Organography of plants, especially of the Archegoniata and Spermaphyta

Part I General Organography

WITH 130 WOODCUTS

Oxford

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Preface To The German Edition

If the botanical historian should ever write an account of that branch of the Botany of our times which is usually termed Morphology, he will probably look upon the period embraced within the last few years of the present century as one of transition. The conspicuous feature of transitionperiods is the suppression of directive influences which have been dominant for a time because they are found to have served their purpose and lead no further. Newer views naturally conflict with the old and even with one another ; things are found not to be so simple as was believed, and the old formularies cease to be applicable.

This change which has been in progress in Morphology is a consequence of the recognition by botanists that the configuration exhibited by plants is a part of their life-phenomena, and is not merely a hypothetical construction as it was made by the older idealistic Morphology. All the phenomena of life have a definite relationship to environment, and therefore, as I shall endeavour to show in this book, the consideration of the configuration of the organs of plants is not merely a comparative historical criticism, but must take into account all the conditions of environment which we find at the present day. Morphology has to determine in what degree the formation of organs shows an adaptation to external relationships, and to what extent it is dependent upon these and upon internal conditions ; and even if the subject be studied from the purely phylogenetic standpoint, such determinations are a necessity, inasmuch as the historical development would be constantly influenced by environment. Phylogenetic speculations are doubtless more attractive than the grappling with the facts, often obscure and apparently insignificant, of the relationships of configuration of the plants around us. It appears to me, however, that to recognize the factors which bring aj^out the development of say a leaf with one side larger than the other is infinitely more important than to construct a phylogenetic hypothesis unsupported by facts.

VI

PREFACE TO THE GERMAN EDITION

Hofmeister, Sachs, and Herbert Spencer have contributed in an especial degree during the later decades to the advancement of this modern line of study which we may designate Organography. It is yet in its infancy ; a wide and little explored, yet promising field lies before us. The greatest difficulty I have found in preparing this work has been in stating the problems correctly in the part which is now issued dealing with the general aspect of the subject ; and it is only after some hesitation that I have made the attempt. Although it does not claim to be an exhaustive treatment of the subject from every point of view, yet if it furnishes material for a subsequent treatise on general organography its appearance may be of use, and the more so, if it shows that the same problems are repeated in the most different cycles of affinity. This of course cannot be brought out so clearly in a special account of the different groups.

The object of the book may be best gathered from its contents. I may here only point out that it deals with the Archegoniatae and Spermaphyta alone ; such mention as there is of the Thallophyta will be found in the general part, and there only for comparison with reference to these groups. I have not attempted completeness, and this is the case particularly in the statements of opinions different from my own ; these are not referred to further than appeared to be necessary for the immediate purpose in view. The results of many investigations which have not yet been published have been made use of throughout this volume.

The authors — Strasburger, Noll, Schenk, Schimper — of the Textbook of Botany which has appeared through the same publishing-house as this book have granted the use of a number of figures ; when these are original they are indicated by ‘ Lehrb.’

Dr. Arthur Weisse has prepared the account of the mechanical hypothesis of the position of leaves and he alone is responsible for the statements in it.

An index to the whole work will appear in the second part ; in this first part an extended list of contents is given.

K. Goebel.

Ambach,

September I, 1897.

Prefatory Note To The English

EDITION

This edition of Professor Goebel’s interesting work brings within reach of English students the only book of recent years upon the subject of Organography of Plants, and is a valuable addition to the series of standard botanical books issued from the Oxford Press. Some of the interpretations and views to which Professor Goebel gives expression may not find unqualified acceptance, but the criticisms they will evoke and the experimental research which is sure to be stimulated by the Author’s suggestive statement of problems requiring solution should further in no small degree the progress of our knowledge of the phenomena of life exhibited by plants.

The portion of the Special Part of the Book dealing with the Bryophyta, recently published in Germany, has been translated and will be issued along with a translation of the concluding portion of the volume when this has been completed by the Author.

