention, can only yield a satisfactory result when artificially
nourished, if they are enabled, by being strongly illuminated, actually to make use of
the carbon dioxide present in the atmosphere. Without the co-operation of light of
sufficient intensity, the carbon dioxide of the atmosphere is as good as non-existent,
so far as the nutrition of the plant is concerned. The light, so to speak, affords to
the cells containing chlorophyll the forces necessary for separating the carbon of the
carbon dioxide from the oxygen, and uniting it at the same time to other elements,
and transforming it into organic plant-substance. Hence it has been said, and
correctly, that it is the sun which maintains vegetable life on the surface of the earth ;
and since the entire animal world derives its food from plants, it may be said that
it is the energy contained in the sun's rays which developes itself in the vital
movements of all terrestrial organisms — animals as well as plants. The mechanical
force contained in the solar rays acts in the chlorophyll-containing cells of the plant
like the movements of a watch-key, by which the spring of the clockwork is wound
up, and the vital processes of animals and plants resemble the slow running down
of the clockwork ; for by these vital processes the organic substance produced in the
chlorophyll-containing cells by means of the light is gradually destroyed again, and
finally re-transformed into carbon dioxide and water, until, on the commencement of
a new period of vegetation, the clockwork is wound up anew.

Referring once more to our experiment on vegetation, after these comprehen-
sive considerations suggested by the results, from another side, we have still to
consider whether the experimental plants, nourished in an aqueous solution, grow
as normally and vigorously as if we had brought them up in good vegetable
soil. The experiment is easily arranged : it suffices to cause several seeds,
of the same species as those employed for the water-culture, to germinate and
develope in vessels containing an equal volume of good vegetable soil, and to
expose these near our experimental plants, and to the same light. It is found
by this that the vegetation is in both cases approximately equal, but in neither
case so vigorous as when the plants are cultivated in the ordinary manner in the
garden. They there meet not only with more direct light, and other advantages
which growth in the open affords, but also, and above all, the roots are able to
develope to more advantage in the open soil, becoming much longer and much more
copiously branched than in our water-culture; moreover they are in contact partly with
air, which they respire, and only partly with fluid, and their root-hairs attach them-
selves, as we already know, to the particles of soil, from which they take up absorbed
food-materials. This is simply the normal condition of true land-plants ; and their
stay in a nutritive solution must necessarily affect their function abnormally.
Nevertheless, the results of our water-culture are scientifically of value ; since they
show that although the roots are compelled to take up the food materials under
abnormal conditions, nevertheless with their help much vegetable substance is formed,
which behaves normally so far that with its aid the whole process of development
of a plant may be completed, including the formation of seeds capable of germination.

LECTURE XVIIL

THE PRODUCTION OF THE ORGANIC SUBSTANCE OF
PLANTS— ASSIMILATION.

Having learnt in the last lecture what are the materials which a normal green
plant must absorb from without, in order to build up its substance from
them, we will now examine the question how and under what conditions the
organic substance is manufactured. Since we know almost nothing as to the ways and
means by which the constituents of the ash co-operate in these processes, we may limit
our considerations to four points. In the first place, the decomposition of carbon
dioxide may be treated of. This furnishes, as we know already, the whole of the
carbon necessary for the manufacture of the substance of normal plants with green
leaves. In the second place, proof is to be given that the cells containing chloro-
phyll— or, more properly speaking, the chlorophyll-corpuscles only — are the organs
by means of which organic substance is manufactured from carbon dioxide. Thirdly,
it is to be observed that the chlorophyll is only effective as an instrument of assimila-
tion when the vibrations of the ether which are perceptible to our eyes as light
penetrate into its substance, and impart to it the forces necessary to produce organic
substance from carbon dioxide and water. Finally, we wish to know what is the
immediate result of the decomposition of the carbon dioxide in the chlorophyll
under the influence of light.

Concerning ourselves first with the decomposition of carbon dioxide, we may
again confine our remarks, for the sake of clearness, to the artificially nourished
plant referred to in the previous lecture. The substance of this plant is combustible,
and after combustion it leaves behind a small quantity of ash, consisting of the
salts taken up by the roots, and which we know to be indispensable for the
production of organic substance. The fact that the organic substance of the plant
is combustible, however, essentially implies that it consists of chemical compounds
poor in oxygen, which are in a condition to become oxidised on the entrance of
atmospheric oxygen at a red heat, and hence to be transformed for the most part
into carbon dioxide and water. Just as organic plant-substance is produced from
compounds rich in oxygen — carbon dioxide and water (with the co-operation of
other compounds rich in oxygen) — so also they may be brought back again into
the form of the same compounds by oxidation. The fact that plant-substance is
combustible, itself implies that in its production from non-combustible compounds

THE EVOLUTION OF OXYGEN BY PLANTS.

297

containing much oxygen a considerable quantity of oxygen must have been hberated,
since otherwise the organic substance could not be combustible.

It is seen from this simple consideration that somewhere and in some manner
oxygen must be separated in nutrition ; and long before these reflections were estab-
lished this evolution of oxygen had been, as a matter of fact, observed. Towards
the end of the last century, Priestly, Senebier, Ingenhouss, and later, with classical
completeness, De Saussure, established the fact that the green organs of plants evolve
oxygen under the influence of light, if they are at the same time in a position to take
up carbon dioxide from without. It is of importance to the history of botany to
make clear how this important fact was gradually established and explained by the
men referred to, and how, for forty years subsequently, almost inconceivable
misapprehensions again obscured the clearly established fact, until more recently
it came to be counted among the indisputable acquisitions of science.

As an essential point, it is at the same time to be insisted upon here that the
volume of oxygen evolved is equal to the volume of the carbon dioxide taken in,
as De Saussure and, later and more exactly, Boussingault, have already established.
This signifies, in other words, that the chemical processes in the cells containing
chlorophyll run their course as if all the oxygen of the carbon dioxide taken up
was evolved. This fact is of the utmost importance, because further consequences
respecting the processes in the assimilating cells can be concluded from it.

Since we are here treating of those chemical processes by means of which the
production of organic substance from inorganic materials (carbon dioxide and water)
originally takes place in the natural course of events, and since this organic substance
affords the constructive material not only for plants but for the whole animal kingdom
as well, it must interest the student of vegetable physiology to observe the process
for himself. This can, of course, only be effected indirectly, and this most simply by
placing water-plants such as Ceratophyllum, Myriophyllum, Udora (Eloded), Potavio-
gelon, etc., in water containing carbon dioxide, and taking care that a smooth cross-
cut is made at the lower part of the leafy shoot-axis. If the vessel filled with water
containing carbon dioxide, in which the shoot is submerged, is placed at a window on
which the sun is shining, a stream of small bubbles of gas is at once seen to spring
from the cut end of the shoot-axis, and to escape upwards. If a glass tube previously
filled with water is inverted over the surface of the water in the vessel, the gas evolved
by the water-plant may be collected, and it is then shown that this gas, if the experi-
ment has been properly made, is very rich in oxygen, but contains also carbon dioxide
and nitrogen. These two latter gases, however, really owe their presence to a falla-
cious arrangement of our experiment : for the bubbles of oxygen evolved by the plant
ascend into the collecting vessel through water laden with nitrogen and carbon dioxide,
and, according to the general laws of diffusion of gases, they must take up carbon
dioxide and nitrogen from the water during the ascent. Although this experiment
with the above water-plants is particularly suitable for ocular demonstration of
the work of plants, a more circumstantial and less obvious mode of procedure
is more convenient for rendering clear the true process of the decomposition of
carbon dioxide. For this purpose, it is better to employ leaves, or portions of the
leaves, of land-plants ; i)ushing them into a glass tube which is cylindrical and gradu-
ated below, and dilated above. Into this tube, closed below with mercury, an accurately

298 LECTURE XVIII.

measured quantity of carbon dioxide gas is placed, together with atmospheric air, and
the whole apparatus is then placed in the sunlight. After some time, perhaps an
hour, the leaf is withdrawn, and the carbon dioxide and oxygen contents of the air in
the glass tube examined, according to known gasometric methods. It now turns out
that the quantity of carbon dioxide has diminished, while the proportion of oxygen has
increased ; and, moreover, it is shown that for each cubic centimetre of carbon dioxide
which has disappeared, a cubic centimetre of oxygen has become free. Such an
experiment, it is true, is little suitable for demonstration in the lecture room, since it
demands at least three or four hours of work, besides corrections and calculations : it
supplies, however, incontestable proof that carbon dioxide is decomposed by means
of a green leaf, under the influence of light, and that an equal volume of oxygen is
evolved in its place. What we here observe, however, in the case of a single small
leaf, or portion of a leaf, is proceeding more or less energetically during the whole
day in all green leaves of plants in the open, even when the sun is overshadowed
by clouds ; and what appears in our experiment as the decomposition of a few cubic
centimetres of carbon dioxide, is represented in the case of a meadow, a corn-field, or
a forest, by hundreds or thousands of litres of this gas.

That it is by means of the chlorophyll of the plant that the absorbed carbon
dioxide is decomposed and its oxygen evolved, follows wth certainty from the fact
that it is only and exclusively those parts of plants which contain chlorophyll that
are able to eifect this decomposition \ No organs devoid of chlorophyll, such as
roots, subterranean tubers, floral leaves and stamens ; no chlorotic white leaves, or
yellow etiolated leaves grown in the dark; and again, no non-green parasites or
Fungi, yield oxygen in the experiment just described: on the contrary, they
continually evolve carbon dioxide from their interior, though in relatively small
quantities. In such organs devoid of chlorophyll, therefore, no reconstruction of
combustible organic substance poor in oxygen can take place; and thence follows
at once that in all plants containing chlorophyll, the organs which are not green
consist of organic compounds which have been produced in the organs containing
chlorophyll, and in like manner that plants generally which contain no chlorophyll,
such as true root-parasites and all Fungi, are necessitated to absorb organic sub-
stance poor in oxygen from without, because they lack the organs for decomposing
carbon dioxide and producing organic substance. Since, however, all animals are
likewise devoid of these organs, and are thus unable to form organic substance
from carbon dioxide and water, although they build up their bodies from such
substance, it follows obviously that the substance of the bodies of all animals is
originally produced in the chlorophyll-cells of plants. The few lower animals which
apparently contain chlorophyll — certain Infusoria, Sponges, and Plaiiarice — contain

' That the green-coloured protoplasm, the chlorophyll, is the organ of assimilation, was first
expressed definitely by me in my ' Experimental-Pkysiologie' (1865, p. 319), in opposition to the
utterly indefinite statements of earlier physiologists, according to which green plants possess a
daily respiration with the evolution of oxygen, and a nocturnal respiration with the formation of
carbon dioxide, and after the doubts which even the distinguished Theodore de Saussure entertained
as to the signification of chlorophyll. I expressly insisted upon the point that ' it is a striking
phenomenon in the history of vegetable physiology that the chlorophyll has not long ago been
distinguished definitely as the instrument for the separation of oxygen.'

CHLOROPHYLL THE AGENT OF ASSIMILATION. 299

chlorophyll as a matter of fact, not as a proper constituent of the body, but, as
Brandt has recently shown, have vegetable cells (Algae) containing chlorophyll in
their bodies : by means of the assimilation of these green bodies, such animals may
be nourished under certain circumstances^.

It matters little in what form the chlorophyll is contained in the cells : whether,
as is usual, in the form of soft granules, or, as in the Conjugatese (a group of Algae),
in the form of green bands or plates, or even simply as masses of protoplasm tinged
with green, as in the Palmellacecp. and some other Algae. It also makes no essential
difference whether or no other colouring matters besides the chlorophyll are present
in the cells, as in the red Algae {Floridece) or the yellow-brown Fucaceae. It was
a fatal mistake of De Saussure's to argue from the red colour of many leaves, such as
the red garden Orache {Airiplex hortensis), that chlorophyll is unnecessary for the
decomposition of carbon dioxide : Cloez, in 1863, first insisted upon the presence of
chlorophyll even in such cases. The whole literature on the nutrition of plants, even
of twenty or thirty years ago, shows how ill established was the knowledge of the
fact that it is the chlorophyll only which effects the decomposition of carbon dioxide :
it was always the custom to say cautiously that green plants decompose carbonic
acid, without more exactly indicating which organ effects this decomposition. In
general, definite recognition of the fact that it is the chlorophyll which is the agent
for the decomposition of carbon dioxide has only made way since the appearance of
my ' Handbook of Experimental Physiology' in 1865 ; and when a more recent author
finds that, properly speaking, it is not the chlorophyll but the cell containing chloro-
phyll which is the organ of assimilation, because the one is contained in the other, it
is somewhat equivalent to saying that the eye is not properly the organ of sight, since,
when taken out of the head, it is no longer capable of seeing. Since no cell assimi-
lates so long as it possesses no green chlorophyll, but does so as soon as it is provided
with it, we are justified in distinguishing the chlorophyll-body itself as the organ which
decomposes carbon dioxide, and consequently assimilates the organic substance. The
most definite proof, however, is afforded by the fact, to be stated more exactly further
on, that the first recognisable product of assimilation appears not in any haphazard
place in the cell containing chlorophyll, but in the chlorophyll-body itself.

I have repeatedly taken the opportunity of referring in these lectures to
the essential co-operation of light in assimilation in the chlorophyll. We are
here in fact concerned with the dependence of plant-life upon the external world,
and in reviewing the different phenomena of vegetation this must never be lost
sight of, if the most mischievous errors are to be avoided. The great importance of
the fact that a plant containing chlorophyll, so long as it does not meet with light
of sufficient intensity is not able to decompose carbon dioxide, and, therefore, is not
capable of producing organic substance of any kind whatsoever, must always be
kept in view. For those not familiar with vegetable physiology a misleading diffi-
culty arises in that plants, in spite of being in dark or feebly illuminated situations, are
able, according to circumstances, to continue living and even to grow vigorously for
some time, particularly when they are well provided with assimilated reserve-materials.

» K. Brandt, ' Uchcr das Ztisammenleben von Thicnn und Algen' (Verhandl. der physiol.
GescUsch. zu Berlin), Dec. 2, 1881,

300 LECTURE XVIII.

The uninitiated student easily draws thence the erroneous conclusion that the growing
plants must also be nourishing themselves. In the following lecture I shall show
more in detail that growth of the organs and assimilation in the chlorophyll are two
processes mutually independent in a high degree. In order that a plant, or certain
of its parts, may grow, it suffices that organic matters assimilated by the previous
activity of chlorophyll are present; and, conversely, a plant may be assimilating
vigorously without growing at the same time. When, therefore, plants grow in the
dark, or with insufficient illumination in general, this takes place at the cost of such
assimilated substances as have previously been stored up in the tissues of the seeds,
tubers, bulbs, root-stocks, or in the cortex of the branches of trees, etc., and which
are now utilised in growth. Moreover, growth with insufficient illumination destroys
considerable quantities of organic plant-substance, in consequence of the respiration
connected with it, as I shall demonstrate in detail in a later lecture. Hence it comes
about that the dry weight of plants growing in the dark is diminished, and this
growth in the dark can therefore only continue until the existing reserve-materials
have been used up.

For the sake of completeness, this opportunity may be taken of incidentally
touching upon another point. As the instrument of assimilation can only have
its assimilating activity set at work under the influence of light of sufficient intensity,
so also it is in most plants only brought to complete development by the light.
The leaves of flowering plants, especially when they are formed by growth in the
dark, produce chlorophyll-grains in their cells, it is true, but these are not
green; they remain yellow ^ tinged with a colouring matter which is but little
diff'erent from the green colouring matter, but which is unable to communicate
to the chlorophyll-grain the power of decomposing carbon dioxide. Such yellow
etiolated leaves do not assimilate, as experiment shows ; exposed for some time to
light, however, even though feeble, they become green, and are then able to
decompose carbon dioxide. Nevertheless, as I established twenty-three years ago,
the development of green chlorophyll is not in every case connected with the
influence of light : the primary leaves of the seedlings of Conifers develope their
normal chlorophyll even in profound darkness, and the leaves of Ferns behave
similarly. This dependence of the formation of chlorophyll upon light (not, as we
see, without exception) often led formerly to the erroneous assumption that the
greening of leaves is itself a process of assirpiladon, and connected with the evolution
of oxygen. A direct refutation of this view, however, lies in the observation that
very feeble light enables the yellow leaves produced in the dark to become green,
although this feeble light is far from sufficient to cause the green chlorophyll-grains
to assimilate. This important fact, much too little noticed, may be very easily
confirmed by allowing seedlings of Beans, Maize, Tropceolum, etc., to germinate
in pots situated at the back of an ordinary dwelling-room. Their first leaves
become green: that no assimilation follows, however, is shown by weighing the
dried plants, which yield a smaller weight of organic substance than was contained

* On the behaviour of chlorophyll in the dark, cf. my ' Expcriniental- Physiologie' p. 317, and
Pfeffer, ' Pßanzen-Physiologie,' p. 221.— On the dependence of the formation of chlorophyll upon
temperature, see Sachs, Flora, 1864, p. 497, and 1862, p. 129.

ASSIMILATION AND LIGHT.

301

in the seed. That the greening is for and by itself no assimilation process, however,
and that it takes place without the decomposition of carbon dioxide, may be de-
monstrated further with the aid of the apparatus here figured. The yellow leaves,
previously grown in the dark, are placed under the bell-glass, and, when the apparatus
stands in the light, become green, even if the plate is filled with a strong solution
of potash which completely absorbs the carbon dioxide under the bell-jar.

Finally is to be mentioned the fact that the organ of assimilation, even when
it has been previously normally developed in the light, can dispense with the
light for a time (in the normal course of things it does so every night) without
taking any harm thereby; but more permanent darkness, or even only deep
shading (i. e. for several days or weeks), brings about a disease of the previously
green cells, their chlorophyll-grains become destroyed, and the leaves turn
yellow and finally perish. This even occurs in the feeble illumination in
which, as mentioned above, the same leaves previously became green. Never-
theless there are plants, the green organs of which can dispense with light, even
for months together, without dying. I found this to be the case for instance with
some species of Cactus and Selaginella.
In the main, the dependence upon light
here referred to is particularly true for
quickly growing summer-plants.

All the relations between the organs of
assimilation and light here mentioned, as
well as the independence of growth with
regard to the latter, must be carefully ob-
served, if the important fact that the decom-
position of carbon dioxide and assimilation
in the chlorophyll are a function of light is
to be properly understood. It follows

from what has been said, that not light of '^^' ^'^'

any haphazard intensity will do what is

necessary. Unfortunately we lack photometric methods to enable us to distinguish
those intensities of light which come into consideration in assimilation with the same
precision, and generally intelligible exactness, as is possible with the thermometer
with respect to temperature. The most exact photometric methods, and especially
the method proposed by Bunsen, for instance, only give us information as to the
intensity of the strongly refractive rays of light, the so-called chemical rays; but
these, as I shall show subsequently, only come incidentally into consideration.
We must therefore adopt entirely different methods with respect to statements
concerning the intensity of light necessary for assimilation, and which will not be
here given in detail. Only so much is obvious, that, for the decision of certain
questions, use may be made of the law that with double the distance of a
surface from a luminous point, the intensity of illumination of the latter sinks
to I ; at three times the distance to \, &c.; and that the intensity of illumination
of a leaf-surface at the same time depends upon the sine of the angle of incidence.
It would cost us too much time, and would moreover lead to no satisfactory result
in the end, to enter more in detail into these matters. It must therefore suffice, that

302 LECTURE XVIII.

assimilation by means of the decomposition of carbon dioxide, in most plants, and
especially in the case of meadow and cultivated plants, trees, and garden-plants of
the most various kinds, only takes place with normal vigour and productiveness
when the ordinary strong daylight of summer is at the disposal of the plants. The
much feebler light in greenhouses, or even in ordinary dwelling-rooms, suffices, it is
true, with most plants to bring about a less productive assimilation in the green
leaves; but the sickliness of the plant shows how feeble is the nutrition under
such circumiStances. It is also to be observed that a pot plant standing close to
a window, under the best of circumstances only receives the light radiating from
half the sky, and only meets with the direct rays of the sun occasionally. If the
plant stands somewhat further removed from the window, it is only necessary to
imagine straight lines running from the plant or a leaf to the edges of the window,
and thence direct to the sky, to find the extent of that part of the latter the rays
of which fall directly on the leaves : it is then perceived that a plant removed but
a few metres from the window, only receives a very small proportion of light from
the sky, and as a rule meets with no direct sunlight at all. Accordingly, the
nutrition of plants in the middle of a room is extremely poor, and sooner or
later they inevitably perish. On the other hand, however, it is also to be observed
that while there are many plants which only flourish well in places which receive
the full light from the sky, and the direct rays of the sun, others exist which
prefer the shade of woods, or even the feeble illumination in the interior of deep
caverns. Here belong, for example, besides some species of Pyrola, many
Mosses, and especially Liverworts ; those Algae which grow exclusively in the
depths of large seas, and are thus feebly illuminated, also show that they find
the conditions of their existence in less intense light. Just as for each mani-
festation of life in plants there is an upper limit of temperature, which cannot
be passed over without injury, so also there is certainly an upper limit of intensity
of the light, at which the chlorophyll-grain can no longer accomplish assimi-
lation. Of course this limit of the intensity of light cannot be exactly given,
in the absence of suitable photometric methods ; and when Pringsheim makes
circumstantial statements concerning the behaviour of cells containing chlorophyll
in the focus of a lens, or in the sun's image, as he terms it, these purely
pathological processes have about as much physiological value as if, for any
reason whatever, a so-called sun's image M^ere allowed to act on the retina of the
eye through a burning-glass. Much better are the statements of several ob-
servers who, employing direct light, allowed the evolution of oxygen of one and
the same plant to take place under various degrees of shading, and so established
that a maximum effect at a light-optimum exists for this function also. In the
absence of photometric measurements of general value however, I pass over these
statements also.

We have much more information as to the various effects of the individual con-
stituents of sunlight, than with respect to the cardinal points of the intensity of the light
concerned in assimilation. As is well known, the light of the sun, like that of most
incandescent bodies, is a mixture of very different luminous rays, which are distinguished
by their refrangibility, i.e. by the amount of divergence which they undergo on entering
another medium, as well as by their chemical effects ; and obviously the question must

ACTION OF LIGHT OF VARIOUS COLOURS.

S'^S

force itself upon the investigator whether, and in what manner these different rays
of Hght, of which daylight is made up, influence assimilation in the chlorophyll.
For the preliminary guidance of those not quite familiar with the physical knowledge
appertaining here, the following remarks may be made. If the sun's rays are
allowed to fall through a narrow slit in the shutter of a room, they pro-
ceed through space in the form of a straight band, which can easily be seen as
luminous strise in the dusty air: if these luminous striae or bundles of rays are
allowed to pass through a triangular glass prism, the edges of which we suppose
vertical, two results follow. First, the ray of light is diverted from its straight path —
it falls on quite another spot on the hind wall of the chamber than was the case
in the absence of the prism ; and secondly, instead of the one bright stripe which
the solar rays originally formed on the hind wall, there now appears a horizontal
coloured band, the so-called solar spectrum, in which the colours of the rainbow, red,
orange, yellow, green, blue, and violet, follow one another in such a way that
the red portion is least, and the violet most strongly diverted from the rectilinear
path of the beam of light. In this spectrum, by proper management, a number
of black hues appear, running perpendicularly in the horizontal band of colours;
these are the so-called Fraunhofer's lines, which, as Kirchoff and Bunsen have
shown, are produced by the absorption of certain rays of light by the incandescent
vapours of certain metals in the solar atmosphere. From these fixed lines in the
solar spectrum, the most evident of which are distinguished by the letters A, B, C, —
If, it is possible to determine exactly the place where definite effects occur. The
refrangibility and colour of the different parts of the spectrum are a consequence, as
the science of optics teaches, of the different wave-lengths in the vibrations of the
luminous aether, of which the light consists.

If now the solar rays, passing through the slit, are allowed to traverse a glass
vessel with parallel walls containing a dark blue solution of ammoniacal oxide
of copper, the whole of the red and yellow^, and part of the green bands in the
spectrum disappear ; the blue solution has absorbed, kept back, or destroyed
these constituents of the sun-light. If a vessel with a concentrated solution of
bi-chromate of potash, which appears to our eyes of a deep orange colour, is
placed at the same spot, just those parts of the spectrum are cancelled which
previously passed through the blue solution — i. e. we now see in the spectrum the
red-orange, yellow, and a part of the green, while the blue and violet have
disappeared. We have thus in these two fluids excellent means for cutting out
the one or the other half of the solar light; and we can therefore, with the aid
of these two solutions, experimentally answer the question, what effect does the
red-yellow or the blue-violet half of the spectrum respectively exert in the de-
composition of carbon dioxide ? After the preliminary and less instructive researches
of Daubeny (1836), I made in 1864 a detailed investigation wdth regard to this
question ^ In a glass cylinder filled with water containing carbon dioxide a
water-plant was placed ; at the cut surface of the stem of this the oxygen evolved
under the influence of light escaped regularly in the form of bubbles. This cylinder

' Sachs, ' IVirkimgcn farbigen Lichtes auf Pflanzen' Bot. Zeit. 1864, p. 353, wliere the older
literature also is collected.

304

LECTURE XVIII.

was placed in a second, wider cylinder, and the space between both filled with
one or other of the solutions previously mentioned, or with pure water. After
careful consideration and preparation, I employed as a measure of the decomposition
of carbon dioxide in the plant, the number of bubbles which escaped from the cut
surface of the stem in one minuted It was now possible to conduct the investigations
m such a manner that the plant could be observed alternately for one minute re-
spectively in white complete light, in red-yellow, or in blue-violet light, one
immediately after the other, and the gas-bubbles counted. It turned out that in
the blue-violet light only very little carbon dioxide was decomposed, while (having
regard to accessory circumstances) the effect in red-yellow light was nearly as

strong as in the full light which passed
through pure water. This result, as well
as the observations previously made by
Daubeny, Draper (1844), Cloez and Gra-
tiolet (1851), contradicted the prevailing
view of the physicists and chemists, that it
is the blue-violet part of the spectrum
which almost alone brings about photo-
chemical effects. The decomposition of
carbon dioxide in the plant evidently de-
pends upon a photo-chemical effect; and
yet we here see that that portion of the
spectrum which is distinguished by phy-
sicists as the one chemically effective, is
relatively inactive, while the other half
of the spectrum is here the effective
one. I directly confirmed this apparent
contradiction, again, by placing in the
upper part of the glass cylinder contain-
ing the plant a small apparatus which
enabled me, while observing the separa-
tion of oxygen, simultaneously to observe
the effect of the coloured light on pho-
tographic paper. When the light passed
through the blue-violet solution, the evolu-
tion of oxygen in the plant was extremely
small, while the photographic paper became deep brown; w^hen, on the other
hand, the red-yellow solution was interposed, the plant evolved large quantities of
oxygen, while the photographic paper reacted but little and feebly.

It may here simply be remarked that it was an inaccurate generalisation on
the part of physics and chemistry to designate the blue-violet portion of the spectrum
as the part chemically active, simply because the corresponding rays of light
cause silver salts to decompose and a mixture of chlorine and hydrogen to form

FIG. 220.— The glass cylinder Ci contains the plant p
in water, in which the thermometer t is immersed. The
cylinder Ci is then placed in the largei cylinder Ca, and
the interspace y filled with the coloured liquid. A sup-
porting hooks. A disengages carbon-dioxide, which is
washed in water in the flask B.

* With respect to the method of counting the bubbles employed by me, and much discussed
later, cf. Pfeffer, ' Pflanzen- Physiologic^ p. 215.

EFFECTS OF VARIOUS COLOURS OF LIGHT.

305

hydrochloric acid. The action of the red-yellow light on the decomposition of
carbon dioxide which we have established contradicts no fact, but only a false
generalisation; since it shows that other chemical processes which take place in
the chlorophyll are brought about by other rays of light, namely the red-yellow.
As regards the method of experimenting, I subsequently devised a more con-
venient arrangement. Glass vessels with parallel walls, and as large as possible
(so-called Cuvettes), were filled with the solutions, and fixed somewhat like windows
into opaque boxes ; in these, by means of a door at the back, the plant to be
observed is placed in water containing carbon dioxide.

So long ago as 1844 Draper had observed the evolution of oxygen from green
leaves, by placing them, enclosed in small glass vessels, in various regions of the
solar spectrum, and determining the quantities of oxygen given off, and thus
learning the effects, of the different coloured parts of the solar light. He found

"■■■■■■■■■/

'

\

/■ "s.

/

/■
,/■

^■' Assimilation.

/

//

\

/ Heat.

^- Chemical action.

/

■1

II

1

l-i ;"!~"!"!:^ :"''■' ""'

FIC. 221.— The curves of assimilation, temperature, brightness, a
are represented on the spectrum of chlorophyll (AH) as an abscissa.

that the effect in the red portion is extremely feeble, rising quickly in the red-
orange, and reaching a maximum in the yellow-green ; it falls again in the blue
of the spectrum to an extremely small quantity. This obviously agrees with the
researches described above, where the light passed through coloured solutions.
If we suppose the solar spectrum to be an abscissa line, on which the effects
discovered by Draper are erected as ordinates, the relative quantities of oxygen
evolved in equal times in the various regions of the spectrum appear in the
form of a curve, which begins in the red, reaches its maximum in the yellow-
green, and then quickly sinks again towards the blue. This Draper's curve,
as we may term the law of the dependence of assimilation upon the colour of
the light,, was tested more exactly by Pfeffer in 1870 by means of the eudio-
meter, and then, in 1872, by counting the bubbles, a method employed by me
previously \ The result of his observations may be understood most simply from

I

' Investigations on assimilation in coloured light are collected in Pfeffer's ' Pflanzen- Ptiysiologie,

l,p. 211.

[3]

3o6 LECTURE XVIII.

the accompan}-ing figure, in which are represented the effects of the different parts
of the spectrum, not only on assimilation in the chlorophyll, but also on the heating
of a thermometer as well as on the decomposition of silver salts, and, finally,
on the eye. It is seen at once that the evolution of oxygen, or Draper's curve,
cannot possibly be due to heating, or to the ordinary chemical effect of the
spectrum ; since the former reaches its maximum beyond the red end, and the
latter in the violet. However, the remarkable fact that the curve of brightness in
the spectrum nearly coincides with Draper's curve, has led some to the erroneous
assumption that Draper's curve itself is an effect of brightness ; such an idea is
simply without meaning, since by the word brightness we mean in this case nothing
more than the action of light on the human eye, an effect which may possibly
be quite different on the eyes of different animals. It is obvious that the action
of light upon our eye cannot possibly be the cause of the action of light on
cells containing chlorophyll. The entire assumption depends therefore upon
want of thought. I expressed the matter of Draper's curve correctly in the third
edition of my ' Text-book ' in the following words : the evolution of oxygen
brought about by the chloroph\ll is a function of the wave-length of light, so
that only light of wave-lengths which are not larger than xö^^-ö-ö rn"^- ^■i^d not
smaller than j-jj^^öö mm. bring about the evolution of oxygen. Proceeding from
both extremes, the effect of light on the evolution of oxygen ascends when its
wave-lengths approach xö^^öö "^™- where the maximum effect lies. Or, if we
start with the medium wave-lengths of the coloured regions of the spectrum,
measured in hundred-thousandths of millimetres, the evolution of oxygen is
effected by light-waves the minimum length of which begins at about 40; it
increases as this ascends to about 59, and decreases again as the wave-length
increases, becoming almost nil at a wave-length of about 69. We have thus a
phenomenon resembling the case of the curve of temperature, first established by
me : as in that case the functions of the plant which depend upon temperature only
begin with a certain intensity of the heat-vibrations, and ascend as the intensity
increases, until at an optimum temperature the maximum effect occurs, falling
again to zero with a further increase of temperature ; so also in the case of
Draper's curve, we see that the evolution of oxygen in plants begins at a certain
small wave-length, and that a wave-length of tüotö^ ™''^- represents about the
optimum, wehere the maximum effect occurs, and that with a further increase
of the wave-length the physiological effect falls to zero.

Although these results have apparently only a purely theoretical interest, it
is nevertheless not to be forgotten that they may be of practical value in the
criticism of certain vital phenomena of plants. Plants for example which grow
in the deep shade of woods, meet with light which has to a great extent passed
through the translucent leaves of trees, and which has lost a great part of its
yellow, blue, violet, and ultra-violet rays in the chlorophyll of the latter ; and in
like manner plants growing in the depths of large seas receive light of totally
different composition from that received by terrestrial plants, and to which the highly
refrangible rays especially are wanting. Again, it is futile to attempt to improve
greenhouses with blue glass, proceeding from the erroneous assumption that the
plants will thus have chiefly blue light conveyed to them, and from the further

THE FIRST VISIBLE PRODUCT OF ASSIMILATION. 307

erroneous assumption that this bhie light must be useful to the plant because it
belongs to the so-called chemical part of the spectrum. It has been shown above
that the latter is not the case ; and moreover blue glass at most transmits as much
blue light as colourless. That it appears blue is simply because it destroys a large
proportion of the yellow light; and we have learnt that it is just this yellow light
which is the effective constituent of daylight in the nutrition of the plant. The
effect of glazing greenhouses with blue glass is therefore directly injurious : it
depends on false conclusions from inaccurately established facts.

Finally, the question as to the action of different coloured light on the nutrition
of plants comes into consideration when artificial terrestrial sources of light are
employed for the illumination of plants instead of ordinary sunlight. This has
hitherto taken place, of course, only in the interest of scientific research. It is
theoretically certain, and has been established by experiment that the electric light
— i. e. the light from incandescent electrodes — as well as lamplight, &c., when
sufficiently intense, can effect the evolution of oxygen from organs containing
chlorophyll. Here, however, it is not to be forgotten that every such source of
light possesses a different mixture of rays of different refrangibility, or, in other words,
a different spectrum ; and, from what has been said previously, each source of light
particularly rich in yellow rays will cause more vigorous assimilation.

We pass, finally, to the fourth question mentioned in the introduction, which
may be shortly answered thus : the first definitely established product of assimilation
in the chlorophyll-corpuscles is starch (Amylum), or some other substance equivalent
to it. Very indefinite ideas were formerly entertained in this connection. It was
supposed, as resulted from the statements of the older physiologists and chemists,
that an indefinite primordial slime, perhaps composed of various substances and
impossible to characterise in detail, arises in the green organs of plants : it was
believed that this substance was again recognised on its way in the living cortex
of trees, and it was assumed that it penetrated into the various organs and tissues,
there to break up into the numerous and various chemical compounds which
are found in different parts of the plant. In a long series of micro-chemical and
experimental researches on the distribution of starch and sugar, of proteid substances
and of fats, and their use in growth, I came to the conclusion in 1862^ that
the enclosed starch, which had already been observed in the chlorophyll-corpuscles
by Naegeli and Mohl, is to be regarded as the first evident product of assimilation
formed by the decomposition of carbon dioxide. I said to myself, if this view
is right, the formation of starch in the chlorophyll-corpuscles must cease on
the exclusion of light, since the decomposition of carbon dioxide can then no
longer take place ; and that in like manner renewed access of light to the chloro-
phyll-corpuscles must also bring about a renewal of the formation of starch in them.
These and similar deductions were confirmed by appropriate investigations; and
yet more — I had already concluded from my previous micro-chemical analyses

' The foundations of my theory, that the starch in the chlorophyll is the first evident
product of assimilation, were first laid in the following of my works: — ^ Flora,' 1862, Nos. 11 and
21 ; and 1863, p. 33; 'Bot. Zeit: 1862, p. 366; ' Jahrb. für wiss. Bot: 1863, III, p. 199; ' Exp.-
PJiys: 1865, p. 320.

3o8 LECTURE XVIII.

of entire plants, in the most various stages, that the starch and sugar etc. which
pass from place to place and are used up in the petioles, stems, growing buds, etc.,
were derived from the green assimilating leaves ; and, consequently, when assimi-
lation ceases in these, the starch must also disappear from the chlorophyll-corpuscles
themselves. This conclusion also proved correct. This is not the place to repeat
in detail the numerous proofs, in part direct, in part indirect, which I brought
forward during the years 1862-1865, to establish the fact that the starch-grains
observed in the chlorophyll-corpuscles of normally vegetating plants are products
of assimilation, and that, after their origin under the influence of light, they are
dissolved and conveyed from the leaves, through their petioles, into the shoot-
axes, and thence into the buds and apices of the roots, to provide the material
for the growth of the organs; and that a portion of this original product of
assimilation is made use of in metabolism for the formation of proteid substances,
while, on the other hand, fats may arise by relatively slight changes from the carbo-
hydrates, and therefore, ultimately, from the assimilated starch. The thought arose
that, in the nutrition of plants, it is only necessary in the first place to decompose
carbon dioxide under the influence of light in the cells containing chlorophyll,
with the co-operation of certain mineral matters absorbed by the roots, and to
produce at the cost of its carbon an organic substance—starch — which then repre-
sents the starting-point, so to speak, from which all the other organic substances
of the plant proceed by progressive chemical changes. This conclusion has in the
course of twenty years become more and more established as the correct one.

LECTURE XIX.

ORIGIN OF STARCH IN THE CHLOROPHYLL, AND IN THE STARCH-
FORMING CORPUSCLES: FURTHER BEHAVIOUR AND FATE
OF THE CHLOROPHYLL.

It was stated at the end of the foregoing lecture that the starch arises in
the chlorophyll by means of the process of assimilation— i. e. by the decomposition
of carbon dioxide under the influence of light — and that it represents the first
product of assimilation hitherto known with certainty. The attempt may now be
made to establish these facts more definitely.

If seedlings of the Gourd, Sunflower, Maize, or Garden-beans, or the sprouts
of Dahlias or Helianthus tuberosus are allowed to grow in the dark at a suf-
ficiently high temperature (i5°-'2 5°C), until development finally ceases, the tissues
of these etiolated shoots, as well as those of the roots, are at length entirely emptied of
assimilated substances. The plants in these cases consist almost solely of cell-walls,
protoplasm, and water. The most important fact, however, is that the small yellow
chlorophyll-corpuscles in the leaves contain no trace of starch, either in the chlo-
rophyll or in the other organs and tissues. If these plants, already green, or
even those taken immediately from the dark, be placed at a light window,
starch arises in the chlorophyll-corpuscles (which have previously become green) at
first in exceedingly small quantity, which rapidly increases in strong light. In the
course of several days, 6 — 20 according to the temperature, abundance of starch is
found in the chlorophyll, and subsequently also in certain layers of tissue in the
petioles and shoot-axes; this may be followed into the buds, the young leaves
of which now begin also to grow anew, since the starch produced in the chlo-
rophyll again supplies material from which the tissues of new organs may be
formed. By means of such investigations I first succeeded, in 1862, in experimentally
proving that the starch formed in the chlorophyll under the influence of light,
is the first visible product of assimilation \ In 1863 and 1864 I convinced myself^
that the starch contained in the chlorophyll of normally developed leaves may
disappear again in long-continued darkness ; and that it is possible to bring about

' Sachs, ' Uebcr den Einßuss des Lichtes auf die Bildung des Amylum in den Chlorophyll-
körnern^ Bot. Zeit. 1862, p. 365.

^ Sachs, ' Ueber die Auflösung und Wiederbildung des Ain^luin in den Chlorophyllkörncrn bei
wechselnder BeleuchtuJig.' Bot, Zeit. 1864, pp. 289 and 322.

310 LECTURE XIX.

the renewed formation of starch by a second illumination. I made these experi-
ments with Begonia, Tobacco, and Geranium peltaiwn, placing the entire plants in
the dark until the starch had disappeared from the chlorophyll of the leaves;
then, the plants being again exposed to the light, a renewed formation of starch
resulted. In the first experiments with Begonia, certain portions of the leaves were
covered with black paper on both sides, with the result that the starch disappeared
from the chlorophyll of the leaves only at these places. I now employ this form
of experiment in my lectures on vegetable physiology, in order to demonstrate
the influence of light on the formation of starch ; or, better, of darkness on the
disappearance of starch. It suffices for instance to fasten a broad band of tin-
foil or lead in summer on plants with conveniently large leaves and growing in pots
— e.g. Tobacco, Maize, Canna, etc., without depriving the plants of light. After
a few days, the leaves so treated are cut off", and thrown for a few minutes into
boiling water in order to kill them, and to cause the starch in the chlorophyll
to swell. They are then placed for some hours in strong alcohol, which removes
the chlorophyll colouring-matter, and the now colourless leaves are finally placed
in a vessel containing a weak, pale brown, alcoholic solution of iodine. After
a short time, the parts of the leaf which were not shaded from the light appear
blue-black, owing to the formation of iodide of starch : the place shaded by the
band of tinfoil, on the other hand, remains colourless, simply because the chlorophyll-
corpuscles there contain no more starch.

This experiment demonstrates yet another fact, viz., that the action of light
is strictly local, and is not transferred to neighbouring shaded places. Moreover
the action of carbon dioxide is also strictly local, as Moll showed in 1878'.
The leaves produce starch only at the places directly in contact with air con-
taining carbon dioxide. If, for example, one half of a leaf is placed in a
space the air of which is deprived of carbon dioxide, while the other half
remains in the ordinary atmosphere, both halves being equally illuminated, starch
arises in the chlorophyll only in the last-named half, the other producing none.
Again, if the leaves of a plant rooted in garden soil are placed in a space
which contains no carbon dioxide, no starch is produced in the chlorophyll,
even with favourable illumination. These experiments prove that the view
entertained until quite recently by the older physiologists, that carbon dioxide
may be conveyed from the roots into the leaves, and there assimilated, is
absolutely wrong.

In some cases it is impossible directly to observe starch as the product of
assimilation in the chlorophyll-grains. I found this to be the case in the
leaves of our common Onion {Alliimi Cepa), where, however, large quantities of
glucose (sugar) are to be recognised as the result of assimilation. As a rule
this plant does not form starch, and the reserve-material in its bulbs also con-
sists of glucose. It was observed later, that in the leaves of Sirelilzia and Musa
also fatty oils are found as a rule in the chlorophyll instead of starch. The
assumption that the first product of assimilation of the chlorophyll is in these

' Moll, * Ueber die Herkunft 3es Kohlenstoffs in der Pßanze.' Arbeiten des bot. Inst, in Würz-
burg, II, 1878, p. 105. See also Vines, ibid., p. 121.

FORMATION OF STARCH-GRAINS IN THE CHLOROPHYLL. 311

cases not starch, but fat, was refuted however by Holle and Godlewski\ who
showed that, even in these plants, the decomposition of carbon dioxide yields
a volume of oxygen equal to that of the carbon dioxide employed, which could not
possibly be the case if fat were immediately formed ; and that under particularly
favourable conditions of assimilation — viz., on supplying larger quantities of carbon
dioxide to the surrounding air, and using a stronger light — starch can actually
be detected in the chlorophyll. It thus appears that in many plants the starch
produced in the chlorophyll may be at once transformed into fat, as may also
be the case with some species of Vaiicheria : this process presents nothing
at all surprising, since I showed so long ago as 1859 and later, by numerous
examples ^ that the transformation of fat into carbo-hydrates and of carbo-hydrates
into fat, is a very common phenomenon in plants ; and with respect to the
glucose in Allium, it is simply to be noticed that it matters little for the plant
whether the chemical processes produce starch or sugar, as we shall see again in
the following lecture.

The formation of starch in the chlorophyll may be more easily observed in
the simply organised Algae than in the leaves of the higher plants. Kraus found,
in 1867, that in Spirogyra, previously kept in the dark and thus deprived of
starch, the formation of starch was to be recognised, on illuminating it under the
microscope, even after five minutes in direct sunlight, and in the course of two
hours in diffused daylight ; and similar results were obtained with the leaves of
a Moss {Funarid), and the water-plant Elodea. It being, further, established that
assimilation is much more energetic in yellow than in blue light, Famintzin also
succeeded in demonstrating, in 1867, that starch is formed more rapidly under the
influence of yellow than of blue light.

Although my above-cited researches (collected in my Ex per imcjital -physiologic)
admit of no doubt that starch, or a substance equivalent to it, is to be regarded
as the first visible product of assimilation, it was nevertheless a welcome con-
firmation of my theory when Godlewski, in 1873, demonstrated, by experiments
as ingenious as they were simple ", that in an atmosphere devoid of carbon dioxide
no starch is produced in the chlorophyll-corpuscles, even in the light. He also found
that the starch produced in the chlorophyll disappears, not only in the dark but
even in intense light, when the surrounding air contains no carbon dioxide. Of
particular importance is the fact, established by Godlewski, that on increasing the
amount of carbon dioxide contained in the air to 8%, the formation of starch is
four or five times more energetic in a strong light, while in diff'used light the action
is much feebler. Very large quantities of carbon dioxide in the air, however,
prevent the formation of starch ; and the more so the feebler the light *. These
statements are the more valuable since Godlewski had previously established.

' Emil Godlewski, in Flora, p. 215, and Holle, ibid., p. 113.

"^ Sachs, ' Uebcr das Auftreten der Stärice bei Keimung öllialtiger Samen^ Bot. Zeit. 1859,
p. 177.

=* Godlewski, ' Abliängiglieit der Stärliebildung in den CJilorophyllJwrnern von dem Koiilen-
säurcgeiialt der Luft.'' Flora, 1873, p. 378.

* Godlewski, 'Abliängiglieit der Sauerstoffabscheidung der Blätter von dem Koldensäuregehalt
der Luft.' Arbeiten des bot. Inst, in Würzburg, 1873. Bd. I, p. 343.

312 LECTURE XIX.

by a detailed investigation in my laboratory, that on increasing the carbon dioxide of
the air -to 5—10% leaves (of Glyceria spedabilis, Typha latifolia, and Oleander)
evolve the maximum amount of oxygen in intense light.

On the basis of all these facts, there cannot be the slightest doubt that the
starch in the chlorophyll-corpuscles is to be regarded as the first evident product of
assimilation ; and that it condnually undergoes solution in the light as well as
in darkness, and becomes distributed in some form from the assimilating organs
into the tissues of the plant. There it is either used up immediately for the growth
of new organs, or is stored up as reserve-materials in seeds, tubers, bulbs, root-stocks,
and the cortex and wood of trees ; or it provides the material for the formation of
other organic compounds, and especially for the synthesis of the proteid substances.

Since, according to my theory, the basis for the whole of the organic substance
of a normal plant is provided by the formation of starch in the chlorophyll of
the organs of assimilation, and, above all, the whole of the carbon, in whatever
organic combination it may be found later, occurs originally in the form of starch,
it is particularly interesting to enquire how great the work of the assimilating chlo-
rophyll may be under definite, and especially under normal conditions of illumination.
The question, however, can so far only be approximately answered, since we cannot
employ individual chlorophyll-corpuscles for such observations — nor even individual
cells, the unicellular Algae not being suitable for quantitative determinations.
Moreover, it is not impossible that the quantitative activity of chlorophyll-corpuscles
similar in size and colour may be peculiar for each species of plant ; and
some observations make such an assumption almost probable. Since in the
course of about 100 summer days a huge plant, the dry-weight of which
may attain two or three kilograms, is developed from the tiny embryo of
a Tobacco seed, while from the much larger seed of a Fir, for instance, there
arises in the same time a small plantlet, the dry-weight of which only amounts
to a few grams, it may be supposed that, among the very numerous causes
which may effect this difference, the activity of the chlorophyll-substance itself is
possibly much more energetic in the first example than in the latter. This, how-
ever, is not the place to enter more deeply into so difficult a question. But it
is desirable to know, even if only approximately, how much starch may be
assimilated in the chlorophyll under favourable circumstances. Such consider-
ations impelled me in 1878 to persuade Dr. Weber, then my assistant, to
undertake an experimental investigation, in the first place to decide only the
question how great is the quantity of starch assimilated in a given time (e. g.
ten hours of daylight) in the summer and with favourable illumination, by a
certain amount of leaf-surface (e.g. a square metre ^). Even in this very simple
form the answer to the question presents great experimental difficulties. This is
particularly the case because the starch at first assimilated by the few leaves of the
experimental plants is at once used up again for the production of new organs
of assimilation ; the area of the leaves enlarges from hour to hour, and from day
to day ; and older leaves cease to assimilate, and younger ones begin to perform

* Weber, ' Ucber spccifische Assiinilationsciicrgic^ Arbeiten des bot. Inst, in \Vürzburg, II,
p. 346.

SPECIFIC "ENERGY OF ASSIMILATION. 313

that function. Hence repeated measurements of the leaves must accompany
the growth of the plant — an exceedingly troublesome task. We may of course
also proceed by preventing a plant, already provided with a large assimilating
surface, from increasing the latter and using up its products of assimilation in
the shoots, which develope in the dark and thus take no part in assimilation.
But even here many difficulties arise. According to the first method, Weber
found that in four species of plants distinguished by their thin and relatively large
leaves, and by their rapid growth and considerable increase in weight in a short
time, a square metre of leaf-surface produces the following quantities of starch
in ten hours of daylight : —

TropcEohim majus . . . . . . 4*446 grams.

Phaseolus inultifloriis . . . . . . 3*2 15 „

Ricinus conwiwiis ...... 5*292 „

Helianthus amiuus ...... 5'559 „

It is to be remarked, however, that the ash contained in the dry substance should
be deducted from the calculation, and the loss of organic substance by respiration
added to it; since the increase in weight of a plant in organic substance is, as a
matter of fact, only the difference between the quantities gained by assimilation
and the quantities, of course much smaller, lost in respiration. This opportunity
may be taken for making the additional remark that the quantities of starch present
for the time being, which may be detected incidentally in the chlorophyll-corpuscles,
is only transitory, since the starch arising in the chlorophyll by assimilation
is continually being dissolved in the light, as well as in the dark, and conveyed
into other portions of the plant. During vigorous assimilation its accumulation
in the chlorophyll predominates : in continuous darkness, or feeble illumination,
on the other hand, the solution and translocation of the starch prevail, and it is
this which renders it possible to deprive the chlorophyll-corpuscles of starch for experi-
mental purposes.

Weber's experiments were made, however, not in the open air but in a
greenhouse, the illumination of which was very strong, it is true, but still not normal,
as in the open. We may therefore assume that in the latter case the assimilation
would have been more energetic, and the more so since plants in the open can
also develope their roots more normally than in Weber's experiments, where they
were confined in pots. This supposition is to a certain extent confirmed by
Kreussler's measurements, which, to be sure, were made by very different and less
exact methods. Maize plants, according to these experiments, yielded as much as
seven grams of starch per square metre in ten hours, under the above conditions ;
and observations made later in my laborator}- on Gourd plants, rooted in the open,
yielded a probable number of eight grams of starch per square metre of leaf-
surface per day, which nevertheless would give in 100 days only 800 grams of dry
substance, while in a sunflower {Helianthus amtuus) I obtained in 100 days more
than 1400 grams of dry substance, although the foliage during the first fifty days
was certainly less than half a square metre, and even later did not amount to a
whole square metre. It is thus to be expected that the energy of assimilation
may, under certain circumstances, be considerably greater than eight grams per day

314

LECTURE XIX.

per square metre ^ At an}- rate, the numbers mentioned give an approximate idea of
the enormous amount of work which the crown of foliage of a tree, such as
a Horse-chestnut, Walnut, etc., must do in the course of a summer. In these
cases the main mass of the products of assimilation is deposited in the form of
wood; whereas in Peas, Beans, and cereals, a great portion of it passes into
the fruits and seeds ; while we find the products of assimilation of the Potato-plant

Fig. 122.— a cells with cliloropliyll-corpuscles, from the FiG. 223.— Two filaments ot Spircffyi-a in conjugation ;

leaves of a Moss {Funana kygrometrica). B isolated the band of chlorophyll containing groups of starch-grains,

chlorophyll-corpuscles, a b with enclosed small starch-grains ;
<-rf« the same grown larger. *'*" a chlorophyll-corpuscle
dividing, y one swollen in water, s 'he same dissolved, the
starch alone remaining.

to a large extent stored up in the tubers, and those of the Beet in the huge root
abounding in sugar.

Having thus learned to understand the amount of work performed by chlo-
rophyll, first with reference to the external circumstances and then to the quantity,
we may now submit the question, what takes place in the chlorophyll-corpuscle itself
in these processes ? Here a distinction is to be made between the changes im-
mediately perceptible with the aid of the microscope and micro-chemical reactions,
and the chemical processes .which take place in the substance of the chlorophyll-

' [ Sachs has since proved this to be the case. See Arbeiten des hot. Inst, in IViirzlnirg, B. Ill,
where experiments show that as much as 20-25 grams of starch per day may be formed by
I square metre of leaf-surface. An abstract of this paper appeared in 'Nature ' for April 10, 1884.]

FIRST VISIBLE CHANGES IN CHLOROPHYLL-CORPUSCLES.

?>^5

corpuscle, and which cannot be seen but can only be inferred from their consequences
and conditions. Confining ourselves first to the visible changes ; the old ob-
servations of Naegeli and myself show that in the primitively quite homogeneous
green substance, starch-grains, at first extremely small, become visible, usually
distributed in twos, threes, or more in the mass of chlorophyll of the corpuscle. These
enlarge and, as they meet one another during growth, become flattened and
applied close to one another with plane surfaces, while their free sides remain
rounded and become arranged more or less according to the form of the chlo-
rophyll-corpuscle; occasionally, however, when they arise at the circumference they
protrude from the chlorophyll-corpuscle. I also observed, almost twenty years ago,
that, under certain circumstances, when leaves (e. g. of Tobacco and Pea) turn

FIG. 224.— .■/ cells from an etiolated leaf of Dahlia variabilis. B the same, but older and with yellow chloroplijil-
corpuscles. C the yellow chlorophyll-corpuscles turned green and further developed in light. D commencement of starch-
formation. H advanced stages in the formation of the starch. F from the leaf of Tropaoliim majns. G and H de-
struction of the chlorophyll (in H yellow oil drops only remain) as the leaves turn yellow in autumn. K a cell from the leaf
of yicia Faba after contraction of the lining protoplasm (primordial utricle), ph the cell wall.

yellow without being diseased, the starch-grains grow so vigorously in the chlo-
rophyll that the latter becomes, so to speak, entirely displaced by them, and finally,
in place of a chlorophyll corpuscle, there Hes a starch-grain compounded of several
grains (cf. Fig. 224).

Very valuable contributions to the question with which we are here con-
cerned were made by A. F. W. Schimper in i88o\ I must refer to these because
they throw a new light on the chemical processes presumably taking place in
the chlorophyll. In the first place, it follows from Schimper's description that

' Schimper, ' Unicrsuchungcn über die Entstehung der Stürkekörner: Bot. Zeit. iSSo, p. SSi.
— Arthur Me^er, Bot. Zeit. 18S0, Nos. 51 and 52.

3l6 LECTURE XIX.

the chlorophyll-corpuscles in the green parts of the stem behave differently from those
in the assimilating lamina of the leaf: this, according to my view, is also demon-
strated by the fact that during growth in the dark, the otherwise green parts
of the stem and petioles remain white, while the assimilating laminae appear yellow
in the etiolated state. Schimper found that in the stems of many plants the
starch-grains do not arise at any haphazard points of the chlorophyll-corpuscle,
but exclusively close beneath its surface, so that they soon protrude from its
substance. Hence the following distinction results. The grains which remain
entirely in the chlorophyll-corpuscle are rounded, and possess a concentrically layered
structure, because they are nourished on all sides from the chlorophyll ; whereas
the starch - grains growing out from the chlorophyll - corpuscl s are stratified
excentrically, since they grow much more strongly and exhibit more numerous
layers on the side connected with the chlorophyll — a proof that they are nourished
from this side.

A long series of older inaccurate observations are rectified by Schimper's research,
in that the starch-grains so common in organs other than the assimilating organs
— petioles, stems, subterranean tubers and roots, and young leaves which do not yet
assimilate — are produced by peculiar little bodies which Schimper terms ' starch-
forming corpuscles ' or Amyloplasts {Slärkebildner). These starch-forming corpuscles
are protoplasmic, usually colourless structures, which become differentiated from the
protoplasm just like the chlorophyll-corpuscles themselves, either in the neighbour-
hood of the cell-nucleus, or also in the rest of the protoplasm. These starch-forming
corpuscles now produce starch-grains, either in the interior of their substance
{Colocasia and endosperm of Melandryuni), or only at their peripheral portions
[Philode7idron, Aviomum, Phajus, and Camta), and thus in the same way as the
assimilating chlorophyll-corpuscles in the leaves. The peripheral mode of origin
appears to predominate however in these non-assimilating starch-forming corpuscles.
According to Schimper's statements also the protoplasmic substance of the starch-
forming corpuscle is more quickly and essentially altered, or used up, during its
functional activity than is the case with the assimilating chlorophyll-corpuscles. The
similarity of the starch-forming corpuscles to the assimilating chlorophyll-cor-
puscles is, however, still further increased, in that the former can in most cases
develope into chlorophyll-corpuscles under the influence of light. In this process,
they increase considerably in size, and, their starch-grains dissolving, they turn
green at the same time — a process long known, for example, in the cortex of
potato-tubers allowed to lie in the light for some time, but incorrectly explained.

The great difference between the starch-forming corpuscles and the assimi-
lating chlorophyll-corpuscles, however, is established by Schimper himself in the
following words — 'Etiolated plants which have not yet exhausted their stores of
reserve-materials contain, as is well known, no starch in the mesophyll of the leaf,
but often have abundance of it in their stems and petioles, and in the vascular
bundle-sheaths of their leaves. This starch, which of course can only be a product
of metastasis and not a product of assimilation formed there, is produced by
starch-forming corpuscles. Good examples of this are afforded by the leaves of
Hyacinihus, the stems of Begonia cuciiUata and Oxalis Orfgiesn, and the cortex
of the stem of Philodendron grandifoUiim. These starch-forming corpuscles

STARCH-FORMING CORPUSCLES. 317

(Schimper here somewhat inaccurately terms them Leucophyll) are all only very
feebly coloured yellow, or are not coloured at all. In the cases investigated they,
like the i bloroi hyll-corpuscles which the same cells would have contained under
normal circumstances, produced the starch -grains at their peripheral portions.
Not less important, with respect to the difference between starch-forming corpuscles
and chlorophyll-corpuscles, is Schimper's further statement, that in the case of a plant
{Tradescanh'a rubella) from the assimilating chlorophyll of which the large starch-
grains had been allowed to disappear entirely in the dark, and which was now
placed in a feeble light, no starch was produced in the normal chlorophyll-corpuscles
of the leaves here developed ; but, on the other hand, abundance of starch arose
in the starch-forming corpuscles — i. e. in the pseudo-chlorophyll-corpuscles of the
vascular bundle-sheaths of the leaves and stem. Schimper draws from these facts
the following conclusion. The fact that the development of starch in the meso-
phyll of the leaf depends on the same conditions as assimilation, while in other
parts of the plant it occurs independently of light, so long as reserve-materials
are present, can only be explained by assuming that in the first case it is exclusively
a product of the assimilation of the chlorophyll-corpuscle in which it appears, while
in the second case it must have in part (I think entirely) a different origin.'

Finally, Schimper expresses himself as follows with respect to the invisible,
purely chemical processes : — ' It is clear that the starch which appears as the
first evident product of assimilation (in the true chlorophyll-corpuscles) does not
originate directly from carbon dioxide and water, but that more or less numerous
intermediate products, still unknown or at any rate known with less certainty,
are interposed. We may assume that the substances conveyed to the chlorophyll-
corpuscles are identical with one of these intermediate products, or are at
any rate very similar to them ; and hence the transformation to starch of the
assimilated matters formed there and then, and of those conveyed from other
organs, is effected by one and the same process' (probably similar processes
is meant).

I cannot here avoid repeating my view on the formation of starch in the
assimilating chlorophyll, already expressed in the '■ Experimenial-physiologie' (1865);
and so much the less, because it has been in the meantime grossly misrepresented,
and falsely quoted, by writers unacquainted with vegetable physiology. I there
said (p. 327), 'If, after all, I regard the starch in the chlorophyll as one of the
first products of assimilation, it is not therefore to be said that carbon dioxide and
water unite forthwith to form molecules of starch within the chlorophyll-substance,
oxygen being evolved. It is not even necessary that any carbo-hydrate whatever
should arise immediately ; it is possible, and probable, that the process accompanied
by the evolution of oxygen is a very complicated one, from which the formation
of starch results only by numerous chemical metamorphoses. It is, indeed, not
impossible that certain more immediate constituents of the green plasma itself take
part in the process — that decompositions and substitutions, for example, take place
in the molecules of the green protoplasm. This possibility obtains some probability
from the observation that in many (not all) cases the chlorophyll-substance gradually
decreases in quantity and at length disappears entirely, while the starch-grains are
growing in it,' etc. With reference to Schimper's results, I am now strongly inclined

3-«

LECTURE XIX.

to assume that both in the assimilating chlorophyll-corpuscles and in the non-
assimilating starch-corpuscles, the material for the formation of starch consists of
sugar. This latter is conveyed to the starch-forming corpuscles of the organs
which do not assimilate : it is formed, on the other hand, in the assimilating
chlorophyll-corpuscles by assimilation. The question now remains, therefore, how
does the sugar itself arise by assimilation ? I hold it as probable, even now, that
in this process the proteid substance of the assimilating chlorophyll itself co-
operates, and undergoes a change. Whether it is right to claim, with Berthelot
and Kekuld (1861), formic acid or some other member of the formyl group as
the first product of assimilation, on account of its simple constitution, I hold as
at least very questionable ; and it has hitherto been proved by nothing. I lay
still less value on Pringsheim's so-called Hypochlorin ; a substance, the chemical
nature of which is not established, the genetic relation of which to the starch in
the chlorophyll is not known, and the significance of which for the processes
of growth has not been investigated. Hypochlorin appears to me, rather, to belong
to the same category as the INIyelin of the animal physiologists, with which the
latter likewise do not know what to do ^

At any rate, even after all the recent researches, the fact, which I established
twenty years ago, that the starch in the assimilating chlorophyll is to be regarded
as the first distinctly recognisable product of assimilation, remains unaltered. Even
then I left the way open for further knowledge, since I laid stress on the fact
that it was a matter of the first distinctly visible product, and that probably other
products, hitherto not distinctly recognisable however, precede the formation of
starch. Hence, even if as a matter of fact formyl aldehyde or vegetable myelin
(I mean Pringsheim's hypochlorin) were actually an earlier product of assimi-
lation, from which the starch is developed in the chlorophyll, which is not the
case, there would still not be the smallest item to alter in the statements which
I have made since 1862.

Having learnt to understand the most important function of chlorophyll, I
will now complete the natural history of this remarkable body by a few passing
statements as to the rest of its behaviour in different phases of Hfe of the organs
of the plant. It has already been mentioned that leaves which develope from
the buds in the absence of light, with a few exceptions, produce the first
rudiments of chlorophyll-corpuscles, it is true, but these remain small and usually
yellow, and likewise that the green chlorophyll-corpuscles can increase in number
by division when the cells grow.

I convinced myself, in 1863, by a detailed investigation of the changes which
take place in the assimilating parenchyma of leaves in Autumn, before their falP,
of the fact that a structure so valuable with reference to the life of the plant as
the chlorophyll-substance, is not destroyed and chemically decomposed at once,

* With respect to Pringsheim's Hypochlorin, Pfeffer expresses himself in his ' Pflanzen- Physio-
logie^ (B. I, pp. 194, 195, and 209) in very measured terms, it is true, but still protesting and
dissentient. Pringsheim's treatise ' Ueber Lichtivirkiing und Chlorophyllfitnk/ion in der Pflanze"
(Jahrb. f. wiss. Bot., Bd. XII, p. 288) is thoroughly examined in Hansen's paper on the ' Geschichte
der Assimilationstheorie' (Arbeiten des bot. Inst, zu Würzburg, B. II, p. 606).

2 Cf Sachs, ' Entleerung der Blätter im Herbst' (Flora, 1863, p. 200).

CHANGES IN THE CIILOROPIIVLI. OF LEAVES TN AUTUMN. 319

at the end of a period of vegetation ; but that it is conveyed intcj the persistent
reservoirs of reserve-materials, which put forth shoots again at a later period of
vegetation. The process does not always run the same course in trees of different
species. In the Horse-chestnut, for example, as well as in Dioscorea balaias, the
form of the chlorophyll-corpuscles is destroyed at the same time as their green
colour, atid the chlorophyll disappears simultaneously with the starch contained in it.
In the Vine, I found that the form of the chlorophyll-corpuscles is first destroyed,
while the starch disappears, the green colour of the amorphous chlorophyll being
maintained however for some time. In Sambuctis Popiilus, and Robinia, on the
other hand, the starch first disappears from the chlorophyll-corpuscles, while the
form and green colour persist for some time longer. In the Mulberry {3Iorus
alba), the form of the chlorophyll-corpuscles is first destroyed, then the green
colour disappears, and finally the matrix together with the starch contained
in it. These observations do not preclude that in the same species of plant the
processes in the cells containing chlorophyll may take their course sometimes in
the one, sometimes in the other way. It is not always possible to see from the
outside whether the autumnal emptying of the leaves has already commenced.
When the leaves become pale, however, the destruction of the chlorophyll has
already begun ; and when they have turned yellow it is completed. On the other
hand, in leaves which are still green in September and October the form of the
chlorophyll- corpuscles may be already destroyed, as in the Vine, Poplar, Robmia, and
Sambucus. With the general disappearance of the cell-contents, the dissolution
of the cell-nucleus and protoplasm, the peripheral chlorophyll-corpuslces lose their
normal outHnes, assume irregular forms, the contained starch disappears, and their
colouring matter undergoes changes : they become pale green. Oil-drops often
a[)pear in the cells, the quantity of chlorophyll perceptibly diminishes, the de-
formed corpuscles become smaller, and, when they have finally disappeared entirely,
a large number of very small granules remain behind in the cell-sap : these refract
light strongly and are coloured bright yellow, but they are to be in no way com-
pared with the immature yellow chlorophyll-corpuscles of leaves grown in the dark.
They often flow together into large oily drops, and evidently form a residuum of
no further use in the economy of the plant. It is these yellow granules which
cause the autumnal yellow colouring of so many leaves, and which also remain
behind in the emptied cells of leaves which turn red in the Autumn ; in this case,
however, they lie in a homogeneous red cell-sap.

The cell-contents of the leaves of plants placed subsequently in the dark
undergo very similar changes, which are particularly rapid at high temperatures.
And various other circumstances which disturb nutrition, such as persistent drought,
or lack of nutritive matters generally, bring about the same processes even in
bright light. In all these cases the process described commences in the oldest
leaves, and advances to the younger ones ; the leaf-cells remain distended with
sap but their volume appears notably to diminish. The emptied cell-skeletons
are finally cast off for the most part, since, as Mohl showed in detail, a new
layer of cells is formed cutting across the base of the petiole, and preparing
the petiole for the fall. Then, with the advent of the first frosty nights at the
end of October or the beginning of November, a plate of ice is formeil in this

320

LECTURE XIX.

separating layer, which melts in the rays of the morning sun, whereupon the leaves
suddenly fall in numbers from the trees.

There can be no doubt that the contents of the assimilating parenchyma
of the leaves before the fall are taken up during the changes described into
the persistent parts of the plants — into the cortex or young wood of the branches.
I was able by micro-chemical methods to follow distinctly the travelling of the
materials, especially of the starch, out through the tissues of the petiole into the
shoot-axes ; and moreover the ash-analyses of assimilating green leaves, compared
with those of fallen ones, show that the most valuable mineral constituents of the
leaves, specially the potash and phosphoric acid, also pass out through the leaf-
stalks simultaneously with the organic substances, and back into the parts of the
shoot which survive the winter, evidently to serve, like these, as nutritive matters
for the newly sprouting shoots in the next period of vegetation.

While this process repeats itself each Autumn in perennial plants, it only
takes place once in the annual summer plants, and, generally, in those plants
which only fruit once. In our cereals, for example, and other cultivated plants,
all the still useful materials M'hich were contained in the leaves and shoot-axes
are collected finally, when the fruit is maturing, in the ripening seeds, either in
the endosperm or in the large cotyledons of the embryo, to be employed later,
on the germination of the latter, as materials for the construction of the growing
parts of the seedling. Hence the vegetative organs of such plants after the ripening
of the fruit (generally named straw, etc.) contain only exceedingly small quantities of
material capable of being employed for further growth : they consist of emptied
net-works of cell-walls, with slight remnants of other matters. Of ash constituents,
the whole of the silica, as well as the greater part of the lime in the form of
calcium oxalate, remains behind, as a rule, in the emptied organs of assimilation.

The processes are somewhat different in the chlorophyll-corpuscles of those
organs which are not to be regarded like the leaves as assimilating organs in the
proper sense, and the chlorophyll-corpuscles of which are probably only green
starch-forming corpuscles in the sense of Schimper. I found, for example, that the
chlorophyll-corpuscles in the antheridia of Niiella and Chara, as well as those of
some Mosses, which enclose starch in the unripe state, assume a red colour when
the antherozoids are mature, but maintain their form, and the enclosed starch does
not disappear : profound chemical alterations of the plastic substances do not, how-
ever, take place here. Much more thorough is the destruction of the corpuscles,
at first green, in the pericarp of those berry-like fruits which appear red or deep
yellow in the ripe state, e. g. Lycium and various species of Solatium. The chlo-
rophyll-corpuscles of these pericarps, as they turn yellow and red, change their
form also : they become angular, two and three pointed, and finally break up into
small granules. In conclusion, the remark may be made here that the bearers
of the yellow colouring matter to which many floral leaves owe their yellow
colour (e. g. the corolla of Cuairlila), resemble chlorophyll-corpuscles, or better,
Schimper's starch-forming corpuscles.

When organs of assimilation become dormant at the conclusion of a period
of vegetation, to renew their activity in the following year, alterations take place
in the contained chlorophyll also : under certain circumstances these however

THE COLOURS OF LEAVES IN WINTER.

321

are less profound. The green spores of many Algae, for instance, become deep
red, without any essential changes of the plastic material occurring at the same
time : with the commencement of the new period of vegetation, the red colour-
ing matter disappears, the chlorophyll again begins to assimilate, and the cells
to grow. In some Phanerogams, the leaves which persist through the winter
behave similarly in many respects : here, however, the intense cooling in winter,
particularly by radiation, co-operates. After Mohl had remarked these processes
in 1845, they were more closely studied by Kraus in 187 1 and 1872. These
winter-leaves either become simply discoloured and . brownish, yellow, or red-brown,
as in Taxus, Pinus, Abies, Juniperus, and Biixus, or of a decided red on the
upper side, as in Sedum, Sempervivum, Mahonia, and Vaccinium. The dis-
colouration of the first group depends, according to Kraus, on an alteration of
the chlorophyll : the chlorophyll-grains lose their shape and distinctness, and form
an ill-defined cloudy mass of protoplasm of a red-brown or brown-yellow colour,
while the nucleus of the cell remains colourless. The winter-leaves which are
coloured red or purple-brown on the upper side owe this colour, according to
Kraus, to a rounded, hyaline, strongly refractive mass, in which tannin predominates,
situated in the upper part of the palisade-cells : the chlorophyll-grains themselves
are intact, and of a fine green, and are all crowded at the inner ends of these cells.
In the spongy parenchyma of the mesophyll, a red or colourless globule of tannin
is found in the centre of each cell, and the chlorophyll-grains, likewise intact,
are collected into lumps on the sides, sometimes at one, sometimes at several places.
In all winter-leaves, as well as in the green parts of the cortex, Kraus found that
the chlorophyll-grains had passed from the walls to the interior of the cells, and
were there aggregated in clumps. In the spring, in sufficiently warm weather,
the normal condition is restored : the red colouring matter disappears, and the
chlorophyll -grains again assume their normal distribution on the cell-walls. If
branches of the first-named group of plants are cut off in the cold of winter and
brought into a warm chamber, they assume their normal green colour after a few
days, even in the dark.

It has already been mentioned that the green colouring matter of the chlo-
rophyll-grains can be extracted by strong alcohol, aether, chloroform, and also
by fat oils, the colourless protoplasmic matrix remaining behind. Chemists
have taken much trouble to investigate the chemical nature of this extracted
colouring matter, without having obtained so far any satisfactory result. It
is not even established whether the chlorophyll colouring matter, for the
production of which small quantities of iron are necessary, as I have shown,
itself contains iron. The view proposed by the French chemist Frdmy, that
the green colouring matter is a mixture of a yellow and a blue, has not been
confirmed, since it is scarcely doubtful that the apparent breaking up of the
colouring matter into two others is simply a chemical decomposition \ It is not
necessary, however, to go further into this discussion; since the views so far
proposed as to the chemical nature of the chlorophyll colouring matter have no
reference to the physiological functions of chlorophyll. The same is true of the

' See my 'Lehrbuch der Bot' IV. Aufl. 1S74, p. 731.
3]

322

LECTURE XIX.

spectrum of the chlorophyll colouring matter, so very remarkable in itself. If
a parallel-walled vessel filled with an alcoholic solution of chlorophyll is placed in
the path of a beam of light (p. 303) which affords a spectrum band by means of
a prism, the whole of the violet and blue disappears from the spectrum, as do also
those ultra-violet rays which become luminous when falling upon a solution of quinine.
At the same time, a broad, black stripe becomes visible in the red part of the spectrum,
and in the yellow region also a feeble absorption of light occurs. The green colouring
matter is also strikingly fluorescent : if the focus of a burning-glass is allowed to fall
in a solution of chlorophyll, the white sunlight appears blood-red. As Stokes showed
long ago, this phenomenon is caused by the fact that the highly refrangible
light-rays become changed into lowly refrangible red ones. By an erroneous
inversion of the correct state of affairs — that the light-rays eflfective in the as-
similating chlorophyll must be destroyed as such — Lommel arrived at the fallacy
that the rays destroyed in the chlorophyll spectrum are those active in assimilation.
In this he left out of account that the rays absorbed in a solution of chlorophyll
are the same as those absorbed in a living green leaf. In a solution of chlorophyll,
however, no separation of oxygen from carbon dioxide takes place : according
to Lommel, also, it must be the most strongly absorbed red rays which bring about
assimilation, whereas all direct observations show that the maximum evolution of
oxygen takes place in yellow light ^

On the whole, investigations on the spectrum of chlorophyll have hitherto
yielded no facts of any physiological value — i. e. we should know quite as much
of the physiological function of chlorophyll if its spectrum were absolutely un-
known to us.

^ Cf. Sachs' 'Lehrbuch,' p. 732.

^^-^ LECTURE XX.

CHEMICAL METAMORPHOSES OF THE PRODUCTS OF ASSBHLA-
TION. PHYSIOLOGICAL CLASSIFICATION OF THE PRODUCTS
OF METABOLISM.

Starch being the sole first visible product of assimilation, it follows obviously
that all the other organic compounds of the plant must be produced from it by-
chemical changes. This statement offers in fact no difficulties even from the chemical
side, so far as the majority of the constituents of the plant are concerned, i.e. the
carbo-hydrates, of which cellulose, sugars, and inulin especially come into considera-
tion here. These substances are chemically so nearly allied to each other, that not
only starch and inulin, but even cellulose may be transformed by the simplest
chemical reactions into sugars ; and from the physiological processes it may be
demonstrated with certainty that starch and cellulose are produced from sugar and
inulin within the living plant.

Besides these carbo-hydrates, however, the fats also play a prominent part in
the economy of the plant. Perhaps there is no living protoplasm which contains
no fat : but its physiological significance becomes much more evident in that the
great majority of ripe seeds contain large quantities of fat together with starch, or
very great quantities of fat without the latter. They may contain as much as 600/0
and more of their weight of fat; and even the spores of very many Cryptogams
abound in it. That sugar and starch are fortned during germination at the cost of
the fat which is accumulated in the seeds and then used up, I demonstrated in detail
in 1859 on a series of oily seeds ^ ; and it was already known that, before maturity,
such seeds contain no fat, but only starch and sugar. Such unripe seeds (e. g. of
PcEonia) may be detached from the mother-plant, and allowed to lie in moist air, with
the result that the starch disappears and is replaced by fatty oil. From such obser-
vations it follows with certainty that the fats of the plant may be produced from
carbo-hydrates, and that the latter are formed at their expense. It is practically
immaterial for us how far chemical formulas are able to afford information as to the
processes here taking place ; in any case we arrive at the result that the carbo-
hydrates, as well as the fats of the plant, derive their origin from the starch assimilated
in the chlorophyll-grains.

The derivation of the third most important group of the materials of the plant,
viz. the proteid substances, from the original product of assimilation, does not appear

^ On the formation of starch during the germination of oily seeds, cp. Sachs' Bot. Zeitung,
[859, p. 178.

3^4

LECTURE XX.

quite so simple. It depends here no longer merely upon various transpositions of the
atoms of carbon, hydrogen, and oxygen in the molecules, as in the case of the carbo-
hydrates and fats : the proteid substances from which protoplasm is formed contain,
besides these elements, also a considerable quantity (about 1 5%) of nitrogen, and a
small quantity (about lo/o) of sulphur. It is obvious that if proteid substances are
to be derived from the first product of assimilation, starch, this can only take place
in so far as, in the course of metabolism, carbo-hydrates, or perhaps also fats, afford
the one constituent of proteid matters, while the nitrogen and sulphur are derived
from other compounds which the plant has taken up through the roots. It is certain
from what has been said concerning the nutritive materials of plants, that the nitro-
gen of the proteid substances is entirely derived from salts of nitric acid, in the more
highly organised plants at least ; while the sulphur is provided by salts of sulphuric
acid. It is essential, therefore, in order that proteid substances may arise in the plant,
that, on the one hand, starch is produced in the chlorophyll, and that, on the other,
salts of nitric and sulphuric acid are taken up by the roots. These ingredients must
somewhere and in some manner come together in the plant : chemical decomposi-
tions must take place in which, above all, the salts mentioned yield up their bases, so
that the nitrogen and sulphur, after certain unknown processes, may finally come into
combination with the elements of a carbo-hydrate or fat in order to produce a proteid.
Unfortunately, organic chemistry here leaves us unaided, so far as regards the question
of the chemical processes in the formation of proteids from the ingredients mentioned
above. The accuracy of our deduction is by no means enfeebled by this, however ;
and we will now see how far we can proceed by purely physiological methods.

It is, in the first place, a very important question whether the whole of the proteid
substances of an ordinary plant is or is not produced during the process of assimila-
tion in the cells containing chlorophyll : at any rate, sulphates and nitrates can
penetrate as far as the green leaves, and there is scarcely anything absurd a pri'orz in
the idea that the formation of proteids perhaps begins with their aid, even during
the process of assimilation in the chlorophyll. But we have certain facts which show
that, although in general only certain non-nitrogenous organic compounds, and espe-
cially sugar, exist in them, even cells devoid of chlorophyll, and which therefore do
not assimilate, are in a position to produce proteid substances on the addition of
nitrates and sulphates. This proof Pasteur aff"orded by nourishing yeast-cells with
sugar and the salts named ; an experiment which may be easily repeated, and which
yields the result that by this means a few yeast-cells give rise to millions of new ones,
each of which is filled with living protoplasm. This is simply possible from the fact
that the yeast-cells are able to produce proteid substances from the materials — sugar
and salts of nitric and sulphuric acids — off"ered to them. If Yeast and other Fungi
are able to do this, however, there is no ground for assuming that cells devoid of
chlorophyll in the highly organised plants are not also able to produce proteid sub-
stances, taking the ingredients for this from their own carbo-hydrates, and from the
salts above mentioned which are absorbed by the roots. On the other hand, again,
this does not prove that in special cases proteid compounds may not also arise in cells
containing chlorophyll. The so-called unicellular Algse, the Palmellaceae, Conjugatese,
etc., consist indeed only of cells containing chlorophyll, and these, so long as they
vegetate, increase the quantity of their protoplasm (that is, their proteids) in pro-

FORMATION OF PROTEIDS. 325

portion as their cells divide. Whether this occurs simultaneously wiih assimila-
tion in the light, or subsequently during the following night, is of course not
known. At any rate, the facts brought forward teach that proteid-like compounds
from which protoplasm arises can be formed independently of assimilation, provided
only that the necessary constituents are conveyed to the cells concerned. Among
these constituents an organic carbon compound is essential ; and this, in its turn, is
of course to be finally derived — though it may be by the most various and round-
about ways — from the starch assimilated in the chlorophyll.

In the case of highly organised plants I have already sought to render it pro-
bable that the formation of proteid substances takes place in the sieve-tubes of the
vascular bundles. The sieve-tubes in the younger shoot-axes and leaves contain an
amorphous proteinaceous slime, which is by no means to be confounded with pro-
toplasm. It consists, to use a modern expression, not of organised or living, but
of circulating proteids, evidently being conveyed through the sieve-tubes to the younger
growing organs. We have many indications, however, that it is probably also formed
in these sieve-tubes : in the first place we find in the neighbourhood of the sieve-
tubes of the organs in question, regular layers of cells which contain fine-grained
transitory starch and sugar, and extremely fine-grained starch is frequently found in the
slime of the sieve-tubes themselves. I regard this starch as one of the constituents
from which the proteinaceous slime of the sieve-tubes is to be formed. Holzner
long ago gave expression to the idea that the formation of oxalic acid in the plant
is for the purpose of decomposing the calcium sulphate taken up by the roots,
and thus setting free the sulphuric acid for the formation of proteid substances
which contain sulphur ; and it is certainly an observation favourable to this sup-
position that, very generally, layers of cells in which calcium oxalate crystallises out
run in the neighbourhood of the bundles of sieve-tubes, from which indeed it may
well be concluded that the sulphuric acid in the calcium salt may be employed
here or in the neighbourhood, or even in the sieve-tubes themselves, for the
formation of proteid substances. With respect to the third ingredient, a salt of
nitric acid, we know that saltpetre is distributed in the tissues of a normal plant.
The constituents are thus given, and the result, the proteinaceous slime in the sieve-
tubes also. The reply to these suggestions, that the quantity of calcium oxalate
does not accord with the amount of sulphur contained in the proteid substances in
question, appears to me of little weight, since, on the one hand, a decomposition of
calcium sulphate by means of oxalic acid may take place also for other purposes
in the plant, and thus under certain circumstances (e. g. in some species of Caches
and Aroidege) an enormous quantity of calcium oxalate be accumulated, while in
other cases, on the other hand, the decomposition of salts of sulphuric acid may
occur without the formation of calcium oxalate.

However, the formation of proteid substances in the plant may also take
place in another way, and perhaps in any parenchyma cell whatever, since it may
be that another body abounding in nitrogen, one of the group of amides, is first pro-
duced, namely, asparagin, a crystallisable substance soluble in the cell-sap, but w^hich
contains no sulphur. Asparagin, as recent researches, especially those of Borodin '

Borodin, Bot. Zeitung, 1878, p. 801, 'lieber die Verbreitung des Asparagins'.

326 LECTURE XX.

have shown, is widely spread in the vegetable kingdom, especially in growing shoots
and perennial root-stocks and tubers. It is of course still questionable whether the
asparagin arises in these organs by synthesis, to be converted by further synthesis and
the assumption of sulphur into proteid substances. That something of the sort
actually occurs under certain circumstances Pfeffer has demonstrated with seedHngs
of Lupins, etc. In these cases, however, the asparagin itself had previously arisen
by a splitting up of the proteid substances of the seed ^ At any rate the possibility
exists that in many cases asparagin may be formed in the first place from carbo-
hydrates and nitrates, to be converted later, with the assumption of sulphur from the
decomposition of a sulphate, into proteid substances. That, conversely, asparagin
arises from the proteid substances of many germinating seeds (e. g. Luptnus), root-
stocks, and tubers (e. g. Potato), to be subsequently reconverted into proteid, when the
green organs begin to assimilate, cannot be doubted from the researches of Pfeffer,
Schultze, and others.

It is now decided that all the various carbo-hydrates, fats, and proteid substances
of the plant are derived from the starch assimilated in the chlorophyll. This is
practically equivalent to saying that all the organic substances necessary for the
construction of the cells and organs of the plant, are to be referred to the activity
of the assimilating chlorophyll, since the plant requires for the construction of
cells and organs generally only these three groups of materials. I have consequently
classified them as the constructive materials of the organs, in contrast to all other
organic substances of the plant. Seeds or tubers, bulbs, rhizomes, buds of trees and
even spores — the unicellular reproductive organs of Cryptogams — may be made
to develope new roots and shoots, and occasionally even flowers, by growth, though
no other materials are placed at their disposal than pure water and the oxygen of the
atmosphere : the latter however is only necessary for respiration, by which means
a considerable portion of the organic substances is completely destroyed. Since now
all the parts of plants above named — which we term reservoirs of reserve-materials,
because the materials for germination in the widest sense are stored up in them
— contain essentially only proteid substances, carbo-hydrates, and fats, or it may
be only proteids and fats, as in many seeds {Ricinus, Brassica, Cucurbita, Almond,
etc.), it is obvious that the new roots and buds are able to develope as the
substances mentioned are used up in growth. This is still more obvious when
the development of seedlings is allowed to take place in profound darkness, where
assimilation cannot come into consideration at all ; or when the developing seedlings
are left in an atmosphere from which every trace of carbon dioxide has been withdrawn.
At the conclusion of such a process of germination, the reservoirs of reserve-materials
are emptied, and when the growth finally ceases, no nutritive matters being admitted
from without and no assimilation taking place, we have before us a new plant which
consists of cellulose walls and relatively small quantities of protoplasm, cell-nuclei,
and chlorophyll-grains (etiolated or not). These constituents of our young plant
have been produced from the reserve-materials, proteids, carbo-hydrates, and fats, with
the aid of the water and oxygen taken up. Of the two groups last named, how-
ever, a considerable quantity has been completely destroyed during the growth, by

Cp. Sachs' ' Lehrbuch der Botanik^ IV. Aufl., p. 690.

DEVELOPMENT AT THE EXPENSE OF RESERVE-MATERIALS.

327

respiration, so that the dry organic substance of the young plant weighs less than
that of the reservoirs of reserve-material

These observations, easily made with a little experimental adroitness, show
moreover not merely that the three groups of substances mentioned completely suffice
for the construction of the organs of the plant ; but we obtain from them another
important piece of information. In order to render this quite clear, we may suppose

FIG. 22S.—Pintis rinea. I Median longitudinal section of the seed, the micropyle end being at j/ ; // commencement of germina-
tion, the radicle extruded ; /// end of germination after the endosperm has been absorbed (the seed lay too near the surface of the
soil and has therefore been carried up by the cotyledons as the stem elongated). A shows the ruptured seed-coat j; B shows the
endosperm e after the removal of one half of the seed-coat ; C longitudinal section of the endosperm and embryo ; D transverse section
of the same at the commencement of germination—«: cotyledons, -u' primary root, x the embryo-sac pushed aside by the primary root
(ruptured at x in Ä), h c hypocotyl, iu' lateral rootlets, r red membrane within the hard testa.

seeds of the Pine, which contain only proteid substances and fat, or those of the
common garden^Bean, in which only very small quantities of fat but large masses of
starch are deposited in addition to the proteid substances, to be allowed to germinate,
until all the reserve-materials are consumed. In the seedling of the Pine we then
find resin-passages filled with balsam, and tannin may be detected in various cells ;
in that of the Bean are found numerous longitudinal rows of cells, densely filled with.

328 LECTURE XX.

mixed solutions of tannin and red colouring matter. Chemical analysis would
in both cases detect small quantities of various other chemical compounds. It
is clear that the substances mentioned, resin and tannin, as well as the vegetable
acids which are never wanting in growing organs, must also have arisen from the
reserve-materials ; and since it is known that the quantity of proteid substances
is not lessened in germination, it follows that the tannin and resin, vegetable acids,
and other non-nitrogenous constituents also of the seedling must have arisen in
the Pine from the fat, and in the Bean from the grains of starch. Among these
materials, however, the resin and tannin present yet further the remarkable peculiarity
that they do not subsequently disappear from the reservoirs in which they are
collected during germination; they, as well as the calcium oxalate, remain lying
unemployed where they have once arisen. What has here been said of the Pine and
Bean, for the sake of example, obtains moreover generally. The resins, ethereal
oils, most tannins, as well as various (probably not all) gum-like contents of
the reservoirs of secretions described previously, are substances which are withdrawn
from metabolism, and find no further use in the nutrition and in the growth of the
organs ; and they are thereby essentially distinguished from the three groups of con-
structive materials. It is probable that the vegetable alkaloids are also to be counted
among these useless secretions, and the colouring matters so frequently appearing
in the vegetating organs certainly are. How and why all these substances, which are
very different in different plants, originate is not known ; for us, however, the im-
portant point remains that they simply find no further use in metabolism.

Finally, it is to be mentioned here that some organic substances do not arise
in the metabolism which accompanies growth, but are formed by the later meta-
morphoses of previously organised parts of cells, and then likewise find no further
use in metabolism. To these belong, chiefly, bassorin, gum-tragacanth, gum-
arabic, linseed-mucilage, and similar bodies, all of which arise by means of sub-
sequent chemical metamorphosis of the cellulose of certain cell-walls : like the
secretions, they may, under certain circumstances, be of use for the plant, but they
take no further part in metabolism itself. That protoplasmic structures also suffer
subsequent degradation, and either wholly or in part find no further use in meta-
bolism, follows from what has been said in the last lecture on the destruction of
chlorophyll ; and it appears also that old cell-nuclei, as well as the thin protoplasmic
utricles of old parenchyma cells, often remain as inert masses until the destruction
of the organ. The share in metabolism taken by some other organic compounds,
such as pectinaceous substances, the so-called glucosides, and some still dubious
tannins, is still very doubtful. The same is also true of the vegetable acids, malic
acid, citric acid, tartaric acid, formic acid, acetic acid, etc. ; the wide distribution
of which is no doubt usually the cause of the acid reaction of the cell-sap in the
parenchyma. According to a view recently expressed by Hugo de Vries\ the
significance of these acids in growth is chiefly mechanical, in so far that they increase
the turgescence of the growing cells, and so contribute to the extension of the
growing cell-walls. It has already been mentioned above that the oxalic acid (which

' On the significance of acids in turgescence and in the growth of vegetable cells, cp. De Vries,
Bot Zeitung, 1879, P- ^47-

SECONDARV USES OF EXCRETA, ETC. 329

likewise belongs here) is, as a rule, of no further use in metabolism, especially when
it crystallises out as calcium oxalate; nevertheless, under certain circumstances,
calcium oxalate becomes again dissolved, and occasionally the acid is found com-
bined with alkalies, as in the Sorrel and the Wood-sorrel. In these cases it is
possible that it is not yet entirely withdrawn from metabolism. That the vegetable
alkaloids when once formed do not again enter into metabolism, as a rule at any
rate, appears probable from what is known as to their occurrence.

The main point in determining the physiological significance of a vegetable sub-
stance is always whether, when it has made its appearance anywhere in the tissues, it
then again disappears from its receptacle during further growth and progressive meta-
bolism, or remains lying inert in it, in which case a further share in metabolism is
obviously excluded. The latter is the case with nearly all the substances contained in
the so-called receptacles for secretions. Of course substances which find no more use
in metabolism may be incidentally of the most varied significance for the maintenance
of the individual, as well as for reproduction. Thus, for example, the lignin which
permeates the cell-walls of vessels, wood-cells, and sclerenchymatous fibres, is of
considerable importance for the solidity of such cell-walls and the conduction of
water in them; and likewise the wax in the epidermis and on the cuticle affords
a means of protection against evaporation, and at the same time against the entrance
of water; and acids at the surfaces of the roots, as was shown above, aid in the
taking up of absorbed nutritive matters. The colouring matters of flowers are
assumed to play a great part in pollination, in so far that insects are allured by
them to the organs of fertilisation ; the large quantities of sugar and organic acids
which accumulate in the flesh of fruits, — e. g. Grapes, Oranges, etc., — are also of
course lost to metabolism, but they may be of use to wild plants in so far that
birds and other animals devour such fruits, and subsequently rid themselves of
the undigested seeds. That the formation of any substance in the plant may also be
essentially influenced by culture and the artificial selection of varieties is shown by
the ever-increasing number of the various colours of garden flowers, and the great
number of different flavoured varieties of orchard fruits and Grapes, all of which have
been produced by cultivation.

Meanwhile, however, these are for us but matters by the way, since we are now
concerned only with establishing that all the different substances of the plant are
derived, directly or indirectly, from the starch arising in the assimilating chlorophyll ;
and that, in the first place, only a very limited number of products of metabolism —
namely the carbo-hydrates, fats, and proteid substances — come into consideration
as the constructive materials of the growing organs of the plant. All other
substances, occurring as secretions or as products of degradation, are derivatives
of these.

When now starch arises in the leaves under the influence of light, it, as well as
the other plastic substances produced at its expense, is conveyed through the petioles
to the shoot-axes, to proceed in the interior of these organs partly downwards to
the growing apices of the roots, partly upwards to the developing buds, and to supply
at both places material for the growth of the organs. Hence, as occurs particularly
in annual summer plants, assimilation and the employment of the products of
assimilation for growth may proceed simultaneously, without any considerable storage

33°

LECTURE XX.

of unemployed plastic substances taking place. In most cases, however, and particularly
towards the conclusion of the period of vegetation, the quantities of such materials
used up for the purposes of growth are smaller than those produced by assimilation.
The superfluous products of metabolism thus arising are then stored up in the tissues
of the plant, to be preserved until the beginning of the next period of vegetation.
In this condition they are termed reserve-materials, a name which was introduced by
Theodore Hartig some years ago. The reserve-materials may be accumulated in
the parenchyma and phloem of all ordinary perennial organs, particularly in trees
and other woody plants ; the twigs, branches, roots and stems of which have their
cortex and the parenchymatous cells of the alburnum filled during the summer and

-A Potato plant developed from seed, r r the roots ; c cotyledons ; ff leaves ;
which produce the tubers tb (after Duchartre).

' subterranean shoots

autumn with reserve-materials, which are used up when the buds put forth shoots
in the spring. If the formation of seeds takes place at the end of the period of vege-
tation, large quantities of reserve-materials are accumulated to a remarkable degree in
these, all non-plastic compounds being excluded ; and in annual plants the seeds are
in fact the only reservoirs of reserve-materials. In all perennial plants, however,
it is chiefly the subterranean root-stocks, rhizomes, bulbs, and tubers, etc., which
especially fulfil the function of storing the substances assimilated during the period of
vegetation ; and on the formation of the germinal shoots and roots at the beginning
of the next period of vegetation, they yield them up to these growing parts. In general,

RESERVOIRS OF RESERVE-MATERIALS.

33^

organs of the most various kind may be employed as reservoirs of reserve-materials.
In the so-called evergreen plants, even the green leaves serve during the winter
as accessory organs for the storage of the products of assimilation; and in the
Algae it is a very common phenomenon that cells abounding in chlorophyll become
densely filled with reserve-materials at the conclusion of the period of vegetation.
What has here been shortly said is only to serve generally as indicating the most
important points, since the vegetable world is simply inexhaustible in the variety of
phenomena of organisation connected with the storage of reserve-materials, and their

ii ^ii

I'IG. 227.— Subterranean parts of a flowering plant of ColcJticum autumnale. A external view from the front, k the corni ; s' and s"
scale leaves enveloping the flower-stalk ; luh base, from which the roots (lu) spring. B longitudinal section of the preceding (plane of
section perpendicular to the paper), hh a brown membrane enveloping all the subterranean parts of the plant ; st the stalk which bore
flowers and foliage during the previous year — it is dead, and only its basal portion, k, swollen into a conn still exists as a reservoir of
reserve-materials for the Jiew plant now in process of flowering. This flowering plant is a lateral shoot from the base of the corm k ;
it consists of the axis, from the base of which spring the roots W and the middle portion of which k' swells into the corm for next year,
the old corm it disappearing in the meanwhile. The axis supports the sheathing leaves s s' s", and the foliage-leaves /' /" ; in the axils
of the uppermost foliage-leaves are the flowers * *', between which appears the free apex of the axis itself. The foliage-leaves are still
small at the time of flowering ; they, together with the fruits, protrude above the surface of the earth during the following spring.
The portion of the axis i' then swells up into a new corm, on which the axillary bud ü" developes into a new flowering axis, while the
sheath of the lowermost foliage leaf becomes converted into the enveloping brown membrane.

subsequent employment. Even Fungi, the nutrition of which we shall take more
closely into consideration later on, occasionally form reservoirs of reserve-materials —
so-called sclerotia. The entire mode of life of a plant is connected most intimately
with the ways and means of employing the persistent organs as reservoirs of reserve-

332

LECTURE XX.

materials. Very frequently, at the end of the period of vegetation, nothing more
remains of the entire richly-leaved and rooted plant developed in summer, than simply
a reservoir of reserve-materials, in connection with one or more buds. This is the
case for example in species of Ophrys, in Aconitum napellus, and in most plants with
bulbs and tubers (e.g. Colchicum : Fig. 227). The main mass of such a perennial
body consists of parenchymatous tissue filled with plastic reserve-materials ; while
the germinal shoots to be developed later persist through the winter in the form of
small buds. On the whole, the principle is the same in seeds also ; the parts of the

embryo properly capable of development
(i. e. the plumule and radicle) which they
contain are extremely small, even in very
large seeds, while the mass of the reserve-
materials is incomparably larger. These
reserve-materials are either deposited in the
endosperm, from which the young plant
subsequently absorbs its nutritive matters
on germination ; or the first two leaves
of the seedling, the so-called cotyledons,
themselves grow to an enormous size, and
become filled with the reserve-materials,
which are then conveyed on germination
forthwith into the tissues of the growing
organs. For the elucidation of the matters
of organisation which, though closely con-
nected with nutrition, are only slightly
touched upon here, I must refer to the
figures and explanations distributed through-
out the text of this lecture.

The form in which the plastic materials
are preserved in the reservoirs of reserve-
materials may be very various; the main
point always being that, in addition to

FIG. 228. -Bulbs of FHtiUaria <'"PerMis in November. pfOtcid SUbstanCCS, nOU-nitrOgCnOUS COm-

A longitudinal section of the entire bulb (recUiced) ; z z the » » o

united lower portions of the bulb-scales, * * their free upper pQUudS alsO arC prCSeUt, thc maSS of which

portions— they surround a cavity / which contains the remains ^ '■

of the decomposed flower-stalk. In the axil of the innermost USUallv far prepOUderatCS. ThcSC lattCr, SO
leaf of the bulb is the bud k which will develope in the follow- J i" i" )

ing year. Its first leaves will form the new bulb, while its far aS TOOt-StOcks, bulbS, tuberS, aud Other
stem developes as the flowennfr-stalk— the root -w springs from '

the axis of this bud. 5 longitudinal section of the apical succulcnt rcscrvolrs of rcscrvc-materials are

region of the next year's bud; s apex of the stem, * l> bb"

youngest leaves. concemcd, arc usually carbo-hydrates, with

a more or less considerable percentage of
fat. In seeds, on the other hand, it is more frequently the case that the starch
entirely disappears on ripening, and, together with the proteid matters, large
quantities of fat only remain over as reserve-materials.

The form in which the proteid substances come to rest in the reservoirs of re-
serve-materials may be that of protoplasm itself, at any rate in succulent organs, such
as bulbs, tubers, and rhizomes ; at least in many such cases considerable quantities
of living protoplasm are found in the reservoirs of reserve-materials, which, when

FORM IN WHICH RESERVE-MATERIALS ARE STORED.

333

the reserve-materials are consumed, are likewise made use of. This is very distinctly
the case, for instance, in the scales of the common kitchen onion and the tuber of the
potato. In such cases, however, albumin or a soluble proteid substance appears to
be present in the cell sap ; in rarer cases a portion, always relatively small, of the
non-organised proteid substance may exist in the form of crystalloids. This is best
known in the tuber of the common potato, where, especially close beneath the skin but
occasionally also in the whole parenchyma, magnificently developed colourless cubes
or even tetrahedra are found lying in the protoplasm in autumn and winter. The

Fig. 229. — A crystalloid of the potato tuber,
altered by alcoholic solution of iodine and gly-
cerine : in the fresh condition it is a perfect cube.
A spherical layer enclosing a small cube has be-
come separated in the

Fig. 230.— a few cells from d very thm section
through the cotyledon of Pisitm sati-jum still in the ripe
seed. The large grains St with concentric layers are
starch-grains (cut through) ; the small granules a are
aleurone-grains consisting chiefly of legumin, with a
little oily matter ; i intercellular spaces.

FIG. 231.— Cells from the cotyledon in a ripe seed of
Lupiniisvarius. A in alcoholic solution of iodine ; .ff after
the destruction of the grains by sulphuric acid, Z cell-
wall ; * protoplasmic matrix with little oily matter ; y
aleurone grains ; 0 drops of oil separated from the matrix
under the influence of the sulphuric acid ; m cavities
from which the aleurone grains have been dissolved (Soo).

careful studies which have been devoted to these crystalloids, and others to be men-
tioned later, leave not the slightest doubt, on the one hand, that they consist of proteid
substances, and, on the other, that they resemble true crystals in all points, but with
the single difference that they are capable of swelling by imbibition ^ The proteid

5 The crystalloids were discovered by Theodore Hart ig (^c?^. Zeit., 1856), and further investi-
gated by Radlkofer in his book 'Über Krystalle proteinartiger Körper" (^ Leipzig, 1859). Bailey
discovered the crystalloids in the potato tuber i^Flora, 1874, p- 415)- O"'' piesent views on the
nature of crystalloids were founded by Nacgeli, ' Bot. Mitthcilungeu; I, p. 217, 1862 (^Sitzungsbcr.

334

LECTURE XX.

substances of ripe seeds behave differently, according as the other reserve-material
is starch or fat. In starchy seeds (e. g. Beans, Peas, the cereals, Chestnuts, etc.) the
cavity of the cell is for the most part filled with starch-grains, the interspaces between
these containing proteid substances in the form of small, spheroidal granules, which
in the Leguminoseae consist of vitellin, in the cereals of a mixture of various proteid
matters, and in the Wheat particularly of gluten. In fatty seeds (as those of the Coni-
ferese, Palmese, Cucurbitaceae, Umbellifer^, Solaneae, Euphorbiacese, etc.) matters are
quite different. Here, in thin sections through the tissue of the reservoir of reserve-
materials (endosperm or cotyledons) the cell-cavities are seen to be filled for the most
part with rounded brilliant granules, the so-called aleurone-grains, which always consist
of proteid substances. The interspaces between these are chiefly filled with amor-
phous fat, after the careful removal of which a scanty net-work, poor in substance
and of protoplasmic nature, is found, as shown in Fig. 232. The aleurone-grains

are soluble either in pure water or in very
weak alkaline solutions, or also in a iqo/o
solution of common salt. They, in their
turn, contain so-called globoids — i. e. sphe-
roidal, brilliant, minute bodies, said to con-
sist of phosphates of lime and magnesia,
and which at any rate probably always
contain a magnesium salt \ In some
cases, but by no means very frequently, a
crystalloid of proteid substance is con-
tained in each aleurone-grain : these crys-
talloids are particularly fine in Ricijius and
other Euphorbiacese, and also in the coty-
ledons of the Brazil nut {Bertholeiia ex-
celsa), one of the Myrtaceae, and in the en-
dosperm of Musa Htllii, and many other
seeds. These crystalloids remain behind
when the amorphous proteid substance
surrounding them has been dissolved by
water : they are thus less soluble than the latter. According to Schimper's very
careful researches, all the crystalloids of proteinaceous substance hitherto known
belong either to the regular system, as the cubes in the Potato and the octahedra
in the aleurone-grains of Ricinus, or to the hexagonal system, as the large
rhombohedral crystals of the Brazil nut, and those of 31usa Hillii and Spar-
ganium ramosum. This is not the place to enter more in detail into the crys-
tallography of these remarkable structures : it appears more important for the
moment to note that in no case does the whole of the proteid substance of a

Fig. 233.— Cells from the endosperm of Ricinus cotnmunis
(800). A fresh, in undiluted glycerine ; B in diluted glycerine ;
C warmed in glycerine ; D after treatment with alcoholic solution
of iodine, the aleurone-grains have been destroyed, by sul-
phuric acid ; the proteid substance of the matrix remains be-
hind as a network. The globoid and (in B and C) the crystalloid
are seen in the aleurone-grains.

der k. bayer. Akademie). On the distribution of crystalloids, see Klein xa. Jahrb. für wiss. Bot.
(B. XIII. p. 60). What is said in the text is founded particularly on Schimper's works, ' Über die
Kry stalk der eiweissartigen Substajizen' in Zeitschrift für Krystallographie (Leipzig, 18S0), and,
further, ' Untersuchtingen über die Prote'inkry static der Pflanzen'' (Stiassburg, 1878). Cp. also
Pfeffer m Jahrb. für wiss. Bot., B. VIII (1872).

1 Cp. Pfeffer's statements in my 'Lehrbuch,' IV. Aufl., p. 55.

ALEURONE-GRAINS AND CRYSTALLOIDS. 335

seed or other reservoir of reserve-materials pass into the crystalline form, but that
amorphous substance is always present in addition ; and that, as it appears, no crys-
talloids are met with in the majority of seeds, and certainly not in the large majority
of succulent reservoirs of reserve-materials. Where they occur, they become dissolved
during germination, and, like the amorphous reserve- substance as well as that existing
as protoplasm, are conveyed into the growing parts, there to serve for the formation
of protoplasm. Like starch-grains and other reserve-materials, crystalloids of pro-
teinaceous substance occur also in organs which cannot be regarded as reservoirs of
reserve-materials in the narrower sense : here they are met with rather as temporary
deposits of plastic substance for special purposes of metabolism. Here belong above
all the crystalloids discovered by Radlkofer (1858) in the nuclei of cells in almost all
the parts of Lathrcca Sqtiatnari'a, as well as those found by Julius Klein in the nuclei
of the cells of Pinguicula and Utricularia. Klein found crystalloids, not in the
nuclei, but in the living cell-contents of numerous marine Algae, especially the
Siphoneae {Acetahulan'a, Bryopsis, Codium, etc.) and many Florideae {Bometia,
Ceraniiiim, PoIysipho7tia, etc.). Finally, the Rhodospermin crystals discovered by
Cramer, which arise from the cell-contents of the Florideae under the influence of
salt solution, alcohol, or glycerine, may be referred here. Klein also found crystal-
loids in the fructification of Pilobolus, a fungus closely allied to Mucor.

The various carbo-hydrates are better known chemically than the different
kinds of proteid materials. From a physiological point of view, however, it may
be shown that even the most different members of this group — starch, the sugars,
inulin, and cellulose — are in so far of completely equal value that they can replace
one another as reserve-materials. While in most tubers and other succulent
reservoirs of reserve-materials, in the cortex and wood of trees, and in many seeds,
starch is usually stored up as the reserve-material, we find exactly the same purpose
served in the perennial organs of Compositse, in the tubers of Dahlia and Helianthus
tuberosus, and in the roots of Liula Hellenium, Taraxacum officinale, etc., by a
concentrated solution of inulin ; on the formation of new germinal shoots this is
made use of in exactly the same manner as starch is elsewhere. In the beet-root,
again, cane-sugar is accumulated in the parenchymatous tissues towards the end
of the first period of vegetation ; and in the bulb-scales of the kitchen onion a
mixture of grape-sugar and other kinds of glucose occur, and here play the same
part as does the starch in the bulbs of the tulip and crown imperial. But even
cellulose itself, which generally occurs only as one of the final products of meta-
bolism, may play the part of a reserve-material. The endosperm of the Date and
of some other palms, as well as the large endosperm of the seed of Phytelephas
(vegetable ivory), consists of exceedingly thick-walled tissue, the laminated and
pitted cell-walls of which are formed of pure cellulose : this is dissolved on germina-
tion, and passes over into the seedling in the form of sugar, thus playing exactly
the same part as starch, inulin, and sugar do elsewhere.

A more detailed description of the species of sugars would here lead us too
far into the province of chemistry. In plants they always occur dissolved ; and when
obtained in the soHd form it is either as true crystals, as in the case of cane-sugar,
or as crystalline, crumbling aggregates, as in the case of the glucoses. The latter
are chemically distinguished particularly by the fact that they easily reduce copper

?>3^

LECTURE XX.

salts in an alkaline solution; whereas cane-sugar only gives a blue fluid with this
reagent. Inulin is still more characteristic. It is dissolved in the fluids of living
cells : if expressed from these, however, it is precipitated in the form of minute
white granules which possess a crystalline structured The crystalline nature of
inulin becomes much more evident, however, when tubers containing inulin are
allowed to lie for a long time in alcohol : there are then formed in the tissues so-
called sphere-crystals — i. e. crys-
talline aggregates of knobby,
rounded form, which in their
turn consist of radially disposed
elements proceeding from a com-
mon centre. Such aggregates not
rarely embrace a considerable
extent of tissue. The sphere-
crystals in the dark field of the
polarising microscope exhibit a
bright cross, corresponding to
the crystalline structure. On
heating up to 50-60° C. the
sphere crystals are again dis-
solved ; they are also formed
under the influence of frost in
the cells containing inulin. The
cellulose which occurs as a re-
serve-material shows no essential
difl'erences from that seen else-
where in strongly thickened cell-
walls, except that it never be-
comes lignified. It is here to be
added, however, that the middle
lamellae of such tissues persist
on the dissolution of the thicken-
ing layers, and are not dissolved
with them.

We must devote somewhat
more time to the consideration
of the starch^. This always oc-
curs in the plant in the form of
rounded, solid, hard granules,
which split on the application of sufficient pressure, and which may grow from a size
scarcely visible even with the highest powers of the microscope to grains o"2 mm.
in diameter : generally, however, the grains possess a diameter of only a few

Fig 233. — Spliere-crystals of inulin. A from an aqueous solution laid
aside for aj months. At a the action of nitric acid is commencing. B cells
of the root-tuber of Dahlia ■variabilis— a thin section after lying 24 hours in
90 n/o alcohol was then immersed in water C two cells with half sphere-
crystals which have their common centre in the middle of the intervening
cell-wall ; from an internode 8 mm. thick at the apex of an older plant of
Helianthus tuberosui wliich had Iain for some time in alcohol. £> fragment
of a sphere-crystal. E a large sphere-crystal embracing several cells (from a
large piece of the tuber ai Helianthus tuberosus after lying some tmie in
alcohol). F inulin after evaporation of the water from a thin section of the
tuber of Helianthus tuberosus (X S50 : £ not so much magnified).

> With respect to inulin cp. vSachs' Bot. Zeitung, 1864 (p. 77), and Dragendorff, "■ Materialen zu
einer Monograpliie des Inulins,'' (^Petersburg, 1870).

^ The chief work on starch-grains is Naegeli's extensive description in the ' Pßanzen-physiolo-

gisclic Untersuchungen'' of Naegeli and Cramer, 1858.

STARCH-GRAINS.

337

thousandths or hundredths of a mm., and an average specific weight of i-^6. In
consequence of these properties, and since they are insoluble in cold water, it is easy
to obtain the starch-grains in quantity from starchy tubers, seeds, and other organs :
it suffices to rub or otherwise crush such parts of the plant, just as ordinary wheaten
flour is obtained by grinding grains of wheat, and then to remove the crushed masses
of tissue with abundance of water, to' obtain pure starch-meal as a sediment. On
observation with the microscope the somewhat larger starch-grains in the cells are
conspicuous by their strong refractive power and corresponding lustre. On the addi-
tion of solutions of iodine they become coloured, according to the quantity of iodine
in each case, of various shades from bright blue to blue-black. The structure of
the starch-grain has been investigated very
thoroughly, especially by Naegeli (1859), from
whose researches a deeper insight into the
innermost structure of organised bodies was
first obtained. It is not necessary, however,
to go further into these matters here. In the
meantime it suffices to bring forward the fact
that starch-grains may possess the most various
shapes, and indeed that every plant produces
them of characteristic form. These character-
istic forms, however, only*appear in the large,
completely developed grains ; granules which
are still young and very small being mostly
spherical, or having the form of segments
of spheres, and only developing the charac-
teristic shapes on subsequent growth.

To illustrate a few of these only, I may
refer in the first place to Fig. 234, which
shows various forms of starch from the tubers
of the Potato. The large and simple grains
are here approximately ovoid, with an eccen-
tric hilum. The starch-grains of the Bean,
Pea, and other Leguminosese, on the other
hand, are ellipsoidal, with a fissure running
in the middle of the substance of the grain
in the dry state. The starch-grains of the

Rye and Wlieat have the form of bi-convex lenses (Fig. 235 5); those of the
tubers of Ctoruma are elliptical tablets, which possess a sort of peg or handle at
one end, in which the so-called hilum lies. Where the starch-grains are aggre-
gated in large numbers and predominate in the cells of dry seeds, as in the
Maize and Rice, they may be polyhedral, and so densely crowded that only very
thin spaces filled with proteinaceous substance are left between them (Fig. 235 0).
These are all simple grains.

Compound grains consist of several or many, or even thousands of individual
starch-grains, which, however, are so closely connected that they form together a
roundish grain, which with sufficient pressure breaks up into the individual granules ;
[3]

Fig. 234.— starch-grains from the tuber of a Potato
(800). ^ an older simple grain ; B a semi-compound grain ;
C, D compound grains. E an older grain the hilum of
which is divided ; a a very young grain, i an older one
c one still older and with divided hilum.

33^

LECTURE XX.

this is the case, for example, in the endosperm of the Oat, of Mirabilis Jcilappa, and
other seeds. Naegeli terms such forms as those in Fig. 234 B, C, and D semi-com-
pound : in these cases two or three starch-grains lie not only with their surfaces
closely appressed to one another, but they are also embraced by common en-
veloping layers.

According to Schimper's more recent observations, compound starch-grains
arise as follows. In the interior of a starch-forming corpuscle are formed starch-
grains, at first like mere dots, then increas-
ing in size : these finally come into contact
with one another, and flatten, the surface
of the whole mass, corresponding to the
circumference of the protoplasmic starch-
forming corpuscle, remaining rounded — a
process which also takes place in the form-
ation of starch in chlorophyll-corpuscles (cf.
Fig. 222). As regards the semi-compound
starch-grains, it results from Schimper's
description that the grains are produced at
two, three, or more points of the surface
of a starch-forming corpuscle, and grow
most vigorously at those surfaces which are
in contact with the starch-forming cor-
puscle, whence the hilum of each comes
to lie eccentrically and towards the outside ;
and when the substance of the starch-
forming corpuscle itself is at length com-
pletely consumed, the partial grains are

mutually in contact at their surfaces of

^y W°J!;'f ^ m 1 growth. All simple starch-grains with a

central hilum and concentric structure
originate, according to Schimper, entirely
in the interior of a starch-forming cor-
puscle ; and are therefore enveloped by its
substance, from which they are nourished.
The eccentrically constructed grains, on
the other hand, arise beneath the surface
of a starch-forming corpuscle, and break
through this as they become larger ;
they then grow at the side connected
with the starch-forming corpuscle, so that the hilum is pushed outwards eccen-
trically.

To the best known peculiarities of starch-grains belongs their stratification.
Around the hilum already mentioned the substance of a starch-grain is not homo-
geneous, but consists of alternating layers of different density, surrounding the hilum
concentrically from within outwards : in grains with eccentric structure, only a cer-
tain number of such closed layers envelope the hilum, while more or less numerous

Fig. 235.—^ a cell from the endosperm of Zea Mais, filled
with closely packed and therefore polyhedral starch-grains ;
between the grains are thin plates of dry fine-grained proto-
plasm. Small cavities and fissures have arisen in the grains
owing to the drying. A^£ starch-grains from the endosperm
of a germinating seed of Maize. />' lenticular starch-grains
from the endosperm of a germinating seed of Triticnrn vitl^
gare : the commencing action of the ferment makes itself
evident in the first place by the stratification becoming more

STRUCTURE AND COMPOSITION OF STARCH-GRAINS.

339

layers lie on the growing side and thin off towards the hilum, as seen in Fig. 234.
Naegeli first showed that the whole of the visible internal structure of the starch-
grain depends upon an unequal distribution of water and starch-substance. The
hilum always consists of a watery portion poor in substance : the layers of the grain
are formed of starch-substance containing much and little water alternately, and this
in such a manner that the outermost layer of a grain is always one poor in water
and rich in substance, and dense. Moreover, the proportion of water of all the
layers increases from the circumference towards the hilum ; whence is to be
explained the fact that, on the desiccation of the starch-grain, fissures arise in the
hilum, and radiate from this towards the periphery.

The chemical substance of a starch-grain, however, apart from this varying
diff'erence in the proportion of water, is composed of two substances, chiefly distin-
guished by their solubility: these are termed granulöse and starch-cellulose. The
first predominates by far in quantity, and is at the same time the one which effects
the blue colouration of the starch-grain with iodine. Being the more easily soluble,
it may be removed by various solvents from the starch-grain, the starch -cellulose alone
at length remaining behind, in such a manner, however, that it still forms a skeleton
exhibiting the essential structure of the starch-grain. If large starch-grains are
macerated in fresh saliva at 35-55° C, or, according to Franz Schulze, digested at
60° in a saturated solution of common salt containing io/q of hydrochloric acid, the
whole of the granulöse is removed sooner or later : i. e. it is chemically altered and
converted into sugar, while the starch-cellulose remains behind in the form of a
laminated skeleton. This latter is coloured copper-red with iodine, or not at all,
and represents only a small proportion of the mass of the entire starch-grain. The
different solubility of the two substances is also made evident on germination by
the behaviour of the starch-grains in the reservoirs of reserve-materials. The starch-
grains in the living cell, however, may be dissolved in a very different way. In
some cases the solution begins with the removal of the granulöse, while the cellulose
remains behind. This however often takes place only here and there : the extraction
proceeds at isolated spots from without inwards. The extracted places are coloured
copper-red with aqueous solutions of iodine, the rest of the mass blue. This I found
to be the case in the endosperm of germinating Wheat (Fig. 235 ^). In other cases the
solution begins likewise at single spots on the circumference, but the entire substance
is progressively dissolved : cavities are produced, and the grain here also finally breaks
up into pieces, as seen in Fig. 235 a-/. In the cotyledons of the germinating Bean,
the solution of the ellipsoidal grains begins from within ; before they break up into
pieces, the granulöse is often so completely extracted that the grains assume a copper-
red colour with iodine, bluish here and there. Subsequently all is dissolved. In the
germinating Potato, and in the root-stock of Canna, on the other hand, the solution
of the grain proceeds from without inwards, layer after layer being removed. It evi-
dently depends here, as with the action of saliva, upon whether the solvent acts slowly
and first extracts the granulöse only, or energetically attacks and dissolves the sub-
stance as a whole.

When the solution of starch or granulöse is here spoken of, a chemical con-
version into sugar at the same time is always to be supposed. Starch-grains as such
are not soluble simply in water. If they are crushed in cold water, however, a small

z 2

340 LECTURE XX.

portion of the granulöse appears to pass into solution, and is precipitated by iodine
as fine-grained blue pellicles. Starch-grains ground up with fine sand likewise
appear to yield an actual solution of granulöse in cold water.

To the very characteristic properties of starch belongs the formation of paste.
Water of at least 55° C. causes large, watery starch-grains to swell up vigorously :
in the case of small denser grains this only results, according to Naegeli, at 65° C. If
sufficient water is present at this temperature the watery inner portion of a grain swells
first, and then the outer : the outermost layer scarcely swells at all, it is rather blown
up and remains behind in the paste produced by the swelling, in the form of pelli-
cular shreds. A dilute solution of potash or soda produces a similar effect in the cold.
The volume of a starch- grain may in this manner increase a hundred-and-twenty-five
fold, and so much liquid may be taken up that the swollen pasty grain only contains
from 2 to 0-50/0 of substance. With a sufficiently large quantity of hot water the
paste is so distributed that it looks like a homogeneous mass, and this on cooling con-
geals to a translucent jelly. After long boiling the starch paste loses the property of
gelatinising after cooling : the starch is then converted into a modification soluble
even in cold water.

Large, completely-formed, and especially simple starch-grains are found only in
the ripe quiescent reservoirs of reserve-materials ; in growing or vigorously vegetating
shoots and roots, and other organs in general where they are evidently being con-
tinually formed and dissolved, they are found only in small and often exceedingly
minute granules. In such cases however even the smallest quantities of starch may
easily be detected under the microscope, if thin sections of the portion of the plant
in question are either warmed in potash solution or left for some time lying in it in
the cold, then carefully washed out with water neutralised with acetic acid, and very
dilute iodine solution finally added. The swollen-up minute starch-granules now
appear as voluminous blue grains. I have employed this method thousands of times,
particularly in investigations upon the origin of the starch in the chlorophyll as well
as its translocation during the growth of the organs, and much information as to the
processes which occur during germination and the passage of material from place
to place, to be described in the following lecture, has been obtained in this way.

LECTURE XXI.

RENEWAL OF ACTIVITY OF RESERVE-MATERIALS. FERMENTS,
DORMANT PERIODS.

The constructive materials of the organs in the reservoirs of reserve-materials, as
well as those within other organs in their temporary conditions of rest, exist in forms
not suited for their immediate co-operation in growth. This is in part because they are
not soluble in the watery cell-sap, and are therefore incapable of being transported
— e. g. starch-grains, many aleurone grains, crystalloids, and slimy proteid substances.
It is above all necessary that these materials should diffuse from cell to cell and
travel from the places where they are deposited to those where they are used :
this is not possible in these conditions.

Another point also comes into consideration here. Even cane-sugar, and similarly
inulin, though always dissolved in the cell-sap, only occur as reserve-materials, and
are not in a fit condition for direct employment in growth ; since when it is necessary
to make use of these matters in the growing germinal shoots, they are previously con-
verted into another chemical form, namely into a sugar which reduces cuprous oxide
in an alkaline solution, and which, neglecting further differences, we shall term
generally glucose.

Since now cane-sugar, inulin, and to a certain extent even dissolved proteids,
occur in a transportable condition, and more or less capable of diffusion, it must in
this case be not simply a matter of making them capable of diffusion, but we may
assume that the chemical alteration also serves to render them in a condition
in which they are suited for direct employment in growth. In these changes of
the reserve-materials, therefore, it is not simply a matter of making them soluble and
transportable, but evidently of also converting them into a form directly serviceable
for growth.

We may perhaps shortly formulate the matter thus. The plastic substances
present two conditions : in the one condition they appear passive, inactive, dormant,
whether in the solid or in the dissolved form ; in the other they are not only always
dissolved and movable, but are also in such a condition that they can directly take
part in the nutrition of growing cells. In this state they appear active, in contrast to
their passive condition in the reservoirs of reserve-materials.

In both animal and vegetable organisms, organic compounds of peculiar kind are
produced, under the influence of which plastic materials arc converted from the passive

342 LECTURE XXI.

into the active and movable condition; these are the Ferments ^ They appear to
originate from proteinaceous substances, always however in extremely small quan-
tities only, so that their existence in most cases can only be inferred from their
conspicuous action ; this action consists in their being able to convert very large,
perhaps indeed unlimited quantities of reserve-materials into the active state. The
ferments affect the latter in a certain sense as stimuli do irritable organs. In what
way this action takes place however is still questionable, though it differs from ordinary
chemical actions in that the ferment itself is not essentially altered during the process,
and does not even enter into a chemical combination. It must be added here,
however, that similar alterations are produced on carbo-hydrates, proteid matters,
and some other peculiar organic compounds (especially the so-called glucosides)
by acids, alkalies, and even by water at higher temperatures. Hence it is not
always certain in the present state of our knowledge whether, in a given case not
more exactly investigated, the changes of the reserve-materials and glucosides in
question are produced by actual ferments, or by certain vegetable acids or alkalies.
In this province of vegetable physiology, which has only been opened up during the
last few years, there is still much that is problematical and uncertain, so that we
are necessitated to depend to a large extent on the experience of animal physiology.
Nevertheless the facts already known concerning vegetable ferments quite suffice
to show that in this connection also, as in so many others, important agreements
exist between the vital processes of animals and plants, especially where fundamental
phenomena are concerned.

With regard to the results of their action, two main categories of ferments may
be distinguished. In the one case the ferment effects only an insignificant chemical
change, as when starch, cane-sugar, or inulin are converted into glucose, or proteids
into peptones. In other cases, on the contrary, the ferment action consists in a very
profound chemical alteration, of such a nature that from one complicated organic
compound two or three simpler compounds very different in kind are produced.
The first category may be distinguished as merely alterative ferments, the second as
splitting ferments. To the latter belongs, for example, the Emulsin of the Almond,
by means of which its amygdalin becomes decomposed into glucose, prussic acid,
and oil of bitter almonds, and which is also able to break up numerous other
glucosides into glucose and other products. The case is similar with Myrosin, which
splits potassium myronate into glucose, oil of mustard, and potassium sulphate. Never-
theless these ferments, as well as the matters upon which they act, are of subordinate
importance, since both occur only in individual families or species of plants, and
because their significance for metabolism, for the purposes of growth itself, is in these
cases very questionable.

The fact is quite otherwise with the first category of ferments, which I have
distinguished as merely alterative. These re-invigorate the plastic reserve-materials
distributed throughout the vegetable world ; and although we have as yet by no

^ More details respecting the ferments are to be found in Schiitzenberger's book, ' £>ie
Gährungserschcimwgcn^ (Leipzig, 1876); and also in VitKex's Pßaiizeti-pJiysiologie, §§ 47, 56, 59.
Both, however, are wanting in criticism of the phenomena. In this connection the best is im-
doubtedly Naegeli's ' Theorie der Gähning' (München, 1879).

nr ASTATIC FERMENTS. 34^

means sufficient information concerning them as to whether these changes are
always effected by ferments, or do not occasionally occur by other means, the great
importance of this category of ferments nevertheless lies in that they are known
in very many cases to re-invigorate the plastic substances.

In this category, alone to be considered here, two chief kinds of ferments are
to be distinguished, namely, the diastatic ferments and the peptonising ferments.
The former convert various carbo-hydrates into glucoses, while the different proteid
substances are transformed by the latter into peptones. As a third subdivision may
be added the emulsifying ferments which act upon fats, and which will be spoken
of subsequently.

The longest and best known diastatic ferment in plants is Diastase, produced
on the germination of the seeds of barley and other grasses : it may be extracted
from the seedling by water or glycerine, and is capable of transforming enormous
quantities of starch into glucose, especially at higher temperatures below 70° C. A
few drops of an extract obtained in this way added to a considerable quantity of
boiled solution of starch, transforms it completely into sugar in a few hours; and
fresh starch-grains at the ordinary temperature (15-25° C.) are in a short time
corroded and finally dissolved entirely, as previously described. Since, now, in the
germination of all starchy seeds, tubers, bulbs, and other reservoirs of reserve-
materials, exactly the same alterations of the starch-grains take place, and the starch
disappears with the formation of sugar, it may be supposed ä priori that in all these
numerous cases, diastase, or at any rate similar diastatic ferments, produce the effect
mentioned. And, as a matter of fact, Baranetzky has shown that in all cases where
he looked for the diastatic ferment in starchy seeds, tubers, stems, and even leaves, it
was to be found ; and even in organs which contain no starch but abound in sugar,
as in the Carrot {Daucus carota) and Turnip {Brassica Rapa), the same observer
found a diastatic ferment. In seeds containing starch, the ferment may appear before
the commencement of germination ; generally, however, it only appears when growth
begins.

It was formerly the custom to distinguish, as Invertin, a substance produced
by the Yeast-plant, Bacteria, and Mould-fungi, which splits cane-sugar into dextrose
and levulose (two species of glucose which reduce cuprous oxide), which then
undergo further decompositions under the influence of these Fungi. That a similar
ferment occurs also in the wintering Beet-root, may be concluded from the fact that
the cane-sugar collected as a reserve-material in its tissues is transformed into glucose
in the following spring, when the flowering shoots are developed ; and a similar phe-
nomenon occurs on the development of the fruiting spikes of Indian corn, the
parenchyma of the stem of which previously contained large quantities of cane-
sugar. The inulin in the tubers and perennial root-stocks of the Compositae, more-
over, as I showed twenty years ago, is transformed into glucose when the shoots
begin to grow, whence we may conclude that here also a diastatic ferment resembling
invertin co-operates.

The ferments appear to be always produced by the growing parts of the
seedlings and buds themselves, and to penetrate from these into the reservoirs of
reserve-materials, there to dissolve or make active the constructive materials. This
is particularly evident in the case of seeds containing endosperm. If the young

344

LECTURE XXI.

seedling (embryo) is removed from the seed of the Indian corn (Maize), Barley or
other plant, and the endosperm alone laid in moist warm earth, its starch is not
dissolved and transformed into sugar. Still more evident than in such cases
appears the action of the growing seedling on the reservoir of reserve-materials, in

the germination of the stone of the
Date ; for here the non-nitrogenous
reserve-material consists of hard
cellulose, which is deposited in the
endosperm in the form of thickened
|\ ' ( i§]^T\ 11 IT'' ^- I cell-walls, and which constitutes the

great mass of the date-stone. The
embryo, at first very minute, pro-
trudes its root and plumule at the
beginning of germination (as seen in
Fig. 236), and only the uppermost
portion of the primary seed-leaf,
which now gradually develops into
a cup-shaped absorbing organ, be-
coming larger and larger, remains
within the endosperm. This organ,
consists of very delicate parenchyma,
and excretes ferments which dissolve
the hard endosperm in its immediate
neighbourhood. The products of
solution are absorbed by the organ,
and then conveyed into the growing
parts of the seedling ; until finally
the whole of the hard date-stone is
dissolved, and its cavity occupied by
the developed absorbing organ. The
seed of Phyfelephas (known under
the name of Vegetable Ivory), which
is at least a hundred times larger
than the date-stone, and the endo-
sperm of which consists of much
harder cellulose, behaves similarly.

The action of those Fungi which

destroy wood and kill trees may be

compaied directly with the action of

such seedlings on their endosperm ;

the thin mycelial threads of these

Fungi, as has been shown by Robert

Hartig in his magnificent work, penetrate into the alburnum and heart-wood of

trees, evidently because they excrete ferments at their growing apices, which dissolve

the hard cell-walls of the wood.

Attention was first drawn to the occurrence of peptonising ferments in the

FIG. i-^f^.—C&xmmTi'CmaoiPlKenixdacfylifira. /transverse section
of the resting seed ; //, ///, /f stages in the germination (/Knat. size) ;
A transverse section of the seed IVsXxx; B transverse section of //•" at
xy ; C ditto at zz; e the horny endosperm ; s sheath of cotyledon ; s/ its
petiole ; ^ its apex, developed into an absorbing organ which gradually
exhausts the endosperm, and at length replaces it ; w the primary root ;
iv' lateral roots ; d'i" the leaves which succeed the cotyledon, 6" becomes
the first foliage leaf; in B and C its folded lamina is seen in transverse

PEPTONISING FERMENTS. 345

vegetable kingdom by the remarkable phenomena observed in the so-called insec-
tivorous plants, of which I shall speak more in detail shortly. My earlier studies
on the germination of various seeds left no doubt that seedlings dissolve and make
active their proteinaceous reserve-materials by means of peptonising ferments. Gorup-
Besanez was, however, the first to detect peptonising ferments in seeds. This he
did in the seeds of the Vetch, Hemp, Flax, and Barley ; and Krukenberg found an
energetic peptonising ferment soon afterwards in the protoplasm of a Myxomycete,
namely, in the yellow plasmodium of the 'flowers of tan' {vEthaliuvi septicum). More
recently, a very energetic peptonising ferment in the latex of Carica Papaya has
attracted particular attention, and a similar ferment has been detected in the latex
of the common Fig {Ficus Caricdf. As we come to know the proteinaceous reserve-
materials of plants better, and if we follow their behaviour in the animal body also,
it can scarcely be doubtful that, in spite of incomplete knowledge, the assumption
is nevertheless warranted that peptonising ferments are perhaps universally dis-
tributed in plants ; moreover, peptones, the result of their activity, have actually been
detected by Schulze in the seedlings of the Lupine. The peptonising ferments in
the animal body, where they occur particularly in the mucous membrane of the
stomach, are much better and more generally known than in plants. Exceedingly
small quantities of them are able to lignify and dissolve coagulated egg-albumen,
blood-fibrin, or muscle, in the presence of an acid, especially hydrochloric acid, with
great energy, and so convert them into a condition in which the proteid matters,
while retaining their essential chemical properties, will diffuse through closed tissue
cells, to be reconstructed into organised proteid substances at suitable spots in the
organism, and to serve for the construction of tissues.

In the peptonising of proteid substances, as in the action of diastatic ferments,
it is a matter of a relatively slight chemical change. A much more profound
decomposition of the proteid bodies takes place, on the contrary, when they are con-
verted into Asparagin ^ and other split products. Among the latter, in any case, a
sulphur compound, probably sulphuric acid, must occur. But, besides asparagin,
Tyrosin, which arises in the artificial splitting of proteid bodies, has also been
observed by Borodin in many cases. It is not yet known whether or how far any
ferment action occurs in this case. In any case, however, the formation of asparagin
has a similar significance for the transport of nitrogenous substance that the formation
of sugar from starch-grains has ; since by means of the conversion of proteid bodies
(which are only slowly and with difficulty diffusible even in the dissolved state) into
asparagin, which is a cryslallisable substance forming solutions which readily diffuse, a
means is obtained for transporting the nitrogenous substance in the closed parenchyma
to the places where it is made use of. Consequently asparagin is formed on the
germination of seeds, tubers, root-stocks, and when the winter-buds of woody plants
put forth their shoots ; this penetrates from the reservoirs of reserve-materials into
the young growing parts, and is there employed for the formation of protoplasm,

' On the ferment in the latex oi Fiais Carica cf. Hansen, ' Sitzungsber. der physical, med. Soc.
zu Erlangen' (Nov. 8, 1880).

^ The literature on Asparagin is citeil in Pfeffer's "■ Pflanzen-physiologic'' (§ 59). Borodin's
treatise, the one chiefly made use of in the text, is in Bot. Zeitg. 1878 (Nos. 51, 52).

346 LECTURE XXI.

a restitution into proteid having previously taken place, and non-nitrogenous sub-
stance (in the first place probably glucose) being made use of in the process.

This significance of asparagin was clearly perceived by Theodore Hartig,
so long ago as 1858, as is seen from his statement as follows : 'This apparently
general occurrence of this crystallisable substance in all the young cell-tissues,
indicates that its solution is the form in which the nitrogenous nutritive substance
of plants, formed from the reserve-materials, is moved from cell to cell.' The fact
that Hartig's important discovery remained so long in abeyance, is to be ascribed
to the circumstance that this able investigator here, as in many other cases, employed
a very peculiar and often unintelligible nomenclature, and termed asparagin ' Gleis.'
Our knowledge of this body was only carried further in 1872, by an investigation of
Pfeffer's : he proved that the accumulation of asparagin during germination in the
dark, is due to the fact that in this case there are not sufficient carbo-hydrates present
to afford the material for the construction of proteids out of asparagin. If seedling
shoots grown in the dark and abounding in asparagin are exposed to strong light,
and a fresh supply of carbo-hydrates produced by assimilation, the asparagin dis-
appears, and proteid substances are formed at the same time.

Moreover, in the formation of asparagin, the end appears to be not simply the
rendering the nitrogenous substance transportable. The fact, discovered and insisted
upon by Borodin, that asparagin arises even in cut-off buds and older shoots
which are allowed to grow, and therefore in the cells which are then beginning
to grow, proves that in the formation of asparagin it is not simply a matter of
conveying proteid-forming substance into the young organs, but that here, as in the
formation of peptone and in the inversion of cane-sugar and inulin, we may assume
that it is a matter of giving the plastic substance a form in which it is directly suited
for the nutrition of growing protoplasm ; or, as I expressed it in the beginning, to
convert the passive form of the reserve-material into an active one.

The universal occurrence of asparagin asserted by Hartig, but disputed later
by Pfeffer, was again established by Borodin in 1878 in a detailed treatise, going
more exactly into the conditions under which asparagin is to be detected.
Under normal conditions of vegetation during vigorous growth, and especially
under the influence of intense light, the asparagin is usually employed, according to
Borodin, for the formation of proteid, and for growth, as quickly as it is itself pro-
duced— an amount sufficiently large for detection only rarely occurs during normal
vegetation. Hence, to demonstrate the presence of asparagin in growing parts, ab-
normal conditions must be induced ; either by allowing the plants to grow in the dark
or in a feeble light, or, better, by placing branches in water, and employing the same
methods as before. The surest method, according to Borodin, is to make single
buds or the young apices of shoots grow in the dark, or in a feeble light ; for by
this means, in part, the addition of carbo-hydrates from older parts, or even from
reservoirs of reserve-materials, is prevented, and in part the new formation of carbo-
hydrates by assimilation is excluded, and thus the reconstruction of asparagin into
proteid made impossible. Asparagin is therefore accumulated under the conditions
named, and can be demonstrated in sections under the microscope by the addition
of alcohol, in which it crystallises in a characteristic manner. In this way Borodin
succeeded in effecting the accumulation of asparagin, and in demonstrating its

ASPARAGIN: FATS. 347

presence in numerous cases where none at all was to be detected under normal con-
ditions of vegetation. It is still to be mentioned that the formation of asparagin
takes place also in young flower-stalks and buds, young fruits, and growing seeds ; on
the ripening of the seed, however, it disappears in the reservoirs of reserve-materials.

It is worthy of remark that Borodin succeeded in demonstrating the formation
of asparagin in all cases where he investigated the growing parts of plants for that
purpose. Unfortunately, there is only a single observation on a Moss : the question
still remains to be answered also how the matter stands with respect to Algae and
Fungi.

Finally are to be mentioned the fats. I showed, so long ago as 1858, that in
the germination of seeds containing fat a transference of the fatty oils from the
cotyledons, or from the endosperm into the growing parts of the seedling, appears
to take place ; and a few years later this was confirmed by chemical analyses
by Peters ^. When the roots and shoot-axes of the seedlings of Ricinus, Gourd, and
Almond have already grown to a considerable size, we find in their parenchyma for
some time longer considerable quantities of fat, which have passed in from the coty-
ledons, or the endosperm, and only disappear later in metabolism. It thus appears
that the fats can pass through the closed tissue-cells as such ; though of course
the greater part of them is transformed into starch and sugar for transport and use.
Similar phenomena with respect to fats occur moreover in the animal body, where
the fats entering into the stomach are in the first place emulsified by the secretion from
the pancreas, that is, they become converted into exceedingly fine drops and then
saponified. This again is brought about by a special ferment. According to
Schützenberger, a similar ferment actually exists also in fatty seeds : if such seeds
are ground up in water, an emulsion is produced in which, as he says, glycerine
and free fatty acids appear forthwith. According to this, however, the presence
of fats in the seedling can only be explained by assuming that glycerine and fatty
acids travel from cell to cell, and are continually becoming re-united for the forma-
tion of fat, a process moreover which presents a certain similarity with the move-
ments of transitory starch. For starch also is found at places in the tissue
where it has neither been originally produced nor is employed, and thus in a con-
dition of translocation towards the places where it is made use of; in petioles and
the older internodes of growing plants, this fine-grained transitory starch is to be
found very generally. It is obvious that it does not pass as such through the closed
cell-walls, and its translocation is only intelligible if we assume that the small starch-
granules in one cell become dissolved and the solution penetrates into the next cell,
there to become separated again as granules by the starch-forming corpuscles of
Schimper, the process being repeated from cell to cell.

As a rule, the remarkable decompositions of organic compounds brought about by
the Ferment-fungi (organised ferments) have also been regarded hitherto as ferment-
actions, it being assumed that in these Fungi — Yeast, Bacteria, INIould-fungi, etc. —
peculiar substances belonging to the category of ferments are produced, which act
upon the saccharine or proteinaceous media as do the above-named splitting fer-
ments. Thus, for example, grape-sugar is broken up by the Yeast- fungus into

On the translocation of fats, cf. my ' Handbuch der Exp.-phys.' (1865, p. 304).

348

LECTURE XXI.

alcohol, carbon dioxide, gl}-cerine, and succinic acid, and proteid matters are split up
by Bacteria into a long series of chemical compounds. But Naegeli, in his ' Theory
of Fermentation' (1879), has given sufficiently strong reasons for believing that the
fermentation and putrefaction due to the action of organised ferments are processes

essentially different from
ordinary ferment actions.
Above all, Naegeli insists,
and rightly, that the hypo-
thetical ferment which the
ferment-fungi and putrefac-
tive fungi are supposed to
excrete has not yet been
extracted from the cells and
described. On the con-
trary, the cause which ef-
fects the fermentation is
inseparably connected with
the substance, and indeed
with the protoplasm of the
living cells of the Fungus.

' Fermentation by means
of Fungi/ says Naegeli, 'only
takes place in immediate
contact with the protoplasm,
and so far as its molecular
action extends. If the or-
ganism needs to affect
chemical processes in areas
and at distances where it
is able to exert no power
by means of the molecular
forces of the living sub-
stance, it excretes ferments.
The latter are particularly
effective in the cavities of
the animal body, in the
water in which Fungi live,
and in vegetable cells poor
in protoplasm. It is even
very questionable whether
the organism ever forms
ferments which shall be ac-
tive within the protoplasm; since here it does not require them, much more energetic
means for chemical action being at its disposal in the molecular forces of the living
substance itself.'

I find with Naegeli a further particularly striking point of difference in the fact

Fig. 237.— Seedling of tlie Gourd, -w primarj' root ; « « lateral roots ; l> hypocotyl
(■cotyledons. At this time the cotyledons contain starch and sugar in addition t(
much fat : the same is also the case in the hypocotyl, less so in the root. The hypo
cotyl and root also contain fatty oil derived from the cotyledons.

DIFFERENCES BETWEEN ORGANISED AND UNORGANISED FERMENTS. 349

that (apart from split-ferments such as Emulsin and Myrosin) the plastic matters are
transferred by the action of unorganised ferments from the passive into the active
condition. Fermentation by means of Fungi has, as Naegeli insists, just the opposite
character; its products are, without exception, less nutritious compounds, and it
destroys especially the most nutritious substances.

' The contrast,' says Naegeli, ' appears most striking in the case of carbo-hydrates
and proteid substances. While the action of unorganised ferments produces from
them glucoses and peptones, fermentation by means of Fungi breaks up these com-
pounds into alcohol, mannite, lactic-acid, and into Leucin, Tyrosin, &c., — in some
cases several fermentations follow one another ; their products then become less
nutritious step by step. We may say, generally, that the Yeast-fungi render the
medium in which they occur chemically less suitable for nutrition by every process of
fermentation which they effect.'

In fermentation by means of unorganised ferments the chemical transformation
proceeds smoothly and completely : dextrin is entirely transformed into grape-sugar,
cane-sugar entirely into inverted sugar, and albuminates entirely into peptones. In the
alcoholic fermentation, on the other hand, the products of which alone have hitherto
been quantitatively determined, only the greater part of the sugar is broken up into
alcohol and carbon dioxide, while, according to Pasteur, about 5% of the sugar is
decomposed by the way into glycerine, succinic acid, and carbon-dioxide. It is like-
wise certain that in the lactic acid fermentation, not all the sugar is decomposed
into lactic acid. Carbon dioxide especially appears to be a by-product of all fermen-
tations, due to the action of Fungi and of all putrefactive processes.

The alleged difference between fermentation by means of unorganised ferments
and that due to the action of Fungi would be unintelligible if both processes were due
to the same cause. Every difficulty vanishes, on the other hand, if fermentation
by means of Fungi is caused, not by a contact substance (ferment), but by living pro-
toplasm. We then apprehend that while the ferment as a simple chemical compound
alters another chemical compound in a simple and equable manner so that all mole-
cules suffer the same kind of decomposition, an organised substance with its various
molecular movements and molecular forces produces more complicated decompo-
sitions.

In this connection, Naegeli brings forward the fact that, as already remarked
above, the effects of proper ferment actions are also produced by acids, alkalies, and
even by water, especially at higher temperatures : the fact is quite otherwise with the
fermentations which are due exclusively to living Fungi, Naegeli puts his view of
fermentation due to organisms as opposed to that produced by unorganised ferments
in the following statement : —

' Fermentation is therefore the transference of the movements of the mole-
cules, atomic groups, and atoms of various compounds composing living protoplasm
(which remain chemically unaltered in the process) to the fermentable material, by
which means the equilibrium between its molecules is destroyed, and it is broken up.'

The older theory of fermentation, which classes it with the action of unorganised
ferments, has to assume that the various fermentations are effected by just as many
ferments in the Fungi producing them ; whereas Naegeli's theory seeks to explain the
difference between the fermentations as due to the different internal organisation of

350 LECTURE XXI.

the protoplasm of the Fungi which produce them. So much for general guidance as
to the unorganised ferments and their differences from the exciting agents in alcoholic
and similar fermentations.

Coming now to speak of the hitherto entirely unexplained dormant periods of
plants, in connection with what has been said with respect to ferments, it may appear
at first sight that no conceivable connection exists. I also by no means wish to
maintain that the view which I propose here is sufficiently proved, though I do
maintain that it opens up for us in the meantime a way to further investigation
of one of the most general phenomena of plant life.

Even under the most favourable conditions of vegetation, dormant periods
occur in the course of the life of the plant. Under circumstances Avhen the plant
would be in a condition to grow most vigorously, because it is provided with reserve-
materials, and water and oxygen are at its disposal, and a sufficiently high tem-
perature might be expected to call forth the internal movements, every externally
perceptible vital motion nevertheless ceases, and it is only after some months of rest that
the growth commences anew, and this frequently under circumstances which appear
far less favourable — especially at a conspicuously lower temperature. This periodic
alternation of vegetative activity and rest, is in general so regulated that for a given
species of plant both occur at definite times of the year, leading to the inference that
the periodicity only depends upon the alternation of the seasons, and therefore chiefly
upon that of temperature and moisture. Without wishing to deny the co-operation
of these factors, a closer consideration shows however that this matter must depend
chiefly upon changes which take place in the resting plant, independently of external
influences, or only indirectly affected by them. To render the facts in question
intelligible to the reader, however, I will in the first place select from the enormous
mass of material a few well-known examples.

The leaf-shoot and flowers contained in the bulb of the Crown Imperial com-
mence to grow vigorously in the spring time with us, even at the beginning or
middle of March, when the soil in which the bulb has passed the wdnter possesses a
temperature of 6-io° C. ; the leaf-shoots protrude forcibly from the cold earth to grow
vigorously in the but slightly warmer air. There would be but litde to surprise us
in this if we did not at the same time notice the fact that a new leaf-shoot is already
formed in embryo in the subterranean bulb in April and May : this shoot, however,
does not grow to any extent in the warm soil during the summer and autumn. On
the contrary, this favourable period of vegetation passes by, until at the end of the
winter an inconsiderable rise of temperature above the freezing-point suffices to in-
duce vigorous growth; and, as is well known, the same is the case with most
bulbous and tuberous plants, many of which, as the Meadow Saffron, possess two
active periods, the flowers developing in the late autumn and the foliage-leaves
belonging to them only in the next spring. The best known examples, however,
are afforded by the common potato and the kitchen onion. I have many times
attempted to induce the tubers and bulbs ripened in the autumn to put forth their
germinal shoots during November, December, and January, by laying them in moist,
warm, loose soil; but in the case of the potato as well as in that of the kitchen onion,
no trace of germination appeared. If, on the other hand, the attem.pt is repeated
in February, or still better in March, the germinal buds begin to grow vigorously

DORMANT PERIODS. 35 1

even in a few days. At this time of the year, indeed, it does not even require
a favourably high temperature and an external supply of water : the shoots begin
to develope at much lower temperatures, and this, even when the potatoes and bulbs
do not receive the addition of water from without. Not only so, they will put
forth germinal shoots when suspended in dry air and shrivelled up by the loss of
water. It is evident that some internal change must have taken place in the tubers
and bulbs during the winter months when it is impossible to bring them into activity
from their state of rest, since no such change is perceptible externally, and the
behaviour described appears inexplicable otherwise.

The behaviour of the Water-nut, the fruit of Trapa natans, is perhaps still
more striking. If at the end of August, or in September, when they are ripe,
one of these is placed in a glass full of water, no germination occurs either
during the autumn or winter, even in a chamber where the water is constantly at
15-20° C; but in March or April germination begins although the water is at
a temperature of only 8-10° C. The most striking examples of periodic rest
and activity are probably afforded however by the majority of woody plants,
especially those which produce winter-buds clothed with scales, such as the Horse-
chestnut, ordinary fruit-trees, and species of Pinus and Abies. As soon as this
year's foliage and flower-shoots have become unfolded in the spring from the winter-
buds of the previous year, the winter-buds for the next year are formed in embryo ;
the future shoots or even flowers develope slowly within the bud envelopes, but
remain in an embryonic condition, and it is impossible by any means to cause these
embryonic shoots to develope in the autumn or the beginning of winter. On the
other hand, they develope in January, or better in February, if branches furnished
with buds are cut off and allowed to stand in water in an ordinary warm dwelling-
room — i.e. at a temperature of about 15-20° C. The winter-buds of trees thus
behave just like subterranean bulbs and tubers.

It is by no means to be denied that in many of these cases, especially in those of
the winter-buds of trees, some bulbs {Hyacinthus, Crocus), etc., a considerable time
is necessary in order that the embryonic rudiments of the leaf-shoots and flowers
within their envelopes may be first so far prepared by slow growth that they are
suited for rapid development subsequently. The decisive point certainly does not
lie in this, however, as may be seen in the behaviour of the Water-nut. We find
the most evident and instructive cases with reference to the question here alone
concerned, however, in the spores of many Cryptogams. The majority of the
spores of Algae and Fungi, especially those produced by sexual fertilisation,
are distinguished as 'resting spores,' because after their formation in the spring
or summer they remain dormant for 8-10 months, either in water or dry, without
germinating, whereupon they then put forth their germinal shoots at a lower tem-
perature in the following spring. This peculiarity of resting spores is so much
the more striking since many of these plants produce simultaneously or previously other
forms of spores, which are capable of germination immediately after their origin.
As examples may be mentioned the zygospores of the Mucorini, which require
rest, and the conidia of the same fungus which can germinate at any time.

It appears certain that the internal changes which take place in the resting
tubers, bulbs, buds, and spores, during the long pause, are promoted, in many cases

^^2 LECTURE XXI.

at least, by a not too excessive drying-up. Numerous experiments with Algce and
Fungi show that by means of desiccation (which must not be carried too far however)
the dormant period can be shortened in many cases, and some observations not yet
sufficiently confirmed allow of the conclusion that even in the winter-buds of trees
something of the same kind occurs. But in all cases where the resting spores, or in
Phanerogams the seeds, as those of Trapa, go through their dormant period at
the bottom of deep water, we cannot speak of the co-operation of desiccation. All
this appears to indicate rather that in every case it depends upon chemical changes
in the dormant parts of the plant; changes which proceed only extremely slowly, and
generally require months for their completion. These chemical changes do not
affect the proper reserve-materials as such, however, or at least not throughout their
mass, as follows from the fact that in potato tubers, which have been so very
frequently investigated, no striking difference has ever yet been found in the
chemical composition in the autumn, before the dormant period, and in the spring
following it. In the same way the numerous observers have perceived no changes
which immediately affect the reserve-materials in the Algae and Fungi.

On the contrary, all vigorous chemical changes only appear at the commence-
ment of germination, and we know already that these changes of the reserve-
materials are in great part effected by means of ferments. From these considera-
tions I come to the conclusion that, in the case of dormant periods, it may be a
matter of a very slow production of ferments, which are formed in the buds capable of
growth; and that the possibility of putting the existing reserve-materials into the active
state in which they are directly suitable for the requirements of growth, only appears
when they have been produced in sufficient quantity. That this production of
ferments may be in its turn favoured by various external circumstances during the
dormant periods — e. g. in many cases by desiccation or the cold of winter — is not to
be denied. On the other hand, in cases where spores, buds, tubers, and seeds are
capable of germination immediately after their production, it may be assumed that
they take up the necessary quantity of ferments or similarly effective matters at the
time of their origin, from the mother-plant.

Since at the present time the attention of vegetable physiologists is being
directed towards ferments, it is to be hoped that researches will not be wanting to
confirm or contradict the hypothesis here put forth. In any case the periodic
alternation of rest and vegetative activity, as one of the most general phenomena in
the life of the plant, is worthy of greater attention than has hitherto been accorded
to it\

^ Pfeffer has collected the literature on the annual periods of vegetation in his ' Pßanzen-
physiologie^ pp. io6, &c.

LECTURE XXII

PASSAGE OF THE PLASTIC MATERIALS THROUGtl THE TISSUES.

That the materials serving for the construction of the organs must travel in the
growing and assimilating plant over distances more or less extensive, results as a
necessary consequence from all the facts hitherto stated. Growth takes place at the
apices of the roots on the one hand, and in and beneath the buds on the other, and
plastic substances are of course employed there : at the time of germination they are
contained in the reservoirs of reserve-materials, and must travel from thence to the
places mentioned. In fully-grown plants with assimilating foliage leaves, the latter
behave like reservoirs of reserve-materials ; the plastic substances necessary for the
growth of the buds and of the distant roots pass out from them. That the
path which individual molecules of starch and sugar and the substances forming
protoplasm pass over may, under certain circumstances, be very long, is obvious
on reflecting that the materials of which the subterranean roots of a tree (e. g. a
Palm) 20 metres high are formed, have been originally produced in the crown of
leaves, 20 metres or more distant from the roots.

But it is also easy to convince ourselves of this fact experimentally. If, for
instance, the bud at the end of a well-developed Gourd plant is directed through
a narrow hole into the dark cavity of a wooden box, as in Fig. 238, it is obvious
that all those materials which are now employed in the growth of the parts in
the dark must necessarily have been derived from the leaves which assimilate
under the influence of light outside the box. The figure represents a definite,
observed case. The Gourd plant made use of in the experiment already possessed
thirteen large leaves on its main stem, the total assimilating area of which amounted
to about 1-5 square metres when, on the 25th of July, 1881, the terminal bud of the
stem was directed at d into the box K. All the buds and leaf-shoots in the axils of
the leaves had been previously cut aw-ay, in order that all the products of assimilation
of the thirteen leaves should pass into the bud which was now in the dark. Inside the
box, which was about two metres high, one metre broad, and one metre long, there
developed, in the course of the next four or five weeks, an extremely vigorous system
of etiolated shoots, with white stems and leaf-stalks, and yellow leaves. On the ist
of September, that is, after five weeks, the terminal bud of the etiolated main stem
was conducted to the exterior through a hole e in the roof of the box ; and the shoot
which had grown in the dark inside the box, and which possessed structural peculiarities
[3]

354

LECTURE XX ir.

accordingly, now developed further in the Hght. The whole of the part C, up to
the 7th of October, was about 95 cm. long, and possessed eight fine green leaves ;
all the organs, flowers, tendrils, etc. being normal in structure. In the meantime,
however, from a small flower-bud which had been conducted into the box with the

. Gourd plant, the main shoot of which was t'lrst developed in the dark space,
and subsequently again led to the exterior above (cf. the text).

rest on the 251h of July, a Gourd fruit had been formed by the artificial impregna-
tion of the flower subsequently developed : on the 8th of October this fruit possessed
a circumference of 59 cm., and weighed about 3 kilograms, and contained 195
seeds, a third of which were shown to be capable of germination. It is evident that
all the substances which had served for the construction of the organs in the dark

TRANSPORT OF CONSTRUCTIVE MATERIALS. 355

had been conveyed from the organs outside the box to the etiolated shoot inside :
the necessary water and ash ingredients coming from the roots, and the organic
substance from the green leaves. The fruit alone contained about 250 grams of
organic substance, and if we assign to the other etiolated organs in the box — the
shoot-axes, leaves and tendrils — the very slight value of 50 grams of organic sub-
stance, we have about 300 grams of organic substance, conveyed in the course of
seventy-four days from the green leaves into the parts growing in the dark. Up
to the ist of September the substance was provided exclusively by the thirteen leaves
situated outside the box, and they possessed an assimilating surface of i"5 sq. metres.
The shoots which subsequently grew out, beyond the roof of the box, were able to build
up their constructive matters themselves: only the water and mineral matters had to
be conveyed to them from the roots through a distance of 4-5 metres. If we had
made our experiment with the difference that, after conducting the bud into the box,
all the leaves outside were cut away, or themselves placed in darkness in the same
way, the bud would then have grown in the box for a few days longer, and then all
the parts in the dark would have perished. The incidental abnormalities of the
organs developed in the dark come no further into consideration here. If during the
period of vegetation, however, we had investigated the etiolated organs in the box
micro-chemically, we should have found sugar and starch-grains in the parenchyma of
the shoot-axes and petioles, proteinaceous matters in the sieve-tubes of the vascular
bundles, protoplasm in all the cells, and etiolated yellow chlorophyll-grains in the
yellow leaves ^

If we now enquire as to the forms of tissue^ in which the plastic substances from
the reservoirs of reserve-materials are conveyed to the growing parts, the answer may
be given generally that the albuminous matters move in the phloem of the vascular
bundles, while starch, sugar and fats, and the nitrogenous substance asparagin,
belonging to the protoplasm-formers, are conveyed in the parenchymatous tissues of
the shoot-axes, petioles, roots, etc. The materials mentioned may be recognised at
all times on these routes, so long as the conditions for so doing exist — i. e. if it is
observed whence the matters in question come, and where they are made use of.
Apart from a few special cases, it is shown that every portion of an organ about to
begin to grow vigorously, becomes in the first place filled with proteid matters (or
asparagin), and with fine-grained starch and sugar, which are conveyed to it from
the reservoirs of reserve-materials, or from the assimilating leaves. Then, when any
particular part of a root, shoot-axis, or leaf, etc. is fully developed, the materials
become used up and disappear.

It is not to be forgotten here, however, that by means of growth itself, the
organs already developed come to lie between those parts which supply the materials

* With respect to the behaviour of plants in the dark, cf. my treatises ' Über den Einßuss des
Tageslichtes auf Neubildung und E7itfaltung verschiedener Pflanzenorgatie'' (Bot. Zeitg. 1863), and
'Die Wirkung des Lichtes auf die Blut hefibildnng unter Vermittlung der Laiibblättcr' (Bot. Zeitg.
1865, No, 15, &c.).

^ My first publications directed against the old theory of the descending sap are, ' Über die
Leitung plastischer Stoffe durch verschiedene Gewebeformenl Flora, 1863 (p. 33), and the chapter on
the translocation of plastic substances in my ' Experimental-physiologie' (1865, p. 374\ where the
older literature is also cited.

35^

LECTURE XXII.

and those where they are made use of; so that with the progressive growth of the
shoot-axes and roots, the distance through parts already fully grown which the forma-
tive substances must traverse on their way to the parts which are growing and which

I'IG. 239.— Diagram of a dicotyledonous plant. J young- embryo in the seed; // the same somewhat older ;
/// the plant after the conclusion of germination when commencing to assimilate independently. The black
portions indicate the growing points, containing chiefly proteid materials ; the grey ones are the parts which
are elongating, where the proteid matters are less abundant, but into which starch and sugar have passed to be
employed in growth. The parts left white are fully grown ; and c and b are carrying on the function of

make use of them becomes greater and greater. Figure 239 may serve to
illustrate this, since it represents diagrammatically the distribution of the states of
growth of a young Dicotyledon. All the organs already fully developed, and

EMPLOYMENT OF CONSTRUCTIVE MATERIALS IN GROWING ORGANS. 357

therefore no longer growing, are simply in outline and not shaded ; those which
are actively growing are shaded ; and the very slowly developing growing points
are represented black. Let us now assume that this diagrammatic plant has
completed its germination, and is already nourishing itself by means of assimila-
tion. The fully developed cotyledons c, which we suppose to contain chlorophyll,
and the fully developed leaf d, do duty as organs of assimilation. From these
the products of assimilation and metabolism have to travel on the one hand down
to the roots w w' w" , and also, and to a still greater extent, up to the young
parts of the shoot which are shaded dark. It is at once seen that between the
places where the consumption of material occurs, and the organs of assimilation,
parts are intercalated which are no longer growing, and thus do not use up the
materials. Nevertheless the plastic substances must travel through these fully de-
veloped parts to the (darkly shaded) growing portions. This takes place, so far
as the albuminous matters are concerned, in the phloem of _ the vascular bundles
which are indicated in the figure : starch, sugar, and asparagin, on the contrary,
travel in the layers of parenchyma, and, as micro-chemical investigations show,
chiefly in those layers which immediately surround the vascular bundles. This refers
especially to the transitory starch, as to the movement of which a few words more
may be added here. Where a young organ commences to grow, its small paren-
chyma-cells become filled with fine-grained starch, and then sugar appears in ad-
dition. As the part goes on growing, the starch and sugar increase ; they being
supplied in quantities greater than those consumed. Subsequently, the relation is re-
versed, and more is consumed than is supplied, and when the organ in question is
fully developed, the starch and sugar have disappeared. This takes place not
only when the supply of food is derived from the starch-forming organs of assimi-
lation, or from reservoirs of reserve-materials abounding in starch, but also when the
reservoirs of reserve-materials previously contained no starch at all, but cane-sugar
as in the root of the Beet, inulin as in the tubers of the Dahlia, or even fat as in
most germinating seeds, or, finally, cellulose as in the Date. In a word, apart
from rarer cases of plants in which the formation of starch is almost entirely
suppressed, as in the common Onion, the various kinds of substances which
form cell-walls, before they are directly made use of in growth, are at least in part
transformed into starch, which is then only used up directly as the cell-walls de-
velope. We have as yet no satisfactory explanation of this remarkable fact. Although
no doubt whatever now exists that the starch which enters the young parts of the
growing shoot (in a germinating Oak for instance, or the bulb of a Tulip just putting
forth shoots), and which at first increases but then always disappears at the conclusion
of the growth of the part, is derived from the starch contained in the reservoirs of
reserve-materials — the cotyledons or bulb-scales; nevertheless it is likewise certain,
on the other hand, that these small starch-granules are for and by themselves im-
movable, and that no single starch-granule is itself in the act of travelling. This
follows, on the one hand, from the fact that the transitory starch which has passed
into the growing parts consists of very small granules, whereas that in the reservoirs
of reserve-materials is large-grained and difi'erent in shape ; moreover it would
be quite impossible for solid starch-grains to pass through the closed cell-walls of the
j)arenchyma, while the necessary moving forces would also be wanting, ^^'hen in

358

LECTURE XXII.

spite of this we speak of a translocation of the starch, an assumption is necessary in
the first place to complete the direct observation. This is in general to the
effect that the wandering starch undergoes continual solution and re-formation
progressively from cell to cell. In this the activity of Schimper's starch-forming
corpuscles co-operates in the cells in question, or, in the younger portions, the change

is effected by the protoplasm itself It is
not necessary to enter more into detail
respecting this problem, however, it being
only intended to do away with a possible
error which might arise from the use of the
expression ' transitory starch.' Perhaps
a similar assumption is also to be made
respecting the translocation of fat in the
closed parenchyma. Within the parts of
the shoot-axes, petioles, and leaf-veins
which are already fully developed, the
starch, then, passes from the reservoirs of
reserve-materials or organs of assimilation
to the growing parts, chiefly or exclusively
in that layer of parenchyma which imme-
diately surrounds the vascular bundle, or,
in many Dicotyledons, separates all the
leaf-traces in the shoot-axis from the cor-
tex as a closed layer. This is the layer
which we have previously learnt to know
as the endodermis, and which, on account
of this very function, I introduced into
physiology long ago as the starch-bearing
layer. If a more extensive movement of
starch occurs, it takes place also in the
external layers of the pith, internal to the
vascular bundles ; and, in the case of a
very vigorous transport of starch, as when
the leaves of trees are being emptied in
the autumn, even the phloem of the vas-
cular bundles may take part in it.

In the growing points themselves, i.e.
in the extremely small-celled tissue of the
apices of the roots and shoot-axes, I have
never succeeded in detecting transitory
starch or sugar. Both appear in these cells only when they pass over from the
embryonal stage into that of extension — of more rapid growth — and as soon as
intercellular spaces arise in the young parenchyma. The extremely small quantity
of cell-wall forming substance which is made use of in the embryonal tissue of the
growing point is in accordance with this. In the growing point itself growth is ex-
ceedingly slow, the developing cell-walls immeasurably thin, and the embryonal tissue

Fig. 240.— Longitudinal section of a germinating bulb of
Tulipa pmcox. h brown membrane enveloping the bulb ;
k the discoid portion of the stem of the bulb, which supports
the bulb-scales sh, sh ; si the elongated portion of the stem bear,
ing the foliage leaves /' /', and passing above into the terminal
flower ; c ovary, a anthers, / perianth ; 2 a lateral bud (young
bulb) in the axil of the youngest bulb-scale ; >• the apex of the
first leaf of this lateral bud. This bud becomes the bulb of
next year. 7v the roots which arise on the fibro-vascular strands
of the discoid stem.

MOVEMENTS OF CONSTRUCTIVE MATERIALS.

359

itself densely filled with proteid substances, in which the extremely small quantity
of cell-wall forming substance eludes direct recognition.

So far as we are informed with respect to the corresponding matters in trees and
other woody plants, the movements referred to take place here also practically
along the paths indicated, though with some slight modifications. Here, however,
the additional fact comes into consideration that the parenchymatous tissue
system extends also into the wood ; from the cortex the horizontal medullary rays
lead directly into the interior of the compact mass of wood, and the wood paren-
chyma distributed between the proper wood-fibres and vessels is proved to be in
continuity with them, so that medullary rays and wood-parenchyma may be regarded
to a certain extent as fine ramifications of the cortical parenchyma, by means of
which it extends into the compact mass of
wood. Accordingly, during the vegetative
period this parenchymatous part of the
wood also becomes filled with reserve-
materials ; chiefly, and usually in our trees,
with starch, in some cases also with other
cell-wall forming substances- -in the North
American Sugar-maple for example with
cane-sugar. That these reserve-materials
stored up in the wood are dissolved when
the buds are put forth in spring, and
carried away and used up in growth, fol-
lows from their disappearance at that time;
this proceeds from the thinner bud-bearing
twigs to the older branches, and then
into the stem, as stated some time ago
by Theodore Hartig. This distinguished
observer found also that when in the
spring a ring of cortical tissue is taken from
the stem, and the whole of the ordinary
conducting organs thus interrupted, the
tree nevertheless puts forth shoots, and
makes use of the starch deposited in the
wood parenchyma beneath the annular
wound in the process, whence it follows that this starch can be conveyed through
the wood parenchyma itself. It is easy to demonstrate by micro-chemical investiga-
tions as well as by simple experiments^ that the buds of trees behave exactly like
the germinal shoots of tubers, bulbs, and seeds, with respect to the phenomena here
spoken of.

I have hitherto taken into consideration only the vegetative organs — roots and
shoots. That exactly the same principles hold good for the nutrition of flowers and
fruits, so far as the employment and the travelling of plastic substances in growth are
concerned, is a fact of which I afforded a series of detailed proofs in my researches

Fig. 241.— Winter-bud of the Horse-chestnut in longitu.
dinal section, r cortex, k wood, ;« pith of the previous year's
shoot-axis -vr. s scales, I foHage I
tains the inflorescence *.

I'es of the bud which (

* See note 2, p. 355.

360

LECTURE XXII.

cited at the beginning ^ ; and the very configuration of the parts of the fruit and
seed allows in many cases the relations between plastic matters and growth to be
perceived particularly clearly. I must here omit the detailed enumeration of these
matters, only finding space for the remark that the materials for growth conveyed
to the flowers and young fruits are obviously carried, in green plants which possess
foliage at the same time, from the assimilating leaves through the flower-stalks.
Nevertheless, the shooting of Tulip and Hyacinth bulbs in spring shows that even
in the reservoirs of reserve-materials substances serving for the formation of flowers
may be stored up ; and the same is true of ordinary fruit-trees, Horse-chestnuts,
etc., the flowers of which unfold in the spring before or simultaneously with the
appearance of the green leaves, employing for that purpose the plastic materials
reserved in the wood and cortex.

We now come to the further question, by means of what mechanical arrange-
ments are the materials put in motion at the time of growth ? It is simply a matter of
the transport of particles the inertia of which has to be overcome, and which must be
set in motion by particular forces. Here again, however, the facts are not so simple
that they can be rendered clear in a few words. At any rate two essentially diff"erent
mechanical processes have to be distinguished from one another, inasmuch as under
certain circumstances pressure is able to effect the normal movements of material,
though of course as a rule it is the forces of diff'usion which are in play.

Regarding in the first place those phenomena where pressure comes into con-
sideration as the cause of movement, the sieve-tubes and laticiferous vessels may
first be mentioned.

The albuminous substances contained in the sieve-tubes are but little suited for
movements of diffusion, hence the transverse and longitudinal walls of the sieve-tubes
are provided with fine perforations, and there is no doubt whatever that though these
pores are so extremely fine, very considerable quantities of these substances can be forced
through them by pressure. If a fresh stem of the Gourd plant, or other very suc-
culent shrub, is cut across, the alkaline contents of the phloem-bundles exude in the
form of drops which slowly increase in size, it may be till they become as large as a pea,
and then (at least in the Cucurbitacese) coagulate. Now it is clear that these reladvely
large masses of albuminous substance could not possibly be contained in the segments
of the sieve-tubes which have been incidentally cut across, but that the contents of
sieve-tubes far distant from the section must here come into view. Taking into
account the anatomical constitudon of the sieve-tubes, this exudation of the contents
is only possible by means of some relatively strong pressure, which must be exerted
on their walls. Such a pressure actually exists, moreover ; it is brought about by the
turgescence of the succulent parenchyma, which strives to extend in all directions.
In exactly the same manner as the epidermis in a living shoot is passively distended
by the succulent parenchyma (as described in Lecture XII), so also must the soft-
walled sieve-tubes be compressed by the parenchyma. Of course so long as they
are filled with fluid, in the intact plant, this supports the pressure of the parenchyma,

^ Details as to the behaviour of the matters forming the cell-wall in fruits are found in my
treatise, ' Über die Stoffe, '<.vckhe das Material zur Bildung der Zellhäute liefern,' in Jahib. für wiss.
Bot. (1863, pp. 270, &c.).

MASS MOVEMENTS OF PLASTIC MATTERS. 36 1

since there is no passage for it to esca|)e. In the neigiibourhood of the growing buds,
however, the tension of the tissue is much smaller, and the pressure of the paren-
chyma on the young sieve-tubes there situated feebler, whence it follows that even
in the intact plant the contents of the sieve-tubes are forced towards the buds by the
pressure of the parenchyma of older parts of the shoot, and this, in accordance with
growth, need only take place slowly. It is not impossible that yet more special
relations of organisation co-operate here ; and the emptying of the sieve-tubes, or,
better, the dilution of their contents by water in older shoot-axes, and perhaps also
the formation of the Callus (in trees, especially in the autumn), indicates something
of the kind. Nevertheless, scarcely anything definite can at present be said in this
connection.

With respect to the pressure of the surrounding parenchymatous tissues, the
laticiferous vessels ^, where they exist, are in the same position as the sieve-tubes.
The simple fact that every wounding of a fresh turgescent plant which contains latex
causes the immediate production of a thick drop of lalex, and that in old, large
specimens of succulent Euphorbias even streams of latex exude, and in a few
seconds yield several cubic centimetres of liquid, alone suffices to demonstrate that the
latex is extruded from wounds with great force. That this is not in any way a mere
outflow, follows at once from the fact that after cutting across a milky stem, not only
the lower cut surface of the apical portion but the upper one of the root-stock also
extrudes the latex. Besides, the laticiferous vessels are extremely narrow capillary
tubes, the normal terminations of which in the buds, leaves, and root-apices are
closed : how could fluid flow out at all on cutting such capillaries closed at the
ends.'' It would scarcely have been worth while to introduce these self-evident
reflections, were it not for the fact that even celebrated botanists have doubted the
action of the pressure which I rendered evident so long ago as 1865 {Experivmital-
physiologie, p. 386). The laticiferous vessels as well as the sieve-tubes behave in this
connection exactly like the blood-vessels in the human body : when we wound our-
selves the blood does not simply flow out, it is driven out.

On now enquiring what significance this pressure, exerted from all sides on the
laticiferous vessels, may possibly have in the economy of the plant, it is to be noticed
that the pressure of the tissues diminishes in the neighbourhood of the growing buds
and other young organs, and that therefore the stronger pressure of the parenchyma
in the older succulent parts must drive the latex towards the younger growing parts ;
though this, of course, cannot be regarded as exhausting the mechanics of the subject.
Nevertheless, we know at present no more than what has been said. However,
that a continuous movement of latex from the older and especially the assimilating
organs to the young growing parts also comes into consideration for the supply
of plastic substances, follows from the fact, confirmed by numerous observers, that
larger or smaller quantities of proteid substances, fats, and carbo-hydrates (in the

' After Schiiltz-Schultzenstein had previously ascribed to the latex an unduly high significance
for the life of the plant, and had at the same time connected with it the most confused errors, in a
prize essay rewarded by the Parisian Academy, the importance of the latex was again far under-
estimated by Hugo von Mohl in a critical reply. I led the way for the correct appreciation of the
significance of latex and its receptacles for the nutrition of the plant in my ' Expcrimcnlal-
phys to logic' (1865, p. 386.

362 LECTURE XXII.

Euphorbiace» particularly starch-grains) are as a rule contained in the latex.
Knowing how economical the plant is with these — its constructive materials — it will
scarcely be assumed that these substances are contained in the laticiferous vessels for
any other purpose than to be conveyed to the growing organs. Of course it is not
to be forgotten that the laticiferous vessels contain at the same time considerable
quantities of secretions — caoutchouc, resins, ethereal oils, alkaloids, etc. ; that is,
substances not employed in growth but produced as bye-products. The presence of
these substances in the laticiferous vessels by no means proves that the constructive
materials likewise present are useless, however, any more than the presence of
carbon dioxide and various products of the decomposition of the tissues in the veins
of animals warrants us in regarding the useful constituents of their blood as useless.
With this view, which I put forth in 1865, a series of experimental researches by
Faivre ^ on Ficus elastica, Morus alba, and Tragopogon agree. Nevertheless, here
again a rich field still lies open for experimental and microscopical investigation, and
its fruiifulness is perhaps enhanced by the fact that very active peptonising ferments
have already been discovered in the latex of some plants, e. g. Carica Papaya and
Ficus Carica.

The pressure of the tissues, as evinced by the outflow of fluids from wounds, is
observable, however, not only in the case of sieve-tubes and laticiferous vessels : even
the succulent turgid parenchyma itself exhibits an exactly similar phenomenon. Any
transverse section made with a sharp knife through a succulent stem, or petiole, shows
that far more sap exudes from the cut surface of the parenchyma than could have
been contained in the few cells incidentally opened by the section : the greater part
of this exuding sap is evidently pressed out from parenchyma cells some distance
removed from the section. Here, however, the fact is not so simple as in the case
of the sieve-tubes, since it depends upon the exudation of sap from closed living
parenchyma cells, where the expressed fluid must filter, not only through the cell-
walls, but, what is more, through their protoplasmic linings, the pressure evidently
being afforded by. the strong turgescence of the parenchyma cells themselves.
However profitable it might be to enter more in detail into this remarkable phe-
nomenon and its consequences, the mere mention of them must suffice here.

We have to seek the most general cause of the movements of the materials in
plants, however, in diosmotic processes. These consist in the attraction between
water and soluble substances, which in the case before us undergoes important modifi-
cations from the fact that the processes in question must take place through cellulose
walls which are clothed with living protoplasm. The most essential facts on this
subject have already been mentioned (Lecture XII). Since space does not allow of
our entering specially upon the mechanics of the diosmotic processes coming into
consideration here, I must content myself with a few general remarks ^

Every process of diff'usion or of diosmosis between a living cell and its environ-
ment must, sooner or later, if undisturbed, pass over into a certain condition of

1 Cf. Faivre on Latex, 'Ann. des Sc. Nat.' 1866, vol. vi (p. 33), and 1869, vol. x (p. 97),
' Comptes retidtcs,' 1S79 (P- 3%) > ^"^ Pfeffer, ' Pßanzen-physiologie'' (p. 325); Hansen, Sitzungsber.
d.ph. med. Ges., Erlangen, 8 Nov. 1880.

^ With regard to the theory of diosmosis I refer to the detailed account in Pfeffer's ' Pflanzen-
physiologie,' chap, ii, and Sachs' 'Experimental-physiologic,' 1865 (pp. 157, &c.)

TRANSLOCATION BY DIOSMOSIS. 363

equilibrium, in accordance with the diosmotic conditions just given, and then all
further movements of diffusion cease. If, for example, a cell is able to take up sugar
by endosmose from its environment, the absorption ceases so soon as an equal con-
centration occurs within and outside the cell, or even earlier. If, however, the sugar
which enters into the cell is made use of, to form starch for example, then so long
as the formation of starch continues more sugar will be able continually to pass into
the same cell ; and it is obvious that the chemical metamorphosis of the diosmotic
body here prevents the establishment of an equilibrium of diffusion, and contributes
to the continual transport of larger quantities into the cell, which thus behaves towards
its saccharine environment as a centre of attraction. Something very similar will
happen if proteid-forming substances diffuse into a cell, and are there employed in
the formation of crystalloids. In like manner it is obvious that when starch-grains
are transformed into sugar in a cell, and neighbouring cells take up the sugar by
diosmosis, equilibrium can only be established when the formation of sugar ceases in
the cell containing starch ; or, in other words, a cell in which a soluble substance
arises behaves in diffusion as a centre of repulsion.

