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ALBERT R. MANN
LIBRARY

AT

CORNELL UNIVERSITY

The natural history of plants, their for

Cornell University

The original of this book is in
the Cornell University Library.

There are no known copyright restrictions in
the United States on the use of the text.

http://www. archive.org/details/cu31924011933219

PLATE I.

SWARM-SPORES AND ZYGOSPORES. FORMS OF
CHLOROPHYLL- BODIES.

Printed from the originals by the BIBLIOGRAPHISCHE | ~

PLATE J.

a—d
e—h

SWARMSPORES AND ZYGOSPORES.

FORMS OF CHLOROPHYLL-BODIES.

Development of swarmspores in the tubular cells of Vaucheria clavata.

Swarmspores and resting-cells of “red-snow” (Sphaerella nivalis), mixed
with pollen-grains of Pines.

Forms of Chlorophyll in cells of Desmidiex (i. Closterium Leibleinii ;
k. Peniwm interrupium).

Formation of zygospores and spiral arrangement of Chlorophyll-bodies
in cells of Spirogyra arcta.

Star-shaped Chlorophyll-bodies in cells of Zygnaema pectinatum.
Glococapsa sanguinea.
Protonema of Schistostega osmundacea.

Transverse section of the foliage-leaf of Cress.

r Transverse section of the leaf of the Passion-fiower.

Relative positions of laticiferous tubes and palisade-cells in the leaf
of a Spurge (Euphorbia Myrsinites).
All the figures greatly magnified.

THE

NATURAL HISTORY OF PLANTS

THEIR FORMS, GROWTH,
REPRODUCTION, AND DISTRIBUTION

FROM THE GERMAN OF

ANTON KERNER von MARILAUN

PROFESSOR OF BOTANY IN THE UNIVERSITY OF VIENNA

BY

F. W. OLIVER, M.A., D8.

QUAIN PROFESSOR OF BOTANY IN UNIVERSITY COLLEGE, LONDON

WITH THE ASSISTANCE OF

MARIAN BUSK, BSc. anp MARY F. EWART, BSc.

WITH ABOUT 1000 ORIGINAL WOODCUT ILLUSTRATIONS AND SIXTEEN PLATES IN COLOURS

HALF-VOLUME IL

NEW YORK

HENRY HOLT AND COMPANY
1895

®

PLATE I. SWARM-SPORES AND ZYGOSPORES.

Bm w bh et

—

Om co poe

be

NO oT wR wh

CONTENTS OF HALF-VOLUME L

LIST OF ILLUSTRATIONS.

ForMS oF CHLOROPHYLL-BODIES,

PAGE

Frontis. 374
» II. Insecrrvorous PLAnts: SUNDEW AND BUTTERWORT, to face 142

» I. Tropica, EprpHytes IN CEYLON,
Illustrations in the Teat—Fig. 1 to Fig. 99.

INTRODUCTION.
The Study of Plants in Ancient and Modern Times,

THE LIVING PRINCIPLE IN PLANTS.

. Protoplasts considered as the Seat of Life,

. Movements of Protoplasts,

. Secretions and Constructive Activity of Protoplasts,

. Communication of Protoplasts with one another and with the outer world,

ABSORPTION OF NUTRIMENT.

. Introduction, -

Absorption of Inorganic Substances,

. Absorption of Organic Matter from decaying Plants and Animals,
. Absorption of Nutriment by Parasitic Plants,

. Absorption, of Water,

. Symbiosis,

Changes in the Soil incident to the Nutrition of Plants,

CoNDUCTION OF Foon.

. Mechanics of the Movement of the raw Food-sap,
. Regulation of Transpiration,

. Prevention of Excessive Transpiration, ;
Transpiration during various Seasons of the Year. Transpiration of Lianes,

Conduction of Food-gases to the Places of Consumption,

FORMATION OF ORGANIC MATTER FROM THE ABSORBED INORGANIC Foon.

. Chlorophyll and Chlorophyll-granules, ~

The Green Leaves,

222

371
396

THE
NATURAL HISTORY OF PLANTS.

INTRODUCTION.

THE STUDY OF PLANTS IN ANCIENT AND IN MODERN TIMES.

Plants considered from the point of view of utility.—Description and classification of plants.—
Doctrine of metamorphosis and speculations of nature-philosophy.—Scientific method based on
the history of development.—Objects of botanical research at the present day.

PLANTS CONSIDERED FROM THE POINT OF VIEW OF UTILITY.

SomME years ago I rambled over the mountain district of North Italy in the
lovely month of May. In a small sequestered valley, the slopes of which were
densely clad with mighty oaks and tall shrubs, I found the flora developed in all
its beauty. There, in full bloom, was the laburnum and manna-ash, besides
broom and sweet-brier, and countless smaller shrubs and grasses. From every
bush came the song of the nightingale; and the whole glorious perfection of a
southern spring morning filled me with delight. Speaking, as we rested, to my
guide, an Italian peasant, I expressed the pleasure I experienced in this wealth
of laburnum blossoms and chorus of nightingales. Imagine the rude shock
to my feelings on his replying briefly that the reason why the laburnum was so
luxuriant was that its foliage was poisonous, and goats did not eat it; and that
though no doubt there were plenty of nightingales, there were scarcely any hares
left. For him, and I daresay for thousands of others, this valley clothed with
flowers was nothing more than a pasture-ground, and nightingales were merely
things to be shot.

This little occurrence, however, seems to me characteristic of the way in which
the great majority of people look upon the world of plants and animals. To their
minds animals are game, trees are timber and fire-wood, herbs are vegetables (in
the limited sense), or perhaps medicine or provender for domestic animals, whilst
flowers are pretty for decoration. Turn in what direction I would, in every
country where I have travelled for botanical purposes, the questions asked by the
inhabitants were always the same. Everywhere I had to explain whether the
plants I sought and gathered were poisonous or not; whether they were efficacious

as cures for this or that illness; and by what signs the medicinal or otherwise
Vou. I. ;

2 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

useful plants were to be recognized and distinguished from the rest. And the
attitude of the great mass of country folk in times past was the same as at the
present day. All along anxiety for a livelihood, the need of the individual to
satisfy his own hunger, the interests of the family, the provision of food for
domestic animals, have been the factors that have first led men to classify plants
into the nutritious and the poisonous, into those that are pleasant to the taste and
those that are unpleasant, and have induced them to make attempts at cultivation,
and to observe the various phenomena of plant-life.

No less powerful as an incentive to the study of herbs, roots, and seeds, and to
the minute comparison of similar forms and the determination of their differences,
was the hope and belief that the higher powers had endowed particular plants with
healing properties. In ancient Greece there was a special guild, the “ Rhizotomoi,”
whose members collected and prepared such roots and herbs as were considered
to be curative, and either sold them themselves or caused them to be sold by
apothecaries. Through the labours of these Rhizotomoi, added to those of Greek,
Roman, and Arabic physicians, and of gardeners, vine-growers, and farmers, a mass
of information concerning the plant-world was acquired, which for a long period
stood as botanical science. As late as the sixteenth century plants were looked
upon from a purely utilitarian point of view, not only by the masses but also
by very many professed scholars; and in most of the books of that time we find
the medicinal properties, and the general utility of the plants selected for descrip-
tion and discrimination, occupying a conspicuous position and treated in an
exhaustive manner. Just as men lived in the firm belief that human destinies
depended upon the stars, so they clung to the notion that everything upon the
earth was created for the sake of mankind; and, in particular, that in every plant
there were forces lying dormant which, if liberated, would conduce either to the
welfare or to the injury of man. Points which might serve as bases for the
discovery of these secrets of nature were eagerly sought for. People imagined they
discerned magic in many plants, and even believed that they were able to trace
in the resemblance of certain leaves, flowers, and fruits to parts of the human body,
an indication, emanating from supernatural powers, of the manner in which the
organ in question was intended to affect the human constitution. The similarity
in shape between a particular foliage-leaf and the liver did duty for a sign that
the leaf was capable of successful application in cases of hepatic disease, and the
fact of a blossom being heart-shaped must mean that it would cure cardiac com-
plaints. Thus arose the so-called doctrine of Signatures, which, brought to its
highest development by the Swiss alchemist Bombastus Paracelsus (1493-1541),
played a great part in the sixteenth and seventeenth centuries, and still survives
at the present day in the mania for nostrums. The inclination of the masses is
now, as it was centuries ago, in favour of supernatural and mysterious rather
than simple and natural interpretations; and a Bombastus Paracelsus would still
find no lack of credulous followers. In truth, the great bulk of mankind regard
Botany as subservient to medicine and agriculture, they look at it from the purely

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 3

utilitarian point of view in a manner not essentially different from that of two
hundred—or even two thousand—years ago, and it may well be a long time
before they rise above this idea.

In addition to the botanical knowledge thus initiated by the necessities of life,
a second avenue leading to the same goal was early established by man’s sense of
beauty. The first effect of this was limited to the employment of wild flowers
and foliage for purposes of ornament and decoration. Later on, it led to the
cultivation of the more showy plants in gardens, and ultimately to the arts of
gardening and horticulture, which at different periods and in different countries
have passed through such various phases, corresponding to the standards of the
beautiful which have prevailed.

THE DESCRIPTION AND CLASSIFICATION OF PLANTS.

A third path leading to botanical knowledge springs from the impulse which
actuates those who are endowed with a keen perception of form to investigate
structural differences down to their most minute characteristics. Workers in this
field arrange and classify all distinct forms according to their external resemblances,
give them names appropriate to their position and importance, catalogue them, and
keep up the register when once it has been started. Many people possess, in addi-
tion, the remarkable taste for collecting, which causes them to find pleasure in
merely accumulating and possessing enormous numbers of specimens of the particu-
lar objects on which their fancy is fixed.

This tendency of the human mind has played a very important part in the
history of botany. The first traces of it can be ascribed with certainty to a period
long before the commencement of our era; for such descriptions and other notes as
are contained in the Natural History of Plants, written by Theophrastus about the
year 300 B.c., are founded, for the most part, on the observations and experiments
of “Rhizotomoi,” physicians and agriculturists, and it is obvious from the text of the
book that in some cases those authorities did seek out plants, and learn to distinguish
them for their own sakes, and not solely for their economic or medicinal value.

At the time of the Roman Empire and in the Middle Ages, it is true, no one
troubled himself about plants other than those known to be in some way useful.
But there was a revival of the practice of hunting for plants for the purpose of
describing and enumerating all distinguishable forms, at that great epoch when the
nations of the West began to study the treasures of Greek thought, endeavouring
to adopt the point of view of antiquity, and to harmonize their own circumstances
with it. It was at this same period that art too shook itself free from the tradi-
tions of the Middle Ages, and became actuated by a new ideal based on the study
of the antique; but science, particularly natural science, has as good a claim as
art to regard that memorable time as its period of renaissance. Although the
ancient Greek writings on natural history, to which people turned with such
youthful enthusiasm in the fifteenth century, could not satisfy their thirst for

4 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

knowledge, yet there is no doubt that, as in art, the effect was to stimulate and
reform; and that this study led up to the source, so long forgotten, whence the
ancients had themselves drawn their knowledge, that is, to the direct investigation
of nature, which has invariably given to every branch of human knowledge new
and pregnant life.

As regards botanical knowledge in particular, the study of old Greek writings
on the part of western nations in both Northern and Southern Europe had the
immediate effect of instituting an eager search for all the different kinds of
indigenous plants; and, besides arousing a passion for investigation, it evoked un-
tiring industry in this pursuit, the results of which preserved in a number of bulky
herbals still excite our wonder and respect. If these folios, dating for the most part
from the first half of the sixteenth century, are perused in the hope of their reveal-
ing some guiding principle as a basis for the arrangement of the subject, the reader
will no doubt be obliged to lay them aside unsatisfied. The plants were described
and discussed just as the authors happened to come across them; and it is only
here and there that we find a feeble attempt to range together and make groups of
nearly-allied species. Only cursory attention was paid to the facts of geographical
distribution. Plants native to the soil, herbs which flowered in gardens and had
been reared from seed purchased from itinerant vendors of antidotes, and plants
whose fruits were brought to Europe as curiosities from the New World recently
discovered—all these were jumbled together in a confused medley. The whole
endeavour of the time was directed to the enumeration and description of all such
things as possess the power of producing green foliage and maturing fruit under
the sun’s quickening rays.

Owing to the fact that researches were then limited to the native soil of the
student, most of the botanical authors of that day had but dark inklings of the
extent to which the floras of various latitudes and areas differ. They assumed that
plants of the Mediterranean shores, which had been described centuries before by
Theophrastus or Dioscorides or Pliny, were necessarily the same as those of their
own more inclement countries. The German “Fathers of Botany” (Brunfels, born
about 1495, died 1534; Bock, 1498-1554; Fuchs, 1501-1566, are the best known)
applied the old Greek and Latin names without scruple to the species growing in
their own localities. They were so firmly convinced of the identity of the German,
Greek, and Italian floras that even the numerous inconsistencies occurring in the
descriptions did not disconcert them, or prevent them from discussing at great
length whether a particular name was intended by Theophrastus and Dioscorides to
indicate this or that plant. It was by slow degrees that botanists first began to
abandon these fruitless debates concerning the Greek and Latin names of plants,
with which it had been the custom to fill so many pages of the herbals. Step by
step they became conscious that although the yellow pages of the ancient books
deserved all gratitude for the stimulating influence they had exercised, yet the
green book of nature should be set above them. This led to their devoting
themselves entirely to direct researches in the subject of their native floras. The

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 5

herbal of Hieronymus Bock, which appeared in 1546, and in which “the herbs
growing in German countries are described from long and sure experience,” contains
a passage treating of the controversy of the day as to whether the Latin name
Erica was applicable to the German Heath or not; and in the midst of the discus-
sion the author expresses the opinion that “the plants we know best were the least
known to the Latins;” and at last he exclaims: “Be our heath the same as Erica
or not, it is in any case a pretty and sturdy little shrub, beset with numerous brown
rounded branches, which are clothed all over with small green leaves; and its
appearance is like that of the sweet-smelling Lavender Cotton.” And again in a
number of other places, after making lengthy philological statements relating to the
old names, he ends by losing patience and declaring that the proper thing would be
to lay aside all disputes concerning this nomenclature.

At length a Belgian, Charles de l’Ecluse (1526-1609), whose name was latinized
into Clusius, emancipated himself entirely from the hair-splitting verbal contro-
versies of the day. He was also the first to abandon the utilitarian standpoint;
and in his extensive work, which appeared at the end of the sixteenth century,
he was guided solely by the desire to become acquainted with every flowering thing.
He therefore endeavoured to distinguish, describe, and where possible to draw the
various forms of plants, to cultivate them, and to preserve them in a dried condition.
It was just at that time that collections of dried plants began to be made. Such a
collection was at first called a “hortus siccus,” and later on a “herbarium.” All
museums of natural history were forthwith furnished with them. Moreover,
Clusius, actuated by the wish to see with his own eyes what the vegetation on the
other side of the mountains looked like, was the first man to travel for the purpose
of botanizing. In order to extend his knowledge of plants he roamed over Europe
from the sierras of Spain to the borders of Hungary, and from the sea-coast to
the highlands of the Tyrol. Journeys of this kind in pursuit of botanical know-
ledge were by degrees extended to wider and wider limits, and thus an abundance of
material was brought together from all latitudes and from every quarter of the globe.

An immense number of isolated observations were accumulated in this way, till,
at length, in the first decades of the eighteenth century, the desirability of sifting
and arranging this chaotic mass became urgent. When, therefore, the Swedish
naturalist Linnzus (1707-1778), by the exercise of unparalleled industry, mastered
in a fabulously short space of time the detailed results of centuries of labour, and
afforded a general survey of all this scattered material, he obtained universal
recognition. Linneus introduced short names for the various species in place of
the cumbrous older designations, and showed how to distinguish the species by
means of concise descriptions. For this purpose he marked out the different parts
of a plant as root, stem, leaf, bract, calyx, corolla, stamens, pistil, fruit, and seeds.
Again, he distinguished particular forms of those organs, as, for instance, scapes,
haulms, and peduncles as forms of stems, and in addition also the parts of each
organ, such as filaments, anthers, and pollen in the stamens, and ovary, style, and
stigma in the pistil; and to each one of these objects he assigned a technical name

6 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

(terminus). With the help of the botanical terminology thus formulated it became
possible not only to abridge the specific descriptions, but also to recognize species
from such descriptions, and to determine what name had been given them by
botanists, and to what group they belonged.

Linnzus selected as a basis of classification in the “System” established by him
the characteristics of the various parts of the flower. In this system the number,
relative length, cohesion, and disposition of the stamens formed the ground of
division into “Classes.” Within each Class, “Orders” were then differentiated
according to the nature of the pistil, especially the number of styles; and each
Order was again subdivided into more narrowly defined groups, which received
the name of “Genera.” To the 23 classes of Flowering Plants (Phanerogamia)
Linnwas added as a 24th Class Flowerless Plants (Cryptogamia), which were
divided into several groups (Ferns, Mosses, Alge, and Fungi) in respect of their
general appearance and mode of occurrence.

This system took immediate possession of the civilized world. Englishmen,
Germans, and Italians now worked in unison as faithful disciples of Linneus.
Even laymen studied the Linnzan botany with enthusiasm; and it was recommended,
especially to ladies, as a harmless pastime, not overtaxing to the mind. In France
Rousseau delivered lectures on botany to a circle of educated ladies; whilst even
Goethe experienced a strong attraction to the “loveliest of the sciences,” as botany
was called in that day. Linneus had introduced for the first time the name
“flora” to signify a catalogue of the plants of a more or less circumscribed district.
He had himself written a flora of Lapland and Sweden, and by doing so had
stimulated others to undertake the compilation of similar catalogues; so that by
the end of the 18th century floras of England, Piedmont, Carniola, Austria, &c.,
had been produced. By this means a certain perfection was attained in that field
of botany which has only in view the examination of the fully-developed external
forms of plants, together with the distinguishing, describing, naming, and grouping
them, and the enumeration of species indigenous to particular regions. Later on,
unfortunately, botanists lost themselves in a maze of dull systematizing. They
either contented themselves with collecting, preparing, and arranging herbaria, or
else devoted their energies to endless debates over such questions, for instance, as
whether a plant, that some author had distinguished from others and described,
deserved to rank as a species, or should’ be reckoned as a variety dependent on its
habitat or on local conditions of temperature, light, and moisture. They took delight
in now including a group of forms as varieties of a single species, now dividing
some species as described by a particular author into several other species. For
this purpose they did not rely upon the only sure method, the determination by
cultural experiment of the fact of the constancy or variability of the form in
question; nor did they, in general, adhere to any consistent principle to guide them
in this amusement.

Aberrations of this kind constituted, however, no serious barrier to progress,
On the contrary, the passion for collecting continued to extend its range. The

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 7
vegetation of the remotest corners of the earth was ransacked by travelling
botanists without any material advantage being gained, though they not infre-
quently ran considerable risk to their health, and sometimes sacrificed their lives.
As one generation succeeded another thousands of students of the “scientia ama-
bilis” made their appearance in every country. Swept along by the prevailing
current of thought they devoted themselves to the examination of native and foreign
floras, or to a detailed study of the most insignificant sections of the vegetable
kingdom. Those who are not under the spell of this passion cannot conceive the
joy experienced by the discoverer of a hitherto unknown moss. To such it is
inexplicable how anyone can devote the labour of half a lifetime to a classification
of Algz or Lichens, or to a monograph of the bramble-tribe or orchids. The pro-
gress achieved eventually in this department of botany is best appreciated when
the wide difference in the numbers of species described in botanical works of
different periods is considered. Theophrastus in his Natural History of Plants
(about 300 B.c.) mentions about 500 species, and Pliny (78 a.D.) rather more than
1000; whereas, by the time of Linnzus, about 10,000 were known; and now the
number must be all but 200,000. It should be remarked, however, that half the
plants described since Linnzous lived fall into the category of Cryptogams, or non-
flowering plants, the examination of which was first rendered possible by the wide-
spread use of the microscope in recent times.

The microscope led also to discoveries concerning the internal architecture
of plants. A faint attempt in this direction, made 200 years ago, had died away
without leaving any trace behind; but at the commencement of this century the
“inward construction of plants” was studied all the more eagerly by means of the
microscope. In buildings belonging to different styles of architecture it is not
only the forms of the wings, stories, rooms, and gables that differ, but also and
in no less degree those of the columns, pilasters, and decorations. The same is the
case with plants. They possess chambers at different levels, vaults, and passages.
They have pipes running through them, and beams and buttresses, some massive
and some slender, to support them. The pieces of which they are built vary in
size, and their walls are sculptured in all kinds of ways. It was the business of the
vegetable anatomist to dissect plants, to look into all these structures under the
microscope, to describe the various component parts as well as the ground-plan and
elevation of the plant-edifice as a whole; and to name the different forms of struc-
ture after the manner of Linnzus when he invented terms for the different forms
of stems and leaves, and for the several parts of the flower and fruit.

DOCTRINE OF METAMORPHOSIS AND SPECULATIONS OF
NATURE-PHILOSOPHY.

Side by side with this immense volume of research, which was directed to the
separation, description, and synoptical arrangement of mature forms only, there
arose about the year 1600 another school which considered vegetable forms from

8 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

the point of view of their life-history, and endeavoured to trace them back to their
origin. Tracing the development, from one stage to another, of all the different
species, of the multitudinous forms of leaves and flowers, and of the various kinds
of cells and tissues, the student of this school has to detect identity in multiplicity,
to show that the connection between forms which have arisen from one another is
in accordance with fixed laws, and to express those laws in definite formule.

The attention of botanists was in the first place directed to the wonderful series
of changes in the form of the leaf which occur in all phanerogamic (.e. flowering)
plants as the delicate seedling gradually turns into a flowering shoot. At the circum-
ference of the stem which constitutes the axis of the plant, foliar structures are
produced at successive intervals. All these structures are essentially the same; but
they exhibit a continuous modification of their shape, arrangement, size, and colour,
according to their relative altitudes upon the stem. To discover the causes of this
structural variation was an attractive problem, and very diverse theories were
suggested for its solution. The earliest explanation, which was given by the Italian
botanist Cesalpino in 1583, is founded rather on superficial analogies and remote
resemblances existing between tissues than on careful observation. According to
this theory the stem is composed of a central medulla highly endowed with vitality,
and surrounded by concentric layers of tissue, those namely of the wood, the bast,
and the cortex. Each of the foliar structures put forth from the axis is supposed to
originate in one of the above-named tissues, the idea being that the green foliage-
leaf and calyx grew out from the cortical layer, the corolla from the bast, the
stamens from the wood, and the carpels from the medulla. It was believed, also,
that the outer envelope of a fruit arose from the rind of the fruit-stalk, the seed-
coats from the wood, and the central part of the seed from the medulla.

Early in the eighteenth century there came to be connected with this theory the
doctrine of so-called “ prolepsis,” which was founded on more accurate comparative
observations. It was thought that the medulla of the stem breaks through the rind
at particular spots to form at each a bud, which subsequently grows out into a side
branch. Owing to this lateral pressure of the medulla the ascending nutrient sap
becomes arrested beneath the rudimentary bud, and, in consequence, the cortex
develops under the bud into a foliage-leaf. In the bud the different parts of the
future annual shoot are already shadowed forth in stages one above the other; and
each is produced always by the one beneath it. As soon as vegetative activity is
resumed after the expiration of the winter rest, the bud sprouts. If only that part
of it develops which constitutes the first year’s rudiment, a shoot furnished with
foliage-leaves is produced. But the embryonic structures belonging to succeeding
years, which are concealed in the bud, may also be stimulated to development; and
when this happens, these premature products do not appear as foliage-leaves, but
in more or less altered forms as bracts, sepals, petals, stamens, and carpels. If no
such anticipatory activity has been excited, the rudiment which in the previous
case would have developed into a bract does not appear till the following year, and
then as a foliage-leaf; whilst that which would have formed a calyx in the first

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 9

year lies dormant till the third year, when it too emerges simply as a leaf. This
transformation of the leaves, or metamorphosis as Linneus called it, is, therefore, the
result of anticipation; and it was assumed by the Linnean school that the cause of
this metamorphosis or hastened development was a local decrease in the quantity
of nutriment. The idea was, that in consequence of the limited supply of sap the
incipient leaves were not able to attain to the size of foliage-leaves, but remained

/ Se

if) | nel?
e . - : IVers
Fig. 1.—Seedlings with Cotyledons and Foliage-leaves. rn
1Cytisus Laburnum. 2 Koelreuteria paniculata. % Acer platanoides.

rudimentary, as is the case with many bracts; and further, that the axis was
no longer capable of elongating, so that the leaves proceeding from it remained
close together, became coherent, and thus formed the calyx. The supporters of this
explanation relied particularly on the experience of gardeners, that a plant in good
soil with a liberal supply of nutriment is apt to produce leafy shoots rather than
flowers; whereas, if the same plant is transferred to a poorer soil, where its food is
limited, it develops flowers in abundance.

But yet a third attempt was made to explain this process of transformation, by
the theory that parts which are identical so far as their origin is concerned, subse-
quently receive the stamp of distinct foliar organs. The diversity in the develop-
ment of parts, originally alike, was supposed to depend on a filtration of the nutrient

10 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

sap, the idea being that identical primordial leaves issuing from the axis of a ak
cular plant were fashioned with more and more delicacy as the sap became clarified
and refined in its passage through the vessels. This explanation of metamorphosis
was first given by Goethe (1790) in a treatise which was much discussed, and which
exercised a most important influence in initiating researches of a similar nature.
Goethe’s interpretation of metamorphosis may be briefly reproduced as follows. A
plant is built up gradually from a fundamental organ—the leaf—which issues from
the node of astem. First of all, the organs which are called seed-leaves or cotyledons
(tig. 1) develop on the young plant as it germinates from the seed; they proceed
from the lowest node of the stem, and are frequently subterranean. They are of
comparatively small size, are simple and unsegmented, have no trace of indentation,
and appear for the most part as thick, whitish lobes, which are, according to Goethe’s
expression, closely and uniformly packed with a raw material, and are only coarsely
organized. Goethe explains these leaves as being of the lowest grade in the evolu-
tionary scale. After them and above them the foliage leaves develop at the suc-
ceeding nodes of the stem; they are more expanded both in length and breadth;
their margins are often notched, and their surfaces divided into lobes, or even com-
posed of secondary leaflets; and they are coloured green. “They have attained to
a higher degree of development and refinement, for which they are indebted to the
light and air.” Still further up, there next appears the third stage in foliar evolu-
tion. The structure called by Linnzus the calyx is again to be traced back to the
leaf. It is a collection of individual organs of the same fundamental type, but
modified in a characteristic manner. The close-set leaves, which proceed from
nodes of the stem at what is, in a certain sense, the third story of the plant-edifice
as a whole, and which constitute the calyx, are contracted, and have but little variety
as compared with the outspread foliage-leaves,

On the fourth rung of the ladder by which the leaf ascends in its effort to perfect
itself, appears the structure named in the Linnean terminology the corolla. It
consists, like the calyx, only of several leaves grouped round a centre. If a con-
traction has taken place in the case of the calyx, we have now once more an expan-
sion. The leaves which compose the corolla are usually larger than those of the
calyx. They are, besides, more delicate and tender, and are brightly coloured; and
Goethe, whose mode of expression is here preserved as far as possible, supposes them
to be filled also with purer and more subtle juices. He conceives that these juices
are in some manner filtered in the lower leaves and in the vessels of the lower
region of the stem, and so reach the upper stories in a more perfect condition. A
more refined sap must then, he says, give rise to a softer and more delicate tissue
(fig. 2). Above the corolla and at the fifth stage of development there follows the
group of stamens, structures which, though not answering to the ordinary conception
of leaves, are yet to be regarded again simply as such. In the circle of the corolla
the leaves were expanded, and conspicuous owing to their colour; on the other
hand, in the stamens they are contracted to an extreme degree, being almost fila-
mentous in part. These leaves appear to have reached a high degree of perfection,

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 11

and in the parts of the stamens termed anthers “ pollen-grains” are developed “in
which an extremely pure sap is stored.” Adjoining these pollen-producing leaves,

Fig. 2.—_Metamorphoses of Leaves as exhibited by the Poppy.
1Germinating plant with cotyledons. 2 and 8 The same plant further developed and with foliage-leaves; in ® the
cotyledons and lowest foliage-leaves are already withered. ‘The same plant with a flower-bud showing the closed
sepals. 5 The bud open and with petals, stamens, and carpels (pistil) developed.

where contraction has reached its extreme limit, is the sixth and last story, which
is composed of leaves, once more less closely-set, and exhibiting a final expansion
on the part of the plant. These are the carpels, which surround the highest part

12 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

of the stem and inclose the seeds, the latter being developed from the tip of the
stem. Thus the plant accomplishes its life-history in six stages. It is built up of
leaves, the “intrinsic identity” of which cannot be doubted, although they assume
extremely various shapes corresponding to the six strides towards perfection. In
this process of transformation or metamorphosis of the leaf there are three alter-
nate contractions and expansions, whilst each stage

is more perfect than the one next below it.

Whilst seeking to explain metamorphosis in
this manner, and endeavouring, with greater per-
spicacity than all his predecessors and contem-
poraries, “to reduce to one simple universal prin-
ciple all the multifarious phenomena of the glorious
garden of the world,” Goethe conceived the notion
of a typical plant, an ideal, the realization of
which is achieved in nature by means of a mani-
fold variation of individual parts. This abstract
notion of a plant’s development with its six stages
corresponding to “three wave-crests” or expan-
sions (Leaf, Petal, Carpel) and “three wave-
troughs” or contractions (Cotyledon, Sepal, Sta-
men) is expressed graphically in figure 3. It still
holds its ground at the present day under the
name of Goethe’s “ Urpflanze,” and the credit of its
invention is entirely his. But it is not quite right
to claim for Goethe, in addition, the title of

<p founder of the doctrine of vegetable metamor-
piers phosis; for in reality he only offered another inter-
; pretation and mode of representation of a pheno-
menon already included by Linnzus under the
term metamorphosis. Linnzus had instituted a
comparison between the metamorphosis of plants and that of insects; in particular,
he likened the calyx to the ruptured integument of a chrysalis and the internal parts
of a flower to the perfect insect (Imago). He also made many different attempts to
establish analogies between the development of plants and that of animals; and in
so doing he opened up a wide field for the speculations of the “nature philosophers”
in the earlier part of the nineteenth century.

An extensive study of this subject now commenced; and writers on nature-
philosophy worked indefatigably at the amplification and modification of this
theme, first broached by Linnzus.

“A plant is a magnetic needle attracted towards the light from the earth into
the air. It is a galvanic bubble, and, as such, is earth, water, and air. The plant-
bubble possesses two opposite extremities, a single terrestrial end and a dual aérial
end; and so plants must be looked upon as being organisms which manifest a

Fig. 3.—Goethe’s “ Urpflanze.”

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 13

continual struggle to become earth on the one hand and air on the other, unmixed
metal at one end, and dual air at the other. A plant is a radius, which becomes
single towards the centre, whilst it divides or unfolds towards the periphery; it is
not therefore an entire circle or sphere, but only a segment of one of those figures.
The individual animal, on the contrary, constitutes of itself a sphere, and is there-
fore equivalent to all plants put together. Animals are entire worlds, satellites or
moons, which circle independently round the earth; whereas plants are only equal
to a heavenly body in their totality. An animal is an infinitude of plants. A
blossom which, when severed from the stem, preserves by its own movement the
galvanic process or life, is an animal. An animal is a flower-bubble set free from
the earth and living alone in air and water by virtue of its own motion.”

‘Page after page of the writings on Natwre-philosophy of Oken (1810) and
other contemporary naturalists is filled with interminable statements of the same
kind. At the present day it seems scarcely credible that such propositions were
then received with admiration as profound and ingenious utterances, and that they
were even adopted as mottoes for botanical and geological treatises. For example,
it is worthy of record that as late as the year 1843 the Austrian botanist Unger
made use of the last of the flowers of rhetoric above quoted from Oken’s Nature-
philosophy as a motto for one of his first works on the history of development,
the title of which is Plants at the Moment of their becoming Animals.

The general divisions or systems of the vegetable kingdom which were evolved
by adherents of the school of Nature-philosophy were, as may be imagined, just as
absurd as the speculations on which they were based. In his Philosophical Systems
of Plants Oken develops in the first place the idea that the vegetable kingdom
is a single plant taken to pieces. Inasmuch as the ideal highest plant is composed
of five organs, there must likewise be five classes: root-plants, stem-plants, leaf-
plants, flower-plants, and fruit-plants. The world is fashioned out of the elements:
earth, water, air, and fire. Hereupon is founded a classification of root-plants into
earth-plants or lichens, water-plants or fungi, air-plants or mosses, and light-plants
or ferns. Proceeding from the assumption that all the groups are parallel and that
the principle of classification for each group is always given by the one preceding
it, we have next, to take one instance, the second class—that of stem-plants—
divided (in accordance with the subdivision of earth into earths, salts, bronzes, and
ores) into earth-plants or grasses, salt-plants or lilies, bronze-plants or spices, and

ore-plants or palms.

SCIENTIFIC METHOD BASED ON THE HISTORY OF DEVELOPMENT.

Though as we see the doctrine of metamorphosis, with its conception of a
typical plant, degenerated thus into the most barren of fancies, still from it originated
the line of research based on the history of development which has since borne
fruit in every department of botany. Observers arrived at the conviction that
every living plant undergoes a continuous transformation which follows a definite

14 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

course, and that accordingly every species is constructed on a plan fixed within
general limits and exhibiting variation in externals only. These, it is true, are often
more conspicuous at first sight than the direction and disposition of the parts which
are really fundamental, and secure the stability of the entire structure. But in
order to ascertain the plan of construction it was found necessary to go back to the
very first visible appearance of each organ; to determine how the original rudi-
ments of the embryo and the beginnings of roots, stems, leaves, and parts of the
flower are formed, and to see what rudiments succeed in opening out, branching and
dividing, and what remain behind to perish and be displaced by organs growing
vigorously in close proximity to them.

These researches into the course of development of the separate parts of flower-
ing plants, and to a still greater extent the observations of the development of
eryptogams or spore-plants (rendered possible by improvements in the construction
of microscopes), led naturally to a study of the history of the elementary structures
of which all plants are composed. Previously three kinds of elementary organs had
been supposed to exist, utricles, vessels, and fibres. The observations of Brown and
Mohl (1830-1840) resulted, however, in the identification of the cell as the common
starting-point of all these elementary organs. This led to the further discoveries
that protoplasm is the formative and living part of a cell, and that each cell is
differentiated into a protoplasmic cell-body and a cell-membrane. It followed
that the envelope of the protoplasmic body, the cell-membrane, which had hitherto
been considered the primary formation, was in reality a product of the protoplasm
enveloped by it, and this discovery resulted in a complete revolution in the con-
ception of cells generally. Further investigation led to the conclusion that the
various modes of growth and multiplication depend on definite laws. That even
in the mode of juxtaposition of daughter-cells arising in reproduction, a certain plan
of construction may be distinguished in each species which must stand ultimately
in some causal relation to the structural system of the whole plant. ‘The progress
achieved along these lines in the course of a few decades has been extraordinarily
great, no doubt due to the peculiar fascination which the study of the life-histories
and transformations of living organisms and the observation of mysterious processes
invisible to the naked eye have had for the mind of the inquirer.

In that group of plants which includes the forms classed together by the earlier
botanists under the name of Cryptogamia an altogether new world was revealed.
An undreamed-of variety was discovered to exist in the processes of propagation
and rejuvenescence of these forms of plants by means of single cells or spores.
Objects which, having regard to their external form, had been assigned to widely
different groups, were found to be connected with one another as stages in the
development of one and the same species; and one result of these discoveries was
the establishment in this division of the vegetable kingdom of an entirely new
system of classification based on life-histories. The systematic arrangement of
Flowering-plants or Phanerogams also underwent essential alteration. The Linnean
system, founded on the numerieal relations between the different parts of the flower,

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 15

had indeed already been displaced by another method of classification, that of the
French observers Jussieu (1789) and De Candolle (1813), who framed systems said
to be natural when contrasted’ with the artificial system of Linneus. At bottom,
however, these classifications only differed from the Linnean in the fact that they
multiplied and widened the grounds of division. The main division of Phanero-
gamia into those which put forth one cotyledon (or seed-leaf) on germinating
(Monocotyledones) and those whose seedlings bear two cotyledons (Dicotyledones)
is the only one that could serve as a starting-point for a system based on the history
of development; but when we come to the grouping of Dicotyledones into those
destitute of corolla (Apetale), those with the corolla composed of coherent petals
(Monopetale), and those with the corolla composed of distinct petals (Dialy-
petala), we have already to admit something forced, and a reliance on characteristics
merely external.

The system which is the outcome of the study of development starts with
the idea that similarity between adult forms is not always decisive evidence of
their belonging to the same group, and that the relationships of different plants
is much more surely indicated by the fact of their exhibiting the same laws of
growth and the same phenomena of reproduction. Plants exhibiting widely
different external forms in the mature state are nevertheless to be looked upon
as closely allied if they are constructed according to the same plan, and vice versd.
There can be no question that a system based on these principles means a material
advance. At the same time it cannot be overlooked that great difficulties are
involved in hitting upon the right selection from among the number of phenomena
observed in the course of a plant’s development, and in determining which of these
phenomena are to be referred to a mode of construction common to a number
.of plants, and therefore treated as fundamental properties, and which should be
esteemed merely as outcomes of the conditions of life affecting the existence of the
plant in question.

OBJECTS OF BOTANICAL RESEARCH AT THE PRESENT DAY.

DEscRIPTIVE Botany only concerns itself with the configuration of a plant.
COMPARATIVE MorRPHOLOGY endeavours to trace back to a single prototype the
extremely various forms exhibited by mature plants. The history of development
deals with the growth and differentiation of such forms. But all these paths of
research shirk the problem of the biological significance of the different forms.
The line of investigation starting from the conception of a plant’s life as a series
of physical and chemical processes, and which attempts to elucidate the configura-
tion of a plant in the light of its environment, could not be developed with the
slightest prospect of success until physics, chemistry, and other allied sciences had
reached a high degree of perfection, and till botanists had become convinced that the
phenomena of life are only to be fathomed by means of experiment.

The earliest attempts to define the biological significance of the several parts of

16 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

a plant do, it is true, take one back as far as Aristotle and his school ; but the ideas
of vegetable life entertained at that time are scarcely more than fantastic dreams,
and the recognition now accorded to them springs rather from a reverence for
antiquity than from any intrinsic merit which they possessed. The its experi-
mental investigations into the vital phenomena of plants were published ty
_Stephen Hales in 1718; but it was not till a hundred years later that this kind
of research really came into vogue. It brought with it the conception of a cell
as a miniature chemical laboratory, and looked for mechanical interpretations of
the phenomena of nutrition, sap-circulation, growth, movement—in short, all vital
processes—and for some connection between these processes and the external form.
Whereas, in the case of descriptive and speculative botany, and in the study of
development, the entire plant was first taken into consideration, next its several
parts, and lastly the cells and protoplasm; in the new department of inquiry, on
the contrary, the complete histories of the ultimate organs were studied first
of all, then the significance of the different forms of the several members, and lastly
the phenomena occasioned by the aggregate life of all the various kinds of animals
and plants.

Modern science, governed as it is by the desire to lay bare the causes of all
phenomena, is no longer satisfied with knowledge concerning the existence of cells,
the arrangement of the different forms of cell, the development of their contents,
and the changes undergone by cell-membranes. At the present day we inquire
what are the functions of the various bodies which are formed within the proto-
plasm? Why is the cell-membrane thickened at a particular spot in a particular
manner? What is the meaning of all the tubes and passages which exhibit such
great diversity of size and shape? What part is played by the peculiar mouths of
these channels, and why do they vary so greatly in shape and distribution in plants
which are subject to different external conditions? We are no longer content to
determine in what manner the rudimentary organ of a plant is produced, or how
it expands in one case and frequently divides, or else is arrested in its growth and
shrivels up; but we inquire the reason why one rudiment grows and develops
whilst another is obliterated. For us no fact is without significance. Our
curiosity extends to the shape, size, and direction of the roots; to the configuration,
venation, and insertion of the leaves; to the structure and colour of the flowers;
and to the form of the fruit and seeds; and we assume that even each thorn,
prickle, or hair has a definite function to fulfil. But efforts are also made to
explain the mutual relations of the different organs of a plant, and the relations
between different species of plants which grow together. Lastly, this department
of research (the rapid growth of which is due to Darwin) includes amongst its
objects a solution of the problem of the ultimate grounds of morphological variety,
the causes of which can only be sought for in a qualitative variation of protoplasm.
Specific relationship is explained by attributing it to similarity in the constitution
of the protoplasm of allied species, and the affinities exhibited by living and extinct
plants are used as means of unfolding the hereditary connection between the

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 17

thousands of different sorts of forms, and of tracing the history of plants and
vegetable life all over the earth.

The various lines of botanical research described in the foregoing pages, with
their particular problems and objects, have but slight connection one with another.
They run side by side along separate paths, and it is only occasionally that a
junction is apparent which establishes a communication between one path and
another. The subject-matter, however, is always the same. Whether we have
to do with the perfected form or with its growth, whether we try to interpret the
processes of life or to trace the genealogy of the vegetable kingdom, we always
start from the forms of plants; and the ultimate result is never anything more than
a description of the varying impressions which we receive at different times from
the objects observed, and which we endeavour to bring into mutual connection.
All the different departments of botany are accordingly more or less limited to
description; and even when we endeavour to resolve vital phenomena into
mechanical processes we can only describe, and not really explain, what happens.
The processes which we call life are movements. But the causes of those move-
ments, so-called forces, are purely subjective ideas, and do not involve the concep-
tion of any actual fact, so that our passion for causality is only ostensibly gratified
by the help of mechanics. Du Bois Reymond is not far wrong when he follows
out this train of thought to the conclusion (however paradoxical it may sound)
that there is no essential difference between describing the trajectory (or particular
kind of curve) in which a projectile moves on the one hand, and describing a beetle
or the leaf of a tree on the other.

But even though the ultimate sources of vital phenomena remain unrevealed,
the desire to represent all processes as effects, and to demonstrate the causes of
such effects—a desire which is at the very root of modern research—finds at least.
partial gratification in tracing a phenomenon back to its proximate cause. In the
mere act of linking ascertained facts together, and in the creation of ideas involv-
ing interdependence among the phenomena observed, there lies an irresistible charm
which is a continual stimulus to fresh investigations. Even though we be sure
that we shall never be able to fathom the truth completely, we shall still go on
seeking to approach it. The more imaginative an investigator the more keenly
is he goaded to discovery by this craving for an explanation of things and for
a solution of the mute riddle which is presented to us by the forms of plants.
It is impossible to overrate the value and efficiency of the transcendent gift of
imagination when applied to questions of Natural History. Thus when we inquire
whether certain characters noted in a plant are hereditary, constant, and inalienable,
or are only occasioned by local influences of climate or soil, and hence deduce
whether the plant in question is to be looked upon as a species or a variety; when
we conclude from the fact of a resemblance between the histories of the develop-
ment of various species that they are related, and place them together in groups
and series; when we unravel the genealogies of different plants by comparing

forms still living with others that are extinct; when we try to represent clearly
VoL. I. 2

18 THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES.

the molecular structure of the cell-membrane by arguing from the phenomena
manifested by that membrane; when we investigate the meaning of the peculiar
thickenings and sculpturings of the walls of cells, or when we discover the strange
forms of flowers and fruits to be mechanical contrivances adapted to the forms
of certain animals, and judge the extent to which these contrivances are advan-
tageous, or the reverse, to the plants—in all these and similar investigations
imagination plays a predominant part. Experiment itself is really a result of
the exercise of that faculty. Every experiment is a question addressed to nature.
But each interrogation must be preceded by a conjecture as to the probable state
of the case; and the object of the experiment is to decide which of the preliminary
hypotheses is the right one, or at least which of them approaches nearest to the
true solution. The fact that when the imagination has been allowed to soar unre-
strained, or without the steadying ballast of actual observations, it has frequently
led its followers into error, does not detract at all from its extreme value as an
aid to research, notwithstanding the fact that it is responsible for the wonderful
fantasies of nature-philosophy of which a few specimens have been given. Nor
should we esteem it the less because enlargements of the field of observation and
improvements in the instruments employed have again and again led to the sub-
stitution of new ideas for those which careful observers and experimentalists had
arrived at by collating the facts ascertained through their labours.

For the same reasons it is unfair to regard with contempt the ideas of plant-
life formed by our predecessors. It should never be forgotten how much smaller
was the number of observations upon which botanists had to rely in former times,
and how much less perfect were their instruments of research. Every one of
our theories has its history. In the first place a few puzzling facts are observed,
and gradually others come to be associated with them. A general survey of the
phenomena in question suggests the existence of a definite uniformity underlying
them; and attempts are made to grasp the nature of such uniformity and to define
it in words. Whilst the question thus raised is in suspense, botanists strive with
more or less success to answer it, until a master mind appears. He collates the
observed facts, gathers from them the law of their harmony, generalizes it, and
announces the solution of the enigma. But observations continue to multiply;
scientific instruments become more delicate, and some of the newly-observed facts
will not adapt themselves to the scheme of the earlier generalization. At first
they are held to be exceptions to the rule. By degrees, however, these exceptions
accumulate; the law has lost its universality and must undergo expansion, or else
it has become quite obsolete and must be replaced by another. So it has been
in all past times, and so will it be in the future. Only a narrow mind is capable
of claiming infallibility and permanence for the ideas which the present age lays
down as laws of nature.

‘These remarks on the limitations of our knowledge of nature, the importance
of imagination as an aid in research, and the variability of our theories are made
with a view to moderate, on the one hand, the exuberant hopes raised by the belief

THE STUDY OF PLANTS IN ANCIENT AND MODERN TIMES. 19

that the great questions connected with the phenomenon of life will be solved,
and to correct, on the other, the habit of not appreciating impartially the various
methods which have been and are still employed by different botanists. In our
own time, adhering as we do to the principle of the division of labour, it has become
almost universal for each investigator to advance only along a single, very narrow
path. But owing to the fact that one-sidedness too often leads to self-conceit, the
lines of study followed by others are not infrequently despised, just as overweening
confidence in the infallibility of the discoveries of the present day leads to deprecia-
tion of the labours of former times.

For the building-up of the science of the Biology of Plants everything relating
to the subject has its value, and is capable of being turned to account. Whether
the materials are rough or elaborated, massive, fragmentary, or merely connective,
howsoever and whensoever they have been acquired, they all are useful. The study
of dried plants made by a student in a provincial museum, the discoveries of an
amateur regarding the flora of a sequestered valley, the contributions of horticul-
turalists on subjects of experiment, the facts gleaned by farmers and foresters in
fields and woods, the disclosures which have been wrested from living plants in
university laboratories, and the observations conducted in the greatest and best
of all laboratories—that of Nature herself—all these results should be turned to
account. Let us take for the motto of the following pages the text:

“Prove all things; hold fast that which is good.”

THE LIVING PRINCIPLE IN PLANTS.

1, PROTOPLASTS CONSIDERED AS THE SEAT OF LIFE

Discovery of the Cell.—Discovery of Protoplasm.

DISCOVERY OF THE CELL.

What is life? This ever-interesting question has seemed to approach nearer
solution on the occasion of every great scientific discovery. But never did the hope
of being able to penetrate the great secret of life appear better founded than at the
time when, among other memorable developments of science, it was discovered that
objects could be rendered visible on an enlarged scale by the use of glass lenses, and
the microscope was invented. These magnifying glasses were expected to yield,
not only an insight into the minute structure of living beings which is invisible to
the naked eye, but also revelations concerning the processes which constitute life
in plants and animals. The first discoveries made with the microscope, between
1665 and 1700, produced a profound impression on the observers. The Dutch
philosopher Swammerdam became almost insane at the marvels revealed by his
lenses, and at last destroyed his notes, having come to the conclusion that it was
sacrilege to unveil, and thereby profane, what was designed by the Creator to remain
hidden from human ken. The observations of Leeuwenhoek (1632-1723) with
magnifying glasses formed by melting fine glass threads in a lamp, were for a
long time held to be delusions; and it was not till the English observer Robert
Hooke had confirmed the fact of the existence of the minute organisms seen by
Leeuwenhoek in infusions of pepper, and had exhibited them under his microscope
in 1667 at a meeting of the Royal Society in London, that doubts as to their actual
existence disappeared. Indeed a special document was then drawn up and signed
by all those who were satisfied, on the evidence of their own eyesight, of the accu-
racy of the observation; and this clearly shows how greatly people were impressed
with the importance of these discoveries. Of the different forms of the tiny
organisms, amounting to nearly four hundred, which were at that time distinguished,
and all included under the name Infusoria, because first seen in infusions of pepper-
corns, some only are at the present day reckoned as animals. In many cases it
has been ascertained that they are the spores of plants, whilst others again belong
to the boundary-land where the animal and vegetable kingdoms are merged.

The presence or absence of movement used to be considered as the most decisive
mark of the difference between animals and plants, and, accordingly, all the minute

22 DISCOVERY OF THE CELL.

beings which were seen bustling about in watery media were described ao.
as animals. No movement was found in the higher plants which were studie es :
the microscope about the same time by Dutch, Italian, and English observers; -
on the other hand, these investigations led to a recognition of the quite ne
peculiarities of such structures as leaves and stem, wood and pith. These parts 0

plants appeared under the microscope like honey-combs, which are built up of a

SS

N
N

yyy
Lis

LY

VS

Wf

Fig. 4.—Vegetable Cells (from Grew’s Anatomy of Plants).

1 Longitudinal section through a young apricot seed. 2 Transverse section of the petiole of the Wild Clary.
3 Transverse section of a pine branch.

great number of cells, some empty and some full of honey. From this similarity
the term “cell” arose, which later was to play so important a part in botany. In
the drawings of parts of plants as seen under the microscope the resemblance to a
honey-comb is very apparent; indeed, it is sometimes rather more striking than
when seen in reality, as, for instance, is the case in the above reproduction of three
engravings from Nehemiah Grew’s fine work published in London, 1672. It was
also noticed that, besides the structures which resembled honey-comb, there were
little tubes and fibres which were distributed and aggregated in very various ways,
and were bound up together into strands and membranes, and into pith and wood;
further, all these things were seen to increase in size and number in the growing

DISCOVERY OF THE CELL. 23

parts of plants. How growth and multiplication took place, and where exactly the
seat of a plant’s life lay, remained, of course, obscure. It was, however, natural to
assume that the walls of these small cells constituted the essential part and living
substance of plants, that they drew materials from the fluids which rose by suction
in the tubes, and so increased in size and were renewed.

It was as yet hardly suspected that the slimy substance which filled the cells
of a plant, like honey in a honey-comb, was the basis of life. The observation made
again and again at the beginning of the nineteenth century, that the cell-contents
of certain alge are extruded in the form of globules of jelly, and that each globule
moves independently and swims about in the water for a time, but then comes to
rest and becomes the starting-point of a new alga, might undoubtedly have led
to this conclusion. The accounts of these occurrences were, however, considered
incredible by the majority of contemporary observers; and it was not till recently,
when Unger established the phenomenon as an indubitable fact, that a proper
estimation of its value was accorded. In the year 1826 this botanist investigated
under the microscope a water-weed found at Ottakrinn, near Vienna, which had
been described by systematic writers as an alga, and named Vaucheria clavata.
To the naked eye it appears like a dense plexus of dark-green irregularly branched
and matted filaments. These filaments, when magnified, are seen to be tubular cells
which wither and die away at the base whilst growing at the apex, and developing
sac-like branches laterally. (Pl. I) The free ends of these tubes are blunt and
rounded. The substance they contain is slimy, and, though itself colourless, is
studded throughout with green granules; whilst near the blunt end of each filament
these green particles are so closely packed that the entire contents of that part
appear of a dark-green colour.

Now, there comes a time in the life of every one of these filaments when its
extremity swells and becomes more or less club-shaped. The moment this occurs,
the dark-green contents withdraw somewhat from the extremity, leaving it hyaline
and transparent. Almost simultaneously the contents of the swollen part of the
tube nearest the apex become transparent, whilst further down the colour becomes
very dark. (Pl. I, fig. a.) Twelve hours after the commencement of this change,
that portion of the tube’s contents which occupies the club-shaped end separates
itself entirely from the rest. A little later, the cell-wall at the apex of the tube
suddenly splits, the edges of the slit fold back, and the inclosed mass travels
through the aperture (fig. c). This jelly-like ball, having a greater diameter than
the hole, is at first strangulated as it struggles forward, so that it assumes the shape
of an hour-glass and looks for an instant as if it would remain stuck fast. There
now arises, however, in the entire mass of green jelly an abrupt movement of
rotation combined with forward straining, and in another instant it has escaped
through the narrow aperture and is swimming freely about in the surrounding
water (fig. d). The entire phenomenon of the escape of these bodies takes place
between 8 and 9 a.m., and, in any one case, in less than two minutes. When free,
each individual assumes the shape of a perfectly regular ellipsoid (fig. d), having

24 DISCOVERY OF THE CELL.

one pole of a lighter green than the other; it moves always in the direction of the
former, so that the lighter end may be properly designated the anterior. At first
the ball rises to the surface of the water towards the light, but soon after it again
sinks deep down, often turning suddenly half-way round and pursues for a time a
horizontal course. In all these movements it avoids coming into collision with the
stationary objects which lie in its path, and also carefully eludes all the creatures
swimming about in the same water with it. The motion is effected by short pro-
cesses like lashes or “cilia,” which protrude all round from the enveloping pellicle
of the jelly-like body and are in active vibration. With the help of these cilia,
which occasion by their action little eddies in the water, the whole ball of green
jelly moves in any given direction with considerable rapidity. But at the same
time as it pushes forward, the ellipsoid turns on its longer axis, so that the resultant
motion is obviously that of a screw. It is worthy of note that this rotation is
invariably from east to west, that is, in the direction opposed to that of the earth.
The rate of progress is always about the same: a layer of water of not quite two
centimetres (1°76 cm.) is traversed in one minute. Now and then, it is true, the
swimming ellipsoid allows itself a short rest; but it begins again almost immediately,
rising and sinking, and resumes its movements of rotation and vibration. Two hours
after its escape the movements become perceptibly feebler, and the pauses, during
which there is only rotation and no forward motion of the body, become both longer
and more frequent.

At length the swimmer attains permanent rest. He lands on some place or
other, preferably on the shady side of any object that may be floating or stationary
in the water. The axial rotation ceases, the cilia stop their lashing motion and are
withdrawn into the substance of the body, and the whole organism, hitherto ellip-
soidal and lighter at its anterior end, becomes spherical and of a uniform dark-
green colour. So long as it is in motion the gelatinous body has no definite wall.
Its outermost layer is, no doubt, denser than the rest; but no distinct boundary is
to be recognized, and we cannot properly speak of a special enveloping coat. No
sooner, however, is the ball stranded, no sooner has its movement ceased and its
shape become spherical, than a substance is secreted at its periphery; and this
substance, even at the moment of secretion, takes the form of a firm, colourless, and
transparent membrane. Twenty-six hours afterwards, very short branched tubes
begin to push out from the interior, and these become organs of attachment. In
the opposite direction the cell stretches into a long tube which divides into branches
and floats on the water. After fourteen days the free ends of this tube and of its
branches swell once more and become club-shaped; a portion of their slimy contents

is, as before, separated from the rest and liberated as a motile body, and the whole
performance described above is repeated.

Si ct

DISCOVERY OF PROTOPLASM. 25

DISCOVERY OF PROTOPLASM.

The study of Vaucheria led, then, to the discovery that there are plants which,
in the course of their development, pass through a motile stage, propelling them-
selves about the water as tiny halls of jelly with ciliary processes, and giving
exactly the same impression as infusoria. Hand in hand with this discovery went
the further observation that a portion of the plastic cell-contents in all plants lies,
like a lining, in contact with the inner face of the cell-walls, so that we find that
these latter, at a certain stage of maturity, are made up of two layers lying close

Fig. 5.—Protoplasm inclosed in Cells.

1 Protoplasm in cells of Orobanche. 2 Streaming protoplasm in cells of Vallisneria. 8 Streaming protoplasm
in cells of Hlodea.

together, the outer one firm and the inner soft. The name of “primordial utricle”
was given to this inner layer. On further investigation it turned out that this
primordial utricle belongs to a body of gelatinous, slimy consistency which lives in
the cell-cavity like a mussel or a snail in its shell. At first it is shapeless and fills
the whole cavity with what appears to be a homogeneous mass; but later on it is
differentiated into a number of easily-recognizable parts—vie. into the above-
mentioned lining towards the inner surface of the cell-membrane, and into folds,
strands, threads, and plates stretching across the interior of the cell. (See fig. 5.)
Mohl of Tiibingen, the discoverer of these facts, applied in 1846 the name of proto-
plasm to the substance of which the cell-contents are composed.

It is possible for protoplasm, under certain conditions, to exist for a time without
any special protective envelope; but, as a general rule, it secrets at once a firm,

26 DISCOVERY OF PROTOPLASM.

continuous coat, and, so to speak, builds itself a little chamber wherein to live. We
may therefore distinguish naked protoplasm from that kind which inhabits the
interior of a cell of its own creation, and compare the former to a shell-less snail,
and the latter to a snail that constructs the house in which its life is spent. Still
better may we compare the firm and solid cell-membrane with which the protoplasm
clothes itself to a protective coat, a garment fitted to the body; and, following out
this analogy, the protoplasm must be designated the living entity in the cell, and
the secreted envelope must be considered as merely the skin of the cell. Conse-
quently, although this cell-wall was the part which was first revealed by magni-
fying glasses, and was called a cell on account of its form, this is not the essential
formative element, which has the power of nourishing and reproducing itself.
It is the body within the cell, the slimy, colourless protoplasm in full activity within
the surrounding membrane made by itself, which must be taken to be the essential
part of the cell and the basis of life.

The term cell had become so naturalized in the science that protoplasm which
had escaped from a cell-cavity was also called a cell, and the unfortunate name of
“naked cell” was brought into use to designate it. More recently many of these
older designations have been abandoned as unsuitable. We now include under
the term “protoplasts” all these individual organisms, consisting of protoplasm,
which occupy little chambers made by themselves, living either alone like hermits or
side by side in sociable alliance in more or less extensive structures, able under
certain circumstances to leave their domiciles, laying aside their envelopes and
swimming about as naked globules.

Only when the protoplasts live in innumerable little cavities congregated close
together in colonies, and when these cavities are bounded by even walls and are for
the most part uniformly developed in all directions, does the part of a plant com-
posed of them look under the microscope like a honey-comb, and each cavity like a
cell. But even in these cases of external similarity there is the essential difference
that in a honey-comb each of the walls separating individual cells is common to both
the adjacent spaces, and, accordingly, the cells of the comb are like excavations in a
continuous matrix; whereas, in sections of cellular plants, every cell possesses its own
particular and independent wall, so that in them every partition-wall between
neighbouring cavities is composed, properly speaking, of two layers (fig. 6).
These two layers are scarcely distinguishable in the case of delicate cell-membranes
newly secreted by the protoplasts. Later on, however, they are always to be made
out clearly (fig. 62). F requently the layers separate one from another at certain
spots, and thus channels are formed between the cells (fig. 6"); these are called “ inter-
cellular spaces.” One often sees cells, too, whose entire surfaces are, as it were,
glued together with a kind of cement, and then this substance which is stored
between the two layers is called “intercellular substance” (fig. 6 *).

By loosening the intercellular substance, where present, by mechanical or chemi-
cal means, we can easily separate adjacent cells from one another; the two layers
of the partitioning cell-walls come asunder, and then each separate cell exhibits a

DISCOVERY OF PROTOPLASM. 27

complete envelope. The individual cell-cavities are often elongated and shaped like
either rigid or flexible tubes; or the wall of such a cavity may become very thick
and encroach to such an extent on the cavity that the latter is scarcely recognizable.
Cells of this kind look like fibres and threads, groups of them look like bundles
and strands, and do not resemble even remotely the cells of a honey-comb. The
term “cellular” is hence no longer suitable in the case of these structures.

The expression “cellular tissue” is calculated also to occasion a wrong idea of
the grouping and connection of the single cell-cavities. By a tissue one would
surely understand a collection of thread-like elements so arranged that some of the
threads run parallel to one another in one direction, whilst similar threads crossing

Fig. 6.—Cell-chambers. Showing Intercellular Spaces (1 and 2) and “ Intercellular Substance” (3) in the
Partition-walls of the Chambers.

the first at right angles are interwoven with them. In such a tissue, as of woven
silk or the web of a spider, the threads are held together by intertwining; but this
is by no means the case with the collections of cells which have been called cell-
tissues. Even where the parts of a so-called tissue of cells are tubular, thread-like,
or fibrous, they lie side by side and are joined as it were by a cement, but are never
crossed or twisted together like the threads in a woven fabric.

Again, cells have been compared to the bricks of a building, but this analogy is
not exact. The process of formation of a cubical crystal from a solution of common
salt may perhaps be compared to the piling up of bricks; but when a leaf grows the
process is not for one layer of cells to be superimposed from the outside upon another
previously deposited. The development of new cells proceeds in the inside of exist-
ing cells and ensues from the activity of the protoplasts inclosed within the cell-
walls; and these protoplasts not only provide the building materials, but are them-
selves the builders. It is in this very fact indeed that we grasp the sole distinction
between organic and inorganic structures, and on this account especially the above
analogy is inadmissible and should be avoided.

Cells and cell-aggregates may be conceived most clearly by considering their
analogy to the shells of living creatures, as we have already done more than once in
the foregoing pages. Protoplasts are either solitary, inhabiting isolated cell-cavities;
or else they live in associated groups, the cells being crowded close together in great
numbers and firmly attached to one another—each cavity being inhabited by one
such protoplast. When the latter is the case, division of labour usually takes place

4
u

28 SWIMMING AND CREEPING PROTOPLASTS.

in a plant, so that, as in every other community, some of the members undertake
one function, some another. The older cells in these plants often lose their living
protoplasts, and then, for the most part, serve as an uninhabited foundation to the
entire edifice, which may thus be penetrated by air and water channels. The proto-
plasts have meanwhile erected new stories for themselves and their posterity on
the old deserted foundations, and are pursuing their indefatigable labours in the little
chambers of these upper stories. This work of the living protoplasts consists in
absorbing nutriment, increasing their own substance, maturing offspring, searching
for the places which offer most favourable conditions with a view to an eventual
transmigration and to colonization by their families; and lastly, securing the region
where all these tasks are performed against injurious external influences. The
sequence of these labours is always governed by conditions of time and place.
Many of them are only to be observed with difficulty in their actual performance
and are first recognized in their perfected products, while others are attended by
very striking phenomena and are easily followed in their progress.

2, MOVEMENTS OF PROTOPLASTS.

Swimming and creeping protoplasts.—Movements of protoplasm in cell-cavities.—Movements
of Volvociner, Diatomaces, Oscillariz, and Bacteria,

SWIMMING AND CREEPING PROTOPLASTS.

Among the most striking phenomena observed in connection with living proto-
plasts are, without question, the temporary locomotion of the protoplast as a whole
and the displacement and investment of its several particles. The freest motion is
of course exhibited by protoplasts which are not inclosed in cell-cavities, but have
forsaken their dwelling and are wandering about in liquid media. Their number,
as well as the variety of their forms, is extremely great. These naked protoplasts
are evolved by several thousands of kinds of cryptogamic plants, at the moment of
sexual or asexual reproduction in these plants, The escape from the enveloping
cell-wall alone takes place in countless different ways, though the process, as a whole,
is conducted in the manner already described in the case of Vaucheria clavata.
Sometimes a single comparatively large protoplast glides out of the opened cell by
itself; at other times, before the cell opens the protoplasmic body divides into several
parts—often into a great number—and then a whole swarm of protoplasts struggle
out.

These swarming protoplasts differ considerably in form. Usually their outline
is almost ellipsoidal or oval; but pear-shaped, top-shaped, and spindle-shaped forms
also occur. Often the body of the protoplast is spirally twisted like a corkscrew,
and has in addition one end spatulate or clavate. Thread-like processes, definite in
number and dimensions and arranged variously, according to the kind of protoplast,

SWIMMING AND CREEPING PROTOPLASTS. 29

project from the surface of its body. In some instances the whole surface is thickly
covered with short cilia, as in Vawcheria (fig. 71); in others the cilia form a close
ring behind the conical or beak-like end of the pear-shaped body, as in @dogonium
(fig. 7°); and in others again, one or two pairs of long and infinitesimally thin
threads, like the antenne of a butterfly, proceed from some spot, generally the
narrow end (fig. 7° and 7*). Many forms are provided with a single long lash or
flagellum at one extremity (fig. 77), and yet others are spirally wound and are
beset with cilia, thus presenting a bristly or hirsute appearance (fig. 7").

These ciliary processes have a combined lashing and rotatory motion, and by
their means the protoplasts swim about in water. In many cases, however, swim-

Fig. 7.—Swimming Protoplasm.

1 Vaucheria; ? @dogoniuwm; 3 Draparnaldia; 4 Coleochete; 'and7 Botrydium; 6 Ulothriz; 8 Fucus; ® Funaria;
10 Sphagnum; 11 Adiantum.

ming is hardly an appropriate expression; certainly not if one associates the term
with the idea of fishes swimming with fins. In point of fact there is, associated
with progression in a particular direction, a continuous rotation of the protoplast
round its longer axis, and on this account its motion may be compared to that of a
rifle-bullet, since in both cases the movement of translation takes place in the
direction of the axis round which the whole body spins. The movement in question
is not unlike the boring of one body inside another; according to this, the soft
protoplasts bore through the yielding water, and by this action make onward
progress.

The microscope magnifies not only the moving body, but also the path
traversed; and when one contemplates a protoplast in motion, magnified, say,
three hundred times, its speed appears to be three hundred times as fast as it
really is. As a matter of fact, the motion of protoplasts is rather slow. The
swarm-spores of Vaucheria, described above, which traverse a distance of 17
millimeters in a minute are amongst the fastest. The majority accomplish an
advance of not more than 5 m.m., and many only 1 mm. per minute.

30 SWIMMING AND CREEPING PROTOPLASTS.

As was mentioned in the description of Vaucheria the locomotion of ciliated
protoplasts lasts for a comparatively brief period. It gives the impression of
being a journey with a purpose: a search, as it were, for favourable spots for settle-
ment and further development; or else a hunt after other protoplasts moving
about in the same liquid. Green protoplasts always begin by seeking the light,
but after a time they swim back into the shadier depths. Many of these, especially
the larger ones, avoid coming into collision, and are careful to give each other
a wide berth. If numbers are crowded together in a confined space, and two
collide or their cilia come into contact, the motion ceases for an instant, but in a
few seconds they free themselves and retire in opposite directions.

Contrasting with these unsociable protoplasts are others, which have a ten-
dency to seek each other out and to unite; and protoplasm acts in many cases
on protoplasm of identical or similar quality, perceptibly attracting it and deter-
mining the direction of its motion. It is very curious to watch the tiny pear-
shaped whirling protoplasts of Draparnaldia, Ulothria, Botrydium, and many
others, as they steer towards one another and, upon their ciliated ends coming
into contact, turn over and lay themselves side by side (fig. 7°); or, to see one
pursued and seized by another, the foreparts of their bodies brought into lateral
contact, and, finally, the two, after swimming about paired for a few minutes,
fusing together into a single oval or spherical protoplast (fig. 7°). Even the
minute fusiform protoplasts which are moved by cilia proceeding from the sides
of their bodies (fig. 7°), as well as the spirally-coiled forms (igs, They
endeavour to unite with some other protoplast. They always move towards
larger protoplasmic bodies at rest, cling to them closely, and at last coalesce with
them into single masses (fig. 7 8),

As a rule no striking change is to be perceived in the inside of motile proto-
plasmic bodies during the rotatory and progressive motion caused by their cilia;
and the granules and chlorophyll-corpuscles dotted about in the body of the
protoplast seem to remain, throughout the period of locomotion, almost unchanged
as regards both position and shape. It is only in the vicinity of certain little
spaces, called “vacuoles,” in the substance of the protoplasm, that changes in
many instances are observed, which indicate that, during the motion of the whole
apparently rigid mass, slight displacements may also occur in the interior, some-
what in the same way as, when a man walks, the heart inside his body is not still
(relatively to the body), but continues to pulsate and cause the blood to circulate.
The changes observed in vacuoles have, moreover, been described as pulsations,
because they are accomplished rhythmically and manifest themselves as alternate
expansions and contractions of the vacant space.

In each of the motile protoplasts of Ulothria (fig. 8) there is found, near the
conical end, which is furnished with four cilia, a vacuole which contracts in from
12 to 15 seconds, and dilates again in the succeeding 12 or 15 seconds. In the
swarm-spores of Chlamydomonas and those of Draparnaldia two such vacuoles
may be observed close together, whose rhythmic action is alternate, so that the

SWIMMING AND CREEPING PROTOPLASTS. 31

systole (contraction) of the one always takes place synchronously with the diastole
(expansion) of the other. The contraction often continues until the cavity entirely
disappears. It must depend, as also does the expansion, on a displacement of that
part of the protoplasm which immediately surrounds the vacuole. But such a
motion as this in the protoplasmic substance, even if only visible in a small part
of the whole body, can scarcely be without its effect on other more distant parts;
and it may, therefore, be concluded that the interior of a protoplast, endowed with
ciliary motion, rotatory and progressive, does not remain quite at rest relatively,
as seems on cursory inspection to be the case.

Protoplasts whose motion is effected by means of cilia have no more need of
their vibratile organs when once they have reached their destination. The cilia,

Gomel?
Tver

ize Tit
Fig. 8.—Pulsating Vacuoles in the Protoplasm of the large Swarm-spores of Ulothria. [bere Ne

ies ZENS if

RES a

whether numerous or solitary, whether short or long, first of all become stationary
and then suddenly disappear. Either they are drawn in or else they deliquesce
into the surrounding liquid. Whether the motile protoplasts have come to rest
because they have reached a suitable place for further development, as happens
in Vaucheria, or because they have united, like with like, into a single mass,
the form taken by the resulting non-motile body is always spherical. The final
act is the development around itself of an investing cell-membrane, so that its
soft and slimy substance may be protected by a firm covering from external
influences.

Essentially different from the motion just described is that of certain proto-
plasts which are unprovided with cilia, but perpetually change their outlines,
thrusting out considerable portions of their gelatinous bodies in one direction or
another, and at the same time drawing in other parts. At one moment they
appear irregularly angular, shortly afterwards stellate; then, again, they elongate,
become fusiform, and gradually almost round (fig. 9). The protruded parts
are sometimes delicate, tapering off into mere threads; sometimes they are com-
paratively thick, and have almost the appearance of arms and feet in relation
to the principal mass. The motion is not in this case like boring, but is best
described as creeping. As one or a pair of foot-like appendages is thrown out

32 MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES.

in one direction, others on the opposite side are retracted, and the protoplast as
a whole glides over the intervening space like a snail without its shell. The
analogy is all the more exact since the protoplast, as it glides onward, leaves a
slimy trail in its wake, so that the latter is marked by a streak resembling the
track of a snail. When two or more of these creeping protoplasts, or plasmodia,
meet, they merge into one another, flowing together somewhat in the same way
as two oil-drops on water coalesce into one—leaving no distinguishable boundaries
between the united bodies. Thus, slimy lumps of protoplasm, which may attain
to the dimensions of a closed or open hand, result from the coalescence of great
numbers of minute protoplasts. And it is a very remarkable fact that these
plasmodia can themselves change their form, putting out lobes and threads, and

7s
se

Fig. 9.—Creeping Protoplasm.

creeping about in the same way as the single protoplasts from whose fusion
they have arisen.

Creeping masses of jelly sometimes move in the direction of incident light; at
other times they avoid light and hide in obscure places, wriggling through the
interstices of heaps of bark or into the hollows of rotten trunks; or they may
creep up the stems of plants, or glide over the brown earth in a viscous condition.
On these occasions they resolve themselves not infrequently into bands, cords, and
threads, which surround fixed objects, divide, and combine again, forming a net-work
of meshes, or else perhaps frothy lumps like cuckoo-spit. If foreign bodies of small
size are enmeshed by.the viscous threads of the reticulum, they may be drawn
along by the protoplasm as it creeps; and if they contain nutritive material, they
may be eaten up and absorbed. Plasmodia are, for the most part, colourless, but
some are brightly tinted; in particular may be mentioned the best-known of all
plasmoid fungi, the so-called “Flowers of Tan” (Fuligo varians), which are yellow,

and Lycogala Epidendron, which comes out on old stumps of pines, and is vermilion
in colour.

MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES.

In the case of a protoplast which is not naked, but clothed with an attached
cell-membrane, the movements are limited to the space included by the membrane,
that is to say to the cell-cavity. Until the protoplasmic cell-body is differentiated
into distinct individual portions no very lively motion can in general take place
in the coated protoplast; though it is not to be assumed that it abides completely

MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES. 33

at rest at any time, except perhaps during periods of drought in summer and of
frost in winter, and in seeds during their time of quiescence. This applies par-
ticularly to immature cells. In them the protoplast forms a solid body whose
substance entirely fills the cell-cavity. The young cell, however, grows up quickly,
its cavity is enlarged, and the space, hitherto filled by the protoplast, becomes two
or three times as large as before. But the increase of volume on the part of the
protoplast itself does not keep pace with the enlargement of its habitation. It is
true that it continues to cling closely to the inner face of the cell-wall, thus forming
the primordial utricle; but the more central part of its body relaxes, and in it are
formed vacant spaces, the vacuoles above mentioned, wherein collects a watery
fluid known as the “cell-sap.” The portions of protoplasm which lie between
the vacuoles resolve themselves gradually into thin partitions bounding them; and
lastly, these partitions split up into bands, bridles, and threads, which stretch across
the cell-cavity from one side of the primordial utricle to the other, and are woven
together here and there where they intersect. With these protoplasmic strands we
have already become acquainted.

But the protoplasm in the interior of a growing cell, whilst relaxing and
breaking up, also becomes motile if the liquid attains a certain temperature, and
then the appearance presented is like that of a lump of wax melting under the
action of heat. These movements may be observed very clearly under the micro-
scope in the case of large cells with thin and very transparent cell-membranes,
especially when the colourless, translucent, and gelatinous substance of the proto-
plasm—not always sharply defined in contour—happens to be studded with
minute dark granules, the so-called “microsomata.” These granules are driven
backwards and forwards with the stream, like particles of mud in turbid water, and
their motion reveals that of the protoplasm wherein they are embedded. Seeing
particles gliding in all directions through the cell-cavity, arranged irregularly in
chains, rows, and clusters in the protoplasmic strands, we are justified in concluding
that this motion takes place in the substance of the strands itself. The movement,
moreover, is not confined to isolated strands, but occurs in all. Granular currents
flow hither and thither, now uniting, now again dividing. They often run in
opposite directions even when only a trifling distance apart; sometimes two chains
are drifted in this way when actually close together in the same band of proto-
plasm. The streams pour along the primordial utricle and whilst there divide into
a number of arms, meeting and stemming one another and forming little eddies;
then they are gathered together again and turn into another strand of the more
central protoplasm. The individual granules in the currents are seen to move with
unequal rapidity according to their sizes; the smaller particles progress faster than
the larger, and the larger are often overtaken by the less, and when this happens
the result often is that the entire stream stops. If so, however, the crowded
particles are suddenly rolled forward again at a swifter pace, like bits of stone in
the bed of a river as it passes from a level valley into a gorge. The course of the

streaming protoplasm remains throughout sharply marked off from the watery sap
Vou, I.

34 MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES.

in the vacuoles, and none of the granules ever pass over into the cell-sap from the
protoplasm.

Larger bodies, such as the round grains of green colouring-matter or chlorophyll,
are in many instances not carried forward, but remain stationary, the protoplasmic
stream gliding over them without altering them in any way. Further, the outer-
most layer of the protoplast, contiguous with the cell-membrane, is not in visible
motion in most vegetable cells. On the other hand, occasionally the entire pro-
toplast undoubtedly acquires a movement of rotation, and then the larger bodies
imbedded in its substance, z.¢. chlorophyll corpuscles, are driven along like drift-
wood in a mountain torrent (fig. 5? and 5%). On these occasions a wonderful
circulation and undulation of the entire mass takes place: chlorophyll grains are
whirled along one after the other at varying speeds as if trying to overtake one
another; and yet another structure, the cell-nucleus presently to be discussed, is
dragged along, being unable to withstand the pressure, and, following the various
displacements of the net-work of protoplasmic strands in which it is involved, is at
one moment pulled alongside of the cell-wall, at another again is taken in tow by a
rope of central protoplasm and hauled transversely across the interior of the cell
(fig. 5°).

When the rate of the current itself is estimated by the pace at which the gran-
ules are driven along, results which vary considerably are obtained, depending chiefiy
ona qualitative difference in the protoplasm, but secondarily also on temperature and
other external conditions. A rise in temperature up to a certain point as a general
rule accelerates the rate of the stream. Particles of protoplasm in particularly
rapid motion pass over 10 m.m. in a minute; others in the same time traverse from
1 to 2 mm; and some, in still less haste, advance only about a hundredth part
of a millimeter. Larger bodies, especially the bigger chlorophyll grains, move
slowest of all. So it is often hours before chlorophyll grains lying near one side of
a cell are pushed through the protoplasm over to the other side, a distance only
equal to a small fraction of a millimeter.

The minute granules, as well as the larger grains of chlorophyll and the cell-
nucleus, are entirely surrounded by protoplasm; and the protoplasm, whether in the
form of bands or threads, whether a peripheral lining or an indefinite mass, must
be conceived as always composed of two layers, the outer “ectoplasm” being tougher
and denser than the inner “endoplasm,” which is softer and somewhat fluid. The
former is homogeneous and non-granular, so that it is the more transparent and
has the effect of a skin clothing the inner, softer layer, which is granular and
turbid. It would be incorrect, however, to think of this as a very strongly-marked
contrast, sufficient to mark off one layer clearly from the other. In reality there
are no such sharp boundaries, and the tougher ectoplasm passes gradually into the
softer and more mobile endoplasm. Of course the granules and corpuscles which
one sees drifting in streaming protoplasm are situated within the more yielding
endoplasm. It is true, minute particles often appear to glide from one side to the
other upon a delicate protoplasmic strand as if it were a tight-rope; but on closer

MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES. 35

study it is apparent that the granules which seem to be travelling on the proto-
plasmic thread are covered by a delicate and transparent protoplasmic peilicle.
Thus, these granules imbedded in the substance of protoplasts have no independent
motion, but are pushed along by the spreading protoplasm.

Each stream of protoplasm is shut off from its environment and limited by
a layer tougher than the rest. But this does not prevent the currents, with their
crowds of drifting granules, from changing their direction. In fact we have
only to follow for a short time the course of one such granular stream to remark
a continuous series of changes: a current from being in a straight line bends
suddenly to one side, it broadens and contracts again, now it runs close alongside
another channel, now breaks away once more, divides into two little arms, and
loses itself finally in the primordial utricle. On the other hand, fresh folds start
from the primordial utricle, stretch and grow until they have pushed across the
cell-cavity to the other side in the form of bands, or the protoplasm may be
drawn out into threads, which elongate until they encounter other similar strings
and form a junction with them. The same processes then that are observed in
free creeping protoplasts take place to some extent here. Imagine a protoplast
captured whilst on its travels—creeping along the level ground—and imprisoned
in a completely closed vessel; it would spread itself out over the inner surface
of the vessel, would branch and creep about and have just the same appearance
as the protoplasts, just described, which inhabit cell-cavities from their earliest
youth. This is but the converse of the power possessed by a protoplast set free
from its cell, which enables it to move, stretch out, and draw in its various parts,
and so to effect locomotion.

Another motion, differing from the creeping, gliding, and streaming action
of protoplasts, manifests itself in the so-called swarming of granules contained
in the protoplasm. It may be best observed in the cells of the genera Peniwm
and Closteriwm, both of which are shown in Plate I, figs. 7, k, though
the same phenomenon is to be seen in many allied forms, living in lakes and
ponds either singly or congregated in colonies, and remarkable for their bright
green colour. The above-mentioned genus Closteriwm includes delicate unicellular
forms having a curved or scimitar shape unusual in plants, whence one of its
species, in which the semi-lunar form is most striking, has been named Closteriwm
lunula. The cell-membrane in all these little water-plants is clear and quite
transparent. The greater part of the cell-contents consists of a dark-green
chlorophyll body longitudinally grooved; but the protoplasm which is visible in
the two sharply tapering ends of the cell-cavity is colourless, and embedded
within it is a swarm of microsomata. These granules or microsomata appear to
be in a most curious state of motion so long as the protoplast lives. They are
to be seen plainly within the limits of the tiny cavity, jumping up and down,
whirling, dancing, and rushing about without really changing their position. One
is reminded of the apparently purposeless journeyings to and fro within reach
of their homes of ants or bees, and the movement has been called not inaptly

36 MOVEMENTS OF PROTOPLASM IN CELL-CAVITIES.

“swarming.” It is difficult to imagine the kind of motion possessed by the
protoplasm in which these swarming microsomata are embedded; but however
closely it is confined, there must be continual rapid displacements in 1ts substance,
which is very fluid, and it may be assumed that here again it is not so much
the tiny grains that bestir themselves as the protoplasm which holds them.
Probably the protoplasmic matter spreads and stretches out and rotates, and
individual granules are carried about by it. This, of course, does not exclude
the possibility of the granules possessing a vibratory motion of their own within
the mass of protoplasm.

Similar, but not identical, is the swarming movement of protoplasm observed
in cells of the Water-net (Hydrodictyon utriculatum), and in several other plants
allied to it. Hydrodictyon looks like a net in the form of a sac, and composed
of green threads. The meshes of this net, which are generally hexagonal, consist,
however, not of filaments but of slender cylindrical cells joined together by threes
at their extremities, somewhat in the same way as are the leaden frames of the
little hexagonal panes of glass in gothic windows. The protoplasmic body of
one of these cells in due time breaks up into a great multitude (7000-20,000) of
tiny clots, which begin to move and swarm within the cell-cavity in what appears
to be a disordered medley. In half an hour, however, the excited mass is again
restored to rest: the minute particles take form and arrange themselves in definite
order, each having two others at either extremity, making an angle of 120° with
it; and, lastly, all unite to form a single tiny net having exactly the same shape
as the one whose component cell constituted the arena of this process of construc-
tion. The miniature water-net so formed then slips out of the cell, the latter
opening for the purpose, and in from three to four weeks it grows to the same
size as the parent plant.

In the above we have an instance of a protoplast producing a whole colony
of cells, which are obliged to leave their home for want of space. In cases
previously considered we have found the protoplast stretching and elongating
in all directions, drawing itself out into bridles and spreading as a delicate lining
to walls, and so endeavouring generally to expand and present the greatest surface
possible. Again, we have seen it wandering freely, creeping, swimming, and
rotating, and by this method also covering as much space as it can. But, con-
versely, there is a time when a protoplast tends to the other extreme; the
expanded mass of its body gathers itself together again, contracts more and
more, and at length becomes a resting sphere, that is to say, it assumes the con-
figuration which exposes the least surface to the environment.

This process exhibits itself with particular clearness within the cell-cavities
of the green alge known by the name of Spirogyra, a species of which is
represented, magnified three hundred times, in Plate I, fig. 2. In this alga
the protoplasm in each mature cell-cavity forms, as a general rule, a very deli-
cate parietal lining wherein green chlorophyll bodies are embedded, arranged
in a spiral band. All of a sudden, however, this lining strips itself off the inner

MOVEMENTS OF SIMPLE ORGANISMS. 37

face of the cell-wall and shrinks together so as in a short time to present the
appearance of a sphere occupying the middle of the cell-cavity. Again, just as
this contraction is an instance of a special form of protoplasmic motion, so also
the further change which the contracted protoplast in a cell of Spirogyra under-
goes is reducible to displacements in its substance, and must be mentioned as
a special kind of protoplasmic movement. For the conglomerated protoplast
remains but a short time in the middle of the cell-cavity. It leans almost
immediately to one side, thrusting itself into a protuberance of the cell-mem-
brane, which is concurrently developed, and which, when further developed, forms
a passage leading over into another cell-cavity. Its body becomes longer and
narrower, and at last slips through the passage into the next cavity, where a
second protoplast awaits it; and the ‘two then unite, fusing together into one
mass. It is not premature to remark that all these displacements and invest-
ments of the protoplasmic substance in cells of Spirogyra, including the pheno-
mena of contraction, as well as those of pushing forward, escape, and coalescence,
are not produced as the results of a shock, impulse, or stimulus from without,
but are to be looked upon as movements proper to the protoplasm, and resulting
from causes inherent in the protoplasm.

MOVEMENTS OF VOLVOCINEA, DIATOMACEA, OSCILLARLZ
AND BACTERIA.

Very remarkable is the movement of those wonderful organisms which are
comprised under the name of Volvocineew. One species, Volvow globator, was
known to so ancient an observer as Leeuwenhoek; but he, and after him Linnzus,
took it to be an animal on account of its extraordinary power of locomotion, and it
was named the “globe-animalcule.” A Volvox-sphere consists of a large number of
green protoplasts living together as a family and arranged with great regularity
within their common envelope. They appear to be disposed radially, and to be
linked together and held firm by a net-work of tough threads, their poles being
directed towards the centre and the periphery of the sphere respectively. From
the peripheral extremity, which in each protoplast is marked out by a bright red
spot, proceed a pair of cilia, and these protrude through the soft gelatinous
envelope of the whole sphere, and move rhythmically in the surrounding water.
A Volvox-globe rolls along in the water propelled by regular strokes, like a boat
manned by a number of oarsmen, as soon as the protoplasts, which form the crew
of this strange vessel, begin to manipulate their propellers. The effect is exceed-
ingly graceful, and has justly filled observers of all periods with astonishment;
indeed no one seeing for the first time a Volvox-sphere rolling along can fail to be
impressed and delighted.

Another plant allied to the foregoing, the so-called “red-snow,” has always
excited wonder in no less degree from the remarkable phenomena of motion which
it exhibits, but also because of its characteristic occurrence in situations where one

88 MOVEMENTS OF SIMPLE ORGANISMS.

might suppose all vital functions would be extinguished. It was in the year 1760
that De Saussure first noticed that the snowfields on the mountains of Savoy were
tinged with red, and described the phenomenon as “red-snow.” Once on the look-out
for it, people found this red-snow on the Alps of Switzerland, Tyrol, and the district
of Salzburg, on the Pyrenees, the Carpathians, and the northern parts of the Ural
Mountains, in arctic Scandinavia, and on the Sierra Nevada in California. But red-
snow has been seen on the most magnificent scale in Greenland. When Captain
John Ross in 1818 sailed round Cape York on his voyage of discovery to Arctic
America, he noticed that all the snow patches lying in the gorges and gullies of the
cliffs on the coast were coloured bright crimson; and the appearance was so start-
ling that Ross named that rocky sea-shore the “Crimson Cliffs.” On the occasion of
later expeditions to the arctic regions, red-snow was observed off the north coast of
Spitzbergen, and in Russian Lapland and Eastern Siberia, but never in such sur-
prising luxuriance as on the Crimson Cliffs of Greenland.

If a snow-field coloured by red-snow is examined near at hand it is found that
only the most superficial layer, about 50 millimeters in depth, is tinged. It is also
present in the greatest quantities in places where the snow has been temporarily
melted by the heat of summer, particularly therefore in depressions, whether big or
little, and towards the edges of the snow-field, where the so-called snow-dust or
Cryoconite extends regularly in the form of dark, graphitic smeary streaks. Exam-
ined under the microscope, the matter which causes the redness of the snow
appears as a number of spherical cells having a rather substantial colourless cell-
membrane and protoplasmic contents permeated by chlorophyll. The green colour
of the chlorophyll is, however, so disguised by a blocd-red pigment that it is only
possible to detect it when the latter has been extracted, or in cases where it is
limited to a few definite spots in the cell. These spherical cells do not move, and
so long as the snow is frozen they show no sign of life. But as soon as the heat of
the summer months melts the snow, these cells acquire vitality, visibly increasing
in size and preparing for division and multiplication the moment they have
attained a certain volume. The growth, so far as it depends on nutrition, takes
place at the expense of carbon dioxide absorbed by the melted snow from the
atmosphere and of the inorganic and organic constituent parts of the dust. We
shall frequently have occasion to return to this dust, but at present it is only neces-
sary to observe, for the comprehension of the drawing of red-snow as seen under
the microscope (Pl. IL, figs. e-h), that in the Alps, amongst the organic materials
which constitute the dust, pollen-grains of conifers occur with great frequency,
especially those of the fir, arolla, and mountain pine. These pollen-grains have
been swept up into the high Alps by storms, and are already partially decayed.
In all the material that I investigated I found the red-snow cells mixed with
pollen-grains of the above-mentioned conifers. The pollen-grains are oval in cross-
section, of a dirty yellow colour, and swollen laterally into two hemispherical wings,
as is shown in Pl. I, figs. e-h.

As has been stated, the red cells are nourished by the constituent elements of

MOVEMENTS OF SIMPLE ORGANISMS. 39

the dust, which are dissolved in the melted snow. They grow and at last divide
so as to form daughter-cells, usually four in number but often six or eight and
less frequently two only (Pl. L, fig. f, g). As soon as the division is accom-
plished, the daughter-cells, so produced, free themselves, assume an oval shape, and
display at their narrower extremity two rotating cilia by means of which they
move about in snow-water with considerable vivacity. The interstices of the still
unmelted, but now granular, snow, are filled with water from the melted parts, and
through these the red cells swim away and are thus diffused over the snow-field.
At the moment of escape and first assumption of movement the cell-body appears
to be uninclosed. But it soon clothes itself with an extremely delicate, though
clearly discernible skin, which, curiously enough, does not lie close to the proto-
plasm, which is withdrawn slightly and inclosed as in a distended sac (see
Pl. I, fig. e). Only in front, where the two cilia carry on their whirling motion,
does the skin lie close to the body of the cell; and it must be presumed that the
cilia, which are simply extensions of the protoplasmic substance, are projected
through the envelope. The swarm-spores afford an example of an unusual type of
protoplasts, namely of those that move about singly in the water by means of cilia
and at the same time carry their self-made cell-membranes with them.

How long the motile stage lasts under natural conditions has not been deter-
mined for certain. On the mountains of central and southern Europe, where hot
days are followed, even in the height of summer, by bitterly cold nights, causing
the melted snow which has not run off to freeze again in the depressions of the
snow, the movement no doubt is often interrupted. On the other hand, in high
latitudes, where the summer sun does not set for weeks together, such interruption
would be exceptional. In any case, however, the locomotion of the red cells with
their hyaline cell-membranes is not limited to so short a period as is that of naked
ciliated protoplasts. Moreover they have the power of nutrition and growth like
the red resting-cells from which they originate, and they have been observed, in a
culture, to increase in size fourfold within two days. When at last they come to
rest they draw in their cilia, assume a spherical shape, thicken their cell-membrane,
which now once more lies close to the protoplasmic body, and divide anew into two,
four, or eight cells (Pl. I, fig. f, g). The fusion of the protoplasts of the red cells in
pairs, and their sexual propagation, which has been observed in addition to the
above-described asexual multiplication, will be the subject of discussion later on.
At present we need only add with reference to this remarkable plant that it was
named Spherella nivalis by the botanist Sommerfelt, and that not only in mode
of life, but also in form and colour, it most closely resembles a kind of blood-red alga,
which makes its appearance in Central Europe in little hollows temporarily filled
with rain-water in flat rocks and slabs of stone, and also inside receptacles exposed
to the open. This alga has received the name of Spherella pluvialis, and also
that of Hamatococcus pluvialis.

Lastly, we have to consider the mysterious movements exhibited by many
Diatomacex, and by the filamentous species of Zonotrichia, Oscillaria, and

40 MOVEMENTS OF SIMPLE ORGANISMS.

Beggiatoa. As regards the Diatoms, some of them are firmly attached to a
support, and are not generally capable of locomotion; but others are almost in-
cessantly in motion, and these little unicellular organisms steer themselves about
with great precision near the bottom of the pools of water in which they live.
Their cell-membrane is transformed into a siliceous coat, and this coat, which is
hyaline and transparent, but very hard, consists of two halves shutting together
like the valves of a mussel. The entire cell thus coated has the form of a gondola
or little boat, with a keel either straight or curved (Pleurosigma, Pinnularia,
Navicula), and is provided with various bands, ribs, and sculpturings on its
siliceous walls. Driven by inherent forces, these little protected cruisers pursue
their way at the bottom of the water or over objects which happen to be in the
water. They either glide evenly over the substratum, or else proceed by fits
and starts at rather long intervals, and apparently with difficulty. For some
time they may hold a straight course, but not infrequently they deviate side-
ways without apparent cause, and after deviating return again. They double
round projecting objects or push them out of the way with one of their hard
points, which are often thickened into nodules, and cause the obstructing objects
to slip by alongside the keel of the little vessel. Yet no paddles or cilia are to
be seen projecting from it, as in the case already described of Volvocines; nor
does the siliceous coat exhibit any sort of motile processes whereto the move-
ments might be attributed. But the strong analogy between the structure of
these Diatomacese and that of mussels seems to justify the assumption that the
two siliceous valves, which are fast shut during the period of rest of the Diatoms
in question, move a little apart, so that the protoplast living within can push
out one edge of its body and creep along over the substratum by means of it.

The movements of the filaments of Beggiatoa, Oscillaria, and Zonotrichia
are explained in a similar manner. These filaments are made up of a number
of short cylindrical or discoid cells, and are attached by one end, but with the
other execute most striking movements. They stretch themselves and then
contract again, coil up and straighten out like snakes, and, most characteristic
of all, make periodic oscillations in the water. The belief is that the mechanism
of this motion is similar to that of the preceding, that infinitesimally fine fila-
ments of protoplasm inserted spirally penetrate the cell-walls, and that these act
like the propeller of a ship.

On looking back over the multifarious examples of movement that have been
described, the conviction that the capacity for motion is inherent in all living
protoplasts is difficult to resist. In many cases, of course, the displacement and
replacement of the substance no doubt takes place so slowly that it is scarcely
possible to express its amount numerically. Movement may even entirely cease
for a time; but, as necessity arises, and under favourable external circumstances,
the protoplasmic mass always becomes mobile again—the direction of its motion
being determined by inherent forces. There is still much to learn, no doubt, con-
cerning the objects and significance of the different movements of protoplasm;

CELL CONTENTS. 41

but in this connection we are justified in assuming that all these movements
have to do with the maintenance and multiplication of the protoplasts. For
instance, amongst the objects of the various movements are the search for food, the
elimination of useless material, the production of offspring, the discovery of the
rays of sunlight necessary to the existence of chlorophyll-bodies and of suitable
spots to colonize. This conception has been brought out frequently in the course
of the foregoing description, and will again engage our attention in succeeding
pages.

3. SECRETIONS AND CONSTRUCTIVE ACTIVITY
OF PROTOPLASTS.

Cell-sap.—Cell-nucleus,— Chlorophyll-bodies.—Starch.—Crystals.—Construction of the Cell-wall and
Establishment of Communication between Neighbouring Cell-cavities.

CELL-SAP.—CELL-NUCLEUS.—CHLOROPHYLL-BODIES.—_STARCH.—CRYSTALS.

In addition to the powers which the living protoplast possesses of shifting
its parts, of expanding and contracting, of dividing and of fusing like with like,
it has also the properties of adapting different parts of its body to particular
functions, of building up various chemical compounds, and of separating them out
when necessary. As the protoplast stretches and expands, spaces and depressions
arise within it, and these form ultimately, when the protoplast is limited
to a peripheral layer lining the walls of the cavity, a single central vacuole.
In the spaces there is secreted, in the first instance, the cell-sap, a watery fluid
containing a variety of substances either suspended or in solution, of which the
chief are sugar, acids, and colouring matters. Moreover, in the interior of the
protoplasm itself, structures with quite different forms occur, and are easily recog-
nizable by their contours; these are the cell-nucleus, chlorophyll-bodies, and starch-
grains.

The principal feature of the cell-nucleus is that, although the substance of
which it is composed is only slightly different from the general protoplasm of
the cell, yet it is always clearly marked off from the protoplasm. Jn the un-
developed protoplast the nucleus is usually situated in the middle, but in mature
protoplasts it is either pressed against one wall of the cell or suspended in a sort
of pocket of protoplasmic filaments in the interior (fig. 51 and 5%). It may
be pushed along by the streaming protoplasm and dragged into the middle of
the cell, and in that case its shape is sometimes altered and it becomes for a time
somewhat elongated and flattened. The nuclear substance, which, as has been
already mentioned, differs but little from ordinary protoplasm, is colourless, and
studded with microsomata, and is liable to internal displacements similar to those
of the entire cell-body. When a protoplast divides, the nucleus plays a very

42 THE CELL-WALL.

important part in the process, and it will be necessary later on to discuss its
significance in this connection.

The chlorophyll-bodies, mentioned already more than once incidentally, are
green corpuscles, roundish, ellipsoidal, or lenticular in shape, and grouped in a
great variety of ways (Pl. I, figs. i, &, 1, m, p). They are produced generally
in great numbers by the protoplast in special sac-like excavations in its body,
but nowhere except where they are necessary, that is, in those cells wherein
the transmutation of inorganic food-stuffs into organic matter takes place. This
transformation, so important to the existence of the organic world, will be con-
sidered in detail later on. Chlorophyll-corpuscles are not, as regards their material
basis, essentially different from the substance of the protoplasm in which they
are formed, and in which they remain embedded for life, but their green colour
distinguishes them very clearly from their environment. This greenness is due
to a colouring matter stored in the protoplasmic substance of the corpuscle; and
our ideas of plant-life are so intimately associated with this remarkable pigment,
that a plant that is not green seems to us to be almost an anomaly.

Besides the nucleus and the chlorophyll-bodies or corpuscles, protoplasts pro-
duce starch-grains, aleurone-grains, crystals of oxalate of lime, and drops of oil, all
of which will be dealt with presently in their proper place. They are evolved in
accordance with the requirements of the moment and with the position held in the
edifice of the plant by the cells concerned. Moreover, the walls of the cells them-
selves are the work of the protoplasts, and it is not a mere phrase, but a literal fact,
that the protoplasts build their abodes themselves, divide and adapt the interiors
according to their requirements, store up necessary supplies within them, and, most
important of all, provide the wherewithal needful for nutrition, for maintenance,
and for reproduction.

CONSTRUCTION OF THE CELL-WALL AND ESTABLISHMENT OF CONNECTIONS
BETWEEN NEIGHBOURING CELL-CAVITIES.

Of all these performances, the construction of the cell-wall shows the greatest
variety from the nature of the case. For the envelope with which each individual
protoplast surrounds itself serves at once as a protection for the delicate protoplasm,
and as a firm support for structural additions; and, at the same time, it must not
impede the reciprocal action between the protoplasts and the external world, or the
intercourse between those living in adjoining cavities. These cell-walls are accord-
ingly very wonderful structures, and we shall often have occasion to discuss them,
especially with reference to the significance of variations in their structure in
particular cases. At present it is sufficient to remark that the original envelope
which is secreted from the body of a protoplast and which appears at first as a
delicate skin, is made of a substance composed of carbon, hydrogen, and oxygen,
belonging to the class of carbohydrates.

The name of cell-membrane, usually applied to the original envelope formed by

THE CELL-WALL. 43

the cell-body, is one quite suitable for the purpose. But this earliest covering under-
goes many modifications. The protoplast is able to store up in it suberin, lignin,
silica, and water in greater or smaller quantities, and by this means it either makes
the envelope more flexible than it was in the first instance, or else hard and
stiff, converting it into a shell-like case. Even the shape is seldom preserved as it
was originally. The solitary protoplast surrounded by its cell-membrane is gener-
ally in the form of a roundish ball, and its envelope, which is closely adherent,
exhibits a corresponding configuration. Young cells, aggregated together, have
outlines too which remind one of crystalline forms, such as dodecahedra, cubes,
and short six-sided prisms. But when a protoplast has produced its first delicate
covering it does not come to rest, but goes on working at the membrane, distending
and thickening it, transforming a cavity which was originally spherical or cubical
into one of cylindrical, fibrous, or tabular shape, and strengthening its walls with
pilasters, borders, ridges, hooks, bands, and panels of various kinds. Where a
number of protoplasts work gregariously at one many-chambered edifice, cells of
most diverse forms are produced in close proximity to one another. These
varieties are, however, never without method and design, but are invariably such
as to adequately equip each cell for the position it holds and for the particular
task allotted to it in the general domestic economy.

The volume attained by cell-cavities in consequence of the expansion of their
walls varies within very wide limits. The smallest cells have a diameter of only
one micro-millimeter, i.e. the thousandth part of a millimeter; others, as for example
yeast-cells, measure perhaps two or three hundredths of a millimeter; and yet
others have outlines perceptible to the naked eye and have a volume amounting
to one cubic millimeter. Tubular and fibrous cells often stretch longitudinally
to such an extraordinary extent that some with a diameter of scarcely the hun-
dredth part of a millimeter reach a length of one, two, or even as many as five
centimeters. An instance may be seen in the filaments of Vaucheria clavata
(Pl. I., figs. a-d), and again in the fibrous cells from which our linen and cotton
fabrics are manufactured.

The enlargement of a cell-cavity, or, in other words, the growth in area of
its walls, ensues in consequence of the intercalation of fresh particles between
those which, by their mutual coherence, form the delicate skin of the protoplast
—the earliest stage of the cell-wall. When these intercalated particles are situ-
ated in the same plane as are those already deposited, the cell-wall resulting
from this method of construction will increase in area without adding to its
thickness. But when once the cells are full-sized, the constructive activity of
the protoplasts has to be directed in many cases to the strengthening and thick-
ening of their walls, so that later on they may be able to perform special duties.
From the appearance of this thickening one would judge that a number of layers
were deposited on the thin original wall according to requirement, and in many
instances no doubt the process corresponds to this appearance; but, as a rule, the
thickness of the wall is increased by intercalation, on the part of the protoplasts, of

AA THE CELL-WALL.

additional material between the original particles, a process which has been termed
“intussusception.”

The appearance of stratification in thickened cell-walls is naturally moss strik-
ing where substances of different kinds have been deposited alternately in the
different parts of the wall, and when successive layers take up unequal quantities
of water. The thickening may at length result in such an extreme restriction of
the cell-cavity that its diameter is less than that of the inclosing wall. Sometimes
nothing remains of the cavity but a narrow passage, and then the cells are like
solid fibres. Formerly they would not have been classed with cells at all, but
would have been distinguished under the name of fibres, from the forms resembling
honey-comb cells. The protoplasts in these contracted cells languish and often die,
especially when the walls of the self-made prison are greatly thickened and do not
allow of intercourse with the world outside. But generally a protoplast takes care,
in constructing its dwelling, not to close itself in entirely, nor to cut itself off
permanently from the outer world. It either makes from the very beginning little
windows in the walls of its house, leaving them quite open or closed only by thin,
easily-permeable, membranes; or else, after constructing a completely closed enve-
lope, it redissolves a piece of it, thus making an aperture through which in due
time it is able to effect its escape. The scope of this work does not admit of an
exhaustive treatment of the formative power possessed by protoplasts needful for
these results; it will be sufficient to give a general description of some of the
more important processes which have for their object the establishment of a
connection between adjacent cell-cavities and of communication with the external
world.

The new particles of material, or cellulose, which are to strengthen the
delicate original cell-membrane, are in many instances not deposited or intercalated
evenly over the entire surface of the protoplast. Little isolated spots are left
unaltered, and these may be compared in a way to the small glazed windows in a
living-room, or cabin port-holes closed by thin panes of glass. The part of the
thickened wall which immediately surrounds the little window, and which so to
speak constitutes its frame, has, besides, often a very characteristic structure,
being elevated so as to form first a ring-like border, and eventually a hood,
arching over the window and perforated in the middle (see fig. 101). A comparison
of this structure, arched over the thin spots in a cell-wall, to the iris spread in
front of the crystalline lens in an eye would be still more appropriate. A similar
annular border projects likewise from the window-frame on the other side,
facing a neighbouring cell-cavity, so that the window appears symmetrically
vaulted on both sides by mouldings with round central. apertures (fig. 10°).
Supposing someone wanted to pass from one cell-cavity to the other he would have
in the first place to go through the hole in the moulding on his side. He would
then find himself in a roomy space, which we will call the vestibule, and would
next have to break through the little window, which is somewhat thickened in
the middle, but elsewhere is as soft and thin as possible. On the further side

THE CELL-WALL. 45

again would be a vestibule, and it would not be until he had emerged from this
through the aperture in the second moulding that he would reach the interior
of the adjoining cell. Seen from in front, the outline of one of these windows,
or rather the outline of the common floor of the vestibules, appears as a circle,
whilst the aperture or opening in the moulding—which is exactly in the centre
of this circle—is seen as a bright dot or pit encompassed by the circle which
defines the limits of the vestibule. Hence these curiously protected window
structures are named bordered pits. They are shown in fig. 10' and 10%, and
are to be seen in great perfection in the wood-cells of pines and firs.

Whenever bordered pits are formed, the thickening of the cell-membrane is
comparatively slight; the frame of the window in the cell-wall is never more than

\ a)

tthe

Fig 10.—Connecting Passages between adjacent Cell-cavities.

1, Bordered pits, 2, Section of a bordered pit. 3, Mode of connection of adjacent cells in the bundle-sheath of Scolopendrium.
4, Sieve-tubes. 5, Group of cells from seed of Nua-vomica, the protoplasts of adjoining cell-cavities connected by fine
protoplasmic filaments.

five times as thick as the window-pane itself. In other cases, however, the cell-wall
becomes twenty or thirty times as thick as it was at first, and the interior of the
cell is thereby seriously diminished in size. But even if, little by little, the cell-wall
augments in thickness a hundredfold, any spot where thickening has not taken place
from the first, and where, accordingly, a little depression occurs, is not subsequently
covered with cellulose, but is' carefully kept open by the protoplast as it builds.
A greatly thickened wall of this kind resembles a fortification provided here
and there with deep, narrow loopholes. Where two cells thus provided adjoin one
another, the windows in the one occur, normally, exactly opposite those of its
neighbour, and the result is the formation of canals, very long relatively, which
penetrate through the two adjacent cell-walls and connect the neighbouring cell-
cavities together (fig. 107). A canal of this kind is still closed, it is true, in the
middle by the original cell-membrane as though by a lock-gate; but this slight
obstruction may be removed later by solution, and the contiguous cells have then
perfectly open connection through the canal.

Very frequently provision is made in the very first rudiments of a cell-mem-

46 THE CELL-WALL.

brane, destined to constitute a partition-wall, for open communications such as the
above. For segments of the wall of various sizes are made from the beginning with
sieve-like perforations, as is shown in fig. 10‘, which represents diagrammatically
portions of tubular cells called “sieve-tubes.” The pores are crowded close together
on the perforated areas of the walls of the sieve-tubes, and their dimensions are
relatively broad and short. Thus, when two neighbouring protoplasts reach out to
one another through these pores, that is to say, when there is continuity of the
protoplasm of the two cell-cavities, the connecting filaments, which pass through the
pores and which fill them completely, are short and thick and have the appearance
of pegs or stoppers.

But in many cases the pores through which adjoining cell-cavities communicate
are drawn out to a great length, forming infinitesimally slender passages. They are
situated close together in great numbers and penetrate transversely through the
thick cell-walls (fig. 10°).. Neighbouring protoplasts may be brought equally well
into mutual connection by means of these canals, or perhaps it would be better to
say that their connection may be equally well maintained. For it is very probably
the case that in the first rudimentary partition-wall, which is produced between the
products of division of a protoplast, minute spots remain open and are occupied by
connecting threads common to both halves of the protoplasm as they draw apart.
Then in proportion as the partition-wall between the two protoplasts, produced by
the division, becomes thicker, the openings take the form of fine canals, and the con-
necting filaments are modified into long and exceedingly fine threads which fill the
canals. These protoplasmic threads pierce through the thickened cell-wall in the
same way as a dozen telegraph-wires might be drawn through a partition from one
room into another. Often a number of protoplasts living side by side and one
above the other are linked together by filaments of this kind, which radiate in all
directions.

This species of connection, of which an intelligible idea is given by fig. 10°,
escaped the notice of observers in former times owing to the extraordinary minute-
ness of the canals, and delicacy of the protoplasmic filaments. Another method of
communication between protoplasts in adjoining cells has, on the other hand, been
long known and often described, its phenomena being very striking and visible when
only slightly magnified. The connection referred to is that which is afforded by
the formation of so-called “vessels.” By vessels the older botanists understood
tubes or utricles, arising from the dissolution of the partition-walls between a series
of cells. Hither the partition-walls in a rectilineal row of cells vanish, in which
case long straight tubes are produced; or portions of the walls of cells arranged at
different angles to one another are dissolved, and then tubes are formed having an
irregular course, and sometimes branching or even uniting, so as to make a net-work.
In instances of the first kind the lateral walls of the series of cells which are to lose
their transverse partitions are previously thickened and made stiff by the proto-
plasts, which also provide them with various mouldings and panellings, and above all
with bordered pits. This task accomplished, the protoplasts forsake the tubes, whose

TRANSMISSION OF STIMULI, AT

function thenceforth it is to serve as passages for air and water; thus the con-
tinued presence of the protoplasts is no longer advantageous. On the other hand,
in the second class of vessels the lateral walls of the cells, which have coalesced
to form them, exhibit no thickening, but are soft and delicate, and resemble
flexible tubing. These tubes, moreover, are not deserted by their protoplasts; but,
after the coalescence of a number of cells into a single duct has taken place, the
protoplasts in the cells are themselves merged together, and the entire tube is
then occupied by an uninterrupted mass of protoplasm, which generally persists
as a lining to the wall.

As the initiation and construction of cell-walls are the work of the living proto-
plast, so also is their removal. The home it has made for itself the protoplast can
also demolish—either partially or completely. But this demolition is preluded by
the importation of particles of water into the portions of the wall which are to be
destroyed. The introduction of water brings the wall into a gelatinous condition;
the cohesion of its constituent particles is loosened, little by little, and at length
completely abolished.

4, COMMUNICATION OF PROTOPLASTS WITH ONE ANOTHER
AND WITH THE OUTER WORLD.

The transmission of stimuli and the specific constitution of protoplasm.—
Vital Force, Instinct and Sensation.

THE TRANSMISSION OF STIMULI AND THE SPECIFIC CONSTITUTION
ra OF PROTOPLASM.

As has been already intimated, the breaking down of individual cell-walls and
the formation of the various pits, sieve-pores and fine canals in thickened mem-
branes, in the manner described in preceding pages, are processes of great import-
ance to the life of protoplasts. In the first place, many of the resulting structures
are the means of preserving the possibility of intercourse with the outside world.
In a space inclosed by evenly thickened walls, the absorption of air, water, and
other raw materials from the environment would be very difficult if not impossible;
the protoplast inside would soon lack the provisions needful for further development,
and would at last die of starvation, drought, and suffocation. But the little win-
dows, whether open or closed by thin permeable membranes, enable it to supply
itself with all necessaries of life. Another advantage is derived, in the case of many
of these structures, inasmuch as the protoplasts on occasion escape through the open
doors and settle down in some other part of the cell-colony, where they are able
again to make themselves useful. Lastly, one of the most important benefits of all
is due to the fact that mutual intercourse between protoplasts, living together as a
commonwealth, is rendered possible by the canals which join them together. And

48 ‘TRANSMISSION OF STIMULI.

such an intercourse must of necessity be presumed to exist. When one considers
the unanimous co-operation of protoplasts living together as a colony, and observes
how neighbouring individuals, though produced from one and the same mother-cell,
yet exercise different functions according to their position; and, further, how uni-
versally there is the division of labour most conducive to the well-being of the whole
community, it is not easy to deny to a society, which works so harmoniously, the
possession of unity of organization. The individual members of the colony must
have community of feeling and a mutual understanding, and stimuli must be pro-
pagated from one part to another. No more obvious explanation offers than that
the protoplasmic filaments, which run like telegraph-wires through the narrow
pores and canals in the cell-walls (see fig. 10°), serve to propagate and transmit
stimuli from one protoplast to another. These threads of protoplasm may indeed
be likened to nerves which convey impulses determining definite actions from cell
to cell.

Imagination takes us further still, and raises the cell-nucleus to the position
of the dominant organ of the cell-body. For the nucleus not only determines
the activity of the individual protoplast within its own cavity, but continues in
sympathetic communion with its neighbour by means of all the threads and liga-
ments which converge upon it. This last idea in particular derives support from
indications that the filaments uniting neighbouring protoplasts have their origin
in specific transformations in the substance of the nucleus itself. When a proto-
plast living in a cell-cavity is about to divide into two, the process resulting in
division is as follows:—The nucleus places itself in the middle of its cell, and at
first characteristic lines and streaks appear in its substance, making it look like
a ball made up of threads and little rods pressed together. These threads gradu-
ally arrange themselves in positions corresponding to the meridian lines upon a
globe; but, at the place where on a globe the equator would lie, there then occurs
suddenly a cleavage of the nucleus—a partition-wall of cellulose is interposed in
the gap, and from a single cell we now have produced a pair of cells. In this
way, from the nucleus, and from the protoplast of which the nucleus is the centre,
two protoplasts have been produced, each having a nucleus of its own, and they
thenceforth live side by side, each in its own chamber. It has been proved that
im this process of division the substance of the nucleus is not completely sundered
by the partition as it grows, but that, as we have already mentioned, minute
pores are kept open in the cellulose wall, and that the pair of protoplasts continue
joimed together by threads running through these pores.

When we realize that every plant was once only a single minute lump of
protoplasm, inasmuch as the biggest tree, like the smallest moss, has its origin
in the protoplasm of an egg-cell or a spore; and when we consider how, by growth
and repeated bipartition, thousands of cells are evolved, step by step, from a
single one, whilst their protoplastic bodies still remain united by fine filaments,
we arrive of necessity at the conclusion that the whole mass of protoplasm, living
in all the myriads of cells whose aggregation constitutes a tree, really is, and

TRANSMISSION OF STIMULI. 49

continues to be, a single individual, whose parts are only separated by perforated
sieve-like partitions. Every member of this community occupies a particular
compartment or cavity, and is governed by a central organ, the cell-nucleus; but
being linked to its fellows by connecting threads of protoplasm, a mutual under-
standing is thus established among them.

The physical basis of such an understanding may in this manner be represented
with tolerable certainty. But it is extremely difficult to throw light upon the
process of this mutual intelligence, the actual method whereby the cell-nuclei
not only govern within their own narrow spheres, but also co-operate harmoniously
for the good of the whole. And yet the problem involved in this unanimity of
action, with a view to a systematic development of the plant in its entirety, is.
of such extreme importance that we cannot evade it even if, in the endeavour
to solve it, we have to move altogether in the region of hypothesis.

In every attempt at explanation of the kind we must, at all events, bear in
mind that the agreement in question, as well as the processes which take place
in pursuance of this agreement, such as the nutrition, growth, and the organization
of the entire plant, are reducible to the subtlest atomic agencies in the living
protoplasm. They may be resolved into the motion of minute particles, into
attractions and repulsions, oscillations and vibrations of atoms, and into re-arrange-
ments of the atomic groups called molecules. Again, these movements are the
result of the action of forces, especially of gravity, light, and heat. As regards
gravity and light, experiment shows, however, that, when acting on living proto-
plasm, they give rise to varying effects even under the same conditions; and this
fact, which will be discussed frequently later on, indicates that these forces are
at any rate only to be conceived as stimulative and not coercive, and that they
have no power to determine the kind of form. It is characteristic of the processes
set up by gravity and light, especially when they take place in the continuous
protoplasm of a great cell-community, that the.coarser movements visible to the
naked eye are often manifested in members comparatively remote from the part
immediately affected by the stimulus. We cannot well represent this to ourselves
except by supposing that the stimulus, which is the cause of the movement, is
propagated through the threads of protoplasm from atom to atom, and from
nucleus to nucleus. But the great puzzle lies, as already remarked, in the circum-
stance that the atomic and molecular disturbances occasioned by such stimuli and
transmitted through the connecting filaments are not only different in the proto-
plasm of different kinds of plants, but even in the same plant they are of such
a nature, according to the temporary requirement, that each one of the aggregated
protoplasts in a community of cells undertakes the particular avocation which is.
most useful to the whole, the effect of this joint labour conveying the impression
of the presence of a single governing power of definite design and of methodical
action.

That a stimulus causes different occurrences in different species of plants, and,

more especially, that cell-communities arising from different egg-cells develap into
Vou. I.

50 TRANSMISSION OF STIMULI.

different forms, though under identical conditions and subjected to the same stimuli,
are phenomena which have parallels in the inanimate world. A different sound
is produced by striking the key of a piano which is connected to an A-string from
that resulting from the transmission of a similar impulse to an F-string; and the
difference depends on a difference of structure and an inequality of tension in
the strings. Again, solutions of the sulphate and of the hyposulphite of sodium
in similar glass vessels are indistinguishable at sight, both being colourless and
transparent. These solutions will preserve their liquid condition when cooled
down gradually to below freezing-point if they are kept absolutely still; but the
moment the vessels are touched and a vibration thereby transmitted to the contents,
they freeze. Crystals are formed in the apparently identical liquids, but crystals
of different kinds, Glauber’s salts in the one case, hyposulphite of sodium in the
other. The variety of form depends simply on the sort of atoms, and on their
number and mode of grouping.

In a similar manner must be explained the variety of forms in many plant-
species developed under the same conditions and affected by the same stimuli.
Dozens of kinds of unicellular Desmids and Diatoms are often developed at the
same time in a single drop of water in close proximity to one another. Although
the protoplasm in the spores of these different species is absolutely identical to
our vision, aided by the best microscopes, yet the mature cells exhibit a multiplicity
of form which is quite astonishing to the observer on first inspection. One cell
is semi-lunar, another cylindrical, a third stellate, a fourth lozenge-shaped, and
a fifth acicular. In one specimen the cell-membrane is smooth, in another it is
beaded; some are provided with siliceous coats, whilst others have flexible envelopes.

The same thing holds good with respect to the vegetable structures, which are
composed of myriads of cells, and develop into huge shrubs or tall trees. The
protoplasm in the egg-cell of an oleander is produced close to that of a poplar on
the same river-bank, and under exactly the same external conditions. The cells
divide, and partition-walls are introduced in the proper direction in either case,
according to a plan of structure which is adhered to with marvellous precision
by the protoplasts engaged in the work of construction. In each species, stem,
branches, foliage, and blossoms have invariably a particular form and arrangement,
have the same colour and smell, and contain the same substances. How utterly
different are the mature leaf, the opened flower, and ripe fruit of the oleander from
the corresponding parts of a poplar. Yet both were nourished by the same earth,
were surrounded by the same atmosphere, and encountered the same rays of sun-
shine. We cannot otherwise explain it than by the supposition that, in a case
like this, the difference of form in the perfected state is based upon a difference
in the self-developing protoplasm, and that the atoms and molecules of this proto-
plasm, which appears to us to be uniform, vary in kind, number, and grouping
in the two species of plants. Consequently, we must assume that every vegetable
organism, every species of plant that appears invariably in the same external
form when mature, and develops according to an invariable plan, has a protoplasm

VITAL FORCE, INSTINCT, AND SENSATION. 51

of its own of a certain specific constitution. And, further, we must assume that
this specific protoplasmic constitution is transmitted from one generation to another,
so that the protoplasm of the oleander, for example, had exactly the same constitu-
tion thousands of years ago as it has to-day. Lastly, we must assume that each
special kind of protoplasm has the power to reproduce its like, ever anew, from
the raw materials occurring in its environment.

