Library
of the

University of Toronto

f '•

i:OMPARATIVE ANATOMY

OF THE

PHANEROGAMS AND FERNS

DE BARV

a

Uonljon

HENRY FROWDE

OXPOED UNIVEESITY PEESS WAEEHOUSE

AMEN CORNER

COMPARATIVE ANATOMY

OF THE

VEGETATIVE ORGANS

OF THE

PHANEROGAMS AND FERNS

BY

Dr. a. DE BARY

PROFESSOR IN THE UNIVERSITY OF STRASSBURG

TRANSLATED AND ANNOTATED
BY

F. O. BOWER, M.A., F.L.S.

LF.CTURER IN BOTANY AT THE NORMAL SCHOOL OF SCIENCE, SOUTH KENSINGTON

AND

D. H. SCOTT, M.A., Ph.D., F.L.S.

■ASSISTANT TO THE PROFESSOR OF BOTANY IN UNIVERSITY COLLEGE, LONDON

WITH TWO HUNDRED AND FORTY-ONE WOODCUTS

AT THE CLARENDON PRESS

1884

All rights reserved \

Digitized by the Internet Archive
in 2017 with funding from
University of Toronto

https://archive.org/details/comparativeanatoOObary

TRANSLATORS’ PREFACE.

-<e

In producing an English translation of the Comparative Anatomy of the
Phanerogams and Ferns, by Professor De Bary, an attempt has been made to
meet two requirements, which have long been felt. In the first place, those
English students who do not read German will now gain access to the most
exhaustive work hitherto published on that subject. Though, through unavoidable
circumstances, a considerable interval has elapsed between the publication of the
original and that of the translation, the book deals so largely with established
facts, and in so much less a degree with matters of controversy, that the delay
affects its value but little. The Translators have however inserted references to
the more important memoirs published since the original was produced.

In the second place, by means of this translation it is hoped that suitable
English equivalents will have been supplied for numerous technical terms which
have not hitherto been translated. Thus, in that part of the book which deals
with the arrangement of the vascular bundles (pp. 232-315), the introduction
of new English terms has been especially necessary, since this part of the science
has not hitherto been treated at length in any English text-book.

In conclusion, the Translators wish to record their thanks to Mr. W. T.
Thiselton Dyer, Assistant Director of the Royal Gardens, Kew, and to Dr. S. H.
Vines, Fellow and Lecturer of Christ’s College, Cambridge, for valuable advice
and assistance ; also to Mr. W. B. Hemsley, for the good judgment and care
with which he has prepared the index.

April 22, 1884.

TO THE READERS.

The present volume will complete the Handbook of Physiological Botany, which,
since its commencement in 1865, has been edited by the late Prof. Hofmeister. As
stated in the Preface to Vol. I, the plan of the book was drawn up in the year 1861,
and the sections, which were to be treated according to the state of the science at
that time, were distributed into four volumes, and among six contributors. Arrange-
ments were made beforehand, so that the Volumes might be expected to appear in
quick succession. Two of the contributors retired at the outset, so that, in. the year
1866, the fourth volume having first appeared, and then the first section of the
second, the programrne in the Preface of Vol. I. assigned all the volumes to four
contributors, and those still remaining to three. Of these another subsequently
retired. Nevertheless, the undertaking was not given up, the preparation of the
remaining parts being undertaken by Hofmeister and by the author of this volume.

At the beginning of^ last year Hofmeister was attacked by severe illness, to
which he succumbed on the 12th of January of this year. After his death the
question of the fate of the Handbook presented itself to the surviving contributors.
Among the papers of the deceased were found, it is true, drafts and beginnings of
the parts he had undertaken. But they have so much the character of incomplete
sketches and fragments, that it was obvious to the undersigned that their publication
.would neither answer the purpose of the Handbook, nor the intentions of their
author. The case standing thus, the remaining parts would have to be undertaken
by another contributor. Supposing some one to be ready at once, he would have
to begin on his own account at the beginning, and the continuation of the
Handbook would at best be delayed for years. Nevertheless, if there were a real
want, the attempt to continue it would be made. But, in the sixteen years which
have passed since the Handbook was planned, the position of our Science has
altered. In face of the Literature of to-day, a new work comprising ‘ the Mor-
phology of the Vascular Cryptogams,’ and ‘the Sexual Reproduction of the
Phanerogams,’ can be dispensed with, while a separate treatment of the ‘Algae,’ as
at first intended, is hardly possible. On these grounds it has been determined to
close the Handbook. As it stands at present, it is arranged as follows : —

Vol. I. ist Part. ‘Die Lehre von der Pflanzenzelle.’ By W. Hofmeister.

2nd Part. ‘Allgemeine Morphologie der Gewächse.’ By the same author.

Vol. 11. ‘Morphologie und Physiologie der Pilze, Flechten und Myxomyceten.’

By A. de Bary.

Vol. III. ‘Vergleichende Anatomie der Vegetationsorgane der Geflisspflanzen.’

By the same author.

Vol. IV. ‘ Experimentalphysiologie der Pflanzen.’ By Julius Sachs.

A. DE BARY. J. SACHS.

Sl'RASSBURG AND WURZBURG, Jlinc, 1877.

PREFACE.

The preparation of the present volume was begun by the author in the year
1865, after the other contributors to the Handbook, who had originally undertaken it,
had retired. It was fairly far advanced, when in 1867, by reason of other necessary
business, it had to be put entirely on one side for almost two years. It also suffered
frequent and long interruption at a later time through changes in the official engage-
ments of the author.

The object of the work, as stated in the programme of the Handbook, was an
epitome of the present knowledge of Hhe Anatomy of the Vegetative Organs of
Vascular Plants.’ From the very first, the necessity of numerous confirmatory in-
vestigations was apparent, since the descriptions at hand were written at very
different times, and by very different authors, and it was only possible to judge
of and sift the differences necessarily present in these by actual and personal
observation. This led to many researches of my own : new results and new
questions appeared. The work soon extended itself beyond the limits originally
intended. When one section was successfully finished, and others were in hand, new
publications appeared which demanded fresh alteration of what had already been done.
Therefore, in order that at least something might result, the necessity finally arose
of bringing this work of the Danaids, this supplementary patching and correcting,
to a definite conclusion, and of finally closing the work. This was done about three
years ago. Since then nothing of importance has been done beyond finishing the
revision.

That such was the progress of the work may be some explanation and excuse
for the frequent unevenness of the performance. Further, the, so to speak, forced
hurry of the conclusion necessarily imposed limits. With regard to the contents,
the exclusion in the first place of all Palaeontology and Pathology, the latter
including the phenomena of wounding and healing by Callus, &c., is understood.
Also the small sections on the throwing off and fall of the Leaf, &c. were omitted
as of minor importance. Further, it was unavoidable that the use of the newest
Literature should be limited. Much that has appeared in latter years has, to my
sorrow, been consciously and intentionally left unused. On the ground of the above
explanations, I particularly beg to be excused for this.

From the older Literature I have perhaps cited too much for many, and for
others too little. But here also arose the necessity of keeping a definite limit, in
order to bring something to completion. On the anatomy of plants such an
indescribable amount has been written, that, in a comprehensive treatise, one or
many authors might be cited in reference to every word. To carry this through,
even to the extent to which it is done in the section on Epidermis, makes the

PREFA CE.

viii

description exceed the bounds of convenience and overstep the limit of human
power and endurance. I have therefore placed a check upon this also, and, once
for all, I will say in answer to possible claims, that every word in this book has
had a previous author, printer, and publisher. I hope that I have as a rule referred
sufficiently to fundamental works : still it may be emphatically stated that the chief
sources and foundations of my work were the writings of Mohl, Nageli, Sanio,
Th. Hartig, and in the latest times of van Tieghem, although in some cases I may
have omitted to cite them expressly. I presuppose a knowledge of the Text-book
of Sachs. When this is quoted, and no further information is given, the fourth
edition is always meant.

The old Literature is only cited when absolutely necessary, since it lies outside
the purpose before us, to write a history of the anatomy of plants. In Sachs’ History
of Botany, Trevir anus' Physiology, and Meyen's Phytotomy and System of Vegetable
Physiology, the reader will find what is wanting here.

The plan and course of description are more exactly indicated in the Intro-
duction. The book deals in the first place with the aclual maiure structure of
the higher plants, and touches upon the history of development only by way of
assistance. We do not thereby ignore the fact that the description of the mature
condition must necessarily be based upon the history of development, since that
which is termed mature is nothing more than a further advanced part of the whole
course of development of the individual. It must therefore always be referred as
a matter of course to earlier stages of development, and be coupled with them.
But it was the more the object of this work to put that stage of development
which is called mature to the fore, since the present overruling preference for the
earlier stages has often brought it about that in the ‘ voir venir,’ the things them-
selves, which are to be produced, are neglected.

I know only too well how far the book falls short of the object indicated in
the title. The name ‘Vorarbeiten,’ or ‘ Prodromus,’ of a comparative anatomy
would better correspond to the result. That title was only rejected for shortness,
sake, and on the consideration that every work should be the predecessor of a
better.

Most of the figures were drawn by the author on wood, from nature. In case
of those copied and borrowed from other books the source is given in each case.
I am specially thankful to my respected colleague Sachs for the permission to use
the woodcuts of his Text-book, and I should have made still further use of them
had not a number of the figures here given been already cut before the earlier
editions of the Text-book appeared. I may offer this expression of thanks without
presumption not only in the name of the author, but also in that of the reader.
Also in the name of both I may add thanks to Dr. von Rostafinsky of Krakau, who
has constructed the index of names.

A. DE BARY.

Strassbürg, June 15, 1877.

TABLE OF CONTENTS.

PAGE

Introduction i

PART I.

THE FORMS OF TISSUE.

CHAPTER I.

Cellular Tissue.

Sect. I. General Introductory Remarks 27

Division I. The Epidermis.

„ 2. General Definitions 29

I. Composition of the Epidermis.

„ 3. Enumeration of the Component Parts ....... 30

„ 4. Epidermal Cells ............ 30

„ 5. Stomata 34

„ 6. Air- and Water-stomata . 45

„ 7. Air-stomata 45

„ 8. Water-stomata 50

„ 9. Gaps in the Epidermis 5-4

„ 10. Hair-structures ............ 54

2. Structure of the Epidermal Elements.
a. Protoplasm and Cell-contents.

„ II. Epidermal Cells 66

„ 12. Stomata 67

„ 13. Hair-structures 68

b. Structure of the Walls.

„ 14. Cellulose-Membranes 70

„ 15. Intramural and Superficial Deposits ........ 73

„ 16. Mucilage, Cuticle, and Cuticular Layers . 73

„ 17. Wax , 82

X

TABLE OF CONTENTS.

PAGE

Sect. i8. Restio diffusus 88

„ 19. Dermal Glands ............ 88

„ 20. Pulverulent Hairs 99

„ 21. Digestive Glands 100

„ 22. Silicification, Calcification, Cystoliths ....... 102

„ 23. Deposits of Lime 106

Division II. Cork.

„ 24. Origin and Structure of Cork 108

Division III. Parenchyma.

„ 25. Forms with Thin Walls 115

„ 26. Collenchyma, Sclerotic Cells 119

„ 27. Endodermis (protective sheath) . . .121

CHAPTER II.

Sclerenchyma.

Sect. 28. General Observations 126

„ 29. Short Sclerenchyma, Stone Sclerenchyma 127

„ 30. Sclerenchymatous Fibres 128

CHAPTER III.

Secretory Reservoirs.

Sect. 31. Synopsis 135

„ 32. Sacs containing Crystals 137

„ 33. Sacs containing Mucilage 143'

„ 34. Sacs containing Resin and Gum-resin 145

„ 35. Sacs containing Tannin 153

CHAPTER IV.

Tracheae.

Sect. 36. Synopsis 133

„ 37. Fibrous Thickening of the Walls 156

„ 38. Bordered Pits 138

„ 39. Transverse Bars 163

„ 40. Tracheides 164

„ 41. Vessels 163

„ 42. Contents of the Tracheae, Thyloses 169

CHAPTER V.

Sieve-tubes,

Sect. 43. Angiosperms

„ 44. Gymnosperms and P'erns 179

TABLE OF CONTENTS. xi

CHAPTER VL
Latieiferous Tubes.

PAGE

Sect. 45. The Latex 183

„ 46. The Tubes 186

„ 47. Articulated Tubes 189

„ 48. Non-articulated Tubes 190

„ „ History. General Observations 192

CHAPTER VII.

Appendix. Intercellular Spaces.

Sect. 49. General Observations. Development 200

„ 50. Intercellular Secretory Reservoirs 201

„ 51. Intercellular Reservoirs containing Air or Water ..... 210

„ 52. Diaphragms 217

„ S3. Internal Hairs 220

PART II.

ARRANGEMENT OF THE FORMS OF TISSUE.

FIRST SECTION. PRIMARY ARRANGEMENT.

Sect. 54. General Observations. Epidermis, Hypoderma 224

CHAPTER VIII.

Arrangement of the Traehese and Sieve-tubes.

I. Outside the Vascular Bundles.

Sect. 55. Scattered Tracheides 226

„ 56. Sheath of Tracheides in Aerial Roots 227

„ 57. Scattered Sieve-tubes 231

2. Vascular Bundles.

„ 58. General Observations 232

A. Arrangement of the Vascular Bundles.
a. hi the Root.

»59 232

TABLE OF CONTENTS.

xii

II.

III.

IV.

V.

VI.

VII.

b. In the Individual Leafy Stem.

Sect. 6o. General Rules

„ 6i. Dicotyledonous Type. Dicotyledons

Gymnosperms

„ 62. Anomalous Dicotyledons .

Medullary Bundles ....

„ 63. Cortical Bundles ....

„ 64. Palm-type. Simple Form .

„ 65-67. Modification of the Palm-type
„ 68. Monocotyledonous Seedlings. Aroidete
„ 69. Type of the Commelinege .

„ 70-71. Anomalous Monocotyledons .

„ 72. Phanerogams with an Axile Bundle .

„ 73. Fern-like Plants. General Observations

„ 74. Equisetum .

„ 75. Osmundaceas

„ 76. Isoetes

„ 77. Psilotum and Lycopodium

„ 78. Selaginella .

„ 79. P'ilices and Hydropteridese

„ 80-83. Axile Bundle. Tube of Bundles
„ 84. Concentric Rings of Bundles

„ 85-87. Medullary and Cortical Bundles

c. Course of the Bundles m the Leaves and Foliar Expajisions.

„ 88. Nodes, Stipules . ... j

„ 89. Petiole

„ 90. Lamina. Nervation ....

„ 91. Superficial Divarication of the Bundles

„ 92. Position as seen in Vertical Section .

Note. Distribution of the Forms of Nervation

d. Connection between the Bundles of different Orders of Shoots and

Branches.

„ 93. General Considerations ........

1. Similar Branches of Leafy Stems.

I. Normal Branches.

„ 94. Type of the Dicotyledons and Gymnosperms ....

„ 95. Other Phanerogams

,, 96. Fern-like Plants

II. Adventitious Shoots.

„ 97.

„ 98.

11. Roots.

B. Structure of the Vascular Bundles.
„ 99. General Observations

PAGE

• 233

• 235

• 245

. 248
. 248
. 256
. 261
. 264
. 267
. 269
. 274

■ 277

. 278
. 279

• 279

. 280

. 280

. 282

• 283

• 283
. 289
. 291

. 296
. 298
. 298

• 299

• 305

• 306

307

307

311

315

• 315

316

TABLE OF CONTENTS.

Xlll

I. Bundle-trunks.

PAGE

Sect.

too.

Synopsis

317

lOI.

Collateral Bundles

319

102.

Bundles of the Leaves of Cycadeae and Isoeteas

335

J5

103.

Bicollateral Bundles

338

„ 104-105. Concentric Bundles

339

106.

Concentric Bundles of the Ferns

342

5)

107.

Radial Bundles

348

108.

Radial Bundles of Typical Roots

351

109.

Root-bundles of Abnormal Structure

364

>7

no.

Imperfect and Rudimentary Bundle-trunks ....

366

2. Ends and Connections of Vascular Bundles.

77

1 1 1.

Endings in the Cortex, and in the Foliar Expansions

371

77

1 12.

Peculiarities of Leaves of the Coniferre .....

378

77

II3-

Terminations in Roots and Haustoria .....

383

77

1 14.

Connections of Bundles

385

C. Development of the Vascular Bundle.

77

II5.

Development of the Individual Bundle

388

77

II6.

Development of the Bundle-system in the Stem. Succession

of the

Bundles. Their Morphological Position ....

392

77

II7.

Development of the Lateral Roots

397

Concluding Remarks

399

CHAPTER IX.

Arrangement of the Primary Parenchyma.

Sect,

II8.

General Considerations

402

7?

1 19.

Pith, Medullary Rays. External Cortex

402

77

120.

Petioles, Ribs of the Leaf

405

77

I2I.

Lamina of the Leaf (Mesophyll, Diachyma) ....

406

77

122.

Cortex of the Root, Root-cap

412

77

123.

Parenchymatous Sheaths, Endodermis, Starch-layer. — Plerome-sheath .

414

CHAPTER X.

Selerenchyma and Sclerotic Cells.

Sect.

124.

General Considerations

417

77

125.

Fibrous Layers and Strands

417

77

126.

Isolated Fibres

423

77

127.

Short Sclerenchymatous Elements

425

77

128.

Thorns, Prickles, Warts

425

77

129.

Sclerotic Elements of the Ferns

426

130-

chapter XL
Secretory Reservoirs.

• 431

XIV

TABLE OF CONTENTS.

4*

CHAPTER XII.

Laticiferous Tubes.

PAGE

Sect. 131 432

CHAPTER XIII.

Arrangement of the Intercellular Spaces.

Sect. 132. Spaces containing Air 440

„ 133. Intercellular Secretory Reservoirs 440

SECOND SECTION. SECONDARY CHANGES.
CHAPTER XIV.

Secondary Growth in Thickness of Normal Dicotyledonous Stems

and Roots.

I. Cambium. Secondary Thickening.

Sect. 134. Origin of the Cambium in the Stem. Intermediate Bundles . . . 454

„ 135. Subdivisions of the Secondary Thickening in the Stem .... 458

„ 136. Cambium and young Secondary Growth 461

„ 137. General Arrangement of the Secondary Elements as seen in Transverse

Section 469

„ 138. Their Longitudinal Course 470

„ 139. Cambium and Secondary Thickening of Roots 473

II. The Wood.

I. Distribution and Form of the Zones of Secondary Growth.

» 140 475

2. The Tissues of the Secondary Wood.

„ 141. Synopsis 478

„ 142. Trachea 478

„ 143. Woody Fibres 481

„ 144. Cells . . 483

„ 145. Crystal-sacs. Laticiferous Tubes 487

Critical Note ............ 487

3. Distribution of the Tissues in the Wood.

„ 146. General Considerations 488

a. Medtcllary Rays and Medullary Spots.

„ 147. Medullary Rays 489

„ 148. Medullary Spots 492

TABLE OF CONTENTS.

XV

b. Ligtieous Bundles.

PAGE

Sect. 149. Normal Ligneous Bundles 493

„ 150. Peculiar Forms 498

c. Chattges of the Tisssues in the Annual Ring.

„ 151 500

d. Normal Differences of Successive Zones of Thickening.

„ 152. Innermost Ring. Medullary Sheath 504

„ 153. Successive Increase in the Size of the Elements 505

„ 154. Alburnum and Duramen 507

e. hidividual and Local Deviations.

„ 155. Differences according to the Thickness of the Annual Ring . . . 511

„ 156. Rings with Indistinct Limits 513

„ 157. Individual Variations 514

f. Differe^ices between Non-equivalent Members.

„ 158. Stem, Branches, Roots 515

„ 159. Fleshy Roots 516

III. The Bast.

„ 160. General Considerations 519

„ 161. Forms of Tissue of the Bast 520

„ 162. Parenchyma. Sieve-tubes 521

„ 163. Laticiferous Tubes 525

„ 164. Secretory Passages and Sacs 525

„ 165. Sclerenchyma. Bast-fibres . . • 526

„ 166. Crystal-sacs 529

„ 167. Variations according to successive Zones, Individuals, &c. . . . 530

CHAPTER XV.

Secondary Changes outside the Zone of Thickening.

Sect. 168. Pith 533

„ 169. Parts lying outside the Cambial Zone 535

„ 170. Epidermis 535

„ 171. Cortical Parenchyma. Dilatation. Secondary Sclerosis . . . 536

„ 172. Sieve-tubes, Sclerenchyma, Secretory Sacs 541

„ 173. Phenomena of Disorganisation 543

Periderm.

„ 174. Development and Differentiation of Periderms 544

„ 175. Superficial Periderm 547

„ 176. First Internal Formation of Periderm 551

„ 177. Repeated Internal Periderms. Bark 554

„ 178. Combination of the different Peridermal Formations in Woody Plants . 558

„ 179. Lenticels . , 560

XVI

TABLE OF CONTENTS.

CHAPTER XVI.

Anomalous Thickening in Dicotyledons and Gymnosperms.

PAGE

Sect. i8o. General Observations. Eccentric Stems and Roots .... 567

„ 181. Synopsis of the Anomalies of Thickening 567

„ 182. Anomalous Distribution of Tissues with Normal Cambium. Senega-

root ............. 569

„ 183. Climbing Bignoniaceas 570

„ 184. Phytocrene 575

„ 185. Malpighiaceas, Apocyneae, Asclepiadeas, Celastrus, Tournefortia . . 577

„ 186. Sieve-tubes in the Wood ; Strychnos, Dicella ...... 577

„ 187. Cambium inside the Ligneous Body : Tecoma radicans .... 580

„ 188. Partial Cambiums and Woody Rings of the Sapindaceae .... 581

„ 189. „ „ in the Calycantheae and Melastome« 584

„ 190. „ „ in the Rhizome of Rheum . . 585

„ 191. Successive renewed Zones of Thickening 586

„ 192. Chenopodiaceae, Amarantaceae, Nyctagineae, Mesembryanthema, Tetra-

gonieas 590

„ 193. Anomalous Dilatation of the old Parenchyma. Intercalary Zones of

Thickening in Stems of Lianes 601

„ 194. „ in Roots, Convolvulaceae, &c 606

„ 195. Cycadeae 608

„ 196. Welwitschia . . . . . . . . . . .614

CHAPTER XVII.

Secondary Thickening of the Stem and Roots of Monocotyledons

and Cryptogams.

Sect. 197. Stem of Dracaeneae, Aloinem, and Beaucarnea 618

„ 198. Tubers of Dioscoreaceae 622

„ 199. Roots of Dracaenem 622

„ 200. Stem of Isoetes 623

Index 625

ERRATA.

P. 41, 1. "i,for Scheidei read Schiedei.

P. 4r, 1. %,for aculenta read aculeata.

In description of Fig. IS, for purpurasceus read
purpurascens.

P. 77, 1. I
P. 1 18, 1. I
P. 89, 1. 20
P. 94, 1. 30

P. 95, description of Fig. 38
P. 144, 1. for Conocephalous read Conoce
phalus.

for Sanseviera read Sansevieria.

ifor Patschouli read
Patchouli.

P. 185, 1. *l,for Haucornea read Hancornea.

P. 228, 1. Anselia read Ansellia.

P. 230, 1. 32, for Homalonema read Homalo-
mena.

P. 248, 11. 38 and 4r,yö?- Tladiantha readf\A^-
diantha.

P. 253, 11. 32 and 34,_/ör of read from the.

P. 477, 1. <^,for Red Fir read Spruce.

P. 510, 1. 16, for Drybalanops read Dryoba-
lanops.

INTRODUCTION.

Thk body of the plant is composed of parts with definite position, succession,
structure, and direction of growth. These we call its members when we refer only
to their share in the structure. Investigation teaches us to recognise members of
different, and, in plants of complicated structure, numerous ranks. Roots and leafy
shoots; internodes, leaves, segments, and layers of meristem, masses of cells ; finally,
the single cell, which may again be separated into members.

Each member, of whatever rank, is adapted according to its development to
definite physiological work. It becomes the instrument, or orgati of this work. As
was the case with members, there may also be distinguished organs of different rank —
simpler, and gradually more complicated. According as an organ adapts itself to a
definite function, it attains properties of form and structure, which are definite, and
differ from those of other organs.

The description and explanation of the collective phenomena of form and struc-
ture constitute the task of morphology. According to the two points of view now put
forward, we must distinguish between the morphology of members, and the morpho-
logy of organs. The former deals exclusively with the phenomena and laws, according
to which the organism is compounded of the members of different rank ; the morpho-
logy of organs with the properties of structure and form, by which the members
become organs, and with the distinction of organs of different rank, according to
those properties. Strictly speaking, the morphology of the organ presupposes a
knowledge of that of the member, since the origination of a member must precede
its evolution into an organ. As a matter of fact, a sharp separation of the two
disciplines can hardly be carried out, since both work with the same material, which
extends from the realm of the one to that of the other without any definite break.

The subject of this book is a part of the morphology of the organs of Plants,
which is limited for convenience sake. According to the programme of the Hand-
book, of which it is a part, it should treat of ‘ the anatomy of the Vegetative Organs of
Vascular plants it is occupied therefore only with the Phanerogams, and Pterido-
phyta, i. e. the Fern-like plants in the widest sense of the word. Further, it pre-
supposes such knowledge, gained from other sources, of the outward form of organs
of higher rank (e. g. foliage-shoots, roots, &c.) as can be acquired without anatomical
investigation, and treats only of their internal structure. Lastly, it is limited to the

Sachs, Textbook, second English edition, p. 149.

B

2

INTRODUCTION.

vegetative organs. Having regard to the points of view established above, as well as
to considerations of the available space, the work has further to presuppose a know-
ledge of the morphology of members — otherwise called the general morphology of
plants, and general doctrine of the cell — and only to touch on these subjects as far
as may be necessary.

Since the investigation extends over three great sections of the vegetable king-
dom, it will be our task to describe comparatively those phenomena, in which the
representatives of these sections correspond, or differ : that is, to offer a comparative
anatomy of the vegetative organs.

Under the term vegetative organs we include all those organs of the plant which
are not organs of reproduction, i.e. which do not, sexually or asexually, serve in the
production or direct preparation of germs : the term includes therefore those organs
which undertake the entire work of preservation of the physiological individual, and
which may take different parts in this work.

In the plants in question, which always have members of many grades, those
of every sort and rank are to be found developed into vegetative organs : both those
belonging to the highest ranks, which are externally apparent, such as roots, leafy
shoots, with their internodes and leaves ; and of successively lower ranks, such as
definite groups of cells, and lastly single cells, or the products of their metamorphosis.
But investigation shows that the adaptation to, and participation in vegetative duties,
that is, the development into organs of definite function, and corresponding structure,
is far the most commonly and definitely carried out for members of lower ranks, i.e. for
cells, and groups of cells, or the products of their metamorphosis. It is these which in
the first instance share among them the vegetative work, and assume a correspondingly
characteristic form and special structure. A member of higher rank composed of
them is only a vegetative organ of higher rank, inasmuch as it consists of them. The
structure characteristic of such an organ is determined by the structure and distribu-
tion of the organs of lower rank, which compose it. The vegetative structure in ques-
tion is not universally connected with definite members of higher rank. Equivalent
members, it is true, very often develope into equivalent organs : the functions of the
leaf, assimilation of carbon, transpiration, &c. are, for instance, usually deputed to
leaves : most roots are equivalent to one another in both relations. But, on the other
hand, the converse is not uncommon, that non-equivalent members are equivalent
organs. In many plants besides the leaves the internodes also, which are connected
with them, take part in the functions of the leaf : in others, with ‘ leaf-like ’ stems,
the function and corresponding structure, which the leaves have lost, are transferred
to the stems. Trapanatans has part of the petiole developed into a swimming organ :
in the floating species of Desmanthus internodes of the stem, and in species of
Jussiaea certain roots assume this function, and corresponding structure.

This being the case, the description of the structure of the vegetative organs
must start from the consideration of simpler forms, i.e. the cells. Since the investi-
gation of these separately is necessarily followed by that of their connection with
others, and of their arrangement into tissues of various grades, the structure of the
organs composed of these gradually becomes apparent.

Those cells or their derivatives, which have the character of definite vegetative
organs, seldom occur, in the plants in question, singly between dissimilar ones ;

INTRODUCTION.

3

usually similar cells are connected so as to form large groups or masses. An aggre-
gation of cells growing in common is termed generally a Tissue (Tela, contextus; in
compound words Each tissue, which is characterised by definite properties,

and distinguished from others, is called a tissue-form or, better, a sort of tissue. For
the single cells which belong to a tissue, or for each elemental form derived from
such a cell, the term iissue-eletnent may be reserved. Tissue-elements which occur
singly between dissimilar ones (Idioblasts, after Sachs’ terminology) usually cor-
respond in their properties with others which occur in connection with similar
elements. They are then to be reckoned as of the same sort of tissue as the latter.
In like manner, finally, such tissue-elements as occur only as Idioblasts (for instance
many laticiferous tubes) will form together with one another a special sort of tissue.
All tissue-elements, which correspond in definite similar properties, are therefore
termed collectively a sort of tissue, whether they be Idioblasts, or are connected with
like elements.

The course of description which is followed in this book may be gathered from
what has been said. The first subject is the characterising and distinguishing of the
sorts of tissue, which serve as vegetative organs; then the grouping and arrange-
ment of these in building up the members or organs of higher rank. In this course
of description a difficulty certainly arises : this can only be overcome by the establish-
ment of a limit, which is to a certain extent artificial. Those tissues, which act
functionally as vegetative organs, are often continuous in the plants in question with
those higher members, which are according to their most important adaptation
reproductive organs. The member of many Ferns which acts functionally as a
Prothallium is principally composed of chlorophyll-containing Parenchyma, similar
to that of foliage leaves. This sort of tissue, together with vessels, vascular bundles,
&c., takes part in the construction of the parts of the flower of many , Phanero-
gams, &c. Many peculiarities of vegetative tissue, which appear in these parts, depend
much less, it is true, on the properties of the single tissue-elements, than on their
arrangement. Since these peculiarities are directly connected with adaptation to the
generative process, the study of them should also be in connection with it, and must
be excluded from this treatise. In cases of sharply-defined phases of development,
the boundary, which must needs be drawn, is evident at first sight. For instance, no
one will expect the Fern-Prothallium to be treated of here. But among the Pha-
nerogams there often occurs a gradual transition between purely vegetative and
reproductive organs. To satisfy in the present case the necessity of a definite limit
to the subject in question, all that falls under the definition of flowers, specialised
inflorescences, or parts of inflorescences, will be excluded from consideration.

As has been already intimated, the differentiation of the sorts of tissue is a
phenomenon which accompanies the development of a part to maturity. Originally
the cells of a part differ, it is true, in certain relations, both in form and direction of
division ; but they correspond in structure, and in the fact that, while they increase
slowly in size, they divide repeatedly ; and thus finally produce cells, which develope
into tissue-elements. From those phenomena of division, such masses of cells are
termed Meristem ; and when they form the first foundation of a member primary

' Compare Unger, Anatomie, p. 138; Sachs, Textbook, English edition, p. 70.

B 2

4

INTRODUCTION.

meriste7n'^. In structure the cells of meristem are characterised by having a delicate'
homogeneous membrane (only in certain exceptional cases thickened and with
flattened pits), and homogeneous, finely granular protoplasm with a nucleus, but
with no further recognisable structural elements. By reason of their constant
division, they are throughout in uninterrupted connection with one another.

In each system of the meristem the divisions pass through a definite number,;©!^
stages, till they gradually cease. According as this happens, the cells, first formed as
members of the meristem, assume those properties, by which the further sorts of tissue
are distinguished : great increase in volume takes place, and changes of structure and
form ; while the latter may result in a partial loosening of the original uninterrupted
connection, i.e. in the appearance of intercellular spaces.

As compared with the meristem, the tissue elements derived from it attain a
great constancy both of form and structure. They have accordingly been termed
permanent tissue, fixed tissue, or mature tissue. If the idea of tissue be understood in
the general sense stated above, the meristem also is naturally included : we distinguish
then on the one hand meristem, or formative tissue, on the other permanent tissue, as
the two main categories of tissue. Merely for shortness of expression, however, the
term tissue will be used also for permanent tissue in opposition to meristem. In this
sense, and according to the previous explanation, the following pages will deal with
the vegetative tissues, which serve as vegetative organs.

Comparative investigation shows that the cells of the meristem universally cor-
respond very closely in the general character of their structure, and the same may be
said of the main phenomena of the vegetative process in the plants we are engaged
with. Answering to this correspondence of origin and functional adaptation, the
forms of tissue in the whole group on which we are engaged correspond in their
main properties, notwithstanding frequent modifications to suit special cases, and the
same few tissue-forms everywhere occur.

Tissue-elements of every sort are derived from the cells of the meristem, each
has originally the properties of a cell. Accompanying definite development there first
appears the fundamental difference, that certain elements retam during their life the
structure and all the characteristic properties of typical cells, others lose the cell-nature.
The former are composed of a completely closed cell-membrane and active proto-
plasm, with a nucleus and cell-contents ; they retain the power of independent growth,
and remain capable of division ; in consequence of this property a meristem may
again arise from them, and this, in antithesis to the original, is distinguished as
secondary meristem (Nageli, 1. c.). The latter lose with their development the power of
division, and of independent growth ; usually they cease entirely to grow, but in many
cases a real lasting growth of such elements occurs, resulting from their nourishment
by adjoining cells. In their structure, the loss of the cell-nature is indicated either
by the complete disappearance of the protoplasmic body, its place being filled by
other bodies, usually air or fluids ; or by its suffering characteristic changes, which
vary according to the individual cases. The latter observation is made with special

' Nägeli, Beiträge, I. p. 2. — Schleiden (Grundz. 3 Aufl. I. p. 253) and Karsten (Veg. Org. d.
Palmen) include these masses under the wide term Cambium-, Unger (Anat. und Physiol, p. 180)
calls them formative cells: Schacht (Pflanzenzelle, p. 165) primary parenchyma.

INTRODUCTION.

5

reference to the sieve-tubes ; it remains doubtful whether the contents of these are
protoplasm or not. The cell-walls of the elements in question are wholly or mostly
retained.

According to the differences already mentioned, which will be further followed
out in the subsequent special observations, the tissues divide themselves into those
which consist permanently of cells (cellulse), and those whose elements are descend-
ants, derivatives, or products of change or metamorphosis of cells. According to their
form and other properties these are termed tubes (Tubi, Tubuli), sacs (Utriculi),
fibres (Fibrse), and are distinguished from cells.

Most tissue-elements, of whatever sort, are formed directly and quickly by the
metamorphosis of meristematic cells. Exceptions from this rule only occur in certain
cases when cells, after they have acted as such for a long time — even for years — may
secondarily pass over to another tissue-form. This takes place in the secondary
development of Sclerenchyma, which will be described in chapters II. and XV.

From this secondary metamorphosis of tissue we must distinguish death, and the
changes eventually connected with it, which appear in certain other cases in the
tissues, such as the dying off of old hairs, cork-cells, cells of the pith of many plants,
of the elements of bark, and of the old wood of Dicotyledons, &c. ; appearances
which can usually be distinguished with ease from metamorphosis of tissue by the
commencement of rotting, weathering, &c.

In accordance with the preceding considerations, the distinction of the forms of
tissue which act as vegetative organs, and the classification of the study of them,
must in the first place be founded upon their structure, that is, on the structure of
the single tissue-elements, and the connection of these with one another— whether
they be connected with like or with unlike elements. It is obvious that, in organized
bodies, certain peculiarities and varieties of structure are connected with certain
phenomena of development. And it is not less obvious that varieties of structure
are also as a rule correlated with eertain varieties of form of single tissue-elements.
But experience teaches that between form and structure a constant relation does not
exist, or at least not universally ; and that, contrary to the older classifications, which
regarded in the first place the form of the elements, this is of only secondary im-
portance in the distinction of the tissues.

According to the principles now laid down, the following main forms of
vegetative tissue may first be distinguished : —

I. Cellular tissue, i. e. that which consists of permanent typical cells ; with the
main subdivisions, epidermis, cork, parenchyma. II. Sclerenchyma. III. Secretory
structures. IV. Vessels. V. Sieve-tubes. VI. Milk-tubes. Separate notice of the
Intercellular spaces may, with advantage, though perhaps not necessarily, be appended
to the study of the tissues.

The study merely of the divisions and subdivisions of tissue-form is at all
points closely connected with the arrangement of these into combinations of
different rank — so as to form vegetative organs of successively higher rank — so far
so, that the two modes of study can never be completely separated the one from
the other.

Aceording to their form the combinations of any rank may be distinguished as
Layers, Bundles, Masses {Groups, Nests) — terms, the meaning of which is obvious from

6

INTRODUCTION.

the ordinary use of the words : a sharper definition of them is here neither necessary
nor possible. When a layer (simple or compound) surrounds a tissue, which differs
from it, it is termed relatively to the latter a sheath. Tissues of whatever sort or rank
may be mutually continuous for long distances, or throughout the whole plant. When
this is the case, they are said collectively to form a System. A system of any rank
may be composed of systems of lower rank, or may join with others to form a
system of higher rank. For instance, the Vascular system is composed in most
plants of that of sieve-tubes, and that of vessels ; each of the latter is a system of
itself ; the two combine as a rule to form the above-named system of higher rank ;
while often the system of Sclerenchyma-fibres joins them as a third constituent of the
joint system.

According to the point of view from which one starts, one may therefore
distinguish systems in the most various sense, as will appear in the later chapters ; for
instance, in the Dicotyledonous stem we may with equal right distinguish a Vascular
and an Epidermal system, or a Woody and Cortical system, whereas the latter includes,
besides the Epidermal tissue, a part of the Vascular system, and other tissues besides.

Sachs (Textbook, Eng. ed. 1882, p. 79, etc.), in the exposition of the anatomy of the
higher plants, starts from the definition of three systems of tissue, which he terms Dermal,
Fascicular, and Fundamental tissue. Under the first term he includes those tissue-forms,
which limit externally such plants as have their cells aggregated in three dimensions
of space, as a matter of fact. Epidermis and Periderm (cf. § 2, 23 and chap. xv).
His Fascicular tissue corresponds in the main to the previously mentioned Vascular
system (chap. viii). The name Fundamental tissue includes what remains after the
separation of the other two. However much this distinction may be fitted to guide
beginners, still, in my opinion, it does not answer its purpose, which is to serve as a basis
for a uniform exposition of the various differentiation of plant-tissues. For the names
Dermal and Fascicular tissue indicate in Vascular plants systems of tissue, which are
positively characterised by definite tissue-forms : but the name F undamental tissue im-
plies the remainder, and this may just as mueh consist of different positively characterised
tissue-forms, and tissue-systems, which are equivalent to the Dermal and Fascicular
systems. But if it is necessary, in the description of the Dermal and Fascicular systems,
to make use of a short general term for the tissues over and above these, the terms
Fundamental tissue, or 'Fundamental mass, or Intercalary mass, are very suitable ; just
as in Nägeli’s treatise on the vascular bundles, or fibro-vascular masses, was his distinc-
tion of these from the rest (‘Proten’), or as was Schwendener’s general term for the
parts of the vascular bundle, which, in his exposition of the meehanical adaptation, did
not bear upon his point. And indeed in describing a form or system of whatever rank,
some such method must always be used. I think, however, that the distinction of the
forms of tissue must first serve as a foundation for the uniform exposition of the subject
which now engages us, and for the choice of terms; then only should follow the investi-
gation, how far these forms of tissue take part in the formation of combinations, and
systems of higher rank.

Although on the one hand the course of exposition above brought forward has
a definite justification in all cases, and, on grounds which need not be repeated, will
be in the present case pursued, and although further the distinction of single forms of
tissue must be carried out under all circumstances with regard to the structure of the
elements only, still the question arises, on the other hand, if the distinction of
definite systems could not be drawn more naturally on other than the purely histo-
logical grounds above stated ; that is, on such as are derived from the morphology

INTRODUCTION.

7

of differentiation (i.e. the history of development). Investigation has taught that the
original meristem, from its first (embryonic) beginning onwards, maintains a well de-
fined differentiation into different — but always meristematic — layers, or segments; and
that, in the plants on which we are engaged, in many cases definite sharply marked
masses of tissue (I may mention provisionally the axile vascular bundles of many
roots) are derived from certain of these layers, others from other layers, just as the
different tissue-systems of the animal body are derived from the different germinal
layers. The question may therefore arise whether, over and above the distinction of
forms of tissue according to definite structure, the exposition of the tissue-systems,
and of the construction of the members of the highest rank from them, must not refer
to that differentiation of meristem, and take it as the foundation. The determination
on this point will depend upon the answer of the further question, whether the origin
of each, or of single tissues or tissue-systems, from one and the same portion of the
primary meristem, can or cannot be generally proved. For if the latter is the case, if
parts of the same tissue-system originate from unlike parts of the meristem, the
course of exposition which we are considering must be put aside as impracticable.

In order to gain clearness on this question, a survey of the original modes of
differentiation of meristems is necessary; and though this lies beyond the strict limits
of the subject of this book, still it may with the more reason be here inserted, since
in the following chapters reference must constantly be made to that differentiation h

I. As Hanstein ^ has shown, the young embryo of the Angiospermous Phanerogams
separates, while still consisting of few cells, all of which are meristematic, into three
layers, or groups of cells, which differ in their arrangement and direction of division ;
these were termed by their discoverer, Dermatogen, Periblem, and Plerome ^ At
the end of the root there is besides these a fourth, the origin of which in the embryo
remains to be further investigated. This was called by Janczewski ^ the Calyptrogen
layer : we shall return to this later. The dermatogen layer is separated by a single
tangential division of the few cells, which form the original rudiment of the
embryo, as a simple peripheral layer of cells. It remains a simple layer of cells, since
all the subsequent meristematic divisions which occur in it take place only by walls at
right angles to the surface. It is only at the future apex of the root that other pheno-
mena appear, which need not be more fully discussed at present. Further divisions
of the cells enclosed by the dermatogen separate an axile longitudinal cylinder, the
plerome, from the periblem, which is a zone of tissue lying between the plerome and
dermatogen. The plerome consists of cells in which longitudinal divisions pre-
ponderate, and which have a corresponding arrangement; the periblem of cells
distinguished from the former by more frequent and irregular transverse divisions.

' [See further Sachs, Ueber die Anordnung d. Zellen in d. jüngsten Pflanzentheilen, Arb. d. Bot.
Inst, in Würzburg, Bd. II; also, Ueber Zellanordnung u. Wachsthum, ibid. — Haberlandt, Scheitelzell-
wachsthum b. d. Phanerogamen, Botan. Zeitg. 1882, p. 343. — Nägeli, Scheitelwachsthum d. Phanero-
gamen, Naturforscher Vers, zu München, 1877.]

^ Hanstein, Botan. Abhandl. I. On the details of the origination of the embryo, and its great
differences in those Di- and Monocotyledons which have been investigated, the reader must be
referred to this treatise and to Sachs’ Text-book, English edition, 1882, pp. 585-593. Further,
Fleischer in Flora, 1874, p. 369, &c. ; Hegelmaier, Bot. Zeitg. 1874, p. 631, &c.

^ Hanstein, Die Scheitelzellgruppe im Vegetationspunkt der Phanerogamen, Bonn, 1868.

* Ann. Sei. Nat. 5 serie, tom. XX.

8

INTRODUCTION.

This differentiation of the meristem is retained at the punctim vegetationis of the
first stem and of the tap root ; further it appears at the puncium vegetationis of all
lateral stems and secondary roots. The differentiation certainly varies greatly in
distinctness in different cases, and is complicated in roots by the presence of the
Calyptrogen -layer. It corresponds exactly to the scheme given above in the apex of

the stem of many thin-stemmed water plants,
as Elodea, Hippuris, &.c. ; in these Sanio ^
first discovered the appearances in question,
which were afterwards more fully worked out
by Hanstein The rounded conical apex of
the punctum vegetationis (Fig. i)

is covered by a single layer of dermatogen {e).
Then, passing inwards, follow usually five
regularly concentric layers of isodiametric

cells, these constitute the periblem which

surrounds the plerome-cylinder {p-p). The
fig. I. -(225) Hippuris migaris. Median longritudinai latter has a bluntly conical apex, where it is

section through the apex of quite a young leafy shoot, ^ ^

which had just begun to deveiope the second whorl of often terminated by 3. single Cell, whilc below

leaves. In older shoots the apex is much more elongated. ^ ^ d o

cf. Sachs, Textbook, Engl. ed. 18S2. p. 154. For further it widens out. In most Other cases the num-

explanation see Text

ber of periblem-layers is smaller, only 2-3.

The three zones of meristem, though they remain distinct, equally take part in
the acropetal growth in length of the end of the stem. Each is continually renewed
by divisions in the group of cells (or single cell) which forms their apical portion,
while as the tissues are removed from the apex the transition from meristem
to the definitive tissue takes place. Each is continuous downwards into definite
tissues or tissue- systems, which will be mentioned hereafter. That apical group of
cells, or (as in the plerome of Hippuris) single cell, which renews the layer and
which hence always introduces the further cell-divisions in it, is called the Initial cell
or Initial group of the layer.

In all Angiospermous plants the dermatogen layer is marked off with similar
sharpness from the tissues below it, and is distinguished by its division walls being
arranged only at right angles to the surface ; with this restriction they run in all
directions. But the separation of plerome and periblem does not appear in all
cases so sharply marked as in the foregoing instance. Especially where the apex
of the stem is broad and flat, it must often be left undecided whether both do not
originate from a single common initial group, and are first clearly separated at some
distance from the apex, on their gradual transition to definite systems of tissue.

The origination of the normal lateral branches of the end of the stem (i. e. of
the leaves and lateral shoots) as emergences of the surface begins beneath the
apex by the outgrowth of definite groups of meristem, which were not previously
to be distinguished by any special character. These are the initial groups of the
emergence. As a matter of fact, both elements of the dermatogen-layer and of the
periblem lying under it take part in the origination, the growth and divisions of both
proceeding simultaneously (comp. Fig. 1) ; but the cells belonging to the dermatogen

' Bot. Zeitg. 1864, p. 223.

^ [Kny, Botan. Zeitg. 1878, p. 760.]

INTRODUCTION.

9

during all their further growth are divided only perpendicularly to the surface, so
that the dermatogen-layer is continuous as a single layer also over the branches.
The plerome-cylinder of the mother shoot, as far as investigation extends, takes no
part in the origination of a branch. In the lateral shoots thus founded the separa-
tion of the meristematic mass, covered by dermatogen, into periblem and plerome
first appears after some time. Both in this case originate from a common initial
groupTxvhtciris derived from the periblem of the mother shoot.

In the punctum vegetatiojiis of the root of the Angiosperms ^ the same differen-
tiation of the meristem often appears, as in the stem, but sometimes much more
definitely. It should be described in the same terms as the latter, so far as the
correspondence is exact. To the meristem, from which springs the body of the
root, is added in all roots the conical cap, made up of layers of cells, which is known
as the Root-cap (calyptra). This covers the meristematic punctum vegetatmiis, and
is increased by it, according as the cells on its outer surface die off. Since this
accession originates in certain cases from a special layer of meristem, the latter is,
according to Janczewski, to be distinguished as the calyptrogen. As has already been
said above, we cannot here discuss what
genetic relation this bears in the main root
of the embryo to the first meristem-cells
of the hypocotyledonary stem. We must
also take no notice here of the origination
of the layers of meristem of lateral roots,
our knowledge of which is for the most part
due to Janczewski.

At the active punctum vegetationis of
the roots of the Angiosperms, which has
already begun to grow in length, four dif-
ferent cases of differentiation, which vary
according to the species or groups, have
been made known to us by Janczewski :

1. The meristem at the apex is diffe-
rentiated into four sharply marked off layers :
plerome-cylinder, periblem, dermatogen, and
lastly the calyptrogen-layer, which covers the
latter, and which soon disappears owing to
the short duration of its activity. This diffe-
rentiation is found only in two Monocotyledonous water-plants, viz. Hydrocharis and
Pistia stratiotes^ (Fig- 2).

2. Sharply defined plerome-cylinder, and calyptrogen-layer. Between the two, at
the apex of the punctwn vegetationis, is an initial group only one layer of cells thick,
which splits immediately behind the apex into periblem and dermatogen (i. e. cortex
and epidermis). This is the case in most of the Monocotyledons which have been

FlC. 2. — (450) Pistia stratiotes. Median longitudinal sec-
tion through a young lateral root, h Root-cap, including
calyptrogen-layer; e dermatogen; pc outer layer of the
plerome-cylinder (pericambium) ; v foundation of an axile
vessel in this. Between e and p c the periblem, which consists
at the apex of only a single layer.

^ [See further Holle, Botan. Zeitg. 1876, p. 241; 1877, p. 537. — Eriksson, Botan. Zeitg. 1876,
p. 641. — Schwendener, Scheitelwachsthum d. Phanerogamen- wurzeln, K. Acad. Wiss. Berlin. 1882;
Botan. Zeitg. 1882, p. 687. — Flahault, Ann. Sei. ’Nat. ser. 6, Bot. tom. VI. pp. 1-229.]

“ [Kubin, Hanstein’s Abhandl. Bd. III. Heft 4, 1878.]

10

INTRODUCTION.

investigated; e. g. species of Allium, and Canna, Hordeum vulgare, Triticura vulgare,
Zea Mais (Fig. 3), Stratiotes aloides, Alisma Plantago, Acorus Calamus, (Janczewski).

Treub^, as the result of his extended researches, ascribes this differentiation to Jun-
caceae, Haemodoraceae, Cannaceae, Zingiberaceae, Typha, Cyperaceae, Gramineas, Com-
melyneae, Potameae, Juncagineae, Sagittaria, Limnocharis, Stratiotes. But he differs
from Janczewski with regard to Allium, Acorus, and Alisma, since he does not allow the
presence of a calyptrogen-layer in the families Liliaceas, Astelieae, Xerotideae, Aspidis-
trejE, Ophiopogoneae, Amaryllidaceae, Hypoxideae, Dioscoreae, Taccaceae, Bromeliaceae,
Musaceae, Orchideae, Palmae, Pandaneae, Cyclantheae, Aroideae except Pistia, further in

FIG. 3. — Median longitudinal section through tlie apex of the root of Zea Mats, from Sachs’ Textbook, a— a outer,
i inner layers of the root-cap ; s calyptrogen layer ; m gf/ plerome ; g rudiment of a vessel ; x, r — r periblem, or the
cortex which has developed from it ; e Epidermis, or dermatogen layer (■z/=the thickened outer wall of its cells). Above
the apex of the plerome cylinder, easily seen between in and tlie dermatogen and periblem layers are reduced to two
initial layers, which occupy the depressed centre. According to Janczewski the initial group should consist of a single layer.

the Iridaceae, Pontederiaceae, Sparganium, Butomus, and doubtfully in Alisma. He finds
rather, covering the sharply defined apex of the plerome, a group of common initial cells
two layers thick, from which originate root-cap, dermatogen, and periblem. Hence, the
last named families should represent a special type, differing to a certain extent from
those first named.

* M. Treub, Le meristeme primitif de la racine dans les monocotyledones, Leiden, 1S76.

INTRODUCTION.

II

3. (Fig. 4), Plerome-cylinder and periblem sharply defined, the latter overlying
the apex of the plerome, and covered by a common initial layer for dermatogen
and root-cap. The divisions of the initial layer, which are parallel to the surface of
the bluntly conical apex, add on the one hand new cells to the root-cap above the
apex, and on the other hand renew the initial layer itself. As the distance increases
from the apex of the periblem, which by its growth in length is constantly advancing,
the divisions become rarer, and at last cease. The last of these separates the
initial cell into two, one of which is added to the root-cap, the other to the
dermatogen as a permanent member of it. We may therefore say with Janczewski
that root-cap and dermatogen arise in this case from the calyptrogen-layer. The
cells of the dermatogen and root-cap, which owe their origin to the division just
described, divide further by walls perpendicular to the surface ; from each therefore
is produced a section of a layer
consisting of several or many
cells. In the root-cap each of
these sections is so arranged
relatively to the similar ones la-
terally next it, and to others
which have arisen above the
apex, as to form a conical hood
one layer of cells thick ; and the
whole root-cap is built up of such
hoods fitting one into another.

The cells of the sections of the
dermatogen undergo extension
perpendicularly to the surface,
in such a measure that each sec-
tion remains for a time consider-
ably less extended in that direc-
tion than its predecessor, which
is farther from the apex. The
surface of the dermatogen layer
therefore becomes narrower, as
the apex is approached, by steps
which occur at definite distances from it. Each step is covered by that section of a
layer of root-cap which originally corresponded to it, while the edge of the latter
abuts on the next lower, that is, the next older step.

A not unimportant variety occurs, according to Janczewski, within the type of
differentiation in question. In the majority of plants which have been investigated,
the periblem consists at its apex of a single initial cell (Fig. 4) or of two such,
which lie side by side in one layer ; it is below the apex that it first increases to
several layers. But in one case, namely, Linum usitatissimum, the apex of the
periblem consists of two initial layers. One of these, the inner or lower of the two,
behaves as in the first case just described. The other, the outer one, belongs to a
layer of cells, which clothes the whole periblem. This, like the dermatogen, divides
only perpendicular to the surface, and therefore always remains a single layer.

Fig. 4. — (210) Polyf^omim Fagopyriiin. Apex of root in median lonfjitudinal
section, pericambiurn, the outer boundary of the plerome-cylinder; ^ der-
matogen or epidermis, between /c and e periblem, h root-cap.

12

INTRODUCTION.

To this type belong the majority of the Dicotyledons. Helianthus annuus, Fago-
pyrum, Raphanus sativus, Myriophyllum, species of Salix, Casuarina stricta, Linum
usitatissimum, and Primulacete ^ have been carefully investigated.

4. The fourth Angiospermous type (Fig. 5) is observed in those of the
Cucurbitacese (Cucurbita) and Papilionaceae (Pisum, Phaseolus, Cicer) which have
been investigated.' Here a common initial zone extends transversely over the
punchmi vegelationis ] by the divisions in this arise, on the side towards the

Fig. 5. — (210) Pisum sativum. Median longitudinal section through the apex of the root, after Janczewski. p—p plerome,
p — r periblem, y common transverse initial zone, c its lateral continuation,

root-cap, successive layers, which are added to the conical middle portion of the
latter. On the side of it facing the body of the root arise a massive plerome-cylinder,
and a periblem many layers of cells thick, having approximately the form of a hollow
cylinder open towards the initial layer. From its margin the transverse initial zone
of meristem curves itself round, so to speak, over the adjoining outer surface of
the periblem, and acts here for a further space as initial layer, on the one hand

^ Kamienski, Zur vergl. Anatomie d. Primeln, Strassburg, 1875.

INTRODUCTION.

13

for the peripheral part of the root-cap covering the sides of the apex of the root,
on the other for the dermatogen-layer of the root. The origin of these, i. e. the
lateral part of the root-cap, and of the dermatogen, takes place similarly to that of
the corresponding parts in the third type.

II. In the Gymnosperms the differentiation of the meristem at the pimcium
vegetationis of the root^ is essentially different from the types described for the
Angiosperms (Fig. 6). A plerome-cylinder with sharp contour occupies the centre
(/-/)). The longitudinal rows of cells which compose this converge at the rounded
apex towards a small initial group of cells. The plerome is surrounded by a mantle of
periblem consisting of many (e.g.
in Thuja occidentalis of 12-14)
concentric layers arranged with
considerable regularity. Each one
of the innerlayers(inThuja,8-io)
of this mantle has its initial group
above the apex of the plerome.

The division of these cells per-
pendicularly to the surface (i.e. ra-
dially) brings about the increase of
the surface-elements of the layer.

At the same time successive divi-
sions parallel to the surface, that
is, a doubling of the layers, takes
place in the apical region (Fig.

6, {). Since the radial walls of
the successive layers fit almost
exactly one on another, the cells
of the periblem mantle are ar-
ranged above the apex in cor-
respondingly regular rows. As
the layers are pushed outwards
above the apex by their succes-
sive doubling, division ceases in
them, and increase of volume of
the cells takes place; those which
happen to be outermost at the
apex become gradually loosened,
and pushed off as a root-cap. Here then it is not possible to distinguish a layer of
calyptrogen or of dermatogen ; the outermost periblem acts as root-cap covering the
meristematic apex. The radially arranged apical prolongation of the periblem is in
all cases relatively strongly developed, its height is usually equal to or greater than the
whole diameter of the root, rarely (Taxus, Cycas circinalis) it is smaller. According
as it is more strongly developed, the arrangement of the cells in rows is more clearly
apparent ; e. g. Pinus, Ephedra, Zamia integrifolia.

Fig. 6. — (190) yimipei-ns Oxycedrzts. Median longitudinal section through
the apex of a lateral root, p—p plerome, surrounded by about sixteen layers
of periblem, the outermost of which represent the root-cap ; i the initial region
for periblem and plerome.

* Strasburger, Die Coniferen, &c., p. 340. — Reinke, Morphclcg. Abhandl. p. i.-r-Janczewski, l.c.

14

INTRODUCTION.

The differentiation of the meristematic apex of the stem of the Gymnosperms '
shows a varying character, which couples it on the one hand with that which
appears in the typical Angiosperms, on the other hand with that in the Lycopo-
diacese, while in Araucaria brasiliensis, also in A. Cunninghami, Dammara, and
Cunninghamia, the dermatogen, periblem, and plerome remain clearly distinguished,
in the extreme apex ; in the Abietineae and Cycas these layers run together into
a common initial group, which occupies the extreme apex ; a separation into the
three layers first appears at some distance beneath this in Cycas, and more clearly in
the Abietineae. Ephedla is specially interesting, for here, in the same species
(E. campylopoda) and apparently fluctuating in the same shoot, the character varies
between the two extremes described. At one time there is a dermatogen-layer,
sharply defined throughout its whole course, covering the two inner layers which
in the extreme apex are more or less clearly separate ; in other cases both merge
with the dermatogen into a common superficial initial group. A series of similar
cases, some corresponding to Araucaria, others approaching the other extreme, were
made known by Strasburger’s researches on Taxus, Podocarpus, Saxegothea,
Ginkgo, Thuja, Cupressus, Sequoja, and Cryptomeria.

Here, as in the Angiosperms, the dermatogen and periblem alone take part in
the origination of leaves and normal lateral shoots, and in most cases also in the
same fashion as there. In the Abietineae, however, there occur also divisions parallel
to the surface in the dermatogen of the young leaf.

III. As already intimated, the differentiation of the meristematic apex of the
LycopodiacecE ^ corresponds closely with that of the Gymnosperiris. The extreme
apex of the stem is occupied by a group consisting of 2-4 prismatic cells with their
longer axis at right angles to the surface. This is the initial group for the periblem
and dermatogen, or rather for a superficial layer which corresponds to the latter.
All these initial cells divide by walls perpendicular to the surface, and the products of
this division, as they are removed from the apex by the advance due to the growth
of the latter in length, divide again parallel to the surface, forming thus the initial
layers of dermatogen and periblem. A plerome-cylinder, which is limited laterally by
the periblem, elongates independently through the activity of an initial group or single
cell of its own, which occupies the centre of its conically tapered apex, and lies imme-
diately beneath the initial group of the outer layers. As Hegelmaier has already as-
serted, conditions are to be found, which point to the origin of the initial cells of the
plerome from the common initial group at the surface of the apex (by transverse divi-
sion). It is thus possible that a fluctuation of the definition of layers occurs here simi-
lar to that in the Coniferse. The formation of the leaves starts in the Lycopodiacese
from one cell of the outermost (dermatogen) layer, which, after arching outwards,
divides first parallel to the surface, then further. The differentiation of the meristem-
atic apex of the root is, according to the investigations of Strasburger and of Bruch-
man, similar in the Lycopodiacese to that in the first type of the Angiospermous roots.

' Strasburger, l.c. p. 323, Taf. 22, 23, 25. — Pfitzer, in Pringsheim’s Jahrb. VIII. p. 56.— [Dingier,
Botan. Zeitg. 1882, p. 795.]

^ Cramer, PHanzenphysiol. Unters. Heft III. p. 10.— Strasburger, Coniferen, p. 336. — Hegel-
maier, Bot. Zeitg. 1872, p. 798 ff., 1874, p. 773. — Bruchmann, Ueber Wurzeln von Lycopodium mid
Isoetes, Jena, 1874. — Compare also Russow, Vergl. Unters, p. 176.

INTRODUCTION.

15

The investigations of Bruchman assign to the root of Isoetes a conformity with
the third type of Angiospermous roots. According to the results of this observer,
which harmonize with those of Hegelmaier ^ with the exception of one not very im-
portant difference, the apex of the stem of the Isoeteae is occupied by a small group
of initial cells, which are common to the whole meristem. Longitudinal divisions of
these form the mother cells of the peripheral layers of meristem, and renew the
initial cells ; transverse divisions of them supply new elements to the central part
of the meristem. A division into the three distinct layers cannot be seen.

Very similar to the structure of the apex of the stem of Isoetes is the
differentiation of the meristem peculiar to some Selaginellas, and the roots of Marattia.
It should therefore be mentioned here, but for the sake of shortness the description
of it will be supplied further on

IV. The stem of Isoetes and the above-named Selaginellas and Marattiacese
form the transition between the forms of differentiation of the meristem already
described, and that which prevails for the great majority of the Pteridophyta (comp.
Fig. 7-9). The characteristic in these cases is this, that the entire meristem of the
apex originates from one single common initial cell, which is called, from its position
at the apex of stem and root, the apical celP. Successive biparlitions divide the
apical cell in each case into an apical part, which retains the original position and
form, this being compensated again by growth, and remains as the apical cell ; and
a basal inferior part, which is added to the growing meristem. The latter part is
termed the segment-cell Further divisions of the segment-cell form the meristem
and later tissues. Each portion of meristem, which originates from a single segment-
cell, is called a segment. In roots, besides these processes, there is in addition the
formation of root-cap, which also originates from the apical cell ; this must now be
provisionally ignored.

The apical cell (Fig. 7-9) has in most of the present cases the form of a three-
sided pyramid, with convex base, which is the apical surface (i.e. the outer wall) ; while
the sides are sunk in the meristem. This is the case in all roots of the plants in ques-
tion (except those of the Selaginellas, in which the form of the apical cell is doubtful),
and in the majority of the apices of stems. In other cases the apical cell has the form
of a two-edged wedge, the arched base and the point having otherwise the same ar-
rangement relative to the other tissues as in the cases with the three-sided cell; apex of
the stem of Salvinia, Azolla, many Selaginellas (S. Martensii Kraussiana), and Poly-
podiacese (Pteris aquilina). Polypodium rupestre. Lingua, aureum, punctatum, phyma-
todes, Platycerium alcicorne, stolons of Nephrolepis undulata according to Hofmeister®.

In the stolons of the last named species, as the apex becomes stronger, the
apical cell assumes the three-sided pyramidal form. In Polypodium vulgare it alter-
nates between the two (Hofmeister).

In the seedling of Selaginella Martensii the apical cell of the main shoot,

* Bot. Zeitg. 1874, p. 481. [Holle, K. Ges. d. Wiss. zu Gott. 8 Jan. 1876.]

^ Nägeli, Zeitschr. f. wiss. Bot. II. p. 121 (1845), III. p. 157.

* Pringsheim, Jahrb. f. wiss. Bot. III. p. 491.

° [M. Treub, Recherches sur les Organes de la Veg. du Selaginella Martensii, Leide, 1877.]

® Hofmeister, Beitr. zur Kenntniss der Gefässkryptogamen H. Abbandl. d. k. Sachs. Gesellsch.
d. Wissensch. Bd. V.

I NT ROD UCTION.

i6

and of the two branches of the first bifurcation, has, by reason of corresponding
divisions, the form of a four-sided pyramid, which however soon passes back to the
two-edged form

Each segment is separated from the apical cell as a tabular cell by a division
wall, which is approximately parallel to one of the sides of the apical cell. This wall is
called the principal wall of the segment Each segment has two principal walls,
one (acroscopic) by which it was separated from the apical cell, the other (basiscopic)
that which adjoins an older segment. Its outer wall is that part of the free wall
of the apical cell which is cut off by the line of junction of the acroscopic principal
wall ; its lateral walls are the parts cut off by the lines of junction of the same
principal wall from the principal walls of the segments, which border it laterally.

The principal walls, which cut off the successive segments from the apical cell,
are parallel in regular succession to the sides or principal walls of the latter. Thus in
the case of a two-sided apical cell they oppose the one and the other side of it alter-
nately, each fronting the older principal wall ; in the case of a three-edged apical
cell, they oppose the three sides successively in spiral sequence, each facing the third
oldest principal wall, and being attached laterally to the two younger principal Myalls.
All the segments therefore of a meristematic apex are arranged (if we ignore sub-
sequent displacement) in as many straight rows parallel to the axis as the apical
cell has sides.

The principal walls of a segment, which has recently been cut off, are inclined
to the theoretically straight perpendicular axis of the meristematic apex at an acute
angle, which varies according to the form of the apical cell. As growth proceeds,
the form of the segment alters, and with it the direction of the principal walls (or
rather the planes in which these lie), so that, with reference to a perpendicular axis,
they assume a horizontal position. Eor a thorough discussion of these phenomena,
and of the growth of the apical cell itself, cf. Nageli and Leitgeb, 1. c. p. 91.
The figure 7 A, which is borrowed from these authors, may present the process
to the eye.

The segments cut off from the apical cell become gradually divided up into
masses of meristem consisting of several or many cells. As the result of the changes
of form and position already mentioned as accompanying the joint growth, each
of these masses represents part of a more and more horizontal disc, and meets
the similarly shaped segments next in age to it at the central line. A transverse
section cut at some distance from the extreme apex includes so many united seg-
ments as there are straight series of these, i.e. where the apical cell is two-sided, 2 ;
where it is three-sided, 3. The divisions proceed rapidly, and if, as must be done
in this case, one disregards lateral outgrowths, they proceed in the same direction and
in the same succession in the successive segments. One thus finds the segments 6f
each transverse section in about the same stage of division.

In those cases where the successive divisions have been successfully and
accurately followed— apex of the stem of Equisetum, Azolla, Selaginella Martensii,
partially also in Salvinia, and especially in the roots of Equisetum, Azolla, and many

* Pfeffer, Entw. d. Keims v. Selaginella. — Hanstein, Bot. Abhandl, Bd. I.

“ Cramer, lieber Equisetum in Nageli und Cramer, Pflanzenphysiol. Untersuchungen, 3 Heft,
p. 21 (1855)-

INTRODUCTION.

17

Filices and Marsileacese — we may distinguish in the first stages of the further
development of a segment, three sorts of division, differing in their direction and
results. They are : —

(1) Divisions into flats (Etagentheilungen), that is, splitting of the segment into
similar stories one above another by division walls at least approximately parallel to
the principal walls.

(2) Radial halving, division of a segment into halves lying side by side, but
never quite alike, by an approximately (but not exactly) radial wall : in the case
of segments arranged in two series, that is, which correspond in the circular transverse
section to semicircles, this radial halving divides the section into (unequal) quadra7tls;
in case of segments in three series, into sextants; the walls in question are named
accordingly. In the first case the division into quadrants is followed either by
a second halving by octant-walls (stem of Salvinia, Azolla) or only each larger
quadrant is again halved, so that each segment is divided by two radial walls into
three cells (stem of Selaginella Martensii).

(3) Division into strata (Schichtentheilung), that is, division by tangential walls
into concentric strata parallel to the surface.

These three modes of division, which appear as the first successive stages of
division, are followed by further divisions in the three principal directions in each
story, and in each stratum. These divisions alternating variously according to the
species, finally result in the definitive composition of the segment.

Of the above three first stages of division, that marked (3) is seldom the first.
Using these figures as above to express them shortly, they usually appear in the
succession i, 2, 3 (apex of the stem of Equisetum, Salvinia), or 2, 3, i (root of
Ferns), 2, i, 2, 3 (apex of stem of Azolla) ; only in the root of Azolla the suc-
cession 3, 2, &c. was found by Strasburger. Relatively to the future layers of
meristem, i.e. to the later developed tissue-layers, the first products of division of
the segments are thus, with the exception of the last mentioned case, still common
initial cells.

From the division into strata, marked (3) above, arise layers of meristem, which
correspond in their arrangement to the three principal layers of the root of the
Angiosperms, i. e. plerome, periblem, and dermatogen. In many cases, though not
in all (e. g. in the roots of Ferns and Equisetum), these undergo a similar develop-
ment to that of the similar layers of corresponding members of the Angiosperms.
They are usually sharply defined, since the walls separating them (like other longitu-
dinal walls) in the successive segments fit pretty accurately one on another. As is
evident from what has been said, we have to deal, in the phenomenon in question,
with more than one, at least two, successive divisions.

As is evident from the foregoing account, many differences peculiar to special
cases obtain in the very first stages of division. This is the case to a still greater
extent in the later stages, which bring about in the segments their definitive compo-
sition. To enter with uniform minuteness into the peculiarities of individual cases
would lead us much too far. After referring to the special literature, and particu-
larly to Strasburger’s description of the many peculiarities of Azolla, we need cite
here only a few examples, keeping an eye at the same time upon many relations of
form, which have not been touched upon in what has gone before.

c

INTRODUCTION.

In the roots of the Equisetums, Polypodiaceae, and Marsileaceae (Fig. 7, 8) the division
of each segment {h, h, Fig. 7) begins with the appearance of the sextant wall s. This
stands vertically, and, as before stated, is nearly, but not exactly radial ; it is attached to
the middle of the outer wall of each segment, its inner edge, however, does not extend

A

Fig. 7. — Scheme of the succession of cells in the 'tr pex of the root of Equisetum hiemale, after Nageli and Leit-
geb. A longitudinal section ; B transverse section at the lower end oiA\h principal walls, s sextant walls, c (‘ cambial
wall ') the first, e {epidermal wall) the second, r (cortical wall) the third tangential wall ; the successive further tangential
divisions between c and r are figured i, 2, 3.

In A the figures I — XVI denote the successive segments ; the letters 'k, I, m, n, p, the successively alder
portions of the root cap ; 0 epidermis (dermatogen). From Sachs’ Textbook.

to the central angle of the latter, but, curving slightly, meets the central part of the
lateral wall further from the centre than the angle. The convexities of successive sextant
walls are as a rule, but not always, homodromous, and turned toward the ascending side
of the segmental spiral. The sextants of one transverse section are therefore alternately
unequal in form and size, according to the distance of the point of junction of the sextant
walls from the angle of the segment ; among the cases observed this inequality is greatest
in Equisetiun, least in the Marsileaceae. The inequality of the quadrants, octants, etc. of
a transverse section from the above-named plants with two series of segments depends

PIQ, 9. — (250) A longitudinal section through the apex of the root of Pteris hastata. B transverse section through
the apical cell of the root and the neighbouring segments of Athyrium filix femina, both after Nägeli and Leitgeb ;
V apical cell ; the other letters and figures as in Fig. 7. From Sachs’ Textbook.

upon similar conditions. Each sextant is in the second place divided by a tangential wall
(c) into an inner cell which is usually small, and a larger outer one : the difference in size
between the two is greater the thinner the root is, but, as stated, always so that the outer
cell has the advantage. The inner cell is the initial cell of the plerome, the outer is in the

INTRODUCTION.

19

simplest cases further divided by a tangential wall (e) into two cells, of which the outer is
the initial of the dermatogen, the other of the periblem. The dermatogen remains in the
simplest cases a simple layer, since its cells suffer only radial divisions alternately hori-
zontal and vertical, but it may also be once more divided tangentially. The two other
layers are further divided and developed by successive vertical walls in definite order,
which need not now be followed ; later these are accompanied by transverse divisions
into flats (Etagentheilung). In roots which increase much in thickness each outer cell,
after the appearance of the wall c, may further divide by a radial vertical wall into two,
in which the division by the wall e then takes place.

Fig. 9. — Apex of the stem of Equisetum. A {550) longitudinal section through a strong shoot of E. telmateja,

B view of the apex of such a shoot from above ^Sachs); C, D, E, from E. arvense, after Cramer ; C a diagrammatic
ground plan of the apical cell and of the youngest segment, D optical longitudinal section of the apex of a stem,

E transverse section through I \n D, S in all cases the apical cell. The Roman figures I, II, . . . (in B read IV for
VI) indicate in a'l cases the segments; the Arabic figures i, 2, 3 . . . the successive division walls within a single
segment ; the letters a, d, ... in C and D the successive principal walls. In A, x,y indicate the highest, youngest
rudiment of an annular swelling which will develope into a leaf-sheath ; bb the same older, bs apical cells of a still
older leaf-rudiment, £ rows of cells from which the -vascular bundles are derived, i the lowest layers of cells of the
segments, r the rudiment of the cortex of the intemodes ; the broad central band between g and g the plerome.

From Sachs’ Textbook.

In the apex of the stem of species of Equisetum (Fig. 9) each segment-cell is, according
to Cramer, Reess, and Sachs, divided first of all parallel to the principal walls into two nearly
similar stories (5, C, D), then follows in each of these the division into alternately dissimilar
sextants (F) as in the root ; abnormally in many cases (comp. Reess, Pringsh. Jahrb. VI, PI.
X. 8) two sextant walls curved in different directions appear in one segment. The next
division in the sextants is either a tangential perpendicular one into an outer and an inner
cell, corresponding to the scheme for the root, or one not exactly radial and perpen-
dicular, which is then followed by the tangential division (/?). Both cases happen side

c 2

20

INTRODUCTION.

by side; the arrangement of the inner cells, which may be called initial cells of
plerome, and which suffer thenceforth divisions alternately in all directions, is therefore
often irregular. In the outer cells rapid divisions now follow, sometimes parallel to
the principal walls, sometimes radial, and tangential-perpendicular. The definite ar-
rangement of these is not ascertained. From these, only when they have attained
a very advanced stage of development, a superficial layer is marked off, which may
be called dermatogen.

In the three first stages of division of the segments (which only are well ascertained
in this case), the apex of the stem of Salvinia corresponds, according to Pringsheim’s
statement, to that of Equisetum, with such differences as follow from the rows of seg-
ments being two in number.

For the majority of the apices of stems of the Ferns ^ it is doubtful, and requires
further investigation, whether and how far the first stages of division of the segments
correspond to the scheme derived from the simpler cases above described. At all
events, we know from the older statements of Hofmeister (Beitr. II) that the segments
undergo directly many and repeated divisions, both in directions parallel to the prin-
cipal walls, and radial and tangential. By these the growing meristem is cut up into
many layers and rows of cells, which are arranged similarly to the segments, but in
which the boundaries of the single segments are not clear. A permanent layer of
dermatogen is first distinguishable after numerous tangential divisions ; a boundary
between plerome and periblem is for the present doubtful.

The formation of the leaves begins, in plants which grow with an apical cell,
from an initial cell cut off from a segment; and the leaf itself grows, at least in its
early stages, with an apical cell which forms segments (Fig. gA, b s).

In the roots of the ferns in question, the formation of the cap starts also from
the apical cell, and begins by the cutting off of a cell from the otherwise unaltered
apical cell near its apical surface, by a wall perpendicular to the axis. This cell has
the form of a flat segment of a sphere, and is the first cap-cell (Fig. 7, 8, A, k). This
is the initial either of one of the simple layers of cells or sheaths (/, vi, n, p), from a
combination of which the root-cap is built up, or, by undergoing a subsequent
transverse division, it is the initial of a pair of such sheaths. Each primary cap-cell
is immediately divided by longitudinal walls at right angles to its outer surface,
which becomes more and more convex as the growth at the apex of the root
proceeds. It is divided first by a median longitudinal wall into two equal halves,
these being again divided into four quadrants by a wall at right angles to the first.
Each quadrant cell is again divided into two unequal parts by a longitudinal wall,
which halves the outer walls, or divides them into unequal parts, and then taking a
curved course inwards attaches itself to one of the lateral walls. The further divisions
which appear in the eight cells of the sheath thus formed become successively more
irregular, and may be followed up in the work of Nageli and Leitgeb. Where the
primary cap divides into two, this happens after the first three longitudinal divisions
are completed.

According to Hofmeister, Hanstein, Nageli, and Leitgeb, the rule is that a
primary cap-cell is cut off from the apical cell after each cycle of segments, which go

’ On the phenomena in Ce7-atopteris, which differ from those in the other Fcr.,s in the narrower
sense, compare Kny, Entwickl. d. Parkeriaceen, Abhandl. d. K. I.eop. Acad. Bd. 37 (1875).

INTRODUCTION.

21

to form the body of the root. In longitudinal section therefore in the younger
transverse zones, each successive segment is laterally overlapped by a new cap (or,
as the case may be, by a pair of them). There are, however, exceptions to this
rule. Further, each cap-cell abuts on the principal walls of the cycle of segments
cut off immediately before it, and this condition remains often for a long time
recognisable by this character, that each cap rests with its margin upon the step-like
outer walls of two successive segments. This arrangement is obliterated sooner or
later by the smoothing down of the steps resulting from growth.

We must now return to the phenomena in many Selaginellas, and in the
Marattiacese, which were before left unexplained.

As before stated, a number of species of the first-named genus have on the
stem a two-sided apical cell, which forms segments from its two sides. Russow ^
first drew attention to the fact that in many species — e.g. S. arborescens, Pervillei,
Wallichii, Lyallii — there is not a single apical cell, but an apical group of common
initial cells. Strasburger^ has carefully investigated 6’. Wallichii, and found that here,
in place of 07ie apical cell, two are present, which form segments in conjunction with
one another. Each of them has the form of a wedge with narrow rectangular
section, and is bounded by five planes ; i. e. two nearly equal lateral planes, in form
of isosceles triangles, the bases of which are the long sides of the rectangle presented
by the cell in transverse section ; two narrow-rectangular lateral planes, and a fifth
also narrow-rectangular, which is the free apical plane : the lateral planes, like those
of simple apical cells, are sunk in the meristem. Both cells are joined by one of
their broad triangular lateral faces into a double wedge of corresponding form, and
the relative position of this is such that the two triangular lateral faces are
perpendicular to the dorsal and ventral faces of the (bilateral) stem, while the
joint wall of the pair of apical cells is in a median position. One may therefore
shortly name the broad triangular faces lateral, and the narrow rectangular ones the
upper and lower. Segments are formed similarly in each of the two apical cells in
the following succession. First a principal wall parallel to the lateral faces cuts off a
segment almost similar to the apical cell, then two narrow segments of rectangular
section are cut off by principal w'alls parallel to the upper and lower sides. After
these follow again lateral segments, &c. Thus four straight rows of segments arise,
as from a single four-sided apical cell, the series right and left being wedge-shaped,
while the upper and lower segments are rectangular, and are arranged in each case
in a double series. The latter form the ventral and dorsal portions of the stem,
the former the lateral portions.

In the Marattiacese the apex of the stem is as yet but little investigated.
Hofmeister (Beitr. II) ascribes to Maraitia cicutcofolia a three-sided apical cell.
The meristematic apex of the root of these plants, as shown by Harting and more
carefully described by Russow (/. c., p. 107), consists of a numerous group of large,
polygonal, pyramidal, common initial cells ; cap-cells are cut off from these by trans-
verse walls near their broader, outer, or apical face: from these the root-cap is formed:
near their inner face, cells are cut off which as initial cells form the plerome cylinder.

' Vergl. Untersuchungen, p. 176. [Cf. Schwendener, über Scheitel wachsthum mit mehreren
Scheitelzellen, Botan. Zeitg. 1880, p. 716.J “ Botan. Zeitg. 1873, p. 115.

^ De Vl iese et Harting, Monogr. des Marattiacees, p. 41, Taf. 4.

22

INTRODUCTION.

Further they divide by longitudinal walls, which are similarly directed to their lateral
faces but are otherwise apparently irregularly arranged, into daughter cells, of which
those nearest the apical point always retain the properties of the common initial cells,
the others, as they retreat from the apical point, form the peripheral layers of raeri-
stem, dividing first by numerous repeated tangential longitudinal walls, which are fol-
lowed by others in radial and transverse directions. In this case also a separation
between periblem and dermatogen appears first in an advanced stage of development.

Lastly, we may notice shortly the meristematic apex of Psilolum, which, according
to Strasburger, shows according to the quality of the shoot, either a simple apical cell,
or an initial group consisting of many members. Reference may be made to the
investigations of Nageli and Leitgeb, and of Strasburger^.

The foregoing summary shows, first, that the similar differentiation of the
meristem at the apex of stems and roots originates in a different way, that is, from
different first beginnings, in the different groups of the vegetable kingdom, and in
such groups as the Selaginellas and their allies, which are intermediate between the
larger divisions ;■ it originates differently even in the single species.

Returning to the question, whether in all cases only definite zones of meristem
give rise to definite sorts of tissue, the most general answer, according to our present
knowledge, is a distinct negative^. To be sure this negative does not hold for all
single cases. For instance, for the large majority of roots, not only does each of the
different layers of meristem correspond to a definite section of a definite system of
tissue, but even the separate parts of each of these sections may often be traced back
to its separate initial cells in the apical meristem. It is therefore in this case not only
allowable, but preferable, for the sake of clearness, to term the layers of meristem
directly the initial layers of the axile vascular strand and its parts, or of the Epidermis,
&c,, instead of using the terms selected above. But even in Roots exceptions occur.
The Epidermis, for instance, in the Gymnosperms does not originate from a distinct
dermatogen layer, so' that we should properly speak of a Pseudo-epidermis, if we
regard as true Epidermis only the layer of cells derived from a distinct dermatogen
layer. In the aerial roots of most Orchids there arises from a distinct dermatogen
layer, as will be hereafter shown, a sort of tissue different from Epidermis.

The negative, however, of the constant genesis of definite sorts of tissue, or
systems of tissue, from definite zones of primary meristem, holds to a much greater
extent for leaf-forming shoots. Here also it is true there are such relations. The
system of vascular bundles of many stems of Phanerogams, for instance, is derived
exclusively from the plerome cylinder. The plerome cylinder of the Lycopodiaceae
is transformed into the axile vascular cylinder ; dermatogen means in the Phanerogams
nothing further than young Epidermis, &c. But exactly the opposite also occurs.
The plerome cylinder arising from the inner cells of the segments develops in Azolla
(and Salvinia ?) into the vascular bundle of the stem. In the stem of Equisetum ® it
develops into the — chiefly transient — axile cylinder of Parenchyma, and the system
of vascular bundles develops, according to the data at hand, exclusively from the zone
of periblem. And the whole of the tissues, and tissue-systems of the leaves, which

* Botan. Zeitg. 1873, p. 118.

^ [Cf. Haberlandt, Entwickelungsgeschichte des mechanischen Gewebesystems d. Pflanzen, 1879.]

® Compare Sanio, Botan. Zeitg. 1864, p; 224.

INTRODUCTION.

33

are continuous with the similar and synonymous tissues of the plerome of the stem
are formed, according to the data at hand, outside the plerome, being derived, as is
the whole leaf, from the periblem and dermatogen, or from the layers of meristem
corresponding in position to these. From all this we see then, that definite relations
between the original differentiation of the meristem and the formation and arrange-
ment of the definitive tissues obviously exist, but that these are not universally the
same. If then the course of treatment is to be uniformly arranged, we must for the
time being regard especially the distribution of tissues, while still keeping an eye
upon the differentiation of the meristem.

In opposition to the foregoing view, another has lately been asserted, since Famintzin^
has undertaken to prove that in the Angiosperms definite systems of tissue, namely
besides the Epidermis especially the system of vascular bundles, are universally, i. e. in all
parts of the plant, each derived from the same primary layers of meristem, which are
separated even in the embryo, and develop further independently near and between one
another like the germinal layers of the animal body. The layers of meristem in question
are fundamentally the same above distinguished by us. On the share taken by the
dermatogen in the development of tissues there can be no two opinions; the main
question therefore is whether the system of vascular bundles arises universally, i. e. in
the whole plant from the same primary layer of meristem. If we ignore isolated cases
of controversy, the plerome or a certain region of it is in stem and root the initial part
for the system of vascular bundles, or for the greater part of them. The question there-
fore is whether the parts of the system of vascular bundles, which pass from the stem-
system into the leaves, and which belong to the latter, also arise from the plerome at the
apex of the stem. This could not be otherwise effected than by outgrowths of the
plerome pushing between the other layers of the young forming leaf, and growing with
these, as was above asserted for the common growth of dermatogen and periblem.
Other investigators do not find this; they rather say that the vascular bundles in the leaf,
like the other inner parts of it, are derived from the primary periblem, since definite
bands of the latter show the corresponding differentiation ; and that they are connected
with the system of the stem by reason of the relative position of the said bands
of periblem ^ Famintzin’s researches certainly afforti valuable conclusions on certain
processes, but no new result on the main question. When he proves that in foliage
leaves, especially in the Papilionacex, the parts of the vascular bundle always arise from
quite definite layers of the meristem, he says nothing new ; for as the mature vascular
bundles in the leaf have a definite position, this must hold also for their younger stages.
He does not produce proof that these bundle-forming layers arise as branches from the
plerome layer in question of the stem, and push themselves between the other tissues of
the leaf, and this proof he should have brought in order to establish his view ; he com-
municates rather observations, which lead to the contrary result. He asserts that the
leaf of the Papilionaceae mentioned, e.g. species of Trifolium, at a certain age consists of
five layers of meristem ; the outermost is dermatogen or epidermis, of the four inner
only the two innermost are points of origin of the vascular bundles. He further asserts
that in an earlier stage within the dermatogen lies only one layer of meristem cells — which,
according to our preceding statement, must be derived from the periblem of the punctum
•vegetationis ; and that the four later layers arise from division of the cells of that one.
It is clear that thereby the postulated pushing in is excluded, and on this the theory of
germinal layers must be founded.

’ Botan. Zeitg. 1875, p. 501. — Beitr. zur Keimblatt-theorie im Pflanzenreich, Mem. Acad. St.
Petersbourg, 7 serie, tom. XXII. — Compare also Botan. Zeitg. 1876, p. 540. [Compare further
Famintzin, Embrj’ologische Studien, Mem. Acad. St. Petersbourg, tom. XXVI. No. 10, 1879.]

^ Compare especially Sanio, Botan. Zeitg. 1864, l.c.

24

INTRODUCTION.

With the differences of differentiation of the meristem are always connected
those of the mature structure : one may say obviously so, since the causes of the
development of the mature structure, which are involved in the properties of the
meristem, are different in every case.

But while the differences in the differentiation of the meristem correspond in
each case exactly to systematic divisions', as distinguished principally on the ground
of other phenomena, and especially so in the greater groups — for instance, all Ferns
and Equisetums correspond just as closely, and are distinguished from other classes
just as much by the differentiation of the apex of their stem and root, as by their
reproductive and embryonic processes — the case is often different with the mature
structure. The structure of the full-grown stem of Equisetum has no more
resemblance to that of a Fern than to that of any Angiospermous plant however
distantly related, as regards both external members and internal structure. Similar
divergences are found on all sides between the characters of mature plants, and
the embryonic or meristematic stages which indicate their relationships. Conversely,
there is equally often to be found a convergence of properties of distantly connected
species : and this is clearly expressed externally in the similarity of the most
heterogeneous plants which live under like conditions, such as water-plants, the
vegetation of steppes, and shores, &c.

The reason of these phenomena is easily understood when seen from the point
of view of the theory of Descent, and has often enough been stated. The existing
form of a species is determined by the inheritance of the properties of its ancestors,
and by the changes w'hich these properties undergo through the influence of the
environment, i.e. the adaptation to the latter. The inherited properties must remain
most clearly retained in those stages of ontogenetic development which through all
generations are most independent of, that is most protected from external influences,
and this is the case with the embryonic and meristematic stages. These recall in
each species most completely and clearly the whole series of its reminiscences of
descent, or, what is the same, they are more plainly different according to the
divisions of the natural system than the later stages. Certainly these are also
influenced by heredity, but the results of this may be obliterated and diverted from
the original direction by successively accumulated adaptation.

As in the external conformation, which must at most be only incidentally touched
upon here, so in the internal structure and arrangement of the tissues we may accord-
ingly distinguish two series of phenomena. On the one hand those in which we recog-
nise the direct effects of the environment {^phenoincna of direct adaptatioii), since they
appear in plants of the most different affinity, as soon as they are adapted to like con-
ditions of life ; and since they may change with these conditions of life even in the
same individual. It is hardly necessary to cite as proof the different forms of growth,
which recur in the most unlike circles of affinity, and the anatomical peculiarities con-
nected with these ; or the remarkable similarity -between species of the same habitat,
which are systematically as far apart as possible : of the latter we may quote as the most
prominent examples, in the first place, the water-plants, the similarities of which are
independent of their systematic position, and will be often referred to in the follow-
ing chapters. In the amphibious species the most remarkable varieties of general
conformation appear, according as an individual, or even a part of one, lives in or out

INTRODUCTION.

25

of the water. Then we may refer to the almost identical form and structure of the
roots of the large majority of plants however different systematically, and to the
peculiarities, which at once appear in these, where a special adaptation takes place, as,
for instance, in the aerial roots of epiphytic orchids, the prop-roots of the Pandanaceae,
Iriarteae, &c.

On the other hand, there are often to be found phenomena in the structure as well
as in the form of the vegetative organs, which may also be derived from adaptations,
which have happened in some epoch or other of the phylogenetic development, but
which cannot now be certainly referred to their causes ; properties, which were acquired
at an unknown time, and through unknown causes, are handed down to definite
series of successive generations, and at the present time are characters of Species,
Genera, Orders, and Classes, these corresponding to those taken from the formation
of flowers, embryos, &c. Of the more obvious phenomena of this category, we may
mention for example the arrangement of the vascular bundles in the stems and leaves
of most of the Monocotyledons and Dicotyledons, the structure of the vascular
bundles of the Ferns, of the wood in the Coniferse, and in most Chenopodiacese, &c.

According to the terminology, which calls the properties, by which the divisions
of the system are characterised, its characters, we may term the (unexplained) pro-
perties of this category (unexplained) anatomical characters.

Since the existing anatomical structure of a species is obviously the product of
the combination of the two categories of properties, it is to be expected beforehand
that, as with external form, in different species, it wfill be more identical the nearer is
their affinity, and the more similar their adaptation. There are instances enough
where this is the case. The Coniferse, Filices, Chenopodiacese, Cucurbitacese may be
again cited on the one hand as groups which have from every point of view a similar
structure with very similar adaptation ; other groups, wffiose genera and species have
very different adaptation, show accordingly very different structural phenomena, for
instance, the Ranunculacese (Ranunculus, Batrachium, Thalictrum, Clematis, &c.),
the Primulacese (Lysimachia, Cyclamen, Hottonia, &c.)

To this rule however any fairly extended investigation brings to light numerous
exceptions, viz. single species, genera, or groups, which, within their narrower or
broader circle of allies, which follow the rule, are characterised by definite peculiarities
of structure ; these must, it is true, be regarded as inherited consequences of the
adaptations of the special ancestors of the plants in question, which however cannot
be referred to direct adaptation. Among the numerous cases, which belong to this
category, and which will be mentioned in the following chapters, we may cite as ex-
amples— the structure of the stem of the Auriculas (Primula auricula, &c.), w'hich
differs so remarkably from that of the other Brimulas, whose adaptations are however
not very different ; the wood of Strychnos, Wintera, &c. Examples of this sort show
how cautious one must be in adducing and using anatomical characters for the
greater systematic groups ; how one must take care not to found such ideas upon
the structure of a couple of casually chosen species.

The frequent occurrence of such exceptional cases makes the series of phe-
nomena, which are to be treated comparative!)', highly complicated, and makes useless
the attempt, which at once suggests itself, to arrange the single sections, which treat'
of the forms of tissue and their distribution, rigidly either according to the different

26

INTRODUCTION.

adaptational forms, or according to the systematic divisions. Whether this attempt
can eve,r succeed, will depend upon the results of further investigations, which shall
have e.xtended over whole families and classes, having regard more comprehensively
and completely to a/l the questions concerned, than has hitherto, as a rule, been the
case. According to the present state of our knowledge there remains for the state-
ment of the anatomical peculiarities of the groups, which may be distinguished
according to natural relationship or direct adaptation, only this one tolerably con-
sistent and practicable course, to start from the tissues and their arrangement, and
to introduce into the general consideration of these the rules and exceptions which
obtain for the two kinds of groups above named.

PART I.

THE FORMS OF TISSUE.

CHAPTER I.

CELLULAR TISSUE.

General Introductory Remarks.

Sect. i. The general properties of cellular tissue are indicated by this name,
which has been interpreted above. A knowledge of the structure of the cell is here
presupposed.

The sorts of cellular tissue are the Epidermis with its individual components,
the Parenchyma with its subdivisions, and the Cork.

These are distinguished in the first place by their structure, further by the
form, arrangement, and mutual connexion of their cells.

In earlier periods of vegetable anatomy the form both of the cells, and of the
tissue elements, which are not here included under the term, was exclusively or par-
ticularly regarded, and according to it were distinguished two main categories of
cellular tissue (or of tissues generally) ; Parenchyma, parenchymatous tissue with cells,
that is, elements (parenchymatous cells) of nearly isodiametric form ; and Prosen-
chyma, Pleurenchyjna with particularly long elements, which are connected with one
another laterally, and with their obliquely tapered or spindle-pointed ends (Prosen-
chymatous cells). Among the former were distinguished a number of subordinate
forms according to the special shape, as, Merenchyma, tabular, and stellate paren-
chyma, &c., the detailed enumeration of which would now be purposeless k

One may, as is often the case, retain these names to indicate the forms; however
it may be better to choose for these forms purely descriptive terms as wanted, and
from this point of view to term the two above-named main categories of form on
the one hand isodiametric cells, on the other elongated or fibrous cells.

With reference to the structure of the cells, besides the special properties,
according to which the distinctions between them will hereafter be drawn, a difference

* Compare Meyen’s Phytotomie, and Mohl, Vegetab. Zelle, p. i6.

28

CELLULAR TISSUE.

often occurs, which concerns the relative development of mass, on the one hand of
the cell walls, on the other of the protoplasmic body and the contents. On the one
hand, we have cells with a relatively thin wall, and richly developed protoplasm and
contents, characterised by the components of the latter — chlorophyll, starch, sugar,
inulin, &c. — as the specific organs of assimilation, and of metabolism, or chiefly con-
taining watery cell sap. On the other hand we find cells whose protoplasm and
contents are reduced relatively to the strongly thickened, and often lignified mem-
brane, and which accordingly, without giving up the properties of typical cells, or
their part in the process of assimilation, obviously participate in the mechanical
functions, i. e. the strengthening of the parts to which they belong. The ‘ Collen-
chyma ’ of the cortex of herbaceous plants and the sheaths of the vascular bundles
of many monocotyledonous roots are examples of the latter condition. One can
accordingly distinguish two extreme forms of structure, and call them shortly thin-
and thick-ioalled cells, names which are explained by what has gone before. When
with the thickening of the wall there appears a process of lignification — which in itself
still needs to be more carefully studied — and a hardening of the wall thus occurs,
this process will for the future be indicated by the term Sclerosis.

The different grades of wall-thickening are not generally confined to a definite
cell-form, or to any one of the sorts of tissue here distinguished on completely dif-
ferent grounds ; there exist isodiametric and fibrous cells with thin, and with sclerotic
walls, sclerotic Parenchyma, Epidermis, and Cork cells, &c. But besides this, as
may be concluded from what has been already stated, there is no sharp limit between
the two main forms, even if one ignores the following fact, which should be brought
prominently forward, that sclerosis is the commonest phenomenon of secondary
metamorphosis which appears in cells.

In the large majority of cases, the species and varieties of cellular tissue are dis-
tinctly different from one another, and the treatment must start from these cases of
marked differentiation and division of labour. But since all are derived from funda-
mentally similar meristem, and the properties of the cell remain to all alike, there
appear also cases of less complete differentiation and division of labour, and tran-
sitional forms, to the existence of which attention must be directed from the very
first, and which raise permanent difficulties in many single cases in the way of a sharp
division of tissues.

From the non-equivalent sorts of tissue which originate by metamorphosis of
cells, the cellular tissues are, irrespective of their common origin, usually quite clearly
distinct. But there are two exceptions to this. Firstly, a sharp limit cannot always
be drawn between scleroiic cells and sclerenchyma, which has lost the cell-quality.
The secondary sclerenchyma-metamorphosis, which often appears in cells, must
lead to transitional forms ; and practically it is often impossible to distinguish
whether the cell quality remains, or is lost. In many cases therefore the question
arises whether a separation of the sclerenchyma from the cellular tissues is to be
attempted at all, and to be as far as possible carried out. The frequent occurrence
of sharp differentiation answers the question, I think, in the affirmative.

Secondly, intermediate cases exist between cells and the secretory reservoirs, in
so far as the bodies termed secretions, which fill the latter, as oxalate of calcium,
resinous bodies, &c., frequently appear also as constituents of the contents of typical

EPIDERMIS.

29

cells, and these, as the quantity of the secretion increases, become like those reservoirs.
For judgment upon these intermediate forms, and the possibility of carrying out the
separation of the forms of tissue, the same reflections hold good as for the scleren-
chyma. The difficulties of distinguishing them in practice are, besides, much smaller
in this case than in that of the sclerenchyma.

SECTION I.

EPIDERMIS.

Sect. 2. Epidermis, outer skin, is the name given to the layer of cells which is
covered by and produces the cuticle. It constitutes the surface of such plants as
are several layers of tissue thick, from the beginning of the differentiation of tissues
onwards throughout their life, or till the beginning of the development of cork, which
takes its place.

On the stems and leaves of Angiosperms the Epidermis is sharply marked off,
even in the young embryo, while still consisting of few cells ; in this case, as long
as it remains in the meristematic condition, it is termed the Dermatogen layer. This
grows, as stated on p. 7, with the stem, leaves, and branches, covering them as a
cellular mantle, one layer of cells thick. It remains in the large majority of cases
throughout its life a simple layer of cells, with exception of hair structures. In
relatively few Angiospermous plants divisions of the young epidermal cells appear
parallel to the surface, and then in a rather late stage of development ; from a simple
layer of cells two or several are thus formed. These assume an essentially identical
structure, and are then termed many-layered epidermis.

Where the differentiation of the apical meristem is other than that characteristic
for the stem of the Angiosperms, a permanent outer layer of meristem, derived by
successive divisions from initial cells common to it and to other layers, assumes the
properties of the epidermis. In plants which grow with an apical cell, definite peri-
pheral products of division of the segments serve this purpose ; in Gymnospermous
roots the transverse portions of the successive layers of periblem from time to time
laid bare by the separation of the root-cap, &c. Comp, above, p. 14. In these cases
we cannot speak of a many-layered epidermis in the same sense as in the stem and
leaf of the Angiosperms, since the genetic relations characteristic of those cases are
different ; that term can at most be conventionally used for single cases which in fact
scarcely ever occur. In single special cases also in the Angiosperms, the epidermis
originates from other elements than the dermatogen. The perforations (and indeed also
the lacinise) in the leaves of many Aroideae originate by the early dying off of circum-
scribed portions of the young leaf, the original epidermis dying off with them h Since
the edge of the mature perforations is clothed with epidermis, this must be completed
from the inner layers of the young leaf, which point moreover remains still to be more

^ Compare Treciil, Ann. Sei. Nat. 4 ser. tom. I. p. 37.

CELLULAR TISSUE.

50

closely investigated; Similar relations, which however also require further observation,
may hold for the margins of the leaf-segments in the Palms, since these segments
* originate by the splitting of the continuous young lamina In one or few-layered
parts, like the leaf-surfaces of the Hymenophyllacese ^ and Hydrillese either there
is no differentiation of the epidermis from the parenchyma, or it has been obliterated.
One can in this case speak of epidermis only on the ground of the cuticular covering,
which is present, or as in the two-layered leaf lamina of the Hydrillese, on the ground
of genetic relations. In the many-layered parts also of submerged water-plants the
differentiation of epidermis and parenchyma often becomes less plain, as will later be
described.

In the overwhelming majority of cases the epidermis is sharply distinguished
from the cells which it surrounds.

1. COMPOSITION OF THE EPIDERMIS.

Sect. 3. The following kinds of cells or cell-groups are to be distinguished as
parts of the Epidermis : — ■

(1) Epidermal cells.

(2) Stomaial, guard-cells, pairs of which enclose a slit-shaped intercellular space,
and together with this form the stoma.

(3) Hair-slruciures (Trichomes).

Sect. 4. Epidermal cells in the strict sense is the name given to those cells
of the epidermis whose lateral walls are in uninterrupted connection with one
another and with stomatal cells. Exceptions to this occur only in the slightly
differentiated epidermis of the base of the leaf of the Osmundaceae and Isoetes
(comp. Sect. 9). The term lateral walls is here used for all those which are at
right angles to the surface. With reference to the longitudinal axis of growth of the
member of highest rank to which they belong, we may therefore speak of superior
or inferior lateral walls, and of side or flanking walls; and, in obvious contrast
to these, of outer and inner walls. The direction at right angles to the surface,
i. e. that of the lateral walls, may be called the height of the cell ; length and breadth
will be used in the same sense as for the whole organ of highest rank, to which
they belong.

Form of the Epidermal cells (comp. Figs. 10-20, which follow below).

a. Epidermis one layer of cells thick.

The general form of the epidermal cells is endlessly various according to
the special cases. As a rule the diameters in the two directions parallel to
the surface are equal, or but slightly different, in parts which grow slowly and
equally in two or three dimensions, e. g. lamina of many leaves : but the longitudinal
diameter is greatly developed in longitudinally extended organs, as most stems,
roots, narrow linear leaves, especially of the Monocotyledons, also on the nerves

* Compare Mohl, Verm. Schriften, p. 177.

“ Mettenins, lieber d. Hymenophyllaceen, in Abhandl. d. sachs. Gesellsch. d. Wissensch. IX.
p. 403.

^ Caspary, in Pringsheim’s Jahrb. I. p. 49.

EPIDERMIS.

3*

and ribs of slightly elongated leaves. The opposite, that is to say, a great
transverse extension, occurs but rarely in the case of the epidermal cells in longi-
tudinally extended parts, as for instance in the leaves of Cycas, Encephalartos, Tra-
descantia, Crassula, Campelia, Dichorisandra’, many Bromeliaceae (Pholidophyllum
zonatum), and as a peculiarity of stems with clearly marked nodes, as Arceutho-
bium, Salicornia.

The height is as a rule either much smaller than the larger, or than both of
the diameters parallel to the surface, the cells are thus of the form of flat plates
overlying the surface ; or it is the greatest diameter of all, the cells are then prisms
arranged perpendicularly to the surface ; intermediate forms between these two
extremes are common enough. The lateral faces are flat, and cut one another with
sharp corners, so that the single cell has the form of an angular plate or prism.
In other and not less common cases they are wavy and folded, in which case the
hollows and protuberances of neighbouring cells fit exactly into one another ^

The extent of the waving may vary, or undulated and flat lateral walls may
both occur in equivalent parts of one and the same species, according to the
adaptation to different surrounding media. Meyen ^ noticed this relation (which
remains to be further followed in other connections), somewhat indefinitely it is true,
for ‘a large number of species of Gentiana,’ in which he found the cells more
wavy ‘ the damper the region of the atmosphere in which the plant was grown.’
Conversely Askenasy^ found in Ranunculus aquatilis and divaricatus, on the
submerged form that the epidermal cells of the leaf have flat sides, on the land-
form strongly undulated sides. Also on the amphibious leaves of Marsilea and
Sagittaria ® differences occur in the above-mentioned relation.

The undulation usually extends equally over the whole height of the lateral wall,
but often, e. g. in leaves of grasses and Equisetum only over the strip along the
outer edge, while the inner part is flat. The outer and inner surfaces of the epi-
dermal cells are flat, or to a variable extent convex ; the latter either over the whole
extent of the cell, or at one (e.g. leaf of Aloe margaritifolia), or two, or several
(Equisetum hiemale) relatively small circumscribed spots.

Other forms than those possible within the limits laid down are more rare ;
e. g. spindle-shaped, elongated, on the leaves of Torreya, Ceratozamia (Kraus, /. c) ;
the often peculiarly formed subsidiary-cells surrounding the stomata, together with
the hair structures, must be specially mentioned below.

One and the same epidermal surface often shows only epidermal cells of nearly
similar form — e. g. many smooth stems. Much oftener however considerable
differences occur on the same surface, (a) In relation to the relief of the surface,
and (often connected with this) in the distribution of stomata and hairs ; in angular
and ribbed stems in relation to the angles, or ribs on the one hand and the faces
or furrows on the other ; in flat leaves and leaf-like organs in relation to the ribs

* Kraus, Bau d. Cycadeenfiedern.— Pringsheim, Jahrb. II. p. 318.

“ Treviranus, Verm. Sehr. IV. p. 16. — Meyen, Phytotomie, p. 94.

^ Phytotomie, p. 95.

^ Botan. Zeitg. 1870, No. 13.

^ Hildebrand, ibid. No. i.

® Mohl, Verm. Schriften, p. 262. — Mettenius, Hymenophyllaceen, p. 444.

32

CELLULAR TISSUE.

or nerves, and the spaces between these ; it is a general rule, that here the epidermal
cells over the stalk and ribs are longitudinally extended, and with straight sides, but
between the ribs the form and direction of the predominant elongation often alter
further differences occur in relation to thorns, prickles, teeth, &c. The leaves
with stomata arranged together in groups (Begonia, Saxifraga sarmentosa) will
be noticed below.

(/3) Independent of relief, and of distribution of stomata and hairs. To this
category belong a number of very different special cases. In the bands of epidermis
free from stomata of the leaves and green stems of most Graminese the epidermis
consists of longitudinal rows of cells, of which some are elongated; others,
alternating pretty regularly with these, are short, that is, broader or at most as
broad as they are long. The short ones stand singly or by twos or threes between
two long ones ; in the two latter cases a further inequality often occurs, in that
the upper, or as the case may be the middle cell differs in form and structure
from the others ^

In the bands of epidermis, without stomata, which cover the peripheral
bundles of fibres in the stem and leaves of the Cyperacese, Douval-Jouve found one
to two longitudinal rows of epidermal cells distinguished from the rest by a less
prominent outer wall, and instead an inner wall projecting inwards in form of a
strongly thickened cone

The cystoliths scattered in the epidermis of the Urticaceae and Acanthaceae
(Sect. 2i), the elongated sac-shaped cells rich in tannin scattered or arranged
in rows between the isodiametric sinuous elements which Engler^ found in the
epidermis of Saxifraga cymbalaria and its allies and of Sedum spurium, the
large solitary cells in the small-celled epidermis of the leaf of Cymodocea nodosa,
and rotunda®, are to be registered as further kindred peculiarities. Then the
‘ Interstitial-bands ’ on the under side of the lamina, between the nerves of the
floating leaves of most if not all species of Marsilea, must be mentioned. They
consist of at most three to five rows of epidermal cells, which are distinguished from
the ordinary cells with undulating colourless walls by more elongated form, smaller
size, deep golden-brown colour of the wall, and homogeneous fluid contents. Many
appearances, to be described below with glands and hairs, are directly connected
with these.

b. An epidermis, more than one layer of cells ihick^, appears in the simplest cases,
by the division of each original epidermal cell by one or more tangential walls into
chambers, which exactly fit one on another. In many cases this happens one
may almost say casually to single cells, while the neighbouring cells, which resemble

* Compare Kraus, /. c. p. 309.

“ Compare Bot. Zeitg. 1841, p. 149, pi. I. figs. 10, ii (Coix), 12 (Sorghum). — Pfitzer, Pringsh.
Jahrb. VII. p. 555. Here the descriptions of the older writers and the discovery by Treviranus (Verm.
Sehr. II) and Meyen (Phytotomie, p. 312, Taf. III. 2, 3) are cited.

“ Douval-Jouve, in Mem. de I’acad. de Montpellier, 1872, p. 227. The phenomenon appeared
in all the species of the family which were investigated, i. e. of the genera Cladium, Rhynchospora,
Fuirena, Eriophoruin, Schxtiiis, Sdrpns, Galilea, Cyperus, Carex, Kyllingia, Hypolytrum, Diplasia.
^ Botan. Zeitg. 1871, p. 886.

® Magnus, ibid. 1871, p. 210.

‘ Treviranus, Verm. Schriften, IV. p. ii. — Pfitzer, in Pringsh. Jahrb. VIII. p. 16, Taf. VI.

EPIDERMIS.

33

them in other points, remain undivided, as for instance in the case represented
below in Fig. 29 of Klopstockia, and that cited by Pfitzer of Tradescantia zebrina;
or divided and undivided cells (i. e. one or several layered epidermis) stand side by
side in about equal quantity, as on the under side of the leaf of Passerina ericoides,
and the examples cited by Pfitzer of the leaf of Pittosporum Tobira, undulatum,
of the stem of Elegia nuda. Ephedra altissima, monostachya. The upper surface
of the leaf of Arbutus Unedo has two layers with their cells fitting one upon another
(not taking into account single cells, which remain uniseriate, and undivided), those
of Begonia manicata 2-3 (Pfitzer, 1. c.), the stem of B. tomentosa 2 that of Pe-
peromia blanda 2 In the families to which the three last-named plants belong, the
Piperaceae and Begoniaceae, and further many species of Ficus (Fig. 18), there
is formed on the leaves a much stronger many-layered epidermis, which is divided
and developed in a much more complicated way.

Pfitzer describes for Begonia sanguinea, ricinifolia, and peltata an epidermis of 4 to 5
layers, while that of B. Dregei and Fischeri on leaf and stem is simple, that in B. Dregei,
however, consisting of very large cells. The petiole of B. manicata has a simple epider-
mis, with only solitary tangentially-divided cells ; the lamina has on the upper surface
2-3 layers with the cells fitting one on another ; the inner of these is much higher than
the outer; on the under surface (Pfitzer, /. cjTaf, VI. 9) it has two layers, the cells of
the inner being more than double as high and broad as those of the outer — this results
from the fact that after the tangential division, which separates the two layers, further
radial division goes on in the outer, while in the inner only growth of the cells, without
division, takes place.

In the Piperaceae the upper surface of the leaf of all Peperomias in which the point
has been investigated^ (P. pellucida, magnoliifolia, blanda, pereskiifolia, rubella, galioides,
polystachya, incana, arifolia, obtusifolia, argyracea) is provided with an epidermis of
more than one layer, while that of the under side is a single layer. In P. arifolia it has
two, in others, e.g. P. blanda 2-4, in P. incana 7-8, in P. pereskiifolia 15-16 layers.
The high number of layers, and, in those cases where the number is smaller, the con-
siderable size of the cells in the inner layers, gives to the epidermis in question a vast
thickness, so that it is even in P. incana thicker than the whole remaining mass of the
thick fleshy leaf ; in P. magnoliifolia and rubella, it exceeds several-fold the rest of the
substance of the leaf, and in P. pereskiifolia it exceeds it seven-fold.

According to the species the cell-division and growth either proceed simultaneously in
all the layers, so that all fit with their cells one upon another ; this is the case usually in
those with two layers, but also in the many-layered P. pereskiifolia, where only the
outermost layer is, as the result of divisions perpendicular to the surface, smaller-celled
and differently arranged from the numerous inner ones (Pfitzer l.c. Taf. VI. i) ; or {e.g.
P. incana) the outer layers become smaller-celled than the inner layers, owing to numerous
divisions perpendicular to the surface, and the arrangement of the cells corresponds less
in the successive layers. ' -

Of the other Piperaceae a two-layered epidermis on the upper side of the leaf was
found by Treviranus in Chavica maculata, and by Payen in Artanthe colubrina. Miq.

As in the Peperomias, so in many species of Ficus, the many-layered epidermis of both
surfaces of the leaf is produced by the division of an originally single layer. This stratum
becomes smaller-celled as one proceeds from the innermost to the outermost layer.

It has been described for F. bengalensis elastica, ulmifolia, pectinata, ferruginea,

* Hildebrand, Unters, über d. Stämme d. Begoniaceen, p. 20, Taf. IV. 4.

^ Sanio, Botan. Zeitung, 1864, p. 213.

^ Treviranus, Verm. Sehr. IV. p. li ; Physiol. I. p. 449.— Pfitzer, i.c. p. 26.

* Treviranus, Verm. Sehr. IV. p. ii (1821).

D

34

CELLULAR TISSUE.

Carica, laurifolia, Neumanni, nymphaeifolia, australis, lutescens, salicifolia Its thickness
varies according to the species, and is on an average less on the under surface than on the
upper. Certain individual cells of the original epidermis remain undivided, and grow to form
the sac-shaped cystolith-cells, which project deep into the inner tissue of the leaf (§ 21).
Ficus lutescens and F. ulmifolia have on the upper surface of the leaf an epidermis of
two or three layers, on the under surface of only one layer (Schacht. 1. c. p. 142, Fig. 10).

A many-layered epidermis has further been described by Nicolai^ and Pfitzer (/. c.) in
the roots of Crinum bracteatum and C. americanum.

Lastly, it occurs closely connected with hair structures, on many glandular spots, to be
described later, in Passiflora, on the ends of the leaf-teeth of Drosera, etc. Comp.§ 18, 20.

Sect. 5. Stomata® (comp. Fig. 10-18). Between the cells of the epidermis
there lie definitely distributed pairs of cells, whose sides opposed to one another are
concave, and between these a slit is left open. The slit extends through the whole
height of the epidermis, forming an open communication between the surrounding
medium and an intercellular space inside it, which is called the respiratory cavity
(Athemhöhle) h The apparatus consisting of the pair of cells with the slit is called a
pore or stoma^ {Spaltöffnung, Porus, stoma), and the cells bordering on the slit
stomatal-, pore- or guard-cells.

The general form of the mature stoma is in surface view (with medium
turgescence) usually nearly elliptical; rarely relatively narrow, usually widely-elliptical
(in 162 out of 174 cases observed by Weiss) ; further it is in some few cases almost
circular the special forms being endlessly various according to the species. The
irregular three- to four-cornered stomata in Salvinia and Azolla form a remarkable
exception to the rule : each guard-cell corresponds to one half (in case of the usual
elliptical form, a longitudinal half) of the whole form ; both are, under medium
turgescence, curved in a half-moon or sausage-shape ; they are directly connected
by their ends, and by the ends and the convex sides they are joined uninterruptedly
with the surrounding epidermal cells. The concave sides are turned towards
one another and bound the slit, which is usually elongated in the direction of the
division-wall by which the ends of the guard-cells touch one another ; in Azolla on
the contrary (Slrasburger, /. ci) at right angles to this direction. The transverse
section of the guard-cell (Figs. 10, ii) is generally round or forms an ellipse

^ Meyen, Phytotomie, p. 31 1. — Mliller’s Archiv. 1839, p. 264. — Payen, Mem. present. I’acad.
des Sciences, tom. IX. — Schacht, Abhandl. Senckenb. Gesellsch. I. — Unger, Anatomie u. Physiol,
p. 190. — Hofmeister, Pflanzenzelle, p. 180. — Weddell, Ann. des Sei. Nat. 4 ser. tom. II. p. 2yi. —
Pfitzer, l.c. p. 25.

^ Schriften der Physic. QScon. Gesellsch. z. Königsberg, VI. p. 73.

^ [Cf. further, L. Reinhardt, Einige Mitt. ii. d. Entw. d. Spalt., in Russian, Ref. Bot. Jahresber.
1879, p. 30. — Schwendener, Botan. Zeitg. 1882, p. 233. — Tschirsch, Beitr. z. vergl. Anat. d. Spalt. Ap-
parats. Verhandl. Bot. Ver. Prov. Brandenburg, Ref. Bot. Centralbl. 1881. Bd. VI. p. 341. Sachs,
Vorlesungen, 1882, p. 395.]

* Unger, Exantheme d. Pfl. p. 43.

® Spaltöffnung, Sprengel, Anleitg. z. Kenntniss. d. Gewächse ; Bau und Natur d. Gewächse,
p. 180. Poren, Hedwig, Zerstr. Abhandl. p. 116; Rudolphi, Moldenhawer. Stomata, De Candolle,
Organograph. vegetale, I. p. 78. Stomatia, Link, Gnmdlehren, p. 108. The name dermal glands
(Hautdrüsen), later resumed by Link and Meyen, has hardly any further historical interest. — For the
history of these parts, so often mentioned since Malpighi and Grew (Anatomy of Plants, pi. XLVHI),
compare Treviranus, Physiol. I. p. 462 ; Meyen, Phytotomie, p. 97 ; Pflanzenphysiol. I. p. 271.

^ For details compare A. Weiss, in Pringsheim’s Jahrb. IV. p. 123, &c.

’ Compare Strasburger, Pringsheim’s Jahrb. V. Taf. 36 : idem, Ueber Azolla, Taf. HI.

EPIDERMIS.

35

variously inclined to the slit, or bluntly angular ; it has usually at the united ends of
the cell a different form, and also larger diameter than at the part bordering on thö
slit. Examples, Persoonia myrtilloides and other Proteacese^, Cycas^, Psilotum,
Equisetum, Coniferse, Restiacese, Grasses, Calycanthus Scirpus, Iris, &c. Along
the slit, but at some distance from it, run in most cases on each guard-cell two
ridge-like protuberances (belonging to the membrane, see Sect. 14), one on the outer,
the other on the inner surface, the corresponding ones being continuously
connected at the ends of the slit. The ridges are channelled and concave on
the side facing the slit, and convex on the other side, at the free edge they
are sharp, and thus appear in the transverse section in the form of sharp teeth.
The outer aperture, the entrance (Eingang), and the inner, the exit (Ausgang) of the
slit are thus bordered by the sharp edges of the ridges ; through the edge of the
entrance one enters into the front cavity (Vorhof), which widens out between the
channelled faces; through the edge of the exit into the similarly formed but usually
much smaller back cavity (fimierhoi) ■, the (Spaltendurchgang ^), which

Fig. II. — Cross section through the leaf of
Pinus Pinaster, s guard-cells ; f passage of the
stoma ; v the furrow, limited internally by the
stoma ; c cuticular layers ; a limiting lamellce be-
tween the epidermal and the hypodermal scle-
renchymatous cells ; si chlorophyll-parenchyma
(800). From Sachs’ Textbook.

Fig. 10. — Hyacinthus orientalis, leaf, cross section, e — e
epidermal cells ; entrance of the stoma, which has been
cut through transversely in the middle ; i respiratory
cavity between the parenchymatous cells,/. {800). From
Sachs' Textbook.

widens towards both cavities, leads between the parts of the section of the guard-
cells which are broadest, and nearest to one another, from the front to the back
cavity. The ridges of exit and of entry are extremely various in form and size,
(comp. Sect. 14), they are not uncommonly very small, especially the ridge of exit,
and therefore easily overlooked. It is rare for both or for the ridge of exit to
be really absent. The latter alone is absent in Elymus arenarius, Bromelia Caratas,
Hakea saligna, ceratophylla, Banksia sp.; both in most observed Coniferse® (Fig. i r),
Cycadese®, Ephedra, Psilotum, Azollah

* Mohl, Verm. Sehr. p. 248. ^ Kraus, l.c. p. 320. ® Pfitzer, t.c.

* ‘Eigentliche Spaltöffnung,’ Von Mohl, Bot. Zeitg. 1856, p. 697, Taf. XIII. Here the subject
is explained. Many good drawings by Strasburger in his Beiträge z. Entwickelungsgeschichte d.
Spaltöffnungen, Pringsh. Jahrb. V. p. 297, Taf. 35-42.

^ Hildebrand, Bot. Zeitg. i860, Taf. IV. — Strasburger, toe. cit. fig. 145.

® Kraus, l.c. — Strasburger, fig. 143. ^ Strasburger, Ueber Azolla, Taf. HI.

D 2

3^

CELLULAR TISSUE.

The size of the mature, full-grown stomata is usually smaller than the average
size of the adjoining epidermal cells, often extremely small in comparison with these,
e.g. Salvinia; on the same surface, e.g. the leaf-surface, it is in the majority of
cases on the whole uniform with slight variations. The absolute size of the
space which they occupy in the epidermal surface lies, according to the measure-
ments made byA. Weiss^on 150 plants, between o'oooii“®n (Amarantus cau-
datus ; length and breadth =o-oi6“i'^) and o-oo459™i“a (Amaryllis formosissima,
length 0 074, breadth 0-079““), most cases between 0.0002““ □ and o-ooo8““d .
The size of the open slit apparently bears an almost constant relation to that of the
whole apparatus, but exact measurements of this have been made only for few cases.

The size and form of the slit as well as of the guard-cells vary regularly in
the same stoma, according to the turgescence and tension of the membranes of
the guard-cells themselves and of the surrounding epidermis ; this turgescence and
tension depending upon the supply of water, and the effect of light and heat.
The curvature of the side of the guard-cells next the slit, and accordingly the
opening of the slit, may in each special case increase to a definite maximum, and
on the other hand diminish till the slit is completely and firmly closed. With
these changes of curvature changes in the general form of the guard-cells are
in each case connected. According to H. v. Mohl insolation and supply of water,
according to N. Müller heat and supply of water, bring about the widening of the
slit’“. The very large stomata of Lilium martagon, candidum and bulbiferum,
widen the slit, according to Mohl, to a breadth of Vno“™ Vi24“"^ uninjured

leaf, at the margins of separated pieces of epidermis to Vgg““ ; on the uninjured
leaf of Zea mais to Vns““! ^^e separated epidermis of Amaryllis formosissima
to Vsg““. The slit remains meanwhile always at least six to seven times longer than
broad. Unger® quotes the size of the open slit in Agapanthus umbellatus at
0-000047““ □, of -Ajuga genevensis at 0.0000137““ □ .

The water-pores to be described below (Sect. 8) assume much larger dimensions,
as also the stomata on the leaf of the Kaulfussias. The latter are visible to
the naked eye as round holes, which are surrounded moreover by a pair of
guard-cells apparently incapable of change of curvature.

The absolute height of the guard-cells, after what has been already said,
requires no description. Compared with the epidermal cells, or the many-layered
epidermis of the same surface, the height of the guard-cells is usually insignificant,
often very small; at most they are of equal height with them (e.g. Hyacinthus
orientalist), Lilium candidum®, Helleborus niger. Fuchsia® (Fig. 10). The position
of stomata relatively to the outer surface of the epidermis is closely connected with
these differences. When the height of the guard-cells is equal to that of the
epidermal cells the outer surfaces of both lie approximately in the same plane. The
same occurs in a series of cases where the height is unequal ; here the respiratory

* Pringsheim’s Jahrb. IV.

“ Compare on the mechanism, which must not here be discussed, and which is not even yet
fully explained, the fundamental work of Mohl, Botan. Zeitg. 1856, p. 697; Sachs, vol. IV of this
Handbook, p. 255 ; N. Müller, in Pringsh Jahrb. VIII. p. 75. [Schwendener, I c.]

® Anat. und Physiol, p. 334. * Strasburger, l.c. fig. 14.

“ Mohl, l.c. fig. 6. ** Unger, Anat. und Physiol, p. 190.

EPIDERMIS.

37

cavity situated beneath the stoma is directly bounded by the lateral walls of the
neighbouring epidermal cells, e.g. the leaves of Orchis latifolia (Von Mohl, Lc), the
very large-celled epidermis of the leaf of the Commelinacese (Strasburger, /. c. Fig.
150), Claytonia perfoliata (1. c. Fig. 120), and many others.

More commonly where the height is unequal, the guard-cells lie so that their
inner walls fall approximately" in the same plane as those of the epidermal cells
(comp. Figs. II, 18, &c.). They form then the bottom of a depression, through
which one approaches the stoma from without. This is surrounded by the neighbour-
ing epidermal cells, and is often over-arched at its outer margin by outgrowths
of these, so that the mouth is considerably reduced. This is the case in the majority
of tough-skinned leaves and green stems ; leaf of Polypodium lingua Equiseta
cryptopora (comp, our Fig. 23, Sanio, Linnsea 29, 385, Taf. III. Milde, Mono-
graphia Equisetor.), Coniferse ^ Cycadese (Kraus. /. c.), Monocotyledons, as Aloe
Agave Dasylirion, Hechtia ®, Iris Allium, Orchidacese, &c., and Dicotyledons, as

Fig. 12. — Pholidophyllum zonatum, adult leaf, under surface, y/ superficial view of a piece of Epidermis with a stoma and its
subsidiary cells, £ median transverse section through a stoma; the guard-cells are pushed omtwards by the lateral subsidiary
cells, which have been pushed down beneath them (390),

Ficus elastica'^, australis, Proteaceae®, Nelumbium’’, Dianthus Caryophyllus, and
many others.

Independently of this relation of height the case occurs that the surrounding
epidermal cells are so pressed against the stoma that the latter rises a greater or less
distance into the air above the outer surface of the epidermis, e. g. leaves of
Chrysodium vulgare Aneimia Phyllitidis, hirta Pholidophyllum zonatum (Figs.
12-16), Nerium Oleander, many Proteaceae Helleborus foetidus^®, Rhinanthus,
species of Primula, many Labiatae, Pyrethrum inodorum, &c.

‘ Rauter, Entw. d. Spaltöffn, von Aneimia, u. Niphobolus. Mittheil. d. natur. Vereins f. Steier-
mark. Bd. II. Heft 2 (1870).

^ Hildebrand, Bot. Zeit, i860, Taf. IV.

^ Schacht. Lehrb. Taf. III. p. 24, — Strasburger, l.c. figs. 114, 115.

^ Moldenhawer, Beitr. p, 103. — Oudemans, Comptes rendus, Acad. roy. Amsterdam, vol. XIV
(1862).

® .Schacht, l.c. Taf. IV. pp. 9, 12. — Unger, Anat. u, Phys. p. 192.

® Unger, 1. c. p. 191. — Mohl, Verm. Sehr. Taf. VHI.

’’ Strasburger, /. c. fig. 133.

® Von Mohl, Ueber d. Spaltöff. d. Proteaceen, N. Act, Acad, Leopold. XVI. H, and Verm. Schrift,
p. 245, Taf. VH. VHI.

* Schleiden, Grundzüge, 3 Aufl. i. p. 278.

Strasburger, Lc. figs. 47, 48. “ Lc. figs. 50, 57.

Von Mohl, Spaltöffn. d. Proteaceen, l.c. Von Mohl, Lc. figs. 20, 21.

3»

CELLULAR TISSUE.

From these examples, which might easily be multiplied, the conclusion is drawn
that the superficial position of the stomata is the rule for herbaceous less thick-skinned
parts, and the depressed position for leathery, succulent, and thick-skinned parts ;
but that this is by no means the case without exception. Further, that corresponding
parts of plants of the same family, otherwise of like nature, such as the firm leaves
of the Proteacese and Bromeliaceae, may show the most extreme diversity. As an
instructive example the tender-skinned leaves of Salvinia natans may here be cited,
the small stomata of which are inserted about half way up the epidermal cells, which
are eight to nine times their height h

It is obvious that when the lateral wall of an epidermal cell abuts on a guard-cell, it
must present a difference of form and direction, which is in many cases very slight,
from those lateral walls which do not border on stomata. The relations of height of
the abutting face follow from what has been said above. The abutting face is in the
one series of cases nearly plane, and set perpendicular to the surface, or inclined
obliquely to it, in such a way that it converges with the corresponding face on the
other side of the stoma, towards the inner face. Both arrangements occur in
stomata which are even with the surface, the latter especially in stomata which
project outwards. Still cases occur of stomata seated in deep hollows, which abut
on their subsidiary cells with a plane perpendicular face In other cases the
abutting face is concave towards the stoma, and the guard-cells are fitted into the
hollow with their convex side, and are therefore more or less completely enclosed by
their neighbours. With this is always connected a depression of the stoma (often
only slight) below the outer surface; Iris, Amaryllis formosissima Gramineas, &c.
In deeply depressed stomata (cf. the examples given above), also in Iris and similar
cases, it often happens that the abutting faces are inclined obliquely towards the
outer surface, so that they diverge inwards on both sides of the stomata. In this
case it comes about that the guard-cells lie mainly on the inner side of the neigh-
bouring cells (compare below. Fig. 24, Equisetum).

Irrespective of the faces just described abutting on the stoma, the neighbouring
cells are in many cases of fundamentally similar form to those epidermal cells of the
same surface which do not abut on stomata, e.g. Lilium, Orchis ^ Hyacinthus,
Helleborus, Poeonia, Vicia Faba, Sambucus nigra, many Ferns, Salvinia, and many
other plants from the most different families But in a large number of epidermal
layers, especially of foliage leaves, each stoma is on the other hand bounded by one
or two or several epidermal cells, differing in form and size from the rest which do not
abut on stomata r these not unfrequently resemble the guard-cells themselves.
These peculiar neighbouring cells of the stomata are termed its subsidiary cells, or
subsidiary cells of the pore

Their superficial form is generally Intermediate between that of the guard-cells
and the epidermal cells, or they completely resemble the first. In the latter case the

^ Strasburger, l.c. Taf. XXXVI. figs. 29, 30.

“ Restio diffusivs, fasciculatus, Pfitzer in Pringsheim’s Jahrb. VII, Taf. XXVII. figs. 1-5.

® Von Mohl, Botan. Zeitg. 1856, Taf. XIII. figs. 2, 4.

^ Von Mohl, Botan. Zeitg. 1856. ® Compare Strasburger, l.c.

® Cdhtla laterales. PI. Krocker, de pi. epidermide. Pfitzer, Pringsheim’s Jahrb. VII. p. 536. —
Compare also Botan. Zeitg. 1871, p. 133; Pliilfsporenzellen, Strasburger,./. r.

EPIDERMIS.

39

arrangement is such that the whole convex side of each guard-cell is bordered by one
subsidiary cell ; the stoma thus appears to be surrounded by two pairs of cells, one
pair bounding the slit, and one peripheral to these (e.g. Gramineas, Proteacese, and
the other examples of two lateral subsidiary cells to be cited below) ; often even by
three pairs, since the first pair of subsidiary cells is often surrounded by a second
similar pair (Hakea ceratophylla, saligna &c.).

If there be a difference of height between the guard-cells and the epidermis,
the subsidiary cells often hold an intermediate position also in this respect ; where
the difference in height is great, they are of equal height to the guard-cells, or a little
higher, and with them are fitted either in the outer surface, or at the bottom of the
depression. Rarely the subsidiary cells are much higher than the epidermal cells ;
this is the case in the Scitaminese (Strelitzia ovata, Heliconia farinosa, cf. Bot. Ztg.
1871, Taf. 1. and our Fig. 28 B), where they connect the stoma with the Epidermis
and Hypoderma.

The arrangement of the subsidiary cells may be most intelligibly described in connec-
tion with the history of their development, and that of the stoma ; this shall therefore
here be given.

The stoma itself makes its appearance by the bisection of a cell of the epidermis,
which may be called its Mother-cell"^. The two products of division are the guard-cells.
When they separate from one another, as will be described below, a chink appears
between them.

The development of the stomata takes place in the epidermis at the close of its meri-
stematic (dermatogen-) stage and not quite simultaneously in neighbouring parts, so that
one may find the most different stages of development close side by side.

The origination of the stomata begins thus: The hitherto almost similar polyhedral
cells of the meristematic dermatogen are arranged in longitudinal rows, or irregularly :
either all, or the majority or only single ones of these divide into two dissimilar daughter-
cells. One of these becomes the Initial cell of the stoma, the other an Epidermal cell.
Where the dermatogen-cells form rows, it is as a rule ^ always the apical, or peripheral
part of the cell, which becomes the Initial cell. Exceptions to this are only known
among those abnormalities or deformities which will be described below as twin sto-
mata. Where the serial arrangement of the dermatogen-cells is absent, the relative
position of the initial cells is also indefinite.

The wall which cuts off the initial cell is perpendicular to the Epidermis, or originally
only slightly oblique ; it either stretches as a plane (transverse-) wall between two lateral
walls of the developing dermatogen-cells ; or it is curved in surface view to a U-form,
and then with its tw'o ends it is attached either to one or tw'o lateral faces of neigh-
bouring epidermal cells, or (as a rule in Aneimia) it has the form of a closed ring, which
touches no lateral wall. In the last case the initial cell is surrounded laterally by a ring-
shaped cell, in the preceding case by a cell of more or less horseshoe shape.

In the further growth the three following chief cases occur: —

I. The Initial cell is the direct Mother-cell of the stoma, and the epidermal cells
undergo no further division. This is the case in Iris, Hyacinthus, Orchis, Sambucus
nigra, Ruta graveolens, Salvinia natans, Selaginella denticulata, Asplenium furcatum ;
Silene inflata, Chrysodium vulgare, the two last have a U-formed wall, the others a plane
division-wall^; further Aneimia has as a rule an annular wall.

' Von Mohl, Spaltöffn. d. Proteaceen, l.c.\ Strasbiirger, l.c.
“ Specialmutterzelle, Strasburger, /. c.

^ Strasburger, /. c. ; Pfitzer, 1. c.

* Compare Strasburger, /. c.

40

CELLULAR TISSUE.

2. The Initial cell is the direct Mother-cell of the stoma. Soon after it is marked off,
along each of its sides a narrow piece of the neighbouring epidermal cells is cut off by
walls running nearly parallel to each of the sides: and this takes place —

a. Once in each of the contiguous cells : of these there are four, two bordering on the
ends, and one on each of the flanks of the stoma : there are therefore four subsidiary
cells corresponding in arrangement to these: Tradescantia species of Commelyna
(Fig. 13), Pothos crassinervia (usually), Pholidophyllum (see Fig. 12), Heliconia farinosa^,
Araucaria imbricala®; or 4, 5, and more: Ficus elastica (4-5), Coniferae®, Cycas, etc.;
also Strelitzia ovata (Fig. 28 A).

b. Once in each cell bordering the flanks, so that the stoma is enclosed on either side
by a subsidiary cell similar to the guard-cells. This is the case in Gramineae (probably

Fig. 13. — Commelyna coelestis ; leaf, development of the stomata and subsidiary cells, surface view very young,
B older stage ; s in both the initial- and at the same time mother-cell of the stoma ; C mature, 5 guard-cells. From
Sachs’ Textbook.

all^) and the foliage parts of other grass-like plants (Carex, Cyperus, Scirpus, Juncus
lamprocarpus, effusus, Luzula maxima), Stanhopea, Aloe soccotrina, nigricans, Musa
sapientum®, Claytonia perfoliata, Proteaceae (Protea®), Grevillea robusta®, Lomatia
longifolia, etc.

c. By repeated division of the subsidiary cells separated as in a and b, there arises
in many cases a double zone or a pair of subsidiary cells on each side. The former is
the case in Dioon’, the latter in Maranta bicolor, Commelyna communis, Pothos
argyraea, Hakea saligna, ceratophylla, and other Proteaceae (Strasburger /. r.).

3. The Initial cell is not the Mother-cell of the stoma, but divides further, once or
several times in succession, and the result of these divisions is a Mother-cell of the stoma,
and one or several subsidiary cells. The chief types of this are : —

* Strasburger, l.c. — Moldenhawer, Beitr. Tab. V. p. 94. — Meyen, Physiol. Tab. V ; Phytotomie,
Tab. III. pp. 4, 5. — Schleiden, Grundz. 3 Aufl. I. p. 277.

^ Botan. Zeitg. 1871, Taf. I.

® Strasburger, l.c. — Hildebrand, Botah. Zeitg. i860, Taf. II.

^ Pfitzer, in Pringsheim’s Jahrb. VII. p. 533, &c.

° Strasburger, l.c. ® Mohl, Spaltüffn. d. Proteaceen, l.c. ’’ Kraus, l.c. p. 335.

EPIDERMIS.

4^

a. The Initial cell, bounded by a curved, or even U-shaped wall, is again divided by
a wall almost similar to the latter into a Mother-cell and a horseshoe-shaped subsidiary
cell (Asplenium bulbiferum', Pteris flabellata (Fig. 14), cretica^); or successively by 2-3
curved walls, which alternate in two directions in the surface, and cut one another, into
a mother-cell, surrounded by a zone (or in parts a double zone) of half ring- or horse-
shoe-shaped subsidiary cells. The longitudinal axis of the subsequent slit is parallel to
the chords of the previous curves of division: Cibotium Scheidei (Hildebrand, I.c. Fig.
37-39), Mercurialis perennis, ambigua, Pharbitis hispida, Basella, Pereskia aculenta; or

Textbook.

it cuts them at right angles : Thymus serpyllum, Physostegia virginiana, and other Labiatae
(Strasburger, 1. c.). In the last category but one are also the Equiseta.

b. The Initial cell is divided successively by walls arranged in three directions in the
surface into a simple or multiple zone of subsidiary cells, and a mother-cell surrounded

divided ; the numbers indicate the successive division walls. B almost mature, e and numbers as in A. From Sachs’
Textbook.

by it. With few subsidiary cells: Papilionaceae, Solanacese, Asperifolias, Crucifers;
with a large number of them: Crassulaces (Fig. 15), Begoniaces®, also Cactaces,

* Strasburger, I.c. figs. 36-41.

^ Hildebrand, Botan. Zeitg. 1866, Taf X, fig. 20-23.

^ Strasburger, 1. c. — Compare for details this work so often cited ; also the not always precise
statements of Karelstschikoff, Zur Entw. der Spaltöffnungen, Bull. Soc. Imp. de Moscou, 1866.

42

CELLULAR TISSUE.

c. Initial cell divided by an annular wall into Mother-cell and annular subsidiary cell:
Polypodium lingua (Rauter, /, r.).

According to what has been said, subsidiary cells of special form originate in all the
cases given under 2 and 3 : in those under i only when the U- or annular-form of the
first boundary wall necessitates special peculiarities of form. The mode of formation
may be always recognised in the known cases in the mature state, but with varying
sharpness, according as the subsequent growth of the cells in surface and height sharpens,
retains, or obliterates the original distinctions.

Oscillations and transitions between the related types are by no means rare. For
details comp. Strasburger and Pfitzer /. c. As regards occasional malformations, we
must here again return to the twin stomata, i. e. those which appear in contiguous pairs,
and refer to Pfitzer’s detailed statement according to which these may arise by means
of many different anomalies of division.

Two normal exceptions must here be somewhat more carefully described. First that
of Aneimia, discovered by Link, later for a long time much discussed, and misunder-
stood, and finally explained by Rauter, who showed that the same was the case in

Fig. 16.— Aneimia hirta; leaf, epidermis, b mature; a surface view, b section perpendicular to the surface,
median through the stoma (375). c, d very young {6co) ; c surface view with one fully-developed stoma, and five
mother-cells as yet undivided; the protoplasm of these has contracted from the delicate membrane through the
process of preparation ; fi unicellular hair ; d perpendicular section through a mother-cell of a stoma with the sur-
rounding cells.

Polypodium lingua. In these cases the stoma is surrounded by one annular Epidermal-
or subsidiary cell

The remarkable point in this phenomenon is nothing more than that the wall of the
mother-cell in normal cases has the form of a ring set at right angles to the surface.

* Pringsheim’s Jahrb. VII. p. 551.

^ Link, Ausgewählte anatom. Abbildungen, Heft III. Taf. IV. 8. — Oudemans, Bulletin du
Congres de Botanique, &c. ä Amsterdam, 1865, p. 85. — Hildebrand, Botan. Zeitg. 1866, p. 245. —
Strasburger, in Pringsheim’s Jahrb. V. l.c. ; also VII. p. 393, Anm. — Rauter, l.c.

EPIDERMIS.

43

between the outer and inner wall, which touches no lateral wall, and which diminishes
conically inwards. In Aneimia (Fig. i6) it, or rather the stoma, is therefore sur-
rounded by an annular epidermal cell. Almost the same applies for Polypodium
lingua (see above, 3. r) ; the annular cell is in its turn as a rule surrounded by a horse-
shoe-shaped neighbour, from which it was originally separated by a U-shaped wall. But
often (Rauter, Fig. 18) also this wall is not U-shaped, but annular, the stoma is thus sur-
rounded by two concentric annular cells. In Aneimia Phyllitidis, and hirta, as in Poly-
podium lingua, it happens exceptionally, and in Aneimia villosa (according to Stras-
burger) it is the rule that the typically annular walls are U-shaped, and attached to a
lateral wall. Those of the mature parts are arranged accordingly. Further it occurs
not unfrequently that from one or from both ends of the stoma (and in the former case,
according to Strasburger, always the peripheral end) a membrane runs bridge-wise to the
nearest lateral wall (Fig. 16, c). In face of the many attempts to explain and interpret
this phenomenon it may be remarked, that from the first there is nothing more than the
appearance shows at once, that is a membranous band, arranged as described, growing
with the other membranes, and requiring an explanation of its appearance no more and
no less than any other membrane.

The second case, which is to a certain extent peculiar, but which otherwise belongs
to the group (3. a), is the formation of the stoma of the Equiseta. It is here stated
according to Strasburger (/.r.). The Initial cell, the first appearance of which was not
observed, is nearly cubical, the two flanks being parallel to the longitudinal axis of the
stem. Near to its own longitudinal axis, thus defined, there appear symmetrically, right
and left, two nearly radial longitudinal walls : both are concave on the sides facing one
another, and contiguous at their upper and lower ends. The initial cell is thus divided

FIG. 17. — Development of the stoma of Hyacinthus orientalis. On the left the mother-cell, just divided ; B successive
further stages of development; on the right the formation of the slit is complete (/); the other letters as in Fig. lo, which
should be compared (800). From Sachs’ Textbook.

into one central biconvex-, and two lateral plano-convex daughter-cells ; the two latter
lessening wedge-wise inwards, the central one outwards. The central cell is the mother
cell of the stoma (it divides later by a longitudinal wall into the two guard-cells), the
two lateral ones are the subsidiary cells. The latter assume a form exactly similar to
the guard-cells, and over-arch them, so that they cover their whole outer surface, and
only leave a narrow space free above the true entrance of the stoma. Hence the form
of the double pair of guard-cells apparently covering one another. In the Equiseta
cryptopora of Milde the matter is further complicated by the depression of the stoma
with its subsidiary cells (comp, below. Fig. 23).

To form the stoma, the mother-cell divides — after, rarely before the completion of
the last division, which produces subsidiary cells — into two halves, which are the guard-
cells ; and the slit appears thus : the division wall between the two splits in its central
part into two lamellae which gradually separate from one another (Fig. 17). This
separation proceeds from the middle towards the ends, and from the entrance and exit

44

CELLULAR TISSUE.

towards the passage of the future slit^. The free edges of the ridges of exit and
entrance correspond to the inner and outer edge of the original wall of division. The
origin of the respiratory cavity by separation of the sub-epidermal cells precedes the
formation of the slit.

The mother-cell of the stoma and the products of its division are of equal height
with the other epidermal cells, and lie in the same plane as they. The subsequent
various unevenness of height and position of epidermal- and subsidiary-cells and of
stomata arises through the growth of the cells subsequently to their division. During
growth all cells without exception increase in volume. But the passive tension by
the internal tissue, which the epidermis of growing, as also of adult parts undergoes,
finally brings about, as Pfitzer has shown in the stomata of the Grasses^, a considerable

Fig. i8.— Ficus elastica ; leaf, transverse section, e^e in each case the thickness of the epidermis ; A (600) upper
side, A[ (390) under side'of the same very young leaf ; in j a mature stoma, which remains superficial, and a (tran-
sitory) hair; in A two cystolith-cells, recognisable by their thickened outer wall, epidermal cells still undivided.

B {600) upper side, (390) under side of a somewhat older leaf, epidermal cells dividing. In B, x \s 3. younger
cystolith-cell, and X[, an older one which shows the peg-shaped outgrowth of the wall. C (390) older leaf, under side ;
division of now three-layered epidermis is ended, stoma depressed, but the final size and form of the parts is not yet
attained. E upper side of a mature leaf, four-layered epidermis, cystolith-cell (375).

diminution of the absolute height and breadth of the part of the stomatal cells which
border the slit. (In Zea Mais the breadth soon after the appearance of the slit amounts
to 1 1.4 ju, later to u.6 p, in the mature state to 5.4 jx.) The same will apply for the other
cases above alluded to, in which the part of the guard-cells bordering the slit is narrower
and smaller than the connected ends.

All the phenomena of development here touched upon are the same, whether the
epidermis consists of a single layer, or of several. Only in the latter case (Fig. 18)

^ Von Mohl, Verm. Schriften, pp. 2^4-2^’]-. — Strasburger, l.c. p. 508. — Pfitzer, Pringsheim’s Tahrb.

VII. /. r. Pringsheim’s Jahrb. VII. /. r.

EPIDERMIS.

45

tangential divisions accompany the extension of the epidermal cells perpendicularly to
the surface, either successively from within outwards (Ficus), or the converse (Begonia,
Peperomia). The extension and division of the epidermal cells above mentioned always
begins after the differentiation of the initial cells of the stomata. The stoma itself re-
mains, so to speak, always a single layer, the same is the case with the cells immediately
surrounding it (subsidiary cells) in Begonia \ but in Ficus, tangential divisions appear
also in the subsidiary cells, which arise according to the type (2, a) ; hence a ring of sub-
sidiary cells two or three layers deep^ In the growth of the epidermis perpendicular
to the surface this difference here occurs, that single stomata fully developed at first,
before the tangential division begins, remain at the surface ; the majority are matured
later, and are deeply depressed below it (comp. Fig. 18). The superficial stomata first
developed are surrounded by several partitioned zones of subsidiary cells.

For the relative position of the stomata the rule holds, that in elongated parts all
the slits run parallel to the longitudinal axis. In parts which do not grow specially
in length the slits are arranged apparently without rule in different directions.
Exceptions to this rule are the stomata on the stems of Viscum album Cassytha,
Thesium, Choretrum, Mida, Myoschilus, Anthobolus, Exocarpus, Arceuthobium,
Antidaphne, Loranthus, Lepidoceras, Nuytsia^ Colletia®, Santalum album, Sali-
cornia Casuarina Staphylea pinnata, on the under side of the leaf of Philesia
buxifolia. Here the slits run perpendicular to the axis of the whole organ, often
(e.g. Salicornia, Arceuthobium, Colletia, Philesia) the epidermal cells are at the
same time extended transversely.

Sect. 6. According to special differences of form, structure, and arrangement,
two varieties of stoma may be distinguished, which may briefly be termed air-pores
(or stomata), and water-pores. Both may occur separately, or side by side in one
piece of Epidermis.

Sect. 7. The air-pores show the slit itself, during normal vegetation, filled
with air ; they lead from the surrounding medium directly into the respiratory cavity,
which is also filled with air. Its guard-cells are, with exception of the abnormal
case of Kaulfussia, we may say always capable of change of curvature, and the slit
therefore of variable dilatation. They represent accordingly to a certain extent
openings in the epidermis which are capable of closing, through which the air
enclosed in the plant communicates with that surrounding it. Their arrangement,
presence, and absence are thereby generally defined.

Air-pores, and all stomata whatever, are completely absent in roots. Of
the other parts of the plant hardly one can be named on which they may not, at
least in many cases, be observed

' Pfitzer, Ueber d. Mehrschichtige Epidermis, &c., Pringsheim’s Jahrb. VIII. /. c.

^ Strasburger, t.c. Tab. 41, figs. 135-138, and our fig. 18, both of E'iats elasfica.

^ Von Mohl, Botan. Zeitg. 1849, Tab. IX; Chatin, Anatomie comparee des Vdgetaux, Plantes
parasites, Tab. 80, 82.

* Chatin, l.c. Tab. 5, 6, 57, 58. 59, 64, 69, 70, 72, 77, 78, 87, 109, no.

^ Pfitzer, Pringsh. Jahrb. VII. p. 549.

® Duval- Jouve, Bulletin de la Soc. Bot. de France, XV. (1868) p. 139.

’ Loew, de Casuarinearum caulis foliique evolutione et structura, p. 35.

® Rudolph! (Anat. p. 91) speaks of stomata on the anthers of Lilium biilbifc7-um ; Unger
(Exanth. p. 127) on those of Capsella bursapastoris, ‘in a pathological state’; on the integument
of Canna, Schleiden, Beitr. p. 10 ; also on the outer margin of the seed in Tulipa, Czech, Botan.
Zeitg. 1865, p. 10). — They exist on Perianths, both with and without chlorophyll, in many

46

CELLULAR TISSUE.

The chief place where they occur is the green leaf, surrounded by air, especially
the leaves of land plants and floating water plants. Certain land plants destitute of
chlorophyll, viz. Monotropa Hypopitys and Neottia Nidus avis^, have no stomata
at all. With the exception of the pistil Lathraea squaniaria is without stomata
On the contrary, on the leaf of Lathrsea clandestina as also of the Orobancheae *
and Lennoaceae they occur in considerable numbers, on that of the Cuscuteae * at
least here and there.

On Rhizomes® growing in the ground they are not uncommon, at least in
isolated cases ; e.g. the young potatoe before formation of the cork-layer the
tuberous stem of Herminium Monorchis.®, the rhizome of Epipogon, &c.

In parts which are submerged air-pores are as a rule completely absent, but
here also exceptions occur. They are to be found regularly on the submerged
primordial leaves and the germinal leaf of the Marsiliaceae on the submerged leaves
of the Calitrichinese, Sect. Eucallitriche Askenasy found single ones on the
cotyledons of Ranunculus aquatilis normally unfolded under water. The statement
of H. Weiss on their occurrence on submerged parts of Najas and Potomageton is
not confirmed.

In water-plants whose leaf can vegetate either submerged or in the air, as
Ranunculus aquatilis, the Callitrichinese, Hottonia, Myriophyllum, Marsilia, &c., the
occurrence or distribution of air-pores varies according to the above-stated habit.

The air-pores occur (perhaps with exception of single cases of their solitary
appearance on submerged parts) only where intercellular spaces, containing plenty
of air, are present in the tissue covered by the epidermis. Still stomata are not
always present where the latter is the case. Where tissue rich in air alternates with
tissue with little or no air (Sclerenchyma, Collenchyma) there is as a rule in the
epidermal tissue covering them a corresponding alternation of spots with and without
stomata^*. Connected with these are the universal phenomena of absence of stomata
on the nerves of leaves ; their occurrence near and between these, their absence on
the channels and edges of channelled leaves, petioles, stems, and their presence
in the surfaces, or furrows alternating with these ; (e.g. leaves of Bromeliacese, Phor-
mium. Grasses, stems of the Umbelliferse, Equiseta, &c. : stomata-bearing bands
and spots on the young shoots of Hedera, Juglans, Populus on the sides, and at the

plants, in others they are absent. Compare Rudolphi, Anatomie, pp. 85-91 ; Treviranus, Verm.
Schriften, p. 50 ; H. Krocker, de Plantar. Epidermide (1833), p. 16 : A. Weiss, Verhandl. Zool. hot.
Vereins in Wien, 1857; and especially Hildebrand, Einige Beobachtungen aus der Pflanzen- Ana-
tomie (1861). ^ Rudolphi, Anatomie d. Pfl. (1807), p. 66.

“ [Krause, Beitr. z. Anat. d. Vegetationsorg. d. Lathraea Squamaria. Diss. Breslau. 1879.] Bow-
man, Trans. Linn. Soc. London. XVI.

^ Duchartre, Sur la Clandestine de l’Europe. Mem. de ITnstitut de France, 1848.

* Unger, Exantheme d. Pfl. p. 49.

^ H. Graf zu Solms-Laubach, Die Lennoaceen (Halle, 1871).

* Hohnfeldt Botan Zeigt. 1881, p. 38. ’ Caspary, Botan. Zeitg. 1857, p. 117.

® Prillieux, Ann. sei. nat. 5 Ser. IV. p. 265, pl. 15.

® A. Braun, Monatsbr. d. Berlin. Acad. 1870, p. 665.

Hegelmaier, Monographie der Gattung Callitriche, p. 10. “ Botan. Zeitg. 1870, p. 198.

[Cf. Potonie, Beziehungen zw. d. Spaltöffnungsystem u. d. Stereom. b. d. Blatt-Stielen d.
Filicineen, Ref. Bot. Centralbl. 1881. Bd. 8. p. 70]

Compare Trecul, Comptes rendus, torn. 73, p. 15.

EPIDERMIS.

47

base of the petiole of the Ferns (comp, below, Chap. IX). The occurrence of
stomata in hollow depressions on the under surface of the leaf of many species of
Banksia, and Dryandra ^ is a special case of the same thing, which derives its peculiar
appearance only from the strong outgrowth of the nerves on the under side of the
leaf. On the under surface of the leaf of Nerium oleander there alternate, between
the nerves, spots with and without stomata. The latter occur in depressions of the
leaf-surface, which are deep and narrow-necked, and covered thickly with hairs

On the part or band, which bears stomata, the air-pores are besides in rare
cases limited to circumscribed spots, separated by intermediate areas without stomata :
the spots are then also characterised by a special form of the epidermal cells. Thus
on the flat underside of the leaf of Saxifraga sarmentosa numerous stomata are
collected in circular groups, removed some distance from one another^; on the under
side of the leaf of many (but not all) Begonias, e.g. B. manicata, spathulata, Drhgei,
heracleifolia, two to six or more stomata stand side by side over a great common
respiratory cavity ^

As a rule there is over large surfaces and bands an almost uniform distribution
of air-pores. Their number, both relatively to the number of the epidermal cells and
to a definite superficial space, varies within wide limits according to the organ and
species, and to some extent according to the condition of the surrounding medium.
In the first relation we have, on the one hand, one stoma to almost every epidermal
cell, e.g. in leaves of Monocotyledons, as Iris ; on the other hand, as in the stems of
many woody plants, Cuscuta, &c., there is one stoma to many hundred epidermal
cells®. In the other relation, the maximum numbers found for were 625

(under surface of leaf of Olea Europsea ®), and 716 (under surface of the leaf of
Brassica Rapa''). For most foliage leaves the number lies between 40 and 300,
rarely higher or lower As above stated, on the stems of many woody plants the
stomata lie several millemeters, or still further apart, as is conspicuously shown on
the formation of lenticels of Sambucus, Acer, &c. (Chap. XV). A like stage of de-
velopment being of course assumed, there may be laid down for each part of each
species a definite average number, which is, it is true, liable to not inconsiderable
individual variations. Karelstschikoff communicates examples of individual variation.
On an equal surface (the same field of the microscope, which was not measured)
six leaves of Viola tricolor, each taken from a different stock, had on the under
surface between 21 and 43, the majority between 30 and 40; on the upper side
o to 14, the majority between 9 and 13.

* Von Mohl, Spaltöffn. d. Proteaceen, Verm. Schriften, p. 245.

^ Amici, Ann. Sei. Nat. XXL p. 438. — H. Krocker, l.c. p. 13. — Meyen, Physiol. I. p. 291. —
Compare Pfitzer, Pringsheim’s Jahrb. VIII. p. 49.

® Treviranus, Verm. Schriften, IV. 30.

* Viviani, Della striittura degli organ, element, tom. I. fig. 4, p. 151, quoted by Treviranus,
Physiol. I. p. 466. — H. Krocker, l.c. p. 13, fig. 39. — Meyen, Physiol, I. p. 280, Tab. V. — On the
development of the groups, compare Pfitzer, Pringsh. Jahrb. VII. p. 551.

® Compare the figures of Strasburger, Pringsheim’s Jahrb. V ; Hildebrand, Botan. Zeitg. 1870,
Taf. I.

® Weiss, Unters, über die Zahlen- und Grössenverhältn. d. Spaltöffnungen. Pringsheim’s Jahrb.
IV. p. 124 ff.

’ Unger, Anatom, und Physiol, p. 193.

Compare Weiss, /. c.

48

CELLULAR TISSUE.

Of the members surrounded by air, stems bearing chlorophyll are rich in
stomata if leaves are absent : e.g. Equisetum, Salicornia, Casuarina, Colletia,
Cactaceae, &c. There are eighteen stomata on in Cereus speciosus (Krocker).

Leafy stems also, whose own foliage-surface is relatively very large, are rich in
stomata, e.g. Campanula patula, linifolia. Salvia glutinosa. Polygonum aviculare, Vicia
Faba, segetalis, Epilobium palustre, Capsella Bursa Pastoris, Möhringia trinervia,
Linum catharticum, Potentilla aurea, and many others (Unger, Exanth. pp. 98-137).
Unger ascribes numerous stomata to the green branches of ligneous plants, such as
Vaccinium Myrtillus, Rhamnus cathartica, and Frangula. Morren found in Prunus
Mahaleb 18, and in Rosa damascena 36 on each Similar large numbers

are found in related species, in Viburnum opulus, &c. (Stahl, Botan. Ztg. 1873,
p. 578). In very many plants, on the other hand, very scattered stomata occur on
the stems ; e.g. in Prunus domestica seven, in Solanum tuberosum four on each
I ram □ , or still fewer ; only in rare cases there are none at all.

From occasional observations on the petiole similar results are obtained as for
the stem.

Numerous observations of their number and distribution have been made on the
parts where they occur in largest numbers, viz. the laminae of green foliage leaves of
land and aerial plants. The older observations of Hedwig, Von Humboldt, Sprengel, the
copious works of Rudolphi, and other more scattered notices, have been followed more
recently by the works of H. Krocker, Unger, A. Weiss, E. Morren, Czech, and Karelst-
schikoff*. The very full statistics of Weiss inform us that of 157 species of land plants
investigated, the mature foliage leaves have on an average on the space of i'"“ □
less than 40 stomata in 12 Species
40 — 100 „

100-

200-

-200

-500

300 — -400

550

more than 600

42

38

39
12

The distribution of the air-pores over the surface of the leaf is in land plants directly
connected with that of the air-containing intercellular spaces. It differs therefore ac-
cording as the leaf shows a bifacial or centric arrangement of the chlorophyll-con-
taining parenchyma, and depends in individual cases upon the number and width of the
lacunae in this tissue. (Comp. Chap. IX.)

Herbaceous, flat, horizontal leaves with bifacial arrangement of the Parenchyma have
usually stomata on both surfaces. Of 466 such species Karelstschikoff found this to be
the case in 450. But of these 37 have on the upper surface only very few, often only
solitary ones, lying near the nerves : and the majority are much poorer in stomata on
the upper than on the under surface.

Firm, leathery, horizontal, also bifacial leaves, with smooth, shining upper surface, as

^ K. Sprengel, Anleitung z. Kenntn. d. Gewächse, I. — Unger, Exantheme der Pflanzen (1833).

Anat. und Physiol, d. Pfl. pp. 193, 334 Compare on the older literature, Meyen, Phytotomie,

p. 108 ; E. Morren, Determination des Stomates de quelques vegetaux, Bullet. Acad. Bruxelles,
tom. XVI (1864) ; Czech, Ueber Zahlenverhältnisse und Vertheilung d. Spaltöffnungen, Botan. Zeitg.
1865, p. loi; A. Weiss, Ueber die Zahlen- und Grössenverhältn. d. Spaltöffnungen, Pringsheim’s
Jahrb. Bd. IV; Karelstschikoff, Ueber d. Vertheilung der Spaltöffnungen auf d. Blättern, Bulletin
Soc. Hist. Nat. de Moscou, 1866. For many details we must here refer to these works, which do not
by any means coincide on all points.

EPIDERMIS.

49

Abies pectinata, Nerium, Rhododendron, Ilex, Ficus, Begonias, and many others, have
stomata as a rule exclusively on the under surface : the same is the case with many firm
herbaceous leaves, as in Glechoma hederacea, Asperula odorata, Trollius europseus, &c.
(Karelstschikoff), Betula alba, Pirus communis, Carpinus, &c. (Morren). Rarely the
relation is reversed, and with it the inner structure of the leaf also : there are herba-
ceous, and even leathery leaves (Pinus sylvestris and its allies, Eryngium maritimum L.,
&c.) with more stomata on the upper than the under surface ; or with the upper
surface exclusively bearing stomata, the under surface without them, as Pinus strobus.
Thuja spec., Passerina hirsuta filiformis, ericoides, and many Graminese with a deeply-
grooved upper side of the leaf, e. g. Aira flexuosa, Calamagrostis epigeios, Stipa pennata,
&c., which will be mentioned below. Flat leaves which stand vertically, and most
fleshy juicy ones (Crassulaceae, many Monocotyledons), bear stomata as a rule, though not
without exception, on both sides ; either they are equally numerous on both sides, or they
preponderate on one side or the other. In this they answer to their centric structure.
Leaves, which float on the surface of the water, have stomata exclusively on the upper
side, or at least chiefly so, as Callitriche (Hegelmaier l.c.), the floating leaves of Sagit-
taria^. Ranunculus sceleratus®. Further general rules, or laws for their distribution and
their relations as to number, cannot for the present be laid down. No general decisive
dilferences, either according to natural affinity and habit, or other conditions of structure
of the epidermis, hold good throughout. Further, the proposition that, the more sto-
mata there are on a surface, the less their size, and 'vice 'versa, is not without exceptions.
Of the observed cases of variation and conformity many cannot be referred directly
to immediate adaptation. For instance, of the two Lathraeas, above cited, which are of
the same habit, with similar members and structure, the one has many stomata on stem
and leaves, the other none.

But on the other hand, the occurrence and distribution of air-pores yields many re-
markable examples of the change of structure by direct, often individual adaptation.
This is especially the case for the amphibious water plants, and indeed all these, though
they belong to the most different families and genera, as Marsilia, Sagittaria, Polygonum,
Callitriche, Myriophyllum, Hottonia, Nasturtium, Ranunculus, show the same beha-
viour, viz. that where numerous stomata are found on surfaces developed in the air,
corresponding surfaces developed under water have fewer stomata or none at all.

Marsilia quadrifoliata and other species of the genus * have, according as their habitat
is submerged or not, floating leaves, with their upper side only exposed to the air and
borne by thin delicate stalks, or leaves, borne on short stout stalks, rising into the air.
These aerial leaves have on both surfaces almost equally numerous stomata, sunk slightly
beneath the outer surface, between the strongly sinuous epidermal cells. In the floating
leaves only the upper surface bears stomata, and on the same area of surface more than
double as many as the aerial leaf. They lie between less sinuous epidermal cells (comp,
above, p. 31), and in M. quadrifoliata, pubescens, diffusa, Ernesti, not depressed ; in other
species, as M. Drummondii, macra, they are depressed like those of the aerial leaves.

A similar difference exists between the aerial and floating leaves of Polygonum
amphibium, and Nasturtium amphibium®. The petiole and laciniae of the cut leaves of
Ranunculus aquatilis, divaricatus, Myriophyllum, and Hottonia®, which in their normal
submerged state are without stomata, form numerous stomata when they develop in the
air (Land form).

' Carnel, Nuovo giomale botan. Italianc, I. p. 194.

^ Hildebrand, Botan. Zeitg. 1870.

® Ascherson, Botan. Zeitg. 1873, pp. 422, 631.

* Hildebrand, Botan. Zeitg. 1870, p. i, Taf. i. — A. Braun, Monatsber. d. Berlin. Acad. 1870,
p. 670. — On the inconstant or exceptional behaviour of M. JEgyptiaca and some others, compare
the same.

® Hildebrand, /. c. ; Karelstschikoff, /. c. ^ Askenasy, 1. c.

E

50

CELLULAR TISSUE.

Sagittaria sagittaefolia has stomata on both sides of the aerial leaves, 4-5 times as
many below as above, on an equal area of surface ^ : on the floating leaves they are very
rare on the under surface, but numerous above. — In the terrestrial forms of the Eucalli-
triches the stem and both leaf-surfaces are rich in stomata^, on the submerged forms they
are absent on the stem, and occur only solitary on the leaves, on the floating leaves they
are numerous on the upper side. A similar relation to that in Sagittaria occurs in the
aerial, and the casually or abnormally produced floating leaves of Ranunculus sceleratus.
An old statement of De Candolle, according to which leaves of Mentha developed
under water have no stomata, is doubtful, and decidedly opposed by Rudolph! These
facts are in accordance with the constant absence of stomata on certain submerged
species, and their presence on closely-related, terrestrial species, e. g. in the genus
Isoetes.

How far the finer gradations of distribution are directly caused by the mode of life
and condition of vegetation requires careful investigation ; in which, besides experi-
mental treatment, it is important to compare, not, as has hitherto usually been the case,
a large number of casually selected plants, but such as are closely related. By the latter
method Pfitzer^ has obtained the following result for a large number of indigenous
grasses : that for these plants the number and distribution of the air-pores, together
with the form of the surface and internal structure of the whole leaf, stand pretty
generally in definite relation to the wetness of the locality. On both flat leaf-surfaces are
numerous stomata in all marsh- and water-grasses observed (9 species, e. g. Phragmites
communis, Alopecurus geniculatus): in numerous meadow- and weed-grasses (34 species,
e. g. Alopecurus pratensis, Anthoxanthum odoratum, Hordeum murinum,Triticum repens):
among the latter Festuca elatior is a remarkable exception, in that stomata appear only on
the upper side of the leaf. Almost all grasses inhabiting very dry localities have leaves
with well-marked longitudinal folds; the surface is therefore marked with long and narrow
furrows, and the stomata are almost exclusively on the sides of the grooves of the upper
side of 'the leaf (12 species, e. g. Aira caryophyllea, flexuosa, Elymus arenarius, Stipa pen-
nata). Koeleria cristata and Agrostis vulgaris have, with leaf structure otherwise re-
sembling the latter category, numerous air-pores also on the under side of the leaf.
The remaining 14 investigated species inhabit bright glades, sunny hills and grass plots ;
they have leaves flat on both sides, and, like the above meadow-grasses, some of them
have stomata on both sides (Avena pratensis, Holcus mollis, Phleum Boehmeri, Poa
bulbosa, compressa, nemoralis. Milium effusum), others — perhaps, with exception of
Triodia, plants which live only in shady situations — have them only on the upper
side (Brachypodium silvaticum, Festuca gigantea, heterophylla, Melica nutans, uniflora,
Triodia decumbens, Triticum caninum). Milium is an exception as compared with
these.

Sect. 8. Numerous phanerogamic plants, of the most various adaptation, have
usually besides the air-pores other stomata different from these, which may be
called Water-siomata or -pores^, since, under definite normal conditions, they serve as
points of exit for excreted drops of water. These drops in many cases hold in
solution large quantities of calcium carbonate, which dries into small scales. These
differ accordingly from the air-pores by the slit (and the respiratory cavity below
it) being, at least at times, filled with water. They are further characterised, as far
as investigations extend, by their guard-cells being immovable, that is, they are
incapable of independent intermittent widening. In many cases this is beyond

* Karelstschikoff, /. c. ^ Hegelmaier, /. c.

’ Anat. d. Pfl. p. 69. * l.c. Pringsheim’s Jahrb. VII.

^ [Cf. Langer, Botan. Zeitg. 1879, p. 511, — Gardiner, Quart. Journ Micr. Sd, 1881, p.407.]

EPIDERMIS.

5^

doubt, since here the guard-cells die off at an early stage (e. g. Tropseolum, Colocasia,
Aconitum, &c.), or disappear altogether (Hippuris, Callitriche) ; other cases certainly
require confirmation. Finally, there is often besides this a considerable difference of
form and size from the air-pores, which sometimes occur on the same epidermal
surface with them.

The water-pores always lie over the ends of the vascular bundles, the structure
of which is described in Chapter VIII ; and therefore usually near the margin of the
leaf, on the teeth, and, in most known cases, on their upper side: more rarely on
other parts of the leaf-surface, singly or in groups, in the latter case often between
epidermal cells, which differ from the rest in special form and (smaller) size. Also
in closely related species there is, according to the species, in one case a single pore,
in another a group of pores. The higher their number at one place, the smaller on
the average is their absolute size, and also the difference in size between them and
the air-pores connected with them. The absolute size is in extreme cases very
considerable, by far exceeding the maxima for the air-pores.

According to their shape, one can distinguish two extreme forms of water-
pores ; on the one hand those with almost semicircular guard-cells, and with a
slit always quite small and skorl (Crassula, Ficus, Saxifraga) ; and on the other hand
those with a very large, long slit which is always found open, e. g. the huge
stomata on the leaves of Aroideae, Papaveracese, and Tropseolum. The largest
of the latter are not uncommonly examples of the early death of the guard-cells pre-
viously mentioned.

The occurrence of water-pores is a very widespread phenomenon, to which the not
very lucid statement of Trinchinetti on ‘Glandulae periphyllae’ refers^. Recently Met-
tenius and after him Rosanoff Borodin *, and Magnus ®, have paid especial attention
to them.

The form v. ith relatively longer slit is known among land plants in the case of the
water-dropping apices of leaves of the Aroideae; in Colocasia antiquorum®, Caladium
odorum and C. esculentum there are two or three enormously large, wide, open
pores. 'J'he water-dropping spot on the middle of the under side of the hair-like leaf-
apex of Richardia aethiopica has numerous widely open stomata, which are larger and
rounder than the air-pores. Further, of Dicotyledons, the following cases, mostly on the
authority of Mettenius, are to be mentioned.

One relatively very large, wide, open pore is to be found at the apex of the leaf-teeth
of the Fuchsias (Fuchsia globosa, &c.). Primula sinensis (rarely 2), (comp, below. Chap.
VIII); on the upper side of each tooth (and of the apex of the leaf) in Saxifraga
orientalis, cuscutaeformis, punctata, Heuchera, Mitella, Soldanella Clusii, Primula auri-
cula, marginata, acaulis, species of Aconitum and Delphinium, Eranthis ; one or two in
the same position in Sambucus nigra, Valeriana sambucifolia, Doronicum Pardalianches,
Ribes triste, Prunus Padus; three in Cyclamen; a group of 3-6 of them in the same
position in Ulmus campestris, Carya amara, Cratsegus coccinea, Helleborus niger,
Geranium macrorhizum; of 6-8 in Crepis sibirica, Helenium autumnale, Verbesina
virginica ; an about equal, but not quite definite, number of them at the same point in

* Linnsea, Literaturblatt, pp. 11, 66. Filices horti Lipsiensis, pp. 9, 10.

® Botan. Zeitg. 1869, p. 883. ■* Ibidem and 1870, p. 841.

* Botan. Zeitg. 1871, p. 479.

® Duchartre, Ann. Sei. Nat. 4 Ser. tom. XII. p. 264, pi. 1 7.

’ E, de la Rue, Botan. Zeitg. 1866, p. 321. ® Mettenius, l.c.

E 2

52

CELLULAR TISSUE.

Hieracium sabaudum, Eupatorium verticillatum, Platanus occidentalis, Corylus Avellana,
Claytonia linoides, Escallonia spec., Aralia racemosa, Ferula tingitana. A numerous group
of pores is to be found at like points in Tommasinia verticillaris, Archangelica officinalis,
Smyrnium perfoliatum, Heracleum flavescens, Eryngium planum, and other Umbelli-
ferae ; Cerastium glabratum, Geum agrimonioides, Aremonia, Potentilla Thuringiaca, and

Fig. 19. — Tropaeolum Lobbianum; upper surface of leaf. ./I (150) epidermis, from the margin, over
the end of a strong vascular bundle, with three large water-pores; s air-pore. B, C perpendicular
section through' water-pores and their immediate vicinity (250}. In C some of the cells surrounding
the wide respiratory cavity have grown out into large papillae rising into the cavity.

other species ; Alchemilla vulgaris. Ranunculus lanuginosus, and other species ; Physos-
tegia virginica, Lycopus exaltatus, Hieracium Pilosella, denticulatum (apex of leaf),
Rudbeckia speciosa, Senecio vulgaris, and other Compositae; Valeriana Phu., Brassica
spec., &c. In the examples given, the upper side of the leaf has air-pores also. Galium

EPIDERMIS.

53

Mollugo and Rubia tinctorum have practically no air-pores on the upper side, at the
apex they have a group of water-pores.

In Papaver orientale, somniferum, and other species, 2-3 large pores lie in a small
cowl-like depression on the under side of the teeth of the leaf.

Tropaeolum majus, Lobbianum and other species have over each nerve-ending at the
margin of the peltate leaf one very large water-pore, near this 2-3 or 4-5 (Tr. Lob-
bianum) additional ones which are rather smaller (Fig. 19). I have not found the
pores described by Rlettenius and Rosanoff on the callous middle portion of the leaf.
Nelumbium speciosum has a group of several pores in the last-named spot.

Among submerged water plants we know from Borodin that in Callitriche verna one
large open pore lies over the end of the vascular bundle, on the upper surface of the
leaf. In Callitriche autumnalls there lie at the same spot on the young leaf a, group of
3-8 open stomata ; in the mature leaf the guard-cells of these break down, so that there
remains a wide hole in the epidermis. In Callitriche verna also this phenomenon ap-
pears in the older leaf : nevertheless I found the guard-cells still intact on leaves several
months old. The apices of the leaves of Hippuris behave similarly to those of Callitriche
autumnalis (Borodin). On the segments of the young submerged leaves of Ranunculus
aquatilis, divaricatus, Hottonia palustris, Askenasy' found several stomata, which die off
with the whole apex before the complete maturity of the leaf. It is doubtful whether
these belong to the category in question.

Water-pores with a short slit are known in the case of a number of species of Crassula,
and Rochea and many species of Saxifraga and Ficus with depressions on their leaves.

The leaves of the above Crassulaceae ^ have round
spots or depressions easily seen with the naked eye,
either on both surfaces (Crassula portulacea, Lam.,
arborescens, cultrata, tetragona, lactea) or only dis-
tributed on the upper side (C. cordata, perforata) ;
or forming a row just within the margin of the leaf,
either on both surfaces (C. lactea, ericoides, Rochea
coccinea), or only on the under surface (C. lycopo-
dioides, L., C. spathulata), in the latter, one at the
base of each notch between two teeth. An ending
of a vascular bundle expands beneath the epidermis
covering the depression. Scattered between the
small, delicate cells of the latter lie, in most species,
several (5-8, in C. lactea up to 25) stomata with short
slits, which are smaller than the air-pores of the
same leaf. In C. perforata and Rochea coccinea
(Fig. 20, comp, also below. Chap. VIII) the whole
depression consists of one stoma, exceeding the air-
pores in size, and somewhat sunk. The air-pores
are present in most species in large numbers be-
tween the large cells of the epidermis of both sur-
faces of the leaf. In C. cordata they are absent from
the upper side, which alone bears depressions.

The leaves of the Saxifrages of the division Euaizonia have depressions on the notches
of their margin, those of the division Kabschia (Engler) and Porphyrion on their upper
side. In these depressions lime is excreted always, or at least while the leaf is young.
The base of these, towards which an end of a vascular bundle runs, is constructed simi-
larly to the spots in Crassula, delicate and small-celled epidermis with two (S. crustata),
or 2-4 (S. Aizoon, longifolia, Rocheliana) large stomata, or one large stoma (S. retusa,
oppositifolia, caesia) forming the base of the depression.

Fig. 20.— Rochea coccinea; small piece of
epidermis from margin of leaf. water-pore ;
j air-pore, with subsidiary cells. The scat-
tered spots are wart-like outgrowths of the
outer wall.

’ Botan. Zeitg. 1870, p. 235.

Magnus, l.c.

54

CELLULAR TISSUE.

The depressions on the upper surface of the leaf of some species of Ficus (F. neriifoiia,
diversifolia, Porteana, Cooperi, eriobotryoides, leucosticta, &c.) have in the main the
same structure as in Crassula.

The openings which TrecuP describes on the large prickles on the leaf-nerves and
petiole of Victoria regia may be here supplementarily mentioned, being doubtful as
regards their structure, and requiring further investigation. These prickles enclose a
thin vascular bundle, which ends under their apex, and at the apex itself is to be
found a depression with one circular opening (ostiole).

Finally, while referring to later chapters, it must be remarked that the excretion of
water or solutions of lime over the ends of vascular bundles is not always connected
with the presence of water-pores.

Sect. 9. Gaps in the epidermis other than stomata and their modifications
occur normally only in rare exceptional cases. In connection with the water-pores
there may here first be mentioned the cracks, which occur regularly at the apex of
leaves of the grasses (seedlings of Zea, Secale, Triticum, &c.) ; from these drops of
water are expressed. They arise by irregular tearing of the originally cowl-like apex
of the leaf, when this spreads itself out flat as it unfolds. Gaps of another sort, as
found by Milde and King occur on the middle part of the winged base of the leaf
of Osmunda regalis, cinnamomea, Claytoniana, Todea rivularis, and on the ligule of
the base of the leaf of Isoetes lacustris. The undulating lateral walls of the epidermal
cells leave intercellular spaces between them, which are elliptical or circular in surface-
view, and are often as large as the cells themselves. Their distribution is irregular :
often many are near one another, even two between two cells, often there are none
for a width of several cells. They pass through the entire thickness of the epidermis,
and open into the intercellular spaces to be found below them. They are filled either
with air or with a colourless jelly of unknown origin.

No further examples can be here adduced of gaps in the epidermis, which are not to
be classed with stomata; mistakes formerly made with regard to Salvinia and Azölla
have been corrected ; the supposed round pores of the Pleurothallideae, again reproduced
by Unger®, have been proved to be the insertions of sunken hairs*; and Luerssen has
recently shown that the large pores, visible with the naked eye in the leaf of Kaulfussia,
are typical stomata, of huge size and wide cavity, with collapsing guard-cells, and sur-
rounded by 2-3 rings of subsidiary cells

Sect. 10. Such outgrowths above the outer surface of the epidermis as do not
belong to the cell wall alone are termed, in the plants with which we are concerned,
Hair-struciures {Trichomes, appendages of Ihe Epidermis). These spring from cells of
the epidermis, and are derived from them.

We may distinguish as typical forms of hair-structures, Bladders (Papulse),
Hairs (Pili, Setse), Scales (Squamae, Lepides, and Paleae), and Shaggy hairs (Villi),
Warts, and Prickles. These forms are characterised by simple relations of shape,
which mostly explain themselves according to their meaning borrowed from the
language of ordinary life, and by equally simple differences of structure : Bladders

* Ann. Sei. Nat. 4 Ser. I. 156, p. 13, fig. 10.

“ Milde, Monogr. generis Osmundse, p. 86.

® Anat. und Physiol, p. 194.

* Meyen, in Wiegmann’s Archiv, 1837, I. p. 419; Schleiden, ibid. 1838, I; Beitr. p. 5.

= De Vriese et Harting, Monogr. des Marattiacees, p. 14, Taf. V. D. — Luerssen, Botan. Zeitg.
1873, No. 40.

EPIDERMIS.

55

are isodiametric, usually unicellular bodies ; Hairs are sac- or thread-like bodies,
unicellular, or consisting of a row of cells, simple or branched ; Scales are flat
membranous structures, always consisting of many cells, arranged in one or several
layers ; Shaggy hairs are thread-like bodies, consisting of two or many layers or rows
of cells ; Warls and Prickles are of similar constitution, but are not thread-like, but
massive and hard, the warts are blunt, the prickles pointed. Intermediate forms and
combinations of these types are common, and may of course be easily named
after them.

On each hair-structure may be distinguished the body and the foot. The former
is the part which protrudes outwards above the epidermal surface. The foot is the
part which lies within this ; it is rarely similar in form to the epidermal cells, especially
often it exceeds them in height, as it not uncommonly extends inwards far beyond
the inner surface of the epidermis, into the sub-epidermal tissue.

The epidermal cells, which surround the foot, may resemble those not bordering
on a hair ; very often they are quite different from these, and may then be termed
subsidiary cells of the hair. Of the various forms of these, that of an annular or
rosette-like girdle of subsidiary cells surrounding the foot of the hair recurs especially
often (Fig. 21 B).

Around the foot of many hairs, or below it, the subepidermal tissue, covered by
the epidermis, bulges outwards, so that the foot is borne by an emergence of that
tissue. This may be limited to a slight excrescence, upon which, as its ‘bulbus,’ the
hair is seated, or to a small, stalk-like outgrowth, which in multiseriate shaggy hairs
is with difficulty distinguished from the hair itself ; but, on the other hand, it
may attain considerable dimensions, as in the prickles of Dipsacus^, and species
of Solanum, &c., which bear a hair on their apex, or the fringed scales of Begonia
manicata^

The converse condition of the origin of a hair, in a more or less deeply hollowed
depression of the surface, is not less common.

Small hairs do not always overtop the edge of the depression in which they
stand. They fill it completely, or only partially, as those on the leaves of the
Pleurothallideae (Pleurothallis, Stelis, Physosiphon, Nephelaphyllum, Octomeria),
which (comp, page 54) were wrongly described by Meyen as cavities of the
Epidermis.

The direction of the body of the hair, as regards the surface which bears it,
varies extremely between that at right angles and that parallel to it.

The hair- structures of one and the same surface are in the minority of cases all
alike, if slight individual differences be disregarded. As examples may be named
all known cases of Root-hairs, Leaf of Elseagneae, Bromeliacese, Leaf and stem of
Convolvulus Cneorum, &c. Much more commonly one and the same surface bears
hairs of different properties, two to five sorts often occurring close to one another.
Comp. Fig. 21.

If we disregard the root-hairs, which with very few exceptions (Elodea, Lemna,
Ophioglosseae) are universally distributed, and reproductive organs, which are not to

' Schleiden, Grundz. 3 Aufl. I. p. 281.

^ Compare Weiss, in Sehr. d. zoolog. hot. Vereins. Wien, 1838.

56

CELLULAR TISSUE.

be specially noticed here, some few families are distinguished by complete or almost
complete absence of hair-structures, as the Equiseta, the Coniferse, the Potamese, and
Lemnaceae.

d'hey occur in the majority of genera and species, though certainly to a very
variable extent.

Different vegetative adaptation does not determine the presence or absence of
hair- structures ; they occur under all states of adaptation, even in submerged species,
as Callitriche, Nymphaea, and species of Ranunculus. On the other hand, their
number and development seems certainly to be influenced by the nature of the
environment, since observation shows that in allied species, and in individuals of the
same species, the hairiness increases with the sun-light, dryness, and airiness of the
spot. But there is no safe foundation for a definite assertion on this point.

As regards the distribution of single forms of hairs through families and genera,
the case is similar to that of the forms of foliage leaves. On the one hand there is
great uniformity of the majority of species and genera of one family, at least as regards
07ie characteristic form of hair, so that one may speak, for instance, of the bristles of
the Borraginese, the short (glandular) capitate hairs and scales of the Labiatse, the
stellate hairs of the Cruciferse, the tufted hairs of the Malvacese, the multiseriate
shaggy hairs of the Melastomese, the delicate branched hairs accompanying the
capitate hairs of the genus Lavendula, the three characteristic forms of hair of most
of the Hieracia, &c. On the other hand, in natural families (e. g. Compositse,
Labiatae), and even genera (e. g. Solanum), the most various forms exclude one
another ; or one characteristic definite hair-form recurs on corresponding parts in the
most remote genera and families, as the stinging hairs on the leaf of Urticacese and
Loaseag, the shield-like stellate hairs or scales on those of the Oleaceae, Elaeagneae,
and species of Solanum, Croton, Bromeliaceae, and Ferns ; the spindle-shaped,
appressed hairs, with central attachment of the Malpighiacese and Cruciferse, &c.

The development of hair- structures, both uni- and multicellular, begins, in all
certainly investigated cases, from one epidermal cell, as Initial cell. This cell protrudes
beyond the outer surface of those surrounding it : the part within this surface
develops- into the foot, the protruded portion into the body of the hair. The growth
which ensues is, according to the special case, acropetal, basipetal, or intercalary, as
regards the hair itself (Rauter). It is obvious that in forms consisting of more than
one cell, divisions accompany growth, and the successive division-walls appear in
definite number and position for each case ; further, that the definite form and
articulation depend upon the successive divisions, and the growth of the cells after
the division is complete. In 2- to 4-seriate shaggy hairs, scales, &c., in which the
rows of cells are continuous into the foot, and are there represented by two or many
cells side by side in the epidermis, e.g. Hieracium aurantiacum and its allies, division
of the initial cell perpendicular to the epidermal surface begins almost simultaneously
with, or very soon after the protrusion of the body outwards. The development of
an emergence bearing a hair begins later than the origination of the hair itself by
local growth of the subepidermal meristcm, and of the epidermal cells surrounding
the initial cell of the hair.

The origination of the hair-structures begins on stem and leaf at a very early age,
on the former however, as a rule (but not alwa,ys), not above the point of insertion of

EPIDERMIS.

57

the youngest leaf^. On the same surface, their formation begins at an earlier stage
of development than that of the stomata. ‘ The succession of appearance of the hair-
structures follows the development of the part of the plant which bears them, but not
so thoroughly that the hairs, in their successive appearance, arrange themselves
exactly according to the direction of the advancing growth of the leaf which bears
them. Not uncommonly new hairs grow out between those already formed.’ Most
hairs on the parts named attain their full development with or before the complete
unfolding of the bud. The thick covering of hairs, scales, and shaggy hairs in the
bud-condition, is generally known. As the bud unfolds, the thickness of the covering
decreases, partly as a result of the separation of the persistent hairs on the growing
surface ; but partly through the disorganisation of hairs present in the bud during the
unfolding, so as to leave behind only rudiments on the unfolded parts, or hardly that^.
Even parts, which after unfolding are completely bare, may be hairy in the bud, e. g.
the leaves of Ficus elastica*. (Comp. Fig. i8A, p. 44.)

We may accordingly distinguish between evanescent, tra?tsiiorj/ hairs which are
peculiar to the hud, and persisteiit hairs. Among the latter we may again distinguish,
as will be shown below (Sect. 13), between such as persist as lijing hairs, and
others which are adherent but dry.

In roots the case is different from that described. It is a universal rule,
that here the hairs always appear on that part which is just ceasing to unfold,
i. e. to extend.

The above sentences will give the general points of view for the anatomical con-
sideration of the differentiation of the hair-structures. Under this head are ranged
structures rich in peculiarities, which have been the object of many works, and therefore
have a huge literature to show. In older times more especially the forms, articulation,
and functions of the hairs, which do not here concern us, were dealt with^; in more
recent, and the latest times, investigations on the history of development are considered
of more importance ^ I cite below for the time up to 1867 only the chief works, and

* On this fact, which need not be further noticed here, compare Hofmeister, Die Lehre von der
Pflanzenzelle, pp. 411, 545, and Rauter, Entwickl. einig. Trichomgebilde, p. 33. [Further, cf. Von
Hbhnel, Botan. Zeitg. 1882, p. 145.]

“ Compare Haustein, Botan. Zeitg. 1S67, P- 697 ff.

® Schacht, Abhandl. d. Senckenbergischen Gesellsch. I.

* Guettard, Memoires sur les glandes des plantes, &c. Eleven treatises in the Memoires de
I’Acad. Royale des Sciences, Paris, 1745-1759; altogether 560 quarto pages. Compare A. Weiss,
l.c. — F. V. P. Schrank, Von den Nebengefässen d. Pflanzen, Halle, 1 794, 8vo., with 3 plates. — Rudolphi,
Anatomie, p. 117 ff. — P. de Candolle, Organographie vegetale, I. p. io8. — B. Eble, Die Lehre von
den Haaren in der gesammten organ. Natur. Bd. I, Wien, 1831 (only known to me from references).
— Meyen, Secretionsorgane d. Pflanzen, Berl. 1837. — Physiologie, Bd. land 11 (1838-1839). — Bahrdt,
De pilis plantarum, Diss. inaug.; Bonn, 1849. — A. Weiss, Die Pflanzenhaare (Abdr. aus Karsten’s
Botan. Untersuchungen, Bd. 1) ; 306 pages, 13 plates, 8vo.

^ Haustein, Ueber die Organe der Harz- und Schleimabsonderung in den Laubknospen. Botan.
Zeitg. 1868. — J. Rauter, Zur Entwickelungsgeschichte einiger Trichomgebilde; Wien, 1871, with
gf plates (from Denkschr. d. Wiener Acad. Bd. XXXI). — J. Martinet, Organes de secretion des vege-
taux; Ann. Sei. Nat. 5 serie, torn. 14 (1872), pp. 91-232, pl. 8-21. — O. Uhlworm, Beitr. z. Entw.
der Trichome, Botan. Zeitg. 1873. — Further, N. Kauffmann, Ueber die Natur der Stacheln; Bullet.
Sei. Nat. de Moscou, torn. XXXH. p. 301 (1859); Warming, Sur la difference entre les trichomes
et les epiblastemes d’im ordre plus eleve (Abdr. aus Kopenhagen. Videnskab. Meddelelser), Copen-
hague, 1873. — C. Delbrouck, Ueber Stacheln und Dornen. Diss., Bonn, 1873. — S. Suckow, Ueber
Pflanzenstacheln, etc. Diss., Breslau, 1873. [Further, Reinke, Anatomie d. an Laubblättern vor-
kommenden Secretionsorgane, Pringsheim’s Jahrb. X. p. 119.]

CELLULAR TISSUE.

.'iS

refer for the complete enumeration of them to the works quoted, especially those of
Weiss and Martinet.

In face of the various facts and opinions it is our first business to determine what one
understands by hair-structures or Trichomes. There are two opposed opinions on this
point. The advocates of the one apply this name only to outgrowths belonging to and
derived from the epidermis — in the sense indicated in Sect, i — ; others apply it to all
characteristically formed outgrowths of the plant, to which the conceptions or traditional
terms stem, leaf, root cannot be applied, whether these protuberances belong to the
epidermis alone, or whether the subepidermal cells, and even the vascular system take
part in their formation, e. g. the prickles of the Roses, of species of Smilax and Solanum,
and of the Thorn-apple, &c. The foundation of the latter view seems to me to He less
in observable facts, than in the historical fact that outgrowths stich as prickles and warts
were formerly included among hair-structures, since it was thought that they belonged
to the Epidermis b If w'e deviate from this view, which is now proved to be incorrect
for the majority of cases, the term trichome must also be restricted, and all outgrowths
must be excluded from it, which include in themselves more than epidermis. Otherwise
a quite unnecessary confusion would be brought into well-founded views and relations,
since, if one includes among trichomes all outgrowths of the surface of stem or leaf, one
must also include those of the leaf margin, i. e. all leaf-teeth. If we adhere to the
anatomical and developmental facts which are clearly before us, we easily obtain the
definition here given of the idea of the hair-structure or Trichome as equivalent to an
outgrowth of the epidermis, and the distinction of this from those outgrowths in which
more than the epidermis takes part, for which the term Emergences proposed by Sachs
(Textbook, 2nd Eng. Ed. p. 161) is suitable.

The distinction between hairs and peculiarly formed epidermal cells may present
difficulties in many single cases, e. g. in the genus Mesembryanthemum, where large cells
scattered in the epidermis bulge outwards in M. crystallinum as huge bladders, while in
other species they scarcely rise above the surface. But it is just the same whether one
calls them hairs or not. The case, described by Uhlworm (/. c., Fig. 28-30), for the warts
of Gunnera scabra, which are covered by a piece of epidermis consisting of cells elongated
perpendicular to the surface, may be denoted, as above, or one can speak of a group
of laterally-united prismatic unicellular hairs, or one may (with Uhlworm) term the whole
piece of epidermis a multicellular trichome, which in that case forms an exception from
the rule of the origin of each trichome from one initial cell.

Starting, as is always necessary in defining types, from clearly characterised forms,
the above established leading types of hair-structure may easily be separated according
to their external development, and they are as a rule easily distinguished, without very
exact investigation, by habit and consistency. Their distinction is therefore to be
recommended for use in systematic Botany, which has as yet made use of these relations
less than they might be employed. Intermediate forms are by no means absent. But
these may easily be subordinated, or appended to the types. It is however often
indifferent to which of the types a special case is appended, and this is defined as
convenience may dictate. One may, for instance, call the flat horizontal appendages of
the Elaeagneae, or of Polypodium Lingua, stellately branched, multicellular hairs, just as
well as stellate scales ; or a capitate hair with a large compound head may just as well be
termed a long-stalked scale or a dermal wart.

Within the main limits, special forms are incredibly various as regards form, special
articulation, and direction, &c. The detailed description of them is the subject of
the most special systematic study, and their minute classification, though it might have
a significance at the times of Guettard and Schrank, can only be idle play at the
present time. Here, therefore, we may give only a few details and one or two drawings

* Compare e.g. Schleiden, Grundz. 3 Aufl. I. p. 2^1; Unger, Anat, und Physiol, p. 188.

EPIDERMIS. 59

(Fig. 2i) as examples of the leading forms, and for further information refer to the
literature above cited, or to any handful of plants.

Fig. 2t. — Examples of forms of Hairs. A transverse section through a young leaf of Plectranthus fruticosus.
a conical multicellular hairs, b small capitate hair, c short glandular capitate hairs (150). B Cajophora lateritia;
transverse section through the carpel; explanation in the text (150). C Hieracium piliferum; leaf, longitudinal
section, a thread-like, c short capitate shag-hair; b multicellular hair, with irregular stellate terminal cell (90).
D Cheiranthus cheiri ; under-surface of leaf, longitudinal section. Explanation in the text (150). E Polypodium
lingua ; under-surface of leaf. Stellate hair, a, in surface view (90) ; b transverse section (150).

6o

CELLULAR TISSUE.

I. Hairs, elongated cells or rows of cells, simple or branched. Free ends not much
widened, or tapering conically: Filiform and conical hairs; or widened into a head.
Capitate hairs {Pili capitati). In the latter case the head is often articulated as a cell-
surface, or cell-body; these are transitions to the leading forms II and III, and may be
named according to convenience.

I. Filiform and conical hairs.

(a) Unicellular and unbranched forms belonging to this group arise, by the arching
outwards of the whole or part of the outer wall of one epidermal cell, so as to form a
cylindrical or conical protuberance above the neighbouring surface. The whole hair is a
single cell, of which a sac-like part of variable size protrudes as the body, the rest is
embedded in the epidermis as the foot.

To this type belong all root-hairs. They are as a rule partial protuberances of the
outer wall of one epidermal cell ; when freely developed they are bluntly cylindrical, but
by application to the solid particles of soil they assume Irregular forms and curvatures
rarely they are branched (in Brassica Napus observed by Sachs, Textbook, 2nd Eng. Ed.
p. 100), or they may arise in pairs from one epidermal cell. Only in Lycopodium^ can special
hair-cells be distinguished from the other epidermal cells on the root. From many of
the original similar polyhedral cells, a part of the lower end is cut off by an oblique
wall as a small cell, which divides further into 2-4 cells : each of these grows out into
a hair : the hairs therefore in the mature root are arranged in groups between the
elongated epidermal cells.

On the foliage-leaf are to be found innumerable further examples. As a remarkable
form may be mentioned the conical hairs of many Borraginese, Loaseae (Fig. 21, B),
Hydrophylleae (Wigandia), Urticeae, many Cruciferae, Biscutella, Draba aizoides, Sinapis,
Brassica spec.), also of latropha urens, and napaeifolia. In the stronger forms of this
category, whether they sting or not, the base of the conical hair is swollen, and
encroaches on the surrounding tissues. It is usually borne on a more or less protuberant
emergence, and is surrounded by a rosette of peculiarly formed subsidiary cells. To
certain of these hairs (Loasa, Nettles, latropha spec.), which are characterised as a rule
by a button-shaped rounding-off of the upper end, and by the nature of their walls and
contents (Sect. 13), but by no further anatomical peculiarities of the hair itself or its
surroundings, the name stinging hairs (Stimuli) has been given. Compare the figures of
Meyen (Secretions-organe), Weiss, Martinet, and Rauter, and the more or less successful
figures of the stinging-hairs of the nettle in most text-books. Forms of this nature are
especially various in the Loaseas (Loasa bryonisefolia, Cajophora lateritia). On the leaf
and the carpels of the latter (Meyen l.c. Tab. VIII, B in our Figure 21) are seated
two sorts of conical hairs borne by slight emergences, and with their swollen bases
.surrounded by subsidiary cells, (i) long, smooth, blunt, stinging hairs (a), and (2) shorter
ones, having the point oblique to the surface, with a thicker wall and numerous whorls of
short points turned upwards (b) ; further (3) small thin hairs, with a circle of reflexed
spicules at the blunt end, and many such laterally, these have a tapering base inserted in
the epidermis (c) : lastly (4) small 2- to many-celled capitate hairs (d).

The term unicellular branched hair may be applied to those described second in
Cajophora (Fig. 21, also c), inasmuch as the spicules or little hooks are short branches.
Transitions from the unbranched to the branched form are to be found in the Cruciferae ;
in Draba aizoides, D. hispanica, Boiss., side by side with the above-named simple conical
hairs, occur others which, though otherwise of like character, are once branched at an
acute angle. More richly branched hairs, with many modifications and complications,
are the prevailing form for the leaf of most Cruciferae, though they are not exclusively
present. The body of these unicellular branched hairs rises from the expanded foot.
After a short distance, through which it remains undivided, it splits into 2-4 equal

^ Sachs, Exp. Physiol, p. 186.

' Nageli und Leitgeb, Bau und Wachsthum der Wurzeln, p. 124.

EPIDERMIS.

6i

diverging branches, which may themselves be repeatedly forked — often, as in Matthiola
arborescens, with cymose unequal continuation of the successive forked branches. In
the forms that are felt-like to the touch, as Farsetia incana, Matthiola arborescens,
Alyssum petraeum, Draba spec., the branches rise obliquely from the epidermis up-
wards. In others they are parallel to the epidermis, and lying close to it they spread
out like a flat star : stellate hairs, e. g. Capsella bursa pastoris, with 2-4 simple rays,
Alyssum petraeum, with 3-4 rays once or twice dichotomised. If the body of the hair
divides close above the outer surface of the epidermis into two conical limbs, both of
which are directed in one line parallel to the surface, the form is attained of a spindle
lying parallel and close to the epidermis ; this at its middle passes over into the foot,
which is inserted in the epidermis. Such spindle-hairs, with their longer axis as a rule
parallel and close to the part which bears them, are characteristic for Cheiranthus cheiri
(Fig. 21, D) and Erysimum canescens ; they are also to be found among 3-4 rayed
stellate forms in Capsella, Erysimum cheiranthoides, &c.

Similar forms occur in other families : unicellular, appressed, very regular stellate
hairs, with sharply conical, short, undivided rays, e. g. on the leaves of Deutzia scabra, 3-6
rayed on the upper, usually 9-10 rayed on the under surface.

In the Malpighiaceae * there is a similar series of forms, though these are less various
than those in the Cruciferae : simple erect conical hairs, and forked, stellate, and spindle-
shaped hairs. The erect branched hairs are simply two-forked, with equal or very
unequal branches; many-rayed stellate hairs occur in the genus Thryallis; specially
large and remarkable, but otherwise of fundamentally similar form to those of Cheiran-
thus, are the unicellular appressed spindle-hairs in this family which are termed by de
Candolle (Organogr. p. 103) Malpighiaceous-hairs.

Another often-described case of the last-named form are the spindle-hairs climbing
hairs’) of Humulus lupulus, with their ends curved like a hook, and borne on an
emergence. Further, Weiss (/. c. p. 528) mentions similar appressed spindle-hairs for
‘many species of Galega, Astragalus, Acer, Verbena, and Apocynum.’

(3) Most conical and filiform hairs are multicellular. In the simplest case they are
two-celled, so that one transverse wall separates a foot-cell from one cell of the body ; in
other cases they consist of more, and even numerous cells (Fig. 21, A a). As regards the
form, the same forms appear again, as in the unicellular hairs. One may even say that
the same hair may be uni- or multicellular, i. e. that the formation of transverse walls is
of minor importance ; thus the long conical hairs on the leaf of Pelargonium zonale are
sometimes unicellular, sometimes they have i or 2 transverse walls ; in the latter case they
are somewhat thicker-walled than in the former. Unbranched, multicellular, filiform,
and conical hairs are the commonest form of all. Examples: leaf of Gucurbitaceae,
Solanum tuberosum, and its allies ; most Labiatas (Stachys, Salvia, Thymus, Plectranthus,
and others, but not all genera); many Compositae (Helianthus, Cnicus, &c.) ; Trades-
cantia spec.; the huge yellowish-brown hairs, up to 3 cm. in length, on the base of the leaf
of several species of Cibotium, which appear in the shops as Pingawar Djambi, Pulu, &c.^

Among the branched forms, in the first place, those described under the unicellular
hairs recur as pluricellular. Hairs of the form of a T, that is, stalked spindle-hairs, with
pluricellular stalk and unicellular cross-piece in the Anthemideae (Pyrethrum roseum,
Tanacetum Meyerianum Sz., Artemisia absinthium, A. camphorata, according to Weiss,
/. c.). Stellate hairs with unicellular, often rather irregular star, or even two stars, one
above another, on a pluricellular stalk : Hieracium Pilosella and its allies. (Fig. 21, Cb,
comp. W'^eiss, Rauter, 1. c.)

Polypodium lingua has stalked, umbrella-shaped, very regular stellate hairs, in which
the foot, the erect stem, the centre, and each ray of the star, are single special cells (Fig.
21, E). In the Hymenophyllese^ are found pluricellular forked- and stellate-hairs. As

‘ A. de Jussieu, Monographie des Malpighiacees, p. 96, pi. II.

^ Compare Flückiger, Pharmakognosie des Pflanzenreichs, p. 142.

^ Mettenius, Die Plymenophyllaceen, p, 65.

62

CELLULAR T LS SUE.

examples of the latter may be named the small hairs of Verbascum the thin-stalked stars
of Lavendula Stoechas, &c. Also the short stalked, two- to many-armed hairs of
Utricularia^ and Aldrovanda in which each arm is a blunt cylindrical cell, belong partly
to this category, partly to the tufted hairs to be named below.

Hairs not forked, but monopodially branched, are (if we disregard cases like that
described in Loasa) always pluricellular. Thus those with scattered, and sometimes
repeatedly branched arms in Nicandra physaloides (Meyen, Weiss, l.c.), Lavendula
elegans, Rosmarinus officinalis (leaf), on the inner surface of the bud-scales of Platanus
(Hanstein, /. c.), those with whorls of branches on the leaves of Lavendula vera, species of
Verbascum (e. g. V. phlomoides, the larger hairs). Also those demonstrated by Schleiden'*,
which cover the leaf of Alternanthera spinosa, belong to this group. Not only is the lower
part, which is attached to the foot, composed of 4-5 disk-shaped cells, one above another,
but also the upper richly branched part is composed of as many cells as it bears whorls of
main branches. The cells are separated from one another by transverse walls folded in
deep waves, and each bulges out immediately above the transverse wall, which limits it
below, into a whorl of pointed branches, and here and there, on the rest of the lateral
wall, into a single branch. The form often cultivated as Alternanthera amoena shows
the same structure in its scattered hairs, but with only weak development of the
branchlets.

The bodies described by Weiss (/. c., Fig. 76) as branchlets on old hairs of Verbesina
gigantea, I was unable to find either in this plant, or in a member of the same genus, and
cannot make anything of them.

Under the name of tufted hairs, already often used, Weiss has judiciously separated a
form allied to those under consideration from the forms usually included in the term
‘stellate hairs.’ It arises by the division of an initial cell of a hair by a number of
successive walls perpendicular to the epidermal surface, and each of the cells thus
produced grows like a simple conical hair, the body of which diverges from the others,
while the basal parts remain firmly united. The history of their origin justifies the
position of these bodies here, side by side, with the branched, pluricellular hairs, though,
as far as the mature state is concerned, one might just as well speak of a tuft of diverging
simple hairs. The tufted hairs are either seated in the epidermal layer, or are borne by
a thin stalk-like emergence (e. g. felty species of Solanum, as S. marginatum, verbasci-
folium, species of Correa), or on the apex of a multiseriate shag-hair (therefore a
transitional form); this is the case in many Melastomese (Tetrazygia elaeagnoides®,
discolor, angustifolia). Further examples are furnished by very many (all?) Malvaceae®
Cistineae ; among the Labiatae, Marrubium ; species of Croton, e. g. Cr. tomentosus,
J. Müll ; species of Quercus, Platanus (comp. Weiss, Ranter, l.c.). The single rays of a
tuft are usually unicellular, in Marrubium pluricellular.

2. Capitate hairs ; erect hairs of various forms, whose free end is swollen to form a
round or disk-shaped head, the transverse section of which usually exceeds that of the
stalk. The head may be part of a cell, or of a unicellular hair (Fig. 21, B, d, glandular
hair of Aspidium molle) ; or it may be itself a single cell (Fig. 31-34), or be 2- to multi-
cellular, with the cells arranged in the most various ways in one or several layers one
above another. Capitate hairs are in the large majority of cases simple. Branched ones
are only known where certain branch-endings of ramifying conical hairs bear a head
(hairs of the bud of Platanus'^). The stalk bearing the head may be reduced to a
minimum, to the form of a very small disk — e. g. the glandular hairs of many Labiatae
(Pogostemon, Plectranthus, Molucella, &c. ; Fig. 2 1, A, b, c, 38).

* Weiss, /. c. fig. 184.

^ Meyen, l.c.\ Benjamin, Botan. Zeitg. 1848, p. 58; Schacht, Beiträge, p. 28,

® Caspary, Botan. Zeitg. 1859, p. 128, Taf. IV.

* Grundz. I. 3 Aufl. p. 280. ® Rudolphi, Anatomie, p. 113.

® Compare Sachs, 2nd Eng. Ed. pp. 43, loi. Hanstein, l.c. fig. 96.

EPIDERMIS.

^3

Capitate hairs occur on most leaf-forming plants, especially Dicotyledons and Ferns,
as a rule in company with non-glandular hairs. It is true they are absent from many
large groups ; e. g. (all ?) ’Gramineae, Cyperaceae, Palms, most Cruciferae. To this category
belong in the first place the great majority of the universally distributed glandular hairs :
in our consideration of these we shall have to betake ourselves to single examples
(Sect. 19). Meanwhile we need only remark here, that the glandular hairs are
characterised by no special form, but rather by definite properties of the cell walls ;
therefore the terms capitate and glandular hair are not equivalent. In the case of many
capitate hairs, it is as yet uncertain whether they possess the characteristic properties of
glandular hairs, since in the investigation of them no attention was paid to the
fundamental point, and since their external development shows no difference from
that of glandular hairs. Such cases may therefore remain unnoticed here, and only a few
typical examples be cited of non-glandular capitate hairs. The family of the Cheno-
podiaceae furnishes the longest series of these : they are short hairs with a uni- or
pluricellular cylindrical basal portion, which acts as stalk, and bears a relatively large
bladder-like apical cell, usually of a round shape, but often irregular. They occur
scattered on the leaf of many species of Chenopodium and Atriplex (e.g. Ch. album,
Quinoa, Atriplex hortensis '), especially while these parts are young : later the bladder-
like terminal cells are easily detached, and then together form a friable ‘meal.’ In
other Chenopodiaceae, whose leaves have a permanently white or gray surface, these
hairs are so closely packed that their terminal cells (which dry up on mature parts) touch
and overlap one another, forming a continuous layer over the epidermis, which does not
fall off, e. g. Obione portulacoides, Atriplex rosea, A. nummularia. Hort.

Non-glandular capitate hairs occur elsewhere, e. g. on the leaf of the Pelargoniums.
The petiole of Pelargonium zonale shows side by side five sorts of hair ; two are sharply
conical (comp, above, p. 61), the one more delicate, without septa, the other stronger
and with one septum ; besides these there are three sorts of capitate hairs, (a) glandular
with short, usually 2-3 celled stalk, and large unicellular, globular, glandular head^; (b)
short-stalked, with inclined, obliquely obovate terminal cell, perhaps also glandular ; and
(f) elongated hairs bearing on a usually three-celled stalk a large oval or pear-shaped
head-cell, not glandular (comp. Weiss, l.c., Fig. 367). Non-glandular capitate hairs with
a short 1-2 celled stalk, and a globular head composed of two cells standing perpendicularly
side by side, are very common among the Labiatae, together with glands and conical
hairs. On the whole, they seem to occur very often as inconspicuous structures.

II. Scales. Of the flat outgrowths of epidermis composed of. one or few layers of
cells, two forms may be distinguished, those which are scutiform, and those which are
attached laterally.

The former consist of a short stalk or foot, standing perpendicular to the epidermal
surface, and a more or less round, umbrella-like disk, attached by its middle to the stalk.
This is usually so short that the disk lies almost on the epidermis. It is either wholly a
hair-structure, unicellular (e. g. Oleacese) or pluricellular ; or is formed, at its insertion,
from a small emergence ; or (Shepherdia and other Elseagneae) it is wholly an emergence,
i. e. the round scale is seated at its centre directly upon a short emergence. The scale
itself consists of radially arranged cells or rows of cells, which arise by corresponding
divisions (i. e. arranged, as regards the hair, radially and perpendicularly). The number
of these varies greatly, from four (Jasminum) to very many. In scales where the
number is large the arrangement is often irregular, especially at the centre, by reason of
tangential divisions, which appear in addition to the radial ones. At the periphery the
cells usually grow out radially like hairs, so that delicate stellate shapes are produced.

It is obvious from what has been said that the more simple forms of this category can
hardly be distinguished from stellate hairs, such as those of Polypodiurn lingua (Fig. 21,

^ Meyen, Secretionsorgane, Taf. II. fig. i; Weiss, l.c. p. 559, fig. 198.
Hanstein, 1. c. p. 745 .

64

CELLULAR TISSUE.

E), Platycerium, and from capitate hairs. The families Oleacese and Jasmineae' yield an
especially complete series of forms, from the 8-celled shield, produced by triple radial
division of the initial cell (Syringa), or a i6-celled shield (Fraxinus), to the 30-32-celled
star (Olea Europaea). Further examples of the forms of this category are the above-
named Elaeagneae, single species of Solanum (S. argenteum. Dun., and allied ‘lepidota’),
Croton (Cr. pseudo-china, nitens), Gapparis Breynia, Andromeda calyculata, Myrica
cerifera^ Further the leaves and stem of Callitriche and Hippuris®, and the long-
stalked scales on the leaf of Pinguicula*. Large scutiform scales with pluriseriate,
multicellular central part, and radial multicellular margin, cover the leaf of most
Bromeliaceae, e. g. Hechtia planifolia, stenopetala, Tillandsia usneoides®, Pholidophyllum
zonatum, Billbergia clavata, Bromelia bracteata ; the young leaves of many Palms, e. g.
Klopstockia cerifera ®, with scales several layers of cells thick in the middle.

As regards their external development, there further belong to this category the
circular, shield -shaped, glandular scales of many plants, consisting of few cells (e. g.
Thymus, Salvia), or of many arranged in several series (Rhododendron ferrugineum,
Humulus lupulus, Ribes nigrum, &c.). The peculiarities of their structure will be
treated of later (Sect. 19).

Of scales attached at one side the Ferns yield the richest and best known examples, in
their so-called chaff-scales, or Paleas. Among these occur various intermediate forms
between purely single-layered hairs, such as are many-layered at their insertion, uni- and
multiseriate hairs, and shag-hairs. Their relations of size, form, and structure, so often
made use of for descriptive purposes, may with a reference to the descriptive literature
be here left untouched Those large branched scales on the stem of Hemitelia capensis,
the similarity of which to leaves of the Hymenophyllums caused them to be described as a
species of Hymenophyllum, are not to be included under epidermal structures, since
they have vascular bundles, and an epidermis with stomata Uhlworm mentions in the
case of Alsophila aspera thorn-emergences, which bear on their apex a large scale.

In the Phanerogams examples of this category may be sought among those forms
which form shag-hairs, inasmuch as these bodies are often developed chiefly in the direc-
tion of one transverse diameter, i.e. into many-layered elongated scales. This is the
case on the leaf-endings and margins of species of Papaver, in the Melastomese, as species
of Lasiandra, Melastoma malabathricum &c. To this category belong also the dermal
scales, borne on scale-like emergences, of Begonia manicata and its allies. As a special
very simple form allied to stellate, tufted, or capitate hairs, may finally be mentioned the
scales occurring in the axils of the leaves of Hippuris and Callitriche (Hegelmaier, 1. c.,
Rauter, 1. c.). These are borne on a short simple stalk-cell, and appear as a circular fan
one layer of cells thick, which is composed of radially arranged elongated cells, or
(Pseudo-callitriche) of rows of cells similarly arranged.

111. On Shag-hairs (Zotten) (Fig. 21, C, a, c) little need here be added to what

’ Prillieux, De la structure des polls des Oleacees et des Jasminees, Ann. Sei. Nat. 4 Ser. V.
p. 1, pi. 2-3.

“ Rudolphi, /. r. p. 114, where generally are very numerous details, though there is occasionally
a confusion with tufted hairs.

® Hegelmaier, Monogr. d. Gatt. Callitriche, p. ii ; Rauter, l.c. p. 6.

^ Schacht, Pflanzenzelle, Taf. VII. p. 16. — Lehrbuch, I. p. 280. — Grönland, Ann. Sei. Nat.
4 Ser. III. p. 297, Taf. X.

^ Compare Schacht, Lehrb. I. Taf. IV. pp. 10, ii ; Pflanzenzelle, Taf. VII. pp. 17, 18.

® How far the scaly or fibrous covering of the unfolding palm leaves consists of hair structures,
or of effete drying masses of tissue, requires more complete investigation in special cases. Compare
Mohl, Verm. Sehr. p. 177, Structura palmarum, § 82.

’ On their development, compare Hofmeister, Vergl. Unters, p. 85.

“ Compare Mettenius, Filices horti Lipsiensis, p. in.

® Rudolphi, l.c. p. 115.

EPIDERMIS.

Ö5

has been already said. The shape of their body repeats that of all single hair-forms,
from which it differs only by its articulation — being pluri- or multiseriate. It terminates
in a head, or is simply conical, or resembles a tufted hair ; the latter for instance in the
shaggy hairs of the leaves of Leontodon hastilis and incanus, which fork at their ends into
2-5 stiff conical hairs, in the shaggy hairs of the above-cited species of Solanum, Croton,
and Correa (p. 62), and the Melastomeae, where they end in a rich tuft of hairs; to these
may be added Osbeckia canescens, and Medinilla farinosa. Its lateral margin is smooth,
or toothed and zigzagged by the outgrowth of conically elongated cells; e. g. the conical
shaggy hairs of the Hieracia, species of Papaver, and Mimosa ; or it even bears tufts of
hair (Correa speciosa). In shaggy hairs the foot is very often seated on an emergence.
Compare e. g. the capitate glandular shaggy hairs of the leaves of Ribes (Haustein,
Rauter, Martinet, /. c.). The family of the Melastomaceae has unusually numerous forms
of shaggy hairs with the most various transitions to scales and tufted hairs.

IV. In the simplest case Bladders differ from unicellular hairs only in form, and
might be called by the same name, were it not too contradictory to the original meaning
of the word to call a spherical body a hair. Such unicellular round bladders, with a
broad foot penetrating far below the epidermis, or borne by an emergence, are known on
the foliage-leaf of Mesembryanthemum crystallinum, Tetragonia expansa and echinata,
and Oxalis carnosa On the whole leaf-surface of Rochea falcata * and longifolia
cylindrical tough bladders rounded at the top arise between the small epidermal cells ;
they are provided, above their broad foot, with several blunt outgrowths, which almost
touch the epidermal surface. They are all of the same height, and are in close juxta-
position, so as to form an almost complete covering of the epidermis. In R. coccinea
the margin alone is fringed by a single row of such bladders, which are rather elongated
to the form of a short thick hair.

The herbaceous stem, petiole, and under surface of the leaf of many Piperaceae —
Piper nigrum Hort, Enkea glaucescens, Artanthe elongata — are often but not always
covered in the young but almost fully unfolded state by scattered spherical bladders, as
large as a pin’s head, which shine like transparent pearls. These prove to be unicellular
hair-structures, with a very small foot inserted in the epidermal surface, or projecting
further inwards. On older parts they burst, and dry up to inconspicuous black-brown
specks. Besides these there occur in the same epidermis very numerous hair-cells,
which only differ from the large bladders by their small foot-cell protruding above the
outer surface as an inconspicuous papilla : it may therefore be said that many hairs re-
main inconspicuously small, while the minority swell to form the transparent bladders.

Just the same appearance for the naked eye, with the same transitory nature, is seen
in the round or oval bladders, as large as a millet seed, which Meyen ® discovered in
Begonia plantanifolia, vitifolia, Cecropia palmata, peltata, Pourouma guianensis, Urtica
macrostachys, in all cases distributed as above in Piper : further in Bauhinia anatomica,
especially on the stem when several years old. These he named pearl-glands. Such
structures are often observed also on Vitis, Ampelopsis* (A. quinquefolia, Veitschii),
Cissus velutina, also on Pleroma macrantha (Melastomacese). These pearl-bladders
(those of Pleroma have not been investigated) coincide with those mentioned for Piper in
this point, that they are chiefly composed of very large bladder-like cells, which are thin-
walled, and contain, besides radially-striated protoplasm, much watery fluid, and a number
of brilliant colourless globules of resin or oil. In other points their structure differs. In
the Begonias, according to Meyen, they are hair-structures which are allied to capitate
shaggy hairs. The body of the pearl consists of about a dozen cells of the above

’ Meyen, Secretipnsorg. Tab. VII. figs. 8-16, 38, 39. — Weiss, L c.

® K. Sprengel, Anleit. z. Kennt, d. Gew. 2 Aull. 1. p. 113, Taf. III. — A. Brongniart, Ann. Sei.
Nat. I Ser. XXI. p. 453, Taf. 10.

5 Secretionsorgane, p. 45, Taf. VII.

* Hofmeister, Handb. Bd. I. p. 545.

66

CELLULAR TISSUE.

character, arranged in two irregular rows, and is borne by a bi-seriate shaggy hair,
as stalk. As in Piper there are also found small club-shaped shaggy hairs, from the
swelling of which the pearls might have been derived.

The pearls of the above-named Ampelideae are, on the other hand, emergences. They
consist of several large cells of the character above-stated, and are covered by a protrusion
of the epidermis consisting of numerous relatively small hyaline cells. On or near the
summit of the body is a stoma, which is widely open, and on old specimens is further
extended by rupture at the corners of the slit. Young speeimens are seated on the
surface as blunt warts, with broad base. In old specimens the upper part swells so much
that the point of insertion appears as a relatively small stalk. The pearls of Urtica
macrophylla, and, according to Meyen’s statement, of the other Urticaceae, have in the
main a like structure, with the difference that they are without the stoma. The pearls
of Cecropia and Bauhinia are, according to Meyen, similarly composed ; they are also
without the stoma, and differ further in their tissue consisting throughout of small very
numerous cells.

V. Prickles and Warts. It was above stated that the massive outgrowths termed
prickles and warts are mostly emergences, in the formation of which epidermis and
subepidermal tissue conjointly take part. For the majority of these structures, as the
prickles of species of Dipsacus, Rosa, Gunnera, Smilax, Solanum, and Ribes, the
Cactaceae, &c., this is thoroughly proved by late investigations of the history of their
development (Ranter, Kauffmann, Warming, Delbrouck, Uhlworm). Delbrouck and
Uhlworm, however, have both shown that exceptions to the predominant law exist,
since the prickles of the investigated species of Rubus (R. caesius, idaeus, Hofmeisteri)
and those of the petiole of Chamaerops humilis belong to the epidermis, and differ only in
form and consistence from shaggy hairs. Further that the warts on the foliage leaf and
carpels of Bunias Erucago are at least chiefly derived from the epidermis. If once an
anatomical distinction is adopted, it is necessary to separate the above outgrowths of
epidermis from emergences of the same or similar form, however closely they may
correspond to them— and to many sorts of hair-formations of most simple structure —
as regards their physiological or teleological significance.

The often-described oval warts of Dictamnus^, which bear on their apex a short,
septate hair, are connected with the above forms. They will be described more in
detail below (p. 69).

2. STRUCTURE OF THE ELEMENTS OF THE EPIDERMIS.

(a) Protoplasm and Cell-Contents.

Sect. 11. The wall of the Epidermal cells both in one-layered and many-
layered epidermis encloses as a rule a delicate protoplasmic sac with distinct nucleus,
and within this clear transparent cell-sap, which is either colourless, or tinted with
dissolved pigments (Erythrophyll, &c.). It is to this condition (and the colourless
membrane) that most epidermal layers owe their great transparency.

In the majority of cases chlorophyll and starch are absent from epidermal
cells This is the case without exception in land plants where the tissue is very
thick-walled, and surrounded by air ; often also in thin-walled cells occurring under
similar conditions. But in not a few other land plants more or less numerous
chlorophyll grains, eventually with included starch, lie in the peripheral protoplasm.

1 Meyen’s (Secretionsorg.) ‘ Mützenförmige Drüsen.’ Compare Hofmeister, Pflanzenzelle, p.
259; Ranter, l.c. Taf. V, VI. ^ [Cf. Stöhr, Bot. Ztg. 1879, p. 581.]

EPIDERMIS.

6j

If one surveys the cases in which this occurs, it will be seen that the foliage of plants
with delicate leaves, which live in shady places, is especially concerned, such as that
of most Ferns, further Impatiens nolitangere, Melampyrum sylvaticum, Galeopsis
tetrahit, Ranunculus Ficaria, Epilobium roseum ■, also Listera ovata, and Staphylea
pinnata * may perhaps be added. On the other hand, however, the, same phenomenon
occurs also in inhabitants of sunny places, as Mercurialis annua, Lamium purpureum,
Caltha palustris, to which examples many others might easily be added. Epidermal
cells of parts growing under water are, on the contrary, rich in chlorophyll grains
and the bodies included in them, richer even than any other tissue of the species.
Thus in the leaves of Ceratophyllum, Aldrovanda, Ranunculus aquatilis, Potamogeton,
Hydrilleas &c. In Elodea canadensis, and its allies, the chlorophyll-containing leaf
consists in the main of only two layers, which originate, like scales, from the epidermis
of the stem. Brongniart ® already showed that in species typically submerged,
but which also occur as land plants, such as .Ranunculus aquatilis, the submerged
epidermis is rich in chlorophyll, while that of the land form is without it, and that an
intermediate condition occurs on transition from one habit to the other. But the
rule just given is not general for all water plants. Both the amphibious species of
Callitriches and C. autumnalis which only occurs submerged, have an epidermis
without chlorophyll

Sect. 12. In contrast w’ith the epidermal cells, the guard-cells of the stomata
are always very rich in protoplasm, chlorophyll, and the bodies included in
the latter, especially starch grains ; in colourless plants only the last-named bodies
are present. The subsidiary cells of the stoma resemble the epidermal cells as
regards the properties in question. No peculiar phenomena, i.e. such as do not
belong generally to the different cells of the plant, are known for the cell-sap of the
epidermal and guard-cells, and the bodies which occur dissolved and suspended in it.
It is true they have as yet hardly ever been carefully investigated. This assertion is
only confirmed by the casual statements made about oily drops suspended in cell-
sap, and masses or drops containing tannin in the Cycadeae (Kraus), about tannin
generally in the Crassulacese, Rosa, Ficus, Camellia, the Saxifragas &c. : also about
more or less solitary crystals of Calcium oxalate in the leaves of Tradescantia
discolor. Begonia manicata, argyrostigma, and Hakea saligna, octohedral crystals in
Asplenium Nidus, klinorhombic crystals which completely fill the small cavity in
scattered or grouped cells of the leaf of Ilex paraguayensis

Thomas describes in the leaves of Pinus Pumilio, Pinaster, and austriaca,
epidermal cells whose contents are dried up, and replaced by air in consequence
of rupture of the membrane. It may, however, be conjectured that this description
refers to abnormal conditions.

* Sanio, Botan. Zeitg. 1864, p. 196. Compare also Kraus, in Pringsheim’s Jahrb p. 314.

“ Caspary, Pringsheim’s Jahrb. I. p. 348. — Botan. Zeitg. 1859, P-

® Ann. Sei. Nat. i Ser. tom, XXI. (1830) pi. 17, figs. 3 and 6. — Further, Askenasy, Botan. Zeitg.

1870, l.c. ^ Hegelmaier, Monogr. p. 9.

® Compare Sanio, 1. c. ; Kraus, 1. c. ; Wigand, Botan. Zeitg. 1862, p. 1 2 1 ; Engler, Botan. Zeitg.

1871, p. 888.

® Kraus, l.c. — Meyen, Physiologie, I. p. 227. — Goldmann, Botan. Zeitg. 1848, p. 557.

’ Pringsheim’s Jahrb. p. 26.

r 2

68

CELLULAR TISSUE.

Sect. 13. The cells of Hair-structures are, while young, provided like other
young cells with a well-developed protoplasmic body, and many while in this condition
quickly attain a great size, so that they are specially suitable and easily obtained
objects for the study of the protoplasmic body. Mature hairs behave in two different
ways as regards their protoplasmic body and their contents. Those of the first
category resemble, in short, the epidermal cells, having a permanent protoplasmic
body, usually in the form of a delicate sac-like lining to the cell-wall ; more rarely
the protoplasm persists for a longer time in considerable quantity (stinging hairs of
Urtica, Hairs of Cucurbita, &c.). The cavities in the protoplasm are permanently
filled with watery cell-sap {Sap-containing Hairs). In hairs of the second category
the protoplasm and cell-sap dry up when growth stops, and are replaced by air.
These persist as air -containing hair-structures. The capitate hairs containing
mucilage in Osmunda regalis are hitherto unique, and will be described below
(Sect. 19).

All root-hairs and a large number of the hair-structures which occur on foliage
leaves contain cell-sap. They can be distinguished at once from those of the other
category by their transparency. Their protoplasmic body and contents show the
same series of various modifications of special character as is the casein the epidermal
cells. Most of them, e.g. all root hairs, all (.?) stinging hairs, &c., are devoid of
chlorophyll. Others have more or less abundant grains of chlorophyll and allied pig-
ments. The correspondence with the epidermal cells extends also, as far as is known,
to the substances mixed with the cell-sap (comp. Weiss, /. c. 645).

The contents of the often-described stinging hairs have special peculiarities,
which are also said to occur in many hair-structures described as glandular hairs of
various categories.

We know of the erect stinging hairs of the Urticacese, Loaseae, and other plants
named above (page 60), which resemble one another so remarkably in structure and
form, that the brittle point (Sect. 22) breaks off when touched, and that a fluid issues
through the hole thus made, which causes more or less slight inflammation when
applied to the human skin, especially if it enters the small wounds caused by touching
the hair itself. It is further known of this fluid that it has, like most cell-fluids, an
acid reaction, not alkaline as stated formerly On the fact that by distilling the
nettle plant with sulphuric acid formic acid is obtained, the conjecture has been
founded that the latter substance causes the phenomena of stinging^. But as a matter
of fact nothing is known of the active substance, not even whether it is to be sought
for in the acid fluid, or in the protoplasm

The apical cells of capitate hairs are often distinguished by very dense proto-
plasmic contents, in which resinous substances may be shown to be present. Hanstein
(Bot. Ztg. /. c. p. 748) states that in the multicellular capitate hairs of Salvia all the
cells may finally be united by the solution of their membranes into one fluid mass
(containing Resin or Balsam) which is surrounded by the bladder-like cuticle. In the

’ P. de Candolle, Physiologie, übers, v. Roper, I. p. 193.

“ Von Gorup-Besanez, in Joum. f. pract. Chemie, XLVIII. p. 191.

* Compare the neat paper of Duval-Jouve (which however gives no new information), ‘Sur les
stimnles d’ortie,’ in the Bulletin Soc. Bot. France, XIV. p. 36.

EPIDERMIS.

69

A

often described club- or egg-shaped warts of Dictamnus, which end in a short hair,
there are found cell-contents of a character exceptional for epidermal structures. The
cells finally fuse to form intercellular balsam-containing cavities’. These, as Rauter has
carefully shown, are multicellular bodies derived from a single epidermal cell, consist-
ing of a permanent peripheral layer of epidermal cells with scanty contents, running
out into the terminal hair, and an inner multicellular mass. The cells of the latter
contain, about the end of their growth, at first chlorophyll ; then there appear drops
of resin and ethereal oil in increasing quantity; these finally unite to large drops,
which fill the cavity formed by the solution of the inner cell-walls (comp. Fig. 22).
Beneath the epidermis, but derived in part from it, there arise in Dictamnus similar
cavities containing ethereal oil. Martinet
{l.c. page 176) describes in Cuphea lanceo-
lata long, multiseriate, shaggy hairs con-
sisting of elongated cells : in the broad base
of each of these is enclosed a central round
body consisting of many small isodiametric
cells. This resembles the central tissue of the
warts of Dictamnus in its position and its
contents, which apparently include drops
consisting of ethereal oil : but the solution of
the cell-walls, which occurs in the latter case,
was not observed in the former. Peculiarly
formed groups of cells, characterised by their
dense contents, which turn brown on drying
or treating with alkalies, and which pro-
trude little or not at all above the outer sur-
face, and only slightly inwards, occur in num-
bers in the epidermis of the tubular leaf of Sar-
racenia ^ They are globular, or flask-shaped,
with the neck directed outwards, and consist
of about 16 small cells derived apparently
from the division of one epidermal cell. Their
structure has been well described by Vogl
from dried material. Their origin and sig-
nification remains still to be investigated.

Those cells and cell-groups belonging to

I Fig. ^22. — Dictamnus fraxinella; oil-containing dermal
warts, cut perpendicular to the surface. A youngest stage
of development; B rather older; C {220) median section

the epidermis, which have contents of peculiar 'trough a mature specimen. After Rauter. from Sachs’

This

nature, such as in the above examples, are

often described as glands if they are also characterised by special form,
term will be discussed in Sect. 19.

Air-coniaming dry hairs, scales, or shaggy hairs, form dry opaque coverings
which appear of different colour according to the character of the membranes and

^ Meyen, Secret! onsorg. Taf. I. pp. 28, 29. — Unger, Anat. und Physiol, p. 212. — Hofmeister,
Handb. I. p. 259. — Rauter, Martinet, l.c.

^ A. Vogl, Phytohistolog. Beiträge. Silzgsber. d. Wiener Acad. Bd. 50 (1864).

70

CELLULAR TISSUE.

of the remnants of the cell-contents, and present a lustre which varies according to
their form and position and the character of their surface. They preponderate in
very hairy plants. Thus the dense white felt on the foliage of many Labiatse
(Stachys, Teucrium, Salvia, &c.), Compositae (e.g. Gnaphalium), the Verbascums,
Banksias, Rubus idseus, &c. ; the silvery white or brown peltate scales of the above-
named Elseagneae, Bromeliacese, Croton, Solaneae, Olea spec. ; the rustling ‘ Paleae ’
of the Ferns ; the white crust consisting of dried capitate hairs in the above-quoted
(p. 63) species of Atriplex, and Obione, and other Chenopodiaceae.

(b) Structure of the walls of the Epidermal Elements.

Sect. 14. The wall of the epidermal cells is, in very delicate parts, a thin
cellulose membrane developed pretty equally all round. In rather more firm parts,
in such as are termed herbaceous, and to a greater extent in very tough parts, such
as stems and branches of smooth-barked ligneous plants, leathery and fleshy leaves,
it is strongly thickened. In rare cases the thickening is almost equal all round,
e.g. leaves of Ceratozamia mexicana^, Pinus sylvestris, and its allies^ (Figs, ii, 27 ;
in this case the lumen almost disappears), or is much less on the outer surface than
on the lateral and inner ones, as is the rule in the Bromeliaceae (Fig. 1 2, p. 37)
Also in the epidermal cells containing mucilage, which will be described below,
the inner wall is of considerable thickness, often exceeding that of the outer
wall. In an epidermis one layer thick and in the outer layer of a many-layered
epidermis the outer wall is usually thicker than the lateral or, inner walls. In the
above-named tough parts, such as leathery and fleshy leaves, old branches of Viscum,
Ilex, Laurus, Menispermum canadense. Palm stems, &c., it is often thickened to such
an extent that it occupies the greater part of the whole volume of the cell. The thick
outer wall is either sharply marked off from the thin lateral walls or graduates gently
into them. The walls of the inner layers of a many-layered epidermis all resemble
in the main the lateral and inner walls of the single-layered epidermis as regards
strength and structure, with the exception of isolated peculiar cases which must be
mentioned as being extraordinary.

The thickened walls have generally the well-known structure of cell-membranes,
stratification, striation, and pitting, but never fibrous thickening of the walls. The
phenomena connected with special peculiarities of substance — cuticularisation,
formation of Cystoliths — will be treated of later. There occurs sometimes on the
wavy lateral walls (e.g. under surface of the leaf of Helleborus foetidus*) at the
bottom of a depression a local thickening in form of an excrescence resembling
a fold or doubling of the membrane, which protrudes inward, at right angles to
the surface.

Pits of the usual form, corresponding on opposite sides, are very common on
the lateral and inner walls. As a rule they do not occur on the thick outer walls,

* Kraus, Cycadeenfiedem, 1. c.

“ Thomas, l.c. p. 25. — Hildebrand, Botan. Zeitg. i860, Taf. IV.

® Von Mohl, Verm. Schriften, Taf. X. 33. — Schacht, Lehrb. 1. Taf. IV. fig. 10.

^ Von Mohl, Verm. Sehr. Tab. VIII. fig. 21 ; Vegetab. Zelle, p. 14. Compare also Cohn, Nov.
Act. Acad. Leopold, vol. XXII. pars 2.

EPIDERMIS.

71

Still they are present in a considerable number of exceptional cases. Thus on
the foliage leaves of Coffea, Viburnum Avabaki, Cocculus laurifolius, Cinnamomum
aromaticum, Camellia japonica and of Grasses where some of them are arranged
perpendicularly to the outer surface ; but on the undulating corners they are directed
obliquely outwards from the lumen of each cell, and facing the neighbouring cell, so
that those of two neighbouring cells cross. They occur also in Abies Cycas^
Lycopodium pinifolium^, and Equisetum hiemale (comp. Fig. 24 B"^). The walls of
the elongated epidermal cells of the upper side of the leaf of Acropteris australis
show a spiral striation, as the result of peculiar pitting (comp. Sect. 30). The free
surface of the outer walls is often quite smooth : but is not uncommonly uneven
by reason of small thickenings protruding outwards : short warts, e.g. in species of
Equisetum, leaves of Sparganium ramosum. Aloe verrucosa, Radula, Crassulacese
(comp. Fig. 20, p. 53), &c. : bands, which are relatively broad and blunt, e.g. leaf of
Helleborus niger, foetidus Dianthus Caryophyllus, plumarius, or thin and sharp, as
in very many leaves and petioles, e.g. Allium Cepa, Eucomis, Rumex Patientia
obtusifolius. The bands often run nearly straight and parallel, and are then usually
longitudinal relatively to the whole body, rarely (Eucomis) they are transverse ; not
uncommonly they are wavy and branched (e.g. Helleborus, Pirus communis), and in
the majority of cases they are continuous from one cell to the next.

The wall of the stomaial cells^ is usually, but not always, thinner on the average
than that of the adjoining epidermal cells. It is in most, and one may say in regular
cases, unequally thickened in such a way that a strongly thickened ridge runs along
the entrance and exit of the slit (Fig. 23). These ridges protrude on the free sur-
face as the above described ridges of en-
trance and exit, which are sharp-edged and
concave towards the slit; rarely both are
almost equally strong (Lilium candidum.

Ficus elastica); usually the ridge of entrance
is much stronger than the other, and in su-
perficial stomata of tough leaves it often
takes the form of a high and thick wall, e.g.

Clivia nobilis, many Proteaceae, Pholido-
phyllum zonatum (Fig. i2,p. 37), Epidendron
ciliare, Octomeria, Sarcanthus rostratus, &c.

The ridge of exit is often extremely small
(leaf of Pholidophyllum, Dianthus caryophyl-
lus, Lomatia longifolia, Sparganium ramosum), or is not present at all (comp. p. 35).
The thickened ridges either protrude into the cavity of the cell as flattened swellings,
or not at all. The remainder of the wall of the guard-cell, that is the convex side

' Kraus, l.c. p. 318. 2 Von Mohl, Verm. Sehr. Taf. IX.

® Thomas, Hildebrand, l.c. * Von Mohl, l.c. Taf. X.

® Sanio, Linnsea, 29, p. 169. ® Von Mohl, Verm. Sehr. Taf IX. 6-8.

’Von Mohl, l.c. figs. 3-5.

® Compare the papers quoted above, sect. 5, especially Von Mohl, Spaltoffn. d. Proteaceen ; idem,
Botan. Zeitg. 1856, l.c.\ and the large series of good representations in Strasburger’s work, Pring-
sheim’s Jahi b. V.

FIG. 23. — Hyacinthus orientalis ; leaf, transverse sec-
tion ; e — e epidermal cells ; 3" entrance of the stoma,
which is cut in median transverse section; t the respi-
ratory cavity, between the parenchymatous cells f,
(800). From Sachs’ Textbook.

72

CELLULAR TISSUE.

which is turned from the slit, the united ends, and the strip of the concave side
which borders on the slit is much less thickened. The last-named strip is seen,
in those cases where ridges of exit and entrance protrude far into the cavity, as a
channel on the inner surface of the wall, which appears in the transverse section of
the stoma like a broad pit. When the ridges of exit and entrance are very broad,
the middle of the convex side assumes a similar appearance (e.g. Ficus elastica,
Fig. 1 8 C). The various modifications of this plan of structure, depending upon the
absolute and relative strength of the thickening and its varying protrusion inwards
and outwards, require no further description in detail; some of them are evident
from the above figures. Further, as regards genuine exceptions, which occur especially
in the Coniferse and Cycadeae, we may refer to the special literature above-quoted
(comp. p. 35). Still the structure of the walls of the stomata of Equisetum, which is
anything but clearly explained in the writings of Duval-Jouve and Milde must not
be entirely passed over (Fig. 24)^ The guard-cells themselves are here in no way

Fig. 24. — Stem of Equisetum hieinale ; stoma with the surrounding tissue (390). A view from the inner surface.
The pair of guard»cells is surrounded by the superposed edge of the pair of subsidiary cells. B transverse section
of the stem passing in a median direction through a stoma, which lies in a depression of the surface. The narrow
entry of the slit is bordered by two fiat guard-cells and the subsidiary cells which surround them ; each of the latter
has a curved pit, turned outwards. Further explanation in the text. The cells of the single-layered epidermis and
of the hypoderraal schlerenchyma below it have numerous pits. C silica residue of a fragment of epidermis with a
stoma, after maceration in Schultze’s mixture and subsequent ignition ; seen from outside. The curved figures are
the outlines of the prominences of the outer surface.

remarkable except in their form, which is flattened obliquely outwards, and in the slight
difference in thickness between the ridges of exit and entrance and the other bands
of the membrane. But the subsidiary cells, which completely surround them (comp,
p. 43), have a thickening on the wall bordering on the guard-cell, in form of bands
protruding into the cell-cavity. These diverge in a radiating manner from the slit.
Hence the beautiful radiate-marking seen in surface view. The number, breadth,
and frequent branching of the radial bands vary according to species. In Milde’s
Equiseta phaneropora (at least E. limosum) each band runs over the whole radiate

' Milde, Monographia Equisetorum, Nov. Act. Acad. Leopold, tom. XXIV. pars II. — Duval-
Jouve, liistoire nat. des Equisetum de France, Paris, 1865.

“ Sanio, Linnsea, Bd. 29, p. 389, Taf. III. — Strasburger, l.c.

EPIDERMIS.

73

surface, the slit is therefore surrounded by a series of radiate bands. In Milde’s
E. cryptopora (at least E. hiemale) the striated wall is traversed about half way
between the slit and the convex outer border by a narrow oblique furrow, which is
almost parallel to the slit, and this divides two concentric series of radial bands, one
of these next the slit, the other on the side opposite to it (Fig. 24).

What has been said of the walls of the epidermal cells holds in the main for
those of hair-structures. Those walls which separate the cells of multicellular hairs
resemble on the whole the lateral and inner walls of the epidermis. Their details of
structure are, if possible, more various than their modifications of form. Pro-
jections of the outer surface, in form of ridges, warts, or even of those sharp
prickles represented in Fig. 21 B, appear in hairs more commonly than in the epi-
dermal cells. The stiffness of the hairs depends upon the thickening of the walls,
which may proceed till the lumen is obliterated. A hard, rigid hair or shaggy hair,
is called a bristle or seta. If it is also conical and sharp one can prick oneself
with it, as with the horizontal hard bristles of Malplghia urens, or the rigid hairs of
the Borraginese and Cucurbitaceae. In this property, so disagreeable to men, lies
the ground for the often-asserted similarity of the hairs of the Malpighiacae to the
stinging hairs, and of puncturing bristles to prickles.

Sect. 15. The cell-walls of the epidermis are cellulose membranes; a number
of other bodies are embedded in, or superposed on this: Cuticular-substance or
cutin, wax, resin, volatile oils, gum and hassorin, compounds of silicon, and
lime sails, bodies with whose presence remarkable peculiarities of structure are
connected.

Sect, i 6. Of the relatively pure layers of cellulose of epidermal membranes it
may be said that in the majority of cases, especially in herbaceous parts, they appear
similar to that watery highly refractive modification of cellulose membranes which
is characteristically developed in the Collenchyma, to be described later. The
detailed investigations necessary for an exact statement on this point have not
been made.

Allied to these watery cellulose layers are the parts of the membrane of
epidermal cells, which have been altered to vegetable mucilage and bassorin ;
Radlkofer ^ has lately drawn attention to the frequent occurrence of these substances
in foliage leaves. The thickened inner wall of these epidermal cells consists, espe-
cially in their inner layers, in the mature condition, of a vegetable mucilage, which
swells in water till its identity is lost, like the mucilage of Linseed, &c. These
layers of mucilage are developed especially strongly on the leathery leaves of the
Cape Diosmeae (Diosma alba, Agathosma spec., leaves of Buku*), where they are
found on the upper side of the leaf, which has no stomata, and in the parts of the
under surface where stomata are absent. The cells in these parts are of a great
height, their outer half has the usual structure of tough cuticularised epidermis. The
whole of the inner half, which is often large, is filled with the stratified mass of
mucilage, which is limited on the exterior by a level surface. This body swells on
addition of water or glycerine to such an extent that it lifts the whole outer parts of

* Monogr. d. Gattung Serjania, p. 100 (1875).

^ Compare Fliickiger, Schweizerische Wochenschrift f. Pharmacie, Dec. 1873.

74

CELLULAR TISSUE.

the epidermis far from the inner tissues of the leaf, and appears itself like a special
mucilaginous layer of tissue.

The same phenomenon, but apparently always developed to a less extent, was
found by Radlkofer in the foliage leaves of numerous Dicotyledons ; e.g. Sapindaceae,
species of Salix, Daphne, Quercus pedunculata, Betula alba, Erica carnea and
Tetralix, species of Prunus, Genista, Cytisus spec., &c., of Ferns in Botrychium
Lunaria. As shown by Radlkofer’s comparative review of the cases investigated,
the phenomenon is by no means generally distributed, nor is it generally peculiar to
definite forms of leaf or systematic groups ; it is absent, e.g. in Salix alba, amygdalina,
Betula fruticosa, Prunus Padus, &c. In the Sapindaceae investigated by Radlkofer
it is often only single cells, or groups of cells, which show the phenomenon
in question.

Cutin occurs in form of the cuticle, and in the cuiicular layers of the cellulose
membrane k

Cutin is a non-nitrogenous carbon compound, which is completely combustible ;
it is dissolved, or destroyed by boiling solution of potash, and by Schultze’s mixture ;
it can therefore be completely removed from the epidermis by these reagents. It is
only slightly attacked by mineral acids, especially sulphuric acid, which destroy
cellulose. It remains unaltered in ammoniacal sub-oxide of copper after previous
treatment with acids ; the same is the case with water, alcohol, and ether. It resists
rotting far longer than cellulose. By these reactions the means are supplied for
isolating the cuticle and the cuticular layers. Iodine preparations, with or without
the assistance of sulphuric acid, colour the cuticle and cuticular layers yellow or
brown. Aniline dyes are quickly taken up by them in large quantity, and deep
coloration is the result. From a mixture of aniline red and violet, the latter is often
(not always) taken up more abundantly

The cuticle covers the whole outer surface of the epidermis, including the hairs,
as a thin, always closely applied hyaline skin. It appears, secreted on the outer surface
of the cellulose walls, on the embryo while it consists of only few' cells, and covers it
henceforward as well as the punctum vegetationis of the stem and all the members
which appear on it, always following the growth of these by means of corresponding
surface-growth, w’hile its increase in thickness is infinitely less. This continues till
the epidermis is eventually thrown off. In rare cases, when the original epidermis
is destroyed early, and is then replaced by new elements (the leaves of the Aroidese
mentioned on p. 29, and perhaps also of Palms), there appears also over the latter a
new cuticle. It is w'anting at the punctum vegetationis of the root, but appears
behind it, where the differentiation of the epidermis begins. The cuticle is con-
tinuous over the surface of the guard-cells, through the stoma, into the respiratory
cavity, from the moment when the formation of the slit begins by separation of the
two cellulose lamellae. It usually continues over the walls of the respiratory cavity
as far as these are formed from the epidermal cells. It becomes gradually thinner
as it proceeds inwards, till it ceases where the respiratory cavity is laterally bounded
by subepidermal cells. It thus forms at each stoma an open tube which passes

1 [Cf. Von Hohnel, über die Cuticula. — Ref. Bot. Jahresbericht, 1878, I. p. 16.]
Hanstein, Botan. Zeitg. 1868.

EPIDERMIS.

15

from the slit inwards. In the Cactese, it extends from the slit onwards over the
whole wall of the spacious respiratory cavity, and sends tubular branches with open
ends into the intercellular spaces of the neighbouring chlorophyll-containing paren-
chyma *. It is wanting as a rule on the inner surface of the epidermis. It is rarely
continued from the slits onwards over the whole inner surface of the epidermis, as
far as this borders on intercellular spaces, as a lamella, which is interrupted by the
surfaces of insertion of the subepidermal cells (v. Mohl, /. c.). This is the case on
both the stomata-bearing surfaces of the leaf of species of Armeria, especially
A. plantaginea, on the under surface of the leaf of Betula alba, Dianthus caryophyllus.
Euphorbia Caput-Medusae, and the stomata-bearing bands of the leaf of Asphodelus
luteus. In Helleborus niger and viridis the inner cuticle extends from the stomata-
bearing under surface, over the upper side of the leaf, which has no stomata. (On
the occurrence of cuticle in the deeper-seated intercellular spaces, comp. Chap. VII ;
on the peculiar phenomena in Restio diffusus, see Sect. i8.)

In the well-established cases, the cuticle cannot by the means of investigation
at present in use be separated either mechanically, or optically, into separate parts
or segments corresponding to the neighbouring cells. By careful maceration with
potash or dilute acids, it may be separated as a continuous skin from large tracts of
the underlying cell-membranes. It appears by the action of the above reagents to
swell more strongly in the direction of the surface than those membranes. By
boiling solution of potash, or Schultze’s mixture, it is transformed into a tough shiny
mass, and then completely destroyed without leaving a cellulose residue. It is in
most cases very thin, especially on submerged parts and roots ; on aerial parts,
not excepting the pimcium vegetaiioms, it is thicker ; only in few cases where it is
specially strongly developed (leaf of Cycas revoluta. Ilex aquifolium), a delicate
stratification can be recognised ; as a rule, there is no sign of this. Its thick-
ness is usually equal all over one and the same surface ; also on the ridges and
warts of the surface so often mentioned the cuticle itself usually runs unthickened
over the corresponding outgrowths of the wall (e. g. leaf of Eucomis, Orchis,
Helleborus, &c. Comp, also v. 'Mohl, Verm. Sehr. Taf. IX. Figs. 7, 8). Projecting
thickenings belonging to the cuticle itself are much more rare ; on the hairs of Mono-
tropa Hypopitys ^ there is the most exquisite example of this ; the outermost layer
of the wall, which is covered with numerous elongated warts, here shows the pro-
perties of the cuticle : it is completely dissolved in boiling potash, and leaves the
cellulose membrane quite smooth.

On some very thick epidermal layers, which form large quantities of wax
(Acer striatum, Negundo, Sophora japonica), the cuticle follows the increase of
thickness of the membranes only a short time, and then breaks up by irregular
splits.

Where the epidermis is delicate the cuticle covers the relatively pure cellulose
membrane of the epidermal cells. But where it is thicker, especially when long-
lived, the part of the cellulose membrane bordering on the cuticle itself also contains
cutin, and consists of layers of cellulose, each of which is permeated by cutin.

^ Von Mohl, Botan. Zeitg. 1845, p. 3. — Unger, Grundzüge (1845), p. 25.
® Schacht, Lehrbuch, I. p. 140.

76

CELLULAR TISSUE.

According as this is the case, the membrane shows the characteristic reactions of
cuticle. Treatment with the reagents, which dissolve cutin, removes it successively
from the persistent cellulose membranes, which retain their original form and struc-
ture, though necessarily with considerable loss of substance (comp. Fig. 25). These
layers containing cutin are called cuticularised, or ciiticular layers

The cuticularisation may extend over all the elements of the epidermis, in-
cluding the epidermal cells, the hair-structures, and the cells of the stomata also.
On the latter the cuticular layers are it is true often thinner, in correspondence
with the smaller size of the cells, but often, especially in the ridges of entry, they are
strongly developed, and are continuous with those of the neighbouring cells, e. g.
leaves of Clivia nobilis Dasylirion Epidendron ciliare and other tough-leaved
Orchideae, Ficus elastica, &c.

The cuticularisation is evenly continuous over the epidermal cells of one
surface, and in the above-mentioned cases over the stomatal cells also, so that
one cell resembles another : thus the cuticular layers, in a simple case, appear in a
transverse section as an even broad band surrounding the whole epidermis. The
cells of the epidermis may be separated from the cuticle, which covers them, both
optically and (by help of the above-named reagents) mechanically. The latter fact
is certain, though it is not always easily done. The form, relative thickness, and
extension of the cuticle over the cell-walls connected with it is no less various and
characteristic in special cases than the other relations of form and structure above
described. The following typical forms may accordingly be distinguished : —

I. The cuticular layers form in the great majority of cases a covering on
the outer side of the epidermal cells, which is sharply marked off internally from the
non-cuticularised membrane. This may be —

(0) A layer of almost universally equal thickness, which follows the surface, and
does not attain a thickness equal to that of the outer wall ; e. g. leaf of Dianthus
plumarius, Caryophyllus, Helleborus foetidus, Vanilla, Galanthus nivalis^, &c.

Or (U) a thick layer, which follows the outer surface, and projects inwards in the
shape of a ridge, of a conical form, in the middle of each lateral wall, and where
several cells join. The projections are usually sharply wedge-shaped towards the
inside, and do not reach as far as the inner wall (Fig. 25). Or they reach as far as
the latter, and are continued into the layer (‘ intercellular substance ’) which marks the
limit towards the subepidermal layer, and which is also cuticularised ; e. g. branches
of Jasminum officinale. Ephedra distachya, leaf of Phormium tenax®. Ilex (Fig. 26),
and Pinus (Fig. 27).

The non-cuticularised layer (coloured blue with Schultze’s solution), which
in all these cases surrounds the cell-cavity, is either relatively thick, and consists of
many layers, as in the leaves of Pinus, Ilex (leaf-nerves), many species of Aloe,

1 Von Mohl, Botan. Zeitg. 1847, p. 502. Von Mohl, Botan. Zeitg. 1856, l.c.

2 Schacht, Lehrbuch, I. Taf. IV. fig. 9.

‘ Von Mohl, Verm. Schriften, p. 260 ff. — Wigand, Intercellularsubstanz u. Cuticula (1850), fig.
96, &c. — Petunikow, Recherches sur la Cuticula, p. 191, figs. 1, 22 (Bulletin Soc. Imp. de Moscou,
1866).

^ Von Mohl, Verm. Schriften, Taf. X. fig. 28, 27.

EPIDERMIS.

77

Agave americana, Epidendron ciliare, Dasylirion, Sanseviera zeylanica, and the
phylloclades of Ruscus aculeatus ; or, in so far as it borders on the cuticular layers,
it is a very thin layer, which in many cases can only be observed with certainty in
very good preparations. This is most frequently the case where the epidermis is
thick. Examples: leaf of Hakea ceratophylla and other species, Ilex aquifolium
(surface of leaf), Hoya carnosa, Taxus baccata (under surface of leaf); one-year-old
branches of Viscum album Taxus, Rosa canina, Kerria japonica. Ilex aquifolium,
Jasminum officinale, Laurus nobilis. Sassafras, Acer striatum &c.

(c) The whole outer wall is cuticularised, the rest of the wall not. The upper
side of one-year-old leaves of Taxus baccata, one-year-old stems of Salix daphnoides,
and, according to v. Mohl (Verm. Sehr. Tab. IX. 15), epidermis of the stem of
Kleinia neriifolia.

2. The cuticular layers and the non-cuticularised part of the wall are not sharply
defined one from the other, but rather —

{a) either the inmost lamella of each cell-wall is not cuticularised, while the
outer layers show the cuticular reaction gradually stronger the further they are from
the inmost layer ; e. g. stem of Psilotum triquetrum, young stems of Selaginella
insequalifolia, Martensii, &c. ;

{b) or the whole wall of the epidermal cells is cuticularised all round : petiole
of Arbutus Unedo two-year-old branches of Nerium oleander leaf of Elymus
arenarius stem of Klopstockia cerifera leaf of Pinus, Abies, Cunninghamia
lanceolata, older stems of the above-named Selaginellas. Further, the brown-walled
epidermis of the stems and petioles of very many Ferns belongs to this category.

3. The epidermis of the pinnae of the leaf of Cycas revoluta may be cited as
an exceptional case The pitted cellulose walls of the epidermis are covered
externally by a thick cuticle, which is stratified, but not separable into cuticular
layers. From the cuticle there run narrow limiting bands of cuticular substance
between the lateral walls of the cells, to the subepidermal tissue.

Where the cuticular layers border on non-cuticularised membrane the limiting
surface is either smooth, e. g. in most epidermal layers of branches cited above
under No. i ; or it is rendered uneven by numerous small processes, which
penetrate the cellulose layer like little teeth. Very small processes of this sort are for
instance to be found on the branches of Taxus the leaf of Hoya carnosa; and larger
sharp teeth on the leaves of many species of Aloe (Fig. 23), and the phylloclades
of Ruscus aculeatus. Epidendron ciliare has numerous fine teeth both on the
surface of the cuticular layers which cover the cellulose wall externally, also on
the wedge-shaped ridges, which protrude into the lateral walls, and thirdly on
their sharp angular pegs, which protrude further into the lateral angles than the

* Compare von Mohl, Verm. Schriften, Taf. IX. X. figs. 12, 14, 23, 26. — Vegetab. Zelle, fig. 40.
— Schacht, Lehrbuch, I. Taf. III. figs. 16, 17, 23-25 ; IV. 9, &c.

“ Von Mohl, Botan. Zeitg. 1849, p. 593.

^ Botan. Zeitg. 1871, p. 596. * Wigand, /. f . p. 78.

® Petunikow, /. r. pp. 19, 20, fig. 21. ' Wigand, l.c. p. 105.

’ Botan. Zeitg. 1871, p. 577.

® Von Mohl, Verm. Sehr. l.c. — Schacht, Lehrb. l.c. —Wigand, /. c. fig. 43.

^ Graf zu Solms-Laubach, Botan. Zeitg. 1871, p. 536,

7«

CELLULAR TISSUE.

ridges. On the epidermis of the branches of Prumnopitys elegans ^ the processed
are very large, in the form of thick plates, which are blunt, often branched, of
unequal size, and irregular curvature; so that in their sections separate pieces,
cut off from these, lie isolated in the non-cuticularised membrane. On the leaves
of many Proteacese (Lomatia longifolia, Hakea ceratophylla, H. Baxter! the pro-

Fig. 25. — (390} Transverse section through the leaf of Aloe verru-
cosa. a lying in water, not acted upon by reagents. The non-
cuticularised parts of the membranes shaded ; outside this the
cuticular layers, broken up by darker limiting lamellae, and covered
by the cuticle, which has a double contour, d after wanning with
potash solution. The cuticle raised from the (shaded) cuticular
layers ; the non-cuticularised (unshaded) inner layers somewhat swol-
len. c epidermal cells after complete removal of the cuticle, and of
the cutin deposited in the cuticular layers, by continued boiling with
potash. They are separated from one another, and are seated upon
the cells of the sub-epidermal parenchyma ; the cuticular layers are
now only distinguishable from the others by more delicate stratifica-
tion.

Fig. 27.— (800) Pinus Pinaster. Leaf, transverse sec-
tion, corner of the margin. cuticularised ; 2 inner
non-cuticularised layers of the epidermal cells ; c' very
large and thick-walled cells lying at the corner; ^—2'
hypoderma ; / chlorophyll-containing parenchyma ;
/r contracted protoplasmic body of the cell with in-
folded walls. From Sachs’ Textbook.

cesses are blunt and swollen, and the spaces between them, which are filled with
cellulose, are narrow slits. These appear in surface-view as bands which are

Fig. 26. — (800) Ilex aquifolium ; leaf, yf transverse
section through the midrib of the underside ; a, b cuti-
cular layers, the inner (b) stains yellow with Schultze’s
solution, and is continued into the limiting lamella,
which reaches to the sub-epidermal layer. The outer
(a) remains uncoloured in Schultze’s solution (only par-
tially true cuticle?), c, c the non-cuticularised portions
of the membranes. From Sachs’ Textbook, compare
p. 35. — B a few cells of the same epidermis, seen from
the outer surface.