Professor Goebel has himself read the English text and added a note.

My assistants, Mr. J. H. Burrage and Mr. H. F. Tagg, have been so kind as to undertake the labour of checking the references in the volume.

Edinburgh,

December , 1898.

I. B. B.

FIRST SECTION.

GENERAL DIFFERENTIATION OF THE PLANT-BODY.

PAGE

I. Introduction. Morphology and Organography .... 3-13

Definition of Morphology 3

Relation to Physiology 4

Views of Metamorphosis ......... 5

Transformations and arrest 6

Metamorphosis as a change of function 6

II. Differentiation K)F Organs in the Spermaphyta .... 13-21

Inefficiency of definitions based only on characters of form . 13

Form is only one superficial and inconstant character ... 13

Definition of organs must be based upon all their characters and

their function must not be overlooked 14

Homologous and analogous organs 14

Homologous organs are not always of common genetic origin . . 17

Polyphyletic divergent development from simple primary forms . . 19

Descendant forms with like developmental capacity .... 19

III. Formation of Organs and Division of Labour amongst the

Lower Plants (Thallophyta) 21-40

Many paths to higher differentiation from simple beginnings . . 22

Differentiation of root and shoot only a special case ... 22

Unicellular and pluricellular plants 22

Monergic and polyergic cells 23

Cell-colonies and cell-dominions 24

I. Colonies or coenobia 25-32

A. Colonies which are not fixed * . . . . . 25

1. Colonies of naked energids 25

2. Colonies of energids invested by a membrane . . 26

a. Protococcaceae 26

b. Volvocineae ........ 27

X

PAGE

B. Fixed colonies ......... 29

Apiocystis 29

Diatomaceae . 29

Licmophora 30

Hydrurus 30

II. Cell-dominions with vegetative points . . . . ' . 33-40

Regular laying down of lateral organs 34

Long shoots and short shoots in different cycles of affinity . 35

Sphacelarieae 36

Florideae 38

IV. Normal Formation of Organs at the Vegetative Point and

Regeneration 41-51

Normal sequence of formation of organs 41

Regeneration, its meaning 42

I. Regeneration of the vegetative point 43-44

In roots 43

In prothalli of ferns 43

II. New formation of organs in regeneration 44-51

Replacement of lost parts 44

Polarity in new formations 44

Sachs’ hypothesis of ‘material and form’ .... 45

Influence upon severed parts of their previous organic

relationships . 45

In Spermaphyta ......... 45

In Pteridophyta 46

In Bryophyta ......... 47

V. Concrescence and Arrest 51-61

Introduction 51

Value of these in comparative organography .... 51

A. Concrescence 52-55

Developmental .......... 52

Congenital 53

Spathiphyllum 55

B. Arrest 56-61

1. Kinds and manner 56

Abortion 56

Suppression 56

2. Causes of arrest 58

a. Through correlation . 58

b. Through loss of function ....... 58

3. Morphological importance of arrested organs .... 60

TABLE OF CONTENTS xi

SECOND SECTION.

RELATIONSHIPS OF SYMMETRY.