VITAL FORCE, INSTINCT, AND SENSATION.

The phenomena observed in living protoplasm, as it grows and takes definite
form, cannot in their entirety be explained by the assumption of a specific con-
stitution of protoplasm for every distinct kind of plant; though this hypothesis
will again prove very useful when we inquire into the origin of new species.
What it does not account for is the appropriate manner in which various functions
are distributed amongst the protoplasts of a cell-community; nor does it explain
the purposeful sequence of different operations in the same protoplasm without
any change in the external stimuli, the thorough use made of external advan-
tages, the resistance to injurious influences, the avoidance or encompassing of
insuperable obstacles, the punctuality with which all the functions are performed,
the periodicity which occurs with the greatest regularity under constant condi-
tions of the environment, nor, above all, the fact that the power of discharging
all the operations requisite for growth, nutrition, renovation, and multiplication
is liable to be lost. We call the loss of this power the death of the protoplasm.
It ensues upon assaults from without if they succeed in destroying the molecular
structure so entirely as to render reconstruction impossible; but, furthermore,
death may take place without external cause.

If cells of the blood-red alga, previously mentioned as allied to the red-snow,
are collected from hollows in stones, casually full of rain-water, and are kept
dry for weeks and then again moistened, the water is found to have a very power-
ful effect. The protoplasm becomes mobile, and swarm-spores are formed which
put forth vibratile cilia, propel themselves about for a short time in the water,
and then settle down in some favoured spot, draw in their cilia, come to rest
and divide, producing offspring which again are motile. This alga may be kept
dry for months, nay even over a year, and still its cells exhibit the movements
above described when put into water. But if a mass of it is preserved under
these same conditions for many years and then moistened, the little cells will, it
is true, take up additional water, but motile cells are no longer formed. The
cells do not move, nor grow, nor divide, but gradually become discoloured; are first
disintegrated and then dissolved. We say then that in them life could no longer
be recalled, and we describe them as dead.

The same thing is observed in great cell-communities. The seeds of many species
of plants preserve the capacity for germination for an incredibly long period, especially
when kept in a dry place. If after ten years such seeds are transferred into

52 VITAL FORCE, INSTINCT, AND SENSATION.

moist earth, the protoplasm in the majority of cases begins to bestir itself and
to move, and the embryo grows out into a seedling. After twenty years, perhaps,
only about five per cent of the seeds preserved would germinate. The rest are not
stimulated by damp earth to further development, their protoplasm no longer
possesses the power of augmenting its volume by absorption of matter from the
environment, or of developing a definite form, but is disintegrated by the influx of
air and water and breaks up into simpler compounds. After thirty years hardly
one of the seeds would sprout. Yet all these seeds were kept throughout the time
at one place and under precisely the same external conditions; nor can the slightest
change in their appearance be detected. Gardeners express the fact by saying that
the capacity for germination becomes extinct in from twenty to thirty years. But
what kind of a force is this which may perish without a physical change of the
substance concerned affording the basis of the extinction? In former times a special
force was assumed, the force of life. More recently, when many phenomena of plant
life had been successfully reduced to simple chemical and mechanical processes,
this vital force was derided and effaced from the list of natural agencies. But by
what name shall we now designate that force in nature which is liable to perish
whilst the protoplasm suffers no physical alteration and in the absence of any
extrinsic cause; and which yet, so long as it is not extinct, causes the protoplasm
to move, to inclose itself, to assimilate certain kinds of fresh matter coming
within the sphere of its activity and to reject others, and which, when in full
action, makes the protoplasm adapt its movements under external stimulation to
existing conditions in the manner which is most expedient?

This force in nature is not electricity nor magnetism; it is not identical with
any other natural force, for it manifests a series of characteristic effects which
differ from those of all other forms of energy. Therefore, I do not hesitate again
to designate as vital force this natural agency, not to be identified with any other,
whose immediate instrument is the protoplasm, and whose peculiar effects we
call life. The atoms and molecules of protoplasm only fulfil the functions which
constitute life so long as they are swayed by this vital force. If its dominion
ceases, they yield to the operations of other forces. The recognition of a special
natural force of this kind is not inconsistent with the fact that living bodies
may at the same time be subject to other natural forces. Many phenomena of
plant life may, as has been already frequently remarked, be conceived as simple
chemical and mechanical processes, without the introduction of a special vital
force; but the effects of these other forces are observed in lifeless bodies as well,
and indeed act upon them in a precisely similar manner, and this cannot be said
of the force of life.

Were we to designate as instinctive those actions of the vital force which
are manifested by movements purposely adapted in some manner advantageous
to the whole organism, nothing could be urged against it. For what is instinct
but an unconscious and purposeful action on the part of a living organism? Plants,
then, possess instinct. We have instances of its operation in every swarm-spore

VITAL FORCE, INSTINCT, AND SENSATION. 53

in search of the best place to settle in, and in every pollen-tube as it grows
down through the entrance to an ovary and applies itself to one definite spot of
an ovule, never failing in its object. The water-crowfoot, in deep water, fashions
its leaves with finely divided tips, large air-passages, and no stomata; whilst,
growing above the surface of the water, its leaves have broad lobes, contracted
intercellular spaces and numerous stomata. Linaria Oymbalaria (see fig. 11)
raises its flower-stalks from the stone wall over which it creeps towards the light,
but as soon as fertilization has taken place, these same stalks, in that very place
and amidst unchanged external conditions, curve in the opposite direction, so as

Fig. 11.—Linaria Cymbalaria dropping its Seeds into Clefts in the Rocks.

to deposit their seeds in a dark crevice. The flower-stalk of Vallisneria twists
itself tightly into a screw and draws the flowers, which previously it had borne
upon the surface of the water, down to the bottom when their stigmas have been
covered with pollen-dust at the surface. These are all cases of unconscious action
for a definite object, that is to say, they are the result of instinct.

If, however, we attribute instinct to living plants, it is but a step further to
consider them as endowed with sensation also. Feeling in animals is the con-
comitant of a condition of disturbance in nerves and brain caused by a stimulus,
which acts on the organs of sense, and is conveyed by nerves to the central
organ. The transmission of the stimulus and the excited state of the brain and
nerves are only molecular movements of the nervous substance, or, let us say, of
the protoplasm, for nerve-fibres and nerve-cells are simply protoplasm developed
in a particular manner. But the state induced by the stimulation of protoplasm,
which is what we call sensation, cannot be essentially different in vegetable
protoplasm from what it is in animal protoplasm, since the protoplasm itself,
the physical basis of life in both plant and animal, is not different. In isolated
plant-cells, indeed, it may amount to such a concentration of the condition of
stimulation as to be called sensation, for the cell-nucleus is to all appearance

54 VITAL FORCE, INSTINCT, AND SENSATION.

a central organ in relation to the protoplast that lives in a solitary cell. It is
not of course to be supposed that within a whole plant-structure, that is in the
community of live protoplasts which constitutes an individual plant, such a con-
centration of stimulation could occur as is the case with individual animals which
have nerve-fibres all converging into the brain; but between the sensation of
animals without nerves and that of plants no essential difference can exist.

Hence we infer that there is no barrier between plants and animals. The
attempt to establish a boundary-line where the realm of plants ceases and the
animal world begins is a vain one. If we naturalists, all the same, agree to
separate plants and animals, we do so only because experience shows that a
division of labour conduces to a speedier attainment of our object. On the
intermediate ground where animals and plants meet, zoologists and botanists
encounter one another, not, however, as hostile rivals with a view to exclusive
possession of the field, but as colleagues with a common interest in the adminis-
tration and cultivation of this jointly tenanted region.

ABSORPTION OF NUTRIMENT.

1. INTRODUCTION.

Classification of plants with reference to nutrition.—Theory of food-absorption.

CLASSIFICATION OF PLANTS WITH REFERENCE TO NUTRITION.

The object of a plant’s vital energy, next in importance to the resistance of such
influences as are likely to bring about the death of the protoplasm, is growth, ic. the
addition of substance to its body, or, in other words, the absorption of nutriment.
A living plant, whether consisting of a single cell or of a vast community of cells,
takes up food from its environment in quantities varying according to the needs of
the moment. But its method of action—how it sets about acquiring possession of
this raw material, how it manages to incorporate the substances absorbed from with-
out, how it contrives to retain only such part as is useful to it, and to reject and get
rid of, like ballast, what does not subserve its own growth—is infinitely varied.
This variety in the processes of food-absorption corresponds, on the one hand, to
differences in the habitat of plants, and, on the other, to the requirements of particu-
lar species, which requirements in their turn depend upon a specific constitution of
the protoplasm in each species concerned. The difference must be very great
between this process as manifested in plants which are immersed in water during
their whole lives and the same as it occurs in plants which live in desert sands and
are not supplied with water for months together. And again, absorption in those
fungi which grow luxuriantly on damp timber in the deep obscurity of a mine must
take place very differently from the corresponding process in the delicate alpine
plants which on our mountain slopes are exposed periodically to the most intense
sunlight, and then, for weeks at a time, are wreathed in sombre mists. So, also,
the reciprocal action between plants and their environment must have a character of
its own in the case of parasitic growths which absorb their food from other living
organisms, and in those remarkable plants, too, which catch and devour small insects,
and in such minute organisms as yeast, the vinegar ferment, and others, which play
so important a part in our daily life, and lastly, in the gigantic trees which form our
forests.

To acquire a general notion of these forms, with reference to their varieties as
regards nutrition, it is best to classify them in the first place in groups according to
their habitat, viz.: into water-plants or hydrophytes, stone-plants or lithophytes,
land-plants, and epiphytes. But here again it is necessary to remark that no sharp

56 CLASSIFICATION OF PLANTS WITH REFERENCE TO NUTRITION.

line of demarcation exists between these groups; all are connected by numerous
intermediate links, and there are forms which belong to one group at one stage of
development and to another at another stage.

The distinctive property of aquatic plants is that they derive their nourishment
either entirely or principally from the surrounding water. Some preserve their
freedom, floating or swimming about in the liquid medium; but the majority are
fixed somewhere under the water by special organs of attachment. Many plants .
that are rooted in the mud at the bottom of pools are able to derive their food from
the water when it is high, and when it is low, from the atmosphere as well: such
amphibious organisms form a transitional group between water-plants and land-
plants. The number of lithophytes is comparatively very small. They include
those lichens and mosses which cling in immediate contact to the surface of
stones and derive their food in a fiuid state direct from the atmosphere. All
lithophytes are so constituted that they can, without injury, dry up and suspend
their vitality for a time when there is a failure of atmospheric precipitation lasting
over a long period or when the air itself is very dry. But not every plant which
grows upon rocks is to be regarded as a lithophyte in the narrower acceptation of
the term. Those that are rooted in earth in the cracks and crevices of the rock
must be classed amongst land-plants. To this class indeed more than half the
plants now in existence belong. Though surrounded by air as regards a part of
their structure they have another part sunk in the soil, and from the soil they take
up water and inorganic compounds in aqueous solution. Plants which grow attached
to other plants or to animals are called epiphytes.

The majority of plants are during the period of food-absorption connected with
the foster-earth and are not capable of locomotion. The plant being fixed to one
spot must therefore sooner or later exhaust the ground in its neighbourhood, and
must require a further supply of nutritive substances. The parts specially devoted
to food-absorption often lengthen out in these circumstances beyond the im-
poverished region, and thus endeavour to bring areas more and more distant within
the range of absorption. Many plants possess the faculty, to which reference has
already been made, of alluring animals and of killing and sucking their juices. Not
only amongst saprophytes and parasites, but also amongst aquatic plants, instances
occur in which certain movements are performed involving the whole body of the
organism, with a view to promoting the absorption of nutriment. Particularly striking
in this respect are many plasmoid fungi (which we may well refer to here, not on
this account alone, but also for the additional reason that they take in nourishment
without the intervention of a cell-membrane). The naked protoplasm in these cases,
which include in particular the class of Amcebe, crawls in its search for food over
the nourishing substratum, and derives from it immediately the materials needful for
growth. Loose bodies are liable to be seized by the radiating processes of the proto-
plasm, which then closes round them and drains them completely of their juices (see
fig. 9, the last figure to the right). These bodies encompassed by the protoplasm, if
small, are drawn inwards from the periphery and are regularly digested in the

THEORY OF FOOD-ABSORPTION. 57

interior. Such parts of foreign bodies as are not serviceable for nutrition are sub-
sequently eliminated or are left behind by the protoplast as it creeps onward. But
this method of food-absorption is limited to amceboid forms belonging to the
boundary-land of animal and vegetable life. The movements of other naked proto-
plasts, such as those which are carried about in the water by vibratile cilia, have
nothing to do with the search for food or with its absorption, but are connected
rather with the processes of distribution and propagation. .

THEORY OF FOOD-ABSORPTION.

In the case of protoplasts inclosed in cell-membranes the food necessary for
nourishment ‘must always pass through the cell-membrane and peripheral proto-
plasmic layer (ectoplasm) into the interior of the protoplasmic bodies. And s0,
conversely, such of the substances absorbed as are of no use in the construction
of the organism or for any other purpose, must be separated and passed out
through these envelopes. The cell-membranes of those protoplasts which are
employed in absorbing food must accordingly have a special structure: the
ultimate particles must be so arranged as to allow of the passage of nutritious
material inwards, and of rejected matter outwards, without prejudice to their own
stability. The passages in cell-walls used for this purpose are very minute, much
smaller at all events than the pore-canals described above as being occupied by
fine protoplasmic filaments; the dimensions are in fact so trifling as to be invisible
even with the best microscopes. Still we are forced to conclude that they exist
by a posteriori reasoning from a series of phenomena, and to assume that the cell-
membrane, like almost every other kind of body, consists not of continuous matter,
but of minute particles, which are termed atoms, and are separated from one
another by infinitesimally small spaces. Various processes and appearances have
also led physicists and chemists to the conclusion that these atoms are not aggre-
gated in disorder, but are always combined together in groups of two or more,
even in the case where all the atoms in a body are of the same kind, «4. are the
same element. If a body contains different elements they are not mixed together
indiscriminately, but are grouped in conformity to a definite law: every group
includes atoms of all the different elements concerned, arranged in a certain in-
variable manner, not only as regards number, but also as regards relative position.
Groups of atoms of this kind are called “molecules,” and the spaces between them
are supposed to be larger than those between single atoms. Further, it is not
improbable that the molecules themselves form groups, each group consisting of
molecules conglomerated in a definite manner, and that the passages separating
these molecular groups are larger again than those separating the single molecules
within each group. ‘These groups of molecules have been called “micelle” or
Tagmata, and they also are supposed to be aggregated together in definite order.

According to this theory the cell-membrane is analogous to a sieve, the pores
of which are grouped in a definite manner, the broadest perforations being between

58 THEORY OF FOOD-ABSORPTION.

the micelle or groups of molecules, narrower apertures between the molecules
or groups of atoms in each micelle, and lastly the finest pores between the atoms
themselves in each molecule. These interspaces are liable to contraction and
expansion, for the union of the molecules is affected by two forces, one of which
manifests itself as a mutual attraction between atoms and atomic groups, whilst
the other tends to drive atoms and molecules asunder. Of these forces the former,
ie. the attractive force existing in all material particles, is called chemical affinity
when it causes atoms of different kinds to unite to form a molecule; and it is called
cohesion when applied to the mutual attraction of similar molecules, and adhesion
where it holds together masses of molecular groups with their surfaces in contact.
The action of heat is opposed to this attractive force, which is only effective at
infinitesimal distances. Bodies are all caused to expand by heat, their atoms, mole-
cules, and micelle being forced apart. Heat is believed to be a vibratory motion
of these ultimate particles, and it is supposed that the greater the vibrations the
greater is the separation of atoms and atomic groups, the interspaces expanding
and the heated body increasing consequently in volume. As is well known, the
atoms and molecules may be forced so far apart by increase of temperature that
cohesion is entirely overcome, and solids are converted, first into liquids and at
last into gases.

The interspaces or passages between the molecules and molecular groups com-
posing a cell-membrane are penetrable by molecules of other substances, provided
always, firstly, that the admitted molecules are not larger than the passages; and
secondly, that there exists between the molecules of the cell-wall and those of the
penetrating body that sort of attractive force which has been designated chemical
affinity. Both premises are satisfied in the case of aqueous molecules, and experi-
ment proves that they are admitted into the inter-molecular spaces of a cell-
membrane with great ease and readiness. The cell-membrane saturates itself with
water, or, to use the technical phrase, it has the tendency and ability to “imbibe”
water. The force of attraction between molecules of a cell-membrane and water-
molecules is indeed so intense that the cohesion of the molecules in the membrane
is partially neutralized, and the imbibed water causes them to move apart. In
consequence of this, the cell-membrane swells up and its dimensions are increased.

It is also supposed that the micelle of a cell-membrane attract and admit water-
molecules to such an extent as to surround themselves with watery envelopes.
Such a condition would no doubt be nothing but beneficial, promoting, as it would,
the interchange of materials through the cell-membrane, and the mixing of fluid
substances situated on either side of the porous membrane. At all events this
mixing process must ensue in the interspaces of the cell-membrane; and, in the
particular case out of which this discussion has arisen, viz. food-absorption, the
interacting substances are, on the one hand, the compounds in the soil outside
the cell-membrane, and, on the other, the organic compounds under the control
of the live protoplast within the cell-membrane. Both the outgoing and the in-
coming substances must be soluble in water, and must, therefore, have an attraction

THEORY OF FOOD-ABSORPTION. 59

for water. But the power of a substance in aqueous solution, whether without
or within the cell-membrane, to permeate the saturated pores, and to mix thoroughly
there, certainly depends also on the degree of chemical affinity and of adhesion
existing between the molecules and micelle of the cell-membrane on the one hand,
and these infiltrating substances on the other. A very complex interaction of
forces takes place which we cannot here investigate any further, as it would take
us much too far afield.

Returning to the explanation of food-absorption, attention must be drawn to
the fact that the mixing or diffusion which takes place through the cell-membrane
differs from the free diffusion which would occur if the cell-membrane were not
present. Experiment has proved that if one side of a cell-membrane is steeped
in a saline solution and the other in an equal volume of pure water, the number
of saline particles which pass through into the water are many fewer than the
number of water-particles which pass into the solution of salt; and, moreover, if
an organic compound, such as albumen or dextrin, is on one side, and water on
the other, water transfuses to the organic compound, whereas no trace of the
albumen or dextrin (as the case may be) passes through to the water. Now this
phenomenon, which is called “osmosis” (“endosmosis and exosmosis”), is of great
importance for the conception we have to form of food-absorption. It is clear that,
whilst water and substances dissolved in water are brought under the control of
the protoplast within a cell through the cell-membrane, as a consequence of the
action of albuminous and other compounds constituting the body of the protoplast,
and of the salts dissolved in the so-called cell-sap in the vacuoles, there is no
necessity for any part of the cell-content to pass out through the cell-membrane.
Thus the protoplasm is able to exercise an absorptive action on aqueous solutions
outside the cell-membrane, and to continue to absorb until the cell is filled. Indeed,
the chemical affinity for water possessed by the substances in a cell may occasion
so great an absorption of water that, in consequence, the volume of the cell is
enlarged and the cell-membrane is subjected to pressure from within. The cell-
membrane is able to yield to this pressure to the extent permitted by its elasticity;
but excessive stretching of the cell-membrane is at length counteracted by cohesion,
and thus a condition is attained in which the cell-contents and the cell-membrane
are subjected to mutual pressure, a state which is called “turgidity.”

The process just described, of the absorption of water in large quantities into
the precincts of the protoplasm without any simultaneous transmission of matter to
the outside, is certainly in no respect an exchange. But it obviously does not
exclude the possibility of a real exchange taking place between substances on either
side of a cell-membrane, é.c. between solutions in the soil and those in the cell-
sap contained in lacune of the protoplasm. Certain phenomena in fact put it
beyond doubt that on occasion a real exchange of this kind does occur. But it
is complicated by the circumstance that substances in process of being exchanged
have to pass not only through the cell-membrane but also through the primordial
utricle; and the primordial utricle consists of molecules of a kind other than

60 NUTRIENT GASES.

those of the cell-wall, having different chemical affinities, and these molecules
again are differently grouped; nor are the passages for aqueous solutions the same.
All this cannot but have an important bearing on the permeating capacity of
the substances that are being interchanged.

Although all these ideas concerning the molecular structure of cell-membranes
and of protoplasm, concerning the intermixture and exchange of materials and
the absorption on the part of cells and their swelling up, have only the value
of theories, still we have good ground for assuming that they are fairly near
the truth. They give us, at all events, an intelligible representation of the inter-
action which takes place between living protoplasts, with their need for food, and
the environment, which supplies the nutriment.

2. ABSORPTION OF INORGANIC SUBSTANCES.

Nutrient Gases.—Nutrient Salts.—Absorption of Nutrient Salts by Water-plants, Stone-plants,
and Land-plants.—Relations between the position of Foliage-leaves and Absorption-roots.

NUTRIENT GASES.

One of the most important sources of the nourishment of plants is carbonic
acid. The living protoplasts appropriate it from water and from air, in the latter
case chiefly by attracting the carbon-dioxide_ This gas penetrates a cell-wall satur-
ated with water more readily than the other constituent gases of the atmosphere
(nitrogen and oxygen). In the wall it is converted into carbonic acid, and it then
passes on into the cell-sap contained in the cavities of the protoplast. Apart from
the effects of temperature and atmospheric pressure, the quantity of carbonic
acid absorbed is chiefly determined by the requirements of the cells whose nourish-
ment is in question. These requirements, however, vary considerably according
to the specific constitution of the protoplasm and with the time of day. During
daylight the need of carbon is very great in all green plants. As soon as the
carbonic acid reaches the cell-sap it is decomposed and reduced by the action of
sunlight, and from it are formed compounds known as carbo-hydrates. The
oxygen thus set free is, however, removed from the cell precincts, and expelled into
the surrounding air or water. In this way the gas when barely absorbed is
withdrawn, as such, from the cell-sap, the carbon alone being retained and the
oxygen eliminated, and a renewed attraction of carbon-dioxide from the sur-
rounding medium ensues. The fresh supply again is immediately worked up in the
green chlorophyll-bodies, so that there is a constant influx of carbon-dioxide, and
therefore indirectly of carbonic acid, from the environment into the interior of
green cells to the part where its consumption takes place. Were it possible to see

1 The atmosphere contains free carbon-dioxide and not carbonic acid. But carbonic acid is formed when the
dioxide is absorbed into water.

NUTRIENT GASES. 61

the molecules of carbon-dioxide in the air, we should observe how much faster they
are impelled towards the leaves and other green parts of plants, where the intense
craving for carbon is localized, than are the other constituent particles of the air.
This impulsion and influx lasts so long as the green cells are under the influence of
daylight. The first thing in the morning when the first ray of sunshine falls upon
a plant the protoplasts begin work in their little laboratories decomposing carbonic
acid, and producing from it sugar, starch, and other similar organic compounds.
And it is not till the sun sets that this work is suspended, and the influx of carbon-
dioxide stopped till the following morning.

The green plants that spend all their lives under water are supplied with car-
bonic acid by the water surrounding their cells, which always contains some of that
material. In the case of unicellular plants of this class, absorption of carbonic acid
takes place through the whole surface of the cell-membrane. Multicellular plants,
with their cells arranged in filaments or plates, only take in carbonic acid through
those parts of the walls of their cells which are in immediate contact with the
water. This applies also to submerged plants composed of several layers of cells
and of considerable dimensions. Thus, in plants of this kind, the cells in contact
with the water constitute the skin. They are always pressed closely together
and squeezed flat, are not thickened on the side exposed to the water, and are
united everywhere edge to edge leaving no gaps. But in the interior of these
water-plants large lacunz and cavities are formed from earliest youth, owing to
the detachment of single rows of cells; and the spaces so formed are filled with
a quantity of nitrogen, oxygen, and carbon-dioxide, that is to say, with a gaseous
mixture not essentially different from atmospheric air. Although this organiza-
tion may have as its primary object the reduction of the plant’s weight as a
whole, it cannot be without a further importance inasmuch as carbonic acid can
be taken up from the air-spaces into adjacent cells. But there is no doubt that,
even in this case (of water-plants provided with large internal air-cavities),
the chief absorption of carbonic acid is through the epidermis, or more precisely
through those walls of the epidermal cells which are in immediate contact with
the water.

The carbonic acid taken up by cells, wholly or partially immersed in water,
is either contained as such dissolved in the watery medium, or occurs in com-
bination with calcium as bicarbonate of lime. Part of the carbonic acid in this
bicarbonate in aqueous solution is susceptible of being withdrawn by water-plants,
mono-carbonate of lime, which is insoluble in water, being then precipitated on
the cell-wall through which the rest of the carbonic acid has passed into the
eell-interior. Accordingly, a large number of water-plants are found incrusted
with lime in both fresh and salt water. We shall return to this important pheno-
menon when we treat of the influence of living plants on that part of the environ-
ment which comes within their sphere of action for purposes of nutrition.

Lithophytes obtain carbonic acid from the moisture deposited upon them from
the aqueous vapour in the atmosphere; and attract carbon-dioxide direct from the

62 NUTRIENT GASES.

air around them. The chief members of this class are those mosses, liverworts, and
lichens which, though clinging to dry rocks, behave just like water-plants as regards
the absorption of carbonic acid. There is no reason to think that these plants
absorb carbonic acid in dry weather; for under the influence of dry air they lose
water fast, and meanwhile receive no compensation from the rock to which they
are attached, and in a short time they become so dry that they crumble into
powder when rubbed between the fingers. Vitality is suspended for a time, and
it is out of the question that there should be any absorption of carbon-dioxide
from the atmosphere under such circumstances. But the moment the plant is
moistened by rain or dew, the cell-walls directly exposed to the air become
saturated, and are enabled to admit water into the interior. Then the lithophytes
suck up water very fast; the dry, apparently dead, incrustations swell up again,
and, together with the rain and dew, carbonic acid is absorbed, it being contained
in all depositions of atmospheric moisture. A tumescent moss tuft can, in addi-
tion, absorb carbon-dioxide direct from the atmosphere through its saturated
superficial cells; but the quantity of carbonic acid thus acquired by a plant is in
any case only secondary. Many mosses, as for example the widely-distributed Grim-
mia apocarpa, are also able to live just as well under water as in air; nor is any
alteration of their leaves necessary in either condition, nor any special contrivance
for the absorption of carbonic acid and water. These substances reach the interior
by similar passage through cell-walls of identical construction, whether the
Grimmia spends its life attached to submerged rocks or in the open air at the
top of a mountain; whence we may infer that there is a greater resemblance
between lithophytes and water-plants as regards nutrition than between litho-
phytes and land-plants.

Land-plants satisfy their need of carbon almost exclusively by withdrawing
the dioxide from atmospheric air. For the purpose of this direct appropriation,
specially adapted structures are found in them. Seeing that these plants are
not able to endure periodic dessication in times of drought, as lithophytes are,
it is necessary for them to be secured against excessive loss of water. Accord-
ingly, the cell-walls in immediate contact with the air, that is to say, the outer
walls of the epidermis, are thickened by a layer (cuticle) which is impermeable
by air or water, and, in general, they are so organized that water cannot readily
escape from the interior of the cells. Obviously, however, a cell-wall which opposes
a strong resistance to the extravasation of water will not give easy admittance to an
influx either, and the conditions for the passage of gases through a cell-membrane,
thickened and cuticularized in this way, would be far from favourable. As a
matter of fact many of the constituent gases of the atmosphere permeate these
thickened walls of the epidermal cells only with great difficulty, and others not at
all. Carbon-dioxide alone has the power of penetrating, but even in the case of
this gas the quantity is not always sufficient to satisfy the demand. To ensure
that so important a form of plant-food should reach in proper amount those cells
lying under the epidermis, which are occupied by protoplasts engaged in the regu-

NUTRIENT GASES. 63

lation of nutrition, there is an adaptation of structure of the following nature.
Among the firmly connected epidermal cells with their thickened outer walls al-
most impervious to air, other cells are interspersed at intervals. They are always
in pairs, are generally rather smaller than the rest, and have a little cleft open
between them. Inasmuch as these apertures (stomata) always exist where passages
and canals, the so-called intercellular spaces, have arisen from the separation of
individual cells of the sub-epidermal tissues, each stoma constitutes the mouth of a
system of channels ramifying between the thin-walled cells of the interior. The
components of the atmosphere, especially carbon-dioxide, are able to reach these
internal passages through the stomata, and in them they travel to the chlorophyll-
containing cells. Through the thin, saturated walls of these cells they are able to
penetrate with ease, and so they reach the living protoplasts, with their equipment
of chlorophyll, whose daily work it is, as already mentioned, to decompose—under
the transforming power of light—the carbonic acid as it reaches the chlorophyll-
bodies, to work up the carbon and expel by the same path as they entered not only
the oxygen but also all other aerial constituents which may have penetrated and for
the moment find no employment.

These ventilation-canals, with stomata as orifices at the epidermis, have other uses
besides the importation of carbon-dioxide (and therefore of carbonic acid) and the
exportation of oxygen. For the same pores, passages, and lacune, as serve for the
influx and exit of carbon-dioxide and oxygen respectively, are the channels of a
plant’s respiration. Moreover, they play a very important part also in the escape
of aqueous vapour, the process known as “transpiration;” and as the variety in
their structure is to be interpreted chiefly as an adaptation to the different condi-
tions under which transpiration occurs, it cannot be profitably discussed until we
treat of that process.

Those saprophytes and parasites which contain no chlorophyll or practically
none, do not absorb any free carbon-dioxide from the atmosphere, but supply them-
selves with carbon from the organic compounds in the nutrient substratum on
which they grow. But saprophytes and parasites, abundantly furnished with
chlorophyll, doubtless do attract free carbon-dioxide in addition. They may do so
either after the manner of water-plants and lithophytes, as is the case with Euglene,
and with mosses growing on the dung of mammalia; or else after the manner of
land-plants, as instances of which the cow-wheat, yellow-rattle, and eye-bright may
be quoted.

It is a very remarkable fact that no plant is known which takes up carbon-
dioxide or carbonic acid from the earth, One might expect that the roots of land-
plants at any rate, ramifying as they do in a stratum of earth saturated with water
containing carbonic acid in solution, would suck up to some extent so important a
food, and that it would be from them conducted to the green-foliage leaves. But
so far as experiments have gone, they indicate that this is not the case.

Equally curious is the circumstance that nitrogen, which is an indispensable
constituent of protoplasm, and therefore a very important means of subsistence, is

64 NUTRIENT GASES.

not absorbed from the surrounding air, although, as is well known, the atmosphere
contains nitrogen to the amount of 79 per cent of its volume. There can be no
doubt that though nitrogen permeates the cell-walls of an air-encompassed plant
much less readily and quickly than carbon-dioxide, yet it is carried from the atmos-
phere into the ventilation-spaces of green foliage-leaves, and further through the thin
cell-walls into the laboratories of the protoplasts, where one would expect it to be
worked up in the same way as carbonic acid. The most careful experiments have
determined, however, that it is not turned to account in this form by the proto-
plasts, but that on the contrary it 1s given back unused to the air, and only such
nitrogen as reaches the interior of plants in combination with other substances is of
any service there.

The principal sources of the nitrogen required by plants are nitrates and
ammoniacal compounds absorbed from the ground; but nitric acid and ammonia
themselves, of which there are traces in the atmosphere and in water, must not be
overlooked. The quantity of nitric acid in air is, it is true, even less than that
of carbon-dioxide; but just as the small amount of carbon-dioxide can be absorbed
from the air with highly productive results, so may also the still smaller proportion
of nitric acid be turned to account. The sources of nitric acid are dead organic
bodies as they decompose and become oxidized. In many ways the process of
formation of nitric acid from decaying bodies may take place so as to produce
ammonia in the first place and from it nitric acid. It would seem possible, though
it is an unproved assumption, that in places where dead bodies of plants and animals,
vegetable mould, manure, and such things are undergoing oxidation, that is to say,
in woods and fields, the small quantities of nitric acid that are given off are imme-
diately taken up by the plants growing there. It must be borne in mind that plants
behave with reference to what is necessary or useful to them like a chancellor of
the exchequer preparing his budget; they take these things where they find them.

The question has been raised, too, as to the source from which the first plants
that appeared on the earth were able to obtain nitric acid. We are obliged to
assume that, at that time before the existence of nitrogenous organisms to supply
nitric acid by oxidation of their dead bodies, all nitric acid, and therefore all the
nitrogen used in the nourishment of plants, was generated by thunder-storms. We
know that nitric acid is formed in the air on occasion of electric discharges and is
deposited on the earth together with rain and dew. This source of nitric acid is
not yet exhausted, and even at the present day it no doubt plays the same part as
in the ages long past at the commencement of all vegetable life.

If nitric acid is used by protoplasts, in the building up of the highly important
albuminous compounds, it is broken up in a manner similar to the decomposition
of carbonic acid to form carbohydrates, that is to say, oxygen is separated out.
In this case, however, sunlight and, therefore, chlorophyll are not immediately con-
cerned. Moreover, the oxygen that is set free is not eliminated, but is used in the

manufacture of other compounds in process of formation in the plant, probably in
that of vegetable acids.

NUTRIENT GASES. 65

Ammonia behaves in relation to plants just in the same way as carbon-dioxide
and nitric acid. It is disengaged from dead decomposing organic bodies, and is
found in traces, either alone or with equally minute quantities of carbon-dioxide
and carbonic and nitric acids in the air, in atmospheric deposits, and in all water
wherein animals and plants reproduce their kind, the old individuals dying and
making way for the young. Water-plants are all limited to this source for acquisi-
tion of nitrogen. As regard lithophytes, it stands to reason that they must derive
their nitrogen from the ammonia contained in the air, in atmospheric deposits,
and from nitric acid. Whence otherwise could a crustaceous lichen attached to a
quartz rock on a mountain supply itself with the nitrogen essential for the growth
of its protoplasm? Moreover, some of the larger lithophytes, especially mosses,
seem to be capable of absorbing ammonia direct from the air. An observation
made in the Tyrolese Alps has some bearing on this question:—The ridges of the
Hammerspitze, a peak rising to 2600 meters between the Stubaithal and the
Gschnitzthal, is, in favourable weather in the summer, the resting-place of hun-
dreds of sheep, and is consequently covered with an entire crust of the excrements
of these animals. A highly offensive and pungent smell of ammonia is evolved, and
renders a prolonged stay on this spot anything but pleasant, notwithstanding the
beauty of the view. Now, it is worthy of note that the mosses, which are produced
in abundance on the rocks above this richly-manured ground, but are not them-
selves actually amongst the sheep-droppings, exhibit a luxuriance unparalleled on
any of the neighbouring summits belonging to the same formation but unfre-
quented by sheep. The gaily-coloured green carpet extends as far as the ammo-
niacal odour is perceptible, and it is natural to suppose that this luxuriant growth
is stimulated by the absorption of ammonia direct from the air.

Land-plants also can take up ammonia from the air. It has been shown that
the glandular hairs of many plants, for instance those on the leaves of Pelargonium
and of the Chinese Primrose, have the power of absorbing traces of ammonia, and
of sucking up carbonate and nitrate of ammonia in water with rapidity. When we
consider that a single one of these primroses (Primula sinensis) possesses two and
a half millions of absorbent glandular hairs so placed as to be able to take up the
ammonia brought to the plant by rain, we are unable to look upon this process as
of altogether trifling importance. It is highly probable that almost all ammonia,
after its formation from decaying substances in the ground, is at once absorbed by
the plants growing in the immediate neighbourhood, and that the relatively small
quantity of ammonia in the upper atmospheric strata 1s referrible to this cause.
The splendid luxuriance of the pelargoniums, thickly studded with glandular hairs,
which one sees in front of cottage windows in mountain villages where a dung
heap is close by, and in the windows of stables, frequently excites admiration and
surprise. Whether it is due to the fact that in these situations there is the possi-
bility of absorbing an unusually large quantity of ammonia is a question which we
will leave undecided.

Vou. I. 2

66 NUTRIENT SALTS,

NUTRIENT SALTS.

If wood, leaves, seeds, or any other parts of plants are subjected to a high
temperature with free access of air, the first changes that occur are in the com-
pounds of nitrogen and of carbon contained in the heated matter. They turn
black, are charred and burnt, and ultimately the products of combustion pass into
the atmosphere in gaseous condition. The incombustible part which remains
behind is called the “ash.” The quantity of this ash, as well as its composition,
varies very much in different species of plants, and even in different parts of the
same plant. Generally the weight of ash is only one or two per cent of the entire
weight of the plant in a dry state before burning. The greatest relative proportion
of ash is that which is obtained from the combustion of those hydrophytes which
live in the sea; and next in quantity is the ash of the family of Oraches which
abound on salt-steppes. On the other hand, the smallest quantity is that afforded by
fungi and mosses, by Sphagnum in particular, and with these must be mentioned
the tropical orchids living on the barks of trees. Seeds and wood yield relatively
much less ash than leaves. But, as above remarked, some ash is formed upon the
combustion of any part of a plant or even of a single cell, and this residue of ash
sometimes allows of our recognizing exactly the size, form, and outline of the cells.
The universal distribution of ash-forming constituents permits us to conclude with
certainty that they do not exist fortuitously in plants, but are essential to them.
That these constituents are indispensable may also be proved directly. If an
attempt is made to nourish a plant on filtered air and distilled water exclusively, the
plant soon dies; but if a small quantity of the constituents of its ash are added to
the distilled water in which the roots are immersed, the plant grows visibly in the
solution, and develops leaves and flowers and even seeds capable of germination.

Experiments of this kind with cultures have been the means of almost com-
pletely establishing the division between those constituents which are indispensable
for all plants, and those which are only necessary under certain conditions and to
particular species, or, still less, only beneficial. Those elements must be regarded
as essential, which are used by plants for the process of construction, and enter
into the composition of the protoplasm or of the cell-membrane—such, for instance
as are essential constituents of proteid substances, or are in some way necessary
to the formation of these products. Amongst these must be included sulphur,
phosphorus, potassium, calcium, and magnesium. Some plants, especially those
that live in the sea, require sodium, iodine and chlorine, and, for green plants, iron
is necessary. Silicon is also very important for most plants in helping them to
flourish in the wild state. Most of these elements are taken into a plant, in the
covrse of nutrition, in a condition of extreme oxidation, that is to say in combina-
tion with a quantity of oxygen; in fact, as a general rule, they are absorbed in
the form of salts, and we may for the sake of brevity include all the mineral food-
stuffs under the name of nutrient salts or food-salts,

NUTRIENT SALTS, 67

It is obvious that food-salts can only pass through cell-membranes and reach
the interior of a plant in a state of solution. On this account the soluble sul-
phates, phosphates, nitrates and chlorides of calcium, magnesium, potassium and
iron, may pre-eminently be called food-salts, Whether an essential element is
absorbed by a plant in the form of one of these compounds or another appears
to be unimportant; phosphorus, for example, may be proffered by the soil in the
form either of potassium phosphate or of sodium phosphate, with like results.
As regards the importance of sulphur to plants, it is at any rate established that
it is necessary for the production of proteid substances. Phosphorus appears to be
indispensable in the transformation of certain compounds of nitrogen. Potassium
is supposed to play a part in the formation of starch. Calcium is introduced into
plants in combination with sulphuric acid as calcium sulphate. This salt is decom-
posed, the lime combining with oxalic acid to form insoluble calcium oxalate, and
the sulphur going to form the sulphuric acid which is used in the construction of
albuminous substances or proteids. Lime is therefore important, inasmuch as
it is a medium of transport for sulphur. Iron certainly participates in the forma-
tion of chlorophyll, even if it does not enter into its composition, as was formerly
supposed. For, it has been proved, by means of artificial cultures, that plants reared
in solutions free from iron were white instead of green, and died at last; whereas,
after the addition of a small quantity of a soluble iron salt, such plants became green
in a very short time, and were able to continue their development. The utility of
most of these elements does not therefore appear to consist necessarily in their
entering into the composition of organic compounds, but in the promotion and
regulation of the constructive and destructive chemical processes.

Silicic acid, which occurs so plentifully in the ash of many plants as to con-
stitute often more than 50 per cent, has a different function. If the minute
unicellular water-plants known as Diatoms are incinerated, or if stems of Equisetum,
Juniper-needles, or leaves of grasses, &., are subjected to a red heat, white skeletons
remain behind which consist almost entirely of silicic acid, and exhibit not only
the forms of the cells, but even the finest sculpturing of the cell-walls. In par-
ticular, the stiff hairs on the leaves of grasses are preserved, and better still the
cell-membranes of diatoms. The latter present very beautiful forms with their
outlines quite distinct, and many structural properties of the cell-membranes,
especially their moulding, striation, and the dots and other excrescences are to be
seen much more clearly after than before ignition, when the transparency was less
owing to the protoplast occupying the interior of each cell. In order to describe
exactly the very varied form of Diatomacezx, specimens are carefully and thor-
oughly ignited, and the descriptions and illustrations of these microscopic plants
are for the most part made from siliceous skeletons prepared in this way. These
skeletons show clearly that silicic acid occurs only in the cell-membrane, and plays
no part as constituent of any chemical compound in the protoplasm; nor does it
appear to be instrumental in the formation of any such compound. The molecules
of silicic acid are so closely packed and so evenly distributed amongst the mole-

68 NUTRIENT SALTS.

cules of cellulose that, even after the removal of the latter, the entire structure is
preserved in outline and in detail. They form, therefore, a regular coat of mail
which may be looked upon as a means of protection against certain injurious ex-
ternal influences.

For a large number of plants living in the sea, sodium, iodine, and bromine also
are of especial importance as food-stuffs. How far fluorine, manganese, lithium,
and various other metals, which have been detected in the ash of some plants, are
of use is not determined, for our knowledge is particularly incomplete with respect
to the various uses subserved in nutrition and growth by the different mineral
food-stuffs. It is worthy of note that alumina, which is so widely distributed and
easily accessible to plants, is only very rarely absorbed. The ash of Lycopodium
is the only kind in which this substance has been identified with certainty in any
considerable quantities.

Lastly, amongst the sources of elements contained in the food-salts, we must
consider the solid crust of the earth. But it is only in the case of comparatively
few vegetable organisms that this earth-crust forms the immediate foster-soil.
The majority derive the salts that nourish them from the products of the weather-
ing of rocks, from refuse and the decaying remains of dead animals and plants,
which, in decomposing, give back their mineral substances to the ground, from
underground waters that filter through fissures in rocks and through the interstices
of sandy or clayey soils soaking with lye, the adjacent parts of the earth’s crust,
and, lastly, from the water of springs, streams, ponds, and lakes, which have come
to the surface holding salts in solution, as also from sea-water with its rich supply
of salts.

The very salts that are needed by most plants are amongst the most widely
distributed on the earth’s surface. The sulphates of calcium and of magnesium,
for example, and salts of iron, potassium, &., are found almost everywhere in the
earth, and in water, whether subterranean or superficial. At the same time it is
very striking that these mineral food-salts are not introduced into plants by any
means in proportion to the quantity in which they are contained in the soil, but
that, on the contrary, plants possess the power of selecting from the abundance of
provisions at their disposal only those that are good for them and in such quantity
as is serviceable. This selective capacity of plants is manifested in many ways, and
we will now briefly consider some of the most important of them.

In the first place we have the fact that plants reared close together in the same
soil or medium may yet exhibit an altogether different composition of ash. This
is particularly striking in water and bog-plants, which, though rooted in close
proximity and immersed in the same water, show very considerable differences in
respect of mineral food absorbed. The result, for instance, of testing specimens of
the Water-soldier (Stratiotes aloides), the White Water-lily (Nymphoa alba),
a species of Stone-wort (Chara fatida), and the Reed (Phragmites communis), all
growing close together in a swamp, was as follows as regarded the potash, soda,
lime, and silicic acid, held by them respectively :—

NUTRIENT SALTS.

69

Water-soldier. Water-lily. Stone-wort. Reed.
Potashivis cesseacassenaaiaeiteieesaiineyi aces 30°82 14:4 0°2 86
Soda,.. 27 29°66 O01 0-4
Lime, sheets 10°7 189 54°8 59
Bilicic ACid,.ssccciissrscuscessssuseeeseces 18 05 03 715

The other constituents of the ash of these plants, in particular iron oxide, mag-
nesia, and phosphoric and sulphuric acids, exhibited less marked differences; but
the inequality in the amounts of potash, soda, lime and silicic acid are so great,
as only to be explicable on the assumption of a power of selection on the part of
these plants. Various species of brown and red sea-weeds, which had been attached
to the same rock and developed in the same sea-water, showed similar variations
in the composition of their ash.

On the mountains of serpentine rock near Gurhof, in Lower Austria, specimens
of Biscutella levigata and Doryeniwm decumbens were collected from plants
growing together, and one above the other, upon a declivity which they clothed.
Their roots, interlaced here and there, were fixed in the same ground, and drew
nourishment from the same store. The following table gives the composition of
the ash in these two species:—

Biscutella Dorycnium Biscutella Dorycnium

levigata. decumbens. levigata, decumbens.
Potash,........0.0000 96 16°7 Silicie Acid,........... 130 63
TAM ey seceisececveriine 14°7 20°9 Sulphur, ...........0.6. 52 16
Magnesia,........... 28:0 19°6 Phosphorus,........... 159 22°3
Tron Oxide,......... 78 2°8 Carbonic Acid,....... 54 97

The differences here seem to be not so great as in the case of the water-plants
previously given, but they are sufficient to prevent our regarding them as merely
the result of chance.

If, on the other hand, we compare the composition of the ash of different
specimens of the same species, which have been reared on similar soils, but at
great. distances from one another, the discrepancies are comparatively slight.
Foliage from beech-trees growing on the limestone mountains near Regensburg
yielded an ash practically identical with that obtained from leaves of beeches on
the Bakonyer-Wald hills in Hungary. The ash of different individuals of a single
species even exhibits the same constitution, in the main, when those individual
plants have obtained their nutriment from soils differing greatly in chemical
composition. Only in cases where the quantity of a substance in one soil is
more abundant than in the other there is generally a greater or less amount of it
to be found in the ash.

That under these circumstances certain substances may replace one another is not
improbable. But such substitution must be confined to those nearly allied com-
pounds whose molecules are capable of being used indifferently by the formative

70 NUTRIENT SALTS.

protoplasm in construction, and in the storage of materials. The annexed table,
which gives side by side analyses of the ash of branches of the Yew (Tawus baccata)
with their leaves attached, illustrates the replacement of calcium by magnesium :—

Ash from branches and leaves of the Yew from
Serpentine. Limestone. Gneiss.
SiGe ACI, cic sacnesencceidansnusd oacsmancaeamecsl 3°8 3°6 37
Sulphurie Aig, sieves scowessanains ssseganasesicaie 19 16 19
Phosphoric Acid, .............ccsseseseeseeeeeres 8°3 55 42
Tron OXIME S.,. osicsentiecantvseckeciennssbeatienas eae aa gee ene
TATA Gs ears dai aust ean aera iattntecaresisieh Oni lost . ‘ . fl $
Magnesia 22°7 f 388 51 } 412 57 } Seo
Potash, ......... 29°6 21°8 27°6
Carbonic ACid, ..........ccceeceeeeeeeee 141 23'1 244
Traces of Manganese, Chlorine, & = — —
Totals; siecceascmsanssaewans 99°6 98'5 98-7

The Yew occurs in Central Europe on very various mountain formations, chiefly on
limestone, but not infrequently on gneiss, and occasionally on serpentine rocks.
On comparing the quantities of calcium and of magnesium in the ash of yews, grown
on lime and on gneiss respectively, with those yielded in the case of serpentine for-
mation, we find that magnesia preponderates considerably in weight over lime in a
yew from serpentine rocks (which are in the main a compound of magnesia and
silicic acid), whilst the proportion between these two salts is reversed in a yew
grown upon limestone. The obvious inference from the table is that, in plants from
a serpentine ground, lime is to a great extent replaced by magnesia. This is fur-
ther supported by the circumstance that if lime and magnesia are counted together
the resulting numbers are very near one another, namely 41:2 per cent of the ash
for limestone, 38°8 per cent for serpentine rock, and 36:3 per cent for gneiss.

But all these phenomena observed in connection with the selection of food-salts
are not nearly so surprising as the fact that plants are also capable of singling out
from an abundance of other matter particular substances, which are of impor-
tance to them, even from a soil containing them in barely perceptible quantities, and
of concentrating them to a certain extent. As has been shown above, nearly a
third of the ash of the white water-lily is composed of common salt. One might,
therefore, suppose that the water in which water-lilies flourish contains a particu-
larly large quantity of common salt. But nothing of the kind is the case. The
bog water which bathed the stem and leaves of this specimen only contained 0:335
per cent of common salt, and the mud through which the roots straggled contained
only 0-010 per cent.

No less astonishing is it to find Diatomacex, with cell-membranes, as above
mentioned, sheathed in silicic acid, existing in water which contains no trace of
silicic acid. Above the Arzler Alp, in the Solstein chain near Innsbruck, there is a
spring of cold water which falls in little cascades between blocks of rock. The

NUTRIENT SALTS. 71

water of this spring is hard, and it deposits lime at a little distance from the source.
Exactly at the spot where it wells out of a fissure in the rock its bed is entirely
filled by a dark-brown flocculent mass which consists of millions of cells of the
beautiful Odontidiwm hiemale, a species of diatom with siliceous coating. These
cells are ranged together in long rows, and are present in numbers and luxuriance
such as are scarcely ever to be observed in other situations. Yet the spring water
flowing round contains so little silicic acid that no trace of this substance could be
discovered in the residue from the evaporation of 10 litres.

An instance similar to this of silicie acid, is afforded by the iodine in the sea.
Most of the sea-wracks inhabiting the North Sea contain iodine, many indeed in
considerable quantity, and yet we have not hitherto succeeded in detecting iodine in
the water of the North Sea. Similar phenomena, sometimes quite baffling explana-
tion, are exhibited by land-plants. The clefts in the rocks of quartziferous slate in
the Central Alps are, in many places, overgrown by saxifrages (Saxifraga Sturmiana
and Sasxifraga oppositifolia) with leaves aggregated together in closely-crowded
rosettes, which are conspicuous from afar, owing to their pale colouring. On
closer inspection one finds that the apices and edges of these rosulate leaves are
covered with little incrustations of carbonate of lime, a substance which will be
frequently referred to in connection with its importance to plants. But one seeks
in vain for any lime compound in the earth which fills the clefts, and the only
traces of lime contained in the adjacent rock itself are those occurring in the little
scales of mica scattered about, and these are not readily decomposable. Yet the
lime incrusting the saxifrage leaves can only be derived from the underlying rock,
just as in former instances the silicic acid in the cell-membranes of diatoms
must be secreted from the spring described, the iodine in sea-weeds from the
sea, and the common salt in water-lilies from the pond where they grow, although in
each case the substance concerned is only to be found, if at all, in scarcely ponder-
able traces in the soil or liquid serving as medium. Facts of this kind have a
special interest, because they prove that plants have the power of appropriating a
substance, if it is important to them, even when it is only present in extremely
minute quantities. Where a plant is surrounded by liquid, we can well imagine
that fresh portions of the medium are constantly coming into contact with its
surface; for, even in water apparently still, compensating currents are con-
tinually being caused by changes of temperature. Thus, in the course of a day,
thousands of litres of sea-water may flow over a sea-weed with a surface of
one square meter, and, even if only a small portion of the substance, traces of
which we are supposing to exist in the water, is wrested from each litre, still,
the absorbing plant might collect quite a profitable quantity in a number of
days. The volume of water flowing over a plant situated in the source of a
spring is still greater, and it is readily conceivable that even the most minute
trace of silicic acid may become of account in course of time. There is more
difficulty in understanding how plants with roots in the earth set about utilizing
substances contained in the soil in scarcely appreciable quantities. These plants

72 NUTRIENT SALTS.

must at all events come into contact with as great a mass of nutrient soil as
possible, and this is effected by means of a widely-ramifying system of roots;
and, in addition, they must assist in making available desirable matter in the
soil by the elimination from themselves of certain substances.

In order to explain the remarkable power that plants possess of exercising
a choice in the absorption of certain food-stuffs from amongst the whole number
presented to them, we must in the first place assume a special structure to exist
in the cells which are in immediate contact with the nutrient medium. To
reach the interior of a cell, the salts must pass through the cell-membrane and
the so-called ectoplasm. We may look upon these walls, that are to be pene-
trated, as filters, or, to abide by our previous simile, as sieves, which allow only
certain kinds of molecules to pass and arrest others. Moreover, just as the
structure of a sieve, especially the size and shape of its pores, has its effect in the
separation of the particles of the matter sifted, so also may the structure of a
cell-wall have a discriminating influence in the absorption of food-salts. It may
be supposed that the cell-wall in one species of plant acts as a sieve capable of
letting through molecules of potash but none of alumina, whilst the cell-wall in
a second species allows molecules of alumina to pass as well, but is impervious to
those of chloride of sodium. This hypothesis would also explain why the absorp-
tion of food-stuffs by plants generally takes place through cell-walls, and why
absorption into the organs concerned by means of open tubes, which would be
at all events a much simpler method, is not preferred. It is, however, necessary
to investigate first the nature of the force which causes molecules of the various
salts to move from the soil to the cell-membranes, which we suppose to be like
sieves, and through them into the interior of a plant. A force acting in this
sense from without is inconceivable, and we must therefore look for the motive
stimulus in the plant itself.

As has been already stated in connection with the absorption of carbonic acid,
it is believed that the cause of this movement is the disturbance of the molecular
equilibrium in the growing vegetable organism. If at one spot in the protoplasm
of a cell a particular substance is altered, and, let us say, converted into an
insoluble compound, the previous grouping of molecules appears to be altered, or
in other words, the molecular equilibrium is disturbed. To restore equilibrium,
there must be a re-introduction of molecules of the material that has been removed;
and the attraction of them from the quarter where they occur in a fluid, that is
to say in a mobile condition, is the more energetic. Supposing, for instance,
gypsum (te. sulphate of lime) is being decomposed within a cell, and the lime
combines with the oxalic acid (set free in the same cell) to form insoluble oxalate
of lime, whilst the sulphur combines with other elements to form insoluble
albuminoids, this use of the gypsum occasions a violent attraction of that sub-
stance from the environment, or, to put it another way, it causes a movement of
gypsum towards the place of consumption. If this latter place is a cell in imme-
diate contact with the nutrient substratum, the absorption of the substance

NUTRIENT SALTS. 73

attracted is direct; but if the cell in which the material is used up is separated
from the substratum by intervening cells, the attraction must act through all those
cells upon it. The substance consumed must be taken in the first place from the
cell adjoining the consuming cell on the side towards the periphery; this cell again
must take it from its neighbour, which is still nearer the periphery, and so on
until the external cells themselves exercise their influence upon the nutrient sub-
stratum. Thus, one may regard the growing cells in which substances are used
up, as centres of attraction with respect to those substances. This also explains
why it is that the influx of food-salts takes place only so long as the plant is grow-
ing; and we see, too, that the direction of the current must vary according to the
position of the growing cells, and according to the degree of their constructive
activity.

But that one plant prefers one substance and another another—that one species
attracts iodine, a second sodium, and a third iron—can only be interpreted as a
result of the specific constitution of the protoplasm. The protoplasm of a growing
cell which contains no iodine does not require that substance either, for the pro-
cesses of transmutation and storage. A protoplast of this kind will not therefore
be a centre of attraction for iodine, but will draw from the environment with
great force substances which are its essential constituents. Having gained this
conception of the absorption and selection of food-salts, we are able to imagine
the possibility of a substance being sought after by one species whilst acting as
poison on another. Iodine itself exercises a prejudicial effect on many plants,
even when present in very small quantities. Cell-membranes in immediate contact
with a medium containing iodine are modified as regards their structure by the
iodine: their pores are enlarged, lose their value as orifices adapted to the admit-
tance of certain food-salts in limited quantities, and they no longer prevent the
influx of injurious substances. Ultimately they die, and by so doing the entire
plant suffers. On the other hand, plants to which iodine is an indispensable
constituent are not hurt in any way by the presence of small quantities of this
substance in the nutrient medium: their cell-membranes are neither paralysed
nor destroyed, and suction is able to take place through them in a perfectly normal
manner. But we must in this case specially emphasize the condition of the amount
being small, for a larger quantity of this substance is positively injurious even to
plants which require iodine.

The general rule for a great number of plants is that they thrive best when the
food-salts necessary to them are supplied in very dilute solutions. An increase in
the quantity of the salts administered not only fails to promote development, but,
on the contrary, arrests it. This is the result even if the salts are such as are
absolutely necessary in small quantities to the plants in question. A very minute
amount of an iron salt is indispensable to all green plants; but, if a certain
measure is exceeded, iron salts have a destructive effect on the cell-membranes and
protoplasm, and cause the plant to die. But at what point the boundary lies
between salubrious effects and the reverse, where the beneficial action of particular

74 NUTRIENT SALTS.

substances ceases and detrimental action begins, is not known more precisely than
has been stated. We only know that different plants behave very differently in
this respect. Suppose, for example, that we scatter wood-ash over a field which is
overgrown by grasses, mosses, and various herbs and shrubs. The result is that the
mosses die; in the case of the grasses growth is somewhat increased; whilst some of
the herbs and shrubs, notably polygonaceous and cruciferous plants, exhibit a strik-
ingly luxuriant growth. If we scatter gypsum instead, the development of clover
is enhanced, and, on the other hand, there are certain ferns and grasses that die
earlier when gypsum is supplied, or, at least, are considerably stunted in their
growth.

The fact that certain plants predominate on caleareous and others on siliceous
ground has been the subject of very thorough investigation; and these researches
were regarded as justifying the assumption that particular species require a more or
less considerable quantity of lime for food, whilst others require similarly silicic
acid. Hereupon was founded a division of plants into those which required and
were tolerant of lime, and into such as required and tolerated silica. The explana-
tion given of these facts does not seem, however, to be satisfactory, at any rate in
the case of siliceous plants. It is much more probable that the so-called silica-
loving plants are produced on ground composed of quartz, granite, or slate, not by
reason of the abundance of silicic acid, but because of the absence of lime in any
large quantity, such as would be liable to injure plants of the kind; for only traces
of lime are found, and its presence to this extent is absolutely necessary for every
plant. This is not of course inconsistent with the fact that individual species
require larger quantities of particular food-salts and only flourish luxuriantly when
these nutritive salts are not meted out too sparingly. In the case of oraches,
thrifts, wormwood species, and cruciferous plants, alkalies, in comparatively large
quantities, are necessary for hardy development. The proper habitat for these
plants, therefore, is on soils which contain an abundance of easily soluble
alkaline compounds, in places where the ground is regularly saturated by saline
solutions, and where crystals of salt effloresce on the drying surface. Such places
are the sea-shore, the salt steppes, and the neighbourhood of salt-mines. The
above plants not only flourish in these localities in great abundance and perfection,
but they supplant all other species on which the excessive provision of soluble
alkaline salts is not beneficial. If the seeds of such plants happen to fall upon the
salt ground they germinate, but only drag out a miserable existence for a short
time, and in the end are crowded out by the luxuriant oraches and crucifers.
Plants which only flourish abundantly on soils rich in alkaline salts are called
halophytes. The same name has also been applied to plants which only thrive in
sea-water. Most of the species used by us as edible vegetables, as, for instance,
cabbages, turnips, cress, &., are really descended from halophytes, and accordingly
require a soil that contains a comparatively rich supply of alkalies. An oppor-
tunity will occur, later on, of returning to the question as to how far agriculture
has gained by all these discoveries, and of considering what processes, based upon

ABSORPTION OF FOOD-SALTS BY WATER-PLANTS. 75

the results of scientific research, have been introduced into practice. Amongst
these processes may be mentioned the rotation of crops, the artificial application of
manure to exhausted land, and the restitution of the mineral food-salts which the
particular plants last cultivated have withdrawn from the land under tillage.

ABSORPTION OF FOOD-SALTS BY WATER-PLANTS.

It is usual to designate all plants that grow in water as hydrophytes or water-
plants. But in their narrower sense these names are only applicable to those plants
which, during their entire lives, vegetate under water and derive their nutriment,
especially carbonic acid, direct from the water. A number of plants have widely
ramifying roots fixed in the earth at the bottom of water, and the lower parts of
their stems, either temporarily or throughout life, immersed in water, whilst the
upper parts of their stems and their upper leaves are exposed to the air and take
carbonic acid direct from the atmosphere, and these should be regarded as marsh-
plants and classed with land-plants so far as regards food-absorption. Reeds and
rushes, water-fennel and water-plantain, the yellow water-lily, even the amphibious
Polygonum and the white water-lily, are marsh-plants and not true hydrophytes.
It is characteristic of all these marsh-plants, that if they are entirely submerged
for any length of time they die, whereas they are not injured if the water’s level
at the place where they grow sinks so as to expose the lower portions of the stem.
In places formerly submerged, but from which, in course of time, the water has
retreated, so that they have been turned into meadows, one may come across not
only clumps of reeds and rushes but even yellow and white water-lilies, flourishing
perfectly on the moist earth.

Water-plants, or hydrophytes in the proper acceptation of the term, perish
if they are kept for a length of time out of their proper medium and exposed to
the air. In most of them death ensues quickly, for their delicate cell-membranes
are not able to prevent the exhalation of water from the interior of their cells;
and, there being no provision for a replacement of the evaporated fluid, the
whole plant dries up. If one supplies aquatic plants, thus desiccated, with
water, though it is indeed absorbed it no longer has the power of reviving them.
Those hydrophytes which occur in the sea, near the shore, are able to stand
exposure to the air for a comparatively long time, and they are regularly sub-
ject to it during ebb-tide. Sea-wracks which at high-tide were floating in the
water are then seen lying on the dry rocks or sand of the shore. But the mem-
branes of the cells forming the outermost layer in all these sea-wracks is very thick.
They retain water staunchly and prevent the plants from drying up, at least until
high-tide occurs again, when they are once more submerged.

Amphibious plants in which the lower leaves are like those of aquatics and the
upper like those of land-plants so far as desiccation is concerned (e.g. several kinds
of pond-weed—Potamogeton heterophyllus and P. natans—and a few white-flowered
Ranunculi— Ranunculus aquatilis and R. hololeucus), exhibit a transition stage from

76 ABSORPTION OF FOOD-SALTS BY WATER-PLANTS.

aquatic plants to land-plants. When the water sinks and they are finally left lying
exposed on the mud or wet sand, to which they appear to be firmly attached by
their abundant roots, it is only the previously submerged leaves that dry up. That
part of the foliage which floated on the surface and was consequently always in
contact with the air continues to thrive, and any fresh leaves that may be developed
adapt themselves completely to the new environment. Similar behaviour is ob-
served in many of the plants which float freely on the surface of water. Such, for
instance, is the case with some species of duckweed (Lemna minor and JL.
polyrrhiza), with Azolla, Pontederia and Pistia; they do not die when the water
sinks, leaving them stranded, but absorb food-stuffs from the wet earth through
their roots, and in this condition are not to be distinguished from land-plants.

Hydrophytes in the narrow sense, 1.c. plants which are entirely submerged and
die if they are surrounded by air instead of water for any length of time, are for
the most part fixed to some support beneath the water. In many cases the
characteristic method of reproduction consists in the separation of special cells,
which then swim about for a time in the water. Sooner or later, however, they
re-attach themselves to some seemingly suitable spot, and the further phases of their
development are again stationary. Comparatively few permanently submerged
species are freely suspended in the liquid medium in every stage of development.
Such free plants are liable to be shifted by currents in the water, but the extent of
their displacement is never very great, owing to the fact that submerged species of
this kind occur almost exclusively in still water. As instances may be mentioned
the ivy-leaved duckweed (Lemna trisulea), the water-violet (Hottonia palustris),
the various species of hornwort (Ceratophyllum), in all of which roots are absent;
and in addition amongst the lower or cryptogamic plants Riccia flwitans, and
many of the Desmidiacex, Spirogyras and Nostocines.

Some of these aquatic plants periodically rest on the bottom of the pond or
lake in which they live. An example is afforded by the remarkable plant known
as the water-soldier (Stratiotes aloides), which, as is indicated by its Latin name,
is not unlike an aloe in appearance. During the winter, this plant rests at the
bottom of the pond it inhabits. As April draws near, the individual plants rise
almost to the surface and remain floating there, producing fresh sword-shaped
leaves and bunches of roots which arise from the abbreviated axis, and finally flowers
which, when the summer is at its height, float upon the surface. When the time of
flowering is over, the plant sinks again to mature its fruit and seeds, and develop
buds for the production of young daughter-plants. Towards the end of August, .
it rises for the second time in one year. The young plants that have meantime
grown up resemble their parent completely, except that their size is smaller.
They grow at the end of long stalks springing from amongst the whorled leaves,
and the stately mother-plant is now surrounded by them like a hen by her chickens.
During the autumn, the shoots connecting the daughter-plants with their parent rot
away, and, thus isolated, each little rosette, as well as the mother-plant, sinks once
more to the bottom of the pond and there hibernates,

ABSORPTION OF FOOD-SALTS BY WATER-PLANTS. 77

Altogether the number of submerged plants which live suspended in water is
very small. As has been said before, by far the greater number are attached some-
where. Seed-bearing plants or Phanerogamia, such as Vallisneria, Ouviranda,
Myrvophyllum, Najas, Zannichellia, Ruppia, Zostera, Elodea, Hydrilla, and several
species of Potamogeton (P. pectinatus, P. pusillus, P. lwcens, P. densus, P. crispus);
as also Cryptogams, such as the various species of Isoetes and Pilularia and sub-
merged mosses, are fastened in the mud under water by means of attachment-roots
or of rhizoids, whilst the almost illimitable host of brown and red sea-weeds are
fixed by special cells or groups of cells, which are often root-like in appearance.
The sea-weeds choose rocks and stones, by preference, for their support, but they
also make use of animals and plants. The shells of mussels and snails are often
completely overgrown by brown and red sea-weeds. Larger kinds of Fucacee,
especially the species of Sargasswm and Cystosira, which form regular submarine
forests, bear upon their branches numerous other small epiphytes, chiefly Floridee,
and these again are themselves covered by minute Diatomacew. Many of the
huge and lofty brown sea-weeds which raise themselves from the bottom of the
sea, remind one forcibly of tropical trees covered with Orchidez and Bromeliacee,
whilst the latter are themselves overgrown by Mosses and Lichens. These epiphytes
are for the most part, however, neither parasitic nor saprophytic. In general
hydrophytes attached by means of single cells or groups of cells derive no
nutriment, 2.e. no food-salts, from the support they rest upon. When loosened from
the substratum they continue to live in the water for a long time; they increase in
size, and if they come into contact with a solid body are apt to attach themselves to
it. In this connection it is well worthy of remark that certain Crustacea have their
carapaces entirely covered by hydrophytes of this kind, and that it takes a very
short time for the plants to establish themselves upon them. For instance, some
species of crabs, such as Maja verrucosa, Pisa tetraodon and P, armata, Inachus
scorpioides and Stenorrhyncus longirostris, cut off bits of Wracks, Floridex, Ulve,
&c., with their claws, and place them on the top of their carapaces, securing them
on peculiar spiky or hooked hairs. The fragments grow firmly to the crabs’
chitinous coats, and far from being harmful to the animals are, on the contrary,
an important means of protection. The crabs in question escape pursuit in con-
sequence of this disguise, and it is to be observed that each species chooses the
very material which makes it most unrecognizable to plant upon the exterior of
its body: those species which live chiefly in regions where Cystosiras are indigenous
deck themselves in Cystosiras, whilst those which inhabit the same places as Ulva,
carry Ulve on their backs. This phenomenon has for us a special interest in that
it shows that the water-plants we are discussing draw no food-salts from their
place of attachment, and that accordingly the chemical composition of the support
is a matter of utter indifference to all these Fucaces, Floridex, Ulve, &c.

There is no doubt that food-salts are absorbed by these hydrophytes from the
surrounding water through their whole surface. Accordingly the structure of their
peripheral cells is much simpler than is the case in land-plants. In the latter very

78 ABSORPTION OF FOOD-SALTS BY WATER-PLANTS.

complicated adaptations are necessary for the extraction of food-salts from the
earth. In particular, the portions which are exposed to the air above ground exhibit
a number of special structures connected with this extraction. These structures
(cuticle, stomata, &c.) are superfluous in the case of aquatic plants, for there is with
them no necessity for raising and conducting food-salts into the parts where they
can be used up. Moreover the absorption of nutritious matter is much simpler,
inasmuch as it is not necessary for the absorbent parts to search for a perpetual
source of the requisite substances. The roots of land-plants have often to range
over a wide area in order to find sufficient nourishment in the earth, and frequently
they have then to liberate it, i.e. bring it into a state of solution. This is not the
case with water-plants. They are completely surrounded by a medium which
is itself to a large extent a solution of food-salts, and no sooner are substances
withdrawn by the absorbent cells from the layers of water immediately bounding
them than those substances are again supplied from the more remote environ-
ment. Constant compensating currents occur in water, and there is, therefore,
scarcely an aquatic plant towards which there is not a perpetual flow of the food-
salts it requires in a form suitable for absorption. In connection with this kind of
food-absorption there is also the fact that the parts by which hydrophytes attach
themselves to a support are relatively small in area. Fucoids, as large as hazel
trees in height and girth, are fixed to submerged rocks by groups of cells perhaps
only 1 cm. in diameter.

The quantity of food-salts absorbed by hydrophytes is very considerable com-
pared with the amounts absorbed by other plants. As has been mentioned before,
soda and iodine play a very important part in the thousands.of different varieties
which live in the sea. If Floridez are transferred from the sea into pure distilled
water, common salt and other saline compounds diffuse out of the interior of the
cells through the cell-membranes into the fresh water around. The red colouring
matter of these Floridez also passes through the cell-walls into the water, proving
that the molecular structure of the membrane is adapted to the agency of salt
water in the osmotic processes of food-absorption.

Plants living in fresh, or in brackish water, likewise absorb relatively large
quantities of food-salts; and this accounts for the fact that water which is very
poorly provided with nutriment of the kind contains only very few vegetable
species.

One would expect that exceedingly abundant vegetation would be evolved in
running water, provided the latter contained food-salts in solution, however small
they might be in quantity. For, in such a situation, it is not necessary to wait for
the salts withdrawn by the plants from their immediate environment to be restored
by the slow processes of mixture and equilibration; the water which has been drained
of nutriment is replaced the next moment by other water bearing fresh food-salts.
Experience shows, however, that flowing water is not so favourable to the develop-
ment of hydrophytes as is the still water of pools, ponds, and lakes. This may
partly depend on the fact that running water is always poorer in food-salts, and

ABSORPTION OF FOOD-SALTS BY LITHOPHYTES. 79

partly also on the circumstance that mechanical difficulties are opposed to the taking
up of saline molecules from water in rapid motion. There are only a few plants
that are able to absorb under these conditions, and these choose, by preference, the
very spots where they are most exposed to the dash of the water. Thus, certain
Nostocinex (Zonotrichia, Scytonema) are to be found constantly in waterfalls at
the parts where the most violent fall occurs. Lemania, Hydrurus, and many
mosses and liverworts, grow by preference in the foaming cascades of rapid
torrents. Amongst flowering plants we only know of the Podostemacez as choosing
a habitat of this kind. Podostemacex are exceedingly curious little plants, which
at first glance one would take for mosses or liverworts without roots. Some of
them, eg. the Brazilian species of the genus Lophogyne and the various species of
Terniola growing in Ceylon, exhibit no differentiation into stem and leaves, but are
only represented by green fissured and indented lobes attached to stones. They
belong without exception to the tropical zone, and occur there in the beds of streams,
attached to rocks, over which the foaming water rushes.

ABSORPTION OF FOOD-SALTS BY LITHOPHYTES.

Nothing would seem more natural, as to the absorption of mineral salts by
lithophytes, than that the stone which constitutes their support should yield the
salts, and that the attached plants should suck them up; but, generally speaking,
the case is not so simple. There are mosses and lichens which cling to the surfaces
of rocks on mountain tops. These rocks are sometimes composed of perfectly pure
quartz, and yet the plants in question contain very little silica; they contain, on
the other hand, a number of substances entirely wanting in the composition of the
underlying rock, and which could not, therefore, have been derived from that
source. For many of these lithophytes the rock is, in the main, only a substratum
for attachment, and in no way a nutrient soil; just as, in the case of many aquatic
plants, the stones to which they cling by their discs of attachment are anything
but sources of nourishment.

From what source, then, do stone-plants of this kind derive the food-salts which
are wanting in their substratum? It may sound paradoxical, but it is nevertheless
the fact, that they obtain those salts from the air through the medium of atmospheric
precipitation. Rain and snow not only absorb carbon dioxide, sulphuric acid, and
ammonia—which occur in air universally, although in extremely minute quantities
—but they also collect, as they fall, floating particles of dust. The opinion is widely
entertained that although the atmosphere is full of dust in the neighbourhood of
cities and human settlements generally, where the soil is laid bare and ploughed
up, and roads and paths have been made for purposes of traffic, and perhaps also
over steppes and deserts where large areas of ground are destitute of vegetation,
yet that there is no dust in the air over land remote from places of that kind or in
the air of marshes, lakes, or seas. This notion has certainly some warrant if we
regard as dust only the coarser particles which are raised from loose earth and

80 ABSORPTION OF FOOD-SALTS BY LITHOPHYTES.

whirled into the air by the wind. Moreover, the quality of the dust will no doubt
be characteristically affected by the vicinity of areas of industry. One has only to
look at the sooty leaves and branches of trees in parks near manufactories to
convince oneself of the reality of this influence. But it would be quite erroneous to
suppose that the air in regions far from land that has been cultivated or otherwise
opened up is free from dust. It contains dust everywhere. There is dust in the air
of the extensive ice-fields of arctic regions and of high mountain glaciers, and there
is dust in the air of great forests and over the boundless sea.

If the rays of the setting sun fall obliquely through a gap between two peaks in
a wood-clad mountain valley, sun-motes may be seen floating up and down and in
circles, just as they do in a room when the last rays before sunset fall through the
window. ‘These motes are of course not usually visible, and they are moreover
much smaller than the particles of dust which are raised by the wind from roads
and then again deposited. Now, when rain falls, it takes the sun-motes from the
air and brings them down to earth, and the air is thus washed to a certain degree
of purity. This nappens still more completely in the event of snow. The latter
acts not unlike a imass of gelatine used to purify cloudy liquids, its effect being to
drag down with it all the particles to which the turbidity is due, leaving the upper
part of the liquid quite clear. Similarly, falling snow-flakes filter the air; and,
mixed with fallen snow, there are accordingly innumerable particles of dust.
If afterwards the snow gradually melts, it dissolves some of the dust, which then
drains away into chinks and depressions; but a portion remains behind undissolved.
This portion is gradually consolidated, and then appears lying on the parts of the
snow that are still unmelted in the form of dark patches, streaks, and binds; often
also it forms a smeary graphitic covering so widely spreading over the last remnants
of melting snow that the latter resemble lumps of mud rather than snow. Accord-
ingly we find it everywhere —in regions cultivated and uncultivated, in tilled
lowlands and on high grassy plains above forest limits, where no tilled land is to be
seen in any direction, and lastly in arctic regions in the middle of glaciers several
miles across.

All this snow dust is not invariably deposited as a result of the filtering of the
air by falling snow-flakes; an additional supply is brought by the winds which
blow across the snow-fields. It is not of rare occurrence in the Alps for snow-
fields to exhibit suddenly, after violent storms, an orange-red coloration. On closer
inspection one finds that the surface of the snow is strewn with a layer of powder,
infinitesimally fine and for the most part brick-red, which has been brought by the
gales. Investigation of this “meteoric dust” shows that it is composed chiefly of
minute fragments of ferruginous quartz, felspar, and various other minerals.
Mixed with these there are, however, sometimes remnants of organic bodies, such
as bits of dead insects, siliceous skeletovs of diatoms, spores, pollen-grains, tiny
fragments of stems, leaves, and fruits, and the like. Once, after a south wind had
prevailed for several days, the snow-fields of the Solstein range near Innsbruck
were covered, at a height of from two to three thousand meters above the sea-level,

ABSORPTION OF FOOD-SALTS BY LITHOPHYTES. 81

with millions of a species of Micrococcus, which lent a rosy hue to vast expanses
of snow.

Most of the dust in the atmosphere originates, doubtless, from our earth. The
air that blows in waves over the earth can carry along with it not only dead and
detached portions of plants, but also loose particles of rock, sand, earth, and dried
mud. If one draws one’s palm across the weather side of a dry rock composed of
dolomitic limestone, gneiss, trachyte, or mica-schist, the surface of the stone always
feels dusty, and the slightest movement of the hand is sufficient to detach a number
of particles which were already separate from the rock and only held in loose con-
nection with it. This dust is liable to be detached and carried away by any strong
gust of wind. Larger and heavier particles are not, it is true, lifted much above the
ground; they are rolled and pounded along and thereby reduced to a still finer
powder. This finer dust may then be scattered afar by gales blowing horizontally,
or even ascend into higher atmospheric strata. The finest dust in particular, how-
ever, is carried up into the higher layers of the air by the currents which ascend
from the earth in calm weather; and this applies not only to the tropics but to the
temperate zones as well, and even to the frigid regions of the arctic zone. When,
therefore, this dust is brought back by rain or snow from the upper aerial strata to
the earth, it but completes a circuit. Indeed it is highly probable that the particles
of dust restored to earth by means of atmospheric deposits recommence their aerial
travels as soon as they are thoroughly dry again, and that there is thus a circulation
of dust analogous to that of water.

There is of course no inconsistency in the fact that meteoric dust, which is
often drifted along in surprisingly large quantities, may originate quite suddenly
during volcanic eruptions; nay, it is even possible that cosmic dust reaches our
atmosphere and thence falls to the earth. Chemical investigation of aerial dust
has, no doubt, yielded in most cases only sulphuric and phosphoric acids, lime, mag-
nesia, oxide of iron, alumina, silica, and traces of potash and soda, that is to say, the
most widely distributed constituents of the solid crust of our earth; but cobalt and
copper have also been found in it, over and over again, and it has hence been
inferred that the dust in these cases was of cosmic origin.

In relation to the question which we have here to answer the above is, after all,
almost a matter of indifference. The only important facts are that dust in a state of
extremely fine division is blown about in the air, that this dust contains the salts
required by plants for their food, that it is carried for the most part mechanically
by drops of water and flakes of snow, condensed in the atmosphere, and is partially
dissolved, that the atmospheric deposits supply lithophytic plants with a sufficient
quantity of nutrient salts, and that the aqueous solution so supplied is rapidly
absorbed by the whole surface of the plants in question. We must not omit to
mention here that the demand of lithophytes for mineral food-salts is not very great.
In particular the protoneme and even the leafy shoots of Grimmie, Rhacomitrie,
Andrewacee and other rock mosses, and the Collemacew and most crustaceous

lichens only contain very minute quantities of these substances. Water containing
Vou. I. 6

82 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS.

the usual mineral salts in about such proportion as is necessary for the cultivation
of cereals in fields has actually an injurious effect on these lithophytes and soon
kills them.

At the end of this section we shall consider what happens to dust which is
brought to earth from the air by rain and snow but is not dissolved, and the
important part it plays in clothing the naked ground and in changes of vegetation.
Here, however, it must be noted that most lithophytes are true dust-catchers, that is
to say, they are able to retain, mechanically, dust conveyed to them by wind, rain,
and snow, and to use it in later stages of development by extracting nutriment from
it. Many mosses are completely lithophytic in early stages of development whilst
later they figure as land-plants.

ABSORPTION OF FOOD-SALTS BY LAND-PLANTS.

In no class of plants is the absorption of mineral food-salts accomplished in
so complicated a manner as in land-plants. Moreover, this absorption is by no
means uniform in different forms of plants, and we must beware of generalizing
with regard to processes which have only been traced and studied in isolated
groups—perhaps only in the commonly distributed cultivated plants. On the other
hand, with a view to synoptical representation, it is not desirable to enter into too
great detail or to attempt to describe all the various differences minutely.

At the outset, it is difficult to give an accurate account of the soil which
constitutes the source of nutriment in the case of land-plants. From the dark
graphitic mass composed of sun-motes, which is deposited in the place of a melted
layer of snow, to coarse gravel, there is an unbroken chain of transition stages;
loam, sand and gravel are only specially-marked members of this chain. Again,
just as earth varies in respect of the size of its component parts, so also it
varies in the mineral salts it contains, in the amount of admixture of decaying
vegetable and animal remains, in the nature of the union of its constituents,
and in its capacity to absorb, to retain, or to yield up water. Compare the sand
composed of quartz on the bank of a mountain stream with that of calcareous
origin which is found impregnated with salt on the sea-shore, or with the sand
at the foot of mountains of trachyte, which has an efflorescence of soda-salts.
Or compare the granite bed of a desert, bare of soil, with the loam on the granitic
plateaus of northern regions where there is an intermixture of the remains of a
vegetation for centuries active. How great is the difference in each case! But
whatever the kind of earth, it is only of value as a source of nutriment for a
plant when the interstices of its various particles are filled with watery fluid
for the time during which the plant is engaged in the construction of organic
substances.

But how is the earth supplied with water?

“Das hat nicht Rast bei Tag und Nacht,
Ist stets auf Wanderschaft bedacht.”

ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 83

Streams fall into lakes, rivers into the sea, and hence the water ascends into the
atmosphere in the form of vapour, and returns once more to earth as snow, rain,
and dew. Through porous earth it percolates until it has filled all the interspaces.
If its further descent be impeded by impervious strata, it spreads literally as sub-
terranean water, or else comes up at some special spot as a spring. Earth which is
richly endowed with decaying vegetable remains is able to absorb vapour in addition
from the atmosphere. When this occurs, carbonic and nitric acids are always
absorbed along with the aqueous vapour. These are contained, as has been mentioned
before, in atmospheric deposits, and another source of these acids is afforded by the
decay of dead parts of plants. Water precipitated from the atmosphere, and con-
taining carbonic and nitric acids, is able by their means to decompose the compounds
in all the rocks which come in its way as it percolates through the ground, especially
when its action is long continued. The siliceous compounds or so-called silicates—
felspars, mica, hornblende, and augite in particular—and quartz, the anhydride of
silicic acid, which form the preponderant mass of the rocks of the solid crust of our
earth, either contain a great quantity of silica, alumina, and alkalies, or if they are
relatively poor in silica they may be rich in iron. The former are found chiefly
in granite, gneiss, mica-schist, and argillaceous slate; the latter preponderate in
serpentine, syenite, melaphyr, dolerite, trachyte and basalt. First the felspars are
decomposed by the acid water. Their alkalies combine with the carbonic and nitric
acids forming soluble salts, and the alumina and silica remain behind as clay. Iron
is also converted into soluble salts. The most difficult substances to decompose are
the mica and quartz, and it is on that account that they so often appear in the
form of glittering scales and angular nodules mixed with the clay produced from
the decomposition of felspar. But, ultimately, even they are unable to withstand
the continuous action of the acidulated water. The result of these chemical
changes is an earth, which, according to the nature of the parent rock, contains
a preponderating amount of clay, of quartzose sand or of mica, which is coloured
in various ways by iron compounds. Of substances useful to plants these
earths yield generally on analysis the following: potash, soda, lime, magnesia,
alumina, ferrous and ferric oxides, manganese, chlorine, sulphuric acid, phosphoric
acid, silica, and carbonic acid, sometimes one sometimes another in greater
proportion relatively, and traces of many substances often so slight as hardly
to be detected.

It ig true that limestone and dolomite, which, next to the above-mentioned
rocks, enter most largely into the composition of the solid crust of the earth,
consist chiefly of carbonate of lime and magnesium carbonate respectively; but
wherever they occur in extensive strata and piles, they always contain in addition
an admixture of alumina, silicic acid, ferrous oxide, manganese, traces of alkalies
in combination with phosphoric and sulphuric acids, &. Of the carbonates of
lime and magnesia a great part is gradually dissolved and carried away upon the
invasion of water containing carbonic and nitric acids, and a proportion also of
the substances mixed with them, as above mentioned, is lixiviated. What remains

84 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS.

behind then consists of an argillaceous, loamy mass, variously coloured by iron and
very similar in appearance to the clay formed from the decomposition of felspar.
According to the quantity of the substances mixed with the carbonate of lime in
the rock, the loamy earth formed from limestone is either abundant or only in
restricted layers, bands and pockets lying on, or intercalated within, the unde-
composed débris of the stone. Chemical analysis has resulted in the discovery
that there are, as a rule, in loamy earth of this kind the same ingredients avail-
able for plants as have been identified in earth produced from silicates; and we
are led to believe that earths, collected in widely different places and covering
rocks of most various kinds, are much more uniform qualitatively than has been
supposed. Only, the relative proportions of the substances forming the mixture
are usually different. Silica and the alkalies are less conspicuous in earth derived
from limestone, and carbonate of lime in that which is formed from silicates.
This difference is particularly striking in instances where the rock consisted
almost entirely either of quartz and mica or of nearly pure carbonates of lime
and magnesium. In these cases the earth formed is not argillaceous, but of loose
consistence, very abundant, and composed, according to the kind of rock, of
quartzose sand and mica scales or calcareous and dolomitic sand.

The conversion of rocks into earths by the action of water from the atmosphere
containing carbonic and nitric acids is, besides, materially modified by the disrup-
tions which ensue from changes of temperature, more particularly by the freezing
of water within the pores of rocks. It is also affected, though more remotely, by
the mechanical action of water and air in motion, and, lastly, by the plants them-
selves, which penetrate with their roots into the narrowest crevices and mingle their
dead remains with the portions of the rock that are decomposed, broken up, or
abraded by chemical and mechanical agencies. The substance produced from a
rock in the manner explained is called earth-mould, or simply earth. The matter
resulting from the decomposition of plants and animals is designated by the term
“humus.” Earth which includes an abundance of decomposed fragments of plants,
ie. has a large admixture of humus, is called vegetable mould. ,

Every kind of earth, but especially earth rich in humus and clay, has the power
of retaining gases, and especially water and salts. When water containing salts in
solution is poured over a layer of dry vegetable mould, it percolates into the spaces
between the particles of earth, and speedily drives out of them the air which has
but slight adhesion, and which then ascends in bubbles. It is not till all the inter-
spaces are full of water, whilst a fresh supply is constantly maintained from above,
that any of the liquid oozes out from beneath the stratum of earth. The water
remaining in the interstices is held there by adhesion to the particles of earth, and
we must conceive each of these particles as surrounded by an adherent film of
water. The inorganic salts, infiltrating with the water, are held with still greater
energy. The water which trickles from the bottom of the earth always contains a
much smaller proportion of salts in solution than that which was poured on above,
whence we conclude that the latter are in part absorbed by the earth.

ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 85

The salts are to be regarded as forming an extremely delicate coating round
minute particles of earth where they are forcibly retained. If a plant rooted in
the earth is to take in these salts it has to overcome the force by which their
molecules are detained. This is effected, however, by means of a very powerful
attraction exerted by the protoplasts of the plant as they grow, carry on the work
of construction, and use up material. What actually happens is an energetic suction
by the cells that are in close contact with particles of earth. This suction depends,
however, upon the chemical affinity between the substances in the interior of the
cells and the salts adhering to the earth-particles, as well as upon the consumption
of food-salts for the manufacture of organic compounds within the green cells. It
is supposed that whenever salts are abstracted from soil-particles by suction, a
restitution of like salts immediately takes place, particles still unresolved in the
immediate neighbourhood being dissolved, and a fresh influx taking place from the
environment. Consequently the concentration of the solution retained by the earth
is always approximately the same, or, at any rate, equilibrium is very quickly
restored. One advantage of this is that the cells in immediate contact with
particles of earth, and their adherent liquid, can only meet with a saline solution of
constant weak concentration, and are therefore secure from injury such as would
result in the case of most plants, from contact with a very concentrated solution.
In other words, the absorptive power of earth acts as a regulator of the process of
absorption of food-salts by plants, and is the means of keeping the saline solution
in the earth always at the degree of strength best suited to the plants concerned.

Naturally, the passage of salts from the earth to the interior of a plant is
dependent on the aid of water containing both the substances composing cell-
contents and the food-salts in solution. The cell-membranes, through which
absorption takes place, are saturated with this solution. The aqueous films adhering
to the particles of earth, the water saturating the cell-membrane, and the liquid
inside the cells are really in unbroken connection, and along this continuous water-
way the passage of salt molecules im and out can take place easily.

The absorption of food-salts directly from the earth by green cells occurs very
rarely. The protonema of Polytrichwm, which spreads its threads over loamy earth
and wraps it in a delicate green felt, and that of the famous Cavern Moss (Schis-
tostega), whose long tubular lower cells penetrate the earth in the recesses of caves,
do undoubtedly suck up their necessary food-salts by means of cells containing
chlorophyll. A drawing of the latter is given in Plate L, fig. p.

The majority of land-plants have, however, special absorptive cells for the
taking-up of salts in solution. These cells are imbedded amongst or lodged upon
the earth-particles, and are usually in intimate connection with portions of them.
Any part of a plant that penetrates into the earth or lies upon it, may, if it performs
the function of absorption, be equipped with cells of the kind. Plagiotheciwm
nekeroidewm, a delicate moss belonging to the flora of Germany, and growing on
earth under overhanging rocks, where it is not exposed to rain, and therefore cannot
receive any food-salts through that agency, develops absorption-cells on the apices

86 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS.

of its green leaflets. So also does Leucobrywm javense, a species native to Java.
Several delicate ferns of the family of the Hymenophyllacee exhibit them on their
subterranean stems. Many liverworts and the prothalli of ferns bear them on the
under surfaces of their flat thalli which lie outspread on damp earth. But most
commonly of all are they to be found close behind the growing tips of roots. Their
form does not vary very much. On the roots of plants fringing the sources of cold
mountain-springs, as on those of many marsh-plants in low-lying land, they are in
the form of comparatively large, oblong, flattened, closely united cells, with thin
walls and colourless contents. In some conifers, whilst having in the main the
shape just described, they differ in that they are arched outwards so as to form
papillae; but in most other phanerogams the external cell-wall projects outwards,
and the whole absorptive cell develops into a slender tube, set perpendicularly to
the longitudinal axis of the root (fig. 12 *).

Seen with the naked eye, or but slightly magnified, these delicate tubes look like
fine hairs, and have received the name of “root-hairs.” The end of a root often
appears to be covered with velvety pile, and the absorptive cells are then very
closely packed; more than four hundred per square millimeter have been occa-
sionally counted. In other cases, however, there are hardly more than ten on a
square millimeter. When in such small numbers they are usually elongated and
clearly visible to the naked eye. Their length, for the most part, varies from the
fraction of a millimeter to three millimeters, and their thickness between 0:008 m.m.
and 0:14 mm. It is only exceptionally that one meets with plants, rooted in mud,
possessing root-hairs 5 m.m. or more in length. The absorptive cells of phanero-
gams are almost always simple epidermal cells of the particular part of the plant
that bears them, and are not partitioned by any transverse walls. In mosses and
fern prothalli, on the other hand, the absorption-cells are generally segmented by
transverse septa and are usually greatly elongated. In those liverworts which
belong to the genus Marchantia they form a thick felt on the under side of the
leaf-like plant, or rather, on such part of it as is turned away from the light, and
some of these tangled rhizoids attain a length of nearly 2¢c.m. The stems of many
mosses also are wrapped in a regular felt. This property is rendered very striking
in the species of Barbula, Dicranwm, and Mnium, and especially in such forms as
have bright green leaves, by the reddish-brown colour of the cells in question.
Sometimes the long capillary cells of which the felt is composed are twisted
together spirally like the strands of a rope. A good instance of this is Polytri-
chum. These fine, hair-like, segmented and branched structures, found on mosses,
variously matted and intertwisted, are called rhizoids. But only those cells which
come into contact with the earth-particles are truly absorbent. The rest do not
serve to imbibe from the ground, but to conduct the aqueous solution of food-salts,
after it has been taken up by the absorptive cells, to the stem and to the leaves.

The tubular cells resulting from the development of a root's epidermis are placed,
as before observed, at right angles to its longitudinal axis. They only grow, how-
ever, in earth that is very damp, and even then their course is not always a straight

ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 87

line, for as a rule they describe a spiral as they elongate. Their movement mel
as though it were for discovering the most favourable parts of the earth for absorp-
tion and attachment. In this manner they penetrate into the interspaces in the
earth which are filled with air and water. They also have the power of thrusting
aside minute particles of earth, especially if the latter consists of loose sand or mud.
If they strike perpendicularly a solid immovable bit of earth, they bend aside
and grow round it with their surfaces closely adpressed to that of the obstacle until
they reach the opposite point on the other side, when they once more resume their
original direction (fig. 12°). When they encounter large grains of earth they

Fig. 12.—Absorptive Cells on Root of Penstemon.

1 Seedling with the long absorptive cells of its root (‘‘root-hairs”) with sand attached. 2The same seedling; the sand
removed by washing. % Root-tip with absorptive cells; x10. 4# Absorptive cells with adherent particles of earth. 5Section
through the root-tip; x60.

sometimes stop and swell up to the shape of a club. The club divides into two or
more arms, which grasp and cling to the granule like the fingers of a hand. Many
fragments of earth remain thus in the grasp of finger-like processes, whilst others
are held fast in the knots and spirals of corkscrew-shaped root-hairs which are
often found tangled together. But the retention of most of the earth-particles
which adhere to a plant, including fragments of lime, quartz, mica, felspar, &c., as well
as plant-residues, is due to the fact that the outermost layer of the absorptive cells
is sticky, it being altered into a swollen gelatinous mass which envelops the
particles. When this sticky layer becomes dry it contracts and stiffens, and the
granules partially imbedded in it are thereby cemented so tightly to the absorptive
cells that even violent shaking will not dislodge them.

In the case of most seedlings, and in that of grasses, the absorptive cells which
proceed from the roots and which are especially numerous in the latter, are generally
thickly covered with particles of earth (see fig. 12‘). If such a root is pulled out
of sandy soil it appears to be completely encased in a regular cylinder of sand (fig.
121), A root of Clusia alba, taken from coarse gravel, had its root-hairs so tightly

88 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS.

adherent to bits of gravel that several little stones, weighing 18 grms., were found
clinging to it when it was lifted. The gelatinous mass, resulting from the swelling-
up of the external coat of the cell, does not in any way hinder absorption or the
passage of food-salts in solution. Nor does the inner coat, the thickness of which
varies between 0°0006 m.m. and 001 m.m., constitute any impediment to imbibition.

In addition to the absorption of nutritive salts by root-hairs, there is also, in
many cases, an interchange of materials; that is to say, not only do substances
infiltrate from the earth into the absorption-cells, and xo onward into the tissues
of a plant, but others pass out of the plant through the absorptive cells into
the earth. Amongst these eliminated substances, carbonic acid, in particular,
plays an important part. A portion of the earth-particles adhering to root-hairs are
decomposed by it, and food-salts in immediate proximity to those cells are hereby
rendered available and pass into the plant by the shortest way.

Having now seen that land-plants take in food-salts by means of special
absorptive cells, it is natural to find that each of these plants develops its
absorption-cells, projects them, and sets them to work at a place where there is
a source of nutritive matter. The parts that bear absorptive cells will accord-
ingly grow where there are food-salts and water, which is so necessary for their
absorption, The Marchantias and fern prothalli spread themselves flat upon the
ground, moulding themselves to its contour. From their under-surfaces they
send down rhizoids with absorptive cells into the interstices of the soil. Roots
provided with root-hairs behave similarly. If a foliage-leaf of the Pepper-plant
or of a Begonia be cut up, and the pieces laid flat on damp earth, roots are
formed from them in a very short time. The roots on each piece of leaf proceed
from veins near the edge, which is turned away from the incident light, and
grow vertically downwards into the ground.

It is matter of common knowledge that roots which arise upon subterranean
parts of stems, like those formed on parts above-ground, grow downward with a
force not to be accounted for by their weight alone. This phenomenon, which is
called positive geotropism, is looked upon as an effect of gravitation. The idea is
that an impetus to growth is given by gravity to the root-tip, and that a trans-
mission of this stimulus ensues to the zone behind the tip where the growth of the
root takes place. It is noteworthy that if bits of willow twigs are inserted upside
down in the earth, or in damp moss, the roots formed from them, chiefly on the shady
side, after bursting through the bark, grow downwards in the moist ground, pushing
aside with considerable force the grains of earth which they encounter. The
appearance of a willow branch thus reversed in the ground is all the more curious
inasmuch as the shoots, which are developed simultaneously with roots from the
leaf-buds, do not grow in the general direction of the buds and branches, but turn
away immediately and bend upwards. Thus the direction of growth of roots and
shoots produced on willow-cuttings remains always the same, whether the base or the
top of the twig used as a cutting is inserted in the earth. A similar phenomenon is
observed if the leafy rootless shoot of a succulent herb (e.g. Sedwm refleaum) is cut

ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 89

off and suspended in the air by a string. Whether it hangs with the apex upper-
most, %.e. in the position in which it grew naturally, or with the apex towards the
ground, it always, in a short space of ‘time, produces roots which spring from the
axis between the fleshy foliage-leaves and bending sharply grow to the earth. Thus
in the former case their direction is contrary to the apex of the shoot; in the latter,
curiously enough, it is in the same direction. If the height at which the shoot is
suspended is only 2 cm. above the earth, the roots growing towards the ground
develop their root-hairs 2 em. from their place of origin. But if the shoot is at a
distance of 10 c.m., the roots only develop their root-hairs when they have attained a
length of 10cm. The rule is, therefore, for the roots to grow until they reach the
nutrient soil without developing absorption-cells, and only to provide themselves
with them when they are in the earth. It is to be observed that these roots are
produced on the suspended shoot at places where, under normal conditions (i.e., if
the shoot were not cut off and hung up), no roots would be developed. Subject
to abnormal conditions and liable to starvation, the plant sends out these roots for
self-preservation.

Phenomena of this kind force one to conclude that a plant discerns places which
offer a supply of nutriment, and then throws out anchors for safety to those places.
This power of detection may, undoubtedly, be explained by the influence which
conditions of moisture, in addition to the action of gravitation, have on the direction
taken by growing roots. The root-hairs can only obtain food-salts when the ground
is thoroughly moist; and whenever roots, or rather their branches, have to choose
between two regions, one of which is dry and the other wet, they invariably turn
towards the latter. If seeds of the garden-cress are placed on the face of a wall of
clay which is kept moist, the rootlets, after bursting out of the seeds, grow at first
downwards, but later they enter the wall in a lateral direction. The longitudinal
growth of the roots is greater on the dry side than on the wet side, and this results
in a bending of the whole towards the source of moisture, in this instance the damp
wall. It has been established that the tip of a rootlet is very sensitive to the
presence of moisture in the environment. Where there is a moist stratum on one
side and a dry stratum on the other, a root-tip receives a stimulus from the unequal
conditions in respect of moisture; the stimulus is propagated to the growing part of
the root, which lies behind the tip, and the result is a curvature of the root towards
the moist side. Thus, the presence of absorbable nutriment, or rather of moisture,
in the ground explains the divergence of roots from the direction prescribed by
gravity.

The extent to which the direction taken by roots in their search for food is
dependent upon the presence of that food, and the fact that roots grow towards
places that afford supplies of nutritious material, are strikingly exhibited, also,
by epiphytes growing on the bark of trees, such as tropical orchids and
Bromeliacew; and again by plants parasitic on the branches of trees, of which the
Mistletoe and other members of the Loranthacew afford examples. Although the
absorption of food by these plants will not be thoroughly discussed till a later

90 ABSORPTION OF FOOD-SALTS BY LAND-PLANTS.

stage, this is the proper place to mention the fact that in them positive geotropism
appears to be completely neutralized. The growing rootlets which spring from the
seed, and the absorptive cells produced from minute tubercles, grow upwards if
placed on the under surface of a branch, horizontally if placed on the side, and
downwards if on the upper surface. Thus, whatever the direction, they grow
towards the moist bark which affords them nourishment.

Positive geotropism seems to be quite abolished also in those marsh-plants
which live under water. When, for instance, the seed of the Water-chestnut
(Trapa natans) germinates under water in a pond, the main root emerges first from
the little aperture of the nut and begins by growing upwards. Soon the smaller
scale-like cotyledon is put forth, whilst the other, which is much larger, remains
within the nut. The whole plant so far is standing on its head, as it were, and
is growing upwards with its principal root directed towards the surface of the
water. Gradually the leafy stem emerges from the bud between the two coty-
ledons, and likewise curves upwards and grows towards the surface, whilst an
abundance of secondary roots is developed at the same time from the main root.
Their function is to absorb nutritive substances from the water around, now that
the materials for growth stored in the seed are exhausted. Finding an aqueous
solution of food-salts everywhere these roots grow in all directions, upwards,
downwards, or horizontally to right or left, forwards or backwards, only they
carefully avoid touching one another or interfering with each other’s sphere of
absorption. It is not till much later that the main root changes the direction of
its apex and bends downward. New roots are then produced from the stem; but
this subject has no further bearing on the problems at present before us.

The movements of roots, as they grow in earth, suggest that they are seeking
for nutriment. The root-tip traces, as it progresses, a spiral course, and this
revolving motion has been compared to a constant palpitation or feeling. Spots
in the earth which are found to be unfavourable to progression are avoided with
care. If the root sustains injury, a stimulus is immediately transmitted to the
growing part, and the root bends away from the quarter where the wound
was inflicted. When the exploring root-tip comes near a spot where water
occurs with food-salts in solution, it at once turns in that direction, and, when it
reaches the place, develops such absorptive cells as are adapted to the circum-
stances.

As has been mentioned before, the roots of most land-plants bear root-hairs on a
comparatively restricted zone behind the growing point (see fig. 12°), and these
hairs have only an ephemeral existence. As the root grows and elongates, new
hairs arise (always at the same distance behind the tip), whilst the older ones
collapse, turn brown, and perish. In ground which contains on every side food-salts
in quantities adequate to the demand, and sufficient water to act as solvent and as
medium for the transmission of the salts, the absorptive cells are rarely tubular, but
exhibit themselves, as already described, in the form of flat cells destitute of outward
curvature. This is the case, for instance, with those Alpine plants which grow in

ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 91

ever-moist hollows and depressions in proximity to springs (e.g. Sawifraga aizoides
and many others). But wherever the substances to be absorbed are not so easily
obtained, the surfaces of the absorptive cells are increased by means of a protrusion
of the outer cell-wall, the whole cell being converted into a tube. These tubular
absorptive cells are most elongated in mossy forests, where rather large gaps occur
not infrequently in the soil. When a root in the course of growth reaches one of
these lacuna, filled with moist air, its root-hairs often lengthen out to an oxtraordi-
nary extent, and sometimes attain to twice the length of those which are in compact
soil. The absorptive cells on the roots of the Water-hemlock (Cicuta virosa) and
the Sweet Flag (Acorus Calamus) do not project at all if the earth in which they
grow is muddy; whilst, if the earth is only slightly damp, and an increase of surface
is therefore advantageous, the absorptive cells become tubular. Plants which grow
in ground liable to periodic drought, and which at these times must secure all the
moisture retained by the earth to save their aerial portions from death by desiccation,
endeavour to obtain as great an area of absorption as possible by the development
of long tubular cells.

The fact must not be overlooked, however, that the form and development of
absorptive cells depend partly on the quantity of water that is given off from the
aerial parts of the plant, that is to say, by the transpiration of the foliage-leaves.
Plants which lose a great deal of water in this way must provide for abundant resti-
tution. They must absorb from as large an area as possible, and enlarge their absorp-
tive surfaces adequately by pushing out the cells into long tubes. For this reason
all plants with very thin, delicate, expanded foliage-leaves, which transpire readily
and abundantly, have numerous long tubular root-hairs. Examples are afforded by
Viola biflora and the various species of Impatiens. On the other hand, plants with
stiff, leathery leaves, being protected by a thick epidermis from excessive transpira-
tion, as, for instance, the Date-palm, exhibit flat, non-protuberant absorptive cells,
because there is a very limited amount of evaporation from these plants, and the
quantity of water to be absorbed to replace what is lost is therefore small. The
same thing holds in the case of evergreen Conifers, in which, owing to the structure
of the stiff needles and to the peculiar formation of the wood, water is conducted
very slowly from the roots to the transpiring green organs. It has been ascertained
that they exhale from six to ten times less vapour than do ashes, birches, maples,
and other flat-leaved trees growing on the same ground.

We shall presently return to the question of the substitution for absorptive cells
in many coniferous and angiospermic trees and in evergreen Daphnacee, Ericacee,
Pyrolacee, Epacridec, &c., of the mycelium of fungi, and shall treat also of the
importance of the form of the absorptive cells, and of the roots which bear them,
in relation to the mechanism of striking root in the ground.

92 RELATIONS OF FOLIAGE-LEAVES TO ABSORBENT ROOTS.

RELATIONS OF THE POSITION OF FOLIAGE-LEAVES TO THAT OF
ABSORBENT ROOTS.

Anyone who has ever taken refuge from a sudden shower under a tree will
remember that the canopy of foliage afforded protection for a considerable time, and
that the ground underneath was either not wet at all, or only slightly so. No doubt
some of the rain flows down the bark of the trunk, and in many species, as, for
instance, the Yew and the Plane-tree, the volume of water conducted down the
trunk is considerable; but in the case of most trees the rain-water which reaches
the earth in this manner is not abundant, and in comparison with that which drips
from the peripheral parts of the foliage its quantity is negligeable. This phenome-
non is dependent upon the position of the foliage-leaves relatively to the horizon.
In almost all our foliage-trees—in limes and birches, apple and pear trees, planes
and maples, ashes, horse-chestnuts, poplars, and alders—these organs slope out-
wards, and are so placed one above the other that rain falling upon a leaf on one
of the highest branches flows along the slanting surface to the apex, collects there
in drops, and then falls on to a lower leaf whose surface is also inclined outwards.
Here it coalesces with the water fallen directly upon this leaf; and so it goes from
one tier to another, lower and lower, and at the same time further and further
from the axis, till a number of little cascades are formed all round the tree. From
the under and outermost leaves of the entire mass of foliage the water falls in
great drops to the ground, and after every shower of rain the dry area at the
foot of the tree is surrounded by a circular zone of very wet earth. It is only
necessary to dig at these places to convince one’s self that the tree’s absorptive roots
penetrate the earth precisely to the wet zone. When a tree is young, its roots lie
in a small circle, and the crown too is not extensive, so that the damp zone is
proportionately restricted. But as the latter is enlarged there is a corresponding
elongation of the roots in their search for moisture, and thus roots and foliage
progress part passw in peripheral increase. It seems not improbable that the
custom amongst gardeners and foresters of trimming the foliage and roots of trees
when the latter are transplanted is to be attributed to the phenomenon above
deseribed. For the rule is observed that the branches of the trunk and those of the
root must be about equally shortened, and accordingly the suction - roots, as they
develop, reach the zone of drip of the growing crown.

A similar method of carrying off water is to be observed in coniferous trees.
Take, for example, the Common Pine. The lateral branches are horizontal near
the main trunk; the secondary branches curve upwards like bows The needles
near the tip of each of the latter slant obliquely upwards from the axis, whilst
the older needles, situated on the under side of the part of the branch which is
almost horizontal and at some distance from its extremity, are directed obliquely
downwards and outwards. Rain-drops striking the upturned needles glide down
them to the bark of the branch in question, and thence to other needles whose

RELATIONS OF FOLIAGE-LEAVES TO ABSORBENT ROOTS. 93

inclination is downwards and outwards. On their apices great drops are gradually
formed, which finally detach themselves and fall on to the mass of needles be-
longing to a lower branch. Thus transmitted, the rain-water travels through
the foliage lower and lower and at the same time further from the axis. This
is also the case with larches. The drops of rain which fall upon the erect needles
of the tufted “short branches” collect and gradually descend to the needles of
the drooping “long branches” on lower boughs. Large drops are always to be
seen on their drooping apices, whence they drip to the earth. Owing to the
pyramidal form of larches, and to the circumstance that the long shoots on each
branch are terminal, almost all the water which falls upon one of these trees
reaches the long shoots hanging down from the lowest branches, which discharge
most of all. Although larches with their tender needles do not look at all as
though they would be any protection against rain, the ground underneath them
keeps dry nevertheless, the principal part of the water falling upon them being
conducted to the periphery. Indeed, the larch belongs to the number of trees
which conduct almost all the rain that falls upon them to a certain distance from
the axis where the absorbent roots lie, and only allow a little to trickle down
the bark of the main trunk.

Many shrubs and perennial herbs also transmit the water, which falls on
their upturned laminz, to parts of the ground where their absorbent roots are
embedded; or, rather, the roots send forth their branches bearing absorptive cells
to the area which is kept moist by drippings from the leaves. Particularly striking
in this respect are the species of the two genera of Aroids Colocasia and
Caladiwm. A specimen of the latter is figured below (fig. 131). If one digs
about individuals of this genus cultivated on open ground, one invariably finds
that the tips of the lateral roots, which proceed in a horizontal direction from
the bulbous root-stock, are buried under the point of the great leaves which slope
obliquely outwards. We must not omit to mention, in addition, that the stalks
of leaves which conduct the rain centrifugally are not channelled on the upper
surface; they are round, and comparable to wires supporting at their upper extremities
the laminz in an outward and downward direction. As instances we may quote
the Horse-chestnut, Maple, and Lime, and many shrubby, suffruticose, and
herbaceous plants, such as Sparmannia, Spirea, Arwneus, and Corydalis, and also
climbing and trailing plants (e.g. Menispermwm, Banisteria, Aristolochia, Hoya,
Zanonia, and Tropeolwm). Whenever a system of grooves is developed on the
surface of an outward sloping leaf, the channels run along the veins and terminate
at the apex of the leaf, or at the apices of the leaf’s lobes, and invariably cause
the water to travel, not to the basal part, but to a spot on the margin whence
it will detach itself in the form of a drop, and fall upon the leaves situated
immediately below and at a greater distance from the axis.

A striking contrast to these trees and shrubs, climbing and trailing plants,
and suffruticose and herbaceous species, with their absorptive roots lying in one
plane, and usually spreading at but little depth, is afforded by plants which possess

94 RELATIONS OF FOLIAGE-LEAVES TO ABSORBENT ROOTS.

bulbs or short root-stocks with deep-reaching suction-roots, and those which have
tap-roots descending vertically in continuation of the main stem, and whose second-
ary roots are short and travel only a little distance from their places of origin.
This other extreme in root-structure, which is represented in fig. 137, has its
counterpart above-ground in the form and direction of the lamine upon which
the rain falls. In all these plants the surfaces of the leaves are not directed

Fig. 13.—Centrifugal and Centripetal Transmission of Water.

1By a Caladium, 2 By a Rhubarb plant.

outwards, but slope obliquely towards the central axis, Their upper sides, more-
over, are concave and exhibit a system of grooves, which conveys the water collected
by the leaf towards the stem, and therefore also, towards the tap-root and suction-
roots. The leaves of bulbous plants, such as the Hyacinth and Tulip, all stand
up obliquely, and their upper surfaces are concave and often deeply channelled.
Along the grooves the rain flows centripetally downwards, and so directly reaches
the part of the earth where the bulbs and suction-roots, which proceed in a tuft
from underneath the bulbs, are situated. The young leaves of Cannacew and of the
Lily-of-the-valley are coiled up like a trumpet; and rain, falling from above
upon the expanded portion, is led along the coiled surface, describing « helix as

RELATIONS OF FOLIAGE-LEAVES TO ABSORBENT ROOTS. 95

it goes, to the earth in the neighbourhood of the absorptive roots, which proceed
from the short root-stock. When the leaves of plants furnished with tap-roots
are arranged in whorls, and are without internodes, and the rosette rests upon
the ground, as is the case in the Mandrake, the Dandelion, and several species of
Plantain (Mandragora officinalis, Taraxacum officinale, Plantago media), there
are always one or more main grooves on the upper surfaces of the leaves, and
the leaves have always such form and position as compel the rain which falls
upon them to flow centripetally, i. towards the tap-root growing vertically
bereath the centre. Plants with petiolate leaves, which conduct rain centyri-
petally, always have on the upper side of each leaf-stalk an obvious groove, the
depth of which is frequently increased by the development of green or (in many
cases) membranous ridges on the two lateral edges. Grooves of this kind are
to be seen particularly well on the petioles of the radical leaves of the Rhubarb
(see fig. 13”), Beet-root, Funkias, and most Violets.

Far more complicated in structure than the radical leaves just described, are
cauline leaves. Leaves proceeding from the stem high above the ground, and
forming receptacles for rain-water, like those of the Rhubarb, are best fitted to
preserve their proper direction when they have no stalks and the base fits directly
on to the stem or passes into it. Cup-shaped lamine, if borne on long erect
petioles, necessitate a great expenditure on supporting-cells, and they are, there-
fore, on the whole, rare. Of the plants we know, only certain Stork’s-bills,
Pelargonium zonale, P. heterogamum, &c., afford examples of cup-shaped, cauline
leaves of the kind, borne on long, rigid petioles. In most cases, therefore, cauline
leaves which conduct water centripetally are either sessile or very shortly petiolate,
have their bases close to the stem, and even extend their edges down it more or
less in the torm of wings and ridges, or surround it in the form of collars, lobes,
and auricles, as in the case of so-called amplexicaul leaves.

When the leaves are in pairs opposite one another and the alternate pairs at
right angles, an arrangement known as decussate, the surplus water is usually
conveyed through two grooves, which run down the intervening piece of stem
from one pair of leaves to the next. Each of these grooves begins in an indenta-
tion between the margins of the bases of a pair of leaves, and terminates above the
midrib of one of the leaves belonging to the next pair. Now, water trickling
down such a groove falls precisely on that part of a lower leaf where the rain
retained by the surface of that leaf is collected; and so the stream of water
becomes more and more copious as it approaches the ground. These grooves
may be seen in many species of ringent Labiate, Scrophulariacee, Primulacec,
Gentianacee, Rubiacew, and Willow-herbs; the best-marked instances are found
in the Knotty Fig-wort (Scrophularia nodosa), the Yellow-rattle (Rhinanthus),
the meadow-gentians (Gentiana germanica, Rhetica, &c.), and the Centaury
(Erythrea). The grooves always possess the property of being wetted by water,
whereas the ungrooved parts of the same stem are not wetted. Sometimes the
grooves are fringed with hairs which absorb the water like the threads of a

96 RELATIONS OF FOLIAGE-LEAVES TO ABSORBENT ROOTS.

wick. By means of both contrivances advantage is ensured in that the water
only oozes quite gradually down the moistened grooves, or else is conducted by
the hairy fringes to the base of the stem, and does not rebound at any spot in
the form of drops. Irregularly bounding drops would be liable to fall on the
ground at spots where no absorptive organs awaited them.

In cases where foliage-leaves, adapted to a centripetal conduction of rain, are
arranged upon a spiral line down the stem, instead of in pairs opposite one another,
the water leaks away along the spiral from one leaf to the next, and finally to
the bottom. Then, again, there are often grooves in the stem along which the
water trickles, as, for instance, in the Common Whortleberry (Vacciniwm Myrtillus).
The erect leaves of this plant conduct the drops as they fall to the branches,
which are deeply furrowed. The water travels through the furrows into those
of lower branches, and finally along those of the main stem of the whole bush
down to the earth. In Veratrwm album each of the concave cauline leaves has,
on the upper surface, a number of deep longitudinal grooves, which all discharge
together at the base of the leaf. The water collected there at length overflows
and runs down the round stem in no particular channel.

The descent of rain-water along a spiral line may be very clearly traced in
many plants of the Thistle tribe. If tiny shot-grains are substituted for rain-
drops in a stiff-leaved plant, the course designed for the drops in that particular
species may be followed with ease. When strewn on a mature plant of the
Safflower (Carthamus tinctorius) or of Alfredia cernua (fig. 141), the grains
of shot roll down the somewhat channelled surface of the highest cauline leaf,
which stands up obliquely, and dash against the stem. The latter is half encom-
passed by the leaf-base, and the shot then roll over one of the basal lobes of
the leaf and travel out of the range of that leaf, falling on to the middle of
the one next below. For the amplexicaul foliar bases are so placed that each
leaf has one of its basal lobes above a concave part of the next lower leaf. In
precisely the same way the shot descend from the second leaf to the third, and
so on until they reach the earth quite close to the stem. The descent reminds
one of the game in which a little ball is made to roll along a spiral groove on
to a board furnished with numbered holes. Rain-drops falling upon thistle-like
plants of this kind naturally follow the same course as the shot. Only, the
additional fact must be taken into account that not only the highest but all the
leaves are adapted as receptacles for the rain as it falls, and that consequently
the drops falling from leaf to leaf are augmented by new tributaries, and become
greater and greater as they descend.

A somewhat different method of water-conduction from that which occurs in
the Safflower and in the nodding Alfredia is observed in the Milk Thistle
(Silybum Marianuwm), in the Cotton Thistle (Onopordon), and in the Mullein
(Verbascum phlomoides). The upper leaves, which have two semi-amplexicaul
lobes, are as nearly erect as those of the Safflower and the nodding Alfredia,
and lead the rain off in exactly the same way. But the leaves in the middle

RELATIONS OF FOLIAGE-LEAVES. TO ABSORBENT ROOTS. 97

of the stem are only erect for about three-quarters of their length; the upper-
most third, including the apex, is bent obliquely outwards and downwards. Drops
of rain falling on this upper third of a leaf would flow ina centrifugal direction,
and do, as a matter of fact, drip down from the apex. Now the leaves in all

Et

Fig. 14.—Irrigation of Rain-water.

1In Alfredia cernua, 2 Ina Mullein (Verbascum phlomoides).

these plants are shorter the higher their position upon the stem, so that the total
contour of the plant may be described as a slender pyramid. In consequence of
this, water dropping from the outward-bent and drooping apices of superior leaves
is arrested by that part of an inferior leaf which shelves towards the stem, and is
thereby conducted centripetally. Thus all the rain-water received by a plant

of this kind at last reaches the immediate neighbourhood of the tap-root, and is
Vou. I. 7

98 RELATIONS OF FOLIAGE-LEAVES TO ABSORBENT ROOTS.

a source of nutriment to the absorption-roots which proceed from it. In the
Milk Thistle (Silybum Marianwm) the margins of the cauline leaves are very
much waved, and, in consequence of this undulation, three or four depressions exist
on each side, through which part of the rain, when there is a heavy downpour,
flows off sideways. But even this water, falling laterally, drops upon parts of
lower leaves, which conduct centripetally, and so coalesces with the streamlets
otherwise produced.

It is very rare for plants which convey water centripetally to have their leaves
arranged in two rows. The most striking example of this class is the Japanese
Tricyrtis pilosa, Its leaves are situated on the fully-developed stem very regularly,
one above the other, in two series. Each leaf has two lobes embracing the stem,
but the base is fixed somewhat obliquely, so that one of the lobes is fixed higher
than the other. Moreover, the higher lobe is closely adpressed to the stem, whilst
the lower forms a channel which discharges exactly above the concave surface of
the next lower leaf belonging to the other side. When rain falls on this plant,
the water, collected by one leaf, flows through the broad exit-channel on to the
leaf below on the other side. Thence a somewhat augmented stream falls upon
a leaf of the first series, and so on, a peculiar cascade resulting, which falls in
a zigzag, from leaf to leaf, until it reaches the bottom, close to the stem.