^ Graf zu Solms-Laubach, 1. c.

^ Nägeli, Sitzungsber. d. Bayr. Acad. 7. Mai, 1864, Taf. II. 19, 20.

EPIDERMIS.

79

arranged irregularly round one central point, or in the case of elongated cells
round two eccentric points. They are often also branched, and connected by
anastomosis. In vertical sections they appear as thin bright radial bands. With
Schultze’s solution they turn blue like outgrowths of the inner cellulose layer, while
the cuticular layers with their protruding swellings assume a brown colour. This
behaviour causes the doubt expressed by Nageli in these cases, as to the nature
of the above-mentioned slits.

As regards the internal structure of the cuticular layers, we need not here
discuss the phenomena of stratification, striation, and areolation, which are general
for thickened cell-walls^. Different successive layers or systems of layers (shells)
are in many cases cuticularised to a different extent, and are therefore of different
refractive power and colour differently with preparations of Iodine ; e. g. in Ilex
aquifolium (comp, above, Fig. 26), Aloe soccotrina, Fourcroya gigantea, Taxus
baccata, (Petunikow, l.c. Tab. Ill, Fig. 3, 4, 7, 9). The same holds in many cases
for the successive striae perpendicular to the surface. Hakea Candolleana Meisn. has
for instance a quite similar striation to that above described for H. ceratophylla.
But the striae lie in the cuticular layers, and do not extend inwards to the cellulose
layer: moreover they turn bright yellow with Schultze’s solution, while the other
cuticular layers turn brown®. Epidendron ciliare shows with Schultze’s solution
broad striae, perpendicular to the surface, of variable and very different intensity
of colour.

Another phenomenon, also of common occurrence in masses of cells, is the
difference in optical and chemical nature of the limiting layers, and limiting lamellcB
of contiguous cells, from the other membranes. And this occurs often very
conspicuously in the cuticular layers. Sharply marked, thin limiting lamellae often
extend from the cuticle, tapering off inwards like wedges. They appear like
continuations of the cuticle inwards, between the cuticularised lateral walls of many
epidermal layers. They also are destroyed by those reagents, which destroy
cuticular substance, and therefore consist entirely or principally of it (comp.
Fig. 25, c. and Fig. 27). In other epidermal layers (e. g. Acer striatum, Dianthus
caryophyllus, &c.) limiting lamellae are not visible or hardly indicated in the fresh
intact preparation. But in these cases also the lateral walls of contiguous cells are
separated by reagents, which destroy the cuticular substance, and this tends to the
conclusion that there is a delicate limiting lamella consisting entirely or chiefly of
cuticular substance.

Ad. Brongniart ® used the term cuticle to indicate, in the first place, that superficial
homogeneous lamella of the Epidermis which alone remains behind when all the rest is
destroyed by rotting or by the action of sulphuric acid. The names of the plants
investigated by him show that he did not distinguish the cuticular layer from the cuticle,
since after the maceration both of these remain, in the leaf of Dianthus caryophyllus, as

^ Compare Hofmeister, vol. I of this Handbook, §§27, 28.

^ The strise described by Nageli, l.c. figs. 14, 15, and Schacht, Lehrb. Taf. HI. 27, 28, for
H. florida, and by von Mohl, 1. c. fig. 18, for H. gibbosa, may be compared with those of H. cera-
tophylla.

^ Ann. Sei. Nat. 2 ser. tom. I. p. 65. Brongniart’s first description was in the Ann. Sei. Nat.
I ser. tom. XXI. p. 427 (1830).

8o

CELLULAR TISSUE.

a connected skin, while in that of Potamogeton lucens only the cuticle remains, since thd
cuticular layer is absent. The distinction of the two parts was not drawn by the
observers of the succeeding period h Mohl first drew attention to their anatomical and
material difference (Bot. Zeitg. 1847, p. 499, &c.).

As regards the cuticular layer, the view, defended earlier by himself and by Meyen, that
the ‘ cuticle ’ is part of the outer membrane of the epidermal cells themselves, was dis-
tinctly proved in this work of Mohl : and the view held by others (Treviranus, Schleiden,
Grundz.), which regarded it as a product of secretory-perspiration, was laid aside. Mohl
says of the cuticle, ‘If any one will ascribe it to a secretion from the epidermal cells, I
have no objection to make to this idea, still it will be difficult to afford a proof of its
truth.’ This he confirms by the striation which occurs in many plants, from which it
may be concluded that there is a definite organisation, and not an origin merely from
hardened excreted fluid. Cohn (De cuticula, in Linnsea, Bd. 23, 1850) then gave a clear
statement of the case, founded especially on investigations on hairs. Wigand ^ also
claimed for the cuticle a (genetic) connection with the cell-membranes of the epidermis,
and defended his view against that of Schacht (Pflanzenzelle, Lehrbuch I), which is
certainly confused, Hofmeister regards it as a part of the cell membrane of the
epidermis'*.

If, in the face of the facts known at that time, most of which are recapitulated
above, the question be raised whether the cuticle consists of parts of all epidermal
cells, or is something distinct from these, the latter alternative must be preferred, even
if anatomical relations alone be regarded, and the difference of material be left on one
side *. Even if Payens’ ® statement were confirmed, according to which the cuticle of
Cereus peruvianus, after continued treatment with boiling nitric acid, water, and
ammonia, if pressed backwards and forwards under the cover slip, separates into
angular pieces, each corresponding to an epidermal cell ; still this is only an isolated
exception. Universally in other cases the cuticle, having been first formed on the
embryo, while still consisting of few cells, grows over the epidermal cells, and uniformly
with them, retaining fundamentally its original properties. Neither in older nor in
younger parts can it be separated into the altered outer lamellae of single epidermal cells,
or groups of cells. Even if it, together with the membranes covered by it, consisted of
pure cellulose, it would be, anatomically considered, an independent membrane, which
belongs in common to the whole member or the whole plant, and must be distinguished
from the walls of the single cells. Its genetic relationship to the contiguous cellulose
layers is not thereby excluded ; it must rather be directly derived from them in all cases
where it grows or is renewed on a free surface, that is, in those cases where the possibility
of an apposition from without is excluded (e. g. such a possibility may be imagined for
the embryo enclosed in the embryo-sac). Further, if we neglect the embryos, pollen-
cells, &c., it is possible on free surfaces — in the development of the stomata — clearly to
observe that first the cellulose membrane alone is present, and later the cuticle appears
upon it. Thus, where reference can be made to the first beginnings of the development
of cuticle (comp. Hofmeister, l.c), at first the cellulose membrane alone is present, sub-
sequently it is separated into cuticle and cellulose membrane. Where the cuticle grows
on a free surface, the material for it can originate only from the cellulose membranes.
It is therefore doubtless a product of differentiation of these, and of the cells to which
they belong.

* Treviranus, Physiol. I. p. 448 — Meyen, Physiol. I. p. 176. — Von Mohl, Linnsea, 1842, and
Verm. Schriften, p. 260, &c.

“ Intercellularsubstanz und Cuticula. Braunschw. 1850, p. 36, &c. — Botanische Untersuchungen
(1854), p. 67. — Flora, 1861, p. 81, &c. — That which is said in these works of the Intercellular sub-
stance does not of course concern the present explanation.

^ Pflanzenzelle, p. 159, 248, 257, &c. * Compare Cohn, l.c. p. 382.

® Compare Hofmeister, l.c. p. 251.

EPIDERMIS.

8l

The cuticle differs from the cell-membranes in its material composition : it consists at
all events in the main of cutin, even though Hofmeister’s ^ observation that the cuticle
of Orchis hlorio and Hoya Carnosa, after maceration for three weeks in potash, be-
came ‘ distinctly ’ blue with solution of I in K I should prove that cellulose was
present {though this point will not be conceded). The question then arises, what is the
origin of cutin, from what material, and where is it produced ? Since it can only
originate primarily from the epidermal cells, and appears originally on the apparently
pure cellulose membrane, and later also is seen within this, in the cuticular layer, and
since it is never found in the protoplasm and cell sap, its origin in the cellulose membrane
and from cellulose is probable, though a safe proof of this cannot be drawn from the facts
at our disposal.

It need not here be emphasised that the cuticle is separated from the very first as a
sharply defined lamella, for which a definite structure from the very first may be
assumed ; and that the old view, which, in cases of excretion or secretion, imagined a
fluid mass filtered out, and hardened sooner or later, is here as little apposite as is the
case with most of the other ‘secretions.’ Comp. Sects. 17, 19.

As regards the origin of the cutin which impregnates the cuticular layer of the cell-walls,
the same holds as has been said for the cuticle. Relatively to the formation and growth of
this layer, it may certainly be observed in many cases (which should moreover be more
carefully studied) that a progressive cuticularisation of the cellulose membrane proceeds
gradually from without inwards ; thus in the above-cited examples of Psilotum and Sela-
ginella. But in the overwhelming majority of cases — on which, however, still more thorough
investigation is necessary — e. g. leaves of Agave Americana^, species of Aloe, branches of
Acer striatum the cuticular layers are first formed with their definitive sharp outline as
thin lamellae, and grow as cutin-containing layers till they attain their definitive thickness,
being always sharply marked off from the cellulose layer which grows with them. It
may be thought that their increase depends on cuticularisation which progresses in-
wards, but this is not evident on observation.

The opinion of Hartig and Karsten according to which the cuticle is a development
from the mother-cel 1-wall of the plant, has hardly any longer a historical interest in the
face of what we now know, especially concerning its restoration, its entry into the
stomata, and concerning the internal cuticle.

The chemical composition of the body called cuticular substance, or, after Fremy, cutin,
which forms the cuticle, and is contained in the cuticular layers, is not yet quite clearly
known. It is indeed very probable that we have not in all cases to do with the same
body, that for instance important differences exist between the deep-brown coloured
substance of the cuticularised epidermis of Ferns, and the colourless or slightly coloured
cuticular substance of the epidermis of Phanerogams. Investigations on the body in
question refer almost exclusively to the latter. On the ground of Payen’s ® works, the
cuticular substances were regarded as nitrogenous bodies — products of the change of
cellulose by combination with a nitrogenous compound.

Fremy ® isolated the cuticle of various leaves, parts of flowers, and fruits (Iris, Camellia,
apples), and found the elastic, extensible skin, which retained its original «structure

^ Pflanzenzelle, p. 257.

^ Compare Oudemans, Memoire sur les stomates, &c. ; C. Rend, de 1’ Acad, d’ Amsterdam,
vol. XIV.

^ Compare Botan. Zeitg. 1871, p. 596, Taf. 11. figs. 29-35.

* Botan. Zeitg. 1848, p. 730. — Compare Hofmeister, l.c. p. 251.

® Details in Hofmeister, 1. c. p. 249, &c.

' Comptes Rendus, tom. 48, p. 669. — Ann. Sei. Nat. 4 ser. tom. XH. p. 331. — On Payen’s
replies to Fremy compare Comptes Rendus, tom. 48, p. 893. Further, the revision of the discussion
in point in Kopp and Will, Jahresbr. ii. d. Fortschritte der Chemie für 1859, p. 529, &c., especially
PP- .536, 539-

G

82

CELLULAR TISSUE.

after boiling with dilute hydrochloric acid, and subsequent successive treatment with
ammoniacal sub-oxide of copper, hydrochloric acid, potash, ether, and alcohol, to be non-
nitrogenous, consisting of the body which he calls cutin, and for which he gives the
following percentage composition — C, 73, 66 . H, ii, 37 . O, 14, 97. The cutin shows
the properties above described (p. 74). When heated it gives fatty acids, on treatment
with boiling nitric acid, suberic acid, and may be saponified by boiling with con-
centrated solution of potash. The cuticle and cuticular layers of the leaf of Agave
Americana, after their isolation by F remy’s method, appeared to me to have retained
their structure completely: they still retained traces of cellulose which could be
determined microchemically. An analysis completed once in Halle, in Professor
Stohmann’s laboratory, showed the substance to be completely non-nitrogenous.

Sect. 17. In company with the cuticular bodies there are usually found com-
pounds of a waxy nature, i. e. carbon compounds, the chemical constitution of which
requires, it is true, more careful investigation, and perhaps is not very closely allied
to that of bees-wax, but which in J;heir physical properties as now known, such as
solubility, fusibility under 100°, &c., resemble the better known sorts of wax, and
therefore may for the present be shortly termed wax. As regards their solubility, all
the allied waxy bodies are soluble with difficulty or not at all in cold alcohol, but com-
pletely in boiling alcohol. Most of them, but not all, dissolve in ether even without
any considerable rise of temperature. According to the incomplete investigations
hitherto made, many sorts of wax found on the epidermis prove to be mixtures of
two or several bodies : with many there is mixed a considerable quantity of resin,
easily soluble in cold alcohol, e. g. with the wax of the stems of Ceroxylon, Klop-
stockia, and Chamsedorea ; a compound of Silicon is mixed with others (stems of
Chamsedorea and Kerria) k

Wax is not present, according to the existing investigations, in the epidermis of
parts. which are submerged, or underground. In epidermal layers however which
are surrounded with air it occurs very generally, perhaps even universally, partly
imbedded in the cell-wall, partly extended on the outer surface of the cuticle as
a wax-covering.

Imbedded wax occurs only in those walls which consist of or contain cutin,
and not in the relatively pure cellulose membrane. It is found both in those which
simultaneously extrude wax-coverings, and also in others in which the latter are
absent or only very slight, such as the stems and branches of Acer striatum, Sophora
japonica, Jasminum fruticans, the foliage leaves of Cycas revoluta. Aloe verrucosa,
Epidendron ciliare, Hoya carnosa. The imbedded wax is disposed in the cuticular
membrane in the form of small bodies, which, in fresh preparations, cannot be
recognised optically. It is determined anatomically, by fusing it out of the
sections by carefully warming them in water. It then exudes from the cuticle
and cuticular layers, in the form of small drops. Boiling alcohol extracts it from
the membranes. The latter, after the wax has been fused or dissolved out, retain
their original structure. But where the infiltration of wax has been excessive (Acer
striatum, Klopstockia) their volume is considerably reduced, the reduction remaining
even after subsequent treatment with water.

^ On the composition of the bodies in question, compare Wiesner, Botan. Zeitg. 1876, p. 225,
postscript.

EPIDERMIS.

83

Wax-coverings are extruded on the outer surface of the cuticle in four chief
forms, which may be described as (i) strata or crusts, (2) rod-like coverings, (3) simple
granular layers, (4) aggregated coverings.

The strata or crusts are superposed on the cuticle in the form of a continuous
membrane. On many epidermal layers they form a clear, smooth, brittle glaze, which,
when the epidermis is stripped off, appears cracked and broken into angular pieces.
It attains a thickness of about Thus on the foliage of Thuja orientalis,

occidentalis, of Sempervivum tectorum, calcareum, the young stem of fleshy Eu-
phorbias (E. Caput Medusae L., E. ornithopus, E. canariensis, piscatoria, balsamifera
Ait.’), of Lepismium paradoxum Salm., Kerria japonica. Very delicate, angular,
homogeneous scales, like the fragments of a delicate skin of wax, are also found as
a rudimentary form of the glaze on many smooth shining epidermal layers : Cereus
alatus, species of Opuntia, leaf of Fuchsia globosa, Taxus baccata, leaf and stem of
Portulaca oleracea.

In other cases the wax-layers attain very considerable thickness and then show
a more or less complicated structure, stratification parallel to the surface, and a
striation or areolation, or both of them together. The wax-crust of Euphorbia
canariensis on old branches attains a thickness of 70/^, and shows obvious stratifica-
tion ; that of Kerria is more than 5jti thick, and is also stratified. The stems of
species of Chamaedorea, especially Ch. Schiedeana, Mart., are covered by a stratified
glaze of wax up to 14/^ thick, which is brittle, and contains silica. The wax
covering on the epidermis of the stem of the wax palms of the Andes, Ceroxylon
and Klopstockia, are much more massive, attaining a thickness of s“““, and show
rich stratification and areolation. The young leaves of Corypha (Copernicia)
cerifera are covered on both sides by a wax-layer which is also striated perpendicular
to the surface, but attains a thickness of only 15-19/i. This having crumbled off
from the dried leaf, is collected as Brasilian Carnauba-wax. The stems and leaves of
Panicum turgidum, Forsk., are covered by a crust of wax up to 30/x thick, which is
brittle and striated perpendicular to the surface.

The thin glaze of wax, of the leaves of Sempervivum glaucum, which is in other
points like that of S. tectorum, is warty and uneven on its outer surface; that
of the leaves of Cotyledon orbiculata, L., is studded with numerous erect converging
out-growths in the form of rods lo/i high, and about ifi thick.

The latter form the transition from the form of wax-covering above described
to the rod-like-covering, which covers the epidermis of the under surface of the leaf of
Aechmea farinosa, but occurs especially in many Scitamineae and Gramineae :
e. g. on the under surface of the leaf, and on the petioles of Heliconia farinosa,
Strelitzia ovata (Fig. 29), Musa spec., on hypsophyllary leaves of species of Canna,
internodes and nodes of Saccharum oflicinarum (Fig. 28), Eulalia japonica, Trin. ;
on the short slightly silicified epidermal cells on the leaf-sheaths and the under sur-
face of the lamina of the latter species, further in the same positions, and on the
stems of species of Sorghum, Coix lachryma, &c. Rods consisting of wax here
stand perpendicularly upon the cuticle, either at relatively wide distances apart
(distances equal to or greater than their width), or so near one another as even

^ Schacht, Lehrb. II. p. 559.

84

CELLULAR TISSUE.

to touch. The rods may be as high as the epidermal cells (Strelitzia, Internodes of
Saccharum, &c.) or in most cases much higher, — the longest, especially those observed
on the nodes of Saccharum, attain a height of more than loo/x and i^Ofx. Their
height is however very unequal. Their thickness reaches usually on the average about
ift, but in the largest often three or four times as much. Their form is cylindrical,
or, in the case of the thickest, more or less angular or compressed like a riband.
They are straight in their lower part, which is attached to the epidermis, but the
upper ends are, in the shorter ones, hooked, in the longer ones very strongly curved
like a crosier, or a cork-screw curl. Their substance is homogeneous, or, when
they are large, longitudinally striated. To the naked eye the rod-like coverings
appear as a white mealy covering of the surface varying in mass according to
the size and number of the rods, and easily scraped or brushed off. It is most

Fig. 28. — Transverse sections through the stem of Saccharum officinarum. A {375} surface of a mature young internode,

B {142} of a similar node.

obvious on the leaves of the Heliconia above cited, and on the nodes of Sac-
charum.

That form of the covering is termed a simple granular layer in which granules
of wax are superposed on the cuticle, side by side in a simple layer, and not heaped
upon one another. The granules have on the average a size up to i/i: they are
spherical, or in the form of very short rods perpendicular to the surface (e. g. Allium
fistulosum, branches of Acer striatum), and form, when of the latter shape, the
transition to the rod-like coverings. They lie either at wide distances apart (e. g.
upper surface of the leaf of Tropseolum majus, Begonia semperflorens and
other spec., Vitis vinifera), or they approach one another leaving small spaces, or
till they touch one another, the latter, e. g. in the mature leaves of Tulipa, Echeveria
pumila, and other species, Dianthus caryophyllus, the red and white cabbage, &c.
When the granules are not too far distant from one another they form the white
or blue, easily removed bloom, to which so many so-called glaucous parts owe their
character. Of the innumerable examples of this besides those already cited may be
named the parts of the epidermis of the above-cited grasses, which do not bear
rods, the leaves of Iris germanica, pallida, Galanthus nivalis. Allium, Brassica
oleracea var., Mesembryanthemum spec., Calandrinia speciosa, the upper part of
the inner side of the leaf-pitcher of Nepenthes (Wunschmann, 1. c).

A large number of glaucous and hoary parts of plants from the most various
families are covered with the wax-bloom of the fourth type, which is termed an

EPIDERMIS.

85

aggregated wax-covering, since it consists of an aggregation of very delicate rods or
granules, which cover the cuticle not with a simple layer, but with several irregular layers.
Examples of the aggregated rods are found on the white Eucalypti (E. globulus, pul-
verulenta). Acacias (A. Hügelii, cultriformis), Lonicera implexa Ait., Andromeda
dealbata ; Secale cereale, Elymus arenarius, Encephalartos horridus ; examples of
aggregated granules are found on Kleinia ficoides, Ricinus communis, under surface
of the leaf of Abies pectinata and its allies. Intermediate forms between the third
and fourth type are sometimes found, e. g. leaves of Agave Americana.

The wax-coverings usually cover the whole epidermal surface evenly, or with
the differences of form described above for the epidermal cells of differing contour.
The granular and aggregated coverings also overlie the guard-cells of the stomata
up to the entry of the slit. The stratified coverings, where observed, leave the stomata
free, in so far as they belong to parts already unfolded, or at least they become so thin
as to be unrecognisable over the guard-cells. On the other hand the wax-layer of the
young not yet unfolded leaf of the Carnauba palm completely covers the stomata also :

Fig. 29. — Strelitzia ovata. A under surface of a leaf still in course of unfolding ; on the epidermal cells are rods of wax in
course of growth j surrounding the stoma is the striated ring — B transverse section of a mature leaf. Epidermis with one stoma
(lying above the wide respiratory cavity S), the rods and the ring. Under the epidermis is the large-celled hypodermal layer ;
below this the chlorophyll-containing cells of the leaf parenchyma (375).

these however are freed when the leaves unfold, since the spontaneous crumbling
off of the wax-layer on the unfolded leaf, though not indeed directly observed, may
still be certainly assumed from the data at our disposal. Certain of the Scitamineae
show remarkable peculiarities in the distribution of the rods. On the leaf of Musa
ornata. the short rods are situated chiefly, if not exclusively on the edges of the
epidermal cells. Strelitzia ovata (Fig. 29) has rods distributed over the whole lower
leaf-surface. Those which stand on the annular rows of cells surrounding a
stoma converge with their hooked ends towards the stoma, and on the edge of
the wall, which separates the two half-moon-shaped subsidiary cells from the
surrounding epidermal cells, stands a ridge consisting of wax, and having the form
of a ring diminishing conically outwards, with its margin curved inwards, and with
radial striation, which gives it the appearance of being composed of many con-
verging crosiers. The outer surface of the subsidiary cells themselves, and of the

86

CELLULAR TISSUE.

guard-cells, is free from wax-covering, the stoma is as it were shut off from this
by the ring. Other species of Strelitzia have a similar ring, though in form and
distribution of rods they differ from S. ovata.

The structure and development of the wax coverings and intramural wax have been
described by the author of this book in a longer treatise (Bot. Ztg. 1871). There also
the very limited older literature on this point is quoted. Later Wiesner made two
additions (Bot. Ztg. 1871, p. 769). For various details we must refer to these special
works. We can here add but little to what has been said above.

The internal structure of the wax coverings, which in the rods is indicated by the
striation, i. e. by the presence of longitudinal layers of alternately unequal optical
properties, attains in the thick layers a complicated differentiation. In the investigated
material of Klopstockia cerifera Karst. (Fig. 30) the covering attained a thickness of

0.66 Above each stoma it is perforated
by a perpendicular canal S-S' which contains
air and fungal hyphae. It is composed of pris-
matic pieces, of which each fits exactly over one
epidermal cell, and which are not arranged sepa-
rately side by side, but are connected directly
by a homogeneous intercalary or fundamental
mass. This appears in thin sections as a trans-
parent limiting band between the sides of the
prisms. Each of the latter shows internally (i) a
rich and delicate stratification parallel to the
outer surface of the contiguous epidermal cell,
dark and clear layers alternating, of which the
latter resemble the hyaline limiting bands; (2)
darker longitudinal bands perpendicular to the
strata and to the surface, these are denser the
nearer they run to the lateral surfaces of the
prism in the hyaline substance; (3) delicate
striations which run from the lateral faces
obliquely at an angle of 3o°-40°, slightly curved,
towards the epidermis and the median line of
the prism, but do not reach the latter. Under a
low power they are not seen, while the longi-
tudinal striations are only slightly visible, the
strata appearing to be continuous from one
prism to the other : as in Fig. 30.

The deposits of wax on species of Chamse-
dorea, Euphorbias, Panicum turgidum, and the
fruits of Myrica cerifera, show striation and
stratification of a simpler sort. The wax cover-
ing of the fruit of Benincasa cerifera is pecu-
liarly complicated; it consists of a reticulate
layer, and bundles of rods united across one another like a trellis.

The layer of wax on the young leaf of Corypha cerifera was first described by
Wiesner (/. c.), and, as I think, not correctly in all the particulars. I find in the material
which I received through the kindness of Professor Wiesner the following structure.
Both leaf-surfaces are covered by a white layer, in which there appear under a higher
power three constituent forms; (i) a clear transparent ground substance; in this (2)
dark (blueish) spots, and (3) much clearer more transparent points in the form of very
various figures, sometimes narrow slit-like, sometimes round. The latter clear spots
appear as cavities in the substance, and may here be so called. Thin transverse sections

FIG. 30. — (116) Klopstockia cerifera; intemode of
stem, transverse section, e—e epidermis ; S~S" stoma,
with two small guard-cells between the two larger sub-
sidiary cells ; 7u wax covering over the epidermis, over
which it fits; 3“ — S' the canal leading to a stoma.

EPIDERMIS.

87

show that they correspond to deep tube-like depressions of the surface, but these are
not complete perforations : it must remain undecided whether such perforations may
occasionally occur. The three constituents are unequally distributed in correspondence
with the longitudinal bands with and without stomata, which alternate in the leaf-
surface. On the former the holes are very near together, and numerous: the spots
between them form a narrow very irregular net, the substance of which has chiefly the
darker, blueish appearance. The network is densest and the meshes narrowest over
the stomata themselves, i. e. over the pair of guard-cells : a narrow slit-like hole
corresponds usually, but not always, to the slit of the stoma. On the bands of the
leaf without stomata, the wax-layer is more homogeneous : it has more solitary usually
slit-like holes.

Corresponding to these appearances of the surface view, the transverse section of the
wax -layer shows straight bands, of alternating unequal refractive power, which are
parallel and perpendicular to the surface. The clearer parts correspond partly to
the holes, partly to the more transparent fundamental mass. On the parts which have
many pores and stomata, the striation is, as might be expected, much denser and more
clearly defined than on the parts with no stomata. But figures, like Wiesner’s Fig. b, I
have never seen, at all events not on clearly cut sections. It must finally be added to
the above typical description, that the regions distinguished are not always quite sharply
limited from one another. Again, Wiesner's description of the wax of the sugar-cane
corresponds so imperfectly with the appearances found on fresh plants, that the idea
suggested itself of different structure of the covering in dil^'erent sorts or varieties. My
observations on material, also kindly supplied by Wiesner, on which his statement was
founded, do not confirm me in such an assumption. The traces of dense, normal rod-
structure may be clearly recognised, and are made indistinct by all sorts of injury
(especially very many fungal hyphje are present), just as one may observe it on the old
nodes of living plants (comp. Bot. Ztg., /. c., p. 15 1). Wiesner’s statement that the wax
layers and rods appear doubly refractive under polarising apparatus is quite correct ; the
same holds for wax-depoSits, which. I investigated on this point: my only reason for not
mentioning this earlier, was that it appeared to me to afford no decided conclusion
concerning the nature of the layer.

As regards the development of the wax- layers, it is proved that they appear on the
persistent cuticle, and that they are extruded, and do not owe their origin to a meta-
morphosis and metacrasis of the cuticle and cell-membrane itself, as earlier writers
had asserted (Wigand, Karsten, Uloth). Wax-layers may arise from very dense simple
granular coverings, by lateral fusion of the granules as they increase in number and size.
The typical layers arise in the first place as such: they may follow, by intussusceptive
growth, the increase of girth of the part which bears them (Kerria, Euphorbia, Ghamae-
dorea). The coverings appear, either when the part which bears them is very young
(Eucalyptus, Acacia, Dianthus, Echeveria, Carnauba palm), or only in later stages
of development, e. g. not till the unfolding of the bud (leaves of Strelitzia, Heliconia,
Galanthus, Tulipa, Cotyledon orbiculata, stem of Saccharum, Chamaedorea, fruits of
Myrica, Benincasa). Gases of the latter sort should be chosen for studying the
development. The extruded wax may first be determined in the cell-wall, or cuticle,
and emerges from the latter on the surface : in no case can a trace of wax be found
previously formed in the interior of the cells which secrete it, so as to be transmitted
outwards through the membrane.

The granular and rod-like coverings form that bloom or rime on surfaces which is so
easily wiped off. After it has been wiped off it can be renewed ’, provided the part of
the plant has not passed a certain stage of development, which must be specially
determined for each special case. As far as experience goes, the renewed covering has
the same structure as the primary one, but is less massive.

' De Candolle, Piiysiol. p. 233. — Treviranus, Physiol. II. p. .(4.

88

CELLULAR TISSUE.

Sect. i8. To the cuticular structures and deposits should be appended the
cellulose covering, which was found by Pfitzer ’ on the stems of Restio diffusus, but
which was absent from other allied species, and of which the development cannot
as yet be explained. On the outer surface of the firm large-celled cuticularised
epidermis of this plant lies a colourless, almost homogeneous, and transparent,
brittle, dry, easily separated layer, as thick as a strong outer epidermal wall. The
layer does not enter the deep hollows in which the stomata lie, but it is rather
curved over the opening like a bridge, and closes it with exception of a very narrow
longitudinal slit with toothed margin. Beneath the layer lies the cuticle : the layer
itself shows an exquisite cellulose reaction. According to careful investigation of
the unfavourable material at his disposal, Pfitzer arrives at the very probable
conclusion that this layer covering the cuticle is the disorganised outer layer of cells
of an epidermis consisting of two layers (excepting in the vicinity of the stomata) :
a view which is especially supported by the fact that the covering appears in surface-
view partitioned off by delicate lines in a cellular manner.

Sect. 19. Dermal glands. In the cell-walls of the epidermis there occur very
generally distributed resins, ethereal oils, and mixtures of both ; and mixed with
these also, though more rarely, vegetable mucilage (Bassorin, which swells greatly or
is dissolved in water), gum and sugar. When free these substances give to the
surface a sticky character; when they are volatile they are perceived as scents.

In contrast to the formation of wax, the occurrence of these bodies is relatively
seldom extended over large epidermal surfaces ; on the other hand they are usually
localised, partly on circumscribed spots in the level epidermis or on teeth or other
emergences, most commonly on hair-structures ^

The spots on which the sticky or volatile bodies in question occur are called in
common speech glands, glatidulce, and the character of surface resulting from these
glandular, the bodies themselves the secretion, or the product of secretion, or segrega-
tion of the glands. But this expression is also used for other very different organs,
which doubtless correspond partially with those in question in the general characters
of the secretion, but differ widely as regards their structure. The organs of the
epidermis, usually termed collectively glands (with few exceptions to be treated of
later), are characterised by very definite anatomical properties, and should therefore
be designated by a definite general name, and be distinguished from the rest. They
may therefore be termed epidermal or dermal glands. According to the above-
indicated difference in the distribution and extent of the glandular properties we
may distinguish between glandular hair-structures {glandular hairs, scales and villi)
and glandular surfaces or areas {glandular surfaces, glandular spots). When the
latter are localised on definite emergences, it is convenient and practical to speak
of glandular emergences {glandular-teeth or -warts, &c.).

The secretion of dermal glands, when it can be recognised anatomically, always
appears first in the walls of the cells, and gives them a peculiar structure. In most
cases this affects the free outer wall of the cells entirely, or partially : here there
appears a bladder-like swelling, hence Uadder-like dermal glands. In other cases the

^ Pringsheim’s Jahrb. Bd. VII. -p. 564.

^ As regards the literatuie, the references on p. 57 should be compared.

EPIDERMIS.

89

secretion appears in the walls between neighbouring cells : these may be called
glands with intramural secretion, or shortly intramural glands. Besides those which
result directly from the presence of the secretion there are in most, but not in all
cases, other peculiarities of form and structure connected with glandular cells.

{a) Bladder-like dermal glands. The peculiarity of structure of the bladder-like
glands consists in the appearance of the secreted body at the limiting surface
between cuticle and cell-membrane. While the form and turgescence of the cell
remain the same, the secretion, as it increases in quantity, raises up the cuticle like a
bladder : either the cuticle grows simultaneously, while its thickness remains the
same or increases, or its surface growth does not keep pace with the increase of
the secretion, and hence it becomes strongly stretched, and at last is easily burst.
The stickiness of the surface arises from the fluid secretion thus set free by the
bursting of the cuticle. The rent cuticle, according to Haustein, may be repeatedly
renewed on young parts.

These phenomena are best seen in the glandular hairs. According to their
external development these belong for the most part to the capitate form, and the

Figs. 31 — 34. — Glandular hairs of the petiole of Primula sinensis. Fig 31 {142). In a the secretion beginning, with a large
secretory bladder; d an old hair, the bladder has burst and the upper part disappeared. — Figs. 32 — 34 (375) Fig. 32=» a of
Fig. 31. a' with intact secretory bladder lying in water ; a" after solution of the secretion in alcohol. Fig. 33. c end of hair,
intact ; c' the same after solution of the secretion by alcohol. Fig. 34 = of Fig. 31 under higher power.

position of the glandular structure and secretion is at the head. When the latter
is the expanded head of a unicellular hair, e. g. on the leaf of Aspidium molle
or the terminal cell of a compound hair, e. g. leaves of Primula sinensis (Figs. 31 to
34), Pelargonium zonale, Pogostemon Patschouli, &c., its membrane is equally thick
all round, delicate and surrounded by the cuticle which is also thin. At the apex
there then begins a thickening of the wall which gradually increases in strength
and in extent from the apex backwards, spreading over the apical half of the
head (Primula sinensis), or even further backwards, and attaining in the Ferns
above-named the strength of a firm cell-wall, and in Primula and Pelargonium the
bulk of a thick cap, which even exceeds the apical cell in size. The thickening of
the wall consists from the first of the resinous body, which, as shown when burst,

’ Also the teeth of the Palete of Aspidium filix mas, and, as an exquisite example, the glandular
hairs w hich occur in this and allied species in the intercellular spaces of the Rhizome.

90

CELLULAR TISSUE.

and treated with dissolving reagents — e. g. alcohol or ether — is intercalated between
cell-membrane and cuticle.

The same phenomenon, in the main, occurs in the multicellular heads of the
bladder-like glandular hairs, villi, and scales. The thickening by the secretion
begins in these cases at a point more or less near the apex of the whole (not on
each or on several single cells) and extends from this point centrifugally, varying
greatly in extent and bulk according to the special cases. The outer walls of
the single cells thus form with one another either a smooth, even, or domed
surface, or they arch outwards like papillae into the secretory mass which overlies
them, and is in its turn bounded by the cuticle. The raised cuticle itself is usually
homogeneous and structureless, in other cases (shield-like scales of Humulus,
Ribes nigrum) it is marked off into areas corresponding to the lateral limits of the
cells. As already intimated the glandular structure in hairs is not always restricted
to the head ; it may also occur on the lateral wall of capitate hairs, and of such as
are not capitate as in Cistus. (Fig. 36.)

From the point of insertion of glandular hairs in the buds of Rumex, Rheum,
Cunonia, Cotfea, Alnus, Carpinus, Corylus, &c., the glandular structure of the walls

extends over the smooth epidermis (Hans-
tein, Bot. Ztg. 1868). This attains thereby
the properties of the glandular surface.
The same occurs in exquisite form on
the sticky young shoots of Betula alba
(Fig. 35), where the glandular nature of
the wall of the shield-like glandular scales
extends over the whole epidermis, as far
as the ridge of entry of the stomata,
where it ceases. The sticky zones under
the nodes of Lychnis viscaria and other
Sileneae appear to be of the same nature, but require more careful investigation.

Limited, bladder-like glandular surfaces, that is glandular spots, in the above-
defined sense (neglecting the nectaries of flowers, which will not be here treated of),
occur in solitary cases on smooth surfaces, as on the under side of the leaf of Prunus
lauro-cerasus, Clerodendron fragrans, or on the ends of emergences without vascular
bundles, as the capitate ends of the villus-like prickles of Rosa the glandular warts
of the branches of Robinia viscosa ^ ; but the principal place of their occurrence is
on leaves and leafy parts over peripheral endings of vascular bundles, sometimes on
the surface, sometimes on parts of the epidermis which overlie emergences and teeth
of various form and arrangement: as examples may be mentioned, the glandular teeth
of the margin of leaf-laminse, e. g. species of Prunus, Salix, &c. the various ‘ glands ’
of the leaf-organs of the Malpighise and species of Acacia ® and many others

* Ranter, l.c. p. 30. Meyen, Secretionsorg. Taf. VI.

^ Compare Treviranus, Physiologie, II. p. 6.

* A. de Jussieu, Monographie des Malpighiacees, p. 92.

5 Unger, Flora, 1844, P- 7°3 > Anat. und Physiol, p. 362.

® Numerous examples of this have been recently described by Reinke, in Pringsheim’s Jahrb,
Bd. 10, p. 1 19 fNachtr. Anm.).

Fig. 35.— Transverse section through a young internode of
Betula alba (375). c—e epidermis ; a respiratory cavity under the
stoma ; c the cuticle raised from by a secretory layer, as far as
the entry of the stoma (secretion removed by alcohol).

EPIDERMIS.

91

On those parts which have a glandular surface only in their younger stage, as
Rumex, Betula, &c., the epidermal cells of the glandular surfaces are not distinguished
by peculiarities of structure. On the sticky zones of the Sileneae between the ordinary
epidermal cells there are others, which are characterised by special form and dark
granular contents, to which alone Unger attributes the excretion of the glutinous
substance. In Silene nemoralis^ these are simple, broader cells, slightly curved
outwards like papillae : in Lychnis viscaria ^ they are very short hairs consisting of one
small pedicel cell, and one roundish apical cell which rises only slightly above the
epidermal surface.

These circumscribed glandular areas have the common peculiarity, that their
epidermal cells are richer in granular protoplasm, smaller, and more delicate than
those on the rest of the dermal surface, and are of the form of elongated prismatic
or narrow pyram.idal bodies with the longer axis standing perpendicular to the surface.
In Passiflora spec, the prismatic cells are divided by a transverse wall into two
nearly equal halves : the glandular epidermis thus consists of two layers. In Clero-
dendron fragrans also there are two layers, the inner consisting of angular tabular
cells with very thick lateral walls, and the outer of narrow prismatic cells, which are
at least sixteen times as numerous. It is obvious at first sight that in all cases the
narrow prismatic cells result from repeated division of the primary epidermal elements,
and that the structures in question are morphologically allied to the scale-like hair-
structures. In fact there exists between the glandular area of Clerodendron fragrans,
and a group of the laterally united top-shaped scales of Hippuris or Catalpa, only
this one important difference, that the latter are placed above, the former in the
epidermis. The glandular area on the leaf-tooth of Mercurialis annua may, as regards
its form and articulation, be also termed a top-shaped scale, &c.

b. The intramural glands, in which the product of secretion appears in the
limiting wall between the cells, are obviously alM'ays multicellular. They rise as
scales, capitate hairs, or villi above the outer surface, or they do not rise above it, but
rather intrude as depressed glands into the subepidermal tissue. The connection with
the bladder-like forms is effected by the villi of buds found by Hanstein in many
plants, in which the resinous secretory layer appears both under the cuticle of the
outer surface, which is raised like a bladder, and on the limiting surfaces of the lateral
and inner cellulose walls (Azalea indica, 1. c., Figs. 93-95). In the typical allied forms
no secretion occurs between the outer wall and cuticle, but only on the limiting sur-
faces of the division walls. The latter are finally separated by a voluminous secretory
layer. The cells, which in the known cases are narrow and elongated, stand in the
secretory mass like the rods of a trellis or the pillars of a vault.

Of the allied hair-structures, which protrude outwards, may be named the large
capitate glandular hairs of Ledum palustre : the flat top-shaped glandular scales of
the under surface of the leaf of Rhododendron ferragineum, hirsutum (Fig. 41),
Caucasicum. Further investigation will probably increase the number of examples
of this class. Those bodies which appear to the naked eye as bright round points
on both surfaces of the leaf of many species of Psoralea (Fig. 42; e. g. P. bituminosa,

Unger, Grundlinien, p. 82.

^ Idem, Anat. und Physiol, p. 214.

92

CELLULAR TISSUE.

hirta, stricta, pinnata, verrucosa, &c. are imbedded intramural glands; to the
structure of these attention had been drawn by Hildebrand ® in a description which
is not quite correct.

If we compare the use of the word Gland by the different authors, and ask ourselves
what is really called a gland, we come to the conclusion that this name implies any part
of the plant from which something does or may come out, or in which something is
contained which, according to the conception of the author, is distinct from those
bodies which together form the generally disti-ibuted plant-substance which no plant is
without, as e. g. cellulose, starch, chlorophyll, &c. A consistent distinction does not
exist, since the point of excretion of the most heterogeneous bodies— air, water, calcium
salts, resin, sugar, &c. — of the most varying structure — epidermis, hairs, stomata, ends of
vascular bundles — and points of deposit, which are as variable in contents and structure,
viz. cells, groups of cells, intercellular spaces, are arbitrarily termed glands or not.
Compare e. g. Meyen, Secretionsorgane ; Treviranus, Physiologie, II. p. i, &c. ; Unger,
Anat. und Physiol, pp. 209-215 ; Martinet, Ann. Sei. Nat. 5 Ser. XIV.

The facts which underlie this confusion of glands are, taken impartially, in the main
the following. At definite points, especially above the ends of vascular bundles, water-
pores, &c., water coming from below filters out on the surface, and bodies, such as sugar,
gum, salts of calcium, may occur in solution in it ; the water gives the surface a moist
character, or evaporates and leaves the dissolved bodies behind as a solid excretion.

Secondly, the surface may be moist, or sticky, or incrusted by bodies, which issue
directly from the epidermis itself, as e. g. the resin of the glandular hairs.

Thirdly, both the above processes may combine on one and the same surface, as e. g.
on the glandular spots of the Acacias.

The same bodies, which make the surface moist, or sticky, occur, in the fourth place,
also in the interior of the tissue, most variously distributed, sometimes in small quantities
in the protoplasm and cell-contents, sometimes filling whole cells, groups of cells, or
intercellular spaces. Thus, e. g. gum, resin.

These four phenomena, that is the morphological facts which underlie them, are not
the only ones, but the most important of those upon which the use of the word gland is
based. It is true that those of the fourth category are distinguished as internal from the
others as external. On the physiological or teleological significance of these internal
glands hardly anything is known, and of the external ones but little. What is known
of the latter is sufficient for drawing a real distinction between definite categories.
A use of the collective name Gland in the sense used hitherto cannot therefore be
justified from the physiological or teleological point of view. Still less can this be
done on anatomical or morphological grounds, as is shown by what is said in this and
later paragraphs. If we wish, therefore, neither to retain the word gland in this sense
of an ever ready makeshift, nor to exclude it from vegetable anatomy, it only remains to
limit its use to definitely characterised anatomical phenomena, and further to distinguish
these in detail by special descriptions. As no one will support the first suggestion, while
the exclusion of the word is obviously impossible — what would become of pili glandu-
losi ? — we must adopt the third method. Since the parts of the epidermis, treated in this
section, on the one hand unquestionably constitute the great majority of the bodies
included in the term gland or glandular, and on the other hand correspond in very
definite anatomical characters with one another, while they differ from other parts, the
term gland, glandular should be applied to them, and in fact to parts of the epidermis
alone, whatever may lie below them. Whatever has not those anatomical properties,
should be called by a different name, which implies its position. Organs of similar
structure to those in question are very rare in places other than the epidermis : still

^ Compare De Candolle, Prodromiis, II. p. 216.

“ Flora, i866, p. 81, Taf. II.

EPIDERMIS.

93

they do occur, as in the parenchyma of Ferns (Sect. 53). The distinction of dermal
glands from the latter is therefore in any case necessary. It may even be retained, if it
be preferred to attach to the term gland a different meaning from our present one.

The anatomical peculiarity of the glandular parts of the epidermis consists in the
appearance in the cell-wall of that body, which is termed the secretion of the gland, as a
part sharply defined from the cellulose layers. The wall grows in thickness at the
glandular spot, by intercalation of a layer between its outer and inner side. The
intercalated mass differs in material from the cellulose- and cuticular-wall, and is termed
a secretion. These are the appearances directly visible. More careful Investigations
are necessary to answer the question as to the appearance and origin of the secretion.
But in any case it is incorrect to imagine a ‘perspiration’ in the sense of a passage of
large optically determinable masses formed in the interior of the glandular cells through
the membrane. Where it is possible accurately to observe glands with resinous secretion
intact, during their most intense secretion, as I can state, e. g. for the glandular hairs of
Aspidium, Cistus, Pelargonium, Molucella, Pogostemon, Primula sinensis, and also for the
depressed glands of Psoralea, and for Rhododendron, there is to be found within the
cell-wall clear cell-sap, and remarkably homogeneous or very uniform finely granular
protoplasm, and no trace of optically visible drops of resin, the presence of which is
assumed in the ordinary perspiratory theory. Hanstein’s description of the visible
passage of previously formed drops of resin through the cell- wall (e. g. in Viola, /. r.)
therefore appears to me to be much in need of confirmation, in so far as it applies to
glandular hairs which have not begun to wither. Existing observations rather favour the
view, that the secretion — at all events the resinous secretion — is, like wax, first deposited
in the wall itself, and perhaps is first formed in the wall, although it must be allowed
that the material for its formation is and must be derived primarily from the protoplasm
of the glandular cells, or of some other cell. In the interior of old glandular cells, in
which the secretory activity has ceased, or is ceasing, larger collections of the (resinous)
secretion certainly do appear. Further investig ition must decide what is the cause of
this phenomenon, which differs from the original and undoubtedly normal condition.
According to the view derived from conditions which we have termed normal, the
secretion found in the old glandular cells might have originated in the wall, and have
passed from this into the interior of the cell.

Of the two main forms of glands above distinguished according to the point in the cell
where the secretion is formed, the bladder-like glands are by far the most widely
distributed; and of these again the various series of glandular hairs, scales, and w7//.
The structure characteristic of these was first clearly figured, but not understood, by
Meyen (Secretionsorgane, Taf. I, Fig. 30, d) for the small glandular scales of Melissa
officinalis. In 1854 J. Personne gave a very good description and figure of the glandular
scales of the Hop (Ann. Sei. Nat. 4 Ser. I, p. 299), which for a long time were not
understood : in 1856 Unger figured the structure of the glandular scales of Plectranthus
fruticosus (Anat. und Physiol, p. 212), and later (Grundlinien, p. 82) the glandular villi of
Cannabis: A. Weiss gave (Pflanzenhaare, l.c,. Figs. 258, 279 and 280, 310, 343, 364) a
number of clear figures of glandular hairs, without rightly seeing the state of the case.
Haustein (Bot. Ztg. 1868, p. 697, &c.) first authenticated the characteristic structure for
a large series of cases, and recently Ranter and Martinet followed him (/. c., comp. p. 57).
From the facts hitherto known, the general conclusion, here repeated, may be directly
drawn, that the glandular or non-glandular structure is connected with no definite form
or articulation of hair-structures. It should be specially asserted for hairs and villi, that
capitate forms should not be identified or confused with glandular. There are glandular
hairs which are not capitate (Cistus): the hairs of the ‘mealy’ leaves of the Cheno-
podiaceae are exquisitely capitate, but not glandular, &c. It is true, however, that the
majority of glandular hairs are also capitate, and that the glandular structure is localised
on the head ; conversely also only the minority of the capitate hairs are not glandular.
A tolerably detailed review of the different forms in question cannot at present be given.

94

CELLULAR TISSUE.

since hitherto the observers, with the exception of Hanstein, have treated the objects as
exceptional, and have not thoroughly considered the points of structure in question. As
examples of glandular hair-structures, which may here be described rather more in detail
than above, we may begin with the bladder-like hairs, and cite once more those of
Cistus creticus. The flat leaf of this shrub is grey with numerous branched hairs, and,
especially at the base, with long pointed unicellular erect appressed hairs. It bears also

short, multicellular hairs, which are capitate and
glandular above ; and lastly, numerous spindle-shaped
large glandular hairs, which consist of a lower broader
part composed of many disc-shaped cells, and a thin
cylindrical, 1-4 celled terminal part (Fig. 36). The
glandular structure occurs on the slightly widened
apex, and often in places on the lateral wall of the
terminal part.

Besides the examples of capitate glandular hairs and
villi, and the scales connected with them already men-
tioned, may be named those figured by Weiss from the
calyx of Maurandia semperflorens (/. c., Fig. 279) and
Antirrhinum majus {Lc., Fig. 310), and by Hanstein
from the leaf-buds of Ribes (/. c., Fig. 30-33), Syringa
(Fig. 68, 69), Helianthus annuus (Fig. 91, 92). The
adjoining figure (Fig. 37) represents a glandular
villus from the petiole of Conoclinium atropur-

Fig. 36. -Spindle-shaped glandular hairs of
Cistus creticus (375). a before the secretion be-
gins. c, (i with secretory bladder on the apex ; in b
the secretion extends from the apex far back-
wards ; c' the apex of c after removal of the secre-
tion by alcohol and ether.

Fig. 37. — Conoclinium atro-
purpureum (142). From a longi-
tudinal section through the epi-
dermis of a young petiole, e — e
epidermis, with a glandular
villus ; c terminal cell of a villus ;
g terminal cuticular bladder,
filled with resin.

pureum, with a non-capitate terminal portion consisting of two rows of disc-shaped cells,
on which the equally high, hemispherical secretory mass, surrounded with cuticle, is
seated. All the Labiatae (comp. Meyen, Unger, Hanstein, Rauter, Martinet / c.) have
besides various other forms of hairs, short glandular hairs, consisting of a pedicel-cell
lying in the epidermis, a short stalk-cell borne by this, and seated on the latter a
glandular head, covered by a great secretory bladder. The head is in the simplest
cases a spherical cell; e.g. Pogostemon Patschouli, Fig. 38: in most cases a spherical
group of four cells (Lamium album, Rauter, l.c.\ Plectranthus fruticosus. Fig.' 21, A, p.
59) ; also, it not uncommonly grows on, with further division, to a multicellular peltate
scale, as in the case of the large, about 12-celled, depressed glands of many species

EPIDERMIS.

95

of Thymus (Fig. 39), Lophanthus, Satureja : also the longer stalked scales of Lavandula
multifida, &c. (comp. Martinet, l.c., Taf. ii). The celebrated glands of the Hop (Fig,
40, Rauter, Martinet, l.c.) are multicellular peltate scales,
depressed in a cup-like or conical manner, and covered
when mature by the bladder-like gland, which is usually
raised conically, and therefore the whole appears as
though it consisted of two cones with their bases in con-
tact. As found by Meyen, the yellow glands on the foliage
of Ribes nigrum closely resemble them in structure, but
are always flatly disc-shaped.

On the extended glandular surfaces, as that of the epider-
mis of the branch of Betula, I have no further details to
adduce. Of the sticky zones of the Sileneae, it may be still
more definitely stated that the rather difficult determina-
tion whether the secretion is formed from the peculiar
prominent epidermal cells alone (Unger), or from the
epidermis of the whole sticky zone, requires further investigation.

Among the circumscribed spots, which are usually called simply ‘ glands,’ or compound
(external) glands, two things may be distinguished from one another. First, closely

FrG. 38. — Pogostemon Patschouli ;
young leaf, transverse section. Piece of
epidermis, with a short glandular hair ;
secretion dissolved by alcohol (375).

Fig. 39.— Epidermis, with glandular scales, of the upper side of the leaf of Thymus vulgaris (375). a surface view.
b transverse section ; secretion removed by alcohol.

circumscribed groups of glandular, capitate hairs, which are closely
congregated. These constitute the round, pale or dark violet-
red spot on the under surface of each stipule of certain vetches
(Vida Faba, sativa, sepium^), which consists in Vicia Faba of
closely congregated, club-shaped, capitate hairs, of equal height,
with short foot or pedicel-cell, and a head consisting of two
pairs of cells one above another : glandular properties have not
been observed in it in this species. The cells of the head con-
tain strongly refractive bodies, forming dense globular aggre-
gations or granules, together with colourless or violet cell-sap.

In Vicia sativa a sugary fluid is excreted, in a manner as yet
not exactly observed Further the ‘ glands ’ on the under side fjg. 40. -Uumuius lupuiusj
of the leaf of Urena sinuata are club-shaped, closely crowded, g'anduiar scales, transverse sec-

* ^ ' tion (142). A before the secretion

capitate hairs, which line a deeply-hollowed depression. Again, begins, the thick cuUcie iirmiy at-
the so-called glands on the under surface of the leaf of Catalpa — B cuticle raised high up by the
syringaefolia and C. Bungei* consist of groups of top- or fan- '"etTo?«™” ved by\S ^
shaped scales with unicellular stalks, similar to the scales

of Hippuris. These, as regards their structure, obviously do not belong to this category,

' C. C. Sprengel, Entd. Geheimniss, p. 356. ^ Compare Fuckel, Flora, 1846, p. 417.

^ Caspary, De Nectariis, p. 40.

96

CELLULAR TISSUE.

but to others treated of above. From these must be distinguished the glandular epi-
dermal spots, which protrude, but not in the form of hairs. A number of these have
it is true the characteristic bladder-like, glandular structure of the wall, as for example
the round glandular spots on the under side of the leaf, especially in the angles between
the three main ribs of Clerodendron fragrans. Vent., and others, but not all species
of the genus ' ; the round spots, 2-3 of which are to be found on each side of the
mid-rib at the base of the under surface of the leaf of Prunus Laurocerasus ^ ; the more
or less depressed cup-like glands forming the end of the conical stalks or teeth on the
base of the petiole of the Passifloras^ (observed on P. atrocserulea, Hort.); the pairs of
round glandular spots, which protrude at the upper end of the petiole in species of
Stigmatophyllum, a genus of the Malpighiacese (S. cristatum, ciliatum) ; and the gland,
the structure of which varies according to species, which lies at the upper margin of the
basal part of the petiole or phyllode in the species of Acacia. The structure and form
of these glandular spots have been given above for Clerodendron and Prunus. In
Acacia marginata, R. Br., Calamifolia, Lindl. lophantha, they appear as convex, callus-
like prominences, with a depression or furrow on the apex: their epidermis has a
glandular structure above the depression or furrow, over the rest of the surface it
is firm and tough-walled. In many other species (A. longifolia, latifolia, melanoxylon,
subulata, longissima, obtusa, myrtifolia, striata *) the glandular spot lies at the base of a
narrow, deep, pocket-like depression with swollen margin. In Acacia pulchella® the
glandular hair lies on the end of a cylindrical stalk, which stands half-way between the
insertions of the two main pinnae. As already intimated, the cuticle in all these glands is
raised up from the membrane by the secretion : in the flat, glandular spots of the Acacias,
Clerodendron, and Laurocerasus, it is raised as a wide bladder, which often bursts in later
stages of development, and in that case is often not to be found in prepared sections.
The very tough cuticle of the stronger glandular spots of Clerodendron usually bursts
transversely over the whole surface, with a gaping slit, which is almost visible with the
naked eye. The bladder-like, glandular structure of leaf-teeth is to be found, e. g. in
Mercurialis annua, Prunus, Salix, and many others

Of the other numerous ‘ glands ’ which occur on leaves and leafy parts, and which were
in part named above, including the numerous glandular teeth of the lamina, the peculiarly
formed glandular teeth of the petiole and leaves of Viburnum opulus, V. Tinus of
Ricinus, and species of Cassia, and the numerous glands of the leaves of most
Malpighiaceae, See., the structure of the surface and the secretion are not yet
thoroughly Investigated : but all that is known of them coincides so closely with
examples which certainly belong to this category, that it is better, at least for the
present, to put them in connection with these.

Intramural glands have not hitherto been described with reference to their characteristic
peculiarity of structure, excepting in the above-quoted account of Hanstein. The flat,
top-shaped scales of the Rhododendrons, which belong to this category (Fig. 41), are
fitted into a depression of the dermal surface, and attached to short 4-5 seriate stalks.
Their free outer surface is roundish and flat. They consist, according to the individuals,
of 60-80 elongated cells, which form a layer, and diverge radially from the stalk. The
ends of all these are directly connected with the stalk, with one another, and with the
cuticle at the outer surface. 40-50 of the cells, in a ring-like series, border on the lateral
periphery of the scale, and are also in immediate connection with one another, and

' F. Fischer, Mem. Soc. des Naturalistes de Moscou, I. p. 246, according to Treviranus, Physiol.
II. p, 35 ; Caspar)’, 1. c. ; von Schlechtendal, Botan. Zeitg. 1844, p. 6.

Caspary, 1. c.

^ Compare Martinet, Lc. p. 191, figs. 238, 239.

* Unger, Flora, 1844, p. 703 ; Anat. und Physiol, p. 362.

® De Candolle, Prodromns, II. p. 455.

® Compare Reinke, /. c.

’ Unger, Caspary, /. c.

EPIDERMIS.

97

covered by the closely applied cuticle. Their outer ends are prolonged, so that they
form a radiating ring round the margin of the flat sui'face. The cells, about 25 in

Fig. 41.— Ä, b Rhododendron ferrugineum ; glandular scales of the under surface of the leaf, a surface view of a larger one ;
b transverse section of a smaller one (142). In the latter the cells are shaded ; the intervening spaces filled with secretion, not
shaded. — c Rhododendron hirsutum ; glandular scales from the under surface of the leaf, transverse section (225). The cellulose
membrane of the scale dotted, cell-contents and secretory space left white ; over the cellulose membrane runs the cuticle, which
is connected with that of the surrounding epidermis. In all cases the secretion has been removed by alcohol.

number, which form the central part of the scale, are much reduced between their two
ends, and the wide interstices thus formed between their lateral cellulose walls (which

turn deep blue with Schultze’s solution)
are filled with the secretory mixture —
resin and ethereal oil.

The almost globular head of the above-
named larger glandular hairs of the leaf
of Ledum consists of 9-10 cells which
diverge from the end of the stalk. The
polygonal outer ends of these are distri-
buted over the periphery of the sphere,
remaining in immediate lateral connec-
tion with one another, and adhering
closely to the cuticle. The cellulose
walls become suddenly reduced inwards
to narrow tubes, which meet at the stalk,
and leave between their sides a wide
space, which is also filled with a resinous
secretion.

The manner in which these cavities
filled with resin arise may be most clearly
observed in the imbedded intra-mural
glands of Psoralea (Fig. 42). At each
of the clear spots on both leaf-surfaces
of the above quoted species lies an al-
most spherical body, which protrudes
into the tissue of the leaf : its outer flat-
tened pole often lies in a slight depres-
sion or excrescence of the outer surface
of the epidermis. In the superficial
view of the epidermis it shows the poly-
gonal outer walls of a group of about
20-30 cells (the number varies according
to the individual), these outer walls
being much smaller than those of the
surrounding epidermal cells, but at least

Fig. 42.— Transverse section of the lamina of Psoralea hirta ; one-
layered epidermis, and the tissue beneath it ; the latter in b and c
distinguished from the epidermis by the shading ; a (375) an almost
mature gland after removal of the secretion byalcohol ; b (600) a very
young stage of a gland, secretion not yet present ; c (600) rather
older, development of secretion beginning.

H

98

CELLULAR TISSUE.

twice as tTiick as in these. The small polygonal outer walls belong to narrow sac-
like cells, of which the peripheral ones (14 to 24 in number) run to the inner pole,
curved like meridian lines. They are uninterruptedly connected laterally, thus forming
with one another the wall of a hollow sphere ; the 5-8 central ones are sinuous or
bowed, and run through the cavity of the hollow sphere : at the inner pole they are
applied with their widened ends to one another, and to the peripheral series, and
thus close the hollow sphere (Fig. 42, a). Between the sides of the central and the
inner surfaces of the peripheral cellulose walls lie wide interspaces filled with large
masses of resin, or innumerable resin drops suspended in transparent fluid (‘ Milchsaft,’
Hildebrand, 1. c.). The contents of the sac-like cells are, when old, of a similar
nature : at first they contain clear protoplasm, which is scarcely granular, and watery
cell-sap.

As distinctly indicated by the mature condition, these glands originate from a single
primary epidermal cell, which bulges inwards, and is repeatedly divided by walls
perpendicular to the surface. The segments elongate in the same direction : their
lateral cellulose walls are at first uninterruptedly connected (3, Fig. 42). During the
elongation resin appears at the limiting surface, at first as an homogeneous narrow and
short intramural layer, at the middle of the cells, forming transverse rings round the
central ones (c. Fig. 42) : gradually it increases in height and thickness, till it attains
the size of the large interstices above described. At first, and during the most active
growth of the resinous layers, the cells contain a very thin transparent protoplasmic
lining, and quite clear colourless cell-sap.

The secretion of the dermal glands of all categories is in most cases resin, or a mixture
of resin and ethereal oil: e.g. Betula, Humulus, Labiatae, Primula, &c. A complete
enumeration of the chemical relations of these bodies cannot be undertaken here. In
other cases it consists of bodies which swell and dissolve readily in water (vegetable
mucilage, gum), as for instance in the buds of the Polygonums (Hanstein, 1. r.) ; or of
mixtures of these and resin, as, according to Hanstein, in most leaf-buds, e.g. Cunonia,
Viola. More rarely, on parts which do not belong to the flower, it consists of mixtures of
gum and sugar: thus in Viburnum Tinus, Clerodendron fragrans, where the sugar may even
crystallise, Prunus Laurocerasus, Acacia, &c. (Von Schlechtendal, Caspary, Unger, 1. c.).
The resinous secretions may be rather solid, e.g. Aspidium ; but certainly in the majority
of cases they are soft and sticky : those which swell and dissolve easily in water are
normally always very watery ; most secretions are therefore naturally moist and sticky.
The character of the surface is not influenced when the moist secretion remains in
the glands, and when only its volatile constituents evaporate through the membrane.
This is the case in all intramural glands, and those with tough resistent cuticle, as in
Humulus and most Labiatae.

In the bladder-like glands with delicate cuticle the latter is either burst by internal causes,
e.g. by increasing aggregation of the secretion, as in the sugar glands of Clerodendron,
Acacia marginata, the glands of many leaf-buds ; or rupture of the cuticle results readily
from external lesion, and the surface thus becomes sticky through the escape of the
secretion. On hair-structures of mature parts the secretion soon ceases after rupture of
the cuticle, and the glandular cells dry up : fresh secretion may in that case proceed from
other later-developed glandular hairs. In the glandular spots, which lie over the ends of
the vascular bundles, the secretion appears in many cases, at all events, to continue long
after the rupture of the cuticle ; still w'e must in these cases distinguish more exactly
how much of that which is excreted is due to secretion itself, and how much to water
which filters from the vascular bundles. In unfolded foliage-buds, according to Hanstein,
the rupture of the cuticle is often followed by its renewal and the formation of new
secretory layers.

Like hair-structures generally, the glandular hairs are also in many cases transitory
organs, which are present in the bud, but disappear after the bud unfolds. The
glandular epidermis of many leaf-teeth also secretes while in the bud, and afterwards

EPIDERMIS.

99

dies off ^ Hanstein has termed these organs which cover the buds with a sticky secretion
‘ Beleimer,’ or Colleters, and their sticky product bud-glue, or Blasto-colla.

There is as yet only one case known to me in which such mucilage, covering buds,
has an origin other than that indicated. The bases of the young leaves of Osmunda ® are
covered with a rich amorphous mucilage. This originates from long septate hairs, with
large bead-like cells, each of which, in the stages of development observed, is completely
filled with a mass of mucilage. The origin of the latter remains to be investigated. On
treating with water, the mucilage swells and escapes as an amorphous hyaline mass from
the burst cellulose membrane.

Sect. 20. The capitate hairs with mealy dust — pili pidverulenti — should at pre-
sent, for the sake of consistency, be separated from the glandular hairs, but they must
be treated in immediate relation to them. To these hairs the under-surfaces of the
leaves of the so-called gold and silver ferns owe their dusty covering, which is white
(Gymnogramme tartarea, calomelanos and other species, Notholaena nivea, Cheilanthes
spec.) or golden yellow (Gymnogramme sulfurea, Martensii, &c., Pteris aurata), while
on the foliage of the mealy Primulas it appears light yellow (Pr. marginata) or usually
white (Pr. Auricula, farinosa, &c.).

The mealy covering of these plants is not excreted like the wax coverings, by
the whole epidermis, but exclusively by the globular apical cells of small hairs, which
are borne on a short unicellular (Gymnogramme) or two-celled (Primula) stalk (Fig.
43)®. It appears on the whole surface of the apical cell in the form of rods or small
needle-shaped crystals. In Gymnogramme
these radiate from the whole surface of the
apical cell ; their length may greatly ex-
ceed the diameter of the latter. In the
Primulas they are irregularly aggregated.

In not quite intact specimens, especially
of the Primulas, they are often shattered
to heaps of small fragments, and scattered
over the whole epidermis. The covering
consists of resinous bodies which are to
a great extent easily soluble in cold alcohol.

In Gymnogramme I found, after solution
in alcohol, a finely granular residue, which
dissolved in ether. Klotzsch ® names the
bodies in question Pseudo-stearoptene : they are, according to his statement,
which certainly requires revision, easily crystallised out from the alcoholic solution
(I obtained only aggregates of very small crystals) ; the crystals are rather hard,
heavier than water, fusible at 50°, sublimated without change when air is excluded,
slightly aromatic, soluble in warm water (.?!), alcohol, ether, acetic acid, and alkalies.

On the foliage of many plants closely allied to those in question — Filices, Primula
sinensis, &c. — instead of the dusty hairs, there are found typical resin-secreting,

' See Reinke, 1. c,

^ S. Milde, Monogr. Generis Osmimdae, Vindob. 1868.

^ Link, leones Selectae, Heft IV. Tab. HI. figs. 7-9. — Mettenius, Filices horti Lipsiensis, p. 42.
* Göppert, N. Acta Acad. Leopold. Carol. XVIII. Siippl. I. p. 206.

^ Monatsber. d. Berlin. Acad. 1851, Dec. Botan. Zeitg. 1852, p. 200.

Fig. 43.— Gymnogramme tartarea. (142) dusty hair from
the under surface of the leaf, floating on water; the round
apical cell covered by the erect radiating rods of resin. — B
(375) a similar hair after momentary action of cold alcohol ;
the rods are for the most part dissolved ; in their place remains
a finely granular layer, which is quickly dissolved in ether. In
the apical and stalk-cells are nuclei ; the larger granules in the
apical cell are chlorophyll grains.

H 2

lOO

CELLULAR TISSUE.

bladder-like glandular hairs. This fact, besides the other points of agreement, shows
how nearly the two structures are allied. In Gymnogramme I always found the crys-
talline covering seated only on the smooth cuticle of the apical cell. On young, as
yet unfolded, fresh leaves of Auriculas, the apical cell of the hair has often a typical
bladder-like glandular structure : so that the cuticle which surrounds the resinous
secretion is extremely thin, and the apical cell bears occasionally one large resin-
bladder, usually two or several small ones. On older leaves I did not observe these
bladders : those which occur on young leaves are replaced after long immersion in
water by the above-described crystals. All these phenomena, which require more
careful observation, point again to the fact that we have here to deal with a peculiar
form of glandular hair and glandular secretion.

Sect. 21. Darwin, in his ‘Insectivorous Plants,’ has drawn attention to organs
belonging to the epidermis, which in their structure and development are closely
allied to the hair structures and dermal glands above described, but are distinguished
by the fact that, at least in the closely-observed cases, when subjected to continued
chemical or mechanical stimulus, they secrete at their surface a fluid, which holds in
solution a free (organic) acid, and a ferment similar in its action to Pepsin. As the
result of the interaction of the two dissolved bodies, they are capable of dissolving
and digesting albuminoid substances, and the solution of these, as well as of Phos-
phates, salts of ammonia, &c., is absorbed by them or — a point which cannot be
considered as generally established — by the surrounding tissue. According to this
digestive function these organs may be termed Digestive glands. Besides the above
solutions there often occurs a rich excretion of mucilage which is sticky, and swells
with water, especially in species of Drosera, Drosophyllum, and Pinguicula. This
appears in the plants named even independently of stimulus, but in this case the acid
reaction and digestive effect are absent. In others the mucilage is not observed.

The organs in question, and the power conferred by them of digesting animal
substance and absorbing it as nourishment, are at present known in the case of the
peculiarly-formed leaves of Droseraceas, especially species of Drosera, Drosophyllum,
Dionaea, and species of Pinguicula. Other plants also absorb dissolved animal bodies
as nourishment by means of their leaves. This is certain in the case of Utricularia,
and very probable in that of Aldrovanda, Nepenthes, Sarracenia, &c. They then
show corresponding organs, which are probably to be regarded as digestive glands,
though they are not at present certainly understood.

The undoubted or presumptive digestive glands have (when they belong to the
epidermis) the position and cellular arrangement either of circumscribed dermal
glandular spots, or of hairs or scales.

They are either situated above the ends of vascular bundles (Drosera, and some-
times in Nepenthes) or have no direct relation to these (e. g. Pinguicula, Dionsea).

Of the allied hair structures, which rise above the epidermal surface, those deli-
cate, long-stalked, umbrella-like scales (p. 64) which are found in large numbers on
the leaf-surface of the Pinguiculas have already been mentioned. They secrete on
their upper surface. The digestive glands on the upper surface of the leaf of Dionsea^
and Aldrovanda are quite short, umbrella-like, stalked, round, multicellular scales,

[C. de Candolle, Ref. Bot. Jahresber. 1876, p. 383, Oct.]

EPIDERMIS.

lOI

without any specially remarkable arrangement of the cells. In the Utricularias the
digestive function may be carried on by the four-armed hairs already mentioned
(p. 62), which are seated in large numbers on the inner surface of the sacs which
catch animals. In the investigated species of Drosera, and in Drosophyllum, the
form and arrangement of the cells on the secreting leaf-teeth is exactly like those of
the circumscribed dermal-glandular spots, occurring especially on the teeth of the
foliage-leaves. The description of them for Drosera will be better given below
(Chap. VIII) in connection with that of the endings of vascular bundles.

The presumptive digestive glands of Nepenthes* so often investigated, but lately
very thoroughly treated of by Wunschmann, have the most peculiar arrangement.
They are situated in these plants on the middle and lower portion of the pitcher-
shaped part of the leaf, on the inner surface, in many species also on the inner surface
of the lid. They belong to and arise from the epidermis, and consist of a disc-
shaped basal portion, formed of one small cubical cell : this bears a rounded head,
composed of prismatic thin-walled cells, arranged in a perpendicular, radiate manner.
Their whole form is thus that of a globular wart. Each of these very numerous
warts, which can even be recognised with the naked eye, lies in the pitcher in a
pouch opening into it : the pouch is formed by extension of the row of epidermal
cells surrounding the upper margin of the wart in the form of a semicircular sharp-
edged band, which is drawn foiward as a cap over the wart. On the lid of the
pitcher this band is equally high all round. In the upper part of the pitcher the
glands are absent from the inner surface : the epidermis is here covered with granules
of wax in a simple layer, and is smooth, excepting that numerous small semilunar
hair-cells (concave on the under side) are inserted between the slightly undulating
epidermal cells. On Sarracenia, compare p. 69.

With all the similarity of structure of the digestive organs in question to that of
the above-described dermal glands, they differ in structure fundamentally from these
in this point, that their secretion — and also, as far as investigation extends, the mu-
cilage of Drosera and Pinguicula — does not appear in the wall, betw'een the cellulose
wall and cuticle, but on the free outer surface of the latter.

Other generally remarkable, anatomical peculiarities cannot at present be brought
forward, without far transgressing the boundaries here imposed, and entering deeply
into physiological details. Here therefore, after this short notice, we can only refer to
the recent literature on insectivorous plants, and a few older works on the anatomy
of the glands in question, some of which have been already cited.

Ch. Darwin, Insectivorous Plants, London, 1875. — J. D. Hooker, Address to the Dep.
of Zoology and Botany of the Brit. Association, Belfast, 1874. — F. Cohn, Ueber die
Blasen von Aldrovanda und Utricularia, Beit. z. Biologie, Heft HI. p. 71. — Treviranus,
Meyen, Oudemans, Wunschmann, l.c., Schacht, l.c., Utricularia, comp. p. 62. — £. Morren,
Note sur le Drosera binata. Bull. Acad. Belg. 1875. — Aldrovanda, Cohn, Flora, 1850. —
Caspary, Bot. Ztg. 1859, P- 1 i7j — Fraustadt, Anatomie d. Dionaea muscipula, in Cohn,
Beitr. z.Biol. d. Pfl. Bd. II. p. 27. — Fr. Darwin, The process of aggregation in the tentacles
of Drosera rotundifolia, Micr. Journ. vol. XVI. N.S. — Warming, in Videnskab. Meddel-
elser fra nat. Forening i Kjöbenhavn, 1872, p. 168, (French Resume, p. 18 ; Drosera).

^ Treviranus, Zeitschr. f. Physiologie, III. p. 73. — Meyen, Secretionsorgane. — Oudemans, De
Bekerplanten, Album cl. Natuur, Groningen, 1863 and 1864. — Wunschmann, Ueber d. Gattung
Nepenthes, Diss. Berlin, 1872.

102

CELLULAR TISSUE.

Sect. 22. Of generally distributed, incombustible constituents of the membrane^
compounds of Silicon, Calcium oxalate and carbonate are often contained in the
epidermis in remarkable quantity and form.

Presence of silica, or silicificaimt, is observed specially in epidermal layers, and is
particularly abundant in the cuticularised outer walls. Highly silicified epidermal
layers are characterised by hardness and firmness. Equisetum hiemale, Calamus
spec., Gramina, leaves of Ficus sycomorus, F. trachyphylla, Deutzia scabra, Celtis,
Ulmus, Davilla brasiliana, Parinarium senegalense, Magnolia grandiflora ; a definite
relation however does not exist between silicification and hardness: the hard epidermis
of the leaves in most Palms, Mahonia aquifolium, Drimys Winteri, Rhododendron,
Hakea spec., Phormium tenax, the phylloclades of Ruscus aculeatus, and in Cycas
revoluta, has no silica. (Mohl.)

Calcium oxalaie is observed in the form of granules, or obvious crystals, in the
epidermis, especially in the cuticular-layers in the leaves of Welwitscbia, of many
Cupressineae, and Taxineae, in species of Ephedra, the leaves of Dracaena reflexa,
arborea, Draco, umbraculifera, Sempervivum calcareum Jord., and species of Mesem-
bryanthemum. When it occurs in large quantity it often gives to the epidermis a
dull white colouring, as in the above-mentioned Sempervivum, Mesembryanthemum
lacerum, incurvum, stramineum, Lehmanni, vulpinum, &c., and the white spots of the
leaves of M. tigrinum.

Calcium carho7iate is contained in great quantity in the membrane of many hairs,
and especially in the peculiar peg-shaped wall-thickenings known under the name of
Cj sloliths, especially in the Urticaceae and Acanthacese.

Silicon-containing epidermal layers have, according to Von Mohl’s widely extended
investigations, been observed in species from 41 families of the most various large divi-
sions of the vegetable kingdom.

The silica is found chiefly in the outer layers of the external wall: but Von Mohl
notes ^ that he had not observed it to be restricted to the outer cuticular lamella alone.
In most cases, when the epidermis is smooth and flat, the silicification extends over the
whole outer wall and the outer part of the lateral wall of the epidermal cells: e.g. in
many grasses and Cyperaceae. More rarely, and only where the silicification is extreme,
the inner wall of the epidermal cells, and the sub-epidermal cells, which border laterally
on the respiratory cavity (Deutzia scabra), take part in it. The guard-cells of the
stomata are silicified all round or partially.

Partial silicification of the cell-wall occurs in other solitary cases : thus it is limited to
the protruding knobs in the median line of the epidermal cells in the stem of Scirpus
palustris and mucronatus. On the subsidiary cells of Equisetum hiemale the transverse
bands on the lower wall are only silicified in their inner part which borders on the slit,
not in their outer part (Fig. 24, p. 72, A, C). Further there belong to this category the
stalks of the cystoliths in the Urticaceae, the silicification of which was proved by Payen,
and cystolith-like outgrowth of the wall of the above-named Borragineae and Compositae
(Onosma, Cerinthe, Helianthus trachelifolius, comp. p. 106). On the stinging hairs of
Urtica dioica the upper brittle part of the wall is very strongly silicified, the lower part
but little (Mohl, l.c., 219).

Very often there are varieties in the silicification of cells and groups of cells of one
epidermal surface, due to the unequal extent of the silicification in closely neighbouring

^ Von Mohl, Botan. Zeitg. 1861, pp. 209, 305, -where also the literature on the subject is
thoroughly treated of; and Hofmeister, Die Lehre von der Pflanzenzelle, p. 242.

EPIDERMIS.

103

parts, or one part is strongly silicified, another not at all. In the epidermis of many
grasses, the upper of the short epidermal cells, which are arranged in pairs one above
another, are distinguished from the rest by specially strong silicification of their wall, the
others are more slightly silicified, and in many cases not at all (Internodes of Saccharum
officinarum). Often the hairs are centres of silicification. They alone may be silicified
(leaf of Campanula cervicaria, Ficus Joannis Boiss., Urtica excelsa, lusitanica, dioica) ; or
the process begins in them, and extends around the base of each hair centrifugally over
the epidermal surface, and spreads evenly or unevenly over it. In the latter case, even
in the mature organ, each hair is seated in the centre of a disc consisting of silicified
epidermal cells, which usually turns white after death. This is separated from other
similar discs by interspaces with weaker silicification {e.g. leaf of Humulus, Ulmus cam-
pestris, Tectona grandis and other Verbenaceae, Cucurbitaceae, Pulmonaria saccharata,
Cerinthe major, Silphium connatum, Helianthus grosseserratus, many Dilleniaceae,
Chrysobalaneae, &c.), or by interspaces w’ithout silicification (leaf of Cerinthe aspera,
minor, Onosma stellulatum, arenarium, Lithospermum officinale, Helianthus tuberosus,
trachelifolius, &c.) Often {e.g. Ulmus campestris) the cell which is the centre of silici-
fication does not grow out to a hair.

In many leaves, especially of Dicotyledonous plants {e.g. Humulus, Morus alba), the
epidermis of the upper side is much more strongly silicified than that of the lower : in
the latter the silicification appears often to be absent, while it is present in the former
{e.g. Helianthus trachelifolius, Heliopsis laevis, Obeliscaria columnaris). In all these
phenomena it is impossible to ignore a certain analogy with the cuticularisation of mem-
branes.

On the intramural deposition of Calcium oxalate, comp. H. Graf zu Solms-Laubach,
Bot. Ztg. 1871 ; Pfitzer, in Flora, 1873, p. 97.

The most remarkable points of deposition of Calciutn carbonate in the epidermis
are those thickenings of the walls termed by Weddell^ Cystoliths.

The Cystoliths of the Urticacese were discovered by Meyen in Ficus elastica: he
described them thoroughly in 1839^: later they, as well as those of allied plants,
were investigated by Payen^, Schacht^, and Weddell (/. c.). Schleiden® contributed
some observations on them, and brought forward a view of their morphological sig-
nificance, which, like some statements of Payen on their structure, have been corrected
by investigations recently conducted

On the still folded leaf of Ficus elastica (Fig. 44), surrounded by its stipular
sheath, the epidermis of the upper side consists, long before the unfolding, of a single
layer of elongated prismatic cells with their longer axis perpendicular to the surface of
the leaf {Ä). These are mostly of equal size and similar shape. They are together
covered by the cuticle, and are mostly provided, below this, with a massive cellulose
outer wall, which exceeds the lateral walls in thickness. Single cells of this layer now
thicken their outer walls four to six or more times as strongly as the rest. The latter
then divide by longitudinal and transverse w'alls, to form the four-layered epidermis.
The cells with strongly thickened outer walls remain undivided, their outer wall grows
only very little further in the direction of the surface of the leaf, so that it soon appears
as a rather stronger band of membrane between laterally adjoining and thickening cell-

1 Ann. Sei. Nat. 4 ser. II. p. 267. “ Midler’s Archiv, 1839, p. 257.

® Memoires sur le developpement des vegetaux. Mem. presentes de I’Acad. des Sciences,
tom. IX.

* Abhandl. der Senckenbergischen Gesellsch. I. p. 133. ® Gnmdziige, 3 Aufl. p. 341.

® [See also Penzig, Verbreitung d. Cystolithen, Bot. Contralbl. Bd. 8. 1881, p. 393.]

104

CELLULAR TISSUE.

walls. The other part of these cells however becomes expanded to form a great oval
bladder which intrudes deeply into the sub-epidermal parenchyma. As soon as the
processes of expansion and division in the neighbouring cells begin, there grows from
the middle of the thickened outer wall, perpendicularly into the interior, a peg-shaped
process (consisting of cellulose), the blunt end of which swells into a knob {B).
When the leaf is fully developed {B) the swelling has the form of an egg-shaped or
almost spherical body, which attains a size filling half or more than half of the cell.
This is the cystolith, which is thickly covered on its exterior with pointed and blunt
radially diverging conical warts, and is suspended in the cavity of the bladder-Kke
cell by an irregularly cylindrical stalk, which is continuous directly into the original
thickened outer wall. The whole body is impregnated with calcium carbonate, and

FIG. 44.— Ficus elastica ; leaf, transverse section, e—e in each case the thickness of the epidermis ; ^ (600) upper
side : .^1 (390) under side of the same very youngr leaf; in A, an already complete stoma, which remains superficial,
and a hair (transient) ; in ^ two cystolith-cells, recognisable by their thickened outer wall, epidermal cells as yet
undivided.— Ä (600) upper, B, (390) under side of a rather older leaf, epidermal cells undergoing division. In B, x is
a younger and jr, an older stage of a cystolith, already showing the peg-like excrescence of the wall.— C (390) older
leaf, under side ; division of the now three-layered epidermis is complete, stoma depressed, but the definitive size
and form of the parts is not yet attained. — E upper side of a mature leaf, four-layered epidermis, cystoiith-celi (375).

the stalk also with silica: it has a homogeneous glassy appearance, and in the pointed
warts there is often stratification and granulation. Acids dissolve the calcium salt,
bubbles of carbonic acid being produced. After solution the stalked body remains
behind with its original attachment ; the stalk is but little altered, the swelling re-
maining as a delicate cellulose skeleton, the outline having become more irregular :
the interior shows well-marked stratification and delicate radial striation, the layers
from the end of the stalk onwards being nearly concentric with the surface.

EPIDERMIS.

T05

The epidermis of the under side of the leaf of Ficus elastica has similar, but
smaller and more scattered cystoliths. The same structures have been proved to
exist in all other species of Ficus which have been investigated on this point, the
form and size varying according to the species. The cystolith-cell has in other
species a much broader outer wall at the surface of the epidermis than that in Ficus
elastica (e. g, F. australis, salicifolia), or its thick outer wall itself rises as a more or
less long hair-like apex above the latter (Ficus Carica, montana, ulmifolia).

Other Urticaceae have similar cystolith-cells, and cystoliths in the epidermis :
species of Parietaria, Boehmeria, Forskahlia tenacissima, Celtis, Morus, Broussonnetia,
Humulus, Cannabis, Conocephalus, Urtica (Payen). A form differing from the round
or oval is shown by the bodies in question in Pilea decora, densiflora? (Weddell),
Urtica macrophylla (Fig. 45). The cystolith is here spindle-shaped, straight or with
two curved legs (Pilea densiflora.'’), it lies in a cell of form similar to itself, of which
the greatest diameter is parallel to the epidermal surface, and it is attached to the
middle of the outer wall of the cell by a stalk, which arises from the middle of one
side of it. The structure of the spindle-shaped cystoliths is the same as that of the
round ones. In the Urticacese the cystoliths are absent in Ulmus and Dorstenia
(Payen).

Fig. 45. — Urtica macrophylla ; piece of the epidermis with cystolith-cells, from the upper side of the leaf ;
transverse section (225).

The cystoliths of the Acanthaceae, which were first found by Gottsche (in
Schacht, 1. c.), resemble in structure those of the Urticaceae : their form is seldom
round (Justicia carnea, Schacht), usually spindle-shaped, or like a transversely
halved spindle. Their attachment by a stalk is also often similar to that above de-
scribed (Justicia carnea, Beloperone oblongata, Schacht, /. c.) : but the half-spindle-
shaped ones are attached by a very thin short stalk, arising at a point of their trun-
cated end, to a lateral wall of the cell which conceals them. Schacht describes
cystoliths in the epidermis of the above Acanthaceae, further of Barleria alba, Ruellia
formosa, livida, Justicia paniculata, to which Eranthemum pulchellum, Goldfussia
anisophylla, and others should be added. He failed however in finding them in
Justicia purpurascens and Acanthus*- mollis.

On the occurrence of cystoliths in the sub- epidermal tissue in Urticaceae and
Acanthaceae, see sect. 32.

‘The knobs which surround the base of the hairs in the Borraginaceae, and many
Synantherae ^ are allied to the cystoliths of the nettle-like plants.’ The base of these
hairs, which is embedded in the epidermis, is surrounded by one or two concentric ring-
shaped rows of cells which are distinguished by their wall, on the whole surface facing
the hair, being covered with a well-marked stratified thickening, which protrudes inwards

Von Mohl, Botan. Zeitg. 1861, p. 229.

io5

CELLULAR TISSUE.

in the shape of knobs. This fills one-half, two-thirds, or often almost the whole of the
cell-cavity. This thickening of the membrane contains (besides a compound of Silicon)
a large quantity of calcium carbonate, partly as a homogeneous infiltrated mass, partly in
the form of very small granules, or often of splitting crystalline masses. Examples,
Cerinthe aspera, major, minor, Onosma stellulatum, arenarium, Echium vulgare, fruti-
cosum, Lithospermum officinale, Anchusa italica, Helianthus tuberosus, trachelifolius,
macrophyllus W., Obeliscaria columnaris, Heliopsis laevish In the multicellular hairs of
Helianthus similar thickenings often occur in the lowest cell, both laterally and on the
under surface of the upper wall. Whether the silica-containing rosettes of cells in
Ulmus, Dilleniaceae, and Chrysobalanese (Von Mohl, Lc.) which surround the hairs or
rudiments of hairs, and resemble the above, also contain calcium carbonate, is not
stated.

In the hairs of many Crucifer se, — Alyssum, Cheiranthus Cheiri, Gapsella, &c. — the
presence of calcium carbonate in very large quantity is shown by reagents. It does not
occur as single distinguishable particles, but is contained mainly (or wholly?) in the
outer layers of the membrane, especially in the wart-like thickenings which protrude
outwards (comp. Fig. 21, D, p. 59). These appear on the fresh hair mounted in water
as strongly refractive, blueish and bright, after solution of the lime-salt very pale and
transparent.

Also other strong hairs (Borraginese, Helianthus) appear to contain large quantities of
calcium carbonate in their lateral walls.

Sect. 23. Incrustations of Lime. Calcium carbonate is often found in finely
granular masses, deposited, as an incrustation, upon the outer surface of the cuticle.

(1) On the epidermis above the ends of the vascular bundles of many land
plants. On these spots lies a white granular scale of lime. This is the case in many
Fern leaves: Polypodium subauriculatum, meniscifolium, repen.s, aureum, sporado-
carpum, areolatum, crassifolium, morbillosum, &c., species of Nephrolepis, Aspidium
leucostictum, albopunctatum, pedatum, Lomaria attenuata and on the leaves of the
white incrusted species of Saxifraga ^ The above-named Ferns show at definite
points on the upper surface of the leaf shallow, in Lomaria attenuata deep, flask-
shaped depressions, in which, when the definitive development of the leaf is attained,
there appears the white scale of lime, which is not renewed after removal. Also in
the Saxifrages the lime scales are excreted in depressions, which lie on the upper
side of the leaf — in the species of Euaizonia on each of the marginal teeth, which
are covered with short blunt hairs — in S. csesia, 4-6, in pairs on the two margins,
and a single one at the end of the middle nerve : in S. retusa and oppositifolia 1-3-5
on the upper side. The depressions are filled up with the mass of lime : their epi-
dermis is distinguished from that of the rest of the leaf-surface by the small size and
thin walls of the cells rich in granular protoplasm, which in Lomaria attenuata grow
out as papillae. Stomata are absent in the depressions in the above Ferns (Mette-
nius), in the Saxifrages the water-stomata described p. 53 are always present.

(2) On the leaves and herbaceous stalks of Plumbagineae ^ (species of Plumbago,

1 Compare von Mohl, l.c. p. 227.

“ Treviranus, Verm. Schriften, IV. p. 66. — Mettenius, Filices horti Lipsiensis, pp. 8, 9.

® Unger, Einfluss, des Bodens, &c., p. 178. — The same, Beitr. z. Physiol, d. Pfl. VIII. — Sitz-
ungsbr. d. Wiener Acad. Bd. 43. p. 519.— Mettenius, l.c.

‘ Braconnot, Ann. Chim. et Phys. LXIII. p. 375. — Treviranus, Physiol. II. p. loi. — Mettenius,
1. c. p. 9.

EPIDERMIS.

107

Statice, Armeria) occur numerous lime-scales scattered over the surface without any
direct relation to the ends of the vascular bundles. Each of these appears on
the outer face of a small group of cells of peculiar form, and exactly similar groups
of cells occur in species in which no excretion of lime occurs, as Armeria vulgaris,
plantaginea, Statice scoparia, latifolia, purpurascens, alata (Mettenius). They consist
of eight cells, arising from one epidermal cell which appears in surface view rounded
and quadratic. This is divided by two walls, perpendicular to the surface and to
one another, into four : each of the latter again divides by a perpendicular wall into
two : one very small one forming the inner angle, and one being peripheral. The
cells of these groups are thin-walled, and contain dense finely granular protoplasm.
Their outer walls lie in many species at the surface : in others, especially thick-
skinned species, they form the base of hollow depressions, e. g. Statice alata,
purpurea, monopetala.

(3) In water-plants, especially submerged ones, the whole epidermal surface is
often equally covered with a thick layer of calcium carbonate. Reinsch^ found
the lime-covering on the upper side of the swimming leaves of Potamogeton natans
continuous over each stoma during the active vegetation of the leaf. In many land-
plants also, which form lime-scales, — Saxifrages, e.g. S. crustata, Statice spec. — the
whole epidermis is covered with a thin crust of lime.

The origin of the lime-coverings remains to be investigated. We may almost
imagine in the case quoted under (3) a precipitate having been formed by the re-
moval of carbonic acid from the lime-containing water, and explain the lime-scales
over the ends of the vascular bundles by the evaporation of drops of lime-con-
taining water, especially since the exit of such drops on the young leaf in the Ferns
and Saxifrages really takes place : and we may explain the incrustation which occurs
near the scales as arising from partial solution of the scales in water containing car-
bonic acid, and subsequent repetition of the evaporation. These are plausible
explanations, for which however the proof is wanting : for the excretion of lime in
the Plumbagineae it is not admissible.

Analyses of the lime-coverings showed in Potamogeton, besides calcium car-
bonate, traces of Silicic acid and oxide of iron (Reinsch), in Saxifraga crustata
(Unger l.c.) to 4'i46 parts calc. carb. 0-817 of carb. of magnesium.

The scales of the Ferns, Saxifrages, and Plumbagineae leave behind a colour-
less gelatinous residue when the lime is dissolved with nitric acid.

Incrustations of carbonates of alkalies are described in the case of the foliage of species
of Tamarix, Reaumaria, shore-plants. Notices on this: de Candolle, Physiol, p. 237;
Treviranus, Physiol. Vol. II, p. loi ; Unger, Anat. und Physiol, p. 369. Definite in-
vestigations do not exist.

1 Flora, 1858, p. 723. •

io8

CELLULAR TISSUE.

SECTION II.

CORK.

Sect. 24. Cork^ is formed in the mature plant as a tissue having the fundamental
physical properties of cuticularised epidermis, the place of which it supplies when
the latter is thrown off in the normal course of development (comp. Chap. XV), or
where the living parenchyma is laid bare by wounding, or where it is to be isolated
from the effects of injuries which have penetrated to the interior. Rarely, as in
many bud-scales, cork appears simultaneously as a strengthening of persistent
epidermis.

All investigated phanerogamic land-plants are capable of forming cork. In the
Cryptogams it is only found in isolated cases, namely on the surface of the Rhizome
of the Ophioglossese

Cork- formation always originates in the epidermis or in living parenchymatous
cells, and indeed in the latter without distinction, to whatever member or region they
may belong. Wound-surfaces, of whatever sort, are closed and healed by it ; and
diseased or dead parts are isolated from those still living. In the normal course of
development (disregarding the surfaces of separation of members thrown off, which
are here to be placed in the category of wound-surfaces) it appears especially on the
surface of such parts as the stems and roots of most Dicotyledons, Gymnosperms, and
a few Monocotyledons, which have a long-continued and abundant growth in thick-
ness, not continuously followed by the successive peripheral layers of tissue (Chap. XV);
it is less generally present on the surface of long-lived, firm, but not continuously
thickening stems and roots of Monocotyledons. Most of the latter retain their
epidermis, but it is replaced by cork in the stem and the roots of Pandanese,
epiphytic Aroidese (Philodendron, Monstera, Anthurium, Tornelia®), the roots and
rhizomes of Dracaenese, Strelitzia, Dioscorea, Zingiberacese. Finally, in rare cases,
formation of cork occurs normally on the surface of leaves S as on the scales of the
winter buds of many dicotyledonous and coniferous trees. Aesculus Hippocastanum®,
Ulmus montana, Populus, Carpinus, Corylus, Abies excelsa®.

The cork-formation begins in a single layer of cells parallel to the surface which
is to be shut off, by the occurrence of divisions also parallel to that surface. This
layer of cells, which relatively to the cork-formation may be called the initial
layer, is the epidermis itself in certain cases of normal development of Dicotyle-
dons, to be more accurately treated of below (Chap. XV) ; in all other cases it is a

* Sanio, in Pringsheim’s Jahrb. II. p. 39. F'urther literature in Chapter XV. [See also F. von
Höhnel, ‘Ueber den Kork und verkorkte Gewebe überhaupt,’ Kaiserl. Akad. d. Wiss. in Wien.
Nov. 1877.]

“ Russow, Vergl. Unters, p. I2i. ® Von Tieghem, Str. d. Aroidees, l.c.

^ [Compare Bachmann, über Korkwucherungen auf Blättern, Pringsheim’s Jahrb. XII. p. 191.]

® Hanstein, Botan. Zeitg. 1868, p. 721.

® Areschoug, Om den inre byggnaden i de trädartade växternes Knoppfjäll. Lunds Univ.
Aisskrift, torn. VII (1870).

CORK.

109

layer of parenchyma lying immediately below this or deeper. In the healing of wounds
it is as a rule that layer of parenchyma which lies immediately within that injured by
the wound : still there occur exceptions to this rule, as a more deeply-seated layer
may become the initial layer of the cork. The forms respectively of the initial cells,
and of the layer of cork which results from their division, depend therefore upon that
of the surface to be shut off. In wounds it may have all possible shapes; in normal
cork formation it resembles the normal surface of the member.

By the divisions parallel to the surface the single layer of initial cells is converted
into a multi-seriate meristematic zone. Of the layers of this zone the outer, i.e. those
opposite the normal surface, or the wound surface, assume at once the properties of
cork-cells and accordingly lose the power of division. But a single layer of cells
bordering the cork-cells internally retains as a rule the properties of meristem, and
therewith the power of continuous division : this is the cork-forming or phellogeneiic
meristem, or the phellogenetic layer.

On parts still increasing in girth, the layer of phellogen follows this growth, and
by divisions perpendicular to the surface can increase the number of its cells, and of
the layers of cork produced from them.

From what has been said it follows that the cells of a mass of cork are arranged
in rows perpendicular to the surface, each of which corresponds to one initial cell.
As the cork-covered surface increases in circumference each series may successively
be doubled. The serial arrangement of the elements of the cork perpendicular to
the surface is always very regularly preserved. The walls parallel to the surface
usually correspond pretty closely in neighbouring series one to another, so that
besides the arrangement in perpendicular series there is also a no less regular
arrangement in layers parallel to the surface.

The succession of the divisions parallel to the surface has been carefully studied
by Sanio in the normal cork formation in the cortex of ligneous plants, especially
Dicotyledons, and will for these cases be more minutely described in Chap. XV In
other cases there exist no detailed investigations of the succession of divisions. But
one will hardly be wrong in assuming as the rule for the great majority of these
cases the simplest of the types of succession distinguished by Sanio, that termed by
him centripetal. In this the initial cell divides in the direction of the surface into two
nearly equal daughter cells, of which the outer becomes directly a cork-cell, the inner
a meristematic cell. In all the successive divisions which follow this, the same
process is repeated, the outer cell always becomes directly a cork-cell, the inner
remains meristematic. For the other types of succession also, which will be described
in Chap. XV, the same general result holds, that at least to every mass of cork which
grows to any considerable extent, new cork-cells are added from the meristem, which
is to be found on its inner surface.

The average number of the strata of cells produced in one cork-layer is in the
majority of cases small, the layer thus appears as a thin skin varying in thickness
according to the special case from two to twenty cells. When this persists for a
long time it retains nearly an equal thickness, since the outer layers die off and peel
away. As this proceeds they are renewed from behind by the zone of meristem.

Compare also there the figures relating to cork formation.

1 IO

CELLULAR TISSUE.

Thicker masses of cork, attaining a thickness of many centimetres, are formed on
the cortex of Testudinaria elephantipes, and especially of the cork trees, which
derive their names from it. These will be treated of in connection with the bark
(Chap. XV).

The cork-cells remain uninterruptedly connected with one another. Only in the
Melastomeae (Chap. XV), which form the first layer of cork on the margin of the
bast, have intercellular spaces between the perpendicular edges of the cork-cells been
observed. The form of the single cell is approximately that of a parallelopiped,
usually with a five^ to six-sided base, parallel to the surface covered by the layer.
The height of the parallelopiped is usually smaller than the diameter of the bases,
and the cells are thus more or less flattened, in extreme cases, to quite flat lamellae,
as on the stem of Fagus, Betula, Tilia, species of Prunus, Boswellia papyrifera, &c.
In other cases the radial and basal diameters are almost equal, or even the former
larger than the latter, as especially in the soft cork of Quercus suber, Acer campestre,
Ulmus, Aristolochia, &c., also in thin layers of cork, e. g. Philadelphus.

The basal diameters are in most cases about equal : in many forms, e. g. old
stems of Betula, Prunus Cerasus, on the other hand, the cells are elongated consider-
ably in a transverse direction.

The cork-cells of the above Melastomese form an exception to these rules, since
they have the shape of elongated four-sided prisms, the sides of which are parallel to
the longitudinal axis of the stem.

The single wall-surfaces remain flat and straight, or show archings and undula-
tions. The latter holds especially for the lateral or radial faces of most of the less
flattened cork-cells : indeed these are usually undulated in the radial plane : rarely
(Pinus sylvestris, Larix ^) in the tangential plane, so that the cell appears in surface
view to have a star-like outline.

The structure and history of growth of the cork-cells are still but imperfectly
known. The following facts, founded mainly on Sanio’s investigations, may at
present be stated on this point.

As regards the structure of the walls, one can distinguish firstly thin-walled
cells with apparently almost homogeneous delicate walls, and others with thickened
walls. Examples of the first are supplied especially by the iso-diametric or radially
elongated cells of the soft masses of cork of the surface of the stem of Quercus
suber, Acer campestre, Aristolochia, and the wide-celled layers of the bark of the
Birch. The flat forms have usually, but not always (Nerium), thickened walls : and
in that case the thickness of the walls is almost equal all round (e. g. Fagus,
Boswellia papyrifera), or the outer (e. g. Salix, Zanthoxylon fraxineum) or the inner
wall (e.g. Mespilus germanica. Viburnum opulus) is specially thickened ; the thicken-
ing mass is either uniform or pitted. Fibrous thickenings are known in the uniseriate
layers of thin-walled cells, which alternate with strongly thickened multiseriate layers,
in the tough corky skin of Boswellia papyrifera The delicate membrane here shows
narrow thickening bands, which protrude inwards, and branch here and there at an
acute angle. Further, Sanio found in the cork-cells of the branches of Melaleuca

* Schacht, Lehrbuch, II. p. 572.

^ Mohl, Botan. Zeitg. 1861, p. 229.

CORK.

Ill

styphelioides a tangential annular thickening, which is uneven arid wavy, and runs
round the middle of the wall.

Corresponding to the usual conditions of structure of membranes, the thicken-
ing mass is in the cases of stronger thickening internally superposed upon a delicate
homogeneous limiting layer (‘ primary membrane ’).

The membranes, which in the young meristematic stage are cellulose walls,
appear in the mature cork-cell always sicher ised to a varying extent, i. e. com-
posed of cork substance. Of the properties of this body we know that the su-
berised membrane shows, in its behaviour under reagents, and its coarser physical
properties, specially its slight permeability for water, completely similar phenomena
to the cuticle and the cuticularised membranes. Its chemical composition, and the
statement that it contains nitrogen, which was found on the analysis of large masses
of cork, are doubtful k When examined microscopically the high refractive power of
the suberised membrane is striking ; its outlines appear dark and sharply marked in
the illuminated field, from the time when the suberisation begins : thicker membranes
(e. g. of the stems of the Birch or Beech) appear brightly luminous ; in thinner
ones one often observes as the microscope is focussed varying phenomena of
colour k

According to the stage of the suberisation we may distinguish totally or partially
suberised membranes, or layers of them. The former behave under reagents like
cuticle (p. 74), cellulose cannot be proved present in them; the latter still contain
cellulose even when old.

Often the wall is suberised all round and throughout its whole thickness: periderm
of Fagus, Salix, and many thin-walled cork-cells on the surface of roots and tubers.
On the other hand, the totally suberised mass often forms an outer lamella of the wall
going all round, and it is lined internally by a layer of a different material,— partially
or often perhaps not at all suberised, — which, after treatment with solution of potash,
shows a clear cellulose coloration with iodine and sulphuric acid, or Schultze’s solution.
This lining is very slightly developed, only as an extremely delicate skin in the
thin-walled cork-cells of Quercus suber and Betula. It is in this case only visible
after not too strong warming with potash : continued boiling with this reagent
destroys or alters it again : in Q. suber its destruction is accompanied by peculiar
phenomena, which still remain to be carefully investigated. In many other cork-
cells the cellulose-containing layer is to be seen, immediately within that which is
totally suberised, as a less refractive mass applied to this internally, forming
sometimes a thin layer equally thick all round, e. g. branches of Nerium, root of
Rheum rhaponticum ; sometimes a stronger thickening mass, equally thick all round
(flat cork-cells of Boswellia papyrifera), or it is thicker on one side (Zanthoxylon
fraxineum, Populus fastigiata, Platanus occidentalis). Without being acted upon by
potash, it does not turn blue : in Platanus it turns blue after warming for a short
time with this reagent ; in the other cases named even after the reagent has acted
for a few minutes without the temperature being raised. Most of the cork-cells

* On the chemical relations of cork, its products of decomposition, &c., compare the epitomes
and citations of the literature in Hofmeister, Die Lehre von der Pflanzenzelle, p. 252 ; Wiesner,
Rohstoffe, p. 479; Gmelin-Kraut, Handb. d. Chemie, VII. i. p. 593; Husemann, Pflanzenstoffe,
p. 1016. “ Compare Sanio, l.c. p. 57.

CELLULAR TISSUE.

112

described by Sanio, which have an inner layer of the thickened wall of low re-
fractive power, may be assigned to this category.

The presence within the totally suberised layer, which shows no cellulose reaction, of
one showing cellulose-blue after treatment with potash, may explain Mohl’s statement
according to which the cork-cells consist of cork substance and cellulose, and the
presence of the latter may be proved by reagents after treatment with potash. The
statement is correct for the cases just mentioned (I have not investigated all those
enumerated by Mohl), but the blue coloration does not affect the whole suberised mem-
brane, as Mohl asserts, but only the above described layer within that totally suberised.
Also after treatment with Schultze’s mixture no cellulose reaction appears in the latter.
Moderate treatment with the mixture causes decolorisation and separation of the totally
suberised membranes from one another. Stronger heating with it gradually changes the
latter into a greasy disorganised mass^. Still stronger treatment with it dissolves it
entirely.

Between the totally suberised outer layers of the wall of contiguous cork-cells there
always lies a very thin limiting layer, the material of which differs in some way from
them. Sanio describes this definitely for Ulmus effusa and Sorbus aucuparia, and
figures the limiting lamella, from which in this case the totally suberised layers often
separate in the section-cutting. It is distinguished from the latter by lower refractive
index, but shows in other respects the same reactions. The fact that cork-membranes
are separated from one another without injury when warmed with Schultze’s mixture
(Quercus suber), or even with potash (Boswellia papyrifera), is further evidence of the
presence of such a delicate and distinct limiting lamella. On the question raised by
Sanio of the possible growth of the membrane by apposition, these facts have at present
no decisive bearing.

With the exception of a few points of difference, what has been above stated
coincides with the facts recently discovered by Haberlandt He found in the cork
of the cork oak, potato, elder, and maple a separation of the cells from one another
after treatment with Schultze's mixture or chromic acid, and appearance of the
cellulose reaction after treatment with solution of potash ; the latter therefore,
according to him, dissolves the cork substance combined with the cellulose, and
the former reagents the ‘ Intercellular substance,’ or limiting lamella, which he
identifies with ‘ ligneous substance.’ Whether the relations are so simple, needs
further investigation, the more after what has been stated, since Haberlandt does
not sharply distinguish the distinct cellulose-containing layer, which is present in
the cell while still part of a continuous tissue.

As Mohl has shown, the fibrous thickened cells in Boswellia papyrifera are
the only case as yet known of silicified cork membranes.

The colour of the cork membranes is independent of the stage and extent of
the suberisation. The totally suberised membranes of old birch bark, of Salix
viminalis, aurita, caprea, are colourless, those of Q. suber, &c. are bright brownish
yellow, the internally thickened walls of Platanus greenish yellow : those of Salix
alba, purpurea, and fragilis yellow ; in general the colour of the membranes them-
selves is always very slight, and the bright brown colour of very many masses of
cork is chiefly due to the cell-contents. The silicified walls of Boswellia, as far
as it is possible to determine, are quite colourless.

1 Botan. Zeitg. 1847, p. 503. “ Compare Schacht, Lehrb, I. p. 14.

2 lieber Nachweisung v. Cellulose im Korkgewebe. Oesterr. bot. Zeitschr. 1874, No. 8.

CORK.

II3

The suberisatioii of the walls begins, in the cases investigated, directly after
the separation of the cork-cells by the divisions of the meristematic layer, and before
the cork-cell has attained, its definitive size and thickness of wall. In the above-
mentioned Melastomacese, according to Vöchting, the wall even of meristematic cells
still capable of division is suberised, provided the first divisions have once taken
place. Where thickening masses slightly or not at all suberised appear within a
totally suberised outer lamella, they appear, according to Sanio, later than the
suberisation in the latter.

In accordance with its continued active growth, the young cork-cell is filled
with protoplasm (nucleus) and cell-sap even after it has become differentiated
by the occurrence of suberisation. Thus a young cork-layer, even when very
completely developed, may remain transparent, and a branch covered by it (e. g.
Tilia) may long appear green to the naked eye, by reason of the chlorophyll of
the cortical parenchyma showing through it. Many cork-cells may remain for a
long time in this condition of independent growth, e. g., those of Sambueus
nigra, which last through the winter, and even form chlorophyll. Finally, and
at most after a year, there appear important internal changes. In one case — ■
especially in the thin-walled forms with large cavities — the contents dry up to in-
significant residues, which often (Betula) attach themselves as granules to the wall :
the space enclosed by the membranes is filled with air. In the other case the cavity
is occupied by a dense, almost homogeneous, more or less darkly brown-coloured
mass : I leave it as undecided whether this completely fills the space, as Sanio states,
or whether air bubbles occur also. This is usually the case in flat, plate-like cork-
cells, like those of the bark of Fagus, Castanea, Tilia, Pirus, &c.

When the air makes its appearance, the death of the cell has begun. The thin-
walled cells and masses of cork, which are filled with air, are incapable of other
than purely passive changes, and sink into gradual decay, e. g. Quercus suber,
Ulmus, Betula, &c. Also in the case of the flat cells filled with brown contents
it is probable that this mass consists of dead protoplasm and contents, and that the
death of the cell is indicated by its appearance. It is at all events beyond doubt
that in the stems, roots, &c. which grow in thickness, the cork-cells in question are
finally torn and decay. On the other hand however these flat cork-cells, as in the
above-cited trees, may be seen to increase for a long period in size in the direction
of the periphery ; there is a decrease it is true of the radial diameter, but not any
great change of structure, or of the thickness of the walls. Destruction begins only
at a later period, and with it the disappearance of the brown contents. Accordingly
the question may be raised, and recommended for further investigation, whether the
increase in size of these cells depends upon a purely passive extension, or is
connected with a real growth, an increase in mass, at least of the walls.

In the larger masses of cork of many plants single cells, genetically equivalent
to the cork-cells, assume the properties of short sclerenchymatous elements
(compare Sect. 29), resembling in all important points the so-called stone-cells.
The cork oak especially supplies examples of this, where such elements occur
singly in all possible places, but especially near the lenticels (Chap. XV), and in the
worse sorts of cork are known only too well by the dealers. Massive irregularly
concentric zones of such stony, brown elements, alternating with numerous thin-

I

CELLULAR TISSUE.

II4

walled layers, characterise the corky masses of old tuberous stocks of Tamus
elephantipes'.

As has often been already indicated in what has been said above, the
successive layers of one corky mass are either formed throughout of almost similar
cells, or concentric zones of unlike properties alternate with one another.

The first case includes most cork-layers consisting as a rule of more or less
flattened cells, which as thin skins cover wound-surfaces, roots, rhizomes, and the
cortex of stems. The second case includes in part equally thin layers, e. g. those
on the branches of Philadelphus, where one layer of cells greatly elongated radially
always alternates with one to two layers of flat cells ; on the other hand more
especially thicker masses of cork, such as those of Tamus elephantipes, and the
stems of dicotyledonous trees. The great flaps of cork on the cortex of Boswellia
papyrifera consist, as already stated, of multiseriate layers of flat cells, which
alternate with uniseriate thin-walled cells, with a fine fibrous thickening and
silicified walls. A similar alternation of multiseriate layers of flat strongly-thickened
cells and wide thin-walled ones occurs in the white corky covering of young
birch stems, in the corky masses on the stems of Quercus Suber, Acer campestre,
Liquidambar, &c. According to Hartig ® and Sanio ® each narrow-celled layer in
the Birch corresponds to the inner limit of one annual increment of growth, as
in the wood, only inverted (comp. Chap. XV) b Also in Quercus Suber® the number
of the concentric zones corresponds with the specified number of years through
which the production of cork on the tree had lasted. ' Whether such relations
between stratification and annual increment are more generally distributed remains
still to be investigated.

It is manifest that the firmness or toughness of a layer of cork must vary according to
the form and thickening of the walls of the cells, even if one assume that the physical
properties of the corky substance are universally similar. As a fact, one finds the flat-
celled, thick-walled layers firm and tough, resisting energetically both the increase of the
enclosed parts and the causes of injury acting from without: the wide-celled thick-
walled forms are soft, more easily burst by the increase of the enclosed parts, and more
easily injured from without. Alternating wide- and flat-celled masses, as especially those
of the Birch and of Boswellia papyrifera, peel off, when old, by splitting of the delicate
wide-celled layers from one another.

According to Mohl’s system it is usual to distinguish the wide-celled softer form as
cork, in the narrower sense, from the tough-walled masses of cork which are termed
Periderm. Since this distinction can never be sharply drawn, it may here be entirely
given up, and the above-described sort of tissue be termed Cork (Suber), while the term
Periderm may be applied to all phellogenetic cortical products (to be more exactly treated
in Chap. XV) of which the cork is a part.

Compare Mohl, Verm. Sehr. p. 190. In the old specimens investigated the hard layers are
much stronger according to Mohl’s statements than in the younger ones.

“ Forstl. Culturpfl. p. 306. ^ l.c. p. 83.

^ Von Merklin, Mel. Biolog. de I’Acad. S. Petersburg, opposes Hartig’s statements.

® C. de Candolle, in Memoires de la Soc. de Physique de Geneve, XVI. p. \ (1861).

® Verm. Sehr. p. 212.

PARENCHI'MA.

^15

SECTION III.

PARENCHYMA.

Sect. 25. The term Parenchyma is here applied to all internal cellular tissue, i. e.
that which is found within the epidermis or cork-layer. Though as a fact it is in the
main identical with Sachs’ ground-tissue (p. 6), in conception it is not so.

It has already been stated (p. 5) that in this limitation and classification of tissues
the distinction is drawn between those elements which retain their cell-nature, and such
as have lost that character. Attention was also drawn to the difficulty in the way of
a generally uniform distinction, partly from the incompleteness of our present know-
ledge, partly from the undoubted occurrence of real intermediate forms between cellu-
lar tissue and many distinct tissues, especially Sclerenchyma. These difficulties did
not come prominently forward in Sections i and 2. Here, in the internal tissues,
they appear frequently, and it is everywhere to be repeatedly pointed out that in the
distinctions about to be drawn, definite types must be indicated, which recur univer-
sally, but are never sharply distinguished from one another. As regards the distinction
of cells from other tissue-elements resulting from the metamorphosis of cells, it may
here be again called to mind that the former are distinguished from the latter by the
permanent protoplasmic body, in which the nucleus also (always?) remains, or
appears temporarily. With these parts, which are directly observable anatomically,
the cells retain the faculty of active growth and of division : it is true that this
faculty is often enough not manifested, but, in the processes of secondary thickening
(comp. Chap. XV), and especially in the phenomena of formation of cork (Sect. 24)
brought about by wounding, it may be so generally observed, that it may serve as
a very useful character. The chlorophyll-containing parenchyma of a foliage-leaf,
for instance, after complete unfolding shows as a rule no further division : the
smallest wound immediately induces it. In very thick-walled, sclerotic cells, the direct
anatomical determination of protoplasm and a nucleus is difficult, and as a fact is
often impossible. Nor is that of the power of division more practicable. In its stead
another phenomenon is to be taken into account, namely the periodic appearance
and disappearance of starch-grains in many elements, which judging from the nature
of their walls may be doubtful. Putting out of account the sieve-tubes (Chap. V),
and certain laticiferous tubes (Chap. VI), in which, at all events, peculiar conditions,
which need not here be touched upon, are the rule, the formation of starch in all
well-known cases is directly connected with an active protoplasmic body. In
doubtful cases therefore it is to be regarded as a character which indicates the
presence of such a body, so long as it is not proved that it can also occur in spaces
without protoplasm, and surrounded by cell-membranes. Abundant starchy con-
tents, and especially periodical changes in their amount, must therefore for the
present be regarded as a criterion of the cell-quality. In Chap. XIV we shall again
return to this subject.

Respecting the structure of the cells of the parenchyma, nothing general need
be brought forward at present, which would not be included in the doctrine of
the structure of the mature vegetable cell, and this we assume to be already known.

CELLULAR TISSUE.

ii6

As was stated in Sect, i, their /orm is extremely various, and we may here
distinguish as the chief types the iso-diametric, or s^or/ forms, and the elongated —
fibrous cells, fibrous-parenchyma (‘ Prosenchyma ’). The further distinction of forms,
to which during a certain period much energy was devoted \ has at the present
day hardly even an historical interest. However certain definite forms, which are
characteristic for definite single cases, must be mentioned.

As is the case in cellular tissue generally, the special structure may be taken
into consideration, and the distinction rnay be drawn between thin-walled and ihick-
ivalled parenchyma, according to the relative development of the membrane on the
one hand, and of the protoplasm and cell-contents on the other ; but this holds only
in extreme cases. In the distinction of subdivisions, the manner of connection of
the cells one with another is taken into consideration as one of the characteristic
special relations of structure.

Thin-ivalled parenchymatous cells are in most plants the organs of the process of
assimilation, and the storehouses of its first products ; besides having a relatively
thin membrane they are therefore usually distinguished by their contents ; viz.
assimilating chlorophyll, and the most widely-spread direct product of assimilation,
starch. According to the preponderance of one or the other of these parts, we may
speak shortly of chlorophyll parenchyma, starch parenchyma, in many other cases of oil-
containing parenchyma, &c. The parts of plants which contain chlorophyll, and in
which reserve products are laid up, e. g. especially the leaf, cortex of stems, and
Rhizomes, are the places where these cells occur in large masses.

In contrast to those characterised by the parts of the protoplasm and contents
appearing as above described, there are other thin-walled parenchymatous cells, in
which, within a protoplasmic sac, which is usually very delicate and slightly de-
veloped, all the solid constituents of definite form diminish till they disappear entirely
before the cell sap ; this sap fills almost the whole of the cell, and is watery,
or contains very thin mucilage. This may accordingly be termed sap-parenchyma.
This is wide spread, and as ‘ aqueous tissue ’ has recently been thoroughly described
by Pfitzer ^ in many thick long-lived foliage-leaves, in which it is situated beneath
the epidermis (hypoderma), forming as it were layers strengthening the latter, as
in the Pleurothallideae, Bromeliaceae, Ilex, Nerium, &c. ; or it appears as a middle
layer of the leaf, and is surrounded by chlorophyll-parenchyma, as in many succulent
plants, e. g. species of Aloe and Mesembryanthemum, and in the leathery leaves of
species of Callistemon, Hakea, &c., which will be more fully described in Chap. IX.
It occurs in specially large masses in parts without chlorophyll which are rich in
inulin or sugar, such as tubers and roots of Compositae, Campanulaceae, Beta, &c.
The cells in question are characterised by their contents, which are almost perfectly
transparent and fluid, being sometimes watery, sometimes (species of Aloe) muci-
laginous. Their chemical constituents are exactly known only in single cases, as
in the above Compositse and Beta, they cannot therefore at present be used in
distinguishing them generally.

^ Hayne, in Flora, 1827, II. p. 601. — Meyeii, Phytotomie, p. 63. — C. Morren, Bull. Acad.
Bruxelles, tom. V. No. 3. — Compare Mohl, Veget. Zelle, p. 15.

Pringsheim’s Jahrb. VIII. p. 16.

PARENCHYMA.

II7

The forms of thin-walled parenchymatous cells are in the main nearly iso-dia-
metric ; but there often occur also elongated-prismatic, spindle-shaped cells, and the
like, examples of which, e. g. in the case of the vascular bundles, will be described
later ; to this category belong also those chlorophyll-containing cells arranged in
many leaves perpendicular to the surface, forming the pallisade parenchyma, to be
described in Chapter IX.

As above intimated, very great variety of shape is found among the iso -diametric
forms. It is only in definite single cases, e. g. in hypodermal sap-parenchyma, that
the cells are of such form that all of them are bounded by flat surfaces and sharp edges,
and therefore are in uninterrupted connection with one another. As a rule the surface
of the parenchymatous cells is more or less rounded, or bears irregular protuberances,
or the protuberances themselves are drawn out into long arms : in this case they are
mutually connected only by definite parts of their surface, which vary in size accord-
ing to the special form. Between them intercellular spaces are left free. Masses of
parenchyma in which the latter (which are then usually filled with air) are developed
to a great extent are distinguished as lacunar parenchyma, or, comparing it with a
bath-sponge, spongy-parenchy7na. Compare Chapters VII and IX.

The walls of the cells of this category are as a rule cellulose membranes, with
ordinary simple pitting. The latter, following the general rule, usually occurs only
on the parts of the surface in contact with that of other cells : in cases then where
the cells show a decided partial rounding off, and only touch one another with
narrowly limited parts of their surface, or only with the ends of protuberances, the
pits lie on these spots, and not on the rest of the wall. As regards the surfaces of
contact, the same may also occur with dissimilar tissue-elements. When similar cells
touch one another by the ends of narrow protuberances,
there is often only a single pit on each protuberance ;
larger circumscribed surfaces of contact appear as pitted
fields on the otherwise smooth wall (Fig. 46). This phe-
nomenon, which was known long ago^, and which occurs
especially often in round-celled chlorophyll-parenchyma
of succulent plants, resembles that of the sieve-plates of
the sieve-tubes (Chap. V) ; but it is incorrect to place it
side by side with this^ since the characteristic structure
of sieve-plates is wanting in the parenchymatous cells,
though the pitted fields also in the parenchyma of the leaf
of Cycads, specially of Encephalartos, are distinguished
from the rest of the wall by brown coloration in Schultze’s
solution, and deep red coloration in solution of Anilin^.

Fibrous partial thickenings of the walls are known
here and there, e. g. in the form of reticulate or spiral
fibres, in the watery hypodermal parenchyma of the leaves
of the Pleurothallideae, and in many roots of orchids ; as reticulate fibres in the

Fig. 46. — A parenchymatous cell
from the cotyledon of Phaseolus mul-
tiflorus, isolated by maceration ; i i
the parts of the wall where it bor-
ders on intercellular spaces, tt the
pitted parts of the surface which bor-
der on neighbouring cells ; the thin-
nest points of the pits are shaded dark
(550). From Sachs’ Textbook.

* See c.g. Schleiden, Grundz. 3 Aufl. I. p. 245.

Areschoug, Botan. Zeitg. 1870, p. 305 ; and Acta Univ. Lund. tom. IV. — Borczow, in Pring-
sheim’s Jahrb. tom. VII. ^ Kraus, C>'cadeenfiedern, l.c.

CELLULAR TISSUE.

ii8

middle layer of the leaf of Sanseviera guineensis ; as longitudinal fibres in the
chlorophyll-parenchyma of the leaf of Cycas They appear in an exquisite form in
the parenchyma of the primary transient cortex of the root of most Coniferae (with
the exception of all the Abietinese); this tissue may be best placed in this category'^.
The cells of the concentric layers of parenchyma, which lie outside the endodermal
sheath (§ 27), are in many forms all finely reticulated (Phyllocladus, Podocarpus sp.),
or thickened with coarse nets and longitudinal fibres (Cupressus spec., Sequoja
sempervirens) ; in Torreya nucifera this thickening is limited to the 2-3 outermost
layers, and the innermost layer bordering on the endodermis. In most of the in-
vestigated forms, as Taxus, Biota, Thuja, only the latter layer has a fibrous thicken-
ing, and, as also in Torreya and Cupressus, each of its radial walls has in its middle
one straight, thick, stratified, half-cylindrical, longitudinal fibre, which is continuous
over the transverse walls into that of the opposite radial wall, and in all cases fits
exactly on to a similar thickening of the neighbouring cell. In Thuja occidentalis
this fibre contains resin according to Reinke. The layer of cells thickened in this
manner appears as a closed sheath, with the exception of Frenela rhomboidea, where,
according to Strasburger, it shows a break opposite both ends of the row of vessels
(Chapter VIII).

As a special case, to a certain extent worthy of mention, the tabular-polyhedral
chlorophyll-cells peculiar to the leaves of species of Cedrus and Pinus® and many
Gramina^ may be further cited: these have narrowly infolded bands of wall, and
from them broad ridge-like thickenings of the wall protrude inwards. Compare
above, pp. 35, 78; figs, ii, 27.

Luerssen ^ has recently proved that partial thickenings of the walls protrudmg
on the outer surface are a characteristic phenomenon for the parenchyma of many
ferns. They occur in the chlorophyll-parenchyma of the leaf of the investigated
IMarattiaceag, and in the parenchyma of the petiole of the same plants, as well as of
numerous investigated Cyatheacese, Polypodiaceae, and of Todea barbara. They
also occur in the stems which have been investigated with reference to this point, e. g.
in Ophioglossum vulgatum, species of Polypodium, and Pteris (Luerssen), Aspid.
filix mas, Onoclea struthiopteris, Cyathea arborea, Imrayana, Alsophylla microphylla;
in Marattiaceae, e. g. M. Kaulfussii, also in the cortex of the root. In most ferns
they appear to be wanting in the chlorophyll-parenchyma of the leaf. The pro-
trusions of the outer surface occur obviously only on those parts of the wall which
border on intercellular spaces, and, as a matter of fact, only on air-containing in-
tercellular spaces. In comparison with the thickness of the rest of the cell-wall they
are always thin, and when slightly developed they appear in the form of small knots,
when better perfected as fine filiform rods, rarely thickened like clubs at their ends ;
the longer ones are not uncommonly branched. In relatively few cases they occur

^ Compare Hofmeister, Pflanzenzelle, p. 168.

^ Van Tieghem, Ann. Sei. Nat. 5 ser. XIII. p. 187. — Strasburger, Coniferen, p. 346. — Reinke,
Morpholog. Abhandl. p. 35.

® Meyen, Physiologie, I. Taf. VI. 17. — Hartig, Forstl. Culturpfl. Taf. 18. — Thomas, in Pring-
sheim’s Jahrb. IV. p. 40. — Compare also Hofmeister, Pflanzenzelle, p. 169.

* Karelstschikoff, Bullet. Soc. Imp. cle Moscou, 1868, No. i.

^ Botan. Zeitg. 1873, p. 641, Taf. VI. — Sitzungsbr. d. naturf. Ges. zu Leipzig, 1877, No. 7.

PARENCHYMA . COLLENCHYMA .

II9

singly, e. g. rhizome of Ophioglossum, petiole of Dicksonia antarctica. Usually they
are numerous and closely aggregated. The elongated rods springing from the dif-
ferent sides of the intercellular space in all directions are irregularly intertwined
between one another, so as to form a delicate framework with air in its inter-spaces.
The single rods sometimes end free, sometimes they are connected by their branches,
or go from one side of the intercellular space to the opposite, and also adhere
to the latter. As regards their material, the above outgrowths of the wall are equiva-
lent or similar to ‘ slightly cuticularised membranes.’ Cellulose colourings cannot
be observed in them, but rather they and the outermost layer of membrane which
connects them both behave under reagents like the limiting lamellae on the surfaces
of contact of the contiguous cells ; they turn yellow or brown with Schultze’s solution,
or with iodine and sulphuric acid, and are destroyed by boiling with solution of
potash. It remains for further investigation to determine how far they may ac-
cordingly be styled parts of an inner cuticle lining the air-passages.

Sect. 26. A definite special form of ihick-walled parenchyma is distinguished by
the name Collenchyma^ . It forms thick bands beneath or near to the epidermis,
especially in stems, petioles, and nerves of the leaf of herbaceous Dicotyledons (e. g.
species of Rheum, Rumex, Beta, and Chenopodium, Aegopodium, herbaceous shoots
of Sambucus, Labiatse, Solanaceae, Begonias, petioles of Nymphsea, etc. ^), and in the
petioles of the Marattias^ In their typical development it is distinguished by the
form and structure of the walls of its cells, which are capable of division and contain
chlorophyll. The cells are in unbroken connection with one another; only in
exceptional cases (stem of Silphium conjunctum
and its allies) are the layers traversed longitudinally
by intercellular canals. In form the cells are
elongated many-sided prisms, with horizontal or
obliquely pointed ends : when they are isolated it
is usually plain that they are derived from elon-
gated mother-cells, with sharply pointed ends,
which are divided by permanently thin transverse
walls, or, as the case may be plainly stated, they are
chambered *. The walls are thin at the ends, and
along the whole of the middle of the lateral sur-
faces of cells, which face similar cells; but along the
angles they are provided with a stronger thicken-
ing, which protrudas into the cavity of the cell so
as to round it off, or it may even project further,
while towards the thin middle band of the wall it
is sloped off, or sharply truncated (Fig. 4^7). In
the stem of the above-named species of Silphium
the thickening extends also over the faces opposite the intercellular spaces. The

’ [Cf. Giltay, Botan. Zeitg. i88l, p. 153; also ‘ Het Collenchym,’ Utrecht. — Ambronn, Pring-
sheim’s Jahrb. XII. p. 473. — Van Wisselingh, ä la conaissance d. Collencbyme, Bot. Centralblatt,

1882, Bd. XII. p. 120.]

^ Compare Mohl, Vegetab. Zelle, p. 20; Botan. Zeitg. 1844, p. 308. — Unger, Anat. und Physiol,
p. 148. — Sachs, Lehrb. p. 24.

^ Kussow, Vergl. Unters, p. 106. * Compare Kraus, Cycadeenfiedern, l.c. p. 310 (6).

Fig. 47. — Epidermis, <?, and collencliyma, r/, of the
petiole of a Begonia. The epidermal cells are evenly
thickened on the outer wall, where they abut on the
collenchyma ; they are thickened like it at the longi-
tudinal angles wherever three cells meet ; chi chloro-
phyll grains; p thin-walled parenchymatous cells
(550). From Sachs' Textbook.

120

CELLULAR TISSUE.

thickened parts of the walls have no pits, and are delicately stratified with very
fine limiting layers (‘Intercellular-substance’); they swell largely with water, but
without becoming gelatinous ; when water is removed they contract greatly in all
directions (measurements are wanting). In the soaked condition they show, in
transmitted light, a characteristic bluish white lustre. With Schultze’s solution they
turn light blue^; after slight warming with potash they immediately turn deep
blue with solution of iodine in potassium iodide (this is the case in Sambucus,
Rumex, Lamium album, Cactacese, Nymphoea).

In the same places which are occupied in many plants by cells thus remarkably
characterised by the above described properties, there are found in many others layers
of cells which differ more or less from these in their form and in the structure of their

t

elements. For instance, the cells of the Collenchyma of the stems of Cacti already
mentioned ^ differ from those described as typical, in their slight elongation, and in
the fact that the walls are thickened strongly and uniformly all round, and coarsely
pitted. Other single forms approach nearer to the thin-walled or sclerotic forms of
parenchyma, without its being possible to carry through any sharp distinction. It is
then to a great extent a matter of taste how far one will extend the term Collenchyma.
It is now used generally for the form of tissue here described as typical, though it
was originally proposed by Link ® for the pollen mother-cells with their gelatinous
membranes, and was then transferred by Schleiden, at first half in joke, to the above-
mentioned collenchymatous cells of the Cactacese.

From the collenchyma must be distinguished the thick-walled forms of paren-
chyma, the membranes of which are more or less lignified, and have thus become
hard and sclerotic. As the most typical representatives of tissue of this sort may be
brought forward the thick-walled cells of the secondary wood of Dicotyledonous
trees, which lay up starch periodically, and often prove themselves capable of divi-
sion in the case of wounds, or rather healing scars. This will be entered into in
more detail in Chapter XIV. In other places than that just mentioned sclerotic
cells are to be found widely spread : together with collenchyma and sclerenchyma
they form the strengthening apparatus of those parts, and they are connected with
both of these tissues by the paost various transitional forms. No general specific
peculiarities of this tissue can be mentioned in addition to what has been already said ;
remarkable examples will therefore for the most part be mentioned in the chapters
which deal with the distribution of tissues. Here we may briefly notice only one series
as being specially instructive, and as presenting difficulties in a sharp classification
of the tissues, viz., that of the sclerotic cells in the Ferns. In the large majority of
these plants there occur in stem, roots, and leaves thick-walled elements, sometimes
isolated, but usually in close and often in uninterrupted connection w'ith one another,
and combined to form uniseriate or multiseriate layers or bundles ; these either lie
near the epidermis, or accompany or ensheath the vascular bundles. In the petiole of
the Marattiaceae they have the properties of collenchyma, as was above stated ; also
many bundle-sheaths, to be cited later, are directly connected with this tissue as regards
their structure. But in the large majority of cases (compare Fig. 48) the walls, which

^ Schacht, Lehrb. p. 195.

^ Compare Unger, Griindziige, p. 25; Schleiden, Anatomie d. Cacteen, p. 14.

^ Grundlehren d. Kräuterkunde, II. p. 199.

PARENCHyMA, SCLEROTIC CELLS. ENDODERMIS.

121

are equally thickened all round, or less thickened on one side, are highly ‘lignified’;
in some few cases they are quite or almost colourless (e. g. stem of Lycopodium),
usually they are coloured a dark brown. On the
chemical property of the characteristic brown substance
nothing certain is known. The sclerotic-tissue elements
are generally of elongated prismatic form, either with
slightly inclined or sharp-pointed ends, in the latter
case they are fibrous cells or fibres. According to the
character of their contents, they must, in compliance
with the fundamental ideas above laid down, be for the
most part assigned to the category of cellular tissue,
since most of the elements, even those with very thick
walls belonging to the dark brown layers and strands
in the ferns, are densely filled with starch grains, which
(as was observed in rhizomes of Osmunda regalis)
gradually disappear as their age increases. It was not
possible by any means to prove that these cells are
capable of division. On the other hand, there occar
side by side with these sclerotic cells, and often con-
nected with them by quite gradual transitions, elements
thickened till the cell cavity almost disappears, and
showing only the last traces of cell-contents. These,
regarded independently, should be accounted as specific sclerenchymatous fibres ;
thus, e. g. in the brown sclerenchymatous sheath of the stem of Marsilia salvatrix.

Sect. 27. The name Endodermis, proposed by Oudemans^ for a special case,
here denotes generally those peculiar limiting layers to which Caspary ^ has given
the name protective-sheath (Schutzscheide)®. They belong to the category of cellular
tissue by reason of the nature of their contents, and their power of independent
growth and division, which is so often to be observed, e. g. in roots of Dicotyledons.

The endodermis is a sheath consisting in all cases of one single layer of cells.
It should also be observed here, that it lies as a rule at the limit between masses of
parenchyma and other systems of tissue, especially vascular bundles, and is then to
be recognised both by its development and its mature properties, as the layer of the
parenchymatous mass bordering on the unlike part. In roots with an axile vascular
cylinder the latter is always enclosed by it. The same is the case in stems with an
axile vascular cylinder, as Hippuris, Callitriche, Ceratophyllum, Utricularia, Elodea,
species of Potamogeton, Corallorrhiza, &c. (compare Chapter VIII), or with a closely
compressed axile system of bundles (species of Potamogeton, Plydrocotyle vulgaris,
&c.): also in stems of Phanerogams with a strongly developed cylinder containing the
vascular bundles, this is marked off from the surrounding mass of parenchyma by
a layer of endodermis, e. g. Tagetes patula, and other Composite^, Cobsea scandens,

■ ^ Ueber den Sitz der Epidermis bei den Luftwurzeln der Orchideen. Abhandl. d. Acad. Am-
sterdam. Math. phys. Klasse IX (i86i).

2 Pringsheim’s Jahrb. I. p. 441 ; ibid. IV. p. loi.

® [Cf. also Schwendener, Die Schutzscheide u. ihre Verstärkungen, Abhandl. d. Königl. Akad.
d. Wiss. zu Berlin, 1882.] * Van Tieghem, Ann. Sei. Nat. torn. XVI. p. 113.

FlG. 48. — Two sclerotic brown ceils from
the hypodermal layer of the rhizome of
Pteris aquilina, isolated by chlorate of
potash and nitric acid. ^4 more strongly
thickened on one side and with branched
pits (550); B less thick-walled; the optical
section of the wall and the pitted wall at
the back are drawn in. From Sachs’ Text-
book.

122

CELLULAR TISSUE.

Primulacese, as Primula sinensis Lobelia syphilitica, Rhizomes of Scitamine®,
Cyperaceae (e,g. Carex hirta), Acorus gramineus.

The same occurs in certain Equiseta. But on the other hand not the whole
body of vascular bundles, but each single vascular bundle is in many cases sheathed
round by an endodermis. This is the case both in the stem and leaf of almost
all ferns and many species of Equisetum, and also in the petioles and leaves (Adoxa
moschatellina, Menyanthes trifoliata, species of Primula), and in many stems of
Phanerogamic plants, as Nuphar, Brasenia peltata, Hydrocleis Humboldtii, Primula
auricula, Menyanthes. Rarely an endodermis occurs in other places than those
named : thus in the parenchyma of the stem of many Equiseta, and in many aerial
roots, especially of epiphytic orchids, the parenchymatous cortex is marked off both
from the vascular bundle and from the tracheal sheath by an endodermis.

The relations in the species of Equisetum may here be described according to Pfitzer'^,
as being specially instructive for the arrangement of the endodermis, which is variable
even in closely allied plants. In the parenchymatous ground-mass of the internode there
is a ring of vascular bundles equal in number to the angles of the stem (comp. Chap. VIII).
In the foliage-stems of E. limosum and E. littorale an endodermal layer surrounds each
single bundle. In E. arvense, Telmateja, silvaticum, pratense, palustre (comp, below,
Chap. VIII), and scirpoides, this sheath is wanting round the single bundle, but sur-
rounds the whole ring externally, curving inwards between two bundles. Besides this
outer general sheath, there occurs in E. hiemale, trachyodon, ramosissimum, and varie-
gatum a similar inner one, i.e. one bordering the whole inner side of the ring of bundles.
In the rhizomes the same phenomena occur on the whole as in the foliage-stem; but in
the same species, as more minutely described by Pfitzer, the rhizome and foliage-stem
may be similar or dissimilar. Finally, at the points of transition between rhizome and
foliage-stems, Pfitzer often found in E. hiemale small strings of parenchyma 1-3 cells
thick, as seen in transverse section, which lay between two vascular bundles, and were
surrounded by an endodermal layer, the latter either arising as a protrusion from the
general sheath, or having no connection with it.

The cells of the endodermis (see Figs. 49 and 50) are nearly of the four-sided
prismatic form, very often flattened in the direction of the tangent of the part
enclosed by them, more or less elongated, with horizontal or oblique ends, and
connected uninterruptedly with one another along their radial lateral faces. Their
membrane is always delicate when differentiation of tissues begins, and often
throughout life it is smooth externally and internally, rarely it is delicately pitted ;
but the radial walls are characterised by a fine and usually irregular wavy transverse
folding, which is continued over the ends from one radial wall to the other. Further,
the undulation extends, according to the special case, either over the whole surface,
or only over a band-like longitudinal strip of it.

The wall of the cells is further characterised by suberisation, which appears
early, i. e. with the first differentiation of tissue : this always affects the undulated
part of the wall, and may also extend, according to the special case, in varying
degree over some or all of the other walls. This is the case in the majority
of Ferns : a good example of the localisation of the suberisation on the undu-
lated bands in the middle of the radial (cellulose) walls is supplied by the root

* Von Kamienski, Vergleichende Anatomie der Primeln, l.c.

^ Ueber d. Schutzscheide der deutschen Equiseten, Pringsheim’s Jahrb. VI.

PARENCHYMA. ENDODERMIS.

133

of Botrychium Lunaria. In the root of Ranunculus Ficaria Caspary found most
of the cells suberised, more particularly at least on the undulated walls ; and, on the
other hand, single cells, with no exactly definable arrangement, equally suberised all
round. The walls in question may be termed suberised on this ground, viz. that
they behave before reagents like the totally suberised membranes of the cork-cells,
or like cuticle. (Comp. pp. 75 and in.) They alone remain behind after the
action of concentrated sulphuric acid, even if the acid has destroyed the surrounding
cellulose walls. More exact investigations of its chemical relations are entirely
wanting. Further, the peculiar refractive properties of corky walls, the dark black
contours when seen by transmitted light, belong to the walls in question. Partly in

Fig. 49. — Ranunculus fluitans ; transverse section through the vascular bundle of a strong old adventitious root (225).
u endoderntis, p pericambium, ^ outer primordial vessels of the diarch uniseriate xylem j'—p' ; between and p the phloem.
Fig. 50. — (375) A piece of the endodermis in tangential longitudinal section.

this circumstance, partly in the wavy folds superposed one on another, in not very
thin preparations, lies the cause of the often-described phenomenon, that the un-
dulated strips of the radial walls appear in transverse sections as dark points or lines.
Another peculiarity, which again recalls the cuticle, is this, that the suberised parts
of the walls swell in sulphuric acid and in potash in the direction of their surface.
The undulations appear after the action of those reagents to become higher ; but
whether this is really the case, or whether they only become plainer for observation,
remains to be investigated.

Like many cork-cells, those of the endodermis often remain thin-walled through-
out life, e. g. in almost all Ferns, the walls being either totally suberised, or (a point
which requires more extended investigation) having a delicate internal cellulose
layer. But on the other hand, there appears not infrequently here also a strong
thickening superposed internally on the original membrane : this occurs especially
in roots of Monocotyledons, the stems of Potamogeton (in many species, as

124

CELLULAR TISSUE.

P. crispus, densus, gramineus, no strong thickening occurs), rhizomes of Cyperacese,
e. g. Carex hirta, exceptionally also in roots of Dicotyledons (Primula Auricula).
Comp. Fig. 51. In many rhizomes of Monocotyledons, e. g. Carices, several thick-
walled sclerotic layers occur in the region occupied in allied plants by the endo-
dermis, when fully developed : it remains to be investigated how far these are
endodermis.

The thickening masses are
as a rule more or less sclerotic —
lignified, or suberised — only in Pr.
Auricula do they consist of cartila-
ginous, gelatinous cellulose. In
relatively few cases they extend in
equal thickness round the whole
cell

epiphytic Orchids, stem of Pota-
mogeton pusillus) ; usually they
are more strongly developed on
the inner side, than on the outer ;
roots of species of Carex, Cy-
perus, Scirpus, Phragmites com-
munis, Triticum repens. Asparagus,
species of Smilax (the so-called
core-sheath (Kernscheide) of Sar-
saparilla roots), Dracaenas, Palms
and in the above-named rhizomes of
Cyperaceas, the stems of Potamoge-
ton pectinatus, lucens, natans, prae-
longus®: comp, below. Chap. VIII.
The thickening masses are stratified and pitted; only in the investigated Dracaeneae
they are not pitted (Caspary). The undulation is not present on the thickened walls,
but it again appears on the original radial walls, if they can be isolated by destruc-
tion of. the superposed thickening masses, e. g. by sulphuric acid. Usually the
thickening and sclerosis extend to all the cells almost equally, but thin-walled cells
often occur between the others. They are found quite solitary, e. g. often in the
root of Auricula (Fig. 51); or numerous, hut alternating not very regularly with
the thick-walled cells, in the root of Strelitzia ovata. But in the vascular-bundle-
sheath of the aerial roots of epiphytic Orchids 1-2 longitudinal series of cells remain
before each vascular group with an unthickened membrane, which turns blue with
iodine and sulphuric acid : in their longitudinal course they are here and there inter-
rupted by thickened cells

The elements of the endodermis are, in all exactly investigated cases, cells in the
fullest sense of the word, with a protoplasmic body; in Equisetum, according to

(root of Pr. Auricula, of many

Fig. 51. — Primula auricula (225) ; transverse section through the heptarch
vascular bundle of an adventitious root and its surroundings. / pericain-
bium ; ^the outer primordial vessels of the vascular rays, which alternate
with the seven phloem-groups, and are separated from these by thin-
walled parenchyma; ti endodermis; outside this is rather thick-walled
cortical parenchyma, with intercellular spaces tetrangular in transverse
section.

* Caspary, l.c. p. 108. — Schleiden, Archiv d. Pharmac 1847. — Berg, Atlas d. Pharm. Waaren-
kunde, Taf. Ill, IV. — Mohl, Palm, structura. — Karsten, Vegetationsorgane der Palmen, Taf. III.
fig. 2.

^ Caspary, Pringsheim’s Jahrb. I. p. 443.

^ Leitgeb, Wiener Acad. Denkschr. Bd. 24,’p. 207.

PA RENCHYMA . ENDODERMIS.

1^5

Pfitzer, they even contain chlorophyll ; further, as in all parenchymatous cells, the
nature of their contents is very various ; many are poor in contents of definite form,
or almost empty; very many have abundant starch grains, and even to a remark-
able degree in comparison with the surrounding parenchyma. Also in strongly
thickened and sclerotic cells there are often abundant starchy contents, as in the
roots of Cladium Mariscus, and Carex arenaria according to Caspary, in the stem
of Potamogeton atans, &c. In single cases, namely, in roots of Ficaria and
Victoria regia, and in stems of Equisetum, Caspary and Pfitzer found the proto-
plasmic body of the cells brown, and contracted to a band stretched between the
undulated walls.

As was already indicated at the outset, the layer of cells limiting the parenchy-
matous cortex from the air-containing sheath, which surrounds it in the aerial roots
of the epiphytic orchids and Aroidese, of Chlorophytum and Hoya carnosa, is
a special case of endodermis. It corresponds in all important points with the
‘ protective sheaths,’ and is generally distinguished by one peculiarity only, that in
each of the longitudinal series, which its cells form, elongated prismatic elements
alternate regularly with short roundish or oval cells. Usually all the cells have thin
walls, and in that case (according to Leitgeb always) they are undulated on their
radial faces ; they have, as far as my investigations extend, a complete suberised
outer layer, and a delicate cellulose inner layer. But in many species the long
cells are strongly thickened and sclerotic, most strongly, and without pits in
Oberonia myriantha (Leitgeb, /. c.). The short cells are always thin-walled. The
long cells contain chiefly watery cell sap. The short ones are characterised by
relatively abundant, granular protoplasm, and a large nucleus. On the structure
of the roots in question, comp. Sect. 56.

CHAPTER II.

SCLERENCHYMA.

Sect. 28. The name Selerencliyma, introduced by Mettenius^ here indicates
those tissue-elements which have not only thickened their walls at the expense of the
cell-cavity, but have also lost the cell quality besides. Together with the sclerotic
cells of the foregoing paragraphs they form the strengthening apparatus. But while
the former, by reason of the nature of their contents, still take an active part in the
processes of assimilation and nutrition, the properties which point to this are wanting
in the tissues in question ; they appear (besides some connection with the transfer of
water) to be in the main only strengthening apparatus, or specific mechanical
elements, to use the terms of Schwendener.

We will not here again return to the practical difficulties in distinguishing this
tissue from the sclerotic cells. Comp. p. 115, and Chap. X.

The general properties of the sclerenchymatous elements consist in this, that as
the thickening and lignification proceed, the protoplasmic body and nucleus dis-
appear, and of these and of the products resulting from their activity only remnants
together with watery fluid remain behind, partly as not clearly defined granular
contents; often however they take the form of rather abundant fine-grained
starch, which apparently has no further use, as e. g. in the fibrous ring of the
outer walls of Aristolochia Sipho, or of crystals of Calcium oxalate, as in many
covering tablets of fibrous bands, to be described below, and in the raphide-
containing fibres of the cortex of the root of Chamcßdorea elegans. According to
Schwendener ® a part of the fluid contents is replaced in the typical sclerenchymatous
fibres by air; they always contain some air in the noruial condition. The structure
of the walls is in general that of strongly thickened cell-membranes, with their
numerous modifications : these will be more readily described in connection with
the single forms.

According to the form, and the definite relations of structure which vary for the
most part with it, we may distinguish two main forms of sclerenchymatous elements,
which, however, are not in all cases sharply defined from one another, viz. (i) short
sclerenchymatous elements, and (2) elongated elements, or sclerenchymatous fibres.

^ Abhandl. d. K. Sachs. Ges. d. Wissensch. IX. p. 432.
^ Das Mechanische Princip, &c., p. no.

S TONE- ELEMENTS.

127

Sect. 29. The term Short sclerenchymatous elements may be applied to
all forms which have not pointed tapering ends; these are sometimes iso-diametric,
sometimes moderately elongated. To this group belong —

(a) The stone-elements (‘ stone-cells ’ of the Pharmacologists), so called after the
stony bodies in the flesh and stalk of many pears, which are composed of them,
are almost iso-diametric, rarely rod-like elongated derivatives of cells (‘ rod-cells ’),
with stratified, very strongly thickened membrane, lignified to a stony hardness :
this wall is perforated frequently by numerous, usually branched pit-canals, of circular
appearance in transverse section (Fig. 52). The narrow internal cavity, v/hich usually
disappears, is occupied by a watery fluid with a few granules,
or often by a reddish brown, apparently formless mass.

Stone-elements of this sort are widely spread among the
Dicotyledons, especially in sappy, fleshy parts ; in the suc-
culent parenchyma they are sometimes isolated, but usually
in uninterrupted connection with one another, forming cir-
cumscribed groups, or masses, of which the elements bor-
dering on the thin-walled tissue may graduate into the latter
by the thickening of their walls at this limit being one-sided
and weaker. In the so-called stout succulent plants, how-
ever, such as the Crassulacese, Cactacese, &c., stony formations
are generally wanting. Exquisite examples are supplied by
the fleshy body of Helosidese, Lophophytum, Langsdorffia^,
fleshy tuberous roots, e.g. Pseonia, Dahlia (Sachs) ; Rhizomes, e. g. Dentaria pinnata,
the pith of Hoya carnosa Medinilla spec.,® and especially the cortex of ligneous
Dicotyledons, in which they are mainly derived from secondary sclerosis of parenchy-
matous cells, as will be more closely described in Chap. XV.

Transitional forms to the sclerenchymatous fibres are supplied by the rod-
shaped stone-elements of many cortical layers, the short pointed fibres of the
Cinchonese, the short and pointed branched stone-elements of the cortex of Firs
and Larches, &c.

In the Monocotyledons the elements of this category are rare ; but we must
include under this head the multiseriate dense layers beneath the epidermis of stems
of Palms *, and elements with large cavity, and large pits, which form in the cortex
of the root of many Aroids (e.g. Tornelia fragrans) 3-4 layers of cells outside the
endodermal sheath of the vascular bundle, and in Raphidophora angustifolia ® also in
the inner cortex of the stem a ring of 1-2 layers in thickness.

Typical stone-elements are wanting in the Cryptogams.

{b) A second form of short sclerenchyma is represented by the peculiar covering
plates which Mettenius® first distinguished in species of Trichomanes under the name

Fig. 52. — Transverse section of
a short sclerenchymatous (stone)
element from the root tuber of
Dahlia variabilis. / lumen, pit-
canals ; sß split, by which an inner
system of layers is separated (‘^oq).
From Sachs’ Textbook.

’ Hooker, Trans. Linn. Soc. vol. XXII. — Graf Solms-Laubach, in Pringsheim’s Jahrb. VI. p.
530. — Eichler, Balanophorese Brasilienses, Tab. II.

^ Mohl, Ranken- und Schling-pflanzen, p. 89. — Ibid. Poren d. Pflanzenzellgewebes, p. 32.

^ A. Giis, Ann. Sei. Nat. 5 ser. XIV. p. 50.

* Mohl, Palmarum structura, pag. VI. Tab. A, C. Verm. Schriften, p. 136. — Botan. Zeitg.
1871, Taf. II.

® Van Tieghem, Struct, des Aroidees, l.c.

Hymenophylleen, l.c. p. 41 8.

SCLERENCHYMA.

ia8

Siegmata, and which, as shown by later investigations of Rosanoff^, occur not un-
frequently among the Monocotyledons also. They always appear on the outer
surface of sclerenchymatous or sclerotic bands of fibres (either such as pursue a
separate course, or accompany vascular bundles), and are applied to these in longi-
tudinal rows, which by the arrangement of their elements lead us to conclude that
they arose by transverse division of spindle-shaped cells. The single elements are
small, and have the form of flat, or (in the Monocotyledons) plano-convex, usually
rectangular plates, with the flat side contiguous with the fibrous band. As regards
their structure they are characterised by unequal thickening on different sides,
usually also by partial silicification of their walls ; they vary extremely in individual
cases according to the species or systematic group. In the species of Trichomanes,
the wall is strongly thickened on one side, i. e. on the inner face, which is contiguous
with the fibrous band. In some few species the thickening is uniform on this surface;
it is equally rare to find it so arranged that it occupies the periphery of the inner
wall in a ring-like manner. Usually there rises from the middle of the inner wall
into the cavity a cushion-like protuberance, hollowed in the middle, or comb-like
bands placed symmetrically near the middle. On the varying special forms of these
outgrowths, compare /. c. Those outgrowths protruding" inwards and the region
immediately surrounding them are distinguished from the rest of the wall, which
shows the cellulose reaction, by their granular appearance and strong silicification.

Similar covering plates, perhaps more properly included under the crystal-
containing structures, since each contains an aggregation of calcium oxalate, occur,
according to a short statement by Mettenius, in certain of the Cyatheacese.

The fibrous bands in the stems, leaves, and roots of Orchideae (Pholidota,
Stanhopea, &c. ^), Palms (Chamserops, Phoenix, Caryota, &c.), of Maranta compressa,
Arundinaria spathiflora, have interrupted longitudinal rows of plano-convex stegmata
on their exterior. The convex outer wall of these is- thin, the inner thickened
to a half-spherical rough body, which almost fills the cavity, and consists mostly of
compounds of silicon. Often 2-3 such silicified bodies occur in place of one.
(Rosanoff found similar silicified bodies also in cells containing chlorophyll and
starch on the fibrous bands in the margin of the leaf of Galipea macrophylla, one of
the family of Diosmese.) The fibrous bands in the lamina of the leaf of Scita-
mineae (species of Maranta, Heliconia, Thalia) show small stegmata, the structure of
which seems to differ from that just described, and remains to be investigated.

Sect. 30. Sclerencliymatous fibres, of elongated spindle-like shape, with
sharp ends, simple or branched, are the form of strengthening tissue which is
universal, especially in Phanerogams ; they are sometimes in uninterrupted lateral
connection, and united, with pectinated ^ ends, into bundles and sheaths ; sometimes
they are imbedded singly in other tissues.

* Botan. Zeitg. 1871, p. 749.

^ Compare Link, Botan. Zeitg. 1849, P- 75°-

^ [It is believed that this translation will convey the meaning intended by the use of the word
‘verschränkt,’ the idea being that of an arrangement similar to the fingers of two folded hands, or of
two combs (pecten) with the teeth of the one passing between those of the other. The word
pectinatory will be used subsequently in describing the course of the vascular bundles (Chap.
VIII, A).]

FIBRES.

J29

The fibres in question are frequently called also Bast-fibres, or Bast-cells, after a
region in which they occur especially often in the Dicotyledons, and, in connec-
tion with these terms, Sanio has called those fibres which occur in the secondary
xylem, and W'hich belong also in part to this category, bast-fibre-like, or libriform
fibres. Comp. Chap. XIV. P. Moldenhawer ^ calls them fibrous tubes.

The name bast, or liber, is at present used for two quite different things. Originally it
was used as a topographic anatomical term, for a definite region of the cortex of the
Dicotyledonous stem, which is, it is true, as much characterised by definite forms of
tissue occurring in it as by its position (comp. Chap. XV). Among these forms of tissue
sclerenchymatous fibres are quite generally characteristic; they are present indeed in
many cases in very large quantity, and are very conspicuous as compared with the other
tissues. On the latter ground, and since the really characteristic structure of this cortical
region was not known, they were considered as the essential tissue of the bast-region, and
the name bast was transferred from the region to the sort of tissue, but later again used
for both without sharp distinction. Since the sort of tissue is by no means limited to
this region, the result was that bast was found at other places than in the bast, or that there
is bast without bast, in other words that doubt and controversy arose. Now it is in itself
indifferent which meaning is attached to the name, and grounds may be brought forward
for authorising both the above uses of it, but it certainly cannot be used for two quite
different ideas. In the choice to be made accordingly it seems to me decisive that the
topographic meaning of the word is the older, and has always been the more usual. Its
use will therefore be here limited to the region to be treated of later, and the fibres in
question will therefore be called East-fibres, wherever they belong to this region.

The form of the sclerenchymatous fibres varies within the above stated limits ac-
cording to species and part. Their transverse section is acutely angular, where they
are closely united into bundles ; it is round in such fibres as lie single and loose in
intercellular spaces, as in many leathery leaves, in the foliage of many Aroideae, &c.
Those firmly connected into bundles are as a rule simple, i.e. unbranched, spindle-
shaped, usually with continued and gradual decrease of transverse section towards
the ends, while the much-elongated forms are usually drawn out at the ends into ex-
tremely fine points. This form — subject it is true to many exceptions — is the rule
also for the fibres occurring in longitudinally elongated parts, but not closely con-
nected into bundles : for instance, for most of the fibres, and even the isolated bast-
fibres, which are scattered in the parenchyma of the roots of many palips (Chamaedorea
elegans), the petioles and pinnae of Cycadeae &c. A remarkable peculiarity of form
is shown by the very long bast-fibres of many Apocyneae, and Asclepiadeae (Nerium,
Vinca, Asclepias spec.), since they are in their longitudinal course alternately nar-
rowly constricted, and then again suddenly distended; the same is the case, in
rather irregular form, in the bast-fibres of species of Sida, Urena, and the species of
Corchorus which yield Jute®.

Even the spindle-shaped fibres, which have just been mentioned as being
usually simple, show not uncommonly, when isolated, shortly- and unequally-
branched ends, or here and there at other points a branch usually of insignificant
size.

’ Beitr, pp. 11-61. , ^ Moldenhawer, Beitr. p. 34.

° S. Wiesner, Microskop. Unters, p. 24 ff., and Idem, Rohstoffe, cap. 11.

K

130

SCLERENCHVMA .

On the other hand there commonly occur in Phanerogams fibres which are
freely and often abundantly branched, and of a form which varies according to the
special place of their occurrence : these usually occur in dissimilar lacunar tissue,
with their branches projecting or pushed into its interstices. Inasmuch as these
project like many branched hairs into wide, air-containing spaces, as in the Nymphae-
aceae, Limnanthemum, Aroideae, Rhizophora, the description of them will be more
clearly given when we treat of these spaces (Sect. 53), and we need only draw atten-
tion here to their connection with the tissues treated of in this chapter. They also
occur more especially in numerous tough, leathery foliage-leaves, though not in the ma-
jority of them ; they push their branches into the intercellular spaces of the parenchyma,
and appear to serve as strengthening apparatus for that tissue. With reference to the
relations of their arrangement, to be treated in Chaps. IX and X, may here be men-
tioned the short-branched fibres in the leaf-lamina of Proteaceae (Hakea nitida, cera-
tophylla, saligna, &c. ^), the long- and finely-branched fibres in the lamina of Olea
europgea, emarginata, fragrans^, the thick, starlike, short-branched ones of Camellia

Fig. S3. — From a transverse section of the leaf of Camellia japonica. P parenchyniatotis cells, with chlorophyll
grains and oil drops ; F thin vascular bundle ; V branched sclerenchyma fibre. From Sachs’ Textbook.

japonica® (Fig. 53), Statice monopetala, the beautiful stellate, many-armed ones in the
lamina and petiole of Fagrsea obovata, and auriculatab Also the leaf-lamina of the
above-named Aroideae, especially the Monsterineae, and the Nymphaeaceae, may be
here again cited. Stellate-branched fibres occur in the foliage-leaf of Sciadopitys,
Dammara, Araucaria imbricata®. Long-branched ones, sometimes of huge size,
form at least half of the substance of the leaf in Gnetum Gnemon, and G. Thoa.

The relation between breadth and length of the fibres varies greatly both ac-
cording to species and in different parts of the same species, and in the self-same part
and the self-same bundle it often varies within wide limits. This is to be taken

' Meyen, Harlemer Preisschrift, p. 84, Taf. V. — Mohl, Verm. Sehr. Taf. VII. fig. 2. — Schleiden,
Grundz. 3 Aufl. I. p. 277.

^ Moldenhawer, Beiträge, p. 61. — Thomas, l.c. p. 32.

® Kraus, Cycadeen-fiedern, l.c. p. 327.

* O. Buch, lieber Sklerenchymzellen. Diss. Breslau, 1872, p. 16.

^ Thomas, l.c. p, 35. — Mohl, Botan. Zeitg. 1871, p. 8.

FIBRES.

,131

into account in the statements of average, which require confirmation throughout.
The published measurements of simple bast-fibres give for the shortest forms, such as
those of Peruvian bark, a relation of about i ; 10 to i : 20 ; for the longest, found
among the Urticacese, a length exceeding the greatest breadth two or three thou-
sand times (to i ; 4000). The branched forms are as a rule relatively short and
broad, e. g. Fig. 53, but much-elongated specimens also occur.

As examples we may cite the following few measurements found by Mohl ^ and
Wiesner®, and in the Quinine bark by VogP, for fibres of bast and bundles; for
further details we must refer to Wiesner’s summary, l.c. Where only the length is
given, the medium of the measurements of breadth given may serve as the breadth.

Le7tgth.

Greatest breadth of fibre.

mm.

mm.

Species of Cinchona, bast

. 0-875 — I'25 . .

. . 0*031

— 0*25.

Tilia, bast

. 0-99 ■ — 2-65 .

. . average 0*015.

Corchorus spec. (Jute), bast .

. 0-8 — 4.1

0-016.

Phormium tenax, leaf . . .

• 2-7 — 5’65. •

• •

0*013.

Linum usitatissimum, bast .

. 20 40

. . 0-15

—0-17.

Cannabis sativa

. 10 & more . .

. . 0-15

— 0-28.

Boehmeria nivea ....

. up to 220 . .

. . 0*04

— 0-08.

iEsculus Hippocastanum . .

. 1-35 —1-8.

Bignonia radicans ....

. 0-6 — 1-35.

Bombax pentandrum . . .

. 2*025 2*92.

Daphne Mezereum . . .

up to 3-375.

Clematis Vitalba ....

. 0-45 —0-85.

Bambusa spec

. 1-8 — 3'°i5-

Cocos botryophora ....

• o’855 — F350-

Lonicera Caprifolium, bast .

. 18-0 — 26-0.

Asclepias Cornuti ....

. up to 26-0.

Urtica dioica

. up to 77-0.

The considerable length of many fibres, together with the occurrence of the
chambered fibres to be described below, has given rise to the view that a fibre does
or may arise by the coalescence of several meristematic cells disposed in a longi-
tudinal series k More exact investigation however can find ä priori no sound ground
for this, and all minute observations have shown that each simple or branched
fibre results from the metamorphosis of one cell

The wall of the sclerenchymatous fibres is thickened to an extent which differs
according to each special case, and usually so that the lumen is greatly reduced
(centripetal) ; the thickening mass is nearly equally thick all round, or in many cases
it projects inwards much more strongly at certain points than at others, e.g. bast-

^ Botan. Zeitg. 1855, p. 876.

“ Mikroskop. Untersuchungen im Laborat. d. polyt. Inst. Wien ; and Rohstoffe, d. Pflanzen-
reichs, cap. II.

^ Die Chinarinden des Wiener Grosshandels, See. 1867.

* Meyen, in Wiegmann’s Archiv, 1838, I. p. 297. — Schacht, in Berl. Acad. Monatsber. 1856, p.
517: Lehrb. II. p. 567. — Hanstein, Milchsaftgef. p. 45.

® Compare Unger, Wachsthum d. Stammes u. Bildg. d. Bastzellen, Wiener Acad. Denkschr.
Bd. XVI; Boehm, Wien. Acad. Sitzungsber. Bd. 53; Sanio, Botan. Zeitg. i860, p. 210. Further,
the statements in Chapter VII upon the intercellular fibres of the Aroidese, and Chapter XIV.

K 2

132

SCLERENCHYMA.

fibres of Corchorus spec., Abelmoschus tetraphyllus, Sida retusa, &c.* The thick-
ening mass is either continuous, as for instance in most fibres used in manufacture,
according to Wiesner^ or in many cases provided with narrow pit-canals, which,
especially in the fibres associated so as to form bundles, have almost always the form
of narrow, rectilinear, longitudinal, or parallel oblique slits like a left-handed screw *

Fig. 54. — Pteris aquilina. A half of a
brown-walled sclerenchymatous fibre
from the stem ; B piece of one of these
more highly magnified (550) ; p profile
view of the slit-like pits ; C transverse
section; a limiting lamella; byC inner
layers of the wall. From Sachs’ Text-
book.

Fig. 55. — Half of a thick
sclerenchymatous fibre, with
crystals of calcium oxalate im-
bedded externally in the wall,
from the stem of Welwitschia
mirabilis. From Sachs’ Text-
book.

(Fig, 54). Still exceptions to this occur in the short fibres, as the simple ones of
the Quinine bark, and the branched ones of the leaf of Camellia : here there are
narrow canals, not slit-shaped, but round in section. The sclerenchymatous fibres
described by Milde * as being spirally thickened, which form a many -layered closed
covering on the upper side and on the nerves of the under side of the leaf of Acro-
pteris radiata, would be better described as having slit- like pits. Their membrane,
which is thickened till the lumen is almost obliterated, has very numerous, regularly

’ Compare Wiesner, Lc.

^ Mohl, l.c. p. 876. — Schwendener, l.c. p. 8.

^ Rohstoffe, p. 305.

* Filices Europas et Atlantidis, p. 40.

FIBRES.

133

oblique slit-like pits, and these alternate with thick bands of wall of equal height.
The epidermal elements overlying this fibrous covering show, as was intimated on
p. 71, the same structure.

For the more minute structure of the thickening masses, those general rules hold
which apply to the structure of the cell-walls In the fibres united into bundles,
and those in the bast, there may often, but not always, be distinguished three dif-
ferent concentric systems of layers, or sheaths, the outermost limiting layer, an inner
layer, and a middle layer, which is usually much broader and softer. The fibres of
the Apocyneae and Asclepiadeae are excellent examples of the striation and areola-
tion of the wall. — The branched fibres in the leaf of Sciadopitys, Dammara, Araucaria
imbricata, Nymphaeaceae, and especially the colossal spindle-fibres made known by
Hooker^, which lie scattered in all parts of Welwitschia mirabilis, are characterised
by numerous crystals of calcium oxalate which are imbedded in the outer layers of
their walls, and which, especially in Welwitschia, attain a considerable size (Fig. 55).

The wall of the sclerenchymatous fibres is lignified, to a very variable extent
according to the special case: of the bast-fibres used in manufacture, e.g. according
to Wiesner, those of Flax, Hemp (light yellow with aniline and sulphuric acid), and of
Hibiscus cannabinus turn blue (of different shades) with iodine and sulphuric acid,
and with aniline and sulphuric acid not at all or hardly yellow ; with the preparation of
iodine the fibres of species of Corchorus, Sida retusa, Urena sinuata, &c. turn yellow
or brown, with aniline and sulphuric acid yellow. In the Ferns and Rhizocarpese the
fibres of this category have also the above-mentioned (p. 121) characteristic dark-
brown colour. In fibres in the bast Sanio ® often found the especially thick inner
layer of the wall cartilaginous and gelatinous, and that it swelled in water, and
turned violet with Schultze’s solution or solution of iodine in potassium iodide
(e.g. in Cytisus Laburnum, Morus alba, Ulmus suberosa, Celtis australis, Ficus Syco-
morus, Robinia pseudacacia, Gleditschia triacanthos, Quercus pedunculata, Passiflora
suberosa) ; this phenomenon also occurs in various modifications in the fibrous
elements of the secondary wood of Dicotyledons, and will be described with the other
properties of these elements in Chap. XIV. Conversely it sometimes happens that
sclerenchymatous fibres develope from originally collenchymatous cells, in which
case the inner layers of the walls become hard and lignified, while the outer retain the
original collenchymatous character : e.,g. in the bands accompanying the vascular
bundles of Eryngium planum, and Astragalus falcatus

As regards the contents of the fibres we must refer to what was above stated
(p. 126) for the sclerenchyma generally. The granular constituents or remnants of
the contents, enclosed by many fibres with a larger cavity, e.g. the enlarged parts of
those of the Asclepiadese and Apocynese, have repeatedly led to the view that the
bast-fibres contain the characteristic latex, which exudes on cut surfaces in the
Asclepiadeae, Euphorbiacese, &c., a false idea, which will be discussed in Chap. VI.

In the majority of cases, and in all those which have been hitherto noticed, the

^ See Hofmeister, Pflanzenzelle, § 27, 28.

“ Trans. Linnsean Society, vol. XXIV (‘Spicular cells’).

® Botan. Zeitg. 1863, p. 105. — Ibid, i860, Taf. VI. 15 and 16.
‘ Schwendener, 1. c. p. 5.

134

SCLERENCHYMA .

cavity of the fibres -is a continuous hollow, though it is very narrow, and often ceases
far from the pointed ends. In the narrow contractions in the Asclepiadese and Apo-
cynese it may, it is true, be doubted whether it is not sometimes completely inter-
rupted by the thickening of the walls. But on the other hand chambered fibres are
often to be found, i.e. such as are cut up into segments or chambers by relatively thin
transverse walls continuous with the inner layers of the lateral walls : e.g. in the bast
of ^sculus Hippocastanum, in the cortex of roots of Palms, as Chamaedorea elegans.
Also the chambered fibres in the bast of Vitis, Platanus, Pelargonium roseum,
Tamarix gallica^, in the cortex of Aristolochia Sipho, &c., which contain starch for a
time, should perhaps be connected with the above, as cases in which the functions
of the cell slowly disappear ; the same may be said of the fibres produced from col-
lenchyma, which are common in the cortex of stems.

The chambered elements of the secondary wood of the Dicotyledons, which are
also connected with the above, will be spoken of in Chap. XIV.

* Compare Sanio, Ueber die im Winter Starke führenden Zellen, See. (Halle, 1858), p. 12 ; Botan.
Zeitg. 1863, p. III.

CHAPTER III.

SECRETORY RESERVOIRS.

Sect. 31. Bodies of a nature similar to the secretions of the dermal glands
(Sect. 19), such as mucilage, and gum, resin, ethereal oils, and mixtures of these
designated balsam, milky emulsions of the bodies of both categories which are
known in the dry state as ‘ Gum-resins,’ are often found laid by in the interior of the
tissues; they occur on the one hand in special Sacs, which develope during the
differentiation of tissues from definite cells of the meristem : these, retaining their
membrane, and growing considerably, are filled completely with the bodies in ques-
tion, and thereby lose their original cell-nature ; or they are found in special Inter-
cellular spaces.

There occur in many plants other sacs, arranged similarly to the above, which
also arise with the first differentiation of tissue from cells of the meristem, and con-
tain as their sole or preponderating contents crystals of oxalate of lime. All these
places of secretion or reservoirs are closely related to one another. The aggrega-
tions of crystals are often associated with large deposits of mucilage in the cavity of
a sac, so that one may speak of mucilage-sacs with crystals (e.g. tubers of orchids)
or of crystal-sacs with mucilage (e.g. Raphide-bearing sacs), according to the pre-
ponderance of one or the other body.

As already stated, resin and mucilage often occur mixed together. The form
of the sacs merges not uncommonly into that of intercellular spaces filled with the
secretory mass, since rows or groups of the former, by absorption of their walls,
coalesce to an amorphous intercellular mass. Further, sacs and intercellular spaces
with like contents often mutually replace each other, since, in the first place, the
same body in different members of the same plant sometimes fills sacs, at other
times intercellular spaces, e. g. the red resin of species of Lysimachia and Myrsine ;
or secondly, of closely allied plants some have sacs, others intercellular spaces filled
with the same secretion, at the same points. Examples of this will be found
below, among the Coniferae, Compositae, &c. Finally, in families more remote from
one another, there occurs only one or the other form of secretion and of the reservoir
containing it.

The mode of formation of the secretion in the interstitial dermal-glands cor-
responds closely with that of the schizogenetic resin-passages, which are to be
described below. We must here refer especially to the depressed glands of Psoralea.

In consideration of its known properties, calcium oxalate can only be regarded
as a body, which is removed from the metastasis of the plant, and is secreted or

SECRETORY RESERVOIRS.

J36

excreted. Direct observation teaches us the same of the mucilage, resins, and ethereal
oils of the dermal-glands. The fact is no less evident that the resins, mucilages, &c.,
which are laid by in circumscribed reservoirs, e.g. in the resin-sacs of the Laurinese,
Piperaceae, Zingiberaceae, &c., after they begin to be secreted in the meristem, remain,
like the calcium oxalate, laid by without further use.

In accordance with all these facts we are bound to regard the whole series of
the bodies in question, like the secretions of the dermal-glands, as bodies excluded
from the constructive metastasis, and to term them, together with these, Secretions.
Their occurrence as admixtures of the contents, or as constituents of the membrane
of active cells, which may be proved for all bodies of this category, is no argument
against this generalisation, since on the one hand calcium oxalate shows plainly that
one and the same body may be excreted both in small quantity in an assimilating
cell, and in large mass in a special reservoir ; on the other hand, in the uncertainty
of our present knowledge, a fundamental difference is always possible between what
is in the one and in the other case termed, for instance, resin. And finally, this
view does not affect that of the application of the secretion to some further uses by
the plant, as, for instance, in the well-known case of the hairs on buds.

On these grounds we group the whole of the above-described reservoirs together
as secretory-reservoirs. There may be distinguished reservoirs of crystals, mucilage,
resin, &c., according to their exclusive or preponderating contents. Since resin
and ethereal oil occur usually as mixtures, and rarely separate, and since we cannot
here enter upon chemical details, which are often uncertain, we shall in the sequel
use the words reservoirs of resin, oil, and balsam without claim to exact indication of
contents, and usually in connection with the meaning customary for each special
case. The term gum-resin is used to indicate, but with stUl smaller claim to accu-
racy, the mixture of watery and resinous secretions, which is milky when fresh.
According to their structure the reservoirs may be distinguished as Sacs, i.e. struc-
tures derived from cells, which retain their walls, and are therefore usually termed
cells ; and intercellidar cavities, which according to their form are termed either
passages or gaps. For many of these forms, which vary in structure and contents, the
term glands, or internal glands, is in use. It will be difficult to banish it, since it has
established itself in the incorrigible terminology of Systematic Botany, although, as
the sequel will show, it is not at all wanted. If it is to be retained, it ought accu-
rately to be used for all secretory-reservoirs, any other use of it is purely arbitrary
and conventional.

With those which certainly belong to this category, we must connect many
doubtful structures, such as many ‘ tannin sacs,’ the ‘ vesicular vessels ’ of the species
of Leek, and others to be named below : this classification may be corrected when
more exact observations have been made.

We have already drawn attention to the alternative occurrence of the different
forms of secretory reservoirs, in different members of the same plant, or in different
genera, or larger circles of affinity.

Similar alternative relations occur here and there between reservoirs and latici-
ferous tubes (comp. Chap. VI). Even if one discounts the Aroidese and Musaceae,
the laticiferous tubes of which should perhaps be enumerated in the present chapter,
the fact is certain that internal secretory reservoirs are absent from all plants which

SACS CONTAINING CRI'STALS.

^37

are provided with laticiferous tubes. In the group of Artocarpese, which in common
with the majority of its allies is provided with laticiferous tubes, these are absent,
according to Trecul, in Conocephalus naucleiflorus, while in their place this plant has
mucilage-containing sacs and cavities. Among those Compositse, which have been
investigated, the Cichoracese are distinguished from the rest by their having lati-
ciferous tubes, and by the absence of the oil-ducts present in the others : only in
Scolymus are both organs developed.

Further, it is often impossible to ignore an alternative relation between the occur-
rence of dermal-glands and internal secretory-reservoirs. In the Cycadese, Coniferae,
Lauraceae, Umbelliferae, Aurantiaceae, and Clusiaceae, which have specially large num-
bers of the latter structures, dermal-glands are absent or rare. For other families,
e.g. the Labiatae, the converse holds. Exceptions, with both sorts of organs side by
side, occur not uncommonly it is true, e.g. Dictamnus, and many Compositae with
glandular hairs and internal reservoirs. And finally, we must not omit to notice
that both organs may be altogether absent, as e.g. in the Gramina, Cyperaceae,
Palms, many Cruciferae, Ranunculaceae, in Taxus alone of the Coniferae, &c. See.

The relations, above brought into prominence, between the different organs
which form secretions, should always be kept in view during their consideration, in
which the first duty is to separate them according to their structure : we will therefore
occupy ourselves first with the sacs containing secretions. The intercellular reser-
voirs will be treated of in Chap. VII, and the intermediate structures will be noticed
in a fitting place.

I. Sacs containing Crys tails.

Sect. 32. It is known that crystals of calcium oxalate are generally distributed
as constituents of the cell contents. In certain sacs they almost exclusively fill the
internal space, and these may be distinguished as crystal-bearing sacs. The crystals ^
consist, as far as is known, entirely of calcium oxalate, which is crystallised either in
the quadratic or klinorhombic system — according to Souchay and Lenssen, when
quickly deposited it takes the klinorhombic form, with the composition

when the crystallisation is slower it forms as quadratoctahedra of the composition

So} C,0,F6H,0.

The fundamental form of the crystals belonging to the quadratic system is the
quadratoctahedron, that of the klinorhombic crystals, which are far commoner in
plants, is the hendyohedron ; derived forms occur of the most various shape, e.g.
klinorhombic columns, klinorhombic plates, twin forms, and blunting of corners. As
specially common forms, which can hardly be accurately defined crystallographically,
may be named the spear- or needle-shaped crystals, elongated and pointed at
both ends, which De Candolle ^ has termed Raphides. They belong most probably,

^ See Holzner, Flora, 1864, pp. 273 and 556. — Ibid. 1866, p. 413.
^ Organographie vegetale, I. p. 126 (pa(fts = needle).

SECRETORY RESERVOIRS.

according to Holzner, to the klinorhombic system. Besides these different, singly-
developed crystals, there often occur others imperfectly developed, and grown
together to angular or stellate groups, which, according to Holzner, may belong as
well to one system as to the other. The form and system of crystallisation is
indefinite in the case of the quite small crystals, which often occur, and appear more
like small granules : on these sharp angles and edges may be recognised with a high
power.

In the sacs the fully developed klinorhombic forms and the groups almost
always occur singly, rarely two together, and fill the greater part of the cell : the
Raphides appear always in larger number ; as a rule they are nearly equally long and
parallel and are closely packed in the sac in a bundle, so that all the ends in the
same direction are in one plane ; more rarely they vary in length and direction, as in
the cortex of many species of Aloe, e.g. Aloe arborescens, in the parenchyma of
Mirabilis, and the very small Raphides in the numerous crystal-sacs of the Cinnamon-
bark of Ceylon. Here the minute granule-like crystals in an innumerable multitude
fill the sac completely, so that in transmitted light it appears to have quite black,
densely granular contents : the same occurs in the herbaceous parts of many Solaneae^,
of Amarantus retroflexus, caudatus, and allies, Sedum ternatum, in the pith and
cortex of Sambucus nigra, the cortex of Betula verrucosa, Alnus glutinosa, Staphylea
pinnata and the bark of the officinal species of Cinchona

The form of the crystal-bearing sacs is closely related to that of the crystals
contained in them, when the latter attain considerable size ; but it cannot at present
be definitely stated whether the form of the crystal is dependent upon that of the
sac, or the converse. The iso-diametric grouped crystals are contained in sacs re-
sembling them in shape, the shorter or longer klinorhombic forms fill sacs of cor-
responding shape, which are even of very much elongated prismatic or spindle form,
e.g. in the rhizome and leaf of species of Iris^ and in the leaf of Aloe Africana. The
sacs containing raphides are elongated in the same direction as the bundle of
raphides where the raphides are very large, as in the cortex of Aloe arborescens, in
the bulb of Scilla maritima they often attain a great length, in the latter case more
than 5“™®.

These phenomena appear very striking in the bast-bundles of dicotyledonous
plants, the tissue-elements of which are derived from elongated spindle-shaped cam-
bial cells. The crystal-bearing sacs arise in this case by transverse division of a
cambial cell (Chap. XIV) ; in those of Guajacum, and Quillaja, which contain a single
elongated klinorhombic crystal, few divisions occur : each of the products of this pro-
cess (? all of them) becomes one crystal-bearing sac. Also it often happens in plants
with small solitary crystals or groups of crystals, that only single products of trans-
verse division may develope to crystal-bearing structures. But in very many ligneous
plants one cambial cell divides by transverse walls into numerous chambers (20-30),
which are hardly or not at all higher than broad, and each of these is filled by a
crystal or a group. The general outline of the original cambial cell is meanwhile

’ Corda, Beitr. z. Kunde d. Kartoffel, (See., in Hlubeck’s CEcon. Neuigkeiten, 1847, Nos. 58-60.

“ Sanio, Monatsbr. d. Berliner Academie, April, 1857.

® Fliickiger, Pharmacognosie, p. 365. ^ Unger, Anat. und Physiol, p. 123.

® Fliickiger, Pharmacognosie, p. 187.

5^05 CONTAINING CRYSTALS.

139

retained, while the whole series of chambers may be isolated, remaining still con-
nected together like a chambered fibre h Ilartig has called these chambered or
septate sacs, crystal-bearing fibres (Krystallfasern). — Similar phenomena occur also
in many woods, e.g. Herminiera Elaphroxylon, and on the outer surface of vascular
and fibrous bundles. The stegmata of Mettenius on the brown fibrous-bands of
Cyatheaceae (comp. p. 128) may perhaps belong more properly to this category.

Sacs with very small and numerous crystals, as those of Solanum, Sambucus, &c.,
usually differ but slightly in form and size from the surrounding cells.

As regards the structure of the crystal-bearing sacs, the bundles of Raphides
lie at first within a protoplasmic utricle : in all, or at least in all carefully investigated
cases, they are enclosed, when mature, by a rather thick layer of homogeneous,
transparent mucilage, which is in its turn surrounded by the slightly thickened
cellulose wall : the mucilage reacts, in a few investigated cases similarly to gum
arable, it swells quickly in water, and disappears (dissolves ?). It remains to be in-
vestigated how far this mucilage belongs originally to the membrane or to the
contents of the cell ; according to Frank’s statements respecting the mucilaginous
sacs, containing a smalt bundle of Raphides, in the tubers of Orchis, the latter is
probable. The presence of the mucilage is the cause of the quick swelling of the
raphide-bearing sacs in water : their membrane bursts, and the Raphides escape with
the swelling mucilage, and scatter themselves through the water. In the elongated
or spindle-shaped raphide-bearing sacs, which are common, e.g. in the Aroidese, the
bursting and escape of the needles usually occurs, as Turpin* has thoroughly de-
scribed, at one or at both ends. Hanstein s* raphide-containing sac-vessels (vesicular
vessels, ‘ Schlauchgefasse ’) are mucilaginous raphide-bearing sacs arranged one
above another in long longitudinal rows.

These series of sacs occur in large quantity in the parenchyma of many Mono-
cotyledons, stem and leaves of Commelineae, Palm stems, e. g. Chamaedorea :
Hanstein found them of large size in the foliage, stems, leaves, and bulb-scales of
many Amaryllideae, of the genera Amarjdlis, Spreckelia, Crinum, Pancratium,
Eucharis, Aloestrmeria, Narcissus, Leucojum, and Galanthus. In these cases they
are found 1-2 layers of cells below the epidermis, and especially in the parenchyma
of the lower (outer) side of the leaf. In the Liliaceae they are less common : they are
strongly developed in the leaves of Hyacinthus orientalis, and also in Agapanthus
(compare Hanstein, l.c.). In the foliage leaves of Scilla, Ornithogalum, Muscari,
there are short series, and isolated sacs, and in the scales of the bulbs of these
plants only isolated ones.

The stems of Commelineae are best fitted for the investigation of the series of sacs
in question. In the growing internodes of these plants, both in the parenchyma of the
cortex and of the middle of the stem, there may be observed single longitudinal rows of
cells, each of which is loosely filled with a bundle of parallel raphides. The cells are at first

'■ Compare Sanio, Monatsbr. d. Berlin. Acad. 1857, p. 261; for further particulars see below,
Chapter XIV.

^ Hilgers, in Pringsheim’s Jahrb. VI. p. 286.

2 Sur les biforines, Ann. Sei. Nat. 2 ser. tom. VI. p. 5.

‘ lieber ein System schlauchartiger Gefässe, etc. Monatsber. Berlin. Acad. 1859, p. 705.

Die Milchsaftgefasse, p. 33.

140

SECRETORY RESERVOIRS.

cylindrical. As the internode extends, the length of the cells and of the raphides increases,
till in the case of the cells it exceeds their breadth on an average 3-4 times. Hitherto a
thin protoplasmic layer, with a nucleus of sharp contour, lines the delicate cellulose wall.
As the internode extends further, the cells which remain thin-walled, become 10-20 times
longer than they are broad, the protoplasmic parts disappear, while round the bundle
of raphides there is seen only transparent mucilage, which shrinks greatly but without
turning misty in alcohol, swells quickly in water till it is unrecognisable, turns yellow
with Schultze’s solution, and is not dissolved in potash. Meanwhile the raphides do not
increase perceptibly in number or size, they form henceforth a relatively small group in
the sac filled with hyaline mucilage. According to Hanstein the members of such a
series of sacs coalesce, at least frequently and partially, to continuous long tubes, by the
breaking down of the transverse cellulose walls which separate them. But the observa-
tions cited in evidence of this are not sufficient to substantiate it. It is true it is often
found in longitudinal sections that the raphides are irregularly displaced, and have bored
through the delicate transverse walls of the series of sacs ; but on the other hand sacs are
also found closed at both ends, and dense bundles of raphides in them. And one can
often directly see the displacement of the raphides and the perforation of the transverse
walls in progress before one’s eyes. The action of water produces this result, the
mucilage swells in fundamentally the same way as in solitary raphide-containing sacs.

I could not prove to myself the occurrence of spontaneous perforation of the transverse
walls, that is of a coalescence of the series to a continuous tube or ‘Vessel.’ Where I
found a perforation already present, it was a gaping rent, such as is seen to be formed
when the wall bursts. F urther, it is not impossible to suppose that even in the living plant,
when too much water is present, walls may burst, and so the same phenomena appear as
are seen in sections. According to all these data, which coincide in the main with the
statements of Hanstein, the structures in question may be regarded as nothing more than
a special kind of raphide-containing sac distinguished by form and arrangement.

Rosanoff ' was the first to find in the pith of Kerria japonica. Ricinus com-
munis, in the sacs which accompany the vascular bundles of the petiole of Aroideaj
(e. g. Anthurium rubricaule, Selloum, Pothos argyrea. Philodendron Sellowianum),
as well as in the parts of the flower of Encephalartos and Nelumbium, that groups of
crystals are connected with the membrane ; either their apices are in close contact
with the lateral wall, or they are suspended by bars of cellulose, which extend from
the wall into the cavity, as far as single points of the group ; these bars are often
branched, and often hollowed like tubes. De La Rue found a similar attachment in
the parenchyma of the leaf of Hoya carnosa in the case of small groups of crystals,
contained in cells which have protoplasm and even chlorophyll (these, however, in
the strict sense do not belong to this category), and also in the leaf and petiole of
Aroidese (Pothos crassinervis, &c.). Further, Pfitzer following up an older observa-
tion of Schacht*, showed that the large solitary Minor hombic crystals contained in the
foliage of Citrus, and those in the cortex of Salix aurita, Populus italica, Celtis
australis, Fagus sylvatica, Rhamnus Frangula, Acer opulifolium, and Platanus
orientalis, are closely surrounded by a cellulose skin, a large part of the surface of
which is attached to the cellulose wall of the sac : this skin arises from the protoplasm
of the young crystal-bearing cell, which surrounds the crystal : at first it lies free,
later it becomes firmly attached to the cell wall. At the point of contact the lateral

' Botan. Zeitg. 1865, p. 32g. — Ibid. 1867, p. 41. — Compare also De la Rue, ibid. 1869, p. 537.
^ Flora, 1872, p. 95, Taf. III.

* Abhandl. Senkenberg. Gesellsch. z. Frankfurt a M. I. p. 150, Taf. VII. fig. 21.

SACS CONTAINING CRYSTALS.

I4I

wall of the sac is often strongly thickened, especially in Citrus, where the crystal
appears inserted in the very thick lateral wall, or in a conical excrescence of it. In
the septate sacs of the wood of Herminiera elaphroxylon ^ there lies in each of the
almost cubical segments one klinorhombic plate, with one side fitted into the strongly
thickened inner wall of the sac, while the rest almost fills the cavity of the segment.
I could not in this case find a membrane surrounding the crystal. The space not
occupied by the crystal in all these sacs is in the mature state apparently filled with
water.

Many crystals of whatever form, with the exception of Raphides, appear to lie
free within the membrane of the sac, being either closely surrounded by it, but
without attachment, or suspended in an apparently watery fluid. Thus, e. g. the large
crystals of Iris, and the crystalline granules. For all these cases, however, it remains
to be more definitely determined whether a gelatinous coat or an attachment to the
wall is present or not. The general occurrence of such a condition is attested by
Payen in his statements on the occurrence of silicious coats round grouped crystals,
and of membranous sheathing layers of a granular substance which turns brown with
iodine. Compare Hofmeister, Pflanzenzelle, p. 393.

Crystal-containing sacs occur in all parts, and in all tissues of plants ; they appear
most abundantly, and often in very great quantity [the stem of Cereus senilis contains
in the dry substance more than eighty-five per cent, of calcium oxalate (Schleiden)],
in the parenchyma of sappy foliage, and in leathery leaves, bordering closely on the
vascular bundles, and arranged in rows which follow these, in the bast and pith of
dicotyledonous plants, often also in the secondary xylem-parenchyma (Pterocarpus
santalinus, Haematoxylon, &c.), and in the medullary rays of the wood (e. g. Camellia
japonica, Vitis ; compare Chapter XIV) : where large air-containing intercellular
spaces are present they are often particularly numerous at the limits of these, and
project into them : e. g. Aroideae, Pistia, Myriophyllum.

They occur in most families, and usually in all genera and species of a family :
in those in which regular crystal-bearing sacs are rare, or absent, the calcium oxalate
is often deposited in the form of small crystals in the contents of parenchymatous
cells, or, as in the Cupressineae, Taxineae, Ephedra, and Welwitschia in the cell-
membranes.

The more generally this rule applies the more worthy of attention is a series of
exceptions. In the Equiseta no oxalate of lime is observed anatomically. The same
is the case in most Ferns, Gramineae, and the Potameae (with the exception in the
Phanerogams of the parts of the flower). StiU exceptions occur in many of the above
families : such as crystals in the Epidermal cells of Asplenium Nidus, in the covering
plates of the Cyatheaceae (comp. p. 128), numerous clusters of crystals in the
parenchyma of the stem of Panicum turgidum.

On the other hand, crystal-containing sacs, or crystals, are not found at all in
certain species or genera of families, the members of which, as a rule, contain
them in large quantity. In Nicandra physaloides and Petunia nyctaginiflora I found
no crystal-containing sacs, while the rest of the Solanaceae, which have been in-

‘ Hallier (Botan. Zeitg. 1864, Taf, III) gives the outline of these cells correctly, but with a
wrong description.

“ Compare Graf zu Solms-Laubach, Botan. Zeitg. 1871.

142

SECRETORY RESERVOIRS.

vestigated, have them in abundance. According to Gulliver, crystal-containing sacs
are wanting in Tulipa silvestris, Fritillaria Meleagris, Lilium Martagon, candidum,
aurantium, whilst most other Liliaceae have them in plenty : Sparganium has many
Raphides, the species of Typha have no crystals. Among the Lemnaceae^, no
Wolffia has crystals ; the Lemnae and Spirodelae have numerous raphide-bearing sacs,
the latter having also many clusters of crystals.

The form of the crystal-bearing sacs, and of the crystals within them, is cha-
racteristic for many divisions, families, and species ^ ; still general and absolute rules
cannot be laid down. In most families of Monocotyledons Raphides occur exclu-
sively, or they preponderate largely, and often occur in vast quantity, e. g. Liliaceae,
Orchidaceae, Bromeliaceae, &c. But in species of Allium there are no Raphides, and,
as far as is known, crystal-containing sacs are entirely absent. Instead of these there
lies in the middle of each cell of the subepidermal parenchyma, on the outer side of
the young scales of the bulb, one prismatic crystal, or several grown together® (this
is specially well seen in A. sativum). In others, e. g. the Aracese, sacs containing
Raphides and clustered crystals occur side by side, often in the same section. In the
Iridaceae only large columnar solitary crystals are to be found. While the Musaceae
have only sacs containing Raphides, there occur in the Marantaceae and Zingiberaceae
only other forms of crystals. In the Dicotyledons there are most frequently found
clustered crystals, or klinorhombic solitary crystals, or both forms together, often
also with granular crystals, while Raphides are entirely absent. In certain cases,
however, in these plants also the latter occur exclusively or in preponderating quantity.
Finally, a few further examples may be added to the above. For further details the
reader must be referred to the authors cited, and for the phenomena in the bast of
Dicotyledons to Chap. XIV.

Clustered Crystals occur exclusively or greatly preponderate in the foliage of Cheno-
podiaceae, Caryophyllaceae, Cactaceae, Lythraceae (Gulliver), and very many other families :
and, according to Sanio^, in the bast of Juglans regia, Rhus typhinum. Viburnum Oxy-
coccos, V. Lantana, Prunus Padus, Punica granatum, Ptelea trifoliata, Ribes nigrum,
Lonicera tatarica.

Solitary klinorhombic crystals in the foliage of Citrus : in the bast of species of Acer, Poma-
ceae, of Quillaja Saponaria, Robinia pseudacacia, Virgilia lutea, Melaleuca styphelioides,
Ulmus campestrls, Guajacum, Berberis vulgaris®, &c. Also in Abies pectinata.

Sanio found solitary klinorhombic crystals and clustered crystals together in the bast of
Quercus pedunculata, Celtis australis, .iFsculus Hippocastanum, Hamamelis virginiana,
Morus alba, Salix cinerea, Fagus sylvatica, in species of Populus, Gleditschia triacanthos,
Carpinus Betulus, Ostrya virginica, Corylus Avellana, Tilia parvifolia, Spirsea opulifolia.

Solitary klinorhombic crystals together with clustered crystals, and sacs containing granules,
are found in the bast of Betula verrucosa, and’Alnus glutinosa (Sanio).

Sacs containing granules alone, in Sambucus nigra.

Raphides are absent in the examples of Dicotyledons hitherto enumerated. They are
numerous and preponderate in the leaves of species of Galium and allied genera, in the

* Hegelmaier, Lemnaceae, p. 33.

“ For details of the cortex of woody dicotyledons cf. Sanio, /. c. ; there are very numerous details
respecting the leaves in Gulliver, Annals and Magazine of Natural History, vol. XI, XII, XIII, XIV,
XV, XVI.

® Hanstein, Milchsaftgefässe, p. 36.

* Monatsber. d. Berliner Academic, April, 1857. ^ Compare Sanio, /. c.

SACS CONTAINING MUCILAGE.

143

foliage of Vitis, Cissus, and Ampelopsis : in Vitis they are found in the wood also, and,
together with klinorhombic crystals, in the bast, in the cortex of Cinnamonmm Zeylani-
cum, and 0!ea Europea: in the foliage (leaves and stems) of species of Impatiens, Me-
sembryanthemum, and Phytolacca, in Nyctagineae, and CEnothereae.

It will be seen from what is above stated that here also differences occur in equivalent
parts of plants closely allied to one another and of similar habit, and that one cannot
designate any form of crystal-containing sacs as a general peculiarity of a family, or as a
phenomenon of adaptation. Among the often-quoted Solanaceae the majority {e.g.
Solanum tuberosum. Dulcamara, species of Nicotiana, Scopolia atropoides, Jochroma
Warczewiczii) have very numerous sacs with granules throughout the parenchyma of
the stem (in the leaves often clustered crystals). Jochroma coccineum has granule-
containing sacs in large quantity in the parenchyma of the pith, but in the cortex only
solitary prismatic crystals : in Atropa Belladonna the granule-containing sacs are entirely
wanting in the foliage : it has been above noted that crystal-containing sacs are com-
pletely absent in Petunia. All these statements hold for true crystal-containing sacs,
and it must always be borne in mind that, besides these, smaller solitary crystals of all
forms may occur in the contents or in the membranes of other tissue-elements.

The crystal-containing sacs appear while the tissues are still young, usually
when the tissues begin to differentiate ^ ; in the leaf of Citrus, according to Pfitzer,
their formation begins when it is about 3 ctm. in length: they are developed in
greater quantity only in the almost fully unfolded, but still tender leaf, when its cells
attain their last definitive extension and thickening of the membranes. They retain
unaltered through life that size and structure which they have attained when the
differentiation of tissues is complete.

In connection with the crystal-containing sacs must be mentioned the occurrence
of cells containing Cystoliths, which are found in the Acanthaceae, and many Urti-
cacese (Pilea), in the epidermis of which they have been described above (p. 105),
also scattered in the parenchyma of the cortex, and even of the pith. As regards
their structure, all that has above been said of the same structures in the epidermis
holds good.

2. Sacs contahiing mucilage.

Sect. 33. When vegetable mucilage and gummy bodies, occurring within the
tissues, do not belong to the cell contents of assimilating parenchymatous cells (as is
the case with the plentiful mucilage in roots of Borraginacese, e. g. Symphytum,
Cynoglossum, &c., or that of mucilaginous sappy parenchyma, e. g. in species of
Aloe, comp. p. 116), they fill completely or almost entirely the cavity of special
mucilage-containing sacs Such sacs occur in the parenchyma of the Malvaceae,
Tiliacese, Sterculiaceae, in the cortex of the officinal Lauracese, the Ulmeae, the Cac-
taceae ^ and of the tubers of Orchis, also in the cortex of the firs (Abies pectinata
and its allies). They are in all cases distinguished from the cells of the surrounding
parenchyma by their greater size, and are distributed between these either singly, or

* Compare Hilgers, l.c.

^ Trecul, I’lnstitut, 1862, p. 314. — A. B. Frank, Ueber die Anatom. Bedeutung, &c., vegetab.
Schleime ; Pringsheim’s Jahrb. V. p. 161, Taf. XV, XVI. — Idem, Zur Kenntniss d. Pflanzenschleime,
Journ. f. pract. Chemie, Bd 93.

® Schleiden, Anatomie d. Cacteen, p. 8, where, by the way, the structure is not riglitly re-
presented.

144

SECRETORy RESERVOIRS.

in groups or rows, usually without any clearly recognisable order. In the tubers of
Orchis they appear fairly regularly as wide sacs completely filling the meshes of a
network, which is composed of plates of starchy parenchymatous cells, one or more
layers thick, which face in all directions ^ When lying in water they appear as
intercellular cavities filled with swollen mucilage, and were described as such by the
older anatomists ^

If the swelling of the mucilage be prevented, e. g. by treatment with alcohol, the
space enclosed by an outer cellulose membrane appears either entirely filled with
the firm mass of mucilage, or partially, so as to leave an unimportant central cavity.
The mass of mucilage shows in the majority of cases — Malvaceae, Cactacese Laura-
cese, — the structure of a very thick, abundantly and delicately stratified cell-wall; it
often has even pits (Malvaceae), and is, as regards its origin and morphological signifi-
cance, nothing more than a cell-wall which has thickened strongly at the expense of
the internal cavity. According to Trdcul’s statements this point may, it is true, be
doubted, and new investigations desired. In other cases, and as types of these the
tubers of Orchis may be named, the mass of mucilage has no such stratification : it
developes from a drop of mucilage, which appears first like a vacuole within the
protoplasm, and surrounds a bundle of Raphides lying near to the nucleus : this drop
as it grows completely displaces the protoplasm and nucleus, while the bundle of
Raphides remains imbedded in the mature mass of mucilage. The sacs in the cortex
of the silver-fir appear to correspond to these in structure, their development remains
to be more accurately investigated.

In later stages of life the sacs often appear swollen up in the living plant to
structureless masses (e. g. in Althaea rosea) : these then fill up cavities in the pa-
renchyma of various form and size, according as they have been derived from the
swelling of one or several sacs. These latter structures may best be connected with
those mucilage-containing cavities, originally derived from a group of swollen
mucilaginous cells, which are described in the parenchyma of the Lime (cortex,
leaves, bud-scales *) : also the ‘ gum ’-bearing cells, and cavities formed by the
swelling of these, which were described by Tr^cul ® in the parenchyma of the branches
of Conocephalous naucleiflorus. This author himself doubts whether the gum-
cavities of the species of Quiina described by him ® belong to this category, or to the
products of secondary disorganisation. Subject to the same doubt, the small masses
of mucilage scattered through the parenchyma of the stem and leaf of Welwjtschia
may be mentioned here.

The sacs with which we are now dealing may be distinguished from those
gummy and mucilaginous products of disorganisation which may be derived in a
secondary manner from the most various tissues, by their originating directly from
the meristem ; they often appear as its first recognisable product of differentiation,

^ Frank, l.c. — Berg, Atlas z. pharm. Waarenkunde, Taf. 23.
^ Meyen, Secretionsorgane, p. 22.

^ Wigand, in Pringsheim’s Jahrb. III. p. 149, Taf. VII. 6.

* Frank, Beitr. z. Pflanzenphysiologie, p. 113.

® Comptes Rendus, tom. LXVI. p. 575 (1868).

® Ibid, tom. LXIII. p. 717 (1866).

’ Compare Hooker, Welwitschia, pp. ii, 19.

SACS CONTAINING RESINS AND GUM-RESINS.

145

differing from the cells of the surrounding parenchyma in their more rapid growth,
and in the absence of any formation of even transitory chlorophyll or starch. It is
plain that the cases of subsequent swelling of sacs are to a certain extent connected
with those of secondary disorganisation. On the other hand, it cannot be denied
that these structures are closely related to mucilaginous epidermal cells (p. 73), and
sacs containing raphides (p. 139), and to sclerenchymatous elements, e. g. those of
the bark of Punica.

3. Sacs containing resins and gum-resins.

Sect. 34. Sacs, which from the moment of differentiation of tissues are per-
manently filled with the above-named bodies, the resin being usually accompanied
by ethereal oil, occur as characteristic components of numerous families, or of single
genera and species ; in the latter case they usually represent at certain places the
intercellular reservoirs, which occur in other parts of the same plant (e. g. Tagetes,
Lysimachia), comp. Sect. 50.

Taking the extreme cases into account, we may distinguish two forms of these
sacs, short and long. The former are of almost iso-diametric and usually roundish
form, and have thin, smooth, homogeneous membranes which, in the cases hitherto
investigated (Taurus, Camphora, Acorus Calamus, Zingiberacese, Canella), give when
mature a yellow instead of a blue coloration with iodine and sulphuric acid, and
are not destroyed by strong action of the acid Protoplasm is apparently absent in
the mature sac, which is completely filled by one homogeneous variously- coloured
drop of resin, or by an aggregation of several of these. Sacs of this category lie
solitary, or in small groups in the parenchyma (primary or secondary), with the cells
of which they are strongly contrasted by their highly refractive contents, and are
often distinguished by their more considerable size, in the Zingiberacese, Acorus,
Piperacese, Lauracese^ Magnoliaceae (Magnolia, Drimys, Liriodendron^) Canellaceae,
in the cortex of Croton Eleuteria, and its allies (Cascarilla bark), Galipea officinalis
(cortex Angusturse ‘), and Aristolochiaceae. In the majority of the above groups
and genera the sacs in question are the only reservoirs of the characteristic secre-
tions. But Galipea has also, according to the statements of Engler ®, intercellular
reservoirs in the primary parenchyma.

In the root of Acorus ® Calamus and gramineus, the inner of the two superficial
layers is composed of regular prismatic resin-sacs. Van Tieghem ascribes a similar
structure to the roots of Xanthochymus pictorius, and Rheedia lateriflora, in which
the intercellular reservoirs present in the stem and leaf are absent, and are replaced
by these sacs (comp, below. Sect. 50). It remains to be investigated whether the
hypodermal layer of tissue containing drops of oil and resin, which is described in
the roots of Valeriana, belongs to this category’.

' [Compare Zacharias, Bot. Zeitg. 1879, p. 617.]

- Unger, Anatomie und Physiol, p. 210. ^ Treviranus, Beiträge, figs, 34, 35.

* Compare the figures in Berg’s Atlas z. Pharm. Waarenkunde, which refer to the above-named
barks and other drugs produced from the families above cited.

® Studien über d. Rutaceen, &c., Halle, 1874.

Van Tieghem, Struct, des Aroidees, Ann. Sei. Nat. 5 ser. tom. VI. p. 175.

’ Compare Meyen, Secretionsorgane, p. 63, Taf. VI. fig. 22.

L

SECRETORY RESERVOIRS.

146

In the primary parenchyma of the roots of many Lysimachias and Myrsinese
also, and in the secondary parenchyma of many Compositse, sacs are found as sub-
stitutes for the intercellular reservoirs which are present in other parts of the same
plants; their mode of occurrence will be more accurately stated in the subsequent
section which deals with these structures.

The development of the short resiniferous sacs, and especially the history
of development of their contents, is still uncertain, and requires thorough in-
vestigation.

I have termed the other category long sacs because they either permeate the
tissues as long tubes, which are simple, i. e. arise from one greatly elongated cell,
retaining its original wall, and are arranged singly or in longitudinal series, or they
form long series, which follow a similar course, though the single members of these
are but little elongated. These two special forms may graduate into one another
in a single plant (e. g. in the Convolvulacese) according to the extension of the
members to which they belong.

Most of the long sacs here grouped together have been only partially investi-
gated, or very unequally in their different relations, it is therefore possible that to
a certain extent quite heterogeneous structures stand provisionally side by side ; the
features common to them all are sufficiently indicated by the present treatment of
them. The contents of the sacs in question, at least when fully developed, usually
consist of a milky mass of resinous bodies (in the widest sense) and watery solutions
or mucilage. Their distribution varies in special cases; the rows of sacs in species
of Allium and the sacs of the Cinchoneae permeate the parenchyma alone. But
most of these structures accompany the vascular bundles or lie in the secondary bast,
being arranged more or less similarly to the laticiferous tubes of various families.
In many plants, e. g. certain Aroideae and Musaceae (comp. Sect. 47), they occupy
exactly the position which is held by the laticiferous tubes in other nearly allied forms,
these being absent in the above plants. All this points to a near relationship, both
morphological and physiological, with the laticiferous tubes, or at least with certain
organs ascribed to this category ; many of the sacs in question are hence frequently
described as laticiferous tubes. It has at all events been assumed for many of them
that they arise, as is the case with articulated laticiferous tubes, by coalescence of
longitudinal rows of cells, a view which is generally incorrect. When however the
sacs of this category are arranged in a linear series, e. g. in Convolvulacese, Acer,
Allium, it appears that such coalescence may occur here and there for a short
distance by perforation of transverse walls, or even of thin pits on the lateral walls ;
it is however always difficult to distinguish (and I have not been quite certain in any
case) whether the perforations observed are spontaneous, or were formed in the
process of preparation.

The organs to be placed in this category are the following : the series of sacs
filled with latex in species oi Allium, and perhaps also of Aloe ; those to be described
below (Sect. 47) in the Aroidecz, and Musaceas ; the series of sacs described as
‘laticiferous vessels’ in the ConvolvulacecB : and closely allied to these in structure and
arrangement are those of the Sapoiacece (Sideroxylon, Bumelia, Isonandra), those
which run along the vascular bundles of many Cynarece, and finally those of Sambucus,
Cinchona, Ladenbergia, and Acer. It must remain undecided whether the resiniferous

SACS CONTAINING RESINS AND GUM-RESINS.

J47

elements in the parenchyma of the secondary wood of Coniferoe belong to this series.
(Comp. Chap. XIV.)

As doubtful cases may here be mentioned the sacs filled with pigment, in
Sangtiinana, Glaticiian, and Macleya, which will be again noticed in Sect. 47 ; the
sacs or cells containing gum-resin which accompany the vascular bundles of species
of Aloe] lastly, the various ‘cells’ filled with peculiar pigments (usually watery
solutions), such as those in the roots of Rheum \ Rubia, &c.

The following particulars may be quoted concerning those organs of this
series which have been more accurately investigated.

I. Hanstein" discovered in those species of Allium in which he sought them (A. Cepa,
fistulosum, ascalonicum, &c.), large wide sacs, which he grouped with the series of
raphide-containing sacs of other Monocotyledons, as sac-like vessels (vesicular vessels).
As regards their form and arrangement they closely resetnble these latter sacs, e. g. those
of the Amaryllidaceae, though they differ from them in structure, and especially in the
character of their contents.

In the scales of the bulb of species of Leek they appear as numerous opaque lines,
which are visible to the naked eye, and run longi-
tudinally like nerves. They lie near the outer
surface of the scale, between the second and third
layers of parenchyma. The single sacs are of
circular transverse section, and wider than the
cells of the neighbouring parenchyma which abut
closely upon them ; they are much longer than
broad ; are often somewhat swollen below their
flattened ends, and are arranged in longitudinal
series one above another (Fig. 56).

At the base of the scale they are often shorter
than above, and not unfrequently bear sac-like
branches, which connect neighbouring series with
one another as transverse or oblique anastomoses:
here also rows of sacs occur in close longitudinal
aggregation.

The sacs are filled with a granular cloudy fluid,
which appears to the naked eye on the surface of
sliced onions as a pale milk, in the sac itself it is
cloudy, but still transparent. The nature of the
constituents of these contents has nof yet been
thoroughly investigated: I have not been able
to substantiate the obvious conjecture that they
especially contain the oil of garlic. Raphides or
other crystals are entirely absent. A large,
slightly elongated nucleus is still to be found in
sacs that are not too old. The walls of the
sacs are colourless and delicate, so that in sec-
tions they are squeezed in laterally by the turgescent parenchymatous cells: where
they touch the latter they are smooth or have solitary small round pits : but over
the whole surface where two sacs are in contact with one another the wall is, like
a coarse sieve, covered with crowded, round — not perforated — pits, while between these
lie rather thick bands of membrane. This is the case both with the transverse walls.

’ Unger, Anat. p. 206.

FXG.s6. — AlliumCepa ; longitudinal section tlirough
a bulb-scale. ^ epidermis, t cuticle, / parenchyma ;
between the first and second layers of parenchyma
ends of two sacs which abut on one another and are
divided by the pitted transverse wall q — g ; they are
represented as cut in half longitudinally ; j’^their con-
tents coagulated by potash ; the longitudinal wall
behind this abuts on another lower sac, and shows
ac^oVdffigly the pitting mentioned in the text. From
•Sachs’ Textbook.

L 2

L.C., compare p. 139

148

SECRETORY RESERVOIRS.

and with the less common lateral surfaces of contact of sacs which are contiguous
longitudinally.

The rows of sacs are continued into the foliage leaves: they have here a similar
position and structure to those in the scales of the bulb, but the sacs are much more
elongated, and the fluid contents less cloudy. Similar sacs to those of Allium have
hitherto been found only in species of Triteleia (Hanstein).

2. The ‘Sap cavities’ in the leaves of officinal and other species of Aloe are of doubtful
relationship to the structures under discussion. They accompany the longitudinal vascu-
lar bundles in the form of a band of prismatic sacs, which presents in transverse section
a semicircular multiseriate appearance : these sacs have flat ends and are arranged one
above another in longitudinal rows. The length of one sac varies, according to Trecul’s
measurements, e. g. in A. vulgaris, from o‘4o“'" to I’jo™“, while the width is consider-
able, being, in the species above named, as great as o’ i o — o’ 1 3 They are thin-walled,
and are filled, according to the species, locality, and time of year, with ‘sap’ of varying
intensity of colour, or even with colourless ‘sap’ (Aloe arborescens, plicatilis), which is
homogeneous, or it contains suspended in it spherical drops, which vary in number, size,
and special structure. It has been asserted that by disorganisation of single sacs, cavities
are formed in the band, which contain the same ‘sap.’ It is not improbable that the
gum-resin, which is the ‘Aloe’ of the shops, is derived from these sacs, but even this is
not certain. The band is marked off from the surrounding chlorophyll-parenchyma by
a layer of apparently prismatic, rather flattened small cells or sacs, which often also con-
tain coloured sap, and in these Fliickiger saw, in A. soccotrina, after slow evaporation of
the ‘ clear, tenacious, beautiful yellow contents,’ distinct yellow plates (of Aloin ?) crys-
tallise out. For further details about these organs, which still require exact investigation,
see Unger, Anat. u. Physiol, p. 206 ; Fliickiger, Pharmacognosie, p. 106, and the detailed
account of Trecul, Compt. rendus, i Mai, 1871 ; Ann. Sc. Nat. 5 serle, t. XIV. p. 80.

Numerous species of Haworthia and Aloe ciliaris have, according to Trecul, no
secretory sacs.

3. In the stems of Sambueus (S. nigra, S. Ebulus) there occur in the cortex outside the
vascular bundles, and especially in the periphery of the pith, longitudinal lines, which turn
dark brown on drying, and in this condition have even been regarded as Fungi h Accord-
ing to Dippel’s exposition^, which I find to be confirmed in all essential points, these lines
consist of elongated, spindle-shaped sacs of very considerable length and breadth, which
are tapered at both ends. The transverse section of these sacs is round, and the breadth
varies between o’o25™“ and o’iöq““ (Dippel). It is stated by Dippel that the
length of the mature sac usually exceeds 18 — 20”'“: the only one isolated by him
without injury was 14 “"i in length. They seem to me to reach a considerably higher
figure, and even to equal the whole length of an internode, that is to attain a length of
20®“ and more: but it is difficult to decide this for certain, owing to the difficulty of
isolating them intact. At all events these lines consisting of sacs, which turn brown on
drying, traverse the whole length of the internodes, and even pass through the nodes
from one internode into the next. The membrane of the sacs is rather thin and
colourless: in older internodes it is thickened and .stratified, and has round or oval,
non-perforated pits. The contents are, when young, a cloudy, finely granular, rather
tenacious mass, which fills the whole cavity. In older stages this mass is often attached,
partially or entirely, to the walls, and the central cavity is then filled with an apparently
watery fluid : in old parts it assumes an homogeneous, firmly gelatinous character, and

' Compare Oudemans, Over eene bijzondere soort von buizen in den Vlierstam (Sambiicus nigra),
tot hiertoe voor een fungus (Rhizomorpha parallela Roberge) gehouden. Verslag. k. Acad, von
Wetenschappen, Natuurkunde, 2 Reihe, tom. VI. (1872).

“ Die milchsaftführenden Zellen der Holunderarten. Verhandl. d. Nat. Vereins f. Rheinland
u. Westphalen, Jahrg. 22, pp. 1-9, Taf. I (1866).

SACS CONTAINING RESINS AND GUM-RESINS.

J49

a red-brown colour. The nature of the material composing this mass is not clear (comp.
Oudemans, l.c.). According to the reaction with salts of Iron it contains much tannin:
it swells in water, alcohol, ether, glycerine, alkalies, acetic acid : it diminishes in volume
in mineral acids and salts of metals : the originally colourless mass is coloured by most
acids (also by iodine and sulphuric acid), alkalies, and metallic salts (with exception of
compounds of iron) a reddish brown, by Schultze’s solution it is coloured blue. Carmine
and aniline colouring matters are taken up by it in very large quantity.

Each of these peculiar structures originates, as Dippel has shown, from one simple
cell, which grows to a great length. The observation of their development in the
youngest internodes leads unmistakeably to this view. In the highest internodes of
Sambucus nigra tangential longitudinal sections, which include the peripheral zone of
pith uninjured, show cells with the above characters of the contents scattered in the
parenchyma : these are easily brought into prominence by their deep aniline-staining ;
they are scattered through the parenchyma : the longest are almost of equal length to
the internode, the shortest are hardly twice as long as broad. In more elongated inter-
nodes, up to 5“m long, the first sacs are already considerably lengthened, while new
cells, some placed alongside, some above the first, assume the same characters. Such
conditions at first sight allow of the assumption that the sacs arise from rows of coalescing
cells, but this is confirmed by no direct observation. In older and quite mature sacs
the mass of contents is easily divided, especially after the action of potash into cylin-
drical pieces sharply limited by transparent bands: these resemble cylindrical cells
arranged in longitudinal series: but from evidence derived from direct observation of
young stages of development they can only be regarded as products of the action of the
reagent. Drawings like Fig. 9 of Oudemans, /. r., represent doubtless something else
than the development of the sacs.

4. The sacs containing gum-resin or latex which, according to Karsten, occur in all
species of the genera Cinchona and Ladenbergia, appear to be closely allied to those
of Sambucus They are found, like these, partly in the periphery of the pith, partly in
the young outer cortex, close to the bast layer. In many species (C. heterophylla,
obtusifolia, &c.) they remain small, and are difficult to recognise even in the cortex
when two years old. But in other barks, such as those derived from Cinch, scrobiculata,
ovata, umbellulifera, &c., they attain a width of 100 /x to over 300 /a, in C. lancifolia (?),
according to Vogl, even of 700 /a, and a length of at least several millimetres. As far as
I could see they have conical closed ends : Karsten’s statement that they arise from the
coalescence of longitudinal rows of cells certainly requires further investigation. The
sacs have a rather thick wall, which shows cellulose-colouring after treatment with potash.
Their contents, which include much tannin, are described as milky in the fresh state ;
in the dried bark they are so shrivelled that the sacs usually appear empty.

5. A great number of Cynareae®, and many Vernoniaeeae, have in the stem, petiole,
and stronger ribs of the leaf on the outer side of the vascular bundles, or of the fibrous band
which limits them, a group of sacs filled with a fluid made milky by numerous resinous (?)
drops: this exudes on cut surfaces in the form of small white milky drops, and is thus
visible to the naked eye. In old sacs the contents coalesce to a very glutinous string.
In many species, e. g. Lappa, Cirsium lanceolatum, the sacs are placed not only at the
outer, but also at the inner margin of the vascular bundles. The sacs themselves have a
spindle-like form ; they are closed at both ends, and attain in the mature plant a con-

' Compare Hanstein, /. r. p. 21.

^ Karsten, Die medic. China-Rinden Nen-Granadas, Ges. Beltr. p. 382. — Berg, Cliina-Rinden
d. Pharm. Sammlg. zu Berlin. — Idem, Atl. d. Pharm. Waarenkunde. — Vogl, China-Rinden d. Wiener
Grosshandels. — Flückiger, Pharmacognosie, p. 566.

” Trecul, Des vaisseaux propres . . . des Cynaiees laiteuses . . . L’Institut, 1862, p. 266. — Vogl,
Ueber Milchsaftgefasse in der Klette; Botan. Zeitg., 1866, p. 193.

SECRETORy RESER VOIRS.

150

siderable length, exceeding 3 — 4 ; they have a moderately thick membrane which

shows no important peculiarity.

The above sacs may occur in certain species of a genus, and be absent in others. Trecul
found them in Cirsium arvense, oleraceum, lanceolatum, anglicum, palustre, prsealtum :
Carduus nutans, crispus, tenuiflorus, Onopordon acanthium : Carlina vulgaris, longifolia,
salicifolia: Jurinea alata, Notobasis syriaca, Tyrimnus leucographus, Galactites tomen-
tosa, Durixi, Silybum marianum, Echenais nutans, Arctium lanuginosum ; Lappa com-
munis:— Vernonia eminens, noveboracensis, preealta: — but they are absent, according to
the same author, in Vernonia flexuosa Sims., and in the Cynarese of the genera Cynara,
Rhaponticum, Acroptilon, Serratula, Carduncellus, Centaurea.

6. The secretory sacs of the species of Acer’, which are usually called laticiferous
vessels from their milky contents, are of cylindrical prismatic form (on the average
about i'"™ long and 50-60 /li broad in A. platanoides), and are arranged in rows longitu-
dinally one upon another. Their colourless cellulose walls are as a rule completely
closed, the terminal surfaces, which fit one on another, are horizontal or oblique, the lateral
surfaces often have short sac-like protrusions, and with these pit-like thinner-walled pro-
trusions they press sometimes between the limiting surfaces of neighbouring parenchy-
matous cells, sometimes on the lateral walls of other similar sacs. In surface view these
thinner-walled protrusions appear as broad, round or transversely elliptical, clearly
marked pits, which are smooth, not latticed. I never saw any perforation of the ends.
The open lateral communications described by Hanstein, between neighbouring sacs, by
means of perforated lateral protrusions, I was also unable to find in sections which had
not been macerated : but they were often found just as represented by Hanstein Q.c.,
Fig. 6) in macerated preparations, even if these (from A. platanoides) had been prepared
only by boiling in water. The contents of two sacs, coagulated by boiling into masses,
then hung together by a short bridge, which loosely filled a corresponding canal. How
far these conditions exist in the living plant, or have arisen as products of maceration,
i.e. by rupture of a closed lateral protrusion due to boiling, I must leave undecided.

The sacs are solitary or in groups of 2-4, surrounded by parenchyma: they lie on the
limit between the phloem of the vascular bundle and the bundle of sclerenchymatous
fibres, which surrounds this externally, at that point in fact where in other plants the
first-formed sieve-tubes stand (comp. Chap. VIII): they are also found in the primary
bark of branches, and in the petiole and ribs of the leaf. They do not extend further into
the parenchyma of the leaf, nor are new ones formed in the secondary bast. Among
the species investigated, they are largest and most numerous in A. platanoides. They
are developed very early in the internodes, and seem to have special significance during
their early stages ; still, according to Hartig, they remain filled with sap in the
branches of A. platanoides for about ten years. In A. saccharinum and monspessulanum
their sap appears, according to Hartig, not to be milky.

7. The peculiar resins of the Convolvulacese ^ occur in sacs, sometimes as nearly
homogeneous masses, but more frequently forming with watery solutions milky mix-
tures: these sacs have usually been called ‘laticiferous vessels’ from the latter peculiarity
of their contents.

The sacs have been observed in all investigated herbaceous species : they are found,
according to the species, in stem, roots, ribs of leaves, or only in certain of these parts :
they occur especially in the parenchymatous cortex, and in the bast of the stem and
roots : they are ranged one above another in rows, which run longitudinally through the
members, and are isolated, or numbers are grouped together: the latter is the case.

^ Hartig, Naturgesch. cl. forstl. Culturpflanzen, p. 545. — Botan. Zeitg. 1862, p. 98. — Hanstein, l.c.
^ Trecul, Des Laticiferes des Convolvulacees ; Comptes Rendus, tom. LX. (1865), p. 825.: —
A. Vogl, Ueber Convolvulus arvensis ; Schriften d. Wiener Zool. Bot. Gesellscli. 1863, p. 258. —
Idem, Zur Kenntn. d. Milchsaftorgane d. Pfl. ; Pringsheim’s Jahrb. bd. V. p. 31.

5'/lC6' CONTAINING RESINS AND GUM-RESINS. 151

especially in the tuberous roots of Ipomoea Purga, where they form numerous annular
zones

Each single sac of a series in slightly elongated members, e.g. in the tuberous roots
cited, is short, being not longer, or even shorter than broad : in elongated internodes
they attain a considerable length, and an extended cylindrical form with flat or slightly
curved ends.

The contents of the sacs are a mass of resin, mixed to a variable extent with watery
fluid, and presenting therefore an appearance which varies in different cases (comp.
Trecul, l.c.) ; in many investigated cases it contains tannin. The walls are thin, homo-
geneous, and apparently soft, and show, as far as investigated, no cellulose coloration.
With iodine and sulphuric acid they turn yellow: long treatment with the acid does not
destroy them.

On the fresh surface of section through the sac-bearing parts the contents of the sacs
exude as ‘latex,’ the more abundantly the longer and more numerous the sacs are. In
fresh plants it often appears that the pressure of the contiguous turgescent parenchyma,
which presses the milky fluid out from the cut sacs, can burst, also the transverse walls,
which are not touched in cutting the section, and press out the contents of more deeply-
seated members of a series of sacs at the surface of section. I was unable to prove to
myself (after investigations on stems and rhizomes of Convolvulus arvensis, Calystegia
sepium, dahurica, Pharbitis hispida) either that there is a perforation or solution of the
septa within the living plant, and a formation in this manner of long sacs by the coales-
cence of shorter ones, or that there is a genetic connection, as stated by Vogl, between
long sacs and sieve-tubes.

8. The reservoirs of the milky secretion in the Sapotaeeae resemble those of the Con-
volvulaceae in many points. Since these are but little known, a report by Herr K. Wil-
helm upon an investigation of them conducted by him may be here inserted. He
investigated especially Bumelia tenax W., and Sideroxylon mastichodendron Jacq., with
which, as far as can be concluded from comparison of dried material, Isonandra Gutta
coincides in the main.

The latex of these plants occurs in completely closed sacs, which are always surrounded
by parenchymatous elements, and differ from these fundamentally only in their contents.
This is literally true for the Inner cortex, the reservoirs of latex here found have exactly
the form and size of the neighbouring parenchymatous cells. In the outer cortex and in
the pith the laticiferous elements are usually distinguished from the rest by their con-
siderable length and breadth, as well as by their arrangement in uniseriate strings, which
run longitudinally through the axis in question, and may be followed to within a short
öxsimce oi the punctum asegetatianis. The outer cortex and pith are thus traversed by
single rows of laticiferous sacs, which are arranged, at least in the youngest parts of the
stem, radially and tangentially perpendicular 4 new elements are constantly added to
these from the apical raeristem. As the rows pass downwards in the stem, their originally
parallel arrangement is disturbed by the increase of the intermediate parenchyma : they
suffer tensions and fractures, — their several parts however remain nevertheless in con-
nection, and at the same time their character as series of distinct sacs is retained. No
single case was observed which necessitated or even supported the assumption that,
in the living plant, a coalescence of neighbouring members of tubes had taken place as a
typical occurrence.

Also in the inner cortex, in the phloem of the vascular ring, it was never possible to
prove with certainty a coalescence of parallel laticiferous sacs, or of such as touched at
their ends, so as to form extensive reservoirs. In tangential sections the primary ar-
rangement shows no regularity: they usually lie scattered and solitary, but sometimes
several occur near to, or one above another, between large parenchymatous cells of
similar form. In radial sections they appear sometimes to form long longitudinal

Compare Berg. Allas, Tab. XXIII.

152

SECRETORY RESERVOIRS.

strings. But careful investigation and comparison of corresponding transverse sections
shows that they never appear in the jame radial planes (or only in rare cases, and then
only few of them), but in the large majority of cases in different planes.

As the laticiferous sacs of the bast-ring draw nearer to the outer cortex, they lose the
milky nature of their contents : they appear constantly more watery, while the sacs them-
selves become gradually more and more compressed, and finally unrecognisable.

The above relations of distribution and arrangement of the laticiferous reservoirs hold
also for the petiole. In the lamina laticiferous sacs appear as elements or concomitants
of the nerves, or are found here and there solitary in the parenchyma, in which case
they are always characterised there by considerable size.

The membrane of the single laticiferous sacs, whether in the outer or inner cortex, the
pith, or the leaf, appears in the large majority of cases of equal thickness throughout.
It is as a rule -very thin, and equal to that of the neighbouring cells of the parenchyma, or
even thinner. However I recognised in many sacs of the inner cortex partial thickenings
of the walls : these appear distributed at many points : they appeared swollen, and a slight
protuberance of the outer surface of the cell usually corresponded to such points. — The
membranes of the laticiferous sacs are colourless ; they give a blue reaction with Schultze’s
solution, but usually the colour is fainter and less pure than that of the surrounding
parenchyma.

The contents of the laticiferous sacs have sometimes the character of an emulsion, and
accordingly appear white by reflected light, showing under the microscope as a finely
granular dark mass — this is always the case in the reservoirs of the inner cortex ;
sometimes they form more or less refractive plugs resembling homogeneous masses of
resin : these are usually colourless, or light yellow, and fill the cavity of the sac com-
pletely. These plugs can easily be isolated from sections under water, when their be-
haviour under solvents can be investigated. They occur chiefly in the outer cortex.
Carbon disulphide, chloroform, and benzol dissolve the mass almost completely: ether
leaves a considerable granular residue. Alcoholic solution of iodine colours it golden
yellow. Addition of alcohol removes the resinous appearance, and makes the mass itself
dark and finely granular. If freshly isolated pieces of latex be exposed to concentrated
sulphuric acid, they are dissolved gradually with a yellow colour ; dilute sulphuric acid
first produces swelling, while homogeneous drops escape from the substance, the outline of
which soon becomes indistinct ; their substance then gradually dissolves in the surround-
ing fluid, and colours it yellow. Meanwhile the original contour of the string of latex is
retained : it appears that one substance is extracted while the other remains undissolved.
The latter was in many cases found still undissolved after long immersion (two days) in
sulphuric acid. Potash produced no apparent change.

In many laticiferous sacs of the outer cortex dusky-looking contents may be found, con-
sisting of numerous drops of the most variable size : they dissolve immediately in water.
One is tempted to assume that the before-mentioned resinous contents develop gradually
from this latex, which is easily soluble— they occur however in the highest regions of the
stem immediately below the punctum vegetationis — nevertheless this phenomenon de-
serves attention, viz. that the resinous plugs in the outer cortex assume, after treatment
with alcohol, an appearance which coincides remarkably with that of the contents of the
inner cortex. The latter may be completely dissolved by warming with dilute potash :
when treated less strongly with this reagent there are sometimes formed from it
numerous small crystals or isolated large ones, which disappear quickly on adding acetic
acid.

The laticiferous reservoirs of the pith were not accurately investigated as regarded
their contents, which coincided exactly in optical properties with that of the sacs in the
outer cortex ^

^ [On Rcsin-sacs in Hypericum, com2:)are v. Höhnel, Bot. Ztg. 18S2, 149.]

TANNIÄ^-SACS.

153

4. Tannin-Sacs.

Sect. 35. The secretion of Tannin in large quantity in the sacs of Sambucus
suggests the idea of placing other sacs or cells with large quantities of tannin in the
category of secretory sacs. It is true the presence of this body in large quantity is
not decisive, since it occurs also in other places, as in the epidermal cells, and in
many plants, especially ligneous ones, particularly in the assimilating, starch-forming
parenchyma, and since, as far as we know at present, it is at least undecided whether
it appears as a secondary product of the constructive metastasis, as is the case with
calcium oxalate, or as an integrating transitional member of it. Further, with the
exception of the tannin, too little is known of the structure, and especially of the
character of the contents of the organs, which are possibly to be distinguished as
tannin-sacs, for us to be able to decide whether, and when, they are to be regarded
as secretory sacs, or only as parenchymatous cells rich in tannin. But there are a
number of organs which, as far as may be concluded from information at hand, have
apparently lost the properties of ceils, and are the points of secretion of mixed
substances requiring further investigation, amongst which tannin takes permancjitly
the most prominent place under the reagents at present in use : these organs
correspond further in many cases, in their early appearance and position with regard
to the vascular bundles, to the secretory sacs of Sambucus, the Cynareae, Aceraceee,
&c., also to many intercellular, secretory reservoirs, and may therefore be substitutes
for these. Awaiting more exact investigation, and excluding such as contain starch
as well as tannin, we may here introduce these organs as Tannin-sacs.

They occur as elongated sacs, especially near to the vascular bundles, in the
parenchyma of the stem and petiole of many Ferns (Marsilia, Polypodiaceae,
Cyatheacese, Marattiaceae &c.).

Among the families of Älonocotyledons, the Araceae and Musaceae should be
mentioned as having those rows of sacs, to be described in Chap. VI, which accom-
pany the vascular bundles. Also the laticiferous tubes of these plants, consisting of
coalesced sacs, would be better placed here than with the rest of the laticiferous tubes
in Chap. VI.

Of the Dicotyledons certain Leguminosae may without doubt be mentioned
here. In Phaseolus multiflorus Sachs ^ found in the phloem of the primary vascular
bundles of the stem and leaves (but not continued into the root) longitudinal rows of
prismatic tannin-sacs arranged singly or in small groups. They form in transverse
section a broken series of curves. A similar arrangement appears in a similar place
in the transverse section of the branches of Robinia pseudacacia ^ The sacs are
here 6-8 times as long as broad, cylindrical, with rounded ends, and only attached
to one another by the flattened middle of the terminal surfaces. A group of rather
wider series of sacs, with longer members filled with tannin, lies in these trees in

^ Von Molil, Baumfarne, Verm. Schriften, p. 113. — Martius, leones pi. Crypt. Brasil. Taf. XXXI
and XXXIII. Compare also Karsten, Vegetationsorgane d. Palmen, /. c. p. 205. — Trecul, Comptes
Rendus, Mai, 1871, and Ann. Sei. Nat. 5 ser. tom. XII. p. 373. — Russow, Vergl. Untersuchungen.

- Unters, über d. Keimung d. Schminkbohne, Wien (Acad.), 1859.

^ Haltig, Forstliche Cullurpfl. p. 546.

154

SECRETORy RESERVOIRS.

the pith, just opposite each of the vascular bundles : besides these, short sacs are
found scattered in the pith.

Many, but not all Leguminoste are rich in tannin, which is distributed in the
tissues in various ways, but in a very constant manner in single species, genera, &c.,
often without doubt in tissues that do not correspond. The same holds for the
Rosifloras. It remains to be investigated whether also, in many of the cases of the
occurrence of tannin enumerated by Trecul. we have to deal with secretory sacs^.

’ Compare Trecnl, Du Tannin dans les Legumineuses ; Comptes Rendus, tom. LX. p. 225. —
Du Tannin dans les Rosacees, Ibid. p. 1035. — See also Sanio, Bern, über den Gerbstoff u. s. Verbrei-
tung, (&c. ; Botan. Zcitg. 1S63, p. 17. — Wigand, Ibid. t862, p. i 21.

CHAPTER IV.

TRACHEA.

Sect. 36. Under the above name all those tissue-elements may be grouped
which have the following characters: (i) their walls become thickened as they
differentiate from the meristem, and lignified to a variable extent, while the
thickening is arranged in a fibrous manner, or with bordered pits, or rarely with
transverse bars ; (2) almost simultaneously the whole protoplasmic body and

organised contents of the cells, which are being transformed, disappear altogether,
their place being taken by air, or by clear watery fluid. The larger and more
elongated tubes falling under this definition were distinguished even by the old
anatomists S as Trachea, Vessels, Tubes (Vasa, Tracheae, Fislulse) ; more recent
investigations ^ have led to the recognition of two subdivisions, which, according to
Sanio, may be distinguished as (i) Tracheides, and (2) Vessels (Vasa) or Trachea, in
the narrower meaning of the word. I shall use the name Tracheae in this book only
as a collective term for both, and specially also in those cases where it is not
certainly decided whether a tube belongs to the one or to the other subdivision.

As will be thoroughly discussed in later chapters, the chief points of occurrence
of the Tracheae are the vascular bundles and woody bodies. It may however be at
once pointed out here by way of explanation that the above localities are by no means
the only ones in which Tracheae are found. Tracheides are found solitary in the
parenchyma (Sect. 55) in many plants, and form the root-sheath characteristic of the
aerial roots of Epiphytic orchids^.

Vessels and Tracheides correspond in the general points of structure : both alike
have very various special forms ; transitional forms are to be found between them,
which will be described more especially in the secondary wood (Chap. XIV). The
distinction between them depends entirely upon the mode of connection of the

' Malpighi, Grew, Anat. Plant. — Compare Treviranus, Physiol. I. p. 82 , Link, Philosoph. Bot.
p. 90, &c.

“ Sanio, Botan. Zeitg. 1863, p. 113. — Caspary, Monatsbr. cl Berliner Acad. July, 1862. — Caspary
terms the organs here called Tracheides, as far as they were the subject of his investigations,
‘ l.eitzellen.’

’ It need hardly be said that in e.xtending the term Tracheae to all tissue-elements which
correspond in the above peculiarities of structure, without regard to their place of occurrence, it
includes also the well-known air- or water-containing, usually fibrous-thickened elements of the leaves
of Sphagnum and Leucobryaceae, they being in fact for the most part Tracheides. Compare on
these elements of the mosses. Von Mohl, Verm. Sehr. p. 294, and Schimper, Monograph, d. Torf-
moose.

156

TRACHE.E.

elements one with another, and upon certain phenomena of structure of the walls,
which will be stated in the description of the latter.

The walls are, as already indicated, always interruptedly thickened,, the
thickening masses following the well-known rules for cell-membranes and being
either pitted, or forming fibrous bands, or both. The form of thickening is either
uniform over the whole wall of an element, and even of many contiguous elements,
or it varies at different points of one wall-surface, or on different sides of one tube,
according to the nature of the neighbouring tissue : these differences are found
to be especially frequent in the secondary wood (Chap. XIV). Vessels in which
these varieties occur have been called mixed (Vasa mixta)^.

That part of the wall of the Tracheae which is slightly or not at all thickened is
always a very delicate, almost immeasurably thin film.

According to the form of the thickening mass there are distinguished —

(1) Trachece. with fibrous ihicke7img hands, under which head are ranged —

(a) TrachecB with spiral fibrous thickening (spiral vessels).

(fi) Trachece with amiular fibrous thickening (annular vessels).

(c) Trachece with reticulate fibrous thickening (reticulate vessels).

( 2) Pitted or dotted Trachece.

(3) Trachece with tra^isverse bars (Tr. trabeculatae).

The Tracheae with spiral fibrous thickening (included in I. d) were terrhed true
tracheae (Trachdes katexochen) by Mirbel and P. de Candolle (Organogr. I), owing
to a false conception of the structure of these and other forms, while the annular and
reticulate vessels were called false Tracheae (fausses trachees) or striped vessels,
Vaisseaux ray^s, the latter being moreover confused with pitted vessels.

The above forms, especially those with fibrous thickening, often merge into one
another, so a's to form ‘ Vasa mixta.’ The pitted vessels show in many cases, as
will be more thoroughly detailed later, protuberances of the inner surface in the form
of fibres usually having a spiral course, more rarely in the form of transverse bars,
which traverse the cavity, and give the character to the form (3).

Sect. 37. In the walls with fibrous thickening the strengthening bands extend
inwards from the unthickened membrane, usually as relatively narrow flattened
ribands, appearing in section of elliptical or rounded-rectangular, or almost quadratic
form : in depth (i. e. perpendicular to the surface of the wall) they are less, or not
more strongly developed than in breadth (comp. Figs. 56*, 57). They are frequently
very flat, broad plates, in the latter case often broken by short, small slits or de-
pressions of the inner surface, e. g. in the spiral or annular tubes of Commelina
tuberosa®; rarely they are deeper than they are broad, e. g. the closely-wound fibres
of the later developed spiral vessels in the stem of Artanthe elongata and other
woody Piperacese, and especially the annular and spiral bands, like sharp fluting,
which protrude far into the cavity in the Tracheae of the stem of many Cacteae and

^ Hofmeister, Pflanzenzelle, § 25. — Sanio, /. r.

^ Compare P. Moldenhawer, Beitr. p. 185 ; Von Mohl, Verm. Sehr. pp. 278, 279.

^ Von Mohl, Ueber den Ban der Ringgefässe, Verm. Schriften, p. 285.

■* Schleiden, Mem. pres. Acad. St. Petersbourg, ser. VI. tom. IV. — Compare Grnndziige I.
(3 Aufl.) p. 239 — Trecul, Ann. Sei. Nat. 4 ser. tom. II. pi. 19.

TRACHEA.

157

of the leaves of many Mesembryanthema, e. g. M. stramineum. A less common form
of the fibres, corresponding to the bordered pits, is that of which the section has the
outline of a short-armed recumbent |— , while the fibre is attached to the thin wall by
the free end of the single (here horizontal) arm. This is the case in the closely-
wound spiral tubes, which show transitional forms to the reticulate, and, in many
woody stems, are first fully formed when the extension of the internode is ended,
as is the case in Artanthe elongata, Nerium, Convolvulus Cneorum. The single arm is

Fig. 56*. — Piece of an annular vessel from the stem of Zea Mais. Jt the thin wall, on which the limits of the adjoining cells are
visible, r annular fibres, r' one of these cut through, iy the strata of the same (550). From Sachs’ Textbook.

Fig. 57. — Saururus cernuus. Piece of a radial longitudinal section through a vascular bundle of the herbaceous stem ; r in-
most, narrow, distorted annular vessel. To the left of this successively (i) spiral vessel, with loosely wound single fibre, which
in two places runs back into itself, so as to form a ring *, the thin wall between the spirals of the fibre has given way ; (2) spiral
vessel with very narrow flattened curve, cut longitudinally in half, with exception of the upper margin ; (3) scalariform reticulate
vessel ; ysclerenchyma (or bast) fibres. The curves of the spiral fibres rise in the drawing in the opposite direction to the course
they really pursue (375).

in these cases almost always smaller than the two others; in the above-named Artanthe
it is very inconspicuous compared with the other strongly-protruding parts.

The Trachea with spiral fibrous thickening show considerable variety in the
number of the fibres, and the steepness and direction of their coils. Their
number is often only 1-2 in the narrow tubes, which are first formed when the
differentiation of tissues begins, in others 4 or more, and it rises in many cases, e. g.
the petiole of Musa, to 16-20. The steepness of the coils is greatest in those tubes
which are developed earliest, before the extension of the part to which they belong
has ceased : since in these the coils are separated from one another by the

TRA CHEM.

158

extension which the tube itself undergoes h By this process a spontaneous separa-
tion (tearing off?) of the fibre from the elongating wall may occur If the tube
developes later, during or after the close of the extension of the part, the coils are
less steep : when several fibres are present they are then arranged at the minimum
distance from one another.

The coils rise in most cases (when seen from without) from right to left, that
is like the thread of a left-handed screw, or, according to the terminology adopted in
Botany, the spiral is right-handed. The opposite direction occurs in Pinus
sylvestris (Mohl) : in the wood of Vitis vinifera, Berberis vulgaris, Artemisia Abro-
tanum, Bignonia capreolata, the inmost first-formed tubes are right-handed, the outer
later-formed ones left-handed. Where the spiral fibre is interrupted, both the
opposite directions of inclination may occur at different heights in one vessel, e. g. the
stem of Cucurbita

Not unfrequently, and especially in the closely- wound forms, the spiral fibres
are branched, or their coils are connected in a bridge-like manner by oblique or
transverse fibrous bands. It is a no less common phenomenon that one fibre at the end
of a tube, or at other points, should run back into itself, thus forming a ring. This
phenomenon characterises the series of numerous transitional forms between the
spiral tubes and the annular and reticulate tubes, while it gives rise in the latter to a
number of special forms of the net. It should be added, in connection with the
annular tubes, that the distance of the rings from one another follows the same rules
as the steepness of the inclination of the spiral fibres. Besides those above mentioned,
there is among the reticulated tubes a varied series of special conformations of the net.
Reticulated tubes, the meshes of which are elongated transversely, and arranged on one
surface of the wall in a row one above another, being thus comparable to the rungs of
a ladder, are called ladder-like or scalariform vessels, and have frequently been con-
founded with the pitted vessels, which show a like surface of wall. (Comp. Fig. 56*.)

Individual peculiarities of the tracheides in the sheath of the roots of Orchidacese
will be brought forward again in Sect. 56.

Sect. 38. It is well known from the general doctrine of cells, that it is only the
relative size of the unequally thickened parts of the membrane which gives rise to a
general distinction between reticularly thickened and pitted membranes, and that
therefore there is no sharp limit between these two forms. The wall of the pitted
tracheides shows sometimes simple pits, i. e. not bordered, sometimes bordered pits^.

The term pit is applied to a gap in the internal thickening of the wall, this gap
being closed externally by a piece of membrane which is only slightly or not at all
thickened. It is in fact a canal varying in length according to the extent of the

' Von Mohl, Veget. Zelle, p. 26.

® Compare Sachs, Textbook, 2nd Engl. Ed.p.go. With this must not be confused that ‘unrolling’
of spiral fibres, which occurs when a part is tom, and the often-cited ‘power of unrolling’ of spiral
vessels. The latter phenomenon comes about simply by the rupture of the delicate unthickened
membrane when the part is torn, while the tough fibre, to which the delicate and easily overlooked
tatters of the ruptured wall are attached, is drawn out.

“ Von Mohl, Verm. Schriften, pp. 287, 321. — Sanio, l.c. p. 124.

^ [Cf. Russow, Zur Kenntniss des Holzes, insonderheit des Coniferenholzes. Orig, communica-
tion to Bot. Centralblatt, Nos. 1-5, 1SS3, in which paper the other principal writings on this subject
are referred to.]

BORDERED PITS.

159

thickening, which traverses the wall transversely. If the canal be equally wide
throughout or narrowing outwards, we have a non-bordered pit. On the other hand,
the term bordered pit is applied to those in which the canal widens suddenly towards
the outside, i. e. towards the non-thickened part of the membrane, so that it is here
broader than at the part of the canal bordering on the internal cavity. In the
surface view of the wall the boundary of the unthickened portion of the membrane

I'lG. 58. — Pinus sylvestris; radial longitudinal section through
the wood of a branch ; a — e ends of tracheides with bordered
pits (/', in surface view ; c b piece of a young wall of a
tracheide, with still immature bordered pits ; further develop-
ment of these, successive narrowing of the canal a — r; d and e
juature condition; st large pits on the limiting surface be-
tween tracheides and cells of medullary rays (550). From Sachs*
Textbook.

Fig. 59. — Ephedra helvetica. Wood (230) ;
a member of a vessel, b tracheides seen from
the radial side, isolated by maceration with
Schultze’s mixture ;ythe oblique ends of the
member of the vessel in surface view, with
two rows of large open bordered pits ; at jc, at
two closed bordered pits. Of the tracheides,
b is only drawn in outline, the surface of the
other is put in. (The direction of the split-
like pits is reversed ; they rise in reality from
left to right.)

may be seen surrounding the limit of the section of the canal like a border or
halo (Fig. 58). The widened outer part of the pit, the limit of surface of which
is the halo, is called the cavity of the pit] in the canal itself may be seen the
outer aperture, which leads into the cavity of the pit, and the inner which borders on

l6o TR A CH EM.

the lumen (comp. Figs. 59-60). The cavity of the pit is in most cases originally and
often permanently of the form of a plano-convex lens (‘ half-lens-shaped ’), since the
outer surface of the thickening of the membrane, which borders it on one side, is
concave, while the unthickened portion of the membrane, which limits it on the
other, is flat. The canal is, according to the extent of the thickening of the
membrane, either extremely short, so that a sharp-edged opening leads from the
lumen of the tube into the cavity of the pit, e. g. in the thin-walled tracheides of
the spring wood of Pinus ; or, when the thickening is greater, it is elongated, and
widens outwards suddenly into the cavity of the pit, e. g. autumn wood of Pinus,
pitted vessels of Nerium, Fraxinus, wood-elements of Convolvulus Cneorum, Pteris
aquilina (Figs. 61, 64), &c.

The above general description of the structure of the bordered pit is said to
hold for those uncommon bordered pits, not belonging to our present subject, which
occur in certain cells^, and for those on the limiting surfaces between Trachese and
other elements ; and it is clear that between these and the non-bordered pits only
the above-described difference of form exists, which corresponds exactly to that
between the flattened and the \ — shaped fibrous thickenings. On the surfaces
abutting on elements of another order, the bordered pits of the Trachese either
correspond to non-bordered pits on the walls of these, or they are opposite to an
unpitted wall. But where Trachese with bordered pits are contiguous with one

another, the bordered pits correspond to
one another in such a way, that on each
limiting surface all the cavities of the pits
of the one fit exactly over those of the
other. The plano-convex cavities are thus
applied to one another in pairs, so as to
form the ‘ lens-shaped pit-cavities ’ (comp.
Figs. 58-62), and each of these is divided
by a thin flat lamella of membrane
(the limiting lamella) into two halves.
This is the case in the first instance in all
investigated cases. Also in mature Trachese
this condition always remains permanent,
as can easily be proved in old wood of
Pinus, Ephedra (Fig. 60 b) ^ As a rule
however the originally plane limiting lamella
grows in surface in such a way, that it be-
comes larger than the median plane of the
lens-shaped double cavity, and therefore
bulges in a convex manner to one side, and comes into close contact on this side
with one of the concave walls of the cavity of the pit (Fig. 60 c) ; meanwhile it
remains a very delicate film, but is always, in the cases investigated, thicker in the
middle than at its margin. In Pinus sylvestris (and its allies), as first shown by
Sanio, the thicker part has the form of a relatively broad plate with a sharply marked

Fig. 60. — Transverse section through the secondary wood
of Ephedra helvetica (600) ; m medullary ray, £—g vessel,
the section having passed through the oblique wall separat-
ing two members, and in fact through the middle of an open
bordered pit (pore), and to the left of this, through the
margin of a pit Besides this vessels and tracheides are
transversely cut at a and b, through the middle of bordered
pits of the lateral walls, which have the limiting lamella
thickened in a knob-like manner on both sides ; at c the
thickening is on one side.

‘ Compare the figures of the endosperm of Phytelephas (?). Schleiden, Grundzüge, 3 Aull. p. 232.
^ Compare Hofmeister, Pflanzenzelle, p. 175.

BORDERED PITS.

l6l

margin ; in Ephedra, which is the most striking instance I know of, it is shaped like
a flat biconvex lens ; in other relatively small, or at least narrow pits (Cassyta pani-
culata, vessels of Nerium, Pteris aquilina, &c.), it appears as a hardly perceptible
swelling. The thicker portion always lies, like a lid, upon the outer aperture of
one of the pit-canals.

The corresponding bordei'ed pits of neighbouring Tracheae are accordingly
closed by the limiting lamella, which occasionally remains plane, but as a rule is
applied to one wall of the pit-cavity. On account of its delicacy, and the small size
of the whole pit, the limiting lamella, in the form in which it usually occurs, has not
hitherto been clearly recognised. In contradiction to Hartig ^ alone, and following
the statements of Schacht and DippeP, the pit-cavity was regarded as being in the
mature state in open communication on both sides with the adjoining cavities of the
tubes, while the few cases in which the limiting lamella was observed were considered
as exceptions. Sanio ® has recently clearly proved that in Pinus sylvestris the case is
as stated above. I find his statements confirmed in all cases subjected to exact
investigation, both in the tracheides of the wood of that tree, and in those of Abies
pectinata, excelsa, Juniperus communis ; also in the tracheides, and lateral walls of
the Tracheae of Ephedra, and Welwitschia : further in the lateral walls of the ‘scalari-
form vessels’ of Ferns (Pteris aquilina) ; the tracheides of the secondary wood of
Dracaena, Cordyline paniculata; the Tracheae of the wood of Convolvulus Cneorum,
Statice monopetala, the large-pitted vessels of the wood of Cassyta (C. paniculata,
R. Br.), Nerium Oleander, &c. Extremely good preparations, which are not always
easy to obtain, always show the case as described : it should then be characterised

at least as the regular condition, and that which is distributed over the most various

divisions of the vegetable kingdom. Further investigations must show whether ex-
ceptions occur.

While the above fundamental conditions of structure remain constant, the
special form of the bordered pit is very variable (compare the Figures 58-62, and
what follows in Chaps. VIII and XIV) ; firstly, according to the length of the canal,

which depends upon the extent of thickening of the walls; this has already been

mentioned above ; secondly, according to the special form of the pit, and of the
canal with its outer and inner aperture, as seen most distinctly in surface view of
the wall, and according to the relative size of the diameter of these parts in each pit.

All these parts have the forms generally characteristic of pits, which (in surface
view of the membrane) vary in individual cases between a circle and a narrow slit.
In the same pit all the parts are alike in form, or very similar, as is the case in the
circular pits of the tracheides of the wood of Pinus, and the slit-like ones of most
scalariform vessels, thus giving rise to the appearance of the bordered pit in surface
view as two or three concentric outlines, which differ only in size (e. g. Fig. 58, 61 B).
On the other hand, the form of the parts may differ in the same pit, either so that they
all differ from one another, or one from the rest, and this may occur in all possible
combinations. Thus there is a narrow elliptical inner aperture, and a circular outer
aperture of the canal, which diminishes greatly towards the outside, with irregularly

* Compare especially Botan. Zeitg. 1863, p. 293.

^ Schacht, De maculis (pits), &c. Programm. Bonn, i860. — Dippel, Botan. Zeitg. i860, p. 329.

^ PringshSim’s Jahrh. Bel. IX. [See also Sachs, in Arbeiten des Bot. Inst, in Wiirzbnrg, II. p, 294.]

M

i62

TRACBEM,

circular outline of the relatively very large cavity in species of Cassyta; a narrow slit-
like inner aperture, and a very small circular outer aperture of the canal, with a
broadly elliptical cavity in Elseagnus acuminata : a long and narrow slit-like inner
aperture, and a short slit-like outer aperture, with a circular cavity in Aleurites
triloba — all these cases occur on the large-pitted vessels of the woodh

As regards the relative diameter of the different parts of a pit it is obvious,
from what has been premised, that it is always larger for the pit-cavity than
for the outer aperture of the canal. The latter is either as large as the inner
aperture, or it is smaller than this, and the canal thus becomes narrower outwards to

Fig. 6i. — Pteris aquiHna Rhizome ; A (t42) end, about ^ of a short member of a vessel ;
the oblique ladder-like end surfaces^ and a part of the lateral wall in surface view; B a
piece oi A atx, magnified 375 times’; C (375) thin longitudinal section through part of a
lateral wall, where two vessels touch one another ; D (375) a similar section through the
oblique waliyand its margin adjoining the lateral wall. At ythe pits are open.

a varying extent, and with a form corresponding to the above description : the inner
aperture, when of a slit-like form, and differing from the cavity, is always narrower,
but often longer than the greatest diameter of the latter. Slit-like bordered pits
placed close to one another may thus coalesce internally, in numbers from 2-6, into
a common slit, as Mohl found (Lc. Figs. 6, 10, 15) in Aleurites and Elaeagnus, and

’ See Von Mohl, Ueber den Bau der getüpfelten Gefässse, Linnsea, 1842; Verm. Schriften,
p. 272, Taf. XII.

BORDERED PITS, ETC.

163

Sanio {l.c. 125) in the wood of Tectona grandis, Fraxinus, Tamarix, &c. This must
originate in the thickening of the membrane lasting longer at the inner side than at
the outer, and altering its original direction at a later period. On the vessels of the
wood of Mahonia aquifolium Sanio found round bordered pits, arranged in left-
handed oblique series, with the inner apertures serially coalescent into long slits,
while between these the thickening of the walls protruded inwards in form of
spiral bands.

The arrangement of the bordered pits on a wall-surface differs in no way from
the known rules for the arrangement of pits generally. They are arranged on a
surface in perpendicular, horizontal, or, especially when of slit-like form, in oblique
spiral series : in the latter case the spirals are almost always left-handed : the number
of these series varies according to the special cases, and on equal areas it varies on
the whole inversely with the size of the pits. We may cite as examples of extreme
cases, on the one hand, the usually loose series of large round-bordered pits on
each radial face of the tracheides in the wood of Pinus, and the several loose series
of large pits on the wide vessels of the wood of Cassyta (Mohl, /. c. Fig. i) ; on the
other hand the walls covered with close and small pits, which are found surrounding
the large vessels in the vascular bundles of the stem of the Cucumber, the tubers
of Dahlia many Dicotyledonous woods, as Quercus, Nerium, &c., &c. ; in these
cases the margins of the pit-cavities are separated from one another by quite narrow
bands or ridges of the wall.

A special case of frequent occurrence may here be mentioned, viz. the trans-
verse, slit-like bordered pits, which are characteristic of almost all Ferns (Fig. 61),
and which also appear in many Dicotyledonous woods, as Cheilanthus arboreus,
Vitis ^ &c., these being arranged like the rungs of a ladder, in one or few longitudinal
perpendicular rows on a single wall-surface. The wall-surfaces on which they occur
may be called scalariform or ladder-like surfaces, while these Tracheae, together with
the similar reticulate tubes above mentioned®, have been termed scalariform or
ladder-like vessels, Vasa scalariformia, also scalariform tubes. It is useful to dis-
tinguish them from the reticulate vessels with non-bordered, transverse pits, either
as bordered scalariform surfaces or Tracheae, or to reserve specially for them the
name of ladder-like or scalariform surfaces.

Sect. 39. As one of the above-described forms of wall-thickening there are
found, in some few cases to be cited below, ingrowths of the thickened portions of
the membrane, which protrude in a conical or bar-like form into the cell-cavity, or are
stretched transversely across it : and those Tracheae in which these are largely deve-
loped may be distinguished by the name, introduced above as No. 3 (p. 156), viz.
Trachece with transverse bars. The bars are very largely developed in the narrow
primary^ tracheides occupying the corners of the vascular bundles of the stems of the
stronger species of Lycopodium, and in the margin of the vascular bundles of the
leaves of Juniperus * (comp. Chap. VIII). They are here somewhat flattened
cylindrical fibres, branching irregularly on all sides, and with the branches connected
on the one hand one with another to form a net spread through the cavity, and

‘ Compare Sachs, Textbook, 2nd English Ed., p. 26. ^ Von Mohl, /. c.

® Compare Link, Elem. Phil. Botan. Ed. i, p. 95; Von Mohl, Veget. Zelle, p. 27; Unger,
Anatomie und Physiologie, p. 172. * Compare Von Mohl, Botan. Zeitg. 1871, p. 12.