This view may be also extended to whole aggregates of tissues and organs,
and at the same time we obtain from it the reason why chemical metamorphoses of
the plastic matters are so generally connected with their transport. To give a few
examples only. — Starch is assimilated in the leaves of the Beet : in the petioles it is
found again in the form of glucose (i. e. a sugar which reduces cuprous oxide). This
glucose now enters the growing and swelling root, and is transformed into cane-sugar
in its parenchyma. As each particle of glucose, which evidently passes from the
petioles and enters the root, is transformed into cane-sugar, it acts, according to the
laws of diffusion, as if it had been annihilated, and so long as the transforma-
tion of glucose into cane-sugar takes place in the root, fresh glucose can continually
flow in. In this manner, in spite of the influx, the root goes on behaving like a
body destitute of glucose, and acts on the product of assimilation of the leaves as a
centre of attraction. The formation of starch in the growing tubers of the potato
plant is another case in point ; here also the product of assimilation of the leaves
is carried through the stem into the tubers in the form of glucose ; in these, however,
it disappears as such, being employed for the formation of starch. An equilibrium
of diffusion does not take place, therefore, so far as the glucose is concerned; it
streams continually into the tubers, because it is there continually being converted
into starch. Chemical metamorphosis acts in the same way in the using up of
reserve-materials in growing seedlings. If the reservoirs of reserve-materials contain
cane-sugar, inulin, or fat, glucose arises in the seedlings at the commencement of
growth, at the expense of these materials, and from this starch-grains, which are
then finally again dissolved and employed for the growth of cell-walls. To mention
one other example only, the genetic relation between proteid substances and
asparagin : in the reservoirs of reserve -materials proteid substances only are
generally present ; at the commencement of the growth of the seedling, however, a
part at least of these is transformed into the easily soluble and diffusible asparagin,
which now passes into the younger buds, and is again transformed into proteid
substance.

We may thus say, to put the matter simply, every growing [lari of the plant

3^4

LECTURE XX 11.

acts on the existing constructive materials as a centre of attraction ; every reservoir
of reserve-materials, and every organ of assimilation, on the contrary, is passive
towards a growing portion, or, it may be, behaves as a centre of repulsion. The
stimulus to the movements of material, however, is always given by the growth of the
young organs. The buds of a tree put forth shoots in the spring by no means because
the nutritive sap enters into them, as people are in the habit of saying ; but exactly
the reverse — the nutritive matters are set in motion because the buds begin to grow.

All the considerations in this and the last lectures have chiefly had re-
ference to the highly organised so-called vascular plants. But in a few words I will
show that the essential points are true also for the simply organised Mosses and

Algae, down to the most simple unicellular forms. We meet everywhere, even in
these lower regions, with the same relations between the metabolism which occurs
during growth, and assimilation: the storing-up of reserve-materials in perennial parts,
e.g. in the tubers so common in true Mosses, and quite generally in the spores,
which, although they consist only of single cells, behave in the main like seeds.
Indeed, the germinating spore of a INIoss or of an Alga affords us the simplest
possible scheme for the process of germination described previously : under the eye
of the observer the reserve-materials disappear in proportion as they are used up
for the growth of the germinal tubes, just as the reserve-materials in the plumules of
the higher plants travel towards the growing ends of the organs, etc.

EMPLOYMENT OF CONSTRUCTIVE MATERIALS IN LOWER CRYPTOGAMS. 365

In the lectures on physiological organography, I frequently mentioned the
remarkable fact that non-cellular plants, such as the Siphoneae among the Algae,
behave, so far as their external organisation and all biological matters connected
therewith are concerned, like ordinary cellular plants, and that the continually pro-
gressive cell-formation of the latter implies no essential difference whatever as opposed
to the processes of configuration of the non-cellular plants. This is confirmed anew
on seeing how, in Caulerpa for instance, the part of the non-cellular vesicle which
corresponds to the stem produces assimilating leaves containing chlorophyll, on the
one hand, and rhizoids on the under side, on the other ; and, so far as we are
informed concerning the assimilation and metabolism, and the movements and meta-
morphoses of material, and the relations between these and growth, essentially the
same processes take place in this non-cellular vesicle (which is nevertheless externally
and internally so sharply differentiated) as were found to occur in the most highly
developed plants with their microscopic cellular structure. Everything, of course,
becomes more and more simple the simpler the growth and mode of life generally
assumed by the Alga.

LECTURE XXIII.

THE ABSORPTION OF ORGANIC NUTRITIVE MATERIAL. PARA-
SITES. COPROPHYTES (SAPROPHYTES). INSECTIVOROUS
PLANTS.

All that has hitherto been said on the nutrition of plants has been with refer-
ence immediately only to those which are provided with abundance of chlorophyll,
and the land-plants provided with large transpiration surfaces were placed especially
in the foreground as the typical chlorophyll-plants, in which all the chemical and
mechanical processes of nutrition are called into play. As to the nutrition of these
normal plants, two chief points were noticed : first, the formation of new organs
takes place at the expense of the reserve-materials which have been produced by
the mother-plant and stored up in the reservoirs of reserve-material ; after these
materials have been consumed, or, as we may also say, at the conclusion of ger-
mination, the second period of nutrition begins, the assimilation of organic plant-
substance by the decomposition of the atmospheric carbon dioxide in the organs
containing chlorophyll, under the influence of light.

There are, however, numerous plants which are entirely devoid of chloro-
phyll, or which possess so little of it that it is scarcely of any importance in
nutrition. Since now the decomposition of carbon dioxide ; and the resulting
formation of organic plant-substance are effected exclusively by means of the chlo-
rophyll, it follows at once that all the plants of this second section are incapable
of exercising the nutritive activity which is characterised by assimilation. Plants
devoid of chlorophyll exist during their whole life in a condition which cor-
responds to the germination of normal plants : like the seedlings of seeds which
contain endosperm, like the germinal shoots of tubers, bulbs and root-stocks, and like
the unfolding winter-buds of trees, plants devoid of chlorophyll take up the whole of
their food for the purpose of growth directly, in the form of organic compounds
of already-formed vegetable substance. They do not produce organic compounds
containing carbon, but they absorb them, and change them in accordance with the
purposes of their own growth.

On the whole, it is practically the same whether the plants devoid of chlorophyll
absorb their organic nutritive matters from other living plants or living animals, or
whether they make use of the dead bodies of plants and animals or their dissolved
constituents for their nutrition. In the first case they are distinguished as Parasites, in

PARASITES AND SAPROPHYTES. 367

the second as Coprophytes or Saprophytes. The comparative insignificance of this
distinction, if the aim of nutrition is exclusively kept in view, is seen at once from the
fact that some Fungi which are usually parasitic can grow in artificially prepared
nutritive solutions — e. g. Agaricus mellcus. Several species of Mucor again are able
to obtain nutriment for their mycelium from the tissues of fresh Apples, although they
also flourish in saccharine nutritive solutions or in gelatine. The main fact is simply
that plants devoid of chlorophyll are necessitated and are able to absorb their car-
bonaceous substance from without in the form of organic compounds. This how-
ever by no means excludes the fact that in general each individual species is met
with in the natural course of things only as a parasite or only as a saprophyte, and
that very often the most capricious choice of nourishing substratum is exerted in the
matter. Most parasites flourish only on perfectly definite species of other plants or
animals ; nay, many of them, particularly parasitic Fungi, can only enter into
perfectly definite parts of their host-plant. Even saprophytes hit upon a careful
choice of their substratum. It also happens, it is true, that certain parasites settle on
the most various host-plants and some saprophytes inhabit any substratum whatever.
It is evident that biological points here come into eff"ect which are connected not
directly and exclusively with nutrition, but with other physiological properties of the
plant in addition.

Although plants which contain very little or no chlorophyll are necessarily para-
sites or saprophytes, it by no means follows that all parasites must be devoid of
chlorophyll. I have already had occasion in Lecture III to mention the Loranthacese,
which abound in chlorophyll and are nevertheless parasitic ; particularly the Miseltoe.
In such cases the parasite really needs to absorb from the host-plant only water and
mineral substances dissolved in it, though it is not excluded that certain organic
compounds of the host-plant may possibly be indispensable to the parasite in very
small quantities. Plants which contain chlorophyll also occur among the sapro-
phytes, as Neottia Nidus-avis, Corallorrhiza, etc. in the family of Orchidese ^ where at
least the flowering-stems which project above the surface of the soil contain chloro-
phyll, of course in quantities so small, and appearing so late in the life of the
plant, that it comes into consideration for the total process of nutrition just as little
as does the chlorophyll developed under the peel of a potato lying in the light for
the development of the tuber.

I have also already spoken in Lecture III of the parasitism of Thesiian and
RInnanthus. These green-leafed plants develope a somewhat richly branched system
of roots in the earth, single threads of which become united by means of small
haustoria with the roots of neighbouring plants containing abundance of chlorophyll.
It is not yet known, however, how far this partial parasitism is of importance for the
life of these plants.

The most remarkable case of supplementary nutrition in plants containing
chlorophyll, by the absorption of organic substance, is offered, finally, by the
so-called insectivorous or carnivorous plants, to which I shall return in detail
further on.

We will, first, once more regard the processes in parasites and saprophytes

' Drude, 'Die Biologie der Monotropa Hypopitys und Neoltia' ^Göttingen, 1S73), pp. 32, 33.

368 LECTURE XXIIT.

devoid of chlorophyll, as presenting the typical case of the absorption of organic
substance. It is at once conspicuous that in the life of these plants it is by
no means simply a matter of the want of chlorophyll, and the compensation
of the chlorophyll-function by the absorption of organic substances : on the
contrary, the internal causal interdependence of all vital phenomena involves that
the whole internal and external organisation of plants devoid of chlorophyll should
deviate essentially from the normal. I have already, in Lectures III and V,
referred in this sense to the organographical peculiarities of parasites and sapro-
phytes, and laid special stress on the fact that with the absence of chlorophyll the
presence of large transpiration surfaces would be superfluous, or even injurious, and
that therefore plants devoid of chlorophyll possess no large leaves, and for the same
reason they form no true wood. Where plants devoid of chlorophyll tend to the
formation of more vigorous vegetative and reproductive organs, these are never
conspicuous for their superficial development, but, on the contrary, for their small
superficies and relatively large mass. Nutrition by the absorption of organic
substance thus reacts on the whole organisation of the plant. No single plant
devoid of or containing but little chlorophyll possesses the ordinary habit, and least of
all the .large leaves and surface development and slender growth generally, of normal
plants. This is so far the case that every one, even those unfamiliar with botanical
matters, recognises in the parasites and saprophytes devoid of chlorophyll organisms
of peculiar structure. No other biological condition effects a change in plants so
deeply affecting the whole organisation as does the want of chlorophyll and the ab-
sorption of organic substance. This goes so far that even the reproductive organs
are influenced by it and degraded to a great extent : all plants devoid of chlorophyll,
even when they are descended from highly organised types of Phanerogams, are
remarkable for their exceedingly small, often almost microscopic seeds, and the
embryos in these seeds often consist of only a few cells, and are not segmented
externally. In the Raillesiacese, Balanophoreae, Orobanchaceae, and Mojioiropa,
the embryos have no trace of radicle and plumule, and in the Cuscute^e there is
only a feeble indication of such.

Parasitism, moreover, works not only to the degradation and alteration generally
of the organisation of the parasite itself, but the living plants attacked by the parasite
are altered by it. In the first place, it is clear that the withdrawal of a certain
quantity of plant-substance, which the parasite puts in requisition for itself, must
enfeeble the host-plant ; and in many cases this goes so far that the latter is prevented
by feebleness from concluding its life in the normal manner. This is the case, for
example, when numerous specimens of Orobanche speciosa attach themselves to the
roots of Vicia Fabia. In other cases, however, the effect on the host-plant goes
further. The parasite induces phenomena of irritability on the part of the host-plant,
bringing about vigorous local growth — for example, the active wood-formation and
production of tuberous swellings in woody plants attacked by Viscum or Loran-
ihus EuropcEus, and many Fungi inhabiting wood, cause the formation of resin in
Conifers, and injure the latter in consequence (Robert Hartig). But even the normal
processes of configuration of the host-plant may be rendered abnormal by parasites.
For example, the ligneous bundles of the roots of the host on which Balanophora is
parasitic grow into the tuberous vegetative body of the latter, and serve to a certain

NOURISHMENT OF PARASITES. 369

extent as vascular bundles for the parasite. Still more striking is the morphological
change suffered by the branches of Pine-trees infested by the fungus jEcidium
ElatinUm; these are known under the name of Witches Brooms, and are distin-
guished by a form of branching otherwise foreign to the lateral shoots of the Pine,
and their needle-like leaves fall off like those of summer plants, to be renewed
annually. Such a branch of the Pine attacked by the Fungus exists on the
mother-branch in the form of a small deciduous Fir-tree, which may become
ten years or more old (De Bary^). Euphorbia cypan'ssias, so very common in
Germany, also has its whole habit altered by a Fungus {JEcidiuni) dwelling in its
tissues.

No less remarkable is the chemical influence of many Fungi on their nutritive
substratum, in so far that they not only extract from it the material necessary for
their nourishment, but, in addition, cause the destruction of organic substances
quite unnecessarily, so to speak, and even to their own injury. The Fungus
{Phytophihora w/estans) which causes the potato disease, for instance, vegetates
in the tissues of the Potato-shoot, in the first place in order to nourish itself:
when its fructification protrudes through the stomata of the shoot, however, a
decomposition of the organic matters in the tissues of the latter is set up, which kills
the Potato-shoot itself, and makes the further nutrition of the parasite impossible.
In the same way, by the mycelium of Mucor Mucedo developing in a fresh Apple, the
tissue of the fruit is not merely extracted for the nourishment of the Fungus, but is
at the same time converted into a soft smeary mass, alcohol and ethereal oils being
formed. The most striking of such cases are the decompositions of the media
not directly serving for nutrition, by the ferment fungi (Yeast) and Bacteria',
while these Fungi take up from their substratum only exceeding small quantities
of sugar and proteids for their own nourishment, they destroy enormous quantities of
these substances by profound decompositions. Many fungi which kill trees by
destroying the wood behave similarly.

Regarding still further the ways and means by which these plants are
connected with their substratum, we find conspicuous differences even in the case of
parasites. In the first place, the parasite may, on the whole, exist and grow entirely
external to the host-plant, only being placed in connection with its tissues by means of
peculiar, relatively small organs, the so-called haustoria, as is to be seen particularly
clearly in the Cuscuteae and in Thesiiim and Rhinanihiis : these haustoria are, as has
already been shown in the organographical introduction, metamorphosed roots
which penetrate into the tissues of the host-plant. According to circumstances, this
metamorphosis is only inconsiderable, as in the Miselto, which abounds in chloro-
phyll, and the whole root-system of which is contained in the host-plant ; or the roots
are converted into haustoria and lose their typical character entirely, as has already
been described in the case of Cuscula, where the tissue of the haustorium breaks up
within the host-plant into single filaments of cells. In saprophytes also, especially
the Orchidese containing little or no chlorophyll, a degradation of the root-system
makes itself evident, in that the roots remain short, are very thick, branch but little,
and form either no root-cap at all or but a very feeble one.

' De Bary, 'Die Erschcimtnq der Symbiose,' p. 27 Strasburg, 1879).

[3] ' ' Bb

?,T^

LECTURE XXIII.

An unusually simple and clear example of the penetration of the haustoria of
the parasite into the host-plant is found in the case of the mould fungus Piplocephalis
Freseniana (Fig. 243). Both the parasite and the Miicor attacked by it are non-
cellular plants, consisting of simple tubular hyphae, various ramifications of which repre-
sent the organs. The figure shows the mycelial tubes m of the Piplocephalis
having become closely attached to the Mticor, M, and put forth root-like haustoria

into the interior of the latter, by
means of which they absorb the
protoplasm of the Miicor and
transfer it to the parasite.

In the form of haustoria the
parasite only penetrates the host-
plant by means of a small part of its
body. In the lectures on organo-
graphy, however, I have already
mentioned that under certain
circumstances the whole of the
vegetative body of a parasite may
be developed inside the host-plant.
In Fungi, e. g. the Peronosporeae,
JEcidiomycetes, etc., this is a very
common occurrence : only the
mature reproductive organs— and
even these not always — protrude
outside the tissues of the host-
plant. Similar cases also occur,
moreover, in Phanerogams which,
as follows from the structure of
their flowers, belong to highly-
organised types. According to the
remarkable investigations of Graf
zu Solms-Laubach, this is the case
especially in the Rafflesiacese,
where the whole of the vegeta-
tive body is developed in the
interior of the cortex and wood
of its host-plant. In these plants {Raßesia, Bnig77iansia, Piloslyles^) parasitism
has had a degrading influence to so great an extent, that not only is the whole
vegetadve body of the parasite enclosed in the tissues of the host-plant, and there
dispenses with any differentiation whatever into root and shoot, only consisting
in fact of growths of tissue between the tissues of the host-plant, but even the
flower-buds are formed in the interior of the host, only bursting through its cortex
and appearing in the open at the very last. In the case of Piloslyles, investigated
with particular care by Graf Solms-Laubach, and which dwells in the foliage shoots

FIG. ■2a,z-—P'plocepJialis Freseniana (after Brefeld). M a portion
of the niyceliuin of Mucor Mucedo from which the mycelium mm of
the Pipiocephatis obtains its nourishment ; h haustoria which have
penetrated the Mucor filaments; c conidiophore ; ss the two con-
jugating mycelial branches which form the zygospore Z.

Graf ZU Solms-Laubach, Bot. Zeug., 1874 (Nos. 4 and 5) and 1876 (p. 449).

ENnOPHVTTC PA RA SI TES.

37*

of species of Aslragalus (Papilionacece of Asia Minor), the entire vegetative body
consists indeed of single, jointed cell-filaments, which this investigator terms simply
mycelium. He follows this up into the growing-point of the host-plant, and shows
how, out of this vegetative body, in the tissue of the leaf-base of the host, cushion-
like swellings arise, from which the flower-buds of the parasite develop, and which
finally protrude right and left out of the base of the leaf.

It has been mentioned above that many Fungi complete their vegetative
development entirely in the interior of their host-plants (and animals), finally
putting forth only their fructifications to the exterior. In this class of plants,
so remarkable also in other respects, we meet however with exactly the opposite

Fro. 244.— Äff Tissues of the
penetrated. Tra

SKS in wliich the vegetative body hit of a Briis;)
-highly magnified (after Graf Solms-Laubach).

extreme, namely, that the host-plant is entirely enclosed within the parasite — that the
parasitic Fungus grows around the Alga which nourishes it on all sides, so that
Fungus and Alga together constitute in a certain sense a single mass consisting of
two forms of tissue, of colourless fungal tissue and of green algal cells. This is the
case in the Lichens.

If we now cast a short glance at the saprophytes, ^\•e find the ordinary case
among the Phanerogamic species to be that the plant, after germination, developes
within the nourishing substratum (mostly loose humus), while its vegetative body
dissolves and absorbs the vegetable remains enveloping it. Only after prolonged
nourishment and invigoration of the plant does the saprophyte send up flowering

B b 2

37

LECTURE XXI7I.

shoots above the surface. This is the case with Neoltia, Epipogon, Corallorrhi'za,
and Monotropa-, and it is still to be remarked that in the case of Corallorrhiza
the vegetative body lying in the humus is a rootless, branched shoot, while in
Monolropa, on the other hand, it is, according to Kamiensky, at first a shootless,
branched root-system, out of which the flowering shoots spring later, like the fructifica-
tion from a mycelium.

With respect to the nutritive processes in question here, only very little is known,
however, in the case of parasitic Phanerogams. In the first place, so much is at any
rate established, that they are not able to produce starch by the decomposition of
carbon dioxide ; for this, not only is the chlorophyll wanting, but light also, during
the greater part of the period of vegetative activity in the majority of parasites and
humus plants; for these plants, apart from Cuscufa, only allow their flowering shoots
to protrude free from the substratum at the very last, when the entire mass of organic
substance is already wholly or for the most part collected. Where then, as usual, we
find considerable quantities of starch and other carbo-hydrates in parasites and
humus plants containing very little or no chlorophyll, this is owing in no case to the
activity of the chlorophyll in the plants in question ; on the contrary, the starch and
other carbo-hydrates are here derivatives of those chemical compounds which have
been dissolved and taken up from the host-plants, or, in the case of the humus plants,
from the remains of dead plants. With respect to the proteid substances of these
plants which contain little or no chlorophyll, these may according to circumstances
likewise be derived from the host-plant, or, as may be supposed, are first formed in
the tissues of the parasite. When, for example, the entire vegetative body, up to the
development of the flowers, developes within the host-plant, as in the Rafflesiaceae,
it may be assumed that here not only the non-nitrogenous but also the nitrogenous
vegetable substance of the parasite is absorbed from the host-plant ; and the same may
be the case in Cusciita, since this parasite, slung around the host-plant, possesses no
roots at all in the soil, and therefore extracts the whole of its food from the host. It
may at least be otherwise in the Orobanchese. These parasites are firmly fixed by
a haustorium to their host-plant, and without doubt extract the whole of the
carbonaceous substance which they require from the host, but they also produce
several roots besides ; these roots are short, it is true, but they penetrate into the
earth, and the surface of the shoot is, like that of the Balanophorese, for a long time
in contact with the surrounding soil : they may thus take up ash constituents and
probably saltpetre also from the earth, the nitrogen of which is possibly employed
for the formation of proteid substances in the tissue of the parasite.

One of the most important questions is, how does the parasitic Phanerogam
commence to absorb from its host-plant the substance assimilated by the latter?
It is to be observed in the first place that the union of the parasite and host is
usually very complete, so much so indeed that it requires the most careful examina-
tion to discover the limit between the tissues of the parasite and those of the
host-plant. The Rafilesiacese, completely enclosed in the tissues of the host-plant,
behave, in fact, as if their vegetative body were simply another form of tissue in
its interior : the sharp distinction between parasite and host only begins on the
development of the flowers. The fact is here of the utmost interest that the nutritive
substances which the parasite takes up from the tissues of the host-plant nevertheless

PARASITES AND SEEDLINGS COMPARED. 373

serve for the production of an organism of an entirely different kind, whence it is
perceived that, in nutrition, it depends by no means simply upon the chemical nature
of the food-materials, but far more upon the inherited nature of the species of plant.
The majority of phanerogamous parasites are connected with their host-plant
only by means of small haustoria : the rest of the body lies outside the host. All the
organic substance from the host-plant must now pass over to the parasite through
this relatively small surface of the haustoria. as is conspicuously the case in the
Orobanchese and Cuscuteae. But it is just in this apparently most difficult point
of parasitism that the plants in question agree with the seedlings of normal
plants : the embryos of seeds provided with an endosperm also take up the whole of
their nourishment from the endosperm by means of special haustoria, which in this
case are always parts of leaves. An uncommonly clear example illustrating what
has been said has already been given in the case of the Date seedling (p. 344). The
same may be said of the Grasses, the haustorium of which is usually termed the
scutellum by botanists: I have referred to this on p. 38 (Fig. 28; cf. Fig. 35).
Moreover, most Monocotyledons afford similar examples of the absorption of the
endosperm by a haustorium of the seedling'. Probably the most striking case is
that of the germinating Cocoa-nut, the tiny embryo of which, when it begins to grow,
developes at the apex of its first leaf a haustorium resembling that of the Date
seedling : this subsequently attains the size and form of a turnip, in order to absorb the
nutritive materials contained in the huge seed. In these cases it is peculiarly organised
haustoria on the leaves of the seedling which absorb the substances of the endosperm
at their surfaces, and transfer them to the seedling : in many other cases, especially
in the Coniferse and some Dicotyledons {Ricitius e.g.), the cotyledons themselves,
which develope subsequently into normal green leaves, accomplish this. In all such
cases, however; the seedling acts like a parasite, which by means of peculiar organs
absorbs its nourishment from a vegetable body (the endosperm) filled with reserve-
materials. The most conspicuous difference, in contrast to the haustoria of parasites,
consists in that the absorbing organs of seedlings are only slightly in contact with the
endosperm, and by no means organically connected with it ; whereas, as already
stated, the haustoria of parasites become so intimately connected with their host-
plants, that it is often difficult to decide where the boundary between the two lies.
Parasites have therefore in this respect an advantage over normal seedlings ; they
behave towards their host-plants rather like the buds of a germinating tuber or bulb.
Just as the assimilated nutritive substances pass over from the endosperm, or from the
tissue of a tuber or rhizome, into the seedling shoots, etc., the assimilated materials of
the host-plant also can pass over into the haustorium of the parasite ; and just as it
is from the seedling of a seed, or the sprout of a tuber, that the force for transferring
the reserve-materials into its tissues proceeds, so the parasite possesses the forces
for extracting and storing up in itself the assimilated materials from the tissues of the
host-plant. If this takes place in the case of seedlirigs by the excretion of ferments,
however, we can scarcely doubt that ferments are also formed in the haustoria
of parasites, and transferred into the tissues of the host-plant. In Cuscuta I have
had the opportunity of convincing myself that the starch stored up in the tissues

1 Cf. my treatise, ' Uchcr die Keimung des Samens von Allium Ce/ti,' Bot. Zcitg., 1S63 (p. 57).

374

LECTURE XXIII.

of the stem of a Linnm attacked by it disappears when a haustoiium penetrates
into the cortex ^

In the case of phanerogamic humus plants which contain Httle or no chlorophyll,
{Neoilia, Corallorrhiza, Epipogon, Monoiropa, Lathrcca, etc.), the relations referred to
are not so clear as in the case of the parasites. Although there can be no doubt
that these plants take up the whole of their organic substance, or at any rate the
greater part of it, from the humus remains of the soil surrounding them, special
haustoria are nevertheless not known at all in these plants. Even in Neotlia, the
relatively small length and number of the roots is noticeable, and in Corallorrhiza
they are entirely wanting. In these cases the root-hairs which spring from the roots
or subterranean shoot-axes appear to serve as haustoria, which are connected with

the still unrotted remains of fallen
leaves or other humous substances ;
and which excrete ferments to dis-
solve them and absorb the pro-
ducts of solution. The generally
very slow increase in mass and
vigour of such plants accords
with this incomplete arrangement.
With respect to the absorption
of organic nutritive materials, the
so-called ' Insectivorous Plants '
are better known than the true
parasites and humus-plants. The
striking adaptations which pro-
mote the absorption of organic
substance in them have for many
years astonished a large number of
observers and led them to active
investigation. It appears to me
that in the case of the insectivorous
plants we meet with a remarkable
case where nature has contrived
complicated mechanisms for the
attainment of an extremely unimportant effect ; for although it cannot be doubted
that the small quantities of proteinaceous substance which the insectivorous plants
absorb from animal bodies are useful for their welfare, the contrast between the com-
plicated adaptations to this end and the evidently very small amount of biological work
which they perform is nevertheless, on the other hand, very striking. Moreover, it is
certainly not doubtful that just the most pronounced insectivorous plants, as Nepenthes
and Dioitcea, can also thrive continuously without this occasional supply of organic
substance. With respect to the absorption of nutritive matters by true parasites

Fig. 245.— A'ziwn« «)«»/;<«;>. / ri
// seedling the cotyledons of which a
pare A and B ; s testa ; e endosperm
primary root ; ?</' its lateral roots ; x ar
to Euphorbiacese. The endosperm ii
proportion as the seedling-leaves wliii
absorbed entirely by them and the rei:

)e seed in longitudinal section ;

re still in the endosperm— com-
c cotyledon ; he hypocotyl ; 7U
appendage of the seed peculiar

icreases during germination in

h it envelopes : it is at length

^ I established exactly the same line of thought as that in the text, in my 'Handbuch der Expert-
viental-physiologie,'' 1865 (§ 55, p. 192). This statement, depending on careful investigations, appears
however to have been completely overlooked by niore recent writers who have been concerned with
the absorption of organic substance by plants, as Pfeffer, Darwin, Drude, and others.

INSECTIVOROUS PLANTS.

?,7ö

it is a matter of life or death, and yet matters are so simple in them ; in the
insectivorous plants it is simply a matter of more or less vigorous flourishing, since,
being possessed of chlorophyll and true roots, they are able to supply themselves with
nutriment, and nevertheless expensive and remarkable mechanisms exist in order to
add a small quantity of proteid substances. Only on knowing that the minute embryo
of a Date-stone or of a Cocoa-nut or of a Ricinus seed absorbs the endosperm, a
hundred or a thousand times larger than itself, is it clearly apparent how extremely
insignificant is the work done by insectivorous plants when they absorb the
contents of small gnats and
flies. Again, there is nothing
peculiar, as many observers
have believed, in the fact that
the insectivorous plants absorb
nutritive substances through
their leaves, since the seedlings
of all Conifers, Monocotyle-
dons, and those Dicotyledons
which are provided with en-
dosperm, do exactly the same.
The marvel of the insectivorous
plants thus lies by no means
in this point, but exclusively
in the fact that leaves which
contain chlorophyll, and are
moreover capable of assimila-
tion, are enabled, by means
of very peculiar reladons of
organisation and irritabilities,
to seize small animals, and
exhaust their proteid sub-
stance. In this again, how-
ever, the important point is
the seizing arrangement, and
not the fact of the absorption
of organic substance; since
numerous Fungi {SaprolegnicB,
SphisricE, etc.) nesde on and

in living animals, kill them, and convert the substance of their bodies into fungus-
substance.

Referring to the works mentioned in the notes ^ as regards the general biology,

FIG. =45 -Leaf of Diomr.j ;
anterior half of the lamina i
internal (upper) side with the
petiole (slightly enlarged).

!LiJ>u/a after the removal of the
the posterior half presents its
'ee sensitive hairs ; c the winged

* Charles Darwin, 'Insectivorous Plants^ (London, 1875). Oscar Drude, 'Die iiisccfeiifrcssen-
dcn Pflanzen," in Schenk's 'Handbuch der Bot.' 1881 (p. 113), where the further literature is intro-
duced. The first experimental researches on the digestion of animal substance were carried out,
according to Drude, by the American Kanby (1866) on Dioncra, and by Mrs. Treat (iS;i) on
Drose)-a. The first comprehensive description was given by Hooker in an address to the British
Association at Belfast (1874). Darwin's very thorough work, cited above, appeared in 1875.

376 LECTURE XXIII.

I will select as examples a few only of the nevertheless considerable number (fifteen
genera) so far proved to be insectivorous.

The ' Venus' Fly-trap' {Dmicea ??iuscipula) is a small member of the Droseraceae,
inhabiting the moors of North and South Carolina. The main shoot is a creeping
rhizome, from which only a few long and thin roots arise, and at the anterior end of
which five or six leaves form a rosette, from the centre of which a vertical flowering
stem shoots up subsequently. The leaves may attain a length of six to eight cm.
and have the form represented in Fig. 246; c c is the petiole, provided with two
green wings, on which the lamina {a b d), consisting of two sharply segmented halves,
is situated. In the figure, the half on the side towards the observer has been removed.
In the non-irritated condition the two halves of the leaf stand so inclined to one
another, that they form almost a right angle at the mid-rib {d b). The lamina
contains chlorophyll, and its venation is visible in the figure. Three long, fine,
easily overlooked bristles stand on the inner face of each half of the leaf; and any
ungentle touch of one of these bristles effects an instantaneous closure of the
two halves of the leaf when the plant is sufficiently irritable, which only occurs
at high temperatures and during the most vigorous vegetation. The two halves then
lie closed like the cover of a thin book. The movement is chiefly effected by the
tissues of the mid-rib and neighbouring parts, and is rapid as lightning in the cases
mentioned : in feeble plants, however, it takes place slowly. By means of this
mechanism small animals in green-houses, such as wood-lice, often a centimetre
long, are seized as they creep over the leaf; and the closure of the two halves of
the lamina is so firm that even so strong an animal does not succeed in breaking out.
Immediately after this closure a second mechanism co-operates in addition— the very
stiff outgrowths {d d) at the margins of the leaf. At the instant of closure, these
organs interlock somewhat as when the fingers of both hands are pushed between
one another, and in this way form a firm closure at the upper convex border
of the coincident halves of the leaf. After the closure described the sensitiveness
slill goes on acting, and the two halves of the leaf now adapt themselves
pliantly around the form of the imprisoned animal, so that its outlines are seen from
the exterior. If the bristles have been irritated with some solid body, or if a
non-digestible mass has been placed between the halves of the leaf, the closure follows
in this case also ; but after some time the two halves of the leaves again diverge from
one another. If, however, a thin lamella of hard-boiled prote'id, or a small piece of
muscle or similar substance is inserted, or if a suitable animal has been seized by
the leaf, the closure becomes more and more complete and tight, and the third and most
remarkable mechanism now comes into activity. Hundreds of glandular hairs bearing
peltate capitula on very short stalks, and which in the quiescent state were quite
dry like the rest of the leaf-surface, now begin to excrete a fluid in relatively very
large quantities : I have occasionally seen it exude in dense drops at the anterior
fissure between the closed halves of the leaf. This vigorous secretion is evidently
a consequence of the stimulus which the animal substance exerts on contact with the
glands. The secretion contains an acid, and, in addition, a peptonising ferment.
The whole process thus reminds one most vividly of the processes in an animal
stomach after a supply of nutritive substance. The ferment, like other peptonising
ferments, may be dissolved out by glycerine, and then acts in a test tube in the

DIGESTIVE FLUIDS OF INSECTIVOROUS PLANTS. 377

same manher, converting bodies of tiie liind mentioned into peptones. Tliis digesting
secretion permeates then the body of the imprisoned animal and completely dissolves
it, so that at last, since the leaf again completely absorbs the secretion together with
the products of solution into itself, only the extremely fine chitinous envelope of the
animal remains behind. This I found to be the case with several vigorous leaves
which spontaneously opened again in from four to six days after the seizure of large
wood-lice, and then continued living and healthy. The quantity of proteinaceous
substance which the leaves obtain in such cases is relatively very considerable, and
in my earlier researches I showed that just those plants which had occasionally
digested two or three wood-lice developed very vigorously, while the majority which
were prevented from seizing animals remained shiall and did not flower. If, on the
other hand, pieces of coagulated proteid scarcely as large as a wood-louse are
allowed to be digested by the leaves, the latter effect the process, it is true, but they
subsequently become discoloured and perish : only very small pieces of coagulated
proteid are digested without injury. The culture of these plants for years has
convinced me that they are able to develope into quite healthy, though small
specimens, without animal food, but that the digestion of animals invigorates them
considerably. Evidently it is here a matter of an addition of nitrogenous substance,
since the leaves are able to produce starch independently by means of their chlorophyll
contents, while the roots, few in number, evidently obtain but very little nitrogenous
substance from the cushions o{ Sphagmwi in which they live; and the same appears to be
the case with all insectivorous plants. The digestion of small animals appears to be
not exactly an absolute necessity for their existence, but an aid to vigorous thriving.

Our native species of Drosera, especially D. rolundifolia, found everywhere in
Sphagnum bogSj and occasionally in other places also, and which may be very easily
cultivated in windows if placed with the substratum in a flower-pot and watered with
pure water, belong to the same family as Dionea, but exhibit quite diff'erent adapta-
tions for the seizure of animals. The plant is small, and consists of a rosette of
8-10 leaves, from the midst of which a copiously flowering stem at length shoots up,
while only very few roots penetrate into the moss or turf. The leaves support, on
a petiole 2-5 cm. long, an almost circular lamina containing chlorophyll, the whole
area of which but rarely attains i sq. cm. At the margin of the lamina, as well as
on the whole of its upper surface, numerous so-called tentacles are situated ; these
are stalk-like outgrowths, each of which bears at its apex a gland of complicated
structure. The tentacles at the margin are the longest, those in the middle of the
lamina the shortest. In the non-irritated condition the tentacles are extended straight
outwards, and each glandular head is enveloped with a slimy secretion, forming a bril-
liant drop. If a small gnat, or fly, or other minute insect now comes in contact with the
slime of the tentacles, it remains hanging to it, and finally, after ineffectual struggles
to free itself, perishes. Here again the resulting phenomena can be called forth by
laying on the lamina a very small piece of hard proteid, flesh, or bread, some 1-2 mgr.
in weight; and here also the stimulus produces two eflfects — movements and chemi-
cal action. The movement consists in that the stalks of the tentacles in the course
of a few hours so curve at their basal portions that the glandular heads come to lie
exactly over the body of the insect ; whether the insect lies at the margin or in the
middle of the lamina of the leaf, the tentacles always curve in such a manner that

378 LECTURE XXIII.

they all become applied to the prey with their heads, and completely envelope it in the
fluid secretion. After some time (10-20 hours) the lamina itself also becomes curved,
and all the movements of the leaf called forth by the stimulus may be approxi-
mately imitated by laying a small body on the palm of the hand while the five fingers
are outspread, and then curving the fingers in such a way that they all touch the
body with their tips, while the palm itself becomes at the same time curved round
the body. The chemical action in the case of Drosera consists in that the secretion
of the glands (which is already present and contains a peptonising ferment, but
was hitherto neutral in reaction) now becomes acid in consequence of the stimulus.
The digestive fluid is thus rendered complete, since it, like pepsin, only acts as
a peptonising agent in an acid liquid. Rees and Will ' were the first to extract the
peptonising ferment out of the leaves o^ Drosera by means of glycerine, and to establish
its digestive action on blood-fibrin — a fact which is the more deserving of mention,
since this was the first demonstration of the existence of a peptonising ferment in
plants. After complete digestion and absorption of the products of solution, the
tentacles of ihQ, Drosera leaf again return to their original position, and the lamina
becomes extended and flat. On the leaves of Drosera in the open the empty remains
of numerous small insects are to be found at any time. Nevertheless the plants
may easily be cultivated in a room under a bell-glass until the ripening of the
seeds, without their catching insects : this, however, does not prove that the latter does
not act favourably for nutrition, and the researches of Francis Darwin and Rees
make it at least very probable that Drosera plants fed artificially flourish more
vigorously than those not so fed.

The course of affairs in our native species of Pinguicula (Utriculariae), which
inhabit moist shady spots, is unusually simple. Here also there is a rosette formed
of a few tongue-shaped leaves, from the midst of which the flowering stalk subse-
quently arises. These plants also may be easily cultivated and observed in pots. The
surface of the leaves, which contain abundance of chlorophj'll, presents a velvety
appearance, owing to very numerous epidermal glands, which possess approximately
the form of a mushroom, some with long and others with short stalks. The capitula of
the glands are moistened by the secretion which they exude, and hold fast any very
small insect, or bread-crumb, or piece of proteid laid on. After one or a few hours
the small body is noticed to be enveloped in a drop of fluid of considerable size, and
with an acid reaction; at the same time the margin of the leaf begins to curve
upwards, and in the course of several hours the digestible body is completely covered
by the leaf margin arched in over it. After a few days, when the products of diges-
tion have been completely absorbed, the leaf again opens out flat.

Finally, we may shortly describe another of the most complex and remarkable
mechanisms for the purpose of digesting insects. The so-called ' Pitcher-plants ' of
the genus Nepenthes, which are extended over Madagascar, Ceylon, and elsewhere in
South-Eastern Asia, are thin-stemmed climbing plants, provided with large simple
leaves abounding in chlorophyll. From the apex of a leaf, a filiform tendril slowly
developes, which, like other tendrils, is able to wind round the branches of neighbouring

1 Rees and Will, 'Einige Bemerkungen über fleischfressende Pflanzen' (Bot. Zeitg. 1S75,
p. 713)-

PITCHER-PLANTS.

379

plants, and so to serve as a climbing organ. At the end of this tendril a peculiar
appendage is noticeable very early; this developes later into the pitcher, so remark-
able in every respect, and the common form of which is shown in Fig. 247. In the
majority of species the pitcher is only 4-10 cm. high, and 1-3 cm. broad; but there
are species with pitchers 30 cm. high and
of corresponding breadth. The pitcher is
provided with a lid, which, however, in
the developed condition never closes the
opening. The whole organisation of this
wonderful organ is calculated to allure
small animals, especially ants, &c., in such
a manner that they are guided by two
wings running outside the pitcher to the
margin dd, where an exudation of honey
invites them further. Immediately be-
neath this margin, and on the interior,
the pitcher is bright and smooth, so that
even the foot of an insect finds no hold :
the animals fall down to the bottom of
the pitcher, and are there received into
a copious fluid secretion. This secretion,
which may easily be poured out of large
pitchers in quantities amounting to several
or many cubic centimetres, is excreted by
some thousands of small epidermal glands
(at 5 in the figure). In the non-irritated con-
dition the liquid has a neutral reaction, but
even then contains a peptonising ferment,
as Gorup-Besanez and Vines have demon-
strated, since on adding an acid the se-
cretion is able to digest fibrin ^ Just as
in Drosera, the acid necessary for diges-
tion is only excreted after an insect has

fallen into the abundant secretion. The great difficulties attending the culture of
Nepenthes in our hot-houses may depend chiefly upon the fact that this addition
of proteid substance in nutrition is mostly deficient.

FIG. 247.— I eaf tendril a of hepeiithes Icevis with tlic
pitcher bisected longitud inlly (the interior of the posterior
lialf of the pitclier is shown) , * basal region with digestive
glands ; c upper smooth portion ; d margin of pitcher ; e lid
(nat. size).

' S. II. Vines, 'On the Digestive Ferment of Nepenthes' (Linn. Soc. vol. xv. p. 427). E. v.
Goriip and II. Will, ' Fortgesetzte Beobachtungen über peptonbildende Fermente im Pflanzenreiche''
(.^ Mitth.) in Berichten der deutsch, ehem. Gesellschaft (Berlin, 1S76). I mixed the secretion of
two pitchers oi Nepenthes Scdeni (amounting to about 12 c.c.) with three drops of hydrochloric acid
and, later, an equal volume of water. This fluid gradually digested enormous quantities of swollen
blood-fibrin, which I estimated at about 8 c.cm.

LECTURE XXIV.

LECTURE XXill CONTINUED. NUTRITION OF FUNGI. LICHENS.

Those parasites and saprophytes devoid of chlorophyll which have been con-
sidered so far, all belong to the highly organised subdivisions of the vegetable king-
dom ; in spite of their not inconsiderable numbers in species, they are, nevertheless,
in contrast to the enormous mass of normal green plants, to be regarded as rarities,
which only come before the eyes of the non-botanical observer exceptionally, and
play a very subordinate part in the economy of Nature ; they belong, however, to
the most various families of flowering plants, and in each small group of these
colourless plants we meet with pecuHar biological deviations from other nearly allied
but typical forms.

Matters are quite otherwise in the case of the Fungi. We here meet with
a class of plants enormously rich in species, all the members of which are devoid of
chlorophyll, and are thus adapted to nourish themselves by parasitism, or at least
by making use of organic remains. The Fungi ^, in their sharply-expressed individu-
ality, so to speak, form an organic kingdom by themselves, to such an extent indeed
that up to the time of Linnseus, and indeed even in the present century, doubts have
been entertained as to their vegetable nature.

The variety of the forms of Fungi is simply enormous. From the simplest Bac-
teria and their allies, distinctly visible only with the strongest powers of the micro-
scope, finely graduated transitions occur to the highly organised forms with massive
bodies, often more than a kilogram in weight and with well-marked differentia-
tion of tissues, the latter being offered us particularly by the Gasteromyces in ex-
tremely remarkable forms. Nevertheless, apart from a few of the very simplest
genera, the body of the Fungus is always constituted of the hyphas already described
— thin colourless threads, usually segmented by transverse septa ; these either live
singly for themselves, as in the Mould-fungi, or are united in large numbers and

* Since I have been compelled several times in the text to refer to mycological facts which may
not be known to the reader, I here cite the most important works for consultation, i. De Bary,
'Morphologie und Physiologic der Pilze, Flechten und I\Iyxomycet£n' (Leipzig, 1864). 2. De Bary
and Woronin, 'Beiträge zur Morphologie und Physiologie der Pilze' R. I-IV. 3. Luerssen, ' Medi-
cinisch-pharinaccutische Botanik oder Handbuch der system. Bot.' B. I (Leipzig, 1874). 4. Goebel's
new edition of the systematic portion of my ' Lehrh. der Bot' (Leipzig, 18S2). 5. Frank, 'Die
Krankheiten der Pßanzcn' (Breslau, iSSoj.

NATURE OF FUNGI. 38 I

form a massive body, as every one has seen in the case of the well-known Mushroom.
But even in these large Fungi the mycelium, the root-like vegetative and feeding
organ, consists very commonly of single branched hyphaj creeping in the substratum.
The non-botanical observer usually sees only the massive fructification of the fungus,
which protrudes above the substratum, because the mycelium remains hidden in the
substratum, or at any rate only grows out from this into the moist air under certain
circumstances, and is moreover only clearly visible with the microscope. So far as the
question of nutrition is concerned, however, it is just the knowledge of the mycelium
and its mode of Hfe which is of special importance ; and what is to be said here with
respect to the Fungi, refers essentially only to the mycelium.

Many Fungi are satisfied with dissolving and absorbing from their substratum
only so much substance as is necessary for their nutrition, either as parasites or
saprophytes. Hence they usually act destructively on their substratum only to a
slight extent ; and even when they inhabit living plants or animals, the injury caused
by them is insignificant — e. g. the majority of Rust-fungi (^cidiomycetes). Others
grow and nourish themselves harmlessly in the tissues of higher plants, finally, how-
ever, when themselves producing fructification, to effect malignant destruction; as
Phyiophlhora infesians, which causes the Potato disease, and the Smut of Wheat, which
at last completely fills up the interior of the grains with its masses of black spores.
To the worst enemies of the higher plants, however, belong the tree-killing Fungi,
the mycelia of which nestle in the wood even of older huge trees, and not only take
from it the nourishment iiecessary for their nutrition, but decompose the masses of
wood until they are converted into soft spongy or powdery humous substances.
Even wood which has already been employed for various kinds of building,
&c., is destroyed by Fungi. Alertdius lachrymans destroys the timber-work of
houses, converting it into a mass of dirt, &c. Even living animals are attacked
and killed by Fungi. Our house-flies are destroyed annually in autumn, with the
exception of a few specimens which live through the winter, by a fungus [Empusa
Muscce) which infests them ; and innumerable caterpillars are killed by SphccricB.
In these cases the whole living substance of the body of the animal is finally con-
verted into a mass of Fungus ; the aquatic Saproleg7iicE in this way consume dead
insects which have fallen into the water, until only the hard chitinous covering
is left.

Nevertheless, these are all only individual phenomena, in contrast to the universal
distribution of other Fungi wherever organic life in general is possible. One may
well say that if there were no Fungi, the entire surface of the earth would be covered
with dense layers of the bodies of plants and animals which had accumulated for
thousands of years ; since it is essentially the Fungi, and particularly the minute
forms of very simple structure, which have year by year in the course of geological
time decomposed the dead plants and animals and again resolved them into carbon
dioxide, water, and ammonia. Only where perished organisms have been protected
from the access of the air, which Fungi require, by being enveloped on all sides by
mud, or sunk in turf, etc., have their remains been preserved, or at least a great part of
their carbon, which could not be oxidised during intramolecular spontaneous de-
composition by the small quantity of oxygen contained in the organic compounds
themselves. This has happened in the formation of coal, and is still taking place in the

382 LECTURE XXIV.

formation of lignite and turf. This enormous labour, however, is chiefly performed
by universally distributed Fungi, which not only absorb the materials necessary for
their nutrition, which would result in effects only extremely insignificant, but destroy
in addition the substances which they do not take up. We find this peculiar
destructive action taking place to the greatest extent by means . of the Fungi of fer-
mentation and putrefaction in the narrower sense. That others also eff"ect similar
work has already been mentioned above, in connection with the Fungi which destroy
wood ; and the mouldering and final disappearance of the masses of fallen autumn
leaves in woods must be ascribed to similar destructive actions of Fungi, the quantity
of Fungus produced itself remaining extremely insignificant.

So far as the Fungi simply derive nutriment from their substratum, their nutrition
resembles in the main that of phanerogamic parasites and saprophytes; what
particularly distinguishes very numerous Fungi, however, is the decomposition of the
organic substratum as already mentioned, the products of the decomposition not being
taken up by the Fungus. Only in this way is it possible for Fungi to prevent the
continual accumulation of organic substances on the surface of the earth ; since if
they were to make use of their organic substratum only for the purposes of their
own nutrition, there would simply result an enormous accumulation of these plants.

If we now attempt to obtain a more exact insight on the one hand into the
nutrition of the Fungi, and, on the other, into the destruction of their substratum
effected in addition by many of them — and thus into the nature of fermentation and
putrefaction — we shall find that the shortest way here, as in the case of the nutrition
of normal plants, is at once to make clear the results derived from the artificial
nutrition of Fungi, and the experimental study of the phenomena of fermentation.

The first experimental investigations into the nutrition of Fungi were under-
taken by Pasteur about thirty years ago. I here pass over the methods of
cultivating Fungi, especially the highly organised forms, in decoctions of fruit and
dung, and in various mixtures of gelatine and sugar, etc., which have meanwhile been
much further developed, because in these methods of culture, highly important
as they are for the biology of the Fungi, it has only been sought to bring various
kinds of Fungi to normal development, in order to make clear their growth
and reproduction and the configuration of their relations of organisation ; the
proper — i. e. the chemical — questions of nutrition not coming further into consider-
ation. Naegeli^ (with the co-operation of Dr. Lcew) has, however, recently made
some very detailed investigations and theoretical considerations on the matter.
Hence we shall obtain the desired glimpse into the processes of nutrition and
ferment action of the Fungi most quickly, if I shortly present the most important
results of Naegeli's work in this province.

The Fungi, like other plants, require in addition to those compounds which
afford them the elements of the proteid substances, fats and carbo-hydrates — viz.,
Carbon, Hydrogen, Oxygen, Nitrogen and Sulphur — also certain mineral matters,
the presence of which is necessary in the chemistry of nutrition and for the mole-

1 The works of Naegeli (and Loew) referred to are to be found in the Sitzungsber. der kgl.
bayr. Akad. under the title ' Ernährung der niederen Pike' (1879), ^"^ ' Über die Fettbildztng der
Gliederen Pilze'' (1878).

NUTRITION OF FUNGI. .^83

cular structure of their organised parts. But the Fungi, according to Naegeli, make
smaller demands than plants containing chlorophyll with respect to their choice of
mineral matters. They can exist provided three elements — viz., phosphorus, one
element of the series potassium, rubidium, and caesium, and, finally, one of the series
calcium, magnesium, barium or strontium — are supplied to them. Since, now, the
other elements here mentioned — viz., rubidium, ccesium, barium and strontium —
certainly do not occur at all places where Fungi grow, we may say they require phos-
phorus, potassium, and, in addition, calcium or magnesium. The difference between
them and normal plants lies in the latter needing calcium and magnesium at the
same time. Iron appears to be necessary only for the formation of the chlorophyll of
normal green plants, and is thus wanting in the Fungi ; and with respect to chlorine
and silica, which Naegeli reckons among the requirements for the nutrition of the
higher plants, the necessary facts have been already stated in Lecture XIV. The
essential difference between Fungi and the normal green plants, so far as the mineral
matters are concerned, would thus lie, according to Naegeli's researches, only in that
the Fungi do not need to take up magnesium and calcium at the same time, but
only require one of the two ; and that instead of this they can also make use of
barium and strontium, and, instead of potassium, of rubidium and caesium. These
are Naegeli's results concerning the elements necessary for the nutrition of Fungi.

Since the Fungi are not able to decompose carbon dioxide and thus to produce
carbonaceous organic substance, because they lack chlorophyll, a carbonaceous
organic compound must be presented in the artificial nutrition of Fungi, in addition
to the compounds of the elements named above. According to all experience
hitherto, it appears that organic carbon compounds of the most various kinds can be
taken up by the Fungi, and then be employed for the formation of cellulose and pro-
toplasm by means of internal metabolism. Of special interest, however, for scientific
insight into the matter is the knowledge of those nutritive substances which are distin-
guished by their pecuHarly simple composition, and the other chemical properties of
which are so exactly known that definite conclusions may be drawn from them
respecting the chemical processes probably occurring in the nutrition of the Fungus.
In this sense Pasteur employed, so long ago as 1858, ammonium tartrate for the
artificial nourishment of the Mould-fungi, Yeast and Bacteria. For the nourishment of
Yeast-fungi, Naegeli distinguishes as well chosen a nutritive solution employed by
Adolf Mayer in 1869. I here subjoin the composition of this, in order to give the
reader a clear idea; but it is not therefore to be supposed that every nutritive
solution in which Yeast is to grow must have exactly this composition — it is simply
an example for illustration. The solution in question was as follows : —

Water 100 Cu. cm.

Sugar 15 grams.

Ammonium Nitrate i

Acid Potassium Phosphate .... o'5 „
Tribasic Calcium Phosphate .... 0-05
Magnesium Sulphate o"2 5

If then this nutritive mixture is a suitable one, a considerable multiplication of Yeast-
cells must be obtained in a short time, if a few of them have previou.s]y been placed

3S4 LECTURE XXIV.

in the fluid by means of a needle point, etc. ; exactly in the same way as we expect,
in the artificial nourishment of a plant containing chlorophyll, that a plant of normal
size with numerous ripe seeds will develope from a seed. This the solution men-
tioned accomplishes ; though there still remain several things to be considered.
Naegeli says, that, theoretically considered, from a quantity of Yeast which cannot be
weio-hed 3-4 grams of new Yeast should be produced in the fluid described, whereas
in reality only i gram is produced. Nevertheless the result must not be under-
estimated, since a single Yeast-cell weighs only about äööö-TjTTiJ*^ of a milligram,
and thus in our solution there would arise from one or a few such cells some 2000
millions of new cells, and a corresponding increment of protoplasm and cellulose also
takes place. However, as already explained, I only introduce these numbers in order
to give a general idea of the matter to those unacquainted with such things. As a
matter of fact. Yeast-fungi, Bacteria and various Mould-fungi can flourish in very
different nutritive mixtures, if only a suitable carbon compound is present in addition
to the mineral matters mentioned.

The most suitable carbonaceous food is always found by these Fungi in sugar
(glucose), and the best nitrogenous food in proteids and peptones. According to
Naegeli the nitrogen may be taken up simultaneously with the carbon also from
Acetamide, Methylamine, jEthylamine, Prophylamine, Asparagin and Leucin ; whereas
Oxaraide and Urea can supply merely the nitrogen, but not the carbon. As sources
of nitrogen the Fungi can also make use of all ammonia-salts, and the Mould-fungi
and Bacteria even of nitrates. Free nitrogen, on the contrary, cannot be used for
the formation of protoplasm by the Fungi : exactly as is the case with the green
plants. In the same way Cyanogen compounds are unfitted for this purpose.
According to Naegeli, almost all carbon compounds are suitable for nutrition as
carbonaceous material, if they are soluble in water and not too poisonous ; but
Cyanogen, Urea, Formic and Oxalic acids, and Oxamide are unsuitable for this
purpose. The best nutritive material in all cases is aflTorded by a mixture of peptone
and sugar.

With regard to the production of plastic matters from the nutritive materials
taken up in the interior of the Fungus-cells, Naegeli has studied chiefly the
formation of fat (starch is as a rule not produced in the Fungi). There is not the
slightest doubt, after what I have stated previously, that fat is formed in ripening
seeds from carbo-hydrates, particularly starch, since this transformation takes place
in the nearly ripe seed even when taken out of the fruit, where no other material
is available under the circumstances for the formation of fat. It results now, from
Naegeli's investigation, that in the lower Fungi also fat is formed from carbo-hydrates,
but also from albuminates and other nitrogenous carbon compounds. If Fungus-
cells are placed in pure water, fat is formed in them at the expense of their proteid
substance, since the protoplasm diminishes in quantity. This is very easily demon-
strated in the case of the Mould-fungi and Yeast. That albuminates and other
nitrogenous carbon compounds can yield material for the formation of fat may
be proved, according to Naegeli, when these substances (with the addition of the
necessary mineral matters) are exclusively employed for the nutrition. The Bacteria
(Schizomycetes) flourish in a solution of proteids, or still better of peptones formed
from proteids ; and the Mould-fungi grow in it when the solution contains a little

FORMATION OF FAT IN FUNGI. 385

phosphoric acid. Now if merely a trace of spores or of P'ungus is sown in the
solution, an increase of the Fungus, and thus of fat, to the extent of more than
a millionfold is obtained. The proteid may be replaced by Asparagin or Leucin
with a hke result. Moreover, sugar and ammonia or ammonic tartrate, with the
addition of ash- constituents, suffice for nutrition. Instead of sugar, mannite
or glycerine, and instead of tartaric acid, acetic or salicylic acid or some other
organic acid may be used. In most cases, again, ammonia may be replaced by
nitric acid as the source of nitrogen. ' If, instead of ammonia or nitric acid,
albumin or peptones are employed as nutriment, the production of fats and
cellulose from sugar or tartaric acid etc. is demonstrated, when only a little of the
former, but a large quantity of the non-nitrogenous compound is present in
the solution. The analysis of the crop in this case shows that only the albuminate
can be derived from the proteid of the nutritive solution, and that the whole, or at
least a great part of the fat and cellulose must be derived from the constituents of the
sugar or tartaric acid. The facts alleged undoubtedly prove that the Fungus cells
can take the material for the formation of fats from the most various nitrogenous
and non-nitrogenous compounds.' Moreover, the theoretical chemical explanation ot
these processes is rendered more difficult by the fact that the chemical constitution
of the nutritive solution appears to be, as Naegeli says, almost without significance
for the formation of fat in the Fungi.

Concerning the relation of the formation of fat to respiration, Naegeli remarks :
' The Mould-fungi only grow when they have access of free oxygen, and abound in fat :
Beer-yeast requires very little oxygen for its development and is deficient in fat, and
the same is true of the Schizomycetes {Bacterid). The Mould-fungi living on the
surface of the nutritive fluid contain more fat than their own submerged budding
forms. Free access of air is necessary for the formation of spores which contain
much fat. The Saccharoniycetes (Yeast-fungi), as is well known, only develope
spores when they are spread out on a substratum and lie in a half dry condition : the
Schizomycetes likewise, as it seems, never produce their spores in the liquid, but
only in the covering at the surface. Mould-fungi living in liquids only form resting
> spores abounding in fat on the hyphse which rise up into the air. Why the Fungi
thus require oxygen for the production of fat, however, remains in the meantime an
open question.'

These details as to the formation of fat in the Fungi, which may probably be
'. extended in essential points to more highly developed forms also, are the more
interesting since the Fungi collectively and individually produce no starch, and even
form true sugars only in small quantity or not at all, though mannite, on the other
hand, often occurs in them. Even the cellulose of the Fungi is in some respects
different from that of other plants, not giving the usual blue coloration with iodine
and sulphuric acid.

It has already been remarked that Yeast and the Bacteria, besides many other
Fungi, not only use up the surrounding substratum to extract from it the constitueivts
of their own nourishment, but that they cause fermentation and putrefaction at the
same time, i.e. decompose the complex molecules of the carbo-hydrates and proteid
substances, and this in far greater quantity than is in any way necessary for the
purposes of their own nourishment. This ferment-action is peculiarly energetic in

[3]

386 LECTURE XXIV.

the case of Yeast, which decomposes sugar into alcohol and carbonic acid, glycerine,
and succinic acid. Naegeli found ^ that i gram of bottom-Yeast (dry weight) in
a 10 per cent, solution of cane-sugar containing tartrate of ammonia, as food
material, and through which air was continuously passed, fermented about 70 gr.
of sugar in twenty-four hours at 30° C, the weight of the Yeast itself increasing to
more than 2-5 gr. during the twenty-four hours. On the average, therefore, 1-7 gr.
of Yeast were effective during twenty-four hours, and decomposed forty times that
weight of cane-sugar. For further criticism of this process, into which however we
cannot here enter in detail, the additional remark may be made that the
volume of a cell of Beer-yeast amounts to something like 10,0^,000*^^^ of ^ cubic
millimetre, which corresponds to a weight of about ,0,000,000^^^ of a milligram.
Naegeli calculates further that in the fermentation of i kilogr. of cane-sugar (or,
after its inversion, of 1-0526 kilogr. of grape-sugar), whereby 0-51 kilogr. of alcohol
are produced, 146-6 thermal units of heat appear. The temperature of a fer-
menting sugar solution may, if no loss of heat occurs, be raised more than i4^C.

The greater part of the sugar is broken up in fermentation into alcohol and
carbon dioxide. Pasteur showed, however, that about 5 per cent, of the sugar
breaks up into succinic acid, glycerine, and carbon dioxide. Although, according to
Naegeli, the ferment-action of Yeast is more energetic on the access of oxygen, it
nevertheless takes place when oxygen is completely excluded from the fermenting
liquid. In the arts this is in fact the common experience : the vats containing the
wine-must are provided with special valves at the bung-hole, which allow of the exit
of the enormous quantities of carbon dioxide, but of no entrance of oxygen ; and in
experiments on a small scale it is also easy to convince ourselves that fermentation
proceeds energetically in a sugar solution utterly deprived of oxygen.

Of the species oi Mucor, the mycelium oi M. racemosus especially breaks up in
a saccarine liquid into spheroidal cells, which multiply by budding like Yeast, and
which, though to a much smaller extent, effect alcoholic fermentation. If a few
spores of Mucor are placed on a perfectly sound apple from a small part of which
the skin has been removed, the mycelium of this Fungus grows through the whole of
the parenchyma of the apple in a few days ; the latter is thereby converted into
a soft and subsequently deliquescent mass, the odour and taste of which demonstrate
the formation of alcohol and ethereal oils.

A peculiar form of fermentation is produced by the putrefactive Fungi (Schizomy-
cetes — Bacteria). These are much smaller than the cells of Yeast, and are spheroidal
or rod-shaped, and remarkable chiefly in that they also decompose saccharine solu-
tions, in the presence of proteid matters, in a manner which differs according to the
species of Fungus. They convert grape-sugar into lactic acid, and the lactic acid into
butyric acid, with the formation of carbon dioxide and hydrogen ; and if some grape-
sugar still remains over in this fermentation, it is converted into mannite. These
decompositions also may run their course without direct co-operation of the oxygen
of the atmosphere. In the formation of acetic acid from dilute alcohol, by means of

' Naegeli's comprehensive treatise (' Theorie der Gährimg^) appeared in Abhandl. der kgl.
bayr. Akad. d. Wiss., B. XII. Abth. 2 (München, 1879), ^^id is indispensable to any one interested
in Fermentation.

BACTERIA. 387

another Bacterium (' mother of vinegar '), the access of oxygen in abundance is
necessary.

Putrefaction in the narrower sense properly affects only the proteid substances
which are decomposed by colourless, yellow, red and blue Bacteria, INIoist bread,
meal, boiled white of t^g, etc. exposed to the air for some time show in the first
place colonies of Bacteria at certain spots, which become extended more and more,
and which, in the case of the pigment Bacteria especially, are very conspicuous. It
is only necessary to touch these spots with a needle point, and then to touch with
the needle an Qgg boiled hard and deprived of its shell, and lying in damp air, to observe
at the infected spot within a few days a colony of Bacteria extending itself with all the
effects of putrefaction. The profound decomposition which the proteid substances
undergo under the influence of the Bacteria is most easily recognised by the escape
of evil-smelling gases — ammonia, sulphuretted hydrogen, and ammonium sulphide.
Finally, the proteid substances are completely destroyed.

It is now scarcely doubtful that common Yeast, as well as Bacteria, are only
special vegetative forms of Fungi which consist of hyphae, just as the IMucor-yeast
is only a peculiar vegetative form of
Mucor racemosiis. Zopf has lately sho\\'n,
especially with respect to Bacteria, that
they arise by segmentation from exceed-
ing fine filamentous Fungi {Beggiatoa,
Cladothrix), and that these forms of Fungi
again proceed from them. The bacteria-
form of these Fungi reproduces itself,
under conditions favourable to nutri-
tion, by continually repeated transverse
divisions. Similar vegetative forms also
occur indeed in many Algae, and even
in the protonema of Mosses. The re- ,^:'"-. =48.-schi2omycetes. r sarar,^. 2. Bacuriu,,,.

tr ^ 3. Vtbrio. 4. Sfirillum (after Cohn).

markable and conspicuous facts in the

Yeast-fungi and Bacteria, however, lie chiefly in their power of decomposing such
exceedingly large masses of carbo-hydrates and proteid substances by means of fer-
mentation, while they themselves make use of only small quantities for their growth.

In the Yeast-fungi and Bacteria we have thus found the ferment-action accom-
panying nutrition proper, increased to a maximum. That a similar thing takes place
in other Fungi also, however, though to a smaller extent, is shown by the behaviour
of the common forms of Mucor, the mycelium of which, penetrating into sound
apples and completely permeating their parenchyma, is by no means satisfied with
extracting the material necessary for its nutrition : on the contrary, the substance of
the apple is decomposed into a soft mass smelling of alcohol and ethereal oils.
Apart however from what has been stated with respect to the ferment-fungi, a closer
insight into the chemical changes efi"ected by Fungi on their substrata is so far
wanting. The best we possess in this connection are Robert Hartig's statements as
to the destruction of wood by tree-killing Fungi. Before I quote his statements on
this subject, however, it will be well to make a few preliminary remarks with regard
to these Fungi.

c c 2

388 LECTURE XXIV.

It has been established by the memorable labours of Robert Hartig ' that the
stalked Fungi growing out from diseased tree-trunks, such as Agaricus melleus and very
many species the fructification of which tends to grow out from tree-trunks in the
form of a sessile fan-shaped structure, such as Tramefes pitii, Polypoms fuhus, P.
vaporarius, P. mollis, and P. borealis, all of which occur on Conifers, as well as Hydtmm
diversidens, Telephora perdrix, Polyporus sulphureus, P. ignartus, P. dryadeus, and
Siereum hirsutum, all of which infest Oaks, and others, are by no means the harm-
less parasites they were formerly considered to be ; he shows, on the contrary, that
the delicate filamentous mycelium of these Fungi penetrates into the roots (Trametts
radiciperda and Agaricus melleus — the latter creeps in the soil and constitutes the
subterranean ' Rhizomorpha '), or often directly into the stems and branches, usually
through wounds previously produced, and vegetates for years in the wood and destroys
it. In consequence of this, even large trees may be killed by the mycelium dwelling in
them, or at least their wood be made useless. From the mycelium vegetating in the
interior of the wood, in all the species named, the fructifications already referred to
at length appear :: these are ofien very large, and in many cases themselves go on
growing further for years (e. g. Polyporus ignarius).

Referring to Hartig's works ^ for the mode of life of these plants, I will here,
where we are concerned with questions of nutritiwi, subjoin only some of his
results with respect to the changes which the wood of Conifers undergoes by the
action of the mycelium. The changes of the cell-walls of the Avood, says Hartig,
which are produced by the Fungus, are of great interest. In the first place, it is
an important fact that the hyphae of the Fungus always vegetate in the interior of
the wood-cells, and only bore through the septa, whereby however it is not pre-
cluded that they occasionally grow forward also laterally or upwards or downwards
in the walls, and become branched in them. The mycelium of Agaricus melleus
lives in fact in the latter form, chiefly in the hard substance of the cell-walls of the
wood. The holes bored in the walls (which either have from the first the same
diameter as the fungus-filament or are often distinctly smaller, because the hyphae
inside the wall penetrated by them thin off) eventually become widened, as a rule
considerably, since the solution of the substance of the cell-wall appears to take place
more rapidly outwards, starting from the perforation. The activity of the fungus-
hyphse in the interior of the woody tissue varies. Sometimes they absorb directly
and unaltered the organic substances met with on the way, and produce fungus-
protoplasm and cellulose from them, with the separation of carbon dioxide : on
the other hand, they extract from the organic compounds at a greater distance
certain substances which they require for their nourishment, and thereby, according
to Hartig, cau-se a chemical alteration of the cell-contents or cell-walls. The fungus-
hyphae penetrating into the sound wood of the Oak absorb the tannin unaltered,
and it can be detected in them by means of salts of iron. The numerous
lateral hyphae, which bore through the walls like haustoria, entirely absorb the dis-
solved substance of the cell-walls of the wood, through their apices; they exert in
addition a very profound decomposing influence on the contents and walls of the

^ Robert Hartig, ' Wichtige Krankheiten der Waldbätime'' (Berlin, 1874), and 'Die Zersetz
iingserscheitningen des Hohes der Nadelbäume und der Eiche' (Berlin, 1878).

TREE-KILLING FUNGI. 389

cells far beyond their immediate neighbourhood, extracting from them certain
matters which serve for the nourishment of the fungus-hyphoe, whereby the most
extraordinary differences, characteristic for each species of plant, appear. In many
cases the wall of the wood-cells again assumes completely the character of cellulose :
it becomes colourless, flexible, capable of swelling, and is coloured a beautiful violet
by chlor-zinc iodine. In other cases, on the other hand, the wood cell-wall becomes
brown and very brittle, its substance largely soluble in ammonia, and yielding a
brown fluid W'ith solution of potash. This, in other words, evidently means that
some of these Fungi take up chiefly wood-substance (xylogen) from the cell-wall
and leave their cellulose behind, while others take up the cellulose itself, and leave
behind the wood-substance impregnating it. The former is the case for example
with Trametes pmi, and the latter with Polyporus mollis. Hartig is of opinion that
the greater part of the organic substance of the wood finally breaks up into
carbon dioxide and water, without being previously taken up into the hyphae. In
very much decomposed wood of Conifers the turpentine appears no longer fluid
but hardened, filling the cavities of the wood-cells in amorphous pieces. In the
wood of Pines much decomposed by Trametes radicipcrda, the tracheVdes, previously
permeated with turpentine, become filled with crystals, the solubility of w-hich in
turpentine admits of the conclusion that they are hydrate of terpin. Obviously
the mycelia of the tree-killing Fungi take up their ash-constituents also from the
wood, and these pass into the fruit bodies situated on the exterior. As in the case of
most other Fungi, crystals of calcium oxalate are here also excreted during nutrition,
both in the nourishing wood and in the organs which bear fructification.

If we now cast another glance backwards on what has been said concerning
the nutrition of Fungi, we find forms which are satisfied with taking up from their
living or dead substrata simply and only so much of what is needed for the construc-
tion of their bodies as is necessary; as well as those which, in addition to absorbing
their nourishment, produce copious decompositions in the substratum and destroy it.
This latter effect, which, as it seems, occurs in the most various degrees down to those
which are scarcely noticeable, we may comprehend in general — even in cases where
the decomposition of wood is concerned — under the extended idea of fermentation.
With respect to the true na'ture of fermentation, and especially with respect to its
essential difference from ferment-actions, I have already said what is necessary
(Lecture XXI). Here it may be still further insisted upon that neither action need
exclude the other. In the first place, we must assume that every Fungus exerts a
ferment-action on its substratum, simply in order to obtain its nutritive material from
it. When a fungus-filament grows through a hard cell-wall, we must assume that a
ferment exists at its surface, by means of which not only is cellulose dissolved, but
also lignine, and, imder certain circumstances, cuticular substance also. In the same
way the germinal hyphae of various parasitic Fungi bore through the coverings of the
bodies of insects, for which purpose (in the same way, and probably necessarily) a
ferment must exist at the surface of the germinal hypha, which in this case, where
the solution of proteid substances and perhaps even of chitin is concerned, may be
regarded as apeptonising ferment. In general it will suffice that the fungus- filament
which penetrates into a substratum contains an exceedingly small quantity of
ferment, since even in the seedlings and buds of Phanerogams only extremely small

39°

LECTURE XXIV.

quantities are produced. Moreover, the action of this ferment at any time need only
bs exercised close to the surface of the growing end of the fungus-filament, so that
at distances measurably removed no further decomposition of the substratum follows.
This at least explains how it is possible for the fungus-filaments to bore smooth
holes (which they completely fill up) through the cuticularised epidermis of living
plants, and through lignified cell-walls, and even through starch-grains. Evidently
this fermenting action of a fungus-filament is to be compared with those which I
have already described (p. 344, Fig. 236) in the germinating Date. As the delicate
soft haustorium of the first leaf of the seedling there grows into the hard mass of
endosperm, because it dissolves and absorbs the hard cellulose as well as the proteid
substances and fat of the endosperm close to its surface, by means of its ferment action,
so, too, we may suppose the fungus-filaments penetrate into their solid substratum,
which is insoluble in mere water.

That the ferment-fungi also exert ferment-actions on their substratum is shown
in the first place by the invertive action of Yeast, since it converts cane-sugar into
glucose, and we may certainly assume that in the nutrition proper of all Fungi which
have a destructive action on their substratum, ferment-actions first take place by
means of which a portion of the substratum is brought into a form capable of
nourishing the Fungus ; besides this ferment action, fermentation proper then comes
into play, by means of which the substratum (occasionally to the injury of the Fungus
itself) is destroyed. The peculiar behaviour of Peroiiospora {Phyfophihora) in/csians,
which nourishes itself for months in the tubers and green shoots of the Potato without
causing injury, and finally, as the fruit-bearing organs develope, rapidly extends and
kills and destroys the nourishing tissue, demonstrates that in this case the same
Fungus in different periods of life sometimes exerts ferment-actions and sometimes
promotes fermentation on its substratum. In contrast to the destructive action which
the Fungi of fermentation and putrefaction in the widest sense exert on their sub-
stratum, we find, however, a large group of Fungi abounding in species, which, on the
contrary, influence their living substratum favourably, even promoting their activity
in order the better to make use of them. This is the case with the Lichens ^ It

' With respect to the history of the discovery of the true nature of Lichens, De Bary expresses
himself as follows in his lecture, ' Über die Erscheinung der Symbiose' (Strasburg, i879\ p. 17: —
' From their (the gonidia) constant occurrence in every Lichen there has long been not the slightest
doubt that they are organs of these otherwise Fungus-like plants : their resemblance to Algoe was
also obvious, and the Lichens were therefore regarded as a group standing between Algae and Fungi,
These views obtained a particularly firm basis by Schwendener's thorough studies into the structure of
the Lichen-thallus, from which it seemed to result that the gonidiaarise as small branches or as the ends
of branches of the filaments devoid of chlorophyll. However, several matters still remained obscure.
Li particular, the first origin of the gonidia-bearing thallus from the typical reproductive organs, the
spores ; since when these were sown, and the sowing kept under rigorous control, there always arose on
germination only those temporary Fungus plantlets referred to in the text, and no gonidia-bearing
Lichen-thallus, and in rare cases where such were obtained on sowing it was not clear whence the
gonidia had come. On the ground of tiiese and similar considerations I first expressed in 1S66 for
certain Lichens the hypothesis (based on extensive unpublished investigations) that they might
possibly result from the union of a certain definite Fungus with an Alga. The extension of this
hypothesis to all Lichens was not admissible by the then existing researches, particularly Schwen-
dener's. Afterwards, when, by the works of Famintzin and Baranetzki especially, the probability
became more and more prominent that the so-called gonidia are identical with Algae which occur
independently, Schwendener was enabled by his later investigations to establish the theory summed

391

^9 3t

i, ^

will be necessary, however, in order to make clear lo the reader the true state of
affairs in connection with these remarkable plants, to refer very briefly to their
anatomical structure. It has long been known that the body of the Lichen consists of
two entirely different forms of tissue : of typical fungus tissue, which also produces
the organs of fructification, and of cells containing chlorophyll, mostly spheroidal or of
some other shape, which present an unmistakeable similarity to simpler forms of
Algae. In the year 1 864 De Bary first pointed out that these gonidia, in a certain group
of Lichens at least (the gelatinous Lichens), may be true Algee, which, interweaved by
the hyphoe of true Fungi, serve to nourish the latter. This idea, suggested by De
Bary, was generalised later by Schwendener; and by means of a scries of experi-
mental works by Baranetzky, Reess, and Stahl, complete certainty was attained that all
Lichens are, as a matter of fact, nothing other than Fungi belonging to the subdivision
of the Ascomycetes, which are accus-
tomed to weave themselves around
Algae, usually microscopically small,
and entirely to envelope them, so
that an organism compounded of a
Fungus and an Alga arises. Figure
250 shows, according to Bornet, a
few examples of the ways and means
by which the germinal filaments of
the corresponding Lichen-fungus lay
hold of and weave themselves around
Algae. The Fungi also germinate,
it is true, in the absence of the
Algae, but they then perish ; and a
Lichen-body only comes into ex-
istence when the germinating Fungi
are able sufficiently early to become
united with the Algae with which
they are adapted to form a Lichen,
since each Lichen-fungus, according
to its specific nature, is confined to
certain forms of Algae.

It has been demonstrated, particularly by Bornet and Stahl, that in consequence
of the union of Fungus and Alga the vigour of both is promoted. Without going
further into detail, it is only necessary to mention that the Algae imprisoned by the

Fig. ■2i<).—Seicta fuliginosa ; transverse section through the
aceous thallus. o upper, u lower epidermal layer : »i m tissue of matted
fungus hypliae ; >■>■ roots ; ss cells of the Alga dividing and. increasing
(highly magnified).

up in the text. He put aside all at once the matters referred to above as hitherto obscure and
doubtful. Moreover, direct proof was forthcoming by the method of synthesis : i. e. by the union
of definite autonomous Algce with suitable Fungi a designed Lichen-thallus was produced.
Reess and Stahl have now shown that this may be accomplished with comparative ease by observing
certain precautions.'

More details on the Lichens are lound in my 'Lehrbuch,' Aufl. IV, pp. 319-330. An interesting
general description also by Kccss, ' Ubcrdie Natur da- Flechten' (Berlin, IIabel\ The highly
interesting work of Stahl's referred to is 'Beiträge zur Entzviekeluugsgeschiehte der Flechten; Heft 2 ■
(Leipzig, 1877, Felix). Among the finest and most instructive works is that of Ed, Bornet,
' Reeherches sur les Gonidia des Lichens; Ann, des sc, nal. p«- scr. Tome XVH, 1" cahier.

39*

LECTURE XXIV.

fungus-tissue, in accordance with the growth of the whole, continually increase
in number by division, and, according to the nature of each Lichen, constitute with
the fungus-tissue a more or less homogeneous mixture (homoiomerous Lichens), or
form a layer within the tissue of the Fungus, beneath the surface of the latter
(heteromerous Lichens).

It is obvious that the green Algas in the body of the Lichen act as organs of
assimilation, exactly as the chlorophyll-cells in the cortex of a green stem or in a
leaf. Their products of assimilation serve as nutritive material for the Lichen-fungus;
while, on the other hand, the ash constituents necessary for assimilation are conveyed
to them by means of the Fungus. By means of this commensalism, however, the

Fig. 250.— Various Algse attacked by Lichen-fungi ; the Fungus hyphae are everywhere denoted by A, the Alga-cells
(gonidia) by ^. ^ germinating spore of Physica parietina, the germinal tube h having fixed itself upon Protococcus ■viridis.
B a filament of Scytonema invested by the hyphas of the Fungus Stereocaulon ramutosum. C from the tissue of the
Lichen Pkysyna ckalagannfn ; the end of a hypha is penetrating into a cell of the Nostoc. D from the tissue of the Lichen
Synnlysitx symphorea; the Alga ^ is a Glococapra. E from the tissue of the Lichen Cladonia /iircata, the host Alga ^
belonging to the genus Prolococcus (highly magnified— after Bornet).

Lichens are now independent of an organic substratum ; while all other Fungi are para-
sites or humus plants, Lichens are able to establish themselves on purely mineral sub-
strata, or even on the surface of crystalline rocks, since the enclosed Alga makes them
independent; and when Lichens exhibit a predilection for the bark of trees, this
certainly does not happen in order to extract their organic nutritive material from
the bark, but for other reasons. While other Fungi decompose organic substrata, we
find numerous Lichens capable of decomposing the inorganic substance of stones,
e. g. granite, in order, like the roots of the higher plants, to obtain those mineral
matters which their chlorophyll-cells (the Algse in their tissue) require for assimi-
lation. Inasmuch as these Fungi thus come into connection with certain Algse, in

COMMENSALISM.

393

order to be nourished by them, they obtain a freedom in the choice of their dwelling-
place which is enjoyed by no other Fungus.

There is however yet another remarkable consequence which results from
the commensalism of Fungus and Alga, in that the external form of the body
of the Lichen generally no longer remains that of the ordinary Fungus, but
behaves rather like that of non-parasitic plants which contain chlorophyll.
It is true there are many so-called crustaceous Lichens which grow closely
attached to the substratum. But where larger growths more separated from the
substratum are formed among the Lichens, the restrictive significance of the chloro-
phyll for the whole configuration of the vegetable world at once makes itself
prominent again. The thallus of the Lichen then becomes developed either in the
form of a flat leaf-like extended plate, as in the so-called foliaceous Lichens, or in

Fro. 251.—^. Usnea !<arba/,i. a fniticnse I.iclien (natural size),
(natural size) seen from below, a fructification ; / the disc by me:

the form of a much branched shrubby plant. In both cases the purpose is attained
of presenting the chlorophyll elements of the Lichen body to the light in thin laminae,
in order to accomplish the function of assimilation ; for, as was shown in the first
lectures, it is this principle which dominates the relations of configuration of all
shoots containing chlorophyll, and as among the higher plants there are massive and
succulent as well as graceful forms, so also among the Lichens succulent forms, the
so-called gelatinous Lichens, are found, the translucent bodies of which allow the
light to penetrate deeper, whereby a looser but more homogeneous distribution of
the cells containing chlorophyll is possible in them. Thus as we have repeatedly
found already in Phanerogams as well as Cryptogams, that in correlation with the
disappearance of the chlorophyll, on the one hand dependence upan organic substrata,
and, on the other, massive forms of body are produced ; so, conversely, we find here
in the Lichens that typical Fungi devoid of chlorophyll, in correlation with their

394

LECTURE XXIV.

commensalism with green Algae, assume forms otherwise proper only to typical chloro-
phyll-containing plants. It was certainly this totally different facies of the Lichens
as contrasted with other Fungi which chiefly contributed to make the new theory
of the nature of the Lichens so unacceptable to the older Lichenologists.

Another highly important lesson may be drawn from the nature of the Lichens
as now understood, namely, that the external form of a plant is not at all de-
pendent upon its histological nature. Evidently it does not lie in the nature of the
fungus-hyphse that the fruticose and foliaceous Lichens assume such characteristic
forms as shrub-like bodies and flat extended surfaces, and still less do the Algae con-
cerned themselves — i.e. when living independently— tend to the formation of bodies
which assume the forms represented in Fig. 251, These forms however may well
result from the union of the two, and this chiefly according to the principle that in
the case of a plant containing chlorophyll it is important to present the green cells
in a suitable manner to the light and air. Finally, again to mention the fact, the
commensalism of Fungus and Alga brings about that the former no longer becomes
differentiated into a mycelium and fruit-bearing organs as is usually the case. In other
Fungi the mycelium is endowed with the properties of true roots, and penetrates into
the substratum in order to take up nourishment : the necessity for this disappears
in the Lichens. The Algae contained in the tissue constitute the nourishing substratum
for the hyphoe, and they must be exposed to the light and air, and thus, instead of a
mycelium, a body is developed which is attached to some solid support by organs of
attachment only at one or a few places, somewhat as in the case of many large Algae.
These organs of attachment in the foliaceous Lichens may assume the most essential
properties of roots.

According to recent investigations, a commensalism similar to that between Alga
and Fungus in the Lichens appears to occur also between certain Algae and various
simply organised animals. It has long been known that some Rhizopods, Paramaecia,
Stentors, Vorticellas, the fresh-water Sponge {SpojigiUa), our small fresh-water Polyp
{Hydra), and various Rotifera ( Voriex) contain in the transparent substance of their
bodies green grains which have been supposed to be chlorophyll-corpuscles. Accord-
ing to a recent investigation of Brandt \ it appears now that these apparent chlorophyll-
corpuscles are small spheroidal algal cells provided with a nucleus, and further that
the animals concerned, when they contain such vegetable cells which are
capable of assimil.xtion, can dispense with the further absorption of food. These
animals are thus nourished like the Lichen-fungi by means of the Algae imprisoned
in them. If on the contrary they contain no Alg^, they are necessitated to feed as
animals in the ordinary manner — i. e. to take up nutritive substances through their
mouths.

* Brandt, ' Über das Zmaminoikbcn von Thicrcii Jind Algcn,' Verhandl. der physiol. Ges. zu
Berlin (Dec. 2, 1881).

LECTURE XXV.

THE RESPIRATION OF PLANTS. SPONTANEOUS EVOLUTION
OF HEAT. PHOSPHORESCENCE.

Plants, like animals, must be continually making exchanges with the atmosphere,
and be able to absorb its oxygen in order to maintain life. The chemical changes
and molecular movements of which the life of plants as well as that of animals
consists, are accomplished only so long as the free oxygen of the atmosphere is able
to enter into them. If the supply of this gas is cut off from them, the internal
movements which effect growth cease, and the streaming of the protoplasm in which
w-e find the most direct expression of life is brought to an end ; the periodic move-
ments of foliage leaves and the parts of the flowers cease, and the organs which
respond to stimuli lose their irritability. If, when the conditions of vegetation are
otherwise favourable, the supply of oxygen is interrupted for a short time only,
the plants still retain their vitality, and the internal and external movements
brought temporarily to a standstill may return as soon as the access of the oxygen is
again permitted. If, on the other hand, the interruption of the vital movements
through lack of oxygen continues for a long time, destructive processes take place in
the cells in consequence of the so-called intra-molecular respiration, to which I shall
return subsequently ; the capacity for living is destroyed sooner or later, and a too
late access of oxygen no longer recalls those peculiar movements which are termed
vital '.

' The most essential facts respecting the respiration of plants and its resemblance to that of
animals had already been clearly recognised by Ingenhouss and Theodore de Saussure before the
beginning of this century. The theory was further developed later by Dutrochet, Grischow, Meyen,
and others. But in consequence of a quite unwarranted dictum of Liebig's, which struck out the
respiration of plants from vegetable physiology, the matter simply passed into oblivion, at least
in Germany, from about 1840, and accordingly the universality of an evolution of heat in living plants
also was apparently put aside. An extreme confusion of ideas was at the same time, in spite of De
Saussure's clever work, brought about by the fact that, by a scarcely conceivable thoughtlessness and
obtuseness, people had accustomed themselves to speaking of a double respiration of plants — of a
so-called diurnal respiration, meaning assimilation, and a so-called nocturnal respiration, by which
was understood the evolution of carbon dioxide which occurs in true respiration. In spite of
Boussingault's splendid work, and Garreau's repeated and accurate putting of the difference between
assimilation and respiration, this confusion nevertheless persisted. By means of the very detailed
collection of the whole of the literature which had appeared up to 1865, and the putting forward of
the radical difference between assimilation and respiration which I accomplished in my 'Handbuch

396 LECTURE XXV.

These statements are now to be supported more in detail by adducing the most
important facts \

That the chemical processes and molecular movements which constitute the
growth of plants only take place when the atmospheric oxygen envelopes them and
is distributed throughout the organs, was first proved by the investigations of Theo-
dore de Saussure in 1804, with the caution and accuracy peculiar to this highly-
gifted experimenter. Dutrochet first showed, however, that air containing oxygen
diffused in the tissues of the periodically motile and irritable organs is a condition of
their motility. On the sensitive leaves of a Mimosa standing beneath the receiver
of the air-pump, exhaustion at first acted like a mechanical shock : in the vacuum,
however, they assume a permanent but rigid position. The periodic oscillations are
suppressed— they are not sensitive to shocks. The irritability and periodic move-
ments of the leaves return, however, when the plants are subsequently exposed to
the air again. In the same way, the periodically motile flowers of Leontodon taraxa-
cum and Sonchus oleraceus became fixed in Dutrochet's vacuum. According to later
researches by Kabsch, the stamens of Äfahom'a and Berberis, which are sensitive to
contact, are rendered rigid and cease to be irritable when the air under the air-pump
is much rarified : the same is the case with the stamens of Hcliayitheinum vulgare.
On renewed access of air — i.e. of its oxygen — the motility of these organs again
returns, Kabsch showed that this is due merely to the oxygen, by allowing the
organs mentioned to remain for some time in pure nitrogen. On again exposing
them to the atmosphere after 10-15 minutes, they regained their irritability; w^hile
they lost it for ever on remaining for a longer time in nitrogen. A stay in pure
hydrogen acted similarly. With respect to the indispensability of an atmosphere
containing oxygen for the maintenance of the streaming of the protoplasm in cells,
as well as in the naked protoplasm of the Myxomycetes, Kühne published detailed
observations so long ago as 1864. This phenomenon also, according to his
observations, disappears on the exclusion of atmospheric air, but returns, if it had
not continued too long, after a few minutes on the access of ordinary air.

I have given prominence to these facts, because they demonstrate directly and
Avithout any need of comment the importance of oxygen respiration. As animals
are suffocated by withdrawal of atmospheric oxygen, so with plants also : their
functions come to a standstill, and if the respiration is not restored at the right
time, the standstill in permanent, and death results. It is true, as w-e shall see
later, the respiration of plants is far less energetic than that of warm-blooded
animals, but probably it can be compared in all respects with that of cold-blooded
animals.

der Exp. Phys.,' the path to a correct understanding of the respiration of plants, lost for twenty-five
years, was first recovered. Numerous more special investigations have since appeared, without,
however, essentially altering the matter itself; only the so-called intra-molecular respiration, occurring
in plants just as in animals, can be regarded as an important addition to the facts long known.

' AVhat here follows in the text is essentially a short extract from the chapter 'Die Athmting
der Pßanzcn, IVärmchildting tind Phosphorescence in my 'Exp. Phys.' (1865, pp. 263-304). The
more recent literature is collected in Pfeffer's ' Pflaiizenphysiologie^ Of the newer works, Borodin,
' Sur la Respiration des Plantcs ' (Florence, 1875), and ' Untersuchtingen über die Pßanzenathtnung''
in Mem. de I'acad. imp. des sc. de St. Petersbourg, VIP Sdr. t. XXVII, No. 4, 1881, are to be
mentioned.

CARBON DIOXIDE FORMED DURING RESPIRATION. 397

Like the respiration of animals, that of plants also consists in chemical
exchanges between the oxygen taken up and the organic compounds of the living
body, so that finally carbon dioxide and water are produced at the expense of these
compounds. The formation of water at the expense of the organic substance can
only be demonstrated indirectly by chemical analysis, as we shall see : on the other
hand, it is one of the easiest experiments in the province of vegetable physiology to
demonstrate the exhalation of carbon dioxide. In general, the evolution of carbon
dioxide is the more energetic the more vigorous the vital activity generally of the
organ observed ; and since the latter increases as a rule as the temperature ascends
to a certain optimum (say 2 50-3 o« C), so also the respiration and formation of
carbon dioxide increase in a like manner. It is particularly in the processes connected
with energetic utilisation of material in the growth of seedlings, unfolding buds, and
especially in flowers, that the evolution of carbon dioxide takes place most vigorously
and can be observed most certainly. The observer only meets with difficulties in
determining the changes in the composition of the air surrounding the plant which
are produced by the respiratory process when he is concerned with organs abounding
in chlorophyll, which are at the same time exposed to the influence of the light,
because in this case carbon dioxide is taken up and oxygen evolved in the process of
assimilation, and thus an exchange of gases takes place which aff"ects the surrounding
air in a manner exactly opposite to that due to respiration. Nevertheless there is no
doubt, cither theoretically or with reference to Garreau's experimental results, that
even green organs occupied in assimilation continually respire in the ordinary
manner like all other living organs. That green leaves, when not assimilating —
e.g. in a feeble light, or in the dark— respire somewhat energetically is established
with as much ease as in the case of organs containing no chlorophyll.

If it is required to show that plants or entire organs convert a definite quantity
of the oxygen of the surrounding atmosphere into carbon dioxide, the simplest
experimental methods suff.ce. It is only necessary, for instance, to shut off with
wetted mercury an ordinary graduated absorption-tube, into which a Bean, Pea,
Acorn, etc. which is commencing to germinate has been placed, to convince
•oneself, by the absorption with potash of the carbon dioxide found in the tube after
a few days, of the fact that the whole of the oxygen has been converted into carbon
dioxide. It is at the same time observed in this simple experiment that the seedling
saturated with water, at first, so long as oxygen is still present in the absorption-
tube, goes on growing, but that after the oxygen is completely used up growth
ceases.

An optical demonstration of the formation of carbon dioxide by respiration may
be provided still more easily in the following manner. A glass cylinder of about 2-3
litres capacity and furnished with a well-fitting stopper is filled with about 400-500
germinating peas in layers alternating with damp filter paper ; or in the same way
several hundreds of unfolding flower-buds of Camellia or of some fruit-tree, or the
unfolding leaf-buds of any plant, may be employed. After carefully closing the
cylinder, it is allowed to stand for 10-20 hours. On then carefully and slowly
raising the stopper, and slowly lowering a burning taper fastened to a wire into the
cylinder, the flame and the incandescent wick are extinguished, exactly as if the
'-vessel had been filled with pure carbon dioxide. Carbon dioxide is, as is well known.

39«

LECTURE XXV.

a very heavy gas which does not hnrriediately pass out of the opened cylinder : hence
the result of the experiment. In exactly the same manner we may convince ourselves
by the same means of the respiration of large developing Fungi, as well as of mould
growing on bread or on a liquid. The enormous energy with which growing plants
absorb the oxygen in their neighbourhood, and give it back as carbon dioxide, is
particularly conspicuous in that the whole of the oxygen in an absorption-tube is
made u?e of for respiration, as follows at once from the fact that the carbon dioxide
absorbed by potash after the experiment corresponds exactly to the volume of the
oxygen originally present, unless, indeed, it happens that by means of intra-molecular
respiration, to be described later, an excess of carbon dioxide is found.

For the purpose of more exact studies as to the carbon dioxide formed by
respiration, the apparatus here figured may be employed. The two bottles/ and g

serve as an aspirator, since the
water flows out fromy down to
g, whence of course the air which
enters at z (on the right) must
pass through the various vessels
of the apparatus. It is first freed
from any small quantities of car-
bon dioxide in the vessel a, which
contains pumice-stone saturated
with potash : if the lime-water in
the flask b remains clear, it proves
that this is accomplished. The
air thus comes into the receiver
c quite free from carbon dioxide.
In this receiver is a trough h
covered with wide-meshed gauze,
which touches the surface of the
water contained in the trough.
On the gauze lie 20-30 germinat-
ing seeds of Peas, Wheat, &c., or
unfolding buds or flowers or suit-
able vitally active parts of plants generally, from which carbon dioxide is evolved
by respiration. The receiver c is fixed air-tight on to the glass-plate k. The
air, now laden exclusively with respired carbon dioxide, streams through the two
flasks d and e containing lime-water. The carbon dioxide is nearly all absorbed
in d — i. e. a white precipitate of calcium carbonate is formed — and usually, when
the air only streams slowly through the apparatus, scarcely any further precipi-
tate of calcium carbonate results in the second flask e. Should this be the case,
however, a third flask must be interposed. The calcium carbonate is now collected
on a filter, and from its weight the quantity of carbon dioxide developed in the
plants is calculated. Another form may also be given to the apparatus by replacing
the tW'O vessels ß 3 as well as the flasks d e with Liebig's bulbs filled with potash
solution. The chief point is that in this apparatus new air containing oxygen is
continually supplied to the plants, and the carbon dioxide formed is removed, so that

Fig. 252.

QUANTITIES OF CARBON DIOXIDE EVOLVED. 39g

the plants can respire in a normal atmosphere, and we are at the same time able
to determine the amount of carbon dioxide respired from time to time, without the
plants themselves being thereby destroyed.

In the course of very numerous investigations on the respiration of plants, various
observers have made use of several different kinds of apparatus, which however
mostly do not offer the advantages mentioned, and do not allow of a long-continued
normal growth of the plants in the apparatus.

It would carry us much too far for my purpose to quote even abstracts of the
experiments, but the general results obtained from the numerous experiments may
well be referred to.

With respect to the absolute magnitude of the respiration — i. e. the quantity of
carbon dioxide which is expired by a definite weight or volume of plant-substance
— Garreau found for example that twelve buds of Syn'nga vulgaris, which when
dried at iio°C. weighed two grams, exhaled 70 c.cm. of carbon dioxide in twenty-
four hours, the leaves having unfolded during the experiment. In the same way five
buds of jEsculus ??iacrßslachya, the dry weight of which amounted to o"85 gram,
produced 45 c.cm. in twenty-four hours, the leaves having unfolded in this case also.
Garreau further sowed seeds in fine sand, moistened with rain-water, and then
brought the seedlings, deprived of the seed-coats, under the receiver, where the
carbon dioxide exhaled at 16° C. was detemiined. Seedlings of Papaver somniferum,
which weighed 0*45 gram when dried subsequently, evolved 55 c.cm. of carbon
dioxide in twenty-four hours; and in the same way seedlings of Sinapis fiigra,
the dry weight of which was 055 gram, evolved 32 c.cm. of carbon dioxide in
twenty-four hours.

Charles Lory investigated phanerogamous parasites — Orobanche, Laihrcea, and
the humus plant Neottia, which contains small quantities of chlorophyll, and found
that they are always taking up oxygen and exhaling carbon dioxide. At 18^ C.
Orobanche Teucrii in full bloom used up its own volume of oxygen in thirty-six hours
— i.e. 4.2 c.cm. per one gram of substance, corresponding to a loss of 2-26 mgr.
of carbon.

The activity of the respiration — i.e. the carbon dioxide exhaled in a given time
by a certain weight or volume of living plant- substance — varies according to the
state of development, the activity of growth, and according to the vitality generally of
the part of the plant concerned. At the commencement of the germination of seeds
and buds, therefore, but little carbon dioxide is at first disengaged : its quantity
increases with progressive development, to diminish again later, when the material
for respiration in the interior of the organ begins to fail. The activity of respiration,
in fact, does not depend upon the total mass of substance which may happen
to be present in the plant, but upon how large a quantity of it is being
immediately employed in growth and other vital processes. When such material
begins to fail, the energy of respiration diminishes also.

Since not only growth but also all other functions which depend upon respira-
tion become more energetic as the temperature rises up to the optimum, so also
the carbon dioxide exhaled increases, other relations being the same, as the temper-
ature rises, reaching its maximum at the optimum temperature.

The energy of respiration of an organ also varies according to its physiological

400 LECTURE XXV.

nature. Even De Saussure found that the respiration of flowers is more energetic

than the green leaves of the same plant, weight and volume being equal. The

respiration of leaves, however (in the dark), again transcends that of shoot-axes and

fruits. To mention a few examples only ; he found that the flowers of Liliiim

candidum consumed five times their volume of oxygen in twenty-four hours, while the

leaves, on the other hand, only consumed 2-5 times their volume. In Passiflora

serraiifoUa the relation of flowers to leaves was 18-5 to 5*25, and so on. Even the

individual parts of flowers respire with different energy ; thus. De Saussure found as

follows : —

Cucurbita Melo-Pepo.

Volume of oxygen consumed
In 10 hoiirs. compared with that of

the organ = i .

Male flowers 76

Female flowers - • • 3'5

Anthers (separated from their bases) . . . . ii-7

Stigma (separated from ovary) . . . . . 47

In general, with normal conditions of respiration and sufficient access of
oxygen, the volume of carbon dioxide exhaled is equal to that of the oxygen taken
in, as De Saussure had already found. The same investigator also showed, however,
as long ago as 1804, that in the germination of fatty seeds this equivalence of volume
no longer exists : the volume of exhaled carbon dioxide is in this case smaller than
the volume of oxygen taken up. A portion of the latter, in fact, is not used for
respiration in the narrower sense of the word, but for the formation of carbo-hydrates
at the expense of the fats present, remaining meanwhile as a constituent of the sugar
in the plant.

It is clear that the carbon contained in the exhaled carbon dioxide can only
be derived from the substance of the plant itself, and that a diminution of the
carbon contents of the plant must thus be eff"ected by respiration. Now this carbon
exists in the plant in the form of carbo-hydrates, fats, and proteid substances. Thus
if a part of the carbon escapes from these chemical compounds in the form of
carbon dioxide, they must suffer a profound decomposition which we may in
general term combustion ; and according to Boussingault's investigations, no doubt
remains that in this combustion, as is to be expected, water also is formed from the
organic substance. The loss in organic substance by means of respiration may,
under certain circumstances (e.g. during advancing germination in the dark, when
no replacement by assimilation takes place), even go so far that more than half the
organic substance is destroyed by respiration. In other words, the organic substance
of a seedling grown in the dark until it is completely exhausted, at last weighs only
half as much as the organic substance of the seed employed for germination, or even
less, and there is no doubt whatever that matters are exactly the same in the shooting
of buds and in growth without assimilation generally.

From the investigation of Boussingault, however, results further the highly
important fact that this combustion effected by respiration concerns only the non-
nitrogenous constituents of the assimilated substance ; and since the fats are con-
verted into carbo-hydrates in metabolism, one may say only these latter are burnt to
carbon dioxide and water in respiration. This conclusion is fully warranted by

TNTRA-MOLF.CULAR RESPIRATION. 4OI

the fact that even in prolonged respiration accompanied by great loss of substance,
no loss in nitrogenous assimilated material — i. e. of proteid ■ substances — is to be
detected. And this result is so much the more remarkable since we have every
reason to assume that it is strictly in the nitrogenous substance of the protoplasm
that respiration directly occurs. Like all the vital phenomena of plants, respiration
also is brought about by means of the protoplasm : however, the protoplasm only
initiates the process, and is not itself injured in constitution by it.

Some light is thrown upon this remarkable behaviour of the protoplasm in
respiration by the recent researches on so-called intra-molecular respiration^
Grischow noticed so long ago as 18 19 that in the respiration of Fungi, more carbon
dioxide is occasionally evolved than accords with the oxygen absorbed. In my
laboratory, also, other researches showed that occasionally portions of plants of
various kinds go on evolving carbon dioxide even when they are unable to take up
oxygen. These facts first gained in general interest however when Pfliiger observed
in 1875 that frogs not only go on living for some time when they remain in an atmo-
sphere devoid of oxygen, but they also exhale carbon dioxide. Pfliiger concluded
from this that both constituents of this gas must be derived from the organic substance
of the frog itself; in other words, that the molecules of the organic substance undergo
a decomposition, even without the access of oxygen, of such a kind that atoms
of carbon and of oxygen come together within these molecules themselves to form
carbon dioxide, which is then exhaled. This process is termed by Pfliiger intra-
molecular respiration. There can be no doubt that the evolution of carbon dioxide by
plants in a space devoid of oxygen, which has long been observed by us, depends On
the same process of intra-molecular respiration ; since from all the facts known to us
the respiration of plants agrees with that of animals point for point. After having
convinced myself that seedlings which remained for days in an atmosphere devoid of
oxygen evolved carbon dioxide and ceased to grow, but on being planted in the earth
again lived and grew vigorously, I requested Dr. Wortmann in 1878 to undertake a
thorough investigation of this question in my laboratory. This was done with skill
and judgment. His experiments with seedlings, flowers, and growing stems were made
for the most part in the Torricellian vacuum, and yielded the following important
results : i. the intra-molecular respiration during the first few hours yields just as much
carbon dioxide as does respiration under the influence of the atmosphere containing
oxygen : 2. the energy of the intra-molecular respiration sinks considerably, even
after a few hours, thus showing an abnormal condition of the plant as contrasted
with normal respiration. This fact is, I believe, decidedly opposed to the view
established by Pfeffer, to the eff'ect that respiration is identical with alcoholic
fermentation. This view is, of course, supported by the fact that the most various
parts of plants on being excluded from oxygen, produce small quantities of alcohol,
besides disengaging carbon dioxide. But, apart from the fact that this production

' On intra-molecular respiration cf. Pfeffer, 'Das Wescti und die Bedeutung der Athmiiiig in
der Pflanze,^ in 'Landw. Jahrb.' (Berlin, 1878). Julius Wortmann, ' Über die Beziehung der intra-
moleculare zurnormalen Athmung der Pflanzen^ in 'Arb. d. bot. Inst, zu Würzburg' (1879), II. B,
p. 500. Eriksson, ' IJber IVärmehildung dureh intraniolcculare Athmung,' in ' Untersuchungen aus
dem bot. Inst, zu Tübingen' ^Leipzig, i88i).

[3] Dd

403 LECTURE XXV.

of alcohol under the circumstances named in the higher plants has never yet been
quantitively determined, emphasis must be laid on the fact that wherever alcoholic
fermentation has been observed in the absence of oxygen (of course apart from
the Ferment-fungi) objects have been exposed which have not merely undergone
intra-molecular respiration for 1-2 hours, but have been cut off from the access
of atmospheric oxygen for days or weeks. Now Wortmann shows (though he
himself inclines to Pfeffer's view), that after the first few hours the intra-molecular
respiration already indicates an abnormal condition of the plant, whence 1 draw the
conclusion (to which Naegeli and Borodin had already arrived in another way), that
the formation of alcohol in the absence of oxygen is an abnormal process throughout,
and has nothing to do with ordinary respiration.

The most important fact, however, and one which must never be lost sight of in
comparing intra-molecular and normal respiration, lies in that intra-molecular
respiration cannot provide the forces necessary for growth and the motility of irritable
organs. So long as the access of external oxygen is excluded, the plants are im-
movable, rigid, and growth comes to a standstill : a point to which Wortmann has
already referred at the conclusion of his excellent work.

The true meaning of respiration, however, still remains unexplained. Never-
theless, so much is established, that it is a function of active living protoplasm.
Since although it results from Boussingault's investigations that carbo-hydrates only
are consumed in normal respiration, this takes place on the other hand only when
they are exposed to the influence of living protoplasm. And thac it is not in any
way the proteid substances, regarded as chemical compounds, which maintain the
respiratory process, results directly from the fact that in non-organised proteid
substances neither normal nor intra-molecular respiration is to be observed. Dormant
protoplasm never respires. It is, on the contrary, a property of active and living
protoplasm to respire; or, perhaps better, the respiratory process is the first and
most fundamental expression of the vital processes in protoplasm. The substance
of what intra-molecular respiration teaches, according to the facts established by
Wortmann, is that it is not the oxygen penetrating from without which gives the
first impulse to the chemical changes of respiration ; but that primarily, and in the
protoplasm in the first place, a decomposition of the molecules of the proteids
occurs, which terminates with the formation of carbon dioxide; that, however, by
means of the entrance of oxygen from without a restilutio in integrum takes place,
when a carbo-hydrate, and especially sugar, is consumed.

These are, moreover, only preliminary attempts to obtain an insight into the
process of respiration. It is certain that numerous patient and laborious investiga-
tions will yet be necessary ere we attain complete clearness as to these matters.

While hitherto the evolution of carbon dioxide, and (as we are warranted in
assuming from Boussingault's researches) the formation of water from the organic
substance, have been placed in the foreground as expressing the activity of respiration,
only the terminal result of the process is intended to be thus characterised. That
a long series of chemical processes in addition are first induced in the plant by
respiration, on which the whole vital process finally depends, there can be no doubt
whatever. We may perhaps regard the formation of those acids which abound
in oxygen at the commencement of germination, and likewise in shooting buds, as

PURPOSES OF RESPIRATION. 40;^

the most obvious indication of the processes of oxidation connected with normal
respiration ; and that these oxy-acids, which evidently arise at the expense of the car-
bohydrates and fats, in their turn constitute important points in the complex of vital
processes, may be concluded from the universality of their occurrence. Moreover,
Hugo de Vries' idea that the vegetable acids play an important part in the turges-
cence of the cells, and therefore in growth also, is scarcely to be put aside. On the
one hand it is a direct or indirect consequence of the process of respiration that in
germination and in the growth of buds also, compounds containing very little or no
oxygen arise from the splitting of carbo-hydrates and fats, since it is only during
normal respiration and the growth depending upon it that resins and ethereal oils
are formed. There is nothing extraordinary in the view that by means of a process
of oxidation (and respiration is a very intense process of oxidation) compounds poor
in oxygen as w'ell as those abounding in it arise ; but of course further investigations
will first have to render clear the details in these processes.

It may appear absurd that plants which decompose carbon dioxide by means of
their organs containing chlorophyll, in order to produce carbonaceous vegetable
substance, on the other hand destroy such carbonaceous substance again during their
whole life by means of respiration, and thus effect a loss of the capital gathered
together by themselves. It may have been this reflection which drove so keen
a mind as that of Justus von Liebig to the quite unwarranted decision that no
respiration at all takes place in plants. But what was said at the beginning of this
lecture leads us to the right conclusion, in opposition to Liebig's, which depended
upon mere perplexity. It is in fact not simply a matter of accumulating a quantity
of organic vegetable substance by assimilation, but rather, this gain in substance is
only to serve the purpose of promoting the vital processes. Starch, fats, and proteid
substances are of course products of assimilation, but by and for themselves they are
inert material, just as bricks and mortar constitute merely the material for the con-
struction of a house. For these to be set in motion, and for the structure actually
to come into existence, moving forces are necessary, and it is respiration which
provides these in the organism. The loss of substance which results in addition
from respiration, serves to develope mechanical forces by means of which the atoms
and molecules of the remaining substance are set in those movements from which
growth and the other functions of the living plant result. In a word, respiration is
the source of the energy from which all the phenomena of life derive their vital
forces; while assimilation in the organs which contain chlorophyll supplies the
materials which are subsequently to be set in motion for the purposes of life.
This, expressed quite generally, is the physiological significance and object of
respiration, and it is certainly not too dearly bought by a relatively small loss of
substance.

In respiration, as we have seen, carbon dioxide and water are produced at the
expense of organic substance. This process, according to a general law of nature,
is connected with the development of heat. As in every other process of combustion
of carbon and hydrogen to carbon dioxide and water, so also in respiration a
definite quantity of heat must be produced, though not exactly the same quantity
as when carbon and hydrogen in the elementary state burn with oxygen, since
a portion of the heat-producing force is destroyed in the former case, because

D d 2

404 LECTURE XXV.

the carbon and hydrogen must first be torn asunder from their molecular combi-
nations. In any case, so much is certain, that heat is produced by respiration in
the plant, just as the heat of the animal body is produced by respiration. While
this remark depends on general natural laws, it may appear somewhat paradoxical
that respiring plants possess either only the temperature of their environment,
or, if living in the open air, may even be cooler than it is. On closer con-
sideration, however, this result is seen to be quite natural, since the temperature
of a body depends not only upon the heat developed in it, but quite as much upon
the causes which carry away this heat, and thus effect cooling. Apart from special
cases, it is the cooling influences which are particularly energetic in plants, because
the very large surfaces and relatively small mass facilitate the exchange of heat
with the environment. In submerged aquatic plants and subterranean organs
every increase of temperature produced by respiration is very easily equalised
by means of the bulky surrounding medium, so that such parts of the plant
exhibit simply the temperature of the environment. Leaves and their shoot-axes
in the open air, however, are more exposed to cooling than these. They lose such
considerable quantities of heat, not only by radiation, but also by the absorption
of heat in the formation of aqueous vapour, that they are usually colder than the
surrounding air. During clear nights the temperature of the leaves may fall by
radiation several degrees below that of the air ; and when the latter is some 2 or 3°
above zero, for instance, the former may cool down 3 or 4° below zero, and the
aqueous vapour of the surrounding air be precipitated in the form of ice crystals
(hoar frost) on the plants. On the other hand, the leaves may be even warmer than
the surrounding air in strong sunshine ; and this of course does not depend upon
respiration. In observing the temperature of the wood of trees earlier observers
allowed themselves to fall into much confusion. They found, for instance, that the
temperature of the wood at night is higher than that of the surrounding air, and
many held this to be an indication of spontaneous heating, although, as later and
more exact observations showed, this phenomenon, as well as the opposite difference
of temperature in the day, depends simply upon the slow conduction of heat by
wood. As a matter of fact,- also, the wood of a tree is the least suitable of all
materials for the observation of the spontaneous heating of plants ; since in the
wood, apart from the relatively small quantity of parenchyma, respiration (and con-
sequently also spontaneous heating) does not occur.

However, if cooling— i.e. the removal of the heat of respiration— is prevented,
it is easy to demonstrate rises of temperature in the most various parts of plants by
means of suitable thermometric observations. The fact longest known in this
connection is the rise of temperature, often very high, of germinating barley in the
preparation of malt : here the respiring seedlings are accumulated in large quantities,
and protected from cooling to any great extent. Göppert's experiments made in
1832 with various seeds, bulbs and tubers, depend on the same principle: the
seeds etc. were accumulated in large quantities, and rose in temperature several
degrees, though great sources of error were overlooked. Evidently it is not sufficient
simply to accumulate large quantities of germinating seeds, bulbs and tubers in
the neighbourhood of a thermometer ; it is also necessary to afford access to the
atmospheric air in the process, in order that respiration may proceed. This may

HEAT DUE TO RESPIRATION.

405

be accomplished by means of the simple apparatus here figured. The bottle / con-
tains a strong solution of potash or soda /, which absorbs the carbon dioxide liberated
in respiration. In the opening of the bottle a funnel r is inserted, in which a large
number of germinating seeds or flower-buds are contained. The bell-jar g is for the
purpose of preventing the radiation of the heat, without however excluding the access
of the atmospheric air, which can enter at the lower margin of the bell-jar, as well as
through the pad of cotton-wool w. A thermometer / is pushed through the latter, so
that its bulb is immersed among the respiring plants. For observation two such
pieces of apparatus should be arranged close together in a chamber, the thermometers
being previously compared. In the second apparatus, the funnel r is loosely filled
with moist filter-paper instead of with portions of plants, etc., in order to establish
similar conditions of evaporation and
radiation. In order still more to regu-
late the evaporation, a divided glass lid,
through the central hole of which the
thermometer passes, is laid on the
funnel r. The apparatus, as well as
the parts of plants experimented with,
should have been already exposed to
the temperature of the space in which
the observation is made for several
hours before the commencement of the
experiment.

By these means, with temperatures
favourable for vegetation, I succeeded in
observing a spontaneous heatingof i'5°C.
in the case of 100-200 germinating Peas
as their roots developed. The anthers
of a Gourd caused the mercury of a
somewhat large thermometer, the bulb
of which touched them on one side
only, to rise through o-8°C. A single
flower - head of Onopordon acanthiuni
showed a spontaneous heating equal to
o"72°C.; and the stamens of a single flower of NympJicEa stellata showed a rise
of temperature of o-6°C. Numerous flower-buds of Anthemis chrysoloica, heaped
round the bulb of the thermometer, were heated through i-6° C. during the unfolding.
In large, vigorously developing Fungi, it sufficed to push a thermometer into their
substance to observe the rise of temperature.

The apparatus for observing respiration figured above (Fig. 252) may also
be employed for these experiments, if a thermometer is inserted in the bell-jar c, so
that its bulb is immersed among the respiring plants.

The idea that the spontaneous heating of plants is a consequence of respiration,
was expressed by Theodore de Saussure so long ago as 1822; although his own
thermometric observations on flowers aff"orded no very satisfactory results, because
they were made in the open air.

Fig. 253. — Apparatus for observing the spontaneous he
of germinating seeds and flowers.

4o6 LECTURE XXV.

The spontaneous evolution of heat is very easily observed in a most instructive
manner in the inflorescence of tht Aroidese. Nature has here brought together
in a confined space a large number of very actively respiring flowers. The
rise of temperature of such inflorescences, especially when large, amounts to
4 or 5 to ID or 12 or even 15 and more degrees Centigrade, and therefore may
be perceived by the senses even without the thermometer. Older observers had
already made close observations on these exceptionally favourable objects as to
the relation between the respiration of oxygen and the evolution of heat : we owe
the most exact of these to Garreau, who has done much for the theory of respiration.
He found in the inflorescence of Arum Italicum, for example, a spontaneous heating
of 3-2°C.; ifi c.cm. of oxygen being respired by i gram of substance in one hour.
The same spike showed a spontaneous heating of 8"3° C, i gram of the spike con-
suming 28-5 c.cm. of oxygen (i.e. converting it into carbon dioxide) in one hour.
These examples at any rate show that the respiration of plants may under certain
circumstances reach an intensity which may be compared with that of warm-blooded
animals.

In large single flowers like that of Victoria regia, as well as in the flowering
spikes of Aroidcce, we find during the period of spontaneous evolution of heat, first
a rise of temperature up to a maximum, and then a decrease in the spontaneous
evolution of heat, evidendy in consequence of the advancing development. Observers
give in addition also periodic oscillations of the spontaneous evolution of heat, the
true nature of which, however, has not yet been explained.

Since respiration, like every other vital process, obtains in intensity as the
temperature increases, until an optimum of the latter is attained, it follows that the
spontaneous evolution of heat at higher but favourable temperatures of the surrounding
air must be more intense than at lower temperatures of the latter; and when the
temperature of the environment is so low that growth and respiration do not occur at
all, it is obvious that no spontaneous evolution of heat is to be expected. These
theoretical results find their confirmation in the observations before us.

The observation of the rise of temperature in the respiration of green shoots is
more difficult than in the cases hitherto referred to. Dutrochet in 1840 employed for
this purpose a thermo-electric pile of great delicacy, by means of which he succeeded
in demonstrating in the interior of individual growing shoots rises of temperature of
J^^th and j^^h of a degree. He found the greatest rise always in the case of
unfolding buds, which of course was to be expected.

From the point of view of purely scientific discovery, however, all these observa-
tions have relatively little value, and I have only introduced them in confirmation of
the theoretical results. Much more important would be the determination of the
quantities of heat, expressed in thermal units, produced by the consumption of a
known quantity of oxygen in respiration.

As already said, the production of heat is a universal and necessary consequence
of respiration. In rare cases, however, the production of light, or phosphorescence,
also occurs. Avoiding here many extremely doubtful statements, I confine myself
to mentioning several cases of luminous Fungi established by good observers, where
it is essentially a matter of showing that phosphorescence may be a consequence of
the respiration of living plants. In this connection, the illumination of the ' Rhizo-

LUMINOSITY OF FUNGI, ETC.

407

morphs,' the mycelium of a tree-killing Agaric {Agan'tus vielleus) has been longest
known. Tulasne mentions, as. examples of spontaneously luminous Fungi, Agaricus
igncus from Amboyna, A. noctilucens from Manilla, and A. Gardneri from Brazil. The
best investigation of the spontaneous luminosity of such Fungi was supplied in 1855
by Fabre in the case of Agaricus olearius. This golden yellow Fungus grows
throughout Provence at the foot of olive trees in October and November. Accord-
ing to Delile and Fabre only its hymenium is luminous, and not the white spores.
According to Tulasne the stem also, at least here and there, is in many cases
luminous ; and even the interior of the Fungus is said to develope light, according
to him. Fabre, who made his observations at a lower temperature of the air,
could not detect this. The observers mentioned, however, agree that the Fungus is
only luminous during its period of vegetation, and that the phenomenon ceases at
death. Even very young specimens are actively luminous, and they retain this
property so long as they live. Fabre describes the light as steady, white and
homogeneous, resembling that of phosphorus dissolved in oil. His observations
were made in November, at io°-i2°C. ; and he first established that the Fungus
is luminous during the day as well as at night, as had already been shown by
Schmitz to be the case in the Rhizomorphae. A previous exposure to sunlight has
no noticeable influence on the subsequent luminosity in the dark, and the degree
of moisture of the air likewise seems to have no perceptible effect. The Fungus is
luminous in rainy weather and in dry, and is just as luminous in air saturated with
vapour. If it is dried to such an extent that death ensues, however, the luminosity
ceases. In the case of the Rhizomorphs, according to Tulasne, this takes place
sooner. At temperatures lower than + 4° or + 3° C. the phosphorescence is very
quickly lost, but returns again when the temperature of the air rises. The maximum
of luminosity is reached at 8°-io° C, and is not increased by further heating. If
plunged into warm water, the Fungus retains its luminosity; but as soon as the
temperature rises to 50° C, the luminosity disappears for ever, the Fungus being
then killed.

In water containing air the phosphorescence is as pronounced as in the air ;
but if a luminous Fungus is plunged into boiled water, the luminosity ceases almost
instantaneously, returning, however, when the Fungus is withdrawn and brought into
the air. The phosphorescence is immediately and completely extinguished in a
vacuum, in hydrogen gas, and in carbon dioxide. After remaining for several hours
in a vacuum or in these gases, the Fungus at once regains its light on being again
brought into the air : a longer stay in carbon dioxide injures it however. In pure
oxygen the light does not become more pronounced : on the contrary, it is enfeebled
after thirty-six hours in this gas. The most important fact discovered by Fabre is
that Agaricus olearius in its phosphorescent condition forms much more carbon
dioxide than when it is not luminous. The pileus with its lamellae in pure oxygen at
i2°C. yielded, in thirty-six hours, 4-41 c.cm. of carbon dioxide for each gram of its
weight : a gram of non-luminous substance only yielded 2-88 c.cm. of carboa
dioxide. On the other hand, on treating a luminous piece of Fungus in the same
way at a lower temperature, where the luminosity ceased, this yielded in forty-
four hours for each gram of its substance, only 2"64 c.cm. of carbon dioxide; and
another piece, not luminous at all, 2*57 c.cm. Hence the substance capable of

4o8 LECTURE XXV.

luminosity, when prevented from emitting the light, exhaled only as much carbon
dioxide as the substance which is not luminous at all.

Fabre concludes his work with the remark that phosphorescence is the eflfect of
the respiratory activity of the Fungus, and depends upon the same causes as the
spontaneous evolution of heat at the time of flowering in certain parts of Phanero-
gams, particularly the Aroidese. Nevertheless, it must be allowed that very peculiar
arrangements must exist for light to be a consequence of respiration in the Fungi,
since the flowers of the Aroidese, and even those of Cucurbita, form relatively far
larger quantities of carbon dioxide, and develope heat without being luminous. The
taking up of oxygen is evidently only one of the various causes the co-operation of
which produce luminosity.

PART IV.

GROWTH.

LECTURE XXVI.

THE DISTRIBUTION OF THE PHASES OF GROWTH IN SPACE
AND TIME.

At the beginning of any new period of vegetation the buds of trees, sub-
terranean rhizomes, bulbs, and tubers put forth new foliage-shoots and often
flowering stems also : in a relatively short time the previously naked trees are
covered with green foliage, and the meadows, gardens, and fields with flowering
plants. The plant-substance, which together with large quantities of water
affords the constructive material for this rapid growth, has been produced in
the previous period of vegetation by means of assimilation, and stored up in
the wintering organs of the plant in the form of reserve-materials, now to be made
use of for growth.

But even the organs themselves, the foliage and flower-shoots which make
their appearance during the first warm days of the spring, had already been produced
in the previous summer and autumn ; but, small and in part even microscopic in
size, had passed into a condition of rest before further development. With the
beginning of the new period of vegetation they renewed their activity ; in a word
the organs themselves, as well as the materials necessary for their further growth
passed the winter in an undeveloped, embryonic condition. It is the same also
in the case of seeds, which contain in addition to the young developino- oro-ans
of the embryo the nutritive materials for their first growth also.

The new germinal and foliage-shoots and flower-stems having been formed
then in the spring at the cost of the reserve-materials preserved through the
winter, and a widely-spread root system having been developed in the soil
assimilation — i.e. the formation of new plant-subslance — is renewed; and accordin«-
to the nature of the plant, this is soon again employed for the formation of new
organs, or deposited in the reservoirs of reserve-materials for the next year, or
both processes are combined in very various ways.

It is easy to conclude from these remarks, which lend themselves directly to
the inference, that nutrition and growth need not coincide either in time or in
space. Nutrition— i. e. the production of vegetable substance— is usually carried on
most energetically at a time when the growth of the organs in the main has already
taken place ; and we find the most vigorous growth taking place at the beginning

41

LECTURE XXVI.

of a new period of vegetation, without nutrition occurring at the same time. This
fact may perhaps be demonstrated still more plainly by simply allowing seeds,
tubers, and bulbs, thoroughly saturated with water, to lie or hang in moist air,
where the germinal shoots and roots make their appearance, and grow up to a
certain point, without taking up nutritive substances from without, but only with
the aid of the respired oxygen and the heat supplied by the environment.

Growth and nutrition thus by no means coincide : but it is self-evident that
growth can only occur if constructive materials — plant-substance capable of
organisation — are already present. It follows from this fact, that we can never
infer simply from the occurrence of growth a simultaneous occurrence of nutri-
tion ; and in the same way it is by no means to be concluded from the fact that
nutrition is proceeding, that the organs of the plant are at the same time growing.
This appears most evidently in the organs of assimilation — the green leaves them-
selves— the nutritive activity of which only predominates when they are themselves
completely developed. If, now, the growth of a single leaf, a shoot, a flower, or of
any other organ is observed from its origin up to the moment when it is completely
developed — the developmental history, as it is usually termed — two points are
noticed, viz. on the one hand the volume becomes larger and larger, until it finally
attains a definitive size and increases no more ; and, on the other hand, the form
of the organ, at first sketched out only in its coarsest outlines, so to speak, becomes
further developed and more finely elaborated during the increase in volume, until
finally its definitive form is perfected.

Growth is thus an increase in volume, closely combined with a change in form.

It may perhaps contribute to the elucidation of the idea if we compare the
'growth of a plant thus defined with that of a crystal, on the one hand, and with
that of an animal on the other.

In the case of the crystal, two points are also to be distinguished in the
growth : the increase in volume, and the formative forces. From the material in
a dissolved, or fused, or even gaseous state, minute and scarcely visible crystals
are formed, which then increase in size. But in their growth the geometrical
form given from the first remains unaltered, and the increase in volume is not, as
in the case of plants and animals, a specifically limited one. Under favourable
circumstances a crystal may go on growing further and further; it is never, like
an organ, 'fully grown.' Moreover, the mode of growth is essentially diff'erent;
for in the first place a crystal takes its origin from amorphous fluid matter, and
then grows by means of the deposition of new, minute, invisible particles on its
surfaces. A vegetable body, on the other hand, never originates directly from
a fluid, but always as part of an existing and already formed plant-organ ; or, if
one will so express it, all crystals arise by ' spontaneous generation,' which is never
the case with organisms. Crystallisable substances may completely give up their
form on solution, fusion, or evaporation, and assume it again under suitable external
conditions : organisms, on the other hand, exhibit an uninterrupted continuity of their
existence. The substance of a plant or of an animal dissolved and in an amorphous
condition never resumes the organised form : this only proceeds from an organised
substance already present.

With reference to the increase in volume, also, a fundamental difierence exists

COMPARISON OF THE GROWTH OF CRYSTALS, PLANTS, AND ANIMALS. 413

between organs and crystals, in that the enlargement of a crystal takes place by the
deposition of new substance at its surfaces ; whereas the plastic substance serving for
the growth of the organ is present in its interior, and is conveyed to it internally from
other organs. Organs grow not by apposition, like crystals, but by intussusception :
their growth is an extension acting from within.

Nevertheless we have cause to assume that, apart from all external matters, even
the growth of the organs of the plant — namely, their elementary structures, cell-walls,
protoplasm, and nucleus — depends fundamentally on forces which correspond essen-
tially with the forces of crystallisation ; only that here, since the supply of substance
capable of being organised takes place from within, complications which are not easily
intelligible present themselves. If the comparison here pointed out meets the case, it
must be mentioned that only the smallest particles of a cell-wall, protoplasm, or
nucleus (the MicellcE of Naegeli) grow after the manner of crystals ; and thus, not the
external form of an entire plant-organ, but the invisible structure of its elementary
constituents might be immediately referred to the forces of crystallisation. Naegeli
has undertaken, in a long series of important researches, to follow up and explain
further these (his own) ideas. His theory of growth by intussusception is concerned
in the first place with the question how the smallest invisible elements of the organised
parts of plants grow\ It is clear, however, that only one who is familiar with all the
details of the internal and external structure of vegetable organs can be in a position
to enter into the questions here raised. Moreover, the theory of intussusception has
by no means as yet attained to that degree of clearness which renders it possible
to explain in detail from it the growth of vegetable organs — leaves, roots, &c, I
renounce, therefore, in the interest of the reader, even an attempt in this direction, in
the conviction that the mere mention of this problem suffices to indicate the difficulty
of the task with which the theory of growth has to do : meanwhile we will keep
more to the surface of the matter, and attempt to make clear those processes of the
growth of the organs which are immedia'tely perceptible to the senses.

The growth of the plant has been so far compared with that of the crystal. In
the essential points of comparison animals agree with plants. Comparisons between
the two kingdoms of organised beings, however, again yield some important differ-
ences. Apart from a few of the most simply organised forms, animals attain by means
of their grow'th a definitive condition, of such a kind that in a fully grown animal
all the organs are fully developed, each contributing by its physiological labour to the
maintenance of the entire body : the completely developed animal consists of com-
pletely developed organs, each performing its functions to the utmost. The case is
quite otherwise with plants. They are never completely developed. It is true we
find on every living plant completely developed parts, but besides these there are
always present in addition the rudiments or beginnings of new organs, capable of
further development.

A plant without those growing-points which effect further development is no
longer a normal plant. Moreover, even the cut-off parts of living plants are very
often able to originate new growing-points of like kind.

The continuity of animal life (apart from some plant-like lower forms) is
brought about by means of continually repeated reproduction, the single individuals
having normally only a limited and often very short period of life. Something

414

LECTURE XXVI.

similar occurs, it is true, in the case of certain plants also, the so-called annuals ;
but the vast majority of plants owe the continuity of their existence chiefly to the
circumstance that, besides organs already fully developed, new growing-points capable
of development are continually being originated, by means of which the life and
growth of the same plant-specimen is continued from year to year, or even from
century to century. It is owing to this fact, also, that the idea of the ' individual '

i.e. that which is only capable of indivisible existence — can find no rational

application whatever in the case of the great majority of plants; since it is mere
trifling with empty terms either to distinguish with Schleiden each individual cell,
or, with Alexander Braun, each shoot-bud emphatically as an individual. At any
rate, no deeper insight into the nature of plants can be obtained in that way.

Plants, therefore, in the condition in which they are usually observed (i.e. apart
from the microscopically minute primary embryonic state), always consist partly of
mature, fully-developed portions which no longer grow, and partly of immature por-
tions ; these latter, as in the case of winter-buds, may be dormant, but subsequently
develope further, or they may be portions which are developing — i. e. increasing in
volume and changing in form. In order to guide himself aright even in the most
elementary doctrines of vegetable physiology, it is absolutely necessary for the
student to acquire the clearest possible idea of these relations : the theory of growth,
especially, has no sense or meaning whatever for him who is not sufficiently familiar
with these matters. I shall therefore attempt to make clear by a few examples the
distribution of the phases of growth in space, as well as their changes in time. It
may be remarked at the outset that I distinguish three phases of growth, which are
continually passing over into one another, but which are nevertheless sharply
characterised.

Organs are first met with in an embryonic condition : growing further, they
enter upon a second phase, that of elongation, by means of which they attain their
definitive volume and their definitive external configuration. Only in a third stage of
o-rowth is the internal structure also of the already elongated organs completed.
I distinguish this last phase of growth as that of internal development, upon which
the condition of being fully grown — the mature state — at last follows.

As an example serving for the illustration of these matters, let us consider Fig. 254,
which represents in the form of a simple diagram the phases of growth of a young,
erect, dicotyledonous plant. In the unripe seed of this plant, at the proper time, an
embryo would be found in the form represented at /. On this are observed the grow-
ing-point (v) of the future shoot (plumule), and that of the radicle {w) : the two pro-
tuberances cc are the two primary leaves (cotyledons) of the seedling in a rudimentary
state. The nearly black shading of v and w is intended to indicate that these parts
still consist entirely of embryonal tissue ; while the lighter shading o{ cc indicates
the second phase of growth, the cell-tissue already beginning to pass over into the
stage of elongation. This condition of development of the embryo has resulted,
however, from one still earlier, where the embryo had almost the form of a sphere,
and where no diff'erentiation whatever, either into various organs or into various forms
of tissue, was as yet to be recognised : the entire embryo then consisted of homo-
geneous, embryonal cell-tissue, the commencing differentiation of which is already
indicated in /.

THE THREE PHASES OF GROWTH.

415

In the ripe seed of the same plant we should find the embryo in the form //.
Between v and 7v is now intercalated a mass of tissue, which is elongating, and
which subsequently forms the hypocotyledonary segment (//) of the shoot. The first
leaves, or cotyledons, c c, have grown considerably. Of the embryonal tissue, of

FIG. 254.— Diagram of the distribution of the

1 pliascs of growth in a dicotyledo

which the entire embryo formerly consisted, there now remain on!}- two portions,
separated from one another ; the upper one of these constitutes the growing-portion
of the future plumule, and the lower that of the radicle {zo).

In /// we have represented the same plant as in //, after the conclusion of

41 6 LECTURE XXVI.

germination. At the uppermost end of the germinal stem, shaded black, is still to be
recognised the growing-point marked v in /; also the growing-point w of the primary
root. New growing-points have now arisen in addition at /t >^ I^\ &c., and have
already in part proceeded to the formation of new lateral shoots. Each of these
new growing-points has originated, during progressive growth, out of the primary
growing-point, v in /. The lateral growing-points k were first protruded in the axils
of the two cotyledons, c, II: the primary growing-point then grew further, and
produced the leaves b, b' , b'\ b"\ Sec, a new growing-point arising each time from
the axil of the leaf, as it was put forth from the growing-point [k' f^\ &c.). In the
course of the growth leaf-buds have already been put forth from these new growing-
points. The leaves and portions of stem also produced from the primary growing-
point V, have in the meantime however grown much more rapidly, and have already
entered into the second and third stages of growth : the leaves, b', b" , b''\ as well as the
parts of the shoot-axis belonging to them, are still found in the elongating condition,
as indicated by the light shading. The cotyledons f, and the leaf b, as well as the
parts of the shoot-axis belonging to them, which are not shaded at all, are on the
other hand already fully developed externally — i. e. they have attained their definitive
volume, and their permanent external form. These parts, however, are now in the
third phase of growth — i. e. their internal development is now being completed ; the
lignification of the vascular bundles, the development of sieve-tubes, the thickening
of the walls of the parenchyma cells, formation of stomata and development of cuticle
on the epidermis are being completed in these portions. All these histological
differentiations have already commenced in the parts of the plants indicated by the
shading ; but it is only after they have attained their definitive volume and permanent
form that the histological development also is completed.

Let us now take into consideration in the same way the subterranean root-system
of our plant (Fig. 254, ///). It is to be noted that the lateral roots w' w" were
produced originally close behind the growing-point {iv) of the primary root. Each of
these lateral roots was originally itself only a new growing-point, by the further
growth of which the filiform lateral roots have been produced ; and the same relation
exists between the lateral roots of the second order and their mother-roots. INIean-
while, it need only here be mentioned by the way that the growing-points of the new
roots originate in the interior of the tissue of their mother-roots ; while the new
growing-points of the shoots are superficial outgrowths of the primary growing-
point.

In the roots also the growing-points are represented black in our figure ; but, it
will be seen, they are surrounded by an additional light zone, the root-cap, which
originates simultaneously with the first rudiment of the growing-point of a root.
Behind each of the growing-points of the root, which are shaded black, is a short
piece marked by lighter shading : this is the elongating portion of the root-fibre, and,
as is seen at once, it is strikingly short in comparison with the elongating region of
the shoot-axis.

A very important point, which will already have been noticed by the way in the
above description, must now be made quite clear. All the growing-points of the plant
///, shaded black, originated in the first place from the two growing-points v and w
of the embryo /; and these themselves are both, as already said, simply remnants of

THE THREE PHASES OF GROWTH.

417

the perfectly homogeneous cell-tissue of which the embryo at first consisted.
All the growing-points, therefore, are directly derived from the primary embryonic
tissue; in such a manner, however, that we have to sup])ose the substance of the
latter as being nourished and continually increasing in quantity. If the growth of the
plant were entirely confined to that of the growing-points, it would consist in an
exceedingly slow increase in volume of
the original embryonic tissue. We might
imagine all these small growing-points,
so to speak, cut off and then united into
one whole : we should then have a very
small structure, consisting of nothing
but embryonic tissue, from which the
individual growing-points would stand
out as protuberances. Having clearly
apprehended this, it is now obvious that
in reality the various growing-points in
the developing plant described above
have been pushed asunder and removed
to a distance from one another; between
each two of them a longer or shorter
piece of shoot-axis (or in the roots a
piece of mature root-tissue) has been
intercalated. This mutual separa-
tion of the growing-points, or interca-
lation of new masses of tissue between
them, is moreover, as is easily observed,
effected in the second phase of growth ;
and this always consists fundamentally
simply in the elongation of those por-
tions of tissue which are situated at the
base of each growing-point.

On reviewing these processes ac-
cording to all that has been said so
far, it must be allowed that the growth
of a plant, even wholly apart from the
processes taking place in the individual
cells, is a very remarkable and extremely
complicated phenomenon, which the
student must try to apprehend perfectly
if he wishes to obtain any clear ideas

at all of the life and being of plants. Considering the difficulty which the right
understanding of these processes must necessarily present to the beginner, it will
not be superfluous to take into consideration yet a few other examples.

Fig. 255 illustrates, also diagrammatically, the processes in question in the case of
an erect monocotyledonous plant. The figure represents a plant of the INIaize (Indian
Corn); but would hold good, with but few alterations, for a Palm, and numerous other

[3] EC

Fig. 255.— Diagram showing the distribution of the phases of
growth in a Monocotyledon.

4I<S LECTURE XXVI.

Monocotyledons. It is advisable, in the first place, once more to regard closely the
germinating stages of the Maize-plant represented in Fig. 28 (p. 38), looking upon the
present figure, to a certain extent, as a continuation of that one. Here again J'Fis the
primary root, from which several thin lateral roots have originated. While, however,
in the dicotyledonous plant of the previous diagram the entire root-system arises from
the primary root, we find in this monocotyledonous plant that the primary root,
with its ramifications, remains small and plays an extremely subordinate part. Of
course as growth proceeds the entire root-system enlarges here also, but this is accom-
plished by new roots 0', <!>", ^''', springing from the lower part of the stem (s) itself,
the process taking place from below upwards : these roots penetrate into the earth
and there branch. The higher up the stem the root arises the more vigorous it is.
Although these secondary roots (0' — ^"') appear at a great distance from the two
primary growing-points of the radicle and plumule respectively, it is nevertheless not
improbable that the rudiments of their growing-points were already formed long
before ; and that on close investigation, which of course entails great difficulties, it
would be possible to detect these growing-points of the roots as direct derivatives of
the growing-point of the shoot of the plant. However, a question is raised here
which will have to be taken more closely into consideration in the next lecture. The
point in view now is rather to distinguish the three phases of growth ; and here again
the growing-points are shaded quite black, the elongating parts grey, and those
which are completing their internal development and are fully grown are left
without shading. The leaves b — b"' of the monocotyledonous plant in question, in
each case envelope the whole circumference of the shoot-axis with their sheath-like
base ; and each younger leaf is, like the younger part of the shoot-axis, completely
enveloped by the sheath of every older leaf. The apex of such a shoot, therefore,
consists mainly of a convolution of leaf-sheaths closely wrapped around one another,
and we have only to imagine these becoming thick and stout to have the whole
presenting the form of a bulb. It will scarcely be necessary to repeat that the
buds k k proceed from secondary growing-points, which in their turn originated in the
primary growing-point of the main shoot of the seedling.

Immediately above the base of each leaf, the figure (Fig. 255) shows, within the
lightly shaded parts, certain darker transverse zones, by means of which a peculiarity
of the growth of this and many other plants is to be explained : for at these
zones the tissue maintains a more or less embryonic character, and its cells situated
further above (in the acropetal direction) gradually pass into the stage of elongation
and definitively complete their development. Such transverse zones at the base of the
internodes may be termed zones of intercalary growth ; the tissue of which they are
composed, however, is directly derived from the embryonic tissue of the growing-
point. The presence of such intercalary zones brings it about that in the plants
concerned the individual segments of the shoot-axis are pushed up from below out of
the older leaf-sheaths ; hence the youngest — i. e. the least developed — portions of the
internodes are situated, not next the growing-point, but next the basal end. This
remarkable state of affairs, by which the processes of growth in a shoot become
still more complicated, occurs not only in many Monocotyledons, particularly
in all Grasses and in the flowering scapes of bulbous plants, but is extremely
clear also in the Horsetails (Equisetaceoe), among which Eqiiisetum hytmale is

THE PHASE OF EMBRYONIC GROWTH. 419

particularly distinguished in that the internodes are each year slightly pushed up
from below out of the older leaf-sheaths. Similar peculiarities occur moreover in
some Dicotyledons — e. g. the Polygonese, Umbelliferge, &c.

In the Grasses and Liliaceae (and in a less degree also elsewhere) there occurs
combined with this intercalary growth of the internodes a very similar process in
the leaves. These also go on growing at their basal parts, which are in the phase of
elongation when the apex of the leaf has long been completely developed : they
become, so to speak, pushed up from below out of the body of the plant, as may
very easily be observed in Hyacinths in the spring.

It will not be difficult for the reader to discover the points common to the two
diagrams described. These points in common, moreover, we may regard as the type
of growth prevailing in the vegetable kingdom. It exists almost without exception in
the Mosses and Vascular plants; and we find it in its simplest features in many
Algae and even in some Fungi. Nevertheless, as follows from what has been said
on p. 84, and may be illustrated by further examples {e. g. Macrocystis, &c.),
entirely different distributions of the phases of growth may occur among the Algae
and Fungi ; these, however, remain confined to a few small groups and will therefore
be passed over here.

After this preliminary sketch, I now proceed to a more exact description of the
three phases of growth.

(i) The condition of embryonic growth in the growing-points is distinguished
chiefly by the following characters : —

The development of new organs takes place exclusively in the growing-points.

The growth of the young organs at the growing-point, both with respect to the
increase in volume and the changes in form, is exceedingly slow.

The mass of a growing-point, from which the very large volume of tissue of an
entire shoot or root-system proceeds finally, is for and by itself extremely small, and
but very rarely amounts to j^ö^h of a milligram.

This small quantity of matter is continually being regenerated by the addition of
suitable nutritive substances ; while the basal portions of the growing-point gradually
pass over into the condition of elongation, and then into permanent tissue.

The growing-points, as well as their outgrowths — i. e. the young organs in
an embryonic condition — consist of a cell-tissue which is usually termed primary
meristem, but is better distinguished as embryonic tissue.

The embryonic tissue of the growing-point and very young organs forms a solid,
somewhat hard, and occasionally even brittle mass, which consists essentially only of
protoplasm and the substance of the cell nuclei; the cell-walls being extremely thin.
These substances are, it is true, permeated with water ; but fluid cell-sap in the form
of so-called vacuoles is not present, or only in very small quantity.

The cells of the embryonic tissue are very small, the nuclei, on the contrary,
being relatively very large, and their rounded form adapted to the shape of
the cells. If the latter are tabular, the nucleus is flattened like a cake ; if cubical,
the nucleus is spherical; if cylindrical or prismatic, the nucleus is elongated also.
The nucleus, however, always forms a very considerable portion of the mass of the
cell ; and thus the embryonic tissue is chiefly characterised by the predominance of
the substance of the nuclei, in which, again, nuclein plays a chief part.

E e 2

420

LECTURE XXVI.

The growing-points, as well as the young organs which are still in an embryonic
state, constitute a mass which is homogeneous in itself; the cells appearing as mere
chambers, the growth of which depends upon the growth of the growing-point as
a whole.

It is not quite correct to say, as is often done, that the cells in the growing-
points and in the young organs — in embryonic tissue generally — are perfectly
homogeneous among themselves. On the contrary, differences in form make
themselves evident even close to the apex of the growing-point, as well as in its
very young outgrowths ; and in the growing-points of roots especially, it is possible
to detect with certainty, even close behind the apex, those cells which will subse-
quently become developed as segments of vessels, or as endodermis, &c. (Fig. 257).
In the same manner, the external layer of the growing-point and every embryonic
organ is easily recognised as what will be the future epidermis. According to the
various form of plant in each case, however, this commencing differentiation of the
tissues in the growing-point is very different. The doctrine of the three so-called
histogenic layers, according to which, besides the future epidermis, a so-called

Fig. 256.— The growing-point of a shoot (winter-bud) of the Pine. 5 apex of the growing-point ;
l> h youngest leaf-rudiments; rr young cortex, and mni pith of the future shoot-axis (highly
magnified). The whole consists of embryonic tissue, of very small cells with relatively large nuclei.

periblem (young cortex) and a plerome are sketched out in the growing-point itself,
cannot therefore be applied universally; although the tissue differentiations referred to
often do occur, particularly in the growing-points of roots.

(2) The second phase of growth, the so-called condition of elongation, is
introduced -by the cells at the base of the growing-point beginning to grow more
rapidly, and to approach their definitive development. The following are the chief
points to be noted here : —

By means of the elongation, the segments of the shoot-axis, as well as the
leaves and the corresponding parts of the roots, attain their definitive size and their
permanent external form.

This process may continue for a longer or shorter time ; and, accordingly, the
elongating piece of shoot-axis or root-fibre is longer or shorter, and attains during the
elongation a more or less considerable breadth and thickness. The length of the
elongating portion in a root-fibre is about 3-10 millimetres behind the growing-point,

THE PHASE OF ELONGATION.

421

in long flower-stalks 10-20, or even 50-80 centimetres. It often happens, however,
that the growth in length generally, by elongation, is very insignificant, particularly
in shoot-axes; so that the fully developed leaf-bases completely cover the shoot-axes,
as in the Fern Asptdiwn filix-mas, and in all cases where so-called radical rosettes

Fig. 257.— Growing-point of a root of Maize (Zea Mays), a n older portion of root-cap ; i younger
part of same ; s apex of growing-point ; e e external layer of tissue of root— the cell-walls af^
thickened {vv); r r young cortex ;yy cells of axial vascular bundle ; g flattened cells of a large
vessel, which will subsequently form long cylindrical segments with bordered pits ; tn parenchyma
of the pith in the axial strand (highly magnified).

are formed by the leaves. If the points of insertion of the leaves are pushed apart,
the shoot-axis forms internodes or interfoliar parts between them.

By the mode of growth of shoot-axes during the phase of elongation, therefore,
a very material influence is exerted on the definitive form of the entire shoot ; in so
far that it depends upon this, for example, whether the leaves are situated close
above one another and form a rosette, or are removed some distance apart, and
arranged in whorls, or are situated alternately or scattered on the elongated stem .

422

LECTURE XXVI.

The leaves which have been developed at the growing-point of a shoot, generally
grow much more rapidly than the parts of the latter situated above their insertion on
the shoot-axis. In this lies the cause of the formation of buds at the end of the shoot
(Fig. 258).

In the leaves also various changes in form are brought about during the process
of elongation. The elongation in the leaves may either begin at the apex and end

at the base, or conversely; and in compli-
cated forms of leaves much more complex
distributions of growth may occur.

This phase of growth by elongation
effects also the most various changes of
form during the increase in volume, espe-
cially in the case of shoots. The entire
form of a mature shoot depends in general
essentially upon the processes of con-
figuration which take place during elon-
gation. Contrasted with the growing-point,
however, this phase of growth is distin-
guished by the fact that no more new
organs are developed : only those already
present obtain their definitive size and
form.

While the extremely slow growth of
the embryonic tissue takes place by means
of a proportionate addition of proteid sub-
stances and nuclein ; elongation, on the
contrary, consists in an enlargement of
the cells which is chiefly effected by the
addition of water. The cells which in the
embryonic condition are solid, and consist
merely of protoplasm and nucleus, change
during elongation into vesicles filled with
water, the volume of which may attain a hun-
dred or a thousandfold that of the original
embryonic cells : in this, however, no pro-
portional increase of the organic substance
takes place, but, in the main, the intercalation of water only. This, however,
does not preclude that the still thin cell-walls also have cellulose deposited in them,
and that the protoplasm may increase slightly in quantity. The enormous inter-
calation of water may, however, go so far that shoot-axes, leaves and root-fibres just
fully grown contain more than 90 per cent., or even more than 95 per cent, of their
weight and volume of water.

During this active increase in volume of the cells, especially at first, numerous
additional cell-divisions take place : subsequently, however, these become rarer, and
gradually cease with the cessation of elongation.

The elongation commences with the differentiation of the primitively homo-

Fig. ■2-:ß.—F,quisetuin arvensc. Longitudinal
subterranean bud in March, s s apical cell of sten
leaves ; K K' two lateral buds exposed by the sectior
subsequently very long intemodes of the shoot-axis are
formed (slightly magnified).

b—gb the

ion. The

yet

THE PHASE OF ELONGATION.

423

■geneous embryonic tissue into the various systems of tissues. Intercellular spaces
soon appear between the cells of the parenchyma ; the different kinds of cells
gradually obtain their characteristic shapes; the cells, at first only parts of the
whole in the growing-point, now become more individualised, peculiar processes of
growth making themselves evident in each one ; the size, external form, and the
differentiation of the contents of each cell are apparently independent of those
of the others — but of course only apparently. The various forms of tissues become
differentiated; the single cells
individualised.

At the time when the
organs, or parts of organs,
have attained their external
form and definitive volume by
elongation, they are still imma-
ture— not yet fully developed
internally. It is true that
the differentiation of the forms
of tissues begins even in the
embryonic tissue of the grow-
ing-point, and continues during
the elongation, so that, at the
end of the latter process, the
epidermis, vascular bundles,
and the various parts of these,
as well as the different forms
of fundamental tissue, are
already to be plainly distin-
guished ; but the develop-
ment, especially of the cell-
walls, is not completed in this
state.

(3) The third phase of
growth makes its appearance
as soon as the organs and their
parts have attained their per-
manent size and external form
by means of elongation, while,
however, still further changes
are taking place in the interior

of the tissue. The lignification in the vascular bundles, the thickening of the
cell-walls, and the various kinds of pitting connected with it ; the development of
the sieve-tubes, the lignification of the sclerenchymatous elements; and more parti-
cularly the further development of the epidermis, especially the stomata, cuticularisa-
tion and perhaps silicification, and the development of hairs, now first attain
completion. The internal development of the tissues, which only occurs late and
after elongation is accomplished, is particularly striking in the case of the roots.

FIG. 259.— Parenchyma cells from the median layer of the cortex of the root of
Fritaiaria impsrialis (longitudinal sections X sso). A very young cells situated
close above the apex of the root, still without cell-sap. B the same cells about
z mm. above the apex ; the cell sap J forms isolated drops (vacuoles) in the proto-
plasm /, plates of protoplasm lying between them. C the same cells about
7—8 mm. above the apex ; the two cells to the right below are seen from in front,
the large cell to the left below in optical section; the cell to the right above has
been opened by the section, and its nucleus is alTectod by the penetrating water
and exhibits a peculiar swollen appearance.

424

LECTURE XXVI.

In those of the Grasses and Liliacece (e. g. Maize, Fritillaria, &c.) elongation is
completed about 8-10 mm. behind the growing-point; but even 50-60 mm. further
back the bordered pits of the large vessels are found to be only just fully developed,
and similarly with other tissue-structures. The internal development and strengthen-
ing of the tissues, which continue for some time after the completion of elongation,
are also plainly to be recognised from the exterior of the organs. Leaves and
shoot-axes which have just finished elongating are delicate, watery, easily torn, and
soon droop ; whereas they subsequently become much stronger, richer in substance,
and more solid.

It would be superfluous to formulate the characteristics of this third stage of
growth, since all that has been said in the lectures on Organography and on the
theory of the tissues, with regard to the external form and internal structure of the
organs of the plant, concerns this last developmental stage. Only one point further

need be mentioned here. With the exist-
^ ence of the cambium in the shoot-axes

and older roots of the Conifers and
woody Dicotyledons, of course a new
process of growth is ushered in, and the
cambium corresponds in many respects
to the embryonic tissue of the growing-
point. It is distinguished from this latter,
however, in that, normally, it only produces
new masses of tissue, but not new organs :
nevertheless the latter process also may
take place under certain circumstances.
In the case of cut-off woody stems, for
example, the cambium swells, grows out,
and forms a so-called callus — i.e. a cushion
consisting of soft tissue — in which growing-
points for the development of new shoots
are developed.

The three phases of growth here dis-
tinguished usually pass into one another without interruption, so that no definite
line can be drawn between the embryonic condition and that of elongation, or
between the latter and that in which the internal development is being completed.
The relation between them is very simple in the roots; whereas the most various
complications may occur in shoots, according to the nature of the plant in each case.
The growing-point of a shoot is only apparently merely the end of the shoot-
axis, as it is generally held to be. The manner in which the leaves and lateral
shoots are developed from the growing-point of a shoot shows, on the contrary, that
the latter is not merely the termination of the shoot-axis, but is the embryonic be-
ginning of the entire shoot, which, in its turn, consists of shoot-axis and leaves.

Just as new growing-points of shoots can arise from growing-points of shoots, so
also on a young leaf, so long as it still consists of embryonic tissue, a large number
of secondary growing-points may arise, which may then produce tertiary growing-
points, and even those of a higher order, as shown in Fig. 261. Hence the sub-

FIG. 260.— Longitudinal section through the apex of an
erect shoot of Hippuris ■vulgaris, s apex of stem ; bdd the
leaves (in whorls); ii axillary buds of the latter and which
all become flowers ; £-—g the first vessels. The dark parts of the
tissue indicate the internal cortex with its intercellular spaces.

THIRD PHASE OF GROWTH.

425

sequent external form and segmentation of a leaf becomes sketched out, and this then,
according to the mode and course of elongation in each case, becomes a branched,
divided, pinnate, lobed, or simply a toothed leaf. In the great majority of cases the
whole of the embryonic tissue of a leaf is finally transformed by elongation into
permanent tissue : the whole leaf is then completely developed. Nevertheless, cases
also occur where the apex of the leaf persists for a long time in the embryonic
condition, while its basal portions become elongated and fully developed. This
happens even in the case of some large pinnate leaves of Dicotyledons — e.g. At7an-
thus, Robinia, &c. : after some time, however, the growing-point of the leaf in these
cases becomes transformed wholly into permanent tissue. In some Ferns, on the
other hand {Nephrolepis, Gleichema, Mertensia, Lygodium), the ends of the leaves, or
of their lateral segments, remain in the em-
bryonic condition for years, so that a con-
tinual or periodically repeated lengthening
of the organ takes place by elongation just
as in perennial shoots.

The growing-points of shoots, how-
ever, may also, on the contrary, become
wholly transformed into permanent tissue,
when they of course cease to be growing-
points : this is the case, for example, in the
formation of the thorns of Rhamnus ca-
tharUca, GleditscMa,2LX\A others. In the latter
instance they are branched shoots, which
only produce very small leaflets, and con-
sist at first of very delicate tissue, which
subsequently, however, lignifies to masses
hard as stone : the points of these thorns
are the previous growing-points, which
are now likewise lignified.

All that has been said so far refers to
the relations of growth in the Vascular
plants and the majority of the Muscineae : c
as already pointed out above, the same °™^''"
processes of growth may also be recog-
nised even in the Algae and Fungi, though often only in a rudimentary form and
taking a much simpler course. Besides these, however, as was stated in the lectures
on Organography, entirely diff'erent types of growth also occur. These may both
be illustrated by a few examples.

One of the most remarkable cases is found in those Algce the growth of
which is not accompanied by cell-divisions, and among which the Caiilcrpa
in Fig. 262 may serve as an example. The figure shows at once that we
have here to do with a plant the whole vegetative body of which is moulded on,
and therefore grows according to, exactly the same plan as that of a creeping
vascular plant. The creeping shoot-axis v s produces branched roots on its
lower side, and leaves containing chlorophyll on its upper side. As in the

lit; leaves of Pastiiiacea :
T/t-,y pinna; ; 1^ pinnae of:

426

LECTURE XXVI.

case of a creeping phanerogamous plant, the roots as well as the leaves
arise from the advancing apex — the growing-point — which is here projecting,
however, so that the youngest leaf and the youngest root are somewhat distant
from the apex : hence no proper bud enveloping the growing-point exists at all.
What is most important for us, however, is the fact that no cell-formation
whatever takes place in the interior of this plant: no cell-walls exist either in
the growing-point or in the completely developed organs. In the cavity of
the thick-walled vescicle, however, the ramifications of which constitute the
entire plant, numerous bars consisting of cellulose are found, which traverse the
lumen of the cell like pillars and rafters, in order to confer greater firmness
on the whole structure : in more slender forms of Cceloblastese, however, as

FIG. it-i.—CanUrfa .
vided into chambers or

'■nssi/olux. The entire plant consists of a vesicle which is not
:ells. i' the growing-point of the creeping dorsi-ventral shoot-
; (nat. size).

in the common Vaucheria, even these are wanting. It is now only necessary,
therefore, to leave out from what has been said above as to the difference of the
three phases of growth all that refers to the cellular structure, which does not exist
here, and all the rest applies even to this non-cellular plant.

A second example may be taken to show the corresponding condition of affairs
in one of the more highly organised Fungi. The accompanying figure (Fig. 263) shows
the development of a Mushroom of the genus Agaricus. At / is represented a small
piece of the myceHum (ot), the thicker filaments of which consist of numerous hyphse
running parallel to one another. The mycelium behaves in all essential points like the
branched root- system of a higher plant ; at the end of each filament there is a growing-
point from which new growing-points are developed as buds. At certain points of this
mycelium, at a and b, organs of reproduction are formed, one of which is represented

GROWTH OF FUNGI.

427

somewhat enlarged at c, to the right above : the apex of this consists of a growing-
point, while the lower portion is already in process of elongation. The somewhat
older fructification represented at II
and /// is commencing to form the
umbrella-like expansion {pikus) at its
apex, the elongation at the top ceasing
meanwhile; hence the mass of tissue
becomes extended radially all round
the growing-point, as shown still fur-
ther developed at IV. The embryonic
character of the fungal tissue now dis-
appears at the apex and on the upper
side generally of the developing pileus,
but is maintained for some time longer on
its lower surface, from which very nu-
merous thin lamellae now grow out, radi-
ating from the stem to the periphery 01
the pileus. From these the reproductive
organs are developed. It is not diffi-
cult to recognise in the processes ot
growth briefly indicated in Agaricus,
relations similar in the main to those
which we meet with in the more highly
organised plants, which is in this case
so much the more remarkable since the
mycelium and the fructification of the
Fungus consists of single filaments or
hyphos, each individual one of which
represents as it were an independent
growing plant, in which growing-points
and the various phases of growth are to
be observed, as in the Mould-fungi.
In the mycelium and fructification of
the present Fungus, however, hundreds
and thousands of such hyphse are com-
bined in such a manner that all their
growing-points become united in the
growing-point of a branch of the my-
celium, or that of an organ of repro-
duction.

It would not be difficult to find
dozens of other cases among the Fungi
andAlgse in which the typical processes of
growth still make their appearance^ it is

true, here and there, but in which they yet are replaced more or less by widely different
arrangements. As a particularly striking example in this direction, I may mention one

Fig. ■zS^.—Agaytcus variecolor. I Mycelium
( w) with young fructification a and * (nat. size) ; c one
of the latter in longitudinal section (magii.). // older
fructification, the pileus is beginning to be formed.
/// the same in longitudinal section. IV the pileus
further developed ; -v velum. The lines in the longi-
tudinal sections indicate the course of the hypliee.

tical section of a young fructification
npudiciis (cf. the text— i nat. size).

428

LECTURE XXVI.

of our most remarkable Gastromycetes, Phallus impudicus, the mycelium of which,
consisting of thick white threads growing in the soil of forests, presents essentially the
same appearance as that of the Agaric considered above. The fructification of the
Phallus arises as a spheroidal protuberance, composed of interwoven hyphae, on
one of the superficial branches of the mycelium. This protuberance while young
may be looked upon as a growing-point, the mass of tissue of which, however, while
growing up to the size of a hen's &%g, undergoes a differentiation quite other than in the
cases hitherto considered ; for although here also the polarity existing between the base
and apex of the structure is not entirely given up, still that process of growth which
may be compared with the elongation of a normal plant affects the fungal tissue, which
has passed out of the embryonic stage, in such a way that concentric layers of tissue
are differentiated. This occurs chiefly as follows : the most external layer, a, and
one lying deeper, ?', attain considerable solidity, while the layer g situated between
them becomes transformed into a soft deliquescent jelly. The portion of tissue

Fig. ■zb'^.—Pediastriim gra7iulatum, after A. Braun (400). A a disc consisting of cells which have grown together.
t^K. g the innermost membrane of a cell is just emerging : it contains the daughter-cells produced by the division of the
green protoplasm ; t various stages of division of the cells ; s p slits in the walls of cells already emptied. B the entirely
emerged inner lamella of the wall of the mother-cell (*) much swollen and containing the daughter-cells^, wnich are
actively swarming. C the same cell-family \\ hours after emission, and 4 hours after the coming to rest of the small
cells— these have arranged themselves into a disc which is already beginning to develope into one like A.

marked st forms a hollow^ cylinder, the apex of which {x) is in connection with the
firm membrane /: in its interior, h, is a hollow cavity. The mass marked sp con-
sists of the spores or reproductive cells of the Fungus. When this complicated
differentiation of tissue (reminding one of the formation of certain berry-like or
plum-like fruits of Phanerogams) is completed, a final act of growth suddenly takes
place in damp weather. The stalk st now becomes extended in a few hours to
a length of 20-30 cm., its diameter being 3-4 cm.: by this the membrane a is
ruptured at the apex, while the membrane /, together with the mass of spores sp,
remain pendent from the top of the elongated stalk, and the base of the latter is
surrounded by the ruptured layers a, g and «, much as if by a broken egg-shell.
The deviations from the normal type of growth prevailing in the vegetable kingdom
go still further in some other Gastromycetes, as in the genera Clathrtis, Geasler, and
Crucibulian (cf. Fig. 162).

But even in the Algae, the majority of which follow the type of growth previously

GROWTH OF ALGJE AND MYXOMYCETES.

429

described, various families are found with very considerable deviations from it. In
the lectures on Organography, I have already referred in detail to the genus
Laminaria, belonging to the subdivision Phaeosporeaj. In La?nmaria, the inter-
calary growing-point of the shoot adds to the length of the shoot-axis downwards,
producing each year a new frond, on the contrary, upwards ; the new frond is thus
interposed between the old frond and the intercalary growing-point.

Referring to p. 70 for further details, I may now make a few remarks on the Algae
in the subdivisions Hydrodictyeae and Volvocineae, where a totally different arrange-
ment in space of the embryonic and later stages of growth takes place, though even here
the sequence with respect to time is still maintained. One of the simplest examples in
this connection is afforded by Pediastriim,
a plant common in our w'aters and the
development of which is illustrated in Fig.
265. The mature plant A consists of a
flat disc, the cells of which are arranged
in concentric circles. In the process of
reproduction the contents of the cells break
up into a large number of small cells,
which, enveloped in a vesicle {B), escape
into the water and there swarm for some
time with a trembling movement. The
young cells, which taken all together have
to be regarded as the embryonic condition
of a new plant, then arrange themselves
spontaneously in the form of a disc, made
up of concentric circles ; and as soon as
this has taken place, as in C, they all
begin to grow, and the very young plant is
now in the stage of elongation. Some
little time before the end of this phase,
however, the third phase of growth — the
completion of development of the cell-
contents and cell-walls — commences, until the stage A is again reached.

Furthest removed from all other modes of growth is that of the Myxo-
mycetes. These consist, in their first period of life — that in which nutrition
occurs — of naked motile masses of protoplasm, creeping forth from the nutritive
substratum as a so-called plasmodium. Such a plasmodium, according to its
specific nature in each case, may assume the most various, and often extraordinarily
beautiful forms. Not rarely it developes at last into a stem, situated on the sub-
stratum, and passing above into a clavate, spherical, or umbrella-like expansion, as in
Fig. 266. So long as this process of construction continues, the entire plant con-
sists of soft protoplasm, and in this condition the Myxomycete may be compared,
to a certain extent, with the embryonic condition of a higher plant, although even
this comparison fails in some respects. Only when the external form is completed
does the outer layer of protoplasm harden to a firm membrane, while its inner
portions develope tubular filaments of the most various forms, the so-called capillitium :

Fig. i(A.—DidyiKium/erinoceum (a Myxomycete) in fruc-
tification : only the solid network produced from the Plasmo-
dium is represented (after Rostafinski).

430 LECTURE XXVI.

the remainder of the protoplasm, that which is capabje of true development and
which is contained between the capillitium, breaks up into innumerable, minute,
rounded portions, which constitute the reproductive cells or spores. Thus we have
here processes of growth where even the last trace of accompanying cell-formation
has disappeared. While in the Coeloblastse the entire plant, if we wish to extend the
comparison to the utmost, may be regarded as a single cell, since it is enveloped by
a firm cellulose membrane, this is no longer possible in the case of the Myxomycetes,
although, as we have seen, they develope definite forms and grow. In this case, an
extremely simple form of cell development appears only when the growth is con-
cluded; this, however, has nothing more to do with the growth itself.

It is important to refer to this point, since for a long time the utterly mistaken
view was held, that the whole configuration and increase in volume of a plant
may be explained from the life of its individual cells. Such is evidently not the case :
just as the growth of the whole plant and of one of its entire organs, so also that of
its individual cells results from general laws of configuration, which dominate organic
quite in the same way as inorganic material.

LECTURE XXVII.

RELATIONS BETWEEN GROWTH AND CELL-DIVISION IN THE
EMBRYONIC TISSUES.

Growth — i.e. the increase in volume and cliange of form— may take
place in plants even without accompanying cell-divisions. In this connection,
I have already repeatedly referred to the non-cellular plants, such as Botrydium,
Caulerpa, Vaticheria, etc., and particularly to the Myxomycetes. It is important
to bear this fact in mind ; because it proves that the formation of cells is a pheno-
menon subordinate to, and independent of, growth. The excessive importance for
organic life hitherto ascribed to cell-formation found expression in this direction also,
in that it was believed that growth depended upon the formation of cells. This is,
however, not the case. On the other hand, however, the fact is of course important,
that while a few hundred simple forms of plants exist in which growth is not accom-
panied by cell-division, in all other plants growth and cell-division are intimately
connected with one another. In attempting, then, to make clear the relations of
the two processes — growth and cell-division — it is above all to be insisted upon
that growth is the primary, and cell-division the secondary and independent
phenomenon.

The following is an epitome of a detailed investigation of the matter which I
published in 1878-79^ The matter here depends upon geometrical, and in part

* We owe the first investigations which laid the foundations as to the relations between growth
and cell-division, as so much of the best of our literature, to Naegeli's ingenious researches ; he
started with a series of treatises on this subject in his ' Zeitschrift für wissenschaftliche Botanik,''
published with Schieiden, in 1844-1S46, a literature consolidated by numerous observers in still more
numerous treatises. All Naegeli's successors, however, held exactly to the scheme established by him,
according to which also Schwendener, with Naegeli's co-operation, further represented the relations
between growth and cell-division in their book ^ Das Mikroskope II. Aufl. 1877, pp. 544, &c.

I established the point of view explained in my lecture, and which has given an entirely different
turn to the ideas concerning this subject, in my treatises, ' Über die Anordnung der Zellen in jüngsten
Pflanzentheilcn' (Arb. des bot. Inst, in Wzbg. B. II, H. i, 1878) and ' Über Zellenanordntmg und
IVachsthum' (ibid. H. 2, 1879).

In the first-named treatise I sought to give precision to my view as to the processes of growth in
growing-points and other embryonic masses of tissue by stating (p. 52), 'If we abstract from the
so-called individuality of the cell, and pay attention only to the course of the laj'ers which cross one
another in three directions, we obtain a structure which may be compared with the internal structure
of a much thickened cell-wall. The three systems of layers in tlie growing-point correspond to the
system of concentric layers and the two systems of so-called striation of the cell-wall, as they have
been described by Naegeli. Stratification and striation of the cell-wall, as is known, depend on a
regular alternation of denser and less dense substance in three directions, which cut one another, as

432

LECTURE XXVII .

purely mechanical, considerations, which can only be rendered quite clear by very
careful thinking ; since, however, I have set myself the task in these lectures of only
expounding what is immediately capable of comprehension, I must confine myself to
making clear a series of the most elementary and easily intelligible facts.

Above all, it is important to point out that, apart from a few exceptions, the
directions in which the new cell-walls of a growing plant-organ appear, depend upon
the internal distribution of growth as well as upon the external form of the growing
organ : the direction of any newly formed division-wall whatever is determined in
advance by these factors. Sections through growing, and especially through young
parts of the plant, always show arrangements of the cells which are quite definite, and
in the highest degree characteristic ; the directions of the cell-divisions are by no
means accidental, and an observer sufficiently acquainted with geometrical and
mechanical science at once recognises in the structures presented by the totality of
cell-walls within an organ, cut in the proper manner, that we have here to do with
a conformity to law, the true meaning of which, however, is difficult to decipher.
It will be well, therefore, in the first place to illustrate the dependence between
growth and cell-division by a few examples of the simplest kind.

The simplest case is presented by thin, filamentous organs, which consist of a
single row of cells, as in the case of many Algae and in the hyphse of Fungi. As a
rule, the cell-walls here occur as transverse septa of the filament — i.e. each new
division-wall cuts the long axis and the circumference of the filament at right angles.
Nevertheless a few exceptions are found even here. In the root-filaments of the
true Mosses, as well as of the Characeae, the transverse septa are oblique to the
long axis, for which in the meantime no satisfactory explanation can be given.
However, in contrast to the enormously large number of cases in which cell-
filaments are divided by transverse walls cutting at right angles, these are but rare
exceptions.

When we take into consideration the growth and cell-division of isolated cells the
volume of which becomes enlarged in all directions, the problem is a more com-
plicated one. We meet with such, for instance, in a large number of simple Algae,

Naegeli aptly remarks, like the three cleavage planes of a crystal. By means of stratification and
striation the substance of a cell-wall is cut up into polyhedral areola, so that the three systems of
densest layers form a network, in the meshes of which the less dense areolae (containing most water)
are enclosed. The substance of a thick cell-wall is built up like the primary meristem of a growing-
point. The cell-walls mutually cutting in three directions correspond to the densest lamellae of
a thick cell-wall, and the protoplasmic bodies of the cells of the primary meristem to the soft areolae.
I will not here follow this comparison (which is without constraint) further, but only remark that
it becomes the more apt the smaller the cells of the primary meristem are.'

Two years after this statement of mine a treatise by Schwendener appeared, ' Über die durch
Wachstimm bedi7igte Verschiebung kleinster Theilehen iti trajektorischcii Curvcn'' (Monatsber. der kgl.
Akad. d. Wiss. zu Berlin, iS8o), where he entirely adopts the new points of view which I had opened
out; only that (p. 426) he believes that he finds an important difference between his own view and mine.
He says, ' According to my (Schwendener's) view the cell-divisions constitute an independent pheno-
menon— and the formation of series of trajectories is everywhere ruled by the same mechanical prin-
ciples which condition the direction of the rows of micella; in starch-grains and thick cell-walls,' &c.

I consider that the quotation from my treatise, which had appeared two years previously, differs
from Schwendener's view just cited only in so far that it puts exactly the same ideas more in detail
and more clearly, and at the same time directly contradicts Schwendener's older views (1877) on the.
relation of growth and cell-division.

DEPENDENCE OF DIVISION UPON THE FORM OF THE CELL.

433

the individual cells of which grow independently after each division ; as in the
genera Chlorococcus, Merismopedia, Telraspora, Glceocapsa, and many others. In
more highly developed plants similar processes take place in the mother-cells of
spores and pollen grains. In these cases the fact is particularly clear, that the
mode in which the cell-divisions follow one another depends by no means on the
physiological or morphological nature of the cells, but upon their mode of growth
and external form — especially upon the latter. Fig. 267, for example, shows six
different forms of cell-division in the pollen mother-cells of one and the same
plant, the Orchid Neolh'a tiidus-avi's. In A the pollen mother-cell had approximately
the form of a circular disc, which, by means of two divisions at right angles to
one another, is cut up into four quadrants. It is to be noticed, however, that the
two vertical division-walls in Fig. A do not exactly meet one another, so that
a small piece of the horizontal division-wall remains intercalated between the points
where they join it : this intercalated piece of the previous division-wall appears like
a breaking of it, and we meet with such interruptions of the walls very generally
in the division of tissue-cells, a point on
which I lay some stress, because in the
more complex cases of tissue-formation
they interfere with the otherwise easily
recognisable regularity of the network of
cell-walls.

The pollen mother-cell C had grown
more vigorously in one direction, and had
become, long and elliptical before its divi-
sions. Accordingly, in the first place two
transverse walls were formed at right
angles to the long axis of the cell, so that
they divided the latter into three approxi-
mately equal parts : the middle one of these
cells has again been divided, however, by a
longitudinal wall. Here again, therefore,

four daughter-cells have been produced by division of the pollen mother-cell, but, as
is seen, in an order different from that in A, corresponding to the elongated form
which the mother-cell had assumed before the division. Whether the form of
division represented in Fig. B was developed from the type A or C cannot well
be decided ; but in B as well as in C we perceive how the primitively straight or
slightly curved walls appear broken after the joining on of a younger wall, somewhat
like stretched threads strung together by another thread. The division-walls which
meet one another correspond with their three angles to a so-called string-polygon, as
also is usually the case in a multicellular tissue.

The dependence of the directions of division upon the external form of the
mother-cell is manifested again in another way in Fig. D. Here the mother-cell
consisted of a sort of cylindrical stalk with a little spherical head : accordingly, two
walls have arisen in the stalk-like portion cutting the external wall at right angles ;
in the hemispherical head, however, the wall is longitudinal. In Fig. E, the moiher-
cell was approximately globular before the division : it is bisected into two hemi-

[3] Pf

Fig. 267.— Differently shaped and therefore differently
divided pollen mother-cells from one and the same anthL-r of
Neottia itidits-avis (after Goebel).

434

LECTURE XXV IT.

FIG. 268.— The

spherical cell divided

spheres by a division wall. Of these, the one has become elongated in a vertical,
and the other in a horizontal direction ; each of the two halves, however, is
divided by a cross-wall produced at right angles to the long axis of the secondary
cell, and thus have arisen two pairs of cells in a crossed position. The mother-cell
F was probably more completely spherical than the previous one, and, accordingly

also, the form of division is different. The
mother-cell has here become divided in a
tetrahedral manner, as we are accustomed
to say — i.e. six division-walls have been
produced simultaneously, in such a way that
the spherical mother-cell has become cut
up into four daughter-cells, each of which
would resemble a tetrahedon if the outer
wall were plane instead of arched out-
wards. The position of these division
walls is illustrated in Fig. 268, and it may
be recognised at once, that in this case
the six walls simultaneously produced do not cut one another at right angles. It
may be shown, however, that this case can be brought with that of the ordinary
rectangular cutting of the succeeding division-walls under a common general
expression.

In all these various cases of the division of a mother-cell, the rule, also common
elsewhere, makes itself evident, that the daughter-cells which arise simultaneously
are equal in volume to one another — i.e. the division consists in a halving of the
mass of the mother-cell. This rule also of course suffers various exceptions in

particular cases : otherwise, however, it is
so comprehensive that it may always be
laid down as the ordinary case.

As the general rules of successive cell-
division in growing organs, therefore, we
can already state (i) that the daughter-
cells are usually equal to one another in
volume, and (2) that the new cell-walls are
situated at right angles to those already
present.

Simple though these two rules may ap-
pear, it is nevertheless difficult to prove
their validity in many instances, according
to the form of the mother-cell in each case.
For example, when the dividing-cell has
the form illustrated in Fig. 269 — i.e. the shape of a tetrahedron with curved
surfaces — and if the successive divisions in it halve the volume each time, and in
doing so stand at right angles on the preceding walls, a structure is obtained like
that shown in the figure, from which the existence of the relations above mentioned
can only be determined by careful geometrical considerations.

That the mode of cell-division depends only upon the increase in volume and

Fig. 269.— Diagram of a tetrahedral apical cell abcd& met
with in Equisetum and elsewhere, seen from above, de, fs,
h k, the walls of three successive divisions ; i the corner where
the three walls cut one another like the sides of a cube.

CELL-DIVISIONS INDEPENDENT OF THE NATURE OF THE ORGAN.

435

the configuration of a growing organ, and not ujjon its morphological or physio-
logical significance, is one of the most general and important facts to which I have
referred in my treatises already quoted. It was formerly supposed to be possible
to characterise the true morphological or phylogenetic nature of an organ by the
way in which its cell-divisions took place, and hundreds of treatises and laboriously
drawn plates were devoted to this purpose. In the exposition which follows, I shall
have plenty of opportunities of showing how erroneous this assumption is. Mean-
while, however, I will refer to a particularly clear case (Fig. 270). It is at once
obvious that Figs. C and D exhibit essentially the same laws of cell-division : in both
cases we have a stalked capitulum, with transverse divisions in the stalk, and in the
head itself transverse and longitudinal divisions which cut one another at right
angles. But C is a glandular hair of the
Gourd plant, and D a very common form
of the embryo of a Phanerogam. In Fig.
A also we at once recognise the type
of cell-division represented in C and D, in
accordance with the outline of the simple
organ ; but here the stalk remains undi-
vided, having only formed the cell marked
/i {Hypophysis). Above this the capitu-
lum becomes divided into sectors, which,
as shown in B, grow in breadth, and so
give to the whole glandular hair the form
of a mushroom. The comparison of the
most various organs of the most different
nature would only afford further examples
of what has been stated above — that the
form of the cell-division depends entirely
upon the growth and outward form (espe-
cially the latter), and not upon its physio-
logical and morphological significance.