It would, however, be wrong to suppose that the above explanation sets forth
the only significance to be assigned to the various arrangements described. To
many plants it is a matter of indifference in what direction rain-water falls from
the leaves. Such, for instance, is the case with all marsh-plants with roots
buried in mud under water, inasmuch as the rain, as it drops, only goes into the
water in the pond or marsh, and could not be conveyed to a definite spot for
the sake of the absorbent roots. In the Water-plantain, the Flowering-rush, and
the Arrow-head (Alisma, Butomus, Sagittaria), accordingly, no relationship between
the form and direction of the leaves and the position of the absorbent roots is
to be discovered.

On the other hand, in arundinaceous plants (Arundo, Phragmites, Phalaris)
an arrangement has been hit upon which is obviously designed to prevent rain-
water from collecting. between the haulm and the leaf. As is the general rule
with grasses, so also in the above-named kinds of reeds, the stem or haulm is
furnished with nodes, and from each node proceeds a leaf the lower part of which
encases the haulm in the form of a tube or sheath, whilst the upper part is expanded
and presents a flat, strap-shaped or concave surface, standing well away from
the stem. The leaves may be folded round the haulm like banners. At the place
where the sheath passes into the part of the leaf which stands away from the
axis at an obtuse angle, one observes on the edge of the leaf close to the angle,
two distinct depressions which represent conduits and convey part of the rain from
the lamina, There is also a very neat contrivance here in the form of an erect dry
membrane which acts as a dam, the so-called “ligule.” This membrane, inserted
upon the leaf-sheath, is, like the sheath, in contact with the haulm. When rain-

SAPROPHYTES AND THEIR RELATION TO DECAYING BODIES. 99

water flows down to this place it is stemmed by the membrane, as by a dam, and
diverted right and left into the two grooves. In this way water is prevented
from accumulating between the leaf-sheath and haulm, where it might do damage.
In many reeds the contrivances for irrigation are even more complete than this.
Sometimes hairs depend from the margin of the membrane in the direction of the
grooves and, like a wick, lead the water in the proper direction.

An opportunity will occur later on of showing how the conduction of rain to
particular spots has an important bearing on the phenomenon of absorption by
aerial parts of plants: and also in the regulation of transpiration; and how, by
means of the apparatus for water-irrigation, not only absorptive cells at the
extremities of roots in the earth, but special organs on the foliage-leaves as well,
are often supplied with water.

3. ABSORPTION OF ORGANIC MATTER FROM DECAYING
PLANTS AND ANIMALS.

Saprophytes and their relation to decaying bodies.—Saprophytes in water, on the bark of trees, and
on rocks.—Saprophytes in the humus of woods, meadows, and moors.—Special relations between
Saprophytes and the nutrient substratum.—Plants with traps or pitfalls for animals.—Insecti-
vorous plants which perform movements for the capture of prey.—Insectivorous plants with
adhesive apparatus.

SAPROPHYTES AND THEIR RELATION TO DECAYING BODIES.

Whenever plants which take up organic compounds formed in the process of
decay are the subject of discussion, the first examples that occur to everyone are
members of the great family of Fungi, specimens of which make their appearance
wherever dead animals or plants are undergoing decomposition. We recall the
moulds, plasmodia, puff-balls, and mushrooms, which grow from dead organic bodies,
and are associated with the unpleasant. mouldy and cadaverous smell always
perceptible in their neighbourhood.

Many of these organisms do, in fact, belong to the class of Saprophytes. Indeed,
one group of them is itself the cause of the chemical decomposition of dead plants
and animals called decay. Their elongated thin-walled cells, the so-called “hyphe”,
thread themselves through dead bodies, and unite to form strands, bundles, net-
works, and membranes, the whole constituting a structure to which the term
“mycelium” is applied. These mycelia are often to be seen, with the naked eye,
covering large areas. For instance, in damp cellars, mines, and railway-tunnels, any
old rotten wood-work is clothed with delicate, whitish reticula and membranes.
The heaps of grape-skins, stalks, and other refuse piled up in the open air by the
side of vineyards after a vintage, are usually so completely overgrown by mycelia

100 SAPROPHYTES AND THEIR RELATION TO DECAYING BODIES.

that their colour is quite altered. The so-called “mushroom-spawn”, used in the
cultivation of mushrooms, is also nothing but a mycelium, which entirely invests
the manure employed in the cultivation of that fungus, and gives it a white
mottled appearance.

In addition to Fungi, however, a number of Mosses, Liverworts, Ferns, Lycopods,
and Phanerogams take up organic compounds from the products of decay to serve
as their food.

In deciding whether a plant takes up only the mineral substances rendered
soluble by the decomposition of the soil, or only organic substances disengaged
by the decay of dead plants and animals, we depend generally on the condition
and appearance of the nutrient substratum, and, in particular, on its composition,
i.e. whether it is exclusively or predominantly organic. But such observations
give a very uncertain indication. For, on the one hand, it is possible for plants
rooted in a substratum of decaying matter to take nothing but mineral salts (ve.
inorganic compounds) from it; and, on the other hand, it frequently happens that
sand or clay, apparently uncontaminated with organic matter, is saturated by
water which oozes from a layer of humus in the vicinity, and brings with it
organic compounds in solution. The following facts are instructive with reference
to the former of these two phenomena. Maize, barley, and other cereals may be
reared in fluids, so prepared as to contain a small quantity of mineral food-salts
dissolved in distilled water (12 mg. potassium phosphate, 12 mg. sodium phosphate,
27 mg. calcium chloride, 40 mg. potassium chloride, 20 mg. magnesium sulphate,
10 mg. ammonium sulphate, and a few drops of iron chloride in a litre of distilled
water), all organic compounds being carefully excluded. When the plants germinate,
they develop roots which descend in the liquid and absorb from it mineral salts
according to their requirements. They produce stems, leaves, flowers, and, ulti-
mately, seeds capable of germination. Other plants of maize or barley reared
simultaneously in richly-manured ground develop likewise leaves, flowers, and fruit.
Moreover, analysis of the ash in both cases reveals the fact that the plants which
took their nutriment from the manure contain the same salts as those reared in the
made-up solution of salts free from organic compounds. Hence, the conclusion may
be drawn that a plant of this kind is capable of obtaining an adequate supply of
food-salts equally well, either from earth free from humus and manure, or from
humus or manure themselves. The experiment further shows that, in the latter
case, organic compounds need not necessarily be absorbed, in addition to the mineral
constituents of humus or manure which are disengaged during decomposition.

We must next refer to a fact in connection with the second point above men-
tioned, viz. that plants rooted in sand or loam devoid of humus may yet have
organic compounds brought to them by water filtering through a stratum of humus
near at hand. The fact in question is, that the very water which one would least
expect should contain organic compounds, that, for instance, of cold mountain
streams, does very generally include traces of such compounds. On looking through
analyses of mineral springs, one finds for the most part, amongst their constituents,

SAPROPHYTES AND THEIR RELATION TO DECAYING BODIES. 101

combustible bodies arising from the dissolution of organic matter. Even the acid
formerly designated by Berzelius by the name of “spring-acid”, is doubtless a pro-
duct of the decay of fragments of plants in the place where the water of the spring
collects. So also is humic acid, a compound produced by decay. The nature of
this acid is not yet, it is true, thoroughly known, and it may be a mixture of several
acids. We know, however, that it is easily soluble in water, and that it forms
soluble compounds with alkalies. Brooks running through woods or meadows,
small mountain lakes adjoining peat-beds, and pools in actual peat, consist of
water, brown in colour, which gives an acid reaction, and contains invariably
organic substances in solution.

The following observations are of great interest in connection with this subject.
In the salt-mine at Hallstatt (Upper Austria) one of the galleries, which is hewn
‘through rock and contains no wood-work of any kind, exhibited (spread out upon
its smooth limestone roof) the mycelium of a fungus (an Omphalia), which
certainly required organic nutriment. There were no decaying animal or vegetable
remains anywhere in the gallery, and the mycelium derived nourishment solely
from water oozing from above through a few narrow cracks in the stone whereby
the surface of the latter was kept moist. This water came from a meadow lying
high above the mine. Between the two was a thick stratum of limestone with
a deep layer of earth resting upon it. The water was clear and colourless, and
contained a certain amount of lime, but no perceptible trace of organic substances.
Yet this water must have brought organic matter from the meadow above into the
mine, and the minute quantity so introduced sufficed to enable the fungus mycelium
to grow luxuriantly.

In the Volderthal, near Hall, in Tyrol, there is a spring of cold clear water
rising out of slate at a height of 1000 metres above the sea-level, which is filled
at its source with a dark thick felt. The felt may be lifted out in pieces the size
of one’s hand, and it is the mycelium of a fungus, probably a Peziza. It clings
to slabs of slate, between which the water trickles abundantly, and its nutriment
can only be derived from this water. There are pine-woods and meadows in
the neighbourhood, but no greater amount of vegetation, humus, or rotten timber
than is found near other springs.

These instances satisfactorily prove that even the clearest mountain springs
contain organic substances in quantities sufficient, however minute, to nourish
fungi. When the origin of springs is taken into account, this result is not really
surprising. They are fed by deposition from the atmosphere. The water thus
deposited percolates into the ground, passing, in the first place, through a layer of
earth-mould which is covered by vegetation, and contains more or less humus in its
upper strata. A small quantity of the products of decay is inevitably absorbed,
and even if they are partially withdrawn again in lower strata of the earth, traces
are still retained by the water in its descent to greater depths, and re-ascent to the
surface in the form of springs. The characteristics of the great veins of water
which ascend in this way are no doubt common to the smaller veins which originate

102 SAPROPHYTES AND THEIR RELATION TO DECAYING BODIES.

in the vegetable mould saturated by snow and rain on the ground of forests or in
the humus covering meadows, and which percolate through into the sand or loam
beneath. Plants whose roots ramify in this deeper layer of earth derive thence the
organic compounds conveyed by the water, and have the additional advantage of
being able to satisfy at the same time their requirements as regards mineral sub-
stances. This circumstance is of importance not only to flowering-plants but also
to many fungi, as, for instance, to all species of Phallus, they having need of a
great deal of lime. An explanation is thus afforded of the fact, formerly difficult to
understand, that in forests and meadows not only the upper black or brown humus
layer, but also the underlying yellow loam, or pale sand, neither of which latter
contains any humus, has mycelia of fungi running through it in every direction,
and weaving their threads over little fragments of rock. Indeed, it sometimes
happens that the lower layer of earth is more abundantly penetrated with plexuses
of hyphe than is the upper layer, consisting of vegetable mould. The greatest
number of saprophytes is to be found therefore at places where the humus layer is
not too thick and loam or sand occurs at no great depth; but where decaying
vegetable remains are piled metres high, as on moors, for example, instead of fungi
being produced in extraordinary abundance, as one might expect, only a few occur.
Pure peat is by no means a favourable soil for fungi, a circumstance which may be
partly due to the antiseptic action of certain compounds developed in it.

It follows from the foregoing observations that a sure conclusion as to the
nature of plants rooted in a particular substratum cannot possibly be derived from
the mere appearance of the substratum. Moreover, the conditions necessary for the
growth of plants requiring organic products of decay as nutriment appear to be of
much wider occurrence than one would suppose upon a cursory observation of the
conditions existing in fields and forests, or, if one considers exclusively instances of
cultivated plants reared on arable land, which is manured and constantly turned
over. The great variety of plants produced on a limited area is also now
intelligible. From the same soil some absorb organic compounds, others mineral
substances only; whilst others again take some organic and some mineral food-
salts. The determining factor is not the amount of a given substance present
in the substratum, but rather the special needs of each species, and ultimately the
specific constitution of the protoplasm in each one of the plants which thus, side by
side, nourish themselves in totally different ways.

If, then, neither the appearance of the ground nor its richness in respect of
humus affords any certain indication as to whether a particular plant lives on
organic products of decay or not, the question may perhaps be solved by the fact
of the plant’s containing or not containing green chlorophyll-corpuscles. We may
take it as proved by many results of investigation, that the decomposition of the
carbon-dioxide absorbed by a plant from the air, and the formation of the organic
compounds of carbon, hydrogen, and oxygen known as carbohydrates (which play
so important a part in vegetable economy), only take place in organs possessing the
green pigment known as chlorophyll. We shall return to a discussion of these

SAPROPHYTES AND THEIR RELATION TO DECAYING BODIES, 103

processes in detail later on, but the fact must be taken into consideration here.
One would suppose, accordingly, that plants able to obtain ready-made organic
compounds from a nutrient substratum could spare themselves the trouble of
building them up, so that the presence of chlorophyll would be superfluous.
This conjecture is in fact supported by the absence of chlorophyll in fungi, which
are typical instances of saprophytes. But, on the other hand, some plants appear
to negative this assumption, or at any rate to deprive it of general application.
In mountain districts, where cattle continually pass to and from the meadows and
alps, one notices on their halting grounds, and along their tracks, moss of a con-
spicuous green colour growing on circumscribed spots. On closer examination we
find that we have here an example of the remarkable group of the Splachnacee,
and that it has selected the cow-dung to be its nutrient substratum. Each growth
of emerald green, Splachnum ampullacewm, is strictly limited to the area of a
lump of dung; no trace of it is to be seen elsewhere. All the stages of development
of this moss follow one another upon the same substratum. First of all the lumps
of dirt which are kept moist by rain or by standing water, become enveloped in
a web of protonema, and their surfaces acquire thereby a characteristic greenish
lustre. Later, hundreds of little green stems, thickly clothed with leaves, emerge,
and the spore-cases, which resemble ‘tiny antique jars, and are amongst the
prettiest exhibited by the world of mosses, become visible as well. Just as
Splachnum ampullaceum is produced on the dung of cattle, so is Tetraplodon
angustatus on that of carnivorous animals, and there can be no doubt that
these, and in general all Splachnacew, are true saprophytes, A similar remark
holds with regard to the green Huglene which escape from Hormidium-cells, and
fill the foul-smelling liquor in dung-pits and puddles near cattle-stalls in mountain
villages, and which multiply to such an extent that in a few days the liquid
changes colour from brown to green.

Thus plants do exist containing chlorophyll although absorbing from the
substratum organic compounds alone, and containing it, indeed, in such quantities
that its presence cannot be looked upon as accidental. It follows, firstly, that
absence of chlorophyll is not the distinguishing mark of saprophytic plants; and,
secondly, that the organic nutriment of the plants above mentioned cannot be used
forthwith unaltered in the building up and extension of their structures, but, like
inorganic material, must undergo various changes, that is, must be to a certain
extent digested before being used for construction. The probability is that green
saprophytes take carbon from their substratum in a form unfitted for the manu-
facture of cellulose and other carbohydrates. Saprophytes that are not green
must obtain carbon from the substratum in the form of a compound, the direct
absorption of which could be dispensed with if chlorophyll were present; but it
does not necessarily follow that all the organic compounds absorbed by non-green
saprophytes are capable of immediate service as materials for construction without
any preliminary alteration.

Impartial consideration of the above facts forces us to conclude that there is no

104 SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS.

well-marked boundary line between plants which absorb organic compounds and
those which absorb inorganic compounds from their respective substrata; and that
there undoubtedly exist plants capable of taking up both kinds of material at the
same time. This conviction is strengthened still further by the circumstance,
which has been repeatedly confirmed by experiment, that plants susceptible of
being successfully reared in artificial solutions of mineral salts—to the exclusion
of organic compounds—do not entirely reject organic compounds when the latter
are tendered to them, but unquestionably assimilate some of them (urea, uric acid,
glycocoll, &.) and work them up into constituents of their own frames.

But, in spite of the impossibility of drawing a sharp line of demarcation
between the two groups, it is convenient to treat of the absorption of organic
compounds separately, because this division of the subject affords the best
opportunity of inspecting in detail, and of surveying generally, the conditions of
food-absorption, the comprehension of which is otherwise difficult. In order to
determine in each individual case whether a given plant lives either exclusively
or principally upon organic food, derived from decaying animal or vegetable
remains, reliance must be placed on experiments with cultures; and, in the absence
of better vantage-ground, the results of the rougher experiments made by
gardeners should not be neglected, always providing that they are accepted
subject to possible correction by subsequent exact experiment.

SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS.

Of the special cases of absorption of organic compounds from decaying bodies,
we have first of all to consider those occurring amongst water-plants. In the sea,
wherever there is an abundance of animal and vegetable life there is also plenty
of refuse, for there death and decay hold a rich harvest. The quantity of organic
matter dissolved in the water is naturally greater in these places than where
vegetation and animal life are less conspicuous. There is a much more varied
flora and fauna to be met with in the sea near its coasts, especially in shallow
inlets, than at a greater distance from the shore; and the number of dead organisms
is also greater near the coast. A mass of organic remains is thrown up by the
tide, and by waves in stormy weather. This mass rots during the ebb. Part of it
is dragged out to sea again by the next high tide, and then flung up once more;
so that the beach is always strewn with dead remains, and the sea near the shore
contains more products of decomposition than in the open.

In the immediate neighbourhood of seaports, moreover, or wherever people
live, the volume of refuse is considerably increased, and the water in harbours and
stagnant inlets behind breakwaters, and at the mouths of canals and sewers, contains
such a large quantity of organic refuse in a state of decomposition that its presence
is revealed by the odour emitted. Now it is just at these places that an abundant
vegetation of hydrophytes is developed. Not only the bottom of shallows, but
stones, stakes, quays, buoys, and even the keels and planks of boats long anchored

SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS. 105

in harbour, are overgrown by Ulve, wracks, filamentous alge, and Floridew. Not
a few, as, for instance, the so-called sea-lettuce (Ulva lactuca), several species of
Gelidium, Bangia, and Ceramium, and the great Cystosira barbata, thrive best
and in greatest abundance in polluted water of the kind; and there can be no
doubt that this is to be accounted for by the presence of a greater quantity of
organic compounds in that water.

It is not only in contaminated sea-water, but also in other collections of water
which contain products of putrefaction in solution, that we find a characteristic
vegetation. We have already alluded to the presence of Euglene in the liquor
of manure-pits. They occur also at the foot of shady walls, in dirty back
streets in towns, in the puddles, and on ground which is saturated with urine and
impurities of every kind. These places are the home of a number of other minute
plants, which stain the polluted ground after rain with the gayest colours. There,
side by side with black patches of Oscillaria antliaria and verdigris-coloured films
of Oscillaria tenuis, are blood-red patches of Palmella cruenta, and brick-red
patches of Chroococcus cinnamomeus. Equally characteristic is the vegetation
which covers the earth at the mouths of drains, and is bathed by the trickling
sewage. Large areas here are overgrown by the green Hormidium murale, which
weaves itself over the mire, and by the dark, actively-oscillating Oscillaria limosa;
and, above all, the curious Beggiatoa versatilis makes itself conspicuous, sending
out from a whitish gelatinous ground mass long oscillating filaments, which emerge
after sundown, and next day split up into innumerable little bacteria-rods. The
red-snow alga, too (represented in Plate I.), lives at the expense of the pollen-
grains, bodies of insects, and other decaying matter blown on to snow-fields;
whilst the nearly allied blood-red alga (Hematococcus pluvialis or Spherella
pluvialis) lives in the water in hollow stones where all sorts of animal and
vegetable remains collect. Leaves blown into deep pools, and lying rotting
at the bottom, are everywhere overgrown by green dogonium, by Plewrococcus
angulosus, and by the amethyst-coloured Protococcus roseo-persicinus. The
bottoms of ditches on peat-bogs, which are full of brownish water containing an
abundance of compounds of humic acid in solution, are covered with this amethyst
Protococcus, whilst a profusion of small filamentous alge, Oscillariz and so forth
(Bulbochete parvula, Schizochlamys gelatinosa, Spherozosma vertebrata, Micro-
cystis ichthyloba, &c.), as well as a group of dusky mosses (Hypnum giganteum, H.
sarmentosum, H. cordifoliwm), all have their home exclusively in still water richly
supplied with organic compounds. When we include also the curious mould-like
Saprolegnie produced on dead bodies floating in water—Saprolegnia ferax and
Achlya prolifera on flies and fishes—some idea is obtained of the great variety
of saprophytes living in fresh water, as well as of those inhabiting the sea.

A much more agreeable and attractive picture than that of these aquatic sapro-
phytes is afforded by plants whose sole habitat is the bark of trees. The dead
bark does not constitute the nutrient base of all the plants which grow from
trunks and branches, or climb up them in the form of clinging and twining lianas.

106 SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS.

Often the trees only serve as supports, by means of which the plants in question
raise themselves out of darkness into light. Such food-salts as they require they
take, not from their support, but from the earth, into which they send absorptive
roots. As years go by, a quantity of inorganic dust collects in the forks of
branches and in the little rents and fissures in the bark of old trees, and this dust
gets mixed with crumbled particles of bark. The clefts, therefore, are more or
less full of vegetable mould, and this forms an excellent foster-soil for a large
number of plants. But it is not necessarily the case that all plants rooting in this
mould take up organic compounds from it. Thus, one finds not infrequently in the
angles of bifurcation of the trunks of old limes and other trees, little gooseberry
and elder bushes, and bitter-sweet plants, which have germinated there from fruits
brought by black-birds, thrushes, and other frugivora. These shrubs, in the forks
of limes and poplars hardly take any organic compounds from the mould in which
they are rooted, but confine themselves to the absorption of such mineral salts as
they may require.

But, with the exception of instances of that kind, the great majority of plants,
nestling in the mould in crevices of bark, do take nutriment from this their
substratum in the form of organic compounds. In cold regions the plants living
in the mould of bark are for the most part mosses and liverworts. They cover
trunks and branches of old ashes, poplars, and oaks, with a thick green mantle, and
grow especially on the weather-side of the trees. In the tropics, on the other hand,
the fissured bark of trees is a rallying ground not only for delicate mosses and
moss-like Lycopodia, but also for a whole host of ferns and vivid flowering plants.
The number of small ferns which develop and unroll their fronds from chinks in
the bark of trees is so great that old trunks appear wrapped in a regular foliage of
fern-fronds. Of Phanerogams, in particular, the Aroidew, Orchidacee, Bromeliacee,
Dorstenice Begoniacee, and even Cactacew (species of the genera Cereus and
Lhipsalis) bury their roots in the mould of bark. It is to be remarked that the
rosettes of Bromeliacew ornament chiefly the forks of trunks, whilst Dorstenia,
Orchidec, and the various species of Rhipsalis grow on the upper side of branches
that ramify horizontally; whilst, lastly, Aroidew and Begonice take root, for the
most part, on the surfaces of huge erect trunks.

Besides the mould collected in crevices and fissures of bark, the bark itself, that
is, the cortical layer, dead but not yet crumbled and mouldered into dust, forms
a nutrient substratum for a whole series of plants of most various affinity.
Many fungi and lichens penetrate deeply the compact bark, and their hyphal
filaments ramify between its dead cells. Other plants, instead of piercing through
the substance of the bark, lay themselves flat upon its surface, and grow to it so
firmly that if one tries to lift them away from the substratum, either part of the
latter breaks off, or the adnate cell-strata are rent, but there is no separation of the
one from the other. If a tuft of moss (e.g. Orthotrichum fallax, O. tenellwm, or
O. pallens), growing on bark, or a liverwort (eg. Frullania dilatata) closely
adherent to a similar basis, is forcibly removed, little fragments of the bark may be

SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS, 107

always seen torn off with the rhizoids at the places where they issue from the
stemlets. The same thing occurs in the case of the roots of tropical orchids
growing to the tree-trunks which constitute their habitat. The majority of these
tree-orchids nestle, no doubt, in mould-filled crevices of the bark, and nourish them-

i in

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selves, besides, by means of special aerial roots which hang down in white ropes
and threads, like a mane, from the places where the plants are situated upon the
trees, and which will presently be described in detail. But a small section develops
strap-shaped roots as well, which adhere firmly to the bark with their flat surfaces.
This phenomenon is most strikingly exhibited by the splendid Phalenopsis

108 SAPROPHYTES IN WATER, ON THE BARK OF TREES, AND ON ROCKS.

Schilleriana, a native of the Philippine Islands; its roots are rigid, compressed,
and about 1 cm. in breadth; the surface turned away from the trunk is slightly
convex, and has a granular structure and metallic glitter like a lizard’s or chame-
leon’s tail. The surface towards the trunk is flat and without metallic glitter,
and upon it, close behind the growing point, there is a whitish fur consisting of
short, thickly packed, absorptive cells. When the tip of one of these roots comes
into contact with the bark it grows so firmly to the substratum by means of the
absorption-cells, that it is easier to detach superficial bits of the bark itself than
the root. The latter, once fixed, flattens out still more and becomes strap-shaped,
whilst creeping outgrowths proceed from it, forming strips which may ultimately
attain a length of 14 metres. The sight of a trunk covered with these long
metallic bands is one that never fails to excite wonder even in the midst of the
world of orchids, wherein, as is well known, there is much to marvel at.

In other species of tropical orchids, eg. in Sarcanthus rostratus (fig. 15), the
roots are not flat from the beginning, but become so when they come into con-
tact with the bark. A root is often to be seen which arises as a cylindrical cord
from the axis, then lays itself upon the bark in the form of a band, and further on
lifts itself once more, resuming at the same time the rope form, as is shown in the
illustration. Here also complete coalescence takes place between the bands and the
bark, and the union is extremely close. Similar conditions have been observed to
hold in many Aroidee living on the bark of trees. The plants in question lie with
their stems, leaves, and roots flat against the trunks, so that they suggest a covering
of drapery. Taking, for instance, the Marcgravie (Marcgravia paradoaa, M.
umbellata), one might at first sight suppose that they adhere to the bark not only
by the roots, but also by the large discoid leaves, which are arranged in two rows.
A very remarkable fact also, in connection with these plants, is that they only
grow on very smooth and firm bark. When transferred to a soft substratum, such
as mould or moss, they languish, because their roots are unable to enter into close
union with a support of such loose texture. This is also true of most tropical
orchids living on bark. When their seeds are transferred to loose earth devoid of
humus, they do indeed germinate, but then perish; whereas when sown on the bark
of a tree, they not only germinate, but grow up with ease into hardy plants.

Where steep rocks occur near clumps of trees it is not uncommon for the same
species of plants to grow on both. Allusion is not here made to kinds which, like
ivy, have their roots in the earth at the foot of rocks and trees, and creep up the
one or the other indifferently, using both merely for support and not as sources of
nutriment, and clinging to them by means of special attachment-roots. The remark
is applicable also to plants which live on the products of the decay of organic
bodies, for example many tropical Orchidew, Dorstenic, Begonic, and Ferns; and in
cooler parts a number of Mosses and Liverworts. It is not difficult to explain this
phenomenon in the case of species which derive their food from vegetable mould.
The crannied wall of rock is, in a certain way, analogous to the rugged bark of a
tree. The holes in the rock are filled in course of time with black vegetable mould,

SAPROPHYTES IN THE HUMUS OF WOODS, MEADOWS, AND MOORS. 109

and plants with foliage, flowers, and fruit of a form adaptable to cracks and holes
are able to establish themselves in the mould there, just as well as in that collected
in crevices of bark. In one respect, indeed, they are even more favourably situated.
For the humus in bark gets quite dry in long periods of drought, because no water
is yielded to the bark by the wood of a tree, even though the latter be abundantly
supplied with sap; whereas, in the case of rocks the probability is, the clefts being
very deep, that even when the top layers of humus filling them yield up their water
to the air, a certain restitution of moisture takes place from the deeper parts, which
are never quite dry. Moreover, plants growing in the mould of rock crevices are
able to send their roots down to much deeper strata than is possible in the case of
bark. This is another reason why deep cracks in rocks, filled with humus, exhibit.
a richer flora, as a rule, than do the much shallower crevices in the bark of trees,
although, as has been said before, the two habitats have many plants i1 common.

It is more difficult to explain how it happens that plants which derive their
sustenance, not from the mould in crevices, but from the substance of the bark
itself, and which lie flat against its surface, are also found adhering to walls of
rock. As an example take Frullania tamarisci, a Liverwort with small brown
bifurcating stems, which bear double rows of leaves and are of dendritic appearance.
This plant grows equally well on the bark of pines or on the face of adjacent gneiss
rocks. At first sight it would seem scarcely possible that a plant of this kind,
clinging to the unfissured surface of rock, should be in a position to obtain organic
compounds from its substratum. This is nevertheless the case. Closer inspection
reveals the fact that the Liverwort does not adhere to blank rock, but to a part
formerly clothed by rock-lichens. This inconspicuous incrustation of dead lichens
is a complete substitute for the superficial layer of bark, and it is into it that the
Frullania tamarisci sinks its roots. Another way by which food is supplied to
plants adherent, like the above, to vertical and unfissured rocks will be discussed
later on.

SAPROPHYTES IN THE HUMUS OF WOODS, MEADOWS, AND MOORS.

Damp shady woods, especially pine woods, are particularly well furnished with
saprophytes. Here again we find representatives of the same families as choose the
bark of trees for their habitat. On the ground of woods, the most characteristic
forms are mosses, fungi, lycopods, ferns, aroids, and orchids. The dark-brown
humus, produced from dropped and decaying needles, is first of all covered by a
rich carpet of mosses, such as the widely distributed Hylocomiwm splendens,
Hypnum triquetrum, and Hypnum Crista-castrensis. The mouldered dust of
dead trees has a clothing of Tetraphis pellucida and of Webera nutans, and
decaying trunks are overgrown by the cushions of species of Dicranwm (Dicranum
scoparium, D. congestum, Dicranodontiwm longirostre), pale feathery mosses
(Hypnum uncinatum and H. reptile) and various liverworts. Everywhere above
the soft, ever-moist carpet of moss rise green fronds belonging to broad-leaved ferns.

110 SAPROPHYTES IN THE HUMUS OF WOODS, MEADOWS, AND MOORS

Woods are also the special abode of fungi, and the damp ground is covered towards
autumn by innumerable quantities of their curious fructifications. Dropped needles
and cones, leaves and sticks strewn upon the ground, fallen trunks, and even the
dark amorphous dust arising from the mouldering of these bodies and of the
numerous roots ramifying in the ground, appear to be perforated by and wrapped
in the protoplasmic threads of plasmoid fungi, or similarly invested by a plexus
of filaments, the so-called mycelia of other forms of fungi. Amongst the scaly
fragments of bark, peeling from the trees, they appear in the form of slimy strings,
or as a dark trellis and net-work, inserted between the bark and wood of the
rotting tree; on the stripped white trunk they are in dark zigzag lines like
those of forked lightning; and between, the white mycelia of huge toadstools and
tremellas are woven in all directions. Here and there large areas of the brown
decaying soil are flecked and speckled by these mycelia, and even the dead stems
of the mosses on the ground are festooned with white fleece, and wrapped round
by hyphee.

It is worth while to glance too at the reciprocal relations of these woodland
plants. We find mosses, lycopods, and various ferns and phanerogams living
upon the fallen twigs and needles, and on the mouldering roots of pines and fir-
trees. The dead remains of those plants afford sustenance to the fungi, which lift
their fructification above the bed of moss. In their turn the rotting fructifications
of the larger fungi form a nutrient substratum for smaller fungi, which cover the
decaying caps and stalks with a dark-green velvet. Lastly, these little fungi, too,
fall a prey to corrupting bacteria, and are resolved into the same simple inorganic
compounds as were absorbed from the air and earth, in the first instance, by the
pines and fir-trees. In the depths of forests there is going on, for the most part
unseen by us, a mysterious stir and strife, accompanied by an uninterrupted process
of exchange between the living and the dead, and a marvellous transformation of
those very substances whose secret we have only partially succeeded in solving.

The results of cultivation have proved that in the group of flowering-plants
belonging to the woodlands of Central and Northern Europe, which derive sus-
tenance partially or entirely from the organic compounds afforded by the humus,
are to be included, amongst others, the various species of coral-wort (Dentaria
bulbifera, D. digitata, D. enneaphyllos), Circea alpina, Galium rotundifolium,
and Linnea borealis, and above all a large number of orchids. Of these, Dentaria
prefers mould produced from the beech leaves, and Circa, Galiwm, and Linnea
appertain to the mould of pine-woods. Of the orchids some are provided with
green leaves, as, for instance, the delicate little Listera cordata, Goodyera repens
remarkable for its villous petals, and the various species of Cephalanthera, Epi-
pactis, and Platanthera; others, such as Limodorum abortivum, the bird’s-nest
orchis, the coral-root, and Epipogiwm aphyllum have none. Limodorwm abortivum
belongs rather to the warmer districts of Central Europe. It has fleshy root-
fibres, twisted and twined into an inextricable ball, and a slender steel-blue stem,
over half a metre in height, bearing a lax spike of fairly large flowers, which

SAPROPHYTES IN THE HUMUS OF WOODS, MEADOWS, AND MOORS. lil

subsequently become paler in colour. The bird’s-nest orchis (Neottia Nidus-avis)
is of wide distribution both in forests of pines and in those composed of angio-
spermous trees. Its stem and flowers are of a light-brown colour, unusual in plants,
but somewhat like that of oak-wood. The flowers have no scent, and the numerous
roots, issuing from the subterranean part of the stem and imbedded in humus,
remind one in form and colour of earth-worms, and together constitute a strange
tangled mass as large as a fist. The latter has been thought to resemble a bird’s
nest, and to this is due the name of the plant. The coral-root (Corallorhiza innata),
unlike the bird’s-nest orchis, has no root at all; but, on the other hand, the sub-
terranean portion of the stem, the so-called rhizome, possesses a distant resemblance
to the root-tangle of Neottia. Pale-brownish branches of this rhizome, which
bifurcate repeatedly at their obtuse and whitish extremities, looking as if they
had been subjected to pressure for a time, and all the short lobe-shaped branchlets
thereby spread out into one plane, lie closely crowded together, sometimes crossing
one another, and so form a body which vividly recalls the appearance of a piece of
coral. This underground coral-like stem-structure develops each year pale greenish
shoots which rise above the ground and bear small flowers speckled with yellow,
white, and violet, and exhaling a scent of vanilla; later, green fruits of a com-
paratively large size develop, turning brown when they ripen.

The fourth mentioned of these pale wood-orchids, the Epipogiwm aphyllum, is
at once the rarest and most curious of them all. Like the coral-root it has no true
roots. Its rhizome so closely resembles the latter’s that it is easy to mistake the
one for the other; but they may be distinguished by the fact that in the case of
Epipogiwm the rhizome sends out. long filiform shoots, which swell up like tubers
at their tips, and may be regarded as subterranean runners. The swollen extremity
becomes the point of origin of a new coral-like structure, which develops at about
the distance of a span from the old one; whilst the latter, usually exhausted after
flowering, gradually perishes. This coral-like stem lives of course underground,
and is not visible till one lifts away the moss from the mould on the ground. It is
often completely imbedded in sandy loam, lying immediately beneath the black
mould. Many years frequently go by without the Hpipogiwm producing flowers.
The plant meanwhile lives entirely underground. In the course of a summer in
which it has not flowered, anyone not having previous exact knowledge of its where-
abouts might pass by without dreaming that the bed of moss and humus on his
path concealed this strange growth. The flowering stems which at length emerge,
when there is a warm summer, are right above the place where they branch off
from the subterranean rhizome. They are thickened in a fusiform manner, and
have, for the most part on one side, a reddish or purplish tinge. Everything
connected with them is tense, smooth, full of sap, and almost opalescent. The
few flowers that are borne by the stem are comparatively large, and emit a strong
perfume resembling that of the Brazilian genus of orchids Stanhopea. The colour-
ing, too, a dull yellowish white with touches of pale red and violet, reminds one of
these tropical orchids.

112 SAPROPHYTES IN THE HUMUS OF WOODS, MEADOWS, AND MOORS.

The sight of the pale-coloured plants lifting their heads, at flowering time, from
the tumid carpet of moss has all the stranger effect because, as a rule, no other
flowering plants are visible in any direction. The flowers are suspended by delicate
drooping pedicels, and owing to their peculiar colour, fleshy consistence, and form
—the erect concave petal like a Phrygian cap or helmet, and the others stretched
out like prehensile limbs—remind one of the opalescent meduse which float on
the blue sea waves. The propriety of the analogy is enhanced by the fact
that the form and colour of other saprophytes produced near Epypogiwm in
woods have a striking resemblance to the animals and wracks which inhabit the
sea-bottom. The fungi, known by the name of club-tops, much-branched, flesh-
coloured, yellow or white Clavarie, which often adorn whole tracts of ground in a
wood, imitate the structure of corals; Hydnew are like sea-urchins, and Geaster
like a star-fish, whilst the various species of Tremella, Exidia, and Guepinia, which
are flesh-pink, orange, or brownish in colour, and the white translucent Tremellodon
gelatinosum, resemble gelatinous sponges. The small stiff toad-stools (Marasmius),
which raise their slender stalks on fallen pine-needles, remind one of the rigid
Acetabularic. Other toad-stools, with flat or convex caps exhibiting concentric
bands and stripes, such as the different species of Craterellus, have an appearance
similar to the salt-water alga known by the name of Padina. Dark species of
Geoglosswm imitate the brown Fucoideew; and one may fancy the red warts of
Lycogala Epidendron, a plasmoid fungus inhabiting the rotten wood of dead
weather-beaten trees, to be red sea-anemones with their tentacles drawn in,
clinging to gray rocks. However far-fetched this comparison between the two
localities may seem at first sight, everyone who has had an opportunity of
thoroughly observing the characteristic forms of vegetable and animal life in
woods, and at the bottom of the sea, will inevitably be convinced of its accuracy.

Meadow-land, rich in humus, is much more sparsely occupied by saprophytes
than the soil of woods. There is no lack of the strange forms of toad-stools and
puff-balls, whose fructifications often spring up in thousands, especially in the
autumn, in company with the meadow-saffron; but in numbers they are not to be
compared with those which occur in the mould of woods. Amongst ferns and
phanerogams, the following species are dependent upon the organic compounds
arising from the decomposition of the humus: Moonwort (Botrychium Lunaria),
numerous orchids, blue and violet-flowered gentians, the famous Arnica, Poly-
galacez, and more especially several grasses, chiefly the Matweed (Nardus stricta)
which, when once it has struck root in the humus, extends in dense masses over
large areas. Several plants, too, adorning alpine pastures, and belonging for the
most part to the same families as the species mentioned above, are to be regarded
as humus-plants. Such are the Alpine Club-moss (Lycopodium alpinum), the
dark-flowered Migritella nigra, and several other sub-alpine orchids; a number of
small, sometimes tiny, gentians (Gentiana nivalis, G. prostrata, G. glacialis,
G. nana, Lomatogonium Carinthiacum), Valeriana, celtica, the Scottish asphodel
(Tofieldia borealis) of the north, a few grasses, sedges, and rushes (eg. Agrostis

RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM. 113

alpina, Carea curvula, Juncus trifidus), various anemones, campions, umbelliferous
plants, violets and campanulas (eg. Anemone alpina, Silene Pumilio, Mewm
Mutellina, Viola alpina, Campanula alpina) and several mosses (e.g. Dicranuwm
elongatum and Polytrichum strictwm) which clothe the humus on stretches of turf
and in inclosures.

Many of the plants also that are native on the black graphitic soil in hollows
of high mountain ridges take up organic food from their substratum. These
include Meesia alpina and various other mosses produced exclusively in places of
the kind; and, above all, numerous Primulacese and Gentianese (Primula glutinosa,
Soldanella pusilla, Gentiana Bavarica). It seems, moreover, to be by no means a
matter of indifference to these plants at what temperature, and in what state of the
air, in respect of moisture, the decomposition of humus takes place. If species which
grow abundantly in these localities are dug up and transferred, together with the
black earth in which their roots are imbedded, into a garden, and are there
cultivated in such a way that the external conditions are as nearly as possible those
of the original habitat; or if young plants are reared from seed in the same black
humus-filled earth, they thrive only for a short time, soon begin to fade, and within
the space of a year are dead; whereas, alpine plants belonging to the same altitude
above the sea, but rooted in loamy or sandy earth, flourish excellently in gardens
as well. Various moor-plants (e.g. Lycopodiwm inundatum, Eriophorum vagin-
atum, Trientalis Europea) only live a short time in a garden even though the
clods of peat, in which their roots are imbedded, are transplanted with them. This
fact can scarcely be explained except by supposing that the organic compounds,
produced by the decay of vegetable remains on alpine heights and moors, are
essentially different from those evolved by similar matter under the changed
conditions of temperature and moisture occurring in a garden at a lower level.
Gardeners say that the peat and black graphitic soil from the slopes of snowy
mountains turn sour in gardens, and they may be to this extent right, that in all
probability the humic acids produced under altered circumstances are different.

SPECIAL RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM.

In the plants under discussion, the cells which absorb organic compounds are,
taken all in all, very similar to those which absorb mineral food-salts. Where there
is no cell-membrane, as in the case of Plasmodia and Euglenex, the food diffuses
through the so-called ectoplasm, or outer layer of the protoplasm, into the interior of
the cell. Saprophytic marine and fresh-water alge are able to absorb the products
of decay in the water around by means of their superficial layers of cells. The
mycelia of fungi have the power of taking in nourishment with special rapidity.
Each hypha, or more accurately, each long, delicate-walled cell of a mycelium is, to
a certain extent, an absorptive cell; its entire surface is capable of exercising the
function of suction and of withdrawing from the environment, along with water,

the very substances which are needed. The coral-like underground stem of
Vou. L 8

114 RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM.

Epipogium aphyllum, as well as that of the “Coral-root”, which is entirely destitute
of roots, develop fascicles of absorptive cells on their ramifications, and on special
little swellings; and the white subterranean stem structures of Bartsia alpina are
also provided with long absorptive cells. The white, fusiform, tuberously thickened,
underground stems of the Alpine Enchanter’s Nightshade (Circcea alpina) exhibit
no roots during autumn and winter, nor until such time as new leafy stems sprout
from them and lift themselves into the daylight; they only have scattered club-
shaped absorptive cells. Yet it is inconceivable that the few absorptive cells meet
the entire requirements of these plants at the season of the development of stems
above ground. Food is absorbed in these cases also by the epidermal cells of the
entire tuber, underground stem, or coral-like rhizome, as the case may be. The
epidermal cells of these subterranean caulomes which lie immediately in contact
with the black mould or humus on the ground of forests, have such thin and tender
walls that they are quite as well adapted to the absorption of nutriment as are the
projecting absorptive cells; indeed the club-shaped absorptive cells on the small
tubers of Enchanter’s Nightshade exhibit somewhat thicker walls than those
forming the general epidermis of the tubers.

We may compare food-absorption as performed by these coral-like and tuberous
structures, imbedded in decaying plant residues, with the action of tape-worms in
process of sucking in through their entire epidermis the fluid filling the intestines
they inhabit. The epidermal cells of the thick tortuous root-fibres of Weottia
Nidus-avis are all capable of absorbing nutriment, though they do not project as
tubes, but are tabular, and have their outer walls, which are in immediate contact
with the nutrient soil, only slightly arched outwards (see fig. 167). The green leafy
orchids rooted in the vegetable mould of woods and meadows are, on the contrary,
furnished with very long tubular absorption cells; and these cells do not wither
and collapse forthwith when the root elongates, but long retain their vigour and
activity. Whereas in the case of land plants adapted to mineral food-salts, the
tubular absorption cells (“root-hairs”) are limited to a narrow zone behind the
growing point of the root and always die comparatively soon; in the case of orchids,
having cylindrical roots imbedded in vegetable mould, these structures appear to be
beset from end to end with long scattered tubular absorption cells, which are
retained even through the drought of summer or the frost »f winter right into the
next period of vegetative activity; and these cells occur most abundantly in parts of
the ground where there happens to be a bed of humus or mouldering remains
particularly amenable to their purpose. Similar relations are found to exist in the
case of the dichotomously-branched roots of the Club-moss. They are twisted in
spirals and bore into the vegetable mould like corkscrews, and their absorption
cells form in some places regular tassels, which are completely cemented over with
fine black mould. The roots of grasses which, like the Mat-grass, live on the
decomposition-products of vegetable mould, are also distinguished by strikingly
long absorption cells, which grow in black or brown humus and there undergo the
strangest bends and contortions. When, for instance, a fragment of a dead root or

RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM. 115

underground stem, peculiarly desirable for absorption, is encountered, it is regularly
embraced by the suction cells, and as great an absorbent surface as possible is thus
brought into contact with the nutritious fragment. Indeed, the development of
suction cells on the roots of many gentians (viz. Gentiana ciliata, G. germanica, G.
Austriaca, and G. Rheetica) is confined to the parts of the root-branches, which, in
the course of their passage through the vegetable mould, have come into contact
with a particularly nutritious portion of it. Wherever there is contact, the root is
thickened, and absorption cells project unilaterally from the epidermis and grow
into the decaying fragment of wood or bark which is to be drained of its nutrient

Fig. 16.—Transverse section through absorption-roots of Saprophytes.
1 Gentiana Rhetica. 2 The Bird's Nest Orchis (Neottia Nidus-avis).

material (see fig. 161). Roots of this kind remind one of the root-structures of
parasites which are furnished with so-called “haustoria”, and which will be
discussed more in detail in subsequent pages. But they are different in that they
absorb food not from living but from decaying parts of the nutrient substratum.

Most plants that grow on the vegetable mould of alpine meadows, and the black
earth deposited by snow-drifts in mountainous regions, develop flat instead of
tubular epidermal cells as suction cells, and in this resemble marsh-plants. In
many of these cases the roots are so abundantly and minutely ramified that they
form a plexus investing the humus. This is likewise true of the absorptive cells on
the rhizoids of mosses.

Plants which lie flat against the bark of trees and have no connection with the
ground, so that they are unable to derive nutriment from it, have a very peculiar
method of maintaining themselves. Their roots, rhizoids, or hyphe, as the case may
be, either grow straight into the bark or are merely adnate to its surface. In
the latter case they are exposed on one side to the open air, and form more or
less projecting lines and ridges ramifying in all directions, often constituting a
regular trellis-work cemented to the bark. Sometimes, too, they are represented

ae

116 RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM.

by thicker ropes or bands which run longitudinally down or encircle the trunk.
These structures certainly serve as instruments of attachment, but at the same time
they also absorb nutriment from the substratum, the decaying bark upon which the
plant is epiphytic. In periods of drought the absorption of food by plants of this
kind is, in general, interrupted and suspended. But when the rainy season
commences and there is a long duration of wet weather, water trickling over the
surface of boughs and trunks washes the bark, cleanses it as it were, and, falling
lower and lower, brings down not only tiny loosened particles of bark but mineral
and organic dust which has been blown into it by the wind; it dissolves all the
soluble matter it finds on its way, and so reaches the roots, rhizoids, and hyphe
which adhere to the bark, in the form of a solution of mineral and organic
compounds, chiefly the latter. The trickling water is in some measure stopped by
the projecting ridges of these adnate structures; here and there also it deposits
particles mechanically suspended in it, and so it conveys to these curious epiphytes
the requisite nourishment.

In the same way, no doubt, epiphytes which grow upon other epiphytes are
nourished. In more inclement regions, the green bark, stem, and, less frequently,
the green leaves of the mistletoe are found to be beset by mosses and lichens; and,
in the tropics it is a common phenomenon for mosses, liverworts, and even small
kinds of Bromeliacez to settle on the green and still living leaves of Bromeliaces,
Orchidex, and Loranthaceex, although they are certainly not properly parasitic, and
only use their absorption cells for the purpose of clinging to the thick epidermis of
the living leaves or stem which support them. The principal part of the liquid
substances absorbed by these plants is conveyed to them by the rain-water that
washes over the substratum.

The species of plants also which have been mentioned as sometimes growing on
smooth vertical faces of rock, though the bark of trees is their usual habitat, are
able to obtain their food-materials in a similar way. If the summit of a cliff is
covered by a continuous carpet of plants, or if ledges and terraces projecting
somewhat from its face support sods of grass, tufts of moss, and various small
kinds of bushes, it must inevitably happen when there is an abundant fall of rain
that the water flowing down the declivity conveys with it organic compounds in
solution. First the sods of grass and moss on the ledges and on the top of the cliff
are wetted, then the humus, which is their substratum, becomes saturated, and such
part of the water as cannot be retained by this humus, or does not percolate into
the cracks and crevices of the rock, trickles down from the ledges and moistens the
face of the rock as it soaks down to the bottom. A rocky declivity is thus washed
in the same way as is the bark of trees, and small fragments of organic and
inorganic bodies must of necessity be rinsed out and carried down by the trickling
water, and then again be deposited in heaps where projecting obstacles are
encountered. It is just in the tracks along which the water flows down steep
rocks of the kind that the plants of which we have made mention are situated.

Associated with the above are generally a number of other plants, for the: most

RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM. 117

part microscopic, all of which cannot be classed as saprophytes, but which, in order
to be able to thrive in the tracks of trickling water, must have the capacity of
surviving desiccation for weeks, and even months, on the barren rock after having
been previously supplied with copious moisture for a time. In the case of lichen-
growths in particular these are very favourite sites; and when the lichens cover
a large area they attract one’s attention from afar. In limestone ranges, the
light-gray rock of steep declivities, interrupted by ledges covered with grass and
low brushwood, is extensively coloured by dark vertical bands and streaks, and the
effect is the same as if a dye had flowed from the ledges over the face of the rock.
These dark streaks indicate the course of the water which oozes from the humus
and renders possible the existence of numberless minute plants on the precipitous
face, in particular several dark crustaceous lichens (Acarospora glaucocarpa,
Aspicilia flavida, Lecidea fuscorubens, Opegrapha lithyrga, &c.).

The quantity of organic compounds brought down in solution by the water
which filters from the layers of humus on rocky ledges, and that which trickles
down the bark of trees, is, however, very small. Still, it is amply sufficient to
meet the requirements of the plants occurring at the spots in question. The claims
made by them upon their nutrient source are very moderate. We may here recall
the instances previously mentioned of mycelia of fungi which have been found
satisfied with the scarcely perceptible quantities of organic compounds in water
filtering into the shaft of a mine, and in the pure water of a mountain spring
respectively. To these instances must here be added the production of mycelia
in the wooden pipes through which the clear water of mountain springs is con-
veyed. After these pipes, which are made from the trunks of pines, have been
used as conduits for years, and their inner layers of wood have long since been
washed out, the mycelium of the fungus Lenzites sepiaria is not infrequently
developed within them, and in such luxuriance, indeed, that it forms great yellowish-
gray flocculent masses, which issue from the pipe’s inner surface, and float in the
stream of running water. In time these flocculent masses increase in the clear
spring-water to such a degree that the pipes become completely blocked, and the
flow of water is arrested. And yet the water conducted through the pipes is so
pure, where it enters into and issues from them, that the residue obtained by the
evaporation of hundreds of litres afforded no trace of any organic matter.

Seeing that most saprophytes absorb only such a comparatively small amount
of organic matter, one is all the more surprised to notice that a large number of
them fall suddenly, at certain times, into the opposite extreme. People speak of
things rapidly produced in abundance as “ mushroom-growths”, and as “shooting up
like fungi”. The fructifications of many fungi are in fact developed with a rapidity
which borders on the miraculous. The various species of Coprinus living on dung
produce their long-stalked, cap-shaped fructifications during the night, and by the
evening of the next day the caps have already fallen to pieces, and are in a state
of decomposition, and nothing is to be seen in their place but a black deliquescent
mass like a blot of ink. The weight of this fructification, thus matured within

118 RELATIONS OF SAPROPHYTES TO THEIR NUTRIENT SUBSTRATUM.

twenty-four hours, is certainly many times as great as that of the entire mycelium
which produced it; and it is quite incomprehensible how this mycelium, which for
weeks only achieves a moderate development, and adds but little to its dimensions,
is in a position suddenly, and in so short a time, to supply the amount of water and
organic compounds requisite for the building up of the fructification. Hpipogiwm
aphyllum exhibits a similar property. After producing nothing for two years
excepting a few branches on its subterranean stem, it develops all at once and in
a very short space of time fleshy stems with large flowers, and one asks with
astonishment how the relatively small coral-shaped stock sets about obtaining the
quantity of nutrient materials necessary for the construction of these flowering
stems. We are here confronted again with the great mystery of periodicity, the
solution of which we must for the present forego.

Saprophytes are much more fastidious as regards the quality of their nutriment
than one might expect. It is true that certain fungi are produced wherever there
are plants in a state of decomposition, and to them it is quite indifferent whether
the mouldered dust, which serves as a nutrient soil for their mycelia, has arisen
from one species or another. Also in the case of orchids imbedded in vegetable
mould, and in that of most of the mosses and liverworts adherent to the barks of
trees, it is, as a rule, of no consequence whether the tree constituting the substratum
is a conifer or a dicotyledon. But a large number of species are associated with the
decaying remains of particular plants or animals only. For example, certain small
species of Marasmius, belonging to the group of the Agarici, occur only on moulder-
ing pine-needles; another small fungus, Antennatula pinophila, is found exclusively
on fallen needles of the Silver Fir; Hypoderma Lauri, which resembles small black
type on rotting laurel leaves, and the tiny Septoria Menyanthis on leaves of the
Bog-bean (Menyanthes trifoliata) lying under water in a state of decay. The
cinnamon-coloured receptacles of Lenzites sepiaria only grow from prostrate
trunks of conifers, and the black fuliginous fructifications of Bulgaria polymorpha. .
only on those of oaks. A small discoid fungus named Poronia punctata, white
with black spots on the top, is only found on cow-dung; another fungus, Gymnoascus
uncimatus on that of mice, and Ctenomyces serratus on decaying goose feathers.

That many mosses are also very fastidious in the selection of their substratum
has already been intimated. Just as in the Alps Splachnwm ampullacewm is only
found growing on the putrefying dung of cattle, so in arctic regions the splendid,
large-fruited Splachnum lutewm and S. rubrum occur exclusively on that of: rein-
deer. Tetraplodon wreeolatus is met with on mountains always with decaying
excrements of chamois, goats, or sheep for a substratum, whilst Tetraplodon
angustatus chooses the excrements of carnivorous animals, and Tayloria serrata
is only seen near cow-chalets on decomposing human faces. The circumstances
of the occurrence of another moss belonging to the Splachnacee, 4c. Tayloria
Rudolfiana is also very interesting. It grows usually on the branches of old
trees, especially maples in sub-alpine regions, and one is tempted to believe that in
respect of its nutrient substratum it is an exception to the rule of the rest of the

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 119

Splachnacesx. But on closer examination there is convincing evidence that this
moss also only lives on animal dung undergoing putrefaction. For remains of
broken mouse and bird bones are invariably to be discovered in the substratum,
and there can be no doubt that the Tayloria chooses for its site boughs of old
trees upon which birds of prey have dropped their excrements. Of the mosses
living on the bark itself, one instance is also worth mentioning. Whereas in the
case of. most species of the genus Dicranwm, the mouldering residues of conifers
constitute the favourite substratum; there is one species, viz. Dicranwm Sautert,
which is found only on the bark of the beech. The weather-worn bark of this
tree is seen, in sub-alpine districts, covered with the most brilliant emerald-green
films of the above-named moss; whilst on adjacent pines and fir-trees no trace of it
can be found.

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

A number of plants exhibit contrivances which obviously have for their object
the capture and retention of such small creatures as may fly or creep on to their
leaves; and it has been ascertained by searching experiments that the majority of
these plants use the animals they capture, in one way or another, as sources of
nutriment. For the most part the animals that, are caught are insects, and hence
the term “insectivorous plants” has been applied to the class in question. The
flesh of the insect being the part of it principally serviceable for food, the name
“carnivorous” or “flesh-eating”, or better, perhaps, “flesh-consuming” plants has
also been used; and seeing that the most important part of the whole process is
really the digestion, or taking in of organic compounds from the captured animals
after they are dead, we might call those plants which are furnished with organs
for the absorption of the dissolved flesh of animals ensnared by them, “ flesh-digest-
ing” plants as well. As will appear from the following discussion of the subject,
no one of these names completely covers the wonderful phenomena in question,
and it is scarcely possible to find a short and not too cumbrous expression which
shall henceforward exclude all misconceptions.

In round numbers we may estimate the plants which capture animals and
demolish them for food at five hundred. Within this comparatively small range,
however, the variety of the mechanism for seizure and absorption of nutritive
matter is so great that in order to give a general picture of them it is necessary to
classify them into several sections and groups. In the first section we have a series
of plant-forms wherein chambers are developed, which admit of the entrance of
. small animals, but not of their escape. The organs of capture and digestion of the
plants belonging to this section exhibit no external movements of any kind, and
are thereby differentiated from the forms belonging to the second section, which
perform definite movements, in response to a stimulus caused by the contact of the
animals, with the object of covering the prey with as great a quantity of digestive
fluid as possible. Lastly, there is a third section wherein the individual forms are

120 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

neither provided with pitfalls nor capable of performing special movements, but
have leaves converted into lime-twigs and on them animals stick and are also
digested.

The first and most extensive group included in the first section is that of
Utricularie or Bladderworts. Their capturing apparatus consists of little bladders
with orifices closed in each case by a valve, which permits objects to penetrate into

Fie r ; re DUVETS ty
Fig. 17.—Bladderworts. ute |
In the foreground Utricularia Grafiana ; in the background Utricularia minor. v we

the cavity of the bladder, but not to issue out of it. The Utriculariz are rootless
plants which live suspended in water, and, according to the season of the year,
either sink down to the bottom or ascend to just below the surface. Upon the
approach of winter, when animal life is gradually disappearing in the chilled and
freezing upper layers of water, the leaves at the extremities of the floating stems
are enlarged and form spherical winter buds; the older parts of the stems together
with the leaves die, their cavities hitherto occupied by air are filled with water, and
they sink to the bottom drawing down with them the winter buds. After the
winter these buds elongate, detach themselves from the old stems and ascend near
the surface, where innumerable little aquatic animals are swimming to and fro, and
there develop two rows of lateral branches in rapid succession. Either all of these
are thickly covered with leaves which are divided into thread-like, repeatedly

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 121

bifurcating, segments, or else only half of them are thus clothed with leaves whilst
the other half bear the before-mentioned bladders. The former is the case in
Utricularia minor, the plant represented in the background of the figure on p. 120;
and the latter in Utricularia Grafiana, which is drawn in the foreground. In
instances of the former kind obliquely ellipsoidal bladders are to be seen on short
stalks on the principal segments of the leaves, usually quite near their angles of
bifurcation. In the smaller species, such as Utricularia minor, they have a diameter
of about 2mm. In individuals of the latter kind the bladders have longer stalks,
and are about 5 mm. in diameter. They are always pale-green and partially trans-
parent. Each bladder is somewhat flattened at the sides and exhibits a markedly
convex dorsal surface and slightly curved lateral surface. An orifice, whose border
is fringed with peculiar stiff tapering bristles, leads into the interior of each of
these stalked bladders. The aperture has four rounded angles and is framed
as it were, by a
pair of lips. The
under lip is strong-
ly thickened, and
is furnished with a
solid cushion projec-
ting into the inte-
rior of the bladder.

From the upper lip Fig. 18.—Traps of Utricularia neglecta.
hangs a thin trans- 1A bladder magnified (x4). 2 Section of a bladder. % Absorption-cells on the internal
surface of the bladder (x 250).

parent, obliquely-

placed valve (see fig. 187), the free edge of which rests upon the inner surface of
the cushion before referred to, and closes the entire orifice. This valve is very
elastic and yields easily to any pressure from outside. A tiny animal is able, by
pressing against it, to force a way without difficulty from the nether lip into the
interior of the bladder, and to slip in through the opening thus made. But as soon
as the animal has got inside, and ceases to press upon the valve, its elasticity brings
it back upon the under lip again. It cannot be opened by pressure from within;
for, resting as it does upon the projecting cushion, it is impossible for the little
prisoner to force it over the latter in an outward direction.

The whole apparatus forms a trap for small aquatic animals, they being able, as
before observed, to slip into the bladder but not to get out again. Most animals
that enter make, it is true, efforts to escape, but they are all in vain. Many perish
in a short time—about twenty-four hours—others live from two to three, or, in
some cases, even as much as six days. But in the end they must suffer death by
suffocation or starvation, and they then decay, and the products of their decomposi-
tion are sucked in by special absorption cells developed within the bladder. These
absorption cells (see fig. 18°) are linear-oblong and somewhat like little rods in
shape, and they line the whole internal surface of the cavity of the bladder. They
are arranged in fours, each group of four forming a cross and being united by a

122 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS

common basal cell. The basal cells themselves are intercalated amongst the cells
lining the bladder. The organic substances from the decaying bodies of captured
animals are sucked up by these stellate groups of cells, and from them pass into the
basal cells, and later, into the other adjacent cells of the bladder and those of the
plant at large.

The majority of the animals caught by the bladders are crustaceans. It is
principally larvee and adult individuals of small species of Cypris, Daphnia, and
Cyclops that fall into the trap; but larve of gnats, and various other small
insects, little worms, and infusoria, are also not infrequently met with imprisoned
in the bladders. The number of animals captured is comparatively large. In
single bladders the remnants of no less than twenty-four small crustaceans have
been observed. The prey secured by Utricularia minor (fig. 17), which lives in
little pools of still water in peat-bogs, is very abundant. The North American
Utricularia clandestina seems algo to use its capturing apparatus with great
success. .

What it is that induces the animals to press upon the valves and so fall into the
trap is not fully explained. We may suppose that they expect to find food in the
bladder-cavity, or that they hope it will afford a shelter where they can rest for
a time and be protected from their pursuers. The last suggestion is especially
supported by the circumstance that the approach to the valve-covered orifice of
the bladder is guarded against the intrusion of larger animals by stiff sharp bristles
which stick out from it (fig. 187). Only very small animals, which can easily slip
in between the relatively large bristles, reach the inside of the bladder, whilst
larger creatures, which would injure the whole apparatus, are prevented from
coming near it. Thus, the most probable explanation is that lesser animals pursued
by greater take refuge in the hiding-places behind the bristles, and so fall into the
trap. Another very striking fact is that the bladders of Utricularia, living in still
water, look delusively like certain Ostracoda, especially species of the genus
Daphnia. The bladder itself resembles the shell-covered body in size and form,
and the bristles the antennz and swimmerets of one of these crustaceans. Whether
there is any significance in this curious similarity of outward appearance must be
left undecided.

The majority of Utricularia live in pools of water beside foot-tracks on moors
and in the little collections of water between clumps of reeds in peat-bogs; and
these are precisely the haunts of the little creatures that are to fall into the traps.
Every handful of water that one scoops up contains hundreds of midge-larve,
water-fleas, Ostracoda, and one-eyed Cyclops, which rush about promiscuously,
pursuing and seizing one another. One species of these plants lives in the moun-
tains of Brazil in the rain-filled receptacles of Tillandsia plants. The Tillandsia
is allied to the pine-apple, and has rosettes of concave leaves, the latter resting one
upon the other in such a way as to form a niche or cavity in front of each. leaf
which fills with rain like a cistern. Many different kinds of small animals are
always swimming about in these little cisterns, and almost every one of the latter

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 123

is the sphere of activity of an individual Utricularia nelumbifolia. This plant
is remarkable also from the fact that long runners are thrown out from its stems,
which grow across, in wide arches, from its cistern to a neighbouring Tillandsia,
where it selects one of the reservoirs in the rosettes as a new site and dips down
into the water—a fantastic method of propagation of which we shall speak again
on another occasion.

A few Utriculariz do not live in water at all, but grow amongst mosses, liver-
worts, and lycopods, in the vegetable mould filling the clefts and crevices of rocks,

and the bark-fissures of old trees. Of ‘this habit, for example, is the pretty
' Brazilian Utricularia montana, which, in spite of the difference of its habitat, is
provided with an apparatus for capturing animals agreeing in all essential respects
with the description already given. The bladders used by these plants for pur-
poses of prey are produced on subterranean filiform stems which thread their way
in the vegetable mould and wefts of decaying moss-stems, and here and there swell
into tubers. The bladders are hyaline and transparent, and are filled with watery
liquid, sometimes also with air. They are only 1 millimeter in diameter, but are
present in large numbers. The entrance into these bladders is much more con-
cealed than in the species that live in water. The dorsal surface of the bladder
being still more highly curved, the position of the orifice is altered so as to be
quite close to the little stalk of the bladder. In addition, the orifice is, as it were,
roofed over, and thereby protected against the possibility of being stopped up by
particles of earth, and the passage leading to it is very narrow. That, in spite of
the difficulty of entrance, a number of minute animals do seek a hiding-place here
is proved by the circumstance that, besides various infusoria, rhizopoda, and
creatures of that kind inhabiting damp earth, species of Acarus and larve of
other animals have been found, both dead and alive, in the bladders.

With this first group of insectivorous plants, wherein the capturing apparatus
includes a valve to prevent the egress of such animals as fall into the trap, is
associated in the first section a second group, viz. that of the ascidia-bearing or
pitcher-plants, in which the foliage-leaves are converted into pitfalls, and the
escape of the captured prey prevented by a number of points lining the inner
wall of the cavity, and directed from the aperture towards the closed bottom.
There is an extraordinary variety in the form of the pitfalls. Sometimes they
are tubular, utricular, or funnel-shaped cavities, sometimes mug or pitcher-shaped,
or urceolate; in some cases these are straight, in others bowed like sickles, or
spirally twisted. They always arise from the part of the petiole upon which the
lamina immediately rests. The lamina is always relatively small, being represented
in the majority of the traps by a scale or lobe, and it only appears to be an
appendage of the large expanded and hollowed-out petiole. In many pitcher-
plants the little lamina looks like a lid placed over the orifice to the pitfall, as,
for instance, is shown in the illustration (fig. 21*), whilst in others (Nepenthes
ampullaria and NV. vittata) it has the form of a handle or stalk, and serves as a
place for animals visiting the pitchers to alight upon.

124 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

In each pitfall there are always three kinds of contrivance to be distinguished:
first, a device for the allurement of animals; secondly, an arrangement for entrap-
ping the prey enticed, which at the same time prevents individuals once imprisoned
from returning and escaping through the entrance hole; and thirdly, a structure
for causing the decay or dissolution of the dead animals at the bottom of the pit-
falls, and for rendering possible the absorption of the products of decomposition
as nutriment. The means of allurement are similar to those which cause the visits
of small creatures to flowers, that is to say, principally honey and bright and
varied coloration, whereby the nectar-secreting spots are recognized from afar,
especially by flying insects. The escape of animals when they have once entered
the cavity of a petiole is prevented, as has been already mentioned, by a fringe
of sharp hairs pointed downwards, or by various spinous structures on the inner

Fig. 19.—Spinous Structures in the Pitfalls of Carnivorous Plants.

1 Genlisea; a piece of the tube seen from inside. 2 Heliamphora nutans; spines on the walls of pitfalls. 8 Sarracenia
purpurea; a piece of the lining of the pitcher near the orifice seen from inside. 4 Sarracenia purpurea; longitudinal
section through the membrane covered with spinous bristles in the lower part of the pitcher. 5 Nepenthes hybrida;
fringe of spines at the orifice of the pitcher. 1, 2, 4, 5 greatly magnified; 8 slightly magnified.

surface of the cavity. The decomposition and dissolution of the prey are effected

by fluids secreted by special cells at the bottom of the utricles and pitchers.

But although in respect of the consecutive and co-ordinate operation of these
three kinds of contrivance, all ascidia-bearing and pitcher-plants resemble one
another, there are considerable individual divergences as to structure and function
that it is well worth while to study in some detail the most noticeable of them.

One of the most noteworthy is the genus Genlisea, which is nearly related to
Utriculariaceze in the structure of its flowers and fruit. It is composed of a dozen
species growing in water and marshy places. Of these one is a native of tropical
and southern Africa, whilst others are found in Brazil and the West Indies. In
addition to ordinary leaves, which in them are spatulate, most of the Genlisex
possess leaf-structures metamorphosed so as to constitute pitfalls. Each pitfall
consists of a long, narrow, cylindrical utricle, which at its blind end is enlarged
into a bladder, whilst the narrow orifice at the opposite end are placed two peculiar
ribbon-shaped processes twisted spirally. The orifice of the utricle is set with very
small sharp teeth bent inwards; and the tubular part of the utricle has its inner
surface lined throughout with innumerable little bristles, which arise from rows

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 125

of cells forming inwardly projecting ridges, and have their sharply-pointed tips
directed downwards (see fig. 191). Amongst these needles are also found, scattered
over the whole internal surface, roundish wart-like glands or papille, composed of
four or eight cells. The bottom of the bladder-like cavity in which the utricle
terminates is destitute of bristles, but provided only with glands arranged in rows.
Small worms, mites, and other segmented animals which enter through the orifice
of the utricle can easily reach the enlarged base. But as soon as they try to com-

i oe
Fig. 20.—Sarracenia purpurea. wee. |

mence the return journey they are opposed by the points of a thousand bristles.
Thus caught they die, and the products arising from the decay of their bodies are
absorbed by the glands situated, as above mentioned, at the bottom of the bladder
and on the walls of the utricle.

As types of a second series of carnivorous plants belonging to the group of
pitcher-plants may be taken Heliamphora nutans, a native of moorlands on the
mountains of Roraima, on the borders of British Guiana, and Sarracenia purpurea
(see fig. 20), which is widely distributed in the marshes of eastern North America
from Hudson’s Bay to Florida. In both instances the leaves metamorphosed into
ascidia are arranged in rosettes, rest their bases on damp earth and thence curve
upwards. They are somewhat inflated, like bladders, at about their middle, but
contract again at the orifice where they pass into the relatively small lamine.
The latter are threaded by red streaks like blood-vessels, have the form of valves,

126 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

and turn their concave surfaces towards falling rain. They serve, moreover, at
least in Sarracenia purpurea, to catch the drops of rain, which then flow down
into the bottom of the ascidia and fill them more or less with water. There is
very little evaporation from the hollow pitchers; and even when there has been no
rain for a week, one always finds some of the previously-collected water at the
bottom. The inner surface of a pitcher is lined by cells arranged like the scales
of enamel on a pike’s back (see fig. 192). The internally-projecting wall of each
of these scales is transformed into a stiff decurved point, and the lower the position
of the cells the longer do the points become. The shell-like lamina again, above
the contracted orifice, bears glandular hairs which exude honey, so that the parts
surrounding the aperture are covered by a thin film of sweet juice.

Many animals are attracted by this honey. Some are winged and alight from
flying; others, being wingless, make use of a peculiar ridge, which projects on the
concave side of the utricle, to help them to creep up the latter. If these honey-
eaters happen to travel away from the lamina to that part of the pitcher which
is lined with the smooth and slippery decurved cells, they are as good as lost.
They slip down over the brink, every attempt to climb up again being rendered
futile by the downwardly-pointing needles which clothe the lower part of the wall;
and ultimately they fall into the water collected at the bottom, where they are
drowned and their bodies putrefy. The products of decay are absorbed as
nutriment by the epidermal cells in this region. The number of animals meeting
with this fate is often so great that an offensive odour, arising from the decaying
bodies, is emitted by the utricles and is noticeable at a considerable distance. In
the wild state, the ascidiform utricles are often half-full of drowned animals and
it is stated that in these circumstances birds also put in an appearance and pick
some of the dead remains out of the utricles.

Whether the liquid filling the bottom of the pitchers consists simply of rain-
water, or whether the latter is modified by a secretion originating in the gland-
like groups of cells there (see fig. 287), is still uncertain. A centipede over
4 centimeters long having fallen into a utricle of Sarracenia purpurea in the
night was found only half immersed in the water. The upper half of the creature
projected above the liquid, and made violent efforts to escape; but the lower part
had, after a few hours, not only become motionless but had turned white from the
effect of the surrounding liquid; it appeared to be macerated, and exhibited
alterations which are not produced in so short a time in centipedes immersed in
ordinary rain-water. When a number of captured animals are undergoing putre-
faction at the same time in a pitfall, the liquid turns brown and has the appearance
of manure-liquor.

There is a great difference between the utricles of Sarracenia purpurea and the
apparatus adapted to the capture of prey in the plants of which we have chosen as
examples, Sarracenia variolaris, a native of the marshes of Alabama, Florida, and
Carolina, and the Darlingtonia Californica, found growing at a height of from 300
to 1000 meters above the sea on moorlands in California from the borders of

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 127

Oregon to Mount Shafta. In both of these the liquid with an acid reaction, which
fills the bottom of each utricle, is certainly only secreted by the cells in the interior
of the cavity itself, and it is quite impossible that a single drop of the rain or dew
deposited upon the plant should reach the interior of the cavity. The hollow
petiole is in both plants, above mentioned, utricular or tubular, and only slightly

q

~ AiR

Fig. 21.—Ascidia-bearing and Pitcher-plants.

1 Sarracenia variolaris, *% Darlingtonia Californica. §% Sarracenia laciniata. 4 Nepenthes villosa, reduced to one-half
* 4
natural size.

enlarged towards the top. The dorsal side of each leaf is, however, at its upper
end hollowed out like a cowl or a helmet, and forms a cupola as is shown in
fig. 21: and 21% The orifice or entrance into the utricle is consequently covered
over and is reduced to a slit or hole under the hood. The lamina is trans-
formed into a lobe, which in Sarracenia variolaris is small and roofs over the
orifice of the utricle, and in Darlingtonia is shaped like the tail of a fish,
and hangs down in front of the aperture. The lower part of the utricle is of a
uniform green colour, but the upper part (7.¢. the cupola and lobe-like appendage)
has red ribs and veins, and here and there is quite purple. Between the veins the

128 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS,

leaf is thin, translucent, and pale-green or whitish; and these clear translucent
patches, framed by purple or green ribs, look as if they were little windows,
especially when seen from within the utricle. The mixture of green, red, and white
gives the upper parts of the leaves such a gay appearance that, from a distance, they
might be mistaken for flowers.

Insects are doubtless attracted by these bright colours, and both round the
orifice, and on the inner surface of the cupola, they find exudations of honey which
they suck or lick up with avidity. In Sarracenia variolaris, honey is to be seen
besides, on the edge of a broad free border which is decurrent along the utricle, and
extends from the ground to the orifice. This border forms a favourite path for
wingless insects, especially ants, which are particularly eager in their quest for
honey. For them it is a sure way to destruction, for when they, gradually
following the honey-baited pathway, arrive at the orifice to the utricle and pass
through it, they inevitably get upon the smooth decurved points of the epidermal
cells, constructed just like those in Sarracenia purpurea, and then, unable to stop
themselves, slip down to the bottom of the pitcher. When small winged insects
alight from flying and fall down the slide into the interior, they make use of their
wings in the hope of saving themselves, but they never succeed in finding the
aperture by which they entered, as it slants downwards and is situated in shadow.
They invariably try to escape through the cupola, mistaking the thin portions,
through which the light penetrates into the interior, for gaps permitting egress.
But just as flies in rooms dash against the windows hoping to pass through them
into the open air, so the small insects in the utricles of Sarracenia variolaris and .
Darlingtonia Californica knock against these windowed cupolas, in their desire
to save themselves by flying through. They always fall down again to the bottom
of the utricle as though into a cistern. If they are immersed in the liquid there
secreted, or only in partial contact with it, they are stupefied, but not immediately
killed. They often live incarcerated for two days, and it would therefore be
erroneous to suppose that the fluid in the pitchers acts on the prey as a deadly
poison. But it assists the decay and dissolution of the captives as they die of
starvation and suffocation, and, as in the case of the utricle-plants previously
described, a brown liquor of very unpleasant odour is produced, and there is a
residue of solid pieces of skeleton difficult to decompose, such as the wing-cases,
claws, and thoraces of various beetles, lice, ants, and other smell insects which have
shared the same unlucky fate.

The number of animals captured is very considerable. The pitchers of
Sarracenia variolaris, which attain to a length of 30 cm., are usually found, when
growing in their natural habitat, filled to a height of from 8 to 10 em. with animal
remains, and even a heap 15 cm. high has been observed. We must here remark
that in the ascidia of Sarracenia variolaris, wingless insects, which creep about the
earth, are found to predominate, whilst in Darlingtonia, on the contrary, most of
the insects are winged. The cause of this is easily understood. The former plant
has honey exuding on the flap or ridge running down from the orifice to the

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 129

ground, and many wingless insects are thus induced to climb up the alluring path -
and to enter the cavity of the pitcher. Darlingtonia, on the other hand, is
destitute of honey on its decurrent ridge, and only provides the sweet meal at the
top in the vicinity of the orifice, where it is available for flying insects, which, as a
rule, only visit nectar-secreting flowers. The purplish-red scale, shaped like a fish’s
tail, and hung out like the sign-board of an inn in front of the entrance to the
pitcher, constitutes an instrument for the attraction, from afar, of these winged
creatures, which are endowed with a vivid sense of colour; and, as experience
shows, it does not fail in its object.

What significance is to be attributed to the spiral torsion of Darlingtonia
leaves (see fig. 21) it is difficult to say. Perhaps the escape of animals once
imprisoned in the depths of a pitfall is hereby rendered still harder. It would at
all events be much more difficult for an insect trying to escape by the use of its
wings to ascend a canal which, in addition to being lined with decurved points,
was spirally wound, than a canal similar but straight and widened towards the top.
We must not omit to mention that a few flies and a smaH-moth-have selected as
their ordinary habitat the pitchers of both the plants just described, in spite of their
being so fatal to most insects. The grubs of a blow-fly (Sarcophaga Sarracenie),
in particular, live in large numbers amidst the heaps of decaying insect bodies at
the bottom of the pitchers, and are there nourished just as are the grubs of allied
species in the rotten flesh of birds and mammals. When mature,the grubs quit the
environment of dead remains, passing through holes bored by themselves in the
side wall of the pitcher, and turn into chrysalises in the earth. But the fly itself
can without danger pass in and. out of the pitfalls, which are so perilous in the case
of other insects, and it is enabled to do this by means of the special structure of its
feet. On the last joint of each foot it has a long claw and sole-like attachment-lobe,
and it is able to push these appendages between the sharp, slippery, decurved hairs
lining the inner surface of the pitcher, and so to hook itself to the deeper strata of
the wall. This apparatus may be likened to the grapple-like climbing irons of
Tyrolese mountaineers, and, thus armed, the fly is in a position to ascend the inner
wall of a pitcher unscaleable by other insects. The case of the small moth
Xanthoptera semicrocea is similar. The tibiee of this insect are armed with long,
sharp spurs, one pair on each of the two middle legs, and two pairs on each of
the two hindermost legs; and, by the help of these spurs it likewise is able to
tread uninjured over the dangerous surface of the wall. Its caterpillars, too, cover
the sharp slippery hairs with a web, and so render them harmless.

The presence of these animals in the death-traps of Sarracenias is of special
interest, inasmuch as it shows that the animals which perish at the bottom of the
pitchers are not exactly digested. If maggoty flesh enters the stomach of a
carnivorous animal, not only the flesh itself but the maggots as well (which, indeed,
immediately die on reaching the stomach) are speedily dissolved by the action of
the gastric juice. Such is also the case with several animal-capturing plants to be
described in the next pages. But the fluid secreted in the pitchers of Darlingtonia

Vou. I.

130 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

and Sarracenia variolaris cannot exercise this digestive action, for if it did the
maggots in the heap of rotting insects could not remain alive and well. Its
action is limited to the promotion of decay and the formation of a foul liquor, in
other words, a liquid manure, which is absorbed as nutriment by the epidermal cells
at the bottom of the pitchers. ;

Another series of pitcher-plants comprises forms in which the petioles are
converted into symmetrical sacs with apertures at the top, and the lamine spread
out over them like lids for protection. Most frequently the pitfalls in plants of
this kind are shaped like pitchers, jars, urns, cups, or funnels; and the lid over the
orifice of each cavity is, for the most part, so placed as to prevent rain-drops from
falling in, but not to hinder in any way the entrance of animals. In this series are
included, firstly, a few species of Sarracenia, viz. Sarracenia Drummondi and
S. undulata, next, the Australian Cephalotus follicularis, and lastly, the numerous
species of the genus Nepenthes, which are designated by gardeners by the name of
“ pitcher-plants” in the narrow sense.

The leaves in both the Sarracenias just named are heteromorphic. Some of them
have acute linear-lanceolate petioles of a uniform green colour, and not hollowed
out; and it is only in the case of from three to five leaves in each individual plant
that the petioles are transformed into tubes with infundibuliform enlargements at the
top. The rim round the mouth of the funnel is somewhat swollen and doubled down
externally; but above the orifice the lamina is arched so as to form a cover to the
pitcher. The margin of the leaf of Sarracenia laciniata, which is shown in fig. 213,
is crinkled and sinuously folded. The cover and also the upper funnel-shaped
enlargement of the pitcher are very conspicuous on account of the contrast of the
colours displayed upon them. The green of the lower part of the pitcher gets
paler and paler above, and merges into a pure white, whilst dark-red veins stand
out from the green and white ground tints, having the effect of a net-work of blood-
vessels, At the mouth of the pitcher, and on the under side of the lid, honey is
secreted in such abundance that little drops of it are not infrequently to be seen
on the swollen rim, and some oozes down into the infundibuliform portion of the
pitcher. But at the very spots where the honey occurs there are also innumerable
smooth conical ¢ells with their solid apices directed downwards; and these cells
become longer the lower their position in the pitcher. When insects, attracted by
the gay-coloured lid, and lured on by the honey, come to the mouth of the
pitcher and tread upon the parts covered with the sharp slippery papilla, they are
drawn into the depths as though by an invisible power. After they have once
alighted on the perilous area, every movement and every effort to climb up against
the points causes them to slide further and further down towards the bottom of
the pitcher, where they are hopelessly lost, being killed within a short time and
ultimately decomposed.

An instance of an exactly similar kind is afforded by Cephalotus follicularis,
which has long been known as a plant native on moorlands in eastern Australia.
It is allied to saxifrages and currants, and is represented on a scale of half the

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 131

natural size in fig. 22. This Cephalotus also has two kinds of leaves, which are
closely crowded in a rosette round the erect flower-stalk. Only the lower leaves
of the rosette are transformed into traps for animals, and these are pre-eminently
adapted for wingless creatures creeping upon the earth. The tankard-shaped
traps all rest on the damp earth, and are furnished externally with borders or
winged ridges, which facilitate the ascent
of crawling animals to the mouth of the
tankard. Flying insects are of course not
excluded, and here again they are made
aware from afar of the feast of honey
provided by the presence of bright colours.
The half-open lid is very prettily adorned
with white patches and brilliant purple
veins, and at a distance is readily mistaken
for a flower.

When small animals, whether with or
without wings, approach to take the
honey, they are so eager in their search
that they get upon the inner surface of
the mouth of the tankard-pitcher, which,
though fluted, is also very smooth and
slippery, and thence they easily slide into
the interior of the cavity The pitchers
being half-full of liquid, most of the un-
lucky creatures die there in a short time
by drowning. But even if this were not
the case they would never succeed in
working their way up to the light of
day. For every animal that wishes to save
itself from a Cephalotus pitcher has three
obstacles to overcome: first, a circular
ridge projecting inside the pitcher; sec- Fig. 22.—Cephalotus follicularis,
ondly, a bit of wall thickly covered with
little papille, sharp, ridged, and pointed downward, the whole being comparable
to a flax-comb; and, lastly, on the involute rim round the mouth of the pitcher,
another fringe composed of hooked, decurved spines which bristle like an im-
penetrable row of bayonets in front of such animals as may have surmounted
the other difficulties. The abundance of the booty found at the bottom of Cepha-
lotus pitchers shows how efficiently these contrivances serve to prevent escape.
Ants, for instance, sacrifice themselves recklessly in their pursuit of honey, and
one often finds great numbers of them drowned in the liquid in the pitchers. The
prey is not in this case converted into a putrid liquor, but is partially dissolved by
a secretion having an acid reaction. This secretion is separated out by special

ie

132 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

glandular cells situated on the lining of the pitcher; and the whole process, wherein
they are concerned, corresponds to that which obtains in the pitchers of Nepenthes,
and which will be more thoroughly discussed in the case of these latter plants.

The species of the genus Nepenthes, of which we know at the present time
thirty-six, are all confined to the tropics. Their area of distribution extends from
New Caledonia and New Guinea over tropical Australia to the Seychelles Islands
and Madagascar, and over the Sunda Islands, the Philippines, Ceylon, Bengal, and
Cochin-China. They only flourish on marshy ground on the margin of small
collections of water in damp primeval forests. There the seeds germinate in
shallow water. The young plants (see fig. 23), which spring from the boggy
ground, have their leaves ar-
ranged in rosettes just like
those of Sarracenias (see fig.
20). They are, too, so nearly
identical in form with the
latter that anyone seeing a
young Nepenthes plant for
the first time, and not knowing
the history of its development,
: would take it for a Sarracenia.

Fig. 23.—Young Nepenthes plants. The leaves, succeeding the

cotyledons and forming a circle

above them, rest their lower portions upon the mud, but their upper parts are

curved upwards, and each carries at its extremity a scale resembling a cock’s comb,

which is, strict speaking, the lamina. This scale roofs over a slit-like aperture, the

entrance to a cavity within the swollen petiole. In addition a green lobe with a few
coarse projecting points is to be seen on either side of the orifice.

Altogether different from the rosettes of young Nepenthes plants are the foliar
structures clothing the stems which subsequently arise from the rosettes (see fig. 24).
In these leaves the lower part of the petiole is winged and flat, has a linear or
lanceolate outline, and resembles the leaf-blade of Dracena; its functions, too, are
those of a green lamina. This expanded section of the leaf-stalk passes next into
a part which is terete and coiled like a snake, and acts as a tendril. Every stem or
branch belonging to a plant, whether living or dead, with which this part of the
petiole comes into contact, is seized and encircled by it; and the third portion of
the petiole, e. the pitcher, being situated at the extremity of this clasping portion,
is thus slung upon the branch of some other plant growing at the edge of a pool
of water. Meanwhile the Nepenthes plant rises higher and higher above the wet
soil where its seeds germinated and the young rosette rested, becomes entangled
with the ramifications of the underwood and with prostrate branches of trees of
the primeval forest; in a word, with everything available as a support, and so not
infrequently climbs, as a true liane, to the tops of trees of moderate height.

The pitcher must be looked upon as an excavated portion of the petiole, and

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

|i

acs
ivuly
VOoremcy

yt

}

134 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

what appears to be the lid of the pitcher is the lamina, as it is in Cephalotus and
the Sarracenias. In this case also the lamina seems to be but little developed in
comparison with the wonderfully metamorphosed petiole. In the majority of the
species of Nepenthes, the mature pitchers are from 10 em. to 15 em. in height. In
the graceful Nepenthes ampullaria they are only from 4 em. to 6 em. high; but,
on the other hand, the species indigenous to the primeval forests of Borneo reach a
height of 30 cm. or even half a meter. The pitchers of Nepenthes Rajah have a
height of 50 cm., and their orifices are 10 cm. in diameter, whilst below the orifice
they expand to 16 cm.; so that if a pigeon were to fly into a pitcher of this kind
it would be completely hidden in it. Immature pitchers are still closed by their
covers. Often they are hairy outside; and, according to the colour and lustre of
the hairs, they may be rusty in tone or glittering like gold; not rarely they look as
if they were powdered with flour (eg. NV. albo-marginata), and sometimes are even
snow-white. Subsequently the lid is raised, and the downy coat disappears either
partially or entirely. Having thus become glabrous, the pitchers display a yellowish-
green ground colour, for the most part flecked and veined with purple; and many
are of a bluish, violet, or rose tint near the orifice, or dark-red as though saturated
with blood. The lid is similarly gaily coloured; and the variety of the tints is
increased by the fact that a pale-blue zone is visible in the interior, beneath the
swollen involute rim of the opening, which is itself brownish, yellowish, or orange-
red. Gaily-coloured pitchers of this kind look at a distance just like flowers,
and remind one, in particular, of the most brilliant floral forms of the liane-like
Aristolochias indigenous to tropical forests. This fact is the more noteworthy,
because the genus Wepenthes is closely allied to the genus Aristolochia in respect
of systematic relations.

The bright pitchers of Nepenthes, visible from afar, are sought, just as flowers
are, by insects, and probably by other winged creatures as well; and this occurs all
the more because there is a copious secretion of honey by the epidermal cells upon
the under surface of the lid, and on the rim round the mouth of each pitcher. The
swollen and often delicately-fluted rim, in particular, drips and glitters with the
sugary juice; and it would be permissible in this connection to speak of a honeyed
mouth and sweet lips in the most literal sense of the words. Animals which suck
honey from the lips of Nepenthes pitchers wander, as they do so, only too readily
upon the interior surface of the orifice. But the inner face is smooth and precipitous,
and rendered so slippery by a bluish coating of wax that not a few of the alighted
guests slip down to the bottom of the pitcher and fall into the liquid there
collected. Many of them perish in a short time; others try to save themselves by
climbing up the internal face of the pitcher, but they always slip again on the
polished, wax-coated zone, and tumble back once more to the bottom. In large
pitchers the involute rim of the aperture is in addition armed with sharp
teeth, which are pointed downwards and bristle in front of such of the unlucky
victims in the pitfal! as try to emerge (see fig. 19°). In a number of species
(N. Rafflesiana, N. echinostoma, N. Rajah, N. Edwardsiana, and N. Veitchii, all

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 135

natives of Borneo) this fringe of sharp teeth looks like the set of teeth of a beast
of prey; and in Nepenthes villosa, of which a pitcher is represented in fig. 214, a
double row of bigger and smaller teeth directed towards the bottom of the pitcher
is developed, and renders the escape of prey, once caught in the trap, impossible.

Most of the creatures that fall into the pitchers are, however, speedily drowned
in the large quantity of liquid at the bottom. For a third part or even a half of
the cavity is filled with liquid. This liquid originates from special gland-cells on
the inner surface of the pitcher, consists mainly of water, and so long as there are
no animals in the pitfall, gives only a very weak acid reaction. But as soon
as the body of an animal reaches the bottom, more fluid is secreted. This has
a distinctly acid taste, possesses the power of dissolving albuminous substances,
such as flesh and coagulated blood, and corresponds, not only in respect of this
action but also in chemical composition, to the gastric juice. For, in addition to
organic acids (malic, citric, and formic acids), an organic body like pepsin has
been detected in it, and nitrogenous organic compounds have been brought into
solution in it artificially as well. If the liquid from a Nepenthes pitcher, which has
not yet captured any animal, is poured into a glass vessel containing a small piece
of meat, the flesh is at first but little affected; but, if a few drops of formic acid are
added, the flesh is dissolved and undergoes the very same changes as it does in the
stomach of a mammal. The process going on in the pitchers of Nepenthes when
animals fall into them is therefore not only analogous to digestion, but may be
properly designated digestion.

The digested portions of the bodies are afterwards absorbed by special cells at
the bottom, and on the lower parts of the lining wall of the Nepenthes pitchers.

The third group included in the first section of carnivorous plants comprises
forms with scale-like leaves, within which are peculiar cavities penetrable by
minute animals only, on account of the narrowness of the entry. Special con-
trivances to prevent the escape of the prey are absent. The animals are retained
and drained of their juices in the cavities by means of protoplasmic filaments
radiating from special cells.

One of the most remarkable of the plants belonging to this group is the Tooth-
wort (Lathrea Sqywamaria), of which we shall repeatedly have occasion to speak.
It is nearly allied to the Yellow-Rattle and Cow-wheat, but it is destitute of
chlorophyll, and lives underground, parasitic on the roots of arborescent Angio-
sperms, except during a brief period annually when it sends up above-ground a few
short shoots covered with flowers. The subterranean stems are white, have a
fleshy, solid, and elastic appearance, and are covered throughout their entire length
with thick squamous leaves placed closely one above the other (see fig. 251 and
fig. 37). In colour and consistence these leaves are like the stem; in outline they
are broadly cordate, and they give the impression of being mounted fairly and
squarely upon the stem by means of the highly swollen and notched basal portion.
But it is only necessary to detach one of the scales from the stem to convince
oneself that this is not the case, and that the part taken at first sight to be the

136 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS

underside or back of the leaf is only a portion of the superior surface. In reality
each of these thick squamiform leaves is rolled back, and in it the following parts
may be distinguished: first, the place of insertion on the stem (fig 25*) which is
relatively small; secondly, the portion taken on cursory examination to be the
whole upper surface of the leaf, and consisting of an obliquely ascending blade
limited by a sharp border; next, starting from this sharp border, the part, which,
owing to its being suddenly bent down at an acute angle and falling away steeply,
is usually taken for the dorsal or inferior surface of the leaf, but which belongs, in
point of fact, to the front of the lamina; fourthly, the free extremity of the leaf in
the form of an involute limb; and fifthly, the true dorsal part, which is very small
relatively and is not visible until the involute tip is removed. Owing to the
involution of the apex, a canal or rather a recess is formed and runs across beneath
the leaf, close under the place where the latter is joined to the stem (see fig. 25”),
From five to thirteen (usually ten) chambers open into these recesses through a
series of little holes. They are excavations in the thickness of the scales and are
probably, in this form at anyrate, unique in the realm of plants. These extraordi-
nary chambers must be described as deep excavations in the foliar substance
proceeding from the back of the leaf. To solve the problem of their significance in
relation to the life of the plant, and to its absorption of nutriment in particular, it
is necessary to examine them somewhat more in detail.

The cavities, varying in number, as has been already mentioned from five to
thirteen, are situated very close together, but are not connected laterally. They are
all deeper than they are broad, and have irregularly undulating walls (see fig. 25°).
Two kinds of structures are conspicuous on the internal surfaces of the walls,
being raised above the ordinary epidermal cells, and projecting into the cavity.
Structures of the one kind are present in large numbers, and each of them consists
of a pair of cells in the form of a little head, borne by a short, cylindrical cell
serving as a stalk. The other variety, which occurs much more sparsely and is
altogether wanting in the folds of the wavy inner wall, is composed of a compara-
tively large tabular cell, roundish or elliptical in outline, inserted amongst the
ordinary epidermal cells and only slightly raised above them, and of two convex
cells, forming a low dome, which rests upon this base (fig. 25+) as though on a salver.
The walls of these cellular structures projecting into the cavity are comparatively
thick, and when the protoplasts living in the cells are stimulated, they send out,
through numerous pores in the thick walls, delicate filaments exactly like the
protoplasmic threads which the coated Infusoria, known by the name of Rhizopoda,
stretch forth through the pores of their armour (see fig. 255).

When small animals penetrate into the labyrinthine chambers of a Toothwort
leaf and touch the organs just described, the protoplasmic filaments are protruded
in rays in response to the stimulus, and lay themselves upon the intruders, They
act as prehensile arms in holding the smaller prey, chiefly Infusoria, and impede
the motion of larger animals so as to cut off their retreat. No special secretion has
been observed to be exuded in the foliar chambers of Lathrwa. But, seeing that

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 137

some time after the creatures have entered the chambers the only remains of them
that one meets with are claws, legs, bristles, and little amorphous lumps, their
sarcode, flesh, and blood having vanished and left no trace, we must suppose that
the absorption of nutriment from the dead prey here ensues through contact with
the extended protoplasmic tentacular filaments as in the case of Rhizopoda, to

2.
Es

a. CX
by Ke ane

Fig. 25.—Capturing apparatus in the Toothwort, Bartsia, and Butterwort.

1 Piece of an underground leaf-shoot of the Toothwort. 2 Longitudinal section through the same; x2. 8% Longitudinal section
through a leaf; x60. 4 Piece of the wall of a cavity; x200. 5 Plasmic threads radiating from the cells of the little heads;
6540. 6 Subterranean bud of Bartsia; natural size. 7 Cross-section through part of this bud; x60. 8 The margin of
a bud-scale in section; x200. 9% Piece of the epidermis of a leaf of Butterwort; x180. 1° Transverse section through the
leaf of a Butterwort (Pinguicula alpina); x50. 11 Transverse section through Butterwort leaf; natural size.

which these organs are so strikingly similar. It is not impossible that the sessile
organs alone have the function of absorption, and that the stalked capitate
structures serve for the retention of the prey; at least, this idea is supported by the
circumstance that the former, which, as already stated, are much the scarcer, have
vessels running to them connected by a peculiar barrel-shaped cell with the large
elliptical tabular cells, and this is not the case with the capitate forms of
structure.

The openings of the chambers into the recess at the back of a Toothwort leaf
being very narrow, only minute animals, such as Infusoria, Amcebe, Rhizopoda,

138 PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS.

Rotifera, small Acarina, species of Aphis, Poduride, &e., slip in. What it is that
prompts them to visit these hidden chambers is as hard to say as it is to give the
reason why the various species of Daphnia and Cyclops make their way into the
bladders of Utricularie. The most probable explanation is that the tiny creatures
push into the cavities in their search for food, and there meet their death.

It has been already stated that Lathreea is a parasite. Although we shall not
discuss the plant in that capacity until later on, we must point out now that the
main part of its nutriment is derived from the roots of deciduous arborescent
Angiosperms by means of special suckers. It only grows in regions where the
activity of trees and shrubs is interrupted by a winter of considerable duration.
As soon as the woody plants on whose roots individuals of Lathrea are parasitic
acquire their autumn tints and shed their leaves, the suckers invariably perish.
When, in the following spring, the ascent of the sap begins in the wood, Lathrwa
sends out new roots, which fasten their suckers underground upon the tree’s roots,
the latter being turgid with sap. The nutriment supplied in this way to Lathrwa
is not essentially different from that taken up by the roots of the tree or shrub in
question from the surrounding earth. It is composed mainly of water holding a
small quantity of mineral salts in solution, a mixture which has been termed
not unsuitably “crude sap”.

Living underground and being destitute of chlorophyll, Lathrea has not the
power of converting atmospheric carbon dioxide, or crude food-sap absorbed by
the suckers from the tree or shrub attacked, into the various organic compounds
necessary for further growth. For this reason, and inasmuch as the quantity of
nitrogenous compounds in the fluids withdrawn from the roots is but small, every
additional supply of organic food, especially of nitrogenous matter, such as is
derived from captured animals, must be exceedingly welcome. Although the prey
that is caught and digested consists for the most part of minute Infusoria, this
addition must not by any means be undervalued. We must take into account the
fact that every one of the innumerable leaf-scales of an individual Lathrea has an
apparatus for capture and digestion, and that this apparatus is active throughout
the entire year. The frost in winter does not reach so deep down in the soil as the
place where the plant is imbedded, so that there, even at a season when above-
ground everything is quiescent, the Infusoria and other little organisms continue
their existence and may be captured by Lathrwa. Thus, the extremely large
number of animals secured in the course of a year is nearly sufficient to maintain
the size of each individual plant.

It is after all anything but strange that a root-parasite, destitute of chlorophyll
and living underground, should make use of traps for animals, besides absorbing
crude sap from other plants; but, on the other hand, we are naturally surprised to
find plants which actually extract food from the earth by means of absorption-
cells, also absorbing through suckers from roots in the capacity of parasites, and,
furthermore, preying upon animals. An instance of such a plant is afforded,
however, by Bartsia alpina. This remarkable organism is distributed in the

PLANTS WITH TRAPS AND PITFALLS TO ENSNARE ANIMALS. 139

arctic region and amongst the high mountain flora throughout almost the whole of
Europe, and is very striking owing to the colour of its foliage being a mixture
of black, violet, and green. The flower, too, is of a sombre dark-violet hue, and
the entire plant, by reason of this peculiar colouring, gives a truly funereal im-
pression. We may remark incidentally that the name Bartsia was chosen by
Linneus for this sad-hued plant as an expression of his own grief at the death
of the zealous naturalist and physician, Bartsch, who was his intimate friend, and
who succumbed at a comparatively early age to the climate of Guiana. Damp
black earth in the neighbourhood of springs constitutes the favourite habitat of
these plants. Upon digging in summer time down to their roots, one sees that
a few suckers proceed from them, and fasten upon the sedges and other plants
growing in the vicinity; but one also discovers subterranean shoots having “ root-
hairs” developed near the nodes, at which are inserted the paired white scales; and
these “root-hairs” have the function of absorption-cells. Towards the autumn, oval
buds, likewise subterranean, are matured, in form not unlike horse-chestnut buds
(see fig. 25°), and composed of etiolated scales arranged in four rows and over-
lapping one another like tiles, so that only the back of the upper part of each
scale is visible, the lower part being covered by the scale next beneath it.

On the visible part of each scale’s convex under surface three sharply projecting
ribs are noticeable near the middle, whilst the two margins are rolied back so as
to form a recess in each case. But, as may be seen in the cross-section of a Bartsia
bud (see fig. 257), one pair of scales lies over the next higher pair in such a way as
to convert the recesses into ducts. Owing to this construction the interior of the
bud is perforated by twice as many ducts as there are covered leaf-scales, and the
orifices of each pair of ducts occur at the spots where the evolute margins of one
scale begin to be covered by the middle of the next lower scale. On one wall of
the ducts, 7.¢. in the recesses, structures like those which occur in the cavities of
Lathrea are developed, i.e. stalked glands, each composed of two cells borne upon
a basal cell; secondly, pairs of hemispherical domed cells; and, lastly, ordinary flat
epidermal cells (see fig. 25°). There can be little doubt that the whole apparatus
acts in the same way as in Lathrea, and is adapted to the capture of Infusoria.

The subterranean buds of Bartsia, just described, are produced late in the sum-
mer, and aerial shoots arise from them in the course of the following spring; and
seeing that the foliage-leaves on these shoots are richly furnished with chlorophyll,
and manufacture organic compounds in the sunlight from the constituents of the
air and from the fluids imbibed by absorption-cells from the ground, the question
arises whether an additional supply of nutriment from the dead bodies of captured
animals can be necessary or even advantageous. We shall, however, taking into
account the circumstances of Bartsia alpina when growing wild, answer this
question with an unconditional affirmative. The plant belongs, as has been said,
to an arctic and high alpine flora, and grows in regions where the activity of
plants above ground is limited to the short period of two months. After the lapse
of this brief vegetative season, the aerial parts of arctic and alpine plants either

140 PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY.

die altogether, or are buried in the snow, retaining their green colour, but suspend-
ing all movement and vital activity for from eight to ten months. The first snow
falls in the districts inhabited by Bartsia invariably before the ground is frozen,
and the wintry covering of snow, which gets deeper and deeper as time goes on,
protects the earth so completely from the cold that even in the superficial strata
the temperature does not sink below freezing point. In the bed thus kept free
from frost neither vegetable nor animal life is quite torpid, and there can be no
doubt that it is only beneficial to Bartsia, during the long interval, for its subter-
ranean buds to obtain an abundance of food from the bodies of captured Infusoria.
The advantage is the more obvious when one considers that the above-ground stem,
with its foliage-leaves and flowers, has to be built up in two or three weeks, in the
ensuing vegetative period, from the organic compounds stored in the cells of the
scales of the subterranean buds, and that both the damp ground in which Bartsia
grows, and also the roots of the marsh-plants to which it is joined by a few suckers,
though yielding water and mineral salts, afford but little material for the produc-
tion of nitrogenous compounds.

CARNIVOROUS PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE
OF PREY.

We have taken Lathrwa and Bartsia as types of the last group of that section
of carnivorous plants which manifest no external visible movement in the pitfalls
for the purpose of capture or digestion. The second section, now to be discussed,
includes plants in which movements of the leaves, or parts of leaves, modified as
organs of seizure and digestion, take place as a result of the contact of animal
bodies—movements which have the common object of bringing about the digestion
of the animals, whilst the retention of the latter is effected in very various ways.

Since in Lathrea and Bartsia the leaves, modified as organs of capture,
exhibit no kind of motion themselves, though movements take place in the proto-
plasm of the capitate pairs of cells in the interior of the cavities, having as their
object the holding of the prey, these plants form, to a certain extent, a link
between the first and second sections. All these divisions are for that matter
merely artificial, and it is not impossible that fresh forms may be discovered and
recognized as intermediate between the groups and series here distinguished,
obliterating the boundaries which have been adopted by us, simply with a view to
obtaining a general survey of the subject.

The first group of carnivorous plants which perform movements for the capture
of prey is composed of the various species of the genus Pingwicula (Butterwort).
Of this stock nearly forty species are known; and they are all much alike. Scarcely
any difference would be detected by an ordinary person between Pinguicula
calyptrata from the mountains of New Granada and Pingwicula vulgaris from
our own hills. In respect of habitat, too, they exhibit close conformity. In both
the Old World and the New they only thrive on damp spots, the neighbourhood of

PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY. 141

springs, banks of brooks, moorlands, and black peat-bogs. In the equatorial zone
they have retired into the cool regions of the higher mountains. The mountain
ranges of Mexico are particularly rich in species of Pinguicula, but all the forms
existing there occupy a circumscribed area. Southern and western Europe also
harbour a few native species whose area of distribution is surprisingly limited.
Thv species occurring in the arctic and sub-arctic zones are, on the contrary, exceed-
ingly widely distributed. One species has been found in antarctic regions at the
Straits of Magellan.

The species best known and most available for study is Pinguicula vulgaris.
The area of its distribution extends over the whole of the arctic and sub-arctic
regions, over the part of North America which lies to the north of the Mackenzie
River, over Labrador, Greenland, Iceland, and Lapland, throughout Siberia down
to the Baikal Mountains, and through Europe to the Balkans, Southern Alps, and
Pyrenees. This graceful plant is represented on its natural scale and growing on
a bog in the annexed Plate IT. entitled, “Insectivorous Plants: Sun-dew and Butter-
wort”. It has bilabiate flowers of a violet-blue colour, with palates covered with
velvety-white hairs, and with a sharp spur at the back. The flowers are borne
singly on slender stalks which rear themselves in an elegant curve from the centre
of a rosette of leaves that rests upon the ground. The leaves of the rosette in
Pinguicula vulgaris, as in all other species of Butterwort, are oblong-ovate or
ligulate and of a yellowish-green colour, and rest their under-surfaces upon the wet
ground, whilst their upper faces are exposed to the sky and rain. Owing to the
lateral margins being somewhat upturned, each leaf is converted into a broad flat-
bottomed trough (cf. the section taken right across a leaf in fig. 251° and 25%).
The trough is covered with a colourless sticky mucilage which is secreted by glands
distributed in large numbers over the entire upper surface of the leaf.

The glands are of two kinds. One variety is distinguishable by the naked eye
as consisting of a stalked head, and looks under the microscope like a tiny mush-
room (see fig. 25%). Its parts are a swollen disc composed of from eight to sixteen
cells grouped radially, and a stalk, consisting of an erect tubular cell supporting this
dise. A gland of the other sort is made up of eight cells grouped in the form of
a wart or knob supported by a very short stalk-cell, and only slightly raised above
the surface of the leaf. For the rest, ordinary flat epidermal cells make up the
epidermis, with here and there interspersed the guard-cells of stomata.

It has been calculated that there are 25,000 mucilage-secreting glands on a
square centimeter of a butterwort leaf, and that a rosette composed of from six to
nine leaves bears about half a million of them. Momentary contact, whether due to
rapid brushing by a solid body or to the incidence of drops of rain, causes no kind
of movement in them. The long-continued pressure of grains of sand or of solid
insoluble bodies in general stimulates the glandular cells to an inconsiderable
augmentation of the quantity of mucilage discharged, but does not cause secretion
of any acid digestive fluid. But as soon as a nitrogenous organic body is brought
into continuous contact with the glands, they are forthwith stimulated not only to

142 PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY.

a more profuse elimination of mucilage, but also to the secretion of an acid liquid,
which has the power of dissolving all bodies of the kind, namely, such as clotted
blood, milk, albumen, and even cartilage. It has been experimentally established
(for example) that small solid bits of cartilage placed on a leaf of Pinguscula
vulgaris, whose mucilage shows no sign of an acid reaction, cause, after ten or
eleven hours, the secretion of an acid liquid, and after forty-eight hours are almost
entirely dissolved by it. At the end of eighty-two hours the bits of cartilage used
in the experiment were completely liquefied, the whole secretion was reabsorbed,
and the glands had become dry. When small insects such as midges alight from
flight on a leaf of Pinguicula they remain glued by the mucilage, and their
struggles to extricate themselves only cause them to sink deeper into it. Thus
they generally perish in a very short time, are digested by the acid juice poured
from the glands in response to the stimulus, and are absorbed with the exception
of the wings, claws, and other parts of the skeleton.

The acid liquid secreted by the glands is viscous, and when a number of glands
are irritated it may exude so copiously as to fill the whole trough of the leaf. If
the margin of the leaf alone is stimulated, as when a small creeping insect, or a
midge alighting from above, gets upon the slightly up-curved margin of the leaf,
not only do the marginal glands, which are comparatively infrequent, discharge
their secretion, but in addition the edge curls over; the object of this movement
being to cover, if possible, the prey whilst it is held fast by the sticky mucilage, or
to push it into the middle of the flat channel, and so, in one way or the other, to
bring it into contact with as many glands as possible. The marginal glands alone
would not produce the requisite quantity of acid liquid to effect solution, and, on
this account, the glands on a wider area are summoned to assist in the manner
described. The involution of the margin takes place very slowly; it is usually
some hours before the animal sticking to the edge is enfolded, or, in the case of
the larger specimens, is pushed into the middle of the leaf. After solution and
absorption are accomplished, usually by the end of twenty-four hours, the leaf
expands again, and its margins assume the position which they had before their
involution.

Besides small insects, pieces of plants, such as spores and pollen-grains brought
by the wind, not infrequently fall on the viscid surfaces of Pinguicula leaves.
These are subjected to the same fate as animal organisms; their protoplasts being
dissolved and absorbed like the flesh and blood of insects.

The action of the acid juice secreted by the glands of butterwort leaves upon
albuminous bodies is identical with that of the gastric juice of animals. We may
presume therefore that there are in it, as in the gastric juice, two kinds of sub-
stance: firstly, a free acid, and, secondly, a ferment completely analogous to pepsin
in its action; for, as is well known, it is by means of this combination that the
juice of the animal stomach effects the solution of albuminoid compounds. Inas-
much as the gland-cells of Pinguicula absorb all the soluble part of the prey, and
re-absorb the solvent previously discharged by them, the action of this plant’s leaves

PLATE I.

1. Drosera rotundifolia. 2. Pinguicula vulgaris. 3. Sphagnum cymbifolium.

PLATE IT,

INSECTIVOROUS PLANTS: SUNDEW AND BUTTERWORT. |

PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY. 143

is exceedingly like that of the animal stomach, and the process may, as in the case
of Nepenthes, be fairly regarded as digestion. Whether, in carrying out this
process, the different forms of glands have also different functions, whether those
of one kind serve principally to secrete and those of the other to absorb, or
whether, perhaps, the one variety only discharges viscid mucilage to capture the
prey, and the other only a liquid containing acid and pepsin, are questions not yet
determined with certainty, although such a division of labour is in itself highly
probable.

The similarity existing between the leaf of Pingwicula and the animal stomach
in respect of their action on albuminous substances was turned to a practical
application in dairy-farming long before the discovery of the relationship by men
of science. The very same changes as are brought about in milk by the addition
of the rennet from a calf’s stomach can be induced by means of butterwort leaves.
If fresh milk, warm from the cow, is poured over these leaves, a peculiar tough
mass of close consistence is formed, the “Tatmidlk” or “Satmidlk” of Laplanders,
mentioned by Linnzus a hundred and fifty years ago as constituting a very
favourite dish in northern Scandinavia. In particular, the fact that by means
of a trifling quantity of Tatmidlk, produced in the manner described, a large
amount of fresh sweet milk may be also converted into Tatmidlk is specially
worthy of emphasis, for we learn from it that the substance generated by
Pinguicula behaves in this respect too, like other ferments. The immemorial use
of Pinguicula leaves by shepherds in the Alps as a cure for sores on the udders
of milch cows is also interesting, inasmuch as the curative effect on the sores is to
be explained by the antiseptic action of the secretion of the leaves in question,
and a method of healing, used empirically two centuries ago, thus finds confirmation
and a scientific explanation at the present day.

Since the curling up and unrolling of the leaf-margin in butterwort is
accomplished but slowly, the process above described is not at all conspicuous.
Moreover, the margin of a young leaf is always incurved, and that of a mature
leaf is also somewhat turned up before stimulation has taken place; so that, strictly
speaking, we only have to do with a greater or smaller degree of involution, and its
nature can only be determined by careful observation.

In the plants which form the second group in this section of carnivorous
plants, and of which the best known representatives are the various species of the
genus Sun-dew (Drosera), the movements, whereby the capture and digestion of
small animals is effected, occur much more rapidly and obviously. These species
are usually rooted in the damp dark soil of moors. They have also the same
habitats as Pinguicule, and often enough sun-dew and butterwort are to be seen
flourishing close together on a strip of boggy ground no larger than one’s hand.
On Plate II. they are shown thus associated. Drosera rotundifolia, together
with Pinguicula vulgaris, is there represented, life size, growing in a bed of
sphagnum amongst sedges on an upland moor. The thing that strikes one most at
sight of the round-leaved sun-dew depicted, and in general of all the forty known

144 PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY.

species of Drosera, is the presence of the delicate wine-red filaments, clavate
at their free ends and each supporting a glistening droplet of fluid, which stand
out from the leaves, and whose function is essentially the same as that of the
glands, stalked and sessile, on the leaf of Pinguicula. These filaments only
proceed from the upper surface and margin of the sun-dew leaf. The under surface
is smooth and hairless, and in many species, including the Drosera rotundifolia
depicted on the plate, it rests upon the damp mossy ground. In this particular,
and also in the circumstance that all the leaves of each individual are adpressed to
the ground and grouped in a rosette or radially around the central slender flower-
ing-stem, there exists a very obvious analogy between Drosera, and not Pinguicula
alone, but many other carnivorous plants, such as Sarracenia, Heliamphora, Cepha-
lotus, and Dionca, the fly-trap presently to be described.

The filaments or tentacles projecting from the upper surface and margin of the
leaf look like pins inserted in a flat cushion and are of unequal size. Those which
stand up perpendicularly from the middle are the shortest, and those which radiate
from the outermost edge are the longest (see fig. 26‘). Between these extremes
are intermediate lengths gradually leading from the one to the other. There are
on a leaf, in round numbers, about two hundred of these tentacles. The clavate
head at the free extremity of each must be regarded as a gland. It secretes
a clear, thick, sticky matter which is readily drawn out into threads, and which
shines and glitters in the sunlight like a drop of dew, whence the plant has derived
its name of sun-dew. Shocks occasioned by wind or the dropping of rain do not
excite any kind of movement in the tentacles. If grains of sand are blown upon
them by the wind, or if little bits of glass, coal, gum, or sugar, or minute quantities
of paste, wine, tea, or any other non-nitrogenous substance are brought by artificial
means into contact with the enlarged extremities of the tentacles, the exudation of
liquid at the places in question is augmented, and the secretion also becomes acid,
but there is no elimination of pepsin, and no change of importance ensues in the
direction of the tentacles, or the attitude of the leaf-margin. But the moment
a small insect, mistaking the glittering drops on the tentacles for honey as it
flies by, alights on the leaf and so touches the glands, or upon the artificial
placing of particles of nitrogenous organic matter, such as flesh or albumen, on the
tentacle-heads, there ensues, as in the case of Pinguicula, an increase in the dis-
charge of acid juice, as well as the addition of a ferment to its composition. The
action of this ferment on albuminous compounds is entirely similar to that of
pepsin, and we may even go so far as to speak of it as pepsin.

The insects that fly on to the leaves and are caught by the sticky juice try to
disencumber themselves by stroking the viscous matter off with their legs, but they
only besmear themselves still more, and are soon plastered all over the body, and
have their movements greatly impeded by the secretion. Their efforts to save
themselves soon cease, the orifices of their respiratory organs are covered with the
juice and choked, and after a brief interval they die from suffocation. All these
phenomena correspond, in the main, to those occasioned by identical causes in the

PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY. 145

case of Pinguicula. But the leaves of the sun-dew are especially characterized by
the movements performed by the tentacles in response to stimulation by animal
matter. These movements are exhibited most conspicuously by the longest
tentacles, which stand out radially from the edge of a leaf. A few minutes after
the gland of one of these marginal tentacles has been excited by a living or dead
animal becoming glued to it, a systematic disturbance is set up in the whole fringe
of tentacles. First, the tentacle bearing the gland originally irritated with the
animal’s body attached to it, bends inwards, performing a movement similar to that

Fig. 26,—Tentacles on leaf of Sun-dew.

1 Glands at the extremity of a tentacle; x30. 2 Leaf with all its tentacles inflexed towards the middle. 3 Leaf with half the
tentacles inflected over a captured insect. 4 Leaf with all the tentacles extended. Figs. 2, 8, and 4x4.

of the hand of a watch. Under peculiarly favourable circumstances it describes an
angle of 45° in from two to three minutes, and an angle of 90° in ten minutes. A
still more intelligible comparison than that of the hand of a watch is afforded by the
human hand. Supposing that the foreign body is glued to the tip of a finger it.
would be moved by the curvature of the finger to the palm in the course of ten
minutes. About ten minutes after the first tentacle has been set in motion, those
standing near it begin to bend also (see fig. 26%). After another ten minutes,
tentacles situated further off follow suit; and in the course of from one to three
hours all the tentacles are inflected and converge upon the body in question.

We must not omit to mention that this object does not always occupy the same
place on the surface of the leaf. Often, no doubt, the prey is exactly in the middle,
and the tentacles then swoop down one after the other to that spot; but often also
the place is elsewhere and yet the movements never fail in their aim. It may
happen that a median tentacle, on repeated excitation, may have to bend now to the
right, now to the left. When little bits of meat are placed simultaneously on the
right and left halves of the same sun-dew leaf, the two hundred tentacles divide
into two groups, and each one of the groups directs its aim to one of the bits of

meat. This happens also if two small insects alight at the same moment on a leaf,
Vou. I. 10

146 PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY.

one on one side and the other on the other. The movement of the tentacles is often
accompanied by an inflection of the whole surface of the leaf, the lamina becoming
concave like a hollow palm, and when, under these circumstances, the tentacles have
converged from the margin on to the concave central part, the leaf has the effect of
a closed fist (see fig. 26 *).

All these movements vary from one case to another and supplement one another
according to the needs of the moment and with a view to immediate advantage.
The one result that is always attained by the combined action is the covering of the
prey with a copious supply of the secretion poured from a number of glands, so that
it is dissolved and rendered fit for absorption and for the purpose of nourishment.
When an insect is caught by one of the marginal tentacles, the secretion there
discharged would not suffice for these purposes. The prey is accordingly trans-
ported as far as possible towards the middle of the lamina, where it comes into
contact with the digestive juice exuded from a maximum number of glands. It is
only when the size of the animal is rather large that the leaf becomes hollow in the
middle like a spoon, with the juice of more than fifty glands concentrated in the
depression. In a case of this kind the tentacles continue inflexed much longer,
because the solution of the prey requires more time. If the captive is very small,
its solution and absorption are completed in a couple of days. Afterwards, the
tentacles lift, straighten themselves, and resume their original positions. The jaws,
wings, compound eyes, leg-bones, claws, &c., of the captured animals are left: behind
undigested; but the flesh and blood are totally absorbed, and the liquid poured out
by the glands to effect solution is also re-imbibed by them. The undigested
remnants being now suspended on dry tentacles are easily blown away from the
sun-dew leaves by the wind. After an interval of a day or two the glands at the
ends of the tentacles, now occupying their original positions, again separate out a
viscid fluid in the form of tiny dewdrops, and the leaf is once more furnished
with the means of securing insects, and is able to repeat the movements above
described.

Amongst the animals which fall victims to the sun-dew the most predominant
are little midges; but rather larger flies, too, ants both with and without wings,
beetles, small butterflies, and even dragon-flies, as they run, creep, or fly past,
adhere ‘to the extended gland-bearing tentacles as though they were lime-twigs.
The larger animals, such as dragon-flies, are secured by the co-operation of two
or three adjacent leaves. Some idea of the large number of captives made by a
sun-dew is given by the fact that once upon a single leaf were found the remains of
thirteen different insects.