M 2

164

TRACHEM.

attached on the other to the thickened lateral wall of the tracheide. In the tracheides
of the leaves of Juniperus (Fig. 62), their points of attachment and origin are
especially the thick swollen margins of the bordered pits, in the Lycopodia the
spiral or reticulate bands with which the lateral wall is thickened. At the corners
of the vascular bundles of the leaves of Biota orientalis the swollen margin of the
bordered pits is often elongated into blunt cones, which protrude into the cavity,
but here end blind, without branching or coalescing with one another or with the
opposite wall. As a rare and anomalous phenomenon Sanio ^ found single simple
bars, stretched transversely from one wall-surface to the opposite one, in single
tracheides of the wood of Hippophae rhamnoides, and Pinus sylvestris ; in the latter
they are stretched between the tangential walls, and where they occur at all they
traverse in the same direction the entire length of long radial series of tracheides,
as far as the cambial zone. (Comp. Chap. XIV.)

p

FIG. 62.— Juniperus communis; leaf,
transverse section (600), p parenchy-
matous cell ; next to it tracheides of the
corner of the vascular bundle, with bor-
dered pits, and reticulately-branched
transverse bars. The parts lying below
the surface of section focussed are
shaded.

Further details on the structure of the walls of the Tracheae will be described in
later chapters, especially the Vlllth and XIVth.

Sect. 40. The wall of the Tracheae, whatever be its structure, is in the one series
of cases a completely closed membrane (Tracheides') ; or it is broken through at the
limiting surfaces between elements placed serially one above another, and originally
completely closed, the series thus coalescing to a continuous tube, which is, called a
vessel. The tracheides therefore differ from the vessels only in the absence of the
holes which occur in certain of the walls, and thus connect the cell-cavities. Tran-
sitions between them occur in the secondary wood of Dicotyledonous plants (comp.
Chap. XIV), e. g. Leguminosse, inasmuch as with otherwise similar characters the
holes are absent in one case and present in another. Holes are also to be found in

Fig. 63. — The same (225) ; vascular bundle, g xylem, c single sclerenchymatous fibre
at the outer limit of the phloem ; t margin consisting of tracheides with bordered pits
and transverse bars. The parenchymatous cells near and between the latter are
shaded with dots.

‘ Botan. Zeitg. 1863, p. 117. — Pringsheim’s Jahrb. Bd. IX. p. 59.

TRA CHEIDES. VESSELS.

165

the elements of the root-sheath of many Orchids (Sect. 56), but these had better be
termed generally tracheides, since the connection in rows which is characteristic of
vessels is absent.

The Tracheides are in some few definite cases — ends of vascular bundles, trans-
fusion tissue, the root-sheath of Orchids — short, even iso-diametric sacs : as a rule
they are of the form of elongated, spindle-shaped, fibrous cells, pointed at the ends,
and with round or polygonal transverse section. They usually remain microscopically
small, their length, which is a large multiple of their breadth, reaches 01 6““ to
about i-oo“™: this is the case in the wood of most Dicotyledons^: or it rises to
4““, as in the later annual rings of Finns ^ : in many cases however they attain
great dimensions : the large spindle-shaped spiral and annular tubes in the stem
and petiole of Musa and Canna® attain a width of o-o8 to and are always

more than in length; the spiral tubes of Nelumbium speciosum have, according
to Caspary, a length of over and width of 0-567™“. The great majority of

Tracheae belong to the category of tracheides: for instance, the tracheal elements of
all peripheral ends and expansions of vascular bundles, of the secondary wood
of the Coniferae, Cycadeae, most elements of the secondary wood of woody Dicoty-
ledons, almost all Tracheae of the Ferns, in the widest sense — vessels are only known
to occur in Pteris aquilina, and in the root of Athyrium filix femina ^ — the Tracheae
of the vascular bundles in stem and leaf of the Cycadeae and Coniferae of many,
though far from all Monocotyledons, and numerous Dicotyledons Many even of
the most striking elements with fibrous thickening, usually described as vessels,
belong to this series. To the already-cited examples of Canna, Musa, and Nelum-
bium we may, according to Caspary’s work quoted above, and referring to this for
further details, add the following examples : the ‘ vessels’ in the vascular bundles of
Stratiotes aloides (stem), Caladium nymphseifolium, Pistia Stratiotes, Acropera Loddi-
gesii, Aerides odorata, Alisma Plantago, Sagittaria sagittaefolia, Hydrocleis Humboldtii,
]\Iusa spec, (vessels in this case in the root), Brasenia peltata, Nuphar luteum, pumilum,
Nymphsea alba, gigantea, Victoria regia, Monotropa Hypopitys. A general view of the
occurrence of tracheides and of true Tracheae will only be possible when the necessary
arduous investigations have been extended over a larger number of cases than has
hitherto been the case.

Sect. 41. A vessel arises from a series of originally-separate cells, placed one
above another, by the perforation, at the close of the process of thickening, of the
division walls between the members of the series, the latter being then termed the
members of the vessel.

The rows of cells, above indicated (pp. 9 and ii, in Figs. 2 and 4) by the
letter v, which extend to the apex of the plerome, and similar ones marked g in
Fig. 3, p. 10, are rudiments of vessels.

The members are also always easily distinguishable in a mature vessel, and are

^ Sanio, Botan. Zeitg. 1863, p. 1 14. “ Sanio, in Pringsheim’s Jahib. VIII. p. 401, &c.

® Compare Unger, Anat. und Physiol, p. 171, und p. 218, Fig. 92 b.

* Russow, Vergl. Untersuchungen, p. 103.

® Mettenius, Beitr. zur Anat. d. Cycadeen, p. 258. [See also V. Höhnel, Ueber das Vorkommen von
Gefassartig-zusammenhangenden Tracheiden-strangen in Coniferen-hölzern, Bot. Ztg. 1879, p. 329.]

* Caspary, Monatsbr. d. Berl. Acad , Juli, 1862.

i66

TRACHEAE.

separable from one another, their limits being marked by their margins, which are
always permanent, often also by other portions of the perforated dividing wall : these
portions have the structure of a thickened double cell-membrane, and consist of two
thickening plates and a simple limiting lamella between them. Schultze’s mixture,
or hot solution of potash, destroys the limiting lamella, and thus separates the mem-
bers from one another.

The form of a member of a vessel is as a rule cylindrical or prismatic, the
breadth being throughout almost uniform, or diminishing quite gradually towards one
end : more rarely each member widens in the middle to a barrel-shape. The length
of a member is usually greater than the diameter : it is very much greater in the
vessels with a loose spiral, or in annular vessels, which develop before the extension
of a part is complete, and thus grow greatly in length as the part elongates. The
members of such vessels as arise after the extension of a portion of a stem or root
is complete are composed of short members, these being barely longer or even
shorter than they are broad, e. g. the wide-pitted and reticulate vessels of old stems
of Cucurbita, Cobsea, Vitis, &c. (comp. Chap. XIV). Successive members of a
vessel are as a rule of almost similar form through long tracts, though they often
decrease gradually in width. The general form of the vessel may be concluded
from these data : such as are composed of short barrel-shaped members were dis-
tinguished by the old authors as rosary-shaped: Vasa monilifonnia.

The walls, by which the members of the vessel are in contact with one another,
are either horizontal, in which case those of the successive members fit exactly on
one another, and together form the septum (comp. e. g. Fig. 3,^), or they are more
or less oblique, and the inclined faces of successive members here also fit exactly
throughout so as to form the oblique septum (Figs. 59-61) : or the ends are oblique
and pointed, and only a part of the opposed faces of successive members is united
so as to form a septum, near and above which the pointed end forms a blind and
often irregularly-formed continuation.

The perforation of the septum is always brought about thus : on the delicate
primary membrane one or several flat large pits are formed by the typical pro-
cess of thickening ; the unthickened parts of the membrane are then at once
dissolved and disappear, while the thickened bands of membrane, connected directly
with the thickenings of the lateral walls, remain persistent. In almost all cases on
horizontal septa, and not uncommonly on oblique ones, there appears one single
pit, or one single round or elliptical opening, which then always occupies the
greater part of the surface of the septum, and often, especially in thin-walled
vessels, the whole surface with exception of a very narrow peripheral band. On
the other hand, strongly-inclined septa, and very rarely horizontal ones (Avicen-
ijia), retain in most cases several or many openings included within the thickened
margin, and separated from one another by thickened bands. These are in some
few cases round, e. g. in the Tracheae of Ephedra^ (Fig- 69)5 usually they have the
form of slits of varying breadth, and arranged parallel in rows, whence the expres-
sion ladder-like perforated septa (Fig. 61). The slits are usually almost at right
angles to the longitudinal axis of the vessel, and the series of them are similarly

* Von Mohl, Ueber den Bau d. grossen getüpfelten Gefässe von Ephedra; Verm. Sehr. p. 268.

VESSELS.

167

arranged : thus the narrow, closely-grouped transverse slits in the ladder-like oblique
walls of the pitted vessels in the wood of the Betulacese, Ericaceae, of Corylus,
Carpinus, Pteris aquilina ; the round openings of the oblique walls of Ephedra
arranged in 1-2-3 rows, &c. Rarely the slits are parallel to the longitudinal axis of
the vessel : e. g. vessels of Hieracium vulgatum, Onopordon Acanthium, in which
irregular reticulate openings also occur. In an Avicennia Sanio found the hori-
zontal septum surrounded by a sharply-marked thickened margin, and the whole
remaining surface covered with many irregular, round or slit-shaped, bordered
openings^. There is no constant relation between the form of thickening of the
lateral wall and the form of perforation. Nevertheless most vessels with fibrous-
thickening have round openings, and very many vessels with bordered pits have
ladder-like perforations. In pitted vessels, however, simple openings are also
frequent, and Sanio found ladder-like perforations in the spiral vessels of species of
Casuarina, Olea europaea, and Vitis.

In thin-walled vessels — such as most of those with fibrous thickening, and
thin-walled pitted vessels, e. g. in the wood of Betulacese and of Tilia — the margin
of the opening of the septum is
smooth and thin, corresponding to
the margin of very flat, not bor-
dered pits. In thick-walled vessels
it is thicker, and has the structure
of a pair of corresponding bordered
pits opened by disappearance of the
limiting lamella, and with but small
difference of width between the pit-
cavity and the wide orifice of the
pit : it consists therefore of two
acutely-diverging lamellae. In many
vessels this structure as of a bor-
dered pit is extremely striking, e. g.
in the large solitary openings of the
pitted vessels in the wood ofNerium,

Eraxinus, Convolvulus Cneorum (Fig.

64), Pirus torminalis in the serially-
arranged round openings of the ves-
sels of Ephedra (Fig. 60, g), and the
small slits of the scalariform ves-
sels of Pteris aquilina (Fig. 61). It
also occurs very plainly in the thick
and closely-wound spiral vessels in the stem of Nerium. In other cases, even when
the margin of the opening is very thick, it is often only slightly indicated by a small
indentation running over the limiting lamella of the margin ; e. g. the pitted vessels
of Cucurbita, Juglans, Acer monspessulanum (Dippel, /. c.). The history of develop-
ment shows, in the cases of the latter category, that the opening arises by the

Fig. 64. — Convolvulus Cneorum, wood, transverse section (600); £'

small vessels cut transversely through the oblique septum. The rest are
tracheides. At t the very delicate closing membrane of the bordered pit
is visible.

* Sanio, Botan. Zeitg. 1863, p. 121. — Von Mohl, l.c.
“ Compare Dippel, Botan. Zeitg. i860, p. 329.

i68

TRACHEA.

disappearance of the limiting lamella in the face of a young pair of wide corre-
sponding bordered pits.

Since the structure of the bordered edges of the apertures corresponds to that
of closed bordered pits, the practical decision whether in a given case an open or a
closed pit is present is necessarily extremely difficult, if the parts of the septum
in question be small, and resemble closely the bordered pits of the lateral walls
in form and size ; and the difficulty is the greater, since in such cases intermediate
forms between the open and closed bordered pits — which are never exactly alike —
occur at the limits between the septum and the lateral walls. The apertures at
the middle of the septum in the pitted vessels of Ephedra (e. g. Fig. 59,y) are
larger than the closed bordered pits of the lateral walls. But at the margin of
the septum there occur not unfrequently round bordered pits (Fig. 59, x), which,
though resembling the apertures in form and size, are closed like the pits of the
lateral wall. In the large scalariform vessels in the rhizome of Pteris aquilina
(comp. Fig. 61) the transverse, bordered, slit-like pits of the lateral walls are always
closed. The strongly-inclined septa or terminal faces of the obliquely pointed
members of the vessels show a quite similar ladder-like series of slit-shaped pits,
like those of the lateral walls, but with the difference that here the slit-like pits
are wider, and the thickened intervening bands which separate them are thinner
than on the lateral walls. In the middle of the septum the slits are open, as is
plainly shown in sections through vascular bundles which have been injected with
glue and then dried, if the sections be then treated w'ith water and the glue dis-
solved. The bands between the slits then separate from one another (Fig. 61, D,/).
Tow'ards the ends of the intervening wall the pits are, on the other hand, of the same
width as the open ones, but closed by a limiting lamella : on the corners of the
lateral walls they become gradually narrower, and like those of the lateral walls.

The vessels are not unfrequently branched, two or more series of members
being attached laterally to one member. When such branches run parallel or
converge, they may attach themselves again laterally to a single series of members ;
a vessel may thus be in its longitudinal course alternately single and double

As regards the absolute size of the vessels, there is nothing to oppose the view
that their length may equal that of the whole plant, or at least may be very great. At
all events, on following the vascular bundles through long distances, member
is found attached to member, while blind-ends are rare, except in the ends of the
peripheral expansions of the plant. The width of the vessels is extremely unequal,
and changes variously according to the point of their occurrence in a given plant,
and according to the single species or genus. It may be said generally that the
diameter does not on the average exceed that of narrow fibrous cells in those vessels
which appear first in stems and roots, before the extension is complete (spiral and
annular), and in those which traverse the nervation of the leaf. Those formed in
stems and roots at the end of the process of extension, or subsequently, may in many
cases attain much greater width, while this does not prevent others of the smallest
calibre occurring with or near the former. For examples of this see Chap. XIV.
Vessels of greatest width occur in the central part of the vascular bundles of many

* Von Mohl, Palm. Stractura ; Verm. Schriften, p. 142.

TRACHEA.

169

Palm stems' (compare Chap. VIII), where they attain a diameter of o-aSomni
(Mauritia armata) to o-562““i (Calamus Draco) ; in the wood of many climbing
and twining plants, e. g. Cucurbita, Cobasa, Phytocrene Ampelideae, in which also
the width may rise to 0-3 — 0-5™», &c.® The vessels of greatest width are always
pitted vessels with short members.

After what has above been said on longitudinal course and branching, it need
hardly be noticed further that in one and the same vessel the width (and with it the
form of thickening of the walls) may often change in successive parts of its course,
i. e. in its successive members ; for instance, Mohl states the diameter of the above
vessels of the Palm-stems at the lower ends of the bundles as o-oii“™.

As regards the material composing the walls of Tracheae and Tracheides, it is certain
that they are, when first formed, cellulose membranes, and that they consist, when
mature, of more or less lignified cellulose. The lignification is present to a very varying
extent according to the special case ; in hard, firm parts more than in soft, sappy parts ;
the Tracheae of delicate foliage-leaves, or of sappy stems, &c., often show an almost pure
cellulose reaction. A very remarkable phenomenon, to which Burgerstein has recently
again drawn attention, is the surprisingly early appearance of lignification in many
vessels. It is beyond the scope of this work to enter minutely into the process of
lignification : it cannot at present be exactly stated how far it shows peculiarities in the
several organs in question. Reference may therefore only be made here to works upon
the subject ; the summary of the older results in Hofmeister, Pflanzenzelle, Sect. 30,
Kabsch, Pringsheim’s Jahrb. Ill, and the newest investigation of A. Burgerstein,
Sitzungsber. d. Wiener. Acad. Bd. 70, July, 1874.

Sect. 42. All Tracheae are alike in the peculiarity that when they are fully
formed the protoplasmic body disappears entirely, without leaving any vestiges
behind, as is the case in dried-up cells. The membrane alone remains of the
components of the cell. The space surrounded by it is filled in the mature tube
with very dilute watery fluids, which may here be called shortly water, or with air,
or with both together. The large majority of Tracheae are entirely or for the most
part filled with air at the time of full development. The extremely thin layer of fluid
on the inner surface, which is always difficult to observe, is often beyond anatomical
demonstration ; even in cases of excessive supply of water in bleeding parts air
bubbles occur in the fluid contained in them It is only in lateral extensions of the
vascular bundles of certain plants (Transfusion tissue, Chap. VIII) and in the endings
of bundles that they are exclusively filled with water. The same holds for the rudi-
mentary Tracheae of many water plants.

A remarkable exception to this occurs very generally in plants which contain
latex, or resinous, or tannin-containing secretions, whether the latter be stored in the
sacs treated of in Sections 33 and 34, or in intercellular reservoirs (Chap. VII). A
greater or less number of vessels are often filled in these plants for a greater or less
distance with latex, or with some such characteristic secretion. No fixed rule is to be
found as to the position of these vessels relatively to the other normal air-containing
vessels, or to the secretory reservoirs. How the secretion gets into the vessels is not

^ Compare Von Mohl, Bau des Palmenstammes ; Verm. Sehr. p. 142.

^ Mettenius, Beitr. zur Botanik, p. 50.

® [Compare Westermaier u. Ambronn, Lebensweise u. Structur d. Schling- u. Kletter-pflanzen,
Flora, 1880.] ‘ Compare Hofmeister, Flora, 1858, p. 2.

170

TRACHEA.

explained in plants without laticiferous tubes, though plausible conjectures may be
made on the subject. The same holds as a matter of fact also for plants with latici-
ferous tubes, but there are controversies on this point, which we shall return to in
Chap. VI.

The frequent filling up of the cavity in the Tracheae, e. g. in old layers of wood
of Coniferse and many Dicotyledons, with resin or resin-like masses is undoubtedly a
phenomenon of incipient degradation and disorganisation. It will be further treated
of in Chap. XIV.

In old or damaged, large, tubular Tracheae the internal cavity is not unfrequently
partially or completely filled with parenchymatous cells (Füllzellen), which in the
wood of the chestnut drew the attention of Malpighi*: they have since been
frequently described, and have been termed by the anonymous writer in the
Botanische Zeitung ^ Thyloses (Thyllen).

They may arise where a Trachea borders on parenchymatous cells, and in fact
from those cells themselves, which grow into it. A small part of the membrane of a
parenchymatous cell adjoining an unthickened point on the wall of a Trachea (as a
rule a pit) grows to an excrescence protruding into the cavity of the latter : it
contains protoplasm, usually with a well-marked nucleus, and expands from a blunt
and short cylindrical form to a round, often voluminous bladder, and finally cuts
itself off as a special cell from the rest of the cavity of the cell which produced it by
means of a division-wall, formed at its point of entrance into the Trachea. Thus there
always arise at first solitary bladder-like cells protruding from the wall into the cavity
of the tube. The process may be arrested at this point : but often the phenomenon
is extended quickly over numerous points of a portion of a tube, so that the latter
gradually becomes entirely coated internally with the cells, and these, as they extend,
gradually fill it up completely. This often happens to such an extent that the tube
is entirely filled by thyloses flattened into polyhedral forms by reciprocal pressure.
Further, a multiplication of them by division has been observed in many cases®.

The parenchymatous cells bordering on a tube take part unequally without
recognisable rule in the formation of thyloses : some throw out thyloses at one
point, others at several, others again not at all. The formation of fresh thyloses may
continue for a long tim.e in a portion of a vessel : in vessels several years old (e. g. in
an eight years’ old layer of wood ofVitis, Reess, l.c.) the first beginnings of new
thyloses often occur in close proximity to others apparently several years old.

The cellulose wall of the thyloses, which is at first delicate, is later thickened in
woody plants, and often has corresponding pits on the surfaces of contact with other
thyloses. In the same plants starch may be stored up in their contents, as is the
case in normal parenchymatous cells.

The formation of thyloses has been observed in Monocotyledons (Arundo

‘ Anat. Plant, p. 9, Tab. VI. fig. 23.

“ Vol. for 1845, p. 225. 0uAA.ts = sac, bag, reservoir. In this treatise the older literature on the
subject is referred to. For more recent information see Reess, Zur Kritik der Böhm’schen Ansicht
über die Thyllen, Botan. Zeitg. 1868, p. i ; Unger, Ueber d. Ausfüllung alternder u. verletzter Spiralge-
fässe durch Zellgewebe, Sitzungsber. d. Wiener Acad. Bd. 56 (1867).

® Trecul, Sur l’origine des bourgeons adventifs; Ann. Sei. Nat. 3 ser. torn. VIII (Maclura). — A.
Gris, Ann. Sei. Nat. 5 ser. tom. XIV. p. 38 (Cissus).

TH VL OS ES.

171

Donax, Canna, Hedychium, Strelitzia, Musa, Palms), and in the wood of very many
Dicotyledons, both in one-year-old stems (Canna, Cucurbita, Bryonia, Cucumis,
Solanum tuberosum), Euphorbia helioscopia, &c., and especially in long-lived stems
of Dicotyledonous woody plants, where they are very widely distributed, and easily
observed phenomena, e. g. in Vitis, Quercus, Sambucus, Platanus, Robinia, &c. But
in the roots of Dicotyledonous trees, which have been examined for them (Quercus,
Fraxinus, Fagus, Betula, &c.), they do not occur, or extremely rarely*: in the roots
of herbaceous plants, however, they occur in large quantity : Pharbitis hispida, young
strong roots of Cucurbita, Urtica, Rubia, See.

The tubes in which thyloses appear are in most cases typical, wide-pitted
vessels : but in Canna (and also in Musa and its allies) they are also the above-
mentioned (p. 165) wide, fibrously-thickened non-perforated, tracheides.

In the pitted vessels of many Dicotyledonous woody plants the formation of
thyloses is a regular phenomenon, which appears in the normal uninjured plant,
though not extending to all pitted vessels. In Robinia pseudacacia it is stated that
the pitted vessels of the wood (and, according to Gris, all the vessels) begin to be
filled with thyloses in the autumn of the year in the spring of which they were
formed, and that these are at times filled with starch. Other woods behave in the
same way as regards the time of first formation of thyloses, but no definite rule has
been recognised for their occurrence or absence ; e. g. in Vitis, Quercus robur,
Platanus, according to Reess. Injuries by which the vessels are opened are, as far
as investigated, without influence on the formation of thyloses in woody plants. In
the large tracheides of the stem of Canna, however, they occur, according to Unger,
if these have been injured, e. g. cut into, and then exposed to air or water. These
facts may afford starting-points for the inquiry into the still unknown causes of the
formation of thyloses, which cannot be further noticed here.

^ Von Mohl, Botan. Zeitg. 1859, p. 294, and earlier.

CHAPTER V.

SIEVE-TUBES.

Sect. 43. The Sieve-tubes, Tubi cribrosi, were first clearly distinguished by
Th. HartigS in the year 1837, as essential constituents of the bast and of the
vascular bundles of Phanerogams, and were in some cases designated by the
above name, while in others they were termed sieve- fibres. After lying unrecognised
for many years, Hartig’s observations were confirmed and extended, especially by
Mohl, Nageli, and Hanstein^

The chief points of occurrence of the organs in question are those above
mentioned ; they are rarely found elsewhere. They are present in both Phanerogams
and Ferns. They have been most thoroughly investigated in the Angiosperms. They
may therefore be treated of first as they occur in the latter plants, and afterwards the
peculiarities to be found in the other divisions may be added.

The articulation of the sieve-tubes in the plants in question is throughout
similar to that of the vessels treated in the preceding chapter. They arise from
longitudinal rows of elongated, cylindrical, or prismatic cells, and these remain
always clearly distinguishable and separable as their me7nhers. On the faces with
which the members are mutually contiguous they come into open communication
through the sieve-plates or sieve-fields, which are circumscribed portions of the wall,
by means of numerous very small, perforated pits, the pores of the sieve.

The form of the members of the tubes is that above stated. Their ends are
limited by one flat, or slightly concave wall (concave on the under side) ; and this is
either almost horizontal, or at most slightly oblique, and in that case as a rule slightly
broader than the middle of the member ; or it is very strongly inclined, and cuts the
lateral-wall on one side at a very acute angle, so that each end of a member is
bevelled on one side like a chisel. The inclination of the terminal surfaces is in
the latter case — though not invariably and exactly — towards the radial plane.

^ Vergl. Untersuchungen über die Organisation des Stammes d. einheim. Waldbaüme; in
Jabresber. üb. d. Fortschritte d. Forstwissenschaft, &c. p. 125. — Compare further Hartig, Vollst.
Naturgesch. d. forstl. Culturpfl. Berlin, 1851 ; Botan. Zeitg. 1853, p. 571. — Ibid. 1854, p. 51.

^ Von Mohl, Einige Andeutungen über d. Bau d. Bastes, Botan. Zeitg. 1855, p. 865. — Nageli,
Ueber d. Siebröhren, Sitzsber. d. Münchener Acad. Feb. 1861. — Hanstein, Die Milchsaftgefässe u.
verw. Organe, &c. Berl. 1864. — Mohl calls the members of sieve-tubes 'latticed, cells’ (Gitterzellen).
P. Moldenhawer had already distinguished them in part as ‘ vasa proprial but confused them with
other elements under this name. [See further Wilhelm, Beiträge z. Kenntniss d, Siebröhren-apparates
Dicotyler Pflanzen, Leipzig, 1880. — Janczewski, Sur les tubes cribreux, Mem. Soc. Cherbourg. 1881.
— Idem, Et. Comp, sur les tubes cribreux, Ann. Sei. Nat. 6 ser. tom. XIV. 1882. — Russow, Sur la
structure et le developpement des tubes cribreux, Ann. Sei. Nat. 6 ser. tom. XIV. 1882.]

SIEVE-TUBES.

173

The first of these two chief forms, which may be termed that with transverse
or flat ends, is by far the most general, and is almost exclusively present in the
‘ primary ’ vascular bundles (Chap. VIII) ; the second, or sharp-ended form, pre-
ponderates, equally in the secondary bast of Dicotyledons. Exceptions are however
found to this rule, e.g. the beautifully-bevelled members of tubes in the vascular
bundles of stems of Calamus, and the roots of Aroideae (e. g. Philodendron Imbe) ;
and on the other hand the flat-ended tubes of the secondary bast of Fagus sylvatica,
Quillaja saponaria. Ficus elastica, Madura, &c.

The size of the members of the sieve-tubes varies, especially in different species,
no less than that of the vessels. The same rules hold for the length of their members
as for those of the vessels : but the maxima of diameter of the latter are not attained
by the widest sieve-tubes. The widest sieve-tubes attain a diameter on the average
not more than 0.02 to 0.08““; Cucurbita; species of Bignonia, Phytocrene,
Calamus, &c. On the other hand, extremely narrow and insignificant ones are to
be found, especially in many, but not all, succulent plants, and in such as have latex
(e. g. Asclepiadacese, Crassulaceae, &c. Comp. Chap. VIII).

A few measurements of large members of sieve-tubes may be given below, but with
the remark that the measurements of length in long members are only approximately
made, or from single specimens, owing to the extreme difficulty of neatly isolating such
delicate organs.

Length. Diameter.

mm. mm.

hitermdes of
Cucurbita Pepo . .

Lagenaria vulgaris
Calamus Rotang . .

Potamogeton natans .
Bignonia spec. (Mohl)

Vitis vinifera, bast .

Root of Philodendron Imbe

0.370 — 0-450

0-125 0-200

over 2™™

0-275
to 1-35
about 0-6
to more than 2'

0045 — 0-050
0-025 — 0-040
0-030 — 0-050
to 0.025
0-450

As regards the longitudinal and lateral connections of the members, and the
branchings which in certain cases are thus produced, the same rules apply in the
main as in the case of the vessels.

The walls of the sieve-tubes are always delicate, not lignified, colourless cellulose
membranes. The sieve-plates characteristic of them only occur on those surfaces
where the members abut on similar elements. The sieve-plate is a sharply-limited
part of the wall, like a large shallow pit, which is originally, and often throughout
life, less thickened over its whole surface than the wall surrounding it. It is thickly
covered over its whole extent with round or polygonal secondary pits, which are
separated from.one another by narrow bands of membrane : it thus resembles a fine
sieve, net, or lattice (Figs. 65-73). The sieve-plates of members of tubes which are
contiguous fit with all their secondary pits exactly on one another, and in them the
intervening wall disappears when the differentiation of tissues begins, so that holes —
the sieve-pores — appear, through which an open communication is established between
the neighbouring members.

The original width of the sieve-pores differs according to the special case. The
widest occur in the Cucurbitace®, where the largest (Cucurbita, Lagenaria) attain a
size of 5 /i and more : most of them are much narrower ; in the above-named Cucur-
bitaceae only 2 n wide ; also for the bast of Bignonia spec. Mohl states it at 2 fi,

174

SIEVE-TUBES.

which would be too high an average for most plants : in many Angiosperms
with small sieve-tubes they are certainly narrower, often being on the limit of clear
recognition.

Further in the same plant, and even in plates lying close together, the width of
the pores is very unequal: in the large tubes of Cucurbita and Lagenaria, where exact
measurement is possible, the diameter of the pores of neighbouring, and otherwise
equally developed plates may differ by three times (comp. Fig. 65). In one and the
same plate the difference in size of the pores is usually small, if at all recognisable :
according to Nageli they are, at least in Cucurbita, wider on the average at the
middle than at the margin of the plate. Very considerable differences in size and
form on the same plate are rare. (Comp. Hanstein, l.c., Taf. III. Fig. 4, Cucurbita.)

i ig. 65

Fig. 65 — 67. — L.ag’enaria vulgaris, mature internode of stem (375). Fig. 65 and 66, transverse sections through one and the
same bundle of sieve-tubes (or vascular bundle) ; nt wide-meshed sieve-plate, occupying the whole horizontal end of a member,
exposed, in surface view ; a similar one with narrow pores. I another, injured at one margin in cutting the section, callous,
the pores still slightly open ; the original cellulose sieve recognisable through the callus ; 0 sieve-plate covered by the contents
coagulated with alcohol ; d delicate parenchyma- Fig. 67, lateral view of two members of a sieve-tube attached end to end, the
plate callous, so as to close the pores completely, the original sieve is recognisable in the middle between the two masses of
callus.

According to the developmental data to hand, which however are not extensive
on this question, the above-described simple structure makes its appearance on all
sieve-plates on their first development. Many retain it long, or even throughout
life ; others alter, by assuming the condition termed by Hanstein callous The change
consists in the thickening of the bands of membrane in all directions. They swell
perpendicular to the surface to three or more times the original thickness, and become
convex on their inner side : in the direction of the surface they expand so that the
original pores are contracted to narrow cylindrical canals, which widen out like
funnels only between the convexities of the inner surface. The gingle bands of
membrane of one plate often take a different share in the callous thickening : this
increases or decreases gradually on one plate from the middle towards the edge ; in
this matter both sides may be alike, or the reverse : the general form of the callous
plate may thus be biconvex, biconcave, or plano-convex, &c. Often the inequalities
of thickening are irregularly distributed over one face. The callous thickening may
lastly extend in the direction of the surface so as to close the canals completely. Sieve-
plates may often be found covered with a thick mass of callus, which is not perforated,

‘ [On the Callus compare Russow, Botan. Zeitg. 1881, p. 723.]

SIEVE-TUBES.

^75

and in which the canals are only indicated by transverse striae, and by funnel-shaped
depressions of the surface : in others even these indications are not noticeable
(Fig. 67, 76).

The callous plate always consists of three lamellae, one central, and two applied
to this laterally, one on each side ; each of these belongs to one of the members of
the tube. The middle lamella is the original cellulose sieve. The lamellae of callus
are in the fresh condition homogeneous, colourless, apparently soft, and having by
transmitted light the peculiar bluish lustre of gelatinous membranes : they are
coloured yellow by solution of iodine in potassium iodide, and by Schultze’s solution
a deep brownish-yellow : in sulphuric acid they swell till
their outline is completely lost. A similar swelling results
from the action of alkalies, especially solution of potash,
and of Schultze’s mixture. By these reagents the callus
mass may be completely removed from the persistent cellu-
lose sieve.

At the margin of the sieve-plate, next the adjoining
membrane, the callus mass stops rather abruptly.

From all these phenomena it is concluded that the
callus mass is formed by apposition upon the original cellu-
lose sieve. The conditions of its appearance and its
physiological significance require further investigation : ac-
cording to some few experiments on Cucurbita and Lage-
naria, the callous thickening seems in these cases to appear
and increase with the age of the sieve-tubes, and in the
first-formed (peripheral) tubes of a vascular bundle it seems to
advance very quickly till the sieve is entirely closed. In many
plants sieve-tubes may be found side by side without callus,
and with callus in the most different stages, e. g. Lagenaria :
in others, e. g. in the bast of Quillaja, only callous sieves
are known, but they are always open. In the bast of many
ligneous plants — Vitis, Tilia — I find all the sieve-plates
completely closed by callus in the winter time; in the height
of summer they are open and not callous. (Comp. Figs.

69, 74, and 76.)

The sieve-plates are always placed on the terminal
faces of the cylindrical tubes. If these faces are horizontal,
or only slightly inclined, each has throughout the properties
of one sieve-plate, which may be termed a simple transverse
plate : this is so in all the above-named cases of the original
vascular bundles and primary bast of Angiosperms (comp.

Figs. 65-67), in the slightly inclined terminal faces of Ca-
lamus, in the secondary bast of Fagus and Quillaja.

On strongly-inclined terminal faces the sieve-plates are arranged like the scalari-
form openings of vessels in series one above another, and are, like these, separated
from one another by narrow bands of membrane : they usually form a single series,
rarely they form here and there several irregular rows. Examples of this are supplied

Fig. 68.— Cucurbita Pepo ; mature
internode, longitudinal section, parts
of two sieve-tubes, with large callous
plates at their terminal faces. T he
contents of both contracted by al-
cohol, but attached to the plates, and
passing through the pores from one
member into the other.

SIEVE-TUBES.

1 76

by the above-mentioned secondary bast of ligneous Dicotyledons, e. g. Phytocrene,
Bignonia, Tilia, Juglans, Vitis (Figs. 69-70), Betula, Populus, Pirus communis \ &c.,
the very oblique terminal faces of Calamus (Fig. 71), Philodendron Imbe, &c. In the
secondary bast of Vitis the occasional horizontal ends of members have also a
ladder-like structure.

Sieve-plates are distributed in different ways, according to special cases, on the
lateral faces of members of tubes, where these adjoin other similar members.

On most forms with simple transverse plates, as Cucurbita, they are not un-
commonly absent on the sides, or they occur irregularly, and are then usually

Fig. 69. Fig. 70. Fig. 71.

Figs. 69, 70.— Vitis vinifera, Bast of a branch several years old, 1®“ in thickness, in Summer (beginning of July). Fig. 69
(145) Tangential section, s, s sieve-tubes, the inclined terminal surfaces, and a horizontal scalariform one, are cut through longi-
tudinally, with the exception of one at the upper edge, vv'hich is seen obliquely in superficial view. 7n, in medullary rays. Fig. 70
Radial section, two scalariform terminal surfaces of sieve-tubes in superficial view, separated from one another by narrow paren-
chymatous cells (375).

FIG. 71.— Calamus Rotang (Spanish Reed). End of a member of a sieve-tube isolated by maceration (375).

relatively small. Where the lateral wall adjoins elements of another category iso-
lated, usually flat, pits are found : in these forms especially the lateral wall is very soft
and extensible ; after maceration in potash its inner layer may be drawn out to some

^ Compare Von Mohl, l.c.: Dippel, Mikroskop, p. 251, &c. — Von Mohl’s fig. ii of Pirus repre-
sents the partial surface-view of three oblique terminal faces.

SIEVE-TUBES OE ANGIOSPERMS.

^11

length. Among these forms the tubes of the secondary bast of Ficus elastica and
Fagus sylvatica appear to be exceptions, since in them the lateral faces, turned
towards the periphery and middle of the stem, are covered thickly with sieve-plates,
which are only separated from one another by narrow fibre-like bands These plates
on the lateral walls are extremely delicate, and it cannot be determined w'hether they
really have pores, or are only portions of the wall having the latticed appearance of
sieve-plates.

In the tubes with ladder-like terminal faces the series of plates is continued,
usually quite gradually, from these to the neighbouring lateral surfaces, and especially
on to the radial ones ; in the bast of the Dicotyledons the arrangement is always such
that the plates on the lateral surfaces are smaller and wider apart than on those
which are terminal.

The contents of the fresh intact sieve-tube as it lies in water appear as trans-

Figs. 72. 73. — Two large sieve-tubes of Lagenaria vulgaris in longitudinal section, where two members join,
after action of alcohol and iodine solution. In Fig. 72^'- is the non-callous widely-perforated horizontal plate,
as seen in longitudinal section ; r the contracted sac-like contents, with the dense aggregation of slime. Pro-
cesses from this traverse the pores on the left side ; on the right side (?') they have been torn out of these in
cutting the section. In fig. 73 the transverse plate {.g) is oblique ; thus the half of it present in the prepara-
tion is seen in section, and obliquely in surface-view. The coagulated slimy contents have been quite sepa-
rated from it in cutting the section, and the plate, which rested on it, has been so turned that its whole sur-
face, which before abutted on the intact sieve-plate, is turned towards the observer. The processes, which
before fitted into the pores, appear upon it as rings (375).

parent as water. More exact investigation shows that the wall of each member of the
tube is invested by a continuous thin layer of almost homogeneous slimy substance
resembling protoplasm. This layer surrounds a central watery fluid, to which must be
ascribed the alkaline reaction ^ characteristic of the contents of bundles of sieve-tubes,
at all events in Cucurbita. At one end, or more rarely at both ends, of the member
it encloses an apparently dense lustrous aggregation of slimy substance, which lies
upon the sieve-plate, either as a thin lamella, or as a plug of relatively considerable

’ Von Mohl, l.c. ; Dippel, Mikroskop, p. 255.
“ Compare Sachs, Botan. Zeitg. 1862, p.' 257.
N

178

SIEVE-TUBES.

height. Usually this aggregation of slime is found at one end only of the member,
and in that case, in Cucurbita according to Nageli, in 4 of the instances (taking the
whole plant into consideration) it is at the upper end, that is, on the under surface of
the sieve-plate. In very many cases numerous very small grains of starch are im-
bedded in the slime, and especially in the terminal aggregations of it Briosi found
these in the stems and leaf-stalks of 129 out of 146 species investigated. At the
sieve-plates the slim}" contents are continuous through the pores from one member
of the tube into the adjoining one. It may be seen, especially in callous plates, and
when coloured yellow with iodine, filling all the pores and demonstrating, like a
natural injection, the open communication through them (Figs. 68, 74). Where
however the size of the parts makes an exact investigation possible, it may be seen
that it does not pass as a homogeneous mass equally from one member into the

Figs. 74, 75. — \'itis vinifera, Bast ; from the same branch as Fig. 69, prepared on Llie satne day (6oo).

Fig. 74. — Tangential section through the ladder-like limiting surface of two members of a sieve-tube A and B. In A the dense
plug of slime, contracted by alcohol, sending blunt processes through all the sieve-pores into 5 ; a a grain of starch.

FIG. 75. — Radial section, after action of absolute alcohol, destruction of the starch by brief action of strong solution of potash
(the latter has caused a slight swelling of the membrane), and subsequent washing out of the potash, and treatment with iodine.
Part of a ladder-like wall in surface view : beneath it the shrunken slimy contents of an adjoining member, which sends capitate
processes — upwards as the preparation lies — through the pores into the other member.

Fig. 76. — Bast from a branch several years old, and I'sc»« thick, of the same plant in winter. Callous closed wall between two
members of a sieve-tube, tangential section {400).

other, but that the peripheral layer of the one member sends processes into the
pores, which they fill, and end blind at the limit of the adjoining member : the
processes either end simply at the surface of the sieve-plate, or are more or less
swollen, and rise above it into the cavity of the adjoining member, while at the point
of transit through the plate they fit into corresponding holes in the peripheral layer
of the member, which they enter (Figs. 72-75). As far as is at present known the
processes always extend on one sieve-plate to one side only, thus from the member
a to b, and not also conversely : further, they extend from the surface on which there is
the larger collection of slime to the other. They are in their turn also filled with slimy
contents. According to Briosi’s statement, that the starch-grains often stick in the

* Briosi, lieber allgemeines Vorkommen von Stärke in den Siebröhren, Botan. Zeitg. 1872, p.
305. — Compare also Sachs, Exp, Physiol, p. 383, &c.

SIEVE-TUBES OF GYMNOSPERMS.

179

sieve-pores, they must often also include starch-grains. Tliis requires more exact
proof. The starch-grains are doubtless densely crowded on the sieve-plate, and
especially so on the pores. But they cannot so easily and generally enter and pass
through the pores, since they are often larger than these ; for instance, in Vitis, at
the period of most active vegetation, they are on the average twice as broad as the
pores. (Fig. 74, rt.)

The structure described above is found in fresh intact sieve-tubes. But it
appears much more plainly after the action of reagents. On treatment with alcohol
the peripheral layer, resembling protoplasm, immediately coagulates along the sides of
the members, separates from the membrane, and contracts to a relatively thin, folded,
but still closed sac, which occupies the middle of the member (Figs. 68, 72, 74).
On the face which is in contact with the sieve-plate, and which is attached by the
processes in the pores, the sac retains its original width, or at least that of the
perforated part of the wall : it thus widens more or less rapidly in a conical manner
opposite these faces : the processes which enter the pores alter their form and position
but little or not at all.

Iodine preparations produce the same changes in form, and colour the whole peri-
pheral layer and the terminal aggregations of slime deep yellow to yellowish brown ;
the starch-grains violet * : this coloration appears much more quickly in the parts in
question than in the callus-masses, so that by this means these two parts may easily be
distinguished from one another : this renders the understanding of the structure more
easy, especially in slightly-thickened and widely-porous sieve-plates, inasmuch as in
this case the figures of the sieve-plates on the one hand, and on the other of the
plates of slime (with their processes), which cover the sieve-plates, are necessarily
similar in the surface-view of the plate, and are often difficult to distinguish at first
sight. According to the above behaviour with alcohol, and preparations of iodine,
and other known chemical reactions^, the slimy contents of the sieve-tubes, i.e. both
the lateral peripheral layer and the terminal aggregations, consist in the main of an
albuminoid substance similar to protoplasm. It is doubtful whether it should really
be termed protoplasm, less because of the slight differences between the iodine
reaction of the slime and of the protoplasm of the surrounding tissue in Cucurbita^,
than because protoplasm is a body which is characterised not only by its material
composition, but also by a definite.organisation or structure, which expresses itself in
protoplasmic movements, differentiation of nuclei, &c., and since phenomena such as
the above have not been observed in the contents of sieve-tubes.

Sect. 44. In the Gymnosperms and Fern-like plants^ tubes are found, in similar
places to the sieve-tubes of the Angiosperms, which, from their great similarity to
these, are doubtless rightly included under the same term, but differ in certain
points from them, and especially in the character of their contents.

The sieve-tubes of those Gyrmiosperms which have been investigated — e. g. Larix,
Abies pectinata, Juniperus, Sequoja gigantea, Salisburia, Ephedra, Gnetum, Encepha-
lartos — are similar in form and average size of the members to those commonly
found in the bast of ligneous Dicotyledons, which have the ends of the members

' Compare Briosi, l.c. “ Compare Sachs, Flora, 1863, p. 38. ^ Nageli, l.c. p. 16.

* [.See Janczewski, 1. c. ; also Riissow, l.c., and Strasburger, Zellhäute, p. 57.]

N 2

i8o

SIEVE-TUBES.

bevelled like a chisel. They may, like these, attain a considerable width, e.g. 0.030™™
in the secondary bast of old roots of Abies pectinata. The oblique terminal faces
are directed, both in the stem and in the roots, towards the radial planes (medullary
rays). Sieve-plates are distributed uniformly in one or two longitudinal rows over
the terminal faces, and the whole remainder of the radial lateral face. They form
roundish spots, separated by high intervening portions, or are rarely elongated
transversely, and separated by narrow ring-like bands : these spots are coarsely
latticed, while in the cavities of the coarse lattice the very delicate
sieve-structure is seen (Figs. 77, 78). Considering their close
similarity to like parts of Dicotyledonous plants, there is no
reason to doubt that the channel through the narrow sieve-pores
is open. But this has not been directly proved, and the proof
has hitherto been impossible, since the tubes in the plants in
question are filled almost exclusively with watery fluid. The
masses of starch-containing slime, giving the reactions of pro-
toplasm, which in the Dicotyledons send their processes through
the sieve-pores, have not yet been discovered in the plants in
question ; on the walls of the tubes there are attached internally
some few very small granules, which turn yellow with iodine.
The nature of the materials composing the fluid contents re-
quires further investigation. Further, I was unable to find a
formation of callus, with the exception of a doubtful case in
the roQt of Abies pectinata.

Among the Ferns a number of plants have relatively large
and wide vascular elements, and among these such as are, from
their position (comp. Chap. VIII), and their structure, to be
enumerated among sieve-tubes. This is the case in many
Polypodiacese, e.g. Pteris aquilina (Fig. 79), Marsiliacese (Mar-
silia Drummondi and its allies), Cyatheace®, Osmundaceae,
Ophioglosseae, according to Dippel in the Equiseta, and at least

Fig. 77.— Sequojagigantea. .i_ 7 t i

End of a member of a sieve- tuC IS-l g’CF LyCOpOOlS, .

tube, from the bast of an old t .lI t? ■ ./. j ^.7 i • 1

stem, presenting the radial In the Equiseta and the Opmoglossese they consist, ac-

wall is shaded. On it are small cording to Dippcl and Kussow, 01 tabular prismatic members,
very‘"fite To7es'’.'*\ndicrted which Stand onc upon another in longitudinal rows with hori-

only here and there by points ,7 n • ti . n 'T'li.i 11

(3„). zontal, callous, sieve-like, transverse walls. I he lateral walls

have no sieve-pits.

In the other cases cited, the members of the tubes are fitted one on another with
pointed ends (in Marsilia also with horizontal ones), and have sieve-plates both on the
latter, and also on the whole of those lateral surfaces which are contiguous with similar
elements. These are usually elongated transversely, forming, according to the width
of the surface of wall, one or several rows : in these rows they are either crowded
closely, and separated only by narrow bands of wall (Fig. 79, F), or they are at a
considerable, and then usually a variable distance from one another. The sieve-

^ Dippel, Bericht d. 39. Versamml. deutscher Naturforscher zu Giessen, 1864, p. 146, Taf, IV.
— Idem, D. Mikroskop, pp. 195, 203. — Russow, Vergl. Unters, pp. 5, loi, 118, 129, 142.

SIEVE-TUBES OF CRYPTOGAMS.

l8l

Fig. 78. — Encephalartos
puiigens. Bast of an old
stem. Part of the radial
wall of a sieve-tube (375).

plates are not callous. Their pores, as far as they can be recognised, are very
narrow and round ; and in Marsilia, according to Russow, very numerous on one
plate; in the cases investigated by me (Pteris aquilina, Cyathea,

Alsophila spec., Osmunda) they are less numerous, and relatively
far distant from one another. The wall of the tubes is thin at
the sieve-plates ; the rest of it is strongly thickened, stratified,
and soft, and apparently swells in water. These tubes contain
a quantity of watery fluid, and a thin peripheral layer, coloured
yellow by iodine, which contains throughout, and especially at
the ends of the members, and on the lateral sieve-plates,
numerous very small granules which adhere closely to the wall.

In dried-up tubes the ends are also found filled with a homo-
geneous brown mass. These granules are not starch : they
turn a deep yellow with preparations of iodine : maceration in
dilute solution of potash destroys them only partially even after
many days. Their dense aggregation and their tenacious hold
on the sieve-plates usually prevents a clear decision on the permeability of the
pores : but I believe that I have clearly
seen in thin longitudinal sections in
Pteris aquilina that the granules of con-
tiguous sieve-tubes are connected by
thin filamentous processes which tra-
verse the transverse pores (Fig. 79, c).

The tubes are not inferior in width
to the medium and thicker tubes of
the Gymnosperms. The length of the
single members is considerable, in the
cases investigated (Pteris aquilina, Cy-
atheaceae) it is 1-3’'^“. In the Mar-
silias they attain, according to Russow,
the length of one whole internode, that
is, of several centimetres, a statement
which may have its origin in the ease
with which the ends of members may
be missed in tubes prepared free by
maceration.

In the larger indigenous Lycopo^
dia (L. clavatum, annotinum) there
occur in the vascular bundles of the
stem organs which, in their position
and width, have great similarity to the
members of sieve-tubes of the above
Ferns. They are prismatic and elon-
gated, so that their pointed ends can
seldom be seen in sections. Their
contents correspond also to those of

Fig. 79. — Pteris aquilina, Rhizome. A end of a member of a sieve-
tube, isolated by maceration {142) ; B part of a thin longitudinal sec-
tion. This has approximately halved two sieve-tubes, Ji and S2,
which are so drawn, that the plane of section faces the observer, and
the uninjured side lies behind. The latter is represented as lighter,
w’hile that which lies in the plane of section is darker. Ji abuts to the
right on parenchymatous cells which have been cut through, to the left
on s2; at the back again, its broad surface covered with sieve-plates,
is contiguous with a sieve-tube, while to the left and at the back
it abuts with a smooth wall on parenchymatous cells. Jg borders with
its whole sinooth-walled posterior side on parenchyma (the nucleus is
indicated in two of the cells), right and left on sieve-tubes ; r, c sections
of walls bearing sieve-pits {375}.

SIEVE-TUBES.

1 S2

sieve-tubes. Neither Hegelmaier^ nor I could find the clearly-latticed sieve-plates,
almost like those of Pteris aquilina, which Dippel described on their lateral walls.
On the other hand I saw on the whole lateral wall numerous small pits, solitary or in
groups, to which were attached those peripheral granules, like those of Pteris
aquilina, which turn yellow with iodine : but from these it could not be determined
w'hether there are open sieve-pores or not (comp, also Chap. VIII). In the smaller
Lycopodia, the Selaginellse, and in very many Ferns with small vascular bundles
composed of narrow elements (comp. e. g. below, Fig. 160, Polypodium vulgare),
the position in which the sieve-tubes occur in the above instances is occupied by
elements of similar form, and general character of contents and walls, but without
distinct sieve-plates or sieve-pores. Whether the latter are really absent, and w'hether
these elements are only the morphological equivalents of sieve-tubes, remains to be
further investigated both in these cases and also in Lycopodium. I am the less
inclined, in the doubtful cases, to deny the presence of sieve-tubes, and prefer the
more to treat the question as an open one, because these very organs counsel one
to be circumspect, for twenty-two years ago no botanist, with the exception of Hartig,
had any idea of the characteristic structure of the most conspicuous of them.

‘ Botan. Zeitg. 1872, p. 778.

CHAPTER VI.

LATICIFEROUS TUBES.

Sect. 45. Certain plants, belonging to families or genera to be named below,
known as plants which produce milk on injury, contain, in tubes of definite structure
and of definite mode of development, a milky opaque fluid, which bears the name of
latex ; after this the tubes themselves may be called Laticiferous tubes

The tubes traverse the parts continuously for long distances, adjoining more
especially the turgescent, usually parenchymatous elements. They are themselves
completely filled with the milky fluid, their walls, though often strongly thickened,
are always soft, and easily compressed. Thus if a tube be injured at any point, the
pressure exercised by the adjoining turgescent tissues forces the milk out through the
opening.

The wall of the milk-tubes is always a soft, apparently watery cellulose mem-
brane, which readily shows the characteristic blue coloration with preparations of
iodine. Details of its structure will be given below.

Within the wall neither protoplasm nor nuclei are to be seen It is true many
forms of coagulated, finely granular latex, e. g. that of the Cichoraceae, resemble
coagulated protoplasm, or there remains here and there, in partially emptied tubes
after action of alcohol, solution of iodine, &c., a coat which looks like a coagulated
protoplasmic lining to the wall. Further investigations will therefore perhaps be able to
prove the presence of a protoplasmic body. Still, as it is difficult to obtain sharp an-
atomical evidence of such a body, and as our present knowledge both of the physio-
logy and chemistry of latex is scanty, we may regard the contents as being fluids.

As the name implies, all latex consists primarily of a watery, in itself transparent
fluid, in which numerous undissolved small bodies are suspended as an emulsion.
In most cases both parts, the fluid and the bodies, are colourless, and the milk white:
more rarely the milk is orange-red (Chelidonium), or sulphur-yellow (species of
Argemone) ; but it is not possible to define exactly in these cases what share each of
the parts takes in the coloration.

The clear watery fluid contains, as is shown by analyses of those sorts of latex
which are used technically, very various bodies in solution ; others, as indicated by the
phenomena of coagulation, in a highly swollen state. In these two forms there gene-
rally occur in latex, varieties of gum, sugar, small quantities of albumen, often Pectic

‘ Milchsaftgefässe, Vasa lactifera, lactea, or Lebenssaftgefässe, Vasa laticis of authors.

“ [On this point compare the papers cited below by Treub, Scott, Bower, and Schmidt.]

184

LATICIFEROUS TUBES.

bodies (said to occur, e.g. in species of Lactuca), further tannin in many plants, espe-
cially the Aroidese, Musa, also in the Cichoracese, and Euphorbia Lathyris : peculiar
constituents soluble in water are found in many sorts of latex used officinally in the
dry state, as e. g. morphin combined with meconic acid in opium ; and lastly, the
greater part of the ash which appears in analyses. As regards the form in which
the latter occur in the living plant it should be mentioned that salts of malic acid,
especially malate of lime, occur in very large quantity in the latex, at least of many
Euphorbias. In the officinal Euphorbia (E. resinifera, Berg) the latter salt is found
in large quantity : in the latex of one-year-old plants of E. Lathyris it occurs in
autumn in so large quantity that when a drop of latex escapes into the air it imme-
diately precipitates innumerable crystals h

As soon as latex comes in contact with the air, and still more quickly on treat-
ment with water, alcohol, ether, or acids, coagula appear in the hitherto apparently
homogeneous, clear fluid itself, and independently of the aggregation of the insoluble
bodies, described by Mohl (Bot. Ztg. 1843, No. 33). The coagula collect together,
and separate with the insoluble bodies from the clear fluid. These phenomena of
coagulation, which appear under the action of so various agencies, point especially to
a complicated composition of the fluid, and deserve further investigation.

The suspended bodies are present in the fluid in varying quantity, and the
cloudiness of it is also variable, according to the age of the part, and according to
the species : e. g. Morus, Nerium, and Stapelia show slight cloudiness ; most species
of Eicus and Asclepias have dense white milk. Excepting the starch-grains of the
Euphorbias, which will be described below, the bodies have the form of round
granules. They are in most cases — e. g. Euphorbia, and all plants with reticulate
tubes — immeasurably small, and, when in free drops, they show active Brownian
movement. The latex of the Artocarpeae and Morese shows larger granules. They
have in Eicus Carica an average diameter of 3jLi(i.4/x to 5 . 6 ju), and show
concentric stratification, as found by Caruel ^ the larger having three layers of
almost equal thickness surrounding a small nucleus, the smaller only two layers.
The outermost layer is sharply distinguished by different refrangibility from the inner
ones. The granules of the latex of Eicus elastica, Broussonetia papyrifera, Madura
aurantiaca, have in the main the same structure ; also, though less distinctly, the very
variously sized ones of Morus nigra : all these granules are soft and sticky, and readily
adhere and collect together after leaving the plant.

The slightly milky latex, which escapes from young petioles of Nerium Oleander,
contains pale, apparently homogeneous, spherical bodies of unequal size, two or more
of which are often adherent to one another : the -larger of these exceed those of the
Eig. in size. Much larger spheres are described in the case of Musa.

On the chemical nature of the g^ranules the existing chemical analyses tell us that
— leaving out of account relatively very small quantities of substances characteristic of
special cases, such as the alkaloids of Opium, which are insoluble in water — they may
be generally designated on the one hand as resins, on the other as consisting of

’ The chemical definition of the crystals I owe to the kindness of Professor Fliickiger.

^ Sur les granules particuliers du sue laiteux du figuier, Bull, de la Soc. Bot. de France, XII
(1865), p. 273.

LATEX.

185

Caoutchouc. There are also found relatively small quantities of fat, and wax-like
bodies : of the latter a large quantity is described only in Galactodendron (Solly’s Ga-
lactin). Resins are abundant, e.g. in the Euphorbias, and in Opium. Caoutchouc, on
the other hand, is stated to exist in the latex of very numerous species, belonging to
the most various families of Dicotyledons. It forms sometimes the large majority of
the constituents which are insoluble in water, as in the Euphorbiacese (species of
Hevea), Artocarpeae {species of Ficus, Castillea), Apocynacese (species of Haucornea,
Urceola, Landolphia, Vahea), which yield the Caoutchouc of commerce, to which,
according to existing statements, might be added the Asclepiadaceae (Calotropis
gigantea) and Lobelia Cautschuk^. In other cases, according to certain unreliable
statements, it occurs in small quantities in many sorts of latex, e.g. in that of Lactuca
virosa an4 Papaver somniferum. It is still uncertain whether the constituent described
as Caoutchouc or ‘india-rubber’ is universally the same chemically distinct body.
Further, it is uncertain whether the body or bodies included under this name are the
sole constituent of the granules of latex, or whether these each consist of a mixture of
different substances. The above-mentioned differentiation of the granules of Ficus,
which certainly consist chiefly of Caoutchouc, into layers of different refrangibility,
makes the latter view more probable in this case.

Besides the bodies described, there are found in the latex of the Euphorbias
numerous starch-grains In the herbaceous (Tithymalus) species they are usually
of the form of cylindrical or spindle-shaped rods, which in E. Lathyris grow to 55/x
in length, and lo/i in thickness, in E. Cyparissias to 40/x in length, and 6/x in
thickness ; more rarely they have a roundish form, or (especially in E. Myrsinites)
rather swollen ends. In the shrubby and succulent species of hot latitudes they are
shaped like a flattened rod, and appear from the narrow side linear-spindle-shaped ;
seen from the broad side they show a massive broad central part, and much widened,
roundish spatula-shaped, often lobed ends Also in others but by no means all of
the Euphorbiacese rod-shaped starch-grains occur in the latex : e. g. spindle-shaped
ones in Excsecaria sebifera, Müll., staff-shaped in Hura crepitans k How far the blue
coloration, which Plartig ® saw appear with iodine in glycerine in the latex of Cheli-
donium, and which Trecul ® saw with iodine in that of Nerium, Cerbera Manghas, &c.,
after boiling with potash, arises from very small starch-granules, remains to be further
investigated.

The above observations, compiled from data at hand, will show sufficiently well how
little is known for certain of the anatomy of latex, which has been entirely neglected
since MohFs work of the year 1843, and how much may be expected from renewed
investigation. The same holds with regard to the chemical conditions. A number of
investigations have been made in this direction on sorts of latex, such as Opium,
Euphorbium, &c., which are used technically or medicinally, without giving any

^ Compare on the plants which yield Caoutchouc, Collins, Report on the Caoutchouc of Com-
merce, London, 1872 ; Wiesner, Rohstoffe des Pflanzenreichs, p. 153.

^ Rafn (Pflanzenphysiol. p. 88) first noted them ; they were first recognised as starch by
T. Hartig, 1835 ; Erdmann and Schweigger-Seidel, Journ. f. pract. Chemie, Bd. V. p. 4.

® Compare Meyen, Physiol. /. c. ; Nageli, Stärkekörner, p. 428; Weiss und Wiesner, Botan.
Zeitg. 1861, p. 41, 1862, p. 125.

* Vogl, in Pringsheim’s Jahrb. V.

® Comptes Rendus, tom. LXI (1865), p. 156.

^ Botan. Zeitg. 1862, p. 100.

LATICIFEROUS TUBES.

I 86

ground for deciding upon the possible chemical changes which result from drying
in the air. Reference must be made to the technical, and especially the pharma-
cological literature, for information on the above-mentioned investigations^. Here
only some few results may be given of analyses of fresh specimens of latex, or of such
as were kept so as to prevent drying up, by way of giving a rough idea.

Faraday* investigated the latex of a rubber-tree of the family Euphorbiaceae— ‘ Hevea
elastica, Siphonia elastica, Pers.,’ probably H. guyanensis — which had been sent to
England in closed bottles. The fluid contained in looo parts —

Water with an organic acid . . . . .

563

Caoutchouc

317

Albumen

19

Bitter substance (with much nitrogen) and some wax

71-3

Bodies insoluble in alcohol, soluble in water

29-1

The preserved latex of Galactodendron utile contains,
100 parts —

according

to Heintz *

Water

57-3

Albumen

0.4

Wax (CagH.eO^)

5-8

Resin (C 35 H 58 0 2)

31-4

Gum and Sugar . . . . .

4-7

Ash ........

0-4

in

Weiss and Wiesner ^ investigated the fresh latex of some indigenous Euphorbias. In the
slightly acid latex of E. Cyparissias they found in loo parts —

W ater

72-13

Resin

15-72

Gum

3-64

Sugar and extractives

4-13

Albumen .

. . 0*14

Ash ....

For comparison with these may be given the composition, as found by Flückiger®, of
Euphorbium, i.e. the fixed residue of the latex of E. resin ifera —

Amorphous resin (C 20 H32 OJ ... 38

Euphorbon (Cjjg H44 O2) . . . . 22

Mucilage® . . . . . . 18

Malates, especially of calcium and sodium * 12

Other constituents of ash . . . 10

100

Sect. 46. The tubes themselves, in which the latex is contained, are all alike in
certain points of structure and arrangement, but may be divided, according to form
and development, into two categories, articulated and non-articidated laticiferous tubes.
Each of these categories is peculiar to definite families, the articulated tubes being

* Compare Wiesner, Rohstoffe des Pflanzenreichs ; Flückiger, Pharmacognosie •, Flückiger and
Hanbnry, Pharmacographia ; Rochleder, Phytochemie, &c.; also Meyen, Physioi. II. l.c.

“ Compare Berzelius, Jahresbericht for 1827 (German by Wohler), p. 246.

^ Poggendorffs Ann. 65 (1845), p. 240. ‘ Botan. Zeitg. l.c.

^ According to Flückiger and Hanbury, Pharmacographia, p. 504.

® Probably inclusive of starch or its products of decomposition.

’ Compare the former statement for E. Lathyris.

LATICIFEROUS TUBES.

187

found in the Cichoracecc, Cafnpanulacece, Lobeliacea (and, according toTrecul, in Gun-
delia Tournefortii, one of the CynaraceoE), the PapayacecB, many Papaveracece (Papaver
Roemeria, Argemone, Chelidonium, but not Glauciuin, hlacleya, Sanguinaria), many
AroidecB, and MusacecE. The 7i07i-ariiculated tubes are found in the Euphorbiacea,
UrticacecE in the wider sense (including the Artocarpeae, and Moreae), Apocynacece,
and Asclepiadacea.

The properties common to all are, firstly, that they traverse the whole length of
the mature parts of the plants as continuous, and, with rare exceptions (Musa, Che-
lidonium), frequently-branched tubes ; and not only do they traverse each single
member of the plant on its own account, but they send branches from these into all
the like and unlike lateral appendages.

Secondly, all laticiferous tubes have, as was above noted, soft, apparently very
watery, smooth or flatly pitted cellulose walls, which often show the characteristic
iodine reaction of collenchymatous walls (cf. p. 120). These are in many cases
very delicate, and without recognisable minute structure: thus, e.g. almost universally
in the Aroidese, in Vinca, Asclepias curassavica, and in other cases in the thin
branches of higher order. The membrane of the stronger stems and branches of
most tubes is thickened, and appears as though swollen in transverse sections, having
delicate stratification, while striation is also seen, especially'in the strongly-thickened
tubes of woody stems (species of Euphorbia, Nerium). The thickening increases
with age. Even in the very thick membranes no sculpture of the surface can be
recognised : on the other hand delicate transversely-oval pits are often seen, e. g. in
the tubes of Plumiera alba, in those of the base of the stem of Campanula Medium
(Trecul), and in old stems of Lobelia syphilitica. In the base of the stem of species"
of Argemone are found crowded together band- and knot-shaped thickenings which
protrude inwards. The wall of the tubes of Plumiera alba may be split, according
to Trdcul, into spiral bands of 10^4-15// in breadth.

Further, pits occur less commonly on the lateral walls than appears at first sight,
especially in articulated laticiferous tubes, since the lateral wall of these, especially
when old, has often very numerous and short protrusions, which give, in surface-view,
the figure of pits with delicate contour. I could never find support for those state-
ments, according to which the pits of the lateral wall have the structure of sieve-
plates.

As will be more completely described in Chapter XII, but must here be only
briefly stated, the laticiferous tubes permeate the whole body of the plant, in most cases
as a continuous system, sending branches from the stem into all lateral members.
These branches often push their way between the elements of all regions, and of all
tissues of other sorts than themselves. As regards the main branches or stems of the
tubes however it is generally the case that they follow the vascular bundles, i. e. the
wood and bast, as concomitants of, or sometim'es even substitutes for the sieve-tubes.
In this course they often approach very closely in point of distance to Tracheae,
especially vessels, and on this circumstance, together with the above-mentioned
(p. 169) occurrence of apparently coagulated latex within the vessels of laticiferous
plants, depend the controversies on the anatomical relations between these two
series of organs. The fact is that the tracheae of the furthest ends of vascular
bundles in the laminae of leaves are often accompanied by branches of laticiferous

i88

LATICIFEROUS TUBES.

tubes, and are in immediate contact with these ^ : further, that in the xylem of the
stem of the Papayaceae the laticiferous tubes are directly and firmly attached to the
large vessels, sometimes throughout their length, sometimes by single ends of their
branches ^ : again, that a similar relation exists between the laticiferous tubes which
accompany the vascular bundles of many Aroidese and the tracheae which belong to
them ®. Finally, it is an indubitable fact that in sections through plants having latici-
ferous tubes, numerous vessels are often to be found filled with coagulated masses,
which appear very similar to the coagulated latex of the plant, and which also
have its characteristic colour, e. g. in Chelidonium reddish yellow b This latter
phenomenon often occurs very conspicuously in roots, and under conditions which
do not allow of the idea of a flow of the latex from a cut surface. Trecul concludes
from this series of facts, observed by him in many cases, that in all plants with
laticiferous tubes at least single branches of the tubes come into direct contact with
tracheae, and open communication is set up by perforation of single portions of the
Avail at the points of contact ^ He even states that he has directly observed the
points of perforation, e. g. in Lobelia laxiflora. Other observers, among whom I
must place myself, according to my investigations up to the present time, have been
unable to find such contact and communication of milk-tubes with the tracheae,
with the exception of the above-mentioned cases of the Aroideae and Papayaceae : on
the other hand, they have seen that where branches of milk-tubes run from the
cortex to the pith, they take a course by the medullary rays through the woody or
vascular ring. Trecul’s statements accordingly require further proof : in the first
place, that on the general contiguity of milk-tubes and vessels, and secondly, that
regarding the open anastomosis of milk-tubes into vessels, which should be confirmed
in the case in which it was observed by him, and, at all events, in the Papayaceae and
Aroideae. According to the present data these anastomoses occur very seldom,
and it is extremely difficult to prove them with certainty by direct observation, and to
distinguish them from non- perforated pits. If they do really occur, still it is not
proved that they are proper to normal tissue, and not really pathological phenomena,
i. e. ruptures in the thin places of contact of the tubes, which arise in the same way,
through the pressure of the turgescent parenchyma, as the discharge of latex on
surfaces of section. The apparent presence of coagulated latex in vessels is evidence
at first sight of the existence of open pores : the normal existence of these is however
made the more doubtful by the fact that, as far as investigated, the occurrence appears
to be quite irregular and inconstant, and that milky or resinous coagula are found in
vessels even in such plants as have no laticiferous tubes, but closed secretory cavities,
without any open connection with the vessels.

If an anastomosis, or even a mere contiguity of milk-tubes with the tracheae,
does not occur in most cases, and I consider this most probable, there is then no ex-
planation of the occurrence of what appears to be coagulated latex in the latter : for
such an explanation it is however necessary, in the first place, to answer the question,

^ Compare, e. g. Hanstein, Milchsaftgefasse, Taf. IX. fig. 13 (Lactuca virosa).

“ Trecul, Ann. Sei. Nat. 4 ser. VII. p. 28g, pi. 12 (1857) ; Comptes Rendus, tom. 45, p. 402.

® Compare Hanstein, l.c. ; Van Tieghem, Structure des Aroidees, l.c. Taf. II. fig. i, pp. 6-8.

* [Compare von Höhnel, Milclisaft in Tracheen, See. Fef. Eot. Jahresbericht, 1878, I. p. 30.]

5 Compare especially Comptes Rendus, tom. LX (1867), p. 78-

LATICIFEROUS VESSELS.

189

whether those coagula. jvre really latex which came as such^fiom the tubes, and not
products of coagulation of fluids which had diffused through the walls of the vessels.

Sect. 47. The distinctions of the two categories of laticiferous tubes depend upon
certain phenomena of their development and form. The ariiculuted series, as types of
which those of the Cichoracese, Papaveraceae, and Papayacese may serve, arise from
series of elongated cells of the meristem (or cambium), which coalesce, by perforation
of their septa, to form continuous tubes. In the simplest case, which occurs in Musa
and Chelidonium (Figs. 80, 81), the tubes remain simple, or are branched, and con-
nected in a net-work only inasmuch as one series of their original members may

FiC. 80. — Chelidonium majus ; tangential section through the
secondary cortex of an old root ; m — m and b — b portions of milk-
tubes between the cells of the parenchyma. At a — a m passes
beneath the parenchymatous ceils (225).

FIG. 81. — Chelidonium majus; stem, cortex, radial
section, part of a milk-tube with a perforated sep-
tum at s (225).

continue its course from any given point as two diverging series, and conversely.
The septa between the original members are here perforated only in the middle by
one or few holes ; their margin is persistent ; occasional large openings in the lateral
wall also occur in rare cases, where two tubes are directly contiguous.

In the majority of really typical cases the septa between the members of each
series soon disappear completely, so that in the mature tube no trace of them re-
mains. Occasionally in such cases single septa may persist through life.

The tube puts out lateral protrusions, usually at numerous points, which force

190

LATICIFEROÜS TUBES.

themselves between th% neighbouring unlike tissue-elements, and grow out to
cylindrical branches, which sometimes remain short, not longer than broad, and
sometimes attain a considerable length. Some of these protrusions end blind, others
join with similar ones from neighbouring tubes, or with the trunks of these, and
open communication is formed by disappearance of the wall at the point of contact.
Where two tubes run longitudinally side by side, they are further directly connected
by numerous large perforations of the wall of contact. Thus there arises a net of
communicating tubes which is usually very complicated, with meshes of most various
form and size, and with blind branches of various length and direction, imbedded in

Fig. 82. — Tangential section from the
cortex of Lactuca viro.sa, with tliree reti-
culately joined milk-tubes (225).

FIG. 83. — Scovzunera hi.-ipanica ; w sliglitiy magnified tan-
gential longitudinal section through the bast of the root. In
the parenchyma the reticulately connected milk-tubes; 13
■ piece of a miik-tube and its stirroundings, more highly mag-
nified. From Sachs’ Textbook.

surrounding — usually parenchymatous — urfljke tissue (comp. Figs. 82, 83). This net-
work, as above stated, extends throughout the whole plant. Non-reticulate articulated
tubes, such as those of Chelidonium, are branched, at least at the points of insertion
of lateral ramifications, and send out branches from these poir.ts into the latter.

Sfxt. 48. The non-articulated latieiferous tubes do not show a net-like

LATICIFEROUS CELLS.

191

anastomosis in any well-constituted case ; all their brancjies, which are often very
numerous, end blind (Fig. 84). Anastomoses may occur between their branches in
the nodes of many plants (leafy Euphorbias), but this is quite uncertain. Each tube
arises, not from a series of coalescing cells, but from one single cell of the meristem,
which grows so as to form a long
branched sac, and forces its branches be-
tween the other tissue-elements. The
statements concerning their first develop-
ment differ greatly. According to Schmal-
hausen’s investigations on species of
Euphorbia and Asclepiadacese, to be more
fully stated below, some few cells of the
meristem in the cotyledonary node of the
embryo, outside the plerome, are the
starting-points of the milk-tubes. These
begin, even in the young embryo, to
elongate into cylindrical sacs, and to
penetrate with their growing ends be-
tween the neighbouring cells into the
cotyledons and towards the end of the
root : they have at an early stage oc-
casional branches in the cotyledonary
node. All hides in the primary cortex,
the leaves, and the pith of the mature
plant are branches of these few sacs, which
are present in the young embryo. From
the embryonic stage onwards their ends
extend to close (6-8 cells) beneath the
primary growing points, and grow on-
wards with these, sending branches, which

have the same properties, into the lateral are broken off. .s from the stem of

^ ^ Ceropegia stapelioides, terminal branches of a non-articulated

buds, leaves, and roots, as soon as these milk-tube prepared free, with numerous blmd ends of branches

(145).

make their appearance. Lastly, they

branch further and elongate in the meristem and the young parts so as to form the
final system of tubes. The whole plant, e. g. a shrub of Euphorbia the height of a
man, has thus only few, much-branched milk-tubes, the ends of the branches of
which extend on the one hand into all growing points, and grow with these to
an unlimited extent, on the other hand they are distributed in the mature tissues
in the manner described, and end blind. As a matter of fact pieces of tubes an
inch long, with hundreds of branches, may be teased out of macerated portions of
the stem without finding a single anastomosis, or any other blind ending than
those of small lateral branches (comp. Fig. 84 A).

According to observed facts the possibility cannot be denied, that in later stages
of development of a plant, especially in the nodes, single cells of the meristem develope
into new milk-tubes, and may coalesce as branches with those which originate from
the embryo. However, the occurrence of this phenomenon, if indeed it occurs at all,

Fig, 84. — A part of a trunk of a milk-tube, with its stronger
brandies prepared free and spread out, hardly larger than
natural size, from the stem of Euphorbia splendens. All ends

192

LATICIFEROUS TUBES.

is very restricted, and not clearly proved hitherto by any observation. I was unable,
even in the milk-tubes, which are present in large quantities throughout the secondary
bast of Morus, Ficus, Madura, and Nerium, to prove that they arise successively
afresh from the cambium (Chap. XIV), and are not branches of the original tubes
which thrust themselves into the layers of secondary bast. A positive proof that the
latter is the case has, it is true, not been as yet obtained.

The peculiarities of form and structure of milk-tubes in special cases of their
occurrence will, in order to avoid unnecessary repetition, be described together
with the treatment of their arrangement in Chap. XII.

The peculiar juice which flows from milky plants, and the peculiar tubes which
contain it within the plant, were known to the fathers of vegetable anatomy, but with-
out their having sharply distinguished them from the reservoirs of resin and other se-
cretions, described in Sect. 34, and from the intercellular passages which so frequently
have contents of similar appearance. The terms then in use, viz. Succi proprii, and
reservoirs of such peculiar juices, referred rather to these latter forms of tissue and
cavities, or to their contents After many more or less successful attempts to distin-
guish them accurately and separate them (the history of which may be found in Meyen
and Treviranus), C. H. Schultz-Schultzenstein from 1823 onwards^ drew the attention
of his contemporaries to the tubes with which we are engaged, and earned for himself
in his later works, especially two large publications in the year 1841^, the merit at least
of isolating by maceration the network of tubes of the Cichoraceae, Campanulaceae,
and the tubes of the Euphorbias, &c., and drawing them for the most part correctly
in their main points. It is true his works were rendered unpalatable by his terrible
views on circulation, or, as he calls it, Cyclosis of the milk, or sap of life (Latex) in the
‘Vasa laticifera,’ and that which was really good in his observations was hidden by
the mass of preposterous statements and representations into which he let himself
be led by the idea that networks of vessels of sap of life must be universally present.
Misunderstood sieve -tubes (the structure of these v/as quite unknown before 1837),
fungal hyphre (comp. Cyclosis, Taf. IV), and many objects which cannot be defined ac-
curately from the figures, were confounded with real milk-tubes Almost simultane-
ously with these larger publications of Schultz, and in connection with them, Meyen*
gave, on the whole, very good representations of the structure of a number of latex-
tubes, but without giving any close attention to the most elaborate networks of tubes
such as occur in the Cichoracese and their allies. The knowledge of the structure
and distribution of the tubes was further advanced by Hanstein ®, Dippel ®, and other
authors, to be named later, who have worked at their development ; Hartig'^ having
first sharply distinguished the articulated from the non-articulated forms, and Unger *

' Compare Treviranus, Physiol. I. p. 137, &c.; Meyen, N. Syst. d. Pflanzenphysiol. II. p. 371, &c.
^ Ueber den Kreislauf des Saftes im .Schöllkraut.

“ Die Cyclose des Lebenssaftes in den Pflanzen, Nov. Acta Acad. Leöpoldino-Carolin, vol. 18,
Supplem. II. p. 336 S. 33 Taf. — Memoire pour servir de reponse aux questions de l’Acad. des Sei.
pour l’Annee 1833 ; Mem. pres. ä l’Acad. des Sei. torn. VII. p. 104 S. 23 Taf.

* Die Secretionsorgane der Pflanzen (1837), Physiol. II. pp. 376-386.

° Die Milchsaftgefässe, &c. Berlin, 1864.

® Entstehung der Milchsaftgefässe, Verhandl. d. Bataafsch. Genootschap. &c. te Rotterdam,
torn. XII. p. 3 (1865).

’’ Botan. Zeitg. 1862, p. 99.

Anatomie und Physiologie, p. 157.

LATICIFEROUS TUBES, DEVELOPMENT.

^93

having given a short and clear, though not quite correct review of the leading types.
A great service in advancing the knowledge of these organs was further rendered by
Trecul, who, in a series of papers published since 1862, described his very numerous
observations, and thereby gave the impulse to the new works on the development of
these organs. It is true Tr^cuP inclines again to the old idea of circulation, and places
the milk-tubes nearer other reservoirs of ‘ sues propres ’ than is allowable from an
anatomical point of view.

Finally, VogP has supplied a number of valuable contributions and confirmatory
observations.

The history of the origin of the milk-tubes, so indispensable to a clear under-
standing of their structure, remained long in the dark^. Unger’s view according to
which (from observations on Ficus benghalensis) they arise from rows of cylindrical
cells by coalescence, found no response, and opinions remained undefined, till in
1846 the often-mentioned anonymous writer in the Botanische Zeitung expressed
the opinion, as the result of an extended series of investigations, that each milk-tube is
originally an intercellular passage without a wall of its own, which is subsequently
provided with a membrane peculiar to it, through the agency of the adjoining cells.
The contradiction by Schacht® of this view, which was at first not unfavourably
received, and the subsequent answer by Trecul, called forth the series of newer
works, by which the anonymous writer was refuted, and a clearer knowledge of the
matter was gained, at least in many particulars. Unger formulated his view afresh in
1855®, in these words. The milk-tubes are ‘shorter or longer, cylindrical, irregular
or branched cells, filled with opaque milky or dark coloured juice, which fuse
with one another in longitudinal rows or at their points of branching.’ He lays

‘ From the series of Trecul’s papers which treated of ‘ sues propres,’ those on milk-tubes may
here be named : Des vaisseaux propres en general et de ceux des Cynarees laiteuses en particulier,
L’Institut, 1862, p. 266. — De la presence du latex dans les vaisseaux spiraux .... et de la circulation
dans les plantes, Comptes Rendus, tom. 45, p. 402 (1857). — Des laticiferes dans les Papaveracees,
Ibid. tom. 60, p. 522 (1865). — Sur les laticiferes des Euphorbes, &c.. Ibid. tom. 60, p. 1349. — Lati-
ciferes et liber des Apocynees et des Asclepiadees, &c.. Ibid. tom. 61, p, 1349 ! L’Institut, 1862, p. 215.
— Des laticiferes dans les Chicoracees, Ibid. tom. 61, p. 785 (1865). — Des laticiferes dans les Cam-
panulacees. Ibid. p. 929. — Des vaisseaux propres dans les Aroidees, Ibid. tom. 61, p. 1163 (1865),
et tom. 62, p. 29 (1866). — Matiere amylacee .... dans les vaisseaux du latex de plusieurs Apocynees,
Ibid. tom. 61, p. 156 (1865). — Rapport des laticiferes avec le Systeme fibro-vasculaire. Ibid. tom. 51,
p. 871 (i860). — Rapports des vaisseaux du latex avec le Systeme fibro-vasculaire; ouvertures entre
les laticiferes et les fibres ligneuses ou les vaisseaux; Ibid. tom. 60, p. 78 (1865). — Des vaisseaux
propres et du tannin dans les Musacees, Ibid. tom. 66, p. 462 (1868).— Most of these works were
reprinted in the Annales des Sciences Naturelles; all in Baillon’s Adansonia, tom.VII-IX.

“ Ueber die Intercellularsubstanz und die Milchsaftgefässe in der Wurzel des gemeinen Löwen-
zahns, Sitzungsbr. d. Wiener Acad. Bd. 48. — Beiträge z. Kenntniss der Milchsaftorgane d. Pfl.,
Pringsheim’s Jahrb. V.

^ [On this subject see further Schmalhausen, Beitr. z. Kenntniss d. Milchsaftbehälter d. Pflanzen,
Mem. de I’Acad. Imp. de St. Petersbourg, XXIV. 1877. — P'aivre, Compt. rend. hebd. t. LXXXVIII. —
D. H. Scott, Development of articulated laticiferous vessels. Quart. Joum. Micr. Sei. 1882. — E.
Schmidt, Botan. Zeitg. 1882, p. 435. — Treub, Comptes rend. 1 Sept. 1879, Archives Neerlandaises,
t. XV].

* Annalen des Wiener Museums f. Naturgesch. bd. II (1840), p. ii, where it is attempted to
adduce a proof, weak enough even for that time. — Endlicher u. Unger, Grundzüge (1843), p. 40.

® Botan. Zeitg. 1851, p. 513. “ Anatomie und Physiologie, p. 157.

o

194

LATICIFEROUS TUBES.

special stress upon the coalescence of originally separate cells, since he places all
milk-tubes among his ‘ cell-fusions,’ and only cites as cases where the cells do not
coalesce to tubes, those of Chelidonium, in which plant he overlooked the perforation
of the septa, and those of Sanguinaria, which must be entirely excluded from the
category of milk-tubes. All investigators acceded in the main to Unger’s view.
Dippel and Haustein made thorough investigations, which proved it clearly for many
cases (articulated tubes). Schacht had already published at an earlier date ‘ an excel-
lent history of the development of the tubes of Papaya from coalescing cells of the
meristem, and greatly shaken thereby his own view repeatedly proclaimed since the
above-cited work of 1851, according to which the milk-tubes are no special form of
tissue at all, but only ‘ Bast cells,’ i. e. sclerenchymatous bast fibres, filled with latex ^

All the later authors mentioned, who expressed their views on the subject,
extended the above theory of coalescence to all milk-tubes, both articulated and non-
articulated. The first objections to this are to be found indicated by Hartig, but have
recently been more clearly brought forward by David As may be seen in the above
statements, the two sorts of tubes should be treated separately.

For the articulated tubes it can no longer be doubted, after the excellent descrip-
tions of their development by Schacht, and particularly those by Dippel, that they
arise, as above described, by coalescence of cells. Without the development having
been traced step by step, this follows in the case of the tubes connected into a net
from the facf that in an earlier stage there are only simple cells of the meristem in the
position afterwards occupied by the network. The fact may be proved most clearly
in the secondary cortex of the Cichoraceae. In the tubes of Chelidonium, which are
not connected into a network, the limits of the original cells remain partially preserved
through life.

The non-articulated tubes offer much greater difficulties. Most authors since
1846 have simply extended to them the same history of development as was observed
for the articulated lubes ; only Dippel and David attempted to get to the bottom of
the question by direct observation. Dippel followed the tubes, in Ficus Carica and
Euphorbia splendens, into the youngest meristem of the growing point, and close to
the latter he found here and there septa in the tubes : he found such septa also in
occasional preparations of old tubes of Euphorbia Cyparissias, Asclepias curassavica,
Nerium Oleander, and Vinca minor, and concludes from this that they arise by
coalescence.

David arrived at a quite different result for the families named in the title of his
dissertation, of which he investigated the following species specially; Euphorbia
splendens, Caput-medusse, Lathyris ; Ficus elastica, Carica ; Nerium Oleander, and
Hoya carnosa. According to him each non-articulated milk-tube is one cell, ‘ a milk-
cell,’ originating at an early stage by the elongation of one single cell of the meristem,
which branches, and thrusts itself between the elements of the surrounding tissue. All
the branches of each of these cells end blind and closed : the cell may grow to a great

‘ Monatsbr. d. Berliner Academie, 1856, l.c.

^ [F. O. Bower, on Gnetum, Q. J- M. S. 1882.]

^ Ueber die Milchzellen der Euphorbiaceen, Moreen, Apocyneen, imd Asclepiadeen ; Dissert.
Breslau, 1872.

LATICIFEROUS CELLS, DEVELOPMENT.

195

length, in E. splendens, e. g. more than 1 2"™^ in E. Lathyris, the length of an inter-
node plus the leaf belonging to it, that is about 20 cm. As the plant grows, new
milk-cells are formed in the growing point of the stem : the tubes of the leaves are
only branches of those which traverse the stem. It is obvious that this view scarcely
differs except in name from Schacht’s theory of bast cells. David’s chief proof of his
statement was obtained by him by macerating sections of the apical meristem with
potash, and then teasing out from them the young milk-cells, which are at first short
and spindle-shaped, but gradually become elongated and branched. In non-macerated
sections also he was able to find the required early stages. The examination even
of the mature specimens shows that David’s view is impossible, for, as above shown,
the tubes, e. g. of the Euphorbias, can be followed for any distance, and numerous
blind peripheral ends of branches may be found in leaves, cortex, and growing points,
but never a completely closed tube which is of less length than the whole plant.
Were the tubes originally formed in succession as single completely closed cells of
the growing point, they must have coalesced with one another to form the mature
tubes.

As a matter of fact those solitary spindle-shaped initial cells of the milk-cells do
not exist. The tubes run continuously into the furthest meristem of the growing
point, their ends can be followed to within 6-8 cells of the extreme apex : their
course is variously curved both in radial and tangential direction between the de-
veloping parenchymatous cells; longitudinal sections must thus cut off portions of them,
which are roundish, or spindle-shaped, or cylindrical, and look exactly like cells of
such form, especially if the section be very thin and transparent, or if the delicate,
very readily swelling membrane be swollen by maceration in potash. David’s young
milk-cells are such sections of tubes : they were represented by Dippel, but were
rightly explained.

Dippel’s view does not rest, like that of David, on mistakes of observation, which
might easily be avoided ; it is, from analogy with articulated tubes, extremely probable :
still I was unable, though I investigated the matter repeatedly, with every prejudice in its
favour, to find the view confirmed by observation. If one examines the ends of stems
in species of Euphorbia, Stapelia, or Ficus, which are elongating, but before secondary
thickening begins, the last ends of the tubes and their branches always extend, as has
been repeatedly stated, into the furthest meristem of the growing point, and of its
youngest leaves and branches, and I was never able to observe in their cavity traces
of septa or of any coalescence of originally separate cells. Where septa appeared to
be present in the young ends of tubes, and this was not rarely the case, continued
and careful observation of the preparation always led to the conclusion that these
were not within the tube itself, but belonged to cells above or below it. The state of
things above indicated is best seen in radial longitudinal sections, which have become
quite transparent by maceration for one or several days in very dilute solution of
potash, without very considerable swelling of the cell-membranes, and which are
thick enough to allow of following the tubes for a considerable distance. Accordingly
I can only explain the septa, which Dippel represents as present in the young ends
of milk tubes, as walls outside the tubes, or perhaps also as the contiguous walls of
two tubes cut obliquely. The septa described by him as occurring here and there, I
also have certainly been able to find not uncommonly at the nodes (but only there), in

0 2

196

LATICIFEROUS TUBES,

Euphorbia Lathyris : they are thick transverse cellulose plates in the main tubes.
They perhaps show, as above stated, that new branches and continuations of the
tubes coming from below arise from cells of the meristem, which coalesce with them.
Perhaps they are subsequently formed structures in the originally continuous sac.

Observations on the growing plant, from the stage of germination onwards,
having constantly proved the existence of tubes which extend continuously into the
extreme meristem, there was reason for supposing that these arise in the embryo in
small number only, and that, once formed, they grow on with the plant in such a
way that the whole system of tubes of the stock, exclusive of the layers of secondary
cortex, is derived from their elongation and branching. In order to test this suppo-
sition, Herr J. Schmalhausen undertook in the laboratory at Strassburg an investiga-
tion of the development of tissues in the embryo of species of Euphorbia (E. Lathyris,
Myrsinites, Lagascae). I append the summary of his results up to the present time word
for word, as it was given to me, with the remark that what is said on the coalescence
of branches has, as above indicated, always appeared doubtful to me.

‘ The first initials of the laticiferous tubes appear at a very early stage of develop-
ment of the embryo of Euphorbia. At that moment, when the cotyledons begin to make
themselves prominent, there are single cells, lying approximately in the same transverse
section of the embryo, which first distinguish themselves from those surrounding
them by a peculiar refractive property of the cell-walls, which gives them the
appearance of being swollen. At this time the plerome cylinder is clearly marked
off at the apex of the root from the three-layered mantle of periblem and dermatogen
by a boundary line, which appears sharply in optical section ; while in the upper
cotyledonary portion of the embryo no arrangement in layers is visible. Where the limit
between the plerome cylinder and cortex of the end of the root ceases at the upper
end of the latter, that is in the part belonging to the cotyledonary octants, the cells
in question may be seen so placed that the line separating the plerome cylinder from
the cortex leads up to the base of these cells. These original cells of the laticiferous
tubes extend greatly at first in different directions, so that they can now be easily
recognised by their great size. As the embryo continues to grow the cells increase
in length, and, thrusting their upper and lower ends between the cells of the sur-
rounding tissue, they put out processes upwards into the cotyledons, and downwards
into the end of the root. Besides this, lateral processes are formed, which compose
a felt in the node of the embryo, and surround its growing-point. Thus the sacs of
the embryo are formed, not by coalescence of cells, but by apical growth of the
processes of the original cells, which force themselves between the cells of the
embryo : where two processes meet with their ends, the wall separating them is
often absorbed, and a coalescence of the sacs takes place, as is the case also in the
nodes and between the branches of the main tubes in young leaves (or cotyledons) :
this occasionally occurs also in the apex of the root.

‘ The best proof that the sacs have an independent apical growth, and do not
arise by coalescence of cells, is obtained at the apex of the root, where they have a
direct course. The processes of the original cells, which grow into the apex of the
root, are arranged approximately in two concentric layers ; one series of them
belongs to the plerome cylinder — they permeate a layer of cells of the apex of tlie
root, which is subsequently found to be within the layer of endoderrais ; the others

LATICIFEROUS CELLS, DEVELOPMENT.

197

are found in the 2nd or 3rd layer of cells beneath the surface. Both series have an
almost straight course, and not unfrequently they may be followed through their
whole length from the node of the embryo to the apex of the root. Before reaching
this, they terminate with diminished diameter, measuring hardly half that of the sur-
rounding cells. Behind the tapering end the lateral walls are bulged out, with
teeth, which fit between the surrounding cells : further back the diameter of the sac
increases, the teeth are smoothed down, and their walls are only slightly sinuous : at
the top, where the diameter of the sac is not less than that of the surrounding cells, or
even greater, its walls are smooth. The appearance of such a sac gives the impression
as though it were only with difficulty that it could find room between the cells to push
in its apex, and that it endeavours, by extension, to fill up all possible cavities. The
surrounding cells may at this time have firmer walls, while those of the tube are
soft — therefore the young milk-tubes cling to the surrounding cells, and push out
teeth between them. Later the walls of the tubes may become firmer, the in-
equalities smooth themselves out, and as the other tissues gain more space, the
walls become quite even. Growing in this manner the tubes permeate the ripening
embryo to the extreme apex of the root, and reach the seat of its future growth, viz.
the limit between the apex of the root and root-cap.

‘ In the germinating seed other remarkable phenomena become apparent.
While the apices of the laticiferous tubes grew up to this point between the cells of
a slowly dividing, and slowly growing tissue, they are now surrounded by a quickly
growing tissue, in the focus of growth of the apex of the root. Accordingly they
enlarge here to the diameter of the surrounding cells, and even exceed them sometimes,
and terminate with bluntly rounded ends, which often appear swollen, at the limit
between the root-apex and the root-cap. The ends of the tubes are easily recognised
by means of their dense contents; their blunt ends — like plastic masses— are easily
found, and clearly marked. But there are never seen even traces of septa in course
of solution, which would certainly be found here, if the tubes were formed by
the disappearance of the walls separating the cells from one another. From the
examination of longitudinal sections through the apices of roots of seedlings, it must
certainly be concluded that the laticiferous tubes of the root of Euphorbia have an
independent apical grow’th, and grow on continuously at the apex with the other
tissues of the root.

‘ At the apical point of the stem the behaviour of the milk-tubes is much more
difficult to observe, since they here take a very irregular, crooked course, and there-
fore cannot be follow'ed for long distances. Terminations of the tubes are some-
times to be found above the youngest leaves, but their connection with tubes lower
down could never be proved b in the nodes a felt of tubes is always formed, from
W'hich branches run up towards the apical cone. There is no evidence against the
tubes having here a growth fundamentally similar to that in the apex of the root, and
none to support the idea that new milk-cells are successively formed in the growing
point, which would subsequently grow out to tubes.

‘ It appears as though all the tubes of the Euphorbia plant owe their origin to a
process of branching of the original cells formed in the embryo.

* I was able to prove this repeatedly in E. splendens and trigona. — De Bary.

19B

LATICIFEROUS TUBES.

‘ In Asclepiadeae and Apocynese (apparently also in Ficus) the same seems to be
the case also in the apex of the root of the germinating seed. The tubes are much
narrower and difficult to follow. In the apex of the root they are distributed
throughout the cortex. My observations on them are in other respects still very
incomplete.’

The origin of the above-mentioned milk-tubes in the secondary bast, which is
formed from the cambium, in Ficus, Morus, Broussonetia, Madura, and Nerium, is
not explained by the above observations ; and I am unable, either from published
accounts, or from my own observations, to propound any well-founded view whether
they arise as branches of those in the primary cortex, and make their way into the
secondary bast, or whether they are successively formed anew from the cambium, and
in that case are in no direct connection with the primary tubes. Madura aurantiaca
may be especially recommended for the further pursuit of this question.

However great the differences in development, above noticed, between the non-
articulated and the articulated tubes, still the frequently abundant branching of single
members of the latter, and perhaps the coalescence of branches of the former to
form anastomoses in the nodes, constitute a transition between the extremes of both
categories; and even without these, other common points of structure and dis-
tribution, and of functions as indicated by the latter, would certainly lead to
uniting both as one sort of tissue.

Brief allusion has above been made, in accordance with the views of Dippel and
Flanstein, to their near relations to the sieve-tubes : this subject will be returned to in
Chap. XII. These relations are mainly physiological, and topographically-anatomical.
The assertions as to close histological relations between milk- and sieve-tubes are, I
believe, wrong, or at least exaggerated ; for instance Yogi’s assertion that both the
resin-passages of the Convolvulacese, which do not belong to this category, and the
true milk-tubes, e.g. in the Campanulaceae, develope from sieve-tubes, oral least may
do so. Such a development never occurs. Again, it is especially stated by Dippel,
both that the septa in articulated tubes — e. g. in Chelidoniura and Papaver — are
perforated like sieve-plates ; and also that the lateral walls (e. g. Papaver, Cicho-
racese, Carica) are often provided with sieve-plates, and that thus there are in these
cases intermediate forms between sieve-tubes and articulated milk-tubes. I could never
find this phenomenon, but rather only smooth, delicately outlined pits on the lateral
walls, or wide, sometimes grouped perforations. Schmalhausen’s investigations on this
point also gave the same result. Further, Dippel’s drawings of these structures show
little similarity to true sieve -plates of dicotyledonous plants. The septa often resemble
them, it is true, inasmuch as they are perforated by more than one pore ; but the pores
are coarse, wide, and irregular, and have none of the other characteristics of structure
of sieve-plates (comp. Figs. 80, 81). On the contrary, the sharp difference of structure
between the sieve- and milk-tubes is always particularly clear, where one w'ould
a priori the most expect to find intermediate forms, i. e. where both are closely side
by side, as is the case in the secondary bast.

The view, so long held, as to the near morphological relations between milk-
tubes and sclerenchymatous fibres — ‘ bast-cells ’ — has been above refuted. It was
e^xcusable that Mirbel at the beginning of this century confused the two organs, and
came in 1835 to the conclusion that all the sclerenchymatous fibres of the secondary

LATICIFEROUS TUBES.

199

bast are ‘ latexiferes b’ But Schacht’s joy at having got rid of the laticiferous vessels
and recognising them as branched ‘ bast-cells,’ which he has repeatedly expressed since
1851, was obviously based upon his failure to recognise the true, delicate milk- tubes
in Hoya and herbaceous Euphorbias, a failure which had less excuse at his time ;
further upon the opinion that the latex of these plants was contained in the thick-
walled sclerenchymatous fibres, which are often branched, and upon cognate and
further confusion of the latter with the well-distinguished thick-walled milk-tubes of
Nerium and succulent Euphorbias.

Finally, it is undeniable that in the family of the Papaveraceae, in the Aroideae,
and Musaceae, there is a near relation between the milk- tubes and peculiar sacs con-
taining colouring matters and tannin. In the first the milk-tubes are absent in the
rhizome of Sanguinaria, in Glaucium, Macleya, and the sacs containing colouring
matter appear in their place : the other above-named genera are without these, and
have laticiferous tubes. All those Aroideae and Musaceae which have been examined
have tannin-sacs, variously distributed; in certain forms there are also found milk-
tubes extremely rich in tannin, in place of which in many Aroideae there are only rows
of tannin-sacs. (On this point compare also Chap. XII.) All these anatomical
relations remain inexplicable, till we know better than at present the physiological
significance of their different contents. Here we cannot do more than draw attention
to them. The anatomical facts above mentioned suggest that under the name Milk-
tubes there are at present united two sorts of structures, which do not correspond in
function, namely, in the first place those of the Aroideae and Musaceae, containing
more especially tannin; on the other those of other milky plants, which have little
or no tannin, and which are closely related to the sieve-tubes.

’ Mirbel, Exposition de ma theorie, &c., Paris, 1809, p. 247, &c. — Idem, Ann. Sei. Nat. 2 ser.
tom. III. p. 143.

CHAPTER VH.

APPENDIX. INTERCELLULAR SPACES.

Sect. 49. Between the elements of mature tissue there are often cavities, which
are grouped under the term intercellular spaces.

These arise in two ways in the original masses of cells, which, at least when in
the state of meristem, are always in uninterrupted connection. Firstly, by separation
of pennanenl tissue-elements, as the result of their unequal surface-growth in different
directions, the original common walls splitting, while the common limiting layer which
was originally present is — perhaps always — dissolved. Secondly, by disorganisation,
dissolving, or in many cases rupture of certain transitory cells, or groups of cells,
which are surrounded by permanent tissues. We may call the first mode of origin
schizoge?ietic, the second lysigenetic, and, if a special term must be adopted for the
mechanical rupture, it may be called rhexigenetic.

According to the stage of development at w'hich the formation of intercellular
spaces takes place, one may, wdth Frank *, distinguish as protogenetic those which are
formed on the first differentiation of tissues, and as hysterogenetic those which appear
subsequently in old mature tissues.

According to their contents intercellular .spaces fall into two categories. The
first are filled with bodies or mixtures of the same sort as the secretory sacs treated
of in Chap. Ill, or the secretory cavities of the epidermis ; they are nearly related to
these anatomically and physiologically, and spaces of these two categories not un-
frequently appear even to act as substitutes one for the other. They may be designated
secretory intercellular spaces, or intercellular secretory reservoirs. In treating of them
reference has often to be made to the other, not intercellular, secretory organs.

The others contain from the very first only air, or in rare cases w'ater. They
form together a special ventilating apparatus for the tissues. The stomata of the
epidermis (p. 34) are a part of this apparatus ; they are a special case of schizogenetic
and protogenetic spaces, which usually contain air, but also water in the special cases
mentioned.

The same intercellular spaces rarely take part in both functions. Thus in the
parenchyma of Lysimachia Ephemerum, where the fixed, red, resinous secretion, to be
more accurately described below, partly covers the wall of the cells bordering on the

* Beitr. zur Pflanzenphysiologie, p. loi.

INTERCELLULAR SECRETORY RESERVOIRS.

201

air space, in some places as a thin layer, in others as thick masses filling the cavity,
while again in other places it is entirely absent.

The intercellular spaces will be here more exactly treated, following the two main
categories as defined by the character of their contents.

INTERCELLULAR SECRETORY RESERVOIRS.

Sect. 50. Hysterogenetic reservoirs of this category arise in old masses of
tissue of long-lived plants from subsequent metamorphosis. Therefore, in order to
avoid repetition, their treatment may be passed over for the present, and be resumed
in Chaps. XIV and XV. We shall then speak here of protogeneiic forms onlyh

These may be distinguished by their contents, and be named as those which
contain resin and ethereal oil, or mixtures of both, or Balsam; further as containing
7nixtures of gum, or mucilage, ox giim-resin.

According to form on the one hand there may be distinguished elongated, tubular
canals or passages, permeating the tissues for long distances, and having a rounded or
angular transverse section ; and short, circumscribed, round or rather long, completely
closed hollows or cavities, the latter being’ also indicated by the word of many
meanings, viz. glaiids (comp. p. 92), and being further distinguished as internal glands,
to avoid confusion with the outer glands belonging to the epidermis.

Between the general quality of the secretion and the form of the reservoir there
is no generally constant relation ; there are passages with balsam or mucilage, and
cavities with the same contents, &c. On the other hand, in both cases there is as a rule
a very constant, similar character of the reservoirs, according to the families, genera,
or species in which they occur, so that they give to these veiy constant anatomical
characters. The Coniferse are a striking exception owing to the variety of form t f
their resin-reservoirs in the different genera. Unimportant exceptions occur here
and there in other families: among the Compositse, e. g. Tagetes patula has short
closed sacs in the leaves, instead of the passages wTiich are found in the other parts
of this plant, and in the leaves of allied species. The same holds for the leaf of
Mammea americana, in contradistinction to the other parts of this tree, and the leaves
of other Clusiacese. Some further cases of this sort will be mentioned in the subse-
quent special descriptions, p. 206.

As already indicated, the reservoirs in question occur only in certain classes,
families, and genera, and especially in those which have no other places for the pro-
duction and storing of those secretions. A comparison of the statements in Chaps. I
and III will make this clear, and will also call to mind the absence of any special
secretory organs in the vegetative body of many plants. We may also shortly
mention the mutual substitution and relation of the reservoirs in question with the
milk-tubes in different genera and species, e. g. in the Compositce and Aroidese : this
has been above indicated, and will be again treated in Chap. XII.

In rare cases a mutual substitution is found to take place between sacs and
cavities with the same secretion, in different parts of the same plant. This also occurs

^ Compare Frank, t.c. — N. Müller, in Pringsheiin’s Jahrb. V. p. 387. — Van Tieghem, Ann. Sei.
Nat. 5 ser. tom. XVI. — [Also Szyszylowicz, Sekret-behalter d. flüchtigen Oele. Ref. Bot. Centralbl.
1881, Bd. 8. p. 259.]

202

INTERCELLULAR SPACES.

most prominently in Myrsine africana, and many species of Lysimachia, where
the characteristic red resinous secretion occurs in sacs in the root, and in roundish
intercellular cavities in the other parts of the plant.

The reservoirs in question may be arranged, according to their form and the
nature of the secretion, in the following systematic groups.

1 . Mucilage- arid gum-passages in the Maratiiacem, many Lycopodia, the Cycads,
species of Canna, and Opuniia, and some AraliacecB. Mucilage-containing cavities
of lysigenetic origin occur in single cases, mentioned above among sacs (p. 144).

2. Resin, ethereal oil, emulsions oi gum-resin of different quality, according to the
special case, and often little known as regards chemical relations, occur —

(a) in passages in the Conifer cb, Alismacecc, and Aroidece, the tubifloral Compositce,
UmbellifercB, Araliacece, Pillosporece, many species of Mamillaria, Clusiacece, Anacar-
diacecE, the genera Ailantus, and Brucea of the family Simarubeae.

(f) In short cavities in the group Rutacece, in the sense of Bentham and Hooker
(exclusive of Simarubeae and Zygophylleae), in species of Hypericum, many species of
Oxalis, MyrtacecB, Myoporece, species of Lysimachia, Ardisia, and Myrsine: perhaps
also in Gossypium.

The wall of all secretory spaces, which is made up of that of the neighbouring cells,
is, with the exception of the above-mentioned special case of Lysimachia Ephemerum,
always completely closed, the lateral walls of the cells, 1. e. those perpendicular to the
surface of the space, being joined together without interspaces. When, as is the case
in the petiole of Marattiaceae and Cycadeae, the original cells, which limit a passage,
are separated laterally from one another by continued growth of the surrounding
tissue in a peripheral direction, the enclosure is completed by the next outer layer of
parenchyma. The number of cells limiting the transverse section of a cavity varies
according to the particular case. The small resin-passages, which run transversely
between the larger longitudinal ones in the secondary cortex of Cussonia, are,
according to N. Müller, at first at. least, slit-like spaces between the partly separated
walls of two rows of cells, and are thus limited in transverse sections by two cells.
Most of these spaces appear in transverse section to be limited by 3, 4, or many
more cells, the number of which may increase as the space widens by divisions
placed radially with regard to the space. Fig. 85.

The cells surrounding the secretory reservoir have generally the properties of
parenchymatous cells. In form they either do not differ, or only slightly, from the
cells of the neighbouring parenchyma, e. g. in the young roots of the Compositse, in
the leaves of Ardisia crenulata : or, as in most cases, they are sharply distinguished,
so that one might speak of a peculiar lining of the wall, or epithelium of the inter-
cellular space. According as the peculiar character extends to one or several layers
of cells surrounding the space, there may be distinguished one-, two-, or several-layered
epithelium. The enclosure of the several-layered epithelium of the resin-passages in
the leaves of Pinus Strobus, sylvestris, Laricio, &c., and of the roots of Philodendron,
in a completely closed sheath of sclerenchymatous fibres, which are wanting in the
homologous passages of other closely allied plants, is worthy of notice. The cells of
the epithelium are generally prismatic in elongated passages ; their greatest diameter
usually lies in the direction of the length of the passage, only in the leaves of Cycads
(Kraus, l.ci) do they lie transversely. Their transverse diameter is usually much

INTERCELLULAR SECRETORY RESERVOIRS.

203

smaller than that of the neighbouring parenchyma, so that they appear very different
from it in transverse sections : in rare cases they are distinguished from it by their greater
width (roots of Composite, branches of many species of Rhus according to Trdcul).
Their inner surface is often slightly convex towards the passage ; in the mucilage-
passages of the Marattiacese it is even elongated and conical ; in those of the leaves of
Lycopodium the celts bulge in a club-like manner. Where an epithelium can be
distinguished in isodiametric cavities (Lysimachia punctata, and its ally, Myrsine)
the cells are flattened towards the surface of the cavities.

The wall of the epithelial cells is delicate, in resin- and balsam-passages often
coloured brown or yellow. In the mucilage-passages of old leaves of Cycas revoluta
alone it is stated by Trdcul* that they are strongly thickened on the side next the
passage.

Fig. 85.— Hedera Helix ; transverse sections of the younj? stem (800) ; resin-passages. These are in A, C
young, and have appeared between four and five rows of cells, in the secondary cortex w b, at t)ie limit of the
zone of thickening c; h wood ; in D and E older, larger passages ; b bast ; rp parenchyma of the outer cortex.

From Sachs’ Textbook.

There is a great lack of investigations on the protoplasm and contents of the
cells of the epithelium, and of the surroundings of the secretory reservoirs generally.
It seems to be certain for all cases that they contain small masses of the secretion,
which then filter in some way in mass through the membrane into the reservoir. The
cells surrounding the young oil passages in the roots of Composite have clear
contents, in which, in Helianthus annuus ^ large quantities of tannin are shown to be
present by reagents; this is also the case with the oil in the passages. In Tagetes
patula at the transition from the root to the hypocotyledonary stem, a clear violet

^ L’Institut, 1862, p. 315.
Sachs, Botan. Zeitg. 1859, p. 183.

^ Van Tieghem, l.c. p. 113.

204

INTERCELLULAR SPACES.

cell-sap makes its appearance in the cells bordering on the passages : further up and
throughout the stem orange-yellow granules, which turn blue with iodine, are also
found fixed upon the wall of the cells next the passage. In the limiting cells of
young passages of the secondary cortex of Pittosporum Tobira, Müller mentions
numerous starch grains with a covering of yellow oil. The epithelial cells of the
reservoirs in the leaf of Ginkgo, in the stems of many Compositse, e. g. Solidago laevi-
gata, contain chlorophyll grains : those which surround the round resin-reservoirs in
the leaf of Ardisia crenulata are, with exception of the necessary peculiarities of form,
in no way different from those of the rest of the chlorophyll parenchyma, to which
they belong.

In many plants which secrete resin (Coniferse, Anacardiacese, Umbelliferse, Ara-
liacese, Compositae) Müller has by staining with Alkanna found drops of resin not
only in the cells bordering on the reservoirs, but distributed widely in the surrounding
tissues. Without wishing in the least to combat the correctness of these observations,
I regard further careful investigation of the contents of these cells as the more
desirable, since very thin sections, which Müller states that he used almost
exclusively, are not the most suitable preparations for the study of protoplasm and
cell contents.

The contents of the intercellular secretory reservoirs form in most cases a homo-
geneous fluid mass, or an emulsion-like mixture without characteristic peculiarities of
structure. Their chemical properties are indicated approximately, and for the present
sufficiently, by the names above used. The red secretion in the cavities of species of
Lysimachia and Myrsinese, which, from its solubility in alcohol, may, till further in-
vestigated, be classed with the resins, is an exception to this rule, since it ^appears in
a fixed form, and with apparently crystalline structure, as will be described below.
The same seems to be the case, judging from incomplete investigation, in many
species of Oxalis.

The large mucilage-passages, wide, of the Opuntias are characterised by
containing numerous and large crystals of calcium oxalate embedded in mucilage.
As regards the material characters of the milky contents of the passages of Mamillaria
angularis, and its allies, which will be more accurately described below, I have come
to no certain conclusion.

The mode of development of the intercellular secretory reservoirs is for the lysi-
geneiic forms generally as follows : that, in a group of delicate cells, which arise by
definite meristematic divisions, and which correspond in form and arrangement to the
future reservoir, the secretion appears at the expense of the original protoplasmic body,
and that subsequently the walls of the cells are dissolved, and the separate secretory
masses coalesce. Of reservoirs of this category, containing gum and mucilage, only
the passages of the periphery of the petiole of the Marattiacese have as yet been
carefully investigated, and in their case it is shown, that the elements of the simple
row of cells corresponding to the subsequent cavity of the passage are filled with the
secretion before they are broken up. Nothing is known of the form in which they
first appear. In the investigated reservoirs of ethereal oil and resin the secretion first
appears as small drops in the protoplasm of the cells about to be broken down.
These increase quickly in size and number, and coalesce after the disappearance of
the walls, to larger masses. Where the original cellular body consists of several

INTERCELLULAR SECRETORY RESERVOIRS. 205

layers, the process of breaking down and of coalescence progresses centrifugally
(comp. Fig. 86, and above, p. 69, Fig. 22).

This mode of development holds for the gum-passages in the periphery of the
petiole of the Marattiacese, for the mucilage-passages of the Opuntias, which require
further investigation, and perhaps also of the Mamillarias ; again for all secretory
cavities investigated, with the exception of those of the Lysimachias, Myrsinese, and
species of Oxalis. Doubtful cases will be named below.

The schizogenelic spaces (comp. Fig. 85, p. 203) appear sometimes between
cells, which resemble those surrounding cells which do not border on secretory
reservoirs, both in arrangement and origin : sometimes they are produced by peculiar
divisions of special initial meristem-cells. The first case is found to occur in the
above-mentioned spaces of Lysimachia Ephememm, and in the slit-like transverse
passages of Cussonia also noted above. Further, the large longitudinal passages in
the secondary cortex of the same plant and of other ligneous plants arise between the
common corners of junction of four rows of cells, which are produced from the
cambium in the same way (Chap. XIV) as its other products. The same holds, with
many special modifications it is true, for the formation of the resin-passages in the
secondary wood of the Abietinese The prismatic longitudinal passages at the inner
limit of the primary cortex in the roots of Compositse are, excepting in the appear-
ance of the characteristic contents, formed in the simplest and most common case, in
the same way as those air-containing cavities found in the external layers of the
cortical parenchyma, viz. at the corners of contact of four rows of cells.

On the other hand, the primary passages in the pericambium of the roots of the
Umbelliferae, which will be described later (Chap. XIII), are the result of special meri-
stematic divisions. The passages in the primary cortex of the Abietineae, and in the
leaves of Cycas and Alisma, may each be traced back, according to Frank and N.
hliiller, to a row of initial cells, which divide longitudinally by successive crossed walls :
the four daughter cells then separate at the angle of contact to form the passage.

As the plant grows there occurs, as has been shortly noted above, a widening of
the passage, and growth of the cells which enclose it, in a direction tangential to its
surface : this is accompanied by increase in number of these cells by radial division,
e.g. from the four original cells to 6 or 8 in Pinus and Alisma (Frank); in cases of long-
continued growth in thickness of the part, and correspondingly great widening of the
passages, the number may reach much higher figures, e. g. cortex of Coniferae, Rhus,
Pittosporum, &c. On the other hand the cells surrounding the space may also divide
in a tangential direction, and the originally single limiting or epithelial layer may be
doubled, or divided into several layers, e.g. in the passages of species of Philodendron,
the cortex of Pittosporum, Hedera (Fig. 85), and the leaves of Pinus. This mode of
origin of several layers of epithelium from the originally simple limiting layer is not
proved for all cases where they occur, and another origin is in many cases quite
possible for the outer layers.

The origin of the secretion contained in the schizogenetic spaces, looking at it
from the purely histological side, and neglecting the chemical questions, is in my
opinion not clear, and requires further investigation. It is obvious that it, or at least

' Sanio, in Pringsheim’s Jahrb. IX. p. 99.

2o6

INTERCELLULAR SPACES.

the material for its formation, must be derived from the cells, which closely or
immediately surround the space. Where it is possible, as e. g. in the case of the
above-mentioned resinous secretion around the reservoirs, to prove its presence in the
cell-contents, it may be assumed that it passes as such from the cells into the reser-
voirs, of course not by filtration in great masses, but, as Müller assumes, by diffusion
through the membranes in successive small quantities. This view is supported by the
fact that, according to Müller in the Coniferse, and Sachs and van Tieghem in the
roots of Compositae, the intercellular spaces are first present without the characteristic
secretion, and that this first appears in them at a later stage.

On the other hand there occur, as above stated, cases where the secretion is also
of a resinous nature, but is not proved to occur as such in the cell-contents in the
vicinity of the reservoir. Further, Sanio has recently expressly stated that the resin-
passages of Pinus are filled with the secretion even from the time of their first origin
So far as my investigations extend, the young stage of the passages of the root of
Compositae, when the secretion is absent, is at best very transitory: the secretion
appears very early, and may be easily overlooked in the narrow young passages : it
may be really absent, especially in transverse sections, since it may have flowed out.

Since, in the dermal glands of the epidermis, secretions, which are in every re-
spect similar to those under consideration, are often to be first observed anatomically
as constituents of the cell-wall, and further since the intra-mural glands (p. 96),
when regarded purely histologically, are merely a special case of schizogenetic
secretory cavities in the epidermis, the question arises, w'hether in all cases the
secretions of schizogenetic reservoirs are not to be regarded as constituents of the
cell-wall. The actual observations supporting this view are of at least equal weight
to those supporting the other, while none of the latter exclude the correctness of the
former view.

All secretory passages, with the exception of the few named on p. 205, are of
schizogenetic origin ; of these however the mucilage-containing passages in Canna
must be more exactly investigated ; also besides those cavities already mentioned in
Lysimachia, Myrsinese, and Oxalis, those which take the place of passages in many
short leaves of Conifers deserve further attention.

Further descriptions in detail of the secretory must so often have reference
to their arrangement, that, to avoid repetition, they must be given subsequently in
Chap. XIII.

Here those which are found in species of Mamillaria will alone be described. In the
literature I find it only briefly mentioned that these plants have latex (De Candolle),
which is contained in passages (Unger). Investigation shows, firstly, that the Mamillarias
are entirely without those mucilage-sacs (p. 143) which occur in the allied genera. I also
find no trace of intercellular secretory reservoirs in small species, as M. glochidiata and
the like, and in an unnamed, very robust species. On the contrary, M. angularis, Hystrix,
and Zuccariniana have a complex system of branched passages. These are limited, as seen
in transverse section, by one 4-5 celled layer, or by two, or even three concentric layers
of delicate cells, flattened tangentially to the passage : the width of the passages is about
equal to that of one large cell of the parenchyma. They contain a thick, uniformly finely-
granular, colourless juice, which emerges on section in large white drops, and hardens

‘ /. c. p. lOI.

INTERCELLULAR SECRETORY RESERVOIRS.

207

quickly in the air without change of colour. This must be a peculiar mixture : water,
alcohol, ether, benzine, and alkalies do not alter it on the whole, though each reagent
may dissolve a small quantity of it. When burnt it leaves behind a very small residue of
ash. I have not obtained a clear idea of the mode of origin of the passages of the
Mamillarias.

The crystal-containing passages of the Opuntias, which may attain a width of
are apparently of lysigenetic origin ; in the mass of mucilage the cells, from the dis-
organisation of which they arise, are still partially recognisable.

Some details concerning the secretory cavities remain to be added, and statements
may also be looked for regarding their occurrence and arrangement.

{a) The Myrtaceae, judging from observations on numerous species of Eucalyptus,
Melaleuca, Callistemon, Eugenia, and Myrtus, are generally provided with oil-cavities.
In horizontal leaves these are particularly numerous on the upper surface, though not
confined to it, and their epithelial layer is contiguous with the epidermis, in which
those cells which touch the epithelium differ from the rest in form and size. In Myrtus
communis, for instance, two semicircular epidermal cells are contiguous with the wall of
the cavity at the upper side of the leaf : these cells are distinguished from the rest by
their lateral walls not being undulated, and by their being only half as high. In the outer
cortex of the branches, according to investigations on species of Eucalyptus, they are
separated by some few layers of parenchyma from the epidermis. They have an
approximately spherical form : the larger ones may be seen with the naked eye as
transparent points, others are smaller, e.g. in the leaves of Eugenia australis. The cavity,
which is filled with mixtures of oil and resin, is limited by a continuous epithelial layer
composed of tabular cells. According to Frank ^ the cavities in the leaf of Myrtus
communis are of schizogenetic origin. A round, thin-walled cell, lying beneath the
epidermis, divides successively into 8 octants ; these separate at their central point of
contact, so as to form directly an intercellular space filled with oil, and this gradually
assumes the form of the spherical cavity ; the original eight epithelial cells meanwhile
stretching tangentially, becoming flattened, and occasionally dividing. This description
is contradicted by Martinet’s short statement, according to which the oil-cavities of the
Myrtaceae arise like those of Citrus in a lysigenetic manner, a view which, though I have
not observed their development myself, I must consider to be correct, from the
agreement of the mature cavities with those of the Rutaceae.

(b) The presence of oil-cavities is a general and characteristic phenomenon in the
members of the group Rutaceas in the sense of Bentham and Hooker, i. e. the families or
divisions of the Rutaceae, Diosmeae, Boronieae, Zanthoxyleae, Flindersieae, Toddalieae
(Skimmia), Aurantiaceae, Amyrideae’*. The Simarubeae and Zygophylleae are excluded
from this group, and have no oil-cavities.

The distribution of the organs in question, and their relation to parenchyma and
epidermis, is, as far as investigated, the same as that in the Myrtaceae. In the stem of
Dictamnus and Correa alba they lie directly under the epidermis, in the leaves of
species of Agathosma and Diosma especially or exclusively on the under side of the
leaf — relations which are found also in the Myrtaceae. They also correspond to these in
average form and size. Their origin is in all cases lysigenetic. Even Frank’s drawing
for Ptelea trifoliata does not contradict this, though according to his description of the
process of development their origin is schizogenetic. Rauter ^ gives a very exact history
of development of the oil-cavities on the upper surface of the leaf of Dictamnus (Fig. 86).
A cavity {Ä) originates from two cells, one an epidermal cell, the other a cell of the
hypodermal parenchyma. The first divides successively into four cells, placed crosswise

’ Beitr. p. 125.

^ Compare Engler, l.c. on Amyris ; also Van Tieghem, l.c. p, 173.
® Trichomgebilde, &c. l.c. p. 21.

2o8

INTERCELLULAR SPACES.

in the epidermal layer; each of these is again divided into an inner cell next the
parenchyma, and a superficial cell {d, c). The superficial cells increase further so as to form
the portion of the epidermis which covers the cavity {B, c, d). The inner ones take a
direct part in the formation of the cavities. The chief part of the cavity originates it is
true from the products of division of the primary parenchymatous cell [A, p, p), which
divides by alternate horizontal and vertical divisions into numerous daughter cells ; these,
together with the similarly formed products of division of the inner epidermal cell, form
a compact round body of numerous small cells. In the protoplasm of all cells of this
body, which are at first very granular, there appear, after the division and growth have

ended, more and more numerous drops of ethereal oil ;
then the delicate membranes are dissolved, and the oil
drops coalesce to large drops (o in C). The process
begins in the middle of the body, and proceeds cen-
trifugally to its surface. The oil-containing cavity
thus formed is limited, with the exception of the
epidermis, by cells of the surrounding parenchyma,
which are more or less flattened towards the surface
of the cavity ; these cells, being in uninterrupted con-
tact with one another, completely enclose the latter.
The cavity in the hairy dermal warts of the Dittany
arises, as described p. 69, in the same way.

The oil-cavities of Ruta have in the main the same
development. That of Citrus differs at most in small
secondary peculiarities, which need not be described
in detail here. Martinet, who described them, has traced
their origin back to a condition in which the transverse
section shows three small cells in the epidermal layer,
with abundant protoplasm, and beneath these three
inner cells. The arrangement of these cells is such, in
these youngest cells and in rather later stages (A c.,
Fig. 234 and 235), that it is probable that their mode
of origin is the same as that described by Rauter for
Dictamnus. It can hardly be doubted, after investiga-
tion of mature or half-formed stages, that those of
the other members of the group of Rutaceae have
fundamentally the same origin.

The cavity is always completely and smoothly iso-
lated by the close connection of the cells of the sur-
rounding tissue, so that in good sections it may be
taken for a single large cell lying between these. As is especially evident in the chloro-
phyll-containing parenchyma, the cells of the limiting layer do not differ fundamentally
in structure from those of the mass of tissue, in which the cavity lies. Obvious remnants
of partially dissolved delicate cell-membranes are not unfrequentiy to be found ; these
form a more or less irregular covering to the wall. It is possible that in many cases
the mass of delicate cells, instead of being dissolved, persists in whole or in part. Many
figures of Engler appear to point to this. But here, even with tolerably good pre-
parations, it is possible to be deceived, since the sections often do not cut the cavity in
a median plane, but lay bare portions of their wall, which then appear in surface-view as
dense, multicellular bodies.

(r) The spots, recognisable with the naked eye as transparent points, in the lamina of
Hypericum perforatum, and its allies, are oil-cavities of flattened spherical form, which
occupy almost the whol^space between the portions of epidermis of both leaf-surfaces
which cover them, and are separated from the lower epidermis by at most one layer of
parenchyma. Their structure, or that of the tissue surrounding them, is fundamentally

Fig. 86. — Dictamnus Fraxinella ; oil-reservoirs
of the upper side of the leaf; transverse sec-
tion ; C (200) mature; .r/ and B successive young’
stages of development (320). Further description
in text. After Rauter from Sachs’ Textbook.

INTERCELLULAR SECRETORY RESERVOIRS.

209

that described for the group of the Rutaceae. Their origin, judging from observations
described, can hardly be other than that stated by Martinet, viz. lysigenetic, though
Frank describes it as schizogenetic. Cavities of the same sort occur in the superficial
parenchyma of the cortex of the stem. In those of the stem of Hypericum balearicum
Unger ^ found papillose and hair-like processes rising from the wall into the cavities.
Many species of Hypericum, e.g. H. calycinum, canariense, hircinum, &c., have no
transparent spots recognisable with the naked eye : it is uncertain whether the oil-
cavities are absent in these cases, or, more probably, are only smaller than in the punctate
species, or are in some way hidden.

((f) The bodies scattered in the parenchyma of species of Hypericum, which contain
violet colouring matter, and are described by the above authors as glands, may here be men-
tioned, and recommended for further research: also the similar bodies in Gossypium.
In the leaves of some species of Hypericum, these consist of spherical, loose aggr^ates
of round cells; the colouring matter apparently lies between them. In the leaves of
Gossypium they are round, undoubtedly lysigenetic cavities, which are filled with the
violet colouring matter, soluble with difficulty in alcohol.

(e) Among the Myoporese the species of Myoporum have numerous round oil-cavities
of unequal size in the leaves and the outer cortex of the branches. The cavities are
superficial, and separated only by one or two layers of cells from the epidermis, which is
arched convexly outwards, e. g. M. parvifolium : in M. tuberculatum, on the contrary,
according to Unger *, they occur in the middle of the chlorophyll-parenchyma of the leaf.
They are surrounded by 1-3 layers of flattened cells. As far as investigated, their origin
appears to be lysigenetic.

(/) In the parenchyma of species of LysimaeMa, of Myrsine africana, and Ardisia
crenulata a resinous body is found in intercellular spaces ; it is soluble with difficulty in
alcohol, easily soluble in ether, of a bright brownish red colour, and forms fixed, often
almost brittle masses : its chemical properties require closer investigation. Its mode of
distribution differs in special cases, and of those plants investigated it is simplest in
Lysimachia Ephemerum. In the root of this plant it appears in most of the usual
prismatic, air-containing, intercellular spaces as a finely granular covering on the wall of
the adjoining cells: here and there it is interrupted, while in thickness it varies from that
of an insignificant layer to such a bulk as to fill up the passage. In the very lacunar
parenchyma of the pith and cortex of the stem, it has fundamentally the same distribution,
but is less regular, owing to the irregular form of the cavities : in neighbouring cavities it
is contained in very unequal quantities, in many not at all. When secreted in quantity, it
forms on the cell-wall a convex covering, striated perpendicular to its surface. Finally,
it is found in the leaves, in the cavities of the chlorophyll-parenchyma, as thick, irregu-
larly shaped masses, with indistinct radial striation : the cavities are surrounded some-
times by ordinary cells, sometimes by a more or less distinct layer of flattened cells.

The secretion is strongly localised in the leaves and the cortex of Lysimachia
punctata “ and of Myrsine, in the leaf of Ardisia, and of most Lysimachias. Here round
reservoirs, which appear as points to the naked eye, are found in the parenchyma : they
are surrounded by about eight flat, closely connected, chlorophyll-containing cells, and
filled with the dense red secretory body, which is in Myrsine radially striated in a remark-
able way. The reservoirs arise schizogenetically, and contain the secretion as soon as
they are visible. In the cortex of branches of the above Ardisia, the reservoirs are
rarely round, usually elongated, spindle-shaped, appearing as little strokes more than
jmni in length.

In the roots of Lysimachia vulgaris and punctata, and of Myrsine, the secretion does
not lie in intercellular spaces, but in single sacs or cells, not distinguished in size or form
from the surrounding cells of the parenchyma : in each of these there is a body of the

' Anatomie und Physiologie, p. 213.

^ P. Moklenhawer, Beitr. p. 162. — Meyen, Sccretionsorgane, p. 61.

P

210

INTERCELLULAR SPACES.

same structure as that in the reservoirs of the leaf, which does not completely fill the
cavity, being surrounded, while young, by colourless granular contents (protoplasm?).
In the roots of Ardisia I did not find the red secretion.

(g) Many, though not by any means all the species of Oxalis from the Cape and
America have, on the under surface of the leaf, and running towards the margin, rather
prominent reddish bands, which are mentioned in descriptions as glands or wales. In the
species — not exactly defined — which I have investigated, these bands are reservoirs quite
similar in the colour, consistency, and radiate structure of the secretory mass, and also in
the structure of the surrounding tissue to those of Lysimachia punctata and Ardisia.
They lie in the chlorophyll-parenchyma, and are separated by but one layer of its cells
from the distended epidermis of the under surface of the leaf. Thorough investigations
were not made ’.

INTERCELLULAR SPACES CONTAINING AIR AND WATER.

Sect. 51. Air- and water-containing intercellular spaces occur, on the one hand,
in many vascular bundles, and these will be treated of in Chap. VIII ; on the other
hand, they are a characteristic component of large masses of thin-walled assimilating
Parenchyma. Intercellular spaces are absent only when the parenchyma forms
definite sheaths.

The cavities in question extend between all cells, so that each one of the latter
borders on one or several. They together form, as will be again mentioned below, a
continuous system throughout the plant, which opens into the stomata, where these
are present. The spaces sometimes contain water in the vicinity of the water-pores,
elsewhere they normally contain air, i. e. a mixture of gases similar to atmospheric
air, in which the proportion of oxygen and carbonic acid varies with the activity of
the processes of assimilation and respiration

The whole volume of the air-spaces varies greatly in special cases, and is often
very large in proportion to the volume of the part of the plant w'hich is not filled with
air. Approximate measurements, which Unger® made on leaves and petioles of
41 species of plants, gave as minimum 77 parts by volume of air to 1000 parts of
the leaf in Camphora officinalis, and as maximum 713 to 1000 in Pistia texensis.
The air of the vessels and that diffused in the cell-sap, which would be also pumped
out, is not taken into account in these statements : but it appears that this is on the
whole of minor importance, and that on the other hand the figures obtained would be
sometimes lower, sometimes much higher, if the single masses of parenchyma com-
posing the leaf were investigated separately.

It was long ago known that the whole volume of the air-spaces in relation to
that of the Avhole plant is largest in plants of all classes and families which grow in
water, or in moist positions, and on the other hand in those which inhabit drj^ places,
as many Compositre, Umbelliferse, Labiatoe, Grasses, &c. with hollow stems or
petioles.

According to their gradual, and not distinctly limited differences of relative

^ [See further, Russow, über sekretführende Intercellular-gänge der Acanthaceen, &c., Dorpat .
1S80. — Bot. Centralbl. i88l, Bd. 5. p. 365.]

** Compare Sachs, Experimentalphysiologie, p. 262.

^ Beitr. z. Physiol, d. Pflanzen. I. Sitzgsbr. cl. Wiener Acad. Bd. XII. p. 367.

SPACES CONTAINING AIR AND WATER.

21 1

width, the air-spaces may be divided into Interstitial spaces, such as are of smaller
volume than the adjoining elements ; cavities, lacimcB, of almost equal or slightly
larger volume; passages, chambers, and hollows of much greater relative volume. The
tissue permeated by air-spaces may accordingly be termed lacunar, chambered, &c.,
and that with only narrow interstitial spaces may be called (relatively) dense.

The air-spaces of dense and lacunar parenchyma always arise schizogenetically
at the first commencement of differentiation of the tissues. In preparations seen by
transmitted light they may be seen close behind the growing-point, by reason of the
air contained in them, as black bands between the cells. Comp. e.g. in Fig. 3, p. 10,
the regions marked m and r.

The miersiitial spaces of dense tissue run as a rule between the rounded corners
of the cells, which are closely connected over the greater part of their walls, as narrow
angular canals, the number of their sides being equal to the number of cells which
border on them. Thus, e.g. the numerous three-sided prismatic interstitial air-spaces
in regular polyhedral cells arranged in alternating rows, the four-cornered ones between
the cells arranged in radial and concentric row’s w'hich are not alternate, in the inner
primary cortex of many roots (Fig. 51, p. 124). More rarely they form narrow slits,
standing serially one above another, and separated one from another by connected
portions of the walls, these slits lying between
the limiting surfaces of two contiguous cells,
as in the dense tissue of the leaf of Myrtaceae,
species of Scirpus, the parenchymatous lamellae
of the cortex of Pilularia and Marsilia ;
this condition is allied to that of the many-
armed lacunar tissue.

Of lacunar parenchyma there may be dis-
tinguished two chief forms, which how’ever
cannot be sharply separated. The first (Fig.

87) arises through the unequal growth in
surface of all cells as they pass from the
meristematic state, in such a way that at
certain points they put out processes which
may grow’ to long arms, at other points these
are not formed. The ends of the processes
of adjoining cells remain connected with one
another ; betw’een the other parts of the sur-
face the intercellular space is formed. Ac-
cording as the processes are short, or are extended as long arms, and are arranged
in one or several radial planes, the form of the cells and of the cavities varies.
Almost all imaginable forms occur in the lacunar part of most bilateral leaves of
all classes, with at the same time various transitions to lamellar parenchyma.
Such parenchyma composed of ‘ stellate ’ cells occurs besides in many Monocoty-
ledons, especially marsh- and W’ater-plants. As examples of this are the dia-
phragms of air-passages, to be mentioned below ; stellate .bands (which will also be
returned to below), alternating with the denser masses of parenchyma which contain
the vascular bundles, traverse the petioles of the Marantaceae longitudinally (species

Fig. 87. — Sag^ttaria sagittzefolia ; root, transverse section ;
I— III and 1 — 3 two radial rows of many-armed cells from
the cortical parenchyma; rr arms ; / cavities (350) From
Sachs' Textbook.

P 2

212

INTERCELLULAR SPACES.

of Canna, Maranta, Phrynium, Thalia), the leaves of many Bromeliacese, e. g. B.
Caratas : cells, branched in a stellate manner in all directions, with long arms, and
small central portion, form the inner part, the ‘ pith ’ of the halm of many species of
Juncus : similar stellate cells, but with a large bladder-like central portion, compose
the massive lacunar inner cortex of the Rhizome of Scirpus lacustris, Sparganium
ramosum, &c.

In Dicotyledonous water-plants, stellate lacunar parenchyma is rare, with excep-
tion of the tissues of foliage leaves ; still, according to Duval-Jouve, it composes the
diaphragms of the air-passages of Limnanthemum nymphoides, and, according to
Planchon^ and Tr^cuD, those of Nelumbium ; also the spongy cortical mass of the
roots of species of Jussisea, and of the internodes of the stem of Mimosese, which
serve these plants as floats.

The spongy air-containing cortex of the adventitious roots, which spring from the
nodes, and which serve as floats for those branches of amphibious species of Jussisea
(J. repens, J. grandiflora, J. natans, J. helminthorrhiza) which grow in water consists of
3-6 armed cells. They are arranged in concentric layers, and each puts out at least
three narrow cylindrical arms from a not distended central portion. Of these the
longest is horizontal and directed radially towards the periphery, and its end is connected
with the opposite face of a cell of the next outer layer. The two shorter are in the
simplest case radially and tangentially vertical, and of equal length, so that the radial
view of the cell has the form of a procumbent H . The end of each is connected with
the corresponding one of the next upper or lower cell of the same layer. A connection of
the cells of a layer in a tangential direction is finally brought about, sometimes by single
tangential arms, sometimes by oblique direction of the halves of the cross-bar of the T.
Radial and transverse sections thus show between the narrow arms very large, four-
cornered air-cavities, which are continuous one with another. The length of the arms
and of the air-cavities is nearly equal in each of the concentric layers, and increases suc-
cessively from within outwards. The radial arms of the outermost layer abut directly
on the epidermis, which soon breaks away and collapses. Also the spongy cortex of
Desmanthus natans W., and apparently other similar Mlmoseae, may be here described,
though from its mode of origin it should be ranged under secondary formations. The
internodes of the stem of this plant, which grow horizontally in the water, are, accord-
ing to Rosanoff at first cylindrical ; their parenchymatous outer cortex consists of an
inner part, which is composed of 3-4 layers of roundish rather large parenchymatous
cells, and an outer consisting of 3 small-celled layers covered by the epidermis. When
the extension of the internode is ended, it swells to a shape like a barrel as the result of
the appearance of the spongy floating apparatus. The formation of the latter begins
with tangential divisions of the third layer of parenchyma from without, which, together
with the changes which follow, are then continued successively in the more internal
layers. The cells formed by the tangential divisions are arranged in radial rows, and
grow to irregular stellate sacs v’ith narrow cylindrical arms, while the central portion is
not distended : the ends of the arms remain connected ; the cells thus enclose wide
lacunae filled with air. Each second or third radial row is branched, and connected
especially in the tangential direction, the i or 2 rows between them are branched on all
sides. The epidermis and the hypodermal parenchyma are torn into narrow rags by the
spongy swelling. Later the whole spongy apparatus is thrown off as bark.

1 Flore des series, tom. VI. “ Ann. Sei. Nat. 4 ser. I. p 168.

5 Ch. Martins, Sur les racines aeriferes des especes aquatiques des Jussisea, Mem. Acad, de
Montpellier, tom. VI (1866). — Frank, Beitr. z. Pflanzenphysiologie, p. 152, fig. 24.

■* Botan. Zeitg. 1871, p. 829, Taf. X.

SPACES CONTAINING AIR AND WATER.

213

If in the stellate lacunar tissue one arm of a cell be separated by a septum
as a special cell from the central portion, as often happens, e. g. in Jussiaea, and
if this cell divides further, the intercellular space is no longer limited on all sides by
parts of branched cells, but, according to the number and direction of the successive
divisions, by whole cells or by rows of cells, or simple or compound layers, or
lamellcB. We may distinguish the more developed forms of this arrangement from
the stellate tissue as laynellar porous parenchyma, though it is obvious that frequent
intermediate forms prevent a general sharp distinction.

In the formation of lamellar parenchyma, unequal surface-growth and divisions
of the cells which are passing from the meristematic stage keep pace with one
another, at least at first, so that the width of the air-cavity and the number of cells
which limit it laterally increase simultaneously : it is only in the last stage that the
cavity is increased by extension of the cells alone It is not certainly proved
whether the other case, which may be observed in the formation of many secretory
reservoirs, and in the secondary cortex of Dicotyledons, viz. that the intercellular air-
space appears after the divisions in its vicinity are ended, occurs also in the primary
parenchyma.

Lamellar parenchyma occurs in certain plants in the same positions as stellate
parenchyma in others ; e. g. lacunar layer of the bifacial leaves of Ilex aquifolium.
Arbutus Unedo, Eugenia australis. Camellia : base of the petiole of Aspidium filix
mas and its allies : cortex of the rhizome of Carex disticha : stems of many Aroideas,
as Acorus Calamus, Calla, Monsterineae, &c. : pith of Saurureae.

As a special intermediate phenomenon between lamellar and stellate parenchyma
may here be named the layer consisting of loose and irregularly connected rows of
cells, which lies in the Selaginellas between the firm bundle sheath and the tough and
dense surrounding tissue. (Comp. Fig. 131, Sect. *78.)

Wide air-containing chambers and canals, the diameter of which greatly exceeds
that of the surrounding cells, are produced in two ways. Those of the one category
arise schizogenetically, and are only distinguished from the lacunae of lamellar
parenchyma by their width. The others are formed lysigenetically, or better rhexi-
genetically : during their development a mass of tissue lying in the direction of the
subsequent cavity ceases to follow the surface-growth of that surrounding it, and, since
growth continues in the latter, the former is ruptured and more or less destroyed.

To the first, schizogenetic, category belong the larger air-spaces in stem, roots,
and leaves of many marsh and water-plants ; Marsiliaceae, Salviniacese, leaves of
Isoetes, Ceratopteris; Potamogeton, Hydrocharideas, Alismaceae, Pontederia, Aroideas,
Lemna ; Papyrus (.^) ; Ceratophyllum, Myriophyllum, Hippuris, Trapa, Hottonia,
Elatine, Utricularia, Menyanthes, Nymphaeaceae, Nelumbium, pith of Desmanthus
natans, &c. Comp. Figs. 88, 112, 122, and 124, Chap. VIII.

To the category of lysigenetic forms belong the air-passages of the Equiseta,
those of the leaves, stems, and roots of most Cyperaceae, and Gramina, the leaves of
Sparganium, Typha, Iris pseudacorus, and its allies, Pandanus, the Marantaceae,
hlusa {?); in part those of the stem of Callitriche, those of the leaves of the narrow-
leaved Eryngia, of Lobelia Dortmanna, Nelumbium &c. ; lastly, the axile tubes of

Compare, e. g. Frank, l.c. — Yochting, Myriophyllum, N. acta Acad. Leop. vol. 36.

214

INTERCELLULAR SPACES.

numerous hollow stems of Equisetum, Grasses, Umbelliferse, Labiatas, Compositse,
&c., of hollow leaves and petioles (Allium, Asphodelus, Umbelliferse, &c.), also the
axile cavity of the internodes of Nelumbiumh

In many cases the origin of the chambers has not been carefully investigated,
but it may be recognised with some certainty from the structure of their mature
walls (which will be described below), in the same way as in the instances marked (?)
above. IMore accurate detail on this subject has but slight interest. On the one
hand the two modes of origin are fundamentally different in extreme cases, usually
also certain differences in structure of the walls of the chambers are peculiar to
each, and lastly they are distributed as a rule in different systematic groups. But on
the other hand all sharp limits are here again obliterated by all sorts of intermediate
cases, while forms differing in origin and structure may be substituted one for the
other, in the same position, in closely allied plants. For instance, the peripheral air-
passages which alternate with the vascular bundles in the stems of the Equiseta are of
intermediate structure and origin: they originate schizogenetically ; finally, some of the
separating cells are broken up and their membranes remain attached to the wall of
the passage The same holds for the large air-passages opposite the two shorter
sides of the quadrangular transverse section of the stem of the Eucallitrichas, while
the smaller ones there and in Pseudocallitriche are all schizogenetic In the
petioles of the IMarantaceae the arms of the very loose stellate-lacunar bands are often
finally torn asunder by extension of the surrounding tissue, so as to form continuous
air-passages, while their torn, often pointed and thick-walled ends rise free into the
cavity. In the halms of Scirpus lacustris* the meristem differentiates first into
prismatic bands of stellate-lacunar tissue, and plates of dense parenchyma, usually one
layer thick, which separate these, and appear in transverse section as a net with
angular meshes. The stellate cells follow the surface-growth of the latter, their arms
elongating greatly, but finally they are for the most part broken up, and only dried
remnants are left behind in the prismatic spaces. For further examples comp, infra.

As examples of the substitution of one form for another in a similar position
in allied plants the above-mentioned halm of Scirpus lacustris and that of Papyrus
may be cited. The latter has air-passages with very similar distribution, but of ap-
parently purely schizogenetic origin ; I have however at hand no direct observations
on their development. Carex arenaria has in the inner cortex of the rhizome a circle
of large air-passages, separated by many -layered, radial lamellae of parenchyma;
these, as is the rule in the Cyperaceae, are lysigenetic : C. disticha has in the same
place 7-10 circles of narrow schizogenetic passages, separated by simple layers
of cells.

As regards the /on7i of the larger air-cavities it has already been indicated that
they are either short polyhedral chambers, e. g. in the leaves of Pistia, the swellings
of the petiole of Trapa, and in the Lemnas, or elongated prismatic canals or passages,
and this is usually the case in elongated parts, such as stems, petioles, or narrow
foliage leaves. The latter but rarely traverse the whole of the elongated part

^ Trecul, l.c. p. 166. “ Compare Frank, l.c.

^ Compare Hegelmaier, Monogr. v. CalUtriclie, p. 24, Taf. I.

* Fiaiik, /, c. p. 147.

SPACES CONTAINING AIR AND WATER.

235

continuously — e. g. the petiole of Nuphar luteum, according to Frank; they are
usually divided into chambers by numerous transverse plates, or diaphragms, which
will be described below, or, in the case of stems, are interrupted, at least at the nodes,
by plates of parenchyma.

The slrticiiire of the walls of large air-cavities shows many notable phenomena.
Stem, leaves, and also roots of many water and marsh plants are traversed by nume-
rous passages or chambers, and these, especially the schizogenetic ones, are separated
in most cases only by plates of parenchyma one layer of cells thick: it is only where vas-
cular or fibrous bundles traverse them that these septa are several or many layers in
thickness. ]\Ieyen^ has drawn attention to the fact that the cells of the one-layered
lateral septa are often quite uninterruptedly connected over large areas, a condition
which seems in fact to be frequent, e. g. in the stems of species of Potamogeton and
Myriophyllum, the stems and leaves of Papyrus, Scirpus lacustris, the petioles of the
Nymphaeaceas, and of Pontederia crassipes ; according to investigations already made,
I cannot join in this statement with full certainty : in the first place, because of the
great difficulty of proving beyond doubt the absence of very small interstitial spaces,
and in the second, because the passages in the petioles of Ceratopteris and Villarsia
parnassifolia communicate one with another by very narrow spaces, and those in
the internodes of species of Marsilia even by rather large ones. An indirect com-
munication of the passages occurs also where the sides are thus completely closed,
through the interstitial spaces in the leaf-lamina, the' nodes of the stem, and in
certain cases through special diaphragms to be described below.

Schizogenetic spaces are limited by the smooth membranes of the cells which
form their wall. In many large passages, e. g. Nuphar or even in cavities,
e. g. Rhizome of Aspidium Filix mas, these are covered by a delicate cuticle.
It remains for further investigation to decide how far this phenomenon is of general
occurrence.

The wall of the lysigenetic spaces is as a rule more or less covered by the
remains of those disorganised cells, at the expense of which the space originated, or
the latter is here and there loosely filled with remnants of cells. Various special
forms originate in connection with this, according as the transitory cells are
mechanically torn asunder, or are broken down, or dried up, or according as
these several phenomena are combined.

In the pith of most internodes of stems, which become hollow, i. e. perforated by a
large, axile, lysigenetic air-passage (Grasses, Umbeiiiferae, Compositae, Equiseta, &c.), the
formation of the hoilow begins by those transitory roundish or poiyhedrai celis, which do
not foiiow the growth of the tissue surrounding them, becoming first separated from one
another, so as to form schizogenetic cavities which gradualiy increase in size. The ceiis
of the tissue thus broken up then graduaily iose their protopiasm, dry up, and coaiesce to
flaky or membranous masses, which are attached to the wall of the cavity. The whole
process begins in an internode, either simultaneously at many points in the pith (e. g.
Phragmites), or in the middle line, spreading centrifugally outwards from it (e. g. Cicuta
virosa, peduncle of Taraxacum ^).

Essentially the same process, on a smaller scale, occurs in the small air-passages which
alternate with the vascular bundles in the internodes of Equisetum : also, according to

Physiol. I. p. 295.

2 Frank, l.c. p. 155.

Frank, l.c p. 145.

INTERCELLULAR SPACES.

Frank, in those which alternate with the bundles in the leaf-sheath of many grasses,
the leaf-sheath and lamina of species of Carex, Luzula maxima, and albida. In the
last-named cases however, a further and slightly different condition seems to obtain,
which may be observed in the passages in leaves of Liliaceae, and Amaryllidaceae, Pan-
danus, and apparently in many other places : the group of cells, which occupies the place
of the future passage, first loses its protoplasm, the membranes become apparently
thinner, are partially dissolved, and are finally ruptured by extension of the surrounding
tissue. Thin very obscure tatters of the ruptured tissue clothe the walls of the mature
passage.

The above-described phenomena are modified in the halms of Cyperaceae (Scirpus
lacustris, species of Heleocharis, and Eriophorum), Juncus effusus, &c., in the leaves of
Iris pseudacorus, Sparganium Typha\ &c., in the following way: the strands of tissue,
which originally occupy the cavity of the air-passages, become at first lacunar, with many
armed cells, and for a time follow the growth of the surrounding tissue, while the arms
of the cells are much elongated : finally they dry up, and are partially broken down. The
extreme delicacy which they finally assume indicates that the membranes are partially
dissolved. As the result of the phenomena described, groups of distorted, more or less
collapsed ‘stellate’ cells are to be found on the walls of these air-spaces, or even, as in halms
of Juncus, the whole cavity is loosely filled with such cells. It is instructive that these
cases correspond closely on the one hand to those described above for the Marantaceae,
where many-armed lacunar bands are partially torn away, though their cells do not die :
on the other that the dried-up lacunar pith of Juncus corresponds to the numerous cases,
where the pith soon dries up to form a cylinder filled with air, without the formation of
large cavities or passages.

Finally, in a number of Cyperaceae those cells, at the expense of which the air-cavity is
formed, retain, at least in part, a firm wall, and these walls approach one another, by
reason of the tension caused by the surrounding tissue, till their lumen disappears.
These collapsed walls are then stretched in the air-cavity like thin plates or threads,
which, as Schwendener says, give quite the idea of a glass tube drawn out in a lamp.
Schwendener^ describes such a structure in the case of the many-armed cells in the halm
of Scirpus maritimus. The same occurs in the cylindrical-prismatic cells of the cortex of
the root of species of Carex and of Cyperus alternifolius : also, though in a slightly
developed form, in the air-spaces of the rhizome of Carex arenaria. In the inner cortex
of the root of Carex folliculata, for instance, numerous radial bands, 1-3 rows of cells
in thickness, continue to be composed of cylindrical-prismatic parenchymatous cells con-
taining starch throughout the tangential extension of the outer cortex : bands, usually of
2-4 rows, alternating with them, are widened out to cavities, in which the membranes
of the transversely distorted cells are stretched as almost solid thin plates in a tangential
direction.

As has been already indicated, and will be further stated in later chapters, layers of
parenchyma alone take part in the formation of the wall of the air-passages in question —
that is, if we leave out of account those hair-like single fibres, which spring in certain
cases from the wall. The only known exception occurs in the petiole of Thalia deal-
bata*, in which each air-passage is traversed throughout its length by numerous thin
bundles of sclerenchyma, which for the most part stand quite free in the passage, and are
only connected laterally with other elements where they run quite straight through the
two sorts of Diaphragms to be described below. Further, it may easily be recognised in
the mature plant that the denudation of the bundles is the result of breaking up of the
lacunar band of parenchyma, which originally fills the passage, this not keeping pace in its
growth with the extension of all the other parts.

' Frank, Lc. p. 148. ^ Meehan. Princip. p. 92, Taf. X. 10.

^ Duval-Jouve, Diaphragmes vasculiferes des Monocotyledones, Mem. Acad. Montpellier, 1873,
p. 168.

DIAPHRAGMS.

217

Sect. 52. During the formation of longitudinal air-passages, whether they are
formed simply schizogenetically or by destruction of cells, transverse zones, which
follow the extension, remain connected at certain places, as diaphrag7?is, which break
the continuity of the passage.

In the first place, they occur in the nodes of all hollow stems, and here they are
deep many-layered discs of dense parenchyma, through which vascular bundles, milk-
tubes, and secretory passages run, and pass out into the leaves, as will be described
in later chapters. In the internodes the large, simple, axile cavity of the hollow stem
is uninterrupted, at all events in most cases, and the same holds for the numerous
peripheral passages in the internodes of many water-plants, as Ceratophyllum,
Myriophyllum, Hippuris, Elatine Alsinastrum, species of Jussiaea, Limnanthemum
nymphoides, Zostera, Posidonia Caulini, Nelumbium : also, with a restriction to be
given below, for the leaf- and flower-stalks of the native Nymphaeacese : and lastly,
for all roots with large air-passages, though these require further investigation in this
respect. On the other hand the air-passages in the internodes, petioles, and leaves
of most Monocotyledons, the petioles of Limnanthemum nymphoides, the internodes
and petioles or conical leaves of the Marsiliacese, the leaves of the Isoeteae, See., are
partitioned by diaphragms.

These are separated by short distances, usually a few millimetres, rarely over

from one another; they are horizontal or oblique; they may alternate, those of
adjoining passages being at unequal heights, or they may be at about the same height,
so that one and the same diaphragm extends evenly over several or many passages.

The diaphragms consist of one, more rarely of several layers of parenchyma-
tous cells, often rich in chlorophyll, between which air-containing interstitial spaces
always lead from one chamber into another : they are sometimes composed of dense
parenchyma with narrow interstitial spaces, as e.g. in the leaves of Luzula maxima
which has diaphragms two layers thick, in species of Carex, Cladium Mariscus,
Scirpus sylvaticus, maritimus, Cyperus fuscus, Veratrum album. Iris pseudacorus,
Posidonia Caulini, Zostera, Caladium, Colocasia and its allies, &c. ; others consist of
many-armed cells, connected by the ends of the arms, and forming a plate with wide
lacunae, as in the leaves and stems of many water-plants, Isoetes, Potamogeton,
Aponogeton, species of Typha, Sparganium, Pontederia, Butomus, Sagittaria, and
Alisma, petioles of Limnanthemum, Strelitzia, halms of Papyrus, Heleocharis palus-
tris, Eriophorum, leaves of Pandanus, &c. In the wide central portion of the halm
of Juncus effusus, glaucus, and their allies, transverse zones of some few layers of
many-armed stellate cells keep their walls relatively firm, and persist as diaphragms,
while the delicate tissue between them, which is also many-armed and lacunar,
collapses.

Intermediate forms, which make it impossible to separate the lacunar dia-
phragms sharply from the dense ones, are not uncommon : for instance, those
composed of short-armed cells, with narrow cavities between, in Scirpus lacustris, in
the leaf-sheaths and leaves of Glyceria aquatica, and Oryza sativa (Duval-Jouve), to
which many of those above-named correspond ; and the narrow-lacunar diaphragms
of the Marsiliacese, the structure of which is not unlike that of the lateral walls of the
air-passages, &c.

Two sorts of diaphragms were mentioned above in the case of the air-passages

2i8

INTERCELLULAR SPACES.

of the petiole of Thalia dealbata. The first are lacunar plates, consisting of many-
armed cells usually one layer in thickness, which have apparently resulted from
transverse rupture of a band of similar tissue, which originally filled the passage.
The others consist of a layer of relatively small-celled, dense parenchyma, which is
covered on each surface by a lacunar layer of many-armed cells. Similar conditions
of undoubtedly similar origin are found in the petioles of species of Musa.

In the leaves and petioles of the Monocotyledons above named, and the halm-
internodes, which resemble them in structure (Scirpus, Juncus, Papyrus, &c.), and
further in the petioles of Nelumbium, the longitudinal bundles are connected by
more or less numerous thin transverse branches (Sects. 66, 91). These run through
the diaphragms, especially where the lateral walls of the air-passages are only a single
layer of cells in thickness, either transversely through their surface, or through their
margin, which abuts on the lateral wall. If the diaphragms are one layer of cells
thick, the vascular bundle appears as a swelling of it, several layers in thickness, which
may protrude either on both surfaces (e.g. Sagittaria), or only on one (e.g. Scirpus
lacustris). Either all the diaphragms contain a transverse bundle (e.g. Papyrus),
or this is the case with some of them and not with others (e.g. Pontederia,
Butomus) : of the two sorts in Thalia and Musa, only the dense ones contain
bundles. Where all the longitudinal corners between the air-passages do not con-
tain longitudinal bundles, as in Papyrus, Sagittaria, and the leaf of Acorus Calamus,
the diaphragms with transverse bundles must be continuous transversely through
several passages, their arrangement is thus to a certain extent dependent upon that
of the vascular bundles.

The form of the cells of lacunar diaphragms wdth many-armed cells, which has
been studied as a hobby by some ’, is particularly various, and often beautiful in
diaphragms one layer thick, in which all the arms of the cells lie in one plane. For
details reference may be made to the works cited, while here some few of the chief
forms from stems and petioles will be noticed.

(1) Rather regular stellate cells wdth long arms, having between them wide lacunae,
which usually correspond to the original corners of contact of three or more cells, e.g.
Isoetes, Villarsia, Nelumbium, Potamogeton natans, Thalia, Pandanus, Pontederia,
Eriophorum, Heleocharis palustris, &c. In the two latter the lacunae are bordered by
the arms of three cells, in outline they are rounded triangular, and contracted in the
middle by the swollen margins of the surfaces of contact of the pair of arms forming
each side : they have thus the form of a gothic trefoil, and the whole diaphragm has,
owing to this, and to the delicate pitting of its thick cell-membranes, a peculiar ap-
pearance.

(2) Short-armed cells {a) with rounded lacunae, corresponding to the original corners
(e.g. leaf of Sagittaria sagittifolia, Butomus).

(b) With a series of small round or slit-like lacunae along the original limiting surface
of two cells : Scirpus lacustris.

(c) With lacunae at both places, those corresponding to the corners being larger than
those along the sides (e. g. Sagittaria indica, lancifolia), or both being of almost equal size
(e.g. Marsilia).

In the diaphragms of Scirpus lacustris the condition described under (c) occurs.

1 Meyen, Phytotomie, pp. 85, 193. — Haarleraer Preisschrifl, p. 138, &c. — N. Syst. d. Pflanzen-
physiologie, I. p. 29.], &c. — Duval-Jouve, l.c.

DIAPHRAGMS.

2\g

especially at the margin, often with relatively very large cavities at the corners (comp.
Meyen, Physiol. I, Taf. II, Figs. 3 & 4). The greater part of the surface of these
diaphragms has, on the other hand, the following structure as a rule. The surface ap-
pears divided into polygonal areas, and each of these is again divided by parallel walls
(which may be termed the inner walls) into 4 cells on the average, of which the central
ones are narrow and quadrangular, the outer are irregular and narrow, with 3-5 corners.
The inner walls of an area are usually not parallel to those of the neighbouring areas.
The walls which limit the areas, and apparently correspond to those of the mother cells
which are subsequently divided by the inner walls, are irregularly undulated and un-
interruptedly connected with those adjoining them. But along the parallel inner walls
each of the cells has a row of usually 5-7 short arms, and between these are roundish
quadrangular cavities.

The several-layered lacunar diaphragms naturally resemble in the main points the
masses of stellate-lacunar parenchyma above mentioned. Those only of the halm of
Papyrus, which run transversely, though slanting and distorted, through many air-
passages, deserve special mention, on the one hand, because of the extremely irregular
form and arrangement of the arms of their cells, on the other, because their lacunar
tissue is also continued transversely through the lateral walls of the air-passages, from
one to the other, and from the outermost into the peripheral chlorophyll-parenchyma.
Since all diaphragms, or at least most of them, are continued transversely through
several or many air-passages, all the passages communicate, by this arrangement, in-
directly one with another, with the air-containing interstitial spaces of the chlorophyll-
parenchyma, and through this with the stomata, though in the lateral walls of the
passages themselves no interstitial spaces are to be found.

In such plants as secrete much calcium oxalate, not only is it often laid up
in large quantity in the form of small crystals in the cells which adjoin the air-
passages, as e. g. in the diaphragms of IMusa and Sagittaria, but the structure of
the walls, both laterally and on the diaphragms, is often complicated by the inter-
calation of crystal-containing sacs in the layer covering the walls, or these are seated
upon the above layer as papillae or small hairs. As far as is known this only occurs
in schizogenetic, not in lysigenetic spaces : it remains for observations of the develop-
ment to decide whether Nelumbium, with its numerous grouped crystals, which
protrude into the air-spaces, is an exception to this. Of the forms of crystal-
containing sacs described in Sect. 32, we have here to deal more especially with
elongated or spindle-shaped sacs with raphides, and spherical sacs, each one en-
closing a single stellate group of crystals.

Such of these as are intercalated in the layer covering the walls require no
further mention here. The sacs with clustered crystals which protrude into the
cavity are always seated on the wall, singly or (Trapa) in groups, as small round
bladders with a broad base. When old their membrane, which is always delicate, is
in many cases extremely thin and difficult to see — it remains doubtful whether it
entirely disappears or not — so that the clusters project or appear to project freely
into the cavity.

The projecting, elongated or spindle-shaped raphide-cells are sometimes attached
to the lateral walls, in wffiich case they either have one of their ends inserted on it,
or are attached by the middle to a narrow surface of a cell of the wall, while the two
ends project freely upwards and downwards into the space. The same holds some-
times for the one-layered diaphragms, and the walls of the chambers (also one layer
of cells thick) in the leaf of Pistia ; sometimes in this plant the raphide-cells have

220

INTERCELLULAR SPACES.

their middle part intercalated in the plate of cells, while their ends extend over it, the
one upwards, the other downwards.

As examples of all these conditions in sacs with clustered crystals may be cited
Myriophyllum, Trapa, Nelumbium : in raphide-sacs, Pontederia^, Scitamineas, Phily-
drum Colocasia odora : in both forms together, many Aroidese, as Colocasia anti-
quorum, Caladium nymphaeifolium Pistia.

In the diaphragms of the petiole of Pontederia (P. cordata and crassipes) there
are found, besides the raphide sacs, others, of spindle-like shape, with their longer
axis placed at right angles to the diaphragm, and with their middle intercalated in it,
so that the ends project upwards and downwards into the space : each of these
contains a single spear-like crystal sharply pointed at both ends. This, together
with the sac which contains it, attains a length, especially in P. cordata, of more
than three times the thickness of the diaphragm. Finally, the membrane of the sac
covering the ends of the crystal ceases to be apparent, so that the latter seems to
protrude freely into the air-space

Sect. 53. The walls of many large lacunae and air-passages are characterised by
projecting cells or portions of cells, which, from their form, are termed hairs. These
may be divided into two categories, namely, glandular hairs ; and non-glandular,
usually firm hairs like sclerenchymatous fibres. The only forms of the first category
are those glandular hairs, first noticed briefly by Mettenius and described later by
Schacht in the air-cavities of the rhizome and base of the petiole of Aspidium Filix
mas. One or more unicellular capitate hairs, which are seated singly on adjoining
starch-containing parenchymatous cells, project into the larger cavities of this plant :
they arise originally as daughter-cells of the latter, or as outgrowths of them.
The small thin cylindrical stalk widens out into a large pear-shaped head, and this
secretes on its surface, as far as the limits of the stalk, a firm, greenish, brilliant, thick
layer of resin. The mode of its secretion and the structure of the whole hair are
the same as above described for the glandular hairs (p. 88) ; glandular hairs, which
closely resemble these intercellular hairs, occur here and there on the surface of the
bases of the petioles of the male fern, as teeth of the base of the Palese. In the
base of the petiole of Aspidium spinulosum — and probably also of other allied species
— such internal, intercellular glandular hairs are found, though in less numbers than
in Filix-mas.

Intercellular hairs of the second category occur especially in the air-passages of
such plants as have no diaphragms : Pilularia, Nymphseace®, Aroidese, Rhizophora,
also Limnanthemum. As may be judged from their structure and arrangement,
they, like the diaphragms, serve as a support. Russow found in the air-passages of
the root of Pilularia globulifera hairs rolled up like watch-springs, with thin mem-
branes, studded with fine external warts : their arrangement is as follows. The inner
cortex contains 1 2 air-passages separated by radial lateral walls one layer of cells in
thickness ; 6 broader ones alternate with 6 narrower ones. Single cells of the lateral

* Meyen, Phytotomie, Taf. V.

^ S. F. Ploffmann, Linnsea, XII. p. 683. ^ Meyen, l.c. Tab. XII.

^ Compare Meyen, Phytotomie, Taf. V. — Duval-Jouve, l.c. p. 166.

° Fil. horti. Lips. p. 92. Pringsheim’s Jahrb. III. p. 352.

’ Vergl. Unters, p. 22.

INTERNAL HAIRS.

221

walls, which adjoin the outer cortex, elongate to form these hairs : they protrude as
blunt sacs, with i| narrow turns, into the small passages, filling up the breadth of
the passage ; they all curve in the same direction. The vertical distance between
two hairs is never less than the greatest diameter of the narrower passages.

In the air-passages of the leaf- and flower-stalks of the NyniphcEacea, in species
of Nymphaea (but not Nuphar), and, according to S. Hoffmann, also in the roots and
Rhizome, there have been known, since the time of Guettard ^ branched ‘ stellate
hairs ’ with pointed arms, and a firm wall, which shows numerous blunt, wart-shaped,
external thickenings, containing calcium oxalate^. In the petioles and peduncles
the hairs arise from the simple perpendicular rows of cells, forming the corners of the
air-passages, which appear polygonal in transverse section : they are at different
heights one above another, often being separated from one another in one corner by
only few cells ; in neighbouring ones they are at different heights, so that on each
large transverse or longitudinal section several may be seen at once (Fig. 88). A
cell of the corner series, flattened
above and below, puts out an arm
into each of the air-passages
(usually 3, more rarely 4) which
adjoin it : these, directly they
enter the passage, divide into the
diverging pointed branches of the
permanently unicellular hair. In
the simple regular case, as is
most common in the petiole of
Nuphar pumilum, each arm
branches once into two almost
equal branches, of which one
turns upwards, the other down-
wards. In N. luteum this vertical
branching is often immediately
succeeded by a second at right
angles to it, so that in the most
regular case each arm runs out into four pointed branches, two diverging obliquely
upwards and two obliquely downwards. But in this, as in other species, it often happens
that, by the absence of one of them, or unequal development of the branches of this
second bifurcation, a more irregular general form is attained : also a more complex
branching may occur. In the larger air-passages the branches are shorter than the
diameter of the passages; they diverge almost at right angles, and as a rule they do not
touch the wall of the passages. In the narrow passages, like those of the periphery
of the petiole of the above species, and, according to Meyen, in all cases in Nymphaea
odorata and caerulea, the hairs are not at all or only slightly smaller than in the wide
ones ; they therefore frequently touch the lateral walls. The angle of divergence of

Fig. 88. — Ntiphar advena; transverse section through the petiole;
vascular bundle, i air-passages, j stellate hairs. From Sachs’ Textbook.

' Compare Meyen, Physiologie, I. p. 311; Phytotomie, p. 200, Taf, IV. — Trecul, Ann. Sei.
Nat. 4 ser. tom. IV.

- According to a note by H. von Mohl, communicated to me.

1222

INTERCELLULAR SPACES.

their vertical branches is often much greater than in the above, reaching not un-
commonly 180'’, so that the hair appears in profile in the form of a letter H.

In the lacunae of the lamina the form of the hairs in the portion near the lower
surface does not differ essentially from that above described. At the limit be-
tween this and the lamellar layer of parenchyma of the upper surface are found
numerous hairs, which put out diverging arms both downwards into the lacunae, and
also upwards : these run straight and perpendicular between the lamellae of paren-
chyma, as far as the inner surface of the epidermis \

In the air-passages of those Nymphaeaceae which have been investigated, at
least in those of the petiole, there occurs another sort of hair formation, different from
that described. Single cells of the lateral wall put out into the passage a sac-like
outgrowth, which branches frequently and irregularly to form many arms, and is
divided by septa into cells, which are also irregular and many-armed. The same
growth continues in these for a long time, so that a small lacunar mass of cells is
formed, which loosely fills up the passage like a diaphragm of many layers of cells.
The cells of these pseudo -diaphragms have permanently delicate, smooth walls, and
scanty protoplasm with some few starch-grains ^

Hard stellate hairs with pointed arms, resembling closely those of the Nymphae-
acece, are found in the air-passages of Limnanthemum nymphoides, and other
investigated species of the same genus, in stems, rhizomes, and petioles They are
always distinguished from those of the Nymphaeaceae by their smooth membranes;
many protrude only into a single air-passage. These stellate hairs have not hitherto
been found either in the true Villarsias, or in other Menyantheae, or in other water-
plants of like habit.

In certain Aroideae, viz. the group of the Monsterineae (Monstera, Tornelia,
Heteropsis, &c.), Pothos Rumphii, and Spathiphyllum ^ numerous hairs of this sort
are contained in the cavities and passages of the lamellar parenchyma. They usually
occur in all parts of the plant, or they are absent in certain parts, e. g. the rhizome
and the roots of Spathiphyllum. They arise by early outgrowth of a cell of the w'all
of the cavity (usually one layer of cells thick) which remains relatively narrow, so as
to form long, thin pointed arms. In the longitudinally elongated cavities of inter-
nodes, petioles, and roots more simple forms occur as a rule : each hair-cell grows
out into one, two, or rarely three of the adjoining passages, forming in each two arms
of equal or unequal length, and tapering very gradually to a point : these diverge
exactly 180° from their point of origin. The hair thus assumes the form of a spindle-
shaped body, with a short blunt transverse appendage, which is fastened in the
lateral wall of a cavity : or of an H with a short transverse portion, which is im-
bedded in the lateral wall betw'een two cavities (Fig. 89). Of the many irregularities
which occur in this type, only one need be mentioned here, viz. that an arm may put
out single lateral branches, which enter like hooks into neighbouring lacunas. In the

1 Meyen, Haarlemer Preisschr. Taf. V; Physiol, l.c. p. 312. — Trecul, l.c. pi. 12, fig. 25.

“ Trecul, l.c. fig. 12. — Frank, l.c. p. 153.

^ Grisebach und Hoffmann, Linn^a, Bd. XII. p. 681. — S. F. Hoffmann, Ibid. Bd. XIII. p. 291
(1839).

' Van Tieghem, Structure des Aroid^es, l.c. p. 137, &c.

INTERNAL HAIRS.

223

short lacunse of the lamina, which communicate in all directions, the branching of the
hairs is more complex and irregular ; they here put out radiating arms diverging on
different sides, which themselves again branch, and may
traverse many lacunae. Each lacuna, especially in stems
and petioles, is thus permeated by numerous hairs; they
may be found in every transverse section, either singly
or many — up to 10 or 20 — in orte cavity, in the latter
case side by side, but always without touching one
another. The size and stiffness of the hairs varies to
a certain extent in special cases : in Spathiphyllum
lancaefolium-van Tieghem found them the longer, nar-
rower, and more thin-walled, the more numerous they
were side by side. Their length is very considerable :
in the last-named plant it reaches 5-7““, with an
average width of o.oin'in_

The membrane of these hairs is always colourless,
quite smooth, more or less thickened, and stratified ;
the inner layers, when the wall is of great thickness,
have shallow pits : their cavity is usually uninterrupted,
rarely it is partitioned by a few thin septa : the con-
tents are transparent and watery, with isolated granules,
and sometimes small crystals of calcium oxalate.

They closely resemble sclerenchymatous fibres, and
were therefore at first described as ‘ bast-cells ’ b

The occurrence of intercellular hairs, quite similar
to those of the Aroideae, in the pith and cortex of
species of Rhizophora, is very remarkable. They are
here usually of the H form, on the whole harder than
in the Aroideae, and their arms are usually solitary,
though sometimes 2-4 occur in one intercellular pas-
sage, filling it loosely.

It is instructive that the above-described many-
armed hard-walled hairs of the Nymphseaceae, Lim-
nanthema, Aroideae, and Rhizophoras are funda-
mentally related to sclerenchymatous fibres, in every respect, and are only special
cases of the latter, distinguished by their form and distribution, and that thus their
earlier designation as ‘bast-fibres’ was not without justification, provided scleren-
chymatous-fibres were really meant by this name. Comp. Sect. 30.

Fig. 89. — Monstera deliciosa; longitu-
dinal section through the petiole, d, d pa-
renchyma ; .f a hair in form of an H. tlie
main arms running perpendicularly through
the air-spaces; at the top to the right is a
small curved branch. From Sachs’ Text-
book.

* Schleiden, Wiegman’s Archiv, 1839, Bd. I. p. 21 1.— Beiträge, p. 42.

PART II.

THE ARRANGEMENT OE

THE

EORMS OE TISSUE.

FIRST SECTION.

PRIMARY ARRANGEMENT.

Sect. 54. The different forms of tissue result from the differentiation of the
primary meristem present in the growing points, and have a definite relative position
and arrangement. In very many instances this is permanent, as is the case in leaves,
and in the stems and roots of plants not belonging to the Dicotyledons and Gymno-
sperms. In other cases, especially in the last-named plants, either new structures,
which arise from secondary meristems, are formed, in addition to the tissues
derived from the primary meristem (p. 4) ; or changes, which are consequences of
secondary formations, appear in the primary tissues.

The masses of tissue directly derived from the primary meristem and their
arrangement are called prwiary to distinguish them from those of later secondary
origin, and from consecutive secondary changes. We shall here study, in the first
place, only the first category. It is, it is true, to be expected a priori, that the
development of the primary masses of tissue takes place not suddenly, but in
definite succession, and that the secondary changes may be closely connected with
it, without there being any sharp limit between the two processes. Nevertheless in
many typical cases a definite limit between them can be found, and, from these as a
starting-point, it can be extended to all.

The course of treatment in this section has been generally indicated by the
distinctions drawn in the first part. Of the forms of tissue there distinguished, the
epidermis will naturally not be treated of further. Its arrangement can be concluded
from Chap. I. i : also the arrangement of single parts of it, which should properly

PRIMARy ARRANGEMENT.

225

be reproduced here, had to be described above, and may be there referred to
if necessary. Peculiarities, which depend upon the tissues covered by it, will be
noticed when the latter are treated of.

As regards the other forms of tissue, to be dealt with in this section, we may
refer in the first place to the method of grouping given on p. 5, which holds for all
tissues. It may here be added that a group of tissues bordering directly on the
epidermis is called from its position hypodernial, while distinct hypodermal layers are
indicated by the substantive hypoderma h

It is best to begin the description of the primary arrangement of tissues with the
tracheae and sieve-tubes, since these are connected, in almost all plants with which
we shall be engaged, into strands or vascular bundles ; and these form a well-marked,
uniformly comparable skeleton,» on and around w'hich the other tissues arrange
themselves.

Pfitzer, Pringsheim’s Jahrb. Bd. VIII.

CHAPTER VIII.

TRACHEA AND SIEVE-TUBES.

I. TrachecE and sieve-lnbes outside the vascular bundles.

Sect. 55. Both the above organs are, as above indicated, united as a rule to
form the vascular bundles. They occur however, in many special cases, external to
and side by side with the latter in other regions also, and otherwise distributed.

Scattered tracheides occur outside the vascular bundles, enclosed in other tissues
in the stems and scale-leaves of species of Salicornia and Nepenthes ^ and in the
base of the leaf of the Isoetese.

In the many-layered, chlorophyll-containing parenchyma of the cortex of the
stem of those species of Salicornia which were examined, Duval-Jouve^ found,
according to the species, cylindrical or spindle-shaped tubes, which have exactly the
structure of air-containing tracheides. They are about as long as the chlorophyll-
containing layer of the cortex is thick, and have their longer axis perpendicular to
the epidermis, which they do not reach, but end one layer of cells further in, close
to one of the very numerous air-cavities of the stomata. Their other end is in
contact with the colourless inner parenchyma of the cortex, but does not extend
to a vascular bundle. In S. sarmentosa, patula ( = S. herbacea of most authors),
and fruticosa, the tracheides are rather regularly cylindrical-spindle-shaped ; their
completely colourless wall is thickened at the sides with a close and fine spiral
fibre, on the blunt ends it is smooth. In S. Emerici Duval-Jouve found the
tracheides but few, and weakly developed. In S. macrostachya they are irregularly
spindle-shaped, with lateral, short, pointed excrescences, and often with hooked ends ;
their membrane is uniformly and strongly thickened, it is smooth or scarcely pitted ;
they remind one to a certain extent of the rod-shaped sclerenchymatous cells in
the leaves of Proteacese (comp. p. 130 and Chap. X).

The tracheides of the species of Nepenthes which are also filled with air, are
almost cylindrical, usually a little tapered at the ends, and of varying length, which
however hardly exceeds that of 10-20 cells of the parenchyma. Their colourless wall
has a close and delicate spiral thickening. They occur in the stem, distributed in all
parts of it, and in large quantity in the parenchyma ; similarly in the petiole and

* [Cf. Mangin, Developpement des cellules spiralees. Ref. Bot. Centralbl. 1882, Bd. XII. p. 85.]

^ Des Salicornia de I’Herault, Bulletin de la Societe Bot. de France, tom. XV. p. 140, pi. i.

^ Korthals, Verhandelingen over de Naturl. Gescliied. d. Nederl. overzee. bezittingen ; Botanic,
p. I. — Compare also Unger, Grundlinien, p. n.

VELA MEN.

237

lamina, and in the pitchers 2-3 layers of cells below the surface. They are never
continuous with the vascular bundles. In the stem they are all arranged parallel to
its axis ; in the leaf, at least in the wall of the pitcher-shaped portion, they point
irregularly in different directions.

In the base of the leaf of the Isoetese ^ are found series of short spiral tracheides,
having the same form as those of the xylem in the stem of these plants : they occur
in the dense mass of parenchyma at the point of insertion of the membranous ligula,
called by Braun the Glossopodium. They extend from the upper and lower margins
of this body almost horizontally to the inner surface of the base of the leaf : those of
the upper side towards the posterior wall, those of the under side towards the memr
branous lip-like lower margin of the depression in which the ligule is seated. They
have no connection with the vascular bundle of the leaf.

Sect. 56. A continuous layer of air-containing tracheides covers, as a sheaih or
velanien, the aerial roots of epiphytic orchids, which
in this respect resemble those of some other plants,
especially Aroidese.

The tracheal sheath of the roots of Orchids
is produced from the layer of dermatogen, which,
according to Treub in Vanilla and Stanhopea, is
differentiated close behind the growing-point from a
common initial group for the root-cap and the body
of the root ; this I found to be the case in Vanda
furva, while in species »of Oncidium (Fig. 90,91)
the dermatogen passes over the growing-point as
a distinct layer between periblem and calyptrogen.

The simple layer of the periblem, adjoining the
dermatogen internally, develops into the endoder-
mis, which consists of longitudinal rows of alter-
nately elongated and short cells (comp. p. 125).

The very delicate cuticle which is present at
first, i. e. where the root emerges from the root-
cap, is absent over the mature outer surface, or
at least cannot be proved to be present as a con-
tinuous skin.

The sheath of tracheides remains in some
few cases a single layer (Vanilla planifolia, aphylla,

Sarcopodium Lobbii, Cirrhopetalum Wallichii) : in
most cases it is cut up by corresponding divisions,
which begin behind the growing-point, into several
layers, numbering according to the species 2, 3, 6, or 18 (Cyrtopodium spec,).
All its elements are in uninterrupted connection with one another (comp. Figs. 90,
91, d). The single tracheides are approximately iso-diametric, or slightly elongated

A

Fig. 90. — Species of Oncidium; aerial root,
slightly magniSed. y/ axile longitudinal section
through the apex, c — c root-cap, p depressed
centre of the growing-point, d sheath of tra-
cheides, A endoderniis, ^vascular bundle, r cor-
tical parenchyma. B transverse section through
a mature part ; lettering as in A.

' Mettenius, Linnoea, 1847, p. 272. — Hofmeister, Beitr. p. 151.— A. Braun, Isoeten d. Ins. Sar-
dinien, Berliner Acad. Monatsbr. 1863, p. 571.

“ l.c., compare p. 10.

9 2

228

PRIMARY ARRANGEMENT OF TISSUES.

in the same direction as the root. Their membranes are usually colourless, thus
the surface of the air-containing layer appears white and shining. In water, which it
quickly absorbs, the layer becomes transparent ; the internal green cortical parenchyma
then becomes apparent. Another cause of the green colour is that in old roots
(Vanda furva and Anselia africana, according to Leitgeb) small green algal cells
sometimes enter the cavity of the tracheides. When old, the air-containing layer
is in many species entirely thrown off (Angraecum subulatum, Cymbidium ensi-
folium, Zygopetalum Mackai, according to Leitgeb ; also Vanda furva), or only the
innermost layer remains : the result of this also is that the green colour of the
cortical parenchyma becomes visible.

In Eria stellata the colour of the aerial roots is brown, since the membranes of
the tracheides assume a brown colour: in Trichotosia ferox the tracheides of the

Fig. 91. — Median longitudinal section through the apex of a young root of the same Oncidium as Fig. 90,
with the same lettering (375).

four-layered sheath are filled with a reddish-brown mass, which gives the roots a
reddish-brown colour; the same sometimes applies to Cymbidium marginatum.
Larger or smaller masses of a loosely coherent black-brown substance were found by
Leitgeb in many cases, especially in the inmost layer of cells : and in specially large
quantity in Renanthera coccinea. According to Leitgeb, those walls which cover the
short thin-walled endoderinal cells always show a limited brown coloration.

With the exception of these last-named cases, air alone, or water obtained from
without, is always contained in the tracheides ; the protoplasm and nucleus dis-
appear entirely during their development, close to the growing-point.

During their differentiation the walls of the tracheides, like those of other tracheal
organs, become lignified to an extent which varies according to the special case. As
regards the form of thickening the greatest variety may be seen, not only in those of
different species often closely allied to one another, but also in those of different layers

VELA MEN.

229

of the same root, and in the several walls of one and the same tracheide. In most
cases the walls are thickened by spiral fibres, which in some plants run exactly
parallel (Sarcanthus rostratus, Gongora Jaenischii, Brassia maculata, Cattleya
IMossias), or leave slits between them (Oncidium pulvinatum, flexuosum, sanguineum),
or form large meshes (Epidendron elongatum, Brassia caudata) : or in other cases
are arranged in band-like groups (Cyrtochilum bictoniense). According as these
fibres are in close juxtaposition (Oncidium flexuosum, sanguineum, Cymbidium
ensifolium), or are further apart from one another (Maxillaria tricolor, Camaridium
ochroleucum), the slits and meshes are smaller or larger. Further, since in many
cases the fibres of two contiguous walls cross, the superposed slits and meshes are
also crossed. Not uncommonly the spiral fibres, which usually traverse the wall
obliquely, but often appear (in transverse sections) radially arranged, run quite
irregularly, in which case they are however sparsely distributed and branch re-
peatedly, and then either continue their course independently, or again unite later to
form broad bands (Renanthera matutina, Phalaenopsis grandiflora, Saccolabium
Blumei). In other cases the spiral-fibrous thickening disappears entirely, and there
are only solitary slits to be seen, which are still arranged in spiral lines (Angrsecum
subulatum, outermost layer) ; but not uncommonly this spiral arrangement also
disappears, and a purely reticulate thickening is found (Dendrocolla teres, Sobralia
decora, Vanda furva). In some though less frequent cases the walls are again quite
regularly thickened, and only present more or less numerous pits (Angrsecum
subulatum, second layer) ; often the thickening layers are only developed at the
corners (Sarcopodium Lobbii, Cirrhopetalum Wallichii), or the walls are without any
thickening at all, and are quite thin (Trichotosia ferox ; Angrsecum subulatum, third
layer, Leitgeb).

These examples may serve to show the variety of the details, while reference
may be made to the further special descriptions and figures of Oudemans and
Leitgeb. What has been said of the triple sheath of Angrsecum subulatum shows at
the same time the variety of form of thickening, which often occurs in successive
layers. Though the conditions in this respect also are very variable, the rule still
holds that both in forms with one and with many layers, at least the outer and the
inner surfaces are characterised by special thickening of the membranes.

At those points -where the spiral or netted fibres separate widely in a slit-like
manner from one another, the wall-surfaces which have no fibres are not uncommonly
perforated whether they be on the free surface or in the body of the sheath. Thus
in the latter case, speaking accurately, the tracheides are united to form vessels.
Where the sheath consists of several layers, the elements of the outermost layer often
grow out as papillae or sac-like hairs, a phenomenon which also appears in the one-
layered sheath. In those cases known, all the elements of the surface are not engaged
in the formation of hairs. The latter sometimes occur on roots which protrude
freely into the air, as Leitgeb found in seventeen species of different genera (in Eria
stellata the felt of hairs, which is usually dense, is quite absent if the roots grow in
moss or earth) : while in other cases the formation of hairs appears only when the

* Von Mohl, Verm. Schriften, p. 322 (Epidendron elongatum).

230

PRIMARY ARRANGEMENT OF TISSUES.

growing root is in contact with a solid (damp) body — Epidendron elongatum, species
of Stanhopea, Oncidium sphacelatum, flexuosum, Maxillaria Harrisonise. The hairs
attach themselves firmly to the contiguous body, while not uncommonly they are
considerably spread out, their free ends even branching in a palmate manner. The
membrane and contents of the hairs resemble those of the rest of the root-sheath of
the species. The membrane separates readily into spiral bands r it is easily ruptured
in many species (e. g. Vanda furva, Sobralia decora), and thus are formed some of
the holes in the outer surface.

The tracheides of the inmost layer are for the most part more elongated than
the rest ; further they are either of fundamentally similar form and structure all round,
or are characterised by peculiarities, where they cover the thin-walled endodermal
cells. As regards the peculiarities of form, they appear as one-, two-, or three-
layered groups of small flat elements, which are evenly fitted at the points indicated
into the other part of the sheath. As regards their stmcture, the special form of
thickening of their walls often differs from that of the surrounding tissue, but within
the rules above laid down for the latter. The brown coloration of the limiting walls,
which Leitgeb states is constant, but which I failed to see in Vanda furva, Oncidium
sphegiferum and Acropera Loddigesii, has been above mentioned. Peculiar thicken-
ings are found in special cases at the points indicated: in Trigonidium Egertoni-
anum a strong stratified swelling intrudes slightly into the cavity of the cell on that
part of the wall which abuts on the thin-walled endodermal cells : in the Sobralias
there is a similar, very protuberant, almost spherical, stratified swelling of a dark
brown colour. The cells of the inmost layer of the sheath are in the latter plants in
all cases of fundamentally the same form ; one of them overlies one of the thin-
walled endodermal cells, or two or three of them are in contact with the latter : since
each of the adjoining cells has a swelling of the wall, 1-3 of the latter overlie one
thin-walled cell.

The aerial roots of many epiphytic Aroidese have a sheath of Tracheides
derived from the dermatogen, and resembling that of the Orchids in all fundamental
points. In Anthurium acaule, egregium, crassinervium, and intermedium, the
tracheides have spiral or reticulate fibres ; the sheath is 4-5 layers thick. In other
species of Anthurium there are 2 or several, in Homalonema cserulescens even 6
layers, but the walls of the tracheides are smooth and thin. A one-layered sheath
consisting of thin and smooth-walled elements occurs in Anthurium violaceum. Philo-
dendron pedatum, and other Aroidese, and again in the aerial roots of Hartwegia
comosa Nees. (Chlorophytum Sternbergianum, Steud.), and Hoya carnosa. In the
latter cases the air-containing elements (which often grow out to hairs or papillae) may
just as well be called dried-up cells as tracheides, or perhaps better. Still it is more
proper to place them here, as incomplete forms, with the sheaths of tracheides,
since in those roots on which they occur (as in all others here cited) an endodermis
structurally similar to that of the Orchids abuts internally on the air-containing
sheath.

The above description of the air-containing sheaths of roots is based on the above-
cited works; also on the investigations of Oudemans, Ueber den Sitz der Oberhaut bei
den Luftwurzeln der Orchideen, Abhandl. d. k. Acad. z. Amsterdam, Math. phys. Klasse
IX. 1861, and especially Leitgeb, Die Luftwurzeln der Orchideen, Denkschr. d. Wiener

EXTRAFASCICULAR SIEVE-TUBES.

23 T

Acad. Math, naturw. Classe, Bd. 24, p. 179 (1864). — Ueber kugelförmige Zellverdick-
ungen in der Wurzelhiille einiger Orchideen, Sitzgsbr. d. Wiener Acad. Bd. 49. — Ueber
Hartwegia comosa, &c., ibid. Bd. 49, p. 138. — Also Nicolai, das Wachsthum der Wurzel.
Sehr. d. Physik. Gesellsch. z. Königsberg, VII. (1865) p, 66. The remarkable white
‘parchment-like’ skin of the Orchids has been known since Link (Elem. philosoph. bot.
Ed. I. (1824) p. 395), and repeatedly investigated by Meyen (Phytotomie, p. 163 ; Physi-
ologie, p. 47) : Mohl, Unger (Anatomie u. Physiol, p. 194) in the Orchids, and by Schleiden
(Grundzüge, Ed. 3, p. 284) in these and the Aroids : but a clear view of the case was
not obtained, since according to Meyen and Schleiden the endodermis was taken for the
epidermis (its short cells were regarded by Schleiden as stomata). Schacht (Lehrb. I.
p. 258) and Oudemans regarded only the simple, outermost air-containing layer as the
epidermis, and the inner layers as an hypodermal ‘ intermediate ’ tissue. The statements
of Chatin, Anatomie des plantes äeriennes de I’ordre des Orchid^es, Mem. Soc. de Cher-
bourg, Vol. IV, 1856, and of Fockens, Ueber die Luftwurzeln, &c., Diss. Göttingen,
1857, have been corrected by Leitgeb and Oudemans in those points in which they
diifer from the above description.

Sect. 57. Sieve-iuhes occur outside the vascular bundles in a relatively large
number of stems of Dicotyledons, and of some Monocotyledons ; they form small
groups or bundles, which traverse the parts longitudinally, and anastomose at the
nodes not only with one another, but also with those of the vascular bundles. The
tubes are always accompanied by the same delicate, elongated cells, as in the vascular
bundles — these will be described when the latter are treated of — often also by
sclerenchymatous fibres and milk-tubes.

Many Dicotyledons have bundles of sieve-tubes at the periphery of the pith, near
to the ring of vascular bundles, many Melastomacese also have them scattered through
the pith. In many plants — Myrtacese, Daphne, Strychnos, Apocynaceae, and
Asclepiadacese, Convolvulaceae, often also in the families to be named below —
they approach so closely to the inner margin of the vascular bundles that they are
better to be regarded as parts of these, and in all cases the bundles of the pith are
related so closely and in such various ways to the system of vascular bundles that the
subject will be returned to when the latter is described : reference may therefore be
made to Sects. 62 and 103. Therefore we need only mention here the bundles of
sieve-tubes which separately traverse the periphery of the pith in the stems of species
of Solanum (S. tuberosum. Dulcamara), Nicotiana, Datura, and Cestrum ; of many
Campanulacese, as Campanula cervicaria, lamiifolia, glomerata, and pyramidalis, but
not C. Medium or rapunculoides ; further, those bundles, accompanied by milk-
tubes, which are found in the same position in the Cynaraceous plant, Gundelia
Tournefortii, and those which occur in many Cichoriacese of the genera Lactuca,
Scorzonera, Sonchus, Tragopogon, Hieracium, but not in Chondrilla, Taraxacum,
and Apargia. In Cichorium the bundles of sieve-tubes are absent in the stem,
but appear in the petiole near to the vascular bundles

In the cortical parenchyma outside the ring of vascular bundles sieve-tubes are
of constant occurrence in thick Cucurbitaceous stems ^ (Cucurbita, Lagenaria,
Cucumis, Eebalium). Here they lie close to the inner limit of the intra-cortical ring

' Hanstein, Die Milchsaftgefasse, pp. 57, 68, &c Trecul, Comptes Rendus, 27 Nov. 1865.

- Sanio, Botan. Zeitg. 1864, p. 227.

232

PRIMARY ARRANGEMENT OF TISSUES.

of sclerenchytna, either singly or 2-3 together, running longitudinally through the
internodes, and often anastomosing at the nodes with the sieve-tubes of the vascular
bundles. Trdcul describes small bundles of sieve-tubes, accompanied by laticiferous
tubes, distributed in the peripheral cortex of Gundelia Tournefortii. Also the bundles
described by Sanio, c., in the cortex of Plantago and Trientalis perhaps belong to
this category.

In many species of Potamogeton (P. natans, lucens, pectinatus) there is in many,
but not all of the bundles of sclerenchymatous fibres which traverse the cortical
parenchyma a small strand consisting of a few tubes, which is surrounded by the
sclerenchyma as by a sheath (comp. Fig. 171). With the above may perhaps be
grouped those bundles which can hardly contain sieve-tubes, found by Sanio * in
the cortex of Elodea. Near to the Epidermis there are in the internode 6 bundles
of some few (usually 5) thin-walled, elongated-prismatic cells, which alternate with
the 6 rows of leaves. They run perpendicularly through the internode, and each
gives off on each side at each node a horizontal branch, which anastomoses with
the rudiment of a vascular bundle on its course into a leaf.

2. Vascular bundles.

Sect. 58. From the very first those bundles which consist essentially of de-
finitely arranged groups of tracheae and sieve-tubes, and which traverse the body
of the plant as a continuous system, with blind endings only in the growing-points
and at the ends of peripheral branches, have been called Vascular bundles, Fasciculi
vasorum. Inasmuch as the vascular bundles are not unfrequently accompanied by
sclerenchymatous fibres, the term Fibro-vascular bundles has in recent times been
frequently applied to them

The general arrangement of the tracheae and sieve-tubes, which are united to
form the bundles, is defined partly by their arrangement in the single bundle, partly
by the arrangement or course of the bundles in the plant. The former, that is the
structure of the single bundle, may, as is shown by experience, change in different
parts of its course. A synoptical description of the structure of the individual
bundles must therefore presuppose a knowledge of their course, and the general
description must deal with this first.

A. ARRANGEMENT OE THE VASCULAR BUNDLES,
a. Course of bundles m the root.

Sect. 59. In the individul root a bundle which terminates at the growing-point,
and which grows with it, runs almost exactly along the longitudinal axis ; in Isoetes ®
it is strongly excentric, and nearer to that side of the root which is opposite the

Sanio, Botan. Zeitg. 1865, pp. 1S6, 191.

^ Nägeli, Beitr. I.

^ Compare Von Mohl, Linnsea, 1840; Verm. Schriften, p. 122, &c. — Hofmeister, Abhandl. d.
K. Sachs, Gesellsch. d. Wissensch. IV. p. 147.

COURSE OF THE BUNDLES.

333

furrow of the stem. In the thick roots of the Pandaneae and the genus of Palms,
Iriartea, there are found a number of parallel bundles, which converge at the growing-
point : as will be mentioned in Sect. 108, it may be doubted whether these should be
called parts of one very large divided bundle, or so many individual bundles. The
tuberous lateral roots of the Ophrydeae, of Dioscorea Batatas, and of Sedum Tele-
phium^, are on the other hand traversed by numerous separate bundles which
converge towards the apex, and are finally united into a short terminal portion.
In the undivided tubers of Ophrydeae they diverge from the point of insertion to the
broadest transverse zone, and from thence towards the apex they curve and converge,
and unite there into a single short apex, which ends blindly. During their course the
bundles, especially the peripheral ones, are united here and there by anastomosing
branches meeting the bundles at an acute angle. The lateral roots of the above
species of Sedum, which require further investigation, appear to behave in a similar
manner, but the anastomoses are absent, and the terminal point is more elongated.
The thick cylindrical adventitious roots of Dioscorea Batatas, the development of
which also requires further investigation, are traversed throughout their length by
very numerous bundles : these are irregularly distributed over the whole transverse
section, have a sinuous course, and are connected on all sides by anastomoses.

b. Course of butidles in the individual leafy stem

Sect. 60. The bundles, which traverse the stem, are separable according to
their course into two categories : firstly, such as always remain in the stem and
grow acropetally with it: they may either have no direct connection with the
bundles of the leaves, or the latter may be attached laterally to them : these are
the cauline bundles, which belong only to the stem : secondly, the bundles common
to stem and leaf, which run for a certain distance in the stem, and then enter a leaf,
and thus belong in one part of their course to the stem, in another part to the leaf.

A stem may contain only cauline, or only common bundles, or both.

The direction of the course of the bundles follows generally the longitudinal
axis of the stem : it is only in the nodes, and in some unimportant connecting
branches, that the direction is exactly transverse. During this for the most part
longitudinal course, their direction with relation to the plane of the surface, which
may be provisionally considered as flat, and to that of a radial longitudinal section
varies ; a bundle may run perpendicularly or obliquely relatively to both ; that is, it
may be radially-perpeiidicular and radially-oblique, iangenlially-perpendicular and tan-
gentially-oblique. By combination of these conditions, curved, A-shaped, and spiral
arrangements may reeult.

After traversing a certain distance, a bundle may connect itself with another
to form a single one. There are accordingly distinguished separate- or individual-
bundles, and united bundles.

^ Irmisch, Botan. Zeitg. 1855, p. 253. — Henry, Verhandl. Naturwiss. Vereins, f. Rheinl. und
Westf. i860.

“ Von Mohl, Palmarum stractura, Monachii, 1831. — Hanstein, in Pringsheim’s Jahrb. I. p. 233.
— Nageli, Zeitschr. f. Wiss. Bot. Heft 3 and 4, p. 129. — Beitr. z. Wissensch. Bot. I.

234

PRIMARY ARRANGEMENT OF TISSUES.

Common bundles rise for a certain distance in acropetal direction through the
stem, and then curve outwards at a node, to enter a leaf inserted at that point.
Their course in the stem is most clearly understood by following them from their
point of exit in a basipetal direction, that is, downwards. The description of their
course in this direction also corresponds best to the facts, inasmuch as at least
in most cases the development of the common bundles begins at the point of exit,
and proceeds on the one hand towards the leaf, and on the other downwards in
the stem.

From the point of exit in the node, the common bundle passes downwards
through a number of internodes, and then affixes itself to, and unites with, another
bundle, usually a common bundle, which makes its exit lower down. The junction
is effected in most cases at or near a node.

The common bundles accordingly represent within the stem the anatomically
demonstrable trace of the corresponding leaves : they are therefore called bundles
of the leaf-trace, and the whole number of those which belong to one leaf form the
trace, or internal trace of it

The number of the bundles of a leaf-trace is constant, within narrow limits
of variation, for each region of the stem of each species : but differs greatly accord-
ing to the region and the species, varying from one to sometimes very high figures :
the leaf-trace thus generally consists of one or many bundles.

A leaf-trace with many bundles may be distributed over a variable portion of
the transverse section, or circumference of the stem, or, as Nageli describes it,
it may be of variable width. The latter may amount e. g. to ^ of the

circumference of the stem. Traces with one bundle, or narrow ones with more
than one, become as a rule smaller, or narrower in a basipetal direction: wide
traces with more than one bundle usually increase in width in the same direction,
so that a lower one is enclosed by the one directly above it.

The number of the internodes, which a bundle of the trace, or a whole trace
traverses before it reaches the point of junction, is constant within narrow limits
of variation for each definite individual case, according to species and region :
further, it is not less various in different individual cases than the above-named
relations.

The individual bundle of a trace either remains undivided during its downward
course, or it may be split into two or more shanks. The bundles of a multiple
trace, as also of successive traces, may descend side by side, being thus concomitant ;
or they are separated from one another by other bundles, which pass between
them, and pectinate ^ with them.

From the above it is plain that where bundles of a leaf-trace are present,
a definite relation exists between the arrangement of the leaves at the periphery,
and that of the bundles of the leaf-traces within the stem. If all bundles of leaf-
traces were separate and concomitant, and had a perpendicular course, their arrange-
ment in the transverse section of an internode would exactly correspond to the
horizontal projection of the arrangement of those leaves, whose traces pass through

‘ Hanstein, 1. c.

^ [Compare foot-note, p. 128.]

COURSE OF BUNDLES IN THE STEM.

2.35

the internode h This may be the case; but in most instances the direct relation
between the two systems of arrangement is obscured and destroyed by oblique
courses, pectinations, splittings, and coalescences.

The special phenomena of the distribution of bundles in the stem, which vary
greatly, subject to the above general rules, may in some few cases be recognised
as direct consequences of adaptation. For the most part they appear as anatomical
characters (p. 25) of the groups of various rank distinguished in systematic botany,
but vary individually within the groups of higher order, as freely as does the external
conformation of individual species ; their differences are often closely correlated with
those of the latter, as might be expected a priori, but not uncommonly they show
unexpected deviations.

In accordance with these facts as at present before us, the following synopsis of
individual phenomena will best be arranged according to the main systematic divisions,
while within these the arrangement will follow' the phenomena of distribution of
bundles. How far more general rules for individual families and genera or for certain
categories of adaptation may be laid dow'n within the main types first mentioned, will
be in part obvious from the description of individual cases ; while in other cases
decision on this matter must be withheld till further investigations have been made,
since in many families, especially of the Phanerogams, the course of the bundles
has been investigated in single examples only, or even not at all.

I. Type of the Dicotyledons.

Sect. 61. By this name is indicated that course of the bundles which is charac-
teristic of the stem of the very great majority of Dicotyledons-, further, of those
ConifcrcB also which have been investigated, and of the Gnetacecs with the exception
of Welwitschia. Among the Monocotyledons many Dioscorese belong to this type,
and of Filicineae, the Equiseta and Osmundaceae ; these will however be treated
in the later sections dealing with these orders.

All the primary bundles of this type are common bundles of the leaf-trace.
They curve inwards into the stem at the node, and from thence they take a radially-
perpendicular course down it, all of them remaining at about the same distance from
the middle and from the surface of the stem. Leaf-traces with one bundle always
pass down through more than one internode : this is also usually the case with those
having several bundles. The insertion of the bundles on such as emerge from
the stem lower down occurs as a rule at the nodes, or close to a node, and in such
a way that they are connected in a unilateral-sympodial manner (Fig. 92), or in a
reticulate manner by means of shanks or diverging limbs, which are attached to the
neighbouring bundles on either side (e.g. Fig. 108).

From this course of the bundles results the characteristic, general, primary
structure of the typical stem of the plants of this category. The bundles are
arranged in the transverse section in a broken, ring-like series, the ring or circle
of vascular hundles. The remaining, chiefly parenchymatous tissue in which they
are imbedded, is separated into an axile, cylindrical or prismatic body, which fills

Compare Karsten, Veget. Org. cfi Palmen, Abliandl. d. Berlin. Acad. 1847, p. 20S.

236

PRIMARY ARRANGEMENT OF TISSUES.

the ring— this is the pith or Medulla ; a mantle, covered by the epidermis, and sur-
rounding the ring externally — this is the outer cortex ; and the bands which lie be-
tween the bundles, and pass radially, as seen in transverse section, from the cortex to
the pith — these are the primary connections with the pith, or primary medullary rays.
The form and number of the latter is defined in each individual case, in the first
instance, by the above-mentioned general rules for the number of bundles, and for
the course of the leaf-traces : their form depends upon the relative width of the
vascular bundles.

In the following paragraphs will be given the most important known individual
cases, following in the main the fundamental investigations of Nageli, while the
most prominent general rules will be more thoroughly exemplified by some few
instances.

A. Dicotyledons.

I. Hypoeotyledonary stem. In most species investigated two bundles of the trace
enter the hypoeotyledonary stem from each cotyledon : these usually unite at the base
of the cotyledon as its central nerve {e.g. Plantago, Urtica, Mercurialis, Antirrhinum,
Impatiens, Tropseolum, Vitis, Lupinus, Lathyrus, &c. ^). In Phaseolus the two bundles
are sometimes separated, sometimes united. In many plants the trace of the cotyledons
consists of but one bundle {e.g. Papaver orientale, Lepidium sativum, Spergula arvensis,
Silene^, Portulaca oleracea, &c.), though it is possible that here also this often arises
through the very early coalescence of two bundles. In Cucumis sativus and Melo 4, and
in Mirabilis Jalapa 5 bundles enter the cotyledons, in Ricinus communis 4 or 5. From
the cotyledonary node the bundles run vertically downwards, and unite at the base of
the hypoeotyledonary part. Where the trace of the cotyledons is a single bundle these
remain throughout separate and undivided. Where it consists of two bundles their
behaviour is not uniform. Either the two bundles of one trace approach one another,
and finally unite as a single bundle ; or the non-equivalent bundles of the two traces
unite, the right-hand bundle of the one with the left-hand bundle of the other. A
transverse section shows in the first case two bundles, the position of which corresponds
to that of the cotyledons (Lupinus luteus, Lathyrus Aphaca, Urtica Dodartii) ; in the
second case two bundles alternating with the cotyledons {e.g. Antirrhinum majus, Tro-
paeolum majus, Impatiens Balsamina, Vitis vinifera). The four-bundled traces of the
cotyledons in Cucumis are united at their margin, since the lateral bundles from
the respective cotyledons coalesce with one another — thus the transverse section of the
hypoeotyledonary stem shows 6 bundles, 4 separate and 2 united : the two latter
separate again at the base into two shanks, and unite with the first four. The 8, 9,
or 10 bundles, which enter the cotyledons, unite in Ricinus to 4, in Mirabilis to 2.

The bundles, which enter from the leaves next above, insert themselves in the cotyle-
donary node on those of the cotyledons, or run down into the hypoeotyledonary stem,
and finally unite with them there : the latter is the case e.g. in Lupinus and Phaseolus
(see below under 4).

II. Region of Foliage.

I . Leaves arranged spirally, leaf-trace a single bundle. ‘ The bundles from the leaves
descend through many internodes, and usually unite with those of definite lower leaves,
so that a transverse section shows the traces in a definite spiral series, which is not
identical with the spiral of the leaves, but is related to it.’

^ Nageli, t.e. p. 61. — Lestiboudois, Phyllotaxie anatomique. Aim. Sei. Nat. 3 ser. tom. X.
p. 19.

“ Rohrbach, Monogr. d. Gattung Silene, p. 22.

COURSE OF BUNDLES IN THE STEM.

237

Iberis amara (Figs. 92,93), foliage shoot. The arrangement of leaves in the terminal bud
is Each bundle descends through 10 or ii, rarely 12, internodes and inserts itself
there on that of the 5th lower leaf. Meanwhile it describes the form of an elongated S,
since it bends from the vertical, first to the ascending or anodic side of the leaf-spiial,
then to the descending or kathodic

side in a tangential direction. The 22

bundles thus run separately through
5, 6, or 7 internodes. By their coale-
scence there are formed 5 sympodial
bundles, which traverse the whole
stem : these complete a circuit in 65
internodes, while the separate bundles
appear as one-sided branches from
them. The oblique course of the leaf-
traces is contrary in direction to the
leaf-spiral; if the latter were right-
handed, the bundles would curve up-
wards towards the left. Oblique con-
necting bundles appear later between
the leaf-traces, originating in the
i4-i8th internode which has vascular
bundles.

T o this type belong further Arabis
albida, Jasminum fruticans, Sarotham-
nus scoparius (comp. Nageli, Hau-
stein, I.C.).

2. Lea'ves spirally arranged. Leaf-
trace of more than one bundle, pect mat-
ing at most with the fifth trace below.

Several (3 or 5) bundles go from
one leaf through the stem, and unite
sooner or later with one another.

These pectinate with the trace of
the fifth or a still lower leaf.

Lepidium sativum. The cotyledons
and the two succeeding almost oppo-
site primordial leaves have traces
with a single bundle. Of the later
leaves, which are all arranged spirally,
some few of the first have 3 bundles,
one strong median bundle, and two
weaker later ones, which unite im-
mediately on entering the stem. In
all the later leaves the median bundle
is divided into 3. At the point of
transition from stem to leaf 5 bundles
are seen, of which the median ones
are formed first, the two marginal
ones last. The 3 median bundles
are rather stronger, and unite above
to form the median bundle of the
leaf. Below, they separate from one
another, and the 2 weaker marginal

bundles unite with them, so that the leaf-trace now descends through the stem as three

Figs. 92, 93. — Iberis aiuata, after Nageli. Fig. 92. Scheme of the
course of the bundles in the young foliage shoot; the ring of bundles
spread out in a vertical plane. The figures indicate the successive
bundles of the leaf-trace at their point of exit from the ring into the leaf.
—Fig. 93 (15). Transverse section through the interuode above the point
of exit of bundle 5 ; meaning of figures as in Fig 92.

23«

PRIMARY ARRANGEMENT OF TISSUES.

bundles. Sometimes only one marginal bundle unites with the median, sometimes neither.
fj s T} T These 3, 4 or 5 bundles of a trace rarely run unaltered through
the stem : the appearance of a trace is varied by occasional
splittings and reunions, so that it may retain 3-5 bundles (with
a width of ^th to .^th of the circumference of the stem), or be re-
duced to 2 or I bundle. The leaf-trace may be followed through
6-8 internodes, but further down it is impossible to recognise
with certainty how many bundles belong to each leaf. Within the
first 5 internodes no crossing or uniting with lower leaf-traces has
been observed. To this type belong Impatiens Balsamina and
Scopolina atropoides (Nägeli).

3. Leaves spirally arranged. Leaf-trace of five bundles,
pectinating with the third or fifth. Cocculus lauri/olius (Nägeli).

4. Leaves spirally arranged. Leaf-trace of three bundles
which pectinate with the second and third.

Lupinus Lehmanni, Hort, and L. luteus, L. (Figs. 94, 95).
In the seedling a pair of opposite primordial leaves, or first
foliage leaves alternates with the two cotyledons. The second
pair of foliage leaves, of which one is inserted rather lower and
develops earlier than the other, alternate with the above, and
are thus opposite the cotyledons. The two leaves of the third
pair are not only at an unequal height, but also show a distinct
horizontal deviation from the opposite arrangement. The
fourth pair is intermediate between an opposite and spiral ar-
rangement ; all later leaves are arranged in a spiral.

Each cotyledon has a leaf-trace of two bundles [a, b), which
become united into one in the lower portion of the hypocoty-
ledonary stem. Sometimes there is found a third weaker bundle
between the two bundles of one cotyledon. All later leaves have
three bundles. Those of the primordial leaves, which will be called
III and IV (f, d, e, f, g, h), are concomitant with the coty-
ledonary traces, so that a transverse section in the upper portion
of the hypocotyledonary part shows 10 bundles, two being op-
posite two, and three opposite three. The median bundles (/, m)
of the second pair of leaves (leaves V, VI) pass downwards
through two internodes : on arriving at a median point above
the trace of the cotyledons they curve, the one to the left the
other to the right, and converging with the lateral bundles of
trace No. Ill, they immediately insert themselves upon them.
Later a second rather weaker shank {y, x) is formed on each :
this curves, above the trace of the cotyledon, to the opposite
side, and is inserted on the opposite lateral bundle of leaf No. IV.
The lateral bundles of the pair of leaves V and VI {/ b, on)
descend through one internode, and cross the lateral bundles from
III and IV at the next node; in the following internode they lie
at the inner side of these, and are inserted upon them at the
cotyledonary node, or rather lower.

The median bundles {p,q) of the third pair of leaves, VII
and VIII, pass through two internodes, and pectinate there with
the traces from III and IV ; ^ lies between the median and the
ascending, or anodic lateral bundle from IV, 9 between the median
and descending, or kathodic lateral bundle from III. They are
inserted on the above lateral bundles in the third internode. The
median bundle from leaf No. IX (r) applies itself to the ascending side of that of leaf
No. VI (m), that from leaf No. X (j) to the ascending side of that of leaf No. V (/).

rig. 94-

rig. 95-

FIGS. 94, 9$. —Lupinus Leh-
manni, after Nagfeli. Fig(. 94.
Scheme of the course of bundles
of a seedling^, the cylindrical sur-
face being exposed in one plane ;
seen from within. — Fig. 95 {10).
Transverse section tlirough the
stem above the cotyledons. The
letters the same in both figures.

COURSE OF BUNDLES IN THE STEM.

239

u and above a and l> are bundles entering from the axillary buds of the cotyledons,
which apply themselves to the lateral bundles from III and IV.

To this type belong also Erythrina crista gallt,
Prunus ansium, Ribes rubrum, Menispermum dauricum.

5. Leaves arranged spirally. Leaf-trace of 3 bun-
dles, pectinating with the ist and 2nd below.

Passiflora Vespertilio, Viola elatior, Trapxolum majus ,
Cucumis sativus.

6. Leaves spirally arranged. Leaf-trace of 7
bundles, all the bundles pectinating with those of the
next trace. Saururus cernuus.

7. Leaves arranged spirally. Leaf-trace of 8 bun-
dles, the lateral ones united. Liriodendron tulipi-
fera, L.

8. Leaves alternating in tvoo rows. Leaf-trace of
3 bundles, pectinating only with the leaf-traces of the
same side. Hertia crassifolia, Nag. l.c.

9. Leaves alternating in two rows. Leaf-trace of
3 bundles, which pectinate with the traces of both
rows : foliage shoots of Aristolochia Clematitis, A.
Gigas, SipLo. (Nägeli, /.c.).

In A. Clematitis (Figs. 96, 97) three bundles enter
the stem from the leaf. The median bundle divides
immediately into two, which traverse the first inter-
node side by side, and, uniting again with one another
at the next node, they pursue a common course
through the next internode. The two lateral ones
pass undivided through two internodes ; at the point
of exit into the base of the leaf they are united
with the two shanks of the mejlian bundle by an
anastomosis. Side by side with the two shanks of
the median bundle lie, on the right and left-hand,
bundles which pass out at the same node, and supply '
the axillary inflorescence : they are therefore axillary
bundles. The whole trace of the lateral structure of
a node thus consists in its own internode of 6, in the
next lower internode of 5 bundles. It encloses in
the former an arc of 215°, in
the latter of 205“.

The median bundle of the
trace {a, f, I, q, v) as it
reaches the four bundles of
the leaf-trace of the second
node curves to one side, and
unites with the lateral bundle
of the next lower trace. The
median bundles of a row of
leaves curve alternately to
both sides, e.g. those of the
traces i, 5, 9 to the right,
those of 3, 7, II to the left. At the point of curvature a second shank is formed later,
which affixes itself to the other lateral bundle.

The lateral bundles of the leaf-trace {bc,gh, mn, rs, xy) pursue an individual course
through their own internode, they then pectinate with the corresponding ones of the next

Fig. 96. Fig 97.

Figs. 96, 97. — Aristolochia Clematitis, after N geli. Fig. 96. Scheme of the course of
the bundles in the stem ; the cylindrical surface reduced to one plane, and seen from
within. — Fig 97 {20). Transverse section through an internode, at the height of the lower
end of Fig. 96 ; further explanation in the text.

240

PRIMARY ARRANGEMENT OF TISSUES.

lower node, they traverse the next internode in conjunction with the median bundle of
the next upper leaf, and each inserts itself in the second node on an axillary bundle of

the next lower trace. The two axillary
bundles, dc, ik, op, tu, take an inde-
pendent course in their own internode,
in the next they combine with the
lateral leaf-bundles of the next higher
trace, and insert themselves in the se-
cond node on the lateral leaf-bundles
of the next lower trace. These con-
ditions are very regular : the transverse
section through an internode therefore
shows regularly 1 1 bundles ; Fig. 97 shows
the arrangement of these, and explains
them by means of the same lettering as
at the lower end of Fig. 96.

10. Leaves alternating in two rows.
The rows are nearer one side in the
terminal bud. Leaf-trace of 3 bundles,
which pectinate with the traces of both
rows.

Medicago sativa, Lathyrus Nhsolia,
L. Aphaca and Pseudaphaca, L. Odoratus,
L. purpureus.

L. Aphaca and Pseudaphaca (Figs.
98, 99). The leafy stems are four-cor-
nered, the corners are sometimes slightly
extended as wings. The transverse sec-
tion (Fig. 99) shows in each of the two
opposite lateral edges a bundle {h, g) and
internally a circle of 8 and more bun-
dles, which later decrease in number by
coalescence. The median bundle of the
leaf-trace divides as it emerges into the
leaf into 3 branches, of which the central
weaker branch enters the petiole, while
the lateral ones form bow-like anasto-
moses with the two lateral bundles : from
these arise the bundles for the stipules.

The median bundle («)/. A 0, r, u)
runs separately down through two in-
ternodes, then curves to one side {u),
and later it forks into two shanks. These
insert themselves on the lateral bundles
of the next lower leaf-trace. The two
lateral bundles {be, gh, mn, pq, st) run
first in the corners through their own
internode : at the next node they enter
the circle of bundles of the trace, where
they pectinate with the lateral bundle of
the next lower trace : each then unites
with one shank of the median bundle of the next higher trace, and running together
with it through two internodes, inserts itself finally in the third internode below on the
lateral bundle of the second lower trace.

Figs. 98, 99. — Lathyrus Pseudaphaca, after Nageli. Fig. 98. Scheme
of the course of the bundles in the apex of the stem supposed to be
transparent ; the bundles on the side further from the observer are
paler, those on the nearside are black.— Fig, 99 (25). Transverse sec*
tion through an internode such as the lowest but one of Fig. 98 ; with
the same lettering as the latter.

COURSE OF THE BUNDLES IN THE STEM.

2,41

If a peduncle is seated in the axil of the leaf it receives two bundles from the stem
(de, ik), which usually run separately through one internode, and affix themselves in the
next node to the lateral bundles of the next higher trace. In this case the transverse
section (Fig. 99) shows eight bundles, or, when the axillary bundles are absent, six bundles
arranged in a circle, and with two more in the corners. Variations from this type are
brought about by the bundles uniting higher or lower, and by variations of the pectina-
tions. The width of the leaf-trace in the first two internodes is 190° to 210°.

II. Leaves alternate, in two rows. Leaf-trace usually of five bundles, the lateral

Fig. 100.

Figs, ioo, ioi.— Fceniculum officinale. Figf. 100. Scheme of the vascular system for leaves with seven bundles,
the cylindrical surface being reduced to a single plane; at the level of the figures i, 2, 3 are the nodes, m always
indicates the median bundle passing out at the node; / the marginal bundles.— Fig. loi {40). Transverse section
through an internode with arrangement and number of bundles corresponding to that of Fig. 100. i. The bundles
of the trace from the next higher leaf, its median bundle ; 1+2 the bundle formed from the marginal bundles of i
and the median bundle of the leaf next but one above. The bundles alternating with those numbered are the united
traces of the two leaves above i ; between the bundles is indicated the cambium zone which connects them. The
small circles outside the stronger bundles are the transverse sections of oil passages; in each of the blunt angles of
the stem is the transverse section of a bundle of fibres having the shape of a .segment of a circle.

bundles of two successive leaves are not completely pectinated. Vitis ’vinifera, Ampe-
lopsis hederacea.

1 2. Leaves alternate, in two rows. Leaf-trace usually of five bundles, the lateral bundles
of two successive leaves completely pectinated.

Phaseolus i’u/garh, Ph. Nägeli, /. r. ; Dodel. in Jahrb. f. wiss. Bot. Bd. VIII.

R

242

PRIMARY ARRANGEMENT OF TISSUES.

13. Leaves alternate, in two rows. Rows approaching one another on one side. Leaf-
trace of 7-9 bundles. All bundles of two successive leaves pectinated. Platanus occi-
dentalis.

14. Leaves alternate, in two or more rows. Leaf-trace of numerous bundles. All
bundles of two successive leaves pectinated. To this type belongs Menyanthes trifoliata,
with leaves in two rows, and a trace consisting of 10-13 bundles, according to Nägeli :
also many Umbelliferae.

According to investigations ^ on ^thusa Cynapium, Phellandrium aquaticum, Hydro-
cotyle vulgaris, and Foeniculum officinale, the Umbelliferae of the usual form have in the
non-flowering shoots a bundle system, which in accordance with their similarity in ex-
ternal conformation is similar in its main features, though it shows individual variations.
The following scheme may be constructed for it (comp. Figs. 100, 10 1). The leaves are
in two alternating rows, or spirally arranged : their base completely encircles the stem,
one margin even overlapping the other ; each leaf-trace has several bundles, its width
being equal to the whole circumference of the stem (1-) ; the leaf-trace, pectinating with
those of the next higher and next lower leaves, and passing downwards through two inter-
nodes, inserts itself in the third node on the trace which comes down from the second
node, and takes up in the second node the trace which emerges next above it. Each bundle
of the trace runs directly downwards from the node (3), in which it emerges, through its
internode ; it takes up in the next node (2) a bundle of the trace which comes from above
(3), and which affixes itself upon it; it passes in (2) between two bundles, which here
enter the leaf, and runs between these down to the node (i). Here turning either right
or left it affixes itself to one, or branching, to two neighbouring bundles, which descend
from node (2). Whether the junction be to the right or left, or branched and on both
sides, seems to be often subject to variation. Not unfrequently the branching arises later
by the subsequent appearance of a second shank on a bundle originally attached on one
side. If the bundles of successive traces are of equal number, and their number = n, the
transverse section of an internode shows 2« bundles of the trace, and of these n are
* stronger and go to the next leaf, n are weaker, alternate with the former, and enter the
second leaf above.

In the three-bundled traces of the creeping foliage-shoots of Hydrocotyle vulgaris
no variation from this type was observed. In other cases, with a larger number of
bundles of the trace, variations are frequent. They depend sometimes on inequalities in
the number of bundles in the successive traces, since as the strength of a shoot increases
the number of bundles of the successive traces enlarges, and then two or three bundles may
pass between two bundles of the next lower trace : sometimes on the fact that the width
of a trace (and leaf-insertion) is less than f of the circumference of the stem, in which
case certain bundles pass down through more than two internodes : sometimes there occur
variations independently of these, in the same species, and possibly even in one and the
same shoot, while many species have constant specific peculiarities. Thus the young
plant of Foeniculum officinale (Fig. 100) has alternating leaves in two rows, their median
planes diverging by 180° : the width of the leaf-insertion is j. or > | of the circumference
of the stem, that of the leaf-trace |. The number/» of the bundles entering one leaf is,
as in other investigated species, an uneven number: 5, 7, to 21 and more. Of the p
bundles of one leaf the two marginal ones, /. 1. Fig. 100, converge as they enter the
node (i), and unite there at once with the median bundle (m) of the next higher leaf,
which descends perpendicularly between them. This united bundle then descends
further perpendicularly through the next internode, and forks in the next node (2), close
above the median bundle, which passes out at that point, and each of its two shanks
unites with the bundle, which descends side by side with it. The course of all other
bundles corresponds to the scheme. The number n of the bundles of one leaf-trace
in an internode is thus in Foeniculum = p—i, while in cases which follow the scheme

^ Carried out in 1873, in the Botanical Institute at Strassburg, by Herr von Kamienski.

COURSE OF THE BUNDLES IN THE STEM.

243

exactly it is = />. The transverse section of the internode of Hydrocotyle shows, e.g. six
alternating unequal bundles, if three enter each leaf : that of Foeniculum shows, when the
number from each trace is equal, twelve (Fig. lor) if seven enter each leaf, and sixteen
if nine bundles enter each leaf. As the result of inequality
in number of successive traces in the growing plant, there
occur also in Foeniculum deviations from the above special
scheme. Those modifications of the course of the bundles,
which certainly occur in the flowering shoots of the Umbelli-
ferae in question, and in many with deviating conformation,
have not been investigated.

15. Leaves opposite, the pairs decussating more or less
exactly. The bundles of one pair pass perpendicularly
down through two internodes, and then curve, in the
second lower node, sometimes converging symmetrically,
sometimes both turning in the same direction, and then
descend further, and unite with those belonging to lower
leaves. This condition is plainly seen in the youngest
stages only, later a second shank is usnally formed at the
point of curvature, so that the bundle becomes branched,
and sits astride of that directly below it (Fig. 102).

Further in many allied cases, the lower ending of the
bundles becomes quite indistinct, by their coalescing
laterally by means of intermediate bundles which appear
very early (Chap. XIV).

To this category belong, according to Nägeli and
Rohrbach (/. c.), Fraxinus excelsior, Vinca minor, Apocynum
hypericifolium, species of Phlox, Veronica incisa, Calluna vul-
garis, Hypericum quadrangulum, Androssemum, Euonymus
europseus, species of Alsine, Spergula, Cerastium, Dianthus
and Silene, also Galium and Rubia. Figs. 102 and 103 may
illustrate the arrangement for the special case of Cerastium.

Fig. 102 is the scheme for the course of the bundles of a
shoot with the cylindrical surface reduced to one plane ;
ah, cd, ef, gh, are the bundles of foliage leaves, the letters
standing at their point of exit from the ring. Below
the node marked dc these bundles of the trace are alone
present. Above dc there are also others, viz. p, 0, n, the
bundles of the terminal flower-stalk, and hi, Im, of which
pairs one enters each of the branches in the axils of the
leaves g and k (comp. Sect. 94). All these bundles have a
place in the ring as shown in Fig. 103, which represents a
transverse section through the internode above ef.

16. Leaves in whorls: traces consisting of one bundle,
which runs down more than two internodes. Trevirania
longifolia, Russelia juncea.

17. Leaves opposite: traces of three or four bundles,
which unite at the second lower node with those of the
next lower pair: not pectinated. Antirrhinum majus, Ruellia
maculata, Bignonia serratifoUa, Tecoma radicans.

18. Leaves opposite and decussate. Traces of 2 bun-
dles, not pectinated. AnagalUs arvensis, Stachys angusti-

folia, Satureja variegata Hort. (Nägeli, 1. c.), and many other Labiatae, Nepeta Cataria,
Melissa officinalis, &c. Two bundles, which are united in the petiole to form one,
separate from one another at once in the stem of the Labiatae (Figs. 104 and 105),

R 2

Fig. 102.

Fig. 103.

Figs. 102 and J03.— Cerastium frigi-
dum, after N'ageli. Fig 102. Scheme of
bundle-arrangement ; explanation m the
text. — Fig. 103 (20). Transverse section
through a shoot in the internode above
e,f oi Fig. 102. The letters indicate the
same bundles in both figures; e, f are
already branched in Fig. 103, in the
sheath-like connate bases of their pair
of leaves.

344

PRIMARY ARRANGEMENT OF TISSUES.

V'

and pass down the corners, between which the leaf is placed, through two inter-
nodes. At the second lower node they unite with those of the next lower trace, after
having traversed one internode close side by side with these. The transverse section
below the apex of the stem thus always shows eight bundles, in groups of two beneath the
corners ; those of each pair are of unequal size, the stronger belonging to the leaf-trace
of the nearer pair of leaves, the weaker to that of the next pair. The bundles of one

corner soon unite, as vessels appear between them.
The transverse section then shows four bundles, which
later unite to form a closed ring (Chap. XIV).

19. Leaves opposite, traces of 3 bundles: the lateral
bundles pectinating with those of the next pair.

Clematis Vitalba, Viticella ; Atragene, Urtica Dodartii,
Lonicera spec., Acer pseudoplatanus, Philadelphus coro-
narius, Tagetes lucida, T. signata Bartl., Humulus Lupu-
lus, Centranthus ruber, /Esculus macrostachya, Euphorbia
Lathyris.

The foliage shoots of the above plants, though
corresponding in the above points, differ in the unequal
length of the course of the traces. The median bundles
sometimes insert themselves at the next lower node,
sometimes at the next but one, sometimes still lower.
The lateral bundles also pass through one, two, or
several sections of the stem. Giving the reference to
Nageli’s work, we will here describe only the very
simple examples of Clematis and Atragene (Figs. 106,
107).

The pairs of leaves decussate at right angles. The
six corners of the internodes, of which two opposite
ones corresponding to the median points of the leaves
are rather more prominent, alternate regularly. The
width of the three-bundled leaf-trace is about 1 1 5°.

The median bundles {ad, gk, qn, xt) pass through
one internode, divide at the next node into two shanks,
and insert themselves with these on the lateral bundles
of the pair of leaves at that point. At first there is
always but one shank present, and the two median
bundles of the same pair have (according to two
observations) a symmetrically converging curvature.
The formation of the second shank often appears at a
late stage in Cl. Viticella, or is entirely absent.

The two lateral bundles of the leaf (äc, ef, hi,
Im, &c.) also pass through one internode, they curve
in a converging manner at the next node, and insert
themselves on those same lateral bundles, with which
the shanks of the median bundle unite. In Cl. Viticella
the leaf-trace in this condition is usually complete : in
Cl. Vitalba a second shank is formed at the point of
curvature of the lateral bundles also : this curves to
the opposite side, and coalesces with a median bundle of the node. The trans-
verse section of the young internode shows six bundles of the leaf-trace (Fig. 107,
p. 246).

The axillary branches have also six bundles in their lowest internode, which unite to
two on entering the stem. These two unite at once right and left with the median
bundle of the leaf which bears them.

1

e

( \

1

1 1

a'

Fig. 104.

Fig. 105.

Figs. 104 and 105.— Stachys angustifolia,
after Nägeli. Fig. 104. Scheme of the vascu*
lar system in the end of the shoot, the cylin-
drical surface being exposed in one plane.
ab, dc,/e, gh, ik, the traces of successive
pairs of leaves, the letters being placed at the
nodes ; from the highest pair only one bundle
of each trace is as yet visible, ik — Fig. 105
(35). Transverse section through a young in-
ternode, corresponding to that above a ^ in
the above figure; the same bundles are indi-
cated by the same letters as in the above.

COURSE OF THE BUNDLES IN THE STEM. 345

20. Leaves opposite ; traces consisting of three bundles, the lateral bundles of the same
pair united from the first. Mercurialis annua and M. perennh.

21. Leaves opposite: traces consisting of five bundles, the two lateral bundles of the
same pair united from the first. Sambucus nigra.

B. Gymnosperms ’.

As has been above mentioned, the vas-
cular system in the stems of the Coniferse
does not differ from that of the Dicoty-
ledons : they will therefore be mentioned
here only as special instances of the
type of the Dicotyledons.

The seedling of most of them has two
opposite cotyledons which grow green on
germination, and rise above the ground ;
rarely (Ginkgo, Araucaria, section Colum-
bea) they remain in the ground. More
than two occur exceptionally in many
genera, and constantly in Taxodium (4
to 9) and in the Abietineae, in the sense of
Strasburger, i. e. Linnaeus’ genus Pinus.
The number of the cotyledons differs here
according to the species, and varies in the
same species within wide limits : e. g. in
Abies pectinata between 4 and 7, in Pinus
sylvestris between 3 and 8, in Pinus Pinea
between 8 and 14. Discounting single
exceptions to be named below, one bundle
enters the short hypocotyledonary section
from each cotyledon ; where there are
two cotyledons both bundles run vertically
downwards, and soon undergo coalescence
to form the root-bundle : where the num-
ber is higher, two or three bundles coalesce
immediately after entering the hypocoty-
ledonary section to form one, so that the
number of the bundles of the trace in the
latter is smaller than that of the coty-
ledons. Statements by Lestiboudois {l.c.,
pp. 25 and 26) lead to the assumption that
in Cupressus pyramidalis and Abies bal-
samea the bundle, which passes from the
cotyledon into the stem as a simple
bundle, splits at the node into two shanks,
and that the opposite shanks of two
neighbouring bundles unite into one,
which (alternating with each pair of coty-
ledons) descends perpendicularly. The
cotyledons of Araucaria brasiliensis ^ have

Fig, 106 (40). — Clematis Viticella, after Nageli. End of abranch
made transparent by removing" the surface and treatment with
potash, showing the course of the leaf-traces. The outgoing ends
of the bundles are somewhat distorted by slight pressure ; the
two highest pair of leaves a ß and y 5 have as yet no developed
bundles.

each 8 vascular bundles, and these each unite in the cotyledonary node to two, so

' INageli, l.c. Lestiboudois, l.c. -A. B. Frank, Botan. Zeitg. 1864, p, 150. — Geyler, Gefass-
biindelverlauf in d. Laubblattregion d. Coniferen, Pringsheim’s Jahrb. VI. — Strasburger, Die Coni-
feren und Gnelaceen. 2 strasburger, 1. c. p. 369.

246

PRIMARY ARRANGEMENT OF TISSUES.

Fig. 107 (20). — Clematis Viticella.
T ransverse section through a young
internode; further explanation in
the text.

A

that from the two cotyledons four bundles of the trace pass down into the hypocoty-
ledonary section.

In all investigated cases the bundles of the trace of the first epicotyledonary leaves
insert themselves on the cotyledonary bundles at or close below the cotyledonary node.
With the single exception of Ginkgo, the trace of foliage and scale leaves passing
through the stem is in the Coniferae a single bundle ; and this is
still the case even where the leaves have several bundles, and where
these arise, as in Dammara and the broad-leaved Araucarias, by
splitting of the single bundle of the trace while still in the node.

In the investigated species of Juniperus, Frenela, Cupressus,
Callitris, Libocedrus, Thuja gigantea Nutt, Chamsecyparis eri-
coides Hort., the leaves are arranged in two- or many-membered
alternating whorls. Their traces consisting of a single bundle
descend undivided through one internode, and fork about the
middle of the second internode into two shanks, which insert
themselves right and left upon the bundles of the trace of this
internode (Fig. 108).

Thuja occidentalis, Th. plicata, and Biota orientalis have on
the other hand, it is true, the same whorls of two alternating
leaves as their nearest allies above named; but the opposite
traces of each pair of leaves pass perpendicularly downwards
through two internodes without dividing ; they then curve over
the leaf-trace, which emerges at the 2nd lower node, both turn-
ing in the same direction (more rarely converging symmetrically),
and affix themselves laterally on the bundles which emerge at the
2nd, 3rd, or rarely the 4th lower node (Fig. 109).

In the numerous Coniferae with spirally arranged leaves,
Chamaecyparis glauca Hort., Wid'dringtonia juniperina, the in-
vestigated species of Taxodium, Glyptostrobus, Cryptomeria,
Sequoia, Cunninghamia, Pinus in the sense of Linnaeus, Podo-
carpus, Saxegothea, Taxus, and Araucaria, the one-bundled leaf-
traces have a course corresponding in its main points to the
scheme described for Iberis (Fig. no). Each bundle descends
independently through a definite number of internodes, and then
curving towards a definite lower bundle, affixes itself laterally on
it, and coalesces with it further down. The number of the
definite lower bundle, on which it is affixed, varies according to
the individual case, but is constant for each single case, and
follows the series 2, 3, 5, 8, 13, 21, ... . The direction in which
the apposition on the coalescing bundle takes place, is also con-
stant for each individual case, and is defined by the number of
the coalescing bundle, so that the apposition on the 3rd, 8th,
or 2 ist lower bundle is in a descending direction, that on the 5th,
13th, or 34th in an ascending direction with reference to the
leaf-spiral (Geyler, /. c.). The same rule above described is
followed as regards the course of the bundles of the trace in
the leaves of Cephalotaxus Fortune!, Torreya grandis, and, as
far as at present known, of Dammara australis: these leaves
appear at first in whorls, but are subsequently displaced spirally. Ginkgo also belongs to
this category, since the two bundles of the trace, after running separately through 1-3
internodes, unite to one, which curves in a kathodic direction above the 5th lower
trace, and inserts itself in about the 8th lower internode in an anodic direction on the
5th lower trace, with which it pursues a united course in the 9-1 1 internode.

Among the Gnetacex, Ephedra vulgaris has in each of the two cotyledons two vascular

li

Fig. 108. — Juniperus nana, after
Geyler. A scheme of the longitu-
dinal course of the bundles, the
cylindrical surface being reduced
to one plane ; the whorls of three
members are slightly displaced spi-
rally ; = bundles of the buds.

B (16) transverse section through a
young shoot ; i, 2, 3, the bundles,
which pass out in the order of the
figures to a whorl; k the bundles
which pass into the axillary buds.

COURSE OF THE BUNDLES IN THE STEM.

247

bundles, these enter the hypocotyledonary stem, which thus contains four bundles of the
cotyledonary trace. The epicotyledonary section contains eight bundles, four opposite
each cotyledon. In the cotyledonary node these eight bundles unite in pairs, each of which
passes down between two cotyledonary bundles, and splits within the hypocotyledonary
section into two shanks, which insert themselves on the next cotyledonary bundles.
Below this point of union the four cotyledonary bundles unite to form the bundle of the
root.

Each of the other leaves of Ephedra, which, as is well known, are scale-like, and are
arranged in accurately alternating whorls of two, also contains two bundles. In E. vulgaris
the two-bundled leaf-trace enters the ring of bundles at its own node (i) ; takes a perpen-

A

Fig. 109,— Thuja plicata, after
Geyler. scheme of the bundle
system, the cylindrical surface being
reduced to one plane. B (16) trans*
verse section through ayoung shoot,
1, 1 the bundles, which pass directly
into a pair of leaves ; outside each,
below the surface, is a resin pas-
sage, k.

Fig. no. — Pinus sylvestris, after Geyler. Scheme of the
vascular system in the young shoot, tlie cylindrical surface
being reduced to a single plane; leaves arranged 8/21, in a
right-handed spiral. The figures indicate the successive bun-
dles of the leaf-trace, which are represented as broad bands.
Each pair of converging bundles (represented as thin lines),
near the emerging bundles 0—9, goes to an axillary shoot.
The traces unite in descending order, each with the eighth
lower one.

dicular and parallel course downwards through two internodes, and inserts itself in the third
node laterally on the trace which emerges at node 2, each bundle joining with that laterally
next it. In the node there appears at an early stage a transverse girdle of tracheides,
which unites the bundles. Strasburger states for Ephedra campylopoda, that between
the two bundles of the trace of each leaf there runs a ‘ complementary bundle ’ which
arises from the girdle of tracheides : this passes through one internode, from the node of
the pair of leaves, to the next transverse girdle. In Ephedra altissima, according to the
same author, the two bundles of the trace of a leaf take a separate course only in their
own internode, and in the next are united to a single bundle. E. vulgaris has therefore
eight bundles of the trace in the internode, of these two opposite pairs belong to the same
pair of leaves : E, campylopoda has ten, E. altissima only six.

The species of Gnetum have on their foliage shoots decussating pairs of leaves, separated
from one another by elongated internodes. Each leaf contains four or five bundles, accord-
ing to the species*. According to some few investigations on Gn. Thoa the leaf-traces
follow the above-described scheme for the Umbelliferae. The ten-bundled trace of each
pair of leaves passes down through two internodes, and unites in the second node with the

' Strasburger, l.c. p. 115.

248

PRIMARY ARRANGEMENT OF TISSUES.

trace of the next lower pair. The bundles of the trace of successive pairs all pectinate
with one another. Besides this, other coalescences must occur at or below the node,
since in the internodes investigated the transverse section showed only eighteen bundles
instead of twenty as assumed according to the scheme, and as actually occurs in Gn.
Gnemon.

On Welwitschia comp. Chap. XVI.

To avoid repetition, the Cycadece will also be described in the chapter cited.

II. Anomalous Dicotyledons.

Sect. 62. A not inconsiderable number of Dicotyledons, some Cycadeae, and
Welwitschia differ in their bundle-system from that which characterises their allies,
in that the primary bundles are not arranged in a simple ring. Either they contain
a ring of bundles arranged according to the usual type, while additional bundles are
found either within these, that is in the pith, or outside them, that is in the outer
cortex ; or the bundles are arranged in several, often not sharply distinguished circles,
or so arranged that they appear in transverse section irregularly scattered between
other tissues, with the exception of the most peripheral bundles, which may be
distinguished as a ring well marked off from the outer cortex.

These more or less remarkable exceptions to the main type either occur in
quite isolated species in typically formed genera and families (e.g. Umbelliferse), or
in numerous species of otherwise typically formed genera (e.g. Begonia), or they are
characteristic of certain genera, or small families (e.g. Nymphseacese, Calycanthaceae,
Podophyllum, Diphylleja), more rarely even of large families, as Piperacese and
Melastomaceae. But even in the latter, exceptions occur to that grouping of the
bundles which holds for the majority of their allies.

The above phenomena are based either upon an oblique radial course of the
bundles of the leaf-trace, or upon the appearance of cauline bundles besides the bundles
of the trace which are arranged in the typical ring. Discounting the Nyctagineae,
which will be described below (Chap. XVI), many Amarantacese, &c. with medul-
lary bundles, the following cases belong to this category.

a. Medullary bundles.

r. All bundles belong to the leaf trace : some on entering the stem are arranged
in the typical ring, and have a radially perpendicular course in it: others, passing
further inwards, are therefore medullary, and are either scattered through the pith or
arranged in rings. To this class belong most Cucurbitacece, species of Amarantus
and Euxolus, Phytolacca diolca, the Piperacece, doubtless also the herbaceous Ber-
beridtce. Podophyllum, Diphylleja, Leontice ; further species of Papaver Thalictrum,
and Actcea.

The bundles of the climbing Cucurbitacese, ^ (Cucumis, Cucurbita, Bryonia, Tladiantha,
Cyclanthera pedata) are arranged in two rings (Ecbalium elaterium, which has no tendrils,
has only one circle of bundles) : those of the outer ring are opposite the corners of the
stem, and are of equal number with these, e. g. five in Cucumis sativus, Cucurbita, Tladi-
antha dubia, Cyclanthera pedata, seven in Bryonia dioica : those of the inner ring alternate

^ Bernhardi, Beobacht, über Pflanzengefässe, p. 20. — Sanio, Botan. Zeitg. 1864, p. 227. —
Nageli, l.c. p. 77.

COURSE OF THE BUNDLES IN THE STEM.

249

with those of the outer, so that their outer portion falls between the latter, but their
number is not always the same as that of the outer series, one being left out (e. g. | in
the specimens at hand of Tladiantha).

The bundles of both rings are, as far as investigated, bundles of the trace, which pass
downwards on the average through two internodes: it remains for further investigations
to ascertain their course exactly, a matter which is made very difficult by the early ap-
pearance of irregular transverse anastomosing bundles in the nodes.

Of those Amarantacex, which have been investigated, some, namely species of Celosia,
Gomphrena, Alternanthera, Froelichia, and Achyranthes, have the primary ring of
bundles, and pith cylinder (the latter without bundles), which are typical of the Dicoty-
ledons. In Amarantus^ caudatus, A. retroflexus, and Euxolus emarginatus A. Br. the
numerous (e. g. n) bundles, which are arranged in the base of the leaf in a curved series
concave upwards, separate from one another in the node, taking however a steep down-
ward course : some form a ring with certain bundles, which descend from above, others
penetrate deeper into the pith. Thus there are formed within the ring of bundles
several irregular medullary rings, in which the bundles belonging to individual leaves
remain grouped together. The middle of the pith has no bundles. The median bundles
of each leaf-trace appear to penetrate deepest into the pith. Lower down the bundles
of one trace unite, after traversing several internodes separately. An accurate investi-
gation of their course remains still to be made. Slender specimens of Euxolus lividus
Moq. show similar, but simpler conditions.

Phytolacca dioica has (according to Nägeli, l.c. p. ri8) three bundles of the trace for
each leaf. The two lateral ones descend in a radially perpendicular direction in the stem
between the pith and outer cortex ; they split first into two, then into several shanks, and
these together form the ring of bundles. The median bundle enters the pith, but hardly
deeper than ^ the radius of the pith, it there descends through 8-12 internodes, and then
again unites with the ring. It describes a curve convex inwards, the strongest curvature
of which is in its upper part : it reaches nearest to the middle of the pith in the 3rd and
4th internodes. The transverse section through a mature internode thus shows 8-12
bundles, which are free within the ring.

The course of the bundles, which in the aerial stem of Podophyllum, Diphylleja, and
Leontice '■* are distributed in the transverse section throughout the pith, almost as in the
hlonocotyledons, and of those suYnch, m Papaver orientale (often also in P. somniferum),
in Actsca racemosa, Cimicifuga fostida, and species of Thalictrum, form an irregular 2- or
3 -seriate zone round the pith, requires further investigation : it can hardly be doubted
that they belong to the leaf-trace.

Further investigation must show whether the medullary bundles, which occur in Statice
or the Plumbaginese.^, belong to this series: I found no such bundles in the species of
Statice, Armeria, and Plumbago which I examined.

Since P. Moldenhawer and E. Meyer it has been known that in the internodes of all
Piperacex which have been investigated, with the exception of Verhuellia, there are
medullary bundles, usually arranged in a circle, within a ring of bundles, which in the
woody species (Pipereae) subsequently undergo secondary thickening. Rarely more than
one inner circle is present, e. g. in Peperomia variegata sometimes 2, in P. incana and
obtusifolia 3-4, in Piper geniculatum 2, in Artanthe cordifolia 4L The number of the

' Link, Grundlehren der Anatom, und Physiol, der Pflanzen, pp. 144, 148. — Unger, Dicotyle-
donenstamm, p. 108.

“ C. H. Schultz, Vaisseaux du latex, &c., Mem. pres. Acad. d. Sciences, VII (1841). — Sanio,
l. c. p. 230.

^ Russow, Vergl. Unters, p. 15,3. — Schwendener, Mech. Princip. p. 143.

* P. Moldenhawer, Beitr. p. 5. — E. Meyer, De Houttaynia et Saurureis, p. 39 Unger, Bau,

&c. des Dicotyledonenstammes, p. 68, &c. — Karsten, Veget. Org. d. Palmen, Lc. p. 148. — C. de
Candolle, Memoire sur la famille des Piperacees, Mem. Soc. phys. de Geneve, Taf. XVIII. p. 2.—

250

PRIMARY ARRANGEMENT OF TISSUES.

bundles, both of the inner and outer circle, varies, at least in many species, in the suc-
cessive internodes of the same shoot. They run perpendicularly down the individual
internode. In most species transverse anastomosis in the node makes it difficult to
follow their further course. Most recent authors have arrived at the result that the
bundles are partly common, partly, and especially the inner ones, eauline : the in-
florescence certainly contains eauline bundles. Karsten alone, in the year 1847, ex-
pressed another view for the Piperaceae, according to which all the bundles are bundles of
the leaf-trace : this view is confirmed and generalised by the more thorough investiga-
tions of Weiss. The latter can only be epitomised here, reference being given to the
original work, since the latter only appeared after this book had begun to pass through
the press.

Peperomia galioides shows the course most clearly. The leaves are arranged in
whorls of five, each has a single bundle. The bundles enter the node in the outer circle,
and pass down in it through one internode: they then curve inwards, and, forming the
inner circle, pass down the second internode : in the node below the latter they insert
themselves on the bundles of the next higher whorl, which here curve into the pith.
The transverse section through the internode shows two concentric series of five bundles
each.

P. brachyphylla has decussating whorls of two leaves each. Each leaf contains three
bundles, one median and two lateral. ‘ The median bundles run in the peripheral circle
through two internodes, then turn inwards, and, after a further course in the pith through
one internode, insert themselves with their tapering ends on a medullary bundle. All
lateral bundles of the trace run through one internode in the peripheral circle, then
curving in the next lower internode into the pith, they run further in the pith through
one internode, and insert themselves, also with tapering ends, on the medullary bundles of
the third internode.’ In each node accordingly six bundles päss inwards; in the pith of
the internode however there are only four: certain bundles must therefore unite on enter-
ing the pith. Weiss found the course similar, but more complicated and less regular, in
P. rubella, and in P. variegata and incana, which have alternating leaves : of these the
former has a leaf-trace of twelve bundles, the latter of seven.

Weiss, in common with Karsten, found in the woody Piperaceae (Piper, Artanthe,
species of Chavica) that the bundles of the many-bundled leaf-traee, which embraces the
stem, descend, where there are one or two medullary circles, through at least one internode
in the peripheral circle, then curving into the pith, they traverse a second internode in the
medullary circle, and finally insert themselves on medullary bundles of a lower internode.
On passing from the outer to the inner cirele two or three bundles may unite, and the
number of the medullary bundles may thus vary with little regularity in the successive
internodes. Where there are more than two circles (Artanthe cordifolia) the medullary
bundles traverse at least two internodes.

It is instructive that the course of the vascular bundles in the Piperaceas closely re-
sembles that of the Commelineae (§ 69).

2. All bundles bcdotig io the leaf -trace. After entering the stem they pass over
into a network of bundles, which branches irregularly on all sides. To this series
belong the Nymphseaceag, Gunneracese, Primula Auricula, and its nearest allies,
perhaps also many Balanophoreae. In the first three groups a definite number of
bundles are seen to enter the stem from each leaf, and immediately after entering
they pass over into a network of bundles, which are irregularly connected both
in the direction of the surface of the stem, and also in radial planes by oblique and

Sanio, Botan. Zeitg. 1864, p. 193. — F. Schmitz, clas Fibrovasalsystem d. Piperaceen, Diss. Essen,
iS?!' — J- Weiss, Wachsthumsverh., &c. d. Piperaceen, Flora, 1S76.

COURSE OF THE BUNDLES IN THE STEM.

25 f

transverse anastomoses, the network being still further complicated by the insertion
of the bundles of roots and buds. Transverse and longitudinal sections through the
stem show ‘ irregularly scattered ’ bundles, cut through in different directions ; the
former thus remind one superficially of transverse sections of stems of Monocotyle-
dons, of which however only those of the Aroidem with irregular reticulate con-
nection of bundles can be more closely compared with them.

The structure of species of Gunnera has been more exactly investigated by Reinke
In G. Chilensis Lam. (G. scabra R. B.) one bundle of the trace enters the short hypo-
cotyledonary stem from each cotyledon : the two unite, after a perpendicular course, to
form the axile root-bundle. The cotyledons are immediately followed by a pair of
almost opposite primordial leaves, which decussate with them : then come the further
leaves in spiral arrangement, all of them being separated from one another by extremely
short internodes. From each of the primordial leaves three bundles, which unite in the
cotyledonary node, enter the hypocotyledonary stem : here the two united traces alter-
nate with those of the cotyledons, and unite with them lower down to form the axile
bundle. Exactly at their point of entry into the centre of the stem they are mutually
united by a horizontal bundle, parallel to the surface of the stem, and by one or a few,
which traverse the middle of the stem obliquely. Branches from the latter pass to the
cotyledonary bundles. From each of the leaves, which next follow the two primordial
leaves, three bundles enter the stem, and from the successively higher ones a larger
number (not exactly stated). Each successive leaf-trace resembles the first, inasmuch as
immediately on entering the stem it is connected with the network of bundles by con-
necting bundles in all directions : only the number of bundles of every sort and direction
increases in proportion as the axis of the seedling 2 mm. thick increases to the swollen
stem of 50 mm. thickness. The appearance of the bundles of all categories takes place
almost simultaneously.

In conformation and structure the following species coincide with G. Chilensis: viz.
G. petaloidea, bracheata, insignis, commutata, peltata, manicata. G. perpensa L., to
which is allied G. macrophylla, has in its stem of in thickness, and with rather longer
internodes, for the most part bundles with a longitudinal course : most of these are united
into a hollow cylindrical net, within the parenchymatous cortex, and have transverse and
oblique connecting bundles, w'hich traverse the pith.

In the short internodes of the leafy stem of G. magellanica, which is 2-3 mm. thick,
3-4 vascular bundles run longitudinally: these are connected, directly with one another,
by convergence, to form pointed elongated meshes, and also at the nodes they are united
by transverse anastomoses, at the points of entry of the three-bundled leaf-trace. The
elongated internodes of the stolons of this plant have usually only one concave band-
shaped axile bundle: sometimes this splits for a short distance into two. Finally the thin
stems of G. monoica and prorepens show in their internodes usually two bundles, which
are here and there united to a single one, into which the (one-bundled ?) leaf-traces run.
In the elongated internodes of the stolons they have one axile bundle.

In Primula Auricula‘s the one-bundled leaf-traces of the cotyledons and of the first
leaves unite, running almost horizontally into the middle of the stem, to form one axile
bundle which traverses it. The bundles which enter from the subsequent leaves run
for a distance — not defined by any constant number of internodes — side by side, and
then unite with one another, or with the axile bundle. As the plant grows stronger the
number of bundles entering one leaf increases to twenty, these pass obliquely down the
stem, and are here united by irregular and oblique branches and anastomoses, which run

’ Morphologische Abhandlungen, Leipzig, 1873, p. 47, Taf. 4-7.

^ Vaupell, Ueber d. peripherische Wachsthum d. dicotyled. Rhizome, Leipzig, 1S55. — Von
Kamitnski, Zur Vergl. Anatomie d. Primeln, Diss. Strassburg, 1875.

252 PRIMARY ARRANGEMENT OF TISSUES.

radially and tangentially. In transverse section there appears a ring of 15-20, widely
separated, stronger bundles, which correspond to the central bundles of the base of the
leaf : surrounding the ring numerous smaller bundles are irregularly scattered ; these are
derived from the lateral ones of the base of the leaf : in the space within the ring are seen
the transverse sections of the connecting branches, which here also run in all directions.
Pr. Palinuri, calycina, and marginata resemble Pr. Auricula. Other species of Primula, as
Pr. sinensis, spectabilis, and elatior, have a typical Dicotyledonous ring of bundles, which
are very soon united laterally by intermediate bundles. On their special course, and the
peculiarities of many species, especially Pr. farinosa, compare Kamienski’s work.

In the Nymphxacex the system of vascular bundles of the stem (Rhizome) is usually a
network of anastomosing bundles difficult to disentangle, from which those for the leaves,
roots, and peduncles branch off at certain places, and which in stronger stems, e. g. of
Nuphar luteum, traverse the whole internal part of the stem, which lies within a sharply
limited cortex, even to its very centre. It may be seen from Unger’s figure* how
chaotic this structure is. Nageli {l.c. 121) tried to explain the matter by the investiga-
tion of weaker rhizomes of Nymphaea alba. I reproduce his description as follows. Inter-
nodes shortened. Leaves arranged spirally. The transverse section shows between pith
and cortex a circle of separate bundles, which is divided usually into three, rarely into four
groups, which can be recognised with the naked eye. The three groups are of unequal
width : they vary continually throughout the length of the stem, and are related to the
arrangement of the leaves. The bundles of the circle are often connected with one
another, so that, when seen in surface view, they appear as a network. The middle of
the pith is traversed by a central bundle, which throws off now and then a branch to
the net-work.

From the base of the leaf five bundles enter the stem ; three of these lie rather higher
and form the true leaf-trace. Their lateral bundles separate widely from one another,
and weave themselves in with the net-like circle at two almost diametrically opposite
points, so that the trace is about 180° wide. The median bundle also loses itself usually
at once in the net. But sometimes it passes inwards through the pith, after forming
some anastomoses with other bundles, and unites with the central bundle. In one stem
it was the 8th and 13th, in another the ist, 6th, nth, i8th, and 32nd leaves, the median
bundles of which passed to the centre, while those of all the other leaves remained in the
outer network. In the first example the 8t’n and 13th, in the second the ist, 6th, nth,
and 32nd leaves were on the upper side of the procumbent stem, the i8th on its lower
side.

An independent growth of the central bundle at its apex was not observed: Nägeli
therefore considers it as a sympodium of median bundles.

I found in weak rhizomes of Nuphar pumilum that the transverse section resembled
that described in the case of Nymphaea : an irregular ring of 8-12 bundles, and a central,
often very eccentric, and frequently branched bundle being seen : the latter is in rare cases
entirely absent from the section. The bundles of the ring form a net with elongated
meshes, the main meshes being limited by the bundles of the leaf-trace, between which
smaller bundles, usually pushed back rather further into the pith, form an irregular net-
work. The leaf-trace consists of three bundles, and is about 120° wide, the median
bundle forks in the node into two shanks diverging at an obtuse angle : each of these in
its descending course is united with the lateral bundle of its own side. I never saw the
median bundle curve to the middle line of the stem, but rather I here saw only a bundle,
which ran irregularly from side to side, and here and there gave off a lateral branch and
anastomosed with the peripheral net. An independent ending of this bundle beneath the
growing-point was not to be found. Besides this I have examined but few preparations,
and wished in the above remarks only to point to N. pumilum as an object well suited for
the elucidation of the structure of the stem in the Nymphseaceae.

* Anatomie und Physiologie, p. 235.

COURSE OF THE BUNDLES IN THE STEM.

253

3. Bundles of Ihe trace and cauline bundles are both present. The bundles of the
trace are arranged in a ring, the cauline hindles are in the pith. To this series belong
species of Begonia, Orobanchese, species of Mamillaria, Melastomaceae, some Um-
belliferse, and Araliaceje : further in the main also Nelumbium.

In the Begonias medullary bundles are common. Hildebrand^ found them in 28
species out of 128: for instance in B. Evansiana, laciniata, Rex, xanthina, etc. Accord-
ing to Hildebrand’s description of stems in the mature state (following the course from
below upwards) these seem to be for the most part cauline. In the first internodes of
the seedling they are wanting, they branch from those of the ring in higher internodes.
They run parallel and perpendicular in the internode, in the nodes they anastomose with
one another and with those of the ring: as a rule none pass through the node without
connection with others. In B. Hiigelii, Muricata, and luxurians Hildebrand saw 1-3
medullary bundles enter, without previous anastomosis with others, directly into the
middle of the petiole, a phenomenon which occurs less frequently in others : e. g.
B. laciniata. From the plexus in the node medullary bundles pass on into the next
higher internode, and other bundles branch off to pass into the ring. In some stems
single bundles in successive nodes pass successively into the pith, and again into the ring,
and finally into a leaf, ‘ but this whole course is rendered very indistinct by anastomoses.’

The many points, which still remain doubtful, arise partly from the difficulty of following
the course of the bundles in the Begonias with accuracy. It is made the more probable
that the medullary bundles are for the most part cauline by the fact that, according to
Sanio’s investigation of B. Evansiana (A c. 224), they appear later than those of the ring^.

Aralia racemosa® and A. japonica have within the typical Dicotyledonous ring a
second, consisting of small, widely separated bundles. In other species, e.g. A. papyrifera,
the second ring is absent. Besides this ring there are, according to Sanio in A. racemosa,
other single bundles scattered in the pith. All these bundles, which run perpendicularly
in the internode, anastomose in the nodes without passing out to the leaves ; their de-
velopment proceeds much later than that of the ring. As regards the distribution of
vessels and sieve-tubes the outer medullary bundles have a position the reverse of that of
the bundles of the ring, the inner ones are irregularly arranged. Comp. §101.