Among the most instructive relations
between cell-division and growth I count
also the fact that the cell-division need not
take place during growth itself, but may

appear only after its conclusion. In this connection, a particularly clear case has
been established by Geyler in the shoots of Slypocaulon, one of the Algae belonging
to the Phseosporeae (Fig. 271). The whole of the apical region of such a shoot,
marked s and z, as well as the region of the lateral shoots marked x and y,
corresponds not only to the growing-point of an ordinary cellular shoot, but also to
the portion which is elongating. Only the parts which have grown to their full
extent, and marked I, II, etc., become gradually converted by division-walls into
smaller and smaller cells, as shown in Fig. 271 ; and Geyler has established that in
the course of these cell-divisions no more growth takes place. We have therefore,
in this case, at the upper end of the shoot growth without cell-divisions, and in the
older portions of the shoot cell-divisions without growth.

F f 2

Fig. 270.-^4, B hairs of different ages from the leaf of
Piiigtticula VKtgaris ; C hair of Cucurbita ; D embryo of Nico-
tiana (tobacco) ; h the so-called Hypophysis.

^.^Ö LECTURE XXVII.

On reo-arding a transverse section of an organ properly taken in accordance
with its relations of symmetry, a characteristic pattern is as a rule presented.
The individual cells lie by no means without order among themselves, but exhibit
definite groupings and arrangements of the most various kinds, and, on a little
reflection, certain arrangements are always visible, which are repeated even in
the most different organs. It is beyond all doubt that these net-works of cells, as
the pattern presented to the observer may most simply be termed, are an immediate
expression of the processes of growth prevailing in the parts of the plant concerned ;

FIG. 271.— A branch of Stypocaulon scoparium
with two smaller branches x and y, and the rudiment
of a third z (after Geyler). All the lines indicate
cell- walls.

Fig. 27s.— Transverse section of a root of the Nettle (Urtica), g the
group (horizontal in the figure) of five vessels which existed at a very early
stage; the other vessels and lignified cells form a double group (vertical in the
figure), c cambium from which the latter and the radially arranged tissue
have arisen. Externally is a layer of cork, beneath which lies a layer of
irregular tissue displaced by the growth in thickness (after De Bary).

and this in a manner exactly similar to what was explained previously by a few very
simple examples, only that in organs which are composed of very numerous cells
the relation between growth and the arrangement of the cells may often be not so
easy to discover.

For the preliminary guidance of the reader the following three statements may
here be made in the first place.

(i) The form of the pattern presented by a network of cells depends chiefly
upon whether, after the successive cell-divisions, vigorous growth of single cells or

GROUPINGS OF CELLS.

437

groups or layers of cells (it may be with subsequent cell-divisions) takes place. In
this case individual cells or groups or layers of cells may grow diflferently, their
volume and form altering in a diflferent way from the remainder, and in this case the
one must necessarily have a definite effect. on the other: mechanical influences of
single cells or groups on those surrounding them must take place; tensions and
pressures must make themselves felt, and only an exact investigation and con-
sideration of the prevailing circumstances can give information as to what
mechanical processes have co-operated in influencing the arrangement of the
cells in the organ. We meet with this case wherever we are concerned with
the external and internal development, and therefore the later phases of growth
of the organs \

(2) The processes are otherwise so long as an organ is still entirely in the
embryonic condition. In this case experience teaches that the entire mass of
embryonic tissue grows as a whole ^, that definite geometrical and mechanical
relations exist between the whole arrangement of cells and the outward form of
the growing organ. Only when the tissue passes from the embryonic condition
into the second phase of growth, does the grouping and differentiation in the
interior of the organ mentioned under (i) begin, and at the same time the pattern
of the cell arrangement becomes essentially different.

(3) The fact that organs of like external form may nevertheless present quite
diflferent patterns of cell net-works in their interior, causes peculiar difificulties in the
understanding of these processes. In other words, the distribution of the processes
of growth in the interior of organs which possess like external form may be very
different.

In our further considerations we will confine ourselves, in the first place, to the
relations presented in the embryonic tissue, and further, to such cases where the
arrangement of the cells of the embryonic tissue are no longer essentially altered by
subsequent processes of growth, as the latter occurs especially in the development
of the woody mass from the cambium, and in the growth of various plants of simple
organisation.

Fig. 273 represents the view from above of a flat extended A\gz {Melobesta),
and for the sake of simplicity we may assume that the entire body of the Alga
consists of a simple layer of cells only (which, as a matter of fact, however, is not
the case). It is easily recognised that the portion of the disc, expanded to a certain
extent like a fan, has been produced by a further growth outwards having taken
place on the one side of the primitively elliptical germinal disc. The cell-network in
this case comes into existence by the flat extended plant-substance having become
divided into small areolae — i. e. cells — by two systems of lines, which correspond to
the cell-walls. Of these systems of lines, or directions of cell-walls, the one series
radiate in a fan-like manner towards the periphery of the body of the plant, which

' I shall refer to this point more in detail in Lecture XXXIII.

" Cf. note I, p. 431. With reference to a statement of Pfeffer in his ' Pßaiizen-physiologk,"
II. pp. 97, 98, I need only remark that my quoted treatise ' Über Zelleitanordiiuug und Wachstlmm '
was only possible from my regarding the cell-wall network as a necessary consequence of
' lVachsthii!>isl>c7vcgting;'' this had not been done before, apart from Hofmeister, with reference
to whose view I have remarked all that is necessary.

438

LECTURE XXVII.

they cut more or less exactly at right angles, and as they become further distant
from one another in this course towards the margin new lines of cell-walls are
continually being intercalated between them. The fan-shaped structure thus
produced is shown particularly clearly on the right side of the figure; and it is
noticed that the very various curvings of the radiating fan-like walls stand in
definite relation to the curving of the line bounding the circumference which is,
as a rule, cut by them at right angles.

Besides this, however, the figure shows, especially on the left side, a larger
number of lines — i.e. cell-walls — which run parallel with the periphery, or, to put
the fact better, conformably with it, and in the main these lines cut the previous
system of fan-like walls at right angles. Geometrically expressed, the two systems of
lines which divide up the surface of the body of the Alga into areolae or cells of
nearly equal size are orthogonal trajectories. The regular pattern of this cellular
structure comes into existence essentially from the fact that only the cells at the

Fig. 2Ti.—Melobcsia Lejoltsii {an Alga of the subdivision Floride«) seen from the
upper surface (after Rosanofl).

external margin of the surface grow radially outwards, and become divided now and
then in the tangential direction. Cell-divisions thus take place only at the margin,
and the cells further distant from it grow no more, either in the radial or in the
tangential direction. Were the latter the case, the cell-network could only maintain
its form by a perfectly equal distribution of the growth : in every other case, however,
it must be altered.

Attention may here be drawn by the way to the further fact that the fan-like
radiating walls have all been drawn, even on the right side of the figure, though in
this portion the majority of the walls running tangentially have been omitted. It is
seen that in this way there arises a cell-network different from that on the left side ;
though the difference only depends on a minor point, viz. that the tangential
division-walls exist in smaller numbers than on the left side, and we could easily
quote examples of other Algse in which the whole cell-network resembles that
represented on the right side of the figure. In such a case, however, the body of

PERICLINES J ANTICLINES ; TRAJECTORIES.

439

the plant gives the impression of having been produced from dichotomously branched
cell filaments, whereas this subjective impression is not produced by the left side of
the figure — unless by peculiar modes of thinking.

From reasons which will become clear in the further course of the exposition,
I distinguish all those cell-walls, or directions of cell-walls, which run conformably
with the circumference of the part of the plant under consideration, as Periclines,
in contrast to the Anticlines which are directed towards the circumference, or actually
cut it, and these terms are now introduced generally into botanical phraseology.
The periclines of our figure are therefore, according to what has been said above,
usually orthogonal trajectories of the anticlines.

Without too great theoretical difficulties, now, we may suppose the above figure
to be the transverse section of an irregularly grown woody body. In this case the rows
of cells lying at the margin, and alone
bringing about the growth in surface,
would correspond to the wood-cam-
bium. The cells proceeding from the
peripheral layer would then be de-
veloped as wood-cells. As in the case
of the Alga, so also the wood-cells ex-
hibit no growth to speak of when they
pass over from the cambial or embry-
onic condition into their definitive form :
the arrangement of the cells produced
by the growth and cell-division in the
cambium likewise undergoes, therefore,
essentially no alteration. The processes
of cell-formation in the cambium of
wood, so far at least as the transverse
section is concerned, are made evident,
however, not merely in the individual
cells, but also in quite the same sense in
the annual rings of the wood and in the
medullary rays or ' silver grain ' which
traverse them. The annual rings appear here as periclinal layers of the mass of wood,
the medullary rays as anticlinal lines of cells representing the orthogonal trajectories
of them. If, as is the case with the inner annual rings in Fig. 274, the mass of wood is
nearly circular in transverse section, the annual rings, if of equal thickness all round,
form concentric circles, and the medullary rays then appear as radial lines, since they
cut these circles at right angles. The figure shows, however, how the woody body has
grown continually more irregular with increasing age : the annual rings (periclines)
having grown much more in thickness towards the side a, than on the side /. Finally,
by a wound, the formation of wood had ceased at a, while it proceeded only so much
the more vigorously at the margins of the wound m, n, p. In accordance with the
course of the periclinal layers of wood thus produced, the medullary rays directed
as anticlines have also now assumed other directions, so that they everywhere cut the
annual rings or periclines at right angles. The peculiar and very various curvatures

Fig. 274.— Transverse section of the wood of a cherry stem
intis cerasifera) which had been deprived of its cortex on the
years previously, whence prominences had been produced
at m n f>. The thick radial lines are fissures produced by the drying
of the wood : their course follows the same law as the medullary
rays represented by thin lines.

(Pru.

siden

440

LECTURE XXVIT.

of the medullary rays are simply nothing further than the expression of the general
rule that the anticlinal cell-walls are the orthogonal trajectories of the periclines.
Supported by this general rule it is also possible in any given transverse section of
wood, the annual rings of which are known, to register at once the course of the
medullary rays; or, when the latter are more distinctly recognisable than the
annual rings, to make out the converse.

In these cases we have had to do with processes of growth where increase
in volume takes place in an easily intelligible manner only at the margin or
circumference, the cells there formed, however, undergoing no further growth worth
mention, although they pass over into the definitive condition of permanent tissue,
in which the second phase of growth— i. e. elongation — is suppressed.

We will now, however, turn our attention to those cases, at first of the simpler kind,
where an organ consisting entirely of embryonic tissue grows throughout its entire

mass ; where increase in volume
takes place not merely at the cir-
cumference but also in the interior,
and accordingly division-walls ap-
pear also in the interior. For
the facilitation of the problem
we will even here only concern
ourselves with transverse and lon-
gitudinal sections, or structures
naturally fiat, because the con-
sideration of proper stereometric
relations would not only cause
great prolixity, but can also be
dispensed with in the meantime
for our purpose. The simplest
case is of course presented by
P,(- 2^j fiat organs which consist of a

single layer of cells only.
Our further considerations will gain in clearness, and will be much facilitated
if we construct for ourselves on paper in advance various possible cases. It is
already clear from what has been said so far that the ordinary case consists in the
rectangular intersection of the periclines and anticlines. We may thus, for example,
imagine an elliptical disc (Fig. 275) consisting of embryonic substance, and propose
to ourselves the question, In what direction are the division-walls formed in it,
if the growing surface (which always remains elliptical) is cut up into a large
number of cells by periclinal and anticlinal division-walls intersecting at right
angles? Now the problem is solved according to geometrical principles in the
elliptical figure given. In the first place, the area is divided into four quadrants by the
major and minor axes of the ellipse, and the two foci are marked//! As periclines,
two other ellipses, P and p, are drawn, possessing the peculiarity that their foci are
likewise situated in //; or, in other words, the three ellipses drawn are confocal.
Instead of three we might draw a large number of confocal ellipses. In order
that the anticlines to be drawn in the figure everywhere cut the periclinal ellipses at

DIAGRAMS AND EXAMPLES.

441'

right angles, they must represent hyperbolas, and in such a way that around each of
the two foci//a larger or smaller number of hyperbolas run, the axes of which coincide
at the same time with the major axis of the confocal ellipses : or, in other words, the
anticlines A and a are confocal hyperbolas. In each case, according as we wish
to have large or small cells in the disc, the number of confocal ellipses and hyper-
bolas may be increased. It is noticed — to remark it by the way — that those
hyperbolas the apices of which lie nearest to the foci X/" there make a strong curve : on
the contrary, the apical curvature of the hyperbolas is so much the smaller the further
they are distant from the iocXff, and the nearer they approach the minor axis of the
ellipse. If we further suppose the whole figure so altered that the two axes become
equal to one another, then the two foci// coincide in one focus, the confocal ellipses

become converted into concentric circles, and the hyperbolas A and a then appear
as straight lines which run from the centre to the periphery, and are thus radii
of the circle.

We have thus, by means of our geometrical construction, obtained a general
scheme for the way in which the cells must approximately be arranged in the
supposed disc, if we divide it up into cells by periclinal and anticlinal partition-walls
cutting one another at right angles : it will scarcely be necessary to say that the
quadrangular portions of surface, or areolae, between the anticlines and periclines
represent the cell-cavities.

Having, then, closely impressed on the mind this scheme, the geometrical
significance of which is readily understood, we find everywhere in such flat objects
the cells arranged in patterns which obviously accord with our scheme. In order

442

LECTURE XXVII.

to demonstrate the validity of this construction on only a few examples taken quite
at random, we may consider Fig. 276. In this case A represents the approximately
elliptical germinal disc of the Alga {Melohesia) which we have already considered
above. We at once recognise the walls corresponding to the major and minor
axes of the ellipse, by means of which the originally unicellular embryo-plant is cut up
into four quadrants : moreover, in spite of a few disturbing interruptions in the walls,
two confocal ellipses are recognised as periclines and two hyperbolas on either side as
anticlines. At the circumference of this disc we of course notice cell-walls which are
not continued inwards ; if, however, they were completed inwards, they too would
form hyperbolas. I may take this opportunity of pointing out that in an object
which is in other respects constructed according to our scheme, individual portions
both of the periclinal and anticlinal lines may nevertheless be wanting, because the cor-
responding cell-divisions have been suppressed : the scheme only implies that when

cell-walls do arise they must lie in the
directions given. That the scheme holds
good for the most various cases, is at once
shown on regarding Fig. 276, Z), which
represents a transverse section through the
slender growing-point of Salvima, and G,
which represents the transverse section of
the vein of a young leaf of a Fern {Tricho-
maiies).

When the elliptical circumference ap-
proaches the circular form, or actually
becomes a circle, as in the Figures C, E,
F, H, K, here also two anticlinal walls
(in this case radial walls) are the first to
appear, by means of which the disc is di-
vided into equal quadrants. If further cell-
divisions now^ follow inside the quadrants,
it would at once contradict the law of rectangular intersection if these new walls
were to run from the centre to the periphery; since these anticlines (radii) would
then meet in the centre of the disc at very acute angles. This never occurs : on
the contrary, all such objects show that the anticlines running inwards from the
circumference make a curve in order to abut laterally on one of the preceding walls
of the quadrants, though the direction in which these curved anticlines run may
be different in each quadrant.

While, in the construction of the foregoing scheme, I started from a geometrical
form with perfectly defined outline — i. e. an ellipse — it was thereby implied, if the
anticlines and periclines cut at right angles, that the periclines must be confocal
ellipses and the anticlines confocal hyperbolas. It would, of course, be impossible to
produce with the same exactness geometrical constructions for every kind of outline
which an embryonic organ can show ; but it is at once clear that even when the out-
line is not actually an ellipse, but only more or less resembles one, the entire cell-
network must nevertheless present a pattern similar to the one which there exists, if the
anticlines and periclines cut one another at right angles. For example, in Fig. 277

Fig. ztj.—A a young, B an older embryo ol Alisma
ptantago. a a the anticlines ; pp the periclines.

THE SEQUENCE OF ANTICLINES AND PERICLINES VARIES.

443

Fig. 27S.— .-? a very young, B an older embryo of Orobanche (after Koch).

A B the resemblance to our scheme is again perceived at once, although the outlines
can hardly pass as ellipses. Besides, we have here the case that the organ which
we are considering as cut up into cells simultaneously with growth is provided on
the one side with a stalk. In such cases a hypophysis (Ji) regularly makes its
appearance at the boundary be-
tween stalk and head. Hanstein ^ B
formerly held this to be an organ
peculiar to the embryos of Pha-
nerogams : it is, however, nothing
further than the expression of the
general law of cell-division for the
case here given.

In these figures we meet with
yet another fact of experience, in
that the sequence in time in the
origin of the anticline and peri-
cline walls, even in very similar
objects, is variable in a high
degree. Sometimes, as in Fig.
277 A, numerous anticlines are

produced, the periclines (/> in E) only following subsequently ; or periclines arise
followed at once by the anticlines, as in Fig. 278 B, But even this in no way alters
the validity of the scheme described above, in the construction of which it is of course
immaterial whether the anticlines or the periclines are drawn first.

As a rule, the external form of very young organs which still consist of
embryonic substance changes as
they gradually develope. Hence
the case occurs not rarely that
walls which have already been
produced undergo displacements
and curvatures during the alter-
ation in form of the entire struc-
ture, in such a manner that they
now become adapted to the new
form of the organ as if they had
only been produced after its at-
tainment. Since this case, hither-
to little noticed, is of peculiar im-
portance for the theory of growth,
because it throws new light on
the intercalations and movements fig. 279.

taking place in the embryonic

mass of tissue, I may here adduce an illustrative scheme and an example. In Fig.
279 to the left is represented a filamentous structure, which is divided by several
transverse walls TT and by one series of longitudinal walls. The figure on the right
shows us the same structure after the filament has become transformed into a stalked

444

LECTURE XXVII.

elliptical disc : one of the preceding transverse walls T is still present as a straight
transverse wall RR, and the series of longitudinal walls referred to above also still
exists as a straight longitudinal wall in the direction of the major axis of the ellipse.
In accordance with the mode of growth mentioned, however, the other walls pre-
viously marked T'have now become converted into hyperbolically curved anticlines A.
This figure is only constructed hypothetically, and agrees with the requirement that
a filament consisting of two longitudinal rows of cells shall become converted into
a stalked ellipse, under such conditions that the walls still cut one another at right
angles.

In Fig. 280, which represents the development of a gemma of Marchantia, there
will be perceived at once, in Figs. I-IV, alterations of the cell-network which agree
in all essential points with the processes illustrated in the diagram (Fig. 279).

On the other hand, however, it also happens that networks of cell-walls undergo

, 280. Development of the geninia oi Mardiaii

displacements by growth, of such a kind that the usually rectangular cutting of the
anticlines and periclines passes over into one more or less oblique. This is occasion-
ally the case in growing-points, the growth of which we shall only consider later on.
It is to be recognised more easily and frequently, however, on transverse sections of
woody bodies with excentric layers. This process also may be made clear most easily
by means of a hypothetical construction. In Fig. 281 I have assumed that a mass of
wood which is circular in transverse section possesses a pith situated very excentrically.
The first ring of wood, /, is equally thick all round ; but all the succeeding annual
rings, II-VII, have grown much thicker on the north side {N) than on the south
side {S). In order to simplify the construction it is assumed that the circumference of
each annual ring is nevertheless circular. The scheme may be so imagined that the
points 2, 3 — 7 on the line NS are the centres of the six annual rings following one

DISPLACEMENTS DURING GROWTH.

445

another. Now if the medullary rays were exact orlhogonal trajectories of the annual
rings, as is the case where the growth of the wood is regular, they must cut the cir-
cumference of the disc of wood at the points r r. But all the medullary rays, on the
contrary, are curved towards the point N, and therefore away from S; or, in other
words, they are driven over towards the line of strongest growth, which reaches
from the pith to N. This scheme is intended to illustrate no theory, but only the
fact, which is very often to be observed on transverse sections of wood, that the
medullary rays are driven towards the side of strongest growth of the wood, and
therefore sacrifice their rectangular intersection with the pcriclines — i.e. the annual
rings. Fig. 282 represents a particularly clear example in the transverse section of an
excentrically grown stem of the Lime. It is easily noticed that the medullary rays s t
are driven from the right as well as from the left towards a, the line of strongest growth.

It is not to be forgotten, however, that we are here concerned with a transverse
section of wood, the growth and cell-divisions of which take place exclusively in an
outer narrow zone of the periphery, viz. in the cambium, whereas in similar cases in
growing-points the entire mass of embryonic tissue with its anticlines and periclines is
growing.

We have hitherto regarded the cell-network as a superficies only, and the
anticlines and periclines as simple lines. If we give a certain thickness to these flat
structures, nothing is essentially altered thereby ; what are mere areolae in the above
figures then behave like the stones of a mosaic, and are actual cells. The matter is
quite otherwise, however, when we come to look upon the figures hitherto considered
as longitudinal or transverse sections of ellipsoidal or spherical bodies, or when the
organ possesses any such form as that of a lens, or a compressed ellipsoid, &c.

446

LECTURE XXVII.

Our space being limited, however, it must be left to the reader to reflect upon the
matters in question. I will only refer to one particular case, since it is common every-
where. When an ellipsoidal, or spherical, or similar body becomes divided, walls

regularly occur in three direc-
tions, cutting one another at right
angles, in such a manner that
the body is first cut by a wall into
two usually equal halves, each of
which is then bisected by a wall
standing at right angles to the
first wall, and then a wall at right
angles to the first and second
cuts the whole into eight octants.
In each of these, anticlinal and
periclinal cell- walls now appear, so
that transverse and longitudinal
sections give figures such as we
have already considered. Here,
as already said, it is of no mo-
ment whether we regard the capi-
tulum of a hair, an embryo, an
antheridium, or any other organ.
For example. Fig. 283 represents
a median longitudinal section (of
course very diagrammatic) of the
embryo of a Fern, produced from
the fertilised oosphere. The anticlines and periclines are at once recognised as we
constructed them in our scheme for an elliptical disc, particularly in the case of the
walls marked Aa and Pp. In order to obtain an idea of the true state of aff"airs in this

case, however, we must suppose
A ^^-^ I ~~~~~-^^ the figure to have made a complete

revolution round the long axis
of the ellipse and thus described
an ellipsoid in space. The draw-
ing then represents only a median
longitudinal section of this ellip-
soid, the entire cellular structure
of which would only be perceived
when also viewed from above,
from below, from behind, and
from before. It would then be
p,G. 283. found, moreover, that the whole

ellipsoid is also completely di-
vided by a wall in the plane of the paper, and thus that not merely four
quadrants are present, as our figure shows, but eight octants. In each of these
octants, again, there has arisen in the first place an anticline, A^ approximately

Fig. 282.— Transverse section of the wood of a 'iJ\w\e{Tilea platyphytlos).
St medullary rays; ap fissures produced by drying; zside of feeblest, a that
of strongest growth in thickness.

ANTICLINES AND PERICLINES IN SOLID STRUCTURES. 447

of the form of a watch-glass, or, since according to our construction the anti-
clines A are hyperbolas, each of the walls A would represent a hyperbolic surface
in the body of the embryo, and in like manner the periclines F and p would
represent portions of ellipsoidal surfaces. The further growth and formation of
organs in this embryo then proceeds by means of the cells 3, sf, and w, which
arise in the octants. In the two lower octants to the left, two cells (as s/) are
formed: these are the so-called apical cells, of which, however, only one is con-
cerned with the further growth, and constitutes the apex of the stem of the young
Fern. This apical cell of the stem has the form of a tetrahedron (cp. Fig. 269,
above) in which, as growth proceeds, new division-walls continually appear parallel
to the anticlines. The same happens also in the apical cell of the first root w,
where, however, in addition to the anticlinal segmentations parallel with A and a,
pericline-walls p are also cut off for the formation of the root-cap. In accordance
with the origin there are properly also two such root-rudiments present in the octants
situated to the right above, but the actual formation of a root takes place in one of
them only. The apical cell 5 becomes united with the corresponding one of the
octant lying next it for the formation of the first leaf of the embryo Fern.

We here become acquainted with a new relation between cell- division and
growth. From what has been said it may be observed that certain cells, previously
determined by the general law of growth in the embryo, make themselves evident as
the points of origin of the new organs of the stem [sf), of the root {tv), and of the
first leaf {ö) ; and with respect to the apical cells si and w which, as stated, possess
the form of a tetrahedron, the important fact may here be brought forward that the
production of these apical cells is a necessary consequence of the law of cell-division
prevailing in the embryo. This observation is of importance, because, until the
appearance of my investigation on the arrangement of cells, the causal relation was
believed to be an entirely different one. Indeed, people went so far as to regard the
fertilised oosphere itself as the first apical cell of the stem ; just as, in general, an
entirely unwarranted importance had been attributed up to that time to the apical
cells which are found in the growing-points of many Cryptogams. This will become
clearer in the sequel.

The subject of the relation between growth and cell-division in the growing-
points of shoots and roots is more difficult than in the cases considered hitherto ; but
even here I have succeeded in making the facts clear on the principles laid down at the
commencement of this lecture, after hundreds of careful investigations on the cellular
network of the growing-point had provided material, welcome and valuable, it is true,
but by no means intelligible. Just as do the organs hitherto considered, which
consist entirely of embryonic substance, so also do the growing-points of roots
and shoots show characteristic cell-wall networks or cell-arrangements on properly
directed longitudinal and transverse sections, and these everywhere, even in the most
different species of plants, agree with the type. This depends essentially upon the
fact that the embryonic substance of the growing-points, as it increases in volume on
all sides, becomes divided up into compartments, or chambers, by cell-walls which
cut one another at right angles. The longitudinal section of a growing-point always
shows a system of periclines, cut by anticlines which in their turn constitute
the orthogonal trajectories of the former. If we arc here concerned with growing-

448

LECTURE XXVII.

points of flat structures, then only these two systems of cell-walls are present ; if, on
the other hand, the growing-point is hemispherical, or conical, or of some other
similar shape — i.e. not merely flat, but forming a solid body — then there exists still a
third system of cell-walls, viz. longitudinal walls running radially outwards from the
longitudinal axis of the growing-point.

It will conduce to intelligibility, however, if we here again confine our
further considerations to a scheme constructed arbitrarily, but according to definite
rules. We may, in the first place, take simply the superficial view of a longitudinal
section through a growing-point. Confining our attention to Fig. 284, the outline
EE of which represents the longitudinal section of a conical growing-point, we may
premise that this outline, which is often nearly realised in nature, possesses the form
of a parabola, and that the chambering of the space occupied by the embryonic
substance of the growing-point again takes place in such a manner that anticlinal
and periclinal walls cut one another at right angles. With this premiss we can now
construct the network of cells in Fig 284 according to a well-known law of geometry.

Given that xx represents the axis, and yy the direction of the parameter, all the
periclines denoted by Pp form a group of confocal parabolas. Similarly, all the
anticlines Aa form a system of confocal parabolas having their focus and axis in
common with the preceding, but running in the opposite direction. Two such
systems of confocal parabolas cut one another everywhere at right-angles.

We may now see whether a median longitudinal section through a dome-shaped
and approximately parabolic growing-point presents a net-work of cells essentially
agreeing with our geometrically constructed scheme ; and we at once find in the
growing-point of the~^Larch, for example (Fig. 285), the corresponding internal
structure, simply noting that in the figure the two protuberances bb disturb somewhat
the symmetry of the figure. These are young leaf-rudiments budding off" from the
growing-point. However, we at once recognise the two systems of anticlines and
periclines, the curvatures of which it can scarcely be doubted cut one another at
right angles as in our scheme above; or the anticlines are the orthogonal trajec-
tories of the periclines. As in our scheme, also, only a few periclines under the apex
^ run round the common focus of all the parabolas ; the others as they come from

^ ^^oulcl be SiwevPl

ANTICLINES AND PERICLINES IN THE GROWING-POINT.

449

below only reach the neighbourhood of the focus. In other words, the corresponding
cell-divisions only take place when the periclines beneath the centre of curvature have
become sufficiently far removed from one another for new periclines to be intercalated
between them ; and the same is true of the anticlines A a. It is easy to see on the
scheme, Fig. 284, that the curvatures of the construction-lines are particularly sharp
around the common focus of all the anticlines and periclines.

Many hundreds of median longitudinal sections through growing-points of
shoots and roots, drawn by very different observers without even the most distant
perception of the fundamental principle, accord with the construction which I have
given, and demonstrate its accuracy; and, further, it may be said that all these
observations have been made under the influence of two false premisses ; first, that
growth — i.e. increase in volume — takes place chiefly at the apex of the growing-point,
and secondly, that cell-divisions are an essential cause of growth. That the latter
is unfounded has already been insisted upon above, and I have shown clearly that
it is just the apical region of the growing-point which is that where growth is slowest

Fig. 285.— Longitudinal section through the growing-point of a winter-bud oi Abies pectinata (x about 200).
s apex of growing-point ; 4* youngest leaves ; r cortex ; m pith.

and increase in volume least. The details of this, however, would here carry us too
far. It need only be mentioned that the mere consideration of the cell-network at
the growing-point and the course of the anticlines and periclines shows most
clearly that growth must be less active at the apex itself than at any point lying
further back. It is only necessary to suppose that in the scheme (Fig. 284) growth
— i. e. the intercalation of mass — is more active in the neighbourhood of the focus
of the anticlines and periclines than further back, to see that, according to the
described laws, the cell-network must assume quite another form — i.e. the course
of the anticlines and periclines must be essentially different.

Perhaps this important and formerly incorrectly apprehended relation will be
rendered sufficiently clear if we here again make use of an arbitrary construction.
We may assume that in Fig. 286 the lower figure represents a square surface
consisting of 36 cells, and, for the purpose of better guidance, the lines qq and vi are
drawn thicker. Let us now suppose that these 36 cells begin to grow as a whole, and
that the upper figure arises therefrom. The line vi then represents the longitudinal

[3] . Gg

45©

LECTURE XX VIT.

axis or axis of growth which divides the entire figure symmetrically in both cases.
The lines ^ q, on the other hand, are intended to show the boundary between two
different forms of growth in the given cell-network. In the cells lying beneath
qSq growth has taken place as in the growing-point considered above; the lines
yy, hh form confocal parabolas. On the other hand, the growth above the line q Sq
has taken place as it was formerly supposed to do — namely, the growth in length
has here been most vigorous at the apex. It is at once obvious that in this way an
entirely different cell-network arises, and that the periclines and anticlines assume
quite other directions than at the apex of an ordinary growing-point. In the case of

I

T

?

I

h

s

1

-h

-

7i

y

Jf

I

J

2

the latter, the pieces X /3 3 and B, intercalated between the periclines, become shorter
in the given direction ; above the line q q, on the contrary, the pieces we e E, inter-
calated between the periclines, increase in length in the order given.

The state of affairs arbitrarily constructed in this scheme, however, is found
as a matter of fact in many roots of vascular plants, as shown in Fig. 287. The
growing-point of the root, the outline of which is here indicated by the Hne K K K,
presents the cell-network as in our scheme Fig. 284, and accordingly as in the portion
situated below q S q in Fig. 286. On the other hand, the mass of tissue marked by
the outline hhh is the root-cap, and it is at once perceived that this corresponds
with that mode of growth which is represented in Fig. 286 above the line q Sq.

CONFOCAL AND CO-AXIAL STRUCTURES.

451

Moreover, such a separation between the growing-point of the root and its cap
does not exist in all true roots ; there may occur quite other distributions of growth
at the apex of the root, and accordingly also very different net-works of cell-walls.

That distribution of growth by
which the intercalation of mass in-
creases from the apex downwards, and
at the same time from the longitudinal
axis outwards, and by means of which,
as we have seen, the anticlines are
caused to radiate like a fan, occurs,
however, not only in root-caps, but
also in many other cases, especially,
for example, in the growth of very
young ovules. Fig. 288 may finally give
another simple scheme for the cell-
wall net-work, and the course of the
anticlines and periclines in such a case.
It must be left for the reader, however,
to picture how the structure B has

arisen from the two cell-series in A fig. 287.

by means of the corresponding pro-
cesses of growth and subsequent cell-divisions, it being simply observed that the
walls marked a and />, as well as those marked i, 2, 3, 4, are in both figures the
same.

Finally, the remark may be added that the first described arrangement of
the cells at the growing-point may be

termed confocal, while the last one may 33

be regarded as co-axial, or fan-shaped,
and that young organs of similar out-
ward form present sometimes the one,
sometimes the other structure : in other
words, the intercalation of mass in the
interior of an organ may accord with
the one or the other type, though the
external form of the organ is the same
in both cases.

If we now return once more to
the parabolic, dome-like growing-point,
from the consideration of which we
started, and the median longitudinal
section of which is here once more re-
presented, we may now assume further f'g. 288.
that the figure included by the line

J^E has made a complete revolution round the longitudinal axis XX. It
is intelligible that then also each of the periclinal and anticlinal lines must have
described a parabolic surface, and if we represent to ourselves the resulting figure,

45^

LECTURE XX VI/.

it consists of a large number of superposed concavo-convex layers, the curvature
of which increases upwards; these are the layers the vertical sections of which
are represented by the anticlinal lines A a of our scheme. Since, however, in the
postulated revolution about the longitudinal axis X X, each of the periclinal lines
Fp has also described a parabolic surface, these concavo-convex layers are divided
up into corresponding rings which run concentrically around the longitudinal axis
X X. Now, in order that these concentric rings may fall into approximately cubical
cells, in agreement with the problem, we must suppose still another system of cell-
walls to be interpolated, which radiate out from the longitudinal axis XX in radial
directions to the surface E E oi the growing-point. These are the radial longitudinal
walls, of which, however, only four can cross one another in the longitudinal axis
XX: the others only commence further outwards, and will, according to our
construction, exhibit approximately the same course as the medullary rays on the
transverse section of a woody body. Or, in other words, the transverse section

of a paraboloidal growing-point shows concentric layers of cells which at the same
time exhibit radial rows, splitting up more and more as we proceed outwards.

Growing-points of this description, as a matter of fact, are very common both
among roots and shoots. They may, however, also be formed quite otherwise, and
especially so that two successive longitudinal sections standing at right angles have
different diameters or parameters — i.e. the growing-point appears compressed on
the one side and dilated on the other, and accordingly the network constituted of the
anticlinal and periclinal walls will then also present different forms. However, here
again this indication must suffice.

In a large number of Algae and most Hepaticse, all true Mosses, Equisetums,
Ferns, and Selaginellas, there is found at the apex of the growing-point of the shoots
and roots a cell, which is usually characterised both by its size and shape, and
which occupies the actual apical region of the growing-point : by the repeated divisions
of this cell into two, as it itself goes on growing, daughter-cells become cut off in
definite sequence, from the further growth and repeated cell-divisions of which the
whole of the tissue of the growing-point in question arises. These daughter-cells cut
off from the apical cell are termed segments, and Naegeli, who first described this

THE APICAL CELL.

453

important fact in 1845, showed how all the tissue-cells, not only of the growing-
point but also all those subsequently produced by their agency, may be compre-
hended as descendants, with definite sequence in space and time, from the segments
of one apical cell. For more than thirty years the most careful investigations
have been devoted to this study, and for this reason alone it is necessary for
us to concern ourselves here somewhat more closely with it, and especially
because serious errors with respect to the relations between growth and cell-division

Fig. 290.— Apex of a shoot of CAar«— longitudinal sectii

have slipped into the subject — errors which have retarded the development of the
whole doctrine of growth. In the first place, however, it is necessary to make the
reader acquainted with the facts.

When an algal filament or a fungus-hypha grows in length, this takes place
usually, but by no means always (e. g. not in the case of Spirogyra and other Con-
jugatae), in such a manner that transverse divisions are formed only at the end of the
filament which represents the growing-point. The cell at the end, which alone grows
forward and divides, may be distinguished as the apical cell : such a cell is also found
at the end of the rhizoids of Mosses and Characese, where, however, the transverse

454

LECTURE XXVII.

septa are placed obliquely. The significance of the apical cell comes out more
conspicuously and characteristically when the products of division (segments) arising
from it not only themselves grow further but also undergo essential changes of form
and corresponding cell-divisions, as is the case in many Algse, and especially in the
Characese : Fig. 290 may serve as a scheme for the latter. The cell marked v, at the
end of the shoot, is in this case the apical cell, from which all the organs and tissue-
cells of this plant can be derived by the formation of segments and the growth
and further division of the latter. Every time this cell elongates a little in the
direction of the longitudinal axis, there appears in it a transverse wall, an anticline,
convex downwards. The cell cut off by this is the segment. This now grows also
in length and in circumference, and after a short time is divided into two cells
by means of a transverse wall convex upwards, viz. into an upper one which possesses
the form of a bi-concave lens and into a lower one shaped like a bi-convex lens.
The growth and further fate of these two daughter-cells of the segment in this case
then differ in important respects. The lower bi-convex cell thenceforward grows
vigorously in length, without undergoing further divisions; the figure shows how
this cell gradually assumes the forms i', i" , i'", i"" , and how the longitudinal axis
of the shoot is produced by numerous such cells. The bi-concave daughter-cell
of the system, on the contrary, soon undergoes vertical divisions, and is trans-
formed into a disc, or so-called node, consisting of cells, since it grows chiefly
in the transverse direction, and only very little in the longitudinal direction of the
shoot. Certain cells situated at the margin of this disc now grow out and project
upwards, U ; these outgrowths then develope further into the leaves b" , b"\ b'"\
and subsequently, by the growing out of certain cells at the nodes, lateral shoots
K, and from the leaves themselves sexual organs a and 0 arise. It will best be
seen what can be produced from a segment of the apical cell by means of the
growth described and the subsequent cell-divisions, on observing that the parts
marked in each case alike with i and b and surrounded by a thick contour have
always proceeded from one segment. It would occupy too much time for our
purpose to show how all the various cells in the tissues of these organs gradually
arise from the originally bi-concave nodal cell, in perfectly definite sequence and
in accordance with the law of cell-division described above, and how these organs
eventually become altered in form by means of elongation and increase considerably
in size.

On account of its simplicity and intelligibiHty, Fig. 291 may be examined in
addition with respect to the apical cell and its segmentation. This represents the
growing-point of an Alga, Dictyoia, which has just split into two similar growing-points,
a and b, by so-called dichotomy. In a we see the apical cell, shaped in accordance
with the slightly arched form of the thin ribbon-like shoot, like a bi-convex lens,
from which, after having elongated a little in the direction of the axis of the shoot,
the segment i has been produced, and this has already become divided by a median
longitudinal wall into two equal halves. The groups of cells marked 2, 3, 4, and 5,
are older segments of the apical cell, in which, as they have grown in length as well
as in breadth, more and more numerous andclinal and periclinal walls have gradually
made their appearance ; and here also it is easily understood how the whole mass of
tissue of the shoot arises from the segments of the apical cell. In b, a longitudinal

SEGMENTATION OF THE APICAL CELL.

455

Fig. 291.— End of a ribband-shaped flat shoot of the Alga Dictyota.

wall has arisen in the apical cell itself, wherewith is given the commencement of
a dichotomous branching of this growing-point which itself proceeded from a
dichotomy. As for the rest, how-
ever, the same relations as before
prevail, and it is at once noticed
in the figure how the two growing-
points a and b show by the anti-
clines further backwards their origin
from a previous dichotomy.

In the two cases examined the
segments are cut off from the apical
cell by simple transverse walls. Fig.
292 may serve as an example of the
case where from an apical cell on
a flat organ (leaf of the Fern Cera-
topteris) the segments are formed
by successive division-walls situated
obliquely right and left {S), whereby
two rows of segments arise, from the
further growth and corresponding
cell-divisions of which the whole of
the tissue of the leaf is produced.
The thick lines in the figure are the

older segment-walls of the apical cell ; and it is noticed, proceeding backwards from
this, how the superficies of the segments have become larger and divided by anticlinal
and periclinal cell-walls in accord-
ance with their age. At Z a lateral
lobe of the leaf is protruding ; this,
as is easily seen, belongs to two dif-
ferent segments, and since numerous
anticlinal walls already exist in it,
which radiate in a fan- like manner
into the lobe and participate in its
whole growth, no apical cell is
formed here.

Such apical cells, segmented in
two rows, are common in flat organs,
as in the leaves and flat shoot-axes
of many Algoe, Liverworts, and in
some dorsiventral shoot-axes of Vas-
cular plants — e. g. in many Ferns
and all Selaginellas. In growing-
points the transverse section of which
is circular and the growth upright,

and especially in the true roots of the Vascular Cryptograms, we find, on the contrary,
at the tip of the growing-point an apical cell from which segments are cut off on three

FIG. 192.— Young leaf of CcratopUris (after Kny).

456

LECTURE XXVII.

sides. This can of course only be recognised with certainty when the apical cell is
observed from above, or in transverse section ; since such a tetrahedral apical cell
seen in longitudinal section presents a very similar appearance to the biseriate
ones described above. This state of affairs will be clear on regarding Fig. 293.
A represents the longitudinal section, and B the view from above of the apical region

Fig. 293.— Apical region of the roots of Ferns. ^ longritudinal section througli tVie end of the root
of Pteris hastata. B transverse section through the' apical cell and the surrounding segments of the
root ai Asi'Uniutnßlix femiyia (after Naegeli and Leitgeb).

of the growing-point of the root of a Fern, v being the apical cell in each case. In B
the numerals /- VIII mark the transverse sections of the concentric segments of the
apical cell v.'mA these are to be recognised in the longitudinal section by their walls
being drawn with thicker contours. In this example there is added a further com-
plication, however, since we are concerned with a root, and accordingly also with a
root-cap. This root-cap is indicated in Fig. 293 ^ by means of the letters k I m n,
and its cap-shaped superposed layers are in fact also segments of the apical-cell v,
which have been produced by successive transverse divisions which have then grown

further. In the growing-point of a shoot
with a tetrahedral apical cell these caps
k Imn would be absent. It is not quite
so easy to see that even such tetrahedral
apical cells obey the law of rectangular
intersection of the division-walls ; and even
the most distinguished observers in this
difficult province have for years erroneously
supposed that the segment-walls of a te-
trahedral apical cell cut one another at
Pj^ ^ angles of 60°, because such appears to

be the case from the view in transverse
section, which represents an equilateral triangle. I have shown, however, that
such an apical cell is correctly apprehended by supposing a corner so cut off from
a cube that the triangular surfaces which meet in the corner are equal to one another,
and, in addition, that the four bounding surfaces are arched outwards, as shown
by the Hues a b c m Fig. 294. This represents the upper view of a tetrahedral apical
cell, and the walls d€,/g, hk are the successive segmentations which appear in it :

SIGNIFICANCE OF THE APICAL CELL.

457

z being the corner in which the three youngest segment-walls always cut one another.
The preceding figure (Fig. 293) shows now in the main for this case also, how
the whole of the tissues of the organ in question arise by successive cell-divisions
from the segments of such an apical cell.

The segment-walls of apical cells with two or three sides are in fact
simply anticlines, and the subsequent division-walls in the segments are some-
times anticlinal and sometimes periclinal, so that the segments, just as we have
seen to be the case before, break up into a network of walls consisting of orthogonal
trajectories. A simple alteration of the scheme given in Fig. 289 here comes in
only in so far that the anticlines under consideration (Fig. 295 A a) do not at once
cut through across the whole apex, but meet one another from two or three sides :
Fig. 2 95-5 shows, however, how each of the anticlinal segment-walls is subsequently
completed by means of a further piece into a complete anticline A a. The
difference, which is of importance, will be evident at once on comparing the
figures A and B. IVIoreover, since tetrahedral apical cells only occur in solid

V X r

growing-points which are circular in transverse sections, and not in flat ones, radial
longitudinal walls also appear in the segments in addition to the anticlines and
periclines, and alternating with them.

Since, as Naegeli showed so long ago as 1845, ^^^ ^^he tissues of a root or shoot
may be derived progressively and in genetic sequence from the apical cell, wall for
wall, the opinion arose gradually that the whole process of growth in the growing
point is ruled by the apical cell itself, which, by means of its segments, adds stone
upon stone to the structure like a builder. Nay, matters even went so far that the
belief arose, without any foundation, that the apical cell is in fact the position of most
rapid and vigorous growth in the embryonic tissue of the growing-point. I have
already referred in my previously quoted treatises to the erroneous character of both
views : the apical cell, as also the apical region corresponding to it in growing-
points not provided with such a cell, is, on the contrary, the place of slowest growth,
and in no part of the growing-point do cell-divisions occur so rarely as the segmenta-
tions in the apical cell. This can be concluded witli certainty from the increase in

4^8 LECTURE XXVII.

volume of the successive segments, and from the number of partition-walls in
segments of different ages.

So far as regards the importance of the apical cell as the ruler of the whole
growth in the growing-point, however, I showed that that cell represents merely
a break in the constructive system of the growing-point^i.e. the apical cell
is that spot in the embryonic tissue in which neither anticlines and periclines nor
radial longitudinal walls have as yet been formed. In order to understand this,
it is only necessary to compare Fig. 295 with the previous scheme, Fig. 289.
There, a few of the periclines run continuously round the focus beneath the apex,
and in the same way the uppermost region of the apex is traversed by anticlines,
so that the proper apical region of the growing-point is occupied by small areolae or
cells. In Fig. 295 yl, which possesses an apical cell, the periclines all cease, on the
contrary, at some distance from the apex ; we need only complete these, however,
into confocal parabolas with the focus S, and suppose in addition a few sharply
curved confocal and parabolic anticlines interpolated above the uppermost anticline
A, and then the impression of an apical cell disappears altogether, and we have a
growing-point such as is found in all Phanerogams and many Cryptogams. Or,
conversely, we need only suppose in our scheme. Fig. 289, the anticlines next the
apex and the upper curved portions of the periclines to be wanting, and we obtain
an apical cell, as in Fig. 295 ^. In apical cells from which segments are cut off
on two or three sides, the relation referred to is, it is true, a little more complicated,
but it is easily noticed on regarding Fig. 295 -5 that here also we are enabled by
properly completing the periclines P, with the help of a few anticlines, to convert the
apical cell into a small-celled mass of tissue. As a matter of fact, corresponding
transformations of apical cells actually occur when the growing-points cease to grow
further as such, e. g. in the prothallia of some Ferns.

By means of this explanation of the apical cell which I have established, the
criticism of the constructive processes in the growing-point obtains as a general
principle. While growing-points with and without apical cells were previously
regarded as essentially different, and the opinion was even entertained that an
apical cell must be found even in all phanerogamous growing-points; according
to my view of the matter, on the contrary, the presence or absence of an
apical cell appears as an interesting but quite secondary point in the growth of the
growing-point. Even the case as it occurs, for example, in the roots of the Marattiacese
among the Ferns, where a group of large apical cells is present, may be
referred without constraint to the principle mentioned. It is to be mentioned on
the other hand that apical cells are only possible, in the sense here employed, when
the periclines and anticlines constitute systems of confocal curves, or, shortly,
in growing-points with confocal structure. If, on the contrary, the growing-point
is traversed by anticlines which run in a fan-like manner, and the increase in
volume is greatest towards the apex, as was shown above in Fig. 288, it is then
possible, under certain circumstances, still to assume the existence of apical cells
in the wider sense, but in these cases authors have not spoken of apical cells, and
so we also may overlook them. In any case, in the older mode of looking at the
apical cell, it must remain enigmatical why this varies so extraordinarily in its
occurrence and even in its form that, as it was formerly expressed, the growth of

GROUPS OF APICAL CELLS. 459

one lobe of a leaf takes place by means of an apical cell, and that of another lobe
merely by so-called marginal cells, as in Fig. 292. From my way of looking at the
relations of dependence of cell-division upon
growth, however, such cases appear thoroughly
explicable.

Flat structures, such as the shoots of
many Algae and Liverworts, and many leaves,
often show on a vertical section through the
flat tissue-body an arrangement of the cell-
walls which will be intelligible from Fig. 296.
A certain similarity of the cell-network with
that of a slender growing-point with an apical
cell impelled the earlier observers to assume
in such cases a special type. They regarded
cells lying beside one another at the margin fig. 296.

as a series of neighbouring apical cells : this,

from a purely formal point of view, is of course possible, but it contributes nothino-
further to the explanation of the processes of growth.

It must now suffice to have brought forward the few cases adduced from the
copious material obtained by observation : a further extension of these considerations
would without doubt presuppose more patience than may be expected from readers
who are not Botanists.

LECTURE XXVIII.

FORMATION OF ORGANS AT THE GROWING-POINT:
BRANCHING.

The growing-points already considered to such an extent in the two preceding
lectures, scarcely ever come under the notice of those not trained in botanical science,
or at any rate they are passed over,' not only on account of their minuteness,
but also because, as a rule, they are hidden from view; those of most roots because
they are in the earth or other substratum, those of leaf-forming shoots because they
are completely enveloped by the young leaves. Nevertheless, it will already be clear to
the reader (and it is one of the most important facts of the whole of the physiology
of plants) that all formative processes are initiated at the growing-points — that all the
organs of a plant arise from these tiny masses of embryonic tissue which we have to
seek in the form of growing-points at the apices of the roots, or at the ends of shoots
inside the buds.

Here again, also, the profound diiference between root and shoot comes out
sharply. Roots produce only organs of like kind, namely, new roots, by means of
which the branching of the root-system is brought about ; whereas the activity of the
growing-points of shoots is entirely different and much more various : they produce
not only growing-points of new shoots, and thus of course subserve branching, but
from them alone also all leaves arise, and together with or on these the true
reproductive organs — sporangia, oogonia, and antheridia, since all reproductive organs
may be referred to these three forms. Thus, when in botanical investigations it is
important to examine organs of any kind whatever in their first stages of develop-
ment— in their primitive mutual relations — it is always the first object of the observer
to investigate the growing-points, and the formation of organs taking place on them.
Owing to the extreme minuteness of these objects, this has always to be done with
the aid of the microscope.

Here again we shall only concern ourselves with the commonest phenomena, or
those which offer features of special interest, and according to the plan of exposition
which I have adhered to so far, I again confine myself in the first place to the
typical and complete forms. We may first regard the growing-points of leaf-forming
shoots as found in all Vascular plants, most Muscinese, and even in many Algae :
the abnormal forms then present no further difficulties. Before entering more closely
upon our proper theme, however, it may be advantageous to make a few preliminary
remarks as to the form of the growing-point, and its position on the shoot.

In leaf-shoots which are growing vigorously in length, the growing-point has

FORMS OF GROWING-POINTS.

461

usually the form of a cone arched outwards like a paraboloid, and which appears
simply as the end of the shoot-axis ; in rarer cases, as for example in the water-
plants Uiricularia and Azolla, which are so remarkable also in other respects,
this cone is considerably elongated and rolled inwards spirally at its anterior end.
When the shoot-axis elongates more slowly, on the contrary, and especially when
a flower or an infloresence is to arise at the apex of the shoot concerned, the
growing-point generally assumes the form of a flat and broad elevation, which
very often becomes eventually extended horizontally, and forms a disc several milli-
metres in diameter. This is well seen at the apex of those leaf-shoots of the
Compositse (e.g. of the Dahlia and the Sunflower) which are preparing for the
formation of flowering capitula : the growing-point, previously conical, is flattened to
an almost level disc, on which the embryonic rudiments of young flowers bud
forth on all sides from the circumference towards the centre — i. e. the apex of the
growing-point. If, on the other hand,
a long flowering axis is to be produced,
to be covered later with numerous small
flowers, as in the Palms, Aroids, and
Grasses, the growing-point becomes
elongated into a_ cylinder, often several
millimetres long, on which 'the lateral
shoots of the future panicle, or even
the flowers themselves, theft arise pro-
gressively from below upwards. The
subsequent form of the whole inflores-
ence is thus already marked out in the
form of the growing-point; of course
the formative processes connected with
the elongation also co-operate later to
an important extent, in the completion
of the form.

Even w'here individual flowers are
to be developed, either as lateral out-
growths from a growing-point or at the end of leaf-shoots, the flower assumes in its
embryonic condition, long before the organs of the flower themselves bud forth, the
form of a hemi-spherical dome or flat disc, or very frequently even that of a hollow
cup, whereby the foundation for the subsequent form of the flower is laid: this
may then of course again undergo the most various transformations by means of
the changes of form connected with elongation.

The category of these modifications and departures from the ordinary type also
includes the far more striking cases of depressed growing-points. A very simple
example is aff"orded by the broad flat shoots of many Liverworts {Meizgen'a, Mar-
chantiecB) and particularly clearly by the prothallus of Ferns. As soon as these
flat leafless shoots attain a certain breadth, the elongating tissue last produced
by the growing-point becomes arched forwards right and left in the form of
two lobes, between which a deep depression marks the place where the embryonic
tissue of the growing-point lies. Fig. 298 v. In this behaviour of leafless shoots,

FIG. 297.— Longitudinal section of the apex of the primary stem
of Helianlhiis aniiuiis, immediately preceding the development of
the flowers. J apex of the broad growing-point ; * 6 youngest leaves ;
r cortex ; m pith.

402

LECTURE XXVIII.

where of course the formation of buds does not occur, we may probably recognise
chiefly a provision for protecting the substance of the growing-point, which is as
delicate as it is important. The same explanation applies to the similar depression
of the growing-point in the neighbouring tissue (in this case surrounding it on all
sides) on the likewise leafless shoots of species of Fucus, where a narrow long slit at
the end of each shoot opens into a narrow cavity, the base of which is occupied by
the growing-point, which here, as Rostafinski has shown, exhibits a series of curious
apical cells. A very similar arrangement also protects the growing-point of the sub-
terranean creeping shoot-axis of the Bracken fern {Pteris aquilind), because here, in

consequence of the extremely sparse pro-
duction of leaves, together with other
circumstances, no leaf-bud exists to enve-
lope the growing-point, and therefore the
leaves cannot protect the growing-point in
the soil.

In Phanerogams which persist by
means of subterranean tubers or bulbs, it
not rarely happens that at the end of a
short (or occasionally long) subterranean
lateral shoot, a body filled with reserve-
materials is formed ; and this contains the
bud with the growing-point at the bottom
of a central cavity. In these cases, how-
ever, peculiar displacements of the leaves
and of the parts of the axis which support
the growing-point usually take place at
the same time. The elucidation of these
processes, however, would carry us much
too far ; the chief points will be clear from
the consideration of Fig. 299.

In the researches, as numerous as they
are brilliant, of Thilo Irmisch, who
has done so much good service for our
knowledge of the subterranean organs of
plants, is to be found abundant material
derived from the observation of cases be-
longing to this category\ Here, again, it is evidently a matter of obtaining the

FIG. 298.— Prothallium of a Fern seen from below, f growing
point ; a antheridia ; iv roots (slightly magnified).

^ It is the phenomena of growth here treated of which especially comprehend the subject
of the very copious morphological literature. A more detailed consideration of the phenomena
belonging here would lead to a morphological review of the natural system ; from the general
point of view which I have attempted to establish in the preceding lecture, however, these
phenomena belong also to physiology ; in fact, they form in a certain sense one of the foundations
of the physiology of growth.

This being the case, it would scarcely be to the purpose to quote this or that treatise for proof;
however, it may be welcome to those readers who are unacquainted with botanical literature
and seek further instruction, to find quoted here a series of the most important works affecting this

DEPRESSED GROWING-POINTS. 463

necessary protection by the depression of a growing-point in a mass of resistent tissue,
which at the same time abounds in nutritive substances ; since such arrangements as
that in the figure occur chiefly in tuberous and bulbous plants, the entire vegetative body
of which perishes annually, leaving behind in the soil only these organs from which
new individuals subsequently arise.

Depressed growing-points are everywhere common in the development of young
flowers, or even young inflorescences, where
it is also usually important that a protective
hollow structure should be formed, in which
the very tender young parts of the flower,
and especially the ovules with their con-
tained embryos, are then to be produced.
A somewhat more complicated case of this
kind is presented in Fig. 300, which repre-
sents the longitudinal section of a young
flower bud of Getwi rivale, a plant belong-
ing to the Rosacea. The growing-point,
X, here projects in the form of a cone from

the bottom of a cup-like hollow structure fig. 299— Development of tubers of G«»^.«/?-^/««-« (after

^ Irmisch). a shoot-axis ; * leaf-sheath, in the axil of which the

_W, the inner side of which, at least at the bud ,4 with the tuber < has been produced.

basal portions, likewise consists of embry-
onic tissue, from which the young stamens a are arising. The elongation which
subsequently sets in, in this case results in that the wall yy of the hollow structure
becomes extended like a flat plate ; in the Rose, however, and in many other cases,
the part yy also subsequently forms a hollow cavity, open above by means of a
narrow mouth only. The depressed growing-point, .r, of the flower, produces at its
outer margin numerous small leaves, which subsequently grow together at their edges
and constitute the so-called ovaries, in each of which an ovule arises.

subject. For this purpose I may mention those from which I have myself derived the best instruc-
tion in years gone by— above all the fundamental works of Naegeli, as follows : —

' Zeitschrift für wiss. Bot.; Schieiden und Naegeli (Zurich, 1844-46), the memoirs on Catilerpa,
Delesseria, laws of growth of Mosses and Liverworts, Polysiphonia, and several other treatises.

' Pflaiizenphysiol. Untersuclmngen; Naegeli und Cramer, Heft I, 1855, growth oi Pterotham-
niuni, Hypoglossum, the leaf of Sphagnum, leaf of Aralia ; also Heft HI, on Lycopoditim and
Equiseiuf/i (by Cramer).

Further, 'Beiträge zur wiss. Bot.; Naegeli, Heft I (Leipzig, 1858), ' C'ber das Wachsthum
des Stammes tind der Wurzeln von Gcfisspflanzen; and H. 4, ' Enstehung tmd Wachsthum der
Wurzeln; by Naegeli and Leitgeb (München, 1867). On roots also, ' Rccherches stir raccroissement
terminal des racines; by E. de Janczewski, in Ann. des sc. nat., Ser. 5 (Paris, 1874).

Also '■Über den Vegetationspunkt der Angiospermenimrzcln; H. G. Holle, Bot. Zeit., 1876,
No. 16.

' Zur Entwicklungsgeschichte des Blattes; Dissertation by A. W. Eichler (Marburg, 1861).

' Die Scheitelzellgruppe im Vegetationspunkt der Phanerogat7ien; J. Hanstein, in Festschrift der
niederrhein. Ges. für Natur- tmd Heilkunde.

' Traite' cTorganogJnie compark de lafleur; by J. B. Payer (Paris, 1857).

' Recherches sur la ramification des Phanerogames; by IVL Eug. Warming (Copenhagen, 1872).

^ Die Coniferen und die Gnctaceen; Strasburger (Jena, 1S72).

That many other treatises may be named in addition to these comprehensive works has already
been slated.

464

LECTURE XXVIII.

FIG. 300. — Longitudinal section of a young flower
of Geum rivals (slightly magnified).

Similar depressions are occasionally met with in leaf-shoots also : this is par-
ticularly clear, for example, in the winter buds of the common Pine, as shown in
Fig, 301. At the end of the shoot of the previous year is situated the still quite

embryonic rudiment 2 of the next year's
shoot, with the growing-point v. This
embryonic shoot, however, is protected by
means of an annular wall for the purpose
of passing the winter. This wall arises
from the tissue of the shoot of the previous
year, and produces numerous bud-scales,
J, which completely envelope the embry-
onic shoot of the next year. These bud-
scales, moreover, as Goebel has shown,
are simply abortive foliage-leaves, which
may be transformed by means of vigorous
nutrition into the ordinary green acicular
foliage-leaves of the Pine.

I may, in conclusion, refer briefly to the
formation of the Fig, as a particularly
remarkable case of the depression of a
growing-point, and of a further departure
from the typical state of affairs dependent thereupon. The Fig, which in popular
language passes for the fruit of the Fig-tree, is as a matter of fact a structure of quite
a different nature from the ordinary fruits of Angiosperms. It is a so-called pseudo-
fruit. The Fig is a hollow structure pro-
vided with fleshy walls, which abound with
sugar. These walls are constituted by a
hollowed-out shoot-axis, on the inside of
which hundreds of small flowers (subse-
quently fruits) are situated : these latter
constitute the hard granules found in the
pulpy mass of a ripe Fig. Fig. 302, which
represents the development of a Fig (/-
III), shows that it is originally an ordi-
nary shoot, the growing-point of which,
however, after giving rise to several leaves
/a, assumes the form of a flat disc. In
the centre of this disc is situated the proper
apex of the shoot, w'hich takes no part in
further growth however : on the contrary,
'''^•3°i- it is the margin of the disc which retains

the embryonic character, and therefore
constitutes an annular growing-point. By means of the tissue formed by this
there now arises a hollow cylinder, /// a, at the upper margin of which the leaves
previously developed are situated, while further below arise new ones in addition,
closing the opening of the cavity of the Fig above.

TRANSFORMATION OF TUE GROWING-POINT INTO AN ORGAN. 465

The cases of depression of the growing-point here mentioned constitute a few
examples only, which might easily be increased by hundreds of others.

We may now return once more to the normal forms of the growing-point, and
to the theme which we are immediately concerned with, the formation of organs upon
it. Two extreme cases occur at the outset. The one consists in that the entire
growing-point, including its apex, is transformed into an organ — into a mass of
permanent tissue — where of course the growing-point as such completely gives
up its existence, and at the same time growth in length ceases as soon as the
elongation of the younger portions is concluded : such a shoot is said to have a limited
(definite) growth in length, in contrast to the other extreme case, where the growing-
point of a shoot remains persistently capable of living, and the tissue proceeding from
it continues to produce new axial portions and lateral organs by elongation. This may
be distinguished as unlimited (indefinite)
growth in length. As a few striking ex-
amples of this case the growth of the main
stem of Tree-ferns, Palms, Cycads, and
species of Adies and Finns may be ad-
duced; the growing-point of these plants
after tens or even hundreds of years is
still the direct prolongation of the grow-
ing-point of the embryo in the seed.

We meet with proved cases of a trans-
formation of the entire growing-point,
including its apex, into an organ of de-
finite physiological function, and consisting
of permanent tissue, chiefly in the forma-
tion of reproductive organs at the end
of the shoot. Although, strictly speaking,
not all the foliage-shoots of Phanerogams
provided with so-called apical flowers be-
long here, there are nevertheless numerous
cases which are certainly to be so con-
sidered ; namely, all flowers with central
ovaries in which is developed a single central ovule, as in the Polygonese, Juglandeae,
Chenopodiaceae, Piperacese, and other families. In Fig. 303 the portion of the young
Rhubarb flower marked k k \% the nucellus of the ovule, in which arises the
embryo-sac with the oosphere. This portion together with the two envelopes
surrounding it constitute the future seed, and from microscopic investigation there
is no doubt that in this and similar cases the nucellus of the ovule has proceeded
directly from the growing-point, which had previously given rise to the carpels/", the
stamens a, and the floral envelopes sp. Equally without doubt is the fact in
question in the formation of the sexual organs of many true Mosses. Leitgeb
showed long ago that in the Bog-mosses {Sphag?ium) at least the first female organ
(archegonium) of the ' flower ' arises directly from the apical cell of the fertile shoot.
Kühn observed later that in the case of another Moss {Andreced) the first arche-
gonium arises similarly from the apical cell, and the following archegonia of the same

[ 3 ] H h

Fig. 302.— Development of the Fig (Ficiis Carica),
after Payer.

466 LECTURE XXVIII.

' flower ' from its last segments : I have myself confirmed this in various other
IMosses. According to Kühn and Leitgeb, moreover, the male organs or antheridia
of Andreaa and of the aquatic Moss Foniinalis arise in the same way as the
archegonia, so that the apex of the growing-point of the Moss may be transformed
into a female or into a male reproductive organ. We may, in addition, also regard
the formation of the sporangium of Mucor at the end of the simple utricular branch
as a case of the formation of an organ directly from the apex of the growing-point :
the end of the hypha, which functions as a growing-point, dilates into a sphere
and produces the spores in its interior.

When in these and other cases the growing-point of a shoot is directly
transformed into a special organ, its previous growth in length is, as already

mentioned, brought to an end. A limi-
tation of the latter may also take place,
however, simply by the embryonic tissue
becoming entirely transformed into per-
manent tissue, as in the case of ill-
nourished Fern prothallia, which then
dispense with their growing-point : with
a proper supply of food-materials, how-
ever, a new growing-point may arise.
The formation of thorns affords an
entirely different example of this class.
The hard point at the end of the small-
leafed shoot of Gleditschia, and in simi-
lar cases, is evidently the growing-point
transformed into lignified permanent
tissue. In other cases, again, the ac-
tivity of the growing-point simply comes
to an end ; it disappears, so to speak,
its tissue becoming merged into neigh-
bouring permanent tissue. This is the
case, for example, in many flowers with
a central ovary but without a central
ovule. Finally, the terminal bud of a
foliage-shoot, together with its growing-point, may periodically die off", the next
lateral bud continuing the growth of the previous axis : in this way are formed
sympodia, the stem of the Lime being an example.

The peculiar characteristic of the growing-point becomes clear, however, in the
cases of so-called indefinite growth. Here the apex continues to grow undisturbed,
while underneath it (or, when the growing-point is flat, in circles around the central
apex) organs of like kind are produced in continuous succession ; these appear at
first as mere protuberances of the embryonic tissue, but become transformed later
wholly into permanent tissue. In this continuous repetition of products of like kind
from the circumference of the growing-point, we have one of the most universal laws
of growth of the whole vegetable kingdom ; organs of the most various kind—roots,
lateral shoots, sporangia, and sexual organs, may spring in this way in continual

Fig. yi-^.— Rheum undulalum. Longitudinal sectior
of the flower bud ; s fi floral envelopes ; a .? anthers ; d 7
glands at base of stamens; y ovary ; n stigma; k k
nucellus of the ovule (slightly magnified).

ACROPETAL VEGETATIVE REPETITION.

467

repetition from one and the same growing-point. This is most distinclly seen,
however, in the formation of leaves.

When organs of Hke kind arise from a growing-point in continual repetition,
this occurs generally, though not without exception, in such a way that the youngest
organ always stands nearer to the apex of the growing-point than any older organ of
the same kind. It follows from this that when we proceed from the base of a shoot
and follow organs of like kind — leaves for example — ascending in series on the
shoot-axis, and necessarily running spirally around the latter, or ascending it in a
zigzag manner, this sequence in space also represents the sequence in time or age in
the production of the organs concerned. Such a sequence in the production of
organs of like kind is usually termed acropetal. This is to be observed particularly
clearly in the case of ordinary foliage-shoots, where the leaves arise without exception
acropetally, as may be demonstrated especially in the case of a young, elongating
shoot, even without microscopic investigation. The numerous lateral roots arising
from a root, again, usually arise in acropetal order, and the same is the case when
numerous roots are produced
close beneath the growing-
point of a shoot, as is the case,
for example, in many Ferns.

The importance of the
growing-point lies, as has al-
ready been mentioned, in that
it consists of embryonic tissue
and gives rise to new organs.
Confining ourselves to this es-
sential character, the growing-
point need not always be situ-
ated at the end of a root or of
a shoot, but, on the contrary,
a portion of embryonic tissue
may be intercalated between
masses of permanent tissue,
noticed on p. 70, Fig. 66

—Median longitudinal section through a young inflorescence of the Sun
• (Hetianthus annittis) the growing point j of which had been injured.

One of these rarer cases has already been
and another above in the formation of a Fig, where
the true growing-point at the base of the hollow structure loses its function as
such, the further growth of the receptacle and the production of leaves and flowers
in its interior taking place by means of an annular zone of embryonic tissue.
Even in such cases the organs further distant from the zone of embryonic tissue
are older than those next it — in fact, a similar relation exists to that found in
ordinary terminal growing-points. This similarity, however, cannot well be expressed
by the phrase acropetal succession, and Goebel has therefore, in order to bring out the
general fact in the various cases, proposed the term progressive series or sequence.
Complicated cases of the progressive formation of organs at growing-points which are
not terminal are frequent enough, especially in the development of flowers. For the
elucidation of this matter, however, I will employ a monstrosity which I discovered
accidentally, and where the fact in question comes out particularly clearly. The
accompanying figure (Fig. 304) represents the vertical longitudinal section through

H h 2

468

LECTURE XXVIII.

a young flower head of the Sunflower {Nelian//ius antiitus). Its true growing-point
was situated in the middle of the disc n n, but the central apex of this growing-point had
been damaged by some means : the young tissue in its immediate neighbourhood
had lost the nature of a growing-point, but had become elevated in the form of
a protuberance a s a, while at the base of the latter 2 s, a zone of embryonic tissue had
been formed. The rudiments of new flowers and the bracts belonging to them only
arise now on this zone. So loug as the central apex of the flowering head was yet
living, the flowers and bracts were produced on the disc in acropetal succession n n,
or, as we may say in this case, in centripetal order: the flowers next the circum-
ference are the oldest, those nearest the centre the youngest. After the destruction
of the central apex, as the protuberance a s a was elevated from the zone z z,
however, the previous centre or apex behaved exactly as if it were the oldest part of
the protuberance : the production of flowers proceeded from the point s of the protu-
berance successively towards the zone z z, and we perceive that at the circumference of

Fig. 305.— Diagram

: the mode in which leaves are developed from the growing-point of a Phanerogam.

this zone the youngest normally formed flower-rudiments are situated, as well as the
youngest abnormally formed ones. The arrangement in both cases is still more
clearly manifested by the relative position of the bracts and the flowers which
belong to them : it is noticed that on the normal portion of the inflorescence each bract
is situated on the outer side of the flower which belongs to it ; on the abnormal portion
as a, on the contrary, the bracts are situated nearer to the former apex s than the
flowers which belong to them. To come back now to the starting-point of our
consideration, we may thus say that the formation of organs before and after the
destruction of the central apex has occurred in progressive order; that is, progressive
towards the true apex before the injury, and progressive towards the embryonal
zone z z after the injury : and it is clear that it v/ould not be quite to the point to
designate the sequence of the formation of organs on the protuberance a s a ^s
acropetal.

The acropetal or progressive order of development is strictly maintained by the
leaves on ordinary vegetative shoots ; at least no exception in this connection is as
yet known. In the region where flowers are formed, on the contrary, cases occur as a

DEVELOPMENT OF LEAVES.

469

matter of fact where organs of leaf-like nature are interpolated between organs already
existing, so that the youngest are no longer next the apex. Payer has detected
such cases in a lafge number of flowers where two or more circles of stamens (that
is, metamorphosed leaves) are present, as, for example, in Dictamnus and Geranium.
He shows that the second circle of stamens arises on the receptacle outside the first,
and therefore further from the apex or centre of the flower. Of course in these and
similar cases the question might be raised whether the true embryonic formative
tissue is not always perhaps to be sought only at the place where the youngest
organs arise — a place which indeed need not always be situated at the apex.
However, further pursuit of this question would involve us in difficulties with which
I need not trouble the reader of this book.

The leaves arise at the growing-point of the shoot without exception as super-
ficial outgrowths. This does not mean that only the outermost layer of tissue of the

Fig. 306.— Apical region of two main shoots of Zar Mais,
s growing-point consisting of very small cells, from which the
leaves *'*"*"' proceed as multicellular protuberances which
soon embrace the stem and envelope it and the young leaves
like a hollow cone. In the axil of the third-youngest leaf *'
the youngest branch-rudiment is seen as a rounded protu-

FIG. 307.— Growing-point at the end of the
leaf-shoot of Hifpitris (magnified).

growing-point is employed for the formation of the leaf ; but the expression signifies
that this layer always takes part in the origin and growth of the young leaf. As
a rule, however, layers of tissue situated deeper also co-operate in the formation
of the leaf. It may thus be stated as the characteristic point that, in the origin of
a leaf, complete continuity of the homonymous layers of tissue of the leaf and of the
shoot-axis exists from the first, as is clear at once from the diagram Fig. 305, where
A is the apex of the growing-point, which, like all growing-points which produce
leaves, possesses the confocal structure described in the preceding lecture. B, C,
and D, mark various stages in the origin of a new leaf, and the cells and periclinal
walls denoted by Roman and Arabic numerals show how the individual cell-layers
of the growing-point take part in the origin of a new leaf. The letters a, b, c, x,
and J', in the figure, show how the existing anticlines and periclines are displaced
in these 'processes of growth. Only when the leaf-rudiment has attained a certain

470

LECTURE XXVIII.

size does the difierentiation of the vascular bundles take place, in the embryonic
tissue of the shoot-axis as well as the young leaf, as mentioned previously, so that the
vascular bundle curving out into the leaf represents the upper end of that which
descends in the shoot-axis. The differentiations of tissue are similar when a normal
growing-point of a shoot arises from a growing-point already existing ; that is, in the
branching of the shoot. The case is quite otherwise in the origin of new roots,
however, as already pointed out in the lectures on Organography. Whether the
roots arise from a mother-root or from a shoot they are always produced in the
interior of the tissue, so that the young root, when elongation begins at the base of
its growing-point, has to break through the external layers of tissue (cortex and
epidermis) of the mother-organ. Of course in this case also a complete continuity of
the homonymous layers of tissue comes to exist subsequently, but only subsequently.

'IG. yJi.—Cnnterfa ,

u/olia. jthesho

growing-point ; h leaves ; •

and the microscopic structure gives the impression that it is here so to speak a matter
of patchwork.

For the subsequent form of the entire shoot it is important what portion of the
circumference the young leaf-rudiment occupies on the growing-point. In most
cases it is only a small part of the circumference, and obviously this is always the
case when two, three, or more leaves are put forth at one and the same transverse
zone, that is, when a whorl of leaves springs from the growing-point. In many
Monocotyledons (Grasses, Sedges, Aroids, Palms, &c.) and even in some families of
Dicotyledons, such as the Umbelliferse and Polygonacese, however, the whole circum-
ference of a transverse zone of the growing-point is occupied by the rudiment of
a leaf. In such cases the base of the leaf appears on further development as a
sheath surrounding the shoot-axis, at the upper edge of which the true »leaf, the

DEVELOPMENT OF LEAVES AND BRANCHES.

471.

so-called upper leaf, buds forth only on one side. If the leaf is subsequently seg-
mented into petiole and lamina, which of course does not always happen, the petiole
is to be regarded as a structure subsequently interposed between the leaf-base and the
lamina, a subject to which we shall return in the next lecture.

In the INIosses and Vascular plants the leaf-rudiments generally arise so closely
above and by the side of one another, that none of the free surface of the growing-
point at all is exposed beneath the youngest leaf. Only through the further course
of development in the second period of growth is it then decided whether the leaves
which arise closely one over the other are separated from one another by the
secondary intercalation of interfoliar parts on an elongating shoot-axis, or whether
this does not occur ; in the latter case none of the free surface of the shoot-axis at all
appears, even in the fully grown condition, because the
leaves, situated close above and below one another,
occupy the entire surface of the shoot-axis, as for
example in the common Fern Aspidium and in the
short twigs of many trees, and the so-called radical
rosettes of many biennial Dicotyledons. Cases where
the leaves at the growing-point are from the first
so far distant from one another that naked inter-
foliar parts of the shoot-axis exist between them, are
probably extremely rare ; as an example may be
mentioned the Bracken Fern {Plej-is Aquilina), where
the growing-point of the horizontal stem annually pro-
duces only one leaf, which requires two years more
for its completion. In the leaf-forming Algae also the
leaves usually arise quite close above and by the side
of one another on the shoot-axis, even though very long
interfoliar parts separate them subsequently, as is very
clearly the case in the Characese (cf. Fig. 95, p. 96).
The genus Caulerpa, on the other hand, presents a
good example of the other case (Fig. 308), where the
growing-point v of the shoot-axis always first elon-
gates to a considerable extent before a new leaf
buds forth from it again. We meet with similar
conditions of affairs in those Algae also where the
lateral outgrowths of the growing-point of a shoot do not develope into flat leaves
in the ordinary sense, and where therefore the shoot constitutes a mere branch
system, as in Fig. 309, where it may be noticed that the youngest branches appear at
some distance apart from the very first.

Shoots, however, produce not only leaves and other organs, but new growing-
points of shoots generally arise again from their growing-points ; that is, they
branch.

Only in very few plants does no branching take place — that is, no formation of
growing-points of secondary shoots. The best known examples of these are found
in some small acrocarpous Mosses, and above all in a few Vascular Cryptogams.
The longest known instance of this is the genus Isoetes, the stem of which grows

'f^^

HG. 309.— Branching of a
leafless shoot of the Alga Gcli-
riiiim (one of the Floridere).

472

LECTURE XXVIir.

for years, and nevertheless always remains short, and only produces leaves and
roots, but no shoot-buds. The Ophioglossaceae and Marattiacege also behave in
exactly the same way, as well as the tall Tree-ferns. In all these cases the single
existing growing-point of the shoot of the unbranched plant, even in advanced age,
is the very same which was developed directly from the fertilised oosphere. Even
the Cycadese, so similar to the Ferns, do not as a rule become branched from the
growing-point, though this may occur in advanced age after the formation of
flowers. It occurs also occasionally as an abnormality in the otherwise so copiously
branched Pines that no secondary growing-points are produced from the original
one of the seedling, and the tree remains for many years without branching. As an
approximation to this, however, we may perhaps consider the case when the primary
shoot developed directly from the embryo grows out into a dominant main stem, the
lateral branches of which always grow considerably less vigorously than the stem itself,

Fig. 310.— Apical region of a primary
shoot of Dictamnus Fraxinella seen from
above, j apex of the primary shoot ; ***the
young leaves; kk their axillary buds. The
two youngest leaves have as yet no a.\illary
buds.

Fig. 311.— Bulb oi Miiscayi botry-
oides. One of the lower bulb-scales
(leaf) is turned back in order to show
the numerous buds situated one be-
side the other in its axil.

as is the case in the Conifers and some other trees. In many species of Cactus a
similar condition of affairs is found in that the shoot-buds, elsewhere developed so
universally in the axils of the leaves of dicotyledonous plants, only rarely attain to
further development: here, also, the growing-point of a shoot which has once
attained the necessary vigour dominates the entire growth, and a lateral shoot is
not easily permitted to make its appearance (Cereus, Echinocachis, Mamillarid).

In contrast to these rarer cases of the absence, or paucity of branching, however,
the tendency prevails in the great majority of plants to produce new growing-points
of shoots from the primary ones, and tertiary ones from the secondary ones ; that
is to develope branch-systems. In Monocotyledons and Dicotyledons it is an almost
universal rule within the vegetative region (that is so long as flowers are not being
formed) that a new growing-point of a shoot is formed in the axil of every leaf, and
in fact very soon after the origin of the leaf itself; this makes its appearance either

AXILLAR}' BRANCHING. 473

exactly in the angle between the leaf-base and the axis, or more on the leaf-base
itself, or even on the shoot-axis above it. Not rarely indeed several growing-points
of shoots arise in the axil of the leaf, and if the base of the leaf is very broad, as
in the Monocotyledons, numerous shoot-buds may arise close to one another, as
for example in the case of many bulbs, and the flowers in the broad bracts of species
of Musa. In Dicotyledons with a narrow leaf-insertion, moreover, two, three, four
or more growing-points of shoots are often produced one above the other from the
interfoliar part which is in process of elongation, as for example in Gleditschia,
Aristolochia, and some other woody plants. This regular production of new growing-
points of shoots in the axils of the leaves obtains however, even in the Monocoty-
ledons and Dicotyledons, only within the vegetative region : in the inflorescences it
is a frequent phenomenon that flowering shoots arise without the previous formation
of bracts {Aroidece, Cruciferce). Conversely, however, no growing-points of shoots
make their appearance in the axils of the foliar structures of the flower itself. The
theory of the older morphology which culminated in the establishment of a so-called
principle of axillary branching, therefore, does not generally obtain even in the
Phanerogams with which that morphology was almost exclusively concerned, and
this is still less the case with the other classes of plants. If we confine ourselves
in the first place to the Coniferse, the nearest related to the Phanerogams, the rule
certainly obtains here also that within the vegetative region shoot-buds are produced
only in the axils of the leaves, but with the difference that although the leaves are
exceedingly numerous, but few of them produce growing-points in their axils : in
spite of the copious and magnificent branching of the Coniferse, it is, from the point
of view of its development, only sparse in comparison with that of the INIonocotyledons
and Dicotyledons. In the Vascular Cryptogams, however, and so far as is known in the
Muscinese and Algae, the formation of secondary growing-points, or the branching
of the vegetative shoot, is entirely unconnected with the axils of the leaves, although
the tendency always exists for lateral shoots to make their appearance by the side
of or under the insertion of individual leaves. However it would be superfluous
for our purpose to treat in greater detail here of the individual cases which have
been more exactly investigated.

We have hitherto confined our attention, with respect to branching, to the ordinary
case, where new growing-points of shoots spring laterally from a growing-point, or
from a shoot-axis, or from a leaf-base, the parent growing-point itself continuing
to grow undisturbed. Much rarer, and confined to a few small subdivisions of the
vegetable kingdom only, is the so-called Dichotomy of the growing-point. We
may take as the characteristic distinction of this that the embryonic tissue of a given
growing-point obtains two apical points, each of which now constitutes the apex
of a new growing-point, and thus the embryonic substance of a growing-point
becomes, so to speak, split into two growing-points, situated close beside one
another and equal in value. This case is particularly clear where an apical cell
exists from which two new apical cells arise, each of which then forms its segments
as I have already explained in the previous lecture (cf. Fig. 291). The accompany-
ing figure (Fig. 312) may serve to illustrate the branch-system of the same plant,
proceeding from the dichotomy. It will be observed that a depression occurs at the
apex of the shoot, and that the prominences which lie right and left of it grow further

474

LECTURE XXVIII.

and constitute branches of the shoot. The behaviour of a dichotomy of course
becomes more compHcated when leaf-forming shoots are concerned, as in the
Lycopodiaceae, where in all the genera and species not only the shoots but also

the roots branch in a dichotomous manner.
In contrast to the ordinary lateral, or as
it is also termed monopodial branching,
there is this characteristic feature, that the
formation of the two new growing-points
from the older one takes place not only
above the youngest leaf (simply because
it takes place in the apex itself) but it also
stands in no relation whatever to the pre-
ceding leaves.

A long misunderstood but nevertheless
very simple case of dichotomy, and one
which is not essentially different from
those hitherto mentioned, is found in those
Liverworts which possess broad band-like
creeping shoots, especially the Marchantiese
and leafless Jungermanniese. If we put
aside the sequence of cell-divisions, which
have been studied in every detail, as well as the presence or absence of an apical
cell, here also the dichotomy consists in that the group of embryonic tissue at the
base of the deep depression previously described becomes slightly extended in
breadth, as shown in Fig. 313; the middle portion of this embryonic tissue then

Fig. 312.— Dichotomously branched shoot of
Dictyota dichotoma, a marine Alga,

Fig. 313.— Flat dichotoninusly branched shoot oi Metzg-eria _furcala
(X about 15) ; )« w mid rib of several layers, dichotomising at^^ ; f J- grow-
inij-points, with projecting wing-like outgrowths//"',/".

grows vigorously, and becomes transformed into permanent tissue, which now
projects in the form of a wedge arched in front, while right and left of it the
rest of the embryonic tissue remains as such and now constitutes two growing-points
developed from one.

PSEUDO-ENDOGENOUS BUDS. 475

The formation of new growing-points of shoots in any of the above-mentioned
ways constitutes, however, merely the ground-plan of the branch-system which will
be ultimately developed, for it is only in rare cases that all the growing-points
of shoots which have gradually arisen from one another, and ultimately form
the primary shoot of the seedling, come to complete development. Very frequently
this happens only in the case of a few, while the others pass over into a dormant
condition, either to attain complete development in the next period of vegetation, or
to remain unchanged for an indefinite time as the so-called dormant eyes of many
forest trees, until by the removal of vigorous shoots opportunity is afforded to
them for further development. The sprouting of numerous twigs from the older
branches and stems of forest trees when the apical parts have been removed
consists in the sudden invigoration of these dormant buds, which have often been
developed many years previously when the corresponding part of the stem itself was
yet in the condition of a bud. Such dormant eyes must not be confounded with
the adventitious shoots to be described subsequently.

Like the leaves, the new lateral shoots usually arise exogenously, or superficially,
so that not only the internal but also the outermost layers of tissue of the growing-
point take part in their production, and thus a complete continuity of the systems
of tissue is brought about, exactly as in the case of leaves. It was for a long
time supposed that the Equisetums formed a peculiar exception to this very general
rule. As a matter of fact we find, on cutting longitudinal sections through the
buds of the haulms of these plants, the lateral buds situated in the axils of the leaf-
sheaths, and completely enveloped by the tissue of the main shoot, much as in the
case of roots ; but more exact investigations have shown that this is only due to sub-
sequent change, and that, even in the Equisetacese, the very youngest bud-rudiments, or
lateral growing-points, proceed from superficial layers of tissue of the growing-point
which gives rise to them. The matter is very similar, though more complicated, in
the case of the two trees Symphoricarpus and Gledilschia, much cultivated in our
gardens. \\c may say a few words with respect to Gledilschia : the thorns on the
leaf-shoots are the original axillary shoots of the leaves, but by the growth of the
mother-axis they become pushed some distance upwards from the axil of the leaf.
A series of further buds then arise progressively towards the axil of the leaf, the
embryonic tissue at the axil evidently persisting as such for a long time. These
later buds remain very small at first, and, as Hansen established, become so
enveloped and overgrown by the cortex of the mother-shoot during the current period
of vegetation, that when they begin to sprout in the following spring it looks as if they
had been produced in the interior of the cortex which they then break through.
As the biology of plants everywhere affords thousands of examples of the fact that
young buds are preserved for future periods of vegetation, and protected and
enveloped in the most various ways, so we find even in some woody plants (e. g.
Virgilia lulea) that the broad base of the leaf encircles its axillary bud so comj)letely
that nothing at all is to be seen of it from without, and it only comes into view after
the removal of the leaf-stalk.

In contrast to all these various cases of an only apparently endogenous origin
of shoot-buds, it appears however that flowers and inflorescences may arise in the
interior of previously more or loss amorphous masses of tissue, particularly in the case

476

LECTURE XXVIII.

of parasitic Phanerogams ; so that in the Balanophoreae, Rafflesiacese, and perhaps
also in the Orobanchese, these reproductive shoots must first break through an
envelope of tissue when they begin to elongate (cf. Fig. i8, p. 28). However, the
matter requires further investigation, and the same holds good probably also of
certain growing-points of shoots in the Jungermanniese among the Liverworts, which
Leitgeb assumes to originate endogenously.

Fig. 314 and 315.— Origin of a lateral shoot of Equüetum (after
Janczewski). Fig. 314 (above) earliest rudiment at ,*. Fig. 315 (below)
further developed bud k with origin of root lu. In both figures st is
the internode above the sheathing leaf sh (highly magnified).

The endogenous mode of origin is, on the contrary, a general peculiarity of the
growing-points of roots; concerning which the most essential facts have already been
stated in the introductory lectures on Organography (p. 15). It is only necessary now
to make the additional remark that the young lateral roots only make their appearance
on the mother-root, as a rule, at a considerable distance from its growing-point.
This, however, does not prove that they did not already exist in embryo in the

ADVENTITIOUS GROWING-POINTS. 477

neighbourhood of the mother growing-point, or even in it. Unfortunately, in spite
of innumerable examinations of the apex of the root, special investigations on this
question are wanting : only so much is certain, that in many cases the first visible
rudiments of. the young growing-points of roots are already to be found a few tenths
of a millimetre behind the apex of the parent-root. It has likewise already been
mentioned in the lectures on Organography that the young roots arise progressively
from a special simple layer of tissue at the circumference of the axial strand of the
mother-root, and this, corresponding to the vascular strands, in two, three, or more
longitudinal rows. Since the first beginnings of these tissue-differentiations are recog-
nisable right up to the apex of the growing-point in the mother-root, the facts given
do not prove that the growing-points of the lateral roots did not already exist in
embryo in the embryonic tissue at the growing-point of the mother-root. While they
then develope very slowly, the apex of the latter grows rapidly in length, and thus
it appears as if they had been produced far distant from the parent growing-point.
In the absence of exact investigations on this point (which entail very considerable
difficulties moreover), we must not overlook the possibility that the first rudiments
of young roots may be formed at a distance from the parent growing-point; here,
however, the further assumption would present itself that the layer of tissue surround-
ing the axial strand, and from which the young roots originate, contains traces of
embryonic substance derived from the growing-point. This layer of tissue would
then, with respect to the formation of organs, behave much as the cambium of
wood-plants with respect to the formation of tissue, since we may regard the cambium
also as a thin layer of embryonic tissue, which, however, only in rare cases produces
growing-points of shoots. This is saying nothing new, since even the tissue of
the growing-point was formerly designated Cambium.

We have hitherto concerned ourselves exclusively with the formation of new
growing-points from growing-points which already exist, or, to speak more generally,
from embryonic tissue already present. In this way arises the ordinary normal
branching of shoots and roots, and the foundation for the complete normal
architecture of a plant is laid. It also happens, however, that new shoots and roots
may originate at places where the tissue has already passed over into the permanent
condition : in fully developed roots, in the interfoliar parts of shoot-axes, and more
particularly in foliage leaves, the tissue of which is already completely diff'erentiated
and developed, new growäng-points of roots and shoots may appear in somewhat
numerous cases. I distinguish these new formations exclusively with the name
adventitious structures — a name which formerly signified many other things also —
thus implying that they constitute something merely additional, something superfluous
for the normal structure of the plant. This is not saying that they are superfluous
for the whole biology and maintenance of the plants concerned : on the contrary,
adventitious roots are often important organs for the support of a plant, while
adventitious shoots serve as organs of propagation which sooner or later become
separated from the mother-plant as so called bulbils, and, forming adventitious roots,
become independent plants.

Adventitious roots may originate, often in large numbers, at the most various
places between existing lateral roots from a main root, or on shoot-axes or leaves, or
even close beneath the growing-point of shoots, and may be indispensable even for

478

LECTURE XXVII T.

the normal maintenance of the plant : this is the case, for example, with many Ferns,
and with the runners and rhizomes of numerous other plants.

The occurrence of adventitious shoot-buds is much rarer. It takes place chiefly
on fully developed or on young leaves. As particularly well-known examples may
be mentioned those in the indentation of the margin of the leaf of Bryophylhmi
calycinum, and the bulbils regularly formed in the angles on the ramifications of the
veins of the leaves of Cardamine pratejisis and Nasturtium officinale. Very striking
examples of such buds are found moreover in Ny??iphcca viicrantha at the centre of
the peltate leaf, and in various Aroideae — e.g. Atherurus ternatiis, where the
adventitious buds have the form of tubers or bulbs, which subsequently become
free and form new plants. In a considerable number of other Monocotyledons
and Dicotyledons also adventitious buds appear on the foliage leaves in the ordinary

course of vegetation: these, however, so
long as the mother-leaf is actively vegetat-
ing, continue in the bud-state, as in the
cases mentioned previously.

Adventitious shoots are particularly com-
mon on the large foliage leaves of the
Ferns, and they appear regularly in the
angles of the pinnatifid laminae of Ce7-a-
topteris thalictroides, and on the rachis
oi Aspleniiim caudatum, A. deciissatum, and
other species of this genus : they occur
also on the under side of the mid- rib of
Woodwardia radicans and others. In these
cases, as in Cardamine, they grow while
still connected with the mother-leaf into
vigorous copiously branched plants, which
sooner or later become free from the
mother-leaf and can at once proceed to
active vegetation. Among the most in-
teresting examples of this kind we have
moreover the formation of adventitious buds on the back of the living basal portions
of the older foliage leaves oi Aspidium ßlix-7nas {i\iQ common Male-Fern), for this
plant developes no lateral buds whatever except these adventitious shoots (which are
moreover very isolated) and possesses no normal branching. It has already been
stated in the introductory lectures on Organography that adventitious shoots are also
developed not rarely from the roots of vascular plants, and new independent plants
then proceed from these : this is the case, for example, in Robinia, Ailatithus, and also
in the Fern Ophioglossimi. To these cases also we may in a certain sense refer the
production of shoot-buds from the growing-points of roots, as mentioned on p. 22.

In opposition to the view formerly held by Hofmeister, that all adventitious
growing-points arise in the interior of the tissue, and that all so arising are to be
designated adventitious, it is to be noticed that in that case almost all roots would
have to be regarded as adventitious. With respect to adventitious shoots it is demon-
strated by recent investigations, and especially those of Hansen on Carda?nine,

Fig. -i^b.—Asplenium dectissatum. Middle portion of a
fully developed leaf the rachis st of which supports the pinnules
// ; at the base of one of these has arisen the bud k which has
already put forth a root (nat. size).

ADVENTITIOUS DUDS.

479

Ndsiurtium, and Alheninis, and is very probable for the remaining cases, that they do
not arise endogenously at all, but exogenously ; in fact Hansen showed that in
the cases mentioned cell-divisions and other changes appear in various directions
at the points in question, in the epidermis and cordcal tissue of leaves which are
already fully developed, thus transforming the permanent tissue into embryonic
tissue, which then forthwith forms a conical growing-point whence leaves and,
subsequently, roots proceed.

If the definition above given is adhered to, to regard as adventitious all
growing-points which proceed from already differentiated permanent tissue, we
must also regard the origin of the leafy stems of a true IMoss as altogether adven-

titious, since, as has already been said in the introductory lectures on Organography
and as is to be seen from Fig. 3 1 7, these arise from certain cells of the protonema
which have already acquired the character of permanent cells. Here, as in other
cases, it simply results that sharp limitations with regard to Nature are always
ineffectual; since, though we see in adventitious structures in the higher plants
something superfluous for their architecture, this no longer holds good in the case
of the Moss.

In a certain sense the formation of so-called gcmmx in the simply organised
Liverworts, Algae, and some Fungi depends upon adventitious budding; the pro-
toplasm of the already fully developed cells of leaves or other organs may become

480 LECTURE XXVIII.

isolated from the remaining tissues, surround themselves with new cell-walls, and
after a period of rest develope into new plants. -

All these phenomena of adventitious budding apparently stand in contradic-
tion to the previous statement that the formation of organs in the vegetable
kingdom proceeds from the embryonic tissue of the growing-point, which we have
in its turn regarded as a derivative from the embryonic substance of the fertilised
oosphere. But considerations from general points of view, to which I shall return
subsequently, render this apparent contradiction irrelevant; as the growing-points
which occur so universally are subordinated to the still more general conception
of embryonic tissue, so also this latter again may be looked upon simply as a
collection of embryonic substance formed during the process of nutrition in the
plant, and collected at definite spots according to circumstances in each case.

In the cases of the adventitious formation of shoots and roots hitherto con-
sidered, it occurs in the particular species of plant at certain definite points, and
belongs moreover to the normal phenomena of life. At another opportunity,
however, and proceeding from other points of view, I shall have to show that
adventitious growing-points may arise at almost any given spot on the older
parts of plants, when these are cut off or injured, and that in this manner a new
shoot provided with roots may arise from the separated portions of shoots and
roots of many plants. It is better to reserve these phenomena for subsequent
consideration, however, since they serve to demonstrate particularly clearly the
influence of external forces on the production of new organs.

LECTURE XXIX.

AXIS OF GROWTH; POLARITY; LATERALITY; RELATIONS
OF POSITION.

The terms at the head of this chapter denote a series of relations in space
within the plant, the consideration of which we cannot here entirely pass over,
because they afford us clear ideas on certain general processes of growth ; and it
is shown at the same time that a series of the most important relations between
growth and external influences — i.e. phenomena of irritability — are only intelligible
if we aljstract from the relations of form evident to the senses, and keep in view
these relations which are only accessible to abstraction.

On every organ it is easy to distinguish two opposite ends, which we may
designate base and apex \ The base is the place where the organ arises from its
parent-organ, and is fixed to it ; the other end, the apex, on the contrary, is free
and movable. As the organ elongates, it is the apex which is pushed forwards in
space by means of the growth taking place between it and the base. When the
bud at the end of a shoot, or the growing-point of a root, gradually traverses a path
of several centimetres, or even of several metres, in a perpendicular, horizontal, or
oblique direction, this is accomplished by means of the growth between apex and
base, by the intercalation of new substance, and, simply because the base is fixed
while the bud with the apex is free, it is the latter which is moved forwards in space.

A line which is supposed to run in the interior of the organ from the base to
the apex gives the longitudinal direction, or the direction of growth in length ; and
a plane which takes in this line is a longitudinal section. Every surface standing at
right angles to this plane is a transverse section.

' In my ' Lehrbuch der Botanik'' (Anfl. II. 1S70, § 26) I have already referred sufficiently in
detail to the significance, not only morphological but also physiological, of the distinction between
apex and base of the organs of plants ; and I pointed out in my treatise ' Über Stoff und Form der
Pflanzenorgane' (Arb. des bot. Inst, in Wzbg., B. II, pp. 452 and 689) that Voechting had subse-
quently, in his book ' Über Organbildung im Pflanzettreich'' (1878), given another inaccurate mean-
ing to the polarity designated by base and apex. In my text-book, as well as in my later treatise,
' Über die Anordnung der Zellen in jüngsten Pflanzentheilen' (Arb. des bot. Inst, in Wzbg., B. II,
p. loi), the previous entirely inaccurate definition of the axis of growth by Hofmeister was also
corrected.

[3] Xi

482

LECTURE XXIX.

If tlie growing-point is situated at the free end or apex of the organ, then
the base of the latter is its oldest part, and every transverse section meets a part
so much the younger the nearer it lies to the apical growing-point. If, however,
the embryonic tissue which represents the growing-point is situated at the base
of the organ, as is the case in many leaves, internodes, etc., then the relative age is
in the reverse direction.

The longitudinal direction does not suffice to fix the position of the axis of
growth in an organ. To define this more exactly the transverse section of the organ
in question must be taken into consideration : in the transverse section of any
organ there is a point which, with respect to the outline and the anatomical structure,
constitutes the organic centre. Every small portion of the surface of the transverse
section exhibits in its structure a side turned towards the periphery, and one turned
towards this organic centre, and is bounded laterally by lines which we may term the

FIG. 318.— Transverse section of the woody body of the stem of a L

radii of the transverse section. Just as the organic centre by no means necessarily
coincides with the geometric centre of the transverse section, so also the radii are not
always straight, but often curved lines, as is clear at once from the consideration of
Fig. 318. Very generally the internal structure of the organ exhibits peripheral
layers and radial arrangement at the same time, as is distinctly perceived on any
transverse section of wood ; but every hair also, and even petioles and leaves, as well
as the transverse section of single cells, may be thus regarded.

If we now suppose the organic centres of all the transverse sections of an organ
(a shoot-axis, the mid-rib of a leaf, or a root, etc.) to be connected together by a
line, this is the longitudinal axis, and also the axis of growth of the organ; and,
according to circumstances in each case, it may be a straight or a curved line, or
if at first straight it may subsequently become curved, and conversely. A plane

THE AXIS OF GROWTH. 483

which contains the axis of growth is an axial longitudinal section. If the axis is
curved in one plane there is of course only one axial longitudinal section ; if, how-
ever, the organ is itself spiral, the axis of growth also describes a spiral line which
cannot be contained in one plane.

As a rule the growth in the direction of the longitudinal axis is more vigorous — i.e.
is more rapid, or continues longer, or both together — than is the case in directions across
the axis, as is at once seen in the case of most shoot-axes, roots, leaves, and hairs.
This character, however, cannot be employed for the general definition of the axis of
growth : there are cases where the growth takes place more energetically transversely
to the longitudinal axis than parallel with it, as, for example, in the stem of Isoe/es, in
the short and broad fruits of some varieties of the Gourd, and in many leaves which
are broader than they are long. However, no doubt can ever arise with respect
to the axis of growth in such cases, since it is always possible to determine with
certainty whether the transverse or the longitudinal section of an organ is under
observation, and with a little practice even
a very small portion of a transverse or
longitudinal section suffices for the re-
cognition of its true nature.

If the growth in the direction of the
axis is unlimited in the sense given above,
the formation of external organs as well
as the internal structural relations repeat
themselves in the same direction ; the
organ becomes segmented into a series

r>f nnrtc vuViiVVi mciv Ka rlictinmiicVif^rl cia Mp- '""'■■ 319.— Anterior portion of a shoot o^ Herposiphoma

01 pans wnicn may oe aistmguisnea as me- ,.^^„,^ („„^ „r the Fiondea). Froir, the shoot-axis, which is

tTTTiprAC cin AvnrAcoir^n olrf>irK' ^^mr^lnv^rl curved forwards and upwards, and the g:rowing-point of which

lamereS an expression aireaa)- empiOyea j^ ^j „^ j^e leaves b'-bs arise ; these are likewise curved.

in Zoology. The individual internodes -/ '-'-1 shoot ;.. root (after coebei).
of a shoot, with the leaves or whorls of

leaves, lateral buds and other organs belonging to them, represent these metameres.
If, however, the free growing-point of a shoot or leaf normally concludes with the
formation of some special organ, metameres may still be present, but they are not
similar to one another; on the contrary, they undergo metamorphosis from the
base towards the apex, as is particularly evident, for example, in the different forms
of the foliage leaves on upright shoot-axes which conclude with a terminal flower
(^Ranunaihis acer, Papaver somniferuni). The shoots of many horizontal creeping
or climbing plants, on the other hand, consist of metameres which resemble one
another, e.g. Lysimachia nummidaria, Cucurbita, Glechoma, Marstlia, etc.

The presence of an axis of growth does not necessarily imply a distinction of
base and apex also, although other cases may occur only among the very lower
Algse. For instance, the filaments of the Algae Spirogyra and Sphccroplea, as well
as the filamentous cell-families of the Desmidiese and Diatomece, have always a
longitudinal axis, though the two ends of the filament exhibit no distinction of apex
and base. With this state of affairs is connected the fact that such plants are not fixed
by the end to a substratum. In the globular cell-families of the Volvocinese we find
no distinct axis of growth at all, but only a middle point around which the whole
organisation is more or less symmetrically arranged on all sides ; and some flat discoid

I i 2

484

LECTURE XXIX.

Algse, as Pediaslrum and Coleochce/e scutata, may be looked upon, so far as their
relations of growth are concerned, as representing a thin transverse disc cut out of
a radially constructed shoot-axis.

The ordinary case, however, is that of polar structure^!, e. the external and
internal organisation exhibit a peculiar arrangement along an axis of growth, in such
a way that all the relations of organisation are directed from the base towards the apex.
This is very evident, for example, in almost all shoots, where it is possible to see with the
greatest clearness even on small pieces which is the anterior and which the posterior
end, or which is the base and which the apex. Doubt could only exist on this sub-
ject, in the absence of exact investigation,
in the case of roots deprived of the apex.
The polarity to which I here refer thus
indicates a progressive differentiation in
the direction of the longitudinal axis,
which may be compared to a certain
extent, at least so far as it concerns the
innermost nature — the capacity for reac-
tion—to that of a magnet.

The polarity appears more sharply ex-
pressed, however, in the development
of a new individual from a spore or
from an oospore, where a distinct con-
trast is at once observable between the
root-end and the shoot-end. This is the
case in the simplest and most elegant
form with the ovoid swarm-spores of many
Algae. The narrower colourless end, which
is anterior and provided with cilia during
the swimming movement, already indicates
the root-pole even before germination ;
while the other end, green and thick,
may be distinguished as the shoot-pole.
The swarm-spore subsequently clings by
means of the root-pole to a solid body,
and the root-like organ of attachment is developed there, while the free shoot-pole
developes into a simple or branched shoot. This is not the place to go into many
other cases, and I may therefore at once refer to the behaviour of the young embryos
of the Vascular plants. Here also, as in the case of a swarm-spore, the con-
trast between root-pole and shoot-pole makes itself evident at an early period,
and when the embryo is to a certain extent further developed, especially in the
ripe seed in the case of the Phanerogams, it already actually consists of a root
at the one end and of the first shoot at the other. That these two poles, which
make their appearance even in the fertilised oosphere, subsequently come to act as
root and shoot, and have an importance which is not merely something external
and formal, but which dominates the whole being of the plant, follows not only
from the different relations of growth of root and shoot, but equally from the

FIG. 320.— Longitudinal section through the grain
and embryo of the Wheat, only the lower part aza
being shown. C lower part of endosperm, y primary
root, ^ its root-cap ; c c the so-called scuielhtni or ab-
sorbing organ of the embryo.

POLARITY.

485

reactions of both parts towards external influences, or, what is the same thing, from
their irritabiUty; and the phenomena of irritability which appear in all parts of
the plant without exception, will only become clear and comprehensible in all respects
when the polarity of the organ is taken into account also. That a tree developes its
crown in the air and its root in the earth is ultimately to be referred to the polarity
of the root-end and shoot-end already present in the fertilised oosphere, by means
of which the capacity to re-act towards light, gravitation, &c. is so distributed that
the upright position of the tree finally results from it. Of course this can
only become intelligible when the phenomena of geotropism and heliotropism
are more closely studied. What has just been stated is only for the purpose of
pointing out that it is no mere play-
ing with empty terms when we speak
of polarity in the plant ; on the con-
trary, polarity denominates its whole
life and growth.

Those plants which creep on
the surface of the earth, or float
horizontally on water, usually under-
go a change of their polarity on
further development. This may
take place to such an extent that
the original root perishes, while
roots make their appearance on
the under or shaded side of the
forward-growing shoot, the upper
side of the same shoot producing
leaves and other organs. In this
way arise dorsi-ventral plants, the
shoot-axes of which possess a ven-
tral side whence the roots spring, and a true shoot-side (as one may best term
it) or upper, light side. There thus appears a polarity transverse to the axis of
growth of the shoot — a polarity which elsewhere exists in the direction of the
shoot-axis itself. However, I shall return to this important point, apart from
which the life of very many plants remains a puzzle.

As a preliminary, it is necessary to make ourselves acquainted somewhat
more in detail with respect to the distribution of the relations of organisation
on the transverse section, or rather around the axis of growth. Unfortunately
there is no satisfactory word to designate what is here referred to ; for lack of a
better the term Laterality may be employed'. It includes the two cases of radial

Fig. 3=1.— Transverse section of the flower-scape of Allium schceno-
pyasum (X about 30). «epidermis; ch chlorophyll cells, r colourless
parenchyma of the cortex ; >n parenchyma of pith ; gg' vascular bundles ;
sy ring of sclerenchyma.

1 The mutual relations of the parts of a plant, designated Laterality in the text, correspond
in the main to what was formerly termed the symmetry of plants. After a very valuable treatise of
Möhl's {Vermischte Schriften, 1845, p. 12) botanists for more than twenty years did not concern
themselves with these very important matters, until I again took up the subject in the second
edition of my 'Text-book,' 1870, p. 26, though proceeding from other points of \äew. In my treatise
Über ort hotrope und plagiotrofe Pflanzentheile' (Arb. des bot. Inst. B. II, p. 226) I then brought
into notice the intimate connection between radial structure and orthotropic growth, and between
dorsi-ventral structure and plagiotropic growth.

^.86 LECTURE XXIX.

and dorsi-ventral growth. These apparently purely geometrical relations deter-
mine, like polarity, in every respect the external form and internal capacity for
reaction of the organ towards external influences. The entire riiode of life of
a plant or of a single organ is essentially dependent upon whether it belongs to the
radial, bilateral, or dorsi-ventral type. Above all it depends upon these properties
in which direction an organ attains its normal position.

The radial structure is most distinctly shown on transverse sections of roots
and perfectly upright shoot-axes. In the transverse section of such an organ the
various masses of tissue appear arranged in three, four, or more directions in such
a way that if we suppose the transverse section cut in half, the one half is, so to
speak, the reflected image of the other, and three, four, or more such divisions may be
imagined. If a radially constructed organ branches, the branches which resemble one
another appear on it in three, four, or more directions. Particularly clear examples
are afforded by shoots with the leaves in whorls; but those with spirally arranged
scattered leaves also belong here. The radial structure of an organ is evinced in
general as much in its anatomical structure as in its capacity for producing out-
growths (roots, leaves, lateral shoots) ; but perhaps the
fact is still more important, and at any rate it stands in
the closest connection with the above, that radially
constructed organs are also sensitive in an equal
degree on all sides of the axis of growth to external in-
fluences or stimuli ; or, as I have designated it shortly,
such organs are orthotropic — i.e. when they are sub-
mitted to a directive force acting from without, as
'^ gravitation or a ray of light, they become curved

FIG. 322. -Very young flower otÄ/!^;»« uutll thc axls of growth of the orgau has acquired

Rhubarb) seen from above : it still consists _ .

entirely of embryonal tissue. A three of the exactly thc dircctiou ot thc actmg lorce. Mcanwhile

six perianth leaves; a a the anthers ar- _ _ .1. i- • -i L

ranged in two whorls ;y the ovary (highly thc objcct hcrc IS ouly to mdicatc prclimmarily the

magnified). . - , . -r-' 1

miportance of this property. ror the moment it
will be well to render the reader somewhat better
acquainted, by means of a few easily intelligible examples, with the meaning of the
word Radial.

Since the expression radial (and the same is true of bilateral and dorsi-ventral)
refers to the arrangement of the relations of organisation around the longitudinal axis
of an organ or of an entire plant, the relations in question are observed most con-
veniently on a transverse section. With respect to the out-shoots from a common
axis, the mere view from above, as in Fig. 322, often sufiices for obtaining a satisfactory
picture of the arrangement. That this flower, still in an early stage of development
and composed essentially of embryonic tissue, is radially constructed is obvious at
once if three straight lines are drawn from the central point of the ovary/" to the
three outer floral envelopes h, and then produced backwards. The whole flower is
divided by each of these three lines into two practically symmetrical halves in each
case, and it is unimportant which of these three bisections is taken into consideration ;
the halves produced in each case are alike in nature. The radial structure with respect
to out-growths also appears very clear on transverse sections of the buds of shoots, as
in Fig. 323. If we suppose straight lines to be drawn from its centre, where the

RADIAL STRUCTURE.

487

shoot-axis is situated, to the points marked i, 2, 3, 4, 5, in the midribs of the leaves,
and produced to the other sides, five divisions of the entire system are obtained, in
which the transverse sections of the subsequent leaves 6, 7, 8, 9, 10 are also arranged;
only care must be taken lest error be made on account of a few minor irregularities,
which depend in part upon the mutual pressure of the organs closely packed together
in the bud-scales I-V, but which are in part artificially produced in the cutting. It is
noticed further that the leaves numbered 1-9 in order of age, in spite of the radial
arrangement, are nevertheless not arranged in circles as in the preceding examples ; on
the contrary, an inspection of the same shoot after its unfolding, when the interfoliar
parts have elongated, would demonstrate that the leaf 2 is situated higher than i, 3 higher
than 2, and so forth. The leaves are, as is usually said, arranged spirally around the

W'P'^^^W //Vi

Fig. 323.— Transverse section through a winK
scales ; 1—9 the pinnate leaves, with

■ bud of Spiraa sorbi/oUa. I—iy bud-
heir stipules {numbered similarly).

axis, that is, if a line is supposed to run round the shoot-axis so that it takes in the
points I, 2, 3-9, it forms a spiral. By means of longitudinal sections such a shoot
with spirally arranged out-growths would never be divided into actual or even ap-
proximately symmetrical halves, as the young flower considered above; nevertheless
it is advantageous on physiological grounds to regard this very common case also as
belonging to the radial type. The off-shoots radiate in five directions, not from the
same but from different levels on the shoot-axis, a fact which is of subordinate
importance for physiological conclusions. Instead of a transverse section then a
so-called diagram serves to represent the arrangement not only on an individual
shoot but even on an entire shoot-system. The whole is supposed to be looked at
from above, and a sufiicient number of concentric circles are drawn on the paper ;

488

LECTURE XXIX.

these circles represent as it were successive transverse sections, and this so that the
outermost circle represents the lowest transverse section, and the following
ones sections at higher and higher levels. On these circles the organs which arise
at the different transverse sections are transferred in the order established by careful
study. In this way is produced in Fig. 324 a diagram of an entire plant oi Euphorbia
helioscopia in flower, with the exception of the root. It is . only necessary to observe
that the leaves cc and i-io, and the central figure B which represents the first
terminal flower, belong to the same shoot-axis. The five figures arranged around the
centre, on the other hand, represent the diagram of five lateral shoots which have been
developed from the axils of the leaves 6-10 ; each of these five shoots has produced
only three leaves and a terminal flower B^ and in the axils of the three leaves appear

Fig. 324. — Diagram of a small plant of Euphorbia helioscopia. c c cotyledons ;
// the first ; i— lo the subsequent foliage-leaves ; 6, 7,B, 9 and 10 form a whorl ; B I
(in the centre) the terminal flower of the primary shoot ; B II the terminal flower of
one of the five axillary shoots ; /// in each case leaves of axillary shoot of second
order.

again three shoots, each with three leaves. Without going more deeply into the varied
relations of symmetry of this entire system, I may simply remark that the shoot com-
mences with two opposite leaves cc (the cotyledons) upon which follow two foliage-
leaves (/) decussate with them. Further upwards, however (i.e. further inwards in
the diagram), the leaves i-io are situated, no longer in pairs but singly, at various
heights on the axis ; at the same time appear lateral displacements, so to speak, the
leaf 3 stands above i, but the leaf 5 stands not exactly over 2, as would correspond to
the original order with which the arrangement of the leaves began, and finally we
find the five leaves 6-10 arranged in a circle, and forming a whorl, whence the shoot-
axis which lower down puts forth leaves radiating in four directions is now surrounded
by leaves equally disposed in five directions. The central flower B also has five more

DORSI-VENTRAL AND BILATERAL STRUCTURE. 489

such enveloping leaves, which alternate however with the preceding leaves 6-10,
i. e. fall in the interspaces between their five radii : finally, the tripartite figure in the
centre of the flower jB is the ovary composed of three leaves. It is thus seen that the
arrangements of the leaves on one and the same axis changes in this as in most
other cases ; moreover, the arrangement of the leaves on the five lateral shoots, as
the diagram shows at once, is again different from that on the main shoot. In
spite of all these irregularities, however, the radial type is nevertheless maintained,
at least on the main shoot of the plant ; the outgrowths radiate in four or five
directions, and there would be no essential difference if they radiated in eight or
thirteen directions. These very few examples will probably suffice to show the
reader what is implied by the term radial.

Radial structure stands in direct contrast to bilateral structure, and here we
have at once to distinguish two different cases, namely, ordinary bilaterality 1, and the
bilaterality which is combined with dorsi-ventral organisation. If, for example, we have
an upright shoot furnished with two opposite rows of leaves, and we suppose this so
bisected longitudinally that all the leaves are at the same time also bisected, the two
halves of the shoot appear nearly symmetrically alike. If, on the contrary, we make
the plane of division at right angles to the previous one, so that the leaves are not
divided, we again obtain in the same way two similar halves, which are now however
conditioned differently from the halves of the first division. In the first case each
half of the shoot has two rows of half leaves ; in the second case each has one row of
whole leaves. Such a structure might just as well be named quadrilateral as bilateral,
or even doubly bilateral. In any case, a symmetrically similar second half exactly
corresponds to each half, and when gravitation, light, and other directive forces act
equally from all sides on such a shoot it places itself vertically — it is orthotropic,like a
radially constructed shoot, and it is physiologically to be relegated to the radial type.

Much more frequent and essentially different from the latter, however, is the
dorsi-ventral bilaterality met with in very many creeping and climbing shoots, in all
leaves, and even in very many lateral shoots which spring from orthotropic and
radial main shoots. The dorsi-ventral 'organisation has always as a consequence
that the organs concerned are plagiotropic towards external influences, i. e. the effect
of gravitation, light, and other directive forces induces such organs (which are
usually moreover extended flat) to place themselves across the direction of gravitation
and of the ray of light. We have to regard as the commonest examples of dorsi-
ventral bilaterality the ordinary flat foliage-leaves. Their bilaterality is at once clear
on observing how two halves of the lamina exist right and left of the mid-rib ; these
are usually nearly symmetrical in leaves on upright shoots ; in those on horizontal
or oblique shoots they are generally more or less unsymmetrical. The leaves of the
Elm and the so-called oblique leaves (e. g. Begonia) afford well-known examples of

» The doubly bilateral but not dorsi-ventral organs are closely connected with radial organs,
both as respects geometric considerations and their reactions towards external influences ; in par-
ticular they are, like those, orthotropic, as is easily perceived in the case of the very excellent
example of the genimtc of Manhantia. Hence, although from a purely formal and geometrical
point of view, the chief contrast appears to lie between radial and bilateral organs; nevertheless, so
far as physiology is concerned, the contrast between radial and dorsi-ventral organs is much more
important.

490

LECTURE XXIX.

this. If the leaf is divided or branched, then in like manner the halves of the entire
leaf situated right and left of the mid-rib are symmetrical or unsymmetrical :
nevertheless they are always equally and oppositely constructed, as our right and
left hand. Fig. 325 shows several foliage-leaves of an Aroid, the orthotropic and
radially constructed stalk of which grows vertically out of the earth, while its bilateral,
symmetrically branched lamina is at the same time dorsi-ventral, and thus situated
oblique or horizontal.

Flat foliage-leaves, as every one knows, are differently constituted on the under
and upper sides — they are dorsi-ventral. The lower side is frequently hairy while the
upper is smooth : the veins project as ribs below, while on the upper side there are
generally corresponding depressions : the under side is dull, the upper side shining
and dark green, and so forth. The difference between the upper and lower sides

appears still more conspicuous
on regarding the transverse
section of a leaf under the
microscope, as represented in
Fig. 326. If we suppose the
mid-rib MN bisected by a ver-
tical line, this corresponds to
the principal section of the
leaf, and divides the whole
bilateral lamina into a right
and a left half. We can ima-
gine no plane of division situ-
ated in any way at right angles
to this, however, by which the
transverse section of the leaf
would be so divided into equi-
valent upper and lower halves.
As the figure shows, the whole
organisation of the leaf is
difFerendy constituted in the
upper and lower halves, though
no very definite boundary line
exists between the two; and it is just in this that the character of the dorsi-
ventrality lies. It must be noticed, however, that the visible anatomical structure
which we here take into consideration, is itself again simply the expression of a
dorsi-ventral constitution situated still more deeply in the invisible organisation.
It can be shown that in every individual cell of the leaf the half turned upwards
is differently constituted and re-acts differently towards stimuli from the lower one.
This remarkable fact comes out still more clearly in the case of those plants which
consist not of cell-tissue but of simple vesicles : in such cases the dorsi-ventrality
is often not to be established by means of the microscope, but by the reactions
towards gravitation and light, which compel us to the assumption that in such
simple utricles the molecular structure presents corresponding relations of direction
across the axis of growth.

DORSI- VENTRALITV.

491

The flat shoots of the Liverworts exhibit a dorsi-ventrality like that of the
foliage-leaves, which they resemble in other respects also; and the same is the case
with many Alg» and the prothallus of the Fern. In all these cases the contrast
between the upper and lower (or dorsal and ventral) side is still more peculiarly
characterised by the nature of their off-shoots : the roots always arise from the

lower or ventral side, which is turned away from the light, while the sexual organs of
the dorsi-ventral Liverworts arise from the upper side; those of the prothallus
of the Fern from the lower side, which is turned from the light.

The relations are more complicated when a shoot possesses dorsi-ventral
organisation, and at the same time a sharp segmentation into axis and leaves •

492 LECTURE XXIX.

and this case is very common. The genus of non-cellular Algae Caulerpa, so often
referred to already, presents a clear example of this. As shown in Fig. 327, the
shoot-axis s v is, sharply segmented off from the roots w and leaves b. The shoot-axis
creeps horizontally, and its dorsi-ventrality comes out far less in its anatomical structure
than in its producing exclusively roots on the under side and exclusively leaves on the
upper side; and when lateral shoots originate from the shoot-axis (which rarely
happens however) these come forth alternately on the right and left flanks. It may
here also be mentioned in anticipation that the roots and leaves which spring from
the dorsi-ventral, and therefore plagiotropic shoot-axis, are, in their turn, orthotropic :
the roots grow vertically downwards and branch radially, the leaves grow vertically
upwards and branch bilaterally. Just as in this unusually clear case, the dorsi-
ventral character of the branching occurs in other Algae which undergo cell-divisions

Wmm

M

FIG. 327. — Canlerpa

as they grow, and even in many highly organised Cryptogams and Phanerogams. The
entire group of Rhizocarpeae behave like Caulerpa, and among the Phanerogams
Utriciilaria especially is to be mentioned, because in it the lateral shoots sprout from
the dorsal side and the leaves from the flanks of the dorsi-ventral axis ; among the
Ferns again the genus Lygodium is especially remarkable, because in it the leaves
arise in one series from the dorsal side of the dorsi-ventral creeping stem.

In these and other cases it is the plumule which, gathering strength as the
main shoot, assumes the dorsi-ventral structure, so that the entire plant is dominated
by the dorsi-ventrality in its organisation. It happens much more frequently, how-
ever, particularly in Phanerogams, that the plumule possesses a radial structure
from the beginning, with its ofif-shoots on three or more sides, and grows

DORSI-VENTRAL SHOOTS.

493

upwards, and then not only all leaves, but also the lateral shoots springing
from the orthotropic main shoot, are dorsi-ventral in structure. This is unusually
clear in some Conifers, especially in the genus Abies: all the lateral branches
springing from the radially constructed stem, which grows perpendicularly upwards,
are here bilateral and dorsi-ventral in structure, and grow out in a horizontal
direction. Even in many Dicotyledons the same occurs, e. g. in the edible
Chestnut, the Red Beech' and others, where the vertical main shoot exhibits
radial phyllotaxis and branching, but the
lateral shoots bilateral and dorsi-ventral
structure — bilateral branching combined
with a tendency to horizontal or oblique
direction.

As follows from what has been stated
hitherto, the capacity of the organs of a
plant to react towards gravitation, light and
other directive forces, so that they grow
in a definite position of equilibrium with
respect to the horizon, co-operates causally
with their radial or dorsi-ventral structure ;
or, as we may also say, the Anisotropy^
of the organs is determined at once by
their radial or dorsi-ventral structure. A
more detailed consideration of this matter
is better relegated, however, to the chapter
on the phenomena of irritability, though
on account of the close interdependence
between growth and anisotropy the subject
could not be entirely passed over here.

For the elucidation of the most es-
sential difference between radial and dorsi-
ventral structure of an organ, however,
the following considerations may serve in
addition. Let us imagine an ordinary
foliage-leaf, in which the upper and lower
sides are sharply characterised, and which is therefore a distinctly dorsi-ventral organ,
so rolled together parallel to the longitudinal axis that its lower side appears as the
convex outer side. As Fig. 329 (C and A) shows, a radially constructed organ is thus
produced from the dorsi-ventral one ; since it is clear that a transverse section of the
rolled-up leaf must exhibit equal distributions of the organisation in all directions.
If, conversely, we suppose any hollow stem whatever slit up in the direction of its
length, and then spread out flat, there arises from the previously radial organ a dorsi-

' I first pointed out generally that Anisotropy, i. e. the different capacity of the organs of the
plant to react towards similar external directive forces (gravitation and light), is a necessary result of
the radial or dorsi-ventral structure, in various editions of my Text-book, and then made it quite clear
in my treatise ' Über orthotropc iind plagiotrope Organe'' (Art. des bot. Inst., B. II, pp. 274, &c ).

494

LECTURE XXIX.

ventral one, one side of which is differently organised from the other. Finally,
we may suppose yet a third construction : assuming some hundreds or thousands of
seedlings, each of which possesses a radicle and a plumule, to be placed beside one
another and parallel in the ordinary direction, and closely connected together, then
these seedlings form a whole in the direction of a line or in a surface, all the roots
being on the one side, all the shoots on the other. We have then a dorsi-ventral
structure, one side of which is composed of roots and the other of shoots. And con-
versely, many dorsi-ventral organs may also be so broken up into individual elements
that each of the latter possesses a root-end and a shoot-end ; if, for example, a piece
is supposed to be cut out of the horizontal flat shoot of a Marchantia, perpendicular
to the surface, by means of a narrow cork-borer for instance, this portion bears on
the under side one or several rhizoids, and on the upper side cells containing chloro-
phyll, and it behaves like a small upright plant. Similarly we may isolate from a
Ma7'silia or Pilidaria, by means of two transverse sections of the shoot-axis, a so-called
node, on which a leaf is situated above, a root below. That such a mode of looking

at dorsi-ventral structure is not an
arbitrary playing with words, but,
on the contrary, accords with the
true being of such a plant, fol-
lows, as I have definitely shown
previously, from the behaviour
which dorsi-ventral organs ex-
hibit towards the influence of
gravitation and light : from every
dorsi-ventral organ which places
itself across the direction of gra-
vity and of the rays of light, and
is thus plagiotropic, a radial or-
gan can be produced simply by
FIG. 329. rolling it together, and this, under

the influence of gravitation and
light, behaves as a radial orthotropic organ. Nature herself makes this experiment
in cases where young foliage leaves are rolled round one another in the bud and
constitute a radial orthotropic structure ; when the older leaves become freed from
the bud and unroll, they then appear as dorsi-ventral organs which place themselves
across the directions of light and gravitation.

From what has been hitherto said it obviously follows that the radial or the
bilateral dorsi-ventral organisation of a plant-organ constitutes a fundamentally primary
property of it, upon which depends not only the reaction towards external directive
forces, but also the ways and means by which new formations — secondary offshoots —
proceed from a given organ. The arrangement of the leaves and lateral shoots on a
shoot-axis is above all determined by whether the latter itself possesses radial bilateral
or dorsi-ventral structure ^ In this simple point lies at once the complete refutation

* The statements of fact here referred to^are borrowed from Goebel's work, ' Über die Ver-
zweigung dorsiventraler Sprosse' (Arb. des bot. Inst., B. II, pp. 353, (!tc.). I need only remark that

REFUTATION OF THE SPIRAL THEORY.

495

of the so-called doctrine of phyllotaxis founded by Schimper and Alexander Braun,
and the spiral theory lying at the foundation of it, and which has dominated Botany
for more than forty years. This theory was derived from the consideration of the
upright radially-constructed shoots of the vascular plants, in which the above-
named spiral arrangement of the leaves commonly occurs. On imagining the
leaves of such a shoot-axis connected together by a line in the order of their age,
this line took the form of a spiral line continually winding round the shoot-axis, and
this was designated the genetic spiral ; in it the fundamental law of growth of the
vegetable kingdom was supposed to be perceived. It was therefore sought, even in
cases where the leaves stand in two straight rows on the shoot-axis, and further,
where they are distributed in decussate pairs or in alternating whorls (Fig. 331), and
even in such cases where they appear on one side only of the shoot-axis, to carry
out the spiral arrangement as the fundamental law of growth, whereby it was
necessary to employ the aid of a series of the most arbitrary assumptions.

An unprejudiced observation of the processes at the growing-point, taking into

Fig. 330.— Diagram of a shoot, the leaves of which
are scattered with a divergence of 2-sths.

Fig. 331. — Diagram of the flowering stem of
Paris quadrifolia. II whorl of large foliage leaves
beneath the flower; ap outer, ip inner perianth;
a a outer, ia inner stamens. In the centre is the
young fruit consisting of four carpels.

account at the same time the physiological relations of growth, demonstrates, however,
as Goebel has already expressly shown, that the spiral theory is not transferable to
dorsi-ventral shoots, because it directly contradicts the facts ; the above-mentioned
relations of growth of Caulerpa, Lygodium, the Rhizocarpese, Liverworts, etc., show-
that not only the places of origin of the leaves but also those of the lateral shoots,
roots, and sexual organs, are to a certain extent determined by the dorsi-ventral or
radial structure of the mother-shoot. There is not the smallest ground for regarding
the single rectilinear series of leaves on the dorsal line of the stem of Lygodium in
any way as an expression of spiral arrangement, and just as little is this possible with
the phyllotaxis and branching of the Rhizocarps, and even the very numerous cases
of two-rowed phyllotaxis, especially for example in the Grasses, and likewise the

I, for my part, in the establishment of the idea of dorsi-ventrality by no means had in view exclusively
the anatomical structure, but just as much, on the one hand, the molecular structure, and, on the other
hand, the capacity to produce different organs in different directions. Only there was no necessity
for me to go more in detail into the latter point, Goebel having done it so well.

496

LECTURE XXIX.

Fig. 332.— Transverse ;

L of a shoot oi Aloe j

numerous cases of decussate pairs of leaves, as in the Labiatas, contradict in every
way even the merely formal arrangement in a so-called genetic spiral running round
the shoot-axis. This latter, moreover, even where it can be carried out, viz. in the
case of radial shoots with scattered leaves, is only subjective and imagined as
belonging to the plant by the observer, and has no significance whatever for the
knowledge of the processes of growth themselves. Innumerable cases may be
adduced from which it results definitely that the arrangement of the leaves and
lateral shoots on a shoot-axis essentially depends upon whether the latter is already

radially or dorsi-ventrally constructed
at its growing-point. A few exam-
ples only will serve to illustrate this.
Fig. 332 represents a transverse sec-
tion through the lateral shoot of an
Aloe. It grew out from the mother-
shoot at first horizontally, and pro-
duced its leaves right and left in two
alternating rows, as may be seen by
the numbers 1-6 : eventually the
growing-point of this shoot took an upward direction, and it became orthotropic
and radial. This change found its expression in the fact that the primitively straight
rows of leaves now pass over into two spirally-wound ones, as is to be seen from the
two dotted lines 7, 9, 11, 13, 15, and 8, 10, 12, 14. The completely erect slioot
bears a rosette of leaves radiating on all sides.

Among the inflorescences of Phanerogams are to be found very many, especially
in the families of the Boragineae, Papilionaceae, and Urticacese, which are termed
unilateral in descriptive botany, that is to say, the shoot-axis from which the more or
less numerous flowers arise as lateral shoots is itself
dorsi- ventral, and therefore produces flower-shoots only
on its dorsal or only on its ventral side. The spiral
theory was compelled, in order to vindicate itself
in such cases, to put forward the most extraordinary
and improbable accessory hypotheses. By means of
careful investigation of the processes at the growing-
point, Goebel showed, however, that it is altogether
impossible to entertain the spiral theory in such cases,
simply because the leaves on the dorsi-ventral parent
axis bud forth, as a matter of fact, only on one side, in
one, two, or more rows. Fig. 333 shows this behaviour
at the growing-point of an inflorescence oi Symphytum,
and the case is exactly the same in the common Forget-me-not {Myosotis). In the
latter plant especially, any one may easily perceive how the axis of the inflorescence,
which is rolled spirally inwards and downwards, bears flowers arranged in two rows
on its convex upper side only : the mature state observed with the unaided eye,
as well as the processes at the growing-point, forbid every attempt to speak here of
a spiral arrangement.

But even in individual flowers the case occurs that the diff"erent phylloid organs,

Fig. 333.— Young inflorescence
of Symphytum, v growing-point ;
* b the young flowers.

THE SPIRAL THEORV

497

which spnng from the growing-point, appear in a sequence and order which excludes
the employment of the spiral. Payer, who has done such excellent service with
respect to the developmental history of flowers, showed that the flower of the
common Mignonette (Fig. 334) is a strictly bilateral structure. From its growing-
point the sepals, petals, stamens, and carpels are put forth in such a manner that
their development begins at one point of the circumference, and then proceeds
right and left on both sides of the young flower towards the opposite point.

Closely connected with the spiral theory, was also the view that lateral shoots
from a main shoot can only arise in the axils of the leaves. But even this so-called
principle of axillary branching presents itself on unprejudiced observation only as
a special case, occurring chiefly among the Phanerogams, and even there only
when lateral shoots arise on erect and radially constructed shoots. Where the
dorsi-ventrality of the mother-shoot is sharply expressed, however, as in the cases
mentioned above, the lateral shoots may arise on the flanks of the mother-axis
while the leaves are situated dorsally, or vice versa. In Ferns and Mosses, the most
careful investigations into the development have shown that normal lateral shoots
may arise on the back or on the
flank of the base of the leaf, or
even quite independently of any
relation to the leaf. Even in the
Phanerogams, from which the older
theory of phyllotaxis started, it is
not always possible to apply the so-
called principle of axillary branching:
the floral region especially aff"ords nu-
merous examples of this. Not rarely, F,c. 334.-Developn,ent of, he flower of /..W..^«,-..., after Payer).

n« in tViP Prilrifpr!*» mnnv Ponilinn '^° "^^ ^'^"- ^ y°"''S. to the right an older bud; the anterior sepals s

as m tne «^rUClIerae, many rapillOn- ^,^ ^^, ^„^y f,„„ ^l,^ l^,,^, t,,^ posterior ones remain, pp petals i

arpn= and T^ono-inpcc. in Ai-nirlpc*! j/ stamens, the posterior ones already advanced in size, the

acese ana UOragmeae, m AlOiaeae, ones not yet developed ; ^carpel-rudimentary fruit.

and others, the flowers arise on the

mother-axis without being preceded by leaves, in the axils of which they might arise.
They are leafless branch-systems ; and in some Boragineae and Crassulacese,
where it is true leaves are formed on the dorsi-ventral axis of the inflorescence,
these are situated with no obvious relation to the flowers, and even the number of
these leaves does not always correspond to the number of the flowers.

The theory of phyllotaxis, with its assumption of the spiral as a fundamental
law of growth, has, to the great injury of all deeper insight into the growth of the
plant, established itself so firmly that even now it is not superfluous to show up its
errors point by point. Thus, among the errors of this theory is the one that the
spiral arrangement of all organs on a common axis must necessarily follow from its
so-called parasiichies. By the term parastichy is understood the serial arrangement
of lateral organs in two or more directions which cross one another ; this appears
very clearly in cases where numerous organs are situated close beside one another on
a common axis. If in such a case the organs have approximately the same form, they
involuntarily present rows to the eye, which it can follow from right to left or from left
to right. The two dotted lines a and I) in Fig. 335 will at once show what is meant.
Now it is of course possible, with certain premisses which the founders of the spiral

[ 3 ] K k

49«

LECTURE XXIX.

theory arbitrarily made, to construct the so-called genetic spiral out of these para-
stichies by certain geometrical artifices. But, on the one hand, direct observation
shows that these premisses by no means always fit the case, and that nevertheless
very fine parastichies arise ; and very often it may be said, on the other hand, that

when numerous figures or bodies similar
to one another are placed close beside one
another on a common foundation in any
sequence whatever in lime, they must
necessarily present to the eye viewing
them series crossing right and left. Even
ordinary wall-papers show such parasti-
chies, and in the same way the arrange-
ment of the scales on the bodies of fishes,
of the hairs on the skin of mammals, and of
the tiles on a roof exhibits such parastichies
clearly enough. They are particularly well
seen in the case of the scales of pine
coneSj and in the numerous flowers of the
capitulum of the Compositae, especially that of Helianthus amiuus, or the flowering
head of Dipsacus and of the Aroidese, and it was these objects particularly which the
supporters of the spiral theory employed by preference, for the purpose of constructing
the genetic spiral from the parastichies. Nevertheless it is very easy to show in
these very cases how uncertain were the facts on which the spiral theory was often
supported. Fig. 336 may serve for the illustration of this. The figure represents

FIG. 335- -The gr

bud of the Larch

Fig. 336.— Arrangement of the young fruits on the conical axis of a flower-Iiead of
Dipsacus fullomim (Fuller's Teazle).

the arrangement of the unripe fruits of Dipsacus fullonum closely crowded on the
conical floral receptacle, but transferred from the natural curved surface to the plane
of the paper : this was attained by blackening the conical flower-head with printer's
ink, and subsequently rolling it on the paper. The figure is thus in all essential
points quite true to nature. We perceive at once two mutually crossing sytems of

DIVERGENCES.

•^99

parastichies ascending right and left : these however exhibit irregularities which are
inexplicable by the spiral theory, but otherwise easily intelligible. The irregularities
simply depend upon the fact that the rhomboidal quadrangular fruits are every-
where nearly equal in size, but the receptacle on which they are situated becomes
smaller above, since it is conical where youngest. Hence the number of the fruits
occurring side by side at the same height decreases upwards; also the number of
parastichies must be diminished by certain of them simply ceasing at different heights.
Thus the series d in the figure ceases between aa^ and cc', and in the same way
several others besides ??i and « terminate on the right of the figure between / /' and o o' .

The doctrine of phyllotaxis based on the spiral theory, which according to the
so-called principle of axillary branching was regarded as dominating the position of
the lateral shoots, laid, further, great stress on the fact that on shoot-axes richly
provided with leaves or ofif-shoots generally, certain ' divergences ' repeat themselves
more frequently than others. By divergence is understood that portion of the cir-
cumference of the shoot-axis which lies between two consecutive leaves; thus, in
two-rowed phyllotaxis it amounts to \, in three-rowed to \, in five-row^ed to f , in
eight-rowed to | of the circumference, and so on. In the case of some axes
abundantly provided with outgrowths (Pine cones, shoots with crowded leaves, &c.)
it is shown now that the divergences on the so-called genetic spiral remain constant
for more or fewer of the segments of the chain, and (a point on which particular
stress was laid) that the very divergences named — viz. \, -i, f, |, -^, &c. — recur
somewhat frequently. These fractions (parts of a continued fraction) appeared to
constitute the expression of a mysterious law which was assumed to dominate growth
in a supposed spiral manner. But it was nevertheless seen to be necessary, in addition
to the relations of phyllotaxis represented by that fraction, to add yet others, which
of course led to any continued fraction whatever, whereby however the point lost in
significance. The numerous cases of dorsi-ventral shoots however remained entirely
outside the system : in these, homologous ofif-shoots arise only on one side of the
axis or on two opposed flanks, and these could in no case be placed on a spiral
running round the axis. Further, it was fatal to the theory that the mysterious
divergences by no means remain constant even on one and the same shoot-axis, but
mostly begin with simple fractions, such as \ or ^, or quite irregularly, and then pass
over into f, f , and so on, probably to be continually returning again to simpler ones,
or yet others. In addition to this it turned out that even in very closely allied plants
the relations of position are often importantly different, and, which I would lay more
stress upon than on anything else, the whole spiral theory, with its divergences and
continued fractions, found no application whatever to the branching of roots— the
roots had no existence, so to speak, for the spiral theory.

Nevertheless the frequent occurrence of the divergences ^, f, |, yV' ^^- ^^
a fact of observation. The secret of this occurrence and the frequent absence of
other divergences is explained, according to Schwendener's investigations^, by

* Hofmeister first attempted, in his '■Allgemeine Morphologie der Gewächse' (i86S), to shake
the foundations of the doctrine of phyllotaxis of Schimper and Braun, but he was himself in the
main still swayed by this doctrine. For my part, I have from the first regarded the theory of
phyllotaxis more as a sort of geometrical and arithmetical playing with ideas, and have especially
regarded the spiral theory as a mode of view gratuitously introduced into the plant, as may be

K k 2

jOO

LECTURE XXI K.

mechanical and geometrical considerations, among which the mutual pressure of
the young organs on a common axis plays an important part. It would require
far too much space here, however, to go more in detail into Schwendener's somewhat
prolix account : it suffices that, as he shows, it is possible to demonstrate by means of
simple models (figures in a displaceable frame) how from purely mechanical causes
arrangements corresponding to the divergence |, for example, must pass over into such
as correspond to | and y%, the transition from one divergence into the other being
accomplished somewhat suddenly. Schwendener's description is based on the
circumstance, previously made evident by Hofmeister and myself, that the laws of
phyllotaxis assumed by the spiral theory only appear at all where the young organs
(leaves or shoots) arise on the growing-point of a mother-shoot in large numbers,
and quite close beside one another, so that at first no free surface whatever of the
axis is presented. By means of this very close position, the place of origin of the
outgrowths newly added acropetally is in part determined, and at the same time,
in consequence of the close packing, pressures and torsions must make their ap-
pearance during growth, by which the relations of position assume their definitive
condition.

Where the offshoots of an axis appear at some distance from one another froin
the commencement, none of the relations spoken of occur. Apart from some Algae
which we might adduce here, I may refer the reader to the known case of the whorls
of branches of the Pines. The leaves of these trees arise close above and beside one
another on the main shoot, and since they are from the first numerous and small
in relation to the growing-point, they present complicated and somewhat constant
divergences : the position of every new leaf which makes its appearance at the
growing-point is simply determined by these conditions. The whorl of branches
developed in any one year on a Pine, on the contrary, only arises after the growing-
point of the main shoot which produces it has grown far beyond the whorl of branches
of the previous year ; the consequence is that the consecutive whorls do not alternate
regularly as elsewhere, but are situated one above another in any direction on the
stem.

Since the older morphology as expressed in the genetic spiral with its parastichies
and constant divergences which were supposed to stand in mysterious connection
with one another, and further with its principle of axillary branching, cannot exist
in the face of an unbiassed criticism of the facts, simply because it sets up a few
relations of position, which only occur on radial orthrotropic shoots, as the funda-
mental law of all vegetable growth, we must now rather acknowledge that there is no
general law which can be formulated for the arrangement of the organs on a parent-

read clearly enough in the four editions of my text-book. That the whole doctrine of Schimper
and Braun depends, not, as it were, merely upon the inaccurate interpretation of single facts, but
that it rather stands in direct opposition to scientific investigation, and was based on the founda-
tion of the idealistic direction of the 'Naturphilosophie^ was clearly expressed by me in my 'Geschichte
der Botanik' (1875). The fundamental ideas of this criticism were also subsequently placed by
Schwendener, in his 'Mechanischen Theorie der Blattstcllungcn'' (1S78), in the foreground of his
considerations. A historical description well worthy of note of all the views on leaf-position
hitherto held, as well as a theory of his own, is contained in Casimir de Candolle's ' Considerations
sur r etude de la phyllota^cie' (Genf, 1881).

THE SPIRAL THEORY.

501

axis ; that on the contrary, according to circumstances in each case, special causes
determine whether the relations of position turn out to be this or that. Among these
causes the most important is the radial or dorsi-ventral character of the growing-point,
which in its turn also may again depend upon whether the growing-point in question has
arisen as a lateral offshoot of another grow-
ing-point. Further, the more or less close
crowding of the young organs at the grow-
ing-point causes new organs to emerge at
definite places, and during the growth dis-
placements, torsions, pressures and strains
take place. In all this, however, there
is still much that remains unexplained —
for instance, that the orthotropic radial
shoots of the Grasses have their leaves
alternating in two rows, while those of the
Labiatae have them in decussate pairs;
why the radial shoot bears in one case
whorls, and in another spirally-arranged
leaves ; why vegetative lateral shoots of the
Monocotyledons mostly begin with one
first leaf turned towards the parent-axis,
and lateral shoots of the Dicotyledons
(Fig. 337) mostly with two, situated right

and left ; why in the true Mosses one leaf proceeds from each segment of the two- or
three-sided apical cell, and so forth. But the chief point is that we feel ourselves
free from the formalism of the doctrine of phyllotaxis, and that these phenomena of
growth have now become transferable to the region of causal investigation — i. e. to
the physiology of growth.

FIG. 337. — A young lateral shoot of Spirsa sorbi/olia.
a the axis of the mother-shoot ; * a leaf developed from this.
■V growing-point of the young axillary shoot, from which the
very young leaf-rudiment ;' is arising, «rrfthe first leaves, and
ir— zthe succeeding ones of the young shoot (somewhat highly
magnified).

LECTURE XXX.

CAUSAL RELATIONS OF GROWTH OF THE DIFFERENT ORGANS
OF THE PLANT ONE TO ANOTHER.

(CORRELATIONS.)

The processes of growth described in the last four lectures must for the time
being be taken simply as facts, the determination of the causes of which is at present
impossible. That the embryonic rudiments of all organs proceed from growing-
points, which are themselves remains or continuations of the fertilised oosphere of
the embryo, that in these the differentiation of the embryonic tissue takes place, that in
post-embryonic development they not only increase in size, but also only attain their
permanent outward form during this period of growth, and so forth — all these matters
of growth appertaining to the development must in the meantime be taken as facts :
that is, we must be satisfied meanwhile to look upon the phenomena in question
as purely objective, and free from preconceived ideas, in order to obtain subse-
quently a glimpse into the causes which bring it about that things go on as they
have been described, and as direct observation shows. Vegetable physiology finds
itself with respect to these phenomena of growth in the same position as is crystallo-
graphy, for instance, respecting the question why common salt, the diamond, copper,
&c., in spite of their differences in other respects, crystallise in the regular system,
whereas graphite, calc-spar, quartz, &c. assume forms belonging to the hexagonal
system.

Nevertheless, in our considerations on growth so far, we have already obtained
here and there glimpses of causal relations by the way. With respect to cell-
formation in the growing-points, the law of the rectangular intersection of cell-walls
presented to us a causal element from which we were able to make intelligible
the arrangement of the cells in the most different embryonic tissues: in the same
way we found in the radial or dorsi-ventral structure of the growing-point the most
immediate cause for the fact of the organs being placed vertically, obliquely, or
horizontally, and producing their outgrowths on all sides, or only on one side, and
so forth. Evidently there lie in these relations, at least the beginnings of a causal
understanding of the matter, although we are by no means in a position to follow
causes and effects step by step according to current mechanical conceptions.

In this and the following lectures we shall be concerned with those phenomena of
growth in which the effective causes or the causal principle are more clearl)' manifested,

CONCEPTIONS OF FORM. 503

though even here we must be satisfied with following the causes and eflfects of growth
in their general outlines only. Even here also it must be left for the future to analyse
the processes further, and to resolve the causal relations into their several factors.

A better survey of the phenomena lying before us will be obtained if we first
distinguish between the internal and external causes of growth. We regard it as the
effect of internal causes of growth when by means of the growth of one organ the
growth of another organ of the same plant is promoted or hindered, thus recognising
a mutual causal relation in the growth of different organs of a plant. This mutual
causal relation has been termed the correlation of the growth of the organs of a plant.
On the other hand, however, experience also shows that the growth of individual
organs, or parts of organs, may be promoted or hindered by purely incidental
external actions, by light, gravitation, contact, pressure, and other influences of the
environment.

Here, however, we enter upon a province where it is very difficult to obtain
certain information, not only because the connection between causes and effects
is in itself extremely complicated in each of the two cases, but still more
because experience teaches that the relations of growth of the plant to the external
world, as well as the correlations, are variable in the highest degree. Processes
of growth which in the one species of plant are established once for all, and appear
spontaneously by the internal connection of development, are only called forth in
other species by definite external influences, and no sharp line is in this respect to be
found anywhere. We are in the habit of saying that such phenomena are in the one
case hereditary and constant, while in the other case they are caused by irritability;
but it must not be forgotten that nothing is explained by such statements, only a
logical classification — i.e. one suited to our preconceived ideas— is obtained.

Here also I regard it as beyond my task to state all that is known with
respect to correlations and external conditions of growth ; but rather, in this and the
following lectures, to make clearer, with a few examples only, the meaning of the
questions here raised, and to show with what problems the theory of growth has
to concern itself, and at the same time I must refer to the fact that it is exactly in the
post-embryonic conditions of growth, with which we have chiefly to do here, that the
organs of the plant are in a high degree sensitive to internal as well as to external
influences. In spite of all difficulties and indefiniteness, however, profound interest
is always attached to such considerations, because the question which here presents
itself is as to what causes determine the internal structure and the external form of
organisms. Only a short time ago the forms or organic bodies were regarded as
something lying beyond the region of causality, and every organism was held to be a
more or less successful realisation of an ' idea.' We here stand therefore on the
boundary between two different modes of viewing the universe, of which the one, the
idealistic — or, to put it in a more concrete form, the platonic — knows and will know
nothing whatever of effective causes in the domain of organic form; whereas my
view starts from the fundamental notion that organic forms, exactly as the configura-
tions of crystals, and all matters of form whatever — it matters not whether we are
concerned with the shape of a drop of water, a planet, a cloud, or any other product
of nature — must be called forth by effective causes, which are determined by the
nature of matter and its forces under the given circumstances.

504 LECTURE XXX.

From this point of view, even the rudimentary beginnings of a causal explana-
tion of the processes of organic growth appear of importance, even though we are not
in a position to explain in any given case the connection of cause and effect link by
link and step by step. It suffices that it can be said at all definitely that this or that
relation of configuration in the province of organic beings may be referred to definite
causes.

As already mentioned, then, we are in this lecture concerned with those causal
factors, which by the growth of one organ of a plant are given for the growth of
another organ of the same plant ; or, to put it shortly, with the correlation of growth
of the different organs of a plant.

A very plastic subject for experimental researches in this province is the
common Potato. On the subterranean portion of the shoot-axis, which developes
above ground as the leaf-shoot, there arise in the axils of small scale-leaves in the
normal course of things, thin filiform horizontally spreading subterranean shoots,
which likewise only produce small scale-leaves, and which finally give rise to the
potato-tuber, by vigorous growth in thickness at the tip of the shoot-axis. If at the
period when the formation of tubers has not yet commenced, the portion of the leaf-
shoot which is above ground is cut off, the terminal buds of the still young filiform
runners become converted into ordinary leafy shoots, which ascend and grow out
above the surface of the soil. Thus by removing the young main shoot it is possible
to cause its lateral shoots, which would otherwise form tubers, to assume an entirely
diflferent form of growth ; and we may therefore infer that in the ordinary course of
the vegetation of this plant, the growth of the foliage-shoot brings it about that its
subterranean lateral shoots become, not foliage-shoots likewise but potato-tubers. It
is also possible to cause the production of tubers at will on the sub-aerial leaf-shoot,
if the subterranean lateral shoots destined for the formation of tubers are carefully
cut away from a vigorously growing potato plant, and the possibility of the formation
of tubers below ground prevented. The materials normally adapted for the formation
of potato-tubers now pass into the axillary buds of the sub-aerial foliage leaves, and
cause their axial portions to remain short and to swell up and thicken, while their
leaves develope but feebly. The existence of the subterranean runners, then, brings
it about in the normal course of affairs that the materials destined for the formation
of tubers do not prevent the development of the sub-aerial buds into leafy shoots.

No less evident and easy to observe is a similar correlation between the apical
shoot of many trees and the lateral shoots beneath the apex. If the terminal shoot
of the Fir A6ies excelsa and several species of Abies allied to it (e.g. A. cephalonica)
is broken off, or destroyed by frost, &c., the horizontal lateral shoots of the upper-
most whorl gradually erect themselves, and occasionally a similar effect is ob-
served even on lateral shoots of the next lower whorl. After 1-3 years one of these
lateral shoots has usually obtained the upper hand, and has not only become erected
vertically, but has also lost its bilateral nature ; the originally horizontal shoot has
gradually become radial and completely orthotropic, and it produces henceforth four-
or five-rayed whorls of branches exactly as the original terminal shoot of the main stem.
We may conclude from these facts that in the normal course of growth a causal
relation exists between the growth of the young lateral shoots and that of the apical
shoot of the main stem ; the growth of the latter evidently brings it about that the

THE EFFECT OF GROWING SHOOTS ON ONE ANOTHER. 505

lateral shoots are dorsi-ventral, grow horizontally, and become branched right and
left chiefly in a horizontal direction, A similar relation, however, also exists between
the various shoots of a whorl : if these are equally strong among themselves they
all rise up after the removal of the apex, but usually the strongest overcomes the
others and obtains the mastery which the true apical shoot exerted previously. In
the Red Pine, however, it happens not seldom that after the removal of the apex of
the main stem two or even three lateral shoots develope into complete apical shoots.
Much less plastic in this relation is the Fir Abies pectinata, and probably also
some of its nearest allies. I have often removed the apical shoot from young trees
of this species, but only seldom, after two or three years, has one of the uppermost
lateral shoots erected itself to develope into a new apex. It is more commonly the
case with this Fir, that close beneath the place whence the apical shoot has been
cut off, or even from the upper side of the base of the nearest lateral shoot, small,
previously unnoticed dormant eyes begin to put forth shoots, sometimes not until
1-2 years after the removal of the apex, of which one then usually grows more
vigorously than the others, and in the course of years is transformed into a new
radially constructed apex.

Very similar mutual relations between main stem and branches also exist in
other wood-plants, and especially many forest-trees, and are made use of in various
ways in forestry, and especially in the art of felling timber, in order so to interfere
artificially with the process of growth as to promote the development of certain buds
and to suppress that of others : exactly the same occurs, however, also in small
herbaceous plants and even in seedlings. If, for example, the common Scarlet
Runner {JPhaseolus multiflorus) is allowed to germinate till the primary root is about
10-12 cm. long, and the young germinal shoot between the two cotyledons, the
so-called plumule, is then carefully cut off, then, as the root-system increases in
strength and activity, vigorous shoots grow out from the axils of the two cotyledons.
These shoots do not usually develope in this plant, because as a rule the normal
primary shoot attracts to itself the whole of the supply of nutriment from the
seed, so far as it is suited for the formation of leaf-shoots. In our experiment,
on the contrary, the shoot-forming substances of the seed penetrate into the growing-
points in the axils of the cotyledons and cause them to sprout vigorously. Not rarely,
however, an abnormality makes its appearance here ; these vigorously growing
axillary shoots of the cotyledons exhibit so-called fasciations — i. e. the shoot-axes
become broad and band-hke, and crooked, and still other abnormalides occur.
Since fasciations not rarely occur in plants of the most different kinds —
e.g. in Willows, Compositae of the Camomile group, &c. — it is at any rate of
some interest to know that it is also possible to produce such abnormaliiies
artificially.

Goebel, a few years ago, in his ' Beiträge zur Morphologie und Physiologie des
Blattes^,' supplied copious and well considered experimental material respecting
correlation of growth ; but I must rest satisfied with shortly reproducing here a few
only of his results. Goebel's researches are chiefly concerned with the correlations

' Goebel, 'Beiträge zur Morphologie und Physiologie des Blattes'' (Bot. Zeitg. 1S80, p. 753. &c.).

5o6 LECTURE XXX.

between the normal green leaves and the scales of winter-buds which envelope the
young foliage-leaves until the next period of vegetation (of. Fig. 301). He shows
in the first place, by simple observation, that the bud-scales as well as the small
scale-like leaves of the subterranean shoots of many plants are according to their
origin and as a matter of fact ordinary foliage leaves, which, however, during further
development are arrested in so far that the blade of the leaf (lamina) very soon
ceases to grow, often even when it is not yet, or is scarcely, visible ; whereas a lower
portion of the leaf, which is but little or not at all developed in normal leaves — the
so-called leaf-base— in many cases grows up vigorously and constitutes the body
of the scale proper.

In order to examine the matter more closely, it must be noticed in the first
place that Eichler established, so long ago as 1861, that two or three stages must be
distinguished in the development of a leaf. The body which comes forth imme-
diately from the growing-point of the shoot as the leaf, is termed by Eichler the
primordial leaf: it appears as a crescentic or annular cushion of embryonic tissue.
This primordial leaf becomes segmented in the first place into two chief portions ;
a stationary zone, which takes no further part in the formation of the leaf, and
a portion which gives rise to the leaf proper. The former is the foliar base, the
latter the so-called upper-leaf, from which the blade of the leaf arises in every case :
when a petiole is developed it is intercalated, as it were, subsequently, between the
foliar base and the young lamina. Goebel shows now that the scales on the winter-
buds of Syringa, Lonicera, Daphne, and others, come into existence by the rudi-
ments of the foliage-leaves being arrested at a middle stage of their development,
the formation of the otherwise normal petiole being altogether suppressed.

A second category of bud-scales is found in the species of Maple, Horse-
chestnut, and other trees. In these cases the bud-scale arises by the above-named
foliar base of the primordial leaf developing vigorously while the lamina of
the leaf, though it exists in a rudimentary state, is soon arrested, and may then
be detected at the apex of the scale by means of the microscope. If sprouting
winter-buds of the Maple and Horse-chestnut are examined in the spring,
intermediate forms between ordinary bud-scales and foliage-leaves may often be
found ; the scale-like portion (that is the developed foliar base) is then smaller,
but the arrested lamina is so large that it can be recognised at once as a
foliage-leaf.

In a third category of winter-buds, it is from the so-called stipules — that is, leaf-like
structures which protrude laterally right and left from the foliar base at the sides of
the leaf proper — that the bud-scales arise. In various species of Alnus and
in the Tulip-tree, the enveloping of the winter-buds is effected simply by the
lowermost, tolerably normally developed foliage-leaf having its two stipules modified
in the form of bud-scales. In our native Oaks also, and in the White Beech and
Red Beech^ it is the stipules of arrested foliage-leaves which envelope the winter-
buds ; in these cases, however, the laminae of the leaves in question are completely
arrested very early, although their true nature can still be distinctly perceived by
means of the microscope. Even the bud-scales of those Coniferge and Cycadese
which form resting-buds are, according to Goebel's investigations, simply modified
foliage-leaves. Of subterranean scales, those on the rhizomes of Dentaria, Chtysos-

goedel's investigations on scale-leaves, etc. 507

pkftium, Anemone hepatica, and other species of Anemone, may here be mentioned.
In all these cases the subterranean and sometimes succulent scales have arisen
by the further development of the foliar base, while the lamina, thvough still more
or less evident, is arrested.

That, so far as these scale-structures are concerned, it is actually a matter
of correlation of growth, Goebel has experimentally demonstrated in the case of
Prunus padus, among others. The growth of this tree proceeds as follows. In the
spring the axillary buds of the shoots of the previous year become unfolded and
again form axillary buds in their turn : these produce in the first place bud-scales,
and pass through the period of rest during the summer and following winter
enveloped in these. The bud-scales which thus arise in the spring come into
existence in Prunus padus by the vigorous development of the foliar base, which
bears above indications of not only the proper lamina of the leaf but also of
two stipules, all, however, very small and only recognisable by means of the
microscope.

On the 14th of April a number of growing shoots and young plants ol Prunus
padus were partly deprived of leaves, and partly lopped at the apex; that is, the
terminal buds were removed. On the loth of May the result was that the axillary
buds which should normally unfold only in the next spring had commenced to put
forth shoots, and vigorous normal leaf-shoots were subsequently developed from
them. Apart from other results of this experiment, it teaches that the leaves impelled
to further growth immediately after their origin did not become developed in the
usual manner into bud-scales, but assumed the form of ordinary foliage-leaves.
The rudiments of foHage-leaves, which when the shoot is left to itself have their
laminae arrested and develope their foliar base as bud-scales, here developed into
normal foliage-leaves; and this because the development of the leaves of the mother-
shoot formed in rudiment in the previous year had been prevented early, by
taking away or lopping the apex. Thus the nutritive materials necessary to the
development of true foliage-leaves were able to reach these young leaf-rudiments,
which, as a rule, develope as scales — or in other words, the growth of the foliage-
leaves of a shoot of Prunus padus which is putting forth leaves prevents the lateral
shoots which simultaneously arise in the axils of its leaves from completing the
development of their foliage-leaves also. This demonstrates the correlation of the
two, and the case is similar in many other plants. This was experimentally
confirmed by Goebel in the case of the Horse-chestnut, and in Maples, Roses,
Syringas, and Oaks. Into Goebel's more involved researches, and those made on
plants which may be less known to the reader, we will not here enter.

The complications which make their appearance in the experimental investiga-
tion of the correlations of growth are chiefly due to the fact, that with the altered
growth of an organ all its reactions towards the environment also become changed :
this shows at the same time, as I stated in the first lectures, that the true physio-
logical nature of an organ is to be sought not so much in its outward form and
anatomically visible structure, as in its irritability or capability of reaction. Even
in the first-named example of the potato, and in the substitution of the removed
apex of the stem of trees, it is clear enough how, with the change of growth
by means of interference with the correlations, the geotropism also— i.e. the

5o8 LECTURE XXX.

capability ot the organ to react towards gravitation — is eo ipso modified ; and
it may be shown in other cases how by such interferences even the reaction
to the action of light of the organ which is being modified becomes changed.

From the small number of examples by which I have sought to illustrate the
conception of correlation of growth, it is not to be concluded that we are here
concerned only with incidental and isolated phenomena : on the contrary, we
have every reason to believe that similar relations between the growth of any one
organ and that of all the others of a plant, however difficult to detect, are very
general. In animals, and especially the more highly organised species, a mutual
correlation, in consequence of the sharply marked individuality, not only exists
between all the organs (which, it is true, are all nourished by the same blood), but is
also more easily intelligible than in the case of plants ; for from daily but superficial
experience, the very common opinion has been formed that in plants every organ
is formed and grows quite independently of the others. The possibility of raising
new entire plants from cut-off" pieces of leaves, shoot-axes, or roots, by the regene-
ration of the missing organs, easily leads to the belief that no intimate mutual
relation of the vegetable organs at all exists. But a deeper insight into the whole
nature of vegetation leads to the opposite conclusion. In the plant also, it is true that
every growing organ derives its constructive materials from the common store of
nutriment which is collected and distributed in the tissues by the assimilation of the
leaves, or has been deposited in quantities in the reservoirs of reserve-materials from
a previous period of vegetation; and it is obvious that when numerous different
organs are simultaneously drawing their materials for growth from the common störe,
one may take what the other needs, and it may happen, as for example the experi-
ment of Prunus padus cited above teaches, that through the growth of a shoot the
simultaneous growth of its lateral buds may be prevented. Above all, from this point
of view, equivalent organs must be regarded as competitors at the common store
of unsorted nutriment : the growth of a shoot will re-act especially on that of other
shoots, the growth of a root chiefly on that of other roots ; since it requires no
proof that the mixtures of materials which the shoots draw from the common store
of nutriment of the plant diff"er in nature from those which afford the constructive
materials of the roots. In the same way experience teaches that in the nutritive
substances produced by assimilation and altered further by metabolism, peculiar
mixtures of materials become differentiated from which the sexual organs and (in
the Phanerogams) the flowers arise. If the first young flower-buds are removed
from a plant, the usual result is that other much younger flower-buds, which would
probably not have developed at all, begin to grow so much the more vigorously ; or
that flower-buds, the rudiments of which are not yet formed, arise at places where
they would not have arisen at all without the interference — a fact upon which the
old knowledge of fruit-culture in part depends. If the flowers, foliage, shoots, and
roots were constructed from the same mixtures of substances, it would not be
obvious why the removal of young flower-buds should not also call forth an
invigorated growth of leaves and roots. The latter indeed occurs under certain
circumstances, but only in consequence of further interdependencies which we
cannot here follow further. Here also we are not further interested in the question
how we are to imagine the production chemically, as it were, of these various

MUTUAL COMPETITION OF ORGANS. 509

organ-forming mixtures of substances. Considerations on this point ' are found in
my treatises ' On the Substance atid Form of the Orgafis 0/ Plants' and later, when
we are concerned with reproduction, I shall probably find opportunity of taking up
the subject again. Here my intention is to mention only one of the most essential
and important factors which co-operate in the correlation of growth, but by no
means to assert that still other causes do not interfere and affect the matter.
Above all there is one point to be insisted upon — that the various organs growing
from the common nutritive material of the plant, and as competitors for that
material, stand opposed to one another in a certain sense as enemies; though,
on the other hand, it is to be noticed that the various organs, especially shoots and
roots, afford support to one another by their functions, and are so far indispensable
to one another. This latter is the case, however, as a rule, only when they are
fully grown and completely developed for their specific function : a mature, fully
grown foliage-leaf promotes the production of new foliage-shoots, because by means
of its assimilation it affords new material for growth, and so forth. Considerations
of this kind, however, w^ould carry us into matters of which I have already treated in
detail in the theory of nutrition.

Again, it cannot be expected that all correlations of growth are so easy to
demonstrate by experimental interference on the part of the observer, as in the
cases considered above : on the contrary, causes may exist in the plant which lead
the result of such experiments into paths quite different from those desired. How-
ever, even without experimental interference we may, supported by what I have so
far stated, obtain a deeper insight into very extensive correlations of the whole
organisation of a plant. As one example I might especially quote the relations,
repeatedly indicated in previous lectures, which exist between the properties of the
chlorophyll and the whole external and internal organisation ; this is of such a kind
that one may regard, without exaggeration, the whole relations of form in the
vegetable kingdom, and particularly the very different aspect of plants in comparison
with that of animals as depending on the properties and activities of chlorophyll. In
this of course we enter upon a province which extends far beyond that of the
correlations of growth shown above, but still, since the whole organisation of a
plant is the result of its growth, we may nevertheless take into our present sphere of
thought what follows.

It is my intention to show somewhat more in detail, that, as a matter of fact, the
most essential relations of organisation of the plant are causally determined by the
properties of chlorophyll.

We may start from the fact that the cells which contain chlorophyll are the only
assimilatory organs of the plant, and that they alone are able to produce, from carbon
dioxide and water, organic and organisable substance suitable for the growth of new
organs, and that for this purpose they require those materials which can only be absorbed
from the soil by means of the roots, the so-called ash-constituents, with which a
nitrogenous compound, nitric acid, is associated for the purpose of forming proteids.

' Sach's ' Stoff und Form der P/Iaiizenorgane' (Arb. des bot. Inst, in Wzbg. B. II, H. 3, 1S80,
p. 452, and H. 4, 1882, p. 689).

5IO

LECTURE XXX.

Experience teaches that even a ver)' thin layer of tissue containing chlorophyll
completely uses up all those rays of light which effect assimilation. A thick layer of
chlorophyll-tissue has therefore no meaning whatever : in fact it would be a waste
of material in the plant. Accordingly, we find that everywhere in the vegetable
kingdom only very thin layers of green assimilatory tissue come to be employed —
layers of one or a few tenths of a millimeter thick — even when, as in the case of
succulent plants, the leaves or soft shoot-axes are very thick and massive, in which
cases the thin layer of green tissue lies as near as possible to the surface of the
organ, in order the better to make use of the incident light.

It is, on the contrary, of the greatest importance for energetic assimilation, or
the production of substances capable of promoting growth, that the thin layers of
green tissue should form surfaces as extensive as possible, if the construction of a
vigorous growing plant is to be at all accomplished.

These reflections then show why, as the organisation of the plant gradually
approaches perfection from its first beginnings, it is above all necessary to produce
organs which are very thin, and possess the largest possible surface of tissue containing
chlorophyll. In the lower Algse this is attained by their assuming the form of thin
and long hair-like filaments, or else that of thin flat lamellae, so that in both cases the
volume of the body remains very small, as compared with its surface. These two
forms, where the entire plant is thin and filamentous and usually much branched,
or else flat and leaf-like, is found again not merely among the highly-organised
Alg£e, but also among the Mosses, and even in some Vascular plants — such as the
leafless shrubs Psilohmi, Spartüan, and others. Here the principle is ever main-
tained that only a very thin peripheral layer of assimilatory tissue, accessible to the
light, exists. But the purpose mentioned is attained much more completely when
the shoots become differentiated into leaves and axial portions, as is the tendency
frequently enough even among the Algas, and almost universally among the Mosses
and Vascular plants. By this means it is possible for the shoot-system to present to
the light (and thus to the nutritive process) a large number of thin lamellae containing
chlorophyll and at suitable distances from one another; and only with such a
differentiation into a support (shoot-axis) and lamellae containing chlorophyll (leaves)
springing from it, does vegetation in general attain to its higher stages of organisation,
and especially to the massive forms inhabiting the dry land, as known to us in the
large Ferns, Palms, Conifers, Forest trees and dicotyledonous shrubs. How
otherwise could the problem be solved so to construct and support a sheet of
assimilatory tissue scarcely 0-2 — 0*3 mm. thick and often many square meters in
area, such as is met with in the crown of a Beech or an Oak with its thousands of
leaves, or in the few but large leaves of a Banana or Palm ? If we imagine a plant,
for instance, of such a kind as Marchafiiia, where the green shoot itself bears the
thin sheet of assimilatory tissue, as large and heavy as a huge Palm or Oak, then it is
obvious what a monstrous organisation we should have, and how contradictory it
would be to the higher types of the vegetable kingdom. Of course leaves are
wanting to plants like the Cacti also : they are satisfied with a thin layer of
chlorophyll at the periphery of the thick shoot, and hence their increase in organic
mass is relatively very slow ; and, what is perhaps more remarkable, their leafless
green shoot-axes assume, even frequently, forms similar to those of leaves — e.g.

THIN ASSIMILATORV SURFACES. 51I

PhyUocactus, many species of Opim/ia, &c. This occurs in a still higher degree in
shrubby plants furnished with Cladodes, as Ruscus, Phyllanl/nis, &c.

The vegetable kingdom, so far as it is self-nourished by means of green tissue, is
solely and entirely, so far as form is concerned, ruled by the principle of developing
on relatively thin supports, or shoot-axes, the greatest possible number of green
organs with the thinnest and largest surface possible. The exceedingly graceful
growth of plants which contain chlorophyll, so familiar to all, is thus due simply to
their containing chlorophyll, because the activity of the assimilating parenchyma only
comes into full effect in this case. The contrast presents itself to us at once in the
case of plants devoid of chlorophyll, the fructifications of Fungi, and parasitic and
saprophytic Phanerogams. It is sjmply the want of chlorophyll which here makes
altogether superfluous the extension of the surface in the form of large leaves; the
body of the plant is therefore developed chiefly as shoot-axes and appears naked,
stout, massive, and ungraceful. To them also, as to all and even highly organised
water-plants, the formation of true wood is wanting, because they do not need it :
since in plants devoid of chlorophyll (which moreover never attain to the size and
massiveness of a tree, or even of a large shrub, but usually remain small and incon-
spicuous) the want of large leaf-surfaces also entails that of organs of transpiration,
and thus the current of water flowing in woody plants from the root to the leaf-
crown is dispensed with — they do not require wood because they possess no leaves
containing chlorophyll. Moreover, true water-plants are devoid of wood for
physiological reasons ; it is true they often possess very large leaf-surfaces, but these
are submerged, or they float on the surface of the water, and can themselves take up
from the environment the water which they require, and thus do not need an active
current of water coming from the roots. We may thus say simply that the formation
of wood and (as must be added from what has been previously stated) the formation
"of numerous thick strands of sclerenchyma is due to the fact that the shoot-axes
of terrestrial plants are elevated above the soil, extend their leaf-surfaces to the air
and to the light, and find in the lignified tissues of the stems and branches not only
elastic and rigid supports, but at the same time the organs which convey to them
from the roots water provided with nutritive materials. I have already attempted to
make clear by what arrangements the current of water ascending in the wood is
conveyed directly into the transpiring green leaves, and how, by means of the
venation or distribution of the vascular bundles in these, this water flows into
thousands of fine canals, from which the chlorophyll-cells of the leaves take up their
nutrient water. I have already shown, moreover (p. 50), how as the size of the
leaf-area increases, and thus the extent of the chlorophyll-tissue, the additional and
important function falls to the venation of keeping the exceedingly thin lamella of the
leaf extended flat, and at the same time of protecting it from lateral rupture.

This consideration, which it is true only brings out the more obvious
features of the organisation of plants, is always traversed by the thought — a guiding
thread as it were— that all these arrangements are intelligible and full of purpose
only because the plants concerned are nourished by the chlorophyll under the
influence of light.

If however we turn again to the current of water ascending in the wood and
distributing itself in the veins of the leaves, we know already that it has fundamentally

512

LECTURE XXX.

only the one purpose of conveying the nutritive substances of the soil to the
assimilating tissue. But these substances are dissolved in very large quantities of
water taken up by the roots in the soil, and I have previously shown in detail what
difficulties the roots of terrestrial plants have to struggle with in order to obtain
these large quantities of water from a soil which is scarcely moist, and that they are
even compelled first to dissolve a portion of the nutritive materials by means of their
innumerable root-hairs, in order to transfer it to the current of sap. The roots,
however, are only adapted for this function when they in their turn obtain not only
the necessary surface, but also a sufficient number of points of contact with the
particles of the soil. The former is attained in the first place by the great length
which the totality of all the root-fibres and their ramifications reach in the soil; by
means of this development in length, and the usually very small diameter of the
absorbing roots, a relatively large surface extension is already attained, but this is
supplemented in an exceedingly advantageous manner by the production of the
multitudinous root-hairs, and their continual new formation behind the forward-
growing apex of the root. All these properties of the roots, which have already
been referred to in the lectures on Organography, are developed only in terrestrial
plants which contain chlorophyll : these alone need a root-system so extensive and
so organised. We must again perceive that it is to the properties of the chlorophyll
and its nutritive activity that the organisation of the roots of terrestrial plants is
due. Land-plants devoid of chlorophyll, since they need no transpiration-current,
also possess but few roots, and these are short and for the most part thick :
water-plants on the contrary are again, for another reason, spared the trouble of
developing an extensive root-system ; since they are in a position to take up water
and nutritive materials at the whole of their surface, or, if the leaves float, at least
at their lower surface, no root-system whatever is needed, or at any rate only an
accessory one, to maintain the nutritive activity of the chlorophyll. In Algse, and"
even in some aquatic Phanerogams, the roots are chiefly or it may be exclusively
organs of attachment to fix the plant in its position, thus again contributing to keep
those parts of the plant which contain chlorophyll in such a position that they
can assimilate under the influence of the light.

Evidently the relations of organisation which are merely sketched here, have
arisen by correlations in the progressive development of the vegetable kingdom ; and
in fact we can see up to a certain point how this has come to pass, so that the
better the chlorophyll-tissue was able to extend itself to the light in thin broad
sheets, the more capable it would be of producing large quantities of vegetable
substance, which would provide the material for the formation of the huge masses of
wood ; and on the other hand we know that the more complete aeration of
the soil as the roots of the land-plants need it, directly and essentially contributes
to promote the lengthening and branching of the roots. In proportion as this
occurs, the roots in the earth are also able to transmit larger quantities of nutritive
water to the chlorophyll, which in its turn produces the material not only for the
conducting wood-body but also for the growth of the root.

We can, however, adduce numerous other relations of organisation as necessary
correlates of the activity of chlorophyll. The Geotropism of the roots as well as
that of the shoot-axes serves (as does also the heliotropic sensitiveness of the latter

CORRELATIONS BETWEEN LEAVES AND ROOTS, ETC. 513

and of the leaves) above all to bring those parts of the shoot which contain chloro-
phyll into a position favourable for their illumination, and the root-fibres into one
suitable for their activity. In many cases, as with twining plants and those \^hich
climb by means of tendrils, peculiar relations of organisation are developed in order
to bring the assimilating leaves into the light in spite of the fact that the shoot-
axes are thin and feeble : in the same way, the numerous leaves endowed with
so-called sleep-movements are always so organised and so react towards the light,
that they present their surfaces to it at the beginning of the day, and when the
light becomes too strong turn themselves so that the rays meet them at small
angles, and are thus rendered harmless. It is obvious that these irritabilities
also have meaning and purpose only in so far as it is important to bring the
assimilating organs containing chlorophyll into the most favourable position possible
for illumination.

Even these correlations, which, as already mentioned, must have made their
appearance in the course of the progressive development of the vegetable kingdom,
may be at least in part subjected to experimental control, and indeed every-day
experiences in the cultivation of plants confirm the above conclusions. Here is one
example only. If a Tobacco-plant, or a Ricinus, or a Sunflower, &c., is allowed to
develope in the open and in good soil, there is formed in the course of 100-120
days a powerful stem, occasionally as thick as the arm, with numerous very large
leaves and an enormous root-system. If, on the other hand, we cultivate such
a plant in a flower-pot filled w-ith three litres or so of the best garden soil, the plant
in its pot standing in the open and being even watered two or three times daily with
proper solutions of nutritive substances, we obtain in 100-120 days a stem of the
thickness of a finger, and a total leaf-area which amounts to scarcely a fifth or a sixth
of that in the previous case, and, shortly, a small insignificant plant, in spite of
all the artificial supply of nutriment at the roots, and in spite of the bright
illumination of the leaves. But the cause exists in the limitation of the space for
the roots in the flower-pot : it is true any one unacquainted with the matter is
astounded at the apparently very large quantity of roots in the ball of earth Hfted out
of the pot, but as a matter of fact this is extremely small in comparison with the
rooting of a similar plant in the open land. Moreover the roots are unfavourably
situated : they are all crowded together, with their growing ends and the parts
specially adapted for the absorption of food materials close to the inside of the pot,
and there forming a dense felt work all round, which not only hinders further growth
and branching, but also completely excludes most of the roots from the soil in the
pot, and even makes impossible the proper employment of the nutritive solutions
with which they have been watered. The result of this inefficient growth of the
roots is a diminished supply of nutritive materials in the assimilating leaves, in
consequence of which their activity remains insignificant, and this again aflccts the
formation of wood in the stem.

In this case the pernicious correlation proceeded from the roots ; but it may start
from the leaves also (even though the roots are fully developed in the earth), when
they are so feebly illuminated that assimilation, although not entirely prevented, is
reduced to a minimum. Together with a somewhat abnormal aspect of the plant in
other respects also, it is then especially found that the stem remains thin, forms but

[3] Ll

514

LECTURE XXX.

little wood, and that this wood is soft. The stem of such a shaded plant may be
easily cut through with a pocket-knife, though this is not the case with a normally
grown plant of the kind mentioned above, on account of the hardness of the wood.
I have already mentioned in a previous lecture and in another connection that the
so-called laying of wheat when it grows too rank is due to the shading of the lower
joints of the haulm, and its deficiency in sclerenchyma.