In order to place in a true light the vast significance of the movements of the
tentacles belonging to Drosera leaves in relation, not only to the nourishment of
that plant, but to plant-life in general, it is necessary to direct attention to the
facts that these movements are accomplished not in the cell directly excited, but in
others, 7.¢. in adjacent cells belonging to the same community; that a propagation of
the stimulus takes place from one protoplast to a second, thence to a third; fourth,

PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY. 147

tenth, and so on, to a hundredth, and that the speed of transmission is susceptible of
measurement. The movements occasioned in protoplasts situated at a distance from
the seat of irritation by the stimulus propagated from its vicinity are, according to
the position of the stimulating object, sometimes in one direction, sometimes in
another, but in every case they are purposeful and for the benefit of the whole
organism.

Investigations with a view to determining the degree of sensitiveness of
Drosera, leaves yielded the following results. A particle of a woman’s hair, 0:2 mm.
long and weighing 0:000822 mg., when placed upon a gland of Drosera rotundifolia,
caused a movement of the tentacle belonging to the excited gland, which manifested
itself externally as an inflection. If so minute a body of the kind is placed on the
human tongue, its presence is not perceived, so that the sensitiveness of the
protoplasts in the glands of the sun-dew is greater than that of the nerve extremities
in the tip of the tongue, though the latter are well known to be the most sensitive
in the human body. A four-thousandth part of a milligram of ammonium
carbonate sufficed to induce motion, as also did s5$o9 mg. of ammonium phosphate,
It would lead us too far to consider all the experiments in detail, but they point to
the conclusion that liquid substances stimulate more strongly than solid bodies, and
that the more nutritious to the plant the material placed upon the gland, the more
quickly does the inflection of the tentacles ensue.

The propagation or conduction of a stimulus from cell to cell, as it takes place
in the cell-community constituting a sun-dew leaf, may be compared to the
conduction of stimulus by nerves from a sense-organ to the central organ, and of the
force of will from the brain to the muscles. This transmission is conceived to be a
progressive movement affecting the ultimate particles of the nerves, and comparable
to the conduction of sound, light, and electricity; but no one has yet succeeded in
making these movements visible. So much the more interesting is it to be able to
see and follow in the glands and tentacles, by the aid of very slight magnifying
power or even with the naked eye, the material change which occurs in the
protoplasts of the sun-dew leaf when they are receiving or transmitting a stimulus.
The pedicel of a tentacle is penetrated by one or two vessels with fine spiral
sculpturing on the inner surface, and around these are parenchymatous cells. The
gland has in the middle a group of oblong cells sculptured internally with very
delicate spiral thickenings (“spiroids”), and the vessel or pair of vessels running
down the middle of the tentacle (see fig. 261) merge into these spiroids. A
parenchyma composed of two or three layers surrounds the median group of
spiroids. In each parenchymatous cell the protoplast is discerned forming a thick
lining to the wall, and having a continuous streaming motion: whilst within
the vacuole is contained a homogeneous liquid of a purple colour. If the minutest
fragment of animal matter, such as flesh or albumen, be placed on these cells it acts
as a stimulant on the contents of the cell-cavities, and the impulse manifests itself
in a division of the hitherto homogeneous purple liquid into dark, roundish, elub-
shaped and vermiform lumps, cloudy spheres, and an almost colourless liquid.

148 PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY.

This change, known as “aggregation”, is propagated from the spot irritated down
from one cell to another through the tentacle, across the leaf surface to adjoining
tentacles, up to the heads of these, and so further and further radiating, so to speak,
in all directions. Accompanying this visible sign of conduction, we have the
bending of all tentacles in which the purple fluid is altered in the way described.
When the source of excitation, the piece of flesh, is dissolved and digested, and the
tentacles resume their original position, the dark lumps and spheres in the cavities

: fi ; Prune
Fig. 27.—Venus’s Fly-trap (Dioncea muscipula). i or

of the protoplasts disappear, and the homogeneous purple colour is restored as it
existed before the stimulation.

The various species of the Sun-dew genus are distributed over all parts of the
world, and are more numerous than those of any other genus of the family of
Droseracee. Most of the other genera belonging to this order (Dionaa,
Aldrovandia, Byblis, Roridula, Drosophyllum) are by no means rich in members.
Each is represented merely by a single or few species, and is found exclusively in
a very limited district. Like Drosera, they are ail insectivorous plants, and all
have the power of dissolving, absorbing, and using as supplementary nutriment,
nitrogenous compounds from dead animals. The most striking of them are Dionwa
and Aldrovandia. They form the very small third group of animal-captors, in
which movements are performed for the purpose of prey, and their apparatus for

PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY. 149

seizure and digestion is one of the most curious adaptations displayed by the
vegetable world.

The Venus’s Fly-trap (Dionwa muscipula), which is represented above (fig. 27),
half its natural size, grows wild only in a narrow strip of country in the east of
North America (from Long Island to Florida) in the vicinity of peat-bogs. The
leaves, like those of many other carnivorous plants, are grouped in rosettes round
the flowering axes, and for the most part rest their under surfaces either entirely
or partially upon the ground. Each leaf consists, first, of a flat, spatulate petiole,
which is, as it were, truncated in front and suddenly contracted to the midrib, and,
secondly, of a roundish lamina. The latter is divided by the midrib into two
symmetrical halves, inclined to one another at an angle of from 60° to 90° like the
leaves of a half-open book. Both margins of the lamina run out into from twelve
to twenty long, sharp teeth, which, however, do not carry either glands or any
other special structures on their tips.

On the central part of each half of the leaf there are three very stiff and sharp
spines, which are always shorter than the marginal teeth, and which stand up
obliquely. They are composed of elongated cells whose protoplasm throughout
life is in very active circulation. At the base of each spinous process is a short
cylindrical pad of tissue formed of small parenchymatous cells, and this pad allows
the spine to be deflected. The spines themselves are rigid, and do not bend in
response to pressure, but are forced down on to the surface of the leaf, the pad of
tissue referred to acting as a hinge. In addition to these processes, glands are
scattered over the whole upper surface of the lamina. They look like the
shortly-stalked glands of a butterwort leaf, are composed of some twenty-eight
small cells, are purple in colour, and capable of secreting a mucilaginous liquid.
Little trichomes, stellate hairs, are also borne on the edge of the leaf between the
sharp teeth and also on the under-surface.

No visible change is produced by a blow or shock or by pressure affecting the
whole plant or leaf, as might be caused by wind or falling drops of rain, nor even
by injuries to the petiole or back of the lamina. But as soon as the upper surface
of the lamina is touched, the two lobes, hitherto at right angles, approach one
another until the sharp marginal teeth are interlocked, and the body touching the
leaf is inclosed between two walls (fig. 287). When the places beset with purple
glands are alone excited by contact with the object, this inflection and closing
follows very slowly; but if one of the six spines projecting in trios from the two
foliar lobes is ever so lightly touched, the leaf shuts up within 10-30 seconds, 4.e.
instantaneously, and the action is best compared to the slamming of a half-open
book. The teeth standing at the edge of the leaf lock into one another on these
occasions like the fingers of clasped hands. The lobes, however, whose: surfaces
were hitherto plane, become at the moment of closing somewhat concave, so that
when approached they do not lie flat against one another but inclose a cavity, the
contour of which nearly corresponds with that of a bean.

The further changes and processes now ensuing depend upon whether the

150 PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY.

sensitive part of the leaf was subjected to prolonged or only momentary contact,
and also upon the nature of the body touching it, whether inorganic or organic,
non-nitrogenous or nitrogenous. When rapidly touched or stroked, the leaf folds
together, but only remains closed for a short time. The lobes soon begin to re-
open, and can be stimulated afresh immediately and caused to shut again. This
is also the case when the disturbance was due to the impact of a grain of sand or
any other inorganic body, and likewise when the stimulus proceeded from an
organic but non-nitrogenous object. But if, on the other hand, the body upon the
upper surface of the lamina was nitrogenous and the contact not too hasty, the
two lobes of the leaf remain closed over the object for a longer period. They also

Fig. 28.—Capturing apparatus of the leaves of Aldrovandia and Venus's Fly-trap.

1 Expanded leaf of a Venus’s Fly-trap. 2 Section of a closed leaf. 8% One of the sensitive bristles on the surface of the leaf.
4 Expanded leaf of Aldrovandia. Section of a closed leaf. 6 Glands on the surface of leaf of Aldrovandia. 7 Gland
from the wall of a Sarracenia pitcher.

become flat and even again, and are pressed together so tightly that intervening
bodies, if soft, are squeezed and crushed to pieces. In addition, the glands, dry
till then, begin to secrete a slimy, colourless, highly acid juice; and this is true
even of those glands which are not at all in contact with the nitrogenous bodies
inclosed, The secretion flows so copiously that it can be seen in the form of drops
if the shut lobes be forcibly separated. It covers the imprisoned body and gradu-
ally dissolves the albuminous compounds therein contained. Afterwards, the
secretion and the matter dissolved in it are re-absorbed by the same glands as
previously discharged the acid liquid, containing pepsin, in response to the stimulus;
and when the trap reopens, the glands are dry. The soluble part of the prey has
now vanished: the six little spinous processes, which were bent in the closed leaf
like the blades of a pocket-knife and lay pressed down upon the surface, stand up;
and the leaf is once more equipped for making fresh captures.

The time requisite for the digestion of a nitrogenous body, resting upon the
surface of a leaf, varies according to the size of the body. The leaf usually remains
closed for from eight to fourteen days, but often even for twenty days. Although

PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY. 151

the larger live articulated animals—earwigs, millipedes, and dragon-flies—caught
upon the upper surface of the leaf, cause the lobes to slam together, they are able
to slip out if part of their bodies projects beyond the toothed margin, for the teeth
are flexible and yield to strong pressure. But small creatures are hopelessly lost
when the lobes have closed over them. They are at once suffocated in the liquid,
which is poured out copiously by the glands, and are then dissolved and absorbed
with the exception of their claws, leg-bones, chitinous rings, &., which are incapable
of being digested.

In spite of the identity of aim and of result, the mechanism of a Dionwa leaf
differs very materially from that of the sun-dew leaf described above. Division of
labour is carried much further in the Fly-trap. The pre-eminently sensitive
structures, viz. the six filaments situated upon the upper surface of the leaf,

Fig. 29.—Aldrovandia vesiculosa.

do not act also as digestive glands. Again, the long sharp teeth at the edge
of the leaf, which correspond in position to the marginal tentacles of a sun-dew leaf,
carry no glands, and only serve to close the trap securely when an animal has been
caught. Accordingly in Dionca there exist special structures for three different
functions, namely, stimulation, seizure, and digestion, whilst in the case of Drosera
all these functions belong to the gland-bearing tentacles alone. The stimulus acting
on the sensitive filaments on the leaf of the Fly-trap is liberated in the form of a
rapid motion of the lobes and a discharge of digestive fluid from the glands, and
this discharge of secretion ensues therefore through the mediation of cells which
have not themselves been directly excited. The process here again is much more
striking than in the sun-dew leaf. The transmission of stimulus, though as a fact
identical in the two plants we are comparing, proceeds at any rate with much
greater rapidity in Dionea than in Drosera.

The analogy existing between these processes, especially the conduction and
liberation of stimulus, and similar phenomena of the muscles and nerves in an
animal organism, has already been brought out in discussing the sun-dew. Itisa
noteworthy fact that, in the fly-traps, actual electric currents have been observed,
which prove that the greatest resemblance exists to muscles and nerves as regards
electro-motor action also. A positive current runs from the base to the apex of the
lamina; another current running in the opposite direction is demonstrable in the
petiole; and the upper layers of cells in the lamina and the midrib are ascertained

152 PLANTS WHICH EXHIBIT MOVEMENTS IN THE CAPTURE OF PREY.

to be the seat of origin of this phenomenon. A great alteration in the intensity of
the current ensues upon each excitation of the leaf; and, inasmuch as this fluctuation
of the electric current precedes the movement of the leaf caused by the stimulus,
it is natural to assume that it depends upon the conduction and liberation of the
stimulus.

Aldrovandia, the plant nearest allied to the Fly-trap in the structure of its leaf,
is a water-plant, which occurs scattered over the southern and central parts of
Europe. It only flourishes in shallow ditches, pools, and small ponds inclosed by
banks of reeds and rushes, where the plants are immersed in clear, so-called soft
water, attaining in summer to a temperature of 30° C., and are exempt from any
incrustation of carbonate of lime, whereby the tender parts of the leaves might
be hindered in their movements. On cursory inspection, one might take Aldro-
vandia vesiculosa, which is represented above full size and in its natural position,
for a Utricularia (cf. fig. 17). It lives, like the latter, floating in water; is destitute
of roots, and has a slender filiform stem with leaves arranged in whorls and ter-
minating in bristles. In proportion as it grows at the apex, the hinder part dies
away and decays. The development of hibernating buds takes place also in
precisely the same manner as in Utricularia. Towards autumn, the stem ceases
to elongate, and the two hundred small and young leaves, which adorn the ex-
tremity of the stem and whose cells are quite full of starch, remain lying closely
wrapped one upon another and form a dark, oval, bristly ball, which sinks at the
commencement of winter to the bottom of the pool or pond and hibernates there
lying upon the mud.

It is not till very late in the following spring, when little midge-larve and
other animals begin to move about in the water, that fresh life is awakened in
these structures. The starch-grains in the leaves are brought into solution and
used for building-material; the axis elongates, and lacune filled with air are
developed, whereupon the plant becomes lighter, ascends, and remains throughout
the summer and autumn floating just below the surface of the water. Although
the little leaves of the winter-buds generally admit of the recognition of their
future form, the apparatus adapted to the capture of animals is but little developed
on them. But when once the leaves are mature, they bear laminw, which are
extremely like those of Dionwa, in shape, and serve, as do the latter, for the capture
of small animals. Each leaf is differentiated, as in Dionea, into a strong, dark-green
petiole expanded and anteriorly clavate, and into a roundish lamina with a delicate
epidermis and with two lobes connected by the midrib and inclined nearly at right
angles to one another (see fig. 28*). The midrib projects beyond the apex of the
delicate lamina in the form of a bristle. In addition, comparatively long, rigid
bristles, tipped with extremely fine spines, proceed from the petiole close to where
the latter is joined to the lamina; and these bristles, which are directed forwards,
give the whole leaf-structure a spiky appearance and prevent the approach of such
animals as are not suitable for prey. The two margins of the lamina are bent
inwards, and their rims are studded with small conical points. On the surface of

CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS. 153

the lamina, especially along the midrib, there are pointed hairs; and a great
number of glands, some larger and some smaller, occur from the midrib to nearly
the middle of each lobe. The larger glands are discoid, and not unlike the sessile
glands on the leaves of Pinguicula. They consist of four median cells with twelve
others grouped in a circle round them, and are borne upon a very short stalk.
The small glands are few-celled, being usually composed simply of a capitate-cell
resting upon a short foot-cell (see fig. 28°), Towards the incurved margin of the
lamina are displayed scattered stellate hairs, i.e. groups of cells so arranged as
to present the appearance of a St. Andrew’s cross when seen from above.

If minute animals or Diatomaces, especially species of Navicula, whilst
swimming about in the water, touch the upper surfaces of the lobes set at right-
angles—in particular, if the hairs in the middle are stroked as they creep
by—the two lobes shut together quickly in the same way as those of Dionca, and.
the animal or Navicula, as the case may be, is then enclosed in a cage between
two somewhat inflated walls. The possibility of an attempt on the part of the
captive to escape by the place where the margins of the lamina meet is met by the
circumstance that the edges of the incurved margins are furnished with sharp
indentations turned towards the interior of the cavity enclosed between the lobes
(see fig. 28°).

Amongst the prisoners we find the same company as in the traps of Utricularia,
namely, small species of Cyclops, Daphnia, and Cypris, larve of aquatic insects, and
not infrequently also species of Navicula and other free and solitary Diatomacee.

How the prey is killed and digested has not yet been ascertained. It dves not
in any case take place so quickly as in Dionwa, for instances have been seen of
animals still living in their prison six days after being caught. But, at last, move-
ments and vital actions cease, and if after a couple of weeks the two lobes of the
lamina are pulled apart, the only contents to be found are shells, bristles, rings, and
siliceous skeletons, whilst everything soluble has vanished, having evidently been
absorbed.

Very similar to the species distributed through Southern and Central Europe
are Aldrovandia australis, a native of Australia, and Aldrovandia verticillata,
inhabiting tropical India. The fact that the remains of small aquatic beetles
and other creatures have been found within their closed laminz, leads us to the
conclusion that they act as entrappers of animals in the same way as Aldro-
vandia vesiculosa.

CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS.

The forms constituting the third section of carnivorous plants neither have pit-
falls nor move in response to the contact of animal matter, but the leaves act as
motionless lime-twigs, their glands having the power of pouring out sticky sub-
stances to capture prey, and juices to digest it, being able besides to re-absorb the
albuminoid compounds dissolved. The most striking representative of this section,

154 CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS.

and the one most accurately studied, is the Fly-catcher (Drosophyllum lusitanicum),
which is indigenous to Portugal and Morocco, and is shown in the illustration on
p. 155. This plant differs from all the carnivorous kinds hitherto discussed in
respect of habitat, inasmuch as it does not grow under water or even in swampy
places but on sandy ground and dry rocky mountains. The stem in robust
specimens is nearly 9 inches high, and bears, on a few short branches at the
top, flowers from 2 to 3 cm. in diameter. The leaves are very numerous and
particularly crowded round the base of the stem. Their shape is linear and
much attenuated towards the filiform tip, whilst the upper surface is somewhat
hollowed so as to form a groove. With the exception of these grooves, the leaves are
entirely covered with beads, which glisten in the sunshine like dewdrops; and it is
to this circumstance that the plant owes its name of Drosophyllum, 1.e. Dew-leaf.
The glittering drops are the secretion of glands, which in form remind one in some
respects of the long-stalked glands of the butterwort, and in others of those of the
Sun-dew (Drosera). They resemble the latter in their red coloration, in the fact
that the pedicel bearing the gland contains vessels whilst the glands themselves
have oblong cells with internal walls thickened by fine spiral ridges, and further,
in the circumstance that the secretion covers the gland with a colourless film in the
form of a drop. But in shape they especially resemble the glands of the butter-
wort, being just like little mushrooms.

Besides these glands, which are borne on stalks of unequal lengths and are
plainly to be distinguished with the naked eye, there are also very small sessile
glands. These latter are colourless, and in particular differ from the stalked variety
in the fact that they discharge an acid liquid only when they come into contact
with nitrogenous animal matter, whereas the secretion in drops on the stalked
glands is secreted without any such contact. This secretion is acid and extremely
viscid. It has the property of adhering immediately to foreign bodies coming into
contact with it, though it is readily withdrawn from the gland itself. When an
insect alights on the leaf, its legs, abdomen, and wings instantly stick to the drop
touched by them. The insect, however, is not held fast by the gland which secreted
that drop, but, being able to move, drags the drop off the gland. Its movements
bring it into contact with other drops, which thereupon are similarly detached
from the glands; and so, in a very short time, the insect is smeared with the
‘secretion from a number of glands. Thus clogged and overwhelmed, it is no longer
able to crawl along, but, suffocating, sinks down to the sessile glands which cover
the surface of the leaf at a lower level. All the soluble parts of its body are then
dissolved by means of the secretion of these glands and are afterwards absorbed.

The glands replace the drops of secretion of which they are despoiled with
great rapidity. The quantity of acid liquid secreted is, in general, very great, so
that it is not surprising to find Drosophyllum covered at the same time with
remains of besmeared dead bodies drained of their juices, and with still struggling
insects which have recently alighted and become clogged. The number of animals
caught by the leaves of a single plant is very great; and even people who are not

CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS. 155

otherwise interested in the vegetable world are impressed by the sight of a plant
with its leaves covered with a number of insects adhering to them as though they
were lime-twigs. In the neighbourhood of Oporto, where Drosophyllum grows
abundantly, the peasants use these plants instead of lime-twigs, hanging them up

in their rooms, and so getting rid of numbers of troublesome flies which stick to
them and are killed.

A number of other plants have the power, though in a less conspicuous degree
than Drosophyllum, of obtaining additional nitrogenous food out of adherent
animals by means of secretory and absorptive glands. Such are many species of
primulas, saxifrages, and house-leeks, which bury their roots in cracks and crevices
of rock (eg. Primula viscosa, P. villosa, P. hirsuta, Saxifraga luteo-viridis, S.
bulbifera, 8. tridactylites, Sempervivwm montanum), secondly, caryophyllaceous

ee SSS / Verity
Fig. 30.—The Fly-catcher (Drosophyllum lusitanicum). padbruncs

156 CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS.

plants and species of the caper order (¢.g. Saponaria viscosa, Silene viscosa, Cleome
ornithopodioides, Bonchea cohiteoides), and lastly, a series of plants which flourish
in peat-bogs and upon deep beds of humus, such as Sedum villoswm, Roridula
dentata, Byblis gigantea, and many others besides.

It would, however, be erroneous to suppose that in all cases where a sticky
coating occurs on leaves and stem a solution and digestion of the insects adhering
to the viscid parts is necessarily denoted. In many instances structures of this
kind, which are analogous to lime-twigs, are a means of protecting honey-bearing
flowers against unwelcome guests belonging to the world of insects, as will be
explained in greater detail later on. Glands secreting a viscid substance may, no
doubt, often possess two kinds of function—they may, on the one hand, prevent
unbidden animals from approaching the honey, and, on the other, turn to advantage
such insects as are tempted by immoderate craving to step upon the perilous path
leading to the honey-receptacles and so adhere to it and die, by dissolving their
flesh and blood with the aid of the secretion and then absorbing them.

Many plants have, on the epidermis of their leaves, structures corresponding in
form to the glands of insectivorous plants, but which do not discharge secretions
either spontaneously or when irritated. On the other hand, these structures have
the power of imbibing water, and are, in this relation, of the greatest importance
to the plants in question. Although the more detailed treatment of them is post-
poned until we have occasion to deal with the absorption of water by aérial organs,
it is advisable to refer now to the fact that chemically pure water only very rarely
reaches the interior of a plant by means of the absorptive organs mentioned.
Sulphuric acid is almost always introduced with atmospheric water, and in some
circumstances ammonia also. However trivial the amount of the nitrogen con-
veyed to plants in this way, it must not be undervalued, at all events in the case
of those which are only able to acquire small quantities of nitrogenous compounds
from the ground by means of their roots. Now, it is very probable that plants of |
this kind do not reject even other nitrogenous compounds which are brought
with the water from the atmosphere to their aérial leaves. The foliage-leaves of
many plants display contrivances whereby rain-water is often retained for a
considerable time in special hollows. In these depressions there is invariably a
collection of dust-particles, small dead animals, pollen-grains, &c., which have been
blown in by the wind, whilst rain trickling down the stem brings very various
objects with it from higher up and washes them into these reservoirs in the leaves.
Sometimes too a few animals are drowned in the water-receptacles, As a matter
of fact, the water in the hollows of the leaves of the Peltate Saxifrage and of
Bromeliaces, in the inflated vagine of many umbelliferous plants, and in the
cups formed by the coalescence of opposite leaves in many Gentianex, Composite,
and Dipsacex, is always brown-coloured, and contains nitrogenous compounds in
solution, derived from the decaying bodies of dead animals which have fallen into
these receptacles.

If absorbent organs are present in the reservoirs in question, the water, together

CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS. 157

with the nitrogenous compounds dissolved therein, is absorbed without delay.
Hollows of this kind occurring in foliage-leaves only differ from those above
described as developed on sarracenias in being destitute of special contrivances for
decoying animals into the traps, and for rendering their escape from the latter
impossible. It cannot be denied that through forms of this kind a gradual
transition has been proved to exist between plants which absorb nearly pure water
by means of their foliage-leaves and those which capture animals. And, further,
amongst the latter we find all gradations of mechanism from Drosophyllum and
the Primulas with their epiphyllous secretory glands up to the Fly-trap (Dionea),
which exhibits the most complex apparatus of all for capturing and digesting prey,
and in which division of labour is carried to its highest development by the com-
munities of cells constituting the foliage-leaves.

It is not surprising that the first apparatus for capturing and digesting insects
to be noticed, to have its functions recognized and to be described, was that of
Dionea. But it strikes one as all the more strange that of late the question has
repeatedly been mooted in the very case of Dionca, as to whether the capture and
digestion of insects is not injurious instead of beneficial to these plants. Gardeners,
who have cultivated Dionwa in greenhouses, have made the observation that
individuals protected from the visits of insects thrived at least as well as those
whose leaves were covered with bits of meat, &c., or, to employ the usual phrase,
were fed with meat. It has also been found that a leaf cannot stand more than
three meals; indeed, it often happens that even after the first occasion of digesting
a bit of meat, the leaf concerned shows signs of having been injured by the
repast. That is to say, a long time elapses before leaves which have digested
a largish alouminoid mass regain their normal irritability; and often they wither
and die. If cheese is placed on Dionca, it is true the leaf closes over it, and there
is a commencement of the process of solution, but before the latter is accomplished
the leaf turns brown and perishes. Yet if Dionca were obliged to lose a leaf after
every meal, the result would be very disadvantageous.

As against these considerations, we have first of all to remark that the
absorption of nutriment takes place in nature in a manner differing materially from
the phenomenon in greenhouses. A leaf of Dionca in the wild state is protected
against the possibility of receiving too plentiful a dose of albuminoid substances at
a time. Insects so large as not to allow the lobes to close together over them slip
out again, and only small ones are caught and retained. When, in the latter case,
one deducts the chitinous coat, and in general all parts not susceptible of being
digested, such a small quantity of alouminoid compounds is left that, compared with
it, the little cubes of meat used in the experiments made in greenhouses must be
looked upon as an exceedingly sumptuous repast. But that so small an amount of
nitrogenous food as is to be derived from a tiny captured insect does. not act
injuriously, follows from the fact that dionzas growing wild flourish excellently,
and do not exhibit the brown discoloration of the leaves which is caused in a
greenhouse by placing bits of cheese upon them. If the absorption of nitrogenous

158 CARNIVOROUS PLANTS WITH ADHESIVE APPARATUS,

aliment from prey were injurious to Dionca, the plant would certainly have died
out long ago. If, therefore, cultivated specimens of Dionca have suffered from
being fed with meat, fibrin, cheese, and other such materials, only this much is
proved, that the nutriment in question was not beneficial. to them owing to its
quality or to its being too concentrated.

As regards the other point, that Dionwa thrives well under cultivation, even
when all visits from insects are excluded, we must, on the other hand, bear in mind
that the successful growth of Dionca, like that of Drosera, Pingwicula, &c., is not
conceivable unless in some way or another the nitrogen indispensable for the
construction of the protoplasm is conveyed to the individuals in question. The
source from which it is taken varies according to the site. If the roots are buried
in deep sods of bog-moss upon a flat expanse of moorland, the supply of nitrogen
from the ground, and also from the air, will be extremely limited, and probably
insufficient, and, in the latter case, the nutriment derived from the dead bodies of
captured insects would be not only useful and beneficial, but may be even essential.
If, on the contrary, the place where the plants have been reared or have grown up
spontaneously is such that they can obtain the requisite nitrogen from the ground
or air, they are able without harm to dispense with the available source of nitrogen
afforded by the capture of insects. It is worthy of notice that insectivorous plants
always grow wild only in places that are poorly supplied with nitrogenous food.
The majority occur in pools fed by subterranean water, whose course lies through
layers of peat, or in the spongy peat itself, or in the sods of Sphagnum. Others
are rooted in deep chinks in the stone on the declivities of rocky mountains, whilst
yet others occur in the sand of steppes. The water available in such situations for
absorption by the suction-cells is, to say the least, very poorly furnished with
nitrogenous compounds; and the quantity of these compounds passing from’ the
ground into the air at the places mentioned is extremely minute and inconstant.
Under these circumstances, the acquirement of nitrogen from the albuminoid
compounds of dead animals is certainly of benefit, and all the various pitfalls,
traps, and lime-twigs are explained as contrivances by means of which this
advantage is secured.

CLASSIFICATION OF PARASITES, 159

4, ABSORPTION OF NUTRIMENT BY PARASITIC PLANTS.

Classification of parasites.—Bacteria.—Fungi.—T wining parasites,—Green-leaved parasites.—Tooth-
wort.—Broom-rapes, Balanophore and Rafilesiacexe.—Mistletoe and Loranthus.—Grafting and
budding.

CLASSIFICATION OF PARASITES.

The ancients understood by parasites people who intruded uninvited into the
houses of the rich in order to obtain a free meal. The designation was first applied
to plants by an eighteenth century botanist, named Micheli, in his work “De
Orobanche” (1720) wherein are described amongst others, many kinds of “plante
secundariz aut parasitices”. Micheli included under the term plants which with-
draw organic compounds from living plants or animals, thus sparing themselves the
labour of forming those compounds out of water, salts, and constituents of the air.
For a long time all epiphytes, including mosses and lichens growing on the bark of
trees, and indeed even many climbing plants, were held to be parasites. Thus, it is not
long ago that Clusia rosea, which occurs in the Antilles, was described as a regular
vampire, in whose embraces other plants met their death; and it has been asserted
respecting a whole series of other plants of the tropical zone, including, for instance,
several species of fig, that they attach their stems and branches to other trees,
divest themselves of their bark, and cause the death of that of the neighbour
attacked as a consequence of the pressure which they exert. The young wood of
the invader would then come into direct connection with the young wood of the
plant assailed, and the possibility would thus be afforded of draining the latter of
all its juices.

These assumptions, at least as regards the exhaustion of juices, have not been
confirmed. When individuals of species of Clusia or Ficus, which have roots buried
in the earth, and are themselves already grown up into stately leaf-bearing plants,
attach their flattened stems and branches to other plants, investing them so
completely as to interfere with the process of respiration; this constitutes, at all
events, an invasion of one of the most important of the vital functions of the plant
attacked, and may ultimately cause its death; but the killing is not under these
circumstances due to drainage of juices, but is brought about by suffocation.
Lichens, too, when they cover the bark of trees with a close-fitting mantle, may
possibly restrict the process of respiration through particular parts of the cortex,
and thereby injure the development of the tree in question; but they are not on
that aceount to be looked upon as parasites any more than the fructifications of the
species of Telephora, and other Basidiomycetes, which grow up rapidly from the
ground, and, spreading out like plastic doughy masses, envelop all objects which
come in their way, and ultimately stifle such as are living, namely, grass haulms,
bilberry bushes, &c. Even creepers, which impose woody stems upon the trunks of
young trees, winding round them like serpents, and restricting their circumferential
growth at the parts in contact with the coils, so that ultimately the latter lie

160 CLASSIFICATION OF PARASITES.

imbedded in regular grooves in the cortex, ought not to be considered as parasites.
The Lonicera ciliosa of North America, represented in fig. 31, may be taken as an
example of creepers of this kind. They only interfere with the conduction of the
constructive materials generated in the green foliage, preventing, in particular, the

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| it

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IA WALN UA S/N Al tte aA
ANN AN \\ ul nh Mh OA i uN j

Fig. 31.—Lonicera ciliosa in South Carolina. ANT,

part of the axis below the strangulating coils from being supplied with those
materials; and so at last they cause the whole trunk, which serves as their support,
to dry up. The assertion may then be made that the young tree assailed has been
strangled or throttled by the creeper, but not that the latter has drained it of juices
and adapted them to its own use. Still less would the statement be applicable to
the numerous brown and red sea-weeds, which settle upon the ramifications of the
great species of Sargasswm, or of the innumerable Diatomace, which often entirely

BACTERIA. FUNGI. 161

cover both fresh and salt-water plants. In still inlets of the sea it is not rare to see
the larger sea-wracks with smaller specimens clinging to them, whilst Floridex are
fastened to the latter, and minute siliceous-coated diatoms to the Florides. Even
in fresh water, eg. in cold and rapid mountain streams, we find little tufts of
Chantransia or Batrachospermum developed as epiphytes upon the black-green
‘filaments of Lemanea, and on the former, again, Diatomacez. One of these
Diatomacez, which, from its resemblance to a scale insect, has received the name of
Cocconeis Pediculus, is especially conspicuous, and is often found by the score upon
the green filaments of Algz. Such a connection does, no doubt, suggest the idea
that the Cocconeis drains the green algal cells of nutriment; nevertheless, such an
assumption is not well founded, and if alge, beset by Cocconeis, derive injury at all
from their presence, it is chiefly owing to a restriction of their absorption of
nutrient substances from the surrounding water and to interference with their
respiration.

The distinctive property of true parasites does not lie, therefore, in the habit of
growing upon other plants and animals, or even in the fact of killing their living
supports, but resides exclusively in the withdrawal of nutrient substances from the
living vegetable or animal bodies which they invade.

The plants and animals attacked and drained of their juices by parasites are
called hosts.

From the point of view of food absorption, true parasites may be classified in
three groups. The first group includes generally all microscopic forms which live
in the interior of human beings and animals, chiefly in the blood; the second
comprehends fungi possessing mycelia, which have the power of withdrawing by
the entire surface of their filamentous cells, or by clavate outgrowths of the same,
nutritive material from the tissues of the host invaded by them; and the
third group comprises flowering plants wherein the seedling, upon emerging from
the seed, penetrates into the host, by means of suction-roots or some other part
which subserves the function of a suction-root, so as to absorb juices from the
host.

BACTERIA. FUNGL

In treating of parasites of the first group, we must, in the first place, refer to
several of the unwelcome visitors known by the name of Bacteria. They appear
to be invariably unicellular, sometimes spherical, sometimes shortly cylindrical or
rod-shaped; some are straight, and others curved in ares or spirals; a few are non-
motile, whilst some are actively motile. The largest forms have a diameter of
sip mm.; the smallest do not measure more than zg45 mm., and are reckoned
amongst the minutest organisms hitherto revealed by the aid of the best micro-
scopes. In liquids of suitable chemical composition and temperature, they multiply
with extraordinary rapidity, reproduction being effected by division. The rod-
shaped cells elongate somewhat and divide into two equal halves, each half, when

grown to a certain size, divides once more into two, and so on without limit.
Vou. I. il

“3

162 BACTERIA. FUNGI.

The process gives the impression of being a repeated splitting of the cells, and
this is the origin of the name of Fission-fungi (Schizomycetes) used to designate
these organisms. It has been observed that within 20 minutes a bacterium-cell
grows enough to be able to divide or split into two, and hence it has been calculated
that from a single cell, under favourable external conditions, upwards of 16 millions
of similar cells are produced in 8 hours; and in 24 hours many millions of millions.

It is this very capacity for rapid multiplication that gives so great an impor-
tance to Bacteria as parasites. For multiplication can only take place at the
expense of the juices and nutrient substratum in which they live. If this nutrient
substratum is to afford materials for constructing the millions of millions of cells
produced within two periods of 24 hours, a far-reaching transformation is inevitable.
Now, for certain bacteria, the blood, with its albuminoid compounds and carbo-
hydrates, is an extremely favourable medium of nutrition; moreover, the tempera-
ture of the blood of men and other mammals (35°-37°C.) could not be more
suitable for the development of bacteria. Hence, it is readily intelligible that if a
single parasitic bacterium-cell gets into the blood, it may be the origin of innumer-
able other cells, and that these are in a position, in a comparatively short time, to
alter and decompose the whole mass of the blood. Owing to their extraordinary
minuteness, bacteria are able to penetrate from outside into the channels of the
blood by a number of spots: every abrasion, pin-prick, and sore place, may become
an entrance-door: so, too, through all the external orifices of the various canals in
in the bodies of men and animals, the bacteria can enter, especially. through the
passages to the respiratory organs—and it becomes more and more probable that
bacteria, diffused in the air, are in the main introduced into the respiratory organs
by the process of breathing, thence penetrating into the finest blood-vessels, the so-
called capillaries, and so pass into the current of the blood.

As regards the parasitic action of bacteria when they have penetrated into the
bodies of men and animals, the supposition is that the protoplasm of each bacterium
works as a ferment upon the environment, splitting up the chemical compounds in
immediate proximity to it, and attracting and incorporating such products of the
decomposition as are necessary for its own growth. Parasites with this method of
operation act, at all events, much more destructively than those which, although
they too absorb part of the host’s juices, yet do not enter upon the necessary
decompositions until the juices have passed into the cavities of their own bodies,
and, therefore, do not alter the constitution of the unabsorbed residue. When the
component parts of the blood are split up and resolved by bacteria, the nutrition of
the host must be especially disturbed, and so must all the functions of the organs
through which the blood perpetually circulates. Ultimately it may culminate in
the organs ceasing to exercise their functions, and in the death of the host. When
one remembers how fast the blood is pumped by the heart’s action into every part
of the body, it becomes intelligible how bacteria, possessing the power of decom-
posing the blood, may also cause the death of the host at very short notice, as we
have occasion to observe whenever there is an epidemic of cholera.

BACTERIA. FUNGI. 163

That numerous diseases affecting men and animals are caused by bacteria is
established beyond question. Indeed, the conviction is gradually gaining ground
that all infectious illnesses are occasioned by bacteria, and that the contagious
matter which used to be called virus or miasma, but as to the nature of which
people formerly had only very confused notions, consists of parasitic bacteria.
Different phenomena in organisms in which illness has been induced by infection
point to differences in the decompositions effected by the bacteria. But a par-
ticular kind of parasitic cell can only set up the same decomposition in any
given liquid. If, therefore, the products of separation or decomposition vary in one
and the same liquid, this can only be attributed to a difference in the impetus
causing decomposition, and therefore to a difference in the parasitic cells; in other
words, we are justified in assuming that every distinct infectious disease is due to
a special kind of parasitic bacterium. ‘This assumption is believed to be warranted
even when no difference in the form of the bacteria is to be discovered which is
discernible to sight or demonstrable by the expedients of research.

Most of the parasitic bacteria regarded as causes of diseases in man and beast
are moreover capable of being very clearly distinguished from one another by the
shape of their cells. The bacterium supposed to be the cause of diphtheria (Micro-
coccus diphthericus) presents itself in the form of minute spherical cells crowded
together in close masses. The bacterium which causes anthrax in cattle (Bacteriwm
Anthracis) has straight rod-like stationary cells. In the blood of people suffering
from relapsing typhus, infinitesimally fine spiral filaments (Spirochaete Obermeieri)
are found during the fever, whilst in the intestines of cholera patients, the comma-
bacilli, so frequently described, occur; and in these cases, likewise, the organisms
are brought into causal connection with the illnesses mentioned respectively. The
answer to the question as to whether parasitic bacteria are developed and propa-
gated in dead bodies also, thus becoming saprophytic, and, in general, the detailed
description of the organisms, which are so important a factor for the weal or
woe of humanity, are reserved for another section.

The second group of parasitic plants, according to the classification above given,
includes several thousands of different kinds of moulds, toad-stools, and Dis-
comycetes, which, notwithstanding great diversity in the conditions of life, dis-
similarity in the history of their development, and endless variety in the form of
their fructifications, yet exhibit great uniformity in respect of food-absorption and
in their methods of attacking and draining their hosts. Spores, conveyed by
currents of air or carried by animals, germinate under the influence of atmospheric
moisture wherever they happen to come to rest. Tubular thin-walled cells, called
hyphe, emerge from them and endeavour to grow into the stems, branches, leaves,
or fruits of the host, sometimes horizontally, sometimes from above downward,
sometimes up in the opposite direction. Many select spots where the resistance
offered is nil or only very weak: they grope about on the surface of the host until
they find a stoma, and then use it as an entrance, and so pass into the passages and
lacune, of which the stomata are the orifices. Others seek out places where the

164 BACTERIA. FUNGI.

surface of the plant serving as host has become broken—wounds occasioned by
animals, violent wind, hailstones, or the weight of superincumbent snow—and use
these as means of ingress. Yet others adopt the shortest route by breaking through
the wall and so effecting an entrance for themselves. The tips of the hyphe and
also of the outgrowths developed by them have the power of decomposing and
destroying the membrane of cells in the living plant serving as their host. At the
spots to which they apply themselves, little gaps are shortly produced in the cell-
membranes, and through them the hyphex penetrate, either in their entirety
or by means of special processes, into the interior of the cells attacked. In
this operation it does not matter whether the hypha concerned has just emerged
from a germinating spore or is a ramification of a mycelium several years old,
which has been quiescent for a time and then begun to germinate again vigorously;
the power of perforating cell-walls is a property possessed by the one as much as
the other.

The aspect of the host’s epidermal cells at the places where the hypha comes
into contact with its victim is, on the other hand, not quite such a matter of
indifference. For plants liable to become hosts are not without contrivances for
protecting themselves against intruders. Thus their epidermal cells have their
external walls greatly thickened and invested with cuticle. Although the main
object of this is merely to afford protection against excessive transpiration and
desiccation of cells filled with sap, a thickening of the kind constitutes also a coat
of armour which is not liable to be broken through by every hypha. Still greater

security is afforded by a double or triple layer of thick-walled cells destitute of “

sap, such as a solid corky bark. Coats of this kind are not penetrated even by the
most vigorous hyphe. In order to gain admittance notwithstanding, many force
their conical tips into the fissures and crannies of the bark, push the peeling scales
apart or even burst them, and so succeed ultimately in reaching parts which are
susceptible of being pierced and allow the hyphe to conduct their mining operations
with effect. In the majority of cases the parasite is not content with perforating
and exhausting the superficial cells alone of the host; its hyphe grow faster as
they penetrate deeper, a process generally accomplished irrespective of the number
or direction of the partition walls in their way. Thus the hyphe of Polypore,
which are parasitic in the wood of living trees, penetrate whole series of cells, now
growing through a bordered-pit, now piercing the uniformly thickened part of the
wall of a wood-cell (see fig. 32%). Others, as, for instance, the Peronospores, prefer
to bury themselves in the passages between individual cells, i.e. in the so-called
intercellular spaces. The hyphz imbedded in this way then develop lateral out-
growths which perforate the walls of the cells adjoining the intercellular space, and
upon entering the interior of the cells swell up to the shape of a club (see fig. 321).
By means of these clavate or almost spherical excrescences, which are named
haustoria, the parasite sucks the substances required for its own nourishment from
the living substance of the penetrated cells.

The hyphe of the above-mentioned parasitic fungi have the peculiarity that in

BACTERIA. FUNGI 165

proportion as the one end elongates the other dies away. Hence the same effect is
produced as if the progressive motion of these hyphe were like that of ship-worms.
This impression is particularly strong in cases wherein one part of the mass of
wood attacked exhibits hyph occupied with their mining operations and growing
through partition walls, whilst the other part has been the scene of past activity,
and exhibits numbers of drilled holes, but no longer any trace of hyphae. The fact
that a plant is thus invaded internally by the parasitic mycelia of fungi is not
always betrayed by its external appearance. Sometimes the hosts remain somewhat
backward in development, but this circumstance might be just as well due to other
causes, perhaps to unsuitability of situation. It is not till the mycelia need once

—u

Fig. 32.—Hyphe of Parasitic Fungi. sc (P
1 Of one of the Peronosporee. 2Ofa Mildew. 3 Of one of the Polyporee. fiversi

more to multiply and distribute their kind that they emerge partially from the
host; they then lift their spore-forming hyphz above the surface, leaving it to the
wind to distribute the spores as they are detached.

This process vividly recalls the similar behaviour of those water-plants which,
in a similar manner, vegetate submerged for months, and only come to the surface
at the flowering and fruiting seasons, in order to expose their flowers to insects,
and their seeds to the breeze. We are also reminded of the saprophytic orchids
already described, which nourish themselves and grow for years imbedded in the
humus of woods, and then seize the opportunity afforded by a favourable summer
to raise up in a few weeks flowering stems above the bed of the forest. Asa rule
the spore-bearing hyphez, emerging from the hosts of parasitic fungi, are highly
conspicuous both in form and colour. As well-known instances we may here
mention the powdery, rust-coloured, chocolate-brown, or coal-black masses of spores,
known by the names of rust and smut; the mealy, orange-coloured masses which
make their appearance on the green stems and fruits of roses (Aicidium stage of
Phragmidiwm subcorticwm), and the discomycetous Peziza Willkommit, which
is parasitic in the branches of green larches, and exposes its fructifications beyond

166 BACTERIA. FUNGI

the bark in the form of small scarlet shields. Again, we have the yellow Poly-
porus sulfwreus with its immense yolk-coloured, bracket-like fructifications, which
in the space of a week grow out from the trunks of larches, although the outward
appearance of the host gives no indication of its being completely occupied
internally by a mycelium. Polyporus betulinus and P. fomentarius likewise
grow to a considerable size, and in both cases it is specially deserving of notice
that the colour and structure of the surface of the fructification is surprisingly
like the bark of the trees upon which they are respectively parasitic; that is to say,
the fructification of Polyporus betulinus strongly resembles the whitish bark of
the birch, and that of Polyporus fomentarius, parasitic on old beech-trees, exhibits
the same pale gray as does the trunk of a beech.

Mildews form in some respects a contrast to these parasites whose hyphe pene-
trate into the interior of their hosts. They attack tender green leaves, stems, and
young fruits, and accomplish their entire development upon the epidermal cells of
the hosts. At first sight the parts assailed appear to be strewn with flour or dust
from the road. But on closer inspection a delicate weft is to be distinguished,
composed of filaments ramifying extensively upon the green substratum intersect-
ing one another, uniting to form reticula, and in parts a regular felt-work covered
at certain spots with the small dark spheres of the sporocarps. Individual hyphe
of this weft adhere closely to the epidermal cells of the host, dissolve the outer
walls of these cells at the points of contact, so as to make little apertures, and then
develop processes which grow into the interior of the epidermal cells in question,
assume a club-like form, and exhaust the cell-contents. The mycelia of mildews
do not penetrate into the host beyond the epidermal cells. Fig. 32? shows a piece
of a leaf of Acanthus mollis attacked by mildew, with hyphal suckers penetrating
into the epidermal cells of the leaf. One of the best-known mildew fungi is
the Vine-mildew (Erysiphe Tuckeri), which weaves itself over the epidermis of
still green and unripe grapes, and has frequently manifested itself through the
districts where the vine is cultivated in southern and central Europe in the form of
a ravaging disease.

The protuberances sent by the hyphe, in the form of clavate swellings, or more
rarely winding tubes, into the cells of the host-plants, correspond to the absorption-
cells of land plants, and the conditions under which suction takes place are
essentially analogous in the two cases. The absorption-cells on the roots of land
plants do not take in all the substances in their nutrient substratum, and similarly
the hyphe only appropriate by means of their organs of suction a portion of the
contents of the cells invaded. They begin by dissolving and breaking up for this
purpose the substances in the infested cells of the host. What compounds they
then select from among the products of decomposition, and what they leave behind,
cannot certainly be specified in detail. It is believed that, in many cases, tannin
is appropriated first of all by parasites. The wood of a healthy oak, for instance,
has a characteristic smell due to the abundance of tannin it contains, whereas the
odour is not emitted by wood attacked by the mycelia of fungi, and this decayed

BACTERIA. FUNGI. 167

wood is destitute of tannin. It is natural to suppose, therefore, that the mycelium
takes away and uses up the tannin. It has also been observed that wherever the
hyphe of the Pine-blister (Peridermiwm Pini) ensconce themselves, the nitrogenous
parts of the protoplasm and the starch vanish, whilst turpentine remains behind,
clinging in drops to the inner walls of the cells. These are, to be sure, very sparse
data; but they show that the entire cell-contents are not absorbed by the parasite
unaltered, or used in that condition as material for the building up of its own body.

Not only the contents of the cells preyed upon, but the walls as well, are partially
used as food by the hyphe which penetrate into the woody axes of arborescent
angiosperms and gymnosperms. The mycelium of several species of Polyporus and
Trametes begins by bringing the lignin in the cell-walls into solution, leaving
nothing but a pale-coloured cellulose wall. Soon afterwards, the so-called middle
lamella, which connects adjoining wood-cells, is also dissolved, and the colourless
wood-cells, now almost like asbestos-fibres in appearance, fall apart at the slightest
touch. When the wood of a larch has been infested by the mycelium of Polyporus
sulfureus, there are always deep furrows running obliquely on the internal walls of
the wood-cells; this loss of substance, too, can only arise from the solution, and
absorption as nutriment, of parts of the walls by the action of the hyphae.

All decompositions and alterations of structure of the above kind within the
precincts of the host’s cells are naturally followed by a disturbance of function, and
ultimately by death. The entire plant is, however, but rarely killed by parasites
belonging to this group. The decomposition by bacteria of a mammal’s blood,
though at first confined to a particular part of the body, spreads in a moment
throughout the whole organism, owing to the heart’s action and the circulation of
the blood. But the decomposition taking place in the manner just described,
through the intervention of hyphz, propagates itself, on the contrary, only very
gradually from the cells immediately attacked to their neighbours, and it gets
weaker and weaker as the distance from the site of the invasion increases, a
circumstance to which we shall recur later on when discussing the phenomena of
fermentation and decay. The nature of the parasite and the power of resistance of
the host have undoubtedly influence on the rate of distribution. In many cases
alteration is limited to the cells attacked and those immediately adjoining, so that
the area destroyed is circumscribed. It is manifested on fresh, green leaves, often
merely in the form of small, isolated, yellow, brown, or black spots and patches,
which only slightly interfere with the activity of the leaf, and do not cause it to
change colour, wither, or fall off any earlier. In other instances, however, the
entire leaves and stem do undoubtedly become flaccid and shrivelled and dried up
into a black mass, looking as though they had been carbonized; or else corruption,
such as that which is excited by bacteria, invades the whole mass,

As above stated, when the wood in the trunks of trees is perforated and
consumed by hyphe it is resolved into fragments. It becomes rotten, takes the
form of an asbestos-like, or crumbling and pulverulent mass, and is then obviously
no longer capable of fulfilling its various functions in the living plant. If the

168 BACTERIA, FUNGI.

invasion is limited in extent, and the host succeeds in surrounding the area of
infection with a rampart of cells capable of resistance, and not liable to be pierced
by the hypha, then the tree may live for years although its trunk is infested, and
in parts rotten. Such is also the case when particular branches of a tree are alone
attacked by the mycelium of a fungus. When, for example, the branch of a larch,
is assailed by the mycelium of the Discomycete, Peziza Willkommit, the fact is first
manifested externally by the fascicles of needles on the branch in question becoming
discoloured in the summer, and acquiring, prematurely, an autumnal appearance; so
that, among the fresh green shoots, individual branches are to be seen bearing golden-
yellow needles. Towards autumn, scarlet cup-shaped fructifications make their
appearance upon the surface of the bark on the branch; in the course of the next
few years the whole branch as a rule dries up, withers, and dies. It is then
broken by the first violent shock of wind and falls to the ground; but the tree,
disembarrassed of the dead bough, continues to grow unharmed, and to put forth
green shoots. It is only when almost all the branches of the larch are infested by
the mycelium of this fungus that the whole tree perishes as a result of the
invasion.

Certain groups of plants are specially liable to be attacked by parasitic fungi,
and there are some conifers and angiospermous trees in which the same stem is
colonized by three, four, or five kinds of parasite. The green foliage leaves of large
numbers of flowering plants are also apt to be selected by parasites, as also are their
roots, tubers, and bulbous structures. Many parasites only attack the anthers in
flowers; others, as for instance the ergot, only the young ovaries. Parasitic fungi
are rarely found on mosses or ferns; whereas a considerable number of parasites
settle upon lichens and even on the fructifications of fungi, even moulds being
infested by other fungi; for example, a fungus named Piptocephalis Freseniana is
parasitic upon the very common mould, Mucor Mucedo.

A fungus known by the name of Cordiceps militaris is parasitic in the cater-
pillars and pups of butterflies and other insects, and its relatively very large
fructification at length bursts out of the body infested by the mycelium in the form
of a club nearly 6 em. long. This clavate structure, built up at the expense of the
insect’s flesh and blood, produces tubular cells in special receptacles, and, inside
these, little rod-like spores, which afterwards fall out and infect other caterpillars,
developing within the bodies of these animals into a hoary mycelium and ultimately
causing their death. The disease of silk-worms, known as muscardine, is likewise
occasioned by a species of Cordiceps. We must also refer here to the widely-
distributed Empusa Musce, a mould which attacks flies and causes every autumn
a regular epidemic amongst house-flies. The flies so often seen at that season
adhering stiff and dead to window-panes are surrounded by a whitish halo, and this
is composed of a conglomerate of spores thrown off by the mould which is parasitic
upon the flies and causes their death. Parasitic fungi have also been observed in
the human skin, and recognized as the causes of skin-diseases, For instance, to the
mould Achorion Schoenleinit is due the disease of the skin popularly known as

BACTERIA. FUNGI. 169

“honey-combed ringworm”, and named Favus by doctors; dandruff (Pityriasis
versicolor) is produced by Microsporon furfur, and Herpes tonswrans by Trico-
phyton tonswrans. The latter has a remarkable effect on the hair, causing it to fall
out and leave the part of the skin affected bald.

Water-plants are attacked by parasitic fungi comparatively rarely, which is the
more noteworthy because such large numbers of non-parasitic epiphytes settle upon
the filaments of green alge, and on the brown Fucoidex, and red Florides. Minute

Fig. 33—Parasites on Hydrophytes.
1, 2, and 8 Lagenidium Rabenhorstii. 4,5 Polyphagus Eugl 6 Rhizidiomy “poph

forms of fungi, invisible to the naked eye, and belonging to the Chytridew and
Saprolegniz, are parasitic upon green algal filaments, especially on the fresh-water
species of the genera Wdogonium, Spirogyra, and Mesocarpus. One of these
microscopic parasites is represented in fig. 33%23, and bears the name Lagenidiwm
Rabenhorstit. It develops non-ciliated, spherical swarm-spores, which lay them-
selves upon the walls of Spirogyra-cells, perforate them, and insert a club-like
process. The protuberance forthwith becomes a tube, which increases rapidly in
size in the interior of the cell, ramifying and completely destroying the bands of
chlorophyll. The branched tubes of Lagenidiwm reproduce themselves in two
ways at the expense of the host’s cells infested by them: they form on the one
hand so-called oospores by means of fertilization, and on the other sporangia. The
latter process is clearly shown in fig. 331425. In this case, one of the tubular

170 BACTERIA. FUNGI.

processes of the parasite fungus pushes out of the cell-cavity of the invaded
Spirogyra into the surrounding water again and there swells up into a spherical
vesicle, within which the protoplasm divides into eight spores. These spores are
then set free as swarm-spores and attack new healthy Spirogyra-cells.

Materially different is the behaviour of the parasite Chytridiwm Ola, which
attacks the green cells of fresh-water Cidogoniz. Its roundish swarm-spores are
furnished each with one long cilium, and swim, searching about in the water until
they meet with an Cidogonium-cell to their taste just occupied in the formation of
oospores. When they find one, they fasten upon it and send infinitesimally fine
hair-like tubes (which have been called rhizoids) into the interior. By means of
these tubes they derive their nutriment from the host. The body of the parasite,
which remains outside the invaded cell, increases in size, and at length grows out
into a sporangium; the latter opens at the top by a lid and once more sets free
swarm-spores into the surrounding water.

Polyphagus Euglene, a member of the Chytridex, is parasitic on the green
cells of Euglene living in water. The swarm spores of this microscopic fungus
(see fig. 334) are oval and furnished, like those of Chytridiwm Ola, with a long
cilium. They swim about the water with the non-ciliate extremity leading, so that
the cilium appears to be a tail at the posterior end. As soon as these swarm-spores
have come to rest, they assume a spherical form and send out in all directions thin,
hair-like tubes, which search for a host. When a tube reaches an Euglena-cell, it
penetrates into the body of the latter, drains it, and, continuing to grow, produces
fresh hair-like tubes, which attack other green Euglenz, often linking together
dozens of them (see fig. 33°). In this way the Polyphagus grows apace and
becomes a comparative large oblong vesicle, whilst the protoplasm within it divides
into a number of parts. These, again, turn into swarm spores, with long ciliary
filaments, and they slip out of the vesicle and may attack fresh Euglene.

Curiously enough, even saprophytic water-plants destitute of chlorophyll are
sometimes attacked by parasites, and that, indeed, by species belonging to the same
group. Thus, for instance, the species of Achlya growing on the dead bodies of
fishes and other animals which have perished in the water, are themselves infested
by small parasitic Saprolegniaceze and Chytrides. The example of these minute
parasites represented in fig. 33° is named Rhizidiomyces apophysatus, and its
host is Achlya racemosa. The swarming spores of the parasite lay themselves,
in the manner described in previous instances, upon the spherical oogonia of Achlya,
and insert extremely fine hair-like tubes into the interior of the cells attacked.
These ramify like roots in the Achlya-cells, exhaust them of nutriment, grow
perceptibly, and at length form spherical swellings, which, after reaching a certain
size, break through the walls of the host-cells, project from the opening, and,
lastly, push out in each case a sporangium. The latter produces a number of
swarm-spores, which escape into the water and are able to seek fresh prey.

We cannot here enter into details respecting the other kinds of reproduction
occurring in the minute fungi parasitic upon hydrophytes. This is the right place,

CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT. 171

however, to mention the fact that the various species of Chytridew and Sapro-
legniaceze do not content themselves with plants that are second-rate hosts, but
exercise a selection amongst the different green algw living in the water. It is
astonishing to find that the swarm-spores invariably swim to cells whose protoplasm
affords the most suitable nutrient basis for them, and attach themselves to those
cells only, and never on other species unadapted to their requirements.

CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT.

The third group into which parasites were divided at the beginning of this
chapter is composed of flowering plants throughout. According to their method of
attacking the host for the purpose of absorbing nutriment from it, they range
themselves in six series. In the following pages we shall discuss the charac-
teristics of each series as manifested in the most remarkable forms belonging
to it.

The first series includes plants destitute of green leaves and of chlorophyll in
general, whose seeds germinate on the ground and send forth each a filiform stem,
which brings itself, by means of peculiar movements, into contact with the host-
plant, coils round it, and develops organs of suction whereby it takes nutriment
from the plant assailed.

To this series belong the genera Cassytha and Cuscuta. The former includes
some thirty species, all of which appertain to warm climates. Most of the Cassy-
the inhabit Australia, where they attack, in particular, the copses of Casuarine
and Melaleuce, fastening their wart-shaped, or, in many cases, shield-like or discoid
suckers upon the young green shoots of those plants. Several species also are
indigenous to New Zealand, others to Borneo, Java, Ceylon, the Philippines, and
the Moluccas. South Africa, too, is the home of a few Cassythe, and one species
(C. Americana) is distributed over the West Indies, Mexico, and Brazil. A
European, seeing these parasites with their twining, thread-like, leafless stems, and
their flowers aggregated in capitula, umbels, or spikes, takes them at first to be
species of the genus Cuscuta, popularly called Dodder. That these plants should
be most nearly related to laurel-trees is the last thing one would expect. Ex-
amination of the flowers and fruit reveals, it is true, a close resemblance to those of
laurel and cinnamon trees, and, therefore, these Cassythe are rightly placed by
systematic botanists among the Lauracez. But in respect of food-absorption, as in
general aspect, they are entirely analogous to the various species of the genus
Cuscuta, which belong to the family of Bindweeds (Convolvulacee). The last-
named genus is even more variously differentiated than the genus Cassytha, and
‘includes about fifty species dispersed pretty evenly over the whole world. Every
part of the world has its own characteristic forms. One group occurs in California,
Carolina, Indiana, Missouri, and Mexico, another in the West Indies, Brazil, Peru, and
Chili, a third at the Cape of Good Hope. Other species are natives of China, the
East Indies, the steppes of Central Asia, Persia, Syria, the Caucasus, and Egypt.

172 CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT.

A comparatively large number of species, i.e. twenty-five, are distributed through
central and southern Europe. A few have been introduced recently for the first
time with seeds from the New World, as, for instance, C. corymbosa, which was
accidentally conveyed with lucerne seeds from South America to Belgium, and has
latterly begun to range over central Europe.

The various species of Cuscuta attack chiefly small herbaceous, suffruticose, and
shrubby plants; but a few American species coil themselves round branches growing
at the top of the highest trees. Notice has been especially drawn to certain
European species on account of their disastrous effects upon cultivated plants. The
most famous is Cuscuta Trifolii, known as the Clover-Dodder, the appearance of
which in clover-fields causes so much anxiety to farmers, and which is so difficult
to exterminate. Another unwelcome visitor is Cuscuta Epilinwm, which coils
round flax stems and hinders their growth, and a third species, Cuscuta Europea,
sometimes ravages hop-plantations. This last is, indeed, the most widely dis-
tributed of all the Cuscutas, and extends from England over central Europe and
Asia to Japan, and southwards as far as Algiers. It is parasitic not only on hops,
but also on elder, ash, and various other shrubs and herbs; in particular it exhibits
a preference for nettles.

The seeds of this species, and of Dodders in general, germinate on damp earth,
on wet foliage undergoing putrefaction, or on the weathered bark of old trunks.
The seedling, which in the seed lies imbedded in a cellular mass full of reserve-
food, is filiform and spirally coiled. It is twisted once, or once and a half, and
is thickened at one end like a club. In true Cuscutas, no trace of cotyledons
is to be perceived, nor does one find vessels in the interior of the seedling; but
chains of cells arranged with great regularity are noticed in the axis of the filiform
body, and are easily distinguished from the surrounding cells. In nature, the
seeds, after falling to the ground and lying there through the winter, do not
germinate till very late in the following year, «.¢. at least a month later than the
majority of the other seeds reaching the same ground simultaneously with them.
Perennial herbs, also, have, by the time that germination takes place, already
developed shoots from their subterranean roots or rhizomes above the surface of
the ground, later a circumstance of great importance to the parasites. If a
Cuscuta were to germinate early in the spring, it would not readily find close by a
support up which to twine; whereas later, there is seldom any lack of annual stems
or shoots of perennial plants in the immediate neighbourhood.

When the twisted embryo germinates, it stretches and at the same time revolves
from right to left, assuming the shape of a screw and pushing its lower clavate
extremity out beyond the coat of the seed (see fig. 3442345) This extremity forth-
with grows into the earth and fastens tightly on to particles of the soil, withered
foliage, and other objects of the sort. The other, attenuated extremity of the
filiform seedling, which is still wrapped in the seed-coat and the mass of reserve-
food, lifts itself up in the opposite direction, avoiding such solid bodies as it may
happen to encounter, and grows in a curve round them. Further growth does

CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT. 173

not take place at either extremity, but always in the median part of the filament.
It is so rapid that by the fifth day after the commencement of germination the
entire seedling has increased fourfold in length. As early as the third day
after the emergence of the tip that fastens itself in the earth, the integument of
the seed, which until then continues to envelop the opposite extremity, is thrown
off and the seedling’s apex is exposed. The reserve-food, given by the parent-
plant to the seedling as provision for the journey, has meanwhile been absorbed
and consumed, so that the seedling is now thrown entirely upon its own resources,
and depends for sustenance upon the earth, to which it is firmly attached, and upon
the surrounding air. Having no chlorophyll, it is not in a position to take up

M E
| t

Fig. 34.—Seedlings of Parasitic Plants. Lt borane

12,3456 The Great Dodder (Cuscuta Europea), 7% % 101.12 4 Broom-rape (Orobanche Epithymum).
8, 14,16 Wood Cow-wheat (Melampyrum sylvaticum).

materials from the air; nor can it derive sufficient nutriment from the earth, even
supposing that water is imbibed by the cells of the clavate extremity. There is no
doubt that it now grows at the expense of the substances contained in the cells of
this club-shaped end. The latter at once begins to shrivel and soon dies, whilst
the upper part of the filament elongates conspicuously. Should this portion of the
seedling meantime come into contact with a neighbouring plant, a rigid haulm,
or anything else that will serve as a support, it straightway coils itself round the
object in question, and its future is then, as a rule, assured.

Failing such a support, the seedling, after the death of the clavate extremity,
falls down and sinks to the ground. In doing so, it almost invariably touches
an adjoining object, whereupon it immediately winds tendril-like round the
support thus afforded. But if there is nothing anywhere around to serve as
a prop, and the young seedling, by this time from 1 to 2 centimeters long,
comes to rest upon the bare earth, all further growth is stopped. It preserves
its vitality, however, for a surprisingly long time, and may remain almost unal-

174 CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT.

tered, lying on the damp earth for four or five weeks waiting for something to
turn up. Not infrequently something of the sort happens, for another plant may
germinate close by or extend a growing shoot from the vicinity and touch the
Cuscuta seedling. In this event, the latter at once seizes the anchor thus thrown
out, and winds round it. But if no support of the kind is to be had, the seedling
must ultimately perish. It is, to say the least, a very remarkable thing that a
filament, capable of developing suckers when adherent to a living plant, is not able
in damp earth to produce any absorbent organs whatsoever.

If the thread-like Dodder plantlet succeeds in seizing a support of any kind,
either during the existence of the swollen extremity, or later, after it has been
absorbed, it makes a single, or from two to three, coils round the prop, raises its
growing point from the substratum, and moves it round in a circle like the hand
of a watch. By means of these manceuvres, which look exactly like a process
of feeling or seeking, the filament is brought into contact with fresh haulms,
twigs, and petioles belonging to other plants. To these it adheres, making once
more two or three tight coils round them. Throughout, it is obvious that the
growing point of the young Dodder rejects dead props, as far as is practicable,
and shows a striking preference for living parts of plants.

At each place where the Dodder is pressed in a coil against the support, the
filament becomes somewhat swollen, and wart-like suckers are developed, which are
usually situated close together in rows of three, four, or five (see fig. 35+).

A piece of stem thus furnished with suckers or haustoria resembles a small
caterpillar creeping up the supporting stem. These haustoria, arranged close
together in rows, and corresponding in origin entirely to rudimentary roots, are at
first smooth, but acquire soon a finely-granulated aspect owing to the walls of the
epidermal cells projecting outwards. With the help of the papille thus formed,
and especially through the action of a juice secreted by them, the suckers fasten
themselves to the host. If the plant has been obliged to clasp a dead object for
support, the wart-like processes flatten themselves against it and assume the form
of a kind of disc, which exhibits no further development, and only serves as an
organ of attachment; but, if the substratum is a living plant, a bundle of cells
forces its way out from the middle of the haustorium and grows into the sub-
stratum direct. The phenomenon here manifested is altogether characteristic.
Each sucker from the time of its production exhibits a kind of core composed of
cells arranged in regular rows, which, together with a few spirally-thickened
vessels, constitute a bundle standing at right angles to the axis of the Dodder's
stem. This bundle now breaks through the coat formed by the rest of the cells
of the sucker and penetrates into the living tissue of the plant attacked (see fig.
35°). Great force is exerted in the penetrating process. The closely-joined cells
of the epidermis, and not infrequently a cortex of considerable density are pierced,
and the bundle of cells often penetrates right into the body of the wood. Having
once reached the interior of the host, the cells, till then bound together in a
bundle, diverge a little, insert themselves singly between the cells of the host,

CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT. 175

and energetically absorb food-materials. They withdraw organic compounds from
the host and convey them by a short route to the strands developed meantime
in the axis of the Cuscuta-stem. When once a union of this kind between
the parasite and the host has been established, the portion of the Cuscuta
situated below the first haustorium gradually dies. The lowest extremity, ie. the
clavate tip, has already perished, so that the Cuscuta-plant is now no longer
in any connection with the ground whereon it germinated, but only remains
rooted to its living host by means of the suckers. If it has had the good fortune
to cling to a host with green foliage, which generates an abundance of organic
compounds, such as the luxuriant juicy stems of the Hop, or the Nettle, with its

Fig. 35.—Cuseuta Europea parasitic on a Hop-stem,

1 Natural size. 2 Section; x40.

plentiful dark green leaves, which are shunned and spared by grazing animals on
account of their unpalatable stinging hairs, the parasite continues to grow with
extraordinary rapidity, and puts forth a number of branches immediately above
the lowest group of haustoria. All these again feel around with their tips, develop
tendrils and suckers, sometimes intertwining and becoming entangled together,
cover an ever-increasing area of the host with their network, and in this condition
fully deserve the name of “ Hell-bind”, sometimes popularly applied to this plant.
Little spheres of rose-coloured flowers are then formed on individual threads of
this tangle, and from them balls of small capsular fruits, which dehisce by means of
lids and have their seeds shaken out by the wind.

The European species of Cuscuta are all annuals. Even when their haustoria
are attached to perennial plants, as, for instance, on young branches of woody
plants, they wither after the seeds have ripened, and nothing is to be seen of them
in the following spring except a few dried tendrils coiled round branches of ash
or willow. But under a tropical sun, perennial species flourish as well. The
suckers of Cuscuta verrucosa, for example, continue to exercise their function

176 CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT.

throughout the year wherever they have once attacked the host. If the woody
branches of the host, with haustoria fastened in them, grow in thickness and
superimpose new wood-cells upon the wood, down to which the absorbent cells of
the haustoria have penetrated, these suction-cells of the Dodder are likewise inclosed
by the wood-cells, and, in proportion to the augmentation of the circumference of
the wood in the branch in question, they also lengthen out so that the bundle of
absorption-cells proceeding from a sucker may, in such cases, be seen imbedded in
the wood to a depth of several annual rings.

The Cassythe, referred to above, behave exactly like the Dodders. In them
also the seedling which issues from the seed is filiform, and lives originally at the
expense of reserve-food stored up within the coat of the seed. So, too, it grows
upward, ramifies, and endeavours, by means of revolving movements of the apex,
to reach a living support, coils round the latter when found, and uses it as a
nutrient substratum. Here, again, at the parts where the tendrils of the filiform
stem are firmly appressed to the living support, rows of wart-like suckers are
developed, and a bundle of absorption-cells grows from each into the host. As in
the Dodder, the lower extremity of the filiform stem then dries up at once, and
connection with the earth is thus cut off. The parasite, once attached by its
haustoria to the host, is able to branch repeatedly, to weave its thread-like stems
over all the branches and to climb to the top of the host, even should the latter be
a tall bush. At some spots everything is entangled to such an extent that one
would think there were birds’ nests amongst the boughs.

The second series of parasitic Phanerogams consists of herbs bearing green
foliage-leaves, whilst the seed contains an embryo furnished with seed-leaves
(cotyledons) and root. The seeds germinate in the earth and there develop seed-
lings without the support of a host; it is branches of the root that first attach
themselves by means of suckers upon the roots of other plants. To this series
belong about a hundred Santalacez, mainly of the genus Thestwm, and many more
than two hundred Rhinanthacex besides. The chief examples of this latter family
are the various species of the Eyebright (Huphrasia), the Yellow-rattle (Rhinan-
thus), Cow-wheat (Melampyrum) and Lousewort (Pedicularis), and also Bartsia,
Tozzia, Trixago, and Odontites. The most extensive genera are Euphrasia and
Pedicularis, the species of which, with few exceptions, are found in the northern
hemisphere, adorning grassy meadows with their pretty flowers, especially in the
arctic zone, and the high mountain regions of the Himalaya, the Altai and Caucasus,
the Alps and the Pyrenees.

Little suggestion of parasitic habit is given in the first stages of development
of any of these plants. A seedling of the Cow-wheat within a week puts forth a
primary root 4 cm. long, from which half a dozen lateral roots ramify at right
angles without there being any attachment to a host to be noted (see fig. 34 181415),
Suckers are never developed until the secondary roots have attained a length of
from 12 to 24 mm., and then only if the latter come into contact with other living
plants to their taste, a circumstance which doubtless is almost certain to happen,

CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT. 177

seeing that the lateral roots are numerous and are sent out in all directions from the
main root, and therefore must inevitably come across the root-systems of other plants.

The seedling in perennial species of Thesiwm develops comparatively slowly.
It reaches a length of from 3 to 4 cm. in the first year, sends a tap-root into the
earth, and puts forth a few branchlets, which do not fasten upon the roots of other
plants by means of suckers until several weeks after germination. These suckers
are relatively large in all species of Thesiwm, and they catch one’s eye the moment
the roots of a plant are carefully divested of earth, They are then recognized,
as may be seen in fig. 361, as little white knobs, which stand out clearly from the
dark earth and are always inserted laterally upon the secondary roots. They are

Fig. 36.—Bastard Toad-flax (Thesium alpinum).

1 Root with suckers; natural size. 2 Piece of a root with sucker in section; x35.

constricted near their insertion, and the strangulated portion often gives the
impression of being a pedicel upon which the knob is seated. This knob is
differentiated into a central core and a multicellular, cortical coat enveloping it.
The cellular coat rests upon the root of the host attacked, and does not merely
adhere to one limited spot, but spreads itself out over the root like a plastic
mass, and forms a cushion surrounding about a fourth or fifth part of the cireum-
ference (see fig. 36?) without, however, penetrating into the substance of the root.
There are in the core two strands or bundles of vessels, and between them small
cells arranged in rows, from which absorption-cells arise at the spot where the
sucker first applied itself to the nutrient root. These absorption-cells grow out
beyond the rind-like envelope round the core, perforate the cortex of the host,
penetrate into the wood at the centre of the invaded root, and there diverge like
the hairs of a dry paint-brush.

The suckers of the green-leaved Rhinanthacew are on the whole similarly
constructed; only they are relatively smaller and more delicate, bemg sometimes

almost translucent, and they are either not at all or only slightly constricted at the
Vou. I. 12

178 CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT.

base. Whereas in Thesiwm they never issue otherwise than laterally from the
ramifications of the roots, in Rhinanthacee they are often terminal. A differentia-
tion into core and rind-like envelope is never clearly marked; a vascular bundle
runs through the middle of the sucker and is surrounded by thick-walled cells.
The absorbent cells are, moreover, shorter than in the Santalacesw. The individual
genera of the Rhinanthacez exhibit amongst themselves only very slight differences
in respect of their suckers. On the roots of Eyebright (Huphrasia), the haustoria
are tiny roundish nodules which rest upon the host’s root without encompassing it.
The absorption-cells are very short, and only just penetrate into the host. The
vascular bundle is either entirely wanting within the sucker, or its place is taken
by a single, comparatively large vessel. On the roots of the Yellow-rattle
(Rhinanthus) the suckers are spherical and of considerable size (up to 3 mm. in
diameter); their margins are swollen and often encompass more than half the
circumference of the roots attacked. The absorbent cells are short but very
numerous. In the Cow-wheat (Melampyrum) the suckers resemble those of the
Yellow-rattle in size and shape and in the shortness of the absorption-cells; but in
the former the margins of the suckers not only embrace the roots of the host, but
cling to them in such a way as to penetrate their substance and form circular
grooves upon them.

All the Rhinanthacee mentioned are herbaceous annuals. Their suckers are
few in number, and therefore easily escape observation. By the time these plants
ripen their seeds any piece of a root that has been attacked has for the most part
already turned brown and been killed, and is in a state of decay. But shortly
afterwards the parasite itself withers. The comparatively large seeds, well-
furnished with reserve-material for the nourishment of the embryo, fall out of the
dry capsules, and generally reach the ground at no great distance from the mother-
plant and germinate there. In the autumn, close to Cow-wheat plants, which are
still green but have already let fall the seeds from their lowest capsules, individual
examples of those seeds may be seen already sprouting in the damp moss and mould
on the ground of woods. If they fall to earth not very far from the parent-plant,
the seedlings may happen to attack the host which has already had one of the
branches of its root sucked and killed by the latter in the previous summer.

Nearly all these annual green-leaved parasites make their appearance in num-
bers close together. If, for instance, a species of Cow-wheat has taken up its
quarters in a particular part of a wood, there are always collections of hundreds
and thousands of specimens to be found together. The small-flowered Yellow-
rattle often grows so abundantly in damp meadows that one might suppose it to
have been sown by the bushel. The large-flowered, hairy Yellow-rattle is
similarly exuberant in ploughed fields, and the Eyebright, with its large number of
species, is produced in such abundance in mountainous districts that, at the season
when its little milk-white flowers are open, regular milky ways seem to stretch
across the green meadows. Millions of them are situated together rooted in the
grass-covered ground, and one would suppose that in course of time the growth of

CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT. 179

grass at such places would be injured. This conclusion appears to be supported
by the assertion of the country folk that after the season when the Eyebright is in
full bloom, the cows yield less milk, a fact which explains the German name of
“Milchdieb” (milk-thief) popularly given to the plant. The diminution in the
quantity of milk yielded is, however, certainly connected with other circumstances.
It depends especially upon the universal abatement of the growth of grasses in
early autumn and the consequent curtailment of the food afforded by the pastures.
The injury done by the Eyebright to its hosts by the withdrawal of nutriment
and destruction of rootlets cannot be very considerable, for the appearance of the
grasses and other host-plants, which are affected, is not noticeably different from
that of the plants of the same kind which escape invasion.

The same statement is true in the case of the various species of Lousewort
(Pedicularis), almost all of which are meadow-plants; that is to say, they are
present in great abundance in upland and alpine pastures without apparently
injuring the species growing in their company and used by them as hosts. Unlike
the species of Cow-wheat, Yellow-rattle, and Eyebright, however, nearly all the
Louseworts are perennial, and accordingly differ from them also in the construction
of their suckers. There is, it is true, no difference in shape between the suckers of
the Cow-wheat and those of Pedicularis, but they are dissimilar in respect of size
and place of origin. The suckers of the perennial Louseworts are barely more
than half the size, and are only developed near the attenuated extremity of a
rootlet. They are very few in number; each of the long, thick, fleshy rootlets,
proceeding from the base of the stem usually produces a single sucker only which
settles upon the root of a suitable host-plant in the same way as the suckers of
Cow-wheat. By the time that the parasite’s fruit ripens, the piece of root which
has been invaded has usually already turned brown and fallen into decay. Now in
the case of Cow-wheat it may undoubtedly be immaterial whether the piece of root
attacked by it is living or not when its fruit is ripening, inasmuch as its own
annual root rots as soon as the seeds have been produced from the flowers above
ground. But with Pedicularis it is different. The perennial roots of this plant.
require a host to nourish them next year, and when the piece of a host’s root which
has been attacked and sucked as a nutrient substratum one year dies, the sucker
belonging to the root parasitic upon it is no longer in a position to fulfil its function
by continuing to absorb fresh juices. Suckers thus reduced to a state of quiescence
soon perish, and only leave little scars to indicate the places where they existed.
The perennial root of the Pedicularis has now to seek a new source of nutriment,
and this is effected by the elongation of its tip, which continues to grow until it
reaches the living root of another plant suitable as host, whereupon it develops a
fresh sucker upon that root. This elongation doubtless requires a large quantity of
plastic materials; but these are found stored in abundance in the older parts of the
parasitic root.

These circumstances explain, at anyrate in part, the characteristic structure and
disproportionate length of the roots of Pedicularis. From all round the short erect

180 CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT.

root-stock, which is generally only from } cm. to 2 em. long, issue fleshy rootlets of
the thickness of a quill, but, in many species, as long and thick as a little finger.
These rootlets are abundantly supplied with starch, and, in course of time, elongate
till they measure 20 cm. They radiate in all directions in the black soil of the
meadow, wherein are buried the root-systems of grasses, sedges, and various other
plants, and fasten on to suitable hosts by means of one or two suckers yearly, and
repeating this process until at length their tips travel into earth devoid of roots,
where no more prey is to be found, and there growth ceases. This explains also
why these long Pedicularis-roots never descend vertically in the earth, but remain
only in the upper strata of soil on a meadow, where a number of other roots are
interwoven together, and where it is most likely that the tapering growing-point
will meet with the root of some new host or other.

The Alpine Bartsia (Bartsia alpina), one of the perennial Rhinanthacee
prevailing in the arctic regions as well as in mountainous parts of Europe on damp,
marshy, grass-covered spots, is distinguished by the sombre dusky violet colouring
of its leaves, and has already been noticed amongst carnivorous plants. On the
secondary roots are suckers exactly like those of the Yellow-rattle (Rhinanthus),
and by means of these organs it clings to the fibrous roots of sedges and grasses, and
sucks their juices. The long, subterranean, runner-like stems, which are covered
with small, whitish scales, also bear, however, elongated absorption-cells (root-hairs),
which are distinctly differentiated, and take up nutriment from the vegetable mould
around. This Bartsia is, therefore, half-parasitic and half-saprophytic, and it is not
improbable that many other perennial Rhinanthacez behave in the same way.

The species of Pedicularis which constitute the most extensive group of
perennial green-leaved and parasitic Rhinanthacee are, it is true, destitute of
tubular absorption-cells (root-hairs) whether on the subterranean stem-structures
or on the root-tip, with the exception of those which develop in the middle of the
suckers. But the construction of the epidermal cells on the roots, and the circum-
stance that these epidermal cells are always in intimate connection with dark
particles of humus, would favour the idea that these plants are capable of taking up
organic compounds from the mould of meadows in addition to the food acquired by
means of suckers from their hosts. This supposition is further supported by the
fact that I succeeded in rearing a species belonging to the Rhinanthacex, namely,
Odontites lutea, from a soil composed of a mixture of sand and humus, in which no
other plants were rooted, so that the possibility of a withdrawal of nutritive matter
from hosts was excluded. It is true that the plants thus reared remained
comparatively small and poor, and only developed few flowers and fruits. But at
anyrate they may be considered to prove that plants exist, which, though normally
parasitic, are yet on occasion able to subsist in vegetable mould without the
assistance of hosts,

The third series of parasitic flowering-plants is very restricted, contrasting in
this respect with the second series, composed of the numerous green-leaved
Santalacew and Rhinanthacex. The species belonging to it differ from those of the

CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT. 181

second series chiefly in their lack of chlorophyll. They all live underground on the
roots of trees and shrubs, develop deep down in the earth a number of flowerless
perennial shoots thickly covered with scales, and, in addition, push up annually into
the light temporary axes bearing flowers, which ripen their fruits and die after the
fall of the seed.

As the best known representative of this series, we may take the Toothwort
(Lathrea Squamaria), which is represented in fig. 37, and has been already
described on a previous occasion as an instance of a plant nourished by capturing

Fe

and digesting infusoria in special receptacles. Like Bartsia, it is a remarkable
example of a plant living on juices in part derived from animals killed by itself and
partly from living hosts. Formerly, the Toothwort used to be included in the
family of Broom-rapes (Orobanchez) on account of the structure of its capsules,
but it is entirely different as regards the form of its seedling. For, whereas the
seedling of a Broom-rape is a thread without any trace of cotyledons, as will be seen
when we study its development and mode of attachment to the host in the next few
pages, that of the Toothwort is clearly differentiated into radicle, cotyledons, and
rudimentary stem, corresponding in this respect entirely with the Rhinanthacee.
Moreover, the Toothwort resembles Rhinanthacee much more than Broom-rapes in
the manner in which it attacks its hosts and withdraws nutriment from them.

182 CLIMBING PARASITES. GREEN-LEAVED PARASITES. TOOTHWORT.

The seed of Lathrea germinates on damp earth. The young root of the seedling
grows at first at the expense of reserve material stored in the seed, penetrates
vertically into the earth and sends out lateral branches, which, like the main root,
follow a serpentine course and search in the loose damp earth for a suitable nutrient
substratum. If one of these meets with a living root belonging to an ash, poplar,
hornbeam, hazel, or other angiospermous tree, it fastens on to it at once and
develops suckers at the points of contact; these suckers are at first shaped like
spherical buttons, but soon acquire, as their size increases, the form of discs
adherent to the host’s root by the flattened side and with the convex hemispherical
side turned towards the rootlet of the parasite. These discoid suckers cling to the
root attacked by means of a viscid substance produced by the outermost layer of
cells. As in the case of the parasites already described, a bundle of absorption-cells
grows out of the core of each sucker into the root of the plant serving as host, and
the tips of the absorbent cells reach to the wood of the root. The shoot extremity
of the seedling, thus nourished by the juices of the host, now develops very quickly,
elongating and producing thick, white, fleshy, scale-like leaves which overlap one
another closely, the whole thus acquiring the appearance of an open fir-cone. The
scaly stems also branch underground, and thus a curious structure is gradually
produced, consisting of crossed and entangled cone-like shoots covered with white
scales, and this structure fills entirely the nooks and corners between the woody
roots on which it preys. Individual plants extending over a square meter and
weighing 5 kilograms are by no means rare. Later on, inflorescences raise them-
selves above the surface from the extremities of the scaly subterranean shoots.
Their axes are at first curved like crooks, but straighten themselves out by the
time the fruit ripens. Whereas the subterranean portions are white as ivory, the
flowers and bracts pushed up above the earth are of a purplish tinge. The roots,
which issued originally from the seedling, and their suckers have long since ceased
to meet the requirements in respect of nourishment of so greatly augmented a
structure, and therefore additional adventitious roots are produced every year,
springing from the stem and growing towards living woody branches of the
thickness of a finger, belonging to the root of the tree or shrub that serves as host.
When there, they bifurcate, forming numerous thickish filiform arms, which lay
themselves upon the bark of the nutrient root and weave a regular web over it.
Sometimes two or three of these root-filaments of the parasite coalesce, forming
tendrils, and the resemblance to a lace-work or braid is then all the more
pronounced. Suckers such as have been described are developed by these root-
filaments laterally, and more especially on the ends of the branches.

Lathreea is interesting in so many different connections that we shall again
return to this plant later on. As has been stated before, it affords a type of a series
of parasites which resembles the species of Cassytha and Ouscuta in the absence of
chlorophyll, Rhinanthacew in the shape and development of the seedling and the
form of the suckers, and the Balanophorex, presently to be described, in being
parasitic upon the roots of woody plants. Lathrea Squamaria, the species repre-

BROOM-RAPES, BALANOPHOREA, RAFFLESIACE. 183

sented in fig. 37, is indigenous to Europe and Asia, its area of distribution extending
from England eastwards to the Himalayas, and from Sweden southwards to Sicily.
Two species are confined to the East, the Crimea and the Balkans, and another
Toothwort (Lathrea clandestina), distinguished by large flowers, but slightly
raised above the earth, extends in western and southern Europe from Flanders over
France to Spain and Italy. This last has the distinctive feature that the discoid
suckers developed on its yellow roots, which latter are of the thickness of a quill,
are as large as lentils and the biggest hitherto discovered on any plant.

BROOM-RAPES, BALANOPHOREZ, RAFFLESIACE A.

The fourth series of parasitic Phanerogamia is composed of plants destitute of
chlorophyll, whose seed contains an amorphous embryo without cotyledons or
radicle. The seed germinates on the earth, and the embryo grows asa filiform body
into the ground and there fastens upon the root of a host-plant, penetrates into
and coalesces with it in growth, forming a tuberous stock, from which, later on,
flowering stems are projected above the earth.

To this series belong the Broom-rapes or Orobanchew and the Balanophoree.
Of the genus Orobanche about 180 species are recognized, which, exhibiting great
uniformity in floral structure and in their general development, can only be
distinguished by minute characteristics. The flowering stem growing up from the
subterranean tuber is, in all the species, rigid, erect, thick, fleshy, and covered at the
top with dry scales. The open flowers, ringent in shape, are crowded together in a
terminal spike, and often emit a strong scent like that of pinks or sometimes of
violets. The colour of the flowers is in one group (Phelypea) mostly blue or violet;
in the rest it is waxen yellow, yellowish-brown, dark-brown, rose-red, flesh-tint, or
whitish. Orobanche violacea and O. lutea, both natives of Northern Africa, have
stems which grow to a height of half a meter and become almost as thick as an arm.
The best-known species is the Branched Broom-rape (Orobanche ramosa), which is
parasitic on the roots of hemp and tobacco plants, and is very widely distributed.
The greatest number of species belong to the East and to Southern Europe. The
extreme north of America harbours one species which bears a single flower at the
end of its stem. In all the species the stem projects only partially above the earth.
The subterranean portion, adherent to the root of a host, is often greatly swollen
and thickened above the place of attachment; in the case of Striga orobanchoides,
which is prevalent in the Nile basin, it is irregularly lobed above the host’s root.
The root of the nutrient plant also is usually somewhat swollen wherever a
parasitic Orobanche has settled upon it, and sometimes it exhibits an irregular
outgrowth inclosing the spot whereto the Orobanche is adnate like a cup. Beyond
the place of attachment of the parasite the root ‘has often the appearance of having
been bitten off, and this is owing to the fact that the particular piece of root has
been killed and demolished by the attack of the parasite. From the base of the
stem, near the point of adhesion to the host, spring short, thick, fleshy fibres, and

184 BROOM-RAPES, BALANOPHOREH, RAFFLESIACEA.

one or other of these bends its tip towards the root of the foster-plant and clings
to it. These fibres are, in many species, very numerous, and are interlaced and
entangled so as to form a reticulate mass, which vividly recalls that of the Bird’s-
nest, and is an instance of the general resemblance existing between Orobanchew
and the Orchidew destitute of green leaves (Neottia, Corallorhiza, Epipogum,
Limodorwm), which have already been discussed.

The establishment of parasitic Orobanches upon the roots of host-plants takes
place in the following manner. The embryo imbedded in the small seed shows no
trace of differentiation into root and stem, possesses no cotyledons, and indeed
consists only of a group of cells; it is surrounded by other cells filled with reserve-
nutriment. When this embryo grows forth from the seed, during which process it
consumes the reserve-food, it exhibits no distinction between root, stem, and leaf,
but is a spiral filament consisting of delicate cells. One extremity, the shoot end,
of this filiform seedling, remains covered by the seed-coat, which looks like a dark
cap (fig. 34°); the opposite extremity is the root.

The seedling Broom-rape stretches downwards just as the Dodder (Cuscuta)
extends upwards. In so doing the descending tip traces a spiral line, and so,
as it were, seeks in the earth for the root of a plant suitable as host. If the
search is fruitless, and if the reserve-material in the seed has meantime been
altogether consumed, the seedling begins to wither and gradually shrivels, turns
brown, and dries up. It lacks the power of nourishing itself by means of the
surrounding earth. But, if the lower, foraging extremity of the seedling succeeds
in finding a live root belonging to a plant able to serve as host, it not only adheres
closely to it, but swells in such a way as to give the young plantlet a flask-shaped
appearance (fig. 34° and fig. 341°). The upper end is still inclosed by the seed-
coat, but in proportion as the lower part thickens, the upper shrivels till no trace
of it is left. The thickened part, on the other hand, which has become attached
to the root of the host, becomes nodulated and papillose. Some of the papille
develop into elongated conical pegs, and the young Broom-rape now rests upon the
nutrient root in the shape of the head of a fighting-club (see fig. 341). At the
place of attachment one of the conical pegs has meanwhile penetrated the cortex of
the root, and there it continues to grow energetically, forcing the cortical tissue
apart, until it reaches the wood. Vessels now arise in the body of the young club-
like plant, and, passing through the middle of the plug, wedged in the nutrient
root, are brought into connection with the vessels of the latter. At the point of
union between host and parasite, a bud is formed, clothed with abundant scales,
which may best be likened to the bulb of the Martagon Lily. Lastly, out of
this bud grows a strong, thick stem, which breaks through the earth and lifts a
spike of flowers into the sunlight.

That portion of the Broom-rape which is buried in the root of the host-plant is so
intimately associated with the separate parts of that root in the development of a
tuber that it is usually difficult to determine which cells belong to the parasite and
which to the host. The degree of union is such that one cannot even state with

BROOM-RAPES, BALANOPHORE, RAFFLESIACEA. 185

certainty where the epidermis of the nutrient root ceases, and that of the Broom-
rape begins. The latter looks as if it were a branch growing out of the root it preys
upon, and this apparent fusion gave some colour to the view of the earlier botanists,
who, ignorant of the life-history of these parasites, believed that they did not arise
from seeds, but were pathological outgrowths of the roots, produced from their
tainted juices; in other words, that they were “pseudomorphs” sprouting from
diseased roots in the place of leafy branches.

It is also deserving of mention that some of the thick, fleshy fibres issuing
laterally from the nodulated seedlings curve towards the host’s root, bury their tips
in the cortex, and thenceforth behave exactly like the peg which was inserted at the
point where the seedling first became attached. We must leave undecided the
questions as to whether the other fibres, which terminate freely in the earth, are
capable of taking up food-materials from that source, whether these fibres are only
present in perennial species and become the starting-points of new individuals, and
lastly, whether they should be looked upon as root-structures or as stem-structures.

In addition, it is noteworthy that in many Orobanchex only those embryos
continue to develop which meet with a plant suitable to be their host. Although it
is not the case that every species of Orobanche adopts one particular species of
plant as foster-parent, yet thus much is certain, that most of them only thrive on
members of a limited circle of species; one lives exclusively on kinds of Wormwood,
a second on species of Butter-bur, and a third on those of Germander. For
example, Orobanche Teucrii prevails on Teucriwm Chamedrys, Teucriwm mon-
tanum, &c., the hosts being invariably species of the genus Teuwerium. Suppose a
hill thickly covered with plants comprising Teucriwm montanwm growing in
company with thyme, rock-roses, globe-flowers, sedges, and grasses, but no great
abundance of the Tewcrvwm, a plant belonging to the species named occurring only
here and there, and let Orobanche Teucrii have established itself at one particular
spot, have attained to flowering and developed fruits, the tiny seeds of which have
been shaken by the wind out of the ripe capsules. Owing to the exceptional
minuteness and lightness of its seeds, every gust of wind will scatter them in
innumerable quantities over the entire hillside and beyond it. The next step is
germination. Filiform embryos emerge from the seeds, in the manner described
above, and penetrate into the earth. Teucriwm montanum being only sparsely
present on the hill in question, comparatively few seedlings will meet with the roots
of that plant, whereas thousands will fall in with the roots of the thymes, rock-
roses, globe-flowers, sedges, and grasses. But, curious to relate, only those seedlings
of Orobanche Teucrit which come into contact with the roots of Teucriwm
montanum establish themselves firmly, penetrate into them, and continue their
development; whilst the numerous individuals which touch the roots of the thyme
and other plants perish. This phenomenon can scarcely be explained in any other
way than by the supposition that the roots of Teweriwm montanum alone, by
virtue of their special structure and quality, afford a suitable nutrient substratum,
and therefore constitute centres of attraction for seedlings of Orobanche Teucrit;

186 BROOM-RAPES, BALANOPHORE, RAFFLESIACEZ.

and that the roots of the thyme, rock-roses, and other plants growing upon the hill
side by side with Teucriwm montanum do not share this property.

Whereas the Broom-rapes constitute a family of plants, the species of which,
though very numerous, are so similar in the structure of flowers and fruit, in the
history of their development and in the general impression they convey, that it is
necessary to discover minute distinctive marks in order to be able to classify them
with tolerable completeness, the Balanophoree, which, together with these Oro-
banchez, belong to the fourth series of parasitic Phanerogams, are related to one
another in a manner quite the reverse. Only forty species of them are known, but
they are so various that, on the basis of the obvious differences, no less than
fourteen genera have been distinguished, among which the forty species are fairly
equally divided. In respect of distribution and occurrence they also contrast
strikingly with both Broom-rapes and Rhinanthacez. The Orobanchex belong in
particular to the Mediterranean flora, and to the East, and the Rhinanthacez, as has
been already stated, adorn chiefly sunny pastures in arctic regions and in moun-
tain districts of the northern hemisphere. Balanophorew, on the other hand,
are only found within a belt encircling the Old and New Worlds, which stretches
little beyond the equatorial zone to the north or south, and they almost all inhabit
the dark bed of primeval forests, where they are parasitic on the roots of woody
plants, beneath a covering of vegetable mould.

The genus of Balanophores named Langsdorfia is confined exclusively to
tropical America. One of its species (Langsdorffia Moritziana) is found native
in the damp forests of Venezuela and New Granada, where it is parasitic on the
roots of palms and fig-trees; a second species (Langsdorfia rubiginosa) occurs in
Guiana and Brazil in the region of the sources of the Orinoco, and a third, the
most common of all (Langsdorfia hypogew) represented in fig. 38, has an area of
distribution extending from Mexico to the south of Brazil. They all avoid the
hottest districts, remaining rather in cool regions; indeed the species first named
has been found at an elevation of from 2000 to 3000 meters. Unlike all the
rest of the Balanophoree, Langsdorfia exhibits a branched, cylindrical stock
ascending from the place of attachment to the nutrient root, more or less felted
externally, and before putting forth any flowers has a remote resemblance to a
doe’s antlers with their winter covering of downy skin. These stems are almost as
thick as a little finger, have a fleshy consistence, and exhibit a clavate expansion
at the base where they rest upon the root of the host. Many of those stems which
bear the male flowers are 30 cm. long; those which bear the female flowers are
usually somewhat shorter. They are all of a pale-yellowish colour; the thickly
tomentose Langsdorffia rubiginosa looks as if it were covered with a yellowish
velvet. At the extremity of each of the ramifications of the stem, which are often
extremely short, having then the form of lobes or knobs, a bud is developed sooner
or later in the lower cortical layer. This bud swells, bursts the outer layer of
cortex, uplifts itself and grows out as an inflorescence between the four lobes
formed by the cruciform rupture of the bark. The inflorescence is surrounded, like

BROOM-RAPES, BALANOPHOREH, RAFFLESIACE. 187

the capitulum of a composite, by a whorl of imbricating scales, of which the lower
are shorter and broader, and the upper longer, narrower, and pointed at the apex.
These scales being stiff, somewhat shiny, and varying in colour from a waxen
yellow to orange or red—in the case of Langsdorffia Moritziana brown-red,—the
whole inflorescence has a vivid resemblance to certain immortelles, namely, the
large species of Helichryswm occurring at the Cape. The inflorescences bearing
male flowers are elongated and egg-shaped, those possessing only female flowers are
shorter and capitulate. The seeds dropped from the nut-like fruits, which are
pulpy internally, have no special integument. The embryo exhibits no trace of

OFF!

Fig. 38.—Langsdorfia hypogeea, from Central America. spiny

cotyledons or radicle, but consists of an undifferentiated group of cells which may
be likened to a tiny bulbil.

Seeds of this kind germinate like those of Lathrwa, and upon meeting with
the root of a tree or shrub suitable for prey, develop into larger tubercles and have
a remarkable effect upon the substratum. The cortex of the host-root is destroyed
at the place of adhesion of the tubercle, and its wood is laid open, lacerated, and
unravelled. The woody bundles are diverted from their previous direction, ascend
towards the parasitic tubercle, which meantime has grown into a full-sized tuber,
and spread out like fans. The cells and vessels of the parasite penetrate between
the ascending wood-fibres, and this results in the formation of a zone at the place
of union of the parasite and root, where cells and vessels belonging to both inter-
lace, traverse, and join one another, coalescing completely in exactly the same way
as happens in the.case of the species of Toothwort. A similar phenomenon occurs
also when one of the wavy stems of Langsdorfia comes into contact with a root
adapted to the purpose. The cortex of the root is demolished at the place of

188 BROOM-RAPES, BALANOPHOREA, RAFFLESIACEZ.

contact; the wood is exposed, split open, and unravelled, whilst the tissue of the
parasitic stem fills up all the interspaces in the upceurved and sundered woody
bundles and fibres, and so intimate is the union thus effected that the stem of the
Langsdorfia might be taken to be a branch of the root of the host-plant which
sustains it. At the point of connection of an already adult Langsdorfia stem, the
hypertrophy of the tissue is not very striking; but the base of each stem of an indi-
vidual produced from a seed presents a highly swollen and clavate appearance. At
first the parasite is only fastened by one side of this thickened base to the nutrient
root, but later on it wraps both sides round the root, and rests upon the latter like
a saddle on the back of a horse.

Between the bundles of a Langsdorffia stem there are passages filled with a
peculiar wax-like matter named balanophorin. The quantity of this substance is
so great that if one end of a stem of Langsdorffia is lighted, it burns like a wax-
taper, and in the region of the Bogota these Langsdorffias are collected and sold
under the name of “siejos”, and are used for illuminating purposes on festive
occasions. In New Granada they have also been employed in the making of
candles; and, although this source of wax is not sufficiently abundant for us to be
able to believe in its consumption and conversion on a large scale, the fact of its
application in this manner shows that the parasite we are discussing must occur in
great exuberance in many tracts of country in Central America.

Much rarer than the parasitic Langsdorffias are the species belonging to the
genus Scybaliwm. Like the former these are confined to the equatorial zone of
America. Two species, viz. Scybalium Glaziovii and S. depressum, flourish in
mountainous districts, one of them indeed occurring only on the mountains of New
Granada; two other species (Scybalowm jamaicense and S. fungiforme) live in the
woods and savannahs of lower-lying regions. The aspect of the last-named species
when seen growing on the ground of a primeval forest, tempts one to suppose it to
be a fungus, and it is easily understood why the first discoverer selected the term
fungiforme to apply to it. Figure 391, representing this rare and marvellous plant,
is taken from the original specimens discovered in the year 1820 by Schott in the
Sierra d’Estrella of Brazil, and brought thence by him to Vienna. We see that, in
this case, instead of the elongated, wavy, branched stem characteristic of Langs-
dorffias, a lumpy, tuberous mass rests upon the root of the host-plant. This tuber
is sometimes rounded and sometimes compressed and discoid; it is nodulated and
often irregularly lobed also, and grows to the size of a fist. It is developed from
a seed which, as is the case in all Balanophoree, is a cellular structure without
integument containing an embryo destitute of cotyledons and radicle, and is best
described as a minute tubercle. The embryo, after emerging from the seed and
finding the living root of a woody plant, increases in volume, and, in the form of
a little knob the size of a pea, exercises the same influence on the plant preyed
upon as has been noted in the case of Langsdorfia. The root attacked is stripped
of bark at the place where the tubercle is attached; the wood is then resolved into
a fringe of fibres which stand straight up, and, diverging like the spokes of a fan,

BROOM-RAPES, BALANOPHOREE, RAFFLESIACEA. 189

distribute themselves in the tissue of the parasite, the latter having in the mean-
time developed into a tuberous stock as large asa nut. These radiating bundles,
issuing from the wood of the nutrient root, come then into such intimate connection
with the vessels formed in the tuber of the parasite, that the one appears to be a
continuation of the other. They are, besides, entangled together, and between them
is intercalated a mass of small parenchymatous cells which also adheres to the yet
unfrayed portion of the foster-root’s wood, and coalesces with it. The tuberous
body of the parasite, which in the first instance is only adnate to the host on one

Fig. 39.—Parasitic Balanophorezx.

1 Scybalium fungiforme, from Brazil. 2 Balanophora Hildenbrandtii, from the Comoro Islands.

side, gradually encompasses it entirely, and the nutritive root then appears to
perforate this irregular tuber. The inflorescences are produced direct from buds,
which are formed under the bark at projecting spots of the brown tuberous stem,
the cortex bursting open and allowing a thick flesh-coloured shoot, closely beset by
ovoid pointed scales, to emerge and grow up into a form resembling a mortar-pestle.
At the summit this shoot expands into a disc, and upon this are borne little capitu-
late groups of flowers, which are inserted amongst innumerable quantities of scales
and hairs. The pistillate and staminate flowers are separated in different inflo-
rescences, whilst the entire structure has an undeniable resemblance when in bloom
to the inflorescence of an artichoke gone to seed, and later on to a toad-stool.

In the eastern hemisphere we find the various species of the genus Balanophora
replacing the Langsdorffias and Scybalia. One of these, Balanophora Hilden-

190 BROOM-RAPES, BALANOPHOREH, RAFFLESIACEZ.

brandtii, which is represented on the left side of the figure 39, occurs in the
Comoro Islands off the east coast of Africa; seven species inhabit the islands of
Java, Ceylon, Borneo, Hong-Kong, and the Philippines, and three species the East
Indies. Balanophora fwngosa, first discovered by Forster, is parasitic on the roots
of Eucalyptus and Ficus, and is indigenous to Australia and the New Hebrides,
The more elevated regions of Java and the Himalaya abound especially in
these singular organisms. Balanophora elongata is so prevalent in Java on
mountains of between 2000 and 3000 metres, that it is collected in quantities for
the sake of the wax-like matter obtained from it. In that island candles are made
from Balanophoras as they are from Langsdorffias in New Granada, or else rods of
bamboo are smeared with the viscid substance, as they are then found to burn quite
quietly and slowly. In the Himalaya, Balanophora dioica or B. polyandra are
the commonest and most widely distributed species, and Balanophora involucrata .
is there met with upon the roots of oaks, maples, and araliads even at a height of
from 2300 to 2500 metres above the sea-level. They possess in almost all cases
very vivid and conspicuous colouring—deep-yellow, purple, red-brown or flesh-tint,
thus resembling the Gastromycetes, Clavariez, and Toad-stools, in whose company
they grow, and with which they manifest an additional uniformity in being all of
fleshy consistence and containing no trace of chlorophyll. At a certain distance,
moreover, the inflorescences rising from the dark ground in a wood, have the
‘appearance of fungi, and all the early observers describe these Balanophorew with
one accord as truly abnormal growths, viz. as fungi which by some marvellous
accident bear flowers. They were also the object of the boldest speculations and
most exuberant imagery on the part of the botanists belonging to the school of the
“ nature philosophers ” of the first decades of this century. Even as late as the forties
a famous German botanist says of them: “They are in the position of a hiero-
glyphic key between two worlds, which intercept and evade one another in an
infinite variety of ways, like dreaming and waking moments”, and the worthy
Junghuhn, who discovered several of these plants in Java, writes: “Those are
words which we may hope will be rightly interpreted thousands of years hence.
Their sublime truth affected me deeply. There, flowerless and leafless, stood the
mysterious plants which afford an instance of the combination of special vessels
in a stalk like that of Balanophoree with the fructification of imperfect Hypho-
mycetes!”

A young Balanophora not in flower is not unlike a Scybaliwm in appearance
at the corresponding stage of its development. It consists of an irregular tuberous
stem, which rests upon the creeping root of a tree or shrub. The exterior of this
structure, which sometimes attains to the size of a man’s head, is uneven, and in
some cases convoluted like the human brain, or it may project in humps and knobs,
or be divided into lobes or short branches like a coral-stem. The resemblance to
the latter is heightened by the fact that the surface is covered by little papilla
shaped like stars or forget-me-nots, which distinguish the genus Balanophora
from all allied genera.

BROOM-RAPES, BALANOPHOREA, RAFFLESIACEA. 191

The seeds settle upon the roots of trees, develop into tuberous axes, and unite
with the nutrient root in the same manner as the Balanophoree already described.
Also the inception of the rudimentary inflorescence beneath the cortex of the tuber
and its eruption are similarly accomplished. In this genus the cortical layer thus
broken through and forced outward always forms a large cup-shaped or crateriform
sheath with an irregularly-lobed margin surrounding the base of the inflorescence.

Fig. 40.—Parasitic Balanophorez.
1 Rhopalocnemis phalloides, from Java. 2 Helosis gujanensis, from Mexico.

The inflorescence itself is spadiciform, and is borne by a thick shaft beset with
large squamous leaves. The spadices growing from a tuber-stock are, for the most:
part, only as long as a little finger, but occasionally they reach a height of 30 em.,
as, for example, is the case in the Balanophora elongata of Java, which is parasitic
on the roots of Thibaudia.

The species of the American genus Helosis, whereof the most common (Helosis
gujanensis) is represented above, resemble those of the genus Balanophora in the
shape of the inflorescence. There is, however, considerable difference in the method
adopted by these Helosis species of settling upon the roots of host-plants and in

192 BROOM-RAPES, BALANOPHOREE, RAFFLESIACEZ.

the whole mode of growth. The phenomena of the swelling of the embryo into a
tubercle after it has chanced upon a nutritive root, the destruction of the cortex,
the exposure of the wood at that part of the root where the tubercle is adnate, and
the derangement of the course of the woody bundles ensue, it is true, in the same
manner as in the other Balanophorex; but the frayed wood-bundles of the foster-
root only form quite short lobules which penetrate but a short distance into the
parasitic tuber-stock, whilst the vascular bundles, formed meantime in the latter,
adhere to them in such a manner that they might be mistaken for direct continua-
tions of them.

When once the parasitic tubers have thus become adnate to a root, and by
means of this union are provided with food, they grow round the nutrient roots in
such a way that the latter appear to perforate or actually to issue from the
tubers. They are always roundish, brown outside, and warty, but without
scales, and they never produce inflorescences directly, but put forth in the first
place several whitish or yellowish runners varying in thickness from a quill toa
finger, which creep along horizontally under the ground, bifurcating, and becoming
interlaced with other ramifications. At the places of contact they coalesce, and so
occasionally form a net-work which is almost inextricably entangled with the root-
system of the plant preyed upon. Whenever a runner of this kind comes into
contact with a living root belonging to the host-plant, the surface of contact at once
swells up. The part affected is converted into a tuberous mass and becomes adnate
to the root, the process being the same as occurs in the case of the tubercle pro-
duced from seed. A net-work of runners thus connected with the root-system of
the nutrient plant at several spots by means of tubers as large as peas might be
compared to the reticulum woven by Lathrea round the roots of its hosts; but,
apart from the size, there is the essential difference that inflorescences are never pro-
duced from the white threads of the ramifying and sucker-bearing roots of Lathrea,
whereas the runners of Helosis afford points of origin for new inflorescences. Warts
are produced on the surfaces of the thicker cylindrical runners, and within these
are developed the buds of the inflorescences. The outer coat of the warts is then
rent open at the top and constitutes a little cup, out of which grows a naked, scale-
less shaft terminated by an oval spadix. Seeing that the runners rest horizontally
under the earth whilst the shafts ascend bolt upright from the ground, the latter
are always at right angles to the runners, of which they are to be regarded as
branches.

The flowers are grouped in capitula, presenting in the spadix a dense mass.
They are protected by peculiar bract-scales, each of which by itself is like a nail
with a facetted head. These heads are in close contact with one another, so that
the young inflorescence seems to be inclosed in a panelled coat of mail, and
resembles to a certain extent a closed fir-cone. By degrees, however, these bract-
scales detach themselves and fall off, and thus the flowers, till then roofed over by
them, become visible. When the seeds are mature, the whole runner concerned in
the production of the inflorescence, and usually also the tuber which served as the

BROOM-RAPES, BALANOPHORES, RAFFLESIACEA. 193

starting-point of that runner, perishes, and another tuber belonging to the net-work
above described, or rather the system of runners proceeding from it, becomes the
basis for the development of new inflorescences. To this extent we may regard
these Helosis species as perennial plants, whereas the majority of the other
Balanophores can lay no claim to this distinction, inasmuch as in their case the
whole plant dies after it has flowered and ripened its seeds. The floral spadices in
Helosis have a purple or blood-red colour, and in Brazil are called “Espigo de sangue”.
Only three species of Helosis have been discovered up to the present time, and
those are distributed over equatorial America, in the Antilles, and from Mexico to
Brazil.

Nearly allied to Helosis is the genus Corynea, which resembles it in having
facetted bract-scales like nails and a cone-like inflorescence, but differs eatirely in
other respects in its mode of growth, especially in being without runners. Four
species of this genus have been discovered in the Andes of South America, in Peru,
Kcuador, and New Granada, where they are parasitic, like the rest of the Balano-
phorez, upon the roots of trees. One of them, Corynwa Turdiei, is worthy of
notice as living on the roots of Peruvian-bark trees, and is rendered conspicuous by
its purple spadix, borne on a white shaft. Rhopalocnemis phallordes (see fig. 401)
is another root-parasite related to Helosis, and the single representative in Asia of
these pre-eminently American groups. It is found preying upon the roots of
fig-trees, oaks, and various lianes, in mountainous parts of Java and the eastern
Himalayas, and is one of the biggest of all the Balanophoree. The fleshy,
yellowish or reddish-brown tuber-stock attains to the size of a man’s head; the
inflorescences, which burst from the protuberances of this lumpy mass and are
from two to six in number, are over 30 em. long and from 4 to 6 cm. thick. The
protuberances are light-brown in colour, and resemble in form a cycad-cone.
Rhopalocnemis, a drawing of which is given in fig. 40+ on a scale of one-half the
natural size, is distinguished, like Corynwa, from Helosis by having no runners
issuing from the tuberous axes.

The Lophophytez are set apart as a further group of parasitic Balanophoree,
and differ from all the groups hitherto described in having their flowers arranged in
separate roundish capitula upon a fleshy rachis springing from the tuberous-stock.
They, again, belong to Central America, and are divided into three genera
‘(Lophophytum, Ombrophytum, and Lathrophytwm) into particulars of which we
cannot enter without exceeding our limits. Only the genus Lophophytwm, which
is in many respects different from other Balanophoree, and in particular has been
more thoroughly studied with reference to its peculiar mode of connection with the
host-plant, demands special consideration. The Lophophytwm mirabile (see
fig. 411) found in the primeval forests of Brazil adhering to the roots of Mimosea,
to those of Inga-trees especially, occurs at some places in such profusion that areas
of ground, occupied by Inga-roots, from twenty to thirty paces in circumference
appear to be entirely overgrown by the parasite. Hundreds of tubers, some large,

some small, rest upon the roots of the trees, covered by fallen leaves and a light
Vou, I. 13

194 BROOM-RAPES, BALANOPHORE, RAFFLESIACEZ.

stratum of vegetable mould. Most of them are the size of a fist, but a few are as
big as a head, and then weigh 15 kilogr. and more. The tubercles formed directly
by the germinating seeds which chance upon the roots are, by the time they attain
to about the size of a pea, already in connection with the wood of the attacked
root. The cortex and a portion of the wood at the place where the parasite is
adnate are absorbed by this root. The tissue of the small tuber-stock is squarely
and firmly inserted into the superficial notch thus made in the root, and short, peg-
shaped bundles, isolated by the loosening of the wood of the nutrient root, appear
to grow into the substance of the parasite. As the tuber increases in size vascular
bundles are developed in it also, and these grow towards the said bundles of the
host and unite with them.

No boundary can then any longer be certainly recognized between host and
parasite, and the strangest fact of all is that we find, in these bundles, cells
concerning which we are not able to decide, even by reference to their shape,
whether they belong to the one or to the other. The cells which belong
undoubtedly to the wood of the nutrient root have dotted walls; the bundles
unquestionably developed in the parasitic tuber exhibit, on the other hand, cells
with reticulate thickening, which, when slightly magnified, look as if they were
transversely striated. Wherever these pitted and reticulate cells meet, cells are
intercalated which do not altogether correspond either to the pitted variety
belonging to the host or to the reticulate cells of the parasite, but display a form
intermediate between the two. Here and there, too, cell-groups belonging to the
parasite are entirely buried in the wood of the foster-root in its growth, and in
the older tubers the cellular elements of the two plants there bound together are so
involved that it is, as has been stated, impossible to establish any line of demarca-
tion between the two.

By the time the tubers have reached the size of a fist their cortical layer is
always solid, corky, and areolated; each of the areas being more or less uniformly
angled, as is shown in the illustration below. Some of the more protuberant portions
elongate and grow out into short, thick stumps bearing scales all round, each of
the little areas having a triangular-pointed scale situated in the middle of it. At
this stage of development the entire Lophophytum plant has an extraordinary
resemblance to the squamigerous rhizome of a fern, or to a dwarf cycad-tree,
stripped of its green leaves; and this likeness is enhanced by the fact that the bark
and scales of Lophophytum are dark-brown in colour. From the centre of each of
these thick stumps, which often reach a height of 15 cm., there now arises a
spadiciform inflorescence. At first it is so thickly covered with ovate lanceolate
scales possessing dark-brown, quasi-horny tips, overlapping one another like tiles,
that the spadix as a whole looks extremely like an erect cycad-cone. Imagine the
surprise of a traveller, who chances upon a spot in the depths of a primeval forest
where the ground is occupied by Lophophytwm, upon seeing hundreds of these
brown, scaly cones grow up suddenly, in the course of a night following some days
of rain, from the subterranean roots of the trees. A day or two later, this’ garden

BROOM-RAPES, BALANOPHOREA, RAFFLESIACES. 195:

of Lophophyta presents an altogether different picture. The brown scales have
detached themselves from the rachis, first those at the base of cone, then also those
on the upper parts. They fall off almost simultaneously, and with them the
envelope which up to that time has concealed the flowers. The erect, fleshy, white,
or reddish rachis bearing the flowers then becomes visible. The female flowers are

Fig. 41.—Parasitic Balanophoree.

1 Lophophytum mirabile, from Brazil. 2 Sarcophyte sanguinea, from the Cape of Good Hope.

on the lower part, and arranged in spherical, deep yellow or orange-coloured
capitula which are packed close together; the male flowers are situated above the
lowermost third of the spadix, and are arranged in looser and less crowded capitula
of a pale yellow colour.

However striking the phenomenon presented by these flowering cones of
Lophophytum mirabile, it is surpassed by another native of Brazilian forests, the
Lophophytwm Leandri. The colouring of the inflorescence in this species cannot

196 BROOM-RAPES, BALANOPHOREA, RAFFLESIACE,

be exceeded in variety, its rachis being pale reddish-violet, the bract-scales
gamboge, the ovaries yellowish, the styles red, and the stigmas white. It is not
surprising that even in Brazil, where there is certainly no lack of curious plant-
forms, they have attracted attention, and that they are used there, as is the case
with all rare plants, for purposes of healing and magic. The tubers of Lopho-
phytum mirabile, which have a disagreeable, bitter, resinous taste, and bear the
popular name of “Fel de terra”, or earth-gall, are employed by quacks against
jaundice, and a belief also prevails that by secretly eating the blossoms youths are
enabled to win the affection of the maidens they admire. The same may be said
of Lophophytum Leandri, and, in addition, there is a tradition that the eating of
it brings luck and agility in hunting, fishing, fighting, and dancing, and for this
reason the Indian youth collect the plants secretly and eat them on particular days.

Of the other parasitic Balanophorez most nearly allied to Lophophytum we will
here only mention in passing the species of Ombrophytum, known in Peru by the
name of “Mays del monte”, which has a yellowish inflorescence over 30 cm. high,
and from 6 to 7 em. thick, somewhat resembling a spike of maize, and lastly, the
Lathrophytum Peckoltiz of Brazil, to which a special interest attaches inasmuch
as it is the sole instance of a flowering plant entirely destitute of all structures of
the nature of leaves, with the exception of the stamens and ovaries. Langsdorffia,
Scybalium, Lophophytum, and even Balanophora, Helosis, and Rhopalocnemis
exhibit scales, which, though transformed in various ways, are yet always in point
of position and form recognizable as leaves; but neither on the tuber, shaft, nor
spadix of this Lathrophytum is any trace of a scale to be seen, nor even a swelling
or rim that might be looked upon as a degenerate leaf.

In comparison with equatorial America with its wealth of parasitic Balano-
phorez the corresponding zone of Africa must be called poor so far as these plants
are concerned. Possibly further explorations may bring to light a few more of
these wonderful vegetable parasites, but it is hardly to be expected that such a
variety as is presented in Brazil, the Peruvian Andes, New Granada, and Bolivia
will be found Only three Balanophorez have been discovered in the Cape regions,
where the flora is well known. One of these, which is represented on the right-
hand side of fig. 41, bears the name of Sarcophyte sanguinea (1.e. blood-red flesh-
plant), whilst the name of Icthyosoma (1.e. fish-carcase) has also been applied to it
because it smells of rotten fish. These names imply that the plant resembles an
animal rather than a vegetable organism. The host-plants adapted to this
Sarcophyte are various Mimosez, especially Acacia caffra, Acacia capensis, &e.
In the first place, as is the case with all Balanophorew, small tubers are formed on
the roots of the above-mentioned woody hosts, and enter into connection with the
wood of the nutrient roots in the manner already described more than once. An
inflorescence then emerges from a bud originating beneath the cortex of the tuber,
and rapidly grows up from out of the cortex, which is rent and pushed up in the
process. The axis of this inflorescence resolves itself into a number of thick,
repeatedly ramifying, fleshy branches, differing in this respect from every other

y
Ui \

\

Fig. 42.—Cytinus Hypocistus on the left; Cynomorium coccineum on the right. im ae "
ihe 1G"

198 BROOM-RAPES, BALANOPHOREZH, RAFFLESIACEZ.

example of the Balanophores. The flowers are arranged side by side on the
branches, staminate flowers on one plant, and pistillate flowers on another, the
latter always grouped in spherical capitula, as is shown in fig. 41%. Reddish-brown
scale-like leaves are situated at the points of origin of the branches, and also at the
base of the entire inflorescence. The general aspect is that of a bunch of verrucose
grapes ascending from the root, or of the fruiting axis of Ricinus, and is very
striking owing to the blood-red colouring of all the parts.