In the stem of some few Umbelliferx*, Silaus pratensis Bess., Peucedanum Oreose-
linum Mch., Opoponax Chironium K., Ferula communis, and an undetermined form of
Taurus, medullary bundles have been observed within the ring: as many as 13 in Silaus,
20 in Opoponax, 82 in the plant of Taurus (Reichardt), at least 100 in the flowering
stems of Ferula communis. They are distributed in the transverse section over the
whole pith: their number varies in successive internodes, e.g. in Silaus, Ex. I. 13, u, 10,
9, 7, 3 ; Ex. II. 10, 8, 7, 7, 6, I ; Ex. HI. 9, 8, 5, 3, i : Peucedanum Oreoselinum, Ex. I.
22, 20, 18, 17, 14, 7 ; Ex. II. 20, i8, 17, 17, 12) 6 ; Ex. HI. 15, 13, 10, 7, 3.

According to the consistent accounts of Jochmann and Reichardt the medullary
bundles do not pass out into the petiole; they are cauline. They take a parallel and
perpendicular course through the Internode, dividing here and there, and united for
short distances : at the node they anastomose by means of connecting bundles with one
another, and with those of the ring : from the anastomosing bundles in the node those
medullary bundles start which traverse the next internode. Those of the lowest inter-
node above the root affix themselves on those of the ring of the same internode, or

‘ Anatomische Untersuchungen über die Stämme d. Begoniaceen, Berlin, 1859.

“ [Cf. Westermaier, Ueber das markständige Bündelsystem der Begoniaceen. Regensburg, 1S79.]
■’ Sanio, 1. c. p. 226.

^ De Candolle, Organographie, I. p. 184, Taf. III. — Jochrnann, de Umbelliferarum structura,
1854. — H. W. Reichardt, Ueber das centrale Gefässbündelsystem einiger Umbelliferen, Wiener Acad.
Sitzungsbr. Bd. XXI (1856), s. 133.

254 PRIMARF ARRANGEMENT OF TISSUES.

rather spring from them ; the same is the case with those of the lowest internode of a'
branch, so that the latter are not in direct continuity with those of the stem (Reichardt).

The occurrence of medullary bundles is a purely specific property. Of eight investigated
species of Peucedanum they appear only in P. Oreoselinum : they are absent in Silaus
tenuifolius. They are not present in the one-year-old seedling of S. pratensis.

Some Mamillarias ^ have, within the typical ring of bundles of the leaf-trace, a second in
the peripheral parts of the pith, this being composed of numerous small cauline bundles.
In M. angularis, and an undetermined similar species, there are about thirty of these.
They ascend the stem parallel to those of the leaf-trace, are strongly undulated in a
radial, and especially in a tangential direction, and anastomose at acute angles. They are
absent in young seedlings and in young shoots, and appear later at some height above
the base of the shoot, springing from the inner side of the bundles of the trace. I was
unable to find anastomoses with the leaf-trace, or the ring of secondary wood, excepting
at their point of origin. In other Mamillarias, as M. pusilla, glochidiata, &c., I looked
in vain for the medullary bundles, even in the mature shoot. Of other Cactaceae, forms
of Echinocactus and thick ones of Cereus (e. g. C. candicans ?) have a medullary system
of bundles, which on account of its peculiar relation to the lateral shoots will be described
in Se.ct. 94.

The small Orobanchex have in their stems only the typical Dicotyledonous ring of vascular
bundles. Strong stems of the more robust forms, as O. elatior Sutt., rubens Wallr.,
caryophyllacea Sm., Rapum Thuill., and Cistanche lutea have,
inside the ring, and scattered in the pith, small bundles, the
number of which is variable, and is large in strong specimens.
They are cauline, and in young specimens take an undulating
longitudinal course through the stem, anastomosing here and
there, and ending blind beneath the apex of the stem. Comp.
Fig. III. In fully developed flowering stems they gradually cease
below the inflorescence, curving outwards, and uniting with the
bundles of the ring. It is only in Cistanche lutea, where they
occur in considerable quantity, that they run freely in large num-
bers to the extreme apex of the inflorescence. In Epiphegus
americanus and Conopholis ® there are found in the base of the
main stem three concentric rings of bundles : in the latter genus
the bundles composing them are contiguous with one another in
radial series. It is uncertain whether these belong to this category,
or to that of radially diverging bundles of the trace, or are pro-
ducts of a secondary cambium.

In the flowering stems of species of Balanophora, according
to Göppert’s^ description, the same or a closely similar arrange-
ment occurs to that in O. Rapum : numerous branched bundles are scattered within a
ring composed of bundles of the leaf-traces.

In the Helosideae the ring of bundles is alone present in the elongated rhizomes, in
the tubers and Inflorescences there are branched, scattered bundles ®.

Two cataphyllary leaves and one foliage leaf succeed one another regularly on the

Fig. m. — Orobanche Rapum
(natural size). Bud of a flower-
ing shoot, median longitudinal
section, b—b ring of bundles,
consisting of leaf-traces ; within
this are the cauline bundles. At
the base, where it has been cut
off. is a network of the latter,
which anastomose one with an-
other, and with tlie bundles of
the leaf-trace.

' Von Mohl, Verm. Schriften, p. 115.

^ Graf zu Solms-Laubach, de Lathrteie generis positione systematica, Diss. Berlin, 1865, pp. 8,
14, and Pringsheim’s Jahrb. VI. p. 522.

^ Chatin, Anat. Comp. Taf. XVIII.

^ lieber den Bau der Balanophoren, N. Act. Carol. Leop. vol. XVIII. Suppl. i . — Compare also
Hooker, Balanophores, Trans. Linn. Soc. London, XXII ; Graf zu Solms, in Pringsheim's Jahrb.
l.c. p. 529.

® Compare Eichler, in Flora Brasiliensis, Fase. XLVII.

COURSE OF THE BUNDLES IN THE STEM.

255

rhizome of Nelumbium speciosum*. The internode between the latter and the next
following scale leaf is elongated (as much as 4 feet), the others are short. The elongated
internode (Fig. 112) has six blunt angles, so that when horizontally placed one surface is
turned upwards (0), another downwards («), and one edge to each side right and left.
It is traversed by 6 large air-canals (/), corresponding to the angles, one small axile one, and
two small ones, corresponding to the upper surface, and in regular cases (slight deviations
occur) by about 252 vascular bundles with a perpendicular course, the arrangement of
which in the transverse section is according to Wigand as follows. Firstly, an inner system
of twelve bundles, disposed in two concentric alternating circles (i and 2) of six each, at
equal distances from one another in each circle, in the inmost circle (i) one bundle is
opposite the upper, and one opposite the lower surface ; secondly, a middle system,
extending to the outer limit of the air-canals : it consists of four concentric rings (circles
3-6); in each of the circles 4, 5, 6, one bundle alternates with two air-canals: in circle 3, in
addition to the otherwise similar arrangement, there are on each side between each two

o

Fig. 112. —Scheme of the transverse section through the intemode of a rhizome of Nelumbium speciosum,
after Wigand, o upper, h lower side ; I air canals. The figures i — 11 indicate the successive circles in which the
bundles are arranged. The bundles of the circles 3 and 5 have an orientation the reverse of that of the others,
which is indicated by direction of the dashes affixed to the round spots.

lateral lower air-passages threebundles, and on each side between the two lateral upper
ones two bundles, that is six more bundles are present than in the other circles : the bundles
of four circles, with exception of the last-named six, form regular radial rows. Thirdly, the
peripheral system, between the outer side of the air-canals and the surface of the stem,
consists of 4-5 concentric circles of bundles (7-1 1), the inmost (7) having 18 bundles in
pairs alternating with the radial rows of the middle series and with the air-passages ;
the next (8) consisting of 45 bundles, of which two are between the two of one pair, and

’ According to Wigand, Nelumbium speciosum, Botan. Zeitg. 1871, p. S16, &c. See also
Trecul, Ann. Sei. Nat. 5 ser. I. p. 162, &c.

256

PRIMARY ARRANGEMENT OF TISSUES.

three between each two pairs of the above ; the bundles of the next (9) alternate with those
of (8), those of (10) and (ii) alternating irregularly with those next within them.

Of these bundles all those of the peripheral series are, according to Wigand, ‘ true
leaf-bundles, since they traverse only one internode and then run into the leaf-organs.’
The course of the rest is difficult to describe with certainty, because of the complicated
branchings of lateral shoots and roots in the node, and it requires further investigation.
The bundles of the inmost circle of the middle series (3) seem to be cauliiie, since ‘it
has not hitherto been possible to prove that they contribute to the lateral organs,’ and
they ‘ apparently always traverse one internode only, and lose themselves in the node,
their place being taken by fresh ones in the following internode.’ The same holds
perhaps for the other members of the middle series, which are radially arranged. Of
the inmost series the four lateral bundles of the inner side (1) give off branches to the
roots, the upper and under ones of the same circle give off branches ‘ directly or in-
directly’ to the leaves. All six bundles of this circle ‘are however distinguished by
the fact that they alone of all the bundles of the stem traverse all internodes and nodes
up to the growing point,’ while on the contrary the six bundles of the same series, which
alternate with them (2), ‘each belong to one elongated internode only, and to the roots
(which arise at each node), and then end in the node, just above the root-region.’

The medullary bundles of the Melastomacex will be described below, p. 259, in con-
nection with the rest of the bundle system of these plants.

b. Corh'cal bundles.

Sect. 63. A relatively small nuinber of Dicotyledons is characterised by having
a bundle system in the internodes arranged typically in a ring, and outside this other
bundles, which traverse the cortex. These cortical bundles are sometimes bundles
of the leaf-trace, which run for a certain distance outside the ring, and later curve
into it : as in the cases of Lathyrus Aphaca, and Pseudaphaca above described, the
Casuarinas, and many Begonias : also the cortical bundles of the Cycadeae, to be
described in detail in Chap. XVI, belong to this category, perhaps also Nepenthes.
Sometimes they are certain bundles, belonging to many-bundled leaf-traces, which
never enter the ring, but form with the similar ones belonging successively to upper
and lower leaves an independent cortical bundle-system only connected with the
ring by anastomoses at the nodes : this is the case in the Calycanthese, many
Melastomaceae, also in Arceuthobium Oxycedri. In many succulent plants with
reduced leaves, as Salicornia, Cactese, they are branches of the bundles of the
leaf-trace, which are branched and distributed like the bundle-expansions of the
leaf lamina, and will therefore be treated of where these expansions are described.
Finally, in the winged Rhipsalidacese, according to Vöchting, the peculiar case occurs
that the bundles of the leaf-trace are mainly cortical, while a ring of bundles,
surrounding a pith, and quite similar to the typical Dicotyledonous ring of bundles
of the leaf-trace, consists at least for the most part of cauline bundles.

The young foliage shoots of the Casuarinas^ (Fig- 113) are covered with whorls of
small leaves, united into a long sheath : the average number of leaves of one whorl
varies according to the species (4-20). The leaves of successive whorls, and the
ridges, which run down the backs of the leaves, alternate in successive internodes. One
vascular bundle enters each leaf. From the point of insertion of the sheath it passes

’ Compare Low, De Casuarinearum caulis foliique evolutione et structura, Berlin, 1865.

COURSE OF THE BUNDLES IN THE STEM.

257

Fig. 113. — Casuarina muricata.
Scheme of the course of the vascu-
lar bundles in the median longitu-
dinal section of a young branch.
I — 4 successive whorls of leaves ;
bundles of whorl, 2 ending in node
a, 3 in 4 in c, &c.

into the periphery of the stem, and there runs, parallel to it, in the cortex as far as the
next node : it then curves inwards, and ranging itself with those of the same whorl of
leaves around a narrow cylinder of pith, it descends per-
pendicularly through a second internode. At the lower
limit of this, i. e. at the 2nd node from the point of exit
into the leaf, it is inserted (according to Low with a short
fork) on the bundles which here pass out into the cortex.

The transverse section of each internode thus shows 2 con-
centric circles of an equal number of alternating bundles;
the one peripheral, composed of the bundles of the trace of
its own whorl of leaves, the other axile (forming the woody
ring at a later period), consisting of the bundles of the trace
of the next higher leaf.

In Begonia angularis Raddi, Hildebrand^ found one bundle
in the outer cortex of each of the six angles of the stem. All
the six in one internode form together the trace of the next
higher leaf, which includes of the circumference of the
stem. They pass perpendicularly downwards in the angles
to the next lower node, and here curve into the ring of
bundles. Their further course in the latter has not been
investigated. Many internodes have less than six angles, and
correspondingly fewer cortical bundles : others may be en-
tirely without either. The same is the case with Begonia
tomentosa, with this difference, that the number of the

cortical bundles is ‘ indefinite ’ and often very large, while some of them often run
through two internodes in the cortex.

Arceuthobium Oxycedri ^ has decussating pairs of leaves, each leaf has three bundles of
the trace, one median and two lateral. The latter converge, and enter the stem : here they
unite and descend opposite those of the other leaf of the pair, and separated from them
only by a narrow band of pith, to the next node, where they insert themselves upon the
bundles which there pass out. The thin median bundle of each leaf pursues an in-
dividual course through the cortex, and is also inserted at the next lower node. The
transverse section through an internode thus shows two decussating pairs of bundles,
one stronger axile pair, and one weaker and peripheral.

In the Calycanthacex^ three bundles pass out into each of the opposite and decussate
leaves, one stronger median, and two weaker lateral ones. The median ones are
arranged in a ring in the stem. Each runs down through two internodes, and then affixes
itself at the node on the median bundles which there pass out. The lateral bundles
(rather later developed) pass perpendicularly down the stem in the cortex outside the
ring of bundles ; at the next node they insert themselves on the cortical bundles which
there pass out. Thus in each internode the transverse section shows the ring of bundles,
and outside it four cortical bundles. In the node each cortical bundle is connected with
the ring by a short radial transverse bundle, and by another with the median bundle
which next passes out, and again, by a stronger horizontal girdle-like bundle, with the
next cortical bundle belonging to the same side of the stem.

In the seedling two bundles enter each cotyledon : they descend with the median
bundles of the first three leaves through the hypocotyledonary axis, the transverse section
of which thus shows six bundles. The cortical bundles of the first two leaves only
extend down to the cotyledonary node.

' I.C., compare p. 253.

^ (traf zu Solms- Laubach, in Pringsheim’s Jahrb. Bd. VI. p. 523.

^ Mirbel, Ann. Sei. Nat. XIV (1828). — Gaudichaud, Archives de Botanique, II. p. 493 (1833).
— Woronin, Botan. Zeitg. i860, p. 177.

S

PRIMARY ARRANGEMENT OF TISSUES.

258

In the internode of Nepenthes ' there is found an inner typical bundle-ring, surrounding
the pith, which later undergoes secondary thickening, and externally other bundles are

Fig. 114. — Centradenia rosea. End
of a shoot, halved longitudinally ; the
three lower pairs of leaves and the epi-
dermis removed, made transparent by
potash, and seen from luithont. The
pairs of leaves and the bundles belonging
to them numbered in order ; in the
median ones, / the lateral bundles. One
leaf of the fourth pair (4) covers the
growing-point. In the node of pair 3 the
transverse girdle is as yet imperfectly
developed. Further description in the
text. Magnified about 25.

Fig. 115. — Osbeckia canescens (magnified about 25). Longitudinal
half of the end of a shoot, prepared like Fig. 114, and seen from without.
Six decussating pairs of leaves numbered in order ; in 5 and 6 as yet no
bundles, in 4 the median bundle is just visible. Further description in
the text.

to be seen in the very broad outer cortex : these are partly bundles of the leaf-trace,
which passing in a radially-oblique direction through the cortex, gradually enter the

^ C. H. Schultz, Vaisseaux du Latex, l.c., compare p. 249. — Korthals, Verhandelingen, l.c.,
compare p. 226.

COURSE OF THE BUNDLES IN THE STEM.

^59

inner ring ; partly small bundles, the origin of which remains to be decided ; these pass
close beneath the epidermis, and are connected with one another by oblique branches.
A thorough investigation is in progress'.

In the Melastomacex, the course of the cortical bundles is partly the same as the
above, and partly similar to that in Calycanthus. It may here be described in connection
with the other peculiarities of arrangement of the bundles in this family (comp. p. 256)''.

The stem has four angles, and bears decussate pairs of opposite leaves : those of one
pair are either equal, or, as in many Centradenias, of unequal size. Each pair faces two
opposed surfaces of the stem : these may be called the surfaces belonging to that pair,
and the others the intervening surfaces. In the simplest case investigated the trace of the
single leaf which enters the node consists of three bundles, one median and two lateral ;
in many species it consists of more than three, through multiplication of the lateral bundles
on each side. The bundles which enter the stem pass, in many species, directly into
the bundle-ring without forming cortical bundles : Sonerila margaritacea, Medinilla
farinosa, Sieboldii, magnifica, Cyanophyllum magnificum, Clidemia parviflora, Miconia
purpurascens, Lasiandra Hoibrenkii^ In the other investigated species the median
bundle always enters the bundle-ring, usually without, rarely after previously giving off
cortical bundles ; the lateral ones either run, as in Calycanthus, down the angles of the
stem as cortical bundles, or they enter the ring, after having given off cortical bundles as
branches. The cortical bundles are always connected at the node both with one another
and with those which pass to the ring, by a transverse girdle of horizontal branches : they
run from this to the next lower transverse girdle, and insert themselves on the latter.
In the ring the bundles of the trace always pass down through several internodes, they
pectinate in various ways with those of lower pairs of leaves, remaining simple, or
splitting into shanks. For numerous differences of arrangement according to the
number of bundles and the species, compare Vöchting, l.c.

The cortical bundles are always derived, as above stated, from the bundles of the
trace. In the simplest case (Centradenia rosea. Fig. 114) they are the lateral bundles
of the three-bundled trace. From the base of the leaf one stronger median bundle
w, — Wj) and two weaker lateral ones enter the node. All the median bundles are arranged
in the ring: they pursue an individual course, directly, or with slight curvature at the
nodes, through three internodes (two according to Vöchting), and then unite laterally with
such as come from lower nodes. The lateral bundles give off one branch on each side
at the node, which runs transversely through the outer cortex, the one to the out-going
median bundle of the same leaf, the other to a similar one, which comes from the
nearest lateral bundle of the opposite leaf. These branches together form the transverse
girdle of the node, which girdle is further connected with the out-going portions of the
bundles by small branches. From the point of departure of the branches of the girdle
each lateral bundle runs perpendicularly through the cortex of the corner of the stem,
and inserts itself on the transverse girdle of the next lower node.

As an example of the other case, i. e. that cortical bundles and transverse girdles are
branches of the bundles entering the ring, Osbeckia canescens (Figs. 115, 116) may be
described. A strong median bundle - nii enters from each leaf, into the corresponding
side of the internode, and here descends perpendicularly, in the bundle-ring : in the
next lower node it splits into two shanks {m^ and m^, which enclose between them the
united lateral bundle, which there enters the stem: these shanks may be further followed
down three internodes. A strong lateral bundle passes from each leaf almost horizontally
through the cortex into the middle of each of the intervening sides : it here unites with

’ E. Zacharias, lieber die Anatomie des Stammes der Gattung Nepenthes, Strassburg, 1877.

^ Vöchting, Bau, &c. d. Melastomaceen, in Hanstein’s Bot., Abhandl. III. Compare also
Crüger, Botan. Zeitg. 1850, p. 178. — Sanio, Ibid. 1865, p. 1 79. — Hildebrand, Begoniaceenstämme, p. i.

^ The names quoted are partly derived from Vöchting’s work, and partly garden names, for the
correctness of which I will not answer.

200

PRIMARY ARRANGEMENT OF TISSUES.

a similar one from the opposite leaf, and the united bundle (/1-/3) then curves into the
ring. Its course in the ring may be gathered from Fig. 115. The lower endings of the
bundles in the ring were not investigated. Opposite its point of exit into the leaf the
median bundle gives off a branch on either side, which takes a curved and almost
horizontal course through the cortex to the nearest angle, and here unites with the
out-going lateral bundle. The transverse girdle accordingly arises from the last-named
branches of the median bundles, and the horizontal portions of the lateral ones. From
the latter sections of the girdle there arise near each angle one or two cortical bundles
(r), which run perpendicularly to the next lower node, and here insert themselves on
the transverse girdle. If two cortical bundles are present, they stand radially before one
another, and one often inserts itself on the other before reaching the lower girdle.
To the above description it may be added that the two lateral bundles which have been
mentioned in each leaf-trace are each formed by the coalescence of two lateral bundles
(/ and p, Fig. 115) of the base of the petiole.

For further peculiarities in other species re-
ference may be made to Vöchting’s work : it may
be added that the examples here selected have
been retained, since the corresponding woodcuts
were finished two years before that work appeared.
Species with wide wing-like projecting angles often
have several cortical bundles placed radially one
before another in each of these angles (e. g. Hete-
rocentron subtriplinervium, Lasiandra macrantha
3-4, Centradenia grandifolia 5-7), these anastomose
nowand then with one another; they originate as
branches either of median or lateral bundles of the
first order, or from lateral bundles of a higher order,
if such are present.

Besides the cortical bundles there are also
cauline medullary bundles in most of the Melasto-
maceas (compare Fig. 116). These are usually to be
found also in those species in which the cortical
bundles are absent. Only Sonerila margaritacea
is without either : it alone of the species investigated
has a quite typical Dicotyledonous bundle-system.
In the simplest case a single bundle is found in the
centre of the pith, e. g. Medinilla farinpsa, Sieboldii,
or this may even be sometimes present, sometimes
absent, as in Eriocnema marmorata and Centra-
denia rosea. In the transverse section of the in-
ternode of other species there are several bundles
which lie in the middle of the pith, e. g. Melastoma
igneum, Lasiandra Maxirniliani 1-3, Medinilla mag-
nifica 2-4, Melastoma cymosum 8-10 ; finally, others
have many of them scattered over the whole trans-
verse section, e.g. Heterocentron subtriplinervium
18, Miconia chrysoneura and Cyanophyllum magnificum 30, 40, and more.

The medullary bundles run perpendicularly through the internodes. In the nodes
they are connected by oblique or transverse branches of varying number one with
another, and with the bundles of the ring. From the network or felt thus formed proceed
those of the next higher internode. In the large majority of cases they arise much
later than the bundles of the leaf-trace which are in the same transverse section, and
they do not pass out to the leaves. The medullary bundles are usually relatively small,
and characterised by peculiarities of structure to be described below (§ 105).

Fig. 116. — Osbeckia canescens. Thick transverse
section through a node, which is rather further
developed than 3 and less than 2 in Fig. 115; it is
transparent, and seen from below. The transverse
sections of the bundles of the leaf-trace which are
nearest the beholder and in the surface of section
are represented as dark, mi the median bundles of
the trace passing out at the node; / the lateral
bundles of the same ; r the cortical bundles which
pass down from the node; 0 those which pass upwards
from it; the median bundles of the next higher
node, which branched above the node; with them
there alternate, opposite each corner, single bundles,
with regard to which it is not quite certain whether
they are the lateral ones of the next higher pair of
leaves, or shanks of the median bundles of the
second higher pair. The lighter spots in the pitli
indicate the transverse sections of the cauline bundles.
In this node they were not yet plain, and have been
drawn hi the figure from another preparation (40).

COURSE OF THE BUNDLES IN THE STEM.

261

Some of the RhipsaUdex.'^ have round stems, others angular and winged ; both forms
have in transverse section a circular or elliptical ring of bundles, which, especially in
the winged species, is surrounded by a very broad succulent cortex. In the round
forms, such as R. Saglionis, salicornioides, the one-bundled leaf-traces pass slightly
obliquely downwards through the co into the ring, which is originally formed from
them alone, but later secondary intercalary bundles also appear (Chap. XIV). In the
winged forms the leaves are seated only on the angles. The bundles of the trace enter
these, and run, in the main tangentially-perpendicular and radially-oblique, down
through the cortex ; they enter the ring about the level of the next lower leaf, and
then descend further in a perpendicular direction. They thus form the portions of
the ring corresponding to the angles. But the portions of the ring between these,
which correspond in the elliptical ring of two angled forms (e. g. Lepismium radicans,
Rhipsalis carnosa), to the broad sides of the ellipse, as seen in transverse section, and
v hich comprise the greater part of the ring, are here composed of cauline longitudinal
bundles, connected here and there by oblique anastomoses: on these the common
bundles insert themselves in the region indicated. These cauline bundles correspond
to the secondary intercalary bundles, which complete the woody ring of typical Dicoty-
ledons, and which will be described in Chapter XIV ; but they are distinguished from
these by their appearing on the first primary differentiation of tissues. Finally, in all
Rhipsalidese branches come off from the bundles of the trace during their course through
the cortex : these with their further branches form a cortical network of bundles (which
is further strengthened by the bundles which come from the axillary buds). The
special form and development of this varies according to the species, in the winged
species it is exclusively or chiefly expanded in the wings, in a radial direction. Compare
Vochting, l.c.

It is not known how far other Cactaceae, which are winged and have a cortical net-
work of bundles, correspond to the winged Rhipsalidese in the course of the bundles
of the leaf-trace and intercalary bundles.

III. The Type of the Palms.

Sect. 64. The stem of most Monocotyledons does not show the bundles
in the transverse section of an internode arranged in a simple ring, but within
a peripheral zone of cortex, which has no bundles, there is a circular surface, in which
either several concentric irregular and interlocking series of bundles are arranged
round a central portion without bundles (pith), as e.g. in many stems of Grasses,
which later become hollow ; or the bundles lie scattered over the whole surface.
Thus instead of the ring of bundles of the Dicotyledons there is here a cylinder,
which contains the bundles ; the zone surrounding the cylinder, which was called
the cortex, corresponds to the cortex of the Dicotyledons : the terms pith and
medullary rays may be used in a comparative sense for the bands of other tissue
(as a matter of fact parenchymatous) which lie between the bundles in the cylinder.

The arrangement of the bundles in transverse section in the Palm type depends
upon the radially-oblique course of the leaf-traces. This must first be demonstrated
for that form which may be called the simple Palm type : with this are further
connected a number of more or less divergent phenomena.

Vochting, Morpholog. und Anat. d. Rhipsalideen, Pringsheim’s Jahrb. IX. p. 326.

263

PRIMARY ARRANGEMENT OF TISSUES.

a. Swiple type of the Palms.

Since INIohl’s Anatomy of Palms ’ the following chief characteristics have been
known for this type.

All bundles of the cylinder (with doubtful, and extremely unimportant exceptions
to be mentioned below) are bundles of the leaf-trace. The base of the leaf encloses
the whole circumference of the stem, or at least the greater part of it. The leaf-
trace always consists of a number of bundles, and usually of many, in strong shoots
of tw'O hundred bundles : its width is ^ of the circumference of the stem, or more, or
not much less. The bundles curve from the base of the leaf into the cylinder, and
pass downwards in the latter ; some at its surface, running almost radially perpendi-
cular; others are radial and oblique, penetrating first in a curve convex upwards
and inwards, towards the longitudinal axis of the cylinder, then turning outwards
and gradually passing towards the surface of the cylinder ; as they approach it, they
assume an almost perpendicular position. All bundles descend through many
internodes, and finally unite in the outer portions of the cylinder with such as
emerge lower down, inserting themselves on these sometimes in a tangential,
sometimes in a radial or oblique direction. Up to this point of insertion at their
lower ends the bundles pursue an individual course. The coalescence of the lower
ends of descending bundles with such as emerge lower down occurs with such
frequency that the whole number of the bundles in equally strong successive inter-
nodes remains about the same. Where the successive internodes and leaves grow
stronger the number of the bundles increases, and vice ve7-sa. The number of the
internodes which a bundle traverses cannot be exactly defined.

Further, the bundles of a leaf-trace, which curve towards the middle of the
cylinder, do not all penetrate to the same depth ; on an average the median bundle
of a series penetrates the deepest, the others less deeply the further they are from
the median one, the marginal ones pass almost perpendicularly down the surface
of the cylinder: where there are several series, those of the inmost penetrate
as a rule deeper than those of the outer, which are equally distant from the
median bundle.

The necessary consequences of the course described are, firstl_y, that the bundles
in the transverse section of an inlernode are more crowded the nearer they are to
the surface of the cylinder, a phenomenon which is especially striking where the
bundles are distributed over the whole surface of section of the cylinder. Secondly,
that the successive traces pectinate, and cross one another with their curved bundles.
Mohl’s celebrated scheme, which is here reproduced in Fig. 117, shows this condi-
tion in radial longitudinal section, but starting from the incorrect assumption that all
bundles of one trace are almost equally curved and are tangential and perpendicular,
that is that they lie at the surface of a cone which has a funnel-shaped opening at
the top. If it is assumed that the leaves alternate with a divergence of | and
embrace the stem, and that the bundles are tangentially perpendicular, their course
in the stem would be represented with greater exactitude by the scheme of a radial

* De Palmarum Structural Monachii, 1831; Verm. Sehr. p. 129; translated in the Ray
Society’s Reports and Papers on Botany, 1849. — Nageli, Beitr. I. l.c.

COURSE OF THE BUNDLES IN THE STEM.

263

section through the median planes of the leaves, Fig. 1 1 8. But the assumption of a
tangentially-perpendicular course only holds good for bundles which are also radially
perpendicular. As Meneghini ^ first asserted, and Mohl also conceded (Verm.
Schriften, p. 160), and as Naegeli proved more accurately, each radially curved btmdle
runs also tangentially oblique, with a spiral curvature, which is stronger the stronger
the radial curvature. Nageli found e.g. that the median bundle of a leaf of Chamae-
dorea elatior Mart, made i ^ revolutions in passing through six internodes : in the
sixth it had not quite accomplished half the distance from the centre of the stem
to the inner surface of the cortex, on its way outwards. In stems with very short
internodes and closely packed bundles the spiral curvature is at once visible in the
transverse section, and very plainly in the bundles of the stem of Xanthorrhoea,

riG. 117. — Mohl’s scheme of the vascular system of
Monocotyledons. Successive leaves, or rather suc-
cessive nodes, are nutnbered in order.

Fig. ii8. — Scheme of the vascular system in the Palm-
type, assuming that the leaves alternate in two rows, and
embrace the stem. Successive leaves numbered in order.
Jn median bundle.

which pass almost horizontally to the middle of the stem ; the peculiar appearance
so often noted ^ in transverse sections of this plant depends upon the above
peculiarities.

Finally, many variations of direction from that hitherto described as uniform
may occur in the course of a bundle, such as curvatures outwards and inwards, &c.,
which never appear to be constant.

The above description holds both for the preponderating number of cases, in
which those bundles which penetrate deepest reach the middle of the stem, and also
for those where, as in the stems of grasses which become hollow, a broad central
part (pith) remains free from bundles. Where the internodes are short, as e.g. in
the well-known preparations by maceration of the stems of Dracsna Draco, the
course can easily be recognised in the main features. Where the internodes are

' Ricerche sulla slruttura del caule nelle plante monocoliledoni, Padova, 1836.

* De Candolle, Organographie, I. Tab. VII. VIII. — Schleiden, Griindzüge, 3. Anfl. II. p. 160.

264

PRIMARY ARRANGEMENT OF TISSUES.

very long and thin, as e.g. in the Grasses, or egg- or spindle-shaped, as in the
so-called pseudo-bulbs of epiphytic Orchids, it appears otherwise at first sight. The
internode appears to be traversed by bundles, which are parallel, or convergent
towards both ends: of these some may be seen to enter the leaf at the node,
while many run into the next internode. One may however be easily convinced
that here also their course is such as that above described. In young stems, which
are still short, no variation from it is to be traced. In the subsequent elongation of
the internodes of the stem of Grasses to 20-50 times their original height or even
more, that portion of all the bundles which turns outwards while descending is so
elongated, that at first sight it does not appear to deviate from the perpendicular
in any one of the six or more internodes which it traverses ; the general view of its
course is rendered especially difficult by a complex felt of transverse bundles in each
node ^ (comp. Sect. 95). In the pseudo-bulbs besides this elongation there is also
a transverse swelling of the internode in the middle, and a consequent curvature of
the bundles.

According to the statements of linger^ and Millardet® there is found in plants of this
category. Grasses, Palms, Dracaena, Yucca, Narcissus, Galanthus, Leucojum, Pandanus,
in addition to the bundles of the leaf-trace a further system of cauline bundles, which
run upwards outside the cylinder, converging towards the growing-point; accord-
ing to the rule which holds for cauline bundles in the Phanerogams, these are formed
later than the bundles of the leaf-trace. Leaving out of account the Commelinaceae,
which will be described below, and the secondary formation of wood in Dracaena, Yucca,
and their allies, I could not convince myself of the presence of such cauline bundles,
which might it is true be easily mistaken for perpendicular bundles of the trace.

As has been repeatedly stated, the cylinder traversed by the bundles is in most
of these cases sharply marked off from the inner surface of the cortex. The cortex
itself is of variable thickness, in Rhizomes it is usually of great thickness, in aerial
stems it is often relatively very thin.

b. Modifications of the type of the Palms.

Sect. 65. According to the scheme laid down in the previous paragraphs all
the bundles pursue an individual course up to their point of final insertion at the
periphery of the cylinder, this being sharply limited from the cortex, which in the
internodes is without any bundles.

In many cases this scheme is subject to the following modifications : (i) the
bundles receive oblique or transverse connecting branches, or anastomoses, during
their course: (2) before they reach the periphery of the cjdinder in their curved
course they unite with others belonging to lower leaves (Sect. 66) : and (3) cortical
bundles appear outside the cylinder (Sect. 67). Each of these phenomena may
occur separately or combined with the others.

Anastomoses of the bundles of the leaf-trace one with another — besides those

1 Von Mohl, Palmarum Structura, Tab Q. — Schleiden, Grundzüge, 3. Aufl. II. p. 158.
^ l.c. p. 54.

3 Mem. de la Soc. des Sciences Nat. de Cherbourg, tom, XI. p. 4.

COURSE OF THE BUNDLES IN THE STEM. 265

which arise through the insertion of bundles which go to the branches and roots —
occur in the greatest abundance, so as completely to mask the typical course
of the bundles, in the tuberous stems of certain Aroidese to be described below.
In other stems with short or slightly elongated internodes they appear as an
unimportant phenomenon which occurs occasionally. They are however numerous
and characteristic in the greatly elongated internodes of the flower-stems and
foliage shoots of many Cyperacese, Scirpus palustris, lacustris, and their allies,
of Papyrus, species of Cyperus, and of Pontederia cordata k These ‘ halms ’ have
this peculiarity, that their longitudinal bundles are connected in a reticulate manner
one with another by small horizontal or oblique branches, like those of the foliage-
leaves of Monocotyledons (Sect. 91). The transverse branches run in the dia-
phragms (but not by any means in all), which separate the air-cavities one from
another. According as the longitudinal bundles are scattered over the whole
transverse section of the halm, or (as in Sc. palustris and its nearest allies) only form
a ring inside the chlorophyll-parenchyma of the cortex, there are found also
transverse branches throughout the whole thickness of the halm, and in the most
various directions, or only in the zone occupied by the ring.

Sect. 66. The phenomenon of insertion of bundles of the leaf-trace during
their curved course through the middle of the cylinder, and before they reach its
periphery, on others which emerge lower down, and of their further descent in union
with the latter, is also of frequent occurrence in the Aroideae to be described below.
Further it occurs in Pandaneae^, Bromeliaceae (Ananassa, Tillandsia acaulis, Hort.),
and apparently also, according to Karsten®, in many Palms, especially Martineza
aculeata. Whether this union of their course follows definite rules for certain bundles
of a leaf-trace remains for more exact investigation to decide.

Sect. 67. The cortex is free from vascular bundles in many of the Monocotyle-
dons of this category, if those bundles be disregarded which pass to the leaves at the
nodes, and those which enter branches and roots ; such bundles must appear in all
or alm.ost all transverse sections where the internodes are very short. On the other
hand a special cortical system of bundles may be distinguished from the cylinder in
certain cases. This consists in the simplest case of bundles of the leaf-trace, w'hich
after their entry into the stem descend first in the cortex through one or several
internodes, and then enter into the cylinder. This is the case in stems of certain
Aroideae, in many Rhizomes, as Carex hirta (but not in C. disticha), where all
bundles run through one internode in the cortex : Scirpus lacustris, Typha, Sparga-
nium, &c. In other cases however it consists of bundles, which do not take part, or
at least not directly, in the construction of the cylinder ; Palms, Scitamineae. and
many Bromeliaceae.

In the cortex of those Palms which have been investigated, small bundles,
which are arranged in irregular concentric rings, are found in the cortex, outside the
dense periphery of the cylinder. P. Moldenhawer has compared the region in which
they lie with the bast of Dicotyledonous trees (Chap. XIV), Mohl has named it the

’ Duval-Jouve, Diaphragmes vasculiferes, l.c.\ compare p. 216.
“ Van Tieghem, Ann. .Sei. Nat. 5 ser. tom. VI. p. 195.

’ Karsten, Veget. Org. d. Palmen, p. 98.

266

PRIMARY ARRANGEMENT OF TISSUES.

fibrous la)'er. It is weakly developed in tubular stems like Calamus (Geonoma, Bactris,
Hyospathe, Desmoncus, Calamus), and in the C3'lindrical stems of Mohl (Mauritia,
Oenocarpus. Kunthia, Astrocaryon sp., &c.) : more strongly in Rhapis flabelliformis,
Phoenix, Jubaea spectabilis ') ; and most fully developed in Mohl’s Cocos-like stems:
Cocos, Leopoldinia, Syagrus, Elais, Corypha sp. Mohl regards the bundles of the
fibrous layer as being, at least in part, bundles of the leaf-trace, the lower ends of
which pass out from the cylinder into the cortex, and pass down to the base of the
stem as fine threads, either undivided, or split up into many thin branches (Cocos).

More recent investigations’^ have shown that the bundles of the fibrous layer are
not the ends of bundles which pass through the cylinder and emerge from it lower
down, but that, as Älohl ^ also allowed to be the case with some of them, they pass
from the base of the leaf directly into the fibrous layer. Here they run almost
perpendicularlj', and frequently show splittings and branchings, or fusions ; the latter
especially in a tangential direction, both in the internodes, and also in the insertions
of the leaves, so that those bundles which enter from a leaf are in direct continuity
with those which descend from higher leaves. Most of the bundles in question in
the fibrous layer consist of a number of sclerenchymatous fibres : in a strict sense
therefore they do not belong to this category. But in others according to Mohl’s
figures, and according to investigations on species of Chamsedorea and Rhapis, there
are solitary small sieve-tubes, and in single cases also one or a few small trachese.
And while some continue as purely sclerenchymatous bundles into the petiole, others,
after their entry into the petiole, assume the structure of fully organised vascular
bundles k Further, according to a note by Schacht, there is a connection with or
transition to the vascular bundles of lateral roots, though the fibrous bundles do not
arise ‘ as branches ’ of these.

The cortical system of bundles of the Palms is accordingly a direct continuation
both of the system of vascular bundles, and of the system of purely sclerenchymatous
bundles in the leaves, and connects them one with another.

Another cortical bundle-system occurs in the stem oi Ana?tassa and Tillmidsia
acaulis Hort. The parenchyma of the thick cortex is here traversed by numerous
bundles of the trace, which pass obliquely downwards from the leaves into the sharply
limited cylinder. Other thin but also complete vascular bundles pass from the base
of the leaf into the cortex : here they pass down, sometimes close beneath the surface,
sometimes deeper, but always far removed from the cylinder, through several inter-
nodes, and then insert themselves on one of the main bundles, and with it enter the
cylinder. The general direction of their course is approximately perpendicular, or
arched according to the dome-like shape of the end of the stem : they are besides
curved in a sinuous manner to a very variable extent. It is doubtful whether the
anastomoses of the bundles of Ananassa, described by Unger®, are these cortical

■ Wossidio, QuEedam additamenta ad Palmarum anatomiam, Diss. Vratisl. i86a, and Nova
Acta Leop. Carol, vol. XXVIII.

“ Schacbt, Lehrbuch, I. p. 327. — Nägeli, l.c. p. 132. — Wossidio, l.c,

® Palmarum Structura, p. xviii; Verm. Schriften, pp. 155, 184.

* Mohl, Wossidio, l.c.

5 Dicotyledonenstamm, p. 5c, figs. 23. 24.— Anatomic und Physiologie, p. 232.

COURSE OF THE BUNDLES IN THE STEM. 267

bundles, or the above-mentioned isolated communications which occur within the
cylinder.

In most Scitamineje (Musaceae, Zingiberaceae, Cannaceae ’), as far as descriptions
go, the vascular system in the cylinder is of the Palm type ; but outside the cylinder
there is a system of complete cortical vascular bundles. According to Wittmack’s
description of Musa Ensete they are ‘ completely limited to the cortical layer, and
have a very sinuous, almost zigzag course, especially in the lower part. Owing to
their crowded arrangement and their frequent crossing, it was very rarely possible to
follow their traces far. But in favourable cases it appeared that they approach rather
near to the epidermis, and then proceed upwards parallel to the surface : but when-
ever they thus approach the base of a leaf, they curve inwards, and form anastomoses
with the chief vascular bundles (bundles of the trace), till finally they themselves
enter one of these leaf-bases, together with the large bundles which pass into it
from the interior of the stem. They here usually turn to the outer or inner wall of
the leaf-sheath, rarely they may be seen penetrating into the central regions, which
are traversed for the most part by the main bundles.’ Wittmack found the same
arrangement in all the nine species of Musa investigated by him, in Strelitzia reginae
(weak), in the rhizome of Curcuma Zedoaria, in the flower-stalk of Phrynium viola-
ceum, and Calathea grandiflora: Meneghini found it previously in species of
Ravenala, Hedychium, and Canna.

Sect. 68. As far as hitherto investigated, the single vascular bundle of the
cotyledon in the seedling of the Monocotyledons of the above types runs directly into
the axis of the main root, e. g. Allium Cepa^: or the cotyledon contains several
bundles, and these unite in the cotyledonary node, and then similarly pass into the
bundle of the root : e. g. Palms The bundles of the leaves which follow after the
cotyledons show the typical course, with such modifications as result from the small
number of bundles and the shortness of the internodes to be traversed. They unite
in the cotyledonary node with those of the cotyledon and of the root.

The elongated internode between the insertion of the scutellum and the first sheathing
leaf in the seedling of Zea Mais shows an abnormal arrangement. It contains a ringlike
mass of vascular tissue, which surrounds a wide pith : this ring is continued at the point
of insertion of the scutellum into the bundle of the first root. The annular mass originates
by the coalescence of the lower ends of the numerous bundles of the first foliage leaf-
traces with the trace of the first sheathing-leaf. The latter contains as a rule two bundles
situated right and left and in front of the median line. In the node they both bend in-
wards, and somewhat to the rear, and anastomose on reaching the node by means of a
curved connection ; they then spread out and pass down in the ring. A branch goes also
from the curved connecting bundle perpendicularly downwards. The traces of the foliage-
leaves consist of many bundles, and are as wide as the whole circumference of the stem :
the lower ends of the lowest pass beneath the node of the sheathing-leaf between and
alongside of those of the latter, and with them they form the ring. According to investi-
gations hitherto made — which require further completion — it appears that the individual

‘ Meneghini, l.c. — M’ittmack, Musa Ensete, Halle (Linnsea), 1867.

^ Sachs, Botan. Zeitg. 1863, Taf. III.

® Mohl, Palm. Struct, p. xliv. Tab. P. — Sachs, Botan. Zeitg. 1S62, Taf. IX. — Compare also the
data on this point in van Tieghem, S)mmetiie de Structure, &c., Ann. Sei. Nat. 5 ser. tom. XIII.

26«

PRIMARY ARRANGEMENT OF TISSUES.

bundles of the trace cannot still be distinguished in the ring. More rarely there are in
the sheathing leaf in addition to the two lateral bundles two other smaller bundles
placed symmetrically in the posterior half, near to the median line of the sheathing-leaf.
Thei;e was found in one case in the transverse section of the internode, in the centre of
the anterior side and outside the ring, a small isolated bundle, the origin and course of
which remained doubtful.

The terms anterior and posterior are in all cases to be understood here, so that the
side next the scutellum is the posterior.

In conclusion, a short connected description may here be given of the peculiarities of
the bundle-system in the Aroideae and Pandanaceae, so often alluded to above'.

A number of forms do not differ from the Palm type, except in the fact that in many
of them the bundles run for a long distance in the cortex before entering the cylinder.
A second category is distinguished from the first by the bundles being united on their
descending curved course within the cylinder, and at a considerable distance from its
surface. In transverse sections therefore there are found within the peripheral layer
‘ compound ’ bundles, i.e. such as are cut through at the points of coalescence or separa-
tion. Finally, in a third group the bundles are not only united on their entry into the
middle of the cylinder, but are connected by anastomoses in all directions, and in such a
way in specially good cases that, as in the Nymphaeaceae (p. 252), there is close beneath
the growing-point a complex network, which branches in all directions, and takes up the
leaf-traces as they enter, so that the typical bundle system can only be recognised by
slight indications.

To the first category belongs in the first place the rhizome of Acorus gramineus and
A. Calamus: the majority of the bundles descend obliquely, as in the above-mentioned
Cyperaceae, through several internodes in the thick cortex, and this, as is especially well
seen in A. gramineus, is traversed by bundles arranged in several rings in transverse sections.
Further a number of epiphytic forms with elongated internodes belong here : all the
Monsterinese investigated by v. Tieghem (species of Heteropsis, Monstera, Raphidophora,
and Scindapsus), with bundles of the trace, which sometimes enter the leaf at once at the
node, but for the most part traverse the cortex through two internodes before passing
into the leaf: further the investigated species of Anthurium and Pothos, in which also
cortical bundles are present, which vary in number and distribution according to the
species, with the exception of A. Miquelianum, which belongs to the simple Palm type.
The second category is connected with the above forms by the investigated species of the
genus Philodendron. As in the former, some of the bundles traverse the cortex through
(two) internodes before they pass out into the leaf. In Ph. micans all the bundles take
an individual course through the internodes, and are only united at the nodes, the joint
bundles then pass downwards, and end at the periphery of the cylinder. In other species
(Ph. Rudgeanum, hastatum, tripartitum) the points of coalescence are within the
cylinder in the internodes also, so that the transverse section often shows ‘ compound ’
bundles side by side with simple ones. According to v. Tieghem this arrangement is
found, subject to many modifications according to the elongation of the internodes, the
presence or absence of cortical bundles, &c., and specific peculiarities, in all investigated
Aroideae with moderately elongated internodes and unisexual flowers (species of Homa-
lonema, Aglaonema, Dieffenbachia, Syngonium, and others to be named below), and of
those with bisexual flowers in Calla palustris, Lasia ferox, and Spathiphyllum. To this
series belong also the stems of the Pandanese (Pandanus javanicus, pygmaeus). The thick,
cylindrical (Alocasia), oralmost tuberous stems, with short internodes, of other unisexual
Aroideae, Alocasia odorata, Colocasia antiquorum, Caladium esculentum, Dracunculus,
Arum, Richardia aethiopica, &c. belong to the third category, since their bundles are not

' See van Tieghem, 1. c.

COURSE OF THE BUNDLES IN THE STEM.

269

only frequently united within the stem, but are connected in a reticulate manner by
anastomoses. Many forms, such as Syngonium, are transitional between the second
group and the third. In small branches of rhizomes of Richardia sethiopica also the
typical curved course may be easily recognised, especially in the ends of the branches; in
Alocasia, Dracunculus, Caladium esculentum the system of vascular bundles forms ‘ a
complicated net, in which it is impossible, by the most careful dissection of a macerated
stem, to trace one bundle with certainty even for a short distance.’ Still even here it
may constantly be recognised, especially at the ape.x of the stem, how the bundles pass
from the base of the leaf with a curved course to the middle of the stem, and from thence
downwards and outwards.

Fic. 119. Fig. 120.

Figs. 119 and 120. — Tradescantia albiflora. Fig". 119. Outline and vascular bundle-system of the end of a stem cleared by
potash, and divested of part of the cortex and cylinder by longitudinal sections. The surface of section is turned towards the
observer. Those bundles which in this position run in a higher plane are drawn darkly, those in lower planes more lightly. 'I he
successive leaves are indicated by the figures 1—5 ; leaf 6 is just formed at the growing-point ; opposite the median bundle of r
is an axillary bud. The bundles of leaves i, 2, and 3 are visible ; y the separate, / the united portions. From 3 are to be seen
four marked y; two of these (right of the median line) run into the leaf; two others, median and anterior, are cut off at their
point of exit. From leaf 2 are three marked y; the middie one cut off. The lines s show the course of the cauline bundles.
Magnified about 25.

Fig. 120 (40).— Transverse section through a young internode. In the middle the four united bundles; externally eight
separate ones, the three principal ones marked d ; then the circle of ten cauline bundles.

IV. TyPE OF THE CoMMELINACE^.

Sect. 69. The bundles in the stem of those Commelinaceae which have been
investigated, and of many Potamogetons, have a course which differs from that of
most Monocotyledons, and resembles rather thqt of the Piperacese (p. 249) and
Mirabilis (Chap. XVI). This is seen particularly clearly in the plant widely known

270

PRIMARY ARRANGEMENT OF TISSUES.

in gardens as Tradescantia albiflora, and will be described first in that example
(Figs. 1 19 and 120).

From the sheath-like base of the alternate distichous leaves, which embraces the
stem, as a rule eight bundles curve into the node (i), and thence descend perpen-
dicularly to the next node (2). In the internode they are at about equal distances
laterally from one another, and at varying distances from the middle of the stem, but
at least ^ of the radius from it. Close above node (2) they curve towards one another
and the middle of the stem, and unite in the node itself in pairs to form four bundles.
Thcsejbur bundles are stronger than the original eight : they are arranged crosswise
near to the middle of the stem, and run perpendicularly downwards to the next node
(3) where each inserts itself on the point of junction of two which pass out at node (2).
Each internode accordingly shows in transverse section (Fig. 120) in the first place
twelve bundles, four inner ones, arranged crosswise, and around them an irregular
circle of eight weaker ones (d). Besides these twelve bundles of the leaf-trace there
are also usually 11-12 bundles (of subsequent development) which are arranged in a
circle outside the eight external bundles of the trace. This circle, together with the
rather small-celled parenchyma between the bundles, marks off the cortex from the
cylmder which contains the vascular bundles. Some of these bundles lie also further
inwards, between the eight outer bundles of the trace. These i r-12 bundles do not
pass out into the leaves, but run up into the youngest internode, passing almost per-
pendicularly through the internodes, curving slightly inwards at the nodes, and passing
near to the outgoing bundles of the trace. Irregular short transverse bundles connect
them in old nodes with one another, and with the bundles of the trace.

Deviations from the above numbers are often found, e.g. in the lower internodes
of lateral shoots, where there are often in all only 18-19 bundles visible in the
transverse section— e.g. 3-1-6 bundles of the trace, and 10 cauline bundles.

I found fundamentally the same arrangement both of the bundles of the trace
and of the cauline bundles in all the other Commelinacese investigated : Commelina
agraria Kth., C. procurrens Schl., Tradescantia zebrina, virginiana, Spironema fragrans,
Dithorisandi'a thyrsiflora, D. oxypetala, Maravelia zeylanica. But in all these the
number of bundles of each category is higher than in Trad, albiflora, especially in
the last-named six species with a Thick stem, and leaves with many bundles. The
arrangement of the separate and united bundles is accordingly more complicated and
requires further investigation. '

Poiamogeion natans (Fig. 121) häs alternating leaves in two rows: these are
ofien displaced from this arrangement (by torsion of the stem .?) : the leaf-trace
consists of three bundles, the width of the leaf-trace is about 180°. The three
bundles of the latter curve towards the middle of the stem, and pass separately down
one internode, the stronger median bundle being nearer the middle line than the two
lateral ones. In the next node they all three coalesce to a single bundle, which then
passes down to the second, node, and here inserts itself at the point of coalescence of
the next lower trace (rarely one of the lateral bundles continues a separate course up
to this point of insertion, Fig. 12 1, x). In the internode there appears accordingly
in the bluntly rectangular transverse section (Fig. 122) of the ‘cylinder’ which con-
tains the bundles, one large bundle at each end of the smaller diameter ; these are
opposite one another: one of these (i) is the united trace of the second higher leaf.

COURSE OF THE BUNDLES IN THE STEM.

2’]l

the other rather smaller one {2m) is the median bundle of the next higher leaf. At
each end of the longer diameter is a small bundle (2, 2): these are the lateral
bundles of the last-named leaf. On both sides of each lateral bundle, that is opposite
each corner of the rectangular transverse section, there is further a small cauline
bundle (j, s). The median bundles appear first, then the lateral ones, and the
cauline bundles much later. At an early stage irregular anastomoses appear in the
nodes between all of them, the small cortical bundles of sieve-tubes and fibres also
taking part in these (p. 232).

Among the other Potamogetons which have been investigated, P. perfoliatus
has fundamentally the same bundle-system. Also the rhizome of P. pectinatus

Fig. I2T.— Potamogeton natans {40). End of the stem, made transparent ; the outer layers of tissue removed in
the lower part by longitudinal sections; successive leaves and their belongings numbered in order, m median
bundle ; / lateral bundles of the leaf indicated by the figures ; / the united portions ; x a bundle of leaf 4 which runs
an exceptional distance separately. The median planes of the leaves i — 3 lie alternately right and left in the plane of
the paper (opposite the middle of 2 is the axillary bud belonging to it). Above 3 is a rotation, so that the median
plane of leaf 4 is at the front, while that of 5 is at the back. The median bundle of 5, $771, therefore runs deep down
into the preparation. The lateral bundles of and the cauline bundles are (as yet) not visible ; 0 = the stipiilar sheath
of the base of the leaf.

appears, according to an incomplete investigation, to resemble it. In other species
the structure resembles that described, but is simpler, and reduced in proportion to
the average decrease in size of the leaves.

P. lucens and P. gramineus have a leaf-trace consisting of a single bundle, which
does not divide into three bundles till its exit at the node into the leaf. Each passes

2/3

PRIMARY ARRANGEMENT OF TISSUES.

down, near to the middle of the stem, and close to that of the next lower leaf, through
one internode, and then unites with the latter in the node (comp. Fig. 124). In a
transverse section of the internode there are accordingly two bundles of the leaf-
trace, which are close to the centre in the diameter between the median lines of the
two rows of leaves. A small vertical cauline bundle appears at a later stage than the
leaf-traces and near to them, and this lies in the radial longitudinal plane at right
angles to the plane of the median lines of the leaves : in the nodes transverse and
oblique anastomoses appear at an early stage, as in P. natans.

P. densus shows fundamentally the same structure, with the striking difference
that each bundle of the leaf curves almost at right angles into the middle of the stem,
and inserts itself in the next lower node directly on the bundle which there passes

Fig. 122 (i4f). — Potainogeton natans. Axile body of the internode, containing the vascular bundles ; transverse
section. endodermis thickened on one side (with starch) ; outside this the lacunar cortical parenchyma (with much
starch) ; I air cavities. Explanation of the figures in the text. The groups of delicate tissue of the numbered areas
are the phloem portions ; the wide meshes in them are the sieve-tubes of the bundles ; the areas in wliich the figures
stand are their vascular portions (for the most part converted into cavities). Between tlie bundles is starchy
parenchyma, and sclerenchymatous fibres, with a narrow lumen, which appears as a dark point.

out, so that only one axile sympodium of the leaf-trace is present besides the
two cauline bundles. In the upright stems of P. pectinatus (Fig. 123), in P. pusillus
and Zanichellia palustris this axile sympodium is alone present, without the two
cauline bundles, to which fact we shall return later.

P, crispus shows a somewhat different arrangement, which will be described
below.

It is not improbable that this type of vascular bundle-system is allied to that of
Hydrocharis, Stratiotes, and their allies, but further investigations are wanted on this
point.

The transverse section of the stolons of Hydrocharis Morsus Ranae ’ shows four bundles

^ Rohrbach, Beitr. zur Kenntniss einiger Hydrocharideen, Abhandl. d. Naturf. Ges. z. Halle,
Bd. XII. p. 75.

COURSE OF THE BUNDLES IN THE STEM.

273

arranged crosswise, two larger and two smaller, the similar ones being opposite one
another. A few layers of cells below the epidermis a circle of eight to ten small
bundles is found, which run perpendicularly and separately through the internode.

In the short thick stem of Stratiotes abides^ the bundles — all of which descend
from the leaves — unite ‘after numerous anastomoses to one central bundle and eight

FiG. 123.— Potamogeton pectinatus ; end of the shoot (40). Thick median section, cleared with potasli,
parallel in the lower part to the median planes of the two rows of leaves ; higher up, from leaf 5 onwards,
this plane has rotated through almost 90®. Successive leaves numbered successively i — 6 ; v the opposite
sheathing part belonging to the correspondingly numbered leaf; s sqjtamzcla intravaginalis of leaf 3.

In the middle is plainly seen the vascular portion of the sympodium of vascular bundles ; the upper-
most bundle, which is clearly seen, goes to leaf 6 (which lies at the back of the section) ; that which goes
to 5 is cut transversely, since 5 was turned towards the observer, owing to the above-noted rotation. In
the axils of each of the leaves (up to 5) an axillary bud is plainly seen. That belonging to leaf 2 lies deep
in the section ; its parts are indicated by ky and the vascular bundle which enters it by + . That belonging
to leaf 3 is still small; but the development of the first vascular bundle which enters it is beginning in the
angle where the bundle 3-1- (which runs into leaf 3) unites in the node with tlj^t which comes from 4. In
the axil of leaf i is a strongly-developed axillary shoot ; «1 and 711. its scale leaves, l\ and ii its two first foliage
leaves ; the vascular bundle which enters li is just beginning to be developed. In the axil of «z a secondary
axillary shoot is appearing; its first vascular bundle is beginning atyC

or nine peripheral ones. In the thin stolons these bundles run perpendicularly through
the internodes.’

^ Rohrbach, /. c. p. 94.

T

274

PRIMARY ARRANGEMENT OF TISSUES.

V. Anomalous Monocotyledons.

Sect. 70. Under this term may be grouped some examples of vascular bundle-
systems, which differ fundamentally from that of the very great majority of Mono-
cotyledons. Some of these are found in certain water-plants, the rest in certain
Dioscoreas, the bundle-system of which approaches very closely to that of the Dico-
tyledons.

Potamogeton crispus, while it approaches very closely in other anatomical
properties to other members of the genus, is distinguished from them by the course
of the bundles in the stem. Comp. Figs. 124, 125.

Fig. 124. — Potamogeton crispus (40).
End of a shoot. Longitudinal section
parallel to the median planes of the two
rows of leaves, cleared by potash. The
successive leaves numbered i, 2 — 10;
7'i, V2 the sheaths of the corre-

sponding leaves; the sheaths of the
upper leaves were obscured in the pro-
cess of preparation, and are partly
omitted in the drawing. The median
vascular bundles of leaves 9 and 10 are
just beginning to develope ; the seven
highest leaves are still without bundles.

l 7n i

Fig. 125.— Potamogeton crispus (40). End of a shoot. Thick
median longitudinal section perpendicular to the median plane
of both rows of leaves, cleared by potash; the sheaths were
obscured by the process of preparation, and are omitted in the
drawing. The successive leaves are numbered in order. The
series of unevenly numbered leaves are uppermost, nearest
the observer; the even numbers are below. The same is the
case with the median bundles, which go to the respective series
and unite in the axile bundle The median bundle is plainly
seen as far as the sixth leaf from the apex ; the lateral bundles
united to form the two bundles /, /, beginning at the eleventh
leaf from the apex (5) in the node ; the development of 4 and 5
is not yet completed downwards through the internode. In the
internodes the air-cavities are first formed from below upwards,
and from the outside inwards.

In each of the sheathing leaves, which alternate in two rows, the bundles pass
out at the node. The median bundles run down through the internodes in the
manner described for P. lucens and gramineus (Fig. 124). The lateral ones (Fig.
125) pass on each side almost perpendicularly from a bundle which traverses the
stem perpendicularly, and corresponds exactly in position and relatively late appear-
ance to the cauline bundles of the other species, so that the arrangement of the

COURSE OF THE BUNDLES IN THE STEM.

V5

bundles in a transverse section of the internode is the same as in them. The lateral
bundles of the stem of P. crispus are however not cauline. The development of
their tracheae begins at the nodes, and proceeds from each of these towards the
middle of the next upper and lower internodes (Fig. 125, 4, 5).

The course of the three bundles in the stem of Zostera marina ^ is just the same
as the above : one axile bundle is built up sympodially from the median bundles of
the leaves, while two lateral ones, which lie in a plane cutting the median-plane of
the bi-seriate leaves at right angles, give off lateral bundles to the leaves ; these
however require further investigation. The arrangement of the bundles in the
transverse section of the internode of Zostera differs from that in Potamogeton,
since in the latter the lateral bundles are close to the central bundle, while in the
former they run at some distance from it, near to the surface of the stem.

In Cymodocea eequorea Koen.^ the median bundle of the seven bundles of the
trace runs obliquely down to the middle of the stem, it there passes perpendicularly
to the next node, where it joins the median bundle, which there passes out. The
transverse section of each internode thus shows a central bundle. Near to the
periphery of the stem 20-25 small bundles (in weak stems fewer) arranged in two
concentric circles pass perpendicularly through each internode. Each of these, ac-
cording to Bornet, divides at the node into two, of which one ascends into the next
higher internode, the other either curves out into the leaf, or unites either with a
neighbouring peripheral bundle, or with the axile bundle. Besides this a complex net-
w'ork of anastomosing bundles is formed in the node between the different bundles.
The peripheral ones appear to be cauline, but this requires to be further investigated.
As far as may be judged from transverse sections Cymodocea isoetifolia exactly re-
sembles other species.

After what has been said it need not be stated in detail how Hydrocharis and
Stratiotes might, as far as our present knowledge goes, belong equally well to this
as to the preceding section : this will have to be decided by further investigation.

Sect. 71. In the foliage-shoots of Tamus and Dioscorea Batatas the vascular
bundles are arranged according to the Dicotyledonous type, that is in a ring sur-
rounding the pith. It is true that here the bundles pass an unequal distance into
the pith, but this also occurs in typical Dicotyledons.

Nägeli (/. c. p. 1 2 3) gives the following description of Dioscorea Batatas. The
leaves are sometimes spirally arranged, sometimes in decussating pairs. The leaf-
trace consists of three bundles. When the arrangement is decussate (Figs. 126,
127) its width is about 120°. If their course in a tangential direction be first con-
sidered (Fig. 126), the six bundles of one pair of leaves pass nearly straight down
tw'o internodes, the lateral bundles (be, ef\ hi, Im ; op, rs ; uv, yz) pectinating at the
first internode w’ith the lateral ones which there enter. Above the bundles of the
second lower node the tw'O lateral bundles of one trace converge towards one
another, and insert themselves on lateral bundles of the next lower pair; but the
median bundle (a, d-, h, g ; t, .x) divides into two shanks which unite with the same

' Compare ^lagnus, Botan. Zeitg. 1872, p. 216.

Bornet, Recherches sur le Phucagrostis major, Atm. Sei. Nat. 5 ser. tom. I. Compare
especially p. 39, pi. 6. fig. i, and pi. ii, fig. i.

276

PRIMARY ARRANGEMENT OF TISSUES.

lateral bundles. The leaf-trace is here much contracted, and usually forms by
coalescence one single mass. The six bundles of one pair of leaves thus go through
only two internodes before they coalesce with lower ones, and the transverse section
through an internode shows twelve bundles (Fig. 127), of which six pass out at the
next node and six at the node above it. These twelve bundles would be arranged in
a circle if they had a radially perpendicular course. But this is not the case : they
penetrate further into the pith as they pass down lower. Their radially oblique course
is however restricted almost entirely to the nodes : the same bundle thus appears at
unequal distances from the centre in the two internodes. The lateral bundles, which
in their first internode are nearer the centre than the median ones, differ still more from
them in this respect in the next lower internode. The transverse section through an

V t V y a: X

Figs. 126, 127. — Dioscorea Batatas, with decussating pairs of leaves; after Nägeli. Fig. 126. Scheme of the
vascular bundle-system in the end of a shoot, the cylindrical surface being reduced to a single plane. Fig. 127.

Transverse section through an internode, the same bundles being indicated by the same letters as at the bottom
of Fig. 27 ; further explanation in the text.

internode therefore shows four bundles which project further inwards and form a
rectangle, — these are the lateral bundles of the next higher pair of leaves, — and eight
outer bundles.

On the longitudinal course where the leaves in Dioscorea are arranged spirally,
and on Tamus, compare Nägeli, l.c.

The course of the bundles in Monocotyledonous stems has recently been the
subject of extended researches by Falkenberg h The chief result of that work which

' P. Falkenberg, Vergl. Untersuchungen über den Bau der Vegetationsorgane der Monocotyle-
donen, .Stuttg. 1876.

COURSE OF THE BUNDLES IN THE STEM.

277

concerns us here was briefly stated in 1874’ : it is the discovery of a new form of
bundle system in the foliage- or flowering-stems of Lilium, Tulipa, Fritillaria, Cepha-
lanthera, Epipactis, and Hedychium. The course of the bundles is such that the
bundles of the leaf-trace penetrate downwards, and for different distances inwards to-
wards the middle of the stem, and then affix themselves on corresponding bundles
of lower leaves, withoiii having previously curved outwards. For the rest we may here
refer to Falkenberg’s comprehensive work, and print off the above paragraphs with-
out alteration as they were written down about four years ago, since they coincide in
the main with his work.

VI. Phanerogams with an Axile Bundle.

Sect. 72. A number of plants with reduced foliage and roots living in water or
marshes, and some in damp humus, belonging partly to the Monocotyledons, partly
to the Dicotyledons, show the bundle-system of the stem united into one bundle,
which is surrounded by a thick cortex, and traverses the middle of the stem longi-
tudinally : from it bundles pass at the nodes into the leaves. With this simplicity of
course there is usually connected a considerable simplification in the structure of
the bundles, which always shows peculiarities ; we shall return to them in Sects. 105
and no.

As regards the coarser structure, and especially the relations to the bundles
of the leaf-trace, which it may be said require more exact study in many cases,
there may be distinguished two chief forms. Firstly, axile bundles, formed or
developed sympodially from weak bundles of the trace, which approach closely to
one another, and coalesce longitudinally : thus they do not differ in their first origin
from the typical bundle-systems of the Phanerogams : secondly, such as are cauline,
and grow acropetally with the end of the stem ; the bundles which run to the leaves
are given off from the former at the nodes, or apply themselves to them as branches :
thirdly, those cases in which the axile bundle is built up of longitudinally coalescent
bundles of the leaf-trace together with cauline bundles are connected with the two
former cases as intermediate forms.

To the first category belong the following Dicotyledons: Bulliarda aquatica ac-
cording to Caspary’s account^, Hottonia, Elatine Hydropiper, hexandra, also E.
Alsinastrum and probably Trapa natans : of Monocotyledons, Potamogeton pectina-
tus and pusillus, to which may be added Zanichellia^ and Althenia®; also Ruppia® and
its allies. To. the second category the following Dicotyledons belong: Aldrovandia

* Botan. Zeitg. 1874, p. 732.

' Schriften d. Physical, oconom. Gesell, zu Königsberg, Bd. I. i860.

^ [Comp. Friedrich Müller, Struktur einiger Arten von Elatine, Flora, 1877, p. 481.]

* Schleiden, Beitr. p. 215. — Caspary, Pringsheim’s Jahrb. pp. 383, 440.

* Prillieux, Ann. Sei. Nat, 5 ser. torn. II.

® Compare Irmisch, lieber einige Arten d. Familie d. Potameen (Abhandl. d. Naturwiss. Vereins
f. Sachsen und Thüringen, 1858), p. 44.

’ Caspary, Botan. Zeitg. 1859, P- ’^6, Taf. V. Ibid. 1862, p. 193.

278

PRIMARY ARRANGEMENT OF TISSUES.

Hippuris', Callitriche^ Myriophyllum ^ Ceratophylliim probably Utricularia, and
the non-aquatic genus of Piperacese Verhuellia® : of Monocotyledons, the Hydrillese,
Elodea canadensis, and Hydrilla verticillata “ ; Najas’’’, and the rhizomes of the root-
less Orchids which grow in humus, Epipogon Gmelini, and Corallorrhiza innata In
Corallorrhiza and the stolons of Epipogon a branch passes from the axile bundle
into each of the biseriate scale-leaves : in the coral-like rhizome of Epipogon, which
has short internodes, the branches which pass to the leaves are absent, according to
Reinke.

According to the structure and development of the bundle the following plants
may be placed in the third intermediate category ; perhaps Myriophyllum, Hippuris,
and Elatine Alsinastrum : further the larger Potamogetons belong to this series :
their original arrangement of cauline bundles and bundles of the trace has been
described above on p. 272. In the series of described' species of this genus, and of
allied forms such as Zannichellia and Althenia, with which Elodea, Najas, &c. are
also connected, there are to be found all stages of simplification of composition (and
of structure) of the axile bundle : leaf-traces consisting of one or more bundles,
running side by side with cauline bundles through the internodes, are found in the
stronger, more leafy forms, at one end of the series, while at the other a single cauline
bundle is present, which unites at the node with the bundles from the leaves.

VII. Fern-like Plants.

Sect. 73. In the young seedlings of all the investigated forms of this series the
bundle-system of the stem is a sympodium of leaf-traces consisting of one bundle
(which with the exception of Equisetum developes in an acropetal direction). The
first bundle, which usually ends blind in the foot of the embryo, curves after a very
short course through the stem into the first leaf; from the point of curvature the
development of a bundle, which runs out into the second leaf, begins. In the case
of the subsequent leaves the same conditions prevail.

In Isoetes, Equisetum, and Osmundacese, this construction of the bundle-system
out of distinct leaf-traces is permanent even in the mature stem. The same holds
perhaps for many Ferns with a simple axile bundle. In the Lycopodiums, and
Selaginellas, the axile bundle which traverses the stem, or the tw'o or more bundles

' Nageli, Beitr. l.c. p. 56. — Sanio, Botan. Zeitg. 1865, p. 191.

^ Nägeli, /. c. — Hegelmaier, Monogr. d. Gattung Callitriche. Idem in Martins, Flora Brasil,
fasc. 67.

® Vöchting, Zur Histologie u. Entwicklungsgeschichte v. Myriophyllum, Acta Acad. Leopold.
XXXVI (1872).

* Schleiden, Beitr. p. 216. — Unger, Anatom, und Physiol. 198. — Sanio, Botan. Zeitg. 1865,
p. 192.

® Schmitz, Flora, 1872.

•' Caspary, in Pringsheim’s Jahrb. I. Idem, Verhancll. d. Naturforscher u. Aerzte z. Königs-
berg, i85o.

’’ Compare Magnus, Beitr. z. Kenntniss d. Gattg. Najas, p. 48.

® Irmisch, Beitr. z. Morphologie und Biologie d. Orchideen. — Schacht, Pdanzenzelle, p. 268.
Idem, I.ehrbuch, II. p. 21. — Reinke, in Flora, 1873.

COURSE OF THE BUNDLES IN THE STEM.

279

of many Selaginellas, may, as regards development, be considered as a cauline
bundle, the corners of which are composed of the sympodially united leaf-traces of a
single bundle. On the other hand, the Lycopodiaceous plant, Psilotum triquetrum,
has only a cauline bundle without leaf-traces. Also in the case of Marsilia and Pilularia
a similar view may be held, in common with Nageli, according to the development of
the bundles. In the majority of the Ferns there is an obvious connection between
the form and arrangement of the leaves, and of the bundles which enter them ; in a
number of cases, especially in those forms, to be described later, with a reticulate
stem-system, and one bundle for each leaf, the stem-system may be recognised as
being composed of the constituent leaf-traces h But in very many cases such a
separation cannot be carried out according to our present knowledge without arbi-
trary treatment, but rather a bundle-system of varying form and complication may be
distinguished in the stem, from which bundles for the leaves are given off at certain
points. The following description must accommodate itself to the facts : in each
case those stem-systems, which may arbitrarily be recognised as being composed of
leaf-traces, will be associated with those with which they correspond most closely in
their real structure.

There may accordingly be distinguished on the one hand the types of Equi-
setum, Osmunda, Isoetes, on the other the various series of types of the Ferns, which
are connected by numerous intermediate forms : under the latter the Lycopodiums
and Selaginellas may be ranged as peculiar instances, and are here co-ordinated
merely for synoptical reasons.

Sect. 74. Equisetum^. The weak bundles of the stem are arranged in a ring
separating the pith and cortex. From the median line of each tooth of the leaf-
sheath one bundle enters the stem, it here passes perpendicularly down one inter-
node, and then divides, at the next lower node, into two short shanks, each of which
affixes itself on the nearest lateral bundle whieh here passes out. Where the
number of teeth of successive sheaths is the same the bundles of successive inter-
nodes alternate as they do.

Sect. 75. Osmundacese Comp. Figs. 128-130. The mature rhizome of
Osmunda regalis has leaf-insertions arranged with a divergence of and short
internodes. Its centre is occupied by an irregular bluntly five-cornered prism, with
a thickness of about 6“™ in strong specimens : this consists of a vascular- bundle-
cylinder (Ring), a narrow sheath of delicate-celled parenchyma surrounding the
latter, and a parenchymatous pith surrounded by the ring of bundles, and with brown
sclerenchymatous cells scattered through it. The prism is enclosed by a cortex 2-5™rr
in thickness, which is dark-brown and sclerotic, but contains much starch : this
is traversed by vascular bundles, also surrounded by a thin sheath of delicate
parenchyma, on their oblique upward course from the ring into the leaves (Fig. 128).

’ See Holle, Botan. Zeitg. 1875, P- 265, &c.

“ Nägeli, Zeitsch. f. wiss. Bot. 3. p. 143; Beitr. l.c. p. 57. — Cramer, in Nägeli und Cramer,
Pflanzenphys. Unters. Heft 3, p. 21. — Hofmeister, Vergl. Unters, p. 93. — Duval jouve, Hist. Nat. des
Equisetuin de France, 1864.

* Giippert, Flora, 1848, Taf. IV. A. — Unger, Denkschr. d. Wiener Academic, Math.-Naturwiss.
Classe, Bd. VI (1853). — Milde, Monogr. Osmunda;, p. 32.

PRIMARY ARRANGEMENT OF TISSUES.

280

Fig. 128.

Fig. 129.

Figs. 128 and 129. — Osmunda regalis. Fig. 128. Transverse
section through a strong stem, seen from above, i. e. from the
apex of the stem ; magnified about twice, t lowest bundle of
leaf-trace, from which a root-bundle passes through the cortex.

Fig. 129. — Sketch of the bundle-ring in the former figure,
more strongly magnified, i. Bundle of the lowest trace cut
through just at its point of entry into the ring, and with one of
the two root-bundles, which are here attached to it. The figures
1 — 13 indicate the bundles of the trace of the thirteen successive
leaves, which are visible in the transverse section; No. 10 is
abnormally united with 2 Compare Fig. 130.

One bundle enters each leaf : the arrangement of the bundles in the cylinder is quite

similar to that of the Coniferse with alter-
nating leaves with a single bundle (Fig.
130). From one leaf« one bundle enters
the cylinder, and runs almost perpendi-
cularly downwards, as a rule through 13 in-
ternodes, and then, when close to the leaf
«-13 perpendicularly below it, it curves
towards the ascending side of the bundle
belonging to the leaf «-8, and unites with
it. In the cases investigated the insertion
and coalescence took place occasionally
even after a shorter course, e. g. in the
case of the bundle 10 in Fig. 130, eight
internodes below the point of exit. The
bundles are strongest at their point of exit from the cylinder,
and are of horseshoe shape. In the petiole they retain
this form, or are at least half-moon-shaped. In the cylinder
of the stem, as they pass downwards, they decrease in thick-
ness at first gradually, and then quickly, and assume a
wedge-shaped transverse section. Here they are separated
from one another by narrow bands of parenchyma (medul-
lary rays). The structure of the whole transverse section
(Fig. 128) may be deduced from the above description. —
The bundles, which pass into the first leaves of the seedling,
unite to an axile bundle without any pith : this gradually
extends into the ring of bundles surrounding the pith.

In Todea africana and T. hymenophylloides are seen
phenomena exactly similar to those in Osmunda, which need
not be described in detail here.

Sect. 76. While thus the bundle-system of these plants,
and of the Equiseta, may well be ranged under the type of
Dicotyledons, and is specially allied to that of the Conifers
(juniperus, Widdringtonia ^), the species of Isoetes have in
their extremely shortened tuberous stem an axile bundle
without pith, as is the rule in submerged water-plants : this
bundle is built up sympodially by the coalescence of the
inner ends of the one-bundled leaf-traces. Phylloglossum
may also belong to this category ^

Sect. 77. Psilotum and Lycopodium. The leafy stem
of Psilotum triquetrum® has one vascular bundle with 2-8
corners which project more or less from the surface. It is
cauline throughout, the small leaves have no vascular

Fig. 130. — Osmunda regalis.
Scheme of the vascular system in
the stem, the cylindrical surface
being reduced to a single plane ;
arrangement of the leaves s/13.
The bundles of the leaves num-
bered in genetic series at their
point of exit ; each has two roots
inserted close to the point of exit,
and indicated by short transverse
strokes. On 2 and 10 is to be
seen the anomaly observed in the
specimen upon which the scheme
is founded, viz. that 10 inserts
itself on 2 close to the point of
exit of the latter, instead of con-
tinuing its course to 2 — 5.

^ Compare above, p. 246, and Geyler, l.c., especially Taf. IV.

Compare Mettenius, Botan. Zeitg. 1867, p. 98. ^ Nageli, Beitr. l.c. p. 52.

COURSE OF THE BUNDLES IN THE STEM.

281

bundles. Slill, according to Nageli there is a relation between the corners of the
bundle and the insertions of the leaves. ‘ At some distance (about 3-8>nm) per-
pendicularly below each leaf one corner of the bundle projects very strongly, and
gradually loses itself below, but rather more quickly above. The corners of the
bundle are therefore the more numerous in a portion of a stem, as the vertical rows
are more numerous of the otherwise irregularly arranged leaves, which can only
with difficulty be referred to a cyclic arrangement.’

The leaves of the Lycopodia ^ are arranged, according to the species and indi-
vidual, in alternating whorls of two or more members, or spirally with a divergence of
f, jj, &c. Each contains one thin vascular bundle. The stem is traversed by
one strong, almost cylindrical axile bundle, in which the symmetrically distributed
bands of tracheides, to be described in Sect. 107, form external protrusions, which,
like the bundles passing into the leaves, and the above-mentioned corners in Psilotum,
consist of narrow spiral tracheides, and may like them be briefly termed corners.
The bundles of the leaves insert themselves (when followed from the base of the
leaf) after a curved downward course through the cortex, on the corners of the
axile bundle. At the beginning of the differentiation of tissues, there is at first a
bundle of spiral tracheides at the corner, which forms a direct continuation of that
which passes into the leaf : it passes down through some internodes, and then
inserts itself on the point of curvature of one which passes out lower down. It is
only later that the more internal masses of larger tracheides are developed.

From these facts, and according to the phenomena of development, the axile
cylinder may be characterised as a cauline bundle, on the corners of which the
sympodially united bundles of the leaf-trace are directly inserted. The same facts,
however, admit equally well of our speaking of a polyarch (Sect. 107) axile bundle,
which gives off branches from its corners into the leaves.

The origination of the bundle which passes into the leaf follows very soon after the
protrusion of the young leaf itself. The development of each bundle of spiral tracheides
begins where the bundle inserts itself upon the point of curvature of a lower one, and
proceeds towards the apex of the leaf in question : then from the point of curvature it
proceeds again in the same direction to a higher leaf. This happens very rapidly, at
least in the stem itself, so that only in favourable cases (in L. alpinum) could Hegelmaier
find a bundle of spiral tracheides, of which the portion passing through the cortex to the
leaf was not already equally de\ eloped with the lower part, which passes down the
corner; and Cramer ascribed to L. Selago a simultaneous development of the whole
bundle from its lower point of insertion to the apex of the leaf — while Hegelmaier found
a basipetal direction of development in the leaf itself.

The corners of the axile bundle, as also the rows of leaves, differ greatly in number in
different species, individuals, and shoots of different rank of one individual, and correspond
to one another as a rule neither in number nor arrangement in one and the same shoot,
while apparently the correspondence is less close the greater the number of both. It is
true Hegelmaier found a correspondence of both in 75 per cent, of the last branchings of
L. alpinum which are covered by four rows of leaves in decussating pairs, and in about
60 per cent, of the branches of L. complanatum. But in most cases the number of

* Nägeli, Zeitschr. f. wiss. Bot. Heft 3 and 4, p. 132. — Cramer, in Nageli unci Cramer, Beitr.
Heft 3. — Hegelmaier, Botan. Zeitg. 1872, p. 78g, &c. — Sachs, Textbook, 2nd Eng. Ed. p. 468.

2^2

PRIMARY ARRANGEMENT OF TISSUES.

corners is smaller than that of the rows of leaves : in L. Selago, e.g. with whorls of five
members (that is ten rows), there are 4-6, in L. inundatum with f arrangement of the
leaves 4 or 5, in the above-mentioned branches of L. alpinum 3, &c. From Hegelmaier’s
statement that in the main vegetative axes of L. clavatum and L. annotinum, with a diver-
gence of I, there were found 10-17 corners, it would appear that there is a higher

number of corners than of rows of leaves.

Where the rows of leaves correspond exactly to the corners, all the bundles of one row
of leaves insert themselves on the same corner. In other cases one corner may take up
bundles from one row only, but must also often take them from more than one row.
Usually it only takes up the bundles of two neighbouring rows, but sometimes also single
bundles of more distant rows^. The bundles insert their inner end irregularly, some-
times on the right, sometimes on the left, and sometimes on the inner side of the next
lower bundle.

Sect. 78. Selaginella. A number of species, forming no doubt the majority,
such as S. Martensii, S. helvetica, pubescens, rupestris, &c., have in each shoot one
axile, ribband- or plate-like vascular bundle, the faces of which in relation to the
ground are directed upwards and downwards, while the margins are lateral to the
right and left : in some, as S. pubescens, the bundle is provided at the middle line of

its under surface, and near to each
lateral margin with a sharp band-like
process. The leaves have each one
small bundle, and these behave in
their course and insertion one on
another, and also on the cauline por-
tion, similarly to the leaf-bundles of
Lycopodium. They are inserted on
the bundle at its margins : in the
species with two double rows of
leaves, one facing towards each side,
the bundles of the two corresponding
rows (that is those from one row of
upper and one of lower leaves) insert
themselves on each lateral margin : in
S. rupestris with leaves in many rows,
the bundles of several rows pass to
each margin. S. Kraussiana,Galeottii^
and most other articulatae® have, in-
stead of one axile bundle, two which
run near the middle line, each follow-
ing one double row of leaves : each
takes up the leaf-bundles on its own
side, viz. those from one upper and one
lower row of leaves, at its outer margin,
with the same arrangement, as in the first-mentioned series of cases.

‘ Compare Cramer, /. r. p. 14, Taf. 30, 31.

2 Nageli, Beitr. I.c. p. 53. — Hofmeister, Vergl. Unters.

^ A. Braun, Monatsbr. d. Berliner Academie, 27 Apr. 1865.

COURSE OF THE BUNDLES IN THE STEM.

283

Other species of the genus show a different arrangement of the bundles in the stem,
such as ‘ 2 median, 3 median, 3 forming a triangle, or numerous scattered bundles h’
S. insequalifolia shows three median ones (Fig. 131). S. Lyallii has in its strong main
shoots, which emerge above the ground, ten or twelve bundles distributed in the trans-
verse section in three parallel equidistant rows in an almost quadratic surface; where
the number is ten they are so arranged that three roundish ones form two opposite
sides of the quadrangle, while one transversely extended one occupies each of the two
other sides, and two other roundish ones lie in the middle of the quadrangle and
form, with the two transversely extended ones, a row of four parallel to the rows of
three. Other transverse sections of the same shoot show in place of one of the
transversely extended bundles two contiguous round ones, which are doubtless products
of its division. The course of the bundles and the insertion of the bundles of the
leaves have not as yet been investigated in those shoots which have other than one
axile bundle or two lateral ones. In S. spinulosa, which has homomorphous leaves
in many rows, there is a single axile bundle of roundish transverse section (and with
a structure differing from that of other species) : the leaf-bundles insert themselves
on it on all sides.

Filices axd Hydropteride.e^

Sect. 79. It has been already stated that there is always in the seedling of
these plants one axile bundle composed of the single, acropetally developed bundles
of the leaves. In many cases each lateral shoot begins with one such bundle.

In a number of forms this structure is persistent in the mature stem. In the
large majority the bundle extends itself, and forms a tube, which surrounds a paren-
chymatous cylinder of pith, and is enclosed in a parenchymatous cortex. At the
insertion of each leaf the tube has a gap, the foliar gap, from the margin of w'hich
the bundles start, which go into the leaf ; at other points it is closed, or reticulately
perforated. Of this simple hmdle-lube — or ring of bundles as it appears in transverse
section — several special forms may be distinguished.

In relatively few cases there are in addition to the simple tube accessory, me-
dullary, and cortical bundles, or there appear several concentric tubes or rings.

a. Axile bundle and simple bimdle-iube.

Sect. 80. One axile bundle, from which a branch goes to each leaf, traverses,
as in submerged Phanerogams, the floating stem of Salvinia and Azolla. It occurs
also in the rhizomes of Pilularia minuta, exceptionally also in weak plants of P.
globulifera^ in the investigated stems of species of Hymenophyllum Gleichenia,

* Compare A. Braun, /. c.

Mohl, Structura cauclicis filicum arborearum, &c. in Martius, Ic. Plantar, crypt. Brasil. Tab.
29-36 — Verm. Schriften, p. 108. — Hofmeister, Beitr. zur Kenntniss d. Gefasskryptogamen, II. —
Abhandl. d. K. Sachs Gesellsch. d. Wissenschaften, V. p. 602. — Stenzel, Ueber d. Bau u. d. Wachs-
thum d. Farne, Nova Acta Acad. Leopold. Bd. 28. — Mettenius, Ueber den Ban von Angiopteris,
Abhandl. d. K. .Sachs Gesellsch. d. Wissensch. IX. p. 500. — Trecul, Ann. Sei. Nat. 5 sdr. tom. X.
p. 3^4, and tom. XII. p. 218.

® Russow, Vergl. Unters, p. 13.

* Mettenius, Hymenophyllere, l.c. (compare above, p. 126);

284

PRIMARY ARRANGEMENT OF TISSUES.

Lygodiuin\ and also of Schizaea, and the leafless stolons of Nephrolepis. The
bundle has usually a circular transverse section, in Salvinia rotundifolia it is horse-
shoe-shaped.

Sect. 8i. In many Ferns the original axile bundle widens out as the stem grows
stronger into a tube, which is for the most part closed all round, and has only at
each node, below the insertion of the leaf, a relatively small slit or foliar gap, through
which the medullary parenchyma is connected with the cortex, and from the margin
of which one or several bundles pass into the leaf. To this series belong for the
most part forms with a thin creeping rhizome, and leaves alternating in two rows :
the investigated species of Marsilia, normal specimens of Pilularia globulifera ^ with
a very small foliar gap, from the lower margin of which a foliar bundle arises. Most
species of Dennstaediia (D. tenera, scandens, davallioides, punctilobula) have a tube,
which is closed as far as the foliar gap ; the bundle which enters the leaf arises from
the whole margin of the gap as a continuous concave plate, which is only occasion-
ally split up at its base for a certain distance into several bundles lying side by side.
‘ The same structure is found in all species of Microlepia and Hypolepis, in the
species of Phegopteris, which are allied to .the latter genus, and in the species of
Pteris of the section P. vespertilio, aurita;’ further in Polypodium Wallichii, and
conjugatum, to the bundle-tube of which attention was first drawn by R. Brown,
and in which a bundle passes into the leaf from each side of the narrow slit-like
foliar gap Of the Hymenophyllaceae, Loxsoma has a closed tubular bundle ^ the
foliar gaps of which have not been described. Of the Schizseacese ® the species of
Schizaea may perhaps be mentioned here : but for reasons which will be given below
(Sect. io6) Russow has correctly placed them in our previous category: otherwise
they have been as yet but little investigated. Among the Ophioglossaceae the above
described structure occurs in the rhizome of Botrychium Lunaria®. Hofmeister’ found
in Ophioglossum vulgatum the network of bundles of the rhizome, which belongs
to the next category, sometimes coalescent for a certain distance to form a closed
tube.

Sect. 82. Most Ferns with an ascending or upright rhizome or stem, with leaves
in many rows, and but slightly elongated internodes, are distinguished fundamentally
from the type just described in this point only, viz. that the foliar gaps are relatively
large, and the bands of the bundle-tube, which separate them, are relatively narrow.
The tube has accordingly the form of a Net, the meshes of which are the foliar gaps.
From the margins of the meshes branch the foliar bundles, which there run obliquely
upwards through the cortex to the insertion of the leaf. The bundles of the meshes
of the stem are, according to the species, relatively narrow, of round or elliptical
transverse section, or, as in stems of the Cyatheaceae, broad, or band-shaped
plates, with their margins often curved outwards: the bundles which pass into the
leaf show the same varieties of form : their number for each leaf is constant within
narrow limits in the mature plant of each species, but varies in different species

^ Russow, /. c. ^ Russow, l.c. ® Mettenius, Angiopteris, p. 544,

* Mettenius, Hymenophyllacese, p. 418.

® [Comp. Prantl, Morphologie d. Gefässkryptogamen, Heft 2, Leipzig, 1881.]

® Russow, /. c. p. 117, &c. r Beitr. III. p. 664.

COURSE OF THE BUNDLES IN THE STEM.

285

from one to very high figures. Where several bundles pass out, they often anastomose
with one another in a reticulate manner immediately after leaving the foliar gap :
this is especially the case in the Cyatheacese. From the various combinations of
these different relations result the most various individual forms of the net, and of the
grouping of the bundles in the transverse section of the stem : a cylindrical pith is
always surrounded by them.

To this type belong numerous Polypodiacese, a number of the Cyatheacese, of
the Schizaeaceae, Aneimia, of Ophioglossacese, Ophioglossum (O. vulgatum, O. pe-
dunculosum). Peculiarities may be subsequently described in a few examples.

The seedling of Aspidium Filix mas begins with leaves arranged with l divergence : their
solitary bundles are united sympodially in the stem to one axile bundle. Above the
5th-6th leaf the stem increases greatly in thickness, the ^ arrangement passes over to
and from the point of outward curvature of the bundle of the highest leaf of the ^
arrangement the formation of the reticulate bundle-tube begins. Each leaf receives
one bundle from the lower angle of the rhombic mesh or foliar gap, upon which its base
is seated : or, in other words, two bundles run into each leaf, arising from the point of
exit of those which pass into the two next lateral older leaves : these converge acutely
towards their own point of exit, and are there united into a single bundle. By the repe-
tition of this formation the network of rhombic meshes is built up. Where the arrange-
ment is ^ there pass up to leaf 9 one bundle from 6 and one from 7, to leaf 7 one from
4 and one from 5, &c. In the second year the plant becomes much stronger, the leaf
arrangement passes over to which divergence is retained in the mature plant, or
passes over into

Each leaf now receives several bundles from the margin of its foliar gap, at first five, in
mature and strong stocks seven : one arising from the lower angle, and six from the sides
of the mesh ; of the latter two weaker ones on each side, belonging to the lower half of the
mesh, and one stronger one belonging to the upper half^ The structure of the meshes
is the same with /g- arrangement as with f ; at the lower angle of each, where the
median bundle passes into the leaf, two bundles which descend from the centre of the
two next lateral older leaves come into contact — that from the one side following the
parastichies composed of every third leaf, that from the other the parastichies composed
of every fifth leaf (comp. Fig. 132). The transverse section of the mature stem thus cuts
eight vascular bundles (where the arrangement is 10-12 bundles), which form a circle
round a wide pith : outside this in the cortex are seen the bundles which pass obliquely
into the base of the leaf, arranged in different number and order according to the position
of the section. The vascular bundles of the stem are weak compared to the mass of
the parenchyma, in transverse section they are roundish or flattened externally and
internally (Fig. 133).

According to the numerous investigations of Hofmeister, Stenzel, and Mettenius,
fundamentally the same structure— even the narrow, reticulate bundles, which are weak
in arborescent stems — is found in Onoclea, Struthiopteris, in all investigated species
of Blechnum (inch Lomaria), Woodwardia, Asplenium, Phegopteris, species of Aspidium
with a stem having leaves in more than two rows, in Ophioglossum, and Aneimia. The
individual peculiarities depend partly upon the form of the meshes corresponding to the
elongation of the internodes — thus very elongated meshes are found in the runner-like
branches of the rhizome of Struthiopteris, Aspidium cristatum, in the creeping stems of
A. Thelypteris, quite short and broad ones in Aspl. Filix foemina — partly upon the
number and arrangement of the leaf-bundles which arise from the margin of a gap. Most

' A. Braun. .Schuppen d. Tannenzapfen, Nova .Acta Leop. vol. XV. p. 278.
^ Hofmeister, Beiträge II. — Stenzel, l.c.

286

PRIMARY ARRANGEMENT OF TISSUES.

of the investigated species of Aspidium have according to Stenzel’s account three or five
bundles for each leaf, one median, one from the lower angle, the others arising in pairs
from the sides of the mesh : in Aspidium Thelypteris the median one is absent, according to
Stenzel (Tab. V. i8), one bundle passing into the leaf from each side at the middle of the
very elongated mesh. Blechnum Spicant (Stenzel, Tab. II. 5) has two lateral ones, one
arising on each side, close to the lower angle of the mesh, Bl. brasiliense however has seven,
one median and three pairs of lateral ones, arising from the lower half of the mesh. In
Asplenium Filix fcemina, the mature rhizome of Struthiopteris^, in Aneimia, and Ophio-
glossum the same arrangement is found, which appears only in the young plant of Filix mas,
viz. each leaf receives only one median bundle from the lower angle of its gap. It is
remarkable, according to Stenzel’s account {l.c. Tab. II. 3), that in the scale leaves of the
elongated runners of Struthiopteris the median bundle is absent, and in its place a bundle
runs on each side from the middle of the long mesh.

Among the Cyatheaceae, Dicksonia (Balantium) antarctica, Karsteniana, Cibotium
Schiedei, glaucescens, Plagiogyria biserrata, Alsophilia pruinata, blechnoides have — in
contradistinction to their nearest allies to be described below — the same structure as is
now under discussion. The appearance of the transverse section of most of these plants,
which differs so strikingly from that of the Polypodiaceae, depends partly upon the form

Fig. 132. — Aspidium Filix mas ; natural size, /'"slightly magnified ; D end of the stem,
the leaves of which are cut off, excepting the highest ones ; b transverse section of a
petiole ; "w roots ; E a similar end of a stem, the network of bundles exposed by paring
off the cortex (.^) ; F mesh of the net, with insertions of the foliar bundles ; C base of the
petiole, with a lateral bud Ä, longitudinal section ; w root. From Sachs’ Textbook.

Fig. 133. — Aspidium Filix mas ;
transverse section through a strong
stem, with 8/21 leaf-arrangement ;
natural size.

of the vascular bundles in the stem itself, these being broad plates, usually curved out-
wards at the margin, with narrow foliar gaps ; partly upon the massive dark brown bands
of sclerenchyma surrounding these bundles; partly upon the large number of thin
foliar bundles, or the presence of one or a few broad channel-shaped ones; finally upon
the very oblique ascent of the foliar bundles through the cortex, and the frequent anasto-
moses in this part of their course between those belonging to one leaf (comp. Fig. 141,
p. 293). If the internodes are short the transverse section shows a circle surrounding
the pith, composed of bundles elongated in the direction of the periphery, or curved out-
wards in a horseshoe-like manner, and with brown sheaths : between these are medullary
rays of unequal breadth, according as the section has passed through foliar gaps at vary-
ing height ; outside the ring of bundles are those bundles which are passing to the leaves ;
where the internodes are elongated and the foliar gaps are of relatively small size, the
transverse section may show one closed annular bundle, which is only broken here and

* Ilofmeisler, l.c.

^ Mettenius, Angiopteris, p. 524.

COURSE OF THE BUNDLES IN THE STEM. 2,8 J

there (by a foliar gap), and opposite these points are foliar bundles in the cortex, such as
Karsten * illustrated in the case of Alsophila pruinata.

Sect. 83. If the structure just described for stems with leaves in several rows be
imagined to be transferred to horizontally growing stems with leaves alternating in
two rows, there are thus obtained foliar gaps in two alternating rows right and left.

FIG. 134- — Davallia dissec-
ta ; rhizon:e ; slightly mag-
nified. ^ vascular bundle-
system, the cylindrical sur-
face reduced to a single
plane; o upper bundle, u
lower bundle, d point of in-
sertion of a leaf, at X point
of origin of a lateral shoot ;
ß transverse section. After
Mettenius.

Fig. 135. — Aspidium coriaceum ;
rhizome; slightly magnified; after
Mettenius. yt bundle-system, the
cylindrical surface reduced to a
single plane; meaning of the let-
ters as in Fig. 134. ß transverse
section.

0

Fig. 136. — Polypodium fraxinifolium ; rhizome ;
slightly magnified ; after Mettenius. A bundle-
system, the cylindrical surface reduced to a
single plane ; o upper bundle, d insertion of a leaf,
X points of origin of the lateral shoots, ß trans-
verse section.

limited by one bundle with a median upper course, and by one median one below, and
by alternating transverse bundles between the two. This arrangement is found in
many forms as described, or with unimportant modifications I Comp. Figs. 134, 135.

* Vegetationsorg. d. Palmen, Taf. IX. fig. i.

* Mettenius, Angiopteris, p. 544. Special deviations and irregularities described by Trecul, /. c.

288

PRIMARY ARRANGEMENT OF TISSUES.

Their creeping rhizome shows a circle of bundles in transverse section. Of
these, one passing along the middle of the upper side (the upper bundle., o Figs. 134,
135) and a second passing similarly along the under side (the lower bundle, u) are
distinguished by their band-like form and greater size from the other weak ones, which
are opposite to the two rows of leaves. Both the stronger bundles are connected
regularly, at distances corresponding to those separating the leaves, by transverse
bundles curved convexly upwards, or bent at an angle, so as to form a net, the
meshes of which are the foliar gaps. From the margins of these arise (in addition
to the bundles for the lateral shoots x') the foliar bundles {F) which converge opposite
the point of insertion of the leaf, which is usually relatively small, and run almost
radially perpendicular in the stem up to that point : these bundles may anastomose
one with another, and with the upper and lower bundles by solitary thin transverse
connections. The transverse sections of the foliar bundles are the small bundles
seen in the transverse section of the stem ; they there form together with the
upper and lower bundles either a circle, as above stated, or in flattened stems
an elliptical figure, which is often compressed in such a way that the upper and
lower bundles have a central position, while the foliar bundles are outside.

The arrangement described occurs in a simple form, with specific modifications
as regards the number of the foliar bundles, the form of the gaps, strength of the
bundles, &c., in Asplenium obtusifolium, A. resectum, Acrostichum brevipes, A.
Lingua, A. simplex, A. Melanopus, Polypodium altescandens, P. tenellum, Nephro-
lepis ramosa, Aspidium albopunctatum, A. coriaceum (Fig. 135). In the Davallias
there arises the further complication, that the branches springing from the margin of
the foliar gap do not run directly or with unimportant anastomoses to the leaf, but
first form a network of fine bundles, which stretches over the foliar gap, and sends
a certain number of branches into the leaf. According to the number of these foliar
bundles (which varies according to the species) the network is simpler (D. parvula,
pedata, heterophylla), or more complicated, and with more numerous meshes (D.
bullata, dissecta (Fig. 134), elegans, pyxidata, canariensis, &c.).

A more considerable deviation from the structure described appears in other
creeping stems of Ferns with leaves in two rows : here, not only is the foliar gap
covered in by a network of bundles, but also instead of the lower bundle tv/o or
more reticulately anastomosing bundles are present, and the lower bundle is as it
were split up into a network of bundles (Fig. 136). Where the number and arrange-
ment of the bundles are very simple, as e. g. in Polypodium aurisetum, piloselloides,
cayennense, or where the upper and lower bundles and their transverse connections
bordering on the foliar gaps are strongly distinguished from the rest by their size, as
in Platycerium alcicorne, the structure may be referred easily to the scheme with
upper and lower bundles. But often the upper and lower bundles and all the anas-
tomoses are of such uniform strength, and the meshes of different order so irregular,
that the foliar gaps can only be distinguished at both sides of the upper bundle,
where the bundles pass out into the leaves. In place of the tube regularly per-
forated by foliar gaps there is in extreme cases as it were a complex irregular net-
work, the relations of which to the more simple type can only be recognised
as indicated by the regularly alternating ‘ foliar meshes ’ b. As extreme examples
may be named. Polypodium vulgare, sporadocarpum, aureum ; numerous species of

COURSE OF THE BUNDLES IN THE STEM.

289

Polypodium and Acrostichum axillare show numerous intermediate forms between
these and the simple scheme with upper and lower bundles. For further details
cf. Mettenius, Angiopteris, p. 552, &c., Taf. VII-X.

b. Several concentric rings of bundles.

Sect. 84. A number of Fern-stems with leaves in many rows — species of Pteris,
and Saccoloma, Marattiaceae, Ceratopteris — show in the transverse section of the
stem several concentric rings of bundles, similar to one another in form and thick-
ness. As far as is known, these cases are connected with those forms above described,
in which the bundles running from the ring to a leaf pass gradually, and for a long
distance obliquely upwards through the cortex, and are connected by anastomoses
both one with another, and also with those belonging to neighbouring leaves. The
middle of the stem is traversed by one axile bundle, or in most cases by a relatively
narrow tube of bundles, surrounding a narrow pith.

From these inmost bundles there arise at regular distances, and in close relation
to the arrangement of the leaves, flattened or narrow bundles, which branch out at
once into broad reticulate layers : these do not pass out directly into a leaf close to
their point of origin, but run upwards and towards the surface of the stem through a
number of internodes, and finally pass out into leaves, or divide into branches, which
pass out in succession. Each of these layers of bundles has the form of a portion
of the surface of a cone, which widens upwards : each is surrounded by a layer of
similar form (and a zone of parenchymatous cortex separating it from the latter),
and arises at its lower end from the inmost bundles. At the points of insertion of the
leaves there are anastomoses between the successive zones, i. e. between those which
are passing out, and the next inner ones, which run further. The rings which appear
in the transverse section are the transverse sections of the conical zones: their
number in any given transverse section depends upon their special course, particularly
upon the inclination of the conical surfaces, which is closely connected with the
elongation of the internodes.

The simplest case occurs in Pteris elata var. Karsteniana, and P. podophylla Sw.’;
also in P. Orizabse, and P. gigantea^. In the two first-named species there is, according
to Mettenius, a second narrow bundle-tube within an outer one, the former being split
sometimes on one, sometimes on two sides. Portions of the latter curve directly out-
wards into the leaves; portions of the first, turning outwards, enter the gaps formed by
the passing out of a portion of the outer ring, and coalesce with the outer tube,
which rises laterally from the base of the leaf.

In Saccoloma inaequale there is a similar arrangement (Mettenius). Saccoloma adian-
toides® shows in transverse section (Fig. 137) at least three closed or split rings repre-
senting so many conical zones, of which the outermost alone gives off broad and flattened
concave portions into the leaves (closely arranged with 57 divergence) ; the middle one,
curving outwards, enters the gaps thus formed ; finally the inner one fills up in the same
way the gaps made in the middle one by this outward curvature.

^ Mettenius, Angiopteris, p. 535, Taf. VI. 12-16.

^ Karsten, Vegetationsorg. d. Palmen, l.c. p. 193.

^ Mettenius, l.c. p. 531, Taf VI. — Karsten, l.c. p. 194 (Dicksonia Lindeni).

u

PRIMARY ARRANGEMENT OF TISSUES.

ago

The inmost of these zones differs in individual cases : in some, investigated by Mette-
nius, it is composed of two flat small bundles, which appear curved in transverse section ;
in others, on which Karsten’s statements appear to be based, there is a solid cylindrical
bundle. Karsten says of this that it ends freely in the pith below : this does not coincide
with the data given above, chiefly after Mettenius, and requires further examination.

Of the Marattiaceae with several rings of bundles as seen in transverse section, the

cylindrical stem of the species of Danaea with on
an average three rings in transverse section, may
according to the notes of Mettenius^ belong to
this category. Of the three rings the outer con-
sists of numerous filamentous bundles, the two
others of broad and flat ones. These stems how-
ever require more exact investigation. The same
may be said of the inverted conical-shaped thick
tuberous stem of Angiopteris evecta^, of which
Mettenius has thoroughly investigated and de-
scribed one large specimen only. Transverse
sections of the stem show 5-6 irregular zones or
rings which pass over one into another. From
an irregular network of bundles surrounding the
pith bundles arise in a succession correspond-
ing to the arrangement of the leaves, and pass
obliquely upwards and outwards; each of these
soon becomes wider, and splits into a rather ir-
regular network rising obliquely outwards and up-
wards, and having the form of a portion of a conical
surface, the bundles of which are reticulately con-
nected with those of neighbouring equivalent por-
tions. A number of branches, derived from that zone which is at the time the outer-
most, enter the base of each leaf (into the back and sides of its basal part), and,
to take the place of those which have passed out, a corresponding portion of the
next inner zone passes upwards into the outward zone from below the axils of the
leaf in question, and of the two next lower lateral leaves. Portions of the third zone
enter the gaps thus formed in the second, and are reticulately connected with the next
outer zone. Further anastomoses between the branches of the successive zones are
formed at the very point of insertion of the leaf, and two bundles belonging to the inner
side of the base of the leaf are given off from the second zone, which enters the gaps of
the outer zone. In the stems investigated the lower zones had small, almost cylindrical
bundles, which formed wide-meshed irregular nets; the upper zones (near to the
stationary end of the dead stem) had broad flattened bundles with narrow reticular
meshes ; the zones of intermediate position were also intermediate as regards the reticular
form ; the bundles which enter the leaves were of similar form to those of the zones from
which they arose. Accordingly transverse sections showed at different heights either
several concentric annular zones, often irregularly connected by oblique bands (the
portions cut through in their course into outer zones), consisting of small roundish
bundles separated by abundant parenchyma — corresponding to the usual arrangement in
the tuberous stems of Marattiaceae; or on the other hand rings, of which the outer at
least consist of broad flattened pieces, separated by some few bands of parenchyma. For
further peculiarities, comp. Mettenius, l.c.

c

Fig. 137. — Saccoloma adiantoides; transverse sec-
tion througit the stem; after Mettenius; natural size.
At a the band-like vascular bundle of a leaf, just
separated from the outer ring; b and c bundles of two
successively higher leaves, appearing as protrusions of
the outermost ring. The bundles which enter the leaves
are finely undulated. The bundles of all rings are sur-
rounded by a sheath of darker, thicker-walled paren-
chyma. In my specimens the inmost ring of bundles is
sometimes narrow, sometimes it is replaced by a single
round bundle.

’ l.c. p. 524. — Compare also Brongniart, Archives du Museum d’Hist. Nat. tom. I. p. 439, Tab.
XXXIII, and Karsten, Vegetationsorg. d. Palmen, Taf. IX. fig. 10.

Brongniart, /. c.

COURSE OF THE BUNDLES IN THE STEM. 2i)\

The investigation of a young stem of Angiopteris show'ed me a completely typical bundle-
tube, interrupted by wide foliar gaps ; two strong foliar bundles arise below at the lateral
margins of the gap, and ascend obliquely through the cortex, within which they divide
into the branches which pass out into the leaf.

The concentric zones of thin bundles in the stem of Ceratopteris thalictroides may
also belong to this category, but require further investigation. Comp. Mettenius, 1. c.

р. 530-

с. Accessory medullary and corlical bundles in addition to the simple tube of bundles.

Sect. 85. Among the Cyatheacece, according to Mettenius, the above-mentioned
forms (p. 286) have only the typical tube, perforated by foliar gaps, and consisting
of flattened vascular bundles, the margins of which next the gaps are curved out-
wards. Other species, including the majority in the genera Cyathea and Alsophila,
have, in addition to the vascular bundles thus disposed, small bundles, which originate
from the foliar gaps and traverse pith and cortex, there forming a delicate open
network. The relatively thin bundles from the margin of the foliar gap, which pass
into the petiole, arrange themselves so that in a transverse section of its insertion,
or in the leaf-scar, they are arranged in a curve, convex downwards, and simple, or
with the ends turned inwards above : it consists of few bundles in small leaves, e. g.
in young specimens of Hemitelia capensis 13-14 bundles, of Alsophila radens Klf. 4,
Cyathea arborea Sm. 13'; on the other hand, in strong specimens or species they
form two curves with the ends curved inwards,
and consisting of many bundles, one of these
curves being convex downwards, corresponding
to the lower edge of the foliar gap and spring-
ing from it, the other convex upwards, and
corresponding to the upper margin. The in-
curved ends of both curves are directed down-
wards and towards the middle of the leaf-scar
so that their bundles form on each side two
nearly parallel series running to the middle of
the scar^ Compare Fig. 138.

Further, in the space surrounded by the single curve or by the upper one, a
relatively small number of bundles pass out into the petiole — e. g. as described by
Mettenius in Hemitelia capensis, Alsophila radens, and Cyathea arborea two each, in
a species of Cyathea 7, in Alsophila Haenkei 4, in Cyathea Imrayana 2 or 4, in C.
ebenina 2. These do not arise from the margin of the foliar gap, but are connected
both inside and outside it with the margin itself, as well as one with another, by
numerous strong anastomoses (usually sheathed with sclerenchyma) : they then run
through the foliar gap downwards into the pith. (Figs. 139, 140, mi)

Immediately after their entry they pass with a steep curve inwards and down-
wards, and divide into branches diverging acutely downwards; these sometimes

‘ Mettenius, Angiopteris, 1. c. Taf. V.

“ Mohl,in Martius, leones, /. c., Verm. Schriften, p. no. — Numerous valuable details in Trecul,
l.c. XII. p. 270.

FIG. 138. — Cyathea Imrayana; natural size. Two
old leaf-scars from a dead stem ; a lower, b higher on
the stem; a with four, b with two medullary bundles
above m.

U 2

PRIMARY ARRANGEMENT OF TISSUES.

293

pass on further in the middle, sometimes at the periphery of the pith, and some
insert themselves at an acute angle on similar branches from lower leaves, others end
blind. At the foliar gap the bundles, which are themselves about as thick as a
bristle, are surrounded by brown sclerenchyma, or supported by it on one side, and
these sheaths of sclerenchyma, which are closed or open on one side, accompany the
bundles for a long distance downwards; sometimes they also anastomose with

Fig. 139. — Cyatliea Imrayana;
natural size. From a dead stem, the
soft parts of which had rotted away ;
the stem is halved longitudinally,
and seen front within. The figure
shows a piece of the tube of vascu-
lar bundles with one foliar gap, /— /,
and a leaf-scar exactly halved longi-
tudinally. From this the bundles of
sclerenchyma which accompany the
vascular bundles may be seen to
branch off, and from there those
which accompany the medullary
bundles arise as branches and pass
down into the pith with many
branchings and anastomoses; sonie
of them have blind pointed endings ;
the one which extends furthest
downwards has anastomosed with
one coming from a lower leaf. Of the
bundles which end at the leaf-scar,
7n originates from the pith, the rest
from the margin of the foliar gap.

s a s' s' a

Fig. 140.— Cyatliea Imrayana; axile longitudinal section through
the same stem as Figs. 141 and 142; natural size. The section is about
3mm. thick, and for the most part transparent; the black bands of
sclerenchyma and pale vascular bundles here represented in one plane
do not all lie exactly in this plane, but are all near it. Certain parts of
the chief sclerenchymatous-sheath s — s' which show through, and at
the bottom two portions of the surface of the stem seen obliquely, are
shaded off as they pass from the surface of section. The letters a, s,
s', /"have the same meaning as in Fig. 141 ; r cortical bundles, b leaf-
scars, bl vascular bundles passing out into leaves, "W insei tions of roots,
7N a foliar bundle running into the pith ; above X blind ending of a
medullary bundle (examined under the microscope).

similar sheaths, which descend from leaves side by side with or below them, some-
times they diminish downwards and end blindly in a point, while the vascular bundles
continue their downward course alone beyond these endings (comp. Fig. 140),

COURSE OF THE BUNDLES IN THE STEM.

293

According as they follow the first or the second course, the sclerenchymatous sheaths
themselves form either a tough net traversing the whole pith, as e. g. in the case of
a stem which is before me under the name of Cyathea ebenina, or they pass from
each leaf inwards and downwards as a sheaf of bundles, with blind and pointed ends,
which show frequent anastomoses one with another, but only fewer or quite solitary
anastomoses with the sheaves of bundles belonging to other leaves : e. g. Cyathea
arborea, Hemitelia capensis (Mettenius),

C. Imrayana, and most other species ;
in Alsophila microphylla and villosa the
vascular bundles in the pith are only
accompanied by isolated spindle-shaped
bundles of sclerenchyma, which are not
connected into sheaths till the foliar gap
is reached (Mettenius). In most of the
dried stems, when subjected to investi-
gation, the delicate unsheathed parts of
the vascular bundles cannot usually be
seen, the tough sclerenchymatous bands
alone being clearly preserved. Since, as
above stated, the course of these bands
is a copy of that of the vascular bundles,
the description given for the latter will
suffice also for them.

IMany but not all species have,
besides the medullary bundles, acces-
sory cortical bundles also. In C. Im-
rayana (Fig. 142) these arise from
bundles which pass into the leaves, and
close above their point of departure
from the foliar gap, and in fact from
most, but not from all those which
branch off from the lateral and lower margin of the gap. They descend with
a steep curve from their point of origin into the parenchyma of the cortex ;
some of them, after pursuing an individual course for a short distance, unite
each with another coming from the same foliar gap ; most of them however pass
almost straight downwards in the middle of the cortex, and in the neighbourhood
of the adjoining lower foliar gaps they either affix themselves at an acute angle on
bundles which there arise, or they end blindly. The cortex is accordingly tra-
versed by a network of bundles with elongated meshes, which are sometimes
quite closed, sometimes open on one side. In the stems in question the cortical
bundles, of the thickness of a bristle, usually have no sclerenchymatous sheath; some
few, especially those which arise from the upper part of the lateral margin of the
gap, are however often accompanied for about from their point of origin by
such a sheath, in the form of a channel, which is open outwards ; comp. Fig. 142. In
a dry strong stem, bearing the name C. Imrayana (not the same used for the accom-
panying figures), a structure similar to that just described can still be clearly seen.

/

Fig. 141.— Cyatliea Inn'ayana: transverse section through
the living stem ; natural size ; seen from above. At ö, c, d,
foliar gaps. All quite black bands and points are transverse
sections of sclerenchyma; all the paler ones are vascular
bundles. In and near the foliar gaps, especially a and^, are
root-bundles going to the periphery; / channels at the base
of the leaf ; a vascular bundles of the main tube ; s outer,
y inner plates of its sclerenchymatous sheath. Inwards
from s* the pith with its bundles ; outwards from s the
cortex with its bundles.

294 PRIMARY ARRANGEMENT OF TISSUES.

But in this case there is this further peculiarity, that from the upper part of each
lateral margin of a foliar gap two or several bundles arise, which unite after a short
course into one, and that these bundles from their point of origin onwards are sur-
rounded by a thick sheath of sclerenchyma. Together with the latter they form on
each side of the foliar gap a cone several millimetres thick at the point of origin ;
they either taper to a point towards the adjoining lower gaps, and end blindly in the
parench} ma, or coalesce with the margin of the adjoining lower gaps, and with a
cone which there arises. The sclerenchymatous sheath of the cone shows slits here

and there, through which unsheathed
branch bundles emerge and turn down-
wards. Cones, fundamentally similar
to those described, the points of which
end blindly in the cortex, and indi-
cate with certainty the presence of a
system of cortical bundles similar to
that in C. Imrayana, were first found
by Mettenius ^ in a dry stem of Also-
phila Haenkei. In other species a
system of cortical bundles is un-
known, partly because of the difficulty
of finding it in dry stems subjected
to investigation ; in many species
however (for instance Cyathea arborea
and Alsophila microphylla may be
named with certainty) it is altogether
absent.

Sect. 86. Most species of Denn-
sicr.dlia have, as above indicated, a
simple tube of bundles which is closed
with the exception of the narrow foliar
gaps. Within this, and near the upper
side in the horizontal stem, there are
in the pith of D. rubigihosa one, in D.
cornuta several small bundles with
circular transverse section, in D. cornuta these form a tube alternately closed and
again split into 2-3 bundles. At the base of a shoot the medullary bundles arise
from the inner surface of the tube, at the foliar gap they approach the latter, and
divide into a few branches, of which some anastomose with the margin of the gap,
others enter the leaf with those which start from the gap, the third series (or single
bundle) ascending further in the shoot as medullary bundles ^

The somewhat more complicated arrangement in Chrysodium vulgare, on
which compare Mettenius, /. c., may be placed in connection with the above.

Fig. 142.— Cyathea Imrayana; piece of a living stem with
four bases of petioles, the outer layers of cortex being peeled
off; seen from without. The margins of four foliar gaps, the
bundles which arise from them and pass into the leaves, witli
the young roots (black) seated upon them, and the bundles
which descend through the cortex, are exposed ; the latter
and the roots are quite free, the rest covered by a little trans-
parent parenchyma, through which they are clearly seen, and
all the parts are held together in their natural position. Natural
size.

' Angiopteris p. 528, Taf. V. A good figure of a transverse section of a Fern stem with cortical
bundles is given by C. H. Schultz, Mem. present, de I’Acad. des Sciences, tom. VII (1841), pi. 22.
Mettenius, Angiopteris, p. 540.

COURSE OF THE BUNDLES IN THE STEM.

295

Sect. 87. While in the last-named cases accessory medullary bundles occur, and
in many Cyatheacese accessory medullary and cortical bundles in addition to a
typical tube of bundles, there is found in Pteris aquilina and Polybotrya Meyeriana a
bundle-tube constructed as in the type with a strong upper bundle, and this is
strengthened by a much divided cortical system of bundles.

In the seedling of Pteris aquilina^, till the development of the seventh or ninth
leaf, there is one axile bundle, which traverses the stem starting from the point of
union of the first leaf wdth the first root : in transverse section it is deeply grooved,
and half-moon-shaped : bundles pass from it into the leaves ; after the formation of
the seventh to ninth leaf ‘ the stem forks.’ Both branches of the fork increase rapidly
and considerably in thickness, and the course of the
vascular bundles in them is altered. The lateral
opening of the axile bundle is widened, then the
upper half is separated from the lower ; there are
now two bundles, an upper and a lower one, which
split now and again into thinner branches, while
these are soon again united. When the branches
of the fork have attained the length of about
and a thickness of about 4““, weaker branches
come off from both the bundles : these run near
the surface (in the cortex), and here form a peri-
pheral network with long and narrow meshes, in
which the upper central bundle is distinguished
from the rest by its greater wddlh. This structure
is retained by the mature rhizome (Fig. 143) : the
number of the peripheral bundles rises to twelve
in the transverse section. Two thick brown plates
of sclerenchymatous fibres lie between the inner and
outer systems of bundles, and are only separated
from one another at the two sides of the stem by
a narrow slit filled with parenchyma : they are often
joined at one side, often even all round so as to form a closed tube. Branch-
bundles from both nets of bundles pass into the leaves and branches ; roots arise
from the outer ones only. At those points of exit, as also at the base of the petiole,
the two nets anastomose by single transverse bundles. Throughout the whole of the
rest of their course they are without connection with one another in many weak
specimens ; in strong rhizomes, according to Stenzcl, thin connecting bundles pass
from the margins of the inner bundles to the lateral outer bundles, while the upper
and lower bundles of both systems are connected by single short branches, which
pass through holes in the bands of sclerenchyma.

In the main axis of Polybotrya Meyeriana*^ an inner network is found sur-
rounding a narrow pith, and composed of 3-7 strong bundles arranged in a
circle when seen in transverse section ; it corresponds in composition to that above

Fig, 143.— Pteris aquilina; stem (rhizome);
slightly enlarged. A transverse section, r
brown sclerenchymatous layer of outer cortex.
p colourless soft parenchyma, ig vascular
bundles of the inner zone, ag the broad upper
bundle of the outer zone, pr the plates of
brown sclerenchyma separating the two zones.
B the upper main bundle of the outer zone of
the stem (si) and its branches (si', si"), with
the branch bundles which pass into a leaf (b)
prepared free ; « — it outline of the stem.

’ Hofmeister, l.c. p. 620. — Mettenius, Angiopteris, p. 561. — Stenzel, Lc.
“ M ettenius, l.c. p. 559, Taf. YII.

296 PRIMARY ARRANGEMENT OF TISSUES',

described for the Polypodiacese with a reticulately divided lower bundle, having long
narrow, rather irregular meshes : foliar meshes and points of origin of lateral shoots
appear only in regular alternation, on both sides of a clearly distinguishable upper
bundle. Outside this net is a peripheral one, showing in transverse section fifteen
to about fifty bundles, which rarely form a single circle, but are usually arranged at
the upper side of the stem in 2-3 irregular series, at the lower side of the stem in a
curve. The bundles of the peripheral net are thin and connected in elongated
meshes, in the arrangement of which there was as little regularity to be recognised as
in that of the obliquely ascending connecting-bundles, by which the outer net is
joined with the inner at many points. Three branches arising from a single mesh
of the inner net and 9-12 peripheral ones enter each lateral shoot ; into the base of
the leaf there pass two or several inner, and 9-24 outer ones. All roots arise from
the outer net.

c. Bundle system in the leaves and foliar expansions.

Sf.ct. 88. The bundles, which leave the bundle-cylinder of the stem and pass
out into the leaves, run as a rule towards the margin and apex of the leaf.

The bundles may run from their point of departure from the cylinder of the
stem onwards, just as they did in the latter, as for instance in a leaf of a Conifer, where
one bundle passes out from the cylinder and runs without division up to the apex of
the leaf ; or divisions of the bundles and coalescences of separate ones may occur at
any point, so that the number to be seen in the transverse section of the leaf is not
the same as at the point of exit itself. Examples of this, when it takes place in the node
itself, have been given above. Sect. 61 ; in the case of their further course the pheno-
menon is universally known, and will be treated in detail in the following paragraphs.

In most cases the bundles at the nodes pass, without division, or after splitting
into branches which run side by side, through the cortex into the leaf. But in certain
individual cases there appear special branches at the node itself, which lie in the
parenchyma of the cortex ; these are here connected into a net or a transverse
girdle ; often branch bundles pass downwards from the node into the cortex of
the adjoining internode.

Branches pectdiar to the node (which do not pass out from it) are often found
where several bundles emerge, forming oblique or curved connections between these.
This is the case both in many traces with numerous bundles, belonging to solitary
(alternating) leaves, e. g. Lathyrus Aphaca (comp. p. 240), where the median bundle
has a curved transverse connection with the lateral ones ; Viola elatior, Platanus,
which will be mentioned below : also in whorls of two or more with leaves having
one or more bundles. In the case of leaves with one bundle Hanstein^ found a
transverse girdle connecting the bundles at the node in numerous Rubiacese with
whorls of two or more members (species of Asperula, Rubia, Galium, Hamelia
chvysantha, Houstonia coccinea, Bouvardia mollis) ; on the other hand, other Ru-
biaceae (Coprosma ligustrina, Exostemma floribundum according to Hanstein) show
no transverse girdle. In whorled leaves with several bundles transverse girdles have

'■ Lieber gürtelförmige Gefässstrangverbinclungen, Abhandl. d. Berliner Academie, 1857, ?• 77-

BUNDLE-SYSTEM IN THE LEAVES.

297

been above described (pp. 257-259) in Calycanthus and Melastomacese. Hanstein
found them in Sambucus, in species of Valeriana, Centranthus, Valerianella ; Scabiosa,
Knautia, Succisa, Dipsacus ; Dahlia, Bidens cernua, and tripartita ; Guizotia oleifera ;
and Nageli in Ilumulus. In most plants with opposite leaves, e. g. the Labiatae,
Asclepiadese, Caryophyllacese, Caprifoliaceae excepting Sambucus, many Compositae
and others enumerated by Hanstein, the transverse girdles do not occur.

Branch bundles passing down through the cortex may here be mentioned, and
have already been described above for many Cyatheaceae. Also in the strict sense
the medullary bundles of these latter Ferns belong to this category, as being ‘ appen-
dices ’ which run downwards from the bases of the leaves. Among the Monocoty-
ledons no examples belonging strictly to this series are known, still the bundles
running through the cortex, as described above for Bromeliaceae and Palms, correspond
in their course to some extent to those under discussion. The same may be said of
the above-described cortical bundles of Melastomaceae and Calycanthus. Among
the Dicotyledons, however, branches passing downwards from the node through the
cortex occur elsewhere ; in the first place in the foliaceous corners of the so-called
winged stems, e. g. in species of Lathyrus (L. silvestris, latifolius, Nissolia, &c.),
secondly, and in the most striking manner, in many (but by no means all) succulent
plants : Salicornia, species of Mesembryanthemum, Cacteae. The course and
ramification of the bundles in these cases and in the winged corners closely resemble
that to be described in the leafy expansions, the cortex here assumes completely the
anatomical (and physiological) properties of foliar expansions.

In the species of Salicornia ' the short scaly leaves are arranged in decussate
pairs. One bundle, which splits up immediately at its point of exit into three branches,
passes from the node into each leaf : of these a single median one runs to the apex of
the leaf-scale, and a lateral one on either side passes perpendicularly downwards into
the cortex. These branches, of which there are six for each pair of leaves, give off
throughout their whole course numerous ramifications, which anastomose frequently in
a reticulate manner. From the apices of the leaves downwards the cortex of the whole
internode, which attains a length of is traversed by a tubular network of bundles,
which is closed (not interrupted as stated by Duval), and which ceases immediately
above the next lower node, without any connection with that which there arises. In
many species of IMesembryanthemum, e. g. M. imbricatum, M. crystallinum, &c., but
by no means in all species of the genus, thin branch bundles, divided into reticulately
anastomosing branches, pass downwards from the node into the cortex ; here again
they do not reach the next lower node. In Cacteae, Epiphyllum truncatum, species
of Cereus, Mamillaria, the above-mentioned (p. 261) Rhipsalidaceae ^ &c., reticu-
lately connected branches come off from the main bundles, which run to the apex
of the rudimentary leaves, and pass through the cortex, forming a continuously
anastomosing network between the neighbouring main bundles.

The bundles, which enter stipules., and other appendages of the base of the leaf
which often have a glandular surface, usually arise as branches from those which
enter the main leaf (e. g. Prunus, Passiflora, Tropteolum, Medicago, Liriodendron

* Duv.al-Jouve, l.c. (see above, p. 226).

® Nageli, Beitr. /. c.

Vöchting, l.c.

298

PRIMARY ARRANGEMENT OF TISSUES.

Coprosma ligustrina, Exostemma floribundum Quercus ; from the anastomoses
at the node ; or from the transverse curves, e. g. in those Rubiacese which have them,
as is seen in an especially striking way in the large leafy stipules of the Stellatse,
and in Sambucus Ebulus (Fig. 44), &c.

In less common cases special lateral bundles of the composite leaf-trace, which
traverses the stem, pass into the stipules In Viola elatior one lateral bundle of
the three forming the leaf-trace passes into each stipule, and in addition branches
from curved transverse anastomoses, which appear in the node between the bundles

of one trace. In Platanus occidentalis the
sheathing stipule has 7-9 bundles at its
base; of these the two stronger lateral ones
arise from the outermost lateral bundles of
the leaf-trace (which has 7-9 bundles), the
rest unite to two, sometimes to one or
three bundles, which separately enter the
bundle-ring of the stem. In Humulus
Lupulus®, of the three bundles of each
leaf-trace which enter the stem, the median
one passes into the petiole, and each lateral
one into a stipule as its middle nerve.
Each lateral one is connected at the node
by a transverse bundle on the one hand
with its median bundle, on the other with
the lateral one of another (opposite) leaf ;
from the transverse girdle thus formed the
lateral nerves of the stipules are given off
in regular succession, so that the girdle
itself is compounded in a sympodial manner of outwardly curving bundles.

Sect. 89. The bundles run as a rule straight through the petiole towards the
lamina. When they are numerous they are here also branched and connected by
anastomoses. The bundles, when more than one, are arranged on the transverse
section either in a curve open upwards, or in a ring, or distributed over the whole
surface of transverse section. Large leaves, e. g. of Leguminosse, Umbelliferse,
Palmse, Aroideae, Cycadeas, Ferns, &c., yield various special examples of these
relations, which have been often made use of in the Ferns for systematic
purposes *.

Sect. 90. In the lamina of leaves, whatever their form, in the peripheral leaf-
members of any stem, and in the leaf-like branches (Phyllodes, Phyllocladia, &c.).

* Hanstein, l.c. ^ A. B. Frank, Botan. Zeitg. 1864, p. 378.

“ Nageli, l.c. pp. 75, 92, 114.

Compare e.g. Grew, Anatomy, Tab. 49. — Presl, Gefässbündelvertheilnng im Stipe's der Farne,
Abhandl. d. k. Böhm. Ges. d. Wissensch. 5. Folge, Bd. V. — Reichardt, in Denkschr. d. Wiener
Academic, Bd. XVIL— Duval-Jouve, Et. sur le petiole des Foiigeres, Hagenau, 1S56. — Trecul, Ann.
Sei. Nat. S ser. X and XII, and the descriptive literature of Ferns. — Reichardt, lieber d. centr.
Gefässbündclsystem einiger Umbelliferen, Wiener Acad. .Sitzungsberichte, 1856. — A. B. Frank,
Botan. Zeitg. 1864, p. 380. [De Candolle, Anatomie comparee des Feuilles, Geneva, 1879.]

FIG. 144. — Sambucus Ebulus, after Hanstein. Sclienie of
the bundle system in two successive internodes : the cylindrical
surface reduced to a single plane. Leaves in decussate pairs ;
each leaf receives one median bundle. Ä, and on either side
two lateral ones, s', s" \ of these the inner stronger one, s',
descends undivided into the internode ; the outer one, s",
divides in the node into two shanks ; one inner one which
pursues a solitary course downwards, and an outer which
coalesces at once with the similar one of the opposite leaf (c^).
By this union in the node the transverse girdle is formed, from
which the bundles n pass into the stipules. The further course
of the leaf-trace of each pair, which consists of twelve bundles,
is obvious on study of the figure, From Sachs’ Textbook.

BUNDLE-SYSTEM IN THE LEAVES.

299

parts which will all be included under the term leaf -expansions, the bundles dis-
tribute themselves along the surface, sometimes ending free, sometimes anastomosing
with one another in a reticulate manner.

The bundles, especially those in the flattened expansions, lie as a rule in the
protrusions or ridges of the surface, which are known as ?ierves, ribs, or veins. The
course of these, the nervation or ribbing, and that of the vascular bundles often
exactly coincide, both phenomena are therefore usually indicated by the same name.
There is no further objection to this convenient use of the term ; but it must be
pointed out that two different phenomena are thus dealt with, one belonging to the
external formation of the parts, and referring to the relief of the surface, and another,
which refers to the inner structure ; and that though the two phenomena are always
closely correlated, they do not coincide always, or in all points.

The phenomena of surface-relief as seen in coarse ribbing may be here
assumed as known, the reader being referred to the literature on this subject,
especially to that of Pteridography and Palaeontology In treating of the arrange-
ment of the vascular bundles we must take into consideration (i) the divarication of
the bundles, that is their course in the direction of the surfaces of the leaf-expansions
(Sect. 91); (2) their position — as seen in a transverse section — within the other
tissues (Sect. 92).

Sect. 91. As regards the divarication of the bundles it must be premised that
for the middle portion (Rachis, Petiolus communis) of compound and deeply-divided
leaves, and for the main ribs of many, especially of large leaves, the same rules
hold good as have been above stated for the course of the bundles in the petiole.
The bundles are continued from a petiole, which contains several bundles, into the
main nerve, and here are arranged in a channel open upwards, or in one or several
circles, and are connected by anastomoses one with another : in their course towards
the periphery some of them pass out into the branches of the nerves, others give off
branches into them, while they decrease in number and size proportionately. The
stronger lateral nerves of a lamina may also contain several bundles, e. g. Quercus
pedunculata. A. B. Frank (l.ci) has given an exact description of the course of the
bundles of the leaf in this plant, and notes on the same in other plants.

The bundles which pass into the foliar expansion, either as branches from the
above-named parts, or directly from the nodes, either remain unbranched, or give off
branches, often up to high orders, their strength diminishing as a rule with each
higher order, but in a degree which varies greatly in each individual case.

The bundles and bi-anches of whatever order either end free in the foliar
expansion, or unite and anastomose with others.

Free ends sometimes lie at the periphery of the foliar expansion, in flat leaves
especially at the margin and point, occasionally also at the surfaces : sometimes they

* As chief works and sources of information the following may here be cited : L. von Buch,
lieber die Blattnerven, and Die Gesetze ihrer Vertheilung, Monatsbr. d. Berliner Academie, 1852,
p. 42. — C. von Ettingshausen, Die Blattskelete der Dicotyledonen, Wien, 1861, fob ; and the following
articles of the same author from the Sitzungsberichten (S.) and Denkschriften (D.)der Wiener Academie:
Apetalae (D. XV) ; Papilionaceoe (S. XII) ; Bombaceee (D. XIV) ; Celastrineas (D. XIII) ; Euphor-
biaceae (S. XII) ; Loranthacese (D. XXXII); Gramineae (S. LIE i).— For the Ferns compare
Mettenius, Filices Horti hot. Lipsiensis.

300

PRIMARY ARRANGEMENT OF TISSUES.

lie internally : internal endings. The bundles which end at the margin and point may
be termed, in accordance with the terminology of the coarser nervation, apically
directed (aerodrome) and marginally directed (craspedodrome). The points of end-
ing of strong marginally directed bundles are often the ends of teeth and laciniae.
Anastomoses may appear between branches of any order, between equivalent and
non-equivalent ones, and at any point of the foliar expansion. They give the bundle
system the form of a net (reticulate veins), which varies greatly in individual
instances. An allied special form, which is especially common in flat leaves, is
found where anastomosing bundles describe curves close within the margin and
following its outline : these are curved (camptodrome) bundles, according to the
terminology of nervation.

The extremely various individual cases, which arise by various combinations of
the above phenomena, group themselves under two main types, namely expansions
with bundles having a separate course, which end free, without anastomoses ; and
such as have anastomosing bundles.

I. Bundles with separate course and free ends are found in the rudimentary and
submerged leaves of many Angiosperms of the most various orders, in the foliar
expansions of all Gymnosperms, with the exception of Gnetum and Stangeria, and in
the leaves of many Ferns. One unbranched bundle, or the ramifications of branched
ones, traverse the foliar expansion, and end free either internally or usually at the
margin.

Rudimentary scale-leaves of Angiosperms often have their vascular bundles thus
arranged, when they are present at all ; the same may be said of the cotyledons of Mo-
nocotyledonous plants with one median bundle, or with two running near to the middle
line, or with more than two. The cotyledons of Dicotyledonous plants have been but
little investigated with regard to the relations in question, many have certainly a reti-
culum of bundles, even when they are ‘ single-nerved.’ These most simple forms
of leaves have been but little attended to in relation to the structural conditions under
consideration. Of larger foliar expansions many submerged leaves of Dicotyledons
(Batrachium, Myriophyllum ^) with one bundle in each segment of a leaf belong to
this series; also Pseudocallitriche with one median bundle^; Elatine Alsinastrum
with one median bundle, which usually gives off some marginally directed branches
into the narrow submerged leaves, &c. ; similarly among Monocotyledons, e. g. the
rudimentary simple median bundles of the Hydrilleae. In the foliage of land-plants
there is one simple apically directed bundle in each of the scale-like rudimentary
leaves of the Casuarinas and of Arceuthobium, as also in those of Equisetum and
Ephedra, which resemble them in habit.

Among Gymnosperms the foliar expansions of all Conifers^ belong to this
category : the leaves of the Cupressinese, of Taxus, Phyllocladus, &c., with one
median bundle ; those of the Abietinese which usually have two very close together,
with median parallel course, rarely (Abies Pindrow) with one simple bundle ; the
double leaves of Sciadopitys with two which have a parallel course near the median

' Aslcenasy, Botan. Zeitg. 1870, p. 19^1. — Vöchting, Myriophyllum, l.c.

“ Hegelmaier, Monogr. d. Gattg. Callitriche, p. 31.

® Geyler, t.c. (see above, p. 245). — Thomas, in Pringsheim’s Jahrb. IV. p. 43. — Strasburger, t.c.

BUNDLE-SYSTEM IN THE LEAVES.

301

line. The leaves of the broad-leaved Araucarias, and of species of Dammara and
Nageia, have more than two bundles, which run unbranched from the base towards
the apex, and end some of them at the apex, others below it. In Ginkgo the two
bundles which pass from the petiole into the lamina, branch repeatedl}' into
marginally directed forks. On Phyllocladus compare Strasburger, L c.

The pinnse of Cycas contain one median bundle, those of most Cycadeae
numerous unbranched bundles, which run parallel or slightly curved from the base
to the apex. In Stangeria they are traversed by one middle nerve, in which 6-8
bundles run side by side ; these give off branches laterally, which are arranged in a
pinnate manner, and sometimes curve towards one another and anastomose close to
the margin k

The leaves of Gnetum have, as far as is known, a typical reticulate bundle-
system ; in the leaf of VVelwitschia there is a peculiar arrangement, which will be
described below.

Among the Pteridophyta, besides the Equiseta already referred to, may here be
cited the awl-shaped leaves of Pilularia, Isöetes, Lycopodium, and Selaginella ; also
the leaves and portions of leaves with fan-like, dichotomous, branched bundles,
representing the Cyclopteris-nervation (e. g. Adiantum, Marsilia), and those with
bundles branched once or repeatedly in a pinnate manner, all being disconnected
and marginally or apically directed, which compose the nervation of Caenopteris,
Ctenopteris, Pecopteris, Taeniopteris, Sphenopteris, Eupteris, and Neuropteris k

2. The foliar expansions with anastomosing bundles may be divided according
to the arrangement of the latter into two subordinate types, which may be termed
the striated and reticulate.

{a) In the striated type numerous bundles run separately and parallel along the
leaf- expansion, the median ones running straight to the apex, the rest diverging the
more from this straight course the nearer they are to the margin, and the more the
boundary lines of the latter depart from parallelism. Most of these bundles curve
towards one another close to the margin, and unite so that each one affixes its acro-
scopically-curved end on the basiscopic side of the one next it in the direction of the
median line. Eree ends are rare. Throughout their course the bundles are con-
nected in a ladder-like manner by thin transverse branches. The former bundles
may accordingly be called shortly longitudinal bundles, in contradistinction to the
transverse branches. This arrangement is found, as far as is known, almost
exclusively in the Monocotyledons and in the leaves of the majority of their
families, also in the phylloclades of Ruscus and Myrsiphyllum, in the latter cases with
transitions to the reticulate form. Some few families of Monocotyledons, such as
the typical Aroidese, Dioscorese, Taccaceae, and many Smilaceae, are exceptions. Of
plants which are not Monocotyledons, the leaves of Welwitschia and of many narrow-
leaved species of Eryngium, as E. pandanifolium, E. junceum, &c., belong to this
category, or are at least allied.

According to the course of the longitudinal bundles we may here again
distinguish two subsidiary forms, which are it is true connected by intermediate

' Kraus, in Pringsheim’s Jahrb. IV. 1. c.

Compare Mettcnius, Filices Horti'ffiipsiensis, p. 2, &c.

302

PRIMARY ARRANGEMENT OF TISSUES.

examples (e. g. in the Dracaenas). In the one, which may be called the longihidinally
striated, all the bundles run, in the manner indicated, separately from the base to the
apex of the leaf or lamina. In the other, with pinnate striation, numerous bundles
enter the midrib of a flat leaf, and pass through it towards the apex. They then pass
one after another from the midrib into one or other half of the leaf, giving off
numerous branches into it ; only one or few of them extend to the apex of the leaf
itself All the bundles and branches which pass into the halves of the leaf are
arranged in a pinnate manner, and have an acroscopically curved direction. The
pinnate arrangement is characteristic of the group of Scitamineae, of the broad-
leaved Dracaenas, Curculigo, many species of Haemanthus (e. g. H. coccineus),
Eucharis amazonica, &c. The longitudinally striated arrangement is characteristic
of the majority of ordinary, linear, tapered leaves of Monocotyledons, also for
the fan-shaped leaves of Palms, and the Foliola of the pinnate-leaves of the same
family.

The longitudinal bundles which traverse the Monocotyledonous leaves of this category
are often of almost equal strength, inasmuch as they enter the leaf from the stem as so
many bundles of the trace, or from the node, as almost equivalent branches of one bundle
of the trace (e. g. species of Potamogeton). On the other hand, it not unfrequently
happens that they are not of uniform origin, some arising as branches from the others,
but all pursuing fundamentally the same course. This has already been noticed above
for the pinnately striated forms ; the same occurs also in the longitudinally striated.

The longitudinal bundles of one leaf are not uncommonly of almost equal strength ; in
other cases of very unequal strength. In Palm leaves Mohl distinguishes bundles of three
different strengths. Frequently one median bundle, which exceeds the rest in strength,
is found in longitudinally striated leaves ; in many leaves of Orchids with five and more
projecting ribs (e. g. Stanhopea, Acropera, Maxillaria squalens) there is one bundle in
each rib, which is distinguished by its size from the rest, which are not prominent.
In the pinnately striated leaves of Heliconia farinosa, the ends of the bundles which
pass out from the midrib are much stronger than their branches which pursue a
similar course, a difference which cannot be recognised in similar leaves of allied plants,
e. g. Phrynium setosum. From the example from the Orchidaceae it cannot be con-
cluded that in striated leaves generally the strength of projecting nerves must cor-
respond to that of the enclosed bundles. In the keel-like projecting midrib of species of
Carex, and in Pandanus pygmaeus, there is a bundle, which far exceeds the rest in strength ;
in the thick midrib of Zea Mais and other large leaves of grasses there are several with the
same arrangement as in the flat halves of the leaf, and resembling those which traverse
the latter, with the exception of the rather stronger median bundle.

The transverse branches which connect the longitudinal bundles like the rungs of
a ladder are often almost equal to them in strength, — e. g. Rhapis flabelliformis, Vanda
furva, — but usually much weaker, being even reduced to a single vascular tube, or row
of tracheides, for instance those in the pinnae of species of Chamaedorea, the leaves of
Curculigo, Zea, &c., which are even hard to find. Their number on a given surface
varies according to the species: on the average the distance between two may be about
jmm, it is often greater, rarely they are much more closely arranged — in Phrynium setosum
on the average 10-12 in a distance of i““. They run either almost exactly transverse to
the longitudinal bundles, so that the whole system of bundles consists of rectangular
meshes; or they have a more or less oblique direction. Further, they pass either from
one bundle to the next, or in very many cases they pass the next bundle, only touching
it externally, and run to the second or third lateral bundle. It is often seen, especially
in leaves with alternating stronger and weaker bundles, that they connect those of equal
strength, running past the intervenirfig ones of unequal strength. The transverse bundles

BUNDLE-SYSTEM IN TILE LEAVES.

above described (p. 265) in the halms of many Monocotyledons
in the leaves.

It is only rarely that in leaves of Monocotyledons single
blindly in the surrounding tissue. It is more commonly the case
to arise as a branch from one of them.

The huge leaf of Wehuitschia is traversed longitudinally
by very numerous strong parallel bundles, these being con-
nected in a ladder-like manner by transverse branches.
The transverse branches either run at right angles from
the longitudinal bundles, or obliquely, and sometimes directly
and simply from one longitudinal bundle to the other, some-
times converging and anastomosing one with another in the
narrow intervening spaces. Here and there a transverse
branch ends freely in the parenchyma, without reaching the
next longitudinal bundle ; and from each of the transverse
connections there often starts one short branch, which also
ends blind in the parenchyma, and is always directed to-
wards the base of the leaf (Fig. 145). The bundle-system is
accordingly similar in most points to that of the longitu-
dinally striated Monocotyledons, but the numerous Internal
free ends correspond to the usual condition in the reticulate
Dicotyledons. — In the leaves of the above-named species of
Eryjigium only transverse branches between the parallel longi-
tudinal bundles are found ; in other similar narrow-leaved
species, as E. aquaticum, there are also free endings and reti-
culate anastomoses.

303

have the same course as

transverse branches end
for a longitudinal bundle

Fig. 145. — Welwilschia mirabilis ;
piece of the network of vascular
bundles in the leaf, prepared free;
magnified about 4. b the edge of
the piece nearest the base of the
leaf.

{V) In the reticulate type (Fig. 146) the bundles which enter the leaf undergo
branching of higher or lower order, and the branches are distributed over the whole
surface, run in different directions, and are sometimes
connected into polygonal or curved meshes, or some-
times end free, internally or peripherally. Meshes of
higher order are enclosed in those of lower orders.

The marginal sides of all marginal meshes form to-
gether in flat leaves a sympodial bundle, which follows
the margin : this is more or less near to the actual
margin, and is not uncommonly situated in the extreme
margin itself (e. g. Quercus pedunculata, Banksia,

Lauraceae, Cocculus laurifolius, and many other cori-
aceous leaves). The bundles which end free internally
arise as branches from the sides of the meshes, and
terminate in the area enclosed by these, often after
repeated short branching.

As far as is known all foliar expansions of the
Dicotyledons, with the few exceptions above quoted,

belong to this type. The leaves of many land-plants, such as those of many species
of Trifolium, which, from the appearance of their coarse nervation, seem to belong
to the type with bundles pursuing a separate course, are still no exception ; nor
are the small linear “ one-nerved ” foliage-leaves of Dicotyledonous plants, such
as species of Erica, Passerina, Fabiana imbricata. Among Monocotyledons the

Fig. 146. — Psoralea bituminosa (40). The
last branchings of the bundles in a piece of
a leaflet ; at r the margin of the latter.

PRIMARY ARRANGEMENT OF TISSUES.

304

Dioscoreae belong to this type, and also many Smilacese, especially Smilax, Taccaceae,
Lapageria, Philesia, &c. ; among the Gymnosperms, Gnetum ; finally, the reticulate
Fern-leaves, which represent in Pteridography the types of nervation of Gonio-
phlebium, Phlebodium, Doodya, Marginaria, &c. The typical Aroidese, the broad-
leaved Potamogetons, and Hydrocharis belong also to this category, but appear
as intermediate forms between the type under consideration and the striated type.

It is well known that the bundles of this type enter the expansion singly or several
together in a midrib, which runs towards the apex, and give off from it branches of
the first order, arranged in a pinnate manner, into the two halves of the leaf (Folia
penninervia) ; or several separate main bundles diverge from the insertion of the leaf,
and are also in their turn pinnately branched (Folia palmatinervia, peltinervia, tripli-
nervia, &c.). The branches of higher orders are also sometimes arranged in a
pinnate manner, sometimes they show (true or false ?) dichotomy. The number of
orders of branching usually ranges in the Phanerogams from five to eight. In the
Ferns the branching is simpler in all respects than in the Phanerogams of this series.

It has already been noted above that branches of each order may end free or
anastomose peripherally or internally. As regards the occurrence of these modes of
ending the following cases occur ; —

I. In rare instances reticulate connection between all branches, free ends
occurring at most only in the apex of the leaf. In many succulent plants — species
of Sempervivum, Mesembryanthemum — all free endings are absent, or at least have
not hitherto been proved to exist : but an exact investigation of this point is still to
be desired. But by no means all succulent plants belong to this type : Salicornia,
e. g. has numerous internal endings in its cortical reticulum of bundles (p. 297), and
the shrubby species of Crassula have numerous peripheral ones. Among Monocoty-
ledons those Aroids which have been investigated (species of Anthurium, Pothos, and
hlonstera, Calla, Richardia) belong to this series, and further Hydrocharis and Pota-
mogeton. All these have a free end at the apex of the leaf. In the two last-named
genera the main bundles have a course similar to that of the longitudinal bundles of
the striated type, the transverse branches which connect them are repeatedly branched,
and the branches are connected into a net with angular meshes. In the Aroideae
also the course of the main bundles reminds us of that of the striated Monocotyle-
donous leaves. Between them branches of many orders form a complex angular
network. Free internal ends are absent, or present only rarely here and there.

2. PTee internal endings within the meshes, and no free peripheral endings. Close
to the margin of the flat leaf runs the sympodial marginal bundle, which limits all
the marginal meshes externally, and gives off no branches in a peripheral direction.
The leaves of species of Ficus and Banksia, of Cocculus laurifolius, Buxus, Quercus
pedunculata', and Psoralea (Fig. 146), may be named as examples of this pheno-
menon ) apparently very many leaves, especially tough long-lived ones, with an entire
margin, resemble these. Still I must cite no further examples, since the existing
works on the coarser nervation do not permit of a certain decision whether short
and thin free-ending bundles pass peripherally from the sympodial marginal bundle
or not.

* Frank, Botan. Zeitg. 186.4. P- 3^°-

BUNDLE-SYSTEM IN THE LEAVES.

3°5

3. Both internal and peripheral ends occur in all (?) Ferns of this category, though
in many(e.g. Ophioglossum vulgatum, pedunculosum, Platycerium) the peripheral ends
are but few ; further in numerous Dicotyledons, and species of Smilax and Dioscorea.
In the flat leaves of the Dicotyledons the terminating bundles sometimes run along
strong (marginally directed) nerves, sometimes they are small short branches which
terminate at the margin, and especially in the marginal teeth. It is a phenomenon
of especially frequent occurrence that two or more branches, one from each side, unite
near to the margin with one stronger, marginally-directed bundle ; they then run
together outwards to the margin, so that in other words the free ends pass off from
the sympodial marginal bundle. This is the case in the leaves of Primula sinensis,
Papaver, Brassica, Fuchsia, Calendula, Cucurbita, Mercurialis, and Camellia japonica.
The leaves of the Cupuliferse Betulacese, Myrica, Planera, Ulmus, species of Tri-
folium, Tropaeolum, &c. are further examples belonging to the present category,
which however have not been exactly investigated as regards the last-mentioned
condition. Small branches go from the sympodial marginal branches towards the
margin, in the investigated species of Smilax and Dioscorea.

Sect. 92. As regards the arrangement of the bundles as seen in a transverse
section of the foliar expansion, they run, of course with the exception of their peripheral
ends, within the other tissues, not superficially, those in the ribs being enclosed by the
collenchyma and sclerenchyma, or by the aqueous tissue (p. 116), which forms the
mass of the projecting rib, or rarely by the chlorophyll-parenchyma, which extends in
greater or less bulk into the rib. For those which do not lie in prominent ribs,
that is, for the smaller branches of most ramifying bundles, and for all the bundles
of many fleshy Monocotyledonous leaves, the rule holds, that they lie close within,
or below the inner limit of the chlorophyll-containing palisade-cells or rows of cells,
which are perpendicular to the surface of the leaf, but are not embedded in that
tissue. Thus in the bifacial leaves (comp. Chap. IX) they lie in the spongy paren-
chyma, where this borders on the palisade layer ; in leaves whose tissues are arranged
on the centric type, at the periphery of the (colourless) middle layer ; in interme-
diate forms, such as Dianthus Caryophyllus and Crassulacese, at the point where the
rows of cells which run inwards perpendicularly from the whole leaf-surface meet at
the middle of the leaf. In the first and last cases the bundle-system is accordingly
extended, in reference to a horizontal leaf, in a horizontal plane, in the second case
on the surface of a much-flattened hollow body.

I know of no exceptions to this rule in bifacial leaves. In concentrically constructed
expansions with a relatively thin middle layer, consisting only of a few layers of cells,
all the bundles are often enclosed within the latter (e. g. leaves of Statice monopetala,
Phyllodes of Acacia marginata), or the stronger bundles are within the middle layer,
and only the thinner branches at its outer limit (Flakea ceratophylla. Acacia longi-
folia, Huegelii). In the leaf of Agave americana the thick middle layer is traversed
by several series of bundles, which, with the exception of the central ones, stop short
towards the margin ; they run parallel to the surface of the leaf, and are connected by
anastomoses : besides these there is an external series which extends round the whole
leaf, at the limit between the chlorophyll layer and the middle layer. In each of the

* Von Ettingshausen, Blattsk. d. Dicotylen, Taf. I, II, &c.

X

3o6

PRIMARV ARRANGEMENT OF TISSUES.

longitudinal lamellag of the thick middle layer of the leaves of Typha and Sparga-
nium, which are separated by large air-cavities, there lie 1-3 longitudinal bundles;
numerous smaller ones are placed at the outer limit, abutting on a bundle of hard
hypodermal sclerenchyma. In the thick leaves of the Mesembryanthema (M. linguae-
forme and its allies, M. barbatum, imbricatum, stramineum, &c.) the main branch-
bundles run longitudinally through the centre of the middle layer, and send out
branches on all sides in an obliquely apical direction, which extend with their reti-
culately connected ultimate ramifications to its outer limit. Also in the thick
leaves of the Crassulacese, and even of the Semperviva, divergences of the bundle-
branches and meshes are found towards the surfaces ; this is most strikingly seen in
the branch-bundles which end at the surfaces in the thick-leaved species of Crassula
to be described below (Sect. 1 1 1).

On the arrangement of the bundles peculiar to the fructiferous leaves of Platy-
cerium, which, from its form, belongs to this category, and upon the peculiarities of
vascular bundles in the sporangium-bearing leaves of Ferns generally, which will not
be further treated in this work, the Pteridographic literature should be referred to,
especially Mettenius, Filices horti Lipsiensis.

It is well known that within all types there is found the greatest variety in the direc-
tion of the bundles, and of the corresponding ribs of different orders, in their divergence,
number, and relative strength. It is the province of the special description of plants to
enter into the details of these conditions of nervation.

The examples of amphibious plants, of leaf-like phyllode branches, of plants with
rudimentary leaves and cortical networks of bundles, show that the bundle-system may
be immediately dependent upon different adaptations. Sometimes it is altered accord-
ing to different adaptations of morphologically equivalent members; the submerged
leaves of the water Ranunculi and of Elatine Alsinastrum belong to the first type with
a separate course of the bundles, while the aerial leaves of the same species belong to the
reticulate type ; sometimes a similar bundle-system appears in morphologically different
members subject to similar adaptation ; phylloclades of Myrsiphyllum, and Ruscus, as
compared with foliage leaves of allied plants.

On the other hand, the different main and subsidiary forms of the vascular system cannot
in most cases be referred directly to adaptative causes. Within a narrower or wider
circle of relationship the same type of nervation occurs, whether the adaptation be similar
or different, and the converse is also the case. Further the nervation is to a great extent
independent of the form of the leaf. After what has been said above it is superfluous to
adduce examples of this.

Among the large divisions of the vegetable kingdom the Dicotyledons show the greatest
uniformity of ground-plan of nervation, since their aerial foliage leaves, with the one ex-
ception of the narrow-leaved Eryngia, all belong to the reticulate type ; and individually
they show the greatest variety, since in this type variations and combinations of dif-
ferent points of detail are possible, and really exist.

Among the Monocotyledons the great majority of forms belong to the striated type,
which shows generally an extraordinary uniformity in the main phenomena. Only the
few above enumerated families and genera appear as remarkable exceptions, since
some of them correspond exactly to the reticulate type of Dicotyledons, while others
approach it.

Among the Gymnosperms Gnetum alone (in accordance with other points in its mor-
phology which approach nearest to the Dicotyledons) has a truly Dicotyledonous nerva-
tion ; the pinnae of Stangeria have only single marginal anastomoses ; all other forms have
bundles with separate course.

CONNECTION OF THE BUNDLE-SYSTEMS.

307

The variety of the vascular system in the large foliar expansions of the Ferns is worthy
of observation: here even in a narrow circle of relationship, e.g. in the genera Polypo-
dium and Aspidium (in the sense of Mettenius, Fil. hort. Lips.), some species have separate
bundles of the most simple arrangement, others reticulate veins ; here also there are
found sometimes in one and the same species, sometimes especially in different species,
all intermediate forms between the most different types. The number of the bundles
on a given area is always small in F'erns as compared with Angiospermous Phanerogams,
but the plan of their distribution, e.g. in Ophioglossuin vulgatum and Platycerium, is
often the same as in the reticulated leaves of Dicotyledons.

d. Confiecit'on 0/ the bundle-systems of shoots and branches of different order.

Sect. 93. The bundle- system of the lateral, similar or dissimilar branches of one
relatively leading axis is continuous with that of the latter, and inserts itself upon it.
The form in which this is brought about depends in the main upon the morpho-
logical quality of the leading and lateral axes, the morphological point of origin of
the latter, and the course of the bundles within the axes under consideration. Specific
peculiarities are found besides in many cases. According to these relations the
following summary may be subdivided thus : —

I. SIMILAR BRANCHES OF LEAFY STEMS.

1. Normal Branches h

a. Dicotyledons a7id Gymtwsperms with a ring of bundles.

Sect. 94. The normal branches of the Dicotyledons and Gymnosperms treated
in Sect. 61-63 are in the large majority of cases axillary; we shall therefore speak
here of these exclusively. There are but few relevant investigations on the rarely
occurring extra-axillary branches, some few of which are referred to below.

The primary bundle-system of the axillary lateral shoots, when it consists of
leaf-traces, shows four main forms of insertion on that of the leading shoot.

In most cases it unites itself at the point of insertion of the branch into two or a
few bundles : these insert themselves, at the node of the leaf which bears the shoot,
on those bundles of the trace of the leading shoot which border on the gap of the
bundle-ring (gap of the leaf which bears the shoot) formed by the exit of the median
bundles of the trace.

In a second, apparently less numerous series of cases, the two or few bundles
of the base of the branch enter the ring of bundles of the leading shoot, at the node
of the leaf which bears the shoot ; they pursue an individual course down to a lower
node, and here insert themselves like bundles of the leaf-trace.

In these two cases the arrangement of the bundles of insertion is always such
that there is direct continuity between the pith of the leading and lateral shoots.

In a third series of cases the bundle-system of the lateral shoot inserts itself

' In the sense of Sachs, Lehrb. p. 174, 2nd Eng. ed. p. 171.

X 2

3o8

PRIMARY ARRANGEMENT OF TISSUES.

externally, at the node of the leaf which bears it, with numerous bundles, on the
many-bundled ring of the leading shoot, so that the pith-cylinders of the shoots of
the two orders are only connected by narrow medullary rays.

The fourth category is represented by many Cactaceae of very peculiar character,
to be described below,

A number of instances of the first and second category have been carefully
investigated by Nageli and others who followed him in investigating the course of
the primary leaf-traces In those belonging to the first series, and these are the
most numerous, the bundle-system of the axillary shoot is united at the point of in-
sertion into two bundles, which may be called the bundles of insertion. These
affix themselves to the bundles of the trace at or immediately below the node of
the leaf, which bears the shoot, either —

(a) Upon the bundles descending from above, which form the lateral limit of the
gap of the leaf which bears the shoot, the one being inserted on the right, the other
on the left — Iberis amara, Lupinus (axillary shoots of the cotyledons. Fig. 94, p. 238),
Passiflora Vespertilio (axillary tendrils). Antirrhinum majus, Urtica Dodartii, also
Pisonia ; Juniperus (Fig. 108, p. 246), and the short shoots (the bundles of needles) of
Pinus (Fig. no, p. 247); or (b) on the bundle or bundles of the trace of the leaf
itself, which bears the shoot — Anagallis arvensis (axillary peduncles), Clematis
(p. 245).

In Satureja variegata both the cases designated {a) and {b) are found to occur.
In Galium and Rubia first the two bundles of the trace of the first pair of leaves, and
then those of the second pair of leaves of the axillary shoot insert themselves, usually
at the node, on the bundle which passes into the leaf bearing the shoot ; the same is
usually the case in Russelia juncea, and sometimes in Spergula arvensis.

In the second series of cases the two bundles of the axillary shoot pass, at the
node of the leaf which bears it, into the bundle-ring of the leading shoot, and pursue
an individual course down through one (e. g. Aristolochia, Fig. 96, p. 239, Lathyrus
Aphaca, Figs. 98, 99, p. 240), twm (e.g. Cerastium frigidum. Figs. 102, 103, p. 243),
and even three internodes (e. g. axillary peduncles of Viola elatior), and then insert
themselves on bundles of the leaf-trace. For further examples and details, see
Nageli, l.c., and above. Sect. 61, p. 235. In the species of Galium, Rubia, Spergula,
and Russelia above enumerated there is, according to Nageli, either direct insertion
at the node of the leaf which bears the shoot, or the bundles pursue an individual
course down through one internode or more. In Vitis vinifera usually three bundles
pass from the axillary shoot, and also from the extra-axillary tendril into the main
shoot, and pursue a separate course through one internode.

The two or three bundles of insertion of the axillary shoot arise either by the
coalescence of the several bundles of the leaf-trace of the lowest internode to two at the
point of insertion, e. g. Clematis ; or these are the single bundles of the trace of the
two lowest leaves, e. g. Galium.

The connection of the axillary bundle-system with that of the leading shoot is
however not limited to the bundles of insertion above described. According to
Frank’s investigations^ on Taxus, Quercus, Bidens, and Solidago, there appear con-

' Compare above, § 61.

Botan. Zeitg. 1864, pp. 154 and 382.

CONNECTION OF THE BUNDLE-SYSTEMS.

309

necting bundles, one in Quercus, in Taxus about three, which run down from the
upper margin of the gap of the leaf-bearing shoot to the bundles of insertion. By
means of these and further ‘ completing bundles ’ the point of connection of the pith
of the main and axillary shoots is soon enclosed by a ring. The completing bundles
doubtless belong to the secondary formations of intercalary bundles (Chap. XIV) ;
the same is probable for the above connecting bundles, but their relation to the leaf-
traces of the axillary shoot requires more exact investigation.

As regards the insertion of the bundles of axillary shoots, two or more of which are
seated one above another, Frank’s statements may here be reproduced word for word
(/. f. p. 388), but the subject is recommended for further research. ‘ In Rubus two buds
are seated in the axil of the leaf closely one above another, their vascular bundle-systems
are united in the lower parts one with another, and are connected with the vascular
system of the stem as though they belonged to one single axillary bud. After both
lateral series (i. e. the branches of the two original bundles of insertion. De Bary) have
united at their anterior ends (i. e. next the leaf which bears the shoot) each separates at
the middle, and the anterior halves close up to form a circular system for the lower bud.
The remaining posterior halves soon unite at their anterior ends and form the vascular
bundle-system of the upper bud. The posterior parts of both circles of vascular bundles
are here also closed by descending bundles, which accordingly arise in the case of the
lower bud from the vascular bundles of the upper. — In the axillary buds of Lonicera
Xylosteum, of which often as many as four are seated one above another, but which are
usually separated some distance one from another, and the uppermost of which can hardly
be distinguished externally from an adventitious bud, the lower parts of the vascular
bundle-systems are also inserted between the members of the vascular bundle-ring of the
stem, but in this case each system is independently connected with the stem, since the
gap of the vascular bundle-ring of the mother shoot closes above each bud, and only
opens again immediately below the insertion of the next, at which point the bundles of
the bud arise on both sides from the margins of the open vascular bundle-ring.’

The third, less common case of insertion of the axillary bundles outside the
closed bundle-ring of the leading shoot occurs in the Umbelliferae, though not in all
of them. The bundles of the leaf-trace of the lowest internode of the axillary shoot
unite in this case at the node of the leaf which bears the shoot, to form one cortical
bundle, which immediately divides into two arms ; these pass off right and left, and
embrace the bundle-ring of the leading shoot transversely like a girdle. From this
girdle branches arise in pairs side by side and pass downwards. Each of these pairs
bestrides one bundle of the trace of the leaf, which bears it, from above and outside,
and inserts itself on its two sides, at the point where it curves outwards from the
bundle-rings of the leading shoot. The bundle-system of the axillary shoot is thus
attached outside the ring of the stem by the pairs which bestride the same number
of emerging bundles of the leaf which bears it. It thus embraces either the whole
circumference of the node, bestriding all the emerging bundles of the stem-embracing
trace of the leaf which bears it — Foeniculum, species of Heracleum, Chaerophyllum,
Myrrhis, and Archangelica ; or a part of it bestriding only some few bundles of the
leaf which bears it — .Tlthusa Cynapium. A continuity of pith between the shoots of
both orders is thus only possible by the narrow medullary rays.

The phenomenon stands thus in the mature plant h The exact study of the

^ If I am not mistaken, it has been long ago described by C. F. Schimper as ‘ Astkorb ;’ I have
not succeeded in my attempts to find the reference.

310

PRIMARY ARRANGEMENT OF TISSUES.

history of development remains still to be made. The arrangement can still be re-
cognised after considerable secondary thickening of the stem ; the stem-embracing
insertions of the branches which are thick, but attached by thin bases, weave in a
basket-like manner round the node of the leading axis : it is especially developed
in the perennial subterranean shoots of Myrrhis, species of Chaerophyllum, &c.

Many Umbelliferse have, as is obvious from the continuity of the pith-cylinder
on both sides, another form of axillary insertion, which remains to be more exactly
investigated : thus Silaus pratensis with its medullary bundles noticed on p. 253.

The same form of axillary insertion as in the above-named Umbelliferae is found
in Aralia japonica ; it remains to be investigated whether the same is the case in
other Araliace®, and in other families in which the leaf-insertion, and perhaps also
the bundle-system, resembles that of the Umbelliferae, e. g. Ranunculaceae with alter-
nating leaves.

The above-mentioned fourth, and very special case of bud-insertion occurs in
species of Echinocactus, and some of Cereus with thick shoots (C. candicans }). Its
development requires investigation. In the mature state the leaf-traces having one
bundle are found united to form sympodial bundles, which are separate and per-
pendicular, and of equal number to the angles of the stem (Echinocactus), or are
connected in a reticulate manner ; between them are broad medullary rays. The
leaf-bundles run slightly obliquely, almost horizontally upwards towards the lower
margin of the spine-cushion, that is towards the point of insertion of the rudimentary
leaves. Just above each foliar-bundle and in a direction almost parallel to it, the
thick cortex of the stem is traversed by some few vascular bundles, which are near
one another, and have their xylem-portions turned towards one another ; these
belong to the axillary bud formed above the rudimentary leaf, and attain a consider-
able strength as soon as the bud developes into a shoot. These bundles of the bud
now pass through the medullary rays, between the sympodia of the leaf-trace in the
stem, into the pith, and there branch freely in all directions, their branches being
united one with another to form an elaborate plexus traversing the whole pith.
This system of bundles of the bud is only directly connected with the sympodia of
the leaf-trace by single short connecting bundles at the points of passage through
the medullary rays. In the Opuntias, Cereus speciosissimus, &c., and also in
the Rhipsalidacese ^ this phenomenon is wanting; the bundles of the bud, as far as
investigated, are inserted, in the manner usual for Dicotyledons, partly on the cortical
bundles, partly on those of the bundle-ring: medullary bundles are altogether wanting.
In the Mamillarias, which have medullary bundles (p. 254), no connection between
these and the young lateral shoots has as yet been discovered.

The above-named plants have accordingly a system of medullary bundles,
which differs fundamentally in its significance from the others described above on
P- 2S3-

Where these latter, and where cortical bundles occur in Dicotyledons, the
axillary insertion occurs, as far as is known, in one (usually the first) of the typical
forms, with the addition of direct connection between the medullary or cortical
bundles of the leading shoot and of the lateral shoot.

Compare Vöchting, 1. c. (p. 261 ).

CONNECTION OF THE BUNDLE-SYSTEMS.

31I

b. Monocotyledons and Phanerogams with axile bundle.

Sect. 95. Among the Monocotyledons belonging to the Palm-type (Sect. 6 5-6 7) the
numerous bundles of the lowest internode of the normal axillary shoot enter, in the
Palms^, Dracaenas, Liliacese, Aroideae, Orchidaceae, &c., at the node into the bundle-
cylinder of the main shoot, and pass obliquely downwards and inwards with the
bundles of the leaf which bears the shoot, inserting themselves successively on peri-
pheral bundles of the latter, without reaching the middle of the cylinder. In many
cases, as in the rhizomes of Acorus, the axillary bundles do not penetrate further
than to the surface of the cylinder of the main shoot, but spread themselves out, with
abundant branching, for a great distance downwards, over the nearer longitudinal
half of the main shoot, and interweaving, and here and there uniting with the bundles
at the surface of the cylinder, they form a dense plexus of bundles, which is sharply
limited on the side next the cortex.

In those forms which have been investigated, Zea, Saccharum, Coix, Arundo
Donax, &c., numerous bundles pass from the lowest internode of the axillary shoot
transversely into the node, and here branch very freely, their branches spreading over
the whole transverse section of the node, but only slightly in a vertical direction, and
thrusting themselves between the bundles of the main axis, which run perpendicularly
through the node, and between one another, and here aird there inserting themselves
on the bundles of the main axis. The whole series of axillary bundles forms at the
node a complex and confused felt, expanded and attached in the manner indicated,
and having the form of a transverse disc, which in the above-named large species
reaches a height of several millimetres ^ ; its origin is not obvious in the mature state,
but it is clearly seen in young stages of development that it is formed by the insertion
of bundles, starting from the axillary shoot.

In the Commelinese with thin stems, as Tradescantia albiflora, Commelina
agraria, several internal bundles are found in the basal internode of the young
axillary shoot (comp. Sect. 69) — e. g. three or four in the Tradescantia named — which
enter the node of the main shoot, and here, turning downwards above the outgoing
median bundle of the leaf which bears the shoot, insert themselves at the point of
union of the inner bundles. In somewhat older axillary shoots there are further
peripheral, doubtless in part cauline bundles, which insert themselves on the cauline
bundles of the main shoot. In thick-stemmed Commelinete, such as Maravelia zey-
lanica and species of Dichorisandra, the internal bundles of the lowest internode of the
axillary shoot unite to a single thick bundle, which passes almost exactly horizontally
into the node of the leaf which bears it, and inserts itself in the middle of this, witli
some few branches, on the internal bundles which descend there. In Tradescantia
virginiana several bundles pass from the axillary shoot into the node of the leaf
which bears it, and there divide into branches, which are interwoven as in the nodes
of the Grasses between the internal bundles of the main shoot, and insert themselves
on them.

* Mohl, Palm. Struct, p. 31. — Compare also Falkenberg, l.c., and the note above on p. 276.
^ Compare von Mohl, /. r. Tab. 9; Schleiden, Grundz. 3 Aufl. II. p. 158.

312

PRIMARY ARRANGEMENT OF TISSUES.

In the node of Potamogeton natans the bundle-system of the young axillary shoot
coalesces so as to form a single bundle, which inserts itself on the median bundle of
the leaf which bears it, at the point where it curves outwards. The other investigated
Potamogetons — P. lucens, gramineus, pectinatus, and pusillus — show a quite similar
insertion, with the difference that the bundle which comes from the axillary shoot is
not inserted on the bundle passing into the leaf which bears it, but on the axile
sympodial bundle which passes downwards at the node. Comp. Fig. 123, p. 273.

As far as is known, the naturally very simple relations of insertion in those
Phanerogams which have an axile bundle resemble those in the last-named Mono-
cotyledons.

c. Fern-like plants.

Sect. 96. Among the Filicinese there is sometimes forked, sometimes Mono-
podial branching; in many species, as in Aspidium Filix mas, Athyrium Filix foemina,
both forms of branching are found side by side.

Monopodial branching occurs undoubtedly in the Salviniacece, Marsiliacece, and
in many Filices. Very many Filices show a shoot-system, apparently composed of a
main axis and lateral shoots, in respect of which it is a matter of controversy whether
it is a Monopodium, or a Sympodium composed of unequally developed successive
forks. It will here be treated as a Monopodium, in accordance with the conclusion
of Mettenius. But I remark distinctly that I only accept this conclusion in order to
simplify the description here to be given, and that I leave the controversy in question
quite undecided. The fact that the insertion of the bundles of the actual lateral
shoots appears in many cases in point to support the conclusion of Mettenius
cannot by any means decide the question, since unequally strong growth of originally
equivalent shoots may also have as its result an originally unequal arrangement of
their vascular bundle-system.

The normal lateral shoots of the plants of this series, with the exception of
many Hymenophyllums and Davallias, which in this point also are the subject of
controversy, are not axillary, nor have they even any other constant relation to the
insertions of the leaves.

They arise either from the stem, and sometimes close to the point of insertion
of the leaf, laterally or at the back of the latter, sometimes at a great distance from it,
between two leaves ; or they arise on the back or sides of the base of the petiole
itself, often, as in the ordinary branchings of Aspidium Filix mas, which may be
accordingly considered as belonging here, at a considerable distance from the point
of insertion, in the above-named example about 2-3 centimetres from it. Comp.
Fig. 132, C, p. 286.

In the undoubtedly Monopodial branching of many Filices with more than two
rows of leaves, the vascular bundle-system of the lateral shoot is united as a rule
towards the point of origin to one thin and not hollow bundle, which is inserted on
one bundle of the main axis. This is the case in the lateral shoots of Aspidium
cristatum, spinulosum, Blechnum Spicant, Athyrium Filix foemina. Polypodium al-
pestre, Alsophila aculeata, &c.^, which appear at or below the back of the base of the

* Hofmeister, Beitr. l.c. — Stenzel, l.c.-, compare p. 283.

CONNECTION OF THE BUNDLE-SYSTEMS.

313

petiole. The single thread-like bundle, which enters the narrow base of the branch,
is inserted^ in the first-named plants, at the lower margin of a foliar gap of the
stem ; in the specimen of Alsophila investigated several bundles are thus inserted,
each of which passes into one of the numerous shoots which arise round the base of
one leaf. The bundle of the lateral shoot of Struthiopteris has the same point of at-
tachment as in Aspidium cristatum ; it is not thread-like, but has the shape of a
plate with a channel-like external groove, which gradually widens as it passes from
the point of attachment, and closes up to form a tube opening obliquely outwards and
downwards. Through the tube and channel the pith of the lateral shoot has con-
tinuous connection with the parenchyma of the main shoot, while where the bundles
are filiform at their point of origin this continuity does not exist. The lateral shoots
seated on the petiole in Aspidium Filix mas have as a rule the same thread-like
insertion of the bundles on one bundle of the petiole. More rarely the bundle-system of
the lateral shoot arises as three bundles from so many bundles of the petiole, or directly
as a tube filled with pith from the margin of a gap in a widened band-like, bundle of
the leaf. Finally, in a species named as Diplazium giganteum, Stenzel found this
latter form of insertion in the branchings arising from the stem below the leaves :
each branch has its own small gap in the network of bundles of the stem, from the
margin of which arises the tube-like system of the branch.

In the Ferns and Rhizocarps with elongated stems bearing two rows of leaves,
and with lateral shoots arising from the stem, not from the petiole, when there is a
simple axile bundle present, there is naturally an insertion of the bundle which
passes to the lateral shoot on that which traverses the main shoot. In the Marsilia-
cese with a tubular bundle, that which enters the branch passes directly off in a
tubular form from the margin of a corresponding gap in the tubular bundle of the
main axis, through which gap the pith of the two axes is continuous.

In the Ferns with clearly marked upper and lower bundles (see above, p. 287),
in most of the described cases the bundle-system of the lateral shoot is united, as
in those ferns with leaves in many rows, into one bundle, which arises from the next
lower transverse bundle separating the foliar gaps ; it usually has the form of a
channel open towards the pith. This is the case in Aspidium albopunctatum, coria-
ceum (comp. Fig. 135, p. 287), Acrostichum Lingua, brevipes, most Davallias, and
other above-named species, on the details of which Mettenius’ description must be
referred to. According to Trdcul’s investigation^, however, it appears that in A.
coriaceum the bundle passing to the branch is inserted both on the transverse and
on the lower bundle, at the angle between the two. Among the Davallias some
species (D. stenocarpa, divaricata) differ from the rest, inasmuch as a closed transverse
bundle limiting the foliar gaps is absent, but in its place, and in the direction which
it has in other species, two bundles run, the one arising from the upper, the other
from the lower bundle ; both converge obliquely towards the apex, and enter the
lateral shoot as upper and lower bundles w’ithout coming into contact. The arrange-
ment in D. chaerophylla described by Mettenius is more irregular still, but should
be placed in this category.

^ Mettenius, Angiopteris, p. 546.

^ Ann, Sei. Nat. 5 ser. tom. XII. p. 242.

3T4 PRIMARY ARRANGEMENT OF TISSUES.

Of the Ferns with divided upper and lower bundles, which are connected by
intermediate forms with those above described, and with those in which there is
only a complex, often irregular network of bundles instead of two bundles (see
p. 288), this only may be stated generally, that in many simply constructed forms,
e. g. Polypodium squamulosum, the bundle-system of the lateral shoots arises as a
simple bundle from a definite mesh in that of the main axis. In the large majority
of these cases several thin bundles, which enter the lateral shoot, arise from the
margin of definite meshes (comp. Fig. 136, p. 287). Their number varies according to
the species, from two to eight, as far as present data extend, and is always smaller than
that which enters a leaf of the same species. Where medullary bundles are present,
and there is continuity between the pith of the main and lateral shoot, branches
separate from the medullary bundles of the former and enter the latter, e. g. Polybo-
trya hleyeriana; where the bundles of the lateral shoots insert themselves as a simple
not hollow bundle, such a branching does not occur.

When the lateral shoots arise from the base of the leaf, modes of insertion of
bundles are found which are similar and subject to similar'variations to those of the
bundles arising from the stem ; this is shown by the above-mentioned example of
Aspidium Filix mas. There are but few thorough investigations on this point.

No exact investigations have been made either on the insertion of the bundles
at the rarely occurring points of branching of the Osmundacete, or on the mode of
origin of these branchings.

In the Equiseta the bundle-system of each branch is united into a single bundle,
and inserts itself externally at the node of the main shoot on the angle of branching
of one of the bundles descending from the next higher leaf-sheath (comp. p. 279)*.

Where the branching appears and remains as a forking of the main axis, the
whole bundle-system also divides into two parts, each of which enters into one
branch of the fork : both systems are fundamentally similar to one another and to
that of the main axis.

Among Ferns the rhizomes of Pteris aquilina^ are — if we disregard for the
present the above-mentioned controversies — an exquisite example of this. Comp.
Fig. 143, p. 295. Also in Athyrium Filix foemina, according to Hofmeister, the phe-
nomenon is frequent ; in Aspid. Filix mas it is rare ; these cases are regarded, it is
true, by Mettenius as an apparent forking, derived from early stronger development
of monopodially formed lateral shoots.

In the Lycopodia and Selaginellae, whose branchings are always forked — though
not always quite equal — the insertion of the vascular bundles which enter the branches
is generally indicated by what has been said above. In the Selaginellae with two
lateral bundles in each shoot, as in S. Kraussiana, Martensii, &c. (comp. p. 2 8 2),' of
the four bundles destined for each branching either three diverge acutely from one
bundle of the main shoot, and the fourth is the continuation of the other which
traverses the main shoot; or each of the latter divides into two branches which are
very close to one another at their entry into the fork. In S. Martensii the former

' See Stenzel, /. c, Taf. IV. Fig. 13.

“ Compare Hofmeister, Stenzel, ll. cc.

CONNECTION OF THE BUNDLE-SYSTEMS.

3^5

case has been observed \ in S. Kraussiana^ both cases are found; in the latter there
is also a more or less complete transverse anastomosis at the point of branching.
The passage into the lateral shoots has not been exactly investigated in those Sela-
ginellae which have several bundles in their shoots.

2. Adventitious Shoots.

Sect. 97. It is a general rule for adventitious shoots that their bundle-system is
always inserted on those vascular bundles, or points of the wood or bast body of the
main axis, which are nearest to their point of origin. Since such shoots may arise
at the most heterogeneous points, and at the most various periods of the life of the
plant, the individual cases show great variety. If the normal course of the bundles
is known, their relative arrangement is completely determined by what has been said
above.

11. ROOTS.

Sect. 98. In the forked roots of the Isoetese, Selaginellse, and Lycopodia, the
vascular bundle forks as in dichotomous stems.

Roots are found as lateral branches on members of their own kind, as well as
on stems, rarely on leaves® ; some appear in definite morphological positions, e. g. at
definite points of the leaf-insertion ; others are without arrangement : the former may
be called normal, the latter adventitious lateral roots.

The invariably endogenous formation of lateral roots takes place in or close to
vascular bundles or masses of wood or bast. Their vascular bundle is inserted
directly and without branching on the nearest one of the main axis, or it divides into
branches, which connect themselves with several bundles of the axis.

The former simple insertion occurs obviously in members with a simple axile
bundle — thus in almost all roots ; also in stems constructed on the Dicotyledonous
type, and in the Ferns.

Splitting of the bundle of the root into several shanks, which insert themselves
on several bundles, is a common phenomenon in the stem of Monocotyledons. It
does not occur however in all species ; e. g. in the rhizome of Carex hirta each root-
bundle inserts itself simply on one peripheral bundle of the stem-cylinder.

The branches or shanks, into which the root-bundles about to be inserted are
divided, separate at the periphery of the bundle-cylinder, they then insert themselves,
in a first series of cases on the bundles which are present at that point, without
penetrating more deeply into the cylinder of the stem : this is the case in the
investigated Orchidem, many Commelineae, Aroideae, Richardia sethiopica, Philo-
dendron spec., with few short shanks, diverging chiefly upwards and downwards ;
Acorus with more abundant branching ; Calla palustris * with a ring of roots, whose
bundle-insertions together form a transverse girdle at each node of the rhizome.

' Compare Nageli und Leitgeb, Entstehung, &c. d. Wurzeln, Taf. XVIII. Fig. ii.

^ Hofmeister, Vergl. Unter. Taf. XXIII. p. 4.

’ [Compare Mangin, Origine et Insertion des racines adventives . . . chez les Monocotyledones,
Ann. Sei. Nat. ser. 6, tom. 14, 1882.]

‘ Van Tieghem, Struct, des Aroidecs, /. r.

3i6

PRIMARY ARRANGEMENT OF TISSUES.

In another series of cases the bundle of the root divides at the outer limit of the
stem-c}'linder into numerous branches, which, diverging in all directions, enter
between the bundles of the stem, and penetrate with a sinuous course to the middle
of it, and then insert themselves, some of them further out, others further within, on
bundles of the stem. This is the case in the Palms \ where the penetrating bundles
do not reach the middle of the stem, in the nodes of the Grasses, and thick-stemmed
Commelineae.

Also in the thick primary lateral roots of Pandanus the bundle-system of the
corresponding lateral axis inserts itself on that of the main axis in the manner just
described, i. e. numerous radiately diverging and undulating branches pass between
and up to the longitudinal bundles, which will be described below (Sect. io8) : also
in the Palms, according to Mohl, the bundle of the lateral roots splits in a similar
way into thin branches, and penetrates between the elements of the hollow cylindrical
bundle of that which is relatively the main root.

The phenomena in the above Commelineae should be rather more exactly described.
Several lateral roots arise at the nodes, especially on the side opposite to the axillary
shoot, and somewhat higher than its point of insertion, and the point of union of the
bundles of the leaf-trace. The bundles of the latter penetrate horizontally into the stem
as far as the cylindrical surface occupied by the ring of cauline bundles (see p. 269).
Here they divide into horizontally diverging shanks, and these together form a slight
transverse girdle passing round the whole periphery. In the thin stems of Tradescantia
albiflora and Commelina agraria this girdle is without centripetal branches. In Tr. zebrina,
virginiana, and Maravelia zeylanica numerous branches pass down from it in a centripetal
direction, they are distributed transversely through the whole node, with sinuous curva-
tures, and anastomose with the descending bundles of the stem, and with those which
enter from the axillary shoot, thus forming a felt, which is less dense and deep than that
which is characteristic of the nodes of Grasses, but is similar to it.

B. STRUCTURE OF THE VASCULAR BUNDLE.

Skct. 99. The vascular bundles are strands which consist of tracheae and
sieve-tubes as their essential parts. Both are accompanied by parenchymatous, and
often by sclerenchymatous elements. The structure of the bundle is determined by
the juxtaposition of all these component parts.

A bundle undergoes in its course more or less considerable changes. Cross-
sections of the same bundle, taken at a distance from one another, may show the
greatest differences in the number and distribution of the individual elements ;
Fig. 147, for example, represents the part of a bundle of the leaf-trace of Acorus Ca-
lamus passing through the leaf, while Fig. 148 shows its lower end as existing in the
stem ; in the intermediate tract the one structure passes gradually over into the other.
Such differences come out most conspicuously on comparing the peripheral ends of
the bundles, where they are spread out in the leaves or periphery of the stem, with
the other parts of them. It is therefore expedient in considering them to distinguish

^ Von Molil, Palm. Struct, p. xix. Tab. I. A, and Verm. Sehr. pp. 157 and 172.

STRUCTURE OF THE VASCULAR BUNDLE.

317

between bundle-ends and bundle-trunks, although a sharp limit between the two
cannot be shown in any case.

To the category of bwtdle-irwiks belong chiefly the bundles which pass through
the stem, roots, leaf-stalks, and thick nerves of the leaf. The description of their
structure must start from these organs.

Most bundle-trunks, however different in details, possess the composition which
has above been indicated in a general way. In comparatively few cases their
structure is simplified by essential organs disappearing or remaining rudimentary. A
distinction must therefore be drawn between incomplete and complete bundle-trunks.
Here we shall first speak of the latter.

I. Bundle-trunks.

Sect. ioo. The essential tissue-elements of the complete bundle are tracheae
and sieve-tubes. The two are always so arranged that one longitudinal portion of

e

b

Fig. 147. — Aconjs Calamus; cross-section through
the periphery of the flower-stalk (145). e epidermis; b
small vascular bundles, with a sclerenchymatous sheath
on the outside. In the middle is a large vascular bundle ;
■zy its phloem ; g outer large tracheae of the xylem ; I
intercellular passage at the inner side of the latter. The
cross-section through the leaf shows the same structure.

Fig. 148. — Cross-section through the
concentrically arranged lower end of a
bundle of the leaf-trace in the stem (145).
The delicate and small-meshed phloem
occupies the middle, and is surrounded
by a ring of scalariform netted tracheides ;
outside this is parenchyma.

the bundle includes all the tracheae, while another, or in rarer cases more than one
other, includes all the sieve-tubes. Thus in every bundle we have to distinguish
between that part which contains the tracheae (tracheides or vessels) and that
which contains the sieve-tubes, or expressed more shortly, between the xylem and
the phloem.

In both parts the characteristic tissue-elements are as a rule not present alone,
but are placed between rows or layers of cells (cf. p. 5) in such a way that all or
most tracheae or sieve-tubes are in contact with the latter at least at one point. It
is true that in the case of very small bundles this intercalation is not uncommonly
absent in the xylem ; but then the few tracheae of which this consists, border, for
the most part at least, on the cells which encircle the bundle. More rarely masses of

PRIMARY ARRANGEMENT OF TISSUES.

318

tracheae, consisting of several or many rows as seen in cross section, without inter-
calated cells, occur in the xylem of thick bundles, as in the Marattiaceas, Osmun-
dacese, and Ophioglosseae and in the bundles of the leaf of Yucca, the stein of
Fritillaria^ &c., to be described below.

The external surface of a vascular bundle is marked off from the surrounding
non-equivalent tissue in various ways, and not uncommonly in such a manner that
the surrounding parenchymatous elements pass over directly and quite gradually into
those which lie within the bundle itself, between the trachete and the sieve-tubes.
A smooth, sharp, external boundary is then not present at all, although the bundle
itself, in so far as it consists of its essential elements, always stands out sharply.
More frequent no doubt is the complete or partial limitation of the bundle by means
of a distinct sheath, strand-sheath, or bundle-sheath in the sense of the word fixed in
general at p. 6. This occurs, firstly, in the form of the endodermis (p. 121), or other
specialised parenchymatous layers (Chap. IX); secondly, in the form of strands or
layers of sclerotic fibrous cells, or especially sclerenchymatous fibres, which border the
bundle on one side or encircle it all round. All these sheaths are as a rule limited
on the inside towards the bundle with the same sharpness as on the outside towards
the non-equivalent surrounding mass ; they can therefore just as well be assigned to
the bundle itself as to its surroundings, its boundary being fixed either at the external
or internal surface of the sheath. The latter is the usual and expedient practice in the
case of parenchymatous, and particularly of endodermal sheaths, especially on the
ground of developmental phenomena in almost all roots ; yet it must not be left out
of consideration that in the case of the endodermis of most bundles in Fern-stems
the history of development rather leads to the opposite result. Cf Sect. 106.

As regards the sclerenchymatous strands and sheaths which accompany the
bundle longitudinally, it has long been customary to assign them to the vascular
bundles, or, as was done by Nageli, to regard them at least as essential concomitants
of the latter, and to call the united strand formed by them and the vascular bundle
the fibro-vascular strand. Both from former sections, and further from the general
comparative consideration of the distribution of sclerenchyma (Chap. X), it follows
that these fibrous sheaths and strands do not strictly speaking belong to the vascular
bundles, but to a special system of tissue, which may or may not have a common
course with the latter over certain tracts.

In spite of all these theoretical considerations, the anatomical treatment of the
vascular bundles cannot by any means leave sheaths of whatever kind unregarded
where they occur.

The elements of the vascular bundle are, as far as investigations reach, almost
everywhere and always in uninterrupted connexion, both among themselves and with
those of the surrounding sheaths. The only not uncommon exceptions are that the
xylem, especially in collateral bundles, shows intercellular spaces containing air at its
inner edge, and that spaces containing secretions lie in the outer regions of the
bundle. Cf. Chap. XIII.

* Compare Russow, Vergl. Unters, p. 117.

“ Von Mohl, Palm. Struct. Tab. G. Fig. ii.

^ C. H. Schultz, Die Cyclose, l.c. (p. 192) p. 246. — Sachs, Textbook (2nd Eng. ed.), p. 124. —
Compare also Russow, Vergl. Untersuch.

STRUCTURE OF COLLATERAL BUNDLES.

319

According to the arrangement of the xylem and phloem, three main forms of
bundles are to be distinguished, which are designated the collateraP, the concentric,
and the radial. One and the same bundle may at different points of its course pass
over from one to the other of these forms (cf. Fig. 147, 148).

I. Collateral Vascular Bundles.

Sect. 101. Collateral bundles are with rare exceptions characteristic of the stem
and foliage-leaves of the Phanerogams, also of the stem of Equiseta, Ophioglosseas,
Osmunda, and Todea (.?)^ Among parts belonging to the category of roots
they occur only in the tuberously developed roots of Dioscorese (D. Batatas),
Ophrydeae, and perhaps those species of Sedum which are related to S. Telephium,
Cf. p. 233.

In the most numerous and the typical cases they consist of a xylem and phloem
portion, each of which borders longitudinally on the other with one surface, and with
the remainder of its periphery on the non-equivalent surrounding tissue. A special
subordinate form, to be called the double collateral or bicollateral, is distinguished
from the usual one by the fact that two phloem groups lie on opposite sides of one
xylem group. These will be described last, and at present only the simple collateral
bundles will be discussed.

The orientation of collateral bundles is in the usual cases, which we may call
normal, always such that the xylem is turned towards the middle, and the phloem
towards the periphery of the whole organ to which they belong. Accordingly we
can use the terms inside and outside as regards the bundle in a general sense, calling
the edge turned away from the phloem the inner edge, and using corresponding
words for the remaining sides. In the bundle-ring of the typically constructed
Dicotyledons and Gymnosperms (p. 235) all the xylem portions lie, in consequence of
the orientation mentioned, in an annular zone which immediately surrounds the pith,
while all the phloem portions occupy a zone concentric with the former and external
to it. The former, together with what is added later by secondary new-formation, is
traditionally known as the woody ring or woody mass, the second as the bast, bast-
ring, bast-zone or inner cortex, and the two parts of the bundle are accordingly called
the woody-part, and the bast-part or cortical-part — xylem and phloe7n the nomen-
clature originally adopted for the Dicotyledons being extended to the structurally
similar parts of all vascular bundles, without regard to arrangement and orientation.
The same orientation is also the rule for the stems of Monocotyledons, and for all
leaves or portions of leaves in which the bundles are placed in a ring around a
central part from which they are absent. Where on the other hand the bundles
in a leaf, or portion of a leaf, have an arrangement other than the annular one just
mentioned, their phloem is turned towards the morphologically lower leaf-surface,
and their xylem towards the upper, thus preserving the same orientation as in the
stem if referred to the latter, the leaf being supposed to be in the erect position.

As regards Dicotyledons with the typical single ring of bundles, no exceptions
to these rules are known, unless perhaps in species of Strychnos (cf. Chap. XVI).

’ Russow, Vergl. Unters.

“ [Compare Haberlandt on collateral bundles in tbe leaves of Ferns, Bot. Ztg. 1881, p. 4(57 ;
idem, 18S2, p. 217.] ® Nageli, Beitr. I.

320

PRIMARY ARRANGEMENT OF TISSUES.

In stems, leaf-stalks, and leaf-ribs with several concentric rings of bundles, or
bundles scattered in cross-section, exceptions certainly occur, though rarely; also
very rarely in the lamina of flat le'hves.

In the positions first mentioned precisely the opposite orientation to that in the
normal case is shown by certain bundles in the stem of Nelumbium (Fig. 112, p. 255),
namely by those of circles 3 and 5 in the intermediate series ; further by the
medullary bundles in the stem of the Araliae mentioned on p, 253, by the four
cortical bundles in the internode of Calycanthus, by those of the middle one of the
three concentric bundle-circles in the leaf-stalk of the Lime and several others.

In stems and leaf-stalks many bundles which are scattered as seen in cross-section
have an irregular orientation, i. e. with the two parts facing neither directly outwards
nor directly inwards. Those especially which anastomose or branch often show
torsions which divert them from the normal orientation, near the points of branching
or of union. Examples of this are found in many leaf-stalks, e. g. Aralia japonica,
Aroidese ; in the pith of Silaus and other Umbelliferse mentioned at p. 253 ; and in
the interior of the stem of Aroideae and Pandaneae (cf. p. 268).

In the leaves of Typha and Agave Americana (p. 305) the bundles running
through the middle layer, which is destitute of chlorophyll, all have their vascular
part turned towards the upper side ; in those on each side which border on the
chlorophyll-parenchyma the vascular part faces the middle of the leaf. Of the
longitudinal bundles lying in one plane in the leaf-lamina of Dracaena reflexa, the
median one has normal orientation, while all the rest have their xylem turned towards
this, and their phloem towards the edge of the leaf.

The general form of the cross-section of collateral bundles is as a rule round
or oblong ; in the latter case the greater diameter passes as a rule through the
middle of the outer and inner edges. In the stems and leaves of several Mono-
cotyledons this unequal extension on different radii of the cross-section amounts to a
marked lateral flattening of the bundle ; e. g. leaves of Scitamineae, Asphodelus
luteus, Hemerocallis, Hyacinthus, Pandanus, leafy-stern of Canna, &c. Other shapes
are rare ; such as the horseshoe-shaped cross-section of the bundles in the stem of
Osmunda (Fig. 128, p. 280), the annular section in the stem of Botrychium (p. 284),
and in the petioles and ribs of several Dicotyledonous leaves, as those of Eriobotrya
japonica, Veronica speciosa^, the pulvinus of the leaf-stalk of Mimosa pudica, &c.
In these latter cases the inner edge of the ring is always the inner edge of the
bundle, not only as regards orientation, but also as regards structure.

Ignoring these annular bundles, and ignoring the places where several bundles
meet, which we shall describe below, both the whole collateral bundle and each of its
two parts form an approximately, though never exactly monosymmetrical body, with
its plane of symmetry passing through the middle of its outer and inner edges. The
arrangement of the elements in the bundle is also in harmony with this approximate
monosymmetry, as will be shown below.

The number of elements in each part, and the resulting thickness of the bundle,
is extremely different according to the individual cases : many sappy herbaceous

’ Compare Frank, Botan. Zeitg. 1864, p. 381.

^ Areschoug, Om bladets inre byggnad. Lund's Univ. Arskrift, tom. IV.

STRUCTURE OF COLLATERAL BUNDLES.

321

plants, especially Monocotyledons, and water and bog-plants, have only a group of a
few (3-6) tracheae, and a phloem portion limited to about 20 or fewer elements; in
the stems and leaf-stalks of many Aroideae the number of the tracheae falls in many
bundles to 2 and i'; on the other hand, the thicker bundles of Monocotyledonous
plants, and above all the thick bundles of the Ferns above mentioned, and of the
leaves of Dicotyledonous land-plants, present very high figures. It is an obvious
a priori conjecture that a definite ratio exists between the size of the bundles,
especially of their vascular parts, and their number, and that both are definitely
related to the extent of the transpiring and assimilating leaf-surface, as well as to the
vigour of the root system and the arrangement of the roots. A number of facts,
which have partly been stated here, partly in Sects. 61-71, and are still to be
considered in Chap. XIV, point to such relations. The comparison of nearly-related
species inhabiting the water and the land respectively, demonstrates among the
former a considerable diminution in the development of the bundles (cf, e. g. Figs.
153 and 154), which may extend to the entire disappearance of the xylem. A
sufficient basis, however, for the attainment of general results, available for more than
individual phenomena, is still wanting, so that we can here only point out these
manifest relations without entering into them more minutely.

The structure of the two parts of the bundle, in so far as it is brought about by
the juxtaposition of the different sorts of tissue, and by their peculiarities, as described
in former chapters, will in the case of the Osmundaceae and Marattiacese be
mentioned further on, under the head of concentric bundles in Ferns (Sect. 106). In
the other collateral bundles, especially those of the Phanerogams when they consist
of more than one or two elements, —

(a) The xylevi is built up of tracheae and (parenchymatous) cells. At its inner
edge lie a few narrow, spirally or annularly thickened tracheae, which are the first
products of the differentiation of the tissues, and are thus to be called primitive
elements (Protoxylem of Russow). For the reasons given at p. 157 it is especially
these primitive elements in which, in the mature plants, the spiral threads are
steeply wound or quite distorted, and the rings widely and often irregularly separated
from one another. The primitive elements themselves are not uncommonly com-
pressed by the expansion of their neighbours, and here and. there manifestly
destroyed. In the Coniferae, Equiseta, and Ophioglosseae, the primitive elements are
tracheides; the same may be the case in the plants or parts of plants mentioned
above at p. 165 as wholly destitute of vessels. In the other cases they are called
vessels, and in most instances no doubt rightly so, although just as in the case
of the primitive tracheae, few very accurate investigations of this really not very
essential distinction have been undertaken. Outside the primitive elements wider
tracheae follow, which are tracheides or vessels according to the individual cases
mentioned in Chap. IV, and especially in Sect. 40 Their development takes place
successively, advancing from the inner edge of the bundle outwards, and as a rule at
a time when the elongation of the entire part to which they belong is nearly at an
end. The thickenings on their walls therefore have a successively denser arrange-
ment : dense spiral and annular tracheae, then reticulated and pitted tracheae follow

Compare van Tieghem, Struct, des Aroidees, 1. c.

“ Compare details in Caspar)', /. c.

323

PRIMARY ARRANGEMENT OF TISSUES.

one another in succession from within outwards, with gradual transitions, or with the
omission of one or the other intermediate form. As regards the occurrence of
particular forms of thickening, it may at any rate be given as a rule that among the
Monocotyledons the series ends with the development of dense fibrous thickenings,
annular and spiral fibres, reticular fibres, and surfaces with non-bordered pits or scalari-
form markings (pp. 158, 163). Bordered pits or scalariform slits here only occur in the
collateral bundles of stems which are long-lived and relatively slow-growing, as those
of many Palms, and Arundo Donax, and many rhizomes. The same often applies
to the stem and leaves of such Dicotyledons as form no secondary wood, though here
exceptions occur even in relatively short-lived parts, as for example in the leafy stems
of Thalictrum flavum and aquilegifolium. Tracheae with bordered pits appear in
the stem of most Dicotyledons and Gymnosperms which form secondary wood, at the
point where the latter joins on to the primary bundles (cf. Chap. XIV), and occur
further in the outer part of the bundle-trunks which traverse the leaf-stalks and leaves.
The elements of this part are in many cases especially among Coniferae, very
similar to those of the secondary wood in the stem generally, both in form and
structure ; yet this minute agreement is by no means of universal occurrence.

As the structure of the walls changes from within outwards, the width of the
tracheae increases in comparison with that of the primitive elements, and this change
may take place according to the individual cases either gradually or suddenly, either
constantly or so that narrower tracheae again succeed the wider.

In very small bundles, containing only one or two tracheae, the latter form a
group which, though varying in individual cases, does not require a further description.
On the other hand, larger bundle-trunks, which consist of numerous elements,
show remarkable differences both in their arrangement and in their gradually
changing width.

The collateral bundle-trunks of most Dicotyledons and Gymnosperms show the
tracheae ranged in rows running from within outwards, which touch each other
laterally, or are separated by rows of non-equivalent elements. In every row the
tracheae become wider towards the outside, and in thick bundles soon attain an
average size, which remains uniform in the whole region (Figs. 157, 158, 183). In
the large bundles of many Dicotyledonous leaves the width of the elements first
increases successively, and then diminishes again to an average size, which further
towards the outside remains constant, e. g. leaves of Camellia, Ilex, Rosmarinus,
species of Eucalyptus, &c. The difference between the narrowest and widest
tracheae is in all these cases moderate, especially in comparison with many Mono-
cotyledons; the largest is 2-3 times as large as the smallest, or even less. The
absolute size of the tracheae is also moderate ; in the leaves they are generally very
narrow, falling far short of the average width of the tracheides and fibres of the
secondary wood. Cf. the dimensions given in Sect. 40 and Chap. XIV.

In the thicker bundles of the Monocotyledons other conditions are the rule. In
most cases the tracheae here form two main rows as seen in cross-section, which
diverge like the limbs of a V. At the point where the two limbs cut one another, or
inside it, lie the primitive elements. The end of each limb is usually occupied by a

^ Compare Frank, Botan. Zeitg. 1864, pp. 167, 393.

STRUCTURE OF COLLATERAL BUNDLES.

323

trachea, exceeding the primitive elements many times in width, with dense spiral, or
narrow reticular thickenings on its wall ; and this either forms the end of a continuous
or interrupted row of successively wider trachete, or it follows suddenly on much
narrower ones. In the middle between the two limbs there are either no vessels
(even the whole Phloem may be included here, e. g. the leafy stem of Asparagus S
and Tamus communis), or the middle is occupied and more or less filled up by a
group of narrow, densely reticulated or pitted vessels, as for example in the Grasses
(Fig. 151) ; and this group may spread out even beyond the external edge of the
large tracheae at the ends of the limbs (Fig. 147). In the laterally flattened Mono-
cotyledonous bundles mentioned above, the tracheae lie in an interrupted single, or
in places multiple row, running from within outwards. In this it is usual for one or
a few narrow primitive elements to be followed on the outside by one or a few
tracheae of considerable width, e. g. by a very large spiral tracheide in the leaf-
stalk of Musa ^ and Canna, &c. ; further outside there are either no more tracheae,
e. g. in the leaf of Pandanus or some relatively very narrow ones, e. g. Musa,
Canna, Heliconia, &c. : the broad bundles in the stem of many Palms, especially
Calamus S though not laterally flattened, also show the same character ; in Calamus
some narrow spiral vessels are followed by a single pitted vessel of enormous width
(cf. p. 169), and there are no others further outside.

The phenomena mentioned give rise to the characteristic habit of most Mono-
cotyledonous bundles, which is especially evident in cross-section. They occur
also among those members of the class which, like the Dioscorese, behave differently
to the others with respect to the arrangement of the bundles (cf. p. 276). On the one
hand, however, they are not confined to Monocotyledons, for the bundles in species
of Ranunculus, and especially of Thalictrum, belong to or are closely connected with
the form first described for Monocotyledons, while those of Nelr.mbium stand in the
same relation to the second form. On the other hand, their distribution is by no
means universal even in those families of Monocotyledons in which they prevail.
The cross-section through the bundle-trunks in the leafy stem of Fritillaria im-
perialis and in the leaf of Phormium tenax ® shows, for example, a triangular group
(widening towards the outside) of moderately wide tracheae, which increase but
little in size towards the outside, and show the regular character and succession in the
structure of their walls. The thicker bundle-trunks in the leaf of Yucca filamentosa
(from which the thinner ones only differ in the number and size of their elements)
show a thick xylem, broadly triangular in cross-section, in which the primitive ele-
ments are succeeded by a likewise broadly triangular group of spiral vessels with a
closely-wound fibre, which are of moderate and tolerably uniform width. With this
group is immediately connected on the outside a zone, consisting of about four cross-
rows of narrower reticulated and pitted vessels, united at all points, which on the other
side borders on the phloem.

The bundles of the Equiseta, to be further described below, agree essentially

' Von Mohl, l.c. Tab. G.

’ Von Meyen, Phytotomie, Taf. VIII.

^ Von Mohl, l.c. Tab. G.

Von Mohl, l.c. Tab. G, fig. 3.
‘ Von Mohl, 1. c. Tab. D, F.

324

PRIMARY ARRANGEMENT OF TISSUES.

in the arrangement of their tracheae with the two-limbed bundles of Mono-
cotyledons.

The arrangement of the parenchymatous cells of the xylem results for the most
part from that stated in the case of the tracheae. In bundles with many rows of
tracheae they form similar rows interpolated between the latter, with the form of
narrow long medullary rays, round which, in a longitudinal direction, the lines of
tracheae run with slight undulations, alternately receding from one another and coming
into contact at the ends of the rows of cells. In bundles of the Monocotyledonous type,
with less regular seriate arrangement, they form single longitudinal rows, or groups of
various form between the tracheae. The cells themselves are elongated in different
degrees, with horizontal or oblique ends, their walls delicate or considerably thickened
and lignified ; in the latter case their distinction from tracheides is often difficult.

(d) The phloem of collateral bundles consists of sieve-tubes, and thin-walled,
elongated, prismatic cells, for which Nageli’s term Cambiform-Q.tW.% is to be reserved.
With reference to the more special structure three cases, not all equally well known,
are to be distinguished.

1. In the more accurately investigated Monocotyledons (e. g. the Grasses, Fig.
150, with which the Equiseta appear also to agree), and no less in very many
Dicotyledons, as Ranunculaceae, Umbelliferse (Foeniculum, &c.), Vitis, Aristolochia,
also Cucurbita, &c., the cross-section of the phloem shows the two kinds of meshes,
which have been known since Moldenhawer, and Mohl’s Anatomy of Palms ; some
wider and polygonal, which are the cross-sections of the sieve-tubes, others narrower,
square or rectangular, or frequently narrow and obliquely four-sided, representing the
cross-sections of the cambiform-cells. The latter stand isolated among the sieve-
tubes, distributed with varying regularity, so that as a rule each sieve-tube borders on
another with one part of its lateral walls, and with another part on a cambiform-cell.
Traced longitudinally the cambiform-cells form rows between the sieve-tubes, and
parallel to them. Regarded individually they are as a rule shorter than, seldom as
long as the joints of the sieve-tubes. Both from their arrangement in cross-section,
and on tracing them in the longitudinal direction, it often has the appearance as if the
cambiform-cells arose with the elements of the sieve-tubes from one mother-cell, the
latter dividing longitudinally into a daughter-cell which becomes the sieve-tube element,
and another which either becomes a cambiform-cell without further division, or is
divided by cross-walls into several of them h On this point, however, more
accurate investigations must be undertaken The cambiform-cells have delicate
non-lignified cell-walls and finely granular protoplasm, with a nucleus elongated in
the longitudinal direction. On the structure of the sieve-tubes nothing need here
be added to what was said in Chap. V.

2. In several, perhaps in numerous Dicotyledons (e. g. in the leaf-stalk of Olea
Europsea (Fig. 156), in the stem of species of Lobelia, Crassulacese, Cactese'^; and in
several, especially succulent, Euphorbise, as E. Caput Medusae), the cross-section
of the phloem shows, among wider thin-walled elements, numerous or sparsely
scattered groups of much narrower cells, each group often appearing from its size

’ Compare Vochting, Melastomeen, p. 16.

* [Compare Wilhelm, l.c. (see p. 172).]

^ Compare Vochting, Rhipsalideen, /. r. Tab. 52.

STRUCTURE OF COLLATERAL BUNDLES.

3^5

and arrangement as though it had proceeded from the longitudinal division of
one of the wide elements. So far as investigations extend, the narrow elements
are here sieve-tubes, which may be accompanied by narrow cambiform-cells ;
the wide ones are cells, whether they be called cambiform or simply parenchyma.
More minute and extended investigations of these elements are wanting ; they are
usually curtly disposed of under the name cambiform or soft bast. In the thick
bundles, often mentioned above, of Dicotyledonous leaf-stalks and ribs, the paren-
chymatous rows, resembling medullary rays, are continued directly outwards from the
xylem into the phloem, and in the latter pass between bands of tissue, appearing
irregularly narrow-meshed in cross-section, which no doubt contain the sieve-tubes.

3. In the primary bundles of the Coniferse, especially of the leaves, where there
is no disturbance from the secondary growth which quickly comes on in the stem,
and also in the large bundles of the leaf of Welwitschia, the cross-section of the
phloem shows regular rows of similar elements with soft membranes, which are in a
very high degree capable of swelling. . They are'either present alone (Figs. 63, 157),
or a single row consisting of relatively wide parenchymatous cells runs here and there
between them, e. g. in the leaf of Dammara alba. The general form of the regularly
serial elements first mentioned is elongated prismatic. How far they are sieve-tubes
or cambiform-cells is undecided.

The development of the elements of the phloem of collateral bundles begins at
the external edge and proceeds towards the xylem, and thus in the opposite order to
that in the latter, i. e. centripetally in the phloem, if the centrifugal direction is
maintained in the xylem. In the middle of the bundle, on the border betw'een the
two parts, active meiistematic division may still be going on when the elements of
the external and internal edge are completely differentiated.

The outermost, first-developed elements of the phloem (Russow’s Protophloem)
are often distinguished from those which follow by their smaller width, and thicker,
apparently gelatinous walls ; as regards their nature, however, they are in the cases
now in question, which admit of more accurate investigation, partly sieve-tubes,
partly cambiform-cells. In the thicker bundles they not uncommonly become com-
pressed in the radial direction, owing to the expansion of the surrounding tissue,
while their walls apparently swell up to the obliteration of their lamina — a pheno-
menon which ensues to a much greater extent in the old sieve-tubes and cambiform-
cells of the secondary bast. (Cf. Chap. XV.)

The boundary between phloem and xylem is in most cases on the whole sharply
marked, owing to the contrast between the delicate and non-lignified membranes on
the one hand, and the characteristically thickened and lignified membranes on the
other. The elements which form the boundary on the side of the phloem, no doubt
always have the characteristics of cambiform-cells ; it is not known that a sieve-tube
ever borders directly on a trachea. In the thick bundles of the great majority of
Dicotyledons the cells on this border-surface long remain capable of division, and
when the differentiation of tissues has once begun at the periphery, their divisions
take place chiefly in the tangential direction, parallel to the outer edge. Owing to
the arrangement of the cells in radial or tangential rows thus determined in the zone
indicated, the boundary is pretty sharply defined, even in cases where the actual
divisions soon come to an end. In the stems of Dicotyledons and Gymnosperms

PRIMARY ARRANGEMENT OF TISSUES.

336

with secondary growth in thickness, this border-zone maintains its capacity for
division, and becomes a portion, or a starting-point, of the cambial ring (Chap. XIV).
In other bundles the cessation of the divisions takes place early in each transverse
portion, simultaneously with the general differentiation of the tissues ; and according
to the degree in which this happens, the radially arranged boundary layer becomes
less clear. It is therefore also, especially in Monocotyledons, the less possible
everywhere to fi.x a sharp boundary between phloem and xylem, the less numerous
and crow'ded the lignified elements in the latter are.

The word Cambiform used above was first adopted byNageli (Beitr. I. p. 4). For the
meristematic strand giving rise to a vascular bundle, and consisting of thin-walled longi-
tudinally extended cells in which longitudinal division goes on for some time, Nägeli
uses the traditional and certainly ambiguous name Cambium, for which Sachs has re-
cently substituted Procambium. The tissue of the phlocTn w'hich has arisen from this
cambium and passed over into the permanent condition is called by Nägeli in its totality
the Cambiform, i. e. cambium-like tissue, as its elements are so similar to that of the
cambium, in their elongated form and thinness of wall, that former observers have
actually identified them with it. Our present knowledge imposes the necessity of severing
the sieve-tubes from Nageli’s Cambiform, as a distinct kind of tissue. The name there-
fore, once being in existence, remains reserved for their characteristic companions, and
may the better be used for them as it is for the most part literally applicable to them,
even when the original meaning, indicated in Chap. XIV, and differing from that above-
mentioned, is restored to the w’ord cambium.

The structure of the phloem of collateral vascular bundles of the main stems has been
described in the preceding paragraphs ; the description was based primarily on the
numerous investigations now existing of such objects as afford a clear and certain
insight, owing to the size of the elements in question. It was already mentioned under
2 and 3, that in certain cases differences occur from the type described under i, and that
doubts exist as to the structure. Besides these definitely characterised cases, there are
many others, especially concerning the smaller vascular bundles, in which we know
nothing more of the structure of the phloem than that it consists of thin-walled, narrow,
and elongated elements, the delicacy and smallness of which, as well as the tendency of
their walls to swell, which interferes with their preparation, is deterrent to accurate
research. Where these difficulties have already been suecessfuily overcome, the struc-
ture described has always manifested itself. An uninterrupted series of transitions
leads from the cases with relatively large, easily intelligible elements, to those where
they are most delicate and difficult. No grounds whatever exist which would compel
us to assume an essentially different structure. I therefore think that I am right in
staling that the structure described exists in all vascular bundle-trunks here in question,
and the more so as it is really not long since the largest sieve-tubes were first clearly
recognised, as remarked at p. 182, and I do not doubt that further investigations, which
are in any case a necessity, will justify what has been said.

In the xylem of many collateral bundles an intercellular passage occurs, which
follows the course of the whole bundle, sometimes next to or within the otherwise
persistent xylem, sometimes so that, though the latter is originally formed, the
trachese become destroyed and degraded as the parts expand.

In numerous Monocotyledons, the Equiseta, and some Dicotyledonous water-
plants, at the inner edge of the bundle, where the primitive tracheae are placed, a
passage is formed by the peripheral extension of the surrounding cells, i.e. schizogene-
tically (p. 200), while the external part of the xylem attains perfect development and
is persistent. This severing of the original continuity of tissue goes on within the

STRUCTURE OF COLLATERAL BUNDLES.

327

wall of the primitive trachese ; the latter are attached to the wall of the passage, and
if its expansion is considerable they may be laterally removed from one another, and
as the separation usually takes place before the elongation of the parts is complete,
they become simultaneously distorted in the longitudinal direction, and reduced to
thickening fibres adhering to the wall of the passage. This process often attacks
annular tracheae, the rings of which, in cases of great elongation, then become
shifted to a long distance from one another. The width attained by the passage is
various, sometimes equal to that of a moderate vessel, sometimes to the cross-section
of the whole persistent part. The cells actively engaged in its formation undergo,
on considerable expansion, divisions which are radial with reference to the passage,
and remain as a rule thin-walled. These passages contain air, with the exception of
some submerged plants to be mentioned below.

All collateral bundles of the stem of Equiseta show a relatively wide passage at the
inner side of the xylem. The same phenomenon occurs very widely in the leafy stems
(halms) and leaves of numerous Monocotyledons, but not in their rhizomes : thus in the
stems of Hydrocharis, Butomus, Sagittaria, Alisma, Juncaceae, Xyris, Cyperaceas, Acorus
Calamus, Leucojum, and Commelineae (Tradescantia albiflora, zebrina, Lyonii^). As the
names show, most but not all of these plants inhabit water or bogs. F urther, the pheno-
menon does not occur in all the bundles of the same part ; e. g. the smaller bundles in
the leaf and scape of Acorus Calamus have no passage, while the larger bundles have a
very wide one ; and further, nearly related plants, agreeing in their mode of life (e. g.
Grasses, Cyperacese, Commelineae), often show a different character with reference to the
width of the passage, and its presence or absence. Of Dicotyledons only some water-
plants belong to this series, namely, the water Ranunculi and Nelumbium, besides those
to be mentioned below. Both have a passage on the inner side of the larger, but not
of the smaller vascular bundles of the stem.

In a number of water-plants the process described extends to the complete
destruction of the whole xylem. The latter originates at an early age in the form of
a few annular tracheides, or of a large number arranged in a bundle, all of which
become both separated from one another laterally, and torn longitudinally, as the part
elongates. The inconspicuous separate rings or fragments of rings remain
adhering to the wall of the passage, the phloem, which is usually strongly developed,
is alone persistent. As far as the investigations extend, these passages contain
water. The leaf-trace bundles of the internodes of Potamogeton natans and its
allies belong to this series; — in their cauline bundles and in the nodes the tracheae
are persistent — also the bundles of the leaf-stalk and flower-stalk of species of
Nymphaea and Nuphar, and of Brasenia peltata^. In many bundles'of the plant last-
named a portion of the vessels are persistent; they thus belong to the former
category ; in their rhizomes no passages occur in the vascular bundles. — On allied
phenomena in the non-collateral bundles of other water-plants, comp. Sect. no.

A formation of passages in some degree different from that described occurs in
the flower-stalks, leaf-stalks, and leaves of Aroidere, especially of those with unisexual
flowers, as Colocasia, Caladium, Richardia In the xylem only a few tracheides —

" Compare Frank, Beitr. l.c. p. 1,^8.

^ Compare Caspary, Berlin. Monatsberichte, 1862, l.c. — Trecul, Ann. Sei. Nat. 4 ser. tom. i.
p. 15'-

^ Duchartre, Recherches sur la Colocase, Ann. Sei. Nat. 4 ser. XII. — Unger, Beitr. z. Physiol.

328

PRIMARY ARRANGEMENT OF TISSUES.

2-4 in each cross-section — are formed, between delicate narrow cells; they are
elongated, and ranged in longitudinal rows, one above the other, like vessels. One
of these rows becomes expanded, according to van Tieghem’s description apparently
passively, to form a passage surrounded by narrow cells, the thickenings on its wall
disappearing in places. The very oblique, fibrously thickened (unperforated ?) end-
surfaces, with which the articulations are in contact one with another, are persistent;
in cross-sections, therefore, the passage often appears divided by a septum into two
unequal portions. The other tracheides, lying partly inside, partly outside the dilated
one, remain narrow and delicate, with annular or spiral fibrous thickening.

In the scape and leaf-stalk the passages increase in width with their distance
from the periphery, the outermost bundles have only a narrow row of tracheides in
place of them. They accompany the bundles into the lamina of the leaf, extending into
the thick strand into which the bundles are united at its apex ; here they lie close
side by side in great numbers, forming the often-described water canals of the
leaves of Aroidese. In the leaves also they are accompanied by rows of tracheides
which are not dilated, with which transverse anastomosing branchlets are everywhere
connected, the petiole not excepted.

In the thicker bundles of the leaf of Sparganium ramosum a wide passage is
produced, according to Franks in the same manner as among the Aroidese. The
contents of the passages consist of air and watery liquid, while among the Aroidese
latex containing tannin is also present in places. Comp. p. 188.

The sheaths of collateral bundles consist either of simple parenchyma, or, rarely,
of the form described as endodermis ; or, lastly, and indeed in the majority of col-
lateral bundle-trunks, of strands of sclerenchymatous or collenchymatous fibres, which
accompany the bundles, whether it be as a tube encircling the whole bundle, or as
a strand which partly surrounds the circumference of the bundle. In the latter case
it rarely borders exclusively or principally on the xylem, but usually on the phloem,
or only on its outer edge. These sheaths and accompanying strands are to be
separated from the bundle itself, and considered in the following chapters. Here
we have only to mention that those which consist of sclerenchymatous fibres may
not uncommonly border immediately on the elements of the bundle, and even insert
their own elements between the latter, so that the limitation, especially as seen in
cross-sections, ceases to be clear, and the arrangement of the specific parts of the
vascular bundle is often influenced in a peculiar manner.

A gradual transition from the sclerenchymatous elements of the sheath to the
cells of the xylem very often takes place in cases where the latter are provided with
strongly thickened and lignifled membranes, e.g. in stout bundles of Monocotyledons.
(Fig. 150.)

The sclerenchymatous sheath is generally very sharply limited on the side of
the phloem ; the latter lies as a uniformly thin-walled mass of tissue between the
sheath and the xylem. It happens in rare cases that here also the sclerenchyma of
the sheath penetrates deeply into the phloem, and is continued as far as the thick-walled

d. Pfl. Wiener Acad. Sitzgsber. Bd. XXVIII. p. in. — De la Rue, Botan. Zeitg. 1866, p. 816. — Van
Tieghem, Struct, des Aroidees, 1. c.

^ Beitr. p. 137.

STRUCTURE OF COLLATERAL BUNDLES. 329

cells of the xylem. In the stem of Rhapis flabelliformis most bundles have a phloem
which is somewhat crescent-shaped in cross-section ; it is surrounded by a thick
sheath forming a bundle of fibres, which projects strongly towards the outside; round
the small xylem, consisting of a few vessels surrounded by thick-walled cells, the
sheath is feebly or not at all developed. In some of the inner bundles a ridge-like
projection about three layers thick passes from the sheath to the thick-walled cells of
the xylem through the middle of the phloem, severing the latter into two symmetrical
halves. Essentially the same phenomenon appears much more conspicuously in the
stem of Calamus. The xylem, as already stated at p. 323, always shows some narrow
spiral vessels, and outside these a very large pitted vessel ; around and between them
lie thick-walled elongated cells. In the smaller peripheral bundles the single phloem,
semicircular in cross-section, lies on the outside of the pitted vessel, and is surrounded
by a strong sheath of fibres, which is continued round the xylem. In most bundles
the phloem is divided by a broad continuation of the sheath, extending up to the
pitted vessel, into two portions lying right and left of the latter. Each of these
portions consists o^ some large sieve-tubes (comp. p. 176) standing in a row parallel
to the circum.ference of the vessel, together with the accompanying cambiform cells.
This row also may once more be interrupted by sclerenchyma, so that individual sieve-
tubes with their accompanying cells stand isolated in the sclerenchyma of the sheath h
In the mature leaf of species of Pandanus, the phloem appears at the first glance
to be wholly absent even in thick bundles, and in its place a strong narrow strand of
sclerenchymatous fibres seems to border immediately on the xylem More accurate
investigation shows isolated sieve-tubes between the sclerenchymatous fibres. In
younger bundles they are easier to
find — perhaps also more numerous ;
it seems as if here, as in other bun-
dles, sieve-tubes present at an early
period were afterwards crushed by the
neighbouring cells and rendered un-
recognisable, but this has still to be
investigated.

As the parts belonging to and bor-
dering on the vascular bundle must
in fact be considered in connection
with one another, it seems more ex-
pedient here, in conclusion, to collect
together some figures referring to
collateral bundles, explaining them
with reference to the paragraphs
above, than to insert them among
the preceding descriptions of the in-
dividual parts.

Fig. 149. Cross-section through a
mature internode of Equisetum

' [Compare Kny, Verhandl. d. Bot. Ver. Prov. Brandenburg. Bd. XXIII, 1881, pp. 94-109;
also his Bot. Wandtafeln, V.]

^ Compare Meyen, l.c. (p. 323). — Van Tieghem, Ann. Sei. Nat. 5 ser, VI.- p. 197.

33°

PRIMARY ARRANGEMENT OF TISSUES.

palustre (145). u Endodermis. i axial air-canal, at x remains of the membranes of
shrivelled pith-cells. In the middle a vascular bundle surrounded by parenchyma, with-
out a distinct sheath. At the inner edge of the bundle lies a wide intercellular passage,
in which the letters r, t, s are written, t an annular fragment, adhering to the wall, of
the membrane of a primitive tracheide for the most part destroyed. r persistent
annular tracheides. g groups of the last developed, likewise permanent, annular and
reticulated tracheides, distinguished from the surrounding tissue by the shading of
their walls, s the phloem ; in this the wider lumina belong to the sieve-tubes (cp. p.
180), the narrower ones, some of which are granularly dotted, to the cambiform-cells.
The double-contoured bands on the outer edge of the phloem, inside the layer of cells
following u, indicate the collapsed primitive elements of the phloem (Protophloem).

Figs. 150 and 1 5 1 represent two cross-sections through a bundle of the leaf-trace of Zea
hlais, at different parts of its course. Fig. 150 (550), from Sachs’s Text-book, is from

I'lG, 150.

the stem, g-g, s, r, I, the xylem, 'v the phloem. In the latter rv, v indicate the sieve-
tubes, between which the narrower cambiform cells stand in regular distribution. At
the outer edge of the phloem are its narrower, thick-walled primitive elements. On the
inner edge of the xylem in an intercellular passage, /, bounded on the outside by the ring,
r, of a primitive annular vessel, partly destroyed by the longitudinal extension, s spiral
vessel, g, g large vessels with (unbordered) pits, or narrowly reticulated. Between the
phloem and s, g and g a transverse group of narrow pitted vessels. A sheath composed
of sclerotic lignified elements goes all round the bundle ; p, p thin-walled parenchyma
outside the sheath, a outer edge, i inner edge of the whole bundle. In the lamina of

STRUCTURE OF COLLATERAL BUNDLES,

331

the leaf and the upper part of the leaf-sheath the bundles are similar to that in Fig. 150,
though on the average smaller.

Fig. 151 (145), on the other hand, is
taken from the place where the bundle
passes through the basal portion of the
leaf-sheath of a young plant. The bundle
itself, with its group of vessels at g, is in
all parts smallerthan that in the preceding
figure, but otherwise similar to it, as is
clear without explanation, even in the
finer points of structure not represented
in the figure. Here however there is a
single-layered sheath, consisting of deli-
cate parenchymatous cells, square in
cross-section, around the whole bundle.
Outside this, and separated by it from the
phloem, is the thick strand of collen-
chyma, sc. Both the latter and the bun-
dle are surrounded by the parenchyma-
tous layer st, which is rich in starch (the
starch-sheath. Sect. 122). e-e Epidermis
of the outer surface.

Fig. 152. Cross-section through a
vascular bundle of the internode of a
creeping stem (runner) of Ranunculus
repens (225). xx annular and spiral ves-
sels at the inner edge, t pitted vessels in
the external region of the xylem ; be-
tween and around the latter delicate-
walled elongated parenchymatous cells,
j phloem ; the larger meshes are sieve-
tubes, the smaller ones, partly dotted,
are cambiform cells. At
the inner border of the
phloem are delicate cambi-
form cells in rows. Ex-
ternally to its constantly
thin-walled peripheral ele-
ments, the bundle is sur-
rounded by a thick scle-
renchymatous sheath, which
is only interrupted on the
outer border of the xylem.

Outside this is large-celled,
thin-walled parenchyma.

The longitudinal section
through the bundle, if not
quite accurately median,
would be similar to that
of Saururus, represented
in Fig. 57, p. 157.

Fig. 153 (225) represents
the cross-section of one of
the larger bundles in the
internode of Ranunculus

Fig. t5i.

Fig. 152,

333

PRIMARY ARRANGEMENT OF TISSUES.

fluitans. The structure is similar to that of the preceding species, but simpler, gx trachea
with annular and loosely spii al fibres, bordering on an intercellular passage in which the
letter is written, g wider tracheae with densely spiral and reticulated wall, s five relatively
large sieve-tubes between narrow cambiform cells. The whole bundle is surrounded
by narrow elongated prismatic cells, showing non-lignified cellulose walls, and these pass
over gradually on the outside into large-celled parenchyma with abundant starch. As
regards the surrounding layer marked u it remained doubtful whether it has an indi-
cation of endodermal structure on those walls which are radial with reference to the
bundle.

Figs. 154 and 155, from the fully elongated hypocotyledonary stem of Ricinus communis
(from Sachs’ Textbook), represent that form of bundle, with radially disposed xylem,
which predominates among Dicotyledons, together with the initial stages of the secondary
growth in thickness, which here follows immediately on the formation of the primary
bundles. In the cross-section (Fig. 154) _§■,/, rare the rows of vessels alternating with rows of
thick-walled cells, beginning at the inner edge of the bundle with the primitive elements,
which are distinguished by the thick shaded walls, t narrower, g wider pitted vessels.
y phloem consisting of sieve-tubes, cambiform and delicate parenchyma, with three bundles
of sclerenchymatous fibres on its outer border, at b. On the boundary between xylem
and phloem, the formation of the zone of cambium and secondary growth (c-r) has begun
by means of tangential divisions, and is continued, starting from the sides of the vascular
bundle, over the parenchymatous zone cb, cb\ cf. Chap. XIV. m parenchyma of the

Fig. 153.

pith, r of the outer cortex. Between the layers containing the letters b and r is the
parenchymatous sheath containing starch-grains (Starch-ring, Chap. IX). Fig. 155 is the
radial longitudinal section through a bundle of the same structure as Fig. 154. The
letters r, b, c, m indicate the same things as in the former figure, p bands of parenchyma
from the phloem, s innermost and narrowest spiral vessel, s' a wider one. / scalari-
form reticulated vessel, t mature vessel with bordered pits ; at q the perforated trans-
verse wall between two elements. F a pitted ve.ssel still immature, the borders of the
pits not yet developed, h, h' thick-walled cells of the xylem ; on the wall of t and t’ the
boundaries of cells which have been cut through are visible, h", h'" narrow tracheides(?).

STRUCTURE OF COLLATERAL BUNDLES.

333

riG. 155.

334

PRIMARF ARRANGEMENT OF TISSUES.

Fig. 156. Cross-section through the vascular bundle in the midrib of the leaf of Olea
Europaea (375)- the phloem, consisting of wide (parenchymatous ?) cells, and scattered
groups of very narrow elements (sieve-tubes?); comp. p. 325. /-/sclerenchymatous fibres,
forming a girdle round the outer edge of the phloem, and occurring scattered on the
inside of the xylem. The very dense xylem borders on the phloem internally; the
primitive elements at its inner edge do not appear clearly ; its larger external portion
consists of radial rows of thick-walled pitted tracheae, which alternate with bands of
parenchyma. The latter are indicated by the granular dotting of the lumen, p paren-
chyma.

Fig. 156.

Fig. 157. Cross-section through a vascular bundle in the leaf of Welwitschia mirabilis
(145). An uninterrupted zone of very thick-walled and elongated sclerenchymatous fibres
surrounds at f the outer edge of the phloem, and at f the -inner edge of the xylem.
Inside the zone y follows the j-hloem, which is crescent-shaped in cross-section, consisting
of narrow, radially arranged, elongated elements ; their structure could not be detected
with certainty, nor are they drawn quite accurately in the figure, because the great
swelling of the membranes makes it impossible to spread out the cross-section of the
phloem in one plane, in such a preparation as that figured. The inner portion of the ^
xylem enclosed by the fibrous sheath f consists of tolerably wide, elongated prismatic
cells, connected without intercellular spaces, with thick almost gelatinously soft mem-
branes. Between them are inserted numerous, very narrow, compressed and distorted,
spiral and annular tracheides, with thick, distorted, fibrous thickenings ; sp ; they doubt-
less represent the primitive elements of the bundle. Further outside follow the persistent
tracheae, placed in tolerably regular rows, here and there alternating with delicate cells,
and in general increasing in width in each row from within outwards ; first annular and
spiral tracheae, with dense and very broad thickening layers, then reticulated and (at g)
large pitted vessels, with bordered pits and round perforations in the cross-walls, t, t
are the rows of pitted and reticulated tracheae in cross-section ; they surround the bundle

STRUCTURE OF COLLATERAL BUNDLES.

335

and are to be described below in Sect. 112. They also occur on the right-hand side of
the figure, and may be known by their notched outline, p parenchyma of the meso-
phyll, with an indication of the small crystals of Calcium oxalate imbedded in the walls.

Sect. 102. The bundles of the leaves of Cycadeae and Isoetes, with which
perhaps those of Phylloglossum agree, are different from the typical, simply collateral
bundles in the arrangement and development of their parts.

Fig. 157.

The peculiarity of these bundles, expressed generally, consists in the fact that
the essential elements of the xylem are reversed in position and order of develop-
ment, as compared with those in the typical cases. In the plants first named there
are the additional peculiarities that tracheides appear at a later stage on the border
of the phloem, and that the bundles are in many cases united in pairs.

The bundles of the leaf-trace of the Cycadese begin in the stem, according to Met-
tenius^ as simple collateral bundles, and run out in the same form into the sheathing
bases of the leaves, but before entering the petiole of the foliage-leaves, or the apex of

‘ Abhandl. cl. K. Sachs, Gesellsch. d. Wissensch. VII. p. 573.

336

PRIMARY ARRANGEMENT OF TISSUES.

the scale-leaves, they assume a changed structure, which they maintain throughout
their w'hole course in the leaf. In the round cross-section of the bundle (Figs. 158,1 59),
a small group of narrow spiral tracheides {s p) (the primitive elements of the xylem)
occupies about the centre. From this an uninterrupted group of large prismatic pitted
tracheides (the inner portion of the xylem) extends towards the inside ; this group
occupies the entire inner side of the bundle, and has in cross-section the form of a

Fig. 158. — Cycas revoluta. Petiole of a small foliage-leaf belonging to a young plant ; vascular bundle, cross-section
(225). Explanation in the text. The spiral trachelde sp is connected with the inner pitted tracheides t by means of a
group of annular and reticulated tracheides. In c here and there fragments of the large crystals of Calcium oxalate.

sector of a circle with the centre at the primitive tracheides. The remaining part of
the bundle, lying outside that described, is principally formed of thin-walled elements,
ranged in radial rows : next the outer edge are several concentric rows of sieve-tubes
{s), separated by delicate parenchyma ; the outermost row which bounds the bundle,
being in the mature condition compressed in the manner often described, and thicker-
walled than the rest, bordering the outer edge as a narrow shining band (/) ; on the
boundary of the xylem, so far as present investigations extend, only prismatic cells
occur, without sieve-tubes. To these parts is finally added an external portion of
the xylem, developed in centrifugal order, outside the primitive tracheides; this forms
a small group of pitted tracheides {a) ranged in irregular radial rows, which are
separated from one another, from the primitive tracheides, and from the inner
portion of the xylem, by thin-w’alled elements. As the bundles become thinner in

STRUCTURE OF COLLATERAL BUNDLES.

337

their course through the leaf, the thickness of the individual portions diminishes,
especially the external portion of the xylem. In many species, at any rate, the bundles
in the cross-section of the leaf-stalk are generally arranged in the figure of an
inverted ß, with the limbs directed towards the upper side. In the constricted part
of the figure either they are separate, the constriction being open ; or they approach
one another in pairs, the inner vascular portions of each pair being turned towards
one another, and the scalariform vessels of each in uninterrupted connection over a
broad surface. In Zamia longi folia one such coupled bundle is present near the
middle of the cross-section ; in Dion there are about six of them forming a row
vertical to the surface of the leafk Other species of Zamia show, according to
Mettenius, a less regular grouping of the bundles and of their connections.

In Cycas revoluta the bundle is encircled by a thick-walled sclerotic sheath,

Fig. 159.— Cycas revoluta (225). Lonjjitudhial section through a similar bundle ot the same leaf-stalk, in the
direction r — r, Fig. 158. The letters have the same meaning as in that figure, n reticulated tracheide with narrow
slits ; the fibres of the narrowest (first) spiral tracheide are distorted.

sharply limited both inside and out, which consists of short, wide, angular elements
{c c), with pitted or narrowly reticulated walls, and cavities often filled up by large
crystals of Calcium oxalate. In the other species investigated, there is no sheath
which can be sharply distinguished from the surrounding parenchyma ; at most
some of the sclerenchymatous fibres, which are scattered through the whole tissue,
stand around the periphery of the bundle.

According to Russow the feeble leaf-bundles of Isoetes closely approach those
of the Cycadeae as regards structure. They are collateral and their orientation is
normal. Their xylem consists of narrow prismatic parenchymatous cells, with some
narrow, spiral, and reticulated tracheides standing betw’een them. The prirhitive
elements appear, according to Russow, on the boundary of the phloem, while the
others, which are nearer the inner edge of the bundle, attain their development later.
On the inside of the phloem thin-walled prismatic elements can be distinguished, but

* Mettenins, l.c. p. 573, Taf. i, fig. 10.

z

l.c. pp. 140, 155.

33«

PRIMARY ARRANGEMENT OF TISSUES.

no evident sieve-tubes ; on its outer border are thick-walled cells, which latter in the
terrestrial species assume the characteristics of tough fibrous cells. On the border of
the phloem and xylem there lies in most species in the middle of the bundle a single
intercellular canal, while in I. Engelmanni there are usually three of them ; their origin
is not clear The radial walls of the layer of cells bordering these canals have in
I. Engelmanni (Russow) and L Durieui the characteristics of the radial walls of the
endodermis. No such structure is present at the periphery of the bundle.

In connection with Isoetes, Phylloglossum may be mentioned, äs the short
description by Mettenius shows at least this one point of agreement, that its vascular
bundles only contain a few delicate tracheides, with annular thickenings, or spiral
fibres which can be unwound. The phloem is at any rate very inconspicuous ;
according to Mettenius it is even in many cases wholly absent.

Sect. 103. The form of collateral bundles above designated as the double or
bicoUateral is distinguished from the simple collateral form by having two groups of
phloem, one being situated as in the latter on the outside of the xylem, and a second
on its inner side. In all other respects they agree with the simple form®.

As the type of this form of bundle are to be mentioned in the first instance all
leaf-trace bundles of the Cucurbitaceae, and in fact of all species investigated b Both
groups of phloem have the typical structure described (p. 324, i), and are especially
remarkable for the size of their sieve-tubes (chap. V). They are frequently connected
by means of a narrow band, fringing the lateral edge of the bundle, and containing
some sieve-tubes, so that in these cases the bundle, strictly speaking, belongs to the
concentric type. The xylem is constructed altogether on the collateral type ; on the
inside are narrow annular and spiral vessels ; towards the outside are reticulated vessels
becoming gradually wider, and finally very large pitted vessels with short articulations.
The latter are surrounded by broad layers of cells, some of which are elongated, with
thick pitted walls, while others are short elements with undulated surfaces fitting into
one another, and round-meshed reticular thickenings on their walls. It remains to
be investigated whether, or how far these elements should be called tracheides.

A bicollateral structure is presented by the leaf-trace bundles of many Dicoty-
ledons, which belong to the ring : Melastomaceae Cichoriacese, Solanacese, Ascle-
piadeae, and Apocynese ; Strychnos, and Daphne. In many of these the inner phloem
is so widely separated from the rest of the bundle, that it may be regarded as a
distinct strand of sieve-tubes ; in other cases distinct strands of sieve-tubes occur side
by side with the inner groups of phloem of bicollateral bundles, e.g. Cichoriacese, Sola-
num tuberosum, and dulcamara; comp. p. 231. Of the Myrtacese already mentioned at
p. 231, Eucalyptus globulus decidedly belongs to this series. All the investigated
species of Eucalyptus, Metrosideros, Callistemon, Melaleuca, and Myrtus, have on
the inner side of the primary bundles a group of tissue consisting of delicate narrow
elements, and, according to what has been found in the case of Eucalyptus globulus,
it is very probable that these groups have the same nature as those in the latter

’ Compare A. Braun, Isoetes-Arten d. Insel .Sardinien, Monatsbericht, d. Berlin. Acad. 18(53.

“ Botan. Zeitg. 1867, p. 99.

^ [Petersen, Ueber das Auftreten bicollateraler Gefässbündel in verschiedenen Pflanzenfamilien.
Engler, Bot. Jahrb. 1882, p. 359.]

* Compare Dippel, Mikroskop, p. 225; Biyonia.

^ Vöchting, 1. c.

STRUCTURE OF CONCENTRIC BUNDLES.

339

plant. No accurate investigations of them however lie before us, and the example
of Welwitschia (p. 335) shows that one must be cautious in coming to a decision on
apparently bicollateral bundles. On the behaviour of Trapa, which here remains to
be mentioned, comp. sect. 105.

2. Concentric Bundles.

Sect. 104. In concentric bundles one of the two parts occupies the middle, and
is encircled by the other.

Of the two cases here possible, the one, namely that in which the phloem occu-
pies the middle and is surrounded by the xylem, occurs in the lower ends of the leaf-
trace bundles of many, but not of all rhizomes of Monocotyledons, where they lie at
the periphery of the bundle-cylinder in the stem, e. g. Iris germanica, Cyperus aureus.
Papyrus’, Carex arenaria^ (but not, for example, C. disticha and C. hirta), Acorus
Calamus and A. gramineus This form arises no doubt from the collateral bundle,
as in its course the xylem gradually surrounds the phloem more and more on both
sides, until the latter is completely enclosed ; where it does occur however this form
must be distinguished from the typically collateral. The structure and the surround-
ing tissues present no generally valid differences from collateral bundles. The phloem,
which is round as seen in cross-section, is as a rule surrounded by a single, or
rarely by a multiple ring of reticulated or pitted vessels, with parenchymatous
cells interspersed between them. Comp. fig. 148, p. 317.

Sect. 105. The other possible case, that the xylem occupies the middle and
is surrounded on its whole surface by the phloem, occurs in individual Dicotyledons
with anomalous distribution of the bundles, also in isolated cases among the Cycadeae,
and is characteristic of the entire group of Ferns, with a few exceptions, partly
mentioned above.

Among Dicotyledons the medullary and cortical bundles of the Melastomaceae'*
may first be mentioned. In these the centre is occupied by a few narrow vessels,
which are scattered among delicate prismatic cells, the vascular group being sur-
rounded by a delicate ring, consisting of sieve-tubes and cambiform cells. In feeble
bundles only one single, narrow, spiral vessel often occurs, and even this may be
absent, so that we then have the sieve-tube bundles mentioned at p. 231.

According to Reinke’s description, all the bundles of the stem of the species of
Gunnera, especially G. scabra, also belong to this series ; so also do those in the stem
of Auriculas (cf. p. 251). In the leaf of the plants last-named the bundles are collateral,
and are arranged in a row as usual among Dicotyledons. The collateral structure
also holds good for the smaller bundles of the stem, even w'hen almost circular in
cross-section ; on the one side is a small group of narrow, primitive, spiral tracheae,
with larger reticulated vessels external to them ; on the other side is the small
phloem, the whole being surrounded by delicate cells, bounded externally by the
endodermis. On the other hand, the larger bundles of the stem of Pr. auricula
show a concentric arrangement ; the narrow primitive elements are in the middle.

’ Link, leones AnatomicEe, Tab. V. figs. 1,9; IX. fig. 6.

^ Treviranus, Physiol. I. p. 195, Taf. III. “ Van Tieghem, l.c,

* Sanio, Botan. Zeitg. 1865 p. 179. — Vöchting, Melastomeen, /. c.

Z 2

34°

PRIMARY ARRANGEMENT OF TISSUES.

surrounded successively by the wider vessels, the phloem, and the endodermis. It
is manifest that this structure may come about owing to the frequent junctions of the
smaller collateral bundles.

The above-mentioned isolated occurrence of concentric arrangement in the
Cycadese was found by me in some of the small bundles in the petiole of Dion ;
they had a round xylem, surrounded by a phloem with its elements in radial rows.

Lastly, the axial bundle (described at p. 277) in the infernodes of several Dicoty-
ledonous water-plants must be placed here : namely, Hippuris, Trapa (?), Callitriche,
Bulliarda, Elatine \ Hottonia, and Myriophyllum. It consists in general of a central
xylem, completely surrounded by a phloem, both parts usually having abundant
delicate parenchyma between the essential elements. In the cases of Hippuris,
Trapa, Hottonia, and Elatine Alsinastrum, the persistent vessels are arranged in an
interrupted ring, which surrounds a relatively thick cylinder of parenchyma (‘ pith ’).
The phloem is bounded on the outside by an endodermis. In the leaves of these
plants, with the exception perhaps of Callitriche, the bundles are collateral, with
normal orientation. The axial bundle of Verhuellia (p. 278) also seems to belong
to this series, though Schmitz could only detect that it consists ‘ of a strand of
prosenchymatous cells, in the middle of which runs a single spiral- vessel.’ All the
cases here cited, with the exception of the three elucidated by Sanio, Vöchting, and
Hegehnaier, require still more accurate investigation.

In Bulliarda aquatica, according to Caspary’s description, the middle of the stem is
occupied by a thin cylindrical strand consisting chiefly of elongated cells, in which, about
midway between periphery and centre, lie two indistinctly separated groups of annular
and spiral vessels, which run out to the leaves. \\\ Elatine Alsinastrum the axial cylindrical
strand consists permanently, as regards its main bulk, of much elongated cells ; a few cell-
layers inside its periphery, one vessel fcr each leaf of the whorl next above it first appears,
and in the node this vessel bends out into the leaf at right angles; or, to express the fact
differently, the vascular elements running into the leaf here abut on the cauline vessels.
The vessel itself appears to be the continuation of the one which has passed out at the
next lower node. Later on other isolated wider vessels are formed side by side with
the original ones; in cross-section all are arranged in an irregular ring. Sieve-tubes
lie in the zone outside the vessels. Axial vessels are not present at the beginning ; after
those of the leaf-trace however 1-2 vessels appear and are permanent. Hottonia appears
(judging from very incomplete investigation) to behave similarly, apart from the obvious
differences due to difference of Phyllotaxis, and with the further distinction that axial
vessels do not occur.

The often investigated axial bundle in the stem of Hippuris ^ shows in its early stages, as
first accurately described by Sanio, annular and spiral tracheae at its centre, which are
scattered among thin-walled prismatic cells, and are cauline, with acropetal growth. At
a later stage vessels are formed at the periphery of the cylinder, and from these the
bundles branch off, which pass transversely through the cortex into the leaves. They
are connected with one another in the node, and in the cross-section of the internode
they represent an irregular, many-layered and often-interrupted ring, in which the
vessels increase in width in the centrifugal direction, according to their order of origin.
Outside the vascular ring lies a many-layered ring of prismatic cells, and between the

^ [F. Müller, Struktur einiger Arten von Elatine ; Flora, 1877.]

Von Mohl, Venn. Sehr.; Palm. Structura, Tab. g, fig. 2. — Nägeli, Beitr. l.c. p. 56. — Sanio,
Botan. Zeitg. 1865, p. igi.

STRUCTURE OF CONCENTRIC BUNDLES.

3^11

latter are small bundles of sieve-tube, each consisting of one, or rarely two sieve-tubes'
surrounded by a layer of cambiform-cells. — Both vessels and sieve-tubes run vertically
and separately in the internode. In the node the equivalent elements anastomose with
each other, and the two kinds of elements unite to form the vascular bundle, and enter
the leaf. The peripheral elements of the axial strand which we have described are
persistent. The cauline axial tracheal elements begin to disappear on the first appear-
ance of the peripheral vessels, and become so crushed by the surrounding prismatic cells
that a mature bundle encloses a dense parenchymatous ‘ pith ’ within the peripheral
vascular ring.

One bundle runs out into each of the leaves, which form multifoliate whorls, and
usually each has a separate course from the rest ; yet it not uncommonly happens that
a common trunk arises from the vascular cylinder of the stem, and then, towards the
periphery of the stem, divides into two or even three leaf-strands.

In the stem of Callitriche'^ the. thin axial bundle, consisting chiefly of delicate elongated
prismatic cells, contains at its apex an axial annular or spiral ves.sel, which grows acro-
petally, and projects far above the last node which contains vessels ; close by this a
second (and a third) soon appear. The first two primordial vessels have a position in
the internode corresponding to the two opposite leaves of the adjoining node; in the
node a small bundle branches off from them for each leaf. As the internodes become
elongated 2-12 wider annular or reticulated vessels appear, by the side of and somewhat
external to the primordial elements ; they are arranged in two irregular groups, and are
persistent, while the primordial vessels soon disappear in the internode, and are replaced
by an axial intercellular passage, to the wall of which their remnants adhere. This pas-
sage may subsequently be filled up again by the luxuriant growth of the neighbouring
cells. In the node the axial passage between the vascular elements, which are here
densely crowded, is absent. The vascular group is surrounded by a small zone of phloem,
consisting of a few rows of narrow elements, and bounded on the outside by the
endodermis.

In Trapa the wide central portion of the axial strand consists, in the fully elongated
internode, of loose, large-celled parenchyma, traversed by numerous longitudinal air-
passages; a relatively narrow, peripheral, annular zone consists of thin-walled prismatic
cells. Among the latter are large vessels arranged in a circle at wide intervals. Their
primitive elements appear to run out into the leaves, but to be distorted and indistinct,
each being in many cases replaced by an air-passage when the internode has attained its
definitive elongation. The large persistent annular vessels are apparently of later origin.
Outside and inside the vascular circle Sanio® found a circle of scattered sieve-tube-
bundles, each of these consisting of one sieve-tube with horizontal cross-walls, surrounded
by a layer of cambiform tissue.

Myriophyllum spicatum * has in the young internode, when elongation begins, in the
middle of the axial cylinder a central group consisting first of one and then of 2-4 spiral
vessels, which are in close contact with each other. This group is cauline and grows
acropetally, and in the node branches grow out from it centrifugally into the leaves,
which are ranged in alternating, usually quadrifoliate whorls. In the leaf the vessels are
united with a small phloem to form a collateral bundle. From the node the four phloem-
bundles of the whorl — which require further histological investigation — run down, as a

' The sieve-tube nature of these elements is disputed by Russow, who however includes them in
his protophloem.

'■* Nageli, l.c. — Hegelmaier, Monogr. d. Gattg. Callitriche. — Idem in Martins, Flora Brasiliensis,
fasc. 67.

^ Botan. Zeitg. 1865, p. 193.

* Vöchting, Zur Histologie und Entwickelungsgesch. v. Myriophyllum, Nova Acta Leop.
XXX VI. 1872.

342

PRIMARY ARRANGEMENT OF TISSUES.

leaf-trace, radially and tangentially vertical, in the periphery of the axial cylinder. Each
passes through two internodes, and at the third node, above one of the leaf-bundles which
pass out here, splits into two short, strongly-diverging limbs, each of which attaches itself
to the nearest of the bundles coming down from the second node. — The main mass of
the axial cylinder consists permanently of thin-walled prismatic cells. When elongation
is complete, the axial bundle of spiral vessels disappears, while round its circumference
larger thick, and usually reticulated, vessels appear, which are scattered and arranged in
irregular rings. Also the number of the supposed sieve-tubes at the periphery increases
with age, so that the original arrangement may become indistinct.

Sect. io6. The vascular bundles in the stem and leaves of the Ferns ^ belonging
to the divisions Polypodiacese, Cyatheaceae, Hymenophyllacese, Gleicheniacese,
Schizaeaceae and Marattiacete, to which the Selaginellae are to be added, are of
various size and form ; in cross-section they may be circular, elliptically trapezoidal,
band-shaped or plate-like ; the outline of the broad ones being even, wavy, folded
in a furrow-like manner, or with the edges bent in ; others are annular or tubular
(e. g. stem of Marsiliaceae, Microlepia, Dennstasdtia, &c., see p. 284), or forming
peculiar symmetrical figures resembling an X, V, U, O, &c. : the bundles of the
leaves may be similar to those of the stem to which they belong, or very different
from them. Comp. Figs. 128-141. Their structure is as uniform as it is distinct
from that of most other forms of bundle. Comp. Figs. 160, 16 1.

The middle is occupied by the xylem, the form of which is either identical with
that of the entire bundle, or similar to it, or in various degrees different ; the former
for example is the case in the annular or band-shaped bundles, and also in the
approximately cylindrical ones of the stem ; while the latter condition occurs
especially in leaf-stalks, in such a manner that the symmetrical figures mentioned
are peculiar to or especially marked in the xylem, the outline of the whole being
simpler. The former may even be severed into two symmetrical groups in one
bundle, as for example in the leaf-stalk of Aspidium molle. Polypodium phymatodes.

The xylem consists, as regards its main mass, of wide, long, prismatic to spindle-
shaped, scalariform tracheides with bordered pits (comp. p. 165), and only in rare
cases of scalariform vessels, with septa perforated in a scalariform manner (Pteris
aquilina, p. 162). Between, or more rarely on the outside of these, lie, at definite
points, some narrow spiral and narrow scalariform tracheides, which are the primitive
elements at the origin of the xylem; from these the development of the wide
tracheides starts, and advances centrifugally with reference to each point of departure,
though, it may be, centripetally with reference to the whole bundle. The position
and number of these primitive groups are different in the individual cases. In
bundles presenting an angular or unilaterally elongated cross-section, one such
group lies at or near each corner, or at each end of the greater diameter of the
section, as in the flat bundles in the stem of most Selaginellas (comp. Fig. 131, p. 282),

’ Von Mohl, .Structura filic. arborearum, l.c. — Link, leones select®, Heft. III. und IV. — Met-
tenius, Angiopteris, l.c. — Karsten, Vegetationsorgane der Palmen, l.c. pp. 117, 130, &c. — Dippel,
Verhandl. der Naturforscher-Versammlung zu Giessen (compare p. 180), and Mikroskop, p. 198 ff. —
Trecul, Sur la position des Trachees dans les Fougeres, &c. ; Ann. Sei. Nat 5 ser. torn. X. p. 344,
torn. XII. p. 219 ff. — Russow, Vergl. Untersuchungen. — With reference to the form of the bundles,
coinpai-e also Presl and Reichardt in the works cited at p. 298.

^ [See Prantl, Unters, z. Morph, d. Gefasskrypt. Heft. H. Leipzig, 18S1.]

STRUCTURE OF CONCENTRIC BUNDLES.

343

where they lie in the corners themselves, and are continued into the leaf-bundles,
which join on here (comp. p. 282) ; and in the band-shaped or symmetrically many-
rayed bundles of the leaf-stalk of Ferns b In bundles which are elliptical in cross-
section, e. g. in the rhizome of Pteris aquilina, their position corresponds approxi-
mately to the foci of the ellipse. Besides these primitive groups occupying the ends
and corners, others may be present in the same xylem, e.g. in the band-shaped,
symmetrically curved bundles from leaf- stalks, represented by Russow; a median
group occurs in Gleichenia vulcanica, Aneimia Phyllitidis, Marsilia Drummondii ;
in Asplenium Filix-femina there is a median group and an intermediate one in each
side between this and those at the margin ; in Balantium Culcita there are two inter-
mediate groups, and so on. In roundish or circular and in annular xylems several
primitive groups are scattered about the cross-section ; five for example in the annular

FIG. 160.— Polypodium vulgare ; rhizome ; cross-section through a weak vascular bundle (225). s phloem region,
sieve-tube structure not clear; narrow spiral tracheides of the xylem, the wider elements which constitute the
majority are scalariform tracheides ; u endodermis apparently derived by tangential division from the same layer of
mother-cells with the parenchymatous layer adjoining it on the inside. Outside u is parenchyma. The pitting on
its cell-walls is essentially the same everywhere ; in the figure it has only been indicated at certain points. Those of
its cells which border on u are thicker-walled on the inside than elsewhere.

bundles of the stem of Marsilia Drummondii ; six, according to Russow’s description,
in the vascular cylinder of Trichomanes radicans, three near the middle in the round
axial bundle in the small stem of Selaginella spinulosa. In very small bundles there
is often only a single primitive group present, which is more or less eccentrically
placed, e. g. small bundles in the rhizome of Pteris aquilina, Angiopteris (Mettenius,
l.c. 517). In the large flat bundles in the stem of Cyatheacese the primitive groups
have only recently been discovered by Trdcul. Here they occur in the form of
narrow reticulated tracheides at the edges bordering the leaf-gap, enclosed among
the scalariform tracheides, or in a narrow notch between them ; and from here they
proceed, or send branches into the bundles of the leaf-stalk. In consequence of early

* Cf. Russow, /. c. Taf. X; especially abundant details in Trecul, Lc.

344

PRIMARY ARRANGEMENT OF TISSUES.

compression and distortion they are only to be detected in the mature stem with
great difficulty, and often there are only traces of them.

The xylem is either composed of tracheides only, without any non-equivalent
elements interposed between them, or there are groups and rows of parenchymatous
cells containing small starch grains, intermixed with them The two conditions are
distributed according to species and perhaps genera, not according to the form of
the bundle. The first, for example, occurs in Marsilia and Pilularia, where the xylem
is an uninterrupted, one to three-layered ring of tracheides ; also in the axial bundles
of the Selaginellas, and in many flat, round, and angular bundles of Polypodiacese,
.e.g. in the stems of Polypodium vulgare (Fig. i6o), P. Lingua, Davallia pyxidata ;
the petioles of Asplenium auritum, Scolopendrium vulgare, and many others
(cf. Russow, Lc.) In the bundles of the Marattiace® also, no parenchymatous cells,
or extremely few of them, occur among the tracheides.