PAGE

I. Introduction 65-73

Polar construction 65

Radial, bilateral, and dorsiventral organs 66

Relationship between symmetry and direction of organs ... 67

Orthotropous and plagiotropous organs 67

Alternation of orthotropous and plagiotropous growth ... 68

Dorsiventrality and plagiotropous growth 70

Radial construction and orthotropous growth 70

Illustrations from lichens 71

II. Position of Organs on Radial Axes 73-8 4

Sketch of the mechanical hypothesis of leaf-position. By Dr. Arthur

Weisse 7 4-84

III. Dorsiventral Shoots 84-114

1. Different structure of the upper and under side .... 84

Epinasty and hyponasty . 85

2. Relationships of position 85-99

Their connexion with the conditions of life ..... 86

A. Creeping and climbing shoots . 90-92

Flattening of their axes ........ 92

B. Dorsiventral lateral shoots 92-99

Brought about by —

1. Change of position of leaves ...... 92

a. In the development 92

(a) By change of the leaf-insertion .... 93

(b) By torsion of the internode or the leaf-base . 93

(c) From the beginning at the vegetative point . 94

b. Hereditary 95

2. Anisophylly 99

C. Anisophylly 99-114

Has varying degrees ......... 99

Historical sketch 99

A. In Musci 100

C. In Pteridophyta 102-107

Lycopodium 102

Selaginella 105

D. In Spermaphyta 107-113

a. Lateral anisophylly . . . . . . . 108

b . Habitual anisophylly 109

1. In Urticaceae 109

2. In Melastomaceae Ill

3. In Acanthaceae 112

4. In Gesneriaceae ....... 113

Conclusions regarding anisophylly 113 xn

PAGE

IV. Relationships of Symmetry of Leaves 114-128

Asymmetry in dorsiventral leaves ....... 114

1. Cotyledons 115

2. Foliage-leaves 116

A. Asymmetry of entire leaves 116-122

Relation to direction of leaf 116

Leaves of Begonia . . . . . . . . n 7

Inequality of sides of compound leaves . . . 120

B. Asymmetry and unequal size of leaflets .... 122-128

a. Asymmetry of leaflets ...... 122

Not caused by gravity 123

Relations to light 124

Asymmetry of stipules 125

b. Leaflets of unequal size 126

Interruptedly pinnate leaves 127

V. Relationships of Symmetry of Flowers and Inflorescences . 128-138

Radial and dorsiventral flowers 128

Asymmetry of dorsiventral flowers 128

Relationship of symmetry of flower to inflorescence ... 128

Flowers which become dorsiventral in development . . . 129

Flowers which are dorsiventral from the beginning . . . 130

a. Essentially ‘ zygomorphous ’ flowers 130

b. Unessentially ‘zygomorphous’ flowers . • . . 130

Origin of dorsiventral ity in flowers 131

2. Inflorescences 134-138

Radial and dorsiventral 134

Inflorescences which become dorsiventral in development . . 134

Inflorescences which are dorsiventral from the beginning . . 134

Biological relationships of inflorescences 134

1. In anemophilous plants 134

2. In entomophilous plants 135

THIRD SECTION.

DIFFERENCES IN THE FORMATION OF ORGANS AT DIFFERENT DEVELOPMENTAL STAGES.

JUVENILE FORMS.

Introduction 141-148

Duration of development depends upon internal and external causes 141

Limitation of development in severed cells or cell-masses often a

consequence of their origination 141

Unfavourable external conditions often shorten the development . 142

Homoblastic and heteroblastic development 143 xiii

PAGE

Juvenile and adult forms adapted to different relationships in

heteroblastic development 143

Juvenile form is primitive or is derived 145

Primary leaves often arrested formations 145

Varying duration of juvenile form 145

Illustrative Examples 148-170

1. Thallophyta 148-150

Lemanea 148

Batrachospermum 149

Dumontia filiformis ......... 149

Polysiphonia Binderi ......... 150

Sphacelarieae 150

2. Bryophyta 15 1

3. Pteridophyta 1 51-15 3

Equisetineae 151

Lycopodineae 151

Filicineae 151

4. Gymnospermae 1 53—1 5 5

Cycadeae 153

Pinus 153

Larix 154

Cupressineae 154

Phyllocladus 155

Sciadopitys 155

5. Angiospermae 1 55-1 70

A. Climbing plants 157-164

1. Root-climbers 157

2. Plants with tendrils 161

B. Aquatic and marsh plants 164-165

C. Xerophilous plants 165-170

1. Acacias forming phyllodes 166

2. Eucalyptus 167

3. Dicotyledonous plants with cupressoid habit . . . 167

4. Plants with arrested leaves 167

Summary 170

Reversions experimentally produced by changes of conditions . . 171

Reversion associated with definite developmental stages . . . 171

Reversion in Bryophyta 17 1

Reversion in Spermaphyta 172

Conclusion of the Development 174

XIV

FOURTH SECTION.