We have already become acquainted with a particularly fine and instructive
example of the influence of chlorophyll on the whole mode of growth of a plant, in our
previous considerations on the nutrition of the Lichens. We found that the Lichens
are true Fungi (Ascomycetes) and nevertheless their bodies have in general forms
quite different from those of all other Fungi. All that appears above the substratum
in the case of ordinary Fungi is usually a fructification; the Lichens on the contrary
are developed entirely outside the substratum, or they send their root-hairs only into
it. The body of the Lichen, instead of showing a tendency to the formation of
thick fleshy or even woody masses, as other Fungi, assumes either the form of
thin leaf-like expansions, or bands, or thin much-branched shrubby structures.
In other words, the tendency prevails among the Lichens to assume the form of
ordinary plants with flat extensions or branched filiform structures, and this, as I
have already said, is solely in consequence of the fact that the tissue of the Lichen-
fungus encloses Algae, which contain chlorophyll and look as if they constituted
a normal anatomical constituent of its tissue. This, however, is equivalent to
saying that since these chlorophyll-cells are simply for the purpose of assimilating
they must be presented to the light in thin but relatively extensive surfaces, and
this is accomplished in the case of the foliaceous Lichens just as in ordinary
foliage-leaves, and in the fruticose Lichens just as with leafless shrubs, where the
assimilating tissue is situated in the external cortex. If the correlation between
chlorophyll and the whole growth of a plant comes out clearly at all, it is certainly
in the comparison of the forms of Lichens with those of the bodies of other Fungi.
Here we have to a certain extent the test for what I have said concerning the
correlation between chlorophyll and the configuration of the plant generally.

LECTURE XXXL

INFLUENCES OF THE ENVIRONMENT ON THE PROCESSES OF
CONFIGURATION IN THE PLANT.

We are now acquainted with a considerable number of cases where, by
means of external influences, and particularly those of Gravitation, Light, Pressure,
and by stimuli due to animals or plants, processes of growth are modified or are
even induced ; this however not simply in the sense of accelerating or retarding
the increase in volume — on the contrary, I have here in view the processes
of configuration itself, the production of new growing points and their embryonic
outgrowths, and further extensive qualitative alterations of organs already existing
in a young state, by means of external influences.

The processes to be described have to be regarded as phenomena of irritability
in the growing parts of plants; since, as has already been incidentally men-
tioned, phenomena of irritability in general are fundamentally nothing but specifi-
cally peculiar reactions of the organism towards external stimuli, and it is just
the cases which are to be considered here which belong to this category. The
province of the phenomena of irritability, however, is so extensive that, properly
put, the whole of physiology is everywhere and always concerned with it, so
that in fact physiology might simply be designated the science of the phenomena
of irritability. But it is exactly on this account that I wish to bring forward the facts
to be described here particularly and by themselves, apart from the endless number
of phenomena of irritability, and not to put them off to the subsequent and more
detailed consideration of the induced movements, because they contribute in an
important manner to render our insight into the nature of the growth of plants more
profound.

Until quite recently the opinion has been retained, dating from times imme-
morial in the period of the development of human thought, that organic forms are
something fixed and unalterable from all eternity, and thus removed from the region
of causality ; and with reference to this mode of viewing the matter it is probably
worth while to convince ourselves, by taking a few well marked cases more
closely into consideration, of the fact that incidental external stimuli are able to
aid in determining the processes of configuration in organisms in very definite
ways. This old mode of representing matters is of course now opposed by

l1 2

-1 6 LECTURE XXX T.

the theory of descent, according to which, organic forms are by no means fixed
from the beginning and simply copies of purely ideal types (Platonic ideas) : on
the contrary, the theory of descent states that every organic form is the result
of a historical process, in which the co-operation of two factors — the transmission
of peculiarities already acquired, and frequent small deviations from these, or varia-
tion— appear as causes of organic form. Of course there lies in this view the
recognition of a creation in time of organic forms, but it leaves the forces therein con-
cerned still in the main undetermined. The theory of descent demands in the first
place only the recognition of the fact that in the course of time organic forms have
been produced by some concatenation of causes ; it leaves to us, however, the
responsibility of the answer to the question what forces have been effective in
determining the production of organic forms in any given case. Some, to be
sure, imagine that heredity and variation are such forces ; but they forget that these
terms are simply words for facts which are not understood, and that heredity and
variation are not forces in the sense of Science in general — i. e. they are not causes
of motion.

I premise these general considerations however, only with a view of throwing
light upon the apparently insignificant and trivial matters with w'hich we are here to
be concerned, and to make their significance clear from a general point of view.
So much is it at any rate obvious, that the processes of configuration in the vegetable
kingdom which are connected with growth, result from the co-operation of two
factors, the one of which lies in the quality of the organic material itself w^hich
is capable of configuration, while the other factor exists in the continuous influence
of universally co-operating cosmic forces, or in occasional accidental stimuli. To
employ an illustrative example taken from inorganic nature, I may remind the
reader how it depends on the physical nature of water that it becomes solidified in
hexagonal columns at low temperatures. In this capability the original nature of
the water is expressed; but it then depends upon accidental external circumstances
whether these hexagonal columns arrange themselves into a solid lump of ice,
apparently without any internal structure, or whether we have innumerable minute
hexagonal ice-crystals joined together on a window pane in the most various forms, as
so called ice-flowers, the growth of which depends upon differences in the external
conditions. As in this case, the organisable substance of the plant also seems to be
subject to certain unalterable formative forces, which however, under the influence
of accidental external causes, may experience the most difl"erent combinations and
variations.

Proceeding now to a somewhat detailed description of some of the most interesting
observations appertaining here, it will be advisable to divide them into two groups, of
which the first comprises those cases where it is a matter of the origin of growing-
points, or of the influence of external forces on the formation of organs on these,
whereas a second group presents phenomena which are called forth by the influence
of external forces on post-embryonic growth.

I. External influences on embryonic configiiraiio7i. I may here in the first
instance describe a case where, by the influence of an external force, gravitation,
the place at which the growing-points of new shoots are formed, is determined.

Thiadiantha dubia belongs to the Cucumber family, and produces on its very

TUBERS OF THLADIANTHA DU HI A. 517

long and thin root-fibres subterranean tubers of a size varying between that of a
Hazel-nut and that of a medium-sized Potato. These tubers are swellings of the
thin root-fibres, and the growth in length of the latter is not interrupted by their
formation. Consequently we find in the middle of a root-fibre 1-2 metres long,
one to three tuberous swellings, the transverse section of each being approximately
quadrangular, with rounded corners ; the tubers are thus four-sided prisms, and
usually lie in the soil in such a manner that one of the flat sides is uppermost, and
the other turned downwards. In Autumn, the plant together with its root-fibres
perishes, and only the tuberous swqllings of the latter persist in the soil through the
Winter. In April we find on these, numerous thin root-fibres proceeding from all
sides, as well as young shoots 1-2 cm. in length : it is with the latter we are here
concerned. If the tuber lies horizontally in the earth, all the shoot-buds arise on
the upper side : if, on the contrary, the acroscopic end of the tuber (i. e. the end
directed towards the apex of the root) is directed upwards they are all situated on this,
around the point whence the root-fibre continued its growth. To me this fact leaves
no doubt that we are here concerned with an effect of gravitation on the production
of the growing points of the shoots, since light was excluded, the object remaining
underground. It should also be mentioned, however, that in all cases the shoot-
buds arising from the tuber are more crowded towards its acroscopic end. A large
number of experiments ^ which I made with the tubers of Thladiantha dubia, yielded
distinctly the result that the shoot-buds are formed exclusively on that side of the
tuber which hes uppermost (towards the zenith) during their development, and that,
moreover, in virtue of an internal disposition the acroscopic end of the tuber is
preferred in the formation of buds. Here, therefore, are two different causes
working together, and determining the place of origin of the adventitious growing-
points of shoots — internal causes bring it about that the end of the tuber situated
towards the apex of the root is especially qualified for the formation of buds, while
at the same time the influence of gravitation determines that the buds arise on the
side of the tuber which is turned away from the centre of the earth.

Not less instructive are my observations on the genus Opuntia, one of the
best known forms of Cactus. In Opuntia ficus Indica and O. crassa, with which
I experimented, the sub- aerial vegetative body consists of elongated flattened
segments of the shoot-axis, on which the positions of the suppressed leaves
are indicated only by tufts of prickles. Under normal conditions of vegetation,
new flattened segments arise chiefly from the apex of the segments which then
happen to be uppermost, and especially from their margins or angles, the angles
directed upwards being always preferred in the process. It is easy to see that
an impulse exists in the plant which leads to the appearance of new shoots
above, and at the same time preferably at the narrow angles of the flat segments :
it is only extremely rarely that a new shoot makes its appearance on the flat
broad side of an old segment. The tendency for new shoots to appear chiefly at
the apical portions of the older ones, is in fact very common in plants generally, but

' More details as to the behaviour here described of the root-tubers of Tliladiantha dubia and
of the shoot segments of Opuntia are found in my treatise ' I'hcr Stoff tttid Form der Pßanzen-
organe' Arb. des bot. Inst, in Wzbg. B. II, 1882, p. 698.

51«

LECTURE XXXI.

in this case experiment teaches that it is an effect of gravitation. In the first place, we
have sometimes an opportunity of observing in gardens where Opuntias are cultivated
the case represented in Fig. 337 a. This figure shows a very large shoot-segment of
the plant, /, which has given rise in the usual manner on the right and left angles to
the lateral segments marked //. After the production of the latter, the segment /
had accidentally bent over at its apex, so that one of its flat sides was turned down-
wards and the other almost horizontal and facing upwards. When new segments
began to shoot forth in the next spring, they appeared not exactly at the angles of /,
but rather more on the uppermost surface of the segment, but nevertheless not far

Fig. 337 A.—Opicntia Fktts indie,

from its two margins, as shown at /// in the figure. Only in the following year did
a further shoot /Farise, quite independent of the two lateral angles, on the up-turned
surface of the mother-segment. It thus required two years to overcome the tendency
to form shoots at the angles, through the influence of gravity, and perhaps with
the co-operation of light, so that the shoot IV could arise at the middle of the
upper side of the shoot /. To see how far the relations of configuration of this plant
are subject to external influences, however, it is first necessary to know that the two-
edged flat form of the shoot-segments is in its turn due to illumination, since the
segments of Opiintia assume this flat two-edged form only under the influence of light,

GRAVITATION AS DETERMINING THE POSITION OF OFFSHOOTS.

5*9

whereas they become more or less cylindrical or prismatic when grown in the dark.
That the normal offshoots thus arise from the margins is an indirect consequence of
light ; that they arise chiefly from the upper surface of the existing segments is a
direct effect of gravitation, which can in its turn conquer the preceding influence of
the light if the segment which produces the new shoots assumes the proper position.
We may easily convince ourselves also by means of an experiment, that the places
of origin of new growing-points are actually dependent on the influence of
gravitation, as illustrated in Fig. 338. A plant which already consisted of the
segments /, //, ///, and growing in the pot c, was placed in an inverted position on
an iron support a, in such a manner that the plant was directed downwards and the
pot c upwards, the falling out of the soil being prevented by the interposition of the
divided cover ö. The experiment began immediately after the formation of a new
growing-point at the apex of the
plant, and this growing-point was
cut off. In the next few weeks
were then produced five new grow-
ing-points, from which the shoots
marked 1-5 in the figure developed.
The origin of the shoots i, 2, and 3
from the segments // and /// sim-
ply present nothing surprising, since
they arise at the apical portion of
the plant. That the segments 4
and 5 however have arisen also
from the old basal segment /,
proves that here, in consequence of
the inversion, an effect of gravitation
has made itself evident : the shoot-
forming substances which previously
streamed towards the apex have now
gone back, in part at least, upwards
into the segments // and /, to pro-
duce the shoots 4 and 5, which in fig. 3^,s.-fl/,u„ü\,ry.us, „.,■:. a.
the normal position of the plant

would certainly not have happened. But the places of origin not merely of
the shoots but also those of the roots are determined in Opuntia by external
forces, and probably chiefly by gravitation. In experiments to elucidate this, how-
ever, difficulties are met with arising from the fact that the plant was previously
growing in its ordinary normal position, and a strong predisposition thus induced.
On cutting off vigorous Opuntia segments and then placing them with their
basal ends in the soil of a pot, and leaving them in the same position as in
Fig. 338, there appear, within a few weeks, from the basal ends (now turned up-
ward) of the segments, vigorous and much-branched roots which fill the soil of
the i)Ot. In this is expressed the tendency of the root-forming substance to be
driven to the basal end even when this is turned upwards ; though here the
moisture and darkness surrounding the basal end co-operate as favourable factors.

520

LECTURE XXXI.

If however cut-off segments of Opunlia are set with the apical end (a piece of which
has been cut off transversely) in the soil, in such a manner that the basal end is again
turned upwards but projects into the air, two to five months pass by before any
roots make their appearance from the apical end of the shoot segment, thus
directed downwards. We may explain the result of this experiment by the assumption
that root-forming substance exists in the shoot-segments of the Opuntia, but that in
the normal position of the plant it always seeks to move in the direction of the base,
in order to strengthen the subterranean root-system of the plant. By this is produced
a predisposition in the tissue of the shoot-segments which facilitates the movement
of the root-building substance towards the base, whence is explained that also in
inverted shoot-segments — but with the base planted in the soil— new roots are pro-
duced rapidly and vigorously at the basal end. If on the contrary the apical end of
the shoot-segment is planted in the soil, and at the same time turned downwards, it
requires a long time before the root-forming substance is able, in opposition to the
previous predisposition and in obedience to gravitation, to make its appearance at
the apical end in the form of roots. That this happens at all in the present
case is essentially aided by the additional influence of darkness and moisture, which
promote the formation of roots at the planted apical end.

In the experiments last quoted we have to do in the main with the reproduction
of organs on separated portions of plants, concerning which numerous experiments
have for a long time been to hand, and which have been recently investigated by
Voechting ^, but in part incorrectly explained. Practical gardening has known from of
old the fact that it is possible to obtain new plants from even small portions of older
ones, roots and shoot-buds being formed on them. For this purpose it is usual to lay
the cut-off pieces in moist sand or soil ; for scientific purposes, however, it is often
better, as Voechting did, to suspend the separated portions in moist air, in order to
attain an equable environment favourable to growth. If these cut-off pieces of the
shoot-axes of woody or herbaceous plants are suspended in a horizontal or vertical
position, the usual result is that roots arise from their basal ends, and shoot-buds
from their acroscopic or apical ends — from which however the true apex has been
cut off. As a rule the behaviour is the reverse in the regeneration of cut-off pieces
of root; here the tendency prevails to produce shoot-buds at the basal end, and
roots at the acropetal end — i.e. the end turned towards the growing-point.
Finally, if the same experiments are made with cut-off leaves, buds and roots arise
at the base of the cut-off petiole, the buds usually being turned upwards, the roots
downwards. Voechting has attempted to bring the results of numerous experiments
in this direction which he has made under a general expression, intended to mark
everywhere a distinction between what he terms the apex and the base of the re-
generating organ. It is obvious, however, that in this sense the facts shortly referred
to above cannot be collated at all. According to my view, on the contrary, it is easy
to get at the heart of the matter if we proceed from certain allowable assumptions, and
keep in view the immediate effective causes which determine that roots and shoots must
arise at definite spots on cut-off parts of plants. In fact, as I have clearly shown in

' Voechting, 'Über Organbildung im Pflanzenreich,'' Bonn, 1878. Compare my reply in the
treatise quoted in note i (p. 517).

MOVEMENTS OF SHOOT- AND ROOT-FORMING SUBSTANCES. 52 1

detail in my treatises on 'S/oß und Form der Pflarizentheile V two causes, which usually
co-operate, come into consideration here : on the one hand it is to be noticed that
the substances for the formation of organs (apart from germinal stages where they
proceed from the reservoirs of reserve-material) are produced in the foliage leaves by
assimilation, and pass out thence into other parts of the plant. So long as a vigorous
main bud of the shoot is present, mixtures of substances suited chiefly for the
formation of shoots pass into it from the leaves, whereas the substances fitted for the
formation of roots flow in the opposite direction, into the roots which already exist.
If then a portion of the shoot-axis is cut off and kept in a moist warm environment,
the substances suitable for the formation of shoots which are already present in it
will, as heretofore, move in the acropetal direction, and those which form roots, on the
contrary, in the basipetal one — i. e. the shoot-buds will make their appearance on a
cut-off piece of stem at the apical end, the young roots at the basal end. Exactly the
reverse must take place, however, in the regeneration occurring in a cut-off piece of an
old root : since here the root-forming substance is in continual movement towards
the apex, the roots produced by regeneration will arise at the acropetal end, and if
any bud-forming substance at all has reached the roots it may come forth at the
basal end of the piece of root, if the second external cause to be mentioned hereafter
co-operates. That roots as well as shoot-buds appear at the basal end of a cut-off" leaf
is explained, according to this view of the matter, simply by supposing that during
the assimilatory activity of the leaf, and in accordance with its normal function,
shoot-forming as well as root-forming substances are continually passing from the
leaf through the petiole into the shoot-axis. If then the petiole is cut off, this customary
movement proceeds no further, being, as in the case of cut-off pieces in general, to a
certain extent checked at the section, and buds as well as roots will grow out from
the petiole near the section. These considerations then show that effective causes
are given in the organisation and vital activity of the plant itself by which the places
of origin of new roots and shoots are determined.

A second cause, however, has already become known to us in the examples
quoted above ; this is the influence of gravitation, which, so far as we are as yet
informed, affects different plants in very different degrees, in that new shoot-buds
arise more easily in cut-off portions on the ends directed upwards, new roots on
those directed downwards. If we suspend a cut-off piece of a shoot-axis in moist air,
with its basal end downwards, two influences are working together to induce roots to
arise at the latter, and at the same time to cause shoots to spring from the acropetal
end. If, however, we give the cut-off piece the reverse position, with the acropetal end
downwards, the two causes mentioned must work in the opposite w^ay : the internal
disposition in this case induces the formation of buds at the acropetal, lower end,
while the influence of gravitation strives, so to speak, to draw the root-forming
substance downwards, and to push up the shoot-forming substance to a correspond-
ing extent, and thus bring about the opposed position of the new organs. It depends
now entirely and simply upon the reactive capacity of the plant concerned, whether

* That in regeneration occurring in cut-off parts of plants an internal disposition depending on
the organisation of the plant co-operates with a direct effect of gravitation and other forces, I have
shown in detail in Arb. des bot. Inst. B. II, pp. 691, &c.

522 LECTURE XXXI.

and to what extent the influence of gravitation succeeds in affecting the position in
relation to it of the new organs.

Since, then, as Voechting had in part found, and as results definitely from my
experiments on Thladianiha and Optmh'a, gravitation acts in such a Avay as to move
the root-forming substances downwards and the shoot-forming substances upwards,
we must assume that this is the case not merely with cut-off pieces, but also with entire
plants living and rooted in the soil ; in this case, however, the internal disposition and
the influence of gravitation work to the same end, at least in so far as plants with
erect stems and down-turned roots are concerned. In accordance with this it is
also the rule that in horizontal creeping or climbing shoots the formation of roots
takes place on the axis progressively behind the terminal bud, on the underside of
the shoot, and in such cases the illumination of the dorsal side of the shoot also
evidently has for its effect that the roots make their appearance exclusively on the
ventral or shaded side.

Instructive examples to illustrate the fact that the light in many cases prevents
the formation of the growing-points of roots upon the directly illuminated side of the
shoot-axis, so that they only appear on the shaded side, may be afforded in the first
place by the leaf-shoots of the Ivy ^. When these climb up on a wall, or on the stem
of a tree, the clinging roots necessary for chmbing arise exclusively on the side turned
towards the solid substratum : in this case, how^ever, the active cause lies, not
simply in the contact or pressure suffered by this side of the shoot-axis, but in the
fact that it is turned from the light. The horizontal pendent shoots of the Ivy teach
this directly : the anchoring roots here are always formed only on the side turned
away from the light. But still more : it is possible experimentally to cause shoots of
Ivy to form aerial roots on what was hitherto the lighted or dorsal side, by illuminating
them for some time on the side which was previously the ventral or shaded side. Fig.
339 illustrates this in the case of an Ivy-shoot, the lower end of which was rooted in a
flower-pot, and fastened upright to a rod. The arrows give the direction of the rays of
light. At A we have the upper part of the shoot with what was hitherto its dorsal side
turned towards the light, and bearing roots only on the side turned from the light ; at
the same time the shoot-axis exhibits a negatively heliotropic curvature towards the
shaded side. A shoot, together with its pot, was now placed at the window so that the
side which bore roots hitherto was turned towards the light. The shoot now made in
the first place the heliotropic curvature represented at C, opposed to the previous one,
and then grew on horizontally : meanwhile new groups of aerial roots arose on the now
horizontal and at the same time shaded side — the previously illuminated dorsal side.
With many other dorsi-ventral shoots of course this reversal of ventral and dorsal side is
not so easy as in the case of the Ivy, since all the influences of external forces here
treated of are dependent upon the specific capacity for reaction of the particular plant.

Another object resembling the Ivy in this connection was found by Voechting
in the broad, two-winged, leafless shoot-axes of a cactus-like plant, Lepismiiim
radicans, in which likewise each of the two flat sides is able to produce roots,
but only when it is turned away from the source of light ^

' The properties of the Ivy shoot here referred to are described in detail in my treatise ' Über
orthofrope und plagiotrope Pßanzentheile' (Arb. des bot. Inst. Wzbg. B. II, p. 257).

^ That the formation of roots is promoted by the absence of light I have already insisted upon

FORMATION OF ROOTS ON THE SHADED SIDE OF SHOOTS.

5^3

So long ago as 1863 I described various examples showing that the formation
of roots upon shoot-axes by darkening them, may be induced at spots where
otherwise no roots arise; here however I need only mention Tropaolum viajus
(Indian Cress). If one of the long thin leafy shoots is bent down as it were into
a depression of the surrounding garden soil, so as not to break it off and so that
an apical portion of the shoot, some 10-12 cm, long, remains upright, and can be
surrounded with damp soil, new roots arise after a few days all around the
vertical piece of shoot-axis, while the older piece of the same shoot-axis, lying
horizontally and likewise covered
with earth, developes roots exclu-
sively on its lower side. This ex-
periment shows not only that the
exclusion of light generally induces
the development of roots on the
shoot-axis, but the simultaneous
influence of gravitation is here very
clearly apparent also : the horizontally
placed shoot-axis forms roots only
on the lower side, the vertically up-
right one on all sides. Of course
in this experiment the growth of
the new roots is promoted by the
moisture of the surrounding soil.
The cause of their first production,
however, by no means resides in that,
since the same result is obtained if a
leaf-shoot of Tropccolum is directed
into the dark cavity of an opaque
box without separating it from its
parent plant, only in this case the
new roots remain very short unless
the air in the box is in some way
kept moist artificially.

These examples refer to the
origin of new growing - points of
roots on shoot-axes ; but, in many
cases, roots may arise in the trans-
lucent tissue of sub-aerial shoot-axes,

that is under the influence of light, although it may be feeble. Thus, in the
creeping or climbing shoots of the Gourd-plant we find a rudimentary root to
the right and left of each leaf, and similarly on those parts of the erect stem of the
Maize plant which are about 10-12 cm. above the surface of the soil: here
the roots come forth in large numbers in a crown above the insertion of the leaf.

shoot {Hedii-a Hc/ix). A has been ill
eral days, B has undergone sini
ater stage of B.

several times in my researclies, ' Über dot Ehifluss des Tageslichtes auf Neubildung und Entfaltung
verschiedener Pflanzenorgane'' (^Bot. Zeigt. 1863, supplement; and the same, 1865, pp. 117, &c.).

5^4

LECTURE XXXI.

In both cases however they remain short ; but it suffices to darken the parts mentioned
to make them elongate. Of course this takes place much more energetically if the
surrounding air is moist ; or best if the parts of the stem concerned are surrounded
with soil, by which means they are darkened and kept damp at the same time.

Much more energetic than in the case of the highly organised vascular plants is the
influence of gravitation, and still more so of light, on the formation of organs in the
Muscinese and other plants of simple structure. Since it is here possible to submit
plants in the very earliest stages of development, as they proceed from the spores or
gemmse, to a definite influence of external forces, the effect begins before the plant
has yet had time to become organised in a manner unknown to the observer. It
is thus possible to allow plants of this category to grow so that one can determine

beforehand and arbitrarily the disposition
of the organs one to another and towards
external influences. One of the most
interesting cases in this connection was
first described by Leitgeb^ in the prothallia
of Ferns. As is already known to the
reader (p. 31) the germinating spore of a
Fern does not give rise forthwith to a Fern
again, but first to a plantlet of quite another
form — the prothallium. The germinating
spore first forms a germinal filament seg-
mented by transverse walls, which subse-
quently spreads out anteriorly, and at length
grows into a very thin leaf-like flat lamella,
which is deeply indented in front and
consists of only one layer of cells contain-
ing chlorophyll : later, however, it produces
in the centre, behind the growing-point,
a cushion, several cells in thickness, from
which the female organs of reproduction
or archegonia arise in large numbers,
while on the posterior portion of the la-
mella, or at its margins, the male fertilising
organs or antheridia arise. Previously to
this, numerous cells at the hinder part of
the prothallus have grown out into long
root-hairs. The whole of this structure
now is most distinctly dorsi- ventral : root-
hairs, antheridia, and archegonia arise exclusively on the under side, if the prothallus
grows on a horizontal substratum and is lighted from above in the usual way.
Leitgeb has shown, now, that the dorsi-ventrality, as such, is in this case an effect of
light, and in the growing parts of the prothallus may be reversed at will if what
was hitherto the shaded side is illuminated, the sexual organs always being formed

Fig. 340.— Prothallus
from below, a antherid
(magnified).

{Ostfutnda regatii

Leitgeb, ' Über Bilateralität der Prothallien,' in Flora, 1879, p. 31

DORSI-VENTRAL STRUCTURE DUE TO LIGHT.

52.5

on the shaded side. Leitgeb demonstrated this highly important fact very elegantly
by sowing the spores of a Fern, Ceratopteris ihalictroides (which, moreover, grows -in
very wet places) on the surface of a clear nutritive solution, where they produce
floating prothallia. If the sowing is illuminated from above, the archegonia and
root-hairs arise on the under side — the latter also from the margins — and both grow
down into the liquid ; if the illumination takes place from below, however, the
prothallia grow downward into the liquid, but become curved, as soon as they begin
to develope the flattened surface, so that the latter is directed at right angles to the
incident light. Here both the surfaces of the plate of tissue are in the water, and
nevertheless the archegonia and roots are developed only on the side turned away
from the light. Prantl has extended this experiment with the prothallia of other
species, and varied it in many ways, with reference to which, however, I cannot
here go more into details, I will only lay stress on one point, that not only the
dorsi-ventrality but also the extension into a bilateral thin plate of tissue is deter-
mined by the light, and that such a plant rendered bilateral and dorsi-ventral by the
light also places itself with its surfaces at right angles to the incident light.

Those Liverworts which consist of flat shoots behave also exactly like the
prothallia ; in them also, as Mirbel partly demonstrated with Marchantia about fifty
years ago\ the dorsi-ventral organisation and bilateral extension are induced by
the light. The germination of the spores of Marchantia runs its course very
much as in the case of the Ferns described above, and if they are sown on the
surface of soil or damp turf, and care is taken that they are lighted from one
side only, it is found that the young, approximately heart-shaped plantlets extend
their surfaces all at right angles to the direction of the incident light, although
their axes of growth may assume the most various positions. The root-hairs
appear exclusively on the shaded side, the stomata only on the upper side. It
is still easier to make use of the gemmce of Marchantia for experiments of this
kind, for example by letting them float on an aqueous solution of nutritive
substances in a transparent glass vessel; if the water is illuminated from below
by means of a large mirror (for which however very intense light must be
employed) while the upper side of the vessel is covered with an opaque lid, the
gemmae grow in the usual manner, and produce broad ribbon-like shoots floating
on the surface of the water, which develope their root-hairs chiefly upwards and
away from the light, and their stomata on the under side which is turned towards
the light. The whole significance of this action of light is only apprehended
however on noticing (see Fig. 341, which represents a piece of a transverse section
at the margin of a shoot) that the structure here concerned consists of several
layers of cells, the relations of growth of which are very complicated, and
that the whole of the organisation, represented in the figure in outline only, can
be brought by the influence of the light into a completely inverted position. If
the Fig. 341 for example is supposed to be illuminated from below, the picture

1 Mirhel's statements occur in his celebrated work, ' Rcchcrchcs anatoniiqucs ct physiologiqiics
stir le Rlarchantia polymorpha'' (Mem. de I'acad. d. scienc. de I'instilut de France, 1835). These
important statements of Mirbel's remained unnoticed till I induced rfcftlr in 1S70 to test them, and
where possible to study the phenomena more closely. The latter published his results in the ' Arb.
lies hot. Inst, in Wzhg! 1!. I, p. 77.

520 LECTURE XXXI.

itself must be completely reversed, i. e. the side marked o would become the
lower side. It is to be mentioned moreover that the Marchantia shoot produces
on the under or shaded side two series of membranous leaves near its middle
line, and very numerous root-hairs. That it is possible to shape this shoot at
will during continuous illumination from one side so that its organic light-side
is turned downwards or upwards, forwards or backwards, is greatly owing to

FIG. 341.— Vertical section tlirouyh tlie lateral port
of upper, u of lower side ; / colourless parenchyma of I
stoma ; i- s boundary between two areolse.

the circumstance that neither the spores nor the gemmae from which the shoots
arise are differentiated dorsi-ventrally. Thus with the beginning of the growth of
the new shoot the influence of the light upon it can come into effect at once,
without being modified by previous influences. In this connection the gemmae
of IMarchantia have already been investigated. They arise from papillae in small
cup-shaped receptacles on the light side of older shoots, and constitute later
approximately lenticular bodies, the two convex sides of which are organised quite
alike and are sensitive to external influences in exactly similar degree. As shown
on the transverse section (Fig. 342) through such a gemma, certain cells a exist

FIG. 342.— Vertical secti<

on of a ripe gemma of Mai-chantia, car:

ried through the

s which develop into rooi

t -hairs ; b cell filled with peculiar conten

ts; «i-wmjiofrec«

e situated (X 165).

> growing

■points, a hyaline
:he growing-points

on both the convex sides, which are destined, according to circumstances in each
case, to grow out into root-hairs ; if both the surfaces of the gemma are equally
illuminated those grow out which are able to follow directly the influence of
gravitation, i.e. those of the lower side, whereas those of the upper side are
prevented from developing. The influence of the light however is stronger than
that of gravitation, since, as Zimmermann has shown \ if the lower surface is
illuminated it is chiefly the roots of the shaded upper side which develope.

' On this point cf. Zimmermann, ' Über die Einwirkung des Lichtes auf den Marchantien-
thaliits' (Arb. des bot. Inst, in Wzbg. B. II, p. 665).

LIGHT AND GRAVITATION, AND DORSI-VENTRAI. STRUCTURE. 527

Moreover, a branch - system consisting of simple cell-filaments can also
behave towards the light like the flat shoot of Marchantia, which consists of
continuous tissue. If the spores of one of our commonest Mosses, Fiinaria
hygro77ieirica, are sown on a brick-shaped piece of damp turf which is artificially
saturated with nutritive substances, and care is taken that the light for weeks and
months always falls only on the same side, there is developed from the spores
the protonema, Fig. 343, already described above (p. 30) ; the filaments of the
primary shoot of this creep on the turf like rhizomes, putting forth roots below and
producing on their upper side erect branched filaments containing chlorophyll, from
which again secondary and tertiary lateral shoots arise. It is shown now that
these upright branch-systems become extended in a plane at right angles to the
incident light, and I have carefully convinced myself of the fact that this is due
not simply to heliotropic curvatures and torsions, but that the bi-serially arranged
offshoots come forth from their parent axes strictly right and left. In other

Fig. 343.— Protonema of Funm-ia hygrometrica. li a creeping- main shoot, from which arise the lateral shoots,
containing chlorophyll and branched in two series. K a bud which will develope into a Moss plant ; tu its first root.

words, the light in this case causes the growing points of the lateral shoots to
arise only on the flanks of the mother-shoots, if these are always illuminated from
one side^

Supposing now that the lateral shoots which arise from the filament b in
the figure, stand so close together' as to be everywhere in contact, we should
then obtain a leaf-like surface of tissue developed at right angles to the incident
light. So it is however as a matter of fact with the flat shoots of Marchantia,
and with those of the Opuntia ; they become narrow and filamentous in the dark,
but in intense light falling from one side develope their normal flat form, in such
a manner that the surfaces stand at right angles to the incident rays.

So far it is gravitation and light which have produced the described efi'ecis

' The relation here described between the protonema of Fiiuaria hygroDutrica and light was
first cited in my treatise 'Über orthotropc und plagioi rope Pßanzcutlicilc' (Arb. des bot. Inst, in
Wzbg. B. II, p. 256). I have completely set aside the doubt I there expressed, by means of later
investigations, and thus established the result expressed in the text.

528 LECTURE XXXI.

on growth, and at the same time we have had to consider the relations of direction
and position under which the embryonic rudiments of new organs arise, and of
the external and internal symmetry of their organisation. These relations are
possible simply because gravitation and light, according to their nature, act on
the plastic substance of the plant in definite directions; and we shall have
opportunities subsequently, when considering geotropism and heliotropism, of
becoming acquainted with other effects of these two natural forces, where it is also
a matter of effects appearing in definite directions, but of effects on organisations
already more or less fully developed, whereas we are here concerned exclusively with
the effects of external forces on the developing organisation itself.

Having thus become acquainted with the influence of gravitation and light on
the embryonic rudiments of the organs, by means of a few examples, I now proceed
to show how the same forces co-operate during the further growth of organs already
established, or, as we may say generally, how they affect post-embryonic growth.

2. Post-embryonic relations of configuration. In this case also, as will be shown
further on, the influence on growing organs in a definite direction may still come into
consideration ; but, in the majority of the known cases, the effects of gravitation and
light here present themselves in far more complicated forms, indeed so that the whole
physiological quality of an organ may be determined by them.

In this sense the behaviour of the subterranean shoots of Draccena and Yucca
will at once illustrate what is meant \ These well-known large Liliacece develope from
the base of their upright stems root-like shoots of a thickness varying between that
of the thumb and that of the arm, which grow down vertically into the earth, and in
their turn produce numerous long roots which feed the whole of the large plant,
but remain thin and filiform. These subterranean shoots produce, it is true,
numerous leaves at their broad growing-points, but these leaves remain rudimentary
and form annular thin membranes. So long as the plant remains undisturbed,
proper foliage leaves never arise from the growing-points of these subterranean
shoots. Their development can be effected, however, simply by completely inverting
such a plant placed on a support, as in Fig. 344 h g, and directing the root-system/
contained in the flower-pot upwards. The growth of the now down-turned bud is
injured if it has no opportunity of curving upwards ; the development of the growing-
points on the thick rhizome shoot a d, however, is promoted. The leaves produced
here develope henceforward no longer as annular membranes, but in the form of
foliage-leaves, finally coming above the soil, and thus leaf-shoots arise directly from
the up-turned rhizomes. It is evident that in the experiment nothing whatever
has been altered except the direction in which gravitation acts on the plant. The
rhizomes in the opaque soil are inaccessible to the light both before and after, the
relations of moisture in the surrounding soil are the same as before, and nevertheless
the post-embryonic development of the leaves arising at the growing-point are forth-
with altered : instead of so-called cataphyllary leaves or scales, vigorous foliage-
leaves are produced, which, however, of course only become green and flat when
they grow forth above the soil. Moreover, it is possible to make the experiment

' What is said in the text as to Yucca and Draccena (sub-genus Cordyline) is described more
in detail in my treatise ' Ubc>- Stoff und Form ' (Arb. des bot. Inst, in Wzbg. B. II, pp. 475, &c.).

INFLUENCE OF GRAVITATION ON POST-EMBRYONIC GROWTH. 529

Still more easily by taking an entire plant of Yucca or Draccena out of the earth and
submerging the apex in a glass vessel full of water, so that the thick rhizomes
are directed upwards. Even in this case, and under the influence of light, the changes
described take place, and after all that has been said, they cannot well be ascribed to

Fig. 344.— yucca g:loriosa. a a rhizcme ; * stem ; cleaves at apex ; rf subterranean leaf-shoots
from the basal part of the rhizome ; ä' leaf-shoots from tlie terminal buds of two shoots of the
rhizome; ir c soil of pot /y'; ^^, /; /i the wooden support for the inverted plant ; ia new root ;
/• and i new buds of the rhizome. The roots are omitted in the figure.

any other influence than that of gravitation, although it is at present quite impossible
to form any clear idea whatever as to the way in which this influence leads to the
final result described. We are here simply in the same position as with almost all
the efi"ects of light and gravitation, pressure, injury, and other external influences :
[ 3 ] ji m

53°

LECTURE XX XT.

we simply perceive the cause and the last effect, while the probably very long series
of intermediate causes and effects remain for the time being unknown. The
same is the case, indeed, with all the phenomena of irritability, among which even
the best-known, both in animal and vegetable organisms, always present ultimately
something entirely inexplicable and unintelligible according to ordinary ideas of
mechanics.

Here again our information as to the influence of light on the development of
organs, the rudiments of w'hich are already formed, is much more abundant, although
here also, as in the whole of this province, we have as yet gained only the first dim
glimpses into very involved processes of Nature. We may in the first place keep
apart two cases ; on the one hand we ask the question, what happens when organs
which normally complete their post-embryonic growth under the influence of light are
necessitated to complete their development in constant darkness ? We have in this
case to do with the so-called Etiolation of plants. In the second place, the case is
to be noticed where organs, especially leaf-forming shoots, which, as a rule, pass
through their whole course of development underground, and therefore in profound
darkness, are compelled to do this under the influence of light.

In spite of the very numerous researches on Etiolation, as it is called — i. e. the
alteration induced by want of light in organs which normally grow in strong light —
it must nevertheless unfortunately be confessed that we here enter upon a province
which until quite recently has been closely bestrewn with gross errors. However,
this is not the place to discuss that matter, and I must be satisfied with acquainting
the reader with those facts which can easily be observed ^

If the seeds of Phanerogams are sown in soil and allowed to develope their
seedlings in the dark, there appear after a short time abnormalities, in general of
such a kind that the shoot-axes are longer by far than is the case when the
illumination is normal, whereas the first leaves of the seedling usually remain much
smaller, and do not complete their extension in one plane, and at the same time
(except in the Coniferse ^) the formation of chlorophyll is suppressed : the leaves

* What is here said concerning etiolation is based on my two treatises, ' C/der den Einfltiss des
Tageslichtes auf Neubildung und Entfaltung verschiedener Pflanzettorgane^ Bot. Zeitg. 1863,
Supplement; and 1865, Wirkung des Lichtes auf die Blüthenbildmig unter Vermittlung der Laub -
blatter, pp. 117, &c. I have of late years carried on in Würzburg similar investigations with the aid
of better methods than were to hand at that time: the results of these have not yet been published
in detail. Gregor Kraus {' Über die Ursachen der Formänderungen etiolirter Pflanzen,' Jahrb. tür
Wiss. Bot. VII, p. 209) and Godlewski {'Zur Kcnniniss der Ursachen der Formänderungen etio-
lirter Pflanzen! Bot. Zeitg. 1879, pp. 81, &c.) have both dealt with the excessive lengthening
of the internodes and the dwarfing of the leaves of seedlings in the dark, but without taking
into consideration my much older statements on the etiolation of shoots nourished by foliage-
leaves exposed to light. Moreover, the text above is chiefly concerned not with the mechanical causes,
but only with the facts themselves, wherefore I need not here go further into the statements of these
observers. Kraus' assumption, in particular, that etiolated leaves remain small because leaves in
general are only able to grow from their own products of assimilation, is strikingly contradicted
by the experiments described and illustrated in the text. That the dwarfing of leaves in the
dark is a phenomenon of disease, and cannot be compared with the growth of leaves under
normal conditions, has been shown by Prantl's investigations in my laboratory, ' Ijber den Einfluss
des Lichtes auf das Wachsthum der Blätter'' (Arb. des bot. Inst, in Wzbg., B. I, p. 371).

2 That the seedlings of Conifers produce leaves which contain chlorophyll even in profound
darkness I first stated in the publication 'Lotos' (Prague, Jan. 1859); ^'^'^^ ^ showed later that

ETIOLATION. 53 1

developed in the dark become yellow, it is true, whereby of course a step towards
the formation of chlorophyll is accomplished, but the greening of this yellow imperfect
chlorophyll is only completed when such etiolated leaves are subsequently exposed to
light. Moreover, the size of a plant thus developed in the dark, as well as the number of
roots and leaves formed, are proportional chiefly to the volume and mass of the seed
or, better, to the quantity of the reserve-materials accumulated in it. From the minute
seed of the Tobacco plant there developes in the dark a correspondingly tiny seedling
with two cotyledons ; from the large seed of a Bean or of a Horse Chestnut, on the con-
trary, there may be produced in the dark a plant of considerable size with copiously
branched roots, and several, though small and yellow, leaves. At length, however, after
a few days, or in the case of very large seeds after a few weeks, the growth in the dark
always ceases, and the whole plant becomes diseased and perishes ; in many cases, as
WiihFhaseolus, &c., not until all the reserve-materials of the seed have been completely
used up for the formation of organs ; in other cases, however, as in the Gourd, growth
and the development of organs cease while the cotyledons still contain considerable
quantities of unused formative substance. The malformation of such etiolated
seedlings, as well as their final perishing, show that they are in a sickly condition,
and this at length affects the processes of growth proper, which alone interest us here.
Although the fact that the shoot-axes become elongated more rapidly in the dark
agrees with the observation that their growth under normal conditions of illumination
takes place more rapidly during the night, or during occasional short periods of
darkness, than under the influence of light — a point to which I shall return in
the next lecture ; yet the dwarfing of the etiolated leaves of seedlings might lead
to the conclusion that their growth is directly retarded or inhibited by deficient
light, and thus might be accelerated by illumination. That is a mistake, however,
since, as we shall see later, healthy green foUage-leaves which have previously been
exposed for some time to the action of light, grow more rapidly during the night or
during other periods of darkness which last for a few hours only. From this it
evidently follows that the dwarfing of etiolated leaves of seedlings must have other
causes, which \ve simply imply by terming the etiolated leaves (and the same is true
of the excessively long shoot-axes) diseased. Of course this appears to be con-
tradicted by the fact that young etiolated leaves when brought into the light grow
vigorously after a few days ; but this growth is not a direct result of the action of
light, but rather a consequence of the fact that, by means of the illumination, in the
first place a normal healthy condition is created in the tissue, and this in its turn
makes further growth possible.

That seedlings which have grown up in the dark soon fall victims to an actual
disease of their tissues, and that their abnormal relations of growth depend essentially
on morbid changes in their tissues, due to want of light, and that consequently points
come under consideration which have hitherto scarcely been noticed, and that, above
all, the supply of suitable constructive materials plays a great part, it is easy to
convince ourselves by a modification of the experiment.

Instead of allowing seedlings to grow up from seed in the dark, I directed

leaves developing in the dark from the older stem of various Ferns form chlorophyll, which, on the
contrary, is not the case with Equisdum.

M m 2

or-

LFCTURF XXXI.

in my experiments in 1863) the 'buds of plants provided with numerous leav'^s
into a dark box, in such a way that the shoots which proceeded thence must
develope inside it, while from numerous leaves, which were as large as possible
and exposed to the most intense light available, the products of assimilation outside

Fir,. 345.—.? B C A Gourd-plant ; A'ATalarge wooden box, in which the part B of the plant i
etiolated in the dark (cf. text on p. 355).

were transmitted to them; thus in proportion as the growth in the dark progressed,
substances available for growth were also formed by the assimilation of the illumi-
nated leaves, and carried to the places where they were used. Fig. 345 illustrates an
experiment of this kind, as I am in the habit of arranging it for the purposes of
demonstration. After all the axillary shoots have been cut away from an already

ETIOLATION.

5?>S

very strong and well-gtown Gourd-plant, furnished with about ten to twelve large
green leaves, the apical portion of the main stem is directed through a hole d, as small
as possible, into a large thick-walled wooden box K, and gently secured ; the box is
provided with a large well-fitting door. The figure shows how from the apical Itud
directed through rt', not only numerous leaves, tendrils, new shoots, and flowers (at B)
have been formell, but also a large fruit /; only these parts were considerably more
numerous than is shown in the figure. When the space became too small for the shoots
growing in the dark, I pushed the active terminal bud through a hole {e) in the roof
of the box out into the open air, and then made the necessary closure. The figure
shows how this shoot-bud which, originally directed into the box at d, had then
developed further in this for several weeks {B), on now being again exposed
to the light, has become quite normally developed again above the roof of the box at
C. The sojourn of this shoot-bud in the dark box was, so to speak, only an episode
in its life, during which it by no means- ceased to produce leaves, leaf-shoots,- tendrils,
roots and flowers in the usual manner. The foliage-leaves, which were of a
clear bright yellow colour, attained a considerable size, as seen. One of the
largest had a lamina 625 sq. cm. in area, while an average-sized leaf of the same
plant previously produced in the light, and dark green in colour, measured 825 sq. cm.
The leaves in the box attained on the average |-f the normal area, although they
w-ere only illuminated once every three or four days for a few minutes for the
purpose of observation, and produced no trace of chlorophyll. Still less was the
effect on the growth of the shoot-axes : the magnificent large tendrils were as
sensitive as it is possible for tendrils to be, the male and female flowers completely
normal in form, size, and colour, and, finally, the fruit/" when ripe weighed 4 kgr.,
and possessed 195 seeds, a third of which were capable of germination ; and, in
a word, apart from the chemical process of greening and a few insignificant
disturbances in the growth of the foliage-leaves, as may be seen in the figure,
the growth in the dark, both in its embryonic and in its later stages, was essentially
normal, and that of the reproductive organs in particular quite normal. On now
noticing further that Gourd-seedlings in the dark exhibit the disease of etiolation
in the highest degree, and that their first leaves especially remain very small, the
above experiment teaches us that in this case it must be a matter not so much of a direct
influence of light on growth, as rather of the production of substances which bring
about growth subsequently. This follows also from the fact that the growth of the
organs enclosed in the dark cavity is so much the more vigorous and norm.al the
larger the assimilating leaf-area exposed to the light is, and the more intense the
light itself is outside the box.

So long as nineteen years ago I obtained results similar to those with the
Gourd, with Phaseolus, Tropceohan (Indian Cress), and Ipomcia purpurea (Red
Bindweed). Of especial interest in all these cases is the unusually vigorous and
normal production of flowers and fruit, so long as the assimilation of the leaves
exposed to the light proceeds energetically.

Since the products of assimilation of the leaves up to a certain extent
accumulate in the latter and in the shoot-axes, before they are made use of for
further growth, it is often possible to obtain vigorous shoots and flowers even in
the dark by simply placing entire strong plants (e.g. Tobacco, Wall-flower, kc>\.

534

LECTURE XXXI.

which are already beginning to form flowers, in the dark : the production, however,
is always more restricted, and the development of normal flowers particularly is in-
significant, or conspicuous abnormalities in the growth of the flowers appear, simply
because the flower-forming substance has not been previously accumulated in suf-
ficient quantity.

These experiments as to the dependence of the formation of flowers on the
preceding or simultaneous activity of the foliage-leaves in the light, now explain
to us also one of the most universal relations of growth in plants. The flowers
appear either singly or several together in the axils of foliage-leaves, evidently because
the formative substance produced in the latter can thus be most easily transferred to
them. In very many other Phanerogams there are formed single or many flowers
(Inflorescences) on long shafts, so that the completely developed flowers are far distant
from the assimilating leaves : this is the case with almost all Grasses, and many Lilia-
cese {Allium, Aloe, Draccrna, &c.). But it must not be overlooked that in such cases
the flower- buds are already fully developed in all essential points while the eventually
very long floral support or shaft is still extremely short, and the entire inflorescence is
thus situated either in the axil of a foliage-leaf or in the midst of a rosette of
numerous leaves, so that in such cases also the formative substances are conveyed
by the shortest route to the young flowers. When then the stalks of the flowers
or inflorescences become elongated, and the flowers removed further away from the
leaves which supply them with nutriment, it is a matter simply of their unfolding, and
this depends essentially upon the necessary supply of water. We may probably
go a step further, and assert that the proper reproductive organs in the narrowest
sense of the word (the spores, male fertilising cells, and oospheres) take up their
formative substance from the assimilating organs, and since only shoots can be
assimilating organs at all, an explanation is oflfered of the fact that all true organs
of reproduction are produced exclusively by shoots, and are usually formed on leaves
or in the axils of leaves, or at any rate in some close connection with them. It
may be objected that in the case of plants which contain no chlorophyll, the repro-
ductive organs are also connected with the shoots. We can however reply that such
plants are phylogenetically derived from plants which contain chlorophyll, and that
it is also advantageous for the reproductive organs in general if they arise on the
shoots protruding above the substratum.

These considerations may at first sight appear scarcely to harmonise with the
fact that bulbs of Hyacinths, Tulips, &c.,when germinating in profound darkness,never-
theless produce magnificent normal flowers, while the leaves at the same time become
etiolated, though not strongly. But this favourable result of growth in the dark
depends essentially on the fact that in the preceding period of vegetation the foliage-
leaves of the bulbous plant were able to assimilate in intense light, and that flower-
forming substance was thus already collected in the interior of the bulb, and then,
during the apparently dormant period in summer and winter, nourished the flower-
buds deep in the interior of the bulb, so that when such a plant is eventually allowed
to put forth its shoots in the dark, it is only a matter of the last stages of develop-
ment, which here also are accomplished chiefly by the addition of water. This
opportunity may be taken of mentioning the fact that many plants provided with
bulbs, tubers, or large rhizomes, and especially phanerogamous root-parasites and

FLOWER-FORMING RESERVE MATERIALS. 535

humus-plants, pass through all their essential processes of development, and
especially all their processes of embryonic growth, in profound subterranean dark-
ness, so that the foliage-leaves only come forth annually and for a short time into the
hght,in order to assimilate and to fill the subterranean parts with nutritive substances j
and the flowers come forth beyond the surface of the earth not because they require
light for their growth, but in order to ensure pollination by the aid of insects.

I have so far, for the sake of simplicity, in considering the subject of etiolation,
always regarded only the contrast between full daylight and profound darkness ; it
must be added, however, that etiolation also occurs in feeble light, and is the more
marked in proportion to the diminution in intensity of the light. Here, however, it
is a matter of one important point chiefly, that is whether the intensity of the light is
at all sufficient to effect assimilation in the chlorophyll ; if this is not the case, as for
example often happens in the interior of a dwelling-room, then though green leaves
may be formed, it is true, so long as the store of nutriment lasts, still, since no
new assimilation takes place, this store is gradually exhausted, and the plants grow
themselves to death — a very common case when they are left on flower -stands
in rooms.

In contrast to the unusually energetic growth which takes place even in profound
darkness when there is a considerable store of assimilated substance accumulated,
either in reservoirs of reserve-materials (seeds, bulbs, tubers), or when it is produced
at the time by assimilating green leaves, are now to be mentioned the spores and
gemmae of the Cryptogams, which so far as is known do not germinate at all in the
dark, although in most cases a feeble light is sufficient to induce their growth.

I must content myself with these few remarks with reference to the questions
connected with etiolation, although much more might be added.

I will now quote a few more cases belonging to the above-named second
category, concerning the fact that organs which normally develope in the dark may
be abnormally compelled to grow in daylight. Here again the common Potato, as
in so many other cases, forms an unusually favourable object for physiological
observations. It may be premised in the first place that potato-tubers lying in a dark
damp place develope, as is well known, 4iumerous etiolated shoots, which are often
very long, and which are furnished with extremely small leaves : in this, after what
has already been said, there is simply nothing peculiar. However, if potato-tubers
are laid on damp sand, for instance, and covered with a transparent bell-jar in the
spring and summer and kept at a bright window, there grow out it is true numerous
thin roots from the so-called ' eyes ' (buds) but the shoots themselves remain for 2-3
months of continued cultivation extremely short, and none of their leaves are
developed — in other words, they are extremely sensitive to the action of light, which
retards their growth to a striking extent. So far as I understand this phenomenon
(which still needs careful investigation), it chiefly depends simply upon the fact that
the potato-tuber in the normal course of life is covered with several centimetres of
soil, and must therefore also be darkened when the sprouting shoots develope from
the eyes. It is chiefly the growth of the 2-3 lowermost internodes of the shoot
which are concerned here, and it is from these that the roots and stolons arise.
These lowermost segments of the shoot are adapted to darkness and can scarcely
grow in the light ; whereas the later segments and rudiments of leaves, on the

536 LECTURE XXXI.

contrary, require the light in a high degree in order to grow normally. This fact is
so much the more remarkable since very many roots and subterranean shoots
exhibit no signs of such sensitiveness ^

While the lower segments of the potato-shoot afford us an example of the fact
that under some conditions the growth of parts of the plant is almost entirely
prevented by the light, an example of the contrary is afforded, according to Goebel's
observations, by the scale-leaves of some rhizomes, which normally grow in subter-
ranean darkness. These if exposed artificially to the light have their growth and
all the processes of configuration promoted, and become transformed into true
foliage-leaves. Goebel ^ cultivated plants of species of Circcea so that the otherwise
subterranean stolons were forced to develope further in the light : their otherwise
scale -like leaves now became transformed into normal green foliage leaves.

The fact established by Stahl that the assimilating parenchyma of the foliage-
leaves undergoes conspicuous changes in the form of its cells according to the
intensity of the light, definite relations between the directions of growth and the
direction of the ray of light being apparent at the same time, also belongs to the effects
of light on post-embryonic growth. As is shown in Fig. 202 (p. 247) the assimilat-
ing tissue of the usually thin flat leaves forms two layers; an upper one turned
towards the light consists of palisade-like upright cells elongated in the direction of
the incident rays of light ; the lower half of the assimilatory parenchyma is composed
of cells which are rounded or often transversely elongated in a plane parallel to the sur-
face of the leaf, and between which there are usually large intercellular spaces. These
layers are distinguished as the palisade layer and the spongy layer of the mesophyll.
In leaves with vertical surfaces, äs the Iris, and in stems like those of Equisetum,
the palisade cells on the other hand lie horizontally, in the former on both sides of the
leaf, in the latter radially directed on the whole towards the periphery. According to
Stahl the palisade cells, however, constitute that form of assimilating tissue which
is especially produced by intense light striking the surface of the leaf directly,
since he shows that in the same species of plant the formation of palisade tissue
diminishes in proportion as the intensity of light decreases. Leaves grown in
the shade possess chiefly or only spongy parenchyma ; those grown in strong light
chiefly palisade tissue. As an example the Beech may be mentioned, as well as
other trees. We have here in view, in the first place, only the fact that light is the
cause of these relations of organisation ; but it must be added that the histological
differences thus produced are in their tiirn again of use for the process of assimilation,
a matter however into which we cannot here proceed further.

Our considerations so far have been concerned with the action of Gravitation and
of Light on growth. The former is always acting continuously on each of the minutest
parts of the plant : light, however, only acts on those parts which are above ground.

^ The action of light on young Potato-shoots described above is the more remarkable when the
nights are taken into consideration : it might be supposed that although the growth is completely
arrested in the day-time, it could nevertheless proceed in the night, and perhaps the extremely slight
growth actually takes place only during the nights. But the growth is so slight that even on
this assumption it seems probable that an after-effect of daylight persists even during the night.

^ Goebel on Ciiraa, in his 'Beiträge zur Morphologie und Physiologie des Blattes'' (Bot. Zeitg.
j88o).

ACTION OF LIGHT ON POST-EMBRYONIC GROWTH. 537

and it undergoes periodic alterations each day. Evidently, however, in both cases the
effects on growth, so far as they occur at all, must be cumulative ; though perhaps in
the course of a few hours or of a day unnoticeable, these actions after months and
years may produce very considerable effects, which however up to the present
are but little known. Since however we already know a large series of such
effects, though notice has only quite recently been directed towards them, it may
be expected that many more will be discovered; and if it is remembered that
while the vegetable world from its origin onwards has been gradually evolving from
its lowest forms the highly organised ones — that during this time all processes of
growth have been continually affected by gravitation, and at least periodically and
partially acted on by light, it may be presumed that almost all relations of organisation
must gradually have been modified to a very large extent by gravitation and light.
In other words, the forms and modes of life presented by plants, and which the
botanist studies, must have been to a large extent induced by the continued action of
gravitation and light. We can now artificially induce or prevent some of these
actions, but others are perfectly hereditary and have become constant, Evidently
there here lies before us one of the most fruitful provinces for botanical investigation.

Finally, it remains to mention still a few other cases of the action of external
forces on the growth connected with configuration ; but this must be done quite
shortly, in order not to spin out this lecture too long. I have already referred to
cases where by the growth of parasitic Fungi the tissue of the host-plant is not
destroyed, but is even enormously promoted in its growth. One of the finest
examples is presented by the formation of the so-called " Witchs' brooms " on Firs ;
— on the horizontal branches permeated by J^cidiiim elatinum are produced abnormally
orthotropic shoots, growing like small primary stems. Another category of growths
abnormal for the plant itself occur in the numerous gall-formations, so common
everywhere ; — insects, especially Gall-flies, lay their minute eggs in the tissues of young
growing parts of plants, and while the small larva is developing from the ^^% the
vegetable tissue surrounding it becomes hypertrophied, and thus forms the Gall.
The common " gall-apple " of the Oak, and the galls of the Rose-bush furnished
with their moss-like appendages, and others are know'n to all. Two points
in the formation of galls are especially worthy of note. First, every species of gall
possesses a definite anatomical structure and external form, as if the gall were an
organism stii generis; and secondly, this complex organisation is induced simply and
only by the development of a specifically distinct insect larva, so in fact that entirely
different galls are produced on the same plant by different insects. It may here be
mentioned by the way that certain aphides produce gall -like structures, among
which the large hollow Chinese galls, and the galls resembling pine-cones produced
by Chermes viridis on our Red Firs, are especially noteworthy. It has lately been
established by Peyritsch, moreover, that some monstrous floral developments, so-
called green flowers {Chloranthy), especially of species of Arahis, can be artificially
induced by placing certain species of Aphis on the still young inflorescences.

In these cases it is abnormal external stimuli which induce abnormal growth. It is
of very common occurrence, however, not only in the vegetable but also in the animal
kingdom, that new growth results in a perfectly normal manner from fertilisation.
Here again the important matter is that a material influence acts on the female organ

^38 LECTURE XXXI.

from without, by means of which the oosphere is transformed into the embryo, and the
latter into a new organism. It will be the object of my last series of lectures, how-
ever, to go into this matter in detail : only the one point need now be noticed, that
not only in the Phanerogams but frequently in the Cryptogams also, the result of the
fertilisation of the oosphere is not merely its development into the embryo, but also
that other parts of the mother plant are thereby incited to further growth. In the
Phanerogams the formation of endosperm in the seed, the swelling of the ovary and
its development into the ripe fruit, are due to the action of the pollen : the infinite-
simal quantities of fertilising substance which are transferred by means of the pollen-
grains, e.g. of a Gourd-plant, to the female flower, are the cause of the development
from the ovary, by growth, of a fruit which is occasionally a hundred pounds in
weight. A still more remarkable case however in this connection, as Hildebrand^
has shown, is presented by the Orchideae : at the time when the pollen reaches
the stigma, the ovules in the ovary, and which are to be fertilised, are either still
incomplete or have even not yet commenced to be developed at all, and if no pollen
is transferred to the stigma these essentially female organs of reproduction de-
velope no further. The growth of the pollen-tubes into the stigma acts as a
stimulus on the ovary, and the latter now begins to grow vigorously, and in it the
ovules only now develope, or even begin to be developed.

In the case of the action of gravitation and of light we had to acknowledge that
the connection between cause and effect is entirely unknown to us, and in the case of
all the phenomena just mentioned we are no less compelled to say that all explanation
as to how the effects perceived result from the causes perceived is wanting.

^ Hildebrand, ' Die Fruchtbildung de7- Orchideen ein Beweis für die doppelte Wirkung des
Pollens' (Bot. Zeitg. 1863, pp. 329, &c.).

LECTURE XXXII.

THE COURSE OF GROWTH DURING ELONGATION.
PERIODIC VARIATIONS.

We have hitherto treated of growth only in so far as it is concerned with
the production of configuration — of the external and internal form of the organs, and
through this of the whole plant. In the present lecture we are to be concerned
with growth from an entirely different aspect. We place before ourselves the question,
in what manner does the growth of any small portion of a plant run its course when
it passes over from the embryonic condition into that of elongation, in order subse-
quently to reach the stage where its external growth ceases ?

Here again, in order at once to keep in view a concrete case, and to avoid the
tedious enumeration of the manifold combinations in the distribution of growth in
different plants, we may first confine our remarks to the typical shoots and roots which
have a terminal growing-point ; and, in order to limit the problem still more, we may
first concern ourselves exclusively with the increase in length of such organs —
i. e. with growth parallel to the axis of growth ^

As the most important result, it is first to be insisted upon that every part of a
plant, and in fact every transverse disc of it, however short, elongates slowly at first,
then, with increasing rapidity, attains a maximum velocity of growth, and then again
grows slowly and still more slowly, until the growth finally ceases altogether. In
order to illustrate this in as simple a case as possible, we may take the young long
primary root, about 2-4 cm. long, of a seedling of Vicia Faba (the common Broad
Bean). Two transverse lines of Indian ink are drawn immediately above the
growing-point, by means of a small sharp camel-hair pencil, so that they are exactly
I mm. apart ; this then denotes the length of a transverse disc of the root. The
seedling being now left in a place where it can at the same time absorb water and
respire energetically, and therefore where .»growth takes its normal course at the
expense of the assimilated reserve-materials, it is only necessary to measure the
length of the transverse disc (originally i mm.) at certain intervals of time, in order to

* The modern fundamental work on growth in length, is my treatise, * Über den Eiiißuss der
Lufttemperatur und des Tageslichts auf die stündlichen und täglichen Änderungen des Längen-
■wachsthums der Internodien" in Arb. des bot. Inst, in Wzbg., B. I, pp. 99, &c., where also all the
then existing literature is thoroughly and critically incorporated. On the growth in length of roots,
detailed statements and quotations are found in my treatise ' Über das IVachsthum der Haupt- und
Nebemuurzeln,' op. cit. p. 385 (1873 and 1874). The periodic variations of growth first brought
into notice in my first-named treatise were subsequently investigated in detail by Oscar Drude
(Leopoldina, B. XXXXVIII, No. 3, Halle, 1881) in the leives of Victoria regia: he obtained no
better information as to the cause of them, however.

540 LECTURE XXXII.

ascertain the elongation or growth which has taken place in the same intervals,
subtracting each time the preceding length from the one subsequently measured, and
denoting the remainder as growth. It is necessary to take care, however, that the
temperature remains constant during the periods of observation, or, since this is very
difficult to attain, that it varies within equal Hmits, and it is also well to exclude
the light from the object under observation, because this also influences the rapidity
of growth; since the point is to observe .the process of growth under constant
external conditions. Thus, I found in the case of a transverse disc, originally i mm.
long, above the growing-point of the primary root of a seedling of Vict'a Faba grow-
ing in moist air, with a daily recurring variation of temperature of i8°-2i'5°C., the
following alterations in a period of 24 hours : —

Growth,

ist day 1-8 mm.

2nd „ 3-7 „

3rd „ iV-5 ..

4th „ i6-5 „

5th „ I7-0 »

6th „ 14-5 • „

7th , 7-0 „

8th ., o-o ,,

Total . . 78-0 mm.

A similar transverse zone of the internode of a shoot-axis, marked off by trans-
verse lines on the youngest part that can be manipulated, behaves also much in
the same way as that on a root. Thus, for example, a transverse zone, 3-5 mm
long, of the first internode of a seedling of Phaseolus mullißorus, was marked bylines,
and the following elongations or growths observed in each period of 24 hours — the daily
variation of temperature being from 10-2°— 11° R.

Growth.

ist day 1-2 mm.

2nd ,, 1-5 „

3rd , 2-5 „

4th , 5-5 .,

5th , 7-0 ,,

6th 9-0 ,,

7th 140 .,

8th lo-o „

9th ,, 7-0 „

lOth , 2-0 ,,

Total . . 59-7 mm.

Numerous other measurements leave no doubt that the growth in length of
transverse discs of leaves and other organs also runs its course in the same way, and
from observations of another kind we may conclude that in organs which consist
only of simple vesicles not divided up by cell-walls, the behaviour is the same.

Every small transverse zone of a growing organ thus exhibits a periodic

TIJF GRAND PERIOD OF GROIVTH. 54I

variation of its growth, which is independent of temperature, Hght, and other forces
which influence growth. Since, however, with the periodical variation of these latter,
still other periodic fluctuations in the growth in length of such a part may make
their appearance in addition, I have, in order to distinguish it more exactly,
designated the phenomenon characterised above as the grand period of groivth.

If we now take such a transverse zone as the above, limited by two lines
as before (but further distant from the growing-point), and then measure its lengths
after certain intervals of time, it is obvious beforehand that this transverse zone,
since it is older and further distant from the growing-point, must be from the
start already in a further advanced stage of growth. The case may occur, for
instance (if we have hit upon exactly the right distance from the growing-point),
that the first measured elongation forthwith coincides with the stage of maximum
growth, so that every further measurement must present decreasing elongations ; or
the zone marked by the transverse lines, if still further from the growing-point
may be found already from the beginning in a condition of decreasing growth, so that
the first measurement yields a small and each succeeding measurement an ever smaller
amount of growth. And if we have taken the marked zone still further removed
from the growing-point, it may happen that it has already ceased to elongate — that
therefore every succeeding measurement gives the same length.

Thus if we had drawn a large number of transverse lines on a root, beginning
from the growing-point, or on a shoot-axis, beginning beneath the bud, in such a way
that the lines are at first equally distant from one another — say i mm. or so — then
on measuring the lengths of these portions after equal intervals of time, we should find
that the portions nearer the growing-point at first show an increasing rapidity of
growth, and that some one of these portions reveals a maximum rapidity, whereas
the transverse zones further distant from the growing-point exhibit in the same period
growths or elongations which are so much the less the more distant they lie from
the growing-point in each case.

In order to illustrate this also by a definite example, the following observation
may be quoted. I had the primary root of a seedling of Vicia Faha subdivided into
transverse discs, each i mm. long, by means of lines of Indian ink, beginning from the
growing-point. After 24 hours (at 20-5° C), the root having grown in moist air, the
marked zones showed the following increments :■ —

Number of transverse disc. Increase.

(Upper) X o-i mm.

IX 0-2 „

VlII 0-3 .,

VII 0-5 „

VI 1-3 M

V 1-6 „

IV 3-5 ..

Ill 8-2 „

II 5-8 „

(Root-apex) I i-5 -.

Total growth in length 23-0 mm.

542

LECTURE XXXII.

Here, then, that transverse disc in which the maximum growth took place was
situated above the second zone from the apex of the root; or, in other words, it was

the third of the marked zones in which
an increment of 8-2 mm. resulted in the
24 hours — the original length being
I mm. Nearer to the apex, as well as
further from it, the amounts of growth
were smaller. We may thus say, shortly,
that at a distance of 2-3 mm. from the
growing-point of this root the growth
had attained its maximum rapidity,
whereas it was very slow immediately
behind the growing-point, and much
further distant from it (10 mm. in the
present example) only amounted to
o-i mm. In transverse zones still
further distant, no further growth at
all was to be detected.

The same is the case also with shoot-
axes. Since in these, however, the
growth is distributed over longer
stretches, it is well to take longer
transverse zones at once from the com-
mencement. Thus, beginning from
the upper end of the first internode
of a seedling of Phaseolus midtiflorus,
a number of equidistant transverse
lines were drawn, marking off pieces or zones each 3-5 mm. long. After 40 hours
the lengths of these portions were measured, and the following growths found to have
occurred : —

Number of zone. Increase.

(Beneath the bud) I 2-0 mm.

Fig. 346.— a seedling of Vicia Faba (Broad Bean),
beginnings of the observation. The plant is fastened
by means of a needle «, its root being in water : th(
region is divided by means of painted lines into transverse zones
(0 — 10). B the same plant after 22 hours, at 21° c. ; the lines, re-
moved some distance apart, show the distribution of the growth.

o a cork
growing

II
III

IV

V

VI

VII

VIII

IX

X

XI

(Lower part of plant) XII

2-5
4-5
6-5

5-5
30
1-8
i-o
i-o
0-5
0-5
0-5

Total growth in length 29-3 mm.
After what has been said above, it is now obvious that the transverse segment
No. IV, if we had observed it 40 hours previously, would have shown an increment of

LENGTH OF THE GROWrNG REGION. 543

only 4-5 mm,, and if we had observed it 40 hours subsequently, an increment of only
5-5 mm. The segment IV simply happened to be in its stage of maximum growth
during the period we observed it ; or, in other words, the segment No. Ill would have
behaved during the next 40 hours exactly like IV, whereas the segment V had been,
during the preceding 40 hours, in the same stage that IV was during our obser-
vation. That is to say, in other words again, the whole growing region of a root, or
of an internode, or of any organ whatever arising from a growing-point, consists of
transverse zones, which, according to the age in each case (i. e. according to the
distance from the growing-point), are in different stages of development : each of
these transverse zones begins to grow slowly, then grows more and more rapidly and
attains a maximum growth, and then decreases and finally ceases to grow altogether.
Although it has been impossible hitherto to draw transverse lines in a similar
manner on the non-cellular utricles of such plants as Vauchena, Mucor and similar
Coeloblastae, we have nevertheless every reason to believe that even in them the state
of affairs is exactly similar.

This method of observation affords us at the same time an opportunity
of learning how long is the piece of a root, shoot-axis or other organ which is
actually growing. Having drawn equidistant transverse lines, starting from the
growing-point of a root, or from the bud of a shoot, and finding at a certain
distance from the growing-point, after some time — say one or two days — that
certain transverse lines have not separated further from one another at all, but
have retained their original distance apart, this signifies that at these places no
more growth in length whatever is taking place, and the distance between the first line
which has not moved and the growing-point gives us the length of the growing
region of the particular organ. Even a glance at the small tables above at once
shows that this length is strikingly different in roots and in internodes; in the
primary root of the seedling of Vicia Faha growth has already ceased at a
distance of lo-ii mm. from the growing-point; in the first internode of the seedling
shoot of Phaseolus, on the contrary, it only ceases at a distance of 12 x 3"5 mm.
(i. e, about 4 cm.) behind the growing-point. The differences, however, may
be much greater still, according to circumstances in each case : in thin lateral
roots the growing portion behind the apex may be 2-4 mm. in length, while it
may attain a length of even 50 cm. in the long flowering scapes of some Phane-
rogams ^. I found, for example, as follows : —

In — Len£^th of growing region beneath the bud.

FritiUaria iviper talis . . . . 7-9 cm. >

Allium Porrum .... about 40 „ / within one internode

Allium Cepa 3° „ ( (of the scape).

A I litem atroptirptircum . . . 50 ,, ^

Cephalaria procera 35 ,, (3 internodes).

Polygonum Sieholdi .... 15 ,, (4-5 internodes).

Asparagus aspcr 20 „ (numerous internodes).

Valeriana Phu 25 „ (4 internodes).

Dipsacus Fullontim 40 ,, (3-4 internodes).

' With respect to the distribution of growth and the lengths of the growing portions in erect
flowering shoots, of. my treatise ' Über Wachstimm und Geotropismtts aufrechter Stengel,' in Flot.i,
1873, p. 322.

544 LECTURE XXXII.

It is seen from this table that the growing region of a shoot-axis may embrace
one or several internodes. In the latter case, especially when the individual
internodes are sharply marked off from one another by large leaves which clasp
the stem, there may be present within any one of them again a transverse
zone situated at the base, or even beneath the node, which behaves just like
a growing-point, and in relation to which the increments of growth increase or
decrease in a similar manner.

It will also not be superfluous to point out that measurements of this kind,
especially on shoot-axes, only give a clear idea when the latter have already attained
a certain length : for at first, when the entire shoot-axis is still short and young,
all the parts of it are growing in length ; only when a certain length (i. e. a certain
age) of the oldest portions is attained, does growth cease at certain places, and it is
then for the first time that we have, at a definite distance from the growing-point,
a region which is no longer growing, as was assumed in our last table. In addition,
however, the case may also occur that the growth as a whole first disappears beneath
the bud of a long flower-scape, while at its base it continues for some time longer,
proceeding from an intercalary zone of embryonic tissue. In this case then the
sequence of the partial growths is reversed, the shoot-axis being pushed up as it were
from its base. The same may take place in very diff'erent ways, even in closely
allied plants ; for instance, I found the usual form of distribution of growth in the
flower scapes of Allium atropurpureum, whereas in those of A. Porrum and A. Cepa
in the later stages of growth there is a basal vegetative zone with the properties
indicated.

What has here been said of the roots and shoot-axes holds good generally
{77iuiatis 7nutandis) of all organs, so long as they possess a growing-point at their apex
or base, since the matter depends simply upon the passing over of the smallest
transverse zones from the embryonic into the definitive condition, and it is to be
remarked especially with respect to leaves, that the processes in question take place
not only in the longitudinal direction, but also in the direction of the lateral veins,
much as in the case of the lateral shoots of a main shoot.

The fact that the maximum elongation in roots, as well as in shoot-axes, occurs
only at a certain and often considerable distance from the growing-point, together
with other observations, warrants the assertion that in the growing-point itself only
an extremely slow growth in length takes place, and that, consequently, no growth
in length occurs at all at the apical protuberance of the growing-point : it results on
careful consideration also that no growth in length can take place at the apical
protuberance, but, on the contrary, always occurs first beneath the apex, and reaches
its maximum at a greater distance from it. This remark is not superfluous,
since for a long time past, with incomprehensible thoughtlessness, the growing-
point, and particularly its apex, was held to be the place of most energetic growth
in length.

If it is observed now how the apex of a shoot, as it grows upwards in the course
of a day, elevates itself several centimetres, or in very rapidly and actively growing
plants some decimetres, or moves forwards in space generally, this movement
must not be ascribed in any way to this so-called apical growth of the shoot :
on the contrary, the bud behaves in the main passively in the process, since it is

MEASUREMENT OF GROWTH IN LENGTH. 545

pushed forwards by the transverse zones which are beneath it on the shoot-axis. The
movement of the shoot-bud in space is the sum of the elongations which the transverse
sections of the shoot-axis beneath the bud are simultaneously undergoing. This is
more especially of interest in the case of the roots: in their case also the so-called apical
growth is a meaningless expression, for the growing-point covered with the root-cap
is driven forwards passively, by the elongation of the older transverse zones of the
root which lie behind it. In this case, however, the length of the growing portion
is remarkably short, only 2-10 mm.: this, however, appears really to be of advantage
when the object is to drive the root-apex forward in the soil. The root behaves in
this respect like a nail being driven into solid wood by the strokes of a hammer.
Where this regard for the mechanical conditions of the forward movement of the
apex of the root is not necessary, we find that the relations of growth are
different even in roots. In the long aerial roots of Aroids, and of a species of
Ct'ssus, I found ^ by measurements that the elongating region situated behind the
growing-point is of considerable length, amounting to several centimeters, as in
the case of shoot-axes ; this is evidently because it would here be superfluous to
concentrate the force which drives the growing-point forward a few millimeters behind
the latter, as is necessary in the case of roots growing in the resisting earth.

Instead of the transverse lines mentioned so often, it is also possible in some
cases to make use of certain natural marks for observing the distribution of growth,
provided of course that there is sufficient ground for the assumption that these natural
marks are equidistant at first, and also stop at equal distances finally. Thus,
Askenasy^ employed the leaf- whorls of the Characeae, or, what amounts to the same
thing, the lengths of the internodes, in order to gain information as to the alterations
in space and time of the growth, from the growing-point to the parts fully developed.
In the same way it is possible in the case of any other chosen stems with numerous
internodes (which, however, must finally attain equal lengths) to find the stages of growth
of the individual internode to a certain extent severally represented in the then existing
stages of growth of all the internodes which are still elongating. It is obvious that
such an experiment will not be possible if the plant is of such a nature that the
various internodes attain diff"erent lengths, or differ in any other way as to their
relations of growth.

In the case of organs the growing-point of which terminates in an apical cell
which forms either one series of segments by transverse walls, or two or three series of
segments by oblique divisions, the length of the surface lying betw^een each two
segment-walls may be similarly employed, like the space lying between two artificially
made transverse lines. Provided (and this is actually realised in most cases,) that
the successive segments have equal lengths originally, the process of growth may
be judged from the relative lengths of the segments lying one over the other. Under
certain circumstances, in the case of very simple organs which consist of one cell

* I described the abnormal behaviour of aerial roots respecting the length of the growing region
in Arb. des bot. Inst, in Wzbg., B. I, p. 593 (1874).

2 The ingenious treatise of E. Askenasy, depending on careful reflection, 'Über cine neue
Methode, um die Vei-thcihmg der Wachsthumsinfensität in zvachsenden Thcikn zu bestimmen,' is
in the Verhandlungen des naturhistor.-med. Vereins zu Heidelberg, N. S. II, 2 II. (Winter). Unfor-
tunately I have not space to go further into the contents of this treatise.

[3] Nn

546 LECTURE XXX TT.

only, it is likewise possible to employ certain equably distributed sculpture markings
on the cell-wall for the criticism of the ways and means by which the growth between
them advances. Nevertheless these natural aids have only been used now and again
so far.

Assuming that in the entire length of the growing region of a shoot-axis or root,
or even in any one portion of this length, at any time whatever, the growth takes place
a little more rapidly on the one side than on the opposite side, it is obvious that
a curvature must thereby be produced, and that this is the more pronounced
the greater the difference in the elongation of the two opposite sides. Such
phenomena now are as a matter of fact to be observed very commonly during the
process of growth in length : they are distinguished as Nutations, and are perceived
the more easily the longer the growing portion and the more energetic the elongation
itself is. Hence nutations are particularly evident in the case of rapidly growing
erect flower - stalks. The leafless flower -scapes of species of Allium — e.g. the
common kitchen onion — are found, so long as they are still growing in length, to be
always bent over towards one side, often to such an extent that they describe more
than a semicircle, and the thin flowering scapes oS. Allium rotmidum and other species
even make loops in this manner of more than a whole circle. Finally, however,
when the growth is ceasing they straighten themselves completely, and stand quite
erect. On observing such nutating flower-stalks from hour to .'hour, now, and
if possible in the dark in order to exclude the heliotropic curvatures due to light, it
is soon noticed that the curvature does not always remain the same, but alters in
such a manner that the side which was previously concave becomes convex after
several or many hours, and therefore, of course, the pendent apex bends over towards
the opposite side. In the intervals also, sometimes one, sometimes another side
of the stem becomes convex, so that the apex is gradually turned towards all
parts of the horizon; and in particularly exquisite cases the change proceeds so
that the apex is gradually carried round in a circle, or to put it better and more
accurately, it moves in the form of an ascending spiral line, because during this
rotating nutation a continuous elongation, and therefore an ascent of the apex in
space, takes place.

After what has been said above, the phenomenon finds its explanation in the
fact that first one and then another side of the organ elongates more rapidly
than the rest. If this takes place alternately on two opposite sides, the apex therefore
bends over at one time to the left, at another to the right ; but if at the circumference
of the organ different sides in succession gradually take their turns in the process,
then the pendent apex must rotate in space. The latter case, however, is regularly
observed to any considerable extent only in strictly radially constructed shoot-axes.
I found such to be the case, for example, in the scapes of Allium, in the
flower stem of Brassica jiapiis (Rape), and in the stem of Liniim usitatissimum
(Flax), as mentioned in my ' Handbuch der Experimenlal-Physiologie ' (1865), p. 514 ^

* The first communication, so far as I know, made on tlie phenomenon which I subsequently
mentioned in my ' Text-book ' as revolving nutation, and which has been lately extended by Darwin
to excessive importance as circumnutation, is found expressed in my '■ Expe7-imcntal-P]iysiologie' 1865,
p. 5 1 4, among the movements due to the process of growth itself, in the words : — ' Here also belongs the
hanging over of rapidly growing leaf and flower shoots, which, without exhibiting torsion, become

NUT A TION,

547

The phenomenon in question, as later observations have shown, occurs very commonly
indeed in orthotropic organs, and those which grow rapidly in length, but it is certainly
an unwarranted exaggeration on the part of Darwin, who designates it circumnutation,
to consider this as a universal property appertaining to all growing organs ; above all,
it is certainly not true of normal and vigorously growing roots. That Darwin
ascribes circumnutation to the latter depends on imperfect observation, since it
may easily be demonstrated that his roots were improperly cultivated and diseased.
I had, long before Darwin's observations, shown that the roots of terrestrial plants
growing in moist air soon become diseased, assume abnormal conditions, and may
then exhibit pronounced nutations which, however, are not to be observed in normal
primary roots of seedlings growing in damp earth or in water.

The nutations which take place during the unfolding of most foliage leaves are
very conspicuous : so long as these belong to the bud at the apex of the shoot,
their posterior side (outer, and subsequently under side) grows more strongly,
a fact upon which the formation of buds depends. When now the oldest external
leaves of a bud are about to unfold, the growth in length commences — and to this
we may in this case consider the growth in surface of the lamina as due — to
become stronger on the inner side (the subsequent upper side) than that of the
dorsal side, and this continues till the leaf has assumed a horizontal or oblique
position, in which it then becomes fixed. In most simple leaves the process
presents little that is remarkable; in the large complex leaves of the Ferns and
Cycads, on the contrary, the young leaves are rolled inwards at the growing-point
like a helix, the lateral parts of the lamina each for itself showing lateral involution
at the same time. With increasing growth the leaf-stalk and midrib, and likewise
the lateral parts of the leaf, now become extended straight, beginning from below
and proceeding upwards, so that in the middle stages of unfolding the upper younger
part of the leaf is still coiled up like a helix, whereas the lower is already unfolded
and extended flat. At last the uppermost part of the leaf also becomes completely
unrolled and extended ; however, in some Ferns the upper end of the leaf continually
retains this involution, because a growing-point exists at its apex by means of which
such a leaf is capable of a so-called unlimited growth in length.

The long, filiform, and even branched tendrils of the cucurbitaceous plants
behave, with respect to their nutation, quite like the leaves of Ferns. Concealed
in the young state between the leaves of the bud, they are coiled up into a helix
with numerous turns; subsequently, proceeding from below upwards, they become
completely unrolled and extended straight. The matter does not end here,
however, but during the growth in length which still continues for several daj'S
nutations make their appearance, causing the tendril to bend over in all directions

bent in turns towards the east, west, north, and south, quite independently of the position of the sun.
and altogether independent of the light (e. g. Allium Porrum, A. Ccpa, Brassica uapKs, Linuin
usitatissinnitn), since it occurs even in profound and constant darkness.' These short statements of mine
were based on very detailed investigations, which, however, have not been published : cf. note 4, loc. cit.
also. Dutrochet, moreover, had already observed nutations in Pisum (' Comptes rend.' T. xvil,
1843, p. 989), Darwin subsequently made evident the significance of the nutation of twining and
tendril-bearing plants, but did not recognise as such the spiral and spontaneous nutation of twining
plants, whence arose his entirely false theory of twining.

N n 2

548

LECTURE XXXII.

successively like the flower-stems described above. The advantage which not only
these but also all other tendrils derive from this nutation we shall consider more
closely later, when considering their phenomena of irritability. There I shall also
show more in detail that the still in part obscure movements of twining shoot-
axes, such as those of the Hop, Bindweed, Aristolochia, Akebia, Menispernnwi and
many others depend in part at least simply on nutations, or are introduced by
such; if tips of shoots, 20-30 cm. long, of twining plants which have not as yet
coiled round a support are cut off, and placed singly in tall glass cylinders
with a little water, they grow on actively for some days, becoming coiled in the
form of a spiral which not rarely exhibits 3-4 or more turns. However, I shall
return to the details, merely mentioning this phenomenon now as a particular case
of nutation^.

As tendrils and twining plants derive advantage from nutation (which is indeed
indispensable for the life of these plants), so also numerous plants derive other

advantages from the often active nutations
of their stamens. It is a very common
phenomenon, especially in large flowers
with long stamens, that the stamens, when
the anthers are about to empty their
pollen, undergo various curvatures as they
grow, as is perceived unusually clearly
for example in the Indian Cress {Tro-
pccoluni), and still more in Diciamnus (Fig.
347). These nutations of the stamens are
frequently accompanied by corresponding
movements of the style, and usually serve
the purpose of giving the anthers that
position in the flower which is necessary in
the transference of the pollen by insects
from one flower to the stigma of another.

The nutations thus present us with
instructive examples of how a capacity of
the plant conditioned by growth, and for and by itself useless, is found to be very
common, and how certain plants then develope this movement further and make it
useful for themselves, sometimes in order to live as climbing plants, sometimes in
order to ensure their fertilisation.

The majority of twining shoot-axes exhibit torsions — i.e. where angles or grooves
project on the segments of the stem and at first run parallel with the axis of growth,
they are subsequently found on the fully grown internodes in the form of spiral
lines running round the shoot-axis. It is easy to see on internodes which are grow-
ing how the torsion gradually arises and proceeds ; but it would be very difficult
without geometrical discursions to show the processes of growth through which the

Fig. 347.— Nutation of the stamens of Diciamnus
fraxinella : those the anthers of which have not
yet opened are curved downwards, those with
opened anthers upwards.

^ Concerning the spiral nutation of twining stems, statements in some detail are foimd in my
^ Notiz über ScJdingpflanzen ' (Arb. des bot. Inst, in Wzbg. B. II, 1882, p. 719) : further on, how-
ever, in the fifth series of the present lectures, I shall make these more complete on the basis of my
most recent observations.

TORSIONS.

549

torsions come into existence. A sufficiently clear idea of the matter may be
obtained by taking a straight piece of caoutchouc tubing or a leaden tube, and
fastening one end of it in some manner, while the other end is seized with the fingers
or a pair of tweezers and then twisted. By this means the superficial layers of the
body will be subjected to elongation, while its axis, or middle line, undergoes no
elongation at all, or if it is at the same time pulled in this direction, only a slight
one. It is obvious that something similar must occur by means of the processes of
growth during the torsion of the stem.

Since then the internodes of a twining stem which are some distance from the
bud are made to undergo torsion by growth, the bud and the youngest internodes
which have not yet suffered torsion must be passively twisted or rotated around their
own axis — a movement which may easily be demonstrated by observing one of the
younger leaves from hour to hour : it is then seen that it is found first on the
upper side, then for example on the left flank, subsequently on the underside, and
finally on the right flank of the oblique or horizontal freely suspended tip of the shoot.
The torsions of twining plants must in some way be causally connected with their
very strong and long-continued growth in length ; since even in shoot-axes which
do not climb, and which in the normal condition are absolutely without torsion, it is
possible to produce torsions by intensifying the growth in length \ I pointed
out twenty years ago in this connection that the seedling stems of the Gourd,
Buckwheat, and other plants, when allowed to grow in profound darkness, where they
attain an enormous length, exhibit very evident torsions, and in consequence of the
movements connected with this occasionally become wound around neighbouring
objects of Hke kind, much as two twisted packthreads lying parallel and close
together spontaneously wrap themselves round one another.

The question has often been raised whether any difference in the growth of the
plant by day and by night exists, and what it is. The question is in itself more of
practical than of theoretical interest. Its experimental treatment, which I accom-
plished in the years 1870-71, gave me an opportunity, however, of going more deeply
into the difficult problem of growth, since it was soon shown that this apparently so
simple question is only to be answered when all the external and internal factors of
growth are known and carefully balanced against one another ; and since my treatise
concerning the matter (' Üder defi Einfltiss der Lufttemperatur und des Tageslichtes auf
die stü7idliche)i und täglichen Änderungen des Längenwachsthums der hiternodien ') first
provided a firm basis for investigations in this direction, and has become the starting-
point of various investigations during the last ten years, it may be well to go some-
what more in detail into the contents of this treatise'^.

' The influence,' I wrote, ' which the varying temperature of the air and the
periodic alternation of daylight and nocturnal darkness exert on the growth in length
of internodes and leaves after they have passed out of the bud-condition, has often

^ I first described the torsions of the stems of etiolated seedlings mentioned in the text in my
treatise ' L/ier den Eitißuss des TageslicJits auf Neubildung und Entfaltung' (Bot. Zeit., 1S63, 2'»
Beilage, p. 16 — on p. 17, to the left, however, the words 'von Cucurbita'' should be added after
' das hypocotyle Glied etiolirter Kci?npflanzen ').

^ The general considerations on growth here given in abstract are contained in my treatise first
cited (cf. note i, p. 539).

^^O LECTURE XXXII.

been the subject of investigation, Christopher Jacob Trew, even so long ago as 1727,
published long-continued daily measurements on the flower-scape o^ Agave americana,
combined with observations on the temperature and weather ; but it was not till a
hundred years afterwards that Ernest Meyer (1827) ^^^d Mulder (1829) gave a new
impulse to investigation in this direction, and they were then followed by Van der
Hopp, De Vriese (1847-48), and others. The important questions, however, were
gone into more thoroughly by Harting (1842), Caspary (1856), and Rauwenhoff
(1867).

' These observations, industriously and perseveringly carried on, led to no definite
answer, nor even to the establishment of an actually useful method, and a careful survey
of them shows that scarcely any two observers came to the same conclusion, and that
the discovery of any causal relation of growth in length to temperature and light was
in fact impossible, since on the one hand the questions to be answered were not put
with suflScient clearness and definiteness, and on the other, the probable sources of
error, and, accordingly, the difficulties of accurate observation, were left more or less
unnoticed. INIeanwhile there appeared another series of publications simply of
repeated measurements of lengths, no regard at all, or not sufficient, being paid to
the external conditions, whence although some idea was obtained of the continual want
of uniformity of growth during different days and at different times of the day, the
causes of these were not registered: som^e observers, in fact, confined themselves
to merely demonstrating the difference between diurnal and nocturnal growth,
not reflecting that ' day ' and ' night ' signify to the plant different and very variable
complications of conditions of growth, and that such a mode of stating the question
cannot possibly lead to the discovery of causal connections, so long as the individual
factors which are contained in the ideas day and night (so far as the plant is con-
cerned) are not known. In this sense the contributions of Seitz, Meyen, Martins, and
Duchatre, for example, are useless for our purpose.

'I have, though with considerable interruptions, been engaged since 1869 with
the completion of more exact methods of observation, and have attempted to classify
and render clear the questions to be answered.'

This was done by means of the following considerations. ' With few exceptions
the majority of observers of growth in length have selected parts of plants which are
remarkable for a very considerable growth in a short time. The huge flowering
stems of the Agaves especially afforded repeatedly, on account of their rapid growth,
the inducement to the making of such observations : such objects were selected
because observers were satisfied simply to measure the growth in length with a
rule laid directly on the part of the plant observed. But even granted that in the
case of quickly growing plants sufficiently exact measurements are to be made in this
manner in intervals of time of from one to several hours, still, other evils make
their appearance of which I will particularise two only. In the first place, plants
which grow so rapidly that even only four to six sufficiently exact measurements
daily can be made, are rarely to be obtained : one is at the mercy of chance,
and a methodical and connected series of observations can scarcely ever be com-
pleted. In the second place such plants (e. g. Agaves, Musaceae, Victoria regia) are
usually so large that it is necessary to undertake the observations in greenhouses or in
the open air, and therefore under conditions where they are exposed to very large and

MEASUREMENTS OF GROWTH. 551

irregular changes of temperature and light, and of the moisture of the air and soil, which
it is quite impossible for the observer to regulate and to overcome. The comparison
of the earlier observations shows that these circumstances have contributed in an
important degree to make the results not only of different observers, but even of the
same observer, conspicuously different and mutually contradictory.

' On these grounds I regarded it as the first problem to discover a method ot
observation which allows of the measurement of any chosen and even slowly growing
small plant with sufficient accuracy, and where possible hourly. Suitable objects,
completely adapted for the purpose, are even in this case sufficiently difficult to
obtain, but still may be prepared by careful cultivation in pots : once obtained,
however, they may be submitted to observation in the laboratory under conditions
which can be varied as may be desired.

' The mode of putting the question here arises, as in all experimental investi-
gations, from the consideration of pertinent phenomena already known, whence may
be drawn the conclusions which may possibly be expected.

' If the object is to gain information as to the process of growth in length
of a part of a plant, in such a way that one obtains not only a connected idea of
it from the beginning to the end, but is also able to criticise the effects which
definite variations of temperature, illumination, and moisture induce, it is absolutely
necessary to measure the growths during short intervals of time — i. e. in one, two, or
three hours — and at the same time to know how the process of growth would have
proceeded if these external causes had remained constant.

' That in the plant itself causes are active which, quite independently of the
variation in external conditions, sometimes accelerate the growth in length, sometimes
retard it, was moreover to be expected, and might be in part concluded from what
was hitherto known. Harting had already found that the stems of Hops grow at
first slowly and then more and more quickly, attain a maximum of rapidity, and then
again grow more and more slowly till growth at length ceases. Munter also,
although his numerous observations were made at very varying temperatures,
recognised this fact, and expressed it in these v/ords : — " In addition to the daily
rhythm, composed of exacerbation and remission, an increase, climax and diminution
{increnmtlum, acme, decremetitum) of the intensity of growth take place. The lengths
produced rhythmically at first increase, ascend to a certain height, and then diminish
down to complete cessation." Rauwenhoff, so far, has most definitely expressed the
fact that in the course of a period of vegetation the growth of the stem first increases,
attains a maximum, and then slowly ceases.'

I then pointed out, from the statements of Harting, IMünster and Rauwenhoff,
the existence of a grand period of growth, and demonstrated it for individual
transverse zones of stems and roots — a fact with which we have indeed already
become acquainted above. The irregular variations of growth, subsequently
mentioned over and over again, although without satisfactory result, by different
observers, and apparently effected chiefly by internal changes, were likewise first
formally expressed by me in this treatise: — 'If then the grand curve of growth
affords an example of how the rapidity of growth of a part of a plant becomes equably
changed indcpendendy of external influences, or even in spite of them, it is to be
insisted upon, on the other hand, that the powerful fluctuations in the growth in length

ss^

LECTURE XXXII.

perceived in half-hourly or hourly observations indicate the existence of other internal
causes which, likewise independently of external influences, aid in determining the
rapidity of growth. I have designated this phenomenon as " discontinuous variations
of growth,"

' I have no doubt that the knowledge of the grand period, as well as that of the
discontinuous variations of growth, will at some subsequent date be of considerable
use for a theory of the mechanics of growth. Here however I have only brought
forward both phenomena because a knowledge of them is absolutely necessary to the
study of the effects of external influences on growth in length, and because the
experimental establishment of causal relations is rendered extremely difficult by
them. Given for example the case where a growing internode is observed in con-
stant moisture and darkness but with variable temperature, then the differences of
growth obtained in considerable periods (several days for example) are not to be
taken forthwith as functions of the different temperatures, since the phase of
the grand period is altering at the same time ; it may happen that the higher
temperature (beneath the optimum) coincides with a smaller hourly or daily growth,
because the internode at this time is in a condition where it is less capable of growth
generally. It is easy to suppose, in order to avoid this difficulty, the plant ex-
posed to different temperatures quickly one after another, and the phase-difference
of the grand period thus reduced to a minimum ; but the discontinuous variations of
growth which make their appearance quite irregularly might sometimes increase, some-
times diminish, the effect of the temperature on the growth, without our being in a
position to decide how much is to be placed to the account of one or of the other. Very
much the same difficulties would repeat themselves when the temperature is constant,
with regard to the action of variable illumination or moisture in short periods of time.

' This complication with internal disturbances, in cases where the object is to
become acquainted with the effects of external agents on growth, makes it not only
necessary to multiply the number of observations enormously, but it also entails
our being but seldom in a position to derive from the numbers expressing hourly
growth any causal connection at all ; in order to ascertain this it is necessary to draw
these numerical values as co-ordinates, and the curves, properly constructed, then
generally reveal the causal relations.' I then showed further in what manner the
moisture of the environment may co-operate in influencing the rapidity of growth, by
means of its influence on the turgescence of the cells, and how in considerations of
this kind this fact has to be borne in mind. In the second place the influence of
temperature on growth required quite special and closer consideration, which I gave
in the following words : — ' That growth only begins when a certain lower temperature
(the specific zero point) is passed, that it is accelerated so much the more the higher
the temperature is, and that at a certain higher temperature, the optimum temperature
(between 20° and 30° C), a maximum rapidity of growth occurs, and that, moreover,
as the temperature rises still further the growths again decrease, I have already
shown to be true for seedlings, and Koppen has confirmed it in his work quoted.
Moreover Harting (1842) had already inferred from his observations a similar
behaviour on the part of the shoots of the Hop, but without contributing convincing
proofs of it.

' These facts are to be realised, so far as the problem lying before us is

GROWTH BY DAY AND NIGHT. ^^^

concerned, only in so far that it must be noticed in the first place that temperatures
below the specific zero-point exert no influence on growth whatever, or, put better,
do not allow it to take place, and that temperatures beyond the optimum act
injuriously. Since, however, in the natural course of things temperatures above
the optimum only seldom occur, and in experiments can be avoided, I shall in what
follows pass over that entirely, and understand by higher temperatures only such as
are below the optimum, and therefore favourable. That even within these limits no
simple relation between temperature and the rapidity of growth exists, follows already
from Harting's investigations, and I have demonstrated it in detail elsewhere in the case
of seedlings; and it is even obvious, because in view of the existence of the grand period
and the discontinuous variations of growth, a simple proportionality between growth
and temperature is inconceivable, when one and the same part of the plant is con-
cerned at different times. In the present state of our knowledge only so much can
be said, that, starting from the specific zero up to the optimum, the rapidity of growth
is the greater the higher the temperature.

' When one speaks of the action of temperature on growth, it is tacitly assumed
that the temperature indicated by the thermometer also actually exists in the growing
part of the plant. Where roots are concerned, which are growing in soil and round a
thermometer stuck between them in the soil, the assumption is certainly warranted ;
such is not the case, however, when the temperature of the air, according to a
thermometer suspended in it, is compared with the growth of a part of a plant in the
air. Since the bulb of the thermometer as well as the part of the plant concerned
owe their temperature to the conduction and radiation of heat, and these are certainly
materially different in the two cases, for this reason alone it will occur but rarely
that the temperature of the growing tissue is exactly given by the thermometer
suspended near it. To this is to be added that in an atmosphere not completely
saturated with aqueous vapour the plant transpires, and thereby becomes cooled, and
this does not take place in the case of the dry thermometer ; on the other hand, how-
ever, it is certain that a wet thermometer will be much more cooled by evaporation
from its surface than the plant, the evaporation from which is much smaller in relation
to the surface and mass. Unless the opportunity occurs therefore to plunge the thermo-
meter into the observed internode itself (and that has never been done so far, and is
impossible in the case of small plants), the thermometer near the plant gives the tempe-
rature of the latter only in a very unsatisfactory manner. If the observation is made in
the open air, in draughts and rapid variations of temperature, or under circumstances
where the plant observed receives the direct rays of the sun, the temperature of the plant
will often be very different from that of the thermometer ; but even this source of error
is reduced to a minimum when the observation is made in a room with still air, slow
and small variations of temperature, and in diffuse light. I shall indicate subsequently
the means which I employed to render this error of observation as slight as possible.

' Quite apart from the fact that under certain circumstances the temperature of a
growing sub-aerial part of a plant may be altered also by the temperature of the water
taken up by the roots, and by the exchange of heat with the soil, the influence of the
soil is of importance in yet another connection. If the air, and with it the part of
the plant above ground, undergoes rapid and large variations of temperature, these
make themselves felt only slowly and to a less extent in the soil and roots ; by this

554 LECTURE XXXII.

means, however, the turgescence of the plant may be altered ; for instance, if the soil
is very warm the roots take up much water and the turgescence is increased, if the
temperature of the air does not suffice to cause active transpiration (such is the case
for example in the evening after a warm day) ; on the contrary, the turgescence is
diminished if, the temperature of the soil being low, the roots take up water slowly,
while a warm wind or sunshine incites the leaves to transpire strongly (as, for example,
after sunrise after a cold night). By the alterations of turgescence thus brought about,
however, the recorded rapidity of growth will be affected at the same time. During
observations in the open, also, these conditions may distort the result as to the effect
of temperature, which is being investigated, until it is unrecognisable, and thus observa-
tion in the laboratory again recommends itself, where the air is quiescent, and very
slow and slight variations of temperature take place, to which the earth in the flower-
pot can adapt itself : although under such conditions the temperature of the latter is
usually several degrees lower than that of the air, the difference is nevertheless small
and almost constant — i. e. the temperature of the air and of the soil (in the pot) if
represenfed as curves almost coincide.'

A third category of influences affecting the daily course of growth occurs in the
alternation of light and darkness — in the decrease and increase in the intensity of the
light. ' Unfortunately,' I said, ' we have as yet no convenient method of so measuring
the varying intensities of light that the measurements are directly applicable to the ob-
servation of plants: measurements of the brightness perceptible by the eye, even if they
could be conveniently determined, would present something other than the required
measurement of those rays of light which influence growth in length; for these, as direct
observation and the heliotropism occurring in coloured light show, are the blue, violet,
and ultra-violet, that is, the inappropriately so-called chemical rays for which Bunsen
and Roscoe have devised a method of measurement, the manipulation of which,
however, for our purposes would entail great difficulties. Since it results from
the determinations made by the observers mentioned, that the " chemical intensity" of
daylight, in general rapidly increases from sunrise to mid-day, to decrease thence
till sunset again with similar rapidity, and since this suffices in the meantime for the
purpose pursued by me, I have not undertaken any photometric measurements.'

From these considerations, now, we may preliminarily, at least presumably,
put together what course the rapidity of growth under the influence of varying
moisture, temperature and illumination may possibly assume. On this subject I
expressed myself as follows : ' If we now attempt, on the basis of the observations
made, to form for ourselves an idea of the course of growth (or of its graphic
representation, the curve of growth) of an internode, which is exposed to the
varying and different causes of growth, especially in the open, it is at once clear
that the curve of growth may assume the most various forms, according as the
different causes work in unison or in opposition, and according as the growing
member is in this or that phase of its grand period. In order forthwith to dispose
of the often raised question whether growth is stronger or feebler during the
night than during the day, and to render its meaning clear, we may attempt an
analysis of the combinations of causes of growth and their effects denoted by the
words day and night. As a rule the average temperature during the day is higher
than the average temperature during the night ; accordingly growth in the daytime

GROWTH BF DAY AND NIGHT. 555

must be more energetic than during the night. Day Hght, however, works in the
opposite way, and the question will therefore arise whether the intensity of the
effective rays suffices to nullify the effect of the temperature. The result also appears
to adapt itself according to the specific nature of the plant, since it is conceivable
that some plants are more sensitive to light than others. In the daytime, also, the
difference of tension of the aqueous vapour in the atmosphere is usually greater than
during the night, and transpiration is thus increased, and it may easily happen that
turgescence during the day is less than at night, and growth likewise may be
retarded thereby. The case might occur, therefore, that the growth during the day, in
spite of the higher temperature, was smaller than at night, and this will certainly be the
case if the temperature during the day is the same as that at night, or lower than that.
If, on the contrary, the excess of temperature of the day as contrasted with the night
is very considerable, it is probable that the effects of light and transpiration will be
overcome, and that the diurnal growth will remain more energetic than the nocturnal,
although the latter is promoted by the darkness and, as a rule, higher turgescence.

' We may further notice yet a few more extreme cases which are here possible.
It might happen that the temperature at night was higher than that of the following
day, and that at the same time rain falling during the night increased the turgescence
to a maximum, while on the following day (the brightness being considerable) for
example a cold wind prevailed ; in this case the nocturnal growth would be the more
intense. In the Spring-time or in Autumn it may happen that the air sinks during the
night below the specific zero-point of the plant, so that the moisture and darkness are
unable to promote the growth ; cessation then occurs, and growlh takes place only
during the day when the temperature is raised sufficiently above the specific zero-point.

' If we further suppose the external causes of growth so distributed that they
would not of themselves effect a too considerable difference of growth by day
and by night, then the difference may be exactly compensated or even reversed by
the different capacity for growth of the plant at different times, e. g. by the influence
of the phase of the grand period. For instance, if an observed internode at night,
when the conditions are otherwise unfavourable, has attained its maximum growth
(the apex of the grand curve), the growth on the following day may still be smaller,
though otherwise the conditions are more favourable.

' These and numerous other combinations are possible, then, when we compare
only the average values of day and night. The number of possible cases is still
larger when the attempt is made to form an idea of the events from hourly obser-
vations. If we suppose the grand curve of growth of an internode to be draw'n, the
hourly alterations of temperature and the hourly alterations in the intensity of the
light, and of the psychrometric difference, will alter the course of the curve sometimes
in one way, sometimes in another: the curve, which during constant external
conditions ascends and descends in the form of a simple arc, will become transformed
into a much and variously zigzagged line, in the steps of which the daily and nightly
up-and-down oscillations of the growth are more or less clearly recognised ; the size,
form and position of these steps results for the time being from the co-operation of
temperature, moisture and light.

'These indications will suffice to show of how little use it is for observers
to simply establish in what relation the nightly and daily growth stand to one

^^6 LECTURE XXXII.

another without exact investigation of the causes of growth. Tliey show, more-
over, how difficult and indeed impossible it is to infer the influence of any single
co-operating factor (temperature, light, moisture, grand-period,' periodic variations)
from observations made in the open air or in green-houses, where all the causes of
growth are at the same time subject to continual and violent variations.

' The object of earnest research in this direction must on the contrary be simply
this — to study the effect of each individual cause of growth in detail by itself:
from this the common and natural course of the phenomena may then be analysed,
combined, and predicted more exactly than was previously possible.'

Since 1869 I had been much occupied with preliminary investigations for the
measurement within short periods of the growth even of ordinary rather slowly
growing-plants, by means of a new apparatus. Of those described in the treatise
quoted I will here however only shortly describe the one represented in Fig. 348,
which I have designated the self-registering Auxanometer, since soon after my
publication this instrument became the model for numerous other instruments for
the measurement of growth, of which I consider only the one constructed by
Baranetzky as an actual improvement.

The apparatus is especially for the purpose of observing the growth in length of
a sufficiently vigorous stem, in such a way that (i) the actual elongations are magnified
for the purposes of observation, (2) a writing or registering of the growth in each hour
is accomplished by the apparatus itself. Of course it is only possible after considerable
practice, and with trained hands, so to use such an apparatus that it yields scien-
tifically useful results.

On a firm table stands a solid iron support A, on which by means of a movable
cross-bar the pulley r is movable : an indicator 2, consisting of a straw, is fastened
to the pulley. The plant y, grown in a pot, is in this case surrounded by a suitable
receptacle B, which can be opened in two halves and has a hole at the top for
the thread : this receptacle is for the purpose of keeping the growing plant in the
dark. Beneath the bud of the plant a pliant silk thread is attached by means of a
loop, and this is hooked on to a piece of thin wire, which is again connected with a
pliant silk fibre above : this thread passes over the groove in the pulley r. The small
weight g hanging on the pulley serves to give the coupling an equal tension.

It is now clear that if the plant / becomes slightly elongated the pulley r will
revolve a little towards the left, and the apex of the indicator z therefore describes a
proportionately larger path : for example, if the indicator is twelve times as long as
the radius of the pulley, as was usually the case in my apparatus, then if the plant
becomes elongated i mm., the indicator must describe a path of 12 mm., and thus
the growth is proportionately magnified for the observer. The apex of the indicator
might of course be allowed to play on an arc of a circle of equal radius, and the
successive growths be observed on the graduations of this arc. To spare the
observation hour by hour, and for other advantages, however, I employed the
hollow cylinder C, made of stout sheet metal, and so fixed on the upright axis a
of the pendulum clockwork JD that it can be easily taken off : zu is the weight and
/ the pendulum of this clockwork, which is for convenience so an-anged that the
cylinder C makes one complete revolution in exactly one hour, and that the clock-
work needs to be wound up only once a day. Previous to the observation a

A UXA NOME TER . ■] 5 7

piece of paper // is so alTixed to the cylinder C that it shows no creases of any
kind : this paper must be as smooth as possible on its outer surface, so as not
to hinder the movement of the apex of the indicator which is to mark on it.

Before the cylinder is fixed on to the clockwork, the attached pajier is moved to and
fro over the smoky flame of an oil lamp, until the smooth surface of the paper is
completely covered with soot. After fixing the cylinder, the indicator z, the apex of

55^

LECTURE XXX IT.

which is provided with a laterally directed style, is so arranged that the style rests
lightly just beneath the upper margin of the paper, and the clockwork is then set in
motion. As the cylinder revolves, the apex of the indicator describes a line s on the
blackened paper ; when the second revolution of the cylinder begins the plant has
grown somewhat during the hour, and the apex of the indicator has therefore fallen,
and thus the second line which is now described on the paper lies at a certain distance
below the first. In this manner, hour by hour, a new line is described, and it is clear
that the distances between these lines can be used for obtaining, by the aid of certain
geometric considerations, an exact determination as to the successive hourly elonga-
tions of the plant. In order to attain this, the clockwork is finally stopped, and the
paper cut off from the cylinder, the observer having first seized the apex of the
depressed indicator and carried it upwards, across the paper on the cylinder, in order
to mark the path on which the measurements of the distances between the lines
are to be taken. Immediately on its removal the blackened paper is drawn through
a bath of a solution of colophony in alcohol, and then dried, when the necessary
measurements ca,n be taken. If the diameter of the cylinder is sufficiently large,
however, and the indicator long enough, the errors of measurement in my apparatus
are so small (as may easily be calculated), that they do not come into con-
sideration at all if the experiment is properly conducted in other respects. More-
over, in my treatise, I explained in detail all imaginable sources of error, which are
to be sought far more in the nature of the plant than in the apparatus, so that
subsequent observers who have given my apparatus a modified form have had the
means of knowing in advance what were the important points.

With the Auxanometer described, and other apparatus, I have for years past
made hundreds of experiments, and thousands of measurements, the results of which,
in spite of a few insignificant differences of opinion, have received confirmation
also from the careful observations undertaken by Baranetzky in 1879, with (as already
mentioned) an actually improved apparatus \

We started above from the question as to how growth proceeds by day and by
night, and Fig. 349 will afford the shortest answer to this question. Since the essential
points are mentioned in the explanation to the figure, I may confine myself simply
to remarking that the rapidity of growth of a normal healthy stem attains a
daily maximum in the morning soon after sunrise, that then the hourly elongation
during the course of the day diminishes until evening, again to increase in rapidity
as the darkness comes on, often even before sunset ; and this increase of growth
continues until after sunrise, when the maximum is again reached. In this con-
nection the fact indicated in the figure by the dotted line 3 z is especially interesting ;
it shows that while the temperature gradually falls during the course of the night,
the rapidity of growth on the contrary may simultaneously increase. The contrary
may also be observed, that while the temperature is slightly rising, a diminution

' J. Baranetzky, 'Die tägliche Periodicität itn Langenwachsthum der StengcP (Mem. de I'acad.
imp. des sc. de St. Petersbourg, Vile serie, 1879). There also is described and figured the improved
Auxanometer referred to. With respect to the view proposed by Baranetzky that the periodicity in
the dark is an after-effect of the preceding illumination, it is absolutely necessary to read carefully
the treatise itself, and particularly pp. 16-18, since his preliminary publication (Bot. Zeit., 1877,
p. 639) omits just the most decisive facts.

DAILY PERIODICITY OF GROWTH.

559

of growth occurs; or, in other words, the daily period indicated, the increase
of growth during the night, and the progressive diminution during the day, cannot
be regarded as a result of the daily variation in temperature. This fact, however,
comes out sharp and clear only when the hourly variations of temperature are by
tenths of degrees, and only amount to a few whole degrees in the course of the day :
where the variations in temperature are large, the movement of the curve of growth
follows in the main the ups and downs of the curve of temperature, and this is the
chief reason why the daily period, which is independent of temperature, was either not

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Fig. 349.— The curve i z represents the hourly gfrowth, and the curve 3« the growth every three hours of a shoot-:
of Dahlia. The curve t indicates the temperature. On the abscissa, 6a denotes 6p.m. ; 12 « ■= midnight ; 6/- 6 a.i
and 12?« = noon. The dark areas correspond to the night, the light ones to the day.

recognised at all or only indistinctly by the earlier observer.s, because their plants
were exposed in the open to variations of temperature which were for the most
part very large.

On the other hand, however, the assumption suggests itself that the daily
period of growth, so far as it is independent of variations of temperature, may
be chiefly induced by the daily alternation of illumination ; for that the light causes
a retardation of the growth of stems and green leaves may be proved in various ways,
and since the plant remains exposed to light from sunrise till evening, its retarding
effects would assert their influence more and more in the course of several days,
i.e. the hourly growths would become smaller anil smaller. When, however, the

560 LECTURE XXXIT.

intensity of the light decreases in the evening, and the darkness of night follows, the
retarding action of the light does not disappear instantaneously, but the forces of
growth, now set free as it were, obtain more and more the upper hand in the plant,
and the growth is accelerated until the morning, attaining a maximum until the
moment when the increasing intensity of the light is again in a position to exert its
retarding influence. A series of observations made by Prantl on normal green
foliage-leaves yielded exactly similar results for these also.

But however satisfactory this explanation appears to be (and it is certain that
the alternations of light and darkness co-operate in the way referred to), further observa-
tions show that even quite independent of the daily variation in illumination, a daily
increase and decrease of growth nevertheless takes place, e. g. in continuous dark-
ness. I had ascribed this fact, which I was the first to observe, hypothetically to the
circumstance that my observing-chamber did not admit of complete darkening ; but
the very detailed observations of Baranetzky allow of no doubt with respect to this
very point, that a periodic up-and-down variation of the hourly growth in length
takes place entirely independent both of temperature and of light. Baranetzky,
however, is inclined to regard this periodic variation, which still exists even in
constant darkness, as an after-effect of the previously existing daily light-period, for
which, moreover, he adduces various apparently plausible reasons ; but, apart from
other objections, Baranetzky himself has brought forward the fact that in the case
of completely etiolated shoots, which had been developed in the dark from the begin-
ning, from tuberous roots of Brassica rapa, the periodicity came out if possible still
more sharply than with normal shoots of the same plant. This single fact suffices
completely to set aside the theory of after-effects, since no one, probably, would
be inclined to believe that we have here an after-effect which has been for a
long time latent, so to speak, in the bud. That would be much as if a pen-
dulum commenced to swing spontaneously after it had been standing still for some
time. Moreover, Baranetzky himself adduces the fact that in different plants of the
same species the appearance of the maxima in the dark is observed at very different
times of the day, although in normal daily illumination this does not occur.

I, on the contrary, am of opinion that in the plant, or at any rate in its grow-
ing parts, periodic variations occur in some way quite independent of variations of
temperature and of light; and these, as I conclude from Baranetzky's observations,
may continue for periods of very different lengths. If now the plant is subjected to
the regular alternation of day and night, and the variations of temperature are very
small, the above-mentioned influences on growth make their appearance, by which its
maximum is transferred to the morning hours, and its minimum to the evening,
the above-mentioned periodicity arising from purely internal causes being concerned
as the weaker factor in a definite daily period of time. So long ago as 1863 I gave
a very similar explanation of the spontaneous periodic movements of foliage-leaves.
There also a periodic up-and-down variation takes place in constant darkness, but
this, through the alternation of day and night, is modified to a daily period. However,
I shall return to this point later on.

Of special interest is the fact demonstrated by Dr. Vines ^ in my laboratory,

^ Sydney H. Vines, ' The Influence of Light upon the Growth of Unicellular Organs ' (Arb.
des bot. Inst, in Wzbg. B. II, pp. 133, &c.).

APPARATUS FOR jMEASURTNG GROWTH.

56]

that in the case of PJiycomyccs nitms (Fig. 3, p. 5) one of the Mucors already so
often mentioned, and a plant not only devoid of chlorophyll but also a non-ccllulnr
one, a direct action of light on the
rapidity of growth occurs of such a
kind that even light of feeble intensity
causes a retardation of the growth in
the course of a single hour, while a
similarly short period of darkness ac-
celerates it. Here also, just as in the
case of the highly organised plants,
it is the more refrangible rays of the
light, the blue and violet, which retard
the growth — a point to which I have
already expressly referred — while it is
chiefly the less refrangible rays, the
yellow, which cause assimilation in the
chlorophyll. Since this is a very im-
portant point in vegetable physiology,
I may take this opportunity of describ-
ing the apparatus which I have con-
structed for the purpose of such measure-
ments of growth on small and simple
plants, and with which Vines made
his observations. The apparatus here
figured (Fig, 350), although constructed
on exactly the same principle, is still
more convenient. On a very strong,
firm, wooden tripod, a thick slab is
arranged by means of tlie triangular
prismatic foot A, so that it can be
moved vertically and fixed. On the
slab stands a very firm cathetometer
E, the horizontal telescope of which
aff"ords an enlargement of about fifteen
diameters, at 12-20 centimeters focal
distance. The apparatus ^ is a very
strong and carefully constructed clock-
work, which is for the purpose of allow-
ing a vertical axis which supports a
horizontal brass plate to make exactly
one revolution in an hour. On this
plate is placed the jilant to be ob-
served, in our case, for instance, a
Phycomyces bearing sporangia, D, which

grows on a piece of bread, and is covered with a glass bell-jar C. The telescope of
the cathetometer, which contains a micrometer scale on which o-i mm. can be read
[3] 00

riG. 350.— Apparatus for measuring the growth in
leiifjth of small plants (Z?) by means of the cathetometer
(k).

502

LECTURE XXXII.

off, is now so fixed that the observer sees distinctly one of the erect hyphag
with its sporangium, and that its apex apparently touches one of the division
lines in the telescope. The clock-work is now set in motion : the plant on the
plate revolves once in exactly one hour, and then comes again into the field of
view of the telescope, and the growth which has taken place during one hour is
now marked by the displacement of the sporangium on the scale ; and so with each
successive revolution. The measurements made hourly on this species then give
an idea of the course of growth in the thin hyphae of this Fungus, each consisting
of a single vesicle only. Since these objects not rarely elongate 2-3 mm. in an
hour, while it is possible to measure less than o-i mm., we obtain an exactitude
of measurement quite sufficient for our purpose. It may now be asked, for what
purpose the revolution of the plant by means of the clock-work is necessary; this
is simply in order that the plants shall grow quite vertically upwards ; for they
would not do this if they were exposed to illumination on one side from a
window, but would bend over towards the window, and make any measurement of
their growth impossible. By means of the slow revolution, however, the observed
part of the plant is successively illuminated on all sides during the course of
the hour, and this acts exactly as if it received equal light from all sides. Hence
no heliotropic curvature takes place. I regarded it as an important advance
when, in the year 1873, I first hit upon the idea of entirely doing away with
the heliotropic curvatures, so exceedingly disturbing in observations on growth, by
means of such revolutions on a horizontal disc.

It is of course obvious that the glass bell-jar C may be replaced by an opaque
vessel, or, what is better, on account of the susceptibility of the object towards the
atmospheric moisture, the glass vessel may be retained and an opaque receptacle (of
cardboard for instance) placed over it. In this way the small plant may be exposed
alternately for one or two hours to the influence of light or of profound darkness, or
different coloured glasses may be interposed to demonstrate the effect of lights of
various colours, and so forth.

Hour of the
day.

Growth
per hour.

Tempera-
ture.

8-9 morn.

2.70

22.9° C.

10 „

2.70

24-3

II ,)

2-30

26-0

12 ,,

I aftern.

2.90
2-70

25-0

25-8

2 „

3-20

2 5. 8

3 „

3-5°

25-2

4 ..

2.90

25-0

5 „

3.20

25.1

6 „

2-80

25-3

am

8 9 10 H

/i

^

2

3

* ä

ff

i.O

"'"■''

ir,l:,l1llli,l| lllllllHi*

■ !

'"

/l\ / i

7

\V^^

^ j/^

/

— L_ 1

-H L_

-- -A

'

1

</J

/

1

1

11

1

1
1

'.1 '1
:,: ; I' ■ '■

Fig, 351.— Tlie thick interrupted line shows the varying
rapidity of growth of a hypha of Miicor in alternate light and
darkness. The thin line marks the temperature.

As an example of the course of such a series of observations I subjoin a table and
the corresponding curves drawn from observations made by Vines (Fig. 351).

LECTURE XXXIII.

MECHANICAL CAUSES AND EFFECTS OF THE GROWTH
OF CELLS AND ORGANS.

When vegetable cells pass over from their young condition into that of more
rapid growth, or we may also say when organs pass out of the embryonic state into
that of elongation, the taking up of water into the interior of the cells plays a
particularly important part, A longitudinal section through a root or a shoot, which
takes in at the same time the growing-point and the older parts of the organ, shows
at once that with the increasing volume of the growing cells the water in their interior
also increases, and exactly the same is observed in isolated living cells of Algae and
Fungi ^ The increase in volume of the vegetable cells corresponds almost exactly
with the quantity of water passing into their interior. It has already been mentioned
that the very young cells of embryos and growing-points, as well as the youngest
organs, are entirely filled with protoplasm and cell-nucleus ; in proportion as they
increase in length, breadth, and volume generally, the water of the cell-sap increases,
and the mass of protoplasm becomes more and more insignificant : the at first
solid protoplasmic body becomes changed into a sac filled with water and which is
closely applied to the cell- wall, as shown in Fig. 352. The cell-wall at first, where the
increase in volume of the individual cell or of a multicellular organ is concerned,
continually increases in extent only, without obtaining essentially in thickness. Only
when the increase in volume of the cells has ceased does the subsequent growth in
thickness of the cell-wall begin : a fact of the highest significance for the theory
of growth generally, and of fundamental importance, since it implies essentially that
growth in surface of the cellulose walls takes place only so long as the latter are
still exceedingly thin and accordingly extensible : strongly thickened cell-walls are
no longer capable of increasing in extent. These facts may be directly established
by microscopic observation, and there can be no doubt as to their mechanical
significance; moreover Gregor Kraus has also demonstrated the fact mentioned

' The contents of this lecture have in the main, but much more in detail than here, been already
published in the third and fourth Editions of my 'Text -book' under the title 'The Mechanics of
Growth,' and in cases where the special literature is not quoted here, I refer once for all to that
chapter. There, for the first time, the growth of the cells and tissues in combination was con-
sidered from the mechanical point of view. With respect to tissue-tension, the last part of my
^Handbuch der Exp. Phys^ (1865) may be consulted.

0 0 2

5^4

LECTURE XXXIIT.

experimentally, by determining the dry weight of growing parts of various ages^
By this means he obtained the following results, agreeing with what has just
been said.

' In a growing shoot or internode, the percentage amount of the contained
water continually increases from the youngest internodes to those which are
older, up to a maximum, and then gradually diminishes again;' this decrease occurs

' only when the growth in
length ceases, and the organ at
the end of this growth contains
most water ; ' i. e. the decrease
of the water per cent, at the
conclusion of growth in length
coincides with the thickening
of the cell-walls which now
makes its appearance, and in
part also with the deposition of
dissolved or solid matters in
the cell-contents. It need only
be added that all this is true
not merely of the internodes of
the shoot, but in exactly the
same way of the leaves, and of
the transverse zones lying one
behind the other in a root-
fibre. The quantity of water
which young organs may have
gained at the end of their
growth in length and surface
is shown particularly clearly
by weighing seedlings, in the
fresh and dry states, at the
time when all the reserve-
materials have been consumed
by growth; it is then found
that such young plants of the
Beet, Bean, Maize, and other
species sometimes contain only
4—5 7o ^^y weight, and
similarly, completely developed Mushrooms and submerged water-plants are also
found to be extraordinarily rich in water in the later stages of growth. I have
found further that pith (parenchymatous tissue) from the interior of various plants,
e.g. Senccio iimhrosus, which at first contained 4-23 7o of solid substance, when

FIG. 352.— Parenchyma cells from the median layer of the cortex of the
root of Fritillaria imperialis (longitudinal sections X 550). A very young
cells situated close above the apex of the root, still without cell-sap. 5the
same cells about 2min. above the apex ; the cell-sap s forms isolated drops
(vacuoles) in the protoplasm /, plates of protoplasm lyin;; between them. C
the same cells about 7—8 mm. above the apex ; the two cells to the right
below are seen from in front, the large eel! to the left below in optical
section ; the cell to the right above has been opened by the section, and
its nucleus is affected by the penetrating water and exhibits a peculiar
swollen appearance.

' Gregor Kraus, ' Über die Wasscrverthcihing in der Pflanze,^ in Festschr. der naturforsch. Ges.
in Halle, 1S79.

TURGESCENCE AS A CAUSE OF GROWTH. 565

laid in water, became very considerably elongated by growth, and at last possesged
only 1-97 7^ of solid substance.

The facts here mentioned allow us to explain in the first place why active
growth in plants is only possible when the supply of water is abundant. When, in the
Spring for instance, the buds of trees, rhizomes, and bulbs unfold, and in the course of
a few days numerous shoots cover the gardens, woods, and fields with their leaves,
this process consists chiefly in an extension by means of water of the small organs
already existing in the winter-buds. Just as a small soap-bubble grows as the air is
driven in, and its volume increases a hundredfold or more, so the cells of these
organs become distended by the water taken up ; but of course even this comparison
falls short of the truth, like others, since the cell-walls distended by water do not
become proportionally thinner as they increase in extent, as does the wall of a soap-
bubble, but grow with the increase in extent in such a manner that they rather
perhaps increase a little in thickness.

A second fact of fundamental importance for the growth of the cells and tissues
is this, that they grow only when turgescent. Turgescence is, if not the only
cause, at any rate one of the most important causes of the grov/th in surface of the cell-
walls ; or in other words cell-walls only grow in surface — i. e. the organs elongate and
become broader — only in proportion as the water contained in the interior of the
cells strives to press the cell-walls outwards, or to stretch them. We have already
(Lecture XIII) learnt to know this condition of the cells as turgescence, and,
to repeat, we understand by this term the hydrostatic pressure which the cell-
sap exerts on the cell-wall, so that the extensible but at the same time elastic
cell-wall becomes distended. It is obvious that by means of the elasticity of the
distended cell-wall a contrary pressure is exerted on the cell-sap which stretches it, or
mutual tension occurs ; it is this condition which we designate turgescence, and
since experience teaches that cells only grow in extent if they are turgescent, we thence
conclude that turgescence is one of the most essential causes of growth : the cell-wall
grows in the direction of its circumference only so long as it is stretched by the
watery cell-sap. The turgescence will cease at once if the growing cell-wall,
by the deposition of new particles of substance in it, actually assumes the form
which has become forced upon it by distension; but as the cell-wall grows in
extent new water is continually being taken up into the cavity of the cell, and thus the
already larger wall becomes again distended, and thus the tension due to turgescence
is maintained even in the growing cell, until finally another condition comes in where
the cell though turgescent grows no more, or where, through the growth of the cell-
wall, its passive extension is compensated without further distension being produced
by the entrance of fresh water. In these two cases the cell, or the growing organ,
is fully grown, i. e. it increases no more in surface, though ihe thickening of the
cell-walls may now begin, and the relations of rigidity especially become altered
as described in Lecture XIII.

It was further stated there that the turgescence of vegetable cells depends upon
three things : — (i) By means of the endosmotic action of substances dissolved in the
cell-sap, water is continually being absorbed into the cell from without ; this is the
moving cause of the whole process; (2) The water forcibly absorbed into the cell, in
spite of the strong pressure which it exerts, must not filter out again, and this is

tfi^ LECTURE XXXIII.

prevented by the peculiar properties of the protoplasmic sac lining the cell-wall ;
(3) Although the sac of protoplasm itself prevents filtration, it is true, it would be
more and more distended by the water forcibly entering by endosmose, were this
not prevented by the firm elastic cellulose wall which surrounds it. The protoplasmic
vesicle does not allow filtration, but it is exceedingly extensible ; a limit is imposed
to its actual extension, however, by the slight extensibility and great elasticity of
the cellulose wall, which however on its part permits filtration to a large extent ^.

It is now important to make out by means of what forces the water is absorbed
from without through the cellulose wall and the protoplasmic vesicle. We know so
far of no other cause for this than the attraction which the substances dissolved in the
water of the cell, and in part contained in the wall itself, exert on the surrounding
water: by means of this attraction the molecules of water are drawn through
the cellulose wall and the protoplasmic sac into the interior of the cell, and increase
the volume of the sap, which therefore presses the walls outwards. Since, how-
ever, the cellulose walls in virtue of their elasticity oppose considerable resistance
to this pressure, the entrance of the water must take place with still more consider-
able force ; or, in other words, the endosmotic attraction must be greater than the
elasticity of the cellulose walls. It has been found that this endosmotic attraction
may attain considerable magnitude, so much in fact that the absorbed water may exert
a pressure of 6-7 atmospheres on the surrounding wall — i. e. the cell-wall under these
circumstances must behave as if a column of mercury 6-7 times as high as the
barometer column were pressing on it from within ^. It was formerly believed that
such powerful endosmotic effects could only arise if the cell contained large quantities
of dissolved substances; and it was also assumed that it is chiefly 'substances of the
nature of gums, sugars, or proteids which bring about the endosmotic action. But
the investigations of Pfeffer have shown that crystallisable salts under the circum-
stances here given exhibit far greater endosmotic force than the organic substances
mentioned; that, for example, saltpetre acts much more vigorously, and that
even very dilute solutions of this and other salts exert vigorous endosmotic action.
Subsequently De Vries pointed out good reasons for believing that in the turgescence
of the growing parts of plants the vegetable acids or their salts are especially
important; in fact, as I had already shown in 1862, all parts of plants which are
actively growing in length have an acid reaction ; and Graham had pointed out that
the organic acids common in the plant, and their potassium salts, are remarkable for
their attraction for water, and according to De Vries sensitive phenomena in motile
organs of plants are accompanied by an increase in the acid reaction, when an
increase of turgescence is taking place at the same time ^.

1 The difference in the significance of the protoplasm and the cellulose wall for the processes of
diffusion in living cells was first recognised and thoroughly worked out by Naegeli, in his treatise
on the ' P7-imordial UtricW (Pflanzen-physiol. unters, von Naegeli und Cramer, H. I, 1S55, p. i).
The principal subsequent workers on this subject have been De Vries {'Sit?- la permcabilite du proto-
flasma des betleraves rouges,' Arch. Neerlandaises, VI, 1871, p. 117), and Pfeffer {'Osmotische
Untersuchungen' 1877).

^ Very detailed statements on the magnitudes of endosmotic force and the resulting turgescence
of the cells are found in Pfeffer's '■Osmot. Unters.,' Leipzig, 1S77.

3 Hugo de Vries, ' Über die Bedeutung der Pflanzensäuren für den Turgor der Zellen,' Bot.
Zeit., 1879, p. 847,

TURGESCENCE AND DROOPING. 567

These statements of Pfeffer and De Vries harmonise completely with my experi-
ments, according to which the strongest turgescence is to be observed in just such
parts of plants, the dry weight of which amounts to only a small percentage, which in
the main are to be put to the account of the cellulose walls and the protoplasm, so that
only a very small proportion can come from the substances dissolved in the cell-sap.

If from our theoretical considerations it results that within a growing cell or a
growing mass of tissue such a tension exists between the sap and the cell-wall, we must
also suppose that on the cessation of this tension the cell-wall must contract, and the
part of the plant concerned diminish in size, and if the extension due to turgescence
has taken place chiefly in the longitudinal direction, a corresponding shortening
must take place on the cessation of turgescence. There is, however, a very simple
means of stopping turgescence : it is simply necessary to prevent the addition of
water to growing parts of plants, and to promote the evaporation of water from the
cells ; in this case, as already explained, parts of plants which are filled with sap
droop and become flaccid, and measurement shows that internodes and roots in this
condition are considerably shortened. The cells which become contracted by the
evaporation of the water of their cell-sap behave like a soap-bubble, which, hanging
from the blow-pipe, contracts and drives back a portion of the inflating air and thus
becomes smaller. This shortening on the cessation of turgescence, then, is effected
by the elastic contraction of the cell-walls which had been previously extended by the
water which had penetrated by endosmose. At the same time, the diminution of
turgescence must cause cells and tissue-bodies to become more extensible, just as
a strongly inflated caoutchouc balloon when it loses air again becomes extensible and
flaccid (cf. what was said in Lecture XIII). It must also be admitted as a further
consequence of our theoretical considerations, that a non-turgescent and drooping part
of a plant cannot grow, since otherwise our theory would be false; but it has
already been stated above that, as a matter of fact, only turgescent organs grow, and
it is easy to convince ourselves by measurements that roots and leaves which have
become flaccid and droop do not grow. Indeed a marked flagging is not necessary
to arrest growth, but only a certain diminution of turgescence : for example, if
plants are cultivated at a window, and the soil in the flower-pots kept continually
nearly air-dry for weeks and months, then although the leaves remain fresh to a certain
extent, the young shoots do not grow ; and something similar is observed in the
open in the Spring when the air and the soil are for some lime persistently dry.

We have to thank De Vries for exact and very careful investigations on the
significance of turgescence for growth in length. From his ' Unter siichiingeti über
die mec ham's cheti Ursachen der ZeUstreckiing ^ ' I may quote the following. If growing
parts of plants are laid in certain salt-solutions, they lose their turgescence completely
in two or three hours, contracting at the same time. Young shoot-axes may
thus lose 4 or 5 % of their length. As solutions particularly suitable for this
purpose De Vries mentions those of saltpetre and of common salt. The shortening
mentioned, accompanied no doubt with a thinning of the part, that is the total diminution

' Hugo de Vries, 'Unters. ül>cr die mechanischen Ursachen der ZeUstreckiing,^ Leipzig, 1S77,
a treatise which must be studied by any one desirous of making himself acquainted in detail with
the mechanics of growth.

568

LECTURE XXXIII.

of volume, indicates how much the cell-walls were extended by the turgescence. As
I have already shown in Lecture XIII, and as is to be observed in the accompanying
figure (Fig. 353), the protoplasm becomes withdrawn from the cellulose wall through
the action of the solution (containing from 5-10 7o of salt), at first here and there, and
then on all sides, since the cell-sap gives up a great part of its water to the salt-
solution ; this latter penetrates through the cell-wall, but cannot enter into the pro-
toplasmic sac, but draws the water of the cell-sap from this : the sac thus contracts,
closely surrounding the cell-sap, which has now become more concentrated. The
pressure which the sap enclosed in the protoplasm previously exerted on the cellulose
wall herewith ceases, and the cell-wall left to itself then contracts back to its natural
size ; its elastic contraction is here certainly less» than that of the protoplasm,
relations which become clear at once on careful consideration of the accompanying
figure, borrowed from De Vries. It is also obvious that by means of feebler
action of a salt solution, or by feeble evaporation which acts in a strictly similar

FIG. 35^—1. Young, half-grown cell from the cortical parenchyma of the peduncle of
C€jjhalaria leiccantha. 2. The same cell in a 4 "/q solution of potassium nitrate. 3. The
same cell in a 6 % solution. 4. The same cell in a 10 % solutioir. i and 4 after nature;
2 and 3 diagrammatic. All in optical longitudinal section, h cell-wall ; / protoplasmic
lining of the wall ; i cell-nucleus ; c chlorophyll-grains ; j cell-sap ; e salt-solution which has
passed through the cell-wall (De Vries).

manner, only a partial stoppage of turgescence will take place, no withdrawal of
the protoplasm from the cell-wall necessarily occurring at first; as soon, however,
as this does occur, even if pnly here and there, turgescence ceases.

According to De Vries, if any part of the plant, young stem, or root, rendered
flaccid by plasmolysis, is again brought into pure water, the penetrated salt-solution
gradually soaks out, the protoplasmic sac again takes up pure water into its
interior, becomes distended, and applies itself on all sides to the interior of the
cellulose membrane, and as this assumption of water and absorption proceed further,
the cellulose membrane also becomes again extended, the condition of turgescence
begins anew, and the part concerned obtains practically the same volume, and
above all the same length, which it possessed before the investigation. That no
injury to the organ takes place here the investigator named has sufficiently demon-
strated, and it is unquestionable that the object again becomes turgescent, and then
begins to grow anew, and that this growth may even be considerable.

The importance of turgescence for growth — m other words, the importance

TISSUE-TENSIONS.

569

of the passive extension of the cell-wall for its proper growth — as I had previously
concluded on other grounds, was demonstrated by De Vries particularly by the estab-
lishment of the proposition, that ' the rapidity of the growth in length in the partial
zones of growing organs rises and falls with the magnitude of the extension caused
by turgescence.' That is to say, the extension due to turgescence first increases in
passing backwards from the apex of a shoot or of a root, attains its maximum in the
region of most active growth, and diminishes thence towards the base, just as does the
partial growth ; at the boundary between the fully-grown and the growing region lies
also the limit of extension due to turgescence. In order to comprehend this matter
quite clearly, it is necessary to refer to what I said in the preceding lecture concerning
the distribution of growth — the partial growths. Finally it need only be mentioned that
the contraction of a growing organ in the salt-solution, and its subsequent re-extension
in pure water, is to be regarded in the first place only as the measure of the existing
extension of the growing cell-walls, but not as a measure of the magnitude of the
force of turgescence. This is at once clear on again referring to a non-cellular
plant — e. g. the Pliycojnyces already mentioned so often ; evidently the turgescence
in the much branched utricle of which this plant consists is everywhere equal in
magnitude, though extension of the membrane is only actually taking place at the
growing ends of the utricle.

So much at any rate follows definitely from the considerations and facts so far,
that vegetable cells only grow when their cellulose walls are passively extended by the
pressure of the cell-sap, and therefore that this extension is an essential mechanical
cause of the growth itself.

It appears, however, that in certain layers of tissue in the more highly organised
plants, the passive extension which is necessary for growth may also be produced in
another way, as we might conclude from the fact of the tension of the tissues, which
has also already been described. From the phenomena which result on separating
the various layers of tissue of a growing shoot-axis, it follows that the epidermis, as
well as the vascular bundles and the as yet non-lignified strands of sclerenchyma,
are subjected to continual passive extension, due to the much more rapid growth
of the soft parenchyma, especially the pith. Although some points still remain
doubtful, we may nevertheless ^.ssume that this passive extension of the epidermis,
vascular bundles, and strands of young sclerenchyma by the growing parenchyma,
is to be looked upon as an important factor in the growth of these passively
extended portions of tissue.

We previously noticed the tensions of tissues only in so far as they affect
the rigidity and elasticity of the young growing shoot-axes and leaves, and it
has only to be added to this that the rigidity even of many fully-grown parts, par-
ticularly of petioles and the ribs of leaves of the Dicotyledons, depends even in the
fully-grown condition on these tissue-tensions.

Now, however, we have to regard the tissue-tensions as a mechanical
property of the growing organs of multicellular plants, and to see in what relation
they stand to growth itself. In the first place, so much is certain, that the parts
of plants concerned only grow at all so long as tissue-tensions exist in them :
if these are destroyed by doing away with the turgescence in the parenchymatous
tissue, the growth ceases also, and the more pronounced the turgescence of that

570

LECTURE XXXIII.

tissue, the more energetic the growth tends to be. On the other hand, however,
it is of importance to note that the tissue-tension itself is called into being by differ-
ences in growth, especially differences in the lengthening of the various layers of tissue.
It begins with their histological differentiation behind the growing-point, and the
further this differentiation of epidermis, vascular bundles, strands of sclerenchyma,
and parenchyma proceeds, the more pronounced also, up to a certain point, is the
of tension, or, put better, the differences in length of the layers of tissue which make
their appearance when the layers are separated. But when the vascular bundles and
layers of sclerenchyma have become lignified, and the growth in length of the
whole organ at this place therewith concluded, the tissue-tension may still persist,
though on the separation of the layers the difference in length is not so great ; for
it is obvious that a very strong tension may be existing in the tissues when only
very small or even unmeasurable elongations and shortenings occur on the separa-
tion of the layers, for this only demonstrates that the extensibility of the tissues in a state
of tension towards one another has diminished. We must not therefore regard the
changes in dimension on the separation of the layers of tissue forthwith and generally
as the immediate expression of the force of tension ; that would only be possible
if the extensibility remained the same at different ages, which is not the case.
In spite of this objection, very important for the specialist, it is nevertheless
necessary for the understanding of the mechanical properties of the growing parts of
plants to return once more to the most important phenomena of the tension of
tissues S even if only to afford the reader a general idea. For this purpose I
may take the following statements from my '■Handbuch der Experimental Physiologie '
(1865). In the first place a portion of the shoot-axis under consideration was
measured, then the tissue-layers mentioned were cut out, and these also measured. In
the following tables the original length of the part is regarded as 100, the shortenings
being denoted as negative and the lengthenings as positive numbers per cent.

Plant.

Number of inter-
node, proceeding
from youngest
to oldest.

Alteration in length of the isolated tissues in
percentages of the parts measured.

Cortex.'

Wood.

Pith.

Nicotiana Tabacum . .

I— IV

— 5-9

-1-5

+ 2.9

V— VII

— 3-1

— I-I

+ 3-5

VIII— IX

-3-5

— 1-5

+ 0-9

X— XI

— 0.5

— 0-5

+ 2.4

Sambucus nigra . . .

I

— 2.6

— 2.6

+ 4'0

II

— 2-0

— 2.8

+ 5-5

III

— 1-5

— o-o

+ 1-5

Similar shortenings of the external layers of tissue, and lengthenings of the

^ A more detailed discussion of the idea of the intensity of the strain in tissue-tension, and above
all as to the fact that the mere changes of length of the separated strips of tissue only present a
measure for the prevailing forces if the extensibility of the compared tissues remains unaltered, as
well as on other considerations appertaining here, are to be found in my 'Lehrbuch^ Aufl. IV, 1874,
p. 763. There also are to be found numerous examples of the behaviour of the separated strips of
tissue of growing parts of shoots.

MEASUREMENT OF TISSUE-TENSIONS.

571

parenchyma, may easily be demonstrated in growing petioles, especially those of
considerable length and thickness, such as those of the Beet, Rhubarb, Philo-
defidron, &c.

If a growing internode or leaf-stalk is split by means of a longitudinal section,
or by two sections at right angles, the parts curve concave outwards, because the pith
side is extended and the epidermis contracts, a state of affairs which must necessarily
lead to such curvature. The phenomenon appears most clearly if a lamella is
taken from the middle of the whole organ by means of two parallel longitudinal
sections, and this lamella laid flat on the table and the pith halved lengthwise ; then,
in proportion as the knife travels forward, the two halves also curve concave outwards.
If now, instead of halving it, thin strips of tissue are separated from this median lamella
of the organ, progressively from without inwards — first one which contains the
epidermis, then one which contains the cortical tissue, and then a further one
containing the young as yet unlignified woody layer — they all curve concavely out-
wards; because the layers which abut on one another are all extended negatively
on the outer side and positively on the inner side, so that on separating them as
above the outer side of each strip must shorten and the inner side lengthen. Here
also I may adduce a few examples from my very numerous measurements.

Name of the Plant.

Length of the
whole inter-
node.

Radius of
curvature of
the sector.

Shortening of
epidermal side.

Lengthening of Half thick-
the concave ness of the
pith side. 1 internode.

SylphiiDH perfoliatwn —

Left half

Right half ....

Older internode of same-
Left half

Right half

Macleya cordata —

(hollow)

69-5 mm.
69-5 »

190-0 „
190-0 „

134-5 „

4 cm.
4 »

3-4 „
3-4 ..

5-6 „

2.8-o/„
2.4 Vo

2-8 o/„
2-6 %

0.74 0/0

9-3 Vo
9-3 Vo

9-5 Vo
IO-8 %

7-1 Vo

3-0 mm.
3-0 „

3-5 »
4-5 „

3-3 ,>

A considerable rapidity of growth in length, with a simultaneous progressive
differentiation of tissue-layers, as met with in upright leaf-shoots, strong petioles,
and tendrils, seems necessary to promote this tissue-tension; since it is not
observed in very slowly growing shoot-axes, and in these cases the tissue-
differentiation also proceeds but slowly, e.g. in the downward growing thick
rhizomes of Yucca and Draccena. Finally, no morphological differentiation of
tissues at all is necessary to produce such tension of tissues : this is proved by
the stalks of growing Agarics, which consist entirely of homogeneous hyphal
tissue, and nevertheless exhibit powerful tensions between the outer and inner
tissue-layers.

When, on isolation, the previously passively extended tissues become suddenly
shortened, and the previously positively distended pith as suddenly lengthens, both
can only be effected by means of a corresponding change in form of the cells ; for
even a merely relatively large alteration in volume is not to be thought of, because
neither the water of the contents nor the membranes saturated with water can

572 LECTURE XXXIII.

change their volume by pressure and traction by means of the forces which are here
acting. Hence we must assume that the strips of tissue which shorten increase in
diameter, and that the elongating pith becomes correspondingly thinner. These
alterations in transverse dimensions are, however, not directly capable of measurement.

At any rate it follows from what has been said that the passive extension
in length makes the cells of the epidermis, the vascular bundles, and the not yet
lignified sclerenchyma strands narrower, and that the epidermis especially, since it
is properly too short for the growing parenchyma, must also be too narrow for it.
In like manner the pith, prevented by the epidermis from extending, must tend to
be distended transversely : since the pith (and soft parenchyma generally) is too long
for the passively extended tissues, it must also at the same time be too thick for
them, and tend to extend them (particularly the epidermis) tangentially. In other
words, it follows at once from the easily measurable tissue-tension in the longitudinal
direction of a growing shoot-axis, that transverse tensions also must exist ; or, since
the passively extended tissues are too short for the soft parenchyma, they are also
too narrow for it, and this conclusion may be directly proved. If thin transverse
discs are cut from the organs in question, and split open by means of a radial longi-
tudinal section, they gape, because the epidermis contracts peripherally, since it was
previously properly too short for the internal tissue — i. e. it was passively stretched.
However, we will not here go further into detail as to the changes in form of split
transverse discs of growing organs, because it would be impossible to avoid difficult
mechanical considerations of various kinds. I will refer only to the one obvious
fact, that the increase in circumference of many internodes and leaf-stalks which
are subsequently hollow is effected by this extension of the external layers
of tissue, while the internal pith is no longer able to grow transversely in pro-
portion; the latter therefore becomes ruptured, its outer layers remaining in
connection with the external tissues which are growing in circumference, while a
cavity arises in the interior. This may be easily observed in the flower-stems of the
Teazel {Dipsacus), the scapes of the common Onion and Dandelion {Taraxacum
officinale), in many Umbelliferge and true Grasses, &c.

The cylinder of pith of a dicotyledonous shoot when freed from its enveloping
layers of tissue is very limp, extensible and pliant, but if laid in water it soon
becomes turgid, stiff, and elastic : it is then longer and apparently also thicker. The
elongation in water may in a few hours amount to 40 per cent, or even more of the
original length. This proves that the cells of the pith absorb with great force the
water surrounding it, and that their cell-walls are still in a high degree extensible, and
thus that they did not possess that degree of turgescence, while yet in the interior of
the uninjured organ, of which they are capable. Further observations on such
isolated cylinders of pith, particularly of Compositae and Solanese (where they often
attain a very considerable thickness and are peculiarly suited for experiments of this
kind), may be employed for further instructive investigations, of which I will only
describe one more in detail here. The isolated pith of a piece of the shoot of Scftecio
umbrosus 235-5 mm. long increased in length 5-7 per cent, at the moment of isolation,
and weighed 5-3 gr. It was divided into three parts by marks of Indian ink : of these
I was the oldest and III the youngest. The lengths were 1= 100 mm., 11= 100 mm.,
111 = 49-9 mm.

BEHAVIOUR OF THE PITH. 57:}

The prism of pith was placed in a wide glass tube, which was then corked
at both ends. In the course of fourteen hours the following elongations had
occurred : —

The part I had elongated 4-5 mm.
„ II „ „ 6.5 „

„ III „ „ 2-0 „ (i.e. 4-i7o)-

The pith had however meanwhile lost 0-15 gr. water.

After another twenty-six hours stay in the glass tube the following fresh changes
had made their appearance : —

The part I was still longer by 2-5 mm.
» II „ „ 0-5 „

„ III was shorter by 0-5 „

During this interval no further loss in weight had occurred, because the air in the
glass tube was now saturated with aqueous vapour, and thus no more evaporation
could take place.

The pith was then laid in water, and in six hours the following changes had
occurred : —

The part I was longer by 1 8 mm.

„ II „ „ 23 „
„ III „ „II „
The pith had at the same time become considerably thicker, and had absorbed 6 gr.
water; though at first, as said, it had only weighed 5-3 gr.

The determination of the dry weight showed that there was in this state only
0-2 2 gr. of solid substance ; this substance after the isolation of the pith was combined
with 5-08 gr. water, and then lost 0-15 gr. At the end of the experiment, however,
it took up 6 gr. more ; or, at first the pith contained 4-23 °/^, at the end only 1-97 °/o
of sohd substance.

It is seen from these statements, as I have already pointed out, that in the cells of
the pith at any rate only an extremely dilute solution capable of causing endosmose
can be present, and that this nevertheless causes a very pronounced absorption of
water, turgescence, and growth. There is one other point in our experiment to be
explained, however, namely, the original elongation of the pith in spite of a loss
(though small) of water. The explanation lies, however, in the very pronounced drying
up of the surface, which could not well be due only to the small loss of water by evapo-
ration ; it is on the contrary probable that the inner cells of the pith took the water
from the outer ones and thus elongated, the external desiccated ones being passively
extended just as the epidermis is elsewhere. That this was actually the case is shown
by the stiffness of the pith in other experiments of this kind ; if the prism of pith which
has become dry on its outside is bisected longitudinally, the parts gape outwards, as
when a living shoot-axis is split longitudinally. I drew from these facts the following
conclusion. If the inner pith cells are able to abstract water from the outer ones, it
may be assumed that the external cells of the pith also are able to abstract water
from the tissues surrounding them, and therefore retard their turgescence, whence
their growth is retarded, and the further consequence is that they become passively
extended by the pith, which robs them of water.

574

LECTURE XXXIII.

It is evident from these observations why longitudinal halves or quarters of
growing shoots laid in water curve outwards to such an extraordinary extent —
a phenomenon which is exhibited particularly well by the flower-scapes of the
Dandelion {Taraxacum officinale) where longitudinal strips lying in water form
numerous coils of a helix.

The passive extension of the epidermis, and of the hypodermal strands of collen-
chyma and sclerenchyma which behave similarly, is thus due chiefly to the vigorous
growth in length of the parenchymatous tissues. From this extension in the
longitudinal direction, however, a simultaneous transverse tension must necessarily
arise. If we suppose a caoutchouc tube extended longitudinally, it is at once noticed
that it tends to become smaller in diameter at the same time, and the epidermis with
its strengthening layers must behave in the same way when it is passively extended
"longitudinally. If, on the other hand, a solid cylinder of caoutchouc is supposed to
be compressed from above downwards, it becomes at the same time thicker

transversely, and the growing pith
or parenchyma generally of a
young stem must behave in just
the same manner. These con-
siderations at once demonstrate
that when the epidermis of a grow-
ing internode is extended by the
pith, and the pith, on the con-
trary, compressed transversely by
the epidermis, transverse tension
must necessarily occur, and it
is at the same time clear that
this latter must be a necessary
consequence of the longitudinal
tension. It has already been men-
tioned that this transverse tension
may be actually recognised.

If, however, at the conclusion
of growth in length of an inter-
in by a cambium zone, a further
Since the wood arises on the inner

Fig. 354.— Transverse section of epidermis and cortex of a Sun-flower,
rapidly growing in tliickness ; the cells have elongated peripherally and then
become divided by radial walls.

node, subsequent growth in thickness is ushered
intensification of this transverse tension results,
side of the cambium, the whole of the cortex surrounding the wood must necessarily be
distended outwards. This tangential extension is perceived with the greatest clearness
by means of the microscope, in the form of the cortical cells in transverse sections
through the cortex of stems of Ricinus, Helianlhus, &c., which are rapidly growing in
thickness. Since, however, the cortex and subsequently even the periderm and bark
of woody stems are very slightly extensible and highly elastic, it results that a strong
opposite pressure of all the cortical layers lying outside the cambium must respond to
the outward pressure due to the growth of the wood; strong tension takes place
between cortex and wood, and the cambium itself lies in the position of greatest
pressure. We shall learn subsequently how this aff'ects the formation of the wood
itself; here it need only be remarked that Gregor Kraus has demonstrated this

MUTUAL PRESSURES OF CORTEX AND WOOD. 575

transverse tension between cortex and wood by means of detailed'investigations'. To
convince ourselves of its existence, it suffices to mark out a ring of cortex from a stem
which is growing rapidly in thickness, by means of two superposed annular cuts, and
then to make a longitudinal cut in this ring, and separate the cortex from the
wood. On now attempting to replace this ring of cortex in its normal position around
the wood, it is found to be too narrow, and gapes ; it would require a very powerful
force so to extend it that the gaping margins again touch one another, and thus
exactly surround the wood. This, however, signifies simply that the cortex was
previously compressed radially, and forcibly distended in the tangential direction by
the growing wood,

Gregor Kraus, moreover, recognised 14-15 years ago^ that a daily periodicity
occurs both in the longitudinal and tangential tissue-tension, such that a decrease
of the tensions is noticeable from early morning to mid-day or afternoon, and
an increase thence till early morning again. Millardet found and confirmed this
periodicity in an entirely different way, which can only be more exactly shown when
we come to the consideration of the sensitive organs. It is scarcely possible to
doubt that these periodic alterations of tissue-tension depend on the same causes as
the similarly periodic variations of growth, the weeping of the root-stock, the daily
movements of leaves, &c.

All these considerations hold good strictly of shoot-axes, petioles, and the
midribs of leaves only. The radical contrast between root and shoot again comes
out here; for in the first place it is important to notice that the conspicuously
short growing portion — 3-10 mm. long — of normal terrestrial roots exhibits a
phenomenon of tissue-tension which differs at least superficially. If the growing
portion of a vigorous root is split by means of a longitudinal section, or by two sections
at right angles, in the direction of the axis, the parts do not gape concave outwards,
and, at first, no separation at all takes place. If roots so treated, however, are left to
go on growing in water, the longitudinal halves become curved convex outwards,
that is in exactly the opposite manner to those of shoot-axes and leaf-stalks.
The phenomenon is explained simply by the fact that the parenchymatous
cortex of the root grows more vigorously in length than the axial strand,
and that the latter is still highly extensible within the growing region. The
tension between the cortex and the axial strand is therefore so small, that the
effect in question only makes its appearance on growth occurring subsequently to
the splitting.

The contrast between roots and shoot-axes, however, is much more conspicuous
in parts of the root which are already fully grown. We are here concerned with jhe
fact already mentioned in the lectures on Organography (Lecture II) that the part of a
root-fibre which is no longer growing in length may shorten itself very considerably
.during a long period, so that seedlings may have their stems eventually drawn
down into the soil. Referring to what I said in the lecture quoted, we are here
only concerned with the causes of this phenomenon itself: this was investigated by

• The transverse tension between cortex and wood was first investigated in detail by Gregc
Kraus, Boi. Zeit., 1867, p. 113.

" Kraus {Joe. dt., p. 121) first described the periodic variation of tissue-tension.

576 LECTURE XXXIII.

De Vries. The shortening of the root, discovered by Filtmann as long ago as 1819^
and then forgotten, and at length again observed by Irmisch and myself, is manifested
in the first place by the production of transverse folds on the surface of those parts of
the root which have for some time ceased to grow in length; the roots of many
marsh-plants, those which grow in water especially, as well as those of Hyacinths,
various species of Ins, &c., easily permit the observation. De Vries made marks at
determined distances on the primary roots of young plants of Red Clover and
Beet, and then allowed them to go on growing in soil or nutritive solution, and
found that after 3-6 weeks the regions marked had become shortened by
10-15 °/a ^'^^ ^^ some cases even 20-25 % ^^ *-^^^^ length. On letting
the main roots of actively vegetating plants of the Caraway, Teazle, and
Artichoke, taken out of the soil and separated from the tuft of leaves, lie for
3-4 days in water, they shortened about 4-8 7o, increasing in thickness the while
(sometimes even 4-8 7o)) and accordingly an increase in volume and simultaneous
stiffening took place. Isolated parts of the tissue in water showed the same
changes : the axial strand as well as the parenchymatous cortex became shortened in
the longitudinal direction and extended transversely. According to De Vries it is
in both the parenchymatous tissue alone which initiates this change, which in fact was
directly measured microscopically by him : the cork envelope as well as the vessels and
bast-like fibres become passively bent, and cause in part the formation of the transverse
folds referred to. De Vries sums up the most important points of his observations as
follows : — ' The contraction (shortening) on taking up water is a phenomenon of
turgescence, and is reversed by all agents which stop turgescence. This is shown most
simply by the fact that the contractUe roots do not shorten on flagging, as growing
root-apices or stems tend to do, but become elongated. In the same way they lengthen
if the protoplasm is in any way killed, or if it is forced to separate itself from the
cell-wall by the action of salt-solutions. On the cessation of turgescence the tissues
become drawn together in the transverse direction.'

The shortening effected by the absorption of water is rendered permanent by
subsequent growth, just as the turgescent elongation of stems and leaves becomes
fixed by subsequent growth. It at once results from this, as De Vries remarked,
that just those roots which have the best marked transverse folds exhibit the most
distinct shortening in water. Thus as we explain the longitudinal extension by
turgescence of parenchyma-cells in shoots, chiefly by the fact that the cellulose
walls are more extensible in the direction of their length than transversely, we must,
on the contrary, assume as regards the root-parenchyma that the cellulose walls
are more extensible transversely than longitudinally, since it is always to be main-
tained that the hydrostatic pressure which causes the turgescence is equal in
magnitude on all sides.

It may be concluded, from a long series of various phenomena in the life of
plants, that the form and size of the individual cells within the tissue depend to
an important extent upon how they are connected with neighbouring cells. In other
words, we may assume that the shape and size of each tissue-cell is determined

' Hugo de Vries, ' Über Verkürzung pflanzlicher Zellen durch Aufnahme von IJasscr," Bot.
Zeit., 1879, p. 650.

PRESSURES AND STRAINS IN WOOD AND CORTEX. 577

chiefly by two factors : on the one hand by the causes of growth which are given in
the chemical and molecular structure of the individual cell itself, while on the other
hand this individual formative effort can only make itself effective in proportion as
the mechanical obstructions permit it. These obstructions, again, are generally of
two kinds ; the individual growing cell is prevented from extending equally on all
sides by the pressure of neighbouring cells, or it suffers, since it is closely con-
nected with its neighbours, passive strains in the longitudinal and transverse
direction. How the relative position of an individual cell or cell-layer will
determine the effects produced by pressure and strain, may be in general deter-
mined beforehand geometrically, with certain premisses; and if it is at the same
time borne in mind how growth in any one direction is usually followed by cell-
divisions at right angles to it, a deeper insight is thus obtained into the causes which
determine the fact that we find the cells on the transverse section of a wood-body
arranged in regular radial and tangential rows, and that the forms of the cells and
their groupings in the cortex of a woody stem which is growing in thickness must be
situated and layered otherwise than before the beginning of growth in thickness, and
so forth. Put shortly, ' the plans of transverse sections of stems and roots in
particular may be understood geometrically, so far as their histological construc-
tion goes, with the aid of the above principles.' This was first done, as regards
the structure of wood, by Naegeli ^ and was then treated generally by Detlefsen ^,
in his treatise ' Üder das Dicke7iwachsthiwi cylindrischer Organe! Unfortunately I
must here pass over the contents of these two unusually instructive treatises, since they
could not be presented without digressions, and especially without geometrical figures
and mathematical formulae which might not be welcome to the reader of this book.

Several other facts however may be without difficulty set forth clearly in words.
It is certainly not uninteresting to learn that a matter of structure so conspicuous and
well known as the formation of the annual rings in wood depends, in the main at
least, on alterations of pressure between wood and cortex. I had expressed this
hypothetically in 1868, since it struck me that the cortical crevices of the older stems
and branches of trees are deepest during the winter and spring, evidently pointing to an
increase of pressure between wood and cortex. In the spring, after these crevices
have been formed, the tension must thus be diminished ; and at the same time the
amount of water used in the unfolding of the leaves may also cause a slight
shrinking of the alburnum. With increasing thickening of the wood, however, during
the vegetative period, and with more pronounced drying up of the external layers of
cortex, the tension between wood and cortex must increase : the layer of wood
formed in the spring is therefore produced under diminished pressure, and consists of
cells which are larger in the radial direction, while with increasing cortical pressure
the extension of the wood-cells and vessels in the radial direction is interfered with,

* The first geometrical and mechanical treatment of the arrangement of cells on the transverse
section of the wood is that of Naegeli, in his treatise ' Über das Dicken7vachsthiiin Jcs Stengels und
Anord7iu>ig der Gefäss-st ränge bei den Sapindaceen,^ München, 1864.

^ Emil Detlefsen, ' Über Dickcnwachsthum cylindrischer Organe' (1878), in Arb. d. bot. Inst,
zu Wzbg., B. II. p. 18. The investigations of Detlefsen referred to further on in the text (' Versuch
einer mechanischen Erklärung des excentrischen Dickenivachsthums verholzter Axen und JVtirzeln,''
Wismar, 1881) is also found reprinted in Arb. d. bot. Inst, zu Wzbg., B. II. p. 670.
[3] Pp

578 LECTURE XXXIII.

and the denser autumn wood thus formed. It is not improbable that other causes
also co-operate here ^ of which, however, we know very little at present ; but in any
case the very extensive investigations of De Vries (1872-76) have shown" that
my supposition was in the main correct. He intensified the cortical pressure in
spring by binding cords tightly round certain spots on two- to three-year old
branches ; the result was that the thickness of the annual ring beneath the ligature
was less than the average thickness of the corresponding ring at some distance above
and below that place. In some branches the difference was so marked, that the
place experimented on was visibly thinner — an impression which was strengthened
by the formation of woody cushions at the upper and lower margins of the
ligature, the latter evidently because at these places cortical tension must have been
lessened by the pressure of the ligature. Moreover, the thickness of the layer of
autumn-wood, which (as usual) ceased to grow in August, was greater beneath the
ligature than in the normal case. The autumn-wood at this place, in very different
species (e.g. Maple, Willow, Poplar, Horse-chestnut, &c.) investigated by De Vries,
was formed of wood-fibres with the transverse section compressed radially, and the
number of vessels was smaller than in the normal wood. These observations show
then, that under increased pressure the formation of autumn-wood begins at a time
when, during normal growth, wood-tissue with wide cells and many vessels would
still be formed. A diminution of cortical pressure is obtained by splitting the cortex
into several parts by means of radial longitudinal sections. The strips of cortex thus
produced contract somewhat in the tangential direction, because they were previously
extended in this direction ; necessarily, the pressure which the cortex exerts on the
cambium and young wood is thus diminished, and this most in the immediate
neighbourhood of the margins of the cuts ; the new wood-tissue now arising close to
these deviates considerably from the ordinary structure of wood. Incisions were
made in June and July, after the formation of the normal autumn wood had already
commenced ; by the middle of August it was seen that the two- to three-year branches
experimented upon had grown considerably more in thickness at the places where
longitudinal incisions had been made, than above and below. On transverse sections
the thickness of the wood-growths was greatest in the neighbourhood of the inci-
sions, and thence diminished constantly up to the middle line between two incisions.
De Vries found in all cases that the newly-formed w'ood was outside the layer of
autumn-wood which had already been formed before the experiment ; therefore
everything produced after the diminution of the pressure was composed of wood-
fibres not at all compressed in the radial direction, and at the same time the vessels
in this wood were more numerous — i.e. a layer of wood had been formed under
diminished pressure, which resembled the spring-wood more than the autumn-wood.
By means of these experiments also Knight's old experiments (1801) find their
explanation. He fixed young apple-trees, with stems about an inch in diameter, in

* E. Russow, in his fundamental investigation, ' Über die Entwicklung des Hoftüpfels, der
Membranen der Hohzellen und des Jahresj-inges bei den Abietineen, in e7-ster Linie von Pinus
silvestris' (134 Sitzung der Dorpater Naturforscher-Ges., Dec. 24, 1881), expresses himself differently
from the view given in the text as to the causes of the difference between spring and autumn wood.

^ Hugo de Vries, 'Über den Einflus's des Rindendruckes auf den anatomisclicn Bau des Holzes,"
Flora, 1S75, No. VII; and, further, 'Über Wundholz,' Flora, 1876.

EFFECTS OF CORTICAL PRESSURE, ETC. 579

such a manner that their lower part (three feet) was rendered immovable, while
the upper part of the stem with the crown could bend under the pressure of the wind.
During the period of vegetation the upper movable part of the stem increased con-
siderably in thickness, the lower immovable part much less so ; this is easily
explained by the fact that the bending to and fro of the upper part of the stem,
under the influence of the wind, causes the cortex to be extended on the convex
side each time and so loosened, and the pressure thus diminished. The growth of
wood in a young tree which was free to move under the pressure of the wind
exclusively north and south, behaved in a corresponding manner : in this north
and south direction the growth of wood was stronger, and stood as regards that
formed east and west as 13:11. Knight himself, however, gave an incorrect
explanation of the phenomena observed by him.

The promotion of the growth of wood by means of longitudinal incisions in the
cortex of the stems of young trees, continued from the crown to the root, has
also been employed for a long time practically, and,, as I have convinced myself for
some years past, with the best results : the stems grow more rapidly in thickness, the
thicker alburnum can convey more water containing nutritive materials from the
roots to the crown of the tree, and thus promote assimilation, and this in its turn
again promotes the formation of wood in the stem.

We owe a new contribution to our knowledge in this direction to an
investigation by Emil Detlefsen (1881), who first pointed out the fact that in
stems and branches with excentric layers (e. g. of the Pine, Maple, Walnut, Vine),
and therefore where, as in Fig. 318 above, the annual rings are thicker on the one
side than on the other, the formation of cortex is also more energetic on the
side of the stronger growth of wood, and the thickness of the cortex more
considerable. The necessity of this is at once obvious, since the pressure
which the cortex exerts on the wood re-acts also conversely from the wood on
the cortex. Detlefsen's remarks on the very commonly occurring causes which
must effect a diminution or increase of cortical pressure are particularly valuable
however. Wherever a branch arises on a stem, and at the places of origin of the roots,
an increased growth in thickness is found. From the places of origin of the thicker
branches descend buttresses of the stem, it may be metres long ; and in like manner
we find at the lower end of the stem the places of origin of the large laterally
spreading roots made recognisable by thick buttresses running upwards. On the
branches, strongly thickened places are found always and only running in the direc-
tion towards the roots, and on the roots, on the contrary, in the direction towards the
stem. Above the places of origin of the branches moreover, and laterally from these,
increased growth in thickness occurs, and conversely in the roots. The explanation of
this universally observable fact is, according to Detlefsen, a very simple one. 'By
means of a branch growing in thickness, and by means of a lateral root behaving
in the same way, the cortex of the organ whence they spring is pushed apart and
the cortical tension thus diminished, and this naturally induces an increased growth
in thickness. The extension of the thickening buttress also is very easy to
understand if tlic anatomical constitution of the cortex is borne in mind. It is
clear that a pull acting on the cortex must make itself noticeable over much greater
stretches in ihe direction of the course of tlie bast-fibres than in the direction at right

p p 2

580 LECTURE XXXIII.

angles to this. The same is of course true also of the diminution of the cortical
tension, and this explains the extension of the thickening buttresses which appear
at the places of origin of lateral organs.

' From the changes in dimensions of the cortex during growth in thickness we
get the following results : —

'(i) The tension of a convex piece of cortex is increased by the growth
in thickness, that of a concave piece of cortex, on the contrary, is diminished
by it.

' (2) The alteration of cortex-tension produced by the growth in thickness
is ccEteris paribus the more considerable the more strongly the cortex is curved.

' For a convex surface of cortex must become enlarged during growth in thick-
ness, a concave one, on the contrary, must diminish, and the changes in size of
the surface produced by equal growth are the more considerable the more con-
siderable the curvature of the surface is.'

Further, the changes, hitherto unexplained, in the direction of the branches on
a tree-trunk are referred by Detlefsen to this principle ; here, however, it is the
weighing down of the branch which depresses it more and more as it increases in
length from year to year. ' Since the weight of the branch forces it downwards,
it becomes, like any bent rod, stretched on its upper side but compressed below ;
the cortical tension of the upper side is thus (and this most evidently at the base
of the branch) increased, that of the lower side diminished. The inequality of
cortical tension at the insertion of the branch, produced by the weighing down, is
often much more considerable than that due to the form of the cortical surface at
this place. In the first case the buttresses which run down from the places of origin
of the branches spring from their under side, whereas otherwise they run down from
above over the two sides of the place of insertion.' For the proper appreciation of
these considerations, what has already been said above must of course be borne
in mind, namely, that every longitudinal tissue-tension must necessarily induce
transverse tensions also, and conversely, every transverse tension must call forth
longitudinal tensions.

Straight branches directed nearly horizontally always show, according to
Dedefsen, a promotion of the growth in thickness of their under side, and this
is in fact usually most evident at the base of the branch, and gradually diminishes
thence towards the apex — branches or stems which stand nearly upright and have
their leaves on one side, and which are situated at the margin of a wood, or which
by prevalent winds in a certain direction have had the development of their crowns
prevented on one side, always show a predominant growth in thickness on the side
where most leaves exist. It is often shown externally by the transverse folds of the
cortex that it is compressed on the side towards which the unilateral weight of the
crown presses down the stem, while the increased tension on the convex side of
such objects is betrayed by the smooth surface of the cortex. * Here,' says Detlefsen,
* where the inequality of the cortical tension is obvious, the usual explanation, that
the excentric growth in thickness is a result of one-sided nutrition, is quite untenable.'
' If the foliage is abundant,' he continues, ' a marked alteration of the form
of the bent branch is produced by it. Thus the older branches of orchard trees are
seen to be bent by the weight of the leaves and fruit, and to sink more and more the

MECHANICAL ACTION OF BRANCHES, ETC. -581.

older they become. While the young branches of the Lime tend upwards, the older
branches of the same tree fall in curves towards the earth. The same is seen in the
older branches of the Pines. The youngest lignified branches at the apex of the
Fir Picea excelsa ascend obliquely into the air, further down the branches are
horizontal, and the lowermost even curve downwards.' This phenomenon is still
more evident in vigorous specimens of Piniis austriaca : the tender annual shoot
at the end of each horizontal branch stands erect, then becomes curved a little
obliquely outwards, and in the course of the year becomes more and more oblique,
and then when the new shoot is developed next year — again vertical — the last
year's piece of branch sinks till nearly horizontal, to become completely so in
subsequent years. Thus it is that a horizontally extended branch of this tree is
composed of distinct annual shoots, each of which at first stood perpendicularly
erect.

In these processes occurring in woody branches and stems we have been chiefly
concerned with a greater or less obstruction of growth, by means of the mutual
pressure of layers of tissue. I may now add a few examples showing how vitally
active cells begin to grow anew on being relieved from pressure.

One of the finest examples in this connection is presented by the formation of
so-called Tyloses ^ in the vessels of the wood of Dicotyledons. If the vessels,
especially the wide ones, of Robinia, the Oak, Vine, and many other woods are
examined microscopically, when they have attained a certain age, they are found
to be entirely filled with a parenchymatous tissue, which was observed and figured
even by the first vegetable anatomist, Malpighi, though of course he had no idea
of its origin. Not before recent researches, especially the careful labours of Reess
(1868), were accomplished, was the remarkable origin of the tyloses rendered clear.
They arise in fact by the very thin closing membranes of the bordered pits, at the
spots where the vessels abut on soft parenchyma cells, becoming forced into the
cavity of the vessel under the turgescence of the latter, and then beginning to
grow vigorously. A club-shaped vesicle is thus formed, which, as it grows,
undergoes cell-divisions, and when such structures protrude from numerous pits,
they fill up the cavity of the vessel and compress one another, and thus produce a
parenchyma-like tissue.

The whole process would be quite impossible if the tube of the vessel itself was
filled with sap and turgescent ; but, as it is, the vessel loses its sap, and the air con-
tained in it is even rarefied, and thus the turgescence of the neighbouring parenchyma
cells must drive forwards the fine closing membranes of the pits into the cavity of the
vessel. I observed something of the same kind, but on a larger scale, in 1854, in the
internodes of Bean seedlings which had become hollow : the pith cells which
surrounded the cavity had grown into it in the form of club-shaped or spheroidal
papillae, and had even undergone division several times. The same cells, however, had
the pith not been ruptured by the stronger tension of the external layers of tissue,
would have remained as polyhedral parenchyma cells compressing one another on
all sides in the usual manner.

* Max Reess, 'Zur Kritik der Böhm' sehen Ansicht über die Entiuickluvgsgeschichte nvd
Funktion der Tüllen' Bot. Zeit. 1868, p. i.

582 LECTURE XXXriT.

The pressure to which the tissue-cells are normally subjected, and which prevents
their free growth, may easily be relieved artificially on one side by cutting across a soft
stem or leaf-rib, or even a root, and then surrounding the cut surface with water or
damp earth ; in this case there is formed, in many species of plants at any rate, if
not always, the so-called callus, which consists essentially in that all the cells of
the epidermis, cortical parenchyma, cambium, &c., which are still vitally active, now
grow forth from the cut surface and divide, and so form a cushion of soft tissue which
at length covers the surface of the section : this cushion is the callus. These callus-
cushions have moreover the highly remarkable property of giving origin to new
growing-points of shoots and roots, from which new plant-individuals become
developed. The callus-cushions can be obtained with the greatest ease if large pieces
of branches, as thick as the arm, of Poplars and many other trees, are cut off and cut
smooth above and below, and kept during the winter in a moist warm chamber : a
thick swollen callus arises at the boundary between the cortex and wood. The same
thing happens if fresh living foliage-trees are cut across above the soil. In such cases
dozens or even hundreds of young leaf-shoots are often seen to sprout forth from the
callus formed on the cut surface of the rooted portion.

Hansen^ has investigated in detail the formation of the callus on the cut-off leaves
and flower-stalks oi Achiinenes grandis, leaves oi Begonia rex and shoots oiPeperomia.
These organs planted with the cut surface in water, or moist sand or soil, develope
the callus there. The section by which the ' cutting ' is separated from the parent plant
is the external stimulus to the commencement of processes of growth on the surface
of the wound. In the first place the layer of cells bounding the cut surface perishes :
this dead tissue is frequently separated from the living tissue still further by the
formation of several layers of cork, a process also common elsewhere, and which may
be produced by wounding masses of tissue which are still living. Beneath this
protection there now begins an active growth of all the tissue-elements still provided
with protoplasm : i. e. the cells of the epidermis, collenchyma, and cortical tissue.
The epidermal cells here perform in addition a peculiar function, growing out into
long root-hairs which supply the young callus with water and nutritive salts. The
callus grows not merely from the cut surface, but also extends laterally, so that the
stem of a leaf-cutting often swells up to double its original diameter, and the callus
as a thick cushion includes the cut end of the petiole. This mass of tissue at first
consists of nothing but similar parenchyma cells ; but after some time numerous
vascular bundles are differentiated in it, and extend in all directions towards the
surface of the callus. At various spots on the tissue of the callus superficial cells now
become rich in protoplasm, divide actively, and produce a small-celled embryonic tissue,
which is conspicuously marked off from the surrounding tissue of the callus. These
embryonic masses of tissue or growing-points soon become elevated, produce leaves,
and thus emerge as young shoots. At the same time embryo roots develope in the
interior of the callus ; these break through the tissues, penetrate the soil, and con-
tribute to the nutrition of the new shoots.

Begonia leaves cut off and laid flat on damp sand exhibit still other curious

* Hansen's researches on the formation of callus are found in his paper, ' Vergl. Unters, über
Adventivöildtmgen,' in Abb. der Senkenbergischen naturf. Ges., B. XII, 1881.

FORMATION OF CALLUS.

5^3

phenomena. In the first place, the section which separates the leaf-stalk not only
induces the formation of callus, shoots and roots at the wound, but also at a place
some distance from it, namely where the petiole passes over into the lamina, and
where at the same time the primary ribs of the leaf arise. If the nerves in a leaf
so treated are cut through here and there, callus-cushions, shoots and roots are like-
wise formed at those places. Moreover isolated cells in the epidermis of the leaf-
nerves, situated at a distance from the sections, are stimulated in a remarkable
way to renewed life. They grow vigorously, division-walls are formed in them in
various directions, and finally the small-celled growing-point of a new shoot is
developed in them.

These and many other observations show that a cut made in an organ of a plant,
when the vegetative conditions are otherwise favourable, is to be looked upon as a
stimulus to growth — as an impulse to extensive and complex activities. In the first
case of course the tissue adjoining the section is simply freed from certain hindrances
to its growth ; and the beginning of the formation of callus depends on this. When

Fig. 355.— a socaiing <ji I uiu F.iIh!, the root ami plumule- of wliieh were sti aioht.
was so placed that the root-apex lay almost horizontal on the surface of the mercury
(black in the figure), and fixed in that portion to the cork i by means of a pin : >i >i
layer of water on the mercury. The figure shows the seedling about 24 hours later.
The growing part of the root has curved sharply downwards, so that the apex
enters the mercury perpendicularly: the resistance which it meets with is expressed
by the form of the root behind the downward curved portion. The stem has
become sharply erected at its basal portion : the nodding position of the bud is a
phenomenon of nutation, independent of gravity.

this has proceeded to a certain extent, however, growing-points arise in the tissue of
the callus, which of course are to be regarded, not as for and by themselves immediate
effects of the section, but as consequences indirectly due to it.