As a final instance of the Balanophorese we may take the genus Cynomorwm,
which was so highly valued in olden times, and is the sole species belonging to this
family of plants indigenous in the south of Europe. A drawing of it is given on
the right-hand side of fig. 42.

Whilst other Balanophores are parasitic on the roots of trees and lianes in the
shade of lofty woods, this Cynomoriwm thrives most luxuriantly upon plants near
the sea-coast, on the roots of Pistacias and Myrtles, and even on actual salt-loving
maritime plants, the various Tamarisks, Salicorniz, Salsolacez, and Oraches, which
are sprinkled with foam whenever the breakers are high. The seed is like that of
other Balanophoresw and those of the Orobanche species, and germinates in the
same way as they do. From the group of cells in the seed which represent the
embryo, a filiform body emerges, and then grows downwards, its upper part
remaining for some time in connection with the other cells in the seed, which are
richly furnished with food-materials. The filiform embryo continues to grow
deeper and deeper at the expense of this nutritive store, and as soon as it reaches a
living root, swells into an oval or irregularly-lobed tubercle, which unites with
the wood of the nutrient root in the manner already described. These tubercles
swell, and from the summit of each a spadix is produced, as in Lophophytum,
which is raised above the surface of the earth. The spadix is clothed with
pointed scales, and is clearly differentiated into a lower stalk-like support, and
a fleshy inflorescence resembling a cone. The small scales are separated from one
another by the process of elongation of the spadix, and some fall off. Others
of them, situated about the middle of the inflorescence, persist, however, until
the time when the entire spadix dries up. The whole of the structure standing
above the ground has a blood-red colour, and when it is injured a red fluid exudes,
which was at one time supposed to be blood. At an age when the peculiar pro-
perties of extraordinary plants were looked upon as an indication given by higher
powers that they were to be used for curative purposes, it was believed that the
spadices of Cynomoriwm, being blood-red in colour, and bleeding when wounded,
had styptic properties. In those days they were even collected for the sake of this
property, and sold in apothecaries’ shops under the name of the Maltese fungus
(Fungus melitensis). Various miraculous virtues were also attributed to this plant,
and the demand for it was so great that it became a regular article of commerce,
its main source being the Island of Malta, whence is derived the name above
referred to.

Of the Hydnorex, which are most properly included in the same series as

BROOM-RAPES, BALANOPHORE, RAFFLESIACEA. 199

Balanophores in consideration of their coalescence with the roots of their hosts,
only three species are known. Two of them (Hydnora Africana and H. triceps)
belong to South Africa, the third (Hydnora Americana=Prosopanche Burmeisteri)
to South Brazil. The tuber is represented by a prismatic body with from four to
six angles furnished with papille along the edges. The flower-buds which burst
from it have at first the form of spherical Gasteromycetes, but gradually elongate
and assume the form of a large fig or upright club. This structure opens at the
thickened upper extremity by three stout fleshy valves representing petals. At
the base of this curious flower no appendage is to be seen that could be interpreted
as a bract or leaf. The fleshy mass of flowers evolves a disagreeable putrid odour,
and in this property the Hydnorex resemble the Rafflesie, which belong to the
next group of parasitic Phanerogams.

The fifth series of flowering parasites is composed of the Rafflesiacex, plants
connected with Balanophoreew and Hydnoreex by their general aspect, the absence
of chlorophyll, and the undifferentiated embryo which consists merely of a group of
cells. They used all to be classed together under the name of Rhizanthess; but the
Rafflesiaceze are now treated as a separate family on account of the characteristic
structure of their flowers and fruit. The formation of these organs will again come
up for discussion later on when we treat of the wonderful structure of the famous
giant-flower Rafflesia; at present we are only concerned with the relationship of
the parasite to the food-providing host-plant. This is, if possible, even more
remarkable than in the case of Balanophoree and Hydnorew. In the latter the
union is effected within a structure like a tuber or a rhizome, the vessels and cells
of the parasite coalescing with the exfoliated and disordered wood-cells belonging
to the root or stem of the host-plant; whereas in Rafflesiaceze the embryo, having
penetrated beneath the cortex of the host, produces a more or less definite hollow
cylinder which surrounds the wood of the host’s root or stem (as the case may be),
and constitutes a sort of vestment intercalated between the wood and the cortex of
the host. There is no production of tuberous enlargements as in the Balanophoree.
The stem or root attacked by the parasite only exhibits a moderate thickening at
the place where the parasite dwells beneath the cortex, and the cortex itself is only
destroyed at the spot where the embryo pierces through it, and where subsequently
the flowers emerge. When roots constitute the substratum whereupon the parasite
has established itself, they are always of a kind that run throughout upon the
surface of the ground; when stems are chosen for attack, they are either the
branches of trees or shrubs, shoots clothed with dead foliage belonging to dwarf
suffruticose bushes, or else woody lianes of tropical forests. The seeds are con-
veyed to the host-plants through the intervention of animals.

Rafflesias are found in the haunts of elephants and along the tracks followed by
those beasts. The Rafflesia-fruits are accordingly no doubt trampled upon and
crushed, and the little seeds imbedded in the pulpy mass of the fruit thus have an
opportunity of adhering to the elephants’ feet. The seeds are afterwards rubbed oft
by projecting roots at places more or less remote from the original locality, and if

200 BROOM-RAPES, BALANOPHOREE, RAFFLESIACEZ.

the root upon which they are detained belongs to a Cissus plant, they germinate.
On the other hand, such Rafflesiacez as occur on the woody branches of trees,
shrubs, and undergrowths, or on lianes, develop succulent fruits, which are eaten
by animals. Their seeds are protected by a horny coat, and preserve their power of
germination unimpaired as they pass through the animals’ alimentary canals and are
deposited with the excrements on the stems of fresh host-plants; or the seeds may
stick to some part of an animal that happens to rub.against them, and be brushed
off later on as being an uncomfortable appendage, and in this way also they may
fall upon the stem of a host-plant. Those Rafflesiacee which occur in Venezuela on
the woody lianes (Caulotretus), known by the name of “monkey-ladders”, owe their
dispersion for the most part probably to monkeys.

Now, if a seed has been deposited in one way or another upon a woody root,
creeping along the surface of the ground, or upon the stem of a woody plant, the
filiform embryo emerging from the seed finds a suitable nutrient substratum present
and it pierces the cortex of the root, and develops beneath it a tissue, which incloses
the wood like a sheath. In Rafflesia and in the Pélostyles parasitic on the
suffruticose shrubs of Tragacanth (P. Haussknechtit, see fig. 431), this tissue consists
of rows of cells, which to the naked eye look like threads. Some are simple and
greatly elongated, others branched, and they are united together to form a net-work,
so closely resembling the mycelium of a fungus as to be readily mistaken for one.
The most complete similarity to these vegetative bodies living beneath the cortex of
a host-plant is exhibited by the mycelia of the toad-stools which spread themselves
in the form of nets and webs between the wood and the cortex of old trunks of
trees. The vegetative bodies of the other species of Pilostyles consist, in each case,
of a tissue composed of many layers of cells forming a parenchyma imbedded
between wood and cortex in the host-plant and including some vessels and rows of
cells capable of being interpreted as vascular bundles. Only in rare instances does
this tissue of the parasite form an unbroken hollow cylinder encompassing the
wood of the host; usually the elements of the host’s tissues penetrate into it and
permeate and split up the cylindrical soma (vegetative body) in the form of bands,
ribs, and fibres. Many elements of the tissues, which the imbedded parasite has
displaced from the living wood, and carries, as it were, on its back, perish; but
sometimes these discarded layers remain in connection with other living tissues and
so preserve their own vitality and power of expansion, and develop layers of wood-
cells covering the parasite. There is then a general confusion and entanglement,
and it is difficult to say what part belongs to the parasite and what to the host.

When the somatic tissue of the parasite has accomplished its connections with
the host-plant in the manner just described, the latter is unable to rid itself of
its occupant. A portion of the juices of the host-plant passes into the parasite’s
cells and the unwelcome guest augments in volume, and endeavours forthwith to
reproduce and distribute its kind by the formation of fruit and seeds. For this
purpose buds are developed at suitable spots in the reticular body of the parasite,
each of which is manifested as a parenchyma of pulvinate appearance, and is

BROOM-RAPES, BALANOPHOREX, RAFFLESIACE. 201

termed a floral cushion. The cells in this cushion, however, now group them-
selves in a definite way; ducts and vessels are produced, and, at the same time,
a differentiation into axis and flowers is exhibited. These members continue their
development, increase in size, and finally the enlarged bud breaks through the
cortex of the host-plant under shelter of which it has been evolved.

In the genus Cytinus alone do we find a stem richly furnished with leaves and
bearing at the top a flattened symmetrical tuft of flowers (see fig. 42, left-hand
side) developed from this bud; in the rest of the Rafflesiaces, the bud, which has

Sn

mcll
Hiwrs {A Fig. 48.—Rafflesiacee parasitic on trunks and branches.

ee . 1 Pilostyles Haussknechtit. 2 Apodanthes Flacourtiana. 3 Pilostyles Caulotreti.

emerged from beneath the cortex of the host, is the flower-bud itself. The axis
supporting the bud is extremely abbreviated and clothed merely by a few scales,
and the flowers are sessile directly upon the root or stem of the host (see fig. 43).
In the case of roots creeping upon the ground, the buds always emerge only on the
side turned towards the light; on lianes, also, they are only formed on the side more
exposed to light where subsequently the opened flowers are easily accessible to
flying insects (see fig. 43°); on upright shrubs and under-shrubs, on the other hand,
they burst forth on all sides upon the branches. Branches of this kind bearing
ubiquitously extruded flowers of a parasite such as Apodanthes Flacourtiana (see
fig. 437) look delusively like the Mezereon (Daphne Mezereum) when the latter is
in bloom in the early spring before the development of foliage-leaves, its woody
branches being similarly studded all round with flowers, which stand out horizontally

202 BROOM-RAPES, BALANOPHOREH, RAFFLESIACEA.

from them; but, in the one case the flowers belong to a foreign parasite living
under the cortex and have broken through it, whereas in Mezereon it is the
flowers of the plant itself that have unfolded. In the case of Prlostyles
Haussknechtii, which is parasitic on the low bushy tragacanth shrubs of the
Persian plateaus, the buds are formed regularly on both sides of the leaf-bases of
the host, so that at the insertion of every one of the older foliage-leaves, one finds
a pair of buds, which subsequently expand into flowers (see fig. 43 1),

Fig. 44.—Parasitic Raffiesiacea (Brugmansia Zipellii) upon a Cissus-root.

Throughout the species of Apodanthes and Pilostyles the flowers are small—
about the size of elder, jasmine, or winter-green blossoms—and by no means
conspicuous. But this is not the case in the genera Brugmansia and Rafflesia.
The Brugmansias, indigenous to Borneo and Java, have very handsome flowers,
as may be seen in the above drawing, which represents on the natural scale
Brugmansia Zipellia parasitic upon the root of a Cissus. But in magnitude they
are far surpassed by the flowers of the Rafflesie, one of which, viz: Rafflesia
Arnoldii, may be described as actually the largest flower in the world. When
open it has a diameter of 1 meter, a dimension exceeding even that of the gigantic
blooms of South American aristolochias. At the period of emergence of the buds
of Rafflesia Arnoldii from the roots of the vines which serve them as hosts, they

BROOM-RAPES, BALANOPHOREA, RAFFLESIACES. 203

are only as large as a walnut and give scarcely any indication of their future
magnitude; but they gradually increase in size, and before opening are curiously
like a cabbage. Up to this time the bracts still inclose the flower proper, and
to them is due the above-mentioned resemblance. They now open back, and
the flower, which, to the last, grows rapidly, unfolds and displays five immense
lobes around a central bowl or cup-shaped portion. The form of the giant-flower
when open is best likened to that of a forget-me-not blossom. The semicircular
outline of the lobes, at least, is similar, and the very short throat of the flower also
exhibits a distant resemblance. At the part where the bowl-shaped centre, which

Fig. 45.—Rajlesia Padma, parasitic on roots upon the surface of the ground.

has the stamens and styles inserted in it, passes into the lobes there is a thick,
fleshy ring like a corona. The upper surface of the lobes is covered with numbers
of papille. The lobes themselves, the hollow central bowl, and the ring, are all
fleshy, and the flower, as a whole, emits an unpleasant putrescent smell. This
floral prodigy was first discovered in the year 1818 in the interior of Sumatra at
Pulo Lebbas on the river Manna, where it occurs parasitic on the roots of wild
vines in places where the ground is strewn with the dung of elephants. It has
never yet been seen anywhere outside Sumatra. Four other Rafflesize have,
however, been discovered, but all in the islands of the Indian Ocean—Java, Borneo,
and the Philippines. In mode of growth, as also in the form of the flowers, they
resemble the species above described, but their flowers are rather smaller.
Raflesia Padma, which occurs in Java, and is represented in fig. 45, possesses
flowers with a diameter of half a meter. The hollow, somewhat ventricose centre
and the ring bordering the floral receptacle are in this Rafflesia of a dirty

204 MISTLETOES AND LORANTHUSES.

blood-red, whilst the verrucose lobes have almost the colour of the human skin.
The flowers are sessile upon roots which wind about upon the dark forest ground,
and a cadaverous smell, anything but pleasant, issues from them. ll these
peculiarities explain the uncanny impression made by the organisms in question
upon their original discoverers and upon all subsequent observers.

Whilst the Rafflesie, as well as the genera Brugmansia and Sapria, belong
to the tropical and sub-tropical regions of Asia, and to the world of islands adjacent
thereto on the south side, the genus Apodanthes is confined to tropical America.
Most of the species of Pilostyles also appertain to tropical America, especially to
Brazil, Chili, Venezuela, and New Granada. One species alone—Pdlostyles Aithio-
pica—has been observed in the mountains of Angola, and another, as has been
mentioned before, in Persia.

The only European representative of the remarkable group of Rafflesiaces is
Cytinus Hypocistus, represented on the left side of fig. 42, but its distribution is
coincident with the entire range of the Mediterranean flora. The roots of cistus
shrubs, plants which are characteristic of the vegetation belonging to the basin of
the. Mediterranean, constitute the nutrient substratum in the case of Cytinus. It is
especially where the layer of earth-mould is not deep, and consequently the roots of
the shrubs in question are exposed, that Cytinus is met with growing in abundance
amongst the under-wood of the cistus plants. The squamous leaves clothing the
stem of this parasite being scarlet, and the plants not solitary but in large numbers,
one sees here and there a flaming red colour glowing in the gaps in the cistus-
groves, and one is thus from far off made aware of the presence of the parasite.
The flowers themselves, which open between the red scale-like bracts, are yellow.
The combination of colour thus afforded is a rare phenomenon in the vegetable
world, and gives a very strange appearance to the plant. Besides the species of
Oytinmus distributed over the area of the Mediterranean flora, there are two other
species in Mexico, and one also at the Cape, which, although not parasitic on Cistus
shrubs but on other woody plants, especially Hriocephalus, yet do not differ from
Cytinus Hypocistus in floral structure or in mode of connection with their host.

MISTLETOES AND LORANTHUSES.

The sixth and last series of parasitic phanerogams includes epiphytes of bushy
appearance with much bifurcated branches, green cortex, green leaves, and berries
containing large seeds, which germinate whilst resting immediately upon the
branches of such trees as are adapted to act as host-plants, and will surrender to the
invader a portion of their nutriment. To this series belong a dozen different species
of the genus Henslowra, belonging to the family of Santalaces, and indigenous
to the South of Asia—chiefly the East Indian Archipelago—and, in addition,
upwards of 300 species included in the family Loranthacee. Amongst the
latter, the plant that is best known and most widely distributed is the Euro-
pean Mistletoe (Viscwm albwm) represented in fig. 46, and as it is also fitted, in

MISTLETOES AND LORANTHUSES. 205

respect of its life-history, to serve as type of the entire series, we will describe it
first of all.

As is well known, the Mistletoe is parasitic upon trees, and these may be either
Angiosperms or Gymnosperms. Most frequently it establishes itself upon trees the
branches of which are coated by a soft sappy cortex—an extremely delicate and
tender cork-tissue in particular —as is the case with silver-firs, apple-trees, and
poplars. The Mistletoe’s favourite tree is certainly the Black Poplar (Populus
nigra). It flourishes with astonishing luxuriance on the branches of that tree, and
wherever there is a small plantation of Black Poplars, the Mistletoe takes up its
abode.

Along the shores of the Baltic and by the Danube near Vienna—especially
in the celebrated Prater from which fig. 47 is taken, one finds, on many of
the Black Poplars, tufts of Mistletoe measuring 4 meters in circumference, and
with axes of a thickness of 5 cm. Birds use their most crowded branches, by
preference, to nest in. In the forests of Karst, in Carniola, and in the Black Forest,
where poplar trees play merely a subordinate part, whilst on the other hand,
quantities of silver firs shade the ground, large numbers of these conifers have
their tops covered with Mistletoe; and in the Rhine districts and the valley of the
Inn in Tyrol, the same parasite occurs as a troublesome visitor upon apple-trees
in the neighbourhood of the peasants’ farms. In localities destitute of these three
kinds of trees, which are pre-eminently the Mistletoe’s favourite host-plants, it puts
up with other trees, and is then usually found on whatever species happens to be
the most common in each particular country. Thus, in the Black Pine district of
the Wiener Wald, it occurs upon the Corsican Pine, whilst on the heaths of the
sandy lowlands of the March, it settles upon the Scotch Pine. Much less frequently
it has been observed on walnut-trees, limes, elms, robinias, willows, ashes, white-
thorns, pear-trees, medlars, damsons, almond-trees, and on the various species of
Sorbus. Mistletoe has also been found by way of exception upon the oak and the
maple, and upon old vines. On one occasion, in the district of Verona, it has been
seen established upon the parasitic shrubs of Loranthus Europeus, that is to say,
one member of the Loranthacez was found parasitic upon another. The birch, the
beech, and the plane, are avoided by the Mistletoe, a fact which no doubt depends
upon the special structure of the cortex in those trees.

The dissemination of the European Mistletoe is effected, as in all the other
Loranthacee, through the agency of birds—thrushes in particular—which feed
upon the berries and deposit the undigested seeds with their excrement upon the
branches of trees. That a preliminary passage through the alimentary canal of
birds is essential to the germination of these seeds is no doubt a delusion, this
assumption of former times being easily refuted by the fact that one can readily
induce the seeds of berries, taken fresh from a tree, and stuck into fissures in the
bark of moderately suitable trees, to germinate; it is, however, true, that in nature,
mistletoe-seeds are dispersed exclusively by birds in the manner above mentioned.
To this method of dissemination must be attributed the phenomenon, which, at first

206 MISTLETOES AND LORANTHUSES.

sight, is surprising, that Mistletoe-plants are rarely seated upon the upper surface
of branches, but very frequently on the sides. For the dung of thrushes, which
live upon Mistletoe-berries, is in the form of a semi-fluid, highly viscid mass, ductile
like bird-lime; and, even when it is deposited upon the upper surface of slanting
branches, it immediately runs down the sides, sometimes extending in ropes
20 or 30 centimeters in length. Owing to the viscous mass thus following the
law of gravity, the Mistletoe-seeds imbedded in it are conveyed to the sides, and
even to the under surface of the bark, and there remain cemented.

Fig. 46.—The European Mistletoe (Visewm album).

It may be a long time before a seed of the kind germinates, especially if it does
not become attached until the autumn. The embryo is completely surrounded in
the seed by reserve food. It is club-shaped and comparatively large, and is dis-
tinguished by the fact that the two oblong cotyledons, which are closely pressed
together, but often somewhat wavy at the margins, are coloured dark green by
chlorophyll, like the environing cellular mass filled with reserve materials. In the
process of germination the axis of the embryo, especially the part lying beneath
the cotyledons, and passing into the hemispherical radicle, lengthens out; the white
seed-coat is pierced, and the radicle makes it appearance through the breach.
Under all circumstances the emergent radicle is directed towards the bark of the
branch to which the seed is adherent. This is the case even when the seed chances

MISTLETOES AND LORANTHUSES. 207

to stick with the radicle of the seedling pointing away trom the branch; the
whole axis of the embryo curving towards the surface of the bark in a very striking
manner. Thus the radicle always reaches the bark, and having done so it becomes
adpressed and cemented to its surface, spreads itself out in the form of a doughy
mass, and so develops into a regular attachment-disc. From its centre a slender pro-
cess now grows into the bark of the host-plant, piercing the latter and penetrating
as far as the wood, but not growing into that tissue. This penetrating process has
been termed a “sinker”, and must be looked upon as a specially modified root.

Fig. 47.—Bushes of Mistletoe upon the Black Poplar in winter.

The development of the first year ends with the formation of this sinker.
When the winter is over, the branch, into which the sinker is inserted so as just
to reach the wood with its point, grows in thickness, a new layer of wood-cells—a
so-called annual ring—being superimposed upon the wood of the previous year.
The increasing mass of wood first surrounds the tip of the sinker with wood-cells,
then forms a rampart all round it, pushing the cortical tissue, wherein that organ
has hitherto been wedged, in front of it in an outward direction, and in this way
the sinker is at length fixed deep within the woody cylinder. The process of
inclosure by the wood-layers, as they are built up, may be compared to the gradual
surrounding of a stake on the sea-shore by the rising tide; the lowermost extremity
is first immersed and then higher and higher parts until the whole is enveloped. The

208 MISTLETOES AND LORANTHUSES.

sinker itself remains, strictly speaking, stationary; it does not grow into the wood,
but the wood overgrows it. But what happens in the following season when a
fresh annual ring is once more added to the wood? If the sinker had entirely
ceased growing it would of necessity be ultimately completely closed by the layers
of wood, as they develop with ever-increasing energy and add to the thickness of
the branch, and at last it would be quite buried. To prevent this result, which
would be fatal to the Mistletoe, a zone of cells is provided near the base of the
sinker, which zone, at the time when the rampart of wood is being raised, adds in
an equal degree to its own height, and causes, of course, an elongation of the sinker
in a peripheral direction. The length of the piece thus intercalated in the haus-
torium is exactly equal to the thickness of the corresponding annual ring in the
surrounding wood of the branch. Thus at length the Mistletoe-sinker is found
imbedded in a number of annual rings, although it has not grown into the latter,
but has been banked up by them year by year.

That zone of the sinker which possesses the capacity for growth, and which is
always to be sought, in accordance with what has been said above, at the outside
limit of the wood of the branch, in the so-called “bast” layer situated on the inner
face of the cortex, produces, in the second year after the adhesion of the Mistletoe-
embryo, lateral ramifications which are called cortical roots. They are thick,
cylindrical, or somewhat compressed filaments, and all run close together under the
cortex in the bast layer of the invaded branch. These rootlets issuing from the
sinkers pursue a course parallel to the longitudinal axis of the branch, whilst the
sinkers themselves are at right angles to the axis (see fig. 48°). If a rootlet springs
from the sinker in a direction transverse to the longitudinal axis it bends imme-
diately afterwards so as to be parallel to the long axis, and adopts the same
direction as the rest, or else it bifurcates just above its place of origin into two
branches which separate suddenly, and in their further course follow the axis of
the branch. Thus it comes to pass that all the rootlets of a Mistletoe run up and
down in the infested branch of the host-plant in the form of thick green parallel
strands, but that none of them ever encircle the branch in the form of an annular
coil. Each of these cortical roots may now develop from behind the growing-point
new sinkers, which are formed in the same way as the first one above described as
proceeding from the actual seedling. They, too, penetrate into the branch per-
pendicularly to the axis, and as far as the solid wood are then encompassed by the
growing mass of wood, but maintain the power of growth in the part close to their
insertions, and in their growth keep pace with the thickening of the wood of the
branch. The fact of the yearly recurrence of this formation of sinkers explains how
it is that those situated nearest the growing-points of the cortical roots are the
shortest, they being the youngest, whilst those which arise near the first sinker are
the longest and oldest. It also accounts for, the former being only inclosed by one
annual ring of the host's wood, and the others being surrounded by an increasing

number of rings the nearer they are to the spot where the Mistletoe-plant first
struck root,

MISTLETOES AND LORANTHUSES. 209

The root-system of the Mistletoe taken as a whole may be described as like a
jaw-bone in shape, or, still better, a rake. The cross-beam of the rake corresponds
to the cortical root, whilst the teeth are analogous to the sinkers; the cross-piece
must be supposed to be parallel to the axis of the branch and lying under the bark,
and the spokes must be thought of as perpendicular to the axis and driven into the
wood.

Whilst the roots of the Mistletoe-plant are spreading in the interior of the branch
in the manner described, the stem is developed outside. At the time when the
process, subsequently to be the first sinker, emerges from the attachment-dise of

Fig. 48.—1 Loranthus Europeus, and 8 Mistletoe (Viscwm albwm)—both parasitic on branches of trees, and seen in section,
2A piece of the wood of a Fir-tree perforated by the sinkers of a Mistletoe.

the embryo and pierces through the bark, the cotyledons are still covered by the
white seed-coat, which rests upon them like a cap. But when once this first sinker
is firmly fixed and in a position to take up nutritive juices from the wood of the
host, the seed-coat is thrown off; the apex of the stem, which is still very short, is
raised; the cotyledons are detached, whilst close above them is produced a pair of
green leaves. Thenceforward the development of the visible portion of the Mistletoe-
plant outside the bark keeps pace with that of the roots underneath the cortex, and
is moreover dependent upon the quantity of food taken up by the sinkers from the
wood. Where there is an abundant supply of nutriment, as in the case of poplars,
the growth of the Mistletoe is correspondingly exuberant; where the flow of juices
is scarce, the parasite is stunted in its growth, and often develops only small

yellowish sickly-looking tufts. If the foster-plant is of a lavish nature, adven-
Vou. I. 14

210 MISTLETOES AND LORANTHUSES.

titious buds are produced regularly by the cortical roots to which the absorbed
nutriment is first of all conveyed from the sinkers. These buds occur on the side
of the rootlets nearest the exterior of the bark, and later they burst through the
rind, and develop into new Mistletoe-plants.

These outgrowths are analogous to the adventitious shoots produced from the
subterranean roots of the Aspen, and this comparison is rendered all the more
appropriate by the fact that the removal of the tuft of Mistletoe encourages the
sprouting of adventitious root-buds just as in the case of the Aspen, the growth of
shoots from the roots is promoted by the felling of the trees to which those roots
belong. If a large Mistletoe-bush, growing in solitude on a Black Poplar, is removed
from the tree with the intention of freeing the latter from its parasite, the hopes
entertained by the operator are disappointed; for, an outgrowth of shoots from the
cortical roots ensues at a number of different spots, and in a few years’ time the
poplar in question is the prey of a dozen Mistletoe-bushes instead of one. Inasmuch
as these bushes, produced from offshoots, are able, under favourable conditions, to
send out fresh roots, and these again may develop shoots, a good host of the kind
will at last have all its boughs from top to bottom overgrown by Mistletoes. In
the Prater at Vienna there are poplars beset by at least thirty large Mistletoe-
shrubs, and double that number of small ones, and if one catches sight of such a tree
at some distance in winter-time when the branches have lost their leaves, one takes
it to be a Mistletoe-tree, for almost the entire system of branches is mantled in a
continuous tangle of evergreen bushes of Mistletoe, which are in a state of parasitism
upon it.

Sinkers of the Mistletoe, 10 cm. in length, and inclosed in forty annual rings,
have been found in the wood of the Silver Fir, whence we may conclude that the
Mistletoe may live for forty years. A greater age could scarcely be attained
by one and the same bush of the parasite. If the Mistletoe dies, the rootlets and
haustoria survive for a time, but at length moulder and fall to pieces, whilst the
wood in which they were imbedded remains unaltered. The affected parts of the
wood exhibit in that case numerous perforations, and look just like the wood of a
target which has been fired at and struck by shot or small bullets (see fig. 48 *).

A small plant belonging to the Loranthacee and named Juniper-Mistletoe
(Viscum Oxycedri or Arceuthobiwm Oxycedri) occurs on the red-berried juniper
bushes (Juniper Oaxycedrus) of the Mediterranean flora. It is very different from
the common European Mistletoe, as is obvious at first sight, its foliage-leaves being
reduced to little scales, which gives a characteristic jointed appearance to the rami-
fications. A whole series of leafless forms allied to this species is found to exist in
India, Japan, Java, Bourbon, Mexico, Brazil, and at the Cape. They are nearly all
small bushes which project from the boughs of host-plants and sometimes clothe
the latter so thickly that the boughs in question serving as nutrient substratum are
entirely enshrouded by the parasitic growth. The Juniper-Mistletoe is only from
3 cm. to 5 cm. tall, and the branchlets are not woody, but soft and herbaceous;
the fruits are blue oblong berries, almost destitute of succulence. The latter are

MISTLETOES AND LORANTHUSES. 211

dispersed by birds like the berries of the common Mistletoe, and the way in which the
parasite settles upon and clings to branches of the host-plant is the same as in that
species. It also develops sinkers and cortical roots, but these root-structures are
not by any means so regularly arranged as in Visewm album, but form an inex-
tricable web of strands and filaments pervading the internal layers of cortex, and
resolving itself into finer and finer groups of cells, which end by looking not unlike
a mycelium, and also remind one of the suction-apparatus possessed by Rafflesiaces.
Such of these strands and cellular filaments as are imbedded in the wood of the
juniper do undoubtedly play the part of suction-organs. They are present in large
numbers, and some of them are occasionally encompassed by several annual rings.
They possess no special zone of growth. The elongation necessary to prevent their
being enveloped and overwhelmed by the wood, as it adds to its thickness, is effected
by the division of individual cells and groups of cells. The outgrowth of shoots
from the root is much more exuberant than in the common Mistletoe; but the
death of the original plant takes place much earlier, and close to yellowish-green
bushes of various degrees of smallness, one finds very regularly dead or dying
shrublets already turned brown, all growing promiscuously over the somewhat
swollen branches of the red-berried Juniper.

The behaviour of Loranthus Ewropeus, which is parasitic on oaks and chestnuts
in the east and south of Europe, is altogether unique. The mode of its attack upon
the branches of oaks is, it is true, similar to that of the two other Loranthacee just
described. The yellow berries, which are grouped in graceful biseriate racemes, are
eaten with avidity by thrushes in the autumn and winter, and the undigested seeds
are deposited with the dung of those birds upon the branches of trees. The embryo,
on emerging from the seed, bends towards the bark and sticks to it, at the bottom
of little rifts and crevices, for the most part, by means of the radicle, which becomes
an attachment-dise. A process grows in the next place from the centre of the
attachment-disc, and pierces through all cortical layers of the oak-branch as far as
the interior of the young wood, as if it were a small nail driven in. This process
increases in thickness at the expense of the nutriment it withdraws from the young
wood, and from it are developed one, two, or three branches, which, however,
invariably run downwards beneath the bark, that is to say, in the direction opposed
to that of the stream of sap ascending in the oak-wood, and never produce the
sinkers so characteristic of the Mistletoe. Each of these roots is shaped like a
wedge, even from the rudimentary stage, and acts, too, in the manner of a wedge,
penetrating between the yet soft and delicate cells of the cambium, which were
formed in the spring at the periphery of the solid older wood of the previous year,
and were destined to constitute a new annual ring, splitting and tearing in the
process that cell-tissue. Such of these tender cells as lie outside the wedge die, those
situated within become lignified and altered into solid wood, to which the wedge-
shaped root firmly adheres. Beneath the apex of the wedge, the lignification of
cambium cells naturally extends much further towards the exterior, because there
it is not at all broken or dead. In front of the apex of the wedge, therefore, there

212 MISTLETOES AND LORANTHUSES.

is, presently, solid resisting wood. The root being no longer able to split the tissue
with its point, is stopped in its growth at this spot. But there is nothing to pre-
vent its continuing to grow along a course somewhat nearer the periphery, and
outside the limit of the new annual ring of solid wood, where a fresh development
of soft and tender cells has taken place in the cambium, and this indeed actually
happens.

Thus, every addition to the length of the Loranthus-root, as it grows onward
between the wood and the cortex of the oak-branch, is further removed from the
axis of the branch; or, in other words, the surface of contact between root and
wood has the conformation of a flight of stairs, of which the lowest step constitutes
the base, and the uppermost the apex of the root (see fig. 481). These steps are
very small, their height varying from about 5 mm. to 7 mm., but they may be
distinguished quite clearly in longitudinal sections, on account of the darker colour
of these roots contrasting with the lighter oak-wood. Nutritive fluids are imbibed
by the Loranthus-root from the wood of the oak at the surface of contact, and it
is probable that this absorption takes place especially at the notches forming the
steps. The root can only elongate, naturally, during the period when there is a
young and fragile cell-layer superimposed upon the solid wood, whence it follows
that in Loranthus the continuation of the root’s growth is more dependent upon a
particular season and upon the annual progress of development of the host than is
the case with the Mistletoe. There may be some connection between this circum-
stance and the fact that the Mistletoe possesses evergreen leaves, whilst Loranthus
is green only in summer, acquiring fresh green foliage in the spring in the very
same week as the oak does, and casting its leaves in the autumn simultaneously
with the tree it infests.

The stem which issues from the embryo of a Loranthus-seed grows away from
the oak-branch into the open air, and develops with great rapidity at the expense
of the nutriment absorbed from the host’s wood, and conveyed to it by the root
above described, into a dense, dichotomously-branched bush. In summer it is not
unlike a Mistletoe-bush, but in autumn, when it has cast its leaves, it acquires a
totally different aspect owing to the dark-brown branches and the conspicuous
yellow clusters of berries.

Bushes of Loranthus grow to a greater size even than those of the Mistletoe;
their stems attain not infrequently a thickness of 4 em., and clothe themselves with
a blackish, rugged bark, the older stems of this kind being then usually studded by
an abundance of lichens. At the spots where stems of Loranthus spring from an
oak-branch they are always surrounded by a great rampart of wood belonging to
the oak, and the base of the stem is often fixed in a deep symmetrically-rounded
bowl reminding one vividly of the similar structures out of which the stems of
Balanophorew arise. But whereas in Balanophorew this bowl-shaped rampart
appertains to the parasite, in Loranthus it is formed from the wood of the host-
plant, i.e. the oak. It must, in the case we are considering, be interpreted as an
exuberant growth of wood-cells and compared to the hypertrophies called galls,

GRAFTING AND BUDDING. 213

which will be treated of in detail in a subsequent part of this book. On old oaks
in the east of Europe these growths round the bases of Loranthus-plants sometimes
reach the size of a man’s head. In the case of a bush of Loranthus nearly 100 years
old, from the Ernstbrunner Wald, in Lower Austria, which had reached a height of
1:2 m. and a circumference of 5°5 m., the hypertrophy in question measured 70 cm.
round. It is not only the base of a bush that is overgrown by wood-cells, but the
older portions of the roots described above are frequently walled in and partially
inclosed by the wood of the branch as it becomes thicker. They may often be
seen fixed deep in the wood, yet still preserving their freshness and vitality, and
this is to be explained by the fact that they retain connection with other parts of
the roots by means of isolated ledges and bridges. Indeed an adventitious shoot
may develop from a piece of a root thus deeply wedged in the wood of the oak,
and this shoot then grows so outwards and breaks through all the layers lying above
it and originates a young bush, which pushes roots under the host’s bark and
afterwards behaves in exactly the same manner as a plant produced from a seed
cemented to the oak-branch.

The Loranthus chosen here for description (L. Europeus) has only small
inconspicuous yellowish flowers; on the other hand, under the tropical sun of
Africa, Asia, and, above all, Central America, the parasitic species of this genus are
amongst the most splendid-flowered of plants. There are species in the tropics—
eg. Loranthus formosus, L. grandiflorus, and L. Mutisii—whose flowers attain a
diameter of 10, 15, or even 20 centimeters, and are besides clothed in the most
gorgeous purple and orange colours. Many Loranthi are like small trees grafted
upon other trees. The host-plants of these Loranthi are principally angiospermous
trees; members of the genus have also repeatedly been met with parasitic upon one
another—as, for instance, Loranthus buwifolius upon L. tetrandrus in Chili. The
fact has been already mentioned that the European Mistletoe has been observed near
Verona parasitic upon Loranthus. It is also worth noticing, in order to complete
the account of the complex relationships between parasites, that one species of
Viscum has been found in India parasitie upon another, viz..— Visewm moniliforme
on V. orventale.

GRAFTING AND BUDDING.

Parasitism of one woody plant upon another, such as occurs in the case of
Loranthacex, calls to mind certain modes of organic union between woody plants
that are artificially effected by gardeners. From ancient times gardeners have
performed special operations which are known as processes of “ennobling”, and
consist in the transference of the branch or bud of one plant on to another plant as
substratum, and the inducement of organic union between the two. The plant from
which the branch or bud is taken is perhaps a valuable variety of fruit-tree, or a
handsome specimen of an ornamental shrub, whilst for the purpose of a substratum
a robustly-growing individual belonging to a wild species of shrub or tree is selected

214 GRAFTING AND BUDDING.

as a rule, and constitutes the so-called wild “stock”. The branch which yields the
bud for the operation or which is itself transferred in its entirety to the wild stock
is named, in the terminology of horticulture, the noble “scion”.

The process of ennobling is effected either by grafting or by budding. In
grafting the stem of the stock is cut off transversely, an excision is made at the
periphery of the surface of the section and the scion is inserted in this opening.
The scion must be previously trimmed to fit; in preparing it care must be taken
that it bears a pair of healthy buds, and that the end to be inserted is cut so as to
correspond to the form of the fissure made in the stock. In inserting it one must
see that, as far as possible, the bark, bast, and wood of the one come into contact
with the corresponding parts of the other. The wounds of the stock caused by the
operation are then covered by a mass of putty, wax, or some other protective
medium, and the chances are that the branch thus introduced will contract an
organic union with the substratum, that nutritive matter will be supplied it by the
substratum, and that new branches will sprout from its buds. In this case there-
fore the nutriment taken from the ground by the stock passes into the grafted
scion, and the scion, which develops branches from its buds, and ultimately may
become a densely ramifying tree-top, behaves as a parasite, whilst the stock plays
the part of host.

It not infrequently happens that a substratum supporting at its summit the
branches of a grafted scion develops subsequently branches of its own lower down
as well, and the curious sight is then afforded of a tree or shrub bearing different
foliage, flowers, and fruit on its inferior parts from those of its upper regions. If,
for example, the stem of a Quince is used as substratum, and Medlar branches are
grafted upon it, the result may be a bush or tree which exhibits below branches
with the round leaves, rose-coloured flowers, and golden “pomes” of the Quince,
and above branches with the oblong leaves, white flowers, and brown fruit of the
Medlar. Gardeners, of course, do not willingly allow this to happen, but carefully
remove the branches belonging to the stock in order that all the food materials
may fall to the lot of the grafted plant, and the latter thrive as vigorously and
luxuriantly as possible.

The same result is obtained by budding as by grafting; but here a single bud of
the scion, instead of an entire branch, is transferred to the stock. This is accom-
plished in the following manner:—Two incisions at right angles forming a T,
are made in a branch of not too great age belonging to the plant employed as
substratum. These cuts are carried through the bark as far as the wood. The two
lobes of bark, formed by the T-shaped incision, are then carefully raised from the
wood, and the bud to be transplanted is pushed in under them. The bud which has
previously been taken away from the scion must have retained in that process a
portion of bark, and usually the bit of bark peeled off is given the shape of a little
shield. This shield, carrying the bud that is to be transferred upon it, is now
introduced between the two lobes above mentioned, and the lobes are folded over
it in such a manner as to allow the bud to project freely from the slit between the

GRAFTING AND BUDDING. 215

lobes. Besides this, the whole is held together by a bandage, the shield in particular
with its bud being pressed firmly on to the new substratum, and thereupon, as a
rule, coalescence takes place at once, and the inserted bud grows out into a branch
which stands in exactly the same relation to the stock as a Loranthus to the oak
whereon it is parasitic. All the branches belonging to the substratum, that is to
say, to the wild stock, may then be removed, leaving only the one branch that has
sprung from the stranger-bud, the result being that all the juices absorbed from the
ground by the substratum are concentrated in this branch and cause it to grow
with the greatest exuberance.

There is between this process of budding and the settling of a parasite a further
resemblance in that shrubs and trees cannot all be made to unite at pleasure one
with the other. A successful result of grafting or budding can only be counted
upon when nearly allied species, belonging to the same genus or family, are
employed for the purpose. Almonds, peaches, apricots, and plums can be grafted
the one upon the other; so also can quinces, apples, pears, medlars, and white-
thorns. But we must relegate to the realms of fiction such assertions as that
peaches might be successfully grafted upon willow stocks, or that the Siberian Crab
(Pyrus salicifolia) has sprung from the grafting of branches of the Pear upon the
Willow and other tales of the sort. Whether it is possible by grafting or budding
to produce new forms, or at least hybrids, is a question which will claim our
attention in connection with the problem of the origin of new species. The only
additional remark to be made here is that notwithstanding the undeniable simi-
larity between grafted or budded plants and the parasitic Loranthacee, a very
essential difference exists in the circumstance that the latter develops roots which
continue to grow year by year, and are always penetrating into new layers of the
host’s tissues, whereas this is never observed in the case of grafted or budded
plants. When the branch of a Peach is grafted on an Almond-tree, there is, it is
true, an organic union of the two at the place of contact, and the juices from the
wood of the Almond stock are conducted direct into the grafted Peach-branch; but
neither roots nor sinkers ever arise from the base of the adnate branch or penetrate
into the stem of the Almond-tree.

216 IMPORTANCE OF WATER TO THE LIFE OF A PLANT.

5. ABSORPTION OF WATER.

Importance of water to the life of a plant—Absorption of water by Lichens and Mosses, and by
Epiphytes furnished with aérial roots—Absorption of rain and dew by foliage-leaves—Develop-
ment of absorptive cells in special cavities and grooves in the leaves.

IMPORTANCE OF WATER TO THE LIFE OF A PLANT.

In the building up of the molecules of sugar, starch, cellulose, fats, and acids, of
proteids, and, in short, of all the important substances of which a plant is composed,
atoms of water have to be incorporated as constructive material, and without water
no growth or addition to the mass of a plant whatsoever could take place. From
this point of view water must be considered just as indispensable an item in the food
of plants as the carbon-dioxide of the air. But water plays, in addition, another
important part in plant-life. The mineral salts which serve to nourish hydro-
phytes, land-plants, and lithophytes, as also the organic compounds which are the
food of saprophytes and parasites, can only reach the interior of plants in the form
of aqueous solutions. They can only pass through a cell-wall when it is saturated
with water, and, having reached the interior of a plant, they can only be conveyed
to the places where they are worked up through the medium of water. In con-
nection with the discharge of these functions in a living plant, water must be
regarded as a dynamic agent. Just as a mill on a stream only works so long as its
wheels are kept in motion by the water, and stops at once if the latter fails, or flows
by in insufficient quantity, so the living plant, as it nourishes itself, grows and
multiplies, needs a continuous and abundant supply of available water to render
possible the performance of the complicated vital processes within it. This avail-
able or organizing water is not in chemical combination like that which is present
as food-material, and is, in general, not permanently retained. On the contrary, we
must conceive it as perpetually streaming through the living plant. In the course
of a summer, quantities of water, weighing many times as much as the plant itself,
pass through it. The total amount of water in chemical combination in the organic
compounds of a plant is very trifling compared with this, though it often happens
that the weight of the latter in a particular plant is greater than that of all the
other substances put together.

Inasmuch as this water evaporates from plants in dry air, and that it may also
easily be withdrawn by alcohol or other means, very simple experiments suffice to
give an idea of the great bulk of free water in any plant. Berries, fleshy fungi,
succulent leaves, and things of that kind, if left in alcohol, are reduced in a short
time to barely half their size in the fresh state. The Nostocinex, which are gela-
tinous when alive, and many fungi (e.g. Guepinia, Phallus, Spathularia, Dacryo-
myces) shrivel us so stringently in drying, that a piece possessing an area of
1 square centimeter when fresh leaves only a dry crumbling mass covering scarcely
3 square millimeters. A Nostoc, which weighed 2224 grms. in the fresh state only

ABSORPTION OF WATER BY LICHENS AND MOSSES. 217

weighed 0°126 grm. after desiccation, so that when alive it must have contained
94 per cent. of water. Bog-moss, weighing 25°067 grms. before the abstraction of
the water was reduced to 2535 grms. afterwards, showing that the percentage of
water was 90. Similar results are obtained in the cases of succulent leaves and
stems of flowering plants, Cucurbita, and other fruits. The least proportion of
water is contained by mature seeds, solid stony seed-coats, wood, and bark; but even
in these an average proportion of 10 per cent of water has been detected. We shall
not go wrong in assuming, on the evidence of the weights determined, that most parts
of plants, when fresh, consist of dry substance only as regards a third, and as
regards two-thirds, of water of imbibition, which passes over into the surrounding
air in the form of vapour when desiccation takes place.

From all this it follows that water is absolutely necessary to plants as food-
material, that it is indispensable as a medium of transport of other substances, and
that the demand for water on the part of all plants is very great. Further, we may
infer that the importation and exportation of water must be regulated with exacti-
tude if the nutrition is not to be disturbed and development hindered.

Water-absorption is at its simplest in hydrophytes. In this case it coincides
with the absorption of the rest of the food-materials, and there is therefore nothing
material to add to the statements already made on that subject.

As regards land-plants, lithophytes, and epiphytes, we may likewise refer to
what has been already said in so far as these plants suck up water at the same time
as food-salts, by means of absorption-cells, from the substratum to which they are
attached, or the earth in which they are rooted; but to the extent that they take
also water direct from the atmosphere, and have the power of absorbing that water
immediately they require it, must be discussed in the following pages.

ABSORPTION OF WATER BY LICHENS AND MOSSES, AND BY
EPIPHYTES FURNISHED WITH AERIAL ROOTS.

The plants which absorb water direct from the atmosphere may be classified in
several groups with reference to the contrivances adapted to the purpose. Of all
plants lichens are most dependent on atmospheric moisture. Many of them,
especially the Old Man’s Beard Lichens, which hang down from dried branches of
trees, and the gelatinous, crustaceous, and fruticose lichens, which cling to dead
wood, and on the surface of rocks and blocks of stone, do in fact derive their
necessary supply of water entirely from the atmosphere, and that by absorbing it,
not in a liquid but in a gaseous form. The latter circumstance is of the greatest
importance to those species in particular which occur on receding rocks, or on the
under face of overhanging slabs of stone. Rain and dew cannnot reach such places
directly, but only by some of the water trickling down from the wet top and sides
of the rocks on to the receding wall, and this happens but seldom. Accordingly,
lichens occurring in situations of the kind are entirely dependent upon the water
contained in the air in the form of vapour. Lichens, however, are also, of all plants,

218 ABSORPTION OF WATER BY LICHENS AND MOSSES.

the best adapted for the absorption of aqueous vapour from the air. If living
lichens, which have become dry in the air, are left in a place saturated with mois-
ture, they take up 35 per cent of water in two days, and as much as 56 per cent
in six days. Water in the liquid form is naturally absorbed much more rapidly
still, When Gyrophoras, which project in the form of cups after a long continuance
of dry weather, are moistened by a fall of rain, they swell up completely within
ten minutes, and spread themselves flat upon the rocks, having in that short
space of time absorbed 50 per cent of water. The saying, “ Light come, light go,”
is no doubt true in these cases. When dry weather sets in, evaporation from the
masses of lichens goes on at a pace corresponding to the previous absorption. In
the Tundra, the lichens, which form a soft tumid carpet when moistened by rain,
are liable to be so powerfully desiccated in the course of a few hours of sunshine,
that they split and crackle under one’s feet, so that every step is accompanied by a
crunching noise.

In the power of condensing and absorbing the aqueous vapour of the atmos-
phere, lichens are most analogous to mosses and liverworts, and to those pre-
eminently which live on the bark of dry branches of trees or on surfaces of rock,
covering places of the kind with a carpet which is often enough interspersed and
interwoven with lichens. Like the latter these mosses and liverworts are able to
remain as though dead in a state of desiccation for weeks together, but as soon as
rain or dew falls upon them they resume their vitality; and similarly if the air is so
damp as to enable them to derive sufficient water of imbibition from that source.
A specimen of Hypnwm molluscum, a moss which covers blocks of limestone in the
form of soft sods, was after a few rainless days detached from the dry rock and
placed in a chamber saturated with vapour, and it was found that after two days
it had absorbed water from the air to the extent of 20 per cent, after six days 38
per cent, and after ten days 44 per cent. Many mosses condense and absorb water
with the whole surfaces of their leaflets, others—as, for example, the gray rock-
mosses clinging to slate formations (Rhacomitrie and Grimmis)—do so especially
with the long hair-like cells at the apices of the leaflets, whilst others again only
use the cells situated on the upper saucer-shaped or canaliculate leaf-surface.

In some bearded mosses (Barbula aloides, B. rigida, and B. ambigua) chains of
barrel-shaped cells occur closely packed together upon the upper surface of the leaf
and at right angles to it, which to the naked eye have the appearance of a spongy
dark-green pad. The terminal cells of these short moniliform chains have their
upturned walls strongly thickened, but the other cells have very thin walls and
take up water rapidly. It is the same with the various species of Polytrichwm,
which are provided on their upper leaf-surfaces with parallel longitudinal ridges
likewise composed of thin-walled, highly-absorbent cells. The rhizoids also play
an important part in these processes. These brown, elongated, thin-walled cells
entirely clothe the moss stems, usually in the form of a dense felt, and often pro-
ject from the under surface of the leaves, whilst in a few tropical species they make
their appearance, strangely enough, in the form of little tufts at the apices of the

ABSORPTION OF WATER BY LICHENS AND MOSSES. 219

leaflets. In many instances this felt of rhizoids does not come into contact at all
with the soil, rock, or bark (as the case may be), but is surrounded by air alone,
and is able to condense or attract, to use a common expression, the aqueous vapour
of the air like a piece of cloth or blotting-paper. In dry weather, it is true, mosses,
like lichens, lose their water, but they part with it much more slowly than the
latter. This is chiefly due to the fact that the moss-leaflets at the commencement
of a drought wrinkle, curl up, become concave, and lay themselves one above the
other, so that the water is retained at the bottom for a longer period.

A very remarkable contrivance for the absorption of water from the atmos-
phere is also exhibited by the white-leaved Fork-mosses (Leucobrywm) and Bog-
mosses (Sphagnacex). Although they possess chlorophyll, and assimilate under the

ne G yi
CeO
:

Fig. 49.—Porous Cells.
10f the white-leaved Fork-moss (Leucobryum); x550. 2 Of the Bog-moss (Sphagnum); x230. 8 Of the root of an Orchid

(Lelia gracilis); x310.
influence of sunlight, yet they look like parasitic and saprophytic plants destitute
of chlorophyll. They are of a whitish colour and always grow in great cushion-
like sods, so that the spots where they grow are deficient in verdure, and stand
out conspicuously from their surroundings in consequence of their pale tint.
Microscopic investigation at once explains this appearance. The cells containing
chlorophyll and living active protoplasts are relatively small, and, as it were,
wedged and hidden between other cells many times as great, which have entirely
lost their protoplasm by the time they are mature, and then cause the paleness of
colour appertaining to the plant as a whole. The walls of these large colourless cells
are very thin, and in the Bog-mosses have spiral thickening-bands running round
them, being thus secured against, collapse. After remaining for a time in a dry
environment they are full of air only; but the moment they are moistened they
fill with water. If there were an actively absorbent protoplast at work in the
interior, the water would be able to pass into the cell-cavity through this easily
moistened wall, as in the case of other mosses, owing to the delicacy of the cell-
membrane. But the air which fills the cells is not absorptive, and in the case of
Lewcobryum and Bog-mosses the water reaches the interior, not in consequence of

220 ABSORPTION OF WATER BY LICHENS AND MOSSES.

a chemical affinity on the part of the cell-contents, but solely by capillary action.
All the cell-walls are perforated and furnished with pores, and through these the
water rushes into the interior with lightning rapidity.

This extremely rapid influx of water into an air-filled cavity leads us necessarily
to the conclusion that each cell has a number of pores in its walls, and that in
proportion as water enters through one of the small apertures the air can escape
equally fast through another. This is in fact the case. The large cells not only
have pores on their external walls, but communicate one with another by similar
holes, and the water soaks in from the one side as it does into a bath-sponge, whilst
the air is at the same time forced out on the other. This absorptive apparatus is
exceptionally elegant in Lewcobrywm, which grows abundantly in many woods.
In it, as is shown in the illustration above (fig. 491), the adjacent prismatic cells
communicate by highly symmetrical, circular gaps made in the middle of the
partition-walls, whilst in the Bog-mosses (the various species of Sphagnwm), they
are to be seen scattered here and there between the thickening bands on the cell-
walls (see fig. 497). Now these porous groups of cells possess not only the power
of taking up water in the liquid state, but also that of condensing it when in the
form of vapour. There is no need of any more proximate proof of the fact that
the cells previously mentioned as containing chlorophyll, and lying imbedded
between the large perforated cells, take up water supplied by the latter, or
perhaps it is better to say that the large perforated cells suck in the water for
the living green cells. We have only to ask why it is, then, that these small green
cells do not absorb water themselves direct from the environment, as is done in
the case of so many other mosses and liverworts. It is difficult to answer this
quite satisfactorily, but thus much seems certain, that the large porous cells, when
full of air, afford a means of protecting the small living cells from too excessive
desiccation, and that they are in addition preservative of the chlorophyll in the
small cells, a matter to which we shall return presently.

A certain resemblance to these Leucobryums and Sphagnums, in respect of water-
absorption, is exhibited by a few Aroidex, and more especially by a whole host of
Orchidaceee. Of the 8000 different orchids hitherto discovered, a good proportion,
it is true, are rooted in the earth. But more than half these wonderful plants
flourish only on the bark of old trees, and most of them would quickly perish if
they were detached from that substratum and planted with their roots buried in
earth. A double function appertains to the roots of these Orchides which inhabit
trees. On the one hand they have to fix the entire orchid-plant to the bark, and,
on the other, to supply it with nutriment. When the growing tip of an orchid’s
root comes into contact with a solid body, it adheres closely to it, flatiens out more
or less, sometimes even becoming strap-shaped (see fig. 15), and develops papilli-
form or tubular cells, which grow into organic union with the substratum, and
might conveniently be termed clamp-cells. In many cases these cells creep over
the bark, divide, interlace, and form regular wefts. The organic connection with
the substratum is so intimate that an attempt to separate the two usually results

ABSORPTION OF WATER BY EPIPHYTES. 221

in a detachment of the most superficial parts of the bark, but not of the tubular
cells. Now, if a root, after having sent out cells of this kind which contract an
organic union with the substratum, reaches into the open, beyond the limit of the

Ty

Fig. 50.—Aérial Roots of an Orchid epiphytic upon the bark of the branch of a tree.

substratum, it immediately ceases to develop clamp-cells, loses its ligulate shape,
and hangs down from the tree in the form of a sinuous white filament. A few
root-fibres are as a rule sufficient to fix the plant to its substratum, the bark of the
tree, and the rest of the roots put forth by the orchid grow from beginning to end

222 ABSORPTION OF WATER BY EPIPHYTES.

freely in the air. They are not infrequently to be seen crowded together in great
numbers at the base of the plant, forming regular tassels suspended from the dark
bark of the branches as may be seen in fig. 50, where an Oncidiwm is represented.

Each of these aérial roots is invested externally by a white membranous or
papery envelope, and it is the cells of this covering that own the resemblance, above
referred to, to the cells of Lewcobrywm and Bog-mosses. Their walls are furnished
with narrow, projecting spiral thickenings and therefore do not collapse, notwith-
standing their delicacy or the circumstance of their inclosing at times an air-filled
cavity; they are further abundantly perforated, two kinds of apertures indeed
being found. The one variety arises in consequence of the tearing of the portions
of the cell-wall situated between the rib-like projections and consisting of extremely
thin and delicate membranes (see fig. 49°); the existence of the other variety is due
to the detachment of the cells which protrude in the form of papille, the result
being, in this latter case, the formation of circular holes very similar to those
already described as occurring in Leucobryum. The cells resembling papille have
the peculiarity that they roll off when they get old in the form of spiral bands.
The holes, of course, can only occur on the external walls of the outermost cells
which border upon the open air, whilst in the interior the communication between
the cells themselves is established by means of the rents previously referred to.
The entire covering thus composed of perforated cells may be compared to an
ordinary sponge, and, indeed, acts after the manner of a sponge. When it comes
into contact with water in the liquid state, or more especially when it is moistened
by atmospheric deposits, it imbibes instantaneously its fill of water. The deeper-
lying living green cells of the root are then surrounded by a fluid envelope and are
able to obtain from it as much water as they require.

But these roots also possess the power of condensing the aqueous vapour
contained in the air. They act upon the moist air in which they are immersed in
exactly the same way as spongy platinum or any other porous body. If the aérial
roots of Oncidiwm sphacelatwm are transferred from a chamber full of dry air to
one full of moist air, they take up in 24 hours somewhat more than 8 per cent of
their weight of water, those of Epidendron elongatum absorb 11 per cent, whilst in
the case of many other tropical orchids the amount thus imbibed is doubtless much
more considerable still.

The power of condensing aqueous vapour, and other gases as well, is of the
greatest importance to these plants. The tree-bark serving as their substratum, to
which they are fastened merely by a few fibres, is anything but a permanent
source of water. Such water as the bark does contain reaches it, not from the
interior of the trunk and indirectly from the soil in which the trunk has its roots,
but from the atmosphere; that is to say, from the very source whence the
epiphytes upon the bark must also derive their supply. Now, when on the occa-
sion of a long-enduring uniform aérial temperature, there is a failure of atmos-
pheric deposits, which is a regularly recurring circumstance in the habitat of the
orchids in question, the sole source of water left is the vapour in the air, and the

PLATE Ill,

1. Saccolobium guttatum. 2. Dendrobium nobile. 3. Phajus Wallichit.

PLATE Tif,

TROPICAL’ EPIPHYTES IN CEYLON.

Printed from the originals by the BIBLIOGRAPHISCHES INSTITUT, Leipzig.

ABSORPTION OF WATER BY EPIPHYTES. 223

only possible method of acquiring that vapour is the condensation of it by the
porous tissue investing the roots. In the event of the air around the orchid-plant
containing temporarily but very little moisture, the porous tissue dries up, it is true,
very quickly; its cells fill with air and their function as condensers is interrupted.
But these air-filled cellular layers then form a medium of protection against
excessive evaporation from the deeper strata of the root’s tissues, which might be
very dangerous in the case of this kind of epiphyte. There is a wide-spread
impression that the tropical orchids grow in a perpetually moist atmosphere in the
dark shade of primeval forests, and this preconception is fostered by pictures of
tropical orchids representing these plants as living in the most obscure depths of
woods. In reality, however, the orchids of the tropics are children of light. They
thrive best in sunny spots in open country. Those species in particular which have
their aérial roots invested each by a thick, white, papery, porous covering belong to
regions where a long period of drought occurs regularly every year, and where, in
consequence, vegetative activity is subject to periodical interruption, as it is in the
cold winter season of the more inclement zones.

For epiphytes inhabiting these regions of the tropics a more expedient structure
of root cannot easily be imagined. In the dry season the papery covering reinforces
the safeguards against too profuse transpiration on the part of the living cells in the
interior of the root, and in the wet season it provides for the continuous supply of
the requisite quantity of water. In this sense the porous layer is to a certain
extent a substitute for wet soil, or, in other words, the concealment of the living
part of an aérial root in the saturated envelope is analogous to that of the root-
fibres of land-plants in the damp earth. The manner in which the water reaches
the inner cells of an aérial root from the saturated envelope is also quite
characteristic. Under the porous tissue lies a layer composed of two kinds of cells
of different sizes. The larger cells are elongated and have their external walls,
which are adjacent to the porous tissue, thickened and hardly permeable by water.
Between these lie smaller, thin-walled, succulent cells, which admit the water from
the porous envelope, and should therefore be regarded as absorption-cells. It is also
noteworthy that the porous, paper-like covering is discarded as soon as an aérial
root is placed in earth. Most orchids with aérial roots perish, it is true, when they
are treated like land-plants and planted in soil; but a few species, on occasion, bury
their aérial roots spontaneously in the earth and push off their envelopes, and then
. the imbedded parts exercise the same functions as in the case of land-plants.

We have already mentioned that, in addition to thousands of orchids, several
Aroidex exhibit the porous, papery covering on their aérial roots. But still more
frequently the air-roots of Aroids, which live as epiphytes upon trees, are furnished
with a dense fringe of so-called root-hairs in a broad zone behind the growing-
point. The hairs project on all sides from the roots, which are surrounded by air;
they are crowded very closely together and give the parts affected a velvety.
appearance. Besides several Aroides, one of which (Philodendron Lindeni) is
drawn on the left side of fig. 51, many other epiphytes, such as the South

ABSORPTION OF WATER BY EPIPHYTES.

224

lynacesx, represented on the
-ferns, display this velvety

Y

American Campelia Zanonia, belonging to the Comme
right side of the same figure, and also several tree

on the left Philodendron Lindeni, on the right Campelia Zanonia.

Fig. 51.—Aérial Roots with root-hairs ;

ferns are short, but spring in

The roots of the tree
thousands from the thick stem, and are so closely packed that the whole surface is

clothed as it were by a woven mantle of rootlets.

coating on their aerial roots.

After some time these aérial

roots turn deep brown, whilst the hairs collapse and die, and both are converted

ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES. 225

into a mouldering mass. But as soon as they perish other new air-roots, covered
with golden-brown velvet, make their appearance and take their place. These aérial
roots never reach the ground or adhere to any substratum, so that their hairs
cannot contract an organic connection with a solid body. It is consequently also
impossible in this case for the root-hairs to draw moisture from the soil in the
capacity of absorption-cells.

These root-hairs, however, are scarcely ever in a position to take up even the
atmospheric deposits. The various species of Philodendron and the other epiphytes.
referred to, have large leaves which cover the air-roots hanging from the stem like
umbrellas, and every tree-fern also bears at the top of its stem a tuft of great
fronds, which prevents falling rain from wetting the aérial roots. Moreover, the
very plants whose air-roots exhibit a velvety coating occur in woods where the
tops of the trees arch over the ground in lofty domes, and form a sheltering roof
against deposits from the atmosphere. On the other hand, the air within these
forests is saturated with aqueous vapour, and it is certain that the velvety roots
have the power of condensing vapour, and that the root-hairs instantly suck up the
condensed water and convey it to the deeper-lying layers of cells. The truth of
this has been established by the results of repeated experiments. Thus, air-roots of
the tree-fern Todea barbata, after being transferred from moderately damp air
into a chamber full of vapour, condensed and absorbed in the space of twenty-four
hours water amounting to 6'4 per cent of their weight. There is, therefore, no doubt
that water may be acquired in this way also by plants, even though the instances
may not be very numerous. All plants in which this kind of water-absorption has
been hitherto observed grow in places where the air is very moist the whole year
round, and where there is also no risk of the temperature falling below freezing-
point. Under other conditions, especially in places where the air is periodically
very dry, these plants would not be able to survive; for, although they possess
organs for the condensation and absorption of water, they have no means of protec-
tion against the desiccation of these organs.

ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES.

The idea that plants absorb with their roots such water as they require is so.
intimately associated with our whole conception of plant-life, that this process is
commonly adduced for the purpose of analogies of the most various kinds, and one
looks upon the water-absorption effected by aérial roots in the manner just described
really as a thing to be expected, notwithstanding the fact that in this case, as the
above account shows, the phenomenon is not so simple as is usually supposed. We
now turn to the consideration of land-plants. If the leaves of plants cultivated in
pots become flaccid, water is poured as quickly as possible upon the dry soil with a
view of supplying the roots which ramify in it with moisture. Nor does the result
fail to be produced. In a short time the foliage becomes fresh and elastic again,

the roots having discharged their function. Even in the open air, it is especially
Vot. I. 15

226 ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES

the soil in which the roots are imbedded that a gardener waters on dry days,
although incidentally he may pour the water over the aérial parts of the plants.
He sees, however, that the water which falls in the form of rain or dew upon the
foliage and stems normally runs off them at once, or else collects in drops, which
trickle down whenever the plant is shaken by the wind, and are sucked up by the
thirsty ground. This phenomenon must be due to the possession by the leaves of
special contrivances to prevent their being wetted. It does not in any case support
the idea that foliage is as well adapted for the absorption of water as experience
has proved subterranean roots to be. This train of thought, which forces itself
upon every unbiassed observer of the processes as they take place in nature, is
certainly warranted in the majority of cases. Each absorption-cell on the roots
buried in the earth has an easily permeable membrane, and, as is well known, water
passes from damp earth through the cell-membranes into the interior of a plant
with great rapidity. The water in the interior of the plant would be equally easily
withdrawn through these cell-membranes by dry surroundings, but, as it is, this
scarcely ever happens, in consequence of the roots being situated underground. In
the case of aérial parts, especially the foliage-leaves, the circumstances are quite
different. The leaves have to yield up to the air a portion at least of the water
conducted from the roots, because, as will be more thoroughly explained later on, it
is only by means of this evaporation that the entire machinery in the interior of the
plant can be kept in motion. But this evaporation must not go too far; it must be
in proper relation to the absorption of water by the subterranean roots, and be
regulated to that end if the plant is not to run the risk of drying up altogether at
times—an occurrence which flowering plants are unable to survive, although the
mosses described in former pages have that power. Accordingly, in the case of the
foliage-leaves of flowering plants, evaporation is confined to certain cells and groups
of cells, and these, in addition, have contrivances by means of which evaporation
can be entirely stopped on occasion of great drought. It stands to reason that all
contrivances which make it impossible for water to pass from the interior of the
leaves through the walls of the superficial cells into the surrounding air also hinder
the entrance of water into the leaves from the atmosphere.

It would be altogether inconsistent with the system of arrangement of the sub-
ject adopted in this book if we were to discuss here all the contrivances serving to
regulate the exhalation of water by leaves, and we must, therefore, confine ourselves
to referring, by way of introduction, quite briefly, to the following facts, namely,
that those pores on the surface of leaves which are known by the name of stomata,
and are used as doors of egress by the exhaled water, do not admit rain or dew, or in
general, any water in the liquid state; that the so-called cuticle covering the exter-
nal walls of the epidermal cells in leaves is an additional barrier to both egress and
ingress of water; that when, in particular, this cuticle is furnished with a wax-like
coating, water does not adhere to the surface of cells so protected; and, lastly, that
atmospheric moisture can only penetrate into the interior of the plant at parts of
the leaves where the waxen incrustations are absent, where water remains adherent

ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES. 227

to the leaf-surfaces, and they are distinctly wetted. But even cells and groups of
cells of this kind usually act but for a short time as absorption-cells, and only when
the necessity and craving for water is very great, or when there is an opportunity
of acquiring nitrogenous compounds at the same time as the water; and here,
again, special contrivances are always present which regulate this kind of water-
absorption, and render it impossible whenever it is not truly advantageous.

At first one would suppose that amongst the cells composing the epidermis of
foliage-leaves, those are best adapted to the absorption of water from the atmosphere
which take the form of hairs. The superficial area being as great as possible, and
the contained matter relatively little, one can scarcely in fact conceive a conforma-
tion better suited to the purpose of water-absorption. As, moreover, the area of
contact between the cells of the leaf and of a hair is small, there would afterwards
be but very little evaporation through the surface of the hair, of the water once
sucked up by it and conducted into the interior of the leaf. In a word, these hairs
on the surface of a leaf appear to be peculiarly adapted to the taking up of water, and
not at all favourable to its exhalation. The hypothesis based on these observations
is indeed entirely applicable to the case of hairs occurring on the leaflets of mosses,
as has been already stated. But it does not hold in the case of the hair-like struc-
tures which spring from the leaf-surfaces of flowering plants. These are frequently
not wetted at all by water; rain and dew roll off them in drops, and cannot, there-
fore, be absorbed by them. This is true even of many soft trichomes (hair-structures)
which form investments upon leaves, and which seem to be more than any fitted
for the absorption of water. For instance, experiments upon the woolly leaves of
the Great Mullein (Verbascum Thapsus) have shown that they neither condense
aqueous vapour nor take up water in liquid drops. Small importance must be
attributed to the thickness of the cuticle, for sometimes it is the very cells which
are equipped with a cuticle of considerable stoutness that are adapted to admit
water, under certain circumstances, through their walls. On the other hand, much
depends upon the presence of wax in the cuticle and upon the contents of the cells;
that is to say, upon whether those contents in particular have a strong or weak
affinity for water. If the cells of the hairs are full of air they are not adapted to
the absorption of water.

If a hair is septate, z.e. consists of a simple series of cells, only the undermost or
else only the uppermost cells of the series absorb water. Instances wherein it has
been observed that the lowest cells alone in hairs of the kind become absorption-
cells are afforded by the Alfredia, represented in fig. 14, by Salvia argentea, and
several other steppe-plants. The same statement is made concerning the widely-
distributed Stellaria media, the common Chickweed. This last has hairs on the
internodes of the stem, running down in ridges from node to node. Usually only
one side of the stem exhibits a ridge of hairs of the kind, and the ridge always
terminates at the thickened node, whence springs a pair of opposite leaves. The
stalks of these leaves are somewhat hollowed out and have their edges beset with
hairs like lashes. The hairy ridges on the segments of the stem are readily wetted

228 ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES.

by rain and retain a considerable quantity of water. The water that they cannot
hold they conduct downwards to the ciliate axils of the next lower pair of leaves, ©
where it is drawn through the lash-like hairs in due course and collected into a
ring of water surrounding the node (see fig. 52°). If this accumulation of water
becomes so voluminous and heavy that it cannot any longer be retained by the
fringe of lashes, the surplus glides on to the unilateral ridge of hairs on the adjacent
internode down to the pair of leaves below. Accordingly, after a shower every
node from which leaves arise is seen to be inclosed in a water-bath, and the hairy

Fig. 52.—Hairs and Leaves which retain Dew and Rain.
1 Dwarf Gentian (Gentiana acaulis). 2Lady’s Mantle (Alchemilla vulgaris). 3 Chickweed (Stellaria media).

ridges also are so soaked with water that they look like edgings of glass. All the
individual cells in each of the hairs are full of protoplasm and cell-sap, but only the
lowest, which are very short, really act as absorption-cells. When these cells
become at all relaxed in dry air, the fact is indicated by the appearance on the
external cell-wall of fine striz (see fig. 53 and 532). The protoplasts inhabiting them
attract water, and after being relaxed in the manner referred to the cells regain
their turgidity on being wetted, whilst the fine wrinkles on the outer membrane are
in consequence immediately smoothed out. Although the upper cells of the hair
possess a less thick cuticle, they, on the other hand, seem not to absorb any water,
but to serve rather to conduct it by their surfaces.

This case is, as we have said, comparatively rare, and the corresponding absorp-

ABSORPTION OF RAIN AND DEW BY THE FOLIAGE-LEAVES. 229

tion of water is not very considerable. But it often happens that the uppermost
cells of a septate hair are developed into absorption-cells. The terminal cell is then
usually spherical or ellipsoidal and larger than the rest, or else this cell is divided
into two, four, or a greater number of cells, which together form a little head, whilst
the lower cells constitute a stalk supporting it (see fig. 533 and 53%). In botanical
terminology structures of this kind are named capitate or glandular hairs. The
protoplasm in the cells of the head is, for the most part, of a dark colour, and the

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Fig. 53.—1 Cauline hairs of Stellaria media; x110. 2 Lowest cells of the same hairs; x200. 8 Capitate hairs of
Centaurea Balsamita; x150. 4Capitate hairs of Pelargonium lividum; x 150.

cell-membranes are readily permeable by water, which is attracted with great
energy by the cell-contents. The cell-membrane is often very thick, it is true, but
as soon as water comes into contact with it the outer layer is discarded, the inner
layers swell up and the water passes through these swollen layers into the interior
of the cell. This happens, for instance, in many pelargoniums and geraniums,
wherein the capitate cells go through a process of excoriation on every occasion of
the imbibition of water (see fig. 53+). In other plants the walls of the capitate
cells-are everywhere thin, and not only do the cell-contents consist of a viscid gum-
like mass, but the external surface of the wall is also covered by a layer of viscid
excretion. In many cases the viscid matter excreted by the glands spreads over the
entire surface of the leaf, so that the latter feels sticky and looks as if it were

230 ABSORPTION-CELLS ON LEAVES.

coated with varnish. Many plants which have their roots buried m crevices of
rock and no small number of herbaceous steppe-plants are quite thickly covered
with glandular hairs of the kind. Centawrea Balsamita (see fig. 53°), a plant
occurring on the elevated steppes of Persia, may be selected as an example of the
latter group. The advantage of the structure of capitate hairs is not far to seek.
In dry weather the thick cuticle (Pelargonium) or the varnish coating (Centawrea
Balsamita), as the case may be, prevents desiccation of the cells and groups of cells
in question. But as soon as rain or dew falls, the cuticle and the coat of varnish
take up water, and it is by their instrumentality that water reaches the interior of
the cells. Thus, whilst the exhalation of water is hindered, its absorption is not.

Other epidermal cells of foliage-leaves besides trichomes are capable of acting as
absorption-cells, although this action, for reasons already given, is very restricted,
and is only had recourse to when the turgidity of the cells of the foliage-leaves has
diminished, and the water exhaled by those cells is not being restored by the
ordinary apparatus of conduction from the roots. If branches are cut from plants
which bear no glandular or other form of hair on their leaves or stems—as, for
instance, the leafy stem of Thesium alpinwm—and the cut ends are closed with
sealing-wax, and the branches left to wither, and, when quite withered, are
immersed in water, they freshen up speedily and the leaves become tense again, the
cells having recovered their turgidity. Here, then, decidedly absorption has taken
place through the ordinary cuticularized epidermal cells. Certainly these epidermal
cells in Thesiwm are not protected against wetting. Wherever the epidermal cells
are not susceptible of being wetted owing to a coating of wax or any other
contrivance there could naturally be no question of water being absorbed. This
very circumstance, however, leads to the supposition that an important part in
water absorption is to be attributed to the alternation of wettable and non-wettable
parts on one and the same leaf. In the case of many foliage-leaves one can see that
only those cells of the epidermis which lie above the veins of the leaf retain the
water which comes upon them, that is to say, are wetted by it, whilst the water rolls
off the intervening areas of the lamina. Indeed, there are in many instances
contrivances obviously designed for the purpose of conducting water from parts of
the epidermis not liable to be wetted to parts that can be moistened.

DEVELOPMENT OF ABSORPTION-CELLS IN SPECIAL CAVITIES AND
GROOVES IN THE LEAVES.

The contrivances last described are all only adapted to rather a casual appropri-
ation of water from the atmosphere. But besides these we find a number of other
contrivances, which render it possible for every rolling dewdrop and every
passing shower to be made of use to the utmost extent. These contrivances
consist of a variety of depressions and excavations, in which rain and dew are
collected and protected against rapid evaporation. Some species have deep hollows
or channels, others little pits, whilst others again have basins, vesicular or bowl-

ABSORPTION-CELLS ON LEAVES. 231

shaped structures, to collect and absorb the water; and the construction of the
protective apparatus, which prevents too rapid evaporation into the air of water
that has once flowed into the depressions, is as various as the form of the depressions
themselves. A short account of the most striking of these structures will now be
given.

Such water-collecting grooves as are closed, so as to form ducts, occur principally
in petioles and in the rachises of compound leaves. For instance, in the Ash the
leaf rachis, from which the leaflets arise, is furnished with a groove on its upper
surface. Owing to the fact that the edges of this groove, which are strengthened
by a so-called collenchymatous tissue, are bent up and curved over the groove, a
duct or conduit pipe is produced, and this duct only gapes open at the places where
the leaflets are inserted upon the rachis, and where, therefore, the drops of rain to
which the leaflets are exposed flow off into the groove (see fig. 541). The simple
hairs and peltate groups of cells developed in the grooves and duets (fig. 54? and
54°) are not merely transiently moistened, but inasmuch as the water is retained
there for several days after a fall of rain, they are during that time immersed in a
regular bath of water, and are able to absorb the moisture very gradually.

In many Gentianeze—most conspicuously in the large-flowered Dwarf Gentian
(Gentiana acaulis)—the decussate pairs of radical leaves form a loose rosette (see
fig. 52'). The larger dark-green blade of each leaf is flat and even, and only the
pale-coloured base is fashioned into a groove. This groove is made deeper by the
tissue of the leaf being puffed up round it, and as all the leaves of the rosette
arise close together, the groove of each leaf is covered by the lamina above it.
The rain or dew accumulated from the blade remains standing in this concealed
nook for some time without evaporating, so that absorptive apparatus with the
power of taking up water has plenty of time for the purpose. In this case the
absorptive apparatus is in the hindmost extremity of the groove, and consists of
long, club-shaped structures composed of extremely thin-walled cells (see fig. 54*),
and these act so energetically that if leaves are cut off and left to fade, and if the
cut surfaces are stopped with sealing-wax, and the whole then bathed with rain-
water, they take up in twenty-four hours about 40 per cent of their weight of
water. A similar phenomenon occurs in the case of a number of Bromeliacez
which adhere by a few roots to the bark of trees in the tropics, and have grooved
rosetted leaves, the latter covering one another, and being arranged in such a
manner as to form a regular system of cisterns. At the bottom of each cistern
there are special groups of thin-walled cells which suck up any water that flows in
when rain falls.

On the under surface of the leaves of the Cow-berry (Vaccinium Vitis-Idcea)
little depressions are formed, and in the middle of each depression there is a club-
shaped structure composed of small thin-walled cells, which contain slimy, viscid
substances and act as absorbent organs. The rain which falls upon the upper
surface of the leaf gets drawn over the edges on to the under surface, fills the small
depressions occurring there, and is taken up by the absorptive apparatus. A

232 ABSORPTION-CELLS ON LEAVES.

similar contrivance is also exhibited by the leaves of alpine roses and those of the
American Bacharis. For instance, on the under surface of the leaves of the Alpine
Rose (Rhododendron hirsutum) there is a large number of discoid glands (fig. 54°),
each of which is supported on a short stalk and sunk in a little hollow (fig. 54°)
The cells composing the gland are arranged radially, and contain slimy, resinous
matters capable of swelling up. These contents are also excreted, and then cover
the entire glandular disc, aud often even the whole surface of the leaf in the form

Fig. 54.—Absorption of Water by Foliage-leaves.

‘Grooved rachis:of the ash-leaf. 2Section through the same; x30. 8 Peltate group of cells from the groove. 4Section
through the base of a leaf of the Dwarf Gentian; x20. 5Under side of a leaf of Rhododendron hirsutum; x30. Section
through a leaf of Rhododendron hirsutum,

of a light-brown crumbly crust. When drops of rain fall upon Alpine Rose leaves,
the whole of the upper surfaces, in each case, is in the first place anoistened; but
without delay, and partly through the action of the hairs fringing the margin, the
water soaks on to the under side of the leaf. ‘As soon as it reaches the glands it is
taken up by the crumbly incrustation mentioned above, which swells up in con-
sequence. The little cavities in which the glands are situated also fill with water,
and each gland is then immersed, as it were, in a bath, and able to absorb as much
(moisture as is required. Owing to the glands being invariably developed above
the vascular bundles of the leaf (see fig. 54°), the water that is absorbed can be
conducted without delay by them to the places where it is required. As soon as
the leaves of alpine roses become dry again, the mass of resinous mucilage again

ABSORPTION-CELLS ON LEAVES. 233

forms a dry crust over the glands and protects their tender-walled cells from too
great evaporation.

Very remarkable also are the structures adapted to absorption on the leaves of
saxifrages belonging to the group Aizoon, and on those of a large proportion of the
Plumbaginew. The saxifrages in question have little depressions visible to the
naked eye upon the upper surface of the leaves behind the apex, and along the
margins. When the margin is dentate or crenate, as, for instance, in Saxifraga

ENLOnCat
( Sin
ae

. SCOO) TOs

ENN SES aR
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\ Ee

NS

Fig. 55.—Absorptive Cavities and Cups on Foliage-leaves.
Leaf from a shoot of the Aspen. 2The base of this leaf; x3. %Section through an absorption-cup; x25. 4 Leaf of
Acantholimon Senganense. 5 Section through part of this leaf; x110. 6 Leaf of the Evergreen Saxifrage (Sazifraga

Aizoon). 7 Two teeth from the margin of this leaf. The absorptive cavity in the upper tooth incrusted with lime; the
lower one with the incrustation removed. ®Section through a tooth from the leaf and its absorptive cavity; x110.

Aizoon (see fig. 55°), one of these cavities occurs in the middle of each tooth.
The cells forming the outer edge of the tooth or scallop are always much
thickened, firm, and rigid; but the median portion of the leaf as a whole is fleshy,
and composed of a bulky large-celled parenchyma. The vascular bundle, after
entering the leaf at its base, divides into a number of lateral bundles which either
run towards the margin without further ramification (as in Sawiccesia), or else
form a net-work by uniting one with another in their course (as in Saaifraga
Aizoon). These lateral bundles terminate in the marginal teeth of the leaf and
immediately beneath the little cavities which occur there, whilst the extremity of
each bundle swells into a knob or pear-shaped enlargement strongly resembling
the roundish groups of spirally-thickened cells in the tentacles of the Sun-dew

234 ABSORPTION-CELLS ON LEAVES.

(cf. fig. 261). The bottom of each depression is made up of cells with very thin
external walls, and the function of these cells is to suck up the water that flows
into the cavity. It is obvious that the absorbed water passes thence into the
enlarged extremities of the branches of the vascular bundles, and may then be
conducted to other parts of the leaf. Seeing that all these saxifrages have their
habitat in crevices of rocks on sunny declivities, they are much exposed to
desiccation in times of drought. The epidermal cells of the medial area and those
of the extreme edge are no doubt protected by a very thick cuticle (see fig. 55°);
but in the case of the thin-walled cells at the bottom of the depression there is the
danger of as much or even more water escaping through them, in the form of vapour,
than has been previously taken in during the prevalence of rain.

In order to prevent this loss of moisture recourse is had to a very remarkable
contrivance for closing the cavity, viz. an incrustation of carbonate of lime. In
many saxifrages this crust covers the whole face of the leaf, in others only the
margin, or the spot where the depression occurs. In the latter case it looks like a lid
over the cavity. At that spot the crust is always thickened, and sometimes it forms
a regular stopper which fills up the entire cavity. It rests upon the epidermis of
the leaf, but is not adnate thereto, and may be removed with a needle. When a
leaf is bent the crust is ruptured and breaks up into irregular plates and scales,
and a strong gust of wind would then easily strip off the fragments and blow them
away. In species subject to this danger, as, for instance, Saaifraga Aizoon, in
which the rosetted leaves curl strongly upwards and inwards in dry weather, the
crust of lime is held fast by peculiar plugs which arise from individual epidermal
cells projecting above the rest in the form of papille (see fig. 55°). These plugs
are found principally on the side walls of the cavities, but are also scattered every-
where on the epidermis of the margin of the leaf. They are so incrusted with the
lime that the latter cannot easily fall off, and a comparatively strong pressure must
be applied with the needle to detach it from the substratum. The calcium carbonate
of which these crusts consist is excreted in solution by the plant from pores occur-
ring at the bottom of the depressions. The pores are constructed like ordinary
stomata, but are, as a rule, somewhat bigger, and it is not improbable that, when
once the lime crust has formed from the excreted solution, they take part in the
function of transpiration.

There is scarcely any need for further explanation of the manner in which the
apparatus here described acts. When rain or dew falls on a saxifrage leaf the
whole upper surface is moistened directly, whilst the water soaks under the crust
of lime, and, diffusing itself there, fills in a moment the depressions, and is taken
up by the absorption-cells situated at the bottom of the latter. The calcareous
stopper imbedded in each cavity is only upheaved by this process to a trifling
extent. In dry weather the crust is appressed closely to the epidermal cells, and
the stopper descends again and impedes the evaporation of water from the thin-
walled cells within the cavities.

The absorptive organs on the leaves of Acantholimon, Goniolimon, and a few

ABSORPTION-CELLS ON LEAVES. 235

other Plumbaginez, resemble in an extraordinary degree those pertaining to saxi-
frages. The depressions are here found uniformly distributed over the entire sur-
face of a leaf, and when they are closed by a crust or scale composed of calcium
carbonate, the leaves are dotted with white spots, ag may be seen in the drawing
of a leaf of Acantholimon Senganense given in fig. 554. Upon the caleareous scale
being removed, a little cavity is revealed beneath, and one observes that the floor of
this cavity is composed of from four to eight cells, separated by radial partition-
walls, and with exceedingly thin and delicate outer walls. The other epidermal
cells adjoining the cavity are, on the contrary, always furnished with a thick cuticle
(see fig. 55°). Whenever water is being copiously supplied to the roots, and the
turgidity of the cells in the leaves is great, the cells forming the floor of the cavity
excrete bicarbonate of lime in solution. Part of the carbonic acid escapes into the
air, and the insoluble mono-carbonate of lime in the water then forms a crust, which
fills and covers the cavity, and often even spreads over the whole leaf, constituting a
coherent calcareous coat.

All Plumbaginesws which exhibit this contrivance—that is to say, the various
species of Acantholumon, Goniolimon, and Statice—inhabit steppes and deserts,
where in summer no rain falls for months together, and the soil becomes dry to a con-
siderable depth, so that extremely little water is available for the roots. Although
the rigid leaves are protected by a thick cuticle, and by crusts and scales of lime
against excessive evaporation of their aqueous contents, still it is difficult to avoid
some slight loss of water, especially when the noon-day sun beats down upon the
steppe, and, owing to the extremely arid nature of the soil, it is scarcely possible to
replace: this loss, however small it may be, by absorption from the earth on the part
of the suction-cells on the roots. All the more welcome to plants of the kind is the
dew which sometimes falls copiously on steppes and in deserts in the course of the
night; it wets the rigid leaves, and, soaking immediately underneath the crusts and
scales of lime to the thin-walled cells at the bottom of the cavities, is absorbed with
avidity by them. When drought returns with the day, the scales of lime close
tightly down like lids on the epidermis beneath, and, so far as possible, prevent
evaporation. In particular, they impede the exhalation of water from the thin-
walled cells at the bottom of the cavities—a loss which would otherwise be quite
inevitable, and would be followed by a rapid desiccation of the entire plant. To
prevent the calcareous lids from dropping off, there are either, as in Saaifraga
Aizoon, papilliform or conical projections from cells in the immediate vicinity of
the cavities, which projections often have hooked ends and confine the crust of
lime, or else each cavity is somewhat contracted at the top and enlarged below, so
that the lime stopper, being shaped according to the contour of the cavity, cannot
fall out.

A significance similar to that attributed to calcium carbonate excretions belongs
also to the saline crusts which are found covering the leaves of a few plants grow-
ing on the arid ground of steppes and deserts in the neighbourhood of salt lakes
and on the dry tracts of land near the seashore. Owing to the fact that in these

236 ABSORPTION-CELLS ON LEAVES.

situations crystals of salt are sometimes to be seen separated out from the soil, and
lying as a white efflorescence upon the ground, it used formerly to be believed that
the salt incrusting leaves and stems was derived, not from the plants in question,
but from the soil around, and had only spread from there over the various plant-
members. But this is not the case. Asa matter of fact, the salt observed on the
leaves and stems of Frankenia, Reaumuria, Hypericopsis persica, and a few species
of Tamariz and Statice, is produced from the substance of the leaves. It is excreted
in just the same way as the crust of lime, above described, is from the leaves of
saxifrages. To the naked eye the surfaces of the leaves in all the plants enumerated
have a punctate appearance. On closer inspection, it is evident that, corresponding
to each dot, there is a little cavity, the deepest part of which is constructed of cells
with extremely delicate external walls. In quite young leaves only a single thin-
walled cell of the kind is to be seen at the bottom of each shallow depression. But
this divides, and, by the time the leaf is full-grown, from two to four cells are seen
to have arisen by division of the one cell. Stomata are, in addition, intercalated in
the membrane in the neighbourhood of these thin-walled cells, and, in the rainy
season, when there is no lack of water in the habitats of the plants in question, a
watery juice, containing a large amount of salts in solution, exudes from these
stomata. The saline solution soaks over the whole surface of the leaf, and in a dry
atmosphere crystals form from it and adhere to the leaf in the form of little gland-
like patches or continuous crusts.

If these tamarisks, frankenias, and reaumurias are observed during a rainless
season, the crystals of salt are seen under the noon-day sun glittering on the leaves
and stems, and may be detached in the form of a fine crystalline powder. But if
the same place is visited after a clear night, no trace of crystals is to be seen; the
little leaflets have a green appearance, but they are covered with a liquid with a
bitter salt taste,’ and are damp and greasy to the touch. The crystals have
attracted moisture from the air during the night, and have deliquesced, and the
saline solution not only covers the whole of the leaf, but also fills the little cavities
visible as dots to the naked eye. The thin-walled cells at the bottom of the cavi-
ties differ from the rest of the epidermal cells and the guard-cells of the stomata, in
that they are susceptible of being wetted, and they may act as absorption-cells, and
allow the water, attracted by the salts from the air, to pass through their thin
walls into the interior of the leaves.

When the air dries under the rising sun, crystals are again formed from the
solution of salts, and, covering the leaves once more in the form of crusts, fill up the
depressions and protect the plants during the hot hours of the day from excessive
evaporation. Whilst, therefore, in the dewy night these plants are indebted to
their salt crusts for water, they are in the day-time preserved from desiccation by
the action of the same contrivance.

1 The salt incrustations which were removed from plants of Frankenia hispida, collected on a Persian salt-steppe,
consisted principally of common salt (chloride of sodium). They contained in smaller quantities, gypsum, mag:
nesium sulphate, calcium chloride, and magnesium chloride.

ABSORPTION-CELLS ON LEAVES. 237

It is also worthy of mention that papille are developed near the absorption-
cells, with a view to the retention of the salt crystals, similar to those which hold
the calcareous incrustations on the leaves of saxifrages and Acantholimon. The
leaves of plants covered with crystals of salt are also for the most part furnished
with little bristles, to which the salt adheres so firmly that it 1s not readily detached,
even by violent shaking.

But however striking the analogy between the development and significance of
lime crusts and salt crusts respectfully, there is the essential difference that the
former have not, like the latter, the power of attracting moisture from the air.
And on this particular stress must be laid. In the hilly and mountainous tracts
on the shores of salt-lakes or of the sea, where tamarisks and frankenias are
especially wont to live, the sandy ground dries up to such an extent in the height:
of summer that it is scarcely conceivable how plants growing in it are able to
preserve their vitality. The proximity of the sea has no immediate effect on the
moisture of the ground in such situations. The sea-water does not penetrate into
the ground far beyond the high-water line, and it is out of the question that the
layers of soil serving as substratum to the frankenias and tamarisks should be
irrigated by subterranean water. When in summer there is an absence of rain for
months together, these plants—even though in close proximity to the sea—would
necessarily perish of drought. Only the circumstance that they turn to account the
moisture of the atmosphere by means of the excreted salts renders it possible for
them to flourish in these most inhospitable of all inhospitable sites.

Many plants which are periodically exposed to great dryness have the tips of the
teeth on the leaf-margins thickened into little cones or warts. They also glitter
somewhat and at times are sticky. The glitter and viscidity are due to a resinous
slimy substance, which often contains sugar and tastes sweet. This substance
covers the teeth and sometimes spreads from the teeth inwards to a great dis-
tance over the face of the leaf in the form of a delicate film-like varnish. The
greatest resemblance exists between this varnish (sometimes known as “balsam ”)
and the secretions of the glands on the leaves of the Alpine Rose and of the
glandular hairs on those of Centawrea Balsamita. It is excreted by special cells,
which are intercalated in the epidermis of the foliar teeth, and are at once marked
out from the other cells of the epidermis by the facts that their protoplasm is of a
brownish colour and that their external walls are easily permeable by water. The
excretion of the varnish-like layer takes place at a time when the entire plant is dis-
tended with sap, chiefly, therefore, in the spring. When summer is at its height
the varnish dries and thenceforward affords an excellent preservative from the risk
of too much evaporation from the cells it covers, and especially from those situated
on the teeth of the leaves by which it was excreted. But if this dried film of
varnish is wetted it saturates itself quickly with water and renders moisture
accessible to the cells beneath it. Thus its value is similar to that of the crusts of
lime and salt on the leaves of the plants above described. When moist it effects
the absorption of water, when dry it guards against desiccation.

238 ABSORPTION-CELLS ON LEAVES,

The reason for the contrivance just described being exhibited especially by the
marginal teeth of the leaf, lies in the fact that dew is deposited particularly at those
spots. If one looks at the leaves of the dwarf almond and plum trees in the
steppe-districts, after clear summer nights, one finds a dewdrop suspended to every
tooth on the margins; but by noon all the teeth are dry again and protected from
loss of water by the coat of varnish. Moreover, not steppe-plants alone, but very
many plants which grow in poor sandy soil on the banks of streams and rivers,
exhibit this contrivance for the direct absorption of water from the ‘atmosphere.
Instances are afforded by the Sweet Willow, the Crack-willow, Poplars, the Guelder-
rose, the Bird-cherry, and many others. It is at once evident that this contrivance
is observed chiefly on the leaves of trees, shrubs, and tall herbs, whilst incrustations
of lime occur only on shorter plants with rosulate leaves spread out on the ground,
or with rigid acicular leaf-structures. The grounds of this distinction may well
reside in the fact that the weight of a crust of lime is many times as great as that
of the dry film of varnish. A load capable of being borne without hazard by the
leaves of a Statice plant, they being spread out on the ground, or by the rosettes of
Saaifraga Aizoon, would be unfit for the leaves of a Cherry or Apricot tree, or for
those of the Sweet Willow, or the Crack-willow; indeed the branches of these trees
would break down under the burden if their leaves were incrusted with lime.

In many cases only a few of the marginal teeth of the leaf are transformed into
absorbent apparatus, and special contrivances then always exist to convey rain and
dew to those teeth. The Aspen (Populus tremula) serves as a very good example
of this. This tree has, as is generally known, two kinds of leaves. Those arising
from the branches of the crown have long petioles and lamine of roundish outline and
with somewhat sinuate margins; those which are borne by the radical shoots have
shorter stalks and larger sub-triangular lamine sloping outwards; and the whole
leaf is so placed and its margin so curved as to oblige the rain which strikes the
upper surface in its descent to flow down towards the petiole (see fig. 551). Now,
situated exactly on the boundary of lamina and petiole are two cup-shaped structures
(fig. 55) originating from the lowest teeth of the leaf, and so arranged that every
drop of rain descending from the lamina must encounter their shallow cavities and
fill them with water. These cups are brown in colour and the size of a grain of
millet; and the cells of their epidermis are furnished with a thick cuticle. Only
the cells lining the shallow depression of each cup have thin walls, and they excrete
a sweet-tasting, slimy, resinous substance which in dry weather films over the
cavity like a varnish, and protects, at all events, the cells lying beneath it against
an injurious desiccation. When, however, this coat is itself in contact with water
it swells up, and the moisture is then absorbed by the cells in the pit-like depression
and is transmitted to the vessels running underneath the cups (see fig. 55°).

A number of tall herbs, principally of the group of Composite, have, like the
Aspen, leaf-teeth which are developed at the part where petiole and lamina join and
act as organs of absorption. In some, besides, the margin of the green laming
extends in the form of a narrow ridge down the pale canaliculate petiole; and, when

ABSORPTION-CELLS ON LEAVES, 239

this is the case, teeth of the kind are found on this narrow green ridge which runs
along the groove. In Télekia, a handsome herbaceous plant of wide distribution in
the south-east of Europe, these teeth—conical or club-shaped—springing from the
margin of the petiole-groove are incurved, and are in general so placed that their
blunt apices prcject into the groove. But precisely on these obtuse tips of the
teeth are situated cells with very thin outer walls easily permeable to water, and
having contents with a strong attraction for it. Thus, as soon as the groove of the

Fig. 56.—Water-receptacles.

1In a Teasel, Dipsacus laciniatus. 2In the American Silphium perfoliatum.

petiole is filled with rain, collected from the surface of the leaf, the tips of the
conical teeth are moistened, and they suck up the water.

Lastly, we have to mention the curious receptacles appertaining to foliage-
leaves in which water from the atmosphere accumulates and continues to stand for
weeks without being protected from evaporation by the excretion of special
substances. Any region or portion of the leaf may participate in their construction.
In Sazxifraga peltata the lamina is shaped like a shield and forms a shallow plate
with the concave surface turned to the sky. In the Cloud-berry (Rubus Chame-
morus) the formation of basins is brought about by the margins of the reniform
lamina being superimposed over one another as if to make a spathe. In the various
species of Winter-green, especially in Pyrola wnijlora, the pale cauline leaves,

240 ABSORPTION-CELLS ON LEAVES.

inserted above to the green leaves, are metamorphosed into little saucers. In one
species of Teasel, Dipsacus laciniatus (see fig. 561), and in the North American
Silphium perfoliatum (fig. 56 *) the two sheathing portions (vaginz) of every pair of
opposite leaves are connate and form comparatively large and deep funnel-shaped
basins, from the middle of which rises the next higher internode of the stem. In
several Meadow-rues (Thalictrwm galioides and T. simplex) the secondary leaflets,
which are opposite one another and shut close, almost like the valves of a mussel, are
moulded so as to form cavities for the retention of water, and in many Umbellifere,
such as Heraclewm and Angelica, the vagina of each individual leaf is ventricose
or inflated, thus forming a sac enveloping the segment of the stem which stands
above it.

These basins, saucers, and dishes are always so placed, relatively to their
surroundings, that the water derived from rain and dew is directed into them from
the surfaces of the leaves, or by the segment of the stem which rises from their
centres, and thus it is that the depressions are filled. Whether in all cases much of
the water accumulated is absorbed is certainly open to doubt. In the case of the
leaves of the Alchemilla (fig. 527), which exhibit the phenomenon so conspicuously
that the plant has received the popular name of Dew-cup; the absorption of water
is, at anyrate, very inconsiderable, and here the retention of the dew secures
advantages of a different kind to which we shall presently have occasion to return.
On the other hand, it is established that in the case of basins belonging to tall
herbaceous plants, particularly such as grow on steppes and prairies where often
no rain falls for a long interval, the water collected is absorbed by the glandular
hairs and thin-walled epidermal cells developed within them. The fact of this
absorption may be proved by a very simple experiment. Let a stem of the
Silphium, represented in fig. 562, be cut off beneath the pair of connate leaves, which
form a basin by their union, and let the cut surface le closed with sealing-wax, so
that no water can be taken up by the stem from below. If the water accumulated
in the basin is now emptied out, the leaves shortly become flaccid and droop; but if
the basin is left full of water, the leaves preserve their freshness a long while and
do not begin to wither until all the water has evaporated and disappeared from the
basin. If oil is poured upon the collection of water in the basin, so that evapora-
tion from the latter is impeded, a constant diminution of the water in the basin is
observed notwithstanding; this leads to the conclusion that the water in question is
really taken up by the absorption-cells at the bottom of the basin and conveyed to
the tissue of the leaf.

The first thing that strikes one on surveying once more all the plants possessing
on their aérial organs special contrivances for water-absorption is that a large
proportion of them have taken up their abode in swamps and on the banks of
rivers and streams, or if not there, at all events in situations where no danger
exists of the ground being thoroughly dried up. No doubt this appears to be
inconsistent. How are we to explain the fact that Gentianes, ashes, willows, alpine
roses, bog-mosses, &c., are still in need of water from the atmosphere, when they all

ABSORPTION-CELLS ON LEAVES. 241

grow either in damp meadows, peat-bogs, on the borders of never-failing springs, or
in ever-moist ravines, where their requirements in respect of nutrient water and
imbibitious water can be supplied all around by means of the roots? A glance at
the company in which these plants occur may perhaps lead to a solution of the
problem. In the damp meadows and along the margins of springs where gentians,
the Sweet-willow, and plants of that kind are found, the Butterwort (Pinguicula),
which has been described in earlier pages amongst carnivorous plants, is never
absent; whilst wherever the pale cushions of the Bog-moss spring, there also the
Sun-dew is certain to spread out its tentacles for the capture of prey.

With reference to community of site the assumption is warranted that all these
plants which flourish under identical conditions of life endeavour to acquire the
same material by means of their aérial parts. Now, this material cannot well be
other than nitrogen, of which they do not find a sufficient store in the substratum.
What then is more natural than that those plants, which are not adapted to the
capture of animals, should use their aérial organs, when these are moistened with
rain or dew, to take up direct nitric acid and ammonia, which are contained—
though in small traces only—in the atmospheric deposits, instead of waiting till
compounds of such great importance to them penetrate into the ground where they
may chance to be detained at spots whence the roots could only obtain them after
long delay and by a highly complicated process? When one considers that plants,
growing amid the sand and detritus of steppes, on ledges, and in crevices of steep
rocks, or epiphytic on the bark of trees, are also able to acquire little or no
nitrogenous food from the substratum by means of their roots, their especial equip-
ment with apparatus for the absorption of atmospheric water becomes explicable on
the ground of the latter being the medium of solution and transport of nitrogenous
compounds. In the case of epiphytes and of plants growing on steppes or rocks,
there is the additional consideration that a supply of pure water, supplemental to that
which can be withdrawn from the substratum, must be very welcome to them in
dry weather, and that at such times it is a great advantage for the atmospheric
water to be absorbed directly by the aérial organs instead of reaching them in a
roundabout manner through the substratum.

If this idea is justified, the atmospheric moisture taken up by the aérial organs
with the help of the above-described contrivances, would be of value to the plant
chiefly in being a carrier of nitrogenous compounds, and in this acceptation would
have to be looked upon as water of imbibition. Whether it is also used, at least in
part, as food-material can neither be asserted nor controverted. A separate absorp-
tion of water which serves only for motive power, and of that which is in addition
employed in the construction of organic compounds does not take place in a plant,
it is not possible to make any a priori statement concerning the moisture taken
up, as to which part it has to play in the plant. Most probably the allotment of
functions is not at all uniform, but varies considerably according to conditions of
time, place, and requirement.

On a former occasion it has been mentioned that small animals are not
Vot. I. 16

242 ABSORPTION-CELLS ON LEAVES.

infrequently killed accidentally in the water filling the larger kinds of basins
formed as parts of foliage-leaves, that pollen, spores, and particles of earth also are
blown by the wind into these basins, and that, after the ensuing solution and
decomposition of the organic and mineral bodies in question, the water exhibits a
brownish colour and contains organic compounds as well as food-salts in solution.
It is not necessary to repeat that these compounds are able to pass into the interior
of the plant with the water through the action of the absorption-cells which are
never absent from the bottom of the basins; but it seems proper to consider
specially in this connection the most conspicuous cases of the phenomenon which
have been observed. The greatest quantity of matter, dissolved and undissolved, is
found in the flat, saucer-shaped lamine of Saaifraga peltata, which grows on the
sites of springs in the Sierra Nevada of North America. The water in these saucers
is sometimes coloured quite a dark brown by the presence of decayed beetles, wasps,
centipedes, fallen leaves, and animal excreta; and when it evaporates a regular crust
is left behind at the bottom of the reservoir. Three days after rain I still found in
the inflated vagina of Heraclewm palmatum, a species of cow-parsnip, a pool of
brown water 2 cm. deep, and at the bottom a deposit of blackish, oily mud in which
the remains of decayed earwigs, beetles, and spiders, were still recognizable. The
same thing is observed in the cisterns of Bromeliaceew and in the water-basins of
Dipsacus laciniatus and Silphiwm perfoliatwm (fig. 56), and it is interesting to
find there are cells also at the bottom of the basins of the Dipsacus in question from
which protoplasmic threads radiate forth, as in the case of the chambers of the
Toothwort, and that numberless putrefactive bacteria always make their appearance
in the water in these basins. The quantity of organic residue is less considerable
in the saucer-shaped leaves of pelargoniums, but, on the other hand, earthy particles
are frequently met with in them to such an extent that, when the water has
evaporated, the concave surface of the leaf is covered with an ashen-gray layer
of earth.

Observations of this nature establish the conviction that no sharp line of
demarcation exists in respect of the absorption of water either between carnivorous
plants and land plants, or between land plants and saprophytes, or between
saprophytes and carnivorous plants; and they lead further to the conclusion that
water, mineral food-salts, and organic compounds are susceptible of being taken up
not only by subterranean but also by aérial absorptive apparatus

LICHENS. 243

6. SYMBIOSIS.

Lichens.—Cases of symbiosis of Flowering Plants having green leaves with the mycelia of Fungi
destitute of chlorophyll Monotropa.—Plants and Animals considered as a vast symbiotic
community.

LICHENS.

In describing the vegetation of a limited area botanical writers are apt to desig-
nate the various species of plants as “denizens” of the country in question. The
conditions under which the plants live are likened to political institutions, and the
relations existing amongst the plants themselves are compared to the life and strife
of human society. By no means the least important factor in the suggestion of
these analogies is the circumstance that often as a matter of fact one has
opportunities of seeing how the species of plants which live together in a locality
are dependent in various ways upon one another; how they exist in continual con-
flict for the food, the ground, for light and air; how some are preyed upon and
oppressed by others, whilst others are supported and protected by their neighbours;
and how, not infrequently, quite different species join together in order to attain
some mutual advantage.

As regards the preying of one upon another the subject has been treated in
detail in a previous chapter, and it was also stated then that the term parasite can
only be applied to those plants which withdraw materials from the living parts of
other organisms without rendering a reciprocal service in return. The host attacked
by a parasite supplies food and drink without being in any way compensated. One
might suppose that nothing would be simpler and easier than to ascertain the
existence of this relationship, and yet many difficulties are encountered in the
determination of parasitism in individual cases. The main difficulty is due to the
fact that one cannot always say with certainty whether the host does not perhaps
get some advantage from the parasite which drains its juices. Should this be the
case, however, the latter would be no longer a parasite, and the relationship between
the two would rather be that of simple commerce and mutual assistance, an ami-
cable association for the benefit of both.

Whilst discussing the second series of parasites, the fact was mentioned that the
plants upon which the various species of Eyebright fasten their suckers suffer no
apparent injury as a consequence of this connection. The rootlet organically
united to the suckers does, it is true, die away in the autumn; but the Eyebright
also withers at that season, and it is not inconceivable that the useful substances
existing in the green leaves of the Eyebright may be transferred, shortly before the
latter withers, to the host-plant and deposited there at a convenient time in the
permanent part of the root as reserve-material, and that in this way the host-plant
ultimately derives benefit from the so-called parasite. The idea here suggested as a
possibility for the case of Eyebright and the grasses connected with it is an ascer-
tained fact in the case of some other plants. For plants are known which unite to

24,4 LICHENS.

form a single organism and thenceforward so co-operate in their functions that
ultimately both derive advantage from the arrangement. The one takes food-stuffs
from the substratum and from the air and transmits them to the other; whilst, in
the green cells of the other, the raw material is worked up, under the influence of
sunlight, into organic compounds. The organic compounds thus created are used
by both for the further production of organs, and therefore a connection such as
this must be looked upon as a true case of symbiosis, 7.¢. associated existence for
purposes of nutrition.

The first place amongst social communities of the kind must be assigned to
Lichens, a section of Cryptogams possessing an extraordinarily large number of
species and differentiated into thousands of forms, representatives of which are

Fig. 57.—Gelatinous Lichens.

1 Ephebe Kerneri; x450. 2% Collema pulposum; natural size. 8 Section through Coll pul 3 xX 450

DP

everywhere distributed, from the sea-shore to the highest mountain peaks yet
scaled by man, and from the tropics to the arctic and antarctic zones.

The partners in the Lichen communities appear to be, on the one hand, groups
and filaments of round, ellipsoidal, or discoid green cells belonging to plant species
included under the general name of Algw; and, on the other hand, pale, tubular
cells or hyphe, which are destitute of chlorophyll, and pertain to species of plants
comprised under the general name of Fungi (see fig. 58).

The form assumed by a large proportion of these lichens is that of incrustations
on stones, earth, bark, or old wood-work; the entire structure of the lichen is either
ensconced and imbedded in the depressions of weathered surfaces of stone, or else
between the cell-walls of dead fragments of wood and bark, so that it often happens
that attention is only drawn to its presence by the altered colour of the substratum,
or by the fructifications which lift their heads above the substratum.

Lichens of the kind are termed Crustaceous Lichens, and the wide-spread
Graphic Lichen (Lecidea geographica) may serve as an example. A second great
group nearly allied to the first is that of Foliaceous Lichens. The form of the

LICHENS. 245

vegetative body in these is best compared to the foliage-leaves of the Curled Mint,
with their corrugated or sinuate margins, or to those of Malva rotundifolia. It
may also be described as a number of lobes radiating irregularly and bifurcating
repeatedly, and only lightly joined to the substratum by root-like fringes, and there-
fore capable of being readily loosened and detached. The light-grey Parmelia
sawatilis, which bear brown saucer-shaped fructifications, may be taken as a repre-
sentative of these Foliaceous Lichens. The Fruticose Lichens are distinguished as a
third group in which the thallus rises from the ground in the shape of a shrub,
whilst the cylindrical, fistular, and ligulate stemlets, which ramify profusely, are
only adherent to the substratum by a very small surface at the base. With these
are associated the Beard Lichens, which hang down from the bark of old trees in
the form of pale, copiously-branched filaments. Lastly, there is a fifth group, the

Fig. 58.—Fruticose and Foliaceous Lichens,

1 Stereocaulon ramulosum in conjunction with Scytonema; x650. 2 Cladonia furcata with Protococcus; x 950
8 Coccocarpia molybdea; section, x650 (after Bornet).

Gelatinous Lichens, which when moistened look like dark, olive-green, or almost
black lumps of wrinkled and wavy jelly or as if composed of variously-divided
bands and strips packed together into little cushions.

In the gelatinous expansions last mentioned the algal cells are arranged in
moniliform rows and are interwoven with the hyphal filaments of the fungus
throughout the entire thickness of the thallus, as in Collema pulposwm (see
fig. 57? and 573), or else they form regular ribbon-shaped double rows, interwoven
with few hyphe, as in Ephebe Kerneri (see fig. 571). In crustaceous, foliaceous,
and fruticose lichens, the algal cells constitute a disorderly heap and are crowded
together in the middle stratum of the thallus, where they are imbedded between
an upper and a lower layer of densely felted hyphal threads, as in Coccocarpia
molybdcea (fig. 58°).

Seeing the wide distribution of lichens it must be assumed that both partners
occurring in the lichen-thallus are able to range about with extraordinary ease and
latitude. When one observes how patches of the most various lichens are produced
in a few years after a landslip on the freshly-broken surfaces of the stones which

246 LICHENS.

have fallen down into the valley beneath, one can only explain the phenomenon by
supposing that the algal and fungal cells concerned have been blown together, and
that the opportunity has been afforded them on the blocks of stone of contracting
a union. Now, so far as regards one of the two partners, viz.: the one devoid of
chlorophyll, and known as a fungus—the idea that everywhere in the air spores of
fungi are swarming about is so familiar to us that the supposition of an occasional
stranding of individual spores, which are being blown about by the wind, upon the
moist broken surfaces of stones can encounter no opposition. Respecting those
spores in particular which are ejected from the aérial fructifications of lichens, the
discussion of their life-history and distribution must of course be reserved for a
later section; but it is necessary to make here the one statement that provision
exists for the most profuse and distant dissemination of these spores.

Thus, in the case of one of the partners, there is no difficulty in realizing its
ubiquity. But when one comes to the Algw, the name at first calls up to mind the
green filaments which occupy our pools and ponds, or the brown wracks and red
Florideze of the sea-shore, and we ask ourselves how it can be possible for these
plants to occur on fractured surfaces of stone, especially on the débris of mountain
sides. Indeed, it is certainly not Alge of these kinds that take part in the
construction of Lichens. The name Alge is properly only a general name for all
Thallophytes containing chlorophyll, and it is applied to many small organisms
besides those mentioned above, namely, to numbers of Nostocines, Scytonemee,
Palmellaces, Chroolepides, and these are the kinds which fall in with the cells of
fungi and form lichens in conjunction with them. Owing to their minute size,
they are apt to escape observation, and, in general, only attract attention when
myriads of them clothe the bark of trees, cliffs, stones, or earth. In these situations
they need but little moisture, and it is not necessary for any of them to live under
water like other alge; they become desiccated without sustaining the slightest
injury and make their appearance on the substratum occupied by them at the first
stage of their development, as powdery coats, and, in this condition being extremely
light, are liable to be blown away by a wind of moderate strength, and so
distributed over mountain and valley.

That this dissemination is not merely hypothetical but an actual fact has been
susceptible of easy proof by the following experiment, made in a mountain-valley in
the Tyrol. A plane surface covered with white filter-paper, which was kept moist,
was exposed to a south wind; in the course of a few hours numerous particles, like
dust, adhered to the paper, and amongst them cell-groups of Nostociness and others
of the above-mentioned alge occurred regularly, in addition to organic fragments
of the most various kinds, such as pollen-grains and spores of all sorts of mosses
and fungi. All these bodies were deposited in the little depressions on the sheet
of paper, and in the same way they rest in the grooves, cavities, and cracks in the
surfaces of stone, bark, and old wood-work, where they succeed in reaching a
further development as soon as the requisite quantity of water is provided. Now,
if at these places the little algal cell-groups meet with hyphx belonging to the

LICHENS. Q47

other potential partner, the latter embrace and enmesh them, as is shown in the
above figures, and thus is produced the confederacy called a Lichen. The member
destitute of chlorophyll takes up nutriment from the external environment; it
possesses, in particular, the property of condensing aqueous vapour, and has, besides,
the power of bringing the solid substratum partially into solution by means of
excreted substances; it effects adhesion to the substratum, and, in a majority of
cases, determines the form and colour of the lichen-thallus as a whole. The second
member, whose cells contain chlorophyll, undertakes the task of producing organic
matter, under the influence of sunlight, from the materials conveyed to it; by this
means it multiplies the number of its cells and increases in volume, whilst, at the
same time, it yields to its mate so much as is necessary in order to enable the latter
to keep pace with it in growth.

The number of alge which enters into a partnership of this kind is, in any
case, much less considerable than that of the fungi, and it must be assumed that
one species of alga may unite with the hyphe of different lichen-fungi. The
extreme variety, moreover, in the combinations of the two sorts of confederate
occurring on a very small area is obvious from the circumstance that it is not
rare for half a dozen different species of lichen to spring up side by side on a patch
of rock no bigger than one’s hand. Whether they all achieve an equally hardy
development, or whether some perchance are not crowded out and overgrown
by others depends on various external conditions—on the chemical composition
of the substratum, and particularly on the conditions of moisture and illumination
of the site in question. Lichens are very sensitive in this respect, and the different
sides of a single rock often exhibit quite different growths of lichens. A very
instructive example of this is afforded by a marble column near the famous castle
of Ambras in Tyrol. This column is octagonal, and has been standing in its place
for more than two hundred years, with all its sides exposed to wind and weather.
Lichens have settled on all the eight faces, and, indeed, are present in such abund-
ance that the stone is quite covered by patches the size of a man’s hand. Many
of these growths are but poorly developed, and not susceptible of being identified
with certainty; but altogether on this column there must be over a dozen different
species, the germs of which can only have been brought by winds. These species
are, however, by no means uniformly disposed; some prevail on one side, some
on another, and a few are confined exclusively to one of the eight faces. Of three
species of Amphiloma, the one named A. elegans is restricted to the warmest side,
ae. the face exposed to the south-west; a second, Amphiloma murorum, is to
be seen on the upper part of the southern face; whilst Amphiloma decipiens
occurs on the same face, but only near the ground. On the side with a northern
aspect Endocarpon miniatum predominates, and on the north-west face Calopisma
citrinwm and Lecidea are the prevailing forms.

What thousands of spores and algal cells must have been blown on to this
pillar to enable all these combinations to arise! What complex processes must
have gone on before the selection of lichens best adapted to each different quarter

248 LICHENS.

of the compass was effected on this little marble column! It is necessary to add,
however, that lichens growing on stone, bark, or any situation of the kind do
not in all cases owe their original appearance on the substratum to a fresh union
of Algee and Fungi, but that there is a second mode of distribution of lichens. This
method consists in the transportation by air-currents of already completed social
colonies to places often situated. at a great distance from the spots where the
initial union between Alga and Fungus was contracted. The process is as follows:
—in the interior of an old, large, and fully developed lichen-thallus certain groups
of cells separate from the rest, each group consisting of one or more green algal
cells enmeshed in a dense weft of hyphw. When a sufficient number of these
daughter-associations has been formed the thallus of the parent lichen is ruptured
and the little miniature social-groups, which are termed “soredia”, come to the
surface. To the naked eye a single soredium is only visible as a bright dot, but
all together they have the appearance of a mass of powder or meal lying loosely
upon the old lichen-thallus. In dry weather this mealy efflorescence is easily
blown away with other organic particles. If, then, a soredium thus removed
comes to rest in the crack of a rock or on any suitable substratum, the alga and
hyphe composing it continue to develop, and the organism grows into a larger
lichen-thallus, which is able to repeat the process just described. In regions where
lichens abound, soredia of the kind are found regularly amongst the elements
of the organic dust, and occur, indeed, mixed with fungal spores and algal cells,
so that it certainly happens not infrequently that two spots close together in the
same cranny of stone exhibit both sorts of lichen-growth, the one newly produced
by the concurrence and union of algal and fungal cells, the other a daughter-
association which has arisen from an old lichen, as a soredium, and is continuing
its development.

Another case of symbiosis allied to that of lichens is manifested by certain
Cryptogams which live socially together under water and have received the
systematic names of Mastichonema, Dasyactis, Enactis, &. In them also a plant
containing chlorophyll, and belonging to the group of Nostocines, appears as one
member of the partnership; whilst the second is some species of Leptothria or
Hypheothria. The green moniliform rows of cells of Nostocines are enmeshed
and wrapped round by the delicate, filamentous cells devoid of chlorophyll of the
Leptothria or Hypheothrix; and later, by repeated processes of division, whole
colonies of green cell-filaments ensheathed in this manner are produced, which
to the naked eye appear as small soft tufts, usually clinging to porous limestone
in the spray of waterfalls. In many cases the filaments destitute of chlorophyll
rest upon the moderately thickened cell-membranes of the green alge, whilst
in other cases they insinuate themselves into the thick cell-membranes, permeate
them with their webs, and form in conjunction with them the sheathing envelope.

SYMBIOSIS OF PHANEROGAMS AND FUNGI. 249

SYMBIOSIS OF GREEN-LEAVED PHANEROGAMS WITH FUNGAL MYCELIA
DESTITUTE OF CHLOROPHYLL.—MONOTROPA.

Another instance of symbiosis is observed to exist between certain flowering
plants and mycelia of fungi. The division of labour consists in the fungus-mycelium
providing the green-leaved Phanerogam with water and food-stufts from the ground,
whilst receiving in return from its partner such organic compounds as have been
produced in the green leaves.