The other case occurs, for example, in the relatively thick cylindrical xylem of

the rhizomes of Trichomanes radi-
cans, Gleichenia, and Lygodium, in
the annular bundle in the rhizomes
of Microlepia and Dennstsedtia, in the
round or flat bundles of the stems
of Pteris aquilina (Fig. i6i). Poly-
podium fraxinifolium, 'Platycerium
alcicorne, Alsophila microphylla, Cy-
athea Imrayana, and arborea ; in the
bundles of the leaf-stalks of Tricho-
manes, Aspidium Filix mas, molle,
Lygodium and many others (comp.
Russow, /. c.). The axial bundle in
the stem of the Schizseas is also
placed in this category by Russow,
and no doubt rightly, for although a
many-rowed uninterrupted ring of
tiacheides appears to surround a
thick axial pith-cylinder of paren-
chyma, yet it is not separated from
the latter in the manner to be de-
scribed below, which is characteristic
of all other bundles of Ferns belonging
here, but the two are in immediate
contact, so that the strand in ques-
tion must naturally be regarded as a
xylem with a coherent axial cylinder of parenchyma. In the leaf-stalk of Tricho-
manes pinnatum and elegans^ and in that of species of Aneimia, Gleichenia and
Schizaea, very thick-walled, lignified, sclerenchymatous fibrous cells are added to the
tracheae ; there is a thick bundle of them in each corner of the V which the xylem

FIG. i6i.— Pteris aquilina. A quarter of the cross-section through
a large vascular bundle of the stem ; cf. Fig. 143» P- 295. 5 spiral

tracheide, wide scalariform vessels {cf. p. 162), sp sieve-tubes, b the
protophloem of Russow, endodennis, p the parenchyma surrounding
it, containing starch-grains ; K thickened portions of the wall of the
vessels between the rows of scalariform pits. Between b and and
in the xylem, especially round .S“, are thin-walled parenchymatous
cells, containing starch. From Sachs' Textbook.

‘ Russow's ‘companion cells’ (Geleitzellen).
^ Mettenius, Die Hymenophyllaceen, p. 421.

STRUCTURE OF CONCENTRIC BUNDLES.

345

forms in Trichomanes, and of the approximate T which it forms in Schizsea pectinata ;
in Gleichenia dichotoma and polypodioides they lie isolated and often separated
from the tracheides by parenchymatous cells along the edges of the V-shaped xylem.

The xylem is surrounded in all cases, in annular bundles both within and without,
by a many-layered complexus of tissue, which is to be regarded as the phloem, Fig. 1 6 1 .
One or a few layers of parenchymatous cells, containing starch, and similar to those
of the xylem, border immediately on the latter. Outside the parenchymatous layer
comes an annular zone, which contains the sieve-tubes, though in the smaller bundles,
as already mentioned at p. i8t, the latter are certainly not always to be clearly dis-
tinguished. When distinctly developed they form a usually single, but in some
places double annular ring, and are in contact with each other by means of those
walls which are radial with reference to the centre of the bundle. This zone is then
followed all round on the outside by a likewise annular zone of elongated fibre-like
elements with narrow lumina, characterised by thick, brilliant, and soft walls. These
are called by Dippel bast-fibres, and by Russow protophloem, because they appear as
the primitive elements of the phloem, and here also it remains doubtful whether they
are to be reckoned among the sieve-tubes, or regarded as special organs. They are
partly in immediate contact with the indubitable sieve-tubes, and even frequently in-
serted in the same circle, while in other cases they are separated from them by small
parenchymatous cells. Finally, a one or few-layered sheath of parenchymatous cells
containing starch, which are often tolerably wide, and always differ from those outside
the bundle in their form and their (smaller) size, completely surrounds the zone of
sieve-tubes and fibres, and, apart from a few exceptional cases to be mentioned below,
this is in its turn enclosed by a single-layered endodermis, which limits the bundle
sharply on the outside. This consists of prismatic, usually inconspicuous cells greatly
flattened from without inwards, with a moderately thick, usually brownish membrane,
which soon becomes cuticularised, and which in the radial walls is easily torn across,
so that in sections the whole endodermal sheath is often split, and difficult to recog-
nise. In especially favourable cases (e. g. species of Polypodium) every cell of the
endodermis stands exactly in front of a cell of the parenchymatous layer adjoining it
on the inside, so that the common origin of the two from one layer of mother-cells is
recognised at once. Even where the latter is not the case, the origin of the two is
the same, at least among the true Filices and Marsiliaceae k

Among the plants belonging to this series the Marattiacese and Selaginellae are
destitute of an endodermis The bundles of the former appear simply inserted in
the parenchyma, and this applies both to the petiole and to the stem. So at least
I found it in young stems of Angiopteris, and I can only suppose that the figure
cited by Russow from De Vriese and Harting’s Monogr. des Marattiacdes (Taf. VII.
Fig. 3, 4), according to which the case would be different in the stem of Angiopteris,
represents the section of a root passing through the stem, for in the root the
endodermis is always present In the Selaginellae the phloem is surrounded by a
dense layer of small-celled parenchyma.

* Russow, l.c. p. 195.

^ [Compare Treub, Recherches sur les Organes de la Veg. du Selaginella Martensii, Leide,
1877.] ^ Compare Sachs, Textbook, 2nd Eng. ed. p. 420, supplementary remarks.

.346

PRIMARY ARRANGEMENT OF TISSUES.

In petioles, when the xylem is concave or notched, strands of cells 3-4 rows thick,
which are distinguished from the rest of the parenchyma by their very wide lumen,
are to be found in its depressions and furrows, sometimes just in front of the primitive
tracheides ; ‘ in longitudinal sections they are conspicuous from the fact that their
walls are irregularly bent in and out, and are connected with those of the neighbour-
ing cells in such a manner that large cavities or intercellular spaces arise; in old
bundles it is usually found that the walls have become brown.’ Russow calls them
cavity-parenchyma (Lückenparenchym). The tracheides bordering on them usually
have very irregularly developed spiral bands. Examples : species of Asplenium,
Cyathea microlepis (Dippel), /. c., Aspl. Filix femina, Cyatheacese e. g. Cyathea
medullaris. The wide cells in Osmunda regalis to be mentioned below may also
belong here.

From its general distribution among the Fern group in the widest sense, one
may term the structure of the bundle just described the Fern-type. At the same
time different degrees of deviation from the type occur within this group. Those de-
scribed in the Marattiaceae and Selaginellae are trifling. The Lycopodiaceae, of which
we shall treat in the next section, are closely connected with the Selaginellae in the
structure of the bundle, as well as in other points. In this respect the Equiseta are
most widely different from the Fern-type ; their strictly collateral bundles, w’hich
most closely resemble those of Monocotyledons, were described above (p. 329).
Besides these, the Ophioglosseae, and in part at least the Osmundacese, have colla-
teral bundles. The tw'O parts have normal orientation in the round or flat bundles,
the xylem is similar to that of Ferns, with some narrow spiral tracheides (primitive
elements) at its inner edge; its main bulk consists of large prismatic tracheides,
which in Ophioglossum (pedunculosum and vulgatum) show narrowly scalariform
reticulate thickening without pit-borders, while in Botrychium they have very thick
reticulate fibres, with elliptical bordered pits in the meshes of the reticulations.
Parenchyma is present in the xylem of the annular bundle in the stem of Botrychium
rutsefolium, in the form of radial bands resembling medullary rays ; I could not
find any in the examples of B. Lunaria, which I investigated. The phloem appears
very similar to that of the typical form; the wide elements presumed to be sieve-
tubes still require more exact investigation (p. 1 80). The bundles of the petiole, and
the small bundles of the stem of Ophioglossum, which in cross-section are arranged
in a circle, are not bounded externally by any kind of distinct sheath. The bundle
of the stem of Botrychium Lunaria and rutsefolium, which is annular in cross-section,
is encircled on the outside by an endodermis, the cells of which do not differ from those
of the surrounding parenchyma, except in the exquisite undulating longitudinal bands
in the middle of their radial side-walls.

In Osmunda (compare p. 280) the bundles of the stem are collateral. The xylem,
where it enters the circle of bundles, is horseshoe-like in cross-section, and during its
descending course becomes narrowed to a wedge-like form ; internally it borders
directly on the parenchyma of the pith ; it has the same structure as in the typical
Ferns, and has hardly any parenchyma inserted among the scalariform tracheides.
The groups of xylem are separated from one another in the whole longitudinal

‘ Russow, l.c.

STRUCTURE OF CONCENTRIC BUNDLES.

347

course of the bundles, by medullary rays 6-io cell-layers in breadth. Round this ring
of separate groups of xylem runs a common annular phloem- region, which is similarly
constructed to that of the typical Fern-bundle : outside each group of xylem are first
some layers of small-celled parenchyma, then an almost uninterrupted zone of large
sieve-tubes running round the whole stem ; this zone is usually one layer thick outside
the xylem-groups, while in front of the medullary rays it is many-layered, and projects
into them like a wedge. The layer of sieve-tubes is next immediately bounded on the
outside by a layer of transversely elongated, partly thick- walled elements, which in their
turn are separated from the brown sclerotic tissue of the stem by a many-layered zone
of parenchyma. Outside the transversely elongated zone runs an endodermis, which
in the mature condition can be recognised by the brittleness of its radial walls. In
the petiole of Osmunda the runnel-shaped x} lem is surrounded by a similarly-formed
zone resembling the phloem of typical Fern-bundles, which in the mature state is
bounded on the outside by a very indistinct endodermis; this zone however, as also
stated by Dippel, only contains sieve-tubes in its broader convex half. On the con-
cave side it is parenchymatous, and distinguished in cross-section by 10-12 small
groups of conspicuously wide cells, which still need investigation k In the stem of
Todea Africana and T. hymenophylloides the structure of the vascular bundle is
like that described for Osmunda, only the form of the xylem is in some degree dif-
ferent in consequence of the fusions of laterally adjoining bundles. In the lowest
part of the leaf-bundle, which has the same shape as in Osmunda, sieve-tubes are, in
T. Africana at least, present on the concave side as well. In the leaf-stalk of
T. Africana I found the endodermis scarcely recognisable, while in T. hymenophyl-
loides it is very clear.

The axial strand, which the collateral bundles in the stem of Isoetes unite to
form, consists of a roundly angular mass of short and irregularly spindle-shaped,
reticulated and spiral tracheides, and of thin-walled parenchymatous cells irregularly
distributed between them, these elements together forming the xylem. The latter is
completely surrounded by a transparent mantle of shortly-prismatic or tabular cells,
with contents clear like water, and a strongly refractive membrane, which is provided
with broad and very delicate pits, but no clear sieve-pores. Russow is no doubt
right in considering this mantle as a peculiarly imperfect phloem of the axial strand,
especially as the equivalent parts of the leaf-bundles pass over into it directly.
With reference to its phenomena of growth it will have to be further spoken of in
Chap. XVIII.

This may probably be the most fitting place to mention the axial bundle, which tra-
verses longitudinally the leafless stolons of Nephrolepis tuberosa, N. acuminata, and
N. exaltatak In the structure, form, and centripetal development of its xylem this agrees
completely with the 5-6 rayed radial bundles in the roots of Ferns to be described below.
Here also, as in the latter, phloem-groups alternate with these rays, and appear to con-
tain relatively wide sieve-tubes, but I am doubtful whether the narrow primitive elements
of the phloem do not also completely surround the rays of the xylem. At any rate the
whole inner part of the bundle is surrounded by about tw'o layers of very narrow
elements, and the latter usually by two layers of wider parenchymatous cells, on which

' Compare Dippel, Russow, l.c.

“ Trecul, l.c. — Russow, l.c. p. 100.

PRIMARY ARRA.NGEMENT OF TISSUES.

the endodermis borders externally. The xylem consists in the middle of wide scalari-
form tracheides, and interstitial bands of Parenchyma. According to all the data, which
however require to be more exactly established, the bundles described may represent
an intermediate form between the concentric and radial Fern-bundles. Aceording to
Russow the likewise axial bundle of the stolons of N. pectinata and rufescens has
not the structure described.

3. Radial Bundles.

Sect. 107. The radial bundles are closely connected with the concentric by
means of those in the stem of Lycopodium, and by the diarch forms, which occur in
many roots. In the typical cases they are distinguished by the fact that in the radial
bundle the xylem forms several bands running out radially from the centre, between
which lie the same number of groups or bands of phloem alternating with them. In
all radial bundles the development of the characteristic elements, both of the xylem
and phloem bands, begins at the periphery, and proceeds thence tvith varying
celerity towards the middle. The primitive elements, which in the xylem are here
also distinguished by their narrowness, form the peripheral ends of the rays. As
these thus form the points of departure of the development of xylem, it is usual
to speak of the number not of the rays, but of the starting-points — of di- to
polyarch bundles

Radial bundles occur in the stems of Lycopodiacese, and in the filiform stolons
of Nephrolepis; and in all roots, with a few exceptions mentioned at p. 319.

The axial strand which traverses the middle of the stem of Lycopodiaceae agrees,
with the exception of its radial structure, with the bundles above described in the
stern of the Selaginellse (with the exception of S. spinulosa), which, in the struc-
ture and development of their xylem, correspond to the diarch or oligarch radial
forms.

In the stem of Psilolum ^ this strand is cauline, not receiving or giving off leaf-
strands. In the branches which appear above ground the cross-section of the whole
is almost circular, bounded on the outside by an endodermis, which only differs from
the surrounding parenchyma in its undulating radial walls. The xylem is triarch to
pentarch and octarch ; its not always equidistant rays are separated from the endo-
dermis by one or a few layers of relatively narrow, elongated, prismatic parenchymatous
cells, and consist at their peripheral ends of a group of narrower reticulated tracheides
(1 did not find spiral tracheides), and towards the centre of a few rows of scalariform
tracheides ; these rows do not reach to the middle of the bundle, but abut on a strand
of elongated prismatic, pointed sclerenchymatous fibres, which traverse the middle.
The rest of the substance of the bundle consists of thin-walled prismatic parenchyma,
in which, especially at its periphery, are scattered some few-celled groups of somewhat
narrower and thicker-walled sieve-tubes. This designation is at least justified by the
appearance of the smooth lateral walls, agreeing with that in the Ferns, by the
granular contents adhering obstinately to the walls, and by the absence of nuclei,
which is very conspicuous on comparing these elements with the surrounding cells ;
on the thin oblique terminal-surfaces of the articulations I believe that I have directly

' Nägeli, Beitr. 1. c. p. 10.

- Nägeli, l.c. — Russow, l.c. p. 131.

STRUCTURE OF RADIAL BUNDLES.

349

seen delicate sieve-pores. In the subterranean shoots of the rhizome (Nageli and
Leitgeb’s rhizoides) the bundle is very weak and rudimentary in its development ; I
find only a flat or three-cornered xylem, consisting of a few, frequently only 3-6, re-
ticulated and scalariform tracheides, separated here and there by thin-walled elements;
the peripheral tracheides are but little narrower than the internal ones ; the xylem is
completely surrounded by 2—4 layers of delicate spindle-shaped cells. I could see
nothing of any sieve-tubes. The vascular bundle of Tmesipteris appears from
Russow’s statement to have a similar structure to that of Psilotum.

In the cylindrical axial strand of the stems of Lycopodium (comp. p. 281) the
xylem consists of a number of plates or bands, the peripheral corners of which are
each formed by a group of narrow tracheides (comp. p. i(?3), the above-described
points of attachment of the leaf-trace bundles, while the larger inner part consists
of wider scalariform tracheides. (Comp. Fig. 162.) The number and arrangement
of the plates and their relations to
the rows of leaves vary with the
species, and with the vigour of the
individual shoots. Of the latter
relations we have already spoken
above. As regards the other con-
ditions which come under con-
sideration \ among the native species
investigated L. inundatum has 3-5
plates, united in the middle to form
a body which has a stellate cross-
section, thus constituting a tri- to
pentarch radial .xylem. The latter
is however even here not unfre-
quenlly in so far irregular, that one
or the other plate separates from
the rest, so as to have an isolated

course for some distance, and then again to unite with the others. Four radial plates
united in the middle are present in the ultimate ramifications of the heterophyllous
species, as L. complanatum, and as a rule L. alpinum, though here deviations occur in
20-30 per cent, of the cases. In the stouter axes of the last-named species, as well as
in L. clavatum, annotinum, and Selago, the number of the vascular plates is higher,
being in proportion to the thickness of the shoots ; — in stout main stems of L. com-
planatum and alpinum, for example, it amounts to ii and 13, in those of L. anno-
tinum and clavatum to 17, but diminishes again in the weaker ramifications to 4 and 3.
In these cases the plates are only partially, or scarcely at all radially convergent ; most
of them rather form separate bands in the decidedly bilateral prostrate main-shoots
of all species possessing them (Fig. 162), these bands being slightly curved, with
their convex surface always directed towards the lower side of the stem, and their
corners lying chiefly right and left ; they are further united with each other in a great
variety of ways, sometimes radially, sometimes so as to form loops. Their union and

Fig. 162. — Lycopodium Chauitecyparissus. Cross section of a shoot,
magnified about 100 times. In the middle the cylindrical vascular strand ;
in the dense cortex to the right tlie cross-section of a bundle running into a
leaf. From Sachs’ Textbook.

^ Hegelmaier, /. c. p. 790.

350

PRIMARY ARRANGEMENT OF TISSUES.

separation vary in successive sections of their longitudinal course. Feeble branches
of higher order once more show a more radial arrangement and mode of union. In
the non-bilateral stems of L. Selago radial union of all the 4-6 plates is, according to
Hegelmaier, the more frequent case, while irregular winding and grouping are more
rare. For further details compare the treatises cited at p. 281. The intermediate
spaces between the vascular plates, which are usually smaller than the latter, are
occupied by the one or more masses of phloem of the bundle, each constituting a
correspondingly-shaped group of elongated prismatic parenchymatous cells with
oblique ends, and apparently oily contents, among which lies a usually simple
interrupted row of wider sieve-tubes, represented by the wider, somewhat more
strongly contoured meshes of Fig. 162 (comp, p, 181).

The walls of all the elements of the phloem are soft, swell strongly in water, and
become blue with solution of iodine in potassium iodide. Between the peripheral
angles of the vascular plates, and alternating with them, lies in each phloem-portion
a small group of thick- walled, narrow, elongated fibrous elements, — the primitive
elements of the phloem. Round all the corners runs a zone of prismatic parenchyma,
usually tw'O cell-layers thick, of the same or similar cell-form and structure to that
of the phloem, but in most species (L. clavatum, annolinum) distinguished by inter-
cellular spaces, and loose, easily separable connection of the cells. A sheath, con-
sisting on the average of two layers of tangentially elongated cells, possessing thin
walls, cuticularised according to Russow, and not undulating, surrounds the whole
vascular bundle, and unites it with the inner cortex, which according to the species
is parenchymatous or sclerenchymatous.

The stout roots of Lycopodium clavatum \ Alpinum, and species of similar
growth, have essentially the same structure as the stems. In the two species men-
tioned the xylem is hexarch to dekarch, very often heptarch, and then so arranged in
the simplest most regular case as to form three separate plates, one being diametral,
while two stand symmetrically in front of the two surfaces of the first ; these two are
concave, with U-shaped cross-section, and with the concavity turned towards the
periphery. Every plate diminishes in breadth in the centripetal direction, and often
consists in the middle of only a single scalariform tracheide. Irregularities and inter-
ruptions of the plates occur similar to those in the stem. In the heptarch or octarch
examples of L. clavatum investigated, I almost always found one of the concave
plates larger, and of narrow horseshoe-like cross-section, the other smaller and much
flatter, with a separate, in cross-section elliptical or wedge-shaped, vascular strand
(in itself monarch), lying in front of its slightly concave outer surface. Other arrange-
ments however occur, and these are sometimes most irregular and involved. The
structure of the surrounding tissue and of the spaces between the vascular plates is
the same as in the stem. In the branches of these roots the number and arrangement
of the plates become reduced and simplified as the thickness diminishes ; their last
ramifications — and in L. Selago and inundatum all roots of every order of ramifica-
tion— have only a vascular group surrounded by a phloem, which is perhaps only
parenchymatous (.?). In the branches of the root of the stouter species first-named, the

* Nägeli und Leitgeb, Entstehung, &c. der Wurzeln, p. 117, &c. — Van Tieghem, Ann. Sei. Nat.
^ ser. tom. XIII.

STRUCTURE OF RADIAL BUNDLES.

351

former consists of a few small vessels lying on one side of the cylindrical bundle. In
the roots of L. Selago and inundatum a strongly-curved, diarch vascular plate, sickle-
shaped in cross-section, lies, according to Russow’s description, inside the cylindrical
phloem, the sieve-tubes being situated between its limbs.

Sect. 108. In the great majority of roots the axial bundle which traverses them
is of very regular radial structure, which in its principal characters is uniform in all
cases

The approximately cylindrical bundle is surrounded by an endodermis, which is
either permanently undulated, or is only so at first, becoming sclerotic in the mature
condition. According to its origin the endodermis is not to be assigned to the
bundle, but forms the innermost (limiting) layer of the surrounding cortex. The
xylem is according to the particular case diarch or polyarch, and its starting-points,
corresponding to what are afterwards its peripheral corners, all lie at equal distances
from one another: in diarch bundles at diametrically opposite points of the circular
cross-section ; in all other cases removed from each other by the fraction of the
periphery determined by their number (^, &c.). From the starting-points vascular

plates develope in a radial direction, and in centripetal order of development ; and these
either meet in the centre or do not reach it, but remain separated by a parenchyma-
tous or sclerenchymatous mass, which permanently occupies the middle of the bundle.
The same number of phloem-groups alternate with the vascular plates, to which they
thus correspond in number and arrangement.

The xylem and phloem-rays are separated from each other by delicate paren-
chymatous cells, and in fact two layers of the latter may as a rule be distinguished
between each xylem and the next phloem group ; more rarely only a single layer is
present, or there are more than two. On the outside an uninterrupted zone of
parenchyma, which usually forms a single layer, more rarely two layers, and rarely
several, constitutes the limit of the whole bundle towards the endodermis. In the
case of the Ferns, Nageli and Leitgeb have called this limiting layer the Pericambiwn,
a name which it may here bear generally, even where, as in Equisetum, its origin is
different from that (n the cases for which it was first introduced. In Monocotyledons
however cases are not rare, where the outermost vessels border directly on the endo-
dermis, and the Pericambium is thus interrupted at every xylem-plate, and only
surrounds the phloem-rays.

Van Tieghem calls the whole of the cells, which are interposed between the groups
of xylem and phloem, and thus unite them into a dense cylinder, conjunctive tissue
(tissu conjonclif). The latter forms, as has been said, the usually two-layered
bands between the xylem-plates and phloem-groups, and is continued inwards between
the former in cases where they do not meet. Externally it borders on the pericam-
bium. The latter is called by Van Tieghem in the case of the Phanerogams the
rhizogenic layer, from the function which it performs in the origination of lateral
roots.

’ Nägeli, Beiträge, / c. p. 23. — P. van Tieghem, Recherches sur la symmetric cle structure dans
les Plantes vasculaires. I. La racine. Ann. Sei. Nat. 5 ser. tom. XIII. — Nageli und Leitgeb, Ent-
stehung d. Wurzeln, München, 1S67. — Nicolai, l.c. (compare p. 231). — See also Link, leones
anatomicse. — Schacht, Lehrbuch, p. 167, etc.

35^

PRIMARY ARRANGEMENT OF TISSUES.

For the structure of the individual parts few general rules are to be given,
except those which hold good generally for vascular bundles and their sheaths. The
xylem-plates consist of one or more rows, which, according to the particular case,
are uninterrupted in the radial direction, i. e. one trachea follows on another ; or
they are interrupted by the interposition of non-equivalent (parenchymatous or
sclerenchymatous) elements. For the special nature of the tracheae, i. e. whether
they are vessels in the strict sense or tracheides, the rules and difficulties stated at
p. 1 64 apply. The first-formed vessels or tracheides, which occupy the corners, are
always narrow, the later ones, following in a centripetal direction, become suddenly or
successively wider. The latter are always pitted or reticulated vessels (or tracheides);
the narrow peripheral ones are as a rule also reticulated or annular vessels, with
dense and fine thickening fibres, the prevalent direction of which is transverse. For
short distances however the fibre has not uncommonly in these cases also a simply
spiral course. Closely wound spiral fibres, which can be unrolled for a long distance,
occur more rarely, e. g. in the roots of Tornelia fragrans, Cucurbitaceae, Anthriscus
Cerefolium (Van Tieghem), Phaseolus (Dodel), Cycadese (Mettenius), and Coniferae.

The structure of the phloem-rays, where they are well developed, is essentially
the same as in the typical collateral or concentric bundles. In feeble roots of Mono-
cotyledons they are not uncommonly reduced to one sieve-tube with nan'ow- celled
surrounding tissue (e. g. Triglochin maritimum, Aponogeton, Hydrocleis Humboldtii,
Potamogeton lucens, comp. Van Tieghem, 1. c. Taf. VI), this being of typical structure,
only small. It is therefore to be supposed that the typical structure belongs to them
generally, though they still require more exact investigation, especially in the small-
celled bundles of Dicotyledons. I should also wish to extend the last remark to the
roots of Coniferae, in the primary bundles of which, according to Janczewski’s more
recent statement', sieve tubes are said to be wholly wanting.

The number, and with it also the arrangement and relative breadth of the xylem
and phloem-rays, the extent and distribution of the tissue occurring around and
between them, lastly the special structure of the particular forms of tissue, and thus
the entire structure of the root-bundle, vary, sometimes in differeqj roots of the same
species, sometimes according to the species and the larger systematic divisions. In
the former relation the general rule holds good that as the thickness of the roots
diminishes, not only does the number of the tissue-elements in the bundle diminish,
but also the number of its radial plates, if in the thicker specimens this exceeds two.
Further slight individual differences, which cannot be referred to difference in thick-
ness, occur among members of the same species. In the other relation, besides the
obvious identity or similarity of structure of closely related forms with similar adap-
tation, the great conformity of structural plan in all divisions of vascular plants is to
be emphasised. For none of them can a special structure be stated as everywhere
characteristic of the group. Van Tieghem’s first plate shows the almost identical
cross-sections of young roots of Cyathea medullaris. Allium Cepa (main root of the
seedling), Taxus, and Beta. Smaller differences between subdivisions of the larger
classes are often more sharply expressed. The existing investigations give rise to the
following rules : —

* Ann. Sei. Nat. 5 ser. tom. XX. p. 31.

STRUCTURE OF RADIAL BUNDLES.

353

T. In almost all Dicoiykdom where the point has been investigated, the original
bundle of the root is oligarch, usually with 2, 3, or 4 rays, more' rarely with 6 or 8,
while higher numbers occur exceptionally. In the main-roots the xylem-plate is usually
diarch-diametral, triarch, or tetrarch ; higher numbers occur rarely, whether in single
individuals (as 5-7 in specimens of Vicia Faba, and perhaps even 12^, instead of 4),
or as the rule for certain species, as most Amentacese (Quercus sp. 6-8, Alnus 5-6,
Castanea 6-12, Fagus 8, Carpinus 4), tEscuIus (6), Coffea (8), &c. None of these
numbers are constant absolutely and without exception even for the particular
species. Whether a definite number can be characteristic of one of the larger
genera or of a natural family (apart from occasional individual variations) is not
to be decided from the existing data. At any rate this is the case in several families
of which a dozen or half-a-dozen representatives have been investigated. Diarch
xylem-plates occur, for example, in the main root of all investigated Cruciferae

Fig. 163. — Ranunculus fluitans. Cross-section tlirougli the vascular bundle of a strong old adventitious root {225).
u endodennis, p pericambium, g external primordial vessels of the diarch uniseriate xylem g—g ; between g—g and
p is the phloem.

(Brassica, Raphanus), Fumaria, Caryophyllaceae, Vitis, Urtica, Umbelliferae (Anthris-
cus Cerefolium, Foeniculum, Petroselinum sativum, Carum Carvi, Coriandrum,
Daucus, Pastinaca sativa v. Tieghem), Chenopodiacese (Beta, Atriplex, Spinacia),
Mirabilis, Centranthus, and Valeriana, and in Tagetes erecta among Compositae ;
tetrarch xylems as a rule occur in the investigated Cucurbitaceae (Cucumis, Cucur-
bita, Lagenaria, Luffa), Euphorbiaceae (Euphorbia, Ricinus, Mercurialis sp.), Tropae-
olum majus, Convolvulus tricolor ; generally the numbers 2 and 4 appear to be
predominant. But on the other hand considerable differences between the forms
investigated occur in the case of the higher numbers of the Cupuliferae above

f

' Compare van Tieghem, l.c. p. 223. In the case cited it was doubtful whether the main root
or a strongly developed lateral root was in question.

A a

354

PRIMARY ARRANGEMENT OF TISSUES.

mentioned ; and among the Papilionacese, of which more numerous representatives
have been investigated than of other families, a considerable variety of the conditions
in question is to be recorded ; as a rule the xylem-plates are diarch in Lupinus varius
and Trigonella, triarch in Pisum sativum, Lathyrus sativus, Orobus vermis, Vicia
sativa, Ervilia villosa, Ervum Lens, Hedysarum coronaiium, Onobrychis sativa,
and Medicago sativa ; tetrarch in Phaseolus, Dolichos lignosus, and Cicer arietinum :
lastly, higher numbers than fcur occur, as mentioned above, in Vicia Faba.

In the branches of the root the numbers remain as a rule the same, or diminish if
they were greater than two. In subsidiary roots springing from the stem they often
increase, in correspondence with the thickness of the roots ; amounting, for example, to
7, 9, II in Cucurbita maxima, 5, 6, 8 in Lagenaria and Luffa (van Tieghem), 4-5 in
Phaseolus, 5-8 in Valeriana; the adventitious roots on the rhizome of Nymphsea alba
have 6-10 rays; in Nuphar luteum there are as many as 27; in an aerial root of
Clusia flava van Tieghem found 13 rays, and so on. The converse case, however,
also occurs ; there is a diarch xylem-plate in all the adventitious and lateral roots
of Tropseolum majus, the main root being tetrarch.

The orientation of the parts in tfie cases investigated is such that, in the case of
diarch and tetrarch structure of the main root, the surface or one of the two inter-
secting surfaces of the xylem-plates always coincides with the median plane of the
two cotyledons, which diverge at an angle of 180°. In the triarch main roots of
Pisum, and the other triarch Papilionacese mentioned, the planes of two xylem-plates
fall according to van Tieghem in the median planes of the two cotyledons, which
only diverge at an angle of 120°. For higher numbers exact statements are wanting.
In all Phanerogams the plane of the diarch xylem-plates of lateral roots always lies
in the median plane of the main axis from which they arise, and the same applies, so
far as investigated, to one of the planes in tetrarch xylems.

The original structure of the individual bands of tissue shows — within the
general plan of structure of root-bundles — few peculiarities characteristic of Dicoty-
ledons. As regards the xylem-plates the usually very gradual increase of the width
of the vessels in the centripetal direction is worthy of remark. Only as an exception,
in the polyarch subsidiary roots on the rhizome of Primula Auricula and Nym-
phaeaceae does the case usual in Monocotyledons occur, namely, that the short row
of vessels, which does not reach to the centre, consists of a few narrow peripheral
vessels, and then of one or several which are very wide (Fig. 164). In most cases
-belonging to this series the one- or few-rowed plates constitute radial bands, narrow
in cross-section, separated by relatively very broad interstices. These bands either
meet in the middle, or they are separated, or connected together, by means of a
parenchymatous axial strand. In stout polyarch subsidiary roots, and in the upper
part of stout main-roots, where they pass over into the hypocotyledonary stem, this
axial parenchymatous mass, the ‘ pith ’ of the root, is of considerable thickness.
Rarely, among Dicotyledons, the axial parenchyma connecting the xylem-rays is
represented by a strand of sclerenchymatous fibres, e. g. in the subsidiary roots of
Stachys sylvatica, Mentha aquatica, and Hedera Helix (v. Tieghem).

A peculiarity, which so far as I am aware only occurs among Dicotyledons, is the
presence of a bundle of sclerenchymatous fibres, roughly crescent-shaped as seen in
cross-section, on the outside of the phloem-groups of triarch and tetrarch roots of

STRUCTURE OF RADIAL BUNDLES.

355

Papilionacese (Pisum, Phaseolus). The fibrous bundle lies inside the pericambium. In
other respects the phloem still requires more exact histological investigation in these
cases.

In all roots of Dicotyledons investigated a pericambium, consisting of one, or in
many cases of several layers, completely surrounds the xylem-plätes. Those peculi-
arities of its structure which are related to the development of lateral roots, the resin-
canals which sometimes occur in it, and other points connected with it, will have to
be discussed below (Sects. 117 and 133).

This original structure of the roots of Dicotyledons is, however, permanent in but
few cases ; in most cases, and in
many species immediately after its
origination, it is altered by the
secondary growth in thickness,
starting from the inside of the
phloem-rays, of which we shall
treat in Chap. XIV ; comp. Fig.

165. Hence result essential and
actual differences from other roots,
especially those of Ferns and Mono-
cotyledons, in which, with the ex-
ception of many roots of Dracaena
these secondary changes are want-
ing. It must, however, be expressly
stated that the changes due to
secondary growth in thickness occur
in by no means all roots of Dicoty-
ledons, and thus do not establish
any generally valid distinction be-
tween these and the others. Apart
from those cases where, as in the
subsidiary roots of Stachys silvatica, Mentha aquatica, Lysimachia nummularia, Myrio-
phyllum, and Hippuris, the secondary growth in thickness is infinitesimally small, and
as such even doubtful, because in other cases also the innermost vessels uniting the
plates are developed very late, this secondary growth is completely absent in a number
of subsidiary roots. For instance in those of Gunnera^ the Nymphaeaceae, Ficaria
ranunculoides, and Primula Auricula, to which cases it may be anticipated that more
extended investigation will add others. The fact that a rudimentary secondary growth
in thickness occurs at the points of insertion of the roots in question (in Ficaria and
Nuphar®) has no efifect on the condition of their much greater portion.

The fact that in roots of Dicotyledons sclerosis of the endodermis but rarely
occurs no doubt stands in the closest relation with the occurrence of secondary
growth. Such sclerosis, however, occurs for example in the adventitious roots on the
rhizome of Primula Auricula and Ranunculus repens ; comp. Figs. 164 and 165.

Fig. 164. — Primula Auricula (225). Cross-section through the heptarch
vascular bundle of an adventitious root and its surrounding tissue. / peri-
canibiuni ; £ the external primordial vessels of the xylein rays, which
alternate with the same number of phloem groups s, and are separated
from the latter by thin-walled parenchyma ; a endodermts, outside which
is tolerably thick-walled cortical parenchyma, with intercellular spaces
quadrangular in cross-section.

1 Compare Caspary, Pringsheim’s Jahrb. I. p. 446. — Falkenberg, Lc. p. 197.

“ Reinke, Morpholog. Abhandl. p. 58. ^ Van Tieghein, I.c, p. 266, &c.

A a 2

356

PRIMARY ARRANGEMENT OF TISSUES.

2. The axial bundle of the root in the Gymnosper ms'- is in general similarly con-
structed to the ordinary one of Dicotyledons. Its original structure is always altered
very early by secondary growth from the cambium ; the sclerenchymatous fibres in the
periphery of the phloem region of Dion described by Reinke may have owed their
origin to this. Over the angles of the xylem-plates the pericambium is single-layered
in Taxus, Thuja, and Biota; many-layered (from 3 and 4 to 7 cell-layers thick) in
species of Podocarpus, Pinus, and in the Cycadese investigated.

The xylem-plates consist at their outer corners of tracheides, with the fibrous
thickenings generally characteristic of this region ; in their internal, later developed
portion they consist of pitted tracheides, such as are characteristic of the wood
of Gymnosperms.

Among the Coniferae the Gupressineae and Taxineae have diametral and diarch xylem-
plates in roots of all degrees, or more rarely triarch ones. In the Abietineae higher

Fig. 165 (145). — Ranunculus repens. Cross-section through the vascular bundle of an old adventitious root.
u endodermis, p pericanibial layer, g external primordial vessels of the tetrarch xylem, r large axial pitted
vessel. In the pitted vessel X surface-view of a roundly perforated cross-wall. A narrow zone of secondary
wood has been deposited on the primary xylem plates extending from gto r; the cells between this and the
phloem groups are tangentially divided ; cf. Chapter XIV.

numbers and with them greater individual variations are the rule, though here no con-
stant relation exists in the main root between these variations and the number of the
cotyledons, which, as is well known, is likewise variable and always more than two. In
Abies excelsa, for example, van Tieghem found in 13 seedlings a triarch root-bundle, the
cotyledons numbering 7, 8, 9, or 10 ; in a specimen with 6 cotyledons the bundle was
diarch, in one with 8, tetrarch. The numerous investigations of the observer mentioned
established similar relations for the species of the genus Pinus in the narrowest sense
(P. Pinea, halepensis, sylvestris, &c.). The number of the xylem-plates here amounts

’ See van Tieghem, L c. — Strasburger, Coniferen und Gnetaceen, pp. 340, 360, &c, — Mettenius,
Beitr. z, Anatomie d. Cycadeen, p. 595, &c. — Reinke, Morpholog. Abhandl. I.

STRUCTURE OF RADIAL BUNDLES.

357

to 3-6, rarely 7. They are distinguished from those of the closely-related Abietinese by
their form, which may be compared to that of a Y. Each of them is, literally speaking,
diarch; and begins externally with two rows of about five narrow tracheides, touching
the pericambium at two separate points ; they converge towards the inside and abut on
each other. From their point of junction a radial row of tracheides, 1-2 layers thick,
extends in the centripetal direction, without reaching the centre of the root. In the
angle of the Y lies a resin-canal surrounded by delicate cells.

The roots of Ephedra which have been investigated have diametrally diarch xylem-plates.

Among the Cycadeae the xylem in the subsidiary and branch-roots, which have
been investigated in numerous species, is usually diametrally diarch, the two original
plates meeting in the middle, or being separated by parenchyma. The same holds good
for the investigated main roots of Cycas revoluta and Zamia furfuracea. More rarely,
in the thick subsidiary roots of usually diarch species, the bundles are three-rayed. In a
hybrid Ceratozamia van Tieghem found three or four xylem-plates, and in a specimen of
Zamia muricata Mettenius found six in the main root. In the subsidiary roots of Cycas
revoluta, when the centripetal development of the plate is already advanced, some
scattered narrow reticulated vessels appear at the sides of its peripheral corners, as if
secondarily ; whether these constitute the first beginnings of the secondary growth re-
mains to be decided.

3. Among the Monocotyledons there are first of all many thin main roots of the
seedling, which in the structure of their axial vascular bundle are indistinguishable
from those of Dicotyledons and Gymnosperms. In the case of Allium Cepa, with a
diametrally diarch, and sometimes triarch xylem-plate, this has already been mentioned
above ; A. Porrum and Lilium Martagon are characterised by a similar structure of the
main-root, Tulipa Gesneriana shows the deviation that its pericambium consists of
two layers instead of one. Bulbine annua has three xylem-plates, which do not meet ;
Iris Monnieri has four. The weaker roots of all degrees are essentially similar to
those just described.

Stouter main-roots, such even as those of species of Asphodelus, Ganna, j\nd
Asparagus officinalis, and then those of the Palms (Phoenix, Seaforthia elegans), and
above all the subsidiary roots springing from the stem (which in this class, as is well
known, usually far exceed the main-roots in thickness), though in the great majority
of cases they maintain the typical plan of structure, yet become polyarch as their
bundles increase in bulk, and also show a more varied differentiation, due to differ-
ences of many kinds in the structure of the tissue-elements. Comp. Figs. 166,
167, 168.

First of all, as regards the number, arrangement, and form of the groups of tissue
of these typical roots of Monocotyledons, the number of the xylem and phloem rays
rises from 5-10 up to 20, 50, and more. The thick roots of Iris, Asparagus,
Smilax (Sarsaparilla), Palms \ &c., are examples of a high degree of polyarchy. The
phloem-bands are always small, consisting of relatively few elements, their cross-
section being roundish or radially elongated. The xylem-bands, consisting of one
or a few rows of elements, usually begin at the periphery with a short uninterrupted
radial band of narrow tracheae, which become gradually wider towards the inside.
These are suddenly followed in the centripetal direction by one or a few, very wide,
reticulated or pitted vessels. The latter are usually separated from the peripheral

‘ Von Mohl, Palni. Struclura, Diplotheinium maiilinium, Tab. I.

358

PRIMARY ARRANGEMENT OF TISSUES.

part of the row by one or more layers of interstitial cells. In the thicker polyarch
bundles these large vessels are often confined to certain rows, while in the others,
which alternate irregularly with the former, they are absent ; or the case frequently

Fig. i66.— Acorus Calamus. Cross-section through a vascular bundle and the neighbouring part of the cortex of
an adventitious root, s cndoderniis; /,/ narrow primitive vessels; ^larger internal vessels, not j'et completely
developed ; ph phloem-groups. From Sachs’ Textbook.

Fig. 167. — Very thin cross-section through the vascular bundle of an older adventitious root of the same plant
(145). s endodermis, g primitive vessels, iv phloem groups. The axial mass of cells, which is still thin-walled in
Fig. 166, is here sclerotic, and the internal vessels are completely developed.

occurs that two neighbouring rows converge at an acute angle towards a large vessel,
forming in cross-section the figure of a V, in the angle of which the large vessel lies.
Xylem-plates do, however, occur among Monocotyledons also, in which the elements

STRUCTURE OF RADIAL BUNDLES.

359

become wider quite gradually in centripetal order, e. g. in many Orchids, as Stan-
hopea sp., Epidendron ciliare, &c. Here also the number of the vessels of a plate
following one another in the radial direction is small, on the average 4-6, not un-
frequently still fewer. In the Carices investigated the row usually consists of a single
narrow peripheral vessel, or two lying side by side in the tangential direction, and
one wide internal pitted vessel, the latter being separated from the former by at least
three layers of parenchymatous cells. A second narrow pitted vessel may lie between
the two. The peripheral vessels often occur without the corresponding wide one, so
that a row can no longer be spoken of.

In smaller roots or bundles, as in the main roots mentioned above, the weaker
roots of all degrees among the Grasses (Secale, Triticum), and in weak adventitious
roots of Tradescantia virginica, the xylem-rows either meet in the middle of the bundle,
or converge towards one or two wide vessels passing through the centre of the bundle,
which originate very early, but attain their development very late. The xylem-rows
sometimes come into direct contact with these, or are sometimes separated from
them by a few interstitial cells. In the thicker typical roots of Monocotyledons the
radial xylem-plates do not nearly reach the centre. The latter is occupied by a
thick cylinder of parenchyma or sclerenchyma, at the circumference of which the
system of xylem-plates often forms a relatively narrow ring.

In the great majority of roots of Monocotyledons the xylem-ring is surrounded
on the outside by an uninterrupted pericambium, which is one layer thick over the
xylem-plates, and the outside of which borders on the endodermis. It rarely con-
sists of two layers over the xylem-plates, as in the main roots of Tulipa Gesneriana
mentioned above, and in roots of Sarsaparilla. All the roots of Graminete investi-
gated form a remarkable exception to this rule (Oryza^, Secale, Triticum, Zea, Coix,
Sorghum, Hordeum, and Paspalum spec. ^), as in these the pericambium is, as a
rule, interrupted by the rows of vessels which thus border directly on the endodermis.
Even here, however, a small pericambial cell often lies between the endodermis and
the outermost vessels, e. g. in Maize. The same occurs among the Cyperacese in
species of Carex. In certain cases the narrow pitted vessel borders closely on the
endodermis, e. g. in C. foenea, folliculata, divulsa, and hirta ; or both this and the usual
arrangement, in which a pericambial cell is present outside the vessel, may occur in
different parts of one and the same cross-section. According to van Tieghem,
other species of Carex, such as C. brizoides, only show the latter typical arrange-
ment ; the same is the case in species of Cyperus, as C. longus and C. alternifolius.

In these typical roots the structure of the single tissue-elements shows a variety
of individual differences, both as regards the vessels, and no doubt the sieve-tubes
also, though the latter in most cases still require more accurate investigation. Into
these differences we cannot here enter at all minutely. The mass of cells, forming
simultaneously longitudinal and concentric rows between and inside the xylem-plates,
shows sometimes a typically parenchymatous, sometimes a typically sclerenchymatous
structure, or a form intermediate between the two. And indeed these peculiarities
either extend uniformly to the whole interstitial mass of tissue in question, or are

^ Nägeli und Leitgeb, l.c.

^ Van Tieghem, l.c. [See further, Klinge, Vergl. hist. Unters, d. Gramineen u. Cyperaceen-
wurzehi, Mem. de I’Acad. Imp. St. I’ctersb. VII. Ser. Tom. XXVI. No. 12, 1879.]

360 PRIMARY ARRANGEMENT OF TISSUES.

different in certain zones and groups. Of the combinations which are here possible
the following usually occur: (i) the entire interstitial mass of cells, including the
cylindrical axial portion, remains thin-walled and parenchymatous, e. g. adventitious
roots of Tradescantia virginiana. Curcuma longa, and Clivia nobilis (Acorus Calamus is
intermediate, i. e. its parenchyma has very strong walls). (2) The whole mass of cells
mentioned becomes sclerenchymatous, e. g. Carex divulsa, Cyperus alternifolius, and
no doubt most adventitious roots of Cyperacese and Grasses; also Curculigo recur-
vata. (3) The tracts of cells between the xylem- and phloem-plates are sclerenchy-
matous, forming together with them a dense firm ring around an axial strand of
parenchyma, with intercellular spaces containing air : e. g. roots of Smilax (Sarsa-
parilla) with very extensive parenchyma containing abundance of starch, most aerial
roots of Orchidacese (e. g. Epidendron ciliare, Oncidium sphegiferum), and many
Palm roots (cf. Mohl, /. c.), in which scattered sclerenchymatous fibres may occur
again inside the axial thin-walled parenchyma, e. g. Chamsedorea elegans.

The pericambium remains in most cases thin-walled and parenchymatous, even

where it borders on sclerenchyma,
though it may itself eventually be
involved in the sclerosis, either wholly
or in part ; the former, for example,
is the case in Sarsaparilla roots, the
latter among Orchids, e. g. Epiden-
dron ciliare, where opposite each
xylem-plate two rows of its cells usu-
ally remain very delicate, while the
others, like those adjoining them on
the inside, become greatly thickened.
The usually one-sided sclerosis of the
endodermis, which is very frequent,
though by no means universally dis-
tributed among long-lived Mono-
cotyledonous roots, has been dis-
cussed in Sect. 27.

Deviations from the type of
structure of Monocotyledonous roots
hitherto considered occur in dif-
ferent degrees as regards the ar-
rangement of the forms and regions of tissue, though their structural conditions
remain the same.

In thick root-bundles, some or all of whose xylem-plates converge in pairs in
the form of a V, the groups of phloem lying inside a V are often smaller than those
between two V’s. The latter are frequently large, radially-placed plates, while the
former are roundish groups. This occurs in an exquisite form in the aerial roots of
an Aroid cultivated in Halle under the name of Philodendron Imbe (Fig. i68), and
also in Palms ; cf. Mohl’s figure of Diplothemium maritimum already cited. In
Chamaedorea elegans this inequality goes further. In the corner of the V lies a
small roundish group of phloem ; between every two V’s a similar group lies towards

Fig. j68.— Philodendron Imbe Hort. Halens. Cross-section
through a thick subsidiary root, slightly magnified. The axial
vascular bundle, and on the right the entire cortex, are shown ; «
external limit of the xylem-rows. The obliquely shaded radial
bands tv are the phloem-groups; /> periderm; b fibrous bundle
surrounding an intercellular passage containing latex.

STRUCTURE OF RADIAL BUNDLES.

361

the outside, while further inside a second occurs, which is elliptical in cross-
section, and is’ separated from the outer group by interstitial sclerenchyma, in which
vessels frequently lie. Thus both an inner and an outer row of phloem-groups are
here present in the otherwise typical bundle.

It has been mentioned above that the xylem-plates not uncommonly converge
towards one or two axial vessels, though it may be without coming into immediate
contact with them. Such axial vessels frequently occur isolated, in the middle of a
thick parenchymatous or fibrous cylinder, and are separated from the inner edges of
the radial plates by many layers of cells. This kind of structure is found here and
there as an individual peculiarity of many roots, as for example in the Sarsaparilla
of Veracruz^; in Carex folliculata I found, on the same stock, roots of the structure
usual , in Carex, with a thick dense sclerotic axial-cylinder, and others in which the
middle of the latter is traversed by about 5 moderately large, prismatic, pitted vessels,
which are in contact with one another.

These trifling forms of deviation constitute the transition to those more con-
spicuous cases, in which numerous vessels, as well as groups of sieve-tubes, occur
scattered in the whole of the cylinder inside the radial ring, a phenomenon which is
characteristic of the thick adventitious roots of many epiphytic Aroideae, of those
Musaceaj which have been investigated, of the Dracsenere, Pandaneae, (Pandanus,
Freycinetia, Cyclanthus), and of the Palms Iriartea exorrhiza and I. praemorsa.

All the investigated roots of terrestrial Aroidese, as well as those of many epiphytic
species, present the usual typical structure; but in the thick aerial roots of other
forms, scattered wide vessels, and very large sieve-tubes, isolated or occurring in pairs,
and accompanied by cambiform tissue, are distributed throughout the wide and constantly
sclerenchymatous cylinder inside the radial ring. The two kinds of elements do not
lie in the radial rows; Tornelia fragrans, Heteropsis ovata, Monstera surinamensis,
Adansonii, Raphidophora angustifolia, Scindapsus pictus. Philodendron micans, and
Anthurium digitatum are examples of this structure.

The same occurs in species of Strelitzia, and no doubt in other Musaceae

Essentially the same arrangement is present in roots of Dracaenae and Pandaneae, with
the sole difference that the axial tissue in which vessels and sieve-tubes are distributed
is not homogeneous, but around the vessels and small phloem-groups consists of scleren-
chymatous fibres, while between them it is formed of parenchyma, in which, in the case
of Pandanus, lie wide intercellular passages containing air and scattered bundles of fibres.
The ring has likewise sclerotic interstitial tissue between the radial xylem- and phloem-
groups, the number of which even in moderately thick (1.5 cm.) roots of Pandanus
amounts to nearly 200 of each. The cross-section of such roots therefore presents first
the typical, relatively narrow ring, surrounded by a many-layered pericambium and an
endodermis, and then, inside this, a wide space filled by parenchyma, in which numerous
thick strands run longitudinally. Each of these strands consists of a many-layered mass of
sclerenchymatous fibres, in which are enclosed one or more isolated wide vessels or groups
of them, and one or more small phloem-groups separated from the vessels, while more
rarely only one, or neither of the two forms of tubes are present. The position of the
two in the stiand varies irregularly. The distribution of the strands in the parenchyma
appears in equivalent roots to be somewhat different according to the species. Among

' See Berg, Atlas d. pharraac. Waarenkunde, Taf. III. g.

- Van Tieghem, Lc. p. 149.

^ Compare Wittmack, Musa Ensele, Halle (Linnoea), 1867, p. 62.

362

PRIMARY ARRANGEMENT OF TISSUES.

the Pandanese, for example, I find them, as seen in cross-sections, isolated and irregularly
distributed, in the thickest roots of Freycinetia nitida of the Berlin Gardens; in Pandanus
pygmaeus (graminifolius of gardens) they are arranged in transverse rows (parallel to a
diameter), which are separated from one another by broader bands of parenchyma. In
P. odoratissimus, two or more strands separated by narrow bands of parenchyma are
placed together in groups, and the groups scattered between broader masses of paren-
chyma. As the thickness of the roots diminishes the conditions of structure described
become simplified. A branch-root of Pandanus pygmaeus, 1-2 mm. in thickness, has, for
example, inside the radial ring about 2-3 large vessels, and the same number of phloem-
groups, enclosed in homogeneous fibrous sclerenchyma, which is directly continued into
the ring. Branch-roots of Dracaena reflexa about i mm. in thickness have a thoroughly
typical structure, the radial ring surrounds a thin-walled axial cylinder of parenchyma.
It is only in thicker roots that an irregularly placed strand of sclerenchyma containing
vessels appears, such strands becoming very numerous as the roots increase in thickness.

The roots of Iriartea, finally, which are an inch in thickness, are distinguished from
those last described, first by the fact that their bulky vascular mass is not cylindrical, but
deeply furrowed, having in cross-section the form of a star with about ten blunt and
usually bifid rays ; further by the fact that the radial ring also is divided up into scleren-
chymatous bundles, enclosing the vessels and phloem-groups, and radial bands of paren-
chyma, which are sometimes narrow, 1-2 layers in thickness, sometimes many-layered,
and which separate the bundles frcm one another. The middle of the star also consists
mainly of thin-walled parenchyma, often with lacunae, which is directly continued into
the radial bands of the ring, and in which bundles of sclerenchyma, each containing one
or more vessels and phloem-groups, lie scattered. Inside each sclerenchymatous bundle
the vessels are surrounded by 1-2 layers of parenchymatous cells, those of them which
belong to the ring standing in direct connection with the many-layered pericambium. An
end odermis, which is thickened here and there, appears according to Mohl’s figure to
surround the star. Finally, in the entire parenchyma, both of the star and of the cortex
which surrounds it, numerous small bundles of sclerenchymatous fibres lie, each enclosing
in its centre 1-2 thin-walled elongated elements (perhaps sieve-tubes?). The xylem-
plates in the ring appear short and irregular in cross-section, their radial arrangement and
alternation with the phloem-plates is according to Mohl’s figure often indistinct, though
in general to be recognised. The development of the elements, both in Iriartea (Karsten)
and in the roots of Pandanus, begins at the periphery of the ring, and in general proceeds
centripetally. According to all these phenomena, the series of large roots just described
are immediately connected with the type of Monocotyledons as special cases, in which
the anatomical differentiation becomes more varied, with the more considerable size.

The system of bundles, which traverses the tuberous roots mentioned at p. 233, is
entirely different in structure from the bundles last mentioned. In Dioscorea and
Sedum all the bundles are typically collateral. The same holds good for the Ophrydese,
with the limitation that the vessels are only very sparingly developed. Each bundle is
surrounded by a separate endodermis.

4. In the Filices in the widest sense, the Marsiliacese, and the Equiseta, with a
few exceptions to be mentioned below, the axial cylindrical bundle of the roots does
not deviate in its differentiation from the types hitherto regarded b Its xylem is in
the great majority of cases, with the exception of the Marattiacese, diametrally diarch,
beginning externally on each side with some narrow, fibrously-thickened tracheides
lying side by side, which are succeeded in the centripetal direction by one or a few
rows of wider, often large scalariform tracheides, of the structure usual in Ferns ;
(true vessels only occur in Athyrium Filix femina ^). Cf. Fig. 1 69. In Botrychium, the

* Compare Nageli und Leitgeb, van Tieghem, Russow, ll. cc.

^ Compare p. 165.

STRUCTURE OF RADIAL BUNDLES.

363

tracheides, which form several rows, are of a different structure, similar to that
described in the case of the stem and leaf at p. 346, and are all of approximately
equal and relatively small width. Triarch and tetrarch bundles sometimes occur in
thick roots of species, which are usually diarch ; triarch-bundles have been observed
in Pilularia, Equisetum, Botrychium, Blechnum brasiliense, and Cyathea medullaris,
tetrarch in Equisetum, the Blechnum above mentioned, and Cyathea. In the species
of Trichomanes^ investigated triarch to octarch bundles usually occur, diarch bundles
being rare, while, on the other hand, the latter are characteristic of the roots of Hymeno-
phyllum. On the monarch bundles of some species of Trichomanes, see below.

FIG. 169. — Adiantuin Moritzianuin (225). Old root, cross-section, h — h hairs of the epidermis cut through, ti endodermis,
pc pericainbium, pr primitive tracheides of the diarch xylem alternating with two phloem groups.

The xylem-plates are in most cases united in the middle, in the thinner bundles
often by means of a very large vessel (e. g. Equisetum), or of a row consisting of
two large vessels, crossing the diametral pair of plates at right angles (Eig. 169).
In other respects various subordinate differences of form occur, e. g. a regularly
elliptical cross-section of the diametrally united plate in Osmunda, Todea, &c., &c.

The arrangement of the phloem-groups corresponds to the general plan of

^ Meltenius, Ilymenopliyllaceen, l.c, p. 420. — Russow, l.c. p. 95.

3^4

PRIMARY ARRANGEMENT OF TISSUES.

root-structure ; their histological peculiarities are essentially similar to those of the
stem of the same plant, and like the latter still require more exact investigation.

The pericambium appears, as a rule, as a single layer all round, but it also
occurs with two layers ; this is the case only outside the phloem-groups in Aspidium
Thelypteris, and all round in Polypodium ireoides^; in Osmunda and Todea there are
several layers all round. In the Equiseta, in contrast to the other forms belonging
to this series, all the cells of the pericambium stand precisely in front of those of the
endodermal sheath, and together with these have arisen from the division of the
innermost cortical layer. In the other vascular Cryptogams the latter forms the
endodermis only, while the pericambium originates by tangential division of the
plerome-cylinder surrounded by the cortex.

In the endodermis, apart from various subordinate differences of form, the cells
lying in front of the corners of the xylem-plates are the initial cells of the lateral
roots, and are often distinguished from the others by their more considerable size.
The structure of the endodermis is otherwise essentially the same as in the bundles
of the stems of the same plants.

In the roots of Cryptogams, the orientation of the diametrally diarch xylem-
plates is always such that their surface cuts the median plane of the next higher
order of ramification at right angles. Those arising on the stem appear, according
to species, either to have the like orientation with reference to its median plane, or
to have their surface coincident with the median plane of the stem.

The axial root-bundles of the Marattiacese ^ are distinguished from those of the
other Ferns, with which their structure otherwise agrees, by their tetrarch or polyarch
xylem The number and length of the radial plates increases, in the same species,
with the thickness of the roots; the former may amount to 18-20. In the thicker
roots we frequently find them converging in pairs, and as seen in cross-section united
to form the figure V, In the roots occurring above the ground the xylem-plates do
not reach to the middle of the bundle ; in the thin, 4-5 rayed branches underground,
they meet, according to Russow, in the middle.

The very thin root-bundle of Azolla which differs in its development from
that of the Ferns, has, according to Strasburger, a usually triarch xylem, consisting
only of spiral tracheides. Besides this there are only some inconspicuous elements
lying inside the pericambium, and constituting a doubtful phloem.

Sect. 109. A structure departing from the general radial type of root occurs in
the rhizophores of Selaginellse in the true roots of the same plants, in the thinner
roots of Lycopodia, and in the roots of Isoetes and Ophioglossum ; with the excep-
tion of the rhizophore of Selaginella Kraussiana, the peculiarity of this structure
consists in the fact that the usually monarch xylem either occupies one side of the
bundle, the phloem lying on the other — the arrangement being thus collateral — or
that the former is at least strongly approximated to one edge of the phloem which
surrounds it. Most roots or rhizophores belonging to this series are dichotomously

' Van Tieghem, l.c. — Compare also Nageli und Leitgeb, p. 83.

^ Meyen, Haarlemer Preisschrift (1836), Tab. VIII.

“ [Cf. Holle, Königl. Ges. d. Wiss. zu Gött. Jan. 8, 1876.]

'* Strasburger, Ueber Azolla, p. 48.

* [M. Trcub, Recherches sur les Organes de la Veg. du Selsginella Martensii. Leyden, 1S77.]

STRUCTURE OF RADIAL BUNDLES.

3Ö5

branched, and show a definite orientation of the parts of the bundle in the successive
bifurcations. The structure in question might therefore be regarded as characteristic
of dichotomous roots, if it were not that those of Ophioglossum are always entirely
unbranched for there is no basis of fact for van Tieghem’s supposition, according
to which this unbranched root would be the favoured branch of a root which has
already undergone bifurcation while still inside the cortex of the stem which pro-
duces it, its other branch not coming to development.

Among the dichotomous rhizophores^ and roots of the Selaginellae, in the first place,
the rhizophores of S. Kraussiana are distinguished by cylindrical vascular bundles, in
which the middle of the central and centrifugally developing xylem is occupied by the
narrow primitive tracheides, while the periphery is formed of wider scalariform tra-
cheides. The phloem completely surrounds the xylem as a many-layered small-celled
zone; histologically it still requires more exact investigation®. To form the bundles of
the first pair of roots, the bundle of the rhizophore is uniformly severed into two halves,
in which a group of narrow primitive tracheides occupies one edge of the xylem, while
the development of the elements proceeds from this point towards the other broader
side. The xylem is thus monarch, similar to the usual collateral bundles, from which
those in question are distinguished by the fact that the phloem completely surrounds the
whole xylem.

The structure last described belongs to all the investigated roots of Selaginella, and to
the rhizophores of S. Martensii. These bundles also divide at the dichotomies in such
a manner that the plane of division passes through the primitive group and the edge of
the xylem lying diametrically opposite to it. In primary axes arising from the stem, the
orientation of the bundles with a unilateral group of primitive tracheides is such that
that group faces the base of the stem. In the dichotomous branches it always lies on the
inner side, turned towards the other branch of the pair. At every bifurcation therefore
each bundle proceeding from the division of the main-bundle undergoes a torsion of 90°,
and indeed this takes place gradually inside the main axis, in such a way that the two
bundles run on side by side from the point where they separate, which is above the point
of bifurcation of the root, as far down as the latter. Only in the first dichotomous
branches of the rhizophore of S. Kraussiana does the same orientation come about
without torsion.

The feeble bundles in the roots of Isoetes * show in the general structure of their uni-
laterally monarch xylem, and in its orientation in the dichotomous branches, the same
behaviour as those of Selaginella. As regards the elementary composition of this part
they are distinguished by the fact that it consists only of a few rows of annular and
reticulated tracheides, without scalariform vessels. So far as investigations extend, the
phloem is feebly developed, and confined to the side remote from the primitive tra-
cheides ; as seen in cross-section it has the form of a narrow crescent-shaped band, wh ile
its histological structure is indistinct. The position of the bundle in the root is
from the first slightly eccentric, and in the dichotomous branches always approaches the
other branch of the pair. The eccentricity, which is caused by a mainly one-sided
extension of the cortex and of its large air-cavities, increases with the thickness of the
roots. According to Mettenius’ short statement®, the structure of the xylem and the
eccentric position of the bundle in the roots of Phylloglossum are similar to those in Isoetes.
The bundle approaches that side of the always unbranched root which is basiscopic with
reference to the stem.

^ Compare Holle, Botan. Zeitg. 1875. Holle’s observations are not in agreement with van Tieg-
hem’s statement that the roots of Botrychium, with typically radial bundles, are dichotomous (p. 315).

“ Nägeli und Leitgeb, l.c. p. 124. ® [Cf. Treub, /. c.]

‘ Hofmeister, Beiträge z. Kenntniss d. Gefasskryptogamen, I. — Nägeli und Leitgeb, l.c. p. 131.
= Botan. Zeitg. 1867, p, 99.

366

PRIMARF ARRANGEMENT OF TISSUES.

The thin roots of Lycopodium, already described at p. 350, are immediately related
to the above.

Lastly, the roots of Ophioglossum are to be mentioned here In the circular cross-
section of the axial bundle it may be seen that the half, which with reference to the parent
axis is basiscopic (lower), is formed of tracheides, which are united together without inter-
cellular cavities, and are similar to those of the stem (p. 346). The upper edge of the bundle
is formed by a half-ring, usually two layers in thickness, of relatively large, wide sieve-tubes.
Between this phloem and the xylem lie some layers, — numbering on the average three, — of
delicate prismatic, narrower elements, destitute of starch, the sieve-tube nature of which
is doubtful ; as a rule a layer of delicate cells separates the xylem from the endodermis,
while the sieve-tubes border immediately on the latter. According to van Tieghem the
last statement often holds good also for the two middle tracheides of the lower edge.
The development of the tracheides begins at one corner of the segment of the circle,
and proceeds from this point round the convex edge, and from this again towards the
phloem. The endodermis, like that of the stem, only differs from the other cortical
parenchyma in the undulating bands on its radial walls.

4. Imperfect and rudimentary Bundle-trunks.

Sfxt. 1 10. The vascular bundles described above occur in land plants possessing
foliage which is rich in chlorophyll, and also in the stems and leaves of parasites
which contain no chlorophyll, or only traces of it, as in the Orobanches, Cuscutas,
Lennoaceae, &c. As indicated at p. 321, they are cceieris par thus, as a rule, all the more
developed the greater the development of the leaf-surface.

Conversely, the development of the vascular bundle-system diminishes in every
respect with that of the leaf-surface exposed to the air ; and this is the case firstly
with regard to its differentiation into individual bundles and their branches, as is
clearly shown by its reduction to an axial strand in the stems of many submerged
plants (cf. pp. 277, 321), and by its simplification in the submerged leaves of am-
phibious plants (p. 306); and secondly as regards the anatomical differentiation of the
individual bundle. In the latter, while the plan of structure remains the same, a
diminution of the essential tissue-forms may be recognised, as, for example, the com-
parison of the bundles in the stem and root of Ranunculus repens (Figs. 152, 165) with
those of R. fluitans (Figs. 153, 163) teaches ; and in fact the diminution is chiefly in
the xylem, while the phloem remains the same, or is less reduced. Further, as the
characteristic elements, and especially the tracheae, progressively diminish, deviations
from their usual typical arrangement also occur. Again, there may be complete dis-
appearance of the tracheal elements, and finally of the sieve-tubes also, so that the
entire bundle is replaced by a strand of uniform elongated cells. Lastly, we find the
absence even of any rudimentary indication of a vascular bundle, as in the tiny
swimming frond of the Wolffias, which is a large-celled mass of parenchyma, covered
by an epidermis, which has stomata on the surface in contact with the air’*.

These cases of imperfectly developed bundles are to be contrasted with the
complete ones hitherto considered. They are divided into two main categories,
namely, those which originate as complete bundles, and then by disappearance of
the xylem become more or less incomplete — being thus bundles with a transitory

^ Van Tieghem, Russow, /. c.

Hegelmaier, Lemnaceen, p. 31.

IMPERFECT AND RUDIMENTARY BUNDLES.

3^7

xylem, — and secondly, those which re7?iaiti imperfect from the beginning. Both
forms are connected step by step with the complete bundles by various intermediate
forms, which have already often been mentioned above.

This especially holds good of those bundles which become imperfeci by disap-
pearance of the Tracheae. In many herbaceous plants with collateral bundles an
intercellular passage appears in place of the primitive vessels when the development
of the tissues is complete, as was described at p. 327.

In a series of other plants, which are submerged or partly submerged aquatics,
all the vessels in most of the bundles disappear at once throughout a long part of
their course, after they have originated as annular or spiral vessels. In place of the
xylem an intercellular canal (filled with water) occurs in the mature bundle, and on
its walls the remnants of this thickened membrane may remain preserved. On the
other hand, the phloem of the bundles is persistent, and, in many of the cases in
question, very well developed. These phenomena present many variations according
to the particular cases, and are especially conspicuous in the stems of the Potamo-
getons, and of the submerged plants connected with them, which have an axial
bundle, or a very simple bundle-system Even in those among these forms in which,
as in P. natans, distinct bundles of the leaf-trace and common bundles can be dis-
tinguished, the want of vessels on the one hand, and on the other hand the crowded
position of the bundles, often gives the whole bundle-system a structure which at the
first glance is difficult to decipher, and into this we here have to enter somewhat
more minutely.

The course of the leaf-traces, and of the four cauline bundles in the stem of Pota-
mogeton natans and perfoliatus, was described above at p. 272. All the bundles are
at their origin collateral, with normal orientation.

In the node all their parts are persistent, and soon become irregularly united by
anastomoses. In the whole internode, on the other hand, the entire xylem disappears
from the bundles of the leaf-trace with the beginning of the more intense elonga-
tion, and is replaced by an approximately cylindrical narrow intercellular passage,
bordered by narrow elongated cells In P. perfoliatus the same holds good also of
the four cauline bundles ; in P. natans, on the other hand, the few (1-3) reticulated and
annular tracheae of the latter are usually persistent. The phloem-portions of all the
bundles are very well developed and persistent. All the bundles are further closely ap-
proximated to one another, being only separated by a few layers of parenchymatous
cells containing abundant starch, and traversed by small groups of sclerenchymatous
fibres. The bundles are grouped to form an axial strand, rectangular as seen in cross-
section, which is marked off from the lacunose cortical parenchyma by an endodermis,
which becomes sclerotic subsequently (Fig. 170). Inside this, one bundle faces
each longer side of the rectangle ; opposite one of these two sides is a larger bundle,
the sympodial one, descending from the second leaf above : facing the other side is
a somewhat smaller bundle, the median one of the next higher leaf, belonging to
the internode. One of the lateral bundles of this leaf faces each of the shorter sides ;
the four cauline bundles face the four angles. A group of sieve-tubes lies at the

' Compare p. 277, and the literature there cited.
^ A. 11 Frank, Beitr. z. Pflanzenphysiol. p. 135.

368

PRIMARV ARRANGEMENT OF TISSUES.

outside of the wide intercellular passage of the sympodial bundle, while two some-
what smaller groups stand symmetrically right and left of the centre of its inner side.
In the remaining bundles the intercellular passage or vascular group is bordered on
the outside by an arched group of sieve-tubes.

In the other Potamogetons investigated (lucens, gramineus, densus, crispus, pec-
tinatus, and pusillus), in Zanichellia, Althenia, Cymodocea, and Zostera, the tracheae
in the node are persistent, while those in the internode are all transitory. To every
bundle an intercellular passage corresponds, which is surrounded on the outside by
phloem. Where several bundles traverse the internode, they approach each other
closely in a manner similar to that described for P. natans ; in the case of the two
leaf-trace bundles in the internodes of P. lucens and gramineus this goes so far, that

FIG. 170 (145V — Potamogeton natans. Axial mass of tlie internode, containing the vascular bundles ; cross-section.
n unilaterally thickened endodermis, containing starch; outside the latter lacunose cortical parenchyma, with
abundant starch ; I air-cavilies. Explanation of the numerals at p. 272. The delicate groups of tissue of the
numbered circles are the phloem ; the wide meshes in the latter are the sieve-tubes of the bundles ; the circles in
which the numerals stand are the xylem portions, usually converted into cavities. Between the bundles is parenchyma
containing starch, and sclerenchymatous fibres with a narrow lumen appearing as a darker point.

their intercellular passages, which are turned towards each other, are only separated
by one layer of cells, or in most cases are united to form a single passage.

The single axial sympodial bundle, which (without cauline bundles) traverses
the internode in the upright stem of P. pectinatus and pusillus (Fig. 171), has, in the
manner of a concentric bundle, a central intercellular passage replacing the xylem-
group, and this is completely surrounded by a relatively bulky phloem, containing
large sieve-tubes, and externally limited by the endodermal sheath, wTich eventually
becomes sclerotic.

The bundles in the stems of Zanichellia and Althenia behave quite similarly to
the forms last mentioned, only with the difference that the phloem is very delicate
and slightly developed, and consists of elongated cells with a few indistinct sieve-tubes.
Elodea and Hydrilla, of which we shall speak later on, are also connected with these

cases.

IMPERFECT AND RUDIMENTARY BUNDLES.

369

The vascular groups of Cymodocea sequorea ^ and Zostera behave, both in the
nodes and in the internodes, like those of the Potamogetons. The intercellular canal
derived from the xylem lies in the small peripheral bundles on the inner side ; outside
this is a radially elongated phloem-group containing two or three large sieve-tubes ; in
the thicker axial bundle the intercellular space occupies the middle, and in the case of
Cymodocea has at its periphery four sieve-groups placed cross-wise in the transverse
section ; in Zostera it is completely surrounded by a broad phloem, as in Potamo-
geton pectinatus.

The small stems of the Hydrillese
and of Aldrovanda vesiculosa consti-
tute transitional forms between the
bundles which become incomplete and
those which reinain rudimentary.

Elodea canadensis and Hydrilla
verticillata have an axial bundle of es-
sentially similar structure to that of Zani-
chellia. The one or two axial tracheae
present in the young rudiment of the
stem, which send off a branch into each
leaf, are incomplete from their first origin;
their walls are only thickened with seg-
ments of rings, and disappear every-
where— even in the node — on the com-
mencement of active extension^. Ac-
cording to Caspary, Aldrovanda shows
an axial bundle of 8-9 annular tracheae,
which, together with their branches
going to the leaves, are persistent in
the nodes, but disappear in the inter-
nodes during extension, and are here re-
placed by a passage surrounded by
delicate-walled elongated elements,
which have not been more minutely
investigated. Of bundles which remain
rudmenlary, those in the small stem
of Ceratophyllum and Najas are im-
mediately related to those just described. According to Sanio the former are
bundles which are at all times destitute of vessels, and consist of a mantle of narrow
sieve-tubes and elongated cells, between which lie several small intercellular pas-
sages, each produced by the absorption of a row of cells®; and inside this
mantle are some layers of parenchyma, which surround an axial passage derived
from the absorption of a multiseriate strand of cells. In the case of Najas the
mature structure is similar to that of Elodea; sieve-tubes have not been described, and
are doubtful; the axial canal arises from the absorption of a row of meristematic cells.

Fig. 171. — Potamogeton pectinatus (80). Cross-section through
an internode of the upright stem, ^intercellular passage replacing
the xylem which has disappeared; n unilaterally thickened endo-
dermis ; between it and ^ the phloem, with wide sieve-tubes.
Between u and the epidermis e is the lacunose cortex ; ö bundles
of sclerenchymatous fibres ; a a similar bundle, with a small group
of sieve-tubes in the middle.

^ Compare Bornet, l.c. ^ Caspary, Sanio, l.c. (p. 278). •’ Compare Frank, Beitr. p. 143.

B b

3/0

PRIMARY ARRANGEMENT OF TISSUES.

A Structure deviating from that described occurs in the bundles of the small
submerged stems of the floating Utricularias, the Lemnacese, the Podostemacese,
Vallisneria spiralis, and in the rhizomes of Epipogon and Corallorhiza, which inhabit
humus, all of these likewise remaining rudimentary.

In Utricularia vulgaris^ an axial, approximately cylindrical bundle is present,
and gives off a branch into each of the so-called leaves. Its smaller half, which with
reference to the horizontally floating stem is the upper, consists of elongated prismatic
cells, usually with flat ends and thick collenchymatous walls ; the lower half consists
chiefly of thinner-walled elements, namely, of wide sieve-tubes and numerous narrower
prismatic cells. Near the boundary between the upper and lower half, but within the
latter, next the centre, lies a single row of very long, wide tracheides, with their
pointed ends applied to one another, showing alternately annular and spiral thicken-
ing. In the small and younger specimens no other tracheides occur. In veiy thick
stems, on the other hand, I found a second single or double row of annular tracheides, /
lying at the side of those mentioned, near the periphery of the bundle. These are
similar to the former in structure, but only about half as wide. Their development
seems to take place very late. All the tracheides are persistent, no intercellular
spaces whatever are present in the bundle.

The bundles in the frond of Lemna^ consist in the native species of a thin row
of annular tracheides, surrounded by one or a few layers of elongated cells.
Spirodela polyrrhiza has several parallel rows of tracheides instead of one ; in Lemna
valdiviana, on the other hand, the development of tracheides is suppressed. The
absence of any vascular bundles in the Wolffias has already been mentioned above.

The bundles of the Podostemacese, which are still much in want of accurate
investigation, consist, according to Trecul’s short statement®, of a bundle of fibres and
some small annular vessels, the latter being often absent in old stems and replaced
by a cavity *. Vallisneria also requires more exact investigation ®.

The a.xial bundle in the rhizome of Corallorhiza contains in the middle two
multiseriate strands of reticulated tracheides with narrow transverse meshes, and
from these the simple bundles branch off for the distichous leaf- rudiments. It
further consists of elongated, usually thin-walled, elements, which still require more
exact investigation. In the rhizome of Epipogon the bundle consists, so far as
investigations extend, only of uniform, moderately elongated cells, with oblique ends
and thin walls.

In the roots of those aquatic plants, which in the stem possess incomplete or small
bundles containing passages, the bundles, which here, it is true, are always feebly de-
veloped, may be complete, and provided with a persistent xylem constructed according
to the radial type, e. g. Potamogeton lucens ®. As a rule, however, they here also either
become incomplete by disappearance of the vessels, intercellular passages appearing in
their place, or they remain rudimentary.

' Compare van Tieghem, Ann. Sei. Nat. 5 ser. tom. X. p. 54.

^ Hegelmaier, Lemnaceen, p. 48. ® Archives du Museum d’PIist. Nat. tom. VI. p. 4

■' [Compare Warming on Podosteraacese, Mem. Acad. Roy. Copenhagen, 1881, See. ; also Cario,
Tristicha hypnoides, Bot. Ztg. 1881, p. 25.]

“ [Compare Franz Müller, die Entwickelung von Vallisneria spiralis, Hanstein’s Abhandl.
Bd. III. Heft. 4, 1878.]

^ Van Tieghem, Ann. Sei. Nat, 5 ser. tom. XIII. p. 164. pi. VI.

ENDS AND CONNECTIONS OF THE BUNDLES.

371

The former process goes on as a rule more slowly in the roots, the vessels persist
longer, than in the stems of the same plants. The structure of the root-bundle (peri-
cambium, alternating xylem and phloem rays) here preserves its typical character,
although the number both of the individual rays, and of the elements composing each of
them, is low, the former being reduced to four or to two, the latter often to one. The
mature root has therefore the typical bundle-structure, with the exception that in the
place of the 2-4 xylem rays, each of which is represented by a vessel, and in place of
the large central vessel, which in many cases unites the rays, an intercellular passage is
present; e. g. Aponogeton, Alisma, Hydrocleis (van Tieghem).

In the root of Elodea canadensis, as in the stem of the same plant, the disappearance
of the 4-5 peripheral vessels, and of the larger central vessel, which is separated from
them by an annular layer of parenchyma, takes place immediately after their first,
incomplete formation.

Among those root-bundles which, on the other hand, remain rudimentary, those of Najas
belong to this category. They consist of two layers of elong.ited delicate cells, and
these surround an axial passage, which arises by the absorption of a row of meristematic
cells.

In the root of Vallisneria, according to van Tieghem, there is only an annular layer of
elongated cells enclosing an axial passage, and surrounded by the endodermis, and this
constitutes the rudiment of the bundle. The delicate bundle in the root of species of
Lemna shows in cross-section essentially the same structure ; the middle is according to
Hegelmaier occupied by a row of cells (not by a passage). Spirodela polyrrhiza shows the
same character, but with the difference that the row of cells in the middle is developed
into a persistent row of narrow annular trachei'des.

II. Ends and Connections of the Vascular Bundles.

Sect. iir. The ends of the vascular bundles, as was shown above at Sect. 91,
lie in the foliar expansions, and in the cortex of many plants, either as mternal ends
ceasing in the parenchyma, or forming anastomoses, or as peripheral ends at the
edge or surface of the leaves ; in the stems of Cyatheaceae, described at p. 291, they
also end in the interior of the pith.

With the ultimate degrees of ramification the thickness of the bundles usually
diminishes, both the number and the size of their elements being reduced ; at the
extreme free ends they often, but not always, show a terminal dilatation. Xylem and
phloem do not behave alike in this respect. Clearly characterised sieve-tubes are,
it is true, often still present in the thicker bundles of the foliage leaf, e. g. in those of
the leaf-nerves ; in the last orders of ramification they are no longer to be found,
the latter consist either of tracheae alone, or of these and of delicate elongated cells
accompanying them, the sieve-tube nature of which is no longer recognisable.
Where and how the sieve-tubes cease and end has not hitherto been clearly made
out in any case, and this point deserves more accurate investigation. The tracheae
always form the direct continuation of the xylem of the thicker bundles.

The ultimate internal ends and anastomosing branches of the bundles consist
only of one or a few rows of short tracheides with fine spiral thickening, or reticulate
thickening with narrow transverse meshes ; their wall is often quite smooth in parts,
appearing as if immature, e.g. in the leaf of species of Chamaedorea, and Zea Mais
(Fig. 175). Whether vascular perforations occur in these rows is at least doubtful,
and not easy to decide.

B b 2

37«

PRIMARY ARRANGEMENT OE TISSUES.

The dilatation of the terminal branches above-mentioned comes about either
by dilatation of the individual tracheides, or by increase in the number of rows.
The end surfaces of the tracheides, bordering on the parenchyma, are usually cut off
sharply, either transversely or obliquely. Characteristic differences corresponding to the
several main forms of distribution of the bundles, or to the larger systematic divisions,
are not to be observed, except that in general the thickness of the single terminal
branches diminishes where the branches are numerous. Thus in the Ferns investi-
gated, the relatively few final ramifications are comparatively thick, consisting of
several rows, while in the foliage of Dicotyledons, where the reticulations are
abundant and internal ends are present, the bundles are at last, as it were, broken up
into single, or in places double tracheal rows, which end free with short branches
(Figs. 172, 173). The transverse branchlets of most Monocotyledons consist of

V

Fig. 172. — Psoralea bituminosa (40). Ulthnate
ramifications of the bundles in a piece of a
leaflet ; at is the edge of the latter.

Fig. 173 (225).— Ultimate ramifications of vascular
bundles from the leaf-lamina of Psoralea bituminosa ;
branched rows of tracheides, the ends marked x being
torn off, the others terminating free. The whole
branched row is surrounded by large cells containing
chlorophyll; outside these are the circular transverse
sections of some cells of the dense palisade parenchyma
of the leaf.

one, or of quite a few rows of tracheal elements (Figs. 174, 175). The leaf of
Welwitschia is to be mentioned as an exception to the rule, for its very numerous
transverse branchlets (comp. Fig. 145, p. 303) have, so far as can be determined, the
structure of complete vascular bundles provided with a multiseriate xylem and
phloem, and it is only the short, thick, free-ending branches, springing partly from
the angles of the cross-branchlets and partly direct from the longitudinal bundles,
which consist exclusively of tracheides, the latter being inserted between the elements
of the surrounding parenchyma.

As follows from what has been stated, the ultimate vascular branches often
border directly on the parenchyma which in the laminse of foliage-leaves contains

ENDS AND CONNECTIONS OF THE BUNDLES.

373

Chlorophyll. Its elements, in so far as they stand in immediate connection with the
tracheae, approach on the one hand the latter, on the other hand the typical paren-
chymatous cells in their form. This is the rule for the leaves of Monocotyledons and
Dicotyledons ; the exceptional case in which even the ultim.ate transverse branchlets
are completely enclosed by stout
sclerenchymatous sheaths occurs
rarely in thick leaves of Mono-
cotyledons, e. g. Rhapis, Vanda
furva. In the leaves of Ferns the
branches of the bundles are, so far
as investigated, always ensheathed
by one or a few layers of elongated
cells destitute of chlorophyll', ot
which the outermost often has the
structure of an endodermis up
to the immediate neighbourhood
of the free ends. At the free
ends themselves the rows of
tracheides pass over into the
chlorophyll-parenchyma, through
the intervention of some elon-
gated smooth-walled cells.

In the case of many ends of
bundles, which according to their
local position must be called peri-
pheral, no essential differences
from the internal ends are to be
mentioned. On the other hand,
the structure shows peculiarities in
those numerous cases, where the
bundles run to parts of the epi-
derniis described in Chap. I, which
are distinguished by water-pores
and water-filtration, by excretion
of lime, or by glandular structure
and secretion.

Of the cases belonging to
this series, in the first instance, the
ends of the bundles in the furrows
of Fern-leaves excreting water and
lime (p. io6) are closely similar to
the internal terminations in these
leaves'*. They show a knob-like swelling, in consequence of a sudden increase in
the number and size of the tracheides, the latter being very short, with narrow

FIG. 174.— Zea Mais. Cross-section through a feeble (lower) leaf-
sheath, about 2 cm. higher than Fig. 151, p. 33:. e epidermis of the
outer surface, bordering internally on a hypodermal bundle of scleren-
chymatous fibres, st. On the inside of the latter one of the smaller lon-
gitudinal vascular bundles abuts ; r annular vessel, ar air-cavity, t—t
pitted vessels ; outside /, r, t are first narrow reticulated and pitted
vessels (with a darker outline), then the phloem rv\ q transverse
branchlet springing from the tradiere of the vascular bundle and
consisting only of a few rows of tracheides.

Fig. 175. — Cross-section through the lamina of the leaf of a young plant of
Zea Mais. 0 epidermis of the upper, u of the lower surface ; h hypodermal
strands of sclerenchyma ; and two small longitudinal vascular bundles in
cross-section; with three, ^>2 with two narrow vessels; both with a small
phloem, consisting in gT. of only three elements ; gi transverse connecting
branch between g\ and gi, consisting of a row of tracheides with partial
fibrous thickening, apd, like the two longitudinal bundles, enclosed directly
in parenchyma containing chlorophyll.

* Cf. Mettenius, Fil. hort. Lips. p. 9.

^ Mettenius, /. c.

374

PRIMARY ARRANGEMENT OF TISSUES.

reticulations and pits, or with spiral fibres ; one or two layers of delicate cells ensheath
the whole end of the bundle, and separate it from the thin-walled epidermis of the
furrow.

The ends of the bundles of the leaf-teeth of Drosera, and of the inner surface of
the foliar pitchers of Nepenthes, are likewise placed close under specialised portions
of the epidermis, and in point of structure stand next to those just described.

The leaf of species of Drosera (in particular of D. rotundifolia) has at its edge and
on its entire upper surface numerous filiform teeth with broadened ends’. Those of
the surface are, apart from differences of length, similar to one
another ; they are filiform processes, somewhat conically
tapered, but swollen at the end to form an approximately
ovate head. They consist of some layers of elongated cells,
in the middle of which are one or rarely two narrow spiral
vessels or rows of tracheides (I will not state definitely which)
branching off from the net of bundles of the leaf-lamina, and
having a straight course ; the whole is covered by a single epi-
dermal layer, the cells of which are likewise elongated. In the
middle of the knob-shaped end the spiral vessel enters a group
of closely-connected, short, reticulated and spiral tracheides,
which has, as a whole, an ovate form, and constitutes the main
bulk of the terminal portion. The layer of cells surrounding the
vesfels terminates below the middle of the group of tracheides,
in the form represented in Fig. 176. At the point of transition
to the head the epidermis first becomes short-celled, and then
suddenly passes over into the three-layered covering of the sur-
face of the knob, which, as Warming has shown, is derived partly
from the primary epidermis, partly from the layer of meristem
lying below the latter. The innermost layer of this covering
forms a bell-shaped single stratum, consisting mostly of elon-
gated cells, which is in immediate contact above and at the
sides with the group of tracheides, while below, at the edge
of the bell, it ends in the outer surface of the epidermis.
The membranes of its cells are smooth and firm, similar in
their reactions to those of an endodermis, the walls vertical
to the surface being undulated. From the edge of the bell
onwards the surface of the knob is covered by two layers of
cells, which are thin-walled, and are distinguished in the fresh
condition by their dense, intensely red contents. The inner
of these two layers does not reach quite to the edge of
the bell, and consists of small, isodiametric, polyhedral
cells, which are very delicate, and are in uninterrupted connection one with another.
It is everywhere covered by the outermo.^^t layer, which is continued immediately
from the edge of the bell over the whole surface of the knob, and consists of
polygonally prismatic cells in uninterrupted connection. The diameter of these cells ver-
tical to the surface increases successively towards the apex of the knob ; here it is about
twice as long as the diameters coinciding with the surface, while at the base it is about
equal to them. The delicate outer walls of, these layers, which are covered later on with
the sticky secretion (p. loi), show a very delicately undulated outline at the edges. — The
teeth of the edge of the leaf are expanded at their ends to the form of a spatula or long

^ Compare Meyen, Secretionsorgane, p. 51. — Trecul, Ann. Sei. Nat. 4 ser. tom. III. — Nitschke,
Botan. Zeitg. 1S61, Nos. 22, 23, &c. — Martinet, Ann. Sei. Nat. 5 ser. tom. XIV. — Warming, /. r. ;
compare above, p. 57. — Darwin, Insectivorous Plants.

Fig. 176 {145). — Drosera rotundifolia.
End of a tooth from the upper side of
the leaf ; axial longitudinal section.
s — s the bell-shaped layer directly sur-
rounding the end of the bundle ; its
lowest, very long cells, j, standing with
their nan ow outer wall in the epidermal
surface, belong, as shown by Warming,
and as their appearance indicates, to
the primary epidermis ; all the others,
occurringabove the apex, proceed from
the subepidermal meristem. The same
holds good of the cells of the second
layer occurring above the apex; in so
far as they border on the apical cells
of the innermost layer they proceed
from the sister-cells of the latter.
Those, on the other hand, which belong
to the lower edge of the knob, as well as
the entire outermost layer, are derived
from the primary epidermis.

ENDS AND CONNECTIONS OE THE BUNDLES.

375

spoon, and have on the upper, somewhat concave surface of the latter the same three
superficial layers as the middle teeth, while the edge and under-side consist of an ordinary
one-layered epidermis. Under the three-layered portion of the surface lies a group of
tracheides, corresponding to it in position, which are of the same structure as those of
the middle teeth, and are connected >vith the network of bundles of the lamina by means
usually of 2-3 spiral vessels.

On the inner surface of the pitcher of Nepenthes branches 1-3 vessels thick, coming
obliquely from the network of bundles, end directly beneath the basal part of the
digestive glands considered at p. loi, but by no means beneath all of them. Most of
these organs, on the other hand, have no bundle-ends extending to them like those of
other plants mentioned above.

JMost bundles, which terminate beneath water-pores or glandular parts of the
epidermis ^ consist of rows of tracheides, perhaps also of vessels, which run parallel
towards the point of termination, diverging directly beneath the latter at greater or
less angles, and then ending blind. The structure of the terminal elements is the
same as in the internal ends ; in species of Crassula reticulated tracheides with
unusually large meshes occur. In all cases investigated, except Crassula, rows of
delicate, smooth-walled cells, elongated in the same direction as the tracheae, lie
among the latter without any definite arrangement. These become more numerous

Fig. 178.

I’lg. 177.

Figs. 177, 178.— Primula sinensis. Fig. 177 (40). Outline of a tooth of the leaf with its branches of the bundles. The thick-
est of the latter ends below the water-pore/.— Fig. 178 (145). Longitudinal section vertical to the surfaceof the leaf, through
the middle of a similar tooth. 0 upper, u lower surface of the leaf; J> water-pore, below which is the air-cavity, and then the
epithema, the cells of which are drawn somewhat too large. It contains a little chlorophyll throughout, and the bundle of
tracheides,^, ends in contact with it. Under the epidermis on both sides is parenchyma containing abundant chlorophyll.

as the tracheae between them terminate or diverge, and pass over gradually into
a group of small delicate cells, which covers the ends of the vessels ; these are in
their turn immediately covered by the epidermis, and with reference to the former
relation may be called the Cover, Epithetna of the end of the bundle. Either a single

^ Compare p. 50, § 8, and p. 90. Most of the literature there cited refers also to the subject
treated of here. [See also Gardiner, on the development of the water glands in Saxifraga crustata, -
Quart. Jour. Micr. Sei. July, 1881.]

“ kniörjfia, the cover.

376

PRIMARY ARRANGEMENT OF TISSUES.

bundle ends in one group of epithema, or two or more converge, so as to end in a
common group. The former, for example, is the case in the large leaf-teeth of
Fuchsia, Primula sinensis (Figs. 177, 178), and Cucurbita, where the bundle-end is
a thick, short strand, derived from the union of several convergent bundles inside
the edge of the leaf ; also beneath the furrows of the surfaces of the leaf of species of
Crassula, in the glandular spots of the surfaces of the leaf of species of Malpighia, &c.
The latter is the case in many teeth and crenations of the leaf, especially the broader
ones, e.g. Brassica’, Papaver, Tropseolum (Fig. 179), and many others. The arrange-
ment of the parts is here such that one bundle coming from the middle of the leaf,
and one or more marginal ones on each side, converge towards the epithema, and
end at its circumference. Both cases, namely common and special epithemata, often
occur side by side, e. g. in leaves of Crassula, in the marginal furrows of the leaf of
Saxifraga Aizoon, elatior, and their allies (comp. Unger, I.c.)

Fig. 179. — Tropreolum majus (40). Course of the ends of the vascular bundles in a piece of the edge of the leaf at its median
indentation. At e epithema, above which lie the water-pores figured at p. 52, fig. 19.

In the furrow's or spots (mentioned in Sect. 8) of the surface of the leaf of certain
species of Ficus, which have incorrectly been supposed to be without vascular bundle-
terminations, in the glandular ends of the petiolar appendages of Passiflorse, Mal-
pighiaceae (e. g. Stigmaphyllum), and Amygdaleae, and in the glandular prominences
or depressions of Acacias, divergent rows of tracheae w'ith short articulations run
into an epithema lying below the epidermis. These rows may belong, according to
the particular case, to one or more bundles, or in several cases may equally well be
considered as one bundle or as several.

In the furrows of the leaves of Ficus, according to investigations on F. neriifolia and
diversifolia, a disc-shaped group of epithema lies below the epidermis, either over a knot
of the network of vascular bundles, or over a single bundle ; in the former case the vascular
bundles meeting in the knot, break up, as it were, towards the side of the furrow, into
numerous short rows of tracheides directed towards the latter ; in the second case a
bunch of tracheides branches off from the bundle and enters the epithema.

* Compare Unger, I.c. (above, p. 327), Taf. 2, fig. 17.

ENDS AND CONNECTIONS OF THE BUNDLES.

377

The glandular spots on the under side of the leaf of Primus Laurocerasus also lie over
a knot or a narrow mesh of the net of bundles, and from this some vessels or tracheides,
which are not numerous, branch off into the epithema lying below the glandular
epidermis.

In the petiole of Passiflora cxrulea and its allies a bundle ending below the epithema
enters the cylindrical appendages or teeth, which end with a concave glandular surface.
The conditions are similar in the appendages of the petiole of Amygdalece.. Several
bundles running to the glandular terminal surface enter the broad round appendages of
the petiole in Stigmaphyllum.

In the Acacias the glandular spots of the appendages of the petiole behave very dif-
ferently according to the species (cf. p. 98). A number of small bundles, here and there
reticulately connected, enter the elongated, wart-shaped projection of the base of the
petiole of A. lophantha, running towards the free surface, and here ending in the epi-
thema. Below the flat wart-like prominence on the upper edge of the base of the
phyllodes of A. marginata, and A. calamifolia, numerous isolated short tracheae branch
off from the neighbouring strands of the net of bundles, and without being united into
distinct bundles turn towards the epithema and there end. The same is to be seen here
and there at the base of glandular pocket-like depressions of A. latifolia and its allies,
but the vascular ramifications are here sparse and very short, while, on the other hand,
the thick xylem groups of the bundle-net often border directly on the epithema.

As regards the characteristics of the group of epithema, in many cases it
scarcely deserves a special name, as it is nothing but a small-celled parenchyma,
which, on the one hand, passes over immediately into the other larger-celled
parenchyma of the organ, and, on the other hand, into the interstitial cells of the
end of the bundle. So, for example, in the glandular appendages of the petiole of the
Passiflorse, and in most ends and teeth of leaves. Here the epithema is distinguished
from the lacunar chlorophyll-parenchyma by the smaller size of its cells, and by
their containing little or no chlorophyll, the places occupied by the epithema thus differ-
ing in their pale colour from the green surface of the leaf. The epithema passes over
quite gradually oir all sides into the large-celled chlorophyll-parenchyma ; the water-
pores of the epidermis lead immediately into the intercellular passages of the latter.
In the leaf-teeth of Papaver orientale even all transitional forms are present between
the small parenchymatous cells of the epithema and the tracheides of the bundle-ends.
These epithemata have a very different form and extent, according to the shape and
size of the bundle-ends and teeth of the leaf ; in the narrow ends and teeth of the leaves
of Fuchsia, Callitriche^ and Primula sinensis, for example, they are quite small bodies,
showing only a few cells in section, and lying immediately below the large stomatal
cavity, which belongs to the water-pore, situated at the end. In the broad teeth of
the leaf of Papaver and Brassica, and in the crenations of Tropseolum, the group of
epithema, which receives several ends of bundles, is a many-layered small-celled
parenchymatous mass, approaching i™™ in breadth.

On the other hand, however, many ends of bundles run out into epithemata, which
are sharply distinguished and limited. The furrows of the leaves of species of Ficus,
Crassula, and Saxifraga, the glandular petiolar appendages of the Acacias, and many
other cases, are examples of this. At the parts of the leaves indicated at p. 51, of
species of Crassula, and of Rochea coccinea (Figs. 180-182), a thick bundle runs

* Borodin, /. r. ; compare p. 51.

37^ PRIMARY ARRANGEMENT OF TISSUES.

vertically to the surface under each epidermal furrow, ending some distance within
the latter in a conical or hollow-conical expansion. Upon or in this expansion rests
an epithema, of oval or elongated form according to the species, and extending to
the epidermis, which contains water-pores. Its cells are on the average about one
fifth the size of those of the surrounding chlorophyll-parenchyma, they are roundish or
slightly elongated in the same direction as the vascular elements, with watery, colour-
less contents. They are almost uninterruptedly united with each other, and even the
cavities below the water-stomata are small. In the furrows of the leaf of the Saxifrages
mentioned above, the end of the vascular bundle is expanded into a large epithema
of an approximately conical form, with its base resting on the epidermis of the furrow.
Its structure is very similar to that of Crassula, its cells are elongated in the same
direction as the tracheae, and the whole body, like the vascular bundles themselves,
is ensheathed by a layer of cells which are very rich in tannin. The epithemata in

FIG. 180. — Crassula arborescens. Longitudinal section, vertical to the leaf-surface, through the apex of a leaf.

Magnified 30—40. e — e' epidermis; ^vascxilar bundle, divided into two branches, whicli terminate below a small-
celled epithema in broadly conical ends consisting of short tracheides ; the branch at / on the upper, that at
on the lower surface of the leaf.

the furrows of Ficus have an approximately discoid general form, are round-celled,
and in other respects are also similar in structure to those of Crassula. The same
also holds good in general of those epithemata which lie below the glandular
portions of the epidermis. How far the nature of the contents of their cells shows
remarkable peculiarities, still remains to be more exactly investigated.

Sect. 112. In the leaves of the Conifer a, as stated above, the finer ramifications
of the bundles are absent ; the leaves are traversed either by a number of bundles
of approximately equal thickness, or in most cases by a single median one ; in
most Abietinese by a median pair of bundles running close side by side, only
separated from one another by one or two layers of elongated cells (e. g. Abies
excelsa, pectinata, Pinsapo ; Cedrus Libani ; Pinus Pinaster, Laricio), or by a thick
strand of sclerenchymatous fibres. The bundles are collateral, and their orientation is

^ Botan. Zeitg. 1864, p. 167, Taf. IV.

“ Ibid. 1871, p. 10. Mohl calls the tracheide-border Transfusion-tissue. [See also Zimmer-
mann, über das Transfusions-Gewebe, Flora, 1880, p. 2.]

It is attached to the xylem by means of one or two longitudinal rows of tracheides,
more or less frequently interrupted by parenchymatous cells, and from here projects
on each side into the surrounding parenchyma ; in most of the species it has the
form of a plate, which is either plane or a little curved, and approximately follows
the direction of the leaf-surfaces (Taxus, Cephalotaxus, Torreya, Taxodium semper-
virens, Cunninghamia (Fig. 183), Juniperus (Fig. 184), Thuja, Thujopsis, Gingko),
or is curved from each side round the body of the vascular bundle, and is separated
from the latter and from the plate on the other side only by a few rows of parenchymatous

AND CONNECTIONS OF THE BUNDLES. 379

normal. Towards the end they are tapered, and the xylem and phloem diminish in such
a manner, that here also the ultimate termination consists only of one or a few rows
of short tracheides. They are distinguished by the fact that within the lamina of the
leaf, as if to replace the finer branches, the edge of the xylem is expanded throughout
its entire length into a border, consisting of rows of short tracheides inserted in the
parenchyma of the leaf. This fringe of tracheides, which was first accurately described
by Frank in the case of Taxus', and afterwards more generally demonstrated by MohF,
is absent, or at least extremely feeble, in Larix europaea alone of forms known to me :
it arises, in the case of the Abietineae, exclusively from the outer, remote edges of
the pair of bundles, in the other cases investigated from both sides of each of them.

Fiif. 181. Hig. 182.

Figs. i8i, 182. — Rocliea coccinea {200). Fig. i8r. Fragment of the epidermis from the edge of the leaf. S water stoma ;
s air stoma, with subsidiary cells. '1 he scattered dots are wart-shaped projections of the outer walls. — Fig. 182. Section through
the edge of the leaf vertical to the surface, e — e epidermis, .S' water stoma, n subsidiary cell, b somewhat thick vascular bundle
in cross-section. The meshes with thicker and double outlines are the sections of bundles of trachese which run to neighbouring
bundles ; the more delicate outlines are those of the accompanying elements. A short strand goes off from b and runs towards 5“ ;
the tracheides of which it consists diverge and embrace the delicate-celled group of epithema lying between and S. All
round is large-celled chlorophyll-parenchyma.

Fig. 183.— Cunninghamia sinensis. Cross section through the leaf {220). it lower, o upper surface; h resin
passage; y_s sclerenchymatous fibres of the hypodcrma, s those scattered in the parenchyma; xylem of the median
bundle; ^ tracheide border of the latter. Below, towards the resin-passage, is the thin-walled phloem; the white
band at its boundary towards the parenchyma surrounding the resin passage is the compressed primordial tissue of
the phloem ; q transversely elongated parenchymatous cell of the middle of the leaf.

cases mentioned, except Abies excelsa and the Pines, they are thicker on their outer
edge, i. e. that remote from the vascular bundle, than on the inner attached edge, in
consequence of increase in the width and number of layers of their elements ; this
often takes place to such an extent that the cross-section becomes wedge-shaped,
e. g. Taxus and Podocarpus Thunbergii.

The tracheides of the border are in general arranged in fairly regular rows both
in the direction of the length and of the breadth of the leaf; these rows often show
interruptions, which are filled up by parenchymatous cells, but all are at some points
in immediate connection with one another. At the inner edge, which is attached to
the xylem of the bundle, their form is similar to that of the tracheides of the latter.

380 Pli/MARr ARRANGEMENT OF TISSUES.

cells. The border is in fact curved round the xylem in Sciadopitys, Araucaria
brasiliensis, Cryptomeria, and Dammara ; round the phloem in Abies pectinata and
Pinsapo. In Abies excelsa and the Pines (P. silvestris, Laricio) it is split as it
were into two plates on each side at its place of attachment, which, in a manner still
to be more accurately described, are bent, the one round the xylem, the other round
the phloem, so that the pair of bundles is completely surrounded by the border
of tracheides.

The plates of tracheides are in many cases, especially in Podocarpus Meyeriana
Endl., of nearly equal thickness throughout in every cross-section; in the other

ENDS AND CONNECTIONS OF THE BUNDLES.

381

namely elongated ; but they are on the average shorter and wider, and have terminal
surfaces, which are but little oblique, and may even be horizontal. As their distance
from the inner edge increases, their length rapidly diminishes, while their width
increases, so that in the outer part of the border they are not longer, and are often
even shorter than they are broad, being similar in form and size to the neighbouring
parenchymatous cells.

These conditions appear in a special form in Podocarpus Meyeriana, Thuja
gigantea, and in the Pines and Abies excelsa. In the two first-named the border is
very broad, it projects deeply into the middle of each half of the leaf in the form of a
flat wing. Its tracheides, with the exception of the innermost, are, with reference to
the diameter of the leaf, much broader than long, thus having their greatest diameter
directed towards the edge of the leaf ; they form rows running towards the latter,

FIG. 184. — Juniperus communis (225). Median vascular bundle of the leaf. ^ xylem ; c isolated sclerenchymatous
fibres at the outer border of the phloem; i border, consisting of tracheides with bordered pits and cross beams.

The parenchymatous cells occurring near or between the latter are dotted in a granular manner.

M'hich are often interrupted, but are in contact with one another, and might be
termed a narrow-meshed net of uniseriate vascular bundle terminations.

In the connate sheathing-base of the
flat pair of leaves of Thuja gigantea, the
border of tracheides of each leaf is expanded
into a low wing, which runs to meet that of
the opposite leaf, and unites with it to form a
transverse girdle.

In the Abietinese last-mentioned the
pair of vascular bundles lies in an approxi-
mately cylindrical central portion of the leaf,
which is free from chlorophyll, and is se-
parated from the surrounding chlorophyll- Pinal'e";’
parenchyma by a parenchymatous sheath

with somewhat stouter walls (cf. Fig. 185). the two vascular bundles.

See Harlig, Naturgesch. d. forstl. Culturpflanzen.

382

PRIMARY ARRANGEMENT OF TISSUES.

Throughout the entire central portion are distributed rows of tracheides, which
in all directions are alternately in connection with one another, and interrupted by
rows of parenchyma, or by isolated sclerenchymatous fibres; they are connected with
the edge of the vascular bundle in the manner described above.

In the Abietinese last-mentioned the breadth and thickness of the border of
tracheides cannot be more exactly defined than by the statement, that the colourless
central portion of the leaf is 2-5 layers of cells thick all round the pair of vascular
bundles. In the other cases, where it is more sharply limited, its breadth may be
stated as on the average equal to that of the vascular bundle, the number of rows of
tracheides in the direction of its breadth being about 5-8. In the direction of
thickness, vertical to the latter, the number of rows in the thickest external part is
usually on the average 4-5, more rarely, when the border is broader, only 1-2 (Cun-
ninghamia, Cedrus, Juniperus communis). In Gingko and Prumnopitys the border
is quite feeble, showing only 1-3 tracheides in cross-section. On the other hand, in
Podocarpus IMeyeriana, so often referred to, it is 1-2 layers thick, but on the average
1 5 rows broj^i, while in the above-mentioned transverse wing of the sheathing base
of the leaf of Thuja gigantea it is 25-30 rows broad. At the end of the bundle
the short-celled external edge of the border passes over directly into the short
tracheides which form the ending of the bundle itself.

The structure of the tracheides themselves is in general that given in Chap. IV,
for this form of tissue. In the living leaf I always found them to contain water, not
air. Their lignified walls agree generally in structure with those of the spindle-
shaped tracheides which form the edge and the external region of the adjoining
xylem, and in structure are identical with or similar to those of the secondary wood
of the same species. In most cases they have the large round bordered pits universally
distributed among the Coniferse (cf. Fig. 183). In many forms, on the other hand, they
are smooth and thin, as in Abietinese, species of Cunninghamia and Thuja, often also
in Araucaria imbricata and brasiliensis, and in Sciadopitys. Many tracheides in the
three species last-mentioned, and all of them in Taxus, Dammara, Gingko, and
species of Podocarpus, have reticulated and spiral thickening in addition to more or
less numerous often isolated bordered pits, in varying distribution, which for most
species requires to be more accurately ascertained. In the leaves of the species of
Juniperus investigated (J. communis, Oxycedrus, oblonga, macrocarpa, Mohl, /. c.,
also J. sabina) the tracheides of the border are distinguished by the transverse
bars described at p. 163, in Biota orientalis by the projecting pegs mentioned at
p. 164 (Fig. 184).

The reticulated elements at the circumference of the vascular bundles of the
leaf of Welwitschia, represented at p. 335, Fig. 157, must be reckoned among the
borders of tracheides in question, so far as the dead material admits of a decision.
They are usually quadrilaterally prismatic, short, almost cubical, while some are
elongated and even have tapered ends. Their structure is that of tracheides with
a narrow-meshed, reticulated membrane, and with isolated bordered pits between
the threads. They are arranged in longitudinal rows, which follow the vascular
bundle, and are often irregularly Interrupted by parenchyma, but at many points
stand laterally in direct connection both with each other, and also with the xylem
of the bundle; the latter is the case both at the sides of the longitudinal bundles.

ENDS AND CONNECTIONS OF THE BUNDLES. 38^5

and more especially at the free ends of the transverse branchlets (cf. Fig. 145,
P- 303)-

Sect. 113. In typical roots, it is simply the meristematic group of the growing
point which forms the end of the vascular bundle.

Those parts of Phanerogamic Parasites which are developed inside the host,
(intramatrically), as well as their hausioria, behave differently. According to the
existing investigations, for the details of which the special works are to be compared^
these organs are traversed by vascular strands, which are quite similar in structure to
the ends of bundles in the foliar expansions ; they form strands of vessels with short
elements, usually reticulated or pitted, accompanied by rows of moderately elongated
thin-walled cells, with acute ends ; or they are isolated vessels with very short, and
then usually very irregular elements, traversing the parenchyma, and often actually
interrupted by parenchyma, thus constituting isolated portions of vessels, or
tracheides, enclosed in the latter. These strands are, on the one hand, connected
with the xylem of the bundles of the extraniatrical organs of the parasites ; on the
other hand, they extend at definite points to the limiting surface between the intra-
matrical tissue of the parasite and the tissue of the host, here entering into close
connection with the latter, and in fact becoming as a rule closely attached to the
equivalent tissue, i. e. to the vessels and woody elements of the host. The intimacy
of the connection may go so far that it becomes difficult to determine the boundary
where the vessel of the parasite begins, and that of the host terminates. Sieve-tubes
have not been found in company with these vessels and strands of vessels.

The following may be mentioned as examples, reference being made to the
numerous individual forms described in the special works.

(i) The haustoria of the Cuscutas, Cassythse, Rhinanthaceae, and Santalaceae.
Tn the first two groups they arise from the twining stem, and penetrate the stem and
foliar organs of the host, round which it is twisted. In the two families last-
mentioned they arise on the roots, penetrating the roots of the host. In most cases
they have the general form of conical warts, with the base adhering firmly to the
host, and from the middle of the attached surface an approximately cylindrical or
flat peg, the sucker, penetrates the tissue of the host. In the haustorium an axial,
broad, small-celled strand of parenchyma, the core, is to be distinguished from a
large-celled cortex ; the core is continued directly into the sucker. In Cuscuta the
extramatrical portion is very little developed, the haustorium may be said to consist
only of the swollen sucker, which springs from the surface of the stem adhering to
the host. Vascular strands branch off from the xylem-groups of the bundles of the
main axis, so as to pass through the core usually until they reach the inner surface
of the sucker where it meets the vascular bundles or the wood of the host. In the
normally developed haustoria of Thesium, Santalum, and Osyris there are two thick
flat strands, which first diverge, with a curved course at the periphery of the flask-
shaped core, and converge again in the sucker ; in Cuscuta, Cassytha, and the
Rhinanthacese there is an axial strand, usually penetrating to the wedged-in end of

’ Graf zu Solms-Laubach, Bau und Entwickelung der Ernährungsorgane parasitischer Phanero-
gamen ; Pringsheim’s Jahrb. Bd. VI. — Id. Das Haustorium der Loranthaceen, &c., Plalle, 1875. —
L. Koch, Entw. d. Cuscuten, in Hanstein’s Botan. Abhandl. Bd. II. Heft 3.

3^4

PRIMARY ARRANGEMENT OF TISSUES.

the sucker. The vessels of the strands consist, in all these haustoria, of short
elements communicating by wide perforations ; in the sucker they are usually more
elongated, the reticulate thickenings of their membranes being often imperfectly
developed at the surface of the process.

(2) The intramatrical ramifications of the haustorium of Viscum and Phora-
dendron, which have been described as cortical roots, are traversed by an irregular
axial vascular strand, which ends in the apical meristem, and is developed slowly and
relatively late. In the ‘borers,’ which, starting from the cortical root, penetrate the
wood of the host, like wedges, the middle is occupied by a relatively thick mass of
vessels, the remainder of the body consisting of large-celled parenchyma. The
vessels do not reach the edge of the borer.

In the broader outer portion, the bundle is irregularly branched, and from the
branches numerous vessels with short elements run generally with a curved course
towards the lateral surfaces of the borer, and here attach themselves to the elements
of the wood of the host. In very old borers, which have finished their growth, the
vessels are continuous with those of the cortical root. So long, however, as the
borer is still growing, they are separated by a zone of meristem, which carries on
the growth, and lies in the cambium of the wood of the host (Chap. XIV). The
vascular body of the borer may be massively developed, while as yet no vessels have
been fully formed at its point of origin in the cortical root. Arceuthobium Oxycedri,
which is much smaller in all its parts, has no vessels at all in the smaller branches of
its cortical roots, or in its small ‘ narrow ’ borers ; the latter simply consist of a few
rows of large parenchymatous cells. The thicker cortical roots and borers are
similar in structure to those of Viscum album, but are simpler, as corresponds to
their small bulk. Similar conditions to those in Arceuthobium recur in the intra-
matrical thallus of Pilostyles, with specific variations.

(3) The flat intramatrical body of Cytmus Plypocistis (cf. Graf Solms, l.c.,
Taf. 36, 37), which is irregularly discoid, and on its lower surface wedged with
irregular protrusions into the wood of the host, is traversed throughout by isolated,
very irregular vessels, which are abundantly branched and reticulately connected, and
which in the wedged protrusions are attached to the ligneous elements of the host.
The elements of the vessels are as a rule irregularly rounded, with a reticulated wall,
and are in communication by means of large round holes.

(4) In the tuberous regions of attachment of the Orobanches, the vascular
bundles, both of the parasite and of the host-root bearing it, are as it were broken
up into a loose web of numerous vessels with short elements, those of the parasite
standing in immediate continuity with those of the host. A sharp boundary, whether
between the vessels or between the parenchyma of host and parasite, is often only to
be recognised in the youngest stages

In the tuberous regions of attachment of most genera of Balanophoreae (Helosis,
Lophophytum, Scybalium ^), the conditions of structure are similar to those in
Orobanche, except that the vessels are clearly distinguishable from those of the w'ood
of the host-root, as far as the point of attachment to the latter. Essentially the same
holds good of the Rafflesiacese.

* Caspary, Flora, 1854, HI*

Eichler, Balanophorse brasilienses, l.c. (p. 254).

ENDS AND CONNECTIONS OF THE BUNDLES.

385

(5) In the regions of attachment of Balanophora and Langsdorffia, according to
the authors mentioned, and the earlier investigations of Göppert^, an additional phe-
nomenon occurs, which likewise belongs to this series. In these cases thick, variously
ramified vascular bundles, arising as branches from the wood of the host-root
attacked, grow into the parenchyma of the tuber of attachment, their branches
having broad blind ends in the parenchyma of the tuber. According to the existing
investigations, a direct connection between these excrescences and the parasites’ own
bundles does not take place, or is at any rate doubtful. The bundles of the ex-
crescence are thick strands attaining more than in thickness, and their branches
have broad ends, which may even be swollen in a club-like manner. They consist
of thick vascular masses, which are accompanied by delicate elongated elements re-
quiring further investigation ; they are penetrated by narrow divaricating bands of the
thin-walled parenchyma belonging to the parasite (cf. Graf Solms, /. c).

Sect. 114. Co7ineciio7is of BuTidles. Where one vascular bundle-trunk branches
off from- another, or, otherwise expressed, where it attaches itself to another, the
equivalent regions and elements of the two are in continuity. Where the arrange-
ment and orientation of the parts of the bundles in question are similar, as is the
case in most stems, and in the lamina of foliar expansions, the above fact indicates
the structure of the region of union in the most essential points ; various individual
differences follow from the general principle that the special structure of every
bundle may change in successive transverse sections.

Where the arrangement of the parts and the orientation of the bundles is
different, torsions and displacements, both of the individual portions of the bundle,
and of any strands or sheaths that may accompany it, must take place towards and
at the place of union, in order to establish continuity of the equivalent elements ; and
with these torsions other changes of structure may be connected, besides those
relating to the orientation of the parts.

Those cases of the union of bundles of unlike orientation which are remarkable in
these respects fall under two main categories, namely, connections between bundles
belonging to the same axis, and connections between the bundles of a main and a
lateral axis.

I. Of the former category we have here first to mention the connection of
the bundle-system of the stem with the radial bundle of the main root in the typical
Dicotyledons and Gymnosperms ^ The hypocotyledonary stem of these plants
contains, as described above, two or more separate, collateral bundles of normal
orientation, and towards the main root these approach one another so as to unite to
form its axial bundle. In the hypocotyledonary stem the primitive tracheae lie at the
inner edge, the phloem at the periphery of each bundle. In the axial bundle of the root
the primitive vessels occupy the outer edge of every xjdem-plate, and the phloem-
bands alternate laterally with the xylem-bands. The investigation of the longitudinal

’ /. c. at p. 2 54,

“ Mettenius, Anat. d. Cycadeen,*/. c. p. 602. — Dodel, DerUebergang des Dicotyledonen-Stengels
in die Pfahlwurzel, Pringsheim’s Jahrb. Bd. VIII. — Strasburger, Die Coniferen und die Gnetaceen,
p. 360. — Van Tieghem, Canaux Secreteurs, l.c. — S. Goldsmith, ‘Beitr. zur Entwickelungsgeschichte
d. Fibrovasalmassen im Stengel imd in der Hauptwurzel der Dicotyledonen,’ could not be made use
of for the present work. [Gerard, Recherches sur le passage de la racine ä la tige, Ann. Sei. Nat.
6 ser. tom. ii. 1881.]

386

PRIMARY ARRANGEMENT OF TISSUES.

course of the bundle shows that the primitive vessels extend continuously from the
bundles of the stem into that of the root, and thus gradually become displaced, on their
way to the root, from the central into the peripheral position. This is accompanied
by a corresponding gradual displacement of the other parts of the bundle, in such a
direction that, on their union to form the root-bundle, the radial structure of the latter
is also attained. Together with these displacements, division and re-union and
disappearance of certain parts of the bundle may successively take place. It is
obvious that a variety of particular cases are possible according to the different
number of the bundles in the hypocotyledonary stem, and of the rays in the bundle
of the root.

Among the particular cases which have been more accurately investigated, that described
by Strasburger in the case of Biota orientalis, which presumably often recurs elsewhere, is
especially simple and intelligible. The hypocotyledonary stem contains in its upper part
two bundles descending perpendicularly from the two cotyledons ; the bundle of the main
root is diametrally diarch. In each of the cotyledonary bundles as they gradually ap-
proach each other a radial division of the phloem into two halves begins. Further below
the two halves become more and more distant, and are shifted into the same tangential
plane with the xylems ; each then approaches that coming from the same side of the other
bundle and unites with the latter to form one broad phloem-group. The xylem-groups of
both bundles go through the above-mentioned displacement or torsion in the same part
of their course ; the two broad phloem-groups therefore alternate with the xylem-plates
of the root-bundle, which have their primitive vessels directed towards the outside.

In the Abietinea with many cotyledonary bundles and a polyarch bundle in the main
root (cf. p. 356), as many cotyledonary bundles as the bundle of the root has xylem-plates
undergo torsion during their downward course through the hypocotyledonary stem; their
phloem-groups shift laterally into the space between the xylem-groups, the primitive
vessels of the latter shift from the central to the peripheral edge. The xylem-groups
of the remaining cotyledonary bundles gradually disappear, while the phloem-groups
amalgamate with those which undergo the deviation described.

In Phaseolus four decussate pairs of bundles traverse, in the simplest case, the hypo-
cotyledonary stem. The xylem-groups of the eight bundles are separate, the phloem-
groups are united to form four broad curved bands, which together form a ring only
interrupted between the two bundles of each pair. The xylem-groups of the bundles
leave the cotyledons with normal orientation, but then go on changing their direction in
such a manner that the primitive vessels of each come to stand in one tangential row with
the others ; and in fact the bundles of each pair here turn their primitive vessels towards
each other. At the boundary of the main root the xylem-groups of each pair revolve
round the primitive vessels in such a manner that the latter come to lie outside, the re-
maining vessels towards the inside ; the further down, the more acute is the angle which
the bundles of a pair form with one another, until they finally become parallel, and fuse to
form one of the four xylem-plates of the root-bundle. The phloem-strands alternating
with the latter are the direct continuation of the four broad bands of the hypocotyle-
donary stem. Both in the latter and in the root they are each supported externally by a
thick strand of sclerenchymatous fibres, which however is interrupted for a short distance
at the point of transition to the root. In other less simple cases, additional intermediate
pairs of bundles lie between the above-mentioned four pairs of bundles of the hypocotyle-
donary stem. These usually end blind at the lower boundary of the stem ; one of them
however frequently enters the root, so as to form a fifth xylem-plate of the pentarch
root-bundle in the same way as the four main pairs.

2. The united portions of the bundles in the stem of certain Aroids and
Pandanese (p. 268) are distinguished by a distribution and orientation of their

ENDS AND CONNECTIONS OF THE BUNDLES.

387

elements, with reference to which van Tieghem^ has appropriately termed them
compound bundles. The bundles, which on their downward course from the leaves
are at first collateral with normal orientation, usually unite to form a body of round or
irregular cross-section, their phloem-groups uniting either directly or by means of an
intermediate bundle of sclerenchyma to form a joint strand, at the periphery of which
lie the xylem-groups, which towards different sides are sometimes separate and
sometimes fused. The number and structure of the latter and the configuration of
the whole bundle change, both in successive cross-sections of the same individual
and in the ditferent species. For details, van Tieghem, l.c., is to be consulted.

Similar, less conspicuous phenomena no doubt occur here and there in other
stems of Monocotyledons, and in stems and petioles with medullary bundles.

II. The following rules, which are to be stated with reference to Sect. 108, and
to the developmental data given below in Sect. 1 17, hold good for monopodially
branched roois with regard to the attachment of the vascular bundles of lateral
branches to that of the main axis.

Apart from a few exceptions to be mentioned below, every lateral root arises on
the outside of the vascular bundle of its relative main root, in front of the centre of
the outer edge of a xylem-plate. Accordingly, in the case of diarch and many
polyarch roots, their xylem-plates are attached to the corresponding outer edge of
the bundle of the main root, their phloem-strands to those alternating with it.

In the polyarch bundles of many Monocotyledons, the xylem-plates are attached
not only to the corresponding plates of the main root, but also to the two neigh-
bouring lateral ones, while the phloem-bands are attached to those bands in the
main root which alternate with the three xylem-plates mentioned ; van Tieghem
found this to be the case, for example, in the ramifications of the adventitious roots
of Iris Germanica, Asphodelus ramosus, and Asparagus. The division of the lateral
bundles, in the case of Pandanus and the Palms, into branches which penetrate
further and deeper, was mentioned above at p. 316. In the Grasses, on anatomical
grounds, which have still to be explained below. Sect. 1 1 7, the lateral roots do not
stand in front of the xylem-plates, but in front of the centre of the phloem-strands of
the bundle of the main root ; accordingly their xylem-plates are attached to the two
next lateral plates of the main-root, their phloem-groups to the corresponding ones of
the latter. A similar relation obtains, for similar reasons, among the Pittosporeae. As
will also be further explained in Sect. 117, each lateral root in the Umbelliferse and
Araliaceae is placed between a xylem and phloem-group of the main root ; its xylem
is accordingly obliquely attached to that surface of the corresponding plate of the
main root which faces it The same position of the lateral roots and insertion of
their bundles occurs, according to van Tieghem in Lycopersicum, while nearly
related plants, e. g. Solanum tuberosum, show the usual behaviour of oligarch roots
mentioned above.

In the case of the other connections of bundles between main and lateral axes,
nothing essential need be added to what has been said in earlier paragraphs, and at
the beginning of this, respecting the orientation of the parts of the bundle.

‘ Structure des Aroidees, /. c, ^ Van Tieghem, Symmetrie de Structure, 1. c.

^ l.c. 226; cf. also § II 7.

C C 2

388

PRIMARY ARRANGEMENT OF TISSUES.

As regards the special structure, the general rule holds good, that at the points
of attachment of lateral organs on stems and roots the elements of the vascular
bundle are short in comparison with those of the main bundles, because they in some
cases arise at such places as never undergo more than a slight elongation, and in
others attain their development after the elongation of that portion of the main axis
which bears them is far advanced. This phenomenon is especially familiar in the
case of the nodes of vascular plants ; it appears the more conspicuously the more
abundant the subdivision and ramification of the bundles where they are attached
(comp. Sects. 94, 95). The lateral walls of the short joints of vessels or of tracheides
in these regions are in the very great majority of cases pitted, or reticulately thick-
ened, and in the latter case they usually have low, transversely elongated meshes.
Spiral and annular threads seldom occur, and then usually pass over within a short
distance into cross-meshed reticulate thickening, e. g. in the node of many Comme-
lineae. The tracheal elements of one bundle as a rule attach themselves to the equi-
valent elements of another with tapered, acute ends, which adhere for some distance
to the lateral wall of the other ; the attached ends are rarely cut off transversely.

The attachment of the sieve-tubes to one another appears to take place in a
similar manner to that of the vessels, especially according to isolated observations on
the Cucurbitaceae. More accurate investigations on this point do not however exist.

C. DEVELOPMENT.

Sect. 115. The vascular bundle is formed from a strand of meristematic cells,
which, in accordance with the definitive form of the bundle-elements, assume a form
elongated in the longitudinal direction of the bundle, and undergo divisions in the
same direction excepting where they give rise to the shorter parenchymatous
cells belonging to the bundle. These initial strands of the bundles are therefore dis-
tinguished, in a degree corresponding to the progress of histological differentiation,
from the surrounding non- equivalent tissue, and especially from the initial layers
of those masses of parenchyma which continue short-celled owing to persistent
transverse divisions. To this is further added the usually smaller growth of the ele-
ments of the bundle in a transverse direction, as compared with the short-celled
surrounding tissue ; the initial strand has narrower cells than the surrounding tissue.
Essentially the same phenomenon appears when other strands consisting of elongated
elements, especially for example strands of sclerenchymatous fibres, become differen-
tiated out of the primary meristem, and from tissue which remains short-celled, what-
ever may be the anatomical definition of the region in which they arise. Therefore
where a fibrous- strand immediately accompanies a vascular bundle, it is not to be dis-
tinguished from the latter in the initial stage, or at least not sharply.

The long-celled initial strands of the vascular bundles, and in certain cases
the fibrous tissue accompanying them, were called by Nageli^ Cambium-strands, in
partial agreement with the older terminology, the term Cambium being used for
the long-celled initial strands, as distinguished from the short-celled ‘Meristem.’

* Beitr. I. p. 2. Compare the footnote at p, 4.

DEVELOPMENT OF THE VASCULAR BUNDLES.

389

Sachs' has introduced the name Procambium for the former, because the term
cambium, if applied to them, would be ambiguous, meaning two different things,
namely, on the one hand the strands in question, and on the other the initial layer
of secondary growth, which has been traditionally designated by this name, and of
which Chap. XIV will treat. We here entirely pass over the use of the word cambium
mentioned at p. 4, introduced by some authors for all those groups of cells which
are called INIeristem in this book. Finally, Russow® call the strands in question
Desmogen.

It is essentially a matter of indifference which name is used, provided it be known
what it means ; that is, in the present case, that we are dealing with groups of cells
which fall under that general conception of Meristem — gradually passing over into
differentiated tissue — to which we have adhered in this book, and that we are
dealing with strands which are different from the cambium to be treated of in
Chap. XIV, although they may actually stand in the closest anatomical and genetic
relation with it. It may well be advisable, however, to avoid any term which would
recall a different idea, or which would seem to imply more than the simple fact that
the strands in question are the young vascular bundles, i. e. the beginnings of the
latter ; and for this reason we may apply to them the term initial strands or initial
bundles, in agreement with the other terminology used in this book. The initial
strands arise from and consist of initial cells, of successively different degree and
value, from which the elements of the vascular bundle are derived.

In typical roots the axial meristematic strand, termed the plerome in the Introduc-
tion, is the initial strand for the vascular bundle. Its differentiation from the surround-
ing meristematic layers, the degree of completeness of which differs according to the
particular types, has been described above in the Introduction; in the case of lateral
roots arising on a main root, this will still have to be discussed below. Here we
have only to repeat that the individual parts of the vascular bundle are already at an
early stage to be distinguished as special layers of the initial strand. In Fig. 186,
reproduced here, and in Figs. 2 and 4-6, above, the layer or row marked p and pc,
the same which in Fig. 3 is marked f next to x — x, is the pericambial layer, which
may be followed up to the common initial cells lying at the apex of the plerome
body ; in the same figures v or g marks in each case an initial row for a vessel or
row of tracheides, and of this initial row the same holds good as of the pericambium.
In the case of the root-bundle of the Ferns and Azollas, similar conditions to those
in the Phanerogams may be demonstrated with still greater sharpness, as shown by
Nageli, Leitgeb, and Strasburger.

In stems with a simple axial bundle the primary plerome body, like that in the
root, has the significance of an initial strand (Fig. i, p. 8).

In stems with a complex bundle-system, and in leaves, the conditions are different.
We will first consider simply the earliest appearance of the vascular bundle, without
reference to the question as to its special place of origin. The development begins
with the fact that a single row, or more frequently a strand consisting of several rows,
of the originally uniform primary meristematic cells, lying in a position correspond-
ing to the course of the bundle, undergoes divisions by means of longitudinal walls,

‘ Textbook, 2nd Eng. ed. p. no.

^ Vergl. Unters, p. 178.

390

PRIMARY ARRANGEMENT OF TISSUES.

while growth in the transverse direction is relatively slight. These divisions are
repeated with a frequency varying according to the particular case. In thick, and
especially in collateral bundles, they often still go on for a long time on the boundary
between phloem and xylem, when at the edges of the bundle the differentiation of the
tissues is already completed. In this case the later-formed septa in the border-zone
just mentioned assume the tangential direction spoken of above (p. 325); the earlier
stages of division always take place in all directions successively.

The completion of the definitive elements of the bundle begins in each trans-
verse section with those of them
which have been described above
as the primitive elements, and
thus in the regions occupied by
the latter. It spreads centri-
fugally from each of these over
the cross-section, when they are
situated in the interior of the
strand. If they occupy one edge
of the strand the development in
general proceeds towards the op-
posite edge; in radial bundles,
as described above, it goes on in
the centripetal direction; in col-
lateral bundles it is centripetal
in the phloem and centrifugal
in the xylem ; in concentric bun-
dles, and in the special case of
leaves of Cycads described above
at p. 336, the development goes on
in different directions according
to the number and position of
the primitive groups, the direction being generally determined by the rule first
stated. Where the edge of the phloem is broad the development extends along
it from the first points of development in the same order. The development always
begins with that of the first primitive elements of the phloem, Russow’s proto-
phloem ; the primitive elements of the xylem appear later in course of development.
Where a bundle is accompanied by sclerenchymatous fibres the development of
the latter takes place later, often much later, than that of the two primitive groups,
and independently of their appearance.

With reference to the longitudinal course of the bundle, both the definitive
completion and the first origination may proceed in the acropetal, or conversely in
the basipetal direction It is further at least conceivable that in the same bundle the
progression of the two processes may take place in unlike directions, or even that
each of them goes on in a different direction at different parts of the course of the
bundle. Sanio’s statement ^ that, according to his investigations, carried out in the

Fig. i?6 (aro).— Polygonum Fagopyruin. Root-apex, median longitudinal
section, pericambium, outside boundary of the plerome strand ; v rudiment
of a vessel ; e dermatogen or epidermis ; between f>c and e periblem ; h root-
cap.

* [Compare Trecul, Ann. Sei. Nat. 6 ser. tom. XII. p. 251, 1881.]

’ Botan. Zeitg. 1864, p. 194,

DEVELOPMENT OF THE VASCULAR BUNDLES.

39 i

case of numerous Monocotyledons and Dicotyledons, those bundles in any cross-
section, which are the first to originate, are also the first to receive the definitive
primitive elements of phloem and xylem, does not exclude these possibilities, which
are rather to be tested by further investigation.

So far as is known at present, both origination and completion proceed in the
acropetal direction in all root-bundles, in all cauline bundles, and in those in the stem
of Filices and Marsiliaceae. The leaf-trace bundles of the Phanerogams certainly do
not all show the same behaviour. In Tradescantia albiflora, and in species of
Potamogeton — with the exception of the lateral bundles of P. crispus — they are both
originated and completed in acropetal order of succession, so far as my investigations
extend. In Cordyline and Chamsedorea many bundles of the leaf-trace appear, accord-
ing to Nageli \ to take the same course of development. Falkenberg ^ makes the
same statement for all Monocotyledons investigated by him.

According to Schmitz ® the median bundle of the leaf-trace of Berberis vulgaris
shows the same acropetal progression, at least as regards its origination ; this takes
place by means of longitudinal division of a strand of primary meristematic cells,

‘ starling ’ from the point where the median strand of the leaf perpendicularly below
(in f Phyllotaxis, the sixth) curves outwards. The same holds good, according to
NageliS for the external leaf-trace bundles of Bougainvillea spectabilis, as to which
however it is doubtful whether they belong to this series. Frank’s statements
according to which the same conditions are said to exist in the origination and
completion of the trace-bundles in Taxus, Quercus, and ^sculus, require testing.

On the other hand, it has been shown by Vöchting in the case of the bundles of
the leaf-trace of the Melastomacese, that with regard to both of the processes in question
they grow downwards in the basipetal direction from the point of exit in the node.
The same conclusion follows from Sanio’s investigations in the year 1864 of the bundles
of the Piperaceas, if these, as Weiss® maintains, are bundles of the leaf-trace. The de-
finitive completion, at any rate, takes place in very many bundles basipetally from the
node of exit; Nageli states this fora portion of the trace-bundles of Chamsedorea and
Cordyline, and for those of a very large number of Dicotyledons and Conifers ; and it is
not difficult here, in growing ends of stems, directly to see the basipetally progressive
completion of the tracheae; comp. e. g. p. 258, Fig. 1 15, m^. Thus, as above remarked
in demonstrating the course of the bundles in these numerous cases, as we follow them
downward from the node, we at the same time follow their order of development.

The possibility that the development of a leaf-trace bundle may follow different
directions in different parts of its course, has been discussed by Mohlj’^; in the case of
the leaf-trace bundles of the Palm-type, he states that they are developed basipetally
from the node in their upper part, but does not consider that this excludes an
acropetal development of the lower part. Neither do Nageli’s and Falkenberg’s
observations, so far as they have been published, appear to me to afford a solution
of this question. Development proceeding in two directions is indubitable in the

'■ Beitr. l.c. p. 37.

^ /. c. (see below, p. 393), p. 30.

^ Botan. Zeitg. 1864, pp. 180, 41 1.

“ l.c. p. 162.
^ /. 0 p. 121.
® Cf. p. 250.

’’ Verm. Schriften, p. 181.

392

PRIMARY ARRANGEMENT OF TISSUES.

case of the lateral leaf-trace bundles in the stem of Potamogeton crispus. It begins
here, as already indicated at p. 275 and in Fig. 125, with the appearance of the first
tracheides in the node, and advances from the latter, on the one hand, towards the
leaf, on the other hand basipetally in the adjoining internodes. Soon after the latter
basipetal development has begun, there appear, above the node, in connection with the
tracheides in it, the first tracheides of the portion of the lateral strand running
through the next internode, and these continue their development in the acropetal
direction. The two processes go on in opposite directions in every internode,
and meet about half-way up the latter, thus bringing the strand of primitive tracheides
into continuity ; the acropetal part starts earlier and grows slower than the basipetal.
The first origination of the bundles takes place, so far as could be decided, in
the acropetal direction.

The completion of the leaf-trace bundles in Equisetum ^ goes on, in a certain
sense, conversely to that in the case just described. It begins in the internode while
it is still very short, and in fact the primitive elements of the phloem first appear
rapidly in basipetal succession, then the first tracheides appear almost simultaneously
throughout the entire length of the internode. At a later period the development
of the primitive elements proceeds further and in fact upwards into the leaf, and
downwards to form the limbs, which are attached to the strands of the next lower
internode.

For the more complicated phenomena in the Phanerogams and Lycopods
possessing an axial bundle, and leaf-bundles laterally joining on to it, comp.
Sects. 70, 77, and 78, 107, 109, no; Niigeli, l.c. pp. 38, 53, 56.

Sect. 116. If a leaf-trace consists of more than one or two bundles, and if, as
in the great majority of cases, a median bundle is to be distinguished from lateral
ones, it is the rule for the median bundle to be first originated and completed, the
lateral bundles being the later developed the more distant they are from the median
one. Examples of this have already been given in Sect. 61. The converse order of
succession occurs but seldom ; as in the leaf-traces of Humulus and of Phaseolus,
which have three bundles. Leaf-traces of Monocotyledonous plants with very
numerous bundles ranged in several rows in the node, behave, according to Nageli’s
investigation of Chamaedorea and species of Cordyline, in agreement with the main
node ; but as regards the succession of the rows they differ in particular cases. Not
unfrequently the formation of the lateral bundles of a leaf-trace only starts when the
median bundles of several upper leaves are already present. If bundles of the leaf-trace
and cauline bundles are both present, the origination and completion of the former
takes place as a rule earlier in the same cross section, than that of the cauline
bundles. A conspicuous exception to this rule is presented by the Rhipsalidese
with winged stems

In recent times the question has been discussed, with reference to the original
differentiation of the meristem spoken of in the Introduction, what is the morpho-

1 Hofmeister, Vgl. Unters, p. 93. — Cramer, Pflanzenphys. Unters. Heft. 3, p. 26. — Nägeli,
Beitr. I. p. 38. — Russow, Vgl. Unters, p. 145.

“ Nägeli, 1. c. p. 1 14.

® Vöchting, l.c.

DEVELOPMENT OF THE VASCULAR BUNDLES.

393

logical region of development of the vascular bundles in the stemh For the axial
bundles, as has repeatedly been said above, it is established that their initial strand is
the plerome cylinder ; the bundles running from them into the leaves are formed out
of the surrounding periblem. For the cortical bundles the latter also holds good,
at least in the great majority of cases. The same has already been stated at p. 22
for the entire bundle-system of the stem of Equisetum.

It follows first of all from these facts, in accordance with the principle more
generally enunciated in the Introduction, that in the stem the formation of vascular-
bundles is not everywhere connected with one and the same primary layer of meristem.
This, however, does not yet answer the question how far other bundles or systems of
bundles, occupying a definite position in the stem, may arise from definite zones
of differentiation of the primary meristem. This is especially the question in the
case of the bundles of the ring and cylinder of the stems typical of Dicotyledons and
Monocotyledons. For reasons given at p. 20, the Ferns must be passed over for the
present.

In stems of the Dicotyledonous type the bundles obviously have their origin in an
annular zone corresponding to their definitive arrangement. This zone, when the
formation of the vascular bundles begins in it, is distinguished by the rapidly
succeeding longitudinal divisions, which give rise to them, and by the small
width of the cells, at least at definite points, corresponding to the commencement
of the bundles. In the adjoining zones, which become converted into pith and
cortex, the longitudinal divisions, from an early period of development onwards,
take place more rarely, and cease sooner, if those cases where groups of sclerenchy-
matous fibres are formed be left out of consideration ; the cells, which for the most
part become developed into parenchyma, follow the general growth in the transverse
direction chiefly by increase of volume without divisions ; and this in general takes
place in such a manner that the more considerable increase of volume begins in the
middle of the pith. The rapid longitudinal divisions of the bundle-ring always
begin, on any cross-section, at those points where, in correspondence with the
general rules of succession, the first vascular bundles originate : thus, for example, in
a young internode, in the position of the single, or of the median trace-bundle going
to the next leaf above. Here there first appears in cross-section a small group of
narrow cells, proceeding from the division of two, or not many more, original cells,
which gradually increases by means of farther divisions corresponding at each time
to the thickness of the initial strand, the cross-section of which is represented by
this group.

At the side of, or between the first initial bundle-groups of any cross-section
the commencements of new initial bundles then appear, in the same form, and in the
order corresponding to the general rules for the succession of bundles, and to the
course of the leaf-trace in the particular case, until the definitive number holding good
for the bundle-ring in question is complete. In particular cases the rapid longitudinal

^ Santo, Botan. Zeitg. 1863, p. 356 &c. ; 1864, p. 192 &c.; 1865, p. 165 &c. — Hanstein, Die
Scheitelzellgruppe, &c. (1868); cf. p. 7. — Russow, Vergl. Untersuchungen, p. 177 &c. — Vochting,
Melastomeen und Khipsalideen ; compare pp. 259 and 261. — Schmitz, Entwickelung d. Sprossspitze
der Phanerogamen, Halle, 1874. — Falkenberg, Monocotyledonen, l.c.

394

PRIMARY ARRANGEMENT OF TISSUES.

division remains confined to the initial bundles of the leaf-trace ; broad bands of
meristem lying between them take little part in the division, and follow the general
growth chiefly by increase in volume of the cells, as in the case of Cucumis, accord-
ing to Sanio ; in the Ranunculi, often mentioned above, the same conditions may
exist. In most cases however the cells of the whole bundle-ring, including the
bands lying between the leaf-trace bundles (primary medullary rays), remain or
become narrower on the average than those of the pith and cortex; the rapid
longitudinal divisions extend from the lateral edges of the initial bundles sideways in
the direction of the ring, so that bundles of the leaf-trace, which arise later, may be
differentiated within a small-celled annular zone already undergoing active longitu-
dinal divisions ; e.g. in the Melastomacese. In the numerous cases belonging to this
category where the bundles of the leaf-trace are early united by numerous intermediate
bundles, the rudiments of the former, in consequence of rapid longitudinal division
spreading laterally, amalgamate at once at their edges to form together, as it were,
a narrow-celled ring, in which the intermediate bundles successively differentiate
themselves from the medullary rays which separate them. The origination and
completion of the bundles of the leaf-trace here passes over continuously and in-
sensibly into that of the intermediate bundles. Comp. Chap. XIV.

For the origination of the groups of sclerenchymatous fibres the same holds
good as for the vascular bundles, in respect of the rapid longitudinal divisions going
on in the cells of the primary meristem, and the small width of the elements result-
ing from this. Where the two occur in compan)^, as so often happens, the breadth
of the narrow-celled-ring is essentially influenced by this fact.

To the narrow-celled initial bundle-ring, the not very happily chosen name
of Thickening-ring has been applied by Sanio.

With reference to the question put more generally above, we have now further
to ascertain what is the morphological region in which (to speak shortly) that initial
ring appears. Sanio, on the basis of careful investigations of successive cross-sections,
has propounded the doctrine that close below the growing-point the originally
homogeneous meristem first becomes differentiated into an axial strand, the 'primary
pith' which developes into the pith-cylinder of the shoot, and is distinguished
by relatively rare longitudinal division and rapid increase in size of its cells, and
secondly an external zone surrounding this. The latter again becomes differentiated
into a peripheral zone, which forms the outer cortex together with the epidermis,
and an inner zone, its thickening-ring. The outer zone, divided into the two layers
mentioned, is further that from which all foliar structures are derived. Russow main-
tains essentially the same view, calling Sanio’s primary pith Endomeristem, the zone
surrounding it Exomeristem ; the latter is divided into the Mesomeristem, which is the
inner layer producing the vascular bundles, and the Perimeristem, which is the outer
zone, forming the external cortex and the Dermatogen. Endistem, Existem, Mesistem,
and Peristem are abbreviated expressions for these successive layers. Thus, according
to this view, the entire system of leaf-trace bundles, together with the outer cortex of
the stem, as well as the leaves, would proceed in typical Dicotyledons from the
existem, or the zone surrounding the primary pith, as is actually the case in
Equisetum. Hanstein’s discovery of the separation of the primary meristem in
the growing-point into the distinct layers called Dermatogen (Epidermis), Plerome,

DEVELOPMENT OF THE VASCULAR BUNDLES.

395

and Periblem (p. 7), was unknown to Sanio, although he himself first clearly
described this phenomenon for some special exceptional cases ; Russow opposes
this view, and especially the further conclusions drawn by Hanstein, chiefly on the
ground of the phenomena in Equisetum.

Hanstein, in his paper of the year 1868, founded the doctrine that the leaf-trace
bundles of the typical Dicotyledonous stem are formed in the periphery of the
plerome-cylinder, while its central part becomes the pith; and that the periblem
(together with the dermatogen) forms all the lateral members, especially the leaves
and the outer cortex, as well as the portions of the bundles, which pass through the
latter on their way to the leaves and branches. According to this, every leaf-trace
bundle is derived in its cauline portion from the plerome-cylinder, in the portion
going out into the leaf, from the periblem mantle. Considering the contradiction
between this view and that of Sanio and Russow, and in view of the fact already
mentioned at p. 8 that the differentiation of plerome and periblem in the growing-
points of stems cannot always be traced up to the extreme apex, it might at first be asked
whether Hanstein’s view of the matter be not incorrect, in so far that the boundary
between his plerome and periblem might run, not between the primary meristematic
layers above-mentioned, but between those which are only secondarily severed from
one another, which Russow calls Peristem and Mesistem. This question or con-
jecture is however negatived, even wholly apart from stems with an axial bundle
ending in a sharply defined plerome-apex, as Hippuris (p. 8), on inspecting longi-
tudinal sections of many growing-points of stems.

In the growing-point of Berberis vulgaris, where the plerome is well distinguished
from the single-layered periblem, Schmitz has recently investigated the origination of
the leaf-trace bundles, and proved that this takes place in the outer layers of the
plerome, but not even at its outer boundary, as the one or two outermost layers take
part in the formation of the external cortex. In the apex of the shoots of Meni-
spermum canadense, according to the same observer, it is the external surface of the
plerome bundle, here also surrounded by an originally single-layered periblem, in
which the vascular bundles originate. In Ephedra, according to Schmitz, a certain
decision of the present question is not attainable, on account of the less sharp
original severance of the primary layers of meristem.

There are thus indubitable cases among stems of the Dicotyledonous type where
the portion of the leaf-trace bundle running through the stem arises from the
plerome, and these cases are without doubt numerous. Although, considering the
different behaviour of Equisetum, it may be open to" objection to base a principle of
general application on the existing results, before more numerous special observations
have been undertaken, yet provisionally, and in reservation of further researches, a
generalisation of the results obtained for cases with a similar type of structure and
growth is demanded. For the Dicotyledons in question, therefore, the origin of the leaf-
trace bundles, and of the bundle-ring generally, will have to be referred to the outer
part of the plerome strand ; and, according to Schmitz’s observations on Meni-
spermum and Berberis, this will also apply to the fibrous strands and rings which
accompany the bundles. As however the bundle-rings of the Dicotyledonous type
may no doubt be regarded as everywhere morphologically homologous — at least
until the contrary should be proved — and as further the vascular strand, or, which is

39Ö

PRIMARY ARRANGEMENT OF TISSUES.

the same thing, the plerome strand of the main root is continued directly through
the hypocotyledonary axis into the bundle-ring of the stem, we can go a step
further, and, in agreement with van Tieghem’s view^, may draw the external
boundary of the plerome strand close outside the external boundary of the bundle-
ring, even in those cases where the former cannot be distinctly traced into the apex
of the primary meristem. According to this view the plerome would be the axial
cylinder, which in the first series of cases only becomes marked off from the periblem
by the differentiation of the initial cylinder in its external part, and which in the other
series of cases is continued with a sharp boundary into the extreme apex of the stem,
where it shows no differentiation other than the primary layers of meristem. Among
the Angiosperms it appears to be chiefly plants with a very flat apical meristem,
forming new internodes at definite intervals, which belong to the first category, while
the second includes those with a more elongated apex. With the latter are further
connected the elongated apices of plants with one axial vascular bundle, which
arises from a sharply defined plerome-bundle.

Medullary bundles situated inside a distinct ring require, after what has been said,
no further explanation with reference to their origin from the primary rows of meristem.
For the net of bundles in the stem of Gunnera the same holds good, according to
Reinke as for the ring of typical Dicotyledons. The Nymphseaceae and Auriculas
require still more accurate investigation with reference to the question under discussion.

For Monocotyledons'^ the question before us may be generally answered by the
statement that the cylinder containing the bundles, defined in this sense above at
p. 201, is derived from the plerome strand. This is often to be traced into the
extreme apical meristem, e. g. Grasses, Polygonatum, Ganna, Potamogeton, Trades-
cantia sp.. Asparagus or it may only be differentiated below this, e. g. Epipactis, &c.,
according to Falkenberg. The outer layer of the bundle-cylinder, in which the lower
ends of the bundles lie, falls in the outer boundary of the plerome, or lies within it.
The succession of development of the bundles follows the general rule here also.
In consequence of this, and of the course of the bundles described above, we find
them, in the case of the Palm-type, appearing in centrifugal order as seen in suc-
cessive cross-sections of young internodes, the median bundles which penetrate most
deeply appearing first, and so on. The completion of the tissue surrounding the
bundles, especially the cessation of longitudinal divisions, and the increase in volume
of the meristematic cells which become converted into parenchyma, proceed in the
same order ; in the external region, partly for the reasons already given, relatively
more abundant longitudinal divisions and smaller growth of the elements in the
transverse directions take place. As long as the longitudinal divisions, which cease
in centrifugal order, persist, the periphery of the cylinder is occupied by a meristematic
narrow-celled ring, which Sanio identifies with his thickening-ring demonstrated in
the case of the Dicotyledons, a view which is so far, but only so far correct, as both
constitute that zone of the plerome body which is engaged in differentiation and in
the formation of vascular bundles.

* Ann. Sei. Nat. 5 ser. tom. XVI. p. 112, note. Morpholog. Abhandl. p. 67.

^ [Compare Guillaud, Ann. Sei. Nat. 6 ser. Bot. tom. V. pp. 1-176.]

* Hanstein, l.c. — Falkenberg, l.c.

DEVELOPMENT OF THE VASCULAR BUNDLES.

397

A necessary consequence of the view here propounded is that in the Monocoty-
ledons also every leaf-trace bundle arises from the plerome as regards that portion
which passes through the cylinder, but from the periblem as regards that portion
which runs out into the leaf.

As regards the origination and completion of the individual bundles in the leaves
themselves, essentially the same rules hold good as in the case of stems. That their
initial strands of meristem must have a definite position and orientation, and that the
latter must correspond to that of the mature bundle, is obvious h The longitudinal
progression of their origination and completion is guided by the direction of growth
of the leaf, which, as is well known, varies in the particular cases.

That the bundles of lateral buds connected with the system of the main axis
originate later than the latter, scarcely requires to be mentioned ; their development,
or at least their completion, either proceeds centrifugally, i. e. from the point of
attachment onwards into the lateral shoot, e. g. Potamogeton, Fig. 123, p. 273; or in
the opposite centripetal direction, e. g. in the node of Zea and Saccharum.

Sect. 117. The development of normal lateral roots on a relative main root
is so immediately connected with the development of the vascular bundles of
roots, at least in the case of the Phanerogams, that it cannot be wholly passed over
in this place, although, strictly speaking, it belongs only in a small degree to the
subject of this book.

In the Phanerogams, as shown by Nageli and Leitgeb, Reinke and Janczewski^,
from the last of whom the following summary is principally taken, the young peri-
cambial layer is the chief seat of formation of normal branch-roots ; neighbouring
layers of cells may take part in the process to a varying extent. How far the datter
takes place is determined according to the particular cases to be adduced, and these
correspond in a great degree, though not always with accuracy, to the types of
differentiation of the apical meristem enumerated at p. 9, &c.

We have here to assume as already known, that the rudiment of monopodial
branch-roots always arises upon the axial bundle of the relative main root, that it here
acquires its characteristic structure, especially the differentiation of its growing-point,
and then, boring through the peripheral layers of tissue, reaches the surface.

Among the Phanerogams investigated, Janczewski distinguishes five types of
the processes of development in question.

In the first of these, to which only Pistia belongs, the plerome-cylinder and the
periblem of the lateral root are derived from the growth and corresponding divisions
of the single-layered pericambium ; the root-cap, or calyptrogenic layer, and the
epidermis, are derived from the endodermal layer. In the second type, represented by
Alisma Plantago, Sagittaria, and Zea, the entire lateral root, including the calyptro-
genic layer, arises from the pericambium, the divisions of which begin irregularly.
The endodermal layer only forms an exterior coating on the cap, and, in the case of
Zea, also the epidermis and outermost cortical layers at the base of the lateral
roots. With reference to the participation of the layers of the mother root,

^ Compare p. 23.

^ Nageli and Leitgeb, Entstehung u. Wachsthum d. Wurzeln, Beitr. zur Wissensch. Botanik,
Heft 4. — Reinke, in Hanstein’s Bot. Abhandl, Heft 3; id. Morpholog. Abhandl. p. i. — Janczewski,
Ann. Sei. Nat. 5 ser. tom. 20.

398

PRIMARV ARRANGEMENT OF TISSUES.

Janczewski’s third type (Fagopyruin, Raphanus, Helianthus) stands near the second,
inasmuch as the entire rudiment of the lateral root arises from the pericambium, with
little or no participation of the endodermal layer. It is distinguished from the second-
type by the regularity of the initial divisions. The cells of the segment of the peri-
cambial layer concerned, become elongated in a radial direction, and divide once
tangentially. The inner of the two layers derived from the latter division forms the
commencement of the new plerome bundle ; the outer is divided again tangentially
into an outermost layer, which is the calyptrogen, and an inner, or middle layer, which
forms the commencement of the entire cortex. In Helianthus the endodermal layer
of the mother root forms a many-layered external covering to the root-cap, over the
apex of the rudiment of the lateral root ; in Fagopyrum it only grows to form a
single-layered envelope of the latter ; and in Raphanus it remains quite passive.

In the fourth type, which embraces the Cucurbitaceae and Papilionaceae
mentioned at p. 12, the rudiment of the lateral root is derived from the common
growth of the portion of the pericambium in question, of the endodermal layer, and
of the r-2 cortical layers adjoining the latter externally. The former constitutes the
plerome bundle, the latter conjointly form the surrounding portions of the rudimentary
root, under the apex of which the common initial zone is differentiated at a relatively
late period.

The roots of Pinus constitute the fifth type. The whole rudiment of the lateral
root, which soon assumes the differentiation described at p. 13, proceeds from the
many-layered pericambium; the endodermis and the layers adjoining it externally
remain passive. In the Cycadese, in Taxus, and in Sequoia, on the other hand, the
latter, according to Reinke and Strasburger, take part to a small extent in the
formation of the most peripheral layers of the rudimentary root h

In opposition to the Phanerogams the lateral rudimentary roots, in the case of the
monopodially branched roots of all investigated Ferns and Marsiliacecs take their
origin from the endodermal layer which surrounds the pericambium ; in the Equiseta
they arise from the layer lying within the endodermis (comp. p. 351). In many
Cyatheacese and in Marsilia the longitudinal rows of endodermal cells, which may be
shortly described as rhizogenetic rows, and which, as corresponding to the places
of origin of the lateral roots, face the xylem-plates, are distinguished by their
wider and shorter cells from the other rows of the same layer ; frequently those
rows of the next outer cortical layer, which lie in front of the rhizogenetic rows,
show similar relations of size, and also have their walls less thickened than the other
rows belonging to the same layer. The layers however which lie outside the
endodermal layer take no active part in the origination of lateral roots. On the
contrary, every lateral root proceeds from a cell belonging to the rhizogenetic layer,
which directly, or after a few irregular preliminary divisions, assumes the character-
istics of the apical cell of the root, as described at p. 18. The pericambium of the
mother-root is only concerned in the formation of lateral roots, in so far as the
connecting piece between the vascular bundles of the two orders is formed in it.

As already indicated, the place of origin of a lateral root in the Ferns and
Equisetum is always a circumscribed portion of tissue lying in front of a xylem-plate.

Strasburger, Die Coniferen, &c., p. 348.

^ Nageli und Leitgeb, /. c. p. 88.

DEVELOPMENT OF THE VASCULAR BUNDLES.

399

In those Phanerogams, in which the pericambium extends over the external corner
of the xylem-plates, the same holds good, with the exception of Lycopersicum. In
these cases therefore a longitudinal row of the rudimentary roots faces each
xylem-plate, an arrangement which is always conspicuous even externally in the
case of oligarch roots ; the attachment of the vascular bundles takes place in
the form described above at p. 387. Where, on the other hand, the xylem-plates
of the roots of Phanerogams directly abut on the endodermis, as is the rule among
the Grasses, the rows of rudimentary lateral roots alternate with the xylem-plates,
and lie opposite the centre of the phloem-bands, as was stated at p. 387.

According to van Tieghem’s investigations the rhizogenetic longitudinal bands,
and the products of their development, have the same position in the case of the
Pittosporeas, because a group of oil passages here lies in the pericambium in front
of each angle of the xylem, as is to be stated in Chap. XIIL As also described in that
chapter, in the case of the Umbelliferse and Araliaceae, e.g. Hedera, an annular row of oil
or resin passages likewise lies in the same position, but here a quite similar passage
is also present in front of each phloem-group. It is therefore the rule for these
plants, apart from individual exceptions described by van Tieghem, l.c. p. 149, that
on each side of every angle of the xylem, alternating with the latter and the next
phloem-group, there lies a rhizogenetic pericambial band ; thus the number of these
bands, and the rows of lateral roots is double that of the xylem-plates, and the
attachment of the bundles of the latter takes place as described at p. 387 k In
Lycopersicum, as already stated, the same condition occurs, without any assignable
anatomical ground.

At the conclusion of this chapter it will be convenient to justify some of the opinions
here expressed, in opposition to other views.

In the first place, ivith a view to simple matters of fact, every distinct strand, which is
separate from others of identical or similar structure, and which consists of tracheae or
sieve-tubes, has been designated by the name vascular bundle ; to the typical complete
strands of this kind, those which remain or become incomplete were appended. This
usage is based on the nature of the object, for the fact is established that the two es-
sential forms of tissue, tracheae and sieve-tubes, together form these bundles. If the
principle maintained in this book, of distinguishing and regarding the systems of tissue,
with reference to the forms of tissue which compose them, were carried out with
extreme consistency, the phloem of the bundles would no doubt have to be considered
separately from their xylem, while the further elements belonging to the bundle would again
demand a distinct treatment. It will scarcely however be disputed, that this would not
only confuse the description, but also misrepresent the relations which actually exist.
If the bundle formed of tracheae and sieve-tubes as its essential constituents, or even
each of its two parts, containing one of the essential forms of tissue, be regarded as a
whole, it is obvious that the non-equivalent organs, such as parenchymatous cells, &c,,
which occur in it, must also be reckoned among its elements. If the comprehension of
non-equivalent elements is thus once conceded, it may further be extended to those
which lie outside, that is, at the periphery of the essential tissues. As soon as this has
been done, strict limitation according to the principles above laid down is, it is true,
given up ; the actual boundary of the vascular bundle becomes conventional, though this
conventional limitation may be based upon good grounds derived from other considera-
tions, as from that of the primary differentiation of the meristem. It will be so far
admissible as, on the one hand, the weight of these reasons, and on the other, advantages

* See van Tieghem, l.c.\ and Canaux secreteurs, Ann. Sei. Nat. 5 ser. tom. XVI.

400

PRIMARY ARRANGEMENT OF TISSUES.

in comparative anatomical description, can be brought to bear in its favour. The limi-
tation of many vascular bundles by means of the endodermis, to which we have adhered
in preceding paragraphs, that is to say, the view that the boundary of the bundle is to be
drawn at the inner surface of the latter, is, for example, conventional in the sense indi-
cated. In the bundles of roots this limitation has a genetic basis, in so far as the endo-
dermis is the innermost layer of the periblem, while that which it encloses proceeds from
the plerome. In other cases, as in the leaves of the Primulas, this developmental ground
is absent; in the case of many bundles of Ferns, developmental considerations in them-
selves undoubtedly lead to the opposite result, as the endodermis and the adjoining layer
of the bundle are here derived, relatively late, from the division of one layer of
mother-cells. Nevertheless, on more extended comparison, the limitation of the bundle
on this principle will hardly be disputed. It is further purely conventional, but, as in
the example previously adduced, justified by developmental considerations and in the
interests of clearness, when the pericambium and the parenchymatous layers within the
endodermis of concentric bundles in Ferns are included in the bundle and not treated as
separate sheaths.

The same, or at least quite similar, considerations may be applied to the fibrous
strands, the ‘ bundles of bast-fibres,’ which accompany many vascular bundles. There is
simply no decisive reason to be found for placing the boundary of a bundle, which they
accompany, at their external, or at their internal surface. Where they accompany the
vascular bundle, they form with the latter one whole, and the fact that they are com-
pleted later than the first, or than many of the elements of the vascular bundle, is in
itself no ground for separating them from the bundle, for the essential elements of the
latter also attain completion in a definite succession. It is here also conventional, if, as
was done above, the fibrous strands are separated from the vascular bundles, and if the
boundaries of the latter are drawn on the inner side of the former. The ground for this
conventional limitation, however, lies in the fact that the accompanying fibrous strands
consist of a sort of tissue different from the essential tissues of all vascular bundles, that
they are absent from very many vascular bundles, and are thus non-essential for the
vascular bundle generally ; and lastly that they, in a still higher degree than the endo-
dermal sheaths, belong to a form of tissue, which in itself forms a system quite inde-
pendent of the vascular bundles, and which, in its development, does not stand in any
constant relation to the primary differentiation of the meristem. They are portions of a
system which may, but which need not, directly accompany the vascular bundles, and they
must be included in this system ; it is therefore well to separate them once for all from
the vascular bundles. These considerations may also throw light on the discussion
which has often been carried on in recent times, and which very clearly illustrates the
confusion of ideas now prevailing in the field of anatomy, as to whether the accom-
panying fibrous strands belong to the ‘ fascicular tissue ’ or to the ‘ ground tissue'.’

The limits between the vascular bundles themselves can be fixed with less difficulty
than those between the vascular bundle and its surrounding tissue. If we have once
distinguished as a vascular bundle that distinct strand which is formed of tracheae and
sieve-tubes definitely grouped, this distinction must be carried out universally, both for
the sake of consistency and in the interests of clearness, and every distinct group of the
two kinds of organs in question, which forms a united whole, must be called a vascular
bundle. What the special grouping of the essential organs may be in these cases,
whether the bundle has arisen from the union of several, whether in any one case it
corresponds as regards position and origin to a system of numerous bundles which occurs
in other cases, these are in themselves questions which have their importance, but they
do not touch the anatomical distinction under consideration. On these grounds, the axial
bundle of roots, and of the stems of Lycopodiacese, has been treated as one vascular bundle.
It is here also conceded that the subject maybe regarded from other points of view, andthus,
for example, we may cease calling the axial bundle of roots a vascular bundle, and regard it

' [Compare Giiillaiid, 1. c. (see p. 396).]

TERMINOLOGY OF THE VASCULAR BUNDLES.

401

rather as a central cylinder or plerome-strand differentiated into tracheal plates, phloem
strands, pericambium, &c. But this method, if consistently carried out, must lead at
once to that general breaking-up of the vascular bundle, which, as already stated above,
though completely justified in principle, is certainly not desirable in the interests of clear
description.

With reference, secondly, to the Terminology employed, I have, as will be granted, aimed
at the greatest possible simplicity and clearness of expression, and endeavoured, as far as
possible, to preserve or to restore names of long standing. I must apologize for the
single really new word, Epithema ; it is not pretty, but I could not find a better one.
The reasons for keeping or not keeping most of the names which come into question follow
partly from what has just been said, partly from the necessity of being consistent in expo-
sition, while they have in part been expressly stated in the latter. It would lead us too
far, and would not be interesting, to discuss them all severally in this place. My motive for
substituting the old expression vascular bundle for the term fibro-vascular bundle, which
has lately become more usual, has likewise been generally stated above, partly at pp. 232
and 318, partly in the earlier portion of this note. In fact vascular bundle, in the above
description, denotes something different from Nägeli’s expression Fibro-vascular strand,
inasmuch as the latter comprehends the accompanying fibrous tissue, which has been ex-
cluded from the vascular bundle above. A vascular bundle may appear as an essential
part of a fibro-vascular strand, but it is not necessarily, and very often not actually com-
bined with accompanying fibrous tissue, and the two things must therefore be dis-
tinguished. I have preferred the expression vascular bundle to the more recent
‘ conducting bundle,’ because it is the traditional term, which is also preserved in the
phrase fibro-vascular strand, and because no decisive ground appears to me to exist for
laying aside the old convenient word, which correctly indicates the principal point, or
for limiting its application.

Caspary * has adopted the latter course, and has distinguished the bundles containing
tracheides, under the name of ‘ cellular conducting-bundles,’ from those which contain
vessels, and comprehended both under the name conducting-bundle, because he assumed
a great difference between tracheides and vessels, like that between cells and vessels.
As the distinction between the tvizo forms of organs is actually a very trifling one, the
ground for the sharp severance of the two kinds of bundle, and thus for the change of
name, in my opinion disappears. It is no doubt an inaccuracy which is permissible in
the interests of simplicity of expression, to speak of vascular bundles without vessels, i.e.
bundles in which the latter are replaced by tracheides.

Berliner Acad. Monatsber. 10 July, 1862, p. 453.

CHAPTER IX.

ARRANGEMENT OF THE PRIMARY PARENCHYMA.

Sect. ii8. The primary parenchyma, in so far as it is not a constituent of the
vascular bundles, forms, as often indicated in earlier chapters, the principal contents
of the space enclosed by the epidermis, where it is left free by the vascular bundles.
Within this space it is traversed by other, non-equivalent forms of tissue, as will be
stated in succeeding chapters. The ‘regions characterised in the preceding pages as
external cortex, pith, and medullary rays, as well as the leaves and foliar expansions,
are therefore, as regards their main mass, built up of parenchyma.

Within the limits of this general plan, however, definite rules exist for the dis-
tribution and arrangement of the particular forms of parenchyma distinguished in
Chap. I, Sect. 3, and these rules will be here given. In doing so, attention will chiefly
be directed to the relatively thin-walled forms, and we shall return to the sclerotic
forms in the next chapter, on account of their close anatomical and physiological
relations to the sclerenchyma. As the occurrence and distribution of the different
individual forms is chiefly determined according to the various organs of the highest
degree in which they occur, and to their anatomical regions as distinguished in the
previous chapter, it is expedient to classify the description with reference to these
organs and regions, in such a manner that first the parenchymatous masses charac-
teristic of each will be described, and then their limiting and sheathing layers.

Sect. 119. The parenchyma of the pith and of the himdle -cylinder of the stem
consists in general of cells arranged chiefly in longitudinal rows, without any very
remarkable anatomical peculiarities ; when young they contain products of assimila-
tion, and in foliage shoots frequently chlorophyll, and they either preserve this
condition of their contents throughout life, or, as is especially the case in Dicotyle-
dons, they soon dry up and die off. In the pith of many Dicotyledons, the two
conditions occur in different rows of cells, which then also differ with respect to the
form and size of their elements, as is shown by the examples of ligneous plants
investigated by A. Gris, which are to be cited below. From the earliest period of
development onwards, intercellular spaces containing air appear in the pith, which
sometimes persist in the form of narrow interstices, sometimes form wide lacunae,
while in the numerous cases of stems which become hollow in the internodes, owing
to their cells dying off, they are widened to form the axial air-canals described above.
Cf. Sects. 51, 52.

In most stems belonging to the Dicotyledonous type, the elements of the pith,
as stated in Sect. 116, become narrower towards the inner boundary of the ring of
bundles ; together with the innermost portions of the vascular bundles, they here con-

PR IMA RY PARE NCHYMA . PITH.

4^3

stitute the zone distinguished as the medullary sheath, to which we shall have to return
in Chap. XIV. In like manner the parenchyma of the cylinder of bundles in Mono-
cotyledons becomes as a rule narrower-celled towards the outer surface of the latter.
In those stems of Dicotyledons and Equiseta which become hollow, the narrower
peripheral elements of the pith are persistent ; the case is the same in the Monocoty-
ledons for the elements of the entire zone which surrounds the cavity and contains
the bundles.

According to the comparative investigations of A. Gris \ the pith of Dicotyledonous
ligneous plants at first consists entirely of parenchyma, in which crystal-sacs appear con-
stantly, and sclerenchymatous elements not unfrequently. On the complete development
of the one-year-old stem or branch, the cells of the parenchyma sometimes become
emptied of their contents, and then dry up, so as to contain air (empty cells, ‘ cellules
inertes ’ of Gris) ; sometimes they remain as active parenchymatous cells (cellules actives),
which alternately store up and give back products of assimilation, especially starch and
tannin, according to the periods of vegetation. According to Gris, this activity continues
for years ; in the case of Platanus occidentalis, Gleditschia ferox, Betula alba, Quercus
Robur, and Fraxinus, it has been traced as far as the 20th year of life. The active cells
are, as a rule, distinguished from the empty ones by their smaller size, and their thicker,
finely-pitted walls.

Only in a few woods does the pith become entirely empty and dried up : Sambucus
nigra. In most cases it either consists of active cells, with crystal-sacs which are isolated
or distributed in larger groups, or of these elements and empty cells in varying arrange-
ment. The former is termed by Gris homogeneous pith. This occurs with a relatively
small number of isolated crystal-sacs in very numerous ligneous plants ; as in the investi-
gated species of Pyrus, Cydonia, Aronia, Quercus, Fagus, Betula, Ainus, Platanus, Ilex,
Prinos, Buxus, and many Ericaceae ; it is interrupted by larger scattered groups of thin-
walled crystal-sacs, which are isolated, or arranged in a reticulate manner as seen in cross-
section, in Pernettya, species of Rhododendron, Calluna vulgaris, Andromeda polifolia,
Cladothamnus, &c.

Pith, which is compounded of active and empty cells, is called by Gris heterogeneous.
It is composed either of an empty central and an active peripheral portion, as in Lonicera
fragrantissima, Abelia rupestris, Symphoricarpus vulgaris, Ligustrum, Ornus, Syringa vul-
garis, Berberis vulgaris, Ulmus campestris, Celtis, and Rhamnus sp. ; or this arrangement
is complicated by the occurrence of active bands in the central empty portion; Pyrus
Malus, Sorbus Aucuparia, Aria torminalis, Crataegus Oxyacantha, Amorpha glabra ; or it
is everywhere made up of alternating bands of empty and active tissue, in which case the
latter chiefly forms longitudinal rows: Viburnum Tinus, and Lantana; or bands anasto-
mosing in all directions in a reticulate manner : Rubus, Rosa and Clethra ; or diaphragm-
like transverse plates : Magnolia, and Liriodendron. In the nodes and at the boundary
of successive annual shoots, the pith is on the whole tougher, and in the heterogeneous
forms has a larger proportion of active elements.

With regard to further details, for which reference is to be made to Gris, a great con-
stancy of structure exists for each species. The examples mentioned of Rosiflorae and
Ericaceae show that species of like habit, belonging to closely-allied genera of a family,
may behave differently.

In the parenchyma of the primary medullary rays essentially the same conditions

^ Sur la moelle des plantes ligneuses, Ann. Sei. Nat. 5 ser. tom. XIV. p. 26, pi. 4-7. — Nouvelles
Archives du Museum d’Hist. Nat. VI. p. 201.

^ Compare Mold, Poren d. Pflanzen-Zellgewebes, p. 27, figs. 27, 37, 38.

D d 2

404

PRIMARY ARRANGEMENT OF TISSUES.

prevail as in that of the periphery of the pith. The peculiarities which come under
consideration in the case of stems with secondary formation of wood, are to be
compared in Chap. XIV.

In most cases, and especially in those cases forming the rule, in which the stem
bears well-developed foliage-leaves, the outer cortex of the stem is built up of two more
or less distinct parts ; one, the Hypoderma ^ bordering directly on the epidermis, and
consisting of thicker-walled, closely united elements, which are often collenchymatous
or sclerotic, and in the latter case are still to be described in the following chapter ;
the other, a thinner-walled, internal mass of parenchyma, the cells of which as a rule
become successively wider towards the interior, and always leave between them inter-
stices or lacunae filled with air. Both parts show a different arrangement according
to the particular cases. For the Dicotyledonous stem, Schleiden’s^ classification may
be adopted, and the following principal forms may be distinguished.

{a) The hypodermal layer surrounds the whole stem as a distinct, closed, multi-
seriate (collenchymatous) layer, which is only interrupted where stomata are present
by small gaps leading to the latter : many Cactese, Melianthus major. Euphorbia
splendens, Syringa vulgaris. Begonia macularis, Ailantus glandulosa, Rosa, Aris-
tolochia Sipho, Piper rugosum, Cacalia ficoides, and Cotyledon coccinea.

(<5) The collenchymatous or sclerotic hypoderma forms longitudinal bundles,
which alternate with longitudinal bands of thin-walled parenchyma, destitute of
collenchyma, reaching to the epidermis. The former usually occur in more or less
projecting corners of the stem, while the parenchymatous bands lie between them,
as in many angular stems, Umbelliferse, Chenopodiaceae, Malvaceae, Solanaceas,
and Sambucus ; in other angular stems, e.g. Labiatae, the collenchymatous hypo-
derma is massively developed in the angles, between them it is at least weaker in
every respect.

{c) The epidermis is bordered by a collenchymatous hypoderma, which towards
the inside gradually passes over into the thinner- walled, loose mass of parenchyma,
and is broken up into isolated masses by the thin-w^alled parenchyma which extends
to the epidermis at the points where stomata occur. So in the primary cortex of
most Dicotyledonous woody plants, e. g. Pyrus, Jüsculus, Salix, Cupuliferae, Betula,
Acer, Hedera, Tilia, etc., with more or less sharp limitation, and a varying number
of strata in the hypodermal layer.

As follows directly from what has been said, the limitation and bulk of the
firmer hypodermal layer is very various, even in the preceding typical cases. In
weak stems, as in those of many water-plants, it often entirely disappears, and is
only indicated by the smaller width and closer connection of the cells bordering
directly on the epidermis, as compared with those which lie deeper.

The converse case is more rare, namely that in w'hich thin-w'alled, loose paren-
chymatous layers border immediately on the epidermis, while an inner zone forms
a many-layered sheath — then always more or less sclerotic — which surrounds the
ring of bundles ; e.g. species of Papaver and Thalictrum.

On the occurrence of stomata, and their relations to the structural phenomena in

’ Compare p. 225. The word Hypoderma was introduced by Kraus; Cycadeenfiedern,
Pringsheim’s Jahrb. IV.

^ Grundzüge, 3 Aufi. 11. p. 152.

PRIMARY PARENCHYMA. CORTEX. PETIOLES.

405

question, Sect. 7, p. 45, is to be compared; on the structure of the collenchyma,
comp. Sect. 26, p. 1 19. The thin-walled parenchyma of the external cortex contains
chlorophyll, except in the few plants which are altogether destitute of it, and the
nearer the surface, the greater on the average is its amount. As already stated, the
parenchyma is always traversed by spaces containing air, and often by wide lacunae.
Also the large lacunae, chambers, and air-passages in the stems of water-plants
(comp. p. 2n) lie in the internal parenchymatous mass, and in fact are so distributed
that they form a circle, interrupted by radial, usually one-layered lamellae, between a
denser zone immediately surrounding the ring of bundles or axial strand, and the
hypodermal zone ; or they may be situated in the same region in two or more
alternating circles. The former is the case in Myriophyllum, Ceratophyllum, Elatine
Alsinastrum, Callitriche, and Utricularia vulgaris ; the latter in Hottonia, Trapa,
Hippuris, etc.

Generally similar phenomena, which cannot be more fully described here, and
to some of which we shall have to return in the next chapter, are shown by the
cortex of Monocotyledonous stems. Comp. Fig. 120, p. 269; Fig. 125, p. 274;
Fig. 17 1, p. 369.

In order to avoid repetitions we will return in the following chapter to the cortex
of the Fern-like plants, in so far as it belongs to our present subject.

The stems of those chlorophyll-containing plants in which the foliage-leaves are
feebly or not at all developed and in which the stems themselves undertake the function
of the green foliage ’, and likewise the numerous Monocotyledons widi ‘ halms ’
similarly constructed to the foliage-leaves of the same plant, behave differently from
the ordinary leafy-stems, inasmuch as their cortical parenchyma assumes the structure
of those layers of concentically constructed laminse which contain chlorophyll ; this
structure will be described in Sect. 121. Examples of this will be given at the place
indicated. Comp, also p. 265.

Sect. 120. Petioles and stout ribs of the leaf %\\0'x in general a similar structure
of the parenchyma surrounding the vascular bundles to that in the stem of the same
plants ; the layers and bands of thin-walled or collenchymatous parenchyma, and in
many cases of sclerotic elements also, are often continued into them from the stem ^
In the smaller petioles and ribs the collenchymatous or sclerotic elements occur for the
most part, or exclusively, around the vascular bundles. Aqueous- and chlorophyll-
parenchyma take part in the composition of the larger ones in very various forms.
The large petioles of Ferns are especially to be mentioned as examples of the band-like
or isolated distribution of air-containing parenchyma, covered by an epidermis contain-
ing stomata, and lying between masses — here usually sclerotic — covered by an epidermis
destitute of stomata. Most frequently the air-containing parenchyma first-mentioned
extends down each side of the petiole from the base of the lamina, in the form of a
longitudinal band ; in species with a creeping stem, e. g. species of Hypolepis and
Pieris, the bands are often continued along its lateral surface throughout its length.
In the petioles of Tree-Ferns the bands are frequently interrupted, forming a longi-
tudinal row of short strips; e.g. Cyathea medullaris. On the persistent base of the

‘ [Compare Pick, Beitr. z. Kenntn. d. assimilirenden Gewebes armlaubiger Pflanzen. Diss.
Bonn, Ref. Bot. Centralbl. 1881, Bd. 6, p. 234.]

^ Compare Kraus, Cycadeenfiedern, in Priiigsheim’s Jahrb. IV.

4o6

PRIMARY ARRANGEMENT OF TISSUES.

leaf, the pulvinus of the Cyatheacese, the air-containing parenchyma appears on the
sides and on the dorsal surface in the form of round or oblong, sharply circum-
scribed, isolated groups, with a breadth and depth of a few millimetres, which con-
sist of a mass of cells shaped like many-rayed stars, leaving between them wide
lacunse, and covered by a portion of epidermis containing stomata. At first narrow in-
terstices containing air lead from the lacunse into the deeply-seated tissue. At a very
early stage of development, in the cases investigated before the petiole and lamina
begin to unroll and to unfold, the epidermis and the mass of stellate cells covered
by it die off, while the walls of the latter become thickened, and assume a yellowish
brown colour ; the dead and crumbling mass leaves a furrow behind, filled by its
powdery remnants, and this becomes sharply limited owing to the sclerosis of the
surrounding multiseriate cellular layer. The latter becomes attached, all round the
petiole, to the tough, sclerotic, brown, peripheral cortical layer, and appears in
the mature condidon as an indented portion of the latter. Comp. Figs. 140 and 141
(p. 292), or Fig. 189 at f'.

We need not enter here into further details of structure of petioles and ribs,
but may refer to the next chapter for some special points still to be brought forward.

Sect. 121. The space in the lamina of the leaf 'Vih.ich is left free by the ribs and
vascular bundles, is mainly occupied by parenchyma, which is simply called leaf-
parenchyma or in the special case of flat foliage-leaves Diachyma or Diploe
according to Link Mesophyll according to De Candolle In the little-developed
scales and cataphyllary leaves, especially those which are destitute of chlorophyll, it
shows no general anatomical peculiarities worthy of remark. In the case of the
green foliage-leaves special phenomena of structure and arrangement are known,
especially since the works of Treviranus‘ and Brongniart Organs of a different
morphological value, which in certain plants assume the functions of foliage-
leaves — Phyllodes, Phylloclades, Halms, &c. — behave essentially like leaves as
regards the structure of their parenchyma, as has already been to some extent
noticed above; for this reason they are likewise to be discussed here.

The parenchyma of the organs in question is, in great part at least, chlorophyll-
parenchyma^, in the sense of the word indicated at p. 116; its character and distribu-
tion primarily determine the conditions which are to be considered here. With
reference to these, two main types are to be distinguished, though they are united by
intermediate forms.

I. In the first type, which may be called the centric, the chlorophyll-parenchyma
is uniformly distributed around the entire organ— if intercalated bands of non-
equivalent tissue be left out of consideration. In flat horizontal parts slight
differences occur in relation to the upper and lower side.

To this type belong the leaf- like branches and ‘Halms,’ flat foliage-leaves
which are not horizontally placed, and many which are both flat and horizontal.

* Mettenius im Bericht d. 34, Versamml. deutscher Naturforscher, zu Carlsruhe, p. 99. — Von
Mohl, Baumfarne (/. o, see p. 291). — Karsten, Veget. Org. d. Palmen, l.c.

^ Philos. Botan. ed. 1, pp. 176, 188. ^ Organographie, I. p. 271.

‘ Verm. Schriften, I. p. 184; Physiol. I. p. 443.

® Recherches sur la structure et la fonction des feuilles, Ann. Sei. Nat. i ser. tom. XXI (1830),
p. 420, pi. 6-18.

® [Compare Haberlandt, Vergl. Anat. d. ass. Gewebesystems, Pringsh. Jahrb. vol. XIII. p. 74.]

PRIMARY PARENCHYMA. LAMINA.

407

The cells of the chlorophyll-parenchyma are arranged below the epidermis in
radial and tangential rows; between them there are always spaces filled with air,
usually forming narrow interstices. According to the particular cases, to be
exemplified below, their form is rounded- polyhedral, or elongated transversely, i. e.
parallel to the surface ; or elongated-prismatic, or cylindrical, and extended vertically
to the surface of the entire organ. According to their shape and arrangement the
cells of the latter form have been not inappropriately designated palisade-cells, or
palisade-parenchyma^ (comp. p. 117). These cells are of approximately equal height
in each of the layers parallel to the surface ; between their lateral angles are inter-
stices containing air, which either run without interruption along the whole angle,
or form those rows of narrow slits described at p. 211.

The seriate arrangement of the chlorophyll-parenchyma usually becomes less
regular at an increased distance from the surface; the palisade-form often passes
over into the roundish form.

According to the form in which chlorophyll-parenchyma, and in certain cases
non-equivalent masses of tissue, take part in the composition of the entire organ, two
principal modifications of the centric type are distinguished, between which here
also intermediate forms are found.

(fl) In many leaves the whole parenchymatous mass is chlorophyll-parenchyma;
towards the middle of the leaf this gradually becomes larger-celled, poorer in
chlorophyll, looser, and is often even traversed by large lacunae containing air. To
this category belong the leaves, consisting of a few layers of parenchyma, of species
of Potamogeton, Ranunculus aquatilis ; the leaves of Chamaerops, Copernicia,
Klopstockia, Physosiphon, Vanda, Cypripedium spec.; many Grasses, as Secale,
Elymus arenarius, Triticum vulgare ; Yucca filamentosa, with a strongly lacunar
central portion; species of Crassula (Fig. 180, p. 378), and Dianthus Caryophyllus.
The leaves of the species of Isoetes ^ are also to be mentioned here, with their four
air-passages distributed symmetrically over the cross-section, in •!. Hystrix and
Durieui extending to the epidermis (comp. sect. 51). The leaf of Acorus Calamus,
with round-celled chlorophyll-parenchyma in the periphery, and a widely lacunar
central portion, may here be mentioned as a transitional form to the second
modification. The flat leaves of the first modification described behave in many
respects intermediately between the second modification and the bifacial type.

((5) The entire organ is built up of a many-layered peripheral zone of
chlorophyll-parenchyma, and a dissimilar middle portmi or middle layer, more or less
sharply marked off from the former. In the leaf-like stems and halms, all of which,
according to what has been said above, belong to this category, this middle portion
consists of the ring or cylinder of bundles, to which may be added a surrounding
inner zone of large-celled cortical parenchyma, containing little or no chlorophyll
(e. g. Salicornia) ; the chlorophyll-parenchyma forms the peripheral cortical zone ;
e. g. the halms of Cyperaceae and Juncaceae, Acorus, Casuarina, Bossiaea, Miihlen-
beckia platyclados, Colletia horrida, Cacteae, &c. ; the Equiseta are also to be
mentioned here. In numerous foliage-leaves and phyllodes, especially those which

Schacht, Lehrb. II. p. 118.

^ A. Braun, Monatsber. d. Berliner Acad. 1863, p. 154.

PRIMARV ARRANGEMENT OF TISSUES.

408

are succulent and leathery, a middle layer is present, similar to the entire organ in
form, which fills up the internal space, and is enclosed by the chlorophyll-
parenchyma as by a cortex; in many species of AI06 (A. tesselata, cuspidata,
atrovirens, &c.) it breaks through the latter, as it were, in places, so as to reach
the epidermis h It consists, as a rule, of relatively large, colourless cells, destitute of
chlorophyll, which essentially contain water or sap ; in succulent plants, e. g. many
species of Aloe (A. soccotrina, plicatilis, arborescens), in other Monocotyledons and
in Mesembryanthemum, they contain abundant thin mucilage (comp. p. 116); in
sappy leaves they have soft walls, in tough leathery leaves thicker pitted walls. On
the course of the vascular bundles within, or at the boundary of the middle layer,
comp. sect. 92, p. 305, and sect. 112, p. 378.

Examples of this structure besides those already mentioned, are afforded by

Fig. 187. — Cross-section through the leat of Welwitschia mirabilis (40). 0 epidermis of the upper, u of the
lower surface ; in their depressions are stomata. ^ trunks of vascular bundles, small branch bundle ; on the
trunk marked a similar one is shown branching off. .r thick sclerenchymatous fibres containing crystals.

Hooker’s spicular cells; yhypodermal and internal strands of long sclerenchymatous fibres; c — c delicate
palisade-parenchyma, containing chlorophyll. Between c and c is the wide-meshed tissue of the middle layer.

Compare Fig. 55, p. 132, Fig. 145, p. 303, and Fig. 157, p. 335.

the foliage-leaves of many Coniferae, e. g. species of Podocarpus, Araucaria with
flat leaves, Pinus (Fig. 185, p. 381), the leaf of Welwitschia (Fig. 187), the leaves of
Myrtaceae (Callistemon, Eucalyptus Gunnii, Melaleuca tetragona, linearifolia), Pro-
teaceae (Hakea spec.), Statice monopetala, purpurea ; the phyllodes of Oxalis
fruticosa and many Acaciae, &c. ; lastly, numerous leaves of Monocotyledons.

Among the special phenomena, which are extremely various in different species,
we have first to mention, with reference to the middle layer, that in many flat leaves
of Monocotyledons this is divided into as many longitudinal bands as there are
longitudinal bundles running through the leaf (p. 301), each of these bundles

* Pfitzer, in Pringsheim’s Jahrb. VIII. p. 64, fig. 20.

^ [Compare Briosi, Contribuzione alia Aiiat. d. foglie. Fe/. Bot. Centralbl. Bd. ii, 1882, p-55.]

PRIMARY PARENCHYMA. LAMINA.

409

being inserted in a lamella of round-celled parenchyma containing chlorophyll;
these lamellae unite the chlorophyll-layers of the two surfaces of the leaf, and stand
at right angles to the latter ; e. g. Hemerocallis fulva, Narcissus pseudonarcissus,
Hyacinthus orientalis, Ornithogalum umbellatum, Phormium tenax, many Iridese,
Bambusese^, &c. Secondly, in the leaves of many Monocotyledons, narrow-leaved
Eryngia, Lobelia Dortmanna, &c., the middle layer is soon destroyed to form lysi-
genetic air-passages. In the round-leaved species of Allium and Asphodelus, in
which the middle of the leaf is destitute of vascular bundles, a wide tubular cavity
traversing the leaf arises in this way; in leaves where the vascular bundles are in-
serted in persistent plates of parenchyma, there are several passages alternating with
the latter; they are numerous in the Narcissus mentioned above, and in Grasses,
Cyperaceae, Sparganium, Typha, Pandanus, and Eryngia; in Lobelia Dortmanna
there is one on each side of a median persistent plate.

In tough leaves of Monocotyledons, the more or less complete interruption of
the chlorophyll-parenchyma by longitudinal strands of fibrous sclerenchyma, usually
containing the longitudinal vascular bundles, is also of frequent occurrence, as
will be described in the following chapter ; and there are many other special
phenomena of alternately dissimilar longitudinal bands, which respectively contain,
or are destitute of bundles, e. g. in the leaves of Grasses, large Bromeliaceae, &c.,
the detailed description of which in this place would lead us too far.

With regard to the chlorophyll-parenchyma are to be mentioned, as examples of
the palisade form, the leaves of the Myrtaceas mentioned, Proteaceae, species of
Statice, Welwitschia, stem of Casuarina, Salicornia herbacea, Colletia horrida,
Scirpus Holoschoenus, lacustris, palustris, Papyrus, Juncus effusus, &c. The leaves
of many Monocotyledons, Mesembryanthema, the stems of Bossiasa, Miihlenbeckia
platyclados, Cacteae, Juncus glaucus, &c., may be mentioned as affording examples
of the round-celled form.

Cells elongated in the transverse direction, i. e. parallel to the surface, are
peculiar to the sword-shaped leaves of many Iridese, as Iris Germanica, Gladiolus
imbricatus, and especially Tritonia deusta. In species of Pinus and Cedrus the cells
of the chlorophyll-parenchyma are characterised by the tabular polyhedral form,
and the infolding of the walls, mentioned above at p. 118. (See Figs, ii, p. 35,
and 27, p. 78.)

2. To the second type belong only flat horizontal leaves, and in fact the
majority of them. It is characterised by the fact that the chlorophyll-parenchyma
forms the entire substance of the leaf, and is severed into two different layers, each
of which corresponds to one surface of the leaf. It may accordingly be termed the
two-sided, the bifacial type. The difference between the two horizontal layers may
be generally expressed by the statement, that the one, that namely belonging to the
surface which is turned towards the light, is denser than the other, being furnished
with less wide interstices, and therefore appearing of a darker green, even when the
amount of chlorophyll in each individual cell is the same.

The usual form is such that the denser layer consists, according to the thickness
of the leaf, of one or more layers of palisade-cells, the other layer consisting of

* Kareltschikoff, l.c. ; compare p. 118.

410

PRIMARY ARRANGEMENT OF TISSUES.

many-armed or lamellar cavernous parenchyma (comp. p. 211), which from this
spongy character has also been called ‘ spongy parenchyma.’ Yet we often find
the cells of both layers of irregular form, and differing only in the size of their
protrusions, and of the air-cavities thus produced, e. g. in many leaves of Ferns,
as Scolopendrium vulgare, Aspidium falcatum, Filix mas, &c. It is superfluous to
mention further examples, but we may refer to the flat foliage-leaves of Dicotyledons
and Monocotyledons (Lilium bulbiferum, Aroideae, &c.), which are dark-green above,
and dull-green below.

The dense parenchymatous layer is as a rule less thick than the lacunar one,
in many leathery leaves scarcely half as thick, e. g. Malpighia macrophylla. The two
are usually sharply marked off one from another, yet there are also cases of quite
gradual transition. In the leaves of species of Podocarpus, Cunninghamia sinensis.
Sequoia sempervirens, Cephalotaxus, and in many Cycadese, as Encephalartos and
Zamia integrifolia, the dense, more or less decidedly palisade-like layer is internally
bounded by a few layers of loose parenchyma, consisting chiefly of transversely
elongated cells containing chlorophyll, which must be distinguished from the trans-
verse elements of the border of tracheides (p. 380) h

As mentioned above, the dense parenchymatous layer always corresponds to the surface
turned towards the light, the loose layer to the other side. The former is, as a rule,
both morphologically and actually the upper surface of the leaf, the latter the lower.
These conditions are reversed in the case of the upright leaflets with the edge rolled
inwards of Passerina hirsuta filiformis, and ericoides, where lacunar parenchyma lies on
the densely hairy concave upper side, and remarkably dense palisade-parenchyma on the
convex lower side. A similar relation exists, though in a less marked manner, in the
leaves of Juniperus communis and nana®. In the flat foliage leaves of Allium ursinurn,
Alstroemeria, Geitonoplesium, Eustrephus, and also in many Grasses, the morphologically
upper side becomes turned downwards, by torsion of the petiole or of the base of the
leaf ; this side here has the loose parenchyma, while the side which actually faces upwards
has the dense tissue

On the relation of the smaller vascular bundles to the two layers of bifacial leaves,
comp. p. 305.

As has already been indicated, intermediate forms between the main types distinguished
are not wanting, and among them the greatest variety in detail prevails.

As regards the distribution of the two main types according to the forms of leaves and
systematic divisions, no general rule, as far as our present knowledge extends, is to be
added to the few that have been adduced above. The principles brought forward that
leaves which are not flat and not horizontally placed always have centric chlorophyll-
parenchyma, while the latter is bifacial in horizontal and flat leaves only, hold good in
this form for all the groups and families which have come under consideration. They
cannot however be extended further, nor does the converse hold good, for among flat
and horizontal leaves the greatest differences occur in closely related forms, e. g. bifacial
structure in Dianthus barbatus, Statice latifolia, Melaleuca hypericifolia. Eucalyptus pul-
verulenta; Allium ursinurn, Epidendron ciliare; centric structure in Dianthus Caryo-

‘ See Thomas, Pringsheim’s Jahrb. IV. p. 37. — Kraus, ibid. pp. 323, 333, &c.

^ Caruel, in Nuov. Giorn. bot. Italiano, I. p. 194. Already indicated in De Candolle, Organo-
graphie, I. p. 274. Compare above, p. 49.

® Thomas, /. c. p. 39.

* Treviranus, Physiol. I. p. 445. — Irmisch, Knollen- und Zwiebelgewächse, p. 4. — A. Braun,
Botan. Zeitung. 1870, p. 551.

PRIMARY PARENCHYMA. LAMINA.

411

■ phyllus and plumarius, Statice purpurea, Melaleuca linearifolia, Eucalyptus Gunnii ;

Allium nigrum, and the thick-leaved Epiphytic Orchideae, mentioned above at p. 407.

In addition to the parenchymatous masses described above, there is present in
many leaves, and especially in tough ones, a single- or many-layered Hypoderma
which is, no doubt, always a continuation of the equivalent layer of the stem or petiole.
It consists sometimes of elements to be described in the next chapter, sometimes, as in
many cases belonging to this category, of thin-walled aqueous parenchyma, the cells
of which are in uninterrupted connection with one another and with the epidermis, and
in many-layered hypodermata increase in size towards the inside, thus corresponding
in all these relations, as well as in the nature of their contents, to the elements of a
many-layered epidermis, from which they are only to be distinguished by their origin.
Comp. p. 32, and Fig. 44, p. 104. They thus form a strengthening layer to the
epidermis. As regards the nature of their walls they often approach the collen-
chyma, so that in many cases a sharp distinction from the latter is impracticable, and
in this respect also they correspond to many epidermal ceils. In the leaves of the
Pleurothallidese, mentioned below, their walls have reticulate or spiral thickenings,
in Physosiphon, the Bromeliaceas to be mentioned, and in ^schynanthus the lateral
walls vertical to the surface of the leaf are folded, with undulations parallel to
the latter.

INIany tough Fern-leaves afford examples of this phenomenon, as Polypodium
Lingua, Aspidium coriaceum ; many Commelineae, e.g. species of Tradescantia ; all
Scitaminese where the point has been investigated, as Musa, Strelitzia (Fig. 29,
p. 85), tieliconia, Canna, Costus sp. ; many Palms, as Chamserops, Caryota, &c. ;
many Grasses, e.g. Arundo Donax ; many Bromeliacese (Bromelia Caratas, Pholi-
dophyllum zonatum, .^chmea, Hechtia, Tillandsia spec.), Orchideae of the group
Pleurothallideae : Pleurothallis ruscifolia, Lepanthes cochlearifolia, Physosiphon
Loddigesii, Octomeria graminifolia, Stelis spec. ; species of Pandanus ; Aroide^
(Philodendron cannifolium, Anthurium membranuliferum) ; ^schynanthus spec. ;
Veronica speciosa, Lindleyana ; Stenocarpus sinuatus ; Mahonia Fortunei ; Quercus
glabra; Ilex aquifolium, ovata, and other species; Rosmarinus officinalis, Nerium
Oleander, &c. &c.^

The hypodermal layers in question consist of one, of several, or of many strata
of cells ; they are as a rule extended uniformly over the whole surface of the leaf,
and only interrupted at the stomata — either by a simple intercellular air-cavity
(e.g. Fig. 29, Strelitzia), or by chlorophyll-containing parenchymatous cells sur-
rounding such a cavity. On the upper surface of the leaf they are usually more
bulky than on the lower, or they are exclusively present on the former. In many
Bromeliacese (B. Caratas, Hohenbergia strobilacea), Orchideae, and .Plschynanthus,
they there attain a thickness which amounts to J or even more than | of the entire
thickness of the leaf. In the leaves of many Grasses, as Arundo Donax, the thin-
celled hypoderma is limited to narrow longitudinal bands, alternating with the
vascular bundles, and covered by large, bladder-like epidermal cells ; on the upper

* Kraus, Cycadeenfiedern, l.c. — Pfitzer, Pringsheim’s Jahrb. VIII.

* Compare Treviranus, Physiol. I. p. 450. — Thomas, Coniferen- blatter, l.c. — Kraus, Pfitzer, l.c.

413

PRIMARY ARRANGEMENT OF TISSUES.

side of the lamina of the leaf of Chamaerops humilis it is thick, large-celled and
many-layered over the main ribs, single-layered and small-celled elsewhere.

Sect. 122. The primary cortical mass of the rooi^ consists as a rule ex-
clusively or chiefly of parenchyma, the cells of which, corresponding to the original
arrangement of the meristem, remain arranged in vertical longitudinal rows. (Com-
pare pp. 9-13.)

In the roots of Phanerogams the cortex is more or less sharply severed into two
layers, an outer and an inner. The former consists of cells which are usually,
though not always, narrower, and are uninterruptedly, or at least very closely, united ;
in the thicker roots they show, as seen in cross-section, a polyhedral form, are
arranged in several concentric, but not in accurately radial rows, and not unfrequently
have somewhat thick, collenchymatous walls, which often become sclerotic. In
thin roots, e. g. Hordeum, Elodea, Lemna, &c. it is a single hypodermal layer of
cells. The inner layer consists in very thin roots, as in those of Lemna minor,
of two concentric strata of cells as a minimum, usually of several such strata, while
in thick roots they are very numerous. (Fig. 168, p. 360.) The innermost of them
is always the endodermis, surrounding the axial bundle. Further, as often mentioned
in former paragraphs, the cells of the successive strata are as a rule permanently
arranged in radial rows, which are very regular, especially in Monocotyledons.
Between their rounded corners they leave spaces containing air, which have the
greatest average width in the middle portion of the layer. The larger lacunae and
passages containing air which occur in roots (comp. Sect. 51) belong to the inner
layer, or in thick roots to its middle portion. The fibrous thickenings on the walls,
described at p. 117, as occurring in Orchideae and Coniferae, always belong prin-
cipally or exclusively to the inner parenchymatous layer of the root.

The successive concentric strata of cells of the inner layer arise, wdth few
exceptions, from the initial cells by tangential divisions in centripetal order, in Acorus
Calamus and other Aroids centrifugally ; in several cases (Zea, Helianthus, Palms)
the later tangential divisions appear in less regular succession. Where the whole
cortex proceeds from a single initial layer, the first tangential division of the latter
(leaving the epidermis out of consideration) seems always to cut off the outer layer,
which then either remains undivided, or undergoes a few further tangential divisions,
which in Stratiotes take place in strictly centrifugal succession. In Linum (p. 12)
a special initial layer for the single external layer of the cortex extends round the
meristematic apex. In the type of root described at p. 12, with a transverse
common initial zone at the growing-point, the succession of the tangential divisions
likewise appears not to be quite regular.

The single layer of cells bordering on the epidermis, which, as already stated above, is
often distinguished by a special structure, and always by uninterrupted lateral connection
of its cells, is named endodermis by Nicolai. As in the present work this word has been
used for layers of cells which, without reference to the place of their occurrence, are
characterised by a definite structure, this term cannot be maintained. According to its
position and origin, this layer, which in agreement with the terminology here constantly

* Compare the works of Nicolai, Nageli and Leitgeb, Van Tieghem, Janczewski, &c., which
are cited at pp. 7, 351, 356. — Reinke, in Hanstein’s Botan. Abhandl., Heft 3. [Also, Olivier, Re-
cherches sur I’appareil tegumentaire des Racines. Ann. Sei. Nat. .Ser. 6, Tom. ii, 1881.]

PRIMARY PARENCHYMA. ROOT.

413

used is to be called the hypodermal layer, is in most cases equivalent to the outer endo-
dermis of the aerial roots of orchids (pp. 125 and 227). Its structure also appears in many
roots to be actually that characteristic of the endodermis, as in Elodea according to Nicolai,
and also in Acorus Calamus, so that the occurrence of a hypodermal endodermis would
thus have a wider distribution than was stated above. Further investigations of this
point are to be undertaken, for the existing data do not appear to me to afford sufficient
certainty. The roots of Orchidese investigated also have a denser, smaller-celled paren-
chymatous layer below their endodermis, and in the case of most roots the endodermal
structure of the hypodermal layer is at least very doubtful.

Corresponding to the conformity prevailing in the other conditions, and apart
from differences in the first origination, and in the phenomena of sclerosis to be
described later, the structure of the cortex of the root in Filices, Hy dr opteridece and
Equiseta, is similar to that in the Phanerogams. In every sextant of the apical me-
ristem (p. 18) the initial cell of the periblem, which after severance of the plerome-
cylinder and the epidermis remains between the two, is divided by a tangential wall
into an outer cell which is the initial cell of the outer cortical layer, and an inner initial
cell of the inner cortical layer ; the former increases the number of concentric layers
by tangential divisions advancing chiefly in centrifugal direction, the latter by those
advancing in centripetal direction ; the number of members of each layer increases
by radial divisions. In the Filices and Hydropterideas it is the innermost layer of
cells, in the Equiseta it is the second layer from inside which becomes converted into
the endodermis. In thin roots the tangential divisions are not very numerous, the
radial ones in many cases are even wholly omitted in the inner cortical layer, so that
the axial bundle, as seen in cross-section, is only surrounded by six endodermal
cells, the latter by the same number of cortical cells, which are relatively very large.
(Fig. 169, p. 363.)

A noticeable phenomenon, which is apparently of frequent occurrence among the
Polypodiaceae and Osmundaceae, is that the entire cortex soon begins to show dark
brown membranes, and uninterrupted connection between them, although the cells
do not cease to contain starch, a condition which, however, is by no means common
to all Ferns, e. g. it is absent in Scolopendrium, Ophioglossum, Marattiaceae, &c.
The species of Isoetes and Selaginella and the Lycopodiaceae agree in the main with
the forms described, as regards the conditions here under consideration. For details
and the history of development we may refer to the special works cited.

The root cap always consists of parenchymatous cells, which in the younger stages
contain abundant starch, and are uninterruptedly connected one with another. In old
age they die off, as is well known, successively, in many cases after breaking up into
layers or rows (exfoliation), a phenomenon which has its anatomical basis in the fact
that the limiting layers of the walls become disorganised to an amorphous mucilage.
In most roots the number of the successive layers of the cap is large, and the layers
as they peel off are constantly replaced by renewed ones, the end of the cap having
the conical form represented above, pp. 9 and 12. In other cases the renewal of the
layers soon ceases ; e. g. according to Caspary and Nicolai in the Nymphaeaceae,
.älsculus, Najas, Lemna, Hydrilla, and Hyacinthus ; and according to janezewski in
Pistia and Hydrocharis. In the Nymphseaceae, except Nuphar, the cap is persistent,
in the other cases mentioned, especially in the thinner, few-layered caps of the
water-plants mentioned, the layers successively die and peel off. The main root of

414

PRIMARY ARRANGEMENT OF TISSUES.

Trapa natans has no cap, although, according to Reinke, it shows at least an in-
dication of one in the form of isolated tangential divisions of the epidermal cells.

Sect. 123. As often had to be mentioned in former sections, the limiting
layers, where parenchymatous masses adjoin non-equivalent groups of tissue, are
often developed in the form of distinct layers, which stand to the latter in the
relation of sheaths. The hypodermal layers of foliage-leaves already described, as
well as the limiting layers between the sharply-bounded central portion and the chlo-
rophyll-parenchyma of centrically constructed leaves, belong essentially to this cate-
gory (comp. Fig. 185, p. 381). But the endodermis of many aerial roots, already
considered in Sects. 27 and 56, and the parenchymatous sheaths, which stand in
immediate relation to the vascular bundles or to definite groups of them, still require
to be especially mentioned here. According as the one or the other of these latter
relations exists, and thus, according to their position, we have first to distinguish
those parenchymatous masses, each of which surrounds one vascular bundle, — and then
frequently also the fibrous strand which may accompany it;— and secondly others,
which mark off the bundle-ring or cylinder from the surrounding tissue. The de--
signation of the two forms distinguished according to their position, follows obviously.
Inasmuch as in roots and stems, the axial vascular bundle, or the bundle-ring or
cylinder, corresponds to the primary plerome, while the surrounding parenchymatous
sheath answers to the innermost layer of the periblem, the name plerome-sheath ^ is
appropriate for the latter, even in cases possibly occurring in which the name chosen
according to the primary differentiation of the meristem may not apply quite exactly.

An internal sheath, occurring on the side of the bundle-ring next to the pith,
exists only in some species of Equisetum, mentioned at p. 122.

The plerome-sheath of the stems of Phanerogams also lies outside the fibrous
strands accompanying the peripheral bundles of the cylinder, where such strands
are present. According to Van Tieghem^ it is always separated from these or from
the phloem-portions of the vascular bundles by a single or few-layered zone of
parenchyma, and this forms the continuation of the pericambium of the main root,
while the plerome-sheath itself is continued directly from the latter into the stem.

Sheaths of the plerome and ring, as well as those of single vascular bundles,
appear in two principal forms, namely in the form of the endodermis, or in that
of a single layer of cells, which agrees with the endodermis in the close lateral
connection of its elements, but is destitute of the characteristic structure of its walls,
being only distinguished from the surrounding tissue by less conspicuous differences
of cell-form, and by permanently containing abundant small starch-grains. From
its latter characteristic it has been called the starch-layer by Sachs. That the endo-
dermis is also often distinguished by containing abundant starch, was already stated
at p. 125. Other peculiarities of the cell-contents, as abundant tannin, characteristic
pigments, &c., also frequently occur in both cases. Apart from the close relation be-
tween the starch-ring and the endodermis already indicated by the above statements,
forms occur as to which it is doubtful whether the one or the other term is appro-
priate, and which are thus intermediate, e. g. the plerome-sheath in the stem of

' Sachs, Textbook, p. 124, 2nd English edition.
^ Ann. Sei. Nat. 5 ser. tom. 16, p. 1 12.

PRIMARY PARENCHYMA. SHEATHS.

415

Lactuca virosa, and the bundle-sheath in the stem of Ranunculus fluitans ; comp, the
explanation of Fig. 153, p. 332. The two forms of sheath may mutually replace
one another in the same region, according to the species ; the plerome-sheath, for
example, appears in the hypocotyledonary stem of Helianthus annuus as a starch-
layer, in that of Tagetes patula as an exquisite endodermis. The thin vascular
bundles, especially the bundle-ends of the foliar expansions, are usually surrounded
by a parenchymatous sheath different from the two distinguished above, and con-
sisting of elongated elements passing over gradually into the epithema at the peri-
pheral ends. (Comp. Sect, in.)

The sheath of single trunks of vascular bundles appears in the form of an
endodermis, around the axial bundles of roots and of most stems which have them,
around the bundles of almost all Ferns, and those in the stem and leaves of certain
Phanerogamic plants. To the cases and descriptions already given in Sect. 27, and
Chap. VIII, we have here to add the bundles of the leaves of Hottonia, Cortusa,
Dodecatheon, Cyclamen, Soldanella, Trientalis^; those of the stem of Caltha
palustris, which, according to Russow, are surrounded by a wholly or partially
sclerotic endodermis ; and especially, according to the statements of the latter
author^, and of Schwendener®, the bundle-trunks in the leaves of Cyperacese,
Juncaceae, and Grasses, in which the endodermis, which usually soon becomes
thickened by sclerosis, lies between the vascular bundle and the strand of scle-
renchyma ensheathing it.

An endodermis ensheathing the whole ring of bundles is present both on the
cortical and the medullary side of the latter, in the species of Equisetum enumerated
at p. 122. In other species of this genus, e. g. E. palustre, Fig. 149, p. 329, it only
extends around the cortical side of the ring of bundles, and the same is the case
with the endodermal plerome-sheath which surrounds the ring or cylinder of the stems
of Phanerogams. As selected examples for the occurrence of this, have already
been mentioned (p. 121) the stems of Cobsea, Tagetes, Lobelia spec., the rhizomes of
Scitamineae, Cyperacese, &c. According to the investigations of Dr. v. Kamienski, which
have been privately communicated to me, numerous Dicotyledons are to be added
to this list ; Linaria, Pedicularis spec., Camelina, Capsella, Atriplex patula, Euphorbia
spec., Mercurialis, all Primulaceoe ; and according to Vöchting* the Melastomacese.
Further investigations will have to decide how far the endodermal structure of the
layer surrounding the plerome and vascular bundles in stems and leaves generally
is of usual occurrence.

The plerome-sheath is developed in the form of the starch-ring or starch-layer^ in
the stem of most Dicotyledons, as far as can be decided from the existing investigations.
Sachs, for example, finds this to be the case in seedlings, especially in the hypo-
cotyledonary stem of Helianthus annuus, Cucurbita, Phaseolus, Iberis, Raphanus,
Prunus, Amygdalus, Convolvulus, Quercus, Acer, Ricinus, (comp. Fig. 154, p. 333),
in the stolons of the potato, and in the mature stem of Dahlia and Ricinus. The starch-
layer is in these cases either uniformly developed around the whole ring of bundles,

* Kamienski, private communication. ^ Vergl. Unters, pp. 169, 170.

2 Das median. System, p. 17. * l.c.\ compare p. 259.

“ Sachs, Botan. Zeitg. i8,t;9, p. 177, Taf. VIII. IX ; Pringsheim’s Jahrb. III. p. 194.

4t6

PRIMARY ARRANGEMENT OF TISSUES.

e.g. Fig. 154, or it only contains starch opposite the vascular bundles; e. g. in the
stem of Brassica oleracea; or only opposite the medullary rays of the ring (shoots of
Atragene alpina). In the latter cases, the layer, in so far as it contains no starch
grains, is difficult to distinguish.

A sheath developed as a starch-layer is present around the single bundle-trunks,
and then also embraces the accompanying fibrous strand, in the leaves of Dicoty-
ledons, and in the stem and sheath of the leaf of Monocotyledons (e. g. Grasses, as
Triticum, Zea). In the former cases it embraces the outer edge of the bundle, in
the Monocotyledons the inner (comp. Fig. 151, p. 331).

As regards the sheaths of the ultimate ramifications and ends of the vascular
bundles, the latter, as already stated in Sect, in, are surrounded in the foliar
expansions of Angiospermous plants by a layer of parenchymatous cells elongated
in the direction of the course of the bundle ; these cells are closely attached to the
bundle, and are laterally connected one with another without interruption. Their
surfaces remote from the bundle often border on the air-containing interstices of
the surrounding parenchyma, and where they abut on many-armed lacunar tissue
their walls are often folded like those of the latter. In many leaves they resemble
the surrounding cells in containing abundant chlorophyll, e.g. Fuchsia, Papaver,
Primula sinensis, Zea, and Triticum (comp. Figs. 173, 175, 178, pp. 372-375); in
thick leaves they often contain little or no chlorophyll.

CHAPTER X.

ARRANGEMENT OF THE SCLERENCHYMA
AND SCLEROTIC CELLS.

Sect. 124. As shown by their anatomical characteristics, and as Schwendener
has demonstrated, sclerenchyma and sclerotic cells form the strengthening apparatus
of the plant; the former is specifically and almost exclusively adapted to the
mechanical function mentioned, the latter serves other functions simultaneously in
different degrees ; the two are connected one with another by various transitional
forms. Accordingly they may both bear a common name, viz. stereides, or steren-
chyma ; and the groups they form may with Schwendener be termed stereome. The
collenchymatous masses also are immediately related to the sterenchyma as transi-
tory strengthening apparatus which often passes over into sclerenchyma, and hence
we must often take them into consideration in treating of the latter. The same ap-
plies to sclerotic endodermal layers, which have been discussed in former chapters.

Schwendener, in his excellent work which has been so often mentioned ’, has
treated both the arrangement of the sclerenchymatous masses and their physiological
relations so minutely and comprehensively, that any detailed exposition of the former,
within the space here available, must be a mere extract from his work, or must seem
like one. I therefore think it best to refer once for all to the book mentioned,
merely giving short indications respecting the masses of sterench)'ma which serve to
strengthen entire organs, as stems, petioles, &c., and only entering more minutely
into some of the points which have received less attention in Schwendener’s work.

In the vegetative organs of the Phanerogams, to which, among the groups of
Filicineae, the Equiseta and the non-aquatic species of Isoetes are in this respect
especially related, the sclerenchyma is usually more sharply differentiated throughout
than in the majority of the Ferns. The latter may therefore be treated subsequently
by themselves, and for the present be left out of consideration.

Sect. 125. Sclerencliymatous fibres, or sclerotic elongated cells which
approach them very nearly, occur generally in parts which are exposed to bending or
tension, and are united to form strands, layers, or sheaths, which traverse the entire
organ longitudinally.

As regards their arrangement, we find in the first place strands and layers in
the hypodermal region, partly in direct contact with the epidermis, partly separated
from it only by one or a few layers of cells. It is a phenomenon of wide occurrence

* Das mechanische Princip, &c., Leipzig, 1874. [See also Habeiiandt, Entwickelungsgeschichte
des mechanischen Systems; 1879. — Westermaier, Z. Kenntniss des Meehan. Gewebesystems; Bot.
Ztg. 1882, p. 174.]

E e

PRIMARY ARRANGEMENT OF TISSUES.

41 8

that both in angular and round stems and petioles longitudinal hypodermal bands
of fibres are found alternating with chlorophyll-parenchyma ; the former project
toward the inside in a more or less convex form or as a strong ridge. This is the
case in numerous ‘halms' of Cyperacese, species of Juncus, and Panicum turgidum.
In angular or channelled stems of Dicotyledons, the corresponding longitudinal
bands are usually collenchymatous ; they become sclerenchymatous in many Um-
belliferse (Chaerophyllum bulbosum), Papilionaceae (Spartium monospermum), and
the Casuarinae ; further in Ephedra and the Equiseta The same phenomena
appear in very wide distribution in tough hard leaves. Thick hypodermal ridges
of sclerenchyma, often projecting deeply inwards, traverse the lamina in Cype-
raceae, Typha, Sparganium, Dasylirion, Phormium, Palms, &c.^; in conjunction
with the vascular bundles inserted in them, to be mentioned again below, they often
form in flat leaves vertical plates passing through from one surface to the other.
Similar conditions are shown by the ribs of many tough Dicotyledonous leaves, as a
fibrous bundle projects into them from one or both sides, upon or between which the
vascular bundles are attached, e. g. Eriobotrya japonica, Theophrasta ornata, Laurus,
Passerina filiformis, Rosmarinus, and many others. Here also intermediate forms
between collenchyma and sclerenchyma frequently occur. Numerous plates of
sclerenchymatous fibres projecting deeply inwards on both sides traverse the leaf of
Welwitschia (Fig. 187, p. 408). In the leaves of the terrestrial species of Isoetes®
a hypodermal strand of fibres passes along the anterior and posterior surface of the
middle line, and in each of the two marginal angles ; between these four, additional
smaller ones may appear according to the species.

In most stems and leaves the fibrous strands are sharply limited laterally
towards the heterogeneous bands alternating with them, quite up to the epidermis.
In other cases they are connected with one another by means of a continuous hypo-
dermal fibrous layer consisting of a few strata, into which their lateral edges are
extended, and this layer follows the entire inner surface of the epidermis, and is
only interrupted at the stomata, e. g. stem of Equisetum hiemale, and Casuarina.

To the latter arrangement are related the hypodermal fibrous layers, often
present in tough leathery leaves, which, with an average thickness of a few strata of
cells, run with approximate uniformity around the entire surface, becoming thicker
at the edges and angles, but constantly interrupted at the stomata, and rarely by
small gaps at other places. Such fibrous layers * occur in numerous tough leaves of
Bromeliaceae ® (Ananassa, Bilbergia zebrina, Bromelia Caratas, &c.) ; in the leaves of
certain Orchids (Vanda furva, Renanthera coccinea) ; in the pinnae of many Cycadese,
e.g. Cycas, Encephalartos, extending sometimes all round, sometimes only on the upper
side; and in most leathery leaves of Conifers ® (Fig. 183, p. 380; Fig. 185, p. 381; and

‘ Compare Pfitzer, in Pringsheim’s Jahrb. VIII. p. 60. — Mettenius, Hymenophyllacese, p. 439.
. — Jochmann, Umbelliferarum structura, p. 8.

^ Compare Pfitzer, I. r.— Mohl, Palm, struct. Tab. K, b. — Karsten, Veget. Org. d. Palmen.

^ A. Praun, Monatsber. d. Berlin. Acad. 1863, p. 588.

* Cf. Pfitzer, i.c.

^ Mohl, Verm. Schriften, p. 265. Taf. X.

“ Compare Kraus, Cycadeenfiedern, I.c. — Von Mohl, Botan. Zeilg. 1871, p. 7. — Thomas, in
Pringsheim’s Jahrb. I. c.

SCLERENCHVMA AND SCLEROTIC CELLS.

419

Fig. 27, p. 78). They form a thick uninterrupted layer on the upper side of the leaf
of Jacquinfa r.uscifolia, an often interrupted layer on the lower side of this leaf, and
on both sides of that of Theophrasta ornata and species of Olea. This form of the
strengthening apparatus is absent, however, in most leathery leaves, and in fact
this is the case even in such species as are closely related and similar to those
mentioned, e. g. most leathery leaves of Orchids, Pholidophjdlum zonalum, Zamia
integrifolia, Taxus, Cephalotaxus spec., Tsuga Canadensis, Abies amabilis, &c.

A continuous two- or many-layered fibrous investment further appears in the
aerial roots of Philodendron Imbe, Rudgeanum, and other species k In the
Cyperacese, e. g. species of Carex, the outer layer of the cortex of the root is often
sclerotic in a high degree, and throughout many strata of cells.

The second form of distribution of sclerenchymatous masses is that in which
they lie at a greater distance from the epidermis, in the inner regions, being united
so as to form either a continuous annular layer or isolated strands.

The former arrangement occurs in a number of stems, in such a form that the
fibrous ring lies in the external cortex, bordering within and without on parenchyma-
tous layers : shoots of Berberis vulgaris ; Caryophylleae, as Dianthus plumarius,
Gypsophila altissima, Silene Italica; Cucurbitacese ; and climbing Aristolochiae, as
A. Sipho Usually, however, the fibrous layer lies on the outer border of the
bundle-ring or cylinder, in such a manner that it includes the vascular bundles, or
in the latter case the outermost of them, or that they rest against it. In this case,
and in Berberis and doubtless also in the other plants mentioned with it, the ring
belongs, according to its origin, to the outer layer of the plerome, it marks, more or
less sharply, the outer boundary of the plerome. This phenomenon is of most
frequent occurrence in Monocotyledonous stems. It occurs in the halm of many
Grasses and of many Cyperaceae and Juncacese, and in fact sometimes in combina-
tion with the occurrence of hypodermal fibrous ridges, which, as processes of the
ring, unite the latter with the epidermis, e. g. Piptatherum, Molinia, Bromus spec. ^ ;
or penetrate from the outside close up to the ring without reaching it (Alopecurus
pratensis, Panicum turgidum, Juncus paniculatus) ; or, lastly, show sometimes one,
sometimes the other condition (Cladium Mariscus). Other plants belonging to the
families mentioned show the sclerenchymatous ring connected only with isolated
ridges of the epidermis, or without this connection, and only with projecting ribs, which
correspond to the insertion of vascular bundles, on its outside ; e. g. Rhynchospora
alba, Juncus bufonius, Pennisetum longistylum. To the latter cases is related the
smooth and sharply limited sclerotic ring, which, with numerous individual modifica-
tions, forms the outer boundary of the cylinder, and includes or supports the
peripheral vascular bundles in most Monocotyledonous foliage-stems : Restiacete,
Eriocaulone* paitly, Commelinete, IMelanthaceae, Liliaceae, Smilacese, Tamus, Irideae,
Orchideae, Alismaceae, Typhaceae, &c. ; and in most rhizomes, also belonging to the
families mentioned above.

The same phenomenon of a sclerenchymatous ring directly supporting or

’ Van Tieghem, Struct, des Aioidees, l.c.

^ Compare Treviranus, Physiol. I. p. 209. — Caspary, Pringsheim’s Jahrb. I. p. 444. — Sanio,
I’otan. Zeitg. 1864, p. 222. — V.on Mohl, Palm. .Struct. Tali. H. — Metteuius, l.c.

® Compare Schmitz, l.c. (p. .^93). ‘ Compare Schwendener, l.c.

E e 2

420

PRIMARY ARRANGEMENT OF TISSUES.

including the bundles, recurs in numerous Dicotyledonous stems : Caryophylleae, e.g.
Silene catholica, Saurureae, Podophyllum, species of Thalictrum (also in the petiole),
Papaver, Plantago, Trientalis, Hypochseris radicata, &c. ^

The converse case, that a continuous layer of sclerenchyma supports the whole
inner side of the ring of vascular bundles, is rare in Dicotyledonous stems. This is
the case in woody Piperaceae — Artanthe, Chavica spec. and is especially characteristic
in the shoots of Bougainvillea spectabilis.

In tough firm organs, longitudinal fibrous strands, which are not distributed
in the hypoderma, and which in the whole or a part of their course stand in no direct
relation to the vascular bundles, are more frequent than continuous annular layers.
Examples of this are afforded by the cylindrical or prismatic strands in the
parenchyma of the leaf and petiole of Marantacese, Palms, Dracsense, and Pandanus,
which, as regards their position, are connected with the hypodermal strands by various
transitional forms ; e.g. the strands in the interior of the leaf of Welwitschia (Fig. 187),
and in the cortical parenchyma of Ephedra, the strands in the internodes of many
Potamogetons, mentioned at pp. 232 and 271, and represented in Fig. 171, p. 369 ;
the little bundles occurring in the parenchyma of the stem-cylinder of many Palms (As-
trocaryum. Cocos, Leopoldinia, Lepidocaryum spec. between the vascular bundles
and those which traverse the cortex of most Palm-stems ; the numerous strands
in the rhizome of Acorus in the cortex of the root of Phoenix, Cocos spec. and of
the Pandaneae, and in the axial cylinder of thick roots of the same (comp. p. 361) and
of the Iriarteae.

In many of the cases mentioned, the fibrous strands have an isolated course,
without connection with the fibres accompanying the vascular bundles ; e. g. in the
roots mentioned, in Ephedra and Welwitschia; also in the leaves of Dracaena, as far
as my experience goes.

The leaf of D. reflexa ®, as seen with the naked eye, is traversed longitudinally by
more than thirty nerves, of which about eighteen are stronger and darker, anastomosing
here and there by means of fine transverse branchlets ; these are the vascular bundles
lying in the middle lamella of the leaf (comp. p. 320), and paler nerves alternate
with them. The latter are simple small bundles of fibres. They do not lie in the
middle lamella of the leaf, but below the two surfaces, separated from the epidermis
by one or two layers of chlorophyll-parenchyma. In the interval between two vascular
bundles they usually lie 3-5 together (next the edge i-o), so that the total number
amounts to nearly fifty, which cannot all be clearly distinguished with the naked eye.
The smallest are only 5-7 fibres thick, the strongest contain about three times that
number. The bundles taper off gradually and terminate below the apex of the leaf,
and immediately above its base, and do not anastomose. Nor did I see them enter
the cortex of the stem. In the case of other fibrous bundles, the anatomical relation
to the sheaths of the vascular bundles still needs investigation. As regards those of

* Compare Schwendener, /. r., p. 143, and the references in the notes, pp. 248-250.
^ Sanio, Botan. Zeitg. 1864, p. 214.

^ Von Mohl, Palm. Structura; Verm. .Schriften, pp. 155, 170.

* Van Tieghem, Struct, des Aroidees, /.r.

Von Mohl, Palm. .Struct, p. xx.

® The determination of the species not quite certain.

SCLERENCHYMA AND SCLEROTIC CELLS.

431

the Potamogetons it has already been stated (p. 232) that they anastomose in the node
with one another and with the vascular bundles. Those in the cortex of Palms,
as already described at p. 266, after running through many internodes, are sometimes
prolonged into purely fibrous bundles, which pass out into the leaves, and sometimes
pass over into the fibrous investment of vascular bundles, which likewise make their
exit into leaves. Connected with this is the phenomenon likewise mentioned above,
of the occurrence, in the plants last named, of intermediate forms between purely
fibrous strands and those which contain small complete vascular bundles or single
sieve-tubes.

It may here be the best place to recall to mind the fibrous strands which sur-
round a secretory passage in the leaves of Pinus and the roots of Philodendron.
Comp. p. 202, Fig. 185, p. 381 ; Fig. 168, p. 360.

The phenomena hitherto described show that the arrangement of fibrous strands
and fibrous sheaths is in a high degree independent of the course of the vascular
bundles ; but that, on the other hand, there are also close relations between the two.
A further phenomenon, corresponding to the attachment of the vascular bundles in
stems to the rings or sheaths described, or their inclusion in the latter, occurs widely,
especially in the Monocotyledons mentioned above, inasmuch as the vascular bundles
are attached to the inner edge of the hypodermal fibrous ridges which project in-
w'ards, or are included in the latter. In flat leaves with fibrous ridges passing verti-
cally through them from one side to the other, the latter often have a bundle inserted
in the middle, or several one above another near the middle. Side by side with these,
other ridges or strands often occur, at least among Monocotjdedons, wTich are quite
similar to the former in structure, but contain no vascular bundles, whether they are
so far related to one of the latter that they stand opposite to it, or whether even
this relation is absent.

In so far as the vascular bundles are attached to or included in those of the
sclerenchyma, the latter stand to the former in the relation of sheaths. The same
relation, as already stated above (Sect. 99 et sq.), is also of general occurrence in the
case of those vascular bundles which are not attached to continuous sclerenchyma-
tous sheaths or to hypodermal strands ; the sclerenchymatous fibres follow their
course as bundle-sheaths (p. 318), which may serve both as a local protective ap-
paratus for the single bundle, and as a strengthening apparatus for the entire organ.
Together with the vascular bundles they form fibrovascular bundles (p. 318). Sclerotic
endodermis may take part in this function, as was stated in former paragraphs.
Between sheaths which simply follow the bundle, and attachment to sclerenchymatous
bundles the position of which is otherwise determined, the most various intermediate
forms occur, as shown in detail by Schwendener.

Apart from these relations of position, the fibrous sheath of the bundle is either
closed all round, or partly interrupted, or only partial, i. e. limited to a relatively small
portion of the circumference. The first-mentioned relation of complete sheathing
obviously occurs in those of the cases above-mentioned, where the bundles are com-
pletely inserted in a closed ring of sclerenchyma. It further exists in other forms
of insertion or attachment, and in the case of individual fibro-vascular bundles,
especially, but not exclusively, in Monocotyledons. In isolated collateral fibro-
vascular bundles the fibrous sheath is then rarely of approximately equal thickness

PRIMARY ARRANGEMENT OF TISSUES.

42 a

all round, e. g. in the rhizome of Carices. In the majority of cases it is thicker on
the outside, where it embraces the phloem, than on the inner border of the bundle;
the converse relation occurs more rarely, e. g. in the rhizome of Scirpus palustris, in
the periphery of the stem of Saccharum officinarum, Bambusa spec., and other
species described by Schwendener. And further its thickness usually diminishes at
the lateral edges of the bundle, so that here, especially next the limiting surface of
xylem and phloem, it is often only 1-2 layers of fibres in thickness, while at the
outer or inner edge its thickness amounts to many layers. This is the case, for
example, in most bundles of Acorus and Zea (comp. Fig. 147, p. 317; Fig. 150,
p. 330), and many other Monocotyledons; in the stem of Saururus and its allies, in
the leathery leaves of species of Melaleuca, Eucalyptus, Eugenia, Callistemon, &c.

To the latter condition is related the most frequent form of local inter-
ruption of the fibrous sheath, which consists in the presence of a gap of greater
or less extent, filled up by comparatively thin-walled parenchyma, next the lateral
edges of the limiting surface of phloem and xylem. Such gaps, or ‘ avenues ’ as
Schwendener calls them, from the surrounding parenchyma to the vascular bundle,
are a widely distributed phenomenon in the region indicated, among the bundles of
tough parts of Monocotyledons, e. g. in the stem of Canna, the leaves of Typha,
Musa, Yucca, and Phormium spec. The bundle of Ranunculus repens represented in
Fig. 152, p. 331, is a good example of this, as is also that of Welwitschia, Fig. 157,
P- 335 ; the same condition obtains in the bundles of many tough Dicotyledonous
leaves, e. g. species of Hakea and Lomatia. Lateral avenues also appear to occur
occasionally in the leaves of the Myrtacese mentioned above.

In a species of Bambusa investigated by Schwendener, an avenue exists on the
inner side of the internal bundles of the stem, in addition to the two lateral ones.
The fibrous strand, which is of immense thickness on the inner edge, is here divided
by a transverse lamella of parenchyma into a narrow, thicker-walled section bordering
directly on the xylem, and a broader, thinner-walled peripheral section. The former is
usually interrupted by two short bands of parenchyma, which lead from -the transverse
lamella to the xylem.

The partial fibrous sheath of collateral vascular bundles usually occurs in such
a form that the phloem is supported in its whole extent, or only at its outer edge, by
a more or less strongly developed fibrous mass, often only by a small group, or even
by single scattered fibres. This is the prevailing rule in the leaves and stems of
Dicotyledons (Figs. 154, 156, pp. 333 and 334), and is besides not uncommon in
Monocotyledons, e. g. in the leaf of species of Crocus, Agave, and Dracaena, in the
sheath of the leaf of Zea (Fig. 151, p. 331), and in the stem and petiole of Aroidese,
as Arum and Colocasia. Comp, also the small bundles of Acorus, Fig. J47, p. 317.

The converse condition, that the partial fibrous sheath embraces the inner edge
of the xylem, is more rare : it occurs in the smaller bundles in the periphery of the
halm of Papyrus, the halm of Cyperus vegetus and other Cyperacese (comp. Schwen-
dener, Lei).

The fibrous strands which appear in company with the vascular bundles often
occur, as show n by the examples of Grasses and Cyperacese adduced above, side by
side with those otherwise arranged, but in very many cases they are present alone.
The latter holds good for most of the Dicotyledons which are not expressly

SCLERENCHYMA AND SCLEROTIC CELLS.

423

mentioned above as examples of the contrary. Among the tough stems of Mono-
cotyledons, the Bambusese investigated show the same behaviour. In many stems
of Monocotyledons the sclerenchymatous strands occur chiefly, though not ex-
clusively, in company with the vascular bundles ; this is the case in the thick stems
of Zea, Saccharum, &c.. Palms, and Pandaneae ; also in the stems and petioles of
Aroidese, as Colocasia, Arum, and many others. It is the rule in Monocotyledonous
stems, and petioles resembling them in structure, that both the relative and absolute
thickness of the strands accompanying the vascular bundles, as well as the thickness
of the walls of their elements, increase as they approach nearer to the periphery of
the bundle-cylinder. In the Aroidese mentioned, the bundles, with the exception of
the outermost circles, are without a fibrous covering. In most Palm stems, the
periphery of the bundle-cylinder, which is surrounded by a narrow cortex, is formed
of massive fibrous bundles, separated by narrow bands of parenchyma ; on the inner
side of each of them a small vascular bundle is attached or inserted ; this region
therefore consists chiefly of firm masses of sclerenchyma, while the bundles in the
interior of the stem, as follows from their general course (p. 262), stand farther apart,
and have in every respect a weaker fibrous investment.

Finally, it scarcely needs to be especially stated, that in the case of vascular
bundles w'hich are accompanied by fibrous strands and w'hich gradually become
longitudinally united a union of the fibrous strands also takes place. If the latter
happens earlier than the union of the vascular bundles themselves, the latter, as seen
in cross-section, appear inserted, two or more together, in one fibrous strand, as is
conspicuously evident in the periphery of Palm-stems, and especially in the stems of
the Pandaneae.

Lastly, we must here once more call attention to the fact that the fibrous
strands are often derived from collenchymatous elements. Those strands, or those
sections of them, which belong to parts characterised by long-continued capacity for
growth and curvatures due to growth, show collenchymatous properties as long as
this capacity is maintained, or they show an intermediate character between
sclerenchyma and collenchyma ; e. g. the base of the sheaths of leaves in the
Grasses (Fig. 15 1), and the above-mentioned stems of Aroidese.

Sect. 126. In certain relatively rare cases, isolated sclerenchymatous fibres, or
fibres united through part of their course to form small bundles, occur in the
parenchyma, external to, and usually side by side with, the strands and layers
described. To this category belong, in the first place, those branched elements
projecting into the air-spaces, which under the name of internal hairs have already
been minutely described above (Sect. 53, p. 221) in the case of the Nymphjeacese,
Limnanthemum, Rhizophoreae, and many Aroidese. Elements more or less similar
to those mentioned occur elsewhere fixed in dense parenchyma. As isolated cases
of this sort may shortly be mentioned the unbranched fibres in the cortex of the
root of Chamsedorea elegans, already dealt with at p. 129; also the branched fibres
in the pith of Carissa arduina^ and the often branched fibres, 6-14™'« in length,
which Tr^cuP- found in the cortex of Euphorbia rhipsaloides, and in the pith and
cortex of E. xylophylloides.

‘ A. Gris, I.C., compare p. 403.

" Comples Kcnclus, LX. p. 1349.

424

PRIMARY ARRANGEMENT OF TISSUES.

Isolated fibres, sometimes ramified, sometimes unbranched, occur as a widely
distributed and characteristic phenomenon in the parenchyma of the cortex and
leaves of many Gymnosperms, and to this mode of occurrence, as well as to that
described in the case of the Nymphseaceae and Aroideae, their appearance in a
number of tough Dicotyledonous leaves is related.

Many of the phenomena belonging to this category, and the literature referring
to them, have already been mentioned in Sect. 30 (p. 130), to which reference must
therefore be made.

Among the Gymnosperms, many Cycadex (Dion, Ceratozamia, Encephalartos, See.),
and several Coniferx (e. g. Cunninghamia, Fig. 183, p. 380), show longitudinal unbranched
fibres, occurring isolated or in small groups, in the parenchyma of the petiole and leaf.
The same applies to the cortex of Ephedra. Stellately-branched fibres lie scattered in
the chlorophyll-parenchyma in Sciadopitys, Dammara, and Araucaria imbricata. The
latter recall the fibres, differing from those of the hypoderma, by which the entire paren-
chyma of Welwitschia, even including that of the floral organs, is abundantly permeated,
the resemblance consisting especially in the presence of great numbers of crystals of
calcium oxalate deposited on their surface : they are thick and short spindle-shaped
elements, with short protruding branches here and there at their pointed ends, and with
a very thick, much stratified, lignified wall (comp. Fig. 55, p. 132, and Fig. 187, p. 408).
In the stem these fibres are directed without order towards different sides. In the leaves,
where they are on the average somewhat narrower than those of the stem, they lie as a
rule, not without exception, in the middle lamella, parallel to the surface of the leaf, with
their longitudinal axis sometimes directed longitudinally, sometimes transversely or
obliquely with reference to that of the leaf ; they usually stand about at right angles to
the surface of the leaf, on both sides of the middle lamella, in the chlorophyll tissue tra-
versed by fibrous bundles ; they reach the middle lamella with one end, and the inner
surface of the epidermis with the other, and often have a hook-like bend at the latter, or
are even wedged in between the inner parts of the epidermal cells. In the parenchyma of
the third genus of Gnetacese, Gnetum, at least in the species investigated, sclerenchyma-
tous fibrous cells are no less abundant than in Ephedra and Welwitschia; in Gnetum
Gnemon they are present in the entire parenchyma of the external cortex, here running
longitudinally, and branching rarely or not at all, also in the pith of the nodes, and in the
leaves near their surfaces, especially the upper, to which they are nearly parallel;
in Gnetum Thoa they occur in the same way in the outer cortex, but especially in the
pith of the nodes and in the leaves; in the regions last mentioned they are abundantly
and variously ramified, in the leaf their size is very unequal, sometimes very considerable.
Comp. p. 130.

The leathery leaves of all these Gymnosperms are thus strengthened by a complicated
sclerenchymatous frame-work. Among Dicotyledons the leaves of Camellia and Fagraea
are characterised by numerous fibres scattered in the parenchyma, which are abundantly
and irregularly branched (Fig. 53, p. 130) ; this also applies to the leaf of species of Olea.
In Olea europaea the fibres are very irregular in their ramification and direction, sending
out branches on all sides as far as the under surface of the epidermis; in O. fragrans
‘they extend, usually without branching, right across the whole leaf at right angles to
the surface, and branch in a more or less pedate manner at the upper and lower epider-
mis, so that they act as columns, connecting the two epidermal layers together.’ (Thomas.)
In the Proteaceae mentioned at p. 130, rod-shaped, more or less ramified sclerenchymatous
fibres stand between the palisade-like chlorophyll-cells, at right angles to the inner surface
of the epidermis. They are as high as, or somewhat higher than the parenchymatous
layer on either side which contains the chlorophyll, and with their usually branched ends
they adhere on one side to the epidermis, while on the other they are attached to or
wedged into the middle layer of the leaf, which in the thick-leaved species is destitute of

SCLERENCHYMA AND SCLEROTIC CELLS,

425

chlorophyll. Still stouter, shortly-branched fibrous cells stand in the palisade-parenchyma
on either side of the leaves of Statice purpurea, approximately at right angles to the
surface, but without reaching the epidermis.

Sect. 127. Short sclerenchymatous elements occasionally appear in the primary
structure of the Phanerogams, united, like the fibres, to form hypodermal strands
or sheaths, or as portions of them. This is the case with the hypodermal layers
in Palm-stems (Cocos, Elais, Astrocaryum vulgare, Mauritia armata, Klopstockia,
Chamaedorea Karwinskiana) mentioned at p. 127, which are interrupted under every
stoma by thin-walled parenchyma, and also with the annular layers also mentioned
at p. 127, in the stems and roots of Aroidese, and with the stegmata. Some special
phenomena to be placed in this category are described by Pfitzer ^ in the case of the
foliage-stem of Restiacese. Restio diffusus has single-layered double longitudinal
rows of rod-shaped, radially elongated sclerenchymatous elements, which alternate
with longitudinal bands of chlorophyll-parenchyma.

Willdenowia spec, and Leucoplocus show the same structure, with the difference
that the hypodermal layers are broader, consisting of 3-4 rows. In Elegia nuda and
species of Restio (R. tectorum, paniculatus, incurvatus, &c.), Thamnochortus, Will-
denowia, Hypolaena, Ceratocaryum, and Leucoplocus, the very large air-cavities under
the superficially situated stomata are bordered by a ring of sclerenchymatous elements,
elongated and placed at right angles to the epidermis, and converging in a curve
towards the inside ; they are in uninterrupted lateral connection with one another,
with the exception of slit-like interstices, by means of which communication between
the stomatal cavity and the intercellular spaces of the neighbouring parenchyma is
effected.

In the external cortex of Dicotyledonous woody plants, short sclerenchymatous
elements often appear in conjunction with fibres, as annular sheaths, the formation
of which, on account of their connection with the processes of secondary growth, will
be dealt with in Chap. XV. Their occurrence in sappy masses of parenchyma and in
the pith of Dicotyledonous plants has been already spoken of on p. 127. The small
groups or nests which lie scattered in the nodes of many Potamogetons (P. crispus,
densus, gramineus, perfoliatus, &c.), near the vascular bundles where they anasto-
mose and pass out into the leaves, furnish an isolated case of the occurrence of
these elements.

Sect. 128. Among sclerenchymatous masses, which strictly speaking belong to
the category of hypodermal tissues under consideration, those remain to be specially
mentioned to which massive hard emergencies owe their strength : e. g. tough
prominent warts, as those of Aloe verrucosa, thorny teeth of leaves, as in Ilex
Aquifolium, Agave, and Aloe, and spines and thorns of the different morphological
categories. The epidermis itself no doubt always takes part in the sclerosis. The
sclerenchymatous elements are in several cases short, e. g. warts of Aloe verrucosa,
thorns of roses; usually they are elongated. The sclerenchyma, together with the
sclerotic epidermis, either forms the entire mass, or it surrounds other internal tissues.
E. g. the sclerenchymatous cylinder of the stem of Berberis vulgaris ^ (p. 419) sends a

Priiigsheim's Jahib. VII. p. 561.

Mettenius, Ilymenopbyllaceac', /. c, p. 439.

426 PRIMARY ARRANGEMENT OF TISSUES.

massive branch into the thorn-leaf, which in the broad base of the thorn forms as a
thick plate the larger lower half ; the narrower upper half is thin-walled parenchyma,
in which the vascular bundles lie. As the thorn tapers off the hypodermal scleren-
chyma increases in relative extent, at the cost of the other tissue, in such a manner
that in the cylindrical upper part there are only feeble vascular bundles in the
middle, enclosed in scanty parenchyma, and completely surrounded by the scleren-
chymatous mass, while the apex is formed of the latter and of the epidermis
exclusively. The ends of the petiolar, stipular, and branch thorns of Astragalus
aristatus, Halimodendron, Robinia, Madura, Crataegus, and many thorny teeth of
leaves, show a similar structure. For further details compare the works cited at
p. 57, by Delbrouck, Uhlworm, Suckow, &c.; v. Mohl, Palm. Struct, p. 7 ; Lestiboudois,
Comptes rendus, Tom. 61, pp. 1034, 1093.

Sect. 129. The sclerotic elements of the Ferns and Hydropteridem ^ are, as ex-
plained in Sects. 26 and 28, sometimes fibrous cells containing starch, sometimes
specific sclerenchymatous elements ; the division of labour between the two forms
is not however strictly carried out, and a sharp severance of the two is not
possible. Their arrangement corresponds on the whole to the rules stated
for the Phanerogams in preceding paragraphs ; even the peculiarities to which
attention is to be called can generally be brought under these rules as special
cases.

Sclerotic hypodcrnial masses of tissue are absent in the stems or rhizomes
of many Ferns, the hypodermal zone being only distinguished from the internal
parenchyma, into which it gradually passes over, by the
closer connection, smaller width, and somewhat thicker walls
of its cells ; e. g. Polypodium vulgare, pustulatum, Davallia
elegans, Acrostichum vexillare, Angiopteris ; Marsiliacem
with a many-layered dense hypodermal zone of parenchyma,
which passes over internally into the inner zone, which
is traversed by a circle of wide air-passages. Many stems,
and especially the thicker ones, have on the other hand a
distinct hypodermal sclerotic zone, consisting of several or
many layers of uninterruptedly united elements, which in
the true Ferns always have brown membranes ; in most
cases this zone does not border directly on the epidermis, but is separated from it
by some layers of thin-walled parenchyma. This is the case, for example, in the
Cyatheaceae, Polypodium Lingua, Platycerium, Davallia pyxidata, &c. The scleren-
chymatous ring, which lies in the cortex of the thin stem of Hymenophylleae, may
just as well be mentioned here as among the instances of sclerenchyma accom-
panying the vascular bundles, to be brought forward later. The same applies to the
dark-brown mass of sclerotic elements containing starch, which in Osmunda regalis
and Todea hymenophylloides forms the principal part of the stem, and which is every-
where sharply marked off from the relatively small colourless tracts of parenchyma
containing the vascular bundles. Comp. pp. 346 and 279, and Fig. 188. Sclerotic
annular layers bordering directly on the epidermis occur more rarely, e. g. in the

FiCi. 188. — Osmunda regalis;
cross-section ofavigorousstem,
seen from above, i. e. from the
apex of the stem, magnified
about 20. i lowest leaf-trace
bundle; a root-bundle given off
from this is seen passing through
the cortex.

* Compare the literature cited above, §§ 73-87, on the structure of the Fern-stem.

SCLERENCHYMA AND SCLEROTIC CELLS.

427

rhizome of Pteris aquilina and Polybotrya Meyeriana. The sclerosis which occurs
at an early period in the cortex of the root of many Ferns was mentioned above
at p. 413.

In the petioles and ribs of the leaves in Ferns, it is a general rule that a more or
less strongly developed sclerotic hypodermal layer, often interrupted by the bands
and islets covered with an epidermis without stomata mentioned at p. 405, lies
directly beneath the epidermis, which itself not unfrequently takes part in the
sclerosis. The collenchymatous zone in the petiole of the Marattiaceae is more
deeply seated, and separated from the epidermis by several layers of thin-walled
parenchyma (comp. p. 120).

According to Mettenius, strands usually branch off from the hypodermal
sclerenchyma in the petioles and ribs, which accompany the finer ramifications of the
vascular bundles into the lamina of the leaf.

Among strictly hypodermal sclerenchymatous masses in the lamina of the leaf,
the continuous layer of Acropteris australis has already been described at p. 132.
Such masses occur in the form of nerve-like bands in many leaves of Marsilise
and Ferns.

In the lamina of the aerial leaves of Marsilia coromandeliana, trichopoda, muscoides,
and distortah narrow colourless bands of sclerenchyma run between and in a similar
direction to the nerves ; some of these are small strands or even isolated fibres, adjacent to
the epidermis of the lower surface of the leaf; others are thicker, extending through the
entire thickness of the leaf, from one epidermal surface to the other. Hypodermal
strands distributed like nerves are described by Mettenius as occurring in the segments
of the lamina of the leaf of Todea Hymenophylloides, Polypodium solidum, Pteris
pinnata, Davallia elata and elegans. In other Ferns the edge of the leaf or of its
segments is entirely or partially rimmed by a hypodermal (many-layered) strand
of sclerenchyma, which is prolonged continuously into the sclerenchyma of the
petiole. This is the case in Polypodium Lingua, sporadocarpum, Brownianum, Asple-
nium lucidum, Polybotrya cervina, Meyeriana, Aspidium falcatum, Adiantum denti-
culatum, &c.^ The strands described by Mettenius as Nervi spurii, in the leaves of
many species of Trichomanes, may also be mentioned here, although, as running
through a usually single-layered lamina, they do not strictly belong to this category.
They consist of one or a few rows of elongated elements, which are usually accom-
panied by stegmata (p. 128).

Around the vascular bundles in the stem, roots, petioles, and the stouter ribs
of the leaf, sclerenchyma and sclerotic cellular tissue is in many cases entirely absent,
the bundle or its endodermis is surrounded by thin-walled parenchyma, which differs
scarcely if at all from that lying further away from it, and is never sharply marked
off. This is no doubt the case in most roots ; in the stem and petiole of the
Marattiae, in the rhizomes of Aspidium Filix mas, Onoclea Struthiopteris, Polypodium
vulgare, Davallia pyxidata, &c. Pteris aquilina may also be mentioned here. In
several roots on the other hand, and in most stems and petioles of Ferns, distinct
sclerotic sheaths and strands occur in company with the vascular bundles. This
is the case, for example, in the roots of many Polypodia, as P. ireoides, vulgare, &c.,

^ A. Braun, Monatsber. der Berlin. Acad. 1870, p. 693.
^ Mettenius, Hymenophyllacesc, p. 438.

42^

PRIMARY ARRANGEMENT OF TISSUES.

Blechnum occidentale, and Scolopendrium vulgare \ in such a form that one or more
layers bordering directly on the endodermis acquire brown walls, strongly thickened
chiefly on the inner side, so that these layers form either a sheath going uniformly
round the whole axial bundle, or one which is interrupted, or at least thinner, over
the corners of the xylem-plates.

The sclerotic sheath in stems and petioles has, in the first series of forms, the
same position relative to the endodermis as in the roots. And indeed the sclerosis
very often at first affects only the inner walls bordering on the endodermis, and the
lateral walls of the layer of cells which is in direct contact with it ; the outer walls
of this layer, like those of the surrounding parenchyma, are not sclerotic, e. g. stem
and petiole of Polypodium Lingua, and pustulatum, and the stem of Davallia elegans.
The converse condition in the thickening of the walls occurs rarely : petiole of
Blechnum brasiliense ; or the sclerotic thickening of the wall exists all round, though

it may be weakest on the outside :
Polypodium Phyllitidis^. In other cases
the endodermis is surrounded by an
uninterrupted sclerotic sheath, consist-
ing of one or more layers (rhizome of
Polybotrya Meyeriana and Hymeno-
phyllese), or by an interrupted sheath,
i. e. by one or several many-layered
strands of sclerenchyma adjacent to it.
This is the case, for example, in the
rhizome of Platycerium alcicorne, and
in very many petioles. In the very
frequent case where the bundles in the
latter have projecting corners and de-
pressed incurvations of their surface, a
definite relation between the latter and
the strands of sclerenchyma exists ; e. g.
they lie on the concave side of the
runnel-shaped bundles in the petiole of
Balantium Culcita and Cyathea medul-
laris, in the corners of the figure x
which the bundle shows as seen in
cross-section, in the petiole of Scolo-
pendrium vulgare, &c. (Comp. Rus-
sow, /. c. and the special pteridological descriptions.)

In a second series of forms, the sclerenchyma accompanying the bundle is not
adjacent to the endodermis, but is separated from it by a zone of delicate parenchyma,
usually consisting of many layers of cells. This is the case in the stem of Todea
africana, but more especially in that of the Cyatheacese. In most of the latter, e.g. Cy-
athea arborea, Imrayana (Fig. 189), Alsophila microphylla, and many others, the band-

FIG. 189. — Cyathea Imrayana; cross-section through the living
stem, natural size ; seen from At d, c, rf, foliar gaps. All the

gitiie black bands and points are cross-sections of sclerenchyma;
the paler ones those of vascular bundles. Within or adjacent to the
foliar gaps, especially d and b, are root-bundles on their way to the
periphery; y furrow of the base of the leaf; a vascular bundle of
the main tube ; s outer, j' inner plate of its sclerenchymatous sheath.
Inside ^ is the pith, outside s the cortex, with their small bundles.

^ Van Tieghem, 1. c,, p. 66, Taf. 5.
“ Russow, 1. c., p. 81.

SCLERENCHYMA AND SCLEROTIC CELLS.

429

shaped main bundles of the stem are immediately surrounded by a many-layered
zone of delicate parenchyma ; this- is completely enclosed by a stout sheath, likewise
consisting of many layers, and reaching a thickness of above i™“, which consists of
acutely spindle-shaped, closely connected fibrous cells. From this sheath strands of
various dimensions branch off, which accompany the medullary and cortical bundles
(with the exception of the thinnest unsheathed branches), and those which pass out
into the leaf ; they seldom enclose these completely as they do the main-bundles ;
usually they are open and runnel-shaped, and placed on the inner, medullary side of
the bundles, from which, however, they are separated by a broad layer of parenchyma
(comp. Sect. 85). In many Cyatheacese, as Alsophila pruinata, blechnoides, and
species of Cibotium, all the bundles of the stem are accompanied only by open strands
or plates of sclerenchyma, which lie opposite their inner side.

Connected with these are the two thick brown plates of sclerenchyma, often
fused to form a tube wdth only a narrow opening on one side, which run longitudinally
through the rhizome of Pteris aquilina in the middle of the parenchyma, between the
outer and inner tube of vascular bundles (Sect. 87). To this category belongs also
the axial strand of sclerenchymatous fibres, running inside the annular vascular
bundle, and often, it is true, bordering directly on the endodermis, in the rhizome of
species of Pilularia and Marsilia. In the larger species of Marsilia, as M. Drum-
mondii, and salvatrix, the outer side of the axial annular bundle is also sur-
rounded by a tough brown sheath of sclerenchymatous fibres, which passes over
internally into thinner-walled layers of cells containing abundant starch. In
M. quadrifolia this sheath is replaced by a many-layered annular zone of cells
bordering on the endodermis, which are rich in starch, and have brown but thin
walls.

Isolated small bundles of sclerenchyma with brown membranes, or even isolated
fibres, are observed here and there in the parenchyma of Ferns, e. g. the very hard
small bundles in the pith of many Cyatheacese, where, it is true, they are often con-
nected with those accompanying the vascular bundles as their ramifications, though
they may often have an independent course, e. g. Alsophila microphylla. On the
other hand, they are entirely absent in several species, e. g. in Alsophila pruinata,
blechnoides, species of Dicksonia and Cibotium, according to Mettenius. Small
strands, consisting of only a few fibres, ending blindly in the parenchyma above and
below, or isolated fibres, traverse the parenchyma longitudinally in the rhizome of
Pteris aquilina, Polypodium Lingua, Osmunda regalis, &c. In the winged edge of
the base of the petiole of Osmunda and Todea similar brown fibrous tracts and
fibres are arranged so as to form pinnate bands k

As regards the distribution of sclerotic elements in the small stem of the Lycopodia
and SelaginellcB, similar general rules and also similar variations prevail, as in the
case of the thinner stems of Ferns. In the stouter Lycopodia, as L. clavatum, alpinum
and Chamaecyparissus (Fig. 162, p. 349), a bulky, many-layered ring of fibres
surrounds the axial vascular bundle. A narrow annular zone of slightly sclerotic
elements lies in the middle of the thin- walled cortical parenchyma of the foliage- stem
of Psilotum. In the rhizomes of this plant and in the stems of Lycopodium Selago

Compare Milde, Monograph. Generis Osmundae ; Vindob. 1868.

43°

PRIMARY ARRANGEMENT OF TISSUES.

sclerotic layers are absent. In the stems of Selaginella the sclerosis is sometimes
limited to the epidermis (S. spinulosa) ; in most species it further affects a hypo-
dermal zone, which gradually passes over internally into thin-walled parenchyma ;
in S. rupestris the entire tissue of the stem is in the highest degree sclerotic, with the
exception of the zone of lacunar parenchyma, which in this, as in all other species
investigated, directly or indirectly surrounds the vascular bundles. (Comp. Fig. 131,
p. 282.) The roots of the Selaginellse and Lycopodia present essentially similar
phenomena to those of the stem, with reference to the conditions in question.

CHAPTER XI.

ARRANGEMENT OF THE SECRETORY
RESERVOIRS.

Sect. 130. The primary arrangement of the secretory reservoirs presents little
of interest, and it has often been impossible to avoid the mention of it in preceding
sections. The present Chapter, which is necessary simply for the sake of consistency,
must therefore be a short one, and confine itself chiefly to references to former
paragraphs.

The sacs contammg crystals lie, in the plants enumerated in Sect. 31, to which
they are peculiar, in the primary parenchyma, sometimes ‘ scattered ’ between its
cells, sometimes more regularly distributed, or at definite places in definite grouping.
On their distribution in the pith of Dicotyledonous woody plants, see p. 403 ; on
their accumulation on the wall of the air-passages of Water-plants, Aroideae, &c.,
comp. p. 219; on the series of raphides accompanied by mucilage in the Mono-
cotyledons, see p. 139. It may here be mentioned more definitely than was done in
Sect. 31, that the vascular bundles are occasionally, but by no means universally
accompanied by series of elements containing crystals; e. g. the petiole of Cycas
revoluta (p. 336) ; and the medullary bundles of Melastomacese, as Heterocentron
and Centradenia spec., with longitudinal rows of sacs containing crystalline aggrega-
tions on their outer side, &c.

The sacs containmg mucilage lie in the primary parenchyma of the plants men-
tioned at p. 143, and in fact principally in the foliage and cortex, usually scattered
without any generally determinate order; their more regular distribution in the
tubers of Orchis has already been mentioned at p. 144.

The same scattered position in the primary parenchyma of the foliage, pith,
and especially of the cortex, prevails in the case of the short sacs containing resin and
gum-resin, of the families mentioned at p. 145.

The long sacs of this category, and the tannin-sacs, have already been discussed
at p. 146, &c. Comp, further Sect. 48, especially p. 199, and Chap. XII.

CHAPTER XIl.

COURSE OF THE LATICIFEROUS TUBES.

Sect. 131. The laticiferous tubes in most plants which are characterised by
their occurrence, traverse the entire body of the plant as a continuous system.
Exceptions to this rule seem, however, to occur ; in the roots of Asclepias
curassavica, and Cornuti, and of Periploca, I could not find them, but will not assert
their absence with complete certainty ; in the roots of Ficus elastica I only find them
in the secondary bast.