MALFORMATIONS AND THEIR SIGNIFICANCE IN ORGANOGRAPHY.

PAGE

Introduction 1 77-179

Limitation of the notion of malformation 177

I. Significance of Malformations in Organography . . . 179-183

Malformations not without a rule 179

Abortion of sporangia in phyllody of stamens and carpels . . 179

Such structure not atavistic, but diseased 183

II. Etiology of Malformations 184-200

A. Many malformations can be inherited and may be latent until

definite external conditions evoke them 184-187

1. Fasciation 185

2. Obligate torsion 186

3. Sterility of Maize 186

5. Other cases 187

B. Malformations caused by external conditions .... 187-200

In Fungi 187

In Algae 188

In Spermaphyta 188

Peloria. Peyritsch’s observations 188

Changes in flowers 191

Effects of attack of fungi 192

Effects of attack of insects ....... 193

Double flowers .......... 193

Formation of galls 193

III. Significance of Malformations in the Theory of Formation of

Sachs’ hypothesis of material and form ....... 201

Growth-enzymes 202

FIFTH SECTION.

THE INFLUENCE OF CORRELATION AND EXTERNAL FORMATIVE STIMULI UPON THE CONFIGURATION

OF PLANTS.

Introduction 205

I. Correlation 206

Notion of correlation 206

Quantitative and qualitative influence of correlation .... 207 xv

PAGE

1. Quantitative influence of correlation or compensation of growth 207-214 The struggle for existence the cause of abortion of flowers and