While the preceding considerations and facts show that the tissue-cells undergo
passive strains and pressures during their growth, on the one hand, and on the other
hand are compelled to act in this way mechanically on their environment, it is
implied at the same time that by means of the processes of growth work is done (in
the mechanical and physical sense of the word) in the interior of the tissues. The
store of energy of the growing cells is not entirely exhausted by this work, however,
as follows at once from the fact that growing plant-organs can also exert powerful
pressures externally, on bodies which come in contact with them. This takes place
very generally when the apices of growing roots are necessitated to drive their way
into the resisting soil ; they not only have to glide between the small particles of soil,
but to push them asunder. The elongating region immediately behind the growing-
point of any root pushes forwards the apex, clothed with its root-cap, and thus drives
it between the closely packed particles of soil much as a nail is hammered into a

584 LECTURE XXXIII.

board. A very clear idea of this external work which, together with the internal work,
is performed during growth, is obtained by allowing the thick primary roots of active
seedlings to grow so that the apices are compelled to penetrate into mercury. This
fluid, which is nearly fourteen times as heavy as water, and therefore about fourteen
times as heavy as the growing-point itself, of course affords a very powerful resist-
ance to the entry of the latter ; nevertheless, the apex of the root penetrates to
a depth of 1-3 cm., and would certainly penetrate still deeper if the mercury lying
in close contact did not prevent respiration and poison it.

The external effects in the case of growing tree-trunks with a hard bark are much
more powerful, however. I have myself had opportunity of seeing how in still weather
a solidly built stone-wall was overthrown simply by the growth in thickness of an old
tree. To the more frequently observed phenomena in this direction belongs the slow
elevation of huge, powerful, heavy trees by their superficial roots, which are endowed
with vigorous growth in thickness. It is a very common occurrence that old Poplars,
Pines, and Larches have their upper horizontal roots subsequently protruding above
the surface of the earth ; the under side of these roots, which were at first entirely
concealed in the soil, bears up against the substratum, which is continually becoming
more resistant, and as growth goes on not only are the roots themselves forced to
project from the soil, in consequence of this resistance, but also to carry up at
the same time the stem, which often weighs many thousands of kilograms. W. S.
Clark, who has investigated this phenomenon, reminds us especially of the well-
known fact that the delicate seedlings of Beans, Oaks, &c. often push up clods of
earth of large size, and states that in England a boundary stone weighing eighty
pounds was pushed aside by three large Fungi growing up beneath it ; and the case
is known of a Hazel in England which had accidentally grown through the central
hole of a mill-stone, completely filled it up, and then being hfted up by the
growth in thickness of its roots also carried the mill-stone up with it. In order to
study more closely this external work of growth, Clark placed a sort of iron waist-coat
(which need not here be described in detail) on a young Gourd fruit, and so arranged
matters that a weight of more than 4000 pounds could be brought to bear on the
surface of the fruit ; even this did not entirely prevent the growth. Unfortunately
the experiment is not described with sufficient exactness to admit of a very clear
judgment, but a calculation based on probable data yields the result that a pressure
of only eighteen pounds to the square inch was here obtained— little more than
the pressure of one atmosphere.

PART V.

IRRITABILITY.

LECTURE XXXIV.

GENERAL CONSIDERATIONS ON IRRITABILITY.

With the word irritability I designate the mode of reaction to stimuH,
which is peculiar to living organisms. This is of course in the first place a
mere arbitrary definition of a word; but such definitions of terms are necessary
for mutual intelligibility, although they are unfortunately often neglected in the
province of natural science. That such a bare definition of the term — which
moreover I have come to regard as right only after many years of thought — has its
uses, is illustrated directly by the following remarks. I say the mode in which living
organisms only react to stimuli, is irritability : by this I imply at the same time that all
irritability in the tissues is due to the protoplasm. For there is no difference of
opinion as to this one point, that all the processes of life depend upon proto-
plasm, and that where no protoplasm is present no vital processes can occur. It
follows thence, however, that the essential cause of all phenomena of irritability
must be situated in the protoplasm, because we regard irritability as a property of
living organisms only. And as a matter of fact, in all cases where the phenomena of
irritability have been successfully and sufficiently analysed, the investigations have led
to the result that living protoplasm plays a chief part in the matter; though this
is not intended to imply that a phenomenon of irritability is conditioned entirely and
solely by the properties of protoplasm. On the contrary, mechanisms which are
influenced in a secondary manner only by the stimulated protoplasm determine to a
large extent the final external character of a phenomenon of irritability, especially in
a complex organ. At the same time it follows from what has been said, that all
living cells, cell-tissues, and living organs of the plant must be irritable, according to
our definition ; for they all contain protoplasm, and there is no doubt that all proto-
plasm is irritable, at least in certain conditions — i. e. it reacts to external stimuli in a
way which lifeless bodies do not as a rule do ; although, as we shall see later, reactions
occur even in crystals which present a very striking similarity to certain phenomena
of irritability.

On the other hand, our definition of irritability excludes the assumption that
every reaction of organised bodies whatever may be regarded as irritability. For, in
the first place, organised bodies, quite apart from their specific peculiarities, are also
at the same time physical objects, and must react to external influences according to

588 LECTURE XXXIV.

the laws of physics : for example, when the growing haulms of cereals, and other
shoot-axes, bend beneath the pressure of the wind and then again become erect, this
is simply in consequence of their elasticity, and thus of a physical property ; and in
like manner when the cell-membrane of a turgescent cell becomes extended, or
contracts on the cessation of turgescence, that also is an action of a physical
nature. If a long-stemmed plant is laid horizontally and left free to move, the stem
at first bends more or less downwards, because it is flexible and elastic — that is one of
its physical properties ; but if we allow it to stay quietly in this position, we find after
several or many hours that the still growing apical portion of the stem now
ascends until it stands perfectly erect — and this is not a physical but a physio-
logical phenomenon, which orily occurs in a living plant, and only there when
the part of the stem referred to is still growing. If possible, the accuracy of the
proposition expressed above comes out still more distinctly in the case of some
movements of the parts of plants, which in fact present great external similarity with
some phenomena of irritability, but are nevertheless purely physical and mechanical
actions, since they occur in dead though organised bodies. The upper leaves of the
common involucre of Carlina acaulis, a kind of Thistle, radiate outwards and down-
wards when the plant has long been dead, and remain in this position as long as
it is dry ; but on plunging such a dead flower-head in water, or letting it remain
in very damp air, the leaf structures referred to become directed upwards and
inwards, and the whole of the dried flower-head closes up ; and if desiccation again
follows later, it reopens— i. e. the involucral leaves again project outwards and down-
wards. These changes ensue still more rapidly in the upper leaves of the involucre
of another composite, Myriocephalus. In both cases the movements mentioned are
eff"ected by short transverse zones on the lower part of the dried involucral leaves :
the cell-walls on the lower side at these places swell up more strongly when they
imbibe water than those on the upper side, and when they dry they contract more
strongly than those, thus causing the movements described.

The various movements of dry fruits and their parts, which, as has already
been pointed out on pp. 2 10-2 11, may sometimes be very complicated, depend
on similar phenomena due to the change in volume of cell-walls on the absorp-
tion and rejection of water. In many other cases also something similar occurs.
I will mention only the so-called Rose of Jericho, a plant which belongs to the family
Cruciferee, and which grows wild in Egypt, though it may be easily cultivated in
Europe. Its radiating branches, on which are situated the ripe fruits, spread them-
selves out on the ground, and when the whole plant is dried and dead throw themselves
inwards, very much as the five extended fingers of the hand are laid together in
forming a closed fist. If the dried plant is moistened, the branches thus rolled
together into a ball open out again. This process is found to occur in some other
plants also, e. g. an American species of Lycopodium ; and again, the hygroscopic
movements of the peristome of the Moss capsule (p. 150) depends on similar pro-
cesses, and so with many other phenomena in the vegetable kingdom. In all such
cases, however, it is not phenomena of irritability but purely physical actions which
come into play, such as the imbibition of water and the alteration in volume of the
cell-walls concerned.

We now come to another very characteristic point, which distinguishes all phe-

DISPROPORTIONALITY BETWEEN STIMULUS AND EFFECT. 589

nomena of irritability, viz. the disproportionaliiy extsliftg between the external stimulus
arid the ultimate actioti. In the cases of movements of parts of plants due to
imbibition and desiccation as just considered, there is an easily intelligible propor-
tionality between cause and effect : when a certain quantity of water penetrates into
the walls of cells, the latter become distended according to the volume of water, and
upon this the movement depends. In like manner the bending of a flexible haulm
of straw, or of a woody branch, corresponds to the pressure which acts from
without and gives rise to the curvature, and we have the simplest case of such purely
mechanical effects, for example, when an elastic ball is driven against another one of
equal size and elasticity ; as is well known, the former comes to rest because it has
given up the whole of its momentum to the latter. All this has no resemblance to
phenomena of irritability; on the contrary, a very characteristic point of the latter
lies in the fact that neither the quantity nor the quality of a phenomenon of irritability
need have any kind of similarity or proportionality to the stimulus, and it is
simply on this that the peculiarly wonderful and even startling effects of irritability
depend ; and here probably we have the reason why until quite recently the pheno-
mena of irritability, which are at bottom identical with those of life, were set off
against those of the rest of nature as something entirely different, and regarded as the
effects of a special force — vital force. A calm criticism of the prevailing circum-
stances, however, leads to an entirely diff'erent conclusion. The conspicuous
inequality in kind and in degree between the cause and the effect arises rather
from the fact that in the living irritable organ a series of causes are already in
existence, which taken together with the external stimulus give rise to the effect.
The disproportionality between cause and eff'ect in irritability is simply and only
apparent, not real, as will be brought forward still more clearly from subsequent
considerations. In the first place, however, the main fact with which we are con-
cerned may be illustrated by a few other examples.

If growing stems or petioles are illuminated from one side for some time, they
generally become curved in such a manner that the apex bends over towards the
source of light. There is no doubt that it is the rays of light which effect this
curvature, and it is just as little doubtful that the mechanical energy of these light-
rays would be far too insignificant to cause such a curvature of the parts of the
plant ; on the contrary, very peculiar mechanisms must exist in the latter which, on
being stimulated by the rays of light, bring about this curvature. Still more striking
in this connection are the geotropic curvatures. If an upward-growing stem is laid
horizontally, it curves upwards ; if the same is done with a primary root, its apex
curves downwards. The simple change of position of the axis of growth of these
organs with regard to the radius of the earth and the direction of gravitation eff"ects
changes in the growth in length, which stand in no comprehensible mechanical
relation whatever to the other effects of gravitation. That it is here, however,
entirely and simply a matter of adjustment conditioned by the organisation of the
plant, follows at once from the above-named circumstance, that the one part of the
plant becomes curved upwards and the other downwards when its position with
respect to the radius of the earth has been altered.

Similarly we meet with the disproportionality between cause and effect in cases
where, by means of a slight touch on the under side of the motile organ, a leaf of

590 LECTURE XXXIV.

Mmosa suddenly falls down limp, and its parts become folded together, or
when a similar effect is produced simply by sudden darkness. And matters are
similar in all phenomena of irritability, not only in plants but also in animals,
and in our own bodies. What qualitative similarity or quantitative proportionality
exists, for example, between the vibrations of the luminiferous ether and our
sensation of sight, the vibrations of the air and our sense of hearing ? What I
am here insisting on is still more evident in the innumerable reflex actions of the
human body.

It will lead to clearness if we again try to render intelligible the meaning of
certain words, because by an inaccurate use of them at this stage the greatest
confusion may be introduced into the matter, against which we must guard our-
selves so much the more since the difficulties in the subject itself are sufficiently
great, and should not be rendered still greater by indefinite language. It will be
well to keep sharply apart from one another the three ideas, ' Stimulus,' ' Stimu-
lation,' and ' Irritability ' or ' irritable structure.'

Stimulus is the name which I give to any alteration in the environment of the
irritable organs by means of which stimulation is caused. Experience shows that
changes in the intensity of the light, variations of temperature, alterations of elec-
trical conditions, instantaneous shocks, sudden pressure, etc. may act as stimuli.
Constant illumination, or constant temperature, etc., on the contrary, is not to be
regarded as a stimulus. However, it is conceivable that even with constant external
conditions of life, the internal states of the irritable organs undergo change, and that
thus their capacity for reacting towards unaltered external influences undergoes
differences, and this must then have the same effect as if the influence of the external
circumstances had been altered. The main point is, that any change whatever,
whether it originates from within or from without, is to be regarded as a stimulus ;
since if both the external and the internal conditions remain constant, no stimulus
whatever appears to occur. In most cases where phenomena of irritability are con-
cerned, the accuracy of these reflections is at once clear ; but there is a series of vital
phenomena very frequently occurring, which can scarcely be grouped elsewhere than
among the stimulations, but which nevertheless make their appearance with especial
clearness just where the external circumstances are constant. These are the move-
ments which I have previously designated autonomous or spontaneously periodic,
and to which we shall return later still more in detail.

It has already been said that a conspicuous feature of the phenomena of irri-
tability lies in that they correspond with the stimuli neither qualitatively nor quantita-
tively, and it is in this very fact that the essential distinction between phenomena
of irritability and simply mechanical, physical, and chemical actions lies. It has
moreover already been shown above in what the explanation of this remarkable
circumstance essentially lies, viz. in the irritable structure of the organ.

It will perhaps contribute to the intelligibility of this statement if we here
again figure to ourselves a few examples from inorganic nature. Something similar
to a phenomenon of irritability occurs, for example, in the case of a heated
steam-engine the valve of which is suddenly opened. Previously at rest, the
mere opening of the valve puts the engine in motion and accomplishes a definite
amount of work. The possibility of this was already given in the tension of the

.S" T I MULUS, S TIM ULA 77 ON, 7R R I TA ItIL I Tl '

591

Steam and in the internal structure of the engine ; there was wanting simply the
external impulse, by means of which the valve was opened, to call forth the working
of the engine, the movement of which however essentially depends on the con-
struction and putting together of its parts. It is obvious that neither tlie mere
movement of the hand in opening the valve, nor the mere tension of the steam
thereby set in action, constitutes the cause of the working of a steam engine, but
that this is rather to be sought to a great extent in the internal structure of the latter ;
and in this sense we have also to regard the internal structure of the plant, which is
set in motion by opportune external changes, as the essential cause of phenomena of
irritability.

Perhaps we obtain the clearest expression for the internal condition of an irritable
organ by saying, that I'/s parts are in a condilion of tin stable equilibrium, with the
addition, however, that every disturbance of this unstable equilibrium will become
sooner or later again compensated, whereby the irritable condition again reappears ;
since the peculiar characteristic of irritable organs consists in that, in consequence of a
stimulus, a stimulation is in fact called forth — i. e. a new condition, in which the same
stimulus can no longer be effective — but that after some time the stimulation ceases,
and the organ again returns into its original state, whence it can then again be driven
to the same stimulation by the same stimulus. The peculiarity of irritable organs lies
less in the fact that their parts can be set in motion in virtue of the unstable
equilibrium, than in the fact that they subsequently again resume their irritable con-
dition— their unstable equilibrium — spontaneously.

Let us dwell a little longer upon these matters. We may look upon a house of
cards, as built by a child, as a very well-known example of unstable equilibrium : a
slight push suffices to make the whole of the artificial structure tumble down.
Here also we have a strikingly large effect consequent on a small cause, as is
usually the case with phenomena of irritability ; but the fallen card-house does not
rebuild itself again on its own account, and is thereby distinguished from an
irritable organ. Something very similar occurs with crystals. The same chemical
compound, the same salt may often crystallise in two different forms, but so
that the one crystalline form arises only under very definite and narrowly cir-
cumscribed external conditions, and corresponds to an unstable equilibrium of the
molecules ; while the other crystalline form of the same salt is stable. The one
unstable arrangement of the molecules can therefore be transformed by insignificant
external alterations into this second stable form, and this occurs in a manner which
calls to mind very strikingly the phenomena of irritability of organisms. Among the
numerous examples known I will only dwell upon one, because I have myself had
the opportunity of becoming more closely acquainted with it. It was first, however,
described in Gmelin's ' Hatidbuch der Chemie,' B. I., 1843, P- 95- 'Potassium nitrate
(saltpetre) usually crystallises in prisms of the arragonite-form. If, however, a drop of
the potassium nitrate dissolved in water is allowed to evaporate on a glass plate, and
the crystals observed under the microscope as they form, it is noticed that in addition
to crystals of the arragonite form, obtuse rhombohedrons of the calcspar form
also are formed at the edges of the drop. On further growth, as the two kinds of
crystals come into proximity, the rhombohedra become rounded off and dissolve,
because they are more easily soluble, while the prisms of the arragonite-form go on

59!Z LECTURE XXXIV.

growing. If the two kinds of crystals come into direct contact, tlie rhombohedral
ones instantly become cloudy, the surface uneven, and prisms soon grow out
from many points of their margins. On contact with other solid bodies also, the
rhombohedra are transformed if still moist. If the liquid is in very thin films
and dries up around the rhombohedra before they are destroyed, the crystals retain
their form for weeks without efflorescing, and endure moderate pressure of foreign
bodies without change ; but with stronger pressure or scratching, as well as on
mere contact with a prismatic crystal of saltpetre, they become transformed into
the prismatic form — a delicate cloud spreads over the surface, starting from the
point of contact, and they now behave to solid bodies like a heap of fine dust,
remaining at the same time transparent however.'

I myself observed in a group of rhombohedra of potassium nitrate, in mutual
contact in a drop under the microscope, that when a prismatic crystal of the same
salt was pushed into the drop and came in contact with one of the rhombohedra,
the latter not only at once became cloudy, but the rhombohedra which were in
contact with one another were also progressively rendered cloudy. This clouding,
moreover, indicated the breaking up of the rhombohedron into innumerable small
prisms of the arragonite form. The most remarkable point in the matter is,
that it is only contact with saltpetre crystals of the arragonite form which is able to
call forth this alteration in the rhombohedric saltpetre crystals ; for the quadratic
tablets of potassium ferrocyanide and the triclinic prisms of potassium Chromate
may be in contact with the rhombohedral saltpetre crystals in a common mother-
liquor, without any change whatever occurring.

These processes have one point in common with the phenomena of irritability
in organisms ; viz. that an entirely specific and at the same time mechanically
insignificant external influence produces an effect which corresponds to it neither
quantitatively nor qualitatively. And similarly also in many other cases. Thus
Gmelin says (loc. cit.), ' Mercuric iodide (Hgig) crystallises at ordinary temperatures
in red quadratic tables ; on sublimation at a high temperature, on the contrary, in
yellow rhombic tables. The red crystals whenever they are heated become yellow,
and again turn red on cooling. The yellow crystals obtained by sublimation remain
unaltered on cooling, but on rubbing or contact the point touched turns red, and this
colour extends itself through the entire group of crystals, with a movement as if the
mass were animated. The external form of the yellow crystals is here maintained,
while the molecules take up the position of the other crystalline system. Each time
these are heated they become yellow, and again turn red on cooling.'

Among numerous other examples I will only quote the behaviour of sulphur,
mentioned by Roscoe and Schorlemmer. If a hot solution of sulphur in alcohol, oil
of turpentine, or carbon disulphide is quickly cooled, there separate out first a few
monoclinic and then rhombic crystals. The rhombic ones correspond to the stable
equilibrium at ordinary temperatures, since Mitscherlich found that monoclinic sulphur
(which is formed on quickly cooling from a high temperature) slowly passes over at
ordinary temperatures into rhombic, the monoclinic crystal being transformed
into a group of rhombic crystals. This transformation proceeds more rapidly if the
monoclinic crystals are shaken or scraped, or exposed to the sun, and a definite
quantity of heat is set free in the process.

RIGOR IN IRRITABLE ORGANS. 593

I have quoted these remarkable processes occurring in crystals in illustration
of the fact that the same chemical substance is able, according to circumstances,
to have its molecules arranged either in unstable or stable equilibrium, and that
by means of certain external impulses the unstable arrangement may pass over
into the stable one. The majority of the phenomena of irritability, however, give the
impression that in their case also conditions of unstable equilibrium are transformed
into conditions of stable equilibrium by means of small external influences. This
comes out with especial clearness in such cases as the irritable leaves of the Mimosae,
where a light touch or other mechanically insignificant influence suffices to transform
the state of unstable equilibrium of the organ suddenly into a new state, which
we might regard as the stable one ; only of course with the difference already men-
tioned, that in this case, when the stimulus has ceased to act, the new condition returns
again to the previous one — the stable condition returns to the unstable one. There
is, however, yet another series of facts which support this view. Just as the unstable
crystalline form of certain salts continues only within narrow limits of external
conditions, passing over into the stable form when they are transgressed, so also
the irritable condition of an organ exists only within certain Hmits of external
influences, during specially favourable circumstances ; if these are transgressed the
irritable — i.e. the unstable — condition ceases to be, and the organ loses its irritability,
i.e. it assumes a condition of stable equilibrium. It is of special interest for an insight
into the phenomena of irritability to take these facts somewhat closely into con-
sideration: this I did in the year 1863 in a treatise entitled ' Vorübergehende Slarre-
zustände periodisch beweglicher und reizbarer Pflanzenorgmie!

From among the details in the work just mentioned I may take the following state-
ments as examples : —

(i) Temporary cold-rigor occurs in the motile organs of Alimosa pudica, the
conditions being otherwise favourable, when the temperature of the surrounding air
remains for some hours below 15° C. The lower the temperature falls below i5°C.
the more rapidly the rigor sets in ; the first to disappear is the irritability for contact
and shaking, later that for the influence of light, and finally even the spontaneous
periodic movement. When the temperature of the air is below 2 2° C. the lateral
leaflets of Hcdysarum gyrans are rigid, according to Kabsch.

(2) Temporary heat-rigor occurs in Mmosa, in moist air at 40° C. within
I hour; in air at 45° C. within 30 minutes; in air at 49-50° C. within a few minutes.
The irritability returns after a few hours in air at a favourable temperature. In
water the cold-rigor of Mimosa occurs even at a higher temperature (viz. within
15 minutes at 16-17° C.) and the heat-rigor even at a lower temperature (viz. at
36-40° in 15 minutes) than in the air. During the heat-rigor, in air as well as
in water, the leaflets are closed, as after stimulation ; but the stalks are directed
steeply upwards, whereas in the stimulated condition they point downwards.

(3) Temporary dark-rigor. If plants with periodically motile leaves, which
are also irritable for light or shaking, such as Mimosa, Acacia, Trifolium, Phascolus,
Oxalis, are placed in darkness, the spontaneously periodic movements occur, apart
from the changes in position effected by the light stimulus, only so much the more
distinctly, and also the irritability for contact remains at first undisturbed. But this
motile condition disappears completely when the darkness continues for one or several

[3] Qq

594 LECTURE XXXIV.

days ; rigor due to darkness sets in. If then a plant which has passed into the
state of rigor due to darkness is again placed in the light, the motile condition
reappears after several hours, or, according to circumstances, only after several days'
exposure to the light.

Very profound darkness is by no means necessary for inducing this condition
of rigor, however; on the contrary, it sets in when a plant which requires much
light, as Mimosa, remains exposed for several days to deficient illumination, such as
prevails at a distance from the windows in the interior of an ordinary dwelling-room.

In contrast to the rigor due to darkness I have designated the normal condition
of motility caused by the alternation of day and night as Phototonus. From what has
been said, then, such a plant after it has been placed in the dark is found for some
time longer (several hours or even days) in the condition of phototonus, which then
gradually disappears ; in the same way the plant under normal vital conditions is in
the condition of phototonus during the night. A plant which has passed into the
state of rigor due to darkness, on the other hand, retains this rigor for some time
(hours or even days) after being placed in the light. Both conditions of the plant
therefore pass over into one another only slowly.

Even on the setting-in of rigor due to darkness, in the case of jSIimosa the
irritability for shaking disappears first, then the periodically spontaneous movement.
In like manner a Mimosa which has passed quite into the state of darkness-rigor
regains first its periodic movements, and then the irritability.

The position of the various parts of the leaves of Mimosa in rigor due to darkness
is different from that produced by darkness on phototonic plants, and even diff"erent
from that in heat-rigor : in rigor due to darkness the leaves are fully expanded, the
secondary petioles directed downwards, and the primary stalk almost horizontal.

Alterations in the intensity of the light act as stimuli to movement only on healthy
plants which are in a state of phototonus: leaves in a state of rigor due to darkness do
not react to variations in the intensity of the light, until owing to long-continued illu-
mination they have regained the phototonus, when they then become stimulated to move-
ments by changes in the intensity of the light. I convinced myself of this in the case
of Acacia lophaniha. A specimen had been left for five days in the dark, where
for forty-eight hours it had given up nearly every trace of its spontaneous periodic
movements. It was then placed in a window where, the sky being cloudy, it
placed its leaflets decidedly downwards within two hours, and then small changes of
position took place in the secondary petioles also ; in this condition, however, the
plant was nevertheless in a state of darkness-rigor, for when it was placed in the
dark about 12 o'clock (noon) with another plant of the same kind which was in
phototonus, its leaves did not change their position, and its leaflets remained open,
whereas the other one within an hour assumed the most pronounced nocturnal
position and its leaflets closed. Both were then placed in the window, where the
plant in a state of dark-rigor opened its leaves in one hour, the sky being cloudy.
In the evening of this day the lower six leaves remained still rigid and open, but the
upper eight or nine leaves were closed : next morning, however, all the leaves again
expanded to the normal diurnal position. Trifolium iticarnaium behaves in a similar
manner, although differing in details.

It is to be noticed that in the case of the plants which I have observed, the

RIGOR DUE TO DARKNESS, DROUGHT, ETC. 595

positions of the leaves in rigor due to darkness resemble the diurnal position much
more than the nocturnal position of those of phototonic plants.

(4) Temporary drought-rigor. I have only observed this in plants of Mimosa
pudica. If the soil in the pots in which they are growing is left for some time
without watering, the irritability of the motile organs visibly diminishes as the
drought increases, and an almost complete rigor then sets in, the primary petioles
stand horizontally, and the leaflets are expanded. In this case the leaves which have
become non-irritable to stimuli are neither withered nor drooping ; but watering the
soil causes a return of irritability within two or three hours.

(5) Temporary rigor due to chemical influences. In this category I include
particularly the condition termed by Dutrochet Asphyxia, which occurs in Mimosse
when they remain for a time in the vacuum of the air-pump. During the exhaustion
the leaves become folded together, probably in consequence of the shaking ; then the
leaflets expand, the petioles become erect, and while the leaves assume a position
similar to that in rigor due to darkness, they remain rigid, and are neither periodically
motile nor irritable to shaking. When brought into the air the plant again becomes
motile. It can scarcely be doubted that the vacuum causes the rigor essentially
by withdrawing the oxygen of the air and therefore suspending respiration.

Kabsch confirmed these statements, and showed that the stamens of Berberis,
Mahonia, and Helianthe?}ium also lose their irritability in a vacuum, regaining it
in the air.

The disappearance of irritability of the stamens mentioned, in nitrogen
and hydrogen, is according to Kabsch probably also to be referred to the
mere suspension of respiration: the irritability returns on the access of air. It
must, on the contrary, be, regarded as a positively injurious chemical eff'ect —
poisoning — when the irritability of the stamens of Berberis disappears in pure
carbon dioxide, or in air containing more than 40 °l^ of that gas, as the same
observer states to be the case. If they remain for three or four hours in carbon
dioxide the irritability returns only after several hours in the air. Carbonic oxide
gas mixed with air to the extent of 20-25 % 'annihilates' the irritabiHty; whereas
nitrous oxide gas behaves indifferently. The stamens placed in nitric oxide,
on the contrary, become bent after i| to 2 minutes towards the pistil, and lose
their irritability. Ammonia gas appears to produce a transitory condition of rigor
after a few minutes.

Even in pure oxygen, according to Kabsch, a condition of rigor sets in
after ^-i hour, from which the stamens recover subsequently in the air.

The vapours of chloroform and ether suspend the irritability of the motile
organs (for variations of light also?) without destroying the hfe, if the eff'ect
does not continue too long. If entire plants or separated branches of IMimosa
are placed in an atmosphere highly charged with these vapours, the irritability
may disappear even in a few minutes. If the organs have been previously
stimulated, they nevertheless now rise up (without being irritable), at the same
time becoming more rigid. The action of the vapours of chloroform or ether
is a purely local one, only the organs directly exposed to them losing their
irritability.

(6) By means of frequent stimulations (vibrations) repeated at short intervals

Q q 2

596 LECTURE XXXIV.

the pulvini of Mimos» are put into a condition in which they do not respond
to stimuh, although they rise during the continued stimulation, and assume a
position of rest as if they had been left alone after the first shock. The irritability
for new shocks — i. e. descent under their influence — does not begin to be restored
until 5-10 minutes after the cessation of the shocks.

(7) Temporary rigor due to electric influence was found by Kabsch to occur
in the gynostemium of Stylidium. A feeble current acted as a stimulus like
vibrations; a stronger one caused a loss of irritability, which returned again,
however, after half-an-hour. In Hedysarum gyrans, on the other hand, the leaflets
which had become immovable in cold-rigor (at 22° C.) were set in movement by
the action of induction currents.

There are two other observations regarding stimulation which have led to
particular remark, and even astonishment; these are the propagation of a stimulus,
and the after-eff"ects of it. Careful consideration, especially based on the phenomena
described at the beginning of the present lecture, shows, however, that both the
propagation and the after-effect of a stimulus can be by no means accidental
peculiarities, but that both must be necessary characters of all phenomena of
irritability.

The propagation of a stimulus, regarded in the first place merely as matter
of fact, is particularly well seen in the Mimosoe and in tendrils. If, for example,
any one of the small leaflets of a leaf, on a shoot of Mimosa with five or six leaves,
is stimulated by means of the hot focus of a burning glass, all the other leaflets of
the same leaf gradually fold together, and after a short time the large motile
organ at the base of the main petiole also becomes bent, and again after a few
seconds the stimulation extends to the nearest neighbouring leaf, then to the succeed-
ing one, and so on, till at last all the leaves of the shoot have made the movement :
this of course only happens if the Mimosa is in a condition of the highest irritability.
In the case of long tendrils the propagation of the stimulus is effected in such a way
that a curvature results first at that point of the tendril which is immediately in
contact with a solid body ; the curving action proceeds from this point, however,
both upwards and downwards in the tendril. The latter then forms, in the course of
a few minutes or hours, a wide loop which narrows more and more, and closes on
the thin support : then follows the free end of the tendril, which winds itself fast
round the support, and finally, after several hours, the stimulation makes itself
evident also in that part of the tendril which lies between the clasped support
and the base of the tendril. This portion becomes rolled up spirally like a
corkscrew.

These examples illustrate particularly clearly both the propagation in space and
the after-effect in time of a stimulus. As another example, especially concerning the
after-effect, I may refer in anticipation to the heliotropic and geotropic curvatures.
If, for instance, an erect shoot-axis growing vigorously in length (e.g. seedling
stems of Buckwheat, Indian Cress, Mustard, the flowering scape of the Crown
Imperial, &c.) is illuminated from one side, even for only one or two hours, when
there is at first no very noticeable curvature, and if the plants are then left for
a time in profound darkness, the curvature towards the previous source of light
already introduced continues for some hours, and may become very considerable.

PROPAGATION OF A STIMULUS ; AFTER-EFFECT, ETC. 597

The case is very similar with geotropic cftects: if an erect growing stem (e.g. of
Dipsacus) is allowed to remain for one or two hours in a horizontal position, here
also the curvature is as yet scarcely noticeable. If the shoot is now placed erect,
and thus brought into a position where the stimulation due to gravitation ceases, the
stimulation already introduced during the horizontal position nevertheless goes
on acting; the stem continues to curve for several hours, the concavity becoming
more and more pronounced on that side which was turned upwards in the previous
horizontal position. And no doubt in the case of every phenomenon of irritability
there is not only a propagation in space of the stimulus, but also an after-effect in
time, although not always so evident as in these cases.

As regards the explanation of these phenomena, it may be said quite gene-
rally, first, respectfng the after-effect in time, that every effect whatever in Nature
occupies a certain time — in fact Time itself is nothing more than the course of
natural phenomena. When a cannon is fired off at 1000 meters distance, the flash
of light is seen almost simultaneously, because the movements of light travel so
exceedingly rapidly'* the report however is not perceived until several seconds later,
because the waves of sound travel much more slowly. The interval of time lying
between cause and effect depends on the properties of the medium in which the
effect is propagated. And the same is the case with the phenomena of irritability ;
it depends on the irritable structure of an organ whether the external stimulus or
shock sets it in motion suddenly — i.e. in a very short time — or whether the given
shock requires a longer time for its action. If the leaf of a Mimosa is irritable in a
high degree, a vibration acts almost instantaneously, and the same is the case with a
leaf of DioncEa; but if these organs are feebly — i. e. less— irritable, then the movement
caused by the shock is slow. Or, as we may also say, the more unstable the
equilibrium in the molecular structure of the irritable organ is, the more rapidly will
any shock acting as a stimulus set the organ in motion. Nevertheless it may also
depend upon other circumstances how much time intervenes between the external
influence and the subsequent visible stimulation : the more complicated the mutually
acting causes and effects are which finally bring about the visible stimulation move-
ment the longer will be the time consumed in the matter. Since then the effects of
stimulation in general usually depend upon very complicated chains of causes, they also
as a rule make their appearance slowly, and this is especially true of most of the stimu-
lations occurring in plants. It was this very slowness which caused the general fact
of the irritability of plants to be so long overlooked, and subsequently under-estimated.

If the effects induced by contact, vibration, alterations of light and tem-
perature, &c. occurred with the same rapidity in plants as do the corresponding
stimulations in animals, plants would appear no less irritable than animals ; and if we
imagine the stimulations which are continually occurring in plants to take place a
hundred times as rapidly as they really do, our gardens, fields, and meadows would
present very strange and uncanny movements.

The rapidity with which stimuli call forth their corresponding effects is in
animals, as a rule, much greater. Between the opening of the closed eye and the
perception of light scarcely any time at all seems to pass, and in like manner a sudden
\vDunding of the finger seems to call forth its reflex motion instantaneously. But no
"physiologist doubts that in both cases a small interval of lime elapses, and compared

59B

LECTURE XXXIV.

with the enormous velocities with which cause and effect may follow one another in
the domain of pure physics and chemistry, even the perceptions of our senses are but
very slow processes.

Respecting the propagation in space of stimuli in plants, then, we may here
refer to quite similar relations among purely physical processes, A physical etfect is
scarcely ever confined exactly to the spot where the effective cause acts, and this is to
be explained simply from the fact that the material parts of the body in which the
action takes place are mutually held together and arranged by means of forces. If a
stretched string is jerked at one point, the whole string vibrates ; if one end of a long
train of gun-powder is set on fire, the whole train explodes progressively, and so on.
And we have to conceive of the propagation of a stimulus in an organ in a similar
manner. If, for example, a single place of an irritable tendril is stimulated by contact
with a solid body, this means that at this spot the unstable equilibrium in the molecular
structure of the organ is disturbed ; but the neighbouring parts are also connected with
those immediately disturbed, and stand in a certain equilibrium with these, so that if this
equilibrium is disturbed at one point, the disturbance is propagated cto the neighbouring
points also, and the propagated disturbance acts again and again on neighbouring
points, so that at last parts of the organ far distant from the points originally
stimulated are set in motion; and it is just in these things that the previously
mentioned disproportionality between stimulus and stimulation comes out particularly
clearly. But in this respect also the effects of stimulation in animals usually surpass
those in plants. In tendrils, irritable Mimosae, and in many other cases, the propagation
of the stimulus proceeds only exceedingly slowly : several seconds, minutes, or even
hours pass before the local stimulation has traversed a path of lo to 20 or 30 cm.
This takes place much more rapidly in the case of animals, where, moreover, special
organs exist — viz. the nerves — for the rapid propagation of the stimulations : these
are wanting in plants. But even in the nerves of man himself the stimulus advances
only about 30 meters per second, and this must be termed an exceedingly slow
movement, compared with the inconceivable velocity of an electric current or of a ray
of light, or even compared only with the velocity of sound in air.

We may, as already mentioned, regard the numerous periodic movements in the
vegetable kingdom as a special category of phenomena of irritability. In cases
where the external conditions are quite constant, where the temperature, light or dark-
ness, or the degree of moisture is perfectly constant, where no vibration occurs, and
so forth, periodically motile organs, as found particularly often among leaves, perform
movements of such a kind as to bend somedmes to one, sometimes to the other side,
either with considerable velocity or very slowly. Thus we find continual vibrations
to and fro in the case of the small lateral leaflets of the compound leaf of Hedy-
sarum gyrans (an Indian papilionaceous plant allied to our Sainfoin) when the
temperature and illumination are high and constant; and in the same way the
leaves oi Mimosa, Oxalis, Trifolium, Acacia , and other plants go on moving in constant
darkness — i. e. their motile organs slowly bend upwards and downwards by turns.

The most astonishing point in such cases, to those unacquainted with science,
lies in the fact that effects are here taking place for which no apparent causes are
present. When a movement is induced by a mechanical shock, however slight, or by
a mere alteration in the intensity of the light or of the temperature, &c., this obviously

SPONTANEOUS PERIODIC MOVEMENTS. 599

accords with our requirements of causality ; but in the processes mentioned that is
simply not the case, and it is just this which must impel us to seek for the causes
or shocks which produce the movements named. At present, however, we can only
offer suggestions in this connection. We may, if need be, imagine that by means
of the continually recurring chemical processes in the living parts of plants,
changes are produced in the state of the protoplasm, and by this again in the
turgescence of the cells, and if the latter are more pronounced sometimes on the one,
sometimes on the other side, the parts of the organ concerned must bend alternately
to and fro. Now this is of course simply a hypothesis, because we are seeking
a cause which renders these phenomena explicable. We have it is true already
met with the fact that the excretion of water in decapitated root-stocks
exhibits a periodically varying course, which is apparently independent of ex-
ternal impulses, and in the same way we found a periodicity in growth in length ;
and this presents very great similarity with the periodic movements referred to,
i. e. the periodically varying nutations of growing stems, tendrils, and so forth, for
which likewise external impulses are wanting, or at least appear to be wanting. We
thus find ourselves here on the boundary of a still very obscure domain of science, and
in such cases it is often an advantage even to be able to avoid the grosser errors. But
such an error would decidedly exist in the assumption that periodic phenomena must
necessarily be due to periodically varying causes — an error which, as it almost seems,
prevails in the whole literature of this province, although there is by no means a lack
of examples showing that, in the domain of mechanical and physical science, periodic
movements are produced by constant causes which do not vary periodically. The
periodic vibrations up and down of the haulms in a field of corn are due to the
pressure of a constant current of wind and the constant elasticity of the haulms,
and thus we have the interesting phenomenon of wave-motion in a field of corn. In
the same way the periodically varying action of a hydraulic ram is effected by the
constant flowing of water into the machine itself, other circumstances also being
constant. Nay, even the periodic alternations of day and night and of winter and
summer arise from the constant revolution of the earth, and its constant course
round the sun.

It would lead us into very tedious and difficult developments of ideas if I
were to attempt to show how periodically varying phenomena must proceed from
causes which act constantly; it suffices here to make the fact evident as such,
because it protects us from the error of regarding it as necessary to refer the periodic
movements of many organs of plants to a periodic variation of their causes. There
are, however, also many other likewise periodic changes in the vegetable kingdom,
which can be easily referred to definite periodically varying causes : for instance, the
so-called sleep-movements of leaves, and the periodic opening and closing of m.any
flowers. In the case of the phenomena here under consideration it is not so,
however.

Nevertheless, we are warranted in regarding the so-called spontaneous or
independent periodic movements as phenomena of irritability, just as animal phy-
siologists place the periodic pulsations of the heart in the series of phenomena of
animal irritability. While it is in other cases the first object of research to under-
stand the course of a manifestation of irritability from a known external impulse.

600 LECTURE XXXIV.

in the present case on the contrary the first problem is to seek for the impulse,
because we premise on the basis of the principle of causality that such an impulse
must actually exist. The preceding considerations will at least have made it clear
that the constant course of molecular and atomistic processes which constitute
life in general may give rise to periodic alterations in the interior of the cells,
and we may regard these latter again simply as stimuli, from which the visible
periodic movements arise as effects.

I have repeatedly had cause to refer to certain resemblances between the
phenomena of irritability in the vegetable kingdom and those of the animal body,
thus touching a province of investigation which has hitherto been far too little
cultivated. In the last instance, indeed, I might say animal and vegetable life
must of necessity agree in all essential points, including the phenomena of irri-
tability also, since it is established that the animal organism is constructed entirely
and simply from the organic substances produced by plants, and ultimately it is
simply from the properties of these substances that all vital movements both of
plants and animals are to be explained. Since it is possible for not only other
animals but even man to be nourished by seeds and tubers of plants, and the
substance of the human body produced by this nutriment is able to carry out all
sensitive perceptions, all the periodic motions of the heart, and finally also the
functions of the entire nervous system, including those of the brain, we must refer
it to the properties of those substances which have been produced by plants from
mineral matters, water, and carbon dioxide under the influence of sunlight.

Returning from these general considerations to definite comparisons between
the animal and the plant, I would make special mention of that exceedingly
remarkable phenomenon in animal hfe, termed by its great discoverer, Johannes
Müller, the specific energies of the sensory nerves. As is well known, we under-
stand by this the fact that for instance the optic nerve responds to any given
excitation whatever with the sensation of light : true, this sensation is as a rule called
forth by the vibrations of the luminiferous ether, but even electric currents or mere
concussion or diseased conditions impel the optic nerve to the sensation of light.
In the same way the auditory nerve is impelled to the perception of sound not
merely by waves of sound, but by every change which aff"ects it, and similarly
with the remaining organs of sense.

Now 1 pointed out several years ago that even the organs of plants are
provided with similar specific energies. Irritable organs in plants are indeed, like
the sense-organs of animals, sensitive to a definite category of stimuli, but they
can very often be affected by other stimuli also, and in this case the stimula-
tion is always the' same. This appears most distinctly, for example, in the case
of growing internodes and leaves. If they are illuminated from one side they
become curved, and if brought out of their normal position they are caused to
make exactly similar curvatures : the one mode possible for responding to any
stimulus whatever is simply this curving. The matter only obtains its full signi-
ficance by means of the fact, which I rendered clear, that every individual plant-
organ responds to the influence of light as well as to that of gravitation in a manner
specifically peculiar to it, and it is upon this that the previously mentioned ani?otropy
of the parts of plants depends. No less clear is the specific energy of tendrils : as

SPECIFIC ENERGY OF IRRITABLE ORGANS. 6oi

a rule their irritable movement results from gentle contact with a solid body, but
vibration as well as cutting off or burning the apex of the tendril causes its upper side
to curve, since it grows more rapidly than the lower side, and these curvatures only
differ in the different cases because external mechanical conditions influence the
course of curvature. The identity of the effect of stimulation in cases where totally
different stimuli act on the growing root-tips is particularly striking : illuminadon from
one side, geotropic action, pressure on a solid body, the influence of adjacent moist
surfaces, and, as it appears from Elfving's recent researches, even constant electric
currents produce exactly the same kinds of curvatures. The organ possesses only
this one mode of responding to stimuli of the most various kinds. In our subsequent
consideration of the phenomena of irritability in Mimosse, when the details are
more closely examined, we shall moreover be permitted to obtain a deeper insight
into the specific energy, since we shall there be in a position to understand the
mechanism of stimulation generally, and therefore also to comprehend why entirely
difterent stimuli must call forth the same effect in the organ.

In concluding these considerations there are still a few remarks to be made on
the significance of irritability for the life of the plant generally. Clothed in words
somewhat different from those employed at the beginning of the present lecture,
we may define irritability as the mode in which a given • organism, or a definite
organ belonging to it, reacts to the external world. On this continuous reaction
between the external world and organic structure depends generally the life not
only of plants, but of animals also, as was described in detail in Lecture XII.
The organism itself is only the machine, consisting of various parts, and which
must be set in motion by the action of external forces : it depends upon its structure
what effects these external forces produce in it. It would betray a very low
level of scientific culture to see in this comparison a degradation of the organism,
since in a machine, although only constructed by human hands, there lies the result
of the most profound and careful thought and high intelligence, so far as its struc-
ture is concerned, and in it there subsequendy become effective the same forces of
Nature which in other combinations constitute the vital forces of an organ. The
comparison of organic life with inorganic processes can thus only be held as a
debasement of the former, if one has become so foolish as to look upon the latter as
something low and common, whereas the incomprehensible magnitude and all-per-
vading power of Nature is equally evident in both cases.

If then irritability is, as said, fundamentally nothing other than the reaction of
the organs towards the outside world, determined by means of their structure,
which in fact argues by itself a similarity in kind of both, it follows thence directly
that the phenomena of irritability both in the vegetable and animal kingdom must in
the main be full of purpose. What I understand by this has already been stated at
the conclusion of Lecture I : all those adaptations in the organism are purposeful,
which contribute to its maintenance, and insure its existence. This is not to say
that every individual irritability is at once or absolutely decisive for the life of an
organism, since organisms are fortunately not so perfectly adapted to external
conditions ; but we may probably say that, on the whole, the possibilit}- of life
depends on the co-operation of the irritabilities of the various organs. To
emphasise one point only; how could an ordinary terrestrial plant live if its roots

5o2 LECTURE XXXIV.

were not impelled by various kinds of irritabilities — heliotropism, geotropism, sensi-
tiveness to contact, damp surfaces, &c. — to penetrate into the soil in order to collect
nutritive substances and to obtain a hold-fast, while the assimilating shoots and organs
of reproduction are impelled by irritabilities of other kinds to grow forth above
the substratum and receive the vivifying influence of light, in order to assimilate
and bear fruit?

By this example we are led on to yet another point of great importance,
namely, the fact that by means of the same external influences reactions
exactly opposite in kind, though otherwise similar in nature, may be called
forth, so that we may in fact speak of a positive and a negative heliotropism for
instance ; and a similar contrast is noticeable in the case of other reactions also.
Of course the facts here coming into consideration can only be rendered clear in
succeeding lectures ; but so much may be said even here, in anticipation of more
exact knowledge of them, that if the same external cause induces exactly opposite
effects in the organs, the explanation of this must simply be sought in the diff"erent
structure of the organs. If one organ when illuminated from one side becomes curved
so as to be concave on the side turned towards the source of light, while another
becomes convex on that side, the cause can only lie in the internal structure of the
organ. But it is just on such diff"erences of structure that the great variety of
reactions which the most diff"erent plant-organs exhibit towards the same external
influences depends ; and, fundamentally, all that we term Biology — the modes of life
of organisms— depends upon the fact that diff"erent organs react diff"erently towards
the same external influences, and these reactions diff"er not only qualitatively but also
quantitatively, the finest gradations existing in both cases.

LECTURE XXXV.

IRRITABILITY AND MOBILITY OF PROTOPLASMIC STRUCTURES.

The phenomena to be described here have ah-eady been in part superficially
examined in Lecture VI, but only a few of the chief points, were there touched upon ;
this was necessary in order to give those still unacquainted with the nature of the
vegetable cell an approximate idea of the nature of protoplasm. What has there
been said may therefore serve as an introduction to the present subject, and I may at
once pass on without preliminary explanations to the subject itself. Here also there
is no lack of bewildering variety in the phenomena, and this affects the treatment of
the subject the more, since -we are as yet by no means in a position to refer the
irritabilities and movements of protoplasm to any general principle whatever. At
present it is scarcely possible to speak of a comprehension of the processes from the
point of view of mechanics, indeed in many cases the merely sensible apprehension
of the processes, which are for the most part microscopical in range, presents great
difficulties even to a trained eye. Since in these lectures I avoid on principle entering
into difficult questions with discursive and technical discussions, and while with
respect to this point I refer to the notes at the end of the lecture, I shall select from
the variety of phenomena only a series of such as we may regard as different types.
This will perhaps best conduce to the reader obtaining a clear idea of the matter.

The protoplasmic structures of which the movements and irritability are particu-
larly conspicuous to the observer are the swarm-spores of many Algae and some
Fungi already occasionally referred to, and the antherozoids (also in other respects
similar) of the Mosses and Vascular Cryptogams. Swarm-spores are naked, sharply-
defined protoplasmic bodies, usually of the shape of a fowl's egg : the larger thicker
half, in the case of those Algae which contain chlorophyll, being green, the anterior
narrower moiety colourless. Deviations from this common form, however, appear in
various degrees : the size also differs extremely, the swarm-spores of some Vaucherias,
for example, being visible in a good light even to the unaided eye of a sharp observer,
while the smallest forms can only be seen with high powers of the microscope.
Between these extremes are found all possible degrees of bulk. It may be noted
by the way, though it need concern us no further here, that many swarm-spores are true
sexual organs, while others are not. Some of the minute organisms belonging here
are during their whole period of life in oscillatory motion ; in others this motile stage
is very soon over, they become fixed to some body by the end which swims forwards,

6o4

LECTURE XXXV.

and which now developes into an anchoring root, and then begin to grow. This ques-
tion also has no interest for us just now — we are concerned only with the mobility and
irritability. That this can in no way be due to the contained chlorophyll, follows at
once from the fact that there are also swarm-spores — those of the Fungi — which
contain no chlorophyll ; and in like manner the antherozoids or spermatozoids, like
them so far as motility and irritability are concerned though usually very different in
shape, are devoid of chlorophyll. Here also I exclude those structures which in other
respects resemble swarm-spores in origin and function, in which the movements
of translation are produced partly or wholly by alterations in the form of the body,
and where we have thus to do with creeping movements, the typical form of which,
moreover, we shall subsequently consider in the case of plasmodia. Still more
definitely to be excluded here are the movements of the Oscillatorise, Bacteria, Bacil-
lariae or Diatomacese, and similar cases, since we
are still utterly in the dark even as to the preli-
minaries with respect to the nature of their move-
ment.

The true swarm-spores and spermatozoa thus
do not change their form and size during the
movement. They owe their locomotion to certain
very minute organs which even at the best and
with very high powers are visible only with diffi-
culty : these are the so-called Cilia, which may
be compared as to form and mode of motion to
an oscillating whip-lash, and that is about all that we
perceive in them. As a rule, the cilia are situated
at the narrower anterior end, singly, or more
often in pairs, or fours ; in (Edogoiiiiwi (p. 83,
Fig. 79) a circlet of numerous cilia exists at the
boundary between the hyaline and green portions.
The large swarmers of Vaucheria (p. 108) are
covered with innumerable very short cilia, as with
a dense pile of velvet. Some spermatozoids also, especially those of Equiseium
and the Marsiliacese, possess very numerous cilia, which are also very long. In some
small swarmers, e. g. the spermatozoa of the Vaucherise, one cilium is situated at the
side, the other on the pointed anterior apex, and in the case of the Euglense so
common in foul ponds, and others, there is only a single very long active cilium
at the pointed anterior end : in Chytridium, on the other hand, the single cilium
is at the posterior end, like a rudder. As an especially interesting case again,
mention may be made of the Volvocinese, a subdivision of the Algae : since here-
entire cell-families, consisting of 4, 8, 16, 32 or more individuals formed exactly
like swarm-spores, make their movements in common because they are held
together by a common exceedingly watery delicate cellulose envelope, so that the
family constitutes a quadrangular tablet, or a sphere, or an ellipsoid, from the
surface of which the long whip-like cilia of the individual elements project in pairs
into the water.

The various arrangements of these minute motile organs, which however

Fig. 356.— Swarm-spores (Zoospores) a b
Acetabularia mediltrranea ; c d of Botrydii
grattulatutm (see Fis 2, p. 4) ; ef of Uloth
zonata. a b d/zxe. sexual zoospores, or so-tal
gametes.

SWARAf-SPORES AND ANTHEROZOI DS, ETC.

605

always produce essentially the same kind of movement, shows already that a com-
plicated problem of mechanics is here concerned, since we know almost nothing
further of the movements of cilia than that they make serpentine or lashing move-
ments in the water, from which the motion of the entire body then results. Where
numerous cilia move simultaneously in this way, it is even questionable whether their
oscillations are always simultaneous and alike in character; in fact it is not
necessary to assume this in the case of the production of the ordinary rotating move-
ment of swarmers, since we know from Thuret's investigations that the relatively
large oospheres of the Fucacese are set in rotating motion by the convulsive
movements of the numerous spermatozoa clinging to its periphery — this rotation
evidently results from the very irregular strokes of these extremely minute (in
comparison with the oosphere itself) bodies.

Among the most significant mechanical points is the always considerable size of
the swarm-spores in comparison with the extreme minuteness of the motile organs.

FIG. -iSl-Stephattosphaera pluvialis (after Colin and Wichura). / restinjj spore; II—VIII swarm-spores
produced by the division of /. l^III—X the family produced by division of one swarm-spore ; A'l a mature
family rotating; ..i'//—^/K microspores produced by division of the cells oi XL

the cilia. Of course their work is essentially facilitated by the fact that the swarm-
spore possesses nearly the same specific gravity as the water; but it is obvious that it
must always be a little heavier, owing to the specifically heavier proteid substance
of the protoplasm. Dead swarm-spores therefore always sink to the bottom. The
swimming itself, so far as it only concerns suspension in the water, is thus work
done by the cilia, but the friction which must necessarily take place during the
rotation and progressive motion at the surfaces of the swarm-spore is perhaps
greater even than this. From all that is known so far we must probably ascribe
to the cilia also the irritability of which we shall consider the different manifestations
later on.

The form of movement which all the objects here under consideration exhibit con-
sists in that in the first place they rotate on their own longitudinal axes, usually swim-
ming forwards at the same time; the movement is thus approximately that of a planet
with its diurnal rotation and simultaneous flight away into space, or that of a shot

6o6 LECTURE XXXV.

which is discharged through the air from a rifled barrel. Naegeli seems to have been
the first not only to exactly study the motion of swarm-spores, but also to criticise
them from the physical and mechanical point of view. It may therefore be of service
to the reader to be made acquainted with his exact expressions as to the apparent
and true velocity in the progressive movement of swarm-spores.

' The movement of swarm-cells,' says Naegeli ^ ' is usually described as very
active, and the rapidity with which they bustle about is no trivial reason for their
having been designated animals. In this it has often been forgotten that one is
looking through the lenses of the microscope, and that the swarm-cells are in
reality much less active than they appear to be. When we observe them with a
power of 300 diameters, it is not only that the cells appear to us 300 times
larger, but the motion also appears 300 times more rapid, since the space
traversed in a given time is in- fact also increased under the microscope 300
fold. In a watch laid under the microscope, with a power of 100 diameters we
see the apex of the long hand engaged in fairly rapid trembling and jerking move-
ments, while the apex of the short hand proceeds extremely slowly, the movement
scarcely perceptible. I observed the slowest continuous movement of the cell
contents in Char a when the temperature of the water was i°C., the most rapid
in the same plant when the temperature of the water was 37°C. The length of
one foot, if the motion is calculated to that measure, would be passed over in
the first case in fifty hours, in the second case in thirty minutes. Diatomacege
at the ordinary temperature of the laboratory traverse one foot in 14-21 hours;
they thus move about six times more slowly than the apex of the long hand of a
watch. Swarrn-cells take mostly about one hour, the most rapid only a quarter
of an hour to pass over a distance of one foot. The most nimble equal in their
movements the apex of the long hand of a clock, the face of which has a
diameter of one foot and a-quarter, and would be left far behind by the laziest of
snails^. Without magnification, even if the little plants were quite visible, their
movement would not be seen, on account of its slowness. — The Infusoria move
but little faster than the vegetable cells. Instead of the active animal movement
of the latter, it would be more correct to speak of the slow plant-like movement
of the former.

' Whether the movement of a body appears to us rapid or slow, however,
depends also on the relation between its size and the space passed over in a
definite time. If an elephant and a mouse traverse an equal distance in the
same time, we call the first slow, the second quick. A man in walking passes
over somewhat more than half his length in one second. The most rapid swarm-
cell traverses in the same time a distance which is two and a half times as great as its
diameter; the Diatomacege only the one-tenth part of their length, and short fila-
ments of Oscillatoria merely the one-hundredth part of their length, longer ones
much less.'

According to Naegeli, swarm-cells have three forms of movement; in many

1 Naegeli, 'Die Bewegung im Pflanzenreich^ in his ' Beitr. zur wissensch. Bot.,' Heft. 2, i860,

P- 13-

^ The movement of swarm-spores is thus slower than that of a particle of combined water and
lithium which ascends in the wood during active transpiration.

THE RAPIDITY OF MOVEMENT OF SWARM-SPORES, ETC. 607

of them the anterior end (hyaline and provided with vibrating ciha) as well as
the posterior green end of the axis of the body remains exactly in the path of
progressive motion, be this straight or curved — they swim forwards rigid, and without
deviations. Others describe a direct or more or less curved spiral line, a rotation
on the axis of the body always corresponding to a spiral revolution, so that the same
side of the cell is always turned outw^ards while the axis of the body runs parallel with
the axis of the spiral path. Thirdly, there are swarmers the anterior end of which
describes a spiral line, while the posterior end describes a straight one, or a spiral
with a smaller diameter. These forms of motion are only to be detected when the
rotation and progression are slow. The motions of spermatozoids essentially agree
with these, according to Naegeli, and he is convinced that if the form were regular
and the mass distributed symmetrically, the cells would swim in a direct line in a
homogeneous medium, and that all deviations from this and the simple rotation round
the proper axis of the body depend upon the fact that the moving bodies are not sym-
metrically constructed, that their centres of gravity are not in the centre, and that they
do not experience equal friction all round. The direction of the rotation is usually
constant for each species, genus, or family ; nevertheless there are exceptions, as in
the tablets of Goiiium. It is often impossible, however, to convince one's-self of the
direction of rotation in the case of unicellular swarmers ; and this, as Naegeli shows,
depends on a peculiar optical illusion not yet understood {Tetraspora lubrica). The
end which carries the cilia usually goes forwards, but it may also go backwards, and
it then rotates in the contrary direction {Uloihrix speciosa). This reversion occurs
when the swarmers rebound : they then rotate for a time at one spot, stand still,
and go back (without turning round the body). The reversal of the rotation only
takes place, however, in so far that the end bearing the cilia is always regarded as
the anterior one ; if, on the contrary, the end which is in front during the pro-
gressive movement (even w^hen reversed) is termed anterior, then the direction of
rotation is always the same, the backward movement always continues for a short
time only, and is soon exchanged again for the usual one.

The progressive and rotating movements stand also with respect to their velocity
in a relation not exactly determined, and both are, according to Naegeli, apparently
due to the same cause. As a rule the one increases or decreases with the other ; but
if a swarm-cell strikes anything it remains at the spot, but continues to rotate, and
occasionally one may be seen to move forwards without revolving. In the absence of
all obstructions, however, both movements appear to be always combined. On the
other hand, cells are also seen which, with an equal number of revolutions in the unit
of time, swim forw^ards with unequal rapidity, or with equally rapid progress revolve
with unequal rapidity. — In these matters there are evidently individual differences
(depending upon the organisation). Swarm-cells occurring in the field of view at the
same time, and thus exposed to the same external conditions, move with unequal
velocities: the cells oi Tetraspora lubrica, for instance, traverse at i4°C. a space of
one-fifth of a mm. in I-2-2-4 seconds, and revolve, striking against the upper or lower
glass plate, once in 0-3-1 -8 seconds. — Heat also accelerates these movements of
protoplasmic structures. According to Unger, the swaim-sporcs of Vaucheria
traversed a distance of one inch in 63-65 seconds.

The fact must not be overlooked that swarm-spores and spermatozoids com-

6o8

LECTURE XXXV

mence their movements before they are yet free. In dividing Palmellaceaa I have
seen the movements begin in the early morning long before the division was finished ;
the daughter-cells still hanging together in the middle trembled actively and
began to swarm inside the mother - cell. The swarming is no doubt on the
whole the same movement as that of the free swimming cells, only restricted by

the continual impact of the cells
one on the other within the confined
space of the mother cell-wall. A
similar phenomenon exists in the
movement of the spermatozoids
within their cell-wall, before they
break through it. Thuret says of
those of the Characege, ' The anthero-
zoids are seen to be agitated and
to twist in all directions within the
segments (cell-segments of the an-
theridia) which enclose them ; after
several more or less prolonged
efforts, they escape to the exterior
by a sudden movement, like the
elastic rebound of a spring.' In
the case of Pelh'a, the Ferns, and
Piltilana, according to Hofmeister,
the spermatozoid also revolves before
it escapes from its cell-membrane.

The movements of swarm-spores,
since they are true vital pheno-
mena, only take place between cer-
tain limits of temperature, and that
an optimum temperature must exist
for them at which the maximum
of motility occurs, is only a special
case of what was stated generally
in Lecture XII: I shall therefore
not go further into that matter.
The irritability of swarm-spores for
light, however, belongs not only to
the most remarkable phenomena of
irritability in the vegetable king-
dom, but also, from the most
recent investigations of Stahl and
Strasburger, to those best known.
Before giving the most important results, however, I have still to mention a
fact which comes of necessity into consideration here, and which was established
by me about six years ago : this concerns the purely passive movements which
the swarm-spores undergo under certain circumstances, simply from currents in

F'g- 35^5 — 'Warm-cliamber for the examination with the microscope
of the movements of protoplasmic structures at different and constant
temperatures. The chamber is constructed of zinc, and possesses double
walls below and at the four sides, the space between being filled with
water ; it is open above to facilitate the insertion of the microscope m,
the fine adjustment-screw of which appears at s; dd lid; / window
which takes coloured or colourless glass ; / thermometer.

SWARM-SPORES AND EMULSION-FIGURES. 609

the water, and the result of which was formerly also ascribed to their active
movements '.

In stagnant pools, ponds, hollows in stones, &c., the water is often found,
especially in the spring time, to be coloured of a more or less intense green by
innumerable swarm-spores. If such green water is poured into an ordinary plate
or other shallow vessel, and placed in the neighbourhood of a window, it is
observed after some time that almost all the swarm-spores are collected at the margin
of the vessel turned towards the window; occasionally, as shown in Fig. 359^,
there appears at the same time, floating in the middle of the liquid, a green figure,
the apex of which is turned towards the window. If the fluid contains at the
same time large and small swarm-spores (macro- and micro-zoospores), e. g. of
Hcemafococciis plmn'alis, &c., all the micro-zoospores are found after a little time at
the margin of the water turned towards the window, and also at the surface ;
while the macro-zoospores are collected at the margin of ihe vessel which is turned
away from the window, and also at the bottom of the water. If, on the contrary,
the vessel referred to is allowed to stand in the middle of a room where the
temperature is equable, or even outside the building where the temperature is
everywhere alike, cloud-like aggregations of swarm- spores are formed in the centre
of the liquid, and these may present the most various shapes, differing, however,
from the preceding in that neither the outer nor the inner margin of the vessel
seems to be preferred. On the contrary, the swarm-spores form cloud-like col-
lections, as rounded groups extending from the surface to the bottom of the
liquid, or as concentric circular clouds coexistent with radiating and rounded
groups, as in Fig. 359 B, or even as reticulate figures extended over the whole
vessel.

I found now that both kinds of aggregation of swarm-spores may occur in the
fluid in the most varied manner, and that they appear also when the vessel is covered
with a glass bell-jar or with an opaque card-board receptacle. Light, as such,
exerts no influence on the formation of these figures ; on the other hand, it is the
currents formed in the water by the warming and cooling which produce the
aggregations of swarm-spores referred to. If the vessel in question stands at a
window, and especially when it is cool or cold outside while the chamber is heated
by a fire, then a continual streaming of the water goes on in the plate or vessel,
so that the cooled water at the margin of the vessel next the window sinks to
the bottom, flows thence to the opposite margin, and then ascends and streams
on the surface as warmer water to the margin of the vessel next the window again.
This continually circulating current of water carries the swimming swarm-spores
with it, and causes, in combination with their active movement, their collection in the
manner described, either at the surface at the colder margin next the window, or at
the bottom of the vessel at the warmer margin turned towards the chamber. That
it is hexe actually and simply a matter of a diff"erence in temperature between the
two margins of the vessel, and not of illumination, follows not only from the above-
mentioned fact that the phenomenon occurs even under an opaque receptacle, but

* Sachs' ' Über Emiihioiisßgurcn und Grtippiniug der Schwärinspcreii im U'asser,' Flora,
1876, p, 241.

[3] Rr

6io

LECTURE XXXV.

also from the fact that the aggregations of swarm-spores at the margins, just
mentioned, can be induced by placing the vessel with the one edge on a cold body
and the opposite one on a warm body: it matters not w-hether the light here
cooperates at all or falls in any given direction, or not, the superficial collection
of swarm-spores at the margin is always formed where the water sinks in the

vessel at the colder place. In like
manner the rounded groups mentioned
above, networks, concentric clouds, &c.
(Fig. 359 B), are produced quite in-
dependently of the light, when the tem-
perature of the vessel is the same on
all sides, by the bottom of the vessel
having a different temperature from its
surface, and particularly if the latter
is uncovered and evaporation takes
place ; vertical currents are thus pro-
duced, ascending and descending in
the liquid, and the contained swarm-
spores collect at the relatively still
places. I was able to produce all
these phenomena in exactly the same
way by employing, instead of water
containing swarm-spores, a mixture of
water and alcohol in which coloured
olive-oil was distributed in the form
of very small drops, simply taking
care that these fine drops of oil
were a little lighter or a little heavier
than the mixture of alcohol and
water.

Quite apart from these currents pro-
duced by differences of temperature in
the liquid, and by which the swarm-
spores are passively carried and caused
to form the above-named cloud-like ag-

FlG. 359.— Emulsion-figures. Oil coloured with alkanet was tlio- _ _

roughly shaken up with a mixture of water and alcohol of nearly grCgatioUS in thc VCSScl, thc SWarm-

equal (somewhat greater) specific gravity, so that a fine emulsion was "

produced ; and this was poured into a plate. W arrangement of SpOreS thcmSClvCS pOSSCSS a pCCUHar
the oildrops after a short time when one edge of the plate (the _ _ _ _ _

upper in the figure) is cooler than the other. B when the plate is irritability for light, wWch COUSlstS in that
equally warm on all sides, but the bottom of the liquid warmer than ...

the upper surface. in their swimming movements, which

moreover occur also in the absence of
light, they pursue a definite course, and this so that they either move away from the
source of light, or in the contrary direction, and in the line of the rays of light.
If a large number of similar swarm-spores are contained in a drop of water under
the microscope, and if they are suddenly illuminated from one side, they all swim
towards the source of light or away from it. This is the phenomenon which
has been studied in detail by Stahl and Strasburger, and I may take yet another

EFFECTS OF LIGHT ON SIVARM-SPORES. 6ll

series of special facts from the statements of the latter \ He investigated chiefly
the swarm-spores of Hcematococcus lacusin's, Uloihrix zona/a, Chceiomorpha cerea,
and of Ulva^ Botrydnim granulatum, Bryopsis, and other Algae, and especially those
of Chytridium, an Alga devoid of chlorophyll. Walz had already discovered in 1868,
and Cornu had confirmed the discovery, that the final formation and expulsion of the
sv^^arm-spores and antherozoids from their mother-cells is dependent not only upon a
proper temperature, but also upon a sufficient quantity of oxygen dissolved in the
water ; but light also promotes the birth of these small organs, as Thuret had
already discovered, so that, in fact, by keeping the Algae concerned in the dark and
then suddenly illuminating them, the moment of swarming can be artificially in-
duced. In the natural course of events, the dependence upon light referred to causes
the swarm-spores to escape from their receptacles and commence to swarm in the early
morning, as a rule at definite hours — i.e. when the light has attained a certain intensity.

Strasburger describes the action of the light as follows : if particularly striking
phenomena are desired, it is well to begin the experiments with the swarm-spores of
Botrydhan gramdahim, the Alga figured on page 4 (Fig. 2). A preparation made one
day before-hand by sowing the spores, when brought from the dark into the light,
shows that at the moment of observation, all the swarm-spores are equally distributed
in the drop; nevertheless they instantly become directed with their anterior ends
towards the source of light, and now rush towards it in straight but otherwise fairly
parallel paths. After a few — usually ig to 2 — minutes, almost all the swarm-spores
have arrived at the illuminated side of the drop, and here swarm together in and
out. If the preparation is turned round so that its illuminated side is now away from
the source of light, all the swarm-spores which are still in motion instantly leave the
margin of the drop, now turned away from the source of light, and rush to what is
now its illuminated margin.

For the sake of brevity the margin of the liquid which is turned towards
the source of light may be termed positive, and the opposite one negative.

' If the swarm-spores of Ulothrix are selected for observation, the phenomenon
becomes in a certain sense still more striking. These also rush rapidly and in
almost straight paths to the positive margin of the drop, but it is only seldom that all
do this ; in most preparations, on the contrary, a larger or smaller portion of them
are seen to move equally rapidly in the opposite direction, and therefore towards the
negative margin. It is possible then to see a curious spectacle — the swarm-spores thus
rushing in opposite directions, and therefore with apparently double velocity, past one
another. If the preparation is turned round through 180°, the swarm-spores collected
at what was previously the positive side are seen to rush at once to the negative
side, and those previously collected at the negative side to rush to the positive side.
Arrived here, the swarm-spores move about in and out, keeping more or less close to
the margin, according to the preparation. One continually notices also, both at the
positive and at the negative side, individual swarm-spores suddenly abandon the margin
and rush directly through the drop to the opposite margin. Such an interchange
continually goes on between the two margins : nay, it not rarely occurs that single
swarm-spores just arrived from the opposite margin again return thence. Yet others

• Strasburger, ' Wirkung des Lichtes und der JVän/ie auf Sch'wäriits/'orcii,'' Jena, 1S78. Stahl
in ' Verhandl. der med.-phys. Ges. Wzbg.,' 1879.

R r 2

6l2 LECTURE XXXV.

remain stationary in the middle of their course, and then rush back to the point they
started from, eventually repeating the play to and fro for some time, like a pendulum.'

The phenomena are usually less striking in the case of Hccmatococcus. In this
form, as well as in Ulothrix, however, Strasburger noticed that the smallest swarm-
spores exhibit the most rapid movement, and the colourless ones of Chytridium vorax
behaved exactly like those of Hccmatococcus., whereas other colourless swarm- spores,
such as those of the Saprolegniae, were not affected by light.

One of the most important results of Strasburger's and Stahl's investigations is
the discovery that there are swarm-spores w-hich constantly rush towards the source
of light only, as well as such which move towards or from it, the ciliated end moving
forwards in the one or the other direction. In this matter the intensity of the light,
again, plays a very important part ; usually so that when the intensity of the light is
less the swarm-spores move towards the source of light, when stronger away from it.
This may be confirmed either by the observer moving his preparation further and
further from the window, or by remaining there and interpolating translucent shades.
If the light is strong, and the swarm-spores are collected at the negative margin of
the drop, a point at which the light is less intense is thus attained, at which they begin
to swim towards the positive margin. Another property of swarm-spores here comes
into action, which Strasburger designates their Phototonus — a property which even
with swarm-spores of the same species may vary with the time, and which consists in
that the reactions mentioned come into play only when the light reaches a higher or
lower intensity. If I understand Strasburger's statements correctly, however, this
phototonus is fundamentally nothing other than a lower or higher degree of irrita-
bility, of such a kind that swarmers which are adapted to low degrees of light are
sensitive to light of low intensity.

The rapidity of the movement of swarm-spores, as Naegeli had already stated,
appears not to be influenced by light ; but Strasburger asserts that the swarmers move
in lines the more direct the more intense the ray of light which directs them is.

Although the mechanics of the movement in all these cases, just as in the case
of the heliotropism of unicellular and multicellular plants, remains unknown to us,
it is nevertheless to be insisted upon that even in the latter, exactly similar degrees
of sensitiveness to light appear, and especially I have long ago established that
shoot-axes of the same plant — e. g. Tropccolum ??iajus — may be negatively heliotropic
in intense light, and positively heliotropic in feeble light.

It should not be omitted that, according to Strasburger, the influence of light
in directing the movements of swarm-spores is the less the larger they are. To the
largest known swarm-spores in particular, he denies all irritability to light whatever,
as Thuret had already done also. Nevertheless there are exceptions : the small
yellow swarm- spores of Bryopsis appear not to be sensitive to light, whereas the
large green ones are very sensitive.

With respect to the refrangibility, colour, or wave-lengths of the effective rays
of light. Strasburger here also confirmed the law which I had already established,
that the mechanical actions of light on plants are due chiefly or exclusively to the
highly refrangible, blue portion of the spectrum. When he allowed the light acting
on the swarm-spores to pass through a dark solution of ammoniacal copper oxide,
which transmits the blue and violet light, the swarm-spores reacted exactly as

INFLUENCE OF THE COLOUR AND INTENSITY OF THE LIGHT. 613

in ordinary white daylight. When, on the contrary, the light passed through a
concentrated solution of potassium bichromate, which transmits green, yellow, and
red rays, the swarm-spores did not react at all, and the same was the case
when the light was allowed to act through a ruby glass, which transmits essentially
only dark-red and green light, or when he allowed the nearly pure }ellow light
of incandescent sodium vapour (of a sodium flame) to act.

Finally, as regards the intensity of the light, it is to be remarked in the first
place that the swarm-spores in the dark move in all directions, in curved and often
sinuous paths, and only come to rest when they perish, or in any way become
fixed. . The particular intensity of the light which influences the direction of
their movements at all differs, according to Strasburger, in different species.
Chilo?)ionas ciirvata on cloudy days did not collect at all at any particular margin,
but remained distributed throughout the entire drop; when the sky brightened,
however, they travelled to the light side of the drop. The swarm-spores
of Bcemaiococcus, on the contrary, became aggregated at the illuminated
margin of the drop, when the light
was scarcely bright enough to read
printed matter by.

Amoeboid movement is the name
given to the locomotion of small free-
living protoplasmic bodies, which, ac-
cording to their nature otherwise, might
also be fitly designated swarm-spores,
and in fact some of them proceed from
such as later stages of development,
as shown in Fig. 360. Here are to be
mentioned particularly, the so-called
Myxamoebae — i. e. the early develop-
mental stages of the Myxomycetes.
In contrast to the true swarm-spores,
the relatively rigid bodies of which,

as it appears, are made to move forward passively, by means of the vibrations
of the cilia, the amoeboid movement consists in the protoplasmic body (which is
sharply defined, however) adhering to a solid substratum, altering its external form
and creeping along the substratum, the peripheral portions of protoplasm being
put forth at some parts of the margin, and withdrawn at others ; and so the mass
slowly moves from place to place. This form of movement will perhaps be best
illustrated by comparing figures 9-12 in Fig. 360. As to the mechanics of such
movements there is practically nothing known, although for twenty years repeated
attempts have been made to explain them ^

album (after Cienkowski). i, spore ; a,
3, the free contents; 4, s. the same as
swarm-spore with one flagellum ; 6, 7, the same after losing their
flagella; 9, :o, 11, fusion of amcebae; 12, a small Plasmodium.

1 That an explanation of the amoeboid movement of protoplasm and its circulation in the cells
is not obtained by ascribing ' contractility ' to the substance was insisted on by Hofmeister (' Flora,'
1865, p. 8). I attempted at the same time in my 'Handb. der Exp.-phys.'' (1865, p. 454) to replace
the indefinite idea connected with the word contractility by more definite assumptions, which of course
did not reach beyond the region of mere hypothesis, though they have so' far not been replaced in
their turn by better. I therefore quote the most important ones. ' I imagine that living protoplasm
consists of molecules of definite form (not round), but not capable of imbibition ; these have a

6i4

LECTURE XXXV

The movements of Plasmodia are directly connected with these amoeboid
movements \ Plasmodia arise by the fusion of numerous myxamoebse, whereby
a laro-e, and sometimes even immense protoplasmic mass is formed, as to the nature
and movements of which I have already stated what is necessary (p. 83, Fig. 80).
The creeping amoeboid movement is altered in the case of plasmodia, according to
their size, into a more flowing one, looking in the main very like the flowing of
a thick tenacious slime. Of course this is only an external appearance, since, while
in the case last mentioned the slimy mass passively obeys external forces, particularly
gravitation, we are in the case of plasmodia concerned with vital movements, and
the mutual displacements of their smallest particles which are effected by internal
forces. Inside a coherent matrix, sharply defined externally, but with its outlines
continually changing, there are formed more fluid portions containing numerous
granules, which are in active streaming movements, the direction and intensity
of which vary (see Fig. 80 A, p. 83), though no external stimuli produce these
changes in any way. I demonstrated the irritability of the plasmodia to the in-
fluence of light fifteen years ago. If large heaps of tan, in which numerous small
yellow Plasmodia of the so-called ' flowers of tan ' {jElhah'nmii sepiicuni) are
contained, are placed on a plate and kept in the dark, all the plasmodia creep
forth in a few hours on to the surface of the tan, and there fuse into large yellow
masses. If the preparation is placed at the right time in a light chamber, the

powerful attraction for water, and hence surround themselves with relatively thick watery envelopes,
so that the mutual attraction of neighbouring molecules, which diminishes with the distance
more slowly than their attraction for water, can effect only a feeble holding together of them (a slight
cohesion). Under the influence of these two forces an unstable equilibrium of all the molecules of a
mass of protoplasm will be set up, and these will at the same time be displaceable by slight external
shocks, and the whole exhibit many of the properties of a fluid ; it is also to be seen why different and
feeble influences so easily deprive the protoplasm of a part of its water and strengthen its cohe-
sion. It may then be further supposed that the molecules, in virtue of their mutual attraction,
strive to lay themselves next one another in such a way that they turn their smallest diameters
towards one another, because this position ensures the closest approximation of their centres of
gravity. In this attempt, however, they will be in part prevented by the aqueous envelopes, and, on
the other hand, it may be assumed that the molecules are endowed with directive forces, so that, for
example, they seek in virtue of these latter to turn their longest diameters towards one another.
One may here indeed suppose an electric polarity to exist in the molecules. Evidently, through the
play of three attractions which are independent of one another in value, it would be possible for a
position of equilibrium to be set up in which relatively considerable quantities of energy are present
in the form of tension ; the most insignificant shock might here disturb the equilibrium, and a dis-
turbance occurring at one point must at once be propagated to the neighbouring molecules, and
the movement must gradually affect places further and further distant from the starting-point. If
the question is now raised as to the shocks which are able to set free the forces in tension in
this molecular system (when it is itself left in equilibrium) very different ones may be supposed.
Within the living protoplasm chemical processes are continually going on : these may at individual
places alter the molecules chemically, or increase or diminish their attraction for water, their mass,
or their polarity. Independently of the chemistry, moreover, small thermal, electric variations,
imperceptible vibrations, &c., will affect sometimes one, sometimes another part of the protoplasm
and disturb its unstable equilibrium.'

* The path to our knowledge of plasmodia, so exceedingly important for the theory of proto-
plasm, was first opened by De Bary, in his celebrated treatise, ' Über die Mycetozoen,' in the
'Zeitschr. f. wissench. Zoologie' (1857, B. 10"), and a second edition, reprinted separately (1864).
Cf. further, Cienkowski in 'Jahrb. f. wiss. Bot.' B. III. pp. 525 and 500. Also Strasburger,
'Studien über Protoplasma^ Jena (1876) ; Sachs, 'Lehrbuch^ IV Aufl., p. 265.

PLASMODIA: CIRCULATION OF PROTOPLASM. 615

Plasmodia again disappear from the surface, creeping into the interior of the tan,
to come forth once more on the renewal of darkness : this may be repeated
several times during the course of the day. According to the statements of
Baranetzky \ it appears also that plasmodia which have crept up glass plates, and
there become extended in the form of elegant networks, withdraw themselves from
those places which are intentionally illuminated, and become collected at the
shaded ones. This irritability to light, however, only exists during a certain
condition of life of the plasmodia; when their internal development is so far
completed that they are proceeding to the formation of sporangia, &c., they make
their appearance on the surface of the tan even when the light is strong. I have
also already nodced the remarkable creeping of the plasmodia up the stems of
plants, up flower-pots, and other objects, sometimes very high. They may
be impelled to do this very easily, by placing moistened glass plates vertically
in the tan, which contains young plasmodia just about to creep on to the sur-
face ; in the course of a few hours the reticulated masses ascend the glass plates up
to the highest points, and they may now be removed and placed directly under
the microscope, in order to observe their internal movements more in detail. There
is probably little doubt that this impulse to creep upwards is to be regarded as
a geotropic stimulation — i.e. that a still unknown action of gravitation on the
molecular structure of the protoplasm so affects the displacements of the molecules
that the result mentioned follows. It is, however, scarcely necessary to say that
the individual mechanical processes in the matter are entirely unknown.

In all essential points the so-called Circulation of the protoplasm - in the interior
of living cells agrees with the movement of plasmodia ; only in this case the
extent of the space within which the motions of the protoplasmic body can occur
is once for all determined by the resistant cell-wall. It has already been described
and illustrated by figures on pp. 80-82 how this comes about, when the cells
at first entirely filled with protoplasm grow by absorbing water, whereby in the
first place small water cavities or vacuoles appear in the interior, and how the
water or cell-sap then increases more and more, so that in the subsequently much
enlarged cell the protoplasm constitutes a more or less thick sac, everywhere
closely applied to the inside of the cell-wall and enveloping the cell-sap, traversing
which are strand-like or plate-like filaments of protoplasm, and how all is in move-
ment. The accompanying figure, reproduced from my Handbook, will contribute
still more to give the reader an idea of the protoplasm thus in circulation. The
latter is represented in the figure as a finely punctated mass in which lie larger
bodies also, especially chlorophyll granules which contain starch ; in one place
also is to be seen a small crystal of calcium oxalate. It is practically the move-
ments of these small granules or microsomes and the larger bodies which betray
the streaming movement of all parts of the protoplasm; only an outermost layer,

* Baranetzky, '■Influence de la hitnicre sin- /es Plasmodia des Myxoiuycetcs,' Mom. de la soc.
nat. des scienc. nat. de Cherbourg (T. XIX. 1876).

^ Dr. Klebs has carefully collected the literature on circulation and rotation as well as on all
the movements of the protoplasm here referred to in the 'Biologisch. Ccittralblatt^ ed. by Rosenthal,
1881 (Nos. 16, 17, and 19).

6i6

LECTURE XXXV.

closely applied to the cell-wall, remains as it appears relatively at rest. Having

already {/oc. ctt.) characterised in the
main the form of movement, and
since a detailed description would
not give the reader any notion of the
motile forces, I may here confine my-
self to mentioning briefly the few facts
which we know concerning the irrita-
bility of the circulating protoplasm, and
fundamentally this will apply also to
the rarer form of movement, the Ro-
tation of the protoplasm, which is also
mentioned in the same place. Whether
and how far gravitation may in any
way influence the movements is not
known. That rotation and circula-
tion take place even in profound and
continued darkness, and likewise in
coloured light, and that by illumina-
tion under the microscope at least no
striking change is undergone, is well
known ^, although this does not ex-
clude the possibility that more exact
studies in this direction may demon-
strate an irritability to variations in
the light. Considering the great sen-
sitiveness of swarm-spores and plas-
modia for light, it is hardly credible
that the protoplasm within the cells
should be indißerent towards it; more-
over all heliotropic organs, which are
thus sensitive to light, of course con-
tain protoplasm in their cells, and
we have every reason to believe that
the light-stimulus in heliotropic organs
affects principally their protoplasm,
and that the corresponding alterations
in the cell-walls are initiated by this.
It is thus obvious (and the reason-
ing applies to geotropic organs also)
that all protoplasm enclosed in cells
is irritable to gravitation and light,
only of course in a manner not

Fig. 361.— Optical lonsfitudinal section of the middle cell of
hair of the Gourd (from the calyx of a young flower-bud). Cell-
wall simply in outline— the fine granules in the protoplasm
drawn too coarse. The central vacuolated clump encloses the
nucleus of the cell. The streaming filaments, everywhere in
active movement, carry chlorophyll -corpuscles (containing
starch) in their substance : at one place (to the left) a crystal
is also carried along.

' In rhe Bot. Zeitg. 1863 (Supplement, p. 3) I stated that the protoplasm ciiculates even in the
cells of etiolated organs, e. g. hairs of Cucurbita : in fact, I prefer to employ wholly or partially
etiolated plants for the demonstration of protoplasmic movements, on account of several advantages.

CIRCULATION AND ROTATION OF PROTOPLASM. 617

directly perceptible by means of the microscope : moreover it need not be simply
in an alteration in the circulation and rotation that the irritability of the proto-
plasm to gravitation and light has to express itself; and in the subsequent con-
sideration of the movements of the chlorophyll-corpuscles we shall see that these
are probably to be referred to the stimulation of colourless protoplasm by light.

Heat acts very energetically as a stimulus, and this so that the streaming
movement is accelerated as the temperature increases up to an optimum. Tem-
peratures beyond this optimum, again, retard the movement, and finally (at about
45° C.) the filamentous network of circulating protoplasm contracts into a clump;
however, if this action has not continued too long, the normal configuration
of the protoplasm may be restored after a short time at a lower and more
favourable temperature '. Strong electric sliocks act similarly, and it is here to
be especially insisted upon that pressure on the cell-wall occasionally induces
similar effects, and in some cases at least a complete arrest of the movement :
subsequently it returns. Hence it happens that in recently made preparations of
hairs or internal cells abounding in protoplasm, the circulation and rotation are not
observable at all during the first few minutes, or it may be for some time, whereas the
same preparations often exhibit vigorous movement a few hours later. According to
Dehnecke's recent observations, moreover, lying in water for some time, and there-
fore probably a certain diseased state of the protoplasm, acts so as to accelerate
the movements, so that they are often much more vigorous a long time after the
preparation was made than in the normal condition'^. It should be mentioned
here that the rotating and circulating protoplasm continues its movements even
when the water of the cell-sap is affected exosmotically by the action of sugar-
solution, the protoplasm separating from the cell-wall on all sides and contracting ;
in fact in the case "of the root-hairs of Hydrocharis it is sometimes even possible
to break in pieces the contracted protoplasm within the cell-wall, and each of the
pieces forms a thick-walled vesicle of protoplasm, the substance of which continues
to rotate actively for some time longer.

So long ago as 1861 I had called attention to the remarkable fact that the
leaves of the most various plants appear bright green in very intense sun-light,
and dark green in the shade, and that it is possible accordingly to obtain a sort
of light-picture by laying a strip of black paper on a leaf on which the sun is
shining : on removing the paper the shaded part appears dark green, the parts
exposed to the light bright green. I did not at that time succeed in obtaining
the right explanation of this fact, but now, from later researches by Famintzin,
Frank, Borodin, and especially by Stahl, it has been discovered : they showed that
the chlorophyll-corpuscles under the influence of the light assume different positions
within the cells, and this necessarily gives to the whole leaf the appearance described
above. Since these phenomena pave the way to still more general points of view
and accurate ideas as to heliotropism proper, it is worth while to enter into

' Details 011 Ihe^e matters arc found in my treatise, ' Über die obere Temperatur-grenze der
Vegetation,' Flora, 1864 (p. 37). Veiten, ' Einwirkung der Temperatur atif Protoplasma-be'wegtmg^
Flora, 1876 (Nos. 12-14).

- Dehnecke, in ' Flora,' 1881 (Nos. i and 2).

6l8 LECTURE XXXV.

them more in detail, and in doing so I shall make use of Stahl's very thorough work '.
He studied particularly the filamentous Alga Alesocarpiis, in which the phenomena
in question are particularly clearly shown. ' The elongated cylindrical cells of this
Alga are connected into long filaments, and contain an axial band of chlorophyll
running the whole length of the cell, and the margins of which now and then
extend all round to the protoplasm lining the cell-wall ; in this case the whole
cell is divided by it into two approximately equal halves. The band of chlorophyll
is usually extended in one plane : on observing it from the surface the whole cell
thus appears uniformly green ; if the Alga is turned round through 90° so that
the band of chlorophyll is seen no longer from the surface, but in profile, the
otherwise transparent cell is traversed along its whole length by a dark-green
thin longitudinal strip.' If various filaments of this Alga are placed beneath the
microscope the observer sees these plates of chlorophyll from the surface, or in
profile, or in positions midway between these. ' Left to itself and undisturbed,
such a preparation presents after some time a different aspect — for it is found
that in the straight filaments all the bands are arranged in one plane. The
orientation of the plates is the same in all those which lie parallel to one another,
but differs in those which cross one another.' For the sake of simplicity we may
here take into account only those filaments which lie horizontally, and receive
the daylight (from a window) at right angles to their long axes. In this case it
is easy to demonstrate that the plates of chlorophyll all turn their broad surfaces
to the light, so that they receive its rays perpendicularly. If the direction of the
incident rays is altered, the plates of chlorophyll slowly turn, so that they always
present their surfaces at right angles to the rays of light ; in warm weather actively
vegetating plants complete these movements in a few minutes. Direct sunlight,
on the contrary, effects after a short time an entirely different disposition of the
plates of chlorophyll : they turn one edge towards the sun, or in other words
place their flat surfaces parallel to the incident rays. ' Light thus exerts a directive
influence on the chlorophyll apparatus of Mesocarpus. In feebler light it is disposed
perpendicularly to the path of the light (this is termed by Stahl the plane position),
in more intense light its plane coincides with the direction of the rays (he terms
this the profile position).'

In the vesicles of Vaucheria (p. 108) there are rounded chlorophyll-corpuscles
embedded in the protoplasmic fining to the wall. According to the intensity of
the light these also are arranged in a manner which is understood most simply by
regarding the chlorophyll-corpuscles themselves as parts of the plate of chlorophyll
described in Mesocarpus. If the intense light persists for some time, the chlorophyll-
corpuscles become collected into distinct heaps — a process which De Bary saw
occurring with great energy in the vesicles of Acetabularia (a marine Alga), and de-
scribed as follows : — ' If actively growing vesicles a few mm. long receive the rays of
the sun directly, the protoplasm which carries the chlorophyll instantly rounds off
and contracts into irregular clumps. The individual grains are seen to leave their
place rapidly, and as it were tumble up against one another, becoming collected

' Stahl, 'Über den Eiiißiiss von Richtung und Stä>-ke der Beleuchtung^ in Bot. Zeit.,
(p 297), where the literature of this subject is also given in detail.

MOVEMENTS OF CHLOROPHYLL-CORPUSCLES.

619

into balls at certain points ; these aggregates thicken as new corpuscles are con-
tinually being added, and obstruct the vesicle as they swell up into clumps
filling up the whole diameter, all the chlorophyll disappearing from the inter-
vening portions. After a few minutes, therefore, the previously uniformly
green tube appears to the unaided eye, or under a lens, divided into unequally
large and irregularly arranged dark green zones alternating with colourless trans-
verse ones.'

We may now turn to the behaviour of the chlorophyll-corpuscles in ordinary
cells united into tissues, and first consider an example where these cells form a
single layer only, as in the case of
the leaves of the Moss. In ordi-
nary diff"use daylight the chloro-
phyll-corpuscles in the leaves of
the Moss, as well as in the similarly
constructed prothallia of Ferns,
lie only on the outer surfaces of
the cells, the walls which separate
any two cells from one another, and
which are situated at right angles
to these not being furnished with
chlorophyll-corpuscles. In Funa-
ria (p. 85, Fig. 82), according to
Borodin, even a short exposure to
darkness suffices to alter this dis-
position ; the chlorophyll-corpus-
cles abandon the free outer walls
of the cells, and travel over on
to the side walls, so that after a
time the upper and lower sur-
faces of such a leaf, and of any
one of its cells, are completely
deprived of chlorophyll. If the
leaves are exposed to the light
the previous arrangement very
soon re-appears. In the simi-
larly one layered parenchyma of

Lemna irisuka (one of the Duckweeds) the free superficial walls of the cells are
likewise found to be furnished with chlorophyll-corpuscles in ordinary daylight ;
but if such a plant is exposed to the action of direct sunlight there takes place,
as Borodin, found, a rapid alteration in the distribution of the chlorophyll-corpuscles.
After 10-15 minutes they cover the side walls evenly, as shown in Fig. 362 B ;
seen in plan the chlorophyll-corpuscles form in this condition a regular network, the
meshes of which correspond to the individual cells. After exposure to continuous
sunshine for some time this is no longer the case ; the corpuscles now form irregular
groups, which occupy the angles where several cells are contiguous. Continuance
of the illumination results in no further change ; but if the plant is removed

Fig. 362.— Transverse section of the leaf of Lemna trisidca (after Stahl).
A plane position (in daylight), B arrant,'enient of the chlorophyll-corpuscles
in intense light, C position in the dark.

620

LECTURE XXXV.

from the sunlight and exposed merely to diffused daylight, the chlorophyll-
corpuscles abandon the above position and spread themselves on the outer walls of
the cells. The changes in position described can be called forth at will, and
often by alternately intense and feeble illumination.

Of Stahl's numerous observations on the chlorophyll-tissue in the leaves of
Phanerogams we may here refer further to his statements with respect to Oxalis
Acetosella (Wood- sorrel). ' The cells of the uppermost layer next the epidermis
are developed into more or less obtuse cones, the bases of which are situated
on the epidermis. The two lower layers of the mesophyll consist of flat
stellate cells as in Fig. 363, Healthy leaves of this plant were laid flat on
a plate and exposed to the rays of the sun falling perpendicularly on them. By
pouring fresh water over them the leaves were prevented from becoming too
warm. Individual leaflets were protected from the direct rays of the sun by
means of paper shades. After an hour the marked leaflets were placed in
alcohol, in order to fix the cell-contents in their position. The decolorised leaflets
were so transparent that mere observation with transmitted light was sufficient to
demonstrate the diflferent distribution of the chlorophyll-corpuscles in the shaded

FIG. 363.— Cells of the lo\
direction at right angles to t
liijht ; b profile position after a

•erniost layer of spongy parenchyma from the leaf of Oxalis acetocella, seen in a
le surface of the leaf a plane position of the chlorophyll-corpuscles in diffuse
short exposure to the sun ; c after longer insolation (after Stahl).

leaflets and those exposed to the sun. Fig. 363 a shows the surface view of a
stellate cell in a shaded leaf; the chlorophyll-corpuscles are distributed approximately
uniformly on the walls parallel to the surface of the leaf. In Fig. 363 h
the grains have passed over on to the walls which are perpendicular to the
surface of the leaf ; this distribution is found after insolation which has not
been continued too long. If the leaves have had the sun shining on them for
a longer time — an hour or more — the arrangement of the chlorophyll represented
at c is met with ; the corpuscles are collected into clumps, lying on the walls common
to two neighbouring cells.

I may here interpolate, with reference to what has been said above as to
the alteration in colour of the leaves in intense and in feebler light, that it is
now easily intelligible from the examples given, how the different positions of the
chlorophyll- corpuscles must effect alterations in the intensity to the unaided eye of
the green colour of the leaves ; it is obvious that a leaf, the chlorophyll-corpuscles
of which all lie on the side walls of the cells, must appear to the eye of a brighter
green than one where they lie on those walls which are parallel to the surface
of the leaf

With respect to the mechanics of these phenomena, I have already expressed

CAUSES OF THE MOVEMENTS OF THE CHLOROPHYLL-CORPUSCLES. 62I

the opinion, based on Frank's observations, that the chlorophyll-corpuscles are passive
in the matter, and that the movements referred to belong to the protoplasm itself
in which they are embedded. Frank and Stahl agree with this view : if it is right,
however, as is hardly to be doubted, then all the statements so far made as
to the chlorophyll-corpuscles apply properly to the protoplasm itself, thus confirming
and more exactly characterising its sensitiveness to light.

Again, the change in form of chlorophyll-corpuscles in varying illumination,
already demonstrated by Micheli in a piece of work done with me in 1866, was
confirmed and more exactly described by Stahl. In the Moss Fimaria he found
that in diflfuse daylight the chlorophyll-corpuscles are disposed on the outer surfaces
of the cells neariy touching one another, and separated only by narrow strips of
colourless protoplasm. The grains are in this condition flat and polygonal. On
exposure to direct sunlight they withdraw their projecting angles and become
rounded and smaller in circumference, and thus further apart. Similar phenomena
were also observed by Stahl in the so-called palisade parenchyma of the leaves of
Phanerogams, and it is only necessary to add here that in sunshine, as in shade,
the chlorophyll-corpuscles of these cells have the so-called profile position — i. e. are
situated on the walls parallel to the rays of light ; in the shade the chlorophyll-
corpuscles are in this case approximately hemispherical, in the sun more flattened
and discoid. Stahl demonstrated similar changes also in the assimilatory paren-
chyma of those Liverworts which have flat extended shoots, and was able to detect
the phenomenon generally wherever he sought for it. At the same time it should
be remarked that in some leaves, clump-like aggregations of the chlorophyll-corpuscles
are formed in the palisade cells by continued insolation.

From all his numerous observations Stahl concludes then as follows : ' In
feeble illumination the largest surface of the chlorophyll-corpuscle is turned towards
the source of light ; the light is absorbed as much as possible. An opposite
behaviour is noticed in very strong illumination : a smaller surface is presented
to the light. One and the same end is attained in utterly different ways ; the
chlorophyll-bodies sometimes protect themselves from too intense illumination
by turning round (^Mesocarpus\ and sometimes by travelling, or by alterations in
shape. These phenomena in the chlorophyll-apparatus proceed, again, hand in
hand with the position assumed by entire leaves in very strong or feeble illumin-
ation ; this also was investigated in detail by Stahl. As we shall see later on,
many leaves have the power of placing themselves in feeble light by means
of peculiar mechanisms, in such a position that the rays of light fall upon their
surfaces perpendicularly, whereas intense light induces them to assume the profile
position — i. e. to turn one edge to the sun, and thus place their surfaces parallel
with the sun's rays, thereby avoiding a too strong influence of the latter. Arrange-
ments of the most various kind combined with corresponding irritabilities for light
thus exist in plants in order to bring, not only the assimilatory apparatus, such as a
whole multicellular organ, but also its individual chlorophyll-bodies into particular
positions, which we must in any case regard as favourable for the employment
of the light.

With reference to the heliotropic phenomena to be described later, and parti-
cularly with reference to my theory of heliotropism, I must notice two other important

623 LECTURE XXXV.

facts already contained in what has preceded. In the first place that in all the
phenomena here described it can only be a matter of the direction of the rays
of light. No one will assume that a swarm-spore swimming towards the light
does this because its anterior end is more strongly illuminated than its posterior.
Still less will it be supposed in the case of the chlorophyll-plate of Mesocarpus,
when assuming its position in plan or in profile, or in the corresponding move-
ments of chlorophyll-corpuscles, that the movements of the protoplasm referred to
can in any way be produced by the one side being more strongly illuminated
than the other ; on the contrary, it can only be a matter of the direction in which
the ray of light falls upon the irritable protoplasm.

In the second place I would lay stress on the fact that both in the case
of swarm-spores and in that of the movements of protoplasm containing chloro-
phyll, entirely different, or even opposite effects occur, according as the incident
hght is feeble or very intense. This phenomenon also we shall again meet
with later in the heliotropism of shoot-axes and leaves. We shall see that
many such organs become curved concave on the illuminated side in a feeble
light, and in a strong light convex.

I shall return again to these facts when treating of heliotropism.

LECTURE XXXVI.

THE PERIODIC MOVEMENTS OF LEAVES AND FLORAL
ENVELOPES (SLEEP-MOVEMENTS).

On glancing at the general aspect of the plants in a garden or a meadow
shortly after sun-down in the summer, it is obvious that the leaves of many of the
plants have assumed attitudes and positions different from those assumed in the
daytime. This is very striking, for example, in the case of plants like the Clovers,
and in all trees and shrubs {Robinia, Colutca, &c.) with similarly organised compound
leaves, especially where three or more leaflets are attached to a common leaf-stalk.
It is then found that the leaf-stalks are more erect or more pendent at night than
during the day, and the leaflets which they bear are drawn closely together, whereas
during the day they are usually extended horizontally. Similarly also with other
compound leaves, particularly those of the Clovers, &c., and, generally, all those (of
Cryptogams as well as Phanerogams) in which the laminae of the leaves are con-
nected with the shoot-axis or leaf-stalk by means of a special cylindrical peculiarly
organised portion. It scarcely needs mention that such plants cultivated in pots
and standing at the window or in a green-house, also exhibit the same phenomena.

In these cases, then, in all Leguminosae and Oxalideae, and many less well-
known plants, it is always by means of peculiar organs at the base of the petiole,
or at the place where each lamina is connected with the stalk, that the changes
in position of the leaves are effected. Beside^ these organs, which put in motion the
parts situated on them by means of their up and down curvatures, all other
parts of the leaves, the laminae as well as the proper petioles, are non-motile —
i. e. they are only passively directed upwards and downwards by the motile organs
mentioned.

As a rule the change consists in the leaves which are extended flat during the day,
and which present their chlorophyll-surfaces perpendicularly to the incident light where
possible, becoming folded together at night. With the commencing light of morning
the motile organs become curved, so that the laminae of the leaves situated on them
again assume the above-named extended diurnal position, while in the evening an
opposite curvature of these organs produces the nocturnal position. This phe-
nomenon has been termed the waking and sleeping of leaves ^

* The older literature as to the periodic movements of leaves and flowers may be here passed
over, inasmuch as it is treated in detail and criticised fully in Pfeffer's works. I shall therefore

624

LECTURE XXXVI.

But the foliage -leaves of very many other plants also, in which no special
motile organs are observable, make daily movements resulting in the production of
diurnal and nocturnal positions : the growing petioles undergo curvatures by which

FIG, :ß^.—Des»wdi

ryrans. A slioot during the day ; B shoot
From reduced photographs (Darwin).

the laminae situated on them are presented to the light during the day, and at night
are directed upwards or downwards.

mention only a few more recent works which will at once place the beginner on the right path in
this province.

Sachs, ' Über das Bczuegitngsorgan und die periodische Bewegungen der Blätter von Phaseolus
und Oxalis,' Bot. Zeit. 1857 (p. 793).

Sachs, 'Die vorübergehenden Starrezustände periodisch beweglicher jind reizbarer Pßanzen-
organe,' Flora, 1863 (Nos. 29, &c.).

Paul Bert, 'Memoir, d. l. soc, d. scienc. phys. et naturell, d. Bordeaux' (1866).

Millardet, ' Nouv. recherches sur la pcriodicitc d. l. tension^ 1869 (Mem. de la soc. natur. de
Strasbourg).

Batalin, ' Über die Ursachen der periodischen Bezuegungen der Blumen und Laubblätter ^ Flora,

1873 (P- 433)-

Pfeffer, 'Physiologische Untersuchungen' Leipzig (1873 and 1875).

Sachs, 'Lehrbuch der Botanik,' IV Aufl., 1874 (pp. S44-869).

I cannot agree with the nomenclature introduced into this subject by Pfeffer. His ' Peccptions-
bewcgmigen ' are simply ' Peiz-bewsgungen,' the peculiarity of which, that they take place only while
the organ is in a condition of phototonus, I characterised in 1S65 by the expression ' pai-atonic stimu-
lation,' an expression moreover which Pfefter accepts, though it makes the term ' Peceptions-bezvegung'
which in fact can only hold for this case, superfluous. Again, I can by no means agree with Pfeffer's
designation of the periodic movements of non-articulated foliage-leaves as movements of nutation,
although he himself as well as Batalin demonstrated their dependence on variations of light ;
for the term ' nutations,' previously introduced by me, applies simply to inequalities of growth on
different sides of an organ, which are not induced by external influences. It would be much to
be deplored if in this difficult province, where Nature herself presents plenty of confusion, still
further difficulties should be produced by an indefinite nomenclature.

MOVEMENTS DUE TO CHANGES IN THE INTENSITY OF LIGHT. 6%$

Similarly with many flowers, the corollas of which open in the morning, rarely
in the evening, while they close again in the evening or morning respectively, definite
hours not being followed as a rule.

These are the phenomena with which we shall now be concerned more in
detail. One of their characteristics is that they vary with the alternations of day and
night, and thus constitute daily periods. Now since in the morning, when the leaves
and flowers open, the temperature and dryness of the air usually increase, and in
the evening when they close the temperature falls, and the moisture of the air there-
fore increases, it might be supposed at first, that these points must be of special
importance for the movements of sleeping and waking of leaves and flowers. As
a matter of fact, the opening and closing of many flowers, as the Tulip and Crocus,
are directly dependent on changes of temperature; and it cannot be denied that
in other cases also changes in temperature and moisture aff'ect the phenomena to a
more or less subordinate extent. Nevertheless, so much is certain, that the daily
periodic movements are called forth chiefly and almost exclusively by alterations
in the brightness of the light, particularly in the morning and in the evening : the
nocturnal position of the leaves and flowers is due to the darkness which supervenes as
the sun goes down, and the diurnal position is equally a consequence of the morning
brightness. That temperature and moisture are entirely subordinate and unim-
portant in the matter is conclusively shown by means of a very simple experiment.
A small plant of the common Garden Bean {Phaseolus) or of the Wood Sorrel
{Oxah's), rooted in a flower-pot, and with the leaves extended in the diurnal position
in the forenoon, may be completely submerged in a large glass vessel filled with
water. If the illumination remains approximately as before, the leaves maintain
their diurnal position, although the water in which they are submerged is many
degrees colder than the air which previously surrounded them, whence it follows
that neither change of temperature nor of moisture produces an observable alteration
in the diurnal position. If, however, such experimental plants, when they have
assumed their diurnal position — it matters not whether they are in the open air or
under water — should be suddenly darkened, e.g. by covering them with an opaque
box, or with a cylinder of wood or card-board, the leaves assume the nocturnal
position after a short time, say half an hour to an hour ; and on again letting in the
hght during the day, the leaves would again take up their ordinary diurnal position.
By means of such simple experiments therefore we can convince ourselves con-
clusively of the fact that the matter is one of stimulations due to variations in the
light — illuminating and darkening.

With the establishment of this fact, however, we have for the first time obtained
a foundation on which a deeper insight into these phenomena may be based by
means of further research. Dwelling for the moment on the most general relations,
it is above all to be insisted upon that, with respect to organisation, it is always
organs with dorsi-ventral structure which are here concerned, as is clear from the
fact that we have to do in all these cases with true leaves, or at least with metamor-
phosed foliar organs, which always possess dorsi-ventral structure — i. e. the parts
capable of curving and of receiving the light-stimulus are organised on the lower
side more or less diff"erently than on the upper side. Since tlien all these
organs, simply because they arc leaves, are situated on a shoot-axis, antl since the

[3] ss

'626 LECTURE XXXVI.

upper and lower sides of the leaves always stand in definite relation to this, it
results that the upward and downward curvatures of the motile parts take place
in a plane which may at the same time be regarded as a longitudinal plane of the
shoot-axis— i. e. the motile foliar organs approach the part of the shoot-axis
situated either above or below their origin, or, regarding the apex of the shoot as
the centre, we may say the movements take place in a centripetal or centrifugal
direction. This is particularly well shown in the case of flowers, which are in
fact metamorphosed shoots with very short axes : while in the case of ordinary
foliage-leaves we distinguish an up and down movement, we have in flowers to
do with an in and out movement. This rule, however, only applies to entire leaves ;
in compound leaves the individual leaflets move upwards and downwards with
reference to the common rachis, or they make at the same time twists directed
forwards or backwards.

Although broad, thin, flat parts of leaves are not entirely exempt from sleep-
movements, it is nevertheless found that in the more exquisite cases, where the alter-
nation of diurnal and nocturnal positions is very marked, and the light-stimulus thus
acts with great energy, the organs or parts in question assume a more or less
cylindrical shape, as is particularly conspicuous in the motile organs of the Legu-
mimosse and Oxalideae, and true compound leaves generally. As a rule, a motile organ
of this kind is a more or less elongated cylinder consisting of succulent parenchyma,
in the axis of which runs a non-lignified and very flexible strand of vascular bundles.
Since the movements of these organs consist in up and down curvatures, it is obvious
that sometimes the upper, sometimes the lower side of the succulent envelope of tissue
must be lengthened, though the axial strand need not undergo either elongation or short-
ening, because it lies in what may be called the neutral axis of the motile organ. It
is sufficient if this strand is flexible and not rigid ; its length need not alter during the
movement. Where the up and down curvatures of foliage-leaves are eff"ected by
means of ordinary petioles, or parts of the laminae, of course vascular bundles traverse
these parts as usual ; but they also abound in succulent parenchyma, and the movement
continues only so long as no lignification has as yet occurred in the vascular bundles.
These points are especially to be borne in mind with respect to the mechanics
of such movements, to be described more in detail subsequently. However, before
we go into the mechanics of the movements, it is necessary for us to become some-
what better acquainted with the movements of the leaves themselves.

Our present theme is the so-called sleep-movements, caused by the daily varia-
tion of the light at sunrise and sunset. Now we shall at once see that even these
movements for and by themselves are combined of two kinds of actions, namely of a
direct stimulation, and of after-effects induced by the stimulus itself. It was, however,
by no means easy to establish this apparently simple fact, since other movements are
combined in the most various ways with the proper daily periodicity, though they
have absolutely nothing to do with it. This is very evident when such a plant as a
Mimosa, Bean, Oxalis, &c. is suddenly excluded from the light : there then occurs at
once, in fact, a sleep-movement ; in from half an hour to an hour the leaves assume
the nocturnal position. On again looking at it the next morning, however, the leaves,
although they have not received any light at all, are found to be in their diurnal
position, with their laminae extended : next evening they fold up in the nocturnal

COMBINATIONS OF STIMULI. 627

position again. This suggests the question whether there is any light-stimulus at all
in the matter. We have here, however, to do simply with an after-effect of the
previous light-stimulus, which after a few days ceases in constant darkness just as it
does also in constant light.

If, on the other hand, the Field Clover {Tnfolium pratense) is used in the above
experiment, or T. wcarnaium, Oxalis aceiosclla and some other plants, they would
be found to be in continual movement, so that at any time after the course of
a few hours an apparent nocturnal position alternating with an apparent diurnal
position occurs. In this case, however, we have to do with a movement independent
of the variation of light, and therefore not to be regarded as an after-effect of it. From
internal changes, as yet not understood, the leaves make up and down movements in
periods of time of a few hours. Since these leaves also are sensitive to light,
however, and have true sleep-movements, this independent so-called spontaneous
or autonomous periodicity is hardly noticed under ordinary circumstances, simply
because the influence of the light is stronger than the spontaneous movement.
Nevertheless the converse occurs also ; this in a very striking degree, for ex-
ample, in the case of another clover-like plant
{Hedysar 11711 gy rails), the leaf of which is here || .fW

figured. The two small lateral leaflets of this (|| _,^^;#^*^^X
make in the course of a few minutes periodic m^^^^^/i \' \h>\
oscillations, it matters not whether they are in 'I -^[^ u / V--^^\
light or darkness, if only the temperature is ^ \| //\"--i\

rather high — at least 22° C. \ /\^\

These spontaneous movements, not called V /\^\

forth by external changes, must therefore be M /Vx\

sharply distinguished in the first place from the M ( / V^]

sleep-movements, with which they are more or \j [\v)

less combined, since the point to be established
is that the daily periodicity of sleep-movements
is due to variations in the light, and is thus causally essentially different from the
spontaneous movements.

But we have to do with yet other complications, not less calculated to lead
to error in the study of daily periodicities and their causes. The leaves in question,
or rather their motile organs, are also heliotropic, i.e. dependent on light in quite
another way : in the case of that stimulation of light which produces waking and
sleeping, the stimulus lies in the variations of the intensity of the light — it is not
the light as a constant force which effects these movements, but the varying
intensity; the increasing intensity in the morning induces the waking and exten-
sion of the leaves, the decrease of the light in the evening the nocturnal position
or closing. In the heliotropic curving of the motile organs, on the contrary, it is the
constant influence of the light which effects the curving, just as in the case of
heliotropic stems and roots. If one of the above-named plants stands for some time
undisturbed at a window, all the laminae become turned towards the light ; if the plant
is turned round, the motile organs make other curvatures until the laminae again
turn their upper sides to the light. The movements of waking and sleeping go on
undisturbed in such heliotropically curved organs. A further great difference between

s s 2

638 LECTURE XXXVI.

these heliotropic curvatures and those which bring about the sleep-movements, hes
in the fact that the organs can make heUotropic curvatures in all directions, e.g.
left and right, so that the one flank of the motile organ becomes convex and the
other concave, according to which side the light falls on. The movement of waking
and sleeping, on the contrary, only takes place as already mentioned in one plane,
which divides the leaf and the motile organ symmetrically; and it is thus unimportant
here in what direcdon the rays of light fall upon the motile organ, but only important
that Hght is present at all and increases or decreases in intensity.

The above will suffice for the distinction of the movements of waking and
sleeping from the heliotropic curvatures. In order to obtain a shorter expression
for the former, however, I have since 1865 proposed to term those effects of light
which cause the opening of the leaves as the intensity increases, and their closure as
darkness comes on, Paraionic effects ; because they only take place if the leaves are
in the normal vital condition which I term Phoiotonus. If such leaves are endrely
excluded from the light for several days, they pass into a condition of rigidity due to
the darkness — i.e. they are no longer capable of being set in movement by variations in
the light, though this recurs after long exposure to light, by which means the phototonus
is restored. We may thus say shortly, the movements of waking and sleeping are
called forth by paratonic light-stimuli, whereas the spontaneous movements of the
same leaves are independent of any light-stimuli, but probably depend on phototonus ;
heliotropic curvatures, on the contrary, have nothing to do with phototonus.

If we now wish finally to distinguish the daily periodic movements induced by para-
tonic light-stimulus, from the various other movements of the same leaves, often combined
with them, I must, in conclusion, notice a phenomenon which is perhaps the most
confusing one of all in this province. I have so far assumed that the waking leaves, in
the diurnal position, only receive ordinary bright daylight, or if the direct rays of the
sun only temporarily and not too strong. However, if they are exposed to very
intense insolation, especially about noon, the leaves close up and assume what looks
like the nocturnal position. We have here, in fact, again to do with the profile position
characterised by Stahl, which I have already described in detail in the previous
lecture as regards the chlorophyll-grains, and have referred to as regards the leaves.
Not only leaves which are paratonic and irritable, but others also have this pecu-
liarity, that under very strong illumination they undergo curvatures or torsions, by
means of which the lamina is so placed that the fierce rays of the sun are parallel to
it, or at any rate fall upon it at a very acute angle ; by this means a too intense action
of the light is avoided.

It will readily be admitted that it is no easy task to detect the differences
between all these various influences which cause alterations in the positions and dis-
position of the leaves, and to keep them apart and refer each to its causes, and
eventually to recognise in its undisguised form the true daily periodicity which may be
combined with all these variations. I first succeeded in 1863 in separating the spon-
taneous periodic movements from the paratonic ones, and in distinguishing between
phototonus and rigidity due to darkness; and Pfeffer in 1875 first recognised the true
daily periodicity as a phenomenon made up of a direct paratonic action and its after-
effect, previous observers having already established the fact that the leaves endowed
with daily periodicity are sensitive to mere darkening and illumination.

STRUCTURE OF MOTILE ORGANS.

629

We have now only to do with the true movements of sleeping and waking, which
are thus induced by paratonic light-stimuli and their after-effects; and in the first
place we shall consider those leaves which we may look upon as the most completely
organised in this direction, namely, the compound leaves of the Leguminosae,
Oxalidece, and others of that type. It will then be relatively easy to make intelligible
the phenomena in question in the case of other leaves where special motile organs
are not developed, and in the case of flowers.

It is in the first place important to obtain a clear idea of the motile organs them-
selves, and this is probably to be obtained in no way better than by examining the
common Garden Bean {Phaseolus ??iultißorus and P. vulgaris), a single leaf of which
is represented in Fig. 366 in the nocturnal position. At a is the motile organ of
the leaf-stalk proper, by means of which it is connected with the stem ; at b and c

FIG. 366.— Leaf of llie Scarlet Runner {Phaseolns mullißoriis) in the nocturnal position ; a the large motile organ at the base
of the leaf-stalk d d ; *<- the small motile organs of the three leaflets eee.

are the motile organs of the individual parts of the leaf, the so-called leaflets. The
portions d d oi the leaf-stalk are stiff, and immovable on their own account, as are
also the laminae of the leaflets e e. It is clear that if the motile organ a, which is now
in the nocturnal position, elongates a Httle on its upper side, it must necessarily curve
downwards, and the stalk d d naturally falls ; if at the same time the motile organs
b and c undergo a slight elongation on their lower sides, and accordingly an upward
curvature, the individual leaflets e e become raised, and we will assume this to occur
so that they all come to lie in one and the same plane. In this case, then, the leaf
attains the diurnal position, and it may be incidentally mentioned that the change here
described is induced by the increasing light in the morning ; the opposite change,
which carries the leaf back to the sleeping position again, results from the decrease
in the brightness of the light in the evening. Although to superficial observation,
it is the movement of the petiole and of the lamina which is most striking, this is

630

LECTURE XXXVI.

nevertheless only passive, and is due to the curvature of the motile organs. Fig.
367 may serve to illustrate the true process more exactly; after the removal of the
laminae there remain only the three motile organs of the leaflets on the common
leaf-stalk, as represented. In A these are very decidedly in the diurnal position :
the leaflets are no longer all extended in one plane, but are more upright ; at B
these motile organs have assumed their nocturnal position. It is hardly necessary to
add that as the light varies, and from its after-effects, these organs may assume all
possible positions and curvatures between those of A and B.

The coarser anatomical structure is illustrated in Fig. 368. C is a transverse
section through the rigid portion of the leaf-stalk itself; as in most other stiff petioles
which are channelled above, there are several vascular bundles G arranged
approximately in a circle, in addition to two more slender ones {g) running in the
edges of the channel; the remaining tissues are green cortex c, and soft pith 7«. The
appearance is quite otherwise in the transverse section D of the motile organ,

Fig. 367.— Upper portion of the leaf-stalk of the
Scarlet Runner, with the three motile organs of
the leaflets. -4 diurnal position ; B nocturnal
position.

Fig. 368.— C transverse section through
the rigid portion of the leaf-stalk of the Scarlet
Runner. D transverse section of a motile
organ (slightly magnified).

although it is fundamentally nothing more than a modified portion of the leaf-stalk
itself. The cortex c is here much more strongly developed, and consists of very succu-
lent, strongly turgescent, parenchyma, a little thicker on the lower side than on the
upper, though otherwise there is no important difference on the two sides. It is im-
portant to mention this point, since in this tissue we have the active motile substance
of the organ, and we shall afterwards see that the movements and curvatures of the
latter depend essentially only upon a different reaction of the upper and lower paren-
chyma c c towards variations in the light. Here again, therefore, the matter depends
not on visible relations of organisation, by means of which irritability is to be explained,
but on invisible molecular structure ; however it must by no means be forgotten that in
discussing the mechanics of the movements themselves, the coarser structural relations
are also to be kept in mind. In the middle of this mass of active parenchymatous
tissue runs a strand G, which, on being highly magnified, is at once seen to be com-
posed of a large number of fused vascular bundles ; it has to be supposed that the

TTSSUE-TENSIONS IN THE MOTILE ORGANS. 63I

Strands marked G and g, in Fig. C, here come close together, leaving a channel
above which is filled up with the pith m. As regards the finer anatomical structure
of the motile organ, reference may be made to Fig. 371 below, which represents the
transverse section of the motile organ of Oxalis, the relations in both cases agreeing
in all essential points. The epidermis of the motile organ is relatively insignificant
and not strongly cuticularised, but furnished with hairs which, however, do not interest
us further.

A property of the motile organ of the Bean which is important for our
purpose, and which also is again met with in all other similar motile organs, consists
in the tension of the tissues ; for the movements are practically nothing but alterations
in the tissue-tension as a whole, and their relative sizes on the upper and lower side
of the organ. In this is conspicuous above all the extraordinary magnitude of this
tension, which depends on the one hand on the strong turgescence of the irritable
parenchyma, and on the other hand on the toughness and elasticity of the non-lignified
strand. By means of the one the thick parenchymatous envelope tends to be
forcibly extended, and this is prevented by means of the other. The obvious
consequence is, as explained in Lecture XIII, that the entire motile organ, although
consisting of eminently succulent tissue, nevertheless possesses a very high degree of
rigidity, which is also absolutely necessary in order that it may support the

^J Bl/

Fig. 369.— Transverse and longitudinal plates from the motile organs of the leaf of the Bean, lying in water in order to show the
changes in turgescence.

weight of the leaf-stalk and leaves, the centre of motion of which lies just in
this organ. If, by the decrease of turgescence in the parenchyma, the organ
became limp, the partial loss of rigidity would result in the attached leaf being
depressed by its own weight: with the changes produced in the parenchyma
by variations in the light, alterations in the turgescence are, as a matter of fact,
connected.

Since it is absolutely impossible to obtain a clear idea of the facts here under
consideration unless this point is kept in view, I may attempt to illustrate the above-
mentioned properties of the motile organ still further, by means of Fig. 369.
A represents a longitudinal section (or better, a longitudinal plate) of the organ,
cut off transversely above and below. It is noticed how the two halves jj of
the parenchyma swell and project above and below, because the axial strand g
is too short for them. In B the right half of the swelling parenchyma s has
been separated from the axial strand by a longitudinal section, and it at once becomes
curved in the manner shown at j on the right. By means of a second section, the left half
of the swelling tissue has been first bisected, and then the inner part separated from
the strand ; this became curved concave towards the strand, while the external half
became curved concave outwards. From B then, it is clear that in the parenchy-
matous envelope of the organ the external layers are likewise in a state of tension
towards the internal lavers, but so that the entire mass of tissue tends to become

632

LECTURE XXXVI.

convex on the outside. This kind of tension is still more clearly shown in C,
where only the one side of the swelling tissue is separated from the strand, and
the latter now becomes itself curved by the extension of s ; had the right side

^rG. 370.— Leaf of Oxalis

diurnal, 2 nocturnal positii

of the parenchyma not been cut away from C, but had it instead lost only a part
of its turgescence, then also the organ would have been compelled to make a
similar curvature to that in C. With the longitudinal tension thus shown, the

S^CÄ

Fig. 371.— Transverse section of the motile organ of a leaflet oi OxaHs i

corresponding ' transverse tension also is connected, as is obvious at once from
D and E. Z> is a transverse plate of the organ, and in E it has been
divided into two halves by a longitudinal section: it is noticed in F how the

HISTOLOGICAL STRUCTURE OF MOTILE ORGANS.

633

tissue s projects in both halves beyond the strand. If the observations are carried
out in the way here described, the parts cut out of the organ must be laid in
water on a glass plate, in order to avoid drying up, since that would destroy
the turgescence and tissue-tension. That the relations described exist in the living
organ in exactly the same way, however, is at once perceived on making proper
sections through such an organ still attached to the stem.

It will certainly not be superfluous to illustrate the relations of organisation
again in the case of Oxalis. Fig. 370 shows a leaf, at / with its three leaflets
in the diurnal position, at 2 in the nocturnal position. Further description of
these is unnecessary. It is noticed, however, that the motile organs which connect
the three leaflets to the top of the petiole are in this case very small, and in

Fig. 372.— Longitudinal section of the motile organ H i o! a leaflet of Oxalis carnea. ~"

fact broader than they are long. The transverse section of such an organ is
represented in Fig. 371, somewhat highly magnified: beneath the epidermis 0
there is here again a relatively thick mass of parenchymatous tissue, which has
distinct intercellular spaces only between the cells of its innermost layers ch : its cells
contain chlorophyll-corpuscles as in the swelling tissue of the Bean, and in other such
cases. Within the vascular bundle-sheath s lies the axial strand G, which here also is
to be regarded as a union of numerous vascular bundles running together in the
petiole and separated in the laminae. Here, also, this strand consists of thick-walled,
very firm, but by no means lignified tissue ; since it is important that while it is very
elastic, it shall yet be exceedingly flexible, for it has to undergo very strong curvatures,
both towards the upi)er and towards the lower side, according to the degree of

634 LECTURE XXXVI.

illumination, when either the lower or the upper side of the swelling parenchyma
is much expanded.

Fig. 372 may afford an approximate, though not an exact idea of the changes
in dimensions of this parenchyma in the alternating diurnal and nocturnal positions.
It represents a longitudinal section through the uppermost portion of the common
leaf-stalk, on which, to the left, is situated one of the three motile organs, with the
mid-rib // of the corresponding leaflet. Beneath the epidermis p the parenchyma
of the petiole as well as of the mid-rib consists of large thin-walled cells; in the
centre the vascular bundles G G ascend in the petiole, with the pith m between
them ; but as soon as they enter the motile organ, they close together and form an
axial strand devoid of pith, which eventually enters the lamina of the leaflet
to form the mid-rib with its lateral ribs. In making this preparation, care was
taken to make the section somewhat thicker on the lower side of the motile
organ (at b b) than on the upper side, where the plate is very thin : hence the
turgescence of the lower side and its tendency to expand were increased (the
preparation lying in water), and those of the lower side lessened. The organ
has, therefore, become curved upwards, just as if it were in the diurnal position,
and the thicker lower side b b presses the axial strand, by means of its tendency
to expand, concavely upwards; and this pressure is at the same time so strong,
that the swelling tissue of the upper side is very strongly compressed, as seen.
If the section is made so that the swelling tissue of the upper side remains thicker
than that of the lower side, a preparation is obtained which represents approximately
the nocturnal position : the swelling tissue of the upper side, when the preparation
lies in water, is then expanded, that of the lower side compressed.

I think these somewhat prolix descriptions will sufficiently prepare the reader
to understand what now follows. I shall here rely partly on my own older
observations, but chiefly on a detailed investigation by Pfeffer.

Proceeding first then from the easily demonstrated fact that by darkening an entire
plant or even one leaf only, the nocturnal position results after from half an hour to an
hour, and when this has occurred the diurnal position is restored on the subsequent
access of light ; the next question is naturally. What changes in the swelling tissue call
forth these two antagonisdc movements ? Of course it is at once clear that in a down-
ward curvature the upper side must become longer than the lower, and that in an
upward curvature the contrary must take place. Provided that a fully developed motile
organ is under consideration, it may be established by observation and calculation
that even after numerous up and down curvatures no permanent elongation of the
entire organ, or, what amounts to the same thing, of the axial strand, results. From
this, then, it is to be concluded that when one side of the organ has become convex, i.e.
longer than the other, or again relatively shorter when the curvature is in the opposite
direction, a corresponding increase or decrease of the water in the half of the organ
concerned must have taken place, because no other change whatever is conceivable.
For the present we may leave out of account the question how this is accomplished, and
use this conclusion simply to obtain an expression for the mechanics of the movement
itself, for we may now say, as darkness comes on, that side of the organ which is to become
convex in the first place obtains more water — i. e. its turgescence and its volume
increase and become greater than on the opposite side. This is induced by the change

CHANGES TN THE AMOUNT OF WATER CONTAINED. 635

in the intensity of" the hght. There is still, however, a very important point to be
observed here, namely that in darkness an increase of rigidity of the whole organ and
therefore an increase in the amount of water which it contains occurs, while on illumin-
ation a diminution of the expansive tendency of the parenchyma and thus a decrease
in the quantity of the water and rigidity of the whole organ takes place ; in the nocturnal
position, the entire organ is more rigid than in the diurnal position, as Brücke, and
subsequently Pfeffer, proved. Moreover, Millardet showed, and Pfeffer confirmed, that
in darkness an increase of turgescence appears simultaneously in the upper and lower
half of the organ, and in hke manner, on illumination, diminution of rigidity occurs in
both simultaneously; but although these changes occur simultaneously they do not occur
with equal rapidity on both sides of the organ. That side in which the increase of tur-
gescence results more rapidly thus first becomes convex, and the other side is passively
compressed by its extension ; meanwhile, however, the same change takes place, though
slowly, in the now concave other half, and this begins to extend and then presses the
organ over towards the other side. This process requires from a few to as many as
twelve hours. Then, according to Pfeffer's description, the reverse change begins
spontaneously : the side of the organ which has at last become concave, again becomes
convex, and this process also is then reversed, and this goes on, the movements be-
coming less and less marked, till after a few days the organ comes to a stand-still.

This result, i.e. this long-continued after-effect, only occurs in an undisguised form,
however, when the plant remains for several days in the dark, after having been exposed
to the normal alternation of day and night, and has assumed its nocturnal position ; or
when, normally vegetating, it has assumed its diurnal position and is then constantly
illuminated for some days. In both cases the after-effect, swinging to and fro like a
pendulum, dies away, but with a different final effect in each case; for if the plant re-
mains in constant darkness until the after-effect ceases, it is then in a state of darkness-
rigor, and is no longer sensitive even to sudden strong illumination, though if con-
tinuously illuminated it may again return to the condition of phototonus ; if, on the
contrary, the movement ceases during constant illumination, the motile organs are at
once irritable for a subsequent darkening, and assume the nocturnal position.

From the behaviour here described, as was first shown by Pfeffer, the ordinary
daily periodicity now arises, the sleeping and waking, the daily recurrent variations of
light combining with the after-effects — i.e. when the leaves have assumed their nocturnal
position in the evening, there still follows during the darkness of the night, as an
opposite effect, a tendency to assume the diurnal position. If then in the morning
the light falls on such a leaf, this also induces a tendency to the diurnal position: after-
effect and the influence of direct light thus combine, and since these changes in the
organs, which have produced the diurnal position, are now followed in their turn
again by the tendency to attain the nocturnal position coming in spontaneously, the
latter is aided by the darkness which comes on in the evening. It is clear that the
periodic variations caused by the after-effect need not always exactly coincide with
the daily variations in brightness, and that therefore several kinds of differences
in the combined actions must result.

The above, then, embraces in the main the theory of the sleeping and waking
of periodically motile leaves which possess special motile organs. I may still add that
Pfeffer, by means of ingenious researches, has also measured the magnitude of the

62,6

LECTURE XXXVI.

forces at work in the motile organs : in the case of the common Garden Bean, for
instance, the increase of the expansive force in the upper half of the organ in the
evening may equal a pressure of five atmospheres, so that the total expansive
force then amounts to a pressure of more than seven atmospheres — i. e. if we
suppose the transverse section of the motile organ to measure i sq. cm., a pressure of
more than seven kilograms would be acting on it. Of course the transverse section
of such an organ is usually only about i sq. mm., for which the pressure would
amount to only the hundredth part of the above.

The reader will no doubt be desirous of having the theoretical statements made
above in the abstract, illustrated clearly by means of a few examples. I will therefore
next quote a series of observations which I made' with Acacia lophantha, ' On the
20th April, 1863, a young plant of Acacia lophantha furnished with new, fine, very
healthy looking leaves, was placed in a wooden case, in which was also a thermometer
hanging close beside the plant.

' The observations were here made hourly, but I will include in the table only
those observations where some change is shown, in order not to make it inconve-
niently long; wherever 4-10 hours are passed over in the table, it signifies that there
was no noteworthy alteration in that time.

X

Hours.

C».

Acacia lophantha in the dark.

20

9 p.m.

17-5

Nocturnal position.

21

6 a.m.

17-5

Leaflets opened 90"^.

8 „

17-5

Opened further.

12 noon

18.0

Leaflets opened 180°.

6 p.m.

17-5

Leaflets opened about 60-70'.

9 »

17-0

The older half-opened, the younger quite closed.

22

6 a.m.

170

Leaflets opened about 1 30°, lateral stalks irregularly downwards.

8 „

17.8

Leaflets opened about 180°, secondary stalks irregular.

1 2 noon

18.0

The same.

4 p.m.

18.7

Leaflets beginning to close.

9 »

18.0

Lower leaves open, upper half-open.

23

7 a.m.

17-7

All leaves regularly expanded flat.

12 noon

16.2

The same.

10 p.m.

16.2

The same.

24

6 a.m.

15-6

The same— inner ones not quite flat.

12 noon

16.2

All leaves quite open (180°).

10 p.m.

15-6

The same — upper leaves irregular.

25

6 a.m.

15.0

All open and expanded flat.

'The plant had thus for 48 hours discontinued its periodic movements all but
mere traces. After the last observation it was placed in the window, where, the sky
being cloudy, its leaflets placed themselves decidedly downwards (the angle of opening
being far beyond 180°) within two hours, and then appeared slight changes in position in
the secondary stalks. About 12 o'clock (noon) iX-ivs, Acacia sufi'ering from rigor due to
darkness together with one in the normal condition were placed in the dark : the
leaflets of the former did not alter their position but remained open, while those of the
latter, on the contrary, assumed a pronounced nocturnal position within an hour.

pfeffer's experiments. 637

Both were then placed at the window, where the leaves of the rigid plant again
maintained their position unchanged, whereas the previously closed leaves of the
normal plant opened again in an hour, the sky being cloudy. In the evening of this
day (about 5 o'clock) the lower six leaves still remained stiffly open, the upper ones
(8 and 9) closed : next day the periodic movement was completely reinstated. The
plant had taken no harm, and is now vegetating actively.'

In the year 1870 Paul Bert, as Pyrame de Candolle had already done at the
beginning of this century, exposed Mimosas to continuous illumination with lamps
during the night and ordinary daylight during the day, and found that the extent
of the movement of the principal leaf-stalk gradually diminished, but subsequently,
under the influence of the daily variation of light, was again restored. The effect of
continuous illumination was also investigated in detail by Pfeffer, who employed
for this purpose two Argand gas-lamps, which, however, were likewise employed
only for illumination during the night, the plants being exposed to diffused bright
daylight during the day. Pfeffer expresses himself as follows respecting an investi-
gation made in this way with Acacia lophantha : —

*In continuous illumination the amplitude of the daily periodic movements
gradually decreases, and if no autonomous movements affect the experimental
objects, the leaves at length become motionless, but are at the same time perfectly
sensitive paratonically. In an experiment made with Acacia lophantha, a small pot
plant bearing four leaves was kept in diffused light on 13th June, 1873, and
illuminated in the evening. On this day the leaflets closed completely ; on the
next almost completely; on June 15th the amplitude of the movement of each
leaflet was about 70°; on the i6th June 15-35°; ^^ the 17th June 5-20°; and
then on June 19th, variations in the brightness being avoided as much as
possible, the amplitude was at any rate less than 5°. The periodic movements
had thus practically ceased, the leaflets on the older leaves being expanded nearly
flat, those on young leaves being inclined towards one another as much as 130°.'

Plants which have become motionless in the light are just as sensitive to
darkness as are plants which have been submitted in the usual way to the daily
variation of illumination.

' In the leaves of the above-named plants,' he continues, ' no autonomous
movements can be demonstrated with certainty, but where such exist they continue
even when the iliumination is constant, and apparently do not lose in amplitude.
The autonomous movements of the leaflets of Trifolium prate7ise are very considerable;
the terminal leaflet may accomplish a movement of 30-120 degrees in the course
of 1^-4 hours. If such a plant, previously exposed to daily variation of light, is illumi-
nated in the evening and thenceforth kept continuously in the Hght, no further
closing movement corresponding to the daily period was to be observed even on the
following evening, apparently because it was obscured by the autonomous move-
ments, which went on with an amplitude of as much as 100 degrees, and with
a rhythm of about two hours. These autonomous movements continued also un-
changed while the plant was kept constantly illuminated for two days longer. Au-
tonomous movements of less amplitude and shorter persistence occur in the
primary leaf of Hcdysanim gyrans, in which occasionally an amplitude of vibration
of only 8 degrees in a period of ten to thirty seconds is observed. If this plant

638 LECTURE XXXVI.

is submitted to artificial illumination, in the manner just described for Tn/olium,
it is possible on the evening of the following day still to perceive clearly the
sinking corresponding to the after-effect of the daily period, the autonomous
movements going on all the time. The independence of this from the daily
periodic movements here comes sharply into view, and the same is the case when
the plant is kept in the dark, where it behaves much as in continuous light. This is
true also for Trifolium prafense — i. e. even on the first day of its being kept in the
dark the daily period is no longer perceptible, on account of the great amplitude of
the autonomous movements.'

In agreement with the fact which I had previously established, that movements
in the vegetable kingdom which are induced by light depend upon the strongly
refrangible or so-called chemical rays, it is also found that the paratonic stimulation
of periodically motile leaves is caused by the light of the blue half of the spectrum.
I demonstrated so long ago as 1857 that in the case of the leaves of the Bean and Wood
Sorrel, when in their diurnal position, covering them with a dark blue glass bell-jar which
excludes all the yellow, green and orange light, produces no change whatever in the
position of the leaves, although to our sense of sight a strong darkening is connected
with it ; to the plant, however, it reacts not like darkness but like complete light. If
the plant, or a single leaf of it, is covered with a bell-jar of ruby coloured glass
which transmits only red and a trace of green light, then the effect is exactly as
if an opaque receptacle had been employed : the leaf in a short time assumes its
nocturnal position, and thus reacts to the red light as towards darkness. It
would be a mistake to suppose from what has been said above, however, that only
the plant covered with the blue cobalt glass would awake again next morning ; for
the one under the ruby-coloured glass does so also, and we know indeed that
it would do so even if it remained in profound darkness.

Attacking at last the question as to what the first effect of the stimulus
properly consists in, which an alteration of the intensity of the light produces, we
must adhere to the conclusion already arrived at and drawn directly from
the facts (p. 634) that it concerns changes in the turgescence in the two halves
of the tissue of a motile organ, and that these under the prevailing circumstances
can only depend upon the addition and abstraction of water. Darkening thus
causes an increased flow of water into the whole motile organ, but more rapidly
into the one half than into the other ; increased brightness of the light, on the
other hand, must cause an abstraction of water from the entire organ, because
it becomes more flaccid, and this again in the one half more quickly than in
the other. This water, however, is inside the cells, surrounded by protoplasm
and cell-walls, and in part is also present in the protoplasm or in the cell-walls
themselves. Two possibilities thus present themselves. Either the light acts on
the imbibition-forces of the cell-wall, increasing them by darkening and diminish-
ing them by brightening ; or the variation of light influences directly the properties
of the protoplasm itself, and induces in this changes by which the turgescence
may be increased or diminished. For the first of these assumptions no certain
analogy is known, and on closer reflection it only leads to new difficulties.
On the contrary I shall show in the next lecture that we can analyse certain
phenomena of irritability of the organs of Mimoseae and others, so far as to say

MECHANISM OF THE MOVEMENTS. 639

that the stimukis first produces a change in the molecular structure of the protoplasm,
which thereby becomes more capable of filtration, i. e. allows water to escape,
whence it penetrates also through the cell-wall, and consequently the cell
becomes smaller. In this manner curvatures, exactly like those here considered, are
produced in the similarly motile organs of the leaves of Mimosa and other plants
by means of mere vibration. Supported by this analogy then it is probable
that we may decide in favour of the second of the two alternatives mentioned
above. We may assume as probable that an increase in the intensity of the light
makes the protoplasm in the cells of the periodically motile organs a little
more capable of filtration, so that the already very high pressure of the cell-sap on
the wall causes a small quantity of water to filter out and enter the neighbouring parts
of the stem or petiole ; whereas on darkening, the capacity for filtration of the
protoplasm is increased, making possible a greater turgescence and extension of the
cells.

This is of course, in the meantime, only a conclusion based on analogies ; but it
appears to be supported by all that we know otherwise of irritability in the vegetable
kingdom, and as to turgescence and the properties of protoplasm. However, I must
refer to the next lecture for more particulars.

In conclusion, before dismissing the subject of daily waking and sleeping, and the
paratonic irritable movements of the leaves due to light, so far considered, I may make
a few short remarks on a point concerning the mechanism of these movements, which
Pfeffer first insisted upon and investigated in detail. When, in the case of a com-
pound leaf, like that of the Bean, the motile organ is situated at the base of the petiole,
the size of the angle between the petiole and the shoot-axis is altered with its move-
ments; the entire leaf rises and falls — in the Bean, for example, it rises in the
evening and sinks in the morning — in the evening the apex of the angle is smaller,
in the morning larger. Simultaneously with these changes, however, the position of
the three leaflets at the other end of the leaf-stalk is also changed ; in the evening
they all curve downwards, and especially the most anterior leaflet. By this, how-
ever, the moment of rotation which the weight of the leaflet on the petiole exerts
as a lever on the lower motile organ becomes smaller. This favours the erection of
the whole leaf-stalk, or the diminution of the angle between it and the shoot-axis, and
the reverse must occur on the access of the light in the morning ; by the direct
stimulus of the light the chief organ of the leaf-stalk not only becomes curved
downwards, but also a little more flaccid, and the weight of the three leaflets
causes a further depression, and in addition the leaflets now become expanded
and thereby exert a greater moment of rotation on the motile organ, and this
also acts again in the same sense that the leaf-stalk falls somewhat more than
would have happened simply by the stimulus of its motile organ by light. In
this case, then, the changes of position of the leaflets in connection with their sleep-
movements, acting on the petiole as a lever, produce a similar effect to that of the
paratonic stimulations on the lower motile organs of the leaf-stalk itself. In the
Mimosa, on the contrary, Pfeffer showed that the sleep-movements of the two or
four secondary petioles at the end of the main stalk produce lever-actions on the
latter which influence the paratonic movements of the large motile organ at the
base of the leaf-stalk in the opposite sense ; since, on darkening, the secondary

640 LECTURE XXXVI.

petioles and leaflets lay themselves together, the lever on which their centre of
gravity acts becomes longer, and thereby the moment of rotation of the leaf-
stalk so increased, that it sinks in the evening, and its motile organ curves
downwards, although it strives, in consequence of the action of the light in itself,
to erect itself as in the case of the Bean. I give these remarks on Pfeffer's
authority, not having yet had an opportunity of making decisive observations for
myself.

There are perhaps not many opportunities where it is possible to make so
clear to the non-physiologist the extraordinary difficulties which are often con-
nected with the investigation of the phenomena of life as here, in the case of
the sleep-movements of compound leaves, and it is chiefly on this account that
I have dwelt on them at so great a length.

As already stated, there are also numerous foliage-leaves, at the base of
which no specially developed motile organ exists, and where also the indi-
vidual parts of the leaf are not sharply separated, and not connected with the
stalk or mid-rib by means of special motile organs, but in which, nevertheless,
very evident or slightly perceptible sleep-movements as well of the whole leaf
as also of its parts take place, and this again in such a way that the entire
leaf rises and falls, approaching or retiring from the upper part of the shoot-
axis, while in many cases the lateral ribs of the lamina curve at the same time
upwards or downwards, leaving the lamina flat or curved. However, only the
movements of whole leaves produced by upward and downward curvatures of
the petioles, or the lower parts of the laminse, have been investigated in detail ;
and the following statements refer to these only. According to Batalin and
Pfeffer the movements in question are very evident in the following well-known
plants. Several Balsaminese {Impah'ens noli-77ie-ta7igere), Chenopodieoe, Atriplicese,
Solanege, Mimulus, Mirabilis Jalappa, species of Silene and Alsine, some Com-
positse, Malva rotundifolia^ (Enoihera, Porlulacca, Linum grandiflorum, species
of Polygonum, Senecio vulgaris, Ipomcea purpurea (Blue Bindweed), Brassica
oleracea (Cabbage); I may also add the leaves of Helianihus annuus (Sunflower),
and particularly the leaf-like cotyledons of many dicotyledonous seedlings.

From the investigations of the observers mentioned, the movements of such
leaves agree with those of the group described in detail above, in all essential
points except one ; these leaves are not furnished with special motile organs, and are
found to be in periodic movement only so long, and are only so long para-
tonically sensitive for variations of light, as they are still growing, and in fact
the irritability begins to make its appearance when the young leaves come
forth from the bud ; the irritability increases, and the magnitude of the daily
movements likewise, in proportion as the growth is accelerated, in accordance
with the grand period of growth ; when the growth becomes slower again, the
irritability and motility also decrease more and more, until at length, on the
cessation of growth, the sleep-movements also disappear. Here, also, the motile
part, capable of curvature, need not remain the same; if the zone of strongest
growth advances along the petiole, or forwards from the base of the lamina, the
curvatures also occur at corresponding places.

With regard then especially to the mechanism of this movement,, practically

OPENING AND CLOSING OF FLOWERS. 64t

all that has been saitl of the proper motile organs, applies to these cases also, at
least according to Pfefifer's account, only with the difference that the important
matter here is not, as there, a periodic lengthening and shortening of the upper
or lower side of the organ, but each elongation with its consequent curvature is
permanent ; when the other side becomes elongated, with a corresponding curvature
in the opposite direction, this elongation is also permanent, and so on — i.e. the
upward and downward curvatures are here caused by first the one and then the other
side growing more vigorously.

Fundamentally, however, there is no difference whatever in principle, as
is evident from the fact that even in the case of the leaves of the group first
considered the periodic movements commence before the motile organs are fully
grown. Siill more important, however, is the following consideration ; it has
already been shown in previous lectures that growth in general depends upon
the turgescence of the cells, and that as the turgescence of the tissue increases
its growth is accelerated, and as it diminishes, it is retarded. If now changes in
turgescence are caused by illumination and darkening, just as in the motile organs
of the first group, so also changes in the rapidity of growth must occur in growing
organs. In this case, for upward and downward curvatures to result, the alter-
ations in turgescence here also must take place more quickly on the one side of the
organ in question than on the other.

From Pfeffer's descriptions it is particularly clear in the case of the young leaves
of Impatiens noli-me-tangere, which are very sensitive to variations of light, that they;
agree in every respect, apart from the difference just explained, with the leaves
of the group first considered above.

Pfeffer established in 1873 that those corollas which exhibit so-called sleep-
movements — i. e. which open and close at certain hours of the day — likewise owe
this periodic movement to periodic alterations in the growth in length of the outer
and inner sides of the petals. Since, therefore, we are here no longer con-
cerned with a new principle, but only with peculiarities of floral structure, and
specific differences, I shall desist from descriptions in detail, and only dwell on
one point.

While in the case of the foliage-leaves of the first group, the effect of changes
of temperature is of entirely subordinate importance, compared with the paratonic
action of variations of light, in the case of the foliage-leaves of the second group,
where growth is the prominent feature, the variations of temperature exert
great influence. But it is in periodically motile flowers that sudden variations
of temperature induce the most active movements, and of such a kind that
an elevation of temperature causes the flowers to open, and thus produces an
outward curvature of the petals, while sudden cooling effects their closure,
that is an inward curvature, by the more active growth of the outer side.
In addition to the flowers of the Crocus and Tulip, Pfeffer mentions as very
sensitive to changes in temperature the corollas of Adotiis vertialis, Oniithogalum
umbeUatum, Colchicum aiiUimnale — i. e. particularly flowers which are apt to open
in the Spring or late Autumn, when the temperature of the air is low, and
which are only occasionally warmed by the sun. Less sensitive arc those of
Ficaria rannticiiloidcs, Anemone ncmorosa, and Milopc Irifidci. all of which execute
[3] Tl

642 LECTURE XXXVI,

movements at all limes of the day under the influence of changes of tempera-
ture, but the more energetic the longer the interval since the last movement was
accomplished. This is still more conspicuous in the case of Nymphcea alba, Oxalis
rosea and O. valdivicnsis, ]\Iese7nhryanlhemiim, and the motile flowers of Compositse.
These close in the evening, and then even increments of temperature to the extent of
10-28° C. scarcely effect their opening; in the morning, on the contrary, a
rise of temperature causes these flowers to open even when it is dark.

The reader may easily see for himself these effects of variations of tem-
perature by taking Crocuses and Tulips cultivated in flower-pots, in the Spring
when the weather is cool and the flowers outside are closed, and placing
them simply in a warm room, where they will often open after a few minutes.
The observation may be made even more simply : if a closed Tulip-flower
is placed with its stalk in warm water at 20-25° it opens visibly, and this in
fact was the experiment by which Hofmeister first detected the influence of
variations of temperature on the opening and closing of flowers. In the case
of the Tulip and Crocus, Pfeffer w^as able by warming and cooling the flowers
to make them open and close eight times in one day; but even in these cases
the opening was more energetic if the flowers had been closed for some time,
and conversely. Particularly sensitive Crocus flowers may be made to open and close
completely by a variation of 5° C. in eight minutes; wäth variations of i2-20°C. this
resulted in as little as three minutes. Pfeffer found, moreover, that Crocus flowers
are sensitive to even ^° C, and those of the Tulip to variations of 2° C.

Those flowers which are sensitive to variations of temperature are more-
over sensitive to variations of light. Nevertheless the sensitiveness is greater
for the one or the other according to the species of plant : the flowers of
the Crocus and Tulip which are so very sensitive to variations of temperature,
become closed when suddenly placed in darkness, and opened on being illuminated,
and this indeed with an energy which is capable of overcoming the effect of opposite
though feebler stimuli due to temperature. On the other hand greater variations in
temperature are again able to reverse the opening or closure effected by light
and darkness. In Oxalis, Nymphcea alba. Taraxacum, &c., on the contrary, the
closure in the evening cannot be arrested by raising the temperature, and just
as little can lowering the temperature in the morning prevent the opening. If
however these flowers are kept closed during the day, they can be opened in
the evening by raising the temperature, and so on.

From this different susceptibility for variations of light on the one hand, and
for variations of temperature on the other, we obtain the simplest explanation of
the fact that some flowers in the open exhibit a marked daily periodicity, while others
close and open at any time of the day on sudden changes in the weather ; evidently
the former are sensitive for the daily variation of light more than for variations of
temperature, the latter, on the contrary, are very susceptible to sudden warming and
cooling.

Finally it has yet to be mentioned that some flowers close when the light is
too strong, as well as when the temperature is too high, thus resembling periodically
motile foliage-leaves — with w-hich, however, this only happens when the light is too
intense. Pfeffer observed this in the flowers of Oxalis valdiviensis, Calendula,

USES OF SLEEP-MOVEMENTS, ETC. 643

&c., when in the direct rays of the sun. Still it is of course doubtful whether in
such cases the effect is to be ascribed to the light or to the heat.

If in conclusion we raise the question as to the purpose and uses for the
plants concerned of all the periodic movements and changes directly resulting from
stimuli here described, of course only a special consideration of the mode of life
of each individual species could afford the answer in detail ; still their utility can
be recognised in a general sense even without that. That flowers with rare
exceptions open in the morning, as the brightness and warmth increase, and close
in the evening, is evidently connected with the function of pollination — i. e. with the
transference of the pollen from one flower to the female organ of another flower
of the same species. For this transference is effected by insects, which as a rule only
visit the flowers in bright and warm weather : at night, when the conditions are
different, the sexual organs of the flower are protected by the closure of the
corolla against excessive cooling by radiation and wetting with dew, and probably
from several other dangers. The opening and closing of foliage-leaves may in some
cases enhance the above described protection of the sexual organs, though as a rule
it may be assumed that by pronounced erection or depression of the laminae of
foliage-leaves in the evening, the excessive cooling of the highly important
chlorophyll-tissue during the night is avoided ; on knowing that these thin tissue-
lamellae may be cooled by mere radiation at night, particularly when the sky is clear
and serene, to 5-8" C. below the temperature of the surrounding air, it is obvious
that even during nights when the air is at 5°-6° C. such a cooling may fall below
the zero point and lead to the danger of freezing. The extended diurnal position
of the foliage-leaves is simply and only adapted for temperatures and lights favour-
able to vegetation, and only useful for assimilation within these limits ; wherever
the conditions are unfavourable in this respect, particularly in the case of thin
delicate foliage-leaves, they become closed or assume the profile position, as well in
excessive sun-light as in the darkness and cold of the night.

T t 2

LECTURE XXXVn.

THE IRRITABILITY OF MIMOSA AND OTHER PLANTS.

Like several other Mimosai and Oxalidae, the Sensitive-plant {Mimosa ptidica)
is noted for the fact that the motile organs of its leaves are irritable not only in
response to the various stimuli mentioned in the previous lecture, but also to small
vibrations and other disturbances. These are the most easily perceived and therefore
the longest known phenomena of irritability, and indeed up to the present century
they were regarded as the only ones in the vegetable kingdom. The effects of
stimulation and the mode in which those effects are produced in this case have now
been studied so often, so thoroughly, and with such good results, that we may regard
them as affording so far the most solid foundation in the whole province of the
phenomena of irritability, so that not only a series of organs which behave similarly,
but in fact almost all other phenomena of irritability, become more or less in-
telligible by means of them.

Mimosa pudica, a Leguminous plant belonging to the widely spread tropical
family Mimoseae, is a native of Brazil, and is now commonly met with in the East
Indies and other tropical countries ; even with us it produces abundance of seeds
capable of germination, and this not only in pots and cultivated in the window, but
even in the open, so that anyone may grow this remarkable plant, and with little
trouble observe the phenomena of irritability now to be described. Growing in
the open, especially in bright sunshine, it forms several vigorous foliage-shoots,
often 60-80 cm. long, which lie prostrate on the soil; in a chamber, on the
contrary, i.e. in a feebler light, the principal shoot grows erect, and only a* few
lateral shoots come out obliquely from below. On each shoot-axis there are
6-10 doubly compound foliage-leaves; on a petiole 4-8 cm. long, we find two,
or a couple of pairs of secondary petioles each 4-5 cm. in length, and each of
which bears 15-25 pairs of small leaflets. Each leaflet is about 5-10 mm. long
and 1-5-2 mm. broad (see Fig. 373 A). All these parts are mutually connected by
well developed motile organs. Each leaflet is situated directly upon a motile
organ, o-4-o-6 mm. in length, joined to the secondary petiole; the four secondary
petioles in their turn are connected to the end of the primary leaf-stalk by a some-
what larger organ which is 2-3 mm. long and about i mm. thick. The base of the
primary leaf-stalk itself is transformed into a motile organ 4-5 mm. long and 2-2-5
mm. thick.

The structure of these motile organs in general agrees with that of those of the
Bean and Wood Sorrel which have been described in the preceding lecture : each

TUE S'ENXITIVE-PLAXT.

^4r>

motile organ consists of a thick covering of parenchyma enveloped by a feebly
developed epidermis devoid of stomata, and which surrounds an axial, flexible,
but only slightly extensible strand of vascular bundles ; the individual bundles of this
strand arise from the vascular bundles of the shoot-axis, and at the other end of the
organ where they enter into the leaf-stalk they become again isolated, as has been
described in the case of the Bean, The parenchyma consists of rounded cells which
in the neighbourhood of the vascular strand enclose rather large intercellular spaces
containing air; these spaces are much smaller, however, in the 10-20 outer layers
of the parenchyma, and are altogether absent in the neighbourhood of the epidermis.
These air-cavities between the cells communicate with one another from the strand
to the middle layers of tissue ; the very small intercellular spaces of the outer layers
of cells, on the contrary, appear as separate three-cornered lacunae, and, in specimens
under the microscope, are filled with water. The cells of the lower side of the organ
are thin-walled, those of the upper side have walls about three times as thick, which like
the former are traversed by numerous pore-canals. In addition to abundance of

-Leaf of Mimosa pniiica, half i
position, or after

ural size. A the diurna
itation by a shock (afte

1: n the noctii

protoplasm, nucleus, and small chlorophyll-corpuscles and starch-grains, the cells
contain each one large spherical drop suspended in the sap-cavity, and which consists
of a solution of tannin surrounded by a delicate pellicle, Unger also found similar
structures in the motile organs of Desmodmm gyratis and Glycyrrhtza, which, how-
ever, are not sensitive to contact and vibration. Again, the organs of Mimosa are
irritable even in the young state, when the cell-walls in the upper half of the sheath of
parenchyma are not yet thicker than those of the lower, and the spheres referred
to are still wanting. Too great importance therefore is not to be attributed to these
anatomical matters, in respect to the specific phenomena of irritability ; and this so
much the less, because the corresponding anatomical characters of the irritable tissue
in other organs to be considered later, differ in several respects, although the
phenomena of irritability are the same in all essential points.

When a Sensitive-plant is left to itself during the day, its petioles are
directed obliquely upwards, the secondary petioles with their leaflets being extended
almost exactly in one plane, as in Fig. 373^. Any vibiation (unless very gentle) affects'

646 LECTURE XXXVII.

the whole plant, so that the motile organs of all the primary petioles curve
downwards, those of the secondary petioles forwards, and those of the leaflets
forwards and upwards, as shown in Fig. 373 ^. This condition resembles
outwardly that of a leaf in the nocturnal position, or the one induced by sudden
darkness ; internally, however, it differs from it, because vibration still induces
stimulation in leaves which are already in the nocturnal position, as is particularly
evident by the relaxing of the lower large motile organ. Mention may here be
made of the very important fact, that the nocturnal position induced by dark-
ness is connected with an increase of turgescence and rigidity of the organ,
whereas it suffers a very marked relaxation under the influence of vibration, so
indeed, that a INIimosa leaf stimulated by contact or vibration, may swing loosely to
and fro until it again assumes the irritable condition. In the day-time this follows
after a few minutes, the leaves assuming the position represented in Fig. 373 ^,
whereupon they are again sensitive to vibration.

In the case of the motile organs of the primary petiole and secondary
petioles, in very irritable IMimosae, a slight touch of the hairs on the lower side
suffices to induce the movement ; and the lightest touch of the glabrous upper
side causes the movement of those of the leaflets.

The sensitiveness of the Mimosaä depends to a large extent on the elevation
of the temperature, and upon the moisture of the air : as these increase, the water
contained in the whole plant increases, and particularly the turgescence of the
motile organs. With the air at 25-30° C, and sufficient moisture, the irritability
of the Mimosa is so great that the mere shaking as one passes by the plant acts as
a shock sufficient to irritate it, and its leaves fall and close up ; it is thus almost
impossible to lift up and replace a plant in a flower-pot, even with great care,
without inducing the effects of stimulation.

Of pecuHar theoretical interest also is the unusually clear propagation
of the stimulus in the Mimosae, which is effective over distances 50 or more
centimeters in length. If, for example, one of the terminal leaflets is snipped
off, or its motile organ touched, or the sun's rays concentrated to a focus
on one of these leaflets, a movement is at once produced, and the opposite
leaflet rises almost simultaneously, the neighbouring leaflets then following in
pairs, and so on with leaflets further and further away, down to the base
of the secondary petiole : after a short pause the lowermost leaflets of a neigh-
bouring secondary petiole then begin to fold together, and this proceeds from
below upwards, and is repeated by the leaflets of the other secondary petioles,
and finally (often only after a long time) the primary petiole of the leaf also falls
downwards. If the plant is only irritable to a moderate extent this is all that
happens, and the stimulated leaf resumes its normal position after a few minutes ;
but in very irritable and quite healthy plants, the irritable movement of the first leaf
is followed after a few seconds by the sudden falling down of one of the nearest
leaves situated higher or lower on the shoot, and thence onwards, proceeding
in series, the irritable movement of all the leaves of the same shoot following as
if the plant had been shaken. Thus, in the course of a few minutes all the leaves of
a shoot of a vigorous Mimosa may be set in movement, although only a single
leaflet had been stimulated originally ; occasionally it happens that individual

EXPULSION OF WATER ON IRRITATION. 647

motile organs are passed over, only to be put in motion later. If the plant is
left to itself, the leaflets and secondary petioles again expand after a few
minutes, the primary stalks become erect, and all the motile organs are then again
irritable.

Just as a motile organ can be stimulated by cutting into a small leaflet,
or by burning it, so too it is possible, especially in the case of very turgescent,
and therefore highly irritable Mimosas, to stimulate the leaves from the inter-
nodes of the shoot-axis. Taking care that the shoot-axis is firmly fixed, and
then placing the edge of a very sharp razor carefully and without shaking
on the epidermis of the shoot-axis, and cutting gently into the succulent cortex
until the resistance shows that the razor is penetrating the wood, a drop of
water at once oozes out, especially if the razor is at once removed, and very
soon one of the neighbouring leaves (or even several of them) is set in motion
and relaxes. This experiment was known to the older vegetable physiologists,
Dutrochet and INIeyen, and is exceedingly instructive : it shows that the mere
movement of water within the tissue induces the position of the leaves produced
by irritation. This conclusion is the more certain, since the incision as far as the
wood only induces the effect described, when a drop of water wells out from the
wound : if the tissue is not sufficiently turgescent to exude water after the
wounding, no movement of the leaves in the neighbourhood is induced.
Evidently the incision of a leaflet with a pair of scissors, or burning it in the
focus of a lens, produces nothing more than a sudden movement of the water in
the tissues, which is propagated into the irritable organ, and induces the actions
yet to be described. In 1865, in my 'Handbook', I drew from these facts the
conclusion, that in the irritability of the Mimosse the only essential factor must be
the movement of water in the tissues, and corresponding changes in turgescence
in the motile organs.'. Further proofs of this, and a more exact insight into
the processes occurring on stimulation were obtained by Pfeff"er in 1872 ^ By
means of linear measurements on the organ, at first not stimulated and then
stimulated, he established in the first place that the volume of the lower parenchy-
matous half, which becomes concave in the curvature due to irritation, diminishes,
and that of the upper, as it elongates, increases ; but the increase in volume of the
upper half is much less than the decrease in volume of the lower one, whence it
follows that the whole motile organ becomes smaller — decreases in volume — as
it curves down in consequence of stimulation. From all the facts, it follows that
the decrease in volume of the lower parenchyma can only be due to the escape
of water from the tissue, and Pfeffer demonstrates this in the following manner,

' I gave a comprehensive description of the irritable movements of the leaves ai Mimosa on the
basis of my own extended investigations and with reference to what was then known in my ' Expcri-
mcntal-physiologie' 1865 (p. 479), and there first laid stress on the fact that displacement of water
in the tissues is of importance in the matter.

^ Very thorough investigations not only into the leaves of Mimosa and Oxalis, but also into the
stamens of the Cynarc(Z and Bcrhcris were given by Pfeffer in his ' P/tysiologischcti Uniersuchutigcn,''
Leipzig (1873). With respect to his results mentioned in the text, I then gave a renewed
description of the phenomena of irritability generally in my ' Lc/ir/nnJi' IV Aufl., 1 874 (pp. 850-869),
and the preceding lecture contains essentially only an extract from that work ; to which I may refer
the reader as regards several points which can only be briefly touched on lu re.

648 LECTURE XXX VIT.

and I have myself repeated the experiment several times. After cutting across the
motile organ at the bomidary of the petiole, where the axial strand is still undivided,
the organ is at first in the highest degree stimulated, and thus curves downwards.
If the plant is now left in an atmosphere saturated with moisture, under a large
glass bell-jar for instance, the motile organ thus freed from its leaf-stalk erects
itself again, and after a short time becomes again irritable : on now attentively
regarding the cut surface, and irritating the lower side of the organ by means of
a somewhat rough touch with a blunt needle, the organ curves downwards, and
a drop of water escapes immediately from the section. This water, as Pfeffer
showed, comes from the parenchyma itself, and almost exclusively out of that
which surrounds the axial strand and contains large intercellular spaces ; occa-
sionally, however, I have seen the transverse section of the strand itself become
moist. If the parenchyma of the upper side is removed from a motile organ, and
the rest of the organ makes a vigorous movement of irritation, it is occasionally
possible, according to the above observer, to see water escape from the horizontal
longitudinally cut surface of the motile organ.

It is thus certainly established that in the movement of irritation water escapes
from the lower parenchyma. The small increment of volume of the upper paren-
chymatous half during the curvature above mentioned, shows moreover, that part of
the water enters into this tissue ; the decrease in volume of the whole organ, as
well as its becoming flaccid during the movement, as already mentioned, show just
as definitely that a portion of the water expelled from the lower parenchyma must
flow somewhere else, probably at the same time into the rigid tissue of the petiole
and into that of the shoot-axis ; probably a very small quantity also passes into
the axial strand of the organ.

The importance of the subject may justify further reference to Fig. 369,
and to the changes in turgescence in the motile organs, especially the large
ones at the base of the petiole in the Mimosse.

On cutting away the parenchyma of the upper side of the large motile
organ, as far as the axial strand, the petiole not only erects itself again subse-
quently, but it becomes in fact more erect than usual, and the organ operated upon
retains a certain degree of irritability. If, on the contrary, the parenchyma of the
lower side is removed, the petiole falls sharply downwards, and the organ operated
upon exhibits no further irritability. Thus it is the lower side only which is
irritable, the parenchyma of the upper side being only accessory in the movement.

If one of the large motile organs is cut away close to the shoot-axis,
without being separated from its petiole, it becomes curved as usual, a drop of
water escaping from it at the same time. If it is then split by a longitudinal section
which divides the axial strand into an upper and a lower half, the former curves
still more strongly downwards, but the lower one becomes almost straight or
only a little bent downwards. If, further, the upper and lower parenchyma are
separated from the axial strand by means of two longitudinal sections, the former
becomes curved strongly downwards and the latter a little upwards, both
elongating at the same time, so that they project considerably beyond the axial
strand.

These and other experiments show that considerable tension exists between

THE MOVEMENTS A CONSEQUEXCE OF THE EXPULSION OF WATER. 649

the parenchyma and the axial vascular strand, even in the stimulated organ which
has lost water, and that the tension in this condition is greater between the paren-
chyma of the upper side and the strand, than between that of the lower side and
the strand.

If an organ prepared as above and still attached to the petiole is laid in water,
in order to replace the loss of water consequent on the operation, and thus to
produce a condition which approaches the normal one, the downward curva-
ture of the upper half becomes still more pronounced; but now the lower
half curves strongly upwards, and its tissue, previously flaccid, becomes very
tense and almost as hard as cartilage, as in the other half. This shows that
the turgescence of the parenchyma of the lower side was more diminished in the
operation causing the loss of water than that of the upper side, and that by
the re- absorption of water it increases in a higher degree than that does. In
other words, the irritable lower side gives up its water more easily than the
upper side, and takes it up again with greater energy ; the upper parenchyma is
alwa}'S tending to press down the axial strand, while the lower only tends to
curve it strongly upwards when it is very full of water. The latter is the case
with the parenchyma, however, only w'hen it is not irritated ; the stimulation
consists simply in the strongly turgescent parenchyma of the lower side expelling
water.

Lindsay noticed a long time ago that the irritated side of a motile organ
becomes darker in colour; Pfeffer fastened the non-irritated petiole so that the
organ could not become curved on stimulation : on then touching a spot on the
irritable side he saw a darker colour extend with lightning-like rapidity from that
spot. He concludes from this that the air is driven from the intercellular spaces,
and replaced by water expelled from the stimulated parenchyma-cells, since the
darkening only appears to be thus explicable.

Putting together all that has been mentioned, Pfeffer also eventually comes to
the conclusion already arrived at by Dutrochet and myself, that the propagation
of the stimulation in Mimosa is effected by means of the vascular bundles. So
long ago as 1865 I supposed the manner of this to be somewhat as follows. The
water contained in the irritable parenchyma as well as in the vascular strand and
in the wood of the shoot-axis, is to be regarded as a continuous mass, which, in
the non-irritated condition of the plant is in a state of relative rest ; every dis-
turbance of this equilibrium, every movement of a portion of this water, brings
about an expulsion of water, chiefly from the lower half of the motile organ, and
it is obvious that the movement of a leaf must necessarily effect a disturbance of the
equilibrium mentioned, and even over wide distances, and each organ entering into
the irritable condition must in its turn at once cause a new- disturbance of this
unstable equilibrium.

Most of the other leaves which are irritable to shocks and vibrations are far
less sensitive than those of Mimosa pudica. In the case of the leaves of the
False Acacia {Robitu'a) and of the Wood Sorrel {Oxalis acetoselhi) really powerful
vibrations are needed to produce movements. However, so far as the mechanics
have been investigated in these cases, they depend in all essential points on
exactly the same processes as in I\fi??iosa. Among the organs most irritable

650 LECTURE XXXVII.

to contact or shocks are the leaves of Dioncca wuscipii/a, ah^eady described
in Lecture XXIV^ as there stated, the laminae clap their two halves together
with lightning-like rapidity when one of the six hairs of the upper side is roughly
touched. From Batalin's detailed researches^ it may be concluded that in the
case of Dioncca also it depends essentially upon similar changes in the tissue
of the mid-rib, and in part of the lamina, as in Mimosa ; but several new points
with respect to the relative arrangement of the active parts necessarily come in, and
a lengthy description would be necessary to give the reader a really clear idea
of the mechanism of the leaves of Dioncca. It suffices for the purpose of this book
to have shown in one example what is most important concerning the phenomena
of irritability produced by contact and vibration.

I will here refer to one point only by the way. Burdon- Sanderson, by
employing the well-known very sensitive electrical apparatus used by animal physio-
logists for the detection of electrical changes in nerves and muscles, found that,
on stimulation, electric currents arise in the leaves of Dioncca, and considering
the general ignorance which prevails as to botanical matters it can scarcely be
wondered at if the conclusion was drawn from these observations that something
of the nature of animal nerves exists in the leaves of Dioncca, as appeared more-
over to accord excellently with the insectivorous propensities of these plants. Our
ideas of the irritability of plants, explained with so much trouble and labour, will one
day be applied to the utterly obscure views as to the so-called negative variation
in animal nerves. Without entering more in detail into the criticism of the
matter, it may simply be mentioned that at my suggestion, and in my laboratory.
Dr. Kunkel ^, well versed in the technicalities of electrical investigations, established
the fact that each movement of the water in the tissues of a plant induces feeble
electrical currents in it. In the case of a slight bending of a shoot-axis or of
a leaf-stalk, which must entail movement of the water in the tissues, electrical
disturbances can at once be detected with delicate instruments. Since now, as
has been shown above, every movement of irritation of the leaves is connected with
what amounts to a very considerable displacement of water in the tissues, this
also must produce electrical disturbances, and conversely, it is also obvious that
electric disturbances acting from without must act as stimuli to movement. These
have been longest known in 3Iifnosa. In any case, then, we have no necessity to
refer to the physiology of nerves in order to obtain greater clearness as to the
phenomena of irritability in plants; it will, perhaps, on the contrary eventually result
that we shall obtain from the processes of irritability in plants data for the explana-
tion of the physiology of nerves, and this, although it is as yet a very distant hope,
gives a special attraction to the study of the irritable phenomena of plants. I shall
therefore invite the reader to consider with me, somewhat in detail, the more im-
portant phenomena of irritability in the stamens of the Cynarese,

The Cynarese are a subdivision of the great family of Composite, in which

■ Batalin, ' Mcclianik der Bezvegungeti der insektenfressenden Pßanzen,^ Flora, 1877 (pp. 33,
&c.), where Drosera and Dioncea are treated with especial care.

' Kunkel, ' Über elektromotoi-ische Wirkungen ati unverletzten lebenden rßanzentlicilen^ in Arb.
des bot. Inst. Würzbg.,' 1878, B. II (p. i) ; and further, ' Über einige Eigenthiimlichkeiten des elek-
irischen Leitungsverniögetis lebender Pßanzentheiles^ 1879 (ibid., p. 333).

IRRITABLE STAMENS.

651

more or less numerous small flowers are situated on a common broad floral
receptacle, surrounded by an involucre of many small leaves : the whole
impresses the non-botanical observer as a single flower. Those not skilled in
botanical matters will easily see their way when I add that the common blue
Corn Flower and its allies {Centatirea), the common Thistle {Carduus) and the
well-known Artichoke {Cynara), together with many other genera, belong to the
Cynarese. Each individual floret contained in the flower-head of these plants
consists of an inferior ovary, from which rises a long narrow flower-tube which
suddenly opens out above, assuming the form of a bell, the margin of which has five
spreading teeth. At the place where this widening of the corolla occurs five stamens
arise, the anthers or pollen-sacs of which are in aU Compositce so coherent laterally
that they form a tube, through which the upper part of
the style projects, arising from the inferior ovary at the
base of the corolla-tube.

Now the five stamens referred to are the objects with
which we have to do here\ These are fixed by their
lower ends to the corolla-tube, as shown in Fig. 374, and
at the upper end to the tube formed by the anthers.
Left to themselves, and before the emptying of the pollen
from the anthers, the five stamens are strongly curved
convexly outwards. If one of them is touched, as with
the point of a blunt needle, it extends itself straight,
i. e. it becomes proportionally shorter ; it may then
happen that, in consequence of the curvature experienced
by the style passing between the stamens, the other
stamens also become dragged or pressed on to the corolla-
tube, and this acts on them as a stimulus, whence they
also contract, and the anther-tube with the style going
through it becomes curved towards the other side again.
The whole sexual apparatus of such a tubular flower may
thus be put into a condition of pendulous movement to and fro.

For the purpose of more exact studies it is well to take single florets out of the
capitulum, and to cut away the corolla down to the origin of the filaments, or
to cut across the corolla-tube, stamens and style, above the insertion of the
filaments, and fix the freed sexual apparatus in damp air by means of a needle.
When the filaments have recovered from the stimulation due to the operation, they
stand sufficiently convex outwards — concave towards the style — for free movement.
The filaments are not round : the radial diameter (with reference to the flower) is
considerably smaller than the tangential one. Each consists of an envelope of
3-4 layers of long, cylindrical parenchyma cells, separated by thin straight trans-
verse walls, and surrounded by a layer of similarly shaped epidermis-cells (with
a strong cuticle), which in many places grow out into hairs, each of which is

Fir,. 374. — Stamens of Centatirea
jact-a freed by removal of the corolla.
--/ in tlie non-irritated, B in the con-
tracted state (magnified), C corolla,
tube ; s filaments ; a anthers coherent
into a tube ; ^ style.

' As to the stamens of the Cynarece, the reader may compare, in addition to Pfcffer's work
already quoted, Franz linger, ' L'ber die S/ni/ctiir einiger reizbaren P/innzenf/ieile," Bot. Zeit., 1872
(p. 113)-

6^2 LECTURE XXX VIT.

divided by a longitudinal wall. Between the parench3'ma-cells lie spacious inter-
cellular passages; the centre of the parenchyma is traversed by a delicate fibro-
vascular strand, which, like the epidermis, is strongly extended by the turgescent
parenchyma.

If in the first-named preparation one of the filaments, curved convexly outwards
and fastened to the corolla below and to the anther-tube above, is touched, it becomes
straight and thus shorter, and applied to the style; if this happens with all the
filaments, their shortening is rendered noticeable by the downward withdrawal of the
anther-tube. After a few minutes the filaments again elongate, becoming arched
convexly outwards, and are then again irritable. If the second kind of preparation
is employed, where the filaments are cut off and freely movable below, it is easy to
convince one's self that every time they are touched a rapid movement follows : if
the outside is touched this becomes first concave and then convex, if the inside is
touched this becomes concave and then occasionally likewise convex. The shorten-
ing of the irritated filament begins at the moment of contact, and soon reaches its
maximum, whereupon the elongation begins again at once, and this proceeds
rapidly at first and then more slowly.

As to the mechanics of these movements, we have an investigation of Pfeflfer's
in which the filaments oi Cynara Scolymus and Centanrca jacca were chiefly employed.
The following embraces the more important results.

The filaments of the species mentioned are 4-6 mm. long : the tangential
diameter of those of the Artichoke (Cynara) is about 0-42 mm, the radial diameter
o'2 mm.; in Caitnurea about o'24 and o"i4 mm. The axial vascular bundle is thin
and delicate, the irritable parenchyma-cells in Cynara 2-3 times, in Cen/aurea 4-6
times as long as broad. Their transverse walls are at right-angles to the longitudinal
axis: all the cell-walls, even those of the strand, are thin, only the outer walls of the
epidermis being thickened to any considerable extent. The very abundant cell-sap
of the parenchyma-cells is surrounded by peripheral protoplasm, which is relatively
abundant, and in which Hes a nucleus; the protoplasm exhibits rotation. In the cell-
sap a little tannin and a good deal of glucose are dissolved.

The filaments are irritable along their whole length, i. e. they can shorten them-
selves in consequence of contact anywhere. By means of special apparatus Pfeffer
succeeded in measuring the shortening under powers of 100 or 200 diameters. The
shortening may amount to 8-22 % of the length in the non-irritated condition, a
thickening of the filament occurring at the same time, which is, however, far too
slight to correlate the shortening with mere change of form ; it points rather to a
very considerable diminution in volume. This decrease in volume is due to the
escape of water from the cells into the intercellular spaces; it wells forth from these
spaces on transverse sections of the filament just as in the organs of the Mimosge, as
Pfeffer directly observed. If the intercellular spaces are filled with water by injec-
tion, the filaments are still irritable, and the expulsion of water at the transver?e
section in consequence of a stimulus is then still more evident.

The filaments are very extensible and at the same time perfectlv elastic ; they
may be stretched to double their length, and yet they will contract again to their
original dimensions.

In the irritable condition the axial strand and the epidermis are strongly

CAUSES OF THE EXPULSION OF WATER. 653

extended by the turgescent parenchyma, and even in the irritated state after the
contraction there still exists a similar though much feebler tension.

Having now made ourselves acquainted with the facts of the movements due to
irritation so far as is accessible to direct observation, the question arises, in what does
the effect of stimulation essentially consist? As in the previous lecture, where we
were concerned with the stimulus of light, so also here we must again keep in view
as the chief point, that the irritable organ, or we may say each of its irritable cells, is
turgescent in a high degree, and that in consequence of a touch or shake this turge-
scence is suddenly diminished, producing a sudden escape of water from the interior
of the cells. The question thus resolves itself into the following : — How is the
sudden expulsion of water from the cells produced in consequence of a stimulus ?