As regards their relation to the other tissues, we may call them, as already indi-
cated above, companions, or in some places even representatives, of the sieve-tubes.
The latter relation is especially manifest in the secondary bast of many plants, to
which we shall have to return, below, Sect. 163. In the primary groups of tissue the
laticiferous tubes are distributed —

(a) In the roots within the phloem of the vascular bundle. Only in the case of
the Euphorbise investigated do others occur in addition, which arise as branches
from those of the cotyledonary node, and lie close under the epidermis, separated
from the latter only by a few layers of cells {comp. p. 196)^

(3) In stems, petioles, and ribs of the leaves, the main courses or main trunks
of the tubes lie chiefly in the tissue surrounding the phloem -portions of the vascular
bundles, following the longitudinal course of the latter, and, as seen in cross-section,
scattered without strict regularity among the surrounding parenchyma. If the phloem
is sheathed by a strand of sclerenchyma they lie outside the latter. In addition to these
tubes, other smaller ones occur in certain cases, e. g. Cichoriaceae and Papaver,
which run in the phloem itself. In milky plants provided with phloem-portions
towards the pith, or with separate medullary bundles of sieve-tubes, these also are
accompanied by laticiferous tubes (comp, p. 338).

In the foliar expansions, the tubes on the one hand still follow the higher orders
of ramification of the bundles ; on the other hand, in the majority of cases, they send
out branches which leave the paths of the vascular bundles, force their way in all
directions between the cells of the parenchyma, and end blindly, sometimes in the
interior of the latter, sometimes at the inner surface of the epidermis. In the case
of Siphocampylus manettiseflorus, Trdcul even states that the ends of the branches
pass between the cells of the epidermis, as far as its outer surface. In many milky
Dicotyledons, branches of the tubes also traverse the cortex of the stem, partly in the
internal parenchyma, partly hypodermally. In the succulent Euphorbise possessing

' See Chapter VI, and the literature there cited.
“ [Compare Scott, /. r., p. 143. (See p. 193).]

COURSE OF THE LATICIFEROUS TUBES.

433

rudimentary or very fugacious leaves, and in the Asclepiadeae, they branch off at all
points from the main trunks, and are distributed through the massive cortical paren-
chyma in all directions as far as the epidermis, having an oblique and crooked course.
In non-succulent stems with well-developed, persistent foliage leaves, longitudinal
tubes occurring in the hypodermal cortical layer, and branched off from the main
trunks in the nodes, are, judging from Tr^cul’s statements and the phenomena in
Euphorbiae to be communicated below, at any rate much more frequent than has
been represented by most previous observers.

In many cases also, branches, which are usually thick tubes, pass from the
main trunks into the pith; in the case of medullary bundles of sieve-tubes, they
branch off from the main-stems which accompany the latter (Hoya, Asclepias
spec.) In plants without medullary sieve-tubes, as Ficus and Euphorbise, they arise
as branches from the main trunks, principally, but as shown by the succulent
Euphorbise not exclusively, in the nodes. According to the particular case, they are
either inserted in the parenchyma throughout the entire thickness of the pith, e. g.
Ficus, or they are confined to its periphery, e. g. Euphorbise.

According to the particular families, and even the species, the general rules stated
above are subject to manifold variations. The most important details will be given
below, reference being made to Chap. VI, and the special works there cited. Some
data referring to the secondary wood and secondary bast, belonging to Chaps. XIV
and XV, may here be anticipated.

I. ARTICULATED LATICIFEROUS TUBES.

I. CichoriaeesB. In the stem of those species investigated, which have ordinary col-
lateral bundles within the boundary of the plerome, the tubes in the first instance lie around
the phloem of each of these bundles. Their longitudinal main trunks here form, as seen
in cross section, a single curved row, often interrupted by parenchymatous cells, at the
boundary towards the cortical parenchyma ; their numerous transverse anastomoses pass
along the outer surface of the approximately semi-cylindrical phloem, which is usually
destitute of any sclerenchymatous support. These peripheral tubes are the largest. In
the stems of Chondrilla, Taraxacum, Apargia, and Cichorium, no other laticiferous tubes
are present. The bundles in the lower part of the stem of Sonchus tenerrimus, Picri-
dium tingitanum, and, in a slight degree, of Lactuca virosa, have sieve-tubes on the inner
side of the xylem also, and are then accompanied here also by laticiferous tubes, which
are connected with those outside by branches passing right round the vascular bundle
(Trecul). Finally, where, as in the investigated species of Lactuca, Sonchus, Scor-
zonera, Tragopogon, and Hieracium, small medullary bundles of sieve-tubes are present,
the latter each contain some laticiferous tubes, which run parallel to the sieve-tubes, and
anastomose with one another between them, without however having an open com-
munication with the sieve-tubes. According to Trecul, anastomoses take place along
the entire intemode between the nets of tubes accompanying the different vascular
bundles. In all plants of the family in question, they are especially numerous in the
nodes, both between the peripheral tubes accompanying the vascular bundles, and be-
tween these and the medullary tubes, the latter occurring together with the anastomoses
described at p. 231, between the groups of sieve-tubes belonging to the two regions.
In the nodes the nets of laticiferous tubes of the stem are further continued into those
of the petioles and of the axillary branches.

In the petioles and ribs of the leaves, the nets of tubes accompany the vascular
bundles with the same arrangement as in the stem, and finally their blanches, which end

F f

434

PRIMARY ARRANGEMENT OF TISSUES.

blindly, either terminate together with the last vessels in the parenchyma of the leaf,
or similar terminal branches are sent out as far as the epidermis.

For the course of the tubes in the peduncle and receptacle, essentially the same rules
hold good as for the stem ; some branches of them accompany the small vascular bundles
which traverse the pistil, corolla, and stamens (Hanstein).

In the roots the tubes lie in the phloem-groups of the original vascular bundles, and
thus within the pericambial zone — at least in Tragopogon and Scorzonera hispanica.

They never enter the xylem groups unless it be in the ultimate ramifications of the
bundles, where they are in close contact with the tracheides, and in the nodal anasto-
moses, which pass through the medullary rays, between the medullary and the peripheral
bundles, where they may come to lie close to the vessels.

2. The nets of laticiferous tubes of the Campanulaeeas and Lobeliaceae are in
general quite similar in form to those of the Cichoriacese. In their distribution a differ-
ence is in so far observable, that the chief seat of their occurrence is the internal phloem-
region, which lies towards the xylem-portions of the bundles. In the periphery of the
phloem and the parenchyma of the external cortex they are in many cases entirely
absent, or very isolated (Lobelia inflata, syphilitica, urens, Adenophora Lamarckii, Phy-
teuma Halleri, spicata. Campanula sibirica, medium, ranunculoides, grandis, lamiifolia) ;
more rarely they are abundant (Tupa salicifolia, Isotoma, Centropogon, Piddingtonia
spec.), and especially in Tupa Feuillei, Ghiesebrechtii, and Musschia aurea, where they
penetrate up to the epidermis. In the case of Siphocampylus manettiaeflorus, Trecul
states that individual ends of branches penetrate as far as the surface of the epidermis,
and even project there as small papillae.

In those Campanulaceae which have sieve-tubes on the inner side of the woody ring, or
in the medullary bundles, the phenomena above mentioned are complicated by the occur-
rence of laticiferous tubes accompanying the latter, as in the case of the Gichoriaceae with
similar structure. They are absent in the xylem, in the secondary ring of wood, and
in the parenchyma of the pith in all Campanulaceae investigated, and in many Lobeliaceae.
In other plants of the latter family on the other hand, e. g. Centropogon surinamensis,
Tupa salicifolia, Ghiesebrechtii, Feuillei, Siphocampylus manettiaeflorus, microstoma, and
Lobelia laxiflora, Trecul found them scattered at the periphery, and more or less deep
in the interior of the pith, and found that these medullary tubes are in communication
with the cortical ones by means of branches traversing the woody ring.

3, The laticiferous tubes of the Papayaeeae, investigated in Papaya vulgaris, Vascon-
cellea monoica, cauliflora, and microcarpa, form in the stem of these plants an abundantly
ramified and anastomosing net-work of tubes, extending both through the highly paren-
chymatous primary and secondary wood, and also through the medullary rays and the
bast-region. The main trunks have an approximately vertical course and form interrupted
rows approximately concentric with the circumference of the stem ; the separate portions
of these rows everywhere alternate variously in wood and bast, with parenchyma, vessels,
and sieve-tubes. The neighbouring tubes are connected laterally with each other by
means of exceedingly numerous wide anastomoses. Similar anastomoses occur in the
most various particular forms, between the different groups and rows, both in radial
and tangential direction, the radial connections being often effected by long transverse
branches with an approximately horizontal course. Blindly ending branches and branch-
lets further occur with varying frequency. The tubes are usually separated from the
vessels by at least one layer of parenchyma, some however are in contact with them, and
as discussed above (Chap. VI) there is perhaps open communication between the two.
Into the pith of the internodes, which soon disappears, the tubes do not enter; they
form, however, a complex network, anastomosing on all sides in the medullary disk,
which is persistent in the nodes. In the primary cortical parenchyma, Dippel alone
found isolated tubes, accompanied as a rule by parenchymatous cells containing crystals
near the outside of the bundle of bast-fibres, and connected with the internal tubes by
very elongated horizontal transverse branches.

COURSE OF THE LATICTFEROUS TUBES.

435

In the leaf-stalks and ribs of the lamina they follow the vascular bundles, often accom-
panying and touching sieve-tubes and vessels. In the parenchyma of the leaf they end
with numerous anastomosing branches.

After the commencement of secondary thickening the root has a similar structure, and
a similar distribution of the laticiferous tubes to that in the stem.

4. Among the milky Papaveraceae two types of laticiferous tubes are to be dis-
tinguished. The one is represented by the investigated species of Papaver, Roemeria,
and Argemone, and shows tubes which arise from elongated elements, but only rarely
allow traces of the original cross-walls to be recognised in the mature condition ;
these are connected into a net by more or less numerous anastomoses. In the stem
and petioles they lie in tangential curved rows in the phloem of the vascular bundles,
in each one of which they anastomose in the transverse direction, though there are no
anastomoses between those of the different bundles of an internode. In the (secondarily
thickened) root, in the cortical parenchyma, and especially the bast layer, and also in
the parenchyma of leaves, pericarps, &c., they end in an abundantly ramified net.

The other type is represented by Chelidonium, and is characterised by the facts that
the cross-walls of the elements merely have one or more large perforations in the middle,
while on the other hand their edge remains preserved, and that reticulate connections
do not occur. On account of the partial persistence of the cross-walls, the tubes appear
at the first glance like rows of cells, the articulation of which comes out all the more
sharply, as they are usually somewhat constricted at the cross-walls (cf. Figs. 80, 81, p. 189).
Sometimes when two tubes are in direct lateral contact, perforations appear to occur
in the lateral wall also. In the older roots the tubes are often branched, owing to the
fact that a series of elements is continued from one point into two diverging series, which
meet at an acute angle ; and the individual elements are short, on the average 2-4 times
as long as broad, being of about the same length as the neighbouring parenchymatous cells,
and elements of the sieve-tubes. In the parts of the plant above ground, on the other
hand, the elements are very elongated, so that their ends more rarely come into view
in preparations. In the (older) roots, the tubes in the bast-layer are so distributed in
groups, forming concentric, irregularly interrupted rows, that every group usually lies in
the neighbourhood of a small group of sieve-tubes, surrounded by masses of parenchyma
containing starch. In the stems and petioles narrow laticiferous tubes lie scattered in
the vascular bundles within the phloem, and at the periphery of the xylem ; they further
occur externally to the vascular bundles, on the outside of the fibrous bundles bordering
on the phloem, and also isolated in the peripheral (cortical) parenchyma. In the lamina
of the leaf and the parts of the flower the system of tubes ends in the reticulate form
described above for other cases.

In other Papaveracese, especially Macleya cordata and species of Glaucium (I investigated
Gl. luteum), no doubt also in Eschscholtzia (which, however, requires further investigation
with reference to the statement of the anonymous writer in the Botanische Zeitung,
1846), and in the Fumariaceae, no laticiferous vessels whatever are known. A red sap,
which is on the whole clear, and mixes both with water and alcohol without turbidity,
appears conspicuously on cut surfaces of the rhizome of Sanguinaria : it is contained in
large, thin-walled, roundish or shortly cylindrical cells or sacs, which are abundantly dis-
tributed through the whole parenchyma, sometimes isolated between (starch-containing)
parenchymatous elements, sometimes, and especially in the cortex, forming continuous
longitudinal rows (cf. Hanstein, l.c., Taf. I). In the stem and petiole, which I have
not investigated, these sacs are elongated cylindrical or prismatic. Neither their walls
nor their contents show the properties characteristic of laticiferous tubes ; they were
therefore mentioned above at p. 147.

The same holds good for the sacs filled with a clear reddish-yellow sap, which are
scattered through the parenchyma in the root of Glaucium luteum, and which, according
to the form of the contiguous elements, are more or less longitudinally extended. In
the stem and foliage of species of Glaucium (cf. Trecul, l.c.) they are absent. In the

F f 2

43^

PRIMARY ARRANGEMENT OF TISSUES.

rhizome and stem of Macleya cordata, similar sacs, sometimes very much elongated,
are scattered in large numbers about the periphery of the ring of vascular bundles, and
in the medullary rays. As the parts grow old the reddish-yellow colour of the sap dis-
appears. The elongated sacs bordering on the fibrous bundles of the bast then acquire
thickened walls, like sclerenchymatous fibres.

5. According to their structure, the latex- and tannin-tubes of many Aroideae^ belong
to this part of our subject. Their main trunks lie in the periphery of the phloem of the
vascular bundles, usually two or more together, in collateral bundles situated as a rule
symmetrically on each side, in the ‘compound’ bundles less regularly distributed. In
their most perfectly developed form, in Caladium and its allies, Alocasia, Xanthosoma,
Syngonium, &c., they constitute thin-walled sacs, about equal to the sieve-tubes in
width, following the longitudinal course of the bundles ; at the limiting surfaces of the
surrounding parenchymatous cells they send out numerous, pointed or blunt, blind
protrusions, which penetrate between the latter. Other protrusions are extended to
form longer tubular branches, penetrating between the surrounding tissue-elements, and
these also sometimes end blindly, often showing somewhat enlarged ends, while some-
times they meet similar branches of neighbouring trunks, and enter into open communi-
cation with them. The network of tubes thus formed is extended throughout the
parenchyma, not only between the vascular bundles, but also not uncommonly in the
parenchymatous cortex, as far as the under side of the epidermis.

The branches of the network of tubes also run towards the tracheae, attach their ends
to the latter, and, as stated above, frequently enter into direct communication with
them.

The contents of these tubes consist of a finely granular fluid, which, according to
Tr^cul, is milky in species of Syngonium and Xanthosoma, in other cases only a little
turbid, and very rich in tannin, so that after the action of iron salts or potassium bichro-
mate the net-work of tubes comes out darkly stained.

Other Aroideae (Richardia africana. Arum vulgare, Dracunculus, Aglaonema simplex,
Dieifenbachia Seguine, and species of Philodendron) have no laticiferous tubes, but contain
longitudinal rows of elongated cylindrical or prismatic sacs in the periphery of the phloem,
with the same arrangement as the trunks of the net-work of tubes described above.
They contain the same turbid contents as the tubes described (in Dieifenbachia Seguine
according to Trecul they are without tannin), but they are separated by cross-walls
and are without lateral anastomoses, only sending out short blind protrusions between
the limiting surfaces of the neighbouring parenchymatous cells.

A third category — Heteropsis, Lasia, Scindapsus, Monstera, Anthurium, Acorus, &c. —
is wholly destitute both of the tubes described, and of sacs.

That these net-works of tubes arise from the union of originally separate, branched
cells, is shown both by the history of their development, and by comparison with the
sac-tubes of the second category.

6. In the species of the genus Musa, the vascular bundles in the stem, the petiole, the
midrib, and the lamina of the leaf (especially also in the fruits) are accompanied by wide
laticiferous tubes which are arranged symmetrically, 2-6 around each bundle, and in fact
around both phloem and xylem ; usually, however, they are not in direct contact with
the bundle, but are separated from it by 1-2 layers of parenchymatous cells. The tubes
in the stem and petiole are unbranched, and each consists of a row of cylindrical sacs
standing vertically one above another, which are about four times as long as broad, and are
united to form a continuous tube by means of a wide round opening in every cross-wall.
Round every cross-wall the tube is somewhat constricted, as was well represented even
by P. Moldenhawer. The tubes contain large, homogeneous, strongly refractive spheres

* Karsten, Monatsber. d. Berliner Acad. 1857; Gesammelte Beiträge, p. 253 — Trecul, Van
Tieghem, Hanstein, /. c.

ARRANGEMENT OF THE LATICIFEROUS TUBES.

437

(of resin ?), suspended in a fluid, which is almost always in the highest degree rich in
tannin; in M. zebrina alone it is frequently destitute of tannin, according to Trecul.
After the action of alcohol or potassium bichromate, a very sharply defined coating of
the wall appears, resembling a contracted primordial utricle, and this likewise shows the
tannin reaction (investigated in M. Cavendishii).

Besides that in the laticiferous tubes, tannin also occurs as a principal constituent of
the contents in single scattered, short, parenchymatous cells, and in isolated cambiform
cells in stem and petiole.

The other Musacese investigated have no laticiferous tubes, in spite of the fact that in
the rest of their structure they agree entirely with Musa. In place of them spaces usually
filled with tannin are found in cross-sections of Urania speciosa and Strelitzia, in the
neighbourhood of the thicker vascular bundles; these appear to be the laticiferous
tubes ; the longitudinal section, however, shows that they are only parenchymatous cells
or sacs with the contents indicated, which do not even form uninterrupted longitudinal
rows one with another. Those which succeed one another at different heights rather
belong, sometimes to one and the same, sometimes to various other rows of parenchyma.
In the rest of the parenchyma, and in the phloem portions of the vascular bundles,
scattered tannin-sacs occur, as in the equivalent parts of Musa. Heliconia speciosa
and H. Bihai, according to Trecul, never show cells or sacs filled with tannin, with the
exception of single scattered ones in the phloem of the vascular bundles. The same
applies to H. pulverulenta Lindl. Finally in Ravenala madagascariensis, &c., Trecul could
not find tannin at all, only the wall of individual cells of the leaf-sheath showed an indi-
cation of blue colouring after the action of iron sulphate for twenty days.

II. NON-ARTICULATED LATICIFEROUS TUBES.

7. Eupliorblaceas. Of this family a number of species of Euphorbia have been in-
vestigated with accuracy. In the shrubby, more or less succulent forms of hot countries,
as E. splendens, E. Caput Medusae, canariensis, rhipsaloides, &c., the stem shows a
relatively thin ring of vascular bundles, or of wood and bast, which encloses a bulky
succulent pith, and is surrounded by a layer of cortical parenchyma, which is also thick.
The stouter, thick-walled, main trunks of the laticiferous tubes run close to the outside of
the ring of bundles ; they are scattered in the cortical parenchyma, either singly or in small
groups. Their course is in general longitudinal, though not rectilinear, but undulating
both in the radial and tangential direction. They give off numerous branches which are
further ramified through several degrees (cf. Fig. 84, p. 191). Those of the first degree
do not differ from the main trunks, their direction also is the same. The branches of
higher degrees become successively narrower and thinner-walled, those of the last degree
have blunt blind ends. 1 he direction of the higher degrees of ramification is very
various ; sometimes their course, like that of the primary tubes, is longitudinal ; some-
times they penetrate in a curved and undulating course between the cells of the cortical
parenchyma, sometimes individual branches pass through the latter towards the surface,
and reach the inner side of the epidermis, where they end blindly, either at once, or
after running for some distance below it. Other less numerous branches pass through
the medullary rays into the pith, in the peripheral region of which they divide into
numerous thick branches, each of which usually has an isolated longitudinal course.
We therefore find laticiferous tubes scattered in the parenchyma, on the medullary
side of the wood also. Reticulate anastomoses do not occur. Where an H-shaped
connection between two tubes is found, this depends merely upon the form and direction
of blindly ending branches. In those species which have developed leaves (E. splendens)
the laticiferous tubes enter them, at first following the vascular bundles, and then sending
out numerous branches from these, through the parenchyma, which run in the most
various directions, and at last end blindly.

438

PRIMARY ARRANGEMENT OF TISSUES.

The tubes are uninterruptedly continuous throughout the entire plant. No trace of
an articulation ever occurs (with exception of the formation of isolated cross-walls,
mentioned at p. 196). It has never been found possible to dissect out from a piece of a
stem, a tube of which even one of the main branches or trunks was closed blindly at
both ends. Trecul isolated a portion of a tube from the stem of E. globosa, the united
ramifications of which were together 93'5™™ in length, and had 120 points of branching ;
yet 7 main branches and many smaller ones were torn off. I have isolated many main
branches from the stem of E. splendens up to 50-70™“ in length, without ever finding
a blind end to them (i. e. apart from the shorter lateral branches). In the growing-point
of the stem, branches, and roots, and in young rudimentary leaves, the laticiferous tubes
extend close up to the extreme apex, even before the development of the first elements
of the vascular bundle, and are always to be clearly recognised as branches which start
from the tubes in older parts, and penetrate into the meristem in course of formation.
Even in the seedling (investigated in the case of embryos of E. resinifera Berg.) the
laticiferous tubes behave as described above, with the sole difference that their ramifi-
cations are as yet less abundant than in the older plant. It follows from these facts, that
all the tubes of the plants in question are branches of a few primary ones, which,
originating in the embryo, grow on and on with the stock, and put out their branches
into the newly formed meristems and tissues. Cf. Chap. VI.

The embryo and seedling of the native, herbaceous Euphorbiae (Tithymalus, Klotzsch

and Garcke) show the same points of origin
of the tubes as the species discussed above,
while the mature plant shows the same con-
tinuity of all of them as branches of a few
primary trunks, which first appear in the
cotyledonary node (cf. p. 196). In corre-
spondence with the entire growth and struc-
ture, the course of all their branches is different
from that in the succulent shrubby forms,
especially in the stem. In the internodes of
E. Lathyris (cf. Fig. 190) the thicker main
branches of the tubes lie in the parenchyma
of the cortex, outside the fibrous strands which
support the phloem of the vascular bundles,
occurring Isolated and in small numbers
between the parenchymatous cells. They
here have a fairly straight course through the
internode, and are little, if at all, branched.
In the node, on the other hand, abundant
ramifications, with a variously curved and in-
tertwined course, occur (Fig. 190), sending
out from here further branches, some of
which ascend, with the arrangement de-
scribed, into the cortex of the next higher
internodes, some enter the leaves and axillary
buds, at first following the vascular bundles,
while others finally, in these regions, pass between the vascular bundles into the pith of
the stem, and then descend scattered singly between the parenchymatous cells of the per-
manently succulent peripheral portion. The inner part of the pith, which soon dries up,
contains no laticiferous tubes. Finally, numerous thin branches proceed from the node ;
these pass along the internodes near the epidermis, and together form a hypodermal
system of tubes. They ascend vertically from the node into the internode next above,
and usually run between the first and second hypodermal layer of parenchyma, more
rarely directly below the epidermis, with a straight upward course and ramifying here and

Fig. 190. — Euphorbia Lathyris; radial longitudinal section
through half the node and the neighbouring portions of the inter-
nodes of a mature stem, together with the base of a leaf ;
slightly magnified ; 7n pith, b portion of a bundle of the leaf-
trace, h secondary wood, / bast layer of the stem. The dark
lines marked r are the laticiferous tubes ; the short black lines
and points in the node are fragments of the web of laticiferous
tubes occurring there. From this two laticiferous tubes belonging
to the bast, and two hypoderinal ones are seen passing upwards,
while four pass towards the pith ; r hypodermal laticiferous
tubes.

ARRANGEMENT OF THE LATICIFEROUS TUBES.

439

there at an acute angle. At the next point of insertion of a leaf, which they reach on
their way, many of these branches, preserving the same position relatively to the surface,
bend out into the petiole, pass through this to the lamina, and here divaricate near the
epidermis of the lower surface. The great majority of the branches, which are dis-
tributed and terminate in this region, certainly belong to the hypodermal system in
question. The latter stands in no other connection with the main trunks belonging to
the bundles, than that which depends on its points of origin in the nodes ; it is separated
from them in the internodes by many layers of lacunar cortical parenchyma. Its presence
may always be recognised, even on the most superficial observation, by the fact that from
almost every slight prick into an internode, though far from penetrating the cortical
parenchyma, drops of latex exude. The great majority, at any rate, of the Tithymalus-
Euphorbise have essentially the same arrangement of the laticiferous tubes; e. g. E.
Cyparissias, sylvatica, Characias, Peplus, Lagascae, also E. Myrsinites, which is charac-
terised by especially large and numerous tubes; many differences occur, it is true,
according to the particular species, which chiefly affect the greater or less frequency
of hypodermal tubes, the presence or absence of medullary ones, &c., and may here be
passed over.

No accurate investigations of the laticiferous tubes of other Euphorbiaceae exist.
Hanstein says : ‘ Where the development of the latex itself is inconsiderable, as in
Ricinus, Mercurialis, and other genera, there we find its vessels also less abundantly
distributed, and less conspicuous. They possess but scanty ramifications and anasto-
moses, but on the other hand they have more strongly thickened walls.’ Vogl mentions
the laticiferous tubes in the outer and inner cortex of Hippomane Mancinella, and calls
attention to the great similarity of those of Hura crepitans with those of the succulent
Euphorbiae.

8. The laticiferous tubes of the Urticacese, Apoeyneae, and Asclepiadese, agree, so
far as investigations extend, with those of the Euphorbiae in all essential points, as regards
both their form, structure, and ramification, and their development and distribution.
This agreement becomes especially conspicuous if the succulent leafless Asclepiadeae, of
the genera Ceropegia and Stapelia, be compared with the Euphorbiae of similar habit.
On the average the laticiferous tubes of the families in question are narrower and
thinner-walled than those of the Euphorbiae, but very thick ones occur, e. g. in species
of Nerium and Ficus. With respect to the abundance in which they occur in the cortex
and pith of the stem, the divarication of their branches in the parenchyma of the leaf,
&c., the same differences between individual species of each family prevail, as within the
genus Euphorbia. While, e. g. in thick-leaved species of Ficus, they extend their
branches abundantly through the parenchyma of the leaf, up to the epidermis, in the
leaf of Humulus, according to Hanstein, they are confined to the vascular bundles, and
do not extend into their ultimate ramifications. Especially abundant subepidermal rami-
fications are described by Trecul in the leaves of the Asclepiadeae, Echites peltata, and
Arauja sericophora.

Among Asclepiadeae and Apocyneae, a larger number of forms have been investigated,
especially by Trecul, e. g. Hoya carnosa, species of Asclepias (A. Cornuti, curassavica,
&c.), Physostemma, Centrostemma, Cryptostegia, Stapelia, Ceropegia, Echites, Arauja,
Nerium, Vinca, Apocynum, Plumiera, Tabernaemontana, and many others.

Among the Urticacese, more accurate investigation has been chiefly confined to species
of Fieus (F. carica, elastiea, repens). Minute comparative investigations on the course
and development of the tubes are however still to be desired, not only for the orders
last mentioned, but also for the Asclepiadese and Apocynese (ef. p. 198).

CHAPTER XIII.

PRIMARY ARRANGEMENT OF INTERCELLULAR

SPACES ^

Sect. 132. The air-containing, and sometimes water-containing intercellular
spaces have been described in Sect. 51, and in the paragraphs treating of the stomata,
the parenchyma, and the structure of the vascular bundle, and their arrangement was
described at the same time. Here it is only necessary again to bring forward the
fact already mentioned on p. 210, that all the air-spaces in question together form
a connected system of communicating tubes throughout the whole plant. Where
there are stomata this system opens first into the ‘ respiratory cavities ' below them,
and then through the slits themselves it communicates with the outside, and thus in
the terrestrial and swimming plants, which we have more especially under con-
sideration, it also communicates directly with the surrounding atmospheric air

Sect. 133. Of the intercellular secretory reservoirs the short cavities have also
already been dealt with as regards their distribution, on p. 207.

It therefore remains still to treat of the arrangement and course of the secretory
passages and canals, and at the same time to take into consideration many pheno-
mena of their structure, which were passed over before (comp. p. 206), with constant
reference of course to Sect. 50.

The secretory passages traverse the members of the plant longitudinally at first
as prismatic tubes, which usually acquire a round or elliptical transverse section:
rarely they appear as more or less elongated sacs with both ends closed blindly, as
in the exceptional cases of Tagetes and Mammea, quoted on p. 201, and in many
Coniferae. In the great majority of cases they are in open communication throughout
the plant, forming a system of tubes, which branches and anastomoses — especially,
but not exclusively, at the nodes — and may send out blind, or also anastomosing
branches into the foliar expansions.

Their arrangement in the members varies greatly according to the groups and
even the species, nevertheless it is regular and constant within each of these circles
of relationship. Besides those which appear constantly in one species or group of
different rank, there are in many cases accessory passages, which may occur in
varying number according to the individual or species, or may even be absent,
e.g. leaves of Pinus, medullary passages of the Terebinthacese, Coniferae, &c.

* [Compare v. Höhnet, Verhältniss der Intercellularräume zu den Gelassen, Bot. Ztg. 1879, p. 541 ;
also, Ueber Harz-räume im Kork-gewebe, Bot. Ztg. 1882, p. 161, and über gefässführende Hölzer mit
Harzgängen. Ibid.]

^ Compare Sachs, Experimentalphysiulogie, p. 254.

INTERCELLULAR SECRETORY RESERVOIRS.

441

Any other form of tissue, and any region may contain secretory passages, even
the primary xylem of the bundles. Still in this relation also the phenomena are con-
stant, according to species and groups, while in individual cases all possible com-
binations occur, as is obvious from the following synopsis of the most important
known examples, and from the works therein cited, which are to be compared for
further details. For the sake of clearness, and in order to avoid repetitions, the
secondary changes belonging to the subjects of Chaps. XIV and XV will often be
mentioned in the following pages.

The mucilage-canals of the Marattincea^ traverse the parenchyma of the pith and cortex
of the stem in great numbers, and branch and anastomose frequently. They are con-
tinuous from the cortex into the roots, in which they pursue a directly longitudinal course
towards the apex, and here end in the meristem ; and into the foliar organs, having a
similar straight course in the petiole and rachis, with few branches and anastomoses.
Their endings in the foliar organs have not been exactly investigated ; thorough descrip-
tions of their course in the segments of the lamina are also wanting.

The leaf of Lycopodium inundatum (also of L. alopecuroides) ^ is traversed on the pos-
terior side by a mucilage-canal, which runs from apex to base : here it enters the cortex
of the stem for a short distance, and there ends blindly. The mature canal is limited, as
in Marattia, by closely-connected cells of the adjoining parenchyma, but irregular club-
shaped cells are seated upon the latter, which project like hairs into the cavity of the
canal. In the young leaf these cells form a strand 4-5 rows thick, where the future
canal will be, and they have the form of angular meristematic cells ; as the leaf unfolds
they separate from one another, while the surrounding tissue extends to a corresponding
extent, and they elongate to a club-shape, while the mucilage appears between them.
They thus constitute the epithelium of the passage, which is still more dissociated than
that in Marattia. A similar small passage is found in the marginal expansions on the
dorsally winged ridges of the leaves of the spike in L. annotinum.

In the stem of the Cycadeac mucilage-passages are also distributed through the paren-
chyma, in specially large numbers in the cortex : they have branchings and anastomoses.
But branches from those of the stem always pass into the leaves, and there end. They
traverse the petiole and rachis of the pinnate leaves longitudinally, in varying number
according to species and individual — in one small leaf of a seedling of Zamia longifolia
I found, e. g. in the petiole only two, in the larger leaves of stronger plants they are
numerous, and their distribution in the parenchyma is generally irregular. They enter
the pinnae only informs of Dion, Encephalartos, and Stangeria®, in the first genus running
sometimes above the vascular bundles, in Encephalartos between them, and indeed alter-
nating regularly with the parallel vascular bundles in the same plane with them and at
equal distances ; in the pinna of Stangeria they lie above and below the vascular bundles
of the rib, without passing out laterally into the lamina. In the specimen examined one
central bundle, and one lateral one near it on either side ran into the rib. One mucilage-
passage lies between the middle bundle and the upper epidermis, and one on each side
between the lower epidermis and the gap which separates the middle one from the
lateral bundles.

Among the Conifera all investigated species, with the single exception of Taxus, have
resin -passages or resin-reservoirs, which vary in distribution and number according to the
species.

Starting from the leaves *, in those examples which have one median vascular
bundle or pair of bundles — as in the investigated species of the Cupressineae, Sequoieae,

' Harting et de Vriese, Monogr. des Maratt. — Frank, l.c.

“ Hegelmaier, Botan. Zeitg. 1872, p. 844.

’ Kraus, Cycadeenfiedern, l.c. p. 328. * Thomas, Coniferenblatter, l.c.

442

PRIMARV ARRANGEMENT OF TISSUES.

Taxineae, the genera Saxegothea, Dacrydium, Podocarpus (excepting the section Nageia),
and Tsuga with exception of T. Douglasii Carr. — there is one constant resin-passage
between the bundle and the epidermis of the lower surface of the leaf, either close to the
latter, often as a keel or ridge projecting outwards as in species of Juniperus, Thuja, and
Biota, or deeply embedded near to the bundle, as in Cunninghamia (Fig. 19 1). Besides
these there are in many species (e. g. Cryptomeria) accessory passages corresponding in
their arrangement to those which are constant in the Abietineae. One of these lies
(with the exception as above of Tsuga) at each lateral margin of the leaf close to the
upper surface ; either this alone is present, e. g. always in Larix and Cedrus, or three
are also accessory ones in the hypoderma, the number and arrangement of which vary

Fig. 191. — Cunninghamia sinensis ; transverse section through the leaf (220). it lower, 0 upper surface ; h resin
passage, y.s hypodermal fibres, s sclerenchymatous fibres scattered in the parenchyma, g xylera of the median
bundle, t its border of tracheides. Below, and towards the resin passage, is the thin-walled phloem ; the white band
at its margin towards the parenchyma surrounding the resin passage is the compressed primordial tissue of the
phloem, q transversely-elongated parenchymatous cells of the middle of the leaf.

according to both species and individual: e. g. in the needles of Pinus sylvestris to such
an extent that 1-22 of them have been observed.

In the leaves of Sciadopitys 4-10 passages lie under the epidermis, which are dis-
tributed in varying symmetry over the margin and lower sides in relation to the simple
or double leaves.

The leaves of Araucaria, Dammara, and Ginkgo, with several vascular bundles, are
traversed by at least as many passages as bundles ; they alternate with the latter in almost
the same plane.

The passages in most cases pass continuously through the elongated leaves from the base
upwards, and end blindly at a distance from the apex, which varies sometimes with the
individual, sometimes with the species. Thus the median passage in species of Podocarpus,
stops far below the middle of the leaf. In the lamina of Ginkgo, in place of the

INTERCELLULAR SECRETORY RESERVOIRS.

443

uninterrupted canals, there are between the vascular bundles short cylindrical sacs i™“
or more in length, which are closed blindly at both ends. In the scale-like leaves of
many Cupressineae, as Thuja, &c., the passages are of course short, and relatively broad,
and may more properly be termed gaps or cavities.

The passages and cavities of the leaves are continuous from the insertion of the latter
into the primary cortex, and there pass perpendicularly downwards. In the transverse
section they form a ring lying in the cortical parenchyma, and are generally grouped
according to the arrangement of the leaves. At any rate in a large number of forms
they end blindly above the insertion of the lower leaves, without open communication
with other passages. Thus in the investigated Cupressineae with whorled leaves, as
Thuja, Biota, Juniperus. In J. communis e.g. a large passage enters the stem from
each leaf, and there runs downwards, in one of the three angles, to a point close above
the plane of insertion of the next lower whorl, and there ends.

On the other hand, in Pinus sylvestris, Abies excelsa, and, according to Mohl’s state-
ments in the Abietineae generally, the passages which come from the leaf, after descend-
ing through numerous internodes, open into others belonging to lower leaves; the point
of confluence corresponds to a widening of the passage on which the higher one is Inserted.
The passages of the primary cortex are therefore connected into a system of communi-
cating canals. The composition of this, as well as its distribution in individual forms,
remains to be more exactly investigated.

The great majority of the Conifers have no others in the primary tissues of the stem
besides the cortical passages above mentioned. This is the case" in all investigated
Taxines except Ginkgo; most Cupressines, Podocarpus, Cedrus, Abies, Tsuga, Pseudo-
larix. — Araucaria Cookii and Brasiliensis, and Widdringtonia cupressoides, have besides,
according to van Tieghem, a passage in the phloem of the primary vascular bundles,
which stops short before the exit of the bundle into the leaf.

In the species of Pinus s. str., Larix, Picea, Pseudotsuga there is also a passage which
is not continued into the leaf, but it does not lie in the phloem, but in the xylem of the
primary bundles.

Finally, Ginkgo biloba has large passages in the pith in addition to the cortical ones.
In transverse section there are one or two present, and so arranged that they correspond
to the insertions of the next higher leaves. Nevertheless they end blindly both down-
wards and towards the petiole, though the canal situated in the latter above the vascular
bundles lies in the ideal prolongation of the medullary canal situated opposite the leaf
in question.

In the Root the passages of the primary cortex are wanting in all investigated Coniferje,
and in most cases also those of the vascular bundles. In the latter, however, they are
found in certain species or groups ; thus, according to van Tieghem, in Araucaria Cookii
and Brasiliensis there are five in each phloem portion of the diarch bundle, in Widdring-
tonia cupressoides one in the same position. The Cedars and Firs (Cedrus Deodara,
Abies pectinata, balsaminea, Brunoniana) and Pseudolarix Kämpferi have a canal in the
middle of the radical bundle. In the Pines (Pinus s. str.) and Larix one passage lies
between the two shanks of eaeh vascular plate, described on p. 357.

Alismaee® and Butomese. For Alisma Plantago Meyen, and especially Unger have
given exact descriptions of the canals with milky contents, while Frank has cleared up
the history of their development. According to Unger’s description the passages are
absent from the roots, but are distributed throughout the rest of the plant. In the
rhizome they traverse the parenchyma, forming a network branching in all directions,
and with a course independent of the vascular bundles. Those which enter the petiole
and peduncle branch off from this network, and then take a longitudinal course, being

^ Botan. Zeitg. 1859, p. 333.

^ Meyen, Phytotomie, Taf. XIV. — Unger, Das System der Milchsaftgänge in Alisma Plantago,
Denksch. d. Wiener Acad. Bd. XIII. 1S57. — Van Tieghem, Lc.

444

PRIMARY ARRANGEMENT OF TISSUES.

connected on the way by occasional transverse anastomoses. Those of the peduncle only
occur in the hypodermal parenchyma. In the lacunar tissue of the petiole lie numerous
small vaseular bundles at the periphery, and five arranged in a curve in the middle. Ex-
ternally to each peripheral bundle, and between it and the hypodermal layer of cells, lies
one passage; one wider one, with its epithelium abutting directly on the epidermis,
alternates with each pair of peripheral bundles. Around the inner bundles there is in the
layer of parenchyma surrounding them, one passage opposite the points of insertion of
each of the plates of parenchyma separating the air cavities. In the lamina of the de-
veloped foliage-leaves the passages appear on both sides immediately beneath the
epidermis : their main trunks accompany the chief vascular bundles of the leaf : their
very abundant branches together form a completely closed net, the meshes of which do
not coincide with those of the network of bundles. The linear primordial leaves of the
young plant have only three passages, which accompany the three vascular bundles, and
only come together at the apex of the leaf. Those which enter bracts and sepals are
only connected one with another at the base of these organs by anastomoses, and then
run parallel towards the apex, and end blindly short of the latter.

In Sagittaria sagittifolia the passages are arranged in the cortex of the stolons in two

circles, the one peripheral, the other
internal, near to the cylinder of vas-
cular bundles ; in the petiole, which has
a structure like that of Alisma, pas-

sages are found between the epi-
dermis and those vascular bundles
which do not abut directly upon it ;
there are others, which are arranged
between the bundles, and singly at the
points of union of the plates of paren-
chyma which separate the air cavities
(van Tieghem).

3

Similar conditions to those in the
Alismaceae described are found in Hy-
drocleis Humboldtii, for the details of
which reference must be made to
Schleiden ^ and van Tieghem. Here
also the passages are wanting in the
roots.

“ Among the Arcideoi, according to

FIG. 192. — Philodendron Imbe Hort. Halens ; transverse section of a the obsCrVationS of Tr4cul^ and VaU

Philodendron, Homalonema, Schisma-

toglottis, and gum pass iges in many species of Aglaonema. The other genera of the
family in question, as far as investigated on this point, have no passages : this applies
on the one hand to all those which have true milk-tubes, and on the other to those whose
vascular bundles are accompanied neither by milk-tubes nor by rows of tannin-contain-
ing sacs, comp. p. 436. The resin passages of numerous investigated species of Philo-
dendron traverse all the members of the plant longitudinally as narrow canals, apparently
(but nothing is stated explicitly on this point) in such a way that they are all connected
one with another at the nodes and other points of insertion. In the lateral roots, the
stem, and petiole, they are scattered in the parenchyma, forming in the cortex of the
root 3, 4-5, or even 8 (Ph. Melinoni) more or less regular concentric rows (Fig. 192) ;

Grundzüge, 3 Aufl. I. p. 267.

2

Comptes Rendus, tom. LXII. p. 29 (1866).

^ Structure des Aroid^es, 1. c.

INTERCELLULAR SECRETORY RESERVOIRS.

445

and occurring in stem and petiole either in the peripheral zones of parenehyma only,
and even within the hypodermal collenchyma, or also (Philodendron hastatum, tripar-
titum, micans) internally, between the vascular bundles. In the leaf-lamina they run in
the parenchyma between the tertiary branches of the bundles and parallel to them ; either
about the middle-plane of the leaf (e. g. Ph. micans, lacerum, crinipes, Imbe, &c.) ; or
near to the lower surface of the leaf, and only separated from its epidermis by 1-2 layers
of cells (e. g. Ph. eximium, Rudgeanum, Sellowianum, pinnatifidum, cannaefolium, &c.).

The passages of Schismatoglottis, Homalonema rubescens, and H. Porteanum resemble
those of Philodendron in fundamental points of form and course, but with the limitation
that in the stem, of H. rubescens, instead of elongated canals, there are found elliptical
cavities from 0-25“™ to o-so“™ in length and 0-20'"'“ to o-38'“*“ in width. It is remark-
able that, aceording to Trlcul’s account, the canals and cavities are entirely absent in
H. Wendlandii.

The parenehyma of the stem of Aglaonema marantaefolium is traversed throughout its
whole length by gum- (or mueilage-) passages about 0-24’“'“ wide, which, however, are not
continued into the leaves nor into the peduncle. In A. simplex these are absent.

Van Tieghem found similar gum-passages in the petiole and midrib of the leaves of
Monstera surinamensis, in the cortex of the stem, and in the petiole of Rhaphidophora
pinnata, and in the lower part of the petiole of Anthurium erassinervium, while they are
absent in M. .Adansonii, Rh. angustifolia, and Anth. violaceum.

The above-mentioned resin-passages have, as was above indicated, a typical structure;
in the roots of Philodendron their epithelium, which is composed of 2-3 layers, is
sheathed by 2-3 closely connected layers of narrow elongated sclerenehymatous fibres
(comp. p. 202). The resin-cavities in the stem of Homalonema are enclosed by several
layers of thin-walled cells arranged in radial rows (apparently derived by division from
so many primary cells) ; the inmost project uniformly in a convex manner into the
cavities. The gum-passages above mentioned are surrounded by a layer of small cells,
often projecting, as in the Marattiaceas, into the passage: these differ but slightly from
those of the adjoining parenchyma.

In the rhizomes of the investigated species of Ganna, and also in the lower portions of
the flowering stems, there are numerous passages : these are filled by a clear transparent
mucilage, which oozes out in glistening drops when they are eut through. The passages
are absent from the eortex, and are very numerous v\ ithin the periphery of the vascular
cylinder: in the middle they are less frequent. They traverse the rhizome longitu-
dinally, their endings have not been observed ; anastomoses and points of branehing were
found here and there. Their walls are eomposed of small cells with abundant proto-
plasm, which often project as irregular papilla into the passage. There has been as
yet no thorough investigation of their development.

Among the Composite' all investigated forms of the section Tubiflorm have a system of oil
passages characterised by complexity of composition and uniformity of arrangement.
There are no investigations at hand concerning the Labiatiflorx. In the Ligulifloral
Cichoriaceae they are absent, with the exception of isolated cases to be mentioned
ultimately.

In the roots of the Corymbiferm and Cynareee the passages lie in the innermost portions
of the primary cortex, and when typically arranged they form a simple curved series
opposite each phloem group of the axile vascular bundle, thus alternating with two xylem
plates of the latter. According to the usual plan of strueture of the roots, the cells of
the inner layers of parenchyma are in these plants also arranged in regular radial and
concentric rows; between the angles of junction of any set of four there is a 4- or 3- angled
intercellular passage; the inmost layer has the properties of the endodermis. In the
simplest case the angular intercellular passages, lying at the point indicated between the
endodermis and the next outer layer of parenchyma, assume the properties (i.e. the

‘ Van Tieghem, Cananx Secreteurs, /. r.

446

PRIMARY ARRANGEMENT OF TISSUES.

contents) of oil-containing passages. There they together form the curved series, and
are separated from one another laterally by a single cell only. According to van Tieghem
the oil passages, especially in Tagetes patula, arise at the point indicated, between con-
centric layers of cells, which are derived by tangential division of the originally simple
endodermal layer, the latter remaining simple opposite the plates of xylem. In the
seedling of Helianthus annuus both cases may be found side by side in the same main
root. In addition to these normal passages there are not uncommonly other peripheral
ones, which arise in the same way between the layer surrounding the endodermis and
the next outer layer. All the passages are at first narrow, the two lateral and outermost
ones of each normal curved series are triangular, the rest quadrangular. When a peri-
pheral series is present, its members may coalesce with those of the inner series by
splitting of the cell-wall separating them. The number of the passages of any normal
curved series or group shows individual variations even in one and the same root, but
comparison of a number of cases shows that there are different average numbers which
are characteristic for groups, genera, and species.

These numbers are highest in those Cynareae which have been investigated — the
following quotations refer in each case to a single group opposite one mass of phloem; —
there are lo and more in the diarch main roots of Carduus pycnocephalus, Silybum
marianum, Xeranthemum cylindraceum, the diarch or triarch roots of Centaurea atro-
purpurea, Echinops exaltatus ; 15-20 in the diarch main root of Cirsium arvense ; 12-15
in the tetrarch subsidiary roots of Serratula centauroides. Calendula officinalis has
8-10; Venidium calendulaceum 3-5.

In the Senecioneae the number decreases; e. g. Helianthus annuus, in the tetrarch
main root, 5-8’; Gnaphalium in the diarch root, 5-8; Tagetes patul 1, in the diarch
main root, 5-7; Arnica Chamissonis, Tanacetum vulgare, tetrarch, 4-6; Cotula matri-
carioides triarch, 2 ; Achillea millefolium the same, 1-3 ; Senecio vulgaris, tetrarch, 2,
sometimes united into one ; Pyrethrum Parthenium, triarch, i, rarely 3, &c.

Among the Astereae van Tieghem found 6-8 in a triarch root of Inula montana, but
only one in Beilis perennis, Erigeron glabellus. Aster, Conyza, and species of Solidago.
In the latter cases, especially in Solidago limoniifolia, the canal may be greatly widened,
by the separation of the cells which originally limited it externally, so that it extends
as far as the next outer layer, or even continues, by further separation of cells, into
several layers which lie further out.

Among the Eupatorieae a triarch root of Tussilago Farfara showed 5-7 passages; a
similar one of Ageratum conyzoides had 2-3 ; Petasites niveus and Eupatorium aroma-
ticum have i each, which is extended as in Solidago.

As the primary cortex is stretched by the secondary formation of wood and bast, the
primary passages of the root, at least in the investigated Senecioneas, remain in their
place, while they increase to a variable extent in width, and the cells surrounding them
in number by division. Compare the details on Tagetes patula in van Tieghem i.c. and
the drawing of Radix Artemisiae in Berg. Atlas, Taf. XV.

In the stem of the Compositae in question the oil passages are only absent in relatively
few exceptional cases: Echinops exaltatus, Gnaphalium citrinum, according to van
Tieghem. In by far the most numerous cases they are continuous from the root through
the stem with its branches and leaves, but subject to branchings, or increase in numbers,
which will be described below. They are seated in the primary tissue of the stem
always in close contact with the outer side of the plerome sheath, which in the Compo-
sitae (p. 415) may be followed from the hypocotyledonary portion through the whole stem,
covering the outside of the ring of vascular bundles. In the hypocotyledonary stem
the passages have the same structure and arrangement as in the root ; higher up, and
especially from the cotyledonary node onwards, they change their arrangement, accord-
ing to that of the vascular bundles, in a manner to be mentioned immediately; they are

* Compare .Sachs, Botan. Zeitg. 1859, Taf. VIII. Fig. 7.

INTERCELLULAR SECRETORY RESERVOIRS.

447

separated from the plerome-slieath by a special layer, often consisting of numerous small
epithelial cells. In addition to those which traverse the primary cortex, there are in
certain species, but not all, other passages situated at the periphery of the pith, but
these are only to be seen above the cotyledonary node.

As regards the special distribution in the transverse section of the stem van Tieghem
quotes the following special cases.

(1) Only cortical passages are present in contact with the plerome-sheath.

{a) Only one passage in the middle of the outer margin of each main leaf-trace bundle;
Senecio vulgaris, Kleinia ficoides. Cineraria maritima, Flaveria contrajerva, Beilis perennis,
Petasites niveus, Baccharis halimifolia, &c.

{b) The .same, but in addition as many passages opposite the outer margin of each
united leaf-trace (faisceau reparateur) as single bundles of the trace coalesce above to
form the united trace : Aster.

(c) On each side close to the phloem of each main bundle of the trace is one passage :
Tagetes patula, Arnica Chamissoni.s, Tanacetum vulgare, Cotula matricarioides, Ana-
cyclus Pyrethrum, Pyrethrum Parthenium, Santolina Chamsecyparissus, Achillea Mille-
folium, Zinnia elegans, Inula montana, Cirsium arvense, &c.

(4) An unequal number, e. g. 3-5 passages opposite the outer margin of each main
bundle : Centaurea atropurpurea.

(e) A group of passages opposite each lateral margin of the phloem of each main bundle:
Silybum marianum.

(2) Cortical and medullary passages are present, the latter opposite the xylem of the
bundles.

{a) Medullary passages only opposite single bundles, e. g. two : Ageratum conyzoides.

{b) Opposite each bundle of the leaf-trace is externally a cortical, and internally a
medullary passage : Solidago limonifolia.

{c) Opposite each bundle of the leaf-trace one medullary and several cortical passages
occur : Serratula centauroides. Dahlia variabilis.

(4) Near each bundle is one group of medullary and one of cortical passages: Carduus
pycnocephalus, Spilanthes fusca.

(f) Opposite each bundle is a curved series of medullary and another of cortical pas-
sages: Helianthus tuberosus.

The petioles and leaves have no oil passages when they are absent in the stems bearing
them, while they are rarely wanting when the latter do contain passages : Xeranthemum
cylindraceum, Cirsium arvense, radical leaves of Lappa grandiflora. In most cases oil
passages are present in the leaves, and especially as direct continuations or branches
of cauline passages ; often also others occur which may be called accessory. The former
may, however, be called fascicular, since they accompany the bundles, either directly, like
the cortical ones, or in close proximity to the parenchymatous or endodermal sheath, which
each individual bundle takes with it from the stem into the leaf. In single plants, as
Tussilago Farfara and Cineraria maritima, they are intercalated in the sheath itself.
Their number and arrangement about each bundle, as seen in transverse section, differ
similarly according to the species, but still more variously than in the stem, as may
be seen from the examples quoted by van Tieghem, l.c., pp. 118 and 133, See. They
either accompany the branches of the vascular bundles through the lamina (on this point
closer investigation is required), or they are limited to the midrib or rachis, as in the leaf
of Tagetes patula, where they do not enter the lateral segments of the leaf. Their form,
average size, and limitation are, at least at first, the same as in the root.

In addition to these fascicular passages there are in single cases, in those species which
form passages in the secondary bast of the stem, others situated in the last-formed parts
of the phloem of the petiolar bundle : e. g. Helianthus annuus.

Van Tieghem found accessory passages in the leaf of Solidago limoniifolia : beneath the
epidermis of the lower surface, and separated from it by 1-2 layers of collenchymatous
cells, there is a series of 3-5 narrow canals on either side of the mid-rib. Again, in

448

PRIMARY ARRANGEMENT OF TISSUES.

Tagetes patula : neither the cotyledons nor the lateral portions of the lamina receive
fascicular passages in this plant. On the other hand there is on either side along t ie
margin, in the parenchyma of the lower side of the leaf, an unbroken series of oil-con-
taining (schizogenetic) sacs, closed blindly on both sides (comp. p. 201).

Finally, in the Cichoriaceae the oil passages are wanting in all parts in most species
investigated, and in the stems and leaves of all species. But in the root of Scolymus
grandiflorus van Tieghem found them in groups of five, with exactly the same position,
origin, and structure as the primary ones of the roots of the Senecionese. And in the
diarch main root of Cichorium Intybus and Lampsana communis there are also rudiments
of passages, there being the characteristic cell-divisions, but no opening of the passage
at those points of the endodermis where they arise in other Compositae. Scolymus is
therefore the one known member of the Cichoriaceae which really has oil passages in
addition to the laticiferous tubes.

The case seems to be different with the relations between the occurrence of oil
passages and that of the sacs with milky contents, which accompany the vascular bundles
(comp. p. 149). At least van Tieghem found both organs side by side in the upper part
of the*"stem and leaves in Cirsium arvense and Lappa; it is true that in the leaves the
passages soon stop, and the sacs become the more numerous. The same seems to be the
case in other species, e. g. according to Trecul’s and van Tieghem’s statements in
Cynara Scolymus ; but on this point further investigations are wanted.

All investigated Umbelliferx. ’ have without exception a very complex system of longi-
tudinal usually anastomosing sap-passages, the contents of which are ethereal oils with
resin or milky mixtures of these bodies with mucilage and solutions of gum.

In the primary tissues of the root the passages lie exclusively at the periphery of the
vascular strand, directly within the endodermis. Their formation starts from a portion
of the one-layered ring of pericambium situated in each transverse section opposite the
angle of each vascular plate. The number of the cells of this portion is always even ;
e.g. 6, 10, 12 ; the radial wall, which separates the two central ones (it may be called the
middle wall), is opposite the outermost vessel of the plate, in the extension of its median
plane. On both sides of it lies an equal number (e.g. 3, 5 ... ) of passage-forming cells.
These are at first rectangular in transverse section, and somewhat radially elongated.
Each then divides into two cells by a wall, inserted at the middle of its outer wall, in-
clined at about 45° to the radial wall facing the elongation of the vascular plate, and
meeting this further out than its middle ; of these two cells one is large and irregularly
5-angled in transverse section, and one small and triangular. The triangular cells lie at
the outer limit of the pericambial layer, the 5-angled cells extend through its whole
thickness ; the middle radial wall of the portion of the ring is met by two, all the rest by
one of the inclined walls. At the angles between each triangular cell and its 5-angular
sister-cell an oil passage arises by splitting of the walls; this passage is situated at the
middle radial wall, and is quadrangular, being limited externally by the two originally
triangular cells which remain small, internally by two 5-angled cells; at all other radial
walls, however, a triangular passage is formed, limited externally by one triangular, and
internally by two 5-angular cells. Opposite each vascular plate both of two- and of
many-rayed bundles there arises a curved series of passages, the number of which must
always be uneven : one central one and on either side of it the same number of lateral
ones. The absolute number varies in species and individual between five and thirteen
opposite each vascular plate. The middle quadrangular one of the passages of each group
is the largest, and the width of the rest diminishes as they are birther removed from it.

In addition to these passages, which correspond to the vascular plates, one smaller one
appears rather later in the middle of each sieve-group. It is pentagonal in transverse

* Jochmann, De Umbelliferarum Structura, Berlin, 1S54. — Trecul, Comptes Rendus, tom. LXIII.
pp. 154, 201 (18Ö6).— N J. C. Müller, in Pringsheim’s Jahrb. V. /.e.— Van Tieghem, Ann. Sei. Nat.
5 ser. XVI. Compare above, § 50.

INTERCELLULAR SECRETORY RESERVOIRS.

449

section, and is bounded externally by two cells of the pericambial ring, internally by
three cells of the phloem group. The whole number of the passages in one root-bundle
may accordingly be very large, e, g. 2 x 1 1 next the xylem^ plus 2 in the phloem, in the
diarch main root of Pastinaca ; 4 x 5 next the xylem, plus 4 in the phloem, in tetrarch
adventitious roots of OEnanthe pimpinelloides, &c. Adventitious roots like those of the
last-named plant may retain this primary structure for a long time, or even throughout
life. Usually, however, and in all investigated main roots, it is altered at an early stage.
The cambium which appears in the bundle (Sect. 139) produces externally a massive, and
for the most part parenchymatous secondary cortex ; the primary cortex, together with
the endodermis, is simultaneously thrown off with an abundant formation of periderm
(sect. 176) starting from the pericambial cells lying outside the passages, and strong
increase and division of the pericambial cells lying within the passages. The passages
thus come to lie near to the inner side of the periderm, at first with their original
arrangement, later, as the result of the increasing growth in thickness of the cortex,
more and more dislocated. They then represent the canals described by Trecul, which
lie under the peridermal covering of the roots of Umbelliferae.

The hypocotyledonary stem of the seedling retains approximately the structure of the
bundles and arrangement of the passages of the root till close below the cotyledons. One
passage branches out from the hypocotyledonary stem to each of the three vascular
bundles which enter there, and has in the cotyledon a position close to and opposite the
phloem of the bundle.

Though there are no definite statements on this point, it cannot be doubted that a
connection exists in the cotyledonary node between the passages hitherto described and
those found higher up in the stem.

The arrangement of these differs in most cases from those hitherto treated of.

In the outer cortex of the internodes there is as a rule one passage opposite each vascular
bundle, or each of the stronger ones (Fig. 193). Since one angle of the stem or one hypo-
dermal collenchymatous bundle corresponds usually to each of the latter, especially the
stronger ones, the passages are thus, as Trecul states, also opposite the strands of collen-
chyma. Instead of one only, 2, 3, or even 4 passages may be opposite to one broad
strand of collenchyma. According to the species these passages are either near to the
periphery, close to or even in the strand of collenchyma ; or they have a more internal
position in the parenchyma between the latter and the vascular bundle. In the creeping
stem of Hydrocotyle vulgaris there is a passage, at the inner limit of the non-collenchy-
matous cortex, closely opposite each vascular bundle, with its epithelium directly adjoin-
ing the inner face of the endodermis (p.121); the same arrangement appears in the branches
of Bupleurum fruticosum.

In addition to the passages mentioned, which may be called fascicular, there are in
many species more or less numerous ones situated in other positions in the outer cortex;
thus e. g. according to Trecul, in all regions of it, from the epidermis to the limit of the
ring of bundles in Smyrnium Olusatrum, jEgopodium Podagraria, and Sison Amomum.
Trecul distinguishes, according to the conditions of arrangement indicated, ten types,
the number of which may be increased if necessary.

Almost all Umbelliferae have oil passages in the pith of the internodes of the stem ; but
Trecul cites Bupleurum Gerardi and ranunculoides as exceptions to this. Also in Hy-
drocotyle vulgaris and Xanthosia rotundifolia I find no medullary passages. The flower-
ing branches of Bupleurum fruticosum show, according to the same author, in the upper
internodes numerous medullary passages alternating with the inner margins of the vas-
cular bundles ; in the lower internodes the number of them diminishes successively, but
they seem, according to the description, which I cannot fully understand, to be present
originally also at the base of the branch, and to be crushed by secondary extension of the
surrounding cells of the pith. In the seedling of Foeniculum officinale the medullary
passages are often entirely absent in the first internodes (comp. Fig. 193) ; in higher in-
ternodes they appear quite isolated at first, but in large numbers in the stronger plant.

450

PRIMARY ARRANGEMENT OF TISSUES.

Where the pith is permanent, e. g. in the stems of species of Ferula and the rhizome of
Imperatoria Ostruthium the passages may be scattered through the whole pith. In the
numerous species with internodes which become hollow they are limited to the persistent
periphery of the pith (Anthriscus vulgaris, Myrrhus, Carum Carvi, Heracleum sp.) If
they are originally formed in the middle, and this is a point which is not decided, they
disappear with the cells of the pith surrounding them. In some cases, however, the passages
persist in the middle of stems which become hollow, either surrounded by some layers of
pith-cells, and standing freely and singly in the hollow (Smyrnium Olusatrum), or em-
bedded in permanent lamellae of pith, which extend from the periphery into the hollow
(Heracleum Sphondylium).

The passages described run through the internodes as a rule with a straight, longi-
tudinal course, and few branches or anastomoses. There are, however, numerous branches

at and near to the nodes, by which all anastomose one
with another, and are extended into the leaves and
axillary shoots. Blind endings have not been ob-
served. Also the sacs described in the old rhizome
of Imperatoria are only huge dilatations of the passages.

The passages have a similar distribution in the
petiole to that in the stem. Anastomoses, even of
reticulate form, occur at the points of insertion of the
segments of divided or compound leaves. The branches
finally enter the lamina. Here, according to Trecul’s
observations on Angelica silvestris, Opoponax, Impe-
ratoria, Smyrnium, Ferula tingitana, Lagcecla, &c., and
also in Eryngium, they accompany the vascular bundles»
in such a way that they traverse the nerve both on the
upper and on the under side, in the latter position they
are on the average larger and more numerous ; there is
one in each of the smaller nerves, in the larger there are
often several ; further they are connected and in open
communication by means of their ultimate branches so
as to form a network similar to that of the bundles.

The sap-passages of the Araliacex contain resin in
most cases investigated; according to Trecul they
contain gum in Aralia chinensis, spinosa, Panax Lessonii,
P. crassifolium, &c. According to Trecul’s investigations of numerous species of the
genera Hedera, Paratropia, Cussonia, and the plants already named, they are as generally
distributed in this family as in the Umbelliferae, and the general plan of their arrange-
ment and course in root, stem, and leaves, as well as their various modifications
according to single species, correspond so closely to those in the Umbelliferae, that
they need not be thoroughly entered into here, but reference may be made for many
details to TrecuU and N. Müller (/.r.). In the roots of Hedera Helix and Aralia
Sieboldtii van Tieghem found the number and arrangement of the primary groups
near the vascular plates to be not always so very regular as in the Umbelliferae, and
those in the phloem to be either in contact with the pericambium, or completely
enclosed in the phloem. The numerous anastomoses between the radially undulated
passages of the primary (and also secondary) cortex of the branch of Paratropia
macrophylla observed by Trecul may here also be mentioned, and the statement of
the same author, that in the lamina of Panax Lessonii and crassifolium the passages
seem to occur in the mid-rib only, and apparently do not follow the lateral branches
of the vascular bundles.

Fig. 193. — Transverse section through an
ntemode of a young plant of Fceniculum offi-
cinale (40); pith surrounded by the partly num-
bered vascular ring. Between the bundles the
cambium zone connecting them is indicated ;
the small circle outside the stronger bundles is
the transverse section of an oil passage ; in each
of the blunt angles of the stem the transverse
section of a fibrous bundle is indicated in form
of a segment of a circle. Compare p. 241.

’ Compare Berg, Atlas z. Pharm. Waarenkunde, Taf. 22. — Wigand, Pharmacognosie.

^ Des Vaisseaux propres dans les Araliacees ; Comptes Rendiis, tom. LXI. p. 1163 (1865).

INTERCELLULAR SECRETORY RESERVOIRS.

45 i

The family of the Clusiacex * has especially numerous passages containing gum-resin,
and in fact all members of it with exception of the genus Quiina, which is separated as a
special group.

According to Trecul’s and van Tieghem’s investigations there are in the stem and
roots of the true Clusiaceae three chief forms of distribution of the. passages. In the
genus Clusia they lie only in the primary parenchyma, in the stem they occur also in the
pith, but are absent from the vascular bundles and the secondary cortex.

A second category has passages in the phloem of the vascular bundles in addition to
the above-mentioned places ; there being one in each primary phloem-group of the root-
bundle, in each primary bundle of the stem, and further in the secondary bast ; e. g.
Mammea americana : the same distribution, with the exception of those in the primary
bundles of the stem, is seen in Calophyllum Calaba.

Thirdly: the passages, are wanting in the cortical parenchyma of the root, but are
present in that of the stem, and in the primary phloem-groups and the secondary bast
both of the root and of the stem : Rheedia lateriflora, Xanthochymus pictorius. They
are present in the pith of the stem in Rheedia, but not in Xanthochymus.

The passages run longitudinally and anastomose one with another through the whole
plant : more rarely in the internodes, but always, and in various individual forms described
by Trecul, in the nodes. From the latter, branches of the passages go into the petiole,
and through this further into the lamina. In these members they are found in most
species only in the parenchyma and the watery hypoderma. Only in Mammea americana
a number of the vascular bundles entering the petiole take the passage which traverses
their phloem with them out of the stem : in the case of the median bundle it goes nearly
to the apex of the leaf. The number of those which enter the petiole is usually high,
and varies according to species. Trecul states, e. g. that there are 30 in Rheedia lateri-
flora, about 40 in Xanthochymus pictorius, 14-20 in Calophyllum Calaba, and more than
200 in Clusia rosea. Their distribution in the parenchyma varies according to the species.
Finally, in the lamina also they run without relation to the vascular bundles, even cross-
ing these in certain cases, and branched here and there, but without visible anastomoses.
According to the arrangement in the massive leaf-substance there may be distinguished a
system of internal passages situated with vascular bundles in the inner chlorophyll paren-
chyma, and a hypodermal system, of which the passages are always narrower. For details
on these relations see Trecul, 1. c. Of the forms investigated Mammea americana alone
has no passage in the lamina except that which traverses the mid-rib, but has instead a
round resin-containing cavity embedded in the parenchyma in each mesh of the network
of vascular bundles.

Pittos-porex The root of Pittosporum Tobira shows originally opposite each vascular
plate a group of oil and resin passages of similar origin and arrangement to those in
the Umbellifer^. The number of the passages in each group is certainly smaller, and
their arrangement often less regular than in this order : there is a central quadrangular
one, and usually on either side of it a smaller, triangular one. The passages in the
phloem-strands are absent in Pittosporum. By the same process of secondary formation
as in the Umbelliferae the passages are subsequently pushed outwards, under the periderm,
while they widen considerably, and the cells limiting them increase in number. In the
primary tissue of the stem there is only one passage in the outer part of the phloem of
each vascular bundle, and these passages of the stem are continuous with those of the
root at the limit of the two members. The bundles which enter the leaf are each
accompanied by one passage, which retains the same position as in the stem, and divides
in the lamina into branches, which also follow the branches of different rank of the
vascular bundles.

^ Meyen, Physiol. II. p. 384. — Anonymous author in Botan. Zeitg. 1846. — Trecul, in Comptes
Rendus, LXIII. pp. 537 and 613 (1866). — Van Tieghem, l.c.

Muller, l.c. — Van Tieghem, l.c.

452

PRIMARY ARRANGEMENT OF TISSUES.

The same conditions of arrangement as in Pittosporum are found in the primary
vascular bundles of the branches and leaves of Sollya heterophylla, and Citriobatus
multiflorus. Bursera spinosa, however, never has sap-passages, according to van
Tieghem.

CactecB. The- passages containing latex in many Mamillaria (comp. pp. 202, 206)
traverse the whole stem, and are scattered through the parenchyma. They are rare in
the inner pith, but numerous in the zone of parenchyma situated between the ring of
wood and the inner circle of cauline bundles (comp. p. 254), in the whole cortex, and the
mamillae. They are branched in all directions, and all branches communicate openly
one with another; those which enter the mamillae run within them near to the axile
strands of xylem, and give off numerous branches, which are repeatedly branched and
run through the chlorophyll-parenchyma straight towards the surface, many of them as
far as the simple hypodermal layer of collenchyma.

In other Cacteae these passages are not found. Those found by Schleiden ' in Opuntia
peruviana differ fundamentally from them. I have investigated them in O. robusta.
They lie here close to the outer limit of the phloem (not, as Schleiden states, in it) of
the bundles of the trace, which are connected into a net, and follow them in their lon-
gitudinal course. They are apparently of lysigenetic origin, being cavities in the
parenchyma, which attain a width of J““, and are filled with swollen, sometimes still
recognisable cells, and numerous conglomerated crystals of calcium oxalate embedded in
the mucilage.

The investigated Anacardiacea — Schinus molle, Spondias cytherea, Pistacia vera,
Lentiscus, Rhus aromatica, suaveolens, Cotinus, Coriaria, virens. Toxicodendron, typhina,
glauca, elegans, semialata^, and villosa — are, as regards the disposition of their passages
containing mixtures of gum-resins, distinguished by the fact that they are situated in the
stem and leaves in the phloem of the primary vascular bundles. Further, in addition to
this, there are others in the secondary bast of the stem, from which, in Rhus viminalis,
blindly ending branches penetrate here and there horizontally into the medullary rays
of the xylem: finally, in many species (Rhus toxicodendron, typhina, glauca, elegans,
viminalis, semialata, and Spondias cytherea) there are medullary passages.

In the root a relatively large passage lies in the middle of each phloem portion of the
primary, usually 2 or 3 rayed vascular bundle. In the secondary layer of bast new ones
are successively added.

The phloem portion of the primary bundles of the stem is limited from the parenchy-
matous outer cortex by a strong bundle of sclerenchymatous fibres of half-moon shaped
transverse section, and the fibrous bundles are almost in contact with one another at
their margins, thus forming together a ring surrounding the outer cortex. Outside this
there is no resin passage, but there is a thick one immediately within it in the phloem of
each bundle. In the secondary cortex, which appears later internally, new ones are
formed successively in the strands of bast. The medullary passages vary in number
according to the species; Trecul gives as the number, e. g. in a transverse section of
a branch of R. semialata 58, typhina 25, viminalis 5-12 ; the larger number are situated
especially at the periphery of the pith, the minority scattered irregularly. According to
successive transverse sections it appears that a part at least of the medullary passages ends
blind in the pith. The cortical passages, as far as they belong to the secondary bast, are
connected also in the internodes by more or less numerous tangential anastomoses. In
the nodes the cortical passages anastomose both with one another, and also with the
medullary passages by means of branches, which follow the vascular bundles out into the
leaf; from the plexus of anastomoses the passages are continued down into the next
internode and into the leaf.

* Anatomie der Cacteen (Mem. pres. Acad. S. Petersbg. tom. IV), p. 358, Taf. VII. 4.

^ Trecul, Des Vaisseaux propres dans les Terebinthinees. Comptes Rendus, tom. LXV. p. 17,
1867. — Van Tieghem, l.c.

INTERCELLULAR SECRETORY RESERVOIRS.

453

The branches observed by Trecul in Rhus viminalis, which pass into the wood, pass
off nearly at right angles from the cortical bundles, and into the medullary rays, without
reaching the medullary passages.

The vascular bundles passing into the petiole, which are arranged in curves in its
transverse section, and branch in their further course, take with them one passage each
from the stem : these passages have the same position as those in the primary bundles.
The same is the case with the stronger branches of the bundles, while the passage is often
absent in the weaker ones. In addition to these passages there are in the petiole of Rhus
semialata medullary passages also, which lie 1-3 together on the inside of the strongest
bundles ; in the petiole of Spondias cytherea one is placed opposite the inner margin of
the median bundle. A similar arrangement, the details of which may be read in Trecul’s
work, is found in the mid-ribs of leaves and leaflets ; these contain several vascular
bundles, which turn their phloem, and also the resin-passages sometimes towards the
upper, sometimes towards the lower surface of the leaf. All lateral ribs contain but one
passage turned towards the lower surface of the leaf, and even this is absent from the
last branches of the bundles. In Rh. semialata and glauca Trecul saw the passages of
the leaf-lamina anastomosing in a reticulate manner like the vascular bundles, which
they accompany.

According to van Tieghem’s investigations of Bursera gummifera, and the transverse
sections of branches of species of Balsamodendron and Protium figured by Marchand
there is found in these balsam-bearing trees of the order Burseracex a structure of the
cortex similar throughout to that described in the Anacardiaceae, and the same distribu-
tion of the gum-resin passages in the roots, the stems and their branches, and the
petioles.

In the genera Ailantus and Brucea now placed among the SimarubeiB, there are
longitudinally-running sap-passages, as in many species of Rhus, at the periphery of the
pith of the stem ; in Ailantus glandulosa as many as 60 ; in other regions of the stem they
are wanting. They appear, according to accounts at hand, to traverse the successive
internodes, and to give off branches at the nodes into the leaves; this is not, however, dis-
tinctly stated. At all events the passages are again found in the petioles and the mid-rib
of the leaflets, and in the pith-like parenchyma surrounded by vascular bundles arranged
in curves or rings, or lying between these. They are not present in the lateral ribs
which are given off from the mid-ribs of the leaflets.

' L. Marchand, Recherches pour servir ä I’histoire des Burseracees ; in Baillon, Adansonia, tom.
VII. p. 258, pi. VIII, et tom. VIII, pp. 17, 74, pi. II., III.

Trecul, Vaisseaux propres des Terebinth acees, l.c.

SECOND SECTION.

SECONDARY CHANGES.

CHAPTER XIV.

SECONDARY CzROWTH IN THICKNESS OF NORMAL
DICOTYLEDONOUS STEMS AND ROOTS.

I. Cambium. General Arrangement of the Secondary Thickening.

Sect. 134. In the Dicotyledons possessing an axial bundle, in the anomalous
forms enumerated on p. 250, sub. 2, in the Berberideae and Ranunculacese men-
tioned on p. 249, and in the Peperomige (p. 249), the primary arrangement of the
vascular bundles and of the tissues surrounding them in the stem undergoes no
change after extension is complete.

The same statement applies to a relatively small number of forms, the stem of
which possesses a normal ring of bundles, consisting of collateral leaf-trace bundles
of normal orientation, as in Saururese, and species of Ranunculus.

In the very great majority of Dicotyledonous stems, on the other hand, the com-
pletion of the primary groups of tissue is followed by the formation of new additional
elements, in consequence of which secondary changes occur in the pre-existing,
primary tissues (Sect. 54, p. 224).

These changes proceed from the ring of bundles, and that is the case both in the
typical instances where the latter alone is present, and also in other cases where,
besides this, medullary and cortical bundles occur. The changes in question consist
chiefly in the fact, that from a meristematic zone, called the Cambium or Cambial
ring, which passes through the ring of bundles, new elements are added to the
latter in the direction of the transverse diameter of the stem, which thus receives a
(secondary) growth in thickness, through the addition of new elements. In short-lived
stems this growth may soon cease; in long-lived ones, and especially in ‘woody’
plants, it endures throughout life.

As these changes go on, the number and arrangement of the primary leaf-trace
bundles, and of the primary medullary rays which separate them, either remain as
they were originally, or new bundles, the Intermediate Bundles, separated by medullary
rays, appear between the original ones, their appearance sometimes immediately
succeeding the primary differentiation of tissues, sometimes occurring later.

Apart from the numerous modifications of detail which thus become possible,
the origin and position of the Cambium, and the arrangement of the elements pro-

SECONDARy THICKENING. NORMAL DICOTYLEDONS.

455

duced from it, which constitute the secondary thickening, are in the main outlines the
same in the very great majority of stems of Dicotyledons and Gymnosperms. The
phenomena belonging to this category, to be treated of in the present Chapter, may
therefore be termed those of the normal growth in thickness, and the stems in question
be termed the normal stems, while those showing a different behaviour may be
contrasted with them, as a?ioi/ialous forms (Chap. XVI).

I. The origin of the Cambium takes place in cases without simultaneous
or previous formation of intermediate bundles, in the following manner. The delicate
cells arranged in radial rows, lying on the internal border of the phloem of . he leaf-
trace bundles (p. 325), remain meristematic ; divisions by means of tangential walls

Fig. 194. -Cross-section tliroug:h the fully elongated hypocotyledonary stem of Ricinus communis, tgy leaf-
trace bundle; c^cb cambial zone; cb interfascicular segments of the latter, arising by tangential division of the
parenchyma of the medullary ray. For further explanation compare p. 332. From Sachs’ Textbook,

go on in them in the succession to be described below, and the vascular bundle thus
grows in the radial direction. Sooner or later the tangential divisions are continued
from the lateral edges of the phloem-groups, through a band in each medullary ray
connecting these groups with one another, so as to give rise to a meristematic zone
in this band also. (Comp. Fig. 194.) The meristematic annular zone, which is thus
formed from the bundles, and from the segments belonging to the medullary rays, is
the cambium. If the leaf-trace bundles in any cross-section are of unequal thickness,
the interfascicular completion of the cambium begins at the edges of the thickest,

456

SECONDA RV CHANGES.

and then extends, in succession, to those which are less thick. In addition to the
example represented in Fig. 194, the process described occurs in the Menispermeae,
Casuarinse, and Begoniae investigated, in Berberis and Mahonia*, in Aristolochia Sipho
and its allies, Atragene, and the woody Piperacese. Cucurbita is also to be men-
tioned here, in so far as the two concentric rings of bundles in its stem (p. 248)
behave, as regards the relation in question, as a single ring curving alternately inwards
and outwards.

In Cucurbita, as well as in Aristolochia Sipho and Atragene, the behaviour
described is most clear, because it goes on especially slowly. In the former plant
particularly the growth in thickness by means of meristematic new formation first
goes on in the vascular bundles alone, the cells of the parenchymatous medullary
rays following it for a long time only by means of radial extension ; only at a
relatively late period does tangential division begin in the latter, in the order
described.

2. No doubt in the majority of cases, the first appearance of the cambium is
immediately preceded by the formation of cauline iniermediaie bundles, and this
happens as follows : —

{a) In the broad medullary ray between two leaf-trace bundles, distinct collateral
vascular bundles appear, which are again separated from the leaf-traces by distinct
medullary rays. The formation of the cambium then starts from the two kinds of
bundles, in the same way as in (i). This process takes place in a very evident
manner in the internodes of Clematis Vitalba. When the completion of the six leaf-
trace bundles (p. 244) has begun, an intermediate bundle appears between each two
of them in the usual manner (p. 389). The twelve bundles of the ring which are then
present are separated from one another by the same number of radial bands, each
about ten cells broad. The cells of the latter now assume parenchymatous properties,
while the twelve vascular bundles increase and develope their elements in the radial
direction ; finally, the completion of the cambial ring takes place by means of
tangential division of an interfascicular layer of cells, starting from the edge of the
six leaf-trace bundles. Each intermediate bundle runs longitudinally through the
entire internode, and is only attached by its ends to the trace-bundles, in the nodes.
A further case, belonging to this category, and characterised by the very early
appearance of the intermediate bundles, is represented by the Rhipsalidese with
winged corners to the stem, spoken of at p. 261.

How far the same process takes place elsewhere in internodes is not certainly
known at present ; the ‘ complementary bundles ’ of Ephedra campylopoda, noticed
above at p. 247, may belong to the same category. It at any rate occurs very
generally in the nodes, even among forms belonging to category i, in order to
form the oblique or reticulate connections of the trace bundles, which here appear
in all cases at an early period.

(3) Between each two leaf-trace bundles collateral intermediate small bundles
of normal orientation appear, consisting only of one or a few radial rows of
elements, and separated both from the leaf-trace bundles and from one another by
equally narrow, non-equivalent radial bands (parenchyma). Their longitudinal

‘ Sanio, Botan. Zeitg. 1863, p. 373.

SECONDARy THICKENING. NORMAL DICOTYLEDONS.

457

course is undulated in such a manner, that within the internode they show alternately
lateral union and separation, at short vertical intervals, both with one another and
with the leaf-trace bundles, and thus form a net with vertically elongated, narrow
meshes, which are filled up by the bands of parenchyma. The whole process begins
at the lateral margins of the leaf-trace bundles, and is continued from the latter
through the interfascicular segments of the original ring. It may therefore be said,
that the leaf-trace bundles coalesce, by means of successive increase in breadth at
their lateral margins, so as to form a closed ring, which is only traversed by the
narrow radial bands of parenchyma, and in which the primary leaf-trace bundles
only remain characterised by the fact that they project more deeply into the pith
(and by the special structure of this projecting portion). In the typical cases, the
whole body thus formed may actually be termed one collateral vascular bundle, with
an annular cross-sectiom The origin and orientation of the cambial zone is, in the
cases in question, essentially the same as in those brought forward above.

This coalescence of the leaf-trace rudiments to form a ring appears most
clearly in those internodes which contain only a few trace-bundles, with a simple
course. In the internode of Euonymus latifolius ^ the rudiments of the traces of the
adjoining pair of leaves, each consisting of a single bundle, appear first at two
diametrically opposite points, between the pith and external cortex , then the traces
■of the next higher pair appear in the middle of the spaces between the, first. From
the lateral margins of these four bundles, according to their order of origin, rapidly
succeeding divisions extend through the interfascicular bands, so as to form the
small-celled rudiment of the closed ring of bundles ; and finally the formation of the
definitive tissue takes place in the latter, beginning with new divisions and consecutive
differentiation at the points where the formation of the ring started, and advancing
towards completion in the same direction as the latter. The whole ring, including
the original leaf-trace rudiments, consists finally, especially in the xylem, of alter-
nating radial bands of bundles, and of non-equivalent elements, which show the
longitudinal course already stated. Fraxinus ^ behaves quite similarly, and most
Rubiaceae, Asclepiadese, Apocyneae, &c., having a very regular radially arranged ring
of wood, are also connected with this type. Comp. Sects. 6i, 63.

The formation of the closed ring is less evident in the case of internodes
possessing numerous leaf-trace bundles, which from the first are separated by very
narrow interfascicular bands, e. g. Acer, Sambucus &c. ; the result, however, is es-
sentially the same. How far the closing of the ring proceeds exclusively from the
coalescent margins of the leaf-trace bundles, or also from small intermediate bundles
arising like those of Clematis, remains to be investigated for each particular case.

(c) The coalescence of the vascular bundles to form a continuous ring may go
still further, so that no alternating dissimilar radial bands appear between the
original bundles, but the whole of the tissue forming these zones assumes the struc-
ture of a vascular bundle, if this expression be allowed for the sake of brevity ; i. e. it
consists of the elements of vascular bundles, with similar structure and arrangement
to those of the later developed portion, and of the products of secondary growth in

* Sanio, Botan. Zeitg. 1S63, p. 360.
Compare Ndgeli, Beitr. 1. p. 95

3 Ibid. Lc.

45^

SECONDA Ry CHANGES.

the bundles of the trace. The descriptions which follow below will explain this
in greater detail. The formation and orientation of the cambial zone are once more
the same as in other cases. Hartig^ and Sanio state that this condition exists in
the case of Ephedra monostachya, Cheiranthus Cheiri, and Miihlenbeckia complexa,
Hieracium, Pyrethrum, Galium, Plantagineae, and other plants to be mentioned imme-
diately. I find it in Cobsea, Crassulaceae ^ (Sedum spec., Sempervivum arboreum,
Echeveria pubescens), Caryophylleae ® (Dianthus plumarius, Silene italica), Rumex
lunaria. Campanula Vidalii, Lobelia syphilitica, Xanthosia rotundifolia, and Centradenia
grandifolia. Many other Melastomacese, and, according to Chatin’s ® description, the
Rhinanthacese also, appear to behave in the same way, but here it is not admissible
to draw any certain conclusion from one or the other species, as to the behaviour even
of its nearest allies ; thus, for example, Rumex alismifolius, in contradistinction to the
R. lunaria just mentioned, possesses the structure given under {d). Comp, also below.
Sect. 147.

Sect. 135. According to the traditional terminology, the part of the ring lying
on the inside of the cambial zone, and including in itself the whole of the xylem
groups, is called, in stems of the Dicotyledonous type, the wood or ligneous body
(xylem of Nageli), while everything that lies outside the cambial zone is called the
cortex. The latter is divided into the bast zone, bast or liber (phloem) which,
limited internally by the Cambium, includes and is characterised by all the phloem-
groups of the ring, and the external cortex ®, DuhameFs Enveloppe cellulaire, lying
outside this. In the ligneous body the elements of the vascular bundles form strands
with the arrangement described, wood-strands ; the bast has similar bast-strands, cor-
responding in their arrangement to the wood-strands ; or, if one will, the two may
be termed xylem and phloem strands. The bands of non-equivalent tissue — consisting
in the great majority of cases of parenchyma — lying between the strands, and having a
radial course, as seen in cross-section, are called medullary rays. Each of the latter
consists of a portion belonging to the ligneous body, the medullary-ray of the wood
(‘Markstrahl katexochen’ of Nageli), and of a portion lying in the bast-zone
(cortical medullary-ray, cortical ray of Nageli). Those medullary rays which are
formed on the first origin of the woody ring pass through from the pith to the
external cortex. They have accordingly been termed large medullary rays, in con-
tradistinction to those which arise later, and do not reach the pith, and are thus in
this respect srnaller rays. With reference to their origin at the first commencement
of the woody ring, the former have also received the name of the original, primary
rays.

The genetic relations which are indicated by the latter term are not, however,
the same in all cases for the anatomically similar large medullary rays, as follows
from what has been stated above. In the case described under (i) they are identical
with the original rays, and thus the expression primary medullary rays is appro-

‘ Botan. Zeitg. 1859, p. 94.

® Regnault, Ann. Sei. Nat. 4 ser. tom. XIV. p. 87. — Haitig, l.c.

^ Anat. Compar^e, p. 221.

^ ‘ Liber, sen interior corticis amictus, ligno contiguiis, fibris reticulatis .... compositus.’
Malpighi, Anat. Plant, cap. I.

® Compare above, p. 236.

SECONDARV THICKENING. NORMAL DICOTYLEDONS.

459

priate, according to the strict sense of the words. In the condition described under
(2) (a) and on the other hand, the large medullary rays have originated secondarily
from the primary ones, primary rays in the sense of the first case having no more
existence after the completion of the ring of wood. The plants mentioned under
(2) {c) have, according to what has been said above, neither large nor primary
medullary rays ; those which appear later in the wood of Ephedra are all small ones
not reaching the pith.

The cambial zone, finally, is divided into portions belonging to the strands and
to the medullary rays, into fascicular and interfascicular portions, according as it
borders on a medullary ray or on a strand of wood or bast.

For Sanio’s terminology, which differs from the above, and the employment of which
would seem to be attended with almost insuperable difficulties, comp. Bot. Ztg. 1863,
p. 372.

In the cambial zone growth in the direction of the radii of the cross-section of
the stem goes on, with pauses in winter; growth is followed by corresponding cell-
divisions ; of the products of division those bordering on the wood and bast are in
each case added to these as definitive tissue ; a zone lying between the two, however,
remains meristematic, and from this the process of new formation is repeated.

The masses of tissue which are added by this process to the wood and bast are
the secondary wood, and the secondary bast.

In the normal Dicotyledonous type, the differentiation of the two in the entire
secondary growth remains essentially the same as, or at any rate quite similar to, that
of the ring of bundles immediately after the first formation of the cambial zone and the
intermediate bundles. On the side of the wood, new elements, equivalent to the
first, are constantly added to the existing medullary rays, in their original direction,
in such a manner that in absolute dimensions and number of cells they either main-
tain everywhere the same height and breadth, or, as they extend in the radial
direction, they increase gradually, and usually relatively little, in breadth ; the latter
is the case especially in broad rnany-layered medullary rays, as in the stem of
Quercus, Casuarina, Clematis, Atragene, &c. The entire ligneous body accordingly
remains divided into the same number of main strands or main sections as that of the
strands existing between the large medullary rays, on the completion of the ring; and
these strands become successively broader towards the outside, being wedge-shaped
as seen in cross-section. On the one hand, they consist as before of elements
equivalent to their original ones, and the differences successively appearing in them
are to be described below ; on the other hand, radially-arranged plates of non-
equivalent tissue occur within the strands, which are essentially similar to the large
medullary rays in structure and direction : there are the small, short, secondary
medullary rays, which sever the main section or strand into partial sections. In
each successive zone of secondary growth new small medullary rays appear, each of
"v\'hich however, when once started, grows on in the radial direction, like the first
medullary rays. Every main section of the wood is therefore divided up by medul-
lary rays, which become successively more numerous and successively extend less
deeply towards the pith.

On the cortical side completely similar conditions obtain to those in the wood ;

SECONDARY CHANGES.

460

the bast-zone continues to be divided into main sections by the large medullary rays,
which at the boundary of the cambium are continuously increased by equivalent
elements, and here for the time being show the same breadth as in the wood ; each
of these main sections is divided into partial sections by secondary, successively
smaller medullary rays, which are a prolongation of those of the wood. The equiva-
lent rays and sections of the wood and bast fit one on another at the cambial
boundary ; the younger rays, which in the wood penetrate less deeply towards the
pith, penetrate to a lesser distance outwards in the bast ; the wedge-like widening of
the portions of the strands is necessarily in the opposite direction in the bast to that
in the wood, as seen in cross-section. From this distribution, slight deviations, still
to be included under the norm.al type, occur here and there. As such are to be
mentioned —

(a) The discontinuous medidlary rays of Hartig^. In Fagus and in exotic
woods certain medullary rays in the wood do not extend to the border of the cam-
bium, but end externally within the ligneous strand. It remains^ however, to be
investigated, w’hether this phenomenon, which appears on examining cross-sections
through medullary rays of no great height, always depends on an actual termination
of the ray, and a formation of bundle-elements at its exterior limit, or whether it may
not perhaps be due to a vertical upward or downward curvature of the ray, in con-
sequence of which its radial prolongation comes to lie in a different surface of cross-
section from that through which the knife has passed, and in which the inner
portion lies.

(р) The medullary spot, characteristic of many woods, which will be described
below.

(с) The appearance of secondary intermediate bundles arising from the cambium,
inside the older medullary rays. This phenomenon occurs in the internodes of Atra-
gene alpina in a form which quite agrees with the normal processes, especially in the
allied species of Clematis. In the specimens investigated the internodes in their
first year all showed the six leaf-trace bundles only, distributed as in Clematis (p. 244),
separated by six large medullary rays, and, like the latter, furnished with secondary
growth by means of a zone of cambium extending all round the stem. In some of
the older internodes, at least two years old, this structure is permanently maintained,
while the secondary growth continues ; they thus correspond to the type given above
under i. In others, however, intermediate bundles appear, in or after the second
year, and in fact, in the most regular case, one appears on each side of each median
bundle of the trace, so that the total number of the bundles now amounts to ten. In
many cases one, two, or three of the four intermediate bundles are absent, and the total
number of bundles is accordingly nine, eight, or seven. The longitudinal course of the
intermediate bundles is that given in the case of Clematis ; in one case only two of
them occurred between two trace-bundles, in a short internode ; they anastomosed with
each other irregularly in their undulating course. The structure and subsequent
growth in thickness of the intermediate bundles are similar to those of the six trace-
bundles.

Clematis Vitalba frequently shows a similar phenomenon, so that in the twelve

* Botan. Zeitg. 1S59, p. 94.

SECONDARY THICKENING. NORMAL DICOTYLEDONS. 46t

large medullary rays, or in some of them, small intermediate bundles subsequently
appear, which have an undulating course b Here, however, the process appears to
be rare ; I could not find it in my material, even in stems an inch thick. In the
broad medullary rays of the wood of Casuarinse the formation of secondary inter-
mediate bundles seems to occur constantly, and therefore, strictly speaking, separates
the stems in question from the normal Dicotyledonous typeb The inner, oldest
portion of these bundles extends vertically, without interruption, through the whole
internode. After some years intermediate bundles appear further outside, which are
at first very small, but constantly become thicker as secondary growth goes on, and
these frequently anastomose with one another and with the neighbouring main bundles,
in an irregularly undulating course. In the older wood, even in branches one inch
thick, the space corresponding to the original medullary ray is divided up by an
irregular net, with pointed meshes, of small bundles. Menispermum canadense pre-
sents the same phenomenon in a less conspicuous degree. To what extent similar
bast-bundles, corresponding to the secondary bundles of wood may occur, still
remains to be investigated.

Sect. 136. When once formed, the cambial ring constantly increases in thick-
ness and circumference, while successive reciprocal differentiation of wood and bast
goes on. The growth of the transverse diameter of its individual cells takes place
in a much smaller degree than the general growth, though in many cases they show
an increase within definite limits. On the other hand, a continual increase of the
number of cells goes on, by division of the existing ones. The general growth, the
divisions, and the differentiation of the products of the division in the direction of
w'ood and bast, proceed during the periods of active vegetation ; they cease during
the periods of rest in winter. The course of the successive divisions and differentia-
tions is obviously only to be determined by investigation during the period of
vegetation, especially at its commencement. In order to test and confirm the result
thus obtained, a comparison of the resting stage in winter is of service.

In order to explain the general course of the divisions, we may, in the first
instance, consider the transverse section alone, and assume that the cambial layer,
as well as the immediately contiguous youngest layers of wood and bast, consist of
entirely similar cells, ranged in radial rows ; this assumption applies with tolerable
exactness in the case of the bundles of the w'Ood.

The following rule was first established by Sanio ® in the case of Pinus sylvestris.
(See Fig. 195.) All cell-division proceeds from a single, annular, meristematic layer
of cells, one row thick as seen in cross-section, which may be called the Iniiial layer.
In this every (initial) cell divides by a tangential longitudinal wall into two daughter-
cells, one of which once more becomes an initial, while the other becomes a tissue
mother-cell ; the latter change may happen either to the inner of the two daughter-
cells, which is added to the wood, or to the outer, which is added to the bast. In
the quite regular case of Pinus sylvestris, each tissue mother-cell then divides once
by a tangential wall, and the two products of its division become directly converted

* Compare Sanio, Botan. Zeitg. 1863, p. 127.

“ Goppert, Linnaea, XV. (1841), p. 747, Taf. IV. Fig. 7. — Sanio, l.c.
^ Pringsheim’s Jahrb. Bd. IX.

402

SECONDA RV CHANGES.

into elements of the tissue. But even here, in the case of Pinus sylvestris, deviations
from the latter rule may be demonstrated. Of the first products of division of the
tissue mother-cell, one divides once more before passing over into definitive elements
of the tissue : in the case of the wood this is always the outer one ; in the case of
the bast it is usually the inner, more rarely the outer ; or each of them divides once
more. Thus in the former case three, in the latter case four elements of the tissue
are derived from one tissue mother-cell.

The investigation of good transverse sections through the active and the resting
cambial zone of the most various Dicotyledonous and
Coniferous woods confirms Sanio’s main result, ob-
tained in the case of Pinus sylvestris, as regards the
bundles of the wood (comp. Figs. 196, 197). In every
radial row there is one initial cell, dividing tangentially,
from the division of which a new initial cell and a
tissue mother-cell proceed in each case. After one
or Iwo further divisions of the latter, the definitive
elements of the tissue are formed. From what is
known of Pinus sylvestris, very various differences of
detail as regards the divisions by which the latter
elements are produced are to be expected as soon
as more extended minute investigations of this diffi-
cult subject have been undertaken.

In most of the cases investigated the longitudinal
divisions in the tissue mother-cells take place ex-
clusively in the tangential direction, the elements of
the tissue are therefore always originally arranged in
radial rows, and deviations from this rule are the result
of subsequent displacement. Exceptions, however, occur
in the bast of many plants, on the origin of the sieve-
tubes ; here the mother-cell of a member of the latter
is divided by one or more excentric walls, directed
neither radially nor tangentially, into a member of a
sieve-tube, and cam biform cells. Comp. p. 324. This
no doubt applies to all the numerous cases of irregular
grouping of the sieve-tubes ; it is, however, undecided
how far displacements occur here, in consequence of
subsequent longitudinal growth of the elements, which
might give rise to the same result in the arrangement
of the latter.

The variations mentioned in the differentiation
and division of the tissue mother-cells are sufficient to
show that the initial cells of a cambial ring do not
always fit exactly one on another with their radial
surfaces, and that even the equivalent products of their divisions in the entire
series of radial rows form with one another annular zones which are not smooth,
but are often interrupted. Comp. Fig. 196, and Sanio, l.c., Taf. 5-8. In addition

Fig. 193. — Pinus sylvestris; cambial
zone ; cross-section through a radial row ;
after Sanio (650). //^side towards the wood,
i (conjectural) cambial initial cell. On the
side of i towards H are twin-cells of the
■wood ; on the side of «‘towards the bast are
twin-cells of the bast; the cell bordering
on I towards the bast is the still undivided
tissue mother-cell for the bast, if the inter-
pretation of i stated above is correct ;
otherwise the former is the initial, and i
an as yet undivided tissue-mother-cell for
the wood.

SECONDARY THICKENING. NORMAL DICOTYLEDONS. 463

to this, even casual observation shows that the secondary growth on the side of the
wood is almost always far more abundant than on the side of the bast.

Both the successively developed elements of the wood and bast, and the cells of
the initial layer, in certain cases, increase for a time in size, while growth in thickness
proceeds, as will be described in greater detail below ; from a definite period onwards,
however, a constant average size is assumed by all the elements which arise subse-
quently, while in other cases the average size remains approximately equal from the
beginning.

The number of elements in the tangential zones, and hence the number of
radial rows, must therefore be constantly increased as growth in thickness proceeds.

Between P — P and r — r primary zone of bast (phloem) ; c cambial zone ; g, g (pitted) vessels in process of formation ;■ p—P
parenchyma of the pith. The cross*sections with a double outline, at and above are the spiral vessels of a leaf-trace bundle.

FIG. 197. — The radial row x^x from the cambial layer of Fig". 196 (600) ; i seems to be the cambial initial cell, just divided :
h side towards the wood ; r side towards the cortex.

and this takes place by means of radial divisions of the initial cells into two equi-
valent daughter-cells, which then perform the functions of initial cells in the manner
described. Assuming that the woody cylinder within the cambium undergoes no
further enlargement, and that all the secondary elements successively formed are of
equal size, it is shown, by a simple theoretical consideration that for each radial

* Nageli, Dickenwachsthum des Stengels, &c., bei den Sapindaceen, p. 15.

4^4

SECONDA RV CHANGES.

row to divide once into two, it is necessary that as many new elements should be
formed in the radial direction as are already present on the radius of the cambial
ring. ‘ An equal increase in the radial and tangential directions, so that two outer
cells would correspond to every inner cell, could only take place if the diameter
of the cell were equal to the radius ’ (i. e. only in the innermost layer of cells of
a stem assumed to be destitute of pith). ‘ When the radius of the cambial ring
has the length of 50, or 100, or 1000 wood-cells the radial rows must be pro-
longed by 50, or 100, or 1000 cells in order to be doubled once.' The former of
the above assumptions holds good exactly for the boundary of the cambial zone
towards the wood ; the second only applies to particular cases. In other cases, in
which a successive increase of size of the secondary elements goes on, the relation is
still less favourable to the multiplication of the radial rows. In fact, according to
these considerations, the radial divisions in the initial layer must take place rarely in
comparison to the tangential ones, and indeed the former are found to appear here and
there in individual cells, in the course of successive secondary growth, without any
demonstrable order of succession.

In the medullary rays the course of the secondary growth and of the divisions is
in general essentially similar, yet at least in the most frequent case, to be described
below, of radially elongated, parenchymatous elements of the medullary ray, it may
be simpler as regards the divisions, the cells following the growth of the wood-
strands for a longer time by radial extension, and the divisions happening more
rarely than in the strands ; they produce on the one side a new initial-cell, and on the
other a new tissue-cell directly, and without any further previous divisions.

In addition to the divisions by vertical longitudinal walls, which alone have
hitherto been regarded, transverse divisions occur in the formation of the secondary
parenchyma, and no doubt oblique ones also in the origination of small medullary
rays. These can only be discussed below, after describing the conditions of form of
the cambial cells.

If we wish to designate by the name camlium a zone strictly distinguished from
wood and bast, it consists, in accordance with what has already been said, of two, or
rather of three, different layers of cells, namely (i) the single initial layer, and
(2) the tissue mother-cells, including (a) those of the wood-side, and {b') those of the
bast-side. The contingent modifications in the case of the medullary rays need not
be repeated here. On the two layers of mother-cells border the products of their
division, which already belong to the wood or to the bast. As the definitive forma-
tion of these requires some time, and they must at their first origin be similar to their
mother-cells, a sharp distinction between them and the cambial zone is usually very
difficult in practice, even in the condition of winter’s rest; in descriptions they are
therefore usually comprehended under the term cambium. We may distinguish them
from the true cambium 2.1, young wood ox young bast ; in cases, however, where this
distinction is not practicable, or is a matter of indifference, while on the other hand a
distinction is required between the mature wood and bast and the collection of zones
just described, the latter may be included under the general name of the zone of young
secondary growth, or young secondary growth, the \.tTxa. young secondary growth
being understood as opposed to the developed secondary growth, consisting of wood
and bast.

SECONDARV THICKENING. NORMAL DICOTTLEDONS.

4^5

The slrucbire of the cambium, and of the young wood and bast, has been given
above, as regards the arrangement of the cells when seen in cross-section.

In each portion of the cambial zone, the form of the ceils is identical with or
similar to the general form of the elements of that section of the mature wood and
bast which borders on it in the radial direction. (Figs. 198, 199.) Parenchymatous
medullary rays are bordered by portions of the cambium, in which the shape of the
cells is almost identical with that in the rays themselves, except that the relative
radial diameter is on the whole smaller; while the bundles of wood, and the rare
medullary rays composed of fibrous cells, are bordered by cambial cells having the
form of elongated fibrous elements. Regarded more
minutely, the form of these elongated cambial cells is
very uniform in all the known cases. It was first
accurately recognised by A. Braun', though only in-
dicated by him, and has lately been fully described by
Velten It is that of a rectangular prism, of which the
radial transverse diameter is smaller than the tangential
(on the average about half as long), while the eaids of
it, owing to an inclination of the radial lateral walls
to the radial plane, form a sharp edge, lying radially and
almost horizontally. The inclination of the lateral walls
is usually confined to one side, and then directed alter-
nately to the right and left, more rarely (e. g. Caragana
arborescens, Cytisus Laburnum) the two radial surfaces
are inclined to one another like a roof The steepness
of the inclination varies, partly according to the individual
case, partly, in its average degree, according to species;
and is on the whole the greater, the more the cells are
elongated; the relatively short cambial cells of Tilia
parvifolia, for example, have their end-surfaces inclined
at about 45°, the relatively very long ones of Hamamelis
virginiana are quite gradually tapered and pointed, as
seen in the tangential view. Very slightly inclined or
horizontal terminal surfaces only occur in isolated cases,
especially above, or at the side of medullary rays. (Fig.

199.) From the conditions of form described, it follows that the elongated cambial
cells, which, as meristem, are in uninterrupted contact, also form, as a rule, un-
interrupted longitudinal rows, and only form alternating horizontal rows in the case
of uniform inclination of the two radial surfaces.

The absolute average size of the cambial cells varies according to the species, as
will be stated below (Sect. 153)' I'' certain cases it remains on the average the same

during the whole growth in thickness, or increases continuously for a series of years,
until a size is attained which remains approximately constant. And in fact this

Fig. 198 — Cytisus Laburnum
three -year -old branch, during the
winter's rest (March) ; tangential sec-
tion {145). acbdihQ zone of seeondary
growth and cambium, containing a
medullary ray above.and bordering on
the autumn wood, h, of the previous
year.

1 Monatsber. d. Berliner Acad. 7 Aug. 1854, P- 1° d. Sep. Abdr. Anmerkg.

^ Botan. Zeitg. 1875, p, 81 1. — Compare also Sanio, Botan. Zeitg. 1863, p. 108. — N. Müller,
Bot. Unters. Heft IV.

H h

4^6

SECONDA RV CHANGES.

increase extends to all the diameters, so that the general form remains, if not exactly,
yet approximately, the same.

The structure of the cambial cells is given in its most important points, by the

Fig. 199.

Fig. 200,

Fig. 201.

Figs. 199 — 201. — Fraxinus excelsior. Inlernode of the stem, two years old, during the winter’s rest, beginning of March (375).
Fig. 199. Tangential longitudinal section through the layer of the zone of young secondary growth, bordering directly on the mature
wood ; r medullary rays. — Fig 200. Radial longitudinal section. Z>, h mature wood ; c, d limiting layer between this and the young
secondary growth ; the latter has rows of round pits (shown with too dark an outline) on the radial lateral walls, and passes over on the
right into the inner zones of the bast; rmedullary ray — Fig. 201. Transverse section, cc, hh mature wood of the previous year,
shaded ; c — c limit between this and the young secondary growth succeed ng it on the left ; A bast ; r medullary rays.

statement of their meristematic properties. They are furnished with densely granular pro-
toplasm, and with a well-defined nucleus, which is spindle-shaped, and in the elongated

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

467

cells is elongated in the same direction as they are ; in the medullary rays of many
woody plants (Vitis, Begonia) they contain chlorophyll, and in winter small starch-grains
(Vitis, Aristolochia Sipho, &c.). Their cellulose walls are thin and delicate at the
time of active growth, yet even here the difference between the radial and tangential
surfaces, to be mentioned immediately, frequently and perhaps always appears, or is
at least indicated. At the commencement of the winter’s rest, the tangential walls of
the elongated cells remain smooth and relatively thin ; the radial walls, on the other
hand, become considerably thickened, the highly refractive thickening mass being
interrupted by a single longitudinal row of roundish pits. In the medullary rays a
similar thickening takes place, and on their limiting surfaces towards the elongated
cells there is a formation of pits corresponding to that on the latter (Figs. 198, 199-
201). On the recommencement of the period of growth, the thickening mass appears
to be again dissolved, at least in part.

These peculiarities of structure are shared by the cells of the young wood and
young bast, as well as by the cambium. These cells may also enter with the latter into
the condition of winter's rest. If, during the latter period, the zone of young secondary
growth between the mature wood and bast be investigated, concentrically and radially
arranged layers of cells are found, with the radial walls thickened as described ; on
the one hand, they are sharply limited towards the youngest mature wood, which is
distinguished by its thick lignified walls; on the other hand, towards the bast, though
here the limitation is less sharp, at all the points where the mature elements have
slightly thickened and non-lignified walls. The number of the concentric cambium-
like layers varies, frequently even in immediately contiguous radial rows; a fact which
finds its explanation in the want of uniformity in the course of the cell-divisions in
the latter, as mentioned above. In the simplest case only the single initial layer lies
between the mature elements of the wood and bast: in' its perfect form I have ob-
served this only in the case of Juniperus communis (comp, below, Fig. 207). Usually
the cross-section shows 2-4, or even more concentric layers of apparently similar,
tangentially flattened cells, and only very accurate investigation teaches that these are
non-equivalent, one being always the initial layer, while the others are partly tissue-
mother-cells, partly young wood or young bast. The latter fact often appears especially
clearly on the side of the wood, when growth begins anew after the winter’s rest, for
then certain of the cells are found undergoing extension to form members of vessels,
directly and without further divisions (e. g. Vitis vinifera). Thus, even during the
winter’s rest, the layers of the true cambium are not distinguished either among
themselves, or from the young bast and young wood by any characteristic structure ;
on the contrary, the entire zone of secondary growth, whether consisting of all its
possible parts, or of the cambium, or the initial layer alone, may enter upon the
winter’s rest, and then assumes everywhere the same structure.

Having finished the description of the zone of secondary growth, we still have
to return to the transverse and oblique cell-divisions taking place in it, which were
left unexplained above. In the medullary rays these do not occur, or, if they do, are
irrelevant. In the elongated elements, on the other hand, they appear as a constant
and essential phenomenon.

(a) In the tissue-mother-cells they appear universally where short parenchyma-
tous cells which do not belong to the medullary rays, and septate fibrous cells, are

H h 2

468 SECONDARY CHANGES.

formed within the secondary wood and bast; wood-parenchyma, bast-parenchyma, &c.
(Fig. 202.) The transverse divisions take place once or oftener, and accordingly the
height of the products of division is unequal from the first ; from the different inser-
tion of the transverse walls on the lateral walls, and the relative thickness of the two
in the mature tissue, it may be conjectured that the transverse divisions take place in
the one case in the early condition of the tissue-mother-cells (wood and bast paren-
chyma), in the other case in cells already belonging to the young wood or young

bast (septate fibrous cells) ; on this point, however, more
exact investigations have not as yet been made. Comp,
below. Sect. 144.

Transverse divisions of the tissue mother-cells, or
of the young wood-cells, are found in, some instances
before the formation of vessels with short members.
The members of wide vessels with horizontal limiting
surfaces may be shorter than the cambial cells from
which they arise ; the difference in length may, however,
be due to the fact that, as they become wider, their height
is diminished by displacement of the oblique terminal sur-
faces into the horizontal position k In other cases how-
ever, e. g. in that of Vitis observed by Cohn^, and in that
of Acacia longifolia observed by Sanio the shortening of
the members of the vessel is so considerable that it does
not find a sufficient explanation in this displacement, but
rather necessitates the supposition of a transverse division.

{b) In the initial layer those transverse and oblique di-
visions of the elongated cells have to take place, by means
of which new small medullary rays, consisting of short
parenchymatous cells, originate within the bundles of the
wood. This follows almost with certainty from the fact
that the medullary ray, from its first appearance onwards,
extends through the initial layer, towards wood and bast.
The only other possible supposition would be that its
first origination is due to divisions which extend, in the
same direction, through cambium, young wood, and young
bast. The first formation of a small medullary ray from
elongated cambial cells is hard to observe, and is at present not very clearly
known From the position of very small medullary rays, only one cell in breadth,
and one or a few cells in height, as seen in tangential sections through the cambium
and the zone of secondary growth, it may be stated that they originate either by
single or repeated transverse division of an end of an elongated cambial cell, or
by the cutting off of a portion of the radial lateral walj of the latter, by means
of a (mussel-shaped) wall of division, with its concave side turned towards the

* Sanio, Botan. Zeitg. 1863, p. 122.

“ Bericht über d. Verhandl. d. Schles. Gesellsch , Bot. Section, 1857, p. 44.

^ Pringsheim’s Jahrb. IX. p. 56.

^ See N. Muller, Bot. Untersuchungen, IV. p. 181. — Velten, Botan. Zeitg. 1875, p. 842.

Fig. 202.— Cytisus Laburnum ; tan-
gential longitudinal section through
the innermost layer of bast of the
same branch as Fig. 198; magnified
as in the latter, s members of sieve-
tubes ; t a sieve-plate lying deeper
than the surface of section ; fn small
medullary ray, two cells high. The
remaining elements are parenchy-
matous cells of the bast, the origin
of which by transverse division of
the cambial cells becomes clear on
comparison with Fig. 198.

SECONDARV THICKENING. NORMAL DICOTYLEDONS.

469

radial wall in question. (Comp. Fig. 202 m.) In the first cell thus formed, further
transverse divisions may then ensue. In the frequent case where a medullary ray,
originating as described, increases in height in the successive zones of secondary
growth, both as regards its absolute size and the number of its cells, the various
possible modes in which new cells may be added may easily be perceived, but the
actual process has not been established with certainty. Further, it is not clear
whether a small medullary ray may not be formed by repeated transverse division of
an entire elongated cambial cell, or even of several one above another.

Sect. 137. As follows from their mode of development, the secondary elements
of the wood and bast are always at first arranged in radial rows, if we except many
groups of sieve-tubes. A growth in thickness of the masses of tissue lying inside the
actual zone of secondary growth, such as would result in a displacement of the radial
rows, takes place in the case of the wood exclusively during the development of the
innermost layers, which are affected by it in the way of displacement or tangential
extension ; at a later period nothing of the kind occurs ; in the case of the bast the
conditions are no doubt different, owing to the continual widening of the zone of
secondary growth, but the displacements following from this, which are to be described
below, affect only the old external zones to any considerable degree.

The secondary elements must therefore maintain their original radial arrange-
ment :

(1) When the form and length which they had in the cambial stage undergoes
little or no change on their definitive development.

(2) When, although they become larger than the cambial cells, they maintain a
form similar to the latter, and when, in particular, they have terminal surfaces in-
clined only towards the radial plane, where they abut on and penetrate between
one another.

The first case occurs almost without exception in the medullary rays of normal
structure (for the exceptions in the case of Atragene, Casuarina, &c. see below), in
most parenchymatous masses of the wood, and in short tracheides, e. g. those of
Cytisus Laburnum, &c. It is true that the height and breadth here frequently in-
crease somewhat, even in the medullary rays ^ on the transition from the cambium
to the definitive condition, but only in a slight degree, which does not alter the general
grouping.

The second case applies to the elongated elements (fibrous cells, woody fibres,
tracheides) of the wood of many, and the bast of most plants which form wood.
Even, however, in cases belonging to this category, the longitudinal growth of the
parts passing out of the cambial condition is trifling, as will be shown in the following
paragraphs.

The conditions in question are present, almost without exception, in all parts of
the wood and bast of the Coniferae ; in the Dicotyledons they are tolerably general
in the soft bast, with the exception, however, of those cases which are characterised by
irregular groups of sieve-tubes.

The sclerenchymatoiis fibres of the bast maintain their radial arrangement, for
example, in Carpinus, Corylus, Ostrya, Liriodendron, and Magnolia acuminata and

Compare Hofmeister, Pflanzenzelle, p. 164.

4/0

SECONDARV CHANGES.

tripetala'. The above-mentioned elongated elements of the Dicotyledonous wood
preserve their radial arrangement, for example®, in Cunonia capensis. Viburnum
Opulus, Staphylea, Hamamelis, Nerium, many Asclepiadeae, Rhus typhinum, Jatropha
Manihot, Laurus nobilis, Camphora, Aesculus, Verbena maritima, Broussonetia,
Catalpa, Paulownia, Hydrangea hortensis, Justicia carnea. Fuchsia, Melastomaceae®,
&c., which are chiefly, though not exclusively, plants with leaves in whorls ; not but
what with similar phyllotaxis a different arrangement of the elements may occur, as
will be shown by facts to be mentioned immediately.

The original radial arrangement is, on the other hand, disturbed or obliterated :

(1) In groups of elongated elements, which show a great elongation on transition
from the cambial condition to that of mature tissue, in the course of which they insert
their tapering ends, which are the principal seat of growth, between each other, and
acquire terminal surfaces which are inclined not only towards the radial rows, but also
in other directions, or even show curvature and torsion of their ends.

(2) When certain of the originally similar elements undergo considerable growth
in the transverse directions, on attaining their definitive development.

The first case perhaps occurs during the formation of many irregular groups of
sieve-tubes, but this has not been minutely investigated, and is doubtful. It certainly
takes place, however, in the case of those fibrous elements, which in the mature
condition do not show any regular serial arrangement, and which often grow to
many times their original cambial length : as in the groups of sclerenchymatous
fibres in the bast of many Dicotyledons, e. g. Tilia (Fig. 21 1), in the fibrous cells,
fibres, and elongated tracheides in the wood of Leguminosae (Cytisus Laburnum,
Caragana, &c. ; most beautifully, on account of the contrast with the other elements
of the wood, which maintain the cambial form and length, in Herminiera Ela-
phroxylon, comp. Sect. 150), Ulmus suberosa, Morus alba, Celtis australis, Tamarix
gallica, Ilex aquifolium, Cornus sanguinea, Pyrus, &c.

The second case occurs universally in the development of wide vessels. Ori-
ginally similar to the other elements, the members of the vessel often become
expanded to many times their initial size; the neighbouring elements thus become
not only displaced, but often transversely deformed in the direction of the surface of
the vessel, compressed, sor even completely crushed, so that mere rudiments remain*.
According to the degree of expansion, and the number of the wide vessels in any
portion of the transverse section, the general arrangement of the elements is
influenced by them.

It need scarcely be mentioned that all the phenomena described may occur in
various degrees, so that cases intermediate between the extremes may exist.

Sect. 138. The collective zones of secondary growth, cambium, young and
mature wood, bast, &c., which have hitherto been regarded with immediate reference
to a single transverse section of stem and root, if traced longitudinally, are continued,
as uninterrupted layers, both through the successive portions of the same axis, and

* Hartig, Forstl. Culturpfl. p. 256. — Sanio, Botan. Zeitg. 1863, p. 107.
® Compare Sanio, l.c. pp. 107, 115.

3 Vöchting, 1. c.

‘ For a minute description, sec Vellen, Botan. Zeitg. 1875, p. 809, &c.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

471

from the main axis into the lateral shoots ; and in fact each of the distinct layers
of any transverse portion is continuous with the equivalent and simultaneously
formed layer of the successive succeeding portions; the cambial layer of a shoot
formed in one year, or its yearly production of wood being continuous with the like
layers in the next year’s shoot, &c.

The longiltidmal course of the single elements, especially of those which are
elongated, as appearing in the direction of the ‘ long grain ’ of the wood and
bast, presents a series of remarkable phenomena, which, in a certain though not
strict sense, are independent of those hitherto discussed. In treating of them we
will here leave out of consideration the torsion of the entire masses of wood and bast
in twining stems, in so far as this is in immediate relation to the torsion of the whole
twining part, as the description of the latter phenomenon forms no part of the
present work h

In stems with a straight and vertical growth, the elements in question usually
have their longitudinal axis in an oblique position, deviating from the vertical, and
this is the case both in bast and wood, the latter having been the subject of more
exact investigations, which are here to be principally regarded

The deviation from the vertical is usually less conspicuous in the direction of
the radial plane ; although it must take place in this direction in the case of the
above-mentioned bundles with elements penetrating irregularly between each other,
e. g. in the wood of Fraxinus and Cytisus Laburnum. It is clearly seen, even on
observation with the naked eye, in the case of the Guaiacum wood, in which the
fibres of successive concentric layers have their ends passing betw'een each other ob-
liquely in the radial direction (though to a less degree than in the tangential direction).

In many woods the oblique position in the tangential plane appears more
clearly. As a rule its direction is the same for all the elements of each concentric
layer, and on observation of the surface is indicated by an oblique ‘ grain ’ or striation
running round the whole stem. The angle at which the striae cut the vertical varies,
partly according to the species, partly in individual cases. According to Braun it
reaches its maximum — as much as 45“ — in Tunica Granatum; then follow Sorbus
Aucuparia (up to 40°), Syringa vulgaris (up to 30°), JEsculus Hippocastanum (io°-
20°); smaller values are more frequent, e. g. usually 4'’-5°, rarely as much as 10° in
Pinus sylvestris, 3°-4° in Populus pyramidalis, Betula alba, &c. ‘ In many cases,’

says Braun, ‘ especially in Pinus, I have convinced myself that specimens with shorter
internodes usually show greater degrees of torsion, than those with longer ones.’
The inclination is also said to alter with the age of the tree, becoming greater in the
later secondary layers in Tunica, and smaller in Pinus.

The direction of the inclination has been found to be invariably the same in the
case of many trees ; right-handed (in the sense of Mechanics) in JUsculus Hippo-
castanum, left-handed in Populus pyramidalis. Other trees show one direction as
the rule, the other as the exception, e. g. Pyrus communis, Carpinus, chiefly right-

* On this subject reference may be made to H. deVries, in Arbeiten des Botan. Instituts zu
Würzburg, Heft III, and the earlier literature on the subject which is there cited.

^ See A. Braun, Ueber den schiefen Verlauf der Holzfaser, Monatsber. d. Berliner Acad. 7 August,
1854; Botan. Zeitg. 1869, p. 747; 1870, p. 158.

472

SECONDARY CHANGES.

handed Salix alba chiefly left-handed. Further, it either remains the same in the
successive secondary layers of the same stem, or, in many kinds of trees, as Pines
and Firs, the direction changes, becoming reversed after a number of similarly
inclined layers.

Among 167 species of Dicotyledonous woody plants and Coniferae, to which
Braun’s investigations extend, the oblique grain is present in in ; in the rest, e. g.
Pinus Cembra, Populus monilifera, Ulmus campestris and effusa, Fraxinus excelsior.
Clematis Vitalba, it has not been observed.

The arrangement of the fibres in the Guaiacum wood is different from their uni-
form obliquity round the entire stem in the trees hitherto mentioned. Here the grain
curves backwards and forwards with short undulations, in each layer of wood, often
cutting the vertical at 45°, assuming different directions in successive narrow layers,
not in the broad (annual .?) rings. This arrangement, showing a different direction
in every small subdivision of the wood, and the interlacing of the elements,
in addition to the radially oblique position and interweaving above mentioned, are
the causes of the impossibility of splitting the Guaiacum wood in the radial, and the
difficulty of doing so in the tangential direction.

The facts mentioned, and especially the reversal of direction, in successive layers
of wood, are sufficient to show, what all accurate investigation confirms, that the
oblique grain is a purely anatomical phenomenon, independent of the external con-
formation of the plant. It is also only perceptible externally in the case of injuries
which lead to the splitting of the tree in the direction of the grain, such as frost
cracks, splitting of the cortex in the direction of the grain of the bast, e. g. in Tilia,
Syringa, Juniperus, and Thuja, or in the case of an excessive local swelling of the
layers of wood, starting from branches or roots, as in many trees (Punica, Car-
pinus, Populus pyramidalis); this leads to the formation of ridges, which run round
the stem obliquely, in the direction of the grain.

A plausible anatomical explanation of the oblique position of the elongated
elements of the wood is afforded in a general way by their conditions of length. As
will be shown below, the elongated elements in a number of woody plants increase
successively in length for a series of years. As the total length of any portion of the
stern remains unchanged during the secondary growth in thickness, and as, further,
no enlargement of individual cells at the cost of others which become obliterated
occurs, either in the cambium or its products (with the exception of the relatively
inconsiderable phenomena connected with the expansion of vessels mentioned
above), but on the contrary, all the cells of any layer parallel to the periphery
grow, and become larger, or at any rate not smaller, it follows that the progressive
increase in length of the elongated elements must result in their position becoming
oblique ; in the one case this affects the cambial cells themselves, in the other the
fibrous elements in process of differentiation. It may at once be added that in the
case of stems with the fibres in a tangentially vertical position, as, for example,
Fraxinus, the length of those belonging to successive layers must remain the sam.e,
or any difference in length must be equalised by radial obliquity only, a point which
has still to be investigated. These considerations render the phenomenon intelligible
it its main outlines, but by no means explain all the details. It is open to question
whether the above-mentioned differences in length are sufficient by themselves to

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

473

explain the extent of the angle of inclination ; the similar direction of the inclination
in the individual layers, its reversal, and more especLilly its alleged diminution in
the later layers in Pinus, &c., all remain to be explained. The basis of such an
explanation is to be sought in a more complete determination of the size and form
of the organs in question, than has yet been made.

The undulated course of the woody fibres, which appears on cicatrised wounds,
&c., and gives the character to knotted and gnarled wood, may here be excluded
from minute consideration, as it is a pathological phenomenon

Sect. 139. Plants with typical Dicotyledonous structure of the stem, as well as
the majority of those Dicotyledons and Gymnosperms which are anomalous in this
respect, form, with very rare exceptions, a cambial ring in Ihe rool"^ at an early
period, and this, when once present, shows a completely similar growth and new
production to that in the stem, although in each particular case definite special
differences exist between stem and root, which are to be discussed below.

The first origin and orientation of the cambial ring must, on the other hand, be
different from that in the stem, in consequence of the different original general
structure in the root. It originates in the axial vascular bundle. The process
begins with growth in thickness and tangential division of that layer of cells which
borders on the inner surface of the phloem-groups, and in fact it proceeds from the
middle of each of these groups towards its two lateral margins, and thus also towards
the outer corners of the xylem-plates. The products of the tangential division are
cambium and young secondary thickening. In the usual case of abundant growth in
thickness, the tangential divisions eventually reach the pericambial cells lying outside
the xylem-plates, and are continued over the latter, thus uniting the originally separate
portions of the cambium to form a closed ring. As follows from the original
arrangement and form of the xylem- and phloem-groups in the root-bundle, the
general transverse section of the cambial ring, at its origin and before its completion,
is a figure following the outline of the xylem body; in the diarch bundles it is a
narrow ellipse, in those with more than two rays it is a polygon, with as many blunt
corners and concave sides as xylem-plates are present. As, however, the cambio-
genetic production of tissue on the side of the wood takes place unequally all round,
and is the more abundant the nearer it is to the original starting-points of the
formation of cambium, the phloem-groups are rapidly shifted towards the outside,
and the concavities of the ring become flattened, so that its transverse section soon
assumes a permanent, approximately circular form.

It is only in the comparatively rare cases of slight growth in thickness that the
concavities between the original xylem-plates are permanent, and that the union of
the original portions of the ring round their angles fails to take place ; the formation
of cambium is in some few cases entirely wanting, comp. p. 355. In other cases the
tangential division in the neighbourhood of the xylem-plates is conspicuously less
than that opposite the original phloem-groups ; the cells in the former position

* Compare Schacht, Lehrbuch, p. 67. — Göppert, Nachträge zu d. Schrift über Inschriften, &c.,
u. über Maserbildung, Bresl. 1870. — Idem, über die Folgen äusserer Verletzungen der Bäume, Bresl.
1873. — Ratzeburg, Waldverderbniss, I. — Nördlinger, Forstbotanik, I. p. 274.

“ See Van Tieghem, Ann. Sei. Nat. 5 ser. tom. XIII. p. 185, pl. 3, 4, 8.

474

SECONDARY CHANGES.

follow the growth in thickness chiefly by means of radial extension, so that, as seen in-
transverse section, the small-celled ring appears interrupted at the xylem-plates by
rows of larger cells, e. g. Cucurbita, Urtica (Figs. 203, 204).

The production of tissue by the cambial ring or its segments,, when once
originated, maintains the same course as that described in the case of the stem.
Any special differences in the succession of the divisions are, at the present time at
least, unknown. The mass of tissue given off internally is to be termed the wood,
the peripheral mass, the bast. Both are divided into wood and hast bundles, which
correspond to one another in the same way as in the stem, and may be compre-
hended under the name slratids. and into radial bands composed of non-equivalent
elements, which alternate with the strands, and in the root no doubt always consist
of parenchyma ; these are the medullary rays.

The original large medullary rays are obviously excluded in the case of the
root. As regards the arrangement of those rays which are present, and the dis-

Fi". 203. Fig- 204*

Fig. 203. — Urtica clioica; transverse section of a small subsidiary root from the rluzome (80). g original diarch xylein-
plate ; crossing it are two secondary bundles of wood, separated bj' broad bands of parenchyma (medullary rays) ; outside eacli,
at s, is the bundle of bast belonging to it. Secondary cortex is present all round, with numerous scattered sclerenchymatous
fibres, indicated as dark spots; the whole bounded externally by periderm, c layer of cambium or young secondary growth.
The primary cortex has been cast off.

FIG. 204. — Cucurbita Pepo; transverse section of the main root of a young plant (40). ^xylein body of the axial bundle,
its four rays united in the middle by a large pitted vessel ; four secondary bundles of wood alternate with them ; s the original
and secondary phloem-bundles. The primary cortex of the root has been replaced by periderm and cast off.

position of the strands which is determined by them, two main types are to be
distinguished, though these are not sharply contrasted in all cases.

(1) A main medullary ray (usually very broad) appears opposite the angle of
each original xylem-plate, the same number of main strands alternating with the
main medullary rays, e. g. Centranthus, adventitious roots of Tropaeolum, Urtica
dioica (Fig. 203) among diarch roots; the main root of Cucurbita (Fig. 204),
Phaseolus, Convolvulus tricolor, and many others among the tetrarch forms ; adventi-
tious roots of Cereus grandiflorus, Clusia, Cucurbita, and Artanthe among the poly-
arch root-bundles. Comp, van Tieghem, /. c.

(2) The whole periphery of the primary bundle acquires fascicular elements.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

475

between which only small medullary rays exist, with an arrangement not corresponding
to that of the primary xylem-plates. The entire secondary growth thus forms a
cylindrical strand, without main medullary rays, e. g. Taraxacum, Scorzonera his-
panica, Rubia, Thuja, Taxus, Cupressus, &c.

On the connection of the bundles of secondary wood with the primary plates,
see Sect. 152.

For the further growth of the bundles and medullary rays when once formed,
the successive subdivision of the former by new medullary rays, and the structure
of the cambial layer, the same general rules hold good as in the stem.

The divisions in the cambial zone also appear to follow in general the same
rules as in the latter, though this remains to be investigated. The frequent appear-
ance of secondary intermediate strands in the broad medullary rays, especially of
fleshy roots, is worthy of notice.

As a rule, the beginning of secondary growth takes place in the root imme-
diately after the differentiation of the primary tissues ; in the cases mentioned of feeble
development on the other hand, as well as in the adventitious roots of Clusia, Cereus,
and Piperaceae, it begins relatively late, so that each portion of the root at first
remains for some time in the primary condition.

II. The Wood.

I. Distribulion and form of the zones of secondary thickening.

Sect, i 40. In the native Dicotyledons and Conifers the wood acquires in
every period of vegetation an increment of growth, the development of which begins
in spring with the unfolding of the buds, and, with the exception of the roots of
Dicotyledonous trees, reaches its end in autumn, starting afresh after a period of rest
in winter ; in the roots of native Dicotyledonous trees, on the other hand, it continues
to make slow progress throughout the winter, and only reaches its end on the
beginning of a new period of growth, whereupon it immediately begins again

The product of each period of secondary growth, corresponding in our climate
to an annual period, is, as a rule, distinguished from that of the earlier and later
periods, by definite differences of structure in the limiting layers, which are to be
described below. It is therefore called an annual zone, annual layer, or annual ring,
and its limiting layers just mentioned are called spring-wood and autiimn-wood.

The consideration of the zones of secondary thickening may conveniently start
from those which are severed into distinct annual layers, especially as this is by far the
most frequent case.

The form of the annual rings has been investigated in the case of trees and
shrubs. It is well known, and does not require any detailed statement here, that
their average breadth shows great variations in the same individual, according to age,
and to the action of more or less favourable conditions of vegetation^, and that under

' Von Mohl, Botan. Zeitg. 1862, p. 313.

^ Compare the works of Nordlinger and R. Hartig, to be cited below. — Further, H. de Vries,
Einfluss des Druckes auf d. Bau, &c. des Holzes, p. 96; Flora, 1872, p. 241, 1S75.

SECONDARY CHANGES.

475

similar, or equally favourable conditions, the average breadth varies considerably
according to the species. Compare, for example, the broad rings of Paulownia and
Ailanthus, with those of Citrus and Cornus ; Pinus silvestris and Abies pectinata with
Taxus, &c. In the stem of the young tree the breadth of the annual rings increases,
under otherwise similar conditions, for a number of years, then remaining for a
series of years at an average maximum, but decreasing again in advanced age. In
the yearly shoots formed on the thickened stem the average maximal breadth of ring
is attained in the first year, or in the first few years It is shown even by superficial
observation, which may be easily confirmed by more minute research, that the yearly
secondary growth in the lateral branches and roots of a tree is less than that in
the stem.

The breadth of each ring is, in the regularly developed sfem, uniform all round,
though even in this case it may be unequal in consequence of unequal acceleration of
the growth on different sides, the ring thus becoming undulating or eccentric, even to
such an extent as to be wholly absent on the deficient side. The rings of one and
the same cross-section of the stem often show the most various differences in all
these respects, forming, as it were, records of the history of its growth and nutrition ;
in certain woody plants, to be mentioned below, such inequalities of growth are
typical.

Unilaterally unequal development of the rings, and consequent eccentric thickening,
are the rule for the lateral branches of the stem and of the roots ; and in fact in the
lateral branches of most Dicotyledonous woody plants, the upper side is the favoured
one, e. g. Acer pseudoplatanus, Alnus, Carpinus, Cornus, Corylus, Crataegus, Cytisus
Laburnum, Euonymus, Gledilschia triacanthos, Fagus, Tilia, Prunus spec., Robinia,
&c.^; on the other hand, in the Coniferous woods, and according to Nördlinger in
Castanea, the under side is favoured. In small stems also, which for a series of years
have grown upright, and increased' uniformly in thickness all round, but have then
been permanently brought into the inclined position by the pressure of snow,
Nördlinger found that the rings formed from the time when the stems became
oblique, were eccentric, and that in the Pines, Firs, and Larches the under side, in
Oaks and Beeches the upper side is favoured. In the lateral roots of frees at the
places where they arise from the stem, the upper side which is continued into the
latter is the one favoured ; at a greater distance from the stem the under side usually
has the advantage, according to Mohi’s opinion®, but this point has not been decided
for certain. Centrically developed roots of trees are, however, not actually rare.

In the case of our forest trees a series of investigations have been instituted on
the average amount of the annual secondary growth of the stem in its successive
transverse sections from base to apex, which of course always determines the general
form of the stem b In some cases the successive surfaces of the transverse section

1 Nördlinger, Der Holzring, p. 14.

^ Compare Nördlinger, Holzring, p. 20. — Hofmeister, Allgem. Morphologie, p. 604.

3 Botan. Zeitg. 1862, p. 274.

‘ Von Mohl, Botan. Zeitg. 1869, p. r. — Nördlinger, Der Holzring, Stuttg. 1872. — R. Hartig, in
Dankelmann’s Zeitschr. f. Forst- u. Jagdwesen, Bd. HI; and Botan. Zeitg. 1S70, p. 505. — We may
refer to these works for the older, very defective literature, and for many details not strictly be-
longing to our present subject.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

477

of the individual layers has been determined, giving the ‘growth in mass;' while in
others the (radial) diameter, the breadth of ring has been measured, on the successive
dimensions of which the form of the stem depends. The growths in mass and
breadth of ring in a layer do not necessarily correspond one with another, because
the former may be greater in its lower portion, where its periphery is larger, though
the breadth of ring is smaller, than in its upper portion where the ring is broader.
The following rules may hold good as giving the consistent result of the published
investigations, which extend to the Oak, Beech, Alder, Silver Fir (Abies pectinata,
D. C-X Scotch Fir (Pinus sylvestris L.), Red Fir (Abies excelsa Poir.), Larch,
Weymouth Pine (Pinus Strobus L.), &c.

(1) In the shaft, i. e. in the unbranched stem between the basal ‘ stock ’ and the
crown, the annual growth in mass increases in the case of trees which stand free,
from above downwards ; according to Nördlinger the average diameter of the ring
always increases simultaneously, while, according to R. Hartig, this may increase
or decrease, or remain the same, which agrees with the results of the radial measure-
ments made by Mohl on three trees grown in the open, which showed increase in
the upward direction. In closely planted trees, the average breadth of the rings
increases in the upward direction, according to Nördlinger, Mohl, and R. Hartig,
while according to R. Hartig’s statement, which has been reasonably disputed by
Nördlinger, the growth in surface or mass remains throughout approximately the
same. Trees, the crown of which has become stunted in consequence of their being
closely planted, show a diminution of the secondary growth in this direction, namely,
from above downwards, which may extend even to its complete absence in the lower
portion.

(2) In the crown the growth increases in the downward direction, both in the
stem and branches.

(3) In the basal stock a considerable increase in the secondary growth and the
average breadth of ring takes place in the older stems, i. e. in the outer layers ; this
starts from the upper side of the insertions of the roots, and may extend upwards to
a different height (0-3-3 metres and more), according to the particular case. At
the points of insertion of vigorous lateral roots, the secondary growth is locally
increased in such a manner that the well-known projections of the stock, separated
by furrows, and in the case of tropical trees attaining huge dimensions, may arise in
the course of a few years.

The ‘forms of growth’ mentioned under (i) are subject to change in the same
individual, when successively planted free, and in contact with others. For each
particular species of our forest trees one or the other form of growth is the rule, and
this depends on whether they usually grow in close contiguity throughout life, either
wild or in forest culture (e. g. Beech, Silver Fir, Red Fir), or whether in their later
years they become free (e. g. Scotch Fir, Larch, Oak, Alder) k

The dependence of the general form of the stem, whether it be more conical
or cylindrical, on the conditions mentioned is self-evident. In the same way it is
clear, that in the stems of exotic plants which deviate from the cylindrico-conical
form, as the spindle- or barrel-shaped stems of Bombacese X the progression of the

* Compare R. Hartig, Botan. Zeitg. 1870, p 513.

- See e.g. the figure of Bombax Munguba in Martius, FI. Brasil. Tab. physiogn. X.

47«

SECONDARy CHANGES.

annual secondary growth from below upwards must be different from that in our
trees, in so far as the form is dependent on the thickness of the layers of wood,
and not on that of the cortical layers or masses of pith. How far the one or the
other explanation is the right one has not been made out in all cases in these plants.
The Mamillarise, for example, cited by Mohl as examples of barrel-shaped stems,
owe their form, not to the successive increase or decrease in thickness of the layers
of wood, but to that of the cortical masses of parenchyma.

2. The forms of Tissue of Ihe secondary wood.

Sect. 141. The forms of iisstie of which the secondary wood is composed*
belong chiefly to the categories of cells (comp. pp. 5, 115, 121), irachecs, and scleren-
chymatous elements, especially sclerenchymatous fibres. They sometimes have their
characteristic anatomical peculiarities, and the division of labour which these in-
dicate, rigorously developed and carried out; sometimes, however, the division of
labour is arranged in such a manner that while an element has the essential pecu-
liarities and functions of one of these forms of tissue, and must therefore be classified
with it, it further shares in the characteristics of another form.

In the tough strong woods of trees and shrubs, which have been the chief
subject of investigation, the latter holds good of the elements of all forms, in so far
that they are — in various degrees — thick-walled and sclerotic ; a phenomenon which
is not characteristic of the secondary wood generally, but only of that of the ‘ woods ’
so called in ordinary phraseology. In the very hard secondary wood of Convolvulus
Cneorum, for example, all the elements both of the bundles and of the medullary rays
are in the highest degree sclerotic ; in the soft, fleshy, chiefly parenchymatous wood
of the stem of Carica, Cheirostemon, many succulent roots, &c., only certain individual
elements have that character. In the stem of Clematis Vitalba, the parenchyma of
the medullary rays takes part in the sclerosis, in that of Atragene it does not,
and so on.

In many w'oods one or the other form of tissue may be absent, and its functions
be undertaken by others, as will be shown by the examples to be mentioned below.

Sect. 142. The tracheae of the secondary wood appear partly in the form of
vessels, partly as tracheides^.

Of the forms of vessels, as distinguished by the structure of their walls, the
reticulately thickened are present exclusively or principally in succulent soft woods,
as in the stem of the Papayaceae, and in many fleshy roots (Sect. 159). Reticulated
vessels with large meshes are further characteristic of the wood of the Crassulaceae
even of the species with hard wood. Reticulated vessels occur, together with pitted
ones, in the Caryophylleae, and may often be found in herbaceous Dicotyledons,
which have been comparatively little investigated. The wood of the Mamillarise and
of species of Echinocactus and Melocactus contains only spiral and annular trachese,
and in fact both vessels and tracheides : some have feebler thickening fibres, the

’ Sanio, Ueberdie im Winter Starke führenden Zellen des Holzkörpers., Halle (Linnsea), 1858.
— Id Botan. Zeitg. 1863, p. 85, &c. The latter also contains detailed citations of the older literature.
^ Compare Chapter IV.

® Compare Regnault, Ann. Sei. Nat. 4 ser. tom. XIV. p. 87.

SECONDARV THICKENING. NORMAL DICOTYLEDONS.

479

transverse section of which is almost isodiametric, while those of others are ridge-
shaped and deeply projecting as described at p. 156. The former are chiefly vessels,
the latter usually tracheides ; but the separation of the two forms is hard to carry out
with certainty on account of the difficulty of establishing the presence or absence of
the vascular perforations. The species of Opuntia and Cereus have reticulated
vessels, which may be accompanied by tracheae with the ridge-shaped thickenings
above mentioned.

The cases described, however, constitute exceptions as compared with the great
majority especially of arborescent or shrubby Dicotyledons. In the latter the vessels
of the secondary wood are pitted vessels, the wall, apart from the pits, being either
smooth, or with a flue spiral fibre on the inner side (comp. Fig. 205). The pits of
the vessels are bordered and correspond one with another, at least on those surfaces
which border on vessels or tracheides (comp. Sect. 38). On the surfaces bordering
on non-equivalent elements, various conditions occur, which will be stated below,
chiefly according to Sanio.

Where pitted vessels are adjacent to sclerenchymatous fibres with unbordered pits
(comp. Sect. 143), the pitting may be wholly absent (Olea europsea. Fuchsia globosa
according to Sanio) ; in most cases pits are present, though they are always less numerous
than those on the surfaces contiguous with tracheae, and they also differ from the latter
in form. On the surfaces adjoining fibres with unbordered pits, the pits on the wall of
the vessel are bordered, but smaller than those on the boundaries of vessels, in Hedera
Helix, Euonymus latifolius, europaeus, and Syringa vulgaris; they are unbordered in
Sambucus nigra, racemosa, Acer platanoides, Salix acutifolia, hippophaefolia, Populus
pyramidalis, .älsculus Hippocastanum, Rhamnus Frangula, Aucuba japonica, Pittosporum
Tobira. The surfaces adjacent to parenchyma and fibrous cells have sometimes bordered,
sometimes unbordered pits, and sometimes both kinds. The first, for example, is the case
in Quercus pedunculata, Diospyros virginiana, Juglans regia, Porlieria hygrometrica,
Spartium scoparium, Caragana arborescens, Sophora japonica. Acacia Sophora, Morus alba.
Daphne Mezereum, Ribes rubrum, Syringa vulgaris, Casuarina equisetifolia. Hibiscus
Rosa sinensis, Paeonia Mutan, Ficus Sycomorus, Olea europaea, Nerium Oleander, Tamarix
gallica, Punica Granatum, Justicia carnea; these pits are unbordered in Hedera Helix,
Sambucus racemosa, nigra, ^sculus Hippocastanum, Rhamnus Frangula, Syringa Josikaea,
Solanum Dulcamara, Populus pyramidalis, Salix hippophaefolia, acutifolia, Vitis vinifera.
Magnolia tripetala, acuminata. Hydrangea hortensis ; — both kinds occur in Bombax
Ceiba, Ficus rubiginosa, Jatropha Manihot, Fuchsia globosa and Eugenia australis.

As a rule the pits bordering on cells are relatively large ; they are rarely small and
very numerous (Hydrangea hortensis) ; where two kinds occur the bordered often differ
from the unbordered also in their superficial outline. The pits of the vessel always
correspond with the unbordered ones of the non-equivalent elements, and in fact those
of the latter are always of the same breadth as the border of the pits on the vessel.

As an exception we must mention the wall destitute of pits with which the vessels
border on the fibrous cells in Punica Granatum.

Where the lateral walls of the members of a vessel have a spirally thickened inner
layer in addition to the pits, the spirals are always present on the surfaces adjacent to
tracheae, with one exception to be mentioned at the end; in certain cases they also exist
on the surfaces adjacent to all the other, non-equivalent elements (Tilia parviflora, Pitto-
sporum Tobira, Prunus domestica, Laurocerasus). In other cases the spirals are absent
on the surfaces which border on parenchyma, while they are present on the others
(Amygdalus communis and other Amygdaleae) ; or they are absent where vessels border
on one another and on parenchyma, and are only present on the surfaces which are
adjacent to fibres.

SECONDARY CHANGES.

4H0

The different forms of perforation of the transverse wall vary according to the
particular cases (species).

The vessels are relatively thin-walled in most true woods, even in very hard woods,
and are often remarkably delicate (Camellia japonica). More rarely the thickness of
their walls is equal to that of the thickest-walled of the elements which accompany them,
e. g. Fraxinus excelsior, Ornus, Nerium Oleander, Piperaceae, Convolvulus Cneorum.

The Tracheides (Fig. 205) either constitute the only tracheal elements of the
wood (Coniferae, Wintereae), or they occur together
with other tracheal organs, especially vessels. They
are characterised as tracheides by the properties
indicated in Chap. IV. The structure of their wall is
in the case of the Coniferae and Wintereae that of pitted
vessels with bordered pits; in most Taxinese there are
also spirals on the inner side of the wall. In the
second case the same holds good, with the addition
that they then resemble the members of the narrower
vessels of the same wood, either in all points, except
the vascular perforation, or at least in possessing
similar bordered pits to those of the vessels belonging
to the same wood. As regards the spirally or an-
nularly thickened inner layer, they usually agree with
the vessels accompanying them, but not always : in
Pyrus communis, Sorbus Aucuparia, and Staphylea
pinnata the vessels have spirals, the tracheides not, in
Philadelphus coronarius the converse is the case. In
particular tracheides of many plants, we find, as an
exception, isolated thickenings of the v-all, projecting
inwards in the form of blunt cylindrical pegs, or of
bars running transversely from one side to the other.

Fig. 205.— Cytisus Laburnum ; tangential y r i 1 i o • • tt* 1

section through the same autumn wood as Bolh lOriTlS WCrC ODServed by baillO 111 hlippopn^
Fig. 198, p. 465. j intermediate cells, nt—m • i ^ ^ m • 1 j n

medullary ray; in the second cell from the rhamnoides, thc latter iti riRUs silvesii'is , and casually

top is a peg-shaped thickening of the wall. 1 • -i-v • tTcr* • rr»i i_

The medullary ray is bounded on the left by bj Hie also ixi Driniys Winteri. i hc transvei’se Dars

an unperforated tracheide, on the right by a ... tit* t • t^* j

narrow vessel ; at is the perforation of the lie m thc radial dii'cction, at Icast lu Fmus and

transverse wall of the latter. The lower . • i i i_ u

longitudinal wall of the tracheide to the left Wmtera, and are contmucd as a Single bar througn

was preserved in the preparation ; the upper j. i t» i*

one, all but a very small piece, was removed many clemcnts oi a radial row. Pcculiar transverse

in cutting the section; in the Other tracheides i.vo-r 1 i*j c

and vessels the longitudinal wall facing up- Imes, which Samo tound on macerated tracheides ot

wards is drawn, the spiral fibres being shown \ «n • i •

in reversed direction, and the really bordered CaSUarina U,C. p. I I 7), Still reqUirC explanation,
pits with only a single outline. 1 . -i *1 i

Where tracheides occur together with vessels or
sclerenchymatous fibres, or both, two extreme cases may be distinguished as
regards their form, namely, on the one hand, those which on the whole resemble
the members of the smaller vessels in length and width, and abut on one another
with a relatively slight inclination of their terminal surfaces; on the other hand,
more elongated, ‘ fibre-like ’ forms, with long acuminate ends, sometimes even
forked (Hippophae, Casuarina torulosa, Staphylea pinnata), which penetrate between
one another and between the non- equivalent elements.

* Compare above, p. 164.

SECONDARF THICKENING. NORMAL DICOTELEDONS. 4(S r

TJie average length, expressed in hundredths of a millimetre, is, for example, accord-

ing to Sanio’s determination, in

Tracheides most Fihriform

similar to 'vessels. Tracheides.

Fagus silvatica ... 39 75

Cunonia capensis . . 69 97

Casuarina torulosa .45 104

„ equisetifolia 48 75

Hamamelis virginica .70 80

Sheperdia canadensis 19 45

The first category includes those tracheides which also approach the vessels
most nearly in the structure of the wall, while those belonging to the second are in
this respect also less similar to the vessels, and approach the fibres in every point.
If we survey the entire series of the woods investigated, there is on both sides,
as well as between the two principal cases just distinguished, a continuous transition
between the extremes

The average thickness of the walls and the nature of the thickening layers ^
generally vary in accordance with the other differences and similarities, if the excep-
tional cases of very thick-walled vessels already mentioned be left out of account.
The peculiar tubes, containing air, which form the floating apparatus of many
Leguminous woods (Herminiera, &c.) must here be mentioned by way of supplement.
In order to avoid repetitions, the description of them follows below, in Sect. 150.

Sect. 143. The sclerenchymaious fibres of Ihe wood, shortly termed woody fibres
(Fig. 206), are generally distinguished from those elongated tracheides which are
more or less similar to them in form, by the structure of their wall. The latter is
always destitute of the spirally-thickened innermost layer — although the whole wall
may be striated and capable of splitting in a spiral direction, — and its pits, which
are invariably slit-shaped, and lie in a left-handed oblique direction, are always
present in relatively small numbers, often very sparingly, and differ in details from
those of the accompanying vessels. While in many cases they are bordered here
also (Quercus, Daphne, Liriodendron, Fraxinus, &c.), they are usually not bordered
(Sambucus, Hedera, Clematis Vitalba, Syringa vulgaris, Ligustrum vulgare, Euony-
mus latifolius, Celastrus scandens), or so small that the presence of the border is
difficult to determine. Both kinds of pits are mentioned by Sanio as occurring in
Jatropha Manihot, those with a border being the more numerous. .In this plant
Sanio found two kinds of pits on the surfaces of junction between the fibres and the
medullary rays, namely, small slit-shaped ones on the radial lateral surfaces of the
cells of the medullary ray, but large round ones on their horizontal corners, towards
which a small pit runs from each adjoining cell of the ray. In the other cases
investigated, the pits of the fibres are of approximately equal size, on whatever form
of tissue they may border.

The walls of the fibres are thickened, in a manner which accords generally with
the fundamental rules holding good for all cell-walls ; their usually comparatively
thick middle layer® is as a rule homogeneous, at least without any conspicuously

* Compare Sanio, l.c. pp. 117, 118. See Hofmeister, Pflanzenzelle, p. 196.

® Hofmeister, /. c.

48a

SECONDARY CHANGES.

apparent finer stratification and striation’; its thickness, however, is very unequal in
different species and individuals. The walls are as a rule lignified. A not un-
common exception, however, occurs, inasmuch as one of the layers is of a
manifestly soft, cartilaginous, gelatinous consistency, and is then excluded
from the process of lignification, becoming violet immediately on treatment
with preparations of iodine. This gelatinous layer (comp. p. 133) is as a
rule the innermost one ; it surrounds the lumen immediately, either as a
narrow border (Jatropha Manihot, Morus alba), or usually as a thick, ap-
parently swollen mass, filling the greater part of the lumen. In rare cases
a layer enclosed between the lignified ones shows the characteristics in
question; these often extend to the whole of the wall lying inside the
outermost limiting layer'(‘ primary membrane ’). Lastly, the gelatinous layer
may often be distinguished by its refraction, even when it is stained like a
lignified membrane by preparations of iodine

The occurrence of the gelatinous layer is strik-
ingly irregular. Sanio found it especially among
Leguminosae (Gytisus Laburnum, Sarothamnus, So-
phora iaponica, Caragana arborescens, Gleditschia
triacanthos), where it is of quite usual occurrence ;
also in Ulmus suberosa, Celtis australis, Hakea
suaveolens, Morus alba, Broussonetia, Ailantus,

Fuchsia globosa, Eugenia australis, Castanea, Dios-
pyros virginiana, Corylus Avellana, Ostrya vir-
ginica, Populus pyramidalis, Betula alba, Ainus glu-
tinosa, Enckea media. Eucalyptus cordata, Calycan-
thus floridus. Amygdalus communis, Prunus Lauro-
cerasus, Jatropha Manihot, and Ficus Sycomorus, and
he supposes that it occurs much more generally. It
is, however, by no means generally characteristic of
all the fibres of these woods, as it may be present or
absent in different parts even of the same annual
ring ; while its occurrence is often rare, and even so
isolated (Betula, Ainus) that one may repeatedly
investigate a wood without finding it. Nor is its
presence or absence connected with any definite
special form of structure in other respects, or with
the average thickness of the wall. It cannot there-
fore be regarded as a characteristic peculiarity of the
fibres, the less so, as in particular cases (Hamamelis,

Fagus silvatica, Casuarina) it also occurs in elements
which, according to their other properties, belong to
the class of tracheides which resemble vessels.

Fig. 207.

Fig. 206. — Cytisus I^burniini ; three-year*old
branch during the winter’s rest (March). Tangential
section (145)- abed the zone of secondary growth
and cambium bordering on the autumn wood, h of
the previous year, and containing a medullary ray
above.

FIG. 207. — Cytisus Laburnum. Outlines of a
selected short woody fibre, from the youngest annual
ring of the same branch as that from which Fig. 206
is taken (145). )

What was stated above in the case of the
tracheides most closely resembling fibres, applies
generally to the fibres themselves as regards form and size. Sanio (/. c. 106) adduces
many examples of the occasional bifurcation of their acute ends. On the average
their length exceeds that of the neighbouring tracheides the more, the more the latter

* Compare Sanio l.c. p. 105.

For details see Sanio, l.c. p. 103.

SECONDA Ry THICKENING. NORMAL DICOTYLEDONS. 4(S3

resemble vessels, comp. Figs. 206 and 207 ; in extreme cases it may be approxi-
mately equal to that of the tracheides (Syringa), or even somewhat less (Ribes).

The following mean values, as determined by Sanio in hundredths of a millimetre, may
serve to demonstrate the relations of length : —

Sophora japonica . .

Sarothamnus scoparius
Ulex europaeus . . .

Celtis australis . . .

Cordia pallida . . . ,

Rhamnus cathartica
iEsculus Hippocastanum
Tilia parviflora . . .

Salix acutifolia . . .

Rhus typhina . . .

Rhamnus Frangula . .

Quercus pedunculata .
Prunus Laurocerasus .
Populus pyramidalis
Hakea suaveolens . .

Eucalyptus cordata .
Periploca graeca . . .

Daphne Mezereum . .

Spiraea chamaedryfolia .
Syringa vulgaris . . .

Ribes rubrum . . .

Tracheides. Fibres.

16

95

17

56

1 6

103

26

87

27

1 1 1

28

52

26

43

31

46

33

53

32

35

24

44

49

80

56

126

39

45

26

81

34

60

28

36

15

2 1

33

35

50

51

49

47

In the contents of the woody fibres shrivelled remains of protoplasm and of formed
constituents of the contents can only be detected in rare cases where the wall is very
thick and the lumen very narrow, as in the tough fibres of the wood of Viscum, and
perhaps also of Leguminosae, Quercus, &c. Further attention must, however, be
directed to this point, which is difficult to make quite clear, on account of the
scantiness of the remaining contents and the thickness of the wall. Even in the

cases just mentioned air is certainly present, in addition to the remnants of the

contents. In most woody fibres, however, the lumen contains nothing but air and
water. It is manifest that they agree in this point with the tracheides, nor does it

admit of doubt that in so far as this is the case they take part in the functions of the

latter, and thus that we here have a case of the above-mentioned phenomena of
incomplete division of labour. The sharp severance of the two organs cannot
therefore be carried out without violence and uncertainty, especially as the characters
assigned to them, and in particular the bordering of the pits, are on the one hand
variable in different cases, and on the other are difficult to determine in practice, in
the case of very small pits. It will therefore constantly be necessary to speak of
tracheides resembling fibres, and of fibres resembling tracheides. On the other hand,
however, many cases of sharp differentiation exist, as in the Leguminosae mentioned,
Quercus, and many others ; these cases render the distinction necessary, and by
taking these as the starting-point, it can be carried out even in the less clear cases.

Sect. 144. The ceils of the secondary wood may be divided according to their
form, into fibrous cells, and short parenchymatous cells.

I. The fibrous cells resemble the woody fibres more or less closely in form.

I i 2

4«4

SECONDARY CHANGES.

Like the latter they proceed from the longitudinal division of an elongated spindle-
shaped tissue-mother-cell of the cambial zone, without any transverse divisions. In
the thick-walled forms, a subdivision of the lumen into compartments may subse-
quently occur, by means of thin transverse walls, as is the case in the chambered
selerenchymatous fibres (p. 134) : these are septate fibrous cells.

The elements in question, as follows from what has been stated, are products of
the cambium, in which the cellular qualities persist permanently, or disappear slowly.
In their further characteristics they are closely related to the other elements of the
wood, namely, on the one hand, to the woody fibres, and on the other to the short-
celled parenchyma. Of the two subordinate forms which result from these relations,
the former may be termed fibrous cells in the strict sense, while the latter may bear
the name of intermediate cells ^ (Ersatzfaserzellen) given by Sanio.

a. The former agree in their form, and in the structure of their walls, with the
woody fibres, and thus certainly take part in the functions of the latter, into which
they may gradually pass over completely. They are distinguished from them by the
nature of their contents. The latter almost always contain starch (comp. p. 115):
in Spiraea salicifolia Sanio found traces of chlorophyll ; this appears more abundantly
in the septate fibrous cells of the one-year-old wood of Vitis vinifera and Centradenia
grandifolia. Tannin is contained in the fibrous cells of Vitis, and traces of it in that of
Syringa vulgaris, while in the other woods investigated it is absent in these elements,
even where it occurs in other cells.

In the wood of Tunica Granatum, all the elements of which, with the exception
of the vessels, are filled with starch the grains contained in the fibrous cells are
on the average considerably larger than those of the other cells.

Besides the plants already mentioned fibrous cells containing starch occur in the
wood of Berberis vulgaris, Mahonia Aquifolium, Begonia muricata, angularis, Sambucus
nigra, racemosa, Cheiranthus Cheiri, Salix cinerea (root), Ligustrum vulgare, Syringa
vulgaris. Clematis Vitalba, species of Acer, Vitis vinifera, Celastrus scandens, Euonymus
europaeus, latifolius; Hedera Helix, Acacia floribunda, Robinia pseudacacia. Ficus elastica,
rubiginosa, Sycomorus, Rhus Toxicodendron, Tamarix gallica ; fibrous cells with slightly
granular contents occur in Ephedra.

Septate fibrous cells occur in Coleus Macraei, Hydrangea hortensis. Fuchsia globosa,
Aucuba japonica, Celastrus scandens, Euonymus latifolius, europaeus. Spiraea salicifolia,
chamaedryfolia, Ceratonia, Hedera Helix, Pittosporum Tobira, Eugenia australis, Rubus
idaeus, Justicia carnea, Ficus Sycomorus, rubiginosa, elastica, Bignonia capreolata, Tectona
grandis, Rhus Cotinus and Toxicodendron, besides the plants already mentioned ; either
the non-septate predominate (e. g. Spiraea salicifolia), or the septate, e. g. Vitis, Hedera,
and Punica. Starch has always been found in the septate cells, although in small quantities,
except in Punica and Ceratonia, In the case of Justicia carnea only, Sanio states that
he found the cells containing air, ‘no doubt abnormally.’

b. Sanio’s intermediate cells'^ (Ersatzfaserzellen) (Fig. 205, p. 480), agree with the
short-celled parenchyma of the ligneous bundles which will be next described, in all

* [In justification of the use of the term ‘ intermediate cells ’ in place of a more strict translation
of the term ‘ Ersatzfaserzellen ’ introduced by Sanio, it may be pointed out that, though the inter-
mediate cells do replace the short-celled parenchyma in some few cases, that is not their constant
character. It is thought that the use of the term ‘ intermediate cells ’ will avoid this difficulty, while
it brings the real character of the cells more prominently forward, viz. that they are intermediate in form
between fibrous and short parenchymatous cells ; compare Sach’s Textbook, 2nd Engl. Ed. p. 950 ]

“ A. Braun, l.c.\ compare p. 471.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

4«5

properties, with the exception of their shape, and the form of their pits, which in
many species is not rounded but slit-like. Sanio’s term owes its origin to the fact
that they not only frequently accompany the short-celled parenchyma of the bundle,
but in many woods exclusively represent or replace it, as in Viscum, Porlieria, Cara-
gana arborescens, and Spiraea salicifolia.

2. The short parenchymalous cells of the wood present some differences accord-
ing as they belong to the ligneous bundles or to the medullary rays. Thus the
parenchyma of the bundles, usually called 'wood parenchyma^ is to be distinguished
from the parenchyma of the rays.

The parenchyma of the bundles has been principally investigated in the case of
hard woods, and it is to these that most of the following statements apply. They may,
however, with some obvious modifications, be also extended to the soft, fleshy, suc-
culent, or at least loose woods, which consist chiefly of parenchyma, as in the stems
of Papayaceae, Bombax, and Chirostemon, and in many roots.

The cells of the typical parenchyma of the bundles arise by predominant trans-
verse divisions of the elongated tissue mother-cells in the cambial zone. Accordingly
they are arranged in simple, or in some places multiple, longitudinal rows, each of
which has a somewhat spindle-shaped form, resembling that of the mother-cell. This
grouping comes out most conspicuously when they lie isolated in non-equivalent
tissue, less so where they are united to form larger masses. The length of the
spindle-shaped groups is usually less than that of the fibrous cells, more rarely, as in
Vitis, it is on the average equal to it.

The form of the individual cells is that of a more or less elongated prism, with
horizontal or oblique terminal surfaces ; it is obvious that those which form the ends
of a group must further show a conical tapering. Those which border on wide vessels
are often flattened in the direction of the circumference of the vessel, and elongated
transversely, in consequence of the expansion of the members of the vessel at the
expense of their surroundings (p. 470).

In many cases the cells which surround a group of contiguous vessels on oppo-
site sides are connected by means of flatly tubular outgrowths of their lateral v/alls,
which penetrate between two vessels, and fit on to one another at their ends. The
outgrowths are frequently branched, frequently they have blunt ends without fitting
on to others. This phenomenon is explained by Sanio \ and no doubt correctly, by
the unequal growth and partial displacement of single rows of parenchymatous cells
originally lying between the rudiments of the vessels. It occurs in Casuarina, Mela-
leuca imbricata, Platanus occidentalis, Celtis australis. Ficus Sycomorus, Cordia
pallida, and especially in Tectona grandis and Avicennia spec. ; and it has also
been found by Sanio in the intermediate cells of Porlieria.

The wall of the parenchymatous cells of the bundles is in the harder woods always
provided with roundish or elliptical pits which are not bordered ; they are never slit-
shaped, or arranged in regular oblique rows ; on the sides which are in contact with
vessels they are usually larger than on the others, though exceptions to this rule occur
(Betula alba). The pitting goes all round, even over the transverse walls, and the
latter are of equal thickness with the lateral, or the thinner lateral walls — a point of

L.c. p. 94, where the further details are to be compared.

486

SECONDARY CHANGES.

distinction from the septate fibrous cells. The v/alls, though lignified in the mature
condition, are uniformly distinguished from those of the tracheae and fibres of the same
wood by their lesser thickness. Exceptions to this rule are very rare : Magnolia acu-
minata and tripetala, Liriodendron tulipifera, Gymnocladus canadensis, and Amorpha
fruticosa, in which the radial walls of those woody parenchymatous cells which lie in
the autumn wood are not inconsiderably thickened. Spiral or annular fibres are
always absent. The parenchyma of the bundles in soft fleshy woods is in general
only distinguished from that above described, by its usually larger cells, and less
thickened walls.

The nature of the cell-contents is in general characterised by the term paren-
chyma. In most hard woods the starch-grains, which are stored up periodically,
during the winter’s rest, form the chief contents ; chlorophyll and tannin occur here
and there ; the former, for example, in the wood of Cobsea scandens.

The parenchyma of the medullary rays consists, in the great majority of secondary
woods, of cells which have essentially the same properties as the parenchyma of the
bundles in the same plant, without being exactly similar in every point. In woods
which are not fleshy and succulent the walls of the cells of the medullary rays are as
a rule lignified, like those of the woody parenchyma. Exceptions to this rule occur
in many twining and climbing plants, in which the cells of the medullary rays remain
unlignified, delicate, and capable of yielding to pressure and tension, e. g. Menisper-
mum canadense, Aristolochiee, Atragene alpina. The lignified medullary rays of
Clematis Vitalba, which agrees so closely in every respect with Atragene, show how-
ever that this phenomenon is by no means generally characteristic of plants with the
habit mentioned.

The form of the cells of the medullary rays is usually that of a rectangular prism,
often with rounded corners, and roughly comparable to a brick ; in thin medullary
rays, filling up a narrow mesh between the ligneous bundles, the cells which occupy
the angles of the mesh have a corresponding wedge-like form. Usually the cells are
chiefly elongated in one direction, and are either procu7nhent with their greatest
diameter directed horizontally and radially ; or upright, with their greatest diameter
vertical. The former is by far the most frequent case. Cells standing vertically occur,
for example, in Asclepiadeae (Periploca, Hoya, Asclepias curassavica), Nerium, Drimys
Winter!, and Medinilla farinosa. In the medullary rays of Camellia japonica pro-
cumbent and vertical cells occur in groups. Medullary rays with procumbent cells
are always easy to distinguish from parenchyma of the bundles, even where they
traverse the latter, because the longitudinal diameters of the tw'o kinds of cells cross
one another ; in the case of the upright cells this distinction is often less simple on
account of the similar direction of the longitudinal diameters.

But few minute investigations on the structure of the cells of the medullary rays exist,
and many details are still to be discovered. From what is already known, it may
however be asserted that the cells of a medullary ray are as a rule similar to one
another, apart from irrelevant differences, some of which follow directly from what has
been said. But few exceptions to this rule are known. In the medullary rays of Aristo-
lochia Sipho, Sanio' found smaller cells, containing finely granular starch, arranged in

' L.c. p. 127.

SECONDARY THICKENING. NORMAL DICOTYLEDONS. 487

irregular reticulate rows, between larger, empty, dried-up cells, a condition which recalls
the pith of the Roses, &c. In the medullary rays of Atragene alpina annular zones of two
kinds alternate from within outwards, the one consisting of a few rows of relatively narrow,
closely connected cells, the other of somewhat larger cells connected to form an irregular,
coarsely lacunar tissue, but otherwise similar in structure to the narrow cells. This
structure owes its origin to the fact, that with each thickening of the ligneous bundles
the medullary ray receives an increment of growth, which remains on the whole smaller
in the radial direction than that of the ligneous bundles as regards the number and size of
its cells. Certain cells or groups of cells either follow the general growth, or are dragged
apart in a purely mechanical manner in consequence of it, so as to form the lacunar zones.
I n general, though not exactly, each pair of dissimilar zones corresponds to an annual ring.

In the wood also the cells of the parenchyma are no doubt always accompanied
at least by narrow intercellular interstices containing air. In particular cases, to be
mentioned below, they surround wide passages containing secretions.

Sect. 145. Forms of tissue other than those discussed in the preceding para-
graphs are in most woods either absent, or, at any rate, of small importance. The
most widely distributed are the sacs containing crystals, which, wherever they occur,
accompany the parenchyma of the bundles or of the medullary rays; e. g. Legumi-
nosae, as Pterocarpus santalinus, Haematoxylon Herminiera (pp. 139 and 141),
Vitis, &c.

Laticiferous tubes are abundantly developed in the wood of the Papayacese,
which is chiefly parenchymatous. Their reticulately connected branches are here
distributed between the elements of the parenchyma, and are in contact Avith the
vessels. In other plants containing latex, only those branches which run from the
cortex into the pith pass through the secondary wood. No doubt they always exist
earlier than the latter, from the primary differentiation of tissues onwards, and sub-
sequently become enclosed, and also of course stretched, by the secondary growth.
Where one of these connecting branches borders on woody fibres, the latter are not
uncommonly bent inwards so as to follow its course, as is indicated in Fig. 190, p. 438,
by the oblique shading. Comp, also Chaps. VI and XII.

The classification of the elements of the secondary wood given in Sects. 141-145 is
based on that which Sanio, in agreement with T. Hartig^, has given in his fundamental
works cited above, p. 478. It differs, however, from the latter in some points. Apart
from the medullary rays, Sanio classifies the elements of the ligneous bundles as
follows

I. Parenchymatous System.

1. Woody parenchyma.

2. Fibres representative of the woody parenchyma.

II. Libriform System.

3. Simple undivided bast-like wood-cells or wood-fibres: Libriform

tissue.

4. Septate Libriform tissue.

III. Tracheal System.

5. Tracheides.

6. Vessels.

His System II includes both our woody fibres and our fibrous cells, the two are placed
together on account of their form and the structure of their walls, while no primary

* Fliickiger and Hanbury, Pharmocographia, pp. 176, 188.
^ Compare e.specially Botan. Zeitg. 1859, p. 92.

SECONDARY CHANGES.

488

importance is attached to their contents. The intermediate fibres are separated from
the fibrous cells, also on account of the structure of their walls, and placed, with the
parenchyma of the bundles, in Category I. The rest of the classification resembles our
own. In so far as nothing further is aimed at than an intelligible arrangement of the
forms of tissue of the secondary wood, Sanio’s classification is without doubt as intelli-
gible as our own, and perhaps more so. Both also suffer from the same defect, namely,
that the categories distinguished cannot always be sharply separated, and in particular
that intermediate forms occur betw'een fibres and tracheides, &c., as has often been
stated above. Both, however, afford guidance in each particular case, in accordance with
the system adopted. There would therefore be no reason for undertaking alterations in
Sanio’s arrangement, if it were not an essential object to refer the forms of tissue in the
secondary wood to their proper place among those distinguished in the plant generally,
including those outside the secondary wood. It can admit of no doubt that the elements
of the secondary wood are not organs sui generis^ but belong to the forms of tissue which
have been characterised in this book as tracheae, sclerenchymatous fibres, and cells ; the
last being distinguished from the others by permanently containing protoplasm, or in
doubtful cases by their periodically varying store of starch (comp. pp. 5 and 115). The
known phenomena of the secondary wood present, as I believe, no argument against the
general classification of the forms of tissue carried out in this book, for the occurrence of
intermediate phenomena cannot avail as an argument against the distinction of typical
forms. In the face of these facts it was, however, necessary to depart in some points
from Sanio’s classification, which was based on other considerations.

I willingly grant that the sharp separation of the cells from the other elements is often
inconvenient for the practical description or identification of woods, as it is not always
an easy matter to establish the cellular quality. In most cases indeed the presence of
starch is a certain character, both in the fresh and in the dry wood. In its absence,
however, the distinction is in the latter often impossible or only possible with great diffi-
culty. In the wood of Cobaea, for example, which has been preserved dry, it can scarcely
be determined with certainty whether the numerous reticulated elements with large pits
are short tracheides or parenchymatous cells, for in the structure of their walls they
resemble reticulated tracheae, which might occur, and all the formed contents have
become indistinguishable. In the fresh plant, on the other hand, the presence of proto-
plasm and with it the cellular quality may be recognised, at least up to the third year,
by means of the large chlorophyll grains. Experiences of this kind are, as we have said,
inconvenient, though, on the other hand, they are certainly instructive, as they point to
the necessity of investigating woods in the fresh living condition, more than is usually
done. They would, however, only present a serious objection to the classification arrived
at, if the problem of the anatomy of wood were to be sought in the construction of a
convenient ‘ key’ for description and identification.

3. Distribution of the tissues in the wood.

Sect. 146. The distribution of tissues in the wood, and the consequent structure
of the latter, is uniform in the successive annual rings, with the exception of certain
differences to be specially treated of below ; the single annual ring may therefore be
considered first. In the few cases without annual rings the description applies to
the entire woody ring.

As has already been often mentioned, alternate radial bands of unlike structure
are nearly always apparent at the first glance: (i) the medullary rays, and (2) the
ligneous bundles. This fact determines the main subdivisions of the exposition.
Where this alternation of unlike radial bands does not exist, as in the cases men-

SECONDARy THICKENING. NORMAL DICOTYLEDONS. 489

tioned on p. 458, and in many fleshy roots to be mentioned below, the entire mass
of secondary wood is to be regarded as a single cylindrical ligneous bundle.

a. Medullary rays and medullary spots.

Sect. 147. In the woods which have been principally investigated, every zone
possesses a large number of medullary rays of various rank. Since new secondary ones
always arise as the ligneous mass grows in thickness, their number increases with
that of the successive layers. And further, as in the successive layers their size either
remains approximately uniform, or at least increases in a much lower degree than their
number, it may well be assumed that the proportion between the space occupied by
them, and that occupied by the ligneous strands, remains approximately the same in
all the successive layers. This assumption agrees with the observation that where the
increase in number of the medullary rays is very slight, an especially striking dilatation
of the original ones occurs ; e. g. Atragene. No detailed researches on this relation
e.xist.

The number of the medullary rays in the surface of cross-section is apparently
about in inverse proportion to their size, i. e. to their breadth, and no doubt their
height also. Nördlinger ^ undertook a great number of enumerations, which, although
according to his own judgment they are not exactly reliable for woods with numerous
medullary rays, on account of the serious difficulties, and are also taken from casually
selected annual rings, yet give definite proportional numbers for the very different
conditions obtaining in the particular species. He gives, for example, in a breadth
of 5 millim., for Aristolochia Sipho 9, Clematis Vitalba 10, Cytisus Laburnum 19,
Robinia pseudacacia 20, Acer pseudoplatanus 33, Abies pectinata 37, Abies excelsa
44, Acer platanoides 47, Acer saccharinum 53, Quercus pedunculata 64, Alnus
glutinosa 78, iEsculus rubicunda 84, Euonymus europaeus, Punica Granatum 105,
Rhododendron maximum 140 (the highest figure determined). The average
breadth of a medullary ray varies according to the species, by Nördlinger’s measure-
ments (/. c.), from I ram (Quercus Cerris) to 0-015 ™m (^sculus, Buxus, Castanea,
Euonymus europaeus, Hamamelis, Juniperus communis, virginiana, Koelreuteria,
Ligustrum vulgare, &c.). It amounts to about 0-025 according to the same
author, e. g. in Abies pectinata, Pinus, Larix, Taxus baccata, Syringa vulgaris, &c.,
to about 0 05 ““ in Acer pseudoplatanus, dasycarpum, Juglans, Robinia pseud-
acacia, Sambucus nigra, &c., to about o- 1 in Ailantus, Alnus incana, Cytisus
Laburnum, Gleditschia, Platanus acerifolia, &c. Whether the measurements were
made on sections of sufficient thinness for the determination of absolutely exact
numbers may remain undecided. ^

The height of the medullary rays is not less variable in different species than
their breadth, but has received much less attention in the published investigations.
In woods without intermediate bundles, and in Clematis with one intermediate bundle
to each primary one, the height of the primary rays is equal to that of the internodes,
and thus amounts to 100 or 200““. In the smallest secondary rays of the Abietinese,
M'hich are only 1-2 cells high, it scarcely exceeds 0-025

* Querschnitte von Holzarten, Band 2, jj 5.

49°

SECONDA RV CHANGES.

These relative magnitudes may be better determined according to the number
of cells, or layers of cells, which compose the medullary ray in breadth and height,
than according to absolute measurements. Those under 0-025 breadth are no

doubt all of them only one cell broad (at most two or a few in the middle), and are
thus ‘ uniseriate ’ as seen in tangential or transverse section, as, for example, in almost
all Coniferae, and generally in the narrowest of the above examples ; while in like
manner the broader ones are always pluri- or multiseriate. Similar rules obviously
hold good for the relative heights, and here also the number of cells varies, according
to the particular case, from very high figures, down to one or two.

Certain woods possess medullary rays of iwo different sizes, with which differences
of the minute structure are also usually connected; e.g. those of the Abietineae, to be
described below, which differ in the presence or absence of a resin-canal ; the broad
high multiseriate rays, and numerous low uniseriate ones between them in Quercus
and Fagus; small secondary rays, which are triseriate in the middle, between the
much larger primary ones, in Casuarina &c.

The medullary rays are sharply defined, and exactly fill the meshes between the
ligneous bundles, which run in a curved course around them. An exception to this
rule is described by Schacht^ in the case of the wood of the root in Araucaria
brasiliensis, in which the uniseriate rays, consisting of irregularly undulated cells, are
united by rows of similar cells, which run between the tracheides of the ligneous
bundles vertically, from one ray to another, lying above or below it. A further
exception is formed by the medullary spots to be described below. In the great
majority of cases the medullary rays consist only of parenchyma. In many woods
they collectively constitute the main mass of the parenchymatous tissue which is
everywhere distributed between the other elements : in many cases (Wintereae) this is
represented by them alone, in others (Coniferae) at least to much the greatest extent.
The succulent parenchyma, which forms the greater part of the wood of the stem in
Carica and Vasconcella, is principally formed by the large-celled, broad, and high
medullary rays.

Exceptions to this purely parenchymatous structure rarely occur. As such are
to be mentioned in the first instance the medullary rays of many Ahieiinece, all
investigated species of Finns in the narrower sense, Cedrus, Larix, Tsuga canadensis,
Abies excelsa, and balsamea — and of Sciadopitys, which consist of two kinds of
elements, namely, parenchymatous cells, and tracheides of similar form to the latter,
distinguished by Hartig ^ as ‘ fibres.’

Among the Abietineae, the Pines (Pinus), Firs (Picea excelsa), Larix and Pseudotsuga,
have two kinds of medullary rays : larger ones, which contain, in their many-layered
central portion, a resin-canal which runs horizontally into the bast, and is not in communi-
cation with other canals of the wood and bast and smaller single-layered rays, usually
only a few (1-12) cells or elements in height, and destitute of a resin-canal. The other
trees mentioned have medullary rays of only one kind, and of the structure last mentioned ;

* Compare Göppert, LinnKa, Bd. XV. p. 747. — Low, Diss. de Casuarinearum .... Structura,
Berl. 1865.

® Botan. Zeitg. 1862, p. 412, Taf. XIII. 15.

ä Forstl. Culturpfl. p. 13, Taf. V. Compare also his Jahresber. (1837), P- i45-
‘ Ilartig, Naturgesch. d. forstl. Culturpfl. p. 95, Taf. 5. — Von Mohl, Botan. Zeitg. 1S39, p. 334.

SECONDARy THICKENING. NORMAL DICOTYLEDONS.

491

they seldom exeeed the height and breadth stated ; — in Cedrus they are as much as 50
cells in height, and often more than one cell broad in the middle. In the species
first mentioned above, the medullary ray consists firstly of somewhat elongated, pris-
matic, procumbent cells, which, on their surfaces of contact with one another and with
the tracheides of the ligneous bundles, have, according to the species, one or more large,
unbordered pits ; in the latter case they are really not pitted, in so far as the thickening
and the pit, strictly speaking, only belongs to the tracheide

Secondly, tracheides occur in addition to these cells of the medullary ray, which they re-
semble in form and position. Their walls, where they border on equivalent elements, and
on tracheides of the bundles, have bordered pits of smaller size than those of the latter ;
in many species of Pinus (e. g. P. silvestris, and Laricio), and Sciadopitys, they further
have irregular thickening ridges, projecting inwards like teeth, on their upper and lower
sides ; towards the cells of the medullary ray they only have extremely scattered small
pits, which, so far as I could see in P. silvestris, are unbordered. Each of the radial
rows of which the medullary ray consists, is, as far as investigation extends, composed
exclusively of one of these two kinds of elements, and in fact in a ray more than two
elements in height, the upper and lower wedge-shaped edges always consist of one to
three series of tracheides. In the middle of the medullary ray there then lie either rows
of cells only, or rows of tracheides alternating with the latter. E. g. they occur in the
following order, succeeding one another from above downwards (or conversely), the
Roman numerals indicating the rows of tracheides, the others the rows of cells, and the
letters (a), (b), &c. the particular medullary rays investigated.

Pinus silvestris, wood of the stem: {a) II, 4, I, i, II. {b) I, 2, 1, 3, I. (c) I, 3, IV, 3, II.
(d) I, 2, I. (e) II, 4, h &c.

Larix europcea, stem: {a) I, i, II, 6, I. [b) I, i, IV, 9, I. (c) I, 14^ 1^ &c.

Small medullary rays, only two elements in height, are in P. silvestris often composed
of tracheides only^.

The second exception to the usual, purely parenchymatous structure, occurs in
several plants which form but little wood, and consists in the fact that the medullary
ray is not formed of parenchyma, but of elongated, sclerotic fibrous cells. It has
been primarily observed in the perennial stems of the suffrutescent Begonise ^ e. g.
B. angularis, muricata, Hiigelii. The very large and broad medullary rays of the
secondary wood here consist of upright, very much elongated cells, which abut on
one another with oblique, sometimes acute, sometimes blunt, terminal surfaces, very
like the cambial cells of the ligneous bundles, and acquire lignified, sclerotic walls of
considerable thickness, with small pits. The cells have scanty contents, and fre-
quently even contain starch. The broad medullary rays form collectively a tough
ring into which the relatively narrow ligneous bundles are fitted.

A similar structure occurs in many herbaceous stems of Umbelliferae, as in
Chserophyllum, Myrrhis, Seseli, Daucus, and Eryngium ^ though here more minute in-
vestigation is needed, with reference to some doubtful points suggested by Jochmann;
it perhaps occurs frequently in herbaceous Dicotyledons. Of the cases adduced by

^ See Fig. 58, p. 159, Further, Hofmeister, Pflanzenzelle, p. 175. — Sanio, in Pringsheim’s
Jahrb. VIII.

^ For further details see Kraus, Bau d. Nadelhölzer, Würzburger Naturwiss. Zeitschr. Bd. V.^
Goppert, Monogr. d. Fossilen Coniferen, Harlem, 1850. For figures see especially Göppert, l.c.\
also Schacht, Baum, i Aufl. p. 202 ; Lehrbuch, I. p. 233.

® Hildebrand, Begoniaceen-stamme, p. 24.

* Jochmann, Umbelliferarum Slructura, p, 10.

SECONDARY CHANGES.

49i

Schwendener (Meehan. Princip, p. 148, sub. 3), Tropseolum, Impatiens, Centranthus,
and Cachrys, perhaps belong to this category, the others not.

Ill any case all the exceptions of the category last-mentioned constitute transi-
tional forms to those mentioned at p. 458, where there is no sharp lateral limitation
of distinct ligneous bundles by radial bands of non-equivalent structure, and hence
medullary rays cannot be distinguished at all, whether it be that only the large
medullary rays are absent and small secondary ones appear later, as in Ephedra and
Cobsea, or that rays of every rank are absent throughout, as in Crassulacese, Centra-
denia, Rumex Lunaria, and Campanula Vidalii.

Sect. 148. In many woods, e. g. constantly in species of Alnus and Sorbus,
accumulations of parenchymatous cells occur, constituting as it were local hyper-
trophies of the medullary rays ; these were first described by T. Hartig as cellular
passages, subsequently by Nordlinger as medullary spots, and by Rossmassler as
repetitions of the pith. According to the investigations of these authors and of
Kraus ’, these structures appear in cross-section in the form of elongated spots,
usually at the outer side, but not unfrequently in the middle of an annual ring, with
their greatest diameter following the periphery of the ring, while they often form
considerable annular segments extending through 90° and more. In the vertical
direction they extend like passages for distances of several feet, sometimes ending
blindly, sometimes branched here and there, not unfrequently crossing one another in
their irregular course. They are often conspicuous to the naked eye by their brown
colouring, e.g. in the trees mentioned ; in other cases they are colourless, e.g. Populus
monilifera, and tremula. They consist of irregularly polyhedral, irregularly arranged
cells, with thick pitted walls, contents including starch, tannin, &c.; the latter are generally
brown in the dry wood, and chiefly contribute to the colouring of the spots. This may
also partly depend on very thin-walled, compressed (partly disorganised ?) cells at the
circumference of the spot, as described by Kraus in the case of Sorbus torminalis.
They owe the names given them to the similarity of their thick-walled cells to those
of the pith, especially of its periphery. The medullary rays coming from the middle
of the stem enter the inner side of the spots, their cells becoming broader as they
approach the latter, and assuming more and more the characteristics of those of the
spots ; thus the medullary rays pass over from within outwards into the passages ;
they further coalesce with them laterally. On the outside new medullary rays start
from the passages, and as regards their direction these are either independent of those
coming from within, or lie in the same straight line with them. Local swellings of
the medullary rays, due to increased breadth and number of their cells, which latter
may assume irregular forms, are immediately connected in structure with the smaller
spots or passages of this kind; and this also applies to the longitudinal union of
neighbouring rays, by means of small groups of parenchymatous, more or less
irregular cells, which abut on them. Such structures occur in Abies alba, balsamea,
Pichta, in Cunninghamia, Cupressus sempervirens, and frequently also in Abies pecti-
nata. In the Coniferse, and in Liquidambar, hysterogenetic and lysigenetic resin-

t

’ Hartig, Forstl. Cultui pflanzen. — Nordlinger, Querschnitte, Bd. II. — Kraus, Nadelhölzer, /. c.
p. 162.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

493

canals often arise in them*; in the wood of Prunus avium, they are, according to
Wigand a principal starting-point of the disorganisation which produces the cherry-
gum.

The dilatations of the medullary rays
and the medullary spots occur, it is true,
relatively seldom ; they are, however,
characteristic of many woods, both
Dicotyledonous and Coniferous. Ac-
cording to Kraus and Nördlinger they
have been frequently observed in Betula
alba, dahurica, populifolia, Crataegus
oxyacantha, monogyna, pyracantha, cor-
data, Cydonia vulgaris, Pyrus prunifolia.

Amygdalus communis, Cotoneaster mi-
crophylla, Prunus spinosa, Salix aurita,

Caprea, bicolor, Rhus Cotinus, Lühea
grandifolia, Pterocarya caucasica, Vac-
cinium Myrtillus, Vitex incisa, Calluna
vulgaris, Erythroxylon grandifolium,

Guazuma ulmifolia, and Liquidambar
styraciflua, besides the plants mentioned
above ; while they occur rarely in Alnus
viridis, Catalpa, Magnolia acuminata,
and Salix triandra. I leave unmentioned
the cases marked (?) by Nördlinger.

Among the Coniferous woods Kraus
found them in Abies balsamea, Pindrow,

Pichta, Picea orientalis, and Juniperus
excelsa ; in Abies pectinata, Cedrus Deo-
dara and Larix, he only found swellings
of the medullary rays. In Dicotyle-
donous woods the passages occur chiefly
in the lower part of the stem, and are
thence continued into the roots; they
also extend, however, into the apices and
branches, though here they are less
numerous and constant. In the Coni-
ferous woods their course has not been
minutely investigated ; according to

Dippel’s statements (A c.), which concern this particular point but little, those in the
white Fir which contain resin canals may be traced longitudinally for considerable
distances.

Fig. 208. — Pinus silvestris ; radial longitufiiual section through
the wood of a branch, a—e ends of tracheides, with bordered
pits {f, e") in superficial view; cb portion of the young wall of a
tracheide with bordered pits still immature ; the further develop-
ment of the latter and the contraction of the canal in the succes-
sion a — c; e mature condition ; s, t large pits on the boundary
between tracheides and cells of the medullary ray (550). From
Sachs’ Textbook.

b. The ligneous bundles.

Sect. 149. The structure of the ligneous bundle within an annual ring differs
according to the presence or absence of the particular forms of tissue, and according
to the distribution of those which are present, and the form and structure of each

^ Kraus, l.c. — Dippel, Rotan. Zeitg. 1863, p. 253.
^ Pringsheim’s Jahrb. III. p. 118.

494

SECONDARY CHANGES.

element individually. In addition to these there are differences, to be discussed below,
which are presented by the structure of the same form of tissue in the spring and
autumn wood, and by the general grouping of all the elements of the ligneous body.

I . In some few woods the bundles are composed of only one form of tissue, to the
exclusion of all the rest, and they then consist of tracheides, which, apart from the
general differences between spring and autumn wood, have everywhere the same
structure. In these cases the parenchymatous system is represented only by the
medullary rays, which are inserted everywhere in great numbers between the bundles.
Among Dicotyledonous woods the WinterecB belong to this category ; namely, Drimys
Winter! and its allies, Tasmannia aromatica^, and Trochodendron aralioides^, the
systematic position of which is doubtful: among Conifer ce, Taxus baccata belongs
here, according to Sanio, but in the case of this tree Hartig and Kraus state that scanty
bundle- parenchyma is present.

In the other Coniferse the tracheides, of which the main mass of the wood is

FIG. 209. — Juniperus communis; stem ; cross-section tlirough the autumn wood, bast, and cambium during
the winter's rest (end of September), h — h outermost series of the autumn wood; by b series of bast-fibres. At X
there is only one cambial cell between h and b ; m — m medullary rays.

homogeneously composed, are accompanied by bundle-parenchyma, which sometimes
occurs in single vertical rows, scattered among the tracheides, while sometimes, as in
many Abietinese, it forms the coating of the resin-canals.

In the Coniferae (Figs. 208 and 209) and the other plants just mentioned, the tra-
cheides are arranged in radial rovs's; they are quadrangular, as seen in cross-section, when
those which belong to neighbouring radial rows stand opposite to one another, hexagonal
or pentagonal when the radial rows are alternate ; their ends are elongated and sharp,
owing to inclination of the radial surfaces (comp. p. 469). The radial surfaces have large
corresponding bordered pits, which in the Wintereae, the Araucariae, Dammarae, and the
wood of the root in other Coniferae, form two or more longitudinal rows, while in the

‘ Göppert, Linnsea, Bd. XVI. p. 134. — Kraus, l.c.
‘ Eichler, in Flora, 1864, p. 451.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

495

Stem-wood of the rest of the Coniferae they form only a single longitudinal row, apart
from individual exceptions, which are frequent, for example, in Larix. In the Coniferae
pits only occur on the tangential surfaces in the autumn wood. Taxus, Cephalotaxus,
and Torreya further show spiral or annular thickenings on the inner surface of the wall
of the tracheideh Comp. Chap. IV.

The resin-canals, surrounded by parenchyma as epithelium, occur in the ligneous
bundles of the same Abietinae vyhich possess horizontal canals in the medullary rays
(p. 490). They run longitudinally, and, as seen in transverse section, lie scattered in a
ring in the external region of every annular layer. Their number varies according to
species and individual; Von MohP counted, for example, on an equal transverse surface
embracing several annual layers, in Pinus nigricans 190, P, silvestris 124, Larix europaea
128, Picea excelsa 78.

2. The ligneous bundle of all Dicotyledonous woods, except those mentioned
above, and of the Gnetacese (Ephedra, Gnetum) always contains vessels, and at least
one of the forms of cells distinguished ; of the latter bundle-parenchyma and inter-
mediate fibres are usually present together. These are further accompanied, accord-
ing to the species, by some or all of the other tissues characterised in Sects. 14 2-1 44.
Of the combinations thus possible, the following have been demonstrated, according
to Sanio’s statements, in the trees and shrubs investigated.

1. Vessels, Tracheides, Bundle-parenchyma, Intermediate fibres.

(a) Only with bundle- parenchyma : Ilex Aquifolium, Staphylea pinnata, Rosa
canina, Crataegus monogyna, Pyrus communis. Spiraea opulifolia. Camellia, &c.

(b) Only intermediate fibres : Porlieria.

(c) Both forms of cell: Jasminum revolutum, Kerria, Potentilla fruticosa, Casua-
rina equisetifolia and torulosa, Aristolochia Sipho, and many others.

2. Vessels, 'Tracheides, Fibrous cells. Bundle-parenchyma, Intermediate fibres.

{a) Only bundle-parenchyma: fibrous cells non-septate: e. g. Sambucus nigra,
racemosa, Acer platanoides, pseudoplatanus, campestre.

(b) Bundle -parenchyma and intermediate fibres, fibrous cells non-septate :

Ephedra monostachya, Berberis vulgaris, Mahonia’’. •

(c) Bundle-parenchyma, fibrous cells septate and non-septate: Punica, Euonymus
latifolius, europaeus, Celastrus scandens, Vitis vinifera. Fuchsia globosa, Centradenia
grandifolia, Hedera Helix, &c.

(d) All four forms of cell: Miihlenbeckia complexa. Ficus (?).

3. Vessels, Tracheides, Woody fibres. Bundle-parenchyma, Intermediate fibres. This is
the prevalent, one may almost say the typical, combination.

(a) Only bundle-parenchyma : Sparmannia africana, Calycanthus, Rhamnus ca-
thartica, Ribes rubrum, Quercus, Castanea, Carpinus spec., Amygdaleae, Melaleuca,
Callistemon spec., &c.

(b) Only intermediate fibres : Caragana arborescens.

(c) Both forms of cell. Most Dicotyledonous woods no doubt belong to this series,
e. g. Salix, Populus spec., Liriodendron, Magnolia acuminata, Alnus glutinosa, Betula alba,
Juglans regia, Nerium, Tilia, Hakea suaveolens, Ailantus, Robinia, Gleditschia spec.,
Ulex europaeus, &c.

* Compare Hartig, Kraus, Goppert, l.c. — Von Mohl, Schacht, Botan. Zeitg. 1862.
^ Botan. Zeitg. 1859, p. 340.

“ Compare Sanio, in Pringsheim’s Jahrb. IX. p. 5.3.

496

SECOND ARy CHANGES.

4. Vessels, Woody fibres, Parenchyma, Intermediate fibres,

{a) Both kinds of cells : Fraxinus excelsior, Ornus, Citrus medica, Platanus, &c.

(fi) Only intermediate fibres : Viscum album.

(r) Only bundle-parenchyma : Avicennla.

5. Vessels, Fibrous cells, Parenchyma,

Cheiranthus Cheiri, Begonia. Here no doubt also belong many of the Crassulaceae
and Caryophylleae which still need more minute analysis.

6. Vessels, Fibrous cells. Parenchyma, Woody fibres (?).

Coleus Macraei, Eugenia australis, Hydrangea hortensis.

7. Vessels, Tracheides, Woody fibres. Fibrous cells (septate). Parenchyma, Intermediate
fibres.

Ceratonia siliqua, Bignonia capreolata, doubtful however as to the woody fibres
according to the existing data.

Although no strict law holds good, without exception, for the distribution of these
tissues in the woods containing vessels, still certain general rules may be given.

The vessels occur in all the layers of the annual ring, but are usually more fre-
quent in the internal than in the external portion. Only Bombax Ceiba shows the
opposite condition, according to Sanio. It is not uncommon, however, for their
frequency to show no difference from within outwards (Acacia Sophora, floribunda,
Enckea media, Artemisia Abrotanum) ; or only a slight one, the pores, or groups of
pores which they present in cross-section, being scattered uniformly through the wood,
e. g. Laurus nobilis, JEsculus, Acer, Populus. The vessels rarely form the main,
fundamental mass of the wood (Avicennia). As a rule they lie in small groups in the
non- equivalent fundamental mass, and these groups are either isolated or arranged in
more or less interrupted radial bands or concentric zones (Hedera Helix). They are
either everywhere of about the same width, or more usually their width is greater in
the inner part of the annual ring, and diminishes gradually or suddenly towards the
outside. With this difference in size, a difference in structure is in many cases also
united, in so far as the narrow vessels have spiral fibres, while the wide ones have
none (Morus alba, Broussonetia papyrifera, Gymnocladus, Virgilia lutea, Gelds aus-
tralis, Ulmus suberosa, Catalpa, and Robinia pseudacacia) ; or the structure is the same
in all (Quercus pedunculata, Castanea vesca, Fraxinus, Amorpha fruticosa, Sophora
japonica, and Periploca).

In the above-mentioned ‘parenchymatous’ woods, as Bombax, Carica, &c., and
also in the roots still to be discussed below, the hundle-parench\>ma forms the main
mass in which the vessels and other elements are inserted in groups. In the solid
‘ woody ’ woods its arrangement has a regular relation to that of the vessels.

It usually accompanies the latter, either in such a manner that it surrounds each
vessel or group of vessels singly — paratracheal parenchyma according to Sanio, e. g.
Enckea media ; or it forms tangential bands, alternating with similar ones, con-
sisting chiefly of tracheides or fibres, in or at the side of which the vessels stand :
Sanio’s metatracheal parenchyma. The latter, for example, is the case in the spring
wood of Tectona grandis, in the autumn wood of Fraxinus, in the autumn and spring
wood of Amorpha fruticosa, Sophora japonica, Robinia pseudacacia, Gleditschia
triacanthos, Gymnocladus, Virgilia, Caragana arborescens, Paulownia, Morus, Brous-
sonetia, Ailantus, Tamarix gallica, &c. In Casuarina equi.setifolia, torulosa, Hakea

SECONDARF THICKENING. NORMAL DICOTYLEDONS. 497

suaveolens, and other Proteacese, species of Ficus, Cordia pallida and many others,
every annual ring has several concentric bands of metatracheal parenchyma.

According to Sanio, parenchyma alwaj's occurs scattered between the tracheides
in Dicotyledonous woods, with the exception of Casuarina, where it only occurs in the
metatracheal arrangement, and of Rosmarinus officinalis. It is absent between the
typical woody fibres, according to Sanio, with the exception of Edwardsia grandiflora,
Ulex europseus, Celtis australis, Olea europsea, and further of Hibiscus Rosa sinensis,
where it actually forms tangential bands between the fibres. In Tamarix gallica it
occurs even between fibrous cells which contain starch.

In some few cases of the wood of roots, which consists chiefly of masses of
parenchyma, the latter are the seat of passages containing secretions : root of Inula
Helenium^and Opoponax Chironium; Tr^cul’s statement respecting Oenanthe crocata
perhaps also refers to this subject

For the intermediate fibres, the same rules hold good as for the bundle-paren-
chyma, because they occur either accompanying or representing the latter.

Although the woody fibres may occur in all the layers of the annual ring, they
are present in especially large numbers in its central portion, in the case of hard
woods. They here usually form the fundamental mass, in which the other elements,
particularly vessels and parenchyma, are imbedded; in many woods, e.g. Robinia and
Gleditschia, they occur only in the central portion of the ring, and are absent in the
spring and autumn wood. In woods which are mainly parenchymatous (as Bombax
and Cheirostemon), and in that of Avicennia, which chiefly consists of vessels, the fibres
are only imbedded in small groups, or singly, between the elements of the funda-
mental mass.

For the fibrous cells, whether septate or non-septate (Berberis, Clematis, Vitis,
Tamarix, Punica, &c.), the same rule of distribution holds good as for the woody
fibres. As shown by the above statements as to their occurrence, the two elements,
which are similar in form and in the structure of their walls, may mutually represent
each other, indeed it is doubtful whether the case mentioned under 7, of the simul-
taneous presence of both, ever occurs.

The tracheides may likewise form by themselves the fundamental mass of the
wood, representing, as it were, the two tissues last mentioned; this is the case in the
combination mentioned above under i, e.g. Pomaceae, Camellia, &c. They then
always belong to the ‘ fibriform ’ category, resembling woody fibres in shape, and in
the character of their walls. Where, on the other hand, they occur in conjunction with
fibres and fibrous cells, they are present chiefly in the neighbourhood of the vessels :
and in fact they occur next them in small munbers, and isolated, when the latter are
scattered singly or in small groups in the annual ring, and are all alike (e. g. Punica,
Fuchsia globosa, Ceratonia, and Nerium). When, on the other hand, two kinds of vessels,
distinguished by their size, and usually also by special structure, are present, then the
tracheides accompany the small ones, rarely the large ones also (Quercus peduncu-
lata, Castanea vesca, and Periploca). According to the mode of occurrence of the small
vessels, they are then either distributed with the latter in the whole annual ring (Ulex

* See Berg, Atlas d. pharm. Waarenk. Taf. X.

* Trecul, l.c.\ compare p. 448.

K k

498

SECONDARY CHANGES.

europaeus, and Rosmarinus), or limited to its outer portion (Morus alba, Broussonetia,
Catalpa, Paulownia, Sophora japonica, Gymnocladus canadensis, Robinia pseudacacia,
Corylus, Carpinus, and Ostrya). — Secondly, the tracheides are in many cases most
abundant in the outer part of the annual ring, or confined to it, even apart from their
connection with particular vessels. In the annual ring of Ribes nigrum, Syringa
vulgaris, Ligustrum vulgare, Euonymus europseus and latifolius, they successively
increase in frequency towards the outer border of the annual ring, until they form the
fundamental mass in which isolated vessels and fibres, or fibrous cells are imbedded ;
while in the inner portion of the ring the fundamental mass consists of fibres or
fibrous cells, and the tracheides occur isolated, side by side with the vessels. Or the
tracheides occur only at the outermost, autumnal limit of the annual ring, while the
latter otherwise only contains vessels of one kind, as in Tilia, Salix hippophaefolia,
acutifolia, Populus tremula, pyramidalis, Rhamnus Frangula, Juglans regia, cinerea,
Pterocarya, Diospyros virginiana, Betula alba, Alnus glutinosa, Laurus nobilis, Cam-
phora ; Acer pseudoplatanus, platanoides, campestre ; Sambucus nigra, and racemosa.

Sect. 150. Certain phenomena which depart in some degree from the usual
rules, but belong nevertheless to the normal Dicotyledonous type, are here to be
mentioned as special cases by way of supplement ; they chiefly concern the wood of
species or families which are distinguished by special adaptations, and by peculiarities
of form correlated with these. Most of these phenomena still require more minute
investigation, for which the following paragraphs will only contain indications.

We have, first, to return to the woods enumerated on p. 458 and p. 492, which
are destitute of medullary rays. In those cases where only the primary large medul-
lary rays are absent, while small ones soon appear, as Ephedra, Cobaea, and no
doubt Xanthosia also, the above general rules for the structure and distribution of
the organs hold good for the main mass of the wood. As far as the absence of the
medullary rays extends in the cases mentioned, and in cases of their entire sup-
pression throughout the whole secondary wood, the main and fundamental mass, with
isolated exceptions to be mentioned below, consists of thick-walled fibrous cells or
fibres — the latter being gradually developed from the former : these are usually
elongated, though short in Echeveria pubescens, and have sharply pointed ends and
a regular radial arrangement. Leaving out of consideration the xylem portions of
the original leaf-trace bundles, which project into the pith, and belong to the medul-
lary sheath to be described below (Sect. 152), the vessels — perhaps also series of
tracheides of otherwise similar structure — and groups of bundle-parenchyma are
inserted in the fundamental mass in the more strongly developed wood.

The grouping of the vessels and of the parenchyma, and the quantity and
quality of the latter, vary according to the particular case. In the aerial stem of
the CrassulacecB^ investigated, which has a somewhat strongly developed ligneous
body, the parenchyma consists of elongated thin-walled cells, which remain
unlignified, and accompany the vessels in longitudinal rows. The vessels and
parenchyma are either quite isolated and scattered in the mass of fibres, at most
two or three occurring together — Sedum maximum ; or some of them form larger
groups, consisting of as many as twelve elements — S. populifolium, and Echeveria

' Compare Brongniart, Arch, du Museum d’Hist. Nat. tom. I. — Regnault, 1. c.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

499

pubescens ; or they form with the parenchyma extensive groups, constituting trans-
versely elongated, occasionally irregular, interrupted transverse bands — Semper-
vivum arboreum. Lastly, in the small creeping stems of Sedum reflexum, they form,
together with the delicate cells, thick, multiseriate, continuous annular zones, which
alternate with similar fibrous zones. Other species however, even those with thick
stems, as Crassula lactea, Sedum ternatum, and Echeveria pubescens, form mere
traces of secondary wood, which are scarcely worth mentioning.

In the CaryophyllecB investigated^ (Dianthus, Gypsophila, Silene spec., and Arenaria
graminifolia), especially in their rhizomes, thin-walled, long-celled parenchyma, often
forming large irregular islets or annular segments, is inserted between fibrous masses
of similar form, but in the foliage stems it may be absent, e. g. Gypsophila altissima.
The vessels are numerous, and occur among both forms of tissue, often forming
interrupted radial rows ; in the fibrous groups they are often unaccompanied by
parenchyma. In the foliage shoot of Dianthus plumarius the main mass of the very
slight secondary thickening consists of vessels.

In the stem of Rumex Lunaria, the pitted vessels, accompanied by rows of bundle-
parenchyma or by intermediate fibres, lie in very regular but interrupted radial
rows in the fundamental mass, and the latter consists of fibrous cells densely filled
with large starch-grains. With the exception of the starch, the same holds good
for Centradenia grandifolia (comp. p. 484). In Campanula Vidalii the vessels are
very isolated, and scattered in the radial series of fibrous cells, while I was unable to
find any parenchyma or intermediate fibres accompanying them.

The Rhinanthacecß require further investigation.

The secondary wood of the thicker, fleshy stems of CactecB must again be men-
tioned here. The peculiar structure of the ligneous bundles, vessels, and tracheides,
in the Mamillarise, Echinocactus, &c., has already been repeatedly noticed above
(comp. p. 478), and it must here be added, that in the genera mentioned the whole
secondary ligneous bundle consists of the spirally and annularly thickened tracheae,
usually with a deeply projecting fibre, between which very thin-walled bundle-
parenchyma is imbedded, in single longitudinal rows. The medullary rays of all
ranks resemble the latter as regards the delicacy of their cells. In other Cacteae,
especially species of Cereus and Opuntia, the secondary ligneous bundle consists of
tough woody fibres, and the scattered reticulated vessels which are usually accom-
panied by parenchyma (see p. 479). The parenchyma, including that of the medul-
lary rays, may be very thick-walled and lignified, e. g. in Cereus speciosissimus.
In the Opuntise short tracheides are also present, with the above-mentioned
thickenings on the wall, which project inwards in the form of plates, and are usually
annular. These elements are sometimes distributed singly in the interior of the
bundle (O. tunicata, O. robusta), sometimes at its edges (O. cylindrica, ramulifera,
andicola).

Lastly, the wood which forms the floating apparatus of the stems of certain
Leguminosae of the genera JCschynomene and Herminiera, which vegetate on the
surface of the water, may deserve mention. Their structure appears to be very

* Regnault, l.c. p. 118, pi. VI.

^ Compare Brongniart, l.c. — Schleiden, Anat. d. Cacteen, l.c, (p. 156).

K k 2

500

SECOyDARY CHANGES.

uniform in the various species of these floating woods, as far as the existing de-
scriptions extend^. Hallier’s figures (/. c) and Figs. 51-53 in Schleiden’s Grundzüge
(3 Ed.), I. p. 261, which at any rate represent a similar object, may serve to illustrate
its coarser characteristics. The following short description refers especially to the
wood of the Ambatsch of the White Nile (Herminiera Elaphroxylon=.^demone
mirabilis Kotschy : —

The extremely light wood has no distinct annual rings. It consists, in its main mass,
of elements which must be called tracheides (until undried material has been investigated),
because in their condition as observed, they contain only air, without a trace of proto-
plasm or remnants of cell-contents. They stand alternately in radial rows, and have the
form of hexagonal upright prisms, about three times as high as broad, with terminal sur-
faces inclined to the radial plane at an angle of about 45°, either unilaterally, or bilaterally,
like a roof. Their thin colourless membrane is very beautifully thickened on the entire
terminal surface, by a fine delicate network of fibres, while on the radial, and in a lesser
degree on the tangential lateral surfaces, it is provided with small groups of simple
minute pits.

The mass consisting of these tracheides is traversed (i) by very numerous parenchy-
matous medullary rays, containing starch, which are i-io, on the average about six cells
high and one cell in breadth, and further by some larger medullary rays, several cells broad
in the middle; the cells of the medullary rays are elongated and procumbent. (2) By
narrow bands arranged in irregular and often interrupted concentric annular zones,
which are themselves crossed by the medullary rays; these bands chiefly consist of
long-pointed fibres, with their ends overlapping one another, in a radially and tangen-
tially oblique direction. In these bands, or more correctly on their inner (medullary)
side, lie large pitted vessels, usually isolated, rarely forming short radial rows of few
elements, and in both cases at long distances from one another laterally, being separated
by several subdivisions of the wood, as determined by the medullary rays. Every vessel,
or every group of vessels, is partly surrounded by a single layer of intermediate fibres
containing starch, or by parenchymatous cells, with a moderately thickened pitted wall,
2-4 of which stand one above another; and very thin-walled, narrow parenchymatous
cells, likewise containing starch, are continued in a single layer over the inner surface of
each fibrous band. Between these the above-mentioned (p. 141) chambered crystal-sacs
are here inserted. Tracheides, intermediate cells, members of the vessels, and also
the medullary rays of medium altitude, are everywhere of approximately equal height,
and lie with their ends in the same horizontal planes, thus forming regular horizontal
layers. Their form and arrangement (with the obvious exception of the members of
vessels) are similar to those usual in cambial cells, so that it may safely be assumed that
they are derived from a cambial zone consisting of cells similar to them in height and
form. The fibres, on the other hand, are (as estimated) at least double as long as the other
elements mentioned, and must thus have undergone the corresponding elongation (and
displacement) on their differentiation from the cambial zone.

c. Changes of the individual forms of tissue in the annual ring.

Sect. 151. In those of the cases just brought forward, in which the distribution of
the forms of tissue is different in the successive layers of an annual increment of growth,
and where the structure of the autumn-wood of one year is thus different from that
of the neighbouring spring-wood of the next year, a limiting .line must appear between

* Hallier, Botan. Zeitg. 1859, p. 152 ; 1864, p. 93.

^ Compare Schweinfurth, Beitr. z. Flora .(Ethiopensis, p. 9.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

501

the two. In addition to this cause of the demarcation of the annual rings, which con-
sists in the distribution of non-equivalent tissues, and is not always present, there are
two other causes, consisting in differences of form and differences of structure of
equivalent tissue-elements, appearing in the successive, and especially in the extreme
zones of an annual increment of growth.

The first of these phenomena consists universally, in a shortening of the radial
diameter, and thus in a tangential flattening of the elements, at the outer limit of the
autumn-wood, and this is due to increasing cortical pressure h Changes in the
average length may, as has been stated, be connected with this. The second cause,
not present in all cases, consists in an increase in the thickness of the wall, and some-
times in further changes of its structure. These changes, especially the shortening of
the radial diameter, affect both the elements of the medullary rays and those of the
ligneous bundles. They come on either suddenly or gradually, and in fact this latter
difference depends, partly on species, partly on differences in the vigour of development
of the annual rings in one and the same wood.

These conditions are shown most simply and clearly in the woods of the ConifercB'^,
the bulk of which consists only of tracheides and medullary rays. In the procumbent
elements of the medullary rays, the shortening of the radial diameter at the autumnal
limit is not very conspicuous, though it does occur. The tracheides however are rela-
tively wide at the spring limit of each annual layer, usually quadrangular as seen in cross-
section, though sometimes pentagonal or hexagonal, and their radial diameter is equal
to the tangential, or even somewhat greater ; at the autumnal limit, on the other hand,
they are always much flattened, i. e. the radial diameter is shortened. According to
N. Müller a diminution of their length in the autumn-wood is connected with this, in
the case of the Fir. To this is further added an increase, accompanying the flattening,
not only of the relative, but of the absolute thickness of the wall ; and the appearance
of pits on the tangential surfaces of the wall, while in the spring elements, with a wide
lumen, the latter are limited to the radial surfaces.

By way of example, these conditions may be illustrated by the mean magnitudes found
by Mohl^ in the case of a well-grown specimen of Pinus sylvestris, thirty years old ; they

are expressed in Paris lines : —

Spring-wood. Autumn-wood.

{Radial diameter 00204 0-0056

Tangential 0-0142 0-0142

Thickness of wall .... 0-0019 0-0031

{Radial diameter 0-0232 0-0094

Tangential 0-0161 0-0161

Thickness of wall .... 0-00 1 8 0 003 5

The sudden or gradual appearance of these differences, and the relative thickness
of the zones with narrow and wide lumen, here depend in general on the thickness of
the annular zones, as will be shown below.

^ H. deVries, l.c. (p. 475).

“ Goppert, Monogr. d. foss. Coniferen, l.c. — Von Mohl, Botan. Zeitg. 1862, p. 225, &c. — Kraus, l.c.
’ Botan. Untersuchungen, IV. 2, p. 190.

* L.c. p. 237.

502

SECONDARY CHANGES.

Deviations from the typical structure occur, in so far as groups of unusually thick-
walled tracheides may appear in the different regions of the annular ring. These, as
seen in cross-section, form band-shaped, annular segments, of a brownish yellow
colour, similar to that of the autumn wood ; in Pinus sylvestris they always occur in
the innermost annual rings, and often also in the outer ones. Comp. Sanio, Pringsh.
Jahrb. IX. loi.

In the Dicotyledonous woods the demarcation of the annual ring is further influ-
enced by the distribution of non-equivalent forms of tissue, and of the changes which
each of these shows in the successive zones of the annual ring.

The shortening of the radial diameter at the autumnal limit, sometimes occurring
suddenly and sometimes gradually, and either amounting to a decided flattening or
only occurring in a slight degree, according to the special case, is here also general :
leaving out of consideration the vessels, which will be specially mentioned below, it is
approximately uniform in the forms of tissue which occur in contiguity, more rarely
it takes place to a very unequal extent : as in the marked and sudden flattening of
the tracheides of the autumn-wood, while the radial shortening of the fibrous cells which
accompany them is gradual and relatively much slighter, in Clematis Vitalba, and
Mahonia Aquifolium.

With reference to the increase of the absolute thickness of the wall at the
autumnal limit, a distinction must be drawn between the particular forms of tissue,
for the latter differ from one another in average thickness in all the parts of any
wood. In the case of equivalent forms of tissue both conditions occur, according to
the kind of wood, namely, either a marked increase of the thickness of wall at the
autumnal limit or its approximate constancy throughout. The latter, for example,
is the case —

In the bundle-parenchyma of Gleditschia triacanthos, Ailantus glandulosa,
Sophora japonica, and Caragana arborescens ;

In the fibrous cells of Berberis, and Mahonia ;

In the vessel-like tracheides of Betula, Ainus, Populus, Salix spec.. Magnolia
acuminata, Sambucus nigra, &c. ;

In the thick-walled fibriform tracheides of Cornus sanguinea, Syringa vulgaris,
and Buxus sempervirens.

Increase of the thickening of the wall at the autumnal limit takes place for
example in —

The bundle-parenchyma of Gymnocladus, Morus alba, Broussonetia, Paulownia,
and Amorpha fruticosa ;

In the woody fibres of Laurus Camphora, Jatropha Manihot, and Carpinus Betulus;

In the vessel-like tracheides of Caragana arborescens, Carpinus Betulus, and
Ostrya virginica ;

In the fibriform ones of Syringa Josiksea, Philadelphus coronarius, Kerria
japonica, &c.

Contrary to the rule holding good for other woods, Sanio observed a decrease
of the thickness of the wall at the autumnal limit, in the tracheides of Staphylea pin-
nata, both as compared with the inner ones of the same annual ring, and with those
of the spring-wood of the next following ring.

As regards the degree of increase in the thickness of the walls at the autumnal

SECONDARY THICKENING. NORMAL DICOTYLEDONS. 503

limit, no accurate comparisons of the different forms of tissue have been carried out,
but one may assume from the appearances, that it takes place in an approximately
uniform ratio in all. Since then, as was shown above, the different forms of tissue of
the same wood usually have a very unequal average thickness of wall, the total relative
bulk of the walls at the autumnal limit must depend in the first instance on the dis-
tribution of the non-equivalent elements in the annual ring, and secondly on the
increase of thickness, which has just been discussed, in each individual form of tissue.
Thus, where, for example, the autumnal limit consists chiefly or exclusively of paren-
chyma, as in Morus alba, Broussonetia, Fraxinus, Robinia pseudacacia, Caragana
arborescens, Amorpha fruticosa, Virgilia, Gleditschia, Catalpa, Paulownia, Ailantus,
&c., or of vessel-like, thin-walled tracheides, as in Betula alba, Alnus glutinosa, Populus,
Salix spec., and Cytisus Laburnum, while in the inner part of the ring thick-walled
fibres prevail, then the autumnal limit is, on the average, thinner-walled than the next
inner portion of the ring. The manifold possible combinations and modifications in
this respect follow from what has been already stated.

The Vessels in most Dicotyledonous woods behave similarly to the other elements,
in so far as they decrease on the average in width from the spring to the autumn
limit of the annual ring, though without, as a rule, undergoing any conspicuous tan-
gential flattening. With this a decrease in the number of the vessels from within
outwards is often connected. Both differences may appear very gradually, as in the
annual ring of the Salicinese, Pomacese, Fagus, Buxus, Cornus sanguinea, &c., and
may even be scarcely noticeable (Enckea media, Ulex europseus). In other woods, as
Quercus pedunculata, Fraxinus, and Castanea, they appear suddenly, in such a manner
that the first spring-zone of the ring, which is rendered highly porous by numerous very
%\ide vessels, is succeeded on the outside by less numerous and much narrower vessels,
occurring between the other elements. According to the degree in which this peculi-
arity of the spring-wood, which may be shortly characterised by the name ‘ porosity,’
clearly appears, and coincides with other differences of structure between the annual
limits and the middle portion of the ring, the boundary comes out more or less
sharply even as seen with the naked eye. The trees last-mentioned form examples
of especially sharp demarcation of the annular rings, and this is also the case with
Fraxinus, Robinia pseudacacia and Gleditschia triacanthos, as the boundary is marked
both by large vessels, and by the other conditions of structure mentioned above. If,
on the other hand, the vessels, while of approximately equal width, are uniformly
distributed singly or in groups, as in Enckea media, Ulex europseus, and no doubt
Olea europaea also, the limit of the annual rings may be perceptible with difficulty,
or scarcely at all, not only when slightly magnified, but even under microscopic
investigation.

These various degrees in the demarcation of the annual rings, at once suggest the
conjecture that cases of their entire suppression may occur. This unquestionably takes
place as an individual peculiarity. Many plants, e. g. the Araucarise, appear to have
a special tendency to this, and such individual phenomena will be discussed below.
Plants, in w'hich the demarcation of the annual rings is constantly absent as a specific
peculiarity, are certainly rare, and the statements respecting such cases have been so
often disputed, that I am unwdlling to adduce even those plants in which I was myself
unable to find limits between annual zones, — namely the woody Piperacese, Opuntia,

504

SECONDA RF CHANGES.

Mamillariese and Cacteae, and Cobaea scandens, — as certain examples of plants
which are constantly destitute of annual rings b

Whether in tropical trees, as well as in our temperate zones, the annual ring as
anatomically distinguished always represents the yearly growth, or whether there are trees
with half-yearly rings, i. e. such as form two annual rings in the year, corresponding to
the two periods of vegetation falling in one annual period, as has been stated to be the
case inAdansonia digitata®, appears to me to be a question which ought to be tested, but
one that does not belong to the present anatomical exposition. Attention may here,
perhaps superfluously, be recalled to the fact that in woods with alternating concentric
bands of non-equivalent tissue, as Ficus, Casuarina, &c., markings are present, which
resemble annual rings on superficial observation, but from which the true annual rings
are distinguished by the characteristic structure of the autumn wood.

d. Normal differences of successive zones of growth and annual rings.

Sect. 152. The first innermost annual ring of every wood necessarily shows certain
essential differences in its structure from all the later ones. In the stem, instead of
the characteristic spring-wood, it has at its internal limit the groups of spiral, annular,
and reticulated vessels (or tracheides) corresponding to the primary vascular bundles,
but absent from the intermediate bundles ; these usually project more or less into the
pith, and, together with the neighbouring tissue-elements, belonging partly to the
ligneous ring, partly to the pith, the structural peculiarities of which have been often
mentioned in former sections, they have long been distinguished collectively as the
medullary sheath or medullary crown, Corona.

In the root, the internal limit of the first annual ring has a different structure,
corresponding to the conditions described on p. 473. The secondary wood here
closely surrounds the original axial xylem-plates. It is quite a prevailing rule that
these latter remain separated from the secondary vessels and tracheides by at least one
layer of connective cells b No case is known in which the secondary elements border
immediately on the outermost narrow, spiral, and annular vessels of the primary plates.
On the other hand, it often occurs that the inner pitted vessels (or tracheides) of the
original plates stand in immediate connection with equivalent elements of the
secondary wood; this has been observed in Taraxacum and Ranunculus repens
(Fig. 165, p. 356).

The cells which usually border on the xylem-plates consist of the inner layer of
those connective cells which were originally present in this position, while from their
outer layer the cambial ring bordering on the phloem-group has been derived
(pp. 351, 473). In those cases where the pitted vessels are contiguous, it still remains
to be investigated whether the latter are derived directly from the above-mentioned
connective cells, or from a cambial zone which, in the latter case, must originate in the
innermost layer of connective cells. All the tissue-elements of the region in question
usually become very thick-walled, and have relatively narrow cavities, especially in

* Compare Link, Philos. Bot, p. 136. — Meyen, Physiol. I. p. 361. — Treviranus, Physiol. I. 235.
— Unger, Botan. Zeitg. 1847, p. 267. — Schacht, Lehrb, II. p. 62. — Sanio, Botan. Zeitg. 1863, p. 392.

^ Compare Botan. Zeitg. 1844, p. 367.

3 See Van Tieghem, 1. 1.

SECONDARF THICKENING. NORMAL DICOTYLEDONS.

505

true woody plants. The accurate distinction of the individual elements one from
another therefore presents great technical difficulties ; even in good transverse sections
the recognition of the original xylem-plates between their thick-walled next neighbours
is often very difficult, besides which the narrow vessels at their angles often seem to
become indistinct owing to pressure from the surrounding tissues.

In addition to these peculiarities of the internal limit of the wood in stem and
root, further peculiarities exist in the inner secondary ligneous mass itself.

In rare cases, especially in the stem of Mahonia Aquifolium, Berberis vulgaris.
Pelargonium roseum, and Solanum Dulcamara, Sanio found septate fibrous cells in
the first annual ring, while they are absent in the succeeding ones. And further, in
the first and next-following annual rings of the stem and its branches, in many
though not all Dicotyledonous woods, although the elements characteristic of the
species are all present, yet their characteristic arrangement does not appear clearly till
later, as it is merely indicated in the former region. The groups of vessels, and the
parenchymatous zones of Hedera Helix, Quercus pedunculata, Juglans, Casuarina, &c.^,
are examples of this. The characteristic structure of a wood is therefore not always
to be recognised with clearness from its inner rings. Similar phenomena, which have
not at present been accurately investigated, may occur in the case of roots.

Sect. 153. The most remarkable phenomenon of this category is the change in
the average size of equivalent elementary organs, as regards both their width and
length, which accompanies the growth in thickness. In most woods this change
takes place in such a manner that the average size increases during a series of years,
and then attains a definite value which remains constant in the succeeding years.
The change in question takes place in a different manner in the stem, branches, and
roots of the same tree, and in their different transverse zones ; in some cases it
depends on a corresponding successive increase in size of the cambial cells, while in
others it is independent of this.

The most complete measurements referring to this point were carried out
on Pinus silvestris by Sanio who determined the mean length of the tracheides,
and their mean tangential breadth in the autumn wood. He thus sums up his
results : —

1. In the stem and branches the tracheides everywhere increase from within
outwards, throughout a number of annual rings, until they have attained a definite
size, which then remains constant for the following annual rings,

2. The constant final size changes in the stem in such a manner that it con-
stantly increases from below upwards, reaches its maximum at a definite height, and
then again diminishes towards the summit.

3. The final size of the tracheides in the branches is less than in the stem, but
is dependent on the latter, inasmuch as those branches which arise from the stem at
a level where the tracheides are larger, themselves have larger tracheides than those
which arise at a level where the constant size is less.

4. In the gnarled branches of the summit, the constant size in the outer annual
ring also at first increases towards the apex, and then falls again ; but here irregu-
larities occur, which may be absent in regularly grown branches.

* Sanio, Botan. Zeitg. 1863, p. 397.

Pringsheim’s Jahrb. VIII. p. 401, &c.

5o6

SECONDARY CHANGES.

5. In the root the width of the elements first increases, then falls again, and
next rises to a constant amount. An increase in length also takes place, but could
not be exactly determined.

The absolute size of the elements is not the same at the same place in
different individual trees, but these differences do not infringe the general rule. In
order to illustrate the absolute dimensions, some of Sanio’s determinations (cf. /. c.),
taken from a main axis 1 10 years old, may here be cited.

ML. = mean length, MBr. = mean breadth of the tracheides, the latter determined in
the autumn wood.

A.

Disk

21

years

old, from the summit.

B.

35

„ above the thick branches of the crown.

C.

72

„ from the shaft, 36' above the ground.

D.

105

„ close above the ground.

The dimensions are expressed in millimetres.

A.

B.

Annual ring.

Annual ring.

I

MBr.;

: o-oi6

; ML.:

0-78

I

MBr.:

o*oi6 ;

ML. ;

: 0-80

14

M

}>

99

1-74

15

99

99

99

2-6o

18

>>

jj

99

2.21

17

99

99

99

2-74

20

J)

99

99

2'9I

18

99

99

99

2-82

21

0*026

99

2.82

19

99

99

99

2-82

20

99

99

99

2-82

22

99

99

99

2-82

35

99

0*028

99

278

C.

D,

Annual ring.

Annual ring,

I

MBr.

: 0-017

; ML.:

0-95

I

MBr.:

0*01 1 ;

ML.;

; —

17

99

99

2-74

20

99

99

99

1.87

19

99

99

3-13

29

99

99

99

2.48

31

99

99

3-69

30

99

99

99

2-6o

37

J)

99

99

3-87

31

99

99

99

2-65

38

99

99

3-91

46

99

99

99

2-65

39

99

99

4*00

60

99

99

99

2-65

40

99

99

4.04

80

99

99

2.6 J

43

99

99

4*09

105

99

0*028

99

2-65

45

99

99

4*21

46

)>

99

99

4*2 I

72

0*032

99

4*2 I.

In the Dicotyledonous woods investigated *, the conditions in question vary ac-
cording to the species ; some show no increase in the size of the elements in the
successive rings : e. g. Mahonia Aquifolium ; or only an inconsiderable one, as in
Berberis vulgaris, where only the vessels in the spring wood become wider.

In the other cases investigated (Caragana arborescens, Sophora japonica, Saro-
thamnus scoparius, Acacia longifolia, Carpinus Betulus, Quercus pedunculata, Cornus
sanguinea, Rhamnus cathartica, and Ficus elastica), an increase in length takes place
from within outwards, which affects the individual forms of tissue unequally. The

* Sanio, Botan. Zeitg. 1863, Lc. — Pringsheim’s Jahrb. IX. p. 52, &c.

SECONDARY THICKENING. NORMAL DICOTYLEDON^.

507

fibres always increase in length. The cells of the bundle-parenchyma show no
change. The members of vessels and the tracheides behave differently according to
the kind of wood.

By way of example we may reproduce Sanio’s statements regarding the mean
length of the elements in a stem of Quercus pedunculata 130 years old, with very
narrow annual rings. The dimensions are expressed in millimetres.

Members of

Woody fibres. Tracheides. large vessels.

ist Annual ring .... 0-42 .... 0-39 ....

2nd „ .... o-6o .... 0-43 .... 0-25

4th „ .... 0-74 .... 053 .... 0-26

Three outermost rings . . 1-22 .... 0-72 .... 0-36.

An increase in the width, and in the thickness of the walls of the elements,

accompanying the increase of the latter in length, is likewise evident in many

Dicotyledonous woods, especially in the case of the large vessels of the spring-wood.
Quercus pedunculata is a typical example of this. Sanio found the mean radial
diameter of these vessels in the third annual ring =o-o8““i; its definitive dimension,
which is not attained before the sixth year, amounts to 0-31 — o-33“'na. As regards
the change in the definitive constant length of the elements at different levels
of the stem, only one investigation, on a single Birch-stem, exists, which may be
referred to in Sanio’s treatise.

The increase of the ligneous elements in length and width is, in some of the
cases observed, the immediate consequence of a corresponding successive enlarge-
ment of the cambial cells ; while in others this is not so. The former is the case
in the Coniferae, where both the successive enlargement of the cambial cells, and the
relatively trifling increase of size, which the ligneous elements undergo after their
origination, are distinct. In the Pine, for example, the tangential diameter of the
winter cambial cells is more than twice as great (0-02 ö“!“) in a stem a hundred years
old as in a shoot one year old (o-oi2«i“); in a one-year-old apical shoot of the
same species, on the other hand, the length of the cambial cells is 0-87“™, that
of the tracheides in the autumn-wood i-os““. The other category includes,
for example, Rhamnus Frangula, Cytisus Laburnum, Caragana arborescens, and
probably most Leguminosse. As far as the investigations extend the cambial cells
here undergo little or no increase of size, as their distance from the pith becomes
greater. The increase in size is thus due to the ligneous elements themselves,
which have already been developed from the cambium; the woody fibres, which
are especially affected by it, become in Cytisus Laburnum, for example, six times
as long as the cells of the cambium.

Sect. 154. In many woody plants there further arise those differences of physical
properties between successive zones of growth or annual rings, on which is founded
the technical distinction between sap-wood (Splint, Alburnum, Aubier) and duramen
or ripe-wood (Kern, Herz, bois parfait). The sap-wood is wood which has been
completely developed from the cambium, and which possesses the anatomical
characteristics above described, and the physiological properties which depend on
them. It has a light whitish or yellowish wood-colour. In many trees, as for
example Acer pseudoplatanus, platanoides, and Buxus, the alburnum condition

5o8

SECONDARY CHANGES.

undergoes, according to Nördlinger, no change, at least as regards the outward
appearance and coarser physical characteristics of the wood. These are termed
alburnum trees by Nördlinger h In most woods changes of the chemical ^ and
physical character, and in a less degree of the structure also, take place sooner
or later with the increasing age of the successive zones. These changes represent
the beginning of that process of degradation which ends with decomposition, and
as they proceed the wood ceases to be available for its original physiological work®,
with the exception of the purely mechanical function in the case of those woods
which become harder. Externally a darker colour appears, which varies in the
different kinds of wood, and may become deep black, as in the Ebony woods, dark-
green as in Guaiacum wood, or red and violet as in the dye-woods of the Csesalpiniae,
Pterocarpus, Hsematoxylon, &c. With these changes is very often connected an
increase of the specific weight and hardness, and a diminution of the contained
water in a word the technical value of the wood is created or enhanced by these
changes. In this case especially the terms ‘ ripe-wood ’ and ‘ heart-wood ’ are used ; —
the two names being applied rather arbitrarily to the particular cases, or used for
successive stages.

The formation of hard and lasting duramen is however only a special case of
the incipient process of degradation. The latter may also lead quickly to the
opposite result. According to Nördlinger ®, if I understand him rightly, ‘ many soft
woods, e. g. Canadian poplar, and several kinds of Willow,’ have a brown heart-wood,
which is not distinguished from the alburnum either by higher specific gravity, or by
hardness and durability, but, on the other hand, shows a tendency to rapid decom-
position, accompanied by the growth of mould. The complete disorganization
of the wood, which in many cases takes place in old living trees, is an allied
phenomenon which will be discussed below.

Apart from these changes, anatomical investigation does not demonstrate any
alteration in the original structure and the original thickness of wall of the cells and
tubes, on the formation of the heart-wood, but merely changes in the material
properties of the walls, and in the contents. The membranes are infiltrated by
heterogeneous organic bodies, and the latter often appear in the interior of the
elements, which they fill up more or less completely, as well as in any cracks and
crevices that may be present. The colours of heart-woods are those of the in-
filtrated substances.

Qualitatively, the latter are usually combustible organic compounds, showing
extreme variety in detail, if all the cases be taken into consideration. While referring
to the technical literature ®, it is here sufficient to call to mind the dyeing substances
and chromogenetic bodies of the dye woods, and the infiltrations of resin in the
wood of many Coniferae, Guaiacum, &c. As regards these substances, it is by no

* Technische Eigensch. d. Hölzer, p. 28, &c.

^ Compare the summary in Hofmeister, Pflanzenzelle, p. 247, and the technical literature,
ä Rossmässler, Tharander Jahrb. IV. p. 186 (according to Nördlinger).

♦ On these subjects, which do not concern us further, compare Nördlinger, 1. c., and Wiesner,
Rohstoffe, cap. 13.

5 /. c. p. 36.

® Compare the summary in Wiesner, /. c.

SECONDARY THICKENING. NORMAL DICOTYLEDONS. 509

means to be asserted that in the woods in question they alone saturate the heart-
wood, and may not perhaps be mixed with other bodies which have not been
minutely investigated, or that differences may not prevail between the mixtures
which occur in the membranes and those in the cavities. On these questions no
sufficiently accurate investigations exist. In a number of Dicotyledonous woods,
which do not serve as dye-woods (Ailantus, Prunus domestica, spinosa. Amygdalus
communis, Zanthoxylon fraxineum, Rhamnus cathartica, Sorbus Aucuparia, Gle-
ditschia, Periploca, &c.), bodies were found by Sanio ^ both in the interior of the
vessels and in the membranes, which on their first appearance in the interior of
the vessels were colourless, but afterw’ards came to differ from one another in their
yellow or red colour, while they agreed together in their high resistance to all
solvent agents. Caustic potash does not change them ; Schulze’s mixture at boiling-
point first produces discolouration, and then solution.

It is thus quite possible that the appearance of a definite body, or of a series
of substances, which are nearly related, and vary in the different kinds of wood,
may be generally characteristic of the process of metacrasis which produces the
duramen, and that the appearance of resins, of special colouring matters, and
so on, may only be a phenomenon peculiar to particular cases, and accompanying
the former changes.

In Caragana arborescens Sanio found that only air is at first contained in the
vessels of the yellow heart-wood; at a somewhat later stage he found a yellow
body with the properties above-stated ; in the discoloured central portion, which is
limited by a red ring, and even in the red ring itself this body had again disappeared.
This phenomenon can afford no argument against the view that the appearance
of the infiltrating substances in question is characteristic of the formation of dura-
men, but can only indicate that in certain cases they may undergo displacement,
or further processes of rapid decomposition.

Inside the vessels and cell-lumina the infiltrating bodies of whatever kind
appear at first in the form of deposits on the wall, often in two layers; here and
there, where they are accumulated in large quantities, they show hemispherical
prominences, projecting inwards, or sometimes form biconcave plates extending
transversely through the lumen; the masses are homogeneous, rarely granular
(Castanea vesca), frequently with flaws and cracks. All these phenomena point to
the fact that they first appear in the fluid form, and afterwards harden. Very
extensive accumulations fill the lumina completely, as for example in the Guaiacum
and Ebony woods. In the vessels and crevices of the Campeche wood, greenish
crystals (Hsematoxylin ?) sometimes occur ^

It is remote from the purpose of this book to discuss the question, so difficult in
the defective state of chemical knowledge on these subjects, as to the origin
of the infiltrating substances, and more especially to investigate how far they are
derived from the transformation of other pre-existing substances, at the same
places in which they occur, to what extent they have been conveyed to these
places from elsewhere, and in the latter cases what their origin is. Attention may

* Botan. Zeitg. 1863, p. 126. — Compare also Hartig, ibid. 1859, p. 100.
^ FUickiger and Hanbury, Pharmacographia, p. 188.

510

SECONDARY CHANGES.

here only be briefly called to the fact, that the anatomical conditions already
adduced point to the occurrence of somewhat complicated processes, and that
the expressions just used are not intended to prejudge any developmental theory
whatever.

As a last stage of the changes described, and one which leads to a result
opposite to the formation of heart-wood in the technical sense, a displacement
of the normal membranes (extending to their complete disappearance) by masses of
resin and balsam is stated to occur locally in the woods which produce these
substances; in other woods, which do not form resin, a transformation into dis-
organised masses of mucilage and gum takes place. The wood of Pinus Strobus
and Abies peclinata, which is infiltrated with balsam, dissolves away, according to
Wigand \ into a resinous mass, and as this goes on, the walls of the tracheides and
cells diminish in thickness, becoming lost eventually in the structureless mass of
resin. The canals, an inch in width, filled with balsam, which, according to
Karsten^, traverse the wood of species of Copaifera, and similar phenomena
reported in the case of the stem of Drybalanops aromatica® can also, according
to the data before us, scarcely arise otherwise than by partial disorganisation and
solution of the ligneous elements.

The formation of cherry-gum by disorganisation of the wood of the Amygdalese,
as described by Wigand, which no doubt belongs to a great extent to the province
of pathology, starts sometimes from vessels, sometimes from groups of wood-
parenchyma, or medullary spots. Comp. Wigand, I, c.

In addition to the organic infiltrated substances, considerable accumulations of
Silicic acid occur, as found by Criiger * in the old wood of plants which are charac-
terised by extensive silicification of almost all their parts, namely of the Chrysobalanese,
Hirtella silicea, Petrsea volubilis, P. arborea, and of Tectona grandis. They occur in
the lumina of the cells and the vessels (the latter exclusively in Teak wood) as
amorphous masses, filling up the space more or less completely. According to the
published statements they are not present in the alburnum. The data before us do
not allow of any decided judgment as to further differences between alburnum and
duramen, as regards the incombustible constituents which they contain.

In the contents of the cells of the wood, both in the medullary rays and in
the ligneous bundles, and also of the thyloses w'here they occur, a further essential
change takes place on the formation of the heart-wood, in addition to those
described. This consists in the permanent disappearance of those constituents
which characterise the living cell, especially of the universally distributed starch,
and in their replacement by air or infiltrations ®. The disappearance of the starch
from the cells coincides fairly exactly, in the cases investigated, with the appearance
of the other characters of the heart-wood.

^ Pringsheim’s Jahrb. III. p. 165. — On the other hand compare also Dippel, Botan. Zeitg. 1863,
P. 256.

^ Botan. Zeitg. 1857, P- 3i6-

^ Compare Fliickiger and Hanbury, Pharmacographia, p. 202.

^ Botan. Zeitg. 1857, p. 297.

® Sanio, Stärkefiihrende Zellen, p. 19 (1858). — Id. Botan. Zeitg. i860, p. 202. — A. Gris, Comptes
Rendus, 1866, tom. 70, p. 603.

SECONDA RF THICKENING. NORMAL DICOTYLEDONS. 511

As has often been indicated in the preceding pages, the transformation or
degradation of the alburnum into duramen takes place in different species of trees
at a different average age, and in some gradually, in others suddenly. In the
stem of Fraxinus excelsior when forty years old, and in that of the Birch when thirty-
five years old, abundance of starch was found by Gris in all the annual layers ; the
former species is, according to Nördlinger, characterised by a very broad alburnum,
while Betula alba is an ‘ alburnum tree.’ In a stem of Fagus sylvatica ninety-five
years old, investigated by Gris, there was abundant starch in the fifteen outermost
annual rings, then a gradual decrease in its amount from these up to the thirty-fifth
ring, while further inside it was wholly absent. According to Nördlinger, Quercus
pedunculata has 8-13 annual layers of alburnum; in an oak-stem fifty-eight years
old, Gris found that the cells ceased to contain starch somewhat suddenly at the
sixteenth ring, and in one ninety-eight years old, at the twenty-first ring. Robinia
Pseudacacia has 3-5 rings of alburnum, which are sharply marked off from the dark
heart-wood, both by their containing starch, and by their light colour : Castanea
vesca behaves in a similar way (Gris and Nördlinger). According to statements
based on those coarser differences which are of technical importance, the diversity
in this respect is very great.

It is equally evident from the facts already mentioned, that in the same species
of tree, and even in the same individual, stem, or branch, numerous individual
variations occur, within definite specific limits, according to the age and vigour
of development, especially of wood-formation, and at different levels of the stem.
This is the case whether the relative thickness of alburnum and duramen be deter-
mined according to the number of annual rings, or according to absolute measure.
Even on different sides of the same cross-section the number of annual rings
showing the properties of alburnum is often different^.

What has been hitherto said applies to wood enclosed in the uninjured stem. It is
well known that changes not unfrequently occur in the wood in consequence of injuries,
and that the extent of these changes, which are similar to those characterising the
formation of heart-wood, shows a definite relation to the position and extent of the
wound. The processes of decomposition to which they owe their origin may often be
different to those which accompany the formation of duramen On the other hand, the
phenomena occurring in both cases are often so similar — as for example the conversion
into resin in the Coniferae, and the formation of black, hard wood in the Ebenaceae — that
we may suppose the same process, which usually comes on more slowly, to be accelerated
or excited by injury.

e. Individual and local deviations.

Sect. 155. Within the limits of the typical characteristics, which have been
explained in the preceding pages, the structure of the annual ring in the same
species shows definite variations, according to differences, however produced, in the
vigour of its development ; the single feebly developed ring, or even portion of a

* Compare especially the detailed statements respecting the Oak in Duhamel, Physique des
Arbres, I. p. 46, &c.

“ Compare e. g. Nördlinger, 1. c. p. 37.

512

SECONDARY CHANGES.

ring, of a tree which has otherwise grown vigorously, shows essentially the same
character as the whole of the rings of a feebly thickened tree. This character is
different in Coniferous woods, and in the Dicotyledonous woods investigated.

In the former, according to Mohl, the relative thickness of the spring-wood
with wide lumina, and the autumn-wood with narrow lumina, and the more or less
sudden transition from one to the other, varies quite universally, according to thp
thickness of the rings, the variation in the stem being, as a general rule, in the
opposite direction to that in the root, subject to specific or perhaps individual
modifications In the former, the outer zone of the ring, with narrow lumina and
thick walls, forms a larger portion of the whole, and is the less sharply marked off
from the interior the thinner the ring is. In the root it is the more developed the
thicker the ring ; in the feeble annual rings which are prevalent in roots, and which
in the White Fir (Abies pectinata), for example, are on the average only about o-z““
broad, it often consists of only 3-1 layers, and is sharply limited towards the internal
zone, which has wide cavities.

In the Dicotyledonous woods investigated (Fraxinus, Fagus, Quercus peduncu-
lata, Morus, Broussonetia, Rhus, Sophora, Gymnocladus, etc.) the middle portion
of the ring becomes reduced, as the thickness of the whole diminishes, in such a
degree that in extreme cases the ring consists exclusively of spring-wood and the
autumnal limiting layer. This condition appears most sharply in Morus, Rhus, and
the Leguminosae mentioned, where the woody fibres are confined to the middle region
in well-developed rings, but wholly disappear in feeble ones. In this respect the
wood of stem and root shows a general agreement, and as the annual rings, in roots
which have once attained a thickness of two or three inches, are, as a rule, extremely
thin (0-25™“ and less in thickness), it follows at once that there is a considerable
difference in the general structure of the wood of stem and root, which is increased
by further differences in the structure and distribution of the elements, which will be
described below.

As in Coniferous woods the outer portion of the annual ring contains relatively
the largest mass of lignified membranes, and consequently possesses the greatest
density, strength, and hardness, while in Dicotyledonous woods, owing to the structure
and distribution of their elements, this is the case with the middle parts of the ring,
the lesser density and strength of the wood of the root, as compared with that of the
stem, is a necessary consequence of the facts above stated. The stem-wood, however,
as technically made use of, when taken from well-grown trees, for which the excep-
tions to be mentioned below do not hold good, diminishes in the case of the Coniferae
in density and strength as the thickness of the annual rings increases, while in
Dicotyledonous woods the reverse is the case.

These rules, however, undergo a considerable modification in the case of stems,
owing to local changes in the relative breadth of the autumnal wood at different
levels on the stem, as Sanio ® found in some well-grown main axes of Pinus
sylvestris. In these cases the relative breadth of the autumnal wood increases in

^ Compare von Mohl, 1. c. p. 238.

“ Compare von Mohl, Botan. Zeitg. 1862, l.c. — Sanio, ibid. 1863, p. 397.
ä Pringsheim’s Jahrb. IX. p. 115.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

513

every annual ring, independently of its total breadth, from apex to base, so that,
for example, the proportion between autumn- and spring-wood in the same three
outermost annual rings amounts to i : lo ö at a height of 27™, and to i : 2-5 at a
height of I'». The strength and technical usefulness of the wood at different levels
on the stem is thus very unequal. According to the statements of technical
authorities S who value the wood of the upper trunk of Dicotyledons and Conifers
less than that of the lov'er, it may be conjectured that similar conditions to those
found by Sanio in the Pine are of general occurrence in the stems of trees ; yet
these differences in technical usefulness might also have other causes, e. g. differences
in the formation of heart-wood, or the causes might be various in the various cases.
More extended investigations are still required before any general rules or laws can
be laid down.

Sect. 156. The indistinctness of the limits betw’een the annual rings, which in
many woods is typical (comp. p. 503), may also occur in those with a typically
sharp demarcation, as an individual phenomenon, especially where the rings are slightly
developed ; in cases of eccentric ring-formation this often takes place in such a way
that two distinct rings on the stronger side run together into a single one on the
weaker. Such an obliteration of the boundaries of rings has often been observed, in
the case of Dicotyledonous and Coniferous trees, both in the wood of the stem, and
more especially in that of roots Both in the cases of unilateral coalescence of
otherwise distinct rings, and in those where the number of the existing complete
rings is less than the known age of the wood in years, a distinction must be drawn
between the partial or total suppression of the growth in thickness during a period of
vegetation, and the suppression of the demarcation of rings where growth takes place.
Both cases may occur, the former, for example, has been demonstrated in stunted
trees ® ; when a case comes under investigation, it can, as a rule, scarcely be deter-
mined subsequently which of the two phenomena has taken place.

As already indicated, the Araucarias appear, according to the existing data and
controversies to have a special inclination to the individual differences in question
during the occurrence of growth in thickness. Schacht denies the demarcation of the
annual rings in A. brasiliensis, even in the face of Goppert’s reply. Kraus, on the other
hand, describes two pieces of the stem of the same species, one of which had concentric
zones, but not a trace of an annual boundary, while the other had eleven annual rings,
characterised by sharply defined autumnal and spring-wood. Ina portion of the stem of
a well-grown specimen of A. excelsa (cultivated in the open ground) I found sixteen
rings, which were so sharply marked when seen with the naked eye that one is astonished
to hear of its being necessary to seek them with high powers in thin sections. This
is explained by the fact that every ring consists principally of thick-walled, tolerably
uniform tracheides, and only contains a narrow zone of more thin-walled elements
(spring-wood) at the boundary of the next inner ring. These are but little wider than
the thick-walled ones, a sharp boundary between the two being as little observable as a
distinct flattening of the latter. For these reasons the limit between the rings is by no

' Compare Nordlinger, Techn. Eigensch. d. Hölzer, p. 130.

“ Compare T. Hartig, Forstl. Culturpfl. p. 86. — Von Mohl, Botan. Zeitg. 1862, p. 228, &c. — -
Kraus, Bau d. Nadelhölzer, t.c. p. 146. — Nordlinger, Der Holzring, p. 21.

2 Compare R. Hartig, Botan. Zeitg. 1870, p. 527.

^ On these compare Schacht, Goppert, Botan. Zeitg. 1862, and Kraus, t.c.

L 1

514

SECONDARY CHANGES.

means conspicuous under microscopical investigation, while the middle of the thin-walled
zone appears to the naked eye as a sharp line of demarcation. Possibly the same
conditions may exist in the stems of A. brasiliensis, which have been the subject of
controversy. A branch of the same specimen of A. excelsa, about 2®'“ thick, with
narrow rings, shows the same characteristics in some parts, while in others there is a
decided flattening of the tracheides at the autumnal limit.

The question whether the formation of two successive rings in one period of vegeta-
tion occurs as an individual deviation from the rule, even in our woody plants which
typically form a single ring, is essentially foreign to the present anatomical survey. It
must be remarked, however, that this phenomenon is stated to occur as a rare exception,
and in fact as a consequence of the interruption of the summer growth by external
causes (frost, drought, attacks of insects, hailstorms &c.). As regards the anatomical
characteristics of these anomalous double annual rings, no other statements have been
published, than that the boundary between them is usually indistinct ; only in the case
of shoots of Sambucus nigra, interrupted by a hailstorm in very vigorous vegetation in
1846, does Unger mention ‘two distinct woody rings.’

Sect. 157. The existing investigations show that the structure of most woods
remains essentially constant, within the limits defined by the preceding statements.
But exceptions even to this rule occur, the most remarkable of which are afforded
by the Ash (Fraxinus excelsior^). In a well-grown tree of this species, the annual
ring, which is about in breadth, shows on the inside a zone of spring-wood,

consisting of slightly thickened fibres, between which wide vessels surrounded by
bundle-parenchyma are inserted ; then follows externally the thick middle layer,
consisting of thicker-walled fibres, with scattered smaller vessels, likewise surrounded
by bundle-parenchyma ; finally, at the outside, there is the autumnal limiting layer,
consisting of several rows of bundle-parenchyma, with small, very thick-walled
vessels. In very thin annual rings, the reduction of the middle layer takes place as
described above. In the case of very luxuriant young trees, grown on damp soil, with
annual rings over 12™™ thick, V. Mohl found the fibres less thick-walled, and the
vessels, especially the large ones, narrower than in moderately thick rings. Sanio
found a specimen which differed conspicuously from the usual form, in having
concentric zones of parenchyma, containing narrow vessels in the middle layer ; in one
piece only there was an annual ring similar to the usual type. Another stem, which
was stunted, and at an age of fourteen years was only 15111“ thick, showed a strikingly
feeble development, and in the thinnest rings actual suppression, of the charac-
teristic spring layer, in contradistinction to the rule holding good for narrow annual
rings, while the vessels were everywhere remarkably narrow. The mean width of
the largest of the latter in one ring was 0'07iiiiii, that of the large vessels in V. Mohl’s
broad-ringed stems was 0'i7iii‘ii, while in normally grown trees it is about o-aö™“. —
Sparmannia africana, as more minutely described by Sanio, l.c. p, 399, shows a
conspicuously different structure, even in successive transverse portions of the same
stem, or on different sides of the same ring, broad bands of irregular large-celled
parenchyma being sometimes present, sometimes absent.

‘ Unger, Botan. Zeitg. 1847, p. 265. — Nördlinger, Holzring, p. 10.
2 Von Mohl, l.c. p. 269. — Sanio, Botan. Zeitg. 1863, p. 398.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

515

f. Differences of Ihe secondary wood in no7i-equivalent members of the saiiie plant.

Sect. 158. The ligneous body of the stem and its branches has, in the plants in
question, the same structure, within the limits of deviation defined in the preceding
pages. In trees, however, differences in dimensions exist, owing to the fact that not
only the thickness of the annual rings, but also the size of the tissue-elements, is
less in the branches than in the stem. This at least is the case according to the
more accurate investigations before us, which were carried out on Coniferous trees,
and are mentioned at p. 506 h

A far less general agreement prevails between the special structure of the ligneous
body in the stem and its branches on the one hand, and in the roots of the same
plant on the other. Here, on the contrary, there are two different extreme cases ;
plants with the wood of the root completely similar to that of the stem, and others
with the opposite character; between the two extremes there are of course many
intermediate cases.

The first of the two cases occurs in Gymnospermous and Dicotyledonous trees
and shrubs. Although the wood of their roots is never quite similar to that of the
stem in form, structure, and distribution of the tissue-elements, yet the differences
only affect relative dimensions and subordinate variations of structure Differences of
the former class consist, firstly, in a considerable reduction, in the case of the root, of
the average thickness of the entire annual ring, which, though subject to great varia-
tions, may sink to minimal dimensions, in the White Fir, for example, to 0-117'^™,
although, on the other hand, it sometimes reaches 2-3™™ ; in Dicotyledonous woods
it may even be smaller than the average diameter of the vessels present in the annual
ring, in which case the ring must have an undulating outline, widened at the vessels.
Secondly, there are differences in the width and thickness of wall of equivalent
portions of tissue. These affect details in the structure of the walls, and, in the case
of Dicotyledonous woods, the distribution of the non-equivalent forms of tissue in the
annual rings. These relations once more appear most clearly and simply in the
Coniferous woods. According to V. Mohl’s measurements, the tracheides of the
spring-wood in the root of the White Fir are distinguished from those of the stem by
this fact, that their radial diameter is on the average and their tangential diameter
i greater, while their length is also greater; those of the autumn-wood by their
greater radial diameter and wider lumen. To this is often added a diminished
absolute thickness of wall in the tracheides of the root, and, perhaps independently
of this, a greater softness of the wood ; further, the fact that the development of the
thicker-walled autumn wood-cells diminishes with the thickness of the annual rings,
and in very thin ones is almost suppressed. Similar relations, to be compared in
V. IVIohl and Schacht, l.c., reappear in other Abietineae. The differences between
the wood of the root and that of the branches, which Schacht compared with it, are
as regards the width of the tracheides even greater than in the case of the stem-wood,
for reasons stated above. In the wood of the root in Coniferse, the pits on the radial
sides of the tracheides are often arranged in two longitudinal rows, whether the side

‘ Compare also von Mohl, Botan. Zeitg. 1862, p. 461. — Schacht, ibid. p. 409, &c.

^ Von Mohl, Botan. Zeitg. 1862, pp. 225, 269.

L 1 2

SECONDA RF CHANGES.

516

be in contact with one neighbouring tracheide, or with two (as is the case where the
elements form alternate rows), while in the stem the presence of a single row of pits
is the rule. (Comp. p. 494).

The wood of the root in Dicotyledonous woods is generally also distinguished from
that of the stem by its greater ‘porosity’ and softness. On the one hand, this character
is closely related to the changes in the structure of the annual rings which accompany
their decrease in thickness, as described above at p. 512, As in the woods investi-
gated the middle strong part of the rings is reduced in the slightly developed ones to
such an extent that it may even be entirely absent, and these rings thus consist
principally of the wide and relatively thin-walled vessels of the spring-boundary, the
distinction indicated must necessarily result. In themselves the large vessels of
the wood of the root are inferior in average width to those of the stem-wood,
in the case of the Ash and Oak. In the Beech, on the other hand, and in a less
degree in the Birch and Aspen also, the average width of the internal vessels,
even in relatively thick annual rings, is greater than in the stem. This increase in
the relative extent of the total area of the cavities of the vessels is, according to
V. Mohl, the only anatomical cause of the greater porosity of the wood of the root
in the Beech and Aspen. Further, in other cases a more or less considerable
increase in the width of tracheides and cells is found (amounting to in Berberis),
and a corresponding decrease in the average thickness of their walls. Besides
Berberis, this is the case in Fraxinus, Betula, and Quercus.

Sect. 159. The second of the two extreme cases distinguished within the
general plan of structure, namely, an extremely different anatomical composition of the
ivood of the root, as compared with that of the stem and its branches, is of very
general occurrence among herbaceous Dicotyledons, especially perennials and
biennials, the roots of which store up reserve substances such as starch, inulin, &c.,
and apparently large quantities of water. The distinction is no doubt most strikingly
marked in the fleshy tap-roots of cultivated plants, Brassica Rapa and Napus, Daucus,
Raphanus, &c. ; but these are only special cases of a phenomenon which is of very
general occurrence. The most general anatomical character of these roots consists
in the reduction of the specific woody elements as compared with the parenchyma.
This is brought about in different ways : —

(1) By feeble development of the entire ligneous body lying inside the cambium
as compared with —

a. The persistent parenchymatous primary outer cortex, or

b. The relatively very thick, also chiefly parenchymatous, secondary bast.

(2) By the development of a relatively small quantity of specific woody elements,
i. e. vessels and fibres, in the ligneous body, which as a whole is strongly developed.

As examples of ( i ) a. the annual subsidiary roots, approaching 2™™ in thickness, of some
Asclepiadeae are to be mentioned. In the shrubby Asclepias curassavica the wood of
these roots is more than i““ thick, cylindrical, and similar in structure to the wood of
the stem. In the thick roots of the rhizome of A. Cornuti and Vincetoxicum officinale
the greatest diameter of the original diarch xylem-plate is less than 0.3“”“ ; the breadth
of the secondary mass of wood attached to it is only half as much ; all the rest, except the
slight zone of bast, is primary cortical parenchyma. The subsidiary roots of the Piperaceae
further belong to this category, as also do those mentioned at p. 355, which show no
thickening of their vascular bundles, or mere indications of it.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

The far more frequent case mentioned under (i) b., to which we shall have to return
in considering the changes of bast and cortex, occurs, for example, in the root of
Taraxacum, Rubia, and Umbelliferae. A root of Taraxacum which is now before me,
4mm in thickness, has, for example, a ligneous body only about 0-5™'“ in diameter.

Lastly, case (2), which especially belongs to the present subject, occurs in its
most typical form in Brassica and Raphanus. The main mass of the Radish and
the Turnip is formed of the chiefly parenchymatous wood ; the bast and external
cortex are not more than 1-2™"^ thick. Between the typical cases mentioned under
(r)(5. and (2), a number of intermediate forms occur, with but slight difference
between the mass of the wood and that of the bast and external cortex, e.g. roots of
the Umbelliferae, Scorzonera hispanica, Rheum Rhaponticum, &c. But in these
cases, as the relative thickness of the wood increases, the proportion of parenchyma
to the specific woody elements increases in favour of the former, if it is allowable to
enunciate a general rule for cases which show such great variety in detail.

In the roots in question, the tracheae of the wood, in all known cases, are
exclusively vessels, with a wall which is reticulately thickened (and then often with
scalariform transverse meshes), or has bordered pits, the latter structure being not
uncommonly found on the surfaces which abut on other vessels, the formier on those
adjoining non-equivalent tissue. Their average width is considerable ; wider and
much narrower ones frequently occur together. They are always immediately
accompanied by rows of elongated prismatic cells with pointed or horizontal ends,
which may be called fibrous cells in the sense defined above, and the nature of the
contents of which still requires more accurate comparative investigation ; they are
further accompanied by short-celled parenchyma, which in form and position "corre-
sponds to the bundle-parenchyma. Tracheides do not seem to occur, yet this point
also still needs further investigation. Between the ligneous bundles, which are thus
constructed according to the general rule, medullary rays of different orders are
inserted as the wood becomes more strongly developed, in a manner which likewise
corresponds to the general rule. So far as is known these are always parenchy-
matous, and their elements are generally to be distinguished from those of the
bundle-parenchyma by their form and position, and the special nature of their
contents, according to the rules described above; but in the particular cases the
differences may be either very clear or not distinct. Leaving out of consideration
the cases marked (i) where the ligneous body is quite small, the formation of massive
parenchyma is apportioned between the ligneous bundle and the medullary rays in
two principal forms, between which of course there are again many intermediate
cases.