fruits 207

Influence of the sprouting of buds 208

Influence of the size of leaves and leaflets .... 209

Influence of the size of the parts in the flower . . . 21 1

Correlation of vegetative and reproductive organs . . 212

2. Qualitative influence of correlation 2 14-2 17

Relationship of direction 214

Chief roots and lateral roots 214

Chief shoots and lateral shoots . . . . . 214

Aerial and subterranean shoots . . . . . 215

Formation of thorns 215

Configuration of leaves 215

Sporophylls 215

Bud-scales . . . 216

II. Influence of External Stimuli 217-270

A. Influence of gravity 219-226

1. Its relation to the disposition of organs 219-224

In prothalli of Pteridophyta 219

In embryo of Marsilia .... .... 220

In Thladiantha 221

In regeneration 221

In the whole plant 222

2. Qualitative influence of gravity 224-226

The shoots of the rhizome in Yucca and Cordyline . . 224

Difference between chief shoot and lateral shoots . . 225

B. Influence of light 227-259

1. Directive influence of light 227-237

Dorsal and ventral sides of dorsiventral organs . . . 227

Gemmae of Marchantieae 227

Germ-plants of thallose dorsiventral liverworts . . . 228

Prothalli of ferns ; apogamous shoots 228

Polar differentiation in germination of spores of Equisetum

and some Fucaceae 229

Branch-system of Cupressineae 230

Rhizome of Nuphar 231

Distribution of roots in Ivy, Cacteae, and others . . 231

Formation of tubers in potato 232

Branching of plagiotropous mosses 232

Position of leaves in dorsiventral Jungermannieae . . 234

Dorsiventral sporogonia 236

Branching in Algae 237

2. Qualitative influence of light 238-259

(1 a ) Developmental stages in relation to light . . . 238-245

In Algae 238

In Bryophyta 239

Hepaticae . 239

Musci 241

XVI

PAGE

In Pteridophyta 241

In Spermaphyta 242

Vegetative organs 242

Leaves of Sagittaria 242

Leaves of Campanula 242

Reproductive organs 242

Formation of flower dependent on light . . 242

Flattening and increase of surface of organs in relation

to light 245-250

In roots of Orchideae 246

In roots of Podostemaceae 247

In shoots of Cacteae and other plants . . . 247

In Pteridophyta 249

In Bryophyta ........ 249

In Algae 249

(c) Anisophylly in relation to light 250-255

In Abies pectinata 250

In Lycopodium complanatum ..... 252

In Goldfussia and other dicotyledonous plants . . 253

(A) Change of function through light 255-257

In Circaea 256

In Algae 256

(e) Influence of light upon the relationships of configuration

of the Fungi 257-259

C. Influence of the medium 259-268

1. Influence of water and air-moisture 260

A. Aquatic and amphibious plants 260

B. Induction of resting states through drought . . . 261

C. Formation of tubers in J uncus and in Poa bulbosa . 262

D. Formation of thorns and prickles 263

E. Succulence of the leaf 264

2. Halophilous plants 265

3- Fungi 266

D. Mechanical stimuli 268-270

FIRST SECTION r.

INTRODUCTION. MORPHOLOGY AND ORGANOGRAPHY.

The botanical text-books of the day endeavour to arrange all that we know about plants in three sections— Morphology, Anatomy, and Physiology. The first two of these are sometimes combined as the morphology of the external and the internal members of plants. This however runs counter to the original signification of the word ‘ Morphology.’ We owe the term morphology to Goethe. He says 1 : ‘ Scientific men in all time have striven to recognize living bodies as such, to understand the relations of their external visible tangible parts, and to interpret them as indications of what is within, and thereby in some measure to gain a comprehensive notion of the whole. . . . We find therefore in the march of art, of knowledge, and of science, many attempts to found and construct a doctrine which we may name the morphology l It is quite evident then that morphology does not deal merely with the distinction and the naming of the outer parts of plants, although this, which really belongs to terminology* has been in part incorrectly called morphology. Morphology does demand the knowledge of the different appearances of the members of the plant-body, but only as a means to an end ; it requires, not isolated facts, but the relation of facts to one another.

Terminology can be based on the study of dried plants, but morphology has, as Goethe stated, to do with Giving bodies/ which are

Goethe, Bildung und Umbildung organischer Naturen.

involved in changes of a fixed character and are subjected to the influences exercised upon them by the outer world ; it has, in other words, to do with that part of life-phenomena which finds expression in the external configuration.

The life-phenomena exhibited by plants are usually regarded as within the province of physiology, and the distinction which is drawn between physiology and morphology is, that physiology has to do with the functions of the organs of plants, whilst morphology takes no heed of these, but is rather a comparative and phylogenetic study. Sachs x, for example, says : ‘ The parts of plants which we usually speak of as their organs, and which vary so much in form and serve such different physiological purposes, may be considered scientifically from two points of view. We may look at them as adapted by their form and structure for the carrying on of certain physiological processes, and in this case we regard them as agents for work — as organs. Such considerations are a part of physiology. On the other hand we may look at them apart altogether from these functions, and seek to determine where and how they arise, and in what relationships, both of space and time, the origin and the growth of one member2 stand to those of another. This method falls within the province of morphology/

A distinction of this kind is however, as Sachs expressly states, artificial and imperfect, and it may only be maintained so long as it does good service. As a matter of fact, it has finally led to one-sidedness, and its outcome has not infrequently been empty theorizing. In nature the form and function of an organ stand in the most intimate relation to each other ; one is caused by the other. I take exactly the same view as is expressed by Herbert Spencer 3, whose work is far too little valued by botanists. He says: ‘Everywhere structures in great measure determine functions ; and everywhere functions are incessantly modifying structures. In nature, the two are inseparable co-operators ; and science can give no true interpretation of nature, without keeping their co-operation constantly in view. An account of organic evolution, in its more special aspects, must be essentially an account of the inter-actions of structures and functions. . . /

The title of the present volume is based upon the idea so aptly

1 Sachs, Lehrb. der Botanik, ed. 4, p. 15 1. I cite this because it expresses clearly a conception that is even now widely prevalent. It is well known that Sachs himself changed his view. See for this his ‘ Lectures on Physiology,’ and his 1 Gesammelte Abhandlungen.’