According to all that we know as to the condition of turgescent cells, from
De Vries' researches on plasmolysis, Pfeffer's descriptions, and my own con-
siderations and experiments, it can scarcely be doubted that the cellulose walls
themselves are always in a high degree permeable to water, and that the con-
dition of turgescence of the cells depends upon the protoplasmic utricle opposing
the expulsion of the endosmotically absorbed water even under high pressure.
A sudden escape of water from turgescent cells can thus be rendered possible only
by this property of the protoplasmic utricle undergoing some change, or, in other
words, by the hitherto non-permeable protoplasm becoming permeable in conse-
quence of the stimulus, and thus letting water escape.

It must at the same time be added that we can at present form no idea why
this change in the protoplasm occurs in consequence of a stimulus, and with what
molecular changes it is connected ; it must suffice for us meanwhile to know that
the externally perceptible effects of stimulations so far described are caused by the
change referred to in the protoplasm itself, and the question now is how the
mechanics of movements due to irritation are to be understood from this.

It is to be observed, in the first place, that the escape of the water from the
tissues is connected with a proportional diminution of their volume. It follows
thence that the cellulose walls themselves must contract in the movement : in
proportion as the water at a high pressure in the cells filters out through the
irritated and therefore permeable protoplasmic utricle, it penetrates to the exterior
also through the cellulose walls, and these then contract elastically, whence follow
directly the movements described above.

It will be noticed that in this mechanism the extensibility of the cellulose walls
plays an important part : true, neither the extensibility nor the elasticity of the cellu-
lose walls is altered directly by the influence of the stimulus, though both properties
are probably put in action. In order that a movement of the organ may come
to pass in consequence of the movement of the protoplasm, the cells concerned
must in the resting condition be strongly distended by turgescence, so that, when
the protoplasm suddenly becomes permeable, they can contract to a corresponding
extent, since only by this means is the movement itself called forth in consequence
of the stimulus.

We might even suppose that the protoplasm forming a closed sac on the interior
of the cellulose wall, allows water to filter out in consequence of a stimulation,
and that, at the same time, il undergoes contraction, as occurs in the plasmolysis of

654 LECTURE XXXVII.

ordinary non-irritable cells, where the previously slighily extended cellulose wall
contracts, but not to the same degree, so that between it and the contracted proto-
plasmic utricle there arises an interspace filled with water, as in Fig. 189. If we
now suppose that the alteration of the protoplasmic utricle just mentioned actually
occurs, in consequence of a touch, shake, sudden illumination, electrical disturbance,
or any other stimulus, but that the corresponding contraction of the rigid cell-walls
does not occur, then no movement of the organ would be perceived externally,
although the protoplasmic utricles had been irritated and had, in fact, reacted to
the stimulus. We see thence that in the case of the irritable organs of plants two
essentially distinct points come into consideration ; on the one hand the action of the
stimulus on the protoplasm, and on the other the extensibility and elasticity of the
cellulose wall. Nature herself presents us with examples of the case here only
assumed for purposes of illustration. In Spirogyra (Fig. 223, p. 314) the protoplasmic
utricle which was hitherto closely applied to the cell-wall contracts, for the purpose
of subsequent conjugation, into a rounded vesicle, and this is evidently only possible
by the water of the cell-sap escaping out through its substance. The protoplasm,
however, separates from the cell-wall in the process, as in a plasmolysed paren-
chyma-cell (Fig. 189), because the cell-wall itself was only slightly extended pre-
viously, and accordingly only suffered a slight contraction. If the cellulose wall of
such a Spirogyra cell had been previously much distended, and if then the proto-
plasmic utricle became contracted, water being expelled, the cellulose wall would
also contract and the entire cell would act like an irritable organ. Evidently we
might suppose the tissue of an irritable organ of Mimosa, or of a stamen of
Cynara, composed of cells resembling those in Spirogyra, and then the conse-
quence of a stimulus would be that their protoplasmic utricles would contract, but
the cellulose walls would remain unaltered, and we should perceive no external
movement. Indeed it is not at all improbable, that such processes actually occur
in the ordinary apparently non-irritable parenchyma of plants, since we know,
as a matter of fact, that in many Algse, hairs, &c., the protoplasm becomes con-
tracted by mere pressure from without, the cell-wall not contracting to the same
extent.

In conclusion I will mention, simply in passing, the irritable stamens of the
genus Berheris. These, six in number, stand in a circle around the central ovary,
and in the opened flowers are thrown radially outwards. A shght touch on the inner
side causes the filament to dart suddenly inwards, so that the anther comes to lie on
the stigma. If an electric current, generated by numerous small elements, is con-
ducted through the flower in such a way that it runs longitudinally through the
pedicel and ovary to the stigma, or takes the reverse direction, the following remark-
able phenomenon is observed : each time the current runs from the stigma to the
pedicel a stimulation of all the stamens follows, while a current in the reverse
direction causes no stimulation. I discovered this fact so long ago as 1878, but have
not had an opportunity since of pursuing it further; it would certainly be not
uninteresting to know, whether a similar phenomenon occurs in other irritable organs
also.

The irritable stamens of Berheris differ as to their mechanism from those of the
Cynarese considerably, above all in that they are irritable only on the inside and not on

USES OF IRRITABILITl'. 6^^

the outside. It appears more important, however, that the irritable parenehyma
contains no intercellular spaces; the cells, which are moreover thin-walled, have
abundance of so-called intercellular substance between them, which has the property
of swelling up. A touch on the inside of the filament causes it to curve along its
whole length, and Pfeffer succeeded in showing here also that if the filament is cut
across, stimulation causes an expulsion of water to take place at the cut surface.

The point around which physiological interest centres in the phenomena of
irritability so far described, exists of course in the recognition of the internal processes
causing them, and which we have been able to trace back to the protoplasm, so that,
in fact, we perceived that it is the protoplasm which is properly irritable, whereas the
external effect, the irritable movement itself, depends chiefly upon the extensibility and
elasticity of the cellulose walls. But the question as to the uses which the plants
referred to derive from these remarkable adaptations, leads us in an entirely different
direction of physiological investigation. With respect to Dioncca muscipida, and the
obviously very different adaptations in Drosera, the statements in Lecture XXIV
may be referred to ; it was there pointed out that the very complicated effects
of stimulation in these plants seem nevertheless to exercise only a somewhat in-
significant effect on their total nutrition. It is probably otherwise in the case of
irritable stamens, the movements of which are evidently calculated to be started
by insects visiting the flowers for the sake of the honey, whence, in accordance with
the other mechanisms of the flower in each case, the pollen from the anthers has
a chance of sticking to the body of the insect, to be rubbed off subsequently on
the stigma of another flower. Irritability here therefore occurs in the service of
reproduction, while in other flowers other adaptations serve the same purpose.

So far as I am aware, no one has as yet attempted an explanation of the use of
the irritability of the leaves of Mimosa ; but I believe that I am able to afford one.
For I have often had opportunities of observing that after a severe hail-storm, when
plants of the most various kinds, and even robust plants, close to my Mimosas
before the window or in the open, have been dashed and broken by the
hail-stones, the Mimosas, in spite of their delicate structure, have come out quite
uninjured ; a few minutes after the rough weather they expanded their leaves again
entirely unhurt. The matter is easily explained. The blows of the first drops
of rain or of a single hail-stone cause all the leaves of the Jlfimosa to pass into the
irritated condition, the primary stalks to hang down limp, and the double rows of
leaflets to close together like clasp-knives ; the now limp and pendent leaves may be
struck even by large hail-stones without taking harm, because they yield to the blows
like pendent threads, whereas stiff leaves are pierced or their turgescent stalks bruised
simply because they resist. This action is, so far as the laminag are concerned,
promoted to the utmost by the fact that the folded secondary petioles and leaflets
can only be struck by a hail-stone in a particularly unfortunate case. Just as against
hail, this behaviour (reminding us of the contraction of many animals when danger
threatens) may also probably be of use on other occasions, as, for instance, when
large animals invade the place where Mimosae grow, where at the same time the
very strong prickles to the right and left of each leaf-base are of service ; both
arrangements must also render it difficult and distasteful for phytophagous animals
to eat the foliage of the Mimosce. A more exact insight into the matter is of course

6^6 LECTURE XXXVII.

only to be obtained in the original home of these plants by careful observation of
their relations with the external world. Of course the irritability of the foliage leaves
in the cases indicated can only be of use if they are sensitive in a high degree ; it
would be very difficult to detect any such use in the slightly irritable foliage leaves of
our Wood Sorrel, Robinia, and several other plants, which only fold up slowly when
sharply struck or shaken.

(fU^c^^

LECTURE XXXVIIL

THE REVOLVING OF TENDRILS AND TWINING PLANTS.

The great majority of sub-aerial leafy shoots are enabled, by means of their
rigidity and geotropism, to maintain themselves erect so as to spread their organs
of assimilation to the light, to render their flowers visible to the insects which
pollinate them, and to mature their fruits and seeds in the air so that they can be
easily detached from the parent plant and disseminated by the wind or by animals.

In very many other plants, on the contrary, the long shoot-axes are too
slender and too flexible to be able to maintain themselves in this upright position
beneath the weight of the organs which they support, although as a rule the
above advantages accrue to them in proportion to the elevation they attain. They
ascend, however, by quite other means — not by simply growing erect, but by
climbing.

The climbing plants, again, differ in their habit of life, and in their modes
of climbing. The Blackberry, for example, and the Rattan- or Cane-palms {Calamus)
(with stems about as thick as the finger and sometimes loo m. long), fling,
so to speak, their long shoots on the jungle and on the branches of trees,
and hang there by means of their hooks. Much more completely organised,
as climbing plants, are the Ivy and others. The long thin shoot-axes of the Ivy
apply themselves closely to a wall, the stem of a tree, or a rock, &c., and then grow
perfectly erect, or, in the case of lateral shoots, obliquely, on the surface of
these supports, and fix themselves by means of numerous small attaching roots
to the vertical surfaces on which they are climbing.

Twining plants are adapted for climbing in an entirely different manner.
In these, e. g. the common Hop, species of Bindweed {Convolvulus), &c., the axis
of the leaf-shoot, which is at least at first thin and flexible, twines itself
around a body which stands vertical or obhquely upright, and is usually thin,
such as the trunk of one of the more slender trees, a branch of a shrub, or, in the
case of smaller twining plants, such as the common Corn-bind, around the haulms of
ordinary grasses, the erect flower-stalks of meadow plants, &c. But it is the tendril-
plants which are to be looked upon as the most perfect of all climbing plants, having
special organs exclusively adapted for climbing. INIoreover, their peculiarities are
better known than those of any other climbing plants, and they may therefore be first
taken into consideration.

[ 3 ] t- u

658 LECTURE XXXVIII.

Departing from the evcry-day terminology which often designates as tendrils
long shoots like that of the Ivy, botanists understand by this term thin, long
filiform organs, which when typically developed are distinguished by being in a
high degree irritable, especially to continued contact with a solid body. By
means of this property tendrils are enabled to twine closely round a thin rod,
the stem or haulm of another plant, or the branch of a woody shrub, &c.,
much as a cord or thin wire may be wound round a pencil, and thus bind
themselves fast; and, since numerous tendrils on any shoot act in the same way,
they fasten the latter to foreign bodies and enable it to climb upwards. The shoot-
axis is entirely passive in this process : a tendril-plant climbs somewhat in the
same way as a gymnast who, without using his feet, hangs by means of his hands
and climbs up the steps of a ladder or the branches of a tree, only of course
with the great difference, that each tendril when it has once established a hold-
fast remains where it is, new grasping organs being continually developed at the
apical portion of the climbing shoot, to seize hold of steps situated higher and
higher (Fig. 375).

We may regard as the most perfectly organised of all tendril-plants the com-
mon Vine and the whole of the family to which it belongs, as well as the Gourd and
its allies the Cucurbitacese, and the Passifloracese.

On examining a Vine there are found opposite many but not all the leaves, and
springing from the shoot-axis, the dichotomously branched tendrils, which are often
20-30 cm. long and 2-3 mm. thick; these twine and make themselves fast round
any body with which they come in contact, even the foliage-leaves of their own
shoot. The tendrils of the Grape Vine are, like those of the Wild Vine [Ampelopsis
hederaced), practically metamorphosed shoots, for they possess a minute leaflet at
the base of each of their branches. They are, moreover, also remarkable
in that they arise not from the axils of the leaves, but from the side of the
shoot-axis exactly opposite the leaf, a fact which German morphologists have
created unnecessary trouble about because it does not accord with the so-
called principle of axillary branching. But it is a physiologically remarkable
fact that between the tendrils and the inflorescences of the Grape Vine and its
allies, every stage of transitional structure occurs. It is only necessary to investigate
a few dozen Vines to find tendrils which, while maintaining the tendril character
completely in other respects, bear one or two flowers on individual branches ;
in others a portion of the tendril is converted into a small bunch of grapes, near
which one or two filamentous tendrils still remain ; in other specimens, again, all
degrees of metamorphosis to completely developed bunches of grapes are to be
detected. It may thus be said that the tendrils of the Vine and its allies are inflores-
cences which have remained more or less, or it may be entirely barren, and have
assumed the properties of climbing organs; and the same may be said of the
Passiflorse and several less well-known plants [Cardiospermicni). The tendrils of
plants of the Gourd family spring from the shoot-axis to the right or left close to the
petioles, and are distinguished in the Gourd itself, the Bottle Gourd {Lageiian'a), and
in Si'cyos and Bryonia (the White Briony), and others by their enormous length —
occasionally 30 or even 40 cm. — and tenuity. Both peculiarities render them ex-
tremely effective climbing organs. They differ from the tendrils of other plants still

THE ORGANOGRAPHY OF TENDRILS.

659

more by the fact that when ihey arc young and first project from the leaf-lnid of llie
shoot, they are closely rolled together in the form of a helix, and this in such a way
that their outer side is convex ; only on further development does the helix uncoil
itself, progressively upwards from the lower part of the tendril, until it is approxi-
mately straight along its whole length. The tendrils of other plants are more or less
straight from the beginning — i. c. ihcy are not coiled up.

In the examples quoted we have true typical tendrils. In other cases, however,
it is peculiarly developed parts of leaves, specially endowed with irritability and
more or less filiform and sensitive to contact, which assume the chief properties of
tendrils. In many species of Cleviatis, in the Indian Cress ( Tropcrohini), Maiiraiidia,
Lophospermtim, Solanum jasmmoides, &c., the petiole itself acts as a tendril, as shown
in Fig. 376; in the common Fumitory [Fumaria officinalis) and the allied Corydalis
claviculata, ihe whole of a leaf is branched into fine slender filaments, and is irritable
to contact and able to twine its separate parts round thin bodies. In the case
of Ghriosa Blandii and FlagcUaria Indica the midrib projects beyond the apex of
the broad simple lamina and serves as a tendril: and similarly with the Pitcher-plants

I- u 2

66o

LECTURE XXXVIII.

{Ncpoitlies), where, however, the ' pitcher' ah'eady described (Fig, 247, p. 379) only
arises at the tip of the tendril after the latter has wound itself round a support.
In many Bignonias, Cobcea scandens, the common Pea, and its allies the Vetches, &c.,
the anterior portion of the pinnate leaf becomes transformed into very thin, filiform,
and (in the first-mentioned plants) much- branched tendrils; and in Lathynis aphaca
indeed the whole leaf is replaced by a tendril.

These tendril-plants, of which Hugo von Mohl (1827) already knew 465 species,
but the number of which is certainly far larger, occur for the most part among
Monocotyledons and Dicotyledons, although several Cryptogams are possessed of
similar organs. They are met with, however, in most abundance among the Dicotyle-
dons, where in fact the distribution of physiological labour in general attains the
highest development. The formation of tendrils is, as a rule, by no means to be found
in all the members of a group in which it occurs. In the Cucurbitaceae, Ampelidese (the

Vine and its allies), and the Passiflorae this is of
course the case ; but elsewhere it is usually only
individual genera of a family, or individual species
of a genus, which form tendrils. And this,
moreover, is true of all climbing plants. Among
the allies of the Ivy, it is this plant only
which climbs in its peculiar Avay, and that the
same is the case with twining plants is shown
by the very close relationship between the Hop
and the Hemp — the former being a typical
twining plant, the latter a stiff, upright, and any-
thing but a climbing species. Just as in the case
of the climbing species, also, the other physio-
logical properties are independent of the sys-
tematic relationship ; in the genus Ranunculus^
for example, we find submerged or floating
aquatic plants, as well as marsh and terrestrial plants ; among the Orchideae
ordinary land-plants with or without chlorophyll, and in the tropics epiphytes
living on trees, and the aerial roots of which serve as organs of attachment.
These may suffice to show the reader by the way how independent the systematic
or phylogenetic relationship of the physiological adaptations may be, and con-
versely. We may now return to the exclusively physiological consideration of the
tendril-plants.

The distinctive properties of tendrils are the more pronounced the more ex-
clusively they serve the one purpose of organs of attachment for climbing, and
therefore the less they partake of the nature of leaves or parts of stems in other
respects — in a word, the more completely the metamorphosis is accomplished.
Among these are especially to be noticed the simple or branched filiform tendrils
of the Cucurbitaceae, Ampelidese and Passiflorae. One of these typically developed
tendrils is shown at Fig. 375 a in the mature state, after it has embraced a support
with its apical part and then become rolled up. What is here stated applies
particularly to such true tendrils.

7'he characteristic properties of tendrils become developed when they have com-

Fig. 376. — Tropizohtyn 7ninus, The Ions' petiole
a a of the leaf / is sensiti\^e to continued contact,
nd has coiled itself round the support and round its
\vn stem i-/so that the latter is fixed firmly to the
)rmer. z is the axillary bud of the leaf.

TYPICAL TENDRILS. 66l

pletely emerged from the bud state, and have reached about three-fourtlis of their
definitive size ; in this condition they are extended straight, and the apex of the
shoot which bears them usually makes revolving nutations. The tendril itself
also exhibits the same phenomenon, becoming curved along its whole length
(usually with the exception of the rigid basal portion and the hooked apex), in
such a manner that the upper side, the right, the lower, and the left side become
convex in turn; no torsions make their appearance. During this revolving nutation
the tendril is rapidly growing in length and is irritable to contact ; i. e. every
more or less pronounced touch on the irritable side effects a concave curvature
at the spot touched, and this curvature then extends further up and down. If
the contact is transitory, the tendril subsequently straightens out again. The
degree of irritability^ is very different according to the species: in Passiflora
gracilis a pressure of one milligram sufirces to effect the curvature in a short time
(25 seconds) ; with others a pressure of 3 or 4 milligrams is necessary, and the curva-
ture occupies a longer time (after 30 seconds with Sicyos\ The tendrils of other
species become curved within a few minutes after being rubbed slightly at one spot ;
in Dicenlra thalictrifolia after half an hour, in Smilax not till after more than an hour;
in Ampehpsis still more slowly. The curvature on the touched side increases for
some time, then stops, and then after some time (often several hours) the tendril
again becomes straightened, in which condition it is again irritable. Tendrils the
apices of which are slightly incurved are only irritable on the concave lower side ;

1 The following publications treat of both tendril-plants and twining-plants : —

Hugo Mohl : ' Über den Bau und das IVittden der Ranken und Schlingenpflanzen,' Tübingen,
1827.

Charles Darwin : 'The Movements and Hahits of Climbing Plants,' London, 1875.

On the irritability and movements of tendrils the following may be consulted : —

Hugo de Vries: ^ Längenwachst hum der Ober- und Unterseite sich kiünimendcr Ranken^
Arb. d. bot. Inst, in Wzbg., B. I, p. 302.

* Über die inneren Vorgänge bei doi Wachsthuniskrilmmungen mehrzellige Organe^ Bot. Zeitg.,
1879, p. 830.

Casimir de Candolle : ' Ol'servatioiis sur fenroulonent des Vrilles,' Bibliotheque universelle
de Geneve, Jan., 1877, T. LVIII.

Treub describes (Anji. dujardin botanique de Buitcnzorg, vol. iii, pp. 1-87, Leiden, 1882) a new
class of climbing plants, the climbing organs of which are in effect very short, curved, hook-like
tendrils, which on contact with a support grow in thickness to an unusual extent.

On twining plants the following may be consulted : —

Hugo de Vries : ' Zur Mechanik der Bewegung von Schlingpflanzen,' Arb. d. bot. Inst, zu
Würzbg., B. I, 1874, p. 317.

Schwendener: ' Über das Winden der Pflanzen,' Monatsher. der Berliner Akad., Dec, iSSi.

In Opposition to the latter treatise, which depends on very scanty observations, I wrote a note
{^ A'otiz über Schlingpflanzen') in Arb. d. bot. Inst, zu Wzbg., B. II, p. 719.

Schwendener's reply to this note (Jahrb. f. wiss. Bot., B. XIII, II. 2") leaves no doubt that he
was entirely iinacquainted with the most important phenomena described in the present lecture, and
in my note referred to, whence his reply is completely without object. I believe that in this
lecture that, without setting up any theory, I have indicated the path along which the true cause of
twining is to be found ; in the case of phenomena so involved it is above all important to test the
scientific weight of the individual observations, and little is won by making a few obserwitions on
one or several plants and seeking to give to these a particularly important look by means of mathe-
tical formula.

662 LECTURE XXXVIII .

Others, as those of Cohaa and Cissus discolor are so on all sides ; in Mutisia clematis
the lower side and flanks are irritable, the upper side not so.

While in the condition of revolving nutation and irritabilit}', the tendrils attain
their complete length in a few days ; the revolutions cease, the irritability disappears,
and, according to the species in each case, further changes now take place. In some,
the fully developed tendrils become immovable, remain straight, decay, and
fall off — e. g. the Bignonias, Vitis, Ampelopsis. More frequently it happens that
the tendril on the cessation of growth in length becomes slowly rolled up with the
lower side concave, commencing at the tip and proceeding to the base, so that
it at length forms a helix {Cardiospermum, Mutisia), commonly a spiral narrowed
upwards into a cone (like a cork-screw), in which condition it then dries up (Cucur-
bitacese, Passiflorae, &c,).

These processes, however, are to be regarded as abnormal, in as far as the
tendrils have in these cases failed to attain their object, which consists in their
coming into contact with a support by means of their revolving nutation during the
irritable stage and while they are still growing ; if this happens on the irritable side
curvature results at the point of contact, and this extends also to the neighbouring
untouched parts of the tendril, so that after a short time — a few minutes or hours
according to circumstances in each case, a coil is produced. This coil, however,
is by no means closely applied to the support if it is very thin, but touches it
only at one point of the concavity. The continued contact at this point, however,
increases the irritable action more and more, and the coil becomes closer, until
it has adapted itself to the support along a complete turn. By this means of
course the contact-irritation is simply increased still more, and the free end of
the tendril becomes curved still more and goes on closing gradually in new coils
around the support, until even the extreme tip applies itself closely to it. The nearer
the place first touched lies to the base of the tendril, the more numerous the coils
around the support and the firmer the holdfast ; nevertheless even a few coils around
the support with the end of the tendril suffice to fix this latter very firmly.

Those parts of the tendril which are between its base and the point where it is
fixed are obviously not able to coil themselves around the support, although the
stimulus which causes the curvature is propagated to this region ; the effect of the
stimulus is simply that the portion of the tendril lying between the fixed point and the
rigid base becomes coiled in the form of a cork-screw, often with very numerous
coils. This coiling is essentially the same as that which occurs in most of the
tendrils which have matured without meeting with a support ; but it differs in two
respects from that spontaneous coiling. In the first place it always occurs, simply
in consequence of the stimulus in all the tendrils which have become attached to
a support, and not only in some ; further, in that it takes place after a short time
(several hours or a day after the seizure of the support), during a period .when the
tendril is still perfectly irritable, and rapidly growing in length, whereas the spon-
taneous coiling up takes place when growth and irritability have ceased. More-
over, the spiral coiling consequent on the stimulus of contact follows much more
rapidly than the spontaneous one. Both are easily seen on the same shoot, where
it is possible to find older tendrils which are not fixed and still straight, and younger
ones fixed and already coiled up. This coiling up of tendrils fixed to su})ports

THE COILING OF TENDRILS.

663

is thus, in the same sense as the twining round the support itself, an effect of
irritability, and it is only the mechanical impossibility of its twining round the
support which impels the part of the tendril between the support and the base
to coil up in this peculiar way. Like the curvature of a long piece of tendril
in consequence of contact at a single point, this spiral incoiling also is a proof
that the local stimulus is propagated along the tendril for a considerable distance,
down to the rigid basal portion. The after-
eflect of the stimulus is not ended even
with these processes, however ; for tendrils
fixed to a support subsequently grow in
thickness, often even very strongly, they
lignify, become solid, and are endowed with
longer vitality than those tendrils which have
failed in their purpose. This long continued
after-effect is very striking in the case of the
petioles which act as tendrils in Solanmn jas-
viinoides ; the petiole, when it has twined round
the support, swells up to 3-4 times its former
thickness. This effect is less marked in the
case of the tendrils of the Vine, though it
happens not seldom that some weeks after
the fixing of the tendrils the part closely
applied to the support becomes considerably
thicker than the parts not in contact. The
most conspicuous example of such cases,
however, is afforded by the tendrils of the
so-called Wild Vine (Virginian Creeper); if
these fail to attain their object they dry
up completely and fall off, whereas the
fixed tendrils become thicker, lignify, and
after they have perished serve for years to
fix the shoots to their supports. In many
other tendrils, it is true, there is very little of
this after-effect to note, but it must be added
that the activity of the tendrils promotes
the growth and well - being, and therefore
affects the whole of the plant. On cultivat-
ing tendril-plants in the open for many years
one cannot fail to perceive that those speci-
mens which are afforded the opportunity of fixing themselves to bushes, cords,
wires, &c., by many tendrils, grow and thrive much more vigorously than such as
have to clamber up unsuitable supports and are fixed by few tendrils only

The coiling up of the fixed tendrils differs again by another peculiarity from
that of those which are coiled up spontaneously; for in the latter all the coils
of the spiral run evenly in one direction, whereas in the turns of the spiral of a tendril
fixed to a support there are points where the direction is reversed (Fig. 337 w, iv').

Fig. 377.— White Bryony (Bryonia dioica). S a portion
of tlie shoot-axis from which, close to the petiole * and the
bud k, the tendril mu arises. The lower piece of the ten-
dril u is rigid (not tendril-like) ; its upper portion x has
become coiled round a branch. The long middle portion
of the tendril between the rigid base ii and the fixed point
X has become coiled up into a spiral, thus raising the stem
/>. 7v and w' points where the direction of the coils is
reversed.

664 LECTURE XXXVIII.

between eacli two of which there always lie a number of turns in the same direction,
which is opposite to that of those between the next such points. In long closely-
wound tendrils there are often five or six such points of reversal. Darwin has
already pointed out that this is not a peculiarity confined to tendrils, and still less
a specific consequence of the stimulus ; on the contrary, the occurrence of points
of reversal is a mechanical necessity. If a body which tends to coil up is fixed
at both ends so that neither end can twist round, coils in opposite directions must
of necessity occur, in order to compensate for the torsion inseparable from the
coiling up. This behaviour of fixed tendrils can be imitated by cementing a narrow
stretched strip of caoutchouc on to one which is not stretched; on releasing the
former, it contracts and forms the inner side of a spiral, the outer side of which
is constituted by the strip which was not stretched. On now seizing both ends
and extending the double strip straight out, and then again bringing the two
ends closer together, spiral turns will be produced to the right and to the left,
as in a tendril which is fixed. If one end is set free, the strip will twist up and
become coiled into a spiral.

Since all the movements of tendrils here mentioned result from growth,
they only take place when the external conditions are favourable for growth, and the
more energetically the more favourable the conditions are — that is when nutrition is
active, the temperature high, and the plant abundantly provided with sap, caused
by an abundant supply of water when the loss by transpiration is small. Given
these conditions, tendrils can, as I have shown, carry on their nutation and
irritable movements, wind around supports and become coiled up, even in the
dark (e. g. plants of Cucurhiia Pepo, growing with the apical portions in a
dark vessel, and nourished by green leaves exposed to the light).

As regards the mechanism of the irritable curvatures induced by contact (the
twining and coiling up of attached tendrils), as well as the coiling up of free tendrils,
there can be no doubt that it depends upon the process of growth in length
and its modification due to transverse pressure on the side which is growing more
feebly. Tendrils are irritable to contact or pressure only so long as they are
growing in length; a passing curvature due to irritation may, it is true, be equi-
librated again during growth, just as for example the passive curvature of growing
shoots caused by vibration. If the stimulus at the support continues for a
longer time, however, and the tendril twines round it, the diflference in length
. of the convex and concave sides becomes permanent, and can no more be com-
pensated. The cells of the convex side are proportionally longer than those of
the concave side (just as in roots which have curved downwards, and the nodes
of Grasses which have curved upwards) ; in the case of thick tendrils w^ound round
their supports the difference in length is so striking, that it is at once detected by
the eye, without measurement, as I have convinced myself in various cases. Recent
experiments by De Vries, who marked tendrils which were still straight wdth cross
divisions, and measured them after they had twined or coiled up, have shown that
the grow'th of the convex side is more pronounced and that of the concave side
less so, than is the case with regions on the same tendril which remain straight
above and below the curved portion. A tendril of Cucurbita Pepo, for example,
had coiled round a supj)ort i-2 mm. thick; after the completion of the curvature

THE IRRITADILITV OF TENDRILS. 665

llic increment on the curved portion for each mm. of tlic original length amounted
to 1-4 mm. on the convex side, but only o-i mm. on the concave side. The
mean increment on the portion which remained straight on both sides amounted
to 0-2 mm. If the growth of the whole tendril at the time of contact with a
support is but small, a considerable increase of growth in length is found to
occur on the convex side ; but on the concave side there is generally no elongation
at all, or there may be even a contraction ; in the case of a tendril of the Gourd this
shortening amounted to almost a third of the original length.

Similar alterations in the length of the convex and concave sides are to be
observed in the spontaneous coiling up of tendrils, as well as in the coiled up
portions lying between the support and the base of attached tendrils; since in
these cases the growth of the whole tendril is usually slight shortly before, the
contraction of the concave side is also a very general phenomenon. (De Vries.)

The whole of these phenomena and others not here described lead to the
conclusion that the growth in length of the untouched side is increased by the
pressure of the support; this forces over the side which is in contact, and in the
curvature which now follows the concave side is compressed and retarded in growth,
or even shortened. It seems to me probable that at the same time a relaxation
of the parenchyma of the touched side (due to its giving off water to the parenchyma
of the upper side), and a corresponding elastic contraction of its cell-walls co-operate
here; at least, in the case of tendrils which are growing slowly, the shortening
of the side which is in contact appears explicable in no other way. But how the
slight pressure of a light thread or the pressure of the nutating tendril on a support
effects these alterations in the growth, not only at the parts in contact but along the
whole tendril, remains for the time being entirely unknown.

The spontaneous coiling up of tendrils which are not fixed to supports is
probably only due to the fact that the upper side goes on elongating for some time
after the lower side has already ceased to grow ; the cells of the growing upper side
probably withdraw a portion of their water from those of the lower side (as the inner
layers of pith do from the outer layers, cf. p. 573), whence the latter become shorter,
while the former elongate.

According to later investigations by De Vries, the first direcdy perceptible effect
of a stimulus consists in the increase of turgescence on the free untouched upper side
of the tendril, and it is directly in consequence of this that the growth also of this
side is accelerated.

The real problem here again, therefore, as in the case of the motile organs
of leaves sensitive to light and contact, is why the conditions of turgescence
on opposite sides of the organ are modified by the stimulus, and here again we
shall point out that it is evidently the protoplasm which in the first place receives the
stimulus, and then, by alteration of its molecular condition causes the turgescence
of the cells to change. In principle, we may say, the stimulation in the case of a
tendril is the same as in the case of a motile organ of Mimosa^ only that it is here a
matter of a continuous though very slight contact, and the alteration of turgescence
leads to a permanent change by means of growth.

Without going further into the numerous questions of a purely mecha-
nical nature connected with the curvatures of tendrils, it need here simply be

666 LECTURE XXXVIII.

shown why thick tendrils are unable to twine round very thin supports. On comparing
two tendrils, one of which is twined round a slender and the other round a thicker
support, it is obvious that in the former the proportional difference in length of the
outer and inner sides must be greater than in the latter. If a thick and a thin
tendril twined round supports of equal thickness are compared, the proportional
difference in length of the outer and inner sides will be greater in the case of the
thick one than in that of the thin one. If we now suppose the support to become
thinner and thinner, the proportional difference in length increases more rapidly for
the thick tendril than for the thin one, and it then becomes a question whether
the growth in length of the two sides of the tendril can or can not attain any
given value. As a matter of fact, the difference in length attainable by unequal
growth of the two sides of the tendril has its limit, as is shown by experiment. The
thin tendrils of Passiflora gracilis can coil closely round fine threads of silk,
whereas the thick tendrils of Vitis can only coil themselves round supports which
are at least 2-3 mm. thick. The most strongly curved Vine-tendril which I could
find had coiled itself tightly round a support 3-5 mm. in thickness, and this only in
one almost circular coil; the average thickness of the tendril at this spot was 3 mm.
The concave side of one turn was therefore nearly 1 1 mm., the convex outer side
nearly 29 mm. long, and thus the relative lengths of both sides nearly as i : 2-6.
If however this tendril, 3 mm. in thickness, were supposed to coil itself round a
support only 0-5 mm. in thickness, an almost circular coil of it would then have a
length of only i-6 mm. on the concave side, and of 20-4 mm. on the convex side ;
the two sides would then stand as i : 13, and it does not appear that such consider-
able differences in the length of the two sides of a tendril are possible by growth. If
on the contrary the problem were for a tendril which itself is only 0-5 mm. thick to
twine itself round a support 0-5 mm. thick, closely and in an almost circular coil,
the inside of a coil need only be i-6 mm. long, and the outer side 4-7 mm., and thus
the relative lengths of the inner and outer sides as i : 3.

In order that a tendril shall cling firmly to its support, it is not sufficient that its
coils simply lie on the support ; on the contrary they must press themselves closely to
it. That this actually takes place is shown by the fact that if tendrils are allowed to
coil round smooth supports which are then withdrawn, the coils at once become
narrower and their number increases (De Vries). This fact shows at the same
time that the tendril irritated by contact with a support strives to make a curvature
the radius of which is smaller than that of the support, provided that the support
is not too thin and the tendril not too thick.

With regard to the pressure which the coils of a tendril exert on the support,
those cases are very instructive where thin leaves are entwined by strong tendrils, and
are thereby compressed and folded.

Since the biological object of tendrils is to grasp supports — usually other
plants — and thus to enable the thin stems of the tendril-plant to climb, it
becomes a matter of primary importance to bring the tendrils into contact with
supports; this is usually accomplished in a wonderfully complete manner by the
fact that at the time when they are irritable, not only the tendrils themselves but also
the apex of the shoot which bears them are endowed with revolving nutation, with
the result that every object which can be used as a support, and which is by any

BIOLOGICAL ADAPTATIONS OF TENDRILS.

667

means brought wiihin tlie area swept by the tendrils, almost certainly comes in contact
with one. The apex of the shoot bearing the tendrils usually describes elliptical ascend-
ing spirals, the course of which is completed in from i to 5 hours. As with twining

^-^^^^^

-r'' ^-_ ^-

-^ ));

FIG. 378,— Upper portion of the end of a climbing shoot of the Virginian Creeper [Ampelopsis heiUtacea). h a tendril
which has coiled itself in the ordinary manner round a nail ; a c tendrils which have become fixed to the wall by means of
cushion-like outgrowths or clasping organs ; d a tendril which is still nutating— its tips are groping about on the wall, but
are still devoid of clasping organs ; e young tendrils.

Stems so also with tendrils, a pronounced positive heliotropism would often carry them
away from the support, and would therefore be injurious. Some in fact appear to be
not heliotropic at all {Pisiim according to Darwin), in others a feeble positive helio-
tropism makes itself evident by the fact that the circular nutating movement takes
jilace more quickly towards the light than away from it. Some tendrils, particularly

668 LECTURE XXXVIII.

those of Ampelopsis hederacea (the Virginian Creeper), and Bignoin'a capreolata, have
the remarkable property of developing broad cushions of tissue at the apices of their
branches, when they remain for some time in contact with hard bodies : these
cushions apply themselves to rough surfaces like suckers, and thus make it possible
for the plants mentioned to climb up vertical walls, when they find no thin supports
around which to twine. In this case it is evidently important that the tendrils should
turn towards the wall serving as a support, in order to fix themselves to it, and
this is attained by means of negative heliotropism, which drives the tendrils towards
the wall shaded by the foliage, where they then, in virtue of their nutations, make
various, one might almost say groping, movements, and glide over the surface, dipping
especially into depressions and cracks, and then develope their attaching discs.

I now pass on to the twining plants, which have already been shortly characterised
at the commencement of this lecture. In the first place stress is once more to be laid on
the fact that we are here concerned not with special climbing organs springing from
the shoot-axis, but with the shoot-axis itself which supports the foliage-leaves and
flowers, and which is at the same time adapted for climbing up supports. The
function of a twining shoot-axis is to twine round an upright support in the direction
of a spiral line, and to apply itself so close to it that, by means of the mutual
friction it clings sufficiently fast to the support, not to slide off from it again even
under the weight of the appendages ; the latter event does happen, indeed, even
with the best of twining plants, if the surface of their vertical support — a rod for
instance — is too smooth to furnish a strong mutual friction. This explains why most
twining shoot-axes tend themselves to have very rough surfaces, provided by means
of torsion-ridges and furrows, or curved, hook-like, silicified hairs, &c. : never-
theless, there are also twining plants the shoot-axis of which is perfectly smooth,
e. g. Bowiea volubilis.

One of the chief differences which distinguish twining shoots from tendrils, must
be understood from the beginning, and clearly borne in mind ; namely, that
twining plants only coil themselves round and climb up upright supports. This
distinguishes them at once from tendrils, which are able to coil themselves round
horizontal as well as upright supports, and downwards as well as upwards. It
appears to be best that the upright support of a twining plant should stand quite
vertical, though it is not impossible for these plants to twine round obliquely upright
supports : it appears that these may be regarded as still not unfavourably situated
even when the angle with the horizon is as much as 45°. The majority of twining
plants, however, are no longer able to twine actively if their support forms a
smaller angle with the horizon, although it cannot be denied that, under peculiar
circumstances, some climbing plants are able to make a few turns round even hori-
zontal supports. In our further considerations we shall always assume for the sake
of simplicity that we have to do with nearly vertical supports.

A further point, conspicuous even on superficially regarding the matter, lies in
the fact that twining shoot-axes wind themselves round the support in a definite
direction, according to the species of plant in each case. The Hop, the Honeysuckle
{Lonicera capri/olium), and a few less well known plants such as Tatnus elephaiitipcs ,
Polygonum scandens, &c. twine to the right, as we are in the habit of saying — i. e. from
the right below to the left above as one looks at the support ; but the majority of

DIRECTION OF TWINING. NUTATIONS. 669

twining plants twine to the left — i. e. from the left below to the right above when the
plant and its support are looked at from the exterior. The latter is the case, for
example, with the Bindweeds {Convolvulus mid Ipomcea), Arisfohchi'a, the Kidney
Bean {Phaseolus), and a few less known plants such as Thunbergia, Jasininiiim,
Asckpias carnosa, Menispermum canadense, &c. Nevertheless, not all species are con-
stant as to the direction in which they twine ; in the case of Blumenbachia lateritia,
one of the Loasacese, it is easy to observe that not only different shoots of the same
stock twine to the right or left, but it happens here, even commonly, that the same
shoot after having twined to the right for some time, grows straight upwards for a bit
and then twines to the left, and vice versa. According to Charles Darwin, something
of the same kind takes place in the case of Scyphantus chgaiis and Hihhcrtia dentata,
though these are rare exceptions.

The first internodes of twining shoots, whether arising directly from the seed as
in the Kidney Bean, or as lateral shoots from root-stocks as in the Bindweed
{Convolvulus), or from sub-aerial perennial parts as in Aristolochia, are not as yet
capable of twining, but grow erect without supports. , It is not until the following
internodes of the same shoot are developed that it is able to twine round a support ;
these internodes first elongate considerably, the foliage-leaves meanwhile growing
out very slowly, and even 30-50 cm. from the tip the leaves, separated by long
internodes, are still in the bud-state.

In consequence of its own weight the elongated apex of the shoot inclines
to one side, and in this position its revolving nutation begins — a rotatory move-
ment which is produced, without the co-operation of external stimuli, by growth
in length taking place progressively along various longitudinal lines on the surface
more rapidly than along corresponding lines on the sides opposite. By these means the
freely pendent apex describes a curve, commonly in the form of a drawn out S, but
which is properly a portion of a very elongated open spiral. This freely hanging
portion is in constant movement, whence the apex is carried round in a circle or
elhpse. If a plant which twines to the right, such as the Hop, is taken, and a
longitudinal black line painted on this ■ portion of the shoot-axis, so that it lies
on the convex side when the bud is pointing to the South, the painted mark is
found subsequently, when the bud inclines to the West, to lie laterally on the
north flank ; as the bud then proceeds round to the North, the mark comes to lie
on the concave side ; and, later still, when the bud points to the East, it again lies
laterally on the north flank of the shoot-axis. These revolving nutations are com-
pleted in quickly and vigorously growing plants in from i to 2 hours, or sometimes
even in half an hour, so that in long pendent shoot-apices, the circular movement on
a hot summer's day can be directly seen : in other cases, however, it requires many
hours to complete a revolution. As a rule two or three of the younger internodes
exhibit revolving nutation at the same time, and since these are all in different phases
of growth, the curvature of nutation of the older usually does not coincide with that
of the younger one. As new internodes develope from the bud they also begin
to nutate, the older ones ceasing the movement ; a new form of movement then
takes place in the latter, viz. Torsion — i.e. the angles of the older internodes
obtain a spiral twist round the true axis of growth, very much as the in-
dividual fibres of a cord are twisted round its long axis. I niav here mention at

LECTURE XXXVIII.

the same time that, in the explanation of twining, too much importance has been attri-
buted to these torsions, as follows from the fact that some twining plants (e.g. Boivica

volnhilis and species of Ciiscuia) exhibit no such
phenomenon at all; moreover the torsions referred
to only make their appearance as a rule after the
shoot-axis has already become closely wound
round a support, though in other cases it is
true they extend close up to the bud, as in
the Hop and the Bindweed [Cotivolvulus). It
must be admitted that the study of the me-
chanics of the twining of twining plants is ren-
dered difficult in the highest degree by these
torsions ; but on the other hand it is established
that they are to be regarded as a bye-phenome-
non only, as an adaptation the better to fix the
stem to the support around which it has already
become twined, and I shall subsequently refer
to yet another useful eff'ect of the torsions in
shoots which have not yet twined round a sup-
port, as well as to the fact that another torsion,
which is usually scarcely noticed, is necessarily
and obviously combined with twining itself. It
will certainly facilitate the understanding of the
matter, however, if I take no further notice of
all these torsions for the time being, simply add-
ing that the direction of the revolving nutation
always coincides with that of the torsions and
with the spiral lines described round the support.
Let us now return to the revolving nutation
of the free pendent apex. It is obvious that this,
which is usually very long — not rarely 50-80 cm.
— must during its sweeping circular movement
occasionally come in contact with a support, a
thin stem or a stick, &c. ; Hke a tendril which is
still nutating in a similar manner, the free sweeping-
apex of the twining plant behaves much as a man
whose arm is extended horizontally and groping
towards all points of the compass in order to
fasten on a support. In fact, the twining plant
seeks in this manner to reach a support, and
when it meets with such with the anterior portion
of the sweeping apex, the part of the apex lying towards the bud curves
round it, and grows spirally up it. The uppermost coils of the spiral which
the apex of the shoot throws round the supports are usually nearly horizontal ;
but as the apex goes on creeping further up the stem in this form, the internodes
situated further behind undergo still further elongation. On the one hand the

Fig. 379,— Shoot-apex ^rif of the Blue Bindweed
(Ipoinma purpurea) winding— i. e. twining round the
rod a a. Flowers, lateral buds and hairs omitted.

BEHAVIOUR OF TWINING STEMS ON THE SUPPORT. 67 1

uppermost coils of the shoot are passively driven up the support by these lower
internodes, since they only lie loosely on it ; on the other hand the older coils thus
become steeper and more erect. Hence it is commonly found, especially with thin
smooth supports, that the lower already completely developed parts of the shoot-axis,
run round the support in steep long-drawn coils, whereas the uppermost coils lie
nearly horizontally, or obliquely with a slight gradient. Subsequently, as they become
older, they also ascend more steeply. This is particularly clear in the case of
thick shoot-axes, such as tliose of the Hop or the Blue Bindweed [T/wucBa) when
twining round a cord or a wire : where the support is thick, it is due to purely
mechanical causes that even the fully developed older turns are not very steep.

The terminal bud of the twining shoot is often applied continuously close to
the support when the latter is sufficiently thick; but where the support is thinner it
often happens that the uppermost coil of the stem is twined quite loosely round the
support, or presses on it only at one point. In other cases again the bud may be
curved sharply downwards close to the support, or even outwards and away from it, or
upwards, evidently in consequence of nutations and torsions occurring in the upper-
most parts of the twining stem.

As regards the true cause of twining and its mechanism, no completely clear
insight into the matter has as yet been obtained. The twining of twining plants
is not so well understood as the coiling of tendrils, and the views on the matter do
not agree as to whether irritability is in twining plants likewise the important factor.
Where the circumstances are so involved it is perhaps better to put together the most
important results of observation, without adopting any theory.

I lay especial stress in the first place on the fact that in the twining of twining
plants some form of geotropism — i. e. an influence due to gravitation — very evidently
plays a conspicuous part : this results from the following observations. On growing
a twining plant — e. g. a Kidney Bean, Bindweed, or Hop — in a pot until it has
climbed up a rod, and then inverting the whole plant together with its pot so that the
pot is uppermost and the twining apex of the plant lowermost, the youngest two or
three coils of the shoot loosen themselves from the rod, and the terminal bud, first
becoming free, turns sideways, and then erects itself and again grows upwards close
to the rod. Mere up-turning thus reverses the twining round the support which has
already been accomplished so far as concerns parts of the shoot which are still grow-
ing, though the parts which are fully developed and wound round the support are
not further affected.

If a twining plant grown in a pot and provided with a support, is laid horizontally,
the terminal bud in like manner loosens itself from the support and directs itself at
once vertically upwards, and if the pot and support are rotated for several hours in a
direction opposite to that of the coils of the support, those parts of the shoot which
are still capable of growth gradually uncoil themselves. To remark it by the way, it
obviously follows from these observations that twining plants (apart from special
circumstances) are unable to twine round a support either in a horizontal or in an
inverted position, and there is certainly no doubt that this behaviour depends
upon an influence of gravitation which cannot be immciliately idjntifiod with the
ordinary geotropism of orthotropic shoot-axes.

There is a second series of facts of, if possible, still greater significance.

672

LECTURE XXXVIII.

U

since they show that twining plants are able to make spiral curves, even without
a support, very similar to those they make when twining round a support ;
only it is necessary in this case that they are by some means maintained in an
upright position. Hugo de Vries (1873) fastened a fine thread to the terminal
buds of the freely sweeping shoot-apices of Kidney Beans and various other twining
plants {Pharhitis hederacea and Quamoclit Inteola), and, by
means of a small weight of 2-3 grams, carried the thread
over a pulley so that the apex of the shoot was drawn
vertically upwards. In the course of a few days spiral turns
were developed, and after the portion of the apex here under
consideration was fully grown, he observed torsions on it at
the same time (though the number of both was such that
no causal connection between them could be established),
just as if the shoot had twined round a rod. De Vries also
fixed a revolving shoot-apex by gumming to a rod the side
which was posterior during the revolution, and obtained a
half to a whole spiral turn.

The experiment with the thread is particularly instructive,
and I have also obtained excellent results with it : it suffices
to employ a weight just sufficient to pull the thin shoot-apex
upright. With Po!vgo?ium diimetorum and Apios iuhcrosa I
obtained in this manner in 15-16 hours 1-2 complete spiral
turns, as well as torsions. In these experiments, as De Vries
noticed, the essential point is only the prevention of nutations
— i. e. of the circular sweeping movement of the apex. There
are, however, various other causes which promote the develop-
ment of free turns in the absence of a support — above all, the
continued upright position of the growing shoot. This may
be elegantly demonstrated by cutting off the apical portions
20-30 cm. long of shoots of the Hop, the Red Bindweed
[Ipo?Ji(Ea purpurea), Menispermiim canade7ise, Dioscorea ba-
iatus, Sec, which have been grown in the open, and as yet
have not attached themselves to supports, but are nearly
straight, or curved into the form of a long S, and placing them
in a glass cylinder 30-40 cm. high and 5-8 cm. in diameter,
at the bottom of which are a few cubic centimetres of water.
Under these circumstances the shoots grow actively and
elongate 5-10 cm., and in the course of 2-3 days there are
developed 2-4 complete spiral turns, which present exactly
the same appearance as if the shoot had wound itself round a rod of 1-5-3 cm.
diameter. Here also the upper parts of the coiled shoot (cf. Fig. 380) are almost or
quite horizontal. The further down the coiled parts of the shoot lie, and the older
they are, the steeper and narrower they are, exactly as if the shoot had twined round
a thin support. No essential difference whatever is to be found. If the glass cylinder
containing such a shoot is now simply laid horizontally on the table, and rotated
through 90" about every hour, so that every side of the spiral shoot-apex is in turn

FIG. 380.— Three coils, which
have been developed indepen-
dently of the support, of a very
vigorous shoot of Me/n's/ermiiin
cnftadense, which had been cut off
and placed in a glass cylinder-
The coils are represented exactly,
but the leaves, flowers, and hairs
are omitted : the Vme/s is simply
to render the direction of the coils

TWINING WITHOUT SUPPORTS.

^n

directed downwards, the spiral turns uncoil themselves again, and the whole shoot
becomes perfectly straight. It is even possible to produce spiral turns in the shoot
a second time by again placing the cylinder upright.

But even when the plants are left to themselves in the open there are frequently
formed, as previous observers had already found, free turns which embrace no
support. This occurs very often in the manner shown in Fig. 381, when the
twining apex grows out beyond the rod up which it had climbed, if at the same time
the now free shoot-apex is sufficiently light and rigid to maintain its erect position. If,
on the contrary, it is very flexible and grows rapidly in length, it falls into a horizontal
or oblique position, and begins to describe
circular movements which lead to the for-
mation not of close spiral coils but onl)' of
open S-shaped curves.

We now come to the question wh}'
the free, horizontally sweeping shoots
make no spiral turns. In the first place
we have already seen that the apex
of a twining plant coiled round a rod,
spontaneously uncoils itself from the rod
if the plant is laid in a horizontal position
and slowly rotated ; and, as I have said
above, even the spiral coils produced in a
glass cylinder again become straight if the
whole is laid in a horizontal position and
slowly rotated. But a long thin sweeping
shoot-apex finds itself in the same position ;
as its posterior parts make torsions the
anterior pliant apical portion is thereby
passively rotated, as if it had been fastened
to a horizontal rotating axis — a movement
which I shall, for reasons which will appear
later, designate klinostat-movement. By
this means the pendent, freely sweeping
and nutating shoot-apex, is placed in the
same position as a pot-plant twining round

a rod, which when placed horizontally and slowly rotated, again uncoils itself from
the rod.

I am indeed convinced that the prevention of the formation of spiral turns in
free-growing shoots, simply in consequence of this klinostat-movement, is of great
use for the plant; for if every free sweeping shoot were to make spiral turns, it
would be impossible for it to seize a support, whereas if this is prevented by
means of the klinostat-movement the apex of the shoot maintains a form which
enables it to coil round a support immediately it comes in contact with it.
Darwin and De Vries had already mentioned the easily demonstrated fact that
feebly growing shoots of twining plants make fice coils which often have the
greatest resemblance to old, free, coilcd-up tendiils, and I am of opinion that the
[3]

FIG. 33
guinala wli
support anj formed free coil

Apex of a «hoot of Akebia
'as grown out beyond the

674 LECTURE XXXVIII.

cause in both cases may be essentially the same ; as in the case of a tendril which is
still straight but actively growing in length, so also in the case of the shoots of
twining plants, so long as they are still elongating, the tendency to make spiral
coils exists, but the tendency is not exhibited, or only to an insignificant extent,
simply in consequence of the rapid growth. If the growth is enfeebled, however, and
approaches complete extinction, the side which will be concave on coiling first ceases
to grow entirely, while the opposite one continues to elongate for some time longer
as the growing side. It thus happens, and not very seldom, that even long pendent
shoots, which have found no support and therefore have their growth interfered with,
finally make several corkscrew-like turns, and then die off altogether, as I have often
observed in the case of Dioscorea batatiis and D. Japonica. But it happens much
more frequently that feeble shoots before they cease to grow altogether, first give up
their circular nutation, suddenly erect themselves, and then, in the course of several
days, make 2-5 corkscrew-like, and usually very narrow fiat coils, and then cease
growing altogether. In this case there are evidently two factors working together
to the same end ; on the one hand, the tendency already referred to, resembling that of
tendrils, to roll themselves up spirally ; and, on the other, the vertical position due to
geotropic erection, and which even in vigorously growing shoots induces the forma-
tion of free coils.

It will, however, certainly need further and very careful researches to derive the
mechanical theory of twining from the statements just made, and which I put forward
simply as facts.

It is a question as yet undecided whether twining shoot-axes are irritable. The
question was answered in the affirmative on Insufficient grounds by Mohl (1827) to
whom we owe the first useful investigation of twining plants; but subsequently
denied on still less sufficient grounds by Darwin. From De Vries' researches the
question also appeared to me to be decided in the negative ; but a more careful
apprehension of the term irritability gives the matter another aspect. When Darwin
denies irritability to twining plants because they make no twining movements when
slightly pressed or rubbed, it is much as if sensitiveness to light were denied to
the retina of the eye because mere rubbing of the eye-lid does not produce vision.
The better reasons which De Vries adduces against the irritability cannot be so
quickly disposed of, though in my opinion they cannot be maintained.

It is above all important what is meant by the word irritability. I understand
by this term, as already said, that kind of reaction which the living organism ex-
clusively, as such and in consequence of its vital capacity, exhibits towards external
influences. When it is found then that an inverted twining plant spontaneously
uncoils itself again from the support, when the apex coiled round a support and
artificially uncoiled spontaneously straightens itself and nutates, when merely placing
the shoot erect causes it to make spiral turns as if it had a support — I find in all
this the essential characteristics of irritability in the above sense, though of course it
affords no explanation of the true mechanism of twining.

The irritabiUty of twining shoot-axes, however, comes in not merely as one of the
causes of twining. I have for many years observed thousands of twining plants
throughout their lives, and found that vigorous shoots when they grow out beyond
the support or meet with none at all, become moribund; it is easy to observe

TWINING PLANTS COMPARED WITH TENDRILS. 67.5

that a shoot which has been growing for some time without a support, on being
afforded opportunity to twine round a support obtains after a few days a new
lease of life, so to speak, and grows much more actively. Here, then, we have an
effect similar to that met with in the case of tendril-plants, where likewise the whole
plant attains greater vigour when it can employ its tendrils.

With regard to the question as to the irritability of twining plants being a cause
of twining itself, the fact must moreover be mentioned that by no means all twining
plants agree -in this respect. It was long ago made known by ]\Iohl that the
Dodder [Cuscufa) is impelled by mere continued contact with a support to twine
closely around it, exactly like a tendril ; these plants, however, are at the same
time slightly geotropic, and as they twine they thus tend to ascend, in which
they resemble twining plants. A few more words, again, as to the leaf-stalks of
Lygodium ; these, as is usually said, behave exactly like twining shoot-axes, and
in fact exhibit the greatest possible similarity to them ; yet INIohl with equal right
designates them as tendrils. According to my observations, in fact, the leaf-stalks
of Lygodhwi act at the same time both as tendrils and twining plants : tendrils in so
far as they are induced to twine round a support, exactly like true tendrils, only by
continued contact ; while they resemble twining plants in that they run round the
stem only upwards, though in doing this they can, like Bhimcnhachia, alter the direction
of the spirals.

The opposite extreme is afforded by the peduncles of the female inflorescence of
the water-plant VaUisneria, which are often more than a metre long and about as thick
as sewing-thread. At the period when fertilization takes place these long filaments are
extended in order that the female flowers may float on the surface of the water ;
after fertilization, however, the filament contracts in close spirals like a cork-screw,
evidently because the one side shortens or the other lengthens, just as in the case
of the coiling up of tendrils or erect twining shoots which have met with no
support.

Unfortunately there is not space here to render clearer the question as to the
true nature of twining shoot-axes, though I must still mention one or two of the
more important facts.

It has already been stated that the upper end of a spirally wound twining
stem makes flat and nearly horizontal turns, whereas the older turns, further
removed from the bud, are steeper; and the same is the case when no support is
present. In other words, the turns are at first flat and become higher and steeper with
increasing age, especially when the support is thin. It is scarcely to be questioned
that this change is due to the influence of gravitation ; in any case it is of use to the
plant, since when the coils which at first lie only loosely on the support raise
themselves and tend to become straighter, they must at the same time go on clasping
the support more closely. To make this clear, it is only necessary to wind a flexible
caoutchouc tube round a rod in loose low coils, and then extend the two ends of the
tube with the hands ; the coils will then become steeper and apply themselves more
closely to the rod. But even young coils begin to exert pressure on the support; if
the latter is smooth and is then withdrawn from the coils, they becoine closer, exactly
as in the case of coiling tendrils.

I stated above that the numerous torsions observe.! on many twining shoots,

x x 2

676 LECTURE XXXVIII.

and which sometimes in the case of the Hop and of the common Bindweed extend
close up to the bud, have nothing to do with the problem of twining proper,
although they are probably of use for fixing the shoots to the support eventually.
On the other hand a torsion is necessarily connected with the actual twining itself,
even in the case of twining plants in which the above-mentioned torsions never
occur, simply because the laws of mechanics demand it. Since this unavoidable
torsion is usually not visible at all, it will be well for the student to convince
himself of its necessary existence. Suppose a long caoutchouc tube laid on the
table in the form of a helix : hold the outer end of the tube fast with one hand,
and seize the inner end with the other, and raise the arm till the whole tube stands
vertical on the table : supposing a black or white straight line to have been pre-
viously drawn on the tube so that, when the tube lies on the table the line runs,
for instance, on the convex side of the turns of the helix, it is then noticed that, after
the tube has been stretched in the manner described, that the line runs round the
tube in the form of a spiral, and this so that one turn of this torsion line comes on
each turn of the original helix, but in the opposite direction to the original coils of
the helix.

Finally, the additional remark that the numbers of twining plants known are
greater than those of tendril-plants, and, like the latter, they occur in all parts of the
world, but especially in America. Mohl, even in 1827, gave the numbers of twining
plants known to him as 866, and it is certain that more than a thousand could now
be enumerated.

The preceding considerations have always reference to the young shoot-apex
only, and its movement on or without a support. Further considerations might
now have to be extended to the other biological relations of these plants, and in this
connection those twining plants, especially tropical ones, &c., in which the shoot-axis
after having wound round a support then developes wood and secondary cortex by
subsequent growth in thickness, would be of peculiar interest. However, we must
here leave these matters and the conclusions to be drawn from them.

/U^^ ^-t.^^:^^

LECTURE XXXIX.

GEOTROPlSxAI AND HELIOT ROPISM '.

It appears to naive unprejudiced mankind a self-evident fact that a tree, e.g.
a Fir-tree, grows with its stem upright (in this case exactly vertical) and that its apex
always tends upwards again in the same direction ; whereas the primary root in like
manner penetrates the soil vertically downwards. The main branches of the stem,
however, stand horizontally, and put forth horizontally lateral branches, and even the
needle-like leaves of the Fir lay themselves horizontally in the plane in which the
main branches subdivide; the lateral rootlets springing from the main root, again,
have their proper directions with respect to the horizon, they grow horizontally or
obliquely downwards, but produce in their turn lateral rootlets of second and third
order which are able to grow out in all directions. Exactly as in the case of the
Fir-tree, so with many thousands of other plants, although with some subordinate

* The older literature on Geotropism as well as on Heliotropism, up to the year 1865, is
collected and criticised in my ' Handötich der Experimental- Physiologie^ pp. 38, &c., and pp. 88-1 1 2.

Of the more recent works on Geotropism the following will be most serviceable to the
beginner : —

Sachs: ' Längenzüac/isthuiii der Ober- und Unterseite horizontal gelegter, sich anficärts kriiin-
mender Sprosse'' (Arb. d. bot. Inst, zu Wzbg. B. I, p. 193).

Sachs: ' Über das Wachsthtan der Haupt- tmd Nebenwurzeln'' (ibid. B. I, pp. 385 and 584).

Sachs: ' Über IVachsthum und Geotropis/nus aufrechter Stengeln' (Flora, 1873, pp. 321, &c.).

Sachs : ' Über Ausschliessung der geotropischen und heliotropischen Krümtnungen ivährend
des IFachsens' (Arb. d. bot. Inst, zu Wzbg. B. II, p. 209), where I described the Klinostat (the idea
of which had already been indicated in my ' PPandbuch,'' 1865), and the horizontal revolution as
means for eliminating heliotropic curvatures.

Detlefsen's work referred to in the text, ' Über die von Darwin behauptete Gehirnfunktion der
Wurzelspitze7t^ is also found in Arb. d. bot. Inst, zu Wzbg. (B. 11, p. 627).

I refer the reader, finally, to the detailed description of geotropic curvatures in my ' Lehrbuch '
(IV. Aufl. 1874, pp. 811, &c.).

Of recent works on Heliotropism I quote only the following : —

Hermann Midler (Thurgau) : ' Über Helioiropismus ' (Flora, 1876, Nos. 5 and 6). I have here
to remark that my new theory of heliotropism was first published in the introduction to this treatise,
and that Müller expressly mentions this; I may indeed add that the introduction in question
consists of my own words. It is therefore not right that certain more recent authors should speak
of my theory as Miiller's. In sharp contrast to my theory of heliotropism stands that of Julius
Wiesner in his very extensive treatise, ' JJie heliotropischen Erscheinungen im PßanzenreicV
(Denkschr. der kaiserl. Akad. der Wiss. in Wien, B. 39, 1878, and B. 43, 1880). I can with
confidence leave it to the future to decide the matter, and entertain not the slightest doubt that my
theory, as soon as it is but generally understood, will be accepted on all sides.

678 LECTURE XXXIX.

differences as regards the lateral shoots and roots ; the petioles especially of most
plants tend to direct themselves obliquely upwards, and the laminae to place them-
selves so that the rays of light fall perpendicularly on their surface.

There are, however, many other plants the shoots of which do not become
erect, and which have no primary root penetrating vertically into the earth ;
plants which creep horizontally on the soil, or cling close to the oblique or vertical
surfaces of rocks, walls, trees, &c., and extend themselves upwards or laterally.
There are also numerous leaf-forming shoots which, like primary roots, penetrate
vertically downwards into the soil, or permeate it obliquely or even horizontally, and
in general we find that the various organs of one and the same plant assume the
most various directions with regard to the horizon of the situation they may happen
to occupy. Frequently it also happens that an organ grows when young in a different
direction from that followed subsequently; this is very conspicuous with the branches
of most species of Piiius, the spring-shoots of which stand erect and subsequently
assume slowly the horizontal position.

It is clear that the whole aspect of a plant depends essentially upon these
different directions of growth of its different organs, as is at once intelligible if one
supposes all the roots and lateral shoots of a Fir to grow vertically upwards like
the main stem ; we should then have, instead of the beautiful tree-form, an ugly
shapeless conglomerate of organs, and the whole would be entirely incapable of
maintaining its existence, because none of the various organs would then be in a
position to discharge their proper functions.

This very simple reflection shows at once that peculiar causes must exist to
compel the different organs of one and the same plant to assume different and
fixed directions for each organ, with respect to the horizon. One of these causes,
as with all vital phenomena, lies in the nature, i. e. in the internal structure of
the organ itself; another cause is the influence of some external force on this
structure of the organ, and it is in fact, as has already been pointed out, gravi-
tation— the gravitating force of the earth, or the general attraction of mass between
the earth and the minutest particles of the plant-organs — which acts on the latter
in such a way that they are compelled to grow in definite directions with respect to
the horizon, or, what amounts to the same thing, under definite angles with respect to
the vertical at their position. This latter is indeed only the direction of the resultant
of all the attractive forces of the whole earth on a definite point in the organ of
a plant. If it is, however, as we now know definitely, the gravitation of the earth
which causes the organs of a plant to assume their specifically peculiar direc-
tions with respect to the horizon, it follows directly that the specific differences
in their directions of growth can only be due to the differences in their internal
organization : the gravitation of the earth acts on every cell of the plant in the
same direction and with the same force, and if the organs are nevertheless caused
to react differently thereby, the cause can only be sought in a difterence of the
internal structure of the organ itself But as in almost all cases of irritability in
the animal and vegetable kingdoms, it is this very peculiarity of structure con-
ditioning the reaction, which is not perceptible to the senses; even the highest
powers of the microscope teach us nothing as to why the apex of a Fir-stem grows
upwards, and the tip of a lateral shoot horizontally, under the influence of gravitation.

GEOTROPISM.

679

and the short p-rowins;

and perhaps nothing brings out tlie point here at issue so much as the fact that
the non-cellular plants behave, with reference to geotropism, exactly like those with
cellular structure, whence also every explanation of these phenomena which is
based on differences of cellular structure must be at once put aside as false.

The fact here treated of generally may be expressed as follows. If the parts
of a plant are displaced by any cause whatever out of their original customary
position into a different one, they become curved until they again assume
the same inclination towards the horizon as before. This curvature, however, is
caused exclusively by growth, and hence only those organs which are still
capable of growth can regain their normal and original position with respect to the
horizon. If a plant grown in a flower-pot, for instance, is laid with its pot
horizontally, all those parts which are already completely developed retain the
new position, and only those which are still capable of growth commence after
some time to curve : the still growing shoot-axes, if they previously stood
upright, become curved until they again stand uprigh
portion of the primary root curves
until its apex is again directed vertically
downwards. Parts which originally
grew horizontally do not rest until they
have again become horizontal, those
usually oblique become curved until
they have resumed the same oblique
position.

In order to simplify the de-
scription, I will first regard in what
follows those organs which originally
grow vertically upwards or downwards,
and assume that such organs have been
placed in a horizontal position. Fig.
383 will serve to illustrate what then
happens. ^ is a diagram of any seed-
ling whose plumule S originally grew vertically upwards, and its main root vertically
downwards ; this plant is now laid horizontally, care being taken that it can go on
growing. After a short time it is seen that the plumule has become curved upwards
as in ,5", until its apex is directed vertically upwards, and in like manner the primary
root has curved, at the short part which is growing in length, until it can again grow
vertically downwards. It is the custom to designate organs which behave like this
plumule as negatively geotropic, and those which behave like this primary root as
positively geotropic. The lines on m S mark a portion of the shoot-axis which is still
growing, and the negatively geotropic curvature may be especially observed here ; the
posterior transverse section 0 u which lies at the limit of the fully developed basal
portion has not altered its position to any marked extent, the anterior transverse
section ou, on the contrary, has become displaced by the upward curvature into
the position o'u', and it is noticed that the under side of this portion of the shoot u u
has elongated considerably, whereas the upper side 00 has not elongated, or has
even become somewhat shorter. A similar stale of affairs is shown by the lines

tlie shoot at fir:

shoot of Crown Imperial {Frit-
I ; of the bulb s being cut away
■, cr part of the scape d. The
.\\:\ after about twenty hours
cctcd itself through b into tlie

68o

LECTURE XXXIX.

on the root, W, only here the upper and lower sides behave in a manner exacdy the
opposite from the above.

The pordon of the plumule 6" here considered is again represented in another
manner at B, the case here selected being, it is true, not the most usual,
though it is the most instructive: the upper side oo shortens to do, while the
under side elongates much more considerably, as shown by u'u'. The curvature
which necessarily results from the shortening and lengthening is, however, not
indicated in this diagram, so that the relative lengths may come out better on the
straight lines. It is easily intelligible that it would also suffice for the purpose
of curvature if the upper side oo retained its original length, while tiu elongated,
and in fact the Hne o o might elongate if only tl li sho\\^ed a greater elongation :
in this case also such a curvature would result that the upper side oo becomes
concave, and the lower side u u convex. All these three cases may actually be
observed in the upward curvature. Exactly the same applies to the diagram C,

which represents that part of a root in which curvature is chiefly taking place, only
that all the matters relating to the upper and lower sides are converse to those in B,
bringing out sharply the meaning of the expressions positively geotropic and
negatively geotropic : B represents the process of growth in a negatively geotropic
organ, C that in a positively geotropic one.

We might now suppose Fig. A to represent a germinating non-cellular plant,
such as Vaucheria : in this case the whole structure is a continuous utricle, the
cellulose-wall of which is indicated by the outlines. Our observations here simply
refer to two equally long portions o o and u u of the upper and lower sides of the
utricle. We might, however, also assume this utricle to be subdivided by more or
less numerous transverse septa into a series of cells, and that the pordon o o, u n,
represents one of these cells, and it is intelligible that the above considerations would
not be essentially affected thereby. Finally, we may also assume that within the
oudine of Fig. A not only transverse walls but also longitudinal walls were present,
and that thus the whole of the space included by the outline consisted of more or less
numerous layers of cell-chambers. Even in this case the diagrams B and C would
again serve to represent two individual cell-chambers, and we might suppose the cell
B just as well situated on the lower side of the shoot 6" as on its upper side, and
exactly the same would hold good of the diagram C with respect to the root W.

POSITIVE AND NEGATIVE GEOTROPISM. 68 1

I may remark here that the reader can only hope to understand the phenomena
of geotropism if he reflects most carefully on the considerations connected with
Fig. 383, and makes them perfectly clear to himself.

Nevertheless the preceding simply shows how we have to figure the geotropic
up and down curvatures in space; though nothing has there been said as to the
cause of the change. We may now attempt to make this latter clearer.

It is sufficient then, as said, to place parts capable of growth out of their
usual position, the erect one for instance, into another, e. g. the horizontal ; this
suffices as an external impulse or stimulus, which alters the processes of growth as
shown in Fig. 383. By thus placing the perpendicularly (upwards or downwards)
growing parts horizontally or obliquely, however, no more has happened than
a change of their position with respect to the radius of the earth; and the next
question is as to how far this can act as a stimulus on the growing parts.
There is nothing for it here but to suppose the vertical line representing the
radius of the earth as the direction in which a force of some kind is acting,
which influences the growth of the plant-organ, and that the main point is what
angle this force makes with the axis of growth of the organ. Every change of
this angle acts as a stimulus by which the growth is so influenced that the above
described differences between the upper and lower sides appear, until the younger
still growing portions again stand in the same direction to the radius of the earth
as before.

Now there is but one known force which acts everywhere at the surface of the
earth, where plants grow, in the direction of the radius of the earth ; and that is gravita-
tion— the attraction of the mass of the earth. The mere reflection that the relative
directions of the organs of plants of like kind with respect to the earth's radius
at diff"erent points of the earth's surface are the same in every place, shows at
once that gravitation alone can be concerned here ; if at our antipodes, or in South
America, or in Japan, and therefore at the most different points on the globe, the stem
of a Fir grows vertically upwards and its primary root vertically downwards, that means
in other words that at each of these places the apical bud of the stem grows away
from the centre of gravity of the earth, while the tip of the primary root, on the
contrary, behaves as if it were attracted by the centre of gravity of the earth ; or,
both organs behave as if affected by a force supposed to be radiating in all
directions from the centre of gravity of the earth. There is, however, only one
such force, and that is gravitation — the attraction of the mass of the earth — the
force which causes a pendulum to hang downwards, and an air-balloon to ascend
vertically upwards. It is this force, therefore, which affects the growth of the
plant-organ, as is shown particularly clearly when the longitudinal axis of a
plant-organ is placed in a diff"erent direction, with reference to gravitation, than that
in which it had hitherto been growing, and it is well to notice that this stimulus
continues until every organ of the plant has resumed that direction which accords
with its internal nature ; or, we may also say, every plant-organ has its peculiar
proper angle, i. e. the property of growing at a definite angle with respect to
the direction of gravitation, and if this is accidentally altered, of curving until its axis
of growth again forms the same angle with the vertical.

These reflections, with the necessary clearness of thought, should suffice to

682

LECTURE XXXIX.

recognise in gravitation the cause which influences growth in the geolropic
curvatures; but this recognition was, as a matter of fact, obtained in quite another
manner. It is usually by round-about paths that truth is detected, because the
direct path to it in most cases requires greater clearness of thought : so it was here.
The fact that it is gravitation which influences the growth of the plant was demon-
strated in 1806 by an Englishman named Knight, as follows. He exposed seedlings
to the continued action of centrifugal force, by submitting them to rapid rotation

Fig. 384.— The axis a is kept in continual rapid rotation. To it is fixed the circular
disc r r supporting the circular plate of cork k. On this, by means of two needles in each
case, the seedlings A and B are fixed, st the plumule : h the primary root. The lateral
roots are all curved outwards in consequence of the rapid rotation, g s^ glass cover : x axis
of

either in the vertical or the horizontal plane. It was shown that the growing
root-apices behaved in this case exactly as if they were simply projected from
the centre of rotation like the weight of a pendulum made with a thread and swung
rapidly round in the hand ; while the shoot of the seedling behaved in exactly the
contrary manner, and grew towards the centre of rotation.

In the case of centrifugal force, as in that of gravitation, it is also an effect of
mass which comes into play, and it was in this fact that Knight perceived the
proof that the geotropic directions of plant-organs generally are produced simply
by the action of the mass of matter, and such an action could only be referred to
the centre of gravity of the earth. It is clear, however, that this kind of proof

CENTRIFUGAL FORCE. THE KLINOSTAT. 683

necessitates much more complicated intellectual exercise than the reflections given
above, which lead to the same conclusions much more simply.

It will probably be not undesirable for the reader to learn, at least as an
example, how the action of centrifugal force on the direction of growth of the
organs of plants can be demonstrated, and this more conveniently than Knight
did it, I refer therefore to Fig. 384 and its explanation.

If, then, it is gravitadon, which we suppose to be situated in the centre of
gravity of the earth, so to speak, and the action of which takes place in the direction
of the earth's radius, or, what is the same thing, in the vertical line, it must be
possible to nullify this action by compelling growing plants to continually alter their
direction with respect to the vertical, in such a manner that the gravitation acts
on the symmetrically opposite sides of a growing part of a plant for equal periods
in opposite directions. Starting from this reflection I constructed an apparatus
which I called the Klinostat. This apparatus, which may be constructed in very
different ways, has essentially the one object of slowly rotating, by means of clock-
work or other motive power, a solid rod of wood or metal which must be exactly
horizontal, and this so that a rotation is completed in 15-20 minutes. On this rod
{1/ in Fig. 385) growing plants, e.g. seedlings, may be so fixed that they participate
in the rotation of the rod without hindrance to their further growth. It matters not
in what direction the growing organs are fastened on the rotating axis as long as the
rotation is equable, so that every growing part of the plant turns the same side up-
wards as well as downwards during equal periods of time, so that the influence pro-
ceeding from the centre of gravity of the earth must act on the growing portions of
the plant during equal periods in exactly contrary directions. If this occurs, no action
of gravitation whatever can make itself effective on the direction of growth, since a
longer or shorter time is necessary for this, and before the part of the plant has had
lime to make a curvature downwards or upwards, it finds itself already, in conse-
quence of the rotation, again in a position which would necessitate its making the
exactly contrary curvature, and thus no curvature at all is accomplished : it goes
on growing in exactly the direction arbitrarily given to it when it was fastened to
the axis.

After these preliminary remarks, I may now attempt to describe more exactly
the processes which take place during the geotropic curvatures upwards or down-
wards. Here I find myself in the agreeable position of being able to depend, step
by step, on my own very detailed investigations.

We may first consider the upward curvature of shoot-axes which normally grow
erect.

My observations have been made chiefly with the thick, rigid, long internodes of
such flower-stalks as attain considerable heights in short periods, and the smooth
surfaces of which admit of being marked with Indian ink, and of exact measurement
of the marked portions. The measurements on the straight shoots, as well as on the
concave and convex sides of curved ones, were accomplished by means of flexible
rules stamped on stiff paper.

In order to be able to criticise the processes during the up-curving of shoot-axes
it is necessary to be previously acquainted with the distribution of growth in them :
this has already been described in Lecture XXXII. At first the whole intcrnode, as

684

LECTURE XXXIX.

well as the shoot consisting of several internodes, is elongating; subsequently the
growth ceases at the base of the segmented stem, and only a certain portion of it
beneath the apical bud forms the growing region — the region capable of geotropic
curvature. In the case of individual internodes of sharply segmented shoot-axes, the

FIG. ^85.— A klinostat. a the clockwork, with weight and penduUim, which slowly rotates the axis
b b: on this axis is fixed a cube of bread at c on which a Fungus (Pkycomyces) is growing. Tlie middle
portion of the axis is surrounded by a glass box d, which stands on a dish filled with water, in order to
keep the air around the plant moist (about -j-^ nat. size).

subsequent growing region may be situated near either the apex or the base : apical
growth is the usual, basal growth the rarer case.

The length of the growing region, if parts which are already fully developed are
present, is at a certain time at a maximum and then decreases, to fall to zero finally
when the whole stem is fully developed. In that middle period I found, in cases
where the growing region was very long, the lengths quoted on page 543.

DEPENDENCE OF THE CURVATURE ON GROWTH. 685

Within this growing region then, as has ah-eatly been mentioned, the rapidity of
growth is so distributed that it increases from the bud up to a certain distance from
it, reaches a maximum, and then declines further back from the bud, and finally passes
over into the region of fully grown parts. In this connection stems with many joints
which do not form pronounced nodes (e. g. Asparagus) behave like individual long
internodes, such as the scapes of the Leek. If, on the contrary, the shoot-axis is
sharply segmented into separate internodes, as in the case of Polygonum Sicboldi,
each internode exhibits its own curve of partial growths which increase upwards from
the node next below, attain a maximum at some one place, and again decrease up to the
node next above. The form of the geotropic curve in this case suffers interruptions at
the nodes. Apart from these matters, however, all that has been said of single long
internodes or that is to be said of shoot-axes with many leaves and no joints, applies
in general and in detail to a jointed stem.

To understand the form of the geotropic curvature, however, a few other points
have still to be mentioned. In the first place, every transverse zone of a grow-
ing stem first begins to grow slowly, then grows more rapidly and reaches a
maximum, and then slackens in growth till it finally ceases. The more rapid the
growth is at one place, the more pronounced the curvature it experiences through
geotropism. The rapidity, however, with which the geotropic curvature occurs,
depends in addition essentially upon the thickness and elasticity of the part capable of
curvature, since it is obvious that a thick shoot in order to make the same curvature
as a thinner one, must suffer a greater difference in the length of the convex and
concave sides, for which a relatively longer time is necessary, and that this also needs
a greater expenditure of force where the elasticity is greater. Moreover the curva-
ture which actually takes place at any place on the shoot-axis depends upon how
long this has been submitted to the influence of gravity transverse to its longitudinal
axiS; and, further, the after-effect which makes itself especially marked in the case of
geotropic curvature comes into consideration. Shoots or single internodes which have
been lying horizontally for one or two hours without curving perceptibly, may, when
subsequently placed erect or fixed on a kUnostat, become curved eventually in con-
sequence of the geotropic influence to w-hich they were previously subjected.

Finally, in criticising the form of curvature exhibited after some time by an ortho-
tropic shoot laid horizontally, in consequence of geotropism, the circumstance has
to be considered at what angle the vertical cuts the longitudinal axis of the shoot.
Experiment teaches that a shoot which is at first vertical and then placed obliquely,
is less strongly affected geotropically than if it had been laid horizontally : the
geotropic stimulation is the greater the nearer the inclination of the shoot-axis to the
vertical approaches a right-angle. If a shoot with a long growing region is laid
horizontally, and geotropic curvature then begins, the anterior portions situated
nearer to the bud will soon become erect, not only on account of their own geo-
tropism, but also passively on account of the curving of the older portions, and by
this means they come into a position in which gravitation is acting only at an acute
angle, and thus exerts a more feeble geotropic action.

The co-operation of these very different factors brings it about that the form of
the curvature, so long as the geotropic movement is taking place at all, is continually
changing, even in the same shoot, and thai it is different in diflcrent shoots. Only the

6<S6

LECTURE XXXIX.

final effect is the same, i.e. the whole growing region finally erects itself vertically,
and then goes on growing in that direction.

In order to avoid unnecessary details, and still to give an idea of how geotropic
erection of a shoot laid horizontally is finally accomplished, we may suppose that
abed, in Fig. 386, is a shoot which grows at the apex, the bud of which is
indicated by the arrow-head. A few hours after it was laid horizontally, it had
assumed the form efcd\ it is noticed that the fully developed portion cd now, as
also subsequently, undergoes no alteration, whereas the still growing portion efc has
undergone a curvature which is nearly the arc of a circle. Some hours later, again,
this portion of the shoot-axis has assumed the form gh c\ in consequence of the
geotropic after-effect, the region h, which is here that of strongest growth, has
curved so much that the part hg has been carried passively back beyond the vertical.
This phenomenon, however, comes forth clearly only in the case of very vigorously
growing, long, and especially thin shoots ; in those of the Corn-flower {Agro-

FIG 38S.— Diagram of g-eotropic curvatures gradually made by the shoot
abed which has been laid horizontally (see the text).

siemma Gilhagn), for instance, the part^/z may after 6-8 hours be directed not only
horizontally but obliquely downwards and backwards. Fig. 386, however, represents
the curvatures of a horizontally laid flower-scape of a bulbous-plant. Allium atro-
purpureum. When the apical portion ^,^, then, has assumed the position indicated,
it is once more submitted to geotropic influence, which, however, must induce a
curvature in a direction opposed to the preceding one, while at the same time the
part h c goes on further erecting itself in the same way as before ; hence the whole
growing portion now obtains the form ike, and this subsequently passes over into
the still more erect and elongated form m n c, which finally extends itself perfectly
straight. It is to be noticed that at last the persistent curvature of the geotropically
erected shoot lies in the region n c, i. e. in that region where, during the whole pro-
cess, the growth in length was at first slow and then ceased altogether. This region
is, by its connection with the fully developed portion e d, in the most favourable position
to remain nearly at right-angles to the direction of gravity for a long time, and
thus to be submitted to strong geotropic influence during the whole process, although
all other factors in the process are not favourable to the curvature of just this part.

CURVATURES OF SHOOT- AXES DURING GEOTROPIC ERECTION. 687

It may thus be said, in a completely erect shoot the strongest and finally per-
manent curvature is at that place which offers most resistance to the influence of
geotropism.

The erection of a shoot-axis by geotropism is, as we see, the result of somewhat
complicated movements, which are influenced by numerous accessory causes ;
nevertheless the final erection in the case of fairly sensitive shoot-axes is mathe-
matically exact, i. e. even the feeblest curvatures which arise in the course of
the geotropic movement become at last so compensated that the growing parts again
become perfectly straight and upright. This is seen with extraordinary clearness, for
example, in the case of one of our most remarkable water-plants, Ulricularia vul-
garis ; the primary shoot of this plant, with its finely divided leaves, floats horizontal
and quite free on the water, only the flower-scape rising perfectly vertically, 15-20 cm,
into the air, although the slightest obliquity suffices to turn the horizontally
floating main shoot round, so that the flower-scape falls horizontally on the water.

It scarcely needs mention that what has been so fi\r said as to the upward
curving refers to the commonest cases only ; there are several others in addition,
where the behaviour is different on account of the organisation of the parts concerned.
I will only dwell upon two cases, in both of which, however, it is not strictly
the geotropic erection of shoot-axes which is concerned, but of peculiarly organised
parts of leaves. I here refer to the so-called nodes on the haulms of Grasses, and
the motile organs of compound leaves, especially those of the Leguminosae, already
so fully considered. These organs, when their leaves are completely developed,
also cease to grow, it is true, but their anatomical and physiological constitution
betray the fact that they are, so to speak, in a persistent condition of youth ; their
parenchyma is highly turgescent, and the lignification of the bundles is suppressed,
and, above all, these organs (especially the nodes of Grasses) can be impelled to renew
their growth simply by being laid in a horizontal or oblique position.

On the fully developed haulms of the Grasses, to which group our cereals belong
— Wheat, Barley, Rye, Oats, ]\Iaize, &c. — there are to be noticed at considerable
distances (5-30 cm. or more) apart certain knot-like swellings, sharply marked off
from the thin cylindrical parts, and usually coloured differently. If the haulm is
split longitudinally, it is easily seen that the knot in question is nothing further
than the annular and strongly thickened basal zone of a leaf-sheath, which envelopes
the internode of the shoot -axis above the knot as a very thin but stiff inrolled
sheath. In young haulms these internodes are very tender and flexible ; the rigidity -
and elasticity of haulms not yet quite mature depend entirely upon the firmness
of these leaf-sheaths.

On bending a haulm sharply above the soil, so that the whole of its apical
portion, about i m. long, comes to lie horizontally, it is noticed after 2-4 days that
knee-like curvatures have been formed at its nodes, in consequence of which the
apical portion of the haulm has again erected itself vertically; as a rule 2-3 nodes
take part in this change. It is sufficient moreover if pieces of the haulm con-
taining one or a few nodes are cut off and placed horizontally with one end, it matters
not which, in damp sand, to obtain after a few days the same strong knee-like
curvatures. These knee -like formations are produced exclusively by the nodes
referred to, the other portions of the haulm exhibiting no geotropic curvatures what-

688

LECTURE XXXIX.

ever; tlie curvature of the node is due to the fact that its under side when placed in

the unwonted horizontal position becomes
elongated vigorously by means of growth,
while the upper side grows feebly or
not at all, and in fact often shortens
considerably. If the piece of haulm with

Fig. 387.— a piece of the haulm of the wheat ^ '

[^ards""' '"''' '"'"^°"'^">'' "■'"' ''^ """^^^ '^""■'''^ the curved node is turned so that the

convex side comes to lie uppermost, then

Mi^ , / W 1] / jr ^^^^^ was previously the upper side of the

/|i|uP\ iV / V \AvV^ ^ node which had become concave begins

y\ 3t;.^=J::r:^-_ '^^ //^;;?^T / 1 to grow also, and just as strongly ; it

therefore becomes straight and is thus at
last much elongated, whereas a similar
node on a piece of haulm treated in the
same way, but which remains erect, shows
no elongation of any kind. The convex
under side of a strongly curved node ap-
pears smooth, translucent, and bright ; the
concave upper side dark and rough from
minute transverse folds. On this side more-
over there is often perceived a deep in-
folding, looking as if the node had been
artificially bent till it cracked; this evi-
dently depends upon the fact that the
lower side as it vigorously elongates and
curves upwards compresses the tissue of
the passive upper side, a process which is
intensified by the latter in its turn losing
water, as can be concluded with certainty
from the very considerable shortening. Of
my very numerous measurements in this
connection, I will only quote one in illus-
tration of what has been said. A node of
the Maize, about 12 mm. thick, was 5 mm.
long on all sides : after it had lain six days
in a horizontal position the upper side had
become shortened by 0-5 mm. (i. e. y^^th of
its original length), while the lower side
had become elongated by 7*5 mm., i.e.
to 1 2-5 mm. In other cases, however, the
shortening of the upper side was still
more considerable, (e. g. from 4 mm. to
3,) while the length of the under side
increased from 5 mm. to 11. Microscopic
examination shows that the cells of the
epidermis and of the i)arenchyma of the lower half of the node have elongated

Fig. 388. —Portion of a Scarlet Runner
originally growing erect, has been for some hours turned
upside down to show the geotropic curvatures of the
motile organs P, />,, Pi.

MOTILE ORGANS OF LEAVES.

689

to an extent corresponding to the above numbers, l)ut without undergoing transverse
division. As in all similar cases of curvatures due to growth— e.g. even in the case of
tendrils and, as we shall see later, of geotropically curved root-apices— the tissue of
the side which has become convex consists of large cells, containing abundance of
water and relatively little protoplasm; that of the concave side, like very young
tissue, of small cells with little water and much protoplasm.

That the motile organs of periodically motile compound leaves can execute
geotropic curvatures I first showed in 1865 in my 'Handbook/ employing the figure
here reproduced (Fig. 388) : it represents a young Kidney Bean which together with
its flower-pot was placed for about 4-6 hours in an inverted position, with the apex
downwards and excluded from light. The petioles had the directions indicated by
the arrows when the plant was inverted ; but in consequence of geotropism the
motile organs P P^ P,^ became curved as represented in the figure, and thus the
petioles, which are not at all geotropic on their own account, assumed the positions
shown. Pfeffer showed subsequently that in these geotropic curvatures of the motile
organs, it is not a corresponding growth of the down-turned upper sides which
occurs, but only a very considerable extension of the cells, combined with the taking
up of water, and later, when the plant is once more placed upright this is again
completely compensated after about 24 hours. It is not until the plant has remained in
the inverted position for several days that a permanent elongation by growth takes
place on the convex side. The motile organs therefore afford an opportunity of
confirming the theory of growth which I had previously established, in so far that,
according to this theory, any growth of the cell-walls is preceded by marked ex-
tension due to turgescence, and this, as we see, can be again annulled even though
it has attained a very considerable value, because the growth which takes place, in
consequence of the extension due to turgescence, only follows some time after-
wards.

Passing now to a more detailed description of the processes which occur during
the downward curvature due to geotropism, I shall again select an object which, in
consequence of the geotropic stimulation, finally places its free end quite vertical.
This is very generally the case with the primary roots of the seedlings of Dicoty-
ledons, although other organs also, e. g. the first seed-leaf of many Monocotyledons,
such as the Kitchen Onion and the Date Palm, behave similarly, and again many
lateral roots, especially those springing from stems, enter the earth vertically. The
lateral roots which grow obliquely downwards, and other organs which behave
similarly, I shall for the time being leave out of consideration; we are only con-
cerned with rendering clear what happens in the case mentioned.

In such experiments with roots not only is great precaution necessary, but also
the experience of years and an extensive knowledge of vegetable physiology, to
avoid falling into errors, as did Charles Darwin and his son Francis, who, on the
basis of experiments which were unskilfully made and improperly explained, came
to the conclusion, as wonderful as it was sensational, that the growing-point of the
root, like the brain of an animal, dominates the various movements in the root.

It is not necessary here to enter in any way into a refutation of this view, since
this has been done by Detlcfsen in the most forcible manner. It is true that, to
demonstrate the geotropic irriiabilitv of a root from which the growing-point has
[3] ■ vy

690

LECTURE XXXIX.

been cut off", is a matter of some difficulty. But if Darwin's view were correct it is
probable that in the case of geotropic shoots also the growing-point at the end of
the shoot-axis plays a similar part ; this is, however, by no means the case, as I
showed long ago, since pieces of growing shoot-axes from which not merely
the growing-point but even the whole apical portion has been removed, are able to
make vigorous geotropic curvatures, and even thin lamellae prepared by means
of two longitudinal sections from such decapitated shoots are still geotropically
irritable.

For roots also the statement holds good, that only the parts which are growing in

length react geotropically, and are therefore capable of curving, and that the

curvature is due to alterations in growth. Since now the whole of the growing

region, as has already been repeatedly shown, is usually

^ ^ but 8-10 mm. long, even in the case of thick strong

primary roots, and the curvature makes itself evident only

in the middle transverse zones of this portion — i. e. the

zones which are growing most actively — the curvature of

roots in general appears to be much more pronounced than

in the case of long geotropically curved shoot-axes ; or,

in other words, the radius of curvature is much smaller.

The accompanying figure (Fig. 389) aff'ords a
sufficiently clear idea of the manner in which a vigor-
ous primary root, previously growing vertically, when
laid horizontally in loose damp soil, curves down-
wards behind a thin plate of talc. The six marks in-
dicated were drawn on the root with Indian ink, so
that the mark o denoted the growing-point enclosed
in its root-cap, while the others were about 2 mm.
apart from one another. On the transparent plate of
talc, behind which the root was situated, a triangular
index of paper was gummed, as shown in the figure,
and by the aid of this the movements of the root-apex
could be better determined ; the figure shows how, in
consequence of the growth in length, the marks o, i,
2, &c. gradually pass beyond the point of the index,
because they are driven forward by the elongation of the parts lying behind.
The relative displacement of the marks shows at the same time the distribution of
growth within this region; at first it is the portion 2-3 which elongates most,
then the piece 1-2 grows more strongly, and finally the principal elongation takes
place in the youngest portion o-i. The oldest portion 4-5 has grown but little
during the observation, because it was already towards the end of its growing
phase at the beginning of the experiment, and, during the course of the experiment,
soon ceased to grow altogether ; similarly with the portion 3-4. In the figure, A
denotes the condition of the root at the commencement of the experiment ; B
after i hour; C after 2 hours ; D after 7 and Rafter 23 hours.

The curvature is, as observed, after two hours (C) distributed along the whole
of the growing region, though still feeble everywhere ; after seven hours [D) the

Fig. 389.— The growing end of the
radicle of a Bean, curving under the
influence of geotropisni.

DOIVNIVARD CURVATURE DUE TO GEOTROPISM. 6i)l

curvature is most pronounced in the region i, 2, 3, where also most elongation
has taken place. By means of the curvature, again, the piece 1-2, and still more
the piece o-i, has come into a position where it is directed nearly vertically down-
wards, and where the active component of the force of gravity can exert but a very
feeble geotropic influence; the very feeble curvature obtained at the beginning
of the part 1-2 is thus no longer appreciably increased, whereas the part 2-3 in
consequence of its more favourable position, in which it is still cut by the direction
of gravity at a tolerably large angle, becomes still more cur\ed. From all that
has been said, it will be noted that the youngest part o-i becomes directed down-
ward chiefly passively, by means of the curvature of the parts lying behind it,
and when it has attained this vertical direction it simply goes on growing in it.

The careful observation of numerous roots by this and other methods leaves
no doubt that the geotropic processes are here in all essential points the same
as in the up-curving of shoot-axes, only they take place in exactly the opposite
direction, and on this account it is quite correct to distinguish the two phenomena
by means of the terms positive and negative. Moreover, as I have demonstrated,
the down-curving of roots agrees with the up-curving of shoot-axes in that a re-
tardation of the elongation of the axis of growth occurs during the curvature, while
the side which will be convex grows more strongly and that which will be concave
more feebly than would be the case in the vertical direction.

The proof of the identity of the processes resulting in positive and negative
curvature which I brought forward, was important because not only Knight but
Hofmeister also had referred the down-curving of roots to essentially other causes
than those to which the up-curving of shoot-axes is due. In particular, it was
believed that the down-curving of the root must be regarded as the mere sinking
of a viscous pasty mass. This view was opposed by Frank, among others ; and it
is, in fact, utterly erroneous, as can be demonstrated at once by compelling the
down-curving root to set in motion a weight much greater than its own. For
this purpose I have placed vigorous roots horizontally and with their apices sub-
merged in a small shallow vessel full of water, from which a thread, suspending a
weight of i-i'5 gr., was carried over a pulley. Should the root-apex curve down-
wards it must set the weight in motion ; and this actually occurred. The experiment
illustrated in Fig. 390 is simpler and more elegant, but is of course less obvious: the
result, however, is the same. A seedling of the Broad Bean ( Vicni Faba), the root and
plumule of which were perfectly straight, was fixed into the piece of cork k by means of
a needle, so that the root-apex lay horizontally on the surface of the mercury, figured
black: nn denotes a thin layer of water on the mercury. After about 24 hours the
seedling had taken the form represented in the figure. The plumule with its sharply
curved bud had become directed vertically upwards ; but the lip of the root had
become directed vertically downwards in the usual way, having meanwhile
penetrated the mercury (which is about i3'6 times as heavy as the watery substance
of the root) to the depth of about i cm. Thus the part which had penetrated the
mercury had displaced a weight i3'6 times as great as its own. It is obvious that
the weight of the mercury presents an equivalent resistance to the entrance of the
root-apex, and this makes itself evident in the figure by a curvature resembling
a drawn out .V being produced behind the curve due to gcotropism. More-

y }• 2

692

LECTURE XXXIX.

over, the entrance of roots into the earth, and the driving in of the root-apex,
in which process the particles of soil must be pushed asunder, show that the
movement can by no means be compared to that of a flowing viscous mass ; the
root-apex, on the contrary, penetrates the earth much as does the point of a nail
hammered into a board, or as a worm does. In thus entering a heavy or co-
herent substance a point of application must of course exist : this is afforded in
Fig. 390 by the fixing of the seed with the needle. If such a point of application
does not exist the root-apex cannot penetrate into the mercury, but executes sinuous
movements on its surface. This is why it is so important that germinating seeds
should not simply lie on the surface of the soil, but should be covered with
a layer of earth sufficiently thick to enable them to offer sufficient resistance so as
to maintain their equilibrium whilst the root is penetrating into the deeper layers.
Germinating seeds which are not covered, or are too slightly covered, turn their root-
apices downwards, it is true, but they cannot penetrate into the earth, because the
small seed does not offer sufficient resistance. It is to be remarked, however, that

Fig. 390.— a seedling of l'icüi Falm, the radicle and plumii
was then laid horizontally 1

; of which had grov
I the surface of the

: position : the seedlii

seedlings possess manifold adaptations in order that even when they are not covered,
or insufficiently so, they may cling fast to the surface of the soil, so that the root-
apex obtains a point of application on entry ; this is often accomplished by means
of numerous root-hairs being developed very early and fixing themselves into the
soil.

With respect to the phenomena of Heliotropism, I may treat of them much more
shortly than with respect to those of GeotrOpism, because, as we shall see immedi-
ately, they agree completely in all essential points : the effects of heliotropic
stimulation are exactly like those of geotropic, only the stimulus is a different one,
namely, Light. However, it is first necessary to describe generally the phenomena to
be here spoken of. In the case of plants growing in the open or in a garden
there is usually not much to be seen of heliotropic effects, especially if the
plants are nearly equally illuminated on all sides, as happens when the light is
reflected in all directions from clouds, &c. The direct light from the sun of course
produces unilateral illumination, but the source of light revolves, so to speak, around
the plant, and thus a similar effect is produced to that which I have already described
in connection with the apparatus on page 561. Nevertheless, in very sensitive plants,
e.g. young Flax-stems and flower-shoots of the Sunflower (the latter being named

IIELIO TR OP ISM. ßo .3

Sunflower, Tounicsol, on that account), it is possible to notice how its apical parts
follow the course of the sun from morn to eve, always inclining towards it.

The point is, then, that growing parts of the plant should be illuminated
from one side only, or at any rate more strongly there than on the opposite side; hence
in most plants cultivated in a room or in a greenhouse very evident heliotropic
curvatures of the shoot-axes, petioles, and, in part, the lamina itself may be noticed.
Apart from exceptions to be mentioned later, the apical portions of the shoot become
directed towards the window and curved convex on the side next the room : petioles
behave similarly as a rule, and, in general, by means of the heliotropism of both,
the final effect is that the laminae, especially if freely movable on long stalks,
assume such a position that the upper side
presents itself approximately at right-angles
to the strongest incident rays of light. This
betrays at the same time the chief purpose
of the heliotropic curvatures : in combina-
tion with their geotropic properties, the helio-
tropism of the shoot-axes and leaves acts in
such a way as to place these organs in posi-
tions favourable to the well-being of the
plant. In this sense it is also to be under-
stood that some organs, in accordance with
special relations of life of the plant, turn their
free ends away from the source of light to-
wards the darkest side, and thus behave as
negatively heliotropic in contrast to the above
described positively heliotropic ones: this is
the case, for example, with the aerial roots
which serve for climbing in Aroids, &c., as
well as the climbing shoot -axes of the
common Ivy, which, when cultivated in a
room, always tend to curve away from
the window. It is very remarkable also that
roots, which normally grow beneath the
soil, when cultivated in transparent water or
in moist air, prove to be heliotropic, some of them positively, others negatively ; these
roots thus possess in their heliotropism a form of irritability from which, under
normal conditions of life, they derive no use whatever, and which therefore certainly
cannot have been inherited by them according to Darwinian principles.

However, these remarks are merely preliminary. In order to study the pheno-
mena themselves more closely, we may avail ourselves of a very simple experi-
ment. We assume the seedling of the White INIustard {Stnapis Alba), shown
in the accompanying figure (Fig. 391), to have its root bde submerged in clear
water nn, contained in a transparent glass vessel. We further suppose the plant
to be fixed in this position in any convenient manner, and to have been placed
hitherto either in the dark or on a horizontal revolving disc in the light, so that it was
either not illuminated at all or was equally so on all sides. In both cases the primary

Fig. 391.— Seedling of White Mustard {Sinapis
nlha), showinjj the Iieliotropic curvature of the shoot-
axis and root.

694 LECTURE XXXIX.

root grows downwards perfecdy vertically, and the primary stem vertically upwards.
We now place the vessel in the middle of a room, so that the plant is lighted from
the left, as shown by the arrows, and preferably so that the rays of light meet the
root and shoot at right-angles or nearly so. Even after 1-2 hours it will be seen that
the growing parts of the plant have made the curvatures shown in the figure, and
these after a few hours are considerably more pronounced ; the plumule has bent over
towards the window whence the light comes, and the illuminated side has become
concave, i.e. it is positively heliotropic, and since at this time the upper two-thirds of
the length of the organ are still growing, the whole of this region has become curved :
the lower portion, already fully developed, has remained straight.

The root of this plant is negatively heliotropic and has become so curved at d
(since growth was still proceeding at this part), that the convex side is turned towards
the source of light, and the free root-apex e accordingly away from it. The part
of the root d d, which was already fully grown before the experiment began, has
remained quite straight.

The positive and negative curvatures of the heliotropic parts lie in the same
vertical plane, which may be supposed to extend from the source of light to the dark
side of the room.

What has been said sufficiently describes the heliotropic behaviour of most
plants ; of course there are also other cases, just as with geotropism, where, in con-
sequence of the stimulus of light, the growing parts of plants tend to place them-
selves transversely or obliquely to the incident ray of light, but I shall take these
and other cases into account in the next lecture ; meanwhile, we here keep in view
only the typical case illustrated by the figure.

In the plant experimented with in Fig. 391, however, neither the positive nor
the negative heliotropism can properly exert its full effect, since the organs con-
cerned are at the same time geotropically irritable also. If, then, in consequence of
the stimulus of light, the apical part at (7 assumes an oblique or even horizontal
position, the geotropic stimulation comes into play, in consequence of which
it strives to become erect. The curvature which it actually presents after
several hours, therefore, results from two tendencies — the inclination towards the
light, and the tendency to erect itself. The plumule is thus in a position of unstable
equilibrium between two mutually opposing forces : on shading the plant, the
plumule becomes erect because the heliotropic curvature is then feebler ; on
illuminating it more strongly, the heliotropic curvature prevails over the tendency
to geotropic erection, and the same (but with the converse result) would apply to the
negative curvature of the root.

If, therefore, for the purpose of more exact investigations, the heliotropic
curvatures are to be made visible in their pure form, the action of geotropism must
be eliminated. This can be accomplished by fixing the plant to the klinostat described
above (p. 684), and placing the axis of the instrument so that it stands at right-
angles to the window ; on then fixing the plant so that it stands at right-angles to the
axis of the klinostat, and thus revolves in a plane parallel to the window-panes,
the effect of geotropism is eliminated, while the plant is continually illuminated from
one side and can therefore show the heliotropic stimulation free from secondary
influences. In this way Hermann IMiiller (of Thurgau), who was dien my assistant,

DEMONSTRATION OF HELIOTROPISM. 695

made a long series of iiivesligations during tlic summers of 1874 and 1875, wiih
much skill and with excellent results.

The object of his investigation was suggested by theoretical considerations which
I then put forward as to the true nature of heliotropism. It was necessary to refute
the antiquated and no longer serviceable, though in its time very plausible view of
Pyrame de Candolle. According to this, heliotropic curvature was to be looked
upon as the result of one-sided etiolations ; since shoot-axes in the dark or in shade
elongate more rapidly than when illuminated strongly on all sides, he assumed that in
heliotropic curvature the side turned away from the source of light grows more rapidly
than the illuminated side. This explanation seemed quite sufficient for ordinary
cases of positive heliotropism, but various other observers had already sought in
vain to explain negatively heliotropic curvature also on the basis of De Candolle's
theory ; at all events it was possible among other things to assume that negatively
heliotropic organs, the above Mustard root, for example, or the aerial roots of Aroids,
grow more slowly in the dark than in the light. But already observations which had
been made in my laboratory by Wolkoff, contradicted this assumption, and Hermann
INIüller proved that as a matter of fact even negatively heliotropic roots, exactly like
positively heliotropic ones, grow more rapidly in the dark than in the light, and the
same resulted from much older observations by Schmitz on the negatively heliotropic
and spontaneously luminous mycelial strands of the Rhizomorphs. All this shows
that De Candolle's theory, although it seems to explain positive heliotropism, can by
no means be applied to negative heliotropism. But positive and negative organs
agree as to their heliotropism completely, exactly as with respect to their geotropism,
only that the effects themselves are in each case opposite, positive or negative. After
I had previously established this agreement concerning positive and negative geo-
tropism, I could scarcely doubt any longer, from the whole position of affairs (which
of course can only be indicated here), that with respect to positive and negative
heliotropism, exactly the same would apply. It necessarily followed from this that the
standpoint assumed by De Candolle must be abandoned, and that the whole slibject
of heliotropism must be looked at in an entirely different way — a view which
impressed me the more, since according to all the facts then known a striking
agreement exists between heliotropic and geotropic effects, and at the same time
I had even then come to see that geotropism and heliotropism are to be looked upon
as phenomena of irritability. In addition to these reflections, also, I came to the
conclusion that in heliotropic curvatures the important point is not at all that the one
side of the part of the plant is illuminated more strongly than the other, but that
it is rather the direction in which the ray of light passes through the substance
of the plant. This view became probable to me from the fact that even very thin
and in the highest degree transparent organs are able to make heliotropic curvatures
— that is, organs in which the side turned towards the source of light is only a little
more brightly illuminated than the other. Thus, for example, the root-hairs of the
Marchantias are negatively heliotropic, whereas the very thin and translucent sporan-
giophores of Minor are positively heliotropic. Similar examples are to be found
also among highly organised plants : the thin stems of the Balsams are extremely
translucent, and yet they are at the same time strongly heliotropic, whereas, ac-
cording to De Candolle's theory, it must be premised that heliotropism is the more

696 LECTURE XXXIX.

pronounced the less the light falling on one side penetrates into the tissues of the
shaded side.

That in geotropic curvatures the important point is only as to the direction
in which gravitation acts on the part of the plant, and that it is not in any way
a matter of a stronger effect on the lower side and a feebler effect on the
upper side, requires no proof; and these considerations led me to the conclu-
sion that in the case of heliotropic curvatures also, it might not depend upon
a difference in the intcnsily of the acting force on opposite sides of the organ, but
that, on the contrary, the heliotropic influence is due to the fact that the rays
of light enter the tissue of the plant, or even only single cells, in a definite
direction.

This view would be rendered highly probable if it were possible to detect
the same relations between the rays of light and the heliotropic curvature, as I
had established with respect to gravity for the geotropic curvatures. This re-
quired demonstration was afforded by Hermann Müller point by point. But
unfortunately the space here at disposal does not admit of my going more
particularly into the details of his demonstration : I must therefore refer to JMiiller's
excellent treatise mentioned in the notes.

For nearly eight years I have been able to find no fact which contradicts
the theory I have proposed, whereas numerous facts only recently established
are to be pointed to as in its favour. Even in the case of the influence of light
on the movement of swarm-spores, the important point can only be as to the direc-
tion of the rays of light, not as to whether the given swarm-spore is illuminated more
strongly in front or behind. The same applies to the movements of protoplasm,
in consequence of which the chlorophyll-corpuscles travel in the cells. With these and
other movements which are produced in plants by light, moreover, the heliotropic
curvatures agree also in that they are due chiefly to the highly refrangible rays
of light. If seedlings like that in Fig. 358 are placed in a box which only receives
such Kght as has passed through a solution of potassium bichromate, no heliotropic
action whatever takes place ; now this light contains only the red, orange, yellow
and part of the green rays, and appears very bright to the eye. If the plant is
placed in an exactly similar box, and receives the light through a dark blue
solution of ammoniacal oxide of copper, the heliotropic curvatures occur with the
same energy as if the plants had been exposed to full daylight ; but this blue light
only contains the blue, violet, and ultra-violet rays of the solar light. I obtained
exactly the same result when the light fell on the plant through sheets of coloured
glass. Behind a pane of very dark blue cobalt glass, which transmits red rays as
well as the whole of the blue half of the spectrum, the heliotropic curvatures follow
as in ordinary daj-light. Behind a pane of dark ruby-red glass, which permits the
passage of very little besides the red rays, no curvature whatever resulted.

Guillemain examined seedlings in the various parts of the spectrum itself, and
came to the conclusion that the heliotropic curvature takes place under the influence
of all the rays, with the exception of the least refrangible heat-rays. According to
him a maximum effect is produced by the ultra-red and ultra-violet rays. Wiesner,
on the contrary, finds that in the objective solar spectrum all kinds of rays, from
the ultra-red to the ultra-violet, with the exception of the yellow only, exert helio-

EFFECTS IN THE SPECTRUM. 697

tropic effects; the greatest stimulating force always resides at the boundary between
violet and ultra-violet, proceeding thence the effect sinks gradually until the green,
and suddenly disappears in the yellow part of the spectrum, thence onwards it begins
again in the orange and ascends up to the ultra-red, where a maximum is reached
which is smaller than the one first mentioned. In the yellow portion of the
spectrum there is, according to Wiesner, not only no heliotropic effect whatever
to be noticed, but it even appears that it diminishes the influence of the orange
and yellow rays.

The observations behind coloured screens then, do not quite agree with those
in the objective spectrum ; but I have already pointed out in my ' Handbook '
(1865, p. 42) that there are certain sources of error connected with the latter mode of
observation, and we may be sure that more has yet to be done in the matter.

Finally we come to the question in what manner do gravity and the rays
of light act on the processes of growth, that they are able to produce geotropic
and heliotropic curvatures ? In spite of many hypotheses, however, as good as
nothing is known about the matter. That when curvature takes place the
turgescence of multicellular organs increases on the side which becomes convex
is practically obvious; the question is simply why this takes place when gravity
or a ray of light meets a geotropic or heliotropic organ transversely or obliquely
to its longitudinal axis, and why the effect ceases as soon as that axis has assumed
the direction of gravity or of the ray of light. The question, how^ever, only obtains
a perfectly clear and strict form if the geotropic and heliotropic curvatures of the
non-cellular plants also are taken into consideration, where the positive and negative
curvatures of simple vesicles are concerned, and where there can be no question of
a difference of turgescence on the convex and concave sides of the organ.

It would be a poor makeshift to try to explain the geotropic and heliotropic pro-
cesses in mukicellular plants by themselves, and to seek another explanation in the case
of the non-cellular plants, and this for a very simple reason. If we take the case of
a single cell in the primary stem or primary root of the Mustard plant considered
above, which cell is situated in a curved portion, then this one cell behaves exactly
like a curved non-cellular utricle of Mi/cor, Vaucheria, or any other similar structure.

LECTURE XL.

THE ANISOTROPY OF THE ORGANS OF PLANTS'.

I USE the above expression to denote the fact that the different organs of
the plant assume very various directions of growth under the influence of the same
external forces. It must be remembered in this connection that the direction in

' The first attempt to describe and classify the vital phenomena of plants here considered, was
made by Hugo v. Mohl (1836) in his treatise ' Über die Symmetrie der Pflanzen,' contained in his
'Vermischten Schriften botanischen Inhalts'' (Tübingen. 1S45, P- ^2). Since that, scarcely any one
troubled himself further about these matters till I again brought the subject into notice in 1S70 in my
'Lehrbuch,' in the second and subsequent editions, in the paragraphs on the directions of growth.
Meanwhile Hofmeister had introduced great confusion into this province by neglecting all the facts
depending on the internal symmetry and other correlations of growth, and by ascribing the directions
of growth of the organs of plants to the influence of light and of gravitation, and especially in cases
where this had nothing to do with the matter ; but the worst was that Hofmeister, in the considera-
tion of the relations of direction of the lateral buds of woody plants, made a scarcely conceivable
error of observation in describing the buds which stand right and left as arising above and below
on the parent-axis, and drew conclusions therefrom.

The description given in this lecture is based chiefly on my treatise, published in 1879, 'Über
orthotrope und plagiotrope /^^cwr^cw/Z/^z/^ ' (Arb. d. bot. Inst. Wzbg., B. II, 1882, p. 226), where I
first made clear the causal relations between orthotropic growth and radial structure, and between
plagiotropic growth and dorsiventral structure.

More detailed statements, especially as to the directions of growth of the lateral shoots of the
Coniferje and Begonias are found in my ^Lehrbuch'' (Aufl. Ill and IV) in the paragraphs on direc-
tions of growth.

Hydrotropism is treated of in my publication (1872), 'Ablenkung der Wurzel von ihrer
normalen Wach sthuinsrichtung durch feuchte Körper'' (Arb. d. bot. Inst. Wzbg., B. I, 1874); and in
my treatise ' tJber Ausschliessung der gcotropischen und heliotropischen Krihniimngen während des
]Vacksthens' (Arb. d. bot. Inst. Wzgb., B. II, p. 209), I first pointed out that orthotropic shoots and
sporangiophores place themselves vertically with respect to the surfaces of a cube rotating on the klino-
stat, and that this is apparently a consequence of negative hydrotropism. This was then confirmed by
Wortmann in his treatise, 'Ein Beitrag zur Biologie der Mucorini'' (Bot. Zeitg., 1881, p. 368).

The reader superficially acquainted with our literature may be somewhat surprised that I have
made no further mention in the preceding series of lectures of Darwin's book, 'The Power of
Movement in Plants' (London, 1880). I find myself with regard to this book in the most painful
position, and can only regret that the name of Charles Darwin appears on it. The experiments
which he, together with his son, describes, are made without sufficient practical knowledge, and are
badly interpreted, and what little good is found in the book concerning general views is not new.
The main conclusion to which Darwin comes in his book, that all the movements of irritability in
the vegetable kingdom depend upon 'circumnutation,' characterises better than anything else the
position which the two authors occupy. It is unnecessary to say anything on this subject. Detlefsen
has said sufficient as regards the brain-function of the growing-point of the root (Arb. d. bot. Inst.
Wzbg., B. II, 1 88 2, p. Ö27).

THE MEANING OF ANISOTROPV.

699

which any part of a plant grows is determined by its geolropism and heliolropism,
and we shall see further on that other external influences also are effective to the
same end.

Before attempting to understand this phenomenon, which prevails throughout the
vegetable kingdom, however, it will be well to make the fact itself clear by means of a
few examples. In very many land-plants the original plumule grows vertically upwards,
and the primary root vertically down-
wards, and, as we have seen, both in
consequence of their sensitiveness to the
influence of gravitation, so long as one-
sided illumination of any kind does not
deflect the vertical directions of growth
into oblique ones by means of heliotropic
curvatures. But the lateral roots of such
plants — e.g. Sunflower, Ricinus, Fir, &c. —
either grow horizontally or descend oblique-
ly, and the lateral roots of the second
and higher orders which arise from these
may grow out in all directions, according
to their origin. The case is very similar
with the lateral off-shoots of the vertical
shoot-axis which is developed from the
plumule : its leaves assume an oblique or
horizontal position, the upper side being
always turned towards the light, while
the lateral shoots grow horizontally, or up-
wards at an oblique angle.

In other cases, on the contrary, the
primär}- shoot developed from the plumule
at once grows in a horizontal direction,
and forms on its under side roots which
descend vertically, and on its flanks or its
upper side leaves, which bear horizontally
extended laminae on vertically erect leaf-
stalks, as for example Fig. 392, and the
same is true, as has already been stated,
of the common Bracken Fern, Pteris
aquilina, (Fig. 55, p. 60).

In many tuberous and bulbous plants, and particularly in the case of
some Aroids, such as SauromaUan, the strong leaf-stalks spring from the subter-
ranean central body, and likewise become raised vertically upwards, like the stem
of a Fir.

It is otherwise, again, in the case of the Ivy, for instance, the shoots of which,
pressed closely to a wall, rock, tree-trunk, &c., grow vertically, obliquely, or hori-
zontally, while the petioles are turned away from this substratum in order to
present their laminx at right angles to the incident light ; whereas the aerial

Fig. Z'j^.—Marsilia salvatri
;aves (_^ natural size). K term
arps, arising from the petioles

V, anterior portion
nal bud ; bb leave

f stem with
; ff sporo-

LECTURE XL.

roots turn away from the light and attach themseh-es closely to the vertical
substratum.

In Agarics which grow up from the soil of woods, the stalk, which is funda-
mentally nothing other than a shoot-axis, places itself vertically upright, though the
umbrella-Hke pileus is horizontal, and the lamellar, tubular, or conical hymenial pro-
jections on which the spores are produced, and situated on the under side, are directed
downwards. I showed in i860 that this depends on geotropic sensitiveness on the
part of these Fungi ; for if an Agaric which is still growing is placed in a horizontal
position, the stalk erects itself vertically, till the pileus is horizontal ; whereas if this
erection is prevented, or if it does not follow quickly enough, the lamellae, tubes
or conical outgrowths of the hymenium make energetic curvatures downwards like
the ends of primary roots.

Even stemless Agarics, such as those frequently growing out from the trunks of
trees, exhibit a similar anisotropy in their various parts. As shown in Fig. 393, Agarics
grow out from a prostrate horizontal tree trunk from the upper side as well as from

the flanks, but always so that the ste-
rile substance of the pileus itself strives
to assume a horizontal position, while
the spore-forming hymenial outgrowths
are directed vertically downwards.

Exactly similar facts of anisotropy
are met with in non-cellular plants, e. g.
the Mucorini, which are so instructive in
other respects also. If the spores of a
Mucor or Phycoviyces are sown on a cube
of damp bread B, which, as in Fig. 394,
is fixed by means of a needle N' N to
the lid of a large glass cylinder, the
bottom of which is covered with water,
the root-like mycelium extends itself in
the substance of the bread, and after
a few days slender sporangiophores arise on all the surfaces of the cube ;
those on the horizontal upper surface of the cube of bread at once grow verti-
cally upwards, those on the horizontal lower surface and on the vertical sides,
at first stand out at right angles to these surfaces, as will appear later, but sub-
sequently they erect themselves geotropically as shown in the figure at /\ The
mycelium of the Fungus, however, behaves exactly like a root-system : after it has
permeated the nutritive substratum, individual branches of the mycelium also come
out from the lateral surfaces of the cube of bread, but more particularly from its
horizontal lower surface, and grow thence downwards into the damp air, while their
lateral branches assume oblique directions.

Now, whether the parts of plants grow upwards, downwards, horizontally or
obliquely, depends by no means upon their so-called morphological nature. Organs
which, and with good reason, receive the same names in descriptive botany, may
nevertheless assume entirely different directions of growth in different species of
plants. Shoot-axes, as has already been stated, may grow vertically upwards,

Fig. 393. — Transverse section of the trunk of a tree, o
Fungi (of the genus Thelephora) are growing, P—P'

ILLUSTRATIONS OF ANISOTROP}'. 70I

obliquely or horizontally, or even verlically downwards, as happens not rarely with

the lateral shoots of perennial shrubs and water-plants. Thus the first lateral
shoots of some plants with labiate flowers, as well as those of the Equisetinese
or Horsetails, penetrate vertically into the earth
like primary roots, and certain lateral shoots of
the rhizome of Typha, Sparganiiwi, some spe-
cies of Potamogeton, and many others behave
very similarly. It has also been already
mentioned that leaves may grow horizon-
tally, obliquely, or even erect; but there exist
also not a few foliar structures which grow ver-
tically downwards like primary roots, or, put
better, are negatively geotropic. Peculiarly
fine examples of these are found in the first
seed-leaves or cotyledons of some Monocoty-
ledons, e. g. the common Kitchen Onion (Fig.
395) : here, as shown in the figure, the primary
root together with the plumule is pushed out of the testa by

he viijorous eloncralion

LECTURE XL.

of the cotyledon, the filamentous first seed-leaf. If the seed lies on or in the
soil, so that the tip of the root is directed upwards by this elongation, as in the
figure [B), then it is not the root but the cotyledon which curves geotropically
downwards, so that the root-apex is compelled to penetrate into the soil (C and D).

Those as yet unacquainted with such matters may be not a little surprised
to see in the germination of the Water-nut {Trapa natans), the tip of the root (iv
in Fig. 396) of the seedling direct itself vertically upwards instead of penetrating
into the soil, but the anomaly is only apparent, for the part h does not belong to the

primary root at all, but forms the so-called hypo-
cotyledonary segment of the stem, the rudiment
of the root at lu being completely aborted, and
not having grown at all it is thus unable to
accomplish any geotropic curvature downwards.
Moreover, there are other seedlings which
exhibit a similar phenomenon, when taken from
the soil and laid on a damp plate for instance.
If a portion of the plumule is already developed,
but the cotyledons still remain fixed in the seed,
and if the latter is heavy enough, the seed re-
mains quiescent in consequence of its great
weight, whereas, in consequence of the geotropic
up-curving of the seedling-stem, the root-apex
is carried up because it is lighter; in this case
of course if the primary root goes on growing
its end may curve downwards, Geotropism,
exactly like heliotropism, brings about only the
curvature of sensitive organs, as already de-
scribed ; which end of the organ is to be
directed upwards in the process depends entirely
and simply upon the position of the fixed point.

These examples will suffice to show that in
anisotropy we are concerned with one of the
most general properties of vegetable organiza-
tion ; indeed it is quite impossible for us to
have any idea of how plants would look, or
could exist, if their various organs were not
anisotropous, and since their anisotropy is nothing other than the expression of
their different irritability to the influence of gravitation and light, with which are
associated, in some cases, a sensitiveness for unequal distribution of moisture in
the environment, and probably some other less well-known influences, it is at once
obvious that it is the different irritability of the organs from which the external
configuration of plants in general arises — a statement from which I started in the
lectures on Organography. Even the most important differences in the biology
of plants are the expression of the different distribution of anisotropy, of which
innumerable examples may be quoted. To mention one only, the stem of Palms
generally grows vertically upwards, but there are also Palms [Sabal], the primary

FIG. 396.— Germinating- Water-nut (rra/ir natans]
/coats of the fruit ; w aborted root ; h hypocotyl
c petiole of the large cotyledon which is buried in th
seed ; c' the small second cotyledon. Between th
two cotyledons is the bud of the primary shoot.

ANISOTROPY AS INFLUENCING TBE FORM OF PLANTS. 703

Stem of which, wilh its bud and the large foHage-leaves which it produces, bores down
into the earth, and since it cannot there advance, being a thick mass, it pushes
the old, properly lower, part of the stem upwards higher and higher above the
soil; similarly the great contrast between the common Ivy and its allies, the
majority of the Araliacese, lies in their mutual difference in anisotropy: the
shoot-axes of the Ivy climb, whereas the others erect their shoot-axes independently
without climbing. The Ivy teaches us, indeed, yet another important fact, viz. that
the distribution of anisotropy in one and the same plant may alter at different
periods of life. It is well known that the leaf-shoots of the Ivy do not always cHmb,
but that when the plant is old enough leaf-shoots are produced which grow out
independently, and then form leaves differently shaped and placed, and finally
flowers and fruits. Another case of this kind is very common with plants which
form runners or stolons, of which the Stra^vberry affords probably the best known
example. Its sub-aerial stolons grow horizontally, a meter or so in length, and
at last the terminal bud changes its character : it suddenly produces large foliage-
leaves in the form of a rosette, and erect flowering shoots. A very similar
state of affairs exists in the case of many subterranean stolons, e. g. the Umbellifer
uEgopodium podagraria, so common in gardens, and which is on that account regarded
as one of the most troublesome of weeds. To quote a case of quite another kind,
the great difference in habit between the pyramidal Poplars, as contrasted with the
Black Poplar and other species, depends essentially upon the branching of the
crown of the tree : in the pyramidal Poplars all the branches turn sharply upwards,
whereas in the other species they stand off from the primary stem at large angles ;
and just this difference can be produced in the formation of varieties, and there-
fore by very small internal changes. A large number of the most different woody
plants, which otherwise form broad spreading crowns, produce varieties of the form
of the pyramidal Poplar, among others the common Juniper {Junipenis communis),
which even forms a third variety, which is shrubby and has the branches spreading
along the ground.

However, all these statements are merely intended to show what is properly
understood by anisotropy ; and it is now time to examine the facts a little more
closely. In the investigation of natural phenomena which appear under very
different aspects, but in which something common to all is felt to be the important
and essential feature, it is always well first to introduce method so as to refer the
differences to the fewest types possible. This may best be done in the present case
by dividing anisotropic organs into two classes, which I have distinguished as
Orthotropic and Plagiotropic. Orthotropic organs are those which under ordinary
conditions of Hfe, where the surface of the soil is horizontal and the illumination
equal on all sides, grow perfecdy vertical upwards or downwards, as for instance
the primary stems of most trees, particularly of Firs and Pines, and the leafy stem
arising from the plumule of very many annual plants, such as Sunflower, Tobacco,
Flax, &c. ; and, generally, all organs which are positively or negatively geotropic
or heliotropic, in the way described in the preceding lecture. Thus primary roots
which grow vertically downwards and shoot-axes which tend vertically upwards are
anisotropic, but resemble one another simply in that both place themselves vertically
wilh reference to the place where they grow. In this they differ fr(nn all plagiotropic

704

LECTURE XL.

organs, lateral rootlets, side shoots, and leaves, which under the influence of the same
external forces assume directions oblique to the horizon or even horizontal positions,
and at the same time strive to present their flat surfaces at right angles to the
strongest light.

True to the fundamental principle which we have throughout adopted, that when
organs react differently towards the same external influences, this must necessarily
be caused by a difference in their organisation, we now inquire in the first place
whether all orthotropic organs are distinct in their organisation from all plagiotropic
ones. It can be shown that in very many cases the relations of organisation
are such as are indicated by the question; for all orthotropic organs are radially
constructed, and, on the other hand, all dorsiventral structures are plagiotropic.
Nevertheless it must be added that there are also many plagiotropic organs which

Fig. ■>ffj.—Phaseohis miillißorus, growing in damp soil behind a slieet of glass, k primary root ; n lateral roots of the first
order ; n n lateral roots of the second order. The roughly horizontal line denotes the surface of the soil.

as regards their coarser anatomy appear to possess radial structure, such as obliquely
growing lateral roots and side-shoots which arise from orthotropic parent-organs ;
but we may assume in such cases that certain peculiarities of structure not yet known,
and which are not necessarily visible by means of the microscope, determine the
plagiotropism, and at the same time correlations of growth very often appear to co-
operate also. For example, as has already been shown, the horizontal or oblique
growth of the branches of a Fir, depends upon the presence of the orthotropic apex
of the primary stem, and it may be added that a similar dependence exists also
between the primary and lateral roots of many plants. For example, if the tip of the
radicle of a Broad Bean or seedling Oak is cut away, one of the lateral roots situated
immediately above the section grows not obliquely but vertically downwards; it
becomes orthotropic, and from henceforth replaces the orthotropic primary root.
Exactly the same thing occurs in the long aerial roots of tropical Aroidese.

Between anisotropy and the correlations of growth generally, the closest
connection exists. The lateral structures which grow out from a strictly orthotropic

DrFFERENT REACTIONS' OF BORSI-VENTRAL AND RADIAL ORGANS. 705

radial organ — lateral roots, side-shoots, leaves, &c. — are as a rule plagiotropic, and
in many cases, particularly leaves, are also distinctly dorsi-ventral in structure ; and
conversely, orthotropic petioles with radial structure, and, similarly orthotropic
radial side-shoots, very commonly arise from plagiotropic shoot-axes. In the case
of plagiotropic lateral roots this makes itself evident in that they produce lateral
rootlets of the second and thirtl order which are apparently not geotropic at all,
as a rule (see Fig. 397).

The relations between anisotropy and the correlations of growth come out, if
possible, still more clearly, when plants are allowed to grow on the axis of a klinostat,
and at the same time the effect of heliotropism is eliminated. Even in this case, where
gravitation and light can have no effect on the direction of growth of the organs,
they nevertheless grow so that they form certain angles with one another ; but the
questions here opened up still require investigation.

I now proceed to describe the relations between radial or dorsi-ventral struc-
ture on the one hand, and orthotropic or plagiotropic growth on the other, relations
which have hitherto been studied chiefly by myself.

When an orthotropic shoot-axis or primary root is laid horizontally, there
results in the case of the former the upward, and in that of the latter the downward
curvature described previously, and it is wholly immaterial which side of the organ
lies upwards or downwards ; it is just in this that the radial structure of such organs,
even with reference to their irritability, makes itself evident — i.e. they react in the
same way on all sides. Radial orthotropic organs also behave in exactly the same
way towards the light : wlicn illuminated laterally they curve until the free movable
end lies in the direction of the ray of light itself, or, what is the same thing, is
illuminated equally on all sides; this part then grows straight forward and in the
direction of the ray of light, towards or away from the source of light — I am here, for
the time being, excluding certain cases of so-called negative heliotropism.

Dorsi-ventral plagiotropic organs behave quite differently. If a still growing leaf
of a Gourd, for instance, is laid horizontally with the normally upper side downwards,
the whole lamina becomes curved concave upwards; if such a leaf is illuminated
from beloAV it likewise becomes concave. In this way it is shown that the venation of
the leaf possesses the same kind of geotropism and heliotropism as ordinary ortho-
tropic shoot-axes; but a great difference makes itself evident in that this only
happens when the lower side of the lamina is turned upwards, or towards the source
of light. If such a growing leaf is laid horizontally with its upper surface upwards,
or if the light is allowed to fall vertically on its upper surface, the latter does not
become concave ; on the contrary, the lamina extends itself in one plane. It is thus
seen that the lamina of a growing leaf reacts differently according as gravitation or
a ray of light affects it from below or from above, and it is just in this the dorsi-
ventral structure of the lamina with reference to stimulation is made evident.

The dorsi-ventral flat shoot of a Marchantia (pp. 69 and 526) and other similar
flat shoots, and, generally, all the organs with strictly dorsi-\entral structure known to
me, also behave like a leaf.

If now we suppose a jilane thin organ with dorsi-ventral structure, such as
the lamina of an ordinary leaf or the flat shocjl of a Liverwort, rolled togclliL-r
parallel to the direction of its longitudinal axis, so that either the ventral or the

L3J

7o6

LECTURE XL.

dorsal surface of the organ is everywhere turned outwards, the rolled up organ
constitutes a radially constructed body, and certainly the most elegant confirmation
of the above statements lies in the fact that the dorsi-ventral organ in this rolled up
condition, where it has only incidentally become radial, now reacts also towards
gravitation and light as an orthotropic organ. Figure 398 will serve to show what is
meant. C represents the transverse section of a lamina in its extended condition,
and the arrows indicate the direction in which the structure changes, proceeding from
the lower to the upper side. If we now suppose this rolled together either as in A
or £, it is easy to see, and is indicated by the arrows, that what was previously the
lower side now runs round the rolled up organ as an external layer, and that the
dorsi-ventral structure has passed over into a radial one, which now confers on
the rolled up organ the character of an orthotropic one towards gravitation and
light.

It is not practically easy, either in the case of growing leaves or of the flat

riG. 398.— (See tlie text.)

shoots of Liverv/orts, artificially to roll up the already extended organ in the
manner indicated, so that it will go on growing undisturbed, but the converse
change also demonstrates what I wish to prove. The subsequently flat leaves of
very many plants — e. g. of all Grasses, most Liliacege and other Monocotyledons,
and even those of many Dicotyledons, as the Water-lilies {Nuphar, Nymphcea),
Pinguicula, and many others — are in the bud-state rolled together as in Fig. 398
A and B, either each individual leaf by itself {B), or so that the young leaves
envelope one another {A); on subsequent growth the rolled-up margins are drawn
apart, and the leaves become extended flat, as in C. Now so long as the young
leaves remain in the rolled-up condition, in the bud-state, as in A and ^, they
are orthotropic, because they constitute a radial convolute structure; but as soon as
they have extended themselves, they become plagiotropic, and are placed obliquely
or horizontally with reference to gravitation, and extended at right-angles with respect
to the rays of light.

That this is not simply a special peculiarity of a certain class of plants

ROLLED UP PLAGIOTROPIC ORGANS ARE ORTllOTROPIC.

is shown by reference to Fig. 399, which represents the fertile marginal lobes
of the large flat Lichen Pelligera canina. The organisation of the vegetative bod)-,
which grows on the flat surface of the soil in woods, is sharply dorsi-ventral ;
it is green and smooth on the upper side, colourless and furnished with roots
on the lower (cf. Fig. 249, p. 391), and in consequence of this dorsi-ventral structure
it is closely appressed to the horizontal surface. Of the lobes at the margin, those
which bear the fructifications or apothecia, a {r r, Fig. 399), rise up vertically, because
this part of the otherwise flat vegetative body becomes rolled up, as shown
at r r in the figure. In this case it is the upper side which comes to be external
on the rolling up : in the Lichen called * Iceland Moss ' {Cetraria hlandica),
which consists of branched ribband-like shoots, the inrolling takes place in such
a w^ay that the organic under or ventral side comes to be external, and in this
case also the flat dorsi-ventral structure gives rise to a radial one, which, for
that reason, is orthotropic. Yet another Lichen, Cladonia pyxifa/a, well known
from its elegant appearance, may also serve for the de-
monstration of what has been said above. It has two
forms of shoots : the exclusively vegetative shoots are
thin, flat, and dorsi-ventral, and therefore lie closely on
a horizontal substratum ; from these spring the shoots
of the second form, structures shaped like a tall old-
fashioned champagne-glass and circular in section : these
are strictly orthotropic.

It will be seen from the foregoing that the clear
understanding of the relations between radial or dorsi-
ventral structure, on the one hand, and orthotropic
or plagiotropic growth on the other, involves ideas
which present much that is extraordinary and difficult,
and I confess that it was only after many years of
thought that I was able correctly to apprehend the
matter; it must be added that even now several points

still await investigation. This is especially true of the fact that the orthotropic
or plagiotropic growth in some cases is due exclusively to gravitation, in others
to the co-operation of geotropism and heHotropism, and in others again the so-called
hydrotropism co-operates.

I will give one or two examples of each of these cases.

I demonstrated in 1874 that the lateral roots which originate from primary roots
assume their oblique direction solely under the influence of geotropism, and this
was the first case of geotropism of this kind which was known at all. Suppose
Fig. 400 to represent the upper part of the primary root of a seedling of Vicia Faha,
which has been grown behind a pane of glass in a box filled with loose soil, and
has developed numerous lateral roots, which have grown straight out laterally and
downwards at oblique angles with respect to the horizon. At that time it was
doubtful whether the latter happened in consequence of a geotropic cftect-; the
question therefore was, whether lateral roots are geotropic at all. By means
of the apparatus represented in Fig. 384, p. 682, I con\-inced myself in the
first place that the lateral roots are verv cli-arly affected by centrifugal force.

FIG. 399.— .4 a fructification (Apo-
thecium) a of the Lichen Peltigira
canina on the rolled up supportinfr
portion r r, which springs from the
flat vegetative body t. B transverse
section of A.

7oS

LECTURE XL.

cur\-ing away from the centre of rotation, which removes all doubt as to their
geotropism.

When I reversed the box, in which the seedling just referred to was grow-
ing, so that the apex of the primary root was turned upwards, the tip of the
root curved vertically downwards ; the direction of growth of the lateral rootlets
changed also, their apices likewise curving downwards not vertically, but only
obliquely, and this so that they grew on at about the same angle with respect to the
horizon as before the reversal of the box. Those portions of the lateral rootlets
which are quite black in the figure are the portions which were growing during the
inverted position. The box was then placed upright again, and the lateral roots
once more curved obliquely downwards, again to grow straight out at the same angle

with the horizon as before the
first inversion of the box. It has
not yet been explained hrtherto
what relations of organisation cause
these geotropic organs to grow
obliquely to the direction of the
earth's radius. Elfving found sub-
sequently, it is true, that runners
growing horizontally in the soil —
e. g. shoot-axes of Heleoc/ian's, Spar-
ganiian, and Scirpus viaritimus —
owe this horizontal direction of
growth to geotropism ; if they are
placed obliquely or vertically up-
wards they curve under the influence
of geotropism, until the bud lies
horizontally, and it then goes on
growing in this direction.

When sub-aerial shoots with
dorsiventral structure are at the
same time heliotropic and geotropic,
two cases may occur ; either the

positions.

heliotropic influence coincides with
the geotropic one, or each strives to give the organ dift'erent directions, so that, as
a matter of fact, a middle direction results. In order to be correctly understood
in what follows, I may premise that in the case of plants growing in the open the
heliotropic effect is to be supposed as if a vertical ray of light fell on the plant from
a luminous point in the zenith ; since just as we must suppose all the influence of the
gravity of the earth to proceed in a straight line only from the centre of gravity
of the earth to the plant, so we may also imagine the effects of the light reflected
from the whole sky as produced by a single resultant ray. We shall now make use
of this supposition in what follows.

The well-known Liverwort, Marchantia poljnnorpha, forms broad shoots which
are green on the upper side, and on the lower side colorless and provided with
numerous root-hairs : these shoots are also seen to be strictlv dorsiventral structures

Fig. 400.— Roots of Vicia Falia growing in soil behind a pane of
glass, at first in the normal, then in the inverted, and then again in

1 position. The ar
acted with respect '

the different

THE ANISOTROPV OF MARCHANTIA.

709

as regards their internal organisation. In the open, with unlimited Hght on all sides,
these shoots grow with their lower surfaces close to the soil, into which the long
roots penetrate deeply. Subsequently they produce shoots quite different in shape,
long stalks as much as 10 cm. in height, each of which supports cither a lobed
disc with male- organs, or a parachute-like structure with female organs. The
Stalks of these shoots are orthotropic and, when growing in the open, are directed
perfectly upright. Transverse sections show however that they are not properly
radial in organisation, but have become radial by the rolling together of the two
lateral margins of a narrow flat shoot, much as in the case shown at Fig. 398 B.
It is to this that they owe their orthotropism. In the further consideration of
this highly remarkable plant I shall confine myself to the two organs referred to,
although there are many other things which might be mentioned. If the ]\Iarchantia
is cultivated — it is very easily reared from spores, or still better from its gcmm»
■ — on the surface of soil in a
flower-pot in a room, not too far
from a window, the }oung or-
thotropic stalks become curved
towards the window, exactly like
ordinary seedling stems, and
then grow straight on at an
angle of about 45°. The flat
vegetative shoots on the con-
trary behave quite differently :
those which have their anterior
depressions turned towards the
window remain closely appressed
to the surface of the soil; those
on the contrary whose depressions
(in which the growing-points lie)
are directed away from the win-
dow towards the room, raise
themselves from the substratum
until they are directed obliquely

at an angle of about 45°, so that in this case again the anisotropy between the
flat shoots and the orthotropic stalks is approximately at right angles.

This behaviour was for a long time quite inexplicable to me, until I pro-
posed the question, how would matters be if the Marchantia were made to grow
on vertical surfaces of the substratum, with the light falling obliquely from the
window ? For this purpose I employed blocks of turf, carefully cut into the
shape of cubical bricks and saturated with nutritive solutions, and covered with
opaque boxes provided with a pane of glass only on the side turned towards the
open sky. A few gemmae were laid both on the horizontal surface of the block
of turf, and on the vertical sides — on the anterior face turned towards the light
as well as on the two flanks. These were easily retained by the damp sub-
stratum and at once began to grow on it. After 2—3 months, vigorous and even
fructifying plants were produced, and the result of the experiment may be illustrated

xt) A-
Kemi

710 LECTURE XL.

by the diagram in Fig. 401. It represents one of the turf-blocks seen from the
left flank; this presents a square outline, the body of the block being to a certain
extent drawn in perspective, so that the anterior surface turned towards the ray
of light L is to be supposed foreshortened to the observer.

The conformity to law of the phenomena now comes out clearly. The
orthotropic stalks 31 and W of the supports of the sexual organs have placed
themselves tolerably exactly in the direction of the incident ray of light L — a
proof that their positive heliotropism must act far more strongly than their geo-
tropism. On the contrary, the flat vegetative shoots to the right above and to the
left below in the figure are placed nearly at right angles to the incident rays, and
also the flat shoots at h to the left above and /^ would have exactly the same
direction— i.e. the direction h^, h^ and likewise /^, 4 — if they were not prevented by
the resistance of the solid turf. If possible these relations come out still more
clearly in the case of those flat shoots which are growing on the square flank of
the turf-block turned towards the observer, only it must be observed that the shoots
provided with the thick black lines stand out from the plane of the paper at
approximately a right angle. The root-hairs on the under side of the flat shoots
are also seen, indicated by thin parallel lines : they also are orthotropic, like the
supports M and IF, but turned away from the source of light.

After what has already been said of geotropism and heliotropism, the ortho-
tropic organs of this plant need no further description ; as regards the plagiotropic
flat dorsi-ventral shoots, on the contrary, considerable space would be needed to
explain their behaviour. Referring the reader therefore to my detailed treatise
published in 1879, I will only remark that the plagiotropic position of these
shoots results from the co-operation of their heliotropic' and geotropic properties, and
perhaps the reader will most readily apprehend what is intended by supposing one of
the root-hairs on the underside of the flat shoot to represent the primary root of an
ordinary seedling ; it is then to be further supposed that there is in connection
with this one root-hair a very narrow piece of the tissue of that part of the flat
shoot which belongs to it, much as if it had been cut out by means of a cork-
borer. This piece of tissue would then correspond to the shoot of a seedling
Phanerogam which had placed itself in the direction of the ray of incident light L.
Now it is easy to understand that the whole of the flat shoot of a Marchajiiia
with its root-hairs might be imagined as consisting of many thousands of
minute seedlings, laterally connected with one another; or, in other words, the
whole plagiotropic flat shoot consists fundamentally of nothing but orthotropic ele-
ments, which however are combined into a flat surface. However I must unfor-
tunately avoid pursuing this very fruitful idea to its further consequences.

The common Ivy (Hedera Helix) may be adduced as a second example of
plagiotropic growth under the simultaneous influence of different directive forces.
If a seedling or a rooted cutting of the Ivy grows in the neighbourhood of a wall
or a vertical surface of rock, the leaf-shoot applies itself closely to the surface of
the wall, &c., so that the leaves are situated to the right and left ; the free
anterior surface of the shoot-axis bears no leaves, and the surface applied to the
wall produces clinging roots which fix the shoot. Closely applied to the wall up
to the bud, then, the shoot grows up vertically. The lateral shoots which now

THE PLAGIOTROPISM OF THE I VF.

711

arise, however, grow upwards obliquely at an acute angle, but behave otherwise like
the parent shoot. There thus results a fan-shaped radiating system of climbing
shoots, which sooner or later reach the top of the wall. As soon as this happens
the shoot-axes curve over at the angle of the wall, and then go on growing
closely appressed to its horizontal surface, till they come to the other angle
of the top of the wall : here they do not bend sharply downwards, however, to
grow down attached to the hinder surface of the wall, but they go on growing
free straight onwards from the hinder angle of the wall, often quite horizontal
for 50 cm., and then sink down obliquely under their own weight. When the one
vertical side of the wall is thickly covered with such appressed shoots, another pheno-
menon makes its appearance; there arise numerous shoots which grow out away
from the wall freely sweeping in the air, and horizontal, at least at first, until
they bend down obliquely under their own weight. The orlhotropic fruit-bearing
shoots of the Ivy arise usually at the highest point which the plagiotropic climbing
shoots have reached ; the former are distinguished from the latter by their radial
structure, their free upright growth and by their differently shaped leaves, which
are here spirally arranged with a divergence of -|, whereas in the plagiotropic
shoots they stand in two rows on the right and left flanks.

We need however no longer concern ourselves with the orthotropic shoots,
since they behave towards light and gravitation like ordinary seedling stems;
only the plagiotropic shoots climbing up the wall or freely sweeping horizontally
require further consideration. Such shoots, cut off and placed with their lower
ends in soil in flower-pots, where they soon become rooted, and fastened up
to near the apex to vertical rods stuck in the soil, form convenient subjects
for experiment. If such a plant is now placed at a window so that the aerial
roots are turned towards the room, and the upper sides of the leaves towards
the light, the apex of the shoot curves away from the window in a few
days and then goes on growing horizontally, while the long thin leaf-stalks
curve towards the window, because they are orthotropic (cf. Fig. 402 A). The
plagiotropic shoot-axes of the Ivy are thus at any rate in some way negatively
heliotropic ; only it turns out that they continue to curve away from the source
of light only until they become horizontal. If they were negatively heliotropic
in the same way as most aerial roots, or the primary root of the White Mustard,
they would then curve not merely till they assumed the horizontal, but until
they were directed obliquely downwards. That this does not occur is at
least in part due to the geotropism of these shoot-axes ; in proportion as they
approach the horizontal, they come into a more and more favourable position
for the action of the geotropic influence under which they tend to become
erect. We may therefore assume that the horizontal position of plagiotropic
shoots results from the co-operation of light and gravitation ; how this occurs,
however, and particularly in what degree each of the influences is concerned
in the realization of the direction of growth cannot be stated in detail even in this
case.

It is easily intelligible that the power of the climbing shoots of the Ivy to
cling closely to a vertical wall or to a horizontal surface is due to the behaviour just
described — i. c. to their negative heliotropism — while the striving of the primary shoot

■iz

LECTURE XL.

to climb vertically upwards, and that of its lateral shoots to climb obliquel}- upwards
are to be ascribed to their differing geotropism, in so far as this makes itself evident
chiefly at the flanks where the leaves are situated.

It has already been shown in a previous lecture that in the case of jMarchaniia
and similar structures the dorsi-ventral organisation is itself induced by the direction
of the incident light ; this once accomplished, however, the side which was hitherto
ventral and shaded cannot again be transformed, by being subsequently illumi-
nated, into an organically upper side — the dorsi-ventral structure cannot be reversed.

The Ivy behaves very differently in this '
respect— i.e. so far as the shoot-axes of
the plagiotropic shoots are concerned ;
if one of these rooted shoots is fastened
to a vertical rod up to close beneath the
apex, and is placed at a window so that
the side which had hitherto borne roots,
or, what is the same thing, had hitherto
been the shaded side of the shoot-axis
is turned towards the light, as in Fig,
402 B, it is noticed in the first place,
that the backward curving of the apex of
the shoot takes place much more slowly
than in the reverse position, evidently
because the sensitive organisation which
had been induced by the previous rela-
tions is only slowly altered ; but it is
actually altered, for the growing apex
of the shoot becomes not only horizon-
tal, as in Fig. 402 C, but even deve-
lopes roots on the side which w-as
previously illuminated, and which is now
converted into the shaded side.

There is still to be mentioned here
an interesting peculiarity of the plumule
of young seedlings of the Ivy. When
seedlings growing at a window have
emerged from the soil, the first seg-
ment of the shoot (the hypocotyl) is
curved concavely towards the window; it is therefore positively heliotropic. On
further growth, however, the new internodes of the axis curve away from the
window, and the same then takes place with the first one also; its heliotropism,
at first positive, thus becoming negative.

A similar behaviour, but in some respects different, is exhibited also by the
Indian Cress (Tropiwhim viajus). I mentioned this then unknown case so long
ago as 1865. The seedling-stem (here the ejjicotyledonary segment) is at first
decidedly positively heliotropic ; but if the plants remain undisturbed at a bright
window, in summer, the seedling-stem together with the new develoj)ing internodes

Fig. 402.— Plagiotropic

res of tiie shoots of Iv
t).

TROP.^OLUM, GOURD, ETC. 713

subsequently become curved convex outwards, the apex of the shoot thus turning
away from the light, while the leaf-stalks, as in the Ivy and in all similar cases,
are decidedly positively heliotropic, and the laminae of the leaves are placed at
right angles to the incident light. The apices of the shoots turned away from
the light never become quite horizontal, however, probably because their geotro-
pism works to the contrary ; but in the open, where the light is more intense,
the negative heliotropism overcomes the geotropism, and the shoot-axes lie hori-
zontal and close to the earth. It is in consequence of this effect of light a^so
that the plant is able to apply itself closely to the vertical surface of a wall, and if
provided with a trellis in order to hold it fast, to climb up it. If, on the other
hand, the plant develojjcs in Uie shade, where the light is feeble, the shoots grow^
erect, and in fact become even positively heliotropic, and it is furdier to be mentioned
that even those shoots which are curved towards the shaded side when the illumin-
ation is strong on one side possess no permanent dorsi-ventrality. If the plant is
turned round so that what was hitherto the shaded side is now strongly illuminated,
the part of the stem already fully developed maintains its curvature, it is true, but the
younger internodes which are still growing become curved backwards, with the
apex of the shoot turned away from the window. With Tropccolum, however, such
experiments must be made in bright summer weather ; in the autumn, when the light is
feebler, the shoot-axes are always positively heliotropic.

The Gourd plant also behaves in all essential points like the Indian Cress.
That its stem, at tirst orthotropic, subsequently lays itself horizontally and prone on the
earth is an effect due to strong light : plants cultivated in the shadow of a room remain
erect for a long time, or curve but slightly obliquely away from the window. For
these reasons Gourd plants are able to climb up vertical walls provided with a
trellis, the apices of the shoot becoming so to speak pressed to the wall by the light,
and are then able to fasten themselves to the lattice by means of their tendrils. In
the open, where the light is strong, the long shoot-axes of the Gourd lie horizontally
on the earth ; but if the plant grows in the shade of undergrowth, the apex
erects itself and, with the aid of the tendrils, climbs between the branches of the
underwood into the full light. In the case of Gourd-shoots lying horizontally on the
earth, the apical portion bearing the terminal bud is erected in ordinary daylight so
as to assume approximately the form of a horse's neck, the bud, bent sharply down-
wards, representing the head. It is now easy to observe that on very bright days
this horse's neck lays itself flatter on the earth, while in dull weather it is more
erect. If the apex of the shoot is directed through a hole into the dark cavity of a
box, as in Fig. 238 (p. 354), it then goes on growing perfectly upright.

It would be very easy to quote numerous other instances of plants which behave
towards different intensides of light like the Indian Cress and the Gourd : in addition
to the common Vine, I will only name Glcchoma hcderacca, one of the Labiatae. In
connection with the phenomena just mentioned, induced by light in highly organised
plants, however, I must return once more to Marclnviiia, to add that the described
plagiotropic position of the flat shoots in this plant also is assumed only in a
sufficiendy strong light; if the light is unilateral and feeble, the Marchantia
shoots erect themselves, but without losing their typical broad form. When the
light is very feeble, the shoots at length remain narrow and become nearly

14

LECTURE XL.

Stalk-like, losing their plagiotropism and curving towards the light in a way which
shows that they are positively heliotropic.

In all the cases so far considered the point has always been simply that the
organs, under the influence of gravitation and light, make curvatures in a plane which
includes the direction of gravity and of the light. Very commonly, however,
twistings — torsions of the shoot-axes — come into existence, and these have the
effect of £0 turning or twisting parts of the bud which were originally arranged in
a vertical plane, that they finally lie in a horizontal or oblique plane. This occurs
quite commonly in the case of woody plants with erect stems from which horizontal
or oblique branches arise. The buds of these branches produce their very often
biserially arranged leaves in a vertical or at any rate approximately vertical plane, as
in Fig. 403. Now if, on the unfolding of such
buds, all the parts maintained their mutual posi-
tions, the leaves of the shoots of the second order
would be situated above and below on the de-
veloped axis ; the ordinary case, however, as found
in the Limes, Elms, Celtidae and very many other
trees and shrubs, is that the lateral shoots arising
from such buds extend their two series of leaves
in a nearly horizontal plane, which, with reference
to the young states (shown in Fig. 403) is only
possible by a twisting of the young shoot-axis dur-
ing elongation. Not rarely indeed it happens that
shoot-axes with crossed (decussate) pairs of leaves
undergo torsions at their internodes alternately
to the right and left, whence the leaves on the de-
veloped shoot appear to be arranged in two rows
only, instead of in four, and become extended in
a horizontal or oblique plane.

However inadequate this treatment of the
anisotropy of the organs of plants must appear,
owing to limited space, it will nevertheless serve to
show how extraordinarily various are the pheno-
mena which are induced by the action of gravitation and light ; and I may again
insist that for the illustration of general ideas, I have selected a few isolated
examples only. These phenomena are universal in the vegetable kingdom. For
instance, what has been said above as to the influence of light on swarm-spores
may be compared with the phenomena occurring in Tropcrolum and Cucurbita, and
in spite of the enormous diff"erence in organisation a great similarity in their sensi-
tiveness to light is to be found : we saw how most swarm-spores swim towards the
shaded side only when the light is intense, whereas when the light is feeble they
turn towards the source of light, and here we find that for example the shoots
of Tropccohim turn away from a strong light, while they turn their apices towards
the source of light when it is feeble.

I here add, in conclusion, a few words on so-called Hydrotropism, simply
because the experimental material to hand is not suflicient to devote a special lecture

Fig. 403.— \'ertical section of a lateral
bud of a horizontal branch of Cercis cana-
densis in December. i,-2...-j the conse-
cutive leaves with their (similarly numbered)
stipules. The outer bud scales are omitted ;
the inner ones are numbered 3 3. In the
centre is the growing-point of the shoot, b
position of the leaf from the axil of which the
bud arises ; a a.\is of parent-shoot ; zi direc-
tion of gravity.

HYDROTROPISM. 7l5

to it. After the older ve<;etablc physiologists, especially Dutrochec, had suggested
that roots may be caused to curve by the moisture of their environment, but
owing to unsuitable experiments had arrived at no results, I found in 1872 that this
supposition is, as a matter of fact, correct. Fig. 404 will make the facts easily intel-
ligible, a a represents the vertical transverse section of a very shallow cylinder or
ring of zinc, which by means of three threads c c is suspended obliquely at d. The
zinc frame a a had been previously covered with large meshed netting and then filled
with damp saw-dust, the upper surface of which is shown at// Peas or any other
seeds saturated with water were then laid in this saw-dust g g. In virtue of their
strong geotropism, the seedling roots at first grow vertically downwards and at
length come forth into the air between the meshes of the nelling at the oblique lower

Fig. 404.— Apparatus for observing the liytlrotropisin of the primary

surface of the apparatus. If the air is completely or nearly saturated with vapour the
roots grow down vertically into it ; if this is not the case, however, and the air is
only to a certain extent moist, but not saturated, the root-apices projecting from the
meshes curve laterally until they again come into contact with the lower side of the
saw-dust, as at // and i. Very often they grow obliquely downwards closely applied
to this oblique surface ; occasionally the tip of the root again penetrates through the
meshes into the damp saw- dust, at once bending downwards again geotropically,
however, to repeat the same process. In this way a root (w /;/) may fairly sew itself
like a needle and thread passing up and down between the meshes of the netting.

It is certainly, as numerous further experiments showed, only the difference
in the moisture of the surrounding air which produces these phenomena : aqueous

n6

LECTURE XL.

vapour is continually being evaporated from the damp saw-dust, and the air in its
immediate neighbourhood is saturated with it ; a little distance from it, however, the
air is relatively drier, and the root apices thus become curved, as the experiment shows,
so that they are concave on the damper side.

In the experiment described the hydrotropism of the root has to overcome its
geotropism ; but if the seeds are allowed to germinate on the surfaces of a damp
block of peat [T, Fig. 405) which is fastened to the axis ^ of a klinostat and slowly

Fig. 405- -Cubical block of pea

A of the klinostat

rotated, geotropism, as we already know, is rendered ineffectual. In this case
then the roots also grow closely apphed along the damp surfaces, it is true, but
if it accidentally happens that the apex penetrates into the turf itself it then goes on
growing as in /, 0, q, continually deeper into the turf, and, according to circumstances,
probably even again comes out from it, as at /, ;/.

In this, however, there is yet another cause acting on the roots ; for I have
found that growing root-apices, when they are pressed on the one side by a solid
body, behave like tendrils (only much more tardily) and become curved so as to be
concave on the touched side.

This is perhaps also the proper place to mention that the power which roots

POSITIVE AND NEGATIVE IIVDROTROnSM. 717

have of penetrating into the soil, and of continuing their progress in it, does not
depend exclusively on their geotropism, but that the contact-stimulus and hydro-
tropism just described are also effective in llie matter; however, I cannot here go
more in detail into this subject.

The klinostat experiment (Fig. 405) yields, however, yet another result. Apart
from disturbing influences, as at g, h, i, k, the plumules erect themselves so that they
stand out at right-angles on all the surfaces of the rotating cube, as a, c, d,/, for which I
can discover no other cause than this — that they likewise, but in an opposite way to
the roots, are hydrotropic — i. e. as the latter are so to speak attracted by a damp
surface, so die thin seedling stems are repelled by it : this is of course only to be
taken as a metaphorical mode of expression. The result is much more distinct, how-
ever, when, instead of a cube of peat, a cube of bread is fastened to the axis of the
klinostat and a few spores of P/iycimyccs, the Fungus we already know (p. 5, Fig. 3), are
sown on all its surfaces. The root-like mycelium penetrates into the bread antl there
becomes copiously branched, since it is not induced to grow downwards by
gravitation as in Fig. 394 : on the contrary, the sporangiophores (cf. Fig. 3, p. 5)
grow out from the bread, so that they all stand upright on the various surfaces of the
cube, as in Fig. 405 ?n m. If one of them comes out by chance at one of the
corners of the cube (as at m.^ it makes equal angles with the two neighbouring
surfaces.

Wortmann, b}- means of sjiecial investigations with Phycomyces, has subsequently
confirmed my previous supposition that the sporangiophores are repelled by damp
surfaces, and I have no doubt that we may apply this result to the thin seedling
stems of Fig. 405 also.

In concluding the consideration of the movements of plants induced by irri-
tability, I may again remind the reader of what I stated in the first introductory
lecture of this book, and what I have laid at the foundation of all considerations on
the vital phenomena of plants, that irritability is universal in the vegetable kingdom,
and that vegetable life without irritability is just as inconceivable as is animal life
without irritability. Irritability is the great distinguishing characteristic of living
organisms; the dead organism is dead simply because it has lost its irritability.

PART VI.

REPRODUCTION.

LECTURE XLI.

THE ORGANS OF REPRODUC TION.
GENERAT. CONSIDKRATIONS. ALG.E; FUNGI; ARCHEGONIATA.

It will be convenient to defer to the last of these lectures the general consider-
ation of the nature and physiological significance of the reproductive processes ; for to
a reader unacquainted with the organography of reproduction they would be at least in
part, if not entirely, unintelligible. The present lecture therefore will be devoted exclu-
sively to the description of the reproductive organs themselves. It will suffice for our
purpose, moreover, to select from the abundant material which laborious investigators
have accumulated in this province, a series of well-known and particularly striking
cases, from the description of which it will become sufficiently obvious that notwith-
standing all the variety of reproductive organs, there is nevertheless but one essential
point, the organographical differences being fundamentally accessory matters and
concerning externals only.

All reproductive processes serve the purpose of producing new independent
living organisms, and this is always accomplished by a portion of the substance of
an already existing organism affording the material from which the new structure is to
proceed.

Reproduction in this widest sense of the word may occur, especially in the
vegetable kingdom, in extremel}' various forms. In the first place there is the mere
regeneration by means of incidentally separated portions of a plant. It is a fact well
known to every one that, in the cultivation of plants, it is possible to produce new
plants from cut-off pieces of shoots, leaves, and often even of roots, under favourable
conditions of vegetation ; and it has already been explained to a certain degree in a
previous lecture how this occurs. Botanists have also long known that even in the
case of very small and in fact microscopic Algce and Fungi, accidentally isolated
pieces are able to acquire through growth organs which were wanting to them, and
to develope into new individuals. Indeed, cases are even know-n of small lumps of
protoplasm artificially expressed from the cells of Algae surrounding themselves with
a cellulose wall and then continuing their growth, as I have myself had the
opportunity of seeing in Stahl's preparations of Vatichcria.

With such processes is connected the very common phenomenon that in
many plants, as a result of the normal mode of life, the individual shoots which had
previously been developed from common growing-points, subsequently separate from
one another and go on growing independently. This happens very frequently in the
case of species provided with stolons (runners) or rhizomes (crce])ing root-stocks) ;

[ 3 ] 3 A

■22

LECTURE XLI.

the connection of these shoots with their parent shoot is sooner or later destroyed by
the dying off and final rotting of their older parts, so that each individual shoot can
now grow into a new independent plant. Many Mosses, Ferns, Equisetums, Grasses
and Reeds, the Strawberry, and numerous other plants may be mentioned in this
connection.

It very often happens, however, that the shoots destined to become separated
assume special and characteristic forms, so that they may be distinguished in a

Fig. ifify.— Tetraphis pellncidn, a Moss. A a plant bearing
gemma: (natural size). B the same m.agnified ; y tlie cup in
which the genimre collect. C longitudinal section through the
apex of B; d the leaves forming the cup. A' gennna? in various
stages of development ; the older ones are torn off from their
stalks by the later growth of the younger ones, and forced
over the edges of the cup. D a mature gemma (X 550) con-

:ing ;

the

rgin of one,

the

veral

layers of cells.

Fig. 407.— Development of Telraphis from gcmm.ie. A
shows a gemma ifi) torn off from its stalk at a ; the protonema-
filainent xy has been formed by the growing out of a marginal
cell of the gemma, and the flat structure/ has been developed
as a lateral outgrowth of the protonema ; this has also put out
root-hairs -w -w' and -w" (X 100). Bp a flat protonema, from
the base of which a leaf-bud A" and root-hairs 'iuiv' have sprung.
The base of the flat protonema often puts forth a number of new
fiat protonemata before a leaf-bud is formed.

certain sense as reproductive organs. In this connection I need only remind the
reader of what was said, in the lectures on Organography, concerning bulbs and tubers,
adding that some plants are propagated for innumerable generations solely by bulbs or
tubers — the potato for instance — so that reproductive organs in the narrower sense of
the word appear to be superfluous. In the simply organised Cryptogams (Algae and
Fungi) such phenomena are quite general. Deciduous cells, Conidia, are produced
on special supporting organs, usually in great numbers, and sometimes, as in the very
common mould, PenicilUu7n glaiicum, the reproduction takes place only in this way,
unless true reproductive organs arise under very special conditions.

VEGETATIVE PROPAGATION. 723

In the true INIosses almost any cell of the roots, leaves and shoot-axes,
and even of the immature sporogonium, may grow out under favourable conditions,
become rooted, form new shoots, and give rise to an independent living plant.
Besides this, however, there are some species, like the Telraphis illustrated in Figs.
406 and 407, which produce, in addition to true organs of reproduction, pecuharly
formed gemmae in special receptacles, and these contribute essentially to the
multiplication of the individuals. The Liverworts behave similarly : in many of them
not only are the individual cells of the leaves able to separate and germinate, but
here also various species are found which give rise to multicellular complex gemmse
in special receptacles, of which the Marchantia figured on ])age 69 may serve as the
most convenient and best known example.

All these different cases, which could be increased by many others more or less
abnormal, are properly left out of account when referring to reproduction in the
narrower sense : they may be collated under the name of organs for vegetative
propagation, in contrast to the reproductive organs in the stricter sense of the word.
All these vegetative organs of propagation are fundamentally nothing more than
parts of the vegetative body itself, or at any rate are not essentially distinct from
it : their organisation presents, in contrast to the rest of the vegetative body,
nothing essentially new or abnormal, and especially it may here be insisted ujjon
that by means of merely vegetative propagation the properties of the parent plant
are usually transmitted to the descendants much more strictly than is the case
with reproduction in the narrower sense of the word — a point to which I shall
return.

The proper reproductive organs, on the contrary, differ from the vegetative
organs in their whole organisation and development, and particularly in the manner
and means by which they fit into the life-history of the plant.

The true reproductive organs, again, are distinguished from merely vegetative
organs of propagation by the fact that they serve exclusively and onl\- for the purpose
of reproduction. They are in no way concerned (apart from a few exceptions such as
Spirogyra and other Conjugatse) either before or afterwards in the function of nutri-
tion, or in the simple mamtenance of the existing individual : their proper task is
that of reproduction. But of course, here, as in the whole kingdom of organic life, no
perfectly strict distinction can be maintained. Here again our ideas are best de-
veloped in considering the most sharply characterised forms — i. e. typical forms. With
these, just as was found to be the case with the vegetative organs, two categories of
more or less deviating forms of organs are connected ; on the one hand the rudi-
mentary organs, which we may regard as incomplete but archaic, and on the other
the degraded organs which have degenerated, so to speak, subsequent!}- from the
typical height of organisation.

In the case of the vegetative organs, shoots and roots, we had above all to take
cognisance of the remarkable fact that organs of Hke genetic significance may assume
different forms in order to fulfil different functions. In the case of the proper organs
of reproduction we find exactly the opposite : their function, the origination of new-
individuals, always remains the same, since this is, as already mentioned, their single
object. Nevertheless, if we take in review the whole vegetable kingdom, proceeding
from the simplest plants, the reproductive organs assume very various forms: so-

3 A 2

7^4

LECTURE XT.T.

indeed that the reproductive organs of the Flowering-plants give to the uninitiated
the impression of having not the remotest similarity to those of the Cryptogams.
Nevertheless, my short description will show that from the reproductive organs of the
simplest Algae up to those of the most highly developed Flowering-plants all con-
ceivable transitions exist, leaving no doubt whatever, that (with the exception of a few
Algas and Fungi) all the reproductive organs in the Vegetable Kingdom are to be
referred to a single type, the clearest expression of which is found in the IMosses and in
the majority of the Ferns and Equisetums. And what may probably be termed the
most astonishing result acquired during the last forty years is the fact which will
shortly become clear, that the reproductive organs of the Flowering-plants, while in a
certain sense the most highly organised it is true, are in another sense again simply
reduced and degenerated forms.

Approaching the matter more closely, and starting from the clearest cases, as
exempHfied in a large number of highly-developed Algse, some Fungi, all Mosses
and in most Vascular Cryptogams, there are always produced in the course of the
developmental history of such a plant two kinds of organs of reproduction, viz.
sexual organs, and asexual organs (Sporangia).

The sexual organs are male or female ; the male organs produce zoosperms
(antherozoids), the female organs oospheres. In the present state of science it would
therefore be the simplest and most accurate plan to denote all male organs Spermo-
gonia and all female organs Oogonia, in contrast to the Sporangia which produce the
asexual reproductive cells — Spores. However it will scarcely be possible to establish
this simple nomenclature as yet, since a series of different names for the same organs
have become naturalised in different subdivisions of the Vegetable Kingdom.

As already stated, there occur in the life-history of a plant of the above-named
subdivisions both sexual and asexual reproductive organs, and this in such a way
that by the co-operation of the male and female sexual organs, or to put it shortly by
means of the fertilisation of an oosphere, a plant-structure of some kind is produced :
this in its turn gives rise sooner or later, often as the result of very protracted processes
of growth and configuration, to a plant-structure of totally different organisation, on
which at last sporangia again make their appearance. The entire process of develop-
ment of a plant of this kind constitutes an alternation of generations, as it is shortly
termed. This consists then, according to what has been said, in the whole life-history
of a plant being divided into two chief sections. The developmental-history is twice
commenced from a reproductive process ; once with the germination from asexual
spores, the other time with the development of an embryo from the fertilised
oosphere, and — a point of extreme importance — the phase of life of the plant in
question which proceeds from the germination of the spore results in quite other
relations of organisation than the other. As a rule higher organisation is attained
in that phase of life which proceeds from the fertilisation of the oosphere.

Such is, briefly, the meaning of the phrase ' alternation of generations.'

If we now consider the mode of reproduction in some other Algse and Fungi,
there is found more or less agreement in the relations of the alternations of generations
described; some forms, however, can scarcely be referred at all to these schemes.
In the case of a few highly-developed Vascular Cryptogams, to which the Coniferae
and their allies are directly related, on the other hand, we meet with very complicated

ALTERNATION OF GENERATIONS.

725

Structural arrangements, which may perhaps be most easily denoted by saying
that in them the generation which proceeds from the spore remains included in
the spore itself, and no longer enjoys independent life, and in the case of the
Flowering-plants this generation at length disappears so far, that it is only recog-
nisable in its last remnants by careful comparative investigation.

After these prehminary explanations, which could not well be avoided, the
description of a series of examples may now follow.

Among the Algge and Fungi there are many forms, the whole develop-
ment (and especially the reproduc-
tion) of which departs from the
above type sometimes in one way
and sometimes in another. I will
select but two examples. Fig. 408
shows at m a portion of the myce-
lium of a Fungus, Piptocephalis, pa-
rasitic on the Mticor, M, and ^hich
has bored into the latter with its
haustorium at h. As a rule this
mycelium produces conidiophores,
as at c, from the ramifications of _

which small conidia are abstricted ,c*'5'°| s^ J^'*^</n 11 \W1 })

in large numbers, and by means of
which this Fungus, Piptocephalis,
usually propagates itself. Under par-
ticularly favourable circumstances,
however, a second kind of repro-
ductive organs is developed, which
we may distinguish as sexual, since
the chief mark of sexuality lies in
that the contents of two cells fuse
with one another, in order to produce
a product capable of development,
whereas each cell by itself would be
incapable of this. This is also true
of the so-called conjugation of the
Fungus in question : two approxi-
mating or perhaps mutually touch-
ing branches of the mycelium, ss, swell up considerably, become densely filled
with protoplasm, and, after a transverse division has occurred in each, their apices
come in contact and fuse, and the separating walls dissolve, whereupon the fused
portion swells up into a relatively large sphere Z, which becomes segmented ofi" from
the conjugating branches jj as a special cell filletl with protoplasm, and forms a
thick prickly envelope. This so-called zygospore — or, following a new terminology,
zygote — requires, like most sexually produced reproductive cells of the Algce and
Fungi, a long resting period, before it germinates and produces asexual conidio-
phores, whose spores again give origin to mycelia.

l'"IG. af».—Piptocephalis Freseitiana (after Brefeld). M ,t portion
of the mycelium of Mucor Mitccdo by which the mycelium m m of the
Piftocefhalis is nourished ; h the haustoria of the latter penctratinj;
into the hypha; of the Mucor; c a conidiophore ; J- J two conjugating
branches of the mycelium, forming the zygospore Z.

i6

LECTURE XLI.

<k

In Piplocephdiis, as in all JMucorini or Zygomycetes, the result of the fertilisation
is thus a single large cell capable of development (the zygospore or zygote). IMatters
are strikingly different in the case oi Ascoholus, an Ascomycetous Fungus studied in
detail by Janczewski, as shown diagrammatically in Fig. 409. Here also m in are
certain of the ramifications of the richly branched mycelium. The end of a
branch c becomes filled with protoplasm, swells up considerably, and undergoes
several transverse divisions; a neighbouring filament then applies itself closely to
the anterior cells of the carpogonium c by means of its thin branches /. This organ
has been named the poUinodium. Whether a direct fusion of its protoplasmic
contents with those of the carpogonium results, or whether a soluble fertilising
substance passes over from it, is not known ; but analogy with all other known
processes of fertilisation scarcely permits a doubt that one or the other occurs, and
that we here ha\e an actual process of fertilisation. Its effect, however, is here quite
^ different from that in the previous case.

The result is not one resting cell capable
of germination, but, stimulated by the
fertilisation, a fruit-body or fructification is
developed which is very voluminous and
highly organised in comparison with the
mycelium. This body in its turn consists
of two essentially different parts : from
one cell of the carpogonium (i. e. from
the female cell) a large number of
segmented fungus-hypha; .s-j grow forth,
on which the sporogenous asci a at
length arise in great numbers, as terminal
branches. Long before this spore-forma-
tion is accomplished, however, the mycelial
branches in close proximity to the ferti-
lising apparatus have also been stimulated
to renewed, vigorous, and entirely altered
growth ; they envelope the fertilising ap-
paratus, together with the ascogenous filaments s s, with a parenchymatous investing
layer//», rr, within which also the spore-forming asci a arise. Between the latter
are noticed thin barren hyphse, the so-called paraphyses, which originate from the
envelope. Finally, it hardly needs mention that the spores produced in the asci a,
when they germinate on a proper substratum, again give rise to a new mycelium,
on which, so to speak as a second generation in the above sense, the fructification
arises in consequence of the sexual act of fertilisation.

In AscohoJus, as shown in Fig. 409, the cell which is fertilised, i. e. directly
touched by the pollinodium, is not the one which eventually gives rise to the asco-
genous filaments s, and the sporogenous asci on these. The case is exactly similar
in that great subdivision of the Algae, the Florideae, where the female sexual organ
likewise consists of a multicellular body, from which si)rings a single cell, the
trichogyne, in the form of a long filament, which takes up the male fertilising
substance; thus in the Floridece also it is other cells of the carpogonium which

Fig. 409. — Diajjrani of a transverse sect
fructification of Ascobolus /iir/iii-acein, an A
Janczevvski's figures), »t mycelium ; c carpogoniu
linodium ; s ascogenous hyphn; ; a asci ; rp steril
the fructification, from which the paraphyses A ate (

ycete (from

'ssue of
eloped.

ASCOBOLUS — SPIROGVRA.

72;

by means of their furlhcr growth eventually produce the spores, and, usually, also
an investment around them. Here, however, it is to be mentioned that in the
Floridea3 the ferliUsation is accomplished not by iiollinodia, as in the Ascomycetes
considered above, but by means of very small cells which are developed in large
numbers in special male organs ; these cells are set free and passively carried by the
water, and then, coming into the neighbourhood of the trichogyne referred to, fix them-
selves on to it, and empty their contents into it. Stahl has shown that the fructification
of many Lichens also is accomplished in a very similar way.

Although their external ajipearance is extremely different, the Mucorini never-

FIG. 4io.—S/tyo£:)'ra loiioxrta. To llie left several cells of two filanicins wliicli are about to conjugate. Tliey show
the spiral chlorophyll-bands, in which crown-like arrangements of starch-grains are lying, as well as small drops of oil.
The nucleus of each cell is surrounded by protoplasm, from which threads go to the cell-wall. * preparatory to conjugation.
A, to the right, cells engaged in conjugation. The protoplasm of the one cell is just passing over into the other at <» ;
in * the two protoplasmic masses have already united. In B the young zygotes are clothed with a wall.

theless agree essentially with the Conjugatse — a subdivision of Algge — in so far as
in the latter also cells externally ahke in nature, and especially equal in size, combine
with one another and allow their protoplasmic contents to fuse into a single mass, which
then forms a resting zygote and only becomes capable of germination in the following
year. Of this process, which of course presents numerous modifications again in the
various species of Conjugatas, Fig. 410 will serve to elucidate the most important
points. Fig. 410 represents, to the left, short portions of two long filaments of
Spirogyra, found everywhere in May and June, as matted green masses floating
on still water. These two filaments have (within the dense mass of the Alga)
become laid })arallel to one another for some distance, whereupon, in each case

728 LECTURE XLI.

from mutually confronting cells of the two filaments, protuberances have grown out,
as at a and d, which come into contact with one another; at the points of contact
their cell-walls then dissolve, and a joint canal is thus formed between the
two confronting cells, as seen in Fig. 410 A at a. Meanwhile the thin lining of
protoplasm which clothes the cell-wall, and in which the band of chlorophyll lies,
contracts so as to form a spherical mass, a process only possible by the expulsion
of water. Now, however, comes in a distinction between the two mutually con-
fronting protoplasmic bodies : the one remains stationary, whereas the other, as
if attracted by the former, puts out a process into the canal, which comes into
contact with the former, and as soon as this contact is accomplished, the whole of the
movable protoplasmic body glides over to the other, the process resembling the
contact and fusion of two oil-drops floating on water (cf. Fig. 410 A, a).
The union is complete. After its accomplishment no trace is perceived of the
fact that the body 5 (in Fig. 410 ^4) has arisen from two; even the two spiral bands
are said to join ends and become one. The zygote thus produced now surrounds
itself with a membrane composed of several shells, and after the fertilised filaments
of the Spirogyra have sunk to the bottom of the water the zygotes contained in
them remain dormant until the next spring ; they then germinate in the manner repre-
sented in Fig. 24 (p. 33), and, ascending to the surface of the water, again develope
into long filaments divided into chambers by means of transverse divisions.

In Spirogyra, more distinctly than in the examples previously considered, the
fact comes forth that fertilisation consists essentially in the fusion of the contents
of two cells. In contrast to the slow movement, which is here executed by only
one of the two sexual cells (which we may designate male) a great number of Algae
are now known whose sexual cells swim actively about in water like ordinary
asexual swarm-spores, and then, when they come into mutual proximity, attract
each other, come into contact, and then fuse together. To distinguish these from
non-sexual swarm-spores, such sexual swarm-spores are called Gametes, and the
product of their union, which usually moreover has to undergo a period of rest, the
Zygote. I will describe these processes in connection with certain Algae of the family
Volvocinese, in which this form of fertilisation was first observed in 1869 by
Pringsheim.