(1) Narrow ligneous bundles are separated or divided up by broad parenchy-
matous medullary rays. They consist principally of vessels and fibrous cells, the
latter being usually narrow, thick-walled, and lignified ; the medullary rays are thick
masses of parenchyma, their cell-walls for the most part thin and not lignified. The
above-mentioned cases, with strongly developed main medullary rays, belong to this
category, as Urtica, Cucurbita, Symphytum officinale, &c. Comp. p. 474, Figs.
203, 204.

(2) In most really fleshy roots the main mass of the parenchyma in the ligneous
body belongs to the ligneous bundle itself. In its most internal portion, bordering

51«

SECONDARY CHANGES.

on the primary xylem-plates, the latter consists of vessels lying somewhat near
together, and only separated by narrow one- or few-layered bands of usually non-
lignified parenchymatous or fibrous cells. If, as is usually the case here, the main
medullary rays are absent, one can scarcely speak of medullary rays at all, they are
only indicated by single radial bands of parenchyma. In cases with a relatively
small wood (Taraxacum, Rubia, &c. (i) 3.) this condition is permanent. Where, on
the other hand, the wood is largely developed (2), as in Rheum, Scorzonera his-
panica, Pastinaca, and the swollen roots of Brassica and Raphanus ^ the formation
of parenchyma in the ligneous bundle increases with the progressive growth in
thickness. The bundle is principally composed of parenchymatous cells with non-
lignified walls, decided longitudinal extension, and radial arrangement ; and in this
massive thin-walled parenchyma of the bundle lie the vessels, which form closely
united groups, or are rarely quite isolated, and are accompanied by narrow, usually
non-lignified fibrous cells. As seen in transverse section they form interrupted radial
rows, increasing in number in the centrifugal direction, and concentric zones which
are also interrupted, while in their longitudinal course they form a net with pointed
meshes. The medullary rays are inserted between the parenchymatous masses of
wood. Their cells are in many cases distinguished from those of the ligneous
bundle by their form, which is usually radially procumbent, and by differences in their
contents. This is the case in Rheum, where the procumbent cells of the numerous
rays, which are only 1-3 cells in breadth and generally only 6-10 cells in height,
are distinguished by containing an abundant yellow colouring matter (Chrysophanic
acid) from the upright parenchymatous cells of the bundle, which chiefly contain
starch ; also in the cultivated root of the Parsnip, &c., where the uniseriate to
triseriate cells of the medullary rays, densely filled with small starch-grains, contrast
sharply with the narrower, elongated cells of the bundle, in which the starch is less
abundant.

On the other hand, many of the cases referred to above, in which there is no
sharp boundary between the parenchyma of the rays and bundles, belong to this
category. As seen in cross-section, the rays may indeed be distinguished in their
middle part by the greater radial elongation of their cells, by their general course,
&c. ; but they pass over quite gradually into the adjacent parenchyma of the bundle.
So, for example, in Scorzonera hispanica, Raphanus, Brassica, and the fleshy swollen
roots of Daucus.

The preceding short statements and examples are only intended to call attention
to the most remarkable structural phenomena in the wood of fleshy roots. Reference
may be made to the descriptions of officinal roots in the pharmacological literature
(Wigand, Flückiger, and Berg), and especially to the representations in Berg’s Atlas,
for illustrations of special characteristics, which are extremely variable in different
species, and of the no less varied intermediate forms between those ligneous masses
which consist chiefly of parenchyma, and those which are, in various degrees, more
woody, i. e. which agree more in structure with the wood of the stem. The variations
in the structure of the wood, which may occur within the same species in different
individuals, sometimes no doubt as an effect of external conditions, and in the same

* Compare Nageli, Beitr. I. p. 2j.

SECONDARV THICKENING. NORMAL DICOTYLEDONS.

519

individual in roots of different order and thickness, are considerably greater in these
cases than any similar phenomena which have been observed in the wood of the
stem. The most conspicuous examples of this are once more afforded by plants
which, in their wild form, have thin roots, but in many cultivated varieties are pro-
vided with fleshy swollen roots, as species of Brassica, Raphanus, Daucus, &c. In
the main root of the wild Daucus Carota, the ratio of the thickness of the wood
to that of the surrounding cortex (bast-layer), expressed according to the radii of the
transverse section, is 5 ; 3. The somewhat firm wood consists in the bundles partly
of narrow fibrous cells, which are at least 8-10 times as long as broad, and pointed
at both ends, and are provided with a moderately thickened membrane with small
pits, and partly of rather wide vessels, arranged in radial bands, the walls of which
show almost exclusively transverse bordered pits. Between the bundles there are
numerous medullary rays, consisting of one or more layers of large, approximately
isodiametric, parenchymatous cells. Further details, which might be mentioned,
especially as regards the innermost portion of the wood, may here be passed over as
non-essential. In the cultivated yellow carrot, the proportion between the radius of
the cross-section of the wood and that of the surrounding, chiefly parenchymatous,
cortex (bast-layer), is approximately as 1:7. The vessels, at least in the great
majority of cases, are reticulated, with transverse meshes, fibrous cells are wholly
absent, and are replaced by wide, thin-walled, parenchymatous cells, which abut on
one another with horizontal surfaces, and are on the average twice as long as they
are broad. Although indications of medullary rays may be recognised, they are not
sharply marked off from the parenchyma of the bundles.

It seems to me remote from the purpose of this book to give a synopsis of all
the woods investigated, which might serve as a key to their identification. Some
assistance towards the latter object may be obtained from the preceding pages.
Reference may further be made to the literature cited, especially that of Pharmacology;
e. g. to Wiesner’s Rohstoffe des Pflanzenreichs, Hartig’s Forstl. Culturpflanzen, and
his treatise, Zur vergl. Anat. der Holzpflanzen, Bot. Ztg. 1859, p. 93, and more
especially to Sanio, lieber die Zusammensetzung des Holzkörper’s, &c., Bot. Ztg.
1863, p. 401. Joseph Möller’s copious ‘Beiträge zur vergleichenden Anatomie des
Holzes,’ Vienna, 1876, could not be made use of for the present work.

III. The BASxb

Sect. 160. The cambial ring of Dicotyledons and Gymnosperms with normal
growth produces on its outer side the secondary layers of bast ; these are added to
the original bast-zone of the stem, which is represented by the phloem-portions of the
vascular bundles. A similar process goes on in connection with the primary phloem-
groups of the root, in the manner described above. The secondary zones are directly
continuous with the original ones, and form, together with the latter, the entire zone
or mass of bast. Its external limit is formed by that of the primary phloem-groups and
of those portions of the medullary rays which lie between them. By means especially

* [Compare Möller, Anatomie der Baumrinden, Berlin, 1882.]

530

SECONDARY CHANGES.

of the former, it is sharply marked off from the non-equivalent tissues of the external
cortex, more especially in those cases most frequent in stems, where the original groups
of phloem are supported or enclosed on the outside by sclerenchyma. Nägeli^ has
called this external limiting zone of the bast-layer the cortical sheath, a term corre-
sponding to that of medullary sheath, used for the internal boundary of the wood.

The original structure of this limiting zone is evident from the descriptions given
in preceding paragraphs ; the structure of the bast as a whole depends partly upon
the former, partly upon that of the secondary increment of growth to be described
here. A further modification, however, results from the fact that the structure of the
bast must undergo a constant change, so long as the volume of the body enclosed by
it is increased by means of the activity of the cambium ; for each zone thus under-
goes, after its first formation, a constantly increasing extension in the direction of
its surface, by which it must be in some way or other affected. So long as growth in
thickness goes on, a constant transition must of necessity take place between the
original conditions and those which have been modified by the peripheral extension,
and in fact the successive stages of change must always come under observation in
every mass of bast which is regarded as a connected whole. This fact must always
be borne in mind. For purposes of description, however, it is necessary to separate
the initial structure from the changes due to peripheral extension. The former will
be immediately considered here, the latter in the next chapter.

The differentiation of the bast (Sect. 135) is similar to that of the wood, in so
far as it consists of principal and partial strands of various degrees, which are
separated from one another, or divided up by the large and small medullary rays
(shortly termed bast-rays). The equivalent rays and strands of wood and bast
correspond, and fit on to one another in the cambial zone. The original form and
size of the medullary rays, and the consequent course of the strands, are the same
as in the adjoining wood.

Sect. 16 r. Among the forms of tissue, sieve-tubes and parenchyma are charac-
teristic, without exception, of the secondary bast of Dicotyledons and Gymnosperms
with normal growth. They are, at least very frequently, accompanied by sacs con-
taining crystals (comp. p. 141) ; further, by sclerenchymatous elements, and especially
by elongated fibrous cells, the bast-fibres ; not uncommonly also by short sclerenchyma,
stone-sclerenchyma (stone-cells) ; while, finally, laticiferous tubes and secretory reservoirs
characterise the bast of certain species or families. As has already been partly
shown by the descriptions of the forms of tissue given in former chapters, especially
Chap. V., all these elements in the bast, with the obvious exception of the scleren-
chyma, possess delicate, non-lignified, soft walls. Nageli has accordingly introduced
the collective term soft bast for all those portions of the bast which are not
sclerenchymatous. In most cases the elements of the soft bast are originally
narrow, and continue to resemble the cells of the cambium, from which they are
derived; they are often difficult to distinguish from the cambium, and from one
another, especially as seen in transverse section. For this reason, but to a much
’greater extent owing to the fact that the softness of the tissues makes it somewhat
difficult to obtain good preparations, the structure of the bast, and its distinction from

* Dickenwachsthum, &c., d. Sapindaceen, p. 13.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

5^1

the cambium, remained for a long time extremely obscure, and the accurate repre-
sentations given by Th. Hartig as early as 1837 failed to be understood, until Mohl,
in 1855^, brought them into deserved honour. For the same reasons later investi-
gations often leave much to be desired, and the special anatomy of the bast is but
insufficiently treated of by most authors.

Sect. 162. The main fundamental mass of the medullary rays always consists
of parenchyma. The form and arrangement of its cells are identical with, or very
similar to those of the adjacent wood. It likewise occurs in the strajids as a constant
constituent, and, like the parenchyma of the wood, it is usually derived from single
or repeated transverse divisions of the tissue-mother-cells, in those portions of the
cambium which correspond to the strands, and its original arrangement agrees

Fig. 210. — Cytisus Laburnum, tangential longitudinal section through the innermost layer of bast of the same branch as
Fig. 198, under the same magnifying power as the latter ; j members of sieve-tubes, i a sieve-plate lying deeper than the
surface of section, m small medullary ray, two cells in height. The remaining elements are cells of the bast-parenchyma,
the origin of which from the transverse division of cambial cells becomes clear on comparison with Fig. 198.

FIG. 211. — Juniperus communis, small stem. Transverse section through the autumnal wood, bast, and cambium, during
the winter’s rest (end of September) ; h — h external rows of the autumnal wood, b, b series of bast-fibres. At x there is only
one cambial cell between h and b ; m — m medullary rays.

with this mode of development (Fig. 210, comp, also Fig. 198, p. 465); more
rarely it arises without transverse division, from longitudinal division only of the
tissue-mother-cells, and then corresponds to the intermediate cells of the wood.

The Sieve-lubes (Chap. V.) are constant, specific constituents of the strands in
the normal soft bast of Dicotyledons. They are always accompanied by parenchyma,
and in most woody plants are in general so arranged that the sieve-tubes form
single, biseriate, or pluriseriate, tangential rows, which may be interrupted by
parenchyma, and alternate with tangential rows of the same. The original radial

Compare note on p. 172.

523

SECONDARy CHANGES.

arrangement of the secondary elements often continues to be maintained here, or
is at least recognisable ; hence, in every radial row derived from the cambium, one
or more sieve-tubes always alternate more or less regularly with parenchyma.

This arrangement appears with quite diagrammatic regularity in the bast of the
Cupressinese and many Taxineaeh The transverse section of the bast (Fig. 21 1)
here shows regular rows, both in the radial and tangential direction. Every fourth
tangential row consists of fibres ; of the three which lie between two fibrous rows,
the middle one is parenchymatous, while the outer and inner each form an inter-
rupted layer of sieve-tubes. The parenchymatous cells are approximately similar in
width to the sieve-tubes (Juniperus communis), dr are wider (e.g. Thuja occidentalis).
In the stem of species of Pinus (P. Strobus, nigricans, silvestris), and also of Abies
pectinata, irregular tangential rows of wide parenchymatous cells alternate with

Fig. 212.— Tilia argentea. Transverse section through the inner bast, cambium, and autumnal boundary of the
wood of a branch seven years old (cut in November) (220). The wood is drawn without the details of structure of the
membranes; h external limit of the autumnal wood, which is sharply limited by the dark outlines of its tangentially
flattened elements ; c~c cambium and zone of young secondary growth ; m — w small medullary rays ; in the one to the
right are three fibres (y) ; k crystal-containing sacs, with crystals, some of which were broken by the razor. In the
bast-strand, between the two rays, three fibrous bands {/) are drawn ; aiternating with them is the soft bast, consisting
of sieve-tubes j, of granularly dotted (cambiform) cells, containing abundant starch and protoplasm, and of other,
somewhat wider elements, bordering on the fibres, distinguished by clear watery contents and pitted walls.

multiseriate zones of radially arranged sieve-tubes ^ In the old root, but not in the
stem of the White Fir, I often find, between two radial rows of sieve-tubes, radial
bands of parenchyma, which are uniseriate and resemble the medullary rays, but do
not lie in the same straight line with the medullary rays of the wood.

* Hartig, Forstl. Culturpfl. p. 95, Taf. 9, 10. — Von MoM, l.c. p. 891. — Graf zu Solms-Laubach,
Botan. Zeitg, 1871.

^ Von Mohl, /. c. — Hartig, l.c. pp. 13, 35, Taf. 5.

SECONDA RV THICKENING. NORMAL DICOTYLEDONS. 523

In ihe bast of Dicotyledonous woody plants, the arrangement of the two kinds
of tissue, so far as can be decided from the existing data, is less regular than in the
Coniferous woods first mentioned, owing to the fact that the tangential series of the
one form of tissue are sometimes single, sometimes double or multiple, and are
interrupted by interpolated elements of the other form; while the average width
of the adjoining tissue-elements of the two kinds not uncommonly shows considerable
differences, which are usually in favour of
the sieve-tubes, e. g. Tilia and Vitis ; more
rarely in favour of the parenchymatous
cells. The narrower parenchymatous cells
which accompany the sieve-tubes here show
the same arrangement and the same
characteristics as the cambifonn cells de-
scribed at p. 324, in the case of the
primary vascular bundles ; they should
therefore be designated by the same
name. They appear in transverse section
as narrow, three or four-cornered meshes
contiguous with the tubes, sometimes on
one side of them, sometimes on more
than one, but never (.?) on all sides, each
side, however, never having more than one
(Fig. 212).

Traced longitudinally (Fig. 213) they
form, in most cases at any rate, series,
each member of which is many times
shorter than the adjoining member of the
sieve-tube, and is derived from the trans-
verse division of the tissue mother-cell. In
Tilia I rarely found them equal in length
to the members of sieve-tubes. The fre-
quency of the narrow cambifonn cells
seems to be very unequal in the different
particular cases ; for example, I find few
of them in Pyrus and Spirsea ulmifolia,
while they are numerous in Tilia. More
accurate details are to be expected from
further investigations.

In spite of the irregularities described,
the radial and tangential seriation of the elements continues to be maintained in
many cases in its principal outlines, and each tangential row contains for the most
part either sieve-tubes or parenchyma, as is especially 'shown by longitudinal
sections which follow its course. The most various plants, e.g. besides those
mentioned, Populus, Salix, Punica, Ficus, Sambucus, Fagus (Mohl), ^sculus
and Ribes, show the greatest agreement in this respect, although great differences
occur between them in the average size of the elements and the special form of

Fig. 213.— Vitis vinifera; bast of a branch several years old.
I cm. in thickness, in summer (beginningr of July). Tangential
section (145). s, s sieve-tubes, the inclined scalariform terminal
surfaces, and one which is horizontal, being cut through longi-
tudinally, with the exception of one at the upper edge, which is
seen obliquely in superficial view, w, nt medullary rays ; on the
boundary between these and the strand of sieve-tubes are sacs
containing crystals.

524

SECONDA RF CHANGES,

the sieve-tubes (comp. p. 173). It may here not be superfluous to add, that, so
far as my experience extends, the same statement also applies essentially to the
common officinal barks, and that it is only the dried condition in which they
usually come under observation which has hitherto, to some extent, impeded the
clear recognition of the actual conditions.

The elements of the soft bast in woody plants of the families Apocynese (Nerium
Oleander), Asclepiadese (Asclepias curassavica), Convolvulacese (Convolvulus Cne-
orum), and Campanulaceae (Campanula Vidalii), are less regularly arranged than in
the cases hitherto described, and this is no doubt also the case in the Cichoriacese,
and in the plants mentioned at p. 324 (2), which are characterised by very small sieve-
tubes in the primary bundles. Between relatively wide parenchymatous cells, which in
some degree maintain the serial arrangement, groups of narrow elements are found
in these instances, which, as seen in cross-section, show very various, triangular, or
polygonal forms, and have apparently arisen from repeated longitudinal divisions
taking place in every direction in the original mother-cells of the tissue. The narrow
elements which are thus grouped are the sieve-tubes and cambiform cells ; in the
Cichoriacese, Campanula, and Lobelia, the laticiferous tubes also occur in connection
with the groups of narrow elements.

The arrangement of the elements of the bundle in the soft-bast, as just described,
prevails both in the stems and roots of woody plants, so far as the existing investi-
gations extend.

There have been few minute investigations on the stems of herbaceous plants,
but, according to the existing observations, they are not essentially different from
woody plants with reference to the conditions here in question.

Those roots which consist chiefly of parenchyma, with a very thick secondary bast,
as described above at p. 516, show in the latter a distribution of the sieve-tubes
which is similar to that of the vessels in the massive parenchymatous wood of the
root. A strand of bast corresponds to every ligneous strand, and the former contains
relatively small groups of narrow sieve-tubes, accompanied by narrow elongated
cells, and enclosed in a mass of large-celled parenchyma.

The more special distribution, relative amount, and form of the tissues under con-
sideration is very various, according to the particular case, and often even in closely
related plants. In most cases the bundles, as seen in cross-section, form relatively
narrow radial bands, lying in the same straight line with the ligneous strands, between
rays of wide-celled parenchyma, which essentially, though not always quite exactly, form
the continuation of those of the wood. The strands consist of narrow, elongated cells,
which may even be pointed like a spindle (the latter, e.g. in the wild form of Daucus
Carota), and of sieve-tubes lying between them, which are always scantily developed, and
are likewise narrow. The individual radial bands, like the ligneous strands, are either
continuous, or are more or less interrupted by interpolated large-celled parenchyma.
Examples of this arrangement are afforded by many roots of Umbelliferae, Scorzonera
hispanica, Cichorium, Argemone, &c.

In the roots of Rhubarb (Rheum undulatum and Rhaponticum) essentially the same
distribution reappears, but is modified by the forms and relative numbers of the
histological elements under consideration. The uniseriate, procumbent, parenchymatous
rays of the wood are continued without interruption through the zone of bast. The
strands separated by them consist, as regards much the greater portion of their mass,
of large, upright, parenchymatous cells, filled with starch, and between these lie the

SECONDARY THICKENING. NORMAL DICOTYLEDONS. 525

narrow sieve-tubes, which are very isolated, and therefore easily overlooked, and are
accompanied by narrow, elongated cells.

Finally, an arrangement deviating further from the usual rule is represented by the
bast of the roots of Taraxacum. Narrow, concentric, annular zones containing the
tubes, here alternate regularly with similarly arranged, broad zones of large-celled
parenchyma, which are on the average about sixteen layers of cells in thickness. The
zones containing the tubes consist of narrow cells, numerous milk-tubes, and scanty
sieve-tubes ; the large-celled zones consist of thin-walled cells, which are placed in very
regular radial and vertical rows, corresponding to the original cambial form and
arrangement. At numerous points the radial series of these elements pass through the
zones containing the tubes, so as to interrupt them, without however consisting of
special ray-parenchyma distinct in form from that of the annular zone. The roots of
Chelidonium and Papaver are intermediate in structure between those of Taraxacum,
and of the first category.

Sect. 163. In plants which have laticiferous tubes (p. 186), the non-articulated
ones may be absent from the secondary bast ; at least I did not find them in it in
Vinca, Asclepias curassavica, and the Euphorbias; in most cases they are present, and
as regards the articulated tubes especially, this is always the case so far as investigation
extends; they are then characteristic companions or representatives of the sieve-
tubes. The large non-articulated tubes in Ficus, Madura, and Morus follow singly
the lines of sieve-tubes. The articulated, usually reticulate tubes form groups
together with the sieve-tubes, which anastomose, both within each individual
strand of bast, and with those of neighbouring strands, by means of connecting
branches. In the bast of the Papayacese the net of milk-tubes has a relatively
scanty development, at least in comparison to its complexity in the wood. In the
other plants belonging to this category the milk-tubes are always relatively very
numerous, as is especially conspicuous in the shrubby stems (Sonchus pinnatus and
Campanula Vidalii), and in the roots of ’Cichoriaceae, Campanulaceae, and Pa-
paveracese : as their number increases the sieve-tubes become proportionately
reduced. In the strands of bast of the roots of Cichoriaceae (Lactuca virosa.
Taraxacum) only scanty, narrow sieve-tubes are present, as has already been
mentioned above ; in the secondary bast of the root of Platycodon grandiflorus, I
did not find the latter at all, though I do not wish to assert that they are entirely
absent. This mutual representation appears in the most striking manner in the bast of
the roots of Papaveracese : Papaver Rhoeas and Argemone mexicana have only very
isolated sieve-tubes side by side with the highly developed net of laticiferous tubes ;
in Chelidonium majus the former are more numerous, although the milk-tubes pre-
dominate ; Glaucium luteum has no milk-tubes, but, on the other hand, has large
groups of sieve-tubes.

Sect. 164. The occurrence of protogenetic secretory passages in the soft-bast
has already been to some extent noticed in anticipation in Chapter XIII. They only
occur in those plants and parts of plants which also possess them in the primary
tissues, and by no means in all even of those. Their position appears to be always
within the bundles, not in the medullary rays.

Among the Dicotyledonous families in question their occurrence in the
secondary bast of stem and root in the Terebinthaceae, Burseracese, and Clusiaceae
has already been mentioned above. So far as is known they are present in the

526

SECONDARY CHANGES.

same regions in all Umbelliferse, and in the Araliaceae ; where the bast is strongly
developed they form, as seen in transverse section, interrupted radial and concentric
series, differing in arrangement according to the species h

In the secondary bast of the stem and branches of Pittosporum Tobira, the
passages appear relatively late ; Van Tieghem found four concentric rows in a
branch io“>» thick. They were not found in the secondary bast of the root of
this plant.

Among the Compositae which contain passages, some also have them in the
secondary bast, e. g. Helianthus and Centaurea atropurpurea ; in Inula Helenium
the bast of the root contains wide passages, which are closed blindly, so far as is
known, at both ends, and coated by a delicate epithelium ; they resemble those in
the secondary wood. Other Compositae have no passages in the region mentioned,
but, on the other hand, have scattered sacs, filled with secretions, in the parenchyma
of the rays, e. g. Echinops and Tagetes patula (comp. p. 203).

The Coniferae, which have such great numbers of protogenetic resin-passages
in their other tissues, do not form them in the secondary bast, excepf in a few
cases. The e.xceptions include, firstly, the blind ends of the horizontal passages in the
medullary rays of the Abietinese mentioned on p. 490, which extend into the zone of
bast. According to Van Tieghem, longitudinal passages occur in the secondary bast
of Araucaria Cookii and brasiliensis, and of Widdringtonia cupressoides, which were
mentioned on p. 443. The spaces filled with resin which occur in other Coniferse
are subsequent, hysterogenetic products of disorganisation, which will be discussed
in Sect. 173.

Secretory sacs, other than those containing crystals, appear in the soft bast in the
plants mentioned in Sects. 33-35, and are sometimes scattered without any perceptible
regularity, while they sometimes have a definite arrangement, also stated in the
paragraphs mentioned.

Sect. 165. The sclerenchymatous fibres of the bast, bast-fibres, or according to
the older terminology ‘ bast-cells ’ in the strictest sense, have the form and structure
generally described in Chap. II. With reference to the latter, it may further be
mentioned that the lamella which forms their boundary, whether towards elements of
the soft-bast, or towards one another, is in these cases especially often an unlignified
membrane of cellulose, which surrounds the more or less lignified, thick membrane of
the fibre as a distinct sheath I Comp. Figs. 211, 212.

The bast-fibres are entirely absent in the bast- of many plants ; both in its
secondary portion, and at the outer boundary of the primary phloem. This is the
case in the stems and branches of Ribes, Viburnum Lantana®, Pittosporum Tobira,
undulatifolium, Citriobatus multiflorus, Porlieria, Centradenia grandifolia, and Ber-
beris vulgaris, and in the roots of many herbaceous Dicotyledons. Thus they do
not universally form an essential constituent of the bast. In the cases where they
are present, which certainly form the majority, they occur —

^ Compare the figures of roots of Umbelliferse in Wigand, Pharmacognosie, and Berg, Atlas,
Taf. 8, 9, 14, 22.

“ See Graf z. Solms-Lambach, Bot. Zeitg. iSyr, p. 516, &c.

^ Hanstein, Baumrinde, p. 17.

SECONDARY THICKENING. NORMAL DICOTYLEDONS. 527

(1) Only at the outer limit of the primary bundles, surrounding their phloem-
portions (comp. p. 422), and not in the products of secondary growth. This is the case
in the stem and branches of Fagus, Betula, Ainus, Platanus, Viscum, Menispermum,
Viburnum Opulus, Convolvulus Cneorum, Nerium, Cornus, Punica, Camellia japonica,
Drimys Winteri, Ephedra distachya, Abietineae &c.

(2) Both at the outer limit above-mentioned, and also in the interior of the
secondary bast. This latter condition is no doubt the most frequent, especially
among woody plants. With reference to the relative amount and distribution of the
fibres, it presents very various modifications.

In the medullary rays fibres are rarely present, e. g. isolated ones in Tilia,
comp. Fig. 212. The principal forms of their distribution in the bundles are the
following : —

{a) Concentric layers or rows of fibres alternate regularly with similar layers of
soft bast. The layers of both kinds in neighbouring bundles fit approximately,
though not always quite exactly, one on another, so that they form annular zones,
extending round the whole stem, and interrupted by the medullary rays.

This phenomenon occurs with especial regularity, as already mentioned at
p. 522, in the Cupressinese and many Taxineae, where every fourth secondary
tangential row of cells becomes a uniseriate zone of fibres, which separates two
triseriate zones of soft bast from one another. Comp. Fig. 211.

Among the Dicotyledons no such strict regularity exists. The fibrous layers
always consist, on the average, of two or more tangential rows, and the number of
these rows changes in the same individual, both in the successive annular zones and
within each individual portion of the bundle ; as follows from these facts, the thick-
ness of the zones of soft bast is also unequal. The conditions in question are also
extremely various, according to the species. In the case of many species, however,
a regular alternation of concentric zones of fibres and soft bast, of definite average
breadth, takes place within the limits of deviation indicated, the original radial and
tangential seriation of the fibres of each portion of the bundle being sometimes
maintained, as in Vitis, Spiraea ulmifolia, Pterocarya caucasica, and species of Acer,
though in most cases their position as seen in transverse section becomes irregular,
owing to the displacements due to longitudinal extension (p. 470) ; Tilia, Cheirostemon,
Sparmannia, Malvaceae, Medinilla, species of Salix, Ladenbergia globosa^ Vas-
concella monoica, Guaiacum, and Clematis Vitalba ; comp. Figs. 212 and 214.

(3) Concentric zones of fibres, alternating with soft bast, may still be dis-
tinguished generally, and in places are even regularly arranged ; on the whole,
however, they are irregular, as they are both interrupted in each bundle by elements
of the soft bast, and are also unequal in number and dissimilar in arrangement in
neighbouring bundles. This condition,- with numerous variations according to
species and individuals, and in the average number and breadth of the successive
zones, is characteristic of the bast of very many woody Dicotyledonous plants.

* Compare Hartig. Forstl. Culturpfl. pp. 13. 212, 326, &c. — Hanstein, Lc. p. 21.— Schacht,
Der Baum, p. 381. — Von Mohl, l.c. p. 891.

“ Berg, Atlas, Taf. 29.

528

SECONDARV CHANGES.

e. g. Quercus, Corylus, Carpinus, Pyrüs, Juglans regia, Sambucus nigra ^ Daphne
Mezereum, Rhamnus Frangula, Simaruba officinalis'*, Ulmus ^ Glycine sinensis,
Quillaja, Olea europsea, and Populus pyramidalis. In these cases the fibres of each
group are seldom radially arranged ; their arrangement as seen in cross-section is
usually irregular.

{c) The bast of numerous other Dicotyledons contains fibres scattered through-
out the soft bast, singly or in small groups, as seen in cross-section ; traced

Fig. 214. — Sparmannia Africana ; branch, transverse section (80). Below h — h is wood ; above h — h is first
the cambial zone, then higher up and towards the outside is the layer of bast, the outer limit of which lies at b\
tn larger medullary rays; the single radial rows marked X X are the smaller ones. Alternating with the
medullary rays are narrow strands of bast, consisting of alternate groups of fibres and of soft bast with narrow
cavities. On the external boundary of the bast are sacs or cells with stellate crystals, e remains of the epidennis ;
p periderm; s remains of a sac containing mucilage, after the mucilage has been washed out.

longitudinally they are also isolated,- or form narrow strands, anastomosing with
others at an acute angle ; sometimes they are distributed over the tranverse
section in large numbers, as in the external zone of bast of Ladenbergia magnifolia,
and in the bast of most Cinchonas'*, also in Ficus elastica, Morus, and Celtis®;

^ Von Mold, l.c. p. 879. ^ Berg, Atlas, Taf. 28, &c.

^ Harlig, l.c. p. 466. * Compare Berg, Atlas, Taf. 29-35.

^ Hartig, Forstl. Culturpfl, p. 450.

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

529

sometimes they occur in relatively very small quantities, as in the inner zone of bast
of the Ladenbergia last-mentioned, and in the officinal barks of Cinnamomum,
Croton EluteriaS Larix europsea and Mahonia Aquifolium.

As the fibres are included in the general serial arrangement of the bast, it follows
that the more closely they stand, the more distinct in this case also are the inter-
rupted, radial, and concentric zones which they form, as e. g. in Cinchona macrocalyx,
figured by Berg, l.c., Taf. 35, and very beautifully in Laurus Sassafras. In general,
numerous intermediate cases occur between the forms of distribution enumerated
here and under b, as was to be expected beforehand. In this respect the cortices of
Cinchonaceae present an instructive series of gradations. Ulmus also deserves to be
again mentioned here.

The appearance of shorl sclerenchymalous elements {stone-elements) in the bast will
be considered in the next chapter, in order to avoid repetitions.

Sect. 166. Sacs containing crystals are often a characteristic, sometimes even a
predominant, constituent of the secondary bast ; their occurrence however is as far
from being universal as is that of the fibres. They lie both in the medullary rays
and in the bundles, in the latter usually forming longitudinal rows with short
articulations ; each of these rows is derived from a single mother-cell, and may often
be isolated as a connected series; they were termed septate sacs at p 139 (comp.
Fig. 213). Guaiacum and Quillaia have already been mentioned above as forming
at least partial exceptions to this mode of arrangement ; it may be left an open
question how far similar, i. e. short, isolated sacs, may further occur elsewhere in the
bundles, or not ; no minute investigations on this point have been published, and
the statements as to the distribution of crystals, which are chiefly based on trans-.
verse sections, do not admit of any decision on the question. The form of the
crystals is either klinorrhombic, or that of clusters, raphides, or granules ; one or
more definite forms are characteristic of each particular case (comp. p. 142, and
Sanio, /. cl).

With reference to the presence or absence of the crystal- containing sacs, and
their distribution in the former case, the following phenomena have been observed,
which however, at least so far as my own observations are concerned, still require
re-investigation.

I indicate those forms of the crystals, which have been chiefly observed in transverse
sections, in parentheses (K = klinorrhombic, C = clusters, R = raphides).

1. Crystals are absent from the secondary bast: Drimys Winter!, Fraxinus, Syringa,
Jasminum fruticans, Mahonia Aquifolium (?), Laurus Sassafras, Cinnamomum aromaticum
(Cinnamon Cassia), Clematis Vitalba, Atragene, Aristolochia Sipho (?), Camellia japonica,
Sorbus Aria, and, according to Hartig (Forstl. Culturpfl.), also Cornus. As regards the
sacs containing the crystals, the Cupressineae, Taxineae, and other Coniferae, and
Ephedra, are also to be included in this series, as in these the Calcium oxalate is not
deposited in the interior of sacs or cells, but is intercalated in the membranes. In
most of the plants just mentioned I find that crystals are also absent from the primary
cortex.

2. Crystals are contained in the secondary bast (and then usually or always also
in the primary bast, and in the external cortex). In these cases they occur —

‘ Berg, l.c. 36, 37.

M m

Hartig, Forstl. Culturpfl. p. 13.

530

SECONDARY CHANGES.

{a) Both in the medullary rays and in the bundles : Nerium Oleander (K), Simaruba
officinalis (K), Canella (C), Platanus, Cinnamomum zeylanicum and its allies (small
raphides, chiefly in the medullary rays), Juglans regia (G, Sanio), Acer platanoides (K),
Sparmannia Africana (C), Carpinus Betulus, and Corylus Avellana (K, C, Sanio).

[b) In the bundles, exclusively, or to much the greater extent : species of Salix (C, K),
Pyrus communis (K), Punica (C), Ribes (C), Guaiacum (K), Galipea officinalis (R,
K), Madura aurantiaca (K), Ulmus (K), Quillaia (K), .äisculus (K), Rhamnus Frangula
(C), Quercus pedunculata (K, C), Betula verrucosa, Alnus glutinosa (K, C, and
granules, Sanio), and Porlieria hygrometrica (K).

(c) Exclusively, or to much the greater extent, in the medullary rays, and when the
latter are of considerable breadth, most abundantly at their boundary towards the
bundles: Vitis (K, R), Tilia (K, C), Cheirostemon (C), Olea europsea (very small R),
Ficus elastica (K), Croton Eluteria (G), Pistacia Lentiscus (C), Prunus Padus (C, K),
P. avium (G), Kerria japonica (C), Berberis vulgaris (scanty K according to Sanio),
Lonicera tatarica (Sanio), and Sambucus nigra (granules, Sanio).

The crystal-containing sacs, especially those which are septate and contain
klinorrhombic crystals, appear in many cases in company with the bast-fibres, as
pointed out by Schacht^; e. g. species of Acer, Pomaceae, Ulmus, Quercus, Salix,
&c. ^ Clusters often occur exclusively and in large quantities where fibres are
absent, e. g. Punica and Ribes ; though the entire absence of the tissue last-
mentioned may also occur in connection with the entire absence of crystals, e. g.
Drimys Winteri.

No constant relation however exists between the presence or absence of the two
forms of tissue mentioned, or between any definite form of tissue and of the crystals,
as is evident from the facts stated. Thus klinorrhombic crystals are especially often
'present in abundance where fibres are absent in the secondary bast, e. g. Porlieria and
Nerium; while, on the other hand, the klinorrhombic companions of the fibrous bundles
are absent in Juglans regia and many other cases. And further, where the fibrous
bundles are accompanied by crystals, the occurrence of the same or another form of
crystal in the soft bast is by no means excluded.

In the soft bast the rows of sacs containing crystals have in most cases an
irregular, scattered position in the transverse section. On the other hand, they are
often arranged in concentric zones, which alternate regularly with zones destitute of
crystals. This is the case in Punica Granatum, where the whole bast appears
regularly striated in transverse section, owing to the fact that uniseriate zones, con-
sisting almost exclusively of crystal-sacs, alternate with zones each consisting of a
few rows of cells, which are destitute of crystals ; these zones are interrupted by
numerous uniseriate medullary rays, which are also destitute of crystals (comp.
Fig. 215, and the beautiful figure in Berg, Atlas, Taf. 40, which however is not quite
correct in the details). In species of Ribes also, the bundles, which are separated by
broad medullary rays without crystals, consist of multiseriate zones of soft bast, like-
wise destitute of crystals, which alternate regularly with usually uniseriate uninter-
rupted zones, containing clustered crystals

Sect. 167. As regards the changes of structure corresponding to the successive

‘ Der Baum, i Aufl. pp. 228 and 238. ^ Compare Sanio, l.c.

^ Compare Hanstein, Baumrinde, Figs. 15-17-

SECONDARY THICKENING. NORMAL DICOTYLEDONS.

531

zones of secondary growth, to the non-equivalent members, and to individual dif-
ferences, there is far less to be said in the case of the bast than in that of the wood,
partly, no doubt, owing to the really greater simplicity of the phenomena, partly
because it is better to pass over a number of the known changes and to discuss them
in the next chapter, and lastly, in no small degree, because, for reasons already stated
above, there is still an absence of any very minute or extended investigations on the
subject.

In woods, the ligneous elements and cambial cells of which increase succes-
sively for a time in size (p. 505), the same is generally the case in the bast, as would
be expected beforehand, on account of the enlargement of the cambial cells. This is
evident on observation in the case of numerous Coni-
ferae and Dicotyledonous woods, e. g. Tilia, Fagus, and
Nerium, also Vitis and Cobaea. It is obvious that
only those internal zones must here be taken into con-
sideration, which have not as yet undergone any sub-
sequent dilatation. No very accurate investigations
have been published on the degree and the persistency
of the general increase in size, nor on the possibly un-
equal participation of the particular forms of tissue in
these changes.

Between stem and branches differences in the size
of the elements appear to exist similar to those in the
wood, but these also have not yet been more closely
investigated. In the roots of trees and shrubs, so far
as the investigations extend, the special structure of
the bast is very similar to that in the stem of the
same plant, e. g. Vitis, Sambucus, Tilia, and Punica.

Wherein the differences, which no doubt occur, con-
sist, cannot be stated at present. It has been repeatedly
mentioned above (pp. 516 and 524) that a different
relation exists between the foliage stems of herbaceous plants, and the roots which
belong to them, especially when the latter are fleshy.

The thickness of the secondary zone of bast added in a definite space of time
is very variable, both in its relation to the simultaneous growth of the wood, and
according to absolute measure, whether in different species and individuals, or in
non-equivalent members of the same plant. In both relations the extreme cases are
represented on the one hand by the fleshy roots, consisting chiefly of bast (p. 516),
and on the other by the woody stems of trees and shrubs.

Especially in trees with a persistent cortex, which is not thrown off by the
formation of bark (Sect. 177), as, for example, the Fir and Beech, the difference of
thickness between wood and bast is, as is well known, very considerable, and the
absolute thickness of the latter small. In the common Beech with smooth bark
(Fagus silvatica) the entire bast-zone in a stem 100 years old is, according to Hartig
scarcely more than thick. Woody plants which periodically throw off their

Fig. 215. — Punica Granatum (220) ; trans*
verse section through the inner part of the
bast of a branch six years old. c side towards
the cambium ; m, vt two medullary rays ;
between the latter is a strand of bast ; s, r
sieve-tubes. It is uncertain which of the
other cavities belong to elements of the latter
kind. Between the empty cavities of the
elements of the soft bast are transverse zones
of crystal-containing sacs ; the clusters con-
tained in them are only indicated by shading.

‘ Forstl. Culturpfl. p. 212.

Mm2

533

SECONDARY CHANGES.

cortex by the formation of bark show a more vigorous production of bast, corre-
sponding to the activity of this process. This phenomenon appears as an instructive
individual variation in those stems of Fagus silvatica occasionally occurring which
are called Stone-beeches, and are conspicuous from their thick, furrowed bark. The
new production of bast no doubt takes place most abundantly in those stems, which,
like the vine, annually renew their entire bast-zone, and throw off that of the previous
year.

It is remote from the purpose of the present description to enter into the causal
relations of these phenomena.

Generally valid anatomical characters of the boundary between successive zones,
corresponding to the annual rings of the wood, may perhaps still be discovered
in the case of the bast, but cannot be determined from the existing data. Even
in those cases where regularly alternating concentric zones of non-equivalent tissue
appear in the bast, especially fibrous zones alternating with soft bast, their number
varies according to the year, the age, and the individual, and a determination of the
annual boundaries is therefore usually uncertain. As an example, Hartig’s^ statement
may be cited, that in the case of Willows and Poplars with a smooth cortex, the
number of zones of bast-fibres is smaller than the age in years, amounting only to
3-4 for every 10-15 years, while, during the development of thick bark-forming
cortices, 2^ fibrous zones arise annually. The species of Acec^ form in the first
years either one, or (at the base of the annual layer) two, successive fibrous zones, but
even from the sixth year onwards the relation changes in such a manner that often
only 20-25 fibrous zones correspond to 100 years. The numbers are more regular
in Tilia, where, according to Hartig®, four fibrous zones appear at the base and one
at the apex of the shoot in its first year, to which two or three are added in the second
year, and in each succeeding year, on the average, two ; this is also the case in Pyrus
communis, which, according to Mohl ^ forms one fibrous zone annnally.

In those woody plants also, which renew their cortex every year, and in which
the limit of the annual increment of growth is sharply defined by the layer of periderm
formed at its outer side (Sect. 177), similar difiFerences to those just mentioned appear.
Lonicera Caprifolium and its allies annually form one zone of fibres and one of soft
bast ; Clematis Vitalba usually two of each ® ; Vitis vinifera generally forms two
fibrous zones alternating with soft bast, at the close of the first period of vegetation,
while in later years 3-5 are generally formed annually®.

* /. c. p. 444.
^ /. c. p. 560.

“ /. C. p. 547-

^ Botan. Zeitg. 1855, p. 880.

5 Hanstein, Baumrinde, pp. 72, 77.

® Hanstein, l.c, p. 61. — Von Mohl, l,c, p. 879.

CHAPTER XV.

SECONDARY CHANGES OUTSIDE THE
ZONE OF THICKENING.

Sect. i68. In the normal Dicotyledons and Gymnosperms the mass of wood
and bast developed from the cambium is bounded on the one hand by the pith, and
the pre-existing ligneous zones adjacent to it, on the other by the external cortex
and the pre-existing bast. It is clear, a priori, that this surrounding tissue may, and
to some extent must, undergo changes, in consequence of the cambiogenetic
secondary growth.

As stated above, no change in the pith is necessarily involved in the actual
course of the processes of secondary growth described, or as a consequence of
them. As a matter of fact, however, it has been asserted, especially by Duhamel \
that in trees and shrubs the medullary cylinder diminishes in thickness, and
may at last wholly disappear as the secondary growth of the wood proceeds. As
regards the majority of cases, this view has been given up, as depending upon
imperfect observation,' and in fact the only demonstrable anatomical change in
the pith during the phenomena of secondary growth is, that it sooner or later,
rapidly or slowly, dies off and dries up. ‘ The possibility of a change in the
pith caused directly by the growth in thickness is not indeed excluded a priori. For
if wood and bast increase in thickness and circumference, and the external cortex
does not yield to this enlargement of the circumference in a corresponding degree,
an increasing pressure will be exercised on the pith, which may lead to anatomical
changes in the latter. In what cases and in what form such changes may possibly
take place, are questions which have not been investigated, and to the solution of
which there is scarcely any safe clue ; the possibilities will not be discussed here.
That such cases occur is however shown by the change of form in the pith of the
internodes of Aristolochia Sipho, which accompanies the growth in thickness. The
young internode, up to the age of one year, has an approximately circular transverse
section. The pith, which is of the same form, or is broadly elliptical, as seen in
transverse section, consists, like the medullary rays, of permanently delicate and soft-
walled cells, and is surrounded by a circle of 11-13 leaf-trace bundles ^ without
intermediate strands. The external cortex enclosing the latter contains a strong

‘ Physique des Arbres, I. p. 37; detailed discussion in De Candolle, Organographie, I. p. 168.
’ Compare Nageli, Beiträge, p. 82, Taf. VIII.

534

SECONDARY CHANGES.

ring of sclerenchyma (p. 419), and is covered by the very tough epidermis, which is
for a time persistent. The vascular bundles take an unequal share in the cambio-
genetic growth in thickness, which makes a vigorous start in the next year. The
median bundles of the two next higher leaves, occupying two diametrically opposite
segments of the circle, and their next neighbours, grow less strongly in thickness
than those situated in the two other segments ; the increase is greatest in the three
bundles which occupy the middle of each of the latter segments ; it chiefly affects
the xylem. During this unequal growth in thickness no perceptible change at first
occurs in the form of the cross-section of the whole internode, and even at a later
period the change is but trifling. On the other hand, those ligneous bundles, which
grow more strongly than the others, press with their inner edges against the pith, the
cells of which become compressed in the direction of the corresponding radii of the
transverse section, and the general form of the pith is changed in such a manner
that its transverse diameter, lying in the direction of the greatest growth in thickness,
constantly becomes smaller, while the diameter at right angles to the latter remains
unchanged. In an internode five or six years old the pith is merely a narrow band,
the shorter diameter of which scarcely amounts to (original) longer one.

The cause of these phenomena manifestly lies in the fact that the cortex undergoes
too small an extension for the volume of the growing wood, and by its resistance
presses the latter against the pith, and compresses it. Allied species of Aristolochia
behave in a completely similar way.

The disorganisation of the pith in the shrubby species of Astragalus, which
yield gum-tragacanth, may further be mentioned here, though not actually standing
in direct causal connection with the anatomical processes accompanying growth in
thickness h It consists in the conversion of the cellulose-membranes into a mucilage
which is capable of swelling up greatly; the change extends from the pith to the
medullary rays, and chiefly affects the cells of the middle of the latter, and of the
pith, while those which border on the ligneous strands are changed in a less degree,
or not at all. In the living plant the mucilage is already present in a highly swollen
condition, and is kept in high tension by the pressure of the surrounding resistent
tissue ^ On injury to these tissues it flows out, and when a plant is left to itself it
may spontaneously burst the surrounding tissue, and exude from the cracks in the
form of the strings which on drying form the tragacanth of commerce. In many
species, e. g. A. rhodosemius, the mucilaginous disorganisation begins early, even
immediately below the apex of the stem ® ; in others it appears to come on at a later
period.

As regards the gradual dying off of the pith in old woody stems, the disappear-
ance of the cell-contents, and especially of the store of starch, similar rules hold
good to those for the formation of heart-wood. Comp. pp. 403 and 510.

Leaving out of consideration the displacements obviously occurring in the cases
of Aristolochia and Astragalus, no anatomical changes in the wood, directly caused
by the growth in thickness, are known. On the influence of the cortical pressure on
the formation of autumn-wood, which is indirectly connected with the above, comp.

’ Von Mohl, Botan. Zeitg. 1857, p. 33. ^ Fliickiger and Hanbury, Pharmacographia, p. 153.

^ Graf zu Solms-Laubach, Botan. Zeitg. 1874, p. 69.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 535

the remarks and citations upon p. 500 ; on the processes of degradation in wood
when ageing, the relation of which to these phenomena is, to say the least, very
doubtful, see Sect. 154. The primary medullary bundles of the forms here in
question — Piperaceae, Begoniae, Aralise and Umbelliferae, Mamillariae, and Melasto-
maceae (comp. Sect. 62) — undergo, so far as is known, no secondary anatomical
changes.

Sect. 169. All the parts lying outside the active cambial zone, namely, the
entire primary cortex and the secondary cortex for the time being, necessarily undergo
progressive changes with the progressive growth in thickness. These consist in —

(a) Growth of existing tissue-elements, new formation of equivalent ones from
them, and subsequent metamorphosis (p. 5): Sects. 170, 171.

{b) Compression, displacement, and destruction of existing tissues : Sects. 172,

173-

(c) New formation of non-equivalent forms of tissue out of existing ones :
Periderm, Sects. 174-179.

The process indicated by {a) only affects the cellular tissues : Epidermis and
Parenchyma.

Sect. 170. In the majority of the cases of vigorous growth in thickness, the
epidermis is destroyed at an early period, cork or bark being formed ; further details
will be given below. Stems, however, are not wanting, whether with weak or with
very vigorous growth in thickness, in which the epidermis follows the latter by its
own growth during a considerable period. This is the case in many herbaceous
plants, and in woody plants with a smooth, green, cortical surface, as long as the
latter is present. The condition of the surface mentioned is in fact dependent on
the persistency of the epidermis, the cells of which, being filled with sap, allow the
colour of the sub-epidermal chlorophyll to show through. As examples of stems and
branches with vigorous secondary growth of the wood, which, for some years at least,
retain the epidermis, may be mentioned: Viscum alburn^, species of Ilex, the ever-
green Jasmines, Menispermum canadense, Aristolochia Sipho and allies, Sophora
japonica, Negundo, and many others. In Acer striatum the epidermis is still for the
most part present in a living condition, and following the growth, even on stems a
foot thick, and forty years or more old.

The long-lived epidermis of the woody plants mentioned is provided from the
first with thick strongly cuticularised outer walls, which sometimes contain and
excrete a large amount of wax (comp. pp. 76, 82). Its original structure under-
goes relatively unimportant changes during growth. These consist in an increased
thickening of the cuticularised external walls, the surface of which covered by the
cuticle usually remains smooth ; but in Acer striatum as well as in Negundo and So-
phora Japonica it becomes cracked as the thickening proceeds, the cracks penetrating
from outside into the external cuticular layers, which do not follow the growth, and
successively breaking them up into crumbling fragments. In Acer striatum the
cracks coincide with the dilated bands of the external cortex, to be described below ;
and a new excretion of wax rods takes place in each case on the surfaces newly
laid bare by the cracks ; it is on this that the white striation of the cortical surface

' Von Mohl, Botan. Zeitg. 1859, p. 593.

SECONDA RV CHANGES.

53Ö

depends’. In addition to these changes, the growing epidermal cells, as they
become larger in the direction of the circumference, i. e. broader, divide successively
by walls which stand at right angles .to their transverse diameter, and to the surface,
and abut on the inner surface of the original wall. This successive multiplication
of the epidermal cells takes place in such a degree, that the average breadth and
general form of the individual cells remain approximately the same, or only undergo
inconsiderable changes. The epidermal cells of the stem of Acer striatum when
the latter is 2oo“m thick, are, for example, scarcely twice as broad as those of a
shoot one year old, and 5™“ in diarneter.

Sect. 171. Parenchyma forms the main mass of the primary external cortex,
the medullary rays of various degrees in the bast-layer, and the parenchymatous
groups in the bundles of the latter. Until a zone of cortex is thrown off by
the formation of bark, which may take place sooner or later, but in many instances
never occurs at all, and which will be discussed below, the parenchyma follows the
cambiogenetic secondary growth by corresponding growth of its own, in all the
parts in which it exists. The parenchymatous mantle of the external cortex
increases successively in width, while the medullary rays of the bast, and the
parenchymatous elements of the bundles, increase in breadth in the centrifugal
direction (Fig. 214, p. 528). The several portions of the bast do not always
participate in the same degree in this phenomenon, which may shortly be termed
Dilatatmi of the parenchyma. If attention be directed to cases of extreme
difference, it is found that in the one case dilatation of the entire parenchyma
of the bast takes place in an approximately uniform proportion, as each annular
zone becomes shifted outwards. In all the radial bands, and thus most clearly
in the medullary rays of every degree, the parenchymatous cells increase uniformly,
and quite gradually in breadth, in the centrifugal direction. The intermediate non-
equivalent tissues, which do not grow with them, especially sieve-tubes and bast-fibres,
thus become uniformly removed one from another, and the more so the further
they are from the cambium ; as in Salix fragilis and allies, Punica, Rhamnus Fran-
gula^ Spiraea ulmifolia, Pyrus communis, and .^sculus. In the other extreme case
the dilatation is unequal in the various radial bands of the transverse section ;
it amounts to little or nothing in the bundles, and is most active, either in all
the parenchymatous rays, or in some of them. Between the lateral limits of these
dilated rays the arrangement, and lateral distance from one another of all elements of
the tissues, remains approximately the same. This behaviour occurs, firstly, in a
number of stems, which are constructed on the type described at p. 455, and the
large medullary rays of which are broad and multiseriate ; e. g. stems of Meni-
spermum, Aristolochia, and Piperacese ; here the dilatation is brought about, at least
to the greatest extent, by the large medullary rays, and, indeed, the latter all take an
approximately uniform share in the process, in their entire height. The strands of
bast therefore remain similar in form and arrangement to the phloem portions of
the original vascular bundles, from the further development of which they have
arisen. They are not, however, wholly without share in the dilatation, as a slight

' For details compare Botan. Zeitg. 1S71, p. 605, &c.
® Compare Berg, Atlas, Taf, 39, 40.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 537

widening of their parenchymatous elements, and through them of the whole bundle,
takes place here also. This category includes Tilia, and other woody plants provided
with a similarly grouped bast. In j’oung shoots of Tilia the zone of wood and
bast is traversed by numerous large parenchymatous rays extending to the pith,
most of which are uniseriate, but many, e. g. in transverse sections of a branch
of Tilia parviflora now before me, the seventh, ninth, or fourth, and so on, are bi- or
triseriate ; the latter is at any rate the case at the cambial limit, and sometimes as
far as the pith, while in other cases they become uniseriate before reaching the latter.
The dilatation begins in the biseriate and triseriate rays ; whatever strands and
rays lie between them, take, in the first instance, no part in the process. As growth
in thickness proceeds, a constantly increasing number of the original uniseriate rays
take part in the dilatation. Later on successive small secondary rays are also in-
volved in it. When a ray takes part in the dilatation it is, as a rule, bi- or pluriseriate
at the cambial boundary. The result of these phenomena is in the first instance the
severance of the bast-layer into the often described^ groups of bundles with a
wedge-like form widened towards the cambium, as seen in cross-section, and rays,
alternating with the former, rvidened in the opposite direction, and further, the
successively occurring subdivision of the first groups of bundles into more numerous
narrower ones, separated by rays. The number of groups in a transverse section,
rises, for example, in branches of T. argentea now before me, from 45, in an
internode ö“*“! thick, to 138 in one 28™™ thick. In order to complete this
description, which has been given in the first instance with reference to the transverse
section, it must be added that the medullary rays are of considerable height — the
larger ones measuring more than a hundred cells — and their ends situated in quite
different transverse sections all round the stem ; and that the dilatation begins
in every ray about at the middle of its height, and thence advances upwards
and downwards.

Quite similar phenomena to those in the bast of the Lime reappear in many
other plants, and in members of different value, e. g. in the stem and branches of
Hibiscus syriacus, Pterocarya, and Galipea officinalis, and in the highly parenchyma-
tous roots of Umbelliferse (Archangelica, Levisticum, &c.), Glycyrrhiza, and many
others ^

Intermediate cases between the extremes, represented on the one hand by Salix
fragilis. Spiraea ulmifolia, and Punica, and on the other by Tilia and Menispermum,
would naturally be expected to occur from what has been stated. In these every
transverse section shows radial bands of the bast, which are more or less strongly
dilated, in various gradations. Examples are afforded by Sparmannia africana,
represented in Fig. 214, p. 528, with numerous strongly dilated rays, and more slightly
dilated ones in the divisions limited by them ; further, in very various gradations,
by the Quinine barks, Croton Eluteria, Simaruba officinalis, Cinnamomum zeyla-
nicum, &c.^

In the structure of the dilated masses of parenchyma the increase in the

’ Compare e. g. Schacht, Der Baum, p. 198; Lehrb. II. p. 50. — Haustein, Baumrinde, Taf. I.
^ Compare Berg, Atlas, Taf. 37. 6, 8, 9.

® Berg, /. c. Tab. 29-38,

538

SECONDARY CHANGES.

tangential diameter, or breadth, of all cells taking part in the dilatation, is most
conspicuous, and follows obviously from what has been stated. In proportion,
however, to the increase in breadth, successive radial bipartitions take place, by
means of which the original breadth of the cells is approximately restored, and
the number of the cells in each tangential row increased in a corresponding degree.
These phenomena also take place in the endodermis of the stems, mentioned at
pp. I2I and 415, and of the roots, so long as it is not thrown off and thus excluded
from the growth. An increase in the average breadth of the individual cell no doubt
takes place, judging from estimates. It appears to rise rapidly to an approximately
constant value, and then to maintain this during the succeeding divisions, so that
cells of the same layer in a stem a foot thick are no broader than in one as thick as
one’s finger, though they are of course more numerous in a corresponding degree.
The final, constant, average dimensions are relatively little in excess of those
existing originally at the beginning of growth in thickness ; they may be estimated
to amount to scarcely more than two or three times the latter. Accurate measure-
ments are still to be undertaken. Cell-division in directions other than the radial,
resulting in a multiplication of the concentric layers of parenchyma, is rare, at least
in the cortex of those woody plants which have been principally investigated, and
apart from the formation of periderms : those special cases in which it does occur
will be discussed below (Sect. 172); it remains to be investigated whether or not
such division also takes place in many fleshy roots.

The structure of the cell-walls and of the contents, the periodic variation in the
amount of starch in the latter, &c., remain the same in the principal mass of the
parenchyma during the dilatational changes, in certain cases throughout life, in
others for a time. Sooner or later, however, changes may occur, and in fact
{a) dilatational changes of the collenchymatous hypodermal layers (p. 404), and
{b) processes of secondary sclerosis.

The collenchymaious hypodervial layers of the cortex of stem and branches in
woody plants always follow the dilatation uniformly for a time in their whole
circumference, M'hile they maintain their original characteristic structure ; this process
often continues uniformly, so long as they are not thrown off by the formation
of bark ; the question whether, in many cases, their walls decrease somewhat in
thickness, as the extension proceeds, remains to be more accurately investigated.
In some cases, which might no doubt be multiplied by further observations, namely,
in Tilia, Acer striatum, and -®sculus, other conditions prevail. At certain points the
collenchymatous cells show a considerably greater growth in breadth than in the
intermediate tracts, and all their walls, both those originally existing and the portions
added by subsequent growth, decrease considerably in thickness. They permanently
assume the appearance of thin cellulose-walls, and are thus sharply distinguished
from the neighbouring thick, brilliant, collenchymatous walls. With this is con-
nected, at least in the case of Acer striatum, a diminution in the amount of
chlorophyll, which is apparent to the naked eye. The process begins in a few small
portions of each transverse section, spreads laterally from these points, and involves
new regions lying between the first. The thick-walled collenchyma in this way
becomes subdivided into constantly smaller portions, lying isolated among the altered
tissue, and at last disappears, as these portions also become involved in the process

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 539

of transformation. Until this occurs the external cortex appears traversed by the
altered bands, which I have already termed elsewhere ‘ dilatational bands h’
Their occurrence indicates an unequal participation of alternate bands of the
cortex in the dilatation, and this will find its explanation in dissimilar mechanical
conditions, which are still to be more minutely studied. In Tilia the first dilatational
bands of the external cortex correspond exactly to the most dilated medullary rays ;
in Acer striatum they often coincide with the intermediate spaces between the outer-
most bundles of bast-fibres, which are filled with parenchyma, but they do not
occupy this position exactly or constantly.

In the peripheral layers corresponding to the original collenchyma the con-
nection of the cells remains close and approximately unbroken, if the local in-
terruptions due to the lenticels, to be described below, be left out of consideration.
In the inner, lacunar, portion of the external cortex the cavities originally existing
grow in the direction of the dilatation. Further, and often extensive, interruptions of
continuity may be produced both in the region indicated, and also, though less
frequently, in the more deeply situated layers of the bast, as the effect of the tension,
distortion, and pressure which the tissues undergo during growth in thickness.
In proportion to the originally lacunar structure of the external cortex, it therefore
becomes traversed to an increasing extent, as dilatation proceeds, by broad, crevice-
shaped cavities, and is often split up into irregular concentric lamellre, e. g. species of
Prunus and Pyrus, ^sculus, &c. A similar subdivision into lamellae is shown, for
example, by the bast-zone in Berberis and Mahonia. In addition to this, radial
cracks appear in the bast in several cases, especially in the species of Prunus ; they
are situated along the lateral boundaries of the medullary rays, and increase in width
towards the outside, because the cells of the rays, while usually remaining in con-
nection among themselves, grow in breadth to a much smaller extent than the rest
of the cortex, and a severance therefore ensues in the lateral limiting surfaces
between rays and bundles.

Secondary Sclerosis is the name given to the phenomenon that individual cells,
or definite groups of cells of the parenchyma, assume the sclerenchymatous character
after the differentiation of tissues is complete, often thickening their walls to an
immense extent during lignification, at the expense of the internal cavity. During
this process they sometimes approximately maintain their original form (short
sclerenchyma, stone-sclerenchyma), while they sometimes undergo considerable
changes of shape and size (many-armed stone-sclerenchyma).

These phenomena occur principally in very persistent portions of the cortex
of woody plants, in some to a very large extent, in others to a less extent or not at
all. The actual structure and the strength of the older cortices are in a high degree
influenced by this process. Conspicuous as these structures are, and often as they
have been described, yet the history of their origin is still much in want of more
accurate investigation.

The limiting zone between the external cortex and the bast-layer is, in the
first instance, the special region of these stony formations, and is also usually first
in point of time. In numerous woody plants, which in this zone form sclerenchymatous

^ Botan. Zeitg. 1871, p. 605.

540

SECONDARY CHANGES.

fibres — the bast-fibres corresponding to the primary vascular bundles — a one- or
many-layered zone of parenchyma, connecting the former into a closed ring,
becomes converted into stone-sclerenchyma. A ring thus arises, composed partly
of fibrous bundles or scattered fibres, and partly of short sclerenchyma, which
traverses the border-zone indicated, and is either completely closed at an early
period (e. g. Cinnamomum zeylanicum and Fagus), or remains locally interrupted
by thin-walled parenchyma for a considerable time, even for years (e. g. Betula
alba). Examples of this occurrence of the compound sclerenchymatous ring are
afforded, besides the plants mentioned, by Quercus pedunculata, Q. Suber, Carpinus,
Corylus, Fraxinus excelsior, Juglans regia, Gymnocladus canadensis, Koelreuteria,
Negundo, Laurus nobilis, species of Cinnamomum, and many others. In Fraxinus
excelsior a compound sclerenchymatous ring, similar in its composition to the first,
may arise at a later period in the secondary bast.

In Fagus silvatica, Quercus Suber, &c., the first sclerenchymatous ring soon
acquires projections directed towards the larger medullary rays. In the one-year-old
shoot of the tree first mentioned, it shows, opposite each large medullary ray, a
protrusion of the adjoining portion of the ring, which here consists of stone-elements
uniting the fibrous bundles. The protrusion projects inwards like a ridge up to the
neighbourhood of the boundary of the cambium. As growth in thickness goes on, these
projecting ridges of sclerenchyma are not only persistent, but increase in the radial
direction in such a manner, that opposite each medullary ray they penetrate into the
wood beyond the cambial boundary, running between the strands of wood and bast.
The cambial boundary is therefore deeply and sharply indented at the medullary
rays indicated. The elements of the cortical rays produced by the cambium in this
indentation immediately become sclerenchymatous, so that a sclerenchymatous ridge
is fitted into the depression. In the medullary rays successively developed at a later
period the same process takes place, with the sole difference that their sclerenchy-
matous ridges do not extend so far as the external ring. If the cortex of the older
stems be torn off from the wood at the boundary of the cambium, the hard ridges of
the medullary rays stand out on its inner surface like little combs.

Similar sclerenchymatous ridges, projecting into the medullary rays, occur without
original connection with the compound sclerenchymatous ring, in Platanus and
Casuarina. They represent the second case of the formation of stone-sclerenchyma
in a definite region.

In the third case, finally, cells take part in the sclerosis, which may be scattered
in all parts of the external cortex and secondary bast. They sometimes occur
isolated in tissue which remains soft, e. g. in the outer portion of the bast of Punica
Granatum, when six years old ; gigantic scattered sclerenchymatous elements (p. 145)
in the bast of the root of the same tree ; ‘ stone-cells ’ in the external cortex of the
Cinchonse, Simaruba, &c. In other cases they form larger groups, nests, annular
segments, &c., inserted in the soft tissue, the number and size of wFich may increase
in the older parts of the cortex to such an extent that they form its principal mass,
so that the old cortex has been appropriately termed ‘stone-bark’ by Hartig.
Casuarina, Platanus, species of Quercus, Betula, Fraxinus, and Acer, the White Fir,
also .Esculus Hippocastanum, and especially Fagus silvatica, may be mentioned as
typical examples of this structure.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 54 1

The phenomena of sclerosis now described appear in woody plants, to some
extent in immediate continuity with the primary differentiation of tissues, and
coincide with the first beginning of secondary growth in thickness ; these cases,
especially the formation of sclerenchymatous rings at the limit of the external cortex,
could not, if they stood alone, be considered under the head of secondary formations.
Other phenomena, however, are so intimately connected with the former, that they
could scarcely be separated in the description, and these affect tissue-elements
which have often belonged for years to a definite differentiated tissue, namely,
the parenchyma, and only subsequently become involved in the sclerosis. The
first sclerenchymatous ring of the Beech, though it originates in the first period of
vegetation, increases every year in mass as it becomes shifted outwards during the
growth in thickness, owing to the fact that both those parenchymatous cells which
border on it externally, and others which are interpolated between its elements, in a
manner to be explained below, successively undergo sclerosis. The ridges of the
medullary rays arising from it constantly become broader, owing to the same
process, the further they are shifted outwards. According to Mohl and Sanio',
the compound sclerenchymatous ring of Quercus Suber behaves in the same way.
In the secondary bast of the Beech, the White Fir, and other trees, sclerotic elements
are absent for at least 1-2 years. As each zone becomes shifted outwards, these
elements then appear in it in increasing numbers. The external cortex of the Beech,
the Horse-chestnut, and the White Fir is, in the first year, and often no doubt
during several years, free from sclerenchymatous elements, while in later years it is
abundantly permeated by them. Similar conditions prevail in the external cortex
and bast-layer of Drimys Winter!, and many other plants.

So far as the existing data admit of a decision, the elements which are affected
by the secondary sclerosis always belong to the parenchyma, as was stated above ;
they may have discharged the functions of parenchymatous cells for years. Their
form and size appears in many cases not to be essentially changed on the occur-
rence of sclerosis (e. g. cortex of Fagus); in others a considerable alteration of
growth and form appears simultaneously with the commencement of this process.
This is most conspicuously the case with the many-armed, ramified, sclerenchyma-
tous elements of Abies pectinata, the arms of which are closely pectinated, forming
as it were a felt; so far as is known, they originally proceed from polyhedral or
prismatic parenchymatous cells, both of the external cortex and of the secondary
bast.

Sect. 172. Sieve-tubes, milk-tubes, sclerenchymatous elements, and sacs con-
taining crystals or other secretions, are, from their differentiation onwards, incapable
of further growth. They behave passively on dilatation, and become displaced from
their original position by the latter process. The longitudinal secretory canals also
share in the displacement, the tissue immediately surrounding them usually becoming
widened through the dilatation. In so far as the process of dilatation acts alone, the
displacement consists in a progressive lateral separation of the elements mentioned,
or of the strands into which they are united, and of the canals. If, as is often the
case, resistance is offered to the dilatation through the elasticity of the superficial

1 Von Mohl, Verm. Sehr. p. 220. — Sanio, Pringsheim’s Jahrb. II. p. 73-

543

SECONDARY CHANGES.

cortical layers for the time being, e. g. epidermis, external cortex and periderm, the
simple dilatational displacements are accompanied by others, the special forms of
which, varying greatly according to the special cases, need not be more minutely
described.

The resistent, sclerenchymatous elements, and also the crystal-sacs, do not
undergo any essential alteration of their structure during this process. Those
provided with soft walls, especially the sieve-tubes, and also many long secretory
sacs, suffer changes, simultaneously with the displacement, which, in general, consist
in the disappearance of the contents and collapse of the walls, and may be shortly
termed Obliteration. As this takes place under the joint action of the pressure
exercised in the radial and tangential directions, proceeding from the dilatation and
the resistance of the surface, it is obviously suggested to find in this the cause of the
obliteration. It is questionable, however, whether a change in the obliterating organs,
and especially in their contents, independent of the pressure, is not the primary cause
of the phenomenon, and the pressure only a co-operating cause.

The obliterated sieve-tubes appear laterally compressed, even to the disappear-
ance of their lumen. Their structure, including that of the ends of their members
which bear the sieves, becomes indistinct, and may even be wholly unrecognisable ;
their walls appear slightly swollen, but no measurements have been made which
actually prove any swelling. Where the tubes are isolated, they are easily over-
looked after their collapse, seeming, at the first glance, to have quite disappeared.
Where they are united to form larger groups, their membranes appear collectively in
sections, especially transverse sections, as a homogeneous, gelatinous mass (like dry
gristle or horn), in which the compressed cavities are visible as narrow crooked
cracks or marks, the original lateral limits as indistinct lines. Similar appearances
have already been described on p. 325, and represented in Fig. 158, p. 336, in the
case of the primitive sieve-tubes of the vascular bundles. As the phenomenon
described often extends with apparent uniformity over the entire transverse section
of a considerable group of sieve-tubes, the question, how far the cambiform cells
which originally accompany the tubes, also share in the obliteration, remains to be
investigated.

Obliterated groups of sieve-tubes have been described by Wigand^ as ‘horn-
bast,’ while their origin and significance have been clearly represented by Rau-
wenhoff ^

The obliteration of the sieve-tubes begins in the oldest external zones of the
cortex, and advances, with the dilatation, in the centripetal direction. It seems to
come on more or less gradually or suddenly according to the particular case, a point
on which more minute investigations still remain to be undertaken.

The obliteration of the secretory sacs has been described by Vogl® in the case
of the large sacs of the Cinchona-bark. These apparently lose their original contents

* Pringsheim’s Jahrb. III. p. 118.

^ Nederlandsch Kriudk.-Archief, V. p. 23. Compare also the treatise by the same author, Sur
les caracteres et la formation du liege, See., reprinted in the Ann. Sei. Nat. 4 ser. tom. XV., and in
other places.

^ Die Chinarinden d. Wiener Grosshandels, p. 12.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 543

and collapse, and the cavity thus formed is filled up by luxuriant growth of the
surrounding, dividing parenchymatous cells, until the wall of the sac may even
disappear.

It has already been stated that those elements which are passive during dilata-
tion, remain in their original close connection one with another during the process of
separation, if they are united into strands. If, on the other hand, as is in fact often
the case with the sclerenchyma, they form closed annular layers, the latter are burst.
The severance of connection then takes place through the limiting surfaces of the
elements. As soon as this begins at any point of the external surface of a scleren-
chymatous ring, neighbouring parenchymatous cells bulge out and insert themselves
into the gap, which they fill up. They then either maintain the properties of
parenchymatous cells, growing and dividing in proportion to the increasing width of
the gap owing to progressive growth in thickness, or they undergo the sclerenchy-
matous metamorphosis immediately upon their insertion ; the original ring becomes
completed by interpolated short stone-elements, as described above.

The latter process occurs in the stony or compound sclerenchymatous rings,
which, e. g. in the Beech, always remain closed without any growth of the existing
sclerotic elements, while their circumference constantly increases. The former case
may occur in the same rings, and takes place especially in rings of fibrous scleren-
chyma, where the latter are not thrown off at an early period by internal formation of
periderm, as is usually the case ; e. g. the bulky fibrous ring of Aristolochia Sipho,
thick foliage-stems of Gypsophila altissima, and, in a lesser degree, in old thick stems
of the Gourd. In the above-mentioned Aristolochia especially, the ring is first burst at
the places lying opposite the bands of greatest dilatation, and then at a constantly
increasing number of other points, and is thus broken up into segments, which are
always becoming smaller, and finally often consist only of single separated fibres,
the space between them being filled up by thin-walled parenchyma, which follows the
dilatation. In this case also, however, a partial completion of the ring by short
stone-sclerenchyma may take place, owing to secondary sclerosis.

Sect. 173. The generally distributed phenomena of displacement and obliteration
may be accompanied in special cases by processes of disorganisation, and the latter
may extend to tissues of every kind. In the cortex of the Amygdaleae, e.g. Prunus avium,
groups of tissue of varying extent become disorganised, and converted into cavities
filled with gum and bassorin, from which the swelling contents, consisting of cherry-
gum, finally exude through the bursting surface of the cortex. According to Wigand’s
statement', it is chiefly the obliterated sieve-tubes in the cortex from which this disor-
ganisation starts, and then further extends throughout the non-equivalent tissues. On
the other hand, the latter also, especially groups of thick-walled parenchymatous cells
which have been called abnormal, are certainly starting-points of the gummy dis-
organisation. On the very various special phenomena, which may, for the most part,
belong to the province of Pathology, comp. Wigand’s description.

In the older cortex of many Coniferse, besides the protogenetic reservoirs of resin
(p. 441), and to some extent as a substitute for the latter when they have been lost
owing to formation of bark, reservoirs filled with balsam appear, which were termed

' /. c. p. 130.

544

SECONDARV CHANGES.

by Mohl ^ resin-cavities ; their origin is no doubt always lysigenetic, resulting from
a disorganisation of definite groups of tissue. Among the Abietineae investigated by
hlohl their formation is wholly absent in several species, namely, Pinus sylvestris and
nigricans, Abies excelsa and pectinata ; they appear in Larix europaea, Abies sibirica,
and Pinus Strobus ; in the first-mentioned tree in the first year, in the two others not
before the eighth or tenth year of life, in the cases investigated. The seat of their
occurrence is especially, and in Larix and Abies sibirica exclusively, the external
cortex, in P. Strobus the bast also. As years go on they increase in number, and
the pre-existing ones in size; the transverse diameter is, for example, stated by
Mohl, in the case of Larix europaea, at little more than ^ young cavity one

year old, and at almost i*““ in a cavity eighteen years old. According to Mohl, the
form of these cavities in the trees mentioned is at first approximately spherical, and
afterwards passes over into a lenticular, transversely elongated form. Their origin
and enlargement by the solution of definite groups of tissue has not indeed been
described in detail, but scarcely admits of doubt, especially after Wigand’s state-
ment according to which they arise in the bast of Pinus Strobus from the solution
of groups of tissue, which contain obliterated sieve-tubes, parenchyma, and stone-
sclerenchyma.

The mode of origin stated by Wigand to occur in Pinus Strobus, doubtless
applies to the cavities filled with resin, which appear in the form of longitudinal
canals, ending, so far as can be asserted, blindly, in the older bast of the Cupressineae
(Juniperus communis. Thuja, Biota, and Cupressus spec., comp. p. 443). Their
formation begins here at points in the older zones of bast which cannot be more
exactly indicated, while the latter are still turgescent, and still contain non-obliterated
sieve-tubes ; resin first makes its appearance in isolated, otherwise unaltered, parenchy-
matous cells, both in the bundles and in the medullary rays ; the resiniferous cavity
then arises by means of an increase in the amount of resin, and successive solution
of the membranes, which perhaps themselves afford material for the formation of
resin ; this cavity becomes widened by the extension of the same process within a
constantly increasing circuit, and over all the surrounding tissue-elements of the
bast, sieve-tubes and fibres not excepted. In the parenchymatous cells bordering
the passage, a considerable enlargement in the radial direction occurs in the case of
Juniperus, with papillose protrusions towards the passage, while isolated divisions by
tangential walls are also frequent.

Pei'iderm

Sect. 174. In addition to the changes in the growing cortical mantle already
described, more profound ones ensue on the new formation of the phellogenetic,
i. e. cork-producing meristem, and its products. It may be expedient to include all

’ Botan. Zeitg. 1859, p. 333. “ l.c. p. 166. ^ Frank, Beitr. p. 122.

* Von Mohl, Unters, üb. d. Entwickelung des Korkes und der Borke auf der Rinde der bau-
martigen Dicotylen ; Diss. 1836. — Verm. Schriften, p. 212. — Hanstein, Unters, üb. d. Bau u. d. Entw.
d. Baumrinde; Berlin, 1853. — Sanio, Unters, üb, d. Bau u. d, Entw. d. Korkes, Pringsheim’s Jahrb.
II. p. 39. -

SECONDARV CHANGES OUTSIDE THE ZONE OF THICKENING. 545

the latter and their meristem with them under one general name, and to choose
Mohl’s old term Periderm, with a modification of its original meaning, comp. p. 114.

The peridermal structures always arise in a layer of cells which has already
been differentiated. This is their initial layer, and is either formed by the single-layered
epidermis, or by a single sub-epidermal layer of parenchymatous cells, which may
occur at various depths, and runs approximately parallel to the surface of the part.
The periderm consists of the phellogenetic meristem, and the tissues that have been
derived from it, which, in all cases, include a single or many-layered stratum of
Cork-cells or Cork-tissue, to which usually, but not always, phellogenic or peridermal
parenchyma, the Phelloderm of Sanio, is added.

If a layer of cork is formed in the interior of a mass of tissue, the tissue lying
outside it dries up, and is eventually thrown off as bark (Rhytidoma, Mohl). The
formation of bark is the immediate consequence of the internal formation of periderm,
and the name is as a rule employed for the dried-up tissues and the adjacent peri-
dermal layers conjointly; with the formation of periderm that of the lenticels stands in
the closest connection, but the consideration of the latter, on account of certain
peculiarities, will be omitted here, and deferred to Sect. 179.

The general course of the formation of periderm has to a great extent already
been stated in Sect. 24. The initial layer of cells becomes converted by tangential
divisions into a multiseriate zone, the elements of which partly remain meristematic,
and partly pass over into tissue. So far, and so long as the former is the case, they
have the capacity of following the dilatational growth by means of their increase in
size, and successive radial divisions occur in them, as in the cells of the dilated
parenchyma, by means of which the original average breadth is always again
approximately restored. Accordingly, all cells belonging to the periderm are ar-
ranged in radial rows, each of which originally corresponds to one of the initial cells,
and may become successively doubled, and they further form concentric (tangential)
layers, see Fig. 216,

In the course of the tangential divisions in an initial cell and the radial row
derived from it, two extreme forms may in the first instance be distinguished, which,
after Sanio, are termed the centripetal and centrifugal forms ; besides these, there is
the mode of succession which proceeds alternately in the centrifugal and centripetal
directions, and may be generally termed reciprocal.

In the centripetal succession the initial cell is divided into two daughter- cells,
the outer becomes a tissue-cell, the inner remains meristematic, and continues the
same process in such a manner that, in the further successive bipartitions, the inner
cell always remains meristematic, while the outer becomes tissue.

In the second case the order is reversed. Of the products of the successive
tangential divisions the outer cell always remains meristematic, while the inner becomes
an element of the tissue.

In the reciprocal succession (Figs. 216-218) the division begins in one of the
orders mentioned, then changes over into the other, and may then again change back
to the first. At the first origin of the phellogenetic layers Sanio found the following
cases of reciprocal succession : —

I. The first two divisions in centripetal order, then the innermost cell becomes
a tissue-cell, the second from the inside becoming a meristematic cell ; after the third

N n

546

SECONDARY CHANGES.

division they go on in the centripetal order, comp. Figs. 216, 217 (‘centripetal-inter-
mediate order ’).

2. Division begins centrifugally, and then changes over to the centripetal order. If
the latter takes place immediately on the third division, in such a manner that the
second cell from the inside then becomes the meristematic cell, which henceforth
divides centripetally, Sanio calls the process centrifugal-intermediate ; if the reversal
only takes place after later divisions, Sanio calls the order centrifugal-reciprocal.

In the later stages of growth of those phellogenetic meristems which are active
for a long time, the reversal of the mainly centripetal order into the centrifugal, and
its immediate return to the centripetal, take place in most cases from time to time
without strict regularity.

Fig. 217. Fig. 218.

Figs. 216— 218.— Transverse section through the surface of a branch of Sorbus Aucuparia. After Sanio. Origin of
the periderm. — Fig, 216. Four epidermal cells, already divided once tangentially ; in the two pairs to the right the lower
cell is once more divided into a and b. a meristematic cell, b phelloderm. — Fig. 217. Further stage of development ;
a — a meristem, b — b phelloderm. The cork-cells corresponding to the outer halves of the epidermal cells have acquired
strongly thickened walls. — Fig. 218. Still further developed stage ; a — a, b — b, as in the former figure. Outside a— a are
three layers of cork-cells, the two outer ones with thickened walls ; at n, n the inner layer of the wall is partially released
from the limiting lamella. To the right, at a, the radial division of a peridermal row is beginning.

Whatever the course of the divisions may be, it is nearly always only one cell of
each radial row, and consequently only one layer of cells in the entire peridermal zone,
which remains in the meristematic condition, so as to carry on the divisions ; all other
cells directly become elements of the tissue, after they have originated by division in
the meristem. An exception to this rule has only been observed with certainty in the
case of Philadelphus coronarius and in this case the division in general proceeds
centripetally, the innermost layer of cells remaining meristematic, but in each of those
products of division which are given off externally, one or two tangential divisions (in
the latter case proceeding centrifugally) take place, and only after this does the de-
velopment of their products into tissue occur. According to Sanio, a similar process

* ComjDare Sanio, l.e. p. 99,

SECONDARV CHANGES OUTSIDE THE ZONE OF THICKENING. 547

appears also to take place on the formation of the first layer of cork in Melaleuca
styphelioides.

As will be shown in greater detail below, the duration and productiveness of a
cork-meristem, when once formed, are remarkably various in the special cases.
It may remain active for decades of years, and longer, and produce masses of tissue ;
and, on the other hand, its generative activity may come to a standstill after a few
divisions, and it may itself, together with the layers it has produced, become con-
verted into a permanent form of tissue.

It may happen exceptionally, and in no known case as a phenomenon of normal
development, that in layers of meristem with persistent growth the cell which typically
remains meristematic becomes a cork-cell incapable of division. The meristematic
properties and functions are then transferred to the next inner parenchymatous cell.

The origin of the two forms of tissue derived from the phellogenetic meristem,
when the latter remains permanently active, is determined in such a manner, that all
cells formed on its outer side become cork-cells, while those formed on its inner side
become phelloderm. Thus purely centripetal phellogens only form cork-cells, while
reciprocal ones form the latter on the one side, and phelloderm on the other. Per-
manently active, purely centrifugal meristems, if they existed, would accordingly only
form phelloderm. The occurrence of the purely centrifugal course of division is,
however, limited to cases in which activity soon ceases, and in these cases either the
inner of the few layers produced become phelloderm, and the outer cork-cells (Loni-
cera Caprifolium), or they are all converted into cork-cells.

The cells of the meristemaiic layer generally show the structure indicated by
this term; individual peculiarities, e. g. the presence of chlorophyll in Sambucus
nigra, and lateral thickening of the walls in Salix, &c., scarcely need to be mentioned
here. Their form is that of polygonal plates, the transverse and longitudinal seetions
being more or less sharply quadrangular, and the radial diameter, as a rule, con-
siderably shorter than the others.

The properties of the phellogenic cortical parenchyma, or phelloderm, are in all
essential points similar to those of the outer dense parenehyma of the cortex; like
the latter, it shows the phenomena of dilatation and sclerosis. The one generally
applicable distinction from the latter consists in its origin as a supplementary struc-
ture added subsequently by the phellogenetic meristem, and in its radial arrangement
resulting from its mode of origin.

The structure of the cork-cells has been dealt with in Sect. 24, p. 108.

The changes which are produced by the phellogenetic formations in the whole
primary and secondary cortex depend upon their place of origin and special character.
Three principal phenomena are accordingly to be distinguished, namely the formation
of superficial periderm, of internal periderm and bark, and of Lenticels. It follows
from the nature of the subject that these three kinds of structure are nearly relatedjo
one another, and therefore that transitions may also appear between them.

Sect. 175. Superficial periderm. In most stems of woody plants, in tubers, and
also in some few roots, as those of Anisostichus (Bignonia) capreolata, and the
Clusiaceae the epidermis is replaced by a periderm arising within or close beneath

* Van Tieghem, Ann. Sei. Nat., 5 ser. tom. XIII. p. 258.

N n 2

-,4«

SECONDARY CHANGES.

it, the meristem of which remains active for along time, and, as its principal product,
forms a coating of cork. In most cases this formation of periderm begins simulta-
neously with, soon after, or even before the complete extension of the internode ; in
the plants mentioned above, p. 535, with a long-lived epidermis, it begins later, and
often not for many years.

In a large minority of the cases the initial layer of the periderm is the epidermis
itself : Nerium Oleander, Viburnum Lantana, lantanoides, prunifolium, all Pomacese
(Figs. 216-218), Virgilia lutea, Staphylea pinnata, Solanum Dulcamara, all investigated
species of Salix (Sanio), Euphorbia antiquorum Melastoma cymosum, and Centra-
denia floribundum The formation of periderm in Acer striatum, which does not
usually occur for many years, may also be mentioned here. In these cases only the
original external wall of the epidermal cells is cut off by the phellogenetic layer ;
it is burst, and left to peel away gradually.

In the very great majority of the plants belonging to this category the layer of
cells lying immediately below the epidermis is the initial layer for the formation of
periderm (comp. Fig. 214, p. 528, and below. Fig. 223). The whole epidermis
■above it is burst and thrown off. Examples ; Platanus, Acer campestre, Abies pec-
tinata, Hakea florida; — Fagus silvatica, Rhamnus Frangula, Quercus Suber, Q. pe-
dunculata, Castanea, Ostrya, Carpinus, Corylus, Betula, Ainus, Ulmus, Juglans, Celtis,
Sambucus nigra, Plectranthus amboinensis, Crassula tetragona, Acer pseudoplatanus,
A. platanoides, Tilia, Catalpa, Fraxinus, Syringa, Prunus, Amygdalus, species of
Rhamnus, Viburnum Opulus, V. Oxycoccos, Populus ; Medinilla farinosa, Miconia
chrysoneura and many others.

Closely connected with these cases is the appearance of phellogenetic divisions
in the second or third sub-epidermal layer, observed in Robinia pseudacacia, Gledit-
schia triacanthos, and Cytisus Laburnum. In this case one or two external layers
are thrown off, together with the epidermis, as a small bark.

In the simplest but rarer cases only cork-cells are produced in these peridermal
formations, and the phellogenetic layer of meristem is reproduced by divisions in
purely centripetal order, phelloderm not being formed. This is the case in Nerium,
where the latter seems never to appear, and in Viburnum Lantanoides, where, accord-
ing to Sanio, phellodermal cells are at any rate still absent in stems five years old.
Many Coniferae also appear to form no phelloderm, though this still requires more
accurate investigation. In most woody plants phelloderm appears in the normal
course of development, whether immediately after, or almost simultaneously with, the
first cork-cells, or only in later stages of development, after the latter have already
been produced in abundance.

The differences in these respects, which may be referred to in detail in Sanio, /. c., are
partly determined by the species, while within the same species individual variations occur,
which often clearly depend on external causes. As regards the former, it is the rule for
a number of investigated species, that at first only centripetal divisions and production of
cork-cells take place, and only at a later period, often not before the second year, a
layer of phelloderm is formed by a reciprocal division, which is immediately succeeded

• Schacht, Lehrb. I.-p. 287. ® Vochting, Bau, &c. d. Melastomeen, p. 49.

^ Vochting, 1. c.

SECONDARY CHANGES OUTSIDE THE ZONE OE THICKENING. 549

again by those in centripetal order. The latter process may then be subsequently
repeated from time to time. This is the case in most Pomaceae, Virgilia, Solanum
Dulcamara, Hamamelis, Platanus, and Acer campestre. In the majority of the forms
investigated, however, the appearance of at least one layer of phelloderm immediately
after the first divisions is the rule, and division thus takes place in the orders termed by
Sanio centripetal-intermediate (Aronia rotundifolia, Fagus, and Rhamnus Frangula),
centrifugal-intermediate, and centrifugal-reciprocal. The two latter orders of succes-
sion occur, for example, in Staphylea pinnata, and all the species enumerated on p. 548,
after the mark ; — .

Sanio describes in Viburnum Opulus an exquisite example of individual variations. In
summer the first layer of periderm is here formed in the centrifugal-reciprocal order, the
reversal of the centrifugal succession taking place after 3-5 divisions. In internodes,
which only develope their periderm later, in September, the reversal takes place more
rapidly, immediately after the second division (centrifugal-intermediate order) ; those
internodes, finally, which only begin to form cork late in autumn, show a purely centri-
petal succession. The relations which here manifestly exist between the change in the
process of development and the action of external agents (warmth, light, &c.) may be
recommended to more accurate physiological investigation. Various similar changes,
which cannot always be brought into relation with external influences, are described by
Sanio, /. c.

The number of the phellodermal layers, which arise in the structures under considera-
tion, is in most species very small, in comparison with the cork-layers which appear in
the same space of time. In each radial row only one or two phellodermal cells cor-
; respond to numerous cork-cells, even after several years ; in periderms which grow for a
very long time, e. g. Fagus, this relation constantly becomes more striking as time goes
on ; in a branch of this tree four years old Sanio figures two phellodermal layers to more
than seven cork-layers, and found in branches twelve years old only two or three layers of
phelloderm, while the number of the cork-layers, though not exactly stated, had certainly
increased considerably.

Here also, however, numerous deviations from the usual rule occur, generally according
to species and genera. In most investigated species of Salix, each initial epidermal cell
produces in the first year one cork-cell externally, and one phellodermal cell internally ;
between the two there is a central meristematic cell, with its wall thickened on the out-
side, and immediately becoming cuticularised on the external thickened surface. In
this central meristematic cell the same division and differentiation as in the initial epi-
dermal cell is repeated in the second year, and the same process takes' place in each
succeeding year, starting from the meristematic cell for the time being, until the forma-
tion of bark begins at a later period. An abundant formation of phelloderm — six layers
by the third year — was found by Sanio in Quercus Suber, where, however, the formation
of cork-cells is also very abundant. Canella alba and Cinnamodendron corticosum may
also be mentioned here; the thin old cortex of these plants, which is used commercially,
shows (when not externally abraded), in so far as the structure can be recognised from
the seriation of the elements in question, massive zones of phelloderm, which may be
over twenty layers thick ; these lie inside bulky layers of cork, and are separated from
them by a layer of meristem. The elements of the phellodermal rows are almost cubical,
and consist chiefly of stone-sclerenchyma, but with the latter thin-walled, unlignified cells,
sometimes containing starch, sometimes clustered crystals, are intermixed in the most
various ways. My material did not allow of a more accurate determination of the origin of
these suberous and phellodermal structures ; according to observations on single pieces
of cortex their development in the deeper cortical layers is at any rate not improbable.

As is evident from the preceding statements, and from those in Sect. 24, the
quantity of cork-layers originally formed, and of those produced anew by the meristem
to replace the layers which peel off during the progress of growth in thickness, is

550

SECONDARY CHANGES.

very unequal in the different cases. To these differences others are closely related,
which affect the form of the cork-cells and the cohesion of the layers.

According to these differences two forms of the superficial formation of cork
may be distinguished, though they cannot be separated quite sharply: namely,
suberous crusts and suberous integuments. The former consist of numerous layers of
soft wide cork-cells, which alternate with thin flat-celled zones, marking the limits of
the annual production (p. 114). They constitute superficial masses, attaining a
thickness of several millimetres or centimetres, which are soft, and concentrically
zoned internally ; from their origin onwards they are provided with wing-like
projections and deep furrows, because the formation of cork is from the first
unequally abundant in alternate longitudinal bands ; as growth in thickness goes
on they become widely and irregularly cracked. This is especially the case in
Quercus Suber and occidentalis, also Q. pseudosuber, climbing Aristolochise, e. g.
A. cymbifera and A. biloba (cf. Fig. 219), the younger shoots of Acer campestre,

Liquidambar styraciflora, Ulmus suberosa,
Euonymus europaeus ; species of Banksia
and Hakea (Mohl) ; further, in the above-
mentioned cortex of Canella, the phelloderm is
covered externally by thick soft layers of cork.

Suberous integuments, the periderm of
Mohl, consisting of flat cells only, or also
of thin wide-celled layers alternating with the
latter (e. g. Betula, Boswellia, &c., comp. Sect.
24), form those smooth coverings of the cortex
which are present in the great majority of
woody plants. Their bulk in shoots of the
same age is very various in different species,
according to the amount of the annual new-
formation and the extensibility of the walls of the cork-cells. All these con-
ditions may remain the same as long as the superficial periderm exists at all, or
they may change at various periods in the age of a shoot. These circumstances
determine the extremely various character of the surface in those woody plants which
persistently form periderm, to illustrate which some few examples must suffice here.

As already mentioned, many species of Salix (e. g. Salix alba) in their younger years
annually form a layer of cork-cells, all successive layers being of the characteristic struc-
ture described on p. 549 ; the outermost follow the growth in thickness by their extension,
and finally they burst injperceptibly, and peel off, the cork-layer therefore remaining
thin and smooth.

Fagus silvatica forms a highly extensible and firm suberous integument, which, from the
first year onwards throughout life, consists of uniform flat cells with brown contents, and
receives only a slight successive increment of growth from the meristem. The young
stem or branch has therefore a smooth brown surface. The external layers of the
suberous integument burst imperceptibly, and wither, while their contents become
discoloured. Towards the tenth year of life^ the process becomes more active, and
begins to give the smooth surface that dull whitish colour by which it is permanently

Hartig, Forstlich. Cullurpfl. p. 177.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 55 1

characterised throughout life (if rare exceptional cases, in which bark is formed later, be
left out of consideration).

Similar conditions occur in other woody plants with a permanently smooth suberous
integument, e.g. species of Carpinus and Planera. Others, as for example the species of
Prunus, likewise form, so far as is known, only uniform, flat-celled integuments of cork,
but the latter are thicker, and extremely tough, and long resist the action of the
weather. In consequence of the latter properties the outer layers, when finally burst,
adhere to the surface of the cortex as tough membranous flaps.

The shoots of Corylus Avellana form in the first year a suberous integument consisting
of wide thin -walled cells. The latter soon bursts, and forms the yellowish gray, easily-
peeled coating of the one-year-old shoots. Later on, firm, flat-celled, and soft wide-
celled layers of cork (one or two strata of cells thick) are alternately formed ; the latter
burst readily, the firm ones peel off from them in shreds, which remain adhering to the
shoots when the latter are several years old.

Betula alba forms from the first year onwards a firm suberous integument consisting of
uniform flat cells with brown contents. The shoots therefore possess in the first instance
a smooth brown surface. Later on, beginning at about the fifth year, wide-celled, thin-
walled layers alternate with the flat-celled ones, the former being at first simple, but
afterwards consisting of several or many strata of cells. In later years the brown mass
of contents is absent, even in the newly-formed flat-celled layers, the entire suberous
integument becoming colourless. The tearing of the thin-walled layers results in the
peeling off of the corky integument.

The remarkably tough, leathery, thick suberous integuments of Bos’wellia papyrifera
split into sheets owing to the tearing of the thin, brittle, silicified layers, which were
described above in Sect. 24.

Sect. 176. A number of stems and branches of Dicotyledons, and almost all
roots of Dicotyledons and Conifers, undergo a profound anatomical change on the
formation of periderm, inasmuch as the latter takes place in the interior, at a
considerable distance from the surface, and all parts lying outside the periderm, being
cut off by the corky layer from the access of the sap, die away. The masses of tissue
thus cut off are termed the bark (Mohl, 1. c.).

The internal periderms arise round the whole member, by means of the pro-
cess of development generally characteristic of peridermal formations, in a surface
which is everywhere approximately equidistant from the centre, and which follows
the periphery of the bast. Its transverse section, corresponding to that of the
bast, is either circular, or indented opposite the medullary rays. According as the
general surface of the member is similar or dissimilar to that of the bast, the
peridermal layer therefore lies either at a constantly equal, or at a variously unequal
distance from the former. Thus, for example, it surrounds the circular transverse
section of the bast in Thuja and Juniperus as a ring, lying at a great distance from
the surface opposite the corners of the branches, and only separated from it by a
narrow zone of parenchyma between them. Between the corners or projections of
the branches of Casuarina, it may even lie immediately below the epidermis.

With reference to its special position, it will be best to distinguish between stems,
or tbeir branches, and roots.

I.- It has already been stated that in many stems of Leguminosae the initial
layer for the formation of periderm is the second or third layer of cortical cells from
the outside. These cases effect the transition to those, which have to be considered
here, with a more deeply situated initial layer.

552

SECOND A RV CHANGES.

The position of the latter, as is probable a priori from the occurrence of
transitional forms, is not determinate in a sense generally applicable to all cases.
It lies ; —

{a) At a relatively great distance from the bast-layer ; this is conspicuous in
the case of Berberis vulgaris, where it borders immediately on the broad sclerenchy-
matous ring of the external cortex (p, 419), and is separated from the phloem-
bundles by a broad zone of lacunar parenchyma containing chlorophyll. The same
is the case in shrubby Papilionacese like Sarothamnus, Colutea, and Coronilla
Emerus ; also in Ginkgo ; Caragana arborescens ^ and the stems of perennial
Caryophyllacese (Dianthus, Silene spec.) have the cork-layer close inside the
sclerenchymatous ring, which is separated from the bast by a zone of parenchyma.

The formation of periderm in Casuarina
and the Abietinese, which, on account of
some peculiarities, must be more minutely
considered below, may also be placed in
this category.

(3) It lies near the external limit of the
bast-layer, and in fact, in the absence of
distinct fibrous bundles at the outer boundary
of the phloem-region, in immediate contact
with the latter : Lycium barbarum, Cobaea,
Ribes (Fig. 220), Deutzia scabra, species of
Lonicera, many Melastomaceae (Melastoma
Heteromallum, species of Lasiandra and
Heterocentron ^), Thuja, and Juniperus ;
Atragene and Clematis may also be men-
tioned here rather than in the category with
bundles of bast-fibres. Where the latter are
present, the phellogen either lies immediately
outside them (Rubus idaeus, according to
Sanio), or close to their inner boundary
(Punica, Spiraea opulifolia, Philadelphus,
Melaleuca, and Vitis). In the Melasto-
maceae mentioned, the initial layer borders
directly on the endodermis which surrounds
the bast. How far such a relation to a
plerome-sheath occurs elsewhere, remains to be investigated.

The succession of the divisions is, in the cases belonging to this series in-
vestigated by Sanio, centrifugally reciprocal (Berberis, Caragana, Lycium, Deutzia,
Lonicera, Philadelphus, Rubus, and Melaleuca) ; in the Melastomeae and Casuarina
it is centripetal. In the other cases mentioned the order of division has not been
more minutely investigated.

Phelloderm is formed in Ribes, Lycium, Caragana, Deutzia, Lonicera, and
Spiraea, and also, as it seems to me, in the Cupressineae mentioned; in the re-

FIG.’22o. — Transverse section through the surface of the
cortex of a one-year-old branch of Ribes nigrym (500). e
epidermis, Ä hair, /r dried-up and distorted parenchyma of
the external cortex, b external region of the bast ; K peri-
derm, consisting of the suberous integument k. phelloderm
containing chlorophyll pd, and the phellogenetic layer of
meristem at c. From Sachs, Textbook.

* Sanio, Stahl, 1. c.

^ Vochting, l.c. p. 51.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 553

maining cases it has not been found, or is not mentioned. The cork-cells form
collectively thin integuments, consisting of a few layers. The extremely irregular
arrangement, still requiring closer investigation, of the thin suberous integument of
Cobaea is worthy of remark.

As regards the very various details of the first development of the periderm, and the
structure of its mature parts, we must refer to the monographs. Here some special
cases only may further be described as examples.

The above-mentioned Cupressinex, and no doubt their similarly constructed allies also,
have in the young internodes more or less broad, blunt projections of the external cortex,
running down from the leaves, and traversed by vascular bundles or resin-passages
(Juniperus), or both, which pass obliquely to the next leaf above. The outline of the
bast, as seen in transverse section, is circular. The initial layer of the periderm passes
close by the bast, — its position still requires more precise determination, — and is only a
few layers of cells distant from the epidermis between the foliar projections, but is far
removed from it opposite them ; the whole of the projections, with their bundles and
resin passages, are cut off by the layer of cork.

The Abietinem, so far as is known, behave very variously as regards the first formation
of periderm. Abies pectinata, as stated above, forms its periderm in the sub-epidermal
layer of parenchyma. According to Mohl’s statements which however do not enter
very minutely into this point, the same conditions may be assumed to exist in A.
siblrica and Pinus Strobus. A contrast to these is presented by Larix europaea, where
the parenchymatous foliar pulvini, which contain resin-canals, are thrown off by internal
formation of periderm in the first year. Whether the periderm is also deeply-seated
between the pulvini, or whether, as conjectured by Sanio, it is here immediately hypo-
dermal, as in the furrows of Casuarina, is not known. Abies excelsa, Pinus silvestris and
nigricans behave, according to Mohl, in a similar way to the Larch, as regards the first
formation of periderm in the pulvini, with the sole difference that the periderm, especially
in Pinus, is inserted less deeply and lies outside the resin-canals.

Casuarina'^' has large longitudinal projections, separated by narrow furrows, on the
internodes of the branches. The approximately cylindrical bast is surrounded by a
broad zone of parenchyma containing little chlorophyll, which in the furrows reaches to
the epidermis, but in the projections is separated from the latter by the masses of tissue
described already, through which runs the vascular bundle of the next segment of the leaf
above, almost at the level of the base of the furrow.

The formation of periderm begins in the furrows, and in fact in the sub-epidermal
layer of parenchyma. It is continued from each furrow in both directions, through a
layer of the internal parenchyma directed towards the vascular bundle, and finally
through a band of cells running transversely through the phloem of the bundle. Thus
in the furrows the epidermis only is cut off by the peridermal layer, while between them
this is the case with the whole of the foliar projections, including the external part of
their vascular bundles.

2. Internal primary formation of periderm is a quite general rule in those roots
which grow in thickness according to the Dicotyledonous type and its initial layer
is here always the pericambial or rhizogenetic layer of cells, which adjoins the
endodermis on the inside. In the seedling it extends in many cases upwards
from the root, through the hypocotyledonary stem. Its beginning coincides with
that of the more abundant cambial growth in thickness, and through the joint

^ Botan. Zeitg. 1859, p. 337. ' Sanio, l.c. — Low, /.o; compare p. 256.

Van Tieghem, Symmetrie de Struct., l.c.

554

SECONDARV CHANGES.

action of the two processes the whole cortex lying outside the pericambium or
endodermis is as a rule split, and thrown off, suffering immediate decay in the case
of subterranean roots, while it adheres to parts above the ground in the form of flaps,
which gradually dry up. In parts which swell quickly this is very conspicuously ap-
parent ; the separation of the flaps from one another often takes place exactly along
those longitudinal lines in which the cambiogenetic growth in thickness is at first most
vigorous, and consequently in those lying opposite the primary phloem-bands of the
vascular bundle. As in diarch main roots the latter alternate with the cotyledons, in
such cases a separation into two flaps occurs, each of which lies below a cotyledon,
a phenomenon which has often been described in the case of plants forming tuberous
roots which, together with the hypocotyledonary stem, swell rapidly; it was com-
pletely explained by Turpin in 1830^. Where the swelling is less rapid, the process
under consideration, by which the usually relatively voluminous primary cortex is
thrown off, often results in a diminution of the total thickness of the member, which
is only compensated by later secondary growth ; most roots of Dicotyledons and
Gymnosperms are thinner at the beginning of the secondary growth than before,
owing to the loss of the external cortex.

The commencement and progress of the cell-divisions in the periderm of roots,
though it has not been investigated in all its details, yet corresponds, so far as is
known, to the general rules for the formation of periderm. According to the data
made known by Van Tieghem, their products are in all cases, including the Coniferse,
both phelloderm and layers of cork. The former constitutes an external layer of the
parenchyma, and is always of relatively small bulk. The cork-cells generally form
thin integuments, rarely (e. g. Pistacia Lentiscus) more bulky, cracked masses of
cork. In the more or less fleshy roots of herbaceous plants, the continuous
suberous integument is often extraordinarily thin, being reduced to one or two
layers of cells. The peeling off and decay of the outermost layers, for the time
being, are no doubt considerably accelerated in the case of subterranean roots
by the nature of their surroundings.

In the roots of woody plants the general character of the suberous integument
is similar to that of the stem. No minute investigations exist on the special differences
which may occur between the two in the same plant. In many herbaceous plants
the relatively great irregularity in the arrangement of the whole peridermal layer is
conspicuous.

The cells of the endodermis, which are likewise wholly or partially suberised,
often form the outermost stratum of the cork-layer, when the external cortex begins
to be thrown off, and, on further growth, are the first to be thrown off themselves.

Sect. 177. Repeated formation of internal periderms. The first formation of
periderm, whether it be internal or superficial, is in many woody plants the only one,
and the periderm follows the growth in thickness of the parts enclosed by it : Fagus.
The great majority of ligneous plants, however, form on stem and branches new
internal periderms, after the first one, which arise successively in deeper layers of the
cortex, and cut off successively deeper zones of tissue as dry bark. This process

‘ Turpin, Ann. Sei. Nat. i ser. tom, XXL p. 298, pi. 5. Compare Botan. Zeitg. 1873, pp.
129, 297.

SECONDARy CHANGES OUTSIDE THE ZONE OF THICKENING. 555

has also been observed in roots (e. g. Sassafras, Vitis, &c.), but always to a small
extent, and in a manner not differing from the stems, so far as is known. More
accurate investigations only exist as regards the latter.

In species the first periderm of which arises deep in the cortex (Lonicera,
Vitis, Clematis, Cupressinese, &c.) all the later ones assume the same arrangement
as the first, and thus cut off in each case an annular layer of cortex, though this
is not always quite complete and regular {Ring-dark of Hanstein). In species with
a superficial primitive periderm, on the other hand, the successive internal layers
arise in such a form, that they abut on the outermost layer of periderm for the
time being, and cut off scale-like portions of the cortex (Scale-dark). The single
scales have extremely various forms and dimensions even in the same individual.
Their formation begins at points which are not morphologically determinate ; the
first scale is joined, at or below its edge, by the peridermal layer which cuts off
a second scale, next the first, and belonging to the same cortical layer, and the
same phenomenon extends round the surface of the stem without any perceptible
regularity, cutting off the first cortical layer in scales, and then in the same way
attacking a deeper one. The scales which succeed one another at different depths
also differ among themselves in form and size ; they do not fit one on another.

The characteristics of a bark depend, next to the arrangement of its parts as
stated above, on the structure of the tissue which becomes dried up, and on that
of the periderm, especially of its suberous layers. As regards the former, essential
differences are due to the varying hardness and toughness of the desiccated layers
of tissue, determined by the occurrence of fibrous and stone-sclerenchyma between
the softer tissues, which on drying often become brittle and easily crumble. The
thickness of the zone of tissue thrown off each time also has an influence, and is
extremely various according to the special case, examples of which will be found
in subsequent descriptions. In the cork-layers all the above-mentioned differences
of structure occur, and according to their combinations, numerous peculiarities appear
in the various species.

The difference in the cohesion of the membranes of the old cork-cells which
undergo desiccation is of primary importance. If they are thin and not very tough,
they must become torn, on the one hand by the progressive extension of the
periphery of the stem, on the other hand by the shrivelling of the layers of tissue
while drying. If a stratum of cork of this nature coats the inner surface of a
dry layer of bark, the latter breaks off completely. The well-known scaly bark
of the Planes is an exquisite example of this. The stratum of cork bordering
a scale is only a few layers thick, and consists in its outer part, adjacent to the
scale, of thin-walled, brittle cells, while the more internal ones have thick yellow walls,
as mentioned in Sect. 24. The scale is released by the thin-walled zone becoming
completely torn through, the thick-walled zone remains till the next following
desquamation, as a fairly smooth covering of the living tissue. The surface of
the cortex thus remains, on the whole, smooth, with only those flat inequalities which
correspond to the outlines of the desquamating portions, and change from year to
year. The case is similar in Taxus baccata, in the false quinine bark occurring
as China bicolorata, and in the stem of Arbutus Andrachne, A. Unedo, and Salix
amygdalina and its allies ; Pyrus Malus may also be mentioned here. In the

55Ö

SECONDA RF CHANGES.

younger stem and apical branches of Pinus silvestris and its allies, the cork layers
which cut off the young thin scales of bark consist, on the outside and inside
respectively, of a few-layered stratum of thin-walled, easily torn cells, while between
them a stratum occurs w’hich consists of one or two layers of sclerotic elements ;
this does not always extend to the edge of the thin-walled strata. When the latter
become torn the whole bark peels away, and forms partly the somewhat, thin scales
of true bark, consisting of desiccated cortical tissue and adherent layers of cork,
partly those tough feathery sheets, of the thickness of paper, which are the sclerotic,
hard, persistent layers of the suberous zones.

The peeling annular bark of species of Melaleuca (especially M. styphelioides),
Callistemon, Vitis, Clematis, &c., also, strictly speaking, belongs to this category. It is
however held fast for a long time in the form of fibrous flaps adhering to the cortex,
and hindered from falling off by the numerous strands of bast-fibres which belong
to the cortical tissue in process of desiccation; they traverse and support each
desquamated zone surrounding the periphery of the stem, as a strong network
of fibres with pointed meshes.

In cases where the cohesion of the old suberous membranes is greater, the
layers of bark succeeding one another from without inwards adhere more closely
one to another, as a connected crust, which, as growth in thickness goes on, becomes
more and more cracked externally, and gradually suffers decay. Woody plants with
a thick, cracked, and rugged bark, such as Oaks, Birches, Poplars, most Willows,
Robinia, &c., afford universally known examples of this phenomenon; the cortex of the
old stem of Pinus sylvestris may also be expressly mentioned as an example, because
the difference which it so conspicuously shows from that of young stems and branches
depends principally, though not exclusively, on the different cohesion of the above-
mentioned thin-walled layers of cork. The further differences consist in the structure
of the desquamating layer of bast, which is no doubt altered by the extensive
formation of phelloderm, and in other respects still requires further study.

A necessary direct connection between the direction of the cracks in the surface,
and the form, size. See., of the scales of bark cut off, does not exist, at least
not in the typical cases belonging to this series. In those which are intermediate
between the latter and the cases of complete desquamation, represented by the Plane,
such a relation may occur.

Secondly, and independently of the cohesion of the membranes, the thickness and
special structure of the entire suberous layer come under consideration, and in this
respect essentially the same differences exist, as were stated above, with regard
to the formation of cork in general, and to superficial periderms.

In most cases, no doubt, the layers of cork which cut off the bark are mem-
branes, a few (not much more than ten) layers of cells in thickness; the cells
themselves then either belong to the flat form, e. g. Platanus and Pinus sylvestris,
or are wide and even radially elongated (e. g. Melaleuca), the successive layers being
similar or dissimilar.

On the other hand, in many cases of repeated internal formation of
periderm, cork is produced in large masses, forming thick strata, consisting of
very many layers, and perceptible to the naked eye as broad zones. In these cases the
cork no doubt always belongs to the wide-celled, thin-walled form, or consists of

SECONDA Ry CHANGES OUTSIDE THE ZONE OF THICKENING. 557

the latter together with concentric zones of flat elements alternating with it.
Examples of this are afforded by Acer campestre, with broad, soft zones of cork ;
by the older, bark-forming cortex of Betula alba, also with broad zones of cork,
of similar structure to the white superficial periderm of younger stems, but firmer ;
but more especially by the Cork Oaks. The cortex of the stem of Querais pseudo-
suber is covered, in the thick pieces of the stem investigated, which was at least
forty years old, by a crust of cork, attaining zcm in thickness, and cracked ex-
ternally; it looks like bad bottle cork, and has the same structure as the latter, with
reference to the cork-elements. It includes at various depths, numerous scattered
isolated scales of desiccated cortical tissue, which are scarcely more than 2““ thick,
and ic“ to about broad and long. The oaks which yield true cork, especially
Q. Suber, have, in the wild condition, the tendency to a similar formation of bark, but
with a far more abundant production of cork. The latter indeed appears chiefly as
superficial periderm (comp. p. 550); but it may also proceed, in the wholly intact
tree, from internal, repeated periderms, and thus cut off narrow portions of cortex
between broad zones of corkb With the object of obtaining technically valuable
cork, the tendency of the tree to the repeated internal formation of periderm is
made use of artificially in the Cork Oaks and the Birch. The numerous mis-
understandings regarding the modes of procedure and phenomena then taking place,
some of which have re-appeared even in recent times, may justify a short account
of the facts in this place, leaving purely technical matters untouched.

The intact Cork Oak^ forms on stem and branches the superficial periderm above
described, producing the thick mass of cork, which becomes cracked externally. Even
on branches and stems many years old this form only is generally observed, covering the
cortex, the whole of which remains alive, while the internal periderms just mentioned
only occur exceptionally ; this however may depend on the fact that old intact stems
but seldom come under observation. In order to obtain cork for technical purposes,
the almost useless, superficially-formed layer (called the male) is removed from the stem
all round (demasclage) ; this is done carefully and smoothly, but not without everywhere
injuring and exposing the living cortical tissue — in the best case at least the layer of
phellogen and the phelloderm. While the cortical tissue begins to die off on the injured
surface, a new periderm appears, one or two millimeters below the latter, around the
entire stem ; and its phellogen produces a new layer of cork, which cuts off the portion
of cortex lying outside it. This periderm grows quicker than the external male cork,
and is used technically as ‘ female cork.’ The first peeling off of the male cork is under-
taken when the tree is about fifteen years old. A serviceable female layer of cork is
formed in about 10-12 years; a layer now before me, twelve years old, has, for example,
an average thickness approaching 30*“, without having undergone any further dressing.
The female layer of cork, when sufficiently thick, is now peeled off like the male one, in
order to be made use of, and is again replaced by the tree, in the way described above,
by an internal formation of periderm. The same process may be repeated periodically,
until the tree attains an age of about 150 years. If left to grow indefinitely, a female
layer of cork may attain an immense thickness; I have a piece before me thick,
though of course of very bad quality.

A quite similar method to that used for obtaining oak-cork is employed in the northern

’ Compare C. de Candolle, Mem. Soc. Phys. de Geneve, XVI. p. i (1861), Taf. I. Fig. 2.

^ C. de Candolle, /. r. — Von Mohl, Botan. Zeitg. 1848, p. 361. — See also FUickiger, Pharma-
cognosie, p. 334.

558

SECONDARY CHANGES.

provinces of Russia for repeatedly obtaining the suberous integuments of Betula alba.
In this tree also the layer of cork subsequently formed internally is distinguished from
that first formed by its greater softness, which depends on the predominance of delicate,
wide, cork-cells

Sect. 178. If the woody plants investigated be compared, on the basis of the
data given previously, and in Sects. 175-177, with reference to the occurrence and
absence of the layers of periderm described, and the appearance and periodical
changes of the latter according to the time, region, and individual, the following
cases are found to occur.

1. Few Dicotyledonous woody plants, and no Gymnosperms, keep their epi-
dermis, and are destitute of any formation of periderm throughout life, or during a
considerable number of periods of growth. Cases belonging to this category have
been mentioned at p. 535. Among woody plants with abundant growth in thickness
Acer striatum is the most remarkable known ease; in stems a foot thick, at least
forty to fifty years old, I found the epidermis still for the most part preserved, with
only isolated local spots of periderm, as to which it further remains doubtful whether
their origin had not been caused by slight wounds.

2. The great majority of the stems in question form superficial periderm;
a relatively small number of these (Negundo, Ilex, Sophora japonica, &c.) form
periderm only in the second, or a still later period of vegetation of the shoot ; it
is formed in the great majority during the first period of vegetation, after the ex-
tension and primary differentiation of tissues is complete in the internode con-
cerned ; in our climate the formation usually begins between the end of May
(Aesculus) and the end of July (Tilia). Late shoots may form periderm before
extension is completed

Many trees confine themselves to the formation of superficial periderm during
life, or for many years. The periderm follows their growth in thickness. In con-
sequence of this they have a smooth cortex, with a suberous integument, or a
cracked covering of cork where the latter is massively developed (Quercus Suber).
This condition is permanent through life in the common Beech, and for very many
(nearly fifty) years at least in Abies pectinata, Carpinus, the Cork Oak, and many
others.

This too, again, is an exception as compared with the majority of the cases.
Much the greatest number of woody plants of this category form internal periderms
later on in periodical repetition, and throw off the superficial periderm together with
the successive external cortical zones, in the form of scale-bark.

As regards the age at which internal formation of periderm and desquamation
of the bark begin, few very accurate statements belonging to this subject exist.
In many trees it happens quite early ; in Ulmus effusa as soon as the third or fourth
period of vegetation of the shoot, in Robinia pseudacacia, according to Hartig, often
even in the first. According to the same author ^ it begins in the native Oaks in the

* Compare von Merklin, Melanges Biolog. de I’Academie de St. Petersbourg, IV. (1864),
P- 563.

* Compare Sanio, /. c. pp. 41, 58.

® Compare the description of the trees mentioned in his Forstl. Cultui pfl.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 559

twenty-fifth to thirty-fifth year of life ; in the Alders in the fifteenth to twentieth, the
Limes in the tenth to twelfth, in Salix amygdalina in the eighth to tenth ; in other
Willows, with relatively abundant bark, earlier. In the Birch (B. alba), the formation
of bark begins on the stems, from the fifth or sixth year of life onwards, starting at
the base, and gradually advancing upwards, seldom to a greater height than four
metres. Populus tremula keeps its smooth superficial periderm for many years;
in P. nigra and pyramidalis it is soon thrown off by the formation of bark.

According to Mohl * the stem of Pinus sylvestris and nigricans begins the
formation of its thick scale-bark in the eighth or tenth year.

3. A relatively small number of woody plants, mentioned above at p. 552, form
their first periderm deep down in the cortex, and thus cast off the external layers of
the latter at once. In the known cases this always happens during the first period
of vegetation of the shoot, or at the limit between this and the second. In the
further behaviour the following different phenomena then appear.

{a) After the formation of the first periderm the shoot remains covered by it ;
it follows the growth in thickness throughout life, or at least for years, and a renewed
formation of internal periderm and bark only appears at a late period. In the stem
of Cobaea scandens, after the external cortex has been cast off in the first year, and the
bast-layer coated by an irregular periderm, no further internal formation of periderm
has been observed. The latter occurs in Ribes (according to Haustein, also
in Punica Granatum, where the one-year-old branch, after it has become coated with
periderm, and has cast off the external cortex, as mentioned above, may grow for
10-20 years in thickness, before a new formation of bark (no doubt in the annular
form) once more throws off a narrow cortical zone. Among Coniferous trees may
here be mentioned Pinus sylvestris and nigricans, in which the casting-off of bark
begins on the stem at the eighth to the tenth year ; Larix, with a formation of scaly
bark beginning approximately in the eighteenth year, and afterwards frequently
repeated ; and Abies excelsa, where this process begins about the twentieth year

{b) The first annular internal formation of periderm is followed by that of
successive new zones, of similar arrangement, at short intervals, which have not been
more accurately determined, and without exact, or at any rate without exactly
investigated, relations to the annual secondary growth of the bast. So in the majority
of the above-mentioned woody plants with annular bark, e. g. Melaleuca, Callistemon,
Cupressinese, &c., the cortex of which becomes covered within a few years by several
layers of bark.

(c) In every period of vegetation succeeding the first a new zone of bast is
produced, and at the end of the period the whole zone belonging to the previous
year is cast off by formation of periderm : Vitis, Clematis, Atragene and Caprifolium.

4. So far as is known, the woody plants investigated either show the same
behaviour in all individuals, or hereditary individual differences. Examples of the
latter have been mentioned above in the case of the Stone Beech, and the Cork
Oaks which form bark; the variety of Ulmus effusa, known as the Cork Elm,
with large wing-like outgrowths of cork, on the surface of the first periderm of
young shoots, which is cast off about in the sixth year, may also be mentioned here.

* Botan. Zeitg. 1859, P- 33^-

“ Von Mohl, l.c.

56o

SECONDARY CHANGES.

5. The phenomena described usually extend uniformly over the whole of the
stem and its branches ; examples, however, occur of differences in the formation of
periderm in zones at different heights. This is the case in Pinus sylvestris (p. 556),
a tree which is distinguished by the thick bark on the lower stem, and the fine peeling
bark on its apex and branches, from its allies, e. g. P. Laricio, in which the thick
bark extends to the apex. According to Hartig this is also the case in the Birch, in
which the higher portions of the stem, and the branches, independently of their age,
constantly remain covered by superficial periderm only.

Sect. 179. Leniiceh'^. In most woody plants which form periderm the uniform
peridermal integuments hitherto described are interrupted at definite points, to be
more exactly indicated below, by bodies w'hich are as it were inserted in them and
belong to them, to which De Candolle gave the name of Lenlicels., and Du Petit-
Thouars the more descriptive one of cortical pores. Only in relatively few woody
plants, provided with a regularly repeated annular formation of bark, have lenticels
as yet been sought in vain: Vitis vinifera, Lonicera italica, L. periclymenum, Tecoma

j

Fig. 221. — Transverse section through the cortical surface of a shoot of Betula alba one year old (145). e, «
epidermis, stoma, d glandular scale, p—P superficial periderm, with a lenticel interpolated below the stoma.

In the lenticel two firmer, denser tangential bands are visible; but the narrow intercellular spaces containing
air are left undrawn m the whole of the lower part, on account of the low magTiif3dng power.

radicans, species of Clematis, Philadelphus and Deutzia, and Rubus odoratus, while,
on the other hand, they appear in other plants, which are nearly related systematically
to those mentioned, or agree with them in growth, habit, and formation of bark ; as
in those Loniceras which are not climbing, Solanum Dulcamara with annular bark,
Ampelopsis, Periploca, Wistaria sinensis, &c.

Those species which form lenticels anywhere, have them both on the stem and
its branches, and on the root.

According to its structure (comp. Fig. 221, and Figs. 222 and 223 below), the
lenticel may in most cases be appropriately termed a local, bi-convex swelling of the
periderm, often projecting above the surface as well as internally; it is distin^
guished from the rest of the layer by the presence of narrow intercellular spaces

’ Von Mohl, Unters, üb. d. Lenticellen, Venn. Schriften, p. 233; also p. 229. — A. Trecnl,
Comptes Rendus, tom. 73, p. 15. — E. Stahl. Entwicklg. n. Anatomie d. Lenticellen, Diss. and Botan.
Zeitg. 1873. — G. Haberlandt, Beitr. z. Kenntn. d. Lenticellen, Wiener Acad. Sitzgsber. Bd. 72 (1875).
[Also Kreuz, Entw. d. Lenticellen von Ampelopsis hederacea, Wiener Acad. Sitzgsber. Bd. LXXXIII.
1881 ; and Potonie, Lenticellen d. Marattiaceen. Ref. Bot. Centralbl. 1881, Bd. 8, p. 70.]

SECONDARr CHANGES OUTSIDE THE ZONE OF THICKENING. 561

containing air between the rounded edges of its suberous, phellodermal and meriste-
matic cells. By means of these spaces, the intercellular air-spaces of the cortical
parenchyma are in open communication with the external air at the time of active
vegetation, as may be proved experimentally ; during the resting period of vegetation
this connection may be interrupted by an integument of ordinary cork consisting of but
few layers.

In the fully developed lenticel minute investigation shows a phellogenetic layer
of meristem, connected with that of the adjoining periderm, and the former, in so far
as it belongs to the lenticel, either lies in the same surface as the surrounding portion
of the layer, or it bulges out towards the inside, or more rarely towards the outside
(e. g. old lenticels of Ginkgo). The cells of the lenticel are either approximately
similar in form to those of the surrounding periderm, or, not uncommonly they are
narrower in the tangential direction. After its first formation the meristematic layer
of the lenticels behaves similarly to the rest of the phellogenetic meristem as regards
its production of tissue. Like the latter it always forms phelloderm internally, and
in fact in abundance ; the latter may be as much as forty cells in thickness in each radial
row, e. g. in old lenticels of Ginkgo. On the outer surface the elements termed by Stahl
the cornplementary cells of the lenticel are first and principally formed in the same manner
as cork-cells, and like the latter are arranged in radial rows ; they are approximately
isodiametric cells, similar to the cork-cells in form, but varying according to the species,
with a thin colourless membrane, which for a long time shows the cellulose-reaction,
and only at a later time becomes brown (suberized T), and does not otherwise present
any peculiarities of structure ; they have a persistent, colourless, protoplasmic layer
lining the wall, and this layer contains a nucleus and often small quantities of starch,
and encloses cell-sap which is also colourless. A remarkable peculiarity of the com-
plementary cells, especially of the younger ones, which has not been sufficiently investi-
gatedj is their hygroscopicity, if the expression be allowed, i. e. their tendency to take
up water and thus to swell. The often conspicuous, puffy swelling of the lenticels
on the living tree in wet weather depends on this property, and it is further known \
that after dipping in water the younger internal tissue swells up to a white mass,
which exudes from the bursting surrounding tissue, becomes irregularly split up into
tatters and fragments, and finally breaks up on the surface into its separate rounded
cells. During this process a considerable, permanent increase in size of the cells
takes place, at least in many cases, especially in the radial direction ; e. g. the
roundish isodiametric complementary cells of Sambucus nigra become extended into
radially placed cylinders, which may become four times as long as broad.

In the cases of the formation of lenticels below stomata, to be more exactly
described below, the first formed, most superficial complementary cells, differ from
those described in the fact that they are arranged irregularly, and not in radial rows.

The complementary cells, like the phelloderm belonging to the lenticel, are
rounded at their radial angles, and between the latter interstices containing air exist,
which effect the above-mentioned communication of the cortical intercellular spaces
with the surrounding air. The rounding is either confined to the narrow corners.

■ De Candolle, Ann. Sei. Nat. 1826, VII. p. 5. — Von Mohl, Flora, 1832, Verm. Schriften, p. 229.
— Unger, Flora, 1836, p. 577, &c.

O O

562

SECONDARY CHANGES.

the Other surfaces of the wall being flat and in close connection with one another (e. g.
Ginkgo, Sambucus and Lonicera) ; or the walls are rounded off on the greater part
of their surface, and the cells are therefore only in slight connection, forming,
especially when dry, a loose, powdery mass, e.g. Prunus avium, Pyrus malus, Robinia,
Betula, ^sculus, and Gleditschia. In the latter case the mass of complementary cells
is held together, owing to the fact that some layers of flat cells, firmly but not uninter-
ruptedly connected among themselves and with the adjoining loose ones, are always
formed alternately with some loose layers of complementary cells.

In the complementary mass of Quercus Suber, which is also loose and powdery,
I did not find this arrangement ; the cohesion is here due to the fact that the whole
lenticel is enclosed in the tough, firm mass of cork, and thus protected from falling
to pieces, as will appear still more clearly below.

The firmer layers in the loose lenticels appear independently of the limits
between the periods of vegetation; e.g. Fig. 221 shows two of them in a lenticel
taken from a this year’s shoot of the Birch on the 5th of June. In older lenticels,
however, even in those filled with dense tissue, the formation of an uninterrupted
layer of cork over their whole surface occurs in many trees at the close of each
period of growth ; at the beginning of the next period complementary cells are then
again added by the meristematic layer. Where such a layer of cork appears at the
close of each period of vegetation (e. g. Ginkgo) it marks the limits of the annual
zones of growth.

The uninterrupted layers of cork serve to shut off the internal intercellular air-
spaces ; the former therefore constitute the ‘ closing layers.’ The closure is, however,
temporary, as the successive layers of cork are again burst by the subsequent formation
of complementary cells.

In trees which form an autumnal closing-layer, the latter is, according to Stahl,
already present before the time of the fall of the leaf. The renewed formation of cqmple-
mentary cells below it begins with the next period of growth, but need not immediately
result in rupture and the opening of aerial communication, as this obviously depends
on the relation existing between the pressure exercised by the new formation and
the resistance offered by the closing layer. As a matter of fact, judging from Haber-
landt’s experiments^, this usually begins after the trees have completely expanded
their leaves, or even after the close of the flowering season in those in which the
flowers appear later than the leaves, although, according to the author’s own judgment,
no certain conclusion can be drawn from these experiments alone, but more accurate
results must be sought by means of anatomical investigation.

The outermost layers of the lenticel, for the time being, undergo, in proportion
to the progress of growth in thickness and phellogenetic new production, the same
passive changes as the layers of cork, namely desiccation and gradual decay.

The productiveness of the phellogenetic layer in a lenticel is, as a rule, especially
in the centrifugal direction (centripetal with reference to the succession of the dividing
walls), greater than outside the lenticels; the latter therefore project as convex
bodies above the surrounding peridermal surface. Only in those trees, which like
Ulmus, Liquidambar, Euonymus europseus, and Acer campestre, form wing-like.

l.c. p. 16.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 563

projecting outgrowths of cork, does the converse case in a certain sense appear, the
lenticels lying in the depressions between the wings. In the case of the thick corky
layers of Quercus Suber also, the inequalities of which are principally due to
mechanical cracking, the lenticels do not appear above the surface; they traverse
the layers in the form of radial, irregularly constricted cylindrical columns, consisting
of a loose mass of complementary-cells which become brown on drying, and extend
from the withering surface to the phellogen ; they are known to everybody who has
seen a cork stopper as the brown pulverulent stripes, running at right angles to the
annual layers.

A lenticel, belonging to a persistent periderm which increases successively
in circumference with the growth in thickness of the shoot, behaves differently
according to the species, as regards its own growth in breadth. In numerous kinds
of trees, as Prunus avium, Betula, Abies pectinata, and Tamarix indica, every lenticel
apparently increases in breadth in about the same proportion as the circumference of
the shoot. On old stems or branches the lenticels appear as large, transverse
segments of rings. Although accurate measurements are wanting it may yet be
asserted of these cases with approximate exactness, that the portion of phellogen
belonging to the lenticel follows the dilatational growth in the same way as is known
to occur in the rest of the periderm, and persistently forms lenticel-tissue.

In other cases, e. g. Fraxinus excelsior, F. Ornus, Ailantus, and Quercus Suber,
the lenticels show little or no increase of breadth, or even show a decrease as they
become older. In a third series, finally, e. g. Pyrus Malus, Rhamnus Frangula,
Broussonetia, and Tsuga canadensis, as well as Quercus Suber, a lenticel may be
divided up into several smaller ones by dense periderm. The latter case can only
arise owing to the fact that at certain points in the phellogen of the lenticel, cork in-
stead of complementary tissue is formed from a definite point of time onwards (no
doubt from the date of the autumnal formation of the cork-layer). The same process
must go on progressively from the periphery towards the centre of the lenticel, when
the latter diminishes in its superficial extent. Where the latter remains the same or
increases slightly, it is doubtful whether the process just mentioned takes place in
proportion to the dilatation of the phellogen of the lenticel ; or whether, as is less
probable, the latter takes no part in the general dilatation of the cortex, or a lesser
part than the periderm outside the lenticel.

According to what has been stated, the absolute size of the lenticel may change
considerably with age, in the same individual, the transverse diameter attaining the
length of lom and more, in lenticels which participate in the growth for a very long
time. The original superficial size, which remains unchanged in those growing but
little in breadth, may be stated at about In periderms which are quickly cast

off, e. g. that of the Plane, considerably smaller lenticels occur, which can scarcely
be clearly distinguished with the naked eye.

The origin of the lenticels shows differences respecting the region and mode of
their formation, according to the position of the periderm to which they belong.

Where the seat of the first formation of periderm is the epidermis, or the sub-
epidermal layer of parenchyma, or, as in the above-mentioned Leguminosae, a slightly
deeper layer, the lenticels arise below Ihe stomala, namely one under each stoma when
the latter are not very numerous and are uniformly distributed, e. g. Sambucus nigra.

002

564

SECONDARY CHANGES.

Prunus Cerasus, Ligustrum vulgare, Syringa persica, Salix fragilis, Rhus typhinum,
Fraxinus Ornus, Robinia pseudacacia, and many others ; — or, where the stomata are
grouped together, one lenticel is formed under each group, e. g. species of Populus,
Juglans regia, and Hedera Regnoriana ; the stoma under which the lenticel originates
lies over the middle of the latter at a later stage of development, while other neigh-
bouring ones may lie in the periphery, e. g. Euonymus europseus, Persica vulgaris,
and Cornus sanguinea.

Even where the stomata are less numerous, some of them may remain with-
out any share in the formation of lenticels, as is especially conspicuous in the

horizontal shoots to be described below,
in which the number of lenticels oh the
upper side is smaller than that on the
lower side. In the investigated cases
the stomata are here equally numerous
on both sides, and on the upper side at
least are always more numerous on the
same area than the lenticels.

The formation of lenticels below
stomata begins with the growth and
divisions of the parenchymatous cells lying
in this position. (Figs. 222 and 223,
comp, also Fig. 221, p. 560). The
divisions at first take place variously in different directions. Their products, and also
no doubt the cells which have not yet divided, grow chiefly in the direction at right

Fig. 223, — Transverse section through a lenticel of Betula alba; older stage tlian Fig. 222 (about 280).
e epidermis, s stoma, under the latter the complementary tissue of the lenticel; inside this is its phellogenetic
meristem. At the edge of the lenticel the tangential divisions in the hypodermal parenchyma which give rise to
the superficial periderm are beginning on both sides.

angles to the epidermis ; their angles become rounded, the chlorophyll originally con-
tained in them disappears, and the cells acquire the properties of delicate colourless
complementary cells. Similar changes now extend further in area and depth from the
original point of departure. As this goes on the further dividing walls soon assume
a more regular and uniform tangential position, in such a manner that the above-
mentioned phellogenetic layer of meristem, which is usually concave towards the in-
side, finally appears. The mass of complementary cells lying outside this, in which the
divisions soon cease, becomes more and more driven towards the outside, in conse-

Fig. 222. — Transverse section through a young internode of
a branch of Betula alba (375). e — e epidermis; a respiratory
cavity under a stoma ; c the cuticle, which is separated from e as
far as the entrance of the pore by a layer of secretion (removed
by alcohol). At x the first beginnings of the divisions which give
rise to tlie complementary tissue.

SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 565

quence of its own growth, and of constant additions from the phellogen ; it first bulges
out the epidermis, and then breaks through if by a longitudinal crack passing through
the stoma, or through one of the numerous stomata, and then protrudes through the
gap. The complementary cells coming out through the crack, which is constantly
opening wider, then dry up, together with the shreds of epidermis adhering to them ;
they form those elements of the complementary mass which were mentioned above
as not being radially arranged, and as peculiar to stomatal lenticels.

The beginning of the formation of lenticels takes place as a rule on the young
shoot at an early period, before longitudinal extension is complete, and before the
formation of other periderm. Indeed the latter as a rule starts from the edges of the
lenticels, as soon as the phellogenetic layer is formed in the latter, and is continued
thence over the surface of the shoot. Exceptions to this rule are rare, and apparently
peculiar to individuals k It is true that the two processes often follow one another
immediately, so that the appearance of periderm and lenticels may be spoken of as
simultaneous, without any great error. In shoots with a long-lived epidermis on the
other hand (Sophora japonica, Rosa canina, Negundo, and Acer striatum), the lenti-
cels already appear in the first year, and thus long before the further extension of
the periderm.

It is obvious that according to the epidermal or sub-epidermal origin of the
periderm, special differences must occur with reference to the conditions described,
especially as regards the connection of the segments of the phellogen with one another.
For these Stahl’s work may be referred to.

In the superficial periderms, all original lenticels, so far as is known, are formed
below stomata in the manner described.

Secondly, the formation of lenticels occurs independently of stomata, on periderms
either when first developing, or in older stages, owing, as it may be shortly expressed,
to the fact that the phellogenetic layer of meristem forms, on limited areas, lenticel-
tissue instead of ordinary periderm. If this begins after layers of cork are already
present, the latter are burst by the growing lenticel. A more minute description of
these processes is superfluous after what has been stated above ; for some special cases,
and also for the peculiar formation of lenticels on the sites of the insertion of the
leaves in Abies pectinata, a case which does not strictly belong to this category,
reference may be made once more to Stahl’s work.

Lenticels independent of stomata arise on the internal periderms, both on those
first formed and on the succeeding ones, simultaneously with the origin of the rest
of the peridermal layer (with the obvious exception of plants wholly destitute of
lenticels).

According to Stahl’s observation on Pyrus Malus, and Haberlandt’s enumera-
tions to be mentioned below, they may also be successively formed anew, between
the pre-existing ones, on older periderms which have been growing for some time,
whether these be superficial, or of endogenous origin.

The latter new formations increase the number of lenticels on those periderms
which follow the dilatation. Those appearing on successive periderms replace those
lenticels which are lost when the bark is separated. If the bark is thrown off in scales,

^ See Stahl, /. c. p. 23.

566

SECONDARY CHANGES.

as in Platanus and Pyrus Malus, the new ones appear on the surface laid bare
by its rerhoval. In trees with an adherent, longitudinally cracked bark, as Robinia,
Prunus domestica, species of Populus, and Ginkgo, the living lenticels lie at the
bottom of the longitudinal furrows. The first cortical cracks pass through the first-
formed lenticels themselves, and give them the position indicated. New ones then
arise in the peridermal layers which are successively laid bare at the bottom of the
furrow, by the further extension of the crack.

According to the few existing enumerations, and judging from appearances,
the number of the lenticels occurring on the same transverse portion constantly in-
creases with the dilatation, at least in many trees ; and probably to a greater extent,
the smaller the dilatational growth of the individual lenticel.

As follows from the facts discussed above, the distribution of the lenticels on a
shoot is in general determined by that of the stomata, by the structure of the older
cortical surface, the form of the bark, &c. An additional phenomenon, independent
of all these relations, further appears, namely, that while the distribution is uniform
all round on upright shoots, in those growing horizontally the upper side has fewer
lenticels than the lower side. The amount of difference between the two sides varies,
according to the species, and according to the age of the shoots of a tree ; in the
latter respect in such a manner that it is greatest in young stages, and becomes more
and more equalised as growth in thickness proceeds. Of the numbers found by
Haberlandt for this relation, in several species of tree (species of Gleditschia, Tilia,
and Ulmus campestris), some may be given here, which also illustrate the successive
increase in the total number of the lenticels. The number on the upper side is
printed as the numerator, that on the lower side as the denominator of a fraction.

Piece of branch, 20 long, of —

\st year.

‘S-iith year.

10-i^tk year.

Gleditschia triacanthos : ;

Ulmus campestris : ;

5 5 ,
7 0 >

iio •

24 1»

ri6_ •
7 8 }

XS.0
2 9 0‘

9 5

Be"’

CHAPTER XVI.

ANOMALOUS THICKENING IN DICOTYLEDONS
AND GYMNOSPERMS.

Sect. i8o. The secondary growth in thickness of stem and root differs in a
number of Dicotyledons and Gymnosperms from that which is called normal because
of its occurrence in the large majority of these plants ; it is therefore called anomalous
in the cases in question.

In the first place, we may range among anomalous forms those not uncommon
cases, in which by reason of strongly eccentric growth of the xylem, while the growth
in thickness of the surrounding cortex is almost uniform, the stem or root attain
forms, which differ greatly from the ordinary conical or cylindrical form ; but in
which the other relations are normal. An exquisite example of this series is seen in
the upright stem of Heritiera Fomes, described by Schacht^, which, as far as may be
judged from the description, grows at first in a manner in other respects normal, and
uniformly all round, but later grows in thickness more especially at two opposite
corners, so that it attains the form of a board, e. g. 1 1 ft. broad, and only i in.
thick. Such phenomena appear in very striking form in climbing stems : species of
Cissus, and Piper with strap-shaped sterns^; Cassia quinquangulata® with five or more
marked prominences, as seen in transverse section, each of these being opposite to
one of the orthostichies of leaves ; Lantana spec. ^ with four longitudinal prominences
separated by deep furrows, and alternating regularly with the leaf-insertions, &c.
In the above-named climbers there often appears a splitting of the whole stem at the
furrows when it grows old. Again, allied but less regular inequalities occur not
unfrequently in roots, and are characteristic, e. g. in those of Ononis spinosa
described by Wigand®. All these appearances, considered anatomically, are nothing
more than extreme instances of the widespread phenomenon of unequal eccentric
development of the woody layers, which appear as specific peculiarities in the
examples cited, while in other plants they may appear as phenomena peculiar to the
individual, or induced by definite physiological causes. They may therefore be
excluded from the subject now under consideration, reference being made to Sects.
138 and 140.

Sect. 181. A special anatomical treatment is, however, demanded by those

* Lehrb. I. p. 344. “ Criiger, Botan. Zeitg. 1850, p. 121.

® Ibid. 1851, p. 469. ^ Fr. Müller, Botan. Zeitg. 1866.

® Flora, 1856, p. 673.

558

SECONDA RF CHANGES.

anomalies of growth, which differ from normal cases in having some other arrange-
ment of the initial layers which maintain growth, or some other distribution of the
tissues, or in showing special phenomena of dilatation. It must be maintained from
the first that even the anomalies to be described are derived from ordinary initial
structures, which are usually formed according to the normal Dicotyledonous type ;
that in their case also we have to do with a further formation of secondary wood
and secondary bast, which are derived from the same kinds of tissue as the normal
formations ; that their formation arises from secondary meristems and cambial layers,
which, when present, behave fundamentally like, or very similarly to such normal
tissues ; and that, finally, the phenomena accompanying an increase by cambium,
such as dilatation, formation of periderm, &c., also in themselves resemble those of
normal examples. Hence also the same terms will be used, and in the same sense
as in the foregoing paragraphs, except when special modifications are described by
special terms.

The relations now under consideration differ from one another and from normal
cases, both qualitatively and quantitatively, to a very variable degree, and are con-
nected one with another, and with normal cases by various intermediate forms. If
the latter be neglected, we have to deal with the following main phenomena.

1. Anomalous distribution of the tissues in zones of wood and bast, with
normally derived, normally arranged, and permanently active normal cambium :
Sects. 182-186.

2. Abnormal formation and arrangement of cambium, wood, and bast: —

(a) Besides the normal cambial ring a second appears, concentric with the
first, at the inner limit of the ring of wood; Sect. 187.

(d) In place of the one normal cambial ring in the ring of bundles, there
appear round the primary vascular bundles several separate cambiums side by side,
either one round each vascular bundle, or one round each group of vascular bundles.
Their position between the xylem and phloem of the bundle or bundles, and the
arrangement of the secondary products of wood and bast relatively to the latter,
resembles that in normal cases. Sects. 188, 189: rarely it is inverted. Sect. 190.
Their productiveness is (compared with c) permanent. To distinguish them from
the normal general cambiums they may be called partial, and the rings or zones of
secondary growth derived from them partial thickening-, wood-, or hast-rmgs in contrast
to the normal general ring. For shortness’ sake, the woody ring is often simply
spoken of when the whole ring is meant.

(r) Renewed thickening zones. The increase in thickness begins normally, then
stops, and is continued by a new cambial zone, which is formed from secondary
meristem in the parenchyma outside the first, and this, as well as an unlimited
number of successive ones, may behave like the first, and like it be replaced by
a fresh one. The successive zones, or cambial layers are nearly concentric, and
arise in centrifugal order ; their arrangement and productiveness are normal, as long
as they continue : Sect. 19 1.

{d) Extrafascicidar cambium. The cambial zone does not, as in normal cases,
pass through the primary bundle-ring, but lies entirely outside it : the arrangement of
the products of its activity differs from the normal: Sects. 192 and 196.

3. Abnormal dilatation of the internal old parenchyma belonging to the xylem,

ANOMALOUS THICKENING IN DICOTELEDONS AND GYMNOSPERMS. 569

usually connected with the appearance of new zones of inlercalary wood, bast, and
cambium derived from secondary meristem ; Sects. 193, 194.

All these chief phenomena often show not only reciprocal approaches to one
another, but often occur variously combined. The following description of con-
crete instances is arranged as nearly as may be according to the above heads,
but cannot be classified exactly in that way, without relinquishing all clearness,
and incurring endless repetition. The paragraphs specified under the single heads
usually indicate those points only where the most imporiani examples of them are to
be found.

The consequences of increase in thickness as they affect the changes of the
peripheral parts, dilatation, distortions, secondary metamorphoses in the cortex, and
formations of periderm, are in general the same, and varied in the same manner
in anomalous stems as in normal secondary thickening. They have been but little
investigated in detail. They will therefore be only shortly noticed as opportunity
offers in the following description.

Where cambium and bundles of wood and bast develope from secondary
meristem, the latter always arises by division of parenchymatous cells, which are as
usual for the most part relatively short, while the above-named tissues are composed
of elongated elements. Those secondary formations must therefore induce distortion
and displacement of the tissues present. With the exception of the one case, to be
noticed below, of Cocculus palmatus, investigated and discussed by Radlkofer,
Nageli, and Eichler, no exact investigation has been made of these phenomena.

As is the case with almost all anatomical peculiarities the anomalies of secondary
thickening also are in part evident phenomena of adaptation, and may in part even be
explained directly as the outcome of mechanical causes ; they are in part unexplained
anatomical characters, which must be regarded as inherited. To the first category
belong the anomalies of twining and climbing liane-stems from the most different
families, whose non-climbing congeners have normal growth, as in the Bignoniaceae,
Sapindaceae, Leguminosae, Malpighiaceae, and others to be named below. The
Hanes from certain families, especially the Sapindaceae, or at least the majority of
them, show very special peculiarities. On the other hand, a remarkable uniformity
is often seen between those belonging to the most different families, as e. g. Meni-
spermum and Gnetum, Bignonia and some Apocynaceae, &c. To the second category
of unexplained anatomical characters belong the phenomena to be described in the
Chenopodiaceae and their allies, in Strychnos, Avicennia, &c. It is sufficient to refer
shortly to this, and to dispense with further observations, till exact investigations of
individual cases have been published.

Sect. 182. As the first and simplest instance of anomalous disiribution of tissues
where the catnbitun is normal may be mentioned the case, in which the production of
wood alongside of the latter is unequal in successive longitudinal bands, while the
production of bast is more active opposite those bands where the development of
wood is less. The wood, therefore, when considered separately, appears excentrically
unequal in some definite form, or furrowed, in transverse section notched and lobed.
The eccentricity is, however, equalised, and the furrows filled up by correspondingly
large and specially formed masses of bast, and the whole form of the stem or root
differs from that of the wood. Leaving cases of very slight inequality out of account,

57°

SECONDARY CHANGES.

the root of Polygala Senega ' belongs to this series : here, as a rule, the wood grows
strongly in thickness only in one approximate longitudinal half, the bast in the other.
The former, as seen in transverse section, attains the form of a half-circle with the
periphery turned towards its stronger growing side, or of a circle with a broad piece
cut out of it on the weaker side. The transverse section of the bast appears as
a narrow segment of a ring surrounding the more strongly grown half of the wood,
on the other side as a large segment of an ellipse. In the dried root the stronger band
of bast projects as the well-known keel.

Sect. 183. The inequalities in question appear in much more characteristic form
in the stems of lianes with deeply grooved wood, and with bast-plates which project
into the grooves : Bignoniacese, Phytocrene, and others to be named below.

In the cYimhmg Bignoniacece'^, Figs. 224-226, the secondary growth of wood
and bast is derived from a normal ring of cambium, and begins with the formation
of a zone of wood and bast, of normal structure and equal thickness all round.

Fig. 224. Fig. 225. Fig. 226.

FIG. 224. — Anisostichus capreolata Bur. Transverse section through a four years’ old branch, X 3. The annual rings
and medullary rays are represented in the wood by radial bands, the bast-layer and bast -plates by concentric lines. The
elongated figures in the peripheral cortex and the dark lines in the latter and in the four plates represent the fibrous
bundles ; /"the four larger ones. The pith is dotted.

Fig. 225. — Transverse section through a non-identified Bignoniaceous stem (Pleonotoma?)» from Schleiden, Grundz.; nat.
size.

FIG. 226. — Melloa populifolia Bur. (Bignonia No. 17, Fr. Miiller, Botan. Zeitg. 1866). Transverse section through a
branch, X 2. Bast and bast-plates shaded, wood left white. Tlie four first bands of wood, which have remained behind,
marked thus ( ') ; medullary sheath shaded radially, pith white. The dotted parts round the medullary sheath are large
islands of parenchyma, the subsequent dilatation of which splits up and bursts the wood.

i. e. like a ring. The latter is limited from the primary outer cortex by an inter-
rupted zone of fibrous bundles. In each internode there are four bundles of this
zone {/), which are from the first larger than the rest. They lie always, according
to Crüger, in four planes, which alternate regularly with the orthostichies of leaves,
and are perpendicular with reference to the straight internode : in transverse section
they are arranged crosswise.

At the beginning of the secondary growth in thickness, the increase of the wood
in the longitudinal bands opposite the four large crosswise fibrous bundles is left

^ Walpers, Botan. Zeitg. 1851, p. 297; Wigand, in Flora, /. c.; Figures in Wigand, Pharma-
cognosie, and Berg. Botan. Zeitg. 1857, Taf. I ; Atlas, Taf. VIII.

^ Gaudichaud, Rechercbes, &c., sur I’organographie, &c., des vegdtaux (Mem. prdsentees ä
I’Acad. des Sciences, tom. VIII), Paris, 1841 ; Idem in Archives de Botanique, II. (1833). — A. de
Jussieu, Monogr. des Malpighiacees ; Archives du Museum, tom. III. (1843). — Mettenius in Lin-
nsa, Bd. 19 (1847).— Schleiden, Grundzüge, 3 Aufl. II. 165. — Crüger, Botan. Zeitg. 1850, p. 101. —
V. Mohl. Botan. Zeitg. 1855, p. 875, — Bureau, Monogr. des Bignoniacees, Paris, 1864, p. 120 ; Id. in
Bullet. Soc. Bot. de France, 1872, p. 14. — F. Miiller, Botan. Zeitg. 1866, p. 65. [Westermaiern.
Ambronn, Lebensweise u. Struktur d. Schling — u. Kletterpflanzen. Flora, 1881.]

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 57 1

behind by that of the rest of the circumference, while the converse holds in the
production of secondary bast. While in other respects the Avood increases as usual
among the woody Dicotyledons, and remains surrounded by a relatively narrow layer
of bast, it first appears depressed in the four longitudinal bands indicated, and is soon
interrupted by furrows limited by flat lateral surfaces, which increase in depth as the
thickening proceeds : the wood thus becomes four-lobed in transverse section :
the groove is, however, exactly filled up by a bast-plate which runs out externally
into the original bast, so that the cylindrical or ribbed surface of the stem suffers
no important change of form through the unequal growth of the wood. The
cambial zone surrounds, on the one hand, the four prominent portions of the wood
as far as the margins of the bast-plates, and on the other hand, the outer surface of
the four bands of wood of which the growth is suppressed : it forms at both points
both wood and bast, but in unequal quantity as described. On the lateral surfaces
of the bast-plates, from the margins to the inner surfaces of the furrows, it is inter-
rupted; as soon as the inflexion of the wood begins, the cells at the margins of the
grooves lose their cambial properties, and assume those of parenchyma of the
medullary rays. At each margin of each of the eight sections of the cambial zone
thus separated, the cells immediately adjoining it form henceforth only parenchyma
of medullary rays ; those of the margin of the projecting portions of wood form
especially cells of medullary rays of the xylem in centrifugal succession, those at the
base of the groove forming cells of the bast rays in centripetal succession.

The lateral surfaces of each bast-plate are therefore limited by one pluri- or
multi-seriate medullary ray, which may be divided into one radial portion belonging
to the bast-plate, which grows centripetally, and one belonging to the contiguous
projection of wood, which grows in the main centrifugally. As a result of the
non-uniform progression of its growth, a continuous displacement goes on between
the two radial portions, in other words, between the lateral faces of the bast-plate
and the adjoining ones of the woody projections ; the two faces are not grown together
one with another ; in transverse sections, even of fresh internodes, a slit-like space
often appears between the two ; the bast-plate is in close tissue-connection with its
surroundings only at the outer and inner sides. But, as has been already said, there
is also a slow growth of wood from the portions of cambium at the base of the
groove, and accordingly as this goes on, there succeeds a firm coalescence (and
usually lignification) of the radial portions of the medullary ray, as far outwards as
the formation of wood extends.

The finer structure of the wood shows no specially remarkable peculiarities in
those species, which have been more exactly studied. Their bundles consist, e. g. in
Bignonia (Anisostichus Bur.) capreolata ^ of wide-pitted vessels, narrow spirally
thickened pitted vessels, and vessel-like tracheides, wood-fibres, paratracheal bundle-
parenchyma, and intermediate fibres : they are traversed by numerous one- or few-
layered medullary rays, to which may be added the four at the limiting faces of the
four portions of wood which are superseded. These latter portions, besides their
limited development, are distinguished from the rest of the wood in the species
named by their denser structure, resulting from the absence or paucity of wide pitted

^ Compare Sanio, Botan. Zeitg. 1863, p. 407.

572

SECONDARY CHANGES.

vessels. This difference between the superseded and the projecting bands of wood
is absent in other species. The genus Clytostoma is, according to Bureau, distin-
guished from most others by very dense compact wood, with very narrow vessels ;
also, the cortex is generally of normal structure. The bast consists of normal soft-
bast, which is traversed by narrow concentric continuous zones of bast-fibres. In
the bast-plates the first-formed secondary zones of soft bast are specially narrow,
and but little thicker than the zones of fibres alternating with them ; those which are
formed later are often on the average of much larger, but alternately variable, radial
diameter. The soft bast consists, especially in the plates, of sieve-tubes, mentioned
in Chap. V, which are usually wide, and are surrounded in the usual way by delicate
cambiform cells. Here also there are various special differences according to genera
and species.

The phenomena hitherto described are uniform for all climbing Bignoniacese
with four intruding plates of bast. Exceptionally, and as an individual peculiarity,
the number five is found instead of four. As regards further conditions, there are
variations which are in exact correlation with the generic differences founded on the
formation of flowers and fruit.

The number of the superseded bands of wood and intruding bast-plates in a
number of genera remains always limited to four as described. In numerous others,
on the other hand, in addition to the four primary plates there appear successively
new ones in addition as the growth in thickness proceeds : these from their origin
onwards behave in all points like the similar primary ones, and in ordinary regular
cases are so arranged that each bast-plate of next higher order divides the protrusion
of wood, on which it appears, into two almost equal lobes (Fig. 226). All bast-plates
of the same rank arise almost simultaneously, and therefore intrude an almost equal
distance inwards, the lobes of the wood, successively increased from 4 to 8, 16, 32,
therefore show a regular dichotomous division and arrangement. When the stem
has grown very old, these conditions may become less regular.

In many genera each single bast-plate has exclusively the phenomena of growth
above described, and therefore always remains equally broad throughout. In others,
however, the plates become gradually broader outwards in the following manner :
opposite each protruding mass of xylem, after it has grown in thickness to a certain
extent, the radial band limiting the bast-plate itself hangs back in its growth, while
the portion of cambium adjoining it forms a narrow bast-plate in the manner above
described, and is joined laterally on to that first present. Since the same process is
repeated periodically — perhaps with each annual layer — as the growth in thickness
proceeds, each bast-plate becomes broader outwards, by step-like increments on
both sides. Each step-like portion of it, as well as the superseded radial band of
wood belonging to it, has the properties above described for the primary ones.
According to the individual case, each successive order of steps arises at the two
sides of one plate, and of all the plates of one transverse section of a stem at a more
or less equal distance from the middle ; the wEole arrangement of steps is therefore
to a variable extent regular or irregular. Comp. Figs. 225, 226.

The widening of the bast-plates outwards, as described, is continuous, if the
segments of wood, which adjoin the successive steps, widen in their further increase
in such a way that the limiting faces remain exactly radial (Fig. 225). But in

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 573

certain genera it happens that the segments of wood at the sides of the bast-plates
grow during the increase in thickness, not only in a radial direction, but also
tangentially, and become successively broader, so that they press against the bast-
plate, and squeeze it together, displacing and destroying its elements. The same
process is repeated in the successive steps, so that old plates are alternately broader,
and reduced to quite narrow bands by step-like corners, which encroach simul-
taneously upon them, those from opposite sides often coming into contact, and
even overlapping one another. Comp. Fig. 226.

In addition to these diversely varied phenomena there appear finally in the old
stem in certain genera, sometimes concentrically renewed rings of growth, sometimes
splittings of the first by subsequent intercalary growth ; this will be discussed in
later paragraphs.

The roots of the plants in question have, according to Criiger, in many species the
same lobes, with intruding bast-plates as the stems, with this difference, that the number
and arrangement of the lobes and plates are less regular. Bureau’s statements contradict
this, even in Bign. Unguis, which is specially quoted by Criiger, since according to the former
bast-plates do not occur, but the thick masses of the xylem composed of vessels and
wood fibres are only variously cleft by bands of parenchyma, which sometimes intrude
radially, while sometimes they are arranged transversely, and connect the radial bands in
a reticulate manner. In the tuberous sw^ellings of the roots, which are characteristic of the
genera Glaziovia Bur. and Bignonia Bur., the parenchyma is largely developed between
relatively small vascular and fibrous strands. In these cases therefore there is simply the
structure of parenchymatous roots. Also in branch-roots tw'O to three years old of An.
capreolata I do not find the characteristic stem structure. The xylem is in transverse sec-
tion only slightly undulated, composed of sclerotic elements, with medullary rays of one
or a few rows of cells; the ba.st-layer is thin, with few relatively small sieve-tubes, and
small scattered groups of fibres at the outer limit ; the cortex, as noted on p. 547, is per-
sistent, and composed of thin-walled parenchyma surrounded by a superficial periderm.

The generic anatomical characters of the stems of the Bignoniacese, founded by Bureau,
are based partly on the combination of the different phenomena above described, partly
on special conditions of structure of wood, cortex, and periderm; and, finally, on the
general form of the stem. Since it is of interest to have a complete knowledge of the
differences of structure and growth, which obtain within a narrowly circumscribed group
of nearly allied plants, corresponding in their mode of life and adaptation, and which are
in correlation with the generic characters taken from the reproductive organs, we may
here briefly reproduce Bureau’s synopsis^, awaiting further explanation of the cases
marked *.

I. Stem always with only four intruding bast-plates.

A. Bast-plates always equally broad throughout. (Comp. Fig. 224.)

(rt) Without subsequent renewed formation of wood in the cortex.

1. Surface of the cortex with a thin periderm: Arrabidxa DC.

2. Surface of the cortex with a thick layer of cork when old : Paragonia, Bur.
{*b) With subsequent renewed unilateral formation of wood in the cortex and

irregular form of the stem resulting from it : Callichlamys, Miq.

B. Bast-plates widened in a step-like manner outwards. (Comp. Fig. 225.)
a. Stem cylindrical or ridged without channels on the ridges.

(a) Without stony elements in the cortex.

a. Steps of the bast-plates broad, i. e. of the width of several segments of
wood separated by small medullary rays.

Compare Bull. Soc. Bot. de France, /. r.

574

SECONDA Ry CHANGES.

1. Stem cylindrical or hardly four-cornered, bast-plates remaining a long

time of the same breadth, then with high irregular steps : Petastoma
Miers.

2. Stem cylindrical, with fine longitudinal furrows, subsequently thrown off.

Steps of the bast-plates appearing late, broad, and few in number :
Stizophyllum Miers.

3. Stem cylindrical with four narrow furrows. Bast-plates broad. Steps

present from an early age, as broad or broader than high : Cuspi-
daria DC.

4. Stem, at least when old, four-cornered ; cortex uneven, with many

lenticels. Bast-plates with regular steps, and one broad medullary
ray between each two step-like bands : Tjnanthus Miers.
ß. Steps not broader than the space between two medullary rays of the
wood. Stem cylindrical. Bast-plates with a very broad middle lamella :
Fridericia Mart.

(b) Stony sclerenchyma in the cortex.

1. Stem somewhat flattened opposite the bast-plates. Bast-plates short,

somewhat widened. Steps not broader than the space between two
medullary rays of the wood. Stony elements only distributed in the
outer cortex, and in small quantity in the primary bast : Tanxcium Sw.

2. Stem cylindrical. Steps of the bast-plates few. A continuous ring of

stony elements 6-7 layers thick beneath the surface : Adenocalymma
Mart.

b. Stem when young with four projecting ridges, these are later thrown off, and
replaced by channels (comp. Fig. 225). Steps of the bast-plates broad,
irregular: Pleonotoma Miers.

C. Bast-plates partially constricted and destroyed by pressure through tangential
widening of the zones of wood. Young branches octagonal. Angles thrown
off later. The old stem cylindrical. Cortex thick: Pithecoctenium Nlz,rt.

II. Bast-zones successively 4, 8, 16, 32.

A. Wood always without displacement or separation of the older zones through
subsequent intercalary formations,
a. Without subsequent formation of wood in the cortex.

(<2) Young shoots cylindrical, without ridges which may be thrown off.
a. Stony elements in the outer cortex, and very numerous in the outer region
of bast. Bast-plates with very irregular steps : Phryganocydia Mart.
ß. Without stony elements in the cortex.

t The four first bast-plates long and narrow, the rest short and broad.

Cortex with red colouring matter : Cydista Miers.
tt All bast-plates of similar form. Cortex without the red colouring.

1. Stem twisted like a rope, with eight external rounded projections. Bast-

plates of successive age differing little in length, all very narrow and
with few steps ; the four first with broad closely arranged medullary
rays externally. Cortex without sieve-tubes ’ : Pyrostegia Pr.

2. Stem not rope-like. Wood very dense, with very narrow fibres and

vessels. Bast-plates very numerous, steps high, broad, irregular,
medullary rays between the middle and the lateral lamellae of a plate
hardly any broader than the rest. Pith very narrow : Clytostoma Miers.

3. Stem not rope-like, or only slightly so. Texture of the wood loose and

porous. Steps of the bast-plates very few. Medullary rays between

* ‘ Pas cle cellules grillagees dans I’ecorce ’ (? Author).

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 575

the middle and the lateral lamellae broad and obvious. Pith broad,
breaking down in the middle: Anemopxgma Mart.

4. Stem not rope-like. Numerous bast-plates with very narrow steps and a

broad medullary ray on each side of their middle lamella : Lundia DC.

5. Stem rope-like. Wood with wide vessels. Bast-plates variably con-

stricted, and destroyed by pressure by the adjoining steps of wood
(sometimes formation of wood in the cortex, opposite the bast-plates*) :
Distictis Mart.

{b) Young shoots with six ridges thrown off later: old ones cylindrical, cortex
thin. Numerous very unequal bast-plates with irregular steps : Amphi-
lophium Kth.

(b)* Stem with concentric (renewed) rings of wood in the cortex.

1. Stem not rope-like or slightly so. Medullary rays of almost equal breadth :

Haplolophium Cham.

2. Stem rope-like. Medullary rays of very unequal width : Gla%io‘via'Y>m.

B. * Inner layers of wood split up in the old stem by subsequent intercalary forma-
tions of parenchyma, wood, and bast.

{a) Wood with annual rings. Segments of splitting few. In the bast-layer are
wood bundles, at first fan-shaped in transverse section, but soon rounded
off: Anisostichus Bur. (Figs. 224 and 237.)

{b) No annual rings. Segments of splitting numerous, repeatedly cleft, between
them newly formed parenchyma. The segments at the middle of the stem
turned in all directions.

1. Segments of wood at the middle of the stem three-cornered, undivided, all

others dichotomously split: Melloa Bur. (Fig. 226.)

2. All wood-segments, both those in the middle and those at the periphery of

the stem, dichotomously split : Bignonia Bur.

Sect. 184. The structure and growth of the internodes of species oi Phyiocrene^
correspond in their main features with those of the climbing Bignonias. The differ-
ences between the two consist partly in differences of the more minute structure of
wood and cortex, partly in this, that the number of the original protrusions of wood
and intruding plates of bast is not four, but usually eight in the specimens examined,
more rarely thirteen, numbers which appear to vary with the individual. Subsequent
divisions of the original protrusions of wood by secondary plates of bast have not been
observed, but on the other hand there are sometimes pauses in the activity of the first
cambium, and zones of renewed growth then appear in the cortex. Comp. Sect. 19 1.

As regards the relations of structure the following facts may be brought forward,
reference being made to the description of Mettenius, and Mohl’s supplement to it.
(Fig. 227).

The pith is surrounded by a narrow uniformly thick, ring-like zone of wood (me-
dullary sheath), which includes between the slightly thickened cells the primary vessels
arranged in radial rows, usually in pairs opposite the protrusions of wood. This zone
passes over externally into one which is also annular, and is composed of numerous
successively wider pitted vessels, thick-walled fibrous elements, fascicular parenchyma,
and narrow medullary rays. This zone of pitted vessels, retaining fundamentally the

’ Griffith, in Wallich, plant. Asiat, rarior. III. p. 216, after Lindley, Introduction to Botany,
p. 69. — A. de Jussieu, l.c. — Treviranus, in Botan. Zeitg. 1847, p. 400. — Mettenius, Beitr. z. Botaiu
p. 50. — V. Mohl, Botan. Zeitg. 1855, p. 878.

SECONDARY CHANGES.

576

same structure, is continuous externally into the equidistant protrusions h, which
are wedge-shaped in transverse section. With the exception of their inner portion
which belongs to the medullary sheath, the intervening bands of wood, which hang
back in their growth in thickness, consist, on the other hand, for the most part
of thin-walled cells arranged in radial rows, amongst which are scattered isolated
pitted vessels, each being accompanied by a few thick-walled elements. The bast
plates (<5) consist in the main of the large, sharply ended sieve-tubes mentioned
in chap. V ; these lie very regularly in radial or tangential rows, being usually
separated from one arwother in a radial direction by single, triple, or quadruple
concentric series of narrow elements (Fig. 228), rarely they are arranged con-
tiguously in pairs ; in a tangential direction, however, they are usually in immediate
connection, being rarely separated by the narrow elements (Fig. 228). These narrow
elements are for the most part sclerenchymatous fibres, less frequently delicate
cambiform cells, of which usually 1-2 abut on each sieve-tube, as seen in transverse
section. The peculiar distribution of the thick-walled and other elements causes

I

Fig. 227.— Phytocrene spec. Transverse section of the stem, X 2, from the same material as that described by Met-
tenius ; h tlie eight protrusions of the porous wood ; b the bast-plates between them. Laterally from one of them, and to
the right of the wood*protrusion marked k, are two small similar ones. Parenchyma of pith and cortex are left white. Ex-
ternally to the inner circle of growth is one of renewed growth, which shows several bundles, between broad parenchy-
matous medullary rays, opposite each of the inner protrusions of wood; these consist of a relatively large xylem, con-
structed similarly to the latter, and an extremely small bast portion ; and opposite each of the inner bast-plates is one
bundle, or at most two to three, constructed like the latter, which have only a very small xylem at their inner side. The latter
could hardly be indicated in the Fig.

Fig. 228. — Portion of a bast-plate front Fig 227 (X about 140) ; s — s sieve-tubes. The narrow meshes of simple contour
abutting on these are transverse sections of the cambiform cells ; the elements with a strong double contour are bast fibres.

the almost chess-board-like appearance of the plates ; at the outermost oldest parts of
them the arrangement is less regular, the sclerotic elements are in larger proportion.
A multiseriate broad medullary ray, consisting of very delicate cells, limits each lateral
face of the bast-plate, and is continuous towards the pith along-side of the superseded
band of wood. It appears, as in the Bignonias, to consist in not very young specimens
of two radial portions, which undergo displacement. There are at least indica-
tions of a step-like external widening of the bast-plates. Externally from the zone
of cambium surrounding the protrusions of wood, that is in the normally arranged
layer of bast, there are scattered irregularly, here and there, small irregular groups
of the same sort of tissue as that which composes the bast-plates. With the
exception of these groups it appears from transverse sections that the bast layer
in question contains no sieve-tubes in by far its greater part, but is composed
rather of thin-walled parenchyma only, with isolated thin sclerenchymatous fibres.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 577

The above description is derived partly from Mettenius’ account, partly from
the same transverse sections of the stem as the latter was based upon. Some other
transverse sections, and the figure in Lindley /. c., show a structure which differs in
many details from that described. But whether we have to deal with individual or
specific differences is uncertain, since there were no definitions of the species in the
material used. According to Jussieu^ there is also in Phytocrene a formation of
dichotomous lobes of the protrusions of the wood.

Sect. 185. Similar phenomena to those in Bignonias and Phytocrene, with
similar differences to those between these plants, are found developed to a different
extent in the climbing shrubs of different other families : within the cylindrical or
slightly ridged stem are flat, or very deep infoldings of the xylem, and bast-plates of
corresponding form, which fit into these. Disregarding certain of the cases in question
which are not defined with certainty, and are not very typical, we may here name
members of the Malpighiaceae, from the genera Tetrapterys, Banisteria, also Stig-
maphyllon and Peixotoa sp. Apocyneae of the genera Condylocarpon Echites ;
the Asclepiad Gymnema silvestre, a species of Celastrus®, and a Tournefortia®.

The Peixotoa (/. c. Fig. 2) shows eight blunt low protrusions of the xylem, within
the cylindrical outer cortex, which is surounded by great rent masses of cork or bark.
Miiller’s Tetrapterys (/. c. Fig. i) shows very similar phenomena to those in the Bignonias
above noticed, p. 574 under II. In the younger round stems or branches, up to i®"" in
thickness, the xylem has six sharp incisions, filled by bast-plates ; in stronger stems there
appears in addition to these, and accompanying their radial development, a further single
or double splitting of the protrusions of xylem by intruding bast-plates, that is a
formation of dichotomous lobes of the transverse sections of the xylem. The older parts
of the bast-plates are constricted and pressed together by tangential extension of the
adjoining portions of xylem. It is uncertain whether the radial lateral faces of the
bast-plates remain surrounded by a layer of cambium.

In Miiller’s Condylocarpon {l.c. Fig. 4) the transverse section of the xylem of young
branches up to i®™ in thickness is round, and equally thick on all sides. Then begins
the formation of incisions and of bast-plates fitting them — the number of those which
appear at first and simultaneously seems to be three ; in addition to the first, which
grow further in a radial direction, numerous fresh ones of similar character successively
arise, so that the whole system shows numerous irregular dichotomous lobes as seen in
transverse section. Here also there is found in older inner parts, for certain distances,
contraction and complete distortion of the bast-plates by swollen protrusions, which
subsequently appear at the lateral faces of the protrusions of xylem ; here also it remains
to be investigated whether they arise from a cambial zone which is permanent from
the first at this point, or from one which appears subsequently.

Sect. 186. An anomaly in the distribution of tissues, which goes a step further
than those hitherto spoken of, consists in the following peculiarity of certain woody
plants, viz. that they develope rings of wood, cambium, and bast in their normal
position, and without specially remarkable relations of form, but develope no sieve-
lubes in the secondary bast, these being united with delicate parenchyma in bundles,
and enclosed in the xylem.

^ 1. C. p. 12 2.

’’ Fr. Müller, l.c.

•' Jussieu, l.c. p. 117.

“ A. de Jussieu, l.c. p. 106.
* Idem, l.c.

® Criiger, Botan. Zeitung. 1851, p. 468.

P p

57^

SECONDARY CHANGES,

This phenomenon occurs in Slrychnos, in all investigated species of the genus,
both in those which climb and grow with tendrils — S. colubrina, toxifera, multiflora,
and that not exactly defined form figured by F. Müller' — and also in the tree- and
shrub-like species — S. nux vomica, brachiata, and innocua. It appears further, though
in a different form to that in Strychnos, in that member of the Malpighiacese
mentioned by F. Müller ^ as Dicella spec.

The above-named species of the genus Strychnos were investigated in dry
material ; for the investigation of the youngest stages of development a living
specimen was used of the plant, which comes to the shops as S. nux vomica, a
plant which really belongs to another species.

The species of Strychnos have a normally arranged ring of bicollateral bundles
of the trace in the young internode. Their externally directed (external) phloem, in
which, on the first differentiation of the bundles, the first developed elements (Proto-
phloem) may be recognised, consists in the fully elongated internode of small groups
of about 4-6 narrow elements, which are very similar in transverse section to small
groups of sieve-tubes. But I will not say definitely whether they really contain
developed sieve-tubes. After those primitive phloem-elements the first vessels appear
at the inner margin of the xylem, and almost simultaneously with them the phloem-
groups of the inner margin begin to become plain. The further development of the
xylem then proceeds in the manner normal for Dicotyledons : in the inner phloem-
groups the increase and growth of the elements continues for a long time, and they
attain a great size. Numerous intermediate bundles, separated from one another
by medullary rays one layer of cells thick, then connect the bundles of the trace into
a dense ring. The intermediate bundles contain also, at least in part, the small ex-
ternal phloem-groups, but no internal ones : at least they are certainly absent in most
cases. An increase of the elements of the small external phloem, in the same way
as in the typical Dicotyledons it keeps pace with the growth of the xylem, does not
occur in Strychnos ; but rather that layer of cells immediately adjoining those small
external phloem-groups, or at least the next inner one, passing round the stem,
becomes the mother-layer of the cambium. The origin of this, as could be proved
with the small quantity of material available, is fundamentally the same as in normal
Dicotyledons ; the same is the case with the direction of the divisions which pro-
duce the secondary elements. In the succession of these, however, and in the de-
velopment of their products, the peculiarity occurs that they at first proceed almost
exclusively in a centrifugal direction : it is exclusively or almost exclusively secondary
v.'ood that is developed. In a one-year-old shoot, thick, of the living plant,
the xylem-ring of which showed on the radius of a transverse section 10-12 elements,
and among them wide vessels, there lay in each radial row, between the outermost
mature xylem elements and the small external phloem-groups, only two cells,
separated by one tangential wall, or even only one thin-walled cell. These belong to the
cambium and zone of increase, the latter is thus directly contiguous with the primary
zone of bast : it forms with this a narrow ring round the xylem, limited externally by
a dense layer of short sclerenchyma. This condition remains for a long lime; in a
dry branch of S. nux vomica thick I find the bast layer not at all or but little

^ l.c. Fig. 10.

2 1. c. p. 59.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 579

thicker. But later a change takes place : a dry piece of a stem or branch of S. nux
vomica of 135“™ diameter, which was investigated, had a bast zone about 5“'“ thick,
consisting of very many layers. Climbing species, such as S. toxifera, S. brachiata,
&c., seem to form secondary bast in still larger quantities. The secondary layer
of bast has, except in one essential point, the same structure, and shows the same
phenomena of dilatation as in the Dicotyledons with normal growth. Especially in
S. nux vomica, it consists of broad, uniformly delicate, parenchymatous medullary
rays, without stony elements, and between these run narrow bands, corresponding
to the strands of xylem, each consisting of a few longitudinal rows of elongated cells
with oblique or horizontal ends, soft, rather thick, lateral walls, with simple scattered
pits, and delicate transverse walls. The strands are accompanied by numerous
chambered sacs with crystals. Both bast fibres and sieve-tubes are entirely absent.
The sieve-tubes are on the other hand situated in the wood. This has in the main the
normal structure of Dicotyledonous wood.

(i) broad, numerous, medullary rays, com-
posed of procumbent cells; (2) narrow
strands, and portions of strands of different
grades, which are composed of irregularly
alternating broad transverse zones of very
thick and long wood-fibres on the one hand,
and large-celled fascicular parenchyma with
pitted vessels on the other. In the mass
of wood, which by reason of this com-
position appears, in a slightly magnified
transverse section, to be marked with ir-
regular bands, there lie numerous strands,
on an average about 0-30™“ thick, with a
roundish, or broadly elliptical transverse
section. They are scattered through the
whole annual ring, being usually isolated
between two medullary rays, and rather
broader than the section of wood which
bears them, so that they intrude on both
sides into the adjoining medullary rays;
sometimes they also pass transversely
through 2-3 strands of wood, together
with the intervening medullary rays. Their longitudinal course follows in the
main that of the xylem-strands, with this difference, that they show fewer acute
anastomoses than these. These strands are composed of longitudinal rows of
thin-walled colourless cells — and their circumference seems to be covered exclusively
by these, — and numerous large sieve-tubes, with oblique endings of the joints, bearing
sieve-plates arranged in a ladder-like manner. Suitable stages of development show
easily that these phloem-strands in the xylem are derived internally from the cambium
(comp. Fig. 229). Other species investigated show similar, sometimes even larger
phloem-strands in the xylem, and, as far as may be decided from dry material, the
same absence of sieve-tubes in the bast. All investigated species have, at the limit

p p 2

consists in th'e species in question of

Portion of a transverse section through a branch (20) ; m pith,
c cainbial zone, between the two is the xylem with numerous
large vessels; si bundles of sieve-tubes at the limit of the
pith; s the same in the xylem, the two outermost at the
limit of the cambial-zone, just passing off from it.

5^0

SECONDARV CHANGES.

between bast and outer cortex, a massive persistent ring of strong sclerenchyma,
and usually single sclerenchymatous fibres at the outer side of it. In S. brachiata and
toxifera there are besides large groups of strong sclerenchyma in the secondary
bast. The parenchymatous outer cortex forms in all of them a thick periderm
at the outer surface, and especially in S. innocua, a thick soft mass of cork. On the
structure of the wood of other species it need only be remarked, that it is dis-
tinguished from that of S. nux vomica by the relatively much larger quantity of
thick fibres, otherwise it shows no generally remarkable properties. The details
still require more exact investigation. In allied Loganiacese, I found in Logania
longifolia and floribunda bicollateral bundles of the leaf-trace, but normal bast, and
no sieve-tubes in the xylem. Gaertnera longifolia, Sykesia spec., and Fagrsea
lanceolata show a perfectly normal Dicotyledonous structure, and no bicollateral
bundles of the trace.

Of the Dicella above-named only a few dry pieces of the stem about 5-6"'™ thick,
sent by Fr. Müller, were available; some of these were round, some very eccentrically
developed. The round pith is immediately surrounded by a uniform ring, about eight
cells thick, of narrow ordinary xylem elements, arranged regularly in radial rows.
The other secondary wood consists chiefly of thick-walled pitted vessels and fibres,
with uniseriate medullary rays, partly also of masses of thin-walled tissue composed
of wide parenchymatous cells, sieve-tubes, and narrow sacs with conglomerated crystals,
which are marked off from the thick-walled mass by an almost unbroken layer of
partitioned crystal-sacs with small klinorrhombic crystals. This delicate tissue, with
sieve-tubes, is distributed in the thick-walled vascular tissue in the form of anasto-
mosing flat strands, which appear in transverse section as irregular concentric
segments of a ring, of very unequal size and form, often with sinuous curvature,
and frequent anastomoses. These form bands and zones in the transverse section,
which are embedded in the hard mass of xylem, and are, on the average, smaller
and narrower than the hard portions, which alternate with them ; these phenomena
give the whole transverse section a peculiar, finely banded, almost marbled appear-
ance. As far as could be made out from the dry brittle material, the whole mass
of wood described arises on the inner side of a delicate one-layered zone of cambium.
This is surrounded by a thin cortex, which shows no peculiarities worthy of mention,
beyond the fact that in the secondary zone of bast, which contains narrow scattered
fibres, and many small conglomerated crystals, sieve-tubes were not to be found.

Further details of structure have been in part purposely left unnoticed in the
above notes, both in Strychnos and Dicella, in part they require still further in-
vestigation.

Sect. 187. The appearance of a ring of cambimn and secondary thickening on
the inner side of the external, normally growing ring of wood, has been discovered by
Sanio ^ in the stem of Tecoma radicans. The bundles of the leaf- trace of the normal
bundle-ring^, with the exception of those perpendicularly below the next higher pair of
leaves, are bounded, when young, on the side next the pith, and opposite the primordial
spiral vessels, by a small strand of delicate cells, which remain narrower than those
surrounding them. The innermost group of cells of each bundle, which abut on

\ Botan, Zeitung. 1864, pp. 61, 228.

Compare Nageli, Beitr. I. p. 107.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 58 1

the large-celled pith, and are formed by longitudinal divisions facing in all directions,
soon ceases to grow (single rows of them may develope into sieve-tubes). The outer
layers develope into elongated, afterwards thick-walled parenchyma. When the normal
cambium has begun its growth at the outer side of the ring of wood, radial extension
and tangential division begins in a middle layer, and this process is extended, as in
the origin of a normal cambium, from each group laterally over an annular layer
running along the whole inner side of the ring of wood. This now forms, as
in a normal cambium but in a reversed direction, wood and bast, the former being
affixed to the zone of parenchyma which covers the inner surface of the ring,
and pushing the bast towards the pith. Both products of the inner cambium
resemble in their structure the secondary wood and bast of the normal outer ring,
and have, like them, both medullary rays and annual rings. Their naturally very
limited growth compresses the originally broad pith more and more. It remains
to be investigated how long the process lasts, when it is terminated mechanically,
perhaps after the complete compression of the pith, and what relation it has to
the frequent splitting of old stems.

Figs. 230, 231.— Transverse sections through stems of not exactly defined species of Seijania or Paullinia. Natural size.
From Schleiden, Grundz. a b outer rings ; c the main ring of the compound woody body. The cortex is white in both figures,
the xylem (in the strict sense) dotted according to the distribution of the vessels. Medullary rays obvious in I-'ig. 231, in 230
not obvious ; also the pith is not shown in the outer rings of the latter figure.

Fig. 232. — Serjania caracasana. Transverse section through a young internode just above the node (10). After Nageli.
Within the ring of sclerenchyma s are the main ring h and four outer rings ; to the left below is one outer ring opening into
the main ring. The dark prominences of the rings pointing towards the pith are the primary bundles of the leaf-trace.

Indications of a similar formation appear, according to Sanio\ to occur in
Rumex crispus, but further investigations are necessary on this point.

Sect. i88. Several partial cambiums and rings of growth, lying side by
side in the same transverse section, appear in the most exquisite development in the
woody stems, so often described since Gaudichaud of climbing Sapindaceae of the
genera Serja7tia, Paullinia, and Thinouia. The transverse section of the stems in
question shows several separate rings of wood surrounded by a common cortex, and
in most of them a larger main ring occupying the middle, and several smaller outer
rings arranged in a circle round it (Figs. 230, 231) ; more rarely five, or in exceptional

^ Botan. Zeitung. 1865, p. 179.

^ Recherches, Scc.l.c. Tab. XVIII. — Compare further A. de Jussieu, Monogr. des Malpighi-
acees, Lc. — Schleiden, Grundzüge (3 Aufl.) II, p. 166. — Treviranus, Botan. Zeitung. 1847, p. 393.
— Crüger, Botan. Zeitung. 1851, p. 481. — Schacht, Lehrbuch, II, p. 58. — Netto, Comptes rendus,
torn. 57 (1863), p. 554, and Ann. sei. nat. 4 Ser. torn. 20, p. 166. — Nägeli, Dickenwachsthum d.
Sapindac., compare p. 463. — Radlkofer, Atti del Congresso Botan. ten. in Firenze, 1874, P- öo, and
Monographie d. Gattung Serjania, Mimchen, 1875. [Also, Radlkofer, Entstehung d. sccundäreii
llolzköipei im Stamme gew. Sapindaceen NatiWforscher, Vers. z. Mvinchen, 1877.]

582

SECONDA Ry CHANGES.

cases 6-7, peripheral rings without the central main ring. Radlkofer calls the first-
named structure a compound x}dem, the second a divided xylem. In both cases each
woody ring is surrounded by a normal, permanently active zone of cambium, and
also by a normal layer of bast. Each encloses a pith, which is often inconspicuous,
immediately around which Jussieu showed that there were spiral vessels. Following
them in a longitudinal direction, it is found that the wood- and bast-rings run
side by side in the internodes ; at the nodes they are connected one with another,
in a manner to be described below, so that as regards the compound xylem the
opin’on might arise that the small outer ones might be the wood and bast of
axillary branches joined longitudinally to the main stem. The composition de-
scribed is found in the stems from the commencement of differentiation of tissues
onwards. (Fig. 232.)

We are indebted to Nageli’s investigations of the development of a number of
forms with compound xylem for a proper understanding of this structure.

The main results of these are summarised below, partly in the author’s own
M’ords : we must forego even a short statement of the individual phenomena in one
species, and the differences in different species, because of their complication,
reference being made to the original work.

The stalks or stems of the plants in question are ridged from the very first ;
the leaves are spirally arranged ; from the axil of the leaves arise a branch and a
tendril as axillary shoots. The course and development of the primary bundles
correspond to the main rules which hold generally for Dicotyledons (Chap. VIII).
At each node three bundles of the trace, and two axillary bundles of the trace, enter
the stem. They show generally a tangentially-oblique course, which deviates more
or less from that of the angles of the stem. The median bundles have a tendency
to unite into three sympodia. Both sorts of bundles of the trace have usually a
radially-oblique course; from their point of entry into the node to their insertion
on a bundle of a lower node they pass further and further from the surface of the
stem. All bundles are collateral, often perhaps^ bi-collateral.

In many forms, which correspond closely to those here to be discussed, such as
the Cardiospermum, described by Nageli, Paullinia sp., and Serjania Mexicana, the
bundles of the trace appear in the transverse section of the young internode at very
different distances from the middle ; they surround a prismatic pith, which has
projecting angles, and even infolded sides. Nevertheless, they are connected by
a normally arranged, general layer of cambium, and together form a simple wood
and bast body of the Dicotyledonous type, which is normal and remains so, though
of angular and infolded outline.

In other forms, however, with a strongly oblique course of the bundles, the
infoldings of the primary ring of bundles become so deep that single groups of
bundles are as it were nipped off, to continue the metaphor, externally from the ring,
and excluded. The bundles of such a group are at various distances from one
another, and from the middle of the internode. The bundles of the group, which
are external as regards the latter, have their xylem and phloem placed normally

^ Compare Nag’eli, l.c, p. 35.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 583

relatively to it, i. e. the former is turned towards the middle of the stem. In the
others this orientation is so inverted, that all the xylem-portions of one excluded
group face towards one common middle point situated within the group itself. A
bundle may belong successively, in its obliquely longitudinal course, to different
sides of one excluded group, and to the main ring, and must therefore undergo
successive rotations.

These arrangements appear on the first differentiation of tissues ; they even hold
for initial bundles still surrounded by meristem. As the differentiation of tissues
proceeds, the main ring, together with the pith surrounded by it, assumes the above-
indicated normal properties. Each excluded group does the same; the strand
of meristem lying between its opposite xylem-masses developes into parenchy-
matous pith ; at the limit of its xylem and phloem a normal cambium appears
all round. The corresponding narrow bands of meristem between the main ring
and the outer rings assume the structure of (chiefly parenchymatous) cortical tissue,
which is continuous into the general outer cortex. As may be concluded from what
has been said above, the arrangement and form of the rings within this general
sheath vary in successive transverse sections; in general, according to the rule
formulated by Nageli, an external ring, followed upwards and downwards, remains
intact and unaltered as far as the next node. Here it opens into the main ring,
whereby the primary bundles alter their mutual positions, and are continued on the
other side of the node as an outer ring or as a fold. (Comp. Fig. 232.)

The histological composition of the mature parts shows, as far as the not very
thorough investigations extend, no fundamental differences from the normal structure
of wood and cortex. The presence of a many-layered ring of sclerenchyma at the
inner limit of the surrounding outer cortex (Fig. 232) is general. This is at first
completely closed, and is burst by the progressive dilatation, and split with the same
phenomena, as M'ere noticed on p. 543 for Aristolochia Sipho.

In the genus Thinouia in perennial stems there appear, in addition to the
phenomena described, cortical zones of renewed growth, as in the Menispermese and
Leguminosse. According to Crüger’s drawings^, which may here be quoted, these
appear very irregularly arranged, sometimes as portions of concentric rings, some-
times as isolated bundles. If I understand Netto rightly, subsequently renewed
peripheral zones appear even in species with an originally simple ring of xylem.

Radlkofer’s divided xylenis, which are peculiar to a definite group of the genus
Serjania, are distinguished from the described compound ones, as above indicated,
solely by this, that only five, rarely six to seven, peripheral partial rings are formed,
without the middle or main ring. Five (or six to seven) groups of bundles bulging
strongly outwards, which only anastomose at the nodes, are present from the first ;
each grows by means of a surrounding cambium into a ring, which is in itself
normal.

According to Radlkofer's investigations most of the numerous genera of the Sapin-
daceae contain only species which do not climb, and show a normal stem structure.
Twining and climbing species occur in the genera: Cardiospermum with non-woody
stem, and angular but normal xylem ; and Serjania, PanUinia, Ur-villea, and Thinouia with

Botan. Zeitung. lä^si, Taf. VIII.

5«4

SECONDARY CHANGES.

perennial woody stems. But not all the species of these genera are Lianes, nor have all
which are so an anomalous structure ; many have only an angular or lobed, but other-
wise normal xylem.

Of the 145 species of the genus Serjania 84 have the above-described compound xylem,
five have divided xylem. The structure of the xylem, and especially the modifications
of the compound xylem, in number, relative size, special form and structure of the outer
rings, are always correlated with the other characters, according to which the subsidiary
groups of the genus are separated. The five species with divided xylems also form a
natural group according to other characters.

Of the still more numerous species of Paullinia twelve have an anomalous and com-
pound xylem.

Thinouia is characterised by the above-described subsequent appearance of cortical
zones of wood at the periphery of the compound wood ; whether this applies to all the
8-10 species of the genus is not certain.

Of the species of the genus Urvillea, which are also 8-10 in number, anomalous struc-
ture is known only in U. laevis, and here it differs fundamentally from that of the other
Sapindaceae, and will be noticed in Sect. 193.

Sect. 189. In the stem of the CalycanthecB, as described on p. 257, the leaf-trace
bundles form the bundle-ring, while four of those bundles traverse the cortex. The
former developes into a normal wood and bast with normal cambium ; the cortical
bundles grow by partial cambiums into those cortical bundles discovered by Mirbel,
and since his time so frequently described^; these grow in thickness as long as the
stem lives. The cortical bundle is collateral, and is composed at first of a small
xylem and a stronger phloem, extended transversely ; its orientation is inverted,
i. e. the phloem is turned inwards, towards the ring of wood, and the xylem outwards.
The latter directly abuts externally upon the broad, almost even, inner face of a strong
bundle of sclerenchymatous fibres, which is almost half-moon shaped in transverse
section : it is so surrounded by the phloem that the lateral margins of the latter also
are in contact with the fibrous bundle. The limiting layer between xylem and
phloem remains active as cambium, forming wood on the side next the former, and
on the side of the phloem a zone of bast, which embraces the wood. The cortical
bundle grows in this way like a single normal strand of wood surrounded by its
corresponding strand of bast, but retains the inverted orientation. Further, it retains
approximately, in its transverse section, the form of a broad and blunt triangle, which
it has from the first, if the fibrous bundle be included, and leaving out of account
irrelevant changes and irregularities, which appear as the volume increases. In the
first, and onwards to about the fifth year, the xylem of the bundle is extremely small
compared with the phloem, consisting of only few elements, while the section of the
latter already shows radial rows of many elements. Later it increases greatly in
strength ; it assumes the structure of a normal secondary strand of wood, divided by
small medullary rays, and even shows annual rings. The bast-zone surrounding
it only consists of elements of soft bast, and in the main at least of parenchymatous
elements : sieve-tubes remain still to be sought for. Its older layers suffer changes
of dilatation as in the normal bast, according as they pass away from the zone of
cambium ; by their own growth, and by the pressure caused by the layer of periderm

Compare p. 257. A complete list of the literature is given by AVoroiiin, /. c.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 585

surrounding the cortex, they are squeezed into the very lacunar outer cortex of the
stem. The external much dilated bast portion, which is sharply marked off from
the less dilated younger bast, is Woronin’s outer cambial zone. The general cortex,
which surrounds the bundle, follows by dilatation the growth of the latter, and of the
main ring of wood. In comparison with the latter, the growth in thickness of the
cortical bundles is slight : in the stem twenty-three years old investigated by Woronin,
the whole transverse section of the strongest scarcely reached a"'™, while the main
ring of wood was 45-55“™ thick. On the outer surface of the stem the cortical
bundles form blunt weak prominences. In the very old stem investigated by Mirbel,
which was 3“®-, that is about 80“™ thick, the four cortical bundles were as thick
as the little finger.

An indication of cambiogenetic growth in thickness is found also in the con-
centric cortical bundles of many Melastomacece (comp. p. 339), since a zone of
cambium surrounding the xylem increases the number of the elements in a radial
direction. Considerable increase does not take place, at least not in investigated
species, since the bundles are thrown off at an early stage, together with the outer
cortex, as bark.

Sect. 190. Partial cambiums with inverted, orientation of their products
of increase are known to us through the writings of Schmitz ^ in the case of the
rhizome of species of Rheum.

In the tuberous branches of the rhizome of R. officinale the collateral leaf-trace
bundles form a normal bundle-ring, further increased by a normal cambium : it
encloses a large pith, which increases in relative circumference with the strength of
the branch. Through the pith when young there run transverse bundles, in closely
superposed transverse zones, corresponding to the nodes : these bundles connect the
bundles of the leaf-trace one with another, and with those of other traces in a
reticulate manner, partly by transverse branches, partly by vertical ones, which run
especially in the neighbourhood of the woody ring. Each of these connecting
bundles is originally a bundle of elongated cambiform cells and sieve-tubes ; they
are continuous with the phloems of the bundles of the leaf-trace at the point where
these curve out into the leaf. At a very early stage a cambial layer appears round
each such strand of phloem, and forms on the one side strands of wood with much
parenchyma, on the other side strands of phloem corresponding to these, both
strands being split up by medullary rays. Further, the formation of bast is next the
original strand of phloem, the bast-layer is in fact on the inner side of the cambium,
but the xylem on the outer side. This extensive and continued growth produces a
bundle, which grows to a thickness of more than i««, and always retains the
characteristic inverted arrangement of wood and bast. The numerous medullary rays
with the same colouring matter as in the root (pp. 518, 524) give to the transverse
section a marked radial striation. Sections of this sort constitute the ‘ streaking ’ and
radiate circles'^ characteristic of the pieces of rhizome of the officinal Rhubarbs.
The growth in thickness of the streaky bundles continues even after the growth of
the pith has ended ; the result of this is a partial compression of the pith and dis-

Sitzungsber. der naturf. Gesellschaft zu Halle, Dec. 1874. Botan. Zeitung. 1875, p. 260.
^ Compaie Wigand, Pharmacognosie. — Berg, Atlas, Taf. XH.

586

SECONDARY CHANGES.

placement of the bundles. The growth in thickness of the bundles further extends
to their point of insertion on the bundle of the trace as it passes out into the leaf,
and continues at the latter point even though the growing woody ring may have
enclosed both portions. The streaky bundles therefore lie both in the pith and
in the parenchymatous wood ; they run, as the result of their original arrangement
and subsequent displacement, in very different directions, their stellate transverse
sections are therefore found both in sections cut transversely, and in other directions
through the rhizome. The quantity of the streaky bundles is on the average the
greater, the thicker the rhizome. Schmitz found the same phenomena in the
rhizome of Rheum Emodi, but not in that of other species cultivated in this country
(Germany).

Sect. 191. Successively renewed thickening ring’s^ are a rather wide-spread
phenomenon, which appears in very various special forms. The general course of the

type of growth thus indicated is as follows.
Both in the stem and root the secondary
thickening begins in all respects normally,
and often proceeds for a long time thus
normally. Then there appears in the
parenchyma, outside the normal first cam-
bium, a secondary meristem, formed chiefly
by tangential divisions ; it usually starts
from single points, and then extends
laterally : in it corresponding strands of
wood and bast separated by medullary
rays are differentiated. These are them-
selves arranged like a normal Dicotyle-
donous ring of secondary thickening :
wood and bast are permanently separated
by a cambial zone, which has as regards
them a normal orientation : by means of
it they receive further thickening. Their
structure resembles that of the normal
secondary thickening in the same plant in
all important points. Thus spiral vessels
are absent here also from the wood. Almost simultaneously with the appearance
of this second cambium and second ring, the secondary thickening of the first
normal ring ceases, at least in carefully investigated cases. In the same way the
second may cease to grow, and be replaced by a third like it, and the same process
may repeat itself through unlimited series.

The successive rings, consisting of wood, bast, and limited cambium (Fig. 233),
may, as above, be called shortly rings of growth. They are separated from one
another by zones of dissimilar tissue, the special quality of which may vary according

Ftg. 233.— Gnetum scandens. Piece of a transverse sec-
tion of a branch (8) ; w pith ; i, 2, 3, successive rings of growth,
the 3rd at the right just beginning to develope, but already
strongly developed to the left ; r ring of stony sclerenchyma,
at the inside of the outer cortex, which is covered by fissured
cork.— Cortex, medullary rays, and intermediate zones are left
white, also the bast-strands, but the latter are limited by a
single line; the strands of wood united with them are trans-
versely shaded, excepting the transverse sections of the large
pitted vessels.

* Compare Ihe quoted works of Gaudicliaud, A. de Jussieu, Mettenius, Criiger, F. Müller,
Bureau; further those to be quoted in this paragraph, by Decaisne, Nageli, Radlkofer, and Eichler,
and in §§ 192-195.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 587

to the special cases to be described. The successive rings may be briefly termed
concentric, inasmuch as the separate portions of all have a similar orientation
relatively to the organic central point of the whole transverse section of stem or root.
This term is not always strictly accurate. There is often strongly eccentric develop-
ment, or stronger development sometimes on one side, sometimes on another. Not
unfrequently instead of a complete ring only portions of one are formed, and these
are then often so far eccentric that their margins abut on rings or portions of rings
next within them. In flat or winged stems of Lianes, e. g. many Leguminosse one
can in many cases hardly speak of portions of rings at the corners, but rather of
parallel striae or bands.

Since the xylem-portions of the rings usually form the chief mass, as in normal
wood, and the bast-portions are relatively small, it follows that, in portions of rings
which are very closely packed and short, facing alternately to different sides, and with
their margins always fitting closely on the next inner ones, the whole of the xylem
portions in the transverse section of stem or root form a connected mass of wood,
in which the layers of bast belonging to them, together with the narrow intermediate
zones, run as curved, irregularly concentric, and relatively narrow bands. This is very
striking, e! g. in the Securidaca figured by Müller (/. c. Fig. 6), and the Hippocratea-
ceous plant Tontelea, represented l.c. Fig. 7. Not only in the last-mentioned cases,
where the successive portions of rings are in immediate contact at their margins, but
also in the cases of more regular concentric arrangement, the radially successive
similar zones are in direct continuity, either at the nodes alone, or at numerous points
in their longitudinal course.

According to the structure of the individual successive rings, which varies like
that of the normal wood, according also to their relative thickness, and that of the
intervening zones, according to the special structure of the latter, and the different
degrees of concentric arrangement, of length, orientation, curvature, and marginal
connection of successive rings and portions of rings, and finally, according to the
general form of the transverse section of stem or root, the structure of the parts in
question may differ with endless variety. It would lead us too far to enter into all
the individual cases which here present themselves. As opportunity offers therefore,
single examples may be brought forward, partly in this paragraph, partly in those
which treat of Chenopodiaceae, Phytolacca, and Cycadeae. Compare also the descrip-
tion of Fig. 227.

As regards the place of origin of the successive renewed zones of growth two
different cases must be distinguished.

(i) The more rare mode is this, that zones of growth following the normal one
arise in the primary outer cortex. It was discovered by Decaisne ^ in Cocculus
laurifolius, and exactly described later by Nageli% Radlkofer“*, and Eichler®, and occurs
generally in the Menispermecz of this category ; further it is found in the CycadecB
to be described below, and in the peculiarly constructed stem of the Avicetmias.

’ Compare Criiger, Botan. Zeitung. 1850, Taf. III. Figs. 19-21, Rhynchosia phaseoloides.
Mem. sur les Larclizabalees, Arch, du Museum d’Hist. Nat. 1 (1839).

® Beitr. I. l.c. * Flora, 1858, p. 139.

^ Denksch. d. hot. Ges. Regensburg, Bd. V. p. 1.

8S

SECONDARY CHANGES.

In the young branch of Cocculiu laurifolius the narrow pith is surrounded by a normal
ring of wood and bast, as in Menispermum (p. 456), with broad medullary rays, and rela-
tively narrow bundles. Each portion of bast of the latter is limited from the outer cortex
by a many-layered half-ring of hard sclerotic fibres. The outer cortex is chiefly paren-
chymatous, about 7-9 layers of cells thick, including the epidermis, and without special
peculiarities. The normal growth in thickness of the first zone of wood and bast con-
tinues, as far as is known, for 1-2 years. Then the activity of the cambium ceases. A radial
extension now begins in the 2-3 inner layers of parenchyma of the outer cortex, followed
by tangential divisions, the latter proceeding especially, though not exclusively, in a cen-
tripetal direction. The thickness of the parenchymatous cortex thus increases to 18-20
layers of cells. Of these the lo-ii outer ones remain unaltered : 3-4 layers next within
these become hard stony elements, forming a continuous ring. Inwards from this ring
the tangential divisions continue, especially in centrifugal progression. Some of the
inner layer of cells thus formed pass over into the condition of permanent very thick-
walled parenchymatous cells ; they form a continuous zone surrounding the exterior of
the primary strands of bast-fibres ; into this the primary medullary rays, which resemble
it in structure, are directly continued. Outside this layer there remains a ring of
meristem at first only 1-2 layers of cells thick, in which the second thickening ring is now
differentiated. The structure of this is in all respects very similar to that of the first;
but its bast-bundles, which are almost semicircular in transverse section, have no fibrous
sheath. After the cambiogenetic increase of the second ring has continued for a time,
the activity of its cambium ceases; between the outer limit of its bast and the stony ring
appears a new thickening ring of the same sort as the second, and so successively
onwards.

The figure 233 gives a diagrammatic view of the real appearance of the transverse
section of the stem described, and of its congeners, though the representation is not exact
since it is taken from another plant.

In the structure of the stem of the A’vicennias (A. officinalis, nitida, tomentosa) the
soft bast and cambium were not clear in the dry material at my command, I therefore
limit myself to short notes. The first normal ring of thickening surrounds a wide pith: its
irregularly undulating outer margin is marked off by a ring of bast fibres, which shows
but few interruptions, from the usually delicate parenchymatous outer cortex, which con-
tains scattered, short, and elongated sclerenchymatous elements. The wood shows a
normal Dicotyledonous structure, and is arranged in very regular radial rows. The
usually pluriseriate medullary rays consist of rather thick-walled cells, and are continuous
with this structure at many places as far as the ring of bast sclerenchyma. Thus as
thick-walled bands they divide up the bast zone, which consists for the most part of very
delicate elements with isolated fibres scattered here and there, into well-marked sections
of unequal size. The first ring hardly attains a thickness of i“”'. Then follows a second
ring of thickening, which abuts directly within on the above-mentioned fibrous zone, and is
limited externally by a zone of sclerenchyma on an average 1-2 layers thick, the elements
of which resemble those of the fibrous zone in transverse section, but are shorter than
they. To this zone a third, and then numerous successive thickening rings are added,
always in the same manner. The later ones also remain narrow; in a branch 5cm thick
for instance they are not on the average thicker than o-5™>".

The structure of all successive rings is just like that of the first : also the interruption
of the soft bast by thick-walled radial bands always takes place, and in such a way that it
is effected not only by medullary rays, but here and there by strands of wood, with few
vessels, which extend to the outer ring of sclerenchyma. The soft bast is often broken
up into single strands which resemble the strands of sieve-tubes of Strychnos. At the
time when the youngest ring appears the growth in thickness of the next inner one has
in each case already ceased. As regards the origin of the rings it was only possible to
make out that a zone of meristem appears outside the sclerenchymatous limit of the last
mature ring, in the delicate parenchyma of the outer cortex. When this zone is still

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 589

very narrow, and at most only some few cells wide, the sclerenchymatous ring appears at
its outer margin, and then within it the beginnings of bundles and medullary rays, and of
the cambial zone between wood and bast. The outer ring of sclerenchyma is pushed
outwards by the growth in thickness derived from the latter. The often-mentioned
thick-walled interruptions of the bast zone can hardly be derived otherwise than by the
ultimate conversion of cells of the cambium into thick-walled permanent elements.

(2) In the large majority of cases, the bast-zone itself is the place of origin of the
successive rings of thickening ; that is, either its outermost (primary) region, as e. g.
in Phytolacca, or the outer, older zones of the secondary bast : thus, e. g. in Wistaria
chinensis, in stems of Lianes passing under the name Bauhinia, and others cor-
responding to the Leguminous plant called by Criiger Rhynchosia phaseoloides :
further, in the Liane above-mentioned, represented by Müller (/. c. Fig, 6) as Securi-
daca, and (Fig. 7) as Tontelea (.?). The same holds for Gnetum, at least for that form
represented in Fig. 233, for Phytocrene, Fig. 227, according to Eichler for Dolio-
carpus Rolandri, and also for those cases to be mentioned directly, which have not
yet been carefully investigated in connection with the question under discussion.

The examples cited show that these phenomena belong to very different families,
and occur in plants and parts of plants adapted to very different modes of life. In
the first place they appear with a remarkable frequency in stems of Lianes, and more
especially at least in twining stems, not in those which climb with tendrils, as in
Wistaria, Mucuna sp. (F. Müller), and other Leguminosae already quoted ; in
Comesperma and Securidaca volubilis from the Polygalacese ; in Tontelea sp.
(Hippocrateacese) ; Cocculus, species of Cissampelos and other Menispermaceae' ;
Doliocarpus and other Dilleniaceae Phytocrene, Gnetum; rarely also in Aristo-
lochias, as shown by Fig. 219, taken from Schleiden’s Grundzüge, see p. 550.
Lianes with tendrils are not however • sharply excluded from such structure as that
under discussion, as shown by old stems of the Sapindaceous genus Thinouia (p. 583),
and the Bignoniaceous plants Haplolophium and Glaziovia (p. 575). Also the solitary
cortical bundles, which A. de Jussieu describes in Anisostichus capreolata, may belong
to this category. On the other hand, it is to be noted that stems of Lianes, which
are closely related to the species here mentioned, such as Menispermum canadense,
and most climbing Aristolochias have no renewed zones. And I may further mention
the appearance of the latter not only in the stem of the non-climbing Menispermaceous
plant Cocculus laurifolius, but also in others, as far removed as possible from being
climbing forms, such as the Avicennias mentioned above, Phytolacca, and Cycadeae, to
which may be further added Maerua uniflora (of the Capparidaceae) from East Africa,
with a stem structure like that of the above-mentioned Tontelea. In some Lianes,
Wistaria, Menispermeae, and Securidaca volubilis (Crüger), the structure and course of
growth described extends also to the roots; most of them have not been investi-
gated on this point. On the other hand, it is found, as far as is known, as a
general family-character in the roots of species of Phytolacca, Chenopodiaceae,
Amarantaceae, &c., even if the stem shows another character. Comp. Sect. 192.

We shall return in Sect. 194 to the phenomena of growth of the roots of
Convolvulaceae, which also belong to some extent to this category.

* Eichler, l.c., and in Flora brasiliensis. Ease. 38, p. 207, Taf. 50, 51.
2 Criiger, l.c. — Eichler, in Flora brasiliensis, Ease. 31, p. 116.

SECONDARY CHANGES.

.590

Sect. 192. The families of the Chenopodiacece, Amarantacece, Nyclaginece, Mesem-
brya7ithej)ia, and according to Regnault also the Tetragoniece, and as far as is
known all their congeners, show a series of anomalies of growth and structure,
corresponding in certain main features, but with various differences in detail within
the same family and even the same genus. With these may be connected Phytolacca
among the Phytolaccaceae, while in the genus Rivina (R. brasiliensis and aurantiaca),
placed in this family, the stem and root have normal growth and thickening^.

The common peculiarity of structure of the parts in question consists in the
fact that the secondary thickening, within the annular active cambium zone, contains
more than one circle of distinct collateral vascular bundles showing a limited growth :
these are embedded in unlike tissue, which may be generally termed interfascicular
or intermediate tissue.

This structure, in its individual forms, originates in different ways.

1. A primary ring of bundles appears in the stem, and in the roots the primary
axile bundle : in both there is first a normal cambium with normal products. The
activity of the latter then ceases, and around it appear in centrifugal order suc-
cessively renewed and disappearing cambiums, each of which forms a circle of dis-
tinct vascular bundles. This mode occurs in all investigated roots of plants of this
category with the exception of those to be named under (3); further, in the stem of
Phytolacca, and, judging from Regnault’s statements, which require corroboration,
also of Tetragonia and Sesuvium ; according to the individual case the successive
rings are variously complete and regular.

2. In a number of stems there is at first a primary ring of bundles, consisting
of leaf-trace buirdles, and often perhaps of intercalary bundles also (p. 456) ; in
species of Amarantus (p. 249) there are also medullary leaf-traces. While the
development of the collateral bundles is still proceeding there appears round the outer
margins of their phloems a ring of cambium, which is accordingly extrafascicular ;
this remains perma7iently active, and forms on its inner side alternately collateral
vascular bundles and intermediate tissue, on its outer side a thin layer of bast,
consisting only of parenchyma, or even no bast at all. This is the case in the
investigated stems of shrubby Mesembryanthema, the stalks or stems of Mirabilis,
Oxybaphus, and of all Nyctaginese (Pisonia, Boerhavia, Bougainvillea, Neea) ; of

^ Literature: — Chenopodiacece. Unger, Dicotyledonenstamm, l.c., p. 260. — Von Gemet, Ueber
den Bau des Holzkörpers einiger Chenopodiac., Bull. Soc. Imp. de Moscou, 1859, I. p. 164. There
is there quoted : Basiner, in von Baer and Helmersen, Beitr. z. Kenntn. d. Russ. Reichs, Bd. XV. —
Sanio, Botan. Zeitg. 1863, p. 410; 1864, p. 225. — Regnault, in Ann. Sc. Nat. 4 ser. tom. XTV.
(i860).

AmarMitacece. Link, Elem. Phil. Botan. ed. 2, p. 444. — Unger, Regnault, l.c. — Sanio, Botan.
Zeitg. 1864, p. 229.

Nyclaginece. E. Meyer, de Houttuynia, p. 40. — Unger, Regnault, 1. c. — Nägeli, Beitr. I. p. 119.
— Sanio, l.c. 1865, p. 197. — P'inger, Anatomie u. Entw. von Mirabilis Jalapa, Diss. Bonn. 1873. —
Grönlund, Stammens og Grenens anatom. Bygning hos Neea theifera sammenholdt med andre
Nyclagineer. Vidensk. Meddelels. nat. Forening Kjobenhavn, 1872, p. 6o.

Alesembryaiithenmm. Regnault, l.c. — Falkenberg, in Göttinger gel. Anzeigen, 1876, p. 99.
Botan. Zeitg. 1876, p. 317. [Also Petersen, Bot. Ztg. 1878, p. 785.]

Phytolacca. Martins, Revue horlicole, 1855, p. 123 (Botan. Zeitg. 1856, p. 5S2). — Treviranus,
Botan. Zeitg. 1856, p. 833. — Regnault, l.c. — Nägeli, Beitr. l.c. pp. 26, 118.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. ^91

Amarantus retroflexus, Celosia argentea, Alternanthera VerschafFeltii (Amarantacese) ;
of Chenopodium album, Atriplex patula, and Salicornia herbacea.

Between these two chief modes or types the following are intermediate.

3. In the root of Mirabilis the phenomena of secondary thickening are the same
as in I. as far as the appearance of the first renewed zone of cambium. This then
remains permanently extrafascicular and active according to mode 2. The same
behaviour appears to occur in the roots of Oxybaphus ■, those of other Nyctaginete
have not been investigated.

4. The stems of many Chenopodiaceae, as Ch. hybridum, Ch. murale, connect
types I and 2, since a normal cambium and a normal secondary thickening appears
as in I in their primary ring of bundles ; this however soon stops, and the further
growth in thickness is continued, according to mode 2, by a new extrafascicular zone
of cambium, which appears outside the primary masses of phloem. Blitum virgatura,
Gomphrena decumbens and globosa, and Froelichia gracilis behave in this way ;
their slightly thickened herbaceous stems show for a long time only a quite normal
thickening, the bundles situated outside the ring of bast appear in small numbers,
or often not at all.

The relation between i and 4 being very close, it is impossible to distinguish
in the old stem whether the first stages of thickening proceed according to the one
or the other type. Therefore in the case of Atriplex Halimus, Obione sp., Salsola
kali, Arthrocnemum fruticosum, Haloxylon ammodendron, Caroxylon arbuscula,
Alternanthera spinosa, ^rva javanica, Achyranthes aspera, Pupalia Schimperiana,
and according to Regnault’s statements in the Tetragoniese Galenia, Trianthema,
and Tetragonella, of which the first stages were not investigated, it can only be said
that they certainly belong to one of the two types.

This is the proper place to insert a remark upon the terms which have been, and ought
to be employed. According to the terminology which is usual, and has been adopted in
the preceding chapters, each thickening zone with normal orientation and structure is to
be denominated, even in the cases before us, respectively cambium, wood, bast, &c., and
there is no difficulty in applying these terms to successively renewed and arrested annular
zones. But in those cases (2) in which the orientation of the meristematic layer, which
brings about the secondary thickening, is from the first other than normal, the adoption
of the terminology used in other cases may be questioned. Even if intermediate forms
were not present, it is suitable in all cases to call the initial layer of each secondary
thickening here also generally cambium, especially as — leaving out of account special
differences which still remain to be investigated — this layer always possesses in itself the
other fundamental properties of normally arranged cambium. Since a special name is
desired for the special orientation indicated under 2, the term extrafascicular has been
chosen for such cambium as appears outside the primary ring of bundles. Starting from
the above reflections we must consistently apply the term wood in the cases of extra-
fascicular cambium also to the secondary thickening, whenever it appears on the inner
side of the cambium, and in centrifugal succession, and the term bast to that which
appears centripetally on the outside. The bast differs here as in Strychnos from normal
bast in its structure, as does the wood also, reminding one of the phenomena described in
Strychnos and Dicella. Link calls this wood lignum hybridum, bastard-wood ; but it
may be more satisfactory to give up the use of this remarkable name, which was accepted
even by Sanio, and to describe precisely in each instance the real state of the case, since
it is subject to such indefinite variation according to the individual instance.

592

SECONDA JiF CHANGES.

The structure of the individual zones of thickening has, with the exception of
some few cases to be rather more accurately described below, been very incompletely
studied, and further investigations are the more to be recommended, since very
numerous differences appear in the special phenomena, at all events in different
species and genera. What follows can but give some indications of this.

As has been already stated above, the structure of each zone of cambtu77i and
the succession and direction of the divisions in it are in general, and with the
exception of individual characters still to be ascertained, the same as in normal
cambiums. The same holds for the originally radial direction, and the subsequent
displacements of the mother- cells of tissues or elements successively derived from
the cambial layers. For cases of extrafascicular cambium it may be added to these
statements in the first place that the latter is always derived, as far as investigated,
from tangential division of a single layer of parenchyma or meristem in contact with
the outer margins of the masses of primary phloem. The secondary thickening by
cambium begins while the differentiation and extension of the tissues surrounded by
it proceeds, and especially the leaf-trace bundles 7nay continue to grow by for77iatio7i of
7iew elements at the limit between xylem and phloem, after the manner of collateral
bundles (comp. p. 390). Longitudinal divisions of the cambial cells proceeding in cen-
trifugal succession add radially arranged secondary elements to the primary ones ;
most of them pass over directly, or at most after a single further longitudinal division,
into definitive tissue-elements, and are thus young wood-elements, or the direct
mother-cells of these. In this manner arise the elements of interfascicular tissue,
in many cases also those of the xylern of the bundles. But at certain points in the
ring of cambium there appear directly in one or a few tissue-mother-cells lying close
together, and separated off internally from the ring itself, rapidly repeated longitudinal
divisions facing in all directions ; these form an initial strand (comp. Sect. 115) from
which, in the manner described generally for the collateral bundles, either a whole
vascular bundle is derived, or the phloem of a vascular bundle, the xylem of which
had already been begun by the simple tangential divisions in centrifugal succession.
In both cases the simple centrifugal tangential division proceeds further outside the
initial bundles ; outside each one of these interfascicular tissue is again formed, and
the same process repeats itself as the cambial ring is removed from the centre ; it is
repeated in the same transverse section successively on different radii, and in
successive transverse sections with such an arrangement of the initial bundles, that
the longitudinal course of the bundles, to be subsequently described, is brought about.
In the initial bundle, however, or the vascular bundle derived from it, the growth
is usually continued, when the increase at its outer margin has already ceased.
Tangential divisions are often found at the limiting surface between phloem and
xylem of a bundle, when the cambial ring is already separated from its outer
margin by several layers of cells in which division has ceased.

In addition to the centrifugally formed fresh derivatives of the cambium there
are usually also centripetal formations, that is, a formation of bast : thus, e. g. in
shrubby Mesembryanthema, species of Obione, Halimus, &c. But in many cases, as
in the first internodes of the seedling of Chenopodium album, and Mirabilis Jalapa,
I have remained in doubt as to the appearance of centripetal divisions.

Wood and bast consist, as has been stated, of collateral vascular bundles, and

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 593

intermediate tissue. The arrangement of the latter is, at least in extreme cases,
different according to the presence of successively renewed rings of cambium and
thickening, or of a permanent extrafascicular cambium.

In the first case the bundles are arranged like the strands of wood or bast of a
normal Dicotyledonous ring. One strand of bast corresponds to each strand of
wood, the two together forming one collateral vascular bundle. Between two
bundles of a ring lies a radial band of intermediate tissue, which, with reference
to the ring itself, is like a normal medullary ray ; each pair of successive rings is
divided by a zone of intermediate tissue. As will be shown below by examples,
these zones derive their origin from the outer margin of the zone of bast bordering
them internally, or the parenchyma directly adjoining this margin externally. The
bundles of successive rings are connected one with another by branches with an
obliquely radial course, the arrangement of which varies according to individual
cases to be described below. The number of the successively alternating zones
of bundles and intermediate tissue has no constant relation to that of the annual
periods of growth, but rather in one period of vegetation a number of successive
zones without definite limit are formed.

In the second case all bundles lie in the wood, and alternate both in radial and
tangential direction with intermediate tissue, either so that they appear in a transverse
section distributed quite irregularly in the latter, as e. g. in the thick masses of wood
of shrubby and tree-like Chenopodiacese (Halimus, Arthrocnemum), Nyctagineas
(Bougainvillea, Pisonia ^), Amarantacese (Aerva, Pupalia ^), and Mesembryanthema ;
or that they appear in a transverse section arranged in irregularly concentric zones
within the intermediate tissue, as in Salsola, Haloxylon, and Caroxylon ; in the root,
and partly also in the stems of Mirabilis. It is doubtful how far these zones are related
to the annual production in woody stems ; in Mirabilis there is no such relation.
In their longitudinal course the bundles show mutual connections both in a radial
and tangential direction ; in the stem of Mirabilis these are at the nodes ; in the
other investigated cases also at other points, since, in their course, which undulates in
both directions, they alternately approach and recede from one another, thus forming
elongated and pointed meshes.

The histological composition of the vascular bundles is in general like that of
collateral bundles, with the limitation that narrow spiral and annular vessels are
usually restricted to the bundles of the leaf-trace ; only the inmost, apparently
medullary secondary bundles of Mirabilis and other Nyctagineae have, like these,
spiral vessels also. In the other secondary bundles the xylem consists of pitted
vessels, and in sappy and fleshy parts also of reticulate vessels, and of parenchymatous
and fibrous elements which require further investigation. Between the two latter the
vessels are arranged usually in one or a few, more or less regular radial rows ; more
rarely one or another vessel is at a distance from the rest in the intermediate tissue.
In many Mesembryanthema the arrangement seems to be less regular, so that the
vessels appear ‘ irregularly scattered ’ in the intermediate tissue ^ The phloem in

1 [See Petersen, z. Hist. u. Entw. des Stengels der Nyctagineen, Ref. Botan. Zeitg. 1880, p. 509.]
Under the generic names quoted it is intended in each case to include specially the above-
named species belonging to them. ■' Compare Falkenberg, 1. c.

Q q

.594

SECONDARY CHANGES.

the stem of Phytolacca has the structure of perfectly typical soft bast ; in the other
investigated cases there is usually a great preponderance of parenchyma, with only
very narrow and isolated sieve-tubes. These may therefore be easily overlooked,
and are to be further investigated ; where I examined them carefully (Mesembryan-
themum, Atriplex sp.) they had the typical structure. In all cases the elements of
the phloem are united to a narrowly circumscribed group, and not scattered over
a wide area. Sclerenchymatous fibres have not been found in the secondary phloem
in any case investigated.

The intermediate tissue appears in extreme cases in two different forms, namely
as thin-walled large-celled parenchyma, or as sclerotic spindle-shaped woody fibres.
The former is in all cases sappy, rich in products of assimilation, and often shows
an independent growth, which is long continued even at a great distance from the
cambium : this growth is connected with cell divisions, and exercises an important
influence upon the whole structure. Intermediate forms between the two named,
such as thick-walled ‘ xylem parenchyma,’ &c., are not wanting, but require more
exact investigation. The latter remark applies also to the distribution of both forms.
If only the most fundamental phenomena be taken into account, the following
rules may be applied to the occurrence of the two kinds of tissue : —

(1) All intermediate tissue consists of delicate, large-celled parenchyma, rich in
products of assimilation. Fibrous elements do not occur at all (root of Mirabilis), or
only in immediate company with the vessels, that is, as constituents of the strands.
This is the case in most fleshy roots of this category, e. g. Beta, and in the stem
of Phytolacca dioica.

(2) The intermediate tissue consists both of parenchyma and of elongated,
spindle-shaped, sclerotic, fibrous elements ; this is the case in —

(a) The extremely hard woods of the shrubs and small trees of the steppes, of
the genera Halimus, Caroxylon, Haloxylon, &c., and the tougher stems of other
Chenopodiaceae, Amarantaceae, Nyctagineae, and Mesembryanthema, the chief and
fundamental part of which consists of the latter ; parenchyma appears to be distributed
among it in small groups, like the bundle-parenchyma of normal wood, but this
requires more exact investigation. It is often found in radial bands of various size ;
but most especially, almost always, in several layers surrounding the phloem portions
of the vascular bundles externally and laterally. Where the latter are arranged in
concentric zones, it thus forms zones of parenchyma, or consisting mainly of
parenchyma. The cells of this parenchyma which accompanies the bundles are
sometimes, like those of the ordinary fascicular parenchyma of firm normal woods,
very thick- walled, and pitted ; but in most cases the layers immediately surrounding
the phloem remain thin-walled and sappy. Like the phloem-strands of Strychnos,
they with the groups of phloem fill up spaces in the hard mass of xylem, which are
round or oblong in transverse section, and which in dry woods are usually for the
most part empty by reason of the shrinking of the delicate tissue. I have only
rarely found exceptions to this, e. g. in the Salsola investigated. As far as investi-
gation extends, all the elements mentioned are arranged radially with considerable
regularity in the firmer woods in question. Grönlund states that there are numerous
medullary rays in the wood of Neea.

ib) Those cases are but little worthy of remark, in which both thin-walled

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 595

parenchyma and hard fibrous elements form large masses, which appear in transverse
section as alternating, irregular, concentric zones or islands ; this arrangement occurs
e. g. in Chenopodium hybridum, and, according to Falkenberg, in many Mesem-
bryanthema.

{c) The intermediate tissue formed at the beginning of the activity of the
cambium assumes in many cases an exclusively parenchymatous nature where the
cambium is extrafascicular ; its cells are more or less displaced from their radial
arrangement, by reason of the more continued strong extension of the masses of
tissue surrounded by them, and their own considerable growth in the transverse
direction. They are therefore arranged similarly to those of the primary pith, and
further they remain thin-walled like the latter and those of the primary medullary
rays, and therefore form with these two parts a connected mass of wide and delicate
cells. Later, when the transverse extension of the pith and the tissues near it ceases,
the pith-like intermediate tissue is succeeded by a dense tissue, with structure and
arrangement similar to {a). Since the latter together with the vascular bundles
enclosed in it correspond to the usual appearance of dense ‘ wood,’ it alone appears
in transverse section to represent the ring of wood, while the wide-celled internal
tissues surrounded by it, including both primary and secondary bundles, appear one
and all to be medullary. Such apparently medullary bundles are to be distinguished
from true medullary bundles, i. e. such as are within the primary ring of bundles
(comp. p. 248). Their occurrence appears to be not uncommon in the plants in
question, and is subject to two chief modifications. In the first place, a number of
layers of pith-like intermediate tissue are formed outside the primary ring of bundles,
but between the secondary bundles only dense tissue ; the primary ring alone
appears to be medullary in this case — and further is often very near to the dense
secondary ring. I’hus, e. g. in Chenopodium album, Atriplex patula, Celosia, and
Achyranthes ; in species of Amarantus there are also true medullary bundles of the
leaf-trace. In other cases pith-like tissue appears also between the inner secondary
bundles, so that the transverse section shows in addition to the primary ring also
one or several, apparently medullary rings of bundles, as in the stem of Mirabilis,
Oxybaphus, and also other Nyctaginese.

All that is necessary has already been said above about the structure of the
bast in the forms with successively renewed annular zones, for here, if the expression
be allowed, each annular zone has its own layer of bast. For the other forms it
may be again stated that the secondary layer of bast, when present at all, consists,
as far as investigated, only of relatively few layers of radially arranged parenchyma,
in which crystal-containing sacs are not unfrequently scattered. Bast fibres are
found, scattered or forming a dense ring, only at the outer limit of the primary bast-
layer of the stem, and are absent even there in many species ; in the secondary bast
they are found in none of the plants under consideration.

To illustrate the above, two or three examples may be more thoroughly described,
though space does not here allow of an exhaustive description of the somewhat com-
plicated phenomena.

I. Firstly, the shrubby Mesembryanthema. I have specially in view a form named as
M. virens Haw. Other species resemble this, in the main, according to the notes of
Regnault and Falkenberg, while in others again, as the annual M. crystallinum, greater

Q q 2

596

SECONDARY CHANGES.

variations appear to occur. The leaves are arranged in decussating pairs. The course
of the bundles of the leaf-trace, which is certainly very simple, has not been exactly in-
vestigated. In the young, and but slightly extended internode they are disposed round
a narrow pith in a ring, which is in transverse section somewhat bluntly rectangular.
The two shorter sides of the rectangle are occupied by the bundles belonging to the
next higher pair of leaves, the two longer ones by those which descend from higher
leaves. The bundles are collateral. Their phloem portions are surrounded by a zone
of narrow thick-walled collenchymatous elements, which is several layers of cells thick,
and runs round the whole ring : this zone is bordered externally by the thick large-celled
outer cortex, traversed by the network of bundles mentioned on p. 297. The innermost
limiting layer of this (Plerome-sheath) is developed as a starch layer (comp. p. 4 15).
Before the development of the bundles of the leaf-trace is complete, tangential cambial
divisions begin in an outer layer (but not the outermost) of the zone of collenchyma : thus
they are extrafascicular. As far as I could see they begin first on the long sides, then
on the short sides of the rectangle, and are then continued from these four starting-points
over a completely circumferential layer of cells. The tangential divisions and the
arrangement of their products in radial rows are from the first very regular, and remain
so, since the latter and the cambium always uniformly follow the slight expansion of
the internal parts. Externally to the circle of the leaf-trace, in the first place a zone
many layers in thickness is formed, consisting of radially-arranged interfascicular
tissue — in M. virens of sclerotic elements. Further outwards the vascular bundles
appear in the intermediate tissue, which becomes sclerotic with the exception of the
thin-walled portion surrounding the phloem ; in the transverse section these bundles are
arranged in interrupted and irregular concentric annular zones, which are often inter-
locked one with another. The vessels of the bundles are at least for the most part derived
directly from simple cambial tangential divisions. When the xylem of a bundle has
been thus formed, an initial strand is added at its outer side, from the active divisions of
which the delicate phloem is produced; I am not able to say whether the outermost
vessels of the bundle are also thus formed or not. Centripetal divisions of the cambium,
the succession of which has not been exactly traced, produce in M. virens a secondary
layer of bast composed of radially seriate, elongated parenchymatous cells. In stems,
in which the radial rows of the secondary wood numbered over fifty elemen ts, thefe
were only five cells in one radial row of the bast. After the extension of the interno de
is complete, and before the first ring of secondary vascular bundles is formed, the thick
outer cortex is thrown off by a formation of periderm, which starts from the starch-
layer; the layer of cork cells thus derived is several layers thick over the broad sides of
the ring of bundles, being formed by tangential division of the starch-layer; over the
narrow sides, however, it is for certain distances a single layer, and arises apparently
directly by suberisation of the starch-layer, without previous divisions.

The roots of the Mesembryanthema have not been investigated in detail.

2. From each of the leaves arranged in opposite and decussating pairs on the foliage
shoots of Mirabilis Jalapa and longifolia (Figs. 234, 235) three bundles of the trace enter
the stem, one median (m), and two lateral (/). They traverse the internode with a
radially perpendicular course, and at the next lower node each trace unites to form a single
bundle (v) ; this projects rather deeper into the pith, and passes perpendicularly down
the next node, inserting itself at the second lower node on the traces, which there join
into a single bundle ; it curves first to one side and is affixed to one of them, and then by
a second shank, which appears later, it is joined to the one on the other side. The
transverse section of a young internode (see Fig. 235) therefore shows first eight bundles,
ranged round a prismatic pith : on each side, corresponding to the next higher pair of
leaves, are the three bundles of the trace of the leaf above it, and alternating with these
two traces, but rather nearer the centre, are the opposite united bundles from the pair
of leaves next but one higher. Somewhat later new bundles appear in the internode
outside the eight bundles of the ring, and at the same time a ring of meristem, the

ANOMALO US THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 597

extrafascicular cambial ring (c), which is somewhat more external, and consists for the
most part of a single layer, becomes visible : it is sharply distinguished from the paren-
chyma in- and outside it (which has meanwhile become large-celled) by the small size of
the transverse section of its cells. If we first consider the longitudinal course of the
new bundles, which become visible at the same time as the cambial ring, and of those
which appear rather later and muH be treated of subsequently (Fig. 234 a), they pass per-
pendicularly through the internode, being arranged in one or two circles outside the
bundles of the leaf-trace, and with out any quite definite position relatively to the latter.
In the node limiting the internode above and below they are inserted on the outgoing
lateral bundles of the trace, and are also connected at once one with another by a
curved transverse anastomosis. Later, this insertion is obscured by the appearance
of very numerous connecting branches at the node.

Fig. 234. — Mirabilis Jalapa. Seedlingf, first and second epicotyledonary nodes; above the latter is the growings point
with the third pair of leaves just beginning to appear, and seen through the base of the leaf which is opposite the
observer. The preparation was made transparent by potash and glycerine, and consists of one longitudinal half, seen
from without, and with the vascular bundles drawn in. m\ median bundles of the leaves of the first pair; m2 median
bundle of the nearer leaf of the second pair; h, I2 lateral bundles of the trace of the first and second pairs of leaves ;
V united bundle of the trace ; a, a secondary, apparently medullary bundles.

Fig. 235. — Mirabilis Jalapa. Transverse section through the young first epicotyledonary intemode {40). m. median
bundles, v united bundles of the leaf-traces passing through the internode ; between 7)i and v on either side are two
lateral bundles ; c extrafascicular cambium ; the extensions of this protruding inwards are initial secondary bundles ; two
of the latter are separated from the cambium by a layer of parenchyma {left white).

The first one or two of these bundles, which are outside the leaf-traces, always became
visible as young initial bundles at the same time as the cambium, but were separated
from it by a narrow zone of parenchyma : it can hardly be doubted that they, like those
which are subsequently formed, are really derived from the cambium. As soon as the
ring of cambium is clearly seen, successive new initial bundles are formed in it, which

59«

SECONDARY CHANGES.

develope into the bundles arranged round the leaf-traces. One, and perhaps some few
closely grouped cells of the cambial ring divide tangentially ; from the internal cell or
cells an initial strand is formed by longitudinal divisions facing in different directions,
while the outer cells continue the centrifugal tangential division. Those products of the
latter, which adjoin the initial strand externally, assume at once the properties of
relatively wide parenchymatous cells, the initial strand is therefoi'e separated by paren-
chyma from the cambial ring, which successively retreats outwards.

Simultaneously with the first formation of the initial strands, a centrifugally successive
development of parenchyma on the part of the cambium takes place between them.
Since this development continues slowly, the cambial ring, which retreats slowly
outwards, forms a number of new initial strands in the manner above described, at
successively alternating points at its periphery ; this centrifugally progressive activity
accordingly produces those irregular circles of vascular bundles, separated by delicate
and wide-celled parenchyma, which surround the ring of the leaf-traces. The number

of these bundles varies greatly according to
the strength of the internodes ; in weak seed-
lings it is often hardly 8-io (Fig. 236), in strong
flowering shoots 2-3 times that number. During
the process described, the growth in the direc-
tion of the transverse diameter continues in
the middle of the transverse section of the
internode. The vascular bundles increase the
number and size of their elements in the usual
way — especially the united bundles of the
trace (-u) are usually to be distinguished at an
early stage by their considerable size ; the cells
of the whole interfascicular parenchyma in-
crease throughout in width. The cells derived
directly from the cambium must therefore be
displaced from their original radial direction,
and the more so since the cambial divisions
apparently succeed one another relatively
slowly. Even the cells of the cambial ring
itself often show an irregularly distorted ar-
rangement. Finally, a stage is arrived at, in
which the extension of the inner parenchyma ceases, and simultaneously the cell divi-
sions in the cambium succeed more quickly and in greater number without changing their
succession. Thus a ring of thickening is now formed, which consists of relatively
narrow elements, arranged in regular radial rows ; it is composed, on the one hand, of a
fundamental mass of sclerotic elongated cells, on the other, of vascular bundles embedded
in this fundamental mass ; for the origin and arrangement of these, fundamentally the
same may be said (that is, if details be not taken into account) as above for Mesem-
bryanthemum. The old internode thus shows apparently medullary bundles within
a dense woody ring (Fig. 236).

In the thin first internodes of the main stem of seedlings I first saw no bast paren-
chyma derived externally from the cambium. Later this process may begin, and in
strong plants this is always or as a rule the case. Similar, but simpler conditions occur,
according to Nägeli’s description, in Pisonia hirtella; also the more complicated
phenomena which Nägeli describes in the internode of Boerha-via scandens and Bougain-
villea spectabilis are to be connected with the above.

The growth in thickness of the root of Mirabilis begins like that in normal Dicoty-
ledons. Comp. p. 473. The vascular strand is rarely tetrarch’, usually triarch; the

Fig. 236. — Mirabilis Jalapa. Transverse section tlirou^yh
the first epicotyledonary internode of a young specimen,
which is already several internodes in height {15) ; v and 7n as
in Fig. 235, next to m are the lateral bundles ; round the eight
bundles of the trace are nine apparently medullary bundles
in the parenchyma, which is left white; further outwards is
the thick ring of wood ; c cainbial-zone.

^ Van Tieghem, Symmetrie de structure, I.c., compare p. 473-

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 599

latter case only will be here taken into account. Opposite each face of the vascular
plate a strand of wood and bast containing much parenchyma is formed in the normal
way, and before each angle a broad primary medullary ray. In the pericambium a
periderm begins simultaneously to be formed, and by it the outer cortex is thrown off.
The primary medullary ray consists exclusively, and the bast mainly, of radially seriate
parenchymatous cells, -which gradually become greatly elongated in the same direction :
these together form a thick layer inside the periderm. In the peripheral portions of this
layer, near to the phellogenetic zone of the periderm, tangential division begins all round,
at the time when the root begins to swell strongly, and thereby a fresh (renewed)
cambium is started. The formative activity of the first normal layer now falls off, and
soon ceases altogether. The second cambium alone continues the secondary thicken-
ing of the root throughout life. By means of tangential divisions proceeding for the most
part centrifugally, it forms alternately radially arranged parenchyma and vascular bundles.
The latter are connected in radial and tangential direction in acute-angled meshes, and
are arranged in the transverse section in tolerably regular annular zones, which alternate
with zones having no bundles, and differ from the latter also by the less marked radial
extension of the parenchyma ; they are thus very conspicuous.

Fundamentally the same phenomena of thickening appear in the hypocotyledonary
portion of the stem, which takes part in the turnip-shaped thickening of the main root :
the peculiarities of this part are irrelevant, and may pass unnoticed here’.

3. In the root of the cultivated forms of Beta^, a diarch primary vascular bundle was
always found in the cases investigated : the secondary thickening begins fundamentally
in the same way as in Mirabilis, and proceeds for a time in a normal manner. The
cortical parenchyma surrounding the outer margin of the primary groups of bast thus
becomes much thicker opposite the two secondary bundles than opposite the angles of
the primary vascular plate ; I will not say for certain whether this is the result, as stated
by van Tieghem, of a formation of phelloderm, starting from the pericambium which
acts as a phellogenetic layer, or of active growth of the parenchymatous elements of the
primary bast (i.e. of the primary phloem strands) seated within the pericambium.
After a time, in the case of the main root of B. vulgaris when it is about g™“ thick, the
formation of a new cambial zone starts in this peripheral layer of parenchyma on both
sides, by regular successive tangential divisions appearing in an annular layer of cells.
S ince this process is continued, from the middle of the two points indicated, laterally
towards and over the two main medullary rays, a completely closed cambial zone is
formed, which undergoes active reciprocal tangential division : in it are differentiated
strands of wood, and of bast corresponding to them, and medullary rays, which are
further developed after the manner of normal secondary thickening. Later, the growth
of this second thickening zone ceases, and it is replaced by a n.ew one similar to it,
which had already begun to appear at the outer limit of the chiefly parenchymatous
second layer of bast, in the form of tangential divisions, which arise at scattered points,
and extend thence laterally all round. Since the same process is repeated, there arise
in the Beet those well-known concentric rings of wood, broken up by medullary rays,
which alternate regularly with the chiefly parenchymatous zones of bast, the number of
which may be as many as six and more in a strong one-year-old beet. According as the
rings are further from the centre, and therefore wider, the number of their strands of
wood and bast increases.

Between the bundles of successive rings there are obliquely descending radial con-
necting-bundles. Further, instead of a ring, larger or smaller segments of a ring may
appear here and there, which are then connected by their margins to the next inner
rings. Here also the hypocotyledonary axis takes part in the formation of the beet, the
phenomena being similar to those in the main root.

’ Compare van Tieghem, 1. c.

“ [See Droysen, Beitr. z. Anat. u, Entw. d. Zuckerrübe, Halle, 1877.]

6oo

SECONDARY CHANGES.

In addition to the phenomena of secondary cambial formation above described, there
occurs in the Beet the above-mentioned phenomenon of growth of the older paren-
chyma, even at a distance from the active cambium : this process influences the definitive
structure. Even in young roots, in which the formation of the innermost zone of
increase has begun, it may not unfrequently be seen that the connecting cells (comp,
p. 351), which separate the primary plate of xylem from the secondary strands of wood,
are transversely extended, divide, and thus give rise to a band of parenchyma, which
intervenes between the strands of secondary wood and the primary plate, and separates
the two parts one from another. This phenomenon may appear on both or only one
face of the plates. With the further growth of the root the intervening band may
increase to a width of 2™™. It is continuous externally into the simultaneously
growing parenchyma around it, especially of the chief medullary rays. Also the other
internal parenchyma, both that belonging to the xylem and that which forms the greater
part of the zones of bast, has a growth accompanied by slow cell division, and continuing
long after the appearance of the next outer cambial ring: the limits of this growth
have not been exactly ascertained. In consequence of the growth of the zones of bast
in a radial direction, the successive rings of wood are separated more and more from one
another; and the increase in volume of the parenchyma in the individual ring of wood
separates the non-parenchymatous elements of it one from another, both in the direction
of the radius and of the periphery; especially the meshes, which are formed by the
bundles, are widened by dilatation of the medullary rays. It is uncertain whether
meanwhile the non-parenchymatous elements of the wood also grow, as in the cortical
bundles of Cycas (Sect. 195).

4. In the stem of Phytolacca dioica the beginning of the secondary thickening is
naturally different from that in the roots. The first cambial ring appears normally
in the primary ring composed of the bundles of the leaf-trace (comp. p. 249), together
with intermediate bundles. The periphery of this, marked by the outer margins of the
primary phloem-bundles, is immediately surrounded externally by some 2-3 layers of
parenchyma ; further outwards there follows an almost closed ring of bast fibres, next
to which is the large-celled parenchyma of the outer cortex. The ring of bast-fibres is
to be regarded as the limit of the layer of primary bast. The layers of parenchyma just
mentioned, which adjoin it internally, show for the future, at least for the most part,
growth by dilatation like the outer cortex. But the parenchymatous cells, which succeed
them internally, and which accordingly belong to the margin of the phloem-bundle,
begin, when the increase derived from the first cambial ring has gone on for some time,
to extend greatly in a radial direction, and to divide tangentially. These processes
arise at certain not exactly defined points of the periphery, and extend laterally from
them over medullary rays and bundles, so as to form an annular layer showing the above
phenomena. As far as has been investigated, this is derived from only one, or at most
here and there from two layers of cells. After several successive tangential divisions
the division stops at the inner side of the layer in question, their products grow into
parenchymatous cells much extended in a radial direction : these retain with considerable
accuracy the arrangement in the radial series in which they arose. In the outer part of
the annular zone the tangential division continues, the beginnings of corresponding
groups of wood and bast, i. e. of vascular bundles, alternating with medullary rays,
appear arranged in a ring, which, like the first, increases normally by means of a normal
cambium. At the outer limit of this second outer ring there arises later, in the same way
as it arose, a third ring, and a layer of parenchyma separating it from the second ; the
same process may repeat itself through higher orders of rings. In each ring the formative
activity of the cambium ceases almost simultaneously with the appearance of the next
outer ring. Thus arise the often described concentric rings of vascular bundles,
separated from one another by broad zones of parenchyma, the number of which
annually formed may rise in a strong branch or stem to six and more. The rings
are in this case often still more incomplete and irregular than in the root of Beta.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 6oi

Connecting bundles between the successive rings are found, according to Nägeli, only in
the nodes.

Sect. 193. In the anomalies thus far described the increase in thickness some-
times takes place like the normal increase in centrifugal progression ; neglecting
possible pathological states, the growth of the tissue within the cambium, which
is at the time active, is soon ended and disappears. Another condition was
only casually indicated for many of the cases hitherto described. We must now
return to these in connection with others.

There are a number of parenchymatous stems and roots in which the old
parenchyma, far distant from the active cambium, has in all regions of the trans-
verse section, not only the power of growth and formation of new tissue, which it
puts forth when wounded and so on, but also in the uninjured plant it really grows
continuously, increases the volume and number of its cells, and produces secondary
meristem, from which strands of wood and bast, and cambium may be derived.
Leaving on one side the phenomena in the Cycadeat, Sect. 195, which do not
strictly belong to this category, it has never been accurately investigated how far
the mature non-parenchymatous elements follow this growth. As far as appearance
goes, they seem usually to play a passive part.

A number of the most peculiar anomalies owe their origin to these phenomena
of growth and secondary development.

From the first series, viz. the stems of Lianes, Criiger quotes a number of
phenomena of this category, which require more exact investigation. Thus the
zones of intermediate parenchyma between the old successive thickening layers of
Securidaca volubilis are said to grow continuously broader, while room for this
growth is provided by widening of the radial bands of parenchyma in the rings
outside it. Criiger gives the same information for the Dilleniacese.

]\Iuch more extended processes of growth and secondary formation, which
fundamentally alter the whole arrangement of the tissues and even the form of the
stems, occur in the Bauhinise, many Malpighiacese, Urvillea, and Bignoniacese.

The originally regularly formed and continuous xylem is here split into pieces
by growth, i. e. extension and cell division of the bands of pith and xylem-
parenchyma : these pieces have an independent growth in thickness by means
of a cambium surrounding them, which is in part undoubtedly a secondary forma-
tion, and which forms bast also on the side turned away from the wood. In
addition to the repeated splitting of the already formed strands of wood, new
ones with a further individual growth may be formed from secondary meristem.

The splitting of the wood, described above, pp. 574, 375, for certain genera of Bigno-
niaceae, occurs e. g. in the simple and diagrammatic case of Anisostichus capreolata
(Fig. 237), since, after being stationary for years, extension and division of parenchymatous
cells begins in the pith, and, extending from the pith radially towards the four plates of
bast, splits up the wood into its four main segments. The segments of wood, which have
from the first been backward in increase in thickness, are split off from the adjoining
main segments on both sides, or remain connected with one, according as the two broad
medullary rays adjoining them, or only one of them, take part in the extension of
parenchyma. Also the four bast-plates, or their parenchymatous portions, undergo a
process of widening: but they are again narrowed, since the split ofi and superseded

6o2

SECONDA Ry CHANGES.

segments of wood undergo considerable increase at their outer side, and are extended in a
fan-like manner so as to grow into the bast-plates, and since fan-like excrescences grow
out also from the lateral margins of the main segments of wood. Each excrescence
of wood starts from a corresponding segment of cambium, that of the short superseded
segments undoubtedly from the original segments of cambium, that of the lateral margins
of the large segments from secondary cambium ; a further though weak production
of bast by the segment of cambium corresponds to each excrescence of wood. The
process in A. capreolata has not, as it appears from other writers, been observed much
further than here described and illustrated. But in other Bignoniae ^ the splitting of the
wood in the first instance goes further ; it is broken up successively into numerous
segments corresponding in general to its dichotomies: these are separated by radial
bands of parenchyma, and surround a central portion, in the transverse section of which
are seen numerous strands of wood of various form. It is uncertain how far the latter
are of secondary origin, or are thrown off from the old wood by corresponding growth
of parenchyma. That the latter is the mode of origin of some of them, and is provided

for to a certain extent from the first, is shown to be
probable by the islands of parenchyma, which lie near
to the medullary sheath in the young still undivided
xylem of Melloa populifolia (Fig. 226). In the old
stem the xylem is cut up throughout by broad bands
of parenchyma and bast, and especially a number of
central bundles are separated from the peripheral
ones.

The separate segments of wood increase in
thickness on all sides, either by segments of the
original cambium surrounding them, or by secondary
cambium ; and it can hardly be doubted that the latter
always forms new bast elements on the side op-
posite the wood. How great a confusion of seg-
ments of wood, and bands separating them, may
arise by the growth described, and by successive
splitting, is shown by Crüger’s not very accurate
figures of Bignonia Unguis^, and by Fig. 238, re-
produced from Schleiden’s Grundzüge, which Crüger
regards as the transverse section of an old stem of
Bignonia, but Schleiden as a Bauhinia.

Similar phenomena of splitting and of in-
dependent further growth within the simulta-
neously growing parenchyma are showm by the originally more or less lobed
xylem of climbing Malpighiacese Special generic peculiarities cannot at present
be definitely laid down for this family. The splitting of the medullary sheath and of
the rest of the wood by the growth of the masses of parenchyma, the independent
growth in thickness of the separate segments of wood, which have no special
pith of their own, have been already clearly described by Jussieu in this group, if
it be overlooked that he often calls the growing intercalary parenchyma cortical
tissue intruding (from the outside). In a living branch of Stigmaphyllum ciliatum
I™ thick I have myself been able directly to follow the dilatation of the pith and of

' Compare Crüger, Botan. Zeitg. 1850, I. Taf. II.

Botan. Zeitg. 1850, Taf. II.

^ Compare A. de Jussieu, /. c. ; Crüger, /. c. ; and above, p. 577.

riG. 237.— Anisostichus capreolala
L.). Transverse section through an old stem.
Natural size. Compare the young stem, Fig. 224,
p. 570. The four projecting segments of the xylem
are completely split apart from one another to the
middle, by extension of the pith and of the bast-
plates. Three of the small superseded segments of
wood lie also separately in the surrounding tissue,
and are extended in a fan-like manner at the peri-
pheral margin. Two of the four segments of wood
are further radially split, in the case of the lower
one to the left, as far as the pith. From the margins
of the segments of wood numerous excrescences
project in a fan-like shape into the plates of bast
and parenchyma. Bast and parenchyma are sliaded,
the wood is left white, witli the exception of an in-
dication of annual rings and medullary rays. Ex-
ternally the cortex is surrounded by fissured bark,
which is drawn in black.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 603

the radial bands of parenchyma of the highly parenchymatous wood, which extend
from the pith, the consequent splitting and disrupture first of the medullary sheath,
then of the outer regions of wood, the appearance of new cambial zones close to the
separated segments of wood, which form also new bast, and finally also the
appearance of new strands of wood and bast in the pith itself, which are derived from
secondary meristem, and have an independent growth in thickness. The changes
start, in the branch investigated, from a swollen portion twining round a wire and
exposed to strong pressure, a fact which shows that they are at all events advanced
by mechanical causes.

Fig. 238. — Bauhinia spec. Transverse section of a stem, two-thirds the natural size, from Schleiden’s
Grundzüge, a all dotted figures are portion-; of wood, sometimes with remarkably large pitted vessels ;
r bundles of wood, conspicuous by their whitish colour, with radial medullary rays, and arranged in a simple
circle {probably the original narrow medullary sheath, split up). With exception of the circle {c) the rest of
the wood consists for the most part of parenchyma, and the medullary rays take a sinuous course, The
bands left white {b) between and around the woody bundles are masses of parenchyma and bast.

It is doubtless similar anatomical changes which bring about the frequent splitting
up of the xylem in the old stems of climbing Baichimas'^ , especially of the section
Caulotreius^, of which the Fig. 238, which, according to Schleiden, is of a Bauhinia,
may give an approximate representation. In these Lianes however there is, in
addition to the phenomena in question, another also, which has not at all events
been shown with certainty to occur in other forms, viz. a long continued growth in
length of the old layers of wood, which may also be ascribed for the most part

^ Compare Gaudichaud, t.c.. Tab. XVIII. Figs. 2, 3.

^ [See V. Höhnel, Die Entstehung der welligflachen Zweige von Caulotretus, Pringsh. Jahrb. Bd.
13. — Also, Warburg, in Botan, Zeitg. 1883, P- &c.]

6o4

SECONDARY CHANGES.

to the parenchyma, and which must be the cause of the peculiar curling of the
band-like stems or branches of these plants.

According to the known facts, which certainly require to be completed and
tested, the following conditions, which in the strict sense only partly belong to this
category, occur here.

The young internodes, with leaves in two rows, are bluntly four-angled. In trans-
verse section the pith lies in the form of a cross, the arms of which are in the plane of
the two orthostichies of leaves, and in one at right angles to it. It is surrounded by
a normal woody ring (medullary sheath) consisting for the most part of relatively narrow
and thick-walled, radially arranged elements, it is therefore dense : it is itself thicker
between the arms of the cross-like pith than outside the latter, and therefore it is
roundish or bluntly octagonal in transverse section. On this zone which scarcely
attains a thickness of i“™, and sharply marked off from it, a softer wood is deposited by
continued growth in thickness: it consists of wide pitted vessels, narrow hard sclerenchy-
matous fibres, and thin-walled, apparently non-lignified, fascicular parenchyma ; these
elements are variously distributed, according to the species, at least according to the
specimens investigated, the names of which were for the most part not accurately deter-
mined; this need not here be described in detail. Numerous narrow medullary rays, and
here and there single very broad ones, traverse the strands of wood, and are distinguished
in the dry material by the brown contents of Ihelr cells. They are as usual placed with
their peripheral ends perpendicular to the surface, and on the flat portions of the stem
they are also curved near the middle in a curve convex towards the margins. The
body of wood thus arranged now grows in thickness, especially at the two sides alter-
nating with the orthostichies of leaves, so that the whole stem attains the form of a
ribbon-like plate rounded off at the two margins. Perpendicular to the surface of the
plate the increase of wood is generally of much less extent, so that over the pith the
whole thickness of the stem is not greater, but often even less than at the sides.

The thickening is usually stronger on one surface than on the other, so that the pith
remains nearer the latter. Local inequalities of thickening lead in most, but not in all
specimens at hand, to the formation of unequally thick and irregular longitudinal ridges
and furrows. The cortex shows no generally remarkable peculiarities of structure,
it is on the average of equal thickness round the stem, if the numerous local irregularities
be not taken into account.

Older ribbon-shaped branches of Caulotretus, about 40-50™'" broad and 10'"'" thick,
show in their form the peculiarity that their margin is considerably shorter than the
middle, the latter is therefore strongly undulated, and often in a direction very regularly
perpendicular to the surface; the undulation is strongest in the middle around the pith,
at the margin it is not present at all, and towards the latter it gradually diminishes^.
Each crest and each hollow of a wave, i. e. each point of strongest curvature towards the
two surfaces of the stem, corresponds to the insertion of a leaf, or axillary shoot ; the latter
is placed in the cases investigated somewhat higher (in acroscopic direction) than the
last-named point. Young shoots, on the other hand, even those which are already flatly
ribbon-shaped (e.g. one before me 10’"'" broad and 3-4""" thick) show no undulations, or
have them hardly indicated at all ; as they grow older the undulations become steeper.

According to these data the undulation must arise from the fact that with the
successive growth in thickness either the margin of the plate becomes absolutely
shorter, or the middle absolutely longer than at first. The former alternative is
a priori improbable, and is not made more probable even by any direct observation.
The other condition might, with reservation till exact quantitative measurements be
made, be explained partially on the assumption that, as in many normally growing stems
(p. 505), the elements of the successive zones of growth increase in length, but here to

' Compare the drawing in Duchartre, Elem. de Botanique, p. 166, Fig. ']'j.

\anomalous thickening in dicotyledons and gymnosperms. 605

a less extent at the margins of the plate than in the middle. This explanation, however,
is not enough, since the successive elongation of the middle is greatest in the oldest
zone surrounding the pith. The same holds as regards the attempted explanation made
by Netto'. If the assumption of a shortening of the margins of the plate be excluded,
the other alone remains, that in the old zones there is a continued growth in length ;
and since according to other data, known in the case of Angiospermous trees, such a
growth in other elements, though not impossible, is less probable, while phenomena of
growth may be proved to occur elsewhere in the older parenchyma, the anatomical
basis of the elongation must first be sought for in the latter. For the proof of
this assumption, and the demonstration of the anatomical changes which necessarily
follow from it in the old zones, more accurate investigation is wanted, and especially
measurements.

In old stems the swollen projections of the xylem increase in size, and the islands
of parenchyma in width. The final splitting of the former into the pieces and lobes
(Fig. 238) with independent growth can hardly come about otherwise than as above
stated, but this point requires more exact investigation. The anomalies of structure are
continued, according to Criiger, into the roots.

In the stems under consideration the whole parenchymatous tissue surrounding
the isolated segments of wood, together with the periderm, follow their unequal growth
for a long time by means of corresponding dilatation.

Especially in the Malpighiaceae and the Bauhinise, however, the inequalities of
growth, as the increase of thickness proceeds, are so great that the masses of tissue lying
between the ligneous bodies are ruptured. At the surfaces of separation a formation
of periderm takes place ; perhaps also a throwing olf of single bands of tissue
as bark; this however has not been closely investigated. The stem now appears
split into more or less numerous longitudinal strands, each of which is covered by
a special cortex, coated with periderm; these are arranged in the most various
manner parallel to one another, or are interwoven one with another, and separated
for a distance and again united.

The above-mentioned behaviour of the stem of Urvillea Isvis^ belongs to this cate -
gory, because in the second or third year the wood begins, as in the Malpighiacese, to
become three-lobed. By growth in width of the parenchyma of the medullary rays
the three segments of the wood corresponding to the three lobes are separated from
one another, and each of them, with the adjoining third of the pith, is surrounded by
a zone of cambium which thenceforth forms wood and bast, so that the stem contains
three separately growing masses of wood and bast.

As less striking and fundamental internal secondary formations we may connect
with the above those strands of wood which arise subsequently from secondary
meristem, as described by Criiger, /.c., in the pith of Doliocarpus, Argyreia, &c. ; it is
doubtful whether the masses of wood, which appear lobed in transverse section,
and lie, as described by Oliver enclosed in parenchyma in the stem of Acantho-
phyllum (of the family Caryophyllacese), belong to this category.

The formation of bast on the inner surface of old hollow stems of Carica Papaya
should rather be included in the province of phenomena of pathological callus
formation and regeneration.

' Ann. sei. nat. 5 Ser. tom. VI. p. 317, &c.

^ Radlkofer, Atti, l.c., p. 63. — Gaudichaud, l.c.. Tab. XVIII. Fig. 20.

^ Trans. Linn. Soc. London, Vol. XXII. p. 289.