2 The word member (Glied) is the common term for an external organ (Organ) of plants and animals. There are no members which are not organs, leaving out of consideration of course cases of arrest. I am at a loss then to understand such a sentence as this : ‘ The morphology of plants knows no organs (Organe) but only members (Glieder) of the plant-body.’ (Strasburger-, Lehrb. ed. 2, p. 7.)

3 Herbert Spencer, Principles of Biology, vol. ii. p. 4.

INTRODUCTION . MORPHOLOGY AND ORGANOGRAPHY 5 conveyed in these words, and the book which I wrote some sixteen years ago was likewise designedly entitled £ The Comparative History of Development of the Organs of Plants1/ In the following pages the organs of plants are considered as being what they really are, organs or agents of work, but I have not gone into the details of their functions, I have only indicated in a general way the interdependence of form and function.

The first question to which we have to find an answer is : How came it that the functions of organs were entirely divorced from morphology ? It is, and rightly so, one of the fundamental declarations of this study that the function of an organ tells nothing about its £ morphological significance/ or, in other words, the same function may be performed by organs of very different morphological value ; £ homologous ’ organs must be distinguished from £ analogous ’ ones 2. The tendrils of the vine and of the passion-flower, for example, are shoot-axes whose leaves are entirely or almost entirely suppressed ; but the tendrils of the Leguminosae and of other plants, although like in form and function to those of the vine and passion-flower, are transformed leaves ; the tendrils in the two cases are analogous, they are not homologous.

This knowledge is one of the weightiest acquisitions of morphology, but at the same time it has been the cause of an incorrect generalization. Because organs of like morphological significance may take on different functions, the functions which they perform have been considered as of subordinate importance, and therefore of no moment in the determination of the characters of the organs ; hence it has been concluded that they must be entirely neglected in the grouping of the different members of plants in general categories.

This conclusion is erroneous, as will be briefly explained hereafter. If has led to an untenable position, especially in that fundamental problem of morphology which from the time of Goethe has been styled the £ Doctrine of Metamorphosis/ By this we understand the fact, that manifold as are the organs of plants, they can be referred back to a few £ fundamental forms 5 through whose £ transformation ’ the many and different members of the plant-body have arisen.

When we inquire how these primary forms and their transformations have been represented to us, we meet with different conceptions on the part of those authors who have taken pains to reflect upon the idea with which they dealt. In the idealistic morphology, as it was expounded by Goethe, A. Braun, and Hanstein, the doctrine of metamorphosis concerned itself