Pandorina Moruni, Fig. 411 /, is one of the commonest of the Volvocinese.
The sixteen cells forming an individual or coenobium are packed closely together and
surrounded by a thin gelatinous envelope, from which the long cilia project. The
asexual multiplication is accomplished by each of the sixteen cells repeatedly breaking
up into sixteen smaller cells, which group themselves into a coenobium in exactly the
same manner as will be described below for Eudorina. The sixteen daughter-
ccenobia (//) become free by the dissolution of the gelatinous envelope of the parent,
and each of them, again invested with a gelatinous envelope, grows up to the
original size of the parent family. The sexual reproduction also is commenced in
exactly the same way. The gelatinous envelopes of the young families deliquesce,
and the individual cells are thus set free, and each swarms by itself independently
(///). These free swarmers diff'er much in size ; they are rounded and green at the
posterior end, and pointed, hyaline, and provided with a red corpuscle at the anterior
end which also supports the two cilia. During the swarming of these bodies, some

PANDORINA.

729

are observed to approach one another in pairs, as if seeking each other ; they come in
contact at their apices, and fuse into a zygote which is at first biscuit-shaped iJV) and
which gradually contracts into a sphere ( V), in which the two red corpuscles and the
four cilia at the enlarged hyaline spot are at first still perceptible, though they all
disappear soon afterwards. A few minutes after the beginning of the conjugation the
zygote is a spherical cell ( VI), which then remains dormant within its cell-wall for
a long time, its green colour passing over into a brick-red one. On placing the

FIG. 411.— Development of Pandorina Moriim (after Pringsheim)- / a swarming family ; // a similar family divided into
sixteen daughter-families ; /// a sexual family, the individual cells of which are escaping from the gelatinous investment ;
IV, K conjugation of pairs of swarmers ; VI a zygote which has just been completed; /'// a fully grown zygote ; VI tl
transformation of the contents of a zygote into a large swann-cell ; l.\' the same after being set free ; .I'a young family
developed from the latter.

dried up, and meanwhile considerably grown, spheres into water, germination begins
after about twenty-four hours. The external shell of the cell-wall breaks away, and
a layer within it swells up, and now contains one or two, or even three large swarm-
spores, which finally escape ( VIII and IX), and after swarming for a short time
surround themselves with a gelatinous envelope, and, by means of successive
divisions, break up into sixteen primordial cells, which now again form a fomily
as in Fig. /.

Particular stress was formerly laid on the fact that both in Pandorina and in
the Conjugatae (e.g. Spirogyra), the two cells which unite sexually are equal in size

73°

LECTURE XLI.

and are apparently also similarly constituted, and such sexual acts were named
Conjugation, in contrast to the ordinary acts of Fertilisation which exclusively occur
in Mosses and Vascular Cryptogams particularly, where the one of the two sexual
cells, relatively large and non-motile, functions as the Oosphere, and is fertilised by
a relatively very minute and actively moving zoosperm (antherozoid). The investi-
gations of late years, however, have brought to light numerous cases from which
it must be concluded that no essential difference exists between conjugation and
ordinary fertilisation by means of antherozoids ; this may be concluded from the
fact, among others, that both forms of sexual process occur in very closely allied
species. Goebel found, for instance, in the case of one of the Volvocineoe which is

female colony (ca-nobiuni) a
(after Goebt;!).

closely allied to the Pcmdorina described above, and, like that, consists of a revolving
coenobium, that the sexual cells are differentiated into oospheres and relatively minute
antherozoids. The plant in question, Eiidorina elegafis, consists of gelatinous vesicles
of elUpdcal shape, in which are contained i6 or 32 cells, each of which possesses two
cilia, which project through holes in the envelope far into the surrounding water.
Active multiplicadon occurs by the asexual method, each individual cell of a family
becoming transformed by appropriate divisions into a family consisting of 16 or
32 cells again: these young families are set free by the disintegration of the parent
envelope. Sooner or later, however, a sexual difference makes its appearance
between different coenobia : some become male, others female. In the latter, 16 or
32 cells assume the character of oospheres, not conspicuously different from ordinary

EUDORINA.

31

vegetative cells. In the male coenobia, on the other hand, 16-32 antherozoids arise
by the division of each individual cell : these antherozoids are minute elongated bodies
with two anterior cilia, their colour being at first green but eventually passing into
yellow. The antherozoids M^ aggregated into a bundle begin to move while still
enclosed in the cavity in which they have been produced, and then escape and
sw^arm in the open. On meeting with a female coenobium the cilia on both sides
become entangled, and the male coenobium is thus fixed ; it then falls to pieces as
in Fig. 412 yJ/j and M.^, whereupon the isolated antherozoids, which now elongate
considerably, bore through the gelatinous vesicle of the female coenobium {Sp). Here

Fig. ^1.1,.— Fuchs phttycaypus (after Thuret). .-/ the end of a large brancli (natural size) ; // fertile br,
B transverse section of a conceptacle ; d the epidermal tissue surrounding all; n hairs projecting through tlx
of the conceptacle ; * internal hairs ; c oogonia ; e antheridia.

they penetrate as far as the oospheres, and, after groping and creeping around them,
apply themselves (often in some numbers) to these. It may be assumed, as has
been actually observed in many other cases, that one of these antherozoids bores
into each of the oospheres. Here also fertilisation is followed by the development
of two membranes, and the transformation of the green colour into a brick red. in
which condition the fertilised oosphere then passes through a period of rest.

Slight though the similarity of the fertilisation last described to that of the
Mosses and Vascular Cryptogams may appear to be, there exists nevertheless no
real difference as to the main points, since here as there the important fact is the
fusion of a small antherozoid with a relatively large resting oosphere ; a great diflfer-
ence exists only in the foci lIuU in the Mosses and Vascular Cryptogams special

732

LECTURE XLI.

organs — archegonia and antheridia — arise on the vegetative body, in which the
oospheres and antherozoids are developed. But even of this the first (of course
simple) cases are found in the Algae and Fungi. The vegetative body gives rise to
special organs, but of very simple structure, the oogonia, in which oospheres are
produced, and to others, antheridia (which would be better designated as spermo-
gonia) in which numerous antherozoids (zoosperms) are developed. Sexual organs of
this kind occur in a very perfect form in the various species oiFucus (Figs. 413, 414),
which are also worthy of mention here, because in some species two or several oospheres
are developed in one oogonium: these become surrounded by the numerous antherozoids
and fertihsed only after they have escaped from the oogonia, and are floating free
in the water, though they have no proper movement. In some Phycomycetes also
several oospheres are produced in one mother-cell, that is in one oogonium. The

Fig. 414.— Sexual xa^mAnzimw ai Fuciis -'csiciilosus. ,J cell-filaments bearing antheridia £ antlierozo ds. j
nium Og with paraphyses / ; // the outer membrane a of the oogonium has burst exposing the inner one con
the oospheres! ///an oosphere which has escaped, and is surrounded by antherozoids I first diMsion of tin
lised oospore; /F young Fiuus produced by the growth of the latter. (After Thuret—.S X j30 the rest X i6o )

typical cases of the IMuscineae and Vascular Cryptogams, however, are approached
by those Algae which never produce more than one oosphere in an oogonium, which
remains immovable within the membrane of the oogonium, and is fertilised by the
antherozoids entering through a neck-like aperture in it.

One of the clearest and best observed cases of this kind is met with in the
genus of non-cellular Algae, Vaucheria, common everywhere, figured on p. 108 (Fig.
107). On cultivating this Alga in a vessel of water at the window in the spring, it
reproduces itself at first only asexually. At the ends of the tubular branches,
which are abundantly supplied with chlorophyll, and which constitute the shoot of
this plant, a large portion of the protoplasm collects, and it is then separated off
by means of a transverse septum. This mass of protoplasm, which has pre-
viously contracted somewhat, then forces its way through an opening formed at
the tip, and escapes into the water, as shown on p. 108 zi A, The body B thus

VAUCHERTA.

733

set free is an ascxually produced s\varm-s})ore, whicli is provided with a velvet-like pile
of minute short cilia over its whole surface ; it swims about with a rotatory motion for
a short time only after its exit in the early morning, and at length becomes fixed some-
where and then germinates {D and B, p. io8). In the figure referred to is repre-
sented the sexual apparatus also of this Vaucheria, at F in og and //, and the Fig.
415 annexed gives a more exact insight into the processes of fertilisation. It is onl)-
after prolonged cultivation, when numerous generations of plants have been produced
by means of asexual swarm-spores, that sexual organs also appear. These, in
accordance with the simple structure of the Vaucheria, are formed by the develop-
ment of protuberances from the utricular shoot, as in Fig. 415 A at og and h. The

Fig. i^\^— Vaucheria sessilis. A, B development of the antheridium a on the branch k, and of the oog^onium og.
C an oogonium which has opened and sent forth a drop of slimy substance. D antherozoids. £ antlierozoids
collecting at the mouth of an open oogonium. F, it an emptied antheridium ; osji oospore in the oogonium. {A, B, E, F
from Nature; C, D after Pringsheim.)

Stouter shorter protuberance, the future oogonium, fills itself with protoplasm contain-
ing abundance of chlorophyll, and grows up into a body which has approximately
the shape of an oblique lemon {og in B) joined to the tubular vegetative branch by a
very short stalk-like portion. At this place the mass of protoplasm which has
collected within the oogonium becomes separated off from the vegetative part of the
plant by a transverse Avail. At length the apical papilla of the oogonium opens,
the protoplasmic body contracts a Httle, rounds itself off, and expels a drop of slime
through the open neck — a process which occurs in many other cases of fertilisation,
and is perhaps quite general — (C at si). While the oogonium thus prepares for
fertilisation, there is developed on the tubular process //, which eventually becomes
curved like a beak, the antheridium a (in D). In this also protoplasm collects, and
becomes divided off by a transverse wall from the stalk-like portion of the beak,
and then breaks uj) into a very large number of extremely minute corpuscles,

734 LECTURE XLT.

each of which is provided with two ciHa : these are the antherozoids, D in Fig, 415.
About the time when the neck of the oogonium opens, the antheridium also bursts at
the apex, and allows its numerous antherozoids to escape. For reasons which will
appear later it is not very probable that the fertilisation which now follows takes place
between the two sexual organs which, as in the figure, stand close together ; on the
contrary, it is probable that as a rule the antherozoids of another sexual apparatus
eifect the fertilisation of the oosphere in the oogonium. At least one antherozoid
enters the oosphere, whereupon the latter becomes clothed by a thick firm membrane,
and, after the whole oogonium has become separated from the parent plant, the
dormant period sets in, after which the fertiHsed and encysted oospore is in a con-
dition to germinate.

With the reproductive processes last described, although they take place in an
Alga so simple as to be even non-cellular, we have, as already stated, made an
approach to the typical reproductive processes of the Mosses and higher Cryptogams.
In fact the alternation of generations (the first as yet dim beginnings of which, more-
over, are to be recognised in many other Algse and Fungi) here comes more clearly
into view.

We may now turn to the reproduction of the IMoss, and since it is by no means
my object to give a complete review of all the subdivisions of the Vegetable King-
dom, I shall take from the whole of this immense group (passing over the Liverworts)
only one example from the subdivision of the true Mosses, selecting the most highly-
developed forms.

Fig. 416 may serve for preliminary guidance in the matter. It represents one
of our commonest Mosses, Catharinea iindulata, at the height of its development.
There are to be noticed numerous erect shoots furnished with leaves, and bearing at
their apices the ' Moss-fruits ' or Sporogonia ; these are curved capsules on long
slender stalks, each being provided with an anterior beak, and clothed with a cap, the
so-called calyptra. These apparent 'fruits,' however, are something quite different
from the fruits of a Flowering-plant. If it is wished to obtain the latter — e. g. the
cherries from a Cherry-tree — separately and independent, they must be torn or cut away,
for such fruits are part of the plant on which they are situated. The case is quite
otherwise with the Moss, however. With a little force, the stalk of the ' Moss-fruit '
can be easily pulled out from the shoot in which it is situated ; its tissue is not really
continuous with that of the shoot, but the stalk is simply stuck, so to speak, into a
sheath. The developmental history shows that the so-called Moss-fnuit is, as a matter
of fact, not a part of the parent plant at all, but an independent body of quite difi'erent
organisation, which is situated on the green leafy shoot of the Moss-plant like a
parasite, and simply for purposes of nutrition. We see in the figure, in fact, the two
chief phases of the life-history — the two alternate generations. The Moss-plant, con-
sisting of roots and leafy shoots, was developed originally from a spore, and was thus
produced asexually ; on the leafy shoot were subsequently developed the two kinds of
sexual organs, female (archegonia) and male (antheridia). Each archegonium con-
tained an oosphere, and each oosphere, after fertilisation, by means of very slow growth
and numerous cell-divisions, developed into a Moss-fruit, and this in its turn finally
brings forth asexually developed spores in the capsule ; the capsule opens by the anterior
])eakcd portion falling off like a lid, and the s])ores fly out like dust. The Moss-plant

THE MOSS.

735

proper, however, is not developed forthwith from these germinating spores, but the
latter give rise in the first place to a pro-embryo, a preliminary stage of the proper
plant, and which is termed the Protonema ; all that is most essential respecting this
has been stated in Lecture V (p. 68). On this protonema the leafy shoots arise by
altered growth and correspondingly altered cell-divisions. I pay no further attention
to this structure here, although it forms one of the most effective organs of propa-

ne. 4i6.-Crt//i,i

(after Schiniper).

gation, and in some cases, indeed, goes on living for years, whereas the projter IMoss-
plants on it only recur annually as temporary structures.

More detailed consideration of the true reproductive organs may be made in
connection with another Moss, Funaria hygromdrica, already mentioned previously,
which occurs very frequently in dense clumi)s on grass-plots, in woods, &c., and is
conspicuous even to the non-botanical obs^erver by means of the very numerous and
long-stalked bright orange-red ' fruits.' The plantlels themselves are small, oiiI\- a few

736

LECTURE XL7.

millimeters high, the stems being furnished with only a dozen or so of leaves. The
stem bears at its apex groups of sexual organs, and these groups may almost be
spoken of as ' flowers.' The smaller specimens are male, and produce exclusively
antheridia, which stand crowded together in large numbers within a rosette of leaves
surrounding them like a perianth. The female plantlets are larger and bear at the
apex of the stem a dozen archegonia, whicli are surrounded by a more bud-like
and closed perigone.

It has already been pointed out that the archegonia of the Moss and also of
the Vascular Cryptogams are fundamentally essentially the same as the oogonia of

riG. 4iS.—Fii>:nria hygromctrica. A an
which has just burst ; a antherozoids (X 350).
zoids very strongly magnified ; b still enclosed
f a free antherozoid of PolytrichiDii.

FIG. i,\i.—Fiinaria hygrometrica (a Moss),
archegonium before the opening of the cover m
of an old archegonium after fertilisation.

I archegonia ; h enveloping leaves. B a
ventral portion with the oosphere. C the

the Algse, and that in like manner the antheridia are only more complicated in
structure than is the case in the latter.

In Fig. 417 are shown, at A the female flower in longitudinal section, with a
group of archegonia at a, and the perigone-leaves in section at b. B represents the
anatomical structure of an archegonium at the time when the oosphere is not yet
ready for fertilisation, but will be so in a short time. At b is the ventral portion of
the archegonium, which is supported on a short and somewhat thick stalk ; from h to
m is the neck, which is in most Mosses remarkable for its great length, and which
is still closed above, at ;«, by four covering cells. In the ventral portion is seen a
long ovoid cavity, from which a canal runs through the entire length of the neck

FERTILISATION IN MOSSES. 737-

up to the covering cells w. Fundamentally, however, both together form one row
of cells, occupying the axis of the entire archegonium, the protoplasm of which
is contracted in the figure and rendered more visible by the mode of preparation.
The lowest and largest mass of protoplasm constitutes the still unripe oosphere ;
in the central cavity above it lies a smaller cell, the so-called ventral canal-cell, and
the narrow neck-canal is still filled up by thin long-drawn masses of protoplasm, the
neck canal-cells. When the archegonium is quite ripe, and if water gains access
suddenly, the now deliquescent canal-cells swell up strongly and exert a pressure
which causes the neck to open at m, the covering cells parting asunder : the
mucilaginous substance contained in the canal then escapes, and an open passage is
thus formed, at the base of which, in the belly or venter of the archegonium, the
now completely developed and spherical oo?phere lies. In this condition fertilisation
can take place.

The antheridia of the male jilant, which have meanwhile become completely
developed, are, in the mature condition, sac-like bodies situated on thin pedicles.
The wall of the sac consists of a single layer of cells, containing chlorophyll,
the colouring matter of which turns red on ripening. The whole of this enclosed
cavity is filled with innumerable very minute cells, each of which produces an
antherozoid. If the antheridia are completely developed, and a drop of water Hes
on the male flower, the antheridia become ruptured at the apex in consequence
of its absorption, and, as shown in Fig. 418 A (at a), a dense mucilaginous
mass is extruded from the opening, consisting entirely of the mother- cells of the
antherozoids, as shown at B, b. By absorption of water these cells swell up, and
become isolated, and the antherozoids soon escape from their envelopes and swarm
actively in the water. The antherozoids of the Moss are spirally wound filiform
bodies thicker at the posterior end, and pointed and provided with two long cilia
anteriorly (Fig. 418 ^, c).

As in all Mosses and Vascular Cryptogams, the archegonia as well as the
antheridia open after they have become quite ripe, but only if they are immersed
in water. The completion of this process in the case of the Mosses is favoured
by their tufted habit : the plantlets stand close together, and when it rains the tufts,
with their small interspaces between the leaves, act like a sponge, the male and
female flowers become thoroughly wetted, and those archegonia and antheridia
which have just become ripe open : the antherozoids now swim about in the
water which saturates the tuft of Moss, and some of them occasionally come near
the mouth of an archegonium. Then, as if impelled by an attraction exerted by
the oosphere and by the mucilage extruded, they collect together at the aperture of
the neck, some penetrate by their own movements into the canal, and finally one
antherozoid reaches the oosphere, and certainly penetrates into it antl dissolves its
substance in it.

Herewith, then, fertilisation is accomplished, and as usual the immediate efl"ects
are that the oosphere excretes a cellulose membrane, and then slowly begins to grow ;
this growth is accompanied by corresponding cell-divisions, and thus is produced
a multi-cellular embryo (Fig. 419 A,//') in the venter of the archegonium {bb).
During the further growth of the embryo, the venter of the archegonium also goes
on growing vigorously in comj)anv with it, as may be easily seen from Fig. 419

[3]

73»

LECTURE XLI.

B and C. It will be noticed how the neck h of the archegonium, which has now
turned red, and is decaying, is still situated above. This still growing venter,
investing the embryo as a loose sac, is the Calyptra.

Enclosed within the calyptra the embryo of the ]\ross now grows up into a long
approximately spindle-shaped bodyy^ the lower end of which bores into the tissue
of the Moss-stem, but without coming into true organic connection with it. When
this elongated embryo has attained a certain length, the calyptra tears away near its
base : the lower part remains attached to the stem as the so-called vaginula, while

Fig. ^K^—Fiiuarin hygro
ception of the sporogoniuin //' in the ventral
portion bb o{ the archegonium (optical long, sec,
XSoo)- B, C, further stages of development of the
sporogoniumy'and of the calyptra f ; A neck ol
archegonium X (about 40).

Fig, 420. — Fu}taria hygro>netrica. A
leafy stem g with calyptra c. B a. plant .j-
with sporogonium nearly ripe : j seta ; y
capsule ; c calyptra. C longitudinal median
section of the capsule (urn): (^operculum ; a
annulus; p peristome cc' columella; h air-
sp.ace. 5 primary mother-cells of the spores.
The tissue of the columella becomes loose
and filamentous below.

the whole of the upper part of the calyptra is carried up by the elongating embrj'o,
which it invests closely, and by which it is nourished. The upper portion of the still
stalk-like embryo, within the calyptra, now swells out and becomes thicker: this
forms the spore-capsule of the Moss-fruit, the calyptra still remaining on it.

It would require too much space to describe in detail the development of the
Moss-fruit: all that is necessary may be explained by means of Fig. 420. Here is
shown {c in A ) the calyptra still enveloping the whole of the embryo ; at B the latter
has already become developed into the Moss-fruit or Sporogonium, consisting of
a long thin stalk s (Seta), and the spore-capsule / (Theca) on which the calyptra

EMBRVOLOGV OF MOSS. 739

c is situated. C represents a longitudinal section through the spore-capsule, which
consists of various layers of parenchymatous tissue within a very strong and well
developed epidermis, and forms anteriorly the lid (Operculum) d, already men-
tioned. A central mass of tissue {/) composed of large cells, is separated by
means of large intercellular spaces /i from the peripheral layers of tissue of the
cajisule, and is distinguished as the columella. A layer close to the periphery
of the columella produces the mother-cells, each of which gives rise to four spores
by repeated bipartition. It may here be remarked that the spores of the Vascular
Cryptogams, just like those of the Mosses, also arise from their mother-cells as
tetrads (quarters), and that the pollen-grains of the flowering plants do exactly the
same, since they also are in fact nothing other than spores.

When the spores of the Moss-capsule are ripe, the operculmii ^/ (Fig. 420 C),
which consists of epidermis, becomes loosened from the urn-shaped portion of
the fruit at a, the large-celled parenchyma becoming disorganised, and thus the
cavity containing the spores opens. In the more
highly organised Mosses there remains, after the
falling away of the operculum, a simple or double
crown of beautifully formed organs, the Peristome,
composed of solid portions of the cell-walls of the
tissue which fills up the cavity of the operculum ;
this contributes to the dissemination of the spores,
the ' teeth ' of the peristome rolling up and unrolling
hygroscopically. Fig. 421 shows the structure of
a double peristome in a particularly excellent ex-
ample.

The spores when emptied out of the capsule „.eSofli^^^^irL:;:)^

germinate and give rise to the protonema, from the {:'',", 'Some T .>' int^ai

consideration of which we started above. peristome.

I will describe the reproductive organs and
alternation of generations of the isosporous Vascular Cryptogams so far as is needful
for our purpose, in the case of the Ec|uisetace8e or Horse-tails, taking as an example
the largest and handsomest of our native species, Eqtiisetum Telmakia, with which
moreover the common Equisetmii arvense of the fields, abundant ever \- where in
pastures &c., agrees. All the species of Equisetum live chiefly under ground, where
their shoots, which have the form of rhizomes, grow on in part horizontally, in part down-
wards, and often occupy large areas. On the rhizome are produced buds which are
directed upwards from the first, and in the spring grow upwards, often from consider-
able depths, to expose themselves to the daylight ; these shoots, however, are of two
kinds. Some of them, which do not emerge until later, when the weather is warmer,
grow up to the height of a man and produce in the axils of their membranous leaf-
sheaths whorls of thin lateral shoots, which then again branch in like manner; these
are the foliage shoots of these plants, the tissue containing chlorophyll however being
developed not in the inconspicuous leaf-sheaths, but in the cortex of the secondary
and tertiary shoot-axes. Their products of assimilation pass into the subterranean
rhizomes, and these, under favourable conditions, may probabl}' go on living for
hundreds of }"cars.

3 B ^

740

LECTURE XLI.

The other kind of orthotropic shoot, which comes forth into the da}light, makes
its appearance early in the spring, produces no chlorophyll, and has only the one
object of bringing to maturity the asexual spores produced long previously when
the shoot was still under ground, and to scatter them to the wind and so sow them.
These sporangiophores, which only attain a height of 20-30 cm., terminate above in
an ovoid or cylindrical ' fructification,' if one will so term it ; i. e. the apical portion
of the shoot-axis bears, in closely superposed whorls, numerous hexagonal shields
(metamorphosed leaves), fixed on slender horizontal stalks, and bearing on their

Fig, 423. — Development of spores of Eqitisehtin
liinosnm (x 800). yl unripe spore witli three coats, fresh
and lying in water. B the same after lying for two
or three minutes in water : the outer coat has become
raised A large vacuole is seen near the nucleus. C
early stages in the development of the elaters on the
outer coat ^ (^ = i in Figs. .-/ and B), Z?, E similar stages
in optical section, after twelve hours in glycerine : e the
membrane whence the elaters are formed. 2 and 3 the
separated inner membranes. F the outer membrane split
into spiral elaters, which are coloured blue by chlor-zinc
iodine.

Fig. t,-2s.—Equisetitm Tehnateia. A upper part of a fertile shoot with the lower half of the sporiferous spike
(nat. size) : b leaf-sheath ; a so-called ' annulus ' (bracts ; x the pedicel of sporangial leaves which have been re-
moved ; y transverse section of the axis. B peltate sporangi.al leaves in various positions (slightly magnified) : it
pedicel : s shield ; s^ sporangia.

under-side (the one turned towards the axis) a large number of thin-walled delicate
sacs, the sporangia. These points will be rendered sufQciently intelligible, even
without detailed descriptions, from Fig. 422. The ripened sporangia dehisce, each
by means of a longitudinal fissure, and allow their spores to escape. It is only
necessary to tear off one of these sporangiophores in March or April and knock
the head on a plate, for instance, to obtain the spores in large quantities in the
form of an extremely fine greenish powder. On collecting this on a piece of
paper, for instance, and breathing lightly on it, an agitating writhing movement
is noticed in it : this is still more obvious when the same experiment is made
with a few spores on a glass slip under the microscope, with a low power. It

SPORANGIOPHORES, ETC. OF EQUISETUM.

741

is then observed ihat each individual spore is a sj)hcrical cell provided with a thin
tough membrane and with protoi)lasm containing chlorophyll, and has two mutually
crossing long bands attached to it at one point (;/, Fig. 423 A,B). When
the air is dry these crossed bands are thrown widely apart ; but it suffices while
looking through the microscope to simply breathe lightly, and thus supply the
spores with damp air, to put the bands in motion at once, and they then roll
themselves together round the spore with extraordinary ra])i(lity, as in Fig. 423/'',
energetically opening out again at once when
the small quantity of moisture hygroscopically
absorbed evaporates. These movements of
the ' elaters ' take place so energetically and
rapidly that the spores are put into jumping
movements, which will evidently also occur in
the open as the moisture of the air changes,
and may in some way contribute to bring the
spores into suitable places for germination.

The sporangiophores of the above Equi-
sctum disappear after the sowing of the
spores ; other species, however (e. g. ^. //mo-
si/m, which sometimes completely fills up
large swamps) send up shoots of one kind
only above the soil, which contain chloro-
phyll, and bear the spikes of sporangia at
the apex at the same time.

The whole of the Equisetum- plant so
far described is devoid of sexual organs ; the
only reproductive organs it produces are the
sporangia described. The whole plant is
thus quite asexual, and this holds good not
only of the Horse-tails, but in exactly the
same way of the Ferns and Lycopodiaceos
and Selaginellse, &c. In all these cases the
ordinary, long-lived and sometimes (as in
the case of Tree-ferns and extinct Lepido-
dendrd) huge tree-like plant is asexual ; it
always produces as reproductive organs spo-
rangia only.

If now the spores of the Florse-tail are sown on the surface of water, for
instance, on which they float, an opportunity is easily affortled of observing their
early stages of germination. These are represented in Fig. 424. In / and
// the germinating spore is seen to be in the first place segmented into root
(7V) and shoot (/); the root, a simple utricle, turns geotropically downwards,
while the shoot containing chlorophyll (/) floats on the water and undergoes
numerous successive cell-divisions. Germinating on ordinary water, however, the
plantlets usually develope no further than in Fig. 424 VI \ they attain their complete
development, however, if the spores are sown on loamy soil or on the surface

Fig. 424.— First stages in tile
development of the prothalliuni of
Iiquisettitn TehttaUia : w first root-
hair; t incipient shoot. The de-
velopment is in the order of the
numbers I— VI (X about 200).

742

LECTURE XLI.

of a block of turf saturated with nutritive substances. The result is that in the
course of several weeks tiny plantlets are produced, usually in dense tufts, w-hich
on closer investigation are seen to be of two kinds, namely, smaller male plants as in
Fig. 42^ A, and much larger female plants as in Fig. 426. Consideration of the
figures referred to shows that the plantlets produced from the spores (which moreover
are very difficult to cultivate) present not the slightest similarity to an Equisetum
plant. The latter is in every respect a highly organised plant, its roots accord
entirely with the type of the higher plants, its shoot-axes and leaves are clothed
with a very strongly developed epidermis, and the fundamental tissue of the shoot-
axes contains strands of sclerenchyma, in addition to colourless and green paren-
chyma, and the vascular bundles, although thin and delicate, nevertheless possess
all the essential elements of such. Matters are quite otherwise with the plantlet
produced from the spore, the branched shoot of which possesses assimilating
chlorophyll, it is true, but in other respects presents the simplest cellular structure ;
the roots are simple long utricles.

It is already clear that the germination of the spore of a Horse-tail signifies
something very different from the germination of a seed of the flowering plants ;
even the tiniest seed, such as that of Tobacco or of a Campanula, contains a young
plant, an embryo, which on its first development at once reproduces the characters
of its mother-plant. It would therefore be very inappropriate to name the small
plantlets developed from the spores of the Equisetum simply embryos, since they play
an entirely different part in the developmental history ; they are usually termed
prothallia.

The small male prothallia of Equisetum give rise at their margins to several
antheridia which form only a few antherozoids. They consist of a simple layer of
cells and a core of tissue each cell of which produces one antherozoid. The ap-
pearance of the latter is shown in Fig. 425 -5 and D ; like those of the Ferns the
antherozoids of the Equiseteae have also numerous cilia for the purpose of executing
swimming movements, since as in the case of the Algae and Mosses, and as in all
Vascular Cryptogams, fertilisation by means of antherozoids can only take place with
the aid of water, even though the prothallia grow on soil which is only damp,
where the sexual organs become so far ripe that on the opportune saturation of
the whole tuft with water the antheridia and archegonia open, and the antherozoids
can swim from the former to the latter.

The archegonia of the female prothallia are represented in Fig. 426 at a.
They have in the main the same structure as those of the Moss, only they are
more simply organised and their ventral portion is immersed in the tissue of the
prothallium, from which only the neck protrudes. Here also the oosphere arises in
the venter of the archegonium as a rounded mass of protoplasm lying free in the
central cavity. Here also an open canal leads to the exterior from which the
deliquescent canal-cells are expelled at the moment of opening. I am not
aware that any one has as yet observed the entrance of an antherozoid into the
oosphere itself, in Equisetum; but that this occurs there can be no doubt, since
in the case of the Ferns and their allies various good observers have repeatedly
succeeded in directly observing the act of fertilisation.

After fertilisation the canal, as in all archegonia, becomes closed. The oosphere.

EMBRYOLOGY OF EQUISETUM.

743

now invested with a new cellulose wall, and the embryo produced from it are thus
completely enclosed in the tissue of the parent-plant. While the embryo itself
slowly grows up, with accompanying cell-divisions, the tissue investing it also in-
creases in volume, and cell-divisions result in it. The manner in which, not only
in the case of the Equisetaceae but also in the Ferns, the growing oospore (or

Fig. 425.-^4 male prothallus of Equisctunt
arvense, with the first antheridia a (after Hof-
meister, X 200). B—F- antlierozoids of Equisctum
Teimateia (after Schacht).

Fig. 426 \'ertical section of the lobes of a

vigorous female prothallus of Hquisetum at-vensc
(after Hofmeister). At aaa two barren and one
fertilised archegonium; h root-hairs (X about 60).

FIG. 4=7.-Developn,ent of the embryo of H.,uisctum arvense (after Hofn.e.ster). -■' v>:rt.cal section of
archegoniim a with e.l.bryoyiX =00). B a free en.bryo further developed : Ö UK.p.ent «- --« i^-^^-J_
the first shoot (X =00). C vertical section of a lobe of a prothallus// w.th a young i-/«»./;»«. «.its first root,
b b its leaf-sheaths (X 10).

what is the same thing, the young embryo) first breaks up into so-called octants
by means of three walls standing at right angles to one another, which then
become further divided up by means of anticlinal and periclinal cell-walls, and
how in this way there arises at last a tetrahedral ai^cal cell for the first root
and a similar one for the young shoots, and how the first inception of the leal

744

LECTURE XLI.

takes place, have already been indicated at another opportunity (p. 446, Fig. 283)
and it may be at once pointed out that the development of the embryo of the
Vascular Cryptogams generally may be referred to that scheme, although occasional
deviations occur. However, it lies quite beyond our present purpose to enter more
closely into these embryological details. What especially interests us is sufficiendy
explained by Fig. 427. At ^ is seen the neck of the archegonium a, and at/ the
young embryo ; at ^ is an embryo somewhat further developed, of which however
only the shoot-portion can be seen, the growing-point of which is at s and its
first still very young leaf-sheath at b, in the form of an annular wall. In C
the young Equisetum is already growing up ; its shoot K already supports two
leaf-sheaths hb' and the first root w is growing downwards. The base of the plantlet
(and this is here the true embryo) is still inserted into the tissue of the prothallium //>,
by which it is still chiefly nourished. When the whole Equisetum itself becomes
stronger the prothallium disappears; the embryo remains somewhat feeble during

Fig. ^■2Z.—.4dia)itum CapiUus-Veneris. Ve
section through the prothalUis p p and the young Fe
li root-hairs ; a archegonia ; b first leaf, lu first ro
embryo (x about lo).

FIG. i,ir).~.ldHtnti(m CapiUm
first leaf; iV primary i

Prothallium seen from below // with the young I
icondary root ; h root-hairs of prothallus (x about i

the first year, however, though it produces from its lower leaf-sheaths a few lateral
shoots, which penetrate downwards into the soil to continue the growth. next year,
since the plantlet arising directly from the embryo itself perishes in the autumn.

In the Ferns and Lycopods all the essential points of reproduction repeat
themselves as in the case of Equisetum, so that a detailed description would be
superfluous. To present a few points of resemblance only, I may briefly mention
that in our better known native Ferns, and in very many others, the sporangia
arise on the lower side of ordinary large foliage leaves^ or on metamorphosed
portions of them (as in the Royal Fern, Osmiinda 7-egalis) mostly in enormous
numbers, and very small in size; they are stalked capsules, which appear to the
unaided eye as minute granules. On sowing the very resistent fern-spores from
the spontaneously burst sporangia, they germinate after some time and produce
in the normal course of events a prothallium, which possesses approximately the
form of a cordate leaf, 0-5 — 1-5 cm. long and broad, on the lower side of which
are developed antheridia, and archegonia, from the fertilised oospheres of which the
new Fern-plants arise. The most essential points here coming into consideration
are illustrated in Figs. 428 and 429.

LECTURE XLII.

THE ORGANS OF REPRODUCTIOxN {Conliiuud)\
IIETEROSPOROUS VASCULAR CRYPTOGAMS; GVMNOSPERMS; ANGIOSPERMS.

There existed during previous geological epochs Horsetails with two kinds of
spores ; but the species are extinct. Nevertheless we have still two small families of
Fern-like plants, mutually very distinct, which in spite of their great differences
are usually grouped together under the absurd name Rhizocarpe» : these are the
Salvinese and the Marsilieae in which are formed two kinds of spores entirely
different in nature, and the same peculiarity is again met with in the case of the third
great subdivision of the Vascular Cryptogams, namely the Lycopodiacese {Dicho-
tomecp). Here also there are two very different families, the Selaginellas and the
Isöeteai, in which two kinds of spores are produced. I cannot here suppress the
remark that it harmonises little with Darwin's views when we see repeated in three
very different classes of the vegetable kingdom, with otherwise similar spores, a
phenomenon so important as is the production of two kinds of spores with their
consequences. Certainly it cannot be explained by natural selection in the struggle
for existence. This however simply by the way.

The consideration of a few cases where the development of two kinds of spores
takes place is essential to the reader, since a satisfactory insight into the reproduction
of the Coniferse (Gymnosperms) and in fact of the true flowering plants can only be
attained by this means. One of the most magnificent results in the province of Embryo-
logy was Hofmeister's demonstration in 1851 of the fact that the formation of seed in

' As in the first eleven lectures, which are to be regarded simply as an organographical
introduction to the physiology proper, so also in the two preceding lectures my wish has been only
to give a very condensed sketch of the organography of the reproductive apparatus. From the
astounding variety of materials to hand only a few examples could be taken into account ; those
who desire more detailed information concerning the organisation of the reproductive system, and
(what can scarcely be avoided here) as to their phylogenetic or systematic connection in the various
subdivisions of the vegetable kingdom, will find in my 'Text-book of Botany' a rajnd but very
thorough survey in this connection. Still further details, especially with respect to the most recent
observations on the Algte and Fungi, and the remarkable developmental relations of the sporangia
of the Cryptogams to those of the Phanerogams, are to be found in Goebel's new edition of my text-
book, which has just appeared under the title ' Gritndziigc der Systciitatik! There is an English
edition published by the Clarendon Press, Oxford.

746

LECTURE XLII.

the Phanerogams is essentially nothing different from the processes in the
germination of the large spores of those Cryptogams which possess two kinds of
spores. Hitherto it really appeared as if a deep and impassable gap existed
between the reproductive processes of the Cryptogams and Phanerogams ; but
Hofmeister showed thirty-one years ago that this gap does not really exist, that it is
filled up by those forms of Cryptogams which develope two kinds of spores ; or in
other words, certain forms of Phanerogams, particularly the Gymnosperms (Cycadeaä,
Coniferae) approximate in their seed-formation so closely to certain heterosporous

Cryptogams, that we might now include these
plants equally justly with the Cryptogams as
with the Phanerogams. This discovery has cast
an entirely new light on the interdependence
of the whole vegetable kingdom.

In the case of Marsilia, already shown in
Fig. 392 (p. 699), there arise from the petioles
of the foliage leaves stalked sporocarps approxi-
mately of the shape of a Bean. In the interior
of these a large number of delicate-walled spo-
rangia are formed, which at first are in so far
alike that in each of them a large number of
mother-cells are developed, which in their turn
divide each into four daughter -cells, as occurs
generally in the spore-formation of all Muscinese
and Vascular Cryptogams, and in the develop-
ment of the pollen of Phanerogams. From this
point onwards, however, a difference occurs. In
a number of the sporangia all the spores pro-
duced by division into four attain to complete
development; but they remain small and are
therefore called Microspores. In the other spo-
rangia of the same fruits, on the contrary, only
a single one of all the spore-cells already pro-
duced attains completion ; this one, however,
attains such vigour that it fills up the cavity of
the sporangium — it is the Macrospore.

Lying in water the spore-fruit of the l\lar-
silia bursts, and by means of a very remarkable
mechanism the macro- and micro-sporangia
become expelled from the sporocarp (Fig. 430), whereupon the further development
of both kinds of spores begins at once.

The contents of the microspores break up by successive bi-partitions into
sixteen or thirty-two small round cells, in each of which arises an antherozoid which
possesses the form of a cork-screw with many turns. As soon as the antherozoids
are completely developed, the external hard shell of the microspore opens, and an
inner thin membrane swells up as a vesicle, and eventually contains the antherozoids,
which then by deliquescence of the vesicle emerge free into the water, in which

Pig. A^o.—Miii-suiasnlva/yix. A a sporo-
carp (nat. size) ; st upper part of its stalk B
a sporocarp burst in water and extruding the
gelatinous ring. C tile gelatinous ring is
ruptured and extended : sr chambers con-
taining sori ; scli coats of sporocarp. D one
of the chambers containing a sorus. from an
immature sporocarp. F. a similar chamber
from a ripe sporocarp : mi microsporangia ;
7na macrosporangia.

REPRODUCTION OF MARSILIA.

747

they move forwards, iheir spirals rotating : as in many other cases a delicate vesicle
clinging to their hinder end is carried with them (Fig. 431).

In the narrower end of the very large macrosi)ore, which is filled with starch-
grains and invested with a very thick firm membrane, the first rudiment of the
prothallus arises under the apex by the collection and rapid division of a mass of
protoplasm. Thus the prothallium is incepted
in the interior of the spore : it is separated off
from the remaining (far larger) ca\ity of the spore,
by means of a transverse wall, the diaphragm.
Then the apex of the spore opens, and as the
diaphragm protrudes beyond the opening as a
vesicle, the young prothallium is forced out of the
spore-cavity, and thus comes into the lower part
of the so-called ' funnel,' which is formed by a trans-
lucent soft gelatinous mass situated at the anterior
end of the macrospore ; a funnel-shaped canal in
this gelatinous mass leads down to the prothallium.
The latter is, however, very small in comparison
with the very large macrospore, and from its whole
cell-structure it may be looked upon as practically
a single archegonium.

On allowing the sporocarp of a Marsilia to
open in a glass of water, the stages of development
of the macro- and micro-spores described may be ob-
tained after from ten to fifteen hours, the temperature
being suitable. The water swarms with thousands
of the rapidly moving antherozoids, hundreds of
which now crowd into the funnel-opening of the
macrospore, while others bore directly through the
soft jelly ; some of them reach the naked oosphere
lying in the venter of the archegonium, from which
in the course of two days an embryo like that in
Fig. 432 is then developed, which already possesses
the first leaf b, the apical-cell of the shoot-axis s,
the primary root w, and the so-called footyC By
means of this foot / the embryo is attached to
the protruding vesicle of the macrospore c, and is
nourished by the latter. The whole structure at
this stage is strikingly suggestive of the develop-
ment of a Fish still bearing the yolk-sac at its ventral surface. As in the flosses
and Ferns, so also here the archegonium goes on growing for some time longer,
and appears in the figure as an envelope consisting of two cells and investing the
embryo, the neck portion of the archegonium {a) being still quite evident. From
the lower ])art of the archegonium or prothallus {p t) numerous long root-hairs
grow out iwh) by means of which the whole structure is anchored firmly at the
bottom of the water ; since it is one of the first objects of ever}- embryo-plant to

FI(

-. 4:,. - ' "

J < ; I I >

Iv. Upper

fiffuie,

Microbpore

sp »ith ij<.ljtinous en-

velope

si, and ap

icil p-ipilla

protrudinc

into th,

; funnel. In

the papilla is a flattened

yellow drop ; S£r ri

aptured wall

of macro-

sporangium (X about 30). Lower figure,
ruptured microspore after the escape of
the antherozoids : ex epispore ; dl extruded
endospore, containing granules ; r z spiral
antherozoids; yy their vesicles, contain-
ing starchy granules. The gelatinous en-
velope of the microspore has disappeared.
The protuberances on the exospore arc
wrongly represented (X 550).

748

LECTURE XL 11.

obtain a fixed position in order that it may maintain a definite direction with
respect to the external world. Finally, it should be remarked that in Fig. 432
also the gelatinous mass referred to {s I) is still to be seen, enveloping the whole
germinating apparatus.

^^'hen at length the whole of the store of nutritive material in the macrospore is
exhausted, and the young plant has produced several roots and leaves, it becomes

free from the prothallus and other parts con-
cerned in its origin, and developes into a
perennial plant.

We have here a first step in the transition
from the germination of a spore to the develop-
ment of a seed. The two parts prothallium and
embryo represent, as we see, the two alternate
generations which we found completely sepa-
rated in Equisetum and the ordinary Ferns, each
as an independent living plant. Obviously, we
might also suppose the prothallium to be de-
veloped in the interior of the macrospore and not
protruding, and that it, together with the embryo
developed in it, remained enclosed in the thick
membrane of the macrospore ; in this case the
germinated macrospore would constitute the
essential part of the seed of a Phanerogam.

The behaviour on germination of those
Lycopodiacege which possess two kinds of spores,
however, actually accords with the idea just put
forward. Fig. 433 shows at A the macrospore of
the genus Isoetcs and at B the prothallus with its
archegonium <?, developed entirely in its interior.
These points come out still more clearly
in the germination of the spores of the genus
SelagincUa. The sporangia here arise in the
axils of the leaves at the ends of the foliage
shoots. Fig. 434 A shows the fertile shoot of
a Sclaginella looked at from the outside ; B is
the same in longitudinal section. The sporangia
situated in the leaf-axils are obvious at once,
and it is observed that those on the right side
contain only four (only three are visible) large
spores, whereas the sporangia on the left side
If the macrospores and microspores are sown
separately on a suitable damp substratum, both develope, it is true, but no subse-
quent fertilisation occurs : the unfertilised macrospores give rise to prothallia with
archegonia, it is true, but no embryo arises in these. This fact, which can be
confirmed in the case of the most different Cryptogams, is in so far of great value as
it is not always feasible to observe directly the entrance of antherozoids into the

Fig. 432. — Marsilia salvcitrix. Longi-
tudinal section of macrospore, prothallium,
and embryo (X about 60) : am starch-grains
in spore ; i inner membrane, ruptured above ;
ex epispore, with prismatic structure ; c
cavity beneath the protruded diaphragm on
which the basal layer of the prothallium is
situated ; pt prothallium ; ivh its root-hairs ;
a archegonium ; y foot of embryo ; w its root ;
s its stem-apex ; * its first leaf, which distends
the prothallus ; si gelatinous envelope of the
spore ; at first it forms the funnel above the
papilla, and it still envelopes the prothallium
(50 hours after sowing).

contam numerous microspores.

EMBRYOLOGV OF SELAGINELLAE, ETC.

749

oosphere ; the suppression of the fertilisation and development of the embryo when
the access of antherozoids is prevented, however, is a certain proof of the necessity
of the union for the formation of an embryo.

The processes of reproduction which alone interest us here can be rajjidly appre-
hended from Fig. 435. /represents a widely open macrospore ; from the aperture
through the very thick external spore-membrane, the prothallium da projects, bearing
at a an unfertilised archegonium ; at // is seen a young unfertilised still closed
archegonium, in which the lower shaded cell represents the as yet unripe oosphere,
and the conical portion lying above it is the canal cell, which, when the neck of the

Fig. 433.— /wfCt'J' lacuiliii
(after Hofmeister). .-/ macrospore
a fortnight after sowing, rendered
transparent by glycerine (X 60).
B longitudinal section of the pro-
thallus a month after sowing the
spores ; a archegonium (X 40).

Fig. ^^i.—SeliJ^^iiicUa inaequali/olia.
A fertile branch (X 2). B longitudinal
section of apex of A^ bearing microsporangia
to the left and macrosporangia to the right.

archegonium opens, is extruded as mucilage. /// is the archegonium with a
fertilised oosphere already divided by a horizontal wall. While the archegonia are
preparing for fertilisation, preparations for the development of the antherozoids are also
taking place in the microspores A-D ; after a small cell {v in D) which does not
take part in the formation of the antherozoids, has been as it were cut off, further
cell-divisions occur in the remaining cells which constitute together, so to speak, a
reduced antheridium. This is well seen in I) : each of these small cells gives rise to
an antherozoid of very simi)le form, just as in the Mosses.

Reluming to the germinating macrospore again, it is to be noticed that the

750

LECTURE XLII.

true prothallus is separated off from the large spore-cavity by the diaphragm d d: in
this cavity there arises a large-celled tissue, such as we shall meet with later in the
embryo-sac of flowering plants — the so-called Endosperm. Figure /, then, shows at
/ a young embryo, developed from a fertilised oosphere, and at e one already further
developed, the shoot portion of which (only roughly indicated) has bored into the
endosperm tissue, but which subsequently grows out from it again.

In order to proceed hence to the formation of the seed in the Coniferae, we need
only assume that the macrospore does not open at all, the prothallium and endosperm

Fig. 435.— Germination of Setaginella (after Pfeffer). /— /// .? Marlensu, 1 — D S <,!:,Us tiis
I lonffitudinal section of a macrospore filled with prothallus and endosperm (<r aiapnragm) m wmcn
two embryos e e' are developing. // a young archegonium, not yet opened. /Jl an archegonium
containing a fertilised oospore, which has undergone one division. A a microspore, showing the
divisions of the contents. B C different views of these divisions. D mother-cells of antherozoids, in
the mature antheridium.

arising in its interior, and one or two embryos becoming developed. Of course, in
the case of the Conifer® and all Gymnospermous plants, what actually occurs is not
only what has just been mentioned, but moreover the macrospore itself also in which
these processes occur remains lying in the very massively developed sporangium, and
no opening of any kind exists through which antherozoids for instance could penetrate
to the oosphere. Here, in fact, we reach a turning-point in the history of fertilisation ;
in that not only in the case of the Gymnospermous plants (Coniferse, Cycadese), but
even in a still higher degree in the true flowering plants, the oospheres remain com-
pletely enclosed in masses of tissue, and thus the possibility of their fertilisation by

EMBRVOLOGY OF GYMNOSPERMS. 75 1

means of motile antlierozoids ceases. The fertilising material — and this is the
only essential in the matter — is carried to the oosphere in quite another manner,
namely, by the male microspores (which are distinguished in the Phanerogams
as pollen-grains) fixing themselves to a portion of the female organ of reproduction,
and thence emitting a tubular prolongation, which makes its way through the tissue-
masses surrounding the oosphere, and finally penetrates to the oosphere, to which
it subsequently transmits the male fertilising substance. This tube is called the
Pollen-tube.

It is unfortunately necessary to add here a few definitions of terminology, since the
history of our science has brought it about that organs of like kind in diff"erent classes
of plants, in spite of their homology, were previously held to be essentially diff'erent
and have therefore had different names assigned to them. At the present time, when
we perceive clearly the true connection of these matters it w'ould be possible, and is
demanded in the interest of science, to designate the pollen-grams simply microspores,
and the embryo-sac simply the macrospore, in order to indicate their homologies; never-
theless the old nomenclature can now scarcely be entirely put aside, since it is too
deeply fixed in the literature, so that to the uninitiated doubts might arise as to the
meanings of terms in reading different books.

As already mentioned, the gymnospermous plants (Coniferge, Cycadeae, and
Gnetacege) are directly allied in their reproductive processes to the higher Cryptogams
with two kinds of spores : nevertheless this was by no means an easy matter to
establish. The investigations of many years were needed to arrive at this conclusion.
If I quote a few examples, it will be at once seen that the essential and important
points are not directly obvious.

The facts of fertilisation come out particularly clearly in the case of the Yew
{Taxus baccala). Each Yew-tree is either entirely male or entirely female, and
thus it is only trees of the latter kind which bring forth the seeds, which are found
in autumn surrounded by an elegant red thick envelope. The male and female
trees flower in the spring. The male flowers are situated on the under side of the
horizontal lateral shoots of the tree : they are small buds {A Fig. 436) bearing
numerous minute scale-leaves at their lower parts, and at the apex eight to ten
peculiarly formed structures a which remind one forcibly of the sporangiophores
of Eqiiisetum. A stalked disc bears on its under surface four or five pollen-sacs,
which may be forthwith designated sporangia ; when these open, they set free their
microspores, which however are generally known as pollen.

On the female Yew trees also minute bud-like shoots are found in the spring
on the lower side of horizontal branches (C, Fig. 436). Here also numerous
scale-leaves j are present ; anteriorly, however, is a peculiar projecting structure j k.
This is the Ovule, so termed because the seed containing the embryo is developed
from it after fertilisation. Fundamentally, however, this ovule is simply a \c\\
highly developed macrosporangium, as will be more clearly seen later on. It
will also be well to regard more closely the longitudinal section through the whole
female shoot D. Here is seen the ovule at the apex of the shoot, and it is observed
that it consists of two different structures. The approximately hemispherical body
K K '\^ the so-called Nucellus of the ovule, and it constitutes the essential i)art :
this is the proper viacmsporaugiuni, in whicli the so-called embryo-sac or macrospore

LECTURE XLIT.

arises subsequently. The portion i surrounds the nucellus of the ovule as a closely
investing envelope formed of several layers of tissue, and narrows anteriorly into
a canal. This investment is called the Inlegtmieut, and the canal in it just referred
to, which thus leads from the exterior down to the nucellus of the ovule, is the
Microfivh'.

It will be of further service for preliminary guidance if Fig. 436 E is com-
pared with the preceding. The ovule is here represented in a more advanced stage :
I is again the integument, now with closed micropyle, much larger, and closely

investing the nucellus of the ovule k k
on all sides. Within the latter we now
find the part e : this is the embryo-sac
filled with endosperm — or, as we may
also say, the macrospore filled with
the prothallium. In this structure the
young plant, the embryo, arises in con-
sequence of fertilisation. To note it
by the way, the portion marked /;/, in
the form of an annular cushion sur-
rounding the base of the ovule, is that
which in the autumn surrounds the
whole ovule (or, better, what is then
the seed) in the form of a red succu-
lent investment.

In this description, I have for
the time being made no reference to
fertiUsation itself, but have only de-
scribed the complex of organs which
co-opera'.e in that process : in like
manner it will, I think, be advantage-
ous if we look at the result of the
fertilisation, before describing the pro-
cesses themselves. This result is the
development of the ovule into the
seed, capable of germinating, the parts
of which must be understood before
a proper insight can be obtained into its origin and significance. This can be
accomplished with the aid of Fig. 437, which represents the germination of the seed
of the Stone Pine, with which the other Coniferae essentially agree. / is the seed
in longitudinal section, consisting of three parts ; i- the hard thick testa developed
from the integument of the ovule after fertilisation ; c is the so-called endosperm, or
further developed prothallium — the black line separating the testa and endosperm is
to be supposed to represent the membrane of the macrospore, or, what is the same
Üiing, the embryo-sac. Finally, we have lying in the middle of the endosperm the
young embryo : zv is its incipient root, c a whorl of leaves which, owing to an extremely
unhappy idea of the older botanists, it is still the custom to term cotyledons. This
again is an entirely meaningless word, which however unfortunately can now scarcely

Fig. 436 — Tnxits bacrata. A male flower
(enlarged) : a the pol. en-sacs. B an anther
with the pollen-sacs opened, seen from below.
C portion of a leaf-.shoot, with a foliage-leaf
1^, from the axil of which springs the fen-.ale
flower ; j its scales ; sk its terminal ovule. D
longitudinal section of the 'ame (magnified) :
/integument; kk nucelius of the ovule; a- a
rudimentary axillary ovule. D longitudinal
section through an older ovule, before fertili-
sation : i integument ; kk nucellus ; e endo-
sperm; m arillus ; s enveloping leaves.

DIFFERENCES BETWEEN SPORES AND SEEDS,

753

be expunged from scientific language ; it may be translated seed-leaves. It is
simply bad taste however to speak of them as seed-lobes.

The rest of the drawings in Fig. 437, concerning which the explanation of the
figure gives further information, show how the germ-plant (the embryo) contained
in the seed, then developes further ;
how first the root cv and then the
primary shoot-axis x elongates and
emerges from the seed: the radicle
enters the soil, the cotyledons c still
remaining enclosed in the endosperm,
to absorb the nutritive materials there
stored up. It 'is not until this has
been accomplished, and the ex-
hausted endosperm is reduced to a
mere membrane that the plumule
elongates upwards, and the coty-
ledons are withdrawn from the coats
of the seed.

After these preliminary explana-
tions, it is scarcely necessary to point
out more particularly how great is
the difference between reproduction
by means of ordinary spores on the
one hand and by means of seeds
on the other. In the former case
individual cells — i. e. the spores
— become separated off from the
mother -plant, and the new plant-
life begins, so to speak, entirely
anew. Here in the case of the
seed-plants the seeds also separate
from the mother-plant, of course,
to carry on a new plant - life,
but the young plant is already
there, and, in most cases, already
consists of the first rudimentary
organs, a primary shoot and radicle,
and these organs are composed of
innumerable minute cells in which
the tissue-systems are already dif-
ferentiated. The young plant which

lies in the seed is already formed while the seed itself is still part of the parent
plant, and is so far nourishcil by this that subsequently, on the germination of
the seed, litde more than a mere enlargement of the embryo is necessary. Seed-
plants may therefore in this sense be compared with viviparous animals. A
comparison of the Cryi)togams or spore-plants with oviparous animals, however,

[3] ^ 3C

Fig.

I through the

tits Ptjtea, I median longitudinal section
seed, >■ being the micropj'le end. // commencement of germination, the
root emerging, /// conclusion of germination, after the endosperm has
been exhausted (the seed was not deep enough in the soil, and was there-
fore carried up by the cotyledons as the stem elongated). A shows the
ruptured testa at s. B shows the endosperm e after the removal of one half
of the testa. C longitudinal section of endosperm and embryo ; and D
transverse section of the same, at the beginning of germination ; c coty-
ledons ; tu primary root ; .vthe embryo-sac pushed aside by the root (it is
ruptured at ^.v) ; he hypocolyl ; iv' secondary roots; r red membrane
lining the hard testa.

754

LECTURE XLII.

would be but a very lame one, since spores are not eggs at all; it is only in
the lowest regions of the vegetable kingdom, especially in the Fucacese and a
few other forms, that oospheres are separated from the mother-plant.

From these preliminaries we may now proceed to the consideration of the
processes of fertilisation itself. Here however there are concerned so many organs
boxed one within the other, that it is scarcely possible to make clear all the parts in
their mutual relations in any one figure taken directly from Nature. In the interest

of the reader, therefore, I prefer to
illustrate the essential points by
means of a diagram.

Fig. 438 is a diagrammatic lon-
gitudinal section through the whole
ovule of a gymnospermous plant:
a is the integument, and k the mi-
cropyle, which in the case of the
Gymnosperms is relatively wide open
at the time when the pollen-grains
or microspores commence their
business of fertilisation. At this
time also a drop of fluid is
frequently excreted, and exudes
from the micropyle ; the pollen-
grains carried here by the wind
stick to this, and when the fluid
contracts, the pollen-grains also are
drawn in with it and come to lie on
the projection of the nucellus of the
ovule i b. Such a pollen - grain
is seen at h. This process, namely
the conveyance of the pollen-grains
to the female organ in the case of the
Phanerogams, is termed polliriafioti,
and it occurs in many Gymno-
sperms at so early a time that the
true reproductive organs within the
ovule have not as yet even begun
to develope, and in the case of the Pines a full year may pass before the
fertilising tube of the pollen already contained in the closed micropyle reaches the
archegonia.

The embryo-sac or macrospore arises in the Gymnosperms deep in the interior
of the nucellus of the ovule, and grows slowly up to a considerable size; in the case
of the very large-seeded Cycadeae it may attain a length of 2 cm. or more and a breadth of
1-5 cm. or more. It becomes filled with a succulent cell-tissue, the endosperm </ äf, in
which (mostly only after a long time, and sometimes not before the seed appears to be
already nearly ripe) the archegonia e e are formed : these archegonia are of very
simple structure, though at the same time relatively large : in the case of the large-

FIG. 438.— Diagram of the I

FERTILISATION IN GFMNOSPERMS. 755

seeded Cycadeae the central cell is often 3-5 mm. long and 2-3 mm. Inroad.
The neck of the archegonium differs much in structure in the different forms of
Gymnosperms, but there can be no doubt whatever that these organs are in every
respect to be looked upon as archegonia resembling those of the higher Cryptogams
and Mosses.

When the prothallium (endosi)erm) with its archegonia is sufficiently developed,
usually only after some months, fertilisation can follow: the pollen-tube t bores
through the tissue of the nucellus of the ovule i, and applies itself with its anterior
end either as a thin tube, or more frequently as a broad sac, to that part of the
embryo-sac where the archegonia are situated : protuberances of the pollen-tube then
bore their way into the necks of the archegonia and penetrate as far as the central-
cells. It is not yet known in what way the male fertilising material now passes
over into the central cell of the archegonium, filled with the oosphere. It is not
certainly established whether an extremely fine actual opening in the membrane
of the pollen-tube facilitates the direct entrance of the fertilising protoplasm,
or whether the membrane remains closed and the fertilising substance diffuses
over as a true solution.

It is for our purpose practically the same thing whether the whole of the proto-
plasm contained in the large central-cell of the archegonium is to be regarded as
the oosphere, or whether only a certain part of it answers to this designation. It
suffices for us to know that in the lower portion of the central-cell e, cell-divisions
now take place by means of which two or three tiers are produced, each of four
cells placed cross-wise and close together, as iny". This is however, strictly speaking,
not as yet the inception of the embryo. On the contrary, these cells, at first very short
and discoid, grow out to long tubes {/') which bore into the endosperm-tissue
and become curved and twisted in the process. The cellsy situated at the ends of
the tubes, hitherto but litde grown, and w-hich previously occupied the basal region
in the archegonium — i. e. the one turned away from the neck — and which are
pushed out from the archegonium by the formation of the tubes, subsequently
give origin to the embryo g ; this then goes on growing, with continual cell-
divisions, and finally displaces the rest of the parts and occupies the middle
of the endosperm. Among the many peculiarities in the seed-formation of the
Gymnosperms, it is found that in some cases fertilisation takes place, and the
seeds even become fully ripened and fall, without the embryo having developed
further. I observed this remarkable fact in the autumn of 1868 in the case
of the plum-like seeds of the Japanese plant Gwgko biloba. On opening the ripe
fallen seeds in October I found apparently no trace of an embryo in them, and
therefore regarded them as unfertilised. On again examining the seeds after they
had been laid aside for two or three months, there was found in each of them a large
well-developed embryo, and the seeds all proved to be capable of germination.
Strasburger has investigated this fact in his great work on the Gymnosperms,
and we now know that the Cycadege also behave similarly.

The ripened seeds of the Gymnosperms, especially those of the Cycads and
of the Gingko biloba just referred to, often bear very little resemblance to what is
elsewhere termed a seed in the Phanerogams, those of the last-named plants being in
the ripe state like small yellow plums or large cherries ; for ihe integument of

3 c 2

n^

LECTURE XLII.

the ovule has become transformed into a thick pulpy mass which encloses a ' stone '
like that of a plum, in which the endosperm is enclosed ; and matters are similar
in the Cycadese. In Cycas revohda the seeds attain the volume of a medium-sized apple.
In most of the Coniferse, however, the integument becomes transformed into a hard
seed-coat, and the seeds resemble those of other Phanerogams in other respects also.
A remark is still to be added regarding the pollen-grains of the Gymnosperms.
That these, from their developmental history and function and also from their
origin in receptacles which are obviously sporangia, are to be regarded as micro-
spores, has already been mentioned. A further similarity with microspores however
exists in the formation, in the interior of each pollen-grain, of a small cellular
body which calls to mind the sterile cells in the interior of the microspores of
Isbetes and Selaginelta. Fig. 439 shows the structure in question at j^, though it
is not equally well developed in all Gymnosperms.
Besides this structure, there still remains a large
mass in the pollen - grain or microspore, which
alone takes part in the development of the pollen-
tube, as may be seen in Fig. 439 B. This
portion of the pollen-grain, which contains the
fertilising substance, corresponds to the antheri-
dium of the microspore (as in the case of the
Rhizocarp Salvüiia), while the small cellular body j'
may be looked upon as the last remnant of a
reduced prothallium.

With the Monocotyledons and Dicotyledons —
among which are found the most highly organised
plants in the vegetable kingdom, and which, apart
from the Coniferae, practically constitute what non-
botanical people are in the habit of thinking of
under the term ' plants,' — the Gymnosperms last con-
sidered agree as regards their reproduction in that
they give rise to ovules which are rendered fertile
by means of a pollen-tube, and then develope into a seed containing an embryo.
Hence all these plants may be contrasted, as Spermaphytes, with the Cryptogams
or Sporophytes.

Nevertheless the fertilising apparatus of most Monocotyledons and Dicotyledons
appears to be strikingly different externally from that of the Gymnosperms. What
is commonly termed a flower is simply the fertilising apparatus of these plants
(which may therefore be contrasted as Flowering-plants in the narrower sense of
the word with the Gymnosperms), but here again it is of course to be borne in
mind that all such distinctions are only admissible when the typical forms in both
cases are contrasted with one another. It needs only a somewhat broader con-
ception of the idea Flower to justify the application of the term even to a Fir-cone
or even to the very unobvious fertilising apparatus of the Yew. Of course the
distinction between the flower of a Lily or of a Rose, and that of a Fir or Pine
or other Gymnosperm is very great, but between the two, even externally considered,
there are numerous stages of flower-development which show us that the difference

FIG. 439.—^ a pollen-grain (microspore) of
Ceratozamia loiigi/olia, a Cycad. B emergence
of the pollen-tube ps from the ruptured outer
membrane (exline) of the pollen-grain. At j/ are
seen the sterile cells (Juranyi).

FLOWERING-PLANTS. 757

is not one of principle but only of degree. That which conspicuously distinguishes
particularly the large and elegant flowers of true flowering-plants from those of the
Gymnosperms is the floral envelope, which is very often a double one and is then
termed Calyx and Corolla. Even the most magnificent flowers when stripped of these
envelopes so that only the essential organs of reproduction remain, show nothing
more of the immense contrast indicated above, and we have here to do particularly
with these proper reproductive organs only, although I shall show in the next lecture
that the floral envelopes are by no means superfluous for starting the process of
fertilisation (i. e. pollination), and in many cases are in fact indispensable.

In considering the fertilising apparatus of a flower, as I now have it in mind,
it is above all evident that both kinds of reproductive organs, the male and female,
are very generally united in one flower, and therefore stand next one another close
to its growing-point ; or, in other words, most flowers are hermaphrodite, whereas the
floral structures of all Gymnosperms always contain only male or only female organs,
both kinds of flowers being distributed on the same tree or on diff"erent trees of
the same species. These two cases of diclinous flower-development are it is true by
no means rare even in flowering-plants, the Monocotyledons and Dicotyledons, plants
of the Cucurbitacese for example having male and female flowers on the same
foliage-shoot, whereas the Hemp or Hop bears always only male or only female
flowers on a plant. The prevailing rule however is that the flowers are hermaphro-
dite— a fact which is of importance physiologically, because a long series, of the
most remarkable mechanisms for pollination are entailed by it, to which I
shall have to refer laler. Also the two kinds of fertilising organs, as such, are very
different from those of the Gymnosperms. Whereas the male spores or pollen-
grains of the Gymnosperms arise in receptacles which obviously agree with the
sporangia of Cryptogams, and particularly in that they are developed on leaves
which are often only slightly diff"erent from ordinary foliage leaves ; we find on the
contrary that the male fertilising organs of the true flowering-plants, the stamens,
usually have forms which do not easily disclose their true morphological nature.
Very generally a stamen consists of a stalk-like support (the filament) which bears
above the so-called Anther; this again consists chiefly of four sacs joined in two
pairs, or occasionally of only two sacs, in which the pollen-grains or microspores
arise. These sacs are to be regarded as the same as the sporangia, especially
the microsporangia of the Cryptogams and Gymnosperms; but it is shown only after
further examinaion and consideration that the filament and the portion (connective)
connecting the pollen-sacs of an anther are together to be regarded as a metamor-
phosed leaf.

Still more important than these peculiarities of the male fertilising organs, is
the structure of the female organ. Here we meet with a profound diff"erence between
the true flowering-plants and the Gymnosperms. In these latter the ovules arise at
the margins or on the surfaces of foliar organs, and this so that they are cither
freely exposed, as in Cycas revoluta and Gingko biloba, or else so that ihey
are simply concealed between the closely packed leaves : at least the micropyle of
all Gymnosperms is in open communication with the atmosphere at the time of
pollination. Nevertheless in most Coniferce the pollinated ovules usually become so
completely enclosed in enveloping leaves subsequently, that all ommunication with the

758

LECTURE XLII.

exterior ceases, as in the case of Fir- and Pine-cones, or so that even capsular
fruit-like bodies arise, as in Thuja and other Cupressmece.

The case is quite otherwise with the flowering-plants. Here the ovules arise
from the beginning in the cavity of a special receptacle, which (with few exceptions)
completely excludes them from the atmosphere — a receptacle in which they will even
have to be sought out by the pollen-tubes subsequently. Accordingly the pollen-
grains cannot, as in the Gymnosperms, reach
the micropyle of the ovule directly, but are
conveyed on to a special part of the receptacle,
and thence send out their tubes to the ovules.
This receptacle bears the name of Ovary {Ger-
nien) : it is this particularly which distinguishes
the female reproductive apparatus of the flower-
ing-plants from that of the Gymnosperms. Since
the Greek word 'Ayyelov denotes a receptacle,
(in this case the ovary) it is customary to group
the flowering-plants — the Monocotyledons and
Dicotyledons under the name Angiospernis, and
thus contrast them with the Gymnosperms, or
naked-seeded plants.

After these definitions, probably not entirely
superfluous for some of my readers, we may
now enter more in detail into those points which
alone really interest us here ; it is unne-
cessary for our purpose at present to regard
the two classes of Angiosperms separately. I
select therefore an example to hand, the flower
of one of our handsomest Monocotyledons, the
Flowering Rush {Butomiis timbellatus), for the
preliminary description of the reproductive
organs. In Fig. 441 A is represented a flower
in its natural size ; in B the floral envelopes
and the nine stamens are cut away, and the
whole female reproductive apparatus consisting
of six single separated carpels is represented
slightly magnified. Each carpel bears above
a narrow process, the so-called Style, which in
its turn bears at the upper end a brush of
hairs, the S/igma n ; this has the function of holding fast the pollen-grains
which have been carried to it from the opened anthers, in order that they may
germinate there, and put forth their pollen-tubes first in the tissue of the style
and then in the cavity of the ovary.

It is easier in the case of Bidomus than in many other plants to perceive that the
carpel together with its style and stigma constitute practically a leaf with the margins
folded together longitudinally: this is obvious in Fig. 441 C, which represents
three carpels in transverse section. In other cases it is of course not so obvious, though

KiG. 440. — P'ir-cone of Abies pcctinata (after
Schacht). A a leaf separated from tlie female floral
axis and looked at from above : it bears the ovuliferous
scale s with the ovules sk (magnified). B upper part
of the female flower (cone) m the mature state, j/axis
of the cone (floral axis) ; c its leaves ; s the much-
enlarged seminiferous scales. C a ripe seminiferous
scale s with the two seeds sa and their wings f
(reduced).

FLOWERS OF ANGIOSPERMS.

759

the pods of the Bean, Pea, Bladder-Senna {Co/u/ca) and Paeony present easily observed
cases of the same kind. The foliar nature of the ovary is somewhat more difficult to
decipher when (as is in fact the commonest case) two, three, or more carpels are
united to form a simple two or more celled ovary. Suppose the three carpels repre-
sented in C united inwards with one another and with the three which are absent, by
their infolded margins : we should then have a six-celled ovary, consisting of six
carpels. Considering the enormous variety of the flowering-plants, the numerous

Fig. 44i.—Butomus wnbellatiis . A flower !nat. size). B carpels, after removal of the floral leaves and
> (magnified) ; « stigmas. C transverse section of three of the mor.omerous carpels, each bearing numerous
ovules on the inner side. D a young ovule; /: the same immediately before fertilisation, ii integuments, K
nucellus, KS raphe, em embryo-sac. /=" transverse section through the stigmatic portion of a carpel (more highly
magnified) ; pollen-grains are attached to the stigmatic hairs. G transverse section of an anther; it is quadrilo-
cular, but the dehiscence of the lobes ß at s takes place in such a way that it appears bi-locular. //part of an
anther-lobe (corresponding to ß in G) : y the place where it separates from the connective, e the epidermis, x
the layer of fibrous cells (erulntheciuni). / diagram of the whole flower ; the perianth// consists of two alternate
trimerous whorls; the andniicium likewise, but the stamens of the outer whorl are doubled (/), those of the
innery simple and thicker. The gyncccium also consists of two trimerous whorls, an outer one c' and an inner
one c'. There are thus six alternate trimerous whorls, with duplication of the segments of the first staminal

families of which are characterised and distinguished especially by their ovaries,
it scarcely needs mention that what has been said only indicates the most essential
points. I may make one more remark for the sake of those who are not botanists,
however, so that the word ovary shall not lead to error: the ovary is, put
shortly, the young fruit ; or, conversely, the subsequent fruit is the ovary which has
grown up and become ripe, just as the ovules are nothing further than the )oung,
as yet unfertilised seeds.

The ovules of most nowcring-jilants arise at the united margins of the carpels,
as can be seen with the greatest clearness in the half-ripe fruits of the Bladder-Senna

76 O LECTURE XLII.

and of the Pceony. It is only in rare cases that, as in Biitomus, the ovules arise on
the whole of the inner surface of the carpel, and occasionally as may be seen in
Fig. 303, p. 466, an ovule appears to be the true end of the floral axis.

In the main the ovule of the flowering-plants is not essentially different from
that of the Gymnosperms, although it usually diff'ers in several external matters.
Fig. 441 ^ represents one of the commonest forms, an ' anatropous ' ovule : on a
stalk-like funiculus KS is situated the nucellus of the ovule K invested by two integu-
ments I and i' and in such a way that the micropyle comes to lie next the base of
the stalk. There are, however, other forms also : straight or ' orthotropous ' ovules
as in Fig. 303, p. 466, for instance. Moreover there are not always two integuments,
but often (especially in many Dicotyledons) only one. The embryo-sac — and therein
lies an important diff'erence in contrast to the Gymnosperms — often grows up as far
as the micropyle even before fertilisation (e m in E^ ; and indeed cases are not rare
{Pedtcularis and other Scrophularinese, and also Santaluvi) where the anterior end of
the embryo-sac grows out beyond the nucellus of the ovule, pushing its way into the
micropyle or even projecting out beyond it. However I shall here pay no further
attention to such peculiarities.

A veiy important difference between the angiospermous flowering-plants and the
Gymnosperms comes out when the processes in the embryo-sac previous to fertilisation
are considered : the Angiosperms form no prothallium with archegonia, unless we
consider a sort of rudiment of them to exist in three cells which occur very frequently in
the hinder basal-end of the embryo-sac (/,; Fig. 442), and which bear the curious name
' antipodal ' cells — i. e. antipodal with respect to the oosphere. Starting from the con-
sideration of what occurs in the embryo-sac of the Gymnosperms, the proper repro-
ductive apparatus inside the embryo-sac appears to be a mere remnant, as it were,
which still retains only what is most essential and indispensable. Thus, in the anterior
end of the embryo-sac turned towards the micropyle there arise before fertilisation,
with rare exceptions, three naked cells close together. One of these, marked z in
Fig. 442, is the oosphere, from which the embryo is formed after fertilisation; the
two others, marked v, which occupy the proper apex of the embryo-sac, were
designated by Strasburger the SynergidcB {GeJiiilfinnen). Strasburger, to whom we
owe our more exact knowledge of the processes in the embryo-sac, assumes that it
is these synergidae which first take up the fertilising substance from the pollen-tube
and then pass it over to the oosphere.

While these things are undergoing preparation in the embryo-sac, the pollen-sacs
of the anthers dehisce. By means of the wind, and much oftener by means of insects
or other animals seeking honey in the flowers, the pollen-grains are conveyed to
the now moist and usually papillose surface of the stigma, where they remain attached,
and they put forth their pollen-tubes as a rule within a few hours (cf. / Fig. 443).
If there is only one ovule in the ovary, it is sufficient if one of the pollen-tubes grows
down through the style as far as the cavity of the ovary, and finally penetrates the
micropyle m, applying its end to the embryo-sac where it comes in direct contact
with the synergidae. Since, as is seen, each ovule requires a pollen-tube to fertilise
its oosphere, at least as many pollen-grains as there are ovules in the ovary must
reach the stigma and perfect the developement of their tubes. Not rarely however
an ovary contains hundreds and even thousands of ovules, and accordingly the dusting

POLLEN OF ANGIOSPERMS.

761

of the stigma with pollen-grains must be profuse if all the ovules arc to be fertilised,
and since not every pollen-tube accomplishes its end, even a greater number of pollen-
grains are needed.

Also in the pollen-grains of the Angiospcrms there is still found a last remnant
(one might almost say a feeble and indistinct memento) of the cell-formation which
occurs in the microspores of the Cryptogams and Gymnosperms. It is to Slras-

Fig. 442. — Diagram of a very simple flower in longitudinal suction, a transverse section of an anther
before its dehiscence ; t an anther dehiscing longitudinally, with pollen ; c filament ; d base of floral leaves ;
c nectaries ; /wall of carpels ; ir style ; h stigma ; i germinating pollen-grains ; klm ^ pollen-tube which has
reached and entered the micropyle of the ovule j n funicle of ovule ; 0 its base ; f outer, g inner integument ;
.r nucellus of ovule ; / cavity of the embryo-sac ; ic its basal portion with the antipodal cells ; v synergidK ; s

burger's investigations, which have been of such immense service to tlie theory of
fertilisation, that we owe the knowledge of the remarkable fact that the ripe
pollen-grains of the Angiosperms regularly contain two cell-nuclei, and that
sometimes indeed, a division, although transitory, of the contents is indicated.

The pollen-grains of the Angiosperms differ much in form and size. The
usually thick cuticularised external membrane, the so-called Extine, very often shows
beforehand the spots from which the pollen-tube or tubes are to emerge later. As
in the germination of spores generally, it is the second membrane consisting

762

LECTURE XLII.

of cellulose (the so-called Inline) of the pollen-grain which grows out and forms the
pollen-tubes. The places at which this will take place are, as already stated, indicated
beforehand, and sometimes, as in the Cucurbitaceae, circular excised pieces of the
e'xdne are set like lids into the apertures, and are simply pushed aside by the
emerging pollen-tubes, as in Fig. 443. The pollen-tubes have now to make their
way through the tissue of the stigma and style down into the cavity of the ovary ;
inside the latter they either enter directly into the micropyle of the ovule, which how-
ever occurs but seldom, or, as is more usual, they grow down along the walls

of the ovary until they reach the
micropyle. To this end the paths
to be traversed are often indicated
beforehand by special relations of
organisation on the wall of the ovary.
The pollen-tubes of the Angiosperms
are for the most part very delicate,
their walls being relatively thick;
they may thus be easily passed over
in the tissue of the style, and it is
always one of the difficult tasks of
microscopy to discover their fertilis-
ing end penetrating into the micro-
pyle. Sometimes the distance which
the pollen-tubes have to traverse
from the stigma to the micropyle,
is very considerable — e.g. in the
Maize it is 20-40 cm. and in many
other long-styled flowers it is from
3-10 cm.

The effect of fertilisation makes
itself evident chiefly in two points,
in the development of the oosphere
into the embryo, and in the in-
ception of the endosperm.

By means of rapidly repeated bi-
partidons of the nucleus of the em-
bryo-sac, as Strasburger has shown,
there are produced in a short time
as a rule very numerous nucleated
masses which distribute themselves at regular distances from one another in the
protoplasmic lining of the embryo-sac. Around each of these nuclei a portion of the
protoplasm collects as around a centre of attraction. At the boundaries of these
portions of protoplasm arise thin cellulose walls (cf. Fig. 106, p. 106) and thus is
formed a layer of tissue which is at first applied to the wall of the embryo-sac, the ex-
ternal layer of endosperm ; as its cells grow in towards the interior of the embryo-sac,
and undergo transverse divisions parallel to its wall, the cavity of the embryo-sac is
gradually filled up with cell-tissue. If the embryo-sac is very narrow, or even tube-like

Fig. 44^.—^ a pollen-ijram (in section) germinating on the stigmatic
lobes. The outer membrane (extine) has a number of circular openings
closed by the lids d ; the inner membrane of the pollen-grain (;) is thick-
ened beneath the lid— it swells, protrudes through the hole, and thus
pushes aside the lid. One of these swollen cushions of the intine is grow-
ing into the stigmatic tissue as tlie pollen-tube sp, B a piece of the pollen
membrane ; i intine, c extine, a" a lid.

ENDOSPERM. 763

as often occurs, the endosperm presents the appearance of a number of transverse
walls dividing its cavity into chambers ; if, on the contrary, the embryo-sac is very
large, it may happen that the endosperm commences to form on the wall but
never fills up the whole of the cavity. This is conspicuously the case in the
Coco-nut, and is not rare to a slighter extent in other large-seeded plants : the
hard shell of the Coco-nut is clothed internally by the wall of the embryo-sac, and
we have here, therefore, an instance of a cell which attains a volume of 500 c. cm. or
more. The development of endosperm in the Coco-nut however proceeds only so far
as to form a layer of tissue 4-5 mm. thick at the circumference of this enormous
embryo-sac, in which the relatively tiny embryo lies : the whole of the remaining
cavity is filled with the watery cell-sap of the embryo-sac, known by the name of the
milk of the Coco-nut, and employed as a drink. In many other cases also the
embryo-sacs of the Angiosperms are distinguished by their large size (as single
cells) immediately after fertilisation and subsequently : on cutting a half-ripe Bean it
appears as a vesicle filled with water. In like manner in the case of the Walnut, before
the shell of the nut becomes hardened, the large cavity which it surrounds is found to
be filled with watery fluid. In these and many other cases the cavity described is
the hollow of the embryo-sac, which subsequently becomes wholly or pardy filled with
endosperm-tissue.

The endosperm of the flowering-plants is distinguished from that of the
Gymnosperms, as is clear from what has been said above, from the fact that it
only arises after fertilisation. It resembles it, however, in that when the ovule ripens
into the seed it becomes filled with products of assimilation (proteids and starch or
fat) the at first extremely delicate cell-walls of the endosperm often undergoing
enormous thickening, so that the endosperm at last forms a very hard thick mass.
Such is the case for example in CofTee ; the so-called Coffee-bean, as it comes into
the market, being the horny hard endosperm. In the same way a Date-stone consists
entirely of very thick-walled endosperm-tissue (cf. p. 344). One of the most remark-
able examples in this connection is afforded by the large seeds of Phytelephas, a
tropical palmaceous plant, which on account of their enormous hardness and solidity
are worked by turners as vegetable ivory : the whole of the hard mass is thick-walled
endosperm, in which the small embryo lies at one point of the periphery, and
when it germinates it dissolves and absorbs the whole of this hard mass. To
remind the reader of but one more example, well-known to all, it may be stated
that the flour-yielding substance of our cereals is the endosperm in the embryo-
sac of these seeds, in this case consisting of very thin-walled large cells contain-
ing proteids and starch.

There are both among the Monocotyledons (Orchidaceoe) and among the
Dicotyledons various families in which the formation of endosperm is either extremely
scanty or entirely suppressed. Such cases, however, must not be confused with
those where no endosperm is to be found in the ripe seed owing to its having been
absorbed by the embryo during the development of the seed. This is the case
particularly often among Dicotyledons : the kernel of such fruits as Cherries
and Almonds, Apples, and the Walnut and Hazel-nut, Acorns and Beech-nuts,
the seeds of the Sun-flower and of all Compositor, the seeds of Gourds and
Cucumbers, Peas, Lentils, Beans, etc. contain in the ripe state no endosperm

764

LECTURE XLII.

because it has been already absorbed again by the cotyledons of the embryo before
and during the ripening of the seed. In consequence of this the cotyledons in. such
seeds grow so large that they completely fill up the whole of the very considerable
space within the seed-coats, whereas the plumule and radicle of the embryo form
but minute appendages.

In what has been said however I have been anticipating the developmental
processes which occur after the fertilisation itself: some points have yet to be
mentioned concerning the origin of the embryo from the oosphere. As occurs even
in the Selaginellse, and to a much greater extent in the Gymnosperms, so also in the
Angiosperms there is developed from the fertilised oosphere not only the embryo

Fig. ^^i,.— Ricinus commiciiis, I the ripe endosperinic
seed in longitudinal section. // the seedling of the cotyledons
which are still in the endosperm— compare .4 and B. s testa,
e endosperm, c cotyledon, he hypocotyl, lu primary root, tv'
secondary roots, x the caruncle, an appendage characteristic
of Euphorbiacese.

FIG. 445-— Bean H'icia Faba), A the exalbumi-
nous seed after removal of one of the cotyledons ; the
other is still present c. -w radicle, kn plumule of the
embryo ; j testa. S germinating seed ; j- testa, / its
ruptured lobes, ti hilum. st petiole of one of the
cotyledons, k curvature of the epicotyl z'; Ac the very
short hypocotyl ; h primary root ; if .f its apex ; iit bud
in the axil of one of the cotyledons.

proper, but a support or pro-embryo, by means of which the embryo is connected with
the membrane of the embryo-sac. The young embryo hence appears generally as a
sphere on a stalk which may be long or short ; for the fertilised oosphere grows in
the first place more or less in length, forming a tube which becomes segmented by
transverse walls, and finally, the true embryo arises from the most anterior of these
cells, which then breaks up by divisions in three directions situated at right angles to
one another, into octants, and as it grows up becomes further divided in the pericline
and anticline directions. These processes will be suflliciently clear from Fig. 446
I-IV. It is not before the spherical body is transformed into a small-celled mass
of tissue that the inception of the first leaves and primary root begins. All that

EMBRYOLOGY OF ANGIOSPERÄIS.

765

represents the future shoot-axis is practically only the mass of tissue which connects
the parts named.

If, as in Fig. 446 V and VI, two opposite first leaves or cotyledons arise
simultaneously, we have to do with a plant from the class of Dicotyledons ; in the Mono-
cotyledons there is developed first a single leaf, which usually grows round the whole
circumference of the embryo, and which is subsequently followed (as a rule not before
germination begins) by a second likewise sheathing leaf. Usually the growth of the
embryo is very slow, as is that of all masses of embryonic tissue ; hence it is found at

Fig. 446.— Development of the embryo of Capsella hiiysn-pastorts (after Hanstein). The order of development
is from I—Il^(Vb end of root seen from below), i, i— i, 2 first divisions of the apical cell of the pro-embryo ; hh'
hypophysis ; it pro-embryo (suspensor) ; c cotyledons ; J apex of the axis j w root. The dermatogen and plerome

the time when the seeds have already attained nearly their permanent volume, as a
still very small body in the cavity at the apex of the embryo-sac. I may here take
the opportunity of pointing out as a remarkable fact, belonging to the category of
phenomena previously described under the name of correlations of growth in their
dependence upon chlorophyll, that the embryos of plants devoid of chlorophyll
generally remain very small. According to Hofmeister, the embryo of Momtropa
is only two-celled in the ripe seed ; and in the Orobancheae, Balanophorese, and
Rafflesiacese a small mass of cells is formed, it is true, but it exhibits no seg-
mentation into definite organs. The embryo in the seed of Ciiscuta is larger and

766 LECTURE XLII.

Stronger, but even it is devoid of the radicle, and probably also of the first leaves. It
is evidently not parasitism per se which induces this phenomenon, but chiefly the
want of chlorophyll; since the embryo of the Mistleto and other Loranthacese
abounding in chlorophyll attains not only a considerable size but is also provided
with cotyledons and radicle.

It has already been remarked that in all seed-plants the seed containing the
embryo is simply the further developed ovule : the ovule in its turn, however, is as
we saw a macrosporangium, and the embryo-sac a macrospore, in which, after
fertilisation, the young plant — the embryo — has been developed from the oospore.
We might therefore designate the ripe seed still as a macrosporangium; but in
most instances this would not quite meet the case, for in the great majority of
Angiosperms the growing embryo-sac squashes the tissue of the nucellus of the ovule
(i. e. of the sporangium) before or after fertilisation, so that it is finally surrounded
only by the integuments. From these, or from certain cell-layers of them, is
gradually formed the testa, the structure of which may be very different in the
various species.

LECTURE XLIII.

THE ACTION OF SEXUAL-CELLS UPON ONE ANOTHER.
(CONTINUITY OF EMBRYONIC SUBSTANCE.)

So far as our experience goes it appears that organic life never commences
otherwise than by means of small masses of substances separating themselves off
from an organism which already exists. These substances however are not fluid
in nature — not watery solutions — although saturated with water, they are not
crystalline, nor in the ordinary sense of the word solid. Moreover, it is here,
as it appears, never a matter of a simple chemical compound, but always of a
mixture of substances; or, to put it shortly, the material concerned is a minute
mass of protoplasm, which usually, especially in the case of asexual reproduction,
contains in addition certain plastic materials for growth, such as starch or fat.
Nay, many spores take away with them from the mother-plant organs of
assimilation also.

We have every reason to believe, moreover, that most of the visible substance
in a reproductive cell forms as a matter of fact only materials for growth in the
ordinary sense of the word, and that in addition, there is present a perhaps
infinitesimally small quantity of a substance which, though not well understood
itself, constitutes so to speak the primum movetts by means of which the other
substances of the reproductive cell are sooner or later put in motion, and the
processes of growth called forth.

This view of the nature of a reproductive cell obtains a certain degree of
probability from the whole of the pecularities of sexual reproduction. The essence
of the latter lies in that in the course of the development of a plant (or of an
animal) two kinds of cells are produced, each of which is incapable of further
development on its own account, but from the union of the materials of which a
product results which is capable of development. Only this latter, the fertihsed
oospore (or in some lower Algae and Fungi the so-called zygote or zygospore)
is a reproductive cell in the strict sense of the word, since without fertilisation it is
not in a position to give rise to a new organism. It is true it appears exceptionally
that even a non-fertilised oosphere may be capable of forming an embryo; but
the as yet little understood cases of so-called parthenogenesis may turn out to be,
as will be shown in the course of these considerations, rather a confirmation of
what I regard as the essential in fertilisation.

An oosphere (or in the case of the Algae a swarming gamete) is rendered

768 LECTURE XLTII.

capable of development by taking up the fertilising substance which is brought to
it by another cell (or another gamete) generally a zoosperm (antherozoid), or in the
case of the phanerogams by a pollen-tube. Now it would be somewhat absurd to
suppose that this fertilising substance is of exactly the same constitution as that of
the oosphere itself; though something of the sort might perhaps be supposed in such
cases as those of the Conjugateae, IMucorini and some Gamosporeae, where the two
conjugating cells appear to be externally of like constitution.

On this assumption fertilisation would merely consist in a union of the substance
of the reproductive cells. That this is just not the point, however, is shown by all
those cases where a relatively large oosphere is fertilised by a very minute zoosperm,
the whole substance of which scarcely amounts to the thousandth part of the mass
of the oosphere, and the same conclusion follows naturally from all observations
on the behaviour of the pollen-tube when it fertilises the oosphere of a seed-
forming plant. Even the different shapes of the two sexual cells — of an antherozoid
or a pollen-grain compared with the oosphere — indicate definitely that both are
constituted differently as to material, since the external form as well as the internal
structure of any body are the necessary expression of its material constitution.
Difference of form always indicates difference of material substance.

We can thus say, then, the fertilisation of an oosphere (or of a gamete)
consists in that something is added to its substance which was hithe?-to watiting to
it, but which it needs for further development. What this fertilising substance may
be is still in a high degree doubtful ; at any rate it is not the whole mass of an
antherozoid, and much less the whole mass of a pollen-grain which can answer
to the title of fertilising substance. The extremely small quantity of the latter,
however, generally effects conspicuous alterations in the oosphere at once : the
excretion of a cell-wall, growth and cell-division, the formation of the embryo, and
finally all those successive changes which take place in the neighbouring parts of
the mother-plant itself — the completion of the seed and walls of the fruit — and not
rarely the consequences of fertilisation extend over the whole plant, which at length
dies down after the ripening of the seeds.

If in the case of so extremely peculiar a natural process as fertilisation it is
permissible to look for an analogy at all, we might be reminded especially of the
action of the ferments, which at least present a similarity in so far that ex-
tremely small quantities of them are able to put in motion large masses of
matter, and thereby chemically alter the latter. There is not much gained
by this however, since the action of ferments is itself as yet not understood.
Fertilisation also presents similarity with many phenomena of irritability, though that
again does not say much for our point of view, since we may say that fertilisation is
the most pronounced of all phenomena of irritability : I understand irritability to be,
as already stated, the mode of action of a living organism towards all external
stimuli, and therefore fertilisation belongs obviously to this category, the oosphere
being the organism which, under the stimulus from without of the fertilising
substance coming into contact with it, reacts in so astonishing a manner that new
processes of configuration and growth arise from it.

In asexually produced spores, however, we find cells capable of repro-
duction which, without the addition of a fertilising substance nevertheless

CONTINUITY OF EMBRYONIC SUBSTANCE. 769

begin and accomplish further development, and it might almost appear as if our
preceding considerations were completely invalidated by this fact. However, the
logical conclusion seems to be this : since the asexual spores germinate without
fertilisation they simply contain all that is necessary. They do not, as is
the case with the oosphere, lack something which must be first transferred
to them in order that they may become capable of development ; they do not
need fertilisation because they have all they require, and exactly the same con-
clusion may be also employed in the very rare cases of well-established Par-
thenogenesis and Apogamy, where female reproductive organs arise and are able
to form embryos without being fertilised. Here again we may say, since they
are able to do this they contain in themselves all that is necessary for
development, and if this is the case such female organs are also only apparently
female ; since this term should probably be reserved strictly for those cases
where a cell is stimulated to further development only by fertilisation. Those
cells which do not need fertilisation, such as the spores and gemmae of Muscinese,
the conidia of Fungi, and the asexual spores of many Algae, and which give rise to new
plants, resemble in this point ordinary vegetative cells, which in fact are also able
under favourable circumstances generally to give rise to new plants.

The question is thus, how comes it that sooner or later in the course of
development certain cells are produced which have lost the capacity for further
development, per se, but which are therefore in a condition to afford by means
of the union of their substance a product capable in the highest degree of
development. That this is no incidental occurrence, but must be dependent on
something in the deepest being of the organism, I conclude from the elaborate pre-
parations for the attainment of the given purpose, since we may regard the formation
of the various sexual organs in which the sexual cells eventually arise as such
a preparation, — and the higher the organisation the more comprehensive this
preparation for the development of sexual cells appears to be — this segregation of the
organic constructive substances into two different substances, male and female, which
onl}' }ield a vegetative product subsequently by means of their union.

I have already repeatedly taken opportunities of mentioning the hitherto
much too little noticed fact, that the continuity of plant-life is expressed essentially
in the continuity of the embryonic substance. I have set forth in detail that in the
normal course of the life of a plant, even of a tree which lives hundreds of years,
the newly formed growing-points are always the descendants of preceding growing-
points, and that finally all the numerous but small growing-points of a much-
branched plant are derived from the first growing-point of the seedling. But this
is a direct remnant of the substance of the fertiHsed oosphere, or of what I term
embryonic substance. The question is, then, whether the embryonic substance of the
oosphere itself also carries on this continuity, and this question must be answered
decidedly in the affirmative : the numerous careful embryological researches of
the last forty years leave no doubt that the oospheres as well as zoosperms and
pollen-grains arise from mother-cells which are direct descendants of growing-
points, from which the more elaborate sexual organs which give rise to them
proceed ; Goebel's recent investigations especially show emphatically that al-
ready in the earliest stages, the cells from which the proper sexual cells are lo

[ 3 ] 3D

770 LECTURE XLIII.

proceed may be recognised from the material peculiarity of their contents, at a
time when the surrounding tissue still possesses entirely the character of the so-called
primary meristem, or embryonic tissue of the growing-point. The differentiation
of the two sexual products thus begins in the interior of growing-points, and
the product of the sexual union is an embryo whose tissue is identical with
that of a growing-point, and from which the first growing-points of the new plant
are to be derived as remnants. Thus, strictly speaking, sexual reproduction is
no more calculated to produce a new organism than is asexual reproduction ;
the elements from which the embryo arises are simply products of the embryonic
substance of a previous plant, and finally, we may say, that what has since the
beginning of organic life on the earth continually maintained itself alive amidst the
perpetual change of all forms, in the continuous alternation of life and death,
and has continually regenerated itself, is the embryonic substance of the growing-
points, which in certain cases becomes differentiated into male and female, and
these unite again subsequently. In these extremely minute masses of matter, organic
life has continually maintained itself through the slow course of geological epochs ;
those parts of the plant which present themselves directly to the eye — fully
grown roots, shoot-axes, leaves, woody-masses, &c. — all these are products of that
embryonic substance which is continually regenerating itself. These, its products,
it is true outweigh it in mass a million-fold, but they are not capable of regeneration.
It is not in these that the continuity of organic life is maintained, but it is
these, which by means of their work in common, carry on the processes of
assimilation and metabolism, and a very small quantity of the substance which
they do not themselves employ for their growth, is made use of for the nutrition
of the embryonic substance of the growing-points and sexual cells.

After this probably not entirely superfluous digression I now return to my
theme, the mutual interaction of the sexual cells, resuming the discussion from another
side. The reasons have already been given which impel us to the assumption
that in order to make the oosphere fertile, some substance on the part of the male
cells must be added to it which it wanted previously. The most recent investiga-
tions of Schmitz, Strasburger, Zacharias, and others, lead in the first place to
the result that the fertilising substance is to be sought in the nuclear substance, the
nuclein of the male cell. It appears, according to the observers mentioned, to be
estabhshed as regards zoosperms (antherozoids), that their proper body is formed from
the nuclein of the mother-cell, while the part which bears the cilia appears to proceed
from the protoplasm. The nucleus of the mother-cell of the antherozoid enlarges until
it has taken up into itself the whole of the protoplasm of the cell, or nearly so.
The peripheral layer then condenses into an annular or spirally inrolled band, while
the central portion becomes less dense and constitutes the vesicle which newly
escaped antherozoids usually trail at their hinder ends, but which they often soon
lose. Zacharias has attempted to establish by microchemical methods that the
actual body of the antherozoid is identical with nuclein, and has pointed out that
in this respect animals agree entirely with plants.

Thus what is carried into the oosphere by the antherozoids is nuclein, since
we may believe that the only importance of the cilia (which do not consist of
nuclein) is simply that of motile organs.

ZOOSPERMS OR ANTHEROZOIDS.

Ir-

in agreement with this, Strasburger has for several years past laid particular
stress on the fact that in the Phanerogams the two cell-nuclei which were already
present in the pollen-grain pass forward into the growing-end of the pollen-tube,
and are carried with this into" the micropyle and then disappear, thus probably

Fig. 447.— Antheridia of Adiaiitiim Capillits-Veneris (X 550) in optical
longitudinal section. / is still immature ! in // the antherozoids are already
formed ; /// has ruptured by the peripheral cells swelling radially, and the
antherozoids have nearly all escaped. / prothallium ; a antheridiuni ; b its
vesicle, containing starch-granules.

Fig. 448.— Young Archegonia of Pteris ser-
<-u!ata (after Strasburger), e oosphere ; /; h
leck ; A'canal-Lcllb.

Fig. 449.— Germination of microspores and development of antlierozoids of IsocUs laiiistiis (after Millardet).
.■/ and C microspores from the right side ; B and n from the ventral side. A and ß show the development of
the antheridium. SS dorsal cells ; ß ß ventral cells. C and Z) show the development of the antherozoids. 6 and /3 have
disappeared, v in A—D is the vegetative cell (Millardet's prothallium). a—/ development of the antherozoids.
(,A—D, a and d X 580, e andy X 700.)

becoming dissolved, and, hypothetically at least, we may assume that here also it
may be the nuclcin which is passed over to the oosphere in a state of solution
by the agency of the synergidce. In cases where numerous pollen-tubes are
develo})ed from one pollen-grain, the nuclear substance is, according to Strasburger,
previously dissolved in the protoplasm, apparently in order to distribute itself

3 1^ 2

72

LECTURE XLIII.

in the various tubes. Only the mere reference can here be made to the complicated
processes, combined in fact with cell-divisions, which Strasburger describes in the
pollen-tubes of the Gymnosperms.

The inference from these statements, apparently so obvious, may seem
to be contradicted again by other facts. In the first place by the fact that the
oosphere itself already possesses a cell-nucleus, with nuclein in it, before fertilisa-
tion, and in the conjugation of Spirogyra it is expressly stated by Schmitz and
Strasburger that the nuclei of the two conjugating cells simply fuse with one
another, and Strasburger has demonstrated the same in other cases also. When
therefore it comes to answering the question, what kind of substance is transferred
to the oosphere as fertilising material, we must allow that the question is not
yet sufficiently answered by the observations named. As a hypothesis it may in
the meantime be assumed that the nuclein of the two sexual cells is not alike in
constitution, and that the nuclein of the male cell thus carries into the oosphere
something other than it already possesses. IMeanwhile we must leave the problem
here shortly treated of in this incomplete form.

One of the most surprising facts connected with the reproductive processes
is the action at a distance, or mutual attraction of the two sexual cells towards one
another. I select this expression for the facts to be examined because it is short
and at least clearly denotes the matter; though the words 'action at a distance'
and ' attraction ' are not to be understood in exactly the same sense as in physics.

In the numerous descriptions given by observers as to the behaviour of the
antherozoids in the neighbourhood of the oosphere, and of swarming gametes
and even of antheridia in the neighbourhood of oogonia, we meet almost without
exception with most definite expressions of the fact that a certain mutual interaction
of the sexual cells at a certain distance exists, and this always of such a kind
that the union of the two cells is thereby accomplished or promoted. This process
is the more remarkable because immediately after fertilisation has been accomplished
this mutual attraction has disappeared.

From a large number of such cases I will take a few only, as follows.
Speaking of the fertilisation of the Ferns, and especially of that of Ceraiopieris
/haliciroides, so favourable for observation. Strasburger says {^ Jahrb. für zviss. Bot!
vii., p. 402): —

' If a prothallium with ripe sexual organs is laid in water, the antheridia
usually open first, and in favourable cases the archegonia soon after. In the case
of Pieris serrulata one has nevertheless to wait probably about half an hour or
more on the average ; with Ceratopten's thaliciroides, however, rarely more than
twenty minutes.

' The antherozoids, which at first come by chance near the apex of the arche-
gonium, just as near other foreign bodies, here behave very curiously as soon
as the neck has opened. The instant they meet with the slime in front of
the canal their movement becomes slower ; it is seen that they are here held
back and their movements impeded by a resistent medium. Some remain fixed
in the slime, some succeed in freeing themselves and hurry away: in most cases
however, neither of these events occurs, but the antherozoid is so directed
by the slime poured radially out of the canal that it steers, apex forwards,

STRASnURGER ON THE FERTILISATION OF FERNS. 773

into the mouth of the current. A diffusion current cannot here be assumed, and
just as little a vortex suddenly seizing the antherozoids and hurling them
towards the opening, since even very minute granules remain completely at rest
in front of the opening of the canal. The motion of the antherozoid in the slime
is decidedly slower, but it does not cease to revolve about its axis. The slime,
however, leads it into the canal, so that its action here may be compared with
the activity of the stigmatic fluid and the conducting tissue, which conduct the
pollen-tube of the phanerogams to the ovule. At the same time we can here
convince ourselves most decidedly how little warranted is E. Roye's assumption that
it is the vesicle at the hinder part of the antherozoid which contains the ferti-
lising substance. Most of the antherozoids have already lost this vesicle before
they even come at the archegonium ; others which still possess it now lose it in
the slime, and none take it with them into the interior of the archegonium. I
have, among others, exactly recorded a case in Cera/op/ens, where five anthero-
zoids just escaped from their antheridium, penetrated into the central-cell, and
six such vesicles were to be noticed in the slime before the opening of
the neck.'

' Arrived at the interior of the canal, the coils of the antherozoid become
drawn widely apart and, if no other disturbing influences affect it on the way, it soon
reaches the interior of the central cell, where the emptied canal-cells have left a suffi-
ciently large cavity. Here the coils again contract, and the movements again become
freer. The first antherozoid that penetrates usually does not remain the only
one, but others soon follow it : their number inside the central cell may amount to
four or even five, so large is the space here afforded by the canal-cells. They then
move actively in and out and between one another, somewhat like the antherozoids
which remain behind in an antheridium, Antherozoids coming later remain stuck
in the canal of the neck : in Pteris serrtdata their number may increase enormously,
each new arrival screwing itself in between those already present so long as its
movements are still possible, and at last becoming extended quite straight. It thus
happens that in some cases the canal of the archegonium appears as if filled
with long threads. New antherozoids now arriving can no longer be taken in;
I have however observed a few cases in Pteris where they nevertheless screw
themselves with their anterior ends between the previous arrivals, and there is
thus formed a great struggling crowd of antherozoids, extending radially from
the canal.

' In this crowd some of them are seen to revolve about their axes for
some time longer, and sometimes even to get loose again and hurry away. In
Pteris serrulata I have repeatedly counted over a hundred antherozoids in
such a crowd, and even half an hour after the entrance of the first antherozoid
a few still remain fixed in the slime. Such aggregations, it is true, are
exceptional, and the number of the antherozoids which enter into an archegonium
is usually limited to a few only ; these aggregations moreover only occur in the
case of Pteris, they are not possible in the case of Ceratopteris ; not only because the
prothallium here developes relatively few antherozoids, but also because the slime
expelled from the neck of the archegonium diffuses far more quickly in the surround-
ing water, and only holds for a short time the antherozoids which are hurrying by.

774 LECTURE XLIII.

' After what has been said above there can be Httle doubt that it is
actually this slime which acts specifically on the antherozoids. However I was
able to convince myself still more definitely of this action by moving the
cover-slip and thus bringing the slime out of its original place, or even removing
it entirely by means of the needle. The antherozoids then, held by the slime wher-
ever it came to lie, strove to free themselves from it or perished in it after a longer
or shorter swarming, but they were no longer directed towards the opening of
the archegonium-neck.

' We will follow yet further the behaviour of the antherozoids inside the
central cell. Pteris is, as already seen, unfavourable for this observation, so that
among the numberless observed cases I was only able to see this process twice here.
Once on a transverse section which laid bare the archegonium, without injuring
it, and antherozoids from a neighbouring antheridium crowded into it; another
time on a superficial view of the prothallium, where the archegonium on the
declivity of the central cell-cushion came to lie so that I could see its optical
transverse section.

' The whole process may be followed comparatively easily however in Cera-
iopteris ihaUctroides. The prothallium is so translucent that it is very easy on surface
views to see both the whole central cell and its contents, the tendency of the
archegonia to be placed at the inturned margins of the prothallus being still
more favourable for observation. The antherozoids are relatively larger than in
Pteris, and thus can be easily followed.

' The first antherozoid which penetrates into the central cell abuts, generally
at once, or it may be after a short swarming, with its anterior end on the pale
spot (the receptive spot) seen about the centre of the top of the oosphere, and
at once remains fixed at this spot ; it then revolves rapidly round its axis and
its apex sinks slowly into the oosphere. Its movements become slower, and
sometimes cease entirely, it disappears more and more into the oosphere, and its
mass fades in it, till after 3-4 minutes (in all cases) nothing more is to be
seen of it. This process, as I have just described it, I have succeeded in following
quite undisturbed only five times out of the many cases observed, that is when
only one antherozoid had penetrated into the central cell ; and this occurred
mostly only when access to the canal was impeded by means of external dis-
turbing influences, e.g. when air-bubbles or other foreign bodies blocked the
entrance to it. In most cases several other antherozoids follow the first
one, which thus becomes displaced by the new comers, unless it has
penetrated already with its apex to a certain depth. The antherozoids now
move in and out, and it is very difficult to follow the individuals. Often two
or even three antherozoids are found simultaneously hanging by their apices
at the receptive spot : they rotate rapidly on their axes, mutually crowding, till
one obtains the upper hand, and is so far absorbed that its posterior coils
cover the receptive spot. The remaining antherozoids are then never retained
longer and swarm in and out for some time. Often their movements cease for a
time, to be resumed again after a few moments; this continues for 8-10 minutes,
and then all come to rest ; and each individual antherozoid remains at the spot
where it came to rest and is here visible for some time longer. In one case

JURANVI ON THE FERTILISATION OF CEDOGONIUM. -jyr^

where only two antherozoids had entered the central cell, the second only came
after the first had already laid for one and a half minutes on the central receptive
spot, and its anterior coils had been taken into the oosphere. It was not able to
displace this, and remained no longer fixed to the central spot after this first
antherozoid had been entirely absorbed, but remained, after swarming side-
ways for some time, lying on the oosphere. Of the first antherozoid no further
trace was visible after four minutes, the second became invisible only after thirty-five
minutes.'

Juranyi describes the processes in the case of a species of (Edogoninm — a genus
of segmented filamentous Algae — in the following words (Jahrb. für. wiss. Bot. IX,
pp. 7-19):—

' After the swarmers have swarmed for from half an hour to an hour they come
to rest and fix themselves on the oogonia, or at least on the cells in its imme-
diate neighbourhood, and form unicellular dwarf males. The swarmers produced
by the antheridia-cells of the males are thus true androspores. The dwarf males
fix themselves generally 3-6 together round or on an oogonium, though cases
are by no means rare where they surround the female sexual organ in larger
numbers. Thus I have often seen oogonia surrounded by from twenty to forty or
even fifty dwarf males.

' The antherozoids thus escaped behave in a very striking manner. Their
movement is not progressive, but a sort of twitching, or convulsive trembling, in con-
sequence of which they can only change their position clumsily and slowly. By this
means they describe a zigzag line around the oogonium, travelling in this way
until they have found the opening of the fertilising tube, or until they perish. — Since
these antherozoids are contractile to no small degree they continually change their
shape during the swarming ; consequently they are seen sometimes as globular,
sometimes again as ovoid or as acuminate wedge-shaped corpuscles, which some-
times desist in their peculiar movements — as if they would come to rest — for several
seconds. When they remain free their movement may continue for two or three
hours.

' When the antherozoid has discovered the opening of the fertilising tube
it meets with an obstacle which impedes its entrance to the oosphere ; for the size of
the antherozoid surpasses the width of the opening of the neck of the oogonium
so much that it could not freely slip through it without altering its size and
shape. To overcome this obstacle the antherozoids are aided by their extreme
contractihty. Hence the antherozoid is seen to suddenly contract before the opening
of the neck of the oogonium, its form also changing at the same time so that its
anterior portion with the crown of cilia are directed forward towards the opening,
and it becomes wedge-shaped. The narrowed and pointed anterior portion of the
antherozoid is now slowly drawn in through the opening of the neck of the oogo-
nium, and as it enters it becomes more and more elongated at its anterior end, it
is at the same time very clear to see how the cilia of the corona move them-
selves vigorously like whip-lasht-s. This movement of the cilia continues only
until the anterior end of the antherozoid has approachctl so near to the surface
of the oosphere, that the cilia reach it, and, coming in contact with the soft
protoplasmic mass of the oosphere, remain attached to it. Here the antherozoid

776 LECTURE XLITl.

is now held fast and cannot go back again : a sort of convulsion then follows,
and the antherozoid touches the oosphere with its anterior end. At the moment this
contact of the two sexual cells occurs, the oosphere turns pale at the point of
contact and there appears a somewhat large rounded colourless and bright spot —
the receptive spot — which is however somewhat translucent at the edges, and this
indicates the place of fusion of the sexual cells : its appearance and occurrence
in this way have not yet been observed in other plants. Immediately after the
appearance of the receptive spot, the union of the antherozoid with the substance of
the oosphere begins, a vigorous contraction of the latter and the slow gliding in of the
former being very clearly observable. The contraction of the oosphere is so vigorous
that its change of form thus produced induces at the same time a change of form in
the oogonium also.

' The slipping in of the antherozoid through the narrow opening of the neck
of the oogonium, and its union with the female sexual cell remind the observer
involimtarily and in the most forcible manner of those phenomena which have been
observed in the conjugation of Spirogyra, for instance ; since here, as there, we see
that the fertilizing sexual cell, in order to be able to unite with the one to be
fertilised must make its way through an opening which is disproportionately narrow
in relation to its size, and accommodate its form and size to this opening — here as
there contraction of the sexual cells takes place before and during their union, and
just as clearly also, on account of the striking difference in colour and size of the
antherozoids their fusion with the substance of the oosphere may be followed in the
clearest manner.'

Finally may be given the description afforded by De Bary and Strasburger
{bot. Zeil. 1877, P- 74^) of ^'^ behaviour of the sexual swarm-spores or gametes of
Acetabularia, a non-cellular marine Alga : —

'In spite of the fact that the drops used for observation appeared to form a
thoroughly efficient medium for the development and movement of the swarmers,
I nevertheless saw that at first all the swarmers perished, without one of them having
germinated. In doing this the swarmers rounded themselves off, formed vacuoles in
their interior, their chlorophyll-grains became disorganised and emptied of starch-
grains, and soon the whole resembled an amoeba of indefinite outline, which
finally resolved itself into a granular spot.

' The movement of the swarmers in this case continued for twenty-four hours
in the most favourable cases, and I saw swarmers which remained in the
interior of unopened spores often still moving even after forty-eight hours. As
a rule a tendency to disorganisation began after a few hours, however, in some
cases after a few minutes,

' The circumstance that I now also saw two spores, which from the appear-
ance of their vesicles had probably opened simultaneously, suggested to me that
the copulation only takes place between swarmers of different origin. Several
weeks then passed till the fortunate accident which showed me what is now known.
It happened eventually on a very sunny and warm day that numerous spores,
favoured by previous warm weather, emitted their contents.

' I saw about mid-day two neighbouring spores, utterly indistinguishable from
one another, open under my eyes, and the swarmers of both hurry straight to the

DE BARY ON FERTILISATION. ^77

margin of the drop next the window. Here there soon presented itself a sight
quite different from the ordinary one.

' For while I saw the swarmers from one and the same spore evidendy evade
one another and distribute themselves at about equal distances apart, there were
soon formed numerous copulating groups, if I may so say, i. e. collective groups
into which the individual swarmers, so to speak, precipitated themselves head-
long. I now saw continually new pairs of united swarmers leaving these
copulation-centres. Frequently, also more than two were anchored together. The
swarmers as a rule abut on one another with their anterior ends, but at once lay
themselves together sideways, and then fusion follows. It begins at or near the
apex, and soon extends over the whole side. The cilia remain free and active
meanwhile, so that the copulating swarmers go on swarming widi the four cilia,
their movement being particularly tumultuous. The swarmers in the pre-
viously mentioned cases are directed parallel, but in other and by no means
rare cases they are seen to be fused laterally, so that their ciliated ends are turned
away from one another. The union may also first take place at the hinder
portions of the swarmers, so that they diverge from one another at their anterior
ends. Finally, I also saw cases where they were united in a cross-like manner.
This however seemed to me to exhaust the variety of cases of pairing.

' As already mentioned, more than two swarmers may exceptionally fuse
with one another. The simplest case here again was that they laid themselves
together parallel to one another. They then worked on with six cilia, going
forward like simple swarmers. I saw also complexes in which two swarmers
took up one direction, and the third the opposite one, and finally also such
which owed their origin to a large and often indefinite number of swarmers. I
could see from an approximately cuboidal mass a large number of colourless
spots project, each moving its pair of cilia. The whole showed an irregular
rotating movement.

' The red streak of the swarmers may assume any position during the
copulation.

' After swarming for some time, in all cases longer than in the case of those
swarmers which remain free, each copulation-product rounds itself oft". At first
it is possible still to recognise the colourless spots of the copulated swarmers in
the complex, and even the cilia may be still retained on the sphere. The colourless
spots and cilia then disappear, however, and we have a sphere coloured green
with chlorophyll, in which a corresponding number of red streaks are to be
distinguished.'

Sdll more remarkable and astounding than the above statements, which I
have expressly taken verbatim from the original memoir, are perhaps De Bary's
most recent statements in his ' Untersuchungen über die Perotiosporeen und Sapro-
legnieen' (1881), where he devotes a special chapter to the fact, established by
himself, that in these Fungi the oogonia are usually the first to develope, the
male fertilising tubes not having been present meanwhile : these latter however
are developed during a certain stage of development of the oogonium, and ex-
clusively in its neighbourhood, either from a branch which belongs to the same
shoot as the oogonium, or else on other tubes which are not at all connected

778 LECTURE XLUT.

with the oogonium genetically, but lie sufficiently near it to be influenced by it.
Of course the distance is not great, and De Bary states that it amounts to about
the diameter of an oogonium. In other cases again, the young oogonia exert a
modifying effect on the growth of such tubes on which male organs (antheridia) may
be formed. ' As soon as a vigorously growing lateral branch of this kind,' says De
Bary, ' attains a certain distance from a young oogonium, its end is seen to bend
over towards it, and then develope into an incipient antheridium.' He then con-
tinues, ' the divergence of the lateral branch described cannot be referred to any
other cause than special properties of the oogonium.'

To my mind there can scarcely be a doubt that the pollen-tube of the
Phanerogams also is specially influenced by something which induces it to
grow with its end containing the fertilising substance directly towards the
opening of the very narrow micropyle, to penetrate eventually to the oosphere.
It is remarkable that this matter has as yet scarcely been thought worthy of
investigation. When one reflects how extremely inaccessible the micropyle of the
ovule in the cavity of the ovary usually is, how narrow it is, how great the distance
often is which the pollen-tube must traverse from the stigma through a long
style down to the ovary, and when one further considers that for the fertilisation
of any ovary which contains only one ovule, even a very few pollen-grains on the
stigma ensure that a pollen-tube reaches the micropyle (e. g. this is conspicuously
the case with Mirabilis longiflord), further, that on the stigma being sufficiently
pollinated, often hundreds and thousands of ovules in an ovary each receives its
pollen-tube, and when one further observes how the pollen-tubes of some Orchids
grow down free through the cavity of the ovary to the ovules, and so on, it must
be obvious that the entrance of the pollen- lubes into the micropyles can by
no means be a matter of chance, but that definite arrangements must exist
which lead the growing end of the pollen-tube to its destination. It is true the
tissue of the stigma and of the style are especially suited to at least offer no
hindrance to the pollen-tubes on their way to the cavity of the ovary ; moreover,
in many cases special relations of organization exist on the inner walls of
the ovary, which evidently have the purpose to show the pollen-tube the. way, so
to speak, which conducts it to a micropyle. But why do the pollen-tubes when they
germinate on the surface of the moist stigma, grow directly into its tissue ; why do
they bend from the stigmadc lobes into the conducting tissue of the style; and why
do they follow in the ovary the indicated road-marks where space and opportunity
exist for deviations, and so on ? It appears to rie that in addition to the visible
coarser relations of organisation referred to, invisible arrangements and unknown
forces exist which chiefly determine that the pollen-tubes find their way from the
stigma to the micropyle.

QyYvA^^

LECTURE XLIV.

TRANSMISSION AND BLENDING OF THE PATERNAL AND
MATERNAL PROPERTIES BY FERTILISATION'.

(HYBRIDS.)

As a rule the sexual union takes place between two cells which originate either
from the same mother-plant, or from two plants of the same kind (species or variety) ;
each of the two sexual cells thus transmits to the embryo properties of its own kind,
and it cannot be forthwith determined which of these properties the male and which
the female element carries into the new plant. The case is quite otherwise when the
fertilisation takes place between two different species of plants. In this case the male
reproductive cell contributes to the union different properties from the oosphere, and
whenever such a union is possible between sexual cells of different species, if an
embryo capable of development results therefrom, it must be investigated in what way
the various properties derived from the different parents are transmitted to the de-
scendants and combine with one another. It must be investigated whether any and
what properties in the descendant are derived from that plant which provided the male
fertilising element, and which of them have been transmitted to it through the female
cell. Experiments show that plants of different species can be successfully com-
bined sexually; such a union is termed Hybridisation or Bastard-formation, their
product being the Hybrid or Bastard; according as different varieties of a species,

1 The first plant-hybrids were produced and carefully described by Christian Gottlieb Kölreuter.
He was engaged for a very long period with this subject, and this so profoundly that subsequent
investigators were practically unable to add anything essential. His chief work bears the title ' Vor-
läufige Nach?-icht von einigen das Geschlecht der Ffiaiizcn betreffenden Versuchen u>id Beobachtun-
gen'' (Leipzig, 1761, and continuations, 1763, 1764, and 1766).

The most prominent of the subsequent works are : —

William Herbert, ' Amaryllidacea;, preceded by &c., and followed by a treatise of cross-bred
vegetables' (London, 1837).

Gärtner, 'Versuche und Beobachtungen über die ßastarderzeugung im Pfianzen reich ' (Stuttgart,
1849). _ *

Wichura, 'Die Bastardbefruchtung im Pflanzenreich, erläutert an den Bastarden der JVeiden'
(2 tables in Nature-print, Breslau, 1865).

Naegeli in ' Sitzungsber. d. kgl. bay. Ak. der ll'/ss.' (München, 1865, 15 Dec, and 1866,
13 Jan.).

Charles Darwin, • Kesults 0/ Cross- and Seif J-'ertilisalion in the Vegetable A'ingdonV ^London,
1879).

780 LECTURE XLIV.

different species of a genus, or two species from different genera have united
sexually, the resultant hybrid product may be designated as a variety-hybrid, a
species-hybrid, or a genus-hybrid.

Of Cryptograms only a few hybrids are known with certainty. Thuret {Ann. des
Sc.Ä^zt. 1885) obtained hybrid embryos by mixing the oospheres oi Fiicus vesiadostis
with the zoosperms of Fucus serrahis. In a few other groups of Cryptogams forms
have been found which from their properties are referred to a hybrid origin. Thus
A. Braun ( Verjüngu7tg,^.'^2g) mentions hybrids of the Mosses Physcomitriiim pyriforme
with Funana hygromeirica, and Physconiitriuni fasciculare with Funaria hygromeln'ca,
also Fern hybrids of Gym7wgramme chrysophylla and Gynuiogramme cahmelcEna,
Gynuiogramine chrysophylla with G. dislans, and of Aspidiiiin filix-mas with A.
spiniilosum.

The hybrids of Phanerogams obtained by artificial transference of pollen are
however preferably valuable for scientific considerations regarding hybridisation, which
at the same time render clearer the meaning of sexuality generally. Naegeli (i) has
collected the results of many thousands of hybridisations, made by Kölreuter in the
last century, and later by Knight, Gärtner, Herbert, Wichura and other observers.
From this critical survey of Naegeli's I take the following statements as examples.

(i) Only such plants as are systematically nearly allied can form hybrids with
one another. Hybridisation is effected most easily and completely as a rule between
different varieties of the same species ; the production of hybrids is more difiicult,
though in many cases possible, between two different species of the same genus.
Only a few cases are known of hybrids between species which are placed in different
genera, and it is probable that such species, one of which successfully fertilises the
other, should be placed together in the same genus. The ability of species to form
hybrids exists moreover to very different degrees in different orders, families, and genera
of Angiosperms. The following are as a rule favorable to hybridisation : the Liliacece
Irideoe, Nyctaginese, Lobeliacese, Solanacese, Scrophularinese, Gesneracese, Primulacese,
Ericaceae, Ranunculacese, Passiflorese, Cactacese, Caryophyllaceae, Malvaceae, Gera-
niaceae, (Enotherese, Rosacese, and Salices. Hybridisation of species succeeds not
at all, or only exceptionally, among the Gramineae, Urticaceae, Labiatae, Convolvu-
laceae, Polemoniaceae, Ribesiaceae, Papaveraceae, Cruciferae, Hypericineae, Papilio-
naceae. Moreover the genera of the same order or family behave differently.
Among the Caryophyllaceae the species of Dianihus may be easily hybridised, those
oi Silene\M\\}a difficulty; among the Solanaceae the species oi Nicotiana andof Ä7/«ra
are prone to hybridise, but not those of Solanum, Physalis and Nycandra ; and among
the Scrophularineae the species of Verbasciim and Digitalis, but not those of Pcntas-
temon, Linaria, and Antirrhinum ; and among Rosaceae the species of Gcum, but
not of Potentilla.

Hybridisation between different genera has been observed between Lychnis and
Silcne, Rhododefidron and Azalea, Rhododendron and Rhodora, Azalea and Rhodora,
Rhododendron and Kalmia, Rhododendron and Menziesia, Aegilops and Tritician,
Echinocactiis, Cereus, and Phyllocactus, besides a few wild forms which are apparently
to be explained as genus-hybrids.

(2) Besides the close systematic relationship, a certain relation of the plants con-
cerned to one another in addition decides as to the possibility of the formation of

SEXUAL AFFINITY. 78 1

hybrids: this expresses itself only in the success of hybridisation, and may be de-
signated, according to Naegeli, sexual affinity. The sexual affinity does not always
go parallel with the external similarity of the plants ; for instance, no one has yet
succeeded in producing hybrids of Apple and Pear, of Anagal/is arvensis and A.
cartdea, of Primula officinalis and P. elatwr, of Nigella damascena and N. saliva, and
other systematically very similar species of the same genus, while in other cases very
dissimilar forms unite; for example, Acgilops ovala with Triticum vulgare, Lychnis
diurna withL.ßos cuculi,Cereus speciosissimus and Phyllocacius Phyllanthus , Nectarine
and Almond. The difference of sexual affinity and systematic alliance is still more
strikingly demonstrated by the fact that sometimes the varieties of the same species
are mutually sterile in whole or part, e. g. Silcne inflata, var. alpina with var. angusii-
folia ; var. lalifolia with var. litoralis ; &c.

(3) When a sexual union of two species A and B is possible, it usually happens that
A can yield hybrids with pollen from B, as well as B with pollen from A (reciprocal
hybridisation); there are also cases however where the species A can only be the father,
and the species B only the mother, the pollination oi A with the pollen of -5 remaining
without result. Thus Thuret found that, as already stated, oospheres oi Fucus vesicu-
losus yield hybrids with the zoosperms of F. serratus, whereas the mixture of the
oospheres of F. serratus with the zoosperms of F. vesiculosus remains without
result. According to Q'-kxintx NicotiaJia paniculata is very prone to form h\'brid seeds
with the pollen of N. Langsdorfii, whereas N. Langsdorfii forms no seed with the
pollen of N. paniculaia. Kölreuter was able to obtain seeds easily from Mirabilis
Jalappa with the pollen of M. longiflora, but more than two hundred pollinations of
M. longiflora by M. Jalappa during eight years remained without result.

(4) The sexual affinity presents the most various degrees. The one extreme
lies in the complete failure of pollination with the pollen of another variety or species,
so that the pollen-tubes do not even enter the stigma, and the pollinated flower be-
haves as if no pollen had reached it : the other extreme shows itself in the formation
of numerous hybrids which not only develope vigorously, but also reproduce themselves
sexually. Between the two extremes the most various gradations and transitions
occur. The lowest stages of the influence of pollen of another kind consist in that
various alterations take place only in the floral parts of the mother-plant itself, the
ovaries or these and the ovules growing, but without an embryo being developed. A
higher stage of the influence evinces itself in the development of ripe normal fruits
with seeds containing embryos, but the embryos are incapable of germinating; a
further advance then appears with reference to the number of ripe embryos capable
of development in the pollinated ovaries (cf. Hildebrand, ' Bastardirungsversuchc an
Orchideen! — Bot. Zeit. 1865, No. 31).

(5) When different kinds of pollen are simultaneously transferred to the same
stigma, only one kind effects fertilisation, that to which the greatest sexual affinity
may be ascribed. Now since in general pollen acts most favourably on the fertilisa-
tion of another flower of the same species — in other words, since the sexual affinity
attains a maximum between the flowers or individuals of the same species — when
pollination occurs simultaneously with pollen of the same and of another species, only
the former is eff'ective in fertilisation ; since on the other hand hybridisation between
varieties is sometimes more effectual than the fertilisation of a variety by means of its

7 83 LECTURE XLIV.

own pollen, in this case the pollen of the other kind may exclude that of its own kind
from the fertilisation. If various kinds of pollen come on to a stigma at different
limes, and if the later comer is of greater sexual aflfinity, it can only then be still
effectual in fertilisation if that which first entered has not yet effected fertilisation, or
injury ; in Nicotiana hybridisation can no longer be prevented by its own pollen
after two hours, in JSlalva and Hibiscus after three hours, and in Dianihus after five
or six hours.

(6) The hybrid stands, according to its systematic characters, somewhere between
the dift'erent ancestral forms. For the most part it maintains the medium ; more
rarely it resembles one of the two parent forms more than the other, this being more con-
spicuous in the case of hybrids between varieties than with those between species.
Hence it follows that in the case of reciprocal hybrids of species A and B, the hybrid
yl .5 is in general externally similar to the hybrid B A. Nevertheless both may ex-
hibit certain internal differences. Thus, according to Gärtner, the hybrid Nicotiana
panicidato-rustica is more fertile than the reciprocal hybrid A"", rustico-panicidata. An
internal difference between reciprocal hybrids also expresses itself in that the one is
more variable than the other ; thus, according to Gartner, the progeny of Digitalis
piirpureo-hdea are more variable than those of D. lutco-purpurea, and those of Dian-
thus pidchello-arenarius more variable than those of D. arenario-ptdchellus.

When two species A and B form hybrids, and the one species A exerts a greater
influence on the form and properties of the hybrid than the other species B^ the hybrid,
when it and its descendants are fertilised by A, must be transferred into the parent-
form A more rapidly than it passes over into the parent-form B by fertilisation with
B. Thus, according to Gärtner, the hybrid of Dianthus chinensis and D. caryophyllus
when repeatedly fertilised by the latter, was transferred into D. caryophyllus after three
or four generations, whereas fertilisation with D. chineusis yielded descendants of the
form of D. chinensis only after five or six generations.

(7) The characters of the parent-forms are as a rule transmitted to the hybrid
in such away that in each character the influence of both parents is evident: a mutual
fusion of the various characters occurs. In the case of species-hybrids this is more
decidedly expressed than with hybrids between varieties. In the latter certain un-
essential characters of the ancestors occasionally occur separated next one another ;
for instance, various kinds of streaks and spots instead of a corresponding mixture of
colours. A hybrid which Sageret derived from Cucumis Chate (female) with C. Melo
Cantalupus (which possessed a reticulate peel) produced fruit with yellow flesh,
reticulate markings on the peel, and fairly strong ribs like the father, but white seeds
and sour taste like the mother ; another hybrid of these two species had on the
contrary fruit with sweet taste and yellow flesh, like the father, but white seeds
and smooth peel, like the mother. To this category belongs also the hybrid
between Cytisus Laburnum and C. purpureus, the branches of which resemble, wholly
or in part, sometimes the one, sometimes the other parent-form. I found what was
very probably a hybrid o^ Antirrhinurn majus, the inflorescence of which bore on the
one side of the axis only uniformly dark red, on the other side yellow flowers ; between
the two halves of the inflorescence stood one flower which was coloured half red and
half yellow.

(8) Besides the inherited peculiarities, the h}l)iid usually possesses new characters

PECULIARITIES OF HYBRIDS. 783

in addition, which distinguish it from both of its parent-forms. One new property of
the hybrid, particularly of the variety-hybrid, for example, is the tendency to vary more
than the parent-form does : species-hybrids are usually sexually enfeebled, but those
of closely allied species are often more vigorous in their growth than the two parent-
forms, whereas the hybrids of more remote species are more feebly developed. The
luxuriant growth of hybrids between closely-allied species expresses itself in the de-
velopment of more numerous and larger leaves, higher and stronger stems, richer
root-systems, and more numerous shoots (stolons, layers), &c. Hybrids also have a
tendency to greater longevity, perennial hybrids arising from annual and biennial
parent-forms, but this is probably in consequence of the usually scanty seed-formation.
Moreover hybrids are noted for commencing to flower sooner, and for doing this
longer and more copiously than the parent-forms ; they sometimes produce extra-
ordinarily large numbers of flowers, which are moreover larger, better scented, more
intense in colour, and more persistent. The flowers of hybrids have a tendency to
become double, multiplying their carpellary and staminal leaves and developing them
as petals. In addition to this luxuriant growth the sexuality is mostly enfeebled, and
this in the most various stages. 'The stamens are in some cases completely developed
to all appearance, but are wholly or partly barren, since the pollen-grains do not attain
the normal perfection ; in other cases all the stamens are aborted and reduced to
minute rudiments. The pistil (carpels, ovaries) of hybrids cannot as a rule be
distinguished from those of the ancestral species, but their ovules are either incapable
of conception, or are only slightly capable of it ; no oospheres are formed, or the
embryo which begins to be developed from the oosphere perishes sooner or later.
In the most favourable cases, when seeds capable of germination are developed, they
exist in smaller quantities, and evince in their slow germination and in the short
period during which they maintain the power of germinating, a certain feebleness '
(Naegeli). The enfeeblement of the sexuality is in some variety-hybrids hardly
noticeable, in others slight ; it increases as a rule in proportion with the remoteness of
the systematic relationship and sexual affinity of the ancestors. If the species-hybrids
are able to form seeds by self-pollination, the fertility usually decreases, on continued
self-pollination, from generation to generation, a phenomenon which perhaps depends
less on the sexual feebleness of the hybrids than on the circumstance that probably
the flowers of the hybrids have often been self-fertilised instead of being crossed
with other flowers or with other individuals of the same hybrids. According to
Naegeli, the rule may in general be applied that the male organs of species-
hybrids are enfeebled to a greater extent than the female ; nevertheless there are
exceptions.

(9) ' Hybrids generally vary in the first generation the less, the more remote in
relationship the ancestral forms are from one another ; and thus species-hybrids less
than variety-hybrids, the former often being distinguished by great uniformity, the
latter by great variability. If the hybrids are self-fertilised the variability increases
in the second and following generations the more, the more completely it was
wanting in the first ; and three diff"ercnt varieties appear the more certainly, the
more remote the parent-forms are from one another — one, which agrees with the
original type, and two others which more resemble the parent-forms. These varieties
have, however, at least in the earlier generations, little constancy, and are not trans-

784 LECTURE XLIV.

formed into one another; an actual reversion to one of the two parent-forms
(the breeding being purely in and in) occurs chiefly when the parent-forms are very
nearly related, as in the case of hybrids between varieties and species which resemble
the varieties. When it occurs with other species-hybrids it appears to be confined
to those cases where one species has exerted a dominant influence in the hybrid
fertilisation ' (Naegeli).

(10) If a hybrid is sexually united with one of its parent-forms, or with another
parent-form, or with a hybrid of different descent, a dcrivalive hybrid arises, which in its
turn again can be united with one of the parent-forms or with hybrids of other descent.
If the union of a hybrid with one of its parent-forms is accomplished, and if the
derivative hybrid thus obtained is again united with the same parent-form, and this
continued through several generations, the derived descendants acquire more and more
the peculiarities of the one parent-form, and at last completely resemble this, the
derivative hybrid reverting to the parent-form employed in the process. According
as the one or the other of the two parent-forms is used, more or fewer genera-
tions are necessary for the derivative hybrid to become like the one parent-form;
from this behaviour Naegeli has deduced certain numerical expressions (formulae of
inheritance) which give in figures the amount of the influence of one species with
respect to the inheritance of the properties in hybridisation. In proportion as the
derivative hybrid approximates to the one parent-form its hybrid nature diminishes
more and more, and its fertility especially is increased.

If a hybrid is sexually united with a new parent-form or with a hybrid of another
species, there arises a derivative hybrid in which three, four, or more species (or
varieties) are fused; Wichura has combined even six different species of Willow into a
derivative hybrid. Such hybrids, which may probably be better designated compound
hybrids, follow, with respect to their form and other relations, in general the rules
given for the simplest hybrids ; the compound hybrids are so much the more sterile
the more diff"erent parent-forms are united in them, and they are also generally very
variable. Wichura showed from his own and Gartner's observations that the progeny
from the pollen of the hybrids are more variable (more multi-formed) than those from
the pollen of true species.

In the preceding, hybridisation has only been regarded from its theoretical
side : that it has practical value also is shown by the innumerable hybrids of
cultivated flowering-plants which have long adorned our gardens, and which are
increased every year by new forms. Perhaps the most important of all hybrid plants
practically however are the hybrid forms of the Vine ; except the W" illows {Salix)
there is scarcely any other genus of plants whose species are so easily crossed as
those of the genus Vitis. The Vine cultivated in Europe, Vitis znnifera, comes from
western Asia: numerous other species grow wild in N. America. Since the time when
the Vine-insect {Phylloxera) began to devastate the vineyards, especially in France, the
American vines and their hybrids, together and with our old cultivated species, have
obtained great practical importance, because several of them withstand the attacks of
the Vine-insect, and present to the Vine-growers a means for further culture. I\Iy
friend A. INIillardet, Professor of Botany in Bordeaux, has been engaged for ten years
with the study of the devastation caused by the Vine-insect in the French vinejards,
and has especially investigated the question of the resistance of the American species

HYBRID VINES. 785

of Vines and their hybrids, and has at my request furnished me with the following
remarks on hybridisation within the genus Viiis.

' The genus Vi'/i's is one of those of which the sexual intcr-crossing of species is
very extensive and clearly established.

' In N.America the following species of Vine are found : — Vifi's rupestris, eastward
of the Mississippi, and along the banks of the INIissouri as far as Texas : V. ripan'a over
nearly the whole region of N. America from Canada to Texas, and from the Rocky
INIountains to the Atlantic Ocean : V. cordifolia in the centre and south of the United
States, and in Texas : V. monticola in Texas and New INIexico : V. cinerea ( V. aestivalis,
var. cinerea) from Missouri to Texas : V. aestivalis in the centre and south of the United
States : V. Lincecuiiiii, in the south of the Union and Texas : V. labrusca, in the east
of the Union from the Alleghanies to the Atlantic Ocean : V. cafidicans, in the south
of the United States and Texas : V. caribcea in Florida (?) : V. cali/ornica in
California, and V. arizonica in Arizona : V. rotimdifolia in the south of United States.

' A careful study of the wild Vines which are annually imported in millions into
France from over the whole Union (especially from Missouri and Texas) leads me
to the conviction that all these species (except the last four, which I have not hitherto
been able to examine with the necessary care) can be crossed (hybridised) with one
another, and this in the most capricious manner. I have so far become acquainted with
the following crosses: — V. riparia with V. rupcstris (INIissouri?) — V. riparia with
V. candicans (Iowa) — V. rtipestris with V. candicans (Texas) — V. cordifolia with
V. candicans (Indian territory) — V. cordifolia with V. rtipestris (ditto) — V. cordifolia
with V. aestivalis (Missouri) — V. cordifolia with V. cinerea (Missouri) — V. aestivalis
and V. cinerea (ditto) — V. aestivalis and V. candicans (S. W. of Missouri).

' All these hybrids are binary : I have however also recognised the following
ternary ones — the variety ' Solonis ' (Arkansas ?) as a hybrid of V. riparia, V. riipestris
and V. candicans ; V. aestivalis with large berries, which Hermann Jäger found in
the wild state in S. W. Missouri and the neighbouring districts. In some of the hybrids
of V. aestivalis and V. candicans. there is also a strain of V. Lincecumii.

' These are the results of my (Millardet's) investigations on the Vines growing
wild in the United States. The study of the Vines cultivated in the vineyards of the
same country has lead to similar results. Perhaps no one sort of those I have
observed is a pure descendant of a wild species, in spite of all that the growers, ampelo-
graphists and botanists may say: all are products of more or less complicated
crossings, in which the European Vine (the culture of which has been attempted at
various times in the Union) has often co-operated. As examples I will mention
only the most important of these spontaneous wild hybrids.

' The so-called ' Clincton ' is a hybrid of V. riparia and V. lahrusca, and
' Taylor,' 'Elvira,' ' Noah,' ' Franklin,' are of the same composition. ' York Madeira,'
is a hybrid of V. labrusca and V. aestivalis ; and the same is the case with ' Eumelan,'
' Alvey,' ' Morton's Virginia,' ' Cynthiana,' &c. ' Delavare ' is a hybrid of V. labrusca,
V. vinifera, and V. aestivalis. ' Jaquez ' is a hybrid of V. aestivalis, V. vinifera and
V. cinerea (?). ' Cunningham,' ' Rulander,' and ' Herbcmont,' are hybrids of
V. aestivalis, V. cinerea, and V. vinifera. Finally, the Vine now called * Gaston-
Bazille ' (the American name is lost) presents a still more complex composition ; it is
a hybrid of V. labrusca, V. aestivalis, V. rupestris, and V. riparia.

[3] 3E

786 LECTURE XLIV.

' These facts, so interesting scientifically, have at the same time considerable
practical importance, as is seen from what follows : —

' Experiments have shown me that (apart from V. labrusca, V. Lincecumii and
V. candicans) all the species named above absolutely resist the Phylloxera, and that the
resistance of the hybrids depends upon their composition : for example, a hybrid of
V. ripan'a and V. rupestn's is capable of absolutely withstanding the insect, whereas
a hybrid of V. riparia and V. labrusca (' Clinton,' ' Taylor '), or of V. aestivalis or
V. cinerea and V. vinifera (' Jacquez,' ' Cunningham,' ' Rulander ') possesses a
diminished, or insufficient, resistance to the insect which no variety of V. vinifera
withstands.

' Another fact well worth attention is the following : — All the species of United
States Vines (except V. califoniica and V. arizonica) are adapted to a climate which
is much wetter than the wettest that is to be found in Europe ; they are consequently
much more resistent against all attacks of Fungi — Oidium, ' Anthracose,' INIildew
{Peronospora viticold) — than our European varieties of Vitis vinifera. It is found
moreover that those species which are most resistent against these evils are at the same
time those which thoroughly withstand the Phylloxera ( V. riparia, V. rupeslris,
V. cordifolia, V. cinerea, Sec).

' Starting from this experience I (Millardet) was the first to make the suggestion
to employ the hybridisation of our European Vine {V. vinifera) with various
American ones, as a protection both against the Phylloxera and the fungoid diseases
referred to. All these hybrids withstand to a certain extent both the Phylloxera and
the injurious Fungi. It only becomes a matter of seeking out the best and most
resistent ones. Only by this means will the Vine-culture again become possible in
districts with a damp summer climate, where the Peronospora viticola makes its
ravages, as in the valleys of the Garonne, the west coast of Portugal, and in various
places in Italy, Algeria, and Switzerland. The numerous investigations in this
direction which I have made for two and a half years establish provisionally that it is
possible to confer upon our Vineyards the power of withstanding all the above evils,
even Phylloxera not excepted. I now possess more than two hundred new hybrids
which afford conclusive evidence of this.

' It yet remains to be seen however what will be the quality of the fruit ; but even
in this direction I have great hopes of attaining a satisfactory result, thanks to the
localisation of the hybrid characters in the case of the Vine. I am convinced, for
example, that among a certain number of hybrids of ' Chasselas ' and Vitis riparia an
individual may be found with fruits like that of ' Chasselas,' with leaves like those of
V. riparia (resistent to Fungi) and with roots resembling those of the latter species
(withstanding Phylloxera). Analogous cases into which I cannot here go sufficiently
authorise me to make this assumption.

' To sum up, my investigations now warrant me in making two statements : —

(i) All varieties of the European Vine are capable of hybridisation with all the
American species of Vitis without exception. The complexity of these inter-crossings
may probably be very great, since it is just as easy to produce quaternary as binary
hybrids.

(2) Even from the first generation onwards it is possible to obtain hybrids which
are endowed with great power of resisting Phylloxera and Fungi ' (Millardet).

LECTURE XLV.

INFLUENCE OF THE ORIGIN OF THE SEXUAL CELLS OF THE
SAME SPECIES ON THE RESULT OF FERTILISATIONS

The male and female cells, or the organs which produce them, arise either
close to one another or at a distance on the same plant, or they arise on
different individuals of the same species; the sexual cells of the same species of
plant may thus, according to their origin, be more or less closely related,
behaving towards one another as sister-cells, or as cousins, or as their grand-
children and great grand-children, and so on. The question now arises as to
what influence this relationship in the origin of the male and female cells exerts
on the result of fertilisation. At present it is true no general law can be
formulated in this connection, but by far the majority of the phenomena point
to the xiew Ihat flic sexual union of very closely related sexual cells is generally
avoided, and this the more the more advanced the morphological and sexual differ-
entiation. It is only in the case of a few of the lower plants that it happens
that the sexual cells which unite with fertility are sister-cells, e. g. in Rhynchonema
among the Conjugatoe; even in most of the other Algge and Fungi the sexual
cells of the same plant are more distantly related {Spirogyra, (Edogonium, Fucus

* The remarkable relations of insects to the fertilisation of flowers treated in this lecture were
first described at length by Christian Conrad Sprengel in his extremely remarkable and inspired
work, ^Das neu eiitdeclctc Ge/iciniinss der N^atur im Bau uud in der Bcfnuhiung der Bluincn '
(with 25 copper plates, Berlin, 1793), after Jos. Gottlieb Koelreuter (1761) had already pointed out
in his 'Vorlaujigcn N^aclirichtcn, das Geschlecht der Pßanzcn betreffend,' the necessity of the aid of
insects in the pollination of many flowers. Sprengel indeed expressed the fruitful idea, ' since very
many flowers have separate sexes, and probably at least as many hermaphrodite flowers are dicho-
gamous, Nature appears to be unwilling that any flower shall be fertilised by means of its own pollen.'
Sprengel's work remained unnoticed until about twenty years ago ; it was rescued from obscurity by
Charks Darwin, and the doctrine extended by new observations of the latter and incorporated with
the theory of descent. Stimulated by Darwin's work on the fertilisation of the Orchids (,iS6j) and
by other works of the same author, an extensive literature on this subject has been developed, of
which I will only quote one or two titles.

Friedrich Hildebrand, 'Die Geschlcchtsvertheilung bei den PJlanzcn und das Gesetz der vermie-
denen und unvortheithaftcn stetigen Selbstbefruclitung' (Leipzig, 1^67).

The most exhaustive and fundamental work on the facts of this subject is Hermann Miiller's
comprehensive work, ^ Die Befruchtung der Blumen durch Inseiitcn und die gegenseitigen Ati-
passungen beider' (Leipzig, 1873). Translated into English by D'Arcy W. Thompson.

3 E 2

788 LECTURE XLV.

plafjrarpus, &c.), and in all cases in which fertilisation is accomplished by means
of active or passive motile zoosperms, at least the possibility is given of their
meeting with oospheres of more distant origin. Even in the case of Vaucheria,
where the antheridium is the sister-cell of the oogonium, the curvature of the
former and the direction in which the zoosperms are emitted, points to the fact
that fertilisation generally takes place not between the organs which stand next
one another, but between those more remote, or even between those of different
individuals.

The tendency to allow only sexual cells of the most different origin possible
to fertilise one another within the limits of the same species, is evinced by very
various arrangements. The simplest way is by only male or only female organs
being produced on each individual of the plant; thus the whole development of
the two plants concerned lies between the two sexual cells which come into
union, if they originate from the same parent-plant, and a still longer series of
developmental processes if the plants in question themselves originate from
different parent-plants. Now this distribution of the sexes, which we may designate
generally as dicecious, is found in all classes and orders of the vegetable kingdom,
and this extension of it alone indicates that it is an adaptation useful for the
maintenance of the most different species; thus we find dioecism in many Algae
(e. g. most Fucaceaj), in most Characese, many Muscinese, in the prothallia of
some Ferns, and most Equisetacese, and further in many Gymnosperms and
Angiosperms.

If the vegetative body which produces the sexual organs is itself large, or
at any rate much segmented, a distant relationship of the two kinds of sexual
cells is attained by male cells being produced on different branches from the
female ones; this case also, which may be denominated generally as 7noncccism,
is very widely distributed in the vegetable kingdom (some Algse, many Muscinese,
very many Gymnosperms and Angiosperms).

But even the apparently most unfavourable condition as regards the principle
laid down above, that the sexual cells arise close beside one another, is frequently
realised in the vegetable kingdom, the sexual cells being thus of close though not
always of the closest descent : thus the same cell-filament of the algal genus
(Edogonnmi produces male and female cells, the same vesicle of Vaucheria forms
antheridia and oogonia close together, the same receptacle ol Fucus platy carpus gives
rise to oospheres and zoosperms, the oogonium of most Characese arises close beside
the antheridium on the same leaf, the archegonia and antheridia of some Mosses
(species of Bryiim) are collected together in one ' flower,' and the prothallia of
many Ferns produce both kinds of sexual organs in close proximity to one
another. In the flowers of Angiosperms the androgynous sexual apparatus is
typical and very general. But in all these cases where the object is apparently
to promote the union of sexual cells which are closely allied, mechanisms exist
at the same time which prevent the male cells coming into contact with the
female ones produced close beside them, or at any rate care is taken that this
need not always occur, a fact which was first recognised by Kölreuter (1761)
and Conrad Sprengel (1793), and more recently extended by Darwin, Hildebrand
and others. It is just in the case of the hermaphrodite flowers and the similar

DICHOGAMV, UETEROSTYLISM, HERKOGAMF. 789

distributions of sexes in the Cryptogams, that it is most conspicuously shown that
the co-operation of the sexual cells of near relationship must be injurious for the
existence of most plants, since such various and often perfectly astounding means
are employed for avoiding fertilisation within a hermaphrodite sexual apparatus.

One of the commonest and simplest means is Dichogatny — i. e. the non-
simultaneous development of the two sexual organs in one and the same andro-
gynous sexual apparatus, so that the closely related sexual cells produced side
by side, compelled to exercise their functions at different times, and thus cannot
act together. The male cells must therefore unite with the female ones of
another androgynous sexual apparatus. This is very generally the case with
the flowers of Angiosperms, as well as with most Fern-prothallia and those
Characese which are not dioecious, where, although the oogonium arises close
beside the antheridium, it attains sexual maturity later than the latter (very
conspicuous for example in Nitdla flexilis).

In the case of the dichogamous flowers of Phanerogams, insects are employed
for the transference of the pollen on to the stigma of other flowers, and for this
purpose special mechanisms exist in the floral organs : we shall examine these
later on. In the dichogamous Nitellas and Fern-prothallia the movement of the
antherozoids suffices, the close growth of the plants promoting their access to
archegonia of neighbouring prothallia, or to the oogonia on other leaves of the
Nitella, or even of other plants. Whether dichogamy exists in the case of the
Algae above mentioned, and some Muscinese, is questionable, but at any rate the
possibility is given, by means of the motility of the antherozoids and other
conditions which here prevail, that they come in contact with the oospheres of
other plants or of other branches of the same plant.

In the Angiosperms however, in addition to the frequent dichogamy, quite
other arrangements also occur which have exclusively the object of accomplishing
the transference of the pollen of hermaphrodite plants with the aid of insects to
the stigma of other flowers, or even of the flowers of other plants. In most
Orchidese, Asclepiadeae, Viola, &c., tlie sexual organs of each individual flower
are developed simultaneously, it is true, but at the time of sexual maturity there
are mechanical arrangements which prevent the pollen coming on to the stigma
of the same flower {^Herkogamy) : it must be transferred to other flowers by
insects.

In other cases, as in Corydalis cava (pointed out by Hildebrand) the pollen
actually falls on the stigma of the same flower, but it is here without eftect, and
it is only effectual in fertilisation when it is transferred to the stigma of another
flower, and only completely so if it is carried to the flowers of another plant of
the same species. This plant, therefore, is only morphologically androgynous :
physiologically it is dioecious. The Orchid Oticidiiun microchiluin behaves
similarly, according to John Scott, in so far as the pollen placed on the stigma
of the same flower does not fertilise it,, while it is able to fertilise another indivi-
dual, and also that the female organ is fertilised by foreign pollen. Pollen and
stigma of the same flower are thus functionally capable, but only for the organs
of a foreign flower. Similar relations were observed by Gärtner in Lobelia
fulgois and Vcrbascum nigrum, and by Fritz Müller in Bignonias.

790 LECTURE XLV.

No less remarkable is Heferosfylism, in connection with the mutual fertili-
sation of different plants of the same species with androgynous flowers. The
individuals of the same plant in this case differ as regards their sexual organs :
the one individual forms exclusively flowers with a long style (elevated stigma),
and short filaments (depressed anthers), the other individual on the contrary,
flowers with depressed stigma and elevated anthers ; we have in this case, then,
in the same species of plant individuals with macrostylous and others with
microstylous flowers. Examples are Liniim pereime, Primula sinensis and other
Primulaceae. The case also occurs, however, as in many species of Oxalis and
in Lythrum Salicaria, that three degrees of length of the sexual organs are met
with in the flowers of three specimens of the same species ; in addition to those
with macrostylous and those with microstylous flowers, there is found also one
with mesostylous flowers. Now in these cases of heterostylism Darwin and
Hildebrand have demonstrated that fertilisation is only possible {Limnn perenne),
or at any rate only has the best result, when the pollen of the macrostylous
flowers is transferred to the microstylous stigma of another plant, and the pollen
of the microstylous flowers to the macrostylous stigma of another plant. Where
three lengths of the styles occur, fertilisation is most successful, according to the
same rule extended, when the pollen is transferred to that stigma which, in
another flower, stands at the same level as the anthers from which the pollen is
derived.

While in the numerous diclinous and dichogamous Phanerogams, and in those
to be mentioned below, insects carry the pollen of one flower to the stigma of
another, it occurs but relatively seldom that the pollination of one flower by another
is also accomplished without the aid of insects ; e. g. in some Urticaccce, such as
Pilea, and Morese, such as Broussonetia, where the anthers flying suddenly from the
bud scatter their light pollen in the air as delicate clouds of dust, which is blown
to the female organs of other flowers. Still more simple is the case of the Rye ;
the flowers in the ears of the Rye open singly, mostly in the morning, and
the rapidly elongating filaments thrust the mature anthers out of the glumes;
the anthers then hang down on the long filaments, and at once open and let the
heavy pollen fall. It falls on to the stigmas of flowers standing lower down
on the same spike or on neighbouring spikes, the swinging of the haulms
in the wind promoting the process. Rimepau has shown, moreover, that the
Rye is ' self-sterile,' that the individual flower can fertilise neither itself nor the
diff"erent flowers of an ear, nor can the diff"erent ears of one and the same plant
pollinate one another with success, although no mechanical hindrance to this
exists.

Considering the effort so clearly expressed among the Cryptogams, and still
more among the Phanerogams, to avoid fertilisation within the same bi-sexual apparatus
(self-fertilisation) it is a very striking fact that among the Angiosperms several
plants occur which form two kinds of androgynous flowers, namely, large ones which
are generally accessible to fertilisation by the pollen of other flowers, and small,
more or less abortive and occasionally subterranean (cleislogamous) flowers, which
never open, and the pollen of which sends its tubes directly from the anthers to
the stigma and fertilises the ovules ; we have here, therefore, on the same individual

hWSECTS AND FLOWERS. 791

plant, flowers of which the one kind are accessible to foreign polUnation, the
others exclusively to self-pollination. This is the case, for example in Oxah's
acetosella, where the small flowers concealed in the soil appear when the large
flowers are already ripening their fruits ; further in Impalieiis noli me längere,
Lamium amplexicaule, Specidaria perfoliata and many species of Viola ( V. odorala,
elaiior, canina, mirabilis, &c.), Ruellia dandesiina, in several Papilionacese
{Amphicarpcca, Voandzeia), Commelina bengalensis, &c. Where in these cases the
large typically developed flowers are fertile, crossings with other flowers of the
same species can and must occur at least occasionally in the course of the generation,
and then the small aborted self-fertilising flowers appear more as an accessory
arrangement, the purpose and importance of which is quite unknown; but it is
remarkable and apparently contradictory to the general rule that the large typical
flowers have occasionally a tendency to be barren (species of Viola) or are
entirely infertile (Voandzeia), so that the reproduction in such cases depends
chiefly or alone on the self-fertilising abnormal flowers.

In other cases, as in most Fumariacese, Cajina indica, Salvia hirta, Limim
usi/alissimiiin, Draha venia, Brassica Rapa, Oxalis inicrantlia and O. sensitiva, the
pollen, in virtue of the position of the sexual organs, comes (according to
Hildebrand) directly on to the stigma of the same flower, and is also eff'ective ;
but in such cases, since the flowers are visited by insects, an occasional crossing
with other flowers is at least not avoided. Even among the Orchidege, where
the most astonishing mechanisms for the avoidance of self-fertilisation occur,
the case is found in Ccphalanihera grandiflora, according to Darwin, that the
pollen-grains send their tubes out from the anther into the stigma; from
Darwin's experiments, however, the yield of good seeds is smaller when the
plants are left to this self-fertilisation alone, than when they are exposed to
crossing, to pollination with foreign pollen, by the aid of insects.

It comes out in the fertilisation of flowers more than in any other case, how
closely the structure of the organ is adapted to perfectly definite vital relations
of the plant, and to the fulfilment of perfectly definite functions. Each plant
has its own special mechanisms to ensure the transference of the pollen to the
stigma of another flower. It is thus impossible to say much of a general nature ;
but the following points may be noticed.

In the first place it is to be observed that the insects eff"ect the transference
of the pollen involuntarily and unknowingly, as they seek the nectar of the flowers
which is (for this purpose) secreted deep down in the base of the flower ; flowers
which are not visited by insects, and the Cryptogams which do not need them,
excrete no nectar.

The position of the nectaries, usually deep down in the base of the flower,
as well as the size, form, position, and often also the movements of the floral
organs during the period of pollination, are always so calculated that the insect,
and this often of a particular species, must assume a particular position and make
definite movements when seeking the nectar, so that the pollen-masses hang on
to its hairs, feet or proboscis, and it then, by assuming the same positions in
another flower is bound to wipe it off on to the stigma. In the case of dichoga-
mous flowers, the movements of the stamens and the style or stigmatic lobes

792 LECTURE XLV.

come in as additional aids. These movements are often of such a kind that at a
certain time the opened anthers assume the same position in the flower which
the stigmas in a receptive condition have at another time, so that the insect, by
the same movement, brings the same part of its body in the one flower into con-
tact with the opened anthers, in the other with the receptive stigmas, and
wipes off on to the latter the pollen which hangs on to that part of its body. The
same principle is also employed in the case of heterostylous flowers, in so far that
here the pollination is most successful when anthers and stigmas which occupy
the same (permanent) position in the different flowers, co-operate with the aid of
insects.

In addition, however, there are most manifold, often perfectly astounding
mechanical adaptations to ensure the transference of the pollen by the aid of
insects. A few examples may now be cited in illustration.

(i) Dichogamous plants are either protandrous or protogynous. In the
former the stamens are developed first, and their anthers dehisce at a time when
the stigmas are still undeveloped and not yet receptive for pollination : the
stigmatic surfaces do not open until later, mostly not until the pollen from the
anthers of the same flower has been borne av/ay by insects, so that they can
only then be pollinated by the pollen of younger flowers. This is true of species
of Geranium, Pelargonium, Epilohium, Malva, the Umbelliferse, Compositse,
Campanulaceas, Lobeliacese, Digitalis and others. The observation of these
points, and also of the movements of stamens and stigmas mentioned above, is
so easily accomplished in these cases, e. g. Geranium, AltJma, that a detailed
description scarcely appears necessary. In the case of protogynous dichogamous
flowers, the stigma is receptive at a time when the anthers of the same flower
are not yet ripe ; when these dehisce later, and allow the pollen to escape, the
stigma has been already pollinated by pollen from a distance, or has even already
withered and fallen off (e. g. Parieiaria diffusa). The pollen of this flower can
therefore only be employed for younger flowers ; this is the case in Scrophularia
nodosa, Mandragora vernalis, Scopolia atropoidcs, Plantago media, Luzula pilosa,
Anthoxanthum odoratum, and others (Hildebrand). Among protogynous dicho-
gamous flowers Aristolochia Clematitis is distinguished by specially conspicuous
and peculiar adaptations.

Fig. 451^ shows a young flower in longitudinal section; the stigmatic
surface n is just ready for fertilisation, but the anthers are still closed. A small
fly /, which has brought on its back a heap of pollen from an older flower, has
just penetrated through the narrow throat of the flower, and is roaming about
in the flask-like enlargement k. Not uncommonly six to ten such flies may be found
in a flower : they are imprisoned and cannot escape, because the throat r of the
flower is beset Hke a trap with long motile hairs, which offer no hindrance to
the entrance of the flies, but bar the passage out as in a weir-basket. While the
insect is thus wandering around the cavity, it brings its pollen-laden back in con-
tact with the stigmatic surface, and pollinates it, in consequence of which the
stigmatic lobes curve upwards, as in Fig. 451 B 71. As soon as this has taken
place, the anthers, which have been closed hitherto, dehisce, and become freely
accessible, at the same time, by the change in the stigma, and by the collapse

POLLINATION OF ARISTOLOCHIA.

793

of the hairs at the base of the enlargement, which now widens; the flies,
having deposited on the stigmatic surface the pollen which they brought with
them, can now therefore creep under to the open anthers where the pollen of
the latter becomes attached to them. About this time, moreover, the throat r of
the flower has also become passable from within ; the trap-like hairs in it having
perished and dried up as a result of the pollination of the stigma. The insect,
laden with the pollen of this flower, can now escape, and, in spite of its late
experience, it again forces its way into a younger flower, there to give up to the

FIG. ^^o.—Aristolochta Clctiiatitis. Portion of stem st, with
petiole *, in the axil of which are flowers of various ajfes. i, i young
and as yet unfertilised flowers ; 2.2 flowers which have been fertilised
and bent downwards ; k flask-like enlargement of the fluwer-tube r ;
ythe inferior ovaries (nat. size).

Fig. 451. — Aristolochia CUmatttis. Flowers in loH'
gitudinal section ; A before, and ß after fertilisation
(enlarged— see text).

still receptive stigma the pollen it has brought with it. While the above changes
are going on in the interior of the flower, the latter, moreover, alters its position.
So long as the stigma in the young flower is still receptive, the pedicel is erect,
and the perianth open outwards (Fig. 450 i, i,), presenting to the flies a hospitably
open door; but as soon as they have accomplished the j)ollination of the stigma,
the pedicel bends sharply downwards at the base of the ovary, and when the
flies, again laden with pollen, have flown away from the flower, the banner-Hke

794

LECTURE XLV.

lobe of the corolla {B, Fig. 451) closes over the mouth of the throat, stopping
the entrance to the flies, which have now nothing more to do here.

(2) Flowers with simultaneous dehiscence of stigmas and anthers, but in
which self-pollination is rendered impossible or difficult by the position of the
organs, and by mechanical obstructions. The transference of the pollen on to
the stigma is here also usually dependent upon insects, mostly so that a stigma
can only be pollinated by the pollen of another flower ; occasionally however,
(as in the Asclepiadese) pollination by the pollen of the same flower, in addition
to that from a distance, is not excluded entirely. The adaptations here are
extraordinarily manifold, and sometimes so complicated that their explanation can
only be detected by means of close research. To this sub-division, for instance,
belong the species of Irt's, Crocus, Pedicularis, many Labiates ; also Melastomaceae,
Passifloreae, and Papilionaceae. Among the most inter-
jl esting are the Asclepiadese, where these relations, how-

ever, can only be explained with the aid of numerous
figures and prolix descriptions.

The mechanism for the avoidance of self-fertilisation
and for the ensuring of crossing between different flowers
of the same species is unusually elegant and easily com-
prehended in the case of Salvia pratensis (the Meadow
Sage) and some other species of this genus. Fig. 452 yl
shows a flower of this species seen from the side, n
being the bilobed receptive stigma, and the position of
one of the two stamens being indicated by means of a
dotted Hne within the upper lip of the corolla. If a
needle is pushed in the direction of the arrow into the
throat of the flower, both the stamens spring forward
as at A a. If a Bee does the same with its proboscis,
striving to obtain honey thence, the dehiscing anthers
strike it on the back and there deposit their pollen at
a definite spot ; on the insect coming into the same
position on another flower, it rubs its pollen-laden back
FIG. 452. against the stigma and pollinates it. The cause of the

springing forward of the anther is sufficiently clear from
Fig. 452 B ; here are shown the short true filaments _/"_/ fixed by their bases on
to the sides of the throat of the flower, and supporting the long connectives c x,
which can be swung to and fro about the points of attachment. Only the upper long
thin arm of each connective c bears a half-anther a, the lower short arm at x
being devoid of any anther, and so connected with that of the other stamen, that the
two together form a sort of lounging-chair. If then the honey-seeking proboscis
of a Bee strikes this apparatus in the direction of the arrow, the short arm of
the connective is driven down and the upper arm c is moved forwards.

The impossibility of self-fertilisation in Viola tricolor depends upon mechanical
arrangements of quite a different kind. Fig. 453 A and B illustrate the position and
arrangement of the floral parts in this case. The base of the flower is invested by the
floral leaves, and completely filled up by the anthers and the ovary which they surround,

POLLINATION OF VIOLA.

795

with the exception of the saccate appendage (spur) of the lower petal, in which the
nectar excreted by the appendages of the two lower stamens collects. The entrance
to this nectary thus situated behind the sexual organs is only accessible through a
deep furrow beset with hairs, in the lower petal; the lateral and upper petals
converge in front of the ovary surrounded by
the anthers, and over this channel in such a
way that the entrance is quite blocked by the
head of the stigma « (in B). This head is
situated on a flexible style {gr in C) and is
hollow, and it opens by means of a hole turned
towards the hairy channel on the lower petal : the
posterior lower margin of this opening is provided
with a lip-like appendage. The anthers dehisce
spontaneously, and the pollen collects below and
behind the head of the stigma, and forms a
yellow dust between the hairs of the channel
mentioned above. An insect carrying pollen
from another flower on its proboscis, pushes the
latter beneath the stigma -head, through the
channel and behind into the nectary in order to
suck the honey; by this means the foreign pollen
hanging to the proboscis is wiped off on to the
lip of the stigma -head, and it at once sticks
to the viscid stigmatic fluid filling the cavity
of the stigma -head, and subsequently sends
its pollen-tubes down the canal of the style.
INIeanwhile, as the insect is sucking the nectar
in the spur behind, the pollen of this flower
lying in the canal behind the stigma- head be-
comes attached to the proboscis, and when the
latter is withdrawn this attached pollen does
not come in contact with the stigmatic fluid,
because the lip is drawn forwards by the move-
ment of the proboscis, and covers the opening
of the stigma-head from behind and below.
The pollen which it takes with it from this
flower is then wiped off" in the manner already
indicated (by the pushing in of the proboscis)
on the opening of the stigma -head in another

flower. If the insect were to insert its proboscis repeatedly into the nectary of
the same flower the pollen of the latter would thus come into the opening of its
own stigma; but insects, as Hildebrand noticed, generally do not do this (and
the same elsewhere) but only enter once, suck the nectar, and then visit another
flower. It is easy to imitate the manipulations of the insects, and to fill the stig-
matic cavity with pollen (from its own or another flower) by pushing a sharp ihia
needle into the channel under the stigma-head and then withdrawing it.

A longitudinal
S the freed ovary
Iready fertilised
ire ruptured and

Fig. 4S^.—l''io/a tricolor.
section of flower (nat. size). .
and anthers, the former e
and swollen ; the filaments ;
the anthers carried forward by the growing
ovary. C the capitate stigma with its opening
o and lip // ; gr the style (magnified), /sepal ;
Is appendage at base of sepal ; c petal ; cs
hollow spur of the lower petal— it acts as a
receptacle for the nectar \/s the appendages
on the two lower stamens— they project into
the spur and according to Hildebrand secrete
the nectar, a anthers ; n stigma ; v bract on
flower peduncle. D transverse section of
ovary with three placentas sf and the ovules

79Ö

LECTURE XLV

The adaptations for cross-pollination in most Orchids, as various as they are
complicated and ingenious, have been described in detail by Darwin in the book cited
above. One of the simpler, and, in its main features, commoner cases is presented by
Epipactis lalifolia, and may be here shortly described. At the time when the sexual
organs are mature, the flower, in consequence of a twisting of the pedicel, stands so
that what is properly the posterior of the six perianth-leaves hangs forwards and
downwards ; its basal portion is deepened like a bowl and thus transformed into a
receptacle for the nectar which it produces itself (Fig. 454 B, D, I). The sexual
apparatus, borne by the Gynostemium 6" (in C) projects obliquely over this

nectary ; the stigma forms a disk with several
lobes and is deepened and viscid in the centre,
its surface being bent obliquely over the de-
pression of the labellum. Right and left, above,
to the sides and behind the stigma, are situated
the two aborted glandular stamens x x ; above
the stigma, hanging over like a roof, is the
single fertile anther, which is rather large and
is in its turn roofed over by its cushion-like
connective {en). The side-walls of the two
halves of the anther spring open longitudinally
to the right and left, so that the pollen-masses
are partially freed ; the pollen-grains are con-
nected together by means of a viscid material.
In the middle in front of the anther and above
the stigmatic surface is found the so-called ros-
tellum h, a peculiarly metamorphosed portion
of the body of the stigma (cf. A); the tissue
of the rostellum is converted into a viscid sub-
stance which is covered only by a thin mem-
brane. The flower of Epipactis left to itself
does not get fertilised, for the pollen-masses
do not spontaneously fall out of the anthers, and
even if they did would not come on to the
stigmatic surface : they must be removed by
insects and transferred to the stigma of another
flower. The manner in which this is accom-
plished may be rendered clear with the aid of
a sharp-pointed lead-pencil. On pushing the
point under the stigmatic surface in the direction of the base of the labellum, and
then pressing it a little on the rostellum, and again withdrawing it slowly, in the
same position (Z>), the viscid mass of the rostellum — the adhesive-disc — remains
sticking to the pencil, with the adherent pollen-masses. These latter are now
completely extracted from the t\vo anther-halves on the withdrawal of the pencil,
as shown in E and F. If the pencil-point together with the pollinia are again
pushed into another flower in the direction of the base of the labellum, the pollinia
necessarily come in contact with the viscid portion of the stigmatic surface and stick

FIG. ^i^.—Epif'actts latifolia (an Orchid). A
longitudinal section of a flower-bud. B newly-opened
flower after removal of the perianth except the label-
lum /. C sexual apparatus after the removal of all the
perianth-lobes— seen from in front and below. D as
B : the point of a pencil is inserted after the manner
of the proboscis of an insect; E and F pencil-point
with pollinia attached. /K ovary ; / labellum, the bag-
like depression of which functions as a nectary; tt the
broad stigma ; m the connective of the one fertile
anther ; p pollinia ; h the rostellum ; xx the two abor-
tive lateral gland-like stamens; i' insertion of the
excised labellum. -S (in C) the gynostemium.

POLLINATION OF ORCHIDS. 797

fast there ; on again withdrawing they remain fixed there, wholly or partly torn away
from the pencil. In virtue of the form and position of the parts of the flower,
then, an insect which settles on the anterior portion of the labellum can creep
down into the base of the nectary without touching the rostellum ; on creeping
out after sucking the nectar it strikes against it and takes away the pollinia with
it. On creeping into a second flower these pollinia come on to the viscid stigmatic
surface and there remain fixed. In some other Orchids the relations are far more
complicated.

(3) In flowers which are pollinated by insects, the mature pollen must often
remain lying in the already dehisced anthers for some time before it is removed ;
during this period it must neither be dispersed by the wind nor moistened by rain or
dew. Numerous and very various adaptations therefore exist for the protection of the
pollen, further details as to which are given by Kerner in his work ' Die Schutzmittel
des Pollens,' (Innsbruck, 1873).

LECTURE XLVL

THE OBJECT OF FERTILISATION. APOGAMY'.

There are many plants which, like Mammals, Birds, and other highly organised
animals, reproduce themselves exclusively in the sexual way because, in the normal
course of life at least, no other mode of reproduction is open to them ; these com-
prise the majority of the Conifers, especially the Firs and Araucariea;, and among the
flowering-plants probably also many Palms, our cereals. Flax, Hemp, Gourd, and
many others, and some are certainly to be found among the Cryptogams.

The great majority of plants, however, have abundant opportunity of multiplying
and reproducing themselves otherwise than by means of sexual organs. Among the
Cryptogams indeed we may leave the spores proper out of consideration, since it is very
generally the case that gemmae, conidia, or other segregating portions of the shoot are
formed in the alternation of generations both before and after fertilisation ; and a very
large number of Phanerogams, especially those provided with runners, bulbs, tubers,
subäerial gemmae of the most various kinds, for though they produce seeds regularly by
the sexual mode, almost none of these ever succeeds in germinating. It is only here
necessary to think of the Potato, which has for hundreds of years continually been
propagated by its tubers, and thus by the asexual mode. There are in fact, as
we shall see, a somewhat large number of cryptogamic and phanerogamic plants
which have in the course of time either entirely lost their sexual organs or have let
them become functionless, but which, nevertheless, and sometimes in perfectly astound-
ing numbers, multiply and propagate themselves ; and on the other hand, in
some cases with common ubiquitous plants, the development of sexual organs de-
pends upon favourable conditions rarely met with, while vegetative propagation
occurs abundantly. In addition to less known examples the commonest of all moulds,
Penicillium ghmcum, may be quoted as an instance ; and even in the case of the
largest of all the Fungi, the Hymenomycetes and Gasteromycetes it is, according to

1 The most important publications on Apogamy are : —

Anton De Bary : ' Über apogame Fame unci die Erscheimmg der Apogamie im Aligemeinen '
(Bot. Zeitg., 1878).

De Bary: ' Beiträge zur Morpiwlogie und Pliysioiogie der Pilze'' (by De Bary and Woronin in
Abhandlungen der Senkenbergischen naturforsch. Ges. B. XII, pp. 225-370, IV. Reihe, 1881,
section 14, ^ Entstehung und WacJistliumsursaclien von Antheridien und Nebenästen^ is particularly
worthy of note.

Strasburger: 'Über die Befruchtung und ZelUlieilung'' (Jena, 187S, p, 63).

OBJECT OF SEXUALITY. 799

the view of the most prominent mycologists, very probable that they possess no sexual
organs whatever.

The consideration of this fact, which we could easily make good by numerous
other examples, leads directly to the question, what is the precise object which Nature
attains by the production of sexual organs and by sexual propagation ?

This question appears the more pertinent when we see, on the other hand, \\ith
what care (if this picturesque expression is allowable) Nature proceeds in very many
cases to ensure the union of the sexual cells, and the production of sexually produced
descendants : all the marvellous adaptations of Dichogamy, Heterostylism, Herko-
gamy, and other arrangements, which we have studied with the aid of examples in
the preceding lecture, may be looked upon in this sense.

Meanwhile, as De Bary has already stated elsewhere, we shall probably have
to concede that, as matter of fact, we simply know that fertilisation is in many
cases demonstrated by experience to be an indispensable process to the plants
concerned, in many other cases this is simply not so, and we have in the meantime no
ground for assuming that all organisms must behave similarly in this respect. This
somewhat unsatisfactory conclusion, however, does not prevent the assumption that
in^ the many cases where reproduction is regularly attained by fertilisation, special
advantages are connected with it, which in the contrary case are simply attained
in some other manner, or perhaps in certain cases are even not attained. A few of
such probable advantages may here be brought forward.

Darwin, on the basis of the comprehensive results of the artificial breeding of
animals and plants, has demonstrated to realisation the idea already expressed by
Sprengel, that in all sexual reproduction the chief point is to ensure the crossing of
individuals of the same species, of which detailed illustrations have been given in
the preceding lecture. It also results from the experimental investigations of Darwin,
Hildebrand, and others, that just as so-called incest among some domestic animals
results in the production of few or feeble descendants, so also does the continued self-
fertilisation of androgynous flowers, whereas in like species of plants the crossing of
different individuals results in the production of vigorous seeds. Supported by Darwin's
authority, one is now very ready to suppose that incidental abnormalities or diseased
conditions are equilibrated by means of the sexual intermixture, and are rendered
uninjurious in the descendants. In addition to some other considerations, mention
should here be made, however, of the very large number of plants which certainly
maintain themselves asexually through hundreds and thousands of generations
without the slightest ground existing for supposing that they are gradually degene-
rated by the process.

On the other hand, we may here think of the fact, established by artificial
hybridisation, that, by means of the sexual intermingling of individuals from different
sources, the variability of the descendants is increased, and that in this way the number
of the different organic forms may have gradually been multiplied : though even here
also it is not to be forgotten that the formation of varieties sometimes occurs also
during asexual reproduction ; at any rate the majority of the varieties of Potato have
probably been produced asexually.

And nevertheless in all the suppositions there again exists the perception that, with
increasing complexity of organisation both in the animal and vegetable kingdoms, the

8oO LECTURE XLVI.

sexual organs become more and more developed, and the sexual reproduction
gains more and more over the vegetative, or even becomes the predominating
one. It would be possible to believe that with increasing perfection of the
organisation a corresponding division of physiological labour also is given, by
which the vegetative mode of reproduction is limited and the sexual one simply
promoted.

I may finally notice yet another observation, which I have already expressed in
my 'Textbook' (Ed. IV, p. 877). Those Cryptogams which possess pronounced
alternation of generations, particularly the Mosses and Vascular Cryptogams, although
in other respects belonging to entirely different types, nevertheless illustrate repeatedly
and in every class the fact that the highest development of the organisation is always
attained only by means of fertilisation. In the case of the Equisetums, Ferns and
Lycopodiacese this is at once obvious on remembering that the first generation pro-
duced from the asexual spore, the prothallium, is usually a minute, very simple, and
transitory structure, the life and importance of which are closed with the fertilisation
of the oosphere ; whereas the embryo, originating from this act of fertilisation, de-
velopes into a highly organised plant, a Tree-fern, for instance, &c. In the case of
the Mosses, it is true, matters are diff'erent, in as far as here it is the generation which
has originated from the asexual spore itself which we are in the habit of regarding as
the proper plant ; but on comparing the forms of cells and tissues and the histological
structure generally of the 'Moss-fruit' produced by fertilisation with that of the Moss-
plant, there can be no doubt that the former is more perfectly organised than the
asexually produced Moss-plant itself. In the case of the Algse and Fungi it is the
same, although the fact is not so easily made out as in these cases. In those Fungi
in which a sexual apparatus is known, the most perfect product of the whole de-
velopment proceeds from the fertilisation. The asexual spore of the Ascomycetes
gives rise to the very simply organised mycelium, and it is only after the act of fertili-
sation on this that the Fungus-fructification with its complicated structure and high
organisation arises (see Fig. 409, p. 726), and similarly may be said of many Algae ;
even in the case where the result of fertilisation is only a single cell, an oospore or
zygospore (zygote), this tends to exhibit, at least in the development of its wall-layers,
a more perfect organisation than the vegetative parts of the same plant.