The union of the two partners always takes place underground, the absorbent
roots of the Phanerogams being woven over by the filaments of a mycelium. The
first root that emerges from the germinating seed of the phanerogamic plant
destined to take part in the association descends into the mould still free from
hyphe; but the lateral roots and, to a still greater extent, the further ramifications,
become entangled by the mycelial filaments already existing in the mould or
proceeding from spore-germs buried there. Thenceforward the connection
continues until death. As the root grows onward, the mycelium grows with it,
accompanying it like a shadow whatever its course, whether the root descends
vertically or obliquely, or runs horizontally, or re-ascends, as is sometimes necessary
when it happens to be turned aside by a stone. The ultimate ramifications of roots
of trees a hundred years old, and the suction-roots of year-old seedlings, are woven
over by mycelial filaments in precisely the same manner. These mycelial
filaments are always in sinuous curves and intertwined in various ways, so that
they form a felt-like tissue, which looks, in transverse section, delusively like a
parenchyma. As regards colour the cell-filaments are mostly brown, sometimes
they are almost black, and it is rare for them to be colourless. The epidermis of
many roots is covered as if by a spider’s web, whilst the hyphe form a complex
tangle of bundles and strands broken here and there by open meshes through which
the root is visible. In other cases an evenly woven but very thin layer is wrapped
round the root; and in others, again, the fungus-mantle forms a thick layer which
envelops uniformly the entire root (see fig. 59). Here and there the hyphe
insinuate themselves also inside the walls of the epidermal cells, and the latter are
permeated by an extremely fine small-meshed mycelial net (see fig. 59%).
Externally the mantle is either fairly smooth and clearly marked off from the
environment, or else single hyphz and bundles of hyphe proceed from it and
thread their way through the earth. When these branching hyphe are pretty
equal in length they look very much like ordinary root-hairs. And they not only
resemble them, but assume the function of root-hairs. The epidermal cells of the
root, which would in an ordinary way act as absorption-cells, being inclosed in the
mycelial mantle cannot exercise this function, and have relegated the business of
sucking in liquid from the ground to the mycelium. The latter undoubtedly acts
as an absorptive apparatus for the partner on whose roots it has established itself;
and the water in the soil, together with all the mineral salts and other compounds

250 SYMBIOSIS OF PHANEROGAMS AND FUNGI.

dissolved in that water, are caused by the mycelial mantle to pass from the
surrounding ground into the epidermal cells of the root in question, and thence
onward, ascending into axis, branches, and foliage.

Thus the fungus-mycelium not only inflicts no injury on the green-leaved plant
by entering into connection with its roots, but confers a positive benefit, and it is
even questionable whether a number of green-leaved plants could flourish at all
without the assistance of mycelia. The experience gained in the cultivation of
those trees, shrubs, and herbs, which exhibit mycelial mantles on their roots, does
not, at any rate, lead to that conclusion. Every gardener knows that attempts to
rear the various species of winter-green, the bog-whortleberry, broom, heath,
bilberries, cranberries, rhododendrons, the spurge-laurel, and even the silver-fir and

Fig. 59.

1 Roots of the White Poplar with mycelial mantle. 2Tip of a root of the Beech with closely adherent mycelial mantle; x100
(after Frank). 8 Section through a piece of root of the White Poplar with the mycelium entering into the external cells;
x 480.

the beech, in ordinary garden soil are not attended with uniform success. Therefore,
as is well known, soil consisting of vegetable mould from the top layer of earth in
woods or on heath is chosen for the cultivation of species of the genera Hrica,
Daphne, and Rhododendron. But it is not even every kind of forest- or heath-
mould that can be made use of. When earth of that nature has been quite dry for
a long time it is no longer fit for this purpose. On the other hand, it is known that
the above-mentioned plants should be transplanted from their forest-home with the
soil still clinging to the roots, and it is also laid down as an axiom that the roots of
these plants should not be exposed and should be cut as little as possible. The
following reasons account for all this. Firstly, fresh earth from a heath, or mould
recently dug from the ground in a wood, contains the mycelia still alive, whereas in
dry humus they are already dead; secondly, the mycelia woven round the roots are
transferred together with the balls of earthy matter suspended to them into the
garden; and, lastly, any considerable clipping of the roots would remove the
ultimate ramifications which are furnished with the absorbent mycelial manile.
The failure of all attempts to propagate the oak, the beech, heath, rhododendron,
winter-green, broom, or spurge-laurel, by slips or cuttings, if the shoot which is cut

SYMBIOSIS OF PHANEROGAMS AND FUNGL 251

off and used for the purpose is put into pure sand, is explicable in the same way.
Limes, roses, ivy, and pinks, the roots of which possess no mycelial mantle, are
notoriously propagated very easily by putting branches cut from them into damp
sand. Rootlets are at once produced on those parts of the branches which are
buried in the sand, and their absorption-cells carry on the task of taking up
nutriment from the ground. But though cuttings of oak, rhododendron, winter-
green, bog-whortleberry, and broom strike root, no progress in their development is
to be observed, because the superficial cells of the rootlets, in these cases, have not
the power of absorbing food when they are not associated with a mycelium. It is
only when the slips from these plants are put into sand with a rich admixture of
humus, the latter having just been taken from a wood or heath and containing the
germs of mycelia, that some few are successfully brought to further development.
The result is even then often not assured, and the cuttings of several of the plants
enumerated die even in sand mixed with humus before they have produced
rootlets.

Seeing also that the result of attempts to rear seedlings of the beech and the fir
in so-called nutrient solutions, where there could be no question of any union with
a mycelium, has been that the plantlets dragged on a miserable vegetative existence
for a short time and ultimately died, we have good grounds for assuming that the
envelope of mycelial filaments is indispensable for the Phanerogams in question, and
that the prosperity of both is only assured when they are in social alliance.

The facts ascertained in cases of analogous relationship lead one to expect that
the fungus-mycelia also derive some advantage from the flowering-plants, the roots
of which they clothe, and to which they render the service of acting as absorption-
cells. The benefit in question is undoubtedly the same as that derived by the
hyphe of a lichen-thallus from the enwoven green cells. The mycelial mantles
withdraw from the roots of the Phanerogams the organic compounds which have
been elaborated by the green leaves in the sunshine above-ground, and which are
conducted thence to all growing parts, that is to say, downwards as well as in other
directions, to the tips of the swelling and elongating roots. According to this,
therefore, the division of labour between the members of the alliance for joint
nutrition consists in the mycelium supplying the green-leaved plant with materials
from the ground, and the green-leaved plant supplying the mycelium with
substances which have been worked up above-ground in the sunlight.

The range of species which live in a social union such as is here described is
certainly very large. All Pyrolacez, Vaccine, and Arbutez, most, if not all,
Ericacees, Rhododendrons, Daphnoidee, and species of Empetrum, Epacris, and
Genista, a great number of Conifers, and apparently all the Cupuliferz as well as
several Willows and Poplars are dependent for nutrition on the assistance of
mycelia. We find, too, that this condition recurs in every zone and in every region.
The roots of the Arbutus on the shores of the Mediterranean are equipped with
a mycelial mantle in precisely the same manner as those of the low-growing
Whortleberry of the High Alps.

252 SYMBIOSIS OF PHANEROGAMS AND FUNGI.

Special importance is given to the social life by the fact that the chief species of
Phanerogams participating in it are of gregarious growth and cover whole tracts of
country, forming boundless heaths and measureless forests, as, for instance, the
various heaths, the oak, the beech, the fir, and the poplar. The conception of this
subterranean life affecting every moorland and vast timbered tract is one full of
wonder and interest.

We can now see why it is that the ground in woods is the abode of such a
profusion of fungi. No doubt some of these fungi draw their nutriment exclusively
from the store of dead plant-organs accumulated there; but others, as certainly, are
in social connection with the living roots of green-leaved plants. It is true we
cannot yet state precisely what are the species of fungi which contract this sort of
union, or whether generally a definite elective affinity exists between certain fungi
and certain green-leaved plants. There is much in favour of this supposition in
a few cases: but, on the other hand, it is very unlikely that each of the various
Phanerogams occupying a limited area of ground in a pine-forest, where a few
square meters of earth contain so many tangled roots belonging to pines, spurge
laurels, bilberries, cranberries, heath, and winter-green, that they can only be
be separated with difficulty, should select from the great host of fungi growing in
the forest a different partner. In instances of this kind it seems just to suppose
that the mycelium of one and the same species of fungus enters simultaneously into
connection with all or several of the plants growing close together; it is similarly
probable that the mycelia of different species of fungi render to one and the same
flowering-plant the service of absorption according to the locality in which it occurs.
This surmise is supported by the fact that when certain species, brought from distant
parts and regularly exhibiting mycelial mantles on the ends of their roots, are
reared in our gardens and greenhouses from seed, they unite in these abodes with
fungus-mycelia, which certainly do not exist in the regions where the Phanerogams
in question grow wild. Thus, for instance, the roots of the Japanese tree, Sophora
Japonica, and those of the Epacridez of Australia, are found in European gardens
in social union with fungi, which with us are native, but which certainly do not
occur in Japan or Australia; and it is therefore scarcely open to doubt that the
Sophora Japonica, to take one example, associates itself with different fungi in
different regions.

Now that the symbiosis of fungi devoid of chlorophyll with green-leaved
Phanerogams has been discussed, we are for the first time in a position to deal with
that most remarkable of all cases of food-absorption wherein the subterranean roots
of a flowering-plant are completely wrapped in a mycelial mantle, whilst the parts
which shoot up above ground bear no green leaves, and, in general, possess no trace
of chlorophyll. Such is the case of Monotropa, the various species of which are
intimately allied in the structure of flowers and fruit with the Primrose and Winter-
green, and are met with scattered everywhere in shady woods. Their stems, which
are from 10 to 20 centimeters in height and emerge from the mould of the forest-
ground in summer time, are thick, fleshy, succulent, and profusely beset with

SYMBIOSIS OF PHANEROGAMS AND FUNGI 253

membranous and transparent scales, and the extremity of each is bent back like
a hook. The cylindrical flowers are developed at the top of the stem with their
open ends turned to the ground, and are half-covered by the scales. Everything
about this plant (stem, leaf-scales, and flowers) is of a pale waxen-yellow colour,
and the general impression it produces is much more that of a Toothwort, or one of
the colourless forest orchids, than of a species of primula or winter-green. Towards
autumn, when ripe fruits have been produced from the flowers, the hitherto
drooping extremity of the stem lifts itself into an upright position, whilst the
entire aérial portion of the plant turns brown and dries up. Every disturbance
caused by the wind, however slight, shakes out of the spherical fruits many
thousands of tiny seeds as fine as dust, which, like the winter-green seeds, consist of
only a few cells, and do not admit of the recognition of any differentiated embryo
within them. Moreover, underground, the rhizomes, from which the small group of
pale stems have arisen in summer, continue to live through the winter, and a
number of new buds are developed on them. On digging down to the hibernating
plant and removing the mould which conceals it, one finds at a depth of from 10 to
40 centimeters bodies like coral-stems consisting of dense masses of roots crowded
together and ramifying multifariously. All the root-branches are short, thick,
fleshy, and brittle, and are matted together to form turf-like masses, which are not
infrequently interwoven with the rootlets of pines, firs, and beeches, and have all
their interstices filled with humus. Each rootlet is enveloped, right up to the
growing apex, in a thick mycelial mantle. The hyphal filaments of this mycelium
do not penetrate into the tissue of the root of Monotropa, nor do they send any
haustoria into the superficial cells of these roots. The hyphe and the epidermal
cells of the root are, however, in such close and continuous contact that sections
exhibit a complete continuity of the tissues.

Monotropa is therefore only able to withdraw nutriment from the hyphal weft
of the mycelium so far as its subterranean parts are concerned, and, seeing that it
is quite destitute of chlorophyll, and its aérial stem and leaves display no trace of
stomata, the possibility of creating organic matter and of adding in general to its
substance by means of its aérial parts is excluded. It therefore receives all the
materials of which it is constructed from the mycelium of the fungus, whilst it is
not in a position to render anything in return to this mycelium that it has not
previously derived from the latter. If the mycelium subsequently withdraws any
materials whatever from the still living or decaying Monotropa, the process is only
one of restitution and not of exchange. Thus, in this case, there can be no talk of
reciprocity in the processes of nutrition or division of labour such as occurs when
there is symbiosis. The Monotropa grows in height and in circumference entirely
at the expense of the mycelium in which it is imbedded, so that we have here the
remarkable phenomenon of a Phanerogam parasitic in the mycelium of a Fungus.
We so often come across the converse process in our experience that we cannot
easily familiarize ourselves with the idea of a flowering-plant draining the
mycelium of a fungus of nutriment: nevertheless there is scarcely any other inter-

254 ANIMALS AND PLANTS A SYMBIOTIC COMMUNITY

pretation possible in this case, for all the other hypotheses,—such as that Monotropa
enters into connection with the roots of trees, or that it is parasitic in the first
stages of development, but subsequently detaches itself from its host and becomes a
saprophyte,—rest on inaccurate observations, and have long been disproved. Asa
parasite Monotropa ought to have been discussed at the same time as others in
earlier pages, but it was not without intention that the description of this plant was
reserved for this place, for it would have been difficult to state and explain the
method of nutrition exhibited by it before some previous knowledge of the curious
phenomena of union of the mycelia of fungi with the roots of green-leaved
Phanerogams had been acquired.

ANIMALS AND PLANTS CONSIDERED AS A GREAT SYMBIOTIC
COMMUNITY.

If we look back at the cases of symbiosis already discussed and inquire what is
their value, we find it consists in an integration of the functions of plants possessing
chlorophyll and plants not possessing it. The reciprocity here implied is, however,
at bottom, but a copy of the complementary interaction of plants and animals which
takes place on a grand scale in the organic world. The associated plant, destitute
of chlorophyll, in which capacity fungi are always the organisms concerned, really
plays the same part in the social life as is taken by animals in the great economy
of nature, and this is in harmony with the fact that in other respects as well
fungi exhibit so many similarities to animals that in many instances one looks in
vain for a line of division to separate them from animal organisms. Hence there is
no need for surprise when cases come under observation wherein a quite unmis-
takably animal organism enters, instead of a fungus, as one of the partners in a
symbiotic community. Certain Radiolariz have small yellowish spots upon them,
which were formerly held to be pigment-cells, but have proved to be little alge,
with cells furnished with true chlorophyll. Similar properties are exhibited by
the fresh-water polyp, Hydra, and by the marine sea-anemones. Small alge occur
in social union with these also in the shape of cells with membranes made of cellu-
lose and containing chlorophyll and starch-grains in their protoplasmic bodies.
These algze are in no wise injurious to the animals with which they are associated;
on the contrary, their presence is beneficial, their partners reaping an advantage
from the fact that the green constituents split up carbonic acid under the influence
of the sun’s rays, and in so doing liberate oxygen which may be again taken in by
the animals direct, and serve a useful purpose in their respiration and all the pro-
cesses connected therewith. Conversely, the alga, in association with the animal’s
body, will derive a further advantage from the latter, inasmuch as it receives at
first hand the carbonic acid exhaled by the animal in breathing. The small alge
living socially with animals cannot be reckoned as parasites in any case, nor
can the animals be looked upon as parasites of the alge, but we have here the
phenomenon of mutual assistance and of a bond serving for the benefit of both

ANIMALS AND PLANTS A SYMBIOTIC COMMUNITY. 255

parties, precisely similar to that noticed in the case of lichens and in the others
which have been described above.

Several of the liverworts which live as epiphytes on the bark of trees exhibit
on the under surface of their leaflets (which are inserted on the stem in two rows,
and are pressed flat against the bark) little auricular structures, and in species of
the genus Frullania, these take the form of definite hoods or pitchers. The rain
that trickles down the trunks of the trees, washing the bark and wetting the liver-
worts in its course, fills the hooded receptacles referred to with water, and is retained
longer in these protected cavities than anywhere else, if a period of drought ensues
and the liverwort becomes dry again. Now these cowls are the abode of tiny
rotifers (Callidina symbiotica and C. Leitgebii), which live on the organic dust
brought thither with the water. In return for the peaceful home thus afforded
them in the hooded chambers of the leaves, the rotifers supply the liverworts in
question with nitrogenous food. For as such must serve the matter excreted by the
rotifers in the interior of the cowls. Without the intervention of the rotifers, the
living organisms (Infusoria, Nostocinez, and spores) contained in the water could
not be converted into food by the liverworts, whereas the liquid manure arising
from the Infusoria, Nostocinex, and spores, digested in the bodies of the rotifers,
contains highly nitrogenous compounds, which are of great value to the liverworts
in question, as indeed they are to all epiphytes living on the bark of trees. It
stands to reason that the symbiotic liverworts and rotifers derive also a mutual
advantage from the fact that the oxygen set free by the former comes into the
possession of the rotifers and the carbonic acid emitted by the rotifers into that of
the liverworts by the most direct method.

Moreover, these cases of partnerships further remind us of other analogous rela-
tions existing between plants and animals, which it is necessary to refer to now,
although they cannot be treated in detail till later on. A great number of flowering-
plants excrete honey into their flowers, and so attract flying insects to them,
which supply themselves plentifully, and in their turn render to the plants they
visit the service of transferring the pollen from flower to flower, thus making
possible the development of fruits and fertile seeds. Certain small moths which
visit the flowers of Yucca bring the pollen to the stigmas, and force it into the
stigmatic orifices in order that mature fruits and seeds may be produced from the
rudimentary fruits, a result which is indeed a matter of vital importance to these
moths. For the moths lay their eggs in the carpels of Yucca, and from the eggs
larvea are developed which live exclusively on the seeds of this plant. If the
Yucca were not fertilized, and did not develop any fruit, the larvee would die of
hunger. A similar phenomenon occurs in many other cases of the kind, where
both plant and animal reap some benefit. On the other hand, in the formation of
galls, which are produced by animals laying their eggs in particular parts of plants,
the advantage (with few exceptions) is all on the side of the animals, and these gall-
structures might most justly be placed by the side of parasitic structures.

It is obvious from all this that such of the mutual relations of plants and of

256 ANIMALS AND PLANTS A SYMBIOTIC COMMUNITY.

their relations to animals as are occasioned by the endeavour to acquire nutriment
are extremely various and often linked together and complicated or deranged by
one another in the most curious manner. Cases occur of a particular plant being
socially connected with another, and at the same time also beset by vegetable and
animal parasites. The absorption-roots of the Black Poplar are covered with a
dense mycelial mantle, so that this tree is associated for purposes of nutrition with
the fungus to which the mycelium belongs. Such parts of the roots of the Black
Poplar as are left free from the mycelium are fastened upon by suckers sent forth
by Toothwort plants, which withdraw from the roots the juices absorbed by the
latter from the earth through the instrumentality of the mycelial mantles clothing
them. Meantime, in the cavities in the leaves of the Toothwort various small
animals are caught and made use of as nitrogenous food. Again, the poplar-tree
bears Mistletoe on its boughs, and its presence there is due to the missel-thrush.
The thrush takes the Mistletoe-berries for food, and, in return, renders the plant
the service of dispersing the seeds and establishing them on other trees. The para-
sitic Mistletoe takes its liquid nutriment from the wood of the poplar-tree; but, on
the other hand, its own stems are covered with lichens, and these lichens are them-
selves a symbiotic community of alge and fungi. Within the wood of the poplar-
stems spread the mycelia of certain Basidiomycetes (Panus conchatus and Poly-
porus populinus), whilst the foliage-leaves are covered with a little orange-coloured
fungus, Melampsora populina. In addition, no less than three gall-creating species
of Pemphigus live on the leaves and branches of the Poplar, and a number of
beetles and butterflies are nourished by them. Certain lichens, mosses, and liver-
worts regularly settle on the bark of old trunks, and included amongst these may
be the species of liverwort which is inhabited by rotifers. If all the plants and
animals which live upon the poplar-tree, within it or in association with it, are
counted, the number turns out to be not much fewer than fifty.

ACTION OF PLANTS ON THE SOIL. 257

7. CHANGES IN THE SOIL INCIDENT TO THE NUTRITION
OF PLANTS.

Solution, displacement, and accumulation of particular mineral constituents of the soil owing to the

action of living plants.—Accumulation and decomposition of dead plants.—Mechanical changes
effected in the soil by plants.

SOLUTION, DISPLACEMENT, AND ACCUMULATION OF PARTICULAR MINERAL
CONSTITUENTS OF THE SOIL RESULTING FROM THE ACTION OF PLANTS.

Reference was made in the preceding section to a marble pillar on the faces of
which a dozen different lichens have settled in the course of centuries. I again
introduce to the reader’s notice this unobtrusive monument in order to demonstrate
in its case the changes to which stone is subjected by the plants clinging to it or
nestling in its crevices. It may be premised, as a matter of course, that when the
marble column was erected two hundred years ago the eight sides were polished,
and presented perfectly even surfaces. But what is its appearance to-day? The
whole is rough and uneven; in parts it is as though corroded, and there are little
pits clustered together in places. The idea might arise that depressions have been
formed in course of time by the impact of drops of rain, but nearer inspection
shows that there can be no question that the inequalities have been produced in
this way; on the contrary, it is by the influence of the lichens adherent to the stone.
Especially on the two sides of the pillar facing south and south-west, one sees
clearly how each pit corresponds exactly in size to a species of grey lichen there
ensconced, and how this lichen, as it continues to grow and extends radially,
corrodes and etches the marble it touches in ever-widening circles. The expression
“to etch” may here be taken literally, for there is no doubt that the process, the
result of which is manifested in the formation of little pits, is mainly caused by the
excretion of carbonic acid from the lichen’s hyphe, whereby the calcium carbonate
is converted into bicarbonate. The latter, being soluble in water, is, in part, taken
up by the lichen as nutriment, whilst part is washed away by the rain.

In addition to this chemical action, the hyphal filaments exercise also a purely
mechanical influence. A growing hypha penetrates wherever the merest particle of
carbonate of lime has been dissolved and accomplishes regular mining operations at
the spot. Projecting particles of the carbonate not yet dissolved are separated by
mechanical pressure from the main mass; and at the places in question where a
lichen is in a state of energetic growth, tiny loose rhombohedral fragments of the
lime are to be seen, which are washed away by the next shower or else carried off
as dust by the wind. The same process as that which may be so clearly traced on
the marble pillar at Ambras takes place, of course, also on the limestone that has not
been carved or polished, in every locality where lichens exist at all. We notice it
in the case of other kinds of stone as well—in dolomite, felspar, and even in pure

quartz rock—for even quartz is not able to withstand the long-continued action of
: Vou. I. 17

258 ACTION OF PLANTS ON THE SOIL.

carbonic acid and the mechanical operations above referred to in the performance
of which the hyphe act like levers. Some of the powerful iron bands belonging
to the great suspension bridge across the Danube at Budapest afford us the
opportunity of observing the mining operations of lichens on a substratum of pure
iron. Of course in these cases the decomposition and solution initiated by the
carbonic acid varies according to the nature of the substratum; the result is,
however, invariably the same; there is always a loss of substance on the part of the
substratum, and a part of the dissolved matter is always taken up by the adherent
plant, whilst another part is carried away either in solution or mechanically by
wind or rain,

Mosses act in precisely the same manner as lichens. If a tuft of Grimmia
apocarpa is lifted away from the side of a block of limestone, it becomes evident
that in the neighbourhood of the place where all the stemlets of the little moss-
colony meet, the underlying stone is threaded through and through, and rendered
friable. There lie the rhizoids imbedded between isolated particles of lime, which
are as fine as dust, and have been disintegrated by the chemical and mechanical
activity of the organs in question. At spots where plants of Grimmua have died,
the limestone always exhibits an obvious loss of substance in the form of unevenly
corroded depressions.

The fact that the roots of Phanerogams also alter the subjacent stone in a
similar manner may be proved by the following experiment. A polished slab of
marble is covered with a layer of sand, and seeds of plants caused to germinate in
this sand. The roots of the seedlings as they grow downwards come almost
immediately upon the marble slab, and, turning round, creep onward in close
contact with the stone. After a short time the parts of the slab against which the
roots are pressed become rough as though they had been etched; a solution of
individual particles of the carbonate of lime takes place under the influence of the
acid juice saturating the cell-walls of the root’s cells, and this circumstance reveals
itself to the naked eye as a roughness which is readily perceptible.

Whereas the loss of substance affecting the solid substratum of plants may thus
be at once detected by sight, the removal of constituents of the air and of water
eludes direct observation. The ingredients withdrawn by plants are instantly
replaced in water and still more in the air by influx from the environment, and
obviously no holes or pits are the outcome as in the case of a surface of limestone rock.

In the discussions that follow it is important to retain the conception that in
the process of vegetable nutrition certain substances may undergo local displace-
ment, accumulation, and aggregation, and temporary consignment to a state of
quiescence. Ingredients of the earth’s crust are borne upwards into atmospheric
regions, and constituent parts of the air are carried deep down into the ground.
Lime, potash, silicie acid, iron, &., pass from disintegrated rocks into the realms
above ground—into stems and leaves, and to the tops of the highest trees, whilst
carbon and nitrogen pass from the aérial shoots and from the foliage spread out in
the sunshine into the deepest shafts which the roots have bored for themselves in

ACTION OF PLANTS ON THE SOIL. 259

the ground. If one were to mark out the space of ground from which the lime,
potash, and other nutrient salts used in the construction of a birch-tree were
derived, its bulk would certainly be found to be much larger than that of the birch;
and, if we were to try to estimate the volume of air through which the carbon,
which has been converted into organic compounds in the tree, was previously dis-
tributed in the form of carbon dioxide, it would turn out to exceed the volume of
the birch a thousandfold. In this sense, every plant may justly be considered as
an accumulator of those substances which serve for its nutriment. Every plant
continues, so long as it lives, to store them up in ever-increasing quantities in its
own body, and in the case of long-lived plants there is thus collected ultimately
quite a considerable quantity. When the life of an accumulator of the kind is
extinguished, those materials which were taken from the atmosphere are able to
return into the atmosphere; but such mineral food as has been derived from the
ground and lifted into the upper parts of the plant—particularly those above the
ground—and has there been amassed in a confined space, does not return to its
original place. A dead tree breaks down on the first provocation, and the trunk
lies on the ground and rots. Such part of its substance as can pass into the atmos-
phere in gaseous form escapes; but the salts accumulated within it, which it raised
from deep under ground during its lifetime, are retained by the surface-layers of
the soil. Even though some of them are washed out of the trunk by the lixiviating
action of rain-water, the superficial layers of earth operate as a filter, and do not
allow any part to return to the underlying strata. So, too, the nutrient salts which
reach the foliage of plants are added to the top layers of the soil; for fallen leaves
go through much the same process as the trunk which is broken by storms and
undergoes decay as it lies prostrate upon the ground.

Thus, wherever men do not interfere by clearing away the accumulative agents
in question, i.e. plants; where there is no removal of the haulms of cereals from
fields, or of mown grass and herbs from meadows to serve as hay, or of timber from
the forest—wherever, in a word, the vegetable world is left to itself and the
natural progress of evolution is not frustrated by any disturbing element—the
food-salts which have been amassed will accumulate in the uppermost layers of the
earth. Moreover, seeing that, as has been already pointed out, every plant has the
power of possessing itself of substances of value to it, even when they are only
present in the environment of the roots in scarcely appreciable quantities, it is
possible for the top layers of soil to contain a considerable amount of a substance
which only occurs in the subjacent rock in such small measure as to be detected
with difficulty. The percentage of lime yielded by the subsoil on the Blockenstein,
a granitic mountain 1383 meters high, on the borders of Bavaria and Upper Austria,
was 2°7, whilst that of the top layer was 19°7; the percentage on Mount Lusen,
situated to the north of the Bléckenstein, was 1:9 for the subsoil and 8°6 for the
superficial layer. When one considers that fresh plants strike root in the ground
near the surface and these again act as accumulators, and remembers in addition
that snails make their appearance in abundance wherever vegetable food containing

260 ACTION OF PLANTS ON THE SOIL.

lime is to be found, that these snails again are to be reckoned as accumulators, and
that their shells, which consist almost entirely of lime, remain after the animals’
deaths in the top layer of soil, it is not surprising to find that the earth-mould on a
granite plateau contains a proportion of lime not much less than that yielded by
mould resting on argillaceous limestone:

Still more striking than the influence of rock plants and land plants in trans-
posing and accumulating lime is the agency of hydrophytes in causing the same
results. In the trickling springs of mountainous regions as well as in the standing
pools of level country and no less in the depths of the sea, plants occur which
obtain part of the carbonic acid they require by the decomposition of the bicar-
bonate of lime dissolved in the surrounding water. The monocarbonate of lime,
which is insoluble in water, is then precipitated in the form of incrustations upon
the leaves and stems of the plants in question. Many of these hydrophytes take up
carbonate of lime into the substance of their cell-membranes; and in other cases both
phenomena occur, that is to say, not only are they incrusted externally with calcium
carbonate, but the cell-walls are also thoroughly impregnated by the same salt. In
the streams arising from springs loaded with bicarbonate of lime in solution derived
from the heart of a mountain, a number of mosses regularly occur—Gymnostomum
curvirostre, Trichostomum tophacewm, Hypnum faleatwm, and others besides.
These mosses and also several species of Nostocinese belonging to the genera
Dasyactis and Huactis become completely incrusted with lime, in the manner
referred to, but go on growing at the apical end as the older and lower parts
imbedded in lime die off. In consequence, the bed of the stream itself becomes
calcified and elevated, and, in course of time, banks of calcareous tufa are formed,
which may attain to considerable dimensions. Banks raised in this manner are
known which are no less than 16 meters in height; to construct them mosses must
have worked for more than 2000 years.

Numerous Stoneworts (species of Chara or Nitella), the Water-milfoil and Horn-
wort (Myriophyllwm and Ceratophyllwm), Water-crowfoots (Ranunculus divari-
catus and R. aquatilis), and more especially many Pond-weeds (Potamogeton),
which grow in continuous masses in still, inland waters, incrust their delicate stems
and leaves with lime during the summer, but in autumn shrink away, that is to say,
their stems and leaves fall and decay, leaving scarcely any trace of the mass of
vegetation till the advent of the following spring. The calcareous deposits, how-
ever, are preserved, and, sinking to the bottom of the water where the incrusted
plants lived, form a layer which year by year increases in thickness. Anyone who
undertakes the investigation of the sequestered wastes of water in the shallow
lakes of lowland districts will be convinced of the magnitude of the scale on which
this kind of accumulation must take place. As one’s boat glides over places where
there is a luxuriant growth of the lime-incrusted Chara rudis and C. ceratophylla,
there is a crepitating sound in the water like the snapping of dry sticks of birch-
wood. Great numbers of stoneworts are fractured by the boat as it strikes against
them, and if one takes hold of the fragments they feel like a heap of brittle glass

ACTION OF PLANTS ON THE SOIL. 261

fibres. What a quantity of carbonate of lime must be deposited yearly at the
bottom of these lakes and ponds! Amongst pond-weeds, Potamogeton lucens, in
particular, clothes its large shining leaves with a very stout, uniform crust,
which drops off in scales as the plant dries, the weight of which can be exactly
determined in the case of each separate leaf. The result of careful weighing showed
that a single leaf equal in weight to 0°492 grm. was covered with a calcareous crust
weighing 1:040 grm. Now, supposing one shoot of this pond-weed, having five
leaves, and covering an area of 1 square decimeter, decays in the autumn, and lets
its lime sink to the bottom of the pond, the approximate weight of lime deposited
each year on a square decimeter of the ground at the bottom is 5 grms., and, if
this process is repeated every year, a layer is deposited in ten years which weighs
50 grms., and consists of calcium carbonate and traces of iron, manganese, and
silicic acid.

There is no doubt that it is possible for calcareous strata of great depth to be
produced in this way in fresh water. That also in times past lacustrine deposits of
lime have had a similar origin is inferred from the fact that the spore-fruits of
stoneworts (Characez) and the nutlets of pond-weeds have been found over and
over again inclosed in these formations of lime. Calcareous deposits originating
in this manner are, at present at least, less frequent in the sea. Still, the Aceta-
bularie undergo similar changes there, and may be the cause of an elevation of
the sea bottom and of an accumulation of lime. On the other hand, in the sea,
the Lithothamnia and Corallinas play a predominant part, and form—just like
true corals, and often indeed in conjunction with these and other marine animals—
lime reefs of great magnitude.

The agency of plants may occasion accumulations of iron hydroxide, silicic acid,
and salts of potassium and sodium at particular places besides lime. The formation
of meadow iron-ore, spring iron-ore, and bog iron-ore, the construction of tripoli,
agate, and flint, by the conglomeration of siliceous-coated Diatomacez, and the
accumulation of potassium and sodium salts in the superficial strata of salt steppes
are processes which take place essentially in the same manner as the piling up of
carbonate of lime, although upon a more modest scale.

The question now arises, why it is that the substances which are stored in pre-
ponderant quantities in the vegetable frame, which are the main constituents of the
living part of plants, and represent the alpha and omega of plant life, are not pre-
served as well as the mineral food-salts in question. Why do not carbon and
nitrogen, materials so eagerly appropriated by the living plant, compounded by it
with the elements of water, secured in some measure in organic compounds, and
constituting the fundamental mass of the vegetable structure, remain behind in the
same condition after the death of the plant? When autumn comes and the lime-
laden pond-weed dies, only the calcareous crust falls to the ground, and, at the
bottom of the pond, enters upon a period of quiescence. The tissue of the plant

1In the case investigated 96 per cent calcium carbonate, 0-28 per cent iron oxide, 1°51 manganese oxide, and 1°51
per cent silicic acid; the last, from the Diatomaces, settled on the calcareous crust.

262 ACTION OF PLANTS ON THE SOIL.

itselfi—all its carbohydrates and albuminoid compounds—cannot remain dormant,
but are split up without delay into those simpler compounds of which they were
compounded in the summer; and, by the following spring there is nothing more to
be seen of any of the pond-weed’s stems and leaves. Certainly this is only to such
a conspicuous extent true of plants living under water; dead plants buried in earth
or exposed to the atmosphere are resolved less rapidly, and under certain circum-
stances deposits of organic remains on limited areas are preserved even almost
unaltered through boundless ages.

Let us try to obtain a somewhat closer knowledge of these various degrees of
preservation. Thoroughly dried wood, leaves, and fruit, if protected from all but
transient moisture, are capable of being preserved unaltered for long periods of time.
When wood is exposed in a dry place to the sun, it turns brown, and in the course
of years becomes quite black outside, the most superficial layers being regularly
carbonized, as may be seen particularly well in the case of woodwork situated
under the projecting roofs of old mountain chalets. This wood exhibits no sign of
crumbling, mouldering, or rotting. In the dry chambers of old Egyptian graves
fruits, foliage, and flowers have been found which were laid by the side of the
corpses 3000 years ago, and they had not undergone a greater change than if they
had been dried but a few days. Even the colours of flowers of the Larkspur, the
Safflower, and other plants of the kind, were still to be seen, and the separate
stamens in Poppy flowers were in a state of complete preservation. Dryness there-
fore may be looked upon par eacellence as one of the preventives of the decomposi-
tion of organic matter.

The same result as is secured by dryness in the cases cited is brought about in
the ground of moors by humous acids. The dead plants saturated with these acids
are not resolved into carbonic acid, water and ammonia, but preserve their form and
weight almost unaltered, and are converted into peat. Above the mass of peat new
generations of plants continue to spring up and produce ever fresh organic matter,
which, in its turn, becomes peat, and is added to the mass beneath, so that gradually
a very deep bed of organic matter may be accumulated in this manner. In the low
country lying between East Friesland and the Hiimmling, from the river Hunte to
the marshes on the Dollart, there is a stretch of nearly 3000 sq. kilometers covered
with a layer of peat which has an average depth of 10 meters.

Of minor importance is the preservation of dead plants and parts of plants in
snow and ice. The leaves, twigs, and seeds, which are carried by the wind on to
the snow-fields of the high mountains, remain there a long time almost without
alteration in respect of form or size; they only turn brown under the influence of
the intense sunlight, and at last become quite black as though they were carbonized,
which, in fact, they are. So also such insects as meet their death on the snow-fields
are converted there into a black, cindery mass. Indeed, even all the minutest
organic fragments lying on a glacier become carbonized, and this explains the fact
that the so-called eryokone, or snow-dust, which we have already had occasion to
allude to, has a graphitic appearance.

ACTION OF PLANTS ON THE SOIL. 263

Dead leaves, haulms, branches, and tree-trunks, when they rest upon damp
ground, as also lifeless roots, rhizomes, bulbs, and tubers, buried in moist earth, pass
into a state of putrefaction, provided that their temperature does not fall below
freezing-point, that is to say, they are resolved into carbonic acid, water, and
ammonia, the rapidity of the process varying directly as the supply of water and
the degree of temperature to which the dead matter is exposed, and inversely as the
quantity of compounds of humous acid present. If more dead fragments of plants
accumulate within a particular interval of time on one spot than decay, a formation
of vegetable mould takes place there; on the other hand, the ground remains
destitute of humus when the entire accretion of organic matter is quickly decom-
posed as soon as it is dead. The general fact turns out to be that the decomposition
of organic bodies is prevented, or at least limited, by a dry condition, and is
promoted by moisture, and that it can only be prevented in moist surroundings by
the presence of large quantities of humous acids, or by the temperature being low
enough to turn water into ice.

This result directs attention to those inconceivably small animate beings, which,
as has been proved by experience, are arrested in their activity by scarcity of water
and are killed by the antiseptic substances referred to. That they are the cause of
the resolution of dead plants is corroborated by the facts that they are always
present where vegetable putrefaction is in progress, and that, on the other hand,
decomposition can be prevented by rendering the access of these minute organisms
impossible. First in importance in this respect of course are bacteria, the causal
connection of which with processes of dissolution, and especially with those decom-
positions, which are known by the name of putrefaction, is established. Of these
bacteria, Bacteriwm Termo, and several micrococci, bacilli, vibriones, and spirilla,
are the commonest. Their multiplication and the withdrawal for this purpose of
substances from dead plants cause a splitting up of the organic compounds in the
latter. The albuminoid compounds are first of all peptonized; next, tyrosin, leucin,
volatile fatty acids, ammonia, carbon-dioxide, sulphuretted hydrogen, and water
are formed, this stage of the process being accompanied by the evolution of an
offensive odour of decomposition, and later, nitrous and nitric acids are produced by
further oxidation. The carbohydrates, too, chiefly cellulose and starch, are split up,
and the products of this analysis, in so far as they are not used up by the bacteria
for their growth and reproduction, pass in a gaseous condition into the atmosphere,
or into the water surrounding the dead plants. Moreover, the bacteria themselves
do not remain at the spots where they have been battening on vegetable matter, but
swarm away through the water, or else come to rest in a short time, in which case
if the seat of their activity dries up they are blown away by currents of air, and so
conveyed to other dead plants. Similar decompositions can be induced by moulds
(Eurotiwm, Mucor, Botrytis cinerea, Penicillium glawcwm) as well as by bacteria,
and, in addition, the disintegration of wood occasioned by the mycelium of Dry-rot
(Merulius lacrymans), the green-rot: of trunks of oaks, and beeches, caused by
Peziza eruginosa, the mouldering of wood induced by the mycelium of Polyporus

264 ACTION OF PLANTS ON THE SOIL.

sulfwreus and various other fungi, the red-rot, &c., all depend on similar disruptions
of the organic compounds in dead plants, and result in the ultimate dispersal of
these in the air in the form of carbon-dioxide, ammonia, nitric acid, and water.

Thus, ultimately, the exercise of this destructive activity only effects a return of
the compounds just enumerated—the most important to plant-life—to the regions
whence they had previously been withdrawn by the plants when living. Carbon
and nitrogen, in particular, are set free from their bonds and given back to the
atmosphere in the form and combination in which they are capable of being
appropriated anew by living plants as food-material.

Considered from this point of view the phenomena of putrefaction and rotting
appear as important and even necessary incidents in the history of the substances
which are of the greatest importance to plants. Abhorrence of putrefaction is
innate in us all, and everything connected with it—in particular, the entire race of
bacteria—is looked upon with aversion. To estimate these processes according to
their deserts requires a sort of self-abnegation. But when we overcome our
repugnance and weigh the whole subject impartially, we come to the conclusion that
the continued existence of vegetable life and of life in general depends upon the
occurrence of putrefaction. If the untold numbers of plants which die in the course
of a year did not rot sooner or later, but remained unchanged as lifeless forms, a
certain quantity of carbon and nitrogen would be idle, being withdrawn from the

- sphere of activity and locked up, so to speak. Now, assuming this to be repeated
year by year, a time must come when all the carbon and nitrogen would be
imprisoned in dead plants. Thereupon, all life would cease, and the whole earth
would be one great bed of corpses.

Not only putrefaction, but also the minute organisms which excite putrefaction
appear in a more favourable light when viewed from this standpoint. Let such
bacteria as act in the capacity of foes to the human race, ravaging town and
village in the form of infectious diseases, be exterminated if possible; but annihila-
tion of putrefactive bacteria would mean a disastrous interference with the cycle of
life upon the earth. These latter are not to be reckoned as enemies but friends to
human beings. The effect of their invasion of dead plants and animals is certainly
first made manifest, not in the most agreeable manner, for some of the substances
mentioned as being evolved in the early stages of the onslaught, viz.: various
ammoniacal compounds, sulphuretted hydrogen, and the volatile fatty acids, are
disgusting to us; but as decomposition advances these phenomena, which are so
unpleasant to our senses, abate, and the action of putrefactive bacteria becomes
ultimately a beneficent process of purification of the last remnants of dead
organisms. The final result of the decomposition of organic bodies by bacteria has
been termed mineralization. It is a fact that nothing is ultimately left behind, in
the ground or water, of bodies decomposed by the indefatigable exertions of bacteria
excepting some nitric acid and the small quantity of mineral food-salts which has

been taken up by the living organism in its time and are now in the form of dust
and ash.

MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS. 265

By filling with water a glass which contains vegetable and animal remains in
a state of putrefaction and swarming with bacteria, one is enabled to follow this
process of mineralization from day to day. First, a decrease of the organic matter
clouding the liquid, accompanied by simultaneous increase of ammonia and nitrous
and nitric acids, is observed; then, after about two months, a complete clearing up
of the liquid. The water is now colourless and odourless, but a precipitate has
formed at the bottom, which contains, in addition to insoluble food-salts, bacteria in
a state of temporary quiescence on the termination of their task and waiting till
fresh prey becomes accessible. No doubt these processes occur in nature in just the
same manner as in the glass of water, and the so-called self-purification of rivers,
for example, has been rightly attributed to mineralization. It was long ago noticed
that the water of such rivers as flow through great towns and consequently take up
considerable quantities of animal and vegetable refuse contains no discoverable
trace of all these impurities a few miles below the mouths of the drainage pipes and
sewers. The water of the Elbe, which receives the refuse of the towns of Prague,
Dresden, and Magdeburg, is so pure at Hamburg that it is there used for drinking
purposes without protest’. The Seine, after taking up masses of rubbish in Paris, is
already by the time it reaches Meulan, a distance of 70 kilometers, clear and pure
again, and does not even exhibit there any traces of the organic matter received in
the great city. Were it not for the activity of the putrefactive bacteria, this
purification would never take place; and although the statement that putrefactive
bacteria are the best of purifiers sounds at first like a paradox, it must be
acknowledged to be consistent and based on experience.

MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS.

All the alterations hitherto spoken of as being brought about in earth and
under the influence of vegetation subsisting therein are reducible to chemical
transpositions. Added to these, there are always certain purely mechanical changes.
The penetration of the rhizoids of a rock-moss or the hyphe of a crustaceous lichen
into limestone is accompanied, as has been already stated, by a solution of part of
the substratum and a mechanical separation of another part; the rhizoids or hyphe,
as the case may be, becoming imbedded amongst tiny detached fragments of the
underlying stone. When the hyphe and rhizoids die, the corresponding piece of
the substratum is left porous, and admits air and water, whilst other plants are
enabled to settle on it, although they may not perhaps possess the power of eating
into stone and pulverizing it in the same degree as their predecessors. This is also
true of the roots of Phanerogams. The food-seeking root-tips and their absorption-
cells displace particles of earth as they insinuate themselves, and when they decay
later on, the soil at those particular places is intersected by passages of varying size.
No doubt these passages mostly collapse like the abandoned shafts and galleries of a
mine, but some trace of root-action always remains behind in the shape of an

1This was written before the last outbreak of cholera.

266 MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS.

increased looseness of the soil in the locality, a result of the greatest importance,
inasmuch as it enables air and water to permeate to a depth much more easily and
quickly by the ways that the roots have previously opened up. Dead roots rotting
underground constitute, moreover, the source of the carbonic and nitric acids which
help to render available the mineral constituents, and so serve the turn of subsequent
generations of plant-settlers on the same spots, whilst they accomplish fresh disin-
tegration of the substance of the soil.

If, however, the subterranean parts of plants are continually engaged in mining,
and so change in various ways the position of the component particles of soil, the
organs above ground exert an influence in some measure opposed thereto, in that
they retain and bring to rest particles of earth which are set in motion by currents
of air or water. In the section that treats of the absorption of nutrient salts by
lithophytes, attention was directed to the fact that the dust pervading the atmos-
phere, and blown from place to place by the wind, is arrested to a remarkable extent
by mosses and lichens. One need only detach a small tuft of the common Barbula
muralis, which everywhere occurs on walls by roadsides, to convince oneself of the
extent to which dust from the road is lodged amongst the leaves and stemlets, and
of the tenacity of its adhesion. Moreover, not only such dust as rises from roads,
but also that variety which, though not easily observed, yet fills the air of remote
mountain-valleys, of arctic ice-fields, and of the most elevated parts of the earth’s
crust, is arrested in those localities by mosses and liverworts, and by many Phanero-
gams besides, the growth of which is similar to that of mosses. There is not much
less dust clinging amongst the stemlets of the dark Grimmias, Andrezas, and other
rock-mosses, which grow in small cushion-like tufts on weather-beaten mountain
crags, than is attached to the Barbula living by the dusty roadside. If one of the
tufts in question is detached from its substratum, a fine powder composed of mica-
scales, granules of quartz, chips of felspar, and a number of minute organic frag-
ments pours out from between the moss-stems, whilst another portion of this finely
powdered earth is left clinging to the leaves and stemlets, and is found to be regu-
larly adnate to them.

It is never, however, the still fresh and living upper parts of these leafy moss-
stems that arrest and carry dust, but always the older dead parts below. The lower
dead half of the moss, whether still in a state of preservation or already rotting, is
alone capable (in consequence of characteristic alterations in the lifeless cell-tissue)
of holding fast the atmospheric dust. The under part of moderate-sized cushions of
moss constitutes a compact mass composed half of imprisoned dust and half of
brown lifeless moss-stems. These little cushions, clothing rocky crags, become a
favourable site for the germination of a whole host of seeds, which are conveyed
thither by the wind and detained in the same manner as the dust. The seedlings
arising from these seeds send their rootlets into the subjacent portion of the bed
of moss, where the interstices are full of dust or finely-divided earth. Here they
find all the conditions prevailing necessary for their nourishment, and they expand,
and, little by little, crowd out the mosses which received them so hospitably,

MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS. 267

forming ultimately a bed of flowering plants, including in especial abundance
representatives of the orders of grasses, pinks, and composites.

Many water-plants—in particular, aquatic mosses and algee—possess, in an
almost greater degree than lithophytes or land plants, the power of laying hold of
inorganic particles, and thus exercise a far-reaching influence as mud-collectors on
the conformation of the ground. It is wonderful how plants are able to arrest large
quantities of the fine sand hurried along by a flood, although they are exposed
to the violent rush of the water. The tufts of the dark green alga Lemanea
fluviatilis and of the aquatic moss Cinclidotus riparius, which cling to rocks in
the cascades of clear and rapid mountain torrents, are so conglomerated by mud and
sand that they cannot be freed therefrom until the tissue has become dry and
shrivelled. Limnobiwm molle, which grows in the turbid waters from glaciers, has
such an abundance of earthy particles adhering to it that only the green tips of the
leaf-bearing stems are visible above the grey-coloured cushions imbedded in the
mud. ‘The felted masses of Vawcheria clavata, filling the channels of apparently
clear, gently-flowing streams, are so mixed with mud that if a lump of this alga is
fished out, the weight of mud clinging to it exceeds that of the alga itself a
hundredfold. In these cases of submerged plants, it is, again, not the living but the
dead parts which serve to arrest the mud. On lifting up a lump one sees clearly
that only the uppermost and youngest prolongations of the filaments—those situated
at the periphery of the algal cushion as a whole—are living and filled with chloro-
phyll; the fundamental mass has become colourless and lifeless. But these dead
parts, which form a thick felt of interwoven filaments, alone retain in their meshes
the finely-divided mud and sand in such surprising quantities; these particles slip
off the green living parts without adhering to them. An important consideration
in this connection is the fact that the dead cell-membranes swell up and become
slightly mucilaginous, so that fine particles of mud lodge more easily in the soft
‘swollen substratum thus formed. Wooden stakes stripped of bark and fixed in a
strong current show this very clearly, as do also the trunks of trees that are thrown
up by floods and lie stranded on the shore with their bared boughs projecting into
the stream. However strong the current to which wood in that condition is ex-
posed, it covers itself in a short time with a grey coat consisting of earthy particles
brought down by the water. If a piece is cut off and exposed to the air, the earthy
deposit does not become detached until the wood-cells have dried up and shrunk.
As long as they are moist the particles of mud continue to adhere to them.

This mechanical retention and storage of dust by rock-plants and of mud by
aquatic plants is of the greatest importance in determining the development of the
earth’s covering of vegetation. The first settlers on the bare ground are crustaceous
lichens, minute mosses and alge. On the substratum prepared by them, larger
lichens, mosses, and alge are able to gain a footing. The dead filaments, stems,
and leaves pertaining to this second generation arrest dust in the air and mud in
the water, and thus prepare a soft bed for the germs of a third generation, which
on rocks consists of grasses, composites, pinks, and other small herbs, and in water

268 MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS.

of pond-weeds, water-crowfoots, hornwort, and various plants of the kind. The
second generation is produced in greater abundance than the first, and the third
develops more luxuriantly than the second. The third may be followed by a fourth,
fifth, and sixth. Each successive generation crushes out and supplants the one
preceding it.

As on the rocky heights and in the roaring torrents of mountains, so also on the
sandy plain and in the depths of the sea, a perpetual variation in the nature of the
vegetation is taking place. At all times and in all places we see younger genera-
tions displacing the older and building upon the foundations laid by their pre-
decessors. The first settlers have a hard fight with uncompromising elements to
seize possession of the lifeless ground. Years go by before a second generation is
enabled to develop in greater luxuriance upon the earth prepared by the first
occupiers; but there is no cessation in the productive and regulative effects of
vegetable life, and its energy and aptitude in the work result in the erection of its
green edifices over wider and wider areas. New germs are established upon the
mouldered dust of dead races, and others on the plant forms adapted to the altered
substratum, and so, for hundreds and thousands of years, the changes go on, until
at length the tops of forest-trees wave above a black and deep soil, the battle-field
of a number of bygone generations. Thus, the life of plants, like that of the human
race, has its epochs and its history: as in the one so in the other a continual
struggle prevails; processes of ousting and of renovation are always in progress, and
there are ever new arrivals upon and departures from the scene.

CONDUCTION OF FOOD.

1. MECHANICS OF THE MOVEMENT OF THE RAW FOOD-SAP.

Capillarity and root-pressure.—Transpiration.

CAPILLARITY AND ROOT-PRESSURE.

Unicellular plants make use individually of the food material which they
absorb from their surroundings, and work it up into the organic substances which
they require for their structure and increase in bulk, and also for the production of
future generations. In all plants composed, on the other hand, of aggregates of
cells, there is a division of labour. Of the protoplasts occupying the cell-cavities
of such larger plant-structures, one part provides for the absorption of the water
and food-salts, another for the taking in of the gases which are used as food,
and yet another part works up this food into organic substances for construc-
tive purposes. The centres in which these various industries are carried on are
frequently situated at some distance from one another, and it is obvious that
there must not only be some communication between the various regions of activity,
but that active forces must come into play which will effect the transport of the
food from the cells whose function it is to receive it, to those in which it is to be
elaborated into building material. It is evident that the greater the distance is
between the various centres of the plant in question, the more difficult will be the
performance of this task. In aquatic plants and lithophytes, all of whose superficial
cells have the power of taking in nourishment from their environment, these
distances are proportionately small, while they attain their greatest dimensions in
land-plants whose roots are embedded in the earth, and whose leaves are surrounded
by air. In trees the food materials which are taken up by the absorbing roots
beneath the ground must frequently travel far more than 100 metres before reach-
ing the topmost leaves. The path to the summit is very steep, and the fluid in
rising must be able to overcome the force of gravitation, which has no inconsider-
able significance at heights such as these.

Naturally, desire for knowledge has at all times directed attention to this
phenomenon, and the most diverse attempts have been made to explain by what
means the food-sap taken in by the roots of trees is enabled to reach their
summits. It was first considered to be in virtue of capillarity; that just as oil,
alcohol, or water, is drawn up the wick of a lamp, the liquid food can rise in the
delicate tubular cell-formations called vessels, which, united together in groups or

270 CAPILLARITY AND ROOT-PRESSURE,

bundles, traverse the stems and leaves of plants. But the vessels are closed in
above and below, and therefore it is impossible that capillarity should be sufficiently
developed in them. At best it could only raise the sap a trifling distance, and
could never convey fluid to a height of many metres. It is a striking fact that in
many plants the ascent of the sap is most vigorous after the evaporation from the
superficial parts exposed to the air has been weakest. The so-called “weeping” of
vines, 7.e. the outflow of sap from the flat surface of a cut vine-branch, does not
take place in summer and autumn, immediately after the branch has been fully
adorned with foliage, and when its extensive leaf-surfaces have given up large
quantities of moisture to the surrounding air; it occurs at the end of the winter
sleep of the plants, when the brown branches rising above the ground are still in
a bare and leafless condition. The cause of the ascent, or at least of the ascent
in the lower leafless branches, must therefore be sought for in the absorbent roots,
and it may be assumed that here the same causes are at work which induce the
fluid food materials of the surrounding earth to enter the superficial cells at the
root-tips.

It has already been shown that the contents of these cells suck up the water of
the nutritive ground with great force in consequence of the chemical affinity they
have for it, or in other words, that the fluid reaches the interior of plant-cells by
endosmosis; it has also been mentioned that in conscquence of the taking in of
water the volume of the cell-contents increases, producing pressure from within
outwards on the cell-wall, and the cell swells and becomes turgid. From this one of
three cases might be deduced:—first, suppose that the cell-wall is so composed
throughout that it allows the entrance of water into the cell, but not its exit, and
that consequently the cell-contents absorb water, but that a filtration of the same
towards the exterior cannot take place. Granted this hypothesis, the cell-wall by
virtue of its elasticity would yield to the pressure of the cell-contents, but only
within the limits of that elasticity; hence a condition of tension would be produced,
in which the reciprocal pressures of the cell-wall and cell-contents would be in
equilibrium. In the second case, suppose that the pressure of the cell-contents is
greater than the force of cohesion between the molecules of the cell-wall, this
consequently ruptures, and the cell-contents issue from the rent which is formed.
This phenomenon is seen in certain pollen grains when placed in water. In half
a second the cells absorb so much water that they double their volume; the cell-
contents still absorb the fluid, and the cell-wall can at length no longer withstand
the pressure; it bursts, and the contents, from which the pressure is now removed,
pour through the opening, and are diffused in the surrounding water.

There is a third case possible. Suppose that in a given cell the opposite walls
are not of identical structure; that the wall which is in contact with the damp earth
is so organized as to allow the entrance of water, but not its filtration to the
exterior, while the opposite wall offers only a slight resistance to such filtration;
then by the increasing pressure of the cell-contents fluid will be forced through
that wall which offers least resistance, and the greater the affinity of the cell-

CAPILLARITY AND ROOT-PRESSURE. 271

contents for the fluids in the nutritive earth, the more abundantly and energetically
will this be carried on. The phenomenon can be well seen in some moulds,
especially Mucor Mucedo, which makes its appearance in such quantity on
succulent fruits; and in the mycelium of the so-called Dry-rot, Merulius lacry-
mans. Fluids are sucked up by the lower portions of the tubular cells which cover
the nutritive substratum, and expelled again through the walls of upper parts
of the same cells, which project freely into the air. These upper portions of the
mycelium cells appear as though ornamented with tiny dewdrops, which in the
case of the Dry-rot coalesce and attain to a considerable size. Damp woodwork in
cellars, where this fungus has established itself, is often thickly besprinkled with
the drops which have been excreted on the surface, and if a lamp is brought into
the darkness, and the infected places illuminated, hundreds of these tiny drops
sparkle and glitter like the “jewels” in a cave of stalactites. Suppose then that
such a cell, one wall of which allows fluid to enter, is attached by the wall opposite
to that through which the fluid enters, to another cell; then this second cell will
absorb the liquid, and, if tubular, the sap may rise higher and higher in it, and by
the pressure of the liquid continually arising from below, even be forced through
other higher cells which are capable of filtration. Naturally the rising current of
sap thus generated follows the line of the least resistance; if then the cell-tissue
where this action terminates is perforated by canals ending in pores on the surface,
the fluid will emerge from these pores in the form of drops. This actually happens
not only in many large-leaved Aroids, but also in plants growing in the open
country if the air which passes over the leafy parts above the ground be very
humid, and the soil in which the roots are buried proportionately warm. In many
plants with succulent foliage, drops of water may be seen issuing from the thin-
walled cells and pores of the leaves when the almost saturated air becomes cooled
after sunset, while the soil, round about the absorbent roots, having been exposed
all the day to the sun’s rays, retains its higher temperature. Young blades of
corn have rows of such drops, which look exactly like dewdrops, and have often
been mistaken for them. This extrusion of water from the leaves can easily be
produced artificially by placing the plants in a saturated atmosphere, and at the
same time slightly warming the earth round the roots. There is no doubt that
the sap which exudes from the leaf-pores originates in the nutritive soil, and is
taken up by the absorbent cells of the root; from these the vessels and cells of
the main root and stem, through which the sap can filter, carry it up to the leaves.
If, therefore, we cut across a stem a little distance above the ground, we shall see
the sap, which has already accomplished half its journey, welling up as drops on
the cut surface; 7.e. we shall see the remarkable phenomenon called “weeping”,
of which mention has already been made. The quantity of sap which flows
from such a cut surface is in many cases astoundingly great. In Java certain
Cissus plants, belonging to the family of lianes and living in damp woods, are
actually made use of as vegetable springs. The watery sap flows so abundantly
from a cut branch that in a very short time it will fill a glass, and forms a cool and

272 CAPILLARITY AND ROOT-PRESSURE.

refreshing beverage. Many Araliacee also furnish a sap fit for drinking. Some
native Indian genera which are used as vegetable wells have on this account
received the name of “plant springs” (Phytocrene, eg. P. gigantea and bracteata).
If the young flower-stalk of Agave americana, an American plant which is
cultivated in European gardens under the name of the “hundred years’ aloe”, be
cut across, in twenty-four hours about 365 grammes, and in a week more than
2500 grammes of sap will flow out. This exudation continues for four to five
months, and a vigorous Agave will produce in this time as much as 50 kilogrammes
of sap, which will ferment, since it contains both sugar and albuminous substances,
and is indeed used by the Americans in the preparation of an intoxicating drink
called “ pulque”. The quantity of sap which exudes from vines is also very great.
A branch 24 em. thick, cut across 14 m. above the ground, produced within a week
over 5 kil. of sap. In a week, from the cut stem of a rose, more than 1 kil. was
exuded. From maples and birches a proportionately large amount of sap can be
obtained, when the trunks are cut about a metre above the ground. The sap which
flows from species of maple contains pure crystallizable sugar, and in some North
American species this is present in such abundance that it was found to be worth
while to collect the sap, at least in former times.

It should be noticed that the volume of the exuded sap is in all these cases
greater than the volume of the root together with that of the stump of the stem
from which the sap is forced out, and this is a proof that it does not consist only of
the water which was contained in the root and stem stump at the time of cutting,
but that there is a continual upward current of sap, and that the absorbent cells of
the roots, for a long time after the operation, continue to draw up water from their
environment.

An ingenious experiment was performed at the beginning of last century in
order to ascertain the amount of pressure by means of which the sap is forced from
the cut surface of the vine and other stems. A vine stem without branches and
about the thickness of one’s finger was cut across in the spring at a height of about
80 cms. above the ground, and on the root-stock was fixed a glass tube with a
double bend, in such a way that one end fitted exactly over the cut surface of the
stump, and the tube was then filled with mercury. By the pressure of the sap
which welled from the cut surface the mercury was forced up the tube, and in a
few days it actually reached a height of 856 mm. The weight of a column of
mercury 760 mm. high is equal to that of a column of air as high as the atmosphere
of the earth, or of a column of water about 103 m. high, and consequently the
pressure by which the sap is forced out of the vine is considerably greater than the
weight of one atmosphere, or of a column of water of the height mentioned. From
these data it has been estimated that the sap can be raised through 11:6 m. by the
pressure originating in the absorbent cells of the root. The pressure is naturally
greatest in the lower portions of a stem, and gradually diminishes towards the
higher regions; the ascending current of sap to which it gives rise is also not
uniform, but shows daily, and even hourly, fluctuations. Moreover, the quantity of

TRANSPIRATION. 273

sap exuded, neglecting these said fluctuations, is greatest soon after the stem is cut,
and then becomes gradually less until finally the outflow ceases entirely with the
death of the stump.

The magnitude of the pressure, and the quantity of the sap forced up by the
absorptive power of the cells, vary with the circumstances of the plants considered.
The pressure appears to be greatest in species of vine, and in the vine stem, as
already remarked, it will support the weight of a column of mercury 856 mm. high.
In the stem of the Foxglove it equals the pressure of a column of mercury 461 mm.
high; in the stem of the nettle the column is 354 mm.; in the poppy stem 212 mm.;
in the stem of a bean 159 mm.; and in the trunk of the White Mulberry tree 12
mm. high. In the majority of herbaceous plants this pressure is quite sufficient to
drive the sap from the root-tips up to the leaves and top of the stem; but this is
not the case with leafy trees and pines, with palms and creeping and climbing
plants. Although watery fluid can be raised according to the above calculation to
a height of 11°6 m. by root-pressure, there is still a great distance between this level
and the leaves of such trees and climbing plants, which may be as much as 160 m.
high; and the question which presents itself is this: By what means is the sap
carried to the higher regions from this level to which it is raised by root-pressure?

It may be supposed that cells are present at the various heights in the stem to
which the water is driven, which act in a manner similar to those of the root; we.
cells which actively absorb, whose cell-wall on one side only slightly resists filtra-
tion, and which therefore are able to force up the sap a little higher. The results of
the following experiments seem to support such a supposition. If a piece of a
branch be cut from the middle portion of a tree, and the lower end be peeled and
placed in water, sap will flow out from the upper cut surface with considerable
force. The same thing occurs when a leafy branch is placed in water so that its
leaves are submerged, while the upper cut piece of the branch projects a good way
out of the water. In this case the cells of the leaves must function as the absorptive
cells. However, even if, as is probable, parenchymatous cells are to be found at all
levels of the plant stem behaving exactly like the absorptive cells of the root,
this arrangement would scarcely suffice in all cases to carry the sap to its destination.
Atmospheric pressure as well as the rarefaction of the air observed in the vessels of
the stem during the summer have been made use of in explaining the upward
current of the sap, and this réle may actually belong to these factors; but all these
mechanical powers are quite overshadowed by that one which has been termed by
botanists “ Transpiration”.

TRANSPIRATION.

By transpiration of plants we mean the act of giving off aqueous vapour to the
surrounding air—briefly and in plain terms, the perspiring of plants. Vapour
escapes from the cells of the plant which are in contact with the air, the formation

of these cells being specially adapted to the process of evaporation, just as i is given
VoL. I.

274 TRANSPIRATION.

off from moist inorganic bodies and exposed liquids. Of the materials which are
held in solution in the sap of plants, only those which have the property of passing
from the fluid to the gaseous condition, at the same temperature which transforms
water into water-vapour, can evaporate with this fluid. All the others remain
behind, and the natural consequence is that the sap in the transpiring cells becomes
more concentrated. If water, which contains in solution extremely small quantities
of sugar, organic acids, nitric, sulphuric and phosphoric acids, and salts of potassium,
calcium, and iron, be set to evaporate slowly in a shallow dish, it will gradually
come about that only a thin layer of fluid is left on the bottom of the dish; but this
now is seen to consist of a very concentrated solution of the substances mentioned;
we. of the sugar, organic acids, and the various salts. It has also all the properties
of such a concentrated solution, 7.¢. it has the power of sucking in water in the
liquid condition from its surroundings. In the same way the contents of a cell in
contact with the air become more concentrated by evaporation, and thus obtain the
power of abstracting water from the environment of the cell, that is to say, of suck-
ing it up. ‘If two adjacent cells contain sap of the same density, whilst only one of
them has the power of exhaling water, the condition of equilibrium between them
will be destroyed. ' However, the balance naturally tends to be restored, and the cell
whose sap has become more concentrated by the evaporation of water, takes up
watery fluid from the neighbouring cell. Now picture a chain of cells containing
abundance of sap connected with one another by cell-walls through which fluid can
filter, and let them be so arranged that only the uppermost member of the chain is
in contact with the atmospheric air. The sap of this uppermost cell having become
concentrated by evaporation will first of all exert a suction on the cell immediately
below. As fluid is withdrawn from this second cell, its sap also undergoes concen-
tration, and in consequence produces suction on the third cell, the third in like
manner on the fourth, the fourth on the fifth, &., passing from above downwards.
In this way innumerable compensating currents are set up between the adjoining
cells, which, however, never lead to true equilibrium as long as evaporation con-
tinues in the cell in contact with the air, but combine together to form a single
ascending stream.

Such a current actually exists in all living plants which evaporate from the
portions above the ground and in contact with the air, while their lower extremities
are embedded in a damp nutritious soil. This has been termed the Transpiration
Current. Its source is the fluid which has been drawn from the earth by the
absorptive cells and brought within the sphere of the living cells of the plant; we
may retain for this fluid the old and very appropriate name “crude” or “raw sap”.
Its direction and destination are determined by the position of the evaporating cells,
and its path is through the wood, which in tree-trunks is inserted as a huge layer
between the bark and the pith; in lesser stems it passes through the bundles and
strands of woody cells and vessels which traverse them, being connected, deep under
the ground, by groups of parenchymatous cells, with the absorptive cells of the
young rootlets, or with the hyphe of the mycelial mantle, which replace the

TRANSPIRATION.

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276 TRANSPIRATION.

absorptive cells (beech, &.). These bundles pass above into the leaves, forming there
the “veins” of the leaf-blade, which spread out into an extremely fine network of
tiny strands, and terminate quite close to the evaporating cells on the surface. That
the wood actually forms the conducting tissue of the transpiration current is satis-
factorily demonstrated by the existence of old trees whose trunks have long been
hollow, whose pith is disintegrated and fallen away, and which have also been de-
prived of bark around their base. In the olive plantations at Lake Garda, one of
which is reproduced in figure 60, many trees are to be seen in which the lower part
of the trunk is not only hollow and without bark, but is also often tunnelled and
split, so that the upper part of the tree looks as if it were raised on stilts. The only
communication between the soil and the upper part of the tree is by means of these
props, which are continuous with the roots’ below and are composed entirely of
woody cells and vessels. And yet these olive-trees are still vigorous, putting out
new branches and leaves every year, and blossoming and producing fruit; and
they derive their necessary food from the ground by supplies which have no other
upward path than the wood of these props.

Moreover, by repeated experiments it has been proved that the bundles of woody
cells and vessels which are united together into a woody cylinder, inserted between
the pith and the cortex in the trunks and stems of trees and shrubs, serve as con-
ductors of the transpiration current. Ifa ring of cortex is removed from the stem
of a leafy plant, whose leaves are transpiring in dry air, and are supplied with
water from below by the transpiration current, this flow of sap to the leaves will
not be interrupted, and the leaves remain firm and tense. But as soon as a piece of
the wood is removed or the above-mentioned strands are cut through, even though
the cortex be left entire, the flow to the leaves stops immediately, and they become
flaccid and hang down in a withered condition.

The cellular formations of the wood and strands, which function as the con-
ductors of the crude nutritive sap to the leaves, are—as already mentioned—wood-
cells and wood-vessels. Formerly the idea was held that these structures served for
the passage of air, and it was believed that they were analogous to the respiratory
organs—the so-called tracheze—of insects; therefore these wood-vessels were also
ealled “trachese”, and the wood-cells “tracheides”. The wood-cells are elongated
chambers, on an average 1 mm. long and 0:05-0°1 mm. broad, and their walls are
unequally thickened, either by reticulate or annular bands, or spiral threads project-
ing slightly from the inner wall into the lumen, or by so-called bordered pits, which
are represented in fig. 107 and fig. 10°. The wood-vessels are tubular, and very long
in proportion to their width, which is never more than a fraction of a millimetre;
they extend uninterruptedly through stalks, branches, leaves, perhaps even through
the entire plant from the root-tip to the crown. They are composed of rows of cells
whose separation walls have been broken down. The walls of the wood-vessels
exhibit similar thickenings to those of the wood-cells or tracheides. When the
chambers and tubes of the wood, with their bordered pits and projecting bands, are
fully developed, the living protoplasm which carried on the building forsakes the

TRANSPIRATION. Q77

scenes of its activity, and consequently in fully formed wood-cells and vessels living
protoplasmic contents are wanting. They must be regarded in a certain sense as
dead structures, for they have no further power of growth, and the reciprocal
pressure of wall and contents observable in absorptive cells and other cell-cavities
occupied by living protoplasm, which has been termed “twrgescence”, is never seen
in them.

In the walls of the wood-cells as well as of the vessels, woody material (Lignin)
is deposited. It appears to be in consequence of this that they are much less
capable of swelling than are cell-walls which consist chiefly of cellulose. The
amount of sap which presses its way in between the groups of molecules of the
lignified walls, and with which these walls are saturated, is also comparatively very
small. On the other hand, of course, this imbibed sap is conducted much more
quickly through the lignified walls of the cell chambers and tubes than through non-
lignified walls. More fluid is carried up by the intermolecular stream through the
woody walls of the cells and vessels than by the ascension of the raw nutritive sap

| in the interior of the wood-cells and tubes. If no evaporation is going on from the
leaves, or if this is only very slight, the vessels and cells become filled with sap. As
soon as transpiration becomes active, part of the sap is taken up, and if fresh
supplies do not arrive quickly enough a limited amount of air can get in temporarily,
which of course must be in a very rarefied condition on account of the obstacles
which oppose its entrance. The passage of the sap is quicker through the non-
septate vessels than through the much shorter woody cells. The sap on its way
through the latter, to the transpiring leaves, must filter through innumerable trans-
verse walls. This filtration will of course be materially helped by the bordered pits
with which the wood-cells are so regularly provided; for the extremely delicate
membrane which is stretched between the two cavities of such an apparatus at any
rate allows the sap to pass through very easily. The bordered pits are exactly like
clack-valves, and they also appear to regulate the sap-stream, though the way in
which they do this is not yet completely understood. The nearer the path of the
raw sap approaches to the spots in which evaporation is being carried on, the greater
is the number of cells in the sap-conducting strands, while the vessels in the same
become fewer and fewer. The termination of the whole sap-conducting apparatus
consists entirely of cells whose walls are stiffened by spiral bands on the inside.
Between this termination and the transpiring cells some parenchymatous cells with
living protoplasmic cell-contents are interposed, whereas, it must again be insisted,
the tubes and chambers composing the sap-conducting apparatus have no living
protoplasm in their interior.

The whole mechanism for the transmission of the raw nutritive sap may be con-
sidered as a system of tubes and chambers provided with clack-valves, into which
the fluid taken up by the absorbent root-cells is forced, and through which it is con-
ducted to the transpiring cells of the green leaves or of the green cortex, which takes
the place of the green leaves in leafless branches. This does not exclude the activity
of cells at certain levels, as it were at intermediate stages of the road traversed by

278 TRANSPIRATION.

the current, which have the power of invigorating the stream, of hastening it if
necessary, and also of lessening it under certain circumstances. Also it is arranged
that in case of need fluid nourishment in the higher regions of the stem may reach
the leaves by side paths.

The cells which by means of the exhalation of aqueous vapour into the atmos-
phere originates the transpiration-current are, as already mentioned, not far from the
terminations of the sap-conducting apparatus. In some mosses they are freely ex-
posed to the air. In the Polytrichacez and several other mosses (Barbula aloides,
ambigua, rigida) they form short chains of cells like strings of pearls, or bands
projecting from the grooved concave upper surface of the tiny leaves (see fig. 61 ”).
Again, among the liverworts are forms, e.g. Marchantia polymorpha, which contain
large characteristic air-chambers in the body of their green leaf-like thallus (fig.
617). On the fioor of this chamber are green cells which are so grouped together

Fig. 61.—Transpiring Cells.

1 Vertical section through an air-chamber of the Liverwort Marchantia polymorpha; x300. 2 Vertical section through
a leaf of the Moss Barbula aloides; x 380.

as to remind one of the shape of the Prickly Pear (Opuntia). These green cells are
thin-walled, and it is from them that water is evaporated. They are not quite
freely exposed, like those of the mosses mentioned above, since the roof of the
chamber, composed of transparent cells, is extended over them; a chimney-shaped
passage, however, is left open through the roof of each chamber by which the water-
vapour given off from the opuntia-like cells can escape. These Marchantias furnish
a transitional form between the freely exposed transpiring cells on the upper surface
of the leaf of the moss and those of flowering plants. In flowering plants the
transpiring cells are situated as a rule in the interior of the green leaves, and also
in the green cortex of leafless branches, forming a part of that green tissue which
has been termed chlorenchyma, or when in the leaves, mesophyll.

Leaves may be described as consisting of cells filled with leaf-green, or chloro-
phyll, placed closely together and joined into layers above one another so as to
form a soft mass of tissue containing abundance of sap; this green tissue pierced
by the branched water-conducting strands whose ultimate divisions terminate in the
tissue mass; the whole surrounded and shut in by a firm cuticle which is perforated
in many places by stomata. Cellular passages are also regularly arranged for the
purpose of conducting away the organic materials manufactured in the green cells,
whilst groups of cells for the support of the whole, serving as beams, strengthening
props, and the like, are placed at definite points.

TRANSPIRATION. 279

In most thin membraneous leaves the upper and under sides are differently
constructed, and the difference is not confined only to the cuticle, but is also plainly
recognizable in the green tissue. The green cells below the epidermis on the upper
side of the leaf have the form of prisms, cylinders, or short tubes, and are arranged
very regularly in ranks and files. In the leaves of plants belonging to the lily
tribe, they lie with their long axes parallel to the surface; but in most other plants
these cylindrical cells have their smaller side directed to the surface, and stand side
by side like palisades, with only very narrow air-passages between them. Below
these palisade-cells, and bordering on the epidermis of the under side of the leaf, is
another stratum of cells of a much looser texture (see fig. 621). The cells of this
under layer are not so crammed with chlorophyll, and therefore appear a lighter

Fig. 62.—Spongy Tissue.

1 Vertical section through leaf of Franciscea eximia. %Spongy tissue in leaf of Daphne Laureola.—The epidermis and
palisade cells of the upper side of the leaf are removed. The epidermis of the under side of the leaf, with its stomata,
can be seen through the spaces in the spongy tissue; x 320.

green than the palisade-cells. In shape they are elliptical, rounded, angular, sinuous,
or generally very irregular; usually they possess protuberances which project in
various directions, and they are so arranged that the outgrowths of adjoining cells
come into contact with one another. It looks as if the neighbouring cells were
stretching out their arms and extending their hands to one another, and consequently
these cells have been called “ many-armed cells”. When several adjoining stellate
cells are connected together in the manner just described so as to form a tissue,
lacune and passages are seen in the tissue, which are broken through by the joined
arms of neighbouring cells as if by pillars, couplings, and bridges. The whole tissue
has the loose perforated appearance of a bath sponge, and is called accordingly
spongy tissue, or spongy parenchyma, (see fig. 62”).

This spongy tissue is the proper place for transpiration. Nowhere else in the
plant are the conditions governing this process so well fulfilled as just here. The
surfaces of the cells are rendered large in proportion to their size by their out-
growths; and they impinge as far as possible on the larger or smaller lacunae,
gaps, and passages filled with air, which all communicate with one another, thus
constituting an unmistakable ventilating system.

Since the spongy parenchyma in the leaves described does not lie freely exposed,

280 TRANSPIRATION.

but is shut off from the atmosphere by a firm cuticle through which water-vapour
can only penetrate with great difficulty, the aqueous vapour which is exhaled by
the branched and other cells of this parenchyma would saturate the lacune, and
further evaporation would be thereby prevented. There must, therefore, be a
direct communication with the outer air surrounding the leaf; the epidermis of the
leaf must possess apertures through which the water-vapour can escape. The already
repeatedly mentioned stomata are to be looked upon as such apertures.

Stomata arise in this way; in a particular epidermal cell a partition wall first of
all divides it into two cells. This cell-wall splits, and the cleft widens, forming
a short canal which pierces the epidermis, and constitutes a connection between the
outer air and the air-containing lacune in the interior of the leaf. This short canal
is called the pore of the stoma, and the two cells which border it are termed guard
cells. These two cells regulate the outrush of aqueous vapour, 1.e. of that vapour
which has been excreted by the thin-walled cells of the spongy parenchyma, and
passed into the adjoining passages in the interior of the leaf. That cavity which is
placed immediately beneath the narrow, short canal of the stoma, and is connected
by passages with other spaces further within the green tissue of the leaf, is termed
the respiratory cavity.

The number of the stomata or transpiration-pores which pierce the epidermis of
the leaf varies very considerably. In the leaves of cabbages (Brassica oleracea) in
1 sq. mm. of the upper surface there are nearly 400, and on the under side over 700.
In the leaves of the olive-tree, on the same extent of surface of the under side, over
600. Succulent plants have remarkably few stomata. On 1 sq. mm. of the leaves
of the House-leek (Sempervivum tectorum) and of the yellow Stone-crop (Sedum
acre) only 10-20 are to be met with. In the majority of cases, on a similar extent
of surface, between 200 and 800 stomata are to be found. The under side of an
oak leaf, 50 sq. cms. in area, showed over two million stomata. They are in most
cases scattered fairly uniformly over the surface of the leaf; on the leaves of grasses
and pines, as well as on the green stalks of the horsetails, they form straight
regular rows which run longitudinally; on the leaves of some species of saxifrage
(Saxifraga sarmentosa, japonica, &ec.) they appear crowded together in small
isolated groups; and on the leaves of the Begonia they are generally to be seen side
by side in pairs. Obviously they are principally developed just where the epidermis
overlies spongy parenchyma, and as in the majority of cases this parenchyma is
situated towards the under side of the leaf, the greater number of stomata are to be
found on this side.

In most flat membraneous leaves, which have one side directed towards the sky
and one towards the earth, stomata are entirely wanting on the upper surface,
being restricted to the under side. An exception to this is afforded by the orbicular
flat leaves which float on the surface of water, e.g. those of the floating Pond-weed
(Potamogeton natans), of the Frogbit (Hydrocharis morsus-rane), and of the
water-lilies (Vymphea, Nuphar, Victoria). These are covered with stomata on the
upper side, while on the lower side, which is in contact with the water, stomata are

Ansa

TRANSPIRATION. 281

entirely absent. On the upright leaves of flags, asphodels, amaryllis, and various
other bulbous plants, and on the vertical leaf-like structures (phyllodes) of the
Australian acacias, as well as on some of the needle-like leaves of conifers, the
stomata occur on both sides in almost equal number. In the mimosas and various
other plants, having, in common with the mimosas, the characteristic faculty of
altering the position of their leaflets when stimulated externally, numerous
stomata are found on both sides of the leaf. :

Most stomata are elliptical when open; rarely circular or linear. The length of
stomates varies between 0:02 and 0:08 mm., the breadth between 0:01 and 0:08 mm.
Pines, orchids, lilies, and grasses have the largest stomata; water-lilies, olives, and
some fig-trees, the smallest.

The stomata in the epidermis, the passages and cavities below them into which
the thin-walled cells of the green tissue evaporate water, and the strands through
which the sap is conducted from the roots to the green tissue, all work in connec-
tion with one another like the various parts of a machine. Each portion of the
mechanism helps and depends upon the others, the immediate result of the common
work being always the elevation of that nutritive fluid which is brought by the
absorptive roots into the plant. In the main, therefore, the result obtained by
transpiration is the same as that which root-pressure aims at, and it might be
thought (taking for granted the truth of the above statement) that either root-
pressure or transpiration is superfluous. Or perhaps transpiration and root-pressure
work in a complementary manner together. Perhaps the conditions between the
two forces are so arranged that the fluid taken in by the absorptive cells from the
nutritive soil is forced up to a certain level by root-pressure, and from thence is
promoted to still higher levels by means of transpiration? This would suggest a
comparison with the raising of water from a spring situated in a valley-basin sur-
rounded and shut in by mountains. In the depth of the basin exists underground
water which is fed by the subterranean supply coming from the mountains. Ac-
cording to the pressure of this supply; the water in the lower earth-strata of the
basin rises to a certain height. This pressure is not strong enough, however, to
drive the water to the surface of the basin, and in order that it may reach this, it
is necessary to employ a pump, which will reach down to that stratum of earth
which is saturated by the underground water. But the level of this water
differs in summer and winter. It depends also upon the amount of rainfall on the
neighbouring mountains, which may undergo great fluctuations. In some years the
underground water in the spring has almost risen to the upper opening; at other
times only the deepest strata of the valley-basin contain water. The pump, by
which the water has to be raised, must be constructed with all these possibilities in
view, and must be so regulated that the absorbent action is felt as far down as the
deepest position which the underground water is known to take.

Transpiration behaves in like manner in the portions of a plant above ground,
and its action on the fluid food taken in by the roots may be compared with that of
a suction-pump. It would be a quite inadequate arrangement if the sucking action

282 TRANSPIRATION,

produced by transpiration could only reach down to the highest level attained by
the water which has been forced up by root-pressure, and precautions must be
taken that, in case of the abatement of the root-pressure, water would be raised
from the lower positions up to the transpiring cells, and that under certain condi-
tions the action of transpiration should reach even to the absorbent cells at the
root-tips. It has been shown by experiments that plants with large leaves lose in
the summer more water by transpiration than is forced up into the stem by root-
pressure, and yet the leaves do not become faded. The conclusion drawn from this
is that at certain times the effect of transpiration makes itself felt down from the
leaves through the stem as far as the root-tips. It has also been shown that in
many plants, just when the most active evaporation is taking place in the leaves,
none, or only very little sap is forced into the stem by root-pressure. If the stem
of a vine be cut across in the height of summer, when the green leaves have been
unfolded some time and are transpiring actively, no “tears” are seen on the cut
surface of the stump, no drops are pressed out. The vessels contain rarefied air
but no sap, and water can be sucked through the stump by the vessels even in the
direction of the root.

Let us pause here in order to get a clear idea of the relations between transpira-
tion and root-pressure. Given the conditions for an abundant evaporation from
the aérial portions of a plant—ie. a fairly dry air, water, and an appropriate
development of transpiring surface—then the action of root-pressure is diminished,
while that of transpiration is increased, and governs the whole of the movement of
the sap. If, on the other hand, the conditions for evaporation from the aérial por-
tions of the plant are unfavourable—if the air is very damp, or if the branches of
the plant are not yet in leaf—then root-pressure comes into play, and, supported by
cells with absorbent contents which occur in the higher regions of the plant, can
force up the sap to the tops of trees and to the highest shoots of vine-branches
which remain leafless all the winter. So far, therefore, root-pressure can supersede
and replace transpiration, a fact of great importance in places where the air is
sometimes very damp, and in countries where the trees and lianes shed their leaves
in autumn; at the commencement of the next period of vegetation they have not
yet put out their new foliage, and therefore do not possess a sufficiently large tran-
spiring surface. It is very probable that in the autumn, when preparing for the
winter, certain cells in trees and lianes provide themselves with materials by means
of which in the coming spring they may exercise a very strong sucking action.
This would also partly explain how it comes about that in the spring there is such
a strong upward current of sap in the still leafless trees and vine branches, and
that the water is conducted up even to the topmost shoots of lianes 100 metres
long, which have shed all their leaves in the previous autumn.

A perfect substitute for transpiration in the form of the pressure produced by
the absorbent cells is seen in moulds, in the already-mentioned dry-rot fungus, and
generally in leafless cryptogams: possibly also in those orchids possessing neither
green leaves nor stomata, and in other humus plants (saprophytes) such as the

oe

TRANSPIRATION, 283

Monotropa, mentioned earlier on, which stands in such a peculiar relation to the
mycelium of fungi. On the other hand, in most green flowering plants which bear
leaves, a complete replacement of transpiration, continuing for a long time, is not an
advantage. Experience has shown that green leafy plants, when kept for a long
while in an atmosphere saturated with vapour, cease to grow and become unhealthy;
they lose their leaves, and at length succumb altogether. This happens even if the
amount of light, the temperature of the atmosphere and of the earth, the composi-
tion and humidity of the soil, in short, if all the other conditions of life are the
most favourable that can be imagined for the plants in question. It follows from
this that it is not immaterial to leafy plants how the sap reaches the leaves,
whether it is drawn up by transpiration, or forced up by root-pressure. If the leaf
transpires, water, in the form of vapour only, is given off to the atmosphere; all the
materials which have been brought in solution from below to the leaves remain
behind in the cells of the leaf. If, on the other hand, fluid water is pressed from
the pores of the leaves by root-pressure, salts, sugar, and other compounds are always
to be found in the exuded drops, having passed through the cell-wall in solution in
the water. When it is a question of secreting sugar as a means of alluring insects,
or salts for a protective covering, such an exudation cannot advantageously be given
up, but is on the contrary a fundamental part of the economy of the whole plant.
If this is not the case, and if materials which have a part to perform in the leaf by
the formation of organic substances are exuded with the drops of water, and the
drops falling from the surface of the plant trickle to the ground, there is loss of
material, which does not contribute to the advantage, but rather to the detriment,
of the organism.

The signification of transpiration may be explained in this way. By transpira-
tion not only is water brought from below to the more highly situated parts of the
plant, but nutritive salts in solution are also conducted to the green tissue of those
branches and leaves which are exposed to light and air. The greater part of the
ascended water is only used as a medium for the transmission of mineral salts,
which have been taken from the soil into the plant. When it has reached the
leaves, most of the water evaporates into the atmosphere, while the salts conducted
by it into the green tissue remain behind, in order to take part in the chemical
changes by which organic compounds are manufactured out of the raw materials.
These salts are indispensable here, and transpiration is therefore also necessary in a
corresponding degree. Without transpiration, it would be impossible that plants,
whose green branches and leaves are surrounded by air, or that trees, which rank
before all other plants on account of their superior size, could be properly nourished;
consequently transpiration must be regarded as one of the most important life-
processes of terrestrial plants.

284 MEANS OF ACCELERATING TRANSPIRATION.

2. REGULATION OF TRANSPIRATION.

Means of accelerating transpiration.—Maintenance of a free passage for aqueous vapour.

MEANS OF ACCELERATING TRANSPIRATION.

Aquatic plants do not transpire; therefore they do not require either vascular
bundles or stomata. Neither trees nor shrubs grow under water, and even the
largest Floridee and the most gigantic sea-wracks have no wood nor stomata.
These structures are on the other hand very important for land plants, and in these
they are developed in extraordinary variety. When one considers how much the
humidity and temperature of the air, those very conditions which influence the
transpiration of plants, are continually changing, this diversity is not really
surprising. What endless series of gradations there are between the damp air of
a tropical estuary, and the arid wastes in the interior of large continents! What
varieties of temperature in the different zones and regions of the earth, and in the
changing seasons; what differences, even in a narrow space in a single small valley,
between the conditions of moisture of the air and ground in the depths of a shady
glen, and on the sunny, rocky slopes! In the one place the air is so saturated with
water-vapour that even evaporation cannot take place from exposed pieces of water,
much less then from plants; in the other it is so dry and the sun is so strong that
plants can hardly suck up enough from the ground to compensate for the water
evaporated from their surface. In the former case contrivances must be devised
which will promote transpiration as much as possible; in the latter, however, it is
important that too much evaporation, which would cause the drying up and death
of the plant, should be prevented.

One of the most important ways of increasing transpiration consists in the
development of many cells whose surface is in contact to the greatest possible
extent with the atmospheric air, and which are so organized that water in the form
of vapour can be exhaled from them. Further, it is of importance that the access
of air to these cells is not rendered difficult, and that as great a portion as possible
of these cell-groups, which help in transpiration, are reached by the rays of the sun.
It is only in the delicate-leaved mosses, which have no stomata, that the whole of
the cells of a leaf, in contact with the air, give off unlimited water, in the form of
vapour, directly to the atmosphere. In plants possessing leaves provided with
stomata, the outer walls of the epidermal cells, which are directly in contact with
the air, are almost always rather thicker than the inner and side walls; moreover,
the outer wall is overlaid by the already repeatedly mentioned covering, termed
“cuticle”, through which water-vapour can pass only with difficulty. In tropical
ferns, especially in the tree-ferns, which grow in narrow wind-sheltered ravines,
traversed by streams of water, and which spread out their fronds in the still, damp,
warm air, the outer walls are so thin and delicate, and are covered by a cuticle of

MEANS OF ACCELERATING TRANSPIRATION. 285

such tenuity, that if the humidity of the air sinks only a few degrees below satura-
tion point, or if a transient sunbeam enters the ravine even for a short time, they
immediately give off water-vapour.

Apart from such cases, the exhalation of water- -vapour from the superficial cells
is scarcely worth noticing; it is almost entirely restricted to the cells of the spongy
parenchyma. Here are to be found, indeed, very striking arrangements, which must
be regarded as contrivances for increasing transpiration. First of all, where
transpiration is to be accelerated, the green, spongy tissue is very strongly
developed, the air-containing lacunew and passages, which penetrate the net-work of
branched cells like a maze, are enlarged and numerous, and the collective free
surface of all the air-bordered cells in the interior of the leaf has a much greater
extent than the mere outer surface of the epidermis. In the leaves of many tropical
plants which are always surrounded by damp warm air, eg. in those of the
Brazilian Franciscea eximia, of which a section is represented in fig. 621, almost
the entire thickness is made up of loose wide-meshed spongy parenchyma, and it is
evident that water will be exhaled from the cells of this tissue as soon as the
temperature of the leaf is raised even to the extent of a few degrees above that of
the moist surrounding air by the sunbeams falling upon it.

In many such plants which urgently require a help to transpiration on account
of their situation, the cavities of the spongy parenchyma are extraordinarily enlarged
and widened at certain points where the greatest number of stomata are develdped.
The difference in appearance between such places and other parts of the leaf having
dense spongy parenchyma can indeed be recognized by the unaided vision. In such
a leaf looked at from above, the large-meshed portions of the spongy parenchyma
appear as lighter spots in the dark-green grounding; the leaf is flecked and marked
with white. This is not only the case with many plants of damp, tropical forests,
but also in those of temperate zones, such as species of the genus Cyclamen,
Galeobdolon lutewm, the Lungwort (Pulmonaria officinalis), and frequently also in
Hepatica triloba, if they grow in very shady places on the damp ground of a forest.
It must, of course, not be forgotten that all the white spots and markings of green
leaves, which have been named collectively “variegations”, are not due to this cause.
In those nettle-like plants, known as Behmerias, the white spots on the central
part of the leaf lamina are caused by peculiar groups of crystals in the epidermal
cells, the so-called cystoliths, which reflect the light; in some Piperacess they are
due to groups of epidermal cells which are filled with air, and below which the
palisade cells are absent; in other plants, again, they may be caused by the
formation of aqueous tissue, a structure which will be discussed later. In many
of those plants with variegated leaves, which are so extensively cultivated for
purposes of: decoration, the variegation is not normal, but must be considered as
pathological, and is in no way connected with transpiration.

Since, as we know by experience, transpiration of green leaves is increased by
light and warmth, it is evidently an advantage for all those plants to which only a
restricted number of sunbeams can obtain access, if their leaf-blades are very large

286 MEANS OF ACCELERATING TRANSPIRATION.

and have such a form and position that the small supply of light can be utilized to
the full. The resultant action is just the same whether 1000 green cells are only
moderately illuminated, or if 500 cells are illuminated by a light twice as strong.
If this argument will not apply to all plants, it certainly fully applies to some, and
it is a fact that plants growing in damp, shady places are characterized by their
comparatively large, thin, delicate leaves. These leaves are also spread out
horizontally in such localities; they are smooth and not wrinkled; neither rolled
back nor bent up. Suppose we enter a thick wood in the north temperate zone,
perhaps in S. Germany. By the side of delicate-leaved ferns grow species of
Corydalis (Corydalis fabacea, solida, cava), together with species of Dentaria
(D. bulbifera, digitata, enneaphyllos), dog’s mercury (Mercurialis perennis), Isopy-
rum thalictroides, bitter vetch (Orobus vernus), woodruff (Asperula odorata),
Lunaria rediviva, herb Paris (Paris quadrifolia), cuckoo-pint (Arum maculatum),
spurge-laurel (Daphne Mezerewm), and many other species belonging to very
different families, but all having the common characteristic of possessing flattened
leaves and no covering of hairs. If a brook ripples through the shady wood,
growing on its banks will be found the yellow balsam (Impatiens nolitangere),
the broad-leaved garlic (Alliwm ursinwm), Streptopus amplewifolius, and the
butter-burr (Petasites officinalis), with its huge foliage, all again characterized by
their large, smooth, flat leaves. In such places in 8S. Germany are generally to be
found the largest leaves. Those of the butter-burr attain to a length of over a metre,
and are almost a metre broad. The fronds of the common bracken-fern (Pieris
aquilina) are equally large in such situations; and on the ground in damp, shady
alder woods, growing in comparatively cold mountain glens, another fern (Poly-
podiwm alpestre) is to be met with, whose frond is 14 metres long. But they only
possess these extended leaves when growing in the situations described, in the damp
air of cool and shady woods. One would expect that under similar conditions
outside the wood, the leaves would exhibit a more luxuriant growth, and would
attain to a still larger size in consequence of the influence of a higher temperature;
but this is not the case. In the drier air and sunshine on the unshaded banks of a
rivulet, the leaves of the butter-burr are scarcely half as large as those growing in
the neighbouring cold shady glen, from whose dim light the brooklet flows out into
the open country; and on sunny ground neither of the two above-named ferns will
even approximately reach that size to which they grow when surrounded by the
cold, damp air in the depth of the alder wood.

This difference in the relative size of the leaves of one and the same species,
according as to whether they grow in sunny places in dry air, or in shady spots in
damp air, is sometimes carried so far that the whole physiognomy of the plants
becomes altered, and they might easily be thought to belong to distinct species.
Thus species of Convallaria Polygonatum, growing in shady meadows watered
by rivulets, show leaves at least three times as large as those which grow on the
rich damp earth on the steep sides of rocks down which water rushes, where they
are warmed by the sun all the day. This comparison might be illustrated by

MEANS OF ACCELERATING TRANSPIRATION. 287

numerous other plants of the flora of Central Europe, which are sometimes to be
found in damp, shady woods, sometimes in sunny fields; but the above examples
will suffice to demonstrate the fact that in shady places and damp air, in spite of
the smaller amount of heat, and even when the humidity of the soil is less, the
leaves will, notwithstanding, have a greater size than in sunny places where they
are surrounded by a drier air.

An apparent exception is to be found only when these plants are situated above
the tree-line in Alpine regions. On the sunny slopes of Monte Baldo, in Venetia, far
above the wood-line, Corydalis fabacea grows with the same luxuriance as in the
shady forests of the lower hilly regions; and on one place on the Solstein chain, in
the Tyrol, at a height of 1800 metres above the sea, dog’s mercury and Galeobdolon
lutewm, species of valerian, spurge-laurel, and ferns can be seen rising above the
boulders with leaves as large as those growing in the shade of the woods below.
But this exception, as stated, is only an apparent one. Where these plants flourish
on Alpine heights flooded with light, the air is just as damp as in the depth of the
woods 1000 metres lower in the valley. For weeks the mist sways like drapery
around the heights, and the air, consequently, is certainly not drier than in the woods
down in the valley. Indeed, the fact that plants, which one is accustomed to see
inhabiting the shady woods in the depth of the valley, grow in Alpine regions on
unshaded places with leaves of the same size and shape as before, is a proof that:
the large size of their leaves in the dark woods of these lower places is not due to
the absence of light, but to the very moist condition of the air which prevails there.
Plants, whether in the shade of the forests, or on the illuminated heights of the
mountains, endeavour to compensate for the detrimental influence of the greater
humidity of the air by the formation of an extensive transpiring surface.

So far the increase of leaf surface may be considered absolutely as a means of
helping transpiration. This method of increasing transpiration comes into action
in the tropics in a much more striking way than even in the temperate zones.
Especially in the most characteristic plant-structures of the tropics may it be
observed how intimately the size of the leaves corresponds to the conditions of
moisture of the air, and how it is that palms develop the largest leaves just in those
districts where, in consequence of the air being saturated with aqueous vapour,
plants can only transpire with difficulty. In the dampest parts of Ceylon grows the
gigantic Corypha wmbraculifera. A copy of a drawing of this tree, sketched on
the spot by Ransonnet, is given in fig. 63. It towers above the tops of all other
plants, and its leaf-blades are from 7 to 8 metres long, and 5 to 6 metres broad.
In similar situations in Brazil the palm Raphia tedigera spreads out its fronds like
gigantic feathers. The petiole of this leaf alone is 4 to 5 metres long, and the green
feather-like blade is from 19 to 22 metres long and 12 metres broad—the greatest
extent which has hitherto been observed in any leaf. Others palms besides these
giants, whose fronds wave all the year round in a damp atmosphere, are but
little inferior to them. Under one leaf of the Talipot ten persons can easily
find room and shelter, and if the pinnate leaves of the Sago-palm be imagined

288 MEANS OF ACCELERATING TRANSPIRATION.

propped up against the houses in the streets of our towns, their tops would reach to
the second story, and it would be possible to climb up to the windows by them as
if by the rungs of a ladder. Many of these palm leaves if placed in an upright
position would be equal in height to our forest trees. In all these leaves the
epidermis is only slightly thickened, the spongy parenchyma is well developed,
stomata are present in large numbers, and the surfaces of the leaves are so directed
towards the incident sunbeams that they are abundantly illumined and warmed
throughout. The leaves become decidedly heated by the sun’s rays, and thus, even
in the saturated air of the tropics, the necessary amount of transpiration becomes
possible. Arrangements similar to those of the palms may be observed in the
Aroids and Bananas. ‘These also develop their most extended leaves in the
saturated or almost saturated atmosphere on the banks of still or flowing water,
and in the moist heavy air of tropical primeval forests.

It is obvious that means of increasing transpiration are required in those water-
plants whose roots are in the wet mud at the bottom of lakes and ponds, whose
stems and leaf-stalks are directly surrounded by water, and whose leaf-blades float
on the surface of the water, as for example the water-lilies (Vymphea, Victoria),
the Frogbit (Hydrocharis morsus-ranc), and the Nymphea-like Villarsia (V.
nymphoides). The blade of the leaf is disc-shaped in all these plants, and the discs
lie side by side flat on the surface of the water. Frequently large areas of lakes
and ponds are covered with the floating leaves of these plants. The whole of the
upper side of such a leaf can receive the rays of the sun, and the leaf is thus
warmed and illuminated throughout. The under side of the leaf is coloured violet
by a pigment called anthocyanin, which we will consider more in detail later, and
of which it need only be mentioned now that it changes light into heat, and thereby
materially helps'to warm the leaves.

The.aqueous vapour which is in consequence developed cannot escape below
from the large air-spaces which permeate the leaf, because thé under side, which
floats on and is wetted by the water, possesses no stomata. The upper side is so
richly furnished with stomata that on 1 sq. mm. 460 are to be seen, and on a
single water-lily leaf about 2} sq. dms. in area, about 114 millions. This upper side
alone provides a means of exit, and it is therefore important that the passage
should not be obstructed at the time of transpiration. If the rain should fall unre-
strainedly on the upper side of the floating leaves, the collected rain-water might
remain there for a long time, even while the sunbeams breaking through the clouds
after the shower are warming the floating leaves and inciting them to transpire.
In order to avoid this an arrangement is made by which it is rendered an
impossibility to wet the upper side of the floating leaves. The falling rain is formed
into round drops on reaching them, and does not spread over the leaf-surface so as
to wet it. But in order that the drops should not remain long on the leaves in
many of these forms, such as in the widely distributed water-lily (Nymphwa alba),the
leaf, where it joins the stalk, is somewhat. raised, and the edges are bent a little up
and down in an undulating manner. This gives rise to very shallow depressions

MEANS OF ACCELERATING TRANSPIRATION. 289

round the edge of the disc, on account of which the drops of water roll down from
the middle of the leaf to the edge on the slightest rocking movement, and there
coalesce with the water on which the leaves float,
This puckering soe

of the margin of

the leaf is attended
in the water-lilies
by a phenomenon
which, although
not directly asso-
ciated with the
matter in hand, is .
so full of interest
‘that it cannot be
passed ‘ without
notice. If we take
a boat in the bright
sunshine at mid-
day, and float over
the calm inlet of a
lake, whose surface
is overspread with
the leavesof water-
lilies, and if the
water is clear to
the bottom, - we
shall see the sha-
dows of the leaves*
which float on the
surface sketched
out on the ground
below. But we can
scarcely believe
our eyes—these do
not look like the
shadows of the

leaves of water- Fig. 63.—Corypha umbraculifera of Ceylon (after Ransonnet). hi 4 Assi

ee

lilies, but rather

of the fronds of huge fan-palms. From a dark central portion radiate out long dark
strips which are separated from each other by as many light bands. The cause of
this peculiar form, of shadow is to be found in the undulating margin of the floating
leaves. The water of: the lake adheres to the whole of the under surface of the
dise as far as the edge, and is drawn up by capillarity to the arched spoTtons

VoL. 1.

290 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

of the undulating margin, The sun’s rays are refracted as through a lens by this
raised water, and so a light stripe corresponding to each convex division of the
curved margin is formed on the bed of the lake, and a dark stripe corresponding to
each concave part. These are arranged in a radiating manner round the dark
central portion of the shadow.

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

Special arrangements are met with in all plants which possess stomata, in order
that the giving off of aqueous vapour may continue without hindrance. Water
falling on the upper side of the leaf, in the form of rain and dew, threatens to
cause the greatest obstacle to this free passage should it be able to collect directly
in the stomata. The width of an open stomate does not render the entrance of
water by capillarity impossible. As long as light and warmth exercise their power,
as long as the temperature in the neighbourhood of the spongy parenchyma is
higher than that of the surrounding air, and water-vapour in consequence is pro-
duced in the spongy tissue and driven out with force from the stomata, such an
entrance is indeed inconceivable, It is impossible for aqueous vapour to pass out
and at the same time for fluid water to enter by the same passage and through the
same gate. But should the leaf become cooled by radiation after sunset, and dew
be deposited upon it, or should a cold rain trickle down over the leaves, and the
stomata have been unable to close quickly enough, it is quite possible that water
might enter, just as it enters a retort (whose narrow mouth dips into water, and
whose contents have been vaporized by placing a lamp under them), when the lamp
is removed, and the bulb of the retort with its contents becomes cooled. But putting
aside the possibility of water thus pressing its way in, this much is certain, that
the formation of a layer of water over the cells in the immediate neighbourhood
of the stomata would cause great injury to the plants; and this, not only as affecting
transpiration, but also the free entrance and exit of gases. Therefore, the im-
mediate surroundings of the stomata must be kept open as a path for aqueous
vapour, and no water must be allowed to collect and take up a position there.

Stomata are much too small to be seen with the naked eye. However, it can be
ascertained by a very simple experiment whereabouts, on a leaf or green branch,
stomata are to be found. A twig or a leaf is dipped in water, and then withdrawn
after a short time and lightly shaken; some spots will be found wet, while other
places remain dry. Where water remains and spreads out to form an adhering
film, no stomata will be found in the epidermis; but where the twig or leaf is dry,
one can be sure of finding them. In 80 per cent of cases experimented upon in
this way, only the upper leaf-surface became wetted, while the under side kept
dry; in 10 per cent both sides remained dry; and in the other 10 per cent the
upper side kept dry, while it was the under side which was wetted, With this
corresponds the actual fact that in far the greater number of instances the under
side possesses most stomata, while the upper side is free from them. It seems as if

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 291

this circumstance could be explained thus, that the upper side is usually turned
towards the rain, and that the stomata are on this account collected together on
the under side, which is sheltered from it. This explanation, however, which at
first sight seems so plausible, does not quite correspond to the true state of the case.
The consideration of the reasons for believing that it is an advantage for the plant
to have the upper side of the leaf free from stomata will indeed come later, but one
thing must be noted here,—that the side of the leaf turned towards the ground,
which in most cases contains all the stomata, remains anything but dry. Of
course the rain-water only reaches the surface of the horizontal leaf-blade when
the margin is so formed that the adherent layer of water which wets the surface is
drawn over gradually from the upper to the under side, and that is very seldom
the case; but the wetting of this surface by mist and dew is all the more important
on this account. On taking a stroll through fields and meadows on a dewy morn-
ing, as a rule only the upper surfaces of the leaves come into view, and one might
easily be led to think that the dew is deposited only on this side. We constantly
use the expression that the dew “falls”. Underlying this is the idea that the dew
comes down like rain, and that only the upper leaf-surface becomes covered with
dewdrops. But one has only to turn the leaf over to convince oneself that the
lower surface is likewise bedewed, and on a closer examination it will even be seen
that dew is of more importance in connection with the lower than the upper side,
because it remains there so much longer. When the sun is already high in the
heaven, and the dewdrops have long disappeared from the upper surface, and tran-
spiration is in full force, the under side may still be found studded with dewdrops.
If in the majority of cases the stomata lie on the under side, and this side is
exposed to the danger of being covered with water as much as the upper one, it is
evident why contrivances for hindering the access of water to the stomata are
to be found much more abundantly on the under than on the upper side of the
leaf.

The most important of these arrangements are the following:—

First the coating of wax. This is either in the form of a granular covering; or as
a fine crust which fits closely to the epidermis; or, most commonly, as a continuous
thin layer which is easily rubbed off, forming a delicate film popularly known as
“bloom”. A group of primulas, belonging to mountainous districts and to the moors
of low countries, of which Primula farinosa may be taken as the most widely
distributed and best known representative, have a rosette of leaves spreading over
the damp ground, and on the lower side of these leaves is a white coat, which under
the microscope is seen to consist of a collection of short rods and knobs of a waxy
nature. If such a leaf is plucked and. placed in water for a short time, and then
withdrawn, the upper side, which is quite free from stomata, will be moistened by a
layer of water, while the under side, on which are the stomata protected by the
granular coating of wax, remains quite dry. The lower surface of the leaves is
covered with a fine closely adherent wax layer, in many of the willows growing in
damp misty places near rivers (Salia amygdalina, purpurea, pruinosa), as well as

292 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

in a great number of rushes, bulrushes, and reed-like grasses. If when the dew
falls heaviest one roams through a thicket of willows, or across a moor, one may see
plenty of drops hanging from the under side of the leaves, but they do not actually
wet this surface, and on the slightest movement of the leaves they roll off and fall
down. It is, indeed, in consequence of this that one is more likely to get thoroughly
wet by walking through meadows and dwarf willows than by an excursion through
country overgrown with ordinary herbs. The two white stripes, so well known on
the under side of fir leaves, are also formed by a waxy coat, which prevents the
stomata below from being wetted. In species of juniper (eg. J. communis, nana,
Sabina) the two white stripes occur on the upper side of the leaf, and it is interest-
ing to see how the distribution of the stomata again corresponds; for junipers
belong to that group of plants whose under leaf-surface is free from stomata, these
being present only on the wax-coated region of the upper side of the leaves.
Many grasses, to which we shall refer later for other reasons (e.g. Festuca punctoria),
only possess stomata on the upper side of the leaf, and again only where the strips
of wax are situated. Generally speaking, wax is a protection from moisture, and is
most frequently formed when the stomata make their appearance on the upper side
of the leaf. The leaves of peas, nasturtiums, woodbine, poppies, fumitory, many
pinks, cabbages, woad, and many other cruciferous plants, which have stomata on
the upper surface, also produce a covering of wax there. Water poured on the
upper surface of a cabbage-leaf rolls off in the form of drops, exactly as it runs off
a duck’s back, without wetting the surface. In the fronds of ferns (e.g. Polypodium
glaucophyllum and sporodocarpwm), on the upright leaves of Irises (Iris ger-
manica, pumila, pallida), as well as on the vertical leaves and leaf-like branches of
many Australian acacias and myrtles, and lastly in the erect whiplike, leafless or
scantily-leaved papilionaceous plants (Retama, Spartiwm), the stomata are pro-
tected from the wet by a coat of wax.

The formation of hairs furnishes another barrier to the entrance of water into
the stomata. We shall come back again to these structures, which serve so many
different purposes in the plant economy, but here only those hairy and felted
coverings whose task is to hinder the wetting of the stomata will be considered.
Examples of these are furnished by many Malvaceze which grow in marshes and
ditches (¢.g. Althoea officinalis), and also by some mulleins (e.g. Verbascum Thapsus,
phlomoides), whose leaves are provided with stomata on both surfaces, and with
hairy coverings which it is impossible to moisten. In the damp meadows of the
valleys of the Lower Alps grows Centaurea Pseudophrygea, whose large leaves,
hairy on both sides, are very rough and much wrinkled. The stomata are only
situated in the hollows between the ridges. When rain falls, or the leaf becomes
bedewed, the water remains in the form of drops on the hairs of the elevated por-
tions, and the cells in the hollows are not wetted. In many alpine plants, for
example the Hairy Hawkweed (Hieraciwm villoswm), after a fall of rain or dew
the long projecting hairs of the leaves are thickly beset with drops of ea
none of which can reach the stomata on the epidermis beneath.

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 293

It should be particularly noticed here that plants with two-coloured leaves,
‘such as those whose upper surfaces are green, smooth, free from stomata and easily
wetted, while their under surfaces, covered with gray or white hairs, and rich in
stomata, which cannot be wetted, are generally to be found on the banks of rivers
and streams.

In the open woods which skirt the banks of rivers in the valleys of moun-
tainous districts, 7.e. in places where mist rises on summer evenings, and all the
twigs, leaves, and stalks are covered with drops of water, the most characteristic
plants are the Gray Alder (Alnus incana) and the Gray Willow (Saliva incana),
and as undergrowth everywhere the Raspberry—all plants adorned with the two-
coloured leaves just described. Leaving the region of woods growing on river
banks for the neighbouring meadows, through which ripples fresh water from a
spring, and where everything drips with dew from evening until the middle of
the following day, we come to the natural home of herbs and shrubs with leaves
-green on the upper and white on the under sides. There Fuller's Thistles (Cirsiwm
heterophyllwum and canum) grow luxuriantly, and the Meadow-sweet, with its
large two-coloured leaves; whilst the whole course of the brook is bordered by the
Colt’s-foot (Tussilago Farfara) with leaves which may be considered typical of this
group.

What a contrast does this present to the lofty vaults of a dense forest, perhaps
only a thousand paces away, where on the shady ground little or no dew is
formed, and where the leaves which canopy the brown soil are never exposed to a
thorough wetting! No parti-coloured leaf is to be found there, no leaves whose
upper surface is green and smooth, while the under side is covered with white
hairs; and plants which exhibit a thick coating of wax on their under surface, like
the Primula farimosa of the moors, are also absent. On the other hand ferns are
here, as for example the Hard Fern (Blechnwm Spicant), whose leaves are furnished
with stomata which open quite without protection on the tops of projecting undula-
tions. This contrast between the leaves of plants in the open marshy country and
in the interior of forests is found, not only in the colder territories of the north,
but also in tropical districts. Moreover, plants whose leaves are covered with white
hairs on the under surface are never to be found under the close leafy roof of huge
trees which prevent nocturnal radiation and the formation of dew. Here occur,
rather, plants having totally unprotected stomata opening on slightly raised areas
of the surface, as for example in Pomaderis phylicifolia, and on the leaves of the
Pepper family, e.g. Peperomia arifolia (see fig. 643 and 64 *).

A very remarkable contrivance by which stomata are protected from moisture
consists in providing the stomata of the upper surface with countless papille and
cone-shaped projections; between them, of course, being innumerable hollows and
depressions. Falling water-drops roll off such surfaces; the water cannot displace
the atmospheric air in the depressions, and therefore the leaves and stems, in so far
as their epidermis presents the aforesaid irregularities, appear covered with a thin
layer of air. As the stomata are situated in the small hollows, they always remain

294 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

dry, and even if that particular part of the plant is wholly immersed, they do not
come into contact with the water. There are two causes for the unevenness of the
leaves: first, the outer walls of a portion of the superficial cells may become strongly
arched outwards; or secondly, solid peg-like projections may arise from the cuticle,
and to these projections the air adheres so firmly that it cannot be displaced even
by a considerable pressure of water. This protection of stomata against moisture
by papilla-like outgrowths is to be found especially in marsh plants which are
exposed to a changing water-level. On the banks of streams and rivers, and

Fig. 64.—Stomata.

1 Surface view of a portion of the frond of the fern Nephrodium Filiz-mas. 2 Vertical section through this portion.
3 Surface view of a portion of the leaf of Peperomia arifolia, * Vertical section through this portion; x350.

where water welling up from below forms pools and ponds, it may happen that
plants are submerged for a week at a time, and then again remain dry for some
months,

Most of the plants growing in such situations, particularly the sedges (eg.
Carex stricta and paludosa), the rushes (e.g. Scirpus lacustris), most of the tall
fistular grasses (Glyceria spectabilis, Phalaris arundinacea, Eulalia japonica), the
plants which grow with the sedges (eg. Lysimachia thyrsiflora, Polygonum
amphibiwm), and many other marsh plants, are all saved from the danger of
having their stomata wetted during their submersion by the papilla-like out-
growths of some of the epidermal cells, near the stomata, as shown in the figures
on next page.

Bamboos, and the grasses Arundinaria glawcescens and Phyllostachys bam-

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 295:

busoides, which so much resemble the bamboos, besides some sedges (e.g. Carex
pendula), exhibit on the other hand the above-named peg-like projections of the
cuticle; these are shown in the section of a bamboo leaf in fig. 667). On plunging
such a bamboo leaf in water, a surprising sight presents itself, The upper side,
covered by a dark green, smooth, flat epidermis, with no stomata, becomes wet all
over, and retains its dark colour and dull appearance; but the under surface, blue-
green in colour, and beset with stomata and thousands of cuticular pegs, does not
allow the air to be displaced; and this layer of air, spread thin over the surface,
glistens under water like polished silver! The leaf may be shaken under water
to any extent, and may even be left submerged for a week, but the silvery glisten-
ing air-stratum is not dislodged. If such a leaf is now taken out of the water, the
upper surface is quite wet, but the under surface is dry, like a hand which has
been dipped in mercury and then withdrawn, and not the smallest drop of water

ues iz
&

1
eee

Sy
ey

ee

Wien caess PRE FDO
ZS SOD OSS SS see

Fig. 65.—Protection of Stomata from Moisture by Papilla-like outgrowths of the Surface.
1 Vertical section through a portion of the leaf of Glyceria spectabilis, 2 Vertical section through a portion of the leaf of

Carex paludosa; x200.

adheres to it. On placing a vessel of water, in which some bamboo leaves are
half immersed, under the receiver of an air-pump, and then pumping out the air,
numerous small air-bubbles are at once given off from the submerged portions
of the leaves. At length the silvery lustre disappears, and the air between the
cuticular pegs is replaced by water. If now the leaves be completely submerged,
the silver lustre is only shown on those parts which were not previously immersed,
and where water could not replace the exhausted air;—the spaces round the pegs
in this region having been again supplied with air on the opening of the stop-cock
of the pump in order to submerge the leaves. It may be imagined from this
experiment how much the stomata would be damaged by water if the plants
mentioned were not protected from moisture by the pegs to which the air adheres
so strongly.

In many plants which grow in the sunshine, and particularly in those whose
foliage is evergreen and only exposed to moisture at the time of the greatest
activity of the sap (while later it is exposed for months to dry air), the stomata are
to be found surrounded by an embankment, or sunk in special pits and furrows.
Even in the leaves of many indigenous plants, which are green in the summer,
e.g. those of the Carrot (Daucus Carota), the guard-cells of the stomata are so

296 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

over-arched by the neighbouring epidermal cells that a sort of vestibule is formed
in front of the true pore. It can easily be imagined that drops of water which
come to’such places are not able to press out the air from this vestibule, and there-
fore cannot penetrate to the guard-cells of the stomata. In Hakea florida and
Protea mellifera, two Australian shrubs (see fig. 67), similar arrangements are met
with, but here the stomata are still more over-arched, so that they are only visible
to anyone looking at the surface of the leaf through small holes at the top of the
dome. The stomata on the green branches of various species of Ephedra are
‘surrounded by mound-like projections from the cuticle of neighbouring epidermal
cells, and are at; the same time somewhat sunken, so that an urn-shaped space is

acs cp
Cua)

Fig. 66. —Protection of Stomata from Moisture by Cuticular Pegs.

1 Vertical section of a Bamboo leaf; x180. 2 Part of the lower portion of the section; x460. 3 Part of the upper

portion of the section; x 460.

formed above each stoma, from which water cannot dislodge the air. On the
leaves of Dryandra floribunda, one of the Proteacese which grows in the thick
Australian bush, several stomata occur at the bottom of small pits on the
under side of the leaf, and from the side walls of the depression spring hair-
like structures which interlace and form a loose felt-work, easily penetrated
by gases but not by fluids (fig. 68). The stomata on the leaves of the Oleander
(Neriwm Oleander) are similarly situated. These also are at the bottom of deep
pits on the lower side of the leaf, and the entrance to them is beset with extremely
delicate hair-like structures (see fig. 73°). The oleander fringes the banks of
streams in the sunny open country of Southern Europe and the East, and in its
natural position it is most exposed to wetting by rain, mist, and dew, just when
transpiration is an absolute necessity for it. But even when the leaves are covered
on both sides with a layer of moisture, none can force its way into the hair-
lined depressions which conceal the stomata, and consequently transpiration is not
hindered even in the wettest season of the year.

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 297

Stomata, which are spread over the green tissue of stems and flattened shoots,
are frequently sunk in furrows, channels, and pits, in plants whose greatest activity
occurs in the short rainy season, and they are saved from wetting in this position
by the most varied contrivances. On the rocky shores of Lake Garda, and up over
the mountain slopes to the heights of Monte Baldo, grows Cytisus radiatus, a
shrub of unusual appearance (see fig. 691). Its branches only possess rudimentary
green leaves, and are themselves furnished with green tissue, which plays the same
role as that assigned to the mesophyll of the leaf-lamina in normal foliaceous plants.

Fig. 67.—Over-arched Stomata of Australian Proteacex.

1 Vertical section through a leaf of Hakea florida, 2 Surface view of the same leaf; x320. 8 Vertical section of a leaf
of Protea mellifera. + Surface view of the same leaf; x360.