6o6

SECONDARY CHANGES.

Sect. 194. On the other hand, the processes of growth and secondary formation
in the old internal parenchyma of the wood, at a distance from the active normal
ring of cambium, as seen in fleshy roots''-, belong to this class. They have already
been described, p. 600, in Beta. Here they do not extend further than the growth
of the parenchyma itself, and the dislocations of other tissues which result from it.
New formations of secondary meristem and of cambium, and strands of wood and
bast occur, however, as first shown by TrecuH in the old root of Myrrhis odorata,
in the fleshy roots of Convolvulacese, and species of Rumex, according to the
investigations of Schmitz and according to Stahl ^ in the roots of Bryonia, and
may be found more frequently in similar parts. Also the partial rings described
in Sedum Telephium should be connected with the above; the spotted structure
of thick Sumbul-roots may also find its explanation in the appearance of partial
strands of wood in the old originally normal wood.

The roots of Myrrhis odorata always have at first the normal structure and secondary
thickening, and they may retain this through life, and thus attain a great thickness. But,
in most cases, after the normal thickening has continued for a long time, the formation
by means of tangential divisions of radially seriate secondary meristem appears in the
internal parenchyma of the wood at some distance from the middle ; it begins from one
point, and extends through an annular zone, which surrounds the root. This zone
assumes the properties of an independent normal cambium, which, starting from its outer
side, bordering on the peripheral wood, and proceeding inwards, that is in centrifugal
succession, forms strands of bast of normal structure, alternating with medullary rays,
and subsequently on the side remote from the bast layer, that is facing outwards, it forms
strands of wood, which insert themselves exactly on the surrounding portions of the
strands of wood, and increase in a centripetal, that is a reversed direction. This pheno-
menon is found in almost all roots i®'“ or more in thickness. According to Trecul a second
internal layer of secondary meristem or cambium may appear in the same way as the
first : this also, if I understand rightly, forms a new layer of bast in centrifugal succes-
sion. Finally, a new cambium, which also forms a layer of bast, may appear between
the inverted strands of wood and those limiting them externally.

If this has happened, the root then consists of the following concentric layers :
I. Normal cortex with a layer of bast. 2. Normal cambium. 3. A layer of wood, placed
normally. 4. A second internal cambium. 5. A layer of bast. 6. A layer of wood,
placed normally. 7. A layer of bast. 8. A third internal cambium. 9. A layer of
wood, inverted. 10. Cambium, ii. A layer of bast. 12. Axile strand of wood. This
discovery of Trecul’s may be only a special case of the various possible combinations of
concentric zones of secondary formation. The zones that happen to be peripheral
follow the internal secondary formation by dilatation, while the normal increase continues
in the normal cambium. Further, any zone surrounding single strands of wood may
form a cambial zone from secondary meristem, by means of which the single partial
bundle undergoes an individual thickening, and becomes surrounded by bast.

In the roots and also many stems of Convolvulacese^ investigated by Schmitz, various
anomalies appear ; viz. in the first place new strands of wood and bast, which grow
by partial cambiums, and appear first in the parenchyma of the old xylem ; secondly.

' [Compare Weiss, Anat. u. Physiol fleischig verdickter Wurzeln. Flora, 1880.]

^ Comptes rendus, 23 July and 6 Aug. 1866, tom. LXIII.

8 Sitzsber. d. naturf. Ges. zu Halle, July 1874. Compare Botan. Zeitg. 1875, p. 677.

* By word of mouth.

^ [Compare Dutailly, Sur quelques phenomenes dans les tiges et les racines des Dicotyledones,
Paris, 1879.]

ANOMALOUS THICKENING IN DICOTFLEDONS AND GYMNOSPERMS. 607

formations which arise in the cortex, and at first resemble the appearance of successively
renewed cambiums (p. 590), though, as already intimated, they cannot always be included
among these ; and thirdly, combinations of the two processes.

In the old parenchyma of the root new partial cambiums appear, without the activity
of the normal ring ceasing : they are partly in connection with normally-formed vascular
bundles, partly with such bundles as have been formed from secondary meristem in the
parenchyma separate from the former : in both cases they independently produce new
secondary wood and bast in variable quantity, and with the normal succession and
orientation. According as this takes place, the original tissue is displaced and crushed
to such an extent, that finally the whole root may consist of irregularly grouped strands
of wood and bast irregularly lobed in transverse section : each of these has its own ring
of cambium. Still further disarrangement may arise by the original normal ring of cam-
bium ceasing to be active, and becoming quite obscure ; and by the appearance of
repeated newly active layers of cambium in the partial xylem-bodies, as they did before
in the original xylem. These phenomena appear in various individual forms, the de-
scription of which would here lead us too far, e. g. in the roots of Convolvulus Scammonia
and Ipomoea Purga.

In very many roots of Convolvulaceae, both in those which are perennial, and also quite
regularly in the annual roots of Pharbitis hispida Choisy (Ipomoea purpurea), the other phe-
nomenon appears: new strands forming wood and bast by means of their own cambium arise
from secondary meristem, in the parenchyma of the secondary cortex, immediately outside
the strands of bast. Their mode of origin is fundamentally the same as in the formation
of renewed cambial rings at the periphery of old ones that are becoming extinct. The
same process may also be renewed as in the latter case in successive zones lying further
out. A difference however from the typical process of formation of renewed cambiums
is found in the first place in the fact that the production on the part of the normal cam-
bium does not cease with the appearance of the new bundles, but continues ; and in the
fact that, in most cases at least, closed cambial rings or segments of rings, producing
strands of wood and bast alternating with medullary rays, and having a normal orienta-
tion, are not formed by the secondary development; but separate strands, each of which
has its own more or less complete circular cambium, from which is derived an increase
of wood and bast.

The whole transverse section thus resembles, to a certain extent, that of a stem of
the Sapindaceae : there is a central normal round xylem, surrounded by a number of
smaller ones, in simple, later also in compound series. The longitudinal course of the
peripheral strands is irregularly undulated ; they are frequently connected laterally one
with another, and with the normal xylem by anastomoses. The phenomenon described is
found in the permanent stems of several species. Jussieu^ describes in the case of C. ma-
labaricus, in a stem 8<=“ thick, 8-9 irregular concentric rings, and found the same in the
stem of several undefined species. I'he same condition has long been known for the root
of Ipomoea Turpethum, the roots of other species show the same, and in such a way that
the cortical bundles appear only in the root, not in the stem. In the woody roots of the
red garden Convolvulus (Pharb. hispida) they are well developed, but only pass up into
the hypocotyledonary stem, and there pass over with thin ends into the strongly thick-
ened normal xylem.

Both processes, the secondary formation of bundles in the parenchyma of the old wood
and in the cortex, may be combined in the Turpeth root, and in the roots which are
found in druggists’ shops as Mechoacanna and Stipites Jalapae. In these cases the
secondary development of xylem-forming strands goes on, sometimes in the original
normal xylem, while sometimes it may extend also to the xylem-bodies in the cortex.

As stated above on p. 233, the massive starchy parenchyma of the napiform lateral

’ l.c. p. 1 23.

“ P. .S. L. Koch (Verhandl. d. Naturhist. Vereins in Heidelberg, Bd. I. Heft. 4) has recently

6o8

SECONDARY CHANGES.

roots of Sedum Telephium and its allies is traversed by vascular bundles, which are
arranged in the transverse section in a ring. In many species, e. g. Sedum Fabaria,
these bundles are connected throughout by a simple, normal cambial ring, which is but
slightly productive. This is found sometimes also in S. Telephium, and even in certain
roots of those stocks, which otherwise show the condition now to be described. This,
which is the rule in S. Telephium, is as follows. At the point of insertion of the root a
simple, normal, smooth, cambial ring connects the bundles ; further between each pair of
bundles the ring curves towards the swollen centre of the root, and the more deeply the
nearer to the apex, till the stage is reached at which it is divided at the centre into as
many separate partial rings as there were inflections or original vascular bundles. The
separate rings are arranged in the transverse sections in a circle, and are separated one
from another by bands of parenchyma. Each has a closed, and but slightly productive
cambial layer, which both on the side next the wood and that next the bast forms especially
parenchyma ; on the side of the wood facing the middle of the partial ring are isolated
small groups of vessels, on the side facing the bast, and corresponding to the latter, are
those small groups of narrow elements of soft bast mentioned on p. 324. Towards the
apex of the root the partial rings open, and unite again to form a normal xylem body of
the root. Comp. Koch, 1. c.

Sect. 195. The structure of the stem of the Cycadecs^ must here be dealt with
generally, in connection with p. 256, since a separation of the primary arrangement
from the various secondary changes could not be well carried out without too great
sacrifice of clearness.

By way of making matters plain, it may be stated at once that the stem always
has at first the same arrangement of tissues as that typical of the Dicotyledons
and Gymnosperms; i. e. it has a normal ring of wood, bast, and cambium, which
separates the outer cortex from the pith. The two last-named regions are always
relatively strong ; in the old stem of Cycas circinalis, investigated by Miquel,
for instance, the outer cortex was more than 7 cm Jq thickness, and the thickness
of the cylinder of pith 10 cm. They consist like the medullary rays of relatively
thin-walled, permanently starchy parenchyma, and are traversed by the system
of branched gum and mucilage canals, described above on p. 441. The cortex
is covered by a superficial layer of periderm. The two kinds of leaves are, as is well
known, arranged spirally, and are so close, one above another, that the fleshy
scale-like expanded bases of the leaves are in superficial contact one with another.
The numerous vascular bundles of the base of the leaf are united at the surface
of insertion into two bundles of the trace, which enter the stem separately and
symmetrically near the middle of the surface of insertion ; they then diverge directly.

shown that the ring of bundles now under consideration is nothing more than the very isolated vascular
groups of a chiefly parenchymatous xylem of the root, which is derived from a typical radial root-
bundle, the original elements of which had hitherto been overlooked in the massive parenchyma.
The napiform roots of Sedum are thus a special case of very parenchymatous typical dicotyledonous
roots. What is said above should accordingly be corrected.

* A. Brongniart, Rech. sur I’organisation de la tige des Cycadees ; Ann. sei. nat. i ser. XVI. p.
369 ; Id. Archives du Museum, I. — von Mohl. lieber d. Bau d. Cycadeenstammes ; Abh. d. Miinchn.
Acad. I. p. 397 ; Verm. Schriften, p. 195. — Miquel, lieber d. Bau e. Stammes, &c. von Cycas circi-
nalis; Linnsea, Bd. XVIII. p. 125. — Karsten, Organogr. d. Zamia muricata, in Abh. d. Berliner
Acad. V. 1856, p. 193. — Mettenius, Beitr. z. Anat. d. Cycadeen; Abhandl. d. K. Sachs Ges. d. Wis-
sensch. VII. p. 567.

ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 609

the one to the right, the other to the left, and curving downwards through the cortex,
they finally enter the bundle-ring, and there descend further in a radially perpen-
dicular manner. This last part of their course, and their final lower insertion, has
not been thoroughly investigated. The length of the curve, projected in the
horizontal plane, and regarded as circular, which each bundle of the trace describes
through the cortex before its entry into the ring of bundles, is not exactly defined,
and appears not to be always the same, but has been estimated in the case of
Cycas revoluta at least at 145° to 150°, but does not reach 180°, that is the coalescence
of the two bundles. At the beginning of its course, as followed from the leaf-
insertion downwards and inwards, each bundle passes for a long distance imme-
diately below the surfaces of insertion of the leaves, that is just within the surface of
the cortex of the stem, diverging but slightly from it, and running not exactly
horizontally, but descending only slightly. In the last portion of its course, which is
approximately equal in height to that of one leaf-insertion, it runs more steeply

Fig 239. Fig. 240.

FIG. 239. — Cycas revoluta. Course of the bundles of the leaf*trace, in a thick transverse section closely below the
punctum vegetationis of a lateral shoot, made transparent with potash, and seen from its acroscopic surface ; slightly
magnified. The bundles, which run in different, but closely superposed planes, are drawn together in the plane of the
paper, so that the point of exit is kept most dark, and each bundle is drawn from this point to that at which it curves
steeply downwards ; the bundles of the eight successive youngest leaves are successively numbered, i, tliose of the
youngest, highest, &C.; those of the oldest, 9, are not numbered. Between 4, 5, 7, and 8, radial connections are
beginning.

Fig. 240 — Cycas revoluta. Transverse section with six bases of leaves, near to the apex of a small plant raised
from a lateral bud ; natural size. Parenchyma of pith, cortex, and leaves left 'white. The vascular bundles laid bare by
the section are represented as lines, those cut transversely as dots. The inner circle is the still ver\ narrow young ring
of bundles surrounding the pith ; it consists of the lower portions of bundles of the trace descending from above. Out-
side it are the delicate beginnings of the radial connections.

inwards and downwards, till it enters the ring of bundles. The condition described is
accurately represented only while young. A thick transverse section through the
shortly-conical end of the leafy stem, if made transparent, shows the leaf-traces of
the youngest, uppermost leaves in the young cortex as simple symmetrical pairs of
curves, which increase in width the further they are from the apex (Fig. ’239).
Closely below the conically diminished end, the stem and the ring of bundles assume
a more or less cylindrical form ; when, or even before the curves come, by means
of the progressive longitudinal growth of the apex, to lie in the cylindrical region,
the original arrangement is modified by the appearance of connecting branches.
These are in the first place so directed as to follow the course of the curves them-

R r

SECONDARY CHANGES,

6 IO

selves; along the periphery of the stem they connect the two curves belonging to
one leaf on the side opposite the latter into a ring-like transverse girdle, open only
at the two ends which enter the leaf : this girdle may frequently be closed even
between the points of exit by a transverse anastomosis. Simultaneously with the
connections described, anastomoses further appear in the second place between
neighbouring girdles, their number and arrangement not being closely defined ; others
also appear, which run generally in a radial direction from the girdles, inwards and
obliquely downwards, and insert themselves on the bundles of the primary bundle-ring.
In Zamia muricata, according to Mettenius, these radial connections run unbranched
and moderately straight from the ring of bundles to the girdles. In Cycas revoluta,
Dion, and Encephalartos horridus they divide, as they pass from the bundle-ring,
usually into two diverging branches, which branch further, and sometimes anastomose
mutually with their branches, and sometimes pass on to the girdles. The whole of
tliese branches form a complex and irregular cortical net-work of bundles (Fig. 240).
In a specimen of Cycas revoluta Mettenius found that nine to eleven of the bundles
or bundle-trunks that leave the ring belong to one leaf.

The average strength of the girdles, and of the radial connections and their branches,
the special direction of their course, &c., show many variations, which cannot be
enumerated here, partly according to species and individuals, partly, and especially
in Dion, according as they belong to foliage or scale leaves. In strong stems of
Cycas or Encephalartos the absolute thickness of the strongest reaches I™*“ and
more. All the bundles of this original system, which may be called the primary
nel-ivork of bundles, are collateral ; tracheae and sieve-tubes are arranged between
radial rows of parenchyma in radial rows, which are themselves interrupted by
parenchyma ; the tracheal elements at the medullary margin are often subsequently
torn apart by the expansion of the parenchyma. All tracheal elements, as has
already been noted on pp. 165 and 336, are tracheides ; the innermost primordial
tracheides are spirally thickened, the majority have transverse scalariform pits.

As far as is known the primary net-work of bundles is developed in its
full complexity close below the end of the stem. The successive transverse portions
of the stem undergo, it is true, a considerable growth in thickness after the moment
when it is already completed, in the first place by the still continued expansion
of the parenchyma of pith and cortex, and subsequently by the cambiogenetic
increase to be described later. The primary net remains meanwhile permanent ;
both the radial connections, and especially the girdles, must therefore increase
continuously in length. As Mettenius found, the spiral tracheides do not take
part in this, they are torn apart and finally cannot be recognised. The scalariform
tracheides grow, on the other hand, considerably in length; in the girdles of young
leaves the shortest measured 0.09“'»; in those of older stems the length increased
to I ■4™“', in the oldest stems investigated to 4-5'““. The unthickened points of
the walls are thus expanded from the form of narrow transverse slits to broad
elliptical pits.

In the primary ring of bundles a cambial zone arises, as far as is known,
according to the normal mode for Dicotyledons and Gymnosperms, and this also
produces wood and bast generally in the normal way. The strands of xylem and
phloem are situated in the transverse section between broad multiseriate large me-

ANOMALOUS THICKENING IN DICOTYLEDONS AND GVMNOSPERMS. 6ll

dullary rays, which form, when their longitudinal course is seen from without,
acute meshes between the undulating bundles. Through these meshes all the
leaf-traces and radial connections of the primary net-work of bundles pass between
the secondary bundles, in their course from their internal points of insertion into the
cortex. The parenchyma of the medullary rays, which surrounds them, consists
of large radially elongated cells, which undergo only few tangential divisions as
the thickening proceeds, so that one may almost speak of an interruption of the
cambial layer by large-celled parenchyma at the points in question. It may
be concluded even from these facts, and it is confirmed by further investigation,
that the cambial zone arises in the lower ends of the bundles of the leaf-trace,
which descend perpendicularlj^, and that the upper parts of them, which curve into
the ring, take no part in its formation. Further it remains to be more exactly
investigated, how far the radial connections share in the formation of the ring and
cambium, and whether other intercalary bundles are formed in addition.

After it has once appeared the zone of cambium forms wood and bast in the
normal way. Both retain a regular arrangement in radial rows, and both tracheae and
sieve-tubes are disposed in numerous narrow radial rows, consisting of one or more
series, and interrupted by thin-walled parenchyma : these lie between relatively
broad, but small, parenchymatous medullary rays. The tracheal elements of the
secondary wood are exclusively tracheides of moderate width, and having on
their radial walls several rows of transversely elongated bordered pits (Cycas,
Encephalartos), or with a scalariform-reticulate wall (Zamia spec., Stangeria). The
sieve-tubes have been treated of on p. 179. They are accompanied in Cycas,
Dion, and Encephalartos by small groups of hard sclerenchymatous fibres, which are
absent in Zamia and Stangeria ; besides these Mettenius describes isolated chambered
sacs containing klinorrhombic crystals.

The whole mass of secondary wood and bast has accordingly, in the main, the
structure typical of sappy, parenchymatous stems of Dicotyledons. It remains
always narrow in comparison with the pith and cortex ; the bast is usually strongly
developed relatively to the wood, and is often of equal thickness with it.

The investigated species of the genera Zamia, Dion, and also of Stangeria,
show throughout their life the structure hitherto described, i. e. the primary network
of bundles, and the normal ring of secondary thickening, which grows in thickness
slowly and without limit. There are no other parts added to those described. But
in the species of Cycas and Encephalartos the case is otherwise. Firstly, since in all
of them the growth in thickness of the first normal ring is limited, it ceases after
a time not exactly defined, but certainly longer than one period of vegetation, and is
continued by a renewed cambial zone, which appears in the cortical parenchyma
at the outer limit of the bast-layer — fundamentally in the manner described above in
Sect. 191. The renewal of the ring may subsequently be repeated more than once.
IMiquel’s large stem of Cycas, which was certainly many years old, had e. g. 6-8
successive rings.

Both the coarser structure of these rings and their histological composition are
the same as in the first. Also the transit of the radial connections of the primary
net through the medullary rays is conducted in the same way as in the former case.
As in most cases of successively renewed rings, which were described in earlier

R r 2

SECONDA RF CHANGES.

6 1 2

paragraphs, the rings are here also on the whole concentric, but with the same
irregularities as have been repeatedly mentioned above. Usually the different
segments pf one ring are of unequal strength. Here and there the margin of a
segment curves towards the next inner ring, and becomes continuous with it. The
successive zones are therefore in direct connection, in not exactly defined longitudinal
bands. The thickness which the successive rings attain decreases the further they
are from the middle ; they thus remain narrow in comparison with the cortex and
pith. For instance, in Miquel's old stem, which has been repeatedly mentioned, the
whole thickness of the 6-8 rings is hardly one-third of the radius of transverse
section ; the absolute thickness of the single rings varies between O'scm and
A different relation is found at the inverted-conical base of stems which are derived
from lateral buds. A piece of Cycas revoluta before me has, for instance, at the
bottom a radius of transverse section of about 22™“, of which 5“«» are referable to
pith, five to the cortex, and twelve to the four almost equally strong rings of wood ;
130“™ higher up in a radius of transverse section of 29“™, 15mm are referable to the
pith, nine to the cortex, and only five to the two rings of wood, of which the outer is
just beginning to appear.

Both the genera with successively renewed rings have, in the second place, in
addition to those described a further system of accessory bundles, which is in Cycas
a cortical system, in Encephalarios medullary.

The latter consists in the mature stem of numerous bundles distributed through
the whole pith ; these run in an undulating longitudinal course, and are connected
one with another, and with the inner surface of the ring of wood by branches in all
directions. They form an elaborate plexus, which is peculiar to the pith and gives off no
branches into the cortex. According to Mettenius the bundles appear at a late stage,
since they were not present in the whole upper half of a stem of E. horridus of the size
of the fist, which he investigated. They may therefore, for the present, be ascribed
to the category of cauline medullary bundles, wEich were described in other cases on
p. 253. The medullary bundles are collateral; their .xylem, according to Mettenius,
never has spiral or annular tracheides, it has (according to incomplete investigation
of E. Gaffer) fundamentally the structure of the secondary strands of wood of the
rings of the same plant, and seems to have a long continued, though very slow,
growth in thickness. The stronger bundles in the stem of the last-named species
are about i“™ in thickness.

The accessory cortical system of Cycas, which Miquel found in C. circinalis,
and Mettenius in C. revoluta, arises, according to the description of the latter, from
strands of secondary meristem in the cortical parenchyma. Longitudinal rows of
cells of the latter ‘ undergo a division into smaller cells, and become transformed into
cambial strands, which gradually develope into small masses of wood.’ It appears as
though, as the stem grows old, new bundles of this order may be formed successively
for a time; at least they were found in small numbers in young individuals, but in
old ones in large numbers. Still this difference may be purely individual, and all
the genetic conditions require still further investigation.

The arrangement of the bundles in the old stem investigated, in which they
were numerous and strongly developed, is described by Mettenius in the main
as follows. They are arranged in the transverse section in several irregular

ANOMALOUS THICKENING IN DICOTVLEDONS AND ÜEMNOSPERMS. 613

annular zones. They take a sinuously curved course in a vertical direction, thus
crossing the girdles and the radial connections. In an apical direction they may
always be followed as far as the broad base of the foliage leaves. Here they are
seated, and in all cases several of them of unequal strength, sometimes on the two
shanks which pass out from the girdle into the leaf, sometimes on the first branchings
of the latter in the base of tlje leaf itself, the strongest ones as a rule on the first
branching. From this point of insertion they descend through the stem, and after
a course of varying length they either unite, sometimes with similar ones from the
same leaf-base, sometimes with others descending from above, or they insert them-
selves with their lower end, or with a lateral branch, on a girdle or on a radial con-
nection. Free blind ends are not present; nor is there any connection with the
bundle-system of lateral buds or roots.

The structure of the cortical bundles is such that their middle is occupied by
a narrow parenchymatous prism of pith, and this is successively surrounded by a
ring of wood and cambium, and a weak ring of bast. On the part of the cambium
there is a permanent though slow increase, so that in old stems the thickness of the
bundle rises from a“»™ to 5-6“™. Wood and bast are traversed by thin-walled medul-
lary rays, which are successively increased as the thickening goes on : between these
the elements of the bundles are distributed as in the rings. The tracheides of the
W'ood have usually a scalariform pitted thickening in C. revoluta, but few of them have
bordered pits ; spirally thickened ones are absent. At the points of insertion there
is an alteration of the structure, inasmuch as the pith disappears ; the elements of
one bundle then become contiguous with the similar ones of the other.

The secondary thickening of the J^oo/s of the Cycadece'^ which have been investi-
gated always corresponds, at first, like the primary structure described on p. 357, with
that of the typical roots of Dicotyledons and Gymnosperms, and especially with those
which are fleshy and highly parenchymatous. The structure of the secondary ele-
ments of the bundle is fundamentally similar to that of the wood of the corresponding
stem. For their individual characters and distribution, comp. Mettenius, 1. c. For ,
the investigated roots of Encephalartos (E. Caffer, longifolius) nothing of importance
need be added to the above ; at least, roots 3cm thick showed no more remarkable
phenomenon than very considerable extension of the internal parenchyma and
consequent distortions of the strands of wood. Also, as far as the subject could be
investigated, the primary outer cortex is in these plants thrown off at an early stage
by periderm. While accordingly the roots described belong to category (2) of fleshy
roots described on p. 516, that of Cycas revoluta must be placed in category (i)(u:).
The secondary thickening is in this case weak, the starchy parenchyma is still living
in roots as thick as the finger, and is only covered on its outer surface by a layer of
periderm. How long it persists is not known. As the root becomes older, according
to Mettenius, the first cambial ring loses its activity, which is renewed by a peripheral
cambium ‘ and the further growth proceeds exactly as in ihe stem.’

On the bushy dichotomous excrescences of the roots of Cycadese caused by
their penetration by Nostoc, comp. Reinke, l.c.

Mettenius, /. c. — van Tieghem, Reinke, /. c. See p. 356.

SECONDA R Y CB A NGES.

614

Sect. 196, The structure of the stem of Welwitschia mirabilis^ is as wonderful
as its form is peculiar. Moreover, by reason of the difficulty of preparation which
the nature of old dry specimens presents, and of the lack of young fresh specimens,
it remains still uninvestigated in many respects According to data at hand, which
relate almost exclusively to the secondary thickening, it is connected in the matter
of the arrangement of its tissues partly with many anomalous Dicotyledons, partly
with certain Monocotyledons with secondary thickening, and in its histological
structure with other Gnetacece.

The youngest plants, which are known, have a roundish stem, about the size of
a nut, which is continued downwards into a strong tap root, provided with relatively
small lateral branches. The round stem, called by Hooker the stock, has a convex
uneven apical surface, the crown, on which, in the specimens examined, there is no
trace of a true punctum vegetalionis to be found. The blunt margin, by which the
crown passes over into the lateral surface of the stem, is for the most part surrounded
by the almost contiguous surfaces of insertion of two opposite, tongue-shaped leaves,
which are regarded with good reason as the two persistent cotyledons ; each of these
leaves is inserted at the base of a deep ring-like furrow, which is so narrow that it is
loosely filled up by the base of the leaf.

It is known that the plant retains this conformation throughout life, and suffers
only this one important change of form, that the upper part of the stem grows
continuously in width in centrifugal progression, so that it attains the form of an
oblong two-flapped disk, in the blunt, more or less erect, marginal flaps of which are
situated the foliar grooves. These together with the bases of the leaves increase in
girth in proportion to the stem ; the leaves themselves elongate throughout life at
their base, and in a basipetal direction ; the stem, crown, and root increase for many
years in thickness, and attain colossal dimensions.

An anatomical investigation in the smallest specimens has not been undertaken,
but in those which are hardly double their size, and successively in other older ones,
this has been done. They all resemble one another, as far as investigations go,
in the fundamental points of structure. The stem and tap root, with exception of
the grooves of insertion of the leaves to be described below, are covered by a
moderately thick, for the most part brown, fissured, bark-like, and very hard and
brittle cortex. This envelopes a strong internal mass of tissue, which consists of
vascular bundles, pale yellow thin-walled parenchyma, and those huge sclerenchy-
matous fibres described on p. 132, which are embedded in large numbers in the
parenchyma of all sorts, and which point in all directions. As regards the vascular
bundle-system, in the first place, in not too large specimens a large number of
bundles are seen, which run with a radially converging course from the insertion
of the leaf towards the lower end of the stem, or the upper part of the tap root.
They are arranged in one plane, which lies between the surface of the crown and the
outside of the stem, rather nearer the former than the latter ; it has thus approximately

’ J. D. Hooker, On Welwitschia, Trans. Linn. Society, London, Yol. XXIV. — Strasburger,
Die Conifeien u. d. Gnetaceen, p. 374. [Bertrand, Ann. Sei. Nat. Ser. V. vol XX.]

^ [F. O. Bower, On the Germination and Flistology of the seedling of Welwitschia mirabilis,
Quart. Journ. Micr. Sei., vol. XXL New series, June 1881. — On the further development of Wel-
witschia mirabilis. Quart Journ. Micr. Sei., vol. XXI, l88i.]

ANOMALOUS THICKENING IN DICOTl’LEDONS AND Gl'MNOSPERMS. 615

the form of a conical surface or of a disc, according to the inclination of the margins
of the crown ; in other words, the bundles form a middle layer in the stem, of similar
form to the whole crown, and similar in direction to the surfaces of it. Hooker calls
it the vascular stratum. It is, it is true, not directly proved, but hardly to be doubted,
that the individual bundles are directly continuous into the leaves, we may therefore
call it the /aj>er of the leaf-trace. Examined more accurately, it consists of two
layers of collateral bundles lying closely one above the other, and only separated by
narrow bands of parenchyma : the bundles turn their phloem portions, which are
strengthened by strong fibrous strands, towards one another, and their xylem portions
respectively towards the crown and the outer surface of the stem. Connections
between the two layers of bundles are probable, but not observed with certainty.
The bundles of the individual layer are placed with considerable regularity side by
side ; they have an undulating course, and are, as far as can be made out, here and
there laterally connected.

In the central portion of the stratum of the leaf-trace, between the middle of the
crown and the insertion of the tap root, the course of the bundles is less regular,
according to Hooker’s description ; here they form a tangled plexus, from which
descend the bundles of the root, to be more accurately described below.

From the two layers of the stratum of the leaf-trace numerous much smaller bundles
branch at all points : these run in an oblique direction, on the one hand towards the
whole surface of the crown, on the other towards the outside of the stem, being curved
in a sinuous manner apparently irregularly in all directions, and connected by branches
one with another, thus forming a complex tangled net, which may be distinguished as
the peripheral network of bundles. The bundles of the net which run towards the
surface of the crown are often directly connected with those of the inflorescences.

The bundles, which descend into the tap root, are sometimes connected with
those of the central portion of the stratum of the leaf-trace, sometimes with those of
the peripheral net, as branches derived from them : the main direction of their course
is, like that of the root, vertically downwards. In the transverse section of the root
they are arranged with some regularity in concentric rings, separated from one
another by zones without any bundles ; these resemble the rings in stems of Meni-
spermeae and Gnetacese. The rings are moreover the more regular, and the bundles
are larger the nearer they are to the middle. Hooker represents 5-7 such rings in
transverse sections of thin roots; in a good young specimen before me, there are
eight rings at a point close below the stem, and where the radius of transverse section
was 30™"! : of these the outermost are very irregular, and the bundles small. The
bundles of each ring have an undulating course, and often anastomose laterally;
there are also frequent oblique connections between successive rings, especially the
outer ones. The inmost ring surrounds, as a rule, a round pith-like central portion,
apparently without any bundles, which is for instance in the specimen before me
about broad. When carefully examined, however, it is not entirely free from
bundles, but contains a number of small strands of vessels, sieve-tubes, and fibres,
the arrangement of which cannot be more exactly described because of the natui-e of
the material. In some roots Hooker found in place of one inmost ring two eccentric
rings side by side, round which the outer ones were arranged with considerable
regularity, so as to form a simple system of rings.

6i6

SECONDARY CHANGES.

All the vascular bundles above mentioned are, as far as investigated, fully formed
collateral bundles, which are not in contact with active cambial layers; also the
dissimilar tissue, which surrounds them is fully developed. During the huge increase
in mass of the whole plant the structure remains the same in its chief characters, the
parts once present remain fundamentally unchanged, only similar new ones are
added to them externally. This increase in thickness arises from a layer of cambium,
which runs close under the cortex round the whole periphery of the body : it shows
peculiarities to be mentioned below at the insertions of the leaves, and is least active
at the middle of the crown of old specimens. It consists of a few layers of radially
arranged cells, which are distinguished from the isodiametric yells of the adjoining
parenchyma by their arrangement as described, and their thinner walls, the latter con-
taining even here also the universally distributed granules of oxalate of lime ; their
radial is about half as great as their tangential diameter, or their height. Fresh young
divisions may always be observed in 1-3 adjoining cells of one radial row, the initial
layer in any case is thus only at most three layers of cells thick. The outermost
vascular bundles adjoin the cambial layer, others, apparently older, are separated
from it by layers of parenchyma, which are still radially arranged, but have begun to
be displaced. If the description in the case of the Chenopodiacese, Amarantaceae,
and hlirabilis on p. 592 be called to mind, and the phenomena to be described in
Chap. XVII in Monocotyledons, the data brought forward shpw that Welwitschia
has an extrafascicular cambium, forming at its inside vascular: bundles which anas-
tomose in a radial and tangential direction, and alternate with interfascicular tissue.
The whole intra-cambial body is wood in the sense of the word given on p. 591.
Externally the cambium gives off a layer of bast consisting of parenchyma with
scattered sclerenchymatous fibres, again coinciding with the type of Mirabilis and its
allies. The production of this secondary cortex must be abutidant ; the living layer of
cortex, which surrounds the cambium, is, it is true, but thin ; for instance, in a trans-
verse section before me of the upper part of the tap root it is sixteen parenchymatous
cells in thickness; thick crusts of bark, thrown off by repeated internal formation of
periderm, are superposed, as intimated above,' on the older cortex.

Towards the margin of the stem and the leaf-groove the cortex becomes
thinner, and the cambial layer less clear, since the whole tissue of the marginal
portion, as far as the inner surface of the leaf-groove and the insertion of the leaf,
with the exception of the thin outer cortical layer, consists of relatively delicate, regu-
larly seriate cells, apparently still growing and dividing, thus resembling those of the
cambium, between which lie sclerenchymatous fibres and small vascular bundles. On
this delicate, half-meristematic tissue, if the term be allowed, is inserted the
meristematic base of the leaf, which grows on in a basipetal direction. It may
accordingly be said that the extrafascicular cambium passes gradually over into the
half-meristematic tissue of the margin surrounding the leaf-groove : still it appeared
as though even here a cambial layer could be distinguished at a certain distance
from the surface. The doubt on this point must be dispelled by further investi-
gations.

It is evident that the expansion of the margin throughout life is caused by the
growth of the half-meristematic mass of tissue of which it consists. It is also clear
that by means of it the bundles of the peripheral net running towards the margin

ANOMALOUS THICKENING IN DICOTILEDONS AND GYMNOSPERMS. 617

must sulTer a marginal elongation, and that, especially also in the case of the bundles
of the stratum of the leaf-trace, which always extend to the insertion of the leaf,
a seat of intercalary growth must be situated in the latter, in which they undergo
a permanent growth in length. Further, as the base of the leaf and margin of the
stem increase in breadth, the number of vascular bundles lying side by side in it and
in the stratum of the leaf-trace is increased ; new ones must therefore arise suc-
cessively in the latter, which apparently are connected as branches with those
previously present. The mode in which these processes go on remains to be
investigated.

If we start from the well-founded assumption that the mature Welwitschia plant,
with the exception of the successively appearing flowering branches, is simply derived
from the growth of the embryo known as Dicotyledonous, and retains throughout
life its original conformation, the following may with all probability be stated, in
accordance with the known anatomical facts, for the early stages of growth. From
the broad surfaces of insertion of the Cotyledons a large number of original bundles
of the trace converge towards the end of the root, which grows out to the tap root,
and here unite into the axile bundle of the root. No bundles are added to the Cotyle-
donary trace from higher leaves, since there is no leaf-forming punctum vegelatio?ns
at all developed on the axis of the embryo. The ring-like ‘ margin ’ of the axis of the
embryo, which bears the surfaces of insertion of the Cotyledons, increases together
with the latter in breadth by permanent intercalary growth, proceeding in a centri-
fugal direction, so that the apex of the stem attains a discoid form ; its tissue
remains, with the exception of a thin cortical layer, permanently in a half-meriste-
matic condition. The bundles of the leaf-trace undergo an intercalary growth in
length at the insertion of the leaf, according as the marginal expansion progresses,
and new ones are added to those first present; all those which are successively
added arrange themselves in the double stratum of the leaf-trace. Simultaneously
with the beginning of this phenome'non an extrafascicular cambium appears, in a
manner which cannot be exactly stated, both round the whole stratum pf the
leaf-trace, and round the axile radial bundle, which is continued into the half-
meristematic tissue of the margin, and remains permanently active in the whole
remaining circumference of the stem and root as a distinct layer, producing secondary
cortex in centripetal succession, and the secondary wood in centrifugal succession.
The latter consists of the collateral vascular bundles, which alternate with the unlike
tissue, maintaining the arrangement above described.

As regards the structure of the vascular bundles it may be added that they
resemble closely those above* described (p. 334) for the leaf, and are accompanied
also, like them, by similar strands of fibres, but are not exactly similar to them in
other points. Those of the stratum of the leaf-trace and of the inner circle of the
root are, with their accompanying fibrous strands, much larger than those of the leaf,
and have the form of narrow plates, the margins of which correspond to the outer
and inner margins of the bundle. Their structure remains to be investigated in
detail. According to Hooker’s statements, irregularities in the course of the bundles,
and also of the stratum of the leaf-trace, appear in old specimens. It is uncertain
whether we have here to do with fresh intercalary formations in the parenchyma.

CHAPTER XVII.

SECONDARY THICKENING IN MONOCOTYLEDONS
AND FERN-LIKE PLANTS.

Sect. 197. Most stems and roots of Monocotyledons show no secondary
anatomical changes after the primary differentiation of tissue, with the exception
of the formation of a superficial periderm, which often appears, especially in roots
and rhizomes, but is by no means general. Comp. Sect. 24, and Fig. 168, p. 360.
Its origin and properties are, as far as known, the same as described for the cortex
of the Dicotyledons^.

A cambiogenetic secondary thickening, forming wood and bast, is completely
absent in the large majority even of tree-like stems of Monocotyledons and their
roots. After the development of the primary bundle-cylinder the arrangement of
tissues within the epidermis or layer of periderm undergoes no further change. It is
true that there are statements and controversies upon the point that the internodes
of such stems, e. g. of Palms, increase in girth for years after their first differentiation
of tissue and extension, a phenomenon which, if true, depends upon an increase in
volume of existing tissue-elements, not upon a cambiogenetic secondary formation.

Cambium and secondary wood and bast appear, as far as at present known,
only in the more or less arborescent stems 'of Aloineae (Aloe, Lomatophyllum,
Yucca), of Beaucarnea, and the Dracaeneae (Dracaena, Cordyline, Aletris, &c.); in
tubers of Dioscoreaceae ; species of Dioscorea, Tamus, Testudinaria ; and the roots
of Dracaeneae ^

The primary arrangement of the above-named stems is according to the Palm
type (p. 262). When it is at least so far advanced that all the primary vascular
bundles have been begun, and are in course of development, the cambial layer
appears: in a number of species, as Yucca aloifolia, Calodracon Jacquini, Aloe plica-
tilis, and Beaucarnea tuberculata, it appears immediately below the apex of the stem,
even before the differentiation of tissues in the transverse section at that point is
complete ; in most Dracsenese on the other hand — D. reflexa, marginata, Aletris
fragrans — it appears in regions of the stem of considerable age, which have long

’ Compare Sanio, Pringsheim’s Jahrb. II. p. 66.

Treviranus, Physiol. I. p. 197. — Meneghini, Ricerche, l.c.\ compare p. 263. — Unger, Di-
cotyleclonenstamm {l.c., compare p. 249), p. 46. — Schleiden, Grundzüge (3 Aufl.), II. p. 159. —
Schacht, Lehrb. I. p. 329 et passim. — Nägeli, Beitr. I. p. 21. — Millardet, Anatomie, &c., des Yucca
et Dracsena. Mem. Soc. des Sc. Nat. de Cherbourg, tom. XI. — Rauwenhoff, Bydr. tot de Kenntn. v.
Dracaena Draco, Amsterd. 1864 (nach Wossidlo). — Wossidlo, Ueber Wachsth. u. Struct, d. Drachen-
bäume. Progr. Breslau, 186S (see here the older literature). — Falkenberg, Vegetationsorg. d. Mo-
nocotyledonen. Stuttg. 1876.

SECONDA RV THICKENING IN MONOCOTYLEDONS.

619

been differentiated, and are situated 14-18 and 20«>", or many internodes below the
slowly elongating apex. The initial layer of the cambium is a layer of parenchyma-
tous cells characterised by no further peculiarities, which runs round the outer surface
of the bundle-cylinder, and is thus extrafascicular. It is in close proximity to the
outermost leaf-trace bundles, and must doubtless be regarded as belonging to the
jjlerome-cylinder.

Radial growth of this layer and tangential divisions in reciprocal succession
produce in centrifugal order secondary wood, in centripetal direction secondary
cortex, in the same way and with similar
arrangement to the extrafascicular cam-
biums described on p. 591, that \%, secondary
vascular bundles are formed, alternating
with interfascicular tissue, which is always
for the most part parenchymatous in the
plants under consideration. Comp. Fig.

241. The true initial layer remains
meanwhile always as a simple layer of
cells. Its cells, as well as those produced
from it, have the simple structure de-
scribed on p. 465 for cambium and young
secondary thickening. The form of both
is that of erect rectangular prisms, which
according to the individual case are
2-4 times as high as they are broad.

Those sides of their rectangular bases,
which are shorter in a degree which varies
according to the stage of their develop-
ment, are radially directed. In accord-
ance with their mode of origin all the
cells are arranged in radial rows. Those
on the inner side of the initial layer
develope on the one hand in centrifugal
succession, and usually after one or more
tangential divisions, into permanent inter-
fascicular relatively wide parenchym.atous
cells. On the other hand, rapid longitudinal divisions facing in various directions
appear at definite points in tissue-mother-cells, or longitudinal rows of them,
separated internally from the cambium : from these are derived narrow-celled initial
strands, which develope into secondary vascular bundles ; the further develop-
ment of the latter proceeds in centrifugal succession, while at the outer margin of
the strand a formation of interfascicular parenchyma again begins. According to
Älillardet, one to three as seen in transverse section, or as many as nine and twelve
original tissue-mother-cells, according to the size of the bundle, take part in the
formation of an initial strand. When the number is large the cells concerned
always belong to several radial rows. The arrangement and succession of the
initial strands may be concluded from the arrangement of the vascular bundles to be

HIC. 241.— Piece of a transverse section of a stem of a Dra-
cccna, probably D. reflexa, about thick and i‘n high,

slightly magnified, e epidermis; k periderm ; r primary cortex ; b
a bundle of the leaf-trace passing out through the latter; jr pri-
mary bundles of the stem embedded in parenchyma ; ni, x zone of
secondary growth and cambium with initial strands; further in is
mature wood; ^ secondary vascular bundles; st interfascicular
bands resembling medullary rays. From Sachs’ Textbook.

620

SECONDARY CHANGES.

described below. During the development of the latter, as may be here remarked,
there are extensions of the elements to many times the length of the cambial cells,
while the transverse section remains the same or increases, and distortions and
mutual displacements result, which have not been thoroughly studied in the cases
before us, but a general opinion may be formed upon them from the points of view
laid down in Sect. 137.

The secondary vascular bundles are directly connected, at least at the nodes,
with the primary bundles of the leaf-trace, at the point where the latter curve
outwards. They are connected one with another in their longitudinal course by
numerous anastomoses both radially and tangentially, so that they form a net
branched on all sides, the meshes of which are filled up by the interfascicular tissue.
In the Aloinese and Dracaeneae investigated, the meshes of the net are elongated,
pointed and narrow, only as broad as a few interfascicular cells, and the bundles have
a similar undulated course to that of the bundles in the Dicotyledonous stem, and
the wood has a corresponding construction. In the stem of Beaucarnea, on the other
hand, at least in the tuberous swollen base, the meshes are elongated and polygonal,
attaining a width of over inn», and forming a beautiful network, through which at the
broad base of the stem the bundles belonging to the older roots of former years run
in a radial direction. In radial longitudinal section and in transverse section the
bundles in the two cases form more or less regular concentric zones. In the
Dracaeneae these are irregularly pectinated, since the bundles of one alternate with
those of the next inner zone, and project partially into the spaces between them ; in
the other investigated cases, however — Yucca, Aloe spec., Beaucarnea — they are
more regular, and separated one from another by broad interfascicular spaces. The
longitudinal course of the bundles diverges, as has been said above, from the vertical
even in dense narrow-meshed woods. In the Dracaenas, however, it is in the main
perpendicular, if we neglect the undulations. In Yucca aloifolia, on the other hand,
Millardet found the main direction, with many irregularities it is true, to be inclined
strongly to the vertical, as much as 45°, and the direction of the inclination changes
in successive layers; it is usually, but not constantly, directed to one side in each
case for two successive zones, and in the two following zones to the opposite side.

The structure of the secondary vascular bundles is known in the Dracaenas
with some, but not with exhaustive accuracy. While the bundles of the leaf-trace
have the collateral composition and the sheathing usual for Monocotyledons and
contain, according to Caspary only tracheides, there being no vascular perforations
even in the spiral primitive tracheides, the secondary strand is composed of a small
phloem, which occupies about the middle of the bundle, and is surrounded by, on
the average, 2-3 layers of tracheides. The phloem consists, as first pointed out by
Wossidlo, of a small number of sieve-tubes with simple narrow-pored, callous
transverse plates (p. 175); the tubes are accompanied by delicate cambiform cells.
In transverse section the whole number of the thin-walled elements of the phloem is
often very small, hardly as many as six, in other cases certainly larger : its general
outline is accordingly very variable. At its periphery, that is, bordering on the
tracheides, there are thick-walled cells, which have large round non-bordered pits on
thin longitudinal walls. All the tracheides are, as far as known, of the same structure;

* Comp.-ire p. 322.

* I.C., compare p. 165.

SECONDA RV THICKENING IN MONOCOTYLEDONS.

621

they are elongated, and spindle-shaped, with their pointed ends pushed between one
another in various directions, and provided with oblique slit-like bordered pits (comp,
p. 1 61) usually arranged in two irregular longitudinal rows on their very thick
lignified walls. The peripheral ones adjoin the interfascicular parenchyma without
any distinct sheath, and with sparsely pitted surfaces. The number of the tracheides
of a transverse section of a bundle is moderate, it may be on the average according
to the individual and species as high as 25-60. The form of the whole transverse
section of a bundle is more or less broadly elliptical, with the longer axis placed
radially ; the relative breadth of the ellipse appears to be inversely proportional to
the lateral distance of the bundles one from another, and the average distance seems
to differ according to the species.

In the stems of Aloe and Beaucarnea the structure of the secondary bundles is,
according to the data at hand, similar throughout to that described, but it has not
yet been investigated with exactitude. The same holds, according to Millardet, for
Yucca, with the limitation that the small strand of phloem does not lie in the middle,
but at the outer margin of the bundle.

As has already been stated above, the interfascicular tissue is exclusively
parenchyma in the species investigated, that is, not taking into account the crystal-
containing sacs, which are scattered in it often in large numbers. Its cells retain on
the whole their arrangement in radial rows, in which they passed off from the
cambium, though they are often of necessity somewhat displaced around each
vascular bundle. Their form also, and especially their height remains, on the whole,
similar to that of the cambial cells. In radial direction they certainly undergo
a more or less considerable expansion after the division which gave rise to them,
so that their transverse section becomes almost quadratic — but rounded off owing to
the formation of intercellular spaces ; or it retains the form of a similarly rounded
rectangle elongated in a radial direction. This expansion is often specially great in
the rows running radially between the sides of closely-grouped bundles, so that in
transverse section they resemble for a certain distance here and there the medullary
rays with radially-elongated cells found in Dicotyledonous woods; this is very
striking, e. g. in Aletris fragrans.

The structure of the interfascicular xylem-parenchyma shows nothing generally
worthy of remark. In the hard woods of the Dracaenas it is provided with rather
thick lignified walls, covered with numerous round non-bordered pits, in the other
investigated forms it remains thin-walled and sappy.

It is known that the secondary thickening described continues without limit,
and that the old stems of many Dracaenas attain a huge girth as the result of it.
It is uncertain how far the necessary periodical remissions and accelerations of
growth may lead to inequalities of structure in the wood, corresponding to the
formation of annual rings of Dicotyledonous woods.

The secondary formatio7i of cortex, which is derived from the cambium, is not
very extensive, its product is thin-walled cortical parenchyma with crystal-containing
sacs. Its cells after leaving the cambial layer undergo single transverse divisions ;
in Cordyline paniculata these are frequent, in other forms, such as Calodracon,
Aloe sp., and Beaucarnea, they occur here and there ; these cells thus become only
half as high as the cambial cells. According to their origin they are at first always

622

SECOJVDARr CHANGES.

arranged regularly in radial rows ; it has not been accurately investigated how far
this arrangement is disturbed by subsequent dilatation. Sooner or later there
appears in the subepidermal layer the formation of the superficial periderm, already
mentioned above ; this continues through life, and follows the dilatation of the stem.
In Beaucarnea it forms the thick masses of cork which are split up, as the plant grows
old, from without inwards, and form a covering for the tuberous base of the stem.

Sect. 198. Most of the basal tubers of the Dioscoreaceae require more exact
investigation in all points. As far as known at present three categories of them may
be distinguished: viz. (i) tuberous swollen roots^ — Dioscorea Batatas; (2) rhizomes
with scaly leaves, and composed of many internodes — Dioscorea villosa ; (3) leafless
tubers, resulting from the swelling of the first epicotyledonary internode of the
seedling — Tamus communis \ also T. polycarpus Testudinaria®, and many species
of Dioscorea. Only the tubers of category (3) have a cambium, and secondary
thickening; and these resemble in the main phenomena those treated in the above
paragraphs. The first origin of the cambium is unknown. In the specimens
investigated it surrounds the whole lateral surface of the tuber inside a thin paren-
chymatous cortex : where the tuber is seated on the ground, as in Testudinaria, with
a horizontal base, or, as in Dioscorea sinuata Hort., with an oblique flat base, it is
absent towards the base. The form of its cells, the production of secondary wood
and sparing cortex, the permanent arrangement in radial rows of the interfascicular
elements, and the arrangement and connection of the secondary vascular bundles
are fundamentally the same as in the stems described. The interfascicular tissue
consists exclusively of thin-walled, starchy, parenchymatous cells often much extended
in a radial direction, together with sacs containing raphides. This forms the main
mass of the tuber. The thin secondary bundles, which form a net in the parenchyma,
are collateral. Their xylem consists of elongated tracheides — at least I could not
find vascular perforations — their lateral walls being reticulate or scalariform, or being
for the most part closely covered with multiseriate, small, transverse, slit-like bordered
pits; they are, especially in Tamus and Testudinaria, curved in the most various
manner, and rolled one within another. The surface is covered at a very early stage
with a periderm, which persists through life, and follows the dilatation, and which
forms the usually sclerotic corky crust: it has already been mentioned, p. 113, that
in Testudinaria the crust is fissured.

Sect. 19 p. A secondary thickening of Monocotyledonoiis roots is only known in
the Dracsenas, in Dr. Draco, marginata, fruticosa, reflexa and in Aletris fragrans.
While some of the roots of these plants retain the primary structure unaltered
(p. 361), the pericambium of many stronger ones assumes the properties and
functions of an extrafascicular cambium. As far as the few and incomplete investi-
gations extend, the properties of these and their secondary products are exactly the
same as those of the similar parts of the corresponding stem. As far as is known,
the secondary thickening always begins first in the root, when it is of a considerable
age, and when its primary tissue has long been fully formed, and its endo.'ermal cells

^ Dutrochet, 01 s. sur les embryons veg^taux. Nouvelles annales du Museum d’Hist. Nat. IV.
(1835'), p. 169. ^ Compare Unger, Anat. u. Physiol., p. 239.

“ Von Mohl, Ueber den Mittelstock von Tamus Elephantipes. Verm Sehr. p. 186 (1836).

* Caspary, Pringsheim’s Jahrb. I. p. 446. — Wossidlo, /. c. p. 27.

SECONDA Ry THICKENING IN ISOETES.

623

are specially thickened and sclerotic. Accordingly, the endodermis is split longitudi-
nally by the growth in thickness, with the same phenomena as have been described
for the splitting of cortical fibrous rings, p. 543. The primary outer cortex follows
the growth in thickness, at least for a time, by means of growth by dilatation. It is
uncertain whether it may be thrown off later by an internal formation of periderm.

Sect. 200. Among the Feni-like plants now living there are known, among
secondary changes of tissue-distribution, some indications of formation of periderm,
which have already been mentioned on p. 108. A secondary thickening, starting
from a cambial layer, which produces secondary wood and secondary cortex, is only
found in the Isoeteae k The phenomena in question, which there appear, differ in
many points from those known in Phanerogams, but are to be connected with
these as a very simple member of the series, which they themselves form. The
single zones and parts related to the secondary thickening may without difficulty be
compared with those of the Phanerogams, and may be designated by the same
names without altering their meaning. Scruples, which have been felt against this,
are allayed, if, in drawing the comparison, a start be not made from the normal
Dicotyledons alone, but if the whole series of phenomena described in the above
paragraphs be kept in view.

As is known from the descriptions ^ the short simple stem of the Isoeteae is
2- or 3-lobed, in exceptional cases 4-lobed, and the lobes are separated from one
another by longitudinal furrows, from which the roots arise. The middle of the
stem is traversed longitudinally by the axile bundle containing tracheides, mentioned
on pp. 280 and 347, which extends on the one hand to close below the meristematic
group of the flat apex of the stem, and there developes further in an acropetal mariner
as a conical-cylindrical body, according as new leaves appear ; on the other, basiscopic
side it widens out into as many arms or wings as there are furrows in the stem.
Each wing is opposite one furrow of the stem : its generally convex lower margin
and its almost straight upper margin join into a single angle opposite the furrow.
The wings increase in breadth as new roots arise, and the vascular bundles of the
latter are inserted on them. The above data lead to the view put forward by
Hofmeister and Sachs, that the whole axile bundle arises simply from the sympodial
coalescence of the points of insertion, on the one hand of the bundles of the leaf-
trace, on the other of the bundles of the roots.

The upper end of the tracheide-containing bundle, surmounted by the small
group of apical meristem, is surrounded laterally by radial rows of meristem directed
towards the surface, from which the thick parenchymatous primary cortex is derived :
the latter retains the arrangement in radial rows. As far as a point close below the
tracheide-bearing end, the bundle is surrounded by a layer of those tabular cells,
which in I. lacustris are transparent, with lustrous walls; these have been above
mentioned, p. 347, as probable representatives of the sieve-tubes. This layer has,
like the bundle of tracheides, passed over into the permanent condition. While the

‘ [On the occurrence of a cambial ring in Botrychium, cf. Russow, Vergl. Unters, p. 119.]

^ Von Mohl, Ueber den Bau des Stammes von Isoetes lacustris. Verm. Sehr. p. 122. — Hof-
meister, Beitr. z. Kenntn. d. Gefässkryptog. I. Abhandl. d. Sachs. Ges. d. Wissensch. Bd. IV. — A.
Braun, l.c., compare p. 418. — Russow, Vergl. Unters, p. 139. — Hegelmaier, Botan. Zeitg. 1874, p.
481. — Compare also Sachs’ Lehrb. p. 473 [2nd Eng. Ed. p. 483].

<524

SECONDARY CHANGES.

radial rows of meristem at the periphery also develope into parenchymatous cells, a layer
immediately adjoining the tabular cells remains meristematic, and acts as cambium
throughout life. This layer of cambium surrounds the whole axile bundle, with the
exception of the apex, and the place where it is simultaneously perforated by the
insertions of bundles from the leaf and root. It appears in the strict sense to be a
single initial layer of cells, but this is not made out with certainty. The cells are of
similar form to those of the tabular layer, but are on the average shorter in a radial
direction, and are also similar in all points to those of the cambium of Monocoty-
ledonous stems. Like these they show further successive radial extens'on, and
reciprocal, tangential, longitudinal divisions. The products of the latter pass over on
the side next the bundle in centrifugal succession, and on the side next the cortex
in centripetal succession, into a definitive condition of tissue. In the relative extent
of the thickening, however, in the two directions, there is this difference from all other
known instances, that the axile bundle always remains thin and narrow in comparison
with the cortex, and only increases in thickness by a few layers, while the secondary
cortex grows in the course of years hundreds of times as much. The elements
added in centrifugal succession to the axile bundle have the form of the tabular cells
often mentioned. They retain the original radial arrangement and close connection
one with another, and, at least in I. lacustris, are without intercellular spaces. Oblique
divisions occur, deviating from the tangential longitudinal direction, and require more
exact study. The cells retain for the most part in I. lacustris the above-described
brilliant walls and transparent contents ; the latter also require more exact investi-
gation. Between the transparent cells there are isolated cells or longitudinal layers
of them, which contain numerous starch grains. In terrestrial species, as I. hystrix, and
Durieui, as Hegelmaier found, there is a regular alternation between transparent and
starchy concentric layers, so that one layer of starchy cells lies between each series of
3-5 layers of transparent ones. Tracheides are entirely absent in most cases in the
secondary thickening of the bundle j but they have been observed in single specimens
of I. lacustris and Durieui, situated singly or in groups between the tabular elements,
and resembling these in form and arrangement, but similar in structure to the original
tracheides of the bundle. Finally, attention may be called to the fact that it is
uncertain whether the above-mentioned first layer of tabular elements is the first
product of the cambium, or belongs to the primary bundle.

The cambiogenetic secondary cortex consists exclusively of parenchyma, the
cells of which contain according to the species large quantities of starch, or of
starch and oil. According to their origin they remain arranged in radial and con-
centric rows ; as they develope, wide air-containing spaces appear between the
rounded corners, especially in species with an aqueous habit. Like the primary
cortex the secondary thickening is also much more considerable between the furrows
than opposite them. It is known that in each period of vegetation there is a large
production of secondary cortex, which pushes the older parts of the cortex outwards,
together with the old leaf-bases and roots which adhere to it, and with the vascular
bundles belonging to these, which lie internally, and are stretched and finally torn
away by the thickening ; also that the old layers of cortex successively die and rot
off, while their empty cell-walls turn brown ; but they are not thrown off by a
formation of periderm.

INDEX.

Abelia rupestris, pith, 403.

Abelmoschus tetraphyllus,
fibres, 132.

Abies, 77 ; annual rings, 476,
477; cotyledons, 245; len-
ticels,563,565; medullary
rays, 489, 492 ; mucilage,
143 ; periderm, 548, 553,
558 ; pits, 7 1 ; resin-canals,
443, 544; vascular system,
356, 378, 380; sieve-tubes,
179.

Abies alba, 492.

Abies amabilis, 419.

Abies balsamea, 245, 443,
490, 492, 493.

Abies Brunoniana, 443.

Abies excelsa, 356, 378, 380,
443. 489. 490. 544. 553 i
cork, 108 ; growth of
wood, 477.

Abies pectinata, 378, 380,
443) 476, 477, 489, 492,
510, 512, 541, 544, 548,
553, 558, 563, 565 ; crys-
tals, 142 ; growth of wood,
477; mucilage, 143; sieve-
tubes, 179, 180; stomata,
49 ; wax, 85.

Abies Pichta, 492, 493.

Abies Pindrow, 300, 493.

Abies Pinsapo, 378, 380.

Abies sibirica, 544, 553.

Abietineae, 14, 118, 300, 356;
bast fibres, 527 ; connec-
tions of vascular bundles,
379, 382, 386 ; medullary
rays, 490; periderm, 553 ;
secretory reservoirs, 205,
526, 544; wood, 515.

Acacia, end of vascular bun-
dles, 305, 377 ; glands, 90,
92, 96 ; parenchyma, 408 ;
secretion, 98; wax, 85,
87.

Acacia calamifolia, glands,
96, 377-

Acacia cultriformis, wax, 85.

Acacia floribunda, 484, 496..

Acacia Huegelii, wax, 85, 305.

Acacia latifolia, glands, 96,

377.

Acacia longifolia, 305 ; an-
nual rings, 506 ; glands,
96 ; thickening, 468.

Acacia longissima, glands,
96.

Acacia lophantha,glands,96,

377.

Acacia marginata, 305, 377 ;
glands, 96 ; secretion, 98.

Acacia melanoxylon, glands,
96.

Acacia myrtifolia, glands,
96.

Acacia obtusata, glands, 96.

Acacia pulchella, glands, 96.

Acacia Sophora, 479, 496.

Acacia striata, glands, 96.

Acacia subulata, glands, 96.

Acanthaceae, 32 ; cystoliths,
105; secretion, 102.

Acanthophyllum, 605.

Acanthus mollis, 105.

Acer, 457, 484, 496, 532;
cortex, 404 ; crystals, 140,
142, 530; fibres, 527;
hairs, 61 ; lenticels, 562,
565 ; ligneous bundle, 495 ;
medullary rays, 489; peri-
derm, 548, 558 ; phello-
derm, 549; sap-wood, 507;
secretory sacs, 146, 150;
stomata, 47 ; vascular sys-
tem, 244.

Acer campestre, cork, no,
114, 495, 498, 548, 557,
562.

Acer dasycarpum, 489.

Acer monspessulanum, 150,
167.

Acer Negundo, cuticle, 75.

Acer opulifolium, crystals,
140, 142.

Acer platanoides, 150, 479,
489, 495, 498, 507, 530,
548. .

Acer Pseudoplatanus, 489,

495, 498, 507, 548 ; annual
rings, 476 ; vascular sys-
tem, 244.

Acer saccharinum, 150, 489.
Acer striatum, 535, 548, 558,
565; cuticle, 75 ; epider-
mis, 77, 79, 81 ; wax in
epidermal layer, 75, 82.
Acerineae, tannin sacs, 153.
Achillea Millefolium, 446,
447-

Achyranthes, vascular sys-
tem, 249.

Achyranthes aspera, 591,
595-

Aconitum, water-pores, 51.
Acorus, laticiferous tubes,
436; sclerenchyma, 420 ;
vascular bundle, 317 (Fig.
147), 358 (Figs. 166,

167).

Acorus Calamus, i o, 2 1 3 ; air-
spaces, 218; parenchyma,
407, 412 ; sclerenchyma,
422 ; secretions, 145 ;
vascular system, 268, 316,
327, 339, 360.

Acorus gramineus, endoder-
- mis, 122 ; secretions, 145,
268, 339.

Acropera, aerial roots, 230;

vascular system, 302.
Acropera Loddigesii, 165,
230.

Acropteris australis, 427.
Acropteris radiata, fibres,
132.

Acroptilon, 150.
Acrostichum axillare, 289,

426.

Acrostichum brevipes, vas-
cular system, 288, 313.
Acrostichum Lingua, 288,

313-

Acrostichum Melanopus,
288.

Acrostichum simplex, 288.
Actaea, medullary bundles,
248.

s s '

626

INDEX.

Actaea racemosa, 249.

Adansonia digitata, annual
rings, 504.

Adenocalymma, 574.

Adenophora Lamarckii,434.

Adiantum, root structure,
363 (Fig. 169); vascular
system, 301.

Adiantum denticulatum,42 7.

Adoxa Moschatellina, endo-
dermis, 122.

jEclimea, 41 1.

.^chmea farinosa, wax, 83.

iEgopodium, collenchyma,
1 19; sap passages, 449.

.(Egopodium Podagraria, se-
cretory reservoirs, 449.

Aerial root-sheath of or-
chids, 227.

Aerides odorata, 165.

.Erva javanica, 591, 593.

iEschynanthus, parenchyma,
411.

jEschynomene, 499.

Esculus, 108, 470, 496;

cortex, 404 ; crystals, 530;
fibre, 131 ; lenticels, 562 ;
medullary rays, 489; peri-
derm, 558; root-cap, 413;
thickening,47 1 ; tracheides
and fibres, 483 ; vascular
system, 244, 353, 391,
536.

jEscuIus Hippocastanum,
108, 479, 540; fibres, 13;
crystals, 142.

Aüsculus macrostachya, 244.

.Esculus rubicunda, 489.

jEthusa, vascular system,
242, 309.

iEthusa Cynapium, 242.
309.

Agapanthus, stomata, 36.

Agapanthus umbellatus, 36-
139.

Agathosma, mucilage, 73;
oil-cavities, 207.

Agave, sclerenchyma, 422,
425; stomata, 37,

Agave americana, 77; cuti-
cle, 81 ; vascular system,
305 ; wax, 85.

Ageratum conyzoides, oil-
passages, 446, 447.

Aglaonema, laticiferous
tubes, 436; secretory re-
servoirs, 444; vascular
system, 268.

Aglaonema marantaefolium,
445-

Aglaonema simplex, 436,445.

Agrostis vulgaris, stomata,
50.

Ailanthus, lenticels, 563 ;

ligneous bundle, 495; me-
dullary rays, 489 ; resin-
passages, 202, 453.

Ailanthus glandulosa, annual
rings, 476; cortex, 404;
gelatinous layer, 482, 502,
509.

Air-cavities, structure of the
walls, 215.

. Air-pores, 45 ; absence of,
45; on rhizomes, 46 ; on
submerged parts, 46.

Air-spaces, 210; of water
plants, 210.

Aira caryophyllea, stomata,
50.

Aira flexuosa, stomata, 49,
50.

Ajuga genevensis, stomata,
36.

Alburnum, 507.

Alchemilla vulgaris, water
pore, 52.

Aldrovanda, 67 : glands, 100 ;
hairs, 62; vascular sys-
tem, 277, 369.

Aletris fragrans, 618, 621.

Aleurites triloba, 162.

Alisma, 10, 205 ; air-spaces,
217; root development,
397; vascular system, 327.

Alisma Plantago, 10, 165.

Alismaceae, air-spaces, 213;
milk canals, 202, 443 ;
vascularsystem, 371 ; scle-
renchyma, 419.

Allium, air-spaces, 214 ; apex
of root, 10 ; crystals, 142 ;
latex, 146; parenchyma,
409, 410; raphides, 142;
secretory sacs, 146 ; sto-
mata, 37 ; vascular system,
267, 357.

Allium ascalonicum, 147.

Allium Cepa, 71; secretory
sacs, 147 (Fig. 56) ; vas-
cular system, 267, 357.

Allium fistulosum, 147.

Allium nigrum, 41 1.

Allium Porrum, 147, 357.

Allium sativum, 142.

Allium ursinum, 410.

Alnus, 502 ; annual rings,
476; bast fibres, 527;
hairs, 90 ; ligneous bundle,
495 ; medullary rays, 489,
492 ; periderm, 548 ; pith,
403 ; vascularsystem, 353.

Alnus glutinosa, 489, 495;
secretions, 138 ; crystals,
142, 530 ; gelatinous

layer, 482.

Alnus incana, 489.,

Alnus viridis, 493.

Alocasia, vascular system,
268, 269, 436.

Alocasia odorata, 268.

Aloe, cuticle, 76, 81; mu-
cilage, 143 ; parenchyma,
1 16, 408; sap-cavities,

148; sclerenchyma, 425;
secondary thickening,
618; secretory sacs, 147,
148.

Aloe africana, secretions,
138.

Aloe arborescens, 408 ; se-
cretions, 138, 148.

Aloe atrovirens, 408.

Aloe ciliaris, 148.

Aloe cuspidata, 408.

Aloe plicatilis, 148, 408, 618.

Aloe soccotrina, epidermis,
79-148, 408.

Aloe tesselata, 408.

Aloe verrucosa, 425 ; section
of leaf (Fig. 25), 71, 78 ;
wax, 82.

Aloe vulgaris, 148.

Aloineae, 618.

Alopecurus geniculatus, sto-
mata, 50.

Alopecurus pratensis, 419 ;
stomata, 50.

Alsine, vascular system, 243 .

Alsophila, sclerenchyma,42 8 ,
429; sieve-tubes, 18 1 ;
vascular system, 286, 291 ,
312.

Alsophila aculeata, 312.

Alsophila aspera, hair-struc-
tures, 64.

Alsophila blechnoides, 286,
429.

Alsophila Haenkei, 291, 294.

Alsophila microphylla, u8,
293, 294, 428.

Alsophila pruinata, 286, 287,
429.

Alsophila radens, 291.

Alsophila villosa, 293.

Alstroemeria, 139,410.

Alternanthera, vascular sys-
tem, 249.

Alternanthera amoena, 62.

Alternanthera spinosa, hairs,
62, 591.

AlternantheraVerschaffeltii,

591.

Althaea rosea, 144.

Althenia, vascular system,
277, 278, 368.

Alyssum, 106.

Alyssuin petraeum, hairs, 61.

Amarantaceae, anomalous
structure, 590, 593 ; me-
dullary bundles, 248,

Amarantus, anomalous

INDEX.

growth, 591, 595 ; secre-
tions, 138; stomata, 36;
vascular system, 248, 249.

Amarantus caudatus, 36-
138, 249.

Amarantus retroflexus, 138,
249, 591.

Amaryllideae, 10: air-spaces,
216 ; raphides, 139.

Amaryllis formosissima, sto-
mata, 36, 38, 139.

Amentaceae, vascular system,

353.

Amorpha fruticosa, 486, 496,
502.

Amorpha glabra, pith, 403.

Ampelideae, crystals and ra-
phides, 143; vessels, 169.

Ampelopsis, 143; lenticels,
560.

Ampelopsis hederacea, vas-
cular system, 241.

Ampelopsis quinquefolia,
hairs, 65.

Ampelopsis Veitchii,hairs,65.

Amphilophium, 575.

Amygdaleae, 479 ; ligneous
bundles, 495 ; vascular
system, 377.

Amygdalus, periderm, 548.

Amygdalus communis, 479,
493> 509 ; gelatinous layer,
482.

Amyrideae, oil-cavities, 207.

Anacardiaceae, resin-pas-
sages, 202, 204, 452.

Anacyclus Pyrethrum, 447.

Anagallis arvensis, vascular
system, 243, 308.

Ananassa, sclerenchyma,
418; vascular system, 265,
266.

Anatomical characters, 25.

Anchusa italica, secretion,
106.

Andromeda calyculata, hairs,
64-

Andromeda dealbata, wax,
85.

Andromeda polifolia, pith,
403.

Androsaemum, 243.

Aneimia, stomata, 37, 39,
43 ; vascular system, 285,
343-

Aneimia hirta, epidermis,
37, 42 (Fig. 16), 43.

Aneimia Phyllitidis, 37, 43,
343-

Aneimia villosa, stomata, 43.

Anemopaegma, 575.

Angelica sylvestris, oil-pas-
sages, 450.

Angiopteris evecta, scleren-

chyma, 426 ; vascular sys-
tem, 290, 343, 345.

Angiospermae, 12, 14, 23.

Angiospermous phanero-
gams, embryo of, 7.

Angiosperms, punctum vege-
tationis, 9 ; sieve-tubes,
177-

Angraecum subulatum, aerial
roots, 228, 229.

Anisostichus capreolata, ano-
malous wood, 570 (Fig.
224); structure of wood,
547, 575, 601, 602 (Fig.
237).

Annular vessels, 156.

Ansellia africana, aerial roots,
228.

Anthemideae, hairs, 61.

Anthobolus, stomata, 45.

Anthoxanthum oJoratum,
stomata, 50.

Anthriscus Gerefolium, vas-
cular system, 352, 353.

Anthriscus vulgaris, 450.

Anthurium, aerial roots, 228,
230; cork, 108; crystals,
140; laticiferous tubes,
436 ; secretory reservoirs,
445 ; vascular system, 268,
304, 361.

Anthurium acaule, 230.

Anthurium crassinervium,
230, 445.

Anthurium digitatum, 361.

Anthurium egregium, 230.

Anthurium intermedium,
230.

Anthurium membranulife-
rum, 411.

Anthurium Miquelianum,
268.

Anthurium rubricaule, crys-
tals, 140.

Anthurium Selloum, crys-
tals, 140.

Anthurium violaceum, 230,
445-

Antidaphne, stomata, 45.

Antirrhinum, vascular sys-
tem, 236, 243, 308.

Antirrhinum majus, glands,
94.

Apargia, 231, 433.

Apical cell, 15.

Apocynacese, 457 ; anoma-
lous wood, 569, 577 ; bast,
524; fibres, 129, 133, 134;
latex, 185; laticiferous
tubes, 187, 439 ; sieve
tubes, 231 ; vascular sys-
tem, 338.

Apocynum, hairs, 61 ; lati-
ciferous tubes, 439.

627

Apocynum hypericifolium,
vascular system, 243.

Aponogeton, air-spaces, 217;
vascular system, 352, 371.

Apple, cuticle, 8t.

Arabis albida, vascular sys-
tem, 237, 243.

Aracese, see Aroideae.

Aralia chinensis, oil-pas-
sages, 450.

Aralia japonica, 253, 310,
320.

Aralia papyrifera, vascular
system, 253.

Aralia racemosa, water-
pore, 52, 253.

Aralia Sieboldii, 450.

Aralia spinosa, oil-passages,
450.

Araliaceae, medullary bun-
dles, 535; mucilage pas-
sages, 202 ; secretory re-
servoirs, 204, 526 ; vas-
cular system, 253, 310,
320.

Araucaria, annual rings, 513;
cotyledons, 245 ; medul-
lary rays, 490 ; paren-
chyma, 408 ; resin-canals,
443, 526; sclerenchyma,
424; vascular system, 246,
301, 380, 382 ; wood,

513-

Araucaria brasiliensis, 14,
245, 380, 382, 443, 490,
513,526.

Araucaria Cookii, 443, 526.

Araucaria Cunninghami, 14.

Araucaria excelsa, 513.

Araucaria imbricata, 382,
424; fibres, 130, 133;

stomata, 40.

Arauja sericophora, 439.

Arbutus Andrachne, 555.

Arbutus Unedo, 33,77,213,
555-

Arceuthobium, 31: cortical
bundles, 256 ; haustorium,
384 ; stomata, 45.

Arceuthobium Oxycedri,

vascular system, 257.

Archangelica, 309, 537.

Archangelica officinalis,

water- pore, 52.

Ardisia, secretory reservoirs,
202.

Ardisia crenulata, secretion,
202, 209.

Aremonia, water-pore, 52.

Argemone, latex, 183; lati-
ciferous tubes, 187, 435,
525; root, 524.

Argemone mexicana, 525.

Argyreia, 605.

s s 2

628

INDEX.

Aristolochia, 486, 536 ; cork,
no; cortex, 404; lig-
neous bundle, 495 ; scle-
renchyma, 419; secre-
tory reservoirs, 145; vas-
cular system, 239 (Figs.

96, 97), 324-

Aristolochia biloba, section
of stem, 550 (Fig. 219).

Aristolochia Clematitis, 239.

Aristolochia cymbifera, 550.

Aristolochia Gigas, 239.

Aristolochia Sipho, 239, 456,
467, 486, 495, 535 ; crys-
tals, 529; fibres, 134;
medullary rays, 489 ; pith,
533; sclerenchyma, 126,
419 : woods, 583.

Armeria,75 ; lime scales, 107;
vascular system, 249.

Armeria plantaginea, sto-
mata, 75, 107.

Armeria vulgaris, 107.

Arnica Chamissonis, oil-pas-
sages, 446, 447.

Aroideae, i o : aerial roots, 230;
air-spaces, 213; cork, 108;
crystals, 141, 142, 220;
endodermis, 125; fibres,
129, 130; intercellular

hairs, 220, 222 ; intercel-
lular spaces, 213; latex,
146, 183; laticiferous

tubes, 187, 188, 199; oil-
passages, 444 ; parenchy-
ma, 410, 41 1 ; raphides,
139, 140; resin-passages,
444; root-structure, 361 ;
sclerenchyma, 422, 423 ;
secretions, 136 ; secretory
sacs, &c., 146, 153, 201,
202; sieve-tubes, 173; sto-
mata, 51 ; stone-elements,
127; vascularsystem, 265,
268, 298, 301, 304, 311,
315, 320, 327, 328, 361,
386; velamen, 227.

Aronia, phelloderm, 549;
pith, 403.

Arrabidaea, 573.

Artanthe, 474; epidermis,
33; medullary bundles,
249; sclerenchyma, 420.

Artanthe colubrina, 33.

Artanthecordifolia, 249, 250.

Artanthe elongata, hair-
structures, 65 ; spiral ves-
sels, 156; tracheae, 156,
157-

Artemisia, oil-passages, 446.

Artemisia Abrotanum, spiral
vessels, 158, 496.

Artemisia Absinthium, hairs,
61 .

Artemisia camphorata, hairs,
61.

Arthrocnemum fruticosum,
59I) 593-

Artocarpeae, latex, 184 ; la-
ticiferous tubes, 187 ;
secretions, 137.

Arum, sclerenchyma, 422;
vascular system, 268.

Arum vulgare, 436.

Arundinariaspathiflora,scle-
renchyma, 128.

ArundoDonax, parenchyma,
41 1 ; thy loses, 170; vas-
cularsystem, 311, 322.

Asclepiadeae, 457, 470, 486 ;
fibres, 129, 133, 134;

latex, 158; laticiferous
tubes, 187, 439 ; sieve-
tubes, 231 ; vascular sys-
tem, 297, 338.

Asclepias, fibres, 129; latex,
184; laticiferous tubes,
187, 194) 432, 439, 525 ;
root, 516.

Asclepias Cornuti, 432, 439,
516 ; fibre, 13 1.

Asclepias curassavica, 187,
194, 432, 439, 486, 516,
524, 525-

Asparagus, vascular system,
323, 357, 387-

Asparagus officinalis, 357.

Asperifoliae, stomata, 41.

Asperula, 296.

Asperula odorata, stomata,
49.

Asphodelus, air-spaces, 214;
parenchyma, 409.

Asphodelus luteus, stomata,
75; vascular system, 320,
353, 387-

Asphodelus ramosus, 387.

Aspidistreae, 10.

Aspidium, 118 ; glands, 93 ;
internal hairs, 220 ; lime
incrustations, 106; scle-
renchyma, 427; secretion,
98; vascularsystem, 285,
288, 307, 312, 364.

Aspidium albopunctatum,
106, 288, 313.

Aspidium coriaceum, vascu-
lar system, 387 (Fig. i35j,
288, 313, 411.

Aspidium cristatum, 285,3 ’ 2.

Aspidium falcatum, 410,427,

Aspidium Filix-mas, 118,
427; air-spaces, 213, 215 ;
internal hairs, 220;
parenchyma, 410; vas-
cular system, 285, 286
(Figs. 132, 133), 312, 313,
314.

Aspidium leucostictum, 106.

Aspidium molle, 342.

Aspidium pedatum, 106.

Aspidium spinulosum, 220,
312.

Aspidium Thelypteris, 285,
286, 364.

Asplenium, sclerenchyma,
427 ; vascular system, 285,
288, 344.

Asplenium auritum, 344.

Asplenium bulbiferum, sto-
mata, 41.

Asplenium Filix - foemina,
285.

Asplenium furcatum, sto-
mata, 39.

Asplenium lucidum, 427.

Asplenium nidus, crystals,
67, 141.

Asplenium obtusifolium,
288.

Asplenium resectum, 288.

Astelieae, 10.

Aster, 446, 447.

Astereae, 446.

Astragalus, hairs, 61.

Astragalus aristatus, 426.

Astragalus falcatus, fibres,

133-

Astragalus rhodosemius,
pith, 534.

Astrocaryum, 420, 425 ; vas-
cular system, 266.

Astrocaryum vulgare, 425.

Athyrium Filix-fcemina, vas-
cular system, 312, 314,
362 ; vessels, 165.

Atragene, 456,459; crystals,
529; periderm, 552; se-
condary growth, 478 ; vas-
cular system, 244.

Atragene alpina, 486.

Atriplex, anomalous growth,
591, 594 ; hair structures,
Ö3, 7°; vascular system,
353-

Atriplex Halimus, 591.

Atriplex hortensis, 63.

Atriplex nummularia, 63.

Atriplex patula, 415, 591,
595-

Atripiex rosea, 63.

Atropa Belladonna, crystals,
143.

Aucuba japonica, 479,
484-

Aurantiaceae, secretions, 1 37,
207.

Avena pratensis, stomata,
50.

Avicennia, 166, 167, 485 ;
anomalous wood, 569,
588 ; ligneous bundle, 496.

INDEX.

Avicennia nitida, 588.

Avicennia officinalis, 588.

Avicennia tomentosa, 588.

Azalea indica, glands, 91.

Azolla, 15, 17, 22, 34, 54:
apex of stem, 16 ; sto-
mata, 35; root-structure,
364; vascular system, 283,
364.

Baccharis halimifolia, secre-
tory reservoirs, 447.

Bactris, 266.

Balanophora, point of at-
tachment, 385.

Balanophoreas, structure of
point of attachment, 384;
vascular system, 250.

Balantium Culcita, scleren-
chyma, 428; vascular sys-
tem, 343.

Balsamodendron, resin-pas-
sages, 453.

Bambusa, fibre, 131; scle-
renchyma, 422.

Bambuse3e,parenchyma,409.

Banisteria, 577.

Banksia, 70: cork, 550;
stomata, 35, 47; vascular
system, 303, 304.

Bark, peeling, 556 ; ring,
555 ; scale, 555.

Barleria alba, cystoliths,
105.

Basella, stomata, 41.

Bast, 458: crystals in, 529 ;
structure of, 519.

Bast-cells, 129.

Bast-fibres, 129, 526.

Batrachium, 300.

Bauhinia, anomalous struc-
ture, 589, 601, 603 (Fig.
238).

Bauhinia anatomica, hairs,
65.

Beaucarnea tuberculata,6 1 8.

Beech, growth of wood,

477.

Begonia, 467 : collenchyma,
1 19 (Fig. 47); cortical
bundles, 256 ; epidermis,
33 ; medullary bundles,
535; medullary rays, 491;
stomata, 45, 47, 49 ; vas-
cular system, 248, 253.

Begonia angularis, 484, 491 :
cortical bundles, 257.

Begonia argyrostigma, 67.

Begonia Dregei, 33, 47.

Begonia Evansiana, 253.

Begonia Fischeri, 33.

Begonia heracleifolia, 47.

Begonia Huegelii, 253, 491.

Begonia laciniata, 253.

Begonia luxurians, 253.

Begonia macularis, cortex,
404.

Begonia manicata, 33, 47,
67 : hair-structures, 55,
64. /

Begonia muricata, 253, 484,
491-

Begonia peltata, 33.

Begonia platanifolia, hairs,
65-

Begonia Rex, 253.

Begonia ricinifolia, 33.

Begonia sanguinea, 33.

Begonia spathulata, 47.

Begonia tomentosa, 33, 257.

Begonia vitifolia, hairs, 65.

Begonia xanthina, 253.

Begoniaceae, stomata, 41,
456.

Beilis perennis, 446, 447.

Beloperone oblongata, cys-
toliths, 105.

Benincasa cerifera, wax, 87.

Berberidese, vascular system,
248.

Berberis, 456, 497, 539: lig-
neous bundle, 495.

Berberis vulgaris, 484, 505 :
annual rings, 506 ; bast-
fibres, 526; crystals, 142,
530; pith, 403; scleren-
chyma, 419, 425 ; spiral
vessels, 158; vascular sys-
tem, 391.

Beta, collenchyma, 119; pa-
renchyma, 1 16; vascular
system, 353.

Beta vulgaris, 599.

Betula, 17 1 ; cork, no, in,
113, 550; cortex, 404;
glands, 91 ; lenticels, 560,
562; ligneous bundle, 495;
medullary rays, 493 ; pe-
riderm, 548 ; secretion,
98 ; sieve-tubes, 176.

Betula alba, 471, 493, 495,
502, 511: bast-fibres, 527;
cork, 551, 557; cortical
pores, 564 ; glutinous layer,
482 ; lenticel, 560 (Fig.
221) ; mucilage, 74 ; pith,
403 ; secretory-layer, 90
(Fig. 35) ; stomata, 49,
75-

Betula dahurica, 493.

Betula fruticosa, mucilage,
74-

Betula populifolia, 493.

Betula verrucosa, crystals,
142, 530; secretions,

138.

Betulaceae, 167 : vascular
system, 305.

Bidens, vascular system, 297,
308.

Bidens cernua, 297.

Bidens tripartita, 297.

Bignonia, anomalous wood,
569, 572, 575; fibre, 131;

ligneous bundle, 496 ;
sieve-tubes, 173, 176.

Bignonia capreolata, 484 ;
spiral vessels, 158.

Bignonia radicans, see Te-
coma.

Bignonia serratifolia, vascu-
lar system, 243.

Bignonia Unguis, 572.

Bignoniaceae, anomalous
wood, 569, 573.

Billbergia clavata, hairs, 64.

Billbergia zebrina, 418.

Biota, connections of vas-
cular bundles, 386 ; secre-
tory reservoirs, 544 ; vas-
cular system, 246, 356,
386.

Biota orientalis, bordered
pits, 164.

Birch, cork, 114.

Biscutella, hairs, 60.

Bladders, 65.

Blechnum, sclerenchyma,
428; vascular system, 285,
363.

Blechnum brasiliense, 363,
428.

Blechnum occidentale, 428.

Blechnum Spicant, 286, 312.

Blitum virgatum, 591.

Boshmeria, cystoliths, 105;
fibre, 1 3 1.

Boehmeria nivea, 590.

Bcerhaavia scandens, 598.

Bombax, 485, 497 ; fibre,
131.

Bombax Geiba, 479, 496.

Bombax pentandrum, 131.

Bombaceae, 497.

Boragineae, hair-structures,
56, 60, 73 ; secretion, 102,
143-

Bordered pits, 158, 493 (Fig.
208).

Boroniese, oil-cavities, 207,

Bossiaea, parenchyma, 407,
409.

Boswellia papyrifera, cork,
no. III, 112, 114, 550.

Botrychium, 320.

Botrychium Lunaria, endo-
dermis, 123 ; mucilage,
74 ; vascular system, 284,
320, 346, 362.

Botrychium rutaefolium,
346.

Bougainvillea spectabilis.

630

INDEX.

590. 593) 598: scleren-
chyma, 420; vascular sys-
tem, 391.

Bouvardia mollis, 296.

Brachypodium sylvaticum,
Stomata, 50.

Brasenia peltata, endoder-
mis, 122 ; vascular sys-
tem, 165, 327.

Brassia caudata, 229.

Brassia maculata, 229.

Brassica, hairs, 60 ; vascular
system, 305, 353, 376;
water-pore, 52.

Brassica Napus, hairs, 60;
root, 516.

Brassica oleracea, 416.

Brassica Rapa, root, 516;
stomata, 47.

Bromelia, 41 1.

Bromelia bracteata, hair-
structures, 64.

Bromelia Caratas, stomata,
35) 212, 411, 418.

Bromeliaceae, 10, 31, 55, 56,
1 1 6, 212: crystals, 142;
hair-structures, 64, 70 ;
leaf-structure, 411, 418;
parenchyma, 41 1; ra-
phides, 142 ; stomata, 46 ;
vascular system, 265.

Bromus, 419.

Broussonnetia, 470, 496,

502 ; cystoliths, 105 ; ge-
latinous layer, 482 ; latex,
184; lenticels, 563.

Broussonnetia papyrifera,
184.

Brucea, resin -passages, 202,
453-

Bi yonia, thyloses, 17 1 ; vas-
cular system, 248.

Bryonia dioica, 248.

Bulbine annua, 357.

Bulliardia aquatica, vascular
bundles, 277, 340.

Bumelia, 146.

Bumelia tenax, secretory
sacs, 1 5 1.

Bundle-trunks, 517.

Bunias Erucago, warts, 66.

Bupleurum fructicosum, se-
cretory reservoirs, 449.

Bupleurum Gerardi, 449.

Bursaria spinosa, 452.

Bursera gummifera, resin-
passages, 453.

Burseraceae, secretory pas-
sages, 525.

Butomese, milk-canals, 443.

Butomus, I o ; air-spaces, 217,
218 ; vascular system,
327.

Buxus, medullary rays, 489 ;

pith, 403; sap-wood, 507;
vascular system, 304.

Buxus sempervirens, 502.

Cacalia ficoides, cortex, 404.

Cachrys, 492.

Cacteae, annual rings, 504 ;
collenchyma, 120; cortex,
404; crystals, 142; latex
passages, 452; mucilage,
143, 144; parenchyma,
407, 409 ; prickles, 66 ;
secondary wood, 499 ;
spiral bands, 156; sto-
mata, 41, 48, 75 ; stone-
elements, 127; tracheae,
156 ; vascular system, 308,
324.

Caenopteris-nervation, 301.

Gajophora lateritia, hairs,
59 (Fig. 21), 60.

Caladium, air-spaces, 217;
crystals, 220; laticiferous
tubes, 436; vascular sys-
tem, 268, 269, 327.

Caladiumesculentum,water-
pore, 31, 268.

Caladium nymphaeifolium,
165, 220.

Caladium odorum, water-
pore, 51.

Calamagrostis Epigeios, sto-
mata, 49.

Calamus, sieve-tubes, 173 ;
silica in, 102; vascular
system, 266, 323, 329.

Calamus Draco, vessels, 169.

Calamus Rotang, sieve-
tubes, 173, 176 (Fig. 71).

Calathea grandiflora, vas-
cular system, 267.

Calcium carbonate, in epi-
dermis, 102.

Calcium oxalate, in epider-
mis, 102,

Calendula, oil-passages, 446 ;
vascular system, 305.

Calla, 213 : vascular system,
304) 315-

Calla palustris, vascular sys-
tem, 268, 315.

Callichlamys, 573.

Callistemon, cork, 556; lig-
neous bundle, 495 ; oil-
cavities, 207 ; parenchyma,

1 1 6, 408; sclerenchyma,
422; vascular system, 338.

Callitriche, 67 ; cortex, 405;
endodermis, 1 2 1 ; hair

structures,5 6,64; stomata,
49, 50; vascular bundles,
278, 341; water-pores, 51.

Callitriche autumnalis,
W’ater-pore, 53, 67.

Callitriche verna, water-
pore, 53, 67.

Callitricheae, air-spaces, 213;
stomata, 46.

Callitris, medullary rays,
493; pith, 403; vascular
system, 243.

Calodracon Jacquini, 618.

Calophyllum Calaba, resin-
passages, 451.

Calotropis gigantea, latex,
185.

Caltha palustris, 67, 415.

Calycantheae, 35 : anomalous
wood, 584; cortical bun-
dles, 257; vascular sys-
tem, 248, 320.

Calycanthus, gelatinous
layer, 482.

Calycanthus floridus, ligne-
ous bundle, 495.

Calyptra, 9.

Calyptrogen, 7.

Calystegia dahurica, 151.

Calystegia sepium, secretory
sacs, 1 5 1.

Camaridium ochroleucum,
229.

Cambial ring, 454.

Cambiform, 326.

Cambium, growth, 454.

Cambium, extrafascicular,
568.

Camelina, 415.

Camellia, 67, 213, 480: cu-
ticle, 8 1 ; ligneous bun-
dle, 495 ; sclerenchyma,
130 (Fig. 53), 424; vas-
cular system, 305, 322.

Camelliajaponica, 486 ; bast-
fibres, 527 ; crystals, 141,
529; fibres, 130; pits, 71.

Campanula, 116; laticife-
rous tubes, 434 ; sieve-
tubes, 231.

Campanula cervicaria, silicl-
fication, 103, 231.

Campanula glomerata, 231.

Campanula grandis, 434.

Campanula lamiifolia, 231,
434-

Campanula linifolia, stomata,
48.

Campanula Medium, latici-
ferous tubes, 187, 434.

Campanula patula, stomata,
48-

Campanula pyramidalis, 231.

Campanula rapunculoides,
231, 434.

Campanula sibirica, 424.

Campanula Vidalii, 458, 499:
bast, 524 ; laticiferous
tubes, 525.

I NDE X.

631

Campanukceae, 116; latici-
lerous tubes, 187, 198,
434,525; sieve-tubes, 231.

Campelia, 31.

Camphora, air-spaces, 210.

Camphora officinalis, 210.

Canella, secretions, 145.

Canella alba, cork, 550 ; pe-
riderm, 549.

Canna, 212, 320, 41 1 : apex
of root, 10 ; mucilage pas-
sages, 202 ; sclerenchyma,
422 ; secretory reservoirs,
445; thyloses, 17 1; tra-
cheides, 165 ; vascular sys-
tem, 267, 357; wax, 83.

Cannabis, cystoliths, 105 ;
fibre, 131 ; glands, 93.

Cannabis sativa, 131.

Cannaceae, 10 : vascular sys-
tem, 267,

Caoutchouc, 185.

Cap-cell, 20.

Capparis Breynia, hair-
structures, 64.

Caprifoliaceae, 297.

Capsella, hairs, 61 ; secre-
tion, 106, 415.

Capsella Bursa Pastoris, sto-
mata, 48.

Caragana, phelloderm, 552.

Caragana arborescens, 485,
496, 502 : annual rings,
506, 507; gelatinous layer,
482 ; periderm, 552 ; se-
condary changes, 465,479.

Cardiospermum, wood, 582,

583.

Carduncellus, 150.

Carduus crispus, secretory
sacs, 150.

Carduus nutans, 150.

Carduus pycnocephalus, oil-
passages, 446, 447.

Carduus tenuiflorus, 150.

Carex, air-spaces, 214, 216,
2x7; endodermis, 124; scle-
renchyma, 422 ; stomata,
40 ; vascular system, 265,
302, 315, 336, 359, 361.

Carex arenaria, 336 : air-
spaces, 214, 216; endo-
dermis, 125.

Carex brizoides, 359.

Carex disticha, air-spaces,
213, 214, 265, 339.

Carex divulsa, 359, 360.

Carex fcenea, 359.

Carex folliculata, 216, 361.

Carex hirta, 265 : endoder-
mis, 122, 124; vascular
system, 315, 339, 359.

Carica, medullary rays, 490 ;
secondary growth, 478.

Carica Papaya, 605.

Carissa arduina, 423.

Carlina longifolia, secretory
sacs, 150.

Carlina salicifolia, 150.

Carlina vulgaris, 150.

Caroxylon Arbuscula, 591,
593-

Carpinus, 167,469, 471,502:
annual rings, 476, 506 ;
cork, 551; fibres, 528;
hairs, 90 ; ligneous bun-
dle, 495 ; periderm, 548,
558 ; sclerosis, 540; sto-
mata, 49 ; vascular system,
353-

Carpinus Betulus, 108, 502 :
crystals, 142, 530.

Carum Carvi, sap-passages,
450; vascular system, 353.

Carya amara, water-pores,

51.

Caryophylleae, crystals, 142;
ligneous bundle, 297, 496;
periderm, 552 ; scleren-
chyma, 419; secondary
growth, 478 ; vascular
system, 353.

Caryota, sclerenchyma, 128,
411.

Cassia, glands, 96.

Cassia quinquangulata, ano-
malous growth, 567.

Cassytha, bordered pits, 163;
haustorium, 383; stomata,
45-

Cassytha paniculata, bor-
dered pits, 16 1.

Castanea, annual rings, 476 ;
cork, 113; gelatinous
layer, 482 ; ligneous bun-
dle, 495 ; medullary rays,
489 ; periderm, 548 ; vas-
cular system, 353.

Castanea vesca, 496 : wood,

511.

Castillea, 185.

Casuarina, 12, 456, 459, 461,
479) 48s, 497) 504) 505:
cortical bundles, 256 ; ge-
latinous layer, 482 ; lig-
neous bundle, 495 ; pa-
renchyma, 407; periderm,
55 1) 553 ■) sclerenchyma,
418; stomata, 45, 48;
tracheides, 481 ; vascular
system, 300.

Casuarina equisetifolia, 479,
495,496.

Casuarina muricata, cortical
bundles, 257 (Fig. 113).

Casuarina stricta, 12.

Casuarina torulosa, 480, 495,
496.

Catalpa, 470, 496 : glandular
hairs, 91, 95 ; medullary
rays, 493 ; periderm, 548.

Catalpa Bungei, glands, 95.

Catalpa syringaefolia, glands,
95.

Cattleya Mossiae, aerial roots,
229.

Caulotretus, anomalous
structure, 603.

Cecropia palmata, hairs,
65.

Cecropia peltata, hairs, 65.

Cedrus, 118: medullary
rays, 490, 493 ; resin-
canals, 442, 443.

Cedrus Deodara, 443, 493.

Cedrus Libani, 378.

Celastrineas, 495.

Celastrus, ligneous bundle,
495-

Celastrus scandens, pits, 481,
484.

Cell, apical, 10, 15; initial,
8, 39 ; mother, 39 ; seg-
ment, 15.

Cell-contents, 66.

Cell-division, 17 ; into flats,
17; into strata, 17.

Cell-wall, epidermal, 70 ;
stomatal, 71 ; thickness
of, 502.

Cells, bast, 129; epidermal,
30; fibrous, 27; isodia-
metric, 27 ; of hair-struc-
tures, 68 ; palisade, 407 ;
sclerotic, 28, 120 ; sto-
matal, 34; stomatal guard,
34 ; stone, 127.

Cellular tissue, 27.

Celosia, 595 : vascular sys-
tem, 249.

Celosia argentea, 591.

Celtis, crystals, 142 ; cy-
stoliths, 105; periderm,
548; pith, 403; silica in
leaves, 102 ; tracheides
and fibres, 483, 528.

Celtis australis, 470, 485,
496, 497: crystals, 140;
gelatinous layer, 482.

Centaurea, 150.

Centaurea atropurpurea,
446, 447 : secretory pas-
sages, 526.

Centradenia, 492 : ligneous
bundle, 495 ; vascular
system, 259, 260.

Centradenia floribunda,
periderm, 548.

Centradenia grandifolia, 260,
458, 484 : bast-fibres, 526.

Centradenia rosea, cortical
bundles, 258 (Fig. 114).

632

INDEX.

Centranthus, 474 : medul-
lary rays, 492 ; vascular
system, 244, 297, 353.

Centranthus ruber, 244.

Centropogon, laticiferous
tubes, 434.

Centropogon Suriname r. sis,
434-

Centrostemma, 439.

Cephalanthera, vascular sys-
tem, 277.

Cephalotaxus, parenchyma,
410; vascular system, 246,
379-

Cephalotaxus Fortune!, 246.

Cerastium, vascular system,
243.

Cerastium frigidum, vascular
system, 2 43 (Figs. 102,103).

Cerastium glabratum,

water-pore, 52.

Ceratocaryum, 425.

Ceratonia, ligneous bundle,
484, 496, 497.

Ceratonia Siliqua, 496.

Ceratophyllum, 67 : air-
spaces, 213, 217; cortex,
405; endodermis, 12 1;
vascular system, 278, 369.

Ceratopteris thalictroides,
air-spaces, 213, 215; vas-
cular system, 289.

Ceratozamia, 31 ; scleren-
chyma, 424 ; vascular sys-
tem, 357.

Ceratozamia mexicana, 70.

Cerbera Manghas, latex, 185.

Cereus, secondary changes,
474. 475. 479 1 vascular
system, 254, 297, 310.

Cereus candicans, 254, 310.

Cereus grandiflorus, 474.

Cereus peruvianus, epider-
mis, 80.

Cereus senilis, crystals, 141.

Cereus speciosissimus, 310,
499.

Cereus speciosus, stomata,
48.

Cerinthe, secretion, 102,
103, 106.

Cerinthe aspera, 103-106.

Cerinthe major, silicifica-
tion, 103-106,

Cerinthe minor, 106.

Ceropegiastapeiioides,milk-
tube, 191 (Fig. 84), 439.

Ceroxylon, wax, 82, 83.

Cestrum, sieve-tubes, 231.

Chaerophyllum, medullary
rays, 491 ; vascular sys-
tem, 309.

Chaerophyllum bulbosum,

418.

Chamascyparis ericoides,
vascular system, 246.
Chamaecyparis glauca, 246.
Chamaedorea, secretions,
139; vascular system,
266, 302, 360, 371, 391 ;
wax covering, 82, 86,
87.

Chamaedorea elegans, 360 :
fibres, 134; sclerenchyma,
126, 129, 423.

Chamaedorea Karwinskiana,
425.

Chamaedorea Schiedeana,
wax, 83.

Chamaerops, lamina, 407,
41 1 ; sclerenchyma, 128.
Chamaeropshumilis,prickles,
66, 412.

Chavica, epidermis, 33 ;
sclerenchyma, 420 ; vas-
cular system, 250.
Chavica maculata, 33.
Cheilanthes, dusty hairs, 99.
Cheiranthus, ligneous bun-
dle, 496.

Cheiranthus Cheiri, 458,
484: hairs, 59, 61 (Fig.
21); secretion, 106.
Cheirostemon, 485, 497 :

crystals, 530 ; fibres, 527;
secondary growth, 478.
Chelidonium, latex, 183,
185,188; laticiferous root,
525 ; tubes, 189, 194,

435.

Chelidonium majus, milk-
tubes, 187 (Figs. 80, 81),

525-

Chenopodiaceae, 25, 70:

anomalous structure, 569,

590, 593; cortex, 404;
crystals, 142 ; hairs, 63,
93 ; vascular system, 353.

Chenopodium, anomalous
growth, 591 ; collen-
chyma, 119; hairs, 63.
Chenopodium album, 63,

591. 595-

Chenopodium hybridum,
591, 595-

Chenopodium murale, 591.
Chenopodium Quinoa, 63.
Chilianthus arboreus, lig-
neous bundle, 496 ; pits,
163.

China bicolorata, 555.
Chlorophyll, absence of, 66.
Chlorophytum, aerial roots,
230; endodermis, 125.
Chondrilla, 231, 433.
Choretrum, stomata, 45.
Chrysobalaneae, secretions,
510 ; silicification, 106.

Chrysodium vulgare, sto-
mata, 37, 39; vascular
system, 294.

Cibotium, hairs, 61 ; scle-
renchyma, 429 ; vascular
system, 286.

Cibotium glaucescens, 286.

Cibotium Schiede!, stomata,
41, 286.

Cicer, 12.

Cicer arietinum, 354.

Cichoriaceae, latex, 183,
184, 187 ; laticiferous

tubes, 189, 433, 525 ; se-
cretions, 137; sieve-tubes,
231, 524; vascular sys-
tem, 338.

Cichorium, 231, 433: root,
524.

Cichorium Intybus, latici-
ferous tubes, 433; secre-
tory reservoirs, 448.

Cicuta virosa, air-spaces,
215.

Cimicifuga foetida, 249.

Cinchona, 537: fibre, 131,
528 ; secretions, 138; se-
cretory sacs, 149, 542.

Cinchona heterophylla, 149.

Cinchona lancifolia, 149.

Cinchona macrocalyx, 529.

Cinchona obtusifolia, 149.

Cinchona scrobiculata, 149.

Cinchona umbellulifera, 149.

Cinchonese, secretory sacs,
146.

Cineraria maritima, 447.

Cinnamodendron cortico-
sum, periderm, 549.

Cinnamomum, 537 : fibres,
529; secretions, 138.

Cinnamomum aromaticum,
crystals, 529; pits, 71.

Cinnamomum zeylanicum,
crystals, 530 ; raphides,
143 ; sclerosis, 540.

Cirrhophetalum Wallichii,
root-sheath, 227, 229.

Cirsium anglicum, secretory
sacs, 150.

Cirsium arvense, 150: oil-
passages, 446, 447.

Cirsium lanceolatum, secre-
tory, 149, 150.

Cirsium oleraceum, 150.

Cirsium palustre, 150.

Cirsium praealtum, 150.

Cissampelos, 589.

Cissus, anomalous growth,
567 ; raphides, 143.

Cissus velutina, hairs, 65.

Cistanche lutea, 254.

Cistineae, hairs, 62.

Cistus,glandular hairs, 90, 93.

INDEX,

633

Cistus creticus, glandular
hairs, 94 (Fig. 36).

Citriobatus multiflorus, 452 :
bast-fibres, 526.

Citrus, annual rings, 476 ;
crystals, 140, 142, 143 ;
ligneous bundle, 496 ;
oil cavities, 208.

Cladium, air-spaces, 217.

Cladium Mariscus, 217 : en-
dodermis, 125; scleren-
chyma, 419.

Cladothamnus, pith, 403.

Claytonia linoides ; water-
pore, 52,

Claytonia perfoliata, sto-
mata, 37.

Clematis, 25, 459, 460 ; peri-
derm, 552, 555 ; vascular
system, 244, 308.

Clematis Vitalba, 244, 456,
484, 497, 502, 560: crys-
tals, 529 ; fibre, 131, 527 ;
medullary rays, 489 ; pits,
481 ; secondary growth,
460, 472, 478.

Clematis Viticella, vascular
system, 245 (Fig. 106).

Clerodendron fragrans, glan-
dular hairs, 90, 91, 96 ; se-
cretion, 98.

Clethra, pith, 403.

Clidemia parviflora, 259.

Clivia nobilis, 71 : cuticle,
76 ; vascular system, 360.

Clusia, 451, 474, 475.

Clusia flava, 354.

Clusia rosea, 451.

Clusiaceae, periderm, 547 ;
resin passages, 451, 525;
secretions, 137 ; secretory
reservoirs, 201, 202.

Clytostoma, wood, 572, 574.

Cnicus, hairs, 61.

Cobaea, 166, 458, 488, 492,
531 : periderm, 552, 559;
phelloderm, 553 ; vessels,
169.

Cobsea scandens, 486, 498 :
annual rings, 504 ; endo-
dermis, 121, 415.

Cocculus, anomalous wood,
588.

Cocculus laurifolius, ano-
malous wood, 587; pits,
71 ; vascular system, 238,
303, 304.

Cocculus palmatus, anomal-
ous growths, 569.

Cocos, sclerenchyma, 420,
425 ; vascular system,
266.

Cocos botryophora, fibre,
131.

Coffea, hairs, 90; pils, 71 ;
vascular system, 353.

Coix, 31 1 : vascular sys-
tem, 359.

Coix lacryma, wax, 83.

Coleus Macraei, 484 : ligne-
ous bundle, 496.

Collenchyma, 119.

Collenchymatous hypoder-
mal layers, 538.

Colletia, stomata, 45, 48.

Colletia horrida, parenchy-
ma, 407.

Colocasia, air-spaces, 217 ;
crystals, 220; scleren-
chyma, 422, 423; vas-
cular system, 268, 327 ;
water-pores, 51.

Colocasia antiquorum, 220 :
water-pore, 51.

Colocasia odorum, 220.

Colutea, periderm, 552.

Comesperma, 589.

Commelina, stomata, 40 ;
vascular system, 270, 311.

Commelina agraria, 270,
311,316.

Commelina caelestis, sto-
mata, 40 (Fig. 13).

Commelina communis, sto-
mata, 40.

Commelina procurrens, 270.

Commelina tuberosa, an-
nular tubes, 156.

CommellneaE, lo ; paren-
chyma, 41 1 ; scleren-
chyma, 419 ; secretions,
139; stomata, 37; tra-
cheae, 156; vascular sys-
tem, 269, 270, 311, 315,

316, 327.

Compositae, air-spaces, 214,
215; endodermis, 121 ;
hair structures, 56, 61,
70 ; intercellular spaees,
2 10 ; oil-passages, 445,
526; parenchyma, 116;
secretions, 102, 137, 146;
secretory reservoirs, 135,
146, 201, 202, 204; vas-
cular system, 297 ; water-
pore, 52.

Condylocarpon, anomalous
wood, 577.

Coniferae, 25, 35, 56: crys-
tals, 529 ; hairs, 56 ; leaf-
structure, 408 ; medul-
lary rays, 490, 493 ; pa-
renehyma, 1 18, 408; phel-
loderm, 548; resin-canals,
201, 441 ; sclerenchyma,
418; secretory reservoirs,
135, 137, 202, 204, 440,
526, 543 ; stomata, 35,

37, 40; thickening, 469,
472, 475 ; tracheides, 165,
170; vascular bundles,
235; vascular system,
246, 321, 325, 352, 378 ;
vascular system in leaves,
300; wood, 5 1 1, 512.

Conocephalus, cystoliths,
105 ; gum cavities, 144.

Conocephalus naucleiflo-
ous, secretions, 137, 144.

Conoclinium atropurpu-
reum, glandular hairs, 94

(Fig- 37).

Conopholis, vaseular sys-
tem, 254.

Convolvulaceae, anomalous
structure, 606 ; latex,
146 ; laticiferous tubes,
198; secretory sacs, 146,
150; sieve-tubes, 231.

Convolvulus arvensis, secre-
tory sacs, 151.

Convolvulus Cneorum, 478 :
bast, 524 ; bast-fibres,
527 ; hair structures, 55 ;
tracheae, 157 ; tracheides,
161, 167 (Fig. 64).

Convolvulus malabaricus,
607.

Convolvulus Scammonia,
607.

Convolvulus tricolor, 474 :
vascular system, 353.

Conyza, 446.

Copaifera, 510.

Copernicia, lamina, 407 ;
wax, 83.

Coprosma ligustrina, 296,
298.

Corallorrhiza, endodermis,
121 ; vascular system,
278, 370.

Corallorrhiza innata, 278.

Corchorus, fibres, 129, 131,
132.

Cordia pallida, 485 : tra-
cheides and fibres, 483.

Cordyline, 391, 618.

Coriandrum, 353.

Cork, 108.

Cork, structure of, no.

Cork oak, periderm, 557.

Cork on leaves, 108.

Cornus, annual rings, 476 ;
bast-fibres, 527 ; crystals,
529.

Cornus sanguinea, 502 : an-
nual rings, 506 ; lenticels,
564 ; thickening, 470.

Coronilla Emerus, periderm,
552.

Correa, hairs, 62, 65.

Correaalba, oil-cavities, 207.

^34

INDEX.

Correa speciosa, 65.

Cortex, spongy, 212.

Cortical bundles, 256.

Cortical pores, 560.

Cortusa, endoclermis, 415.

Corylus, 108, 167, 469:

annual rings, 476 ; fibres,
528 ; glandular hairs. 90;
periderm, 548 ; sclerosis,
540.

Corylus Avellana, cork,
551; crystals, 142, 530;
gelatinous layer, 482 ;
water-pore, 52.

Corypha, 266.

Corypha cerifera, wax
covering, 86.

Costus, 41 1.

Cotoneaster microphylla,
493-

Cotula matricarioides, 446,
447-

Cotyledon cocclnea, cortex,
404-

Cotyledon orbiculata, wax,
83, 87.

Crassula, 31 : parenchyma,
407 ; periderm, 548 ; vas-
cular system, 376 ; water-
pores, 51, 53.

Crassula arborescens, end
of vascular bundle, 378
(Fig.180) ; water-pore, 53.

Crassula cordata, 53.

Crassula cultrata, 53.

Crassula ericoides, 53.

Crassula lactea, 53, 499.

Crassula lycopodioides, 53.

Crassula perforata, 53.

Crassula portulacea, 53.

Crassula spathulata, 53.

Crassula tetragona, 53, 548.

Crassulaceae, 67, 71, 127,
458, 492 : ligneous bundle,
496 ; secondary gi'owth,
478; stomata, 41, 49; vas-
cular system, 305, 324,

377.

Crataegus, annual rings, 476;
ligneous bundle, 495 ; me-
dullary rays, 493 ; scleren-
chyma, 426.

Crataegus coccinea, water-
pores, 51.

Crataegus cordata, 493,

Crataegus monogyna, 493,
495-

Crataegus Oxyacantha, pith,
403-493.

Crataegus Pyracantha, 493.

Crepis sibirica, water-pores,

51.

Crinum, epidermis, 34-139.

Crinum americanum, 34.

Crinum bracteatum, 34.

Crocus, 422.

Croton, hair-structures, 56,
62, 64, 65, 70.

Croton Eleutheria, 537 :
crystals, 530 ; fibres, 529;
secretory reservoirs, 145.

Croton nitens, hairs, 64.

Croton pseudochina, hairs,
64.

Croton tomentosus, hairs,
62.

Cruciferae, hair structures,
56, 60, 63 ; secretions,
137 ; stomata, 41 ; vascu-
lar system, 353.

Cryptogams, cork, 108 ;
sieve-tubes, 180; stone
elements, 127.

Cryptomeria, 14, 442 : vas-
cular system, 246, 380.

Cryptostegia, 439.

Crystals, 140, 529.

Crystals, clustered, 142, 529.

Crystals, in bast, 529.

Crystals, klinorrhombic,
529-

Crystals, solitary, 142.

Ctenopteris, 301.

Cucumis, sieve-tubes, 231 ;
thyloses, 17 1 ; vascular
system, 236, 239, 248,
353, 394-

Cucumis Melo, 236.

Cucumis sativus, 236, 239,
248.

Cucurbita, 12, 68, 166, 167,
176, 177, 178, 456 : root,
517 ; sieve-tubes, 231 ;
spiral vessels, 158 ; thy-
loses, 171 ; vascular sys-
tem, 248, 305, 324, 353,
376; vessels, 169.

Cucurbita maxima, 354.

Cucurbita Pepo, secondary
growth, 474 (Fig. 204);
sieve-tubes, 173, 175 (Fig.
68).

Cucurbitacese, 12, 25 : hairs,
61, 73 ; medullary bundles,
248 ; root development,
398; sclerenchyma, 419;
sieve-tubes, 231 ; silicifi-
cation, 103 ; vascular sys-
tem, 248, 338, 352, 353.

Cunninghamia, 14; medul-
lary rays, 492 ; scleren-
chyma, 424 ; vascular
bundles, 380 (Fig. 183);
vascular system, 246, 379,
382.

Cunninghamia lanceolata,
epidermis, 77.

Cunninghamia sinensis, pa-

renchyma, 410; resin pas-
sages, 442 (Fig. 1 91).

Cunonia, hairs, 90 secre-
tion, 98.

Cunonia capensis, thicken-
ing, 470; tracheides, 481.

Cuphea lanceolata, hairs, 69.

Cupressineae, crystals, 141,
529; fibres, 527; phello-
derm, 552 ; periderm,
553) 555 ; secretion, 102 ;
sieve-tubes, 522 ; vascular
system, 300.

Cupressus, 14 : medullary
rays, 492 ; resin canals,
443) 544 ; secondary

changes, 475 ; vascular
system, 246.

Cupressus pyramidalis, 245.

Cupressussempervirens, 492 .

Cupuliferae, cortex, 404 ;
vascular system, 305, 353.

Curculigo, vascular system,
302, 360.

Curculigo recurvata, 360.

Curcuma longa, vascular sys-
tem, 267, 360.

Curcuma zedoaria, vascular
system, 267.

Cuscuta, haustorium, 383 ;
vascular system, 366.

Cuscuteae, stomata, 46, 47.

Cuspidaria, 574.

Cussonia, secretory reser-
voirs, 202, 205, 450.

Cuticle, 29, 75 : composition
of, 79.

Cutin, 74 : chemical compo-
sition of, 81.

Cyanophyllum magnificum,
cortical bundles, 259, 260.

Cyathea, sclerenchyma, 428;
vascular system, 291, 292,

293) 344) 346.

Cyathea arborea, 118, 291,

294) 344) 428.

Cyathea ebenina, 291, 293.

Cyathea Imrayana, 118,

428: sieve-tubes, 181 ;
stem-structure, 428 (Fig.
1.89) ; vascular system,
291 (Figs. 138-142), 293,
294) 344-

Cyathea medullaris, 346, 405,
428.

Cyathea microlepis, 346.

Cyatheacese, 118-128, 141,
406 : sclerenchyma, 426 ;
sieve-tubes, 180; tannin-
sacs, 153; vascular system,
285, 291, 297, 342, 371.

Cycadese, 67-129 ; mucilage
canals, 202, 441 ; paren-
chyma, 1 17, 410 ; scleren-!

I N DE X.

chyma, 424; stomata, 35,
37 ; structure of root,
613; of stem, 608; vas-
cular system, 248, 298,
301, 335, 352, 356; tra-
cheides, 165.

Cycas, 31, 118: epidermis,
35 ; vascular system, 301.

Cycas circinalis, 13,608,612.

Cycas revoluta, 14, 102 ;
course of leaf-trace, 609
(Figs. 239, 240); cuticle,
75; epidermis, 77; pits,
71 ; secretory reservoirs,
203 ; stomata, 40 ; vascu-
lar bundle, 336 (Figs. 158,
159), 357 ; wax, 82.

Cyclamen, 25; endodermis,
4J5 ; water-pores, 51.

Cyclantheae, 10.

Cyclanthera pedata, vascu-
lar system, 248.

Cyclanthus, vascular system,
361.

Cyclopteris-nervation, 301.

Cydista, 574.

Cydonia, pith, 403.

Cydonia vulgaris, 493.

Cymbidium ensifolium,
aerial roots, 228, 229.

Cymbidium marginatum,
228.

Cymodocea, vascular sys-
tem, 275, 368.

Cymodocea aequorea, 275,
369.

Cymodocea isoetifolia, 275.

Cymodocea nodosa, 32.

Cymodocea rotundata, 32.

Cynara, 150.

Cynara Scolymus, 448.

Cynareae, oil passages, 445 ;
secretory sacs, 146, 149,
153-

Cynoglossum, mucilage, 143.

Cyperaceae, 10, 32 ; air-
spaces, 213, 216; endo-
dermis, 122, 124; hair
structures, 63 ; paren-
chyma, 407, 409, 415;
scleren chyma, 418; secre-
tions, 102, 137; stomata,
35; vascular system, 265,
268, 327, 339, 359.

Cyperus, 265 ; air-spaces,
216, 217 ; endodermis,
124; stomata, 40; vascu-
lar system, 359.

Cyperus alternifolius, 359,
360 : air spaces, 216.

Cyperus aureus, 339.

Cyperus fuscus, 217.

Cyperus longus, 359.

Cyperus vegetus, 422.

Cypripedium, 407.

Cyrtochilum bictoniense,
229.

Cyrtopodium, velamen, 227.

Cystoliths, 103.

Cytinus Hypocistis, intra-
matrical body, 384.

Cytisus, annual rings, 476 ;
mucilage, 74.

Cytisus Laburnum, annual
rings, 507 ; bast layer, 468
(Fig. 202), 521 (Fig. 210) ;
medullary rays, 489 ; peri-
derm, 548 ; secondary
growth, 465 (Fig. 198),
482 (Fig. 206); tracheides,

480 (Fig. 205); woody
fibre, 482 (Fig. 207).

Dacrydium, secretory reser-
voirs, 442.

Dahlia, endodermis, 415 ;
oil passages, 446, 447 ;
pitted vessels, 163 ; stone-
elements, 127 ; vascular
system, 297.

Dahlia variabilis, scleren-
chyma, 127 (Fig. 52).

Dammara, 14: fibres, 130,
133; resin canals, 442;
sclerenchyma, 424 ; vascu-
lar systen), 246, 301, 325,
380.

Dammara alba, 325.

Dammara australis, 246.

Dansea, vascular system,
290.

Daphne, mucilage, 74 ; pits,

481 ; sieve-tubes, 231 ;
tracheides and fibres, 483 ;
vascular system, 338.

Daphne Mezereum, 479 :
fibre, 131, 528.

Dasylirion, cuticle, 76, 77;
sclerenchyma, 418; sto-
mata, 37.

Datura, sieve-tubes, 231.

Daucus, medullary rays,
491 ; vascular system,
353-

Daucus Carota, root, 516,
524-

Davallia, sclerenchyma, 426,
42 7 ; vascular system, 288,
313, 344-

Davallia bullata, 288.

Davallia canariensis, 288.

Davallia chaerophylla, 313.

Davallia divaricata, 313.

Davallia dissecta, vascular
system, 246 (Fig. 134).

Davallia elata, 427, 428.

Davallia elegans, 288, 426,

427-

^35

Davallia heterophylla, 288.

Davallia parvula, 288.

Davallia pedata, 288.

Davallia pyxidata, 288, 344,
426, 427.

Davallia stenocarpa, 313.

Davilla brasiliana, silica in
leaves, 102.

Delphinium, water-pore, 51,

Dendrocolla teres, aerial
roots, 229.

Dennstaedtia, vascular sys-
tem, 284, 294, 342.

Dennstaedtia cornuta, 294.

Dennstaedtia davallioides,
284.

Dennstaedtia punctilobula,
284.

Dennstaedtia rubiginosa,
294.

Dennstaedtia scandens, 284.

Dennstaedtia tenera, 284.

Dentaria pinnata, 127.

Dermal glands, 89 : secre-
tions of, 98.

Dermatogen, 7.

Descent, in relation to
structure, 24.

Desmanthus, spongy cortex,

212.

Desmanthus natans, 2, 212,

213.

Desmogen, 389.

Desmoncus, 266.

Deutzia,lenticels, 560; phel-
loderm, 552.

Deutzia scabra, hairs, 6 1 ;
periderm, 552 ; silica in
leaves, 102.

Diachyma, 406.

Dianthus, periderm, 552 ;
vascular system, 243; wax,
87.

Dianthus barbatus, 410.

Dianthus Caryophyllus, 71:
cuticle, 76 ; epidermis,
79 ; parenchyma, 407,
410 ; stomata, 37, 75 ;
vascular system, 243, 305 ;
wax, 87.

Dianthus plumarius, 71:
cuticle, 76, 411, 419, 458,
499-

Diaphragms, 217.

Dicella, anomalous wood,
578, 580.

Dichorisandra, 31 : vascular
system, 270, 31 1.

Dichorisandra oxypetala,
270.

Dichorisandra thyrsiflora,
270.

Dicksonia, sclerenchyma,
429 ; vascular system, 286.

636

INDEX.

Dicksonia antarctica, 119,
286.

Dicksonia Karsteniana, 286.

Dicotyledonous woods, ob-
lique grain of, 472.

Dicotyledons, 12, 63: ano-
malous thickening in, 567 ;
bordered pits, 163, 164;
collenchyma, 119; con-
nections of vascular
bundles, 385 ; crystals,
142; endodermis, 415;
fibres, 527 ; medullary
rays, 493 ; origin of vas-
cular bundles, 393 ; peri-
derm, 558 ; pith, 403 ;
raphides, 142; scleren-
chyma, 420; sieve-tubes,
173, 231 ; silicification in,
103; stone-elements, 127;
thickening, 469, 475 ; thy-
loses, 171 ; tracheides,
165,169; vascular bundles,
235, 248; vascular system,
322, 355,393; witharing
of vascular bundles, 307 ;
wood of roots, 516; woods,
512.

Dictamnus, oil cavities, 69
(Fig. 22), 207; secretions,
137; warts, 66, 69.

Dictamnus Fraxinella, oil
reservoirs, 208 (Fig. 86).

DiefFenbachia, vascular sys-
tem, 267.

Dieffenbachia Seguine, 436,

Dilleniaceae, 589, 601 : silici-
fication, 103, 106.

Dioon, anomalous thicken-
ing, 610; sclerenchyma,
424 ; secretory reservoirs,
441; stomata, 40; vas-
cular system, 337, 340,
356.

Dionaea, glands, 100.

Dioscorea, cork, 108 ; struc-
ture of tubers, 622 ; vas-
cular system, 233, 235,

275, 276 (Figs. 126, 127),

301, 304, 319, 362.

Dioscorea Batatas, 233, 275,

276, 319, 622.

Dioscorea sinuata, 622.

Dioscorea villosa, 622.

Dioscoreae, 10, 301, 618.

Diosma, oil cavities, 207.

Diosma alba, 73.

Diosmeae, mucilage, 73 ; oil

cavities, 207.

Diospyros virginiana, 479.

Diphylleia, vascular system,
248, 249.

Diplazium giganteum, 313.

Diploe, 406.

Diplothemium maritimum,
360.

Dipsacus, 297 : hairs, 55, 66.

Distictis, 575.

Dittany, oil cavities, 69,208.

Dodecatheon, 415.

Dolichos lignosus, 354.

Doliocarpus Rolandri, ano-
malous wood, 589, 605.

Doodya, 304.

Doronicum Pardalianches,
water-pore, 51.

Dorstenia, 105.

Draba, hairs, 60, 61.

Draba aizoides, hairs, 60.

Draba hispanica, 60.

Dracaena, endodermis, 124;
sclerenchyma, 42 2 ; secon-
dary growth, 619 (Fig.
241); vascular system, 264,
302, 311.

Dracaena arborea, secretion,
102.

Dracaena Draco, secretion,
102.

Dracaena fruticosa, 622.

Dracaena marginata, 619,
622."

Dracaena refiexa, 619, 622 :
sclerenchyma, 420; secre-
tion, 102.

Dracaena umbraculifera, se-
cretion, 102.

Dracaeneae, 618 ; vascular
system, 361.

Dracunculus, laticiferous
tubes, 436 ; vascular sys-
tem, 268, 269.

Drimys, secretory reser-
voirs, 145.

Drimys Winteri, 486, 494,
541: bast-fibres, 527;

crystals, 529; secondary
changes, 480.

Drosera, digestive glands,
100.

Drosera rotundifolia, end of
tooth of leaf, 374 (Fig.
176).

Drosophyllum, digestive
glands, 100.

Dryandra, stomata, 47.

Dryobalanops aromatica,
510.

Duramen, 507.

Ebenaceae, wood, 508, 511.

Ecbalium, sieve-tubes, 231.

Ecbalium Elaterium, vascu-
lar system, 248.

Echeveria, wax, 87.

Echeveria pubescens, 458,
498, 499.

Echinocactus, secondary

growth, 478 ; vascular sys-
tem, 254, 310.

Echinops, secretory pass-
ages, 526.

Echinops exaltatus, 446.

Echites peltata, 439.

Echium fruticosurn, secre-
tion, 106.

Echium vulgare, 106.

Edwardsia grandiflora, 497.

Elaeagneae, hair structures,
55, 63, 64, 70.

Elaeagnus acuminata, bor-
dered pits, 162.

Elais, sclerenchyma, 425 ;
vascular system, 266.

Elatine, air-spaces, 213, 217;
vascular system, 277, 306,
340.

Elatine Alsinastrum, 217,
277, 300,340: cortex, 405.

Elatine hexandra, 277.

Elatine Hydropiper, 277.

Elegia nuda, 33, 425.

Elodea, 8, 412 : endodermis,
121; root-hairs, 5 5 ; sieve-
tubes, 232.

Elodea canadensis, 67 : vas-
cularsystem,278,368, 369.

Elymus arenarius, 77 : la-
mina, 407 ; stomata, 35,
50 ; wax, 85.

Emergence, 55, 58.

Encephalartos, 3 1 : crystals,
140 ; parenchyma, 117,
410; sclerenchyma, 418,
424 ; secretory reservoirs,
441; sieve-tubes, 179.

Encephalartos Catfer, 612.

Encephalartos horridus,

anomalous growth, 610;
wax, 85.

Encephalartos longifolius,
613.

Encephalartos pungens,

sieve-tubes, 181 (Fig. 78).

Enckea glaucescens, hairs,
65.

Enckea media, 482.

Endistem, 394.

Endodermis, 121.

Endomeristem, 394.

Ephedra, 13, 14, 166, 167,
168, 459, 484, 492, 498 :
crystals, 141, 529; ligne-
ous bundle, 495 ; scleren-
chyma, 418, 420, 424 ;
sieve tubes, 179 ; secre-
tion, 102 ; stomata, 35 ;
vascular system, 246, 247,
357-

Ephedra altissima, 33, 247.

Ephedra campylopoda, 14,
247,456.

IN DE X.

Ephedra distachya, bast-
fibres, 527 ; cuticle, 76.

Ephedra helvetica, bordered
pits, 159 (Fig. 59), 160
(Fig. 60).

Ephedra monostachya, 33,
458, 495-

Ephedra vulgaris, 246.

Epidendron ciliare,7 1: aerial
roots, 229; cuticle, 76,
77; epidermis, 79; paren-
chyma, 410; vascular sys-
tem, 359, 360; wax, 82.

Epidendron elongatum, 229,
230.

Epidermal cell-wall, 70.

Epidermis, 27 : composition,
30 ; gaps in, 54 ; many-
layered, 33 ; of Gymno-
sperms, 22 ; structure, 29.

Epilobium palustre, stomata,
48.

Epilobium roseum, 67.

Epipactis, vascular system,
277, 396.

Epiphegus americanus, vas-
cular system, 254.

Epiphyllum truncatum, 297.

Epipogon, stomata, 46 ; vas-
cular system, 278, 370.

Epipogon Gmelini, 278.

Epithelium, 202.

Equisetum, 16, 17, 22, 35,
56, 71, 141 : air-spaces,
213, 214, 215 ; apex of
stem, 19 (Fig. 9) ; col-
lateral bundles, 329 (Fig.
149); endodermis, 122,
124; hairs, 56; paren-
chyma, 407, 413, 415;
pith, 403 ; root-develop-
ment, 398 ; sclerenchyma,
417, 418; sheath, 414;
sieve-tubes, 180 ; silica in,
102 ; stomata, 37, 41, 43,
46, 48 ; vascular system,
235» 279, 321, 323, 324,
327, 362, 363, 392.

Equisetum arvense, 122.

Equisetum hyemale, 71, 102,
122, 418: root apex, 18
(Fig. 7) ; stoma, 72 (Fig.
24).

Equisetum limosum,72-i22.

Equisetum littorale, 122.

Equisetum palustre, 122,
415.

Equisetum pratense, 122.

Equisetum ramosissimum,
122.

Equisetum scirpoides, 122.

Equisetum sylvaticum, 122.

Equisetum trachyodon, 122.

Equisetum Telmateia, 122.

Equisetum variegatuin, 122.

Eranthemum pulchellum,
105.

Eranthis, water-pore, 51.

Eria stellata, aerial roots,
228, 229.

Erica, vascular system, 303.

Erica carnea, mucilage, 74.

Erica Tetralix, mucilage, 74.

Ericaceae, 167 : pith, 403.

Erigeron glabellus, 446.

Eriobotrya japonica, 320,

418.

Eriocauleae, sclerenchyma,

419.

Eriocnema marmorata, cor-
tical bundles, 260.

Eriophorum, air-spaces, 216,
217.

Ervilia villosa, 354.

Ervum Lens, 354.

Eryngium, air-spaces, 213;
medullary rays, 491 ; oil
passages, 450 ; parenchy-
ma, 409 ; vascular system,
302, 306.

Eryngium aquaticum, 302.

Eryngium junceum, vascu-
lar system, 301.

Eryngium maritimum, sto-
mata, 49.

Eryngium pandanifolium,
301.

Eryngium planum, fibres,
133 ; watev-pore, 52.

Erysimum canescens, hairs,
61.

Erysimum cheiranthoides,
hairs, 61.

Erythrina crista-galli, vas-
cular system, 239.

Erythroxylon grandifolium,
493-

Escallonia, water-pore, 52.

Eschscholtzla, 435.

Ethereal oil, reservoirs, 202.

Eucalyptus, oil cavities, 207 ;
parenchyma, 410, 41 1;
sclerenchyma, 422 ; tra-
cheides and fibres, 483 ;
vascular system, 338; wax,
85, 87.

Eucalyptus cordata, 482.

EucalyptusGlobulus, 85,338.

Eucalyptus Gunnii, 408, 41 1.

Eucalyptus pulverulenta, 85,
410.

Eucharis, 139.

Eucharis amazonica, vascu-
lar system, 302.

Eucomis, 71 : cuticle, 75.

Eugenia, ligneous bundle,
496 ; oil cavities, 207 ;
sclerenchyma, 422.

Eugenia australis, 207, 212,
479. 484-

Eulalia japonica, wax, 83.

Euonymus, annual rings,
476; ligneous bundle, 495;
pits, 481.

Euonymus europaeus, 479,
484,495; cork, 550; lenti-
cels, 562, 564 ; medullary
rays, 489; vascular system,
243.

Euonymus latifolius, 457,
479, 484, 495-

Eupatorieae, 446.

Eupatorium aromaticum,
446.

Eupatorium verticillatum,
water-pore, 51.

Euphorbia, endodermis, 415;
latex, 184, 185 ; latici-
ferous tubes, 194, 195,196,
199, 432, 437, 525; scle-
renchyma, 423 ; vascular
system, 244, 324, 353 ;
wax, 87.

Euphorbia antiquorum, peri-
derm, 548.

Euphorbia balsamifera, wax,
83-

Euphorbia canariensis, wax,
83, 437-

Euphorbia Caput Medusae,
194, 324, 437 : stomata,
75 ; wax, 83.

Euphorbia Characias, 439.

Euphorbia Cyparissias, 194,
439 : latex, 185, 186.

Euphorbia globosa, 438 :
thyloses, 17 1.

Euphorbia Lagascae, 196,
439-

Euphorbia Lathyris, latex,
184, 185 ; laticiferous

tubes, 194,438 (Fig. 190);
vascular system, 244; wax,
86.

Euphorbia Myrsinites, latex,
185 ; laticiferous tubes,
196, 439.

Euphorbia Ornithopus, wax,

83-

Euphorbia Peplus, 439.

Euphorbia piscatoria, wax,

83-

Euphorbia resinifera, 438 :
latex, 184, 186.

Euphorbia rhipsaloides, 423,
437-

Euphorbia splendens, cor-
tex, 404 ; milk-tubes, 191,
194 (Fig. 84), 437.

Euphorbia sylvatica, 439.

Euphorbia xylophylloides,
423.

638

INDEX.

Euphorbiacese, fibres, 133;
latex, 184, 185; laticife-
rous tubes, 187, 437 ; vas-
cular system, 353.

Eupteris, 301.

Eustrephus, 410.

Euxolus, vascular system,
248, 249.

Euxolus emarginatus, 249.

Euxolus lividus, 249.

Excaecaria sebifera, latex,
185.

Existem, 394.

Exocarpus, stomata, 45.

Exomeristem, 394.

Exostemma floribundum,
296, 298.

Fabiana imbricata, vascular
system, 303.

Fagopyrum, 12 : root de-
velopment, 398.

Fagraea, anomalous w'ood,
580; fibres, 130; scleren-
chyma, 424.

Fagraea auriculata, 130.

Fagraea lanceolata, 580.

Fagraea obovata, 130.

Fagus, 171, 531 : annual
rings, 476 ; bast-fibres,
527; cork, no, in, 113,
550; periderm, 554; pith,
403 ; sclerosis, 540.

Fagus sylvatica, 511 : crys-
tals, 140, 142 ; gelatinous
layer, 482 ; periderm, 548 ;
sieve-tubes, I73)i75)i77;
tracheides, 481.

Farsetia incana, hairs, 61.

Fern-like plants, secondary
thickening in, 623 ; vas-
cular system, branching,
312.

Ferns, bordered pits, 163 ;
lime incrustations, 106 ;
root development, 398 ;
sclerotic cells in, 427 ;
sieve-tubes, r8o ; tannin-
sacs, 153.

Ferula communis, vascular
system, 253.

Ferula tingitana, oil pass-
ages, 450 ; water-pore,
52-

Festuca elatior, stomata, 50.

Festuca gigantea, stomata,
50.

Festuca heterophylla, sto-
mata, 50.

Fibrae, 5.

Fibres, 5 ; and tracheides,
483; lengths of, 1 3 1 ; tests
for, 133.

Fibrovascular bundles, 421.

Ficaria ranunculoides, endo-
dermis, 125; vascular sys-
tem, 354.

Ficus, 67 : annual rings, 504 ;
crystals, 530; cystolith-
cell, 104, 105 ; epidermis,
33; latex, 184, 185; lati-
ciferous tubes, 192, 194,
432> 439, 525; stomata,
37, 49 ; vascular system,
304, 376, 377 ; water-pore,
51, 53, 54-

Ficus australis, 34, 37,

105.

Ficus bengalensis, 33.

Ficus Carica, 34, 105, 184,
194, 439.

Ficus Cooperi, 54.

Ficus diversifolia, end of
vascular bundle, 376.

Ficus elastica, 33, 37, 57,
71, 194, 432, 439, 484:
annual rings, 506 ; cuticle,
76 ; cystolith cells, 44
(Fig. 18), 103, 104 (Fig.
44); fibres, 528 ; leaf struc-
ture, 44 (Fig. 18) ; sieve-
tubes, 173, 177; stomata,
40.

Ficus eriobotryoides, 54.

Ficus ferruginea, 33.

Ficus Joannis, silicification,
103.

Ficus laurifolia, 34.

Ficus leucosticta, 54.

Ficus lutescens, 34.

Ficus montana, 104.

Ficus neriifolia, 54, 376.

Ficus Neumanni, 34.

Ficus nymphaefolia, 34.

Ficus pectinata, 33.

Ficus Porteana, 54.

Ficus repens, 439.

Ficus rubiginosa, 479, 484.

Ficus salicifolia, 34, 105.

Ficus Sycomorus, 479, 484,
485 : gelatinous layer, 482;
silica in leaves, 102.

Ficus trachyphylla, silica in,
102.

Ficus ulmifolia, 34, 105.

Fig, cambial layer, 463 (Fig.

197)-

Filices, 17, 20, 56, 63, 67,
70, 74, 81, 99, 141 : bor-
dered pits, 163 ; endo-
dermis, 123; parenchyma,
405,413; root-structure,
362; sclerotic-cells, 120,
426 ; sieve-tubes, 180 ;
stomata, 38, 47 ; tannin-
sacs, 153 ; tracheides,
165 ; vascular system,
235, 283, 295, 300, 306,

312, 342, 345, 362, 373,
,391-

Fir, growth of wood, 477.

Flaveria contrajerva, secre-
tory reservoirs, 447.

Flindersieae, oil-cavities, 207.

Fceniculum, oil-passages,
450 (Fig. 193); vascular
system, 309, 353.

Fceniculum officinale, vas-
cular system, 241 (Figs.
100, lOl).

Forskahlea tenacissima, cys-
toliths, 105.

F ourcroya gigantea, epider-
mis, 79.

Fraxinus, 167, 171, 457, 471,
51 1 : crystals, 529; len-
ticels, 563 ; ligneous bun-
dle, 496 ; periderm, 548 ;
pits, 481 ; pitted vessels,
160, 163.

Fraxinus excelsior, hair-
structures, 64 ; pith, 403 ;
sclerosis, 540 ; secondary
growth, 466 (Figs. 199-
201), 472, 480 ; structure
of wood, 514; vascular
system, 243.

Fraxinus Ornus, 480, 563.

Frenela, vascular system,
246.

Frenela rhomboidea, 118.

Freycinetia, vascular sys-
tem, 361, 362.

Freycinetia nitida, 362.

Fridericia, 574.

Fritillaria, vascular system,
277, 318.

Fritillaria Meleagris, 142.

Froelichia, vascular system,
249.

Froelichia gracilis, 591.

Fuchsia, 470, 479 : ligneous
bundle, 495 ; stomata, 36;
vascular system, 305, 376 ;
water-pores, 51.

Fuchsia globosa, 484, 495;
gelatinous layer, 482 ;
water-pore, 51.

Fumaria, 353, 435.

Gaertnera longifolia, 580.

Galactin, 185.

Galactites Duriaei, secretory
sacs, 150.

Galactites tomentosa, 150.

Galactodendron utile, latex,
186.

Galanthus, crystal sacs, 139;
cuticle 76 ; vascular sys-
tem, 264 ; wax, 87.

Galega, hairs, 61.

Galenia, 591.

INDEX.

Galeopsis tetrahit, 67.

Galipea macrophylla, 128.

Galipea officinalis, 537 :
crystals, 530; secretory
reservoirs, 145.

Galium, 458 : raphides,
142 ; vascular system,
243, 296, 308.

Galium Mollugo, water-
pore, 52.

Geitonoplesium, 410.

Genista, mucilage, 74.

Geonoma, 266.

Geranium macrorhizum,
water-pores, 51.

Geum, water-pore, 52.

Ginkgo biloba, 14: cotyle-
dons, 245 ; lenticels, 561,
566 ; periderm, 552 ;
resin-canals, 204, 443 ;

vascular system, 246, 301,
379, 382-

Gladiolus imbricatus, 409.

Gland, 92.

Glands, dermal, 88 ; diges-
tive, 100; intramural, 91,
96 ; secretions of, 98.

Glandular hairs, 89.

Glaucium, laticiferous tubes,
187, 435 ; secretory sacs,
147.

Glaucium luteum, 435 ;
sieve-tubes, 525.

Glaziovia, wood, 573, 575,
589.

Glechoma hederacea, sto-
mata, 49.

Gleditschia, lenticels, 562,
566 ; ligneous bundle,
495, 497, 509-

Gleditschia ferox, pith, 403.

Gleditschia triacanthos, 496,
502 : annual rings, 476 ;
crystals, 142 ; fibres, 133 ;
gelatinous layer, 482; len-
ticels, 566 ; medullary
rays, 489 ; periderm, 548.

Gleichenia, vascular system,
283, 343-

Gleichenia vulcanica, 343.

Gleicheniaceae, vascular
system, 342.

Glyceria aquatica, air-
spaces, 217.

Glycine sinensis, fibres, 528.

Glycyrrhiza, 537.

Glyptostrobus, vascular sys-
tem, 246.

Gnaphalium, 70, 446.

Gnaphalium citrinum, 446.

Gnetaceas, vascular bundles,
235; vascular system, 246,

247, 306.

Gnetum, anomalous wood.

569; sclerenchyma, 424 ;
sieve-tubes, 179; vascu-
lar system, 246, 300, 301,
304.

Gnetum Gnemon, fibres,
130, 248, 424.

Gnetum scandens, anoma-
lous wood, 586 (Fig. 233),
589.

Gnetum Thoa, 130, 247,
424.

Goldfussia anisophylla, cys-
toliths, 105.

Gomphrena, vascular sys-
tem, 249.

Gomphrena decumbens,
591.

Gomphrena globosa, 591.

Gongora Jaenischii, 229.

Goniophlebium, 304.

Gossypium, secretory reser-
voirs, 202, 209.

Gramineae, 10, 35, 39, 63,
71, 1 1 8, 141 : air-spaces,
213, 214, 215; crystals,
141 ; excretion of water,
54 ; intercellular spaces,
210; parenchyma, 407,
409, 41 1, 415 ; secretions,
137 ; silica in, 102, 103 ;
stomata, 35, 38, 44, 46,
49 ; vascular system, 264,
323, 327, 359, 387; wax,
83-

Grasses, see Gramineae.

Grevillea robusta, 40.

Guajacum, crystals, 142,
529, 530; fibres, 527;
sap-wood, 508 ; secre-
tions, 138; thickening,
472.

Guazuma ulmifolia, 493.

Guizotia oleifera, 297.

Gum reservoirs, 202.

Gum-resins, 145.

Gundelia Tournefortii, 187,
231, 232.

Gunnera, vascular system,
251, 339, 396-

Gunnera bracteata, 251.

Gunnera chilensis, 251.

Gunnera commutata, 251.

Gunnera insignis, 251.

Gunnera macrophylla, 251.

Gunnera manicata, 251.

Gunnera peltata, 251.

Gunnera perpensa, 251.

Gunnera petaloidea, 251.

Gunnera scabra, 251, 339:
hair structures, 58, 66.

Gunneraceae, vascular sys-
tem, 253, 339.

Gymnema sylve.stre, 577.

Gymnocladus, 486.

639

Gymnocladus canadensis,
486, 496, 502 : sclerosis,
540.

Gymnogramme,mea!y hairs,
99, 100.

Gymnogramme Calomela-
nos, mealy hairs, 99.

Gymnogramme Martensii,
mealy hairs, 99.

Gymnogramme sulphurea,
mealy hairs, 99.

Gymnogramme tartarea,
dusty hairs, 99 (Fig. 43).

Gymnosperms, 13, 14: ano-
malous thickening in, 567 ;
axial bundle in root, 356 ;
connections of vascular
bundles, 385; cork, 108;
epidermis, 22; meriste-
matic apex, 14; periderm,
558 ; punctum vegetatio-
nis, 1 3 ; sclerenchyma,
424; sieve-tubes, 179;
vascular system, 245, 300,
304, 306, 307, 322, 356;
with a ring of vascular
bundles, 307.

Gypsophila altissima, 419,
499, 543-

Haemanthus coccineus, vas-
cular system, 302.

Haematoxylon, 487 : crys-
tals, 141 ; sap-wood, 508.

Hsemodoraceae, 10.

Hair-structures, 30, 54 ;

forms of, 56 ; mixed, 55 ;
submerged, 56.

Hairs, capitate, 62 ; con-
taining air, 69; containing
sap, 68 ; dusty, 99 ; forms
of, 60 ; glandular, 89 ; H-
shaped, 222: intercellular,
220 ; multicellular, 61 ;
persistent, 57 ; root, 55 ;
stinging, 68 ; transitory,
57 ; tufted, 62 ; unicel-
lular, 60.

Hakea, 102 : cork, 550 ;
epidermis, 77, 79 ; fibres,
130 ; ligneous bundle,
495; parenchyma, 116,
408 ; periderm, 548 ;
sclerenchyma, 422 ; tra-
cheides and fibres, '483;
vascular system, 305.

Hakea Baxter!, epidermis,
78.

Hakea Candolleana, epider-
mis, 79.

Hakea ceratophylla, 130,
305 : epidermis, 77, 78 ;
stomata, 35, 39, 40.

Hakea florida, 548.

640

IN DE X.

Hakea nitida, 130.

Hakea saligna, 67, 130:

stomata, 35, 39, 40.

Hakea suaveolens, 495 : ge-
latinous layer, 482.

Halimodendron, scleren-
chyma, 426.

Halimus, 593.

Haloxylon Ammodendron,
591, 593-

Hamamelis, medullary rays,
489 ; phelloderm, 549.

Hamamelis virgimana, crys-
tals, 142 ; thickening, 465 ;
tracheides, 481.

Hamelia chrysantha, 296.

Hancornia, latex, 185.

Haplolophium, 575, 589.

Hartwegia, aerial roots, 231.

Havvorthia, 148.

Hechtia, hair stiaictures,
64; parenchyma, 41 1 ;
stomata, 37.

Hechtia planifolia, 64.

Hechtia stenopetala, 64.

Hedera, cortex, 404 ; lig-
neous bundle, 495 ; oil-
passages, 450 ; pits, 481 ;
stomata, 46.

Hedera Helix, 479, 484,
496, 505 : resin-passages,
203 (Fig. 85), 450; vas-
cular system, 354.

Hedera Regneriana, lenti-
cels, 564.

Hedychium, thyloses, 17 1 ;
vascular system, 267, 277.

Hedysarum coronarium,
vascular system, 354.

Heleniumautumnale, water-
pores, 51.

Heleocharis palustris, air-
spaces, 216, 217.

Helianthus, hairs, 61 ; par-
enchyma, 412 ; secretion,
106.

Helianthus annuus, 10:
glands, 94 ; oil-passages,
203,446,526; sheath, 415.

Helianthus grosseserratus,
103.

Helianthus macrophyllus,
106.

Helianthus trachelifolius,
secretion, 102, 103, 106.

Helianthus tuberosus, oil-
passages, 106, 447.

Heliconia, tannin-sacs, 437.

Heliconia Bihai, 437.

Heliconia farinosa, scleren-
chyma, 128; stomata, 39,
40 ; vascular system, 302 ;
wax, 83, 87.

Heliconia pulverulenta, 437.

Heliconia speciosa, 437.

Heliopsis Ixvis, secretion,
103, 106.

Helleborus, cuticle, 75 ; sto-
mata, 36.

Helleborus foetidus, 70, 71 ;
cuticle, 76 ; stomata, 37.

Helleborus niger, 71 ; sto-
mata, 36, 75 ; water-
pores, 51.

Helleborus viridis, stomata,
75-

Helosidex, stone-elements,
127; vascular system, 254.

Helosis, point of attach-
ment, 384.

Hemerocallis, 320.

Hemerocallis fulva, 409.

Hemitelia capensis, hair-
structures, 64 ; vascular
system, 291.

Heracleum, secretory re-
servoirs, 450 ; vascular
system, 309.

Heracleum flavescens,
water-pore, 52.

Heracleum Sphondylium,
450.

Heritiera Fomes, anomalous
growth, 527.

Herminiera, 487 ; crystals,
141.

Herminiera Elaphroxylon,
470, 499, 500 : crystals,
140; secretions, 139.

Herminium Mon orchis, sto-
mata, 46.

Hertia crassifolia, vascular
system, 239.

Heterocentron, cortical
bundles, 260; periderm,
552.

Heterocentron subtripliner-
vium, 260.

Heteropsis, 268 : internal
hairs, 222 ; laticiferous
tubes, 436.

Heteropsis ovata, 361.

Heuchera, water-pore, 51.

Hevea elastica, latex, 185,
186.

Hibiscus Rosa sinensis, 479,
497-

Hibiscus syriacus, 537.

Hieracium, 458: hair struc-
tures, 56, 65 ; laticiferous
tubes, 433; sieve-tubes,
231.

Hieracium aurantiacum,
hairs, 56.

Hieracium denticulatum,
water-pore, 52.

Hieracium piliferum, hairs,
59 (Fig. 21).

Hieracium Pilosella, water-
pore, 52 ; hairs, 61.

Hieracium sabaudum,
water-pore, 52.

Hieracium vulgatum, 167.

Hippocrateaceae, 587.

HippomaneMancinella,439.

Hippophae, 480.

Hippophae rhamnoides,
bordered pits, 164.

Hippuris, air-spaces, 213,
217 ; cortex, 405 ; endo-
dermis, 121 ; glands, 91;
hair structures, 64 ; vas-
cular bundles, 278, 340 ;
vascular system, 340, 395 ;
water-pores, 51, 53.

Hippuris vulgaris, growing
point, 8 (Fig. i).

Hirtella silicea, 510.

Hohenbergia strobilacea,
411.

Holcus mollis, stomata, 50.

Homalomena, aerial roots,
230 ; secretory reservoirs,
445 ; vascular system,
268.

Homalomena cxrulescens,
320.

Homalomena Porteanum,
445-

Homalomena rubescens,
445-

Homalomena Wendlandii,
445-

Hordeum, 10, 412: vas-
cular system, 359.

Hordeum murinum, sto-
mata, 50.

Hordeum vulgare, apex of
root, 10.

Hottonia, 25 : air-spaces,
213; cortex, 405; endo-
dermis, 415; stomata, 46,
49 ; vascular system, 277,
340.

Hottonia palustris, 53.

Houstonia coccinea, 296.

Hoya, 199, 486.

Hoya carnosa, aerial roots,
230; crystals, 140 ; endo-
dermis, 125; epidermis,
77,81; laticiferous tubes,
194 ; stone elements, 127 ;
wax, 82.

Humulus, cystoliths, 105 ;
glandular scales, 95 (Fig.
40); secretion, 98, 103.

Humulus Lupulus, glandu-
lar hairs, 90 ; hair struc-
tures, 61, 64, 90; latici-
ferous tubes, 439; vas-
cular system, 244, 297,
298, 392.

INDEX.

641

Hura crepitans, latex, 185;
laticiferous tubes, 439.

Hyacinthus, raphides, 139;
root-cap, 413; stomata,
39,43 (F'g- 17); vascular
system, 320.

Hyacinthus orientalis, cell-
wall, 71 (Fig. 23); paren-
chyma, 409; stomata, 35
(Fig. 10).

Hydrangea hortensis, 484:
ligneous bundle, 470, 496.

Hydrilla, root-cap, 413 ;
vascular system, 278, 368.

Hydrilla verticillata, 369.

Hydrilleae, 67, 278, 300.

Hydrocharideae, air-spaces,
213.

Hydrocharis, growing point
of root, 9 ; root-cap, 411 ;
vascular system, 272, 304,
327.

Hydrocleis Humboldtii,i65 :
endodermis, 122 ; vascular
system, 371.

Hydrocotyle vulgaris, en-
dodermis, 1 2 1 ; secretory
reservoirs, 499 ; vascular
system, 242.

Hydrophylleae, hairs, 60.

Hydropterides, 283, 413.

Hymenophylleae, scleren-
chyma, 426 ; vascular sys-
tem, 283, 284, 342.

Hymenophyllum, hairstruc-
tures, 64 ; vascular sys-
tem, 283, 363.

Hyospathe, 266.

Hypericum, secretory re-
servoirs, 202, 208, 209 ;
vascular system, 243.

Hypericum balearicum, 209.

Hypericum calycinum, 209.

Hypericum canariense, 209.

Hypericum hircinum, 209.

Hypericum perforatum, oil-
cavities, 208.

Hypericum quadrangulum,
243.

Hypochaeris radicata, 420.

Hypocotyledonary stem,
236.

Hypoderma, 225, 404.

Hypodermal tissue, 225.

Hypolaena, 425.

Hypolepis, petiole, 403 ; vas-
cular system, 284.

Hypoxideae, 10.

Hysterogenetic spaces, 200.

Iberis, 415.

Iberis amara, vascular bun-
dles, 237 (Figs. 92, 93I,
308.

Idioblast, 3.

Ilex, 70, 1 16, 535; cuticle,
76; ligneous bundle, 495 ;
parenchyma, 411; peri-
derm, 558 ; pith, 403 ; sto-
mata, 49 ; vascular sys-
tem, 322.

Ilex Aquifolium, 213, 41 1,
470: cuticle, 75; epider-
mis, 77, 78 ; ligneous bun-
dle, 495 ; sclerenchyma,
425 ; section of leaf, 78
(Fig. 26).

Ilex ovata, 41t.

Impatiens, 67 : crystals,i43;
medullary rays, 492 ; ra-
phides, 143 ; vascular sys-
tem, 236.

Impatiens Balsamina, vas-
cular system, 236, 238.

Imperatoria Ostruthium, se-
cretory reservoirs, 450.

Initial cell, 39.

Intercellular spaces, 4, 135,
200; arrangement of, 440;
containing air and water,
210; hairs, 220.

Inula Helenium, 497, 526.

Inula montana, 446, 447.

Ipomcea purga, anomalous
structure, 607 ; secretions,

151-

Ipomcea purpurea, anoma-
lous structure, 607.

Ipomcea Turpethum, 607.

Iriartea, 233 : sclerenchyma,
420; vascular system, 361,
362.

Iriartea exorrhiza, 361.

Iriartea praemorsa, 361.

Irideae, 10: leaf-structure,
409; sclerenchyma, 419.

Iris, crystals, 141, 142 ;

cuticle, 81 ; secretions,
138; stomata, 35, 37, 38,
39, 47; vascular system,
339) 357-

Iris germanica, vascular sys-
tem, 339, 387.

Iris Monnieri, 357.

Iris Pseudacorus, air-spaces,
213, 216.

Isoetes, 15, 54; air-spaces,
213, 217 ; axile bundle in,
280; parenchyma, 407,
413; secondary thicken-
ing, 623, 624; stomata,
50; tracheides, 226; vas-
cular system, 278, 301,
335) 337) 365-

Isoetes Durieui, 338, 407,
624.

Isoetes Engelmanni, 338.

Isoetes Hystrix, 407, 624.

t t

Isoetes lacustris, 54, 624.

Isonandra, 146.

Isonandra gutta, secretory
sacs, 1 5 1.

Isotoma, laticiferous tubes,

434.

Jacquinia, 419.

Jasminum, 535 : hair struc-
tures, 63, 64 ; ligneous
bundle, 495.

Jasminum fruticans, crys-
tals, 529; vascular sys-
tem, 237; wax, 82.

Jasminum officinale, cuticle,
76; epidermis, 77.

Jasminum revoluturn, 495.

Jatropha Manihot, 470, 479,
481, 482, 502.

J atropha napaEifolia,hairs,6o.

Jatropha urens, hairs, 60.

Jochroma coccineum, crys-
tals, 143.

Jochroma Warczewiczii,
crystals, 143.

Jubsea spectabilis, vascular
system, 266.

Juglans, 167, 505: ligneous
bundle, 495; medullary
rays, 489 ; periderm, 548 ;
sieve-tubes, 176; stomata,
46.

Juglans cinerea, 498.

Juglans regia, 479, 498:
crystals, 142, 530; fibres,
^28; lenticels, 564; scle-
rosis, 540.

Juncaceae, 10 : parenchyma,
407, 415; sclerenchyma,
419.

Juncagineae, 10.

Juncus, intercellular spaces,
212 ; pith, 212.

Juncus bufonius, 419.

Juncus effusus, air-spaces,
216; parenchyma, 409;
stomata, 40.

Juncus glaucus, 409.

Juncus lamprocarpus, sto-
mata, 40.

Juncus paniculatus, 419.

Juniperus, bast (Fig. 211),
521; parenchyma, 410;
periderm, 551 ; medullary
rays, 489; resin-canals,
443 ; secretory reservoirs,
442, 544; sieve-tubes,
179; tracheae, 163; tra-
cheides (Fig. 209), 404;
vascular system, 246, 308,
381, 382.

Juniperus communis, 382,
410, 467, 489; bordered
pits (Figs. 62, 63), 164;

642

I NDE X.

vascular bundles (Fig. 184),
381.

Juniperus excelsa, 382.

Juniperus macrocarpa, 382.

Juniperus nana, 410.

Juniperus oblonga, 382.

Juniperus Oxycedrus, 382 :
apex of root (Fig. 6), 13.

Juniperus sabina, 382.

Juniperus virginiana, 489.

Jurinea alata, secretory sacs,
150.

Jussiaea, diaphragms, 217 ;
spongy cortex, 212.

Jussiaea grandiflora, 212.

Jussiaea helminthorrhiza,
212.

Jussiaea natans, 212.

Jussiaea repens, 212.

Justicia carnea, 470, 479,
484: cystoliths, 105.

Justicia paniculata, 105.

Justicia purpurascens, 105.

Jute fibre, 1 31.

Kaulfussia, 36 : stomata, 54.

Kerria, crystals, 140, 530;
ligneous bundle, 495;
wax, 82, 83, 87.

Kerria japonica, 502 : epi-
dermis, 77.

Kleinia ficoides, secretory
reservoirs, 447 ; wax, 85.

Kleinia neriifolia, epidermis,
77-

Klopstockia, sclerenchyma,
425 ; wax, 82, 83.

Klopstockia cerifera, 33, 77 :
hair structures, 64 (Fig.
30) ; lamina, 407 ; wax
covering, 86.

Knautia, 297.

Koeleriacristata,stomata,5o.

Koelreuteria, 489 ; scle-
rosis, 540.

Kunthia, 266.

Kyllingia, 32.

Labiatae, air-spaces, 214;
collenchyma, 119; cor-
tex, 404 ; glands, 94 ; hair
structures, 56, 61, 62, 63,
70, 94 ; intercellular

spaces, 210; secretions,
98, 137 ; stomata, 37, 41 ;
vascular system, 243, 297.

Lactuca, latex, 184, 185 ;
laticiferous tubes, 433,
525 ; sieve-tubes, 231.

Lactuca virosa, milk-tubes,
190 (Fig. 82), 433, 525;
sheath, 415.

Ladenbergia, secretory sacs,
146, 749.

Ladenbergia globosa, fibres,
527.

Ladenbergia magnifolia,
fibres, 528.

Lagenaria, 173 : sieve-tubes,
174, 231 ; vascular sys-
tem, 353, 354.

Lagenaria vulgaris, sieve-
tubes, 174, 175 (Figs. 72,
73)-

Lagoecia, 450.

Lamina of the leaf, 406.

Lamium album, collenchy-
ma, 120 ; glands, 94.

Lamium purpureum, 67.

Lampsana communis, 448.

Landolphia, latex, 185.

Langsdorffia, point of attach-
ment, 385 ; stone-ele-
ments, 127.

Lantana, anomalous thick-
ening, 567.

Lapageria, 304.

Lappa, oil-passages, 447; se-
cretory sacs, 149.

Lappa grandiflora, secretory,
reservoirs, 447.

Larix, medullary rays, 489,
490, 491 ; secretory re-
servoirs, 442, 443, 544 ;
sieve-tubes, 179.

Larix europsea, fibres, 529;
structure of wood, 491 ;
vascular system, 379.

Lasia, 436.

Lasia ferox, vascular system,
268.

Lasiandra, cortical bundles,
259, 260; hairs, 64; pe-
riderm, 552.

Lasiandra Hoibrenkii, 259.

Lasiandra macrantha, 260.

Lasiandra Maximiliani, 260.

Latex, 146, 182.

Lathraea, stomata, 46, 49.

Lathraea clandestina, 46,
49.

Lathraea squamaria, 46, 49.

Lathyrus, cortical bundles,
256; vascular system, 236,
240.

Lathyrus Aphaca, 236, 240,
256, 296.

Lathyrus latifolius, 297.

Lathyrus Nissolia, 240, 297.

Lathyrus odoratus, 240.

Lathyrus Pseudaphaca, vas-
cular system, 240 (Fig. 98),
256.

Lathyrus purpureus, 240.

Lathyrus sativus, 354.

Lathyrus sylvestris, 297.

Laticiferous tubes, 183 ; ar-
ticulated, 186, 433 ; course

of, 432 ; non-articulated,
186, 437.

Lauraceae, mucilage, 143,
144; secretory reservoirs,
136» 137; vascular sys-
tem, 303.

Laurocerasus, 96.

Laurus, 70: sclerenchyma,
418; thickening, 470.

Laurus Camphora, 470, 498,
502: secretions, 145.

Laurus nobilis, 470, 498 :
epidermis, 77 ; sclerosis,
540.

Laurus Sassafras, crystals,
529 ; epidermis,77 ; fibres,
529.

Lavandula, glandular hairs,
95 ; hair structures, 56.

Lavandula elegans, hairs, 62.

Lavandula multifida, glands,
95-

Lavandula Stoechas, hairs,
62.

Lavandula vera, hairs, 62.

Leaves, tough, structure of,
41 1 ; vascular system in,
296.

Ledum palustre, glands, 91,
97-

Leguminosae, 483: anoma-
lous wood, 569 ; bordered
pits, 164; periderm, 551;
tannin-sacs, 153 ; vascular
system, 298.

Lemna, air-spaces, 213,214;
root-cap, 413 ; root-hairs,
55 ; root-structure, 370.

Lemna minor, 412.

Lemnacese, 56 : crystals,
142 ; vascular system, 370.

Lennoaceas, stomata, 46 ;
vascular system, 366.

Lenticels, 560; number of,
566 ; origin of, 563.

Leontice, vascular system,
248, 249.

Leontodon hastilis, hairs, 65.

Leontodon incanus, hairs, 6 5 .

Leopoldinia, sclerenchyma,
420 ; vascular system, 266.

Lepanthes cochlearifolia,
411.

Lepides, 54.

Lepidium sativum, vascular
system, 236, 237.

Lepidocaryum, 420.

Lepidoceras, stomata, 45.

Lepismium paradoxum,wax,
83.

Lepismium radicans, 261.

Leucojum, 139: vascular
system, 264, 327,

Leucoplocus, 425.

INDEX.

643

Levisticum, 537.

Libocedrus, vascular system,
246.

Ligustrum, pith, 403.

Ligustrum vulgare, 484 : len-
ticels, 564 ; medullary
rays, 489 ; pits, 481.

Liliaceae, 10 : air-spaces,
216; crystals, 142; ra-
phides, 139, 142 ; scleren-
chyma, 419 ; vascular sys-
tem, 3 1 1.

Lilium, parenchyma, 410 ;
stomata, 36, 38 ; vascular
system, 277, 357.

Lilium aurantiacum, 142.

Lilium bulbiferum, 36, 410.

Lilium candidum, 36, 71,
142.

Lilium Martagon, 36, 142,
357-

Lime, excretion of, 53 ; in-
crustations of, 106.

Limnanthemum, air-spaces,
212, 217; fibres, 130; in-
tercellular hairs, 220, 2 22 ;
sclerenchyma, 422.

Limnanthemum nymph-
oides, 212, 217.

Limnocharis, 10.

Linaria, 415.

Linum, fibre, 131.

Linum catharticum, sto-
mata, 48.

Linum usitatissimum, apex
of root, II, 12, 1 3 1.

Liquidambar, cork,i 14,550;
lenticels, 562.

Liquidambar styraciflua, me-
dullary rays, 493.

Liriodendron tulipifera, 486:
ligneous bundle, 495 ; pith,
403; pits, 481; secretory
reservoirs, 145 ; thicken-
ing, 469; vascular system,
239, 297.

Listera ovata, 67.

Lithospermum officinale, se-
cretion, 106.

Loasa, hairs, 60.

Loasa bryonisefolia, 60.

Loasaceae, hairs, 56, 60, 68.

Lobelia, 458: latex, 185;
laticiferous tubes, 187,
188, 434 ; sieve-tubes,

524; vascular system, 324.

Lobelia Dortmanna, air-
spaces, 213 ; parenchyma,
409-

Lobelia inflata, 434.

Lobelia laxiflora, 188, 434.

Lobelia syphilitica, 122 : la-
ticiferous tubes, 187, 434.

Lobelia urens, 434.

Lobeliaceas, laticiferous
tubes, 187.

Logania floribunda, 580.

Logania longifolia, 580.

Lomaria, lime incrustations,
106.

Lomaria attenuata, 106.

Lomatia, 422.

Lomatia longifolia, epider-
mis, 71, 78.

Lomatophyllum, 618.

Lonicera, lenticels, 560; pe-
riderm, 552, 555 ; phello-
derm, 552 ; vascular sys-
tem, 244.

Lonicera Caprifclium, 532,
547 : fibre, 131.

Lonicera fragrantissima,
pith, 403.

Lonicera implexa, wax, 85.

Lonicera italica, 560.

Lonicera Periclymenum,
560.

Lonicera tatarica, crystals,
142, 530.

' Lonicera Xylosteum, 309.

Lophanthus, glands, 95.

Lophophytum, point of at-
tachment, 384 ; stone ele-
ments, 127.

Loranthus, stomata, 45.

Loxsoma, vascular system,
284.

Luehea grandifolia, 493.

Luffa, 353, 354.

Lundia, 575.

Lupinus, vascular system,
236, 308.

Lupinus Lehmanni, vascu-
lar system, 238 (Figs. 94,
95)-

Lupinus luteus, 236.

Lupinus varius, vascular
system, 354.

Luzula albida, air-spaces,
216.

Luzula maxima, stomata,
40; air-spaces, 216.

Lychnis viscaria, glandular
hairs, 90, 91.

Lycium, phelloderm, 552.

Lycium barbarum, peri-
derm, 552.

Lycopersicum, 387.

Lycopodiace3e,meristematic
apex, 14; parenchyma,
413-

Lycopodium, 14, 22: bor-
dered pits, 164 ; hairs, 60 ;
mucilage-canals, 202, 441;
sclerenchyma, 429 ; scle-
rotic cells, 1 2 1 ; secretory
reservoirs, 203 ; sieve-
tubes, 180, 18 1, 182 ; tra-
T t 2

cheae, 163, vascular sys-
tem, 280, 281, 301, 315,

348-

Lycopodium alopecuroides,

441-

Lycopodium alpinum, 281,
349, 350, 429-

Lycopodium annotinum,
181, 282, 349, 350, 441.
Lycopodium Chamaecypa-
rissus, 429 : vascular
strand, 349 (Fig. 162).
Lycopodium clavatum, 18 1,
282, 349, 429.

Lycopodium complanatum,

281, 349.

Lycopodium inundatum,

282, 349, 350, 441.
Lycopodium pinifolium,

pits, 71.

Lycopodium Selago, 281,
349, 350, 429-

Lycopus exaltatus, water-
pore, 52.

Lygodium, 284, 344.
Lysigenetic spaces, 200.
Lysimachia, 2 5 : secretion,
209 ; secretory reservoirs,
135, 146, 202, 203, 204,
205, 209.

Lysimachia Ephemerum,
205, 209 : secretion, 200.
Lysimachia Nummularia,
355-

Lysimachia punctata, 203,
209.

Lysimachia vulgaris, 209.
Lytharieae, crystals, 142.

Macleya, secretory sacs, 147.
Macleya cordata, 435.
Madura, crystals, 530; lati-
ciferous tubes, 192, 525;
sclerenchyma, 426.
Madura aurantiaca, latex,
184; sieve-tubes, 173.
Maerua uniflora, 589.
Magnolia, ligneous bundle,
495 ; pith, 403 ; secretory
reservoirs, 145.

Magnolia acuminata, 486,
495 : medullary rays, 493 ;
thickening, 469.

Magnolia grandiflora, silica
in leaves, 102.

Magnolia tripetala, 486.
Magnoliaceae, secretory re-
servoirs, 145.

Mahonia, 456, 539: ligneous
bundle, 495.

Mahonia Aquifolium, 484,
505 ; annual rings, 506 ;
bordered pits, 163; crys-
tals, 529 ; fibres, 529.

644

INDEX.

Mahonia Fortune!, 41 1.

Malpighia, parenchyma, 4 10;
vascular system, 376.

Malpighia macrophylla, 410.

Malpighia urens, 73.

Malpighiaceae, anomalous
wood, 569, 577, 601 ;

glands, 90, 96 ; hair struc-
tures, 56, 61, 73.

Malvaceae, cortex, 404 ;
fibres, 527 ; hair struc-
tures, 56, 62 ; mucilage,
143, 144.

Mamillaria, 504 ; medullary
bundles, 535; secondary
growth, 478 ; secretory
reservoirs, 202, 204, 206 ;
vascular system, 253, 254,
297, 310.

Mamillaria angularis, 204,
206, 254.

Mamillaria glochidiata, 206,
254.

Mamillaria Hystrix, 206.

Mamillaria pusilla, 254.

Mamillaria Zuccariniana,
206.

Mammea, resin passages,
440, 451.

Mammea americana, secre-
tory reservoirs, 201.

Maranta,sclerenchyma, 128,
212.

Maranta bicolor, stomata,
40.

Maranta compressa,scleren-
chyma, 138.

Marantaceae, 21 1 : air-

spaces, 213, 214 ; crystals,
142.

Marattia, apical cell, 21.

Marattia cicutaefolia, 21.

Marattia Kaulfussii, 118.

Marattiaceae, 15, ii8, 119:
mucilage canals, 202, 203,
204, 441 ; parenchyma,
413; sclerenchyma, 427 ;
tannin-sacs, 153 ; vascular
system, 289, 321, 342,
345. 362, 364.

Maravelia zeylanica, 270,
311, 316.

Marginaria, 304.

Marrubium, hairs, 62.

Marsh-plants, air-spaces,
213 ; stomata, 50.

Marsilia, 31,211, 215; air-
spaces, 213, 215, 216;
sclerenchyma, 121, 427,
429 ; sieve-tubes, 180,
181 ; stomata, 49 ; tannin-
sacs, 153 ; vascular sys-
tem, 279, 284, 301, 343,
344-

Marsilia coromandeliana,
427.

Marsilia diffusa, 49.

Marsilia distorta, 427.

Marsilia Drummondii, 49,
180, 343, 429.

Marsilia Ernesti, 49.

Marsilia macra, 49.

Marsilia muscoides, 427.

Marsilia pubescens, 49.

Marsilia quadrifoliata, 49,
429.

Marsilia Salvatrix, scleren-
chyma, 121, 429.

Marsilia trichopoda, 427.

Marsiliaceae, 17, 18: air-
spaces, 213, 215, 2i6;
root-development, 398 ;
sclerenchyma, 426 ; sieve-
tubes, 180, 181 ; stomata,
46; vascular system, 312,
342, 362.

Martineza aculeata, vascu-
lar system, 265.

Matthiolaarborescens, hairs,
61.

Maurandia semperflorens,
glands, 94.

Mauritia, 266, 425.

Mauritia armata, vessels,
169.

Maxillaria squalens, 302.

Maxillaria tricolor, aerial
roots, 229.

Medicago, 297.

Medicago sativa, vascular
system, 240, 297, 354.

Medinilla, cortical bundles,

259, 260; fibres, 527 ; pe-
riderm, 548 ; stone ele-
ments, 127.

Medinilla farinosa, 259, 260,
486 : hair structures, 65.

Medinilla magnifica, 259,

260.

Medinilla Sieboldii, 259,260.

Medulla, 236.

Medullary bundles, 248 ;
rays, 236, 489; discon-
tinuous, 460 ; spots, 489.

Melaleuca, ligneous bundle,
495; oil-cavities, 207;
parenchyma, 408, 410 ;
periderm, 552 ; scleren-
chyma, 422; vascular sys-
tem, 338.

Melaleuca hypericifolia, 410.

Melaleuca imbricata, 485.

Melaleuca linearifolia, 408,
411.

Melaleuca styphelioides,
no: cork, 556; crystals,
142.

Melaleuca tetragona, 408.

Melampyrum sylvaticum,
epidermis, 67.

Melanthaceae, 419.

Melastoma cymosum, corti-
cal bundles, 260; peri-
derm, 548.

Melastoma Heteromallum,
552.

Melastoma igneum, 260.

Melastoma malabathricum,
hairs, 64.

Melastomaceae, no, 458,
470 : cortical bundles,
259; endodermls, 415;
hair structures, 56, 62,
64, 65 ; medullary bundles,
535 j periderm, 5 52; sieve-
tubes, 231; vascular sys-
tem, 248, 253, 338, 391.

Melianthus major, cortex,
404.

Melica nutans, stomata, 50.

Melica uniflora, stomata, 50.

Melissa officinalis, glands,
93 ; vascular system, 243.

Melloa populifolia, anoma-
lous wood, 570 (Fig. 226),
575-

Melocactus, 478.

Members, i.

Menispermacese, anomalous
wood, 587.

Menispermum, 536 : ano-
malous wood, 569, 589 ;
bast-fibres, 527 ; vascular
system, 239, 395.

Menispermum canadense,
70, 461, 486, 589.

Mentha, stomata, 50.

Mentha aquatica, 354, 355.

Menyanthese, air-spaces,
213 ; internal hairs, 222.

Menyanthes, vascular sys-
tem, 242.

Menyanthes trifoliata, 242 :
endodermis, 122.

Mercurialis, endodermis,
415; vascular system, 236,
245, 353-

Mercurialis ambigua, stoma-
ta, 41.

Mercurialis annua, 67, 245 :
glands, 91, 96.

Mercurialis perennis, 245 :
stomata, 41.

Meristem, 394; primary, 3;
secondary, 4 ; phelloge-
netic, 109.

Meristematic apex, 14.

Mesembryanthemum, ano-
malous structure, 589,
590, 595 ; crystals, 143 ;
parenchyma, 116, 408,

409 ; raphides, 143 ; se-

INDEX.

645

cretion, 102 ; vascular
system, 297, 304, 306;
spiral bands, 157.

Mesembryanthemum barba-
tum, 306.

Mesembryanthemum crys-
tallinum, 58, 297: anoma-
lous wood, 595, 597; hair
structures, 65.

Mesembryanthemum imbri-
catum, 297, 306.

Mesembryanthemum incur-
vum, 102.

Mesembryanthemum lace-
rum, 102.

Mesembryanthemum Leh-
manni, 102.

Mesembryanthemum lin-
guaeforme, 306.

Mesembryanthemum stra-
mineum, 102, 157, 306.

Mesembryanthemum tigri-
num, 102.

Mesembryanthemum virens,
595, 598.

Mesembryanthemum vulpi-
num, 102.

Mesomeristem, 394.

Mesophyll, 406.

Mespilus germanica, iio.

Metrosideros, 338.

Miconia chrysoneura, corti-
cal bundles, 260; peri-
derm, 548.

Miconia purpurascens, 259.

Microlepia, vascular system,
284, 342.

Mida, stomata, 45.

Milium effusum, Stomata, 50.

Milk-tubes, 183; origin of,
193-

Mimosa, hairs, 65.

Mimosa pudica, 320.

Mimoseae, 212.

Mirabilis, anomalous struc-
ture, 590, 593, 596 (Figs,
234-236);secretions, 138,
vascular system, 353.

Mirabilis Jalapa, 236, 596.

Mirabilis longiflora, 596.

Mitella, water-pore, 51.

Moehringia trinervia, sto-
mata, 48.

Molinia, 419.

Moluccella,glands,93; hairs,
42.

Monocotyledons, anoma-
lous vascular system, 274 ;
cork, 108; cortex, 405;
crystals, 142 ; endodermis,
123 ; origin of vascular
bundles, 396 ; parenchy-
ma, 408; raphides, 142 ;
root-structure, 357; scle-

renchyma, 421, 422; se-
condary thickening in,
618 ; secretions, 139 ;
sieve-tubes, 231; stomata,
49 ; stone-elements, 127;
tannin-sacs, 153; thylo-
ses, 170; tracheides, 165;
vascular system, 267, 274,
300, 301, 324, 327, 328,
339) 396; with an axile
bundle, 311.

Monotropa Hypopitys, 46,
165 ; cuticle, 75.

Monstera, cork, 108 ; in-
ternal hairs, 222, 223 (Fig.
89) ; laticiferous tubes,
436 ; secretory reservoirs,
445 ; vascular system, 268,
304, 361.

MonsteraAdansonii, 36 1,445.

Monstera surinamensis, 361,
445-

Monsterineae, 213, 222, 268.

Morese, latex, 184; latici-
ferous tubes, 187, 192.

Morus, cystoliths, 105;
fibres, 528; latex, 184;
laticiferous tubes, 525;
silification in, 103.

Morus alba, 470, 479, 482,
496: crystals, 142; gela-
tinous layer, 482.

Morus nigra, latex, 184.

Mother-cell, 39.

Mucilage, 143.

Mucilage canals, 441.

Mucilage reservoirs, 202.

Mucuna, 589.

Muehlenbeckia complexa,
458 : ligneous bundle,495.

Muehlenbeckia platyclados,
407, 409.

Musa, air-spaces, 213, 218;
latex, 183, 184; laticife-
rous tubes, 189, 436; pa-
renchyma, 41 1 ; scleren-
chyma, 422; thyloses, 17 1 ;
tracheides, 165; vascular
system, 267 ; wax, 83, 85.

Musa Cavendishii, 437.

Musa Ensete, 267.

Musa ornata, 85.

Musa sapientum, 40.

Musa zebrina, 437.

Musaceae, 10: latex, 146;
laticiferous tubes, 187,
199; raphides, 142; se-
cretions, 136, 146; secre-
tory sacs, 153.

Muscari, 139.

Musschia aurea, 434.

Myoporese, secretory reser-
voirs, 202.

Myoporum, oil-cavities, 209.

Myoporumparvifolium, 209.

Myoporum tuberculatum,
209.

Myoschilus, stomata, 45.

Myrica, vascular system, 305 ;
wax, 87.

Myrica cerifera, hair struc-
tures, 64; wax covering,86.

Myriophyllum, 12: air-
spaces, 213,215,217; cor-
tex, 404; crystals, 141,
220 ; stomata, 46, 49 ; vas-
cular bundles, 278, 341.

Myrrhis odorata, anomalous
structure of roots, 606 ;
medullary rays, 491 ; sap-
passages, 450 ; vascular
system, 309.

Myrsine, secretory reser-
voii's, 135, 146, 202, 203,
204, 205, 209.

Myrsine africana, secretion,
209.

Myrsiphyllum, 301.

Myrtaceae, 21 1 : leaf struc-
ture,4o8 ; oil-cavities, 202,
207 ; parenchyma, 408 ;
sclerenchyma, 422 ; sieve-
tubes, 231 ; vascular sys-
tem, 338.

Myrtus, oil-cavities, 207 ;
vascular system, 338.

Myrtus communis, 207.

Nageia, vascular system, 301.

Naias, root-cap, 413; sto-
mata, 46; vascular sys-
tem, 278, 369.

Narcissus, secretions, 139;
vascular system, 264.

Narcissus Pseudonarcissus,
409-

Nasturtium, stomata, 49.

Neea, 590, 594.

Negundo, 535 : lenticels,
565 ; periderm, 558 ; wax
in epidermis, 75.

Nelumbium, 212 ; air-spaces,
213, 214, 217, 218; crys-
tals, 140; sclerosis, 540;
stomata, 37 ; vascular sys-
tem, 253, 255 (Fig. 112),
320, 323, 327.

Nelumbium speciosum, tra-
cheides, 165.

Neottia Nidusavis, 46.

Nepenthes, cortical bundles,
258; digestive glands, 100,
101 ; tracheides, 226 ; vas-
cular system, 374, 375.

Nepeta Cataria, vascular
system, 243.

Nephalaphyllum, hair struc-
tures, 55.

646

INDEX.

Nephrolepis, lime incrusta-
tions, 106 ; vascular bun-
dle in stolons, 347; vas-
cular system, 284, 288,

347.

Nephrolepis acuminata, 347.

Nephrolepis exaltata, 347.

Nephrolepis pectinata, 348.

Nephrolepis ramosa, 288.

Nephrolepis rufescens, 348.

Nephrolepis tuberosa, 347.

Nephrolepis undulata, 15.

Nerium, 116, 129, 167, 470,
479, 486 ; bast, 524 ; cork,
no; latex, 184, 185; la-
ticiferous tubes, 194, 439 ;
phelloderm, 548 ; pitted
vessels, 160, 1 6 1 ; trachea;,
157-

Nerium Oleander, 77, 194,
41 1 : crystals, 530; peri-
derm, 548 ; stomata, 37,
47, 49-

Neuropteris, 301.

Nicandra physaloides, 141 :
hairs, 62.

Nicotiana, crystals, 143 ;
sieve-tubes, 231.

Notholsena nivea, dusty
hairs, 99.

Notobasis syriaca, secretory
sacs, 150.

Nuphar, endodermis, 122;
vascular system, 252, 327,

354.

Nuphar advena, intercellu-
lar hairs, 221 (Fig. 88).

Nuphar luteum, 165, 252,
354 : air-spaces, 215.

Nuphar pumilum, 165, 221,
252.

Nuytsia, stomata, 45.

Nyctagineae, . anomalous
structure, 590, 593 ; ra-
phides, 143.

N y mphaea, collenchyma, 1 1 9,
120 ; hair structures, 56 ;
internal hairs, 221; vas-
cularsystem, 252,327,354.

Nymphaea alba, 165, 252,
354-

Nymphaea caerulea, 221.

Nymphaea gigantea, 165.

N ymphaea odorata, 221.

Nymphaeaceae, air-spaces,
213, 215, 217; fibres, 130,
133; intercellular hairs,
220, 222; root-cap, 4135
sclerenchyma, 423 ; vas-
cular system, 248, 250,
252.

Oak, cork -periderm, 577 ;
growth of wood, 477.

Obeliscaria columnaris, sili-
cification in, 103, 106.

Oberonia myriantha, endo-
dermis, 125.

Obione, 70, 591, 592.

Obione portulacoides, hair-
structures, 63.

Octomeria, hair structures,
55, 71.

Octomeria graminifolia,4ii.

Oenanthe crocata, secretory
passages, 449, 497.

Oenanthe pimpinelloides,
449.

Oenocarpus, 266.

Oenothereae, raphides, 143.

Olea, 70, 324: fibres, 130;
sclerenchyma, 424.

Olea emarginata, 1 30.

Olea europaea, 424, 479:
crystals, 143, 530; hair-
structures, 64; raphides,
143 ; stomata, 47; vas-
cular bundle, 334.

Olea fragrans, 130, 424.

Oleaceae, hair structures, 56,
64, 70.

Oncidium flexuosum, aerial
roots, 229, 230.

Oncidium pulvinatum, 229.

Oncidium sanguineum, 229.

Oncidium sphacelatum, 230.

Oncidium sphegiferum, 230,
360.

Oncidium species, aerial root
sheath, 227(Fig. 90) ; apex
of root, 228 (Fig. 91).

Onobrychis sativa, 354.

Onoclea Struthiopteris, 118,
285, 427.

Ononis spinosa, anomalous
thickening, 567.

Onopordon Acanthium, se-
cretory sacs, 150-167.

Onosma, 102 : secretion, 106.

Onosma arenarium, 106.

Onosma stellulatum, 106.

Ophioglosseae, cork, 108 ; pa-
renchyma, 413; root-
hairs, 53; sieve-tubes,! 80;
vascular system, 284, 285,
305, 318, 321, 346, 364.

Ophioglossum pedunculo-
sum, 285, 305, 346.

Ophioglossum vulgatum, ii 8,
284, 285, 305, 346.

Ophiopogonex, 10.

Ophrydeae, vascular system,

233, 319, 362.

Opium, 185.

Opoponax Chironium, oil-
passages, 450 ; vascular
system, 253, 497.

Opuntia, mucilage passages,

202, 204, 207; secondary
thickening, 479 ; secre-
tory reservoirs, 452 ; vas-
cular system, 310.

Opuntia andicola, 499.

Opuntia cylindrica, 499.

Opuntia peruviana, 452.

Opuntia ramulifera, 449.

Opuntia robusta, 452, 499.

Opuntia tunicata, 499.

Orchideae, 10; aerial roots,
229; crystals, 142; cu-
ticle, 76 ; depressions
with hairs, 55; endoder-
mis, 124; leaf structure,
411 ; parenchyma, 41 1 ;
raphides, 142; root-
sheath, 155, 227 ; scle-
renchyma, 128, 419; sto-
mata, 37 ; tracheides, 165 ;
vascular system, 278, 311,
315, 359 ; velamen, 227.

Orchis, 39 : cuticle, 75 ; mu-
cilage, 143, 144 ; raphides,
139; stomata, 38.

Orchis latifolia, 37.

Orchis Morio, cuticle, 81.

Organs, i.

Ornithogalum, 139.

Ornithogalum umbellatum,
parenchyma, 409.

Ornus, pith, 403.

Orobanchaceae, stomata, 46 ;
vascular system, 253.

Orobanche,haustorium, 384;
vascular system, 254,
366.

Orobanche caryophyllacea,
254.

Orobanche elatior, 254.

Orobanche Rapum, vascular
system, 254 (Fig. iii).

Orobanche rubens, 254.

Orobus vernus, 354.

Or) za sativa, air-spaces, 217;
vascular system, 359.

Osbeckia canescens, cortical
bundles, 258 (Fig. 115),
260 (Fig. 1 16), hair struc-
tures, 65.

Osmunda, 54 : mucilage, 99;
vascular system, 319.

Osmunda cinnamomea, 54.

Osmunda Claytoniana, 54.

Osmunda regalis, 54 : hairs,
68 ; sclerenchyma, 426 ;
sclerotic cells, 121 ; stem
structure, 426 (Fig. 188);
vascular system, 2 80 ( F'igs.
128-130), 319.

Osmundaceae, parenchyma,
413; sieve-tubes, 180;
vascular system, 279, 318,
321.

I N DE X.

647

Ostrya, 469, 498 : periderm,
548.

Ostrya virginica, 502 : crys-
tals, 142 ; gelatinous layer,
482.

Osyris, haustorium, 383.

Oxalis, secretory reservoirs,
202, 204, 210.

Oxalis carnosa, hair struc-
tures, 65.

Oxalis fruticosa, 408.

Oxybaphus, 590, 591.

Paeonia, stomata, 38 ; stone-
elements, 127.

Paeonia Moutan, 479.

Paleae, 54.

Palisade-cells, 407.

Palmeae, 10, 70, 102, 134,
169: endodermis, 124;
hairs, 63, 64 ; parenchyma,
411, 412; sclerenchyma,
128, 418, 420, 425 ; se-
cretions, 137, 139; thy-
loses, 171 ; vascular sys-
tem, 233, 261, 262, 263,
265, 298, 301, 311, 316,
322, 323, 357, 387; ves-
sels, 169 ; wax, 82, 85, 86,
87.

Panax crassifolium, secre-
tory reservoirs, 450.

Panax Lessonii, 450.

Pancratium, 139.

Pandaneae, 10 : air-spaces,
216; cork, 108; scleren-
chyma, 420 ; vascular sys-
tem, 233, 264, 265, 268,
320, 361, 386, 387.

Pandanus, air-spaces, 213,
217; parenchyma, 409,
41 1 ; vascular system, 268,
302, 320, 329, 361, 362.

Pandanus graminil’olius, 362.

Pandanus javanicus, 268.

Pandanus odoratissimus,
362.

Pandanus pygmaeus, 268,
302, 362.

Panicum turgldum, crystals,
141; sclerenchyma, 418,
419; wax, 83, 86.

Papaver, 53: cortex, 404;
hair structures, 64, 65 ;
latex, 185 ; laticiterous
tubes, 187, 432 ; medul-
lary bundles, 248; root,
525; sclerenchyma, 420;
vascular system, 248, 305,
376.

Papaver orientale, vascular
system, 236, 249, 377;

water- pore, 53.

Papaver Rhoeas, 525.

Papaver somniferum, 249 :
latex, 185 ; water-pore,
53-

Papaveraceae, laticiferous
tubes, 187, 189, 199, 433,
525 ; water-pore, 51.

Papaya vulgaris, 434.

Papayaceae, 485 : laticife-
rous tubes, 187, 188, 189,
434. 487-

Papilionaceae, 12, 23: peri-
derm, 552 ; root develop-
ment, 398 ; stomata, 41 ;
vascular system, 354.

Papulae, 54.

Papyrus, air-spaces, 213,214,
215,217,218; parenchyma,
409 ; vascular system, 339.

Paragon ia, 573.

Paratropia macrophylIa,45o.

Parenchyma, 27,1 i5;fibrous,
1 16; of the cortex, 536;
of the root, 413; primary,
arrangement of, 402.

Parietaria, cystoliths, 105.

Parinarium senegalense, si-
lica in leaves, 102.

Paspalum sp., 359.

Passerina, parenchyma, 410;
sclerenchyma, 418 ; sto-
mata, 49 ; vascular system,
303.

Passerina ericoides, 33, 49,
410.

Passerina filiformis, 49, 410.

Passerina hirsuta, 49, 410.

Passiflora, glands, 91, 96;
vascular system, 297, 308.

Passiflora atrocaerulea,
glands, 96.

Passiflora caerulea, end of
vascular bundle, 377.

Passiflora suberosa, fibres,
133-

Passiflora Vespertilio, vas-
cular system, 239, 308.

Pastinaca, root, 518; sap-
passages, 449.

Pastinaca sativa, 353.

Paullinia, anomalous wood,
581, 582.

Paulownia, 496 : annual
rings, 476.

Pecopteris, 301.

Pedicularis, 415.

Peixotoa, anomalous wood,
577-

Pelargonium, glands, 93.

Pelargonium roseum, 134,
505-

Pelargonium zonale, 89 :
hairs, 61, 63.

Pennisetum longistylum,
419.

Peperomia, 454 : epidermis,
33 ; medullary bundles,

249, 250; stomata, 45.
Peperomia argyracea, 33.
Peperomia arifolia, 33.
Peperomia blanda, 33.
Peperomia brachyphylla,

250.

Peperomia galioides, 33,
250.

Peperomia incana, 33, 249,
250.

Peperomia magnoliifolia, 33.
Peperomia obtusifolia, 33,

249.

Peperomia pellucida, 33.
Peperomia pereskiaefolia, 33.
Peperomia polystachya, 33.
Peperomia rubella, 33, 250.
Peperomia variegata, 249,

250.

Percskia aculeata, stomata,
41.

Periblem, 7.

Pericambial layer, 389.
Periderm, 544.

Periploca, 486, 496: latici-
ferous tubes, 432 ; lenti-
cels, 560 ; tracheides and
fibres, 483.

Pernettya, pith, 403.

Persica vulgaris, lenticels,

564-

Persoonia myrtilloides, 35.
Petasites niveus, 446, 447.
Petastoma, 574.

Petioles, structure, 405.
Petraea arborea, 510.

Petraea volubilis, 510.
Petroselinum sativum, vas-
cular system, 353.

Petunia nyctaginiflora, 141.
Peucedanum, vascular sys-
tem, 253.

Peucedanum Oreoselinum,
. 253, 254.

Phalaenopsis grandiflora,
aerial roots, 229.
Phanerogamae, 22 : axile
bundle in, 277; root de-
velopment, 397 ; with an
axile bundle, 311.
Pharbitis hispida, anoma-
lous structure, 607; secre-
tory sacs, 15 1 ; stomata,
4'-

Phaseolus, 12: connections
of vascular bundles, 386 ;
secondary thickening,47 4 ;
vascular system, 236, 241,
352, 354) 386, 392.
Phaseolus multiflorus, 241 :
parenchyma, 117 (Fig.
46); tannin-sacs, 153.

648

INDEX.

Phaseolus vulgaris, 241.

Phegopteris,vascularsystem,
284, 285.

Phellandrium aquaticum,
vascular system, 242.

Phelloderm, 552.

Phellogenetic meristem,
109.

Philadelphus, 560 : cork,
HO, 114 ; periderm, 552 ;
vascular system, 244.

Philadelphus coronarius,
244, 480, 502, 546.

Philesia, vascular system,
304.

Philesia buxifolia, stomata,
45-

Philodendron, aerial roots,
230; cork, 108; crystals,
1 40 ; laticiferous passages,
436, 444 ; sclerenchyma,
421 ; secretory reservoirs,
202; vascular bundles, 360
(Fig. 168) ; vascular sys-
tem, 268, 3 15, 361.

Philodendron cannaefolium,
411, 445.

Philodendron crinipes, 445.

Philodendron eximium, 445.

Philodendronhastatum, 268,
445-

Philodendron Imbe, 419,
445; sieve-tubes, 173, 176.

Philodendron lacerum, 445.

Philodendron Melinoni, 444.

Philodendron micans, 268,
361, 445.

Philodendron pedatum, 230.

Philodendron pinnatifidum,
445-

Philodendron Rudgeanum,
268, 419, 445.

Philodendron Sellowianum,
crystals, 140, 445.

Philodendron tripartitum,
268, 445.

Philydrum, raphides, 220.

Phlebodium, 304.

Phleum Boehmeri, stomata,

50-

Phloem, 317, 324.

Phlox, vascular system, 243.

Phoenix, sclerenchyma, 420 ;
vascular system, 266, 357.

Pholidophyllum, 31, 64, 71,
41 1: stomata, 37 (Fig. 12),
40.

Pholidota, sclerenchyma,
128.

Phoradendron, haustorium,
384.

Phormium, 102 : sclerenchy-
ma, 418, 422; stomata, 46.

Phormium tenax, cuticle.

76; fibre, 1 31; parenchy-
ma, 409 ; vascular system,

323-

Phragmites, air-spaces, 215 ;
endodermis, 124.

Phragmites communis, sto-
mata, 50, 124.

Phryganocydia, 574.

Phrynium, vascular system,
212, 267, 302.

Phrynium setosum, 302.

Phrynium violaceum, 267.

Phyllocladus, 118, 300.

Phylloglossum, root-struc-
ture, 365 ; vascular sys-
tem, 335, 365.

Physosiphon, hairs, 55 ; la-
mina, 407, 41 1.

Physosiphon Loddigesii,
411.

Physostegia virginiana, sto-
mata, 41 ; water-pore, 52.

Phyteuma Halleri, latici-
ferous tubes, 434.

Phyteuma spicata, 434.

Phytocrene, anomalous

wood, 575 (Figs. 227,
228), 589 ; sieve-tubes,
176 ; vessels, 169.

Phytolacca, crystals, 143 ;
medullary bundles, 248,
249-

Phytolacca dioica, anoma-
lous structure, 248, 589,
594, 600.

Phytolacceae, anomalous

structure, 590.

Picea, 493.

Picridium tingitanum, 433.

Piddingtonia, 434.

Pilea, cystoliths, 105.

Pilea decora, 105.

Pilea densiflora, 105.

Pili, 54.

Pilostyles, thallus, 384.

Pilularia, intercellular hairs,
220; sclerenchyma, 429;
vascular system, 211, 279,
283, 284, 301, 344.

Pilularia globulifera, 220,
283.

Pilularia minuta, 283.

Pinguicula, digestive glands,
TOO ; hair-structures, 64.

Pinus, 13, 67, 77 ; bordered
pits, 163 ; cotyledons, 245 ;
cuticle, 76 ; medullary
rays, 490, 491; periderm,
553) 559; root develop-
ment, 398 ; ruptured cells,
67; scale-bark, 559; se-
cretory reservoirs, 202,
544 ; sieve-tubes 522 ;
thickening, 472, 476, 477,

480 ; vascular system, 308,
356) 378, 380.

Pinus austriaca, 67.

Pinus Cembra, 472.

Pinus halepensis, 356.

Pinus Laricio, 202, 378, 380,
491.

Pinus Larix, cork, no.

Pinus nigricans, 495, 522,
559-

Pinus Picea, 443.

Pinus Pinaster, 67 ; section
of wood, 78 (Fig. 27), sto-
mata, 35 (Fig. n) ; vascu-
lar bundle, 381 (Fig. 185).

Pinus Pinea, 245, 356.

Pinus Pumilio, 67.

Pinus Strobus, 202, 510, 522,
544) 553: growth of wood,
477 ; stomata, 49.

Pinus sylvestris, 70, no, 202,
356, 461, 522, 553, 559:
annual rings,476,477, 505;
bordered pits, 164; cam-
bial zone, 462 (Fig. 195);
medullary rays, 491; spiral
vessels, 158, 159 (Fig. 58);
stomata, 49 ; thickening,
471 ; tracheides, 165 ; vas-
cular system, 247 (Fig.
no), 380.

Piper, anomalous growth,
567 ; medullary bundles,

249-

Piper geniculatum, 249.

Piper nigrum, hairs, 65.

Piper rugosum, cortex, 404.

Piperaceae, 456, 536: epi-
dermis, 33 ; hair struc-
tures, 65 ; medullary
bundles, 535; root-struc-
ture, 516 ; sclerenchyma,
420; secretory reservoirs,
136, 145; tracheae, 156;
vascular system, 248, 249,
25O) 391.

Piptatherum, 419.

Pirus, 113,470,539: cortex,
404 ; fibres, 528; pith, 403;
sieve-tubes, 523.

Pirus communis, 71, 471,
480, 536: crystals, 530;
ligneous bundle, 495 ;
sieve-tubes, 176; stomata,
49-

Pirus Malus, 555: lenticels,
562, 563, 565 ; pith, 403.

Pirus prunifolia, 493.

Pirus torminalis, 167.

Pisonia, anomalous growth,
59°) 593 ) vascular system,
308.

Pisonia hirtella, 598.

Pistacia Lentiscus, crystals.

INDEX.

530 ; periderm, 554; resin
passages, 452.

Pistacia vera, resin passages,
452.

Pistia, air-spaces, 210, 214,
219; crystals, 141, 220;
root-cap, 413; root de-
velopment, 397.

Pistia Stratiotes, calyptro-
gen layer, 9 (P'ig. 2), 165.

Pistia texensis, 210.

Pisum sativum, apex of root,
12 (Fig. 5) ; vascular sys-
tem, 354.

Pith, 236, 403.

Pithecoctenium, 574.

Pittosporeae, secretory re-
servoirs, 202 ; vascular
system, 387.

Pittosporum, bast-fibres,
526; resin passages, 204,
451.

Pittosporum T obira, 33,204,
479 : secretory passages,
526.

Pittosporum undulatum, 33,
526.

Piagiogyria biserrata, vascu-
lar system, 286.

Planera, 305; cork, 551.

Plantagineae, 458.

Plantago, sclerenchyma,
420 ; sieve-tubes, 231 ;
vascular system, 236.

Platanus, bast-fibres, 527;
crystals, 140, 530; fibres,
134; hairs, 62; ligneous
bundle, 496 ; periderm,
548, 556; sclerosis, 540;
thyloses, 171 ; vascular
system, 242, 296.

Platanus acerifolia, medul-
lary rays, 489.

Platanus occidentalis, 242,
485 : cork, iii; pith, 403;
water-pore, 52.

Platanus orientalis, crystals,
140.

Platycerium, sclerenchyma,
426 ; vascular system,
305.

Platycerium alcicorne, 15;
vascular system, 288, 344,
428.

Platycodon grandiflorus,
laticiferous tubes, 525.

Plectranthus, hairs, 61, 62.

Plectranthus amboinensis,
periderm, 548.

Plectranthus fruticosus,
glands, 93, 94; hairs, 59
(Fig. 21).

Plenotoma, anomalous
wood, 570 (Fig. 225), 574.

Pleroma macrantha, hair
structure, 65.

Pleurothallideae, 116, 117:
parenchyma, 41 1; pores,
54-

Pleurothallis, hairs, 54 ; leaf
structure, 41 1.

Pleurothallis ruscifolia, 41 1.

Plumbagineae, lime scales,
106 ; vascular system, 249.

Plumbago, 249 ; lime scales,
106.

Plumiera, laticiferous tubes,
187, 439.

Plumiera alba, 187.

Poa bulbosa, stomata, 50.

Poa compressa, stomata, 50.

Poa nemoralis, stomata, 50.

Podocarpus, 14, 1 18 : paren-
chyma, 408, 410; resin
canals, 442 ; vascular sys-
tem, 246, 356, 380.

Podocarpus Meyeriana, 380,
381.

PodocarpusThunbergii, 380.

Podophyllum,sclerenchyma,
420; vascular system, 248,
249.

Podostemaceae, vascular sys-
tem, 370.

Pogostemon, glands, 93 ;
hairs, 62.

Pogostemon Patchouli, glan-
dular hairs, 95 (Fig. 38).

Polybotrya cervina, vascu-
lar system, 295, 427.

Polybotrya Meyeriana, scle-
renchyma, 427,428; vas-
cular system, 295, 314.

Polygala Senega, anomalous
thickening, 570.

Polygonum, secretion, 98 ;
stomata, 49.

Polygonum aviculare, sto-
mata, 48.

Polygonum Fagopyrum,
apex of root, ii (Fig,
4).

Polypodiaceae, 118: apical
cell, 15-18; parenchyma,
413; sieve-tubes, 180;
tannin-sacs, 153; vascu-
lar system, 285, 342.

Polypodium, 118: lime in-
crustations, 106 ; scleren-
chyma, 426, 427, 428; vas-
cular system, 288, 307,
344, 345, 364-

Polypodium altescandens,
288.

Polypodium areolatum, 106.

Polypodium aureum, 1 5,106,
288,

Polypodium aurisetum, 288.

649

Polypodium
427.

Polypodium
288.

Polypodium
284.

Polypodium
106.

Polypodium fraxinifolium,
vascular system, 287 (Fig.
136), 344.

Polypodium irioides, 364,

Brownianum,

cayennense,

conjugatum,

crassifolium,

427-

Polypodium Lingua, 15, 344,
411, 426, 427, 428 : hairs,
59 (Fig. 21) ; stomata, 37,

42, 43.

Polypodium meniscifolium,
106.

Polypodium morbillosum,
106.

Polypodium Phyllitidis, 428.
Polypodium Phymatodes,
15, 342.

Polypodium piloselloides,
288.

Polypodium punctulatum.

15-

Polypodium pustulatum,
426, 428.

Polypodium repens, 106.
Polypodium rupestre, 15.
Polypodium solidum, 427.
Polypodium sporadocarpum,
106, 288, 427.

Polypodium squamulosum.

314-

Polypodium subauricula-
tum, 106.

Polypodium tenellum, 288.

Polypodium, vulgare, 15,
288, 426: concentric bun-
dles, 343 (Fig. 160);
sieve-tubes, 182.

Polypodium Wallichii, 284.

Pomaceae, 497 : crystals,
142; periderm, 548;
phelloderm, 549.

Pontederia, air-spaces, 213,
217, 218 ; crystals and
raphides, 220.

Pontederia cordata, 220 :
vascular system, 265.

Pontederia crassipes, 220:
air-spaces, 215.

Pontederieae, 10.

Populus, crystals, 142 ;
fibres, 528 ; ligneous bun-
dle, 495, 496 ; medullary
rays, 492 ; periderm, 548,
558; sieve-tubes, 176;
stomata, 46 ; tracheides,
and fibres, 483.

Populus fastigiata, cork, 1 1 1.

650

INDEX.

Populus italica, crystals, 140.

Populus monilifera, 472,
492.

Populus nigra, 558.

Populus pyramidalis, 471,
472. 479> 498, 528, 558.

Populus tremula, 492, 498,
558.

Pore, 34.

Pores, air, 45 ; in Pleuro-
thallideae, 54 ; on prickles
of Victoria, 54 ; water,
50.

Porlieria, 485 : bast-fibres,
526; ligneous bundle, 495.

Porlieria hygrometrica, 479:
crystals, 530.

Portulaca oleracea, 236.

Posidonia Gaulini, 217.

Potamese, 10, 141 ; hairs,
56.

Potamogeton, 67, 407 : air-
spaces, 213, 215, 217;
endodermis, 121, 123,

124; lime scales, 107;
sclerenchyma, 420, 425 ;
sieve-tubes, 232 ; stomata,
46 ; vascular system, 270,
302, 304, 312, 367, 368,

370, 391, 392.

Potamogeton crispus, 123,
368, 392, 425 : vascular
system, 272, 274 (Figs.
124, 125).

Potamogeton densus, 123,
272, 368, 425.

Potamogeton gramineus,

123, 271, 274, 425.

Potamogeton lucens, epi-
dermis, 80, 124, 232, 271,
274, 312, 352, 368, 370.

Potamogeton natans, 107,

124, 125, 232 : sieve-

tubes, 173; vascular bun-
dles, 368 (Fig. 170) ; 312,
367; vascular system, 270,
271, 272 (Figs. 121, 122).

Potamogeton pectinatus,
124, 232 : vascular bun-
dles, 271, 277, 312, 369
(Fig. 171); vascular sys-
tem, 273 (Fig. 123).

Potamogeton perfoliatus,
271, 367, 425.

Potamogeton praelongus,
124.

Potamogeton pusillus, 272,

277, 312, 368.

Potentilla aurea, stomata,
48.

Potentilla fruticosa, ligneous
bundle, 495.

Potentilla Thuringiana,
water-pore, 52.

Pothos, vascular system,
304.

Pothos argyrea, crystals,
140 ; stomata, 40.

Pothos crassinervia, crys-
tals, 140; stomata, 40.

Pothos Rumphii, internal
hairs, 222.

Pouroumaguyanensis, hairs,
65-

Prickles, 55, 66.

Primula, dusty hairs, 99,
100 ; secretion, 98 ; sto-
mata, 37; vascular sys-
tem, 250, 251, 252;

water-pores, 51.

Primula acaulis, water-pore,
51-

Primula Auricula, 25, 99:
endodermis 124 (Fig. 51);
vascular system, 250, 251,
254 ; water- pore, 51.

Primula calycina, 252.

Primula elatior, 252.

Primula farinosa, 99, 252.

Primula marginata, 99, 252:
water-pore, 51.

Primula sinensis, 99, 122,
252, 305: end of vascular
bundle, 375 (Figs. 177,
178) ; glandular-hairs, 89
(Figs. 31-34), 93 ; water-
pore, 51.

Primula spectabilis, 252.

Primulaceae, 12, 25: endo-
dermis, 122, 415.

Prinos, pith, 403.

Procambium, 389.

Prosenchyma, 27.

Proteaceae, 71, 77, 497:
fibres, 130; parenchyma,
408; sclerenchyma, 424;
stomata, 35, 37, 39, 40.

Protective sheath, 121.

Protogenetic, 200.

Protophloem, 324, 345.

Protoplasm, 66.

Protoxylem, 321.

Prumnopitys elegans, epi-
dermis, 78.

Prunus, 539: cork, no;
crystals, 530; glands, 90,
96 ; medullary rays, 493 ;
periderm, 548; secondary
changes, 479 ; tracheides
and fibres, 483 ; vascular
system, 297, 377.

Prunus avium, 493, 530,
543 : lenticels, 562, 563 ;
vascular system, 239.

Prunus Cerasus, 1 10 : lenti-
cels, 564.

Prunus domestica, 479, 509 :
lenticels, 566; stomata, 48.

Prunus Laurocerasus, 377,
479: gelatinous layer,
482 ; glandular hairs, 90,

96 ; secretion, 98.

Prunus Mahaleb, stomata,

48.

Prunus Padus, crystals, 142,
530 ; mucilage, 74; water-
pores, 51.

Prunus spinosa, 493, 509.
Pseudolarix Kaempferi, 443.
Pseudotsuga, medullary
rays, 490.

Psilotum, 22: meristematic
apex, 22 ; sclerenchyma,
429; stomata, 35; vas-
cular system, 280.
Psilotum triquetrum, epi-
dermis, 77 ; vascular sys-
tem, 279, 280.

Psoralea, glands, 91, 92,

93-

Psoralea bituminosa, glands,
92 ; vascular bundles, 392
(Figs. 172, 173) ; vascular
system, 303 (Fig. 146).
Psoralea hirta, glands, 92,

97 (Fig. 42), 135.
Psoralea pinnata, 92.
Psoralea stricta, 92.
Psoralea verrucosa, 92.
Ptelea trifoliata, crystals,

142.

Pteris, 1 1 8, 405; vascular
system, 284, 289.

Pteris aquilina, 15, 1 61, 167,
168: bordered pits, 162
(Fig. 61) ; sclerenchyma,
132 (Fig.54),427 ; sclero-
tic cells, 121 (Fig. 48) ;
sieve-tubes, 181 (Fig. 79J;
vascular bundle, 344 (Fig.
161); vascular system,
295 (Fig. 143), 314, 342,
343; vessels, 165.

Pteris aurata, dusty hairs,
99-

Pteris aurita, 284.

Pteris cretica, stomata,
41.

Pteris elata, v. Karsteniana,
289.

Pteris flabellata, stomata, 4 1
(Fig. 14).

Pteris gigantea, 289.

Pteris hastata, root apex,
18 (Fig. 8).

Pteris orizabse, 289.

Pteris pinnata, 427.

Pteris podophylla, 289.
Pteris Vespertilio, 284.
Pterocarpus santalinus, 487 :
crystals, 14 1 ; sap-wood,
508.

INDEX.

651

Pterocarya, 498, 537.

Pterocarya caucasica, fibres,
527 ; medullary rays, 493.

Punica, 497, 536 : bast-
fibres, 527; crystals in
bast, 530, 531 (Fig. 21s) ;
ligneous bundle, 495 ;
periderm, 552, 559.

Punica Granatum, 484 ;
crystals, 142 ; medullary
rays, 489 ; secondary
thickening, 471, 479.

Pupalia Schimperiana, 591,
593-

Pyrethrum, oil-passages,
446, 458.

Pyrethrum inodorum, sto-
mata, 37.

Pyrethrum Parthenium,
446, 447.

Pyrethrum roseum, hairs,
61.

Pyrostegia, 574.

Pyrus, see Pirus.

Quercus, 459, 483 : annual
rings, 476; cork, no,
III, 1 1 2, 557; crystals,
140, 530; fibres, 528;
hairs, 62 ; lenticels, 562 ;
ligneous bundle, 495 ; me-
dullary rays, 489 ; mu-
cilage, ■ 74 ; periderm,

548,558 ; pith, 403 ; pits,

481; pitted vessels, 163;
sclerosis, 540 ; thyloses,
171 ; tissues, 507 ; tra-
cheides and fibres, 483 ;
vascular system, 298, 353,
391.

Quercus Cerris, 489.

Quercus glabra, 4 1 1 .

Quercus occidentalis, 550.

Quercus pedunculata, 479,
496, 497, 505, 540, 548 :
annual rings, 506 ; crys-
tals, 142, 530 ; fibres, 133;
length of elements of
tissue, 507 ; ligneous bun-
dle, 495 ; medullary rays,
489 ; mucilage, 74 ; tra-
cheides and fibres, 483;
vascular system, 303, 304,
308 ; wood, 5 1 1.

Quercus Pseudosuber, 550:
cork, 557.

Quercus Robur, pith, 403 ;
thyloses, 17 1.

Quercus Suber, 540, 548,
550: cork, no, in, 112,
113, 114, 557; lenticels,
562, 563.

Quiina, secretions, 144.

Quillaja, crystals, 142, 529,

530; fibres, 528; secre-
tions, 138.

Quillaja saponaria, crystals,
142; sieve-tubes, 173,175.

Radula, 71.

Rafflesiaceae, point of at-
tachment, 384.

Ranunculaceae, 25, 310, 454 :
secretions, 137; vascular
system, 324.

Ranunculus, 25, 56, 407:
sheath, 415; stomata, 49 ;
vascular system, 306, 323,
327, 366.

Ranunculus aquatilis, 25, 31,
53, 67 : stomata, 46, 49.

Ranunculus divaricatus, 31,
53 : stomata, 49.

Ranunculus Ficaria, 67.

Ranunculus fluitans, 366,
415 ; root-structure, 123
(Figs. 49, 50) ; section of
vascular bundle, 332 (Fig.
153); vascular bundle,
353 (Fig- 163).

Ranunculus lanuginosus,
water-pore, 52.

Ranunculus repens, 366,
422, 504 : section of vas-
cular bundle, 331 (Fig.
152); vascular bundle,
356 (Fig. 165).

Ranunculus sceleratus, sto-
mata, 49, 50.

Raphanus sativus, 12: root,
516; vascular system, 353.

Raphides, 137.

Ravenala, vascular system,
267.

Ravenala madagascariensis,
437-

Reaumuria, 107.

Red Fir, growth of wood,
477-

Renanthera coccinea, aerial
roots, 228, 229 ; scleren-
chyma, 418.

Renanthera matutina, 229.

Reservoirs of gum, 202.

Reservoirs of mucilage, 202.

Reservoirs of resin, 202.

Reservoirs, secretory ar-
rangement of, 431.

Resin of Pinus, &c., origin
of, 510.

Resin-passages, 441.

Resin reservoirs, 202, 441.

Resins, 145.

Restiaceae, sclerenchyma,
419 ; stomata, 35.

Restio diffusus, cellulose
covering, 88; sclerenchy-
ma, 425; stomata, 75.

Restio incurvatus, 425.

Restio paniculatus, 425,

Restio tectorum, 425.

Reticulate vessels, 156.

Rhamnus, ligneous bundle,
495 ; pith, 403 ; tracheides
and fibres, 483.

Rhamnus cathartica, 495,
509 : annual rings, 506 ;
stomata, 48.

Rhamnus Frangula, 479,
498, 5°7, 536: crystals,
140, 530; fibres, 528;
lenticels, 563 ; phello-
derm, 459 ; stomata, 48.

Rhaphidophora, 268.

Rhaphidophora angustifolia,
secretory reservoirs, 445 ;
stone elements, 127 ; vas-
cular system, 361.

Rhapis flabelliformis, vascu-
lar system, 266, 302, 329,
373-

Rhaponticum, 150.

Rheedia, resin passages, 451.

Rheedia lateriflora, secre-
tory reservoirs, 145.

Rheum, anomalous wood,
585; collenchyma, 119;
glandular hairs, 90; se-
cretory sacs, 147.

Rheum Emodi, 585.

Rheum officinale, 585.

Rheum Rhaponticum, iii :
root, 517, 524.

Rheum undulatum, 524.

Rhexigenetic, 200.

Rhinanthaceae, haustorium,
383, 458.

Rhinanthus, stomata, 37.

Rhipsalideae, vascular sys-
tem, 256, 297, 310, 456.

Rhipsalis, cortical bundles,
261.

Rhipsalis carnosa, 261.

Rhipsalis Saglionis, 261.

Rhipsalis salicornioides, 261.

Rhizocarpeae, vascular sys-
tem, 313.

Rhizophora, fibres, 130;
H-shaped intercellular
hairs, 220, 223; scleren-
chyma, 423.

Rhododendron, 102 : glands,
91. 93, 96, 97 ; pith, 403 ;
stomata, ^9.

Rhododendron caucasicum,
glands, 91.

Rhododendron ferrugi-
neum, glandular scales,
91, 97 (Fig. 41); hair
structures, 64.

Rhododendron hirsutum,
glands, 91.

652

INDEX.

Rhododendron maximum,
medullary rays, 489.

Rhus, resin passages, 203,
452 ; tracheidesand fibres,

483.

Rhus aromatica, 452.

Rhus Coriaria, 452.

Rhus Cotinus, 452, 484, 493.

Rhus elegans, 452.

Rhus glauca, 452.

Rhus semialata, 452.

Rhus suaveolens, 452.

Rhus Toxicodendron, 452,

484.

Rhus typhina, crystals, 142,
452, 470.

Rhus villosa, 452.

Rhus viminalis, 452.

Rhus virens, 452.

Rhynchosia phaseoloides,
anomalous wood, 589.

Rhynchospora, 32.

Rhynchospora alba, 419.

Ribes, bast-fibres, 526 ;
crystals, 530 ; glands, 94 ;
hairs, 65 ; ligneous bun-
dle, 495; periderm, 559;
phelloderm, 552; prickles,
66; tracheides and fibres,
483.

Ribes nigrum, cortex, 552
(Fig. 220) ; crystals, 142 ;
glandular hairs, 90, 95 ;
hair structures, 64.

Ribes rubrum, 479: vascu-
lar system, 239.

Ribes triste, water-pore, 51.

Richardia, laticiferoustubes,
436; vascular system, 304,

315, 327-

Richardia aethiopica, vas-
cular system, 268, 269 ;
water-pores, 5 1 .

Richardia africana, 346.

Ricinus, glands, 96 ; struc-
ture of stem, 455 (Fig.
194) ; vascular bundle,
333 (Figs. 154, 155)-

Ricinus communis, crystals,
140 ; vascular system, 236,
353 ; wax, 85.

Rings, annual, 501.

Rivina aurantiaca, 590.

Rivina brasiliensis, 590.

Robinia, 497 : annual rings,
476; lenticels, 562; lig-
neous bundle, 495 ; scle-
renchyma, 426 ; thyloses,
171.

Robinia Pseudacacia, 484,
496 : crystals, 142 ; fibres,
133; medullary rays, 489 ;
periderm, 548, 558; tan-
nin-sacs, 153 ; wood, 51 1.

Robinia viscosa, glands, 90.

Rochea coccinea, vascular
bundles, 379 (Figs. 181,
182) ; water-pore, 53
(Fig. 20).

Rochea falcata, hair struc-
tures, 65.

Roemeria, 187 : laticiferous
tubes, 435.

Root-cap, 9.

Root-hairs, 55.

Root-hairs, absence of, 55.

Roots, vascular system of,

315-

Rosa, cortex, 404 ; glands,
90 ; ligneous bundle, 495 ;
pith, 403 ; prickles, 66.

Rosa canina, epidermis, 77 ;
lenticels, 565 ; ligneous
bundle, 495.

Rosa damascena, 48.

Rosmarinus, vascular sys-
tem, 322.

Rosmarinus officinalis, hairs,
62 ; parenchyma, 41 1 ;
sclerenchyma, 418, 497.

Rubia, 17 1 : root, 517 ; se-
cretory sacs, 147; vascu-
lar system, 243, 296, 308-

Rubia tinctorum, water-
pore, 53.

Rubiaceae, 457 ; vascular
system, 296.

Rubus, 66, 70 : pith, 403 ;
vascular system, 309.

Rubus caesius, prickles, 66,

Rubus Hofmeister!, prickles,

66.

Rubus Idaeus, 484 : peri-
derm, 552 ; prickles, 66.

Rubus odoratus, 560.

Rudbeckia speciosa, water-
pore, 52,

Ruellia tormosa, cystoliths,
105.

Ruellia livida, 105.

Ruellia maculata, vascular
system, 243.

Rumex, collenchyma, 119,
120; glandular hairs, 90,
91.

Rumex alismifolius, 458.

Rumex Lunaria, 458, 499.

Rumex obtusifolius, 71.

Rumex patientia, 71.

Ruppia, vascular system,
277.

Ruscus, vascular system,
301.

Ruscus aculeatus, 77, 102.

Russelia juncea, vascular
system, 243, 308.

Ruta graveolens, oil- cavities,
208 ; stomata, 39.

Rutaceae, oil-cavities, 202,
207.

Saccharum, vascular system,
311 ; wax, 87.

Saccharum officinarum, scle-
renchyma, 422 ; wax rods
on stem, 83, 84 (Fig. 28).

Saccolabium Blumei, 229.

Saccoloma, 289.

Saccoloma adiantoides, vas-
cular system, 289, 290
(Fig. 137).

Sacs, 5, 135: containing
mucilage, 143 ; contain-
ing crystals, 137, 529;
containing gum-resins,
145; contents of, 152;
membrane of, 152 ; tan-
nin, 153.

Sagittaria, 10 : air-spaces,
217,218,219; intercellu-
lar spaces, 211 (Fig. 87);
root-development, 397 ;
stomata, 49, 50.

Sagittaria indica, 218.

Sagittaria lancifolia, 218.

Sagittaria sagittifolia, 165,
218: stomata, 49, 50.

Salicornia, 31 : parenchyma,
409 ; stomata, 45, 48 ;
tracheides, 226 ; vascular
system, 256, 297, 304.

Salicornia Emerici, 226.

Salicornia herbacea, 226,
409, 591.

Salicornia patula, 226.

Salicornia sarmentosa, 226.

Salisburia, sieve-tubes, 179.

Salix, 12, 536; cork, 1 10, 1 1 1,
112, 550; cortex, 404;
crystals, 530 ; fibres, 527 ;
glands, 90, 96 ; lenticels,
564 ; medullary rays, 493 ;
mucilage, 74 ; periderm,
548 ; secondary thicken-
ing, 479; tracheides and
fibres, 483.

Salix acutifolia, 479, 498.

Salix alba, mucilage, 74,
1 12, 472.

Salix amygdalina, mucilage,
74, 555-

Salix aurita, crystals, 140,
493-

Salix bicolor, 493.

Salix Caprea, 493.

Salix cinerea, crystals, 142,
484.

Salix daphnoides, epidermis,

77.

Salix fragilis, 112, 537, 564.

Salix hippophaefolia, 479,
498.

INDEX.

653

Salix purpurea, 112.

Salix triandra, 493.

Salsola, 593.

Salsola Kali, 591.

Salvia, hairs, 61, 64, 68, 70.

Salvia glutinosa, Stomata, 48.

Salvinia, 15, 16, 17, 54 : air-
spaces, 213; Stomata, 34,
36, 38, 39; vascular sys-
tem, 283, 312.

Salvinia natans, 38.

Sambucus, 457, 547 : collen-
chyma, 119; cortex, 404;
fibres, 528 ; ligneous
bundle, 495; pits, 481;
secretory sacs, 148, 153 ;
stomata, 47 ; vascular sys-
tem, 297.

Sambucus Ebulus, 148 : vas-
cular system, 298 (Fig.
144).

Sambucus nigra, 148, 479,
484- 495, 498: cork, 113 ;
crystals, 142, 530; lenti-
cels, 561 ; medullary rays,
489; periderm, 548; pith,
403; secretions, 138, 146 ;
stomata, 38, 39; structure
of wood, 463 ; thyloses,
171 ; water-pore, 51 ;
wood, 514.

Sambucus racemosa, se-
condary growth, 479, 484,
495, 498.

Sanguinaria, laticiferous
tubes, 187, 435; secretory
sacs, 147.

Sansevieria guineensis, n8.

Sansevieria zeylanica, 77.

Santalaceje, haustoria, 383.

Santalum, 383.

Santalum album, stomata,
45-

Santolina Chamaecyparissus,
secretory reservoirs, 447.

Sap cavities of aloe, 148.

Sapindaceae, anomalous

wood, 569 ; mucilage, 74.

Sapotaceae, latex, 146; se-
cretory sacs, 146, 1 5 1.

Sarcanthus rostratus, 71,
229.

Sarcopodium Lobbii, root-
sheath, 227, 229.

Sarothamnus, gelatinous

layer, 482 ; periderm, 552 ;
tracheides and fibres, 483;
vascular system, 237.

Sarothamnus scoparius, an-
nual rings, 506.

Sarracenia, glands, 69, 100.

Sassafras, periderm, 555.

Satureja, glands, 95 ; vascu-
lar system, 243.

Satureja variegata, 308.

Saurureae, 213, 420, 454.

Saururus cernuus, spiral ves-
sels, 157 (Fig. 57); vascu-
lar system, 239.

Saxegothea, 14: secretory
reservoirs, 442 ; vascular
system, 246.

Saxifraga, 67 ; line incrusta-
tions, 106, 107 ; vascular
system, 376, 377 ; water-
pores, 51, 53-

Saxifraga Aizoon, 53, 376.

Saxifraga caesia, 53, 106.

Saxifraga cuscutaeformis,
water-pore, 51.

Saxifraga crustata, 107.

Saxifraga Cymbalaria, 32.

Saxifraga elatior, 376.

Saxifraga longifolia, 53.

Saxifraga oppositifolia, 53,
106.

Saxifraga orientalis, water-
pore, 51.

Saxifraga punctata, water-
pore, 51.

Saxifraga retusa, 53, 106.

Saxifraga rocheliana, 53.

Saxifraga sarmentosa, 32 :
stomata, 47.

Saxifrageae, excretion of
lime, 53.

Scabiosa, 297.

Scalariform vessels, 158.

Scales, 63.

Schinus molle, resin-passa-
ges, 452.

Schismatoglottis, 444.

Schizaea, vascular system,
283, 284, 345.

Schizaea pectinata, 345.

Schizacaceae, 284.

Schizogenetic spaces, 200,

213.

Sciadopitys, fibres, 130, 133;
medullary rays, 490 ; scle-
renchyma, 424 ; vascular
system, 300, 380.

Scilla, 139.

Scilla maritima, secretions,
138.

Scindapsus, vascular system,
268, 361.

Scindapsus pictus, 361.

Scirpus, 35, 211 : air-spaces,

214, 218 ; endodermis,
124; parenchyma, 409;
sclerenchyma, 422 ; sto-
mata, 40 ; vascular sys-
tem, 265.

Scirpus Holoschcenus, 409.

Scirpus lacustris, 212, 265,
409 : air-spaces, 214, 215,
216, 217.

Scirpus maritimus, 217.

Scirpus palustris, 265, 409,
422.

Scirpus sylvaticus, 217.

Scitamineae, endodermis,
122; parenchyma, 41 1;
raphides, 220; vascular
system, 267, 302; wax,
83, 85.

Sclerenchyma, 28, 126 : ar-
rangement of, 417,

Sclerenchymatous fibres,
128.

Sclerosis, 28 : secondary,
539- .

Sclerotic cells, arrangement
of, 417.

Scolopendrium, parenchy-
ma, 410, 413; vascular
system, 344.

Scolopendrium vulgare, 344,
427.

Scolymus, secretions, 137.

Scolymus grandiflorus, se-
cretory reservoir.s, 448.

Scopolia atropoides, crys-
tals, 143 ; vascular system,
238.

Scorzonera, laticiferous

tubes, 433; root, 524;
secondary thickening,

475; sieve-tubes, 231.

Scorzonera hispanica, milk-
tubes, 190 (Fig. 83), 434 ;
root, 517.

Scybalium, point of attach-
ment, 384.

Seaforthia elegans, vascular
system, 357.

Secale cereale, 54, 85 : la-
mina, 407 ; vascular sys-
tem, 359; wax, 85.

Secondary changes, 454.

Secondary changes outside
the zone of thickening,
533-

Secondary thickening in
Fern-like plants, 623.

Secondary thickening in
Monocotyledons, 618.

Secretory canals, 440.

Secretory passages, 440.

Secretory reservoirs, 135.

Secretory reservoirs, ar-
rangement of, 431.

Secretory reservoirs, inter-
cellular, 201.

Securidaca, 587.

Securidaca volubilis, 589,
601.

Sedum, 458, 498 : vascular
system, 362.

Sedum Fabaria, 608.

Sedum maximum, 498.

654

INDEX.

Sedum populifolium, 498.

Sedum purpurascens, sto-
mata, 41 (Fig. 15).

Sedum reflexum, 498.

Sedum spurium, 32.

Sedum Telephium, anoma-
lous thickening, 608 ; vas-
cular system, 233, 319.

Sedum ternatum, 499 ; se-
cretions, 138.

Segment, 15.

Segment-cell, 15.

Selaginella, 15, 213: apical
cells, 15, 21 ; epidermis,
77; root-structure, 364;
sclerenchyma, 430 ; sieve-
tubes, 182; vascular sys-
tem, 282 (Fig. 131), 301,
314» 315» 342> 343) 345)
348) 364.

Selaginella arborescens, 21.

Selaginella denticulata, sto-
mata, 39.

Selaginella Galeotii, 282.

Selaginella helvetica, 282.

Selaginella inaequalifolia,
283 : epidermis, 77.

Selaginella Kraussiana, 15,
282, 314, 364, 365.

Selaginella Lyallii, 21, 283.

Selaginella Martensii, 15,
16, 17) 77) 282, 314,

365.

Selaginella Pervillei, 21.

Selaginella pubescens, 282.

Selaginella rupestris, 282,
430.

Selaginella spinulosa, 283,
343, 43°-

Selaginella Wallichii, 21.

Sempervivum, calcium oxa-
late in, 102; vascular sys-
tem, 304.

Sempervivum arboreum,
458) 499.

Sempervivum calcareum,
secretion, 102 ; wax, 83.

Sempervivum glaucum, wax,
83.

Sempervivum tectorum,
wax, 83.

Senecio vulgaris, secretory
reservoirs, 446, 447 ; wa-
ter-pore, 52.

Senecioneae, 446.

Sequoia, 14, 118: sieve-

tubes, 179 ; vascular sys-
tem, 246.

Sequoia gigantea, sieve-
tubes, 179, 180 (Fig.

77).

Sequoia sempervirens, 1 1 8 :
parenchyma, 410.

Serjania, anomalous wood,

581 (Figs. 230, 232),

583.

Serratula, 150.

Serratula centauroides, 445.

Seseli, medullary rays, 491.

Sesuvium, 590.

Setae, 54.

Sheath, 6, 414.

Sheath of aerial roots of
orchids, 227.

Sheperdia canadensis, hair
structures, 63 ; tracheides,
481.

Sida, fibres, 129.

Sida retusa, fibres, 132.

Sideroxylon, 146.

Sideroxylon mastichoden-
dron, secretory sacs, 1 5 1 .

Sieve-fields, 172.

Sieve-plates, 172.

Sieve-pores, 174.

Sieve-tubes, 172 ; extrafas-
cicular, 231 ; of Angio-
sperms, 177 ; of Apocyna-
ceae, 231 ; of Asclepia-
daceae, 231; of Campanu-
laceae, 231 ; of Cichoria-
ceae, 231 ; of Convolvula-
ceae, 231 ; of Cryptogams,
180; of Equisetum, 180;
of Ferns, 180: of Gym-
nosperms, 179 ; of Lyco-
podium, 180 ; of Melasto-
maceae, 231 ; of Myrta-
ceae, 231; of Potamoge-
ton, 232 ; of Selaginella,
182 ; position of, 226.

Silaus, vascular system, 253,
320.

Silaus pratensis, 253, 254,
310.

Silaus tenuifolius, 254.

Silene, periderm, 552 ; vas-
cular system, 236, 243.

Silene catholica, 420.

Silene inflata, stomata, 39.

Silene italica, 419, 458.

Silene nemoralis, glandular
hairs, 91.

Sileneae, glandular hairs, 90,

95.

Silica in epidermis, 102.

Silicification in leaves, 102,
103.

Silphium conjunctum, col-
lenchyma, 119.

Silphium connatum, 103.

Silver Fir, growth of wood,
477-

Silybum marianum, secre-
tory sacs, 150, 446.

Simaruba officinalis, crys-
tals, 530, 537 ; fibres, 528.

Sinapis, hairs, 60.

Siphocampylus manettiae-
florus, laticiferous tubes,
432) 434.

Siphocampylus microstoma,
434-

Siphonia elastica, latex, 186.

Sison amomum, sap-pas-
sages, 449.

Skimmia, oil-cavities, 207.

Smilaceae, sclerenchyma,
419; vascular system, 301,
304.

Smilax, endodermis, 124;
prickles, 66 ; vascular sys-
tem, 304, 357.

Smyrnium, sap-passages,
449) 450-

Smyrnium Olusatrum, 449.

Smyrnium perfoliatum,

water-pore, 52.

Sobralia, aerial roots, 229.

Sobralia decora, 229, 230.

Solanaceae, collenchyma,

1 1 9 ; cortex, 404 ; crys-
tals, 1 41, 143 ; secretions,
138; stomata, 4 1 ; vascu-
lar system, 338.

Solanum, hairs, 55, 56, 64,
65, 70; prickles, 66.

Solanum argenteum, 64.

Solanum Dulcamara, 143,
338, 479, 505: lenticels,
560 ; periderm, 548; phel-
loderm, 549 ; sieve-tubes,
231.

Solanum marginatum, hairs,
62.

Solanum tuberosum, 338 :
hairs, 61, 143 ; sieve-tubes,
231; stomata, 48; thy lo-
ses, 17 1 ; vascular system,
387.

Solanum verbascifolium,
hairs, 62.

Soldanella, 415.

Soldanella Clusii, water-
pore, 51.

Solidago, oil-passages, 204,
446, 447 ; vascular sys-
tem, 308.

Solidago laevigata, 204.

Solidago limoniifolia, 446,
447-

Sollya heterophylla, 452.

Sonchus, laticiferous tubes,
43 3)525 ; sieve-tubes, 231.

Sonchus pinnatus, 525.

Sonchus tenerrimus, 433.

Sonerila margaritacea, cor-
tical bundles, 260.

Sophora, tracheides and
fibres, 483.

Sophora japonica, 479, 496,
502, 535: annual rings.

INDEX.

^55

506; cuticle, 75; gela-
tinous layer, 482; len-
ticels, 565 ; periderm, 558 ;
wax in epidermis, 75, 82.

Sorbus, medullary rays, 492 ;
periderm, 546 (Figs. 216-
218); pith, 403.

Sorbus Aria, 403 : crystals,
529.

Sorbus Aucuparia, 1 12, 403,
471, 480, 509.

Sorbus torminalis, 403, 492.

Sorghum, vascular system,
359; wax, 83.

Spaces, intercellular, 4, 210.

Sparganium, 10 : air-spaces,
213, 216, 217; parenchy-
ma, 409; raphides, 142;
sclerenchyma, 418; vas-
cular system, 265, 306, 328.

Sparganium ramosum, 71,
212, 328.

Sparmannia, bast, 528 (Fig.
214); fibres, 527; lig-
neous bundle, 495.

Sparmannia africana, 537 :
crystals, 530; wood, 514.

Spartium monospermum,
418.

Spartium scoparium, 479.

Spathiphyllum lancaefolium,
internal hairs, 222 ; vas-
cular system, 268.

Spergula, vascular system,
243.

Spergula arvensis, 236, 308.

Sphenopteris, 301.

Spilanthes fusca, 447.

Spinacia, 353.

Spiraea, ligneous bundle, 495 ;
phelloderm, 552; tra-
cheides and fibres, 483,
527.

Spiraea chamaedrifolia, 484.

Spiraea opulifolia, crystals,
142; ligneous bundle,495;
periderm, 552.

Spiraea salicifolia, 484, 485.

Spiraea ulmifolia, 536 ; fibres,
527; sieve-tubes, 522.

Spiral vessels, 156.

Spirodela, crystals, 142.

Spirodela polyrrhiza, 370,
371-

Spironema fragrans, vascu-
lar system, 270.

Splint, 507.

Spondias cytherea, resin-
passages, 452.

Spreckelia, 139.

Spruce Fir, growth of wood,
477-

Squamae, 54.

Stachys, hairs, 61, 70.

Stachys angustifolia, vascu-
lar system, 244 (Figs. 104,
105).

Stachys sylvatica, 354, 355.

Stangeria, 300, 301, 306 :
anomalous growth, 6 1 1 ;
secretory reservoirs, 441.

Stanhopea, root sheath, 227,
230; sclerenchyma, 128;
vascular system, 302, 359.

Stapelia, latex, 184, 439.

Staphylea, 470 : ligneousbun-
dle, 495.

Staphylea pinnata, 67 : lig-
neous bundle, 495 ; peri-
derm, 548 ; secondary
changes, 480 ; secretions,
138 ; stomata, 45.

Starch, absence of, 66.

Starch-layer, 415.

Starch-ring, 415.

Statice, lime scales, 107 ;
medullary bundles, 249 ;
parenchyma, 408, 410.

Statice alata, 107.

Statice latifolia, 107, 410.

Statice monopetala, 107,
408: fibres, 130; vascular
system, 305.

Statice purpurascens, 107.

Statice purpurea, 107, 408.

Statice scoparia, 107.

Stelis, 41 1 : hairs, 55.

Stellatae, 298.

Stem, hypocotyledonary,
236.

Stems, structure of herba-
ceous, 491.

Stenocarpus sinuatus, 41 1.

Sterculiaceae, mucilage, 143.

Stigmaphyllum, anomalous
wood, 577; glands, 96;
vascular system, 376.

Stigmaphyllum, ciliatum, 96,
602.

Stigmaphyllum cristatum,
96.

Stipa pennata, stomata, 49,
50.

Stizophyllum, 574.

Stoma, 34.

Stomata, 34: absence of, 46 ;
distribution of, 47 ; on
rhizomes, 47 ; in grasses,
50 ; in marsh plants, 50 ;
in water plants, 49 ; on
floating leaves, 49 ; num-
ber of, 48 ; water, 50 ;
cell-wall, 71.

Stone-cells, 127.

Stone-elements, 127.

Stratiotes, 10; vascular sys-
tem, 273.

Stratiotes aloides, 10, 165.

Strelitzia, air-spaces, 217;
tannin, 437; thyloses, 171 ;
vascular system, 267 ; wax
rods, 86, 87.

Strelitzia ovata, endoder-
mis, 124; parenchyma,
41 1 ; stomata, 39, 40 ; wax
rods on epidermis, 83, 85
(Fig. 29).

Strelitzia reginx, 267.

Structure in relation to phy-
sical conditions, 24.

Struthiopteris, 285, 313.

Strychnos, 2 5 : anomalous
wood, 569, 578, 579 (Fig.
229); sieve-tubes, 231;
vascular system, 319,338.

Strychnos brachiata, 578.

Strychnos colubrina, 578.

Strychnos innocua, 578.

Strychnos multiflora, 578.

Strychnos Nux vomica, 578.

Strychnos toxifera, anoma-
lous thickening, 578.

Succisa, 297.

Syagrus, vascular system,
266.

Sykesia, 580.

Syinphoricarpus vulgaris,
pith, 403.

Symphytum, mucilage, 143.

Symphytum officinale, root,
517-

Syngonium, laticiferous
tubes, 436; vascular sys-
tem, 268.

Syringa, crystals, 529;
glands, 64 ; hairs, 64 ; pe-
riderm, 548 ; tracheides
and fibres, 483.

Syringa Josikaea, 479, 502.

Syringa persica, lenticels,
564.

Syringa vulgaris, 484, 502 :
cortex, 404 ; medullary
rays, 489; pith, 403 ; pits,
481 ; secondary thicken-
ingj 471, 472, 479-

System, 6.

Tabernsmontana, 439.

Taccaceae, 10 : vascular sys-
tem, 301, 304.

Taeniopteris, 301.

Tagetes, oil-passages, 440,
446 ; vascular system, 244.

Tagetes erecta, vascular sys-
tem, 353.

Tagetes lucida, 244.

Tagetes patula, endodermis,
1 2 1, 415 ; secretory reser-
voirs, 201, 203, 446, 447.

Tagetes signata, 244.

Tamarix, pits, 163.

656

I N DE X.

Tamarix gallica, 470, 479,
484, 497: fibres, 134.

Tamarix indica, lenticels,
563.

Tamus communis, 323, 622 :
sclerenchyma, 419; se-
condarythickening, 618;
vascular system, 275.

Tamus elephantipes, cork,
1 14.

Tamus polycarpus, 622.

Tanacetum Meyerianum,
hairs, 61.

Tanacetum vulgare, oil-pas-
sages, 446, 447.

Tanaecium, 574.

Tannin sacs, 153.

Taraxacum, 475, 504: air-
spaces, 215; laticiferous
tubes, 433, 525; root,

517)525; sieve-tubes, 231.

Tasmannia aromatica, 494.

Taxineae, bast-fibres, 527 ;
crystals, 141,529; second-
ary changes, 480; secre-
tion, 102 ; sieve-tubes, 522.

Taxodium, cotyledons, 245.

Taxodium sempervirens,
vascular system, 379.

Taxus, 14, 1 18: annual

rings, 476; secretions,
137; secretory reservoirs,
441 ; vascular system, 246,
300, 308, 356, 379, 391,
475-

Taxus baccata, 13 : epider-
mis, 77, 79 ; medullary
rays, 489; periderm, 555.

Tecoma radicans, lenticels,
560; vascular system, 243;
wood, 580.

Tectona grandis, 484, 485,
496 ; pits, 163 ; silicifica-
tion, 103, 510.

Terebinthacese, secretory
reservoirs, 440, 525.

Terminology, remarks on,

591.

Tests for fibres, 133.

Testudinaria, cork, ii; se-
condary thickening, 618.

Testudinaria elephantipes,
622.

Tetragonella, 591.

Tetragonia, hair structures,
65.

Tetragonia echinata, 65.

Tetragonia expansa, 65.

Tetragonieae, 590, 591.

Tetrapterys, 577.

T etrazygia angustifolia,
hairs, 62.

Tetrazygia dissoluta, hairs,
62.

Tetrazygia elaeagnoides,
hairs, 62.

Teucrium, 70.

Thalia, 128, 212.

Thalia dealbata, air-spaces,
216, 218.

Thalictrum, 25: cortex,
404 ; medullary bundles,
248, 249 ; sclerenchyma,
420 ; vascular system, 3 2 2,
323.

Thalictrum aquilegifolium,
322.

Thalictrum flavum, 322.

Thallus, intramatrical, 384.

Thamnochortus, 425.

Theophrasta ornata, 418,
419.

Thesium, haustorium, 383 ;
stomata, 45.

Thinouia, 583, 589.

Thladiantha dubia, vascular
system, 248, 249.

Thryallis, hairs, 6r.

Thuja, 14, 118; periderm,
551 ; secondary growth,

475 ; secretory reservoirs,
442 ; stomata, 49 ; vascu-
lar system, 246, 356, 381.

Thuja gigantea, 246, 381.

Thuja occidentalis, 13, 118,
246 : wax, 83.

Thuja orientalis, wax, 83.

Thuja plicata, vascular sys-
tem, 246, 247 (Fig. 109).

Thujopsis, 379.

Thy loses, 170.

Thymus, hairs, 61, 64.

Thymus Serpyllum, stomata,
41-

Thymus vulgaris, glandular
scales, 95 (Fig. 39).

Tilia, 167 : annual rings,

476 ; bast, 522 (Fig. 212) ;
cork, no, 1 13; cortex,
404; crystals, 530 ; fibre,
1 3 1, 527; parenchyma,
537; periderm, 548, 558 ;
secondary growth, 472;
sieve-tubes, 175, 176, 523 ;
tracheides and fibres, 483,

Tilia argentea, 537.

Tilia parvifolia, 479, 537:
crystals, 142.

Tiliaceae, mucilage, 143.

Tillandsia, parenchyma, 41 1 ;
vascular system, 265, 266.

Tillandsia acaulis, 265.

Tillandsia usneoides, hairs,
64.

Tissue, arrangement of the
forms of, 224; cellular,
27 ; element, 3 ; fixed, 4 ;
form, 5 ; forms of in the

secondary wood, 478;
ground, 115 ; hypodermal,
225 ; parenchymatous, 27;
permanent, 4 ; pleuren-
chymatous, 27; primary
arrangement of, 224;
prosenchymatous, 27.

Tithymalus, 438, 439.

Tmesipteris, 348.

Toddalieae, oil-cavities, 207.

Todea, 118 : sclerenchyma,
426 ; vascular system, 280,
319, 367.

Todea africana, 280, 367.

Todea barbara, 118.

Todea hymenophylloides,
280, 367, 426, 427.

Todea rivularis, 54.

Tommasinia verticillaris,
water-pore, 52.

Tontelea, 587.

Tornelia, internal hairs, 222.

T ornelia fragrans, stone-
elements, 127; vascular
system, 352, 361.

Torreya, 31 : vascular sys-
tem, 246.

Torreya nucifera, 118.

Tracheae, 155, 478: pitted or
dotted, 156; position of,
226.

Tracheides, 155, 165: and
fibres, 483.

Tradescantia, hairs, 61 ; pa-
renchyma, 41 1 ; stomata,
40 ; vascular system, 270,
316, 327.

Tradescantia albiflora, vas-
cular system, 269 (Figs.
119, 120), 311, 316, 327,
391.

Tradescantia discolor, 67.

Tradescantia Lyonii, 327.

Tradescantia virginiana,
270: vascular system, 311,
359-

Tradescantia zebrina, 33,
270, 316, 327.

T ragopogon, laticiferous
tubes, 433; sieve-tubes,
231.

Trapa, air-spaces, 213, 214,
219; cortex, 405; crys-
tals, 220; vascular bun-
dles, 277, 341.

Trapa natans, 277 : petiole,
2 ; root-cap, 413.

Trevirania longifolia, vas-
cular system, 243.

Trianthema, 591.

Trichomanes, sclerenchy-
ma, 127; vascular sys-
tem, 343, 344, 364.

Trichomanes elegans, 344.

INDEX.

Trichomanes pinnatum,344.

Trichomanes radicans, 343.

Trichomes, 30, 54.

Trichotosia ferox, serial
roots, 229.

T rientalis, endodermis, 415;
sclerenchyma, 420 ; sieve-
tubes, 231.

Trifolium, 23 : vascular
system, 303, 305.

Triglochin maritimum, 352.

Trigonella, 354.

T rigonidium Egertonia-

num, serial roots, 230.

Triodia decumbens, sto-
mata, 50.

T riteleia, secretory sacs,
148.

Triticum, 54 : vascular sys-
tem, 359.

T riticum caninum, stomata,
50.

Triticum repens, endoder-
mis, 124; Stomata, 50.

Triticum vulgare, apex of
root, IO ; lamina, 407.

Tritonia deusta, 409.

T rochodendron aralioides,
494.

Trollius europseus, stomata,
49.

T ropseolum, medullary rays,
492 ; secondary thicken-
ing, 474; Stomata, 51;
vascular system, 236, 297,
305 ; water-pores, 51, 53*

Tropseolum Lobbianum,
water-pores, 52 (Fig. 19).

Tropseolum majus, end of
vascular bundle, 376 (Fig.
179); vascular system,
236, 239, 353; water-
pore, 53.

Tsuga canadensis, 419 : len-
ticels, 563 ; medullary
rays, 490.

Tsuga Douglasii, secretory
reservoirs, 442.

Tubes, 5.

Tubes, milk, 183.

T ulipa, vascular system,
277, 357, 359; wax, 87.

Tulipa Gesneriana, 337,359.

Tulipa sylvestris, 142.

Tupa Feuillei, laticiferous
tubes, 434.

Tupa Ghiesbrechti, 434.

Tupa salicifolia, 434.

Tussilago Farfara, oil-pas-
sages, 447.

Tyloses, see Thyloses.

Tynanthus, 574.

Typha, 10, 142, 265; air-
spaces, 213, 216, 217;

parenchyma, 409 ; scle-
renchyma, 418, 419, 422 ;
vascular system, 306.

Tyrimnus leucographus,
secretory sacs, 150.

Ulex europaeus, 497 : ligne-
ous bundle, 495 ; tra-
cheides and fibres, 483.

Ulmus, cork, no, 112;
crystals, 530 ; fibres, 529 ;
lenticels, 562, 566 ; muci-
lage, 143; periderm, 548,
558 ; secretion, 105, 106 ;
silica in leaves, 102 ;
thickening, 470, 472 ;

vascular system, 305 ;
water-pores, 5 r .

Ulmus campestris, 472, 566 ;
crystals, 142 ; fibres, 528 ;
pith, 403 ; secretion, 103 ;
water-pores, 51

Ulmus effusa, 1 12, 472, 558,
559-

Ulmus montana, 108.

Ulmus suberosa, 470, 496 :
cork, 550; gelatinous

layer, 482.

Umbelliferae, 537: air-
spaces, 214, 215; cortex,
404 ; intercellular spaces,
210 ; medullary bundles,
535 ; medullary rays, 491;
root, 517, 524; sap-pas-
sages, 448 ; secretions,

137; secretory reservoirs,
202, 526 ; stomata, 46 ;
vascular system, 242, 248,
253, 298, 309, 320, 324,
353 ; water-pore, 52.

Urania speciosa, 437.

Urceola, latex, 185.

Urena, fibres, 129.

Urena sinuata, glands, 95.

Urtica, 17 1 : cystoliths,

105; hair-structures, 65,
66, 68; root, 517; sili-
cification, 103 ; vascular
system, 236, 353.

Urtica dioica, fibre, 131 ;
secondary growth, 474
(Fig. 203) ; silicification,
103 ; stinging hairs, 102.

Urtica Dodartii, vascular
system, 236, 244, 308.

Urtica excelsa, 103.

Urtica lusitanica, 103.

Urtica macrophylla, cysto-
lith cells, 105 (Fig. 45).

Urtica macrostachys, hairs,
65.

Urticaceae, 32 : cystoliths,
103 ; hairs, 56, 60, 66, 68 ;

657

laticiferous tubes, 187,
439 ; secretion, 102.

Urvillea, anomalous struc-
ture, 583, 601, 605.

Urvillea Isevis, 584.

Utricularia, air-spaces, 213 ;
endodermis, 121 ; glands,
100 ; hairs, 62 ; vascular
system, 278, 370.

Utricularia vulgaris, cortex,
405.

Utriculi, 5.

Vaccinium Myrtillus, medul-
lary rays, 493 ; stomata,
48.

Vahea, latex, 185.

Valeriana, 145, 297 : vas-
cular system, 353.

Valeriana Phu, water-pore,
52.

Valeriana sambucifolia,
water-pore, 51.

Valerianella, 297.

Vallisneria spiralis, vascular
system, 370, 371.

Vanda, aerial roots, 228 ;
lamina, 407.

Vanda furva, root-sheath,
228, 229, 230; scleren-
chyma, 418 ; vascular
system, 302, 373.

Vanilla, cuticle, 76 ; root-
sheath, 227.

Vanilla aphylla, 227.

Vanilla planifolia, 227.

Vasa, 155: mixta, 156.

Vasconcellea, fibres, 527 ;
laticiferous tubes, 434 ;
medullary rays, 490.

Vasconcellea cauliflora, 434.

Vasconcellea microcarpa,
434-

Vasconcellea monoica, 434,
527.

Vascular bundle, axile, 277 ;
structure of, 316.

Vascular bundles, arrange-
ment of, 232 ; collateral,
319; concentric, 339;
connections, 385 ; course
of, 233 ; development of,
388; ends of, 371 ; Di-
cotyledonous type, 235 ;
imperfect, 366 ; of Coni-
ferae, 235; of Gnetaceae,
235 ; of the leaf-trace of
Dicotyledons, origin, 395;
bundles, origin of, 23 ;
origin of in Monocotyle-
dons, 396; radial, 348;
in a ring, 307 ; termino-
logy of, 401 ; various
types of development, 397.

u u

658

INDEX.

Vascular system, anomalous,
248 ; anomalous mono-
cotyledonous, 274 ; Com-
melinaceous type, 269 ; in
leaves, 296 ; of Palms;
263 (Fig. 118); Palm
type, 261.

Velamen, 227.

Venidium calendulaceum,
446.

Verbascum, hairs, 62, 70.

Verbascum phlomoides,
hairs, 62.

Verbena maritima, 470.

Verbenaceae, hairs, 61 ; sili-
cification, 103.

Verbesina gigantea,hairs,62.

Verbesina virginica, water-
pores, 51.

Verhuellia, medullary bun-
dles, 249, 278 ; vascular
system, 340.

Vernonia eminens, secre-
tory sacs, 150.

Vernonia flexuosa, 150.

Vernonia noveboracensis,
150.

Vernonia praealta, 150.

Vernoniaceae, secretory sacs,
149-

Veronica incisa, vascular
system, 243.

Veronica Lindleyana, 41 1.

Veronica speciosa,32o, 41 1.

Vessels, 155, 165.

Vessels, annular, 156.

Vessels, forms of, 478.

Vessels, length of, 168.

Vessels, mixed, 156.

Vessels, pitted, 479.

Vessels, reticulate, 156.

Vessels, scalariform, 158.

Vessels, spiral, 156.

Vessels, width of, 168.

Vessels, width of spring and
autumn, 503.

Viburnum Awafouki, pits,
71-

Viburnum Lantana, bast-
fibres, 526 ; crystals, 142 ;
periderm, 548 ; pith, 403.

Viburnum lantanoides, 548.

Viburnum Opulus, 470, 548 :
bast-fibres, 527 ; cork,
no; glands, 96 ; peri-
derm, 549 ; stomata, 48.

Viburnum Oxycoccos, 548 ;
crystals, 142.

Viburnum prunifolium, 548.

Viburnum Tinus, glands, 96;
pith, 403 ; secretion, 98.

Vicia Faba, glands, 95 ;
stomata, 38, 48 ;■ vascular
system, 353, 354.

Vicia sativa, glandular hairs,
95 ; vascular system, 354.

Vicia segetalis, stomata, 48.

Vicia sepium, glands, 95.

Victoria regia, 165 : endo-
dermis, 125; prickle-
pores, 54.

Villarsia, internal hairs, 222.

Villarsia parnassifolia, air-
spaces, 215, 218.

Villi, 54.

Vinca, fibres, 129; latici-
ferous tubes, 187, 194,
439) 525-

Vinca minor, 194: vascular
system, 243.

Vincetoxicum officinale,
root-structure, 516.

Viola, secretion, 98.

Viola elatior, vascular sys-
tem, 239, 296, 298.

Viola tricolor, stomata, 47.

Virgilia lutea, 496 : crystals,
142 ; periderm, 548 ;
phelloderm, 549.

Viscum, 70, 483, 485 ; bast-
fibres, 527 ; haustorium,
384; ligneous bundle, 496.

Viscum album, 77, 535:
stomata, 45.

Vitex incisa, 493.

Vitis, 166, 467, 479 : bast,
523 (Fig. 213); bor-
dered pits, 163 ; crystals,
141, 530; fibre.s, 134,

527 ; hair-structures, 65 ;
lenticels, 560 ; ligneous
bundle, 495 ; periderm,
555) raphides, 143 ; sieve-
tubes, 175, 176; thylo-
ses, 170, 171 ; vascular
system, 236, 324, 353.

Vitis vinifera, 308, 484 :

sieve-tubes, 173, 175, 176
(Figs. 69, 70) ; sieve-

tubes, &c., 178 (Figs. 74-
76); spiral vessels, 158;
vascular system, 241.

Water, excretion of, 50.

Water-plants, air-spaces in,
210.

Water-pores, 50 ; in sub-
merged parts, 53.

Water-stomata, 50.

Wax, extrusions, 83 ; im-
bedded, 82.

Wax-layers, development
of, 87.

Welwitschia mirabilis, ano-
malous structure, 614 ;
cross-section of leaf, 408
(Fig. 187) ; crystals, 141 ;
fibres, 133; sclerenchy-

ma, 132 (Fig. 55), 420,
422, 424 ; secretion, 102,
144; vascular bundle, 335
(Fig. 157) ; vascular sys-
tem, 248, 3*2^) 325) 372,
382 ; vascular system in
leaves, 303 (Fig. 145).

Widdringtonia, vascular
system, 246.

Widdringtonia cupressoides,
secretory passages, 443,
526.

Widdringtonia juniperina,
246.

Wigandia, hairs, 60.

Willdenowia, 425.

Wintera, 25.

Wintereae, medullary rays,
490, 494 ; secondary

changes, 480.

Wistaria sinensis, anomal-
ous wood, 589 ; lenticels,
560.

Wolffia, 142, 366.

Wood, 475 ; classification
of the elements, 487 ; dis-
tribution of the tissues in,
488 ; heart, 508 ; inter-
calary, 569 ; of dicotyle-
donous roots, 516; sap,
507 ; successive zones of,
and annual rings, 504 ;
ripe, 507.

Woodwardia, vascular sys-
tem, 285.

Xanthochymus, resin pas-
sages, 451.

Xanthochymus pictorius,
secretions, 145.

Xanthosia, 498.

Xanthosia rotundifolia, 449,
458.

Xanthosoma, 436.

Xeranthemum cylindra-
ceum, secretory reser-
voirs, 446, 447.

Xerotidese, 10.

Xylem, 317, 321.

Xylosteum, 309.

Xyris, vascular system, 327.

Yucca, sclerenchyma, 422 ;
vascular system, 264, 323.

Yucca aloifolia, 618, 620.

Yucca filamentosa, 323, 407.

Zamia, mucilage-passages,
441 ; parenchyma, 410 ;
sclerenchyma, 419 ; vas-
cular system, 337, 357.

Zamia furfuracea, 357.

Zamia integrifolia, 13, 410,
419.

INDEX.

659

Zamia longifolia, 337, 441.
Zamia muricata, 357, 610.
Zanichellia, 277, 278, 368.
Zanichellia palustris, vas-
cular system, 272, 277.
Zanthoxyleae, oil-cavities,
207.

^^anthoxylon fraxineum,
cork, no, in; wood,

509-

Zea Mais, 54, 412 : annular
vessel, 157 (Fig. 56) ; apex
of root, 10 (Fig. 3); col-
lateral bundles, 330 (Figs.
150,151); sclerenchyma,
422 ; stomata, 44 ; vas-
cular bundles, 373 (Figs.
174, 175); vascular sys-
tem, 267, 302, 311, 359,
371-

ZingiberacesE, 10, 108 :

crystals, 142 ; secretions,
145 ; secretory reservoirs,
136; vascular system, 267.

Zinnia elegans, 447.

Zostera, air-spaces, 217 ;
vascular system, 275, 368.

Zostera marina, 275.

Zygopetalum Mackai, aerial
roots, 228.

THE END.

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