Moreover the Phanerogams or Spermaphytes only apparently contradict the
above observation; in reality they confirm it in the most astounding manner.
As we have seen, the embryo-sac in the ovule is the true macrospore, in which the
first structure to arise is what is termed the prothallium in the Vascular Cryptogams :
the endosperm is a physiologically and histologically reduced and degenerated pro-
thallium, of which, strictly speaking, only the oosphere and synergidae survive in the
Angiosperms. What we designate generally the plant among the Spermaphyta is the
sexually-produced product, which also here has sprung from the fertilised oosphere,
while the preceding stage of development, corresponding to the prothallium, no longer
exists at all as an independent organism.

In conclusion I may return to the phenomena of Apogamy already referred to.
With this word De Bary distinguishes the case, partly discovered by himself, where
asexual reproduction occurs in the place of sexual reproduction : this may happen in
very different ways — for example, by oospheres, which under normal conditions

APOGAMF.

8oi

require fertilisation, proceeding to form embryos without it, of which the highly-
developed Alga Chara criniia affords the only established (first by Alexander Braun)
example. This plant, which lives at the bottom of stagnant water, is met with
throughout the whole of North Europe exclusively as female individuals, which, how-
ever, and thus without fertilisation, yield abundant and normal germinating fruits. Male
plants of this species are known as isolated specimens from Transylvania, the souih
of France, and the neighbourhood of the Caspian Sea, though their generative power
has not been investigated. It is evident that Chara crim'ia, like all other Charas, was
formerly sexually propagated, and that the power to give rise to progeny from the exist-
ing female apparatus even without fertilisation can only have appeared subsequently.

To a second category belong three cases of Ferns, discovered and investi-
gated by De Bary himself and his pupil Farlow, which have long been known as
common garden plants, and possess the remarkable property of giving rise to new
Fern plants directly from the tissue
of the prothallium by means of
simple budding. In two of these
Ferns {P ten's cretica and a garden-
variety of Asplenhwi filix-femiiia-
cristatuvi) no archegonia whatever
are developed on the prothallium,
although antheridia occur occasion-
ally. In Asple?tmm falcatimi, on the
other hand, there are found pro-
thallia completely devoid of sexual
organs and still capable of propa-
gation by means of budding, as well
as others with a few antheridia, and
finally prothallia which bear anthe-
ridia and archegonia, but neverthe-
less give rise to the Fern by simple
budding. There is still to be added
that in the three Ferns mentioned
this asexual propagation by budding
from the prothallium is the only mode, and that no example of them was found
with the embryo formed from an oosphere, whereas in very many other ferns
which De Bary investigated with this object not a single one exhibited apogamy.
All the facts considered, there can be no doubt whatever even in the case of these
apogamous Ferns that the prothallia formerly gave rise to normal sexual organs and
propagated themselves in the ordinary way, and that Apogamy, the loss of sexuality,
was only, as physicians say, 'acquired' subsequently, and perhaps in this case
it depends upon the fact that the three Ferns in question have been cultivated plants
for a long time.

A point worthy of notice in the case of the Ferns just mentioned is that the leafy
shoot which really replaces the actual embryo, arises at that spot on the prothallium
where the archegonia would be formed in the normal case. In this respect the cases
of apogamy in some flowering-plants observed by Strasburger resemble them. In

[ 3 ] 3 F

Fig. 45S— Development of the adventitious embryos in FuitHa
ovata {after Strasburger X about 150) / apical region of nucellus of ovule
with the oosphere o and one of the synergidae s. II \y\& cells shaded in
/ have developed into the adventitious embryos a e, the oosphere re-
maining sterile. The tissue i belongs to the micropyle portion of the
integument.

8o2 LECTURE XLVI.

Ftinh'a ovafa and Allium /ragrans, two common garden plants, according to his
investigations, no embryo is developed from the actual oosphere in the embryo-sac,
not even when a pollen-tube may have happened to penetrate into the micropyle ;
but close to it cells of the nucellus of the ovule grow out into the embryo-sac
and embryos arise from these cell-proliferations. It is very probable that matters
are quite similar in the case of the Orange-tree also, and in the case of one of the
Euphorbiacese from Australia, Ccelehogyne, of which female specimens alone occur
with us. In all these plants several embryos are produced in the interior of the em-
bryo-sac by budding from the surrounding tissue.

Among the phenomena of apogamy are also to be counted those cases in the
flowering- plants where, though flowers are developed, they are devoid of true
sexual organs, or where no flowers at all are developed. Here again the plants in
question are chiefly ciiltivated plants. Thus Müller designates certain Scitaminese
and Dioscoreae and also the Horse-radish {Armor acid) as entirely seedless, and De
Bary remarks in this connection that also our (of course not cultivated) Ficaria
and Dentaria bulbifera but rarely produce seeds ; among the species of the genus
Allium (Garlic) there are several in which small bulbils arise in place of the
flowers, and among these is also Allium sativum (the Garlic) in which no seeds
whatever are developed.

The view that such apogamous species are undergoing extinction is opposed,
and correctly, by De Bary, with the remark that it is just among most apoga-
mous plants that an excessive production of asexually produced descendants tends
to occur. Sexual propagation is more than sufficiently replaced by asexual pro-
ductivity.

INDEX.

The figures in large type refer to illustrations.

A.

Abies, 321, 351, 465, 493.

— cephalonica, 504.

— excelsn, 504.

— pectinata, 146, 505.

44,449,758.

Abnormal growing points,

468.

Abnormal growth in thick-
ness, 169.

Abnormal growth due to
parasites, &c., 368, 537.

Abnormal types of growth,

425-
Absorbing organ of seedling,

344.
Absorption by parasites, 372.

— by roots, 253, 255.

— by root-hairs, 257, 262.

— by roots as affected by
temperature, 264.

— in water plants, 248.
Absorption of endosperm,

373-

— of gases by leaves, 255.

— of light in chlorophyll,

322.

— of liquids, 210, 274.

— of nitrogen, 292.

— of nutritive materials by
roots, 246, 264, 287, 295.

— of organic nutritive sub-
stances, 366, 374.

— of organic substance, 298,
368.

— of oxygen, 395, 398.

— of Proteids by insecti-
vorous plants, 374.

— of salts by soil, 260.

— of water by soil, 259.

— of water by roots, 246.

— of water by leaves, 254.
Absorptive properties of

soil, 261.
Acacia, 285, 593, 598.

— lophantha, 594, 636, 637.
Acanthacex, 178.
Acceleration of growth, 554.

Accessory cells, 119.
Acer platanoides, 180.
■ — striatum, 166.
Acetabularia, 179, 335, 618,
776, 777.

— mediterranea, 604,
Acetamide, 384.
Acetic acid, 328, 385.

— fermentation, 386.
Achimenes grandis, 582.
Achlya, 98.

Acid, acetic, 385.

— butyric, 386.

— carbonic, 386.

— formic, 384.

— lactic, 349, 386.

— nitric, 385.

— oxalic, 384.

— phosphoric, 385.

— salicylic, 385.

— succinic, 348, 386.

— tartaric, 385.

Acids excreted by root-
hairs, 262.

— fatty, 347.

— vegetable organic, 328,

329, 342, 384, 385, 403,
566.

— vegetable, turgescence

due to, 328, 403, 566.
Aconitum napellus, 24, 332.
Acorn, 397, 763.
Acorus, 144, 180.

— calamus, 16, 135.
61.

Acropetal development, 41 8,
468.

— vegetative repetition, 467.
Action of ferments, 343, 349.

— of parasites, 368, 537.

— at a distance, 772.

— of sexual cells on one
another, 767, 770.

Active condition ofc plastic

substances, 341.
Activity of respiration, 399.

— of roots, 264.
Adaptation, 60 r.
Adiantum, 53.

3 F 2

Adiantum Capillus - Veneris,

744, 771.
Adonis -vernalis, 641.
Adventitious buds, 478, 479.

— embryos, 802.

— growing-points, 477, 480.

— roots, 477.

— shoots, 475, 477, 478.
iEcidiomycetes, 370, 381.
yEcidium Elatinum, 369,

537.
A^gilops, 7 So.

— o'vata, 781.
^gopodium podagraria, 58,

703.
Aeration of roots in soil, 256.
Aerial roots, 12, 14, 23, 522,

523, 660, 693, 704, 711.
^sculus hippocastanum, 44.
■ — macrostacbya, 399.
^Ethalium septicum, 83, 345,

614.
^thylamine, 384.
After-effects, 560, 596, 597,

626, 627, 629, 630, 635,

638, 663.
Agapantbus, 92,
— • umhellatus, 118.
Agarics, 571, 700.

— phosphorescent, 407.
Agaricus, 426, 427, 428.

— campestris, 72.

— Gardneri, 407.

— igneus, 407.

— melleus, 367, 388, 407.

— noctilucens, 407.

— oleareus, 407.

— -variecolor, 427.
Agave, 116, 148.

■ — americana, 270, 550.
Agrimonia eupatorium, 106.
Agrostemma Gitbago, 686.
Ailantbus, 425, 478.

— glandulosa, 2 r .
161, 239.

Air, composition of, 293.

— in soil, 257.

— in wood, 2 3 8, 267, 268,
269.

8o4

INDEX.

Air in wood, contraction and

expansion of, 270.
jiktbia, 548.

— quinata, 56, 673.
Alb'iz%ia I Optant ha, 236.
Albumin, 333.
Albuminates, 384.
Albuminous substances, 79.

— movements of, 360.

Alburnum, 163, 230, 330,

344, 577, 579-
jilchemilla, 278,
■ — alpina, 279.
Alcohol, 348, 349, 369, 386.

— action on tissues, 208.
Alcoholic fermentation, 349,

386, 401.
Aldro'vayida vesiculosa, 43.
Aleurone-grains, 77, 334,

335, 341.

Algae, I, 90, 114, 120, 126,
i55> 199, 283, 288, 299,
302, 311, 312, 321, 324,
331, 335, 347, 351, 364,

' 365, 371, 387, 391, 419,
427, 432, 435, 437, 438,
442, 454, 455, 471, 479,
483, 484, 491, 499, 510,
512, 563, 603, 604, 61 1,
618, 654, 767, 769, 775,
787, 788, 789, 800.

— apical-cell in, 452.

— branching of, 473.

— cellular structure of, 77.

— epidermis of, 121.

— fertilisation of, 108.

— growing points of, 460.
- — growth of, 425, 429.

— non-cellular, 70, 425,
491, 732, 734, 776.

— nuclear division in, 106.

— reproduction of, 721, 722,
724-728, 732, 734, 736,
742.

— roots of, 29, 31, 255.

— sexual reproduction of,
728.

— shoots of, 54, 65, 69.

— tissues of, 149, 151.
Alisma pi ant ago, 51, 442.
Alismaceae, 181.
Alkalies, 79, 253, 342.
Alkaline earths, 253.
Alkaloids, 172, 175, 328,

362.
Allium, 145, 177, 231, 534,
546.

— atropurpureum, 543, 544,
686.

— Cepa, 45, 62, 86, 180.

43, 180,310,543, 544.

— fistidosuni, 180.

— fragrans, 802.

— odorum, 104.

Allium P or rum, 543, 544.

— rotundum, 546.

— sativum, 802.

— Schcsnoprasum, 74, 146,
219, 485.

Almond, 3.

— 326, 342, 347, 763, 781.
Alnns, 124, 506.

Aloe, 55, 116, 141, 148, 270,
496, 534-

— serrula, 496.
Alpine Rose, 123.
A I sine, 640.

Alterative ferments, 342.
Alternation of generations,

724-726, 734, 739, 748,
800.
Althxa, 125, 126, 792.

— rosea, 180.

80, 100, 122, 123.

Amaranthus, 177.
American Vines, 785, 786.
Amaryllideae, 177, 178.
Amides, 325.
Ammonia, 381, 387.

— absorption of by leaves,

255.

— rigor due to, 595.
salts, 260, 292, 384.

— tartrate, 383, 386.
Ammonium sulphide, 387.
Amoeboid movements, 613,

614.
Amotnum, 316.
Amorphophallus bulbosus,^90.
Ampelidese, 660.
Ampelopsis, 56, 661, 662.

— hederacea, 658, 668.

667.

Amphicarpxa, 791.
Amygdalin, 342.
Amygdalus communis, 3.
Amyloplasts, 316.
Amylum, 307.
Anagallis arvensis, 78 r.
• — ccerulea, 781.
Anatropous ovule, 760.
Andrxa, 465.
Androgynous flowers, 788-

790.
Androspores, 775.
Anemone hepatica, 507.

— nemorosa, 641.
Angiopteris evecta, 14.
Angiosperms, 464, 745, 758-

763, 780, 788, 789, 790,
800.
- — embryology of, 764, 765.

— and* cryptogams con-
trasted, 761, 766.

— and gymnosperms con-
trasted, 757, 760-764.

Animal-heat, 404.

life, continuity o.f, 413.

Animal-respiration, 395, 397,
401, 406.

Animals containing chloro-
phyll, 298, 394.

— correlation of functions

in, 508.

— growth of 412, 413.

— irritability in, 597, 600.

— reproduction of, 747, 753,
798.

Anisotropy, 493, 698-704,
709, 713, 714.

— oi Ivy, 710-712.

— of Marchantia, 709, 710,
712.

— of non-cellular plants,
700.

— and correlation of growth,
704.

— independent of morpho-
logical nature of organs,
700.

Annual plants, 330, 414.

— rings, 162, 230, 268, 439,
444, 577, 578, 579.

Annular vessels, 136.
Anona cheirimolia, 165.
Antagonism of heliotropism

and geotropism, 694,

708,713.
Anthemis chrysoloica, 405.
Anther, 548, 651, 757, 760.
Antheridia, 524, 732-734»

736, 737, 742, 772-775,
778,788.

— reduced, 749, 756.
Antherozoids, 603-605, 608,

724, 730, 731, 732, 734,

737, 742, 746-750, 768,
770, 773, 778, 789-

— development of, 770, 771.

— mother-cells of, 737.
Anthoxanthum odoratum, 792.
Anthracose, 786.
Anthriscus, 52.
Anthurium, 25.
Anthurium longifoUum, 2 2 .
AntbylUs vulneraria, 48,

234.
Anticlines, 439, 440, 445,
452, 454, 457, 458, 469,
743, 764.

— in growing-point, 449.

— in solid organs, 447.

— sequence of, 443.

— suppression of, 442.
Antipodal cells, 760.
Antirrhijium, 780.

— majus, 782.
A-nucleate plants, 107.
Apex, 481, 482, 483, 520.

— the region of slowest
growth, 449, 457, 544.

Aphis, 537.

INDEX.

805

Apical-cell, 95,452,453» 454>
458, 473,545,747-

— division of, 452, 455.

— rarity of divisions in, 457.

— significance of, 457.

— tetrahedral, 447,456,743.
456.

Apical cells, groups of, 458,

462.
yipios tuberös a, 672.
Apocynaceae, 172.
Apogamy, 769,798, 800-802.
Apothecia, 707.
Apparatus for measuring

growth, 556-558, 561,

561.

Apple, 141, 367, 369, 386,

763,781.
Apples, fermentation of, 386.
Apposition, growth by, 413.
Aquatic plants, 225, 404,

511, 564, 660.
Aquatic roots, 14, 23.
Aqueous vapour, 228, 248,

264.

— in wood, 268.
Arabis, 537.
Aral to, 123.
Araliaces, 181, 703.
Araucaris, 798.
Arbor vitx, 42.
Archegoniata, 721.
Archegonium, 465, 524, 525,

732, 734, 736, 737, 738,
742, 744, 747-749, 754,
755, 760, 772-774, 788,
789.

Arcyria incarnata, 83.

Area of leaf-surface, 312,

5ir, 513-

— of wood cell-walls, 241.
Argemone mexicana, 175.
Arillus, 752,
Aristolochia, 220, 473, 548,

669, 792, 793, 794.

— Clematitis, 793.
--792.

— sip ho, 244.
Aristolochiea', 180.
Armoracia, 802.
Aroidea", 125, 177, 179, i8r,

279, 325, 406, 408, 473,
478, 497, 498, 704-
Aroids, 12, 114, 278, 461,
470,490, 545, 699.

— roots of, 24, 693, 695.
Arrested leaves, 55, 61, 506,

507.
Artichokes, 576, 651, 652.
Artificial cells, 215, 275.

— cultures, 283, 286, 295.
— • selection, 329.

Anon Italiciim, 406.
Asarwn, 51.

Asarum Etiropn^um, 47.

Ascending sap, 156, 225, 51 r.

Ascent of water in root-
stock, 276.

in stem, 225, 253, 264.

in stem, velocity of,

235.

Asci, 726.

Asciepiadex, 54, 171, 172,

174, 789, 794-
Asclepins, 174.

— carnosa, 669.
Ascoholus, 726, 727.

— furfuraceus, 726.
Ascogenous hyphae, 726.
Ascomycetes, 391, 514, 726,

727, 800.
Asexual reproductive organs,
724, 732, 740.

— reproduction, 728, 770,

798, 799, 802.

— spores, 768, 769.
Ash-analyses, 287, 290, 320,
constituents, 79, 290,

296, 313, 320, 355, 388,
509.
Ash-constituents of fungi,
383.

— of parasites, 372.
Asparagin, 345, 355, 363-

— as food for fungi, 384.

— detection and reactions
of, 346.

— disappearance in light,

346.

— importance of, 325.

— metamorphoses of, 326,

346.

— occurrence of, 347.
Asparagineae, 55.
Asparagus, 61, 685.

— asper, 543.

— officinalis, 6r.
Aspects of vegetation, 702.
Asphalt, 294.
Asphyxia, 595.
Aspidium, 185, 471.

— Filix-nias, 13, 40, 129,

421, 478, 780.

— Filix-mas, 129.

— spinulosum, 51, 780.
Asplenium caudatinn, 478.

— decussatum, 478.
478.

— falcatum, 801.
— filix femina, 456.

801.

Assimilating cells, 24S.

— chlorophyll - c(#puscles,
316.

— surface, 227.

— tissue, 148, 535.
Assimilation, 226, 250, 264,

286, 289, 291, 293, 295,

296, 308, 309, 324, 364,
366, 397, 411, 412, 508,
510, 521, 579.
Assimilation, apparatus for
showing, 304.

— chemistry of, 297, 314,

317.

— curve of, 305.

— energy of, 311.

■ — equal volumes of gases
concerned in, 298.

— experiments on, 310.

— intermediate products of,

317.

— rate of, 304.

— specific energy of, 312.

— a local act, 310.

— and chlorophyll, 299,509,

— independent of growth,
300, 4ir, 412.

— necessity of carbon diox-
ide in, 310.

— quantityof starch formed,

312-314.

— and light, 198, 294, 296,

299, 301, 307, 309, 535.

— in coloured lights, 304.

— in electric light, 307.

— in intense light, 3 1 1.

— in insectivorous plants,

375-

— in Lichens, 392, 393,
514.

— products of, 299, 306,

309, 314, 317, 318, 357,
403, 533, 763.

— chemical metamorphoses
of products of, 323.

Astragalus, 371.

— tragacantba, 57.
Aiherurus ternatus, 478.
Atmosphere, composition o*",

293.
Atmospheric carbon, 292,
294.

— nitrogen, 291.

— pressure, 269.
Atriplex, 640.

— hortensis, 299.
Attachment of root-hairs

to soil, 257, 259.
Attraction of sexual cells

one to another, 737, 772-

778.
Autumn-leaves, 318, 319.
wood, 162, 230, 268,

578.
Auxanomctcr, 556, 557,558.

— 557.

Avoidance of self-fertilisa-
tion, 787, 790, 791, 794-

797, 799-
Axial longitudinal section,
483.

8o6

INDEX.

Axillary buds and branching,
472, 473, 497, 499. 505,
.507.
Axis of growth, 481, 483,

484, 539)545-
A%aka, 780.
Azolla, 461.

B.

Bacillarieae, 604.

Bacteria, 245,343, 347, 348,

369, 380, 383, 384, 385,

386, 604.

— cultivation of, 387.
■ — forms of, 387.

• — pigment-, 387,

— segmentation of, 387.
Bacterium, 387.

Bal anaphora, 368.

— fungosa, 28, 63.
Balanophores, 63, 232, 368,

372,476,765.
Balsam, 124, 175, 179, 184,

199, 327, 695,
Balsamic secretions, 185.
Balsamineae, 640.
Banana, 50, 130, 510.
Barberry, 57.
Barbula, 29.
Barium, 383.
Bark, 156, 166, 226, 246,

574-

— crevices in, 577.
Barley, 343, 344, 345, 404,

687.

Barren flowers, 791.

Basal growth, 419, 544.

Base, 481, 482, 483, 520.

Bassorin, 90, 180, 328.

Bast, 131, 145, 159.

fibres, 160, 163, 576.

Bastards, 779.

Bean, 56, 193, 217,228, 262,
263, 283, 284, 286, 300,
309, 314, 327, 334, 337,

339, 397, 531, 539, 564,
581, 584, 625, 626, 631,
633, 636, 638, 639, 640,
644, 645, 669, 671, 672,
691, 704, 759, 763.

— 542, 631, 690, 764.
Beech, 164, 229, 28S, 493,

506, 510, 536, 763.
Beer-yeast, 385, 386.
Beet, 24, 314, 335, 343, 357,

363, 564, 571, 576.
Beggiatoa, 387.
Begonia, 143.

— 310, 489.

• — cucullata, 316.
■ — rex, 582.

Berberis, 57, 201, 396, 595,
647,654.

Bertholletia ex eels a, 334.
Betula alba, 168.

— — 184.

Bignonia, 660, 662, 789.

— capreolata, 668.
Bilateral structure, 485, 489,

490, 493, 494, 504-

induced by light, 525.

Bindweed, 56,548, 640,657,

669, 670, 671, 676.
Biology, 602.

— of tendrils, 667.
Birch, 165, 266, 270.
Bitter almonds, oil of, 342.
Bituminous substances, 294.
Blackberry, 657.

Black Poplar, 703.

Bladder-Senna, 759.

Bleeding of wood, 266, 270.

Blending of parental charac-
ters in hybridisation, 779,
782.

Blood-vessels, comparison of
latex-vessels, &c. to, 361.

Blue bacteria, 387.

— Bindweed, 670.
671.

— glass, 307.

— light, 303, 306, 311, 638,
696, 697.

— wood, 163.
Blumenbachia, 675.

— later it ia, 669.
Bocconia, 172.
Bog-mosses, 465.

plants, 255.

Boletus ßa-vidus, 113.
Boragineae, 123, 496, 497.
Bordered pits, 137, 160, 237,

424, 581.
Bornetia, 335.
Botrychium Lunaria, 2n.
Botrydiuni, 32, 70, 76, 43 1.

— granulatum, 4, 611.

604.

Bottle-cork, 165.
gourd, 658.

Bowiea volubilis, 668, 670.

Box, 163.

Bracken fern, 60, 144, 146,

462, 471, 699.
Bracts, 473.
Bramble, 58, 125.
Branches, 330, 677, 699, 704,

705,714.

— development of, 471,

— experiments with cut,
277, 278.

• — injection of water into,
279-

— mechanics of, 580, 581.

— of trees, 580.
Branch-systems, 471, 472,

497.

Branching, 460, 470, 473,
477, 485, 492, 493-

— axillary, 472, 497, 499.

— monopodial, 474,
Brassica, 159, 283, 326.

— napus, 546.

— oleracea, 640.

— Rapa, 343, 560, 791.
Brazil-nut, 334.
Brightness, curve of, 305.
Broussonetia, 790.
Brown leaves, 321.
Brugmansia, 370.

— 371.

— Zippelii, 66.

28.

Bryonia, 658.

— dioica, 57, 659, 663.
Bryophyllum calycinum, 478.
Bryopsis, 84, 108, 335, 611,

612.
Bryum, 788.

— roseum, 113, 150.
Bubble-counting, method of,

304.

Buckwheat, 193, 283, 549,
596.

Buds, 41, 186, 309, * 326,
333, 351, 359, 399, 403,
411, 418, 422, 464, 472,
476, 481, 483, 486, 487,
494, 498, 499, 504, 517,
520, 535, 544, 547, 549,
565, 706, 714.

— adventitious, 478.

— axillary, 473,505, 507.

— dormant, 475.

— extra-axillary, 473.

— pseudo-endogenous, 475,

479.

— unfolding of, 547.

— from prothallia, 801.
Bud'scales, 464, 506, 507,

508.

Bulbils, 477, 478.

Bulbs, 43, 59, 61, 300, 312,
326, 330, 332, 335, 343,
350, 357, 366, 404, 418,
462, 478, 534, 565, 722,
798.

Bulb-scales, 335, 357.

Butomaceas, 181.

Butomus umbel latus, 758, 760.

759.

Buttresses on tree-trunks,
579, 580.

Butyric acid, 386.

— fermentation, 386.
Buxus, 321.
Bye-products, 362.

Cabbage, 159, 228, 264, 278,
283, 640.

INDEX.

807

Cactaceae, 780.
Cacti, 510.

— shoots of, 54, 64, 160,
527.

Cactus, 13, 116, 149, 159, 180,

203, 227, 301, 325, 472.
Caesium, 383.
Cajophora lateritta, 121.
Calamine, 288.
Calamus, 180, 657.
Calcareous skeletons, 290.
Calcification, 290.
Calcium carbonate, 178.

— oxalate, 167, 175, 177,
291, 320, 388.

— salts, 202, 226, 262, 284,

287, 289, 291, 383.

— sulphate in metabolism,
325.

Calendula, 642.

Calla ^-Ethiopica, 278.

Callus, 424, 582, 583.

• — • of sieve-tubes, 361.

Calyptra, 734, 738.

Calyx, 757.

Cambium, 44, 155, 424, 437,

439, 445, 477.
' — fascicular, 157.

— of roots, 168.

ring, 157, 42.1, 574, 582.

Camellia, 146, 178, 397.
• — japonica, 147.
Campanula, 742.
Campanulacex, 171,173,792.
Canal-cells, 737, 742, 773.
Cane-palms, 657.

— -sugar, 335,341,343,357,
363, 386, 390.

fermentation of, 342,

349-
Canna, 181, 310, 316, 339.

— in die a, 791.
Caoutchouc, 172, 175, 362.
Capillaries in wood, 238.
Capillarity, 208, 243, 268.
Capillitium, 429.
Capitate hairs, 184.
Capsellu bursa-pastoris, 765.
Caraway, 576.
Carbo-hydrates, 175, 250,

308, 317, 323, 326, 329,
346, 361, 382, 400, 403.

— as reserve materials, 332.

— destroyed in respiration,
402.

— fermentation of, 342, 349.

— in parasites, &c., 372.

— storage of, 335.

— metamorphoses of, 311,
323.

Carbonaceous substances in
parasites, &c., 372.

— food of Fungi, 384.
Carbonate of lime, 179, 290.

Carbon, 216, 290, 312, 324,

382, 400, 403.
- — sources of, 292, 294, 384.
compounds, origin of,

325-

destruction of, 403.

dioxide, 202, 248, 264,

282, 292, 301, 317, 384,

366, 381, 403.

— amount respired, 398, 399.

— decomposition of, 296,
298, 299, 303, 309.

— effect of high percentage,
311.

— essential to form starch,

3ir.

— evolved in germination,

397.

— exhalation of, 397, 401,
402.

— germination in absence of,

326.

— in air, 293, 294.

— local assimilation of, 310.

— necessary for assimila-
tion, 310.

— of respiration, 397.
■ — rigor due to, 595.

— volumes absorbed, &c.,
297.

Carbonic-acid, 386.
in soil, 263.

— oxide, rigor due to, 595.
Cardamine pratensis, 478.
Cardinal points of light, 302.

— of temperature, &c., 193.
Cardiospernium, 658, 661.
Carduus, 651.

Ca rex, 59.

Carica papaya, 159, 172, 345,

362.
Carlina acaulis, 588.
Carmichaelia, 55.
Carnivorous plants, 367,
Carpel, 497, 758.
Carpellary leaves, 758.
Carpiniis, 186.
Carpogonium, 726.
Carrot, 24, 343.
Caryophyllaceae, 780.
Cascarilla, iSo.
Catalpa, 51.
Catbarinea undulata, 67, 734.

735.

Caulerpa, 70, 76, 84, 108,

365, 425, 431, 471, 492,

495-

— crassi/olia, 4:26, ^10,^92.
Caulinevascularbundles, 126.
Causal relations of growth,

502.
Cause of geotropism, 681-
683,697.

— of heliotropism, 697.

Causes of growth, 563, 565.

Cavities of wood, 238.

Cell, 73, 112, 430, 433, 482,
678, 728.

aggregates, 77.

chambers, 76, 94, iii.

colony, 73.

contents, 77.

abnormal, 319.

development, 94, 422.

division, 94, 97, 101, 105,

107, 422, 431, 434, 435,
449, 452, 454, 743-

in filamentous organs,

453-
independent of physio-
logical nature, &c. of
organs, 433, 435.

— — laws of, 434, 435, 454,
502.

rarer in apical-cell than

in segments, 457.
relations to growth,

431, 435, 437, 447, 449,
453, 502, 577-

families, 483, 604, 729,

730.

multiplication, 94, 433.

plate, 105.

republic, 73.

sap, 77, 212, 319, 328,

341, 419, 563, 564-569,

573,581.

proieids in, 333.

skeletons, 319.

wall, 73, III, 212, 242,

289, 290, 309, 413, 419,

423, 563-565, 567-569>

572,638.

crystals in, 177.

elasticity of, 566-568,

653-655-
filtration through, 276,

653, 655-

imbibition of, 208.

pits in, 92, 237.

structure of, 91.

sculpture of, 92.

thickening of, 92, 423.

walls, directions of, 432,

438, 439-459-

pierced by Fungi, 390.

of wooii, area oi, 241.

Cells, artificial, 215.

— assimilating, 248.

— daughter-, equal in vo-:
lume, 434.

— groupings of, 436, 43S,
447, 502, 577.

— growth of, 78, 96, 422,

432, 437, 563, 565.

— isolation of, 75.

— of wood, 238, 439.
volume of, 239.

8o8

INDEX.

Cells, Traube's, 215.

— volume of, 422, 563.
Cellular structure, 73, 77.

— plants, 126.
■ — tissue, no.

Cellulose, 87, 206, 290, 323,
335, 357, 389, 422.

— degradation of, 328,

— as reserve-material, 344.

— in Fungi, 385.

— starch-, 339.
Celtidae, 714.
Celth, 177.
Centaurea, 201, 651.
■ — jacea, 652.
651.

Central cell, 754, 773-775.

• — ovule, 465.

Centrifugal force, 682, 683,

707,708.
Cephalanthera grandiflora,

791.
Cephalar'ia leucantha, 214,

568.

— procera, 543.
Ceramium, 335.
Ceratopteris, 455.

— 455.

Ceratonia siliqua, 88.
Ceratophyllum, 297.
Ceratopteris tbalictr aides ^ 478,

525, 772, 774-
Ceratozamia longifoUa, 756.
Cercis canadensis, 714.
Cereus, 54, 159, 472, 780.

— senilis, 178.

• — speciossissimtts, 780.
Cestrum roseum, 279.
Cetraria Islandiia, 707.
Chcetomorpha aerea, 611.
Chalk-skeletons, 290.
Chamomile, 505.
Chara, 95, 320, 606.

— apical cell of, 96, 453.

— cell-division in, 106.
Characes, 32, 85, 95, 432,

453, 454, 471, 545, 608.

— apogamy ot, 788, 789.
Chara crinita, 801.
Cheilanthes, 186.
Cheiranthus Cheiri, 121.
Chelidonium, 172, 174.

— majiis, 175.

Chemical action, curve of,

305.
• — analysis, 288.

— changes in germination,
328.

— changes due to Fungi, 369.

— compounds, 206.
in soil, 261, 287.

■ — forces in the plant, 202.

— influences, rigor due to,
595.

Chemical light -rays, 301,
304.

— metamorphoses of pro-
ducts of assimilation, 323.

— processes in assimilation,
297, 314, 317-

— processes in chlorophyll,
315.

Chemistryofchlorophyll,32i.

— of respiration, 402.
• — of starch, 339.
Chenopodiacese, 465, 640.
Chenopodium qiiinoa, 124.
Cher me s viridis, 537.
Cherry, 90, 147, 763.
gum, 180.

stone, 218.

Chestnut, 334, 493.
Chickory, 24.
Chilomonas curvata, 613.
Chinese galls, 537.
Chloranthy, 537.
Chlorides, 202, 284, 287.
Chlorine, 383.

— absorption of by leaves,
255-

■ — importance of, 286.

Cblorococcus, 433.

Chloroform, rigor due to,
595.

Chlorophyll, 63, 64, 77, 85,
194, 198, 226, 264, 289,
294, 298, 324, 326, 375,
378, 397, 510, 512, 766.

— the agent of assimilation,
299, 309, 368, 530.

— absorption of light by, 322.

— in animals, 298, 394.

— changes in, 319, 321.

— chemistry of, 321.

— chemical processes in, 315.

— and configuration, 509-
514.

— consequences of its ab-
sence, 368.

— development of, 300, 533.

— fluorescence of, 322.

— functions of, 314, 318.

— in Lichens, 393, 514.

— in thin layers, 510, 511.

— in swarmspores, 603, 604.

— origin ofstarchin, 309,3 10.

— plants devoid of, 367.

— reactions of, 321.
• — spectrum of, 322.
bodies, 85, 299.

movements due to

light, 617-621, 696.

cells, 148.

corpuscles, 85, 148, 250,

285, 296, 307, 309, 314,
319, 326, 628, 633, 645.

assimilating, 316.

changes of, 320,

Chlorophyll-corpuscles, de-
velopment of, 318.

— from amyloplasts,

316.

disorganization of, 319.

division of, 318.

first visible changes in,

315-

pseudo-, 317.

starch in, 338.

plants, 366.

Chlorosis, 285.
Chlorotic leaves, 298.
ChroococcacesE, 107.
Chrysosplenium, 506.
Chytridium, 604, 611.

— vor ax, 612.
Cichoracese, 171, 173.
Cichorium Intybits, 76, 92.
Cilia, 604, 605, 613,728-731,

733, 734, 742, 770, 775,

777.
Cinchona, 164.
Cinnamon, 177.
Circxa, 536.

Circulating Proteids, 325.
Circulation, 82, 615-617.
Circumnutation, 546, 547.
Cissus, 66, 371.

— 545-

— discolor, 662.
Cistus, 185.
Citric acid, 328.
Citron, 183.
Citrus, 177, 183.
Cladodes, 55, 64, 511.
Cladonia furcata, 392.
■ — pyxitata, 707.
Cladophora, 33.
Cladothrix, 387.
Clasping organs, 668.
Classification of ferments,

342.

— of products of metabo-
lism, 323.

CI at hr us, 428.
Cleistogamous flowers, 790,

791.
Clematis, 56, 166, 220, 659.

— -viticella, 46, 127.
Climbing leaves, 675.

— organs, 658.

— plants, 56, 513, 548.

— shoots, 56, 483, 489, 522,

657, 660, 713.
Closing of flowers, 599, 641.

— of stomata, 249.
Clover, 623, 627.
Clusiaceae, 183.
Clusters of crystals, 177.
Coagula of latex, 172.
Coal, 294, 381.
Co-axial organs, 451.
Cobea, 662.

INDEX.

809

Cobea scandens, 56, 660.
Coco-nut, 218, 373, 375, 763.
Coditim, 154, 335.
Ceslebogyne, 802,
Coeloblastcr, 76, 84, 108, 154,

179, 280, 426, 430, 543.
Coenobium, 728, 730, 731.
Coffee, 763.

Coiling of tendrils, 661-665,
Colchicum, 332.

— autiimiiale, 331, 641.
Gold, rigor due to, 593.
Coleochwte scutata, 125, 484.
Collemaceae, 90.
CoUenchyma, 142, 216, 582.
Colleters, 124, 186, 281.
Colocasia, 316.

■ — atitiquorum, 278, 280.

Coloured bacteria, 387,

lights, 303, 696, 697.

• action onswarmspores,

613.

— solutions, experiments
with, 235, 269.

Colouring matters, 176, 180,

299, 319, 320, 328.
Colourless plants, 380.
Colours of flowers, 320, 329.

— of leaves in autumn, 319.

in winter, 321.

Columella, 739.

Colutea, 623, 759.
Combustion, 400,
■ — of plants, 296.
Commelynacefv, 178.
Commelyna bengalensis, 791.

— Ccelestis, 97.
Commensalism, 34, 299, 392.

— in animals, 394.

Comparison between fertili-
sation and ferment-action,
768.

— between growing points
of Phanerogams and of
Cryptogams, 458.

— between Heliotropism and

Geotropism, 692,694-696.

— between irritability of
plants and that of ani-
mals, 597, 600.

— of latex-vessels to blood-
vessels, 361.

— between parasites and
seedlings, 373.

— of plant to machine, 190,

590, 591, 6or.

— between reproductive
organs of plants and of
animals, 747, 753,798-

• — between respiration of
animals and plants, 406.

— between tendrils and
twining shoot-axes, 668,
674, 675.

Competitionbetween organs,
508, 509.

Composita^, 181, 335, 343,
461, 498, 505, 572, 640,
642, 650, 763, 792.

Composition of atmosphere,
293.

— of latex, 362.

— of light, 302.

— of plants, 206, 290.

— of seeds, 290.

— of soil, 263, 287.
■ — of wood, 290.
Compound hybrids, 784.

— leaves, 52, 490, 623, 626,
629, 639, 640, 687.

— starch-grains, 315, 337.
Concentric starch - grains,

316, 338.
Condensation of imbibed

water, 210.
Conditions of assimilation,

295, 296.

— affecting root - pressure,

273.

— affecting transpiration,

246, 251,

— of plant-life, 189, 299.
Conductibility of wood, 243.
Conducting tissue, 773, 778.
Conduction of heat in plants,

404.

— of water in wood, 230,
Cone, 498, 499.
Configuration, 420, 422, 515,

516, 528.

— and chlorophyll, correla-
tions between, 509-514.

— changes in during growth,
443-

— and growth, relations be-
tween, 450.

— influence of environment
o", 516, 537.

Confocal structure, 451, 469.
Conidia, 722, 725, 769, 798.
Gonidiophores, 725.
Coniferae, ascent of water in,
230.

— branching of, 473.

— epidermis of, 122.

— roots of, 23.

— shoots of, 37, 55.

— vascular bundles of, 44.

— venation of, 52.

— resin passages, 181.
■ — wood of, 237,
Conifers, 143, 146, 155, 162,

164, 177, 225, 229, 267,
269, 300, 334, 368, 373,
375, 388, 424, 472, 473,
493, 506, 510, 530, 724,
745, 746, 750-756, 757,
798.

Conium, 52.

Conjugate, 299, 324, 453,

723, 727, 729, 768, 789.
Conjugation, 107, 725, 728-

730, 772, 776, 777.
Connective, 757.
Constructive materials, 326,

329, 341, 355, 357, 362,

41 1, 412, 508,
movements of, 355,

359-
Contact, as a stimulus, 190,

503, 597, 601, 602, 716.
Contents of cells, 77.

— of laticiferous vessels, 172.

— of sieve-tubes, 325.
Gontextus cellulosus, in.
Continuity of embryonic

substance, 417, 502, 767,
769, 770.

— of animal-life, 413.

— of laticiferous vessels, 173.

— of tissues, 159, 457, 469,
470, 475-

Contractile stamens, mea-
surements of, 652.
Contractility of protoplasm,

613,654.
Contraction on drying, &c.,
208.

— of air in wood, 270.

— of cortex, 575.

— of oosphere, 776.

— of tendrils, 665.
Contrast between Angio-

sperms and Gymno-
sperms, 757, 760-764.

— ■ — Angiosperms and Cryp-

togams, 761, 766.

— between radial and dorsi-
ventral structures, 493,
704, 705.

— between plagiotropic and
orthotropic structures,
493, 705.

— between spores and seeds,

753-
Coni'allaria latlfolia, 51.

52, 128.

Conversion of bilateral into

orthotropic organs, 504.

— of bud-scales into leaves,

507.

— of embryonic into per-
manent tissue, 466.

— of growing - point into
ovule, 465.

— of permanent into em-
bryonic tissue, 480.

— of scale-leaves into foli-
age-leaves, 536.

— of starch into sugar, 339.

— oftubers into leafy-shoots,
504.

8io

INDEX.

Convolvulaceae, i8o, 780.
Coniwh'ulus, 56, 657, 669,
670.

— Scammomum, 180.
Cooling of plants by radia-
tion, &c., 404, 643.

by transpiration, 553.

Coprophytes, 366.
Corallinese, 179, 290.
Corallorhiza, 62, 63, 367, 372,

374-
Cork, 45, 88, 156, 246, 582.

cambium, 165.

cells, 165.

oak, T65.

■ warts, 1 66, i68.

Corm, 45.
Corn-Bind, 657.

■ Flower, 651, 686.

CornuJ, 166.

Corolla, 625, 641, 651, 757.

Corpuscles, starch-forming,

309.
Correlations of functions in

animals, 508.

— of growth, 502-505, 507,

508, 512, 765.
and anisotropy, 704.

— between chlorophyll and
configuration, 509-514.

— between leaves and bud-
scales, 506.

— between leaves and roots,

512, 513-
Corrosion-figures of roots,

262.
Cortex, 158, 230, 300, 312,

320, 330, 335, 359, 422,

470, 479, 574, 575, 579,

582, 630.

— contraction of, 575.

— pressures and strains in,

574, 575, 577-579, 580.

— of roots, 169.

rays, 159.

Corydalis cava, 789.

— Cla'viculata, 659.
Cory 1 us, 186.

— colurna, 51.

■ — maxima, 51.
Cotton, 125, 183.
Cotyledons, 4,332, 334, 357,

373, 414, 505, 531, 040,

702, 752, 753, 764, 765.
Couch grass, 59.
Crassulaceae, 55, 148, 227,

497.
Creeping shoot-axes, 60, 4 8 3 ,

485, 489, 492, 522, 678,

699.
Crevices in bark, 577.
Crocus, 45.

— 351, 625, 641, 642, 794.
•^ "vernus, 43.

Cross-fertilisation, 787, 788,

790, 799.

Crown Imperial, 59, 61, 281,
335, 350, 596.

679.

Crucibulum, 428.

— vulgare, 152.
Cruciferae, 123, 177, 473,

497, 588, 780.
Cryptogams, 351, 365, 447,
455, 458, 473, 492, 535,
538, 623, 660, 789, 790,

791, 798, 800.

— hybrid, 780.

— receptacles for secretions
in, 171, 179.

— reproduction of, 722, 724,
734, 746, 753, 755, 756.

— tissues of, 149.

— and phanerogams, homo-
logous structures in, 751,
756, 761, 766.

Crystallisation, 206, 502, 516,

591, 592.
■ — of Inulin, 336.
Crystalloids, 333, 334, 335,

341.
Crystals, 177.

— growth of, 412, 413.

■ — in cells, 77, 291, 388.

— in cell-walls, 177.

— sphere-, 336.

— unstable, 591-593.
Cucumis Chate, 782.

— Mela Cantalupus, 782.
Cucurbita, 52, 320, 326, 408,

483, 714, 763.

— 140, 435.

— Melo-Pepo, 400.

— Pepo, 664.

12.

Cucurbitaceae, 334, 360, 516,

547, 658, 660, 661, 757,

762.
Cultivated Vines, 785.
Cultivation of Bacteria, 387.

— of Fungi, 382.

— of Yeast, 384.

— of plants in nutritive so-
lutions, 283,284, 286,290,
291, 295.

— of plants in pots, 198.
Cupressineae, 40, 166, 181,

758.
Curcuma, 337.
Curvatures, geotropic, 680-

691.

— heliotropic, 693, 694.

— of motile organs, 629,630,
632, 634.

— of nodes, 687, 688.

'' — of tendrils, 661, 662.

— of tissues, 217.
Curve, Draper's, 305.

Curve of growth, 551, 552,
554, 555, 558, 562.

— of temperature, 406.
Curves of assimilation, &c.,

195, 305-
Cuscuta, 27, 35, 63, 369, 372,
373, 670, 675, 765.

— epiltnum, 27.
Cuscutese, 368, 369, 373.
Cut branches, experiments

with, 235, 244.
Cuticle, 47, 89, 1 16, 176, 246,

329, 416.
Cuticular substance, 38S.
Cuticularisation, 423.
Cutin, 116.
Cuttings, 5S2.
Cuvettes, 305.
Cyanogen compounds, 384,
Cyanophyllumformosum, 51.
Cyathea Imrayana, 130, 231.
Cycad, 756.
Cycadeae, 181, 472, 506, 746,

750, 751, 754-756.
Cycads, 143, 465, 547.
Cycas rei'oiuta, 756, 757.
Cynara, 651, 654.

— Scolymus, 652,
Cynareae, 647, 650, 651, 654.
CyperaceiE, 176.

Cyperus, 219.
Cystoliths, 178.
Cytisus Laburnum, 782.

— purpureuj, 782.

Dahha, 24, 309, 335, 357,
461, 559.

— 140, 247, 491, 559.

— variabilis, 90, 315, 336.
Daily periodicity, 199, 625.
of floral movements,

642.

of growth, 558-560.

of leaf - movements,

&c., 625-627, 635-638.

of root-pressure, 273.

of tissue-tensions, 575.

Dandelion, 572, 574.

Daphne, 506.

Darkness, development of

flowers in, 534.

— disappearance of starch
in, 309.

— germination in, 326, 530.

— growth in, 353, 519, 532,

713-

— rigor due to, 593, 628,
635,636.

Date, 194, 230, 335, 344,
357, 373, 375, 763.

palm, 689.

Datura, 186, 279, 780.

— Stramonium, 130.

INDEX.

8ii

Daucuj rarota, 343.

Daughter-cell, 95, 100, 433,
434. 452, 454) 728, 746.

nuclei, 104.

Decomposition of carbon-
dioxide, 296, 298, 299,

303, 309-

— ot Proteids, 402.

— of rocks by Lichens, 392.

— of soil, 263.

— of vegetable matters, &c.,

294,

— ot wood by Fungi, 389.
Decompositions due to

Fungi, 369, 381, 385.
Deep-sea plants, 306.
Definite growth, 465, 483,

544-
Definitive form and size, 412,

413, 420, 421, 423, 439.
Degeneration, 7, 368.
Degradation, 64, 65.

— due to parasitism, 368,

370.

— products of, 328, 329.
Degraded reproductive or-
gans, 723.

— roots, 369.
Delesseria sanguinea, 71.
Dentaria, 506.

— bulbifera, 802.
Dependence of cell-division

on form and growth of
organs, 433,435.

— of geotropic curvature on
growth, 679, 685, 688,
691.

— of plant-life on heat, 197.
on light, 197, 301.

■ on oxygen, 395, 402,

403.
Depressed growing - points,

461, 463, 474.
Derivative hybrids, 784.
Derived forms, 7.
Descending sap, 156.
Descent, 2, 10, 516.
Desiccation, 211.

— of soil, 258.

— of spores, 352.

— of wood, 242.
Desmideac, 483.
Damodium gyratu, 624.
645.

Destruction of organic sub-
stance in respiration, 400,
402.

— of trees by Fungi, 3 88,

389.
Destructive action of Fungi,
369.

in wood, 388.

Development, 412, 414, 419,
524.

Development, exogenous,
475.

— progressive, 467, 468.

— at expense of reserve-
materials, 327.

— of growing-points influ-
enced by environment,
516, 523.

— of organs at growing-
points, 460, 465, 467,
769.

— of antherozoids, 770, 771.
■ — of bark, 166.

— of branches, 471, 475.

— of cells, 94.

— of chlorophyll, 300, 318.

— of chlorophyll-corpuscles,
316.

• — of flowers, 497.

— of flowers in darkness, 534.

— of glands, 184.

— of leaf, 425, 468,471,475,
506, 744.

— of laticiferous vessels, 174.

— of plants in nutritive solu-
tions, 286.

— of pollen, I or.

— of resin-canals, 182.

— of root, 15, 418, 470, 476,
522, 523, 744-

— of shoot, 744.

— of sporogonium, 738.

— of starch-grains, 315, 316,

338.
Dew, 278,
Dextrine, 349.
Dextrose, 343.
Diagram of flower, 761.

— floral, 488.

— of geotropic curvatures,
680, 686.

— of plant, 488, 495.

— of root, 275.

Diagrams of growth and cell-
groupings, 440, 441,
443, 448, 450, 451,
452, 457, 459, 468.

Diameter of tendrils, 666.
DiaTithuj, 780, 782.
■ — caryophyHus, 782.

— chhiensis, 782.
Diaphragm of macrospore,

747, 750.
Diastase, 343.
Diastatic ferments, 343.
Diatomacea:, 288, 483, 604,

606.
Dicetitra thalictrifolia, 661.
Dichogamous flowers, 791,

792.
Dichogamy, 789, 790, 799.
Dichotomex, 745.
Dichotomy, 454, 473.
Diclinous flowers, 757, 790.

Dicotyledon, 356, 415.
• — ascent of water in, 230.

— flowers of, 757.

— glands of, 184.
Dicotyledons, 44, 144, 155,

160, 170, 229, 373, 375,
414, 418, 419, 424, 425,
470, 471, 472, 473, 478,
569, 581, 660, 689, 706,
758, 760, 763, 765.
• — phyllotaxis of, 500.

— receptacles for secretion,
177.

— reproduction of, 756.

— secretion canals, i8r.

• — • secretion vesicles in, 179.

— vascular bundles, 134.

— venation of, 52.

— wood, 162.
Dictamnuj, 548.

— Fraxinella, 183,472, 548.
183.

Die ty Ota, 454.

— 455.

— dichotoma, 474.
Didymium fcrofioceum, 420.

— leucopus, 83.

Diff"erences between organ-
ised and unorganised
ferments, 348.

— between sexual cells, 768.
Diff'erentiation, 420, 423,

437, 470.
Diffusion, 212, 362, 363.
Digestion in Dionwa, 376.
■ — in Drosera, 378.

— in Ntpenthes, 379.
Digestive fluids in insecti-
vorous plants, 377, 379.

— glands, 186, 378.
Digitalis, 271, 780, 782, 792.
Dimorphic flowers, 790.
Dioecism, 788, 789.
Diomm, 374, 376, 597.

— muscipula, 376, 650, 655.
375.

Dioscorea, 56.

■ — batatus, 319, 672, 674.

— Japonic a, 674.
Dioscorex, 802.
Diosmosis, 211, 362, 363.
Dipsacus, 572, 597.

- — fulloniwt, 498, 543.

498.

Directions of cell- walls, 432,

438, 439-459-

— of growth, 698.

— of twining, 668, 669.
Directive action of gravita-
tion, 678.

— ■ — of light on chlorophyll-
corpuscles, 617-621.

— ■ — of light on leaves,
621.

il3

INDEX.

Discontinuous variations in

growth, 551, 552.
Disease, 285, 289, 294, 301,

531, 547.

resisting Vines, 786.

Disintegration of rocks, 264.

Disorganisation of chloro-
phyll, 319.

Displacements of cell-walls
during growth, 445.

— of leaves, 462.
Disproportionality between

stimulus and effect, 589,

595, 597, 598.
Distilled water, germination

in, 284.
Distribution of growth, 411,

415, 437, 438, 544, 569,

683-685.

— of laticiferous vessels, 173.

— of resin- passages, 181.

■ — of water in wood, 241.

Diurnal position of leaves,
&c., 625-630, 634, 635,
636, 638, 646.

Divergences, 499, 500.

Dividing cells, form and vo-
lume of, 434.

Division of cells, 94, 97, 101,
105, 107.

— — laws of, 433, 434, 435.

— of chlorophyll-corpuscles,
318.

■ — of nucleus, 102-105.

— ofpoIlenmother-ceils,433,

434-
Dock, 186.
Dodder, 675.
Dolomite, 263.
Dormant buds, 475.

— condition of organs, 320,
411, 414.

— periods, 341, 350, 534.

— protoplasm, 402.
Dorsi-ventral and radial or-
gans contrasted, 705.

— organs, rolled up, 706,707,
709.

— shoot-axes, 455, 485, 713.

— structure, 485, 489, 490,
491, 492, 493, 494, 496,
499, 502, 524-527, 625,
698, 704, 709.

494.

Double flowers, 783.
Draba -verjia, 791.
Drocxna, 51, 62, 130, 144,
169, 230, 528, 571.

— 170, 231.
Drainage, 256.

water, 259.

Draper's curve, 305.
Drooping, 215, 228, 233,244,

254, 258, 264, 567.

Drosera, 124, 378, 655.
— rotiindifolia, 377.
Droseraccr, 376.
Drought, rigor due to, 595.
Duckweed, 122, 619.
Duramen, 163.
Dwarf-males, 775.

Echinocactus, 54, 472, 780.
Effects of fertilisation, 768.

— of injury, 504, 507.
Elfragnuj, 124.

Elasticity of cell- walls, 566-
568.

— of stems, 234, 589, 599,

— of tissues, 219.
Elaters, 741.
Elder, 77.
Electric-currents in Dionxa,

&c., 650.

lightandassimilation, 307.

Electricity as a stimulus, 6 1 7,

654.

— effect of on plants, 190,

201, 601,

— rigor due to, 596.
Elements composing plants,

290.
Elimination of geotropism,
683, 694, 705, 716.

— of heliotropism, 705,
Elm, 165, 489, 714.
Elodea, 297, 311.
Elongation of cylinders of

pith, 572, 573.

— of organs, 414, 417, 419,
420, 422, 424, 440, 461,
539, 563, 570.

Embryo, 332, 344, 373, 414,
435, 443, 472, 484, 538,
563, 737, 742, 743, 744,
747, 748, 750, 753-756,
760-766, 767, 800.

sac, 465, 750-752, 754,

755, 760, 763-766, 800,

• — - — nucleus of, 762.

Embryology of Angiosperms,
764, 765.

— of Equisetaceae, 743.

— of Ferns, 744.

— of Gymnosperms, 751,
754-756.

— of Mosses, 737-739.

— of Vascular Cryptogams,
747-750.

Embryonic condition, 411,
414, 420, 424, 437, 439,
461, 539, 544, 563.

— growth, 419.

— substance, 447, 480.

— — continuity of, 417,503,
767, 769, 770.

Embryonic tissue, 112, 415,
417-419, 422, 423, 431,
440, 445, 461, 466, 467,
473, 477, 479, 480, 482,
502, 765, 770.

Embryos, adventitious, 802.

■ — of parasites, 765, 766.

Emptying of leaves in
autumn, 319.

■ — of sieve-tubes, 361.

Empusa Muscw, 381.

Emulsifying ferments, 343,
347.

Emulsin, 342, 349.

Emulsion-figures, 609, 610.

610.

Emulsions, 175, 179, 182.

Endodermis, 143, 169, 358.

Endogenous buds, 476.

Endophytic parasites, 371.

Endosmose, 211, 225, 243,
250, 253, 275, 280, 363,
565, 566, 573.

Endosperm, 332, 335, 343,
366, 373, 390, 538, 750,
752-756, 762, 763, 800.

Energy of assimilation, 311,
312.

— of fermentation, 386.

— of irritable organs, 601.

— of nerves, 600.

• — of respiration, 396, 397.

— stored and freed by the
plant, 199, 295, 403.

Enfeeblement of sexuality,-

783.
Environment of the plant,

189, 515, 593, 594, 678.

— influence of on growth,
552, 554, 555-

on origin of growing-
points, 516, 523.

on post - embryonic

growth, 516, 537.

Epicotyl, 712.

Epidermal glands, 175, 183,
185, 378.

— hairs, 44, 48, 122.

— secretions, 184.

— tissue, no, 113, 171.
Epidermis, 45, 115, 159, 166,

216, 226, 246, 288, 329,
416, 420, 423, 470, 479,
631,633, 634, 645, 739.

— of leaves, 47, 97.

— of moss, 149.

— tensions of, 569-572, 574,
582.

Epilobium, 792.
Epipactis latifolia, 796.
Epipogon, 62, 63, 372, 374.
Epithelium, 182.
Equilibrium of diffusion, 363.
• — unstable, 59/, 592, 597.

INDEX.

813

Equisetaceae, 55, 178, 288,
418, 701.

— reproduction of, 739, 742,
743, 788, 800.

Eqttisetuni, ^3, 62, 122, I49,
475, 536, 604, 722, 724,
739-742, 744, 748, 751-

— 476.

— apical-cell of, 452.

— aruense, 739.
43, 422, 743.

— hyemale, 418.

— limosum, 59, 741.
740.

— pal us t re, 59.
— • Tehnateia, 739.

740, 741, 743.

Erection of shoot-axes, 686,

687.
Ericaceae, 780.
Erodium griiinum, 211.
Ether, rigor due to, 595.
Ethereal oils, 124, 126, 175,

183, 328, 362, 369, 386,

403.
Etiolation, 197, 289, 298, 300,

309, 316, 353, 355, 514,

531, 533, 535, 560.
Eudorina, 728, 731.

— elegans, 730.
Euglenae, 604.

Euphorbia, 55, 174, 203, 361.
• — canariensis, 55.

— Caput medusx, 55.

— cyparissias, 369.

— globosa, 55.

— helioscopia, 488.

— spltndens, 174.
Euphorbiaceae, 171, 172,174,

180, 203, 334, 362, 802.
Euphorbium, 172.
Euryaleferox, 51.
Evaporation of water, 227.
Evolution of carbon-dioxide,

397, 401, 402.

— of oxygen, 195, 297, 303,

306, 312, 397.
Exalbuminousseed, 763,764.
Excentric growth, 439, 445.

— starch-grains, 316, 338.

— structure, 579, 580.
Excreta, 175.

— secondary uses of, 329.
Excretion of acids by roots,

262.

— of ferments, 344, 389.

— of nectar, 281.

— of water, 265, 266, 278.

— of water, 278.

by Fungi, 280.

Excretions, 172.
Excretory organs, 175.
Exhalation of carbon - di-
oxide, 397.

Exhaustion of reserves, 284,

326.
Exogenous development, 47 5.
Exosmose, 211, 281, 617.
Expansion of air in wood,

267, 269, 270.
Experiments on assimilation,

310, 353-

— on correlation, 504-506.

— on geotropisin, 691,

— on hellotropism, 693-695.

— on respiration, 397, 399.

— on sleep of plants, 625.

— with coloured solutions,
235, 269.

235.

— with cut branches, 235,

244, 277, 278.
244, 279.

— with lithium nitrate, 235.

— with swarmspores, 611.
Expulsion of water from

motile organs, 647-649.
Extensibility of tissues, 220.
External causes of growth,

503-

— conditions of plant-life,

189.
Extinct Horse-tails, 745.
Extine, 761, 762.
Extra-axillary buds, 473.
Extrusion of latex from

wounds, 361.

F.

Fagus, 177.

Fall of leaves, 319.

False Acacia, 649.

Fasciations, 505.

Fascicular cambium, 157.

Fats, 77, 172, 175, 290, 310,
326, 329, 347, 355, 357,
361, 363, 382, 400, 402,
403.

— as reserve materials, 332,
763.

— in Fungi, 384.

— origin of, 323.
Fatty acids, 347.

Fatty seeds, 334, 347, 400.
Female caniobia, 731.

— plants, 737, 742, 751.

— prothallia, 742.

— reproductive organs, 724,
734, 736, 751, 757, 769-

Fennel, 181.
Ferment-action, 342, 343,

349, 387, 389-
fungi, 347, 369, 382,

402.
Fermentation, 342, 348, 382,

385, 389-

— acetic, 386.

— alcoholic, 349, 386, 401.

Fermentation and fertilisa-
tion, 768.

— and oxygen, 386.

— butyric, 386.

— energy of, 386.

— lactic, 349.

— of apples, 386.

— products of, 348.

— by protoplasm, 349.

— temperature of, 386.

— theory of, 349.
Ferments, 172, 175, 341,

342, 347, 348, 352, 373,
376, 378, 379, 389.

— classification of, 342, 343,

348.

— excretion of, 344.
by Fungi, 389.

— in seedlings, 343, 345, 389.

— organised, 347.

Fern, 13, 85, 129, 137, 156,
421, 442, 446, 447, 455,
456, 467, 471, 472, 478,
492, 497, 510, 525, 547,
608, 800.

— 446, 462.

— apical-cell of, 452.

— embryo of, 446.

— epidermis of, 116, 121.

— leaf of, 455.

— prothallus of, 458, 466,
491, 619, 788, 789.

— ramentaceous hairs, 125.

— reproduction of, 722, 724,
741-744, 747, 748, 772-
775-

— roots of, 31.

— shoot-axis of, 45.

— stomata of, 119.

— vascular bundles of, 134.

— venation of, 51, 52, 128.
hybrids, 780.

Ferns, 144, 178, 225, 300,
425.

— apogamous, 80 r.

— ascent of water in, 230.

— secretion vesicles in, 179.
Ferrous salts, 285.
Fertile shoot, 748.
Fertilisation, 108, 726, 727,

730, 731, 733, 734, 737,
742, 750, 751, 752, 754,
757, 760-764, 767, 768,
771, 772, 779, 799-

— as a stimulus, 537, 768.

— compared to ferment-
action, 768.

— delinition of, 768,

— effects of, 768.

— object of, 798,

— of Ferns, 772-775.

— of flowers by aid of in-
sects, 329, 535, 791, 793-
797-

8i4

INDEX.

Fertilisation of Gymno-
sperms, 755.

— of CEdogonium, 775, 776.

— Peronosporea-, 777, 778.
Fertilising substance, 726,

751, 755, 760, 768, 770,
773, 778.

— tubes, 777, 778.
Fibres, 139, 160, 329.

— libriform, 139,
Fibro-vascular strands, 12.
Fie aria, 802.

— ranunctiloides, 43, 641.
Ficiis, 172.

— acuminata, 50.

— carica, 172, 354, 362.
465.

— elastica, 362.
179,

— religiös a, 50.

— scnndens, 56.

Fig, 21, 345, 464, 467.

— 464.

Filamentous organs, growth
and cell-divisions in, 443,
453.

432.

Filaments, 652, 757.

Filtration, 213, 245, 275, 362,
639.

— in irritable organs, 653.

— through cell-wall, 276,
565, 566.

— through wood, 237.

— of light through leaves,
&c., 306.

F'l", 37, 43, 116, 146, 163,
200, 203,225, 237, 312,
369, 504, 505, 537, 581,
677, 678, 68r, 699, 703,
704, 756, 798.

Fir-cone, 758.

First phase of growth, 414.

Flag, 60.

Flagellaria indie a, 659.

Flat-shoots, influence of light
on, 709, 710.

Flax, 90, 242, 345, 545, 692,
703, 798.

Flexibility of tissues, 220.

Floating leaves, 51, 511.

— plants, 485.

roots of, 255.

Floral diagrams, 488.

— envelopes, 623, 625, 757.
• — nectaries, 281.

— organs, 298.

— receptacle, 498.
Florideae, 114, 179, 299, 335,

483, 726, 727.
Flowering Rush, 758.
Flower, 401, 463, 467, 475,

485, 508, 625, 626, 641,

651, 758, 759.

Flower, development of, 497.
- — diagram of, 761.

— terminal, 465.

— typical, 756, 757.

buds, 397, 461, 463, 50S,

534-
forming substances, 534,

535-
Flowering plants, 225, 724,

725, 750, 757, 758, 760.
Flowers, androgynous, 788-

790.

— cleistogamous, 790, 791.

— colours of, 320, 329.

— development of in dark-
ness, 534,

— dichogamous, 791,792.

— diclinous, 757, 790.

— fertilisation of, 791, 793-

797.

— hermaphrodite, 757, 788.

— heterostylous, 792.

— mechanisms of, 791, 792.

— periodic opening and
closing of, 599.

— protandrous and proto-
gynous, 792.

- — relations to leaves, 534.

of insects to, 655, 657,

760, 789-797.

— respiration of, 400.

— self-sterile, 790.

— sleep-movements of, 199,
623, 625, 641, 642.

— sterile, 791.

— trimorphic, 790.

— of Dicotyledons, 757.

— of Gymnosperms, 756.

— of Monocotyledons, 757.

— of Parasites, 63, 64.

— of Tan, 82, 614.
Fluctuations in root-pres-
sure, 272.

Fluorescence of chlorophyll,

322.
Fwniciilitm officinale, 142,18!.
Foliaceous Lichens, 393.
Foliage leaves, 47, 489, 493,

506, 507, 623, 625, 627,

633, 640.

— shoots, 37.
Foliar-base, 506, 507.
Fontinalis, 465.

— antipyretica, 151, 739.
Food-materials of Fungi, 3 8 2 .

— of parasites, 373.
Foot, 747.

Form, 9, 432, 503, 507, 514,
516, 678, 703.

— and growth, relations to
cell-division, 433, 435.

— change in, during growth,

443-

— definitive, 412, 421, 439.

Formation of fats in Fungi,
384.

— of Proteids, 324, 346.

— of starch, 311, 315, 316,
Formic acid, 318, 328, 384.
Forms of dividing cells, 434.

— of growing-point, 461,

— of hairs, 122, 125,

— of lichens, 393.

— of reserve-materials, 332.

— of starch-grains, 337.

— of tisssue, 110,423.

typical, 115.

Formyl aldehyde, 318.
Fraunhofer's lines, 303.
Free cell-formation, 99, 106.

— nitrogen not assimilated,
291, 384.

Freezing of plants, 192, 242.
Fritillaria, 424, 679.

— imperialis, 78, 332, 423,
564, 679.

— imperialis, 281, 543.
Frog, intra-molecular re-
spiration of, 401.

Fructification, 726, 727, 740.

— of Fungi, 381, 388, 427,
428, 514.

Fruit, 329, 483, 588, 759-

— ripening of, 320.

— respiration of, 400.
Fruits, coloured, 320.
Fruticose Lichens, 393.
Fucaces, 299, 605, 754, 788.
Fuchsia, 279.

Fucus, 32, 70, 462, 732.

— platy carpus, 787, 788.
731.

— serratus, 780, 781.

— I'esiculosus, 732.

780, 781.

Fuller's Teazle, 498.
Fumaria, 56.

• — officinalis, 659.
Fumariaceae, 791.
Fumitory, 659.
Funaria, 311, 619, 621.

— hygrometrica, 527, 735,
780.

30, 68, 85, 120, 150,

314, 527, 736, 738.
Functions of chlorophyll,

314,318.

— of hairs, 123, 125.

— of leaves, 248.

— of petioles, 53.

— of roots, 16.

— of root-hairs, 255.

— of sieve-tubes, 325.

— of stomata, 248, 251, 264.

— of tissues, 355.

— of vascular bundles, 134,

325, 359-

— of wood, 163.

INDEX.

Fundamental tissue, 45, 115,
141, T59, 171, 423.

Fungi, 90, 120, 155, 203, 283,
292, 298, 324, 331, 343,
344, 347, 350, 3=)i, 367,
368, 370, 371, 375, 381,
391, 408, 419, 432, 479,
511, 514, 563, 584, 603,
604, 717, 786, 787, 798,
800.

— ash-constituents of, 383.

— as stimuli, 537.

— calcium oxalate in, 178.

— cell-structure of, 77.

— chemical action of, 369.

- — contrasted with green
plants, 383.

— cultivation of, 3S2.

— decompositions due to,

381, 385.

— destruction by, 369, 388.

— excretion of water by,
280.

— ferment-, 347, 382, 402.

— ferments in, 344, 369,
389.

— food-materials of, 382,

384.

— formation of cellulose in,

385.
of fat in, 384.

— fructification, 153, 381,

388, 427, 428.

— geotropism of, 700.

— growing point of, 427.

— growth of, 425, 426.

— heat evolved by, 405.

— Insect-killing, 381.

— Lichen-, 391.

— luminous, 406.

— metabolism of, 384, 385.

— Mould-, 347, 380, 383,
384, 385, 427.

— non-cellular, 152.

— nuclear division in, 106.

— nutrition of, 367, 380, 381,
382, 383, 385, 389-

— parasitic, 381, 389, 537,
725.

— putrefactive, 382, 386.

— reproduction of, 721-725,
752, 734, 777.

— respiration of, 385, 398,
401.

— root-, 3S8.

— root-pressure in, 281.

— Rust-, 381.

— saprophytic, 381.

— sexual reproduction of,

725.

— shoots of, 54, 71.

— tissues of, 149, 151.

— tree-killing, 344, 369,

381, 387, 389, 407.

Fungi, vegetative forms of,
387.

— variety of, 380.

— Yeast-, 349, 385.

— wood-destroying, 344,381,

387.
Funiculus, 760.
Fim^la ovata, 99, 801.

— — 802.

G.

Gngea pratensis, 463.
Gall-apple, 537.

■ insects, 537.

Galls, 204, 537.

Gametes, 604, 728, 767, 768,

772, 776.
Gamosporeae, 768.
Garlic, 59, 802.
Gases, absorption of by

leaves, 255.

— concerned in assimilation,

297, 298.

— movements of, 251.

— of putrefaction, 387.

— rigor due to the action of,

595-
Gasteromycetes,9o, 151,380,

428, 798.
G east er, 428.
Gelatinous Lichens, 391.
Gelidium, 71, 471.
Gemmae, 444, 479, 524, 526,

535, 709, 723, 769, 798.
Genetic spiral, 495, 498,499,

500.
Gentiana lutea, 2 1 .
Genus-hybrids, 780.
Geometric centre of organs,

482.
Geometry of growth, 438.
Geotropic curvature, 680,

686, 687.
dependent on growth,

679, 685, 688, 691.

— curvatures, measurement
of, 683, 688, 690.

Geotropism, 16, 200, 485,
507, 512, 528, 596, 597,
600-602, 657, 675, 677,
679, 685, 692, 694, 697,
699, 700, 702, 705, 707,
709, 710, 712.

— due to gravitation, 6S1-
683.

— elimination of, 705, 716.

— positiveandnegative,679-

681, 689-691, 703.

— and heliotropism com-
pared, 692, 694-696, 697,

708,713.

— ot Fungi, 700.

— of motile organs, 689.

— of nodes, 687, 688.

Geotropismofplasmodia,6i5.

— of roots, 679, 680, 689-
691, 707, 708.

— of shoot, 679, 680, 690,
691.

— theory of, 691.

— of twining plants, 671,
675.

Geraniaceae, 780.
Geranium, 469, 792.

— peltntum, 310.
Germinating seeds, 398, 400.
Germination, 194, 262, 283,

290, 323, 327, 343, 344,
345, 347, 353, 357, 364,
373, 397, 403, 404, 412,
416, 524, 525, 691, 692,
702, 715.

— 405.

— carbon dioxide evolved
during, 397.

— chemical changes in, 328,

332, 335, 339-

— in darkness, 326, 530.

— in absence of carbon
dioxide, 326.

— ferments in, 351.

— loss of weight on, 400.

— of macrospores, 749.

— of seed, 742, 752, 753,
765.

— of spores, 729, 741, 742,
746, 748, 753.

Gesneraceae, 780.
Gexini, 780.

— ri-vale, 463.
464.

Gingko biloba, 755, 757.
Glands, 183.

— digestive, 378.

— epidermal, 175, 378.
Glandular hairs, 124, 183,

376, 435-
Glaucium luteum, 175.
Glecboma, 483.

— hederacea, 713.
Gleditscbia, 425, 466, 473,

475-
— ferox, 57.

— triacanthus, 57.
Gleichenia, 425.
Globoids, 334.
Glcvocapsa, 433.

— 392.

Gloriosa Blandii, 659.
Glucose, 310, 335, 341, 342,

349, 363, 384, 390, 652.
Glucosides, 328.
Gluten, 334.
Glyceria sped a bills, 312.
Glycerine, 347, 348, 385,

386.
Glycyrrhiza, 645.
Gnetaceae, 751.

8i6

INDEX.

Gold Fern, i86.

Gonidia of lichens, 391.

Goniiim, 607.

Gossypium, 183.

Gourd, 13, 51, 74, 157, I93>
217, 229, 232, 262, 263,
264, 276, 277, 289, 290,

293, 309» 313, 347, 353,
360, 405, 435, 483, 525,

531, 533, 538, 549> 584,
658, 665, 705, 713, 763,
798.

— 12, 76, 148, 348, 354,
532.

Graminese, 176, 780.
Grand period of growth,

541, 551, 552, 555, 556.
Granulöse, 339,
Grape-sugar, 335, 347, 349.

vine, 658.

Grapes, 329.
Graphic method, 194.
195, 305.

— representation of the
effects of light, heat, &c.,
195, 305.

of root-pressure, 274.

Grasses, 51, 128, 134, 144,
178, 219, 276, 278, 279,
288, 343, 373, 418, 419,
424, 461, 470, 495, 501,
534, 572, 664, 687, 706,
722.

Gravitation, the cause of
geotropism, 68 1-683,698,
699, 702, 705, 707.

"-influence of, 190, 196,
200, 485, 486, 489, 493,
494, 503, 515, 516, 519,
521-524, 526-528, 678,
714-

on development of

growing-points, 517-519.

on post - embryonic

growth, 528, 536, 537.

on protoplasm, 616,

617, 620, 621.

Green light, 303, 638, 696,
697.

— organs, respiration of,
397, 406.

— plants, nutrition of, 292.
Greening of leaves, 285, 301,

309.

301.

Grouping of cells, 436, 437,

447, 577.
Groups of apical cells, 458,

462.
Growing plants, work done

by, 583, 584.

— region, 420.

length of, 543-545.

point, 40, 112, 358, 413,

414, 416, 417, 418, 419,
420, 422, 424, 426, 435,
443, 447, 448, 451, 452,
453, 454, 460, 461, 464,
469, 473, 477, 479, 480,
481, 482, 496, 500, 502,
539, 563, 570, 582, 583,
744.

Growing - point, anticlines
and periclines in, 449.

conversion into ovule,

465.

development of new

organs from, 460, 467.

influence of environ-
ment on, 516-519.

intercalary, 429.

metamorphosesof,464.

protection of, 462.

the region of slowest

growth, 449, 457, 544.

relations between

growth and cell-division
in, 447.

of Fungi, 427.

of Phanerogams and

Cryptogams compared,
458.

of roots, 476.

of shoots, 425.

Growing- points, abnormal,
468.

• — — adventitious, 477, 480.

depressed, 461, 463,

474-

forms of, 461.

typical, 460.

with and without an

apical-cellcompared,458.
Growth, 353, 356, 364, 412,

417, 449, 453, 483, 493,

503, 515, 55c.

— abnormal types of, 425.

— apparatus for measuring,
561.

— axis of, 481, 483, 484,

539, 545-

— by apposition and intus-
susception, 413.

— basal, 419, 544.

— causal relations of, 502.

— causes of, 563, 565, 689.

— and cell-division in solid
bodies, 440, 445.

— change of form during,
443-

— correlations of, 502-505,
507, 508, 512, 704, 765.

— curve of, 551, 552, 554,
555, 558, 562.

— daily periodicity of 558-
560, 640, 641.

— in darkness, 353, 531,
532,713.

Growth by day and night,
549, 552, 553, 555, 558,

559, 560.

— definite and indefinite,
465, 466, 483.

— definition of, 412.

— dependence on external
conditions, 194,552, 554,
555.

— dependence on turges-
cence, 215, 565, 567-569,
572, 573, 576, 581, 641.

— diagrams illustrating cell-
division and, 440, 441,
442, 443,448,450,451,
452, 457, 459, 468.

— directions of, 698.

— in distilled water, 284.

— discontinuous variations
in, 551, 552.

— displacements during,445.

— distribution of, 411, 415,
437-440, 544, 569, 683-
685.

— embryonic, 419.

— excentric, 445.

— geometry of, 438.

— grand period of, 541, 551,

552, 555, 556.

— independent of assimi-
lation, 300.

■ of nutrition, 411, 412.

— influence of light on, 554,
555.

of temperature on,

553, 555.

— intercalary, 417, 418, 544.

— internal and external
causes of, 503.

— measurement of, 540-545,
551, 561, 562.

— mechanics of, 432, 437,
563, 570, 572.

— in nutritive solutions, 286.

— on different sides of
organs, 546.

— oxygen necessary for, 397,

399-

— periodic variations in,
539, 541, 549, 556, 559,

560, 599, 640, 641.

— phases of, 411,414,416,
417, 418, 420, 423, 424,
425, 437-

— post-embryonic, 503,516,
528, 536, 537.

— relations to cell-division,
431, 435, 437, 449, 453-

to configuration of

organs, 450.
to geotropism, 679,

685, 688, 691.

— retardation by light, 535,
559, 561.

INDEX.

817

Growth, respiration during,
399-

— slowest at apex, 449, 457,

544-

— in thickness, 483, 574,
577.

— and tissue-tensions, 569-
574.

— types of, 451.

— velocity of, 552, 560, 569,
571.

— zones of, 544.

— of Algse, 425, 429.

— of cambium, 157.

— of cells, 96, 422, 432, 437,
563, 565.

— of cortex, 158.

— of crystals, plants and
animals compared, 412,
413-

— of epidermis, 166.

— of filamentous organs,
443, 453-

— of flat structures, 441,
445-

— of Fungi, 425, 426.

— of internodes, 540-545,
553-

— of leaves, 422, 540, 640.

— of non-cellular plants,
540, 543-

— of orthotropic organs,
698, 709, 711.

— of plagiotropic organs,
698, 711.

— of roots, 17, 539, 541-544,

553-

— of shoot, 41, 421, 422,

540-544.

— of starch grains, 315.

— of tendrils, 664, 665.

— of twining shoot - axes,
669, 674,

— of young organs, 364,
419.

Guard-cells of Stomata, 97,

248.
Guiactttn, 163.

— officinale, 177.

Gum, 90, 124, 172, 175, 180,
328, 566,

— arable, 90, 328.
passages, 181,

— tragacanth, 328.

— vesicles, 179,
Gummosis, 180.
Gummy substances, 182.
Gymnogramme, 186.

— cnlomelanos, 126.

— chrysophylla, 780.

— distans, 780.
Gymnosperms, 155, 178, 745,

746, 750, 751, 754-757,
758, 764, 772, 788.

[3]

Gymnosperms, 754.

— and angiosperms com-
pared, 757-764-

— embryology of, 751, 754,
755.

— fertilisation in, 755.

— flowers of, 756.
Gynostemium, 796.
Gypsum, 252, 262, 28^, 287.

H.

Habitats of Lichens, 392.
Hx77iatococcus, 612, 613.

— lacKstris, 611.

— plui'ialis, 609.

Hairs, 122, 126, 289, 376,
423, 435,482.

— root-, 17-20, 25, 12 3
Halymeda, 154, 179.

— opuntia, 153.
Haschish, 185.

Haulms of Grasses, 687, 688.
Haustoria, 25, 63, 369, 372,

373, 388, 390, 725.
Hazel, 266, 584, 763.
Heart-wood, 163, 230, 268,

344-
Heat, animal, 404.

— as a stimulus to proto-
plasm, 617.

— of combustion, 199.

— of imbibition, 210.

— influence on plant-life,
190, 191.

— loss of, by radiation, &c.,
404.

— of respiration, 403, 405.

— rigor due to, 593.

— spontaneous, 395, 405.
Hedera Helix, 523.
Hedysariim gyrans, 593, 596,

598, 627, 637.

627.

Heleocharis, 708.
Helianthemum, 595.

— t'ulgare, 396.
Heliatjthus, 186, 574.

— annuiis, 199, 236, 271,
273, 313, 468, 498, 640.

42, 95, 112, 277,

461, 467.

— tuber osus, 59, 319, 335.

79, 336.

Heliotropic curvatures, 693,

694.

— eflfects of coloured lights,
696, 697.

Heliotropism, 16, 199, 485,
512, 528, 546, 589, 596,
600, 602, 612, 616, 617,
621, 622, 627, 628, 677,
692-697, 699, 702, 705,
707, 709, 710, 712.

3 G

Heliotropism, antagonism to
geotropism, 694, 708,713.

— elimination of, 705.

— experiments on, 693, 694.

— and geotropism com-
pared, 692, 694, 695, 696.

— positive and negative,
693-697, 701, 703, 709,
711, 712, 713, 714.

— of tendrils, 667.

— theory of, 677, 695, 696,
697.

Helleborus niger, 279.
Hemp, II, 15, 37, 229, 262,

289, 345, 660, 757, 798.
Hepaticse, 31.

— apical-cell of, 452.
Heracleum, 216.
Herb Paris, 60.
Heredity, 9, 503, 516, 723,

779.
Herkogamy, 789, 799.
Hermaphrodite flowers, 757,

788.
Herposiphonia repens, 483.
Heteromerous Lichens, 392.
Heterosporous Vascular

Cryptogams, 745-751-
Heterostylism, 789, 790, 799.
Heterostylous flowers, 792.
Hevia, 172.
Hibbertia dent at a, 669.
Hibiscus, 782.
Hieraceum piliferum, 121.
Hilum, 338.
Hippuris, 44.

— 469.

■ — vulgaris, 40, 424.

Histogenic layers, 420.

Histological diff"erentiation,
416, 570, 571-

Histology of wood, 160.

Hoar-frost, 404.

Homoiomerous Lichens, 39 2 .

Homologies between Pha-
nerogams and Crypto-
gams, 751, 756.

Honeysuckle, 668.

Hooks, 657.

Hop, 24, 56, 124, 185, 242,
289, 548, 551, 552, 657,
660, 668-672, 676, 757.

— 184.
Horse-chestnut, 43,123, 186,

283, 285, 314, 319, 351,
506, 507, 531, 578-

— 359.

radish, 802.

tails, 55, 59,62, 418, 701,

739-742, 745.
Host-plants, 368, 369.
Hoya carnosa, 177.

89.

Humus, 259.

8i8

INDEX.

Humus-plants, 63, 374, 392,

399-
Hyacinth, 43, 360, 419, 534»

576.

— 118.

Hyachithtis, 316, 351.
Hybrid vines, 784-786.
Hybridisation, 779, 799.

— laws of, 780-784.
Hybrids, 779, 780, 783.

— derivative and compound,
784.

— properties of, 782-784.
Hydnora Jfrkana, 65.
Hydnoreae, 63.
Hydnum diversidens, 388.
Hydrate of Terpin, 389.
Hydrccharis, 13, 255, 617.
Hydrodictyeae, 429.
Hydrogen, 206, 290, 293,

324, 382, 386, 396, 403,

595-

— sulphuretted, 387.
Hydrotropism, 601, 602, 698,

707, 714-717.

— 715.

• — positive and negative, 717.
Hygroscopicity, 588, 739,

741-
Hymenomycetes, 798.
Hypericineae, 780.
Hypericum, 183, 184.
Hypha, 152, 380, 381, 388,

426, 432, 453, 571, 726.
Hypochlorin, 318.
Hypocotyl, 702, 712.
Hypoderm, 143.
Hypophysis, 435, 443-

Iceland Moss, 707.
Ilex aqitifolium, 116.
Imbibition, 207, 243, 638.

— heat of, 210.

— water of, 209.

— in wood, 241, 268.
Immature condition, 414.
Impatiens, 177.

— glandulifera, 199.

— noU-me-t anger e, 640, 641,
791.

Indefinite growth, 465, 466,

483.
Independence ofanisotropy

and morphological nature

of organs, 700.

— of cell-division and the
morphological and phy-
siological nature of or-
gans, 433, 435.

— of growth and cell-divi-
sion, 431.

and nutrition, 412.

Indian Corn, 343, 344, 417.

— Cress, 533, 548, 596, 659,
712, 713.

— Hemp, 184.
India-rubber, 172.
Individual, 414.
Inflorescence, 461, 463, 473,

475, 496, 534, 658.

Influence of the environ-
ment, 515, 516, 523, 537,
593, 594, 595, 678.

on growth, 552, 554,

555-

— of gravitation on develop-
ment of growing points,

517-519-

on post - embryonic

growth, 528, 536, 537.

— of light on chlorophyll-
corpuscles, 617-620.

on development of or-
gans, 522, 524.

— • — on flat shoots, 709,
710.

on growth, 554, 555.

on leaves, 617, 621.

on post - embryonic

growth, 530, 536, 537.

— of the origin of sexual
cells on offspring, 787.

— oftemperature on growth,

553, 555.

on protoplasmic move-
ments, 606, 608.

Infusoria, 298, 606.

Inherited disposition, 189.

Injection of water in cut
branches, 279.

Injury, as a stimulus, 583.

— development of growing-
points caused by, 480,
582, 583.

— effects of, 504, 507.
Insectivorous plants, 124,

186, 345, 366, 374.

digestion by, 377.

nutrition of, 374, 375,

parasitism of, 377.

Insect-killing Fungi, 381.

Insects and Flowers, 655,
657, 760, 789-797.

Insects, relations of plants to,
204, 329, 537, 538, 548,
643.

Insect-traps, 379.

Integuments, 752, 754, 756,
760, 766.

Intense light, effect on as-
similation, 311.

Intensity of light, 713. .

influence on swarm-
spores, 613, 621.

Intercalary embryonic tissue,
467, 468.

Intercalary growing - point,
429.

— growth, 417, 418, 481,

544-
Inter-cellular canals, 181.

— gaps, 182.

— spaces, III, 141, 176, 247,

264, 423, 645, 652, 655.
Inter-tascicular cambium,

^57-
Intermediate products of

assimilation, 317.
Internal causes of growth,

503-

— disposition, 522, 678.

— glands, 183.

— parasites, 370.

— structure of plants, 189,

503.
Internodes, 40, 421, 471,
483, 549, 683, 684, 685,
687.

— growth of, 540, 542-545,

553-
Interpolated organs, 469.
Intine, 762.
Intra-molecular respiration,

395, 401, 402.
Intussusception, 413.
Inula Hellenium, 335.
Inulin, 323, 335, 336, 341,

343, 357, 363.

— fermentation of, 342.
Inverted sugar, 349.
Invertin, 343.

Invertive action of yeast,

390-
Iodide of starch, 310.
Iodine, 288.
Ipomxa, 669, 671.

— purga, 24, 180, 640.

— purpurea, 533, 672.
670.

Irideae, 177, 780.

Iris, 60, 144, 536, 576, 794.

— pumila, 102.

Iron in plants, 202, 226, 284,

285, 287, 289, 321, 383.
Iron-wood, 163.
Irritable leaves, 641-649,
655.

— movements, electric cur-
rents during, 650,

mechanics of, 652,653.

— organs, 591, 598, 600,
654.

rigor in, 593-595-

specific energy ot, 601.

— stamens, 396, 595, 650,

651, 654, 655, 790.
Irritability, 251, 368, 395,
481, 493, 503, 507, 513,
515, 530, 548, 566, 589-
591, 593, 594, 599, 601,

INDEX.

819

6^4-656, 678, 6S9, 692,
695, 717.
Irritability, definition of,
587.

— of growing leaves, 640,

— law of, 196.

— of plants and animals, 597,
600.

— of protoplasm, 587, 599,
603, 615, 616, 6x7, 620,
621, 655.

— of roots to contact, 716.

— of swarmspores to light,
608, 610-612.

— of tendrils, 658-666.

— and turgcscence,647,65 3.

— of twining stems, 674,

675.

— uses of, 655, 656.
Isoeteae, 745.

hoetes, 471, 483, 74S, 756.

— by J t rix, 57,

— laciistrii, 749, 771.
Isosporous Vascular Crypto-
gams, 739.

Ivy, 12, 24, 42, 56, 522, 523,
657? 658, 660, 693, 699,
703, 710-712, 713.

— 182, 523.

J.
Jalap, 180.
Jasminium, 669,
Jerusalem artichoke, 59.
Juglandes, 465.
J uncus, 145, 219.
Jungermannie3e,66, 474, 476.
Juniper, 703,
Jiiniperus, 321.

— conwnm'is, 703.
164.

Kalmia, 780.
Kerria, iTj.
Kidney Bean, 194.
Klinostat, 201, 562, 683,

685, 693, 705, 717,
— 684, 716.

movements, 673.

Knight's experiment, 682.

Labellum, 796.

Labiateac, 496, 501, 713, 7S0,

794.
Lactic acid, 349, 386.
— fermentation, 349.
Lagenaria, 658.
Lamina, 47, 471, 489, 490,

506, 507, 547, 623, 626-

630, 633, 640, 650, 693,

705.

Lnminaria, 32, 69, 208, 242,
429.

— Claustoni, 32, 70.
Lnmium amplexicaule, 791.
Lamp-light and assimilation,

307.
Land-plants, 203, 253, 295,
511, 512.

— absorption by, 283.

— leaves of, 47-53.

— roots of, 246, 255.

— transpiration of, 225, 246.

— vascular bundles of, 44.
Larch, 584.

— 498.

Lateral buds, 508, 714.

— roots, 17, 416, 476, 699,
704, 705.

geotropism of, 707,

708.
Laterality, 481, 485.
Latex, 172, 180, 362.

— extrusion of, 361.
cells, 174.

vesicles, 179.

Lathrxa, 14, 63, 374, 399.

— squamaria, 335.
Lathyrus alphaca, 660.
Laticiferous vessels, 171,174,

361.

pressures on, 360.

Laurineae, 180.

Law of irritability, 196.

Laws of cell-division, 434,

435, 442, 454j 456, 502.

— of hybridisation, 780-
784.

Layers of wood, 161.
Laying of wheat, 289, 514.
Leaf, carpellary, 758.

— development of, 425, 469,
475, 506,744.

— primary, 747, 764.

— primordial, 506..

— upper, 506.

base, 471, 506,

buds, 397.

nectaries, 281.

sheath, 418, 687.

tendrils, 659, 660.

traces, 157.

Leaflets, 626, 629, 633, 639,
644,646.

Leaves, 4, 38, 39, 47, 282,
290, 343, 418, 424, 448,
454, 455, 464, 467, 482,
483, 487, 489, 490, 492,
494> 504, 506, 510, 513.
520, 528, 547, 564, 567,
569, 575, 625, 626, 627,
633, 640, 644, 699, 705.

— absorption of water by,
254.

of gases by, 255.

3 G 2

Leaves, area of, 227, 312,
5", 513.

— arrested, 55,61, 506, 507.

— autumn, 318, 319.

— and bud-scales, correla-
tions between, 506.

— chlorotic, 298.

— climbing, 675.

— colours of in winter, 321.

— compound, 52, 425, 490,
687.

— decomposition of carbon-
dioxide by, 298.

— development of buds
from, 478.

— evaporation of water
from, 227, 246.

— fall of, 319.

— floating, 51, 511.

— greening of, 285, 298, 301,

309, 533.
301.

— growth of, 419, 422, 540.

— importance to flowers,

534-

— influence of light on, 617,
621, 623-640.

on structure of, 536.

— irritability of growing,
640.

— mechanics of, 49.

— metamorphosed, 464.

— metamorphoses of, 536.

— motile organs of, 623-
646, 689.

— negatively heliotropic, 701.

— periodic movements of,
623, 627, 628, 635, 636,
640.

— protection of, 50.

^ quantity of starch pro-
duced in, 314.

— radical, 421.

— regeneration from, 721,
723, 582.

— respiration of, 400.

— as reservoirs of reserve-
materials, 331.

— and roots, correlations
between, 512, 513.

— scale-, 504, 506, 507.

— scale-like, 63.
— • sessile, 53.

— sensitive, 641-649, 655.

— sheathing, 470.

— sleep-movements of, 199,
623, 626-629, 635-640.

— structure of, 490, 491.

— submerged, 511.

— tensions in, 49.

— vascular bundles of, 47-

53-

— venation ot, 48,49, 51,

128, 511.

)20

INDEX.

Leaves, vernation of, 547.
Leek, 179, 685.
Leguminoseae, 199, 334, 337,

626, 687.
Lemna, 13, 255.
■ — trisulca, 619,

619.

Lemnaceae, 122.

Length of roots, 13, 512, 513.

Lenticels, 167.

Lentil, 763.

Leant odon taraxacum, 396.

Lepidodendron, 741.

Lephmium radicans, 522.

Leucin, 349, 384.

Leucophyll, 317.

Levisticum officinale, 425.

Levulose, 343.

Libriform fibres, 139, 160.

Liciiens, 90, 153, 178, 225,

371, 380, 390, 391, 392,

393, 514) 707.^

— assimilation of, 393.

— forms of, 393.

• — gonidia of, 391, 392.
392.

— reproduction of, 727.

— roots of, 34, 35, 263, 394.

— symbiosis of, 390,

— thallus of, 393.

— tissues of, 153, 392.
fungi, 391.

Life, 191, 395,717-

Light, 397, 406, 485, 486,
489» 493, 494, 503, 508,
511, 513, 515, 517-519,
522, 524, 527, 528, 531,
714.

— absorption of by chloro-
phyll, 322.

— as affecting the develop-
ment of chlorophyll, 300.

— and assimilation, 148, 282,

294, 296, 299, 301, 304,
307, 309, 510,512.

— bilaterality induced by,

525.

— cardinal points of, 302.

— composition of, 302.

— electric, and assimilation,
307,

— infiuence on development
of organs, 522, 524, 525,
526, 527.

— influence on development
of flat shoots, 709, 710.

— Influence on growth, 554,

555.

on leaves and chloro-
phyll, 617-621, 623.

on life of plants, 190,

195, 197, 301, 593-

on post - embryonic

growth, 530, 536,537.

Light, influence on struc-
ture of leaves, 536.

on transpiration, 228,

251.

— intensity of, 302, 306.

— irritability of plasmodia
towards, 615.

ofprotoplasm towards,

616, 617, 620, 621.

— local action of, 310.

— movements of leaves due
to, 625-630, 634-639.

— refrangibility of, 301.

— retardation of growth by,
535, 559, 561.

— sources ot, 307.

— as a stimulus, 594, 597,
601, 625-628, 638, 653,
692, 694, 698, 702, 705.

— irritability of swarm-
spores to, 608, 610-612,
613, 621, 696, 714.

— variations in intensity of,
541, 551, 552, 554, 556,
559, 560, 594, 613, 621,
623-643, 713.

— of various colours, 303,

305, 311, 638, 696, 697.
• — wave-lengths of, 306.
Lignification, 88, 90, 289,

416, 423, 570.
Lignified elements, 161.

— tissue, 231.

— cell-walls, 242.
Lignin, 329, 389.
Lignite, 294, 382.
Liliaceae, 156, 169, 177, 419,

424, 528, 534, 706, 780.
Lilium, 118.

— candidum, 178, 400.

— Mart agon, 178.
Lily, 51, 756.

Lime, salts of, 79, 159, 254,
290, 320, 334.

— in sap, 274, 280.

— oxalate of, 291,

— 466, 581, 714.

— 445, 446, 482.
Linaria, 780.
Linum, 374.

— grandi/olium, 640.

— pe renne, 790.

— usitatissimum, 546, 791.
Lithium, experiments with,

235, 254, 269.

— spectrum of, 236.
Lithocysts, 177.
Liverworts, 31, 66, 120,

302, 474, 476, 479, 491,
495, 525, 621, 705, 706,
708.

— growing-point of, 461.

— reproduction of, 723, 724.

— tissues of, 150.

Loam, 259.
Loasaceae, 124, 669.
Lobeliaceae, 171, 780, 792.
Lobelia fulgens, 798.
Longitudinal axis, 482, 48 6.

— sections, 447, 449, 481,
483, 563, 571-

■ — walls, 452.
Lonicera, 186, 506.

— caprijolium, 668.
Lophospermum, 659.
Loranthaceae, 64, 367, 766.
Loranthus Europxus, 368.
Luminous fungi, 406, 408,

695.
Lupm, 326, 345.
Lupitiiu -varius, 333.
Lupulin, 185.
Luzula pilosa, 792.
Lychnis, 780.

— diurna, 781.
— flos cuculi, 781.

— "viscaria, 184.
Lycium, 320.
Lycopodiaceae, 52, 18 r, 474,

800.

— reproduction of, 741, 744,
745, 748.

— shoots of, 55, 62.
Lycopodium, 75, 588.

— chamxcyparissus, 1 1 2 . 1 46.
147.

Lygodium, 60, 425, 492, 495,
675.

Lysigenous intercellular spa-
ces, 176.

Lysimachia, 183, 184.

— nutnmularia, 483.
Lythrum Salicaria, 790.

M.

Macleya cordata, 571.

Macrocystis, 419.

Macrosporangia, 746, 751,
766.

Macrospores, 746-752, 766,
800.

Macrostylous flowers, 790.

Macro-zoospores, 609.

IMagnesia, 79, 334.

Magnesite, 263.

Magnesiun salts, 202, 226,
254, 262, 284, 287, 289,
290, 291, 383.

Magnoliaceae, 180.

Mahonia, 321, 395, 595.

Maize, 12, 29, 193, 194, 217,
228, 229, 262, 263, 271,
278, 283, 284, 286, 288,
300, 309, 310, 313, 337,
344, 417, 424, 523, 564,
687.

— 13, 38, 42, 271, 421.
Majanthemum bifoUum, 51.

INDEX.

821

Male ccEiiobia, 731.

— fern, 478.

— plants, 737, 742, 751.

— prothallia, 742.

— reproductive organs, 724,
727, 728, 734, 736, 757,
769, 779-

Malic acid, 328.
Ma I ope trißda, 641.
Malt, 404.
Mahn, 782, 792.

— rotundifolia, 640.
Malvaceae, 123, 180,780.
Mamillaria, 54, 472.
Mandragora vernalis, 792.
Manganese, 288.
Mannite, 349, 385, 386.
Manures, 292.

Rlaple, 163, 180, 506, 507,
578, 579.

— Sugar-, 359,
Marattiacese, 13, 181, 458,

472.

— marginal cells of, 459.
Marchantia, 31, 33, 444, 489,

510, 525, 526, 527, 695,
705, 709, 710, 712, 713,
723,

— 444.

— polymorpha, 708.

69, 91, 120, 151, 526,

709.
Marchantieae, 461, 474.
■ — tissues of, 1 50.
Marginal growth, 440.
Marine plants, 287.
Marsilia, 44, 483, 494, 746,

747.

— salvatrtx, 15, 699, 746,
747, 748.

Marsiliacex, 604.

— reproduction of, 745.
Mastic, 181.

Materials for growth, 329,

411.
Mature condition, 414.
Maurandia, 56, 659.
Maximum point for light,

195, 302.

— for evolution of oxygen,
306.

— for temperature, 193.

— velocity of growth, 552,
560.

Meadow saffron, 350.
Mealy hairs, 1S6,
Measurement of assimila-
tion, 304,

— of contractile stamens,
652.

— of geotropic curva-
tures, 683, 688, 690.

— of growth, 539-545,
551, 561, 562.

Measurement of pressures in
motile-organs, 636.

— of root - pressure, 274,
277.

— of shortening of roots,
576.

Mechanics of branches, 580,
581.

— of flowers, 791-797.

— of growth, 432, 437, 563,
570, 572.

— of insectivorous plants,
376, 377, 378, 379-

— of irritable movements,
652, 653.

— of protoplasm, 614.

— of sleep-movements, 626,
634-640.

— of stomata, 248.

— of tendrils, 664.

— of twining, 670-672, 674-
676.

Median longitudinal sections,

449.
Medullary rays, 159, 359,

439, 452.
Melaleuca, 165.
Melandryum, 316.
Melastomacese, 794.
Melobesia, 437, 442.

— Lejolisii, 438.
Melobesiacea», 179, 290.
Membranes, properties of,

214.
Menispermum, 548.

— canadense, 669, 672.
672.

Mentha, 59.
Menziesia, 780.
Merismopedia, 433.
Meristem, 170.

— primary, 419.
Mertensia, 425.

Merulius lachrymans, 281,

381.
Mesembryanthemum, 55, 177,

642.
Mesocnrpus, 618, 621, 622.
Mesophyll, 47, 321, 536.
Mcsostylous flowers, 790.
Metabolism, 175, 289, 328,

364, 365, 400, 508.

— of fungi, 384.

— products of, 323, 330,
357.

Metameres, 483.
Metamorphosed leaves, 464.

— organs, 7, 483.

— roots, 23, 369.

• — shoots, 54, 475, 658.
Metamorphoses of carbo-
hydrates, 323.

— of fats, 328.

— of growing-points, 464.

Metamorphoses of products
of assimilation, 323.

— of Proteids, 326.

— of reserve-materials, 341.

— of starch, 323, 328.
Metastasis, products of, 316.
Method of bubble-counting,

304.
Methylamine, 384.
Metzgeria, 461.
— furcata, 474.
Micells, 206, 243, 254, 413.
Micro-chemistry, 291, 307,

320.
Microsomes, 79.
Microsporangia, 746,
INIicrospores, 746-751, 754,

. 756, 757.
Microstylous flowers, 790.
Micropyle, 752, 754, 757,

758, 760, 762, 771, 778.
Micro-zoospores, 609.
Middle lamella, 75, 90, 112.
Mid-rib, 489, 490, 650.
Mignonette, 497,
Mildew, 786.
Milfoil, 52.
Mimosa, 201, 218, 396, 590,

593, 594, 596, 597, 598,

626, 639, 644-649, 654,

655,665.

— pudica, 593,595,644,649-
645.

Mimosse, 593, 595, 596, 598,
601, 637, 638, 644, 646,
647, 648, 649, 652, 655.

Mimulus, 640.

Mineral substances, 282,
284, 382.

Minimum point for evolu-
tion of oxygen, 306.

— for light, 195, 302.

— for temperature, 193,

— velocity of growth, 552,
560.

Mirabilis Jalappa, 338, 640,
781.

— longiflora, 781.
Mistletoe, 25, 64, 367, 369,

766.

— 26.

Mniiim horniim, 68.

Mobility of water in cell-
walls, 242.

Moisture as a stimulus, 519,
625, 702, 715.

— effect on transpiration,
228, 251, 278.

— variations in, 551, 552,
625, 646.

Molecular structure, 205.

of wood, 242.

INIolecules, 2.(3.
Monocotyledon, 417.

822

INDEX.

Monocotyledons, 51, 128,
144, 373, 375, 418, 470,
472, 473, 478, 660, 689,
706.

— ascent of water in, 230.

— excreta, 177.

— flowers of, 757.

— growth in thickness, 169.

— phyllotaxis, 501.

■ — reproduction of, 756, 758,
763, 765.

— secretion-canals, 181.
vesicles, 179.

— vascularbundles, 127, 134.
Monoecism, 788.
Monopodium, 60, 474.
Motiotropa, 34, 63, 368, 372,

374, 765.
Monster a, 24, 25.

— dfliciosa, 125.
Moraceae, 174, 790.
IMorphia, 172,
Morphological nature of

organs, anisotropy inde-
pendent of, 700.
■ cell-division indepen-
dent of, 433, 435.
Mortis alba, 319, 362,
Moss, 126, 155, 264, 302,
320, 347, 364, 387, 419,
432, 453, 465, 471, 479,
497, 501, 510, 588, 603,
619, 621, 722, 723, 724,
747, 755, 800.

— 314, 364, 479, 735.

— apical cell of, 452.

— branching of, 473.

— cell-structure, 114.

— chlorophyll - corpuscles,
85,311.

— epidermis of, 121, 149.

— fertilisation, 108.

— growing-points of, 460.

— movements of water in,
225,

— reproduction of, 730-739,
742.

— roots of, 29, 150.

— shoots of, 54, 65.

— stomata, 120.

— tissues, 149.

— tubers, 364.
Mosses, hybrid, 780.
"Moss-flowers," 736, 737.

" Moss-fruit," 734, 738, 800.
Mother-cells, 97, 100, 434,
746, 770.

of antherozoids, 737.

of pollen, 433, 739.

of spores, 433, 739.

Motile condition, 593, 594.

— flowers, 396.

— organs, 396,623-629, 639,
644-646, 665, 687, 689.

Motile organs, expulsion of
water from, 647, 649.

geotropism of, 689.

structure of, 629-634,

645.

tissue-tensions in, 648,

649.

Mould-fungi, 152, 281, 343,
347, 380, 383, 384, 385,
398, 427, 722, 798.

Movements, amoeboid, 613,
614.

— of chlorophyll-corpuscles,
617-621, 696.

— of constructive materials,
359, 361.

— due to changes in the in-
tensity of the light, 625-
630, 634-639.

^ of gases, 251.

— periodic, 395, 590, 593,
594, 598, 599, 623, 627,
628, 635-641.

— of protoplasm, 81, 194,
395, 603, 606, 607, 608,
621.

— of reserve-materials, &c.,
360.

— of root, 21.

— sleep-, 599, 623, 626-629,
635-640.

— of stamens, stigma, and
style, 792.

— ofwater,2ii,225,253,27o.
- — of zoospores, 604-608,

777.

Mucilage, 88, 90, 186, 328.

Mucor, 5, 108, 280, 335, 367,
370, 386, 387, 465, 543,
561, 695, 697, 700, 725.

— curve of growth of, 562.

— Mi4cedo, 369.
370, 725.

— racemosus, 386, 387.

■ yeast, 386, 387.

Mucorini, 152, 351, 700,

726, 727, 768.
Mucus, 175, 179, 180.

■ vesicles, 180.

Mublenbeckia platyclada, 55.
Mulberry, 319.
Multinuclear cells, 86, 106,

109.
Mi/sa, 50, 75, 179, 310, 473.

— E>isete, 220.

— Hin a, 334.

— sapientum, 236.
Musaceae, 550.

Mus cart botryoides, 472.
Muscineae, 120, 126, 171,

425, 460, 473, 524, 732,

769, 788,789.
Mushroom, 71, 75, 152, 426,

564.

Mustard, 596, 695, 697.

— oil of, 342.

Mut is i a clematis, 662.

Mycelium, 34, 71, 152, 344,
381, 388, 407, 426, 428,
700, 717, 725, 726.

Myelin, 318.

Myosotis, 496.

Myriocepbaltis, 588.

Myriophyltum, 297.

Myronate of potassium, 342.

Myrosin, 342, 349.

Myrtaceae, 183, 334.

Myxamoebs, 613, 614.

Myxomycetes, 82, 84, 345,
395, 429, 430, 431, 613.

N.

Nasturtium officinale, 478.
Necessity of oxygen, 396, 611.
Nectar, excretion of, 281.
Nectaries, 281, 791.
Nectarine, 781.
Negative geotropism, 679-
81,691, 703.

— heliotropism, 693 - 695,
697, 701, 703, 711, 712,

713-
■ — hydrotropism, 717.

— pressure in root-stock,

271, 277.

— pressure in wood, 268.
Neottia, 14, 63, 372, 374, 399.

— nidus-ai'is, 22, 367, 433.

— 433.

Nepenthes, 56, 374, 378, 379,
660.

— Ixvis, 379.
Nephrolepis, 425.
Nerves, 600, 650.
Nettle, 271, 274.

— 169, 436.
Nicotiana, 780, 782.

— 435.

- — Langs dorfii, 781.

— paniculata, 781.

— t aba cum, 236, 570.
Nigel la damascena, 781.
— flexilis, 789.

— sativa, 781.
Nitella, 320.

Nitrates, 202, 226, 252, 284,

287, 291, 324, 384.
Nitrate of lithium, 235.
Nitric acid, 324, 385.

— oxide, rigor due to, 595.

— absorption by leaves, 255.

— in rain-water, 263.

— in soil, 292.

Nitrous oxide, rigor due to,

595-
Nitrogen, 206,290, 293, 297,

324, 382, 384, 395, 595.

INDEX.

• 23

Kitrogen in atmosphere,
291,

— absorption of, 292, 384.

— of parasites, 372.

N itrogenous compounds, 291,

345-
• — food of fungi, 384.

— reserve-materials, 401.

— substances in parasites,
&c., 372.

Nocturnal position of floral
envelopes, 625, 642.

— of leaves, 625-630, 634-
638, 646.

Nodes, 128, 454.

— geotropic curvatures of,
687, 688.

Non-cellular Algae, 70, 365,
425, 492, 732, 734, 776.

— plants, 109, 569.

anisotropy of, 700.

geotropismof, 679,680,

697.

growth of, 540, 543.

heliotropism of, 697.

Non - nitrogenous reserve-
materials, 332, 344, 400.

— substances in parasites,
&c., 372.

Non - sexual swarmspores,

728, 733, 739.
Nojtoc, 392.
Nostocaceae, 107.
Nothochlsena, 186.
Nothoscordon fragrans, 86,

104.
Nucellus of ovule, 465, 751,

752, 754. 755, 760, 766.
Nuclear disc, 103.

— division, loi.

— filaments, 102.

— poles, 102.

— spindle, 103.

— substance, 206, 770.
Nuclei, 78, 86, 326.

— crystalloids in, 335.

— of pollen, 761, 771.
Nuclein, 87, 102, 109, 419,

422, 770-772.
Nucleoli, 86-102.
Nucleoplasm, 87, 103.
Nucleus, 73, 77, 87, 319, 413,

419, 422, 563, 645, 652,

770.
-^ of embryo-sac, 762.

— of oosphere, 772.
Number of stomata, 251.
Nuphar, 126, 706.
Nutation, 546-548,671,672.

— of stamens, 548.

— of tendrils, 661, 664, 666,
667.

— of twining plants, 669,
670.

Nutrition, 190, 198, 282, 289,
292, 297, 302, 328, 366,
4ir.

— of lungi, 367, 380-381,

382, 383, 385, 389.
humus-plants, 374.

— independent of growth,
All, 412.

— of insectivorous plants,
374-

— ot parasites, &c., 368, 369,

372.
Nutrition of schizomycetes,
384.

— ot yeast, 386.
Nutritive materials, 252, 282,

287, 383, 508, 511, 512,
579, 748, 753.

— organs, 282.

— salts in soil, 259, 262.

— solution, 228, 284, 286,
291, 383.

— substances, absorption of,
246, 264, 287, 366.

Nycandra, 780.
Nyctagineae, 780.
Nymphxa, 706.

— alba, 51, 642.

— micrantha, 478.

— stellata, 405.
Nymphaeaceae, 125.

Oak, 43, 176, 204, 225, 229,
288, 357, 388, 506, 507,
510, 537, 581, 584, 704-

galls, 204, 537.

Oat, 338, 687.

Object of fertilisation, 798.

Oblique septa, 454.

Occluded buds, 475.

Octant-cells, 446, 743, 764.

Odours, 184,

(Edogojiium, 33,604, 787,788.

— 33, 82.

— fertilisation of, 775, 776.
CEfianthe phellandriuni, i 30.
GZnotbera, 640.
ffinotherae, 780,.

Oidiitm, 786.
Oil, 319.

— of bitter almonds, 342.

— of mustard, 342.
Oleander, 52, 312.

Onci ilium microchilum, 789.
Onion, 43, 59, 62, 310, 333,

335, 350, 357, 546, 572,

689.

— 701.

Onoffordon acanthlum, 405.
Oogonia, 724, 732, 733, 734,

736, 772, 775-778, 788,

789.

Oosphere, 9P, 108, 447, 465,
480, 484, 538, 605, 724,
730, 731, 732, 734, 736,
737, 742, 747, 749-751,
754, 755, 760, 764, 767-
778, 780, 788, 789, 800.

Oospore, 734, 743, 762, 766,
767, 800.

Opening and closing of
flowers, 599, 641.

— — of stomata, 249.

— of flower-buds, 397.
Operculum, 739.
Ophioglosscse, 472.
Ophioglosstim, 478.

■ — I'ulgatum, 37.

Ophrydeae, 24.

Ophrjs, 43i 332.

Opium, 172.

Optimum point for light, 195,

302.
for evolution of oxygen,

306.
for temperatures, 192,

395.

— velocity of growth, 552,
560.

Opuntia, 54, 70, 159, 511,
517, 518, 519, 520, 522,
527.

— crassa, 517.

— Ficus-hidica, 517.

518.

Opuntiae, 181.
Orache, 299.
Orange, 183, 329, 8of.
Orange light, 303, 638, 696,

697.
Orchideae, 62, 367, 369, 538,

660, 763, 789, 791.
Orchids, 25, 114, 177, 433,

778.

— polHnation of, 796, 797.

— saprophytic, 63.
Orchis, 180.

Organic acids, 329, 385.
■ — centre of organs, 482.

— compounds, 206.

— substance, 226, 289, 290,

293, 323, 355, 366, 422.

— nutritive materials, ab-
sorption of, 366.

— substance, absorption of,
298, 368, 374-

— substance destroyed in
respiration, 400.

— substance, formation of,
296.

origin of from starch,

326.

Organic substances, physio-
logical significance of,
329.

Organisation, 509, 510, 704.

824

INDEX.

Organised ferments, 347.

— substance, 206, 312.
Organisms, plasticity of, 515.
Organography, i, 72, 368,

424, 425, 429, 476, 478,
512, 575, 702, 721, 722,
745-
Originof carbon-compounds,
325.

— of fats, 323.

— of Proteids, 324, 325.

— of roots, 477, 523.

— of sexual cells, 787.

— of starch in chlorophyll,
309.

Ornithogallum iimbellatum,

641.
Orobanchaceae, 368,372, 373,

476, 765-
Orobanche, 14, 63, 399,

— 443.

— speciosa, 368.

— Teuer it, 399.
Orthogonal trajectories, 447,

457-
Orthotropic organs, 504, 547,
685, 698, 703-705, 709,
711, 713, 740.

— and plagiotropic organs
contrasted, 705.

Orthotropism, 485, 489, 490,
492, 493, 494, 500, 706,
707, 709.

Orthotropous ovule, 760.

Oscillarise, 107.

Oscillations of temperature,
406.

Oscillatorieae, 604, 606.

Osmometer, 214, 275.

— 276.

Osmose, 211, 28 r.
Osmunda regalis, 51, 744.

31, 524.

Osteolite, 263.

Ovary, 463, 651, 758, 759,
778.

Ovule, 451, 463, 465, 751,
752, 754-756, 757-760,
766, 773, 778, Soo.

— conversion of growing-
point into, 465.

Oxalate of lime, 177,291,320.
Oxalic acid, 328, 384.

significance of, 325.

Oxalidese, 199, 623, 626.
Oxalis, 183, 593, 598, 625,

626, 631, 633, 642, 647,

790.
Oxalis acetosella, 620, 627,

649, 791.

— 620,

— carnea, 632, 633.

— micrantha, 791.

— Ortgiesii, 316.

Oxalis rosea, 642.
■ — sensit iva, 791.

— valdiviensis, 642.
Oxamide, 384.
Oxidation, 296, 381,400, 403.
Oxy-acids, 402.

Oxygen, absorption of, 395,
398.

— evolution of, 195, 248,

264, 297, 303, 397.

— and fermentation, 386.

— importance for growth,
397.

— — in nutrition, 287, 290,

293, 382.

— in plants, 202, 206, 296,

317, 324, 326.
■ — necessity of 396, 595, 611.
respiration, 395.

— rigor in, 595.

— volumes absorbed in re-
spiration, 398.

P.

Pxonia, 323.

Paeony, 759, 760.

Palissade-cells, 321, 536.

Palm, 12, 37, 45, 46, 58, 130,
144, 156, 178, 200, 225,
230, 334, 335, 353, 4i7,
461, 465, 470, 510, 702,
798.

Palm, 46, 127.

Palmellaceae, 299, 324.

Pandorina Morum, 728-730.

— 729.

Papaver Rhceas, 175.

— sominferum, 172, 399, 483.
Papaveraceae, 171, 173, 780.
Papayotin, 172.
Papilionacese, 371, 496, 497,

780, 791, 794.

— leafless shoots of, 55.
Pappus, 125, 147.
Paraphyses, 726.
Parasites, 23,25,63, 203, 251,

253, 292, 298, 366, 367,
392, 534-

— abnormalities induced by,

368.

— absorption by, 372.

— action of, 368.

— compared with seedlings,

373-

— food-materials of, 373.

— internal, 370, 371.

— embryo of, 765, 766.

— nutrition of, 368,369, 372.

— respiration of, 399.
- — starch in, 372.

— substance of, 372.
Parasitic fungi, 381, 388, 389,

725.

Parasitic phanerogams, 370,

372, 476, 5", 534-
Parasitism, 25, 65, 367, 766.

— of Cuscuta, 27.

— degradation due to, 368,

370.

— of Dioniva, 376.

■ — of insectivorous plants,
377.

— of mistletoe, 25.

— of Pinguicula, 378.

— oi Nepenthes, 379.
Parastichies, 497, 498, 500.
Paratonic effects, 628, 629,

637-640.
Parenchyma, 141, 149, 169,
216, 232, 288, 325, 330,
355, 416, 423, 626, 630-
634, 645, 652, 655.

— phloem, 138.

— pressures and strains in,
361, 569-572,581.

— xylem, 138.

Parental characters, blending
of in hybrids, 779, 782.

Parietaria dijfusa, 792.

Paris quadrifolia, 60.

495.

Parthenogenesis, 767, 769.

Partial growths, 544, 569,
685.

Passiflora gracilis, 661, 666.

— serratifoUa, 400.
Passiflore3e,56, 658, 660, 66 r,

780, 794.
Passion flower, 56.
Pastinacea sati-va, 425.
Patschouli, 185.
Pea, 56, 263, 314, 315, 334,

3 37, 397, 398, 405, 660,

759, 763.

— 17.

Pear, 146, 781.
Pectinaceous substances,

328.
Pediastrum, 429, 484.

— granulatum, 428.
Pedicularis, 760, 794.
Pelargonium, 185, 792.
Pellia, 608.

Peltate hairs, 124.
Peltigera canina, 707.

— hori%ontalis, 34.
Penicilliiim glaucum, 75, 281,

722,798.
Pentastemon, 780,
Peperomia, 582.
Pepsin, 378.
Peptones, 342, 345, 349, 377,

384.
Peptonising ferments, 172,

343, 345, 362, 376, 378,

379, 389-
Perennial plants, 330.

INDEX.

825

Periblem, 420.
Pericambium, 169.
Pericarp, 147.
Periclines, 439, 440, 445,

449, 452, 454, 457, 458,

469, 743, 764-

— in solid organs, 447.

— sequence of, 443,

— suppression of, 442.
Periderm, 45, 165, 226, 246,

574-

— of monocotyledons, 170.

— of roots, 169.
Perigone, 736.

Period of vegetation, 411,
Periodic movements, 395,

623, 625, 627, 628, 635,
636, 640, 641.

■ spontaneous, 590, 593,

594, 598, 599, 627, 628,
635-638.

— oscillations of tempera-
ture, 406.

— variations in growth, 539,

541, 549, 556, 559, 560.
Periodically motile floM^ers,

396, 599, 623, 625, 641.
Periodicity, 199, 351, 537,

599, 623.

— of growth, 558-560, 599.

— of movements, 623, 626,
635-638.

— of root-pressure, 273, 599.
Peripheral growth, 158.
Peristome, 150, 739.
Permanent tissue, 419, 425,

440, 465, 467, 479.

conversion of embry-
onic into, 466.

converted into embry-
onic tissue, 480.

Perotiospora I'iticola, 786.

Peronosporeae, 370, 390,777.

Petals, 497.

Petasites albus, 52.

Petiole, 53, 471, 482, 506,
547, 569, 571, 575, 623,

624, 626, 630, 633, 639,
644, 646, 693,

Petioles as tendrils, 659, 663.

Petroleum, 294.

Petunia nyctaginiflora, 178.

Peziza cowvexuln, 98.

Phacosporea", 429, 435.

Phajus, 316.

Phallus impudicus, 428.

— 427.

Phanerogams, 492, 496, 497,
508, 512, 538, 620, 621,
623, 746, 748, 751, 754,
756, 771, 778, 789, 790,
798.

— calcium carbonate in, 1 78.

— —oxalate in, 175.

Phanerogams, cell-division
in, 435.

— and cryptogams com-
pared, 751, 756.

— dormant periods, 352.

— embryos of, 443.

— epidermis of, 11 6.

— fertilisation, 108.

— growing point, 458, 468.

— hybrid, 780.

— laticiferous vessels of, 171.

— parasitic, 476, 511, 534.

— parasitism of, 368, 370,

372.

— receptacles for secretions,

176.

— respiration, 408.

— saprophytic, 371, 511.

— seedless, 802.

— tissues of, 149.

— vascular bundles, 126.
P bar bit is bederacea, 672.
Phaseolus, 56, 179, 193, 283,

531, 533, 593, 625,669.

— multißorus, 90, 136, 168,
629, 704.

194, 313, 505, 540,

542, 543-

— vulgaris, 629.

— 629.

Phases of growth, 411, 414,
416, 420, 423,425,437.

the three, 414, 417,

418,424.

Phelloderm, 166.

■ — of roots, 169.

Phellogen, 165.

Philodendron, 25, 316, 571.

— gratidifolia, 3 1 6.
Phloem, 45, 131, 157, 160,

330, 355, 360.

— general characters of, 133.

— parenchyma, 160, 163.

— rays, 159.

— of roots, 168.
Phcenix dactylifera, 344.
Phosphates, 202, 226, 253,

254, 262, 284, 287, 334.
Phosphorescence, 395, 406.

— its dependence on oxy-
gen, 407, 408.

Phosphoric acid, 79, 260,320,
385.

— in sap, 274.
Phosphorus, 289, 290, 383.
Photometric methods, 301,

554-
Phototonus, 594, 628, 635.
- — of swarmspores, 612.
Phycomyces, 569, 700, 719.

— 701.

— nitens, 561.

5, 280.

Phycomycetes, 34, 732.

Pbyllanthus, 55, 511.
Phyllocactus, 54, 511, 780.

— Phyllanthus, 781.
Phyllocladus, 55.
Phyllotaxis, 483, 486, 487,

493, 494, 495, 496, 499,
500, 501, 714-

Phylloxera, 784, 786.

Physolis, 780.

Physarum album, 613.

Physcia parietina, 392.

Physcomitrium fasciculare,
780.

• — pyrifornie, 780.

Physiological classification
of products of metabol-
ism, 323, 329.

— labour, 171.

— nature of organs, cell-
division independent of,

433, 435-
• — significance of respiration,

403.
Physiology, 189, 414, 515,

650.
Physma chalaganum, 392.
Phytelephas, 335, 344, 763.
Phytophthora itifestans, 369,

381, 390.
Picea excelsa, 581.
Pigment-bacteria, 387.
Pilea, 790.
Pileus, 427, 700.
Pilobolus, 335.
Pilostyles, 370.
Pilularia, 44, 494, 608.

— globulifera, 493.

Pine, 37, 75, 166, 180, 225,
237, 266, 268, 327, 369,
389, 464, 472, 500, 579,
581, 584, 703, 754, 756.

— 420, 464.

cone, 499, 758.

wood, 327.

Pinguicula, 124, 335, 378,
706.

— 'vulgaris, 435.

Pinus, 182, 321, 351, 465,
678.

— austriaca, 581.

— Pinaster, 118, 143.

— Pinea, 327, 753.

— sylvestris, 112.

160, 236.

Piperacex, iSo, 231, 465.
Piptocephalis Freseniana, 370,

725, 726.

370, 725.

Pistacia tcrebinthus, 18 I.
Pision satii'um, 56.

17, 333.

Pitcher-plants, 378, 659,
Pith, 230, 359,564,569,571,

572, 574, 581, 630, 634.

826

INDEX.

Pith, elongation of, 572, 573.
Pit-canals, 92.
Pitted- vessels, 136.
Pits, 92, 423.

— bordered, 137, 160, 237,
424.

Plagiotropic organs, 698,
703-705, 711.

— organs, rolled up, 706,
707, 709.

— and orthotropic organs
contrasted, 705.

Plagiotropism, 489, 492, 494,

709, 710, 712, 713, 714.
Planarise, 298.
Plant ago lanceolata, 129.

— major, 220,

— media, 792.

— psyllium, 90,
Plant-life, external condi-
tions of, 189.

Plants in the tropics, 197.
Plasmodia, 82, 429, 604,
615.

— irritability to light, &c.,
615.

— movements of, 614.
Plasmolysis, 213, 567, 568,

617, 653.

Plastic substances, 326, 329,
413, 767.

active and passive con-
ditions of, 341.

translocation of, 329,

353, 357, 361.

Platanus, 166, 279.

Platycerium IVilUngkii, 32.

Plectranthus friiticosus, 121.

Plerome, 143, 420.

Plum, 90, 147.

Plumule, 3, 41, 332, 344, 414,
492, 505, 680, 694, 702,
712, 717, 753, 764.

Pogostemon patschouli, 185.

Polarity, 481, 484, 485.

Polemoniacea:, 780,

Pollen, 761, 789.

— nuclei of, 761, 771.
■ — protection of, 797.

— grain, 76, 92, 98, 99, 739,
751, 754, 756, 757, 758,
760-762, 769, 778.

762.

development of, i o i .

mother-cells of, 433.

sterile cells of, 756.

sacs, 757, 760.

— -tube, 751, 754, 755, 756,
758, 760, 762, 768, 771-
773,778.

Pollination, 329, 535, 538,
754, 757, 760, 781, 790,
792.

— oi Aristolochia, 792-794.

Pollination of Orchids, 796,
797-

— of Salvia, 794.

— oiFiola tricolor, 794, 795.
Pollinia, 796.
Pollinodium, 726, 727.
Polygonacea:, 55, 186, 419,

465, 470.
Polygonattim, 60.

— miiltiflorum, 61.
Polygonum, 640.

■ — dumetorum, 672.

— fagopyrum, 283.

— scandens, 668.

— Sieboldi, 543, 685.
Poljpodium vulgare, 131.
Polyporus borealis, 388.

■ — dryadeus, 388.

— fulvtis, 388.

— ignariuj, 388.

— mollis, 389.

— sulpbureus, 388.

— vaporarius, 388.
Polysiphonia, 335.

Poplar, 146, 186, 225, 229,
319, 578, 582, 584, 703.

Populiis, iTj, 319.

Porosity of wood, 238.

Porous bodies, 243.

Portulacca, 640.

Position of reproductive
organs, 787.

Positive geotropism, 679-
68r, 689-691, 703.

— heliotropism, 693-695,
697, 703, 709, 712, 713,
714.

— hydrotropism, 717.
Post-embryonic configura-
tion, 528.

— growth, 503.

influence of environ-
ment on, 516, 528, 536,
537.

Potamogeton, 297, 701.

■ — natans, 51.

Potassium myronate, 342.

— salts, 202, 226, 252, 260,

262, 284, 287, 289, 290,
291, 320,383.

— - — in sap, 274.

— sulphate, 342.
Potato, 58, 330, 333, 337.

— 59, 165, 169, 273, 274,
277, 278, 314, 316, 326,
333, 337, 339, 35o, 363,
369, 390, 504, 507, 535,
722, 798, 799.

disease, 369, 381,

Potent ill a, 780.
Precipitation membranes,

214.
Pressureson sieve-tubes, &c.,

360.

Pressures as a stimulus, 503,
515, 617.

— and strains in cortex and
wood, 574-579-

— in tissues, 362, 437, 581,
582.

Prickles, 57, 125, 289.
Primary embryonic tissue,
417.

— leaf, 747,764.

— meristem, 419, 770.

— root, 418, 689, 690, 703,
747, 752, 753, 764-

— shoot axis, 747, 753.
Primordial leaf, 506.
Primrose, 186.
Primula auricula, 186.

— elatior, 781.
— farinosa, 186.

— marginata, 186.

— officinalis, 781.

— sinensis, 129, 184, 279,
790.

184.

Primulaceae, 780, 790.
Products of assimilation, 156,

299, 314, 317, 357, 403,

533, 763-
first visible, 306, 309,

318.

metamorphosesof,3 2 3.

Products of degradation,

328, 329.

— of fermentation, 348.

— of metabolism, 316, 323,

330, 357.
Pro-embryo, 735, 764.
Progi-essive development,

467, 468.
Propagation of stimuli, 596-

598, 646, 649, 662,

663.

— vegetative, 477, 723, 798,
800.

Properties of hybrids, 782-
784.

— of parents transmitted to
off"spring, 779.

— of wood, 234, 243.
Proportions of ash-constitu-
ents, 290.

— of salts absorbed, 287.
Prosenchyma, 146.
Protandrous flowers, 792.
Proteaceae, 146.
Protection of growing-point,

462.

— of pollen, 797.
Proteid reserve-materials,

332,345-

substances in parasites,

&c., 372. _

absorption of by insec-
tivorous plants, 374.

INDEX.

827

Proteids,79,i 7 2, 1 75,206,284,
289, 290, 307, 318, 323,
329, 332, 334. 341, 348.
349, 355, 361, 363, 369,
381, 384, 387, 400, 401,
403, 422, 566, 763.

— circulating, 325.

— decomposition of, 402.

— fermentation of, 342, 345.

— in sap, 274, 333.

— metamorplioses of, 326.

— origin of, 324, 346.

— and respiration, 402.

— in sieve-tubes, 325.
Prothallia, 742, 743, 744,

747-750, 752, 755, 800.
Prothallus, budding from,
801.

— of fern, 45S, 461,466,491,
525, 619, 772-774, 788,
789.

— reduced, 756, 760, 800.
Protococctis I'irulis, 392.
Protogynous flowers, 792.
Protonema, 30, 67, 479, 527,

735, 739-

— 29, 68, 479.

Protoplasm, 73, 78, 289, 290,
309, 316, 319, 323, 332,
355, 402, 413, 419, 422,
613, 614, 615, 638, 639,
645, 652, 654, 767.

— action of water on, 617.

— composition of, 79, 206.

— destruction of, 328.
■ — dormant, 402.

— during division, loi.

— and fermentation, 349.

— irritability of, 587, 599,
603, 655.

to external agents,

616, 617, 620, 621.

— movements of, 81, 194,
395, 603, 606-608, 615-

617, 621.

— regeneration from, 721.

■ — as a regulator of ditTusion,

212.

of osmosis, 275.

— respiration in, 401.

— structure of, 84.
Protoplasmic utricle, 80,

213, 251, 275, 563, 566,
568, 653, 654.
Prunus, 184.

— avium, 281.

— cerasifera, 439.

— lauro-cerasus, 184, 281.
■ — padus, 507, 508.
Prussic acid, 342.
Pseudo-chlorophyll corpus-
cles, 317.

endogenous buds, 475;

479.

Pseudo-fruits, 464.

Ps Hot urn, 55, 62, 70, 510.

Psoralen hirta, 185.

— - 185.

Pteris aqiiilina, 60, 129, 462,

471,699.
60, 87, 89, 129, 134,

138, 144, 146.

— aurata, 186.

— cretica, 801.
—flabcUata, 119.

— hastata, 456.

— serrulata, 772-774.
771.

Pulque, 270.

Purple leaves, 321.

Purpose, 10.

-- of respiration, 403.

Putrefaction, 348, 382, 385,

386.
Putrefactive fungi, 382, 386.
Pyrola, 302.

Quamoclit luteola, 672.
Quantitative experiments on
respiration, 399.

— selection by roots, 287.
Quantity of carbon dioxide

respired, 399.

— of embryonic tissue, 419.

— of starch assimilated, 313,
314.

— of water in plants, 564,
567, 573-

Quince, 90.

R.

Radial cell- walls, 442, 452,

457.

— and dorsiventral shoots
contrasted, 494, 705.

— growth, 438.

— organs, 486, 487, 489,
490, 493, 494, 496, 500,
502, 546, 698, 704.

rolled up, 706-709.

— section of wood, 159.
Radiation of heat, 404, 643.
Radical leaves, 421.
Radicle, 12, 332, 414, 689,

753, 764.
Radii, 482.
Radish, 24, 169.
Raßt'sta, 28, 63, 370.
Rafllesiaceae, 368, 370, 476,

765.
Rain-water, nitric acid in,

263.
Ranunculacca-, 177, 7 So.
Ranunculus, 660.

— acer, 483.

Ranunculus Ficaria, 24.
Rape, 546.
Raphides, 177.
Raphidophora, 25.
Rapidity of growth in length,
569,571.

— of protoplasmic move-
ments, 606, 607.

— of starch-formation, 311.
Rarefaction of air in wood,

267, 269.
Rattan, 657.
Ravenala, 50.
Reactions of aleurone grains,

334-

— of asparagin, 346.

— of chlorophyll, 321.

— to environment, 204.

— of starch, 337, 340.
Receptacles for secretions,

171, 175, 329-

Receptive spot, 774-776,

Reciprocal hybrids, 781.

Rectangular intersection of
cell- walls, law of, 433,
434, 442, 456, 502, 743-

Red autumn-leaves, 319.

— bacteria, 387.

— Beech, 159, 493, 506.

— Bindweed, 533, 672.

— Clover, 576.

• — colouring matters, 320,
328.

— fruits, 320.

— light, 303, 638, 696, 697.

— Pine, 505.

— wood, 163.

Reduced antheridia, 749,
756.

— forms, 6, 23.

— prothallia, 756, 760, 800.

— roots, 23.

— shoots, 54.
Reeds, 722.
Regeneration, 508, 721,723.

— from cut organs, 520, 521,
582,721.

Regulation of transpiration,

247.
Relation of nucleus to cell-

division, 107.

— of roots to leaf-surface,

156.

to soil, 256.

Relations between plants
and animals, 202, 413.

growth and arrange-
ment of cells, 436.

and cell - division,

431, 435, 447, 449, 453-

■ and configuration,

450.

Relations of position, 481,
528,681.

838

INDEX.

Relations between tendrils
and supports, 666.

twining plants and

supports, 668, 671.

Relative lengths of separ-
ated tissues, 570.

Renewal of vegetation, 411.

Reproduction, 9, 721, 723,

749-

— asexual, 728, 798, 799.

— sexual, 725, 728, 733,

770, 798-800.

— vegetative, 769.

— of angiosperms, 756.

— replacement of sexual by
asexual, 802.

Reproductive cells, 724, 732,
767.

mutual attraction be-
tween, 772-778.

— organs, 8, 368, 465, 534,
721-724, 735, 739, 741,
745, 757, 779-

position ot, 787.

Reseda odorata, 497.

Reserve-materials, 24, 156,

176, 284, 299, 310, 312,

317, 319, 326, 328, 332,

341, 345, 357, 363, 366,

373, 411, 462, 508, 521,

531, 534, 535, 539, 564-

employment of, 360,

364.

exhaustion of, 326,

327.

forms of, 332, 344.

metamorphoses of,

341.

re-invigoration of, 342,

352.

in seeds, 332.

storage of, 330, 364.

translocation of, 320,

360.

Reservoirs of reserve-
materials, 326, 330, 335,
340, 343, 358, 360, 508,
535.

Resin, 124, 126, 172, 175.

passages, 181, 327.

vesicles, 179.

Resinous substances, 163,
182.

Resins, 183, 362, 403.

Resistance to extreme tem-
perature, 192.

Respiration, 294, 298, 300,
326, 395, 402, 539, 595.

— 398.

— of animals, 395, 397, 401,

406.

— carbon dioxide of, 397,
399-

— chemistry of, 402.

Respiration, destruction of

organic substance by,

400, 402.
■ — diminution of weight

owing to, 300, 313, 327,

400.

— energy of, 396, 397, 399.

— evolution of heat in, 403,
405.

— experiments in, 397.
in "vacuo, 401.

— in fatty seeds, 400.

— of fungi, 385, 398, 401.

— of green organs, &c., 397,
400, 406.

— and growth, 399.

— importance of, 396.

— intramolecular, 395, 401,
402.

— light produced by, 406,
408.

— magnitude of, 399.

— of parasites, 399.

— in protoplasm, 401.

— quantitative experiments
on, 399.

— significance of, 402, 403.

— specific, 400.

— temperature of, 397.

— water of, 397, 400.
Resting spores, 351.
Retardation of growth, 199.
by light, 535, 559,

561.
Retention of salts by soil,
261.

— of water by soil, 258.
Reticulate thickening, 92.

— vessels, 136, 160.

Reversion of hybrids, 784.

Revolving nutation of ten-
drils, 657, 661, 664, 666,
667.

of twining plants, 657,

669, 670.
Rhamma, 266.

— cathartica, 57, 425.
—frangula, 162, 229.
Rheum, 485.

— undulatum, 466.
Rhinanthus, 28, 367, 369.
Rhizines, 35.
Rhizocarpes, 492, 495.

— reproduction of, 745, 756.
Rhizoids, 23, 29, 35, 365,

453-

Rhizomes, 60, 326, 330, 332,
478, 506, 534, 536, 565,
571, 701, 721, 739-

Rhizomorpha, 388, 407, 695.

Rhizophores, 22, 25.

Rhododendron, 780.

- — ferugineum, 185.

Rhodora, 780.

Rhodospermin crystals, 335.
Rhubarb, 51, 186, 216, 465,
571-

— 485.

Rhynchonema, 787.
Ribes, 124, 186.

— nigrum, 165.
Ribesacese, 780.
Rice, 337.

Ricinus, 177, 181, 227, 271,
326, 334, 347, 373, 375,
513, 574, 699-

— 158.

— co)nmunis, 313.

133, 136, 334, 374,

764.
Rigidity of organs, 217, 631,

657.

— of plants due to absence
of oxygen, 396, 402.

— of tissues, 218, 565, 569.
Rigor in irritable organs,

593-596, 628.
Ring-bark, 166.
Ringing of trees, 229.
Ripening of fruits, 320.

— of seeds, 320, 323.
River-water, 288.
Robinia, 199, 285, 319, 425,

478, 581, 623, 649, 656.

— pseudacacia, 21, 42.
Rocks, disintegration of,

264.

Rolled-up dorsiventral or-
gans react like radial
ones, 706-709.

Root, 3, 113, 266, 275, 284,
298, 329, 330, 335, 344,
416, 417, 418, 423, 424,
447, 451, 456, 460, 467,
474, 478, 482, 485, 486,
491, 492, 505, 508, 512,
513, 520, 528, 547, 563,
564, 567, 569, 579, 582,
583, 678, 689, 690, 703,
723.

— absorption by, 246, 253,
287.

— activity of, 264.
^ adventitious, 477.

— cambium of, 168.

— degraded, 369.

— development of, 15, 418,
470, 522.

— entrance into soil, 256,
545, 583, 692,702.

— functions of, 16, 255.

— geotropism of, 679, 680,
689-691.

— growing-points of, 476,
689.

— growth of, 17, 423, 539,

541, 543, 544, 553-
in thickness, 155, 168.

INDEX.

829

Root, heliotropism of, 693,
694.

— length of, 13, 512, 513-

— movement of, 21.

— primary, 23, 747, 752,

753,764.

— sensitiveness of, 16.

— shortening of, 19, 576.

— structure of, 15.

— tap, 23.

— xylem and phloem of, 168.
Rootsofalgae,29,3i, 33, 255.

— of aquatics, 14, 23, 255.

— of aroids, 24.

— of coniferae, 23.

— of ferns, 31.

— of fungi, 33,

— of hepaticeae, 31.

— of land-plants, 246, 255.

— of lichens, 34, 35, 394.
- — of mosses, 29.

— of parasites, 23, 25, 369.

— aerial, 13, 14, 522, 523,
660, 693, 704, 71 1.

— aeration of, 256.

— development of, 523, 744.

— effect of temperature on,
264.

— hydrotropism of,7i5-7i7.

— irritability to contact,
715, 716.

— lateral, 17, 284, 416, 476,
677, 699, 704, 705.

— geotropism of, 707, 708.

— metamorphosed, 23.

— relation to leaf-surface,
156, 255.

to leaves, 512, 513.

- — relations to soil, 256.

— regeneration from, 721,
723.

— rudimentary, 23.

— selective power of, 287,

— space occupied by, 13,
512.

— tissue-tensions in, 575.

— typical, II.

— vascular bundles of, 126-

135-
Root-cap, 14, 416, 447,451,

456, 545, 583-
fibres, 13, 14, 15, 418,

422.
forming substance, 519,

521, 522.

— -fungi, 388.

hairs, 17-20, 25, 123,

256, 374, 512, 514, 524-
526, 582, 695,710.

absorption by, 257,

262, 274.

action of, 263.

attachment to soil,

257, 259.

Root-hairs, excretion of
acids by, 262.

— — functions of, 255, 275.

• solution by, 262.

Rootless plants, 62.
Root-like shoots, 62.

parasites, 232, 253, 534.

pole, 484.

pressure, 271, 274, 275,

281.

274.

conditions affecting,

273,278.
daily periodicity of,

273.
fluctuations of, 272,

599.

in fungi, 281.

measurement of, 274,

277.
stock, 276, 300, 312, 330,

343, 366.
negative pressure in,

277.
weeping of, 270.

— tubers, 21, 23, 24, 517.
Rosacese, 463, 780.
Rose, 463, 507, 537, 756.

— of Jericho, 588.
Rostelluin, 796.
Rotation, 82, 616-617.
Royal fern, 744.
Rubidium, 383.
Rudimentary tissue, 141,

149.

— organs, 6.

— vascular bundles, 151.

— shoots, 65.

— roots, 29.

— reproductive organs, 723.
Ruellia clandest'ma, 791.
Rumex, 124.

Runners, 58, 478, 504, 703,

708, 721, 798.
Ruscus, 55, 511.

— aculeatus, 55.

— hypoglossum, 49.
Rushes, 145, 219.
Russelia juncea, 55.
Rust-fungi, 381.
Rutaces, 183.
Rye, 337, 687,790-

S.
Sabal, 702.

Saccharomycetes, 385.
Saccharum ofpchiarum, 117.
Sag'ittaria, 43.
Sa^lep, 180.
Salices, 780.
Salix, 177, 184, 784.
—fragilis, 236.

— gramlifoUa, 5 1 .
50, 128.

Salicylic acid, 385.
Saltpetre, 291, 325, 567.
Salts, 79, 172, 202, 226,253,

260, 262, 283, 296, 324,

566.

— absorption by roots, 262,
287.

— absorption and retention
by soil, 260, 261.

— ammonia, 292, 384.

— as affecting transpiration,
252.

— essential in nutrition, 284,
287.

— in sap, 274, 280.

— in soil, 259, 262, 287.
Salvia, 186.

— 794.

— pollination of, 794.

— hirta, 791,

— pratensis, 794.
Sal-vinia, 62, 442, 756.
Salviniaceae, 40.

— reproduction of, 745.
Sambuciis, 186, 319.

— Ebulus, 129.

— 7iisra, 177, 179, 570.
Sand, 259.
Sandarach, 181.
Santalum, 760.

Sap, 274.

— salts in, 274.
Saprolegnis, 375, 381, 612,

777-
Saprophytes, 63, 366, 367,

369, 371.
Saprophytic fungi, 381.

— Phanerogams, 371, 511.
Sarcina, 387.
Sargassum, 69.

— inilgaruin, 71.
Sauromatum, 699.
Saxifrage, 280.
Scalariform vessels, 137.
Scale-bark, 166.
Scale-leaves, 504, 506, 507.

— converted into foliage-
leaves in the light, 536.

Scammony, 180.
Scape, 418, 543, 546.
Scarlet Runner, 505.

629, 630, 688.

Schizogenous glands, 185,

186.
Schizomycetes, 384, 385,

386.

— 387.
Scirpus, 219.

— cladium, 145.

— nwritimus, 708.
Scitaminese, 802.
Sclerenchyma, 144, 181, 219,

230, 423, 511, 569, 570,
572.

830

INDEX.

Scleroblasts, 167.
Scolopendrium, 51.
Scopolia atropoides, 792.
Scorzonera, 173.

— hisfanica, 173.
Scrophularia nodosa, 792.
Scrophularinese, 55, 760, 780.
Sculpture of cell-wall, 92,

137-
Scutellum, 373.
Scybalium, 64.
Scyphantus elegans, 669.
Scytonema, 892.
Sea water, 287-288.
Second phase of growth, 414,

417, 437-
Secondary cortex, 156, 163.

— thickening, 155.

— uses of excreta, 329.
Secretion, 186, 376.
Secretions, 172, 179, 328,

329, 362,

— receptacles for, 171, 175,
329.

vesicles, 176, 179.

Sections of organs, 432, 436,
4395 442, 447-

Sedges, 59, 219, 470.

Sedum, iTj, 321.

■ — purpureum, 119.

Seed-coats, 147, 753.

leaves, 753.

plants, 753, 766.

Seedless Phanerogams, 802.

Seedlings, 366, 399, 401, 414,
531, 539, 564, 575, 581,
584, 694, 702, 710, 712.

— absorbing organ of, 344.

— compared to parasites,

373.
■ — fat in, 347.

— ferments in, 343, 389.

— geotropism of, 679, 692.

— reserve materials in, 363.
Seeds, 300, 312, 323, 326,

327, 330, 332, 334, 335,
357, 364, 404, 4", 414,
465, 485, 538, 745, 748,
750-756, 763-766, 798.

— composition of, 290.

— contrasted with spores,

753.

— exalbuminous, 763, 764.

— ferments in, 345.

— germination of, 398, 742,

752, 753-

— reserve-materials in, 332,

334, 343-

— ripening of, 321, 323, 352.
Segment, 95, 100.
Segmentation, 425, 483.

— of Bacteria, 387.
Segmented laticiferous ves-
sels, 173.

Segments of apical-cell, 452,
453, 454, 455, 456, 473,
545-

Selaghiella, 25, 40, 301, 455.

— 19.

— apical cell of, 452.

— caulescens, 750.

— inxqualifolia, 22.
41, 110, 149, 749.

— Ix'vigata, 22.

— Martensii, 22.
750.

Selanigellae, reproduction of,

741,749, 764.
Selective power of roots, 287.
Self- fertilisation, avoidance

of,787, 790,791,794-797,

799.

pollination, 791.

sterile flowers, 790.

Sempert'tvum, 321.
SeJiecio umbrosus, 564, 572.

— vulgaris, 640.
Sensitive Plant, 644-649.
Sensitiveness, 395.

— of root, 16.
Sepals, 497.

Separating layer of autumn-
leaves, 319.

Septa, transverse, 432.

Septum, 105.

Sequence of anticlines and
periclines, 443.

Sessile leaves, 53.

Seta, 738.

Sexual affinity, 781.

— cells, mutual interaction
of, 767, 770.

— • — differences between,
768.

origin of, 787.

— organs, 467, 491, 508, 724,
728, 734, 741, 769, 799.

— reproduction, 107, 721,

728, 733, 770, 798-800.

of algae, 728.

of Equisetum, 740.

of ferns, 744.

of fungi, 725.

of mosses, 737.

replaced by asexual,

802.

— swarmspores,72 8,733,777.
Sexuality, 780.

— enfeeblement of, 783.

— object of, 799.
Shade-plants, 302, 306.
Shaking as affecting tran-
spiration, 252.

Sheathing leaves, 470.
Shoot-axis, erection of, 686,
687.

— heliotropism of, 693,

— primary, 753.

Shoot, 3, 36, 37, 414, 424,
447, 454, 457, 460, 467,
474, 484, 505, 508, 510,

520, 528, 564, 569, 571,
581,678, 723.

— development of, 744.

— geotropism of, 679, 680,
690, 691.

— growth of, 41, 414, 421,
422, 540, 542-5-14-

— growing point of, 425.

— organisation of, 43, 481.

— regeneration from, 721,
723.

— vascular bundles of, 44,

126.

— -axis, 4, 38, 39, 45, 417,
424, 482, 486, 492, 504,
510, 511, 569, 570, 575,
669, 674, 703.

dorsiventral, 455, 485.

of Fern, 45.

respiration of, 400.

axes, thickening of, 155.

forming substance, 519,

521, 522.

pole, 484.

system, 510.

Shoots,adventitious,4 7 5, 477,

478.

— climbing, 483, 489, 548,
668, 669, 674, 675, 713.

— creeping, 483, 485, 489,
492, 678, 699.

— exogenous development
of, 475.

— metamorphosed, 54.

— of parasites, 63, 64.

— plagiotropic and ortho-
tropic contrasted, 494.

— radial and dorsiventral
contrasted, 494.

— of algae, 54, 65, 69.

— of fungi, 54, 71.

— of mosses, 54, 65.

— of parasites, 63, 64.
Shortening of root, 19, 575,

576.

— of tissues, 570-572.
Shrub, 510.

S icy OS, 658, 661.
Sieve-tubes, 135, 160, 163,
175, 325, 355, 416, 423-

— emptying of, 361.

— as places of origin of pro-
teids, 325,

— pressures on, 360.
Silene, 640, 780.

■ — inflata, 781.

Silica, 88, 163, 260, 288, 289,

320, 383.
Sihceous deposits, 288.

— skeletons, 288.
Silicification, 289.

INDEX.

831

Silver Fern, 186.

grain, 159, 439.

Sinapis alba, 18, 693.

— nigra, 399.
Siphoneae, 179, 335, 365.
SiphoTiocladium, 82.

Sizes of starch-grains, 336.

— of stomata, 251.

— of swarm-spores, 603,
605.

Skeleton leaves, &c., 129.
Skeletons, calcareous, 290.

— cell, 319.

— siliceous, 288.

— starch, 339.

— wood, 159.
Sleep-movements, 513, 599,

625.

— mechanics of, 626, 633-
640.

— uses of, 643.

— of flowers, 199,623,641.

— of leaves, 199, 623, 626-
629, 635-640, 643.

Slow movements of water,

_253.
Smilax, 56, 661.

— sarsaparilla, 51.
Smut of wheat, 381.
Sodium chloride, 287.

— salts, 286.
Soft-bast, 163.
Soil, 256, 292, 295.

— 257.

— absorption from, 255.

— absorption of water by,

259.

— absorptive properties of,
261.

— amount of water in, 258.

— air in, 257.

— carbonic acid in, 263.

— chemical compounds in,

261.

— composition of, 263, 287.

— decomposition of, 263.

— desiccation of, 258.

— nutritive salts in, 259, 262,
287.

— relation of roots to, 256,

512, 545, 583, 692, 702.

— retention of salts by, 260,
261.

of water by, 258.

— structure of, 257.

— sulphates in, 259.

— temperature, &c., of, 553,

554-
Solanaceae, 178, 780.
Solaneae, 177, 334, 572,640.
Solanum, 320, 780.

— jasminoides, 56, 659, 663.

— pyracantha, 58.

— atro-sanguineion, 58.

Solar-spectrum, 303.
Solomon's seal, 60.
Solution, 207.

— of minerals by root-hairs,

262.

— nutritive, 284, 286, 291.

— of starch, 339.

— and translocation of starch,

312, 313.
S one hits ol er a cells, 396.
Sorrel, 329.
Sources of carbon, 292, 294,

384.

— of hydrogen, 290.

— of light, 307

— of nitrogen, 291, 384.

— of nutritive elements, 290,

384.

— of oxygen, 290.

— of sulphur, 291.

Space occupied by roots, 13.
Sparganhtm, 701, 708.

— ramosum, 334.
Spartlum, 510.

— junceum, 55, 70.
Species-hybrids, 780, 783.
Specific energy of assimila-
tion, 312.

of respiration, 400.

ofirritableorgans,6oi.

— energies of nerves, 600.

— gravity of wood, 240.
Spectrum of chlorophyll,

322.
195, 305.

— of lithium, 236.

— solar, 303, 697.
Specularta perfoliata, 791.
Spermaphytes, 756, 800.
Spermatozoa, 604.
Spermogonia, 724, 732.
Sphagnum, 377, 465.
Sphserise, 375, 381.
Sphn-roplea, 483.
S|)here-crystals, 336.
Spindle-tibriliae, 103.
Spiriva sorbifolia, 487, 500.
Spiral theory, 499, 500, 501.
refuted, 495, 497.

— thickening, 92.

— vessels, 136.
Spirillum, 387.
Spirogyra, 104, 31 1, 453, 48 >,

653, 723, 727-729, 772,
776, 787.

— 314.

— Ji/galis, 33.

— Icngata, 727.
Splint-wood, 163.
Splitting ferments, 342.
Sponges, 298.
Spongy mesophyll, 536.
Spontaneous generation, 4 1 2.

— heating, 395, 405.

Spontaneous periodic move-
ments, 560, 590, 593,
594, 598, 599, 627, 628,
635-638.

Sporangiophorc, 695, 700,
717, 740, 741, 751.

Sporangium, 465, 467, 724,
740, 741, 744, 746, 748,
750, 751, 756, 757.

Spore-capsule, 738, 739.

fruit, 746.

— -plants, 753.

Spores, 75, 98, 153, 321,323,
326, 351, 352, 364, 386,
428, 430, 484, 524, 525,
527, 534, 535, 724, 725,
726, 734, 740, 74,-, 761,
769, 798.

— asexual, 740, 768, 769.

— germination of, 729, 741,
742, 746, 748, 753-

— mother-cells of, 433, 739.

— and seeds contrasted, 753.
Sporocarp, 746, 747.
Sporogonium, 149, 723, 734.

— development of, 738.
Sporophytes, 756.
Spring-water, 262.

wood, 162, 230, 268, 578,

St achy s, 59.

— germanica, 124,
Stamens, 469, 497, 652, 757.

— irritability of, 395, 595,
650, 651, 654, 655, 790,
792.

— nutations of, 548.
Stapelia, 54, 203.

Starch, 172, 284, 293, 307,
319, 323, 332, 335, 347,
355, 384, 403, 763.

— carbon-dioxide necessary
for its formation, 311.

— chemistry of, 339.

— direct formation in chloro-
phyll-grains, 3 1 o, 3 1 1 , 3 1 8,
338.

— disappearance in dark,
309.

— fermentation of, 342.

— iodide of, 310.

— origin in chlorophyll, 309,

315-

— the origin of organic sub-
stances, 326.

— in parasites, &c., 372.

— quantity formed, 312,313,
314.

— rare in fungi, 385.

— reactions of, 337, 340.

— in seeds, 334, 343.

— solution of, 339.

— transformations of, 311,

323, 328, 339, 343,

— transitory, 325, 357, 358.

832

INDEX.

Starch, translocation of, 312,
313, 316, 325, 329, 358,
363.

bearing layer, 358.

— -cellulose, 339.

forming corpuscles, 309,

316, 338, 358.
grains, 77, 172, 290, 334,

341. 355, 362, 645,

compound, 315, 337.

concentric, 316, 338.

development of, 315,

316, 338.

excentric, 316, 338.

forms of, 337.

growth of, 315.

penetrated by fungi,

389.

semi-compound, 338.

simple, 337.

sizes of, 336.

stratification of, 338.

structure of, 337, 339.

swelling of, 340.

paste, 340.

skeletons, 339.

Stärkebildner, 316.
Stem, 330, 343.

tendrils, 658.

Stephanosphxra plwvialis^QOb.
Stereocaulon ramulosum, 392.
Stereum hirsiitimi, 388.
Sterile cells of pollen-grain,

756.

— flowers, 791.
Stictaftdiginosa, 391.

— pulmonacea, 393.
Stigma, 758, 760, 762, 778,

789.

— movements of, 792.
Stimulation, 591, 601, 617,

627, 644, 647, 649, 653,
665, 685.

— due to contact, 597, 60 r,
602, 644.

— due to electricity, 601,
654.

— due to injury, 582, 583.

— due to light, 199, 594,
597, 601, 625-628, 638,
653, 692, 694, 698, 702,
705.

— due to moisture, 702,
717.

— due to temperature, 597.

— due to vibration, 597.
Stimuli, 189, 395, 515, 516,

591, 601, 654, 681.

— definition of, 590.

— abnormal growth due to,
537.

— combined action of, 627.

— propagation of, 596-598,
646, 649, 662, 663.

Stimulus, definition of, 590.

— and effect, dispropor-
tionality between, 589,
595, 597, 598.

Stinging-hairs, 124.
Stipule, 506, 507.
Stolon, 58, 703, 721.
Stoma, 249, 250.
Stomata, 47, 97, 115, 117,

167, 226, 279, 416, 423,

525.

— functions of, 248,251, 264.

— mechanism of, 248.

— opening and closing of,
249.

— of parasites, 251.

— sizes of, 251.

— subterranean, 251.

— water, 119, 279.
Stone-cells, 146, 167.
fruit, 147.

— -Pine, 752.

Storage of carbo-hydrates,
335-

— of reserve -materials, 330,
364.

Stratification, 90.

— of starch grains, 338.
Stratiotes, 13, 255.
Straw, 320.

Strawberry, 58, 703, 722.
Stre/iizia, 50, 310.
Striation, 91.
Strontium, 383.
Structure, 189,503,507,678,

— of aleurone grains, 334.

— of flowers, 792-797.

— of laticiferous vessels, 173.

— of leaf, 490, 491.

— of leaf, influence of light
on, 536.

— molecular, 205, 242.

— of motile organs,629-634,
645.

— of root, 15.

— of shoot-axis, 40.

— of soil, 257.

— of stamen, 652.

— of starch-grains, 337, 339.

— of vascular bundle, 131.

— of wood, 237.
Struggle for existence among
» organs, 509.

Style, 548, 651, 652, 758,
762,778.

— movements of, 792.
Sty/idium, 596.
Stypocaulon, 435.

— scoparium, 436.
Sub-epidermal glands 138,
Suberin, 88.
Suberisation, 88, 90.
Substance and form of,

organs, 509, 521.

Subterranean organs, 404.

— plants, 225.

— shoots, 61.

— stomata, 251.
Succinic acid, 348, 386.
Succulence, 362.
Succulent plants, 54, 510.
Suction in wood-cells, 269.
Sugar, 172, 307, 310, 314,

318, 323, 329, 335, 339,
341, 343, 347, 349, 355,
357, 363, 369, 384, 385,
390, 400, 566,

— in fungi, 385.

— in sap, 274, 281.

cane, 117.

Maple, 359.

Sulphates, 202, 226, 253-254,

262, 284, 287, 291, 324,
325.

— in soil, 259.

— of potassium, 342.
Sulphur, 206, 284, 289, 290,

293, 324, 325, 382.

— sources of, 291.
Sulphuretted hydrogen, 387.
Sulphuric acid, 79, 324, 345.
absorption by leaves,

255.

in sap, 274.

Sun-flower, 11, 13, 199, 225,
227, 229, 244, 271, 274,
276, 277, 293, 309, 461,
468, 513, 640, 692, 699,
703, 763.

— 467, 574.
Sun-light, 302.
Supports of tendrils, 666.

— of twining plants, 668,
671.

— twining without, 672, 673.
Suppression of anticlines and

periclines, 442, 458.
Swarm-spores, 82, 107, 199,
429, 484, 603-605, 612,
613, 616, 775.

— chlorophyll in, 603, 604.

— experiments with, 611.

— irritability of to light, 608,
610-612, 613, 621, 696.

— movements of, 604-608,
777-

— phototonus of, 612.

— non-sexual, 728, 733, 739.

— sexual, 728, 733, 777.
■ — sizes of, 603, 605.
Swaying of tree-trunks, 579.
Swelling of organised bodies,

207, 243.

— of starch-grains, 340.
Swollen cellulose, 180.
Sylphium perfoitatum, 570.
Symbiosis, 34, 299, 390.
Symphoricarpus, 475.

/

INDEX.

^33

Symphytum, 496.
— 496.

Sympodium, 60, 466.
Synalissa symphorea, 392.
Synergidye, 760, 771, 800.
Syringa, 124, 1S6, 506, 507.
- — vulgaris, 399.
Systems of tissue, no, ii^
141.

Tamils elepbatitipes, 668.
Tangential growtli, 4 38.

— piiloeni, 164.

— section of wood, 159.
Tannin, 176, 179, 321, 327,

328, 388, 645, 652.

vesicles, 179, 181.

Tap root, 23.
Taraxacum, 642.

— officinale, 21, 174, 335,
572, 574-

Tartaric acid, 328, 384.
Tartrate of ammonia, 383,

386.
Taxinese, shoots of, 55.
Taxus, ITT, iSij 321.

— baccata, 751.

752.

Teazel, 572, 576.

Tectona, 163.

Temperature, as affecting

the bleeding of wood,
266.

evaporation, 228,

movements of water,

270.

protoplasmic move-
ments, 606, 608.

root-absorption, 264,

278.

root -pressure, 278.

transpiration, 252.

weeping of root-stock,

270.

— curve of, 305, 406.

— effect on plant, 191, 194,
593, 646.

— of fermentation, 386.

— of imbibition, 210.

— influence on growth, 553,
555.

— periodic oscillations of,
406.

— of plants, 404.

— of respiration, 397.

— of soil, 553,554.

— as a stimulus, 597, 641.

— effect of variations in,
540, 541, 549, 551-553,
556, 559, 560, 625, 641,
642.

— of wood, 404.
Tendril-plants, 56, 513, 658.

[3]

Tendrils, 378,533, 547,571,
596, 598, 599, 657, 658,
659, 660, 667, 670, 673,
675, 676, 689, 713, 716.

— coiling of, 661-665.

— contraction of, 665.

— curvature of, 661, 662.
■ — diameter of, 666.

— growth of, 664, 665.

• — heliotropism of, 667.

— irritability of, 658, 659,
661-666.

— mechanics of, 664.

— relations to support, 666.

— revolving nutations of,
661, 664, 666, 667.

— thickening of, 663.

— turgescence of, 665.

— and twining shoots com-
pared, 668, 674, 675.

Tension of air in wood,
268.

Tensions and pressures in
cortex and wood, 574,
575, 577-579, 580.

— of tissues, 216, 437, 569-
574, 583.

Terminal buds, 466.

— flower, 483.
Testa, 752.
Tetrads, 739.
Tetrahedral cells, 434.
434.

— apical-cell, 447, 456, 743.
456.

Te trap bis, 68, 723,

— pelluciJa,'l'2.2.
Tetraspcra, 433.

— lubrica, 607.
Thallophytes, 126, 171.

— epidermis of, 121.
Thallus, 8, 390, 393.
Theca, 738.
Thelephora, 700.

— perdrix, 388.

Theory of ascent of water,
238, 241.

— of descent, 516.

— • of fermentation, 349.

— of geotropism, 691.

— of growth, 689.

— of growth by intussus-
ception, ^13.

— of heliotropism, 677, 695-
696.

Thermo-electric observa-
tions, 406.

Thermomctric observations,
404.

Tbt'sium, 27, 367, 369.

Thickening of the cell- wall,
92.

— of roots, 168.

— of tendrils, 663.

3 H

Third phase of growth, 414,

423, 425.
Thistle, 588, 651.
Thladiantba, 522.

— dubia, 21, 24, 516.
Thorn-apple, 37, 130.
Thorns, 57, 425, 466, 475.
I'hree phases of growth, 4 1 4,

417, 418, 424.
Thuja, 40, 758.
Thunbergia, 669.
Thyme, 185.
Thymus vulgaris, 185.
Tilia platypbyllos, 446.
Tiliaceae, iSo.
Tissue, assimilating, 148.
cells, 433,453-

— embryonic, 415, 417, 418,
419, 422, 423, 431, 440,
445, 461, 466, 467, 473,
477, 479, 480, 482, 765.

— fundamental, 141, 423. .

— lenticel-, 167.

— permanent, 419, 425, 440,
465, 466, 467, 479.

— rudimentary, 141.
— • systems of, 141.

• tensions, 216, 437, 583.

daily periodicity of,

575.
and growth, 569-574.

— — in motile organs, 631-

633, 648,649.
in roots, 575, 576.

— — transverse, 572, 574,

580.
Tissues, no, 423, 457, 753.

— continuity of, 159, 469,

475-

— cortex, 158.

— curvatures of, 2 1 7.

— of cryptogams, 149.

— elasticity of, 219.

— extensibility of, 220.

— flexibility of, 220.

— functions of, 355.

— of lichens, 392.

— pressure in, 362.

— of phanerogams, 149.

— rigidity of, 218.

— shortening of, 570-572.

— transport of materials in,

355.

— ot wood, 158.
Toadstools, 71, 75, 152.
Tobacco, ir, 51, 225, 227,

228, 229, 259, 262, 264,
271, 274, 276, 277, 293,
310, 312, 315, 513, 531,
703, 742.

— 435.

Torsions, 548, 549, 669-

673, 675, 676,714.
Tracheae, 137.

«34

INDEX.

Tracheides, 137, 160, 237,

389.
Tradescantia, 105, 126.

— albißora, 114.

— rubella, iif.
Tragacanth, 180.
Tragopogon, 362.
Trajectories, 439.
Trametes pitii, 388, 389.

— radiciperda, 388, 389.
Transformations of aspara-

gin, 346,
• — of carbo-hydrates, 323.

— of fats, 311, 328.

— of Proteids, 326.

— of starch, 311, 323, 328,
343-

Transitory starch, 325, 357,

358.
Translocation by diosmosis,

363.

— of fats, 347, 358.

— of plastic materials, 353,
355, 357.

— of reserve materials, 320,
329, 345-

— of starch, 312, 313, 316,

325, 329, 358, 363.
Transmission of parental

characters, 779.
Transpiration, 226, 244, 246,

276, 277, 511, 512.

— 252.

— amount of, 228.

— conditions affecting, 228,
246, 251.

• — of land-plants, 225.

— light as affecting, 251.

— moisture as affecting, 251.

— purpose of, 253.

— regulation of, 247.

— salts in soil as affecting,
252.

• — shaking as affecting, 252.

— temperature as affecting,
252.

- — work of, 229,
Transverse folds on roots,
576.

— growth, 483.

— sections of organs, 436,

439, 447, 481, 482, 485,
486, 488, 577,

— section of wood, 444.

— septa, 432, 453.

— tensions in tissues, 572,
574, 580,

Trapa, 352.

— not ans, 351.

702.

Traubes' cells, 215.
Travelling of plants, 59.
Tree-Ferns, 128, 144, 465,

472, 741, 800.

Tree-Ferns, ascent of water
in, 230.

killing fungi, 344, 369,

381, 387, 389,407.
.Trees, 510.

— branches of, 580.

— correlations in, 505.
Tree-trunks, swaying of,

579.

buttresses on, 579, 580.

Trichogyne, 726, 727.
Trkhomanes, 442.
Trifolium, 593, 598, 630.

— incarnatum, 594, 627.

— pratense, 627, 637, 638.
Trimorphic flowers, 790.
Triticmn, 780.

— 688.

— repens, 59.

— -vulgare, 33S.
781.

Tropxolum, 56, 263, 279, 300,

533, 548, 659, 713, 714-

— majus, 313, 523, 612,
711.

315.

— minus, 57, 660.
Truffles, 152.

Tubers, 21, 24,59, 169,298,
300, 312, 314, 326, 330,
332, 343, 350, 357, 366,
404, 462, 478, 504, 517,

534, 722, 798-

— conversion into leafy
shoots, 504.

— of moss, 364.

Tulip, 43, 59, 6r, 335, 357,

360, 534, 641, 642.
Tulipa precox, 59, 358.

— silvestris, 178.
Tulip tree, 51, 506.
Turf, 294, 382.
Turgescence, 213, 233, 250,

255, 264, 275, 360, 362,
552,554-

— due to acids, 328, 403,
566.

— and growth, 565-569,

572, 573, 576, 581, 641.

— and irritability, 588, 599,
631, 633, 634, 635, 638,
639, 646-649, 653, 665,
689, 697.

Turnip, 24, 169, 343.
Turpentine, 389.
Twin crystals, 177.
Twining, mechanics of, 670-
672, 674-676.

— directions of, 668, 669.

— without supports, 672,
673-

— plants, 56, 513, 657, 660,

668, 676.
geotropismof,67i,675.

Twining plants, relations to
supports, 668, 671.

revolving nutations of,

669, 670.

and tendrilscompared,

668, 674, 675.

— shoot axes, 548, 668, 669.

growth of, 669, 674.

irritability of, 674,

675.
Tyloses, 581.
Types of growth, 451.

abnormal, 425.

Typha, 701.

— latifolia, 312.
Typical cell-division, 106. ,

— forms, 6.
of tissue, 115.

— growth in thickness, 157.
412.

— growing-point, 460.

— roots, I r .

— shoots, 36.

— tendrils, 659, 660.

— reproductive organs, 723,
734.

— flower, 756, 757.
Tyrosin, 345, 349.

U.

Udora, 297.
Ulmeae, 180.
Ulmus, 279.
Uloihrix, 611, 612.

— speciosa, 607.

— zonata, 6 1 r .

— 604.

Ultra-violet rays, 696, 697.

Ulva, 6ri.

Umbelliferae, 52, i8r, 334,

418, 470, 572, 703, 792.
Unbranched plants, 472.
Unfolding of leaf-buds, 397,

547-
Unsegmented laticiferous

vessels, 173.
Unstable crystals, 591, 593,

593-

— equilibrium, 591, 592,

597-
Upper-leaf, 471, 506.
Urea, 384.
Urticaceae, 124, 171, 17 8,

496, 780, 790.
Urtica, 436.

— dioica, 169.

Uses of irritability, 655,

656.
Usnea harhatn, 34, 153, 393.
Utricular-vessels, 179.
'Utricularia,\i, 335, 461, 492.

— -vulgaris, 687.
Utricularise, 378.

INDEX.

^5

V,

Vacciniuni, 321.
Vacuole, 80, 213, 419.
Vacuum, experiments on
plants in, 401.

— rigidity of plants in, 395,

396, 595.
Vaginula, 738.
Valerian, 58.
Valeriana officinalis, 58.

— Pbu, 543.
Valerianeae, 125.
Valisneria, 675.
Vallonia, 82.
Variation, 9, 516, 799.

— of hybrids, 783.
Variations in growth, 539,

54I5 549, 551, 552, 556,

559, 560.

— in intensity of light, 541,
551, 552, 554, 556, 559,

560, 594, 625-630, 634-
639, 642.

— in moisture, 551, 552.

— of pressure in tissues, 361.

— in temperature, 540, 541,
549, 551-553, 556, 559,
560, 625, 641, 642.

Variety of fungi, 380.

hybrids, 780, 783.

Vascular bundles, 44, no,
114, 126, 157-159, 171,
216, 230, 325, 416, 423,
470, 569-572, 582, 626,
630, 633, 634, 645, 649,
652.

— • — elements of, 132.

■ — — ■ functions of, 230, 355.

of leaves, 49.

of Monocotyledons,

170.

sheath of, 144, 316.

of skeletons, 129.

■ structure of, 1 30.

transverse section of,

13^*

— Cryptogams, 171,455,471,

473, 603, 800.

fertilisation of, 108.

heterosporous, 745-

751.

isosporous, 739.

reproduction of, 724,

730-744.
vascular bundles of,

126.

— plants, growing point of,
460.

Vattcheria, 70, 76, 82, 84, 311,
426, 431, 543, 603, 604,
607, 618, 680, 697, 721,
732,733,788.

— terrcstris, 81.

Vaucheria sessilis, 108, 733.
Vegetable acids, 172, 328,
342, 403, 566.

— ivory, 335, 344, 763.
Vegetation, aspects ot, 197,

411, 702.

— period of, 411.
Vegetative forms of fungi,

387.

— organs, i, 8, 72, 723.

— propagation, 477,723,769,
798, 800.

— region, 473.

— repetition, 467.
Velamen, 25.

Velocity of ascending water,
236.

— of growth, 552, 560.
Venation of leaves, 48, 51,

128, 233, 511, 705.
Venter, 737, 738, 742.
Ventral canal-cell, 737.
Venus' Fly-trap, 376.
Ferbascum, 21, 124, 780.
• — nigrum, 789.
Vernation, 547.
Vessels, 135, 160, 237,274,

288, 329, 424, 576, 578,

58r.

— laticiterous, 171,
Vetch, 56, 345, 660.
Vibrations, effect on plant,

221.

— rigor due to, 595.

— as stimuli, 597.
Vibrio, 387.
Viburnum lantana, 52,
Vicia, 56.

— Faba, 281, 283, 368, 539,
541, 543, 691, 707.

18, 315, 542, 583,

692, 708, 764.

Victoria Regia, 406, 550.

Vine, 52, 56, 177, 270, 274,
278, 279, 319, 581, 658,
660, 663, 666, 713.

Vines, American, 785, 786.

— cultivated, 785.

— hybrid, 784-786.
Viola, 186, 789, 791.

— canina, 791.

— elatior, 791.

— mirabilis, 791.

— odorala, 791.

— tricolor, 794.
795.

Violet light, 303, 696, 697.
Virgilia lutea, 475.
Virginian creeper, 56, 663,
668.

667.

Viscum, 26, 166, 368.
• — album, 25.
26.

Viteliin, 334.

Vitis, i66, 178,279,662,666,
784, 785.

— lestivalis, 785, 786.

— arizonica, 785, 786.

— californica, 785, 786.

— candicans, 785, 786.
^ caribaa, 785.

— cinerea, 785, 786.

- — cordifolia, 785, 786.

— labrusca, 785, 786.

— lincecumii, 785, 786.

— monticola, 785.

— riparia, 785, 786.

— rotundifolia, 785.

— riipestris, 785, 786.

— lünifera, 236, 784, 786.
Voandzeia, 791.

Volume of carbon-dioxide
absorbed, 297.

— of cell, 422.

— of daughter-cells equal,

434-

— definitive, 412.

— increase in, 449.

— of gases in assimilation,
equality of, 298.

— of oxygen absorbed, 398.

— of oxygen evolved, 297.

— of wood-ceils, 239.

— of yeast-cell, 386.
Volvocineae, 483, 604, 730.

W.

Wall-flower, 533.

Walnut, 51, 52, 266, 314,

579, 763.
Warm-chamber, 608.
Warping, 211, 588.
Water, 202, 203, 225, 296,

309, 317, 326, 381, 403,

419, 422, 512.

— absorption of by roots,

246.
of by leaves, 254.

— action of on protoplasm,

617.

— ascent and conduction of,

230, 253, 264, 276.

— of crystallisation, 209.
culture, 228, 271, 283,

290, 291.
284.

— distribution in wood, 241.

— drainage, 260.

— excretion of, 266, 273.
of in fungi, 280.

— expelled from motile or-
gans, 647-649.

— of imbibition, 209, 240.
lily, 126, 706.

— mobility of, 242.

— movements of, 233, 270.

836

INDEX.

Water-nut, 351.
702.

— of organisation, 79, 206.
plants, 126, 155, 203, 227,

232, 253, 255, 287, 297,
511, 512.
absorption by, 247.

— of respiration, 395, 400.

— river-, 288.

— sea-, 287, 288.

— in soil, 257, 258, 259.
stomata, 119, 279.

— in tendrils, 665.

— theory of ascent of, 238,

241.

— transpiration of, 227, 246.

— velocity of movements of,
236, 254.

— well-, 288.

— in wood, 239, 267, 269.
Wave-lengths of light, 306.
Wax, 117, 172, 246, 329.
Weeping of plants, 266, 279,

— of root-stock, 270, 277.
Weight, dry- of seedlings,

&c., 564, 567, 573.

— lossot by respiration, 300,

313, 327, 400, 403.-
of in germination, 400.

— of wood, 240.

— of yeast-cell, 384, 386.
Welavitschia mirabilis, 178.
178.

Wheat, 262, 263, 334, 337,
339, 397, 687.

— 20, 260, 261, 483, 688.

— laying of, 289, 51 4.

— Smut of, 381.
White Bryony, 658.
659, 663.

— Mustard, 261, 711.

— — 693.

Whorl, 470, 483, 486, 495,

500.
Wild vine, 658, 663.
Willow, 218, 505, 578, 784.
Winter-buds, 42, 43, 351,

366, 464, 506, 507.

Winter-leaves, 321.

— -rest, 197.

Witches'-brooms, 203, 537.
Wood, 131, 161, 218, 229,

266, 312, 314, 320, 335,
359, 444, 482, 511, 574,
575, 579* 58f.

— air in, 238, 267, 268.

— aqueous vapour in, 268.

— area of cell-walls of, 241.

— autumnal, 230, 268, 578.
- — bleeding of, 266.

cells, 238, 267, 274, 288,

329, 439-

suction of, 269,

volume of, 239.

— composition of, 290.

• — conductibility of, 243,

— desiccation of, 242.

- — -destroyingfungi, 344,381,
387.

— destruction of by fungi,
388.

fibres, 160, 578.

— filtration through, 237.

— 1 unctions of, 163, 231.

— heart-, 230, 268.

— imbibition of, 241, 268.

— molecular structure of,
242.

— porosity of, 238.

- — properties of, 234, 243,
264.

skeletons, 159.

■ — specific gravity of, 240.

— spring, 230, 268, 578.

— structure of, 237, 439.

— tension of air in, 267, 268,

269, 270.

— tensions and pressures in,
574-580.

— tissues of, 158.

— -vessels, 160.

— water in, 239, 241, 267,
269.

■ sorrel, 329,620, 625, 638,

644, 649, 656.
JVoodivardia radicans, 478.

Woolly hairs, 124,
Work done by growing-
plants, 583.

X.

Xylem, 131, 157, 230.

— general characters, 133.

parenchyma, 160.

rays, 159.

— of roots, 168.
Xylogen, 88, 389.

Yeast, I, 76, 324, 343, 347,
369, 383, 384, 385, 387.

— -cell, weight and volume
of, 384, 386.

— cultivation of, 384.

fungi, 349, 385.

Yellow autumn leaves, 319.

— bacteria, 386.

— fruits, 320.

— light, 303, 311, 638, 696,
697.

— wood, 163.
Yew, 237, 751, 756.
Yucca, 169, 528, 571.

— gloriosa, 529.

Z.

Zea Mays, 118, 130, 236, 283.
38, 42, 79, 91, 111,

132, 141, 145, 232, 338,

421, 469.

Zinc in soil and plants, 288.
Zones of growth, 541, 544,

685.
Zoosperm, 724, 730, 768-

771, 780, 788.
Zoo-spores, 82, 604.

— movements of, 604-608.
Zygomycetes, 726.
Zygospore, 351, 725, 726, 767,

800.
Zygote, 725-729, 767, 800.

(ALFRED HftFNER)
NEW YORK