These green branches bear very numerous secondary branches inserted in decus-
sating pairs. On the secondary branches new shoots develop every spring exactly
similar in form, and arranged in the same manner. At the period when this
development is taking place, the humidity in that part of the Southern Alps, to
which Monte Baldo belongs, is very great. In dull weather, rain and mist, or dew
in fine weather, deposit large quantities of water on the soil, and on the plants
covering it, particularly in the alpine region of the above-named mountains, on the
westerly slopes leading down to the lake, which are thickly clothed with the shrubs
in question. It is therefore important that the rod-like branches of this Cytisus
should be able to breathe and transpire without hindrance, and that the short time
during which the conditions for these vital transactions are favourable, should be
fully and wholly taken advantage of. Here again the point above all others to be

298 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

aimed at is to keep a free passage for the water-vapour which must escape from the
stomata. To bring this about, the stomata are situated in grooves filled with air
which are sunk in the green tissue, and which give a striped appearance to the
branches. Water cannot force out the air from these narrow furrows which run
along the green branches and twigs, six of them to each branch. The branches may
remain submerged in water for an hour without a trace of moisture entering the
furrow. Moreover hairs are present in the furrows as a guard against moisture.
These cannot be wetted, and the air adheres to them just as to the cuticular pegs of
the bamboo leaf. A clear idea of this arrangement is given in the transverse
section of the stem shown in fig. 69° and 694. The adjacent section of the green
branch of the Australian Casuarina quadrivalvis shows that these curious plants
also have exactly the same arrangements, that the stomata lie at the bottom of
2

SSS

ones ee=

LJ

Fig. 68.—Stomata in Pit-like Depressions.

1 Surface view of a leaf of Dryandra floribunda. A portion of the hairs which fill the pit is removed, in order to show
the stomata; x350. 2 Vertical section through a leaf of Dryandra floribunda; x 300.

narrow furrows which run along the green leafless branches, and that peculiar hair-
structures are present in the: furrows, to which the air adheres, forming a barrier
against water, exactly as in those of the Cytisus. The Casuarine, which must
finish their work for the year during the very short rainy period of their native
country, require during this time arrangements providing for unhindered transpira-
tion no less than does the Cytisws in the Southern Alps. Altogether this con-
trivance is found to be present in only a limited number of cases; in perhaps only
twenty papilionaceous shrubs, most of which belong to the Spanish flora, of the
genera Retama, Genista, Ulex, and Sarrothammus, in addition to the Australian
Casuarinas, and in allied species of Cytisus (holopetalus, purgans, ephedroides,
equisetiformis, candicans, albus, &c.). Most remarkably also this arrangement
occurs in a small species of Broom (Genista pilosa), which is distributed over the
mountains of Central Europe, over the heaths of the Baltic Lowlands, Denmark,
Belgium, and England. And the presence of this contrivance here is the more
strange, from the fact that the green branches with their furrows, in which lie
stomata, are not leafless, but, on the contrary, are provided with a comparatively
well-developed foliage.

Among the most peculiar plants whose stomata are concealed in hidden nooks,

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 299

impenetrable by water, are two very small orchids, of which one, Bolbophyllum
minutissimum, grows in company with mosses on blocks of sandstone and on the
bark of trees in the rocky ravines near Port Jackson, and on the Richmond River
on the east coast of Australia; the other, Bolbophyllum Odoardi, lives in similar

>
PQY’
ies

Ea
ie
rr

a)
Tn
poe
ri

re
ae

ihe
ae
oss

Wi

3

Ss
ae
a

Fig. 69.—Stomata in the Furrows of Green Stems.

‘ Branch of Cytisus radiatus; natural size. 2 Portion of a branch; x10. 8 Cross section of this branch; x30. 4 Part of the
same section; x150. 5 Branch of Casuarina quadrivalvis; natural size. 6 Portion of a branch; x8. 7 Cross section of
this branch; x30. 8 Part of the cross section; x130.

situations in Borneo. Both have a filamentous rhizome from which spring rootlets
(from 2 to 5 mm. long and 0°3 mm. thick), arranged in pairs, by which they attach
themselves to the stone and the bark of trees. Above the origin of each pair
of rootlets is a little disc-shaped tuber, from 14 to 3 mm. in diameter, and } mm.
thick, with an aperture on the upper surface, scarcely #5 mm. broad, leading into a
hollow chamber within the disc-shaped tubers, about 0°5 mm. broad and 0'1 mm.
high (see figure 70). The leaves of Bolbophyllum minutissimum are reduced

300 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

to tiny pointed scales about } mm. in length; two of them are situated at the
mouth of each cavity, and are inflected towards one another across it. In Bolbo-
phyllum Odoardi, each of the small tubers bears only one small green leaf, which
is about 14 mm. long and 1 mm. broad, and is placed close to the opening of the
chamber (see fig. 7045), Stomata are found exclusively in the interior of the
hollow tubers. Water cannot enter through the narrow mouth into the air-containing
chamber, and even when, in the rainy season, the whole of the mossy carpet, in
which these smallest of all orchids are interwoven, is saturated with water, their
transpiration continues unhindered, provided that the other conditions on which it
depends are fulfilled. It is obvious that these structures which prevent moisture
reaching the stomata during the wet season of the year can take on another function

Fig. 70.—Orchids whose Stomata lie in Hollow Tubercles.

1 Bolbophylt inutisst 2 A tuber seen from above; x8. % Vertical section through this tuber; x15. 4 Bolbophyllum
Odoardi. 5A tuber; x6. 6 Longitudinal section through this tuber; x6.

in a succeeding dry period, which may follow immediately; but this must be
spoken of again later.

The occurrence of “rolled leaves”, which are observed in so many plants of widely
different affinity, is also connected with the keeping of water from the stomata.
The rolled leaf is always undivided, of small area, generally linear, but sometimes
ovate-linear, elliptical, or even circular in outline; always stiff, and usually ever-
green, and therefore living through two or three periods of vegetation. Its edges
are bent down and more or less rolled back, even whilst still hidden in the bud.
In consequence of this, the lower side which faces the soil is hollowed to a greater
or less extent, while the upper side, turned skyward, is arched. Frequently the
leaf is rolled so as to inclose an actual chamber, which only communicates with the
outer world by a very narrow fissure, as is the case, for example, in the Crowberry
(Empetrum). The rolled-back margins of the leaves in this plant almost touch
one another, and the epidermis of the lower side of the leaf forms the actual
lining of the cavity which resulted from the rolling of the leaf (see fig. 71°).

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 301

If the bent-back margins do not fit so closely together, a groove appears on
the under side of the leaf, which is more or less sunken according to the extent of
the rolling, as for example in the Heaths (Erica caffra, vestita, &e., see fig. 711).
Occasionally a groove is developed which divides into two side furrows running

oo
yh te a
SG WG Sy
SGB a). SO
—CDBOEEK 5
SS eM,
oe, ZS
» ae
eS YZ
a yy :
aL) “A

4
Pat HS

SS
D>
os ty,
SGA
4 \ y Veal Coe
ice

Fig. 71.—Transverse Sections through Rolled Leaves.

1 Erica caffra; x280. 2 Empetrum nigrum; x160. % Andromeda tetragona; x150, * Tylanthus ericoides; x130.
5 Salix reticulata; x 200,

beneath the rolled edges, as for example in the leaves of Andromeda tetragona (fig.
71%), and in those of the Cape Rhamnea, T'ylanthus ericoides (fig. 71‘). The central
portion of the space framed in by the rolled-back leaves is also frequently divided
into two longitudinal grooves, and in such a manner that the tissue below the
midrib of the leaf may project as a broad strong band. On the under side of the
leaf, therefore, are three longitudinally elongated parallel projections, a central one
under the midrib, and two lateral, which have been formed by the rolled-back margins

302 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

of the leaf. On the right and left of the middle ridge lie two deep grooves, which
are apparent to the naked eye as light stripes between the dark green projecting
portions. This is the case, for example, in the leaves of the Azalea procumbens,
also in one of the Ericacee known by the name of Loiselewrea, which covers the
soil with a close-matted carpet wherever it makes its appearance, and is widely
distributed through Labrador, Greenland, Iceland, Lappland, and generally through
the whole Arctic region, as well as over the high mountains of Scandinavia, the
Pyrenees, Alps, and Carpathians. The annexed figure 72 represents a transverse
section through a single rolled leaf of Azalea, a hundred and forty times its natural
size.

Occasionally several strong anastomosing ribs project from the under side of
the rolled leaf, inclosing small pits and depressions in whose depth stomata are
situated, as may be seen in the leaves of the widely distributed Willow, Salia
reticulata (see fig. 71°).

Although all these rolled leaves have an appearance of firmness and solidity,
and frequently remind one of the needle-like leaves of the conifers, they are, unlike
these, filled up with a very loose spongy parenchyma, which takes up far more
room than the palisade tissue lying beneath the epidermis of the upper side. The
upper epidermis of all rolled leaves is easily wetted, frequently uneven and finely
wrinkled, destitute of any waxy covering; the cells strongly thickened on their
outer walls, and pressed closely together, so as to leave no spaces between them. On
the under side it is very different. Here stomata are present in great number, and
the epidermis is either covered with wax, as in the Marsh Andromeda, the Whortle-
berry, and the Reticulate Willow (Andromeda polifolia, Oxycoccos palustris, and
Salix reticulata), or it is clothed with a fine felt-work,as, for example, in Ledum
palustre. Very often peculiar rod-shaped or filamentous projections of the cuticle
are present, which at first sight might be taken for hairs, but which differ from
hairs in being solid, not hollow. Figs. 72 and 71128 show these structures (which
may be considered as counterparts of the cuticular pegs on the bamboo leaf) on
the under side of Azalea procumbens, Erica caffra, and Andromeda tetragona,
as well as on the edges of the fissure which leads into the hollow leaf of the Crow-
berry (Empetrum nigrum). These structures are to be found almost without
exception in the heathers of the northern moors as well as in the Mediterranean and
Cape flora. The importance of this continuous delicate coat lies chiefly in the fact
that air adheres to it as to the cuticular pegs of the bamboo leaf, and indeed so
firmly that even water, under considerable pressure, is not able to displace it. On
placing a leaf of Azalea procumbens under water, two elongated air-bubbles are
seen along the two longitudinal furrows, which glisten like two strips of silver.
Even shaking the leaf to and fro will not dislodge these air-vesicles, and even if the
branch has been left submerged for a week, this air will still cling to the depressions
in whose depths the stomata occur. If the branch be removed from the water it
will be seen that the upper side of the leaves is wet, while water has been kept
away from the stomata of the under side. And as with Azalea procumbens, so is

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR, 303

it with all other rolled leaves, whether they belong to Cape plants or to heath plants
of the Baltic lowlands,

It cannot be doubted that the mechanism of rolled leaves, as just described,
furnishes a protection for the stomata against moisture, and keeps open a passage
for aqueous vapour and excreted gases. The question is now only how it comes
about that this arrangement is to be met with in plants of such widely distant
countries and under such differences of climate?

In order to understand this clearly, let us imagine ourselves in some of the
regions which are specially characterized by the abundance of plants with rolled
leaves. First, on one of the high ridges of the Central Alps, where the low-lying
Azalea spreads a thick covering over the soil, where Erica carnea in great quantity

Fig. 72.—Vertical Section through a Rolled Leaf of the Trailing Azalea (Azalea procumbens); x140.

covers broad slopes, where Dryas octopetala, Salia reticulata, Homogyne discolor,
Saxifraga cesia, and many other plants which possess evergreen rolled leaves
weave their carpet over the stony earth. The ground in which all these plants are
rooted, and from which they draw their fluid nourishment, has many natural dikes
and retains a large quantity of water, not only from the melting of the heavy
winter mantle of snow, but also from the abundant rain of summer. For weeks
together the heights are wrapped in a cold mist which saturates everything with
moisture, and drops of water hang from the stems and leaves, unable to
evaporate as long as the air remains so supercharged with vapour. At length the
sky clears again, and the water on the plants begins to disappear. But even during
the fine night following, all the plants become covered with a very heavy dew in
consequence of their rapid cooling and radiation, and this not unfrequently remains
until the middle of the next day. Transpiration at last occurs when the sun shines,
and particularly if dry winds sweep over the heights. But who knows how long
this state of things will continue? Each moment is precious, and every hindrance
to the evaporation, so important for the plants, would be a distinct disadvantage.
The outlets for aqueous vapour on the under side of the leaves especially should not
be obstructed, and the above described contrivance is arranged with this end in

304 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

view. It can hardly be doubted that the earlier mentioned plants of high moun-
tainous regions cease to transpire for weeks at a time in the wet seasons, when a
thick unbroken mist covers the slopes, and earth, stones, and vegetation are dripping
with moisture; and of course the conduction of food-salts to the green leaves is
interrupted to a corresponding extent. If one considers how short a period is
afforded to plants of high mountain districts in which to perform their year’s work,
it will be understood how the most active means for promoting transpiration must -
be brought into play, and how everything which might interrupt or limit this
process, so important to the welfare of the plant, must be avoided. A few months
after the last snow has melted on the heights, fresh snow again falls, and entirely
prevents growth and nourishment during the long winter.

These climatic conditions account for the fact that so many Alpine plants,
almost all those having rolled leaves, are evergreen. It is necessary that every
sunbeam during the short vegetative period should be utilized, and that the leaves
retained from the previous year should be able to transpire and to form organic
materials on the first sunny day after the winter snow has melted, although the soil
may have become only slightly warmed. It may perhaps be urged against this
explanation that though, in the steppes the period of vegetation is restricted to the
brief space of three months, nevertheless evergreen plants with rolled leaves are
completely absent. But the conditions of moisture on the steppes during this three
months’ vegetative period are essentially different from those of the high mountain
region. In the steppes, transpiration is never brought to a temporary standstill by
too much moisture; evaporation can take place uninterruptedly from the leaves, and
they have to be protected not from moisture, but from over-transpiration. With the
exception of the halophytes and a few other growths which are particularly well
protected, no plants, on account of the extreme dryness of the air, can retain their
green foliage in the height of summer on the steppes.

Some of the plants which adorn the high mountains of southern regions make
their appearance in the lower plains of the extreme north. The same carpet of
Trailing Azalea, Dwarf Willows, and Dryas (Azalea procumbens, Salia reticulata,
Dryas octopetala) is found on the soil underfoot. In addition are other small plants
which remain green during the winter (e.g. Cassiope tetragona), which are similarly
provided with rolled leaves. Even if we were not informed by Arctic explorers
that the number of foggy days in the course of the short Arctic summer is much
greater than on the mountain heights of the south, and that therefore a help instead
of a hindrance to transpiration is required, the utmost use being made of the short
time in which it is possible to draw food-salts from the soil, we might infer this to
be the case from the frequent appearance of these small carpet-forming plants with
their evergreen rolled leaves. Apart from other considerations, and disregarding
the development of the various floral areas in point of time, the above signification
of the evergreen rolled leaves explains the similarity and partial identity of the
arctic flora with that of the heights mentioned.

Let us turn now to the low-lying country along the North and Baltic Seas, and

MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR. 305

to the lowlands, which extend as far as the northern slopes of the Alps. Where man
has not transformed the ground into arable soil, only moor and heath, heath and
moor, are seen in wearisome monotony. On the moors especially are always the
same plants—various Heaths (Calluna vulgaris, Erica Tetralia, Erica cinerea),
Black Crowberry (Empetrum nigrum), Whortleberry (Oxycoccos palustris), Marsh
Andromeda (Andromeda polifolia), Wild Rosemary (Ledwm palustre)—all plants.
with evergreen rolled leaves, just as on the mountain heights. Some of these small
evergreen bushes, viz. the Crowberry and the common Ling (Calluna vulgaris), may
be traced in an unbroken range from the plains up to a height of 2450 metres on
the slopes of the Alps. Strange to say, these plants do not blossom much earlier on
the lowlands than on the high Alpine regions, and it has actually been shown that.
Calluna blooms rather sooner at a height of 2000 metres than in the northern portion
of the Baltic lowlands. How is this? The winter snow has long disappeared from
the lowlands, while the hill-sides above are still concealed under their cold white
covering. The winter snow has gone, to be sure, but not the winter! While every-
thing around is already in blossom, while the ear is already visible on the stalks of
rye, the neighbouring moor is still dismal, waste, and lifeless. A month or so later
there is a stir on the dry soil of the cold moor, and the absorbent roots of the plants
which have evergreen rolled leaves commence their activity. When the warm days
of midsummer arrive and the sun sends down its powerful rays, the temperature of
the soil quickly increases, and indeed rises far more than would be thought possible.
The damp cushion of bog-moss at mid-day feels quite warm; and a thermometer
placed 3 ems. below the surface in the uppermost mossy layer of a moor on a
cloudless summer day (22nd June) showed a temperature of 31° C. while the tem-
perature of the air in the shade was 13°! An unpleasant vapour rises from the damp.
earth, which settles on the surface, and makes a walk over the moor particularly
disagreeable. Scarcely has the sun set in glowing red on the horizon when this
vapour condenses into patches of mist which settle over the dark expanse; stems,
branches, and leaves are covered with drops of water, and next morning everything
is as thoroughly soaked as if it had rained throughout the night. This process,
which is regularly repeated during the fine weather, is only interrupted when a
damp wind from the sea blows, driving masses of cloud over the heath, or when
copious rain saturates the soil. It needs no further showing that under such condi-
tions an abundant and continuous transpiration from plants is impossible, and that
in the short intervals which are allowed to the leaves for transpiration, the outlets
from the wide-meshed spongy parenchyma must not be obstructed; and it does
not need further proof that the evergreen rolled leaf is the form most suited and
adapted to these conditions.

The flora of the Cape of Good Hope may not unjustly be compared with that of
the Baltic lowlands—countless low bushes which are very like Heaths, Wild Rose-
mary, and Crowberry in appearance—all with small rigid evergreen leaves, and entire
rolled-back margins; the upper side of the leaf usually dark green, the under side

having the same arrangements ‘as shown in the rolled leaves of plants growing on
VoL. I, 20

306 MAINTENANCE OF A FREE PASSAGE FOR AQUEOUS VAPOUR.

moors which border the Baltic Sea, and in the cold Arctic tundra. This shrubby
evergreen vegetation of the Cape belongs indeed in part to the same families as
these. Heaths especially are to be found in abundant variety; as many as 400
species can be counted—many more than are furnished by the whole of the rest of
the world taken together. But a great number of species from other families, viz.
Rhamnex, Proteacex, Epacridex, and Santalacew, possess an exactly similar foliage,
and without blossom and fruit are often indistinguishable from the heaths. This low
evergreen bush vegetation is not distributed all over the Cape, but is restricted to
the neighbourhood of the coast, to the country which slopes in terraces down to the
south-west, and to the celebrated Table Mountain, rising abruptly above Cape Town.
The aqueous vapour brought by the sea-winds condenses directly over these regions,
and for five months, from May till the beginning of October, the soil is not only
soaked by abundant rain, but what is perhaps of even greater moment, all the ever-
green bushes are kept in a damp condition by the falling mist, and often are
dripping with water just like the heaths on the moors of the Baltic lowlands.
When the development of vegetation on the lower terraces of the south-west coast
is at a standstill on account of the increasing dryness, the summit of the Table
Mountain is still enveloped in the celebrated mass of cloud known as the “table-_
cloth”, and the plants growing on the ridges and plateaus are during this time as
much saturated as the Trailing Azalea, which robs the south wind of its moisture on
the mountain ridges of the Central Alps. It is, however, in this damp period that
the growth of the plants in question takes place. Most of the plants on the heights
of the Table Mountain blossom and put forth new shoots in February, March, and
April; on the lower terraces from May to September. In the northern regions and
on mountain heights the beginning and end of the year’s work in plants is limited
by the cold, but in the Cape the dryness of the soil is the cause which brings the
upward current of the sap in vegetation to a standstill for so long a time. At the
coast, however, this dryness is never so severe that the plants are exposed to the
danger of withering up altogether, as on the steppes.

As on the south-west coasts of the Cape, so is it round about the Mediterranean
Sea and in the west of Europe, which is swept by sea-winds laden with vapour
from the Atlantic; for example, Portugal and the south-west of France, which are
in like case, characterized by an abundance of low bushes, with evergreen rolled
leaves, and especially by some gregarious heaths. Here also the year’s growth
takes place in the wettest season, and arrangements must be made that during this
period the formation of organic materials, the withdrawal of food-salts from the
soil, and consequently unhindered transpiration may be carried on. Here, too, dry-
ness interrupts the activity of the absorbent roots, and the evergreen vegetation of
the coast-line extends inland as far as the damp sea-winds make themselves felt;
while still further inland a steppe-like vegetation preponderates. The analogy pre-
sented by the Mediterranean flora goes so far that, on the southern point of Istria,
for example, which may be compared as to shape with the south point of Africa,
quantities of the evergreen Erica arborea are only to be found on the south-west

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 307

coast district on a comparatively narrow strip of land; while in the interior of Istria
the waste dry terraces of the Tschitscherboden (which might be compared with the
arid plains of the Cape) show no trace of a heath vegetation.

Why the plants with evergreen leaves which grow in the far north, on the
heights of the Alps, on the Baltic lowlands, on the shores of the Atlantic Ocean, on
the borders of the Mediterranean basin, and at the Cape of Good Hope are not all
of the same species, is a question which cannot be answered here; yet it seems
proper to point out that all plants furnished with evergreen rolled leaves, whose
year’s work is stopped by dryness, would freeze in countries where the earth in
winter is covered with snow, i.e. the molecular structure of their protoplasm would
be entirely altered by the frost, which would kill it; while the protoplasm of the
analogous northern forms would suffer no harm from the cold. It is well worthy of
remark in this connection that some of the last-mentioned plants have an extra-
ordinarily wide distribution; that they may actually be found, quite similar in
appearance, in the bleak north, and in the southern districts, if only those conditions
of moisture which we have shown to account for the form of the leaves obtain in
the places mentioned. Thus the Irish Heath (Daboecia polifolia) may be found
along the Atlantic coast as far as Portugal, and the common Ling (Calluna vulgaris)
grows just as well at a height of 2450 metres above the sea beside the glaciers of
the Ctzthal in the Central Alps, as further south on the Abazzia, surrounded by
laurel groves on the sea-coast of Istria.

3. PREVENTION OF EXCESSIVE TRANSPIRATION.

Protective arrangements on the Epidermis.—Form and Position of Transpiring Leaves and
Branches.

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

The relation of the form of the evergreen rolled leaf to transpiration is anything
but exhaustéd in the foregoing account. The part played by this form of leaf, in
particular during the dry season of the year, yet remains to be discussed. If it is
necessary during the wet period that transpiration should be increased as much as
possible, and that everything which might restrict the exhalation of aqueous vapour
from the stomata should be kept away, it is also of importance that on the appear-
ance of the dry season the equilibrium between the water taken from the soil and
the water excreted by the leaves should not be destroyed, and that an excessive
evaporation from the portions of the plant above ground should be hindered. New
seasons bring new problems to be solved. At the time when the water-current
begins to ascend from the soil saturated by the winter rains, we have an aid to
transpiration; later on, in the dry period, we have a protection against the dangers
which might attend excessive evaporation. It is certainly of great interest to see

308 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

how a whole group of the arrangements discussed above, among which the rolled
leaf is not the least noticeable, serve, at different seasons of the year, and often at
different times of the same day, two distinct purposes, as indicated.

First, the stomata themselves. While the green tissue has need of food-salts
from the soil for the manufacture of organic materials, they cannot be too widely
opened; everything is welcome, then, which promotes transpiration, and conse-
quently assists in the elevation of fiuid nourishment from the saturated soil. But if
the temperature and dryness of the air increase after the green parenchyma has
finished its yearly task, or if the soil from which the absorbent roots have hitherto
derived their supply of fluid become so dry that the water exhaled from the aérial
positions can no longer be replaced, it is of the greatest importance that the stomata
should be closed. This is brought about by the two cells bounding the stoma,
which have been termed the “ guard ” cells.

In order to clearly understand the mechanism of the opening and closing of
stomata, it Is necessary to examine the structure of these guard-cells more in detail.
Both are bean-shaped in outline, their concave surfaces being turned towards the
stoma; they are only connected with one another at their extremities. By their
convex sides they are in contact with ordinary epidermal cells; their outer walls
are in contact with the atmospheric air, and their inner walls with the spongy
parenchyma. Both the innermost and outermost walls of the guard-cell are
strongly thickened, but the wall by which they are connected with neighbouring
epidermal cells, as well as that portion which directly borders the stoma, is relatively
thin, elastic, and extensible. If the figure of two such guard-cells be imitated in
caoutchouc, and they be fitted together like an actual closed stomate—water being
forced into them under considerable pressure—the curvature of the thin and elastic
portions of the walls will be most altered. The side wall in contact with the
neighbouring epidermal cell bulges out, and at the same time the whole cell becomes
elongated in a direction perpendicular to the surface. By this means the two
guard-cells are forced apart. When the water is allowed to flow out of the swollen
caoutchouc cells, they again fall back into position, the two portions of the walls
which border the stoma coming into contact with each other and closing the opening.
The same thing occurs in the actual guard-cells of the living plant. As soon as
they become distended, they separate from one another; when they relax and
resume their original position, they come closely into contact again. This process
bears a strong resemblance to the changes in the cells of the pulvini at the base of
the sensitive leaves of Mimosa, which will be described later, and it is. highly
probable that it may be traced back to a similar stimulation. That the guard-cells
actually separate from one another by swelling up, i.e. by absorbing fluid, and then
close together again in consequence of the loss of water, can be shown by first
supplying water and then withdrawing it by a solution of sugar. In the former
case the stomata open, in the latter they close, and it may therefore be considered an
established fact that a closing movement is brought about by the extra loss of water
in dry air. But if these pores, through which water vapour escapes when the plants

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 309

are full of sap, close as soon as there is a danger of too much evaporation, the
mechanism must be considered as excellently regulating transpiration, and as pro-
viding a true preventative against over-evaporation.

This closure of the exhalent chambers in the interior of the leaf, important as it
is, would alone be sufficient in but a very few cases to ward off this threatened
danger. If the epidermis which stretches over the thin-walled transpiring cells of
the spongy parenchyma is itself thin-walled and succulent, water will be exhaled
from it also into the dry atmosphere; this loss of water from the epidermal cells is
compensated for by water drawn from the neighbouring parenchymatous cells in
the interior of the leaf, and ultimately the leaves would wither up if the supply of
water from the roots were stopped or became insufficient. Therefore the epidermal
cells must be adequately prevented from exhaling. When this is the case, and when
the stomata are closed, the spongy parenchyma, and, generally speaking, all the
succulent cells in the interior, are securely protected.

The walls of the epidermal cells in the first stages of their development are
composed mainly of cellulose, and are uniformly thin and delicate on all sides. The
outer wall, which is in contact with the air, then becomes thickened and divided into
an inner and an outer layer. The inner retains its original character, but the outer
—the so-called “cuticle”—undergoes great modifications. The cellulose becomes
changed, and is replaced by a mixture of stearin and the glyceride of a fatty acid
(suberic acid), forming a tallow-like fat which is termed cutin or suberin. In
consequence of this metamorphosis the cell-wall becomes less and less permeable to
water, and when it has attained a considerable thickness it becomes at length almost
entirely impervious to water and aqueous vapour. Frequently, between the inner
cellulose and the outer corky layer, other so-called “cuticularized layers” are
formed, whose chief constituent is again suberin, and which often attain to a con-
siderable bulk.

Aquatic plants, which are not exposed to the danger of excessive evaporation, of
course do not require this protection. Plants whose leaves are surrounded by air,
on the other hand, can never entirely dispense with it. The thickness of these
corky layers is extremely variable according to the condition of humidity of the air.
Where the air is very damp throughout the year, the outer wall of the epidermal
leaf-cells appears to be only slightly thicker than the inner, and the cuticle only
forms a thin continuous layer. On the other hand, plants which are temporarily
exposed to dry air possess very highly developed cuticular strata. Especially when
the leaves are evergreen and remain several years on the branches, as, for example, in
the Holly (Ilea Aquifoliwm, see fig. 73”), and in the Oleander (Neriwm Oleander,
fig. 73°), the cuticular layers are so strongly developed that the outer wall of the
epidermal cells is many times thicker than the inner wall. Evergreen parasites, as,
for example, the Mistletoe (fig. 731), those tropical orchids and Bromeliaceze which
live epiphytically on the bark of trees and are often exposed to great dryness in the
hot season of the year, cactiform plants, and generally the majority of succulent
plants, possess epidermal cells with very strongly thickened outer walls. This is so

310 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

also in the case of the pines with evergreen needle-shaped leaves, where, as a rule,
the water compensating for that exhaled by the leaves cannot come quickly through
open channels, but only slowly through the woody cells. Usually the cuticle and
cuticular layers are of equal thickness over the whole leaf surface; this is so espe-
cially in smooth, shiny, leathery evergreen leaves. Not infrequently, however, an
irregular thickening is seen, particularly in the neighbourhood of stomata, where
circular ramparts are raised, as in Protea, mellifera (see fig. 67 *), or peg-shaped pro-
jections are formed, as in the Bamboo (see fig. 66), or elongated hair-like filaments
arise, as in the rolled leaves of Azalea and many Heaths (see figs. 71 and 72).

It would, however, be erroneous to suppose that this formation of a thick cuticle
on the epidermis is a peculiarity of evergreen leaves. Plants which are surrounded

Fig. 73.—Thickened Stratified Cuticle.

1 Vertical section of a portion of the leaf of Mistletoe (Viscum album); x420. 2 Vertical section of a portion of the leaf
of Holly (Ilex Aquifolium); x500. 3 Vertical section of leaf of Oleander (Nertum Oleander); x320.

all the year by a damp atmosphere, and are never exposed in their natural condition
to the danger of too much evaporation, very often have evergreen leaves, and yet
the outer wall of the epidermal cells is not at all, or only very slightly, thicker than
the inner; and conversely, plants with apparently thin delicate leaves, which are
green only in the summer, have quite conspicuous thickening-layers. A knowledge
of these conditions is of the utmost importance in plant culture, and gardeners know
very well that many plants, although they appear to be capable of resistance, can
never be removed from the damp air of the greenhouses, because the leaves then
become desiccated like those of aquatic plants which have been taken out of water
and exposed to the air. A species of palm, Caryota propinqua, which is repre-
sented in its native habitat in fig. 74 opposite, was grown in the botanical gardens
at Vienna, and it developed in the damp air a magnificent stem with fine large
leaves. On a summer day, when the temperature of the open air coincided with
that of the greenhouse, this Caryota, together with the tub in which it was rooted,
was carried into the open and placed in a somewhat shady place, but partly exposed

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 311

312 PROTECTIVE. ARRANGEMENTS ON. THE EPIDERMIS.

to the sun’s heat. One day, after a warm dry east wind had swept for only a short
time over the foliage, it became quite brown, and in the evening all the leaves were
entirely dried up and dead. And yet leaf-segments of this palm appear to be firm,
leathery, and dry, and one would have supposed them to be particularly well pro-
tected against drying up. The section of part of a leaf which is represented in
fig. 75, however, corrects this impression. This shows that the epidermal cells are
certainly very compact, by which the firmness of the leaf is materially increased,
but that their walls are not thickened, being only like those of a delicate fern in
this respect. Under these small thin-walled epidermal cells lie large succulent cells
which form the so-called aqueous tissue, the structure of whose walls likewise
cannot limit evaporation; below these are the large succulent cells of the green
tissue. A glance at this leaf section will make it clear that this palm is well

meas@asaeseeseaseeaseaenpoeeaa ea ee eeeeoesaesseaasesaeeseeeeseeeetaoa es
in B=, ‘

Fig. 75.—Vertical section of a portion of the leaf of Caryota propingua; x260.

adapted to its warm damp habitat, where it is never exposed to a strong evaporiza-
tion, but not to the dry, even if warm, air of a Continental climate.

To the wax-like excretions of the cell-wall which form a delicate bloom, easily
rubbed off, on both sides of the leaf, frequently colouring it pale blue, grey, or white
instead of dark green, it has already been stated that the réle is assigned of protect-
ing the stomata from moisture. From what has been said, one would expect that
these waxy coverings, which are especially to be met with in the Cruciferse and
Rutacee of steppes, in many acacias and Myrtacez of Australia, and in the pinks
and spurges of the Mediterranean flora, would also be able to limit transpiration in
the epidermis—that is, in the structures over which the bloom-like covering extends.
Experiments specially undertaken, have also shown that in the same space of time,
and under otherwise similar conditions, leaves whose bloom had been carefully
rubbed off lost almost a third more water than others whose waxy covering had
been left intact.

That the varnish-like covering of the epidermis, composed of a mixture of
mucilage and resin (“balsam”), which is excreted from capitate hairs and other
glandular structures, is able to restrict transpiration has also been pointed out.
These coverings are especially developed in many plants of the Mediterranean flora,
particularly in a whole group of Cistus (C. laurcfolius, populifolius, Clusia, ladani-
ferus, monspeliensis, &c.); further in shrubby plants which develop late, in the
height of summer—as, for example, in Inula viscosa, which is so abundant: on the

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS, 313

coast. Plants of steppes and prairies (eg. Centaurea Balsamita of the Persian
steppes and Grindelia squarrosa in the prairies of North America) are likewise
protected throughout life from over-vaporization by varnish-like coverings of this
kind; while the foliage of Cherry, Apricot, and Peach trees, as well as of Birches,
Sweet Willows, Balsam and Pyramidal Poplars, and the Black Alder, is only covered
with such a varnish while young, when it has just burst from the buds, and the
outer walls of the epidermal cells have not yet become sufficiently thickened; later
on, however, when the cuticularized layers have become fully formed, this covering
which limits transpiration disappears. Only on those places of the epidermis, where
the outer walls of the cells remain very thin and permeable by fluids and gases, is
this coat of balsam retained until the leaf is to be thrown off; but in this case it
probably regulates the absorption of atmospheric water.

How far the incrustations of lime and salt excretions take part in the absorption
of atmospheric water by organs situated above the ground has likewise already been
considered in the section on water absorption. It is obvious that these concretions
and coverings of the epidermis must be capable of restricting transpiration.
Incrustations of lime are principally found in plants which grow in the clefts and
crevices of rocks; excretions of salt are only observed in shore-plants and those of
steppes and wastes, but then always on low bushes and shrubs with small narrow
leaves, and herbs whose foliage rests on the soil. The reason for this is again easily
found. High trees could not support the weight of leaves loaded with incrustations
of lime and salt, even if their trunks and branches possessed the greatest strength
imaginable. .

It has been observed that plants whose leaves are covered by incrustations of
lime and salt, or whose epidermal cells are strongly thickened on their outer walls by
corky layers, are almost always destitute of hairs; while plants, on the other hand,
whose epidermal cells possess delicate outer walls, if they are not surrounded
by a damp atmosphere throughout the year, nor submerged in water, are usually
furnished with structures known as. plant-hairs (trichomes); from which it may be
inferred that the hairy covering of the leaf or stalk in question is able to protect it
from drying up in just the same way as the corky layers. Of course only those
hairs are meant whose protoplasmic contents have disappeared, and which have
become sapless and filled with air; for those hair-structures, which consist of cells
rich in sap and osmotic contents, would not help in preventing evaporation from the
deeper tissue; they are themselves in need of protection, and special protective
arrangements exist for them, as already set forth in the discussion on the absorption
of water by aérial portions of the plant. Such structures would, if unprotected,
give off water to the surrounding air, and continually absorb fluid from adjacent
cells below them. This action does not take place in air-containing cells, and if their
dry membranes, and the air which they inclose, are interpolated between the dry
atmosphere and the succulent tissue below, this latter will be protected from evapo-
ration, like damp earth covered with a layer of dry straw or reeds, or the fluid at
the bottom of a bottle whose neck is closed with a plug of cotton-wool.

314 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

The importance of air-containing cells as a covering for succulent tissue must
also be considered in another relation. It is well known that evaporation from the
surface of fluid or a damp body is much increased by the warmth of the sun’s rays.
On the other hand, if the heating is restricted, so also is the evaporation. If we use
a dry cloth to shade from the sun, we lower not only the temperature, but also the
amount of evaporation from the shaded body. The covering of air-containing hairs
on leaves may be compared to such dry screens, and its action may be demonstrated
by the following experiment :—Take two of the bi-coloured leaves of a Bramble
bush, which are smooth on the upper side, but covered with a white felt-work of
hairs on the lower, and which are exactly similar in size and position with regard to
the sun, being situated very near each other on the stem. If these leaves are
wrapped round thermometers, in such a way that the leaf which covers one thermo-
meter bulb has its white felted side turned towards the sun, that covering the other,
the green hairless side, it will be found that the temperature in the leaf whose
smooth green side is directed towards the sun will in less than five minutes rise
2°-5° above that of the leaf whose white felted side is so directed. If such leaves
are plucked and exposed to the sun, some with the white felted side, others with the
smooth green side uppermost, the latter always shrivel and dry up much sooner than
the former. There can be no doubt, after this, that a dry coat of hair over succulent
plant tissue, which is exposed to the sun’s rays, considerably restricts the heating of,
and exhalation from this tissue.

The significance of the coverings of hair on portions of plants turned away from
the sun, particularly on the under sides of flat and rolled leaves, has already been
discussed. These coverings are only of slight importance as a means of protection
against over-transpiration. In rare cases, indeed, it happens that the hairy covering
on the side of the leaf turned from the sun, the lining of the leaf, so to speak, must
act as a protection, since the flat leaf-lamina is so twisted and turned that the sun’s
rays strike not on the upper but on the under surface. There are certain ferns of
Southern Europe (Ceterach officinarum, Cheilanthes odora, Notochlena Marante),
which, contrary to the habits of most of this shade-loving group, grow on blocks and
walls which are exposed to the burning sun. In these ferns the upper surface of
the leaf is smooth, but the under, on the other hand, is thickly covered with dry
hair-scales. In wet weather the leaves are spread out flat, with the smooth
surface uppermost; in dry weather they become rolled up, and the under cottony
side is then exposed to the sun and to dry winds. Among the low herbaceous
growths of the Mediterranean flora, a like behaviour is shown by the widely distri-
buted Hawkweed, Hieracitwm Pilosella, whose radical leaves, forming a rosette on
the soil, appear green on the upper and white on the under side, by reason of a felt-
work of star-shaped hairs. In places where the ground easily dries up, and when
there have been no showers for a long while, it is usually seen that first the margins
of the leaves turn up, and then by degrees the whole leaf becomes bent and rolled,
so that the lower side is turned towards the sun’s rays, and the white felt of hairs
functions as a protective screen to the whole leaf.

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 315

The relations between the hairy covering on the upper side of the leaf and trans-
piration stand out, most strikingly, in those districts where plants during their
vegetation period are, as a rule, exposed to dry air only for a few hours each day,
and where their activity is not interrupted by a long warm dry period, but by frost
and cold—as is the case, for example, in the Alpine region of mountain heights. On
the Alps, the drying up of flowering plants by the sun only occurs in a very few

Fig. 76.—Edelweiss (Gnaphalium Leontopedium),

cases, viz., where the scanty soil on the narrow ledges of steep projecting rocks, and
crags, and on rocky slopes, &c., is only watered by rain, mist, and dew. If no
showers fall for several successive days, and the south wind blows over the heights
with a clear sky day and night, these scanty layers of soil may dry up to such
an extent that they are unable to supply the necessary fluid food to the plants
rooted in them. Under these circumstances plants growing there have most
pressing need of means of lessening transpiration in the leaves. In places such as
these are to be found, almost without exception, plants whose leaves and stems are
thickly covered on all sides with hairs, together with succulent plants and saxi-
frages incrusted with lime. This is the habitat of the felted Whitlow-grass (Draba

316 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

tomentosa, stellata), of the grey-leaved Ragwort (Senecio incanus and Carniolicus),
of the magnificent silky Cinquefoil (Potentilla nitida), and of the white-leaved
bitter Milfoil (Achillea Clavenne); especially is this the habitat of the most
celebrated Alpine plants, of the scented Edelraut and the beautiful Edelweiss—the
former (Artemisia Mutellina) with a grey shimmering silky coat, the latter
(Gnaphalium Leontopodiwm) wrapped in dull white flannel. On looking at. the
vertical section of the Edelweiss leaf (see fig. 771), one sees that the epidermal
cells with their thin outer walls would be unable to regulate exhalation and drying
in the sun, and that a powerful protection is afforded against too rapid evaporation,
in case of extraordinary dryness, by the possession of a layer of sapless, air-filled,
interwoven hair-structures. The Edelraut, Ragwort, and the other plants named,
which grow on the sunny rocks of the Alps, show these same characters of leaf
structure, and what has just been said about the Edelweiss applies fully to them
also, It should be mentioned that on the heights of the Pyrenees, Abruzzi, and
Carpathians, as well as on the Caucasus and Himalayas, the plants growing on
sunny ridges of rock, where they are exposed to the wind, are covered with silk and
wool exactly after the model of the Edelraut and Edelweiss, and that there is on the
Himalayas an Edelweiss which is wonderfully similar to that of the European Alps.
In the far north, on the other hand, where the flora in other respects has so much in
common with that of the Alps, these plants are absent, and generally a search over
the rocky crags for herbs and shrubs, whose leaves are furnished with silky or felt-
like coverings on the upper surface, is futile. The genera which grow on these
places and form a characteristic feature of the vegetation in consequence of their
great abundance—as, for example, Diapensia Lapponica, Andromeda hypnoides,
Mertensia maritima, Draba alpina, and others, possess remarkably smooth green
leaves. When hairy coverings are present, they are restricted to the under Jeaf-
surface, especially to that of rolled leaves. They are never found on the plants of
rocky slopes, but only on those of damp marshy ground, or by the side of water
which is for a short time free from ice. Here, however, they certainly do not help
to lessen transpiration, but function in the way described above in the discussion on
rolled leaves. It is indeed not too much to connect these facts with the conditions
of the climate, and especially to explain the absence of plants whose foliage is silky
or felt-like on the upper surface, by saying that a drying up of the soil and a
limiting of the water supply never occurs on the narrow terraces of steep rocky
declivities in Arctic regions, and that therefore there is no danger of over-evapora-
tion to plants growing in those regions.

It is in keeping with this explanation that on Central and South European
mountains, on whose heights an Alpine vegetation is to be found, the number of
forms having silky and felted foliage increases as these mountains are situated
further south, and the more they are exposed to temporary dryness. Plants of the
Edelweiss type are still wholly foreign to the Riesen-Gebirge; in the Northern
Alps their number is comparatively small, in the Southern Alps they increase
in a surprising manner, and the summits of the Magellastock, the ridges of

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 317

the Sierra Nevada, and the mountains of Greece are unusually rich in such
forms.

If plants growing in such situations are protected against the dangers of too rapid
and too abundant evaporation, how much more must this be the case in those regions
where, with the increasing warmth of summer, the number of showers steadily
diminishes; and where the soil becomes dried more and more deeply, so that all the
plants whose roots are near the surface are unable to derive a drop more water from
it? All plants which are to survive the dry period in such places must during this
time entirely cease transpiring—they must, as it were, turn into a chrysalis and
sleep during the summer. They actually do this in all sorts of different ways, and
by the most diverse means. One of the commonest and most widely spread
methods is, without doubt, by having the transpiring organs clothed with a thick
covering of dry air-containing hairs. Plenty of examples of this are furnished
by the flora of the Cape, Australia, Mexico, the savannahs and prairies of the New
World, and the steppes and deserts of the Old. In the dry elevated plains of
Brazil, Quito, and Mexico, there are large tracts covered with gregarious spurge-
like growths and grey-haired species of Croton, and when the wind blows, moving
these bushes to and fro, undulations are set up over wide extents of country, the
whole appearing like a billowy sea of grey foliage. A similar picture is presented
by the Painciras belonging to the Composite, or by the Lychnophora, on the high
plains of Minas Geraes in Brazil. Nowhere in the whole world, however, does the
presence of hairs on foliage, as a protection against exhalation, appear in such an
abundant and varied manner as in the floral region surrounding the Mediterranean,
known as the Mediterranean district. The trees have foliage with grey hairs; the
low undergrowth of sage and various other bushes and semi-shrubs (for which the
name “ Phrygian undergrowth”, used by Theophrastus, may be retained), as well as.
the perennial shrubs and herbs growing on sunny hills and mountain slopes, are
grey or white, and the preponderance of plants coloured thus to restrict evapora-
tion has a noticeable influence on the character of the landscape. He who has only
heard from books of the evergreen plants of the Greek, Spanish, and Italian floras,
feels at the first sight of this grey vegetation that he has been in some degree
deceived, and is tempted to alter the expression “evergreen” into “ever grey”. Every
conceivable sort of hair structure is to be met with in these parts—coarse felt-work,.
thick velvet, and white wool mixed in endless variety. Here is a leaf looking as if
covered with a cobweb; there another as if bestrewn with ashes or clay; here a leaf
surface, covered with closely pressed hairs or scutiform scales, glistens like a piece of
satin; and here again is a plant with such a long flock of hair that one might
imagine that sheep in passing had left pieces of their fleece hanging on it. There is
hardly a family in the flora of the Mediterranean district which does not possess
members richly provided in this way. The Composites are the most remarkable in
this respect, especially the genera Andryala, Artemisia, Evax, Filago, Inula, and
Santolina; then come the Labiates of the genera Phlomis, Salvia, Teucrium,
Marrubium.,. Stachys, Sideritis, and Lavandula; rock-roses, bindweeds, scabious,

318 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

plantains, papilionaceous plants, and plants of the Spurge-laurel family—just those
plants which constitute the main part of the vegetation on the shores of the
Mediterranean Sea, and which possess a thickly-woven covering of hair. Indeed
representatives of families such as the Grasses, whose members are usually bare,
here appear to be quite shaggy with hair. It is also very interesting to see that so
many species, which have a wide range of distribution, and which, from Scandinavia
to the coasts of the Mediterranean, have bare foliage, can in the South protect them-
selves from drying up, by developing hairs on their epidermis. For instance, from
Northern and Central Europe as far as the Alps, the epidermis of the stems and
foliage of Silene inflata, Campanula Speculum, Galiwm rotundifoliwm, and Mentha
Pulegiwm is smooth and bare; in the South,—particularly in Calabria,—the leaves
and stems of these species are covered with thick down.

Next to the Mediterranean flora, the neighbouring Egyptian and Arabian desert
regions, the elevated steppes of Persia and Kurdistan, as well as the lowlands of
Southern Russia and the plains of Hungary, show a comparatively large number of
species whose leaves are thickly coated with hairs on both surfaces. Their number
is less than that of the flora of the Mediterranean district, because in the steppes
and deserts the dryness of the summer is greater than in that region, and even
thick hairy coverings are not always a sufficient protection against this dryness, and
also because in some of these districts the dry period passes directly into a severe
winter, and the hairs would offer but a poor protection against the cold. Since on
the coasts of the Mediterranean Sea the winter temperature never falls below
freezing point, evergreen and grey leaves remain there unmolested, and recommence
their activity at the beginning of the next season.

The successive developments of certain plant forms are very instructive with
regard to the relations existing between whole floral regions and transpiration. In
the steppes, Mediterranean district, and at the Cape, bulbous plants and annuals
first make their appearance; then follow the perennial grasses and woody plants;
and finally succulent plants and thickly-haired immortelles. The numerous tulips,
narcissi, crocuses, stars of Bethlehem, asphodels, amaryllises, and all the other
bulbous growths, which begin to sprout immediately after the first winter or spring
rain, always have bare foliage. Their transpiration is very active in consequence of
the rapidly-increasing temperature of the air, but the saturated soil provides a
sufficient substitute for the evaporating water, and also has ready in a free state the
food-salts which are required for rapid growth. The shrubs which sprout at the
same time, the peonies and hellebores, as well as the host of annuals which spring
up, blossom, and fructify in an inconceivably short time, almost all possess bare
foliage, especially in the steppes. Towards midsummer, when the drought com-
mences, all these plants are already in fruit; their foliage, which until now has been
actively at work, begins to turn yellow and to dry up; their succulent tubers and
bulbs are imbedded below the surface in soil which is now as hard as a stone; and
the seeds which have fallen from the annual plants are easily able to survive the
aridity of the summer and the severity of the winter, since they are inclosed in

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. : 319

protective coverings of great variety. Any plants which are still to retain their
activity during the summer on the steppes or in the Mediterranean floral district
would succeed very badly if only furnished with the bare foliage of the spring
vegetation. If such a plant is to be protected from drying up, its transpiration
must be lessened. This is effected by various protective arrangements, but best of
all by a thick coating of hair. The papilionaceous plants and species of Orache,
above all the immortelles and wormwoods (Helichrysum, Xeranthemum, Arte-
mista), which are still in bloom in the height of the summer and can bear the
strongest heat of the sun, are, as a rule, thickly covered with hair, and regions,
which perhaps only a month before were clothed in fresh green, are now shrouded
in dismal gray. With the transition from the wet period of the spring and winter
rains to the dryness of midsummer, there is a corresponding gradual transition from
the green of the bare, succulent hyacinth leaf to the grey of the rigid felt-covered
leaf of the immortelle.

A peculiar appearance is shown in Mediterranean floral districts by many
biennial and perennial plants which one spring give rise to a rosette of leaves close
to the soil, and in the spring following to a stem bearing both leaves and blossom,
which arises from the centre of the rosette. This rosette formed in the first spring
has to live through the dry hot summer, and is therefore covered with felted grey
hairs; the stem formed in the second year which gives rise to the blossom, since it
is formed during the wet period, has no need of the protective hairs, and is there-
fore furnished with green foliage. The Salvia lavandulefolia and Scabiosa
pulsatilordes of Granada, the Hieracium gymnocephalum of Dalmatia, and in the
Mediterranean flora the wide-spread Helianthemum Tuberaria may be mentioned
as examples of such plants. Their appearance is so strange that one involuntarily
asks whether this green leafy stem really belongs to the grey rosette of leaves, or
whether some one has not been playing a joke by putting together the stem and
rosette of two different kinds of plants.

These hair-like structures, called “covering hairs”, whose function is a pro-
tection against excessive exhalation, exhibit a very great variety with regard to
form. Notwithstanding this diversity, however, a certain degree of uniformity
must not be overlooked, inasmuch as in individual species the same kind of hairs
are always present. The coat of hair contributes not a little to the characteristic
appearance of the species, and therefore has always been considered of especial value
in description and discrimination. As a help to description the older botanists
introduced a series of expressions into botanical terminology by which to denote
shortly and tersely the most pronounced varieties, and this seems to be the most
suitable place for explaining these terms—z.e., the forms of covering hairs which are
signified by them. ,

First, those covering hairs consisting of a single epidermal cell, which grows out
beyond the other epidermal cells, must be distinguished and set apart from those
which have become multicellular by the formation of separation walls.

Unicellular clothing hairs in many cases only project slightly above the surface

320 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

of the leaf to which they belong; they bend down nearly at a right angle almost
immediately above their place of insertion, so that the long tapering part of the hair
cell lies on the leaf-surface, as shown in fig. 77%. When such hair-forms, in great
numbers and parallel to one another, entirely cover the surface of the leaf, light is
strongly reflected from them, and the surface looks just like a piece of silk. Such
a covering of hair, which is seen particularly well on the shining foliage of the South
European bindweeds (Convolvulus Cneorum, nitidus, olecefolius, tenurssimus, &c.),
is termed “silky” (sericeus), Two varieties of this may be distinguished, viz. the
more usual case in which all the hairs of the leaf lie parallel with the midrib, and
the rarer case where the hairs assume a different position on the right and left of
the midrib, the whole of those on either side being respectively parallel to the
lateral ribs of their respective sides. The reflected light then only meets the eye of
the observer, in any one position, from one half of the leaf, the other half therefore
appearing dull. In such a case the whole leaf has that peculiar shimmer, changing
on the slightest movement, which we admire on the wings of certain butterflies, and
which is also shown by that variety of silken material known as satin. When the
unicellular hairs do not lie on the surface, but rise up from it, the shimmer is
altogether absent, or is only present to a. small extent. If the hairs are short, very
numerous, and closely pressed together, they are said to be “ velvety ” (holosericeus);
if they are of greater length and situated further apart, the expression “shaggy”
(villosus) is used. Hairs which consist of single elongated air-containing cells,
much twisted and bent, with thin and pliant walls, are called wool-hairs, and the
covering formed by them is said to be “woolly” or “tomentose” (lanuginosus).
Woolly hairs are always twisted spirally, sometimes loosely, sometimes tightly—
frequently almost like a corkscrew. As a rule the spiral is in the opposite direction
to the movement of the hand of a watch, whose direction is said to be to the left.
It should also be noticed whether the elongated twisted cells of the wool-hairs are
circular in cross section, as in the South European Centawrea Ragusina (see fig. 77°),
or whether they are compressed like a ribbon, as in Gnaphalium tomentosum
(fig. 77+). The latter case is by far the most common.

Multicellular clothing hairs originate by the repeated division of certain
epidermal cells caused by the formation of separation walls. These dividing walls
are either all parallel to the surface of the leaf or stem, or some of them are perpen-
dicular to the plane of the leaf. In the first case the cells are usually arranged like
the links of a chain, and are termed jointed or articulated hairs. When such arti-
culated hairs are short and not interwoven—as, for example, is the case in the
beautiful gloxinias (see fig. 77 *), the surfaces clothed with them appear like velvet;
when they are elongated, curved, and twisted and entwined, the leaf appears to be
covered with wool (see fig. 77+), and to the naked eye this form of covering is the
same as that already stated to be shown by unicellular covering hairs. Silky coats
are also produced by multicellular hairs, even by such a peculiar form as is repre-
sented in fig. 78°. These hairs are developed in the following manner. <A super-
ficial cell by the formation of a septum parallel to the leaf-surface divides into two

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 821
\

daughter-cells; the division is repeated and gives rise to a small chain of three, four,
or five short cells which project slightly above the surface of the leaf. The top cell
does not divide further, but enlarges in a striking manner, not, oddly enough,
lengthening in an upward direction, but transversely, parallel to the leaf-surface,
forming a lancet-shaped, rod-like structure, which shades the leaf, and is supported
by its sister cells as if on a pedestal. Thousands of such curious hair-structures,

Fig. 77.—Covering Hairs.

1 Articulated woolly hairs of Gnaphali Leontopodi 2 Articulated velvety hairs of Gloxinia speciosa, % Silky hairs of
Convolvulus Cneorum. + Ribbon-like flattened woolly hairs of Gnaphalium tomentosum. 5 Spiral woolly hairs of Cen-
taurea Ragusina. § Stellate hairs of Alyssum Wierzbickii. 7 Umbrella-shaped hairs of Koniga spinosa; surface view.
8 Vertical section of the same hairs. ° Stellate hairs of Draba Thomasti. xabout 50.

which may best be compared to compass-needles, clothe the surface of the leaf in
close proximity to each other, and when they are arranged in a regular manner,
they reflect the light uniformly, and produce a distinctly silky lustre. If they are
twisted, this lustre is lessened to a greater or less extent. This variety of hairs,
‘called T-shaped, is distributed in a remarkable way. Numerous species of Astra-
galus, the scabious of the Mediterranean flora (Scabiosa cretica, hymeltia, gramini-
a folia), several Crucifers (Syrenia, Erysimum), native on the steppes of Southern

-- Russia, the magnificent Aster argophyllus of Australia, and particularly numerous
- Vou. I. : 21

322 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

species of wormwoods; the South European Artemisia arborescens and argentea,
the Artemisia sericea and laciniata belonging to the steppes and Siberian flora, the
Common Wormwood, Artemisia Absynthiwm, and the frequently-mentioned Edel-
raut, Artemisia Mutellina, growing on the rocky crags of mountain heights—all
owe their silky appearance to these T-shaped hair-structures.

It may also happen that the cell which is elongated transversely (1c. parallel to

gs e

YYW TY

A Wine

Fig. 78,—Covering Hairs.

EEX

<a]
ea Be

=

a

ae

eg°

CESS MB
OS)
GEE NGEES)
eel
ears)
eS |

ene)

FEET)

=

(SEBS
E>

CEES

1 Floccose hairs of Verbascum thapsiforme. % Tufted hairs of Potentilla cinerea. % T-shaped hairs of Artemisia mutellina.
4 Actinia-like hairs of Correa speciosa. §Scutiform scales of Eleagnus angustifolia, %Stellate hairs of Aubretia
deltoidea. x about 50.

the leaf-surface), and which is the uppermost of the small group of cells projecting
above the epidermis, is prolonged in three, four, or even more directions, so as to
have a stellate appearance. Thus the covering of the leaf is seen to consist of three,
four, or many-rayed stars, each supported on a short stalk (see figs, 78° and 77°).
The rays of the stellate cells are frequently forked, as in Draba Thomasti (see
figs. 77°). ‘In rare cases they have a comparatively large central portion, and are
only divided at their circumference into short rays; they then look exactly like
small sunshades spread out over the leaf-surface. This elegant form, which is
represented in figs. 777 and 77%, has a particularly beautiful appearance in

PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS. 823

Koniga spinosa, a member of the Mediterranean flora. All these clothing hairs,
with star-shaped indented upper cells, are grouped together under the name of
“stellate hairs” (pili stellati). In Cruciferss and Malvacess they occur in endless
variety.

When the uppermost cell of the group forming the stellate hair is divided by
separation walls, which in part are placed perpendicularly to the leaf-surface,
branched hairs are the result. In branched hairs the branches, which are almost

Fig. 79.—Flinty armour of Rochea falcata.

1 Section perpendicular to the leaf-surface. 2 Surface view; on the right hand the vesicular distended portion of some
superficial cells is removed and the stomata are brought into view; x 350.

always arranged in a stellate manner and are usually unicellular, can be dis-
tinguished from the part which supports the branches. This portion usually looks
like a pedestal, and is sometimes multicellular, sometimes formed from a single cell.
When the pedestal is very short, and the cell supported by it is divided by several
radiating divergent septa, which are either oblique or perpendicular to the leaf-
surface, tufted hairs (pili fasciculati) are formed. These look like sea-urchins
lying on the surface in close proximity to each other; they vary very much in the
size, number, length, and direction of their branches, and they are particularly
abundant on the cinquefoils (Potentilla cinerea and arenaria), cistus and rock-
roses (Cistus and Helianthemum). A common form is represented in fig. 787.
When the foot-stalk is very short, and the radiating branch-cells borne by it are

3824 PROTECTIVE ARRANGEMENTS ON THE EPIDERMIS.

joined to one another, a star-shaped, ribbed, multicellular plate, indented at the
margin, is produced (see fig. 78°). These plates are generally flat, lie level on
the surface of the leaf or stem, overlap one another with their indented margins,
and cover the green surface of the leaf so completely that it appears to be white
instead of green, and invest it with a bright, almost metallic, lustre. Such leaves
are said to be “scaly” (epidotus). The best known examples of such leaves,
covered with shining silvery hair-scales, are those of Hlwagnus and of the Sea
Buckthorns (Hippophaé). If the plates are bent, irregularly fringed, and lustreless,
the leaf covered with them looks just as if it were strewn with bits of clay, and is
said to be “clayey” (furfuraceus). Examples of this are well shown by the leaf-
coverings of many plants allied to the Pine-apple (Bromeliacez). When the top
cell of the hair is supported on a moderately high pedestal, and is divided into
numerous radiating daughter-cells which diverge from one another, a structure is
produced which is somewhat like a knout, or, if the radiating cells are short, like a
sea-anemone (Actinia). This form of hair is seen, for example, in the Southern
and Eastern European Phlomis, in many mulleins (Verbascum Olympicum), and,
with multicellular pedicels, on the leaves of Correa speciosa, an Australian shrub
(see fig. 784). Occasionally a branched hair produces several whorls of branches
above one another, and then hair-structures are formed which resemble stoneworts
(Characez) or miniature fir-trees under the microscope. When many such tiny
tree-like hairs are placed close together with interwoven branches, they look under
a magnifying-glass like a small plantation, and the analogy is heightened if one-
storied tree-shaped hairs, like the undergrowth in a high forest, occur under the
higher many-storied ones. This is the case in the Torchweed, Verbascwm thapst-
forme, whose hairs are represented in fig. 781. Hair-structures like these appear
to the naked eye like flock, and are described as “floccose” hairs (pili jloccosi).
Many of these have the peculiar habit of rolling themselves together into small
balls, which make the leaf-surface look as if it were bestrewn with coarse white
powder. This is the case, for example, in the mullein known as Verbascum veru-
lentum.

In the crowded condition of stellate and tufted hairs, of branched floccose and
unbranched woolly hairs, it is unavoidable that the neighbouring hair-cells should
cross one another, intertwine, and be more or less interwoven; and thus arises a
felted mass which covers the surface of the organ in question. Such hair-masses
are termed “felt” (tomentwm), and the varieties are distinguished as “felted” (or
“tomentose ”) stellate or woolly hairs, &. Often the felt only forms a thin loose
layer, through which the green of the leaf-surface can be seen; but occasionally it
is so thick that the leaf appears snow-white.

While in all these cases the covering which protects the leaves and stem of the
plant from over-transpiration is woven from air-containing cells, cylindrical and
elongated—usually, indeed, very much elongated—in some thick-leaved plants,
especially in species of the genus Rochea, a native of the Cape, these cells become
vesicular and distended; they are arranged in rank and file adjoining one

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 325

another, so that taken together they form a layer which spreads over the other
epidermal cells like a coat of mail. The ordinary epidermal cells are small and only
slightly thickened on their outer walls, as shown in the illustration above. The
cells which are placed together to form the armour, however, are enlarged in quite
an unusual way; their stalk-like base, looking as if wedged in between the ordinary
epidermal cells, is indeed comparatively large, but the bladder-like swollen portion
exhibits dimensions which are about six hundred times greater than those of the
ordinary epidermal cells. The vesicles are closely packed together, and become
almost cubical by the mutual pressure they exert on each other. Where a space
might occur, the bladders form protuberances and bulgings at the side which fit in
together in such a way that a completely closed coat of mail is the result. The
expression “coat of mail” is the more justified here since the swollen bladder-like
cells of Rochea are as hard as pebbles. Large quantities of silica are present in the
cell-walls, and by burning them a complete skeleton in silica can be obtained, as in
the case of the silica-coated Diatomacee. It needs no further explanation that in
the dry season such a coat of armour affords to the succulent cells it covers an
excellent protection against evaporation.

There is, however, still another point to be considered. The vesicular swollen
cells on fully-grown leaves are still occupied by protoplasm, which forms a thin
layer round the walls, while in the centre is a large cavity filled with cell-sap; it is
only in older leaves that the bladder-like cells become filled with air. As long as
they contain watery cell-sap they serve as reservoirs of water from which the green
chlorophyll-bearing cells below can obtain supplies at the periods of greatest
drought, when all other sources are exhausted. This fact, that the water-reservoirs
are situated on the exterior of the plants, where there exist so many aids to exhala-
tion, shows how well the silicified walls of these bladders function. They may be
compared to glass vessels whose mouths are directed towards the green tissue, and
whose walls allow absolutely no water to pass through.

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

The enlargement of the green leaf-surface has been already explained as a means
of increasing transpiration, which is of special importance when the plants con-
sidered grow in damp air. Similarly a diminution of the green surface signifies a
restriction of transpiration. This relation is illustrated by the fact that in all floral
areas, in which the activity of the vegetation is restricted or entirely stopped by
increasing dryness, the foliage of the plants is not so widely outspread, 1.e. it under-
goes a diminution. It is also a well-known fact that one and the same species,

if grown in a dry sunny position, will exhibit_smaller, and in particular, narrower
leaves than when_i ee in_ a damp situation. This is well seen in

passing from the mountainous districts bordering the Hungarian lowlands to the
plains of the lower regions. A number of shrubs and herbs, Anchusa officinalis,
Linum hirsutum, Alyssum montanum, Thymus Marschallianus, &e., exhibit on

326 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

the dry sands of the plains much narrower leaves than in the valleys of the moun-
tainous regions. In conjunction with the narrowing of the foliage, the wrinkling of
the leaves has to be considered, 1.e. the formation of grooved depressions on the sur-
face. Strictly speaking, there is no lessening of the whole surface of the leaf, but
only of that portion of the surface which is exposed to sun and wind. This is the
point with which we are concerned. With regard to the exhalation of water, only
the extent of the surface directly influenced by the agents for increasing transpira-
tion is to be considered; whilst the extent of the grooved depressions, which are not
exposed to the sun’s rays, nor to dry currents of air, may be in a certain measure
neglected. On the whole, plants with wrinkled and grooved foliage are not very
abundant. For the most part the crumpling is to be seen on quite young leaves
when first they break through the bud-scales, and when their epidermal cells are not
yet sufficiently thickened with cuticular material. Later, when the formation of the
cuticle is advanced, the wrinkles gradually become smooth, and the leaf becomes
flat.

It has already been pointed out that those pit-like depressions, on the floor of
which stomata are concealed (cf. figs. 68 and 73), may also serve to restrict trans-
piration. There is no contradiction in the statement that the same structure at one
time hinders the entrance of water and the wetting of the stomata at the bottom of
the pit, and at another time prevents direct contact with dry winds and consequent
over-transpiration. Each has its turn. When the foliage of the Australian Prote-
acee, during the summer sleep, is exposed for months to the scorching rays of the
sun and to the warm dry air, and when all supplies of water from the soil have
ceased, evaporation from the leaves must be restricted as much as possible; it is then
that the pit-like depressions perform their duty in this respect. When, later on, the
plants are aroused from their long sleep, and have to provide themselves with food,
to grow, blossom, and fructify in an extremely short space of time, while violent
showers of rain are pouring down from the clouded sky, and all the leaves are
dripping with wet; it is then very important that, in spite of these exceedingly
unfavourable conditions for evaporation, an abundant transpiration should never-
theless take place, and that the function of the stomata should be in no way
impaired by the moisture. These pit-like depressions, which in the dry period pre-
vented evaporation, now have to keep moisture away from the stomata.

In many plants evaporation from the superficial tissue is restricted by the close
contact of the leaves to their supports, like the scales on the back of a fish. The
upper side of a leaf in contact with the stem, and frequently adhering to it, is thus
deprived of the means of exhalation, and transpiration can only take place on the
somewhat arched or keeled under side of the green scale-like leaf. This occurs,
for example, in the Tree of Life (Arbor vite), in several species of Juniper, in
Thujopsis, Libocedrus, and various other Conifers. It is not without interest to
notice that in several of these Conifers the little green scale-like leaves only become
close pressed to the stem when they are exposed to the sun, whilst vaey, project
from it if the branches in question are shaded.

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES, 327

A further reduction of the evaporating surface is brought about by the
development of thickened or fleshy leaves. In order to render the points under
consideration as clear as possible, it is perhaps well to insert here the following
observations. By altering the form of a sheet of lead 8 cms. square and 1 mm.
thick into a solid cylinder, the diameter of this cylinder is seen to be only 1 em., and
the whole surface of the cylinder is only one-fifth of that of the previous flat sheet.
The application of these figures to the tissue of a leaf demonstrates how much
smaller is the transpiring surface of a thick cylindrical leaf than of a thin flattened
one. Such thickened leaves, which approach more or less to the cylindrical shape,
are to be found regularly where transpiration has to be reduced for a considerable
time—as, for example, in the mountainous districts of Central and Southern Europe,
in the genus Sedum, growing on sandy soil which easily dries up, and on stone walls
and battlements (Sedwm albwm, reflecwm, dasyphyllum, atratum, Boloniense,
Hispanicum, &e.). They also occur in a striking manner in many tropical orchids
which grow on rocks, or epiphytically on the bark of trees in the East Indies,
Mexico, and Brazil, exposed for more than six months to great aridity (Brasavola
cordata and tuberculata, Dendrobium junceum, Leptotes bicolor, Oncidiwm
Cavendishianum and longifolium, Sarcanthus rostratus, Vanda teres, and many
others); but especially are they found in aloes and stapelias and species of
Cotyledon, Crassula, and Mesembryanthemum, whose habitat is in the dryest
districts of the Cape. Several Umbellifere, Composite, and Portulaces (Inula
erithmoides, Crithmum maritimum, Talinwm fruticoswm) growing on stony places
of the sea-shore in the burning sun, and many salsolas of the deserts and salt
steppes, as well as finally some Proteacez, which for two-thirds of the year are
exposed to the droughts of Australia—all are characterized by their development of
fleshy leaves.

Just as thick-leaved plants have acquired their succulence by a modification
of their foliage, similarly, in the so-called cactiform plants, it is the stems which
become thick and fleshy, and take on the functions of leaves. Here the green
tissue is situated in the cortex of the stem, the epidermis covering it contains
stomata just like the epidermis of foliage-leaves, and the green cortex transpires,
and functions on the whole exactly as the green leaves do. When the stems
of the cactiform plants are richly branched and the branches are short, they
sometimes much resemble thick-leaved plants. Frequently also the separate
portions of the stem and branches take the form of fleshy leaf-like discs, as in the
genus of the Prickly-pear (Opuntia), and such stem-structures are usually mis-
taken by the uninitiated for thick leaves. Gardeners, as a rule, group the thick-
leaved and cactiform plants together under the single term “succulent plants”.
To the cactiform plants belong the opuntias and cacti, species of Cereus, Echino-
cactus, Melocactus, and Mamillaria, which are distributed from Chili and South
Brazil over Peru, Columbia, the Antilles, and Guatemala. These are, however,
especially developed on the high plains of Mexico in astonishing variety of form.
To the cactiform plants belong also the leafless candelabra-like tree-shaped spurges

328 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

of Africa and the East Indies, These plants are exposed, far more than the
thick-leaved plants, for the greater portion of the year to extraordinary dryness.
Their usual habitats are dry sandy and stony plains, waste rocky plateaus, and
crevices of rocks which are almost completely wanting in soil. They always inhabit
regions where no rain falls for about three-fourths of the year, and which usually
belong to the driest parts of the earth. The whole organization of these plants
corresponds to these conditions of their habitat. Dry scales and hairs are produced
instead of foliage-leaves, or these are often metamorphosed into thorns which
project in great numbers from the thick stem-structures, and efficiently protect
them from the attacks of thirsty animals. The epidermis of the pillar-like, disc-
shaped, or spherical stem-portions is thickened on the outer wall, so as to almost
resemble cartilage, and frequently it forms a coat of mail round the deeper-lying
green tissue by the abundant deposition of oxalate of lime (as much as 85 per cent).
Most of the succulent plants, whose cell-walls, which are in contact with the air, are
fortified by oxalate of lime, silicic acid, or suberin, have in their tissue peculiar
aggregates of cells which apparently serve for the storing-up of water for the dry
season, and which have been termed “aqueous tissue”. The water in these reser-
voirs is always so apportioned that it lasts from one rainy season to another; that
is to say, the adjoining green tissue which exhales the stored-up water does not
suffer from drought during the dry season. Also, it is contrived in these plants
that, immediately after the fall of the first rain, the reservoirs are again filled with
water, and that the emptying and filling of the cells and the decrease and increase
of their volume exercise no harmful influence on the adjoining tissue. Succulent
plants have been not inaptly compared to camels, the “ships of the desert”, which
provide themselves with a large quantity of water, and are then able to dispense
with further supplies for a long time without injury. The cells of the aqueous
tissue are comparatively large and their walls thin; the active protoplasm within
forms a delicate layer round the walls—that is to say, a sac whose cavity is filled
with watery, often somewhat mucilaginous, fluid. In the cactuses the aqueous
tissue is hidden as much as possible in the interior of the thick rod-shaped or
spherical stem; also in many thick-leaved plants, such as some of the European
species of the genus Sedum (eg. Sedum album, dasyphyllum, glawecwm); in South
African species of the genera Aloé and Mesembryanthemum (eg. Mesembry-
anthemum blandum, foliosum, sublacerwm), the aqueous tissue is concealed in the
interior of the leaf, and is usually composed of cells surrounding vascular bundles
there situated. In Sedum Telephium, known by the name of Orpine, as well as in
species of House-leek (Sempervivum), and many salsolas growing on steppes, the
ramifications of the vascular bundles are enveloped in a mantle of green tissue,
and the bundles, which are, as it were, overlaid with green cells, are so arranged
with regard to the colourless aqueous tissue, that to the naked eye they look
like green strands in a transparent matrix which is as clear as water. In the
Mexican Echeverias the aqueous tissue is inserted as broad stripes’ in the green
tissue, and in the thick-leaved orchids it appears as if sprinkled between the green

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 329

cells. The epidermis in numerous other thick-leaved plants serves as a store-house
for water in a marvellous way. Individual epidermal cells are then greatly
enlarged and project beyond the others in the form of sacs, clubs, or bladders, as
shown in the picture of Rochea (fig. 79). These bladders either fit together into a
one-layered extended coat of armour, or they are frequently placed irregularly side
by side or above one another. In some instances they form isolated groups or occur
singly, and appear then to the naked eye like protuberances on the green stems and
leaves, where they glitter and sparkle in the sunshine like an embroidery of dew-
drops. Many leaves and branches—as, for example, those of the widely-distributed
Ice-plant (Mesembryanthemum cristallinwm)—have the greatest resemblance to
candied fruit covered with clear, colourless, sparkling sugar crystals.

When the walls of the enormously-distended vesicular or bladder-shaped cells of
the epidermis are silicified, as are those of the repeatedly-mentioned Rochea, it is
easily understood that the watery cell-sap which they contain is not exhaled into
the air; the fluid is, so to speak, inclosed in a glass bottle and can only be given off
in the direction of the green tissue. But when the walls of the bladder-like giant
cells are not silicified, and not even particularly thickened, what is the result?
From the aspect of the Ice-plant one would think that a single warm dry day would
suffice to shrivel and dry up the watery vesicles. But this is certainly not the case.
Leafy twigs cut from the Ice-plant may be left all day on the dry ground in dry air
and sunshine, and the large bladder-like cells on the surface will not lose their
aqueous contents. After a week they become collapsed, having given up their
water, not to the atmosphere, but to the green tissue covered by this swollen coat.
Without doubt this phenomenon is to be associated with a peculiar formation of the
cell-wall; but it is as certain that the constituents of the cell-sap, which fills the
vesicles are also important, and it must be assumed that substances are dissolved in
this aqueous fluid which restrict the evaporation of the water.

These substances, which hold water with great energy, and thereby enable the
plants in question to survive through periods of the greatest dryness, are partly
viscous, gummy, and resinous fluids, partly salts. It is well known that the sticky,
watery pulp of crushed mistletoe berries, used in the manufacture of “bird-lime”,
may be exposed to the air for months without quite drying up, and the mucilaginous
juices of many cactuses and thick-leaved plants behave in a similar manner, espe-
cially those of the Cape aloes, which exhale no water, and enable the plants
possessing them to withstand the drought for months. In the thick-leaved plants
of the salt steppes and deserts, the fluids are rarely resinous or gummy, but they
frequently contain a surprising quantity of salts dissolved in water, such as common
salt, chloride of magnesium, and the like; and these also obstinately retain water in
proportionately large quantities. It is one of the most surprising of phenomena to
see the thick-leaved salsolas rising above the soil of salt steppes, green and succu-
lent, when the ground is at its driest in the height of summer, when for months no
clouds have tempered the sun’s rays and not a drop of rain has fallen, and when
almost all other plants have long ago turned yellow and faded. The large amount

it

330 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

of salts contained in the sap of these plants renders them capable of a resistance
which is almost greater than that afforded by mucilaginous materials and gum-resins.

It must, however, be remarked here that not all green leaf- or stem-cells contain-
ing abundant water have the function of storing it up for a dry season, and that the
aqueous cell-groups and strands adjoining the green tissue, especially the so-called
outer aqueous tissue, in very many cases, has another important function, viz. the
conducting of carbonie acid to places where it can be assimilated, but this will be
described in the next chapter.

‘An extreme reduction of the leaf-surface, combined with a formation of green
transpiring tissue in the cortex of the stem, is also shown in another group of plants
known by the name of “Switch” plants. They are characterized by thin rod-
shaped stems and branches, while the cactiform plants, on the contrary, always have
their axes but little branched, and massive, thickened, fleshy and rigid stem-struc-
tures which are unaffected by the wind. The switch-plants may be subdivided into
those which are flexible, hollow, and only slightly branched—as, for example, the
horse-tails (auisetwm), reeds (Scirpus), rushes (Juncus), bog-rushes (Schenus), and
several cyperuses (Cyperus); and into broom-like shrubs with rigid woody boughs
breaking up into innumerable branches and twigs. The former are distributed over
the whole world; the latter are principally to be found in Australia and in districts
bordering on the Mediterranean Sea. In Australia it is chiefly Casuarinas and
some genera of Papilionacee and Santalacee (Sphewrolobium, Viminaria, Lepto-
meria, Hxocarpus) which take on this odd form, and some of them even attain to
the size of trees. In the Mediterranean flora isolated species and groups from the
families of Asparaginez, Polygalacez, and Resedacew are seen with thin, stiff, rod-
shaped, leafless branches, which project stiffly into the air with green cortex; but
again, most of. these plants belong to the Papilionaces and Santalacese. Several
switch-plants of the papilionaceous genera Retama, Genista, Cytisus, and Spartiwm,
growing together, often cover wide tracts of country in densely-crowded masses,
and thus contribute not a little to the scenic peculiarity of the district. Many small
rocky islands off the coast of Istria are entirely overgrown by Spartiwm scoparium,
which is represented in the illustration opposite. In May large golden flowers,
scented like acacias, appear on the green rods of the Broom, and then for a short
time the dark green of the switch-plant is changed into a brilliant yellow. On
passing near the coast, just at this time, the remarkable phenomenon is seen of
golden yellow islands rising above the dark blue sea. This floral adornment is,
however, but transitory, and nothing more monotonous and desolate than such a
dry unwatered islet, covered with these shrubs, can be imagined

The Spartiwm belongs to those switch-plants which are not entirely leafless, but
which develop little green lancet-shaped leaves at intervals on their long twigs. But
these are of such secondary importance that their green tissue can only form the
smallest portion of the organic substances necessary to the further growth of the
plants, and this duty chiefly falls to the share of the cortex of the switch-like
branches. The cortex is also characteristically formed in accordance with this fact.

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 331

Under the epidermis, whose outer walls are much thickened and coated with wax,
is the green transpiring tissue or “ chlorenchyma”, consisting of from five to seven
rows of cells. This green tissue does not form a continuous mantle round the stem,
but is divided into from ten to fifteen thick strands by strips of hard bast (see
fig. 81). Below this cortex of alternating green tissue and strips of bast are soft
bast, cambium, wood, and a very large pith; but these have no further interest for
us here. It is, however, worthy of remark that in the green strands of the cortex

AIT p87) nS = SSS
Fig. 80.—Switch-plants. fated

Bushes of Spartium scoparium near Rovigno in Istria.

of the Spartiwm, the crowded green chlorophyll-containing cells of the chlorenchyma
closely adjoin one another, and that only very narrow air-passages ramify between
them, so that here there is no formation of a spongy parenchyma penetrated by
wide canals and passages. On the other hand, large cavities occur where the green
tissue touches the epidermis, and these act as substitutes for the wide ramifying
canals. Over each of the cavities a stoma is to be seen in the epidermis through
which the water vapour, exhaled chiefly from the green cells, can escape (see
fig. 812). The stomata are proportionately small, but their number is very great.
Since the guard-cells are not so strongly thickened on their outer walls as are the
other epidermal cells, the stomata appear to be somewhat sunken. By this arrange-
ment, and also by the epidermal coating of wax, they are protected from moisture.

332 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

In the Casuarinese and in Oytisus radiatus (see fig. 69), the green tissue is distri-
puted in the cortex of the branches exactly as in the case just described; but the
strips of green tissue traversing the stem are deeply cut into by longitudinal
furrows. In some other leafless switch-shrubs, such as species of the genus
Ephedra, the chlorenchyma forms a continuous and uniform mantle round the
stem, uninterrupted by strips of bast. But in this case the stomata are distributed
uniformly over the whole surface of the rod-shaped branches, while in the brooms,
Casuarinex, and in Cytisus radiatus they are absent from those portions of the
epidermis which cover the strips of hard bast.

Plants with leaf-like branches or cladodes are distinguished from switch-plants

Fig. 81.—Switch-shrubs.

1 Part of stem of Spartium scoparium cut transversely; x30. 2 Part of the transverse section; x240.

‘by the fact that all their shoots are not circular in section, but some are flattened,
looking as though they had been pressed out. When this flattening is restricted to
the so-called “short branches”, i.e. when on a stem only the ultimate, comparatively
short branches are flattened, the main axes remaining cylindrical, like ordinary
stalks, these structures have quite the appearance of leaves which are sessile on the
rounded stems. This explanation of them, however, given by botanists, is not at
first sight satisfactory to the uninitiated. Why should these flat green structures be
branches, and not leaves? The illustration opposite at once makes the matter clear.
It represents two cladode-bearing plants, viz. two species of Butcher’s-broom (Ruscus
Hypoglossum and aculeatus), each at an early stage of development and also when
fully grown. On the young shoots, which have just made their way out of the soil
(see figs. 827 and 82%), the true leaves can be seen in the shape of small sessile pale
seales on the long, rounded, finely-ridged axis; and from the angles which these scales
make with the long axis arise darker, much thicker organs which rapidly increase
in size, while the supporting covering-scales become dry, shrivel up, and finally

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 333

disappear, leaving no traces. Since the members which arise from the axils of
leaves (whether these are small clothing-scales, or large green laminz does not
matter) are not considered to be leaves, but shoots, the flat leaf-like structures of the
Butcher’s-broom are also regarded as shoots, and are named “flattened shoots”
(cladodes)—or, considering their similarity to leaves, “leaf-branches ” (phylloclades).

Fig. 82.—Plants with Leaf-like Branches (Cladodes).
1 Young shoot of Ruscus Hypoglossum. 2 The same branch fully grown, with flowers on the cladodes. 8 Young shoot

of Ruscus aculeatus. * The same branch with flowers on the cladodes.
This view is strengthened materially by the fact that these leaf-like structures, in
their further development, and in the production of shoots, behave exactly like
ordinary cylindrical axes. That is to say, small scale-like leaves spring from them,
and from the axils of these scales arise stalked flowers (see figs. 82® and 82 *) which
ultimately fructify. Plants possessing such phylloclades are not very numerous on

334 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

the whole. The Butcher’s-brooms, chosen above as examples, belong to Southern
Europe, and occur in large quantities on the soil of dry woods, where everything is
wrapped in deep sleep during the height of summer. In the Antilles, and in the
prairies of the East Indies, are about twenty shrub-like species, belonging to
the genus Phyllanthus of the Spurge-family. New Zealand also possesses one of
these peculiar phyllocladous plants, belonging to the papilionaceous genus Car-
michelia. In the species of both these genera (see fig. 83) the flattened shoots are
exceedingly like lancet-shaped foliage-leaves, and the true leaves are transformed
into small pale scales. These tiny scales are situated on the margins of the
phylloclades, and from their axils arise stalks bearing the flowers and fruit. On
the Andes of South America occur the remarkable colletias, of which a species,
Colletia cruciata, is represented in fig. 83. The leaflets on these extraordinary
shrubs are diminutive, but not pale and scale-like; whilst the green phylloclades,
which play the part of the foliage-leaves, form very strong flattened organs, tapering
to a point, and placed opposite one another in pairs, so that each pair is always at
right angles to the couple next above or below. Yet another arrangement is seen
in Coccoloba platyclada (Polygonaces), a native of the Salomon Islands, and in
Cocculus Balfourii, growing in the island of Socotra. But it is impossible here to
enter into all these variations in detail; it is enough to have brought forward
the most striking forms of phyllocladous plants which are represented in figs. 82
and 83.

If in all these peculiar plants the branches are flattened and spread out, it
cannot indeed be asserted that the surface of their transpiring tissue has undergone
diminution, and thus far of course this strange development has nothing to do with
the restriction of transpiration. The arrangement by which this is brought about
must be sought for elsewhere. It consists in this: the leaf-like shoots are so
directed that their surfaces are vertical and not horizontal. Contrary to most flat
leaves, which turn their broad surfaces fully to the incident light, the flattened
shoots are placed vertically so that at mid-day they only cast a very narrow shadow,
and do not stop the sunbeams on their way to the soil. It is obvious, however, that
such a leaf-like structure placed vertically, as it were on edge, will exhale much less
than a foliage-leaf whose surface is opposed to the mid-day sunbeams. The work
carried on in the green cells, under the influence of light, is not hindered by this
position of the leaf-like organs. If the vertical green surfaces are not so well
illuminated by the sun’s rays during the warmest part of the day, this is abundantly
compensated for by the fact that their broad surfaces are exposed to the light both
of the morning and evening sun. On the other hand, when the sun rises and sets,
the heat is not so powerful, and consequently there is no such rapid exhalation to
be feared as when the sun is in the zenith. To put the matter shortly, transpiration
alone—not illumination—is restricted by the vertical position of the green lamina,
and therefore this metamorphosis has rightly been considered a protective measure
against excessive transpiration,

This arrangement is only found in plants of dry regions, where transpiration

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 335

requires no assistance, but where, on the contrary, the danger is often imminent that
water cannot be drawn from the soil in sufficient quantity to replace that lost by
exhalation.

The phylloclades, moreover, are only a type of a large number of organs which,
in a word, all agree in this; the edge or narrow side of the flattened exhaling
structure, not the broad surface, is turned towards the zenith. In many of the

KG
Fig. 83.—Plants with Leaf-like Branches (Cladodes).

1 Colletia cruciat 2 Carmichelia australis. % Phyllanthus speciosus.

vetches of the Southern European flora (Lathyrus Nissolia, Ochrus), but especially
in a large number of Australian shrubs and trees, principally acacias (Acacia longi-
folia, falcata, myrtifolia, armata, cultrata, Melanoxylon, decipiens, &c.), it is the
leaf-stalks which are extended like leaves placed vertically, and then the develop-
ment of the leaf-lamina is either entirely arrested, or has the appearance of an appen-
dage at the apex of the flat green leaf-stalk, or “ phyllode”, as it is called. In many
Myrtaceee and Proteacez, especially in species of the genera Lucalyptus, Leucaden-

336 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

dron, Melaleuca, Protea, Banksia, and Grevillea, the leaf-blades themselves are not
placed horizontally like those of our maples, elms, beeches, and oaks, but vertically
on edge, like the phylloclades and phyllodes. Imagine now an entire wood of such
eucalypti and acacias, on which the mid-day sun is pouring down its rays. If it is
not exactly literally true to say that each vertical leaf only casts a linear shadow at
noon, it is at least certain that there is not much shade on the ground of such a
wood. The sunbeams find their way everywhere between the erect leaf-blades,
penetrating the depths below, and it is impossible to speak of the dim forest-light
under such circumstances. The Casuarinex, which grow with eucalyptus, acacias,
and Proteacez do not help to make such woods shady. and thus one is quite
justified in speaking of the shadowless forests of Australia.

Although Australia stands alone in the variety and abundance of its plants
possessing vertical leaf-blades, other floral areas furnish numerous and remarkable
examples of this arrangement. One has only to think of the curious shape of the
so-called “equitant” leaves belonging to many plants of the Daffodil family
(Tofieldia, Nartheciwm), numerous irises, and the closely-related genera, Gladiolus,
Ferraria, Witsenia, Montbretia, &e., chiefly natives of the Cape. The leaves
exhibit the peculiarity of being folded together lengthwise, and the sides thus
brought into contact become fused to one another. Only at the point where they
join the stem do the two halves remain distinct, forming a groove in which is
inserted the base of an upper leaf. The formation of such equitant leaves from
ordinary leaf-blades may perhaps be illustrated by taking a strip of paper smeared
on one side with paste and folding it longitudinally so that the pasted sides are in
contact and become joined together. Such equitant leaves are so directed that their
broad surfaces are much less exposed to the perpendicular rays of the mid-day than
to those of the rising and setting sun.

In the Mediterranean fiora, and on many steppes, plants are not seldom to be
met with whose leaves look as if they had not been able to free themselves from
the stem. In such plants the projecting portion of the foliage-leaf is very small,
but the margins are continued for some way down the stem as projecting strips
and wings. Leaves of this kind are termed “decurrent”. They are particularly
abundant amongst Composites, viz. in the genera Centawrea, Inula, Helichrysum;
but they also occur in many Papilionaceous plants and Labiates. The position of
these vertical wings, which traverse the stem, is exactly the same, with regard to
the sun, as that of the phyllodes, phylloclades, and equitant leaves, and they behave
in respect to transpiration in exactly the same way.

In many plants the blades of the foliage-leaves when young have not a vertical
position, but gradually assume it during development, i.e. the blades at first are
turned so that the flattened surfaces are horizontal and face upwards and downwards.
Later they twist round at the point where they are inserted on the stem, so that
their margins become directed upwards and downwards. As already stated, this
peculiarity is observed in many eucalypti and various other Australian trees
and shrubs. But plants in sunny situations in other regions also exhibit this

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES, 337

peculiarity. In the Spanish flora, for example, is an Umbellifer (Buplewrum
verticale) whose leaves are so twisted with regard to the sun that they remind one
forcibly of the Australian acacias. Many Composites, especially the widely-distri-
buted Wild Lettuce (Lactuca Scariola), growing on dry soil in Central Europe

>

Fig. 84.—Com pass Plants.

1Silphium laciniatum, seen from the east. 2 The same plant seen from the south. % Lactuca Scariola, seen from the east.
4 The same plant from the south. Both species are considerably reduced. ‘

exhibit this contrivance in a striking manner. A Composite shrub, Silphiwm.
laciniatum, to be found in the prairies of North America, from Michigan and
Wisconsin as far south as Alabama and Texas, has obtained a certain renown by
reason of the remarkable twisting of its leaf-blades. It long astonished hunters.
in the prairies that in these plants (represented in fig. 84) the leaf-laminz, especially

those springing from the lowest portions of the stem, not only assumed a vertical
Vou. I. 22

338 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

position, but that the broad surfaces of each leaf always faced the rising and setting
sun. Healthy living plants as they’grow in the sunny meadows look as though
they had been laid between two gigantic sheets of paper, somewhat pressed, and
dried for some time in the way plants are prepared for herbariums, and had then
been removed from the press and set up so that the apex and profile of the vertical
leaf-blades point north and south, i.e. in the meridian; while their surfaces face
the east and west. This inclination is so well and regularly observed by the living
plants on the prairies, that hunters are enabled to guide themselves over such
regions, even under a clouded sky, by means of these plants; for this reason Sil-
phiwm laciniatwm has been called a “compass” plant. The life of the compass
plant is assisted by this placing of the vertical leaves in the meridian, in that the
broad surfaces are placed almost at right angles to the incident sunbeams which
illuminate them in the cool and relatively damp morning and evening, while at the
same time they are not too strongly heated nor stimulated to excessive transpiration.
At mid-day, on the other hand, when the sun’s rays only fall on the profile of the
leaves, the heating and transpiration are proportionately slight. It is of interest
that the leaves of these compass plants, as well as those of the above-mentioned
Lettuce represented with the compass plant in fig. 84, show this inclination and
position when they grow on level, moderately dry, unshaded ground, and that in
damp shady places, where there is no danger of over-transpiration from the
powerful rays of the noon-tide sun, the twisting of the leaves does not take place,
and they are not brought into the meridian.

The placing of their leaf-blades parallel to the ground when in the shade, but
vertically when in dry sunny places, is, generally speaking, a phenomenon which
may be seen in very many plants, including shrubs and trees. A species of lime, a
native of Southern Europe, viz. the Silver Lime (Tilia argentea), is particularly
noticeable in this respect. On dry hot summer days the leaves assume an almost
vertical position, but only on those boughs and twigs which are exposed to the sun.
If the tree stands at the foot of a wall of rock, or on the edge of a thick wood, so
that a portion of it is shaded, the leaves on this shaded part remain extended
horizontally. Such a tree then presents a strange aspect, as the leaves are of
two colours—dark green on the upper side, and white on the under surface by
reason of a fine felt-work of white stellate hairs—and it is scarcely credible at first
sight that the shaded and sunny portions of the tree belong to one another.

In the compass plants and also in the Silver Lime the alterations in the
direction of the leaves are brought about by alterations in the turgidity of certain
groups of cells in the leaf-stalk. It is exactly the same cause which produces the
periodic movements of numberless plants with pinnate or palmate leaves, and the
leaf-folding of many grasses; and it is natural to conjecture that these phenomena
of movement are also connected with transpiration. This is in part actually the
case. In consequence of alterations in turgidity of the pulvini, the pinnate leaflets
of the Gleditschias and some Mimosas rise up after sunset, while those of the Amor-
phas fall down, and assume a vertical position during the night; but this is con-

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 339

nected with the nocturnal radiation of heat (as will be explained later) and not with
exhalation. It is, however, equally certain that the placing together and folding up
of leaves and leaflets in many other plants is brought about in order to prevent
over-transpiration and consequent withering up. Many shrubby, thorny mimosas
of Brazil and Mexico, when in their native habitat and position, extend their leaflets
horizontally when evening approaches, contrary to the behaviour of the well-known
Sensitive Plant (Mimosa pudica), and they remain in this position throughout the
night. Next morning they are still widely outspread. As soon as the sun has
risen, and its beams fall on the foliage, the leaflets shut together; the menacing
thorns, which until now have been hidden by the extended leaves, become
apparent; and the leaflets remain in the vertical position during the hottest and
driest hours of the day. Towards sunset they again rise and are extended
horizontally. There is but one exception to this cycle of changes—if the opened
leaf is shaken by the wind, and if the sky has been gray and clouded all day. In
the former case, under the influence of the wind, a rapid closure occurs; in the
latter case, when the weather is bad, they remain open all day: One of the
Rutacer, Porliera hygrometrica, behaves like these mimosas. In Peru, the native
country of these plants, where they abound, the opening and closing of the leaves
has even been made use of for weather predictions, for when the vertical leaves are
closed, dry hot weather can be reckoned upon; when they are open, damp cool
weather. In the cultivated Bean (Phaseolus), moreover, alterations of position in
parts of the leaflets may be observed to take place during the day. When the sun
is powerful, the leaflets assume a vertical position, so that at noon the sun’s rays
only reach a small portion of the blade.

In several species of Wood-sorrel belonging to the South African flora, and
also in the widely-distributed Common Wood-sorrel (Oxalis Acetosella), it may be
noticed that the leaflets, as soon as they are directly struck by the sun’s rays, sink
down, so that their under surfaces—qn which the stomata are situated—face one
another, the three leaflets together forming a pyramid; while these same leaflets
in damp shady places remain extended. The leaflets of the water fern, Mar'silea
quadrifolia, which grows in marshes and is distributed through Central and
Southern Europe, temperate Asia, and North America, are very similar to those of
the Wood-sorrel, but carry their stomata on the upper surface. As long as they
remain floating on the surface of water, these leaflets are extended, but as soon as
the water-level sinks and the leaflets become surrounded by air, they fold together
above in the sunshine, and their position becomes vertical, precisely as in the
compass plants.

As another phenomenon of this kind the periodic folding or closing of the leaves
of grasses must be specially mentioned. It has long been noticed that certain
grasses exhibit a very different aspect according as they are observed on a dewy
morning or in the noon-day sunshine. In the morning their long linear leaves are
fluted on the upper surface, or spread out quite flat. As soon as the humidity of
the air diminishes, in consequence of the higher position of the sun, they fold

340 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

together lengthwise; again after sunset they widen and become flat or fluted. This
process may be repeated twice on a summer's day within twenty-four hours, if a
storm intervenes at mid-day and is followed by a sunny afternoon. How much
this depends upon the conditions of humidity of the air, is demonstrated by the fact
that such grasses, when grown in pots, can be easily made to open and close their
leaves by alternately sprinkling them with water and placing in damp air, and then
for a short time exposing them to dry air. The leaf-folding in various species
of Sesleria is exceedingly quick and also very interesting. The species of this
genus grow principally on the Alps, Carpathians, and Balkans. They always
grow together and often cover wide stretches of hilly and elevated districts with
thick grassy turf. One species (Sesleria cerulea) is distributed over Northern
Europe in Finland, Sweden, and England. The closing of the leaves of these moor-
grasses reminds one strongly of the Venus Fly-trap (Dionwa muscipula), which has
already been fully described. It is indeed an actual shutting together of the two
halves of the leaf. As in the leaf of the “Fly-trap”, the midrib of the leaf of the
Sesleria remains in its original position unaltered; also the two halves of the leaf do
not come flatly in contact, but rise up obliquely so as to leave between them a deep,
narrow, groove-like cavity, widest at its lowest part (see fig. 85”). While the open
leaf turns its upper surface, rich in stomata, towards the sky, the two raised halves
of the folded leaf are parallel with the incident sunbeams, and the folded leaf of the
moor-grass may then be compared to the equitant leaf of an iris. In the cavity
produced by the closing up of the leaf are the stomata, however, and thus the green
tissue next them is excellently protected from the sun’s rays as well as from the
direct action of the wind. The epidermis of the lower surface, which is exposed on
the folded leaf to all the agencies which excite transpiration, possesses no stomata,
but is provided with a thick cuticle.

A leaf-folding similar to that of Sesleria, along the midrib, has been observed in
the leaves of Avena planiculmis, which grows in sunny fields on the Sudetics and
Carpathians. It also occurs in Avena compressa, and many others related to these
species. The folding or closing of the leaves in the large section of fescue-grasses
(Festuca) is carried on somewhat differently. In Sesleria, the opened upper sur-
face of the leaf forms only a single shallow groove, and the folding only occurs at
the midrib; but on the upper side of the fescue-grass leaf several parallel grooves
are to be seen, and the green tissue is divided up by these grooves into several pro-
jecting ridges, exhibiting a very remarkable structure. In each ridge can be dis-
tinguished the base which forms a part of the under side of the whole leaf; then
the apex opposite the base, belonging to the upper surface of the entire leaf; and
finally, the two side portions forming the sloping sides of the grooves which run
between the ridges (see figs. 87 and 88).

The greater part of each ridge consists of green tissue. The stomata on the
ridge only open on the sloping sides facing the grooves. Neither the crests of the
ridges nor the lower surface of the leaf exhibit a single stomate. The apex is without
chlorophyll, and almost always has a border of elongated cells with strong elastic

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 341

walls under the epidermis; the same thing occurs on the under side of the leaf (1.2.
at the base of the ridges), which is formed of one or several layers of cells without
chlorophyll, but furnished with thickened walls. The closing of the leaf is not so
simple here as in the Seslerias. There the leaf-folding only produced a single deep
channel, widened at its base; in the fescue-grasses all the small grooves between
the ridges become narrowed by the closing, i.e. by the upward inclination of the
right and left halves of the leaf, those adjoining the central ridge to the greatest

Fig. 85.—Folding of Grass-leaves.

1 Vertical section through an open leaf of the thin-leaved Moor-grass (Sesleria tenuifolia). 2 Vertical section through
a closed leaf; x40. 4% Portion from the centre of an open leaf; x300.

extent, those in the neighbourhood of the approximated margins in a lesser degree
(see fig. 887). Since the stomata lie on the sides of the ridges, it is obvious that
transpiration is checked to the utmost by the closing and consequent approximation
of the opposite sides of each groove.

In individual cases among various fescue-grasses are to be found manifold
differences in the number and shape of the ridges, also with respect to the formation
of the under surface of the leaf, and most of all in the form assumed by the leaf in
its expanded condition. There is a large group of festucas which are said to be
poisonous by the shepherds in the mountain regions of Spain, and in the Alps, the
Taurus, and the Elbruz. These will be spoken of again later. When open in
damp weather they form only a moderately narrow main furrow, with several

342 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

narrow secondary grooves leading from it, as can be seen in a vertical section of
an open leaf of Festuca alpestris, a plant very abundant in the Southern Alps (see
fig. 865). In Festuca alpestris, the blunt apex of each ridge has a border, three
layers deep, of cells destitute of chlorophyll, and the lower side of the leaf is pro-
vided with an actual armour of thick-walled bast cells, covered by an epidermis,

7

o
2

Fig. 86.—Folding of Grass-leaves.

1 Vertical section through part of the open leaf of Stipa capillata; x240. 2 Vertical section through an entire open leaf.
3 Vertical section through a closed leaf; x30. 4 Vertical section through a portion of the leaf of Festuca alpestris; x 210.
5 Vertical section through an entire open leaf. 6 Vertical section through a closed leaf; x30.

whose outer walls are much thickened. A vertical section through the leaf of
Festuca punctoria, a native of the Taurus, is represented in fig. 88. In this
plant, the leaves, when open, present a fairly shallow depression; the under surface
is clothed with a protective mantle of five layers of strong cells devoid of chloro-
phyll; the ridges are rounded off and possess only a single layer of covering cells,
provided with an extremely strong wax-like coat. The open leaves of Festuca Porcit,

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 343

a native of the Carpathians, are relatively thin (see figs. 874 and 875). Below the
epidermis of the under side is no mantle of bast cells as in the species already
described, but only isolated strands of bast; however, the crest of each ridge is
furnished with a strand of bast cells ; the ridges themselves project very much, and
the whole leaf is traversed by six deep narrow grooves.

In the three fescue-grasses cited here as examples, and in all species of the
genus Festuca, forming the main part of the turf of our fields, a vascular bundle

Ke

Je

dit

Fig. 87.—Folding of Grass-leaves.

1 Vertical section through a closed leaf of Lasiagrostis Calamagrostis. % Vertical section through an open leaf; x24.
3 Vertical section through a portion of the open leaf; x210. 4 Vertical section through a closed leaf of Festuca Porcit.
§ Vertical section through an open leaf; x24. 6 Vertical section through a portion of the open leaf; x210.

surrounded by green tissue traverses each ridge. In the hinged leaves of many
other grasses, the green tissue of each ridge is divided into two portions. The
vascular bundle is bordered above and below by strands of thick-walled cells devoid
of chlorophyll, and thus arises a strong septum in the green parenchyma, beautifully
shown in the transverse section of a leaf of Lasiagrostis Calamagrostis, illustrated
in fig. 87. In the leaves of the Feather-grass (Stipa capillata) are alternating higher
and lower ridges; a vertical section is shown in fig. 86+2% In the higher ridges
oceur septa similar to those in Lasiagrostis, but in the lower there is only a vas-
cular bundle surrounded by green tissue as in the fescue-grasses. No less than

344 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

twenty-nine ridges can be counted on the leaf of the above-mentioned Lasiagrostis,
a plant widely distributed in the valleys of the Western and Southern Alps, where
it clothes the sunny slopes in thick masses. When the leaf folds up, the twenty-
eight grooves between the ridges, on whose sides are the stomata, become narrowed,
and the entire leaf assumes a tubular form, so that transpiration is almost com-
pletely suspended. In Stipa capillata, which is very abundant on clay steppes,
the same thing occurs (see fig. 862). In both grasses the closure of the grooves on
whose sides are the stomata, is completed by short stiff hairs on the summit of the
ridges, which interlock when the ridges approach one another, and so block up
access to the grooves (fig. 86%). It would take us much too far to describe the
numerous other modifications which are to be met with in the structure of hinged
grass-leaves. The examples given suffice to make it evident that the danger
of over-transpiration is avoided by the folding of the leaf, and that amongst
the grasses very many arrangements obtain in order, sometimes, to expose those
green parts of the leaf whose epidermis is supplied with stomata to the rays of the
sun, and at other times to withdraw them, according to the humidity of the soil
and of the surrounding air, thus suitably regulating transpiration to the existing
circumstances.

The mechanism by which grass-leaves open and close may be explained in two
ways—either the process is due to hygroscopic changes, as in the opening and clos-
ing of the “ Rose of Jericho ’, or to alterations in the turgidity of certain groups of
cells, as in the mimosas. If the former alone were the case, a dry, dead grass-leaf
should be still capable of opening and closing in accordance with its damp or dry
condition; but a leaf of any of these when cut off and dried no longer opens, even
after being moistened for a considerable time, and therefore the first explanation
cannot be accepted, at any rate for most of the grasses. Apparently, the mechanism
consists of alterations in the turgescence of those groups of cells situated in the
angle of the grooves. Since the floor of the grooves was frequently found to con-
sist of peculiar thin-walled cells destitute of chlorophyll, and filled with colourless
watery sap, it was concluded that the opening and closing of the grass-leaves was
due to the change in turgidity of these cells. However, this was going too far.
These cells in most instances, for example, in Festuca punctoria (see fig. 88 *), would
be much too delicate to effect, unaided, the closure of the leaf by their loss of
turgidity, or to open it by their increasing turgescence. In many grasses these
cells are completely wanting (eg. in Festuca alpestris and Stipa capillata, fig. 86).
Moreover, it is observed that the opening and closing of the leaf is still carried on
when the thin-walled cells at the bottom of the grooves are destroyed, artificially, by
puncturing with fine needles. The cause of the movement must therefore be looked
for in the alteration of turgescence of other cells below the grooves. When a
mantle of several layers of thick-walled cells is present on the under side of the
leaf, their walls are seen to swell up simultaneously with the alterations of tur-
gescence of the parenchymatous cells. Of course the inner cell-layers of the mantle
must be capable of swelling up to a greater extent than the outer, and this has

FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES. 345

actually been shown to be the case in some species. Moreover, although the thin-
walled cells at the bottom of the furrows are not considered strong enough to bring
about the opening and closing by changes in their turgidity alone, it is by no means
asserted that they have no other part to play. When they are constructed as in
the leaves of the moor-grasses and in the fescue-grass of the Taurus (Festuca
punctoria, figs. 85 and 88), they certainly are not without a purpose. Their advan-
tage to the plant lies in the fact that they can be much compressed without harm
by the closure of the leaf, whereby the neighbouring parenchymatous cells are pro-

Fig. 88.—Folding of Grass-leaves,

1 Vertical section through an open leaf of Festuca punctoria, of the Taurus. 2 Vertical section through a closed leaf; x40.

3 Vertical section through a portion of the open leaf; x 280.
tected from injury; also that by means of these cells, which are filled with watery
sap, carbonic acid from the atmosphere is conducted to the underlying green tissue;
and lastly, that in case of necessity, water can be absorbed from the air. They re-
mind one strongly of the thin-walled groups of cells of foliage-leaves used for the
direct absorption of moisture, and possibly they can function in this way. If,
in places where these grasses grow naturally, a slight shower of rain falls after a
long period of drought, or if dew falls during clear nights, little or none of the
water reaches the roots, since it is retained by leaves overspreading the soil. But
the water easily runs into the furrows of the folded leaves of grass, and since the
large thin-walled cells at the bottom of the grooves can be wetted, they offer to the
water which can pass through them the shortest path to the green cells in the
interior of the leaf.

346 FORM AND POSITION OF THE TRANSPIRING LEAVES AND BRANCHES.

A process, very similar to the opening and closing of grass-leaves, is also to be
observed in the true mosses, in all species of the genus Polytrichwm, and in some of
the Barbulas. The peculiar structure of the leaves of these mosses has been already
treated of. In addition to the description there given, it may be mentioned that the
ridges of thin-walled green cells, which are present on the upper surface of such a
leaf (see fig. 89), only remain exposed to currents of air as long as this air possesses
the requisite degree of humidity; that is to say, the blade of the leaf from whose
upper surface the bands project only remains expanded while that is the case
(fig. 89?).

As soon as the air becomes dry, the lateral portions of the leaf-blade bend
upwards, and envelop the green ridges like a mantle (fig. 89+). These are then

Fig. 89.—Folding of Moss-leaves.

_ Transverse sections through the leaf of a Polytrichum (Polytrichum commune), 1 The leaf dry and folded.
2 The leaf damp and open; x85.

inclosed in a hollow chamber, and only retain communication with the surrounding
air by a narrow slit above, which is left open between the inflected leaf-margins.
But here again it should be noticed that the highest cells in each ridge are strongly
thickened on the part turned towards the opening, which doubtless helps to lessen
transpiration. The opening and closing of the Polytrichum takes place very rapidly.
By repeated hygrometric changes in the air, the process may be performed naturally
several times ina single day. In Polytrichacee, which have been plucked while their
leaves were open, the closure is seen to be completed, in dry air, in a few minutes.
Dead and withered leaves are always closed, and never reopen, even when kept
damp for a long time—from which it may be concluded that the mechanism of the
opening and closing cannot be due toa simple hygroscopic phenomenon. Probably,
the same mechanical forces come into action as produce the folding of leaves of
grasses; but the process in moss-leaves is much more complicated, since it consists,
not merely in the upward inclination of the leaf-edges, but also in an upward curva-
ture and spiral twisting of the whole leaf.

OLD AND YOUNG LEAVES, 347

4. TRANSPIRATION DURING VARIOUS SEASONS OF THE
YEAR. TRANSPIRATION OF LIANES.

Old and Young Leaves.—Fall of the Leaf.—Connection between the structure of the Vascular
Tissues and Transpiration.

OLD AND YOUNG LEAVES.

The various regulators of transpiration, hitherto described, either persist in the
plant-organs in question throughout life, or only remain for a comparatively short
time. They are present throughout life in evergreen leaves, particularly in regions
where wet and dry seasons alternate during the year. In this case the plants
require powerful aids to transpiration in the rainy season, and in the dry season
abundance of protective measures against excessive loss of water. Evergreen leaves
cannot afford to dispense with either the promoting or inhibiting arrangements
after the expiration of the first year, because for several years they still have to
pass through both these seasons. It is otherwise with those leaves whose activity
only lasts for a single summer. These burst from the buds at the beginning of the
vegetative period, and then unfold, transpire and respire for a few months, pro-
ducing organic materials, and conduct them towards the places where they are
required. At the commencement of the drought, however, or on the appearance
of frost, they turn yellow and fade, are detached from the stems and branches
which bear them, and die. In leaves of this kind, an arrangement which is very
necessary during the first season may become superfluous later—it may even
become disadvantageous under changed external influences, and the leaf would then
be benefited by freeing itself entirely from the contrivance. It would often be
useful to the plant to substitute in the place of a protective contrivance, which is
only beneficial at the commencement of the vegetative period, another arrangement
fitted to the new and altered conditions. In the so-called deciduous leaves, 1.e.
in those which throughout the year are only active in the summer, often only
for two months, it is a fact that an alternation of this kind may be regularly
observed in the mechanisms which govern transpiration.

It will be noticed in a young foliage-leaf which has just pierced through the
ground, or in one which is still half-hidden between the cotyledons of a seedling, or
surrounded by the loosening scales of a winter bud, that the development of that
portion whose duty will later on be to transpire and assimilate, is very backward.
The leaf-veins are already very prominent, but the green tissue is in quite a rudi-
mentary condition. It is not only that the extent of the surface is very small,
but that the epidermis which covers it is not yet properly developed; the outer
walls of the epidermal cells are not yet fortified with a cuticle, and are consequently
neither water-tight nor impermeable to aqueous vapour. If exposed to sun or
wind, the green tissue would at once dry up. When the young foliage-leaf has

348 OLD AND YOUNG LEAVES.

forced its way out of the bud above the soil, or from between the cotyledons, the
conditions are still the same, and therefore particularly efficacious protective
arrangements are required that the leaves just merging from the bud, and thus
exposed to the vicissitudes of the weather, may grow up properly, 2.e. that their
green transpiring tissue may be normally developed.

Some of these protective contrivances belong exclusively to the developing
period of the leaves, and are lost when they become fully grown. Others may be
seen in the adult leaves. The most striking instances are perhaps the diminution
of the surfaces directly exposed to the sun and wind, the vertical inclination of
the leaf-blades, and the concealment of the green tissue under a protective mantle.

The diminution of the surface directly exposed to the sun and wind is caused by
the position which the foliage-leaf takes up within the bud. Space is very limited
here, and the youngest and smallest leaves appear to be fitted into the space by
the rolling, or folding, or crumpling of their blades. This diminution is obviously of
great advantage when the leaves open out into the daylight: it constitutes a special
protection against the drying up of the green tissues, and is, therefore, retained
until other protective measures are developed, and in some cases even throughout
life. In many Polygonacee (e.g. Polygonum viviparum and Bistorta), in species of
Butter-bur (Petasites), in some Primulaces, and especially in many bulbous plants,
the green portions of the leaf are rolled. The midrib, and frequently a fairly broad
central strip of the leaf in addition, remains flat, and the right and left halves are
rolled up from the margins, sometimes towards the upper, sometimes towards the
lower surface. The stomata are chiefly, or wholly, to be found on the concave side,
beneath which lies the soft green tissue with its ramifying air-passages. In the
Crocus, the two halves of the leaf are rolled outwards; they are connected together
by a broad, white, central stripe which is not rolled, and is devoid of chlorophyll,
in the Star of Bethlehem (Ornithogalum), whose leaves are traversed by a similar
white stripe, the leaf margins are rolled inwards. In species of Crocus the stomata
are placed in the two grooves on the under surface; in the Star of Bethlehem, in
the grooves on the upper surface. The central stripe of the young leaves in the
plants mentioned always remains flat, but in young fern-leaves, which are also
rolled, the strongly-developed midrib is curled spirally inwards like a watch-spring,
and thus the green feather-like pinne, springing from the rachis, are placed one
above the other. Most ferns in their native habitat rarely require special protec-
tion against over-transpiration during the first stages of development; but when
this is necessary, it is afforded in every case by the form assumed by the young
leaf just described. Moreover, in such instances special protective envelopes are,
as a rule, to be found, which will be spoken of later.

Leaves are not so often crumpled as rolled on first emerging from the bud. In
crumpled leaves the net-work of anastomosing veins forms a strong lattice-work, and
the green leaf-substance, fitted into the interstices of the lattice, is swollen up like
bubbles or sunken into pits, giving the whole leaf the appearance of a crumpled sheet
of paper or cloth. The vernation (or position occupied by the leaf in the bud) is

OLD AND YOUNG LEAVES. 349

therefore aptly termed “crumpled”. Leaves specially noticeable in this respect are
those of the many species of dock (Rwmeza), rhubarb (Rheum), and also of several
spring primulas (Primula acaulis, elatior, denticulata, &c.). Frequently the
crumpling and rolling occur together, leaves with crumpled vernation having their
lateral margins also somewhat rolled inwards.

Young leaves which have just burst from the bud, and still retain the form they
possessed there, are very often seen to be “plaited”. The veins of the leaf form,

Fig. 90. Unfolding of Leaves.

1, ? Wild Cherry (Prunus avium). 8, 4 Walnut (Juglans regia). 5, 6 Wayfaring Tree (Viburnum Lantana).
7 Lady’s-mantle (Alchemilla vulgaris). 8 Wood-sorrel (Oxalis Acetosella).

as it were, the fixed framework, and it is only the green portions between which are
laid in folds. From the multiplicity in form and division of the leaf-veins, the kind
and manner of folding is also very varied. When the leaf-blade is traversed, by
radiating veins, as, for example, in the Lady’s-mantle (Alchemilla vulgaris), shown in
fig. 907, the leaf is folded in vernation just like a fan; the veins which radiate out
in the adult leaf are as yet parallel to one another, and the green portions which in
the fully-formed leaf are stretched between the veins, form deep folds, which are
closely packed together. The same arrangement occurs when each of the radiating

anf)
meer!

350 OLD AND YOUNG LEAVES.

veins becomes the midrib of a leaflet, as in the cinquefoils, and species of clover
and Wood-sorrel. Each leaflet is folded up along the midrib like a sheet of paper,
and the folded leaflets are placed side by side in the same way as folded leaves
in a book.

When the leaves are pinnate, and the leaflets are arranged in pairs on a common
rachis, the latter are folded together along their midribs, and placed side by side, so
as to resemble the pages of a book. This vernation occurs in roses, Mountain Ash
(Sorbus aucuparia) and Walnut (Juglans regia), see figs. 90? and 904% In the
roses the rachis is so short in the bud that the leaflets springing from it appear to
originate from one point, as in the cinquefoils. In most maple-leaves and those of
Saxifraga peltata, the folding takes place not along the radiating veins alone, but
along the short lateral veins which spring from the larger radiating ribs. In this
way small folds are inserted between the larger, and this vernation leads up to that
which was described before as “crumpled”. The leaf-folding exhibited by the
foliage of the Beech (Fagus silvatica, see fig. 92), the Hornbeam and the Hop-
hornbeam (Carpinus and Ostrya), the Oak (Quercus), and many other plants, whilst
in the bud, is very characteristic. Each foliage-leaf possesses a midrib and numer-
ous strong lateral veins, which run right and left from the midrib like the bony
processes from the spinal column of a fish. The green portions of the leaf form
deep folds between these lateral veins, which are as yet very close to one another,
and the folds are thus arranged exactly as in a fan. Yet another method of folding
occurs in the Cherry (Prunus avium). Each leaf, while in the bud, and for some
time after it has burst from it, is folded along the midrib only (see figs. 901 and 902).
The right and left halves are so flatly folded together, and fit over one another so
completely, that at first sight they appear to form only a simple leaf-blade. More-
over, the two halves which are in contact are actually joined by means of a balsam-
like secretion. At this stage of development they are always erect; and this
brings us to another protective contrivance to be observed in young undeveloped
leaves.

It may be stated that, with the exception of a few “crumpled” forms, all young
foliage-leaves when they emerge from the bud-scales, or from between the coty-
ledons, or as they force their way through the soil into the light of day, are so
directed that their blades are not horizontal. In this first stage of development,
indeed, the green transpiring, but still delicate, portions of the leaf have always a
vertical position. Their blades usually exhibit the direction observed in phyllo-
clades and phyllodes, in the equitant leaves of irises and tofieldias, in the leaves of
the compass plants during their greatest activity, and in the leaves of grasses
when folded together in dry air. Sometimes the entire extended or rolled blade is
erect, as in most bulbous plants and grasses; or the midrib is inclined towards the
horizon, in which case the halves of the leaf are folded together and the two
margins come into contact, forming a sharp edge which is turned towards the sun
at noon. This is seen in some grasses (Glyceria, Poa), and in the Cherry (Prunus
avium). If the blade is not erect, the stalk of the leaf is perpendicular while the

OLD AND YOUNG LEAVES. 351

still delicate blade hangs from it like a closed parasol, as in Podophylluwm, Cortusa,
Hydrophyllum, and several Ranunculaces. In the Horse-chestnut (disculus
Hippocastanwm) the folded leaflets are erect when they emerge from the bud; they
then sink down so that their apices point to the ground; and later, when the
epidermis has become more thickened, they again rise until they are almost
horizontal. Leaves of limes (Tilia grandifolia and parvifolia) are vertical when
they first break through the bud, the apex directed towards the ground; it is only
later that they become almost parallel with its surface. The upright leaf-stalk is
often bent like a hook at the end, and the vertical folded leaflets depend from the
hooked portion. This arrangement is shown in the common Wood-sorrel, and
many other plants (see fig. 90),

A third method of protecting these delicate undeveloped green portions of
young leaves consists in the formation of screens and coverings, which exhibit the
greatest variety. The envelope is frequently furnished by the so-ealled stipules.
In many plants two lobes arise on the right and left of the leaf-stalk at the point of
junction of the leaf and stem, and these have been termed “stipules” (stipule). In
figs, oaks, beeches, limes, magnolias, and numerous other plants, the stipules are
membraneous, pale, usually without chlorophyll, and they appear like scales placed
as screens in front of the small, tender green leaflets when they burst through the
bud, and in any case must be considered to protect them from the sun’s rays (see
fig. 92). When once the young leaf has grown beyond the top of these screens and
no longer needs them, they shrivel up, are detached, and fall to the ground.
Millions of such fallen scales, called in botanical terminology “deciduous stipules”,
are to be seen on the ground in oak and beech forests shortly after the leaves have
attained their normal size. The stipules of magnolias, particularly of the Tulip-
tree (Liriodendron tulipifera), a native of North America, but now cultivated all
over Europe, are very remarkable (see fig. 91). They are comparatively large and
boat-shaped, and are always so arranged in pairs as to form a closed cup. Shut
up within this membraneous, slightly transparent cup can be seen the young leaf,
its stalk being bent into a hook, and the two halves of the blade folded together
along the midrib like those of the Cherry. In this position the leaf grows gradually
as if in a small greenhouse; it enlarges, and as soon as the epidermal cells are so
much thickened that there is no further danger of it drying up, the cup opens and
the two boat-shaped stipules separate from one another, shrivel up, and at length
fall off. Only two scars at the base of the leaf remind one that two stipules were
situated here in the spring, whose function was to protect the delicate young leaf
from desiccation.