1 Vergleichende Entwicklungsgeschichte der Pflanzenorgane, forming vol. iii. pt. 1 of Schenk's Handbuch der Botanik.

2 See afterwards on pages 18, 19.

with an essentially theoretical construction, as I have elsewhere explained1. Goethe himself has plainly stated his view as follows : ‘ That which according to our idea is equal, may in reality appear either as equal or as similar, or indeed as completely unequal and dissimilar ; this is the essence of the pliant life of nature.’ In somewhat other form this idealistic notion has been preserved, inasmuch as the history of development was raised by the labours of K. F. Wolff, R. Brown, and Schleiden, to the rank of one of the most important aids to organography. The view which I have called the ‘ Differentiation theory ’ is based, as indeed is the whole of the doctrine of metamorphosis, on the study of the transformations of leaves, the manifold character of which is well known. Had its foundation been the transformation of the root instead of the leaf, the general notion of metamorphosis as merely a hypothetical transformation would have been replaced by the conception of it as an actual transformation — a view which, for many years, and in the face of active contradiction, I have endeavoured to establish. The differentiation theory assumes that at the vegetative point of the shoot indifferent primordia arise which are capable of development according to the needs of the plant in manifold ways, but have this in common — they are ‘ leaves ’ ; the other view assumes a real transformation of a primordium in such a way, that, for example, the primordium of a foliage-leaf, instead of developing actually into a foliage-leaf, can become in the mature condition a leaf of quite a different character, a scale-leaf, say, or a sporophyll ; similarly, the primordium of a stamen might become a petal. It will be seen that here a longer or shorter portion of the path of development is the same in the two primordia, — for instance, of a foliage-leaf and a scale-leaf. We speak of the scale-leaf as a transformation of the foliage-leaf, because in the juvenile condition we often find it to be possessed of parts which unfold themselves in the foliage-leaf, but become arrested in the scale-leaf ; and further, we can experimentally hinder this transformation. The point is so clear in a case such as this that I will briefly illustrate it by an example2 3. Fig. i shows, at /, the outline of a foliage-leaf of Acer platanoides, and at //, the outline of a bud-scale. The two are outwardly very different structures. The foliage-leaf consists of the blade, Z, of the stalk, 5, and of the very small leaf-base, G ; the bud-scale shows no differentiation, nevertheless it is nothing else but the transformed primordium of a foliage-leaf. If we examine more closely, and under some magnification, the small black tip of the bud-scale we find that it possesses a minute leaf-blade, or rather the rudiment of this,

1 Vergleichende Entwicklungsgeschichte der Pflanzenorgane. Schenk’s Handbuch der Botanik iii.

3 See Goebel, Beitr. z. Morphologie und Physiologie des Blattes, in Botan. Zeitung, 1880, p. 753.

INTRODUCTION. MORPHOLOGY AND ORGANOGRAPHY 7 which has not developed further, but has died off. This interesting fact is explained when we follow the history of development of the foliage-leaf, which need only be looked at here in its coarsest features. At a certain stage of development of the foliage-leaf its form is something like what is shown at Fig. 1, IV. We recognize clearly the rudiment of the leaf-blade, Z, and its lobes are already visible ; the leaf-stalk has not yet appeared, but it will arise by the elongation of the zone lying between the leaf-blade, Z, and the leaf-base, G . This stage of development is found in both foliage-leaf and bud-scale. But after this a change sets in : the primordium of the blade, Z, dries up, the leaf-stalk is not developed, whilst the leaf-base grows considerably and itself forms the scale-leaf from which we started.

We are able, as I have already stated, to control experimentally the development of these leaf-primordia ; one that would on account of its position develop normally as a scale-leaf we can force to become a foliage-leaf, and the converse is also true. It happens also, and this will be shown in the special part of this book, that the primordium of the blade may become arrested at a late stage of development. In Brownea erecta, for example, the buds are protected by erect dried-up foliage-leaves, the pinnules and other parts

of which are clearly formed. It may frequently happen that the transformation takes place much earlier and is then of course no longer capable of direct proof, but only to be concluded upon comparative grounds. When now the differentiation theory says — the primordium of a scale-leaf and of a foliage-leaf of Acer are both ‘ leaf-primordia ’ which may sometimes develop in this way and sometimes in that, it entirely overlooks the point that the notion of ‘leaf’ is a purely abstract one, that it is an artificially constructed category which has no concrete existence. What we see is foliage-leaves, scale-leaves, tendril-leaves, sporophylls, &c. That which these organs have in common , and which we endeavour to fix through a general idea, must be something else than their origin from c leaf-primordia.’ Seeing that if there are no leaves 1 there can be no primordia of leaves, either the primordia of foliage-leaves, scale-leaves, &c., are different from the outset, or they are alike for a time and then become different, and there must therefore be an actual transformation, a change in the course of development of one of these primordia out of which then the others can develop. That the primordia of the organs at the vegetative point are not of an indifferent nature, that they do not consist merely of embryonal tissue which is capable of development in any direction, is shown by the fact that the primordia of the leaves and of the lateral shoots are different from one another even at the moment when they appear as unmembered papillae upon the surface of the vegetative point.