One of the most noticeable arrangements for the protection of the tender,
undeveloped green tissue consists in the peculiar grouping of the leaf-veins. This
may be best observed in foliage-leaves which are folded along the lateral veins in
vernation. Each individual leaf is erect, usually a little bent at the apex and
margins, and slightly hollowed so that the upper surface is concave, and the lower
side, which is turned towards the incident light, convex. Since the midrib of the

352 OLD AND YOUNG LEAVES.

leaf is still comparatively short, while the numerous lateral veins, on the contrary,
are already strongly developed, the latter must lie so close to one another that they
actually come into contact. Consequently on the under surface of the arech leaf,
which is turned towards the sun, nothing can be seen of the delicate green tissue;

Fig. 91.—Leaf-unfolding of the Tulip-tree (Liriodendron tulipifera).

1 A twig at the end of which the leaves are beginning to unfold. 2 End of the same twig, the leaves being further expanded.
3 The anterior boat-shaped stipule artificially removed from the upper bud. 4 One of the stipules about to fall off.

only the thick lateral veins, devoid of chlorophyll, stand out side by side like the
supporting framework of a rush mat. The green portions of the leaf, which extend
between the veins, form projecting folds on the concave surface, 4.¢. on the surface
which is turned from the sun. They are thus hidden behind the close-pressed layer
of ribs as if by a roof, and are consequently protected as efficiently as possible from

OLD AND YOUNG LEAVES, 353

the sun and wind. The ribs themselves are composed of cellular structures which
are not open to the danger of over-transpiration, and the epidermis which covers
them is entirely devoid of stomata. When the leaves at the ends of the young
twigs are opposite, erect, and concave, and their margins are in contact, they form
an actual capsule round the apex of the shoot. This occurs in the Wayfaring Tree
(Viburnum Lantana), illustrated in fig. 90°. The small folds of green tissue
project into the interior of the capsule, and the still closely-pressed lateral veins
form the outer wall, and at the same time furnish a protective covering for the
enlarging green portions of the leaf. As soon as these are fully developed, and the

|)
tT
i eS

if ii

h

) aff
Tae ill
eC Z

Fig. 92.—Unfolding of Beech-leaves.

1 The brown bud-scales have been loosened, and the membraneous stipules surrounding the foliage-leaves are visible above.
2 Further stage of development, the folded foliage-leaves being visible between the stipules. % The same twig further
developed. 4 Lower surface of a young folded leaf.. 5 Portion of the same leaf; the depressions caused by the folding
are bridged over by silky hairs. 6 Surface view of an unfolded leaf; the stipules are withered and about to fall.
7 Vertical section of a leaf at right angles to the midrib. 8 Vertical section parallel with the midrib. é

epidermal cells are correspondingly thickened, the projecting folds become smooth,
the veins separate from one another, and the whole leaf becomes flat, assumes a

‘horizontal instead of a vertical position, and turns the upper instead of the lower

surface to the incident light (see fig. 90°).

It has already been repeatedly stated that coats of varnish as protective
coverings are especially to be met with on young leaves, which they guard from
over-transpiration and desiccation during their development, and that when the
leaf-laminze become provided with a cuticularized epidermis, these coats disappear.
Tt has also been pointed out, incidentally, that coats of hairs are of great use as

protections and screens to the young foliage-leaves when they first emerge from the
Vou, I. 23

354 OLD AND YOUNG LEAVES.

buds. The leaves of a great number of plants are only hairy during the commence-
ment of development. Long hair-cells may be seen inserted by their narrow bases
between the flattened epidermal cells; these at an early stage shrink up close to
their origin, and then break off. They may remain hanging to the leaf for a little
while, but afterwards are thrown or pushed off by the enlargement and expansion
of the leaf-blade, or are frequently blown away by the wind. The leaflets, which
were originally quite thickly clothed with hairs, then appear partially or entirely
smooth and green on both sides. A remarkable instance of this is furnished by
Amelanchier vulgaris, whose foliage, early in the spring, is folded along the midrib
and covered with snow-white wool, reminding one strongly of the Edelweiss, while
in the summer no trace of the covering remains. The White Poplar (Populus alba),
pear-trees, and mountain-ashes behave in like manner. Horse-chestnut leaves,
when they make their way through the brown, loosened bud-scales, are thickly
covered with wool, but during the spring they lose this so completely that only
here and there on the fully-formed leaves can remnants still be observed clinging
to the leaf. It is, however, not only woolly coverings that are later either partially
or wholly thrown off as superfluous. On the foliage-leaves of the already-
mentioned Wayfaring Tree (Viburnwm Lantana) appear felted stellate hairs
which fall off as soon as the epidermis is sufficiently thickened. In a species of
Rhubarb (Rhewm Ribes) brittle, candelabra-like, short-branched trichomes are
situated on the edge of the leaf, which is much crumpled at an early stage, and
later, when of no further use, they break away in pieces and fall off. Again, in
many mulleins (e.g. Verbascum pulverulentum and granatense), there are branched,
shrub-like hair-structures which become detached from the surface of the fully-
developed leaves, and are carried away in loose flakes by the wind.

The covering of the young leaves of the Beech (Fagus silvatica) consists of
silky hairs, and the way in which these are arranged and utilized is so peculiar that
it is worth while to inquire further into the details. At first sight, the under
surface of the young beech-leaf appears to be entirely covered with silky hair;
on a closer examination, however, it is seen that the hairs are only inserted on
the margins and on lateral veins, and that the green portions of the leaf are in
reality perfectly smooth and free from hairs. Since the green portions of the
leaf are thrown into deep folds (see figs. 92 and 925), and the veins are still
close to one another, while the tops of the silky hairs springing from these veins
reach far beyond the vein next to them, all the furrowed depressions caused by
the folding are completely covered over. Each groove is bridged over by the
hairs, which are regularly arranged, side by side, parallel to one another; thus
the leaf appears to be clothed completely in a delicate silken coat. There can
be no doubt as to the function of these hairs. The green tissue overspanned by
them is protected from the sun until its epidermis is sufficiently thickened, and
when this is the case the folds flatten out (fig. 92°) and the leaf assumés a
horizontal instead of a vertical position, thus turning the lower surface away
from the sun, and rendering the hairs of no further use. They have become

FALL OF THE LEAF. 3855

superfluous, and. usually fall off—or, if they still remain on the lateral veins, they
are shriveled, insignificant, and meaningless.

The dry membraneous scales seen on young fern-leaves should be mentioned
here. Let us examine a frond of the first wild fern we meet—say of Nephrodiwm
Filia-mas. The young frond is still spirally rolled, although it has forced its way
through the soil, and is now exposed to the wind. Moreover, nothing is to be seen
of the fresh green which later adorns this fern; the lower part of the midrib and
lateral veins appear to be strewn with chaff, being entirely covered with dry
membraneous brown scales and shreds. Later, as the leaf unrolls more and more,
its green fronds also become expanded, but by this time the cell-walls are sufficiently
strengthened, and no longer require the chaffy coat. In ferns which grow in
sunny, rocky situations, and as epiphytes on the fissured bark of old trees in
tropical regions, this coat of chaffy scales is even more noticeable, and, as stated
earlier, in such plants it persists throughout life.

FALL OF THE LEAF.

Just as many phenomena of the sprouting and unfolding of foliage are dependent
upon transpiration at the beginning of the vegetative period, so many processes,
but chiefly that of the fall of the leaf, stand in causal connection with transpiration
at the close of that period. Sooner or later, of course, the activity of each leaf
entirely ceases; it dies, becomes detached from the stem to which it has rendered
service, and falls to the ground, where it decays. In districts where the vegetation
can continue its activity uninterruptedly throughout the year, there is nothing very
noticeable about the fall of the leaf. Asa rule, as the new leaves arise below the
growing apex of the shoot, the lower, older leaves wither up and decay; the fall
is quite gradual, and takes place, like the development of new leaves, all through
the year. In neighbourhoods, however, where the changes of climate prevent the
uninterrupted activity of plants throughout the year, it is essentially different.
Trees and shrubs, and many smaller plants, shed the whole of their foliage in a few
days at certain annually-recurring periods, and then remain with bare branches for
a considerable time, apparently quite lifeless. This is the case in regions where a
long, hot, dry period follows the short rainy season, and also in very cold districts
where the long-continued frost causes an icy winter, and the plants are locked in a
deep sleep. In tropical and sub-tropical regions, where no showers occur for many
months at a time, the branches become stripped of their leaves. Even at the begin-
ning of the dry hot season, they remain apparently dead for months, but again
break out into leaf at the commencement of the cooler rainy season, when invigor-
ating moisture is supplied to the parched ground. On the other hand, in those
regions of the temperate zone in which there is no sharp distinction between the
rainy and dry seasons, and rain falls every month, the foliage is stripped from the
trees at the beginning of the cold period, and after the winter is over, fresh green
leaves once more burst from the buds on the branches.

356 FALL OF THE LEAF.

It certainly appears strange that the leaf-fall should be sometimes connected
with the approach of cold, and sometimes with that of hot weather. And yet this
is the fact. Heat and cold are only the indirect causes; the primary cause of the
fall of the leaf is the danger threatened to the plant by the continuance of transpira-
tion when either heat or cold is excessive. The danger of transpiration during con-
tinued dryness of soil and air scarcely requires much explanation. The conditions
may be summed up in a few words: the throwing off of the transpiring surfaces when
the drought commences, and the temporary stoppage of the sap-current—z.e. the so-
called “summer sleep”—furnish one of the best protective measures in plants sur-
rounded by air against excessive transpiration and withering. It is more difficult
to explain the connection between the fall of the leaf and the commencement of the
cold period. This is best indicated by some culture experiments which illustrate
these relations. When the soil, in which are cultivated plants with actively trans-
piring leaves (melons, tobacco, and the like), is cooled down to a few degrees above
zero, the leaves after a short time become faded, even although the temperature of
the air and the humidity of both soil and air are entirely favourable. By the
lowering of temperature in the soil, the absorbing activity of the roots buried
therein is so reduced that the water which is lost by transpiration from the foliage-
leaves can no longer be replaced. The leaves wither, dry up, turn brown or black,
and appear to be burnt or charred. In the ordinary language of gardeners they are
said to be “frozen”—frozen at a temperature above the freezing point, which
phenomenon is said to be due to the peculiar sensitiveness of these plants. It is
incorrect to speak of freezing in this case, however. The plants are in reality dried
up by reason of the low temperature of the soil and consequent lessening of the
stream of fluid up to the transpiring foliage-leaves. In regions which annually pass
through a long period of cold, the leaves of the plants are as liable to be dried up
by the cooling of the soil round their roots when winter approaches, as are the trees
in the catingas of Brazil when the hot dry season commences. They also denude
themselves of their leafy raiment as these do, since otherwise they would be
unable to make good the water exhaled by the leaves. When the temperature of
the air sinks below zero, frost ensues, and the water in the plant stiffens into ice;
this hastens the fall of the leaf, but it was already partially accomplished before the
frost set in, and where the leaves still cling to the branches, preparations are already
made for their detachment, which is brought about by the limitation of transpira-
tion. It must not be concluded from this that plants foresee the approach of winter,
and that the preparations for the fall of the leaves result from such an intelligent
foresight; the phenomenon is much more easily explained on the assumption that
in a climate which renders necessary a long cessation of transpiration, those plants
flourish and multiply best whose natural characteristic is to follow a period of
energetic work by a long season of rest. The ultimate cause of this instinctively
adaptive periodicity is certainly not yet explained; it is as mysterious as those life
processes and phenomena which regularly recur at certain periods, which are perhaps
hastened or retarded by favourable or unfavourable external conditions,*but cannot

FALL OF THE LEAF, 357

be stopped by them, and which the plant carries out, or endeavours to carry out,
without immediate external stimulus.

It is highly interesting, with respect to the acceleration or retardation of the leaf-
fall, to observe how the same species of plant will behave under various favourable
or retarding external influences; or how, in each region and locality, a selection has
been made to a certain extent of the plants best adapted to the given conditions,
First it is to be noticed that, under otherwise similar circumstances, the foliage
remains green for a longer time, and is retained longer on the branches in places
where the soil and air are more humid. In damp, shady, wooded glens, not only
ferns, but the leaves of birches, beeches, and aspens are still green while on the
sunny hillocks close at hand the brown leaves flutter down on to the withered
fronds of the Bracken Fern.

The most remarkable fact, however, is that in elevated mountain regions a plant
loses its leaves much earlier than does.the same species growing in the lowlands.
From the fact that in the Alps, the larches and whortleberry bushes, on the upper
limits of the woods, put forth their green needles and leaves about a month later
than in the valleys at a height of 600 metres above the sea, it would naturally be
expected that this considerable delay would be compensated for by a corresponding
postponement of the ending of the year’s work, and that the fall of the foliage on
the upper limits of the wood would also be postponed for about a month. But this
is far from being the case. The same species of larch which becomes green a month
later, up on the mountain slopes, also turns yellow a month earlier in the autumn.
While the whortleberry bushes in the depths of the valley are still adorned with
dark-green leaves, the same species growing in the glades on the upper limit of the
wood, already, from the valley, appear to be shrouded in deep crimson. Their leaves
are becoming discoloured above, and are withering and dropping from the twigs.
The explanation of this phenomenon follows naturally from what has just been said.
In the high mountain regions where tall trees find their uppermost limit, the ground
is frequently covered with frost at the end of August; snow falls regularly in the
first half of September, and although this may be melted in sunny places, the soil
is nevertheless thoroughly cooled by the water so produced. The days rapidly
become shorter, and the sunbeams can no longer replace the heat lost by radiation
in the lengthened nights. The temperature of the soil in which the plants are
rooted consequently falls rapidly, and the immediate results are that the absorbent
roots stop working, the decolorization progresses, and the foliage-leaves, which are
no longer able to repair the loss caused by transpiration, wither and fall away.
Accordingly, on this upper tree limit, only those larches and whortleberry-bushes
can thrive which are organized to commence their year’s work a month later,
and to finish it a month earlier, than those which have taken up their position
1400 metres below.

This obviously applies not only to the larches and whortleberries, cited here as
examples, but to all other plants whose range of distribution extends from the
lowlands up to the wood limit on the slopes-of the mountains. It also applies

358 FALL OF THE LEAF.

further to those plants which have a wide horizontal distribution; for example,
to those which grow wild or are cultivated from the lowlands at the northern
foot of the Alps to South Italy, and even further south, on the further side of the
Mediterranean. By journeying southwards, it will be seen that the beeches and
elms which, on the northern foot of the Alps near Vienna, lose their colour in
the beginning of October, are never discoloured before November on the moun-
tains of Madeira, and that whilst the planes already show leafless branches in the
North Tyrolese valleys at Innsbruck, they retain their leaves (although these are
turning yellow) on the mild shores of Lake Garda at the southern foot of the Alps.
In Palermo they are still adorned with dark-green foliage. Planes, indeed, in
certain instances remain green all winter in Greece, and thus far it was no myth
when Pliny spoke of evergreen planes. The Elder, which in the north is a deciduous
plant, in Poti, on the Black Sea, retains its green leaves through the whole winter.
In the oases of the North African deserts the Peach-tree keeps its foliage fresh
and green from one vegetative period to another, and while the blossom of this tree
in Central and South Europe unfolds on branches which have lost their foliage in
the previous autumn, in the oases the flowers are situated amongst the still green
leaves of the last period of vegetation. It may be confidently assumed that here
also the cause is the temperature and humidity of the ground, and that the planes
and peaches, whose roots at the end of autumn and winter are buried in a damp
and relatively warm soil, are the last to throw off their foliage.

From all these considerations it cannot be doubted that the stripping of the
foliage depends upon the stoppage of transpiration, and primarily upon the dry-
ing-up of those sources from which the transpiring leaves derive their water.
Plants which denude themselves of their foliage of course lose with it much organic
material, for whose production they have toiled for months; but this loss will stand
no comparison with the advantages gained by the abscission of the leaves. In
reality, it is only a framework of empty cells—the dead envelopes of the living
portion of the plant—which is thrown away. The protoplasm has opportunely
withdrawn, the plastids which carried on their activity in the cells of the foliage
have migrated thence and taken up winter quarters in other sheltered parts of the
plant—in the stem, roots, or tubers, and have there deposited everything which
will be of use in the following year, such as starch, sugar, &c. The empty cells
can thus be easily sacrificed to the common weal. The leaves fall to the ground,
where they decay and help to form natural mould, of which the posterity of the
deciduous plants reap the benefit. Since, by the formation of albuminous com-
pounds in the leaves, an abundance of calcium oxalate arises which is of no further
use to the plant, and is consequently stored up in such quantity at the end of
summer that it at last becomes burdensome to the plants, the throwing off of the
foliage must really be regarded as a method of removing waste materials, and may
be compared to the excretion of waste which occurs in animals,

Finally, it should be noted that only plants whose foliage lies flat on the ground,
or whose branches and twigs are very elastic and bear needle-shaped leaves, are

FALL OF THE LEAF. 359

unharmed by the pressure of snow. ‘Trees, bushes, and shrubs with broad out-
spread leaves, such as planes, maples, limes, beeches, and elms, are not capable of
supporting the weight of snow lying on their large leaf-surfaces. When, as
occasionally happens, mountain and valley are covered in snow in the autumn
before the leaf-fall has commenced, or when, late in the spring, to the terror of the
farmer, snow falls on wood and meadow after the young leaves have attained to a
considerable size, the devastation produced is fearful. The large-leaved shrubs are
pressed down and their stems broken. Branches as thick as one’s arm and huge
tree-trunks are shattered, and in the woods quantities of maples and beeches are
felled, or even uprooted. Such devastation would recur every year in regions
with snowy winters if the leafy trees did not strip off their foliage in time, and
it can easily be imagined what would happen to the woods after a series of such
catastrophes.

There is, consequently, a widespread idea that the autumnal leaf-fall is brought
about by frost. This idea is founded on the observation that when the temperature
in October and November falls below zero, quantities of leaves drop from the
branches in the early hours following the cold bright nights. Though it can
scarcely be denied that the fall of the leaf is in some measure connected with frost,
still that it is not always the immediate cause, is demonstrated by the fact that
when plants with leafy branches are exposed at the end of August or beginning of
September to a temperature below zero the leaves do not fall immediately; while,
on the other hand, the foliage of limes, elms, maples, cherry-trees, &., is at last
stripped off in the autumn even though no frost has occurred. It can only be said,
therefore, as already stated, that frost is favourable to the fall of the leaf, and
that it hastens the commencement of the process; but not that the detachment
of the foliage is brought about by its sole agency.

The detachment of the leaves from the branches is brought about by the
formation of a peculiar layer of cells, from the co-operation of a special tissue, which
has been termed the layer of separation. As a rule, leaves cannot detach them-
selves without the previous formation of this tissue, not even if they are exposed
for a long time to a very low temperature, and the sap in their cells and vessels is
stiffened into ice. That portion of the leaf in which the separation is to take place
is made up of a strong tough tissue, and the mechanical alterations produced by the
frost would not sufficé to complete the rupture. The separation-layer, on the other
hand, which is formed within this tissue in one or several definite places, consists of
succulent parenchymatous cells, whose walls are so constructed that they are easily
separated by mechanical or chemical agencies, thus rendering possible a disintegra-
tion of the cell-tissue. The incitement to the construction of a layer of separation
is indeed usually the limitation of transpiration by the gradual cooling of the
ground, and the cessation of the absorbing power of the roots in those regions
which experience a cold winter. As soon as this restriction of transpiration
commences—and it varies very much, as shown in the previous discussion, with the
latitude and altitude of the region in question—thin-walled cells arise in the lower

360 FALL OF THE LEAF.

portion of the leaves and leaflets, which rapidly increase by division, and in a short
time form a zone, readily to be distinguished from the thick older tissue by its
lighter tint and by the fact that it is somewhat transparent. Usually this zone
is formed in the petiole, and at those places where the vascular bundles become
narrowed in passing from the twig to the leaf-blade, there to divide up into the ribs
and veins. The growing tissue is inserted just at this place; it actually presses and
tears the other older cells apart, and even causes a rupture between them. As soon
as the separation -layer has attained its proper thickness, its thin-walled cells
separate from one another, but so as not to injure or burst their membranes in
any way. It seems that the so-called middle lamella of the cell-wall is dissolved by
organic acids, and that thus the continuity between the cells of the separation-layer
is destroyed. The most trifling cause will now effect a splitting in the loose tissue
and a fracture between the cells of the separation-layer; and when no other external
shock follows, the detachment ul‘imately takes place of itself, the weight of the
leaf helping to bring about a complete severance. As a rule, however, the fall of
the leaf is hastened by external influences. Every gust of wind brings down the
leaves; the alterations in volume dependent on the frost and chill and the subse-
quent thawing of the cell-sap, aid the severance and also hasten the tearing of
vascular bundles which are still entire; and thus it happens that thousands of
leaves fall to the ground even in the absence of wind, especially when, after a
frosty night, the rising sun illuminates the autumn-tinted leaves, and dissolves the
frozen sap.

The region where the separation is effected is usually sharply marked off, and it
looks as if the leaves and leaflets had been cut through with a knife. The severed
surfaces present a variety of contours, according to the shape of the leaf-stalk.
Sometimes it is horseshoe-shaped, sometimes triangular or rounded, or it reminds
one of a trefoil-leaf, and sometimes it has an annular shape. The stalk of the
plane-tree leaf has at the base a conical swelling which incloses a bud; when the
leaf falls a fissure is formed entirely going round it. Many of the separation
surfaces of the leaf-stalks are like the articular surfaces of the long bones in
the human skeleton (of the radius, tibia, and at the elbow). Vine leaves form
two layers of separation, one close to the stem at the base of the leaf-stalk—
the other at the upper end of the leaf-stalk immediately below the blade. In the
palmate leaves of the Horse-chestnut and Virginian Creeper (A mpelopsis), in the
compound leaves of Spirea Arwncus, in the pinnate leaves of the Chinese Tree of
Heaven (Ailanthus glandulosa), and in the bipinnate leaf of the North American
Gymnocladus Canadensis, a small separation layer arises below each leaflet, and a
larger one, in addition, at the base of the leaf-stalk. Such leaves, consisting of
several leaflets, collapse like houses built of cards when touched, and under the trees
late in the autumn lies a confused heap of leaflets and leaf-stalks, the latter some-
times looking like. long rods (as, for example, in the Ailanthus and Gymmocladus),
sometimes almost like long bones (as in the Horse-chestnut, fig. 93).

Frequently the layer of separation is so situated on the leaf-stalk that after the

FALL OF THE LEAF, 361

detachment a small portion of the stalk remains on the branch. This is the case in
the so-called Syringa, or Mock Orange (Philadelphus), where the scale-like part
which is left has to protect the bud situated just above the leaf-stalk.

In some trees and shrubs defoliation is very rapid, in others only gradual. Tn
the Japanese Maidenhair Tree (Ginkgo biloba), the formation of the separation-layer
and the detachment of the leaves is completed in a few days; in hornbeams and
oaks the stripping of the foliage continues for weeks, and frequently only a portion

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of the dead leaves is thrown off in the autumn, the remainder not until the close of
the winter.

It is also worthy of remark that in some trees the leaf-fall begins at the end of
the branches and gradually proceeds towards the base, while in others the contrary
is the case. In ashes, beeches, hazels, and hornbeams, the apices of the branches are
leafless when the lower parts still bear firmly-fixed foliage; in limes, willows,
poplars, and pear-trees, on the other hand, the lower portions of the branches are
seen to lose their leaves early in the autumn, the denudation gradually extending
upwards; on the extreme ends of the branches some leaves, as a rule, obstinately
remain for a long time, until they also are at length whirled away by the first
snowstorm.

362 THE VASCULAR TISSUES AND TRANSPIRATION.

CONNECTION BETWEEN THE STRUCTURE OF THE VASCULAR TISSUES
AND TRANSPIRATION

It is naturally to be expected that between the contrivances regulating transpira-
tion in the immediate neighbourhood of the green tissue, and those mechanisms which
effect the transport of the crude sap from the roots, through the stem and branches,
up to the region of this transpiring tissue, a mutual co-operation will exist.

Where much water is exhaled from the surface, much water must be supplied,
and in tracts leading to extensive and strongly-transpiring leaf-blades, the fluid
moves more quickly than in a conducting apparatus leading to green tissue, which
transpires but slowly and to a small extent. In pines, whose stiff acicular leaves
transpire but little, the raw food-sap moves much more sluggishly than is the case
with maples, whose flat leaves give off large quantities of water in the form of
vapour. The quickest movement, however, is to be found in twining and climbing
plants, whose stems, a few centimetres in thickness, may attain to a length
exceeding 100 metres. This is the case in those peculiar climbing palms, which at
first wind over the ground in numerous snake-like coils, and then rise to the tops
of the highest trees, and unfold their leaves there in the sunshine. Climbing palms
(Rotang) are known whose stems actually attain a length of 180 metres, and which,
when they have reached the summit of the trees after numerous windings, become
erect and extend their larger pinnate leaves just like the straight-stemmed palms.
The illustration opposite (fig. 94) depicts in the background the edge of a wood up
whose trees have climbed examples of such a species of Rotang.

Many hours of the day may pass, when, on account of a clouded sky and the
great humidity of the air, the transpiration in the wide-spreading leaves above the
tops of the trees will be extremely little; but when the sun shines brightly and the
leaves become thoroughly warmed, a large quantity of water vapour must be
exhaled in a very short time. This quantity of water must be replaced, and very
quickly, but by means of a stem 180 metres long and only some centimetres thick.
In order to render the replacement possible, everything which might hinder the
rapid movement of the water and its dissolved food-stuffs on its long journey,
especially the resistance of the conducting tubes, must be minimized as much as
possible. The forward movement of fluids in a channel is, however, rendered
more difficult as the tube narrows, because in a narrower tube a relatively larger
amount of the fluid adheres to the inner surface, and therefore it is necessary, in
order to obtain a rapid movement, that this adhesion be reduced as far as possible.
This is most simply effected by widening the channel, since the adherent surface
is thus diminished in comparison with the large amount of the fluid passing
through. As a matter of fact, in the stems of climbing palms relatively very wide
tubes are to be seen, through which a large quantity of fluid can be brought
from the roots to the transpiring leaf-surfaces in a very short time, and this
actually occurs. The climbing palm, Calamus angustifolius, has conducting tubes

THE VASCULAR TISSUES AND TRANSPIRATION. 363

of more than 4 mm. diameter, and in the species of Rotang illustrated in fig. 94
they are almost as wide.

Fig. 94.—Indian Climbing Palms (Rotang). From a photograph.

What has been stated here with especial regard to the Rotang or Climbing Palm
applies also to all other twining and climbing plants known by the name of lianes,
and their sap-conducting tubes are the wider, the longer their stems and the larger

364 THE VASCULAR TISSUES AND TRANSPIRATION.

their transpiring leaves. In very many lianes the cavities of the conducting vessels
can be plainly seen with the naked eye. This is the case, for example, in the cross-
section of the liane represented in natural size in fig. 95° A diameter of } mm. is

Fig. 95.—Lianes.

1 Portion of tne suem of a tropical Aristolochi 2 Cross section of a liane-like Aristolochia. 8 Menispermum Carolinianum.
4 Cross section of the twining stem of Menispermum (magnified). 6 Portion of a liane (probably an Asclepiad) gathered in
a tropical forest; nat. size.

not at all rare in passion-flowers and aristolochias, and, generally speaking, in most
twining and climbing plants; whilst in many lianes the conducting tubes have
even been observed to be 0°7 mm. in diameter.

THE VASCULAR TISSUES AND TRANSPIRATION, 365

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Fig. 96.—Aroids (Philodendron pertusum and Philodendron Imbe) with cord-like aérial roots. | gs *,

366 THE VASCULAR TISSUES AND TRANSPIRATION.

A particularly noticeable method of conducting water from the soil to the green
leaf-blades is exhibited by some large-leaved tropical Aroids which climb up trees,
and are provided with aérial roots. These plants have really two kinds of aérial
roots, viz.: shorter ones, which are at right angles to the stem, by means of which
they climb up their support, usually old tree-trunks; and longer ones, passing
down perpendicularly to the ground like ropes or strings. In the Mexican
Tornelia fragrans (Philodendron pertuswm) represented in fig. 96, these latter
roots attain a length of 4-6 metres and a diameter of 1-2 cm. They are of
uniform thickness, brown, smooth, unbranched, and quite straight. As soon as
they reach the ground, the tip bends round almost at a right angle, and sends
a number of lateral roots which are covered with an actual fur of root-hairs into
the soil. The bent end then enters the soil for a short distance, and thus the
entire aérial root is rendered fairly tense. As a rule, two such cord-like aérial
roots originate below each new leaf, and it seems as if this arrangement was
specially adapted to transport the necessary food-sap from the soil to the large
luxuriant leaf above by the shortest path. But it not only seems so, for this is
actually the case, and it is especially remarkable that root-pressure takes a
prominent part in the transport. On cutting through one of these cord-like aérial
roots about a span above the ground, watery fluid is immediately seen to ooze from
the middle of the cut surface. The woody portion of the root, which here forms a
central strand, contains very wide conducting tubes, like those in the stems of lianes,
and the quantity of fluid exuded in thirty-six hours amounts to as much as 17 grms.
It is noteworthy that the root-pressure here, according to all appearances, acts with
the same force all through the year. In the vine this is not the case. Vines which
are cut through in the summer, it is well known, no longer weep; the cord-like
aérial roots of tropical aroids, on the other hand, weep at all seasons of the year
when cut across. Indeed, the vegetative activity is never entirely interrupted in
these plants all the year, and it should be remembered, in connection with this fact,
that they grow in places where the air and soil are always warm, and where their
humidity is only subject to slight variations. It may happen that in damp, warm
places transpiration from the leaves ceases for a time entirely, and then it is very
necessary that the amount of food-sap should be forced up to the leaves by root-
pressure in order that they may be supplied with the food-salts they require. The
water, which contained dissolved food-salts, is of no use when it has given these up,
and it is therefore forced out of the stomata, these in consequence being trans-
formed into water pores.

The aérial roots, which form the shortest and straightest channels for con-
ducting the raw food-sap to the leaves, are, moreover, of great importance to these
tropical aroids, since it not infrequently happens that the lower portion of the
stem’ in an old plant dies off, leaving the upper part, which is fastened to the
trunk of a tree by the earlier-mentioned short supporting roots, and therefore in
no direct connection with the ground. The supporting roots would not be sufficient
to supply the fluid food required, and the whole plant is therefore provided

TRANSMISSION OF THE FOOD-GASES. 367

with this food only through these cord-like aérial roots which are sent down into
the soil.

These few examples are enough to show that the construction of the stem and
roots stands most intimately related to transpiration, inasmuch as the transpiring
green tissue is effected by the structure. But since the construction of these plant
members, 7.¢. the architecture of the stem, is also dependent upon various other vital
processes to be described later, it would not be fitting to discuss their relations here
in detail, and their treatment must be postponed until a later section.

5. CONDUCTION OF FOOD-GASES TO THE PLACES OF
CONSUMPTION.

Transmission of the food-gases in land and water plants and in lithophytes.—Significance of
aqueous tissue in the conduction of food-gases.

It has been repeatedly pointed out that a division of labour occurs in all large
plants, so that one portion of the cells provides for the reception of water and food-
salts, another for that of food-gases, and yet another for the conduction and trans-
mission of fluid and gaseous nourishment to the places where they are consumed.

How the aqueous food-salt solutions derived from the soil are brought to the
green tissue, what contrivances are thereby brought into action, and what
phenomena of plant-life are related to this conduction have been discussed, as far
as practicable, in the previous pages, and it now only remains to describe the
transmission of the gaseous food-materials. This is far more simple than the
conduction of the solutions of food-salts. The most important of the food-gases in
question are carbonic acid and nitric acid. Carbonic acid is continually being
conducted by means of water to the green tissues. The shortest passage is to be
found in aquatic plants whose protoplasm, provided with green chlorophyll and in
need of carbonic acid, is only separated from the surrounding water by a thin

- cell-wall, while this water always contains carbonic acid, though perhaps only in

small quantity. Under the influence of sunlight, the groups of green cells in
hydrophytes form a centre of attraction to the carbonic acid, which is sucked up
with great energy from the surrounding water, passes easily through the cell-wall,
and so comes directly into the neighbourhood of the green protoplasm, 1e. that
place where its decomposition is effected. The green cells of water plants therefore
furnish an apparatus for both absorbing and decomposing carbonic acid, and
usually no further means and no special conduction through other cells are required.

In lithophytes it is otherwise. Here we have the remarkable fact that they are
only active at times; only, that is to say, when they are sufficiently moistened by
rain, dew, and mist, and are to some extent submerged for a time by heavy down-
pours. In dry air their vital activity is suspended; they then adhere to the rocks like

368 TRANSMISSION OF THE FOOD-GASES.

withered turf and dry scales, as if dead. But as soon as they are moistened, or can
condense moisture from the air, they are aroused to renewed vitality, and then suck
up with great eagerness atmospheric water, which always contains small quantities
of carbonic acid gas, and also traces of nitric acid. In the rock-inhabiting mosses
the cells, which absorb water from the atmosphere containing carbonic acid, are also
those in which the decomposition of carbonic acid takes place. In this respect
these mosses behave exactly like aquatic plants; nor is it perhaps superfluous
here again to point out the interesting fact already mentioned, that there are mosses
which permanently live under water, and there behave like true water plants,
though they are able equally to live on rocks, where they remain dried up for
weeks together, and only resume their activity when wetted by rain. It is to be
taken for granted that such damp, water-saturated mosses have the capacity of
absorbing carbon dioxide from the surrounding atmosphere. The carbon dioxide is
changed into carbonic acid by its passage through the cell-wall saturated with
water. Probably it is only when carbonic acid is dissolved in water that it reaches
the active protoplasm in the cells in question. In lichens the carbonic acid which
reaches the protoplasm provided with chlorophyll is also dissolved in water;
however, in most lichens the green cells do not come in contact with the atmosphere,
but are separated from it by a layer of hyphal threads. Thus the conduction to
the green cells takes place by means of the hyphal layer destitute of chlorophyll.
In land plants also the cells which are filled with chlorophyll-bearing protoplasm
seldom come directly into contact with the atmosphere; usually the green tissue is
surrounded with an actual mantle of water. That is to say, the cavity of each
epidermal cell contains very watery fluid, or, in other words, in the fully-formed
epidermal cells the protoplasm constitutes merely the parietal layers without
chlorophyll, their large cavities being filled with water. These epidermal cells fit
closely to each other, and on the upper side of the leaf are only rarely interrupted
by stomata. Usually the epidermis on the upper side of the leaf gives rise to a
layer of cells with clear watery contents, directly bordering on the green palisade
tissue; and as the carbon dioxide of the atmosphere has to pass from the upper side
to this green tissue, it must first of all pass through this watery cell-layer of the
epidermis. There it becomes changed into carbonic acid, and passes from this
epidermal sphere of activity, not in the form of gas, but dissolved in water, to the
cells of the palisade tissue below. Since the green palisade tissue under the
influence of sunlight uses up the carbonic acid in the manufacture of organic
material, it becomes a centre of attraction for this acid as long as the illumination
continues. At first the carbonic-acid-bearing contents of the contiguous cells are
eagerly absorbed, and indirectly carbon dioxide also is drawn from the surrounding
air and made to force its way into the epidermal cells. The cell-wall offers no
great resistance to this entrance. It has been proved that carbonic acid, or rather
carbon dioxide, passes very easily through the cell-wall. According to all this, it is
evident that the small quantity of carbon dioxide is drawn from the air by the
green illumimated tissue of the leaves and stem, that carbon dioxide streams

TRANSMISSION OF THE FOOD-GASES, 369

rapidly towards these parts, penetrates into the epidermal cells, is changed into
carbonic acid, and reaches the green tissue by means of the aqueous contents of the
epidermal cells.

According to the previous statement, which has been discussed in detail, the
epidermis has also to provide for the transmission of the carbonic acid to the places
of consumption, viz. to the green tissue.

In accordance with climatic and other local conditions, and corresponding to the
individuality of separate species, the epidermis presents, as is well known, endless
variations in structure. This variety of formation is concerned chiefly with the
part which it has to play as a protective covering, as strengthener, and the like.
As a conducting apparatus for carbonic acid, that is, in the form of a water mantle
or outer aqueous tissue, it exhibits comparatively little variation. In evergreen
plants which grow in warm, damp situations where transpiration is limited, and
where the water of the soil is often conducted by root-pressure to the large
transpiring leaf-surfaces, as, for examples, in tropical bananas, palms, mangroves,
figs, and peppers, the aqueous cells which lie above the green palisade tissue are
always arranged in several layers. In all those plants also whose outermost cells
in contact with the air have much thickened walls, and consequently a restricted
lumen, as, for example, in the Oleander, which grows on the sides of brooks (see
fig. 73°),and in the proteaceous Dryandra floribunda growing in the Australian,
bush (see fig. 68), the water mantle consists of a double layer of cells. When the
green tissue is penetrated by vascular bundles and groups of strengthening cells
without chlorophyll, the aqueous epidermal layer is also interrupted, and is usually
only co-extensive with the palisade cells. In the leaves of grasses the colourless
aqueous cells form rows which are placed above the green assimilating tissue, and
surround this tissue as an actual mantle.

The demand of the green tissue for carbonic acid regulates itself to the
consumption in the formation of organic substances. But the consumption is at a
maximum at the time of strongest illumination and greatest warming of the green
tissue, and therefore coincides with the most abundant transpiration. At such a
time the carbonic-acid-bearing sap is drawn by the active protoplasm in the green
tissue with great eagerness from the epidermal cells lying above, often so
abundantly that a quick replacement is impossible. But in consequence of this
the epidermal cells lose their turgescence; they collapse, and the hitherto tense
epidermis presents a flaccid appearance. In order that this collapse may take place
without injury, the following contrivance has been devised. The side-walls of those
cells which form the epidermis, i.¢. the outer aqueous tissue, are delicate, thin, and
flexible, and as these cells give up a portion of their sap, their side-walls are folded
together just like a bellows from which the air has been expelled. When the cells
become again filled with fluid, the folds are straightened out as in a bellows filled
with air, and the cells regain their former tenseness.

In the course of the foregoing representation we have only described the

transmission of carbonic acid through the epidermal cells rich in watery Cleap on
Vou. I.

370 TRANSMISSION OF THE FOOD-GASES.

the upper side of the leaf. But it must not be forgotten that the same process also
takes place on the under side of the leaf, particularly when the green tissue is not
divided into palisade cells and spongy parenchyma, and also when the epidermis is
provided with stomata both on the upper and under sides of the leaf. In certainly
70 per cent of all leafy plants the arrangement is such that palisade tissue occurs
beneath the succulent epidermis of the upper side, under this again spongy
parenchyma, and again under this the epidermis of the lower side, which is
abundantly pierced by stomata. It can therefore be asserted, for the majority of
plants with green foliage, that the epidermis of the upper side chiefly regulates the
transmission of carbonic acid to the palisade cells, and that transpiration is chiefly
regulated by the epidermis of the lower side.

It is hardly probable that carbonic acid finds entrance to the green tissue
through the stomata. At the time when the demand for carbonic acid is at a
maximum in the green tissue, a considerable quantity of food-salts must be
delivered to the green cells, and the water which provides for the transport of the
food-salts from the soil up to the small chemical laboratories, as the palisade cells
may be called, is rapidly expelled from the stomata in the form of vapour. But
while water-vapour is streaming out of the stomata, the carbon dioxide of the air
can hardly stream in through the same avenues at the same time, and it may be
concluded that when, generally speaking, this gas is absorbed through the stomata,
the occurrence is exceptional.

Concerning the filling of the epidermal cells with water and carbonic acid, it
should be here again pointed out that in not a few plants the absorption of rain and
dew takes place directly through the foliage-leaves. Since rain and dew always
contain small quantities of carbonic acid and traces of nitric acid, this method of
filling the epidermal cells is so much the less to be undervalued. In very many
green foliage-leaves the continuous epidermis above the palisade cells is capable of
being moistened, while the lower epidermis, rich in stomata, on the other hand, is
kept dry by the most varied contrivances; and it is very probable that in such cases
the water of rain and dew is taken up by the whole epidermis of the upper leaf-
surface, especially when these epidermal cells have a short time previously given up
a portion of their contents to the green tissue, and have become consequently
somewhat collapsed. In many cases it must be concluded, from their shape and
position, that the filling of the epidermal cells is only caused by the watery sap
brought from the roots, and indeed only by means of the green palisade tissue,
ae. of the same tissue which, on occasion, again receives watery fluid from the
epidermal cells. This periodic alternation of absorption and expulsion may be
explained in the following manner. The water arriving from the soil is given off
by the palisade tissue to the epidermal cells at certain times, i.e. when no carbonic
acid is required, in order that carbon dioxide may there be drawn from the air and
changed into carbonic acid. When this has happened, and a demand for carbonic
acid is set up in the palisade tissue, this tissue takes back the water it had
previously given off, now of course accompanied by the absorbed carbonic acid.

FORMATION OF ORGANIC MATTER FROM THE
ABSORBED INORGANIC FOOD.

1. CHLOROPHYLL AND CHLOROPHYLL-GRANULES.

Chlorophyll-granules and the sun’s rays.—Chlorophyll-granules and the green tissue under the
influence of various degrees of illumination.

CHLOROPHYLL-GRANULES AND THE SUN’S RAYS.

In the former section of this book it has been described how everything which
serves as food for plants is conducted to the green tissues. Food-salts, food-gases,
and water arrive at the same goal by the most diverse contrivances—to the green
cells as those places where the raw material is worked up and organic substances
prepared from it; to the place of need where the materials for further building and
development, for rejuvenescence, multiplication, and reproduction of the plants in
question have to be provided. The question how living plants manufacture organic
substances in the green cells from the raw materials which stream to them, particu-
larly from the raw food-sap and carbonic acid, must now be discussed.

First, it should be remembered that the formation of organic materials always
commences with the decomposition of the absorbed carbonic acid. This decom-
position, however, is only carried on by that protoplasm in which are imbedded
chlorophyll-granules. The protoplasm in question can only accomplish the indi-
cated task by the help of these structures, and the chlorophyll-granules are
therefore really the organs on which everything depends. It is in them that
those remarkable processes are carried on, upon which depends the renewal, and
ultimately the existence, of all life. The description of these organs must, there-
fore, precede all further discussion.

Having regard to the importance of their function, the structure of the
chlorophyll-granules appears to be simple enough. It is possible that later
researches, with instruments and methods of observation more perfect than those
now at our disposal, will furnish more accurate details about their minute structure,
and particularly as to their dissimilarity from the protoplasm in which they are
imbedded. In the meantime, only this much is known—that the ground-work of
the chlorophyll-granules differs but little in its structure and composition from the
surrounding protoplasm. Like all sharply-defined protoplasmic bodies, chlorophyll-
granules exhibit a pellicle-like thickened outer layer; the inner portion, on the

372 CHLOROPHYLL-GRANULES AND THE SUN’S RAYS.

other hand, is formed of a porous mass of reticular or scatfold-like strands, which
may best be compared to a bath sponge. The holes and meshes of this spongy
colourless ground substance contains a green colouring matter, which is dissolved in
an oily material, and clothes the continuous small spaces in the form of a parietal
layer. This green colouring matter of the chlorophyll-granules, which has been
called chlorophyll, is easily soluble in alcohol, ether, and chloroform. If green
leaves are steeped in an alcoholic solution, they become blanched in a short time,
and the colouring matter passes entirely into the fluid. The alcohol assumes the
beautiful green colour which the leaves formerly possessed, and the previously
green leaves are now to be seen floating in the green alcohol. In transmitted light
the solution appears a beautiful green; but when observed in reflected light it
appears blood-red, and therefore the colouring matter displays a marked fluorescence.
If a fatty oil is added to the green-tinted alcohol, and the two are shaken up
together, the green colour passes into the added medium, while in the alcohol a
yellow substance remains, which has been termed xanthophyll. The chemical
composition of chlorophyll is not yet so clearly understood as we could wish.
It is asserted that it is possible to obtain chlorophyll in a crystallized form. The
crystals obtained form green transparent rhomboids, which, when exposed to the
light, slowly decompose again. This chlorophyll behaves like a weak acid; contrary
to earlier belief, it is free from iron, but leaves behind almost 2 per cent of ash,
consisting of alkalies, magnesia, some calcium, phosphoric and sulphuric acids. The
fact that the production of these crystals must be preceded by a series of long-
continued operations, together with the fact that chlorophyll is extremely delicate
and easily decomposed, always allows us to suppose that the crystals mentioned are
only a product of decomposition, and do not belong to that chlorophyll which
colours the chlorophyll-granules in living cells. It was previously thought that
chlorophyll was a mixture of two colouring matters, viz. a blue and a yellow, until
later researches demonstrated that this supposition was unfounded, and that a false
impression had been received through observation of the process of decomposition.
A characteristic absorption spectrum has been obtained for chlorophyll, which is
especially useful in all cases where it is a question of demonstrating the presence
of very small quantities of the colouring matter in any parts of the plant. With
respect to this it is enough to say that the whole of the violet and blue and the
ultra-violet rays are cut off from the spectrum, and that it exhibits seven character-
istically distributed absorption-bands. It may be further remarked here that after
treating the chlorophyll with hydrochloric acid tiny crystals arise, which have been
called hypochlorin. The results of all these researches have thrown but little light
upon the part which chlorophyll plays in those processes which commence with the
decomposition of the absorbed carbonic acid in the chlorophyll-granules.

Compared with the size of the whole mass, chlorophyll forms only an extremely
small fraction of the granules it colours green, and when it is withdrawn by the
addition of alcohol, only the colour and not the size of the granules in question is
found to be altered.

CHLOROPHYLL-GRANULES AND THE SUN’S RAYS. 373

Chlorophyll-granules appear to be imbedded in protoplasm from their origin
until their disappearance. When the protoplasm is situated round the wall—or, in
other words, when the central cavity of the protoplasm is large and filled with
watery cell-sap, and the plasma which surrounds the sap-cavity is sac-like and only
forms a thin covering to the cell chamber, then the chlorophyll-granules are usually
imbedded in the middle layer of the parietal plasma, so that they are separated
from the sap-filled central cavity, as also from the cell-wall, by a layer of colour-
less protoplasm. The same thing occurs when the chlorophyll-granules are
imbedded in the plasma strands which are stretched across the cell-cavity (see
figs. 5? and 5°). Frequently the chlorophyll-granules project like warts, and thus
give a knotty appearance to the protoplasmic strands; but even then they are
always covered by a thin colourless layer of protoplasm.

In spite of this close connection, chlorophyll-granules always appear to be
sharply defined, and exhibit in their entire development a certain separateness from
the protoplasm in which they may reasonably be supposed to take their origin.
They enlarge, divide, and multiply, and occasionally in the course of their life alter
their form. With respect to their shape there is little variety in the green tissue
of the stem and leaves of higher plants. The chlorophyll-granules almost always
appear there as rounded or occasionally angular, sometimes even as lenticular or
many-sided grains. A much greater diversity is observed in those simple green
plants which live in water, and have been classed together under the name of
Alge. In the cells of the green filaments of Zygnema, which are represented in
fig. m of Plate I, the chlorophyll bodies are stellate, and are so arranged in
each cell that there are usually two stars side by side. In species of the genus
Spirogyra (Plate I, fig. 1) they form spirally wound, usually knotty, bands, and in
most species of the genus only one band in each cell; but in some species there are
two bands, whose spirals cross one another, whereby very ornamental structures
come into view under the microscope. In species of the unicellular Peniwm
(Plate L., fig. &), the chlorophyll bodies form plates: or bands parallel to the long
axis of the cell, projecting against the cell-wall in all directions. In Mesocarpus a
single green plate is observable, which divides the cavity of the cell into two almost
similar halves; @dogonium exhibits a latticed plate; species of the genus Ulva
have plate-shaped chlorophyll bodies which lie close to the wall; in the cells of
Podosira are seen disc-shaped chlorophyll bodies which jut out in all directions;
and in the liverwort Anthoceros the chlorophyll bodies are in the form of hollow
spheres surrounding the centres of the cells.

The number of chlorophyll-granules in the protoplasm of the cell varies from
one to several hundreds. In the cells of selaginellas there are usually 2-4; in
those of the luminous moss, Schistostega osmundacea, to be described later more in
detail, 4-12 (Plate L., fig. p). The green cells of most leafy flowering plants contain
20-100, many even 200. In the cells of Vaucheria (Plate I, fig. a-d), the proto-
plasm is so crowded with thickly-pressed small green granules as to make one
think that the whole cell-body contained but a single chlorophyll mass. Foliage-

374 CHLOROPHYLL-GRANULES AND THE SUN’S RAYS.

leaves, in which a distinct separation between the palisade and spongy parenchyma
is completed, always show many more chlorophyll-granules in the former than in
the latter. Careful countings have shown that the palisade cells usually contain
three or four times—occasionally even six times—as many chlorophyll-granules as
the adjoining cells of the spongy parenchyma. When the chlorophyll-granules in a
cell are so many that the whole inner wall of the cell can be covered with them,
they arrange and distribute themselves very equally in this manner, and such cells
appear uniformly green. It then seems as if the whole cell-chamber were entirely
filled with chlorophyll-granules, but this is not really the case. The central cavity
of the protoplasm filled with cell-sap never contains a single chlorophyll body.

_ The chlorophyll-granules imbedded in the parietal protoplasm can also undergo
the most remarkable displacements, which we will forthwith describe.

With regard to shape, cells with active protoplasm, containing chlorophyll-
granules, exhibit the widest variety. Especially are all imaginable cell shapes
to be found in the group of Desmids which live in water: rod-shaped, cylindrical
(Plate I., fig. &), crescent-shaped (Plate I, fig. 1), tabular, stellate, tetrahedral, and
many others for which it would be hard to find short and suitable names. The
Algee, which to the naked eye seem composed of green threads, are built up of cells
which are, for the most part, tubular and cylindrical (Plate I, figs. a, b, and l, m).
In Lichens and Nostocacez the cells which form the tissues are spherical; in Mosses
and Liverworts they are pentagonal and hexagonal.

As already mentioned in former sections, the green tissue in the foliage of
Phanerogams is formed, in the majority of instances, of two kinds of cells—of
branched cells forming the spongy parenchyma, and of cylindrical cells which con-
stitute the palisade tissue (Plate I, fig. r). The latter are often short, their length
being not much greater than their width, but usually they are five or six times, and
oceasionally even ten or twelve times, longer than broad. In bulbous plants the
palisade-shaped cells are arranged parallel to the upper leaf-surface, but in the
majority of seed-bearing plants they are at right angles to the upper surface of the
foliage-leaf, as shown in the cross-section of a leaf of the Passion-flower in Plate L.,
fig. r. The green cells below the epidermis of pines and various firs exhibit a very
peculiar form. In contour they appear angular and tabular, and are fitted closely
to one another without intercellular spaces. From the cell-walls parallel to the
upper surface of the leaf trabecule project into the interior, by means of which each
cell is divided up into niches usually of equal size. Such cells remind one of
stables in which the stalls of the different horses are separated by boarded parti-
tions. The projecting trabecule are always so arranged that the entire cell-chamber
appears like a group of palisade cells whose side walls separating one from another
have been interrupted. These partitions, which, as stated, are to be found in many
firs, but also in grasses and many Ranunculacee—especially in the Monkshood
(Aconitum), Peony (Pwonia), and Marsh Marigold (Caltha)—increase the internal
surface of the chamber, and this appears to be advantageous, inasmuch as by
this means many more parietal chlorophyll-granules can find a place than would

CHLOROPHYLL-GRANULES AND THE SUN’S RAYS. 375

be possible in a single cell of equal dimensions, but devoid of such projecting
trabeculae.

It is shown by very accurate investigations that the quantity of organic
substances formed in a cell, by the decomposition of carbonic acid, is greater the
greater the number of chlorophyll-granules, provided that all of them are so
arranged within its protoplasm that they can discharge their functions. A heap of
chlorophyll-granules filling the cell irregularly would be little suited to effect this
result. The small, green chlorophyll-granules must, on the contrary, be so arranged
that no one deprives another of light, and this is most easily possible, especially in
a many-storied plant-structure, composed of numerous cells, when the chlorophyll-
granules are grouped together like the stones in a mosaic, and are arranged along
the walls in this order. When, moreover, the light falls unhindered through certain
portions of wall, as through a window into the cell-cavity, all the chlorophyll-
granules there situated are almost equally illuminated. The larger the extent of
wall surface, the more chlorophyll-granules can be accommodated on it, and there-
fore the more abundantly can the decomposition of carbonic acid be carried on in
such cells. For such green multicellular tissue, whose most important function is
the decomposition of carbonic acid and the formation of organic substances, the
parietal grouping of the chlorophyll-granules, the above-mentioned infolding of the
inner surface of the cells, generally the increase of the inner surface of the cell-
walls clothed with chlorophyll, is accordingly the most advantageous arrangement
for the best possible utilization of the available space. :

When one speaks of the “green” of plants one thinks first of all of the foliage-
leaves, in ‘which that colour is especially noticeable. The name “chlorophyll”
translated by “leaf-green” might lead to the idea that cells and tissues provided
with chlorophyll are only to be found in the leaves; but this would not at all
correspond to the true state of the case. Those plants which are not differentiated
into stem and leaves, especially the many kinds of green water-plants classed under
‘the name of Algz, generally consist entirely of chlorophyll-bearing cells. In those
mutually-nourishing combinations named Lichens, one of the partners is without,
while the other is provided with, chlorophyll.

When stem and foliage-leaves are clearly differentiated, a portion of the tissue is
deprived of chlorophyll while the other portion is more or less rich in the same.
Chlorophyll-containing tissue is found in all the members of these stem-plants, in
roots, in stems, in foliage, in floral leaves, in fruits, and seeds. In tropical orchids
the aérial roots when dry appear white and are seemingly quite devoid of
chlorophyll; but when moistened their green colour is seen, because when the outer
porous covering is filled with water, and its cells become transparent, the colour of
the green tissue-layer below shines through. There are even orchids, eg. Angre-
cum globulosum, funale, and Sallet, which, when not flowering, have no other green
tissue than that in the aérial roots, and in which not only the absorption of food-
materials, but also the working up of the absorbed nourishment, particularly the
decomposition of carbonic acid and the formation of organic substances, is carried

376 CHLOROPHYLL-GRANULES AND THE SUN’S RAYS.

on by the green tissue of the aérial roots. Green tissue is much more frequently to
be met with in stem-structures than in roots. Hundreds of rushes, bulrushes,
cyperuses, and horse-tails, as well the Casuarineze and species of Ephedra, included
under the switch plants, many papilionaceous plants of the genera Retama,
Genista, and Spartiwm, a number of Salicornias, tropical orchids, and cactiform
plants, the Duckweed (Lemna), and all the plants possessing flattened shoots (see
fig. 82), contain green tissue, without exception, in the cortex of their stem and
branches. Also ovaries and fruits which are not yet fully ripe are so universally
coloured green that in popular language green fruit and unripe fruit are synony-
mous. Chlorophyll is more rarely observed in seeds. Those whose embryos are
differentiated into axis and leaf only seldom—as, for example, in the pines—show
green tissue in the cotyledons. The seeds of orchids, especially those living
epiphytically on the bark of trees, behave in a peculiar manner. These are
marvellously small, consist of only a single group of parenchymatous cells, and no
trace is to be seen in them of a radicle or cotyledon. They only retain the capacity
of germinating a short time, and it is important to these seeds, which are poorly
supplied with reserve food, that immediately after leaving the fruit-capsule they
may be able to provide themselves with nourishment from their surroundings, and
to manufacture organic substances from this food. This they can naturally only do
by the help of chlorophyll, and it is interesting to notice that they also are actually
endowed with this substance. Even when they are still inclosed in the capsule of
the parent plant these seeds become green, and when they are carried by the wind
into some cleft on the bark of an old tree-trunk the chlorophyll is able at once to
function. After a short time a small green tubercle grows out of the green seed,
and fixes itself by absorbent cells to the substratum, then very gradually it grows
up to form a large plant-stem.

Large flowers whose petals, from the commencement to the end of the flowering
period, exhibit a green colour are esteemed rarities. On the other hand, small floral
leaves, rich in chlorophyll, are of very common occurrence. The change of the
floral colour also, during the flowering period, from white, red, violet, and brown, to
green, has been frequently observed in small as well as in fairly large flowers. A
very striking example of this is the Black Hellebore (Helleborus niger). When its
flowers open, the outer large leaves situated below the petals (which are transformed
into small nectaries), are snow-white, and show up conspicuously from their darker
surroundings. From afar they attract the attention of honey-collecting insects, by
whom they are eagerly sought out. When, by means of these honey-sucking
insects, pollination is brought about, the small nectaries, as well as the large
dazzling-white outer floral leaves called sepals, become superfluous. The nectaries
forthwith fall off, but the large sepals remain and take up another function.
Chlorophyll is abundantly developed in their cells; the white colour disappears;
fresh green appears in its stead, and the same floral leaves which previously had
attracted insects by their conspicuous colour now function as green leaves exactly
like foliage-leaves. A similar alteration of colour, which also has the same

CHLOROPHYLL-GRANULES AND THE SUN’S RAYS. 377

significance, is observed in many orchids and liliaceous plants, but on the whole
such a change of function in floral leaves is not common. These cursory
observations must show that chlorophyll may appear in all the members of a plant,
although it is also true that foliage-leaves chiefly contain the green tissue, so that
certainly in 90 per cent of all chlorophyll-bearing plants the decomposition of
carbonic acid is carried on in the foliage-leaves.

If, now, after the description of the arrangement, form, and distribution of
chlorophyll-granules, we would also learn something as to how organic substances
are formed in the cell-chambers by means of these structures, we find ourselves
in the position of an inquirer who visits a chemical laboratory without a guide,
and wishes to ascertain in what way some material—for example, a pigment—is
manufactured. He notices apparatus set up there, sees the raw materials heaped
together, and also finds the finished product. If the manufacture is actually
proceeding, he can also observe whether warmth or cold and greater or less pressure
are brought into action as propelling forces, and he can, if intrusted with the
manipulation necessary to the production of such pigment, imagine the relation
of the different parts to the whole. Of the details, indeed, much must remain
incomprehensible, or quite unknown. Especially with reference to the quantity of
the transformed raw material, and with regard to the propelling forces, must the
visitor’s knowledge remain incomplete.

It is not otherwise with us when we would inspect the processes carried on in
the cells where chlorophyll-granules develop their activity. We see the effective
apparatus, we recognize the food-gases and food-salts collected for working up, we
know that the sun’s rays act as the motive force, and we also identify the products
which appear completed in the chlorophyll-granules. By careful comparison of
various cells containing chlorophyll, on the ground of observations which establish
the conditions under which the manufacture of organic substances succeeds best
and worst, having found by experience that under certain external conditions the
whole apparatus becomes disintegrated and destroyed; it is indeed permissible to
hazard a conclusion about the character of the propelling forces. But what is
altogether puzzling is how the active forces work, how the sun’s rays are able to
bring it about that the atoms of the raw material abandon their previous grouping,
become displaced, intermix one with another, and shortly appear in stable
combinations under a wholly different arrangement. It is the more difficult to
gain a clear idea of these processes, because it is not a question of that displacement
of the atoms called decomposition, but of that process which is known as
combination or synthesis. Decompositions and analyses, even of the most
complicated compounds into simple combinations are well understood, but not
so the converse. It is always considered a fortunate occurrence when a chemist
succeeds in producing from its fundamental elements, or from the simplest com-
bination of these, one of those complicated bodies, which are, nevertheless, formed
with such ease in plant cells. When sugar is “made” in a manufactory, it is not
that carbon and the elements of water are used, although these. are so abundantly

378 CHLOROPHYLL-GRANULES AND THE SUN’S RAYS.

at disposal, but only that the sugar is isolated which has been formed synthetically
from these substances in those tiny chemical laboratories, the vegetable cells.
Consequently it is really incorrect to say that sugar is “made” in our manu-
factories; we should only say that there the sugar manufactured by the plants is
separated from other substances and prepared for further use.

Although it is not possible to represent the processes concerned in the synthesis
of organic materials in plant cells as a matter beyond all doubt, one is justified in
taking refuge in hypotheses. And it must be looked upon as an hypothesis when
we consider the movement by which the atoms of the food-gases and food-salts are
displaced by the sun’s rays in the vegetable cells as a transmission of the vital force
of the sun. The atoms have arranged themselves by this movement in a different
order, they hold and support one another, they are stable, and a condition of
mutual tension has been set up. The vital force of the sun has become the hidden
spring of all these changes. The now stable organic material formed by synthesis
is thus equipped with an adequate supply of energy, designated in other words as
latent heat. If the atoms of the organic material from whatever cause again break
loose, abandoning their combination and arrangement, they perhaps so displace and
rearrange themselves that those groups which previously existed are formed again,
and thus the potential energy is changed to vital force, the latent heat to sensible
warmth. When a tree-trunk is consumed, the vital force of the sun, which had
been changed by the formation of cellulose and the other organic materials
composing the wood of that time into latent force, is again transformed into
active energy; and when we burn coals, the sun’s rays, which thousands of years
ago brought about the formation of organic vegetable substances and were
imprisoned in the coal, will again be set free, will warm our rooms, drive our
machines, or propel our steamships and locomotives. Keeping this idea in view, it
is at least possible to imagine the mechanical significance of the sun’s rays in the
formation of organic substances in plants, and it may be reckoned that the
quantity of organic substance produced stands in a fixed proportion (which may be
expressed in figures) to the store of energy in the same.

One circumstance on which particular stress must here be laid is that the
various rays of which sunlight is composed, the rays with various wave-lengths
and refrangibility, which, some of them at least, appear to our eyes as the different
coloured bands in rainbows, play each a very distinct part in the formation of
organic materials in plant cells. Under the influence of the blue and violet rays,
ae. of those which are most highly refrangible and have the smallest wave-lengths,
the oxidation of the organic materials called carbohydrates is assisted, that is to
say, not the formation but the decomposition and transformation of these
compounds are favoured. The red, orange, and yellow, ic. those rays which are
less refrangible and have a greater wave-length, behave quite otherwise. These
favour the reduction of carbonic acid, assist the formation of carbohydrates from
raw materials, and are therefore chiefly concerned in the originating of such
organic substances. When a sunbeam passes through a colourless glass prism a

CHLOROPHYLL AND LIGHT INTENSITY. 379

continuous spectrum is produced—violet, dark blue, light blue, green, yellow,
orange, and red. If the same sunbeam passes through a transparent but coloured
body, which may be either solid or fluid, whole groups of colour absent themselves
from the spectrum. Dark bands appear in the corresponding places, and we say
that the light in question has been absorbed by the coloured body. Now, if
chlorophyll has the property of absorbing those colours of the spectrum which are
not advantageous in the formation of organic substances from raw material, the
role of this chlorophyll cannot be too highly estimated. It must not be overlooked,
moreover, that many bodies have the capacity of absorbing light rays of shorter
wave-length, and, on the other hand, of giving out rays of greater wave-length.
It is precisely those pigments which are distributed in plants, again above all,
chlorophyll, which possesses this property called fluorescence; and we must
therefore also assign this significance to chlorophyll, that it can transform rays of
light which are useless in the synthesis of organic materials into those which show
the best possible action in this respect. If the fluorescing pigments of plants
(chlorophyll, anthocyanin, phycoérythrin) can transform the violet and blue rays
into yellow and red, it is to be supposed that their activity goes further, and that
they will be able to change rays of small wave-length and higher refrangibility into
rays which are found beyond the red, which are imperceptible to our eyes, and
which possess very great heat-giving powers, or, in other words, that they will be
able to transform light into heat. From all this it may be seen that the
significance of chlorophyll in the formation of organic materials would be three-
fold. First, a retention or extinction of those rays which might hinder the
formation of those compounds known by the name of carbohydrates; further, the
transformation of rays with short wave-length into those of longer wave-length,
which, according to experience, most favourably effect the production of sugar and
starch; and, finally, the conversion of light into heat, and ultimately into latent
heat.

CHLOROPHYLL-GRANULES AND THE GREEN TISSUE UNDER THE
INFLUENCE OF VARIOUS DEGREES OF ILLUMINATION.

If it is beyond question that organic materials can only be formed from the
absorbed carbonic acid in the presence of chlorophyll, it is, on the other hand, equally
certain that the sun creates and works through these formative processes by its
rays, and thus, as the propelling force, becomes the fountain of all organic life. The
sun rises and sets, day follows night, and during the night the process just men-
tioned, upon which the existence of the living world depends, is interrupted. But
even in the daytime also, the strength of the sun is very unequal; it is one thing at
mid-day, when the source of light is in the zenith and the rays fall perpendicularly
on the earth, and quite another in the evening, as the illuminating orb sinks
below the horizon and the last rays spread almost horizontally over the surface.
Clearly it is anything but a matter of indifference to the organs possessing a certain

380 CHLOROPHYLL AND LIGHT INTENSITY.

amount of chlorophyll in what manner the sun’s rays light upon them, or what
quantity of vital force is transmitted to them in a given time. Various species of
plants may make very different demands for sunlight, but for each individual
species the need of propelling force fluctuates only within very narrow limits, which
cannot be exceeded without injury. The greatest possible equality in the supply of
propelling force is an indispensable condition of a successful career. In order to
meet the inequality in the flow of light on bright and dull days, and also during
various parts of the day, it is arranged that the green organs can turn towards the
sun, and that according to the hour of the day and the strength of the sun’s rays
at that particular time, they can take up a definite position, and again alter this
position with ease. And, indeed, the green chlorophyll-granules in the interior of
the cells also show this capability of accommodating themselves in accordance with
the demand for light as well as the entire cells, and, finally, even the green leaves,
together with the stems and branches which bear them.

If one would obtain a clear idea of the withdrawal of the chlorophyll-granules
from the sunlight, one must remember, first of all, that these green bodies, what-
ever may be their form, are imbedded in the protoplasm of the cell, and that the
protoplasm is mobile and easily capable of displacement—or, in other words, that
the protoplasm which contains the green chlorophyll-granules twists and rotates
within the cell it inhabits, and can transport the granules hither and thither. Still
more. Chlorophyll-granules can be temporarily heaped up and crowded together in
definite places; they may again be separated from one another, and distributed
equally throughout the whole cell-body. In the tubular cells of Vaucheria
clavata, represented in figure a@ on Plate I, the protoplasm forms a lining layer on
the inner side of the colourless transparent cell-wall, and is so thickly studded
with round chlorophyll-granules that the cell appears of a uniform dark green.
But this is only the case with light of moderate intensity. When strongly illumi-
nated the chlorophyll-granules move apart from one another, arrange themselves in
isolated balls, and in a very short time, in each tubular cell, dark-green spots and
zones may be seen corresponding to the crowded granules, and light, irregular
bands appearing in those places from which the chlorophyll has been withdrawn.
If the intensity of the light diminishes, the green clusters dissolve, and the former
equal distribution and colouring is resumed. In another filamentous green alga,
which lives in water and belongs to the genus Mesocarpus, each of the long
cylindrical cells contains a plate-like chlorophyll body, which in weak diffuse light
turns itself at right angles to the incident rays. In this position the broad side, i.e.
the larger surface of the chlorophyll body, is turned to the sun, and the incident
light is in this way utilized to the utmost possible extent. As the plate-like
chlorophyll body usually extends right across the cell, under the conditions indicated,
the cell appears of a uniform green colour. If the full rays of the sun fall on such
Mesocarpus cells, the plate-like chlorophyll bodies begin to turn so that the plane
of the plate is parallel to the direction of the rays. Now the narrow side, i.e. the
smaller surface of the chlorophyll body, is turned to the rays, and only a dark-green

CHLOROPHYLL AND LIGHT INTENSITY. 381

stripe is to be seen. This turning movement of the chlorophyll body is very
quickly performed, and can be repeatedly effected by darkening and illuminating
the cells of the Mesocarpus filament.

In cells, too, which are joined together to form tissues, this displacement and
movement of the chlorophyll-granules often appears. It has been noticed for a
long time that in the prothallium of ferns, in the leaf-like liverworts, in the
leaflets of many mosses, and even in the large, delicate foliage-leaves of flowering
plants, the green tissue appears to be coloured a lighter or darker green according
to the intensity of the incident light; that under the influence of intense sunlight
they become blanched and yellowish-green, but in weak light assume a darker tint.
If a strip of black paper be placed on a foliage-leaf, illuminated by the sun, so that
only a portion of the leaf-surface is covered by it, and if the paper be removed after
a short time, the portion left uncovered and illuminated by the uninterrupted rays
of the sun appears light green, while that part on which lay the strip of paper, and
from which the sun’s rays were withheld, is dark green. Careful investigations
have shown that this change of colour is due to displacement of the chlorophyll-
granules. In diffuse light the chlorophyll-granules group themselves on those cell-
walls on whose surface the light falls perpendicularly, and consequently in the
cylindrical palisade cells of the foliage-leaf on the small walls parallel to its upper
surface, and it is clear that such cells (and therefore the tissue formed by them)
have a dark-green appearance when looked at in the direction of the incident light.
As soon as they are illuminated by direct sunlight, the chlorophyll-granules retire
from these walls and take up their position on the cell-walls which are parallel to
the direction of the incident light. In palisade cells the chlorophyll-granules group
themselves by the side of the long lateral walls, while the short walls, which are at
right angles to the rays, are colourless and free from chlorophyll. In the branched
cells of the spongy parenchyma the chlorophyll-granules, which in diffuse light
were equally distributed in the cell, heap themselves together in groups in the
branches, while the central portions of the cells become clear and free from
chlorophyll. The whole tissue, however, in which this displacement has been
completed appears much paler than before, and displays usually a decidedly
yellowish-green tint. This change of position of the granules, according to the
intensity of illumination, may be particularly well seen in the very simply
constructed leaf-like duckweed, especially in Lemna trisulca. Three sections of
the green tissue of this plant, vertical to the surface, are shown in fig. 97.

With these phenomena is indeed also connected the alteration of shape which
is observed under varied illumination in chlorophyll-granules. In the leaflets of
Funaria hygrometrica, a moss very common on piles of charcoal, damp walls, and
rocks, the chlorophyll-granules, which are close to the outer walls of the cells, are
flattened out, angular, and comparable to small polygonal tablets, in diffuse light.
They are also so arranged that the entire wall covered by them appears an uniform
green, and only narrow, colourless lines remain between them. As soon as direct
sunlight falls on them they quickly alter their shape, the tablets becoming hemi-

382 CHLOROPHYLL AND LIGHT INTENSITY.

spherical or spherical bodies, which project towards the centre of the cell-cavity.
By this means the area of the chlorophyll-granules attached to the cell-wall is
contracted, and consequently the green of the leaf-surface in question is diminished.
In the leaves of many flowering plants, also, the chlorophyll-granules which are
distributed in the palisade-cells along the elongated side-walls appear, in diffuse
light, hemispherical or even conical, and project towards the centre of the cells so
that they are illuminated to the greatest possible extent by the light rays passing
through. Under the influence of direct sunlight they flatten out, become disc-
shaped, and withdraw to some extent from the bright rays passing through the
centre of the cells. The significance of all these processes, the changes of shape
as well as the displacements of the chlorophyll-granules, is evident when it is
considered that an over-abundance as well as a deficiency of light would be
prejudicial, and that for every species the quantity of the sun’s rays absorbed
by the chlorophyll-granules is definite. Protoplasm, provided with chlorophyll,

Fig. 97.—Position of the Chlorophyll-granules in the cells of the Ivy-leaved Duckweed (Lemna trisulca).

1In darkness. 2In direct sunlight. 3 In diffuse light.

tries under all circumstances to obtain this definite amount. When weakly
illuminated, chlorophyll-granules maintain a shape and position in consequence of
which they present the largest possible surface to the light; when strongly illumi-
nated, they assume a shape and position by which the smallest possible surface is
soexposed. These processes, especially the displacement of the chlorophyll-granules,
obtain a heightened interest from the fact that they can only be brought about by
the streaming movements of the irritable protoplasm. It must be borne in mind
that it is really living protoplasm which displaces the chlorophyll-granules
imbedded in it in order to bring them to the places best suited to the illumination
then existing, and to place them in sunlight or shade; so that it always happens
that the displaced green bodies are neither too much nor too little illuminated.
Many unicellular water-plants, especially zoospores, attain the same result not
by displacement of the chlorophyll-granules in the interior, but by movements of
the entire cells. These green unicellular organisms may be seen swimming towards
the light by means of their cilia, and in this way they take up the position always
best adapted to the given conditions. If many swarm-spores are collected together
in a limited area, it may happen that they all travel to one particular place; there
they swarm about in the water and appear to the naked eye like a little green
cloud. Or they may settle on the bottom of the pool, there arranging them-
selves side by side, so that no one deprives another of light, and they then appear

CHLOROPHYLL AND LIGHT INTENSITY. 383

to the naked eye as green stripes and patches. If swarm-cells of Spherella
pluvialis are cultivated in a flat white china dish filled with rain-water, and one-
half of the dish is darkened by means of an opaque body while the other half
remains illuminated, the whole of the swarm-cells swim from the darkened to the
illuminated water in order to take up a position as favourable as possible with
regard to the light. If now the china dish is turned round so that the hitherto
illumined portion becomes darkened and light falls on the part previously obscured,
the swarm-cells forsake the position which they had recently taken up, swim from
the now darkened place to the illuminated side opposite, and arrange themselves
there according as the illuminating conditions are favourable.

If, instead of the Spherella pluvialis discussed above, clumps of Vawucheria
clavata are cultivated in a china dish filled with water, and the water is again
partially darkened, together with the green tufts growing in it, it will be seen that
the cells, which are elongated and fixed at one end, seek with the other end those
places where they can find the best light. Vaucheria clavata, which has been
repeatedly cited as an example, and which is represented in the middle figure on
Plate I, consists of long tubular cells, frequently bulging and branched, whose
blunt growing ends appear dark green, while the lower dead portions are branched
and coloured yellowish-white. The protoplasm is so richly studded with chlorophyll-
granules that the entire inner wall of the tubular cells appears covered with a green
lining. At the bottom of shallow pools, which is the natural habitat of these
plants, they form hemispherical clumps, and all the tubular cells which compose
the clumps have their green ends directed upwards towards the source of light.
The same thing occurs when the Vaucheria cultivated in the china dish is
uniformly illumined from above; but, if partially obscured, those filaments over
which the darkening shadow is thrown very quickly alter their position. They
bend towards the light side, and then the clump looks just as if its filaments had
been combed in this direction. Moreover, the same thing is also seen when the
china dish containing clumps of Vaucheria (on which until now diffused light has
fallen uniformly from above) is placed at the further end of a one-windowed room,
so that the light can only reach it from one side. Here, again, all the filaments, or
rather, tubular cells of the clump, bend towards the source of light, and if they
continue to grow, the increase in length is universally in a line with the direction
of the incident rays. After a few days these Vawcheria clumps also look as if they
had been combed out.

The green tissues of thallophytes, and the green leaves and stems of ferns, and
phanerogams, 7.c. those extensive combinations of green cells whose function is to
work in a harmonious manner, and to manufacture organic substances for the
plant to which they belong from carbonic acid with the help of other food-
materials; these behave in the same way as the individual green cells which
swim freely in water, and as the tubular cells of Vaucheria, which are attached
at one end. Arrangements are necessary for these likewise, by which they can
always be placed in the most favourable light. Of course, in these plants where

384 CHLOROPHYLL AND LIGHT INTENSITY.

division of labour has been so far developed, the conditions are not so simple
as in those plants which consist only of single cells, and it is naturally to be
expected that, according to the character of the individual species and the places
which they inhabit, the arrangements would be very varied. The fact must also
be kept in mind that each spot on which a plant has settled itself in the course of
time may undergo alterations in consequence of which the amount and strength of
the light affecting that part varies considerably. Long-lived plants, which grow
vigorously in height and breadth, alter in their relation to the sun in various
stages of growth, and must also alter their form in a corresponding manner, or, at
least, must alter the direction and position of their green tissues. All this requires
a multiplicity of contrivances which are, as a matter of fact, innumerable, and the
exhaustive treatment of which is scarcely possible. In order to obtain a general
view, it will be better to pick out some of the most remarkable of the long series
of arrangements whose significance lies in this, that each species of plant receives
for its green organs neither too much nor too little light, and to describe them in
their relations to light as types of smaller or larger groups.

We will begin with those arrangements which are rendered necessary by
a peculiar habitat, and, first of all, we will investigate those plants which have
taken up their quarters in caves or grottoes, and there pass through all their
stages of development. In deep excavations shut off entirely from the light, as
well as in those which have been formed without human interference, and those
which have been dug in order to obtain metal ore, coal, salt, and water, plants
with chlorophyll-bearing cells and tissues are completely wanting. The plants
which we find there consist only of pale fungi, which live on the scanty organic
compounds which the infiltrating rain-water brings with it into the depths from
the surface of the sunny land above, or which have established themselves on
organic decaying bodies brought there by chance or intentionally by animals and
men. It is otherwise in caves, mines, grottoes, pits, and wells, where light is able
to penetrate from above or from the sides, even if only through a comparatively
small aperture. Truly the vegetation developed there is not very luxuriant, but
it is a very remarkable circumstance that there, as a rule, the plants are still green.
What actually astonishes one at first sight of this vegetation, flourishing in caves
illuminated only from one side, is the fact that they exhibit the most beautiful
and vigorous green, a green much fresher, indeed, and more pronounced than
that displayed by the plants outside. Thus the Hart’s Tongue (Scolopendriwm
officinarum), widely distributed in Southern Europe, when adorning the deep shady
walls of rocky ravines is coloured a much brighter green than when it grows on stony
places in the open country where light can reach it from all sides. Also the liver-
worts which cover the damp stones with their leaf-like thallus, in grottoes through
which waters ripple, show there in the half-light a distinctly richer green than when
outside the grotto. But this phenomenon is most striking in the prothallia of some
ferns belonging to the section of the Hymenophyllacex, and in many mosses.

A tiny moss, called popularly the Luminous Moss, but which has received from

CHLOROPHYLL AND LIGHT INTENSITY. 385

botanists the name Schistostega osmundacea, has even attained a certain celebrity
on this account. It is found distributed throughout the Central European granite
and slate mountains, but is only to be met with in clefts of the rocks, caves and
similar spots. As a rule it covers the yellow, clayey earth and the decayed and
disintegrated pieces of stone which form the soil of these caverns and small
grottoes. On looking into the interior of the cave, the background appears quite
dark, and an ill-defined twilight only appears to fall from the centre on to the side
walls; but on the level floor of the cave innumerable golden-green points of light
sparkle and gleam, so that it might be imagined that small emeralds had been
scattered over the ground. If we reach curiously into the depth of the grotto to
snatch a specimen of the shining objects, and examine the prize in our hand under
a bright light, we can scarcely believe our eyes, for there is nothing else but dull
lustreless earth and damp, mouldering bits of stone of a yellowish-grey colour.
Only on looking closer will it be noticed that the soil and stones are studded and
spun over in parts with dull green dots and delicate threads, and that, moreover,
there appears a delicate filigree of tiny moss-plants rising star-like, pale bluish-
green in colour, and resembling a small arched feather stuck in the ground. This
phenomenon, that an object should only shine in dark rocky clefts, and immediately
lose its brilliance when it is brought into the bright daylight, is so surprising that
one can easily understand how the legends have arisen of fantastic gnomes, and
cave-inhabiting goblins who allow the covetous sons of earth to gaze on the gold
and precious stones, but prepare the bitter disappointment for the seeker of the
enchanted treasure; that, when he empties out the treasure which he has hastily
raked together in the cave, he sees roll out of the sacks, not glittering jewels, but
only common earth.

It has been mentioned that on the floor of rocky caves one may discern by
careful examination two kinds of insignificant-looking plant-structures, one a web
of threads studded with small crumbling bodies, and the other bluish-green moss-
plants resembling tiny feathers. The threads form the so-called protonema, and
the green moss-plants grow up as a second generation from this protonema. How
this comes about will be described in another place; here it only interests us that
the gleams do not issue from the green moss-plants, but only from their protonema.
If this is viewed under the microscope a sight is presented like that depicted in
fig. p on Plate I. From the much-branched threads, composed of tubular cells, which
spread horizontally over the ground, numerous twigs rise up vertically, bearing
groups of spherical cells arranged like bunches of grapes. All the cells of a group
lie in one plane, and each of these planes is at right angles to the rays of light
entering through the aperture of the rocky cleft. The grape-like groups of cells
have longer or shorter stalks, but they always appear in rows side by side and
behind one another, placed cup-like, that the anterior groups do not deprive those
behind them of too much of the light which enters the cavity. Each of the
spherical cells contains chlorophyll-granules, but in small number; usually four,

six, eight, or ten and they are always collected together on those os of the
OL. I.

386 CHLOROPHYLL AND LIGHT INTENSITY.

cells which are turned towards the dark background of the cave. There they are
grouped like a mosaic, and usually so that one green granule forms the centre,
while the others surround it very regularly in a circle. Such groups remind one of
the arrangement of the floral-leaves in Forget-me-not flowers, and give a very
ornamental appearance to the cells. Taken together, these chlorophyll-granules
form a layer, which, under a low power of the microscope, appears as a round green
spot. With the exception of these chlorophyll-granules the contents of the cell
are colourless and transparent, and share these characteristics with the unusually
delicate cell-wall. The light which falls on such cells through the opening of a
rocky cleft behaves like the light which reaches a glass globe at the further end of
a dark room. The parallel incident rays which arrive at the globe are so refracted
that they form a cone of light, and since the hinder surface of the globe is within
this cone, a bright disc appears on it. If this disc, on which the refracted rays of
light fall, is furnished with a lining, this also will be comparatively strongly-
illuminated by the light concentrated on it, and will stand out from the darker
surroundings as a bright circular patch. This lining has the power of manufac-
turing organic substances in the spherical cells of the protonema of the Luminous
Moss, and in this way the scanty incident light is turned to the greatest possible
advantage; it is refracted and concentrated on those places where the chlorophy]ll-
granules are situated, and consequently these receive in the dark recesses an
amount of light which amply suffices for their special functions. It is well
worthy of notice that the patch of green chlorophyll-granules on the hinder side
of the spherical cell extends exactly so far as it is illumined by the refracted
rays, while beyond this region, where there is no illumination, no chlorophyll-
granules are to be seen. The refracted rays which fall on the round green spot are,
moreover, only partially absorbed; in part they are reflected back as from a
concave mirror, and these reflected rays. give the cells of the protonema a luminous
appearance. This phenomenon, therefore, has the greatest resemblance to the
appearance of light which the eyes of cats and other animals display in half-dark
places, only illumined from one side, and so does not depend upon a chemical
process, an oxidation, as perhaps does the light of the glow-worm or of the
mycelium of fungi which grows on decxying wood. Since the reflected light-rays
take the same path as the incident rays had taken, it is clear that the gleams of
the Schastostega can only be seen when the eye is in the line of the incident rays of
light. In consequence of the small extent of the aperture through which the light
penetrates into the rock cleft, it is not always easy to get a good view of the
phenomenon described. If we hold the head close to the opening, we thereby
prevent the entrance of the light, and obviously in that case no light can be
reflected. It is, therefore, better when looking into the cave to place one’s self so
that some light at anyrate may reach its depths. Then the spectacle has indeed
an indescribable charm. What has just been said about the isolated cells is
exemplified in groups of cells placed behind one another, of which usually
many thousands are found in a very small area.

CHLOROPHYLL AND LIGHT INTENSITY. 387

Among the mosses which find their home in deep shady places, principally in
hollow tree-trunks, and are noticeable there for their glossy green, Hookeria
splendens is especially worthy of attention. To be sure, its leaves do not shine as
brightly as the protonema of Schistostega, but the appearance is, on the whole, much
the same, and here also a similar development is the cause. The leaves of Hookeria
are comparatively large, but at the same time very thin and delicate. They are
composed of a single layer of rhombie cells, very convex above and below, so that
the whole leaf may be compared to some extent to a window with very small
so-called “)bull’s eyes” in the glass. The chlorophyll-granules are here arranged
with far less regularity than in the protonema of the Luminous Moss, but they
are heaped together just as in that plant on the side of the leaf facing the ground,
that is to say, which is turned from the light. The side which is turned in the
direction of the scanty incident light has no chlorophyll layer. The hemispherically-
convex cells, opposed to this scanty light which falls on one side of the leaf, act like
glass lenses; they concentrate the weak light on the chlorophyll-granules heaped
up on the other side; but, on the other hand, light is also reflected, and this gives
rise to the green lustre with which the Hookeria shines forth from its dim sur-
roundings.

Like those plants which inhabit rocks, grottoes, and stone clefts, and the shady
obscurity of hollow trunks, plants whose habitat is at the bottom of the sea, and
in the depths of lakes and ponds, are only visited by weakened sunbeams. The
illumination becomes the dimmer the deeper the habitat in question lies below the
surface of the water, since the intensity of the light penetrating the water dimin-
ishes with the increasing length of the distance travelled. At a depth of 200 metres
under the sea complete darkness reigns; at 170 metres the intensity of illumination is
like that observed above the water on a moonlight night; such an illumination is
insufficient to enable chlorophyll-bearing plants to manufacture organic substances
from the absorbed raw materials, even although the plants were provided with all
possible aids for the collection of this exceedingly weak light. It is only at a depth
of not more than 90 metres that light is sufficient for the chlorophyll cells to
decompose carbonic acid, and this depth is ascertained to be the lowest limit of
chlorophyll-bearing plants. Moreover, these figures are only applicable in the most
favourable circumstances in broad daylight, and only when the water is very clear
and transparent, which really only seldom occurs, we might even say excep-
tionally. The substratum on which the submerged plants are situated, whether
sand, mud, or rock, is usually sloping, and is most visited by the oblique rays of the
sun. Frequently also small solid particles are suspended in the water, even in water
which in the aggregate appears to be quite clear, and so the light is again con-
siderably weakened. This happens especially in the neighbourhood of steep coasts,
where the seething of the waves works uninterruptedly at the destruction of the
solid shore, and consequently at a depth of 60 metres on such steep declivities,
plants possessing chlorophyll are seldom met with.

Generally speaking, the vegetation in the sea is limited to a zone of about 30 metres

388 CHLOROPHYLL AND LIGHT INTENSITY.

in depth, whose width varies with the steepness of the shore. Below this narrow
girdle, vegetation is practically extinguished, and the depths of the ocean are in all
parts of the globe a plantless waste. This statement is not contradicted by the fact
that sea-wracks have been found showing a length of 100, it is alleged even of 200
and 300 metres, as, for example, the celebrated Macrocystis pyrifera, between New
Zealand and Tierra del Fuego. These sea-wracks do not rise perpendicularly from
the bottom to the surface of the sea, but proceed from steep declivities, and grow
at an angle to the surface, on which account they often take the direction of the
current. One must imagine their position in the water to be almost like that of the
Floating Pondweed, or the Water Crowfoot (Potamogeton fluitans and Ranunculus
flwitans), which occur in brooks only a few decimetres deep, and nevertheless may
attain a length of more than a metre.

It is naturally to be expected that plants which grow in the dim light, deep
under the water on a rocky reef, would behave exactly like the grotto-inhabiting
Luminous Moss; and if the barrel-shaped and spherical cell-structures connected
into chains, the cyst-like and berry-shaped outgrowths of the unicellular Caulerpas
and Halimedas, as well as the facetted cell-walls of those diatoms living in the
abysses of the sea in dim twilight, are accepted as contrivances by which light is
collected and focussed on those places within the cells where the chlorophyll-bodies
are heaped up, then no mistake will be made. Several of the sea-inhabiting Floridec
and sea-wracks belonging to the genera Phylocladia, Polysiphonia, Wrangelia,
and Cystostra, even exhibit under the water a peculiar luminosity which may be
compared with that of the Luminous Moss, although the optical apparatus is here
essentially different, In the superficial cells of the luminous Phylocladias are to be
found plates segregated out of the protoplasm and closely adhering to the outer
walls, which contain a large number of small crowded lenticular bodies. From these
minute lenses the blue and green rays are chiefly reflected, and thus the peculiar
iridescence is produced. But, on the other hand, yellow and red rays are refracted
on to the chlorophyll-granules, and consequently these plates must be regarded as
an apparatus for focussing the light, which, by its passage through the thick layers
of water, has undergone a considerable diminution.

In the depths of the sea, however, another optical phenomenon must be taken
account of, by which the illumination of chlorophyll-granules in the plants growing
there becomes in the end a favourable one, and that is the fluorescence of erythro-
phyll, the fluorescence of that red pigment to which the Florides owe their charac-
teristic colour. In order to make this phenomenon clear, it seems necessary first
of all, to rectify a wide-spread error with regard to the colour of water generally,
and particularly of sea-water. In the very attractively-written work by Schleiden,
The Plant and its Lofe, the seventh chapter, which treats of the sea and its
inhabitants, begins with the following lines:—*O learn to know them, the horrible
deeps, which are concealed beneath the shining treacherous surface. You descend,
the blue of the sky vanishes, the light of day is gone, a fiery yellow surrounds
you, then a flaming red, as if you were plunged into a watery sea-hell, without

CHLOROPHYLL AND LIGHT INTENSITY. 389

glow and without warmth. ‘The red becomes darker, purple, finally black, and
impenetrable night holds you enchained”. This description is founded doubtless
on the account of divers of the olden time, according to which red light should
predominate in the abysses of the ocean. These accounts must, however, be
retained only to the following extent. The cliffs and the rocky bottom to which
the divers descended might have been richly carpeted with red Floride, possibly
also just then the strata of water above were filled with those unicellular red
alge, which cause the so-called “flowers of the sea”. In the neighbourhood of
the mouth of the Tejo at times a superficial area of sixty million of square
metres is coloured scarlet by Protococcus Atlanticus, a unicellular alga, 40,000
of which cover only a square millimetre; and Trichodesmiwm Lrythreum,
another microscopic alga consisting of bundles of delicate articulated threads in
innumerable milliards, fills the watery strata in the Red Sea as well as in the
Indian and Pacific Oceans, so that there immeasurable stretches of water receive
a dingy red colouring. When these alge make their appearance the sea is said
to blossom, and at those times the depths may appear to the diver as shrouded
in a reddish-yellow twilight. At times the same colour has even been observed
in the Lake of Geneva when its waters had been disturbed; it is due to the
fact that the blue rays of the incident light are weakened by the fine atoms
suspended in the water. With respect to this occurrence, we may consider that
the above-mentioned accounts of divers are not the results of intentional decep-
tion, but only refer to particular cases. They cannot be applied universally. As
a matter of fact, the colour of sea-water, in direct as well as in reflected light,
is blue, and the diver who carries on his work at the bottom of the untroubled
and non-blossoming sea, is not surrounded there by red, but by blue light.
The greater the quantity of salt contained in the water, the deeper the blue. This
blue nowhere appears so beautiful and so deep in tint as in the Dead Sea, and in
the region of the Gulf Stream and of the Kurosiur, where the water is particularly
rich in dissolved salts, and also has in the upper strata a comparatively high tem-
perature. The blue colour of the water is explained thus: from among the ‘ays
which are characterized by different wave-lengths and different refrangibility
(which, taken together, form colourless daylight, and which we admire separated
in the colours of the rainbow), the red, orange, and yellow are absorbed in their
passage through the water, and only those rays which are characterized by high
refrangibility, viz. the blue, are allowed to pass through. The rays on the further
side of the red, not perceptible to our eyes, the so-called dark heat-rays, are like-
wise absorbed in their passage through the water, and an object at some depth under
water would therefore only be reached by rays of high refrangibility, particularly
blue rays. The conditions of illumination for plants growing in the depths of the
ocean are consequently in reality quite unfavourable. It is not only that a portion
of the light falling on the surface of the water is reflected, and the other portion is
weakened by its passage through the water, but besides, those rays which are
necessary to the formation of organic matter by the chlorophyll-granules in the

390 CHLOROPHYLL AND LIGHT INTENSITY.

plant cells are abstracted from the light which passes through; for the chlorophyll-
granules need just the red, yellow, and orange rays if they are to perform their
functions; only under the influence of these rays can the decomposition of carbonic
acid, the separation of oxygen, and the formation of carbohydrates, take place.
The blue rays do not assist at all in this respect; they are even hurtful to these
processes, since they assist the oxidation, that is, the decomposition of organic
substance. Consequently, phycoérythrin, the red pigment of the Floridex, now
appears, and indeed so abundantly, that the chlorophyll-granules in the interior
are quite hidden by it. This colouring-matter displays a very marked fluorescence,
that is to say, it absorbs a large portion of the light rays falling on it, and gives
out other rays of greater wave-length. The blue rays are to some extent changed
by it to yellow, orange, and red, and thus the chlorophyll-granules finally receive
those rays which act as the propelling force in the decomposition of carbonie acid.
But this also affords an explanation of the remarkable phenomenon that sea-
plants are only coloured green close to the shore, and only in the most superficial
layers of water, while lower down they appear red. Only quite on the surface the
emerald-like Ulvacess and Entermorphas sway hither and thither, forming thus a
light-green belt; these alge are to be sought for in vain in the depths beneath; of
the plants which flourish below this region it can no longer be said that they grow
green; this mark of vegetation has entirely vanished. Green has given place to
red. All the innumerable Floridee are reddened—sometimes a delicate carmine,
sometimes a deep purple; then again a light brownish-red and a dull, dark crimson,
and as we admire in the bush the innumerable gradations of green colour, so is the
eye delighted in the manifold shades of red, in which the different variegated
species of Floridez, intermixing with one another, display themselves.

Let us now leave the blue twilight of the sea-depths, and set foot on the strand
lapped by the blue waves sparkling with white foam, and climb up one of the rocky
crags rising there above the seething waters. Around us is the bright daylight,
and broad terraces of rock thickly overgrown with plants, all brilliantly ilumined
’ by the unclouded sun. But where is that fresh green which we expect to find up
here according to the foregoing definitions in herbs and bushes? Here are not green,
but grey foliage and branches, white-haired stems and leaves, and the whole
woven and bound together into a carpet, which looks as if it had been strewn with
ashes, or as if the wind had for a week brought hither the dust from the neigh-
bouring streets and deposited it. The plants here on the sunny rocks have pro-
vided themselves with silky, woolly, and felted coverings for the purpose of softening
the too glaring light. In the depths of the sea and in the grottoes of the slate
rocks, the light was too weak; here, however, it is too strong. The chlorophyll-
granules tolerate neither the one nor the other; they require light of a definite
intensity. If the limit, which in this matter is exactly defined for each species, is
overstepped, the chlorophyll is destroyed. Too much light may be no less injurious
to the plants than if the chlorophyll-granules are condemned to inactivity on
account of the want of light.

CHLOROPHYLL AND LIGHT INTENSITY. 391

How quickly a glaring light is able to destroy the chlorophyll can be well seen
in the green Sea-lettuce on the shore below. In a high sea a violent wave tears
fragments of the Ulvaces, known under the name of Sea-lettuce, from the coast-
rocks; a second wave as it rushes up washes the leaf-like structures on to the
shingle of the shore, and there it remains with other débris lying amongst the
stones. The sea now becomes calm, the sky has cleared, the sun’s rays are again
burning with undiminished strength on the shadeless strand. As long as the Sea-
lettuce adhered closely to the rocks below the surface of the water it displayed a
brilliant emerald green; the water in which it was submerged to some little depth,
even at a low tide, sufficed to somewhat temper the sunlight; but the stranded Ulva
is deprived of this light-regulating covering of water, and in a few hours its
chlorophyll is destroyed. It is turned yellow, and looks like a lettuce-leaf which
has lain for a week ina dark cellar. A similar appearance is also seen in confervas
and spirogyras which fill stagnant pools of water with their masses of united fila-
ments. Two decimetres below the water they display a beautiful dark-green colour,
while close to the surface they appear a yellowish-green, and if the pool dries up so
that the masses of filament come to lie on the damp slime, in two days they are
quite bleached; the undimmed sunlight has completely destroyed the chlorophyll in
the cells. In the depth of beech-groves the Woodruff (Asperula odorata) raises its
leaves arranged in whorls on the stem; over it the thickly-leaved branches of the
beeches bend together, forming a roof through whose interstices only here and there
a weak sunbeam finds its way into the depths. In the dim light the leaf-stars of
the Woodruff appear of a deep, dark-green tint. Now the axe of the woodcutter
resounds through the forest—the beeches are felled, the shading roof of foliage is
demolished, and the floor of the wood is exposed to the glaring sunbeams. Within
two weeks the Woodruff can no longer be recognized; it has become sickly and pale;
the leaf-stars have lost their dark green, and the chlorophyll has been destroyed by
the glaring light. The same thing occurs with ferns as with the Woodruff. In the
dimness of the floor of the forest, between steep-walled rocks, and on shady northern
declivities they are tinted dark green; in sunny situations they become pale,
and then are noticeably retarded in growth. All these plants are not organized to
adapt themselves, in the case of an alteration of the illumination of their habitat, to
the new conditions and to protect themselves from the undimmed rays falling on
them. They are only fitted for the shady floor of the wood, and an over-abundance
of light is their death.

But how is the vegetation protected in a habitat where during the whole of
the vegetative period full light predominates, where the sun makes itself felt from
rise to setting with uninterrupted power? It has already been pointed out that the
plants on the broad ridges and terraces of the rocky shores of the Mediterranean are
shrouded in dull grey, clothed in silk or wool, or else overstrewn with chaff-like
scales, and consequently have lost their fresh green colour. In reality it is not
quite correct to say that they have “lost” the green, for their parenchymatous cells,
especially those of the palisade and spongy tissues in the foliage-leaves, are no less

392 CHLOROPHYLL AND LIGHT INTENSITY.

rich in chlorophyll-granules than those of shaded plants, only they have developed
from their epidermal cells those structures which have been previously described
as covering hairs. These cellular structures, devoid of chlorophyll, cover over the
green tissue, and thus give to the leaf in question a grey or white colour. They
play the part of awnings and light-extinguishers, and when they are removed the
leaf appears just as green as one that has been plucked from the shade of the wood.

Silky, velvety, and woolly coats may thus doubtless take. on the function of
extinguishers. We meet, therefore, the same contrivances apparently which already
on a previous occasion have been treated of, viz. when describing the protective
measures against excessive transpiration. Thus through these structures two birds
are killed with one stone. All contrivances which keep off too glaring sunbeams,
and thereby hinder the destruction of chlorophyll, at the same time diminish trans-
piration; and inasmuch as these contrivances perform two such important functions
for the life of plants, their wide distribution and great diversity is accounted for.
Suited to the conditions, adapted to the habitat and season of the year, and in
harmony with other developments, they change in a thousand ways, and thus
display a diversity which can scarcely be treated exhaustively. Besides the
covering hairs which are placed above the green tissue, as a protection and shade
against too intense light, and at the same time against excessive transpiration,
obviously all the other contrivances previously described are to be taken into
account. The development of one or several layers of cells, filled with watery
cell-sap, above the tissue exposed to the sun’s rays, the thickening of the cuticular
layers, the waxy and varnish-like coatings, the lime incrustations and salt
excretions, the diminution of the illuminated portion of the leaf-surface, the
formation of wrinkles, folds, pits, and grooves on the illumined surface of the
foliage—all these are able to interrupt and diminish the rays and to reduce their
intensity to the right degree.

The number of the special contrivances which simply secure chlorophyll from
destruction by too glaring light, without at the same time protecting the green
tissue from excessive transpiration, must indeed be very small. First of all, we
may mention the dry thin-skinned scales which in many plants are inserted between
the green leaves. These are seen, for example, in species of the genus Paronychia,
which in masses have their habitat in sunny places, and produce silver-glittering
transparent scales, devoid of chlorophyll, close to that portion of the stem from
which the small green leaves originate. These scales, which are designated stipules,
and which, here, are usually as large, occasionally even larger, than the green leaves,
take up naturally such a position in the plants growing on shadeless hillocks that
the sun’s rays first of all fall on them, and only reach the green leaflets in a
weakened state.

Another arrangement, which indeed is able to restrict the destruction of the
chlorophyll by the sun’s rays, without affecting transpiration, consists in the
development of a blue or violet colouring-matter in those cells which compose the
superficial covering of the leaves and stem which is directly illuminated by the sun’s

CHLOROPHYLL AND LIGHT INTENSITY, 393

rays. Such an arrangement is found, for example, in the leaves of the aromatic
Satureja hortensis, originally growing wild in the Mediterranean floral district, and
cultivated in gardens under the name of Summer Savory, of which leaves a small
portion is represented in cross section on Plate I., fig. g, printed in colours. Before
the sunbeam reaches the chlorophyll-granules of the green cells in the middle of the
leaf, it must pass through these epidermal cells filled with violet sap, and here it
becomes so weakened and also so changed that an injurious influence on the
chlorophyll-granules is out of the question. We must not omit to notice here that
the violet light-reducing colouring-matter in the epidermal cells is more abundantly
developed the intenser the light to which the plants in question are exposed. If
plants of the Summer Savory grow in shady places, their leaves remain green on
the upper sides, and scarcely any traces of the violet colouring-matter are to be
discovered in the epidermal cells. If, on the other hand, they have germinated in
shadeless districts, both stem and leaves are coloured dark violet, and the cell-sap in
the epidermal cells is then of that deep tint shown in Plate I, fig. g. Some years
ago I cultivated seeds of the Summer Savory in my experimental garden at a height
of 2195 metres above the sea-level in the Tyrol. As is known, the sun’s rays are
much more powerful in the Alpine heights than in the valley, and it was therefore,
indeed, to be expected that the leaves of the germinating plants would be of a much
darker tint than in the shadeless gardens of the valley below. In fact, the colouring-
matter developed in extraordinary abundance; even the stems and leaves actually
became a dark brownish violet. It is, therefore, beyond question that the quantity
of colouring-matter in the epidermal cells directly exposed to the sun increases
with the increase of the intensity of the light. Obviously this protection of the
chlorophyll can only occur when the plants possess the materials for forming the
pink colouring-matter in their green organs. When this is not possible, when the
characteristic constitution of the protoplasm does not permit the development of
the colouring-matter named in the foliage-leaves, the chlorophyll must be pro-
tected against the glaring light in another way, and if the plant species is not
able to ward off the over-abundance of sunlight in the new position, it perishes
entirely. Flax (Linum usitatissimwm) was sown next to the Summer Savory in
the Alpine experimental garden—a plant which bears the direct sunlight quite
well, and flourishes in the valley as well as in the plains in sunny situations.
However, the light of the Alpine region was too brilliant for the germinating flax-
plants; the leaves turned yellow, their chlorophyll was destroyed, and the seedlings
became pale and perished. Flax has not the capacity of manufacturing the
colouring-matter in its superficial cells, and it is also not organized to produce
covering hairs on the leaves and stem, or to thicken its cuticular strata suitably—
in a word, to adapt itself to the position and to provide itself, under the increased
light intensity, with corresponding sun-shades and light-extinguishers. While close
at hand, the Summer Savory, which requires just as much warmth, and an equally
long vegetative period as flax, reached the flowering stage, and even produced ripe
fruits capable of germinating, the flax died before the development of its flowers.

\

394 CHLOROPHYLL AND LIGHT INTENSITY.

From these culture experiments two things may be learned: first, that a very
brilliant light is able to influence the distribution of plants and to set up an
impassable barrier for many of them; and secondly, that many plants have the
capacity of adapting themselves to various degrees of light intensity; but in conse-
quence of this they occasionally develop such a varying character that they might
be mistaken for wholly cifferent species. But I shall return again later when
speaking of the origin of new species to this result of cultivation. Here we shall
only discuss, in order to prove and make clear the connection between certain plant
characteristics and the conditions of illumination, how it happens that the surface
of foliage exposed to the direct rays of the sun is so frequently coloured violet or
red, or is completely covered over with hairs, while the leaves of the same species if
they have been developed on shady soil in dispersed light are coloured green, and
remain almost bare; how it happens that plants of one and the same species in the
deep valleys possess but few hairs, or are provided with but thin cuticular layers,
but on the sunny slopes of high mountains are shrouded in thick grey or white fur,
or appear thick and almost leathery in consequence of strongly-developed cuticular
layers. In order to prevent misconception, it must indeed be pointed out here that
all this only refers to the epidermis over the green tissue which is exposed to direct
or diffuse sunlight, chiefly, therefore, to the wpper side of the foliage-leaf, and that
when the blue colouring-matter and also the covering hairs are developed on the
under side of the leaf, or in floral leaves devoid of chlorophyll, they have then an
essentially different significance, which will be described in the next section.

When describing the protective measures of the green tissues against the
dangers of over-transpiration, the vertical direction of branches, flattened shoots,
phyllodes, and especially of the green leaf-surfaces, was pointed out. The leaves of
irises, and of the so-called compass-plants, the flattened outspread petioles, with
their edge directed towards the zenith, in so many Australian trees and shrubs,
were there more especially described, and finally it was pointed out that the
leaflets of many papilionaceous plants, and the leaves of numerous grasses,
temporarily take up a position by sinking, rising, and folding together, in which
not the broad side, but the narrow edge, is exposed to the vertical rays of the
mid-day sun.

A leaf-surface which assumes one of these positions with regard to the sun
will transpire much less than a foliage-leaf on whose broad surface the mid-day
sun falls vertically, or almost vertically; but by such a position the leaf is also
afforded a protection against the too vivid light of noon. The rays which reach a
vertical leaf-surface at morning and evening are not so intense as to be able to
destroy chlorophyll; they have rather just that intensity which the chlorophyll-
granules require for their activity. Therefore, by this arrangement the function of
the chlorophyll-granules is not restricted, but is actually assisted, and in this sense
the vertical direction of the green surfaces is to be looked upon also as an
arrangement for regulating the activity of the chlorophyll-granules.

It is evident after this explanation that herbs with vertically-directed leaf-

CHLOROPHYLL AND LIGHT INTENSITY. 395

surfaces are never to be met with in shady places. On the floor of a thick wood
grow no irises and no compass-plants; these are at home on the ridges of rocky
mountains, and on treeless prairies, and if it happens that a seed of such a plant
falls into the shade of a wood and germinates there, developing foliage-leaves, then
the leaf-surfaces do not assume a vertical position, and twist and bend themselves
until their broad surface is turned towards the scantily-penetrating diffuse light.
If the light falls from above through the interstices of the leafy covering, the
leaf-surface becomes horizontal and parallel to the ground; if the crests of the trees
close together to form a thick, opaque canopy, and the diffuse light penetrates from
the side between the trunks of the trees, the leaf-laminze bend and turn to the
openings of the wood, giving the impression that they are looking out longingly
to the sunny country which borders the dense, deep-shaded forest.

The same thing is seen under every small shady bush, and, generally speaking, in
all places where dissimilar tall plants overlap one another, and where the leaves of
the lower are arched over by those of the higher plants. If they belong to different
species, they cannot be said to have any consideration for one another. Each looks
out only for itself, and the lofty species do not trouble themselves about the
inferior stuff which arises from the soil under their leaves. If in the depths below
there are plants which find all they require in the diffuse light and the green rays
passing through the leafy roof, very well; if not, these lower plants must perish in
the shade. But it is otherwise if the leaves overlapping each other belong to one
and the same branch, to one and the same plant; where they must co-operate for
the weal of the whole plant, and the whole can only maintain itself in the struggle
for existence by harmonious division of labour. Therefore care must be taken that
no leaf shall take too much light away from another; that one shall protect and
support the other; that neighbours shall not jostle if one or the other has to bend,
turn, and extend itself in order to best adapt itself to the incident light.

‘And this foresight actually occurs. It is exhibited, first of all, in the position
of the leaves on the stem, or in other words, in the regulation of the intervals
between the places of origin of neighbouring leaves; secondly, by the fact that the
stalks of the green leaf-blades have the capacity of rising and sinking, twisting and
bending, and also of elongating if required; and thirdly, through the form which the
leaf-surfaces possess. ,

396 DISTRIBUTION OF THE GREEN LEAVES ON THE STEM.

2. THE GREEN LEAVES.

Distribution of the green leaves on the stem.—Relation between position and form of green leaves.
—Arrangements for retaining the position taken up.—Protective arrangements of green leaves
against the attacks of animals.

DISTRIBUTION OF THE GREEN LEAVES ON THE STEM.

Landscape painters tell us how difficult it is to treat foliage correctly, and at the
same time artistically; how hard, for instance, so to reproduce the leafy crown of
maples, beeches, elms, limes, and oaks that they shall immediately be recognized
for that which they are intended to represent, and at the same time that that effect
and tone shall be produced which is aimed at in the picture. The variety of the
foliage is caused not least by the distribution of the green leaves on the branches,

.and by the branching dependent upon this; things as definite as possible for each
species of tree, and, generally speaking, for every plant.

On cutting various leafy branches and observing the distribution of the leaves
on them, the following differences first strike the eye. In numerous plants it is
seen that two or more leaves originate at the same height on a branch, while in
many other plants, at a particular level of the stem or branch, only a single leaf
is produced. In order to be able to understand these circumstances, it is advisable
to imagine the leaf-bearing shoot or stem as a cone. The apex of the cone
corresponds to the upper end, and the base of the cone to the lower portion, #.e. to
the oldest part of the shoot. The whole shoot is not at any time in a completed”
state; it continues to grow at the apex, and at the upper part is not only younger,
but is also less bulky than the older portions lying nearer to the base. It can,
therefore, indeed be quite well compared to a cone, although this form is only
seldom so noticeably to be met with as in the following diagrammatic figures.

That which applies to the age of the various portions of the shoot naturally
applies also to the leaves projecting from the shoot. That is to say, the lower
leaves of the shoot are the older, the upper leaves are the younger. On looking at
the top of the cone (see fig. 98), the places of insertion of the older leaves appear to
arise, first of all, from the circular disc which forms the base of the cone, while the
younger leaves originate close to the apex, therefore close to the centre. The stem
is to a certain extent divided up by the leaves into sections one above another.
Usually it is somewhat thickened or knotted at those places where the leaves
project from it, and therefore the places of origin of the leaves are designated as
nodes. Each portion of the stem lying between two successive nodes is called an
internode. When two leaves project at the same height from the stem, they are
inserted opposite one another, not unlike the two extended arms of a human body,
and they appear on the cone-shaped stem (whose cross section at all heights
forms a circle) at a distance from one another of exactly half the circumference
of the circle (180°), (fig. 981). If three leaves spring together from the stem,

DISTRIBUTION OF THE GREEN LEAVES ON THE STEM. 397

as, for example, in the Oleander, these are separated from one another in a
horizontal direction by one-third of the circumference of the circle (120°). Several
leaves springing from the same height form together a whorl, and the distance of
the individual members of a whorl from one another is called the horizontal
distance, or the divergence. The divergence amounts to 3 in fig. 981, and } in
fig. 98°, of the circumference of the circle, and can be thus shortly expressed by
means of these fractions.

It is very remarkable that the whorls which follow after and above one
another according to their age on one and the same shoot do not originate at
corresponding places of the circumference, but are displaced regularly with regard
to one another. Thus the point of origin of the second two-membered whorl in

Fig. 98.—Plan of Whorled Phyllotaxis.

1Two-membered Whorl. *% Three-membered Whorl.

fig. 98* is shifted through a quarter of the circumference (2.¢. through 90°, a right
angle) from the point of origin of the first, oldest, and lowest two-membered whorl.
The third whorl is again shifted through a right angle with regard to the second,
and so it continues up the stem as far, generally speaking, as foliage-leaves are to
be found on it. If the stem is elongated in the case described, four rectilineal lines
(orthostichies) appear to be developed on it (fig. 981). If a whorl is composed of
three leaves, and if the successive whorls be displaced through one-sixth of the
circumference, as, for example, in the Oleander (see fig. 987), six rectilineal series of
leaves or orthostichies originate, running parallel to one another down the stem.
The leafy stem can also be imagined as divided into stories, each of which
aisplays the same number, position, and distribution of the leaves, and agrees
completely in the plan of its construction with the adjoining story. In one such
case (fig. 981), each story possesses four leaves in the form of a cross; in another
case (fig. 98”), it possesses two sets of three leaves separated from one another by
a distance of 60°. If the stories standing above one another are separated, they
would be so alike in arrangement as to be easily mistaken for one another. Each

398 DISTRIBUTION OF THE GREEN LEAVES ON THE STEM.

originates below and ends above exactly like the one over and the one under it, and
the only difference rests in the fact that the sections closer to the summit of the
branch have smaller diameters, and often also a somewhat different outline of their
members. The plan of construction is, however, as stated, exactly the same in the
successive stories.

In those instances where each story consists of two whorls of leaves, which are
displaced with regard to one another through a certain angle, especially in the very
common case where the whorl is two-membered, 7.¢. where the leaves are opposite
one another in pairs, and where the successive pairs of leaves are alternately
displaced through a right angle from one another, appearing thus like a cross, the
leaves are said to be decussate. This arrangement is seen especially in maples and
ashes, in lilac and olive-trees, in elder and honeysuckle, in labiates, gentians,
Apocynaces, and numerous other families of plants.

Still more common than this arrangement of the leaves is that which has been
called the spiral. Here at one and the same height only a single leaf originates
from the stem, and therefore all the leaves of a stem are not only shifted with
respect to one another in a horizontal, but also in a vertical direction. If one
imagines the nodes of a stem with decussate leaves to be so arranged longitudinally
that the leaves are inserted no longer at the same heights, but at definite intervals
above one another, then from the decussating, 7.c. whorled, arrangement a spiral is
produced. In many willows (¢.. Salix purpurea), and very regularly also in some
buckthorns (e.g. Rhamnus cathartica), in the speedwells (e.g. Veronica spicata and
longifolia), and also in many composites leaves arranged partly in whorls and
partly in spirals occur on the same axis, and doubtless the one merges into the
other, but for the sake of clearness it is better to keep them distinct, and to draw
a line between them, even though it be an imaginary one.

It may be observed that stems with spirally-arranged leaves are constructed
exactly like those which bear leaf-whorls, and that they consist of many stories
each displaying a similar plan of construction, so that the number, position, and
distribution of the leaves is repeated in each story, and as a matter of fact the
followin;; plans of construction are actually to be found very frequently.

Furst case. In each story only two leaves arise from the circumference of the
stem. These two leaves are displaced with regard to one another in a horizontal
as well as vertical direction, and their horizontal divergence amounts to half the
circumference of the circle (180°) as shown in the plan in fig. 991. If a continuous
line be drawn from the point of insertion of each lower older leaf to the younger
one next above it on the surface of the stem, this will display the form of a spiral.
It has been called the genetic spiral. In the first case here discussed it forms in
each story only a single spiral band. This arrangement is repeated in the second,
third, and perhaps in many other stories which follow successively on the same
axis. In this way the lower leaf of the second, third, or fourth story always lies
exactly above the lower leaf of the first story. The same applies to the upper
leaves of all the stories. Thus two rectilineal lines or orthostichies are formed on

DISTRIBUTION OF THE GREEN LEAVES ON THE STEM 399

the circumference of the stem by the leaves situated vertically above one another.
The two lines are opposite, or, what comes to the same thing, they are separated
from one another by half the circumference of the stem. This arrangement of the
leaves, which may be observed, for example, on the branches of elms (Ulmus) and
limes (T%lia), is called the one-half phyllotaxis.

Second case. Three leaves are developed in one story, each at a definite height,
an under, a middle, and an upper leaf. In a horizontal direction two of the leaves
following one another in age are always shifted from one another through a third
part of the circumference (see fig. 997). If the point of insertion of the lower leaf
is connected with that of the middle leaf, and this again with that of the upper
leaf by a line, and this line is continued to the beginning of the next story, a single
spiral is thus formed surrounding the stem. Now above the story just described,
which we will call the lowest, a second follows, which is again provided with three
leaves arranged in exactly the same way. The lower leaf of the second story is
situated vertically above the lower leaf of the first story, the middle above the
middle, and the upper above the upper leaf, and the same arrangement is continued
through all the stories. In this manner three rectilineal lines, or orthostichies,
arise on the circumference of the stem from the leaves situated above one another,
and each of the lines is separated from the other two by 4 of the circumference.
This arrangement, which is to be found on the upright branches of alders, hazels,
and beeches, is called the one-third phyllotaxis.

Third case. Five leaves originate in each story, which are designated according
to age as the first, second, third, fourth, and fifth, the lowest being the oldest, the
highest the youngest. These five leaves give place to one another in a horizontal
direction, and the shifting, i.¢. the horizontal distance between two leaves next in
age, amounts to 2 of the circumference of the circle (see the plan, fig. 99°). If
the five leaves are joined together in succession according to their age, a spiral
line is obtained consisting of two revolutions, and the “ genetic spiral” consequently
forms two circuits round the stem. If a stem, whose leaves are arranged in this
manner, is built up of two or several stories, then the similarly numbered leaves
are situated in straight lines above one another, the first (lowest) leaves of all the
stories form together one straight line (orthostichy); in the same way the second, the
third, &. Thus five lines are developed on the circumference of the stem by the
leaves situated one above the other, and each line is separated from another by
1 of the circumference. This arrangement, which is found in oaks, round-leaved
willows, and in many buckthorns, is designated the two-fifths phyllotaxis.

Fourth case. Hight leaves are to be found in each story, which may again be
numbered from one to eight according to their age. Any two successive leaves are
separated from one another horizontally by ~ of the circumference (see fig. 99 *).
If a line be drawn starting from the first and lowest leaf, joining all the eight leaves
of the story in the order of their ages, this forms a spiral line, or “ genetic spiral”,
which traverses the stem three times. In a stem consisting of several such stories,
the leaves named by the same numbers are placed in straight lines above one

400 DISTRIBUTION OF THE GREEN LEAVES ON THE STEM.

another, and accordingly eight rectilineal lines (orthostichies) run down the stem.
Each line is separated from its neighbour by } of the circumference. This arrange-
ment, which occurs in roses, raspberries, pears, and poplars, if laburnuns, and in the
barberry, is called the three-eighths phyllotaxis.

Yet a fifth case is very often to be found in trees and bushes with narrow
leaves, viz. in the Almond-tree, in the Goat’s-thorn, in the Sweet Willow, in the
Sea Buckthorn, and many Spirea bushes. Each story contains thirteen leaves.

\
1
‘
‘
'
'
'
1
V
1
1
j

Fig. 99.—Plan for Spiral Phyllotaxis.

1 One-half Phyllotaxis. 2 One-third Phyllotaxis, 8 Two-fifths Phyllotaxis. 4 Three-eighths Phyllotaxis. The conical stem
horizontally projected; the points of insertion of the leaves on the circumference of the stem marked by a dot.

which may be connected by a spiral line, 4.¢. a “genetic spiral”, with five revolu-
tions. The number of the straight lines here amounts to thirteen, and the distance
between two leaves following one another in age is 3%; of the circumference, 4.¢.
138° (see fig. 100).

Not so common, or rather not demonstrable with the same precision, are
instances in which one story shows twenty-one leaves which are connected by a
genetic spiral with eight revolutions; and where a story includes thirty-four
leaves which are connected by a genetic spiral with thirteen revolutions. In the
one case any two leaves next one another in age in a story are separated from